GEOLOGY OF THE ARABIAN PENINSULA Sedimentary Geology of Saudi Arabia
U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 560-D Geology of the Arabian Peninsula Sedimentary Geology of Saudi Arabia
By R. W. POWERS, L. F. RAMIREZ, C. D. REDMOND, and E. L. ELBERG, JR.
U.S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 560-D
A review of the sedimentary geology of Saudi Arabia as shown on USGS Miscellaneous Geologic Investigations Map I 2JO A j" Geologic Map of the Arabian Peninsula
UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1966 DEPARTMENT OF THE INTERIOR DONALD PAUL HODEL, Secretary
U.S. GEOLOGICAL SURVEY Dallas L. Peck, Director
First printing 1966 Second printing 1985
For sale by the Distribution Branch, U.S. Geological Survey 604 South Pickett Street, Alexandria, VA 22304 FOREWORD This volume, "The Geology of the Arabian Peninsula," is a logical consequence of the geographic and geologic mapping project of the Arabian Peninsula, a cooperative venture between the Kingdom of Saudi Arabia and the Government of the United States. The Arabian- American Oil Co. and the U.S. Geological Survey did the fieldwork within the Kingdom of Saudi Arabia, and, with the approval of the governments of neighboring countries, a number of other oil companies contributed additional mapping to complete the coverage of the whole of the Arabian Peninsula. So far as we are aware, this is a unique experiment in geological cooperation among several governments, petroleum companies, and individuals. The plan for a cooperative mapping project was originally conceived in July 1953 by the late "William E. Wrather, then Director of the U.S. Geological Survey, the late James Terry Duce, then Vice President of Aramco, and the late E. L. deGolyer. George Wadsworth, then U.S. Ambassador to Saudi Arabia, and Sheikh Abdullah Sulaiman, then Minister of Finance of the Government of Saudi Arabia, lent their support to the plan. In November of the follow ing year, 1954, Director Wrather approved the U.S. Geological Survey's participation and designated G. F. Brown responsible for the western Arabian shield region in which he had previously worked under U.S. foreign-aid programs. In January 1955 F. A. Davies, Chairman, Board of Directors, Arabian-American Oil Co., approved Aramco's participation and appointed the late R. A. Bramkamp, chief geologist, responsible for compilation of the area within the Kingdom where the sediments crop out. This responsibility fell to L. F. Ramirez following the death of R. A. Bramkamp in September 1958. R. A. Bramkamp and G. F. Brown met in New York in February 1955 and planned the program, including scales of maps, areas of responsibility, types of terrain representation, and bilingual names. Thus there was established a cooperative agreement between the King dom of Saudi Arabia, the U.S. Department of State, and the Arabian-American Oil Co. to make available the basic areal geology as mapped by Aramco and the U.S. Geological Survey. The agreement specified publication of a series of 21 maps on a scale of 1:500,000, each map covering an area 3° of longitude and 4° of latitude. Separate geologic and geographic versions were to be printed for each of the quadrangles; both versions were to be bilingual in Arabic and Enblish. A peninsular geologic map on a scale of 1:2,000,000 was to conclude the project. High-altitude photography, on a scale of 1:60,000, of the Kingdom of Saudi Arabia was initiated during 1949 by the Aero Service Corp. and completed in 1959. Both third-order vertical and horizontal control and shoran were utilized in compiling the photography. This controlled photography resulted in highly accurate geographic maps at the publication scale which then served as a base for the geologic overlay. The topography of the sedimentary areas was depicted by hachuring and that of the shield region by shaded relief utilizing the airbrush technique. The first geographic quadrangle was published in July 1956 and the last in September 1962. While preparation of the geographic sheets was in progress, a need arose for early publication of a l:2,000,000-scale peninsular geographic map. Consequently, a preliminary edition was compiled and published in both English and Arabic in 1958. The second edition, containing additional photography and considerable new topographic and cultural data, was published in 1963. The first of the geologic map series was published in July 1956 and the final sheet in early 1964. The cooperative map project was completed in October 1963 with m IV FOREWORD the publication of the 1:2,000,000-scale "Geologic Map of the Arabian Peninsula" (Miscel laneous Geologic Investigations Map 1-270 A). As work on the quadrangles progressed, geologist, companies, and governments working in areas adjacent to the Kingdom of Saudi Arabia were consulted by Aramco and invited to participate in the mapping project. The number of cooperating participants was expanded to 11, which included the operating oil companies in the peninsula and which are identified elsewhere in this text; the Overseas Geological Surveys, London; the Government of Jordan; F. Geukens, who had worked in Yemen; and Z. R. Beydoun, who had studied the Eastern Aden Protectorate. With the close cooperation of the authors, the new data were added to data already plotted on the base map of the Arabian Peninsula. As the geological coverage of the peninsular map grew, the need for a text to accompany the map became apparent to both the U.S. Geological Survey and the Aramco geologists. Exploratory conversations were begun by Aramco with companies working in the other countries of the Arabian Peninsula for their participation in the preparation of a monograph on the geology of the Arabian Peninsula. Each author prepared a description of the geology of the area for which he was responsible, as shown in the sources of geologic compilation diagram on the peninsular map. The U.S. Geological Survey undertook the publishing of the volume as a professional paper, and the Government of Saudi Arabia was to finance its printing. It was early agreed that there would be no effort to confine the contributions to a standard format and that no attempt would be made to work out an overall correlation chart other than shown on the "Geologic Map of the Arabian Peninsula." Thus, the individual style of authors of several nationalities is preserved. Cooperation and relations have been of the highest order in all phases of the work. The project would not have been possible without the full support of the U.S. Department of State, the Kingdom of Saudi Arabia, and all contributors. In fact, the funds which made publi cation of this volume possible were contributed the Saudi Arabian Government. The data provided by the maps and in the professional paper provide information for an orderly scientific and economic development of a subcontinent.
0. A. SEAGER, Arabian-American Oil Co. (Retired).
W. D. JOHNSTON, JR., Former Chief, Foreign Geology Branch, Z7.S. Geological Survey,. CONTENTS
Page Page Foreword.______iii Stratigraphy Continued Abstract ______Dl Jurassic System Continued Introduction. ______2 Tuwaiq Mountain Limestone Upper Jurassic. . D49 History of exploration in Saudi Arabia ______2 Hanifa Formation Upper Jurassic.____--_--- 53 History of Arabian map project.______4 Jubaila Limestone Upper Jurassic ______57 Acknowledgments _------___-____---______5 Arab Formation Upper Jurassic _ _ _ - 60 Stratigraphy.--_--_------______-_ 5 Hith Anhydrite Upper Jurassic.______65 Outline of major stratigraphic divisions.______5 Jurassic and Cretaceous Systems - ___------66 General features--_----___-__-_-_____-______5 Sulaiy Formation Upper Jurassic(?) and Lower Lower Paleozoic clastic rocks..______6 Cretaceous(?) ______-_------_---- 66 Permian and Triassic clastic rocks.______Cretaceous System. ______-__-_-__------70 Lower and Middle Jurassic clastic and carbonate Yamama Formation Lower Cretaceous - 70 rocks.______Buwaib Formation Lower Cretaceous _ _ _. 72 Upper Jurassic and early Lower Cretaceous car Biy adh Sandstone Lower Cretaceous ______74 bonate rocks.------_-__-__-______--__- 9 Wasia Formation Middle Cretaceous _ _ _ _ _ 77 Late Lower Cretaceous clastic rocks______10 Aruma Formation Upper Cretaceous.__-_--_ 79 Middle Cretaceous clastic rocks__--______.___ 10 Tertiary System_____------_------84 Upper Cretaceous to Eocene carbonate rocks.__ 12 Umm er Radhuma Formation Paleocene and Miocene and Pliocene clastic rocks..______13 Lower Eocene.___.___-____---_---_------84 Paleogeography__. ______13 Rus Formation Lower Eocene.______-_---- 87 Early Paleozoic.______13 Dammam Formation Lower and Middle Cambrian ______13 Eocene.______------_------88 Ordovician, Silurian, and Devonian. ______14 Hadrukh, Dam, Hofuf, and Kharj Formations Permian and Triassic______-______14 Miocene and Pliocene.______--__--_------91 Late Permian.______14 Tertiary and Quaternary Systems ______---- 97 Triassic..__-_--__-______-___-_____- 15 Gravel--.----.------97 Early and Middle Jurassic.______15 Terrace sand and gravel. __-___--__-_------98 Early Jurassic.______15 Marl and sandstone.______------98 Middle Jurassic. ______15 Basalt __-_.------.------98 Late Jurassic and lower Early Cretaceous--___- 16 Calcareous duricrust.______-_---_- 98 Late Jurassic_..____.______.______. 16 Terrace, mud flat, and gypsum deposits. ------99 Lower Early Cretaceous.--.------.-- 17 Lake terraces__.______-_____._----- 99 Upper Early Cretaceous__-_-_-----___-_---_- 17 Marine terraces and raised beaches. ____ 99 Middle Cretaceous______18 Deltaic and tidal mud flats______99 Upper Cretaceous.______19 Gypsum deposits._.______-__---_ 99 Paleocene and Eocene._-______-_--___-______19 Quaternary deposits..------99 Miocene and Pliocene-_---____-----___._--_- 20 Terrace gravel_____.______-_____------_ 99 Cambrian and Ordovician Systems.______20 Sheet gravel..______------_____-__--_ 99 Saq Sandstone and equivalent units Siq, Gravel remnants...--______._--_------_ 99 Quweira, Umm Sahm, and Ram Sandstones.. 20 Gravel, sand, and silt.____-____-____.----_-. 100 Ordovician, Silurian, and Devonian Systems ______22 Sabkhah deposits--.....------100 Tabuk Formation rLower Ordovician to Lower Eolian sand______-_-___---___-----_- 100 Devonian.______22 Structure. __.______.____-_____---__-_- .. 100 Devonian System.____-__-______-_-__-__-____ 26 General features.______100 Jauf Formation Lower Devonian ______26 Stable region.______------_-----_------101 Permian and older(?) Systems-__--_---______- 27 Arabian Shield------101 Wajid Sandstone Lower Permian and older (?) 27 Western Arabian Shield-___-_--__-----__ 101 Permian System.______29 Yemen-Western Aden Plateau.-____--_--_ 101 Khuff Formation Upper Permian ______29 Southern Arabian Shield....______--_--__ 101 Permian and Triassic Systems______32 Arabian Shelf------101 Sudair Shale Upper Permian and Lower Interior Homocline.___-_____------102 Triassic ______32 Central Arabian arch defined by north Triassic System. ______35 ern and southern Tuwayq segments. 102 Jilh Formation Middle and Upper(?) Triassic. _ 35 Minjur Sandstone Upper Triassic.______37 Ha'il arch_____.____-_--__._------. 102 Jurassic System. ______39 Widyan basin margin. ______104 Marrat Formation Lower Jurassic._._____.. 39 Hadramawt Plateau______105 Dhruma Formation Middle Jurassic__ _ __ 42 Hadramawt segment.______----__ 105 VI CONTENTS Page Page Structure Continued Stratigraphic sections.--__-______.____.---__-____ DUO Stable region Continued Tabuk Formation______110 Arabian Shelf Continued Jauf Formation_-_---___-_-_-----.-_------__ 113 Interior Platform._.-_-______.__-_..-. D105 Khuff Formation.______115 Basins_ -______-____-_-____---___-__- 105 Sudair Shale--._____-______... 117 Rub' al Khali basin------..-.---- 105 Jilh Formation.-..----..______---___-_--____-_ 118 Northern Persian Gulf basin ______108 Mmjur Sandstone___-______.-_-_-______-.__ 120 Dibdiba basin_____--_.______._-_ 106 Marrat Formation__-_-______-_-___-_-_-_____ 121 Sirhan-Turayf basin___-_----___--_ 106 Dhruma Formation-______--__-______123 Mobile belt mountains and forelands______106 Tuwaiq Mountain Limestone____----__-__-_____ 132 Mountains. ______106 Hanifa Formation.------.------133 Zagros Mountains...-______._.____ 106 Jubaila Limestone._--_.__-______-__-_.__ 134 Oman Mountains.___--__--______--_--. 106 Arab Formation.______135 Forelands.-._-_---___--_----_--__------____ 107 Hith Anhydrite ------__--_------_-_-_-_-_-.__ 139 Zagros Mountains foreland______107 Sulaiy Formation_-----_-_------__-_-_-___- 139 Oman Mountains foreland______107 Yamama Formation..-.-___-__-______._-____.-_ 140 Huqf-Haushi swell.______107 Buwaib Formation..__-_____---__-______-__ 141 Central Arabian graben and trough system__. _ _ _ _ 107 Biyadh Sandstone.__--_-.-_-__-__-_-.---_____._ 142 Nisab graben______107 Wasia Formation.__---___-_- .-...... __..._.. 142 Sahba' trough-----___---__-____-.______107 Aruma Formation.-_-.-____. .-__-..._-__.-__ 143 Mughrah structural trough______..___-_ 108 Umm er Radhuma Formation. ...-...--_..._.. 143 Awsat graben.______108 Rus Formation.____-__-____. .___._-_-_.--..- 145 Durmd graben-_-______.______108 Dammam Formation_.____. .-_-_--_.____.__ 145 Qaradan graben______109 Hadrukh Formation..---.---. .-.--._-_-....-- 145 Barrah graben______109 Dam Formation______-___. .--....---__---_ 146 Majma'ah graben______109 Hofuf Formation.---_-_____. .--.--._----._._ 146 Khawr Umm Wu'al graben.______110 Bibliography.______--______-_-_--__-______-__ 146
ILLUSTRATIONS
[Plates are In pocket] PLATE 1. Regional stratigraphic section, Ash Sharawrah to Tabuk. 2. Map showing structural provinces, Saudi Arabia and adjacent areas. 3. Tabuk Formation type section. 4. Jauf Formation type section. 5. Jilh Formation type section. 6. Minjur Sandstone type section. 7. Dhruma Formation reference section. 8. Tuwaiq Mountain Limestone reference section. 9. Biyadh Sandstone type section. 10. Umm er Radhuma Formation reference section. Page FIGURE 1. Schematic section showing Cretaceous surface-subsurface relationships_-______-___-______--___------Dll 2. Khuff Formation reference section__-___-______30 3. Sudair Shale type section.______34 4. Marrat Formation reference section.______41 5. Hanifa Formation reference section ______55 6. Jubaila Limestone reference section ______59 7. Arab Formation type and reference sections_____-_-____ 62 8. Sulaiy Formation type section______67 9. Buwaib and Yamama Formations reference sections__ __ 72 10. Aruma Formation type section ______81 11. Dammam and Rus Formations type and reference sections. 89 12. Hadrukh Formation type section.______94 13. Dam Formation type section______95 14. Hofuf Formation type section.______96
TABLE Page TABLE 1. Saudi Arabian outcrop sequence D7 GEOLOGY OF THE ARABIAN PENINSULA
SEDIMENTARY GEOLOGY OF SAUDI ARABIA
ByR. W. POWERS, L. F. RAMIBEZ, C. D. REDMOND, and E. L. ELBEBG, JR.1
ABSTRACT and Triassic sedimentary rocks. The initial deposit, the Khuff Systematic mapping of the sedimentary geology of Saudi Formation, is mostly shallow-water limestone; overlying beds Arabia by Arabian-American Oil Co. (Aramco) began in 1933. are nonmarine elastics except for thick carbonate units in the By 1959, exploration parties of one type or another had surveyed middle part of the section. more than 1,300,000 square kilometers (500,000 square miles) of Above the Triassic System is some 200 to 500 m of Lower and sedimentary outcrop. Middle Jurassic rocks which, near the middle of the escarpment The foundation for sedimentary deposition is the Arabian region, are interbedded marine shale and shelf limestone. These Shield a vast Precambrian complex of igneous and metamorphic grade to sandstone, in part continental, in the northern and rocks that occupies roughly one-third of the Arabian Peninsula southern areas of outcrop. The Middle Jurassic is overlain by in the west and crops out sporadically along the southern coast. a great sequence-of nearly pure carbonate rocks, highly fossilif- Since the outset of the Paleozoic Era the shield has been amaz erous and accurately dated as Upper Jurassic and early Lower ingly stable, subject only to gentle, epeirogenic movement. On Cretaceous. this rigid land mass was deposited an aggregate total of nearly The Jurassic System is spectacularly displayed in central 5,500 meters (18,000 feet) of sedimentary rocks ranging in age Arabia where it forms the backbone of the escarpment region from presumed Cambrian to Pliocene(?). the Tuwayq Mountains. Carbonate sedimentation was inter Paleozoic, Mesozoic, and lower Tertiary strata are magnifi rupted several times in the closing stages of the Jurassic b^ the cently exposed in central Arabia where they crop out in a great onset of evaporite conditions which gave rise to cyclic deposits curved belt bordering the shield. Here the landscape is domi of anhydrite and calcarenite. The resulting sequence the nated by a series of essentially parallel west-facing escarpments, Arab Formation is of prime importance for its porous car each supported by a resistant limestone cap. Exposures are bonate members contain billions of barrels of proved oil reserves. unusually good, and many rock units can be traced without The carbonate sequence is succeeded by a thick body cf late significant interruption for 500 to nearly 1,000 km. Beds Lower and Middle Cretaceous sandstone. (The Middle Creta reflecting buried basement configuration dip gently and uniformly ceous Series and Epoch, as defined by European geologist-, are away from the escarpment region into the Persian Gulf and used in this report.) Late Lower Cretaceous rocks are non- Rub' al Khali basins. marine and appear only in the middle and southern parts of the East of the escarpment belt is a broad expanse of relatively escarpment region. Middle Cretaceous rocks, nonmarine in the low-relief terrain in which Tertiary and younger deposits effec south, become progressively more marine in the north where tively mask older units. Clues to the character of pre-Tertiary they follow a transgressive path northwest across older beds as rocks in this large area, which includes the Rub' al Khali desert far as Jordan. and most of northeastern Arabia, are afforded only by widely Upper Cretaceous and Eocene rocks, almost exclusively in scattered bore holes and oil wells. limestone and dolomite facies, are extensively exposed alorc the In extreme northwestern Arabia, largely lower Paleozoic eastern edge of the escarpment belt and continue northwest into sedimentary rocks are exposed, although a basinal area bordering Iraq. The sequence, with an average thickness of about 5^0 m, the Paleozoic rocks on the north is characterized by thick Upper includes rocks of Upper Cretaceous, Paleocene, lower Eocene, Cretaceous to Tertiary strata. Tertiary to Recent volcanic rocks and middle Eocene ages. cover substantial parts of the area. The stratigraphic sequence above the Eocene consists of 200 In general the older sedimentary rocks are exposed in north- to 600 m of Miocene and Pliocene rocks, mostly of nonn^arine central and northwestern Arabia near the Precambrian basement origin. These deposits a heterogeneous assemblage of marly where as much as 2,000 m of lower Paleozoic rocks are present. sandstone, sandy marl, and sandy limestone blanket the Rub' Although unfossiliferous, the lower 600 m can be equated, at al Khali and northeastern Arabia. least in part, to rocks of certain Cambrian age in Jordan. Above the Miocene and Pliocene rocks are unconsolHated Higher beds contain intervals confidently dated as Lower Quaternary deposits which comprise great sand deserts and Ordovician, Silurian, and Lower Devonian. Lower Paleozoic widespread gravel sheets. Sand of the Rub' al Khali Desert rocks are chiefly coarse-grained sandstone of terrestrial origin, alone covers about 600,000 sq km (230,000 sq mi) or most of although marine shale occurs at several levels and the upper southern Arabia. 300 m is mainly shale with thin beds of limestone. Two major structural provinces are recognized within the Lower Paleozoic strata are succeeded in the central escarpment Arabian Peninsula and adjacent areas. One is the comparatively region by a thick sequence (about 1,000 m) of Upper Permian stable interior region whose rigidity is controlled by the Pre cambrian basement. The other is the great mobile belt of 1 Of the Arabian-American Oil Co. Taurus, Zagros, and Oman Mountains, bordering the stable Dl D2 GEOLOGY OF THE ARABIAN PENINSULA
region on the north and east. Saudi Arabia falls entirely within quences outside Saudi Arabia to depict the distri the stable region. bution, lateral change, age, and gaps in the stratigraphic The interior stable region contains the Arabian shield as well as the Arabian Shelf an extension of the basement thinly record, somewhat tedious but necessary prerequisites veneered with little-disturbed sedimentary rocks. Widespread to unraveling the geologic history of the subcontinent. structural events, presumably related to epeirogenic movement Much less space is devoted to structure, and orly the within the basement, have divided the Arabian Shelf into several major tectonic elements of Saudi Arabia and adjacent distinct and significant structural elements the Interior areas are considered in any detail. Homocline, the Interior Platform, and several basinal areas. This report is fundamentally a compilation e.nd, as Bordering the shield is a great belt of sedimentary rocks whose dip basinward is so slight and uniform as to be impreceptible to such, presents the findings of the Aramco geologic the eye. This Interior Homocline has an average width of group as a whole. A few of the ideas and basic data about 400 km and a persistent dip varying from slightly more are new; most are not, although a number of the rela than 1°00' in older units to less than 0°30' in the youngest. tively broad generalizations have been reached but One structural feature of the homocline the central Arabian recently. Many of the concepts presented here origi arch has greatly influenced the present surface distribution of sedimentary rocks in the interior escarpment region. The nated with individuals, particularly the late Max arch, which affects all rocks from the basement up, marks the Steineke and the late R. A. Bramkamp, former /ramco area of maximum curvature of the homocline in central Arabia. chief geologists. Still other fundamental idear origi Although the arch has a varied history, it is apparently a nated spontaneously with the group rather than with residual high between the periodically sinking Persian Gulf and individuals. The writers, of course, take responsibility Rub' al Khali basins rather than a true independent positive feature. Support for this concept conies from the presence of for the manner in which the concepts are presented a great arc of tensional structures the central Arabian graben and supported here. and trough system near the crest of the arch and along the hinge line between the Persian Gulf basin and the stable western HISTORY OP EXPLORATION IN SAUDI ARABIA block. Bordering the homocline is the Interior Platform, a remarkably References to the geology of Saudi Arabia prior to flat area of varying width in which systematic dip off the crystal the start of oil search in 1933 are scarce. Camel line core no longer prevails. Superimposed on the platform are trips across the interior escarpment by Philby, during several major north-south anticlinal trends which include the World War I and later years, provided the fir*t con great oil fields of Arabia. Adjacent to the platform are several basinal areas that have crete evidence that marine rocks are extensively from time to time received thick sedimentary deposits. Such exposed in central Arabia, (Philby, 1922, 1928 1933, basinal sags have developed on the shelf in the northeastern 1939). Collections made on these early travels in Rub' al Khali, northern Persian Gulf, Dibdibah and Sirhan- dicated the presence of lower Kimmeridgian (Newton, Turayf areas. 1921) and Callovian (Cox in Philby, 1933) carbonates. INTRODUCTION Little else specific was known of the geology of this An impressive amount of geological information on vast region. Saudi Arabia has been collected during Arabian- On May 29, 1933, the Saudi Arabian Government American Oil Co. (Aramco) operations, in the last 30 granted an oil concession to Standard Oil Co. of years. Most of the data are included in company California. Less than four months after the cor cession reports; much of what appears in this paper has agreement was signed, the first geologists, R. P. Miller largely been anticipated in articles by Steineke and and S. B. Henry, arrived at the seaport of Al Jubayl, Bramkamp (1952 a, b), Thralls and Hasson (1956), and systematic surface mapping of the coastal area Steineke and others (1958), Bramkamp and Powers started. Soon these geologists were followed by others, (1958), and Powers (1962). including Max Steineke (Sidney Powers Memorial The objective of this report is twofold: (1) To Medalist, 1951). By 1935, two-man reconnaissance bring together in a single document a comprehensive parties had penetrated south as far as Yabrln (lat summary of what is now known of the sedimentary 23°15' K, long 49°00/ E.), northwest to £Tafar al rocks exposed in Saudi Arabia and (2) to describe the Batin (lat 28°26/ N., long 45°58/ E.), and west across sedimentary geology of Saudi Arabia as depicted on Ad Dahna' to the Precambrian basement. the recently published 1:2,000,000-scale map of the Structure drilling began late in 1936 in the Al Arabian Peninsula (U.S. Geological Survey ^Arabian- Qatlf-Al 'Alah area; seismograph was first used at American Oil Co., 1963). Much of the text is devoted Abu Eadrlyah early in 1937. By this time the initial to a systematic account of the stratigraphic and paleontologic record in the Kingdom. Within indi objective of getting a general picture of the concession vidual stratigraphic units, local rock sequences are had been achieved the succession of geologic strata described in considerable detail. These local sections had largely been worked out, the broad geologic are related to each other and to equivalent rock se pattern of the area deciphered, the major uncon- SEDIMENTARY GEOLOGY OF SAUDI ARABIA D3 formities recognized, and most of the rock units raphy was of particular help in this phase. Between named. 1949 and 1958, 1:60,000-scale photographs cf all Dammam Dome, a salt dome, was the first prospect Aramco's concession and much of the remaining parts to be tested by deep drilling. Dammam well 1 was of the Arabian Peninsula became available. spudded on April 30, 1935. Results were disappoint By 1959, subsurface geological exploration of one ing, and the well was abandoned after reaching the kind or another had reached even the remotest parts Middle Cretaceous. Nine more wells were later drilled of the remaining concession, including the offshore area, at Dammam; although some of these were promising, and party strength was reduced accordingly. Full none proved out on sustained testing. Thus, the Mid gravimeter-magnetometer coverage had been achieved dle Cretaceous zones, productive at nearby Bahrain in 1958. Two land seismic crews, supplemented for a Island, had proved a disappointment in ten wells. short period by a marine unit, were in operation until Dammam well 7 was the first well programmed to test early 1963; three land parties have been active since deeper strata. Drilling of well 7, suspended while the that tune. Middle Cretaceous was tested, was resumed in late An especially interesting feature of the seismic 1937. This well encountered first gas and then large program is that in 1958 Aramco embarked on a refrac quantities of oil in the Arab Formation (late Upper tion survey which was to eventually result in a network Jurassic). The first test to yield oil was made on March blanketing the entire concession. The fortuitous 4, 1938, and Dammam 7 was completed in the C arrangement of a nearly concession-wide, competent, Member of the Arab Formation later the same month. anhydrile refractor immediately above the main Geological exploration was carried forward steadily, producing interval in the Arab Formation made this and by the fall of 1937 nine field parties were working method of structural mapping highly effective. in the concession. Surface mapping, seismograph stud The relatively expensive and shallow-penetration ies, and structure drilling continued, and in 1939 a structure drills were gradually phased out, and the gravimeter-magnetometer survey was started in the last hole of this type was completed in 1961. Deep coastal area. This same general level of activity con stratigraphic drilling was substituted. Wells 5,COO to tinued until 1940 when work was gradually curtailed 10,000 feet deep were spaced uniformly around the with the advent of World War II. When the war forced margin of the concession and scattered through the a general suspension of operations, Aramco geologists interior of the operating area. Information provided had mapped in detail some 50,000 square miles and had by these wells has proved invaluable, bearing heavily completed reconnaissance surveys over an additional on regional correlation, presence and amount of 175,000 sq mi. They had penetrated into the northern truncation of unconformities, facies changes, and portions of the Rub' al Khali, southwest as far as distribution of favorable reservoir rocks. In addition Wadi ad Dawasir, west across the Tuwayq Mountains the deep holes provide critical velocity control for the in several areas, and northwest to Wadi as Sirl^an adja seismic refraction network. Two such stratigraphic cent to the Jordan border. During this period oil was drills were active until mid-1963; one was still in discovered at Abu JJadrryah in 1938 and Abqaiq in operation in 1965. 1940, unsuccessful wildcats were drilled at Al 'Alah in During the early postwar period, wildcat drilling 1937 and Ma'qala' in 1940, and a triangulation network discovered oil at Qatif (Al Qa^lf) in 1945, at Ain Dar was started to provide accurate control for the various ('Ayn Dar) in 1948, and at Haradh (IJaratf) and surveys then under way. Fadhili (Al Fatfili) in 1949. Aramco's first of?hore Exploratory work was sharply limited during the war discovery came in 1951 with the drilling of the Safaniya years, although surface mapping and structure drilling (As Saffamyah) structure in the northwestern Persian continued on a small scale until 1942. An unsuccessful Gulf. Further discoveries in 1951, 1952, and If 53 at wildcat well was started at Jauf (Al Ju'uf) in 1943, and 'Uthmaniyah (Al TTthmanfyah), Shedgum (Shadqam), from late 1944 to mid-1945, two surface-mapping parties and Hawiyah (Al IJawryah) established Ghawer (Al were in the field. Ghawar) field, a single anticline oil-filled from Haradh After the war, exploration once again increased nearly to Ain Dar and Fazran, a distance of nearly 250 to its former level, and eight field parties of various kilometers. Other wells on untested structures and kinds were in operation. Two of these were surface- major exploratory wells in known fields followed in mapping parties which, by the end of 1954, had sur quick succession and resulted in commercially im veyed much of the concession, in part by reconnaissance portant finds at Khursaniyah (Al Khursaniyah) in traverses but mostly in considerable detail. Only 1956, Khurais (Khurays) in 1957, Manifa (Manlfah) limited investigations of special outcrop problems have in 1957-58), Abu Hadriya (Abu gadriyah) in 1959, been carried out since that time. High-altitude photog Fadhili in 1962, Abu §a'fah in 1963, and Qatif in 1963. D4 GEOLOGY OF THE ARABIAN PENINSULA
Wildcats not finding commercial oil were drilled at The first step in the preparation of the 1:500,000- Al 'Ubaylah in the central Rub' al Khali in 1953-55 scale maps was compilation of a topographic base. and El Haba (Al Lihabah) in 1963. Each base was later to be overprinted with the geology. At the end of 1963, Aramco had discovered productive Basic control was provided over most of the area of oil in eleven widely scattered fields and sixteen dif Aramco responsibility by the slotted-template method ferent reservoirs. Total estimated remaining proved of laydown firmly tied to horizontal control points reserves of crude oil were about 55 billion barrels at identified on photographs. In the eastern Rub'al the close of 1963. Khali, control was established by computing geographic coordinates from shoran readings. Prior to trans HISTORY OF ARABIAN MAP PROJECT ferring the data to the geographic base, topographic Aramco's exploration entered a particularly inter features were outlined on individual photographs. esting phase in 1954 when the U.S. Geological Survey In marking the photographs, maximum detail was and Aramco began work on a series of geologic- preserved to show relief, escarpments, drainage, culture, geographic maps covering all the concession and most and other features. Sand-covered areas were c^fined, of the Peninsula. The cooperative effort was spon and dune crestlines were delineated. This material sored jointly by the Kingdom of Saudi Arabia and the was then transferred by sketchmaster to the Isydown U.S. Department of State. The purpose of the base. Hachures were used to represent relief in areas program was to make readily available for scientific of outcrop, and bodies of sand were shown by stippling and economic application the geologic data collected to depict trends and different types of sand ter-ain. by Aramco and U.S. Geological Survey during the Throughout the many years of exploration, an course of their many years' work in the Kingdom. important function of the field parties was to gather Saudi Arabia is one of the few countries in the world place names. Aramco's Arabian Affairs Division, that has been completely covered by high-altitude Government Relations Organization, played a major aerial photography. Aerial mapping began in 1949, role in evaluation of the data collected and in the and 250,000 sq mi of eastern Arabia had been photo selection and proper placement of names used in the graphed by 1951. Remaining parts of the Kingdom map series. Saudi Arabians from all parts of the involving some 550,000 sq mi were covered from 1954 Kingdom were consulted by Aramco's Arabists to to 1959. All photography was flown under strict assure accuracy of transliteration and to evaluate photogrammetric specifications with the necessary old names and obtain new ones. All name^1 were forward and sidelap to give complete stereoscopic reviewed by the Board on Geographic Names in coverage. Washington, D.C. for conformance with the BGN/ While aerial mapping was in progress, Aramco PCGN system of transliteration. was busy on an extensive survey network to provide The geologic map series was compiled in almost the accurate horizontal and vertical control. A third- same manner as the geographic series. The great order triangulation net and a second-order level net amount of field data collected over the years had to be were extended over most of the Eastern Province and compiled, and in the process plane-table traverses, southward into the central Rub'al Khali. Elsewhere, car traverses, and geologic stations had to be identified high-order astronomical observations using meridian on photographs and adjusted to the new maD base. transit stars, recording chronograph, and chronometer Elevations were reviewed, adjusted, and tied to later were established. Over much of the central outcrop level lines. Stratigraphic contacts were identified area this work was augmented with stadia and ba and traced on photographs across areas not covered rometer traverses; control developed over a 15-year by ground traverses. period to accurately tie down the extensive geologic After completion of the l:500,000-scale geologic field work. All previously established control points sheets and the revised l:2,000,000-scale geographic and geologic field stations were identified on photo map, Aramco personnel began work on their portion graphs. The points thus recovered provided the basic of a l:2,000,000-scale geologic map of the Arabian topographic and geologic control needed to map the Peninsula. Contributor geology was generalized on Najd-Tuwayq area. A shoran trilateration net ex the geographic base in close collaboration with the tending southward from the Saudi Arabia-Jordan authors. In addition, hand-colored proof was returned border through northwestern Arabia and into the to each contributor for review and comments before western Arabian Shield provided control over this publication. Concurrently, members of the U.S. area. Photography of the eastern Rub'al Khali was Geological Survey were moving ahead with comDilation also shoran controlled. of the geology of the western Arabian Shield. SEDIMENTARY GEOLOGY OF SAUDI ARABIA D5
ACKNOWLEDGMENTS concepts; indeed, such data have usually servei to As stated before, this paper basically represents a strengthen their predictions. compilation of many geologists' work over a period This acknowledgment would be incomplete without of 30 years. During that time 77 individuals studied mention of S. D. Bowers who read much of the man the Arabian outcrop sequence and were involved in uscript and offered many useful suggestions and the collection and processing of the data and the formu cr'ticisms. Thanks are also due James P. Mandaville, lation of the ideas expressed here. Each of these Jr., Geographical Analyst, Aramco's Arabian Affairs Aramco geologists is cited individually. Other mem Division, for his painstaking research and reading and bers of Arameo's geologic staff made significant con correcting of all Arabic place names appearing in this tribution in the form of paleontologic determinations, paper. The manuscript was typed by Mary E. Archer photogeologic mapping, and other work. It is not who, in addition, thoroughly edited the manuscript possible to acknowledge separately each contribution for place names and technical errors. For all her but it should be noted that the following Aramco help the writers are most grateful. The manuscript specialists have added to our knowledge of Arabian is published by permission of Aramco. geology: P. R. Aswerus, G. R. Ball, T. C. Barger, STRATIGRAPHY H. S. Bennett, Jr., E. L. Berg, S. Bevan, B. Beverly, Jr., G. W. Blakslee, H. W. Blodget, L. E. Bogle, F. OUTLINE OF MAJOR STRATIGRAPHIC DIVISIONS Bourgeois, S. D. Bowers, R. A. Bramkamp, A. B. GENERAL FEATURES Brown, C. W. Brown, H. L. Burchfiel, R. B. Carr, The foundation for sedimentary deposition in £ A. E. Clements, M. C. Coffield, J. S. Cruse, W. Arabia can readily be generalized into two positive DeU'Oro, W. C. Dimmock, J. J. Donohue, D. B. structural elements on the west, the western Arabian Eicher, C. R. Farquhar, O. B. Fenner, F. J. Frank- Shield, a vast dominantly Precambrian complex of ovieh, W. M. Furnish, R. D. Gierhart, T. F. Harriss, igneous and metamorphic rocks; on the south, the D. O. Hemer, S. B. Henry, B. B. Hill, W. H. Hoag, Jr., Arabian part of a crystalline massif extending northeast D. A. Holm, R. W. Holt, J. W. Hoover, B. R. Hudson, from the horn of Africa. R. C. Kerr, H. A. Kimball, T. W. Koch, R. P. Kopp, The western Arabian Shield must be regarded as a N. M. Layne, Jr., R. L. Maby, Jr., R. D. MacDougall, continuation of the adjacent African shield ar*i is H. L. McAlister, H. A. McClure, L. B. Milam, R. P. now separated from it by the Red Sea rift. The time Miller, C. S. Morse, R. L. Myers, O. W. Nine, J. W. of initial separation is still controversial; however, the Northrup, L. D. Owens, K. R. Parsons, A. F. Pocock, Red Sea appears to be primarily a late Tertiary feature. R. W. Powers, L. F. Ramirez, C. D. Redmond, D. O. Western shield rocks can be clearly dated as Pre Reese, W. H. Reiss, S. J. Roach, C. W. Rock, N. J. cambrian by their unconformable position ber°-ath Sander, R. M. Sandford, H. W. Schneider, J. R. unaltered sediments which in Jordan have furnished Schultz, J. B. Sheehan, A. M. Short, W. T. Short, Cambrian fossils. The oldest identifiable fossils so far Max Steineke, J. W. Thomas, F. R. Waldron, J. C. discovered in Saudi Arabia proper are Early Ordo- Wells, A. A. Weymouth, J. S. Williams, and M. P. vician, but a substantial sequence of rocks correlative Yackel. with the Cambrian units in Jordan intervene between Two of the Aramco geologists Max Steineke and these and the basement. Age of the southern Arabian R. A. Bramkamp should be singled out for special complex in Yemen, the Aden Protectorates, and mention. Both were pioneers in the study of the Dhufar also appears to be Precambrian for the most geology of Saudi Arabia. Steineke arrived in the part, although the presence of some early Paleozoic field September 20, 1934, almost one year after oil elements has been determined by radioactive da.ting. exploration had commenced. Bramkamp arrived in Since Precambrian time the massifs have been mid-September of 1936. Most of the work on which amazingly stable, subject only to gentle, epeiroj?enie this report is based was either done by them personally movement. On these rigid peneplaned land masses or under their guidance, for Steineke and Bramkamp was deposited a thick sequence of continental and were Aramco's chief geologists respectively from shallow-water shelf sediments that dip gently into the Mediterranean, Persian Gulf, and Rub' al Khali basins. 1936 to 1947 and 1947 to 1958. Both men were true The opposite flank of the Mediterranean-Persian Gulf scientists and outstanding geologists, and it was during geosyncline is formed by the Russian Platform far to their tenure that the framework of the stratigraphy the northeast. Presumably the Rub* al Khali basin is and structural geology of Arabia evolved in much a reflection of the broad northeast-plunging syuoline its present form. A large amount of new work, much connecting the western Arabian and southern Arabian of it detailed, has done little to modify their basic Shields. D6 GEOLOGY OF THE ARABIAN PENINSULA
Field work in Saudi Arabia in connection with oil of Paleozoic units off the Arabian Shield to tin east, search has now been in progress for 30 years. During northeast, and north outlines the continuation of the this time, Aramco geologists have identified, measured, Interior Homocline west of An Nafud. A basinal area and mapped in considerable detail an aggregate total of bordering the homocline on the north is characterized nearly 5,500 meters (18,000 ft) of sedimentary rocks by a thickened Upper Cretaceous to Tertiary seiimen- ranging in age from presumed Cambrian to Pliocene (?) tary section. In addition, Tertiary to Recent volcanic (table 1, pi. 1). The main rock units have been blocked rocks cover substantial parts of the area. out, and their limits have been tested widely enough to The sedimentary section exposed above tin Pre- assume that they will stand in essentially their present cambrian in Saudi Arabia falls naturally into eight form. As always, much work remains to be done major divisions. Dominated by a characteristic lithol- particularly so far as paleogeography, regional litho- ogy and separated by significant breaks in the geologic facies, and more precise dating of some parts of the record, the divisions are: section are concerned. It is expected, however, that 1. Lower Paleozoic clastic rocks Cambrian (?) through additional work will affect only the detail and that the Lower Devonian; dominantly coarse clastic rocks major framework as now defined will survive. but with some Ihin, carbonate beds in uppermost Paleozoic, Mesozoic, and lower Tertiary sedimentary part. rocks are magnificently exposed in central Arabia. 2. Permian and Triassic clastic rocks Upper Permian Here, rocks crop out in a great curved belt flanking the through Upper Triassic; alternating nonriarine- eastern margin of the shield. Beds certainly reflecting marine units, dominantly clastic but wit! thick buried basement configuration dip gently and uniformly calcareous sections at base and in middle. northeast, east, and southeast. Rate of dip changes 3. Lower and Middle Jurassic clastic and carbonate progressively from slightly more than 1°00' in older rocks Toarcian to Callovian(?); in central Arabia units to less than 0°30/ in the Upper Cretaceous and marine shale interbedded with carbonate grades to Eocene. Low dip of such remarkable constancy gives sandstone in northern and southern ar^as of rise to one of the fundamental structural provinces of outcrop. the Arabian Peninsula a wide Interior Homocline 4. Upper Jurassic and early Lower Cretaceous car bordering the Arabian Shield (pi. 2). Dominating the bonate rocks Callovian through Valantunian; exposed part of the homocline is a series of essentially mostly carbonate but with alternating evsDorite- parallel, west-facing, strike escarpments, each supported normal marine cyclic deposits near end of Jurassic. by a resistant limestone cap. Sandstone and shale, less 5. Late Lower Cretaceous clastic rocks Hauterivian resistant at least so far as arid weathering is concerned, through Aptian; dominantly coarse clastic rocks floor intervening valleys and plains. The west-facing with thin basal carbonate unit. scarps offer fine opportunity for studying the sequence 6. Middle Cretaceous clastic rocks Cenomarian to in that exposures are unusually good and some rock Turonian(?); dominantly coarse clastic rocks. units can be traced without significant interruption over 7. Upper Cretaceous to Eocene carbonate rocks Cam- a distance of 500 to nearly 1,000 km. As the softer panian through Lutetian; carbonate units but in formations between cuestas are very often only partially subsurface lower Eocene includes evaporite Faction. masked by late gravels and sands, even these units are 8. Miocene and Pliocene clastic rocks dominantly fairly well known. sandy limestone and sandstone. East of the homocline is a broad expanse of relatively LOWER PALEOZOIC CLASTIC ROCKS low-relief terrain in which older rocks are buried by OCCURRENCE AND CHARACTER Tertiary and other late deposits. Paleozoic and Mes In northwestern Saudi Arabia at least 1,900 m of ozoic strata do not reappear at the surface until the Lower Devonian and older Paleozoic rocks are eroosed. Oman Mountains in the easternmost part of the penin Exposures extend southeast from the Jordan border, sula and Iran on the far side of the Persian Gulf are form a gently curving arc parallel to the northern mar reached. In this broad area, which includes the Rub' al Khali desert and much of northeastern Arabia, gin of the shield, and disappear eastward unc^r the specific information on pre-Tertiary rocks is available blanket sands of An Nafud. Shallow dips progressively only from a few widely scattered stratigraphic holes shift from northeast to north and northwest to outline and fields where oil wells have been concentrated a separate embayment of Paleozoic platform sedimen sparse control when the size of the area involved is tary rocks west of the 5a'il arch (long 41°30/ E.). considered. Equivalent sedimentary rocks again crop out south of In extreme northwestern Saudi Arabia, largely lower An Nafud. Here they are gradually overlapped by Paleozoic sedimentary rocks are exposed. Gentle dip Permian beds and wedge out completely near lat SEDIMENTARY GEOLOGY OF SAUDI ARABIA D7
TABLE 1. Saudi Arabian outcrop sequence Thickness (Type or Age Formation Generalized lithologic description Major stratigraphic divisions reference section) QUATERNARY AND Surficial deposits Gravel, sand, and silt TERTIARY Kharj Limestone, lacustrine limestone, gypsum, and gravel 28m Sandy marl and sandy limestone; subordinate cal Hofuf 95m iMioceneandEocene Pliocene ( careous sandstone. Local gravel beds in lower part Miocene and Pliocene Clastic Dam Marl and shale; subordinate sandstone, chalky lime CENOZOIC stone, and coquina 91m Rocks TERTIARY Hadrukh Calcareous, silty sandstone, sandy limestone; local 84m chert -^^-^ ^^ ^^^^ ^Nw^^N^^*_^^^>^^^^_-N *~*^**^~S^^^+^S^*^S* ^ '^-'^ ^S_X^*-S_X^**^" Lutetian Dammam Limestone, dolomite, marl, and shale Marl, chalky limestone, and gypsum; common chert Ypresian Rus and geodal quartz in lower part. Dominantly an 56m hydrite in subsurface o a> Upper Cretaceous to Eocene Thanetian Umm er Radhuma Limestone, dolomitic limestone, and dolomite 2§ Montian(?) 243m Carbonate Rocks Maestrichtian Limestone; subordinate dolomite and shale. Lower Aruma part grades to sandstone in northwestern and 142m Campanian southern areas of outcrop ^^*^ -^^^^^^^. "^XS^^W***^ " i"*^**-*** Turonian(?) Wasia Middle Cretaceous Clastic (Sakaka Sandstone of Sandstone; subordinate shale, rare dolomite lenses 42m Cenomanian northwest Arabia) Rocks jCRETACEOUS ^^^ ^^^^-^ >»^^^s^«w'N-'^^>^^^-*^^-^-'''^-''s-^^~^-x> ^s^XS^^^^^^**^ Aptian Biyadh 425m Barremian Sandstone; subordinate shale Late Lower Cretaceous Clastic Rocks Hauterivian Buwaib Biogenic calcarenite and calcarenitic limestone in- terbedded with fine sandstone in upper part 18m ^^^^^^^^-^^ t'*^S*^S**~f**-ii**-*f*** v^ta^Nw^N, t^****^^s^ ^^^^^^^^^-^^^^^ *~>^**-S*l-s***>**-s Yamama Biogenic-pellet calcarenite; subordinate aphanitic 46m Valanginian limestone and biogenic calcarenitic limestone Chalky aphanitic limestone; rare biogenic calcarenite Berriasian Sulaiy 170m 1 i and calcarenite limestone Hith Anhydrite 90m MESOZOIC Calcarenite, calcarenitic and aphanitic limestone, Tithonian dolomite and some anhydrite. Solution-collapse Arab 124m carbonate breccia on outcrop due to loss of inter- Upper Jurassic and early Lower bedded anhydrite Cretaceous Carbonate Rocks Aphanitic limestone and dolomite; subordinate cal Jubaila carenite and calcarenitic limestone. Lower part ±118 m CJ Kimmeridgian sandstone between 20° H. and 22" N. Hanifa Aphanitic limestone, calcarenitic limestone, and cal 113m carenite Oxfordian Aphanitic limestone; subordinate calcarenitic lime Tuwaiq Mountain stone and calcarenite. Abundant corals and stroma- 203m Callovian toporoidsin upper part Caljpvian(?)_ Aphanitic limestone and shale; subordinate calcar Bathonian Dhruma enite. Dominantly sandstone south of 22" H. and 375m Lower and Middle Jurassic Bajocian north of 26° H. Clastic and Carbonate Rocks Toarcian Marrat Shale and aphanitic limestone; subordinate sandstone 103m v'^^^S-*"^*^*' l^S^^f*^*^^*** -^*v ^S^N^^SW^**^-^ Upper Minjur Sandstone; some shale 315m CJ "Middle" Jilh Sandstone, aphanitic limestone, and shale; subordi nate gypsum ±326 m Permian and Triassic Clastic t Lower Sudair Red and green shale 116m Rocks Limestone and shale; dominantly sandstone south of Upper Khuff 171m ct 5 21°N. L u **~*^~^"^S^*^* ^**^»*^S*~s**^^~<**~s****~** *"S_ ^Si^^ «^_-^«^.^^^^ i*Sh^^^« ^^ ^N. |XS_XNfc^SMX>M^S-, Ci_ JE Lower Sandstone, gravel, and basement erratics 950m Wajid (Recognized only in southwestern Saudi Arabia undated and northern Yemen) calculated ^-s~*' " -^^S^S. ^^^^^^"^'N^^ ^s*~~~~-~-~~~-^Pnc imbrian basement complex -~-~^-~~^-^-~^^->- ^ PALEOZOIC) ~^~s ^^^^^^ ^^^>^ '*-S****S***l>*<**S+**^i****^S*+^***r*' \ Jauf Limestone, shale, and sandstone 299m L J ~ Lower Tabuk Sandstone and shale 1,072 m Lower Paleozoic Clastic Rocks o » z = Umm Sahm Ram Quweira Sandstone + 600m 11 ( Siq Precambrian basement complex Compiled by R. W. Powers and L. F. Ramirez, June 3, 1963 D8 GEOLOGY OF THE ARABIAN PENINSULA
24°20' N. Intervals within this sequence have been Permian, probably north of BEa'il, and cut into Lower dated by graptolites and brachiopods as Lower Ordo- Devonian. West of Al Jawf, the .Middle Cretaceous is, vician (Arenig), Silurian, and Lower Devonian. Cam in turn, truncated by pre-Upper Cretaceous erosion brian fossils have not been found, although recent photo- that eventually cut deep enough to completely remove geological work shows the lower part of the section to the Lower Devonian. be equivalent to Jordanian units of certain Cambrian Time represented between basement stabilization and age. Major internal unconformities have not been the beginning of presumed Cambrian sedimentation is detected, but their absence cannot be assumed on the unknown. However, fair dating of units bracketing basis of sparse field work so far done. the upper unconformity shows approximately Middle Silicate clastic rocks dominate the entire lower Paleo Devonian through middle Permian time unrecorded in zoic succession. The lower one-third is almost exclu outcrop. Until recently, rocks of these ages had not sively white, buff, and red, medium- to coarse-grained, knowingly been found anywhere in Saudi Arabia. Now, crossbedded sandstone. A thick section of graptolite- however, spore and pollen analyses show that one well bearing shale alternating with light-colored, micaceous, in northern Arabia (near lat 29°50' N., long 41°50' E.) commonly crossbedded sandstone follows. Capping penetrated shale of Mississippian and PennsyJvanian the sequence is nearly 300 m of shale with subordinate age (Visean to Westphalian). Other wells contain stringers of sandstone and carbonate. Upper Devonian, Carboniferous, and Lower and middle In southwestern Arabia a thick sandstone intervenes Permian rocks, thus filling out the Paleozoic record. between basement and Upper Permian carbonate rocks. PERMIAN ANJ> TRIASSIC COLASTIC ROOKS Unfortunately, this unit has proved barren, and com OCCURRENCE AND CHARACTER plete or even partial equivalency to Paleozoic clastic Nearly 1,000 m of Permian and Triassic sediir«mtary rocks in the northwest has not been established. Flora rocks are exposed in a wide belt that curves around the from adjacent bore holes does suggest, however, that eastern margin of the Arabian Shield. The sequence Lower Permian as well as Carboniferous and even falls naturally into four subequal units which apparently Devonian may be represented. The possible presence represent alternating marine and nonmarine deposition. of still older Paleozoic rocks is not disproved. Exposed At the base limestone and dolomite alternate with red in northern Yemen as outliers on the eroded Precam- and green gypsiferous shale. Next above is unfossil- brian surface, the massive sandstone, in Saudi Arabia, iferous brick-red and green shale, presumably non- forms a wedge of nearly continuous outcrop as far north marine. A thick section of limestone interbedded with as Wadi ad Dawasir (lat 20°30' N.). At this point light-colored sand and some gypsiferous shale follows. the sandstone pinches out between basement and over At the top are highly colored clastic beds containing lapping Permian limestone. Calculated to be more petrified wood as the only recognizable organic con than 900 m thick, the postbasement wedge is mainly stituents, characteristics that suggest the unit is mainly coarse-grained sandstone including common gravel beds continental. As a whole, the Permian and TMassic and some zones of basement erratics. appear to be mixed marine and continental clast;°s, the RELATIONSHIP TO ADJACENT UNITS latter predominating. Precambrian elements of the Arabian Shield were Only the sketchiest time framework has been supplied buckled, firmly stabilized, and had undergone at least to the outcrop by marine fossils. The lower carbonate one and possibly several periods of erosion and pene- unit contains molluscs which have been assi-imed a planation before the first Paleozoic sediments were probable Late Permian age. Higher carbonate stringers deposited. Marked nonconformity is everywhere evi contain fragments of ammonites considered to be dent between basement and overlying sediments. Middle Triassic. Fortunately, nearby bore holos have The lower Paleozoic sequence is terminated at the top yielded rich suites of diagnostic spore and pollen from by definite unconformity. South from An Nafud, pre- equivalent beds. On the basis of these, it is safe to say Permian erosion progressively eliminates Devonian, that the outcrop contains an almost complete Upper Silurian, Ordovician, and Cambrian (?) beds, eventually Permian through Upper Triassic (Keuper) sequence. bringing Permian rocks into contact with basement near RELATIONSHIP TO ADJACENT UNITS the latitude of Ad Dawadimi (lat 24°29' N.). From Regionally important unconformities above end be here the erosion surface can be traced south across the low set the Permian and Triassic strata off from b^acket- basement arch of central Arabia and again effects the ing units. The lower contact is marked by Upper southern wedge of Paleozoic clastic rocks. Permian beds discordantly on rocks as widely vrried in Beneath An Nafud other unconformities come in. age as: Precambrian (across the central Arabian arch); First, pre-Middle Cretaceous erosion eliminated the Cambrian through Early Devonian (north of the arch); SEDIMENTARY GEOLOGY OF SAUDI ARABIA D9 and middle Permian and older (in the south). At the good faunal control shows that uppermost Middle top, Upper Triassic is overlain by beds of Toarcian age Jurassic beds are laterally persistent and are without (upper Liassic), and lower Liassic is missing over the evidence of truncation. Rather than elimination at length of outcrop. In the extreme southern part of the the top by erosion, it seems more likely that the Middle outcrop, upper Liassic rocks are overlapped and Middle Jurassic pinches out to the south by progressive CTilap Jurassic rests directly on Triassic. and loss of successively younger beds at the base. LOWER AND MIDDLE JURASSIC CLASTIC AND CARBONATE Eventually, elimination is effected by complete Upper ROCKS Jurassic marine overlap. OCCURRENCE AND CHARACTER UPPER JURASSIC AND EARLY LOWER CRETACEOUS The Lower and Middle Jurassic are typically repre CARBONATE ROCKS sented in central Arabia by about 500 m of shallow-water OCCURRENCE AND CHARACTER shale and carbonate. In the Riyadh-Purmd area, a Upper Jurassic and early Lower Cretaceous ricks, thin unfossiliferous basal sandstone is followed by alter almost entirely in shallow-water limestone facie's, are nating limestone and shale all highly fossiliferous. spectacularly exposed in central Arabia where they The lowermost carbonate bears Toarcian ammonites stand up as a series of scarps subparallel to the eastern (uppermost Lower Jurassic) and above it, definite edge of the shield. More than 850 m of section has Bajocian and Bathonian and possible Callovian beds been measured near the latitude of Riyadh. CaJcar- have been identified. enitic limestone (lime sand admixed with lime-mud Thin members of clean, current-washed carbonate matrix) predominates, although thick, often laterally occur at various levels, often as units of high lateral persistent beds of calcarenite (clean-washed lime sand persistence that are useful in setting up a framework and gravel) are common at many levels. Lime sands of correlation. Lithologic changes to the south along are of various origins, but skeletal debris and aggregate outcrop consist primarily of gradual replacement of pellets are most common. shale and carbonate by sandstone. The transition The basal 200 m of Upper Jurassic section, composed first takes place in the lower beds just south of lat 24° of abundantly coralliferous and stromatoporoidal 1'tho- N. and progressively affects higher units so that at graphic limestone, is a massive cliff former. This unit lat 21° N. sandstone, apparently continental, makes up stands up in sharp relief for more than 1,000 km to the entire sequence. Hints of similar replacement can form the dominant scarp of Jabal T\iwayq, ore of be seen to the north before interbedded sandstone the major topographic features of the Arabian Penin and shale disappear beneath Recent cover. Although sula. Exposed at the base of the cliff-forming unit the trend toward a continental enviroment in this is a zone rich in fossils considered to be middle Ollo- direction is obvious, it is likely that a considerable vian. Strata representing remaining Upper Jurassic part of the unit is marine where last seen in the vicinity stages and the early Lower Cretaceous through Val- of Az Zilfi (lat 26°18' N.), and the actual strand line anginian are deposited above without apparent must have been even further north. interruption. The only significant change in deposi- RELATIONSHIP TO ADJACENT UNITS tional environment throughout is reflected by the exposure of massive Kimmeridgian Tithonian anhy Beds of definite Toarcian age rest disconformably on drite in a solution sink, Dabl Hit (lat 24°39'18" N.; Upper Triassic nonmarine sandstone throughout the long 47°00'06" E.). Laterally, little is known of the length of outcrop. Although unconformity at this true evaporite rock sequence owing to extensive level cannot be proved by surface relationships, bore removal by solution. Most of the area in which these holes show extensive truncation immediately downdip beds would be expected to crop out is a nearly hopeless and, in some places, the entire Upper Triassic is miss jumble of low hills representing complexly settled rocks. ing only a short distance basinward. South of lat The collapse breccia can be traced from lat 20°45' 22° N. the Toarcian itself pinches out in response to N. to nearly lat 26° N., a distance of about 550 km. progressive Middle Jurassic overlap. Only in the southern part of the solution-collapse zone The transition from Middle to Upper Jurassic, long is anhydrite present and then only in small amount. considered to be unconformable, now is thought to Equivalent rocks are extensively known in the sub take place without interruption or at the most involves surface where they comprise the Hith-Arab evaporite- only slight disconformity. Misleading was the fact carbonate complex. that basal Upper Jurassic rocks do indeed rest with Some lithologic differentiation is apparent in the definite discordance on older units around the southern southern part of the lower Kimmeridgian outcrop, margin of the Rub* al KEali basin. Now, however, including the appearance of a large marine sandstone D10 GEOLOGY OF THE ARABIAN PENINSULA lens in the middle part of the sequence. The sand although compositing of several sections indicate^1 maxi grades to limestone north of lat 22° N. and south of mum sandstone thickness to be about 700 m. lat 20° N. Limited exposures in a downdip direction, Recently an intense search and sampling of the that is, to the southeast, also show sandstone giving basal carbonate unit resulted in a fine collection of way to carbonate. In this same general area, much rock samples, larger fossils, and foraminifera. The of the lower Kimmeridgian above the sandstone is characteristic lithology is calcarenite and calcr.renitic replaced by thickly bedded dolomite. Below the sand limestone, the sand- and gravel-size grains mostly stone Upper Jurassic carbonates show a marked buildup being molluscan debris. Fine-grained sandstone is of clean, current-washed, shelf calcarenite. Progres interbedded with carbonate in the upper part, r.nd the sive increase in calcarenite continues to the south until, unit shows gradational contact with the overlying between lat 18° N. and 19° N., Callovian through continental series. lower Kimmeridgian rocks are dominantly lime sand Although the megafossils have not yet been studied, and lime gravel; presumably all mud-size particles were relationships established by means of Forminifera removed from the scene by turbulent water. indicate that the limestone is probably Hautcrivian. RELATIONSHIP TO ADJACENT UNITS It is undated on outcrop, but comparison with equiva lent subsurface strata shows the sandstone to be mostly The contact between Upper Jurassic and older beds, if not entirely Barremian and Aptian (fig. 1). apparently conformable north of about lat 19° N., becomes unconformable to the south. In this direction, Callo RELATIONSHIP TO ADJACENT UNITS vian limestone first laps over Lower Triassic and then Both the upper and lower surfaces of late Lower successively transgresses older Paleozoic units. Cretaceous strata are demonstrable unconformities. The contact between the Upper Jurassic-early Lower Pre-Hauterivian erosion removed a small sectior in the Cretaceous carbonate succession and overlying strata north but southward cut progressively deepe~ until involves two unconformities. Maximum section is lower Kimmeridgian was involved. The uppe~ sand preserved near lat 24° N. South of this parallel, pre- stone limit was effected by pre-Cenomanian truncation. Hauterivian erosion uniformly cuts successively older Accordingly, Albian strata are apparently not present beds until, at the southern limit of exposure near lat on outcrop (as they are a short distance dovndip), 18° N., lower Kimmeridgian is affected. North of lat and between lat 24° N. and 26° N. the entir** Hau- 24° N., only a limited amount of section was removed terivian to Aptian section is cut out. by this unconformity before pre-Cenomanian trunca MEDDLE CRETACEOUS CLASTIC ROCKS tion took over and rapidly eliminated i emaining Lower OCCURRENCE AND CHARACTER Cretaceous to Upper Jurassic beds. The last vestige The Middle Cretaceous Series and Epoch, as defined of the Tiiwayq escarpment (Callovian limestone) passes by European geologists, are used in this report. In under Cenomanian sandstone near lat 28° N. central Arabia the Middle Cretaceous is represented LATE LOWER CRETACEOUS CLASTIC ROCKS by a thin but highly persistent unit, mainly sandstone. OCCURRENCE AND CHARACTER Shale is commonly interbedded, and thin dolomite Except for a thin, albeit persistent, basal limestone, and limestone layers are present locally. Most of the late Lower Cretaceous rocks in central Arabia are sandstone is brown, and crossbedded; interbedded shale mainly continental sandstone. The basal carbonate and siltstone is varicolored red, purple, anNeogene and Recent again crops out around Sakakah. Relatively small cover. Further south, it appears again in windows at outliers also occur southwest of Sakakah. Until Al Jaladah and Ash Sharawrah. Incomplete exposure recently, age of the clastic section exposed in the north prohibited accurate measurement of a full section, west was the subject of considerable controversy. SEDIMENTARY GEOLOGY OF SAUDI ARABIA Dll
o 228-776 O 66 2 D12 GEOLOGY OP THE ARABIAN PENINSULA Now, however, lateral relationships established by UPPER CRETACEOUS TO EOCENE CARBONATE ROCKS wells show the sequence to be, with little doubt, OCCURRENCE AND CHARACTER equivalent to Middle Cretaceous rocks of Najd. Upper Cretaceous to Eocene rocks are widely exposed Thickness of the Middle Cretaceous rocks south of in Saudi Arabia. In the south where its width is rela lat 26° N. is remarkably uniform averaging about 40 m. tively constant, the belt of outcrop follows the trend of The unit thickens gradually to the northwest, and at older units. North of lat 24° N. it almost exactly paral Sakakah incomplete measurement suggests as much as lels the Middle Cretaceous transgressive sandstone 250 to 300 m of section. which cuts diagonally across the strike of older units. So far as age of the sandstone-shale sequence is con The shift in strike is accompanied by a gradual widening cerned, few fossils have been found on outcrop and these of the outcrop belt and introduction of progressively only in central Najd where thin limestone lenses occur. younger beds at the top. For example, south of Wadi Here, rare ammonites have been dated as Cenomanian. as Sahba', Campanian(?) to Thanetian rocks are ex Downdip in well sections, where more faunal control is posed across a width of 85 km. A short distance north, available, equivalent beds are mainly Cenomanian. A width of outcrop has expanded to 110 km as Ypresian and thin scum of Turonian may reach the surface as well, al Lutetian rocks are introduced. Still further north, pro though proof of this possibility is lacking. gressive reduction in rate of dip is reflected in continued RELATIONSHIP TO ADJACENT UNITS widening of 'outcrop until, at Sakakah, Upper Creta ceous rocks alone are exposed across 160 IOP, and Middle Cretaceous relationship to adjacent units, so younger units lap over into Iraq. far as outcrop is concerned, is fairly well understood; Although the Cretaceous rocks are not exposed east downdip the situation is less clear. The lower contact of the main outcrop belt, the Paleocene and (or) Eocene on outcrop is one of marked unconformity. South of rocks surface again near Abqaiq, at Dammam Dome, about lat 25° N. Cenomanian sandstone rests on what is south of Qatar, and in the southeastern Rub' al Khali. believed to be Aptian sand. North of this latitude, Upper Cretaceous to Eocene rocks exposed in Saudi rapid truncation at least in Saudi Arabia where region Arabia are, for the most part, limestone and dolomite al unconformities commonly eliminate beds at the rate with a few very thin interbeds of marl and shale. Sev of less than 1 m per km superimposes Middle Creta eral of these clay-bearing units show remarkable lateral ceous on all older units down to and including Lower persistence both on outcrop and in the subsurface, and Devonian. At the top, Cenomanian or Turonian(?) is a few have been traced without appreciable change over apparently in disconformable contact over most of the northeastern Arabia and large parts of the B.ub' al length of outcrop with Campanian(?) limestone. Khali. Relationship to the subsurface sequence is still incom South from lat 24° N., the Upper Cretaceous section pletely understood. There is considerable evidence is progressively replaced from the bottom up br sand that surface type sections of Middle and Lower Creta stone; complete replacement is effected just north of ceous sandstone formations do not properly match sub lat 22° N. A similar relationship exists in the extreme surface units bearing the same names (fig. 1). Difficulty northwest near Sakakah where sandstone appears at of detecting uncomformities and lack of fossils the base. within the sandstone sequence play a major role in the RELATIONSHIP TO ADJACENT UNITS uncertainty and inability to resolve the problem. Basi The Upper Cretaceous to Eocene carbonate succes cally, the difficulty revolves around the "pre-Cenoman- sion is separated from beds above and below by hiatus. ian" unconformity, so evident on Arabian outcrop, and At the base, Campanian(?) is in disconformable contact its accurate identification in coastal wells. So far this with Cenomanian and Turonian (?) elastics throughout unconformity has not been pinpointed in the subsur almost the entire length of outcrop. Near Sakakah, the face, although it certainly falls much higher in the unconformity becomes angular; Cenomanian bods are section than the upper surface of Aptian carbonate eliminated near long 39° E., and to the west Upper rocks with which it was previously equated. As is so Cretaceous beds rest on Lower Devonian. often the case, recognition of a problem is one thing; its Although lower Upper Cretaceous stages (Coniacian solution is another. The unconformity separating Mid and Santonian) are not present on outcrop, tHy are dle and Lower Cretaceous (each a single formation) on extensive in the coastal area. Even here, however, outcrop cannot be identified in the subsurface. By the pre-Upper Cretaceous unconformity manifests itself, same token, the boundary as now designated between primarily by truncation of beds on the crests of indi subsurface formations cannot be located in terms of the vidual folds. While strong angular discordance is surface sequence. present on some structures, others show no sig;n of a SEDIMENTARY GEOLOGY OF SAUDI ARABIA D13 break at this level. It is likely that no important time CAMBRIAN gap is involved in off-structure areas, and the uncon Rocks of proved or presumed Cambrian age occur formity fades to a time line possibly falling near or on only sparsely in areas immediately adjacent to Saudi the Turonian-Coniacian boundary. Arabia. Cambrian strata have been described from The upper limit of the Upper Cretaceous through southwestern Jordan where they are exposed in a thin Eocene sequence involves widespread and persistent belt bordering the Dead Sea rift. Much of this section unconformity. On outcrop and in wells drilled basin- is sandstone and gravel, although, opposite to the Dead ward, Miocene and Pliocene sand and sandy limestone Sea, it includes a thin shale-carbonate unit contain rest on lower middle Eocene or older rocks. Extensive ing Middle Cambrian trilobites. Almost certainly, drilling throughout Saudi Arabia has failed to detect the Jordan sequence is a continuation of the chiefly upper Eocene or Oligocene strata except possibly in the continental, but also in part littoral, sandstone exposed SirJian-Turayf basin near Jordan. in Saudi Arabia. MIOCENE AND PLIOCENE CLASTIC ROCKS A complex of strongly folded sediments including OCCURRENCE AND CHARACTER conglomerate, sandstone, shale, tuffaceous rocks and In northeastern Arabia, the stratigraphic sequence two well-defined carbonate units with some gypsum above the Eocene consists of a relatively thin succession occurs in southwest Ha^ramawt. The sequence is (200-300 m) of Miocene and Pliocene rocks, mostly of believed to pass westward into metamorphic rocks; nonmarine origin. These deposits a heterogeneous eastward, the complex has been tentatively equated to assemblage of sandy limestone, calcareous sandstone, a thick series of contorted elastics (including graywacke) sandy clay, and sandstone blanket the Eastern Pro in Dhufar and questionable Lower Cambrian elastics vince. Much of the section contains only sparse and dolomites in Al Buqf area of south Oman. fresh-water fossils that are of no use in dating. In North of Al Suqf, in the featureless Oman I issert, limited parts of the coastal area, however, marine are several salt domes with salt of presumed Cambrian molluscs and echinoids have been found through the age. These domes expose a varied assortment o* rock middle part of the sequence. The fauna indicates types, including some igneous debris, which suggest approximate correlation with Lower Fars of Iraq- correlation with the Middle and Upper Cairbrian Kuwait and is presumably middle Miocene. Because Hormuz Series of southern Iran-Persian Gulf-T^cial of apparent continuity and stratigraphic position, the coast salt domes. closing succession of Arabian Tertiary deposits is Cambrian rocks are widely known in southwestern considered to be early Miocene to Pliocene (?). Iran; many of the occurrences are associated with A lithologically similar, although much thicker, intrusive salt masses. The salt intrusions ard re Miocene and Pliocene sequence covers the Rub' al lated rocks of the Hormuz Complex contain frag Khali. Here, as much as 600 m of sandstone, sandy ments ranging in age from Lower Cambrian to Jurassic. marl and sandy limestone filled and leveled the Rub' Cambrian rocks have also been found in normal outcrop al Khali depression. Only rare nonmarine fossils have along the northeastern margin of the Zagros Mountains so far been recovered from this clastic section in spite as well as within the area of salt plugs. Red shale of a relatively uniform network of bore holes through and sandstone accompanied by subsidiary gypsum and the area. traces of salt represent the Lower and Middle Cairbrian RELATIONSHIP TO ADJACENT UNITS in the northeast, and mainly salt prevails in the routh. Miocene and Pliocene rocks everywhere rest un- In both areas, Upper Cambrian rocks are red and green conformably on Eocene or older beds; Oligocene de shale and sandstone with thin limestone and dolomite posits are unknown. Unconsolidated sand and gravel, beds. presumably for the most part Quaternary and Recent, References to the main occurrences of Cambrian salt are erratically superimposed on the Miocene and have been given in the preceding discussion. Ror^hly, Pliocene sequence. the area involved is southwestern Iran, islands of the PALEO GEO GRAPH Y Persian Gulf north of the Trucial coast, the Tmcial EARLY PALEOZOIC coast, anil southern Oman. A possible outlier is found Data on Middle East Cambrian to Devonian rocks in Jabal Sanam in southern Iraq where rocks of Hcnnuz are too incomplete to furnish any satisfactory picture type are associated with a strong negative gravity of the paleogeography of those times. The problem is anomaly. In Saudi Arabia itself, one structural feature, further complicated by the fact that occurrences of Dammam Dome, offers strong evidence of salt p^erce- these systems are largely inadequately dated and hence ment origin. Other features, although salt movements equivalencies are usually uncertain. may have been involved in their formation, do not D14 GEOLOGY OF THE ARABIAN PENINSULA show sufficiently clear evidences of this origin to use the time interval is represented on outcrop by uncon them to outline the extent of underlying salt. formity. Even in southwestern Iran, middle Paleozoic Outcrop sections in central Arabia are of little aid in time is poorly represented by sediments, although outlining the original extent of the saline basin. As Monograptus, a genus usually considered to be Silurian, the dated portion of the Paleozoic is Ordovician to has been reported from this area. Lower Devonian and no slumping has been reported PERMIAN AND TRIASSIC below this level, it is likely that saline deposits were Permian and Triassic time is fairly well represented not originally present. Thus, the limit of the area in outcrop and bore-hole sections of the Middle East. underlain by salt probably lies between Dammam Actually, it is within these periods that clues to the Dome and the Paleozoic rocks exposed in the interior, outline of the major Mesozoic (Tethys) basin to the the latter probably representing a marginal nonmarine north and east of the Arabian massif begin to emerge. and littoral clastic f acies bordering the evaporite basin. Roughly, the center of the Tethys trough (marked by The Lower(?) Cambrian clastic and carbonate beds maximum subsidence, thickest sedimentation, and (with some gypsum) exposed along the southern fringe deeper water deposition) forms a great arc around of the Arabian Peninsula may also be marginal to and the Arabian Shield. It passes east through Lebanon, represent the opening phase of the Cambrian evaporite Syria, and Turkey; southeast across Iraq; and then, cycle so well recorded to the north. in southwestern Iran, parallels the modern Persian ORDOVICIAN, SILURIAN, AND DEVONIAN Gulf trend. Older Paleozoic rocks may also hav? been So far as the Ordovician in Saudi Arabia is concerned, deposited, possibly in considerable thicknesses, in an about 400 m of elastics has been assigned to this system. ancestral basin of similar position and configuration, Units of purple, olive-green, and gray graptolitic shale although evidence for this deposition is now lacking. alternate with massive beds of reddish-brown to gray, LATE PERMIAN in part crossbedded, and commonly micaceous sand Late Permian time marks a significant change in stone. Cruziana tracks, none of which have been col sedimentation from dominantly clastic to mostly lected from the Ordovician, are found in abundance carbonate. During this epoch, the sea spread over both above and below. The Arabian sequence extends much of the Arabian foreland and there deposited a without significant lithologic change into southern sheet of shallow-water limestone that extends from Jordan. northern Iraq and Turkey to Oman. In each area, the Nearly identical sandstone and shale containing relationship to underlying units is unconformable, and Cruziana constitute the reported Ordovician lithologic the transgression initiates a period of gradual subsi units exposed in northern Iraq and Oman. In addition, dence which continued with periodic interruptions one well in Oman near Wadi al Ghabah (lat 21°20' N., long 57°19' E.) drilled about 2,000 m of Lower Ordovi into the Cenozoic. In central and northern Najd, the Permian is domi cian clastic rocks containing graptolites and Cruziana; nantly limestone interbedded with shale and dolomite. other deep wells in Syria penetrated graptolite-bearing The carbonate units are gradually replaced by sand clastic rocks defined only as lower Paleozoic. stone and shale in southern Najd, most of the change Described Ordovician lithologic units certainly reflect taking place between lat 22° N. and 21° N. Through extensive shallow-water conditions in which several out the length of outcrop a thin unit of massive brick- marine transgressions are recorded by graptolite-bearing red and rarely green shale caps the carbonate-sand shales, trilobites reported from Jordan, and occurrences stone part of the Permian. of Orthoceras sp. in Saudi Arabia and Iraq. Nonfos- The subcrop of apparent-strand-line and near- siliferous crossbedded sandstone and other massive strand-line clastic beds seen on outcrop between lat sands containing abundant pelecypod molds record 18° N. and 21° N. has been traced eastward under the intermittent emergence and near-shore conditions. Rub* al Khali by deep drilling. Roughly north of lat Silurian and Devonian rocks in Saudi Arabia are 21°15' N., from the Arabian Shield to Oman and south dominantly clastic and include some thin beds of lime western Iran, Upper Permian sediments are almost stone in the Lower Devonian. Graptolites and brachio- exclusively carbonate; to the south clastic bed^1 pre pods attest to the marine nature of some of the section; dominate. Contrary to this generalization, s well other parts are certainly continental and transitional. near lat 30° N., long 42° E. found the Upper Permian With the exception of Jordan, equivalent or partially in full shale facies, certainly marginal to limestone equivalent rocks are not exposed elsewhere. In most deposition to the northeast. other areas immediately adjacent to Saudi Arabia such Dolomite, replacing shallow-water clastic limestone, as Iraq, Yemen, the Aden Protectorates, and Oman, dominates many of the well sections of central and SEDIMENTARY GEOLOGY OF SAUDI ARABIA D15 coastal Arabia and the central Rub* al Khali and also There is little doubt that the limestone, now abun is exposed in significant amounts in sections in the dantly dolomitized, is of shallow-water origin. Original Oman Mountains. Included evaporite deposits in clastic textures are commonly preserved even though northern Iraq and Saudi Arabia attest to the periodic dolomite replacement is complete. The commor oc existence of barred-basin conditions. currence of thin evaporite units throughout, as well as TRIASSIC fetid dolomite and dark shale, attests to the fact that Lower and Upper Triassic rocks exposed in Saudi restricted conditions prevailed or constantly threatened. Arabia are respectively red and green, and commonly It is believed that throughout the period the basm of consist of gypsiferous shale and sandstone with vari deposition occupied a relatively constant area with sills colored shale. Fossil wood is present at several levels cutting circulation from time to time between certain in the Upper Triassic, and presumably the series is parts. mainly, if not entirely, continental. Systematic lat EARLY AND MIDDLE JURASSIC EARIY JURASSIC eral variation in lithology is recognized only in the Middle Triassic. Southernmost exposures are mainly So far as known, the passage from Triassic to Jurassic sandstone, probably in large part nonmarine. The is everywhere represented either by unconformity or proportion of limestone steadily increases northward nonmarine sedimentation. Around the margins o* the (from lat 23°30' N), and carbonate rocks dominate the Arabian Shield, dated marine Jurassic beds are sepa sequence near lat 27° N. This regional pattern is rated from undoubtedly Triassic or older units by interrupted near lat 26° N. by the appearance of a generally unfossiliferous sandstone. These nonmarine major lens of bedded gypsum in the upper part. elastics tentatively assigned to the Liassic(?) or the Therefore, in Arabia, rocks of continental and near- Liassic and Middle(?) Jurassic occur in Jordan ea^t of shore facies dominate the Triassic, although at least the Dead Sea and on the opposite side of the rift vr.lley, one more or less persistent marine transgression is in Yemen and in the Aden Protectorates. recorded by Middle Triassic limestone. Although pre-Toarcian Liassic marine strata have Marginal clastic deposits also prevail in Jordan been reported in Iraq, Kuwait, southwestern Iran, and where Triassic rocks crop out near the Dead Sea, Oman, it is with the Toarcian that widespread Jurassic although here also thin limestone and evaporite beds transgression begins. The relative position of non- also occur sporadically through the section. The Jorda marine Liassic (?) and marine Toarcian rocks indicates nian-Arabian exposures almost certainly block out in that an expanded Tethys sea covered the norther^ tip rough form the southwestern and western edges of the of Sinai, northwestern Jordan, Lebanon, much of north Triassic sea. ern Syria and eastern Iraq, and eastern Arabia (a^ far With recent palynological correlations, it now seems south as lat 22° N.) and extended eastward into Iran safe to extend Triassic units from outcrop into the and across northern Oman. coastal and Rub' al Khali areas of Arabia. Similarities Some lithologic differences between Lower and Mid in lithology and stratigraphic relations offer strong sup dle Jurassic are retained over much of the basin be port for this usage. Some reservations should be held cause the Toarcian is partly sandstone and shale in on the age of sediments just below the pre-Jurassic Rub' al Khali and Eastern Province wells and contains unconformity because we cannot be certain that beds significant anhydrite in Kuwait-Iraq. Included lime equivalent to the Upper Triassic on outcrop are fully stones, commonly finely fragmental and oolitic, further represented to the east. In fact, there is some evidence attest to the neritic and lagoonal conditions in which that Upper Triassic rocks are entirely removed at least Toarcian deposition took place. over the southern half of the Rub' al Khali basin and MIDDLE JURASSIC much of the coastal province as well. Bajocian and Bathonian time witnessed further In all coastal and Rub' al Khali wells so far drilled, expansion of the Tethys sea. This expansion is par the lithologic composition of the Triassic is dolomite ticularly evident in southern Arabia where the basal complexly interbedded with anhydrite, limestone, shale, contact of the Middle Jurassic and succeeding Upper and some sandstone. Similar conditions must have Jurassic carbonate units is markedly transgrersive. persisted overmuch of the Arabian foreland for, even Successively younger horizons rest on older rod's as though details of the sequence vary, like rocks are found in the Oman Mountains, southwestern Iran, and north the distance from the deeper part of the basin of ern Iraq. Wells drilled intermediately between the deposition increases. Data from outcrops around the Arabian coast and the outcrop belt penetrated the same shield areas also show a landward shift of the mid- complex of rock types, although shale predominates in Jurassic shoreline relative to that defined by Toarcian the south, and anhydrite, in the north. sediments. D16 GEOLOGY OF THE ARABIAN PENINSULA Two facies of the Middle Jurassic are recognizable in offshore bars, and spread out in sheets over higl' parts the coastal area of Saudi Arabia. From Abu 5adriya of the bottom lands of the Arabian foreland. Lime south, aphanitic limestone, in part dark and argil mud, substituting for clay particles, came to rest in laceous, dominates. Variable amounts of lime sand, sheltered areas or below effective wave base and there usually having lime-mud matrix, are interbedded. formed tight, aphanitic limestone or the matrix of Northward, at Jauf and Safaniya, darker, more argil calcarenitic carbonate rock. laceous, presumably deeper water limestone and gray- During Callovian to early Kimmeridgian time, the black shale occur. In this area, few lime-sand beds are Tethys was a broad shallow sea depositing clastic present, and these beds are admixed with dark impure limestone from central Arabia east to Iran and Oman mud matrix. These basinal, argillaceous limestones and south across Yemen and the Aden Protectorates. and gray-black shales can be traced north into north Similar neritic limestone extends north through central eastern Iraq and across the Persian Gulf into south Iraq. In these areas extensive calcarenite fornation western Iran. and abundant remains of shallow-water organisms Updip, the darker sediments rapidly give way to attest to the fact that shoaling of the sea floor was lighter colored, commonly coarsely clastic, clean- widespread and persistent. washed shelf limestone. This neritic facies has been Perhaps the most significant facies change occurs in recognized in northwestern Jordan and central Iraq; central Iraq where the upper part of the Callovian(?) it probably passes under the Saudi Arabia-Iraq Neutral is represented by massive anhydrite. This change is Zone, includes Khurais, extends south into the western of particular importance for it marks the beginning Rub' al Khali, and then swings east across the Oman of evaporite conditions that were eventually to spread Mountains. over much of the basin of deposition. It is assumed A somewhat thinned group of partly impure lime that conditions essential to the precipitation of calcium stones with some interbedded calcarenite represents sulphate restricted basin where evaporation rtceeds the Middle Jurassic in a central Rub' al Khali well. precipitation and runoff obtained first over central Although of relatively shallow-water deposition, the Iraq and then spread progressively south to include facies represents somewhat deeper water deposition Kuwait. Here, massive salt is interbedded with sub than that of central Najd. ordinate amounts of anhydrite. Deposition of evap As indicated earlier, Middle Jurassic units are conti orite in Saudi Arabia did not begin until near the end nental elastics in southwestern Arabia, Yemen, and the of early Kimmeridgian time. Aden Protectorates. Wells along the southern margin With the approach of evaporite condition, the of the Rub' al Khali show Upper Jurassic directly on Tethys shoaled, and widespread deposition of cal Permian or older strata. Presumably, Lower and carenite began. In addition to covering much of Middle Jurassic rocks in marginal facies are overlapped Saudi Arabia, calcarenite deposited at this time has a short distance north. also been recognized in southwestern Iran, Qatar,and LATE JURASSIC AND LOWER EARLY CRETACEOUS Oman. Essentially equivalent current-washed deposits may occur elsewhere (for example Yemen and the Marine Upper Jurassic and early Lower Cretaceous Aden Protectorates), but available descriptions are too rocks, dominantly in shallow-water carbonate facies, vague to be certain. The clastic limestone represents are extensively exposed in the Middle East. The the transition from continuous carbonate deposition to carbonate succession cropping out in central Arabia is precipitation of nearly pure anhydrite. It also initiated only a small part of a great belt of limestone that widespread cyclic sedimentation which resulted in four extends from north Africa across northern Sinai and main cycles, each starting at the base with normal Lebanon, covers large parts of Syria, Iraq, Iran, Oman, marine limestone and closing with an evaporite member. Yemen, and the Aden Protectorates, and continues The sequence culminated with the deposition of a into east Africa. LATE JURASSIC massive Tithonian(?) anhydrite over much of tb^ area. Recognized in most wells so far drilled in Aral ?a- and The episode of subsidence, expansion, and infilling also present at Qatar and Bahrain, each anhydrite may of the Tethys trough that began in the Early Jurassic represent separate expansions along a similar route from continued without apparent interruption into the Late the north where a more-or-less permanent evaporite Jurassic. Low-relief landmasses adjacent to the shelf basin persisted. contributed almost no terrestrial debris. Lime sand, Conditions sufficiently restricted to bring about derived from local sources, served as a hydraulic sub precipitation of calcium sulphate occurred first over a stitute for quartz grains, and as a result calcarenite broad lagoon generally including the area of outcrop was concentrated along the beach, piled up to form and extending east over southern Ghawar. The evap- SEDIMENTARY GEOLOGY OF SAUDI ARABIA D17 orite basin continued to expand, eventually blanketing Carbonate deposition closed earlier in the west the Arabian coastal area and extending at least as far where precipitation of anhydrite first began. Saline east as southwest Iran. Presumably one or more of conditions moved progressively east and eventually the incursions reached Oman where a limestone breccia- terminated carbonate sedimentation over most of the conglomerate occupies an equivalent stratigraphic Persian Gulf area. interval. The breccia probably represents residual The same general carbonate-evaporite cycle ws to carbonate after removal of interbedded anhydrite. be repeated at least three more times before the close Major exposures of Upper Jurassic gypsum-anhydrite of the Jurassic and open marine conditions again in northeastern Yemen and adjacent parts of the Aden prevailed. Protectorates show that an arm of the saline basin(s) IOWEB EAEIY CRETACEOUS extended southwest into this area as well. Thicknesses With the influx of fresher water and termination of of the evaporite-limestone sequence deposited here are anhydrite deposition, shallow-water clastic limestones similar to those found in wells near the Saudi Arabian were once again deposited limestones which are outcrop. generally indistinguishable from those laid down In Saudi Arabia and Qatar, carbonate members during the Late Jurassic. These early Lower Ceta introducing the four evaporite cycles are, of course, ceous rocks maintain a high degree of uniformity over of prime importance because of their oil content. The much of the foreland area. Particularly noteworthy lowest of these is by far the most important reservoir; is the lateral extent of some of the calcarenite units it contains productive oil in eight widely scattered which must have been deposited literally as great fields. sheets of lime sand. One such Bemasian lime sand Owing to intense interest prompted by oil search, can be traced from Khurais to the Persian Gulf and enough data are now available to reconstruct, on a from the southern end of Ghawar into Kuwait and regional scale, conditions associated with the deposition Iraq. Even though continuity over the entire area of the lower limestone member. The general picture is not proved, concentration of sand on this scale must to emerge is that of sedimentation on a broad shelf. assuredly reflect a huge, remarkably flat, submarine On the west, bordering the present area of Upper platform only just below sea level. Although cal Jurassic outcrop, mainly clean-washed calcarenite and carenite is the most striking lithologic component, the calcirudite accumulated. Presumably these mud-free larger part of the Bemasian and Valanginian section deposits record littoral and near-littoral deposition is aphanitic limestone and calcarenitic limestone. around the edges of a carbonate-covered platform. Similar neritic carbonates occur in the central Presence of wide belt of generally finer calcareous Rub' al Khali, Qatar, Oman, coastal Iran, Kuwait, sediment immediately to the east suggests lagoonal and central Iraq. More basinal calcareous mudstone conditions. The lagoonal sediments are in turn limited and radiolarian shale, marl, and limestone cover by a relatively unbroken sheet of clean calcarenite eastern Iraq. Presumably these indicate approach that blankets northern Ghawar-southern Abqaiq and to the deeper part of the basin. The axis of the extends north probably as far as Manifa. Tethyan trough at this time apparently fell near These lime sands are interpreted here as representing the northeast border of Iraq and extended southeast an offshore bar, or more likely, a group of offshore into Iran along the Zagros Mountains. bars. The absence of reefs cannot be assumed from Early Lower Cretaceous rocks do not occur along the present widely scattered data, although the marked the southern margin of the Arabian Peninsula. The lateral continuity of individual beds, uniform thickness limitation may, however, be the result of truncation of the unit, and uniform sorting of the component rather than nondeposition. Bemasian and Valangin grains suggest that large features of this type are not ian rocks are, for all practical purposes, elimirated likely to be found. Calcarenites here, as well as those south of lat. 20° N. by pre-Hauterivian erosion. What near outcrop, contain abundant dasycladaceous algae is probably an equivalent and still active surface of which certainly mark widespread and persistent unconformity is recognizable far to the south. shoaling of the sea floor into the zone of vigorous UPPER EARLY CRETACEOUS photosynthetic activity. The more muddy limestone of the Dammam-Qatif-Bahrain area is presumed to The upper Early Cretaceous shows rather marked have been deposited in water of deeper but unknown paleogeographic differentiation. The thin basal lime depth. It is tempting to think that this area lay slightly stone so persistent on outcrop gives way to shale east of the mud line of the time; however, regional only a short distance downdip, and this facies prevails considerations indicate that these sediments collected over much of the coastal area. The overlying conti in a rather localized depression. nental, deltaic, and lagoonal sandstones grade east- D18 GEOLOGY OF THE ARABIAN PENINSULA ward into limestone. Sandstone occurs in western in Qatar and Bahrain. Shale begins to predominate Jordan, western Iraq, Yemen and the western part of over sandstone in an easterly and northerly direction. the Aden Protectorates, as well as in Saudi Arabia. The elastics extend north into Kuwait, where sand is The eastern boundary of the area where sandstone still dominant, and on into southern Iraq where shale is dominant follows approximately the Tigris River is the major constituent. Sand is entirely replaced north of Baghdad, passes close to Basra, just east by limestone, anhydrite, and shale in northern Iraq. of Kuwait, west of Safaniya and Abu Hadriya, across Beds of probable Albian age, equivalent to th°s lower northern Ghawar, and thence trends south toward unit of the coastal Middle Cretaceous, apparertly are Al Mukalla on the Arabian Sea. To the east, across not exposed in central Arabia. Oman and the Persian Gulf, neritic, and possibly some From such data it can be inferred that CenoTianian deeper water, limestone dominates. Shale, commonly seas transgressed older beds and onto areas that were dark, is dominant and transitional between sandstone emergent during Albian time. Continued subsidence and limestone in a thin band, in general including and oscillation of the shoreline led to deposition of northern Ghawar, Fadhili, Abu Hadriya, Khursaniyah, thin marine stringers far up on the foreland. Greatest Manifa, Safaniya, Basra, and probably some parts subsidence occurred from the latitude of Riyadh of eastern Iraq. Radiolarian marl, shale, and limestone northward, bringing Middle Cretaceous across the occur along the eastern border of Iraq and suggest beveled edges of progressively older units to eventually basinal conditions prevailed in the area. Other cap Lower Devonian strata in the vicinity of Sf.kakah. radiolarian limestones of this age are known in northern During this submergence, littoral and shelf sani, with Oman. The axis of the Hauterivian to Aptian basin rare thin carbonate intertongues, was spread over the reproduces that of the lower Early Cretaceous, the northern area of Arabian outcrop; coarse elastics of deeper part falling near or just beyond the north probable marine origin continued downdip far enough eastern Iraq boundary and along the Zagros Mountains to blanket Ma'qala' and Khurais and extended across of Iran. the southern tip of Jordan and the southwest border In summary, deposition of a thin Hauterivian of Iraq. carbonate-shale was followed by widespread emergence. South of Riyadh, the marine sand grades laterally Continental and transitional continental-marine con into continental deposits, still mainly sandstone. ditions obtained in the west over a belt almost 200 to Presumably, the continental beds originally ertended 300 km. wide that extends from Yemen to northern north to border the marine sands on the west. The Iraq. To the east, normal marine limestones and nonmarine facies persists to the southern limit of shales were deposited on a shelf gently sloping into exposures near Wadi ad Dawasir. Even further the deeper part of the basin. Maximum subsidence, south, in Yemen, it is quite likely that a thick conti along a northwest-southeast trend, occurred in south nental-type Cretaceous sandstone includes equivalent western Iran and near northeastern Iraq. There is beds, although this has not definitely been established. some indication that subsidiary but large depressions East of the nonmarine beds and near-shore elastics, existed west of the main basin. progressively more lagoonal and neritic section was MIDDLE CRETACEOUS introduced at the expense of the sandstone. PHale is the main rock type over a large area including tH Rub* As seen on outcrop, Middle Cretaceous rocks are al Khali and much of central and northeastern Arabia. mainly sandstone with interbedded shale, at least in Similar politic rocks probably extended into south part marine in the central and northern area, and western Iraq and southern Jordan. Further basin- probably nonmarine in the south. The iron content ward, neritic limestone, in part in rudistid reef facies of the rocks gives relatively strong colors in contrast to and commonly Orbitolina-be&rwg, becomes dominant. the unoxidized drab colors that prevail in coastal well Such shelf carbonates were deposited in a bro«.d belt sections, where the series falls naturally into two lithogenetic units. extending from southern Arabia to Jordan. More The upper unit of the coastal sequence, probably precisely, slow subsidence during Cenomanian and the time equivalent of the thin marine-nonmarine Turonian time permitted accumulation of shallow- water carbonate deposits over southern Arabia generally beds cropping out in the interior, is usually shale and east of Al Mukalla and south of the Rub* al Khali. limestone with little sandstone. The lower unit, The limestone belt continues north across the Oman primarily sandstone with subordinate shale, is a thick, Mountains, flares out to cover the Trucial coast, widespread complex of deltaic, littoral, and other Qatar, Persian Gulf, and southwestern Iran, and then shallow-water deposits. Similar alternating sandstone swings inland to include Kuwait, southwestern Iraq, and shale reflecting a fluctuating shoreline also occur and most of Jordan. Even deeper water glob^erinal SEDIMENTARY GEOLOGY OF SAUDI ARABIA D19 marls were being deposited at the same time in north ally displaced the area of globigerinal marl sedimenta eastern and eastern Iraq. Presumably these continue tion to the southwest. into Iran, but proof of this is lacking. In the Oman mountains, deposition of a thick r^dio- In summary, widespread emergence of the Arabian larite-chert series was followed by large-scale extrusions foreland during the upper Early Cretaceous was of ophiolite. Orogenic movements, initiating the for followed by renewed subsidence in Cenomanian time. mation of the now complex Oman geanticline, began Northern Arabia was particularly active in this respect. during this time. Concurrently, volcanic activity ap Continued subsidence of the area between the central parently commenced in Yemen and the Western Aden Arabian and Ha'il arches permitted widespread Protectorate. Cenomanian transgression over the beveled edges of PAUEOCENE ANT> EOCENE older units. Definite disconformity between Cretaceous and UPPER CRETACEOUS Paleocene can be demonstrated in several areas1 ad When viewed only in terms of Saudi Arabia, Upper jacent to Saudi Arabia. Work with planktonic Fo- Cretaceous paleogeography is relatively simple. Domi- raminifera suggests equivalent hiatus in Arabia as well, nantly shallow-water carbonates blanket the area of although this has not been definitely proved. In any outcrop, and deeper water shale and limestone come in event, Paleocene transgression was widespread and as distance from the basin margin increases. Placed resulted in a thick succession of neritic limestone and in the framework of adjacent areas, the picture becomes more basinal marls. Marginal sandstones of this epoch significantly more variable. are unknown, carbonates alone being exposed alon^ the Emergence, possibly beginning in the Turonian, landward edges of outcrop. Abundantly fossiliferous mainly affected coastal parts of the basin and isolated limestone documents Paleocene submergence of the interior structures. Renewed transgression began in southern half of the Arabian Peninsula generally east the Campanian and reached a maximum during Mae- of long 47° E. One exception to this is the Oman strichtian time. Marginal areas of the basin are defined Mountains where Paleocene and Eocene limestone by sandstones in southwestern Arabia, the Aden Pro overlaps both flanks of the range but does not reach tectorates, and Yemen. Presumably those sandstones the crest. Al 5uqf itself was also incompletely cohered in Yemen are exclusively continental, passing east and although the remainder of the Oman-Dhufar foreland northeast through deltaic and shallow-water facies. apparently was submerged. Similar sandstones occur in northwestern Arabia near North of lat 24° N., the belt of carbonate rocks, Sakakah and southern Jordan. extending from Arabian outcrop to southwestern Iran, Along the southern fringes of the Rub* al Khali, swings northwest and continues into Iraq. Mo^t of shale is the major lithologic component almost to the Iraq was covered by globigerinal marl except near the exclusion of all other rock types. Shale persists into northeast border. Here, flysch-type deposits accumu the central Rub' al Khali and coastal Arabia but is lated in considerable thickness in a trough slightly there confined mainly to the lower part of the Upper offset southwest relative to the Late Cretaceous fore- Cretaceous. The upper part (Campanian and Mae- deep. Paleocene carbonate rocks also covered much strichtian) is dominantly shallow-water limestone of of Jordan. the blanket type. Locally, rudistids as well as other The Ypresian Age of the early Eocene witnessed the heavy-shelled molluscs are abundant. The limestone introduction of persistent and widespread evaporite sheet covers the central Rub* al Khali, northern out precipitation. Anhydrite in considerable thickness was crop area, coastal Arabia and Qatar, Kuwait, south deposited over the Eastern Aden Protectorate and western Iraq, and Jordan. In Jordan, large amounts Dhufar, in the Rub' al Khali basin, across Qatar, the of chert are interbedded. In northeastern Iraq, and western Persian Gulf, and northeastern Arabia and presumably southwestern Iran as well, more basinal continued on into Kuwait and southern Iraq. Normal conditions are reflected by the deposition of a thick marine conditions prevailed in nearby areas except for succession of globigerinal marls. red-bed deposition along the northeastern flank of the Tectonic development during upper Late Cretaceous still subsiding Iraq-Iran trough. Invasion of fresher sea water in the middle Eocene time also played a significant role in basin architecture again brought about widespread deposition of carbon and sediment patterns. Major uplift, generally along ates, both nummulitic and globigerinal. For the most the northeastern side of the Zagros Mountains, con part, pre-anhydrite sedimentation patterns and overall tributed great volumes of flysch-like detritus to a long, distribution of carbonate were reestablished. In north linear trough rapidly developing on the southwest. eastern Iraq, however, clastic deposition ceased, and a Flysch-type sediments continued to pour in and gradu carbonate-depositing sea once again covered the area. D20 GEOLOGY OF THE ARABIAN PENINSULA MIOCENE AND PLIOCENE The type locality of the Saq Sandstone occurr along Widespread emergence of the Arabian platform in a traverse from the base of Jabal al 5anadir (lat the middle Eocene reduced the Tethys to a relic sea, 26°27'24" N., long 43°30'11" E.) to Jabal Saq (lat probably much as it is now. Since then emergence has 26°16/02" N., long 43°18'37" E.) and then southwest persisted, and continental conditions have obtained to the Precambrian basement (lat 26°15'14" N., over Saudi Arabia, with the exception of minor inter long 43°06'21" E.). Calculations indicate that more mittent flooding of the present coastal area in middle than 600 m of sandstone is exposed here. Miocene time. The fauna of these marine beds indi So far as equivalent units in northwestern Arabia cate approximate correlation with the Lower Fars For are concerned, the Siq Sandstone was first described mation of Kuwait, Iraq, and Iran. The Miocene and assigned formational status during preparation sequence in Arabia probably represents in effect a thin of U.S. Geological Survey Miscellaneous Geologic wedge of lacustrine, fluvial, and coastal plain deposits Investigations Map 1-200 A (Bramkamp and others, peripheral to the main area of subsidence in Iran and 1963). The type section, just northwest of lat 28°00/ Iraq where evaporite-forming conditions prevailed. N., long 36°00/ E., is about 25 km east of Sba'lb as Marine sedimentation persisted in Iraq and Iran for Siq, from which the formational name is derived. a brief period following the Lower Fars evaporitic The lower limit of the Siq is marked by obvious phase, but this was brought to a close with the start contact between Precambrian basement and sandstone; of large-scale deformation northeast of the main trough. the upper boundary is not clearly defined. Much Rising mountain folds shed great volumes of clastic additional work needs to be done to properly define debris into adjacent synclines first red-bed (Upper the vertical and lateral extent of this formation and Fars) material and then, with accelerated deformation, to determine its relationship to overlying sandstone coarse sand and conglomerate (Bakhtiari). Combined units. thickness of these two units in some places reached as Type localities of Quweira, and Umm Sahm, Ram much as 4,000 m. Sandstones are in southwestern Jordan where they In spite of the intensity of diastrophism across the were measured and described by Quennell (1951). Persian Gulf, little trace of tectonic activity has been These formations were reliably traced southward found in Arabia. In fact, rocks of presumed Pliocene into Saudi Arabia and mapped there by photogeologic (Bakhtiari) age have a low average dip of 1 to 2 m per interpretation. Although the Umm Sahm an°l Ram km toward the Persian Gulf. How much of this is the Sandstones could have been accurately separated, result of tilting and how much is to be regarded as the two massive sandstone units were grouped for initial dip is unknown. Vast lava fields in the shield convenience and then traced on photographs into areas and in northwestern Arabia do attest, however, Arabia. to considerable volcanic activity at this and later The base of the Saq and equivalent sandstones is times. at the sharply defined unconformity with the under With the Quaternary formation of great sand deserts lying crystalline complex. The top is at the contact and sheet gravels, the landscape of Saudi Arabia took between massive sandstone and overlying grcDtolitic on its present aspect. shale (basal member of the Tabuk Formation). CAMBRIAN AND OBDOVICIAN SYSTEMS OCCUBENCE AND THICKNESS SAQ SANDSTONE AND EQUTVAJLENT UNITS SIQ, QUWEIRA The Saq Formation and equivalent units crop out UMM SAHM, AND RAM SANDSTONES in a narrow band along the northern margin of the DEFINITION Arabian Shield. Except for a short sand-covered The Saq Sandstone was named by H. L. Burchfiel interval west of 5a'il, exposures are continuous from and J. W. Hoover in 1935 for Jabal Saq (lat 26°16'02" the latitude of Riyadh to the Jordan border, a distance N., long 43°18'37" E.) where the upper part of the of nearly 1,200 km. Width of the outcrop belt expands formation is exposed. As originally defined, the Saq from a few kilometers at its southern limit to more was the lower member of the now obsolete Uyun than 100 km near the 39th meridian. The outcrop Formation. R. A. Bramkamp raised the Saq to for- belt again narrows to the northwest and is only 30 km mational status in 1952; formal recognition followed wide where it passes into Jordan. in 1956 (Thralls and Hasson), and type section details The outcrop surface is a rough, hummock^ plain were published in 1958 (Steineke and others). Out which rises imperceptibly toward the basemert com crop suitable for detailed continuous measurement plex. The plain is covered with erratic, wind-scooped is lacking, and, as a result, only the upper part of the basins and wind-sculptured hills, usually with low formation has been adequately described. relief, although a few isolated, precipitous hills rise SEDIMENTARY GEOLOGY OF SAUDI ARABIA D21 abruptly from the plain. Jabal Saq, 152 m high, and to dark-brown and purple, crossbedded sandstone with Jildiyah, 330 m high, are two of the more prominent common quartz pebble zones and locally interbeided features. red, ferruginous, sandy shale. The lower part cor^ists Thickness of the Saq, only a few meters near lat of buff, tan, purple, massive quartz sandstone. Pre 24°22' N., uniformly increases to the northwest. At sumably continental in the lower part, thin lenses of the type section thickness of more than 600 m is in purple shale with Cruziana indicate that the upper dicated by calculation. As fieldwork south and west part is at least partly marine. Further west, near of the Great Nefud is sparse, section thicknesses in BLarrat al 'Uwayri$, lenses and nodules of secondary that area are unknown. Quennell (1951) gives a com chert, hydrated iron, and manganese oxide are common. posite thickness of 850 m for the Quweira, Umm Sahm, In this area, the upper sandstone weathers to pinnacles and Ram Sandstones at the type locality in south and spires (Umm Sahm terrain). Below, a cliff - western Jordan. forming, eolian crossbedded sandstone (Ram terrain) LITHOLOGIC CHARACTER is intricately jointed and on weathering forms bosses Saq lithology is strikingly uniform both vertically and stacks having vertical and overhanging walls. and laterally. The dominant rock type is buff to NATURE OF CONTACT WITH ADJACENT FORMATIONS gray and white, commonly crossbedded, poorly to The Saq Sandstone Ijes with marked unconformity well-sorted quartz sandstone that is often friable and on a mature surface of Precambrian crystalline rocks. commonly case hardened to a black, ferruginous The contact has been observed at several localities, quartzite surface. Color varies locally to mustard although it is usually obscured by a cover of v^hite yellow or a pale brick red. In places a few thin part quartz gravel. Where the contact can be seen, the ings of sericitic siltstone are present. Concentrations basal Saq is conglomeratic, containing boulders, cobbles, of iron along a well-developed set of north-south joints and pebbles derived from underlying basement rock. and a slightly less prominent set of east-west joints The conglomeratic bed shows steep dip in several places, frequently produce low, spinelike ridges which stand but whether the dip is a result of initial deposit'onal a foot or two above the main rock surface. processes or subsequent basement tilting is unknown. At Jildiyah, 68 m of upper Saq is exposed. The The basement surface on which sedimentation first section here is mainly off-white to purple, fine- to began is remarkably flat a peneplain in the strictest medium-grained, poorly indurated, subrounded, well- sense of the word. East and southeast of 5a'il, ero- sorted quartz sandstone with streaks of quartz pebbles. sional relief seldom exceeds 10 m. Here the surface Some intervals weather brown to black to red black, dips uniformly northeast under the sedimentary blanket and many are casehardened. The sandstone is strongly at a rate of approximately 1.5°. The few observations crossbedded throughout and extensively jointed in two that have been made west of 5a'il indicate that the directions approximately at right angles to each other. peneplain extends across this area and on into Jordan A clean contact between the Saq Sandstone and over as well. lying Hanadir member of the Tabuk Formation is Throughout almost the entire length of outcrop, the exposed at Jildiyah. At Jibal a$ Tawal, to the north upper Saq limit coincides with a sharp lithologic and west, about 120 m of Saq is exposed; it is chiefly white topographic break between massive cliff -forming sand to purple to yellow, fine-grained, semifriable, massively stone below and soft, easily eroded shale above. The crossbedded sandstone and is capped by beds of the contact, apparently gradational, persists from Jordan Tabuk Formation. Sandstone with the same charac to just south of lat 26°00' N., where pre-Permian erosion teristics of crossbedding and peculiar weathering per eliminates the overlying shale and brings Upper Per sists southward to the featheredge of Saq outcrop. mian limestone into unconformable contact with the West of Jibal a^ Jawal, Saq outcrop is obscured for Saq. Southward, pre-Permian truncation cuts pro a short distance by Nafiid sand cover. As a result, gressively deeper into the Saq and, near lat 24°22' N., direct outcrop connection between Saq and equivalent has completely eliminated the formation. units to the west has not been observed. However, there is little doubt regarding equivalency for Saq, PALEONTOLOGY AND AGE and overlying Tabuk units can be traced without Age of the Saq Sandstone, previously published as break around the sharpest part of the ^Ja'il arch. In Cambrian(?), has been revised to Cambrian and Odo- addition, there is substantial evidence that the Ua'il vician. The unit, in Saudi Arabia, is essentially bsrren arch is a relatively recent feature and apparently does although tracks of questionable trilobite origin were not affect Paleozoic sedimentation patterns. discovered at §afra' Baqil as early as 1947. ^hey West of An Nafiid the upper part of the Umm Sahm were also observed on the slopes of Jabal Saq and in and Ram is buff, light-gray, red, and brown, weathering other localities northwest. Later, similar molds and D22 GEOLOGY OP THE ARABIAN PENINSULA casts were collected from the Umm Sahm-Ram se an increase in Tabuk Formation thickness and inclusion quence. The tracks, apparently confined to the upper of the overlying Tawil Sandstone as a member within part of the Saq and Umm Sahm-Ram sequence, occur the Tabuk Formation. in local thin lenses of red-purple shale. Some of these The name Tawil Sandstone was originally proposed forms were forwarded to Preston E. Cloud, Jr., for in 1944 by E. L. Berg and others to include the more identification. He referred the specimens (written than 200 m of section exposed in the north-facing commun, Oct. 6, 1960) to the genus Cruziana (con escarpment of At Tawil (lat 29°29' N., long 39C ?0' E.). sidered to be tracks of arthropods) and identified Unfortunately, neither the upper nor lower parts of the Cruziana cf. C. furdfera d'Orbigny and C. huberi unit are exposed at the type section. The lower contact (Meunier) of probable Early Ordovician age (about was designated concurrently with establishment of the Arenig of the British standard succession). Tabuk type section, nearly 250 km west of At Tawfl. The lower part of the sandstone sequence (Siq and The upper limit was defined by work in the vicinity of Quweira) can be assigned to the Cambrian with con Al Jawf (about 60 km northeast of At Tawil) where the fidence. Near the Dead Sea in Jordan, a limestone- Tawil-Jauf contact is clearly exposed. Whe^i aerial shale unit well up in the section contains Middle photography became available in 1958, the Tawil out Cambrian trilobites. Thus, the Saq (Siq, Quweira, crop was photogeologically mapped where possiHe, and, Umm Sahm and Ram equivalent) may well record in recognition of type section shortcomings, a more Early Cambrian to Early Ordovician sedimentation. complete sequence at Ash Sha'ib (lat 28°59'28" N., ECONOMIC ASPECTS long 37°06'42" E.) was designated a reference section. GROUND WATER At the same time, the Tawil was reduced to member status, for both field and photogeologic work indicated Throughout the vast outcrop area of combined Saq accurate separation from older beds of similar lithology and Umm Sahm, Ram, Quweira, and Siq Sandstones was possible in only a few places. only one significant concentration of population occurs; that is in the district of Al Qasim near lat 26°00' N. The basal 104.9 m of the Tabuk type sectior as now Here a number of wells withdraw water from the Saq defined (pi. 3) was measured on a traverse between lat aquifer. 28°33'12" N., long 36°03'17" E. and lat 28°34'10// N., Outside the area of outcrop, several settlements use long 36°14'33// E. To the east, an estimated 212.7 m water derived from the Saq. At Tabuk, an estimated of section is concealed by gravel cover. An eq^iivalent 20 to 30 flowing artesian wells have been completed in interval was studied in a supplemental section southeast the Saq within the last two years. Average depth of of Tabuk near lat 28°19'30" N., long 37°04'03// E. these wells is about 245 m, and water quality is re A minor structural disturbance prevents completely ported as good. At Tayma', three wells, ranging in filling the gap, although there is good reason tc believe depth from 107 to 122 m, produce from the Saq. The that only a few meters at the top remain unmeasured. static water level is 4.5 m, quality being about 1,300 The succeeding 159.8 m was composited from eroosures parts per million of total dissolved solids. Elsewhere, at two locations namely, lat 28°21'00// N., long 37°07'- there are a few widely separated, shallow, hand-dug 00" E. and lat 28°21'12// N., long 37°05'24// E. wells whose productivity varies directly with the annual Another 314.3 m was measured at lat 28°44'54" N., rainfall. These wells produce from Quaternary allu long 36°34'39" E., and the remaining 102.8 m of pre- vium, and the water level fluctuates with the local rise Tawil beds was described in the vicinity of Ash Sha'ib and fall of the water table. at lat 28°59/39" N., long 37°02'36" E. A supplemental measurement for the upper 177 m (Tawil Member ORDOVICIAN, SILURIAN, AND DEVONIAN SYSTEMS reference section) was obtained at lat 28°59/39" N., TABUK FORMATION LOWER ORDOVICIAN TO LOWER long 37°02'36// E. (31.0 m) and lat 28059'28" N., long DEVONIAN 37°06/42// E. (146.0m). DEFINITION The base of the Tabuk Formation is at the sh<\rp con R. A. Bramkamp and others (unpub. data) in 1954 tact between Didymograptits-besanng shale and. under proposed the name Tabuk Formation to include the lying, coarse grained, crossbedded Saq Sandstone. The thick sequence of sandstone, siltstone, and shale crop top is the apparently conformable contact of massive ping out in the vicinity of Tabuk (lat 28°23' N., long Tawil Sandstone Member with shale and limestone of 36°34/ E.). The Tabuk at that time included all beds the Jauf Formation. between the Saq Sandstone and the coarse, crossbedded sandstone of the Tawil Formation. Following formal OCCURRENCE AND THICKNESS definition of the Tabuk and Tawil Formations by The Tabuk Formation crops out in four main areas Steineke and others (1958), new field work resulted in separated from each other by the Great Nefud sand SEDIMENTARY GEOLOGY OP SAUDI ARABIA D23 belt. The first area, extending southeast from An structural trends. Alternating hard and soft elastics Nafud, discontinuously exposes Tabuk beds from Baq'a' of the pre-Tawil part of the Tabuk weather to a to Buraydah, a distance of 280 km. Available thick relatively smooth surface in comparison with the rough, ness data is limited to two sections, each composited hummocky surfaces formed by the strongly crossbrdded from several measured increments. At Baq'a', the sandstones of the Saq below and Tawil Member above. formation is slightly more than 700 m thick. So far The third main Tabuk outcrop occurs at At Tawu< as known, the thickness remains nearly constant to where the upper part of the formation (Tawil Member) lat 27°00' N. Near this parallel the pre-Permian is exposed in a precipitous range of black sandstone erosion surface intersects the Tabuk and, southward, hills, some more than 200 m high. West of At Tawfl> progressively cuts deeper into the formation. At the upper Tabuk is exposed in a sharply dissected Buraydah, the remaining Tabuk passes under the area fringing the southern edge of the Al Burayta' Nafud ash Shuqayyiqah, and final Tabuk truncation gravel plain. takes place somewhere under these sands, probably The smallest area of Tabuk outcrop is a few near the southern edge. kilometers northwest of Al Jawf where the Tawil Tabuk exposures, southeast of the Great Nefud, Sandstone Member is exposed hi the slopes of hills form a complex topography consisting of alluvium- and scarps. Here the massive sandstone is protected filled basins and intervening plains. Topographic by caps of resistant Cretaceous and Eocene limestone. expression is controlled largely by extensive duricrust UTHOIOGIC CHARACTER formation and gravels concentrated during a post- The pre-Tawil part of the Tabuk Formation con Eocene erosional cycle. sists essentially of two major types of cyclic deposits. The second and most extensive Tabuk outcrop area, The first is thin-bedded (less than 1 to 2 inches thick) roughly rectangular in shape, extends from the western sandstone, shaly sandstone, sandy shale, siltstone. and edge of the Great Nefud to the southern Jordan bound shale. Ironstone, hematite, and gypsum occur loyally, ary. In this area (about 340 km long and 240 km usually representing authigenic mineralization along wide) the Tabuk is for all practical purposes unmapped. bedding planes. Color ranges from off white to light Two sections, pieced together from isolated measure purple, buff, brown, and brick red. Vertical tubular ments, indicate that average formation thickness is structures, Scolithus, and worm trails (?) parallel to between 1,000 and 1,100 m. These figures are in bedding planes are extremely common. The second doubt, however, as the monotonous nature of the type of cyclic deposit consists of relatively thick sediments often obscures exact correlation between bedded, (usually more than 5 to 10 ft thick), variably individual sections. The picture is further complicated cemented, massive sandstone that locally is nomally by some faulting and the fact that unconformities, not crossbedded or gradationally bedded. Streaks of g"avel- yet recognized, probably occur at several levels. size grains (4 mm and larger), usually rounded quartz The Tawil Sandstone Member is exposed in a wide fragments, occur locally. Some units host Scolithus. band along the northeastern margin of the area. Because of the cyclical nature of the pre-Tawil, Contact between Tawil and pre-Tawil beds is clearly outcrop characteristics may be expressed in terras of marked along the southern edge of the Al Sufrah thin-bedded and massive units only or with reference plain by a rough, irregular escarpment. West of long to thin-bedded, slope-forming units capped, underlain, 38°00' E., the contact scarp breaks down and loses its or sandwiched by massive, ledge-forming sandstone sharp topographic expression. As a result, the Tawil units. Additionally, surface exposures of the massive exposed between Wadi Fajr and At Tubayq cannot sandstone units can be described in terms of weatl ^ring readily be separated from underlying strata. This characteristics that is, "pinnacle" or rounded "sugar area is indicated on the "Geologic map of the Arabian loaf," mainly controlled by degree of induration and Peninsula" (U.S. Geological Survey Arabian-American cementation (iron or silica) and high-angle frf.ture Oil Co., 1963) by fractional symbol and approximate contact. system. The extensive Tabuk exposure west of the Great The Tawil Member, on the other hand, is char acterized by medium- to coarse-grained sandstone Nefud forms a broad planar surface, in places broken by northwest-southwest structural ridges and drainage with common gravel and conspicuous large-scale channels. Differential relief averages about 50 m, crossbedding. individual hills rising somewhat higher above the The type section of the Tabuk Formation affords general ground level. In some areas, flat-lying, well- one of the most complete and representative de^crip- bedded sandstone is intricately dissected into mesa and tions of Tabuk rocks (pi. 4, section 1, p. 110). The butte topography, channels being controlled by local entire interval was pieced together from several sec- D24 GEOLOGY OF THE ARABIAN PENINSULA tions within an 85 km radius of Tabuk. A complete that is, with cyclic deposition of thin-Vedded, Tabuk section was measured and computed between fine-grained sandstone-siltstone and thick-bedded, Tayma' and Al Huj approximately along long 38° 30' E. coarser grained sandstone-gravel. Poor exposures and complexly interbedded lithologic 3. A "stable" heavy-mineral suite consisting of zircon, units make meaningful subdivisions difficult; however, rutile, B-tourmaline, G-tourmaline, and indicolite five major rock divisions can be recognized (section 2, is common to all samples analyzed. p. 112). 4. With the exception of certain authigenic index miner The most complete exposure of Tabuk beds south als, all heavy-mineral suites were derived from of the Great Nefud occurs in the Qusayba' section preexisting, sedimentary deposits. Accordingly, approximately along lat 26° 55' N. (section 3, p. 113). the provenance of these sandstones consisted of The dominant lithology is crossbedded sandstone, yet older sandstones and (or) unconsolidated shale, siltstone and complexly interbedded sandstone sediments. and shale. Occurrence of graptolites is widespread; 5. The sandstones sampled yielded fractions of very and vertical tubular casts, Scolithus, are common. fine sand and silt. These provided most of the Three shale members within the section form roughly usable heavy minerals. parallel escarpments near the top, middle, and at the Petrological descriptions summarizing the analytical base. Using these shales as dividers, the Qusayba' results for each formation studied follow. section can be separated into six informal units. Aruma Formation (sandstone fades). In the eastern Although the middle and upper shales cannot be traced part of At Tubayq, Upper Cretaceous Aruma sandstone laterally with any degree of certainty, the basal shale unconfonnably overlies the Tawil Member of the Tabuk (Hanadir member) can be recognized along the entire Formation. Essential Aruma minerals include quartz length of Tabuk outcrop. with an accessory heavy-mineral suite consisting of A. F. Pocock and R. P. Kopp in 1949 measured a apatite, biotite, chlorite, rutile, titanite, tournaline, second set of sections across the Tabuk just south of zircon, black opaques, brown resinous opaques, and the the Great Nefud. The line of traverse ran generally index mineral apatite-H. Frequent occurrence of between Jildiyah and Ash Shu'aybah (section 4, p. apatite-H characterizes the Aruma Formation. 113). At this latitude four main subdivisions were Apatite-H is colorless and euhedral, having extremely recognized, including an upper sandstone unit that well developed hexagonal bipyraminds and basal pina- may be equivalent to the Tawil Member further coid or thin hexagonal prism plates. Becaus** basal northwest. hexagonal sections are optically isotropic, ar exact What is possibly an igneous plug intruding Tabuk mineral species identification has not been made strata was observed during the photogeologic delinea Tawil Member. Essential minerals include well- tion of formation contacts. Although the "plug" rounded quartz grains and an accessory heavy-mineral (near lat 27°30' N., long 37 W E.) has not been suite consisting of apatite, biotite, muscovite, chlorite, visited on outcrop, the central, presumably igneous rutile, titanite, tourmaline, indicolite, zircon, leu°.oxene, core, is characteristically weathered and fractured. black opaque authigenic aggregates, D-anatase-X, and Radial drainage is well developed on beds dipping D-anatase-Y. High-frequency presence of D-anatase-X sharply away from the "intrusive" mass. If this and D-anatase-Y is characteristic of the Tawil Member. feature is, in fact, an igneous plug, it is the only such D-anatase-X is green or brown, thick, tabular, worn occurrence known within the sedimentary area of euhedral crystal fragments of anatase. Saudi Arabia. D-anatase-Y is light-yellow and brown, thick, An experimental study to determine the usefulness tabular, worn, euhedral crystal fragments of rnatase. of heavy minerals for correlation within the Saq-Tabuk D-anatase-Y exhibits a striking geometric patterning complex was completed in 1961. The survey indicated significantly different from that of D-anatase-X. that index minerals were present in sufficient number Tabuk Formation (pre-Taunl). The essential miner and variety to permit some regional correlation. As als include fine-grained and silt-sized quartz particles the study was concerned mainly with Tabuk zonation, with possible presence of several clay minerals. The the results are summarized here. The more important pre-Tawil part of the Tabuk hosts a clearly identifiable generalizations include: mineral suite consisting of apatite, biotite, chlorite, 1. Abundant heavy minerals having distinctive charac muscovite, rutile, tourmaline, indicolite, zircon, leuco- ter and frequency were recorded for each formation xene, authigenic aggregates, and A-anatase. High-fre sampled. quency presence of extremely well developed A-anatase 2. Type and frequency of heavy minerals alternate in serves as an index mineral for the thin-bedded Tabuk direct response to changes in sediment texture sandstone units. Locally there is a high frequency SEDIMENTARY GEOLOGY OF SAUDI ARABIA D25 occurrence of sericite. Tawil-type sandstone (massively directly on Tabuk. The Tabuk itself is cut out some crossbedded) is intercalated with thin-bedded sandstone where under the Nafud ash Shuqayyiqah. in certain parts of the pre-Tawil section. Where this PALEONTOLOGY AND AGE occurs, proportion of A-anatase to D-anatase-X in creases with descending stratigraphic position. Steineke and others (1958) considered the age of the A-anatase is colorless, light-green or light-brown, Tabuk Formation to be Ordovician and Silurian. authigenically zoned, thin, tabular euhedral crystal frag Fossils discovered since that time indicate the upper ments predominantly restricted to the 0.062- and part is Lower Devonian. A summary of the criteria 0.031-millimeter fractions. A-anatase also occurs as upon which present dating is based follows. crystal composites associated with authigenic aggregate Ordovician. The lowermost part of the Tabuk For material. mation (Hanadir member) is characterized by the com Saq Sandstone and equivalent units. The essential mon occurrence of graptolites. Graptolites collected in minerals include medium- to coarse-grained quartz par 1947 were identified by A. A. Weymouth as Didymo- ticles and an accessory heavy-mineral suite consisting graptus cf. D. bifidus Hall. In 1960 Reuben J. Foss, of biotite, muscovite, rutile, tourmaline, zircon, leuco- Jr., U.S. Geological Survej, reidentified the same s">eci- xene, A-tourmaline and D-anatase-Z. These heavy mens as Didymograptus protobifidus Elles. Wide lateral minerals separate the Saq Sandstone from the overlying persistence of this fossil serves to date the lower part of Tabuk by a relatively improverished heavy-mineral the Tabuk Formation as Lower Ordovician (Areiig). content, and a high-frequency ratio of zircon with the Diplograptus? sp. and Climacograptus cf. C. brevis Elles presence of D-anatase-Z and A-tourmaline. and Wood, of Upper Ordovician age, occur near the D-anatase-Z is yellow, reddish-brown, corroded, worn, middle of the formation. sometimes euhedral fragments of octahedral crystals. Silurian. Several samples collected from the Tabuk, D-anatase-Z exhibits a persistently worn, octahedral but without specific locality information, were iden form which distinguishes it from D-anatase-X and tified by Reuben J. Ross, Jr. (written commun., 1961), D-anatase-Y. as definitely Silurian on the basis of contained Mono- A-tourmaline is light- to dark-green, highly rounded graptus. Other graptolites and some pelecypods also to subhedral crystal grains of tourmaline exhibiting suggest a Silurian age although, in this case, other extremely well developed to fairly well developed authi periods are not precluded. genic terminations. The .fragility of the authigenic Devonian. A single collection from about 501 m terminations precludes any development other than Saq above the Tabuk base consisted of molds of brachiopods, in situ. pelecypods, and gastropods. With regard to the gas tropods, Ellis L. Yochelson, U.S. Geological Survey NATURE OF CONTACT WITH ADJACENT FORMATIONS (written commun., 1961) states, "The seven gastropod Throughout the length of Tabuk exposure, the base specimens are all referred to the genus Plectonotus. To of the formation is marked by sharp but apparently the best of my knowledge, Plectonotus is limited to the conformable contact between graptolitic shale above Devonian * * *." The pelecypods and brachiopods and massive, crossbedded Saq Sandstone below. also tend to give a general impression of Devonian Tabuk relationship to overlying units is considerably according to J. T. Dutro, Jr., and Ellis L. Yochelson more complex. In two areas west of the Great Nefud (written commun., 1961). Al Huj and Al Jawf the contact is conformable and As brachiopods from the upper part of the overMng upper Tabuk is directly overlain by Jauf Formation Jauf Formation have been tentatively determined as shale. Elsewhere, upper Tabuk beds are missing or Lower Devonian by G. A. Cooper, U.S. National covered. In southern Jordan and along the northern Museum, it is likely that the upper one-half of the edge of At Tawil, Cretaceous and Tertiary units cut Tabuk can be assigned to this epoch. across strike to overlap the entire Tabuk sequence. A comprehensive listing of Tabuk Formation fossils However, a down-dropped block near Umm Nukhaylah includes: Bottia sp., "Camarotoechia", Coelospiral, has preserved a small patch of Jauf and Tawil, possibly Leveneat, Lingula, Machaeraria (cf. "Rhynchonetta" in conformable sequence. formosa Hall), Amplexograptust, Olimacograptus c*. C. Southeast of the Great Nefud, Tabuk is in apparently brevis Elles and Wood, Climacograptus sp., Didymo conformable contact with Jauf limestone as far south graptus protobifidus Elles, Diplograptus, Glyptogrcptus as lat 27°40' N. At lat 27°10' N. the Jauf has been sp., Monograptus, Orthograptus aff. 0. calcaratus Lap- removed, presumably by pre-Permian erosion, and worth var. priscus, Rastrites, Colpomyat, Endoceras?, Khuff Formation limestone (Upper Permian) rests Goniophorat, Modwmorphat, Orthoceras, Paleoneilo?, D26 GEOLOGY OF THE ARABIAN PENINSULA Plaesiomya?, Plectonotus, tracks and burrows [Scolithus The base of the Jauf Formation is at the contact (=Tigittites), Arihrophycus, Cruziana]. between lower Jauf shale and silty limestor^ and underlying crossbedded sandstone of the Tabuk For ECONOMIC ASPECTS mation (Tawil Member). The top is at the uncon GROUND WATER formity between fossiliferous Jauf limestone and West of the Great Nefud, ground-water potential of overlying siltstone and sandstone of the Sakaka the thick Tabuk elastics is virtually unexplored. Con Sandstone (Wasia Formation). sidering its wide lateral extent, comparatively few watering spots exist, although there are shallow, OCCURRENCE AND THICKNESS hand-dug wells at Bi'r Fajr, Al Qalibah, and Tayma'. In comparison with other Arabian units, Jruf ex Some local water has been developed in the "Tabuk posures are small. Significant outcrops occur only in Valley" along the route of the abandoned Al Sijaz three areas. The first is around Al Jawf where the railroad. Agricultural development is notably lacking .complete sequence is cleanly laid out. Her% the except for limited areas in and near the city of Tabuk. formation forms a series of northwest-southeast es Population concentration hi the Al Qasim region carpments, each with a scree-covered face of soft, (near lat 26°00/ N., long 44W E.) has resulted in silty shale capped by a thin bed of flat-lying limestone. extensive water search. An estimated 300 flowing The scarps are generally less than 50 m high al artesian wells produce appreciable quantities of water though some are as much as 100 m high. T" ^ few from Tabuk aquifers. Similar agricultural sites are sandstone units present commonly form moderate found at Qusayba', Al 'Uyun, Baq'a', Al Quwarah and ledges hi the scarp faces, but are not sufficiently Ash Shu'aybah. The prime water source in these resistant to support large mesas. Structural events areas is local hand-dug and shallow drilled wells, all have seriously disrupted areas of Al Jawf exposures, producing from alluvial fill. Water accumulation in and fault and flexure scarps are everywhere consp *cuous. alluvium of larger channels and depressions varies In the region of Al Huj (lat 29°00' N., long 38C30'E.), with annual ram fall. At Buraydah and 'Unayzah, Jauf exposures form a northwest-trending rectangle increased rain and consequent flow through Wadi ar roughly 40 by 80 km. Here, flat-lying beds form Rimah is immediately reflected by higher water levels conspicuous scarp and bench topography and a few in the local wells. The water in this area, although isolated hills. At places the beds are disrupted by plentiful, is extremely variable in quality and is com tensional faulting and isolated down-dropped blocks. monly restricted to agricultural use because of high Main Jauf exposures in Najd (east of lors 42°) ion concentration. form two parallel escarpments, Jal al Hayla' r.ud Jal ash Shu'aybah. Each scarp, capped by a thin bed of DEVONIAN SYSTEM resistant dolomitic limestone, has an average relief of JAUF FORMATION IA>WER DEVONIAN less than 20 m along the entire 50 km of outcrop. Jauf DEFINITION exposures, hi the bottom of channels, can be traced The Jauf Formation was named by E. L. Berg and south from the escarpments almost to lat 27°25' N. others (unpub. data) in 1944 for the town of Al Jawf where Quaternary duricrust completely covors the (lat 29°49' N., long 39°52' E.) near where the entire formation. formation is exposed. The formation was originally Formational thickness ranges from 185 m in Najd to defined by generalized section measurements hi the about 300 m at the type locality. A 206-m section was vicinity of Al 'Abd syncline, 15 km northwest of Al measured at Al IJalwat in the Al Huj region. I .educed Jawf, but detailed plane-table mapping by A. F. thicknesses occur hi other areas where pre-Cre*aceous Pocock and others in 1950 resulted hi a substantially and younger erosion cycles have removed upp^r Jauf improved measurement and description. To obtain beds. a complete succession these geologists pieced together 1ITHOIOGIC CHARACTER 10 isolated measurements within a 30-km radius north The lithologic character of the Jauf Formation and west of Al Jawf. The resulting composite (pi. 4) changes little throughout the area of outcrop. The was adopted as the type section by Steineke and few complete sections that have been measured indicate others (1958). The type locality remains as published that the unit is mainly varicolored silty shale. Numer except for addition of 27.1 m of calcareous shale and ous thin beds of limestone and dolomite occur in the sandstone at the top. Previously these beds were upper one-third of the formation, and a few carbonate considered to be of Wasia (Middle Cretaceous) age; stringers are present near the base. Very thin beds of but Devonian fossils were recently discovered, and fine-grained sandstone occur at several levels. Data the unit is now included with the Jauf. from the three complete sections so far measured indi- SEDIMENTARY GEOLOGY OP SAUDI ARABIA D27 cate that, on the average, about 50 percent of the Elsewhere in this area Jauf beds are overlain by relatively formation is silty shale, 25 percent is sandstone, and recent duricrust and eolian sand. the remainder, carbonate. Significant variations in PALEONTOLOGY AND AGE shale to sand ratio between sections do occur, and Meager faunas have been collected from various levels these are, no doubt, related to lensing and complex in the Jauf Formation. Brachiopods in the upper part, interbedding. including Anathyris and Rensselaeria, have been tenta One of the most accessible and best known sections tively determined as Lower Devonian by G. A. Cooper. of the Jauf Formation is that pieced together in the Uppermost beds have yielded ostracods and fish vicinity of Al Jawf (pi. 4). As this section was measured remains. Knoxietta and fish dermal scutes were con in detail and later designated as the type locality, a sidered in 1960 by R. W. Morris, Aramco, to be complete description is given in section 5 (p. 113). definitely Devonian and probably Lower or Miidle A complete Jauf sequence (section 6, p. 115) was Devonian. measured in the vicinity of Al JJalwat (lat 28°50' N., The lower part of the Jauf Formation was origirally long 38°42' E.). The so-called transition beds of the suspected to be Silurian. However, the discovery of type section as well as the upper part of the Ham- Devonian (probably Lower Devonian) forms in the mamiyat member are apparently missing, owing to underlying Tabuk Formation precludes a Silurian age pre-Wasia erosion. Although some facies changes are for any part of the Jauf. Thus the Jauf can be assigned evident, lower units are still recognizable. a Devonian age with confidence and probably is Lower Cursory examination of Jauf Formation exposures Devonian. A comprehensive fossil list includes: in the At Tubayq-At Tawu< plateau region suggests that "Bythocypris" sp., Knoxietta sp., Anathyris, Linoula, the gross lithologic features of the type locality extend Plectambonites? sp., cf. Pleurothyris, Rensselaeria, into this area as well. Some variations within members Schizophoria spp., Schuchertella spp., Schuchertetta? were observed; however, on the basis of the limited data spp., Spirifer spp., indeterminate strophomenid brrohi- available they do not seem significant enough to warrant opod, Cyaihophyllum? sp., Favosites (s.l.) sp., Favosites? separate description. cf. Orthoceras, Tentacidites sp., Valvata, crinoid rtem A. F. Pocock and R. P. Kopp were the first to recog fragments, spongiostromatids, crustacean and arachnid nize Jauf exposures southeast of the Great Nefud. fragments, fish dermal scutes. Although 185 m of section was measured near Ash Shu'- aybah, the sequence is incompletely known, for Nafud ECONOMIC ASPECTS sand and other late sediments mask the upper and GROUND WATER middle parts. Described lithologic components include Jauf water potential is for all practical purposes red-brown, impure dolomitic limestone and gray-green, unknown. Several wells have been drilled in the Al gypsiferous, silty shale in the lower 30 m and white to Jawf area, but information on these is lacking. gray, fine-grained, crossbedded sandstone with a 1-m limestone cap in the upper 85 m. PERMIAN AND OLDEB(P) SYSTEMS WAJID SANDSTONE IX>WER PERMIAN AND OU>EB(?) NATURE OF CONTACT WITH ADJACENT FORMATIONS DEFINITION In all outcrop areas so far studied the Jauf Formation The Wajid Sandstone was named in 1948 by R. D. directly overlies Tabuk elastics. Definite evidence of Gierhart and L. D. Owens for Jibal al Wajid. Here, the discordance between the two units is lacking, although lower part of the type section (Steineke and otl Najran 228-776 O 66 3 D28 GEOLOGY OF THE ARABIAN PENINSULA (lat 17°35' N.), a distance of more than 300 km. The hematitic bands and some thin quartzitic layers are eastern edge of outcrop closely follows long 44°45' E.; included. Conglomeratic beds, some containing lime the western margin is convex against Precambrian stone pebbles, occur in the middle part basement. Maximum outcrop width of 100 km is In 1950, S. B. Henry and R. A. Bramkamp measured reached near the 19th parallel. 120 m of Wajid at Khashm Khatmah (lat 18°10' N., Smaller isolated remnants capping Precambrian long 45°23' E.) where the uppermost part of the forma basement occur to the southwest and extend into tion is exposed. The upper 50 m is dominantly gray- Yemen. In addition, part of the upper Wajid crops green to red silty shale with subordinate, interbedded, out at the base of the Bam Kha^mah escarpment tan to purple, fine-grained sandstone. White, poorly ce (lat 18°10' N., long 45°23' E.). mented, medium-grained sandstone with pink and The main area of Wajid outcrop south of Wadi ad white quartz pebbles occurs near the top. The lower 70 Dawasir forms a rather extensive pediment which m is chiefly off-white to yellow, coarse-grained, poorly maintains nearly the same gradient as gravel cover to cemented, crossbedded sandstone. Erratics derived the east. Isolated sandstone hills and dissected from granitic and metamorphic rocks occur in two sandstone plateaus and mesas rise about 150 m above zones the lower, 30 m above the base, and the upper, the pediment level. Spectacular hills occur at BanI about 70 m above. The boulders of basement rocks, Mu§ayqirah, BanI Sanamah, Jibal al Wajid, and commonly less than 1 ft in diameter, may be es much Bam Khurb. as 5 ft across. Continuous sand and gravel cover Thickness of the Wajid for sometime was considered separates Bani Kha^mah (upper Wajid) and Bani to be in excess of 300 m (Steineke and others, 1958). Khurb (middle Wajid) exposures. As a result, strati- Calculations based on new data indicate maximum graphic relationship between the two areas is uncertain, outcrop thickness may be as great as 950 m. although lithologic similarity and position relative to overlying beds suggest at least partial equivalency. UTHOIOGIC CHARACTER The intricately dissected Wajid plateau results from NATURE OF CONTACT WITH ADJACENT FORMATIONS erosion of a remarkably homogeneous continental Observations along the western margin of Wajid sandstone. Detailed measurement and description outcrop indicate that the Wajid Sandstone wav" depos of the sequence has not been possible because of ited on a gently undulating basement surface. Invari limited field work and lack of correlative marker beds. ably, a basal quartz conglomerate fills low ar^as and Lithologies characteristic of various parts of the section depressions; basement highs are covered by coarse have been described from isolated localities, however, grained crossbedded sandstone. and a discussion of them follows. Outliers of Wajid occur west and southwest of the The basal unit of the Wajid, a conglomerate im main sediment-basement contact. The largest of these mediately above Precambrian basement, has been cap high mountains in the vicinity of Najran and studied near lat 19°30' N., long 44°00' E., where a extend into Yemen. Few such isolated remnants have typical sequence is clearly exposed. Here, the unit is been visited, and, as a result, they are little known. 11 m of crossbedded sandstone and poorly consolidated, Conversely, inliers of igneous rock have been observed quartz gravel conglomerate. as much as 10 to 15 km east of the main basement In 1949, R. D. Gierhart and L. F. Ramirez measured mass. At several places granite pedestals protrude 150 m of lower Wajid at BanI Sanamah (near lat 20° 15' through Wajid cover and rise several tens of meters N., long 44°18/ E.). The interval is mainly lightgray, above the surrounding sandstone. These inliers, in poorly to moderately sorted, crossbedded sandstone each instance capped by Wajid, are believed to repre grading upward into a well-sorted, friable sandstone sent presedimentation monadnocks. A striking ex with some limonite and hematite concretions and ample of such an isolated hill occurs at IJima (lat a few dolomite fragments. Erratic bands of quartz- 18°14' N., long 44°27' E.) where a large outcrop of pebble conglomerate occur near the bottom, and thin pink granite protrudes from the wad! floor and reaches quartzitic hematite bands are present throughout. the height of flanking sandstone valley walls. A thin bed of white to tan sugary dolomite occurs near Contact between Wajid Sandstone and overlying the top. units is visible in only two areas. In the first, near lat A 365-m section, representing middle Wajid, was 20°00' N., long 44°45' E., small patches of Khuff Forma pieced together in the south-facing scarp at Bani Khurb tion (Upper Permian) limestone rest directly or Wajid. (lat 19°43' N., long 44°41' E.). The section is almost Although contact is certainly uncomformable, it is entirely sandstone, mainly poorly sorted, subangular, not possible to determine whether angular discordance crossbedded, and friable. Many red and purple is involved. The second area of upper Wajid exposure SEDIMENTARY GEOLOGY OF SAUDI ARABIA D29 is in the west-facing escarpment at Bam Khatmah. OCCURRENCE AND THICKNESS Along most of the scarp, Khuff sandstone disconform- Khuff exposures can be traced from. Ban! Kha^nah ably overlies Wajid shale. At Jabal al Qasjbah, Gat 18°00' N.) north to the Great Nefud (lat 28°10' however, Tuwaiq Mountain Limestone (Upper Jurassic) N.), a distance of more than 1200 km. Between the has overlapped the Khuff and lies unconformably on Great Nefud and Qu§ayba' (lat 27°00' N.), only snail Wajid. Alluvial cover prohibits tracing the contact outliers of Khuff are exposed. These are isolated much beyond Jabal al Qa§bah, but bore holes far from the main outcrop area to the south by duricrust to the southeast still show Tuwaiq Mountain resting cover. From Qu§ayba' south to lat 21°30' N., the on Wajid. Khuff crops out in a nearly continuous band. Width PALEONTOLOGY AND AGE of the outcrop belt throughout this distance (about 650 km) is amazingly uniform, averaging about 25 km. For many years, stratigraphic position afforded the South of lat 21°30' N., the Khuff is exposed in patches only clue to the age of the Wajid Sandstone that is, isolated by Quaternary sand and gravel. In this area, pre-Khuff and postbasement. Recently, a bore hole the best and most persistent exposures occur in the Al at Ash Sharawrah (lat 17°30' N., long 47°06' E.) recovered Lower Permian spores from beds probably 'Ariel and BanI Khatmah escarpments. Thickness of the Khuff Formation shows little vari equivalent to the upper Wajid of outcrop. Other ation when length of outcrop is taken into consideration. southern Arabian wells have penetrated Lower Devoni Five measurements between lat 27°00' N. and 24°00' an through Carboniferous spore zones in strata that N. recorded a minimum thickness of 235 m and a m axi- are probably, at least in part, equivalent to outcrop mum of 292 m. South to Wadi ad Dawasir (lat 20°00' Wajid. Age of the Wajid Sandstone is therefore N.), some thinning is evident and the unit range-* in considered to be Lower Permian and older(?). thickness from 195 to 230 m. The southernmost ex ECONOMIC ASPECTS posure of a full Khuff sequence (154 m) occurs in the Al 'Ariel scarp near lat 19°00' N. The reduced sections GROUND WATBK (100 to 0 m) measured further south at BanI Khatnaah Subsurface water potential is virtually unexplored represent truncated rather than true depositional ttick- except for a single penetration at Ash Sharawrah. The nesses. well at this locality was bottomed in the Wajid and LETHOLOGIC CHARACTER now produces potable water from these sands. The lithology of the Khuff Formation changes gradu ally along strike. Generally north of lat 24°00' N., the PERMIAN SYSTEM section is characterized by a natural tripartite subdi KHUFF FORMATION UPPER PERMIAN vision, the lower and upper parts being mostly l:me- DEFINITION stone and the middle dominantly shale. South of the 24th parallel, distinction between the three units be The Khuff Formation was named for Khuff ('Ayn comes less clear as the shale is largely replaced by lime Khuff) (lat 24°55' N., long 44°43' E.) near the Riyadh- stone and much of the section is dolomitized. The Jiddah road. As defined by Max Steineke in 1937, the Khuff, mostly limestone at lat 23°00' N., becomes pro formation included the lowermost carbonate sequence gressively more clastic to the south. At the southern cropping out in central Arabia and was the basal unit end of the Al 'Ari$ escarpment, the formation is almost within the Mustawi Group a now discarded subdivi exclusively in sandstone facies. sion. Although the Khuff was first recognized in publi In 1945, R. A. Bramkamp and others measured and cation by Steineke and Bramkamp (1952b), it was not described in detail the Khuff section near Ar Rayn (lat until later that the succession exposed near Khuff was 23°33' N.). Exposures are much more complete than formally designated as type section (Steineke and at the type locality, and, as a result, the Ar Rayn irfold division of the Khuff Formation was rec 24°53'12" N., long 44°32/48" E. The base is at the ognized (section 8, p. 116) at the type locality itself unconformable contact between a lower sandy phase of (near lat 24°56' N.) in the vicinity of Wadi Maghfb. the Khuff and the underlying massive sandstone of the During 1948 and 1949, the Khuff was mapped nc^th- Saq. The top is placed at the level where Khuff lime ward from the type locality. At lat 26°30' N. the stone and dolomite are in sharp contact with red and Khuff is partly obscured by duricrust, and no complete green gypsiferous shale of the overlying Sudair Shale. outcrop occurs north of this point. A. F. Pocock and D80 GEOLOGY OF THE ARABIAN PENINSULA Location: 23°32'45" 23°43'00" 45°34'30" ° 45°42'00" Generalized lithology Diagnostic fossils Covered (4) APHANITIC-CALCARENITIC LIMESTONE: Yellow and white commonly marly and Anfa//s,Avicu/opecfen spp., Crurithyris? fossiliferous (recrystallized molluscan debris) aphanitic and calcarenitic limestone; occa sional thin beds of sandy limestone and pelecypod-bearing limestone. Brown crys talline dolomite grading to oolite calcarenite caps sequence and forms strong bench. Brownish-gray brachiopodal calcarenitic Derbyia aff. D. cymbu/a, Schucherte/fa? J limestone occurs at bottom of section. (28.2 m) (3) APHANITIC LIMESTONE: Alternating beds of gray, white and brown, rubbly weathering aphanitic limestone; occasional stringers of finely crystalline dolomite. Prominent bed of white, soft, marly limestone occurs in lower part. Tan marly poorly exposed shale occurs near the middle and gray-green and olive-green shale with thin beds of white marly fine-grained limestone makes up basal part of unit. (71.1 m) (2) DOLOMITE AND LIMESTONE: Cream im Dadoxylon indicum pure finely crystalline dolomite; subordinate interbeds of fine-grained limestone and fine grained well-cemented oolite and coquina. jH Antalis, Avicu/opecten spp. , Coe/ogash-oceras aff. C Gray porous partially recrystallized coquina mex/conum, Foordiceras transitorium with abundant small indeterminant fossils caps unit. Gray tight fine-grained to micro- crystalline limestone with brachiopods and some strongly cemented, shelly and oolitic Avicu/opecten, Derby/a? limestone occur in lower part. Basal bed is _ poorly exposed, white, rubbly weathering 1 limestone with foraminiferal oolite near the top. (33.7 m) (1) DOLOMITE AND SHALE: Dominantly tan to white, rubbly weathering dolomite; sub ordinate tan to gray fine-grained limestone. Poorly exposed unit of olive-green, partially gypsiferous shale occurs in upper part. Basal part is covered, but probably tan gypsiferous clay-shale with thin bed of pink and white poorly sorted angular granitic sand and fine conglomerate at bottom. (38.4 m) BASEMENT COMPLEX Z < Vertical scale: 1:1000 UJCT crm Q-S After R. A. Bramkamp, E. L. Berg, < R. L. Myers, and W. T. Short, 1945 o FIGURE 2. Khuff Formation reference section. SEDIMENTARY GEOLOGY OF SAUDI ARABIA D31 R. P. Kopp pieced together a section as far north as strike ridges. This feature the Khuff escarpment is Qu§ayba'; but much of the sequence is covered, and the first of several distinctive benches to be seen in the many of the interval thicknesses were calculated. The region. The upper limestone supports a somewhrt less composite section indicated 255 m of Khuff with 76 m prominent ridge the Khartam escarpment. Between of Khartam limestone, 63 m of Midhnab shale, and the Khuff (i^afraV as Sark) and Khartam escarpments, 116 m of lower Khuff limestone. the surface of the Midhnab member is characterized by Northern area Khuff is best characterized by the a number of small, shinglelike benches. Each beich is section exposed about 30 km north of Buraydah near supported by a thin carbonate bed in an otherwise lat 26°30' N. Here, Pocock and Kopp described an comparatively soft gypsum-shale sequence. Taken as almost complete succession (section 9, p. 117). a whole, the Midhnab has been eroded into a broad South of the Riyadh-Jiddah road, the Midhnab shale asymmetric valley sandwiched between the two larger passes laterally into carbonate. With loss of the shale scarps. The valley is the site of numerous date gardens separator, the three-fold subdivision can no longer be from Khuff to Buraydah. Sandstone and shale occur recognized at the Ar Rayn reference section. About in the truncated Khuff section at Bam Khatmah. 100 km south of Ar Rayn, other significant changes in NATURE OF CONTACT WITH ADJACENT FORMATIONS Khuff lithology can be observed. Near Al Mulayb (lat 22°36' N.), sand and shale intertonguing with car Geometric relationships between the Khuff and bonate give the first hint of the complete replacement underlying units suggest that epeirogenic uplift and by sandstone that occurs farther south. R. D. Gier- massive erosion preceded Khuff deposition. Region hart and others in 1949 traversed and composited the ally, it can be demonstrated that the lower Khuff poor Khuff plains exposures in the Al Mulayb area; a contact is everywhere unconformable. From north to summary of their description is given in section 10 south the Khuff (1) rests successively on Lower Devonian (P- 117). through Cambrian (?), (2) is in nonconformable contact From Al Mulayb south to the Al 'Ari$ escarpment, a with Precambrian basement between lat 24°20' N. distance of about 350 km, the Khuff is largely obscured and 20°30' N., and (3) disconformably(?) overlies by surficial sand and gravel. Because of its low relief Wajid south of lat 20°30' N. The contact between in this area, the Khuff probably is mostly shale and Khuff and older sedimentary units is most commonly sandstone. Investigation of partial exposures west of limestone on sandstone. In the general area where Al Haddar (lat 22°00' N.) tends to confirm this assump Saq Sandstone is overlain, however, the basal Khuff tion, and further support is offered by outcrops of basal is a clastic usually sandstone facies. South of the Khuff elastics near lat 20°00' N., long 44°50' N. 19th parallel the contact is also sandstone on sandstone. At Al 'Ari$, the Khuff sequence is not fully exposed The top of the Khuff is sharply defined along most of at any one locality because of partial scree cover, but the outcrop belt by conformable contact between a complete section has been obtained by moving later limestone below and red and green Sudair Shale above. ally along the scarp. In 1950, S. B. Henry and R. A. South of lat 19°00' N., Sudair is cut out and Tuwaiq Bramkamp measured the upper and middle parts of the Mountain Limestone (Upper Jurassic) rests unconfor- formation respectively 11 and 22 km south of Khashm mably on Khuff Sandstone. The pre-Tuwaiq Moun Sudayr (lat 19°12' N.); the lower part was described by tain erosion surface cuts progressively deeper into the R. D. Gierhart and L. D. Owens in 1948 at Jabal Umm Khuff until the formation is removed entirely ne^r the Ghiran (near lat 18°42' N.). A summary of this de south end of Bam Kha^mah (lat 18°00' N.). There scription is given in section 11 (p. 117). is little doubt that elimination of Khuff in northern A recent reconnaissance traverse near 'Unayzah Arabia is brought about by pre-Wasia truncation, (just south of Buraydah), revealed discrete evaporite although sands of the Great Nefud cover the area where units within the Khuff. The lowest unit, consisting of this occurs. 6 m of gypsum, is 28 m above the base of the formation. PALEONTOLOGY AND AGE A second bed of gypsum (4 m thick) is exposed some The Khuff Formation was considered by Stemeke 90 m above the base. Two other thin beds of gypsum and others (1958) to be of probable Late Permian age. were also observed further south in the area of the type As the only diagnostic fossils found on outcrop had section. been a few poorly preserved nautiloids and brachic^ods, The three major lithologic units identifiable north of some uncertainty was attached to this age assignment. the Riyadh-Jiddah road strongly influence Khuff topog Other fossils are not uncommon, but most are nondes raphy. The lower Khuff limestone forms a prominent cript pelecypods and gastropods. 20- to 40-m-high west-facing scarp, the back slope of Microfossils derived from recently drilled wellr sup which has been weathered to a series of well-developed port a Late Permian age for the Khuff. L. G. Henbest D32 GEOLOGY OF THE ARABIAN PENINSULA in 1957 (written commun.) examined a Khuff core been fully evaluated. Some promise is indicated by from Dammam well 43 (lat 26°20' N., long 50°10' E.). the fact that gas has been fourd at several localities. Several Foraminifera including Colanietta parva? (Co- lani) were identified. Henbest states "Permian is PERMIAN AND TRIASSIC SYSTEMS indicated with fair assurance. According to the SUDAIR SHALE UPPER PERMIAN AND I^OWER TBIASSIC present, rather meager knowledge of the range of DEFINITION Colanietta parva? (Colani) the lower part of the Upper The Sudair Shale is named for Khashm IT idayr Permian is indicated, but question even of Permian (lat 19°12' N., long 45°06' E.) where the lower part versus Pennsylvanian age must remain until this of the unit is well exposed. A complete discussion of evidence is verified with other information." the development of present-day Permian and Tiassic Additional core material from well 43 was sent to nomenclature is included in the definition of tt^ Jilh Richard Rezak for study. He reported (1959) "Algal Formation (p. 35). assemblage dates the Khuff Formation as late middle The Sudair was first separated out as a d^crete Permian or late Permian." unit in 1945 by R. A. Bramkamp. who designated it A shallow hole drilled in 1961 at the base of the the lower member of the now obsolete Es Sirr Forma Khuff escarpment (near lat 22°00' N.) yielded Upper tion. In 1951, R. D. Gierhart and R. A. Bramkamp Permian spores and pollen. Another Upper Permian raised the rank of the Sudair to formation and estab (Thuringian) spore and pollen suite was collected in lished the Al' 'Ari$ escarpment as the type locality. 1962 from a deep bore hole drillled near lat. 21°50' N., long 48°00/ E. Formal publication followed in 1952 (Steineke and Bramkamp, 1952b), and the type section was described On outcrop, the lower Khuff fossils (shown on fig. 2) by Steineke and others (1958). The total type section were collected between 40 and 72 m above the base of the formation. Those listed in the upper Khuff were (116 m) was pieced together from three measured collected from 2 to 28 m below the formation top. sequences: the lower 27.1 m is exposed at K>ashm Although the faunas of the lower and upper Khuff Ghutfayy (lat 19°17'42" N., long 45°06'27" E.), differ somewhat in species, the two assemblages are another 23.1 m was picked up 12 km further north essentially similar generically. Both collections in (lat 19°23'23" N., long 45 WOO" E.), and the upper clude genera that axe not known to occur above the 65.8 m occurs at Khashm Abu Ramatfah (lat 19°36'09" Paleozoic. A comprehensive listing of fossils collected N.,long45°07'21"E.). from Khuff outcrop includes: Crurithyris? sp., Derbyia The base of the Sudair is placed at the top of 8 platy sandstone unit believed to be equivalent to tl ^ top cf. D. cymbula, Derbyia spp., Derbyia?, Meekella? sp. cf. Orthotetes, cf. Rhipidomella, Schuchertetta? sp., of the Khuff Formation in its type section. The upper limit is at the contact between red Sudair shale below Antalis, Antalisl, Aviculopecten spp., Bellerophon spp., Betterophon?, Botulus? sp., Cardinia? sp., Circulopecten and Middle Jurassic sandstone above. sp., Coelogastroceral aff. C. mexicanum (Girty), Cym- OCCURRENCE AND THICKNESS polia, "Dentalium" sp., Dimyopsis sp., Dimyopsisl There are few good exposures of the Sudair. How sp., Foordiceral transitroium (Waagen)?, Hyolithes sp., ever, original extent of the formation can be inferred Nautilus spp., Oxytoma? sp., Pleuromya? sp., Pleuro- from isolated outcrops and, in areas of cover, from the tomaria? sp., Taenioceras? sp., indeterminate pectinids, Sudair's characteristically depressed topographic ex cerithids, and loxonemids, Dadoxylon indicwn, Lepido- pression. Using these criteria, the Sudair c as far south as lat 23°45' N. Between lat 23°45' N. topography. Dark-brick-red gypsiferous shale if the and 19°45' N., a distance of nearly 450 km, Sudair main component above the basal sandstone. Some rocks are exposed only in small windows in Quaternary gray-green shale is present, and three thin, iso^ted cover. Outliers of this type occur near Al Juwayfah. beds of light-gray, finely crystalline dolomite (with Al Haddaar, Khashm Mishlaii, Khashm TTaryan1 and some evaporite laminations) are included withir the Khashm Kumdah. The most southerly and best brick-red sequence. N. M. Layne, Jr., and F. L. exposures of the Sudair are in the northern part of the McAlister in 1957 studied several short intervals of Al 'Ari$ escarpment. Sudair in the same general area. Beds of msssive Poor exposure has permitted accurate measurement gysum from 8 to 13 m thick were observed within the of Sudair thickness only in two places near Buraydah succession of red and purple gypsiferous shale. and at Al 'Ari$. Some idea of regional changes can Outcrops north of Buraydah were investigate! by be obtained, however, by supplementing these figures A. F. Pocock and R. P. Kopp in 1949. A combination with calculated data. Generally speaking, the for of measured sections and calculated intervals indicates mation thins southward. A maximum thickness of a thickness of 198 m of Sudair in this area. Most 198 m was pieced together from exposures north of Buray exposures are gray, rarely yellow, red, and green, dah; a minimum (116 m) was obtained at the type friable, in part crossbedded, moderately sorted, fine- section near lat 19°30' N. An anomalous interval to medium-grained sandstone with common thin of about 180 m occurs between lat 22°00' N. and interbeds of red and gray-green shale and gypsiferous 23°00' N. Whether this represents actual reversal siltstone. Several thick beds of red and green shale in the regional trend or results from error in calculation and thin beds of red, impure limestone are present in and measurement is uncertain. the middle part of the formation. IFTHOLOGIC CHARACTER NATURE OF CONTACT WITH ADJACENT FORMATIONS Sudair lithology is dominantly brick-red and green Surface relationships, supported by subsurface data, shale with subordinate red siltstone, sandstone and indicate that the Sudair Shale conformably overlies gypsum. Several thin lenses of carbonate are inter- the Khuff Formation along the entire length of outcrop. bedded in the extreme northern and southern parts North of lat 22°00' N., the contact is usually shale of the outcrop. on limestone. To the south, Sudair shale rests on The type Sudair section, composited from several sandstone. measurements at Al 'Ari$, is the most complete sequence Between lat 28°10' N. and 22°00' N. the Sudair available (fig. 3). The upper part of the formation is conformably overlain by the Jilh Formation (Middle (65.8 m) was measured by R. D. Gierhart and Triassic). The contact is commonly marked by L. D. Owens in 1948; the remainder, by Gierhart gradation upward from red and gray silty shale and R. A. Bramkamp in 1950. Their description of through ferruginous siltstone to mottled, fine-grained the Sudair is given in section 12 (p. 117). sandstone. Quaternary gravel masks events south Because of poor exposures, the Sudair north of the of lat 22°00' N., but regional geometrical interpretation type locality has not been described in detail. Partial shows that the Jilh should be completely overlapped measurements and limited field observations do suggest, by Dhruma Formation (Middle Jurassic) in the vicinity however, that the lithology of the formation is re of lat 21°00' N. At Khashm Kumdah Gat 20°17' N.) markably uniform. In most areas, dark-brick-red and in the northern part of Al *Ari$, Dhruma c'm be shale predominates. Siltstone, sandstone, and gypsum seen directly on Sudair. occur in nil sections, but substantial amounts are Exposures in the face of the Al 'Ari^. escarpment present only in the north. show the unconformable Sudair-Dhruma contact rising D. A. Holm, A. F. Pocock, and L. F. Ramirez in uniformly to the south at a rate of 00°08' to OC°10'. 1948 pieced together numerous poor exposures south At this rate, the Dhruma is cut out and the Sudan of Wad! ar Rimah. The composite measurement is intersected by the overlapping Tuwaiq Mountain gives some idea of lower Sudair rock types in this Limestone (Upper Jurassic) just north of Khasbm az area. A white to tan, friable, poorly sorted, silty, Zifr (lat 19°30' N.). Continued pre-Tuwaiq Moun and very fine to medium-grained sandstone directly tain truncation eliminates the Sudair a few kiloireters overlies the Khuff. Weathered outcrops of this unit south of Khashm Sudayr. are often permeated with a white, gypsiferous powder, and slabs of satin spar litter the scree surface. Erosion PALEONTOLOGY AND AGE commonly cuts the sandstone into a complex of narrow No fossils have been found in Sudair outcrops. gullies and sharp, low-relief hills reminiscent of badlands Solely on the basis of stratigraphic position, the Sudair D34 GEOLOGY OF THE ARABIAN PENINSULA Location: 19°36'09"-45°07'21" 19°23'23"-45°08'00" 19°17'42"-45°06'27" -IQOO Generalized lithology Diagnostic fossils II SANDSTONE: Brown ledge-forming sandstone o SHALE: Poorly exposed brick- to dark-red massive shale; occasional silty layers. A Surface exposures barren. Spores and pollen thin calcareous siltstone and impure lime in equivalent subsurface section identified stone unit occurs approximately 50 meters as Upper Permian and Lower Triassic above the base. Lower part includes thin beds of friable fine-grained sandstone SANDSTONE: Pink, white and red fine-grained Section composited from 3 locations sandstone. After R. A. Bramkamp, R. D. Gierharv . and L. D. Owens, 1948 and 1950 Vertical scale : 1:1000 FIGTTKE 3. Sudair Shale type section. was originally presumed to be of Permian age (Steineke sphaerid WUsonastrum, closely associated with the and Bramkamp, 1952b) and later, Permian or Triassic Permian and Triassic contact, is also common. (Steineke and others, 1958). Striated bisaccate pollen, so pronounced in the Recent examination of Sudair floras recovered from Jilh (Middle Triassic), also occurs in the upp°ir part wells has yielded new evidence indicating a Permian- of the Sudair. The range of these bisaccate pollen Triassic contact within the formation. In wells where grains begins immediately above rocks that contain the lower Sudair is fossiliferous, an Upper Permian Upper Permian pollen. As there is little variation in (Thuringian) flora is found. (D. O. Hemer, Aramco, the Jilh and Sudair forms, the contact between Middle unpub. data, 1962). Such bisaccate pollen as Hoff- and Lower Triassic cannot be definitely determined. meisterites microdens Wilson and various forms of However, the development of striated bisaccate pollen Lueckisporites have been identified. The hystricho- is so closely related to the Early Triassic that there is SEDIMENTARY GEOLOGY OF SAUDI ARABIA D35 little doubt that the upper Sudair is assignable to this the Jilh is exposed continuously between Wadl Birk epoch. (lat 22°15' N.) and 'Irq al Ma?hur (lat 26°20' N.). To ECONOMIC ASPECTS the north and south however, surficial deposits cover GROUND WATER all but a few, thin, isolated patches. Throughout most of its outcrop, width of the Jilh ranges from 8 to 20 km. For all practical purposes, the ground-water potential The Jilh landscape, for some 70 to 80 km north of of the Sudair Shale is unknown. A number of shallow, Wadl Birk, is dominated by a pair of steep, west-facing hand-dug wells do exist in the Sudair outcrop belt, scarps. Near lat 24°00' N. these scarps break down but nothing is known of their quality, productivity, or and merge imperceptibly into a broad belt of low, actual water source. multiple benches with a persistent regional stride of TRIASSIC SYSTEM N. 29° W. This series of ragged, sand-drifted benches persists north to the Riyadh-Jiddah road and then JILH FORMATION MIDDLE AND UPPER (?) TRIASSIC gradually rises to a bold scarp whose steplike outer DEFINITION profile reaches a relief of 100 m at Ad Duwayjiirah The Jilh Formation is named for Jilh al 'Ishar, a low (lat 26°13' N.). The scarp continues north in bold escarpment across which the type section was meas relief until breached by the wide sand-filled channel of ured. Beds now assigned to the Jilh were first de Wadl ar Rimah. North of the wadi, it become? pro scribed by Max Steineke in 1937 (unpub. data) but gressively less precipitous and all but looses its topo were then included as part of a much larger sequence graphic identity before passing under sands of Trq al the Es Sirr Member. At that time the Es Sirr and Ma?hur. Jilh rocks can be picked up within the 'Irq Khuff Members combined formed the Mustawi Forma where they appear as low benches of slumped ool; te in tion. Later, in 1945, R. A. Bramkamp raised the Es a broad sand-free area Shamat al Akbad. Sirr and Khuff to formational status, separated the Regional change in thickness of the Jilh is fairly well Jilh limestone from bracketing strata, and designated established by sections uniformly spaced along the out the Jilh the middle of three members within the Es crop. From the 26th parallel north there is little Sirr. From top to bottom the three members were: variation from an average thickness of about 2^0 m. (1) Upper Es Sirr Sandstone (now Minjur Sandstone), The formation gradually thickens southward to a maxi (2) Jilh Limestone (now Jilh Formation), and (3) mum of 326 m at the type section (near lat 24°00' N.). Lower Es Sirr Member (now Sudair Shale). Con About 35 m of thinning occurs between the type local currently, Mustawi was raised to group rank. In 1951 ity and a section measured north of Wadl Birk. JTouth R. D. Gierhart and Bramkamp changed the three Es of the wadi, the Jilh thins abruptly, and at the southern Sirr Members to formations and, at that point, allowed most outcrop near Al Haddar, calculations show only the terms Es Sirr and Mustawi to lapse. 130 m of strata sandwiched between the Sudair Shale A list of the Permian and Triassic formational names and Minjur Sandstone. now in use was published by Steineke and Bramkamp LITHOIOGIC CHARACTER (1952b). Formal publication of Jilh and other type section particulars followed in 1958 (Steineke and A characteristic feature of the Jilh Formation is others). The type Jilh section was measured and marked vertical and lateral change in lithology. The described on a nearly straight-line traverse from lat picture is further complicated by the fact that softer 24°03'48" N., long 45°46'00" E., northeast to lat units are often obscured by blocky talus derived from 24°11'06" N., long 45°51 /30// E. limestones capping individual benches. This is par The base of the formation is covered at the type ticularly true in the central area where differential locality, but the contact can be seen 3 km northwest relief between individual benches is seldom more than where friable sandstone and green shale of the Jilh rests a few meters and the absence of crosscutting channels conformably on red shale of the Sudair. The top is at makes it difficult to get a true idea of the rock sequence. the contact between yellow-brown, slabby-weathering, At the type locality, near lat 24°00' N., the J; '.h is fine-grained limestone below and tan, massive, com chiefly a sandstone and shale unit with prominently monly crossbedded sandstone of the overlying Minjur. exposed, but nonetheless subordinate, limestone. Con centration of the carbonate beds at the top and just OCCURRENCE AND THICKNESS below the middle of the section do help to subdivide Outcrops of the Jilh Formation form a narrow, the Jilh into smaller, more natural units (pi. 5). arcuate band from Al Haddar (lat 22°00' N.) to the South of the type locality the formation grades northern edge of Shamat al Akbad (lat 28°10/ N.), a rapidly to sandstone. The isolated outcrop at lat distance of about 770 km. For all practical purposes 22°05' N. is the last one in which the Jilh can be sep- D36 GEOLOGY OF THE ARABIAN PENINSULA arated with some certainty from the overlying sandstone carbonate can be seen along Safra' al Asyab. A meas of the Minjur. urement and description of the Al Qasim sequence was Proportions of shale, sandstone, and limestone remain made near Ad Duwaybirah by A. F. Pococv and relatively constant from the type section north to Ad R. P. Kopp in 1949. The same year, they also compiled Duwaybirah, although some increase in calcareous con other representative Jilh sections near 'Ayn Ibn F ihayd tent is evident. The succession does not, however, fall and Pidah (lat 27°06' N.). Generalized descriptions into any rough grouping similar to the type section, as for these measurements are given consecutively in various rock types are intimately interbedded through sections 15, 16, and 17 (p. 119-120). out. NATURE OF CONTACT WITH ADJACENT FORMATION Significant change in Jilh lithology takes place north Surface-subsurface relationships show the Jilh For of Wadi ar Rimah as shale and sandstone in the lower mation to be conformable with Sudair Shale belcw and two-thirds of the formation are progressively replaced Minjur Sandstone above. Uppermost Sudair is visible by limestone. In connection with this, the sequence on outcrop at only a few localities, and, as a conse near the north end of §afra' al Asyab appears to be quence, the base of the Jilh is commonly taken at the mostly carbonate. Of particular interest are thick lowest Jilh limestone bed. In this case, the contact gypsum beds found in the tipper one-third of the unit falls within a gradational zone of sandstone, shale, and near 'Ayn Ibn Fuhayd (lat 26°46' N.). Beds of gypsum a few thin layers of red marly limestone. As the zone as much as 13 m thick are intercalated with rubbly marl usually does not exceed 15 m in thickness, the accuracy and shale through an interval of 70 m. Lack of ex of the pick probably varies no more than a few meters posures does not permit tracing of the gypsum to the from the precise contact. northwest; however, the fact that the interval is occu The upper Jilh contact is most often marked by a pied by a string of silt flats is, in itself, suggestive that strong topographic break between sandy, commonly evaporites and (or) other soft beds do extend along the oolitic, cliff-forming limestone (Jilh) and medium- to northeastern edge of §afra' al Asyab. coarse-grained, plains-forming sandstone (Minjur). In There is little doubt that Jilh deposits are, for the some areas, a thin, poorly exposed interval of platy most part, continental, with limestones representing marl, gypsiferous shale, and gypsiferous siltstone is individual marine incursions. The general pattern in sandwiched between the uppermost Jilh bench and the dicates that the succession becomes increasingly cal coarse sandstone of the Minjur. careous and more marine to the north. This gradation The Jilh is the lowest of a series of formatiors that is exaggerated by the surficial appearance of the out thin to the south and transgressively overlap each other crop itself. Black iron staining tends to emphasize the onto the Sudair Shale. This transgressive relationship sandstone in the south at the expense of other rock can still be seen in beds well up in the Jurassic. The components. Conversely, sandstone from Wadi ar decrease in Jilh thickness may be due to thinning ? gainst Rimah north seldom weathers dark, and this, coupled the margins of the basin, truncation by succeeding with the blocky limestone talus from numerous low- strata, or a combination of both. The absence of relief benches, accents the carbonate content. obvious unconformity adjacent to or within tH Jilh A section measured across the Jilh al 'Ishar by Max is puzzling; however, conditions of deposition near the Steineke and others in 1945 was, later the same year, shield were remarkably constant, so that breaks within selected by R. A. Bramkamp to serve as the type Jilh. the stratigraphic section would be difficult to detect. As this sequence (pi. 5) remains the most complete and best description of the Jilh Formation available, a PALEONTOLOGY AND AGE detailed description is given in section 13 (p. 118). Fossils collected from Jilh exposures, mostly poorly In 1949, Bramkamp and S. B. Henry traversed the preserved, include Ca,rdinw,7, Myophoria spp., Pseudo- Jilh Formation west of Khashm al Manjur (lat 23° monotis sp., fragments and nuclei of ammonites, a few 31 ' N.). Their work, coupled with limited reconnais amphibian (?) bone fragments, and indeterminate pelec- sance mapping south of Wadi Birk, shows that south ypods. Fossil wood has been observed at several levels, of the type locality the Jilh is mainly in sandstone but the material has not been positively identified. facies. A summary of their description is given in About 40 fragments and nuclei of ammonite^1 (col section 14 (p. 119). lected between 45 and 55 m below the top of tl ?- Jilh The cleanest exposures of the Jilh Formation, other and near the Marah-Buraydah road) were referred by than at the type locality, occur in Al Qasim (near lat Arkell (1952) to the Paraceratites fauna of Middle Tri- 26°15' N.) and §afra' al Asyab. Rock types exposed assic age. The same specimens were later examined in the Qasim closely resemble those at the type section. in 1961 by N. J. Silberling (written commur.); he On the other hand, a distinctive facies shift to increased recognized most of them as clionititids and clydonitids SEDIMENTARY GEOLOGY OF SAUDI ARABIA D37 and their age as Late Triassic. Silberling states his below (Jilh) and tan, commonly crossbedded, non- "best guess as to a more refined stratigraphic assign marine sandstone above. The upper limit is tta un- ment is Norian ((upper Upper Triassic) based on the conformable contact of this sandstone with red shale resemblance of some of the fragments to such genera as of the Marrat. Clydonites, Sandlingites, Steinmannites, and Allocliomtes. OCCURRENCE AND THICKNESS However, assignment to an older part of the Upper Triassic is not precluded." Outcrops of Minjur Sandstone have been idertified Other evidence on the age of the Jilh comes from bore over a distance of 820 km; that is, between lat 21°32' holes where spores and pollens have been recovered N. and 28°07/ N. The southernmost occurrence is an from equivalent strata. The flora of the upper part isolated patch in front of the 'JMwayq escarpment near of the Jilh contains such forms as Cuneatisporites Al Ji'lan (lat 21°30' N.). From Al Haddar (lat 22°00/ radialis Leschik, Pitysporites neomundanus Lesehik, N.) north to 'Irq al Mashur (lat 27° 14' N.) the Minjur Unatexttisporites mohri, Leschik, EUipsovelatisporites is continuously exposed except for breaks at Wadi plicatus Klaus, Ovcdipollis lunzensis Klaus, Enzonala- Birk and Wadi ar Rimah. Minjur rocks again appear sporites sp., thordasporites sinallichorda Klaus, and in an outlier north of 'Irq al Ma?hur between lat Taeniaesporites sp. In the lower part of the Jilh 27°56' N. and 28°07' N. striated bisaccate pollen grains become dominant and The Minjur outcrop measures some 33 km across include Striatites jacobii Jansonius, Striates richteri at its widest point near lat 24°30' N. Narrowing to (Klaus) Jansonius, Platysaccus sp., Veryhacium sp., the north and south, the last complete section are Lueckisporites spp., and Taeniaesporites interruptus less than 10 km wide. Jansonius. According to D. O. Hemer, Aramco, (writ North of the latitude of Khashm adh Dhiti (lat ten commun., February 10, 1963), the bisaccate pollens 24° 13' N.) the Minjur typically weathers to a broad found in the Jilh equivalent in bore holes have pub gravel plain with scattered black hills and low d^con- lished ranges from middle Keuper through Early Tri tinuous benches. Relief is somewhat more pronounced assic (Scythian). Logical interpretation places the toward the northern end of Safra' al Mustawi where varied flora in the Middle Triassic. Jibal Ar Rukhman (lat 26°15' N.; long 44°29' E.) and Although the Jilh is considered here to be Middle Burmah (lat 26°19' N., long 44°27' E.) rise 30 m above Triassic, it is possible that at least part of the unit is the surrounding gravel-covered surface. Wa/ii ar Upper Triassic. Rimah marks the northern limit of the $afra' al Mus ECONOMIC ASPECTS tawi plain, and beyond, the Minjur forms a broad GROUND WATER trough filled with silt flats. From Khashm adh Dhibl south the band of Minjur narrows; the upper part is Aside from a few scattered shallow, hand-dug wells exposed in a steep slope at the foot of Jabal fuwayq, there has been no development of Jilh ground water. and the lower part mostly obscured by gravel cover. MINJUR SANDSTONE UPPER TRIASSIC The Minjur Formation is 315 m thick at KHshm DEFINITION al Khal^a, the type locality. There is little change in thickness as far south as Khashm Mawan (lat 22°50/ The Minjur Sandstone is named for Khashm al N.) where 326 m were measured. South of Khashm Manjur (lat 23°31 / N.) where the upper part of the Mawan some thinning is apparent, presumably owing formation is exposed. Max Steineke (unpub. data, to truncation and overlap by Lower and Middle Juras 1937) did nor originally set the Minjur apart from other sic units. Thicknesses and relationships in this area Triassic units but included it, along with beds now are obscured, however, because of strong littologic assigned to the Jilh and Sudair, in the Es Sirr Member (obsolete). Subsequent subdivision of the Es Sirr and similarity of Minjur with bracketing strata. The Minjur Formation shows definite thinning north evolution of Triassic nomenclature is described in de of the type locality. About 290 m of section was tail under the definition of the Jilh Formation (p. 35). measured at Khashm adh Dhibl, 195 m was measured The type section of the Minjur extends from the and computed near Ar Rukhman and 185 m was cal eastern edge of the dip slope formed by the top bed of the Jilh Formation (lat 23°34'33" N., long 46°07'15" culated north of Wadi ar Rimah. E.) up to the base of the Marrat Formation in the UTHOLOGIC CHARACTER face of Khashm al KhaHa' (lat 23°35'24" N., long The Minjur Formation is essentially a littoral- 46° 10'36" E.). continental sandstone with small amounts of corglom- The base of the Minjur is at the contact between erate and shale. The lithologic character of the brown, sandy, oolite calcarenite with marine fossils unit as a whole is remarkably persistent along strike. D38 GEOLOGY OF THE ARABIAN PENINSULA It is typically a white to tan to brown, medium-grained, break from cliff-forming, sandy and oolitic limestone poorly sorted, crossbedded quartz sandstone that capping the Jilh to deeply weathered, crossbedded weathering commonly alters to jet-black quartzite sandstone of the Minjur. There is commonly a very- masses. The sand is often friable, crushing readily thin, poorly exposed gradational zone of platy-wrather- in the hand. Lenses of conglomeratic sandstone occur ing marl and gypsiferous shale. This contact is con at a number of levels, and some thin beds of red, purple, formable and can be traced with few interruptions and green shale are intercalated. Ripple marks, mud throughout the length of outcrop. cracks, and sandbar structures are common, and fossil The top of the Minjur represents an unconformity. dunes occur in the upper part of the section. No From Khashm adh Dhibi north this contact is between marine fossils have been found, but several forms of maroon siltstone or gray to black-weathering sand fossil plant material are present throughout. These stone of the Minjur and gray to tan, tight limestone of include crude stem impressions and molds of limbs and the basal Marrat. South of Khashm adh Dhibi the trunks of trees as well as silicified wood. basal Marrat grades to shale, siltstone, and evertually The most detailed description and measurement of sandstone near lat 23°00' N., where, because of litho- the Minjur is that made at Khashm al Khalta' in 1951 logic similarity, the break can no longer be recognized by R. A. Bramkamp and others (pi. 6). This section, with certainty. The Minjur south of lat 22°00' N. now designated as the type Minjur, is given in section is obscured by cover; however, geometrical interpreta 18 (p. 120). tion of bracketing units suggests that complete wedge- The lower and upper parts of a complete Minjur out occurs near lat 21°00' N. At least part, and section (327 m) were measured in the face of Khashm probably most, of the cutting takes place at tie top. Mawan (near lat 22°50' N.) respectively by R. D. In connection with this, Dhruma beds (Middle Jurassic) Gierhart and L. D. Owens in 1947 and R. A. Bramkamp overlap th.e Marrat (Lower Jurassic) just south of and H. A. Kimball in 1950. The lithology at this Khashm Mawan, transgress the Minjur and Jilh, and latitude, as well as in the localities studied further butt against Sudair (Permian and Triassic) near Wad! south, is mainly sandstone with subordinate shale. ad Dawasir. The sandstone is off-white to tan to brown, red and Obvious thinning of the Minjur can be seen between black weathering, generally crossbedded, medium to Khashm adh Dhibi and Wadi ar Rimah. Thero is no coarse grained, and commonly friable. The shale, evidence to indicate whether pre-Marrat eros; on or occurring as thin interbeds throughout the sequence, depositional thinning is involved. The last vestiges of is gray, purple and red, and commonly silty or sandy. the Minjur are beveled and disappear beneath the Maximum thickness of any individual shale bed seldom Wasia Formation (Middle Cretaceous) near the south exceeds 5 m. Several thick beds of quartz-pebble ern edge of At Taysiyah plateau (lat 28°05' N.). conglomerate occur, one at the top of the Minjur and others near the base. PALEONTOLOGY AND AGE Traverses north of the type locality also show little Only fossil wood has been found in surface exposures variation in lithology. A. F. Pocock and R. P. Kopp of the Minjur Sandstone. On its basis of the strati- in 1949 pieced together a typical section from intervals graphic position, Steineke and others (1958) considered measured and computed near lat 26°15' N. Most the Minjur to be Upper Triassic or Lower Jurassic. outcrops, through the 196-m succession, are light- Palynological investigation of Minjur sediments re colored, in part black-weathering, fine- to coarse covered from deep bore holes has recently shec1 more grained, poorly sorted, crossbedded sandstone. Thin definitive light on the age of these beds. The lower beds of varicolored gypsiferous shale are scattered part of the Minjur contains a predominance of the through the section, and two weak benches of thin- pollen grains Sulcatisporites interpositus Leschik de bedded, platy marl occur in the middle part of the scribed from the middle Keuper of Switzerland and formation. Pityosporites ruttneri Klaus as described from the In summary, Minjur lithology both vertically and Carnian (or middle Keuper) of Austria. Directly above laterally is a monotonous succession of light-colored, these forms, Minjur strata contain an entirely different crossbedded sandstone with perhaps 10 percent shale monosaccate flora. These in turn are overlain sharply occurring at various levels. Lenses of conglomeratic by a Lower Jurassic flora falling within the 1 larrat material are common. Formation dated by ammonites as Toarcian (Early Jurassic). If the middle Keuper dating of the very NATURE OF CONTACT WITH ADJACENT FORMATIONS small bisaccates is correct, it can be assumed the The contact between the Minjur and underlying Jilh monosaccate flora represents a zone of late Keuper age. Formation is generally marked by a strong topographic That they are Late Triassic is fairly certain. Using SEDIMENTARY GEOLOGY OF SAUDI ARABIA D39 these data, the Minjur can now be dated as Late Marrat can be seen within and along the nortHrn Triassic with some confidence. margin of Nafud Qunayfidhah. The erratic outcrop pattern is, at least in part, related to local fractures and ECONOMIC ASPECTS flexures splayed offshoots from the central Arabian GROUND WATER graben system. In the vicinity of Eiyadh several deep wells penetrate Marrat is again cleanly exposed north of the fault the Minjur Sandstone. These now produce large quanti zone where two continuous, though low-relief, scrips ties of potable water from this aquifer. Elsewhere only are readily recognizable. The benches become pro a few hand-dug wells exist, and the ground-water gressively more prominent to the north and, in the potential is virtually unknown. vicinity of Marah and Shaqra' (lat 25°15/ N.), form a pair of steep west-facing cuestas that dominate the JURASSIC SYSTEM landscape. The upper escarpment, 55 m in height, is MARRAT FORMATION LOWER JURASSIC formed by erosion of soft red shale under a resistant DEFINITION cap of upper Marrat limestone. The lower scarp, 40 The Marrat Formation takes its name from the town m high, is held up by rough, blocky-weathering, lower of Marah (lat 25°04/ N., long 45°29' E.) near where the Marrat limestone. type section was pieced together. Marrat strata were North of Shaqra', the escarpments gradually break at first considered by Max Steineke to be the basal down. The upper bench disappears under the sand of member of the Tuwaiq Formation. In 1945 the Marrat Nafud ath Thuwayrat near lat 25°27' N. and does not was raised to formational status by R. A. Bramkamp crop out again. The basal escarpment, progressively and Steineke, and, concurrently, the Tuwaiq was re- decreasing in height, persists to the northwest (along designated a group. The Tuwaiq Group included the the §afra' al Mustawl), passes under a thin veneer of Marrat, Dhruma, Tuwaiq Mountain, Hanifa, and sand near lat 25°45' N., and emerges a short distance Jubaila Formations. Formal publication of forma north as a low, intensely weathered bench of reddish- tional names followed (Steineke and Bramkamp), gray limestone the Sharabith escarpment. Dunes of 1952b), and, at about the same time, usage of the term Nafud ath Thuwayrat again completely overlap the Tuwaiq Group was discontinued. Finally, Steineke lower Marrat at lat 26°17' N. Further north, along strike, interdune hollows within the 'Irq al Ma?hur and others in 1958 published particulars of the Marrat type section. are floored with highly weathered basal Marrat 1'me- The lower part of the type section was measured in the stone. Another outlier of this limestone, the most escarpment (at lat 25°03'00" N., long 45°25/42// E.) northerly occurrence of Marrat known, has been about 5 km southwest of Marah; the middle part, in identified on the far side of the 'Irq near lat 28°00' N. Jabal Kumayt (lat 25°04'48" N., long 45°28'18" E.) The Marrat Formation reaches a maximum thickness immediately north of Marah; and the upper beds, of 111 m near Marah, the type locality. Uniform southeast of Marah (near lat 25°01'30" N., long thinning occurs to the north and south. A section 45°33'00"E.). composited 15 km north of Shaqra', near where the last complete Marrat sequence is exposed, showed the At the type locality, the base of the Marrat Formation is at the contact between tan, thin-bedded limestone formation to be slightly less than 90 m thick. South of the type locality, sections measured at lat 24°13' N. above and gray, angular sandstone below. The top is at the change from Marrat gray (golden-brown- (Khashm adh Dhibi), 23°57' N. (Khashm al Jufr.yr), weathering) limestone to olive-green, occasionally gyp- 23°40' N., 23°31' N. (Khashm al Manjur), and 23°12' siferous shale of the overlying Dhruma. N. (WadlBirk) were respectively 102, 92, 70, 50, and 41 m thick. The gradual southward reduction in tl ick- OCCURRENCE AND THICKNESS ness defined by these measurements apparently con Discontinuous exposures of the Marrat Formation tinues beyond Wadi Birk, but here unit boundaries are can be traced from Khashm Mawan (lat 22°50' N.) to uncertain. the At Taysiyah plateau (lat 28°03/ N.), a distance of 1ITHOIOGIO CHARACTER more than 650 km. The outcrop band reaches its In the northern area of exposure, where the M^rat maximum width, about 15 km, in the vicinity of was first studied, three natural topographic-lithe?ogic Marah. From Khashm adh Dhibi (lat 24°13' N.) subdivisions are readily visible. Basically the pattern south, the formation narrows and loses topographic is a soft red shale bracketed by resistant limestones. distinctiveness as it dips under the protective ledge In spite of significant facies changes, the three members of the ^uwayq escarpment. In the area immediately can be traced without difficulty south to Khashra al north of Khashm adh Dhibi, isolated remnants of Jufayr. D40 GEOLOGY OF THE ARABIAN PENINSULA The lower limestone member is typically exposed in One such measurement, by Max Steineke and others the Sharabith escarpment west of Nafud ath Thuway in 1945 hi the vicinity of Khashm al Jufayr (lat rat. South of Sharabith, the member crops out almost 23°57' N.), shows gradual introduction of shale at continuously to the §afra' al Mirakah (lat 24°36' N.) the expense of limestone and sandstone at the expense disappears under Nafud Qunayfidhah, reappears at of shale. The sequence is summarized hi section Khashm Dhuwayban, and is again exposed below the 20 (p. 122). Dhibi escarpment from Khashm adh Dhibi south to Still further south, R. A. Bramkamp and others in Khashm Mawan. Mostly limestone in the north, the 1945 measured the Marrat at Khashm al Hanjur. lower part of the member is mainly sandstone and shale At this point substantial changes in lithology can be at Khashm Dhuwayban (24°28/ N.). The upper part observed; most of the section is sandstone and shale, of the member grades to elastics just south of Khashm only a vestige of the basal limestone is present, and al Jufayr. the lower and middle Marrat can no longer \^ sepa The Marrat red shale member is thickest between rated. The sequence is summarized in section 21 Shaqra' and Khashm adh Dhibi. Northwest of Sha (p. 122). qra', sandstone and conglomerate displace shale before Definite Marrat is identifiable as far south as the sequence passes under Nafud ath Thuwayrat Khashm Mawan. The upper formation boundary (lat 25°27' N.). South of Khashm adh Dhibi, the fed cannot, however, be placed with certainty, as the shale member maintains lithologic identity to Khashm contact involves sandstone on sandstone. Bramkamp al Jufayr though a thick wedge of red, silty sandstone and H. A. Kimball described the entire Mawan sequence appears in the upper part. Beyond this point, pro in 1950 and assigned about 37 m to the Mar-at. A gressively more sandstone is added at the expense of summary of their notes is given in section 22 (p. 122). shale. The Marah area affords the best exposures"1 of the The upper Marrat limestone, which caps a strong Marrat Formation northwest of the reference section. escarpment, can be traced with few interruptions from It was there, in fact, that the type Marrat wa-* pieced Nafud ath Thuwayrat to Khashm Qaradan (lat together. The whole sequence does not crop out at 24°45' N.). The limestone again crops out in the face a single locality but must be composited from several of Khashm adh Dhibi. South of Khashm adh Dhibi, sections generally along lat 25°02' N. There, the three limestone is gradually replaced from the bottom up informal niembdrs of the Marrat are typically de by shale until, at Wadl Birk, there remains only a veloped. The lower and middle members w Location: Unit 1 Unit 2, 3, 4 24°13'11"-46°05'53" 24 13'15"-46°06'20" 24°13'06"-46°05'58" 24 13'20"-46°06/41" Generalized lithology Diagnostic fossils GYPSUM: Varicolored thinly bedded gypsum (4) APHANITIC AND CALCARENITIC LIME STONE: Light-brown, moderately porous chalky aphanitic limestone; thin interbeds of brown chalky gastropod calcarenitic lime stone in lower third of unit. Thin shale beds in basal part. Major cliff former Nejdia bramkampi (3) SHALE SILTSTONE, AND SANDSTONE: Hildaites sanderi Dominantly red, rarely green and yellow, shale and siltstone; poorly exposed. Prom inent ledge former in upper part reddish- brown moderately porous fine-grained cross- bedded calcareous sandstone. Thin beds j. a. a. j. of aphanitic limestone and gastropod cal carenitic limestone near top (2) APHANITIC AND CALCARENITIC LIME STONE AND DOLOMITE: Lower half: mostly brown medium-grained crystalline dolomite; cr. some interbedded golden-brown partly dolo- mitized aphanitic limestone. Upper half: o light- to red brown, porous, commonly chalky, interbedded aphanitic and calcarenitic Bou/eiceras nitescens, B. elegans, B, arableum, limestone; lime mud pellets and molluscan B. marraticum, Profogrammoceras macfagascar/ense, debris most common grains in calcarenitic Pecten ambongoensis limestone. (15.0 m) '(I) POORLY EXPOSED: Scarce outcrops indicate complexly interbedded red and green sand stone, siltstone, and shale thin beds of aphanitic, calcarenitic limestone in middle and near base of section. Capping unit reddish-brown, medium- to coarse-grained angular calcareous sandstone. (21.5 m) SANDSTONE: Reddish-brown and white medium- to coarse-grained crossbedded Vertical scale: sandstone 1:1000 ID DC After R. W. Powers and H. A. McClure, 1962 I- FIGUEE 4. Marrat Formation reference section. stone above and gray, friable, crossbedded sandstone of outcrop. A marked topographic break accompanies below. Most commonly, the basal Marrat forms a the lithologic change. Cliff-forming aphanitic lime massive ledge from which large blocks of limestone stone (Marrat) is sharply overlain by soft, olive-green talus slide down over a steep Minjur slope. From shale^or, more rarely, gypsum (Dhruma). The picture Khashm adh Dhuwayban to Khashm al Manjur a 1-m changes in the vicinity of Wadl Birk where the upper thick bed of tan limestone or dolomite marks the basal Marrat limestone and lower Dhruma shale are, in large Marrat. Usually, however, the bed is covered by scree, part, replaced by sandstone. South of Wadl Birlr the and the contact is identified by a change from cross- two formations can no longer be separated on lithologic bedded sandstone below to olive-green shale above. or topographic grounds. As an added complication, it Continuous tracing of beds has established the contact is likely that Dhruma overlap comes into play in this as far south as Khashm Mawan although, at this point, area. In fact, the Marrat is competely pinched out sandstone rests on sandstone. The lower Marrat limit not far south of Khashm Mawan, and Dhruma beds cannot be set farther south. transgress onto Minjur. The Marrat contact with the overlying Dhruma So far as the northern limit of Marrat is concerned, Formation is conformable throughout most of the length mapping shows the formation truncated by Wasia D42 GEOLOGY OF THE ARABIAN PENINSULA (Middle Cretaceous) sandstone just beyond lat 28°00 / ECONOMIC ASPECTS N. GROUND WATER PALEONTOLOGY AND AGE In the vicinity of Shaqra', deep water wells are The Marrat Formation has been dated confidently as currently producing from the Minjur Sandstone. The Toarcian by ammonites (Arkell, 1952). Nejdia bram- holes are probably open to the Marrat as we1!; how kampi Arkell and Hilda/ties sanderi Arkell were collected ever, the quantity of water, if any, being withdrawn from the upper member, the middle member is barren, from this unit is unknown. No other data on Marrat and lower Marrat beds have yielded Bouleiceras nitescens water potential is available. Thevenin, B. elegans Arkell, B. arabicum Arkell, B. marraticum Arkell, and Protogramm&ceras madagascar- DHRUMA FORMATION MIDDLE JURASSIC iense Thevenin. The Bouleiceras fauna is considered DEFINITION by Arkell to be lower Toarcian; the Nejdia fauna is The Dhruma Formation is named for the town of dated by him as early upper Toarcian. Durma (lat 24°36' N.; long 46°07' E.) near the middle Palynological determinations (D. O. Hemer, unpub. of the outcrop belt. Max Steineke in 1937 first defined data, 1963) of surface samples obtained by air drilling the Dhruma as a member of the Tuwaiq Formation indicate that the Marrat-Minjur contact also marks the along with the Marrat, Tuwaiq Mountain, Hanifa, Jurassic-Triassic boundary. The Lower Jurassic flora Jubaila, Arab, and Hith members. In 1945, R. A. consists primarily of the form Classopollis and associated Bramkamp raised each member to formation and trilete spores. Marine microflora are rare, although a raised the Tuwaiq to group. Formal publication of small number of dinoflagellates and hystrichosphaerids formational names followed (Steineke and Branakamp, are present. Abundant Upper Triassic monosaccate 1952b), and, concurrently, the term Tuwaiq Group was pollen grains immediately underlie this flora. dropped. Type section particulars were published by These data, taken collectively, indicate that early Steineke, Bramkamp, and Sander (1958). and middle Liassic time is not represented on outcrop. The type sequence was composited from a reries of A rather impressive and varied collection of mega- successive measurements at Khashm adh Dhibi (lat fossils was made during early field work, particularly 24°12'24" N.; Iong46°07'30" E.) and between Fhashm in the vicinity of Khashm adh Dhlbl. These brought adh Dhibi and Khashm al Mazru'i (lat 24°19/00// N.; about the early recognition of a Liassic (Early Jurassic) long 46°19'36" E.). This succession in the face and age for the Marrat, a determination refined by the later on the back slopes of Khashm adh Dhibi is on°- of the work of Arkell. A complete list of identified fossils most accessible and best exposed. includes: "Rhynchonetta" sp., Spiriferina spp., "Tere- At the type section, the base of the formation is at bratula" spp., Anabacia sp., Monttivaltia sp., Acrosalenia the contact between the gypsum and interbedded shale cf. A. somaliensis Currie, Hemipedina? spp., Bouleiceras and limestone and the underlying cliff-forming Marrat nitescens Thevenin, B. arabicum Arkell, B. marraticum limestone; the top is placed at the contact between Arkell, B. elegans Arkell, Hildaites sanderi Arkell, olive-green shale of the upper Dhruma and light-brown, Nejdia bramkampi Arkell, Nejdia Jurnishi Arkell, soft, chalky limestone of the Tuwaiq Mountain Protogrammoceras madagascariense Thevenin, Ampul- Limestone. letta sp., Cardium (s.l.) sp., Ceratomya spp., Ceratomya? The major subdivisions of the Dhruma Foination sp., Ceromyopsis arabica Cox, Chlamys sp., Discoehlix defined by Steineke, Bramkamp, and Sander (1958) sp., Eligmus sp., Globularia cf. 6. hemisphaerica Roemer, are retained in this report. These are (1) upper Gryphaea spp., Jibula? sp., Lima cf. L. gigantea (Sower- Dhruma here divided into two members, (2) middle by), Lima cf. L. harronis Dacque, Lopha sp., Modiolus Dhruma containing the ammonite zones Dhrumaites, (Inoperna) sp., My a sp., Mytilus (ArcomytUus) sp., Micromphalites, Tulites, and Thambites, and (?) lower Mytilus, Arcomytilus somaliensis Cox, Mytilusf sp., Dhruma with the Ermoceras zone at top, two unnamed NautUus sp., Nerinea sp., Neritaf sp., Ostrea spp., units in the middle, and the Dorsetensia zone at the base (pi. 7). Oxytoma sp., Pholadomya spp., Pholadomya (Homomyd) The upper Dhruma is divided on lithologic and inornata Sowerby, Pholadomya lirata (Sowerby), Pleu- fauna! evidence into two distinct members the romya? sp., Protocardium sp., Pseudotrapeziumt sp., Hisyan above and the 'Atash below. The definition Pseudomelania sp., Pteria (s.l.) Purpuroidea? sp., of these members is published here for the first time. Quenstedtia sp., Thracia sp., Turbinaf sp., Turbo (s.l.); The Hisyan Member is named for Wadi al IJisyan indeterminate lucionids, nautiloids, indeterminate (near lat 24°50/ N.), a major drainage chanrel that molluscs, ostracodes. cuts directly across the unit. Holm, who first proposed SEDIMENTARY GEOLOGY OF SAUDI ARABIA D43 the member in 1947, did not designate a type locality, mouth of Wadi al 'Atk. North of the wadi only a although it is known that some beds he placed at the single bench, probably the middle one, remains as a top of this unit are not now included within it. traceable topographic feature, and the Dhruma forms Type locality for the Hisyan Member is designated a lowland of banded mud flats and low benches1 sand as Khashm al 'Atash (lat 24°10'50" N., long 46°27'53" wiched between the Tuwaiq Mountain Limestone E.) where the complete section (46.5 m) is cleanly escarpment and the Nafud ath Thuwayrat, The exposed. The upper limit is at the lithologic change Nafud effectively covers the lower part of the forma from shale with thin interbeds of limestone below to tion throughout most of the area; middle and upper limestone with thinly interbedded shale above. Litho- beds are commonly disrupted and blanketed by a logically, the upper contact is somewhat gradational; flood of outwash from the Tuwayq front. faunally, it is sharp. The lower limit of the member From Al Furuthl (lat 25°46/ N.) to Az Zilfi (lat is firmly fixed at the break from yellow-gray shale 26°18/ N.), the Dhruma appears only in a few gravel- above to yellow-brown calcarenite of the 'Atash Member strewn windows in surficial deposits. The formation below. is completely overlapped by sands of Nafud ath The 'Atash Member is named for Khashm al 'A^ash Thuwayrat about 20 km north of Az Zilfi. Small (lat 24°10/50// N., long 46°27/53// E.) where the upper patches reappear near lat 27°00/ N. as yellow silt 14.1 m is cleanly exposed. The lower 12.2 m was flats and sandy-marly benches in hollows within the described from samples from a shallow air-drilled hole, 'Irq al Mazhur. T-80, spudded in at the base of the section used to From Khashm adh Dhlbl south, the Dhruma is define the upper 'Atash. The lower limit of the remarkably well exposed in a steplike series of high, member is at the change from chalky calcarenitic limestone-capped scarps and broad dip slopes. Of the limestone above to impure calcarenitic limestone and three benches identified by Holm, only the lower or calcarenite of the Dhrumaites zone below. Dhibi bench has been traced south, and it can be Considerable economic importance is attached to recognized with certainty at Al Haddar (lat 22°OY N.). the 'Atash Member as it is equivalent, both lithologic- Immediately south of Al Haddar, Dhruma limestone is ally and faunally, to the subsurface Fadhili reservoir largely replaced by shale and sandstone. As a conse an important oil-bearing interval in eastern Arabia. quence massive scarps fade, and the Dhruma retreats under a protective cap of Tuwaiq Mountain Limestone. OCCURRENCE AND THICKNESS Maximum thickness of the Dhruma Formation is at Beds of the Dhruma Formation form a boomerang- the Khashm adh Dhlbl reference section where 375 m shaped arc extending from Al 'Ari$l (near lat 19°20' was recorded. Southward the formation thin" uni N.) to 'Iraq al Mazhur (near lat 27°05/ N.), a distance formly to 264 m at Wadi Birk and about 20C m at of more than 900 km. The outcrop belt reaches a Khashm al Juwayfah (lat 22°32' N.). Thicknesses are maximum width of 20 to 25 km in the vicinity of Wadi uncertain beyond this point, as lower Dhruma sand Birk-Khashm adh Dhibi and narrows irregularly to stone is indistinguishable from sandstone of the under the north and south. lying Marrat, Minjur, and Jilh Formations. Tb^s gen Dhruma topography in the vicinity of Khashm adh eral picture, however, is one of continued southward Dhlbl is dominated by three prominent limestone thinning through progressive loss of beds at the base by benches. The lowermost of these is capped by the overlap. Finally the Dhruma wedges out agairst the Dhibi limestone which supports the high outer escarp Sudair Shale near Khashm az Zifr (lat 19°30' N.). ment of Jabal Tuwayq. Just north of Khashm adh The few Dhruma sections available north of the Dhibi, the central Arabian graben system breaks the reference locality indicate less drastic thinning in this topographic continuity of the scarps, but beyond, the direction. Intervals measured and computed near lat benches can again be accurately identified. 25°02/ N., 25°32' N., and 26°02' N., and calculated Dhruma outcrops north of the grabens are generally across the 'Irq al Ma^hur give thicknesses of about 375, poor exposures of intensely weathered shale under 350, 330, and 250 m respectively. capping ledges of limestone or, more rarely, sandstone. Many interbench areas are floored with long broken LITHOLOGIC CHARACTER ribbons of yellow silt. While this makes detailed In the area between Wadi al Eawtah (lat 23°33' N.) lithologic description difficult, it has a decided advan and Ar Raghbah (lat 25°09/ N.), the upper and lower tage so far as precise segregation of fossil horizons is benches mapped by D. A. Holm in 1947 separate the concerned. The benches become progressively more Dhruma into three natural units corresponding to the subdued until they are covered near Al Qas.ab (lat upper, middle, and lower Dhruma as defined by Stoineke, 25°18/ N.) with a flood of silt and gravel from the Bramkamp, and Sander (1958) and as used in this re- 228-776 O 66 4 D44 GEOLOGY OF THE ARABIAN PENINSULA port. Basically, the pattern from bottom to top is: (1) the first to undergo significant change. The Dhibi lower Dhruma shale with some limestone that near limestone persists but underlying beds show slight re Khsahm adh Dhibi contains a thin layer of gypsum at placement by sandstone as far north as Al Jufayr the base and that has an upper bed of massive, cliff- (lat 24°01 / N.). However, most of the replacement forming Dhibi limestone, (2) middle Dhruma limestone occurs between Faridat Bal'um (lat 23°42' N.) and containing beds of clean, current-washed calcarenite Khashm al Khalta' (lat 23°36' N.). at various levels, many as units with high-lateral per The next complete traverse of the Dhruma was made sistence, and an upper oolite bed that forms a prom at Wadl Birk (lat 23°12/ N.) where the lower part of inent cliff, and (3) upper Dhruma shale (Hisyan the formation is mostly sandstone and upper units Member), with some limestone in lower part ('Atash show little change from the reference section. In 1945, Member). R. A. Bramkamp and C. W. Brown described the lower The most important lateral change in the Dhruma Dhruma; the middle and upper members were covered Formation is the southward replacement of limestone by R. D. Gierhart and P. R. Aswerus in 1947. The and shale by sandstone. The first change is in the sequence is summarized in section 25 (p. 129). lower Dhruma which becomes dominantly sandstone in Dhruma lithology at Khashm al Juwayfah (lat the vicinity of Khashm al Khalta' (lat 23°36/ N.); the 22°32' N.) is similar to the Wad! Birk sequence but the middle and upper members give way to sandstone does exhibit some differences. Notable changes in near Al Haddar (lat 22°00/ N.). Change from lime clude: (1) an increase in shale at the expense o.^ lime stone to sandstone takes place abruptly in individual stone, probably involving beds equivalent to the 'Atash beds, usually without passing through a shaly phase. Member, (2) introduction of considerable crossb adding The southward change in facies represents transition and fossil wood in the lower sandstone, and (3) loss of from marine to continental deposition. definition of the Marrat-Dhruma contact, although In the north beyond Ar Raghbah the proportion of bracketing markers permit approximate placement. shale in the Dhruma gradually increases until it domi Further south the contact is lost completely. nates the sequence near Al Jurayfah (lat 25°31 / N.). The Al Juwayfah section was pieced togethe1* from A few thin beds of sandstone also occur here; progres two isolated measurements. Gierhart and L. D. Owens sively more are added northward. Although the trend described the lower Dhruma in the face of the Khashm toward shoreline conditions is obvious, marine beds in 1947; Bramkamp measured the middle and upper still make up a considerable part of the sequence just units opposite Khashm 'Ushayrah (lat 22 °36' N.) in before the Dhruma passes under the Nafud ath 1951. The section is summarized in section 26 (p. 130). Thuwayrat. Sections at Khashm Khar^am and along the northern Selected sections give a more detailed picture of edge of Sha'ib al Haddar (near lat 22°00/ N.) show lateral lithologic changes in the Dhruma. The ex small but significant changes relative to the Al Juv^ayfah cellent exposures at Khashm adh Dhibi are charac sequence described above. For example: (1) a thick teristic of the sequence in the central area of outcrop. lense of impure limestone and calcarenite has been R. W. Powers and H. A. McClure measured this section introduced, presumably in the lower part of the Fisyan in 1962 and prepared a detailed description of the Member in bracketing sections the interval is occu lithologies from polished sections of samples collected pied by shale, (2) interbeds of sandstone occur near at intervals of 1 m or less. The new measurement, the probable level of the 'Atash Member, (3) the Dhibi pieced together in the same general area as the type limestone is present at both localities but has lost its section (Steineke, Bramkamp, and Sander, 1958), is cliff-forming character in the Haddar section, and just herewith designated a reference section (pi. 7). A com to the south it is replaced by sandstone, and (4) beds plete succession was composited from three increments below the Dhibi are sandstone which cannot H dis (1) the lower 120.8 m was measured in the face of the tinguished from underlying Triassic units. Dhibi escarpment between lat 24°11 / 17// N., long Major lithologic variations are evident about 5 km 46°11 / 10// E. and lat 24°11'53" N., long 46°11'30" E., south of Al Haddar and at Khashm ash Shajarl (lat (2) the middle 63.7 m on the back slope of the Dhibi 21°46/ N.). South of Al Haddar, shale dominates the limestone between lat 24°11 /04// N., long 46°17/ 14'/ E. post-Micromphalites interval; remaining section exposed and lat 24°11 / 12// N., long 46°17/ 17// E., and (3) the upper 190.0 m in the face of the Tuwaiq Mountain (only above the Dhibi) is limestone. At Khasbm ash Limestone scarp between lat 24°11 / 19// N., long 46°18/ Shajari, sandstone is the main constituent, only the 50" E. and lat 24°12/34// N., long 46°19/03// E. A full lower half of the Micromphalites zone being in carbonate description is given in section 24 (p. 123). facies. The two sequences are summarized in sections South of the reference locality the lower Dhruma is 27 and 28 (p. 130), respectively. SEDIMENTARY GEOLOGY OP SAUDI ARABIA D45 South of Khashm ash Shajari the Dhruma Formation some shale was discovered in the lower 40 m. An is, with the exception of a several-meter-thick limestone estimated 108 m of covered section intervenes between at the base of the Micromphalites zone, made up of the last sandstone ledge and the projected top cf the sandstone and minor shale interbeds. The above- Marrat Formation. Therefore, the approximate total mentioned limestone has been partly replaced by shale thickness of the Dhruma Formation near Al Gl a$ is at Khashm Mishlafr (lat 21°08' N.) and completely 330 m. Essential details of Dhruma lithology are replaced by shale at Khashm faryam (lat 20°37' N.); unknown to the north. at Khashm Kumdah (lat 20°17' N.) the equivalent interval is entirely represented by sandstone. South NATURE OF CONTACT WITH ADJACENT FORMATIONS ward from this point the Dhruma continues in sand In the central area from Wadi Birk to lat 25°00' N., stone facies, eventually wedging out between Tuwaiq the Dhruma contact with the underlying Marrat For Mountain Limestone and Sudair Shale near Khashm mation is a sharp but conformable lithologic t^eak, azZifr (lat 19°30' N.). a break from shale (or gypsum in the vicinity of Khashm North of the reference section the middle Dhruma is adh Dhibi) above to limestone below. The contact the first unit to show perceptible change. By the time is nowhere exposed north of this area. From Wadi Khashm al IJaysfyah is reached (lat 24°51' N.) sig Birk south, the lower limit becomes less clear as the nificant amounts of shale have been added at the contact disappears in a monotonous sandstone-shale expense of limestone. Essentially the same limestone- sequence. Apparently, the Dhruma unconfornably shale ratio is maintained in the better exposed outcrops overlaps the Marrat near Khashm Mawan (lat 22°50' in front of Khashm at Turab (lat 25°03' N.). N.), then rests on progressively older beds of the Minjur The closest possible approach to a complete Dhruma and Jilh Formations, butts eventually against the sequence (in the northern area) was composited from Sudair (near lat 20°00' N.), and wedges out compVtely three individual sections measured approximately along between Tuwaiq Mountain and Sudair near Khashm lat 25°02' N. Here, D. A. Holm and A. F. Pocock az Zifr (lat 19°30' N.). Loss of Dhruma beds south described the upper Dhruma in 1948; R. A. Bramkamp ward over this distance almost certainly takes place and others described the middle and lower Dhruma at the base as successively younger Dhruma strata in 1949. The sequence is summarized in section 29 lap over progressively older rocks. Further data (p. 131). (from tracing of surface beds and detailed faunal Progressive addition of shale to the north results correlations in nearby bore holes) show conclusively in almost complete replacement of limestone in the that beds are not removed at the top. vicinity of Al Jurayfah (lat 25°31' N.). An almost The contact with the overlying Tuwaiq Mountain full succession, except for the basal part, can be pieced Limestone represents a conformable transition from together from measurements of the upper Dhruma shale to limestone. The upper Dhruma limit is placed made by Bramkamp and others in 1951 and the middle at the change from shale, thin interbeds of calcar PALEONTOLOGY AND AGE (see Steineke and Bramkamp, 1952b), and in view of LOWER DHRUMA the foregoing discussion, it is highly probable that the Arkell (1952) unhesitatingly places the lower part of ,Bathonian-Bajocian boundary does not coincide exactly the lower Dhruma in the middle Bajocian on the basis with the top of the escarpment-forming lithographic of the Dorsetensia fauna. His assignment of the Ermo- limestone on the uppermost Dhibi but falls somewhere ceras-bearing beds in the upper Dhibi limestone is some a short distance below. what less definite, namely "Early-Upper or late-Middle MIDDLE DHRUMA Bajocian." This placement of the entire lower Dhruma within the Arkell (1952) lists the following succession of middle Dhruma ammonite faunas and gives age references. Bajocian presents certain problems when viewed in relationship to the contained foraminiferal faunas. Top: Those faunas of the lower part of the lower Dhruma are Middle (and upper?) Bathonian: Dhrumaites fauna rich in members of the Lagenidae; while not as yet Micromphalites fauna worked out in detail, they carry the typical aspect of Tulites fauna published faunas from Bajocian and older beds through Lower Bathonian?: out the world. On the other hand, the foraminiferal Thambites fauna faunas of the upper part of the Dhibi limestone intro Bottom. duce many of the genera and even a few of the species There are at least three major foraminiferal zo^ies in which characterize the overlying middle Dhruma. in the middle Dhruma, but the exact relationship of Among the forms which appear in the Dhibi lime these zones to the ammonite faunas is not yet com stone are the following: Dhrumella evoluta Redmond, pletely clear. It is known, however, that Dhrumaites Haurania amiji Henson 1948, Haurania deserta Henson occurs within the upper foraminiferal zone and Tham 1948, NautUoculina spp., PJenderina inflate, Redmond, bites, within the lower zone, but the placement of the Pseudomarssonetta primitive, Redmond, Riyadhella elon- middle ammonite zones within the foraminiferal zones gata Redmond (Redmond, 1965). has yet to be established. All the above genera make their first appearance at In the following lists detailing the more conspicuous this point, some of them to persist through lengthy microfaunal elements of the middle Dhruma the words ranges. On the species level the situation is even more "upper", "middle", and "lower" are used purely in the striking because Pseudomarssonetta primitiva is abun spatial sense and are intended neither as formal zonal dant in the lower part of the middle Dhruma and designations nor as litbic subdivisions. Dhrumella evoluta and Riyadhella elongata range even The lower part of the middle Dhruma contains higher, completely through the lower two zones of the Dhrumella evolv-ta Redmond, NautUoculina spp., Pfen middle Dhruma. derina sp., Pseudomarssonetta mcclurei Redmond, Pseudo Hudson and Chatton (1959) report the entrance of marssonetta primitiva Redmond, Riyadhetta elongata the genus NautUoculina in the group b beds of the Redmond. Musandam Limestone, together with Haurania amiji The middle part of the middle Dhruma contains Henson and a Haurania of. H. deserta Henson. After Dhrumella evoluta Redmond, NautUoculina spp., Pseudo suggesting a correlation between the lower part of the marssonetta reflexa Redmond, Riyadhella elongata Red Dhruma Formation and the group b beds on the basis mond, Riyadhetta arabica Redmond, Virgulina spp. of comparable microfossils and similar stratigraphic The upper part of the middle Dhruma contains position, Hudson and Chatton go on to remark that the Flabettammina sp., NautUoculina spp., Pseudcmars- type locality of Haurania amiji and Haurania deserta in sonetta biangulata Redmond, Pseudomarssonetta bipar- Iraq is now considered to be Bathonian in age. In ad tita, Redmond, Riyadhetta inflata Redmond, Riyadhella dition to this they correlate the overlying Haurania nana Redmond, Riyadhella rotundata Redmond. (See amiji-be&riag group c beds with the middle Dhruma. Redmond, 1965.) It would thus appear that the relationship between UPPER DHRUMA the group b beds and the group c beds is exactly anal To date no ammonites have been reported from ogous to the relationship between the lower Dhruma accurately placed positions within the upper Dhruma. and the middle Dhruma. The lower unit in each in Arkell (1952) listed such forms as Gryphaea cos^ttata stance carries in its upper part fauna! elements which Douville, Eligmus rollandi Douville, Eligmus rollandi range on up into overlying beds of undoubted Bathonian Douville var. jabbokensis Cox, Eudesia cardium Lamarck and Eudesia cardioides Douville from this part of the Since the Ermoceras faunas of the Dhibi limestone section and assigned a questioned late Bathoniau age do not appear to range completely to the top of the unit to it. SEDIMENTARY GEOLOGY OP SAUDI ARABIA D47 P. M. Kier, E. G. Kaufmann, and others of the A comprehensive fossil list of the Dhruma Formation Smithsonian Institute are currently studying much includes the following: more complete megafossil collections obtained in the Brachiopoda: recent past. The outcome of their work is expected Bihenithyris barringtoni Muir-Wood to clear up much of the present uncertainty over the Burmirhynchia gregoryif Wier correct age assignment of the various segments of the Charltonithyris bihenensisl Weir Daghanirhynchia daghaniensis Muir-Wood upper Dhruma, and it would be profitless to debate Loboidothyris jubaensisl Weir the matter here. It can be noted, however, that the Parathyrisl sp. rich foraminiferal faunas in the upper part of the "Rhynchonella" spp. Hisyan Member are so closely related to those of the Somalirhynchia bihenesis Muir-Wood immediately overlying middle Callovian lower part Somalithyris aflf. S. macfadyeni Muir-Wood Terebratula respolit Stefanini of the Tuwaiq Mountain Formation as to suggest a superstes Douville" break of less than stage magnitude at the contact. aflf. T. saemanni Oppel This is particularly true of the highly evolved and Coelenterata: closely related Kurnubias occurring on both sides of Amphiastraeaf sp. the contact. In view of these relationships it is sus Anabacia spp. Isatraea? sp. pected that at least the upper part of the Hisyan Isastrocoenial. sp. Member will prove to be no older than Callovian. Milleporidium sp. A partial list of the Foraminifera occurring within Montlivaltia sp. the upper Dhruma (Redmond, 1964a, 1965) is as Stylinal sp. follows: Thamnastraea sp. colonial corals undetermined 'Atash Member ("Indicates occurrence only in lower part of Echinodermata: member): Acrosalenia somaliensis Currie Kurnubia variabilis Redmond* smelliei Currie Nautiloculina spp. spp. Pfenderella arabica Redmond* Apiocrinus aff. A. roissyanus d'Orbigny Pfenderina gracilis Redmond Bothriopneustes somaliensis Currie neocomiensis (Pfender) Smout and Sugden cf. B. orientalis Fourtau trochoidea Smout and Sugden* sp. Pseudomarssonella maxima Redmond Clypeobrissus sp. plicata Redmond Holectypus spp. Sanderella laynei Redmond* Polycyphus sp. Trocholina spp. (including a large, high-spired form with Recrosalenia sp. an acute initial end) cidaroid radioles In the foregoing list the large, high-spired Trocholina crinoid plates and stems Foraminifera: represents the lowest occurrence of this type of Trocho Ammobaculites sp. lina in Saudi Arabia. The genus is sparingly represented Bolivina sp. below this point, but all forms seen to date are small Conicospirillina sp. and low spired. Likewise, Kumubia variabilis is the Dhrumella evoluta Redmond stratigraphically lowest known representative of the Flabellammina sp. Guttulina sp. genus Kurnubia. Haplophragmoides sp. Hisyan Member: Haurania amiji Henson Upper part: deserta Henson Conicospirillina sp. Kurnubia bramkampi Redmond Kurnubia bramkampi Redmond variabilis Redmond spp. spp. Pseudomarssonella media Redmond Lenticulina sp. Riyadhella sp. Marginulina sp. Steinekella crusei Redmond Marginulinopsis spp. Trocholina palastiniensis Henson (1948) Nautiloculina spp. cf. T. palastiniensis Henson, "high spired variant" Opthalmidium sp. (Henson, 1948, pi. 13, fig. 10) Pfenderella arabica Redmond Lower part: Pfenderina gracilis Redmond Kurnubia spp. inflata Redmond Praekurnubia crusei Redmond neocomiensis Smout and Sugden Riyadhella hemeri Redmond trochoidea Smout and Sugden Trocholina sp. (small, flat form with prominent marginal sp. band on ventral surface) Praekurnubia crusei Redmond D48 GEOLOGY OF THE ARABIAN PENINSULA Pseudocyclammina sp. Dorsetensia arabica Arkell Pseudomarssonella biangulata Redmond Eligmus polytypus (Eudes-Deslongchamps) bipartita Redmond rollandi Douville" maxima Redmond rollandi var. jabbockensis Cox mcclurei Redmond weiri Cox media Redmond cf. E. aualites (Stefanini) plicata Redmond Ermoceras coronatiforme Arkell primitive Redmond coronatoides (H. Dou villa") reflexa Redmond elegans H. Douville" Pseudovirgulina sp. magnificum Arkell Riyadhetta elongata Redmond reineckeoides Arkell hemeri Redmond runcinatum Arkell inflata Redmond splendens Arkell nana Redmond strigatum Arkell arabica Redmond cf. E. mogharense H. DouvillS rotundata Redmond cf. E. runcinatum Arkell sp. Eudesia cardiddes Douville" Sanderella laynei Redmond cardium (Lamark) Spiroloculina sp. sp. Steinekella crusei Redmond Ghbularia spp. Trocholina palastiniensis Henson Goniomya? sp. cf. T. palastiniensis Henson Gryphaea balli Stefanini spp. costettata Douville Valvulina sp. Gryphaeat sp. Virgulina sp. Homomya inornata (J. de C. Sowerby) Mollusca: cf. H. gibbosa (J. Sowerby) Acteonina sp. sp. Ampullella sp. Isocyprinal sp. Anchura (Pietteia)! sp. Leda sp. Anisocardia sp. Lima gigantea (Sowerby) Aporrhais sp. harronis Dacque Area sp. sp. Arcomytilus laitmairensis de Loriol Limatula cf. L corallina Arkell somaliensis Cox cf. L. elliptica (Whiteaves) sp. Lopha eruca Cox Astarte scytalis Holdhaus gregaria (Sowerby) Bakewellia wattoni (Lycett) solitaria (J. de C. Sowerby) Bramkampia steinekei ArkeTl sp. Bullarial sp. Lucinal sp. Camptonectes cf. C. brovmi Cox lucinoids undetermined sp. Mactromya aequalis Agassiz Ceratomya wimmisensis (Gillieron) cf. M. crassa Agassiz cf. C. excentrica (Roemer) sp. cf. C. wimmisensis (Gillieron) Micromphalites clydocromphalus Arkell sp. elegans Arkell Ceromyopsis arabica Cox intermedius Arkell cf. C. somaliensis Weir pustuliferus (H. Douville') sp. vertebralis Arkell Chlamys curvivarians Dietrich cf. M. busqueti (de Grossouvre) macfadyeni Cox sp. cf. C. macfadyeni Cox Modiolus imbricatus (Sowerby) sp. (Inoperna) plicatus (J. Sowerby) Clydoniceras pseudodiscus Arkell cf. M. imbricatus (Sowerby) Cochleochilust sp. sp. Corbula sp. Mytilus somaliensis Cox Crassatellites'! sp. sp. Cylindrites cf. C. striatus (Lissajous) Nautilus spp. plicifer Cossman Nerinea (Ptygmatis) aff. brevivoluta Huddleston sp. somaliensis Weir Cylindrites^ sp. cf. N. bathonica Rigaux and Sauvage Dhrumaites cardioceratoides Arkell sp. Discohelix douvillei Cossman Nucleolites sp. elegantulum Nucula cuneiformis J. de C. Sowerby cf. D. elegantulum Douville" cf. N. tenuistriata J. de C. Sowerby Discohelixt sp. SEDIMENTARY GEOLOGY OF SAUDI ARABIA D49 Opis sp. Miscellaneous: Ostrea dubiensis Contejean calcareous sponges undertermined sp. algal balls Oxytoma inequivalve (Sowerby) ECONOMIC ASPECTS sp. Oxtomaf sp. GROUND WATER Paracenoceras sp. Only negligible information has been gathered or the Pholadomya aubryi Douville ground-water characteristics of the Dhruma Formation. lirata (J. Sowerby) somaliensis Cox The most promising area appears to be in the southwest spp. where the formation is in porous sandstone facies A Pholadomya? sp. bore hole downdip from Wadl ad Dawasir yielded large Pinna sp. quantities of fresh water from the Dhruma. Pinna? sp. Pleurotomaria sp. PETROLEUM Pleurotomaria? sp. Protocardia somaliensis Cox The few widely scattered wells that have so far sp. penetrated the Middle Jurassic have passed through Pseudisocardia macfadyeni Cox potentially favorable reservoir units at several levels. Pseudotrapezium sp. The most attractive and widespread of these is the Pteria sp. Pteroperna sp. 'Atash Member which is usually found to be at least Purpuroidea aff. P. perstriata Cossman partly in porous calcarenitic facies. The 'Atash Mem Stephanoceras arabicum Arkell ber (Fadhili reservoir) contains commercial quantities cf. 5. psilicanihus Wermbter of oil in Fadhili, Khurasis, and Qatif fields. Significant Strungia arabica Arkell Teloceras cf. T7. labrum Buckman shows have been detected in other structures. In Thambites mirabile Arkell addition, the 'Atash is oil bearing in several other f Hanif a Formation. D50 GEOLOGY OF THE ARABIAN PENINSULA OCCURRENCE AND THICKNESS sections near Al Ghat (lat 26°02/ N.), Az Zilfi (lat 26°18/ The Tuwaiq Mountain Limestone has been mapped N.), and Qiba' (lat 27°24' N.) measured respectively from Huwaymil (near lat 17°30' N.) to lat 27°30/ N., a 130, 80, and 45 m; and to the south the limestone distance of more than 1,200 km. Exposures are almost measured about 150 m at Wadi Birk, 100m at Al continuous except for sand cover at 'Irq al Mazhur, Al Haddar, 80 m in Wadi ad Dawasir, and 60 m at the Mundafan, and south of Ban! Khat-mah. From a southern end of Bam Khatmah. width of 20 km in central Najd, the outcrop band narrows irregularly to about 10 km at the northern and LTTHOIOGIC CHARACTER southern extremities. Although strike of the beds Steineke, Bramkamp, and Sander (1958) described shifts gradually along the outcrop belt, dip remains the Tuwaiq Mountain Limestone as relatively constant, averaging about 00°40'. a great plate of coral-bearing, dense, pure limestone, at the base Tuwaiq Mountain Limestone dominates both the of which is a thin noncoraliferous well-bedded chalky unit * * *. stratigraphy and topography of central Arabia. Over In typical Tuwaiq Mountain limestone, coral heads are scattered, most of its extent the limestone forms a precipitous commonly in position of growth, but ordinarily they make up only a small proportion of the rock as a whole. Several small west-facing cuesta, gently bowed in the center and reefs up to 15 meters high and about 50 meters in diameter are concave toward the west. Differential relief from the present just west of Riyadh, but aside from these the unit is top of the scarp to the valley floor in front is more than thickly, but well, bedded. To the north as far as its most 500 m in the central area of outcrop. Away from this northerly known outcrops near Aqibba [Qiba'] (about Latitude area, the height of the escarpment diminishes gradually 27° 15' North), the Tuwaiq Mountain limestone shows increased inclusion of soft limestone. To the south of the latitude of until a low weathered bench of a few meters remains in Riyadh, the unit progressively thins, the proportion of calcare- the extreme north and south at Qiba' and Huwaymil, nite increases, and corals become more and more concentrated respectively. The true elevation of the scarp remains into rubbly beds in which coral remains make up a substantial relatively constant (about 1,000 m) in the southern and proportion of the rock. Branching stromatoporoids, few in the central areas. In the north, however, the top of the central area, increase in abundance and, in places in tl e south, limestone cliff uniformly loses elevation and reaches a become rock forming. low of 640 m just before passing under 'Irq al Mazhur The marked lithologic continuity of the Tuwaiq (atlat26°30/ N.). Mountain Limestone is one of the most spectacular For all practical purposes, the western boundary of features of the geology of Arabia. In central Najd, the formation is marked by a vertical limestone cliff the formation is a massively bedded aphanitic limestone which in many places exposes 100 m or more of lower with some calcarenitic limestone and calcarenite in the Tuwaiq Mountain strata. An additional 30 to 40 m of upper part. The lower 25 to 40 m is well-bedded, Tuwaiq Mountain rocks occurs below the cliff-former, highly fossiliferous, soft, chalky limestone with some but this is more argillaceous and simply weathers as an soft calcarenitic limestone, calcarenite, and shale. The extension of the gentle slope established in the under lower chalky unit is gradational from Dhruma shale lying Dhruma strata. The upper part of the formation below to massive limestone at the top and fcrms an forms a strong plateau surface tilted gently eastward. even talus slope at the foot of the vertical clip. The This surface is not a true dip slope but is slightly beveled base of the cliff-forming limestone is not a stratigraphic so that successively younger beds are present downdip. plane. Tracing the "notch" weathered at the junction The eastern margin of the formation is usually marked of the hard and soft beds confirms this observation. by a wide flat valley eroded in front of the Hanifa Where the basal beds of the limestone become more Formation. argillaceous and calcarenitic, the "notch" rises- in the In addition to its occurrence in the Tuwayq scarp, section, and vice versa. The typical litholog^-topo- Tuwaiq Mountain Limestone is preserved in Awsat, graphic pattern of a shale slope at the base, a limestone Durmd, Qaradan, Barrah, and Majma'ah grabens as cliff above, and a broad, bench-ridden back slope cut prominent ridges and erosional remnants. In general, in the upper limestone beds maintains itself throughout only the upper part of the formation is exposed in these the outcrop belt. structures, the main exceptions being Barrah, where the Because of the limestone's tendency to form sheer basal beds crop out at the ends of the graben, and at cliffs, the only easily accessible full sections of Tuwaiq the west end of Awsat*. Mountain Limestone are found along the courses of Thickness of the Tuwaiq Mountain Limestone transverse channels which breach the Tuwayq front at reaches a maximum in the Darb al Hijaz (type locality)- nearly regular intervals. In these, erosion usually re Wadi Nisat (lat 24°15/ N.) area where between 200 duces the vertical cliff to steplike talus slopes of about and 215 m was measured and calculated. The forma 40°. Complete sections have been measured at each tion thins uniformly away from this region; to the north, gap, and a few of these have been selected to illustrate SEDIMENTARY GEOLOGY OF SAUDI ARABIA D51 the lateral changes taking place in Tuwaiq Mountain The Tuwaiq Mountain beyond Al Mundafan is lithology. again typically aphanitic limestone with a basal unit Samples from Riyadh water well 1 (lat 24°36/43 // of sandstone, although important concentratiors of N., long 46°40/38// E.) provide the most complete record calcarenite do occur at several levels. of Tuwaiq Mountain rocks, and the interval between NATURE OF CONTACT WITH ADJACENT FORMATIONS 217.6 and 420.6 m in depth in this well is herein designated as a reference section (pi. 8). A detailed The placement of the lower limit of the Tuwaiq description and lithologic breakdown made by N. M. Mountain Limestone has evoked as much spirited Layne and R. W. Powers in 1957 is given in section discussion as any other aspect of Arabian geoT ogy. 31 (p. 132). In Max Steineke's original description of the formation The first important change north of the reference in 1937, he did not precisely define the Dhruma-Tuwaiq locality is detected in the Al Ghat section where shale Mountain contact, although it appears that he con appears in the upper part and massive calcarenite sidered it to fall at the base of the massive cliff-foming beds in the lower. As a result of the introduction limestone. of softer beds, the sharp contact with the overlying R. A. Bramkamp in 1945 found that the fauna for Hanifa is lost. In 1952 S. J. Roach described and at least some distance below this point was similar measured the Al Gha^ sequence approximately along to that above. On faunal evidence, he lowered the lat 26°02/ N. The succession is summarized in section contact to the upper limit of clay shale which appeared 32 (p. 132). to coincide with the upper limit of Gryphaea costettata. Farther north, in the vicinity of Qiba', the formation In the central fuwayq this results in the inclusion is 45 m thick and can be divided into two units. The of 25 to 40 m of soft, argillaceous limestone (below upper 20 m is mainly soft, chalky limestone containing the main cliff-forming member) within the Tuwaiq a difinitive Tuwaiq Mountain fauna. The highest Mountain Limestone. beds are calcarenitic limestone with abundant silicified Later work by D. A. Holm and others in 1947 coral heads. The lower 25 m consists of platy lime showed the upper range of Gryphaea costellata to be stone, sandy marl, and a small amount of greenish- erratic. Holm then proposed the Hisyan Member yellow shale and, in the lower part, considerable (of the Dhruma) to include all beds between the base interbedded sandstone. This unit grades rapidly of the cliff-forming limestone and the top of what is northward to an argillaceous and arenaceous facies. now defined as the 'Atash Member. A detailed South of the reference locality, the first basic change lithologic-paleontologic analysis of the problem by in Tuwaiq Mountain rocks is seen just beyond Sha'ib J. S. Cruse in 1961 led to the conclusion that Bram al Haddar (lat 22°00' N.) where the basal few meters kamp's placement of the contact (as defined in Steineke of limestone is replaced by sandstone. The sand, and others, 1958) was the most logical choice, and no with some irregularity in thickness but averaging redefinition was necessary. The break is apparent about 10 m, persists to the southern limit of exposure. both on surface and in deep wells. Examination of The first significant change in beds above the sand the contact from Az Zilfl in the north to Sha'fb al is the appearance of a thick dolomite-calcarenite lens Haddar in the south has shown it to be mapp^ble. in sections immediately adjacent to Wadl ad Dawasir. Although the contact is not knife sharp, the trans;tion R. A. Bramkamp, B. Beverly, and S. B. Henry in from upper Dhruma clay to lower Tuwaiq Mountain 1954 described a typical sequence in the area at Limestone takes place in a very narrow zone, anc1 the Khashm Fardah (lat 20°12/ N.). The succession is break can be placed within a few meters with a fair summarized in section 33 (p. 132). degree of certainty. Consequently, the contact pro Flanking traverses show that the dolomite-calcarenite posed by Bramkamp in 1945 and formalized by Steineke lens is restricted to the vicinity of Wadi ad Dawasir and others (1958) is retained here. and is replaced to the north and south by fine-grained Immediately south of Sha'ib al Haddar, the b^.sal carbonate rocks. beds of the Tuwaiq Mountain Limestone are replaced by sandstone, a direct reflection of the shift to sandstone Another major lens of calcarenite, grading in both in the underlying Dhruma. Thickness of the l^,sal directions to typical massive aphanitic limestone, clastic beds never exceeds 17 m and is usually about occurs in the Al Mundafan area. A section described one-half this figure. Interbeds of calcareous sandstone by Bramkamp and H. A. Kimball in 1950 at the and sandy limestone and numerous fossils (mostly southern tip of the Al 'Ari$ escarpment (lat 18°33" N.) Nerinea) serve to separate the Tuwaiq Mourtain is characteristic. The sequence is summarized in from the Dhruma and other sandstones all the way to section 34 (p. 132). Al Mutabattitat (lat 17°47' N.). D52 GEOLOGY OF THE ARABIAN PENINSULA The Dhruma-Tuwaiq Mountain contact is conform uppermost beds are poor in Foraminifera but carry able from Qiba' to Al Haddar. Presumably the contact fragments of a Kurnubia which very possibly represents becomes disconformable between Al Haddar and lat Kurnubia jurassica (Henson). 19°24/ N., where the Dhruma pinches out and Tuwaiq A comprehensive list of fossils recovered from the Mountain rests with low angular discordance on the Tuwaiq Mountain Limestone includes: Sudair Shale. Continued transgression carries the Brachiopoda Tuwaiq Mountain Limestone across the beveled edges Rhynchonella sp. of the Sudair and Khuff Formations, and finally Somalirhynchia bihenensis Muir-Wood brings the formation into contact with the Wajid at Terebratula cf. T. bicanaliculata Schloth Bam Khat>mah. spp. Coelenterata: The Tuwaiq Mountain contact with the overlying Anabacia sp. Hanifa Formation appears to be conformable although Milleporidiumt sp. there is some evidence, at least on a local scale, that an Thamnastraea sp. unconformity separates the two units. Subsurface colonial and solitary corals undetermined data support this possibility as pre-Hanifa erosion is Echinodermata: Apiocrinus sp. demonstrable over several structures in coastal Arabia. crinoid stems and plates On outcrop, the upper limit of the Tuwaiq Moun Foraminifera: tain is placed at the change from light-colored, massive, Conicospirillina sp. resistant, coral-bearing aphanitic limestone below to Guiiulina sp. brown, softer, thin-bedded, noncoraliferous, aphanitic Haplophragmoides sp. and calcarenitic limestone above. South of Wadi ad Kurnubia jurassica? (Henson) wellingsi (Henson) Dawasir, coral heads extend up into the lower Hamifa, spp. but bed tracing, lithologic character, and topographic Lenticulina sp. expression permit placement of the formation boundary. Marginulina sp. In central and northern Najd, the basal Hanifa lime Marginulinopsis sp. stone is more argillaceous and even contains shale at Nautiloculina spp. Steinekella steinekei Redmond various localities. The contact is readily recognizable Trocholina palastiensis Henson as a white band below brown slopes, the bottom of sp. the white band coinciding with the break. The Valvulina sp. contact is excellently exposed on either side of Wadi Mollusca: Hanifah near Jabal al Abakkayn (lat 24°58' N.). Acteonia sp. Aequipecten macfadyeni Cox The Tuwaiq Mountain Limestone is abruptly Ampullella sp. terminated about 20 km north of Qiba' where it inter Anisocardia sp. sects the pre-Wasia erosion surface. Aporrhais sp. Arcomytilus PALEONTOLOGY AND AGE "Cardium" sp. The lower beds of the Tuwaiq Mountain Limestone Ceratomya wimmisensis (Gillieron) spp. were assigned to the middle Callovian coronatum cf. C. plicata (Agassiz) zofte by Arkell (1952) on the basis of contained ammon Ceromyopsis arabica Cox ites. They also contain fairly abundant foraminiferal sp. faunas including Trocholina palastiniensis Henson and Chlamys curvivarians Dietrich several undescribed species of Kurnubia. macfadyeni Cox sp. The middle and upper parts of the Tuwaiq Mountain Cylichna sp. Limestone are lacking in ammonites but contain two Cylindrites plicifer Cossman distinctive Foraminifera: Kurnubia wellingsi (Henson) Discohelix douvillei Cossman and Steineketta steinekei Redmond (Redmond, 1964a). sp. As yet, there is no record of the latter form outside Eligmus rollandi var. jabbokensis Cox sp. of Saudi Arabia, but Kurnubia wellingsi was described Erymnoceras philbyi Arkell from the Kurnub anticline where it occurs in beds cf. E. philbyi Arkell which Hudson and Chatton (1959) consider to be of cf. E. jarryi (R. DouvillS) Oxfordian (Argovian) age. aff. E. triplicatum (Till) Exogyra nana (Sowerby) Where the Tuwaiq Mountain is present in full sp. development, Steinekella steinekei and Kurnubia well Globularia hemisaperica Roemer ingsi do not range to the top of the formation. The sp. SEDIMENTARY GEOLOGY OF SAUDI ARABIA D53 Gryphaea balli (Stefanini) HANIFA FORMATION UPPER JURASSIC sp. DEFINITION Homomya inornata Sowerby sp. The Hanifa Formation takes its name from Vradi Lima harronis Dacque IJanifah where the type section was measured (near sp. lat 24°56' N.). Lopha solitaria (J. de C. Sowerby) Max Steineke in 1937 first defined the Hanifa as a eruca Cox sp. member of the Tuwaiq Mountain Limestone. The Modiolus imbricatus (Sowerby) member was raised to formational status by R. A. (Inoperna) plicatus (J. de C. Sowerby) Bramkamp in 1945 and included in the Tuwaiq Moun sp. tain Group. With formal publication of Arabian Mytilus (Ar corny tilus) somaliensis Cox stratigraphic nomenclature in 1952, the term "Tuv^aiq cf. M. imbricatus (Sowerby) sp. Mountain Group" was discarded, and the term Nerinea cf. N. bathonica Rigaux and Sauvage "Hanifa" was retained at formational rank. Detailed spp. definition appeared later the same year and in 1958 Ostrea spp. in articles respectively by Steineke and Bramkamp Oxytoma inequivalve (Sowerby) Pachyceras cf. P. schloenbachi (Roman) (1952b) and Steineke and others (1958). Paracenoceras sp. The 101 m of Hanifa designated as the type section Pholadomya aubryi Douvill6 was measured over an 8-km traverse along the north lirata (J. de C. Sowerby) side of Wadi IJanlfah (from lat 24°57'24" N., long somaliensis Cox 46°12'48" E. to lat 24°55'12" N., long 46°17'12" E.). spp. Pinna sp. At the type locality, the base is at the contact be Pleurotomaria? sp. tween massive, resistant, coral-bearing Tuwaiq Moun Pseudomelaniaf sp. tain Limestone below and soft, chippy-weathering, Stylina cf. S. macfadyeni Thomas calcareous(?) shale of the Hanifa above. The top i^ at sp. the break from clean-washed, tightly cemented, ooUte- Stylosmilia cf. S. octoradiata Subgrossouvriaf sp. pellet calcarenite of the Hanifa below to slalby- Ostracoda: weathering, aphanitic and calcarenitic limestone of the Bairdia spp. Jubaila Limestone above. Cytherella sp. OCCURRENCE AND THICKNESS Cytherelloidea sp. "Hutsonia" spp. The Hanifa Formation crops out in a narrow, gently Paracyprisf sp. curving belt from Nafud ath Thuwayrat (lat 26°32/ N.) Paraschuleridea sp. Procytheridea spp. to Bam Kha^mah (lat 17°50' N.). Over this 1,100- Plant remains: km distance, width of outcrop varies from 0 to 25 km Bennettitales sp. but averages about 10 km. Exposures are almost Ginkoales sp. continuous north of Wadi al Faw (lat 19°53' N.). Miscellaneous: Below the wadi, however, sand ridges on the back slope calcareous sponges, echinoids undetermined, cidaroid of the Tuwayq escarpment and a major concentration radioles, corals undetermined of sand at Al Mundafan break the outcrop pattern into ECONOMIC ASPECTS a series of isolated strike patches. Continuity of the Hanifa is disrupted in the Wadi PETROLEUM urma area and again near Al Majmr.'ah Commercial quantities of oil has been discovered at (lat 25°50' N.) by elements of the central AraHan two levels within the Tuwaiq Mountain Limestone. graben system. In the central area, prominent lir^ar At Abu Hadriya, oil occurs in the upper 160 ft of the channels of the Nisafr, Awsat; and J)urma grat^ns formation in a porous limestone-calcarenite sequence slice through the backbone of Jabal Tuwayq and offset the various Jurassic units. In the region of Al the Hadriya reservoir. In addition, the basal part of Majma'ah (lat 25°55' N.), the persistent trough of the the formation contains oil on the crestal part of Fadhili Majma'ah graben disrupts the Hanifa as well as field and at Qatif where some porosity develops through bracketing formations. This structure cuts through the introduction of calcarenitic limestone and cal- the Tuwayq escarpment due east of Al Jurayfah and carenite. This porous buildup forms the upper part forms a series of en echelon valleys across the upland of the Fadhili reservoir, a zone falling mainly within surface. The valleys, about 60 m deep in the TwuayQ the upper Dhruma. area, become gradually shallower after breaking through D54 GEOLOGY OF THE ARABIAN PENINSULA the main scarp and lose their topographic distinctive- The golden-brown calcarenite beds, although thin, ness in the Cretaceous northeast of Az Zilfi. are ordinarily resistant and form obvious benhes or The Hanifa is a unit of relatively soft rocks plastered weather to slabby debris. Generally the Hanifa is on the resistant Jabal Tuwayq plateau and protected sufficiently covered with fragments of this calcarenite above by harder rocks of the Jubaila Formation. In so that the entire interval appears brown from a dis the central area, the lower part of the unit weathers tance, in strong contrast to the lighter colored forma to a long shallow trough. The east flank of the trough tions above and below. is marked by a steep scarp in which most of the for Accessibility to Hanifa sections is limited, for the mation is exposed; the west edge is commonly limited most part, to areas where Jabal Ttrw^yq has been by a low irregular scarp cut in basal Hanifa beds. breached by through-cutting channels such f.s Wadi Rounded outliers of Hanifa are scattered on the Tuwaiq ad Dawasir, Wadi al Faw, Sha'ib al Haddar, Wadi Mountain Limestone surface west of the valley. From Birk, Wadi Nisab, Wadi IJamfah, and Wadi al 'Atk. JJuraymila (lat 25°07/ N.) northward, the topography Full Hanifa sequences have been measured at each of is no longer dominated by a single Jubaila-capped these cuts and at a few other places where terrain scarp but gives way to more intricately dissected permits. multiple benches. In the south, the more prominent Characteristics of the Hanifa can be adequately de Hanifa benches break down in the vicinity of Wadi monstrated by only a few sections because of the small al IJamr (lat 22°21 / N.) and grade beyond to a steplike changes in lithology and thickness along strike. In series of low-relief cuestas. the cental area, Jabal al Abakkayn affords the cleanest Thickness of the Hanifa Formation remains remark and most accessible place to study the formation in ably constant throughout and is, without doubt, the its entirety. Here, contacts with adjacent formations most consistent of all Jurassic units. In the 400 km are clearly visible. Jabal al Abakkayn, immediately from Wadi al (1) APHANITIC-CALCARENITIC LIMESTONE AND SHALE: Off-white to gray-brown, com monly chalky, rubbly weathering, fine grained limestone with variable amounts of fine- to medium-grained ooliths and pellets. Softer limestones poorly ex posed and form gentle scree-covered slopes. Poorly exposed interval of brick- red and yellow-gray shale occurs near middle; basal part of section is tan gyp- siferous shale. Thin beds of brown tightly cemented pellet-oolite calcarenite occur at number of levels. Colonial corals common in lower part. (94.4 m) Ceromyops/s somaliensis, Exogyra ncrncr, Gry- phaea bo/li, Lima harronis, Lopha gregarra, L solitaria, Modiolus imbricafus, Mytilus /urensis, M. laitmairensis, Paracenoceras sp., Perisphinctes Cf. P. africanus, Pho/adomya profei, P. somal iensis, Soma//rhyncru'a afr/cana var. meso/oba, Velata inequistriata CALCARENITIC LIMESTONE: Light-gray to brown, tight, in part lithographic, siliceous, Vertical scale: stromatoporoidal-coralline-crinoidal cal 1:1000 After R. W. Powers, L. F. Ramirez, carenitic limestone; forms major dip- and N. M. Layne, Jr., 1953 slope FIGURE 5. Hanifa Formation reference section. Mundafan, the dolomite gradually expands at the than the lower limit; however, it has been fairly expense of other carbonates until, at Ban! Kha^mah, accurately traced on photographs and by following the upper 35 m of an 82-m section is replaced. The beds except at the far ends of the outcrop where lower 47 m shows considerable change as well and is dolomite replacement obscures the break. In Najd mainly aphanitic and calcarenitic limestone; less than the upper contact is relatively obvious. The distinc 15 percent of the sequence is clean calcarenite. tive brown color of the Hanifa rocks, the capping unit of which is a characteristic gray-brown, tf?htly NATURE OF CONTACT WITH ADJACENT FORMATIONS cemented, pellet-oolite calcarenite, contrasts sharply The character of the lower Hanifa contact has with the light-colored, usually cliff-forming, somewhat already been discussed in the section on Tmvaiq more thinly bedded limestone of the overlying Jubaila Mountain Limestone. Limestone. The conformable contact between the Hanifa and The general lithologic-topographic definition of the overlying Jubaila is somewhat more difficult to place upper contact persists to Sha'lb al Haddar. Between D56 GEOLOGY OF THE ARABIAN PENINSULA Al Haddar and Wadi ad Dawasir, a major lens of morrisi closely resemble similarly oriented sections of marine sandstone appears in the lower part of the Kurnubia wellingsi, and since Kurnubia morrisi and Jubaila. Consequently, in this area the contact is Pseudocydammina jaccardi occur together in the Saudi sharply marked by a change from gray-brown, pellet- Arabian section, there is a strong possibility that they oolite calcarenite below to brown, medium-grained, do so also in the group f beds and that the reported moderately cemented, fossiliferous, calcareous quartz Kurnubia wellingsi occurrences in reality represent sandstone above. South of Wadi ad Dawasir, the Kurnubia morrisi. contact is once again pellet-oolite calcarenite overlain This situation would remove the necessity for placing by muddy limestone of the Jubaila. This lithologic the Hanifa Formation within the Oxfordian and open sequence gives way from Khashm al Fara'id southward the way for a different correlation derived from deep- where dolomite replaces both upper Hanifa and lower well information. Jubaila strata and the contact can only be approx Recent work has demonstrated the presence of both imately placed by continuous bed traces. lower Jubaila and upper Hanifa beds at FadhiP field. Presumably, the Hanifa is terminated on the north These are underlain in turn by a series of black shales by pre-Wasia erosion, although the actual line of and black impure limestones which carry an ammonite cutting is masked by the Nafud ath Thuwayrat. fauna unanimously identified as lower Kimmeridgian by W. J. Arkell, R. W. Imlay, and Mueller (ratten PALEONTOLOGY AND AGE commun. in 1951, 1961, and 1960, respectively). There Both Kurnubia morrisi Redmond (Redmond, 1964a) appears to be no question that the black shale and and Pseudocydammina, jaccardi (Schrodt) (=Pseudo- limestone unit represents a facies variant from the cydammina sequana of authors) range throughout the normal lower Hanifa lithology and that much if not all Hanifa Formation and into the lower part of the over of the Hanifa Formation must be placed within the lying Jubaila Limestone. The biozone of Pseudocy- lower Kimmeridgian. cyclammina jaccardi has been established by Maync A comprehensive faunal list for the Hanifa Formation (1960) as upper Oxfordian through lower Kimmerid- includes: gian. At first glance this would appear to fit well with Brachiopoda: Arkell's lower Kimmeridgian determination for am "Rhynchonella" sp. monites in the lower part of the Jubaila outcrop and Somalirhynchia bihenensis Muir-Wood with Hudson and Chatton's (1959) assignment of their africana Weir var. mesoloba Muir-Wood Musandam Limestone group f beds (which they con "Terebratula" spp. sider equivalent to the Hanifa Formation) to the Coelenterata: Milleporidium sp. lower part of the upper Oxfordian. Echinodennata: Although there does not seem to be any reason for Acrosaleniat smelliei Currie challenging Hudson and Chatton's correlation (1959) Goniopygus? sp. of the group f beds with the Hanifa Formation, there Recrosaleniat sp. is some reason to reexamine the evidence which led Foraminifera: Arenobulimina sp. them to assign the group f beds to the Oxfordian. Cyclammina spp. As noted by Hudson and Chatton, the evidence for "Haplophragmoides" sp. this age assignment was entirely indirect; it was Trochammina sp. based upon a supposed occurrence of Kurnubia wettingsi Pseudocydammina jaccardi (Schrodt) (Henson) in the group f beds and a consequent correla Kurnubia morrisi Redmond tion with Kurnubia weWmpsi-bearing beds of known Mollusca: Aequipecten macfadyeni Cox Oxfordian age in the Kurnub anticline. "Area" sp. Hudson and Chatton also list Pseudocydammina Ampullella sp. jaccardi (as Pseudocydammina sequana ) from the Ceratomya spp. group f beds. Inasmuch as the ranges of Pseudo cf. C. telluris (Lamark) Cox Ceromyopsis somaliensis Weir cydammina jaccardi and Kurnubia wettingsi do not spp. overlap each other in the Saudi Arabian section, the Chlamys macfadyeni Cox former occurring no lower than the base of the Hanifa Cosmoserist sp. Formation and the latter occurring no higher than the Diplodontat sp. top of the Tuwaiq Mountain Limestone, the identity Discohelix sp. Exogyra nana (Sowerby) of the supposed Kurnubia wettingsi in the group f Globularia hemisphaerical Roemer beds becomes somewhat suspect. In this respect it Gryphaea batti (Stefanini) may be pointed out that transverse sections of Kurnubia Homomya sp. SEDIMENTARY GEOLOGY OF SAUDI ARABIA D57 Latomeandra sp. kamp, 1952b). They were again published without Lima harronis Dacque significant change by Steineke, Bramkamp, and Sender Lopha solitaria (3. de C. Sowerby) gregaria (Sowerby) (1958). Meandrarea somalica! Thomas The lower part of the type section (about 85 m) Mactromya aequalis Agassiz was measured along Wadi IJanlfah from lat 24°53/48// sp. N., long 46°19/36" E. to lat 24°53/ 12// N., long 4C>°26/ Modiolus imbricatus (Sowerby) 42" E., a point near the town of Al Jubaylah. The Mytilus jurensis Roemer somaliensis Cox upper 25 m was studied in Wadi IJanlfah between Al spp. Jubaylah and Kiyadh. Nerinea spp. At the type locality the base of the Jubaila is placed Ostrea sp. at the level where cream to tan, dense limestone gives Paracenoceras sp. way downward to the gray-brown pellet-oolite calcar Pholadomya protei (Brongniart) sp. enite of the underlying Hanifa Formation. The top Pleurotomaria? sp. is at the change from brown, ledge-forming limestone Pteropema sp. below to gray, softer, rubbly-weathering dolomite and Stylina? cf. macfadyeni Thomas limestone of the Arab Formation above. Stylosmilia sp. Velata inequistriata (d'Orbigny) OCCURRENCE AND THICKNESS ECONOMIC .ASPECTS The Jubaila Limestone conforms perfectly to the GROUND WATER outcrop pattern of older Jurassic units; that r>, it forms a gently curving arc wrapped around the central Shallow, hand-dug wells occur sporadically through Arabian arch, reversing itself south of Wadi ad Dawasir out the Hanifa outcrop belt. For the most part, these to define the southwest margin of the Hub' al Fhall wells produce from surficial alluvial deposits, but, in basin. Kocks of the Jubaila have been mapped over some instances, near-surface permeability caused by a distance of nearly 1,100 km from Al Muntatib weathering has permitted the storage and production (lat 17°41' N.) to Nafud ath Thuwayrat (lat 26°27' N.). of meteoric water. South of Wadi al Faw (lat 19°57/ N.), the Jubaila forms PETROLEUM a series of patches isolated by tongues of sand encroach The upper 200 to 300 ft of the Hanifa Formation ing from the Hub' al Khali; northward, exposures are contains commercial quantities of oil at Abqaiq, Abu almost continuous, although sharply interrupted where Hadriya, Khurais, and Khursaniyah. In each locality, the Nisab, Awsat, and Majma* ah grabens cut through. the reservoir is porous calcarenitic limestone and Width of the Jubaila outcrop belt varies from a few calcarenite. Originally referred to as the "mid-Jubaila kilometers at each end to more than 40 km in the central zone," the productive interval has now been identified area. Changes in width away from the central area as uppermost Hanifa. are irregular but nonetheless show unmistakable tapering overall. JUBAILA UMESTONE UPPER JURASSIC The Jubaila Limestone, an extremely resistant lime DEFINITION stone sandwiched between two softer units, typically The Jubaila Limestone derives its name from the weathers to a series of high, talus-free steps. Lower town of Al Jubaylah (lat 24°53/ N.) near the type beds form a prominent scarp, often more than 75 m locality. high, capping the Hanifa. The upper Jubaila breaks Steineke in 1937 proposed the name to cover the away to a gently eastward-tilted plateau which is a relatively resistant limestone between the softer beds slightly beveled upland surface rather than a true dip of the Hanifa Formation below and the lower Kiyadh slope. Consequently, the western margin of the for Formation (obsolete) above. He designated the Jubaila mation is a sharp scarp; the eastern boundary is less as a member of the Tuwaiq Formation of Burchfiel regularly defined by patches of the Arab Formation and Hoover. In 1945, Bramkamp advanced the rank erratically scattered on the strong Jubaila slope. of the Jubaila and Tuwaiq to formation and group A short distance north of the type locality, the strong respectively. Names of many of the Arabian rock western scarp and high relief is somewhat modified as units, including the Jubaila, were first published in the Jubaila breaks down into two or three Icwer 1952 (Steineke and Bramkamp, 1952a). At this time, benches. The highest of these has an elevation of 31 m the term Tuwaiq Group was dropped from the nomen at Wadi al 'Atk. From here, relief within the forma clature. Particulars of the type section appeared tion diminishes gradually to the northwest. In the almost immediately thereafter (Steineke and Bram south, the Jubaila forms low benches with intervening D58 GEOLOGY OF THE ARABIAN PENINSULA wide, slightly eastward-dipping slopes. Scarps are 1961. A complete description of the Jubaila sequence relatively subdued, usually less than 5 m high, although in Wadi Nisafr is given in section 38 (p. 134). a fairly bold escarpment occurs at Wadi ad Dawasir Significant changes in Jubaila lithology north of the (that is, Khashm ash Shidad, lat 20°26/ N.). reference section first appear in the vicinity of Wadi The Jubaila Limestone shows some more drastic IJuraymila (lat 25°07/ N.). At this latitude, the variations in thickness than other Jurassic units. upper part of the formation is mostly replaced by Maximum thickness of 130 to 145 m occurs in the dolomite, and some clean sandstone occurs near the Sha'Ib al Haddar-Wadi ad Dawasir area; about 85 and middle. In 1952, R. A. Bramkamp and S. J. Roach 55 m were recorded respectively at Wadi al Faw and measured the Jubaila in the wadl; their description is in a bore hole at lat 18°08' N., long 46°05' E. North summarized in section 39 (p. 134). of Sha'Ib al Haddar the Jubaila thins to about 90 m Little additional change takes place between Wadi at Wadi Birk, expands to nearly 130 m at Wadi Nisafr Uuraymila and Wadi al 'Atk, but beyond there is a and Riyadh, and then gradually reduces to 110 m at decided increase in sandstone; dolomite makes- up the Jabal al Abakkayn (lat 24 °58' N.) and 72 m at Wadi remainder of the sequence. About 50 m of Jubaila is ljuraymila (lat 25°07' N.). Partial sections measured present a few kilometers south of Al Ghat. The lower beyond this point indicate only slight depositional thin one-third and a capping 5- to 10-m bed consists of ning to the northwest; however, in the vicinity of Al reddish-brown dolomite; the rest of the section is Majma'ah (lat 25°55' N.), pre-Wasia truncation comes mainly tan to brown, coarse-grained, calcareous into play and upper layers of the Jubaila are planed cemented sandstone. off. The northernmost measurement near Al Ghat South x)f the reference locality, the first notable (lat 26 °02/ N.) shows but 40 m of section between the change in Jubaila rocks takes place near Sha'ib al presumed top of the Hanifa and the Wasia Formation Haddar. The shift can first be seen just north of the (Middle Cretaceous). sha'ib where a few sandstone interbeds are present near LITHOLOGIC CEARACTEB the middle of the formation. South of the sha'ib, near lat 21 "50' N., sandstone dominates the lower part of the The Jubaila Limestone, like the underlying Tuwaiq Jubaila; the upper one-half is partly replaced by thick- Mountain and Hanifa Formations, is basically a shallow- bedded dolomite. Almost complete dolomite replace water carbonate unit. Though it is mostly aphanitic ment of the upper limestone of the Jubaila occurs in and calcarenitic limestone, some highly persistent layers the vicinity of Wadi ash Shutbah (lat 21°30/ N.). The of clean-washed lime sand (calcarenite) are present, lithologic pattern of dolomite above and sandstone particularly in the central and northern areas. Corals below, that persists for at least 150 km, may be charac and stromatoporoids, few in the Riyadh-£)unna area, terized by a section measured along lat 20°37/ N. The become abundant to the north and south. Marked sequence here is summarized in section 40 (p. 135). lateral changes in the Jubaila to the south consist of a The dolomite-sandstone facies can be traced without sudden shift from limestone to dolomite in the upper interruption beyond Wadi ad Dawasir, but near Al part of the formation and limestone to sandstone and 5asl (lat 20 "OS' N.) all sandstone is abruptly replaced back to limestone again in the lower part. Similar, by aphanitic limestone. From this point to where the but less abrupt, limestone transition to dolomite and sandstone also takes place to the north. Jubaila disappears under the Rub' al Khali sard, dolo mite and aphanitic limestone are the only rock types. Jubaila lithology shows relatively little change from Dolomite does shift stratigraphically downward along Sha'ib al Haddar nearly to ljuraymila. Over this 370- strike, however, replacing the upper Jubaila in the km distance, the Jubaila is well exposed in many areas, north and the lower part of the sequence in th°- south. but one of the most accessible and best sections is that at Wadi Nisafr, herewith designated as a reference NATURE OF CONTACT WITH ADJACENT FORUATIONf section (fig. 6) to supplement less detailed work at the Characteristics of the conformable contact between type locality. The whole sequence, however, was not the Jubaila Limestone and the underlying Hanifa measured at a single locality but must be pieced to Formation have been described in connection with the gether from three sections. The lower 21.5 m was described (at lat 24°13/22// N., long 46°41'55" E.) by Hanifa. E. L. Berg and R. L. Myers in 1945, the middle 30.8 m The upper limit of the Jubaila is a sharp lithologic was covered (at lat 24°14'55" N., long 46°44'39// E.) and topographic boundary. A pattern which maintains by R. A. Bramkamp and S. B. Henry in 1948, and the itself for more than 700 km starts at the base with a upper 68.5 m was measured (at lat 24°13"24// N., long thin calcarenite (unit 3, fig. 6) overlain by 6 to 10 m of 46°48'59" E.) by R. W. Powers and H. A. MoClure in gray, tight, chippy-weathering aphanitic limestone SEDIMENTARY GEOLOGY OF SAUDI ARABIA D59 Location: /a x 24°13'22" 24°14'55" 24°13'24' l°' 46°41'55" (b) 46°44'39" 46°48'59' Generalized lithology Diagnostic fossils CALCARENITE: Brown tightly cemented skeletal calcarenite (4) APHANITIC AND CALCARENITIC LIME STONE: Alternating layers of tan to yellow chippy weathering tight aphanitic lime stone and pellet-skeletal calcarenitic lime stone; includes bed of gray-brown tightly cemented calcarenite near base. (6.0 m) Ceromyopsis somaliensis, Pholadomya protei E (3) CALCARENITE: Gray to golden-brown ' tightly cemented fine- to medium-grained rarely sandy calcarenite; in part poorly exposed. (7.0 m) c) (2) CALCARENITIC LIMESTONE AND CAL CARENITE: Complexly interbedded off- white to reddish-brown fine- to medium- grained pellet-skeletal calcarenitic lime stone and cream to golden-brown tightly cemented pellet-skeletal calcarenite; I I Q < several thin beds of brown finely crystal- U CC line dolomite at top. (20.5 m)______I I I I (1) APHANITIC LIMESTONE: Dominantly cream to yellow tight rarely sandy partially 1= dolomitized aphanitic limestone; occa Perisphinctes aff. P. progeron 15'0 sional beds of tan to brown pellet-skeletal calcarenitic limestone and tightly ce mented calcarenite. (84.8 m) .Perisphinctes /ubai/ensis, Lifhacoceras? CALCARENITE: Gray-brown tightly cemented After E. L Berg, and R. L. Myers,1945; pellet-oolite calcarenite. Vertical scale: R. A. Bramkamp, and S. B. Henry, 1948; 1:1000 R. w. Powers and H. A. McClure, 1961 FIGURE 6. Jubaila Limestone reference section. locally with interbeds of calcarenite and calcarenitic phenomenon that does serve a useful purpose, however, limestone (unit 4, fig. 6). The top of this lime-mud as it permits easy photodistinction between the con interval marks the conformable contact of the Jubaila torted beds of the Arab Formation and the flat, un and Arab Formations. Above is a light-colored unit disturbed Jubaila surface. of softer rocks, commonly complexly interbedded Much of the lithologic pattern so characteristic of aphanitic limestone and calcarenite, which is in turn beds bracketing the Jubaila-Arab contact on outcrop overlain by breccia presumably representing the can be recognized in the subsurface sequence. This is residuum after solution of a thin anhydrite bed. particularly true of the aphanitic bed at the top of the The uppermost Jubaila unit of aphanitic limestone Jubaila, which is equally obvious in the surface and sub commonly weathers to a resistant dip slope. At the surface. This unit corresponds to the middle Ar^b-D foot of this gentle slope is an irregular, highly dis member (Powers, 1962) that subdivides the prolific continuous band of softer gray carbonates of the Arab Arab-D reservoir in Ghawar and other coastal oil fields. Formation. Solution alteration of the Jabal TuwaV(l The widespread distribution of this muddy limestone upland surface locally obscures the upper contact, a and the more fragmental carbonate units flanking it 228-776 O 66 D60 GEOLOGY OF THE ARABIAN PENINSULA attest to changes that must have influenced sedimen PETROLEUM tation over much of the basin so far studied. Porous beds within the upper part of the Jubaila PALEONTOLOGY AND AGE Limestone contain prolific amounts of oil in eight widely Arkell (1952) places the Jubaila Limestone in the scattered fields Dammam, Abqaiq, Ghawar, Qatif lower Kimmeridgian on the basis of the perisphinctid Abu §a'fah, Khursaniyah, Fadhili, and Khurais. ammonites Perisphinctes jubailensis Arkell and Peri Stratigraphically, the top of the Jubaila falls near the sphinctes aff. P. progeron von Ammon. middle of the well-known Arab-D reservoir. Kumubia morrisi Redmond and Pseudocyclammina Areas of maximum oil accumulation and highest pro jaccardi (Schrodt) occur at the same level as the am ductivity correspond rather closely to areas of maximum monites. The presence of Pseudocyclammina jaccardi calcarenite deposition. Pores, now oil filled, are essen fits in well with Arkell's determination since this fo- tially those formed between lime sand grains at tl ^ time raminifer is known to range from the upper Oxfordian of deposition and have since undergone little modifica into the lower Kimmeridgian. tion. Little or no pore space is contributed to the Pseudocyclammina powersi Redmond occurs in the Jubaila part of the reservoir by dolomite. upper part of the formation, above the range of Pseudo ARAB FORMATION UPPER JURASSIC cyclammina jaccardi. The beds carrying Pseudocyclam DEFINITION mina powersi likewise fall within the lower Kimmer Lack of suitable exposures precluded choice of a sur idgian, if Hudson and Chatton (1959) are correct in face type locality and the coupling of a geographic name equating the stratigraphically higher group g Ashab to the economically important rock sequence that has limestone with the Cidaris glandarius beds of the come to be known by the general term Arab Formation. Lebanon and Kurnub areas. Beds now assigned to the Arab were originally in A comprehensive fossil list of Jubaila includes: Mil- 1937 included by Max Steineke as part of the Riyadh leporidium, indeterminate colonial coral, Kurnubia spp., Member, the uppermost member of the Tuwain For Kurnubia morrisi Redmond, Pseudocyclammina powersi mation. In 1938, R. A. Bramkamp and T. C. Barger Redmond, Pseudocyclammina jaccardi (Schrodt), Am- studied the upper part of the Riyadh Formation and puUeUat sp., Avicula sp., Brachidontes sp., Ceratomya defined members within it. From bottom to top these spp., Ceromyopsis somaliensis Weir, Ceromyopsis sp., were lower Riyadh (now Arab) Formation, Hith Anhy Globularia hemisphaerica Roemer, Gryphaea ballif drite, Sulaiy Limestone, and Yamama detrital member. (Stefanini), Lithacocerasf sp., Lucina sp., Mytilus The finding of presumed Lower Cretaceous (Neoco- (Arcomytilus) laitmairensis de Loriol, Nautilus? sp., mian) fossils in the Sulaiy led Steineke in 1940 to re Nerinea sp., Parattelodon? sp., Perisphinctes jubailensis strict the Riyadh Formation to include only the lower Arkell, Perisphinctes aff. P. progeron von Ammon, Riyadh formation and Hith Anhydrite and throw the Pholadomya spp., Terebratula spp., and indeterminate Sulaiy and Yamama members into a new formation ostreids. ECONOMIC ASPECTS the Thamama shown to be at least mainly of Lower Cretaceous age. In 1945, Bramkamp raised the P.iyadh GROUND WATER to group and its members to formation; that is. Arab A number of drilled wells in and near the capital city and Hith Formations. of Riyadh have penetrated the Jubaila Limestone. Formal publication of most names of Arabian rock Several of these produce from thick Triassic sand, others units came in 1952b (Steineke and Bramkamp). This derive water from channel fill; the amount actually paper substituted the term Arab Formation for lower obtained from the Jubaila is unknown. Riyadh Formation (obsolete), but retained the P.iyadh For many years Riyadh obtained its water from the Group. Later this too was dropped (Steinel'e and channel gravels in Wadi Hanlfah. This source has now others, 1958), and Arabian stratigraphic nomenclature been supplemented by shallow wells in Wadi Nisafr and assumed its present form. other nearby channels and deep drilling near the city At first the type locality of the Arab Formation was itself. Limited data indicate that hand-dug wells in taken as a general area around the city of Riyadb. Ex the vicinity of Riyadh are bottomed both in the Arab tensive loss of beds and subsequent slumping along the and Jubaila Formations. These two units are relatively entire outcrop prevented accurate measurement of sec tight here, but weathering extends to a considerable tion, however, and forced selection of a subsurface se depth and jointing is fairly well developed. Ap quence to best represent the unit. Detailed correla parently, Jubaila water is mainly derived from openings tions, carried east from the outcrop, have defined the resulting from weathering and jointing, rather than limits of the Arab Formation in coastal wells. These from porosity inherent in the rocks themselves. limits are probably accurate within a few meterr SEDIMENTARY GEOLOGY OF SAUDI ARABIA D61 Because of excellent sample and core coverage, the tains a persistent brecciated zone that suggests some interval from 1,371.6 to 1,499.1 m depth in Dammam loss of section due to anhydrite solution. well 7 (lat 26°19'04" N., long 50°07'38" E.) was chosen Between the Jubaila near Riyadh and the Hit as the type section. The base of the Arab Formation escarpment, east of which slumping dies out, it is is at the contact of clean-washed calcarenite with the estimated that roughly 150 m of beds should be present. tighter, more muddy rocks of the underlying Jubaila. A similar estimate between Dabl Hit (lat 24°2S'18" The top of the formation falls at the change from tan N., long 47°00'06" E.) and Al IJa'ir (lat 24°23' N.) aphanitic limestone below to massive anhydrite of the to the west would suggest about 100 m. Obviously Hith above. these estimates are of limited value, but they do serve Subdivision of Arab rocks on outcrop is impossible, to indicate that the Arab in this area originally H.d a but in the subsurface alternating carbonate and thickness approximating that in coastal Arabia. anhydrite very naturally separate the formation into lithologically and economically significant members. UTHOIOGIC CHARACTER In the subsurface the Arab consists of four normal Lack of good Arab exposures, except for the lower marine carbonate units, each separated by thin, 15- to 25-m interval, has prevented working out a precise laterally persistent beds of anhydrite. Earlier publi lithologic sequence. The little that is known o* the cations (Steineke and others, 1958; Powers, 1962) basic rock types indicates they are similar to those informally designated the carbonate units from top to found in wells to the east. As a result, most informa bottom as the A, B, C, and D members of the Arab tion on the Arab has been obtained from bore 1 oles. Formation; the anhydrite separators were unnamed. In fact, subsurface units, probably not differing greatly Since the carbonates, together with their overlying from the original outcrop sequence, have been tak^n as anhydrite, represent four main cycles of deposition the type and reference sections. and each of these cycles approximates a time-strati- The description of the A, B, and C Members of the graphic unit, it now seems logical to redefine the lower Arab and the capping anhydrite of the D member in three members to include their anhydrite caps (fig. 7). Dammam well 7 by R. A. Bramkamp and H. A. Kimball The much thicker upper or closing anhydrite unit (500 in 1955 is still the best account of the stratigraphic ft in Ghawar) is still considered a separate formation interval. Considerable new information has-been ob the Hith Anhydrite. tained on the D Member carbonate through thin-se".tion Dammam well 7 is herewith designated as the type analysis (Powers, 1962). Consequently, the better locality for the four members of the Arab Formation understood sequence in Abqaiq well 71 (lat 26°lg'28" The A Member, between 1,371.6 and 1,388.4 m depth; N., long 49°45'45" E.) between a depth of 2,223.C and the B Member, between 1,388.4 and 1,399.1 m depth; 2,260.0 m is herewith designated as a reference section the C Member, between 1,399.1 and 1,440.6 m depth; for this unit. A detailed description of the Arab For and the D Member, from 1,440.6 to 1,499.1 m depth. mation type-reference section is given in section 41 In each instance, the contact between members is (p. 135). marked by change from carbonate to evaporite. Thicknesses of individual members change little from OCCURRENCE AND THICKNESS the coastal area toward outcrop. Each anhydrite bed does, however, thicken considerably, a change compen Rocks of the Arab Formation form a thin, essentially sated for by thinning of the underlying carbonate unit. unbroken belt from Al IJasi (lat 20°08/ N.) to Juwayy There is little doubt that, to the west, carbonate beds (lat 25°49' N.), a distance of nearly 700 km. Width of are progressively replaced from the top down by facies the outcrop over this distance is relatively uniform, change to anhydrite. In fact, regional lithofacies averaging about 10 km. studies show that maximum evaporite development, The area of Arab exposure is mainly a broad, gently presumably mainly or entirely anhydrite, occurred rlong undulating plain. A few low, isolated hills ("haystack a north-south line near the longitude of Riyadh, \rhere jibal") stand above the general ground level. Effects the amount of carbonate rocks remaining in the solution- of slumping are everywhere evident a phenomenon collapse zone is small and the interval was apparently that can be readily attributed to the removal of anhy mostly soluble anhydrite. drite beds and the subsequent collapse of the carbonate Steineke, Bramkamp, and Sander (1958), in their units left behind. concise description of Arab outcrop characteristics, No satisfactory estimate of the total thickness of point out that the Arab has been made because of the complex only a basal unit of carbonate rocks 15-25 meters thick regains, slumping. Only the basal 15 to 25 m has been meas and even within this, along nearly the entire length of its outcrop, ured with any assurance, and even this interval con a persistent brecciated zone suggests that at least one thin, D62 GEOLOGY OF THE ARABIAN PENINSULA A. B. Type Section Reference section Dammam well 7 Abqaiq well 71 Location: 26° 19'04"-50°07'38" 26° 18'28"-49°45'45" Generalized lithology Diagnostic fossils ANHYDRITE: White massive anhydrite APHANITIC LIMESTONE: Tan to brown compact aphanitic limestone; thin inter- beds of brown calcarenite; minor anhydrite near top. (16.8 m)______(2) ANHYDRITE: White, massive anhydrite; thin interbeds of tan partly dolomitized aphanitic limestone. (4.6 m)______(1) APHANJTIC LIMESTONE: Tan compact aphanitic limestone; thin interbeds of brown medium-grained calcarenite. (6.1 m)______(2) ANHYDRITE: White to tan massive anhydrite; minor streaks of brown dolo mite and limestone. (12.8 m)______o E 7T.T i q (1) CALCARENITE: Tan to brown generally o *t fine-grained moderately to strongly CVJ cemented partially dolomitized calcar enite; some anhydrite blebs. Rare thin r, i cr beds of tan partially dolomitized aphanitic and calcarenitic limestone. (28.7 m) o: (2) ANHYDRITE AND DOLOMITE: White UJ 2 massive anhydrite complexly interbedded Q_ Q_ «j with dark-brown finely crystalline dolo < mite with relic clastic texture and strongly dolomitized calcarenite. (18.0 m) (1) CALCARENITIC LIMESTONE, CALCAR ENITE, AND DOLOMITE: Complexly inter- bedded gray-brown to brown tight to moderately porous dominantly algal C/ype/ncr jurassica, C cf. hanabatensis, Cy/indro- debris-pellet calcarenitic limestone and porella arabica, Polygonella incrustata, Kurnubia brown weakly cemented fine-grained to Spp., Naufiloculma Spp. gravel-size clean-washed well-rounded to angular mainly algal debris-pellet cal carenite. Occasional thin beds of brown cc weakly cemented gravel-size, "algal" UJ nodule-stromatoporoid coarse carbonate (calcirudite). Sand-and-gravel-size grains are mainly aggregate pellets, algal debris, CC stromatoporoid fragments, "algal" nod Vertical scale UJ ules, "fecal" pellets, and Foraminifera; 1:1000 very rare ooliths occur near top. Brown I moderately porous to tight, finely crystal line dolomite is present in upper part and near base. Some thin stringers of brown tight partially dolomitized aphanitic lime stone occur mostly in upper part. (37.0 m) (After R. A. Bramkamp and LIMESTONE: Brown fine-grained compact H. A. Kimball,(l955) limestone, finely detrital and oolitic at After R. W. Powers, 1960 top FIGURE 7. Arab Formation type and reference sections. SEDIMENTARY GEOLOGY OF SAUDI ARABIA D63 highly persistent evaporite layer was included * * * Above the are as yet unrecognized on outcrop. It is urUkely, basal carbonate unit, little is known of the true rock sequence however, that any great thickness is involved. The as good exposures are few and fragmentary because extensive solution-collapse phenomena have almost entirely eliminated problem is further complicated by the probable occur outcrops of the anhydrite. * * * Dropped masses of the younger rence of Sulaiy rocks that dropped down and admixed Cretaceous rocks occupy the eastern part of the solution-collapse with the older rocks. zone, and dolomite and other carbonate rocks representing thin Because of these complications, the surface juncture carbonate members, possibly equivalent to the "A," "B," or "C between the Arab-Hith sequence and the Sulaiy rocks members" of the Arab of the Eastern Province, show at the surface in the western part. cannot be precisely defined. The contact shewn on the various geologic maps is only approximate. The basal unit of carbonate rocks referred to by Somewhere in the vicinity of Wadi al 'Atk, the upper Steineke, Bramkamp, and Sander (1958) has been surface of the Arab Formation intersects the pre- measured and studied at several localities between Al Wasia erosion surface. Although this unconfo~mable IJasi (lat 20°08/ N.) and Wadi al 'Atk (near 25°30/ N.). contact cannot be traced northward, it is reasonable to Over this entire distance there is little major variation assume that the Arab is progressively truncated and in lithologic content except for increased dolomitization finally overlapped completely near Al Majma'ah in the north and south. A section measured at Wadi (lat25°55/ N.). Nisak is characteristic of the interval. The lithologic PALEONTOLOGY AND AGE sequence here is summarized in section 42 (p. 139). The rather limited fauna so far recorded from surface There is every reason to believe that the basal Arab exposures of the Arab Formation has not proved carbonate unit described above is equivalent to the diagnostic. Except for Diceras, identifiable forms lower part of the type D Member at Dammam; it may range down into the Jubaila below. Consequently, well represent the total D Member not replaced by the subsurface sequence provides the only means of anhydrite in wells as far west as Khurais and Ma' dating Arab rocks. qala'. Kurnubia spp. Nautiloculina, spp., Clypeina juras- Only at the southern end of the solution-collapse sica Farre, C. cf. hanabatensis Yabe and Toyama, zone have beds been found in place above the basal Oylindroporella ardbica Elliott, Polygonetta incrustata unit. Between Sha'ib al Haddar and Al Ijjasi, outcrops Elliott and Salpingoporetta sp. range throughout the of anhydrite rest directly on the laterally persistent Arab-D Member (Powers, 1962). Although some calcarenite that marks the top of the basal sequence. elements of this assemblage extend on into the A These overlying beds have an average thickness of Member, it is only the beds of the D Member that have about 14 and are mainly white anhydrite and gypsum been reliably dated. These can be equated to at least with thin interbeds and caps of brown dolomite. part of Hudson and Chatton's (1959) group g beds Little is known of the Arab sequence on outcrops above through the No. 4 Limestone of Dukhan oil field in this stratigraphic level. Qatar, the direct equivalent of the D Menrber of NATURE OF CONTACT WITH ADJACENT FORMATIONS Saudi Arabia. Hudson and Chatton (1959) also list Calpionetta Characteristics of the Jubaila-Arab contact have alpina Renz, Pseudocyclammina sp., and Buryurdier already been described in connection with the Jubaila steinerae Hudson from group g and equivalent beds. Limestone. The upper contact with the Hith An By comparison with the Alam Abyadh limestones of hydrite is exposed only at a single locality that is, in southwestern Arabia, ddaris glandarius beds of the Datil Hit (lat 24°29/ 18// N., long 47°00'06" E.), a Lebanon and Kurnub, and with other formation, they solution cavity at the foot of the Hit escarpment. The conclude that the faunas are Sequanian (early Kim- contact here, as in the subsurface sequence, is at the meridgian) in age, a dating compatible with other lines relative sharp break from carbonate of the Arab-A of evidence. Member below to massive, white, and blue-gray anhy The post-D Member of the Arab Formation and the drite of the Hith above. Hith Anhydrite have not yet been dated. However, Where Hith Anhydrite is preserved, as in Dafrl Hit, as these beds occur between known lower Kimmerid- the upper Arab contact is conformable. At all other gian and presumed Berriasian (Lower Cretaceous), it places along outcrop, however, evaporates of the Arab- may be assumed that they are, at least in part, Hith sequence have been dissolved, presumably bring Tithonian. ing residual Arab carbonate rocks into contact with ECONOMIC ASPECTS slumped limestone of the Sulaiy Formation. As very GROUND WATER thin stringers of limestone occur locally within the Many of the wells around Riyadh derive water from Hith, it is possible that some of these are present but rocks of the Jubaila and Arab Formations, but present D64 GEOLOGY OF THE ARABIAN PENINSULA data are too sparse to estimate the importance of one with calcium sulfate, a characteristic entirely consistent unit relative to the other. The general character of the with its supposed origin. The chloride content of the Jubaila-Arab aquifer near the capital city is described water is low. Water levels at Sampan and 'Ayn a$ more fully in connection with economic aspects of the Pil' rise and fall very nearly together, and it is there Jubaila Limestone. fore assumed that fairly efficient interconnection Agricultural development of the Al Kharj area, exists between the two pits. around the towns of As Sulaymaniyah and Al Yama- Khafs Daghrah (lat 23°50' N., long 47°12' E.), mah, has largely been supported by water derived from another solution pit similar to those at Al Kharj, is the Arab Formation. Earlier cultivation of this valley somewhat smaller. A less extensive agricultural de was apparently dependent on Quaternary aquifers. velopment has been undertaken using water pumped Three large solution pits Sampan (lat 24°07/ N., from this sinkhole. long 47°16' E.), 'Ayn ad DiT (lat 24°07' N., long 47°16' Dabl Hit (lat 24°29.4' N., long 47°00.1 / E.) ir also E.), and Umm Khlsah (lat 24°07' N., long 47°15' E.) much like the Al Kharj pits in character and geologic supply great quantities of water for the Al Kharj setting except that the elevation is higher and ^ater project. Sampan and 'Ayn ad Dil* have furnished most occurs only in the very bottom. Here again, the Arab of the water in the past and will probably continue to Formation is the apparent source as the uppermost do so. These two sinkholes, with nearly vertical sides, beds are exposed in the pit. In 1961 a two-stage pump are situated at the toe of the low Hit escarpment. was installed in the dafrl. The water is now us**d to Their depth and diameter, which are essentially the irrigate the alluvium-covered plain in front of tita Hit same, are slightly more than 100 m. The origin of escarpment. Umm Khlsah is no doubt similar to the other solution Solution pits are also well developed in the vicinity cavities, although its margins are less abrupt and con of Layla (near lat 22° 15' N.) in Al Aflaj. For unknown siderable vegetation has succeeded in getting a foothold reasons, only limited use has been made of the water. on its banks. Analysis shows high calcium sulfate and low chloride- Geologically, the origin of these pits is relatively ion concentration and, here again, Arab beds are the simple. They occur at the toe of a badly slumped probable source. escarpment which involves mainly Sulaiy rocks but Quantitative estimates of Arab Formation recharge which at places also involves the lowest beds of the are not available; however, a great drainage area dis Yamama Formation. The slumping is reasonably charges across its outcrop. Since the latter is of large regular with rocks draped down over the face of the extent and is characterized by complicated topography scarp and increasing in dip to the base of the slope. there should be ample opportunity for the dowrward On the plateau surface above, and very near the hinge percolation of the infrequent flood waters. Large areas line, prominent tensional cracks, some still open as far of closed drainage in the Arab Formation slump zone down as 10 m, indicate the mechanism by which Sulaiy also greatly aid water to reach the main underground beds have adjusted to the slumping. At the base of the accumulation. Excluding the water which may be scarp, as well as in the walls of the pits themselves, the derived from areas west of Jabal fuwayq, the watershed beds are badly jumbled and brecciated. There is little which is on, or must escape across, the outcrop of the doubt that the slumping has resulted from solution of Arab is of the order of 20,000 sq km. In view of this, supporting anhydrite layers. The Al Kharj pits fit although the amount of water entering the Arab is not well into this general picture and represent solution known, it can be assumed to be large. openings which succeeded in stoping their way upward to the surface. PETBOLBUM The water-bearing member which feeds the Al Kharj Although all members of the Arab Formation contain pits cannot be demonstrated conclusively, although it oil, accumulation in the upper, thinner A, B, and C seems likely that it is the Arab Formation. Depth of carbonates is much less extensive than in the basal D the Al Kharj pits (about 100 m) is only slightly less Member carbonate. Oil occurs in the A and B Mem than the normal thickness of the Sulaiy in this region, bers of the Arab in Abu Hadriya, Dammam, T^hur- and, as the Hith Anhydrite has been removed, the Arab saniyah, Manifa, and Qatif and in minor accumulations can logically be considered the aquifer. In addition, in northern Ghawar and Abqaiq. Arab-D Member oil there is reason to suspect that considerable filling of has been found at Abqaiq, Dammam, Fadhili, Gbawar, the pits with debris has occurred and that the actual Khurais, Khursaniyah, Qatif, and Abu §a'fah. In all source of the water may be somewhat deeper than their these fields, the 25- to 40-m-thick D Member as w41 as present bottoms. approximately the upper 45 m of the Jubaila Limestone A notable feature of the water is its high saturation are oil saturated. SEDIMENTARY GEOLOGY OF SAUDI ARABIA D65 Sparse information on the upper three Arab carbonate limestone presumed to be basal Sulaiy. Ne^ well units indicates that the dominant lithology in each is data show that the fragmental limestone is in fact admixed calcarenite and aphanitic limestone. The C collapse breccia resulting from loss of relatively thick Member, however, appears to have a much higher con interbeds of anhydrite. As such, it is properly part centration of clean-washed calcarenite in comparison of the Hith and is so considered here, raising the total with the more muddy, often dolomitic rocks of the over formation thickness to 90.3 m. lying A and B units. Arab-D rocks are characterized The top of the Hith is at the contact (possil^y dis- by a high percentage of lime sand, commonly skeletal conformable) of limestone breccia with evenly bedded and commonly in the form of massive clean-washed oolite calcarentie of the overlying Sulaiy Formation. calcarenite beds. Calcarenitic limestone is the next The base is taken at the sharp break from anhydrite most frequent rock type; pure aphanitic limestone and above to calcarenitic limestone and dolomite of the dolomite are quite rare. Reef structures have not yet Arab Formation below. been found, and the wide continuity, uniform thickness, and regularly bedded nature of the Arab-D suggest that OCCURRENCE AND THICKNESS large structures of this type are unlikely. The only definitely known exposure of the Hith Specific particle types contributing to rock formation Anhydrite is in Dal^l Hit, a deep, steep-walled, solution of the Arab-D reservoir include calcareous algae, stro- cave about 32 km southeast of Riyadh. Patches of matoporoids, and Foraminifera as the main skeletal gypsum (possibly hydrated anhydrite of the Hith) elements and aggregate pellets, "algal" nodules, and occur below the Sulaiy in the vicinity of Laj'la (lat "fecal" pellets as the most important nonskeletal grains. 22°17' N.) and can be traced as far as lat 20°45/ N. The major oil deposits in the Arab-D accompany In addition, about 5 m of primary bedded anhydrite maximum development of porous calcarenite. It is has been reported about 14 km northwest of Qasr certain that intergranular pore space associated with Eiman (lat 20°49' N.). the rocks at the time of deposition almost exclusively Except for the anhydrite at Dajil Hit, the r.rea on controls present distribution of oil. To be sure, dia- outcrop where the Hith should occur is a rather broad genetic changes including recrystallization, dolomiti- zone of badly slumped beds. The belt of collapsed zation, and cementation have all modified the original rocks can be traced over a distance of some 550 l~m and pore pattern to some extent, but, in each case, the effect involves mostly Sultiiy strata. Slump effects extend has not been drastic. beyond the edge of the Sulaiy, but this is attributed to Several of the Arab units also carry oil outside of solution of anhydrite beds within the Arab Formation. Saudi Arabia. Most notable among these is the Slu'mping in the eastern part of the zone, on th°, other Dukhan oil field in Qatar which produces from lime hand, is probably mainly due to the loss of the thicker, stones equivalent to the C and D Members. more massive anhydrite of the Hith itself. Undoubtedly, the Hith is present in the subsurface MTTH ANHYDRITE UPPER JURASSIC over a considerable north-south distance just a few DEFINITION kilometers east of the solution-collapse zone. Certainly The Hith Anhydrite is named for Dafcl Hit (lat the Hit escarpment, a monoclinal flexure of Sulaiy 24°29'18" N., long 47°00/06" E.), a solution pit in beds between lat 23°00/ N. and 25°00' N., marks the which the type section was measured and which is still western edge of undissolved anhydrite. Drilling sup the only known outcrop. ports this assumption and suggests in addition that Evolution of Hith-Arab nomenclature, from the Hith beds flanks the solution-collapse zone nearly original concept of Max Steineke in 1937 to its present throughout its length. form, is fully described in the definition of the Arab At the type locality the Hith Anhydrite is about 90 Formation. Both units followed the same steps from m thick. Elsewhere, slumping makes accurate meas unnamed beds within the much larger Tuwaiq Forma urement impossible. tion to their modern status of separate formations. LTTHOLOGIC CHARACTER An apparently complete Hith sequence is visible in the walls of Dafrl Hit. The section here was origin The Hith, both at Dafcl Hit and in the subeurface, ally measured by R. A. Bramkamp and T. C. Barger in is mainly massive anhydrite. Well sections usually 1938 and designated the type locality. As defined by contain some interbedded dolomite and limestor e, par Steineke, Bramkamp, and Sander (1958), the Hith is ticularly in the upper part. 71.2 m of bluish-gray anhydrite. The top was placed R. A. Bramkamp and T. C. Barger in 1938 first at the contact between this anhydrite and fragmental described the Dafcl Hit section and recognized it as D66 GEOLOGY OF THE ARABIAN PENINSULA the type sequence. The full exposure in the dalpl, ECONOMIC ASPECTS as well as in the overlying Sulaiy cliff, was again PETROLEUM measured in detail by S. J. Roach in 1952. His Over much of eastern Arabia the uppermost beds of description of the succession is given in section 43 the Hith Anhydrite are commonly in porous calcarenite (p. 139). facies. Recently, oil was discovered in this interval So far as the Hith found in downdip wells is concerned, in the offshore field of Manifa, and, as a result, it has a generalized section from top to bottom is as follows: been appropriately named the Manifa reservoir. Non Aphanitic limestone and calcarenite of the Sulaiy commercial quantities of oil were also detected in the Formation (Upper Jurassic? and Lower Cretaceous?). Thickness same unit at Abu Hadriya. Hith Anhydrite: (meters) Anhydrite with associated limestone and dolomite. 6-15 As a matter of historical interest, a tar seep was Calcarenite, limestone, and anhydrite.______15- 18 discovered at Dabl Hit in 1938. The tar occurs in Largely massive anhydrite in upper part, anhydrite vugs and large pores in a 0.8-m limestone ledge at the interbedded with aphanitic limestone, dolomite base of the Hith. This seepage was extremely interest or calcarenite in lower part__.______90-120 ing as its exact position with respect to both the surface Total thickness of Hith Formation.. ______111 -153 and subsurface sequence was immediately recognized. Arab Formation. Aramco geologists realized at once the great regional significance of the discovery as it showed the probable NATURE OF CONTACT WITH ADJACENT FORMATIONS widespread distribution of this petroliferous group of Contacts of the Hith Anhydrite with bracketing units rocks, a conclusion strikingly borne out by later events. are visible only at Dabl Hit. The lower contact with JURASSIC AND CRETACEOUS SYSTEMS limestone of the Arab Formation (A Member) is sharp SULAIY FORMATION UPPER JURA8SIC(?) AND LOWER but apparently conformable. Subsurface relationships CRETACEOUSasia. anhydrite layers. This zone of transition now appears Max Steineke in 1937, following the early definition, to correspond to the brecciated limestone interval now included in the Tuwaiq Formation a number of yormger included at the top of the formation in Dabl Hit. It beds that underlie the "Nubian Sandstone" (thatis, is likely that the disconformable appearance of the Biyadh plus Wasia) in the central area but that contact in the dabl is due to solution effects. cut out to the north. The additional beds above the Jubaila were referred to simply as "Riyadh chalks PALEONTOLOGY AND AGE and limestone, including Yamama detrital member at The age of the Hith Anhydrite cannot be determined top." on direct evidence, but it obviously represents the In 1938, R. A. Bramkamp and T. C. Barger, after climaxing phase of anhydrite deposition that begins in studying the upper part of the Tuwaiq Formation, the Arab Formation (lower Kimmeridgian). As there separated out the Riyadh chalks and limestones, is little likelihood of a significant stratigraphic break raised this sequence to formational rank, and defined within the Hith-Arab sequence, at least some part of members within it. At this stage, the Riyadh Forma the Arab above the D Member and possibly the Hith tion included from the top down, the Yamama detrital, as well must have been deposited during Late Jurassic Sulaiy limestone, Hith Anhydrite and Lower Riyadh tune. members. In addition, regional considerations suggest that the On the discovery of what was then believed to be overlying Sulaiy Formation is of Tithonian to Berri- Lower Cretaceous fossils in the Sulaiy, the Sulaiy and asian(?) age. If these relationships are valid, the Yamama detrital members were broken out of the Hith-Arab sequence can be dated as early Kimmeridgian Riyadh Formation and placed in a new formation, and Tithonian with some confidence. the Thamama. In addition to the Sulaiy and Yamama SEDIMENTARY GEOLOGY OF SAUDI ARABIA D67 Location: 24°29'18"-47°00'06' Generalized lithology Diagnostic fossils PELLET CALCARENITE: Brown poorly sorted tightly cemented pellet calcarenite; common foraminiferal and molluscan debris D/ceros?, Milleporidium E eg O O APHANITIC-CALCARENITIC LIMESTONE AND CL O. D CALCARENITE: Complexly interbedded W .£ . D light grayish-tan compact rubbly weather * - | I i ing, in part dolomitic aphanitic limestone, £.22-8 pellet-oolite-detrital calcarenitic lime ip j> |j| o stone, pellet-oolite-detrital calcarenite, S £ _P § -g'-ep ct: p LU and strongly recrystallized molluscan 1=9 Z Si P 9* jo coquina. Scattered quartz sand grains OQ 3 are present near base. (170.2 m) I CO Aporrhais sp., Ostrea sp., Nen'neo sp. +-> LIMESTONE BRECCIA: Gray-tan thin to if platy angular blocks recemented in fine grained marl matrix Vertical scale : 1:1000 After S. J. Roach, 1952 FIGURE 8. Sulaiy Formation type section. D68 GEOLOGY OF THE ARABIAN PENINSULA members, a group of stratigraphically higher carbonate The only full measurement of the Sulaiy is at the beds was included within the Thamama formation type section where about 170 m are exposed. An and recognized as constituting a third, as yet unnamed Al Kharj water well, about 40 km southeast, pene member. trated 168 m of Sulaiy. Lack of accurate thickr esses The first formal publication (Steineke and Bramkamp may be attributed to several factors including: (1) 1952b) still included these three units within the the basal Sulaiy is exposed only in Day. Hit, (2) the Thamama, but, by that time, the members had been lower beds are complexly slumped, and (3) middle formally named and raised to formational rank and Sulaiy beds occurring at the foot of the escarpmer t are the Thamama was elevated to group status. From top almost everywhere covered by recent fill. to bottom the three formations were designated as the UTHOLOGIC CHARACTER Buwaib, Yamama, and Sulaiy. Later, the upper limit of the Thamama Group was redefined to include Both surface data and near-outcrop wells shovr the a fourth formation, the Biyadh Sandstone. lithology of the Sulaiy to be remarkably uniform. The The type section of the Sulaiy Formation (fig. 8) main rock type is tan, chalky, massively bedded, aph is in the cliff above Dafcl Hit at lat 24°29'18" N.; anitic limestone. Some thin beds of skeletal and pellet long 47W06" E. The upper limit is placed at the calcarenitic limestone and calcarenite occur through change from chalky aphanitic limestone of the Sulaiy out but become particularly prominent in the lower to the more detrital calcarenitic limestone and cal- 40 m. The basal beds also include moderate amounts carenite of the Yamama Formation. The base is at of fine quartz sand. the contact of the basal calcarenite of the Sulaiy with The sequence in and above Datil Hit was studied the limestone breccia of the Hith. As pointed out in in 1938 by R. A. Bramkamp and T. C. Barger and the discussion of the Hith contacts, the limestone was designated the type locality. About 110 m is breccia (included in the Sulaiy by Steineke and others, exposed in the escarpment and approximately 60 m 1958), is now considered a part of the Hith. on the back slope. In 1952 S. J. Roach reworked the same outcrop; OCCURRENCE AND THICKNESS his description of the sequence is given in section 44 The Sulaiy crops out in a gently curving arc from lat (p. 139). 22°38' N. to Al Khatilah (lat 25°26' N.). Only the The few locations studied south of the structural Nisafr graben and a few thin banks of recent cover complications of Wadi Nisafr show marked lithologic break the continuity of the Sulaiy over this 350 km constancy along strike. In 1962 R. W. Powers- and distance. Width of the outcrop belt varies little H. A. McClure measured the upper 44.5 m and lower between Khashm Khartum (lat 23°24' N.) and Khashm 30.6 m of the Sulaiy respectively from lat 23°54'14" al 'An (lat 24°44/ N.). N., long 47°13'47" E. to lat 23°54/09// N., long 47° The most prominent feature of the Sulaiy landscape 14'17" E. and from lat 23°46/31// N., long 47°01'22" is the 50- to 100-m thick, west-facing slump escarpment E. to lat 23°46/16// N., long 47°01 /23 // E. Recent in the upper part of the formation. Readily recogniz sand and gravel cover the middle part of the formation. able for over 200 km, the escarpment (herein termed The accessible parts of the succession are summarized the Hit escarpment throughout its length) marks the in section 45 (p. 140). boundary between two areas of quite different topo graphic expression. To the east is a gently tilted, NATURE OF CONTACT WITH ADJACENT FORMATIONS steplike, upland surface formed by the weathering of The Hith-Sulaiy contact is visible only at Dafrl Hit. tight, well-bedded limestone. To the west is an Here, bedding planes in massive anhydrite show low undulating surface of contorted limestone beds and low, angular discordance below a plane of truncation. complexly slumped hills. These slump features Until recently, this discordance was cited as evidence characterize lower Sulaiy outcrops even in areas where for a disconformity at this level. Now, however, it upper Sulaiy beds and the escarpment are no longer is believed that the contact is gradational and con exposed that is, generally north of Khashm al 'An formable and that the angular discordance is the result and south of Khashm Khartum. of subsurface solution rather than subareal erosion. A detailed analysis of the origin of the escarpment Elsewhere, along strike, Hith strata are missing, and has already been presented in the discussion of the the Sulaiy has been down dropped against contorted Hith Anhydrite. Suffice to say, the main cause was Arab limestones. removal of underlying Hith evaporite by solution and From lat 23°04' N. nearly to the Darb al IJijaz subsequent draping of Sulaiy beds over the front edge (lat 24°50/ N.) the Sulaiy is overlain conformable by of the remaining thick anhydrite sequence. the Yamama Formation. Throughout this distance SEDIMENTARY GEOLOGY OF SAUDI ARABIA D69 the contact is marked by a change from tan, chalky outcrop and subsurface were uncertain. Deep- aphanitic limestone below to massive beds of clean- well nomenclature, originally based on tentative washed Yamama calcarenite above. Although the comparison with the surface beds, can now be contact is covered north of the road, conformable re shown to be inexact and misleading. For ex lations between the units probably extend nearly to ample, "Yamama detrital" of deep wells in reality Al Khatilah. At about this latitude, Middle and almost entirely falls within the Sulaiy of outcrop. Upper Cretaceous rocks swing across Yamama and Other correlations permit accurate subs^irface Buwaib beds, truncate the Sulaiy, and come in contact definition of the Buwaib and Yamama, and these with the underlying Arab Formation. names will be carried into the subsurface as On the southern end of the outcrop, below lat 23°04' well. N., pre-Buwaib erosion cuts progressively deeper into PALEONTOLOGY AND AGE Sulaiy strata. The formation is completely cut out A partial list of Foraminifera occurring H the near lat 22°35' N., where Buwaib rocks rest directly Sulaiy Formation is given below. All the forms listed on Arab. range to within a few feet of the top of the form ation. Recent sampling of the type Yamama and bracketing Their lower limits are not so clear, however, with the strata by R. W. Powers and H. A. McClure hi 1962 exception of that of the Everticyclammina sp., which permits accurate placement within the type sequence is known to range through all but the uppermost of faunal horizons recognized in adjacent shallow air- beds of the formation. drilled holes. Placement of these horizons affects Foraminifera of the Sulaiy Formation include: not only Sulaiy-Yamama-Buwaib surface correlations Bramkampella arabica Redmond (Redmond, 1964b), but sheds considerable light on their subsurface Everticyclammina sp., Iberina lusitanica (Egger), Nau- relationships as well. tiloculina sp., Pseudocydammina sulaiyana Redmond, C. D. Redmond also in 1962 demonstrated that the Trocholina sp. 1 (a large, high-spired form with a top of the Sulaiy in the area of the type Yamama strongly protruding base and blunt apex), Trocholina should be revised upward approximately 4 m so as sp. 2 (a medium-sized, low-spired form with radial to include the basal member of the type Yamama. fluting hi the inner side of the marginal band). This member represents a thinned-down equivalent Presence of a common NautUoculina sp. gives the of the uppermost 9 to 18 m of the "Yamama detrital Sulaiy faunas a Jurassic aspect although this genus limestone" of deep wells. Concurrently with this is known to extend into the Lower Cretaceous. the designation "Yamama detrital limestone" as used Recent work by Maync (1959) has demonstrated in deep-well terminology is changed to Sulaiy For that Iberina lusitanica has a somewhat extended mation. The advantages of these two related changes vertical range, first appearing in the lower Kimmerdgian are as follows: and extending upward across the Jurassic-Lower 1. The top of the Sulaiy Formation coincides with the Cretaceous boundary into the lower Valang-;man. top of a major faunal division. Pseudocydammina Its presence in the Sulaiy Formation of Saudi Arabia sulaiyana Redmond (Redmond, 1964b), which is helpful only insofar as it limits the possibilities for appears in the Sulaiy Formation and ranges age assignment to either Late Jurassic or earliest upward to the top of the present "Yamama Cretaceous. detrital" of the deep-well subsurface section, Psevdocyclammina sulaiyana Redmond was discussed is thus confined to the Sulaiy Formation. The and figured by Henson (1948) as Pseudocydammina same conditions apply at outcrop where Pseudo- aff. litus (Yokoyama) from Qatar Dukhan well 2, cyclammina sulaiyana ranges to the top of the presumably in beds equivalent to the Sulaiy Formation. basal member of the type Yamama, now redefined Henson listed a stromatoporoid, Cladocoropsis sp., as uppermost Sulaiy. as occurring at the same level and considered tbe age 2. In most deep-well sections, the tip of the revised of the beds to be Late Jurassic. Sulaiy Formation coincides with a major lithologic Until such time as more definite evidence is obtained break accompanied by a marked electric-log it seems best to leave the age assignment of the P^laiy signature pattern; a point corresponding to the Formation open, listing it as Tithonian to early older Sulaiy top would have to be picked on Valanginian. faunal evidence alone. A comprehensive list of fossils found in the Pulaiy 3. The proposed modification of existing stratigraphic Formation includes: Chaetetes sp., Mitteporidiur> sp., terminology reconciles surface and deep-well Aporrhais sp., Diceras? sp., Exogyra couloni d'OrMgny usage. Prior to the recent findings, Lower Nerinea aff. N. blancheti Pictet and Campiche, Nerinea Cretaceous stratigraphic relationships between sp., Ostrea sp., and miscellaneous cidaroid racUoles. D70 GEOLOGY OP THE ARABIAN PENINSULA ECONOMIC ASPECTS with the now accurately placed "pre-Buwaib" surface GROUND WATER of unconformity. At the Yamama type locality this Because of their generally aphanitic and tight char involves lowering the Yamama-Buwaib contact r.bout acter, Sulaiy rocks appear unfavorable as water reser 8 m. The effect on the Buwaib-type sequence has voirs. This impermeability does serve a useful purpose, been to include the lower 23 m in the Yamamr. and however, in that water falling on the back slope of restrict the Buwaib to approximately the upper 10 m. the Hit escarpment drains into and recharges both Evidence for these changes is as follows: the Wadi Nisafr-Wadi as Sahba' channel system and 1. Analysis of the type Buwaib microfaunas by means the Biyadh-Wasia sandstone aquifer. of recently developed species keys shows that PETROLEUM Cyclammina greigi sensu strictu does not range below the top 10 to 12 m of the formation. Also, Porous Sulaiy rocks contain oil in one Saudi Arabian forms which appear in samples immediately be field that is, the offshore field of Manifa. Here, the low that level are those which are known elsewhere producing interval or Lower Ratawi reservoir is about only from the Yamama Formation. Further 250 ft of calcarenite and calcarenitic limestone. As more, the echinoids from the type Buwai? are is true of most Arabian carbonate reservoirs, oil ac associated with these lower forms and appear to cumulation is directly related to development of clean- fall below the lower limit of Cyclammina sreigi. washed lime sand (calcarenite). Other Middle East 2. Specimens of Pygurus cf. P. rostratus from the type fields outside of Saudi Arabia also presumably have Buwaib came from a position high in the forma discovered productive oil in the Sulaiy. tion, as originally defined. This species can now CRETACEOUS SYSTEM be accurately placed within the type Yamama YAMAMA FORMATION LOWER CRETACEOUS microfaunal sequence and can be demonstrated to fall below the range of Cyclammina. qreigi here DEFINITION as well. The Yamama Formation is named for the town of There is no possible doubt that at least the lower Al Yamamah (lat 24°11' N.) about 20 km north of 23 m of the type Buwaib is equivalent to a ronghly the type section and on line with the strike of outcrop. comparable interval in the lower part of the type Development of Yamama nomenclature from an un Yamama and that the echinoid occurrences in the two named part of the Tuwaiq Formation to a formation areas fall very nearly at the same level in time. within the Thamama Group is described in detail in Redefinition of the Yamama now places 45.5 m of the definition of the Sulaiy Formation. strata in this unit at the type locality. The base is The Yamama type section was pieced together from at the contact of chalky aphanitic and calcarenitic a number of short exposures on the Al' Qu§ay'a' upland limestone of the Sulaiy below with golden-t^own surface between lat 24°00/24" N., long 47°15/42 // E. calcarenite of the Yamama above. The top is placed and lat 24°00/24// N., long 47°20/54" E. at the unconformable break from golden-brown pellet- As defined by Steineke, Bramkamp, and Sander calcarenite below to molluscan calcirudite of the (1958), 58 m of section between the dense Sulaiy Buwaib above. limestone below and pre-Buwaib unconformity above OCCURRENCE AND THICKNESS was assigned to the type Yamama. As already men tioned in connection with the Sulaiy, recent work has Outcrops of the Yamama form a very thin, ciirved shown that the basal 4 m of the Yamama type sequence band from lat 23°04' N. to the northern end of Jal belongs logically with the Sulaiy. The lower limit Buwayb (lat 25°24' N.), a distance of nearly 300 km. of the Yamama is herewith redefined to account for In the central area the band reaches a maximum width this change. of about 9 km. Yamama rocks are continuously The picture is further complicated by the fact that exposed from their southern limit to Wad! ?Tisafr many beds assigned to the lower part of Buwaib at where the beds are covered by younger deposits. its type locality further north were also included in The formation reappears north of the wadi, then per the Yamama type sequence. In addition, it is certain sists to the northern edge of the Al Jubayl plateau that the "pre-Buwaib unconformity" was somewhat where it passes under Quaternary cover. It is again misplaced in both areas. To straighten out the re exposed for about 25 km at Jal Buwayb in the steep lationships between the Yamama- and Buwaib-type face of the Al 'Aramah escarpment. sequences and adhere as far as is feasible to older Except for the clifflike exposures at Jal Buv^ayb, concepts, the upper limit of the Yamama has been Yamama rocks usually weather to a gently tiUed, adjusted in the outcrop and subsurface to coincide steplike series of dip slopes and low parallel benches SEDIMENTARY GEOLOGY OF SAUDI ARABIA D71 or hills. These are in turn superimposed on the flat Sulaiy, and thus, throughout the length of outcrop, upland surface dipping east from the Hit escarpment. limestone lies on limestone. Generally, however, the Complete measurement of the Yamama sequence break is marked by a change from tan, moderately has been made at two localities. At the reference porous aphanitic limestone below to brown, tightly section (near lat 24°00/ N.) the formation is about cemented pellet calcarenite above. In addition, Sulaiy 45 m thick; far to the north near Khashm Buwayb bedding tends to be thicker and more resistant to ero (lat 25°15' N.) it is about 25 m thick. Intraformational sion, and, on weathering, these beds form a steilike correlations show that most of the loss of section north series of 2- to 10-m sheer cliffs separated by short inter from the reference area is due to pre-Buwaib trunca vals of dip slope. In the Yamama, on the other h and, tion. short intervals of soft beds form low-angle slopes each About 10 km south of the reference section (near capped by a thin 0.5- to 2.0-m resistant ledge. lat 23°54' N.) another full Yamama sequence (40 m) The contact between the Yamama and overfying can be pieced together from a measured section and Buwaib Formation is uncomformable. In spite of this, two shallow air-drilled holes. As these holes were the break limestone on limestone is inconspicuous, spudded directly on the outcrop, their stratigraphic and exact placement must often await detailed faunal position in terms of the surface sequence is precisely control. Beveling of the Yamama involves only $ few known. At this locality, loss of beds relative to the meters per kilometer, and the unconformity can on'y be reference section can be attributed to pre-Buwaib recognized by the systematic disappearance of units erosion. Beveling of the Yamama continues south over a long distance. For example, from the refei^nce ward nearly to lat 23°00' N. where the entire unit section south, the Buwaib rests on progressively older is eliminated. units of the Yamama, cuts across the Sulaiy, and then IITHOLOGIC CHARACTER rests on Arab beds. Basically the Yamama Formation, like the Sulaiy, Although some pre-Buwaib truncation of the Ya is a nearly pure carbonate unit. Calcarenite, calcare- mama is apparent north of the reference section, surfi- nitic limestone, and aphanitic limestone occur in varying cial deposits cover the units before the Yamama is fully proportions along strike. Clean-washed calcarenite cut out. In fact, it is quite possible that some lower and calcirudite are the main rock types at the type Yamama strata persist beneath the cover far enough locality; away from this area progressively muddier north to intersect the pre-Wasia unconformity near sediments aphanitic and calcarenitic limestone and Wadi al 'Atk. even very thin layers of shale come in. PALEONTOLOGY AND AGE R. W. Powers and H. A. McClure in 1962 measured Occurrences of Pygurus rostratus Agassiz and Trema- in detail a full Yamama sequence not far from where topygus cf. T. grasi d'Orbigny (identified by J. E oger R. A. Bramkamp originally pieced together the type fide Sander in Steineke, Brampkamp, and Sander, (1958) section. Samples collected at closely spaced intervals within the Yamama Formation are indicative of a along the traverse provide the most complete record Valanginian age. of Yamama outcrop lithology now available. As a A partial list of Foraminifera from the Yamama result, the section is herewith designated a reference Formation is as follows: Everticylammina eccentrica locality. Redmond, Evertieyclammina ele.gans Redmond, Pseudo- The reference section (fig. 9) is a composite of three cydammina cylindrica Redmond (Redmond, 19C4b), isolated but overlapping measurements. The lower Pseudocyclammina spp., Trocholina sp. (a large, low- 26.5 m was measured along the back slope of the Hit spired form with the base more convex than the spiral escarpment between lat 24°00'37" N., long 47°15'40" side). As nothing is known of the occurrence of the E. and lat 24°00'13" N., long 47°16'54" E. Another above Foraminifera outside of Saudi Arabia, they can llm was picked up in the side of a low hill from lat have no special significance, in themselves, in deter 24°01'04" N., long 47°19/01" E. to lat 24°00'48" N., mining the age of the formation. long 47°19'08" E. The upper 8 m was described at A comprehensive list of Yamama fossils includes: another hill between lat 24°02'47" N., long 47°20'20" Astrocaeniat sp., MUleporidium cf. sp., Pygurus ros E. and lat 24°02'42" N., long 47°20'27" E. tratus Agassiz, Trematopygus cf. T. grasi d'Orbigny, Rocks found in the reference section are typical of cidaroid radicles, Oyclammina spp., Evertieyclammina Yamama exposures elsewhere; hence, a complete description is given in section 46 (p. 140). eccentrica Redmond, Evertieyclammina elegans Red mond, Pseudocyclammina cylindrica Redmond, Pseu NATURE OF CONTACT WITH ADJACENT FORMATIONS docyclammina sp., Trocholina spp., Haplophragmium The Yamama Formation rests conformably on the sp., Haplophragmoides sp., Trochammina sp., Aniso- D72 GEOLOGY OF THE ARABIAN PENINSULA Locations: (a), (b), (c): Yamama (d): Buwaib (a) 24°00'37"-47°15'40" (C) 24°02'47"-47°20'20" 24°00'13"-47°16'54" 24°02'42"-47°20'27" (b) 24°01'04"-47°19'01" 23°53'51"-47°18'53" 24°00'48"-47°19'08" 23°53'52"-47°19'53" Generalized lithology Diagnostic fossils SANDSTONE: Red medium-grained ferru CO ginous, black weathering sandstone SHALE, DOLOMITE, CALCARENITE AND APHANITIC LIMESTONE: Complexly in- terbedded blue-green and olive-green shale, reddish-brown medium-grained crystalline dolomite, golden-brown, in part Cyclamm/ncr greigi Everficyc/amm/'na hensoni beds sandy, well-cemented pelletdiei calcarenite,caicarerme, * rfterf^rffcrf^rf^rf^rf^^^^^*^*^^^^^^^^^^^^^^^*^^^ and tan to brown chalky aphanitic lime- r Everficyc/am miner eceenfr/co stone. Occasional thin beds}ds of molluscan ) Pseudocyc/ammina cy/indrica horizon calcirudite and quartz sandstone.dstone. (17.7 m) ( V^'-*--" »> * ' (3) CALCARENITE AND APHANITIC LIME STONE: Golden-brown usually medium- Pygurus rosfrafus grained and well-sorted tightly cemented Tremafopygus Cf. T. grasi pellet-Foraminifera calcarenite; poorly ex posed chalky aphanitic limestone inter- bedded in upper part. Single thin bed of Everficyc/amminc e/egans golden-brown fine- to medium-grained calcareous quartz sandstone present near middle of unit. (16.0 m)______(2) APHANITIC AND CALCARENITIC LIME- STONE: Tan moderately porous chalky, Pseudocyc/ammina su/a/'yana horizon sparsely sandy aphanitic limestone inter- bedded with tan to brown molluscan cal- Vertical scale: carenitic limestone. Occasional thin beds 1:1000 of golden brown well sorted tightly cemented Foraminifera-pellet calcarenite. (10.1 m)______(1) CALCARENITE AND COARSE CAR- BONATE: Brown to golden-brown poorly to moderately well-sorted tightly cemented clean washed fine-grained to gravel-size pellet-molluscan-Foraminifera calcarenite and calcirudite. Several intervals poorly exposed, probably chalky aphanitic lime stone. (19.4 m) CALCARENITIC LIMESTONE: Brown and tan sparsely sandy molluscan and foraminif- eral calcarenitic limestone After R. W. Powers and H. A. McClure, 1962 FIGURE 9. Buwaib and Yamama Formations reference sections. cardia sp., Aporrhais sp., Cerithium (s.l.) sp., Corbis sp., This is the only occurrence in this formation within Corbula sp., Chione cf. sp., Cuculaeal sp., Exogyra spp., Saudi Arabia, but other Middle East fields presumably Gyrodesf sp., Gryphaea balli (Stefanini), Homomya sp. have accumulations at this level as well. At Manifa, Lima, sp., Modiolusf, Natica sp., Nwulana sp., Ostrea the effective thickness is small and limited to calcrtrenite sp., Paphia sp., Pholadomya cf. P. decussata, Pleuroto- development in a dominantly aphanitic interval. maria? sp., Siligua sp., Stmparolust sp., Tellina sp., Trigonia sp., Trochus sp., Venus sp., unidentified am BUWAIB FORMATION LOWER CRETACEOUS monite genus showing affinities with Knemiceras and DEFINITION Hypengonoceras, and unidentified small molluscs. The Buwaib Formation is named for Khashm ECONOMIC ASPECTS Buwayb (lat 25°15' N.) near where the type section The offshore field of Manifa contains productive oil was measured. Development of Thamama Group in the Yamama Formation (Upper Ratawi reservoir). (including Buwaib Formation) nomenclature from its SEDIMENTARY GEOLOGY OP SAUDI ARABIA D73 inception to modern usage is covered in detail under unit weathers to a relatively low-relief plain. The the definition of the Sulaiy Formation (p. 66). generally flat surface is broken, however, by several Recent work has shown that some modification of small discontinuous benches that crop out at various the type Buwaib as defined by Steineke, Bramkamp, levels through the section. and Sander (1958) is necessary. It can now be demon In the southern area of outcrop generally between strated that the lower two-thirds of the 33.8-m interval Wad! ad Dawasir and Wadi Birk Buwaib Formation previously assigned to the type Buwaib is, in fact, thickness is relatively constant at 40 to 45 m. There equivalent to the lower part of the type Yamama and, is considerably more fluctuation in sections measured in addition, falls below the pre-Buwaib unconformity. to the north for example, 18 m at the reference section Supporting evidence has already been cited as part of (lat 23°54' N.), 33 m near lat 24°00' N., about 20 m the discussion on the Yamama Formation (p. 70). near the Darb al IJijaz (lat 24°52' N.), and 11 m at the Based on the new data, it now seems logical to type section near Khashm Buwayb (lat 25°15' N.). restrict the name Buwaib Formation to the upper 11.2 m of the type section at Khashm Buwayb and throw LITHOLOGIC CHARACTER the lower 22.6 m with the Yamama. Thus, the Buwaib Although the Buwaib contains several rock types, the would take in all beds carrying Cyclammina greigi and general lithologic character changes but little 8 long would put the base of the formation at the stratigraphic strike. Aphanitic limestone and calcarenitic limestone break that elsewhere has been called the "pre-Buwaib are most common, but invariably the sequence is unconformity" in both surface and subsurface reports. broken with thin intervals of calcarenite, sandstone, and The section measured in the face of Jal Buwayb at shale. Beds of clean quartzose sandstone appear to be lat 25°15'03" N., long 46°38'52" E. is held as the type confined to the upper part of the formation. locality for the Buwaib (restricted). The base is at the One of the most complete sections of the BuwaJb so unconfonnable contact of cream-colored aphanitic far studied is a few kilometers northeast of Khafs limestone above with quartzose sandstone and aphanitic Daghrah (lat 23°50/ N.). Here R. W. Powers and limestone of the Yamama below. The top is at the H. A. McClure in 1962 measured and sampled the highest limestone below the sandstone of the overlying entire sequence which is herewith designated as a Biyadh. reference section (fig. 9). The formation was studied OCCURRENCE AND THICKNESS on a continuous traverse from lat 23°53/51 // N., long Outcrops of the Buwaib Formation have been mapped 47°18/53// E. to lat 23°53/52// N., long 47°19/53// E. in a narrow irregular band from Khashm al Bazum A full description is given in section 47 (p. 141). (lat 20°38/ N.) to the northern end of Jal Buwayb (lat Sections at the extreme northern and southern 25°24' N.), a distance of nearly 600 km. South of end's of the outcrop belt fail to show any significant Khashm al Khufaysah (near lat 23°00' N.) average differences in the lithologic pattern seen at the ref- width of the outcrop is about 18 km; to the north, it is ference locality. For example, the type sequence at about 2 km. Khashm Buwayb may be summarized as follows: Continuity of the Buwaib is broken at several At the base 3.8 m of aphanitic limestone with thin places notably, at Al Aflaj (lat 22°10' N.) where a 50 interbeds of calcarenitic limestone and calcarenite, km wide sheet of gravel cuts through the Lower overlain in turn by i.7 m of friable sandstone, 3.7 m Cretaceous, again at Wadi Nisafr where the central unexposed but probably shale, and 2.0 m of tl inly Arabian graben system funnels through, and finally interbedded sandstone and sandy calcarenitic Pme- just north of lat 25°00' N. where exposures are covered stone. for 30 km by surficial deposits. Southerly exposures are characterized by a section In addition to the main belt of outcrop, several small just north of Qasr IJimam (lat 20°49' N.). The outliers of Buwaib are preserved in the slumped and sequence here may be summarized as given in section brecciated zone just west of the Hit escarpment. The 48 (p. 142). most prominent of these occur in front of Khashm NATURE OF CONTACT WITH ADJACENT FORMATIONS Khartum (lat 23°23/ N.) and Khashm Fazra' (lat The Buwaib Formation rests unconformably on all 23°38/ N.) and west of 'Irq Banban (in the vicinity of underlying units with which it comes in contact. lat 25°00' N.). Certainly Buwaib rocks are down From the vicinity of Jal Buwayb nearly to lat 23°00' N., dropped in other areas as well, but structural complica the Buwaib rests tfn different beds of the Yam ama tions and lithologic similarity have so far prevented Formation. Southward, the Buwaib progressively over accurate sorting of the various units involved. lies the Sulaiy and finally comes in contact with Arab Except for the steep cliff face at Jal Buwayb, the strata about 20 km north of Layla. D74 GEOLOGY OF THE ARABIAN PENINSULA The upper contact with the Biyadh Sandstone is appeared in publication in 1952 (Steineke and everywhere conformable and gradational. It is ex Bramkamp, 1952b). pressed as a low discontinuous bench of Buwaib Truncation and overlap within the Wasia as first limestone capped by a thin veneer of reddish-black, defined had long been recognized. The exact surface ferruginous sandstone of the Biyadh. of unconformity, however, was only recently pinpointed At the southern end of the outcrop, the Buwaib by detailed field and aerial photomapping. With these disappears under gravels dispersed by the Wadi data, it could be demonstrated that the Wasir, con ad Dawasir drainage system, and its ultimate limit is tained two sand units separated by a major uncon unknown. Surficial cover also obscures events on the formity. Consequently, in 1952 Bramkamp tents tively northern end of the outcrop belt beyond Jal Buwayb. removed the beds below the unconformity from the In this place, however, regional geometry suggests Wasia and assigned them to a new formation the that the Buwaib persists to Wadi al 'Atk and is there Biyadh Sandstone. The Biyadh, because of its con truncated by pre-Wasia erosion. sidered Lower Cretaceous or Neocomian age, wr s and still is the upper formation of the Thamama Group. PALEONTOLOGY AND AGE Concurrently, the beds remaining above the uncon Surface exposures of the Buwaib Formation carry formity were assigned to the Wasia (restricted). The Cyclammina greigi Henson, Everticyclammina hensoni new terminology first appeared in a paper by Thralls Redmond, a#d several other lituolids. and Hasson (1956), and formal publication of strati- As the Buwaib unconformably overlies the Val- graphic limits followed in 1958 (Steineke and others). anginian Yamama Formation along the outcrop and The definition of the Biyadh and Wasia Formations underlies the lowermost occurrences of Orbitolina cf. noted by Steineke, Bramkamp, and Sander (195S) has 0. discoidea Gras, 0. discoidea Gras var. delicata Hen- been retained in this paper. There is considerable son, and Chqffatetta dedpiens Schlumberger in sub evidence, however, that the Biyadh-Wasia boundary surface, its position would be within the Hauterivian. on outcrop does not coincide stratigraphically with Buwaib fossils include: Ammobaculitesf sp., Cy the subsurface contact. In fact, there seems little clammina greigi Henson, Everticyclammina hensoni doubt that strata assigned to the upper Biyadh at the Redmond, unidentified lituolids, and unidentified small surface are included within the Wasia in deep-well molluscs. sections (fig. 1). At least one and possibly several unconformities, obscured by sand-on-sand relationship, BIYADH SANDSTONE LOWER CRETACEOUS complicate the picture. DEFINITION The Biyadh section was measured along a two-part The Biyadh Sandstone is named for Al Biya$, an traverse: (pi. 9) (1) the lower part from the b^.se of extensive gravel-covered plain that marks the area of the sandstone at lat 24°06'38" N., long 47°23'53" E. outcrop. Beds now assigned to the Biyadh were first (near Ath Thulayma') east to lat 24°05/28// N., long included by T. W. Koch and C. W. Brown in 1934 as 47°38'07" E. and (2) the upper part from a pcmt at part of a larger unnamed clastic series of presumed lat 24°00'03" N., long 47°35'09" E. (the same strati- Cretaceous age. H. L. Burchfiel and J. W. Hoover graphic level as the top of unit 1) southeast to lat applied the name "Nubian Sandstone" to this unit 23°58/38// N., long 47°41 /37// E. Recalculations in 1935 because of similarity to the Cretaceous section based on new dip data show the Biyadh at the type in Lebanon. locality to be about 425 m thick. Max Steineke in 1938 pointed out that the Nubian The base of the Biyadh is at the top of the highest Sandstone is primarily a facies term with no strati- thin marine limestone beds of the Buwaib Formation. graphic significance. It had been used in Egypt, The top is at the unconformable contact of coarse Sinai, Lebanon, and Palestine as a "catch-all" for quartz sand with quartz pebbles and the overlying arenaceous sediments of widely differing ages. The ferruginous silty sandstone of the Wasia Formation. Arabian sandstone, on the other hand, occupied a OCCURRENCE AND THICKNESS definite stratigraphic position and could only be Sandstone of the Biyadh can be traced rlmost correlative with the upper part of the succession in continuously from Wadi ad Dawasir to Wadi al 'Atk, Egypt. For that reason, the loose term Nubian a distance of nearly 650 km. Only two breaks of any Sandstone was replaced by the name Wasia Formation. significance occur, and these are the synclinal area of The Wasia was bracketed at the top by limestone of Wad! as Sahba' where Wasia and Aruma sedimentary the Aruma Formation and at the base by limestone rocks effectively mask the Biyadh and the gravel sheet that is now Buwaib. The Wasia as thus defined at Al Aflaj (about lat .22°10' N.). Nearly 50 kir wide SEDIMENTARY GEOLOGY OF SAUDI ARABIA D75 at Wadi ad Dawasir, the Biyadh narrows persistently Gravel Stripe." It is distinct both on the ground and northward to Khashm Khanasir al Khafs (lat 25 °35' aerial photographs and can be traced south for several N.) where it pinches out in the face of the Al 'Aramah hundred kilometers, even far beyond where the under escarpment. Southward, the Biyadh disappears be lying calcareous member gives out. The upper limit of neath alluvial gravel and eolian sand of the Rub* al this white gravel stripe is taken as the Biyadh-Wasia Khali. Scattered exposures of combined Biyadh and contact. Wasia are known within the sand-covered area; the The bulk of the Biyadh appears to be of nonmarine most prominent are at Al Jaladah (near lat 18°30' origin, except for the thin carbonate stringer wMch N.) and Ash Sharawrah (near lat 17°15' N.). contains poorly preserved marine molluscs and a poorly From Wadi as Sahba' south, weathering of the preserved nondiagnostic marine microfauna found 25 to Biyadh results in a characteristic landscape made up of 30 m below the carbonate stringer. C. D. Redmond in numerous small dark-colored to black hills between 1962 suggested that the lower marine lithology may which are gently rolling white, gravel-covered plains. represent a western extension of the environment recog The Arabs refer to this belt as Al Biyaol which roughly nized in the subsurface as the Mauddud Limestone translated means "the white country." The same gen Member of the Wasia Formation. Unfortunately, p^oof eral topography picks up again north of Wadi as Sahba', is lacking; if this could be demonstrated, much of the but here the terrain is somewhat rougher and more uncertainty regarding Biyadh-Wasia surface-subsur*ace difficult to traverse. relationships would be eliminated. Gravel littering the Biyadh surface is apparently The type section of the Biyadh is considered to be residual and is composed almost wholly of white, well- that measured about 20 km south of Wadi as Sahba' rounded quartz pebbles and lesser amounts of quartzite by D. A. Holm and N. M. Layne in 1949. Their de and fossil wood. Some igneous and metamorphic grains scription of the rocks exposed (section 49, p. 142) at this also occur, but these are generally concentrated near locality serves as an excellent summary of Biyadh rocks major channels that originate west of Jabal Tuway 228-770 O C D76 GEOLOGY OF THE ARABIAN PENINSULA the presence of any one of these species is sufficient makes it seem most logical to carry the Biyadh as to indicate a position within the formation. Based Barremian and lower Aptian. Clearly, this dating on contained forms and regional considerations, the may not involve the upper 200 m of the type section. subsurface Biyadh is probably Barremian and lower ECONOMIC ASPECTS Aptian. As presently interpreted, these beds would equate approximately to the lower 200 to 250 m of GROUND WATER Biyadh at the type locality. In the outcrop area the Biyadh and Wasia sands, In connection with this, there is some suggestion and to a small degree Quaternary alluvium, corirrigation only at Al Bijadlyah, r.n agri theorize that the Ammobaculites and zone of marine cultural development in Wadi as Sahba' generally molluscs just above represent the westerly extension across the Biyadh-Wasia outcrop belt. A totrl of 54 of this marine environment. nonartesian wells have been drilled. Except for three It is possible that the Biyadh as now defined on wells completed in Quaternary gravel to tho west, outcrop could include beds ranging in age from all the wells are completed in the Biyadh-Wasir, sands. Hauterivian or Barremian to Cenomanian. Such The number of permanent pump installations is un age assignment would be in sharp conflict with the certain; probably less than 30 have been made much better known subsurface sequence where upper The Al Bijadiyah area is underlain by 300 m of Aptian to Turonian strata are included within the highly productive water-saturated sediment that is Wasia. Unfortunately, however, east-to-west gra constantly recharged by fresh meteoric water slowly dation from marine shale and limestone to nonmarine seeping downward through channel fill. To tbe west, sand and gravel is accompanied by facies changes on the south side of Wadi as Sahba' where tta water of such magnitude and rapidity that it has proved table rises above the base of the Quaternary alluvium, impossible to recognize any widespread marker hori the shallow, subsurface, highly permeable gravel zons. Because of this, there is little likelihood that channel(s) which act as conduits for movement of the Biyadh-Wasia contact in the subsurface can ever water down the wadi, can be developed as well. be placed with certainty in terms of the surface se quence, and vice versa. The widespread usage, PETROLEUM ease of recognition, and economic importance of the The main economic interest of the Biyadl is its subsurface division throughout the Persian Gulf area equivalence to the Zubair zone which is productive in SEDIMENTARY GEOLOGY OP SAUDI ARABIA D77 southeastern Iraq. Within Saudi Arabia proper, the the sandstone is preserved in sturcturally disturbed only significant show of Biyadh-Zubair oil is a minor blocks. accumulation in Safaniya. From An Nafud to Ar Rub' al Khali the Was;a is interrupted only by such through-cutting channel" as WASIA FORMATION MIDDLE CRETACEOUS Al Ba^in, Wadi al 'Atk and Wadi as Sahba'. Patches DEFINITION of sandstone bedrock cropping out in windows in the The Wasia Formation is named for Khashm Was!' Rub' al Khali sand mantle have been described as (lat 24°23' N.), where a complete section is exposed. Biyadh and Wasia undivided; Al Jaladah and Ash Changes leading to the present definition of the Wasia Sharawrah are exposures of this type. Actually, have been discussed in connection with the Biyadh however, there is some doubt that the Wasia extends Sandstone (p. 74). that far southwest. Stratigraphic details of the Wasia as now defined The strike of Wasia beds changes sharply in two were first published by Steineke, Bramkamp, and places that is, Wadi as Sahba' and Wadi al 'Atk. Sander (1958). The type section is in the lower slope In each case Wasia exposures turn almost due west, of Al 'Aramah escarpment, extending from the pre- and the wadis cut abruptly down through the section. Wasia unconformity (at lat 24°23'04" N., long Presumably in both cases structural happenings, 47°45/ 12" E.) southeast to low hills near Was!' (lat possibly also related to wadi formation, are respons;ble. 24°22'38" N., long 47°45'49" E.). The base is at the In the central and northern areas of outcrop, Wasia contact of varicolored fine-grained sandstone and silt- beds typically weather to an upward-steepening s^pe stone of the Wasia with underlying light-colored, that terminates in an almost sheer face beneath the pebbly, coarse-grained sandstone of the Biyadh. The scarp-forming limestone of the Aruma Formation. top is at the disconformable contact of Wasia sandstone In the southern area, beginning about 40 km south of with dolomite and limestone of the Aruma Formation Wadi as Sahba', the Aruma progressively loses its above. cliff-forming ability as the formation is replaced from Relationship of the Wasia to a thick clastic unit in the bottom up by softer sandstone. Complete re northwestern Arabia the Sakaka Sandstone has long placement is effected near lat 22°30' N., and, with been controversial. Variously dated as upper Paleo this change, the surface of the Aruma and Wasia breaks zoic and Middle Cretaceous, the only rigid limits were down to about the same general level as the adja^-ent imposed by the underlying Lower Devonian Jauf Biyadh plain. Formation and overlying Upper Cretaceous Aruma Near Sakakah, the formation crops out as a rough, beds. Recently drilled bore holes have, however, now maturely dissected plateau with scattered hills that bridged the gap between the two units and extended follow no particular topographic pattern. Channels the Wasia northward to where there is little doubt cutting the plateau are short, poorly developed, and that it is laterally continuous with the Sakaka. Hence, debouch into small mud flats. West and southvest, the Sakaka is herein described simply as a prolongation outliers of Wasia (Sakaka) form small plateaus or of the Wasia. The term Sakaka, first applied by cap isolated hills. R. M. Sandford in 1950 to cover the doubtful unit, Sparse measurements indicate that the thickness of is herewith considered obsolete. the Wasia increases irregularly but persistently f~om south to north. Some 25 to 30 m thick near Wadi ad OCCURRENCE AND THICKNESS Dawasir, the section expands to about 40 m at V^adi The southernmost occurrence of definite Wasia is as Sahba', 60 m near Khashm Khafs, 80 to 100 m near Wadi Dawasir. From here, the formation con east of Al Majma' ah (lat 25°55' N.), 90 m at T'ba' tinues northward in a great arc to where it passes under (lat 27°24' N.), and reaches a maximum thickness of the western edge of An Nafud (near lat 28°25' N.). 285 m near Sakakah. Exposures west and southwest The unit again emerges around Sakakah (lat 30°00' of Sakakah are incomplete, and the thickest interval N.) where it has been mapped as the Sakaka Sandstone. measured at Al galwat (lat 28°50' N., long 38°42' Total linear distance of Wasia exposure to this point E.) is somewhat less than 90 m. is nearly 1,450 km. A number of small outliers also LTTHOIOGIC CHARACTER occur west and southwest of Sakakah, for example: The Wasia (Sakaka) Formation is a relatively thin (1) In hills which stand up along the north side of sandstone and shale unit which is at least in part the At Tawil fault (near lat 29°22' N.), (2) on the tops marine in the central and northern areas and probably of hills rising above Al Huj plateau, and (3) around nonmarine in the south. In the vicinity of Wasi', Umm Nukhaylah (lat 29°30/ N., long 38°33' E.) where a lenticular bed of red sandy dolomite and a local lens D78 GEOLOGY OF THE ARABIAN PENINSULA of soft nodular limestone containing marine fossils Aruma happenings in the north are almost perfectly occur 25 m below the top of the formation. South of mirrored by events in the south. Just below Vadi as Wadi Sahba' similar lenticular beds of carbonate are Sahba', Aruma limestone is replaced from the bottom known in the same part of the section. Several inter- up by sandstone, thus masking the lower contact. beds of dolomite have also been observed in the se Diligent tracing of beds, including phototracirT, has, quence at Qiba'. however, permitted accurate placement of the Wasia- Wasia rock types south of An Nafud are character Aruma unconformity south to the termination of ized by the type succession at Was!' a succession Aruma outcrops (at Wadi ad Dawasir). measured by R. A. Bramkamp and S. J. Roach in PALEONTOLOGY AND AGE 1955. Their description of the interval is given in section 50 (p. 142). Comments on the ammonite fauna have been supplied A substantial thickness of Wasia (Sakaka) crops by W. M. Furnish (written commun., July 25, 1953). out in northwestern Arabia. Here again, sandstone, A dolomite bed about 25 m below the top of the Wasia siltstone, and shale are the main rock types present. carries the ammonite Neolobites vibmyeanus d'O'Mgny. The 285-m interval exposed in the vicinity of Sakakah A single specimen was collected near Was!'; several was summarized by D. A. Holm and S. D. Bowers in others were found near Qiba'. According to Basse 1952 as given in section 51 (p. 143). (1937), this species is known from many localities In Al Huj plateau, 88 m of Wasia (Sakaka) cap Al around the Mediterranean and is a fair index for upper IJalwat (lat 28°50/ N., long 38°42/ E.), and 71 m are Cenomanian. at the top of Barud (lat 29°03' N., long 38°24/ E.). At Fossils collected just north of Wadi al 'Atk on an both places lower beds are conspicuously crossbedded, extensive low bench formed by the upper dolomite but the upper part is massive bedded and weathers bed of the Wasia consisted almost entirely of poorly to heavy ledges; at Al 5alwat, the sandstone is slightly preserved small gastropods, simple pelecypods and a calcareous toward the top, and there is a mud-pellet solitary coral. None have direct counterpart in any conglomerate 11 m below the summit. collection from the overlying Aruma. Tw> frag mentary, poorly preserved ammonites were also col NATURE OF CONTACT WITH ADJACENT FORMATIONS lected from this locality. The ammonites sho^ some At the base of the Wasia is a profound unconformity differences when compared with Libycoceras from the that cuts into progressively older beds from south to overlying Aruma Formation, and they are not close north around the arc of outcrop. From Wadi ad to the Neolobites found elsewhere in the Wasia. The Dawsir to Rufcayyat al Samra* (lat 24°50' N.) trunca worn suture, however, appears to show a relationship tion of rocks, in this case the Biyadh, takes place at a to the genus Knemiceras s.L, and on this basis the rate of up to 1 m per km. Northwestward, the rate logical deduction would be a Middle Cretaceous age of truncation increases to about 3 m per km and the assignment. Biyadh is completely overlapped near Wadi al 'Atk. Surface-subsurface relationships are not entirely clear From there, the Wasia progressively transgresses and (see discussion of Biyadh Sandstone, p. 74) but it finally truncates and overlaps the entire Mesozoic seems certain that beds now assigned to the T^asia in section and west of Sakakah rests on Lower Devonian central Arabia correspond approximately to the middle rocks. part of the coastal Wasia well succession, an interval The Wasia Formation is everywhere in discon- that roughly involves the Ahmadi and Pnmaila formable contact with the overlying Aruma. From Members which are confidently dated on contained Wadi as Sahba' nearly to Sakakah, the break is clearly microfossils as Cenomanian. It is uncertain whether marked by a change from sandstone below to reddish- the overlying Mishrif Member (Turonian) extends as brown, coarsely crystalline dolomite and dolomitic far as the outcrop area, but it is possible that at least limestone above. part of it does. Consequently, the surface sequence Around the rim of the Sakakah basin (starting near is now dated as Cenomanian and Turonian(?\ lat 30 °00' N., long 40"SO' E.) most of the lower part ECONOMIC ASPECTS of the Aruma limsestone is replaced by sandstone. As a consequence, the top of the Wasia is somewhat GROUND WATER more difficult to pick, but fortunately, a very thin bed The Wasia aquifer is a thick, widespread hydraulic of limestone marks the basal Aruma over much of the unit of Middle Cretaceous age that crops out in central area. In fact, it is the geometry of this bed that shows Najd and dips gently eastward, maintaining its' water the Wasia (Sakaka) to be truncated by pre-Aruma bearing characteristics at least as far as Pahrain. erosion near long 38"SO' N. It is composed primarily of permeable, unindurated SEDIMENTARY GEOLOGY OP SAUDI ARABIA D79 sand which throughout most of the Eastern Province sheets of limestone are sandwiched between thick is considered a single hydraulic unit. Lithologically bodies of complexly lenticular sandstone, siltstor^, and hydraulically, the aquifer becomes more complex and shale. Individual limestone layers are roughly toward the Persian Gulf and in the coastal area is contemporaneous across the structure and over lar:»e separated into two distinct major and possibly several parts of northeastern Saudi Arabia. They serve to minor units. The Wasia has an established hydraulic subdivide and correlate the intervening, often barren, gradient which ranges from water-table conditions at clastic sequences. Although friable sandstone serves and near the outcrop to flowing artesian conditions as the main reservoir rock, significant quantities of along the Persian Gulf. oil have also been discovered in porous limestone beds. The catchment area consists of: (1) an approximately In addition to the Safaniya accumulation, gas occurs 25,000-sq km intake belt, a lowland hydrophysio- in the Wasia at Dammam field and a minor amount graphic province which includes the Wasia and Biyadh of low-gravity oil was detected at Khursaniyah. The outcrop and overlying Tertiary and Quaternary sed Wasia is a highly important producing interval outsiie iments and (2) a nearly 75,000-sq km runoff belt, of Saudi Arabia as well, particularly in Bahrain, a highland hydrophysiographic province which is Kuwait, and Iraq. underlain by relatively impermeable pre-Biyadh car ARUMA FORMATION UPPER CRETACEOUS bonates. Recharge occurs by two methods that DEFINITION is, direct percolation in the intake area and subsurface inflow through the channel systems which primarily The Aruma Formation was named for its occurrerce drain the limestone highlands. The sand- and gravel- in the Al 'Aramah plateau, a broad upland surface filled channels which cross the intake area act as a related to the easternmost of the Najd escarpments. recharging mechanism during the entire year. Limits of the formation stand essentially as they wr^e Wasia water ranges in quality from excellent at originally described by H. L. Burchfiel and J. W. or near the outcrop where it is freshly meteoric to brine Hoover in 1935. toward the northeast where it represents post-de- First formal reference to the Aruma appears in positionally modified connate water. Oil-field struc Steineke and Bramkamp (1952b). This was followed tural features along the coast have a marked effect in 1958 (Steineke and others) by a detailed description upon the gentle, east-dipping hydraulic gradient and of the type locality. The type sequence was compos upon water quality. Restricted permeability on the ited from several sections measured along a traverse flanks of structures tends to divert regional flow and from Khashm Khanasir (lat 25°38'12" N., long 46°£2' restricts flushing by fresher meteoric water, with the 29" E.) northeast to a point on the back slope of Al result that the older connate water is partially retained. 'Aramah escarpment (lat 25°39/ 18" N., long 46°23'30" In a converse manner, recharge funneled along the E.), thence northeast to a promontory (at lat 25°44'35" projected axis of the Wadi as Sahba' trough favorably N., long 46"SOW E.) in which the top of the formation affects the regional gradient and water quality. is exposed. Extraction of Wasia water is confined to three small The base is at the contact of Aruma carbonate rocks areas: (1) At and near the outcrop for irrigation and with underlying varicolored clastic sediments of the limited domestic uses, (2) at Khurays for domestic Wasia Formation. The upper limit is taken at the and drilling water supply, and (3) at North Abqaiq change from yellow-brown dolomitic shale below to for injection of nonpotable water into the oil reservoir. gray crystalline Lockhartia-b&anng dolomite of the The aquifer has a large potential for development Umm er Radhuma Formation above. as a source of supply for domestic use and irrigation, OCCURRENCE AND THICKNESS except where it is poor in quality in the northeastern area, along the Persian Gulf coast, and on structure. Rocks of the Aruma Formation crop out from Wadi The most favorable areas for development are those ad Dawasir to beyond the Iraq-Saudi Arabia border, areas where the aquifer will produce good quality water a distance of more than 1,600 km. Over this distar^-e and the intake area, especially in the vicinity of the the strike of the outcrop belt changes gradually.from transintake channels. northeast to north to northwest, describing a great arc wrapped around the central Arabian arch. As the PETROLEUM belt approaches Iraq it again swings gently nor*h, The Wasia Formation contains commercially pro probably in response to another positive basement and ductive oil at several levels in the offshore field of Paleozoic feature the Qa'il arch. Safaniya. Here Wasia rocks apparently record the Width of the Aruma outcrop band is somewl at growth of a great delta, and thin, laterally persistent erratic, but from a regional point of view it persistently D80 GEOLOGY OF THE ARABIAN PENINSULA increases from south to north. About 20 km wide at to the south. As a whole, the Al 'Aramah plateau is Wadi ad Dawasir, the formation is spread out over well dissected by channels cut perpendicualr to the face nearly 200 km where it passes into Iraq. of the main scarp. This is particularly true in the In addition to the main belt of outcrop described areas of At Taysiyah and Al Widyan where il°> surface above, the Aruma can be traced around the northwest is broken by an intricate drainage complex. margin of the Jawf-Sakakah basin and then almost A series of small faults break through tin Aruma continously along a south-facing escarpment from lat around the structurally active Jawf-Sakakr.h basin. 29°57' N., long 39°44' E. to lat 29°56' N., long 39°18' E. The faults routinely strike about N. 20° W. and appear This scarp roughly outlines the northern limit of the to be either vertical or dip at very high angles. Most A$ Tawfl-At Tubayq plateau. Exposures are again show low-order vertical displacement (probably less picked up in the face of the escarpment between lat than 15 m); horizontal movement is not apparent as 29°41' N., long 39°23' E. and lat 29°32' N., long drag folds, shear fractures, and offset features are 39°19' E. Following the scarp around, Aruma beds lacking. The faults are apparently related to adjust come in once again and can be mapped almost con ments in older beds. tinuously from lat 29°26' N., long 38°30' E. to where The Aruma Formation is calculated to be about 60 m the formation crosses into Jordan near Thanlyat fiirayf thick near where the unit passes under the Rub' al (lat 29°41' N.). Aruma remnants are also preserved Khali sand. This same approximate thickness is main in down-dropped blocks at Umm Nukhaylah and Jabal tained northward nearly to lat 22°30' N. and then the ar Ri'atayn (lat 30°05' N., long 39°41' E.). formation progressively thickens to about 130 m at It should be pointed out that the two patches of Wadi as Sahba' and 140 m at the type locality (lat Aruma shown in the Al Harrah lava field (U.S. Geologi 25°40' N.). The same approximate thickness is ap cal Survey-Arabian-American Oil Co., 1963) lat 30° parently maintained to the north, as an interval meas 46' N., long 39°13' E. and lat 30°40' N., long 39°20' E. ured west of Sakakah indicates about 145 m of Aruma are in error. These areas are, in fact, gravel-covered, in this area. Some thinning is apparent around the and there is no evidence that Aruma rocks were de margin of the Jawf-Sakakah basin, and intervals meas posited here. ured along the northern edge of the A$ TawIkA $ fubayq The Aruma outcrop belt is breached at only a few range from about 5 to 35 m. places by surficial cover. In the south, exposures are masked by gravel outwash from Wadi al Maqran and UTHOLOGIC CHARACTER Wadi al Jadwal (lat 22°00/ N.). In the central area, In the extensive area from Wadi as Sahbf.' to Sak Aruma beds are down dropped into the Wadi as Sahba' akah, the Aruma Formation is mainly shallow-water structural trough and covered by channel fill. Struc limestone, in part replaced by massive dolomite. Sub tural complications also appear to be responsible for ordinate shale and impure dolomite are present, partic interruptions at Wadi al 'Atk and along Al Bu^ayn ularly in the upper 25 to 35 m. Locally, rudHtids and (roughly from Wadi al 'Atk to Umm al Jamajim, lat other heavy-shelled molluscs are abundant enough to 26°52' N., long 45°18' E.). In the latter, a wide, to form biostromes. northwest-trending sheet of locally derived gravel is The rock types and lithologic sequence of the Aruma banked on the east against a sharp monocline involving Formation throughout the Wadi as Sahba' -Sakakah Aruma rocks. Sand of An Nafud severs the outcrop belt are characteristically displayed at the typ*. locality. belt along lat 28°50' N. where it encroaches east nearly This sequence (fig. 10) was measured just south of far enough to merge with sand of Ad Dahna'. Wadi al 'Atk by Roach in 1952. He recognized two The western edge of Aruma limestone is charac units that served to group most of the shale and impure teristically a steep cliff that can be recognized around carbonate into a single member the Lina overlying Sakakah and south from An Nafud to Wadi Jabaliyah a somewhat cleaner carbonate sequence the Atj mem (lat 22°25' N.). Generally the scarp is coincident with ber. This subdivision, never formally adopted, has the western limit of the formation, but south of Wadi some merit, as Redmond has demonstrated a discon- as Sahba' it becomes progressively lower and moves f ormable contact between the two members, a contact eastward diagonally across the outcrop band as the that has widespread subsurface expression with signifi limestone is replaced from the bottom up by less cant angular discordance. Unfortunately, the lateral resistant sandstone. continuity of the two units has never been systemat The back slope of the APAramah escarpment forms ically demonstrated beyond the limits of Al 'Aramah the great plateaus of Al 'Aramah, At Taysiyah, and Al plateau. A detailed description of the type sequence is Widyan north from Wadi as Sahba' and of Huraysan given in section 52 (p. 143). SEDIMENTARY GEOLOGY OF SAUDI ARABIA D81 Location: 25°38'12" . 25°39'18" t 25°44'35" 46°22'29" 46°23'30" 46°30'41" Generalized lithology Diagnostic fossils I DOLOMITE: Gray, granular crystalline dolomite i Possible disconformityi Ai/sfrosphenodisci/s sp., Cyc/o/ifes medlicotti, Elphidiella multiscissurata, Fissoelphidium oper- (4) SHALE AND DOLOMITE: Yellow-brown dolomitic and calcareous shale, olive shale, culiferum, Chrysalidina sp., Loftusia spv and argillaceous dolomite; interbedded lime rudisfids stone and dolomite. (32.9 m) (3) DOLOMITE: Blue-gray to cream dolomite; some limestone near top. (27.6 m) IT) ;o £ CC O (2) LIMESTONE AND SHALE: Cream fine K. grained chalky limestone; thin bed of olive- LU green calcareous shale at base. (41.0 m) Cyc/o//fes medlicotti, C. regu/an's n. n. Orbitoides gensac/cus I Osfrea cf. O. verneu/i, Hemipnei/sfes sp., rudistids (1) CALCARENITIC LIMESTONE: Cream fine grained nodular calcarenitic limestone; abundant organic remains. Lower part con Sphenodiscus acutodorsatus, Osfrea dichofoma, Hemi- tains common impure and sandy layers. pneusfes sp., Meandrops/na vidali, Monolepidorbis Typical basal Aruma unit is red-brown vuggy sancfae-pe/ogiae dolomite. (40.0 m) Nerinea ganesha Sphenodiscus acutodorsatus, Hemipneustes sp., Hem/- asfer sp. SHALE AND SANDSTONE: Red and green silty (O shale overlying light-gray, fine- to medium- LUO grained crossbedded sandstone -flu Vertical scale: QO Q< 1:1000 LU After S.J. Roach, 1952 I 6 FIGTTEE 10. Aruma Formation type section. D82 GEOLOGY OF THE ARABIAN PENINSULA Lateral changes in the Aruma Formation south of NATURE OF CONTACT WITH ADJACENT FORMATIONS Wadl as Sahba' consist mainly of a gradual replacement Over most of the outcrop the Aruma rests discon- of limestone by sandstone from the bottom up. First formably on the Wasia. Between Wadl as Sabba' and hints of the replacement can be seen about 40 km south the vicinity of Sakakah, the contact is easily placed at of the wadl; progressive transition from carbonate to the break from sandstone and shale below to limestone sandstone continues so that in the vicinity of Wadl above. South of Wadl as Sahba', the Aruma ir mainly Jaballyah (lat 22°25' N.) the Aruma is sandstone only. in clastic facies, and the contact is less clear. Even This facies persists to the southern limit of outcrop. here, however, the contact is marked by a charge from In the northern part of the Aruma outcrop, relatively silty sandstone and siltstone below to sandstone above. marked changes occur with the appearance of a major In the vicinity of Sakakah, the Wasia (SHkaka)- lens of sandstone in the lower part of the section. At Aruma contact emerges from under the blanket sand Khashm Zallum (lat 30°18' N., long 40°21' E.) 61 m of of An Nafud and can be traced along the eastern and sandstone overlies an 11-m-thick basal Aruma limestone northwestern edge of the basin. The break between unit. Some investigators have suggested that the top the two units is definitely unconformable, and over this of this limestone corresponds to the top of the Atj distance Aruma limestone rests on successive^ older member at the type locality. This relationship has parts of the Wasia (Sakaka); near Al Mur^yr (lat yet to be proved; if it is correct, then lower beds of an 30°03' N., long 39°55' E.) the Wasia (Sakaka) is over expanded Lina member are involved in the limestone- lapped, and Aruma rests directly on Lower E woman to-sandstone transition. The sandstone informally (Jauf) beds. Not far to the southwest the limestone termed the Zallum sandstone is locally poorly in pinches out. At the next outcrop (at lat 29°57' N., durated, fine to medium grained, and made up of long 39 °40' E.) the Zallum sandstone facies is the only subrounded grains. Striking colors of white, gray, vestige of the Aruma left, and at this point it h lapped brown, and purple prevail, particularly on weathered onto the Tawil Sandstone Member of the Tabik For surfaces. Near the base, concentrations of cobbles mation. Sporadic outcrops of Zallum (to the vest and and pebbles of chert, quartz, and quartzite are locally south along the Al Jayb scarp and west agaii across common. In places, the sandstone is now an ortho- the northern edge of the A$ TawiUAt fubayq plateau) quartzite which weathers into hard, dark-gray boulders. rest on beds of the Tawil Member and, in one area west From Khashm Zallum the sandstone can be traced of Umm Nukhaylah, on beds of the Jauf Formation. southwest in the face of Jal al Amghar to Khashm al Along its eastern margin, thte Aruma is everywhere Makhruq (lat 30°03' N., long 40°40' E.) and then east overlain by the Umm er Radhuma Formation. The for about 15 to 20 km where it thins slightly and under contact has for many years been considered conformable goes a rapid facies change to marly dolomite, dolomite, and gradational, but there is some subsurface paleon- and limestone rocks that are typical of the Aruma tologic evidence that at least locally a disconformity further south. Northeast of Khashm Zallum, the separates the two. Some paleontologists (F.. Page, sandstone crops out in the upper reaches of Wadl (Ar unpub. data, 1959) have suggested that the contact 'ar directly under Aruma limestone and can be traced in the deeply incised canyons as far as Al Mu' tadil over all Saudi Arabia represents a disconfornity or (lat 30°31' N., long 40°27' E.); it cannot be identified more accurately, a paraconformity (contact is a simple with certainty north of this point. bedding plane); for on outcrop and in deep wells, the The Zallum sandstone is not exposed in the Aruma top Aruma unit is a thin bed of blue-gray to bis ck dolo- limestone escarpment northwest of Sakakah but has mitic shale bearing Fissoelphidum opercutiferum Smout, been identified as the caprock on Jabal Qiyal al Kabir Loftusia spp., and Omphalocyclus macroporus Lamark. (lat 30°06/ N., long 40°05/ E.) on the northwestern Immediately above is limestone or dolomite with defi flank of the basin. Zallum sandstone again comes to nite lower Paleocene microfossils. The mechanics of the surface (at lat 29°57' N., long 39°40' E.) in an es formation of a paraconformity of such magnitude and carpment that closely parallels the unconformable con regularity is difficult to imagine. tact between Paleozoic and Eocene or Cretaceous rocks. North of Sakakah, the Aruma Formation is overlain Sporadic outcrops of Zallum, the only Aruma unit ap pearing west of long 39°40' E., have been mapped in by a cherty, nummulitic limestone of Paleocene or the face of this scarp as far as the Saudi Arabian-Jordan Eocene age that was apparently not directly related to border. With the exception of the Zallum lens, Aruma the Umm er Radhuma basin of deposition. Little rock types and fossils in the northern Al Widyan region work has been done in this area, and the exact nature are very similar to those occurring further south. of the contact is unknown. SEDIMENTARY GEOLOGY OF SAUDI ARABIA PALEONTOLOGY AND AGE liostoma sp., Campanile sp., cf. Campanile sp., Card'ta There is some possibility that the lowermost beds of cf. C. beaumonti d'Archiac, Cardium sp., Cardiumf ST>., the Aruma outcrop could be as old as Santonian, as Cerastoderma? sp., Chione? sp., CMamys sp., Chlamyst they contain Meandropsina vidali Schlumberger, a form dujardini (Roemer) Noetling, Corbis sp., Corbula ST>., originally described from the Santonian of Spain. Granisl sp., Crassatetta sp., OrassateUat sp., Gnocero^ However, the uppermost occurrence of this form in sp., Cymatium sp., Cympolia spp., Eocypraea sp., Eu- Saudi Arabia overlaps the range of Monolepidorbis spira sp., Faunus sp., Ficus sp., Olycimeris spp., sanctae-pelagiae Astre, a Campanian form likewise Harpagodes? sp., Hippochrenes sp., Indoceras balusc^i- originally described from Spain. tanensis Noetling, Inoceramus sp., Keilostoma sp., The middle part of the Aruma Formation presents a Lambisl sp., Lima sp., Mitral sp., MytUus? sp., Mono- problem in that it contains beds which fall below the dontat sp., Morganial sp., Nautilus (Heminautilus) sp., level of the lowermost definitely Maestrichtian Foram- Nautilus spp., Neithea guadricostata (Sowerby), Nemo- inifera and above the level of the uppermost definitely cardium? sp., Nerinea ganesha Noetling, Nerita cf. N. Campanian Foraminifera. pontica (d'Archiac) Noetling, Orthechimus aff. 0. creta Immediately overlying this anomalous interval are ceous Cotteau and Gauthier, Orthopsis sp., Ostrea di- beds which carry abundant occurrences of Orbitoides chotoma Boyle, Ostrea cf. 0. verneuli Leymerie, Owda gensacicus (Leymerie) and Omphalocyclus macroporus sp., Pecten sp., Perissolax? sp., Pholadomya cf. P. tigris, (Lamarck), both good Maestrichtian markers with a Noetling, Pholadomya sp., "Pleurotomaria" sp., Plica- very wide distribution in Europe. tula cf. P. hirsuta Conrad, Potomides sp., Protocardium? Omphalocyclus macroporus is known to occur within sp., Pugnettas aff. P. orientalis Lees, Pyrazus spp., a meter of the top of the formation, thus placing the Pyrfuliferaf sp., Pyrina orientalis Cotteau and Gauthier, uppermost beds of the Aruma within the Maestrichtian. Pyrulaf sp., Sassia sp., Scaphetta spp., Spondylus spp., Other forms occurring within the upper part of the Sphenodiscus cf. S. acutodorsatus Noetling, Spheno- Aruma are Elphidietta multiscissurata Smout, Fissoel- discus spp., Sycostoma? sp., Tegula sp., Tibia sp., phidium operculiferum, Chrysalidina sp., and Lepi- Tresinus sp., Trochust sp., Tudoceras? sp., TWo sp., dorbitoides spp. The latter appear to be limited to the Turbo? sp., Turritetta cf. T. quettensis Noetling, TW- middle and lower parts of the range of Omphalocyclus nfeWa sp., Tylostoma sp., Venericardia sp., Umboninm macroporus. sp., Volvulina sp., Xenophoraf sp., ostracodes, and crib It is interesting to note that the outcrop Aruma is claws. completely lacking in pelagic Foraminifera, though ECONOMIC ASPECTS these are abundantly represented in the deep wells of the coastal area. On the basis of its Foraminifera the GROUND WATKB Aruma on outcrop is apparently Campanian and The Aruma aquifer has an established hydraulic Maestrichtian, a conclusion consistent with ammonite gradient ranging from water-table conditions on or and other fossil evidence. near the outcrop to flowing artesian conditions in the A comprehensive fossil list for the Aruma Formation eastern and southeastern Rub* al Khali. Conditions includes: Cyclolites medlicotti Noetling, Cyclolites reg- in northeastern Arabia are less well known. Hydraulic ularis Lamark, Cyclolites spp., Coral spp., Ooniopygus factors are complicated by stratigraphic changes, and sp., Hemiaster sp., Hemipneustes spp., Holectypus sp., water-bearing zones probably constitute only a miror Parapygus aff. P. inftatus Cotteau and Gauthier, part of the total section. Pygurostoma aff. P. morgani Cotteau and Gauthier, Aruma water has been tested for quality and quantity cidaroid spines, Anomalina sp., Anomalinella cf. A. in only a few widely scattered areas. Such sparse rostrata (Brady), Ataxophragmium sp., "Borelis" sp., information suggests that the eastern edge of potable Cibixides sp., Cyclammina sp., Fallotia? sp., Globigerina water will pass south of IJafar al Ba^in, near Al Lihab^.h, sp., Globotruncana spp., Haplophragmoides sp., Lepi- along the western flank of the Ghawar oil field, and dorbitoides sp., Loftusia spp., Meandropsina sp., Om then south through the Rub* al Khali along the 50th phalocyclus sp., Orbitoides sp., Planispirina sp., Prae- meridian. sorites? sp., Pyrgo sp., Quinqueloculina spp., Rotalia The Aruma aquifer has been developed as a per sp., cf. Valvulineria sp., miliolids, Actinophyma? sp., manent water supply source only in a few restricted Agathelia sp., Alaria sp., Amauropsis sp., Ancilla sp., areas. The pipeline stations at Raffia* and Badarah Anomia sp., Aporrhais sp., Area sp., Arrhoges sp., are supplied by wells completed in the Aruma; artesian Asaphis sp., Barbatia sp., Bettardia sp., Bournonia sp., rises are small, and yield of the wells is comparatively Bullaria? sp., Calliostoma cf. C. omanicus Lees, Cal- low. D84 GEOLOGY OF THE ARABIAN PENINSULA TERTIARY SYSTEM Radhuma swings north, cuts sharply across section UMM ER RADHUMA FORMATION PAJLEOCENE AND LOWER and overlaps the Aruma. EOCENE Umm er Radhuma exposures usually form a gently DEFINITION undulating but rough surface spotted with lo^ isolated The Umm er Radhuma Formation was named by hills and weak discontinuous benches. Karrt features S. B. Henry and C. W. Brown in 1935 for the Umm are a prominent part of the upland surface, particularly Ratfmah wells (lat 28°41 / N., long 44°41' E.) that northwest of Al Batin. This area also supports a well- draw water from the upper part of the unit. In 1936, developed system of through-cutting channels that Max Steineke and J. W. Hoover also studied the follow regional dip. formation in this area but, because of difficulties in In the area southwest of the Trans-Arabiar Pipeline, working out a full, sequence, designated a reference between Ash Shu'bah (lat 28°54/ N., long 44°42' E.) section in Wadi al Batin. Umm er Radhuma was and Rafba' (lat 29°39/ N., long 43°28/ E.), the Umm first cited in publication by Steineke and Bramkamp er Radhuma surface is marked by numerous elevated (1952(b)); a later paper (Steineke and others, 1958) river channels which begin at the Linah escarpment and included reference section details. trend northeast to the Umm er Radhuma se^rp where With exception of the uppermost beds, the entire the majority of them are covered. Together with their formation can be seen in the steep walls of Wadi al tributaries they form a dendritic pattern wlich bears Batin. Here, the reference section was pieced together little relationship to the present drainage system and from many short intervals measured in a continuous none to present structure. Their present elevation, 70-km traverse from the top of the Linah escarpment from 2 to 18 m above the local base level, war probably (lat 27°38'03" N., long 44°53 /24// E.) northeast along caused by subsequent deflation of the softer irtervening Al Ba^in to lat 27°59/00// N., long 45°27/48// E. rocks. Miller (1937) discussed these bas-re?ief drain The base of the Umm er Radhuma is at the contact age lines. of Lockhartia-be&rwg dolomite and limestone with The elevated channels and tributaries, each with underlying dolomitic shale of the Aruma Formation. varying thicknesses of duricrust, are separated by low, The top is not exposed at the reference section because enclosed silt basins which collect present runof*. Upper of overlapping Miocene and Pliocene rocks, but it is surfaces of the tributaries slope toward the main chan present a short distance downdip in structure holes nels at gradients ranging from 2° to 4°, whereas beds and water wells where Umm er Radhuma dolomite of channel deposits making up the elevated tributaries containing Lockhartia hunti Ovey var. pustulosa Smout have a more gradual dip (from 00°30' to 1°30') in the is overlain by soft chalky Rus limestone. same direction. Channel deposits are most commonly marl and limestone derived from the Umm er P.adhuma, OCCURRENCE AND THICKNESS but, in the northwest, rocks of the upper Aruma are in Rocks of the Umm er Radhuma have been mapped cluded as well. in a fairly wide belt extending from just south of Much of the area northwest of Al Batin is character Wadi Jabaliyah (lat 22°25' N.) to the Iraq-Saudi ized by a very poorly developed internal drainage sys Arabian border, a distance of about 1,200 km. Width tem related to numerous small sinks and major sharp of the Umm er Radhuma is highly variable, in some scarps, such as Jal Nawazir-Mahshim U'ayvij, 35 km places because of superficial sand and gravel cover, west of RafbaV, and Jal al Batm, 60 km north of Raf^a'. in others because of overlapping Miocene and Pliocene Within Saudi Arabia, the highest of these rionoclinal beds. Prior to such cover, however, the outcrop escarpments Jal al Ba$n extends between Al Liflyah probably was quite regular, measuring 60 km across (lat 30°25' N., long 43°04' E.) and Birkat al 'Aqabah at the south and about 120 km near Iraq, most of the (lat 30°05/ N., long 43°37' E.), roughly a distance of 65 expansion apparently taking place between Khurays km. Although its trend is generally northwest, the face Gat 25°05/ N.) and Ma' qala' (lat 26°32/ N.). of the flexure is quite sinuous and dips variously to the From Wadi al Batin south, the Umm er Radhuma is west, southwest, and south. Regional dip en the up land surface, about 00°10' NE., is maintained to the longitudinally disrupted by the great curved Ad crest of the jal where it abruptly shifts to a maximum Dahna' sand belt, a cover that effectively prevents of 12° in the opposite direction along the frort-face dip study of any complete section throughout this distance. slope. At the foot of the slope the rate of dip gradually At the southern limit of the Umm er Radhuma, upper slackens and merges once again with the regional. The beds are masked. Northward the Ad Dahna' sand greatest relief measured along Jal al Batn from the rim belt shifts, covers the middle part of the formation of the base of the escarpment was 85 m. A similar nearly to Al Batin, and then, as strike of the Umm er flexure Mahshim U'aywij-Jal Nawazir occurs about SEDIMENTARY GEOLOGY OF SAUDI ARABIA 35 km west of Raffra'. Its strike is north-northwest cover and loss of beds at the top through pre-Miocene with the dip slope facing southwest. In many respects erosion prevents accurate measurement. However, it is similar to Jal al Ba^n, although its maximum relief partial measurements supplemented by calculated is only 35 m. Slumping involves Umm er Radhuma as intervals show about 105 m near Wadi al 'Atk and well as a few isolated patches of Miocene and Pliocene. 110 m at Wad! as Sahba'. North of the reference The steepest dips on the face are 10°. section, two sections have been pieced together from Tested by drilling, these two monoclines have proven numerous very short increments measured along high ly to be superficial, primarily related to solution-collapse circuitous traverses. The first, extending generally caused by removal of anhydrite. Shallow holes brack northeast to Ni§ab (lat 29°11 / N., long 44°43' E.), eting the flexure show a thick interbed of Umm er Rad indicated about 210 m; the second (approximately huma anhydrite immediately below the surface on the along lat 30°00/ N.) showed 234 m but did not rearoc^s uncon- measured and described in that channel. Max Steineke formably overlie Zallum where it is present but other and J. W. Hoover in 1936 were the first to work the wise rest directly on Lower Devonian. Al Ba^in section and define it as a reference locality. Contact between the Umm er Radhumr, and the Because rates of dip of the channel bottom and of the overlying Rus Formation is visible only from Wadi flanking bedding are so nearly alike, only 3 to 4 m of as Sahba' north to lat 25°44' N., a distance of 180 km. new beds are exposed per kilometer. It is somewhat Over this distance, dolomite and dolomitic limestone difficult to trace such low-dipping beds in a monotonous of the Umm er Radhuma is conformably overlain sequence and accurately tie together the many small by typical soft, chalky limestone of the Rus Forma increments making up a full section. As a result, W. H. tion. Outside of this area, the Umm er Radhuma is Reiss and R. D. MacDougall in 1953 retraced the orig unconformably terminated at the top by erratic out inal traverse and with plane-table survey remeasured crops of sandy marl and sandy limestone (Miocene the entire outcrop band (pi. 10). Their description of and Pliocene) or surficial deposits. the reference section is given in section 53 (p. 143). Western exposures of Paleocene and Eocene un Like the Umm er Radhuma-Rus-Dammam (Paleo- divided (including Hibr facies) are discordantly over cene to middle Eocene) sequence to which it most lain by sandy carbonates (Miocene and Pliocene) closely equates, the Hibr is also almost exclusively lime or by flows of Tertiary and Quaternary basalt. stone. The unit is composed of light-colored, thin- bedded, chalky and cher.ty limestone and marl. Partial PALEONTOLOGY AND AGE dolomitization is common. Nummulite and other fossil The Umm er Radhuma Formation falls into three zones occur at several levels, but no consistent marker principal subdivisions on fauna! grounds, ^he upper beds have been observed. Much chert is interbedded division is early Eocene in age; it carries such species with chalky limestone in the form of lenses, platy as Sakesaria cotteri Davies, Lockhartia tipperi (Davies), masses, flat ovoid nodules, and spheroidal concretions. and Lockhartia hunti Ovey var. pustvlosa Smout. It Parts of the upper Hibr, for example between Tiirayf is equivalent to the lower Eocene of Qatar described and Tall al Hibr, have been altered to tripoli, a fine by Smout (1954) and the Laki beds of India. powdery form of silica. On weathering, the Hibr forms The two lower subdivisions are Paleocer0* in age, a rather flat, featureless plain. Included chert often the break between them probably corresponding to remains as residual chips after removal of surrounding the contact between Smout's Qatar Paleocene zones softer limestone by weathering and deflation. 4 and 5 (Smout, 1954). These lower subdivisions of the Umm er Radhuma correspond at least in part to NATURE OF CONTACT WITH ADJACENT FORMATIONS the Ranikot of India, but exactly what p<»rt of the The Umm er Radhuma everywhere overlies the Ranikot is represented in the Saudi Arabian beds is Aruma with apparent conformity. Precise character not known. However, the general parallelism be istics of the contact have already been pointed out in tween the beds of the lowermost Umm er Radhuma the discussion of the upper Aruma limit (p. 82). and the uppermost (Maestrichtian) beds of the Aruma The contact of the Hibr rocks with underlying units would not suggest any prolonged time gap at the is less clear. On the east, it overlies the Aruma For contact. SEDIMENTARY GEOLOGY OP SAUDI ARABIA D87 Originally considered Eocene and Oligocene, the or near the outcrop to flowing artesian conditions post-Aruma sediments northwest of SakSkah are now along the Persian Gulf and in some topographically fairly accurately dated as Paleocene to late Eocene. low areas of the Rub' al Khali. The catchment area Nautiloids of Paleocene affinities were collected at the for this aquifer is the belt of Aruma and Umm er base of the section (according to W. M. Furnish, re Radhuma outcrop plus the area mantled by eolian ferring to Comomia sp. cf. C. kugleri Miller). Strong sand of Ad Dahna'. evidence for a Paleocene age was also given by re Water from the Umm er Radhuma ranges in quality covery of Anomalina dorri Cole var. aragonensis from excellent at or near the outcrop to poor generally Nuttall, Bulimina semicostata Nuttall, Globorotalia northwest of the Trans-Arabian Pipeline. Some r.reas velascoensis (Cushman), Loxostoma applinae (Plummer), within the general region of potability, such as Nariya, and Siphogenerina eleganta Plummer from a T?urayf Abqaiq, and Ras Tanura, produce poor-quality water water well. Higher beds in this Turayf well carry because of the presence of dissolved hydrogen sulfide Bulimina jacksonensis Cushman and Hantkenina mexi- gas. In the Rub' al Khali, water with less than 4,800 cana Cushman, forms indicative of either Middle or ppm of total dissolved solids occurs as far east as" the Upper Eocene age. Correlative beds on the outcrop, 51st meridian. East of this line and generally north however, also carry abundant occurrences of Num- of the 21st parallel, total solids vary from 5,001 to mulites gizehensis (Forskel), thus suggesting that 95,000 ppm. most of the higher beds at rj?urayf belong within the Local structural features in the coastal area have Middle Eocene. The presence of Upper Eocene is some effect on water quality and also upon the east- not proved but is postulated on the occurrence of dipping hydraulic gradient. A decrease in permea post-Hibr beds still carrying B. jacksonensis and on bility on the flanks of the features tends to divert the occurrence of beds above the range of B. jackson regional flow and to restrict flushing by fresher meteoric ensis but within the biozone of Guembelina. water. This effect is not as marked, however, as it is A comprehensive faunal list of the Umm er Radhuma in the Wasia. and Paleocene-Eocene undivided (including Hibr facies) The Umm er Radhuma aquifer has been developed is given below. by Aramco as the water supply for many of the company The Umm er Radhuma Formation contains Opis- communities and outlying installations. Potential of saster somaliensist, indeterminate echinoids, Anadara the aquifer is such that it could also handily support sp., Arcal sp., Brachiodontest sp., Buttaria sp., Carditat major domestic and agricultural projects. sp., Cerithium (s.l.) sp., Chama? sp., Corbula sp., BUS FORMATION LOWER EOCENE Cympolia spp., Diplodonta? sp., Euspira cf. E. rod, DEFINITION Homalopoma? sp., Macomaf sp., Nautilus sp., Nemo- cardium? sp., Oliva1? sp., Pentalium? sp., Siligua cf. The Rus Formation is named for Umm ar Eu'us sp., Strepsidura sp., Turritetta sp., indeterminate (lat 26°19' N., long SOW E.), a small hill where the lucinoid, naticoid, cerithiid, and strombid gastropods. type section was measured. R. A. Bramkamp first Foraminifera are poorly preserved and not abundant applied the name in 1946 as a direct replacement for on outcrop. See plate 10 for representative subsurface the term "Chalky Zone" which had been informally forms. used for lower Eocene beds above the Umm er Radtuma The Paleocene and Eocene undivided (including and below the Dammam Formation. The limits have Hibr f acies) contains Amphiope? sp., Echinocyamus not been changed. libycusf Fourtan, Cardita cf. C. asperula, Cassis nilotica?, A table of Saudi Arabian rock units appearing in a Chlamys, Conus, Comomia cf. C. kugleri, Lucina sp., paper by Thralls and Hasson (1956) constitutes first Mesalia hofana, M. locardi, Ostrea pUcataf Solander, formal publication of the term Rus Formation. De Turritella sp., Venericardia sp., shark teeth, Amphiso- tailed information on the type sequence was published rus?, Assilina1?, Buliminaf, Clavulina sp., Globigerina, by Steineke, Bramkamp, and Sander (1958). Addi Nummulites curvispira d'Archiac and Haime, N. spp., tional stratigraphic and paleontologic data was included Operculina cf. 0. discoidea Schwager, QuinguelocuUna, in Sander's (1962) discussion. sp., Textulariaf, Uvigerina, Valvulina spp., Valmdineria. The type section of the "Chalk Zone," now the Rus Formation, was originally established by S. B. Fenry ECONOMIC ASPECTS and J. W. Hoover in 1934 on the southeastern flark of GROUND WATER Dammam Dome in and below Umm ar Ru'us (lat The Umm er Radhuma aquifer is a single thick 26°19'04" N., long 50°07'51" E.). hydraulic unit of regional extent. It has an estab The base is at the contact between brown dolomite lished gradient ranging from water-table conditions on containing Lockhartia hunti Ovey var. pustulosa Sraout D88 GEOLOGY OF THE ARABIAN PENINSULA below and light-colored, chalky, partially dolomitized NATURE OF CONTACT WITH ADJACENT FORMATIONS limestone above. Common leached molds of small The Rus in the vicinity of Wadi as Sahba' represents indeterminate molluscs is a marked characteristic of an unbroken transition from the Umm er Radhuma the basal Rus unit. The upper few meters of the below to the Dammam above. In spite of the uni Rus is not exposed at the type locality but can be formity of section, contacts are reasonable clear cut. picked up clearly in the adjacent rimrock around Brown Umm er Radhuma dolomite is overlain by off- Dammam Dome. Here the break is at the change white, chalky, porous limestone with abundant indeter from light-colored, chalky calcarenite below to yellow- minate molds and casts, which in turn gives way at brown shale of the basal member of the Dammam the top to tan, calcareous, and gypsiferous shale and Formation above. interbedded marl of the basal Dammam. Comparable breaks mark the top and bottom of the OCCURRENCE AND THICKNESS Rus at Dammam Dome. Definite exposures of Rus rocks are limited to two PALEONTOLOGY AND AGE small areas the first, a narrow band extending some 180 km northward from Wadi as Sahba', and the Diagnostic fossils are not known to occur in the Rus second, a nearly circular patch about 10 km in diameter Formation although it is underlain and over'ain by cropping out in the breached core of Dammam Dome. rocks of proven early Eocene age. In terms of standard The Rus north of Wadi as Sahba' is rather feature European stages the Rus is presumed to fall entirely less, without a traceable bed or other reliable guide to within the Ypresian. stratigraphic position. For this reason the full thick ECONOMIC ASPECTS ness can only be estimated as 25 to 30 m. Thickness at the type locality is 56 m. As a dominantly evaporite sequence, the Fus has few porous beds. Limestones at the base and top of A number of isolated patches in the far southeastern the interval are porous and probably water bearing, Rub* al Khali, north of Dhufar, have been mapped as but these have not been selectively tested. Some Rus and Dammam undivided, but the exact makeup shows of low-gravity oil have been found in offshore of these outliers is uncertain. Paleocene and Eocene wells. rocks (Hibr) occurring northwest of Sakakah, certainly in part equivalent to the Rus, have been described in DAMMAM FORMATION LOWER AND MIDDLE EOCENE connection with the Umm er Radhuma. DEFINITION LTTHOIOQIC CHARACTER The Dammam Formation was named for Dr»mmam Dome where the entire sequence crops out. R. A. Rock types in the two areas of Rus outcrop are for Bramkamp in 1941 measured and designated the type all practical purposes identical. Basically, the unit is locality along the old Dhahran-Al 'Alah road from light-colored, soft, chalky limestone and marl. R. A. the point where this road intersects the rimrock Bramkamp's remeasurement of the Umm ar Ru'us (lat 26°19'16" N., long 50°04'50" E.) northwest to sequence in 1946 (fig. 11) resulted in the description the Eocene-Miocene contact. given in section 54 (p. 145), which is typical for other The base is at the contact of calcarenite or limestone exposures as well. of the Rus Formation with the overlying brown shale Subsurface equivalents of the middle Rus (unit 2, and light-gray marl of the basal Dammam. The section 54 and fig. 11) are highly variable both in top is at the unconfonnable contact of pure carbonate lithology and thickness. Most commonly the sequence of any part of the Dammam Formation with the is (1) white, compact, finely crystalline anhydrite with overlying sandy limestone of the Hadrukh Formation. interbedded green shale or (2) gray marl with coarsely Where present in full thickness, the Dammam is crystalline calcite and interbedded shale and limestone. almost everywhere divisible into five members, two Variations in thickness are largely caused by variations of which possess distinctive and laterally persistent in the amount of anhydrite present. As a general basal marl units. Listed in descending orde", these rule, the middle Rus is thicker on the flanks of structures members and their subdivisions are as follows: than it is on the crests. Complexly settled rocks, such Alat Member: as those seen around Dammam Dome, suggest some of Alat limestone the differences in interval are caused by solution of Alat marl anhydrite. In other places, the differences seem to be Khobar Member: related to original depositional thinning over structural Khobar dolomite highs. Khobar marl Rus type section Dammam reference section 26°19'04"-50°07'51" , . 26°16'10" .. 26°19'42" 1 ;' 50°06'26" Generalized lithology LIMESTONE: Compact slightly sandy limestone. LIMESTONE AND MARL: Upper part light-colored chalky porous Alat commonly dolomitic limestone; abundant molds of mollusks and other organic remains. Lower part light-colored dolomitic marl. (15m) LIMESTONE AND MARL: Light- to dark-brown, in part dolomitic Khobar limestone becoming off-white soft marly limestone and marl in \ lower part. (9.3 m) Alveolina Ls LIMESTONE: Tan limestone. (1 m) Saila SHALE: Blue to blue-gray, subfissile shale; weathers yellow-brown Shale Thin basal unit light-gray limestone. (4.2 m) ______Midra Sh SHALE: Poorly exposed yellow-brown shale. (3 m) (3) LIMESTONE: White soft chalky porous limestone; calcarenite beds at top (3.6 m) (2) MARL AND LIMESTONE: Light-colored marl; local irregular masses of crystal line gypsum, occasional thin limestone beds and geodal quartz at several levels. In other areas, unit is highly variable, including as common equivalents: (a) white compact finely crystalline anhydrite with interbedded green shale and minor amounts of dolomitic limestone or (b) gray marl with coarsely crystalline calcite and interbedded shale and limestone. (31.8) (1) LIMESTONE: Gray to buff compact commonly partially dolomitized limestone; minor amounts of soft limestone made porous by leaching of small organic remains. Quartz geodes occur rarely in the lower part, and are typical of upper part. (21 m) DOLOMITE: Contains Locfc/iarfia fipperi Davies * Range of microfossils shown above composited .... , , from subsurface occurrences. I Vertical scale: = 1:1000 Dammam Formation after R. W. Powers, tion H.W. Schneider, and M. C. Coffield, 1953; Rus formation after S. B. Henry and J. W. Hoover, 1934,and R. A. Bramkamp, 1946 FIGURE 11. Dammam and Rus Formations type and reference sections. D90 GEOLOGY OF THE ARABIAN PENINSULA Alveolina Limestone Member At the type locality the Saila Shale Member consists Saila Shale Member of 3.6 m of brownish-yellow subfissile clay shaJe under Midra Shale Member lain by 0.6 m of gray limestone. The top of the member The Alat Member takes its name from Al Alat is placed at the contact of Alveolina Limestone Member well 1 (lat 26°27/49" N., long 49°50'11" E.), where above with blue-tinted shale below. A thin limestone the upper part of the member is exposed nearby and which usually occurs at the base of the shale is grouped the lower part is represented by the drilled interval with it, and the lower limit of the Saila is considered to from 0 to 67 m. Alat beds are best displayed in a fall at the contact between this limestone and under low hill at lat 26°19'42'/ N., long 50°03'24" E., and lying yellow-brown shale of the Midra Member. this section is herewith designated as the type locality. The basal member of the Dammam Formation the Here, as elsewhere, the upper part of the member Midra Shale Member (originally "Shark Tooth shale," is limestone (9.0 m), the lower part, marl (6.0 m). informal and obsolete) is 3.0 m thick at the type The lower marl was for many years informally referred section. The top is picked solely on lithology. It is to as the "Orange marl," but the name is now obsolete. placed at the uppermost occurrence of yellow-brown The top of the Alat is at the unconformable contact earthy clay shale below limestone of the Saila Member. of light-colored, chalky dolomitic limestone below The base is at the contact between yellow-brown shale with sandy limestone of the Hadrukh Formation above. or gray marl above and soft chalky limeston0. or cal- The base is taken at the break from dolomitic marl carenite of the Rus Formation below. above to dolomite or dolomitic limestone (with abundant Nummulites) of the Khobar Member below. OCCURRENCE AND THICKNESS Pre-Alat members of the Dammam Formation are Outcrops of the Dammam Formation are limited to exposed at several places along the rimrock of Dammam a number of small but widely scattered patches north Dome, but one of the best sections is that southwest of Wadl as Sahba', near the coast, and in tH south of Umm ar Ru'us at lat 26°16/ 10" N., long 50°06'26" eastern Rub* al Khali. E. This sequence is herewith designated as the type Dammam beds crop out in a narrow irregular band locality for the Khobar, Alveolina, Saila, and Midra from Wadl as Sahba' to lat 25°41' N., a distance of Members. nearly 180 km. The band, usually less than 5 km wide, The Khobar Member is named for the town of includes a somewhat low discontinuous escarp raent on Al Khubar (lat 26°17' N., long 50°13' E.) on the eastern the west. A probable maximum of 15 m of basal edge of the Dammam Dome rimrock. Characteris Dammam is exposed above the Umm er Radhuma and tically, the upper part of the unit is limestone, and under the Miocene and Pliocene overlap. the lower part, marl; at the type section, thicknesses A full Dammam sequence is exposed in the conspic are 7.8 and 1.5 m respectively. The name of the upper uous oval-shaped escarpment rimming Dammam Dome. limestone was originally proposed as "Khobar dolomite Average inside diameter of the ring of Dammam rocks (and limestone) member," but usage has reduced is about 10 km; average outside diameter is 1£ km. A this to "Khobar dolomite" in spite of the fact that maximum formation thickness of 32.5 m 1 as been limestone and marl are far more typical of this part measured between the Rus center and the surrounding of the section. There are, of course, some places cover. Four small outliers of Dammam h$we been where dolomite predominates, but these places are mapped generally west of the dome and north cf Abqaiq scarce. and another near Saratf (lat 24°08' N., long 49°04' E.) The top of the Khobar is placed at the contact of at the southern end of Ghawar oil field. Nummulites-be&rin.g dolomitic limestone or dolomite below with marl of the Alat Member above. The A relatively large exposure of Dammam, about 20 base falls at the change from Nummulitic marl above sq km, occurs at the base of the Qatar Peninsula. For to Alveolina-be&riug limestone below. the most part the rocks are Dammam, but occasional The Alveolina Limestone Member, only 1.0 m windows of Rus can be seen in low hollows. Thickness thick at the type section, characteristically contains of the section here varies from 10 to 51 m, encompassing common specimens of Alveolina ettiptica (Sowerby) the Midra, Saila, Alveolina and Khobar Members. var. floscvlina Silvestri. The top of the member is Numerous isolated exposures of Dammam and of picked on a lithologic break from marl to limestone Rus and Dammam undivided along the eastern and and is confirmed by fauna. The base is placed at southeastern margin of the Rub* al Khali si ow that the level where tan Alveolina-be&rmg limestone gives Eocene rocks floor a substantial area beneath eolian way downward to blue and gray-blue shale or marl sand cover. Nothing is known of the precise thickness of the Saila Shale Member. or members represented. SEDIMENTARY GEOLOGY OF SAUDI ARABIA D91 1ITHOIOOIC CHARACTER same pattern as that established by the Umm er B ad- Rocks of the Dammam Formation are characteris huma. Recharge to both aquifers is apparently pro tically light-colored limestone, marl, and shale. There vided by downward percolation through Miocene and is no significant variation from the rock types at Dam- Pliocene sands overlying the truncated outcrop. mam Dome, even though the Dammam Formation and Khobar water is used extensively in communities its individual members thicken drastically in the sub and agricultural areas of the Eastern Province. Al surface and extend over most of northeastern Arabia and Qatlf oasis, north of Dhahran, and the towns of Al large parts of the Rub* al Khali. Qa^if and Al Khubar as well as adjacent communities Best exposures are those of the Dammam Formation constitute the most concentrated areas of withdrawal. reference section which also serves as the type locality A few wells have penetrated the Khobar aquifer in and for the separate members. The section was pieced near the oasis of Al Hasa. together by R. W. Powers and others in 1953 from two Major withdrawals from the Alat aquifer are also measurements (fig. 11): the lower covering the Midra- limited to the Eastern Province. The member is the Saila-Alveolina-Khobar interval, the upper including sole source of potable water for the townsite of Ras Alat strata. Reference section particulars have been Tanura and the pump station at An Nu* ayrlyah. given in connection with member definition (p. 88). Locale of heaviest development, however, is in Al Qa^if A detailed description of the rock sequence is given in oasis. Erratic artesian conditions prevail in this area, section 55 (p. 145). and it is necessary in many places to drill through the Alat and complete in the Khobar to obtain flowing NATURE OF CONTACT WITH ADJACENT FORMATIONS water. The base of the Dammam Formation is exposed only HADRUKH, DAM, HOFUF, AND KHARJ FORMATIO] T<-J at Dammam Dome and in the vicinity of Wadl as MIOCENE AND PLIOCENE Sahba'. In both areas, shale of the Midra Member DEFINITION rests conformably on chalky limestone of the Rus For mation. At the top, various members of the Dammam The coastal area of Saudi Arabia has been essentially are in discordant contact with Miocene and Pliocene a broad structural terrace, only intermittently sub or with younger sediments. merged, throughout Tertiary time. As a result, few marine sediments were deposited during that perod, PALEONTOLOGY AND AGE and these only erratically as seas oscillated to and fro From the uppermost beds of the Alat Member, which across the terrace with each invasion reaching varying carry a very large Dictyoconus sp., to the base of the distances inland. The most western margin of marine Alveolina Limestone Member with Alveolina ettiptica sedimentation seems to be generally less than 100 km (Sowerby) var. flosculina Silvestri, the Dammam For from the present Persian Gulf shoreline. mation is clearly middle Eocene (Lutetian) in age. Where marine rocks occur, the Miocene and Pliocene Below this level the presence of a "Nummulites globulus succession can be subdivided into three format;ons Leymerie" of the type figured by Smout (1954) from which from the bottom up are Hadrukh, Dam, and the lower Eocene of Qatar strongly indicates that the Hofuf. Toward the interior, marine marker I 228-776 O 6C TO2 GEOLOGY OP THE ARABIAN PENINSULA The Hadrukh Formation was named for Jabal al monly falls within the Paleocene and Eocene outcrop Haydaruk (lat 27°04'36" N., long 49°11'24" E.) where band; the eastern edge is usually marked by sand of the the upper part of the type sequence is exposed. In Rub* al Khali-Al Jafurah belt and the Persian Gulf 1935 Steineke and Koch measured and defined the shoreline. To be sure, several exposures are outside type locality by taking the upper 51.5 m in the south this general area, but most of these are small an d little face of Jabal al Haydaruk and the lower 32.5 m from a known. Three, however, are large and worth noting: shallow hand-dug well about 1500 m to the southeast. the first forms a thin strip along the eastern edg'^ of Al Generally the basal bed of the Hadrukh is a few Jafurah from Jibal al 'Uqayr (lat 25°34' N.) to the meters of cream, slightly sandy limestone. This rests vicinity of Al Jawb al Hadari (near lat 23°20' N.); the unconformably on nonsandy, marine Eocene limestone. second is an elongated, discontinuous band flooring The top is at the contact of calcareous sandstone below Wadi as Sirban in the northwest (just east of the with Echinocyamw-beaiing limestone and marl of the Saudi Arabian-Jordan boundary) and flanking the basal Dam above. A$ Jawil plateau in the southeast; and the third occurs The Dam Formation is named for Jabal al Lidam as isolated patches on the surface of Al Huj. In (lat 26°21 /42// N., long 49°27'42" E.) where the lower addition, Miocene and Pliocene rocks are exposed in part of the type unit crops out. Formation limits small windows in the Rub* al Khali sand around were set in 1935 by Steineke and Koch who measured 'Uruq ibn Bamudah (lat 18°50/ N., long 51°50' E.) and the lower beds in the east face of Jabal al Lidam and in a wide, east-west belt generally between lat 21°00/ N. the upper part of the 89.8-m interval at Al Umayghir and 22°30' N. and extending from the 49th to the 53d (lat 26°17'15" N., long 49°30/24" E.). meridian. The latter deposits, while at least in part At the type locality, the base of the Dam is placed equivalent to the Miocene and Pliocene of other areas, at the change from sandstone of the Hadrukh below probably contain some Quaternary elements as well to fossiliferous marl with Echinocyamus above. The and, for this reason, have been depicted on Miscel top is at the contact between marl and limestone with laneous Geologic Investigations Maps as Tertiary and marine fossils of the Dam and overlying clay, sandstone, Quaternary marl. and gravel of the basal Hofuf. As stated before, the presence of marine strata near The Hofuf Formation takes its name from the town the coast permits subdivision of Miocene and F1; ocene of Al Hufuf (lat 25°22' N., long 49°35' E.) about 15 km rocks of this area into three formations which ar% from south-southwest of the type locality. In establishing bottom to top, the Hadrukh, Dam, and Hofuf. the type sequence in 1935, Steineke and Koch meas Rocks of the Hadrukh Formation cover a lowland ured the lower 78 m at the southern end of Barqa' belt roughly 100 km wide along the coast from the Rukban (lat 25°30/48" N., long 49°31 / 18// E.) and an Saudi Arabia-Kuwait Neutral Zone almost to Abqaiq. additional 17 m in exposures to the west and north Some outliers occur as far inland as 200 km, for example, west, giving a total of 95 m. near Jabal al Khafaq (lat 26°55' N., long 47 C15' E.) The base of the Hofuf is at the contact of generally and in windows in the band of eolian sand blanketing quartz-pebble-bearing beds above with calcareous the Abqaiq-Al Qafltf area. rocks of the Dam below. The top is at the upper Thickness of the Hadrukh, as would be expected, is limits of the exposures of the Al Hufuf area, commonly highly variable, ranging from 20 m in the Abnaiq-Al an old duricrust-covered surface. Qatif area to 90 m near Jabal al Lidam (lat 26°22' N., The Kharj Formation takes its name from the area long 49 °38' E.). Still greater thicknesses of up to 120 m of Al Kharj in the upper reaches of Wadi as Sahba'. were recorded around Abu Uadriyah (lat 27°20' N., Although R. A. Bramkamp and T. C. Barger studied long 48°57' E.) and to the northwest. Kharj beds as early as 1938, the unit was never formally Residual patches of Dam rocks show that the middle named until 1956 (Bramkamp and others). No Miocene sea transgressed roughly 120 km inland from type locality has been designated; however, the unit the modern coastline and covered an extensive area is typically displayed in a small hill about 3 km north- from south of Qatar to the eastern edge and around northeast of Khashm Khuraysah (lat 23°34' N., long the northern end of Ghawar and then on to Jibal an 47°07' E.). Here, 28.1 m of Kharj rocks was measured Nu'ayrlyah (lat 27°31' N., long 48°23' E.). TTiis is a and described. distance of about 450 km measured parallel to the coast. Around the margins, marine rocks of tl Q. Dam OCCURRENCE AND THICKNESS give way rapidly to continental deposits, complete gra Rocks of Miocene and Pliocene age cover a large dation often taking place in as short a distance ar 10 km. area that extends from WSdi ad Dawasir to the northern Thickness of the Dam Formation varies considerably. boundary of Saudi Arabia. The western edge com About 90 m is exposed at the type locality, fr-xt else- SEDIMENTARY GEOLOGY OF SAUDI ARABIA D93 where as little as 30 m and as much as 100 m have vicinity of Ashqar Maraghah (lat 24°20' N., long been found to represent a full section. 47°06' E.). The largest single exposure of the Hofuf Formation IITHOIOGIC CHARACTER is superimposed on Ghawar field and almost perfectly mirrors the outline of that giant oil-filled anticline. Miocene and Pliocene rocks are characteristically The northern part of the structure is reflected in Hofuf varicolored sandstone, sandy marl, and sandy Irne- beds at the surface by a prominent, dissected plateau stone. Lenses of shale and conglomerate are common, capped by a resistant limestone layer. The preserva gypsum and chert are present locally. tion of the plateau seems to result from carbonate Rocks of the Hadrukh Formation are nonmarine enrichment of its surface over the anticline, this enrich except in a small area outlined by Al Qa^if, Al 'Alah, ment giving it greater resistance to erosion there than Jabal Qurayn (lat 26°39' N., long 49°37' E.) and Al in the synclinal areas. In southern Ghawar, the resist Jubayl al Bam Gat 26°54' N., long 49°38' E.). Oer ant cap is not as thick, and only patches of it remain. this region, a few thin layers near the top of the forma Here, Hofuf beds stand somewhat higher than the sur tion contain poorly preserved marine molluscs. Ha rounding area but are much less prominent topographi drukh rocks are mainly green, grayish green, and gray, cally than in the north. but red, brown, white, and pink colors are also comiron. Outside the immediate Ghawar area important expo Marly sandstone, sandy marl, sandy clay, and sandy sures of the Hofuf (1) extend in a very thin band from limestone make up most of the unit; chert is common at Jibal al Kharaniq (lat 26°13' N., long 48°55' E.) north some levels, and minor amounts of gypsum are also west for about 70 km, (2) follow the Al 'Uqayr escarp present. ment southeast from Jibal al 'Uqayr (lat 25°34' N., The formation is typically exposed at Jabal al Fay- long 50°04/ E.) to the 24th parallel, (3) occur as small daruk, the type locality (fig. 12) established by Max outliers north and northeast of Al Hufuf, and (4) cap Steineke and T. W. Koch in 1935. Their description is Jabal al Kharma' (lat 25 "W N., long 49°38' E.) and given in section 56 (p. 145). several isolated hills to the south. The Dam Formation is pink, white, and gray r^arl At the type locality the Hofuf Formation is 95 m and red, green, and olive clay with minor interbed^ of thick; measurements in other areas show definite varia sandstone, chalky limestone, and coquina. Marine tion from this figure. The thinnest sequence is about fossils are abundant throughout the unit, but the best 30 m, and the maximum, somewhat more than 100 m. marker zones are near the base. Of these, the "Button With the disappearance of the marine deposits of the bed" composed of large numbers of small echindds, Dam Formation toward the interior, it is impossible to Echinocyamus sp., and the closely associated "Archams" subdivide the all-continental Miocene and Pliocene suc sp. bed are undoubtedly the most widespread. They cession. Distribution of the continental series corre have been recognized throughout the area of E "Mn sponds roughly to the extensive As ^umman plateau outcrop. bordering the Paleocene and Eocene outcrop north of The Dam type locality selected by Steineke and lat 22°20' N. Southward, Miocene and Pliocene beds Koch in 1935 serves well to characterize sediments of carry the As §umman surface across the Umm er that formation (fig. 13). A measurement and de Radhuma and overlap onto Upper Cretaceous. scription at Jabal al Lidam is given in section 57 (p. 146). In the north, bore holes indicate that continental In the coastal east-facing escarpments, the marine beds form a wedge increasing from 0 m on the west to Dam Formation is overlain by continental deposit"* of about 100 m where they merge with the marine facies. conglomerate, sandstone, sandy limestone, sandy m arl, Thickening in the south is more drastic, reaching as and sandy shale assigned to the Hofuf Formation. The much as 300 m before passing under the sands of the upper beds consist of sandy fresh-water limestone wl ;<*.h caps not only the coastal exposures but the greater Rub' al Khali. part of the interior As $umman plateau region as well. Irregular patches of the Kharj Formation, roughly The type section measured in 1935 by Steineke and contemporaneous with the Miocene and Pliocene units Koch north of Al Hufuf is as representative a sequence described above, dot the landscape in front of the Hit as can be expected in such a heterogeneous and lenticu escarpment. For the most part, Kharj rocks are con lar body of rocks (fig. 14). Their description is given fined to the Sulaiy outcrop belt between Wadi al 'Atk in section 58 (p. 146). and lat 23°15/ N., a distance of more than 300 km. The most interesting feature of the Hofuf Formation Thicknesses of Kharj deposits measured at two locali is the great area! extent over which gravel deposit^ of ties are (1) 28 m exposed in a small hill a few kilometers this unit and its interior equivalent are found. Ap north of Khashm Khuraysah and (2) about 9 m in the parently at the end of Dam time there was a general D94 GEOLOGY OF THE ARABIAN PENINSULA Location: 27°04'36"-49°11'24" Generalized lithology Diagnostic fossils LIMESTONE: Gray hard fossiliferous sandy limestone______ SHALE, SANDSTONE, AND MARL: Green and grayish-green generally finely sandy clay and green to gray calcareous sand stone; commonly weathers to fine con None cretionary pellets. Minor amounts of cream to gray marl and gypsum are also present. Chert occurs at a number of levels. Basal 5-10 meters commonly cream sandy limestone or marl. (84 m) LIMESTONE: Light colored porous nonsandy UJLJ fossiliferous limestone with molds and Vertical scale: 1:1000 Ouj castb 00 After M. Steineke and T. W. Koch, 1935 FIGURE 12. Hadrukh Formation type section. titling of the Arabian foreland and a rapid erosion of covered plains. These hills commonly contain con the interior region which furnished the gravel incor glomeratic beds, and it is believed that the widespread, porated in the Hofuf. Usually the gravels consist of uniform distribution of the gravels results from desert quartz, various types of igneous rocks, and metamor- erosion of the Hofuf, the heavier, resistant constituents phic rocks. In some places such as the type locality, remaining as residual or lag gravels. the pebbles and boulders are recognizable limestone In the §umman, Wad! as Sirban, A$ fawfl, and Al quite definitely derived from Jabal Tuwav Location: 26°21'42" _ nri 26°17'15" 49°27'42" 49°30'24" UJ I-*-! Generalized lithology Diagnostic fossils CLAY: Red clay overlain by limestone boulder Ill conglomerate CL. MARL, CLAY AND LIMESTONE: Upper part red and green partly sandy and silty clay with minor marl and sandstone beds and O buff to white limestone with yellow, fossil- O iferous limestone and sand; lower part white very calcareous fossiliferous marl; minor sand and clay. (90.8 m) L I- L L L L t- L Echinocyamus sp., Ostrea latimarginata, "Archaias" sp. SANDSTONE: Buff coarse-grained fossili ferous sandstone Vertical scale: 1:1000 O O After M. Steineke and T. W. Koch, 1935 FIGURE 13. Dam Formation type section. now standing amid the deposits as isolated peaks or Sahba', Kharj rocks are normally lacustrine limestone cones. Dip angles suggest that the detritus was origi with associated bedded gypsum and gravel. Discon nally deposited around these cones but that a later pe tinuous exposures of this same general suite of rc^ks riod of erosion cut the land to its present aspect, removed have been located as far south as lat 23°15/ N., east much of the conglomerate blanket, and exhumed the Al down Wadl as Sahba' to just beyond Jabal at Tayah Huj surface. (lat 24°13/ N., long 47°40/ E.); and alongside the Sf.udi These conglomerates have not been found elsewhere Government Railroad from lat 24°17/ N. to lat 24°30 / and apparently are unique to Al Huj. The size of the N. North of lat 24°30/ N., the apparent equivalent of grains varies locally. Close to the high cones the con the Kharj is poorly exposed gravel composed chiefly of glomerates are literally boulder beds, made up of large limestone pebbles and cobbles locally cemented by dark, blocks of sandstone and limestone eroded from the Jauf slightly ferruginous caliche. and the Wasia (Sakaka) Formations. Further away, Lacustrine beds are best developed in the low country the elastics are smaller and deposited at lower dip just southwest of the Hit escarpment roughly between angles. Certainly these deposits can be classed as DaU Hit and Jabal Mabawiz (lat 24°16/ N., long 47°15' fanglomerates. E.). Here there is normally a resistant fresh-w^ter The only other partially stratified rocks considered to limestone about 1 m thick resting on several meter* of be of Miocene and Pliocene age are those of the Kharj soft beds, mainly gypsum and gypsiferous sand. Poorly Formation. In and southward from the area where big preserved fresh-water fossils have been found at several channels from Jabal ^uwayq join to form Wadl as localities. D96 GEOLOGY OF THE ARABIAN PENINSULA Location: 25°30'48"-49°31'18" vaci id aii^Gu iiuiuiugy k'lM^I I\S«?IIW I\S^**U«? SURFICIAL DEPOSITS ^"- "-*" :- ' None; occasional freshwater fossils such as (4) CONGLOMERATE: Gray marly conglom ». '. ''-jf-' "..'"* : --..'.. Lymnoeo and Charo. Closing unit of Arabian erate; limestone boulders in quartz sand a ' ' '"' :f" " I matrix. (9.1 m) j Tertiary deposits; may be either late Miocene :^: -5 '£: or Pliocene &5;S LJ S ;'£' ' S' Z (3) SANDSTONE: Alternating red and white LJ ? argillaceous sandstone. (48.6 m) in S>3 :5I- 01*^ 8 i-w-.-'i' _j c CL O ^.. " ' *-> (0 : .x_ III. a: E :?:;??-,-?:' O £ ^ ^T' ;; 2 LJ <*- Z LJ 1 8 (2) LIMESTONE: Off-white, in part impure sandy limestone. (18.2 m) 5 I 1 ! 1 ! ITTT .*.' :. . .*' (1) CONGLOMERATE: Red and white con glomerate; boulders and pebbles of lime \f : o: ". A "'? ' ;:i'":."-; "; : stone in quartz sand matrix. Basal unit &--:'''::-i': greenish-gray to red, in part sandy marl. : : .' " * ' .. (19.1 m) 4f. -: :- r .: V^>r '^t gV MARL AND CLAY: Gray fossiliferous marl ^3^^ LJ E Z (0 and red and green sandy and silty clay Vertical scaler'l: 1000 LJ a 8 Aftar M Qtoinolro anrl T W Knrh 1 Q^ s: FIGUBE 14. Hofuf Formation type section. Kharj rocks in fresh-water facies were also studied ceous and even Paleozoic beds are affected. The con just north of Khashm Khuraysah (lat 23°34/ N., long tact in each locality, however, is relatively Hmple to 47°07' E.) where 28.1 m was measured and described pick, as the varicolored lenticular sandy carbonates and from top to bottom as follows: 3.4 m tan and dark-gray, sandstones of the Miocene and Pliocene usually contrast fresh-water limestone containing botryoidal structure sharply with the pure carbonates and bedded elastics of suggestive of algal origin; 1.8 m limestone conglomerate older units. with a coarse and angular component derived from the At the top, Miocene and Pliocene rocks ar^ uncon- nearby outcrop of Arab, Sulaiy, Buwaib, and Yamama formably overlain, usually by various surficial deposits, Formations; and 22.9 m poorly sorted, gravel, conglom but in the far northwest by basalt flows. erate, and sand, including weathered limestone pebbles and discontinuously bedded conglomerate with calcar PALEONTOLOGY AND AGE eous cement. The age of the Hadrukh Formation is not specifically established. However, because of its apparent con NATURE OF CONTACT WITH ADJACENT FORMATIONS tinuity with the overlying Dam Formation, the Had Surface and subsurface data show that Miocene and rukh is considered to be early Miocene. Eare non- Pliocene units are everywhere in discordant contact marine fossils including Chara are present, and near with underlying units. Paleocene and Eocene strata the coast a few thin layers near the top of the formation are most commonly involved, but in some areas such as contain poorly preserved marine molluscs and echinoid the northwestern Rub* al Khali and near Jordan, Creta debris. A complete list of fossils recovered from SEDIMENTARY GEOLOGY OF SAUDI ARABIA D97 Hadrukh surface beds includes: Chara, Anomalocar- ECONOMIC ASPECTS dial sp., Cardita cf., Cerithium (BeHardia?) sp., Chlamys GROUND WATBK cf. C. senatoria (Gmelin), dementia papyracea (Gray) ?, The water-bearing properties of the Miocene and Diplodonta cf. D. incerta d'Archiac and Haime, cf. Pliocene rocks, as might be expected, are erratic. Natica, Ostrea latimarginata Verdenburg, Ostrea spp., Apparently the sequence carries water regionally, but Turritella gradata, Turritella sp. indeterminate venerid productive capacity, storage factors, and quality vary pelecypods, Echinocyamus!', crab claws, sharks teeth, sharply both laterally and vertically. Locally, artesian cidaroid spines, and fossil wood. flowing conditions are found, as in a few wells ir the So far as the Dam Formation is concerned, the Rub' al Khali and in the oases of Al Hasa and Al presence of Ostrea latimarginata Vredenburg, Echino- Qat Dahna', one 10 km and the other 25 km north of Wadi showed a phaneritic basalt with zoned pk«noclose as Sahba'. These are well-defined eastward-trending (andesine-labradorite), augite, commonly altered oli gravel ridges leading east from Ad Dahna' to the gravel vine, calcite, and black unidentified opaque minerals, plains around southern Ghawar. These probable links probably ilmenite and magnetite. The thickest se strongly suggest that the channel gravels are remnants quence of younger lava is at Umm Wu'al (lat 31°49' of a group of late Tertiary rivers which brought down N., long 38°53' E.) where about 100 m rests directly the great gravel flood incorporated in the lower part of on the eroded Eocene surface. At other places younger the Hofuf Formation. basalt flows thinly cover Miocene and Pliocene un The relationship of the channel gravels to fresh divided. This fact, coupled with the fact that little water deposits of the Kharj Formation is unknown weathering is evident, has led to a considered Quater even though they are in contact at one locality that nary age for the upper Al Uarrah volcanics. is, near lat 23°16/ N. It is tempting to suggest that Near the fort at Qurayyat al Milb (lat 31°24/ N., the channel gravels may be the result of streams that long 37°29' E.) there are about 35 m of older lava, a discharged overflow from lakes in which the fresh highly weathered alivine-bearing basalt, resting on water sediments were being deposited, but this sug Miocene and Pliocene marl. In the middle part of gestion is pure speculation. the succession, the lava is cut by numerous thin aplite TERRACE SAND AND QRAVEL dikes and at places an amygdaloidal phase r distin guished. In marked contrast to the younger basalts, Terrace deposits of presumed Pliocene age are known the older lavas have been deeply weathered prior to only at the northwestern edge of 5arrat Hutaym (at lat deposition of an overlying sandstone that includes 26°35/ N., long 39°40' E.). Here the rocks are fine basalt boulders in the basal part. A few kilometers grained gray sandstone and marl with a basal conglom west of Qurayyat al Milk similar lavas are interbedded erate. Fresh-water ostracodes are present in the upper with sandstone and marl of presumed Miocene and part. Pliocene age and here are considered to be of the same A similar deposit of light-gray marl with gastropods age. and fresh-water ostracodes has recently been discovered CALCAREOUS DtJRICRUST near lat 27°26' N., long 38°29' E. (not shown on Mis A resistant duricrust carapace masks underlying cellaneous Geologic Investigations Maps). The fauna stratigraphic and structural details over some rather is modern in aspect and can scarcely be older than extensive areas in northwestern Arabia. Although the late Tertiary; it is questionably assigned to the Pliocene. precise chemical conditions that produce this deposit Relationship between this and the -Jarrat Hutaym are poorly understood, it is apparently a product of a patch is still obscure. somewhat more moist, possibly semiarid, climate than MARL, AND SANDSTONE exists in present-day Arabia. During the dr7 season Small outliers of sandy marl, marly sandstone, and movement of ground water is upward to the surface sandy limestone occurring in an east-west belt across where soluble substances are deposited. As evapo the central part of the Rub' al Khali have already been ration proceeds, a calichelike carbonate-enriched crust mentioned in connection with similar Tertiary sediments is built up, mainly of the material on which it forms. which are certainly in part, if not wholly, equivalent Where the underlying rock is carbonate, durcrust is (p. 91). particularly hard and in some instances supports BASALT escarpments as much as 50 m in height. If the under Al Uarrah, the only volcanic complex within the lying rock is sandstone, a much less resistant calcareous, sedimentary area of Saudi Arabia, extends from near gypsiferous, and earthy deposit forms. Ath Thayat (lat 30°32/ N., long 39°20/ E.) north Duricrust is particularly well developed on Pale westward for almost 250 km where it crosses into ozoic and Mesozoic units from Buraydah northwest Jordan. Over this distance, width of the lava field to Al Widyan plateau, south of Al Jawf and ij'^kakah, ranges from 80 to 100 km. Younger basalt components in Al Huj, and on Tertiary beds south and we^t of the of Al Uarrah are tentatively assigned to the Quaternary; Saudi Arabia-Iraq Neutral Zone. Usually it is a older elements, informally known as the Garaiyat lavas, discontinuous mantle of red, brown, gray, tan, and are considered to be Miocene and Pliocene. yellow, resistant, sandy limestone containing frag The younger lavas are chiefly olivine basalt locally ments of the underlying rock. Thickness of the crust with thin aplite dikes. At places they are scoriaceous ranges from a few centimeters to 3 or 4 m. Coarse and vesicular and include bedded pyroclastics, plugs, frosted quartz grains are often present, and aho a few and cones. Petrographic analysis of a sample from fresh-water molluscs are present. In the southwest Qitab ash Shaman (lat 31°36' N., long 37°57/ E.) ern Rub* al Khali the few small patches of duricrust SEDIMENTARY GEOLOGY OF SAUDI ARABIA D99 known are gray to tan, locally gypsiferous, sandy lime Miocene and Pliocene and with Quaternary units. Only stone grading at places into sandstone with calcareous one such deposit has been studied in sufficient detail to cement. permit mapping, and this occurs south from Wadi al As duricrust is not known to have formed in rocks Faw along the foot of the Al 'Ari$ escarpment. Here a younger than Miocene and Pliocene, these carbonate- thin, 60-km-long band of white to tan, powdery to enriched rocks are believed to be Quaternary. patchy gypsum is associated with Quaternary gravel. TERRACE, MUD FLAT, AND GYPSUM DEPOSITS QUATERNARY DEPOSITS LAKE TERRACES TERRACE GRAVEL, Lake beds, now cut into a steplike series of low ter races, crop out in the southwestern Rub* al Khali Outcrops of terrace gravel are limited to two one in around lat 19°05' N., long 46°20/ E. and in the Shaqqat the bottom of Wiidi al Bu^in where several small al Mi'ah (near lat 17°30' N., long 46°35/ E.). These patches occur 60 and 90 km northeast of the Trans- fresh-water sediments are interbedded, white, tan, and Arabian Pipeline and another in Wfidi as Sahbil' where brown, fine-grained quartzose sandstone with mica a few isolated patches are present between the 47th ceous, silty and rarely gypsiferous clay and fossiliferous and 48th meridians. The gravel is mainly limestone silt. and quartz that has been concentrated in terrace A number of fragments of vertebrate teeth and bones deposits obviously related to present or recent drairage. were found within or associated with the lacustrine de SHEET GRAVEL, posits. Age, based on these fossils, is considered to be The largest single occurrence of sheet gravel blankets late Pleistocene (?) to early Recent. the Ad Dibdibah plain a. broad, flat boulder-strewn Elsewhere in the southwestern Rub' al Khali, Pale surface flanking Al Biitin and extending from south of olithic and Neolithic sites have yielded arrow heads,fist the Trans-Arabian Pipeline into Iraq and Kuwait. hatchets, scrapers, and other stone implements. At The sheet represents the residue of a vast flood of rock one locality charcoal, presumably from a Neolithic debris derived from the basement complex and fun- campsite, was dated by carbon-14 as 5,090 ±200 years. neling out through the Wfidl ar Rimah-Al Bfit-in channel Age was determined by T. A. Rafter (written commun., system. The gravel veneer is nowhere more than a June 1960). Field (1958, 1960) has described the stone single pebble thick and is chiefly quartz with varying implements collected from these sites. proportions of carbonate, igneous, and metamorphic MARINE TERRACES AND RAISED BEACHES grains. The plains area around Tabuk is also littered Marine terraces and raised beaches are rather com with a similar gravel. mon along the western Persian Gulf shoreline. Within Another gravel sheet, the As Satin plain (around Saudi Arabia beach gravel and sand occurs on marine lat 30°40' N., long 42°40' E.), includes basalt pebbles terraces a,nd raised beaches from north of Al Jubayl probably derived from Al Harrah lava field. In addi (lat 27°01' N., long 49°40' E.) to the southern tip of tion, a large number of chert granules are included. Dawbat Salwah. The commonest occurrence of the Large, coalescing, deltalike surfaces of outwash g-avel marine Pleistocene is in the form of sand and coquina emanate from the Wadi as Sahba'-Wadi ad Dawasir- terraces 1 or 2 m above mean high tide. These terraces Wadi Najran drainage systems. Present distribution of form low discontinuous mesas along the coastline and a exposures shows that prior to formation of the great short distance inland. Rub* al Khali-Al Jafurah sand desert a vast part of Along Dawbat Salwah well-preserved marine fossils, the area generally west of the 51st meridian was most of which are present-day species, are found in soft, blanketed by coarse rock debris pouring out from these pebbly sandstone and coquina. Maximum thickness channels. At the same time, large quantities of the of this fossiliferous unit is about 8 in. Quaternary debris were diverted (u short distance from the base marine fossils also occur at Sabkhat Sawda' Nathil (lat ment) along a shallow trough scooped in soft Permian 24°30/ N., long 51°04 / E.) about 3 m below mean high beds, thus lining this depression with gravel from the tide. vicinity of Wadi Najran to Khuff (near lat.25°00' N.). DELTAIC AND TIDAL HUD FLATS GRAVEL, REMNANTS Deposits of this type occur only in Kuwait and are described in connection with the surficial geology of Dissected remnants of gravel apparently unreHted that country. to modern drainage occur in two areas: one along the GYPSUM DEPOSITS foot of the Hit escarpment (between lats 24°20' N. Young bedded deposits of gypsum occur in small and 24°32/ N.) and the second in front of Al 'Arf.mah patches at several localities, usually in contact with escarpment near lat 25°54' N. Grains are for the D100 GEOLOGY OF THE ARABIAN PENINSULA most part limestone, but other rock types are included precipitates various salts at and near the surface to as well. form a hard saline crust. During moist periods stand GRAVEL, SAND, AND SILT ing water from tidal invasion or runoff helps recharge the sediments and also by evaporation contributes to Bottoms of channels and undrained depressions are the salt scum. commonly floored with a thin but often extensive layer EOLIAN SAND of unconsolidated, locally derived sediment. Approximately one-half of the sedimentary area of Gravel of this type represents: (1) a recent concen Saudi Arabia is blanketed by eolian sand. The Rub' al tration of coarse debris from the immediately sur Khali contains probably the largest continuous body rounding area, (2) a residual deposit derived from older of sand in the world and covers nearly 600,000 sq km. sediment or rock by removal of intergranular fines, Another 180,000 sq km is covered by interior nafuds and (3) a lag concentrate from an earlier gravel sheet and Ad Dahna'. The largest of these, An Nafud, is or train. roughly 68,000 sq km. Numerous widely scattered, undrained basins have R. A. Bramkamp developed a practical classification in common a flat, firm, silty surface that contrasts of the fundamental types of sand terrain found in sharply with other crusty, brine-saturated depressions Saudi Arabia. He recognized four main classes which (Sabkhah deposits). The silt flats, known by various were adopted and whose distribution was depicted on names depending on soil type, vegetation, and so forth, the Miscellaneous Geologic Investigations Map series. locally include calichelike and gypsiferous deposits. Basic characteristics of these classes are as follows: Other surficial deposits that cannot be subdivided or I. Transverse: Predominantly simple and compound classified for any of several reasons include gravel, barchan dunes in areas of more mobile sand p.nd (or) sand, silt, gypsum, and other associated fine material. simple rounded ridges, oriented transverse to the SABKHAH DEPOSITS prevailing wind direction. Sabkhah (plural: sibakh) is an Arabic term for coastal II. Longitudinal: Primarily dikakah (bush- or grass- and inland saline flats or playas built up by deposition covered sand) and various types of undulating sand of silt, clay, and muddy sand in shallow, albeit some sheets, in general characterized by elongation of the times extensive, depressions. The deposits are com individual forms parallel to the prevailing wind monly saturated with brine and often are salt encrusted. direction, often partly stabilized by sparse vegetation. Indeed, the presence of salt is the distinguishing feature III. 'Uruq: Various forms of long, nearly parallel, of a sabkhah. Other silt-filled areas, usually centers sharp-crested narrow sand ridges and dune chains of centripetal drainage, contain no salt. These are separated by broad sand valleys usually ir eluding more correctly called Ql'an. elements of Type I sand terrain. These fo^Tus are MamlaiiaiL (plural: mamalib), a name often heard the resultant of a system of two dominart wind in the interior, refers to a sabktiah where salt is, or directions and are identical to the sayf dunes of at one time was, mined. Strictly speaking mamlatah north Africa. designates the excavation or area in the sabkhah where IV. Sand mountains: Dominated by large sand massifs salt is being removed. commonly cresting 50 to 300 m above the substratum, Sibakh are, for the most part, concentrated in a often with superimposed dune patterns consisting of narrow belt along the coast from Kuwait to east of various types of complex barchans. Common forms Qatar. Most are found within 60 km of the shoreline, are giant barchans spanning several kilometers from but some are located far inland for example, in horn to horn, giant sigmoidal and pyramidal sand Wadi as Sirtian (at lat 30°50' N., long 38°10' E.), peaks as well as other less common peak forms, and near Baq'a' (lat 27°53' N., long 42°24' E.), southeast giant oval to elongate sand mounds. of Buraydah (lat 26°20' N., long 43°58' E.), near STRUCTURE Al Qasab (lat 25°18/ N., long 45°30/ E.), around Yabrin (lat 23°17/ N., long 48°56/ E.), and over a GENERAL FEATURES wide band along the eastern margin of the Rub' al The structural pattern of the Arabian Peninsula was Khali. set in Precambrian time with stabilization of the Origin of these saline flats is in doubt, but most Arabian shield. These ancient rocks, now lying in workers now agree that they result from sand and silt, tangled confusion, in themselves reveal a complex and commonly eolian, infilling topographically low areas. mobile history. By the outset of the Paleozoic Era, Brine is nearly always to be found within 1 or 2 m of however, they had been fused into a rigid land mass, the surface. Apparently, prolonged evaporation of and their surface had been reduced to a nearly level capillary water during dry periods concentrates and plain. SEDIMENTARY GEOLOGY OF SAUDI ARABIA D101 From the Cambrian on, vast epicontinental seas and largely overlain by younger volcanics in the west. moved back and forth across the lower parts of this Though the shield has acted as a rigid land mass stable basement core and there deposited a compara since the beginning of Cambrian time, consolidation tively thin succession of almost flat-lying strata. The was long in coming. Included within the basement epicontinental seas were in turn flanked on the north complex are great belts of sedimentary rocks now and east by a great sedimentary basin the Tethyan contorted, intruded, and metamorphosed that record trough that occupied a relatively constant area in the growth and decay of former mountain systems. Turkey, northern Iraq, and southwestern Iran. The Presumably, through the eras of Precambrian time, trough remained a negative feature throughout most the shield was subjected not once but several times of the Paleozoic and Mesozoic while many thousands to downwarping, sedimentation, orogeny, injection by of feet of sediment accumulated. Destruction of the plutonic rocks, and leveling before becoming immolate ancient seaway began in the Late Cretaceous with a complex, but little known, structural history com erogenic movement recorded from Turkey through Iran parable with that of other shields and former and extending into Oman. The climax came, however, geosynclinal areas. in the late Tertiary (Alpine orogeny) when rocks from Likewise, the structural situation after the str.rt of the deeper part of the basin were folded and thrust to the Paleozoic is obscure, but some intrusion and local form the Zagros-Taurus chain of mountains. It is faulting is likely. A number of epeirogenic movements during this period that the great rift-fault systems of of the shield have been clearly recorded in the sedi Africa and the Red Sea area also began to take on their mentary sequence, generally in the form of regional present form, although movement on these faults may unconformities. During the Tertiary Period, sub have started somewhat earlier. stantial faulting and other disturbances occurred along Within this broad tectonic framework, two major the margin of the Red Sea depression. structural provinces are recognized (pi. 2). One is YEMEN-WESTERN ADEN PLATEAU the comparatively stable interior region whose rigidity is controlled by Precambrian basement. The exposed In the Yemen-Western Aden area, Precambrian part of the shield, as well as that part thinly veneered basement supports a relatively high, substantially with little-disturbed shelf sediment, make up this faulted plateau. The plateau is capped in northern province. The other major province is the great Yemen by patches of flat-lying Jurassic, Cretaceous, mobile belt peripheral to the stable region and made and Tertiary sedimentary rocks. Massive extrusive up of the Taurus-Zagros-Oman Mountain systems. volcanic rocks of Late Cretaceous to Recent age cover Saudi Arabia falls wholly within the stable region, central and southern Yemen and enclose fault blocks and it is this structural province with which we are of basement and Mesozoic rocks that have been up most concerned here. Consequently, this province and lifted through the volcanic sheet. Less stable than its significant structural subdivisions will be discussed the western and southern part of the Arabian Shield, in some detail; those peripheral to it will be touched the plateau area apparently operated as a Hnge, on but briefly. It should be emphasized that stable is absorbing differential movement between flanking a relative term used here to differentiate an area that massifs. Presumably, Yemen-Western Aden remained has from Precambrian to Recent time been subjected positive until the late Jurassic and then, for the first mainly to gentle warping and tilting. tune, was sufficiently downwarped to permit direct com munication between the Tethyan and east African STABLE REGION seaways. ARABIAN SHIELD SOUTHERN ARABIAN SHIELD Shield rocks, comparatively stable and primarily The structure and configuration of the southern positive since Precambrian time, make up a sizable Arabian Shield is obscured by the Gulf of Aden; how part of the Arabian Peninsula. On the west they form ever, its northern margin can be roughly reconstructed most of Najd and all of Al IJijaz and 'Asir; on the from exposures preserved along the ^Jatframawt, Al southwest they support the highlands of Yemen, and Mahrah, and Dhufar coasts and in the Khuriya Muriya along the southern coast of the Arabian Peninsula Islands. Like other parts of the Arabian Shield, these they can be seen in sporadic outcrops. Until recently, rocks show evidence of strong deformation, intrusion, the Arabian Shield was a projection of the shield of and metamorphism followed by mature peneplaration Africa but is now separated from it by Red Sea rifting. before the first sediments were deposited. WESTERN ARABIAN SHIELD ARABIAN SHEL.F The western Arabian Shield is an extensive area of After solidification and mature peneplanation, the Precambrian rocks, mainly igneous and metamorphic, Arabian Shield was tilted slightly northeast toward D102 GEOLOGY OF THE ARABIAN PENINSULA the ancestral Tethys trough. As subsidence continued, with an average width of about 400 km and a persistent shallow seas advanced across the beveled surface of dip ranging from slightly more than 1°00' in the Per crystalline rocks and buried it beneath thin sheets of mian and Triassic to 0°20' in Upper Cretacecus and almost flat-lying sediment. It is this belt of low- Eocene beds. dipping, relatively undisturbed beds that gives rise to The homocline thus defined marks an area of unusual the second main structural province of the stable tectonic stability. To be sure, significant structural region the Arabian Shelf. It should be kept in mind interruptions do occur, but these appear to result from that the shelf differs from the exposed part of the shield blocks of different sizes and shapes moving independ only in possessing a thin sedimentary cover. ently and without disturbing adjacent parts of the As early as Early Qrdovician, marine water had homoclinal surface. Change in strike of the homocline, transgressed inland nearly to the present limit of sedi related to gentle flexing of the basement, tends to break mentary beds, well documenting the invasion with a the belt into several poorly defined segments. thin but widespread, graptolite-rich shale. Conse quent upon successive rising and sinking of the shield, CENTRAL ARABIAN ARCH DEFINED BY NORTHERN AND SOUTHERN TUWAYQ SEGMENTS seas advanced and retreated. Because the sea floor was so flat, even gentle bending substantially moved One structural feature the central Arabian arch the shoreline and altered the position of land and sea. has had a profound influence on the present surface dis Effects of such epeirogenic movements, clearly recorded tribution of sedimentary rocks. The name applies to in the interior by unconformities, disconformities, and the area of changing strike between the northern alternating marine-nonmarine rocks, fade as the deeper ^uwayq and southern 'J'uwayq segments of the Interior part of the basin is approached. Here, subsidence Homocline. In reality, these segments are an integral apparently persisted with few interruptions from part of the arch which affects all rocks from tie base Paleozoic to relatively recent tune. ment up; its influence can still be seen as far east as Qatar. The arch, although a very gentle feature, Sediments of the shelf are characteristically of shal appears to have considerable importance by marking low-water origin. Limestone, clastic-textured and me the area of maximum curvature of the Interior Homo chanically sorted, is the most striking rock type. Sand cline in central Arabia and by supporting the reversal stone and shale are also present in large amount. All between the north-dipping part of the shelf and its units, regardless of lithology, are usually thin and wide south-dipping part which passes uniformly under the spread, maintaining lithologic character over large Rub' al Khali. areas. Shifts in facies do occur, but these are usually Early work in Arabia led to the assumption that this gradual. Such lithologic constancy, presumably great basement ridge was an ancient and pernanent brought about by continued sorting and reworking of positive feature. It was assumed that bands of sedi the limited amount of sediment introduced into the mentary rock were deposited around this swell in much basin, reflects prolonged stability. their present form and original strike of deposition Not all parts of the Arabian Shelf had the same geo roughly paralleled present strike. Furthermore, it was logic history. Some areas were more active than assumed that the structural form of eastern Arabia was others; some were depressed and some elevated either due to regional and uniform tilting of the shield toward persistently or for short periods. Thus the shelf is the Persian Gulf and Rub* al Khali basins. Sediment divisible into several subprovinces namely, an Inte distribution and geometry of rock units around the rior Homocline everywhere marginal to the shield, an arch have shown this concept to be incorrect, and it is Interior Platform bordering the homocline, and, adja now known that the history of the feature is much more cent to the platform, several basinal areas that have complex. from tune to time collected thicker deposits. In con During early Paleozoic time, the arch was mildly nection with this, the Interior Platform and adjacent basins correspond at least in a rough way to the "un positive, as pre-Permian units seem to pinch out against stable shelf" just as the Interior Homocline appears to it. Strong upwarping occurred in the later part of represent the "stable shelf." the Paleozoic Era (post-Early Devonian and pre-Late Permian), older beds were beveled and stripped from INTERIOR HOMOCLINE the crest of the arch, and later submergence permitted Bordering the shield is a belt of Paleozoic, Mesozoic, deposition of Upper Permian carbonates directly on and Cenozoic strata whose dip basin ward is so .slight basement across the axis and for some distance north and uniform as to be imperceptible to the eye. Plane- and south. table, structure drill, and seismograph surveys over the During Mesozoic time, the central part of the craton area have defined a remarkably constant homocline gradually subsided, and thick sequences of continental SEDIMENTARY GEOLOGY OF SAUDI ARABIA 1103 and shallow marine sediments accumulated in a basin Interior Homocline during Late Cretaceous t;me. whose deepest part apparently fell roughly along the Details of this shift in strike are discussed in connection present axis of the arch. This relationship is par with the formation of the IJa'il arch. ticularly marked in the Jurassic but can also be seen Subsidence in the north continued while Paleo^ene in the rocks of bracketing systems. Detailed measure and Eocene rocks were deposited, thus further increas ments show that the Jurassic is thickest on or near the ing the apparent relief of the arch. The Tertiary pic axis of the arch and thins uniformly and persistently ture is complicated by the fact that the Rub' al Fhali to the north and south. Pre-Wasia (Middle Creta became an area of active subsidence for the first time. ceous) truncation accounts for some of the thinning Downwarping here would also tend to accentuate the in the far north, but not all, for the section is distinctly basement ridge. It is this differential sag, in fact, that condensed when compared with an equivalent sequence may be in large part responsible for the central Arabian near the center of the arch. There is also a correspond arch, probably a residual high between the periodically ing change from deeper water to shallow water and subsiding Persian Gulf and Rub' al Khali basins rs ther even continental sediments away from the axis. Thick than a true independent positive feature. intervals of limestone and shale in the high area are It is doubtful that the central Arabian arch was represented by shale and sandstone on the flanks. It further exaggerated in Miocene and later time, though seems logical to assume from these data that the shore there is a reflection of the swell in the arc of the §umman line of Jurrassic times in central Arabia fell considerably terrace. Miocene sediments of the §umman are ex west of the present outcrop belt and lay approximately clusively continental, however, and as such would be along a line connecting Jurassic exposures on the north much influenced by adjacent topography during their and south where an approach to strand-line conditions deposition. At this time the topography, even as it is evident. does today, must have shown the influence of the arch, The first traces in the sedimentary record which which would exercise a primary control on the drainage would indicate the origin of the central Arabian arch as well as on the form of the surface on which the appear in the Wesia Formation (Middle Cretaceous). Miocene was laid down. On outcrop, this formation is continental in the south Support for this concept of the formation of the cen where it disconformably overlies nonmarine rocks of tral Arabian arch comes from the presence of the ^radi Early Cretaceous age. On and north of the arch, Nisafr-Wadl as Sahba' graben and trough complex however, Wasia rocks become progressively more along the axis of the swell. From a regional point of marine and sharply transgress older strata. The best view, these tensional structural features form b'lt a explanation of this change appears to be that the small part of a major system of grabens that, near the sagging, which resulted in thickened Jurassic and western edge of the Tuwayq Mountains, swings north Lower Cretaceous deposits, was disrupted during pre- west in a great arc along the Tuwayq front and then Wasia deformation with the uplift of the southern again passes through the Tuwayq escarpments west part of Arabia. At the same time, subsidence con of Al Majma' ah. The counterpart of the central tinued in the north. Thus, in contrast to the relatively Arabian graben and trough system can be seen far to stable conditions that prevailed in the southern the northwest as the Khawr Umm Wu' al graben and Tuwayq area, the northern Tuwayq segment and the plexus of faults around Al Jawf-Sakakah. Thus, adjacent parts of the shelf were sites of active depres these tensional features are found about in the place sion. In general, the zone which separates these two they would be expected if the central Arabian arch areas coincides fairly well with the central Arabian formed in the manner outlined above; that is, the faults arch. It is therefore considered that the arch results mark the area of maximum tension along a hinge line from its position between a stable or slightly rising between a stable upland block and a sinking shelf. province and a gradually sinking area, giving rise to Similarly, the age of the graben system fits well into a gentle swell. this general concept in that it appears to have originated During Arum a (Upper Cretaceous) time, this same in mid- or Late Cretaceous time and shows evidence of general pattern of movement persisted, and the continued movement during the Paleocene and Eocene. structural elements inherited from the Middle Creta HA'lL ARCH ceous were still further emphasized. The southern * TU way q-south western Rub' al Khali area remained A shift in strike of the Interior Homocline around emergent, and marine rocks were excluded. Con Ha'il (lat 27°32' N., long 41°43' E.) defines the IJa'il tinued subsidence in the north, quite possibly at an arch, a structural feature similar to the central Arabian accelerated rate, can be assumed from a thickened arch. Lower Paleozoic sedimentary rocks, which have rock sequence and a sharp change in strike of the a consistent regional strike of N. 45° W. in the northern D104 GEOLOGY OF THE ARABIAN PENINSULA fuwayq segment of the Interior Homocline, swing the time of its formation appears to have been mainly gradually west in conformity with the trend of the Cretaceous and early Tertiary. underlying basement. At Jibal a^ f&waL, about 40 km One significant aspect of the ga'il arch is that all north of Ha'il, a uniform strike of N. 70° W. was meas major faults in the area conform to a definite northwest- ured in Ordovician shale. In all probability the strike southeast trend. This trend may be related to persist continues to parallel the basement margin, shifting ent Late Cretaceous and Eocene sinking of the Persian first to west and then west-southwest to define the west Gulf and Sirban-Turayf basins, which caused tensional flank of the arch, although evidence for this is hidden faulting of the residual high between. The rise of the by An Nafud sand. ga'il arch had a profound effect on the architecture Lack of significant change in facies or thickness of of the Tethys seaway. Prior to appearance of the sedimentary units suggests that the arch was merely arch, strike of Paleozoic, and presumably most Mesozoic a stable headland around which Paleozoic and later seas units as well, closely paralleled the northern margin of advanced. The first definite sign of movement comes the shield. With uplift of the arch in the Cretaceous, in the Cretaceous Period. Middle Cretaceous rocks, communication across northwestern Arabia between in sandstone facies, were deposited uninterruptedly the Persian Gulf and Mediterranean seaways was across the crest of the arch, essentially parallel to the broken. Consequent upon this, the Interior Homo strike of the Paleozoic strata. On the other hand, basal cline was split into two segments to the west the Aruma (Upper Cretaceous) rocks show sharp facies Tabuk segment comprised mainly of Paleozoic rocks change from marine carbonate on the east flank of the and to the east the Widyan basin margin made up swell to continental sandstone across the crest. These solely of Cretaceous and younger beds. beds were the last to be deposited over the arch before WIDYAN BASIN MARGIN the strike of the Interior Homocline was shifted north into Iraq by the rising basement. The Widyan basin is the Saudi Arabian part of a Surface mapping north of An Nafud detected the major basin of deposition. As used here, however, the name is applied to the eastern limb of the 5a'il-Putbah prolongation of the 5a'il arch where it is a broad swell in Cretaceous and Eocene limestone, the high part swell. It is an area of Upper Cretaceous and Eocene passing east of Jabal 'Unazah on the Saudi Arabia- rocks having a gentle northeast dip averaging about Iraq boundary. The northeast flank dips gradually 2.5 m per km. toward Iraq; the southwest flank passes gently into the Superimposed on the broad homoclinal framework are Sirfran-Turayf basin. At the crest of the arch is a great many of minor irregularities. The^e are Khawr Umm Wu'al, a trough made up of a series of primarily solution-collapse features of various types, grabens and synclinal depressions. Its relationship to covering a wide range of radius and amplitude from the 3a'il arch approximates the relationship between sinks and residual highs a few tens of meters across and the Wadi Nisab-Wadi as Sahba' grabens and the central 5 to 10 m in height to features several kilometers in Arabian arch. diameter and 10 to 20 m or more in height. Tve size of individual structures in any given area usually falls Structure drilling near the Trans-Arabian Pipeline within a relatively restricted range, as the characteris also confirms the presence of the 3a'il arch at depth and tics of the slumping are presumably determined by the has given a relatively complete transverse section of a depth of the soluble unit whose removal has caused the broad arch between Badanah and Turayf, its highest collapse. point being roughly 40 km west of Al Jalamid (lat Larger structural features are surprisingly few, and 31°19' N., long 40°03' E.). Upper Cretaceous and in fact, only in the far southwestern part of the province Eocene strata thicken and apparently represent a can it be shown conclusively that faulting, originating more complete rock sequence in both directions away from deep-seated causes, follows the trend of the Al from the crest. The swell is also present in the older, Jawf area with only minor displacement and disturbance underlying, presumable Paleozoic rocks. of Upper Cretaceous rocks. Evidence to th^ west The 5a'il arch no doubt continues northward into suggests much larger displacements along these trends Iraq and there joins with the Rutbah high. Infor in pre-Upper Cretaceous units. mation is too incomplete to precisely define the axis of The characteristics and formation of long curved the arch, but it apparently passes west of JJa'il, through monoclines such as Jal al Ba^n and Jal Nawazir have the Al Jawf-Sakakah area, west of Al Jalamid, and been previously discussed in connection with the Umm east of Jabal 'Unazah. The arch may have originated er Radhuma Formation (p. 84). by its own growth or as a residual high flanked by one The Widyan basin margin is a northwestern extension or possibly two areas of subsidence. In either event, of the Interior Homocline, although it shows a certain SEDIMENTARY GEOLOGY OF SAUDI ARABIA D105 amount of sinuosity. In broad plan, strike of Cre platform is less well defined, trending southeast from taceous and Paleocene beds shifts from slightly west of Kuwait into the Persian Gulf, far out around Qatar and north near the Saudi Arabia-Iraq boundary to N. 60° possibly (in one area north of Qatar) reaching east as W. at Darb Zubaydah (long 43°25' E.). Southward, far as the coast of Iran, and passing inland along the the strike reverses to about N. 40° W., and the beds of western edge of Sabkhat Mat-! and around the Rub* al Al Widyan merge with the northern Tuwayq segment Khali basin before terminating against the Oman Moun of the Interior Homocline. tains near lat 23° N., long 56° E. Width of the p] at- form ranges from about 100 km along the southern and HADRAMAWT PLATEATT * * western sides of the Rub' al Khali basin to 400 km or Between the southern Arabian Shield and the sands more across the Qatar Peninsula. The great widtl of of the Rub' al Khali is a high, dissected, generally north the platform in the vicinity of Qatar could conceivably ward-dipping plateau of early Tertiary rocks which reflect the extension of the central Arabian arch. forms the southern margin of the Rub' al Khali basin. The deep structure of the sedimentary rocks of the Basically, the plateau is a broac|, generally east-west- platform must be inferred from structure drill and trending swell extending nearly from the 47th meridian seismic results. These data indicate that the beds are to Dhufar. Superimposed on the main swell are two unusually flat in a geological sense and irregularities smaller east-west arches separated by a low syncline, are limited to low, gentle structural undulations that the western end of which roughly corresponds with the lack any strong orientation. Superimposed on this deep channel of Wad! Hatframawt. are several major north-south anticlinal axes that rise The area lias been affected by some east-west faulting. above the general level of the platform. One of the Such disturbances are relatively minor in the main pla most notable of these is the En Nala anticline, the teau area but are more pronounced along the coast southern part of which includes the oil field of Ghavar. where they give rise to a rugged fault-block topography. The folds of Abqaiq and Qatif are other good examples of large north-south trends. The surviving segments of HADHAMAWT SEGMENT the platform on the two sides of any of these major North of the Haolramawt Plateau and extending east axes seem to have about the same structural elevation. to Oman is the lower part of the Interior Homocline Any conclusions are highly speculative, but the flat, the Hao'ramawt segment. Much of the segment is gently undulating parts of the structural platform are buried beneath the Rub' al Khali where structure drill presumed to be underlain by rigid blocks of basement and seismograph have mapped uniform north dip and that resisted deformation, thus protecting the sedi defined the northern toe. Average dip of Tertiary and mentary blanket from significant disturbance. The Quaternary rocks is about 0°20'. Underlying Mesoxoic origin of the major anticlines is still obscure, but if strata have a more pronounced dip owing to thinning and local complexities are disregarded, the nearly flat crest probable truncation toward the Haolramawt Plateau. and steep linear flanks give strong reason to suspect East of long 4S°, strike of the homocline holds rela that these features are related to some sort of horstlike tively constant at slightly north of east. West of this uplift at great depth. Study of gravity data further meridian, however, strike swings rapidly west and then strengthens this hypothesis. Several concepts of causal northwest to define the southern limb of a broad, north stresses seem equally acceptable simple compression, eastward-plunging syncline which connects the IJaolra- deep-seated differential uplift, and horizontal torsion. mawt segment of the Interior Homocline with the south Only one sharp feature Dammam Dome departs ern Tuwayq segment. This bend, which in effect completely from some expression of north-south outlines the southwestern margin of the Rub' al Khfill elongation. Because of its complex crestal faulting, basin, involves a change in strike of about 120°. Pre strong negative gravity anomaly, and oval shape, it sumably, this area of maximum curvature also corre is regarded as a salt-intrusion structure. sponds to a hinge line or /one of adjustment between BASINS the western Arabian and southern Arabian Shields. Several depressions are superimposed on the Arabian INTERIOR PLATFORM Shelf and at one time or another have received thick Bordering the homocline is the Interior Platform, an deposits relative to adjacent parts of the platform. area of varying width in which systematic dip ofl1 the Such basinal sags have formed in the northeastern Pub' crystalline core no longer prevails. The nearly Hat al Khali, northern Persian Gulf, Dibdibah, and Sirljuln- structural platform is sharply set ofl' from the homo Turayf areas. cline by an abrupt break in slope a hinge line between HUB* AL KHALI BASIN the two structural provinces. The opposite edge of the Seismograph and structure drill work in the Fub' D106 GEOLOGY OF THE ARABIAN PENINSULA al Khali have outlined an elongate basin that plunges Cretaceous and Eocene sediments. The bask opens gently northeast form Bi'r Had! (lat 19°26' N., long to the north and northwest into Jordan. Volcanic 51°02' E.) toward the Persian Gulf. The north rocks, representing the southern end of the major lava eastern part of the trough apparently extends across fields of the Syrian desert, blanket a substantial area Abu Dhabi and Trucial Oman and continues into the along the low part of the trough. Small linear faults Persian Gulf almost as far as the coast of Iran. Width with a north-northwest trend probably reflecting dis of the basin is relatively uniform throughout its length, location in older rocks are common. One structural averaging about 300 km. trough-graben (Khawr Umm Wu'al) occurs in the The Rub' al Khali basin is primarily a Tertiary eastern part of the area. feature. Paleocene, lower and middle Eocene, and The southwest flank of the Sirkan-Turayf l^sin is upper Tertiary sedimentary rocks thicken toward the relatively complex and may even represent a half graben center. This thickening is moderate, however, except that controls the present course of Wadi as Sirhan. in the eastern part of Trucial Oman where very thick The Sirfran depression, a broad elongate trough, is Tertiary deposition took place in the deep sedimentary flanked on the southwest by a uniformly northeast- trough in front of the intensely folded Oman ranges. dipping homocline that is also the southwest flank of Mesozoic rocks show a trend toward shallower water the much larger Sirkan-Turayf basin. A series of nor sedimentation from the center of the basin northwest, mal faults, downthrown to the southwest and largely west, and southwest toward the platform. A similar buried under a mass of extrusive basalts, bound the trend appears to the south and southeast although Sirban depression on the northeast. much of the pertinent evidence for this has been elimi The Sirfran-Turayf basin may have resulted from nated by several Mesozoic unconformities. To the growth of the IJa'il arch to the east during Lr.te Cre northeast, deposition appears to have taken place in taceous and Eocene time. A more likely explanation, as deep or deeper water than in the central Rub* al however, is that the basin developed as a sag in a struc Khali. In general, Mesozoic sedimentation appears to turally flat area. This interpretation is suggested by be related to a basinal area much larger than the comparable elevations of contemporaneous rocks on present depression. If this is true, the Rub' al Khali the two sides of the basin. as an independent basin would be exclusively Tertiary, Later structural deformation of the basin rhows a possibly with some Late Cretaceous history. Appar dominance of tensional over compressional forces ently, the basin is a shallow reflection of the depressed abundance of faults and grabens and rarity of folds. hinge between opposing basement blocks. No thrust faults are recognized. NORTHERN PERSIAN GULF BASIN MOBILE BELT MOUNTAINS AND FOREL4NDS Limited marine seismic work and regional relation Encircling the stable interior region is a mobile belt ships show that a structurally low area covers the head of young mountains and a thin foreland area deformed of the Persian Gulf. Seismic mapping has provided contemporaneously with the main ranges. The various some clue to the geometry of the higher part of its ranges butting against the Arabian shelf the Zagros southwest flank which dips at a nearly constant 2° to of Iran and the Oman Mountains are small links in 4° NE., a steep dip, indeed, when compared to the the much longer Alpine-Himalayan system. None fall adjacent platform and the gentle gradient (0°20' to within Saudi Arabia, and only a brief description of 1°20') of the Interior Homocline. The northeastern them will be given here. margin of the basin is formed by the Zagros Mountain MOUNTAINS foreland. ZAGROS MOUNTAINS DIBDIBA BASIN The Iranian Ranges or Zagros Mountains, the term The term Dibdiba basin is applied to a downwarped usually used for the whole mountain belt of south area between the Jauf-Wafra-Burgan high and the western Iran, is a broad belt of northwest-southeast Interior Homocline. The basin, postulated primarily folded and faulted Paleozoic, Mesozoic and Cenozoic on seismic work which indicates a major thickness of rocks. Thrusting is common in the younger sediments. Cretaceous rocks, extends north between the Neutral Evidence for some tectonic activity is present in Upper Zones into southern Iraq. Cretaceous rocks, but the main orogenic movements occurred in late Tertiary, mainly Pliocene time. SIRHAN-TURAYP BASIN Structure drilling and surface mapping have delin OMAN MOUNTAINS eated in the northwestern corner of Saudi Arabia a Bordering the Gulf of Oman on the southwest is a synclinal basin filled with a thick sequence of Upper complex geanticline of Paleozoic and Mesozo;c rocks SEDIMENTARY GEOLOGY OF SAUDI ARABIA D107 with Cenozoic sediments overlapping the flanks. Oro- bounded on the north and south by normal faults genic movements began at the end of mid-Cretaceous which dip under the down-dropped floor at 60° to 70°. time and reoccurred between the Oligocene and Displacement on the Nisab border faults is hard to Miocene. determine in the eastern part of the trough wher?. the FORELANDS picture is obscured on both sides by the badly slumped ZAOROS MOUNTAINS FORELAND Arab and Sialaiy sequence. It is obvious, however, Southwest of the Zagros Mountains proper is a thin that movement has been at least 100 m and possibly belt of gently folded rocks. Structural features within as much as 300 m as Biyadh and Wasia rocks in the this foreland area are generally elongate, symmetrical, floor of the graben are in contact with Arab, Sulaiy and are parallel and contemporaneous with the main and Yamama beds across the faults. ranges. In the western part of the trough, the amount of OMAN MOUNTAINS FORELAND displacement can be more accurately determined. An area of gently deformed, generally west-dipping Here, lower Arab is downthrown against Hanifa and Mesozoic rocks almost entirely covered by flat-lying upper Hanifa can be seen dropped down against upper Tertiary sediments flank the Oman Mountains. Their Dhruma, which suggests between 200 and 300 m of formation, but to a lesser degree, resulted from the same movement. In addition to the two main bourdary orogenic forces which affected the Oman Mountains. faults, the floor of the graben is broken by subsidiary HUQF-HAUSHI SWELL. faults, satisfactory exposures of which occur only in In southeastern Oman, near the coast, is a broad, a few places. gently uplifted feature the Huqf-Haushi swell. The Drag effects are found in rocks on both sides of the swell, built around a block-faulted core of Paleozoic graben. This sagging is visible as far as 1 km back rocks, trends north-northeast and is terminated on the from the edge of the trough. In addition to this west by a gently dipping scarp that exposes Mesozoic broad, rather gentle drape, local intense drag is found and Tertiary rocks. Paleozoic, Mesozoic, and Tertiary in the immediate vicinity of the fault planes. deposits unconformably lap over the flanks of the Seemingly, the first beds displaced are of Y^asia ancient high. (Middle Cretaceous) age. The Wasia preserved far west in the graben, however, might conceivably repre CENTRAL ARABIAN QRABEN AND TROUGH SYSTEM sent a tongue of sediment deposited in a trough already A notable feature of the Interior Homocline is the in existence before Wasia deposition began. This central Arabian graben and trough system. This concept of pre-Wasia movement is seriously challenged system is a series of grabens and synclinal depressions by one feature that is, the complete lack of limestone forming a broad arc concave to the northeast and debris in Wasia rocks flooring Wadi Nisab. Pre extending west from JJaracJ through Ad Dahna', Al sumably the upthrown sides of the graben would have Kharj, and £>urma, swinging nearly parallel to the shed considerable limestone fragements into the down- fuwayq front for some distance, and finally passing dropped trough. Such is not the case, however. Thus back through Jabal fuwayq a short distance west of on present evidence it is considered likely that g-aben Al Majma'ah. The system is actually a compound formation began sometime after the close of Wasia feature in which the progressive tensional readjust deposition. ments are passed from one graben to another en echelon. The closing age of Nisab movement cannot be fixed In some cases this passage between adjacent grabens directly; however, Eocene rocks are involved in the takes place by scissors faulting and oblique synclinal formation of the Sahba' trough, a direct extension of folds. Six major grabens, two large troughs, and the Nisab graben. Thus, some movement probably numerous small subsidiary grabens and troughs are took place on the Nisab faults as late as the Eocene. related to this structural system which can be traced SAHBA* TROUGH for at least 560 km. Wadi as Sahba', although a much broader feature, NISAH GRABEN is a direct eastward extension of Wadi Nisab and the A well-defined graben coincides over much of its Nisab graben. The great width of Wadi as Sahba', length with the present valley of Wadi Nisab. The particularly in its upper part west of As Salamlyah structure has been traced eastward with few inter (long 47°18' E.), results in an extensive area of cover ruptions from the western edge of labal Tuwav QARADlN GRABEN MAJMA'AH GRABEN The Qaradan structure begins a few kilometers north The northernmost, or Majma'ah graben, is not struc of Khashm Mijfrarah and extends about 18 km N. turally as simple as the other down-dropped blocks but 50° W. to Khashm Qaradan (lat 24°44' N., long 45°54' rather is a system of overlapping faults and en echelon E.). This feature was formerly considered to be a grabens. From its southern end near Al Qasab (lat northwestern continuation of the Purma graben but 25°18' N., long 45°31' E.) the Majma'ah conplex slices is now known to be separated from it by transverse N. 20° W. through the Tiiwayq front and continues faults with a strike of about N. 70° W. If it were not along this trend approximately to the latitude of Al for these transverse dislocations, the Qaradan graben Ghat (lat 26°02' N.). Northward, the structure swings might be interpreted as an extension or an en echelon gradually to N. 04° W. and dies out as a gentle mono part of the Purma structure. cline in Upper Cretaceous rocks at lat 26°36' N. Qaradan graben, like the Purma, consists of an The possible extension of the Majma'ah graben elongate, flat-topped, frequently breached limestone southeastward under 'Urayq al Buldan is suggested by ridge with a relief of between 50 and 100 m. The flanks two outliers of Tuwaiq Mountain Limestone one of the block are made up of a series of overlapping immediately west of Al Qasab and the other, Jab^l al alluvial fans which cover the bounding faults for Fahdah, 12 km south-southeast of Al Qasab. Tiese nearly the full length of the graben. Several low passes outliers, remnants of down-faulted blocks, occur in line which appear to be synclinal sags break through the with the projection of Majma'ah graben and lie rlong ridge of limestone. the northwestern projection of Barrah graben. T1 us, Only the Tuwaiq Mountain Limestone is preserved the length of the Barrah and Majma'ah grabens may in Qaradan graben. At the northeastern end of the be greatly increased beyond the visible parts of their trough this limestone is dropped beside the uppermost remnant ridges and, in fact, may represent parts of a beds of the Marrat Formation, a relationship that single system now interrupted by Quaternary sand. shows 400 m or more of displacement. At the point where the Majma'ah graben intersects BARRAH GRABEN the fuwayq Mountains, the graben changes frcm a prominent obsequent faultline ridge to a long, straight The general outline of another graben can be re slot about 1 km wide walled in by opposing faces of constructed around a resistant limestone ridge cropping Tuwaiq Mountain Limestone. Relief along most of out about 25 km northwest of Khashm Qaradan. this trough is 50 to 60 m, lessening toward the north. A prominent outlier of Tuwaiq Mountain Limestone, East of Al Ghat the trough is a low sag containing a Nasilat a? Zu'aynah (lat 24°56' N., long 45°48' E.), discontinuous ridge of hills formed of Middle and marks the southeastern end of the ridge. The graben Upper Cretaceous rocks. as identified from the block of limestone is 16 km long, averages a little more than 2 km in width, and bears Along the southern part of the graben, Tuwaiq N. 55° W. Its topographic expression is that of a Mountain Limestone is faulted down against Dhrrima, series of deeply dissected limestone ridges separated and Hanifa is down dropped against Tuwaiq Mountain by several passes which may be synclinal in origin. Limestone. At the northern end, a group of Cretaceous Erosion of the limestone is much more complete than hills, Jabal 5a^tabah, stands above the dissected upland in other grabens so far described. Topographic relief surface of Tuwaiq Mountain Limestone. Movement does not exceed 75 m. is calculated to be 150 to 200 m at the southern end of Faults bordering the limestone ridge are covered by the graben, progressively diminishing to 0 m in the coalescing alluvial fans, and their location can only be extreme north. inferred by the topography. The structure is a down- On the evidence available, the age of faulting cf.n be dropped fault block bounded by two or more normal placed only as post-Late Cretaceous. faults, with some minor flexing along the axis. The In summary, there is in central Arabia a complex of normal faults are high angle and expressed as obsequent faultline scarps. Tuwaiq Mountain Limestone forms six grabens and two associated troughs that form a giant the greater part of the ridge. Synclines at each end of arc at least 500 km long and concave toward the north the graben expose rocks of the Hisyan Member of the east. It appears that faulting began in Upper Ceta upper Phrumd Formation. There is no Hanifa ceous time and continued at least through Paleocene remaining above the Tuwaiq Mountain. and probably on into middle Eocene. The grabec sys Although direct evidence is lacking here, displace tem probably marks a hinge line between a relatively ment of Tuwaiq Mountain Limestone from its normal stable land mass to the southwest and a periodically position above is estimated to be from 200 to 300 m. subsiding shelf to the northeast. DUO GEOLOGY OF THE ARABIAN PENINSULA KHAWR UMM WU'AL OBABEN SECTION 1. Tabuk and vicinity Continued Khawr Umm Wu'al, a narrow, linear trough, has Tabuk Formation (top truncated) Continued 7. Tawil Sandstone Member (177.0 m been traced southeast from Umm Wu'al (lat 31°47' N., Thickness long 38°54' E.) to the northern edge of the Al IJarrah thick) Continued (meters) lava field, a distance of some 135 km. The structure, Sandstone, buff, crossbedded, moderately cemented, medium-grained; thin partings 5 to 8 km wide, changes along its length from a simple and layers of white to light-gray platy syncline to a combined syncline and graben. Topo siltstone and fine-grained well-cemented graphic relief on the walls of the trough does not exceed sandstone. Local zones are finely nodular 100m. and ferruginous ______25.0 The margins of the khawr, in addition to being the Sandstone, yellowish-brown, medium- to coarse-grained, in part crossbedded; scat sites of major faults, are also the loci of basalt dikes, tered quartz pebbles, occasional layers of plugs, and flows. Khawr Umm Wu'al graben appar soft purple to greenish-brown sandstone, ently represents tensional adjustment to the growth of local partings of sandstone with ferrugi the 5a' il arch. Presumably the arch is a residual high nous matrix ______9.5 between two areas of subsidence the Sirttan-furayf Sandstone, yellowish-brown to brown, strongly crossbedded, moderately ce basin on the west and the Arabian Shelf on the east. mented, medium-grained; abundant black ironstone partings and ferruginous stain STRATIGRAPHIC SECTIONS ing. A weak sandstone bench at top____ 5.0 Detailed and generalized descriptions of stratigraphic Sandstone, reddish-brown, well-cemented, crossbedded; in part with ferruginous sections cited in text follow. Significant stratigraphic matrix; surface weathers black with local details are shown graphically in figures 2 through 14 and ironstone concretions.______20. 0 plates 3 through 10 for most type and reference sections. Sandstone, black- to brown-weathering, Formations of constant, homogeneous and(or) incom strongly ferruginous, fine- to coarse pletely known lithology not depicted in plates or strati- grained; subordinate quartz conglomerate with ferruginous matrix. Forms thick graphic sections are described in text. These include ledge with prominent bench at top. _____ 5.0 the Saq, Wajid, Hith, Wasia, and Kharj Formations Sandstone, brown to golden-brown anc? and the Tertiary and Quaternary surficial cover. Char coarse-grained, ferruginous; many con acteristic and diagnostic fossils are shown in the type glomeratic layers. Weathers black ir and reference section plates, and complete fossil lists patches.______-___--_-----_ 10.0 Sandstone, yellowish-brown, moderately are included in the text. All section thicknesses are in cemented, crossbedded, coarse-grained; the metric system. Formation names and corres scattered quartz pebbles ______16.0 ponding place name spellings differ where the original 6. Sandstone (286.4 m thick): type locality name was later revised and (or) altered Sandstone, buff to yellow-brown, medium- through transliteration into the BGN/PCGN system. to coarse-grained, massive-bedded, irreg ularly weathering. ______8.7 The original formation name spellings are perpetuated Sandstone, white to light-gray, fine-grained, to avoid confusion in geologic nomenclature. slabby-weathering. ______2.0 Sandstone, pink-gray to buff, irregular TABUS FORMATION SECTIONS patches of purple, well-cemented; commor ironstone concretions in upper part. SECTION 1. Tabuk and vicinity Prominent bench at top_____------__- 2.5 [Tabuk type section description composited from several increments within an 85 km Sandstone, white, gray, and dark-brown; radius of Tabuk, measured and described in 1051 by R. A. Bramkamp, C. W. Brown, 8. J. Roach, and A. E. Clements] many irregular ironstone partings, com monly nodular. Forms prominent ledge. 19. 2 Sandstone of Aruma Formation (Upper Cretaceous). Sandstone, buff, gray, and purple, fine- Unconformity. grained, well-bedded; abundant tubular Tabuk Formation (top truncated): casts______-______-___--.- 144 Thickness 7. Tawil Sandstone Member (177.0 m thick): (.meters) Sandstone, gray to buff and purple, fine Sandstone, light-brown and red, crossbedded, grained; abundant tubular casts______20. 8 medium- to coarse-grained; fine-grained Sandstone, purple-gray to buff, semifriable, ferruginous matrix common and pebble massive; abundant vertical tubular casts, beds abundant, locally hematitic and pisolitic. 56. 0 large sacklike masses of buff well-cemented Sandstone, light-brown, strongly cross- bedded, medium- to coarse-grained, mod sandstone with concentric structure.... 45 erately cemented; occasional purple and Sandstone, gray and purple, well-bedded, light-gray shaly partings, locally contains fine-grained; many irregular, concentri a few quartz pebbles, considerable fine- cally banded masses of fine-grained sand pebble conglomerate in upper part__... 30.5 stone near middle.______--____------3.5 SEDIMENTARY GEOLOGY OF SAUDI ARABIA Elll SECTION 1. Tabuk and vicinity Continued SECTION 1. Tabuk and vicinity Continued Tabuk Formation (top truncated) Continued Tabuk Formation (top truncated) Continued 6. Sandstone (286.4 m thick) Continued (mete?*) 5. Shale and siltstone (290.5 m thick): Sandstone, pink to gray, fine-grained, well- Siltstone and sandstone; complexly alter cemented; many brown ferruginous part nating layers of olive-green to gray mica ings with irregular organic (?) markings ceous siltstone and buff to gray-brown, (tracks?)___.__._____.__....______8. 0 largely micaceous, well-bedded fine Sandstone, purple and gray, medium-bedded, grained sandstone.______34. 4 fine-grained; abundant tubular casts.-. _ _ 6. 4 Siltstone and shale; gray to brown to olive- Concealed..__-_._-___._-_.___-.___.._.. 12. 8 green, slightly micaceous siltstone and silty shale; few thin layers of brown Sandstone, gray, buff, and brown, well- ferruginous fine-grained sandstone, thin bedded, silty, in part micaceous, fine wavy bedding common______9. 1 grained; forms cliff capped by 1.0-m-thick Sandstone, purplish-brown to brown, fer ledge of resistant sandstone.__-_____-_._ 10. 8 ruginous, slightly micaceous, fine-grained. 1.0 Sandstone, buff to pale-green, in part Shale and siltstone; greenish-gray to olive- limonite-stained, friable, silty, fine gray, irregularly bedded, in part mica grained. ______1. 5 ceous, silty shale and siltstone; thin Sandstone, brown-weathering, ledges of layers of pinkish-brown to brownish-gray gray, in part micaceous, silty, fine-grained; fine-grained, slightly micaceous sandstone. ferruginous layers and splotches, rare Thin (0.5 m) sandstone bed at top forms worm(?) trails___.______13. 0 prominent bench.______35.6 Sandstone, tan to white and purple-mottled, Siltstone, greenish-gray, micaceous; subor moderately cemented, fine-grained; nu dinate shale and very fine grained mica merous short straight tubular root(?) ceous sandstone. Thin sandstone bed at casts. A brown-weathering, somewhat top forms prominent bench.______15.8 harder sandstone forms weak ledge at Sandstone, shale, and siltstone; complexly top__-__-_-_-______.__ 7. 1 interbedded layers of brown fine-grained Sandstone and siltstone, purple to olive- sandstone, olive-green micaceous silty green, shaly, fine-grained; forms slope. shale, and siltstone. Forms prominent Cruziana tracks in scree near middle_____ 32. 6 bench______4.4 Sandstone and siltstone; alternating thin Sandstone, brown, speckled, slightly cross- layers of purple, dark-red, greenish-gray, bedded, micaceous, fine-grained------0.3 light-gray fine-grained micaceous silty Shale, gray to olive-green and purple, strongly sandstone and finely sandy siltstone_____ 15. 0 micaceous, silty; few thin sandstone interbeds.. _ __-_____-_-____-__-_-.-_-_ 62.5 Sandstone and shale; olive-green micaceous Shale, siltstone, and sandstone; olive-gray silty sandstone and sandy shale______5. 2 to light-purple, strongly micaceous and Sandstone, purplish-gray, thinly bedded, silty shale and siltstone; thin ripple- fine-grained, micaceous..______0. 2 marked layers of micaceous fine-grained Concealed.______2. 1 sandstone.______--___------_-_____ 14.8 Shale and siltstone; medium-gray micaceous Siltstone and sandstone; complexly mixed silty shale and siltstone; thin beds and and thinly interbedded olive-green to irregular lenses purplish-brown sand gray micaceous siltstone and fine-grained stone. Upper 1.5 m thinly crossbedded sandstone; lower part includes some light- and in part ripple marked.______11. 7 gray and purple-weathering shale. En Sandstone, white to gray and dark-red, tire interval poorly exposed and may be fine-grained, silty, micaceous; rarely ripple slumped______17.5 marked and crossbedded.__..--__-_____ 23. 1 Sandstone, light-gray to dark-brown, thinly Sandstone, gray, fine-grained, silty, mica bedded, in part ripple-marked, fine ceous; platy to finely laminated beds 0.1 grained, ferruginous, micaceous, and silty; to 0.2 m thick in part separated by equally subordinate interbedded shale exposed as thin beds of olive-green micaceous silty a series of weak ledges.______4.0 shale.------_-_-______20. 0 Shale and siltstone; purple-gray and some greenish-gray, limonite-stained, strongly Sandstone, gray and red-brown, thinly micaceous silty shale and siltstone; thin, bedded, micaceous, silty, fine-grained; platy layers of buff to gray micaceous weak bench at top______5. 1 fine-grained sandstone with yellow stain Covered; fragments of dark-green scree ing in upper part______-___-_----- 34.8 suggest this interval largely shale with Shale, brown to gray, strongly flaky, silty minor sandstone______5. 1 and micaceous; poorly preserved grapto- Sandstone, olive-green to brown, ripple- lites at several levels_____---__--_---- 18.3 marked, micaceous, well-laminated, very Sandstone, brown, crossbedded, moderately fine grained; commonly iron stained. __._ 31. 1 cemented, fine-grained______-_---- 0.5 D112 GEOLOGY OF THE ARABIAN PENINSULA SECTION 1. Tabuk and vicinity Continued SECTION 1. Tabuk and vicinity Continued Tabuk Formation (top truncated) Continued Tabuk Formation (top truncated) Continued Thickness 5. Shale and siltstone (290.5 m thick) Continued 2. Sandstone (129.8 m thick) Continued (meters) Shale, purple and gray, silty, micaceous. _ _ _ 0. 6 Sandstone, gray to brown, moderately cross- Sandstone, brown, crossbedded, moderately bedded, commonly ripple-marked; thir cemented, fine-grained. ______0. 3 bedded with alternating hard and soft Shale, gray to brown and purple, in part layers. ______31.0 silty and micaceous; occasional irregular Shale, olive-green, in part micaceous anc1 platy beds of tan fine-grained sandstone. 24. 5 silty; increasing interbedded platy sand Sandstone and shale; light-brown to buff and stone toward top______7.0 yellow thin-bedded fine-grained sand 1. Hanadir member (12.2 m): stone; alternating layers of purple and Shale, purple and gray, thin-bedded, platy, gray silty shale. Weak sandstone bench in part calcareous; several ferruginous at top. ____--_____--______.__-.____. 8. 7 limestone layers. Abundant Didymograp- Sandstone, gray to brown, thin- bedded, tus protobifidus...... 12. 2 fine-grained; some purple and gray mica ceous silty shale _ - ______3. 4 Total thickness of Tabuk Formation_ 1071. 5 4. Sandstone (104.9 m thick): Sandstone, tan to brown, commonly black- Saq Sandstone. weathering, fine-grained, micaceous, in part silty; partings of olive-green shale SECTION 2. Tayma' to Al Huj common. _-_-__-__--_-____-_____-____ 72. 1 [Generalized section of Tabuk Formation from Tayma' to Al Huj, N. M. Layne, Jr. Sandstone, tan to brown, ripple-marked, and D.O. Reese, 1959] fine-grained, micaceous, silty, thin- bedded; exposed as series of thin sandstone ledges Siltstone and limestone of Jauf Formation (Lower followed by poorly exposed slopes of soft Devonian). Thickness sandstone. ______32. 8 Tabuk Formation: (meters) 3. Poorly exposed (70.7 m thick) : 5. Tawil Sandstone Member: Sandstone, buff to Poorly exposed; few coherent sandstone beds red-brown, weathering black and dark-brown form thin resistant ledges separated by medium- to coarse-grained, commonly gravel- scree-covered slopes; digging indicates bearing; conspicuous corssbedding or graded covered intervals mostly brown and pur bedding throughout. Layers of ironstone at ple micaceous silty shale with occasional base and several higher levels. Upper beds usu thin beds of platy siltstone. _____--_.___ 63. 8 ally form cliffs or hills.-..------_..._ 200.0 Shale, dull-gray to brown, white-weathering, 4. Upper pre-Tawil sandstone: Sandstone, gray to silty; Diplograptus 4 m above base of unit. brown, fine- to medium-grained, in part cross- Interval considerably disturbed; probably bedded; vertical tubular structures (Scolithus) slumped ______5. 2 common. Much of interval is poorly exposed Concealed. ______1. 7 and thickness of covered sections computed __ 196. 0 2. Sandstone (129.8 m thick): 3. Upper siltstone: Siltstone, dominantly gray, Sandstone, brown, black-weathering, gray-green and buff; Scolithus common. Silt- medium- to coarse-grained, moderately stone is complexly interbedded with subor cemented; some quartz pebbles. Interval dinate sandstone and shale. Sandstone p not measured; thickness estimated...... 35. 0 variously red, brown, gray, or tan, fine grained, Sandstone, pink, fine- to medium-grained, thin bedded, ferruginous, silty, micaceous, and moderately cemented, crossbedded; con flaggy weathering, with some ripple marks. glomeratic layers with quartz, pink lime The shale is olive green, gray, brown, and stone, and shale pebbles ______2. 1 micaceous.______-____----_------346. 0 Shale and sandstone; red to purple and white, 2. Lower sandstone: Sandstone, gray, brown, red- thinly interbedded silty shale and light- brown, fine- to medium-grained, thin-bedded; gray fine-grained, in part calcareous, mica Scolithus common, occasional beds of gray and ceous sandstone; poorly exposed..---. 20.8 greenish-gray shale and siltstone-______222. 0 Siltstone, pearl-gray, shaly, in part mica 1. Lower shale (Hanadir member): Shale, olive- ceous., -_-_--_____._.-___-_-__-______2. 0 gray to reddish-brown, micaceous and silty; thin interbeds of reddish-brown, fine-grained, Sandstone, pink and tan, moderately ce silty and micaceous sandstone common. Didy- mented, in part crossbedded __-_-_.___._ 2. 9 mograptus protobifidus common ______60. 0 Sandstone, off-white to gray, brown- weathering; massively bedded in lower Total thickness of Tabuk Formation__.._ 1024. 0 part and strongly crossbedded in upper part. Forms promient ledge ______29. 0 Saq Sandstone. SEDIMENTARY GEOLOGY OF SAUDI ARABIA E113 SECTION 3. Qn§aybd' SECTION 4. Jildlyah to Ash Shu'aybah Continued [Generalized section of Tabuk Formation near lat of Qujayba' measured by A. F, Tabuk Formation Continued Pocock and R. P. Kopp, 1949, and N. M. Layne, Jr., and H. W. Blodget, 1968] 3. Upper shale (Qusayba' member): Shale, in part unexposed, tan, gray, greenish-gray, commonly Limestone of Khuff Formation (Upper Permian). scricitic and gypsiferous; interbedded with gray Unconformity. fine-grained thin-bedded sandstone, a few silt- , , T, ^ Thickness Tabuk Formation: (meters) stone stringers, and very thin, rare and impure 6. Upper sandstone: Sandstone, brown, fine- to limestone lenses. ______125. 9 medium-grained, thin-bedded; commonly ripple 2. Middle and lower sandstone: Sandstone, brown marked. A yellow-gray gypsifcrous soft shale and yellow-white, medium- to coarse-grained in bed with stringers of gray griable fine-grained upper part, red to purple and fine-grained in crossbcddcd sandstone and red and yellow lower part; occasionally crossbeddcd and friable crossbcddcd sandstone caps the unit- nodular weathering. Thin scricitic siltstone May include beds equivalent to Tawil Sandstone layers in middle and lower part common. _____ 3^5. 2 Member.-______-__-_____------______- 32. 0 i. Lower shale (Hanadir member): Shale, in part 5. Upper shale (Qusayba' member): Shale, vari poorly exposed, purple to olive-green, brown, colored, but mostly gray-green, laminated; more red, silty, sericitic; graptolitic in basal beds. gypsifcrous and with thin beds of rod, hematitic Several thin brown fine-grained, in part gypsi siltstone in upper part; lower part is graptolitic. fcrous, sandstone beds ______68. 5 Poorly sorted gray medium-grained thin-bedded Total thickness of Tabuk Formation._____._ 714. 8 sandstone and highly weathered calcareous beds Saq Sandstone. at top of unit.______57. 4 4. Middle sandstone: Sandstone, light-red and pale- JAUF FORMATION SECTIONS yellow, medium-grained, distinctively cross- SECTION 5. Jauf and vicinity bedded, mostly well-sorted; nodular weathering; few very thin dark scrcitic shale and silty beds [Jauf Formation type section measured and described by A. F. Pocock, S. Bevera, and near middle. Duricrust locally covers expo II. W. Schneider in 1950 in Al Jawf and vicinity] sures, ___--_-_-_-----______241. 5 Sandstone of Wasia (Sakaka) Formation (Middle 3. Middle shale (Ila'an member): Shale, gray to Cretaceous) . greenish-gray, scricitic; thin interbcds of silt, Uncomformity. silty sandstone, and fine-grained sandstone. Jauf Formation: Several red-brown to gray-brown fine-grained 4. Upper elastics (transition beds, 27.1 in thick): well-sorted sandstone beds are near top, and Sandstone, reddish-tan, very fine grained, greenish-gray to yellow-gray scricitic shale poorly bedded; zones of thinly laminated caps the member. Graptolitcs at basc______67. 0 sericitic sandstone. __..______2. 0 2. Lower sandstone: Sandstone, light-colored (yel Siltstone and silty shale; brown siltstone and low, white, pink), medium- to fine-grained, gray-green to gray silty shale; occasional crossbeddcd; occasional scricitic beds and some laminations of marl and a few thin layers of nodular weathering. Sericitic siltstone in lower platy silty limestone in middle part______10. 7 part______220. 2 Dolomite, reddish-tan, dense, very thin bed 1. Lower shale (Hanadir member): Shale, greenish- ded, microcrystallinc to saccharoidal. _ _ _ _ 0. 1 gray to yellow, in part gypsiferous and sericitic; Sandstone and siltstone, thin, alternating intcrbcdded brown and red-brown fine-grained beds.. ___..__. ..___.----....____- . 5. 0 scricitic sandstone and some well-laminated Sandstone, fine-grained, micaceous; becomes siltstono toward the bottom. Graptolitcs in siltstone at base _ . ______2. 1 lower part______59. 1 Limestone, gray-tan, thin-bedded, platy, impure _ - ______0. 2 Total thickness of Tabuk Formation______677. 2 Sandstone, light-gray to tan, fine-grained, Saq Sandstone. well-sorted, subangular, friable, commonly crossbedded; includes some gray and tan SECTION 4. Jildlyah to Ash Shu'aybah sericitic and silty shale. Interval shows rapid lateral lithologic changc______7. 0 [Generalized section of Tabuk Formation from Jildlyah to Ash Shu'aybah measured 3. Upper limestone (Hammamiyat member, 106.3 m by A. F. Pocock and R. P. Kopp, 1949] thick) : Limestone, gray-tan, detrital; poorly exposed Limestone of Jauf Formation (Lower Devonian). Thickness on beveled dip slope. Entire interval thin Tabuk Formation: (meters) bedded. Some greenish-yellow fine-grained 4. Upper sandstone: Sandstone, poorly exposed, limonite-stained, slightly silty limestone. _. 7. 6 light-brown and gray to white, fine- to medium- Limestone, gray, medium-bedded, finely po grained, mostly crossbedded; greenish-gray rous, in part impure ; weathers to steel gray. 1. 9 thin-bedded scricitic shale near top. May be Siltstone, mustard-yellow, poorly exposed. _ _ 1. 5 all or in part equivalent to Tawil Sandstone Shale, gray, silty; occasional thin beds of red Member.- -_-_---______-____-______125. 2 dish-tan fine-grained sandstone. ______5. 5 D114 GEOLOGY OF THE ARABIAN PENINSULA SECTION 5. Javf and vicinity Continued SECTION 5. Jauf and vicinity Continued Jauf Formation Continued Jauf Formation Continued 3. Upper limestone (Hammamiyat member, 2. Middle shale (Subbat member, 113.4 m thick) Cnntinnpd Thickness 106.3 m thick) Continued ^ommuea. (meters) Sandstone, reddish-tan, fine-grained, semi- Sandstone, light-brown, fine-grained, well- friable, sericitic, highly lenticular. ______2. 3 sorted, friable, crossbedded to massive____ 3. 8 Shale, gray to grayish-green, silty; subordi Shale, gray, silty; common maroon hematitic nate gray fine-grained sandstone and silt- partings. Thin interbeds of brown impure stone- ______10. 7 limestone and fine-grained sandstone__ _- 7. 1 Limestone, gray to tan, thinly laminated, Shale, sandstone and siltstone; alternators platy, impure; grades upward to siltstone__ 8. 7 laminations of gray-green, slightly silty Silt, white to light-greenish-yellow; poorly shale and fine-grained flaky micaceous exposed. __-____---_-_--______6. 3 sandstone and siltstone; interval capped Dolomite, grayish-tan, saccharoidal; thin in- by 1.0-EJa-thick bed of tan, moderately terbeds of slate-gray lithographic limestone hard, impure limestone____----_-_----_ 3. 7 and tan mottled detrital limestone ______0. 7 Sandstone, tan, thin-bedded, fine-graine'l, Limestone, greenish- tan, dense, silty. ______0. 8 friable. _.______----- 0.5 Siltstone and shale; interval includes equal Shale, grayish-green, slightly silty; common amounts of tan thin-bedded sericitic silt- hematitic partings.._-__----_-_----_---- 3. 5 stone and grayish-green silty shale___----_ 7. 0 Sandstone and shale; gray-green silty shale Limestone and siltstone; complexly interbed- overlain by white soft fine-grained sand ded tan marly limestone and gray sericitic stone- _------_-_------___-----_-_----- 2. 1 siltstone; occasional thin beds of cream- Shale and siltstone; gray-green silty shale and colored detrital limestone and fine-grained siltstone; interval capped by a 10-m-thiok sericitic sandstone. ______7.0 bed of gray-brown, very fine-grained, Limestone, yellow to gray-tan, dense, very platy sandstone-_----___--_---_--__---- 17. 6 finely crystalline to microcry stalline ; subor Shale, gray and maroon, very silty ______21. 0 dinate silty limestone. ______1.7 Sandstone, white, fine-grained, well-rounded, Shale, light-greenish-gray, thinly laminated, well-sorted, friable______---_------0.3 marly; thin beds of gray- tan silty, finely Shale, bands of maroon and light-gray, finely detrital limestone near top .__--______.__ 1.5 silty; weathers to nodules. Lower 3.5 m Shale, gray-green, silty; rare laminations of poorly exposed______--_____----- 28. 5 reddish-tan, slightly platy siltstone. Forms 1. Lower limestone and shale (Qasr member, 52.4 m laterally persistent shale zone. ______11. 1 thick): Siltstone, cream to gray, marly ______1.7 Limestone, grayish-tan, dense, thin-bedded, Limestone and siltstone, tan, thin-bedded, fossiliferous; abundant fossil casts give dense limestone; an approximately equal weathered surface pisolitic appearance, amount of yellow-tan thin-bedded siltstone. upper 3.4 ra poorly exposed in dip slopes.- 7. 2 Poorly exposed __ . ______5. 9 Covered; probably yellow, marly siltstone- _ - 4. 0 Limestone, tan (gray-mottled), dense, thin- Limestone and siltstone; yellow-tan thin- bedded, very finely crystalline to micro- bedded impure limestone; poorly exposed crystalline; in part slightly silty. -.______. 6. 5 interbedded soft siltstone ___-______-_- 2.5 Siltstone, cream-colored to gray; capped by Limestone, gray-tan, thin-bedded; poorly 0.5 m of tan and white-speckled fine-grained exposed subordinate yellow-tan platy siTy sandstone. ___--_------_-______10. 5 limestone. ______-_--__----- 2.0 Limestone, yellow to gray-tan, dense, tliin- Limestone, gray-tan, weathering to steel-grsy, bedded, in part impure; lower part poorly commonly thin-bedded; abundant simll exposed. ______4. 7 casts give weathered surface pebble-grained Limestone, brownish-tan, thin- to medium- appearance. Small reef buildups common bedded, dense, finely crystalline; thin bed in this interval..______-----_------2.1 of mustard-tan, slightly silty limestone at Limestone, gray-tan, dense, thin-bedded; 2. 7 abundant weathered gastropod and brachi- 2. Middle shale (Subbat member, 113.4 m thick): opod casts in upper part-_____-__---_--- 1.0 Siltstone, creamish-tan, thinly laminated, Siltstone, green-tan, thin-bedded, platy, sericitic- _-_-____---_-______--__--_____ 0. 2 marly; grades upward into tan, very fine Sandstone, brown, fine-grained, friable; grades fine crystalline marl-______---____--_-- 2. 1 Shale, gray and some maroon, silty; grading upward into brown, thinly laminated, upward to tan silt at top.___-----__----- 6. 0 platy, impure limestone. ______2.2 Shale and siltstone; blue-green shale grading Sandstone, gray-brown, thin-bedded, friable, upward to brown thin-bedded marly si't- fine-grained, ______5. 2 stone___-___-_-___----_--__------5. 2 Shale, gray-green, silty; subordinate maroon Sandstone, pinkish-tan (black-mottled), very and gray-brown siltstone, occasional part fine-grained, well-cemented; grades later ings of fine-grained sandstone. ______17. 7 ally to calcareous siltstone______0. 3 SEDIMENTARY GEOLOGY OF SAUDI ARABIA D115 SECTION 5. Jauf and vicinity Continued KHUFF FORMATION SECTIONS Jauf Formation Continued SECTION 7. Ar Rayn [Khuff Formation reference section measured and described in the vicinity of Ar 1. Lower limestone and shale (Qasr member, 52.4 Rayn by R. A. Bramkamp, E. L. Berg, R. L. Myers, and W. T. Short in I'MS] m thick) Continued Shale (Upper Permian) of Sudair Shale. Thic^nest Shale, gray-green; capped by a thin bed of Khuff Formation: tan marl.______5. 3 4. Aphanitic and calcarenitic limestone (28.2 m thick): Siltstone, light-tan, thin-bedded, slightly Dolomite, tan to brownish-red, blocky-weath- marly______1. 2 ering, medium-grained, crystalline; lo Shale, gray and maroon, silty______4.0 cally grades to strongly recrystallized, Sandstone, brown (black-mottled), fine fine-grained calcarenite. Forms prominent grained, moderately hard, well-sorted, bench______2. 0 block-weathering.______0. 3 Limestone, yellow, limonite-stained, marly, Shale, tan to gray; subordinate thin beds of fine-grained; a few bedding planes in upper tan platy impure limestone. Shale be part with abundant small clams on weath comes more silty toward top. Capped by ered surface__-__------_ -_--__-______9. 2 thin bed of pink-weathering platy impure Limestone, medium-grained crystalline; many limestone______7. 2 limonitic stains and limestone pebbles and Shale, gray and maroon; capped by 4-cm- fragments locally common______0. 4 thick bed of platy impure limestone._____ 2. 0 Marl, yellow, limonite-stained, moderately indurated; in part with questionable molds Total thickness of Jauf Formation.... 299.2 of small clams.______-______---__- 1. 7 Tabuk Formation (Tawil Sandstone Member). Limestone, white, finely crystalline; weathers to heavy ledges with ellipsoidal boulders. SECTION 6. Al Halwat Basal 0.5 m sandy_------_--_-_-__ 12. 2 [Generalized section of Jauf Formation at Al Ifalwat after N. M. Layne, Jr., and Covered______1. 0 D. O. Reese, 1959] Limestone, golden-brown to gray, chippy- to Sandstone of Wasia (Sakaka) Formation (Middle Creta blocky-weathering, tight, finely crystalline; ceous) . brachiopods locally common.______1.7 Unconformity. 3. Aphanitic limestone (71.1 m thick): Jauf Formation: Limestone, tan to light-gray, rubbly-weather 3. Upper limestone (Hammamiyat member, 61.5 m ing, compact, fine-grained; upper part thick): massive and forms moderate bench._____ 4. 1 Shale, dominant, olive and purple, often Poorly exposed; probably marl or marly lime gypsiferous; subordinate interbedded gray stone. ____-_-_-_-__---__--___-_-_-__-_ 3. 5 and tan-brown medium-grained cross- Dolomite, light-gray to tan, compact, finely bedded friable sandstone. Single 5.0-m crystalline___.______-_--______--__ 1. 0 thick bed of tan, finely crystalline, partially Limestone, brown, porous-weathering, fine dolomitized, calcarenitic limestone forms grained; forms weak bench______0. 1 ledjje at top of interval..-______41. 7 Covered; much gypsum in weathered zone.- 0. 3 Limestone, mostly calcarenitic, gray and Limestone, gray-white, rubbly-weathering, reddish-brown, in part impure, partially fine-grained______0. 1 dolomitized; subordinate thin interbeds of Covered; probably shale; much gypsum in brown medium-grained, slightly micaceous, weathered zone.______-___-_--____- 0. 8 somewhat calcareous sandstone ______19. 8 Limestone, white, fine-grained.______0. 2 2. Middle shale (Subbat member) : Limestone and shale; complexly interbedded Shale, green and olive, reddish-brown and white to brown rubbly-weathering fine maroon, commonly gypsiferous; thin inter grained limestone and olive-green shale; beds of gray to brown medium-grained shale intervals poorly exposed with much friable, often silty sandstone. A 1.7-m- gypsum in weathered zone______2. 7 thick bed of off-white to cream impure Dolomite, off-white, marly, finely crystalline. 2. 8 blocky-weathering limestone near base_.__ 106. 7 Limestone, light-gray to brown, rubbly- to 1. Lower limestone and shale (Qasr member): slabby-weathering, compact, fine-grained; Shale, yellow, gray, and purple, chippy- many large limestone pebbles and cobbles weathering, in part silty; banded gypsum (intraformational conglomerate) and a common in lower part. Several thin beds few small subspherical siliceous bodies in of tan impure, in part fossiliferous lime upper 1.0m.______6. 0 stone and gray dolomite. Single thin bed Dolomite, gray-tan, finely crystalline; many of brown to black sandy gypsiferous iron subspherical siliceous bodies, in part solid, stone at base.______38. 7 and in part miniature geodes.______0. 3 Limestone, light-gray to brown, rubbly- Total thickness of Jauf Formation_... 206. 9 weathering, poorly exposed, compact, fine Tabuk Formation (Tawil Sandstone Member). grained; thin interbeds off-white soft marL 6. 1 D116 GEOLOGY OF THE ARABIAN PENINSULA SECTION 7. Ar Rayn Continued SECTION 7. Ar Rayn Continued Khuff Formation Continued Khuff Formation Continued 3. Aphanitic limestone (71.1 m thick) Continued 1. Dolomite and shale (38.4 m thick) Continued Limestone, light-gray to brown, rubbly- Dolomite, off-white to light-gray, rubbly- weathering, compact, very finely crystal weathering, thin-bedded, compact, finely line, partially dolomitized______1.2 crystalline; many veins of clear prismatic Poorly exposed ; probably shale ______6. 0 calcite in lower part. ______8. 4 Limestone and dolomite, tan, poorly exposed, Shale, olive-green, gypsif erous ______5. 0 rubbly- weathering, fine-grained limestone; some marly-weathering, finely crystalline Dolomite, gray, tight, finely crystalline; r_re dolomite_------_---______2. 7 chert near top. ______1.3 Limestone, off-white to gray, rubbly-weath- Limestone and dolomite; tan to gray thick- ering, fine-grained; several thin oolitic bedded compact fine-grained limestone layers. Interval capped by thin oolite- complexly interbedded with gray-brown, coquina beds. ______1. 7 finely crystalline dolomite; several resist Limestone, off-white to gray, soft, rubbly- ant beds with rare indeterminate sim.ll weathering, fine-grained; commonly marly. 24. 0 organic remains. ______8. 8 Shale, gray-green, partly covered. ______5. 1 Shale, poorly exposed, tan, marly, gypsif erous. 10. 7 Limestone, light-tan to light-gray, marly- Conglomerate, pink and white, moderately weathering, compact, partially dolomitized, cemented, poorly sorted, angular granitic finely crystalline. ______0. 8 sand and fine conglomerate. ______1.5 Shale, olive-green, poorly exposed ______1.6 2. Dolomite and limestone (33.7 m thick) : Total thickness of Khuff Formation. _ _ _ 171. 4 Coquina, tan to gray, porous, partially re- Nonconformity. crystallized; abundant indeterminate small Granite of basement complex. fossils _ -_.____-______0. 2 Dolomite, tan, soft, marly, finely crystalline __ 1. 6 SECTION 8. Wadl Maghib Limestone, light-tan to gray, compact, fine grained. ______---____-____.____-____-_ 0. 3 [Generalized section of Khuff Formation in vicinity of W5di Maghib (Vype section Dolomite, tan, light-gray-weathering, soft, locality) measured by D. A. Holm and A. F. Pocock, 1948] marly, finely crystalline; rare fragments of silicified wood on surface, none definitely Shale (Upper Permian) of Sudair Shale in place. ______6. 0 *r, ,- -n ,. Thickness Limestone, gray, flaggy-weathering, compact, Khuff Formation: (meters) 3. Upper limestone (Khartam member): Limestone, fine-grained ______1. 0 Limestone, tan, soft, marly, partially dolo tan, commonly sandy; grades upward into gr>y mitized, fine-grained; a few thin slabby to tan platy limestone with common oolitic gray-brown beds. Indeterminate nauti- texture. Substantial proportion of interval believed to contain marl although softer rock loid at base ______1. 7 Calcarenite, gray-brown, thin-bedded, strongly types are not cleanly exposed-_-______-_-___ 25. 0 cemented, very fine to fine-grained, oo- 2. Middle shale (Midhnab member): Shale, gray-bHe lite(?) ; forms moderate bench______0. 8 to tan, plastic, often delicately banded, gypsif- Shale, olive-green, poorly exposed; many thin erous; subordinate marly limestone and bedd SECTION 9. Near lat 26°SO' N. SECTION 11. Al 'Arid escarpment Continued [Generalized section of Khufl Formation north of type locality near lat 26°30' N. Khuff Formation Continued 1$S£ff measured by A. F. Pocock and R. P. Kopp, 1949] 2. Middle Khuff: Sandstone, white to partly pink and red, massive, fine- to medium-grained; mid Sand and shale (Upper Permian) of Sudair Shale. Thickness Khuff Formation: (meters') dle part of interval is complexly interbedded 3. Upper limestone (Khartam member): Limestone, dark-brick-red silty shale and white silty sand light-brown to gray, poorly exposed; in part stone. ______50.0 oolitic and fossiliferous. Basal part is yellow 1. Lower Khuff: Sandstone, white, gray- and red- to off-white soft porous, rarely oolitic, impure mottled, poorly exposed, in part crossbedded; limestone thinly interbedded with platy crystal some lenses of red and green shale and fossil line limestone.._--__-______.___..__ 81. 5 wood.______56. 0 2. Middle shale (Midhnab member): Upper part is poorly exposed yellow gypsiferous shale and Total thickness of Khuff Formation.______154. 0 siltstone complexly interbedded with fine- to Wajid Sandstone. medium-grained friable sandstone with fine, grained ooliths. Light-gray friable, highly gypsiferous shale and gray subrounded, mod SUDAIR SHALE SECTION erately hard, thin-bedded fine-grained sandstone occur at the base. Lower part is covered by SECTION 12. Al 'Arid duricrust, but probably consists of gray to green, [Sudair Shale type section composited from several measurements made by R. D. finely laminated shale with thinly interbedded Gierhart and L. D. Owens in 1948 and R. D. Gierhart and R. A. Bramkamp in gypsum and marl.______70. 2 1960 along the Al 'Arid escarpment at Khashm Ghudayy and Khashn Abu 1. Lower limestone (Khuff member): Limestone, Ramadah] gray to reddish-gray, hard, microcrystalline; Sandstone of the Dhruma Formation (Middle Jurassic). gray to cream medium-grained, moderately Unconformity. Th'ckness sorted subrounded soft sandstone occurs near Sudair Shale: (waters) the bottom; base of interval unexposed______140. 5 Shale, brick-red, platy, indurated; forms weak cliff_. 2. 4 Total thickness of Khuff Formation. ______292. 2 Shale, alternating brick-red and gray.______-_-_--_ 22. 6 Unconformity. Shale, poorly exposed, apparently brick-red as above. 3. 4 Tabuk Formation. Shale, gray, very sandy; forms weak cliff.______1.7 Shale, alternating brick-red and gray, sandy ___ 1. 7 SECTION 10. Al Mulayh Shale, poorly exposed, probably brick-red as above. _ 34. 0 [Generalized section of Khuff Formation near Al Mulayh (lat 22°36' N.) measured Siltstone and limestone; complexly interbedded light- by R. D. Gierhart and L. F. Ramirez, 1949] gray, strongly platy, slightly micaceous, calcareous Shale (Upper Permian) of Sudair Shale. Thickness siltstone and off-white argillaceous and silty lime Khuff Formation: (meters) stone; forms moderate ledge.-___-___---_------0. 6 2. Upper dolomite: Dolomite, dominant, gray to Shale, dark-brick-red._--_------_-___- 0.8 brown and red, thin-bedded, commonly platy- weathering, occasionally shelly, in part gypsif Siltstone, off-white to light-gray, platy, slightly mica erous; subordinate interbeds of light-colored ceous, calcareous.__.__-__-______--__------_ 0. 6 soft marly limestone, red and green gypsiferous Shale, dark-brick-red._____.__--___-__---_------_ 1.6 shale, red to brown siltstone, and off-white to Siltstone and shale; greenish-gray, thinly platy, brown friable fine-grained sandstone. ______103.0 slightly micaceous, calcareous siltstone and 1. Lower limestone: Limestone, off-white to gray and greenish-white silty shale.______1.5 brown, soft, marly, rubbly-weathering; some in Shale, dark-brick-red; thin layers of greenish-white terbedded red and green shale. Single bed of calcareous shale 1.0 and 2.4 m below top _... 8. 6 dark-gray fine-grained platy-weathering sand Shale, greenish-white, calcareous.______0. 3 stone exposed near base. Nearly one-half of Shale, dark-brick-red; a few 1- to 2-cm layers of interval covered and poorly exposed.______121.0 greenish-white silty shale.______24. 1 Total thickness of Khuff Formation. _-___._ 224. 0 Shale, alternating greenish-white and dark-brick-red; Basement complex. several thin beds of silty friable sandstone. ______3. 5 Shale, brick-red..______1. 1 SECTION 11. Al 'Arid escarpment Shale, alternating greenish-white and red______0. 3 [ Generalized section of Khuff Formation exposed in Al 'Arid escarpment at Shale, brick-red, massive; silty and sandy near base. Khashum Sudayr and Jabal Umm Ghiran. Upper and middle Khuff measured Several bands of green shale. _------6. 2 by S. B. Henry and R. A. Bramkamp, 1950; lower Khuff by R. D. Gierhart and L. D. Owens, 1948] Shale and sandstone; thinly interlaminated greenish- white silty and sandy shale and pink to white fine Shale (Upper Permian) of Sudair Shale. Thickness Khuff Formation: (meters) grained sandstone______1.0 3. Upper Khuff: Sandstone, dominant, white, massive, medium- to coarse-grained; subordi Total thickness of Sudair Shale. 116.0 nate red and green silty shale____------_-__- 48. 0 Khuff Formation. D118 GEOLOGY OF THE ARABIAN PENINSULA JILH FORMATION SECTIONS SECTION 13. Jilh al 'Ishar Continued SECTION 13. Jilh al 1 Ishar Upper Jibl Formation (184.0 m Thick) Continued 4. Sandstone (117.8 m thick) Continued [Jilh Formation type section measured and described across the Jilh al 'Ishar escarp Limestone, brown to golden-brown, slabbj*- ment by M. Steineke, E. L. Berg, R. L. Myers, and W. T. Short in 1945] to blocky-weathering, tight; forms promi Sandstone of the Minjur Sandstone (Upper Triassic). Thicknas nent dip slope. _-___----_--_-______-__ 2. 0 Upper Jilh Formation (184.0 m thick): (metert) Siltstone and shale. _____-----_--__-_-__-_ 2. 0 5. Sandstone, siltstone, and shale (66.2 m thick): Sandstone, gray to tan, friable; small con Limestone, yellow-brown, thinly bedded, cretions and molds of fossil wood. ______11.4 slabby-weathering, fine-grained ; alternating Siltstone and mudstone, greenish-gray; thin with yellow marly limestone. Rare marine limestone bed near middle. ____-.______-_ 3. 5 fossils ______4. 0 Sandstone, gray, friable, fine-grained, cross Shale, yellow; thin streaks of yellow platy bedded ______3. 4 limestone and soft marl. Forms distinc Shale____----_------_------__---- 2. 0 tive marker bed______3. 4 Sandstone, gray, blocky-weathering __ __.__ 3. 6 Covered ; probably shale. ______3. 0 Shale, variegated; grades to red at base and Limestone, gray to dark-brown, sandy; top includes some sands tone____ . ______6. 8 forms prominent dip slope.-. ______2. 0 Sandstone, dark-brown, tightly cemented, Sandstone, friable, fine-grained; common thin fine* to medium-grained, crossbedded; stringers of siltstone. ______2. 0 upper 2.0 m calcareous, forms ledge ard Sandstone, gray, crossbedded, fine- to prominent dip slope. ______12. 2 medium-grained, very friable. ______0. 8 Covered; probably shale. ______3. 0 Covered; probably silt and fine-grained Sandstone, tan, well-cemented, fine-grained; sandstone. ______3. 4 numerous small concretions ______0. 5 Limestone, gray to tan, blocky-weathering, Claystone, yellow_____ .__-_-_____-___- 2. 0 tight______0.3 Sandstone, tan, well-cemented. ______1. 0 Covered ; probably silt and shale. -_-____.__ 8. 0 Shale, olive-green. ____._-_-_-__-__--__--_ 2. 1 Sandstone, fine-grained, in part calcareous. _ 2. 0 Sandstone, tan, fine-grained; occasional thin Covered ; probably silt and shale. -___--____ 3. 4 interbeds of siltstone ______3. 4 Limestone, golden-brown, thinly bedded, tight. 3. 0 Lower Jilh Formation (142.1 m thick): Sandstone, fine-grained; very thin interbeds 3. Limestone and marl (35.7 m thick) : of silt, shale, and golden-brown limestone.- 4. 0 Limestone, tan to brown, tight, brittle, Limestone, golden-brown, tight, slabby- blocky-weathering, fossiliferous; thin bed weathering- -_-__--_.______-___ 0. 5 of calcareous sandstone at top. Caps Shale. -----_-_-_------_._-___--_--._.._. 1.0 escarpment and forms dip slope. ______1.0 Limestone, golden-brown, sandy _.-.______0. 5 Marl, yellow; some shale and rare fossils. ... 5. 0 Siltstone and shale, green to purple, soft, com Limestone, light-brown, tight, blocky- ard plexly interbedded; forms prominent ledge. 2. 0 slabby-weathering; some fossil casts. Covered ; probably shale. ______3. 4 Forms prominent dip slope east of main Limestone, golden-brown, thinly bedded; scarp___--_------_------__---_- 7. 0 upper 0.5 m caps escarpment- ______1. 5 Limestone, yellowish-brown, tight, slabby- Shale, green; subordinate siltstone and some weathering, fine-grained; forms top of JFh soft sandstone. ---__--______.__:__ 3. 5 escarpment. Lower 1.0m sandy. ______3. 7 Sandstone, gray to brown, massive, cross- Sandstone, reddish-brown, in part marl", bedded, medium-grained; occasional lenses commonly crossbedded, fine- to mediuri- of coarse-grained sandstone and gravel. ... 2. 5 grained. ______---__-_---__ 8. 8 Marl and marly limestone. ______4. 0 Siltstone, greenish-gray, soft, friable; some Sandstone, dark-brown, fine-grained, mica shale and sandstone. Forms face of ceous; forms weak ledge______-____--_-_ 0. 3 escarpment. ______4. 0 Marl; thinly interbedded marl and marly Sandstone, light-brown, crossbedded, limestone ______---__----__- 3. 9 medium-grained. ______1. 0 Limestone; alternating gray tight fine-grained Shale, green, commonly sandy ______1.5 limestone and soft shaly limestone; occa Sandstone, thinly bedded, fine-grained, platy. 0. 5 sional bone fragments______2. 0 Shale, green, silty; common partings of 2. Shale (25.6 m thick): purple and gray sandy shale. ______5. 0 Shale, olive-green______._____-___----_- 4. 1 4. Sandstone (117.8 m thick): Limestone, gray-brown, fine-grained ______0. 3 Sandstone, brownish-black, crossbedded, Shale, gray-green______--__--- 3.9 medium-grained; many black iron-stained Limestone, gray-brown, tight, blocky- concretions in marblelike clusters and weathering; fossil casts. Forms wei.k wood fragments throughout. Forms wide bench ______--__--__----_--- 0. 3 valley __ _._..._..______-______._ 50. 0 Shale, green, gypsiferous-_--_----_------3. 4 Sandstone, brownish-black, medium-grained, Gypsum, white to red; streaks of red litre- friable; contains hard ledges. Forms stone__--__-_--_-_--_------0. 5 scattered black buttes. ______9. 5 Shale, green, very gypsiferous ______3. 4 SEDIMENTARY GEOLOGY OF SAUDI ARABIA E119 SECTION 13. Jilh al 'Ishar Continued SECTION 14. Khashm al Manjur Continued Thickness Lower Jilh Formation (142.1 m thick) Continued (metert) Jilh Formation Continued 2. Shale (25.6 m thick) Continued Sandstone, buff to brown, crossbedded, fine- to coarse Shale, green, gypsiferous______1. 7 grained; local layers of ironstone. Upper part Limestone, brown-stained, tight, blocky- strongly calcareous and slabby weathering; middle weathering; forms weak bench______0. 2 part poorly exposed, probably includes silty sand Shale, green, gypsiferous.______2. 2 stone and sandy shale. ______38. 5 Sandstone, well-bedded, fine-grained; base Sandstone, buff to brown, strongly crossbedded, fine- ripple marked______0. 7 to coarse-grained; many well-laminated limonitic Shale, dark-green; some fine-grained sand lenticles and platy masses of black ironstone. stone. Upper 0.5 , gypsiferous______3. 2 Layers with concentrations of quartz pebbles Limestone, brown; fossil casts. Forms prom common. ____-__---_-______---______.____ 27. 0 inent bench______0. 4 Dolomite, golden-tan, strongly ocherous and sandy, Limestone, soft, marly-______1.0 finely crystalline; forms weak bench... ______0. 5 Limestone, brown; fossil casts______0. 3 Sandstone, light-brown, massive, friable, medium- 1. Sandstone and shale (80.8 m thick): grained; common interbeds of red and green, Shale, light-green______3. 6 gypsiferous shale and silty sandstone. Few beds Sandstone, light-gray, friable, fine-grained __ 3. 4 are poorly exposed or covered______.______22.0 Sandstone, iron-stained, ripple-marked, Sandstone, light-brown, crossbedded, medium- to slabby-weathering, fine-grained; small con coarse-grained; few streaks with common quartz cretions common. ______40 pebbles up to 8mm in diameter. Upper 1.0 m Shale; reddish-brown shale and sandy shale._ 1. 7 irregularly stained red. Thin bed of red and green Sandstone, fine-grained.______1. 5 gypsiferous shale near base. ______29. 0 Shale and sandstone; thinly interbedded green Covered ; thickness calculated .-_-______.-______25. 5 shale, finely sandy shale, and fine-grained Sandstone, light-brown to off-white, fine- to medium- sandstone______.______3. 5 grained, crossbedded; exposed in pediment and Shale, light-green, sandy______1. 5 thickness calculated _ _ .______-_--___-_ 12. 0 Sandstone, tightly cemented, medium- Covered; thickness calculated. A few 0.5-m beds of grained______-___-____-____-___ 0. 1 gray to tan, purple-weathering, commonly sandy, Shale, dark-green______1. 2 tight, finely crystalline limestone form very weak Sandstone, soft, shaly______-______- 0. 5 cuestas. Remainder of interval unexposed except Sandstone, slabby-weathering, iron-stained- 0. 1 Shale, light-green______2. 1 for two small ledges of brown well-cemented, fine- Sandstone, well-cemented, iron-stained, me to medium-grained sandstone __ _ .______-___._ 133. 0 dium-grained ; small concretions.___.-_.__ 0. 2 Sandstone, friable, fine-grained; some gypsum Total thickness if Jilh Formation. ______??9. 5 and green shale.______2. 7 Sudair Shale. Sandstone, gray, medium-grained; small con SECTION 15. Ad Duwayhirah cretions. ______0. 3 [Generalized section of Jilh Formation near Ad Duwayhirah; A. F. Pocork and Claystone, green; some finely interbedded R. P. Kopp, 1949] sandstone.______2. 6 Sandstone of Minjur Sandstone (Upper Triassic). m , ,.,, ,. Th'cknest Sandstone, light-gray, slightly iron-stained, Jilh Formation: (rretert) thinly bedded______-______-__-_-.-__ 0. 4 Sandstone and shale; approximately equal amounts of Claystone and sandstone; green claystone and off-white to brown, fine- to medium-grained, mod fine-grained sandstone-______2. 3 erately sorted, commonly crossbedded, rarely Sandstone, black, slabby-weathering, iron- stained, well-cemented; common minute calcareous, gypsiferous, and silty sandstone and ferruginous concretions______0. 5 mustard to greenish-yellow, soft, commonly Shale, dark-green, sandy______2. 0 gypsiferous shale. Thin beds of gray to tan, tight, Sandstone, slabby-weathering, iron-stained, platy-weathering, commonly sandy and impure, medium-grained; lower part ripple marked. 1. 0 very finely crystalline limestone are scattered Sandstone, light-gray, porous, friable, fine- throughout interval.______--_--_-_-__-____-- 120. 0 to medium-grained; small concretions and Limestone and siltstone; approximately equal some greenish clay___--_-__--___-____-_ 4 5 amounts of complexly interbedded, light-brown, Unexposed; probably mainly sandstone.--__ 41. 1 platy, impure and sandy, rarely fossiliferous lime Total thickness of Jilh Formation.______326. 1 stone and gray, yellow and green, in part gypsif Sudair Shale. erous siltstone. Occasional thin beds of gypsif SECTION 14. Khashm al Manjur erous shale and gray sandstone.______-_---_ 48.0 [Generalized section of Jilh Formation west of Khashm al Manjur; R. A. Bramkamp Sandstone, tan, well-sorted, fine- to medium-grained, and S. B. Henry, 1949] angular, slightly gypsiferous______--_-___-_ 12. 0 Sandstone of Minjur Sandstone (Upper Triassic). Shale, gray, friable, gypsif erous. ______7. 0 Jilh Formation: Covered; thickness calculated.______45. 0 Sandstone, yellow-brown, strongly calcareous, medi um-grained, locally oolitic; rare, poorly preserved Total thickness of Jilh Formation_____.____- 232. 0 fossils. Forms main cap of upper Jilh escarpment. 2. 0 Covered. D120 GEOLOGY OF THE ARABIAN PENINSULA SECTION 16. 'Ayn Ibn Fuhayd SECTION 18. Khashm al Khal\a' Continued [Generalized section of Jilh Formation near 'Ayn Ibn Fuhayd; A. F. Pocock and Minjur Sandstone Continued ^£85)* R. P. Kopp, 1949] Sandstone, tan, massive, crossbedded, mediun- Sandstone of Minjur Sandstone (Upper Triassic). grained; some layers with a white earthy matrix -.,, T-, ,. Thickness Jilh Formation: (meters) Common pebble-bearing beds in lower 11 n. Gypsum, gray to white, thin-bedded, in part poorly Forms vertical cliff______21. 1 exposed; occasional thin beds of gray to tan, platy, Sandstone and shale; alternating beds of tan medium- impure, finely crystalline limestone and dolomite. grained crossbedded sandstone and purple ard Some sandstone and shale occurs in upper part. cream sandy shale. _-__-_------______13. 8 Interval is capped by very thin bed of tan to brick- Sandstone, buff, massive, crossbedded, medium- to red oolitic sandstone and oolite same unit typi coarse-grained; common layers with quartz pel - cally caps Jilh at other localities______78. 0 bles_ ------__-_ _- 15. 7 Marl, limestone and shale; in part poorly exposed but Sandstone and shale; complexly interbedded tan largely complexly interbedded tan to gray platy crossbedded sandstone and variegated shale______18. 5 sandy marl, tan to reddish-brown thin-bedded, Sandstone, buff, massive, crossbedded, medium- to commonly detrital-oolitic limestone, and gray- coarse-grained; forms cliff..__-_-__--______12. 3 green gypsiferous shale. Tan to light-gray sand Shale, purple, tan and gray; some shaly sandstone. stone, in part calcareous, occurs at several levels About 600 m west of measured section interval is but becomes more common in lower part. ______166. 0 replaced by coarse-grained sandstone.______10. 5 Sandstone, buff, crossbedded, moderately cemented, Total thickness of Jilh Formation_-____.____ 244. 0 medium- to coarse-grained; forms weak bench..__ 5. 4 Covered. Shale and sandstone; purple, light-gray, and tan SECTION 17. Didah sandy shale and thinly interbedded silty sandstone; [Generalized section of Jilh Formation near Qidah; A. F. Pocock and R. P. Kopp, forms weak ledge. Thin layers of ironstone com 1949] mon in upper part._____--___---_-______-_--__ 9. 5 Sandstone- buff, moderatly cemented, crossbedded, Sandstone of Minjur Sandstone (Upper Triassic). medium- to coarse-grained; forms weak ledge..___ 2. 2 Jilh Formation: Sandstone and shale; light-gray, mottled purpl?, Limestone and shale; alternating 2- to 4-m beds of well-bedded soft silty sandstone and sandy shaH; tan to brown, tight, very sandy limestone and thin scum of ironstone at top----__---___-----__ 3. 5 yellow, poorly exposed silty shale and soft marl._ 22. 0 Covered; probably shale and gypsum(?)______53. 0 Sandstone, light-brown, massive, strongly crossbed Limestone and shale; interval in large part poorly ded, moderately cemented, coarse-grained; streaks exposed but apparently mostly tan thin-bedded with quartz pebbles common.__-_---______---- 12. 3 platy-weathering limestone and soft marly lime Shale and sandstone; tan and purple sandy silty shale stone intimately interbedded with indeterminate and many thin lenses of sandstone; a thin irregular amounts of tan silty shale. A thin bed of red ironstone layer at top. Upper contact shows local friable sandstone occurs near base______178. 0 relief as great as 3 or4m______-----__ 12. 2 Sandstone, buff, pink-weathering, crossbedded; a Total thickness of Jilh Formation.______253. 0 thin bed of ironstone forms weak bench at ton, Crossbedded sandstone of the Sudair Shale. grades laterally to shaly sandstone.______---_- 12. 6 Shale, purple and gray, sandy and silty; thin beds of MINJUR SANDSTONE SECTION ironstone form weak benches 6.3 m above base ard SECTION 18. Khashm al Khalta' at top. Lower part grades laterally to sandstone. 9. 1 [Minjur Sandstone type section measured and described at Khashm al Khalja' by Sandstone, red-brown, pink-weathering, crossbedded; R. A. Bramkamp, 8. B. Henry, 8. J. Roach, R. B. Carr, and M. C. Coffleld in 1951] locally replaced laterally by sandy shale---.---.- 11. 8 Sandstone, red-brown, strongly crossbedded, friabJc, Shale of Marrat Formation (Lower Jurassic). Unconformity. highly lenticular; some argillaceous silt and sand Minjur Sandstone: stone. Thin beds of purple and black ironstone Sandstone, tan, massive; forms vertical cliff. Inter occur at several levels------15. 5 val measured by theodolite-_----_-----_--___--_ 48.0 Sandstone, red-brown, massive, highly crossbedded, Shale and sandstone; complexly interbedded tan to friable'; common layers with abundant small peb purple silty shale and tan sandstone-_-_-_-_.___ 4. 5 bles. In general, strongly lenticular. Light-col Sandstone, tan, pink-stained, weakly crossbedded, ored argillaceous sandstone and silt occur betwe3n medium-grained______7. 0 main lenses. A very thin purple-black ironstone Shale, buff, spheroidally-weathering______1.0 layer caps unit.______-_---_-----_----- 3. 7 Sandstone, buff, massive, crossbedded, medium, Sandstone; interval calculated, mainly sandstorc; grained._-_-----___-______.--_-__ 4. 8 Shale, cream, spheroidally-weathering. ______1.2 some purple and gray shale._____-_-__------48. 5 Shale and sandstone; purple, tan, and gray variegated shale; several beds of buff, massive, crossbedded Total thickness of Minjur Sandstone. 315.0 sandstone in lower part______10. 3 Jilh Formation. SEDIMENTARY GEOLOGY OF SAUDI ARABIA 1121 MARRAT FORMATION SECTIONS SECTION 19. Khashm adh Dhlbi Continued Lower Marrat Formation (36. 5 m thick) : SECTION 19. Khashm adh Dhlbl 2. Aphanitic and calcarenitic limestone and dolomite [Marrat Formation reference section measured and sampled in the face of Khashm (15.0m thick): adh Dhlbl by R. W. Powers and H. A. McClure in 1962] Calcarenitic limestone, reddish-brown, tight, partially recrystallized; 15 to 20 percent Gypsum of the Dhruma Formation (Middle Jurassic). gastropod-pelecypod fragments and com Upper Marrat Formation (24.2 m thick) : mon hexacorals__-__-___--_-___-_--___- 0. 5 4. Aphanitic and calcarenitic limestone: (meter*)* Aphanitic limestone, light-brown, moderately Aphanitic limestone, light-brown, moderately porous, partially recrystallized and parti porous, partially re crystallized, chalky; ally dolomitized; pelecypod-gastropod de forms massive cliff_------_--___-______11. 0 bris common__-___--_----__-_---__-_-_- 2. 0 Calcarenitic limestone, light-brown, moder Aphanitic limestone, light-brown, partially ately porous, chalky; 15 percent medium- recrystallized, moderately porous, rubbly- to coarse-grained lime-mud pellets and weathering, chalky; common gastropod- pelecypod debris.------2. 0 pelecypod debris. ______3. 0 Aphanitic limestone, tan, moderately porous; Calcarenitic and aphanitic limestone, poorly 5 percent pelecypod-gastropod remains exposed, rubbly- weathering ______4. 0 and rare dolomite rhombs______1. 5 Gastropod calcarenitic limestone, brown, Calcitic dolomite and aphanitic limestone; moderately porous, chalky; 30 percent dolomite as below; common patches of gastropod casts and common pelecypod essentially unaltered aphanitic limestone. _ 1. 5 debris____-_----__---______2. 0 Calcitic dolomite, reddish-brown, moderately porous, medium-grained crystalline; 20 Limestone, poorly exposed, chalky, calcareni percent original lime-mud matrix preserved tic and aphanitic_-_--_--______1. 6 in intercrystalline spaces, ghosts of replaced Gastropod calcarenitic limestone, brown, pellets common. ______1.0 moderately porous, chalky; 35 percent Calcarenitic dolomite, dark-gray-brown, medium- to coarse-grained gastropod casts. medium-grained crystalline, tight; ghosts Forms weak bench. ______0. 4 of pellets common. Rock originally cal Shale poorly exposed, mainly yellow, fos- carenitic limestone ______1.0 siliferous-----_-_-__--_-__-____-_._____ 1. 8 Dolomite, golden-brown, moderately porous, Calcarenitic limestone, brown, moderately very finely crystalline, semifriable ______2. 0 porous, chalky; 20 percent pelecypod- Aphanitic limestone, golden-brown, tight, gastropod debris and 10 percent dark-brown partially dolomitized; 5 percent pellets and 60 percent finely crystalline dolomite medium-grained well-rounded lime-mud rhombs scattered uniformly through out. __ 1. 0 pellets. Forms weak bench______0.4 Calcarenitic dolomite, reddish-brown, tight, Middle Marrat Formation (41.8 m thick): medium-grained, crystalline; 20 percent 3. Shale, siltstone, and sandstone: ghosts of replaced pellets "floating" in Shale, red-__-_-______2. 2 dolomite------1- 0 Covered..___-_.--_._-_.______0. 4 Aphanitic limestone, reddish-brown, sandy; Gastropod calcarenitic limestone, brown, tight, 50 percent medium- to coarse-grained, partially recrystallized; 60 percent calcite- angular to subrounded, quartz sand grains in lime-mud matrix. .___ --______-__--_-_ 1. 5 replaced and -infilled gastropod-pelecypod 1. Poorly exposed; sandstone, siltstone, and shale shells set in continuous original lime-mud (21.5m thick): matrix. Caps weak bench.______0.2 Calcareous sandstone, reddish-brown, Aphanitic limestone, light-brown, moderately medium- to coarse-grained, well-sorted, porous, chalky; forms weak bench ______1.0 angular to subangular, quartzose; 30 Shale, red; rare thin layers of green shale____ 7. 5 percent reddish-brown carbonate cement. Sandstone, reddish-brown, moderately porous, Quartz sand grains not in contact with very fine grained, angular to subrounded, each other suggesting sand deposited crossbedded, calcareous cemented, quart- concurrently with intergranular lime-mud 2. 5 zose; forms prominent massive ledge. _ ^ _ _ _ 2. 5 matrix. _____.______-_------Poorly exposed; scarce exposures indicate Shale, red; thin layers of green and yellow complexly interbedded sandstone, silt- shale. ______7. 0 stone, and shale..... _-_-_-__---.-_----- 7. 5 Sandstone, red, silty, massively crossbedded; Aphanitic limestone, golden-brown, tight, forms prominent ledge______1.0 partially recrystallized; forms weak bench. Shale, red; rare green shale.______15. 0 Lateral continuity indicates this bed defi Covered; probably red shale.______5. 0 nitely in place on scree-covered slope. _ _ __ 0. 5 D122 GEOLOGY OF THE ARABIAN PENINSULA SECTION 19. Khashm adh Dhlbl Continued SECTION 21. Khashm al Manjur Lower Marrat Formation (36.5 m thick) Continued [Qeneralized section of Marrat Formation at Khashm al Manjur; R. A. Framkamp, 1. Poorly exposed; sandstone, siltstone, and shale E. L. Berg, R. L. Myers, and W. T. Short, 1945] (21.5 m thick) Continued Covered; presumably shale of the Dhruma Formation (Middle Sandstone, greenish-gray, moderately porous, Jurassic). well-sorted, medium- to coarse-grained, Marrat Formation: subrounded, quartzose-.----______1.0 Upper Marrat (15.8 m thick): Poorly exposed; comparison with adjacent Limestone, tan to yellow, rubbly-weathering, sections shows interval to be mostly green chalky, aphanitic; abundant limonitic and to olive-green shale..______9. 0 phosphatic nodules. Thin beds of tan marly Sandy pellet calcarenitic limestone, golden- shale cap interval. Forms prominent bencl -_ 8. 3 brown, fine- to coarse-grained; 40 percent Shale and limestone; dominant! y green to tan well-rounded fine-grained lime-mud pellets shale with subordinate interbeds of golden- and 30 percent medium- to coarse-grained, brown, ocherous aphanitic limestone; abundant angular to subrounded quartz sand. limonitic and phosphatic ooliths in upper psrt, Forms weak ledge.__--__-_--_____--____ 1.0 rare marine fossils---_------___--_-_--.___ 7. 5 Middle and lower Marrat undifferentiated (34.1 m Total thickness of Marrat Formation. __ 102. 5 thick): Minjur Sandstone. Sandstone, partially covered, reddish-brovn, massive, micaceous and silty, moderately ce SECTION 20. Khashm al Jufayr mented; occasional beds of reddish-brown siltstone______23. 4 [Qeneralized section of Marrat Formation at Khashm al Jufayr; Max Steineke, Shale; mostly covered but local exposures of red R. A. Bramkamp, E. L. Berg, R. L. Myers, and W. T. Short, 1945] silty shale...__-___-__-_-___-__-______-_ 8. 7 Gypsum and shale of Dhruma Formation (Middle Jurassic). Dolomite, yellow to tan, impure, finely crystalline. 2. 0 Marrat Formation: Upper Marrat (22.3 m thick): Total thickness of Marrat Formation.._._-___ 49. 9 Limestone, tan, chippy-weathering; several beds Minjur Sandstone. with abundant limonitic and phosphatic nod ules. Forms prominent clifif.--______-__-__ 8.0 SECTION 22. Khashm Mawdn Poorly exposed; mainly olive-green and tan shale; [Generalized section of Marrat Formation at Khashm Mawan; R. A. Bra-nkamp and well-preserved ammonites 2.0 m from base____ 7. 0 H. A. Kimball, 1950] Limestone and shale; complexly interbedded golden-tan chippy-weathering fossiliferous lime Sandstone and Shale of Dhruma Formation (Midc"e stone and tan to olive-green shale. Basal shale Jurassic). »« a. n j.. Thickness bed contains well-rounded limestone pebbles. _ 7. 3 Marrat Formation: (meters) Middle Marrat (34.5 m thick): Sandstone, red to brown; alternately strongly and Shale, red and brown, rarely green; few very thin weakly cemented; strongly cemented layers form beds of fine-grained sandstone.______4. 5 prominent ledges with intervening softer beds Sandstone, red to brown, thinly bedded, mica recessed..____.______28. 2 ceous and silty; occasional thin beds of red Shale, light-gray, tan, dark-red, sandy.______4. 0 shale.______-__-____-_____--_-____--_--__ 16. 5 Sandstone, brick-red, friable, in part crossbedded, Shale, red, silty; in part poorly exposed.______13. 5 fine- to coarse-grained; rare shale, quartz pebb^s Lower Marrat (35.0 m thick): in lower part______---___------__- 4. 7 Dolomite and limestone; equal amounts of inter- bedded gray to tan, tight, fossiliferous limestone Approximate total thickness of Marrat For and brown, impure, finely crystalline dolomite; mation. _--__----__------_------_ 36. 9 sand grains common in lower bed. Poorly Minjur Sandstone. preserved fossils including ammonites, brachi- opods and echinoids common.______10. 2 SECTION 23. Mardh and vicinity Shale and sandstone, complexly interbedded; red and green shale and golden-tan, ocherous, [Generalized section of Marrat Formation in vicinity of Mar ah; D. A. Holm and limonite-stained, poorly cemented quartz sand A. F. Pocock, 1947 and R. A. Bramkamp, S. B. Henry and N. J. Sander, 1949] stone, shaly sandstone, and sandy siltstone; Shale of Dhruma Formation (Middle Jurassic). Marrat Formation: _., common gypsum veinlets in upper part__... _ _ 12. 2 TT ,_ , ,_._ _ ... , . Thickness Limestone, brick-red, blocky-weathering, slightly Upper Marrat (20.5 m thick): (meters) sandy, tight, aphanitic; forms weak ledge_..._ 0. 6 Limestone, tan, chippy- to slabby-weathering, Shale, green to yellow, occasionally red; thin beds compact, in part fossiliferous and detrital.... 20. 5 of tan, slightly sandy limestone near the middle. 12. 0 Middle Marrat (57.0 m thick): Shale, reddish-brown, in part micaceous and Total thickness of Marrat Formation...... 91. 8 silty; subordinate interbeds of red-bro^n Minjur Sandstone. fine-grained sandstone.-______-__-__--_-- 57. 0 SEDIMENTARY GEOLOGY OF SAUDI ARABIA D123 SECTION 23. Marah and vicinity Continued SECTION 24. Khashm adh Dhlbl and eastward Continued Thickness Marrat Formation Continued (meters) Upper Dhruma Formation (89.0 m thick) Continued Lower Marrat (33.3 m thick): 10. Hisyan Member (64.0 m thick) Continued Limestone, reddish-tan, thin-bedded; forms Shale, poorly exposed, olive-tan, calcareous; caprock of basal Marrat scarp. _.__..______4.4 possible thin interbeds of soft chalky Sandstone, yellowish-green, friable, limonitic.- 1. 1 aphanitic limestone.. ______3. 6 Limestone, tan, thin-bedded, impure.______0. 8 Molluscan calcarenitic limestone, as 1.4-m- Dolomite, light-gray, medium-bedded, sandy. __ 0. 9 thick limestone below__ -_-_--______0. 4 Sandstone and shale; interbedded tan friable, Shale, poorly exposed, tan to olive-tan, cal in part calcareous sandstone and sandy shale. 1. 9 careous, some very thin interbeds of Limestone, yellow, tan, and rarely purple-red, chalky aphanitic limestone ______1. 6 alternating hard- and soft-bedded, in part Molluscan calcarenitic limestone, brown, dolomitic; occasional shale interbeds. ______21. 2 moderately porous, chalky; 25 percent Shale, gray to tan.___----_-__---______-_-__ 2. 4 medium-grained to gravel-size, tabular Limestone, tan, compact, thinly bedded.------0. 6 fragments of pelecypod and gastropod shells. Medium-grained well-rounded ag Total thickness of Marrat Formation...----- 110. 8 gregate pellets common. _---______1. 4 Minjur Sandstone. Shale, poorly exposed, olive-tan, calcareous. 2. 6 Pellet calcarenite, brown, well-sorted, very DHRUMA FORMATION SECTIONS fine to fine-grained, well-rounded, lami SECTION 24. Khashm adh Dhlbl and eastward nated, tightly cemented; composed dom- inantly of fine-grained aggregate pellets. [Dhruma Formation reference section remeasured and described by R. W. Powers and H. A. McClure in 1962 in the vicinity of the original type section. The new Rare pelecypod and echinoid debris __ __ 0. 7 measurement was composited from three increments between Khashm adh Dhlbl Aphanitic limestone, golden-tan, moderately and the Tuwaiq Mountain Limestone escarpment about 21 km eastward] porous, chalky; 5 percent molluscan de Limestone of the Tuwaiq Mountain Limestone (Upper bris and scattered, finely crystalline dolo Jurassic). mite rhombs.. _ __------_-____-_-- 1.0 Upper Dhruma Formation (89.0 m thick): Pellet calcarenitic limestone, tan, moder 10. Hisyan Member (64.0 m thick) ately porous, chalky; 30 percent yellow, in Shale, poorly exposed, olive-tan, calcareous. 1. 8 part superficially coated, medium-grained Pellet calcarenitic limestone, tan, moderately well-rounded aggregate pellets. Mollus porous, chalky; 30 percent very fine can debris and Foraminifera common. _ _ _ 1.0 grained, well-rounded aggregate pellets, Aphanitic limestone, brown, moderately 20 percent algal (?) trash, and 10 percent porous, chalky; rare molluscan debris and calcite-replaced molluscan debris. Foram- aggregate pellets. _____-___-____-_-_-__ 0. 7 inifera common.______0. 7 Shale, poorly exposed, tan to olive-tan. .__. 7. 6 Pellet calcarenite, brown, poorly sorted, Pellet calcarenite, brown, well-sorted, well- clean-washed, tightly cemented; 40 per rounded, very fine grained, tightly ce cent fine- to medium-grained aggregate mented; almost exclusively fine-grained pellets, 20 percent algal (?) debris, and 10 aggregate pellets. Patches of tan chalky percent calcite-replaced molluscan shell aphanitic limestone common. ______0.5 fragments._._-_._.-._---.__.__.______2. 0 Shale, poorly exposed, olive- tan __-__-__-_- 4. 4 Coarse molluscan carbonate, reddish-brown, Mollusc an-pellet calcarinite, dark-red, poorly poorly sorted, clean-washed, strongly sorted, angular to well-rounded clean- recrystallized; 50 percent calcite-replaced washed, tightly cemented; 30 percent re tabular fragments of pelecypod and gas placed molluscan shells set in a strongly tropod shells admixed with 20 percent medium-grained well-rounded aggregate recrystallized matrix of red fine-grained pellets. Foraminifera common___---___ 0.8 well-rounded pellets. Common echinoid Shale, poorly exposed, olive-brown, calcar debris and fragments derived from older eous. ----___------__--__--______1.4 limestone ______-_____-_____-_---- 0. 5 Calcarenitic limestone, as 1.5-m-thick lime Shale, poorly exposed, olive-tan. __.._.__--_ 3. 3 stone below__-_--_--_--______--_._ 0. 5 Calcarenitic limestone, brown, moderately Shale, poorly exposed, olive-tan, calcareous. 2. 6 porous, chalky; 20 percent coarse-grained Pellet calcarenitic limestone, brown, well- to gravel-sized, commonly calcite-replaced bedded, well-rounded, chalky; 40 percent fragments of molluscan shells. Abundant medium- to coarse-grained aggregate pel fine- to medium-grained, well-rounded lets and abundant fragments of molluscan shells and older limestone set in original aggregate pellets. Isolated pockets of fine lime-mud matrix.______1. 5 grained well-sorted, tightly cemented, Shale, poorly exposed, olive-tan__.--____._ 5. 4 clean-washed pellet calcarenite. Abundant Molluscan calcarenitic limestone, as 1.4-m- Gryphaea costettata ... _ _ __-._-____._----- 3. 7 thick limestone below.______0. 5 Shale, poorly exposed, olive-tan. ______8. 6 228-77© G 66 9 D124 GEOLOGY OF THE ARABIAN PENINSULA SECTION 24. Khashm adh Dhlbl and eastward Continued SECTION 24. Khashm adh Dhlbl and eastward Continued Upper Dhruma Formation (89.0 m thick) Continued Upper Dhruma Formation (89.0 m thick) Continued 10. Hisyan Member (64.0 m thick) Continued 9. 'Atash Member (25.0 m thick) Continued (JJ80* Coarse molluscan carbonate, brown, very Oolite-detrital calcarenite, brown, poorfy poorly sorted, strongly recrystallized, sorted, tightly cemented; 30 percent me tightly cemented; almost exclusively pele- dium- to coarse-grained, well-rounded, cypod and gastropod debris. Most original superficial ooliths and 25 percent medium- molluscan shell material now replaced by grained to gravel-sized, angular fragments clear, coarsely crystalline calcite in crystal- of aphanitic limestone derived from older lographic continuity with secondary inter- rock. Common algal(?) debris.______1. 2 granular cement. Molluscan debris com Pellet calcarenitic limestone, light-brow-T, monly preserved as internal molds of lime moderately porous, chalky; 30 percent mud____-______---______0. 5 fine- to coarse-grained, well-rounded ag Covered ; probably olive-tan shale ______2. 4 gregate pellets and 10 percent calcite- Molluscan calcarenitic limestone, reddish- replaced molluscan debris. Rare algal(?) brown, tight, partially dolomitized; 60 fragments and isolated pockets of clean, percent calcite-replaced, medium- to coarse current-washed calcarenite ______2. 0 grained, angular fragments of molluscan Calcarenitic limestone, golden-brown, sand;', shells. Common unidentifiable fine moderately porous, chalky; 10 percent grained organic debris. ______0. 5 medium-grained aggregate pellets ard Covered ; probably olive-tan shale ______1.8 abundant molluscan debris. Gryphava 9. 'Atash Member (25.0 m thick) : costellata. ______2. 0 Calcarenitic limestone, tan, moderately po Aphanitic limestone, tan, soft, rubbl7- rous, chalky ; 20 percent medium- to coarse weathering, chalky; lenses of biogenic csl- grained, angular, calcite-replaced mollus carenitic limestone. Gryphaea costellata..- 3. 2 can shell debris. Rare aggregate pellets. _ 3. 0 Calcarenitic limestone, light-brown, moder Aphanitic limestone, light-brown, moderately ately porous, chalky; 20 percent fine- to porous, chalky; calcite-replaced fragments medium-grained, well-rounded aggregate of molluscan shells common. ______1.5 pellets and 20 percent coarse-grained, we'l- Residual coral, brown, strongly recrystal rounded acorn-shaped algal(?) debrX lized; dominant constituent is in situ Abundant molluscan shell fragments ard hexacorals. Interseptal space packed with rare Foraminifera. Bed contains isolated lime mud and original calcareous shell ma patches of clean, current-washed, tightly terial now altered to clear crystalline cemented pellet calcarenite. Gryphaia calcite. Common mud-filled molluscan costellata...... 2.8 shells-______-__--___-______1. 5 Middle Dhruma Formation (164.7 m thick): Calcarenitic limestone, light-brown, moder 8. Dhrumaites zone (54.0 m thick): ately porous, chalky; 15 percent fine- to Algal(?) calcarenite, brown, moderate^ medium-grained, well-rounded aggregate sorted, well-bedded, fine- to coarse-grained pellets and 5 percent algal (?) and molluscan well-rounded, tightly cemented; 30 percent debris ______2. 0 acorn-shaped coarse-grained algal(?) debris Aphanitic limestone, light-brown, moderately 20 percent fine- to medium-grained, we11- porous, chalky; common angular, fine- to rounded aggregate pellets, and 10 percent coarse-grained calcite-replaced fragments angular to well-rounded fragments of okhr limestone, molluscan debris and Foramhrf- of pelecypod shells and corals. ______2. 0 era common caps prominent bench.--.-- 1. 5 Pellet calcarenitic limestone, reddish-brown, Aphanitic limestone, light-brown, moderately moderately porous, chalky; 25 percent red porous, chalky; 5 percent medium-grained to yellow, very fine- to coarse-grained, well- well-rounded pellets and molluscan debris. _ 1. 5 rounded aggregate pellets, 10 percent Poorly exposed; probably tan soft chalky coarse-grained algal debris and pelecypod aphanitic limestone and marl with thin fragments. Few isolated colonial coral interbeds of calcarenitic limestone.______13. 9 heads__---______-______. ______1. 5 Aphanitic limestone, tan, chippy-weatherir<*, Aphanitic limestone, light-brown, moderately soft, chalky.....-----.-.------3. 2 Molluscan calcarenitic limestone, tan, tight; porous, partially recrystallized ______1.2 50 percent molluscan debris and aggregate Calcarenitic limestone, light-brown, moder pellets. Forms weak bench.___-_____.-. 0. 4 ately porous, chalky; 15 percent fine- to Aphanitic limestone, tan, tight, chippy- medium-grained aggregate pellets and 5 weathering; local layers finely pelleted and percent medium- to coarse-grained, well- littered with organic debris. A weak rounded fragments of older aphanitic ledge at top formed by very thin bed of limestone, molluscan debris and algal trash _ 1. 1 calcarenitic limestone.______-_-____---- 7. 6 SEDIMENTARY GEOLOGY OF SAUDI ARABIA E126 SECTION 24. Khashm adh Dhlbl and eastward Continued SECTION 24. Khashm adh Dhlln and eastward Continued Middle Dhruma Formation (164.7 m thick) Continued Middle Dhruma Formation (164.7 m thick) Continued 8. Dhrumaites zone (54.0 m thick) Continued 7. Micromphalites zone (32.3 m thick) Continued Aphanitic and calcarenitic limestone; alter Oolite calcarenite, golden-brown, moderately nating layers of cream-colored to tan, soft porous, well-sorted, medium- to coarse aphanitic limestone and hard pellet-mol- grained, well-rounded, moderately ce luscan calcarenitic limestone ; weak benches mented; almost exclusively superficial 2.3 and 4.8 m above base ______8. 7 ooliths. Rare Foraminifera. ______1. 0 Covered_-___---____-______._--____ 3. 3 Oolite-molluscan calcarenite, golden-brown, Poorly exposed; scree slope littered with pellet moderately sorted, medium- to coarse calcarenitic limestone; probably much soft grained, well-rounded; 50 percent ooliths marly limestone in interval- ______2. 2 (generally superficial) and 40 percent Aphanitic limestone, reddish-brown, tight, tabular fragments of molluscan shells partially recrystallized. ______0. 5 oriented parallel to bedding. Rare algal Oolite calcarenite, tan, medium-grained, mod debris. Nuclei of ooliths almost exclu erately cemented; dominantly oolitic. sively aggregate pellets and possibly frag Forms gentle slope above bench. ______0. 8 ments of aphanitic limestone derived from Oolite calcarenitic limestone, brown, tight; older rock. Base of massive crossbedded forms prominent bench______1.0 ledge-forming oolite. ______1.0 Aphanitic limestone, tan, tight, partially recrystallized, brittle, fossiliferous; weathers Covered ______11. 7 to small irregularly shaped chips- ______2. 9 Aphanitic limestone, reddish-brown to light- Pellet calcarenitic limestone, reddish-brown, brown, moderately porous, chalky. ______11. 8 moderately porous, chalky; 30 percent fine- Calcarenitic limestone, tan, tight, partially to medium-grained well-rounded aggregate recrystallized, f ossilif erous. ______0. 3 pellets and 10 percent medium-grained Aphanitic limestone, light-brown, moderately angular fragments of molluscan debris. _ _ _ 1. 8 porous, chalky; 5 percent medium- to Pellet-molluscan calcarenitic limestone, red coarse-grained well-rounded pellets and dish-brown, moderately porous, well-sorted, fragments of molluscan shells. ______3.0 medium- to coarse-grained; 40 percent ag 6. Tulites zone (36.1 m thick): gregate pellets and 30 percent molluscan debris set in partially dolomitized lime-mud Pellet calcarenite, golden-brown, moderately matrix. Rare Foraminif era .--___-______1.2 sorted, very fine- to coarse-grained, well- Pellet calcarenite, reddish-brown, poorly rounded, tightly cemented; 50 percent sorted, very fine grained to fine gravel- very fine to fine-grained well-rounded sized, tightly cemented; 25 percent very aggregate pellets, 10 percent coarse-grained fine to medium-grained, well-rounded, to fine gravel-sized well-rounded fragments commonly superficially coated aggregate of older aphanitic limestone, molluscan pellets, 20 percent red coarse-grained well- debris common. Forms prominent bench. 1. 7 rounded composite grains derived from Pellet calcarenitic limestone, golden-brown, older lithified limestones, and 15 percent tight, well-sorted, medium- to coarse fine gravel-sized tabular fragments of grained, well-rounded; 70 percent aggregate molluscan shells.. - _-_-_--__---__-______1.0 Calcarenitic limestone, golden-brown, moder pellets set in original lime-mud matrix. ately porous, chalky; 20 percent fine- to Although individual grains commonly in medium-grained, well-rounded, superficially contact, all intergranular space is packed coated, lime-mud pellets and 20 percent with lime mud. Pellets appear to be medium- to coarse-grained angular frag detrital fragments derived from older lime ments of molluscan shells. Rare Forami- stone. Molluscan debris and Foraminifera nifera______0. 8 common. ______2. 5 Aphanitic limestone, brown, moderately Pellet calcarenitic limestone, reddish-brown, porous, chalky, rarely pelleted. ______1. 7 tight, partically recrystallized; 30 percent 7. Micromphalites zone (32.3 m thick): medium- to coarse-grained well-rounded ag Oolite calcarenite, golden-brown, porous, poorly sorted, weakly cemented; 30 percent gregate pellets commonly with superficial fine- to medium-grained well-rounded oolite coat, 10 percent medium- to coarse superficial ooliths, 20 percent medium- grained well-developed ooliths. Molluscan grained biserial Foraminif era, and 10 per debris common ______-_--___-- 1. 0 cent fragments of older limestone, mol Oolite calcarenitic limestone, brown, moder luscan debris, and echinoid spines. Top ately porous, partially recrystallized, of major ledge-forming crossbedded oolite chalky; 15 percent medium- to coarse bed______-______-__-_---__-__------a 5 grained superficial ooliths- ^---_-_------_ 1.0 D126 GEOLOGY OF THE ARABIAN PENINSULA SECTION 24. Khashm adh Dhlbl and eastward Continued SECTION 24. Khashm adh Dhlbl and eastward Continued Middle Dhruma Formation (164.7 m thick) Continued Middle Dhruma Formation (164.7 m thick) Continued 6. Tulites zone (36.1 m thick) Continued 6. Tulites zone (36.1 m thick) Continued Pellet calcarenitic, golden-brown, moderately Pellet calcarenite, golden-brown, well-sorted, sorted, well-rounded, tightly cemented; 60 well-rounded, fine- to medium-grained, percent very fine to fine-grained well- tightly cemented; 10 percent coarse rounded aggregate pellets and 10 percent grained fragments of pelecypod shells. _ _ _ _ 0. 4 medium- to coarse-grained superficial Calcarenitic limestone, golden-brown, moder ooliths, molluscan debris commonly re ately porous, chalky; 20 percent mediurr- placed by calcite. Rare patches of lime- grained, well-rounded, aggregate pellets mud matrix.______1.0 and 20 percent medium-grained molluscr.n Calcarenitic limestone, golden-brown, mod shell debris.______-__-__-__---______1.3 erately porous, chalky; 10 percent medium- Aphanitic limestone, poorly exposed, tan, to coarse-grained well-rounded superficial soft, chalky______1. 2 ooliths and 10 percent well-rounded medi Pellet calcarenite, tan, well-sorted, very fine um-grained aggregate pellets. Molluscan grained, tightly cemented; mainly aggregate debris commonly replaced by calcite-___-- 1. 5 pellets, superficial ooliths and orgaric Aphanitic limestone, light-brown, tight, par debris common. Forms ledge______0. 5 tially recrystallized; coarse-grained well- Calcarenitic limestone, tan, moderately po rounded aggregate pellets common.------1. 5 rous, soft, chalky; 20 percent superficial Pellet calcarenite, light-brown, moderately ooliths and aggregate pellets.______1.6 sorted, very fine to medium-grained, well- Algal (?)-molluscan calcarenite, golden-brown, rounded, tightly cemented; 60 percent ag very poorly sorted, fine- to coarse-grained gregate pellets and 10 percent calcite re to gravel-sized, well-rounded to tabulr.r, placed molluscan debris-.----_--_-__---- 0. 7 tightly cemented; 25 percent coarse Aphanitic limestone, reddish-brown, tight, grained, irregularly shaped algal(?) debris, strongly dolomitized; 70 percent very finely 25 percent molluscan fragments (pelecypod crystalline dolomite rhombs irregularly dominant), and 20 percent fine- to medium- scattered through lime-mud matrix.______1. 6 grained aggregate pellets.______1.2 Aphanitic limestone, light-brown, moderately Pellet calcarenite, brown, well-sorted, porous, chalky; rare pockets of clean, cur medium-grained, well-rounded, tightly ce rent-washed, tightly cemented pellet cal mented; dominant constituent is aggregate carenite-.___- ______4. 1 pellets, rare well-developed, coarse-grained Oolite calcarenitic limestone, tan, moderately ooliths--______--_-__-______-______-_ 1. 0 porous, strongly recrystallized; 50 percent 5. Thambites zone (42.3 m thick): medium- to coarse-grained well-rounded Covered______1. 1 superficial ooliths and rare molluscan de Pellet calcarenite, golden-brown, well-sorted, bris___---___----______--_.__-___.__- 3. 0 medium-grained, tightly cemented; forms Pellet calcarenite, golden-brown, well-sorted, prominent bench______1. 7 very fine grained, well-rounded, tightly Pellet calcarenitic limestone, brown, chalky __ 1. 5 cemented; dominantly aggregate pellet, Pellet calcarenite and pellet calcarenitic some fine- to medium-grained fragments of molluscan shells and very rare coarse limestone; complexly interbedded, golden- grained, well-developed ooliths.______1.0 brown, well-sorted, medium- to coarre- Pellet calcarenitic limestone, brown, mod grained, tightly cemented pellet calcarenite erately porous, chalky; 30 percent very fine and light-brown aggregate pellet calcareni grained, well-rounded aggregate pellets, tic limestone. Molluscan debris and al rare calcite-replaced pelecypod shells. - ... 1.0 gal (?) fragments common, grains commorly Aphanitic limestone, reddish-brown, tight, coated with single oolitelike layer______1.0 strongly recrystallized, in part siliceous. _. 2. 0 Pellet-molluscan calcarenite, golden-brown, Pellet calcarenite, reddish-brown, well-sorted, moderately sorted, porous, fine- to medium- dominantly fine grained, moderately ce grained, poorly cemented______1. 2 mented and porous; mostly aggregate Aphanitic limestone, brown, moderately po pellets, molluscan fragments common. ... _ 3. 5 rous, soft, chalky; single ammonite------1. 5 Aphanitic limestone, light-brown, moderately Pellet calcarenite, golden-brown moderately porous, chalky; 5 percent calcite-filled out sorted, fine- to coarse-grained, well-rounded lines of monaxon sponge spicules, calcite- tightly cemented; 50 percent lime-mud pel replaced fragments of molluscan shells lets and 20 percent coarse-grained tabu'ar common, rare aggregate pellets and pockets fragments of pelecypod shells, rare super of clean, current-washed, tightly cemented ficial oolite coating on some pellets and tab pellet calcarenite.______1. 8 ular nuclei______4. 5 SEDIMENTARY GEOLOGY OF SAUDI ARABIA E127 SECTION 24. Khashm adh Dhibl and eastward Continued SECTION 24. Khashm adh Dhlln and eastward Continued Middle Dhruma Formation (164.7 m thick) Continued Middle Dhruma Formation (164.7 m thick) Continued 5. Tkambites zone (42.3 m thick) Continued T(mCeterl)S 5. Thambites zone (42.3 m thick) Continued Pellet calcarenitic limestone, light-brown, Pelecypod-pellet calcarenite, reddish-brown, tight, partially recrystallized; 40 percent moderately sorted, dominantly medium- to fine- to medium-grained aggregate pellets, coarse-grained, tabular to well-rounded, abundant Foraminifera, and replaced mol- tightly cemented; 40 percent coarse-grained luscan debris. Patches of clean, current- tabular fragments of pelecypod shells, 30 washed, tightly cemented pellet calcarenite percent fine- to medium-grained, well- common. ______2. 1 rounded aggregate pellets and rare Foram Pellet-molluscan calcarenite, reddish-brown inifera- ______0. 8 poorly sorted, fine-grained to gravel-sized, Aphanitic limestone and pellet calcarenite, tightly cemented; 40 percent fine- to coarse brown, complexly interbedded pellet cal grained, well-rounded lime-mud pellets and carenite and aphanitic limestone-______0. 5 30 percent molluscan shell debris set in Aphanitic limestone, tan, moderately porous, pellet calcarenite matrix, Foraminifera chalky; 5 percent fine- to medium-grained, common______0. 5 well-rounded lime-mud pellets and mollus Pellet clacarenite and pellet calcarenitic lime can debris, rare Foraminifers______1. 4 stone; golden-brown, moderately porous, Pellet calcarenite, golden-brown, well-sorted, fine- to medium-grained pellet calcarenitic fine- to medium-grained, well-rounded, limestone intimately admixed with pellet tightly cemented; 70 percent aggregate pel calcarenite; calcarenitic limestone is soft lets, rare Foraminifera and pelecypod frag and chalky; pellet calcarenite is hard and ments. Forms weak bench______0. 6 tightly cemented. Foraminifera common. 1. 3 Pellet calcarenitic limestone, light-brown, Pellet calcarenite, golden-brown, moderately tight; 40 percent fine- to medium-grained, sorted, fine- to coarse-grained, tightly cemented, well-rounded; 60 percent aggre well-rounded lime-mud pellets, common gate pellets, 10 percent calcite-replaced, pockets of clean, current-washed, well- angular fragments of molluscan shells, and rounded, fine- to medium-grained, tightly rare Foraminifera-______0. 6 cemented pellet calcarenite, Foraminifera Pellet calcarenitic limestone, tan, tight par common, and rare fragments of molluscan tially recrystallized; 30 percent fine- to shells_____-_-__------_-_------0. 4 medium-grained, well-rounded aggregate Aphanitic limestone, brown, tight, partially pellets, abundant Foraminifera and calcite recrystallized; 5 percent very fine to medi replaced pelecypod shells. Isolated patches um-grained aggregate pellets, Foraminifera of tightly cemented, clean, current-washed and molluscan debris.______0. 8 calcarenite common.______1. 5 Pellet calcarenite, brown, moderately sorted, Pellet calcarenite, golden-brown, moderately well-rounded, tightly cemented, fine- to sorted, medium- to coarse-grained, well- coarse-grained; 60 percent fine- to medium- rouhded, tightly cemented; 70 percent ag grained, well-rounded aggregate pellets, gregate pellets, abundant coarse-grained and abundant molluscan debris and Fo fragments of calcite-replaced molluscan raminifera______2. 0 debris______1. 5 Covered____--_--_-----_------__ 3.5 Calcarenitic limestone, brown, soft, marly; Pellet calcarenite, golden-brown, well-sorted, few fossils including ammonite in lower medium- to coarse-grained, well-rounded, part______1. 7 tightly cemented; 60 percent aggregate Pellet calcarenitic limestone, light-brown, pellets. Foraminifera common, algal tight, poorly sorted; 30 percent fine- to debris, and molluscan fragments.______-- 2. 1 medium-grained, well-rounded aggregate Algal calcarenite, golden-brown, moderately pellets, 10 percent coarse-grained, calcite- sorted, fine- to coarse-grained, tightly replaced, angular fragments of molluscan cemented; dominantly medium- to coarse shells, Foraminifera common and pockets grained, well-rounded to tabular algal of clean, current-washed, fine- to coarse debris; 20 percent fine- to medium-grained, grained, tightly cemented pellet calcarenite_ 2. 5 well-rounded aggregate pellets, abundant Pellet calcarenite, golden-brown, fine- to angular fragments of molluscan shells and medium-grained, well-sorted, tightly ce Foraminifera common. _-______--__-- 1.4 mented; 50 percent fine- to medium-grained Covered..------1- 0 aggregate pellets, 10 percent molluscan de Pellet calcarenitic limestone, tan, soft, rubbly- bris, rare Foraminifera______1.3 weathering; forms weak bench.______2. 3 D128 GEOLOGY OF THE ARABIAN PENINSULA SECTION 24. Khashm adh Dhlln and eastward Continued SECTION 24. Khashm adh Dhlln and eastward Cont;uued Lower Dhruma Formation (120.8 m thick) : Lower Dhruma Formation (120.8 m thick) Continued 4. Ermoceras zone (34.5 m thick) : 2. Shale, aphanitic limestone, and calcarentitic lim Pellet calcarenitic limestone, golden-tan, stone (35.6 m thick) Continued moderately porous, chalky; 30 percent Shale, golden-brown, rarely green ______2. 1 fine- to coarse-grained, well-rounded, Aphanitic limestone, gray-tan, soft, chalky __ 1. 3 aggregate pellets, abundant Foraminifera. Covered ______1. 3 Lime-mud matrix shows distinct mottled Shale, golden-brown ______1. 0 "algal" appearance. Top of Dhibi lime Shale, poorly exposed, green, gypsiferous; a stone, caps major cliff, and forms wide a few thin layers of chalky aphanitic limv spread dip slope __ . _____---_-______0. 5 stone____---_- ______5. 2 Aphanitic limestone, light-brown, tight to Calcarenitic limestone, light-brown, mode1*- moderately porous, partially recrystallized, ately porous, chalky; 15 percent badly chalky; abundant medium-grained pellets, fragmented molluscan debris and common gravel-sized algal nodules, and calcite outlines of calcite-filled, monaxon sponpe blebs filling disrupted lime mud, Foram spicules. Caps moderate bench. ______0. 5 inifera common ______5. 0 Pellet calcarenitic limestone, light-brow^, Pellet calcarenitic limestone, light-brown, moderately porous, chalky; 30 percent tight, partially recrystallized; 30 percent pellets and 20 percent molluscan debris. _ _ 1. 5 fine- to coarse-grained, well-rounded, Shale, green, gypsiferous- ______2. 7 aggregate pellets; common "birdseye" Molluscan calcarenitic, brown, poorly sorted, texture with mud apparently of algal fine- to coarse-grained, tightly cemented; origin or algal-bound. Foraminifera and 30 percent gastropod-pelecypod shell debris, stylolites common. ______1.0 and abundant well-rounded aggregate pel Aphanitic limestone, medium-brown, tight, lets. Top of weak bench_____-----_.-_- 0.4 partially recrystallized; common gravel- Shale and aphanitic limestone; complexly sized (2.0 to 8.0 mm) algal nodules with interbedded green gypsiferous shale ard well-developed concentric layering and gray chalky aphanitic limestone..- ______1. 6 common calcite-filled openings, common Calcarenitic limestone, golden-brown, tight; well-rounded, medium-grained pellets, 15 percent pelecypod-gastropod remain*. Foraminifera, and poorly preserved Forms weak bench__-----___------__- 0. 5 brachiopods and stromatopora______3. 0 Shale and aphanitic limestone; complex'y Calcarenitic limestone, tan, tight, partially interbedded green gypsiferous shale ard recrystallized; 20 percent light-brown, gray chalky aphanitic limestone______3. 2 well-rounded, fine- to medium-grained Calcarenitic limestone, golden-brown, moder pellets, Foraminifera and stylolites common. ately porous, chalky; 20 percent strongly Base of Dhibi limestone.. _ -_-_-_-______0. 5 recrystallized pelecypod fragments ______0. 6 Covered __ __--_____-____--___-______18. 5 Aphanitic limestone, tan, tight; rare thin Shale, olive-green and golden-brown, gypsif- interbeds of green shale_____------__- 2. 3 erous__ _ ---_-___--____---_-______6. 0 1. Dorsetensia zone (30.5 m thick) 3. Shale (20.2 m thick) : Sandstone, green, silty, gypsiferous; very Shale, olive-green gypsiferous; single, very rare, tan aphanitic limestone.--. ______1. 5 thin bed of gray-tan, chalky aphanitic Aphanitic limestone, pink, tight, partially 1. 5 recrystallized------0. 3 Shale, olive-green and golden-brown, gypsi Shale, green, gypsiferous------0.1 ferous ______9. 7 Aphanitic limestone, tan, moderately po Impure pellet calcarenitic limestone, brown, rous, chalky______--____--.-_------2.0 soft, chalky; abundant aggregate pellets Shale, green; veined with gypsum. ______2.8 and some fossils______0. 3 Pellet calcarenitic limestone, brown, mode-- Shale, olive-green; some gypsum veinlets____ 4. 0 ately porous, fine- to medium-grained_____ 1.0 Shale, olive-green, gypsiferous; common inter- Calcarenite, reddish-brown, porous, weakly bedded, impure aphanitic limestone, com cemented; dominantiy white, cylindrical mon sponges______4. 7 organic remains of unknown origin. ______1.0 2. Shale, aphanitic limestone, and calcarentitic lime Shale, green and purple, gypsiferous ______1.6 stone (35.6 m thick) : Aphanitic limestone, brown, moderately po Impure aphanitic limestone, gray-tan, chalky; rous, chalky______-_____-__--____-_- 0.2 Shale, green and purple, gypsiferous- ______1.0 sponges common ______2. 3 Pellet calcarenite, dark-brown, porous, Shale, olive-green, gypsiferous- ______3. 0 strongly recrystallized, fine-grained, moc'- Aphanitic limestone, light-tan, rubbly- erately cemented; entire bed recrystallized weathering, chalky; forms weak bench. .__ 4. 0 to finely crystalline calcite mosaic; how Pellet calcarenitic limestone, tan, moderately ever, original texture still readily visible. porous, chalky ; common pellets. ______2. 1 Forms weak ledge_----__-----_-----_--_ 0.7 SEDIMENTARY GEOLOGY OF SAUDI ARABIA D129 SECTION 24. Kha&hm adh Dhlbl and eastward Continued SECTION 25. Wadi Birk Continued Lower Dhruma Formation (120.8 m thick) Continued Upper and middle Dhruma Formation (193.5 m thick) 1. Dorsetensia zone (30.5 m thick) Continued Continued Shale, yellow, calcareous. ______1.3 8. Hisyan Member (about 56.0 m thick) Con. Sandstone, yellow, calcareous; few poorly Shale, gray-green; occasional gypsum veins preserved fossils _ . ______0.2 and very thin beds of limestone. Base is Shale, green, gypsiferous- ______1.0 believed to be approximate level of 'Atash- Aphanitic limestone, brown, moderately po Hisyan contact. ______16. 0 rous, chalky______0.1 7. 'Atash Member (about 19.5 m thick) : Shale, green, gypsiferous- ______0.5 Limestone and shale; complexly interbedded Coarse molluscan calcarenitic limestone, tan to gray, soft, marly limestone and reddish-brown, poorly sorted, sandy, par gray-green shale. ______------_-----_- 9. 5 tially dolomitized; 60 percent gravel- Limestone, gray to brown, thin-bedded, sized, gastropod-pelecypod mixture set commonly marly; a few layers of hard, in an almost completely dolomitized mat platy- weathering, highly fossiliferous lime rix of lime mud______0.6 stone. Fossils are mostly pelecypods.---- 10.0 Covered _ .. ______0.5 6. Dhrumaites zone (about 17.0 m thick): Shale, yellowish-green, gypsiferous ______0.3 Shale, poorly exposed, gray-green, calcareous, Aphanitic limestone, reddish-brown, moder in part gypsiferous ______17. 0 ately porous, partially dolo mitized __ ___ 0.1 5. Micromphalites zone (about 21.0 m thick): Shale, purple, sandy, gypsiferous. . ______0.6 Limestone and shale; alternating beds of Shale, yellowish-green, gypsiferous ______0.6 gray, marly limestone and gray-green, Calcitic dolomite, reddish-brown, very finely calcareous shale. _____-___-______---_ 21. 0 crystalline, moderately porous, calcitic; 4. Tulites zone (39.5 m thick) : 20 percent original lime-mud matrix still Limestone, tan to brown, in part marly, preserved between dolomite crystals. rarely sandy, commonly fossiliferous; thin Forms top of small rounded hill. ______0.2 beds of calcarenite at several levels.------39. 5 Shale, yellowish-tan, in part calcareous _ ___ 4.0 3. Thambites zone (40.5 m thick) : Gypsum, varicolored; subordinate green and Limestone, poorly exposed, off-white to tan, purple shale_____-______1.8 thin-bedded, fossiliferous, aphanitic and Oolite calcarenite, gray-brown, well-sorted, calcarenitic__------_----___------40. 5 tightly cemented, medium- to coarse Lower Dhruma Formation (67.6 re thick) : grained; 60 percent well-developed ooliths 2. Post- Dorsetensia beds (51.3 m thick): admixed with some internally structureless Limestone, buff, tan- to gray- weathering, aggregate pellets______0.1 tight, aphanitic; massive cliff former Gypsum, varicolored ______2.5 Dhibi limestone. Unit plus underlying Aphanitic limestone, golden-brown, partially interval of unknown thickness are lateral dolomitized ______0.2 equivalent of Ermoceras zone ______--- 7. 6 Gypsum, varicolored, well-bedded, crystal Covered; thin (1.0-m-thick) bed of orange- line ______2.3 brown, strongly cemented, oolite calcarenite Coarse pelecypod-pellet carbonate, brown, in lower part______10. 5 poorly sorted, moderately cemented; 50 Sandstone, brown, moderately to well-ce percent disarticulated gravel-sized pelecy- mented, medium- to coarse-grained, com pod shells and some gastropod remains in monly silty__-----_-_------33. 2 a matrix of pink, very fine grained, pellet 1. Dorsetensia zone (16.3 m thick): calcarenite. Forms thin bed in gypsif Shale and sandstone; mainily green, some red erous and shaly unit. ______0.2 and purple shale with interbedded, gray, Gypsum, varicolored, well-bedded, crystal friable sandstone. _____--_-______--.--- 3. 8 line ______1.2 Shale, yellow, red, green, purple, in part silty Total thickness of Dhruma Formation. _ 374.5 and sandy; a thin gray, ripple-marked iron Marrat Formation. stone layer just below top. ______----__- 4. 0 Dolomite, golden-brown, soft, shaly- weath SECTION 25. Wadi Birk ering, finely crystalline; becomes less dolo- [Generalized section of Dhruma Formation at Wadi Birk; measurements by R. A. mitic and oolitic near base __ _ ------2. 1 Bramkamp, 1945, and R. D. Gierhart and P. R. Aswerus, 1947] Sandstone and shale; golden-brown, ferru Limestone of Tuwaiq Mountain Limestone (Upper Jurassic). ginous, silty and coarse-grained quartz Upper and middle Dhruma Formation (193.5 m thick): Thicknett sandstone intimately interbedded with (metert) 8. Hisyan Member (about 56.0 m thick): variegated shale; single thin bed of finely Shale, gray-green; occasional gypsum veins crystalline dolomite near middle __-._ 6. 4 and very thin beds of limestone.______34. 0 Limestone, gray, massive, marly; forms Total thickness of Dhruma Formation._ 261. 1 prominent cliff..______6. 0 Marrat Formation. D130 GEOLOGY OF THE ARABIAN PENINSULA SECTION 26. Khashm al Juwayfah and Khashm ' Ushayrah SECTION 27. Al Haddar Continued [Generalized section of Dhruma Formation at Khashm al Juwayfah and Khashm Upper and middle Dhruma Formation Continued 'Ushayrah; measurements by R. D. Gierhart and L. D. Owens, 1947, and R. A. Bramkamp, 1951, respectively] 3. Approximate equivalent of Dhrumaitea zone, Hiyan and 'Atash Members (about 82 m Limestone of the Tuwaiq Mountain Limestone (Upper thick) Continued Jurassic). Thickness Upper and middle Dhruma Formation (147.0 m thick): (meters) Shale, gray, calcareous, gypsiferous; a fe^r 4. Dhrumaites zone, Hisyan and 'Atash Members very thick bands of fossiliferous limestone. (90.1 m thick): Bracketing marker beds indicate Microm- Shale, olive-gray, in part calcareous; a few phalites-Dhrumaites contact near base of very thin layers of cream-colored to brown, interval.______13. 7 soft, marly limestone in upper part. Poorly 2. Approximate equivalent of Micromphalites zonn. exposed interval near middle, Gryphaea (about 21.0 m thick): costellata and Eudesia cardium common Shale, gray, calcareous, gypsiferous.------6. 0 near base______46. 3 Limestone, tan to brown, chippy- to rubbly- Calcarenite and limestone; intimately inter- weathering, aphanitic; thin 1.0-m-thick bed bedded brown to gray, biogenic and oolitic of reddish-brown, calcite-cemented, silty calcarenite and calcarenitic limestone; com sandstone near middle.__----______-- 15. 0 mon interbedded olive-tan, silty shale. 1. Thambites and Tulites zones (about 26.0 m thick): May be in part equivalent to 'Atash Mem Limestone, brown and olive-green, chalky, ber______-_--.______8. 7 rubbly-weathering, in part fossiliferous; be Shale, poorly exposed, brown; several thin comes sandy in lower part..______24. 0 indurated layers with biogenic debris. _ - __ 24. 6 Not measured; estimated 2.0 m to approx'- Aphanitic limestone, gray-tan, compact, im mate equivalent of top of Dhibi limestone. _ 2. 0 pure.. _--____-_-______---______6.5 Shale, yellow and olive-brown, calcareous. _ _ 4. 0 Total thickness of upper and middle 3. Micromphalites zone (21.8 m thick): Dhruma_-___--__----__--__-.about 129. 0 Limestone, cream-colored to gray, compact, No data. rubbly-weathering, aphanitic; occasional thin beds of porous, tightly cemented cal SECTION 28. Khashm ash Shajarl carenite. Base of unit is base of Microm phalites zone__-____-.-___-______--__- 21. 8 [Generalized section of Dhruma Formation at Khashm ash Shajari; measurements by R. A. Bramkamp and H. A. Kimball, 1950] 2. Thambites and Tulites zones (35.1 m thick): Limestone, cream-colored, soft to compact, in Sandstone of Tuwaiq Mountain Limestone (Upper Ju part biogenic and oolitic; common interbeds rassic). of calcarenitic limestone, single calcarenite Upper and middle iDhruma Formation (about 122.1 m Thickness bed at base______35. 1 thick): (meters) Lower Dhruma Formation (about 50 m thick): 4. Approximate equivalent of Dhrumaites zone, 1. Pre-Thambites beds (about 50 m thick): Hisyan and 'Atash Members (76.9 m thick): Limestone, brown, cliff-forming; Dhibi lime Sandstone, buff to yellowish-brown and stone. ______--___--_.____-__-- 7.0 brown, crossbedded, medium-grained, Sandstone and shale; mainly white, red, and in part ferruginous; common concre brown, coarse-grained, crossbedded sand tionary ironstone masses, slabby black stone; nearly one-third of interval is inter- ironstone, fossil wood, and yellow, pur bedded gray to brown, commonly sandy ple, and green shale.______76. 9 shale; upper part poorly exposed. _ . _ about 43. 0 3. Approximate equivalent of Micromphalites zone (18.3 m thick): Total thickness of Dhruma Forma Shale, yellowish-tan, calcareous, sandy, in tion______about 197. 0 part gypsiferous______6. 7 Marrat Formation. Limestone, cream-colored, chippy-weath SECTION 27. Al Haddar ering, soft, impure; thin interbeds of tan gypsiferous shale. Gryphaea costettata Generalized section of Dhruma Formation at lat 21°55 N. (near Al Haddar); measurements by R. D. Gierhart and L. D. Owens, 1948] near base. Base of unit corresponds to Tulites-Micromphalites contact further Sandstone of Tuwaiq Mountain Limestone (Upper north_-__--__------_-._-_-____-. 11. 6 Jurassic). Thickness 2. Approximate equivalent of Thambites and Upper and middle Dhruma Formation: (meters) Tulites zones (26.9 m thick): 3. Approximate equivalent of Dhrumaites zone, Hisyan and 'Atash Members (about 82 m thick): Sandstone, yellowish-brown, moderately Shale, gray-tan to brown; occasional thin beds cemented, medium-grained; several thin of impure and fossiliferous limestone.__ _ _ _ 39. 3 layers of tan sandy dolomitic shale. _ _ _ _ 11. 1 Sandstone, white to gray, commonly red- and Shale, tan, soft, sandy, dolomitic; common purple-mottled, in part friable; interbedded thin layers of tan, very soft, sandy, gray shale near middle..___--______.__ 29. 0 rubbly-weathering dolomitic limestone. . 15. 8 SEDIMENTARY GEOLOGY OF SAUDI ARABIA D131 SECTION 28. Khashm ash Shajarl Continued SECTION 29. Khashm at Turab Continued Thickness Lower Dhruma: (meters) Lower Dhruma Formation (about 126.0 m thick): l.» Approximate equivalent of Dhibi limestone 2. Post-Dorsetensia beds (about 102.3 m thick): (11.9m thick): Covered______102. 3 Sandstone, brown, massive-bedded, mod 1. Dorsetensia zone (23.7 m thick): erately cemented, medium-grained; forms Shale, poorly exposed, red and green, gypsif cliff__._._-__--_.___._-. 11. 9 erous; occasional very thin beds of tan platy aphanitic limestone and calcarenite. Total thickness of Dhruma Formation. _ 134. 0+ Interval capped by 1.6-m-thick bed of Sandstone of Dhruma Formation and (or) Minjur or dark-brown, fine- to medium-grained, Jilh Formations. tightly cemented sandstone______23. 7 SECTION 29. Khashm at Turab Approximate total thickness of Dhruma Formation______374. 7 [ Generalized section of Dhruma Formation near Khashm at Turab; measurements by D. A. Holm and A. F. Pocock, 1948, R. A. Bramkamp, R. C. Kerr, B. Beverly, Jr., R. D. Gierhart, and W. Dell 'Oro, 1949, and R. A. Bramkamp, S. B. Henry, and N. J. Sander, 1949.] SECTION 30. Al Jurayfah [Generalized section of the Dhruma Formation near Al Jurayfah; measureir ents by Limestone of the Tuwaiq Mountain Limestone (Upper R. A. Bramkamp, 8. B. Henry, M. C. Coffield, 8. J. Roach, and R. B. Carr, 1951, Jurassic). and A. F. Pocock and R. P. Kopp, 1948] Upper Dhruma Formation (about 86.0 m thick): 8. Hisyan Member (59.0 m thick): Limestone of the Tuwaiq Mountain Limestone (Upper Jurassic). Shale, gray to olive-green; occasional beds of Thickness off-white, marly limestone.______--_.. 59. 0 Upper Dhruma Formation (82.9 m thick): (meters) 7. 'Atash Member (about 27.0 m thick): 6. Hisyan and 'Atash Members (82. 9 m thick): Not measured; but dominantly cream-colored Shale, olive-tan, brown, pale red, commonly to gray, marly rubbly-weathering limestone; gypsiferous, rarely calcareous; common few interbeds of olive-gray gypsiferous limonite ooliths in lower part, rare thin shale. Gyrphaea costettata common. _ _ _ about 27.0 beds of reddish-brown oolitic calcarenitic Middle Dhruma Formation (about 162.7 m thick): limestone and calcarenite in upper part-. 82. 9 6. Dhrumaites zone (52.0 m thick): Middle Dhruma Formation (about 147.8 m thick): Covered; includes some olive-tan shale; inter 5. Dhrumaites zone (47. 2 m thick): val capped by 1.0-m-thick bed of golden- Shale, olive-gray, calcareous, gypsiferous; tan, strongly cemented white calcarenite several thin beds of brown, ferruginous, that forms prominent bench.__._-_____-_ 10. 5 ripple-marked sandstone near top______25. 7 Limestone, cream-colored to tan, compact, Covered; probably gypsiferous shale; single soft, rubbly-weathering, aphanitic; common 0.9-m-thick bed of brown well-sorted interbeds throughout of brown medium- medium-grained sandstone exposed near grained, tightly cemented, pellet calcar middle___-___-_-.._____._____.._____ 21.5 enite__-__-___-____-_-_____-_-_-_--_ 26. 2 4. Micromphalites zone (29.2 m thick): Shale, poorly exposed, gray to olive-green, Shale, dominantly olive-gray and yellow- calcareous; common thin interbeds of green, gypsiferous; occasional thin beds of cream-colored soft marly limestone.___--. 15. 3 buff platy limestone in lower part and tan 5. Micromphalites zone (32.7 m thick): well-cemented crossbedded sandstone in Limestone, poorly exposed, cream-colored to upper part.______-_.__-----__-_- 29. 2 gray, soft, marly, rubbly-weathering- _ -. _ _ 9. 0 3. Tulites zone (36. 4 m thick): Calcarenite, tan, massive, medium-grained, Limestone, cream-colored to gray, soft, in tightly cemented; forms prominent bench_ _ 7. 0 part shelly and oolitic; 1.5-m-thick bed of Shale and limestone; poorly exposed, com tightly cemented calcarenite caps interval. plexly interbedded, reddish-gray silty shale About one-third of interval is covered, and gray-tan soft marly limestone. ______16. 7 probably masks shale---.------.--- 20. 0 4. Tulite& zone (38.0 m thick): Shale, poorly exposed, gray, tan, olive-green, Limestone, poorly exposed, gray to tan, soft, gypsiferous; occasional very thin beds of rubbly-weathering, chalky; occasional thin light-tan limestone.______--___-- 16. 4 beds of brown, tightly cemented calcarenite. 2. Thambites zone (about 35.0 m thick): About one-third of interval is covered; may Shale, olive-green to tan, gypsiferous; occa mask shale._-_---__-_____-_-_-__-__-._ 38. 0 sional thin beds of tan soft impure lime 3. Thambites zone (about 40.0 m thick): stone. Base of interval marks approximate Limestone, cream-colored, soft, chalky, equivalent to top of Dhibi limestone. _ about 35.0 rubbly-weathering, rarely calcarenitic____, 23. 2 Lower Dhruma Formation (about 123 m thick): Shale, poorly exposed, olive-green; rare layers 1. Pre-Thambites beds (about 123 m thick): of ironstone and chalky limestone.______12. 0 Shale, poorly exposed, olive-gray, rarely cal Covered; calculated 4.8 m to top of Dhibi careous; essentially equivalent to Dbibi limestone.______..about 4.8 limestone_____--_____---.--_------13. 9 D132 GEOLOGY OF THE ARABIAN PENINSULA SECTION 30. Al Jurayfah Continued SECTION 32. Al Gh&t Lower Dhruma Formation (about 123 m thick) Continued [Generalized section of Tuwaiq Mountain Limestone at Al Ghat; measu-sments by Pre-I7iam6i.es beds (about 123 m thick) Con. S. S. Roach, 1952] Shale and sandstone; poorly exposed but dom inantly red and tan shale with common Dolomite of Hanifa Formation (Upper Jurassic). interbedded yellow-brown to black, medium- Tuwaiq Mountain Limestone: to coarse-grained, in part calcareous, Limestone, cream-colored to tan, gray- and yello-1"- ferruginous sandstone; occasional thin beds weathering, alternating hard and soft, finely gran of gray to tan, tightly cemented, shelly and ular, commonly clastic-textured and oolitic; abun oolitic calcaienite in upper part_-_-______55. 1 dant masses of chert in upper beds______7. 6 Covered; interval calculated-______about 54.0 Marl, tan, brown-weathering, soft, earthy, massive. 8. 3 Shale, brown, fissile, calcareous.______6. 0 Approximate total thickness of Dhruma Limestone, cream-colored, yellow-weathering, cil- Formation______353. 7 carenitic; thin nodular bedding.______10. 0 Marrat Formation. Residual coral, colonial corals in matrix of soft apl -v- nitic and Calcarenitic limestone; forms distinctive brown-weathering band______8. 3 TUWAIQ MOUNTAIN LIMESTONE SECTIONS Aphanitic limestone, light-gray to tan, compact, SECTION 31. Riyadh water well 1 massive, chalky; abundant coral heads throughout. [Tuwaiq Mountain Limestone reference section from Riyadh water well 1 (lat Lower 25.0 m forms vertical cliff. Thin 1.0-ra- 24°36'43" N., long 46°40'38" E.) between drilled depths 217.6 and 420.6 m. Detailed thick bed of tan coarse-grained oolite calcarenite description and lithologic summary by N. M. Layne, Jr., and B. W. Powers, 19571 20.0 m below top of interval is at top of first cMff of Tuwayq escarpment.______56. 4 Impure limestone of the Hanifa Formation (Upper Jurassic). Calcarenite and limestone; dominantly tan coarse Tuwaiq Mountain Limestone: grained, tightly cemented, in part oolitic calcare Aphanitic limestone, gray to tan, compact, slightly nite intimately interbedded with tan chalky apH- impure; thin bed of dark-gray, poorly sorted, nitic and calcarenitic limestone; forms band in skeletal calcarenite at base..______4.9 cliff face--...--.___...... __.__------_ 12.0 Apanitic limestone, light-tan, moderately porous; Aphanitic limestone, light-gray, soft, chalky, spar"<*- Foraminifera common, rare gray to white chert._ 24. 4 ly detrital; common thin beds of tan, tightly ce Calcarenitic limestone, tan, moderately porous, mented calcarenite and calcarenitic limestone. skeletal; thin bed of gray-tan, strongly cemented, Coral heads occur at several levels._____._____-- 23. 2 fine-grained, skeletal calcarenite at base_____.- 6. 1 Aphanitic limestone, tan, slightly impure, moderately porous; common "birdseye" texture, rare white Total thickness of Tuwaiq Mountain Lime chert______9. 1 stone - - -- 131.8 Calcarenitic limestone, gray, partially recrystallized, Limestone and subordinate shale of the Dhruma Fonra- fine-grained, pellet-skeletal.______5. 5 tion. Aphanitic limestone and calcarenite; complexly inter bedded tan compact aphanitic limestone and gray fine-grained well-cemented biogenic calcarenite; SECTION 33. Khashm Fardah common chert__----__-_-_-_-____.______3. 7 Aphanitic limestone, tan, compact; common "birds- [Generalized section of Tuwaiq Mountain Limestone at Khashm Fardixh; measure eye" texture.______4. 9 ments by B. A. Bramkamp, B. Beverly, Jr., and 8. B. Henry, 1954] Calcarenite, dominantly tan, fine- to medium-grained, Dolomite of Hanifa Formation (Upper Jurassic). Thickness well-cemented, partially recrystallized, pellet- Tuwaiq Mountain Limestone: (mettrt) skeletal; common interbeds of tan aphanitic and Dolomite and calcarenite; red, gray, and tan, medium- calcarenitic limestone______-______--____-__- 13. 4 grained crystalline dolomite with abundant ghosts Aphanitic limestone, tan compact, rarely pelleted; of branching stromatoporoids and head corals com commonly mottled with "birdseye" texture; rare thin beds of tan, fine- to medium-grained calcare plexly interbedded with tan, strongly cemented, nitic limestone. Thin bed of gray, fine- to me poorly sorted, fine-grained to gravel-sized, stroicxa- dium-grained, weakly cemented calcarenite at toporoid-coral calcarenite; common interbedf of base__.--____---_----_--__-______----.- 33. 5 tan, partially dolomitized aphanitic and calcaren Aphanitic limestone, tan to gray, compact, in part itic limestone.... . __ _ - _ --- -- 25.2 marly and impure, rarely partially dolomitized; Aphanitic limestone, cream-colored to tan, poorly occasional thin beds of gray compact pellet cal bedded, compact: forms vertical cliff with sonvi ir carenitic limestone. Interval becomes progres regular ledges. Common corals at several levels. __ 38.1 sively more marly, impure, and pelleted in the Sandstone, yellowish-tan, scoriaceous-weathering, basal part__--__--__--____--____-__-____---- 97. 5 well-cemented, massive.__._--_------6.5 Total thickness of Tuwaiq Mountain Limestone. 203. 0 Limestone and subordinate shale of the Dhruma Forma Total thickness of Tuwaiq Mountain Limestone.. 69.8 tion. Dhruma Formation. SEDIMENTARY GEOLOGY OF SAUDI ARABIA D133 SECTION 34. Al 'Arid SECTION 35. Jabal al Abakkayn Continued [Generalized section of the Tuwaiq Mountain Limestone at lat 18°33' N. (Al 'Arid Hanifa Formation Continued (neten) escarpment); measurements by R. A. Bramkamp, 8.B. Henry, and G.W. Blakslee, 1. Aphanitic limestone, calcarenitic limestone, and 1950) shale (94.4m thick): Limestone, cream-colored to gray, soft, marly, Calcarenite of the Hanifa Formation (Upper Jurassic). _, . ,_. . . T . , Thickness in part finely oolitic; forms massive ledge.. 3. 0 Tuwaiq Mountain Limestone: (meters) Calcarenite and limestone; dominantly tan to gray, Limestone, poorly exposed, cream-colored, chalky, in part finely oolitic; forms scree tightly cemented, medium-grained to gravel-sized, slope with abundant fossils, becoming stromatoporoid- coral- oolite Calcarenite with com harder in upper part at base of prominent mon interbeds of cream-colored soft aphanitic and ledge______--. __ 16.1 calcarenitic limestone; thin bed of red, finely crys Limestone, gray to brown, chalky, rubbly- talline dolomite is presently near base.______32.4 weathering, in part finely oolitic and poorly Aphanitic limestone, cream-colored, soft, platy- and exposed; common very thin interbeds of rubbly-weathering, in part poorly exposed, chalky; golden-brown, tightly cemented, coarse common thin beds of tan, tightly cemented, strom- grained oolite calcarenite, abundant fossil atoporoid-coral-oolite calcarenite throughout ______15.1 fragments throughout. Rubbly-weath Sandstone, brown to reddish-brown, well-cemented, ering scree slope covers lower part of medium-grained, calcareous; rare fossil casts------4.1 interval------______18. 0 Limestone, cream-colored to gray, compact, chalky, Shale, yellowish-gray to brick-red, chippy- aphanitic; alternating beds of sandstone in lower weathering; several very thin layers of 3.3 m____--____-_--____--__--__----____---_-_ 6.9 golden-brown, well-cemented, finely oolitic, Sandstone, brown, well-cemented, medium-grained. _ 3.4 slabby-weathering, partly sandy calcar enite__-______10.1 Total thickness of Tuwaiq Mountain Limestone. _ 61.9 Limestone, cream-colored to reddish-brown, Sandstone of the Khuff Formation. soft, chalky and marly, chippy- to rubbly- weathering, in part finely oolitic, fossilifer- HANIFA FORMATION SECTIONS ous; occasional thin beds of dark-golden- SECTION 35. Jabal al Abakkayn brown, tightly cemented calcarenite. Limestone becomes strongly coralliferous [Hanifa Formation reference section was measured in a continuous traverse at Jabal in lower part and cherty at base of interval- 40. 6 al Abakkayn by R. W. Powers, L. F. Ramirez, and N. M. Layne, Jr., in 1953] Shale, yellowish-tan, chippy-weathering, gyp- Limestone of the Jubaila Limestone (Upper Jurassic). TT ..,-,,. ^ vr i Thickness siferous; interval poorly exposed but lith- Hanifa Formation: (meters) ology verified by digging._.--____--_____ 6. 6 2. Calcarenite and calcarenitic limestone (18.9 m thick): Total thickness of the Hanifa Formation. 113. 3 Oolite calcarenite, yellow to reddish-brown, Tuwaiq Mountain Limestone. tightly cemented, in part sandy, slabby- SECTION 36. Vicinity of Al Ghat weathering, coarse-grained; forms major [ Generalized section of the Hanifa Formation near Al GhiJ; measurements I * S. J. bench______2. 5 Roach, 19S2] Poorly exposed; probably yellowish-cream, Dolomite of the Jubaila(?) Limestone (Upper Jurassic). Ti?cknes* chalky, oolite calcarenitic limestone; forms Hanifa Formation: (meters) scree-covered slope.______4. 8 Limestone, tan, granular, chippy-weathering, in part Oolite calcarenitic limestone, dark-gray, hard, calcarenitic; top of interval considered possible in part sandy, slabby to rubbly-weathering, equivalent of Hanifa-Jubaila contact but dolomiti- partially dolomitized; interval capped by zation and lack of marker beds makes placement of very thin bed of golden-brown oolite and Hanifa top uncertain in this area.______3. 3 calcarenite. Equivalent to top of Jabal Dolomite, light-tan, thin-nodular-bedded, finely crys al Abakkayn______1. 9 talline; common relic clastic texture and thin inter Oolite calcarenite, yellowish-brown, well- beds of brown, tightly cemented calcarenite-______9. 0 cemented, sandy, medium-grained; abun Limestone, off-white to light-gray, abundantly corallif dant organic detritus on weathered surface. 2. 0 erous, nodular to indistinctly bedded, in large part calcarenitic; common thin interbeds of tan, tightly Sandstone, yellow, friable, calcareous, fine cemented oolite calcarenite______23. 6 grained, rubbly-weathering ______---.__ 1.6 Calcarenite and limestone; dominantly tan, strongly Oolite calcarenite, golden-brown, well-ce cemented, coarse-grained, oolite calcarenite com mented, medium- to coarse-grained, in plexly interbedded with tan oolite calcarenitic lime part sandy; abundant fossil debris.__..___ 1.1 stone; some thin interbeds of tan aphanitic lime Limestone, cream-colored, soft, chalky, in stone; abundant coral heads throughout-___-__.-- 18. 8 part finely oolitic______1. 6 Dolomite, brown, coarsely crystalline; capped with Poorly exposed; probably limestone as below thin bed of chert______-___-___--_---__-- 5. 0 grading upward into tan well-cemented Approximate total thickness of Hanifa Forma medium-grained oolite; marked color tion.. ..___--._____--...._.___--_-----_--- 59. 7 change at base.._--__--____-_.______3. 4 Tuwaiq Mountain Limestone. D134 GEOLOGY OF THE ARABIAN PENINSULA SECTION 37. Khashm al Fara' id SECTION 38. Wadl Nisah Continued [ Generalized section of the Hanifa Formation near Khashm al Fara'id; measurements Jubaila Formation Continued (meten)* by R. A. Bramkamp, S. B. Henry, and G. W. Blakslee, 1950] 2. Calcarenitic limestone and calcarenite (20.5 m Limestone of the Jubaila Limestone (Upper Jurassic). Thickness thi ck) Continued Hanifa Formation: (meters) Limestone and calcarenite; complexly inte~- Aphanitic limestone, poorly exposed, light-gray, soft, bedded, off-white to reddish-brown, par platy-weathering, compact, partially dolomitized; tially dolomitized, tight, fine- to mediurr- capped by 0.1-m-thick bed of tan, medium- to grained, pellet-skeletal calcarenitic lime coarse-grained, tightly cemented, skeletal-oolite stone and cream-colored to golden-brown, calcarenite-______3. 6 tightly cemented, partially recrystallized, Dolomite, dark-gray, porous, finely crystalline. _____ 4. 6 fine- to medium-grained, pellet-skeletal Covered; 1.0-m-thick bed of tan, coral-bearing, medi calcarenite-______12. 5 um-grained, tightly cemented oolite calcarenitic Calcarenitic limestone, cream to yellow, tigh4;, occurs near middle.--_----_---__-___-______.__ 13. 5 partially dolomitized, fine- to mediurr- Calcarenite and limestone; mainly gray-brown, medium- grained, pellet-skeletal; rare thin beds of to coarse-grained, tightly cemented, coralliferous- aphanitic limestone, common scattered stromatoporoidal-oolite calcarenite and coarse car coarse-grained shell debris at various levels. 7. 0 bonate intimately interbedded with off-white, par 1. Aphanitic limestone (84.8 m thick): tially dolomitized aphanitic limestone ______14. 4 Aphanitic and calcarenitic limestone, appror- Covered.______7. 2 irriately equal amounts of complexly inte1*- Oolite calcarenite, tan, tightly cemented, poorly sorted, bedded, cream-colored to yellow, tight, abundantly coralliferous and stromatoporoidal, commonly partially dolomitized, very rarely coarse-grained to conglomeratic.______12. 5 sandy aphanitic limestone and golder- Calcarenite and limestone; dominantly tan, tightly brown, tight, fine- to medium-grained, cemented, medium-grained to gravel-sized, corallif- partially dolomitized, pellet-skeletal ca1- erous-stromatoporoidal-oolite calcarenite; subordi carenitic limestone; occasional thin beds of nate beds of cream-colored, soft, partially dolomit tan, tightly cemented calcarenite and fine!" ized aphanitic limestone.______27. 3 crystalline dolomite______--__-_--- 22. 0 Aphanitic limestone, cream-colored to tan, Total thickness of Hanifa Formation.______83. 1 chippy- to rubbly-weathering, tight, rarely Tuwaiq Mountain Limestone. chalky, commonly sandy; occasional thin beds of brown, pellet-skeletal calcarenit>, JUBAILA LIMESTONE SECTIONS limestone and tightly cemented calcarenite- 62. 8 SECTION 38. Wadl Nisafy Total thickness of Jubaila Limestone. _ _ 118. 3 [Jubaila Limestone reference section composited from three measured increments in Wadi Nisah. The lower part was described by E. L. Berg and R. L. Myers in 1945, Hanifa Formation. the middle by R. A. Bramkamp and S. B. Henry 1948, and the upper by R. W. Powers and H. A. McClure in 1961] SECTION 39. Wadl Huraymila Calcarenite of the Arab Formation (Upper Jurassic). Thickness Jubaila Formation: (meters) [Generalized section of the Jubaila Limestone at Wadl Huraymila; measurements by 4. Aphanitic and calcarenitic limestone (6.0 m thick): R. A. Bramkamp and 8. J. Roach, 1952] Aphanitic limestone, tan to yellow, tight, Brecciated calcarenite of the Arab Formation (Upper chippy-weathering, partially recrystallized; Jurassic). single thin bed of gray, tight, skeletal cal JubailaT i- M Formation:-m - Thickness(meters) carenitic limestone near middle.______1. 5 Dolomite, poorly exposed, light-gray, finely crystal Calcarenitic limestone, tan to yellow, tight, line; some partially dolomitized aphanitic lime chippy-weathering, skeletal; occasional thin stone__------__-__------_-----.------_-- 3. 7 layers of aphanitic limestone and beds of Aphanitic limestone, light-gray, chippy-weathering, gray-brown, tightly cemented calcarenite tight_...___.--__-- ______-_--_ _. 4. 5 near middle and at base______4. 5 Dolomite, dark-gray, massive, finely to medium- 3. Calcarenite (7.0 m thick): grained crystalline-____------_-_---_-_-----____ 10. 0 Pellet-skeletal calcarenite, gray to golden- Pellet-oolite calcarenite, cream-colored to yellcw- brown, tightly cemented, fine- to medium- orange, tightly cemented, medium- to coarse-grain ed grained, rarely sandy; thin bed of golden- shelly; common coral heads. Interval becomes brown, tightly cemented, partially aphanitic and sandy toward top. Forms con dolomitized, coarse stromatoporoid car spicuous ledge..-______-_---_-__-_-_-----_-_-__ 8. 4 Dolomite, gray-brown, medium-grained crystalline; a bonate near base. Part of interval is few thin layers of aphanitic limestone______8. 9 poorly exposed______7. 0 Aphanitic limestone, poorly exposed, cream-colorod, 2. Calcarenitic limestone and calcarenite (20.5 m soft, chalky; common thin interbeds of calcarenitic thick): limestone and tightly cemented calcarenite _ _ _ _ _ 5. 1 Dolomite, dark-reddish-brown, compact, Sandstone, cream-colored, friable, fine-grained; some finely crystalline.______1.0 interbeds of tan, soft, sandy, aphanitic limestone_ 5. 5 SEDIMENTARY GEOLOGY OF SAUDI ARABIA E135 SECTION 39. Wadl Huraymila Continued SECTION 41. Dammam wett 7 and Abgaiq well 71 Continued Th'~.knest Jubaila Formation Continued Arab Formation Continued (meters) Aphanitic limestone, cream-colored, soft, rubbly- to B Member (10. 7.m thick) Continued chippy-weathering, in part calcarenitic; occasional Dolomite, brown, finely crystalline; relic clastic thin beds of brown, tightly cemented pellet-oolite texture__-----_------_------_---_----- 1.5 calcarenite in upper part______28. 1 C Member (41.5 m thick): Anhydrite, off-white to gray, rarely brown, Total thickness of Jubaila Limestone.______74. 2 massive______12. 1 Limestone of the Hanifa Formation. Aphanitic limestone, brown, moderately porous, impure; rare irregular blebs of anhydrite___ 0. 1 SECTION 40. Widyan al Majami' Anhydrite, off-white, massive______0. 6 [Generalized section of Jubaila Limestone along lat 20°37 N.; measurements by R. A. Aphanitic limestone, tan, porous, chalky par Bramkamp, R. W. Powers, N. M. Layne, Jr., A. E. Clements, and F. R. Waldron, tially dolomitized; some intergrown brown 1953] coarse anhydrite crystals. ______1. 3 Limestone of Arab Formation (Upper Jurassic). Dolomite, tan to light-brown, porous, finely Jubaila Limestone: crystalline; relic clastic texture ______0. 1 Dolomite, off-white to gray-brown, porous, in part Calcarenite, light-brown, moderately cemented, poorly exposed, commonly ledge-forming, mostly partially recry stallized, fine-grained ______2. 2 medium-grained crystalline; capped by 2.0-m- Calcarenitic limestone, light-brown, moderately thick bed of tan limestone that locally grades to porous.___-_-_-____-_-_------______--- 0. 4 black-weathering, strongly ferruginous, fine Calcarenite, tan, partially recrystallized, strongly grained sandstone. Dolomite includes a number cemented, fine-grained; abundant intergrown of coral- and stromatoporoid-bearing biostromal brown anhydrite in lower part.______0. 5 layers.______81. 4 Aphanitic limestone, light-brown, compact, Sandstone, tan to gray-tan, crossbedded, poorly partially dolomitized__-----__-_--_---_.__- 0. 3 sorted, friable, rarely conglomeratic; common Calcarenite, brown, partially recrystallized and fragments of indeterminate marine fossils.______30. 4 dolomitized, fine-grained, moderately porous, Dolomite and limestone; poorly exposed but covered pellet-____-----_-_ -_-----_---_ -_-__ 0. 2 surface littered with red-brown dolomite and gray Aphanitic limestone, brown, tight______---____ 0. 2 and yellow slabs of limestone...-___----______12. 2 Calcarenite, tan, moderately cemented, partially Sandstone, tan, crossbedded, poorly sorted.______4. 4 recrystallized, fine-grained ______0. 1 Limestone, tan, sandy; complexly interbedded with Aphanitic limestone, ligh^-brown, moderately tan sandstone, yellowish-red dolomite, and brown porous to tight; thin interbeds of brown, sandy coquina______10. 2 moderately cemented calcarenite______2. 0 Sandstone, brown, crossbedded, poorly sorted, in Calcarenite, light-brown, weakly to tightly ce part conglomeratic; shell fragments common_ _ _ _ _ 12. 4 mented, partially recrystallized, fine-grained, Total thickness of Jubaila Limestone.______151. 0 commonly foraminiferal______8. 1 Hanifa Formation. Aphanitic limestone, gray-brown, tight, litho graphic------0. 2 Calcarenite, tan, porous, partially recrystallized, ARAB FORMATION SECTIONS fine-grained____--___---_------_------0.3 SECTION 41. Dammam well 7 and Abgaiq well 71 Aphanitic limestone, brown, tight, stylolitic.--- 0. 1 [Arab Formation type-reference section from Dammam well 7 and Abqaiq well 71 Calcarenite, light-brown, moderately to tightly respectively. R. A. Bramkamp and H. A. Kimball in 1955 described the type cemented, fine- to medium-grained, partially section; R. W. Powers in 1960 designated the lowermost 37.0 m of Arab Formation recrystallized; rare blebs of anhydrite in upper from Abqaiq well 71 a reference section based on new D-Member carbonate data developed through thin-section analysis (Powers, 1962)] part and some coarse fragments of older limestone in lower. ______--___--- 2. 0 Anhydrite of Hith Anhydrite (Upper Jurassic). Calcarenitic limestone, brown, porous; grains Arab Formation: Thickness A Member (16. 8 m thick): (meters) set in lime-mud matrix.______---_-_-- 0. 1 Aphanitic limestone, tan to brown.______1.5 Calcarenite, brown, moderately porous; common No recovery______3. 1 clear calcite cement.______--_---_---- 0. 3 Aphanitic limestone, tan to brown, tight______3. 0 Calcarenitic limestone, brown, moderately po Aphanitic limestone and calcarenite; complexly rous, pellet-detrital-____--_------0. 1 interbedded, tan, tight aphanitic limestone and Calcarenite, tan to brown, porous to moderately brown, tightly cemented, medium-grained cemented, in part partially recrystallized, fine- pellet calcarenite____-.___.______9. 2 to medium-grained; small high-spired gastro B Member (10. 7 m thick): pods and indeterminate molluscs rather Anhydrite, white, massive; subordinate thin common____-__-_------9. 0 interbeds of tan, partially dolomitized, ap hanitic limestone.______4. 6 Dolomite, brown, medium-grained, crystalline; Aphanitic limestone, tan, tight; minor thin relic calcarenite texture often with voids in interbeds of brown, moderately cemented pel place of grains. Ghosts of stromatoporoid let calcarenite-______4. 6 fragments.------.------1- 2 D136 GEOLOGY OF THE ARABIAN PENINSULA SECTION 41. Dammam well 7 and Abgaiq well 71 Continued SECTION 41. Dammam well 7 and Abgaiq well 71 Continued Arab Formation Continued Arab Formation Continued D Member (55.0 m thick): ^SSSff D Member (55.0 m thick) Continued ^SSSff Anhydrite, off-white, massive; rare, very thin Calcarenitic limestone, dark-brown, tight, well- laminations of brown dolomite and gray sorted, partially dolomitized and recrystsl- aphanitic limestone-______1.5 lized; 10 percent fine-grained, well-rounded Dolomite, brown, porous, finely crystalline; relic aggregate pellets. Dolomite, mosaic calcit0!, clastic texture and common blebs of brown and coarse authigenic anhydrite scattered and off-white anhydrite______0. 8 uniformly through rock______0. 2 Anhydrite, off-white, massive; rare laminations Coarse carbonate, brown, moderately porous, of brown limestone and dolomite______2. 0 partially recrystallized, gravel-sized, "algal" Calcarenite, gray-brown, strongly cemented, nodule; nodules are composed of lime mrd tight, fine-grained; common crystals of authi- and sand-sized grains of algal trash ard genic anhydrite______0. 5 pellets, concentric layering is well developed. Dolomite, brown, porous, finely crystalline; relic Many grains surrounded by a thin crust of clastic texture.______0. 7 needlelike calcite crystals growing normal to Anhydrite and dolomite; approximately equal the grain surface (drusy coat)______-_ 0.1 amounts, complexly interbedded mottled Calcarenite, brown, tight, poorly sorted, fine- to white and brown anhydrite and dolomite-.-. 0. 5 medium-grained, in part strongly recryst-l- Anhydrite, white to pale-gray, massive; occa lized and partially dolomitized, algal debris- sional irregular stringers of dark-brown limestone. 6. 5 aggregate pellet; large patches have been Aphanitic limestone, gray, compact; some dis recrystallized to very finely crystalline calcite seminated pyrite______0.1 mosaic and original texture is obscure-______0. 3 Dolomite, tan, porous, finely crystalline; relic Calcarenitic limestone, light-gray-brown, tight, clastic texture and some irregular laminations fine- to medium-grained, partially dolomitized of white and brown anhydrite.______0. 8 and partially recrystallized, algal; rare Fon- Calcarenite, tan, moderately porous, strongly minifera-____---____--__----__-____--_-.-- 0. 2 recrystallized, fine-grained; voids and ghosts Calcarenite, lithology as in 0.3-m-thick cal in place of clastic grains.______0. 3 carenite above ______0. 1 Anhydrite, white to pale-gray, massive; rare Calcarenitic limestone, light-gray-brown, mode^- streaks of aphanitic limestone and dolomite. _ 0. 6 ately porous, moderately sorted, partially Anhydrite and Calcarenite; complexly inter- dolomitized, fecal pellet; pellets are fine to bedded white anhydrite and tan, moderately medium grained, oval shaped, and set in porous, strongly recrystallized calcarenite with lime-mud matrix. Irregular flattening su^- relic clastic texture_--______-____-______0. 7 gests pellets soft at time of deposition. Sor^e Calcarenite, tan, moderately porous, fine algal debris and crystals of authigenic an grained, strongly recrystallized; grains are hydrite in lower part; very rare Fdraminifer*. 0. 7 commonly voided-__.______0. 4 Aphanitic limestone, brown, tight, partially Anhydrite, white to pale-gray, massive ______2. 6 dolomitized; a few scattered fine-grained Calcarenitic limestone, light-brown, moderately aggrega te pellets and fecal pellets ______0. 2 porous, poorly sorted, fine- to medium-grained, Dolomite, dark-brown, tight, euhedral to partly partially dolomitized, aggregate pellet; core anhedral, finely crystalline; medium to of pellets commonly voided and filled with coarsely crystalline anhydrite is intimately chert (variety microcrystalline quartz) and intergrown__-_----_------_---_------0.3 authigenic anhydrite. Some algal debris. Calcarenite, brown, moderately porous, we'i- Pore space consists of highly tortuous channels sorted, medium-grained, heterogeneous bnt and unfilled voids in pellets. Rare Foraminifera- 0. 2 dominantly algal debris; 10 to 15 percent Calcarenitic limestone, brown, tight, poorly aggregate pellets and ooliths. Drusy coating sorted, fine- to medium-grained, dominantly is well developed around individual grains. aggregate pellet; very small patches of pellets Interval strongly recrystallized with dolomite show void porosity and anhydrite filling as and large calcite crystals concentrated in above. Rare isolated coarse crystals of auth algal debris which is more susceptible to igenic anhydrite. Foraminifera common. _ _ _ 0. 4 alteration than other particle types. Very Calcarenite, brown, tight to moderately porous, rare Foraminifera___----_---__------0. 1 poorly sorted, medium- to coarse-grained, Calcarenitic limestone, brown, tight, moderately angular- to well-rounded, pellet-agal; more sorted, dominantly fine- to medium-grained, than 30 percent algal debris, mostly dasyclad angular to well-rounded, heterogeneous b'lt forms. About 30 percent coarsely crystalline mostly aggregate pellet-algal; some partially authigenic anhydrite. Foraminifera common. 0. 3 dolomitized, oolite Calcarenitic limestone in Coarse carbonate, brown, tightly cemented, middle part. Much of original mud matrix "algal" nodule; dominantly gravel-sized aggre appears to be algal dust highly susceptible to gates of fine particles with well-developed recrystallization. Foraminifera very com concentric layering______0. 3 mon______-_-__---__-_-_-_------1-2 SEDIMENTARY GEOLOGY OF SAUDI ARABIA D137 SECTION 41. Dammam well 7 and Abgaiq well 71 Continued SECTION 41. Dammam well 7 and Abqaiq well 71 Continued Arab Formation Continued Arab Formation Continued D Member (55.0 m thick) Continued ^S"' D Member (55.0 m thick) Continued 7XSSff Calcarenite, gray-brown, moderately porous, Calcarenitic limestone, gray to dark-brown, well-sorted, medium- to coarse-grained, well- moderately porous to tight, poorly sorted, rounded, aggregate pellet; some algal debris. grains dominantly coarse-grained to gravel- All grains surrounded by well-developed drusy sized, angular to well-rounded "algal" nodule- coat of acicular calcite crystals. Some clear algal-aggregate pellet; rare Foraminifera and calcite cement and coarse crystals of authigenic stromatoporoid fragments. Percentage of anhydrite. Rare Foraminifera______0. 6 different types of sand- and gravel-sized Calcarenitic limestone, dark-red-brown, mod grains quite variable but mud matrix usually erately porous, moderately sorted, fine- to forms 50 percent or more of rock throughout medium-grained, aggregate pellet calcarenitic interval. Pore space most commonly of dis limestone and poorly sorted stromatoporoid, rupted or solution channel type. Channels coarse calcarenitic limestone; 10 percent algal partly filled with clear calcite cement______3. 1 debris, rare Foraminifera. Original mud Coarse carbonate, dark-brown, slightly porous, matrix discontinuous and makes up 20 percent poorly sorted, medium-grained to dominantly of rock. Some authigenic anhydrite crystals. 0. 5 gravel-sized, angular to well-rounded, hetero Aphanitic limestone, brown, tight, partially geneous, "algal" nodule; 10 to 15 percent each dolomitized; 25 percent uniformly scattered, of aggregate pellets, stromatoporoid fragments, finely crystalline euhedral dolomite. Rare and algal debris, rare Foraminifera. About 5 Foraminifera-_-___-_-______..-___ 2. 0 percent mud-sized grains trapped between Calcarenitic limestone, brown, tight, well-sorted, larger clastic particles tends to seal porosity __ 0. 2 well-rounded, aggregate pellet; 50 percent Calcarenitic limestone, dark-brown, poorly finely crystalline dolomite rhombs and 20 sorted, medium-grained to gravel-sized, percent medium-grained aggregate pellets in strongly disrupted and channeled, aggregate original lime-mud matrix. Very rare Foram pellet; 10 percent algal debris and 15 percent inifera..------_---___--_-_-_____---_ 0. 4 "algal" nodules; Foraminifera common._____ 0. 2 Aphanitic limestone, brown, tight, partially Calcarenite, dark-brown, poorly sorted, fine dolomitized______o. 3 grained to gravel-sized, angular to well- Dolomite, brown, finely crystalline; well- rounded, moderately porous, aggregate pellet- developed intercrystalline porosity. Thin bed algal; 20 percent "algal" nodules, Foraminifera near top contains about 30 percent chert common______-_---_-_-__. 0. 2 cement (variety microcrystalline quartz) and Calcarenitic limestone, gray-brown, moderately some authigenic anhydrite that effectively porous, poorly sorted, fine-grained to gravel- plug intercrystalline pore space. Dolomite sized, subangular to well-rounded, aggregate crystals vary in size from 0.06 to 0.15 mm___ 3. 1 pellet; 20 percent stromatoporoid fragments Aphanitic limestone, brown, tight, partially and "algal" nodules occur at several levels; dolomitized-______o. 2 rare Foraminifera. Some channel porosity Calcarenitic limestone, light- to dark-brown, and calcite cement filling intragranular pore heterogeneous, poorly sorted, fine- to coarse space within individual calcareous algae and grained, angular to well-rounded, moderately stromatoporoid grains--...... __-___-_.--. 0. 9 porous, aggregate pellet-algal-stromatoporoid; Calcarenite, brown, moderately to very porous, pore shape highly irregular owing to odd poorly sorted, fine-grained to gravel-sized, particle shape and common disruption of mud algal; about 20 percent each of stromatoporoid matrix. Rare Foraminifera_-___-_-..___. 3.0 fragments and aggregate pellets, very rare Coarse carbonate, brown, moderately porous, Foraminifera. Pore shape highly irregular poorly sorted, medium-grained to gravel-sized, aggregate peilet-algal-"algal" nodule; rare and often channeled wider than original pack Foraminifera. Bed is moderately well ce ing size between grains_____-_____-___--___ 0. 5 mented with medium-grained crystalline, Calcarenitic limestone, medium-brown, porous, clear calcite-______o. 2 poorly sorted, medium-grained to gravel-sized Calcarenitic limestone, dark-brown, moderately stromatoporoid-aggregate pellet-"algal" no porous, "algal" nodule; rare Foraminifera---- 0. 2 dule-algal; nearly equal amounts of the four Coarse carbonate, dark-brown, moderately po main grain types are set in 15 percent original rous to tight, poorly sorted, medium-grained mud matrix, rare Foraminifera. Porosity is to gravel-sized, angular to well-rounded, highly irregular arid involves intragranular as stromatoporoid-algal-"algal" nodule; signifi well as channel type ___--___-_-_-----_ 0. (5 cant amounts of aggregate pellets, Foraminif era common. "Algal" nodules commonly Calcarenite, dark-brown, very porous (20 per composed of 3- to 10-mm fragments of stro cent visual), well-sorted, dominantly medium- matoporoid encased in concentric layers of grained, well-rounded, moderately cemented, lime mud.______0. 7 aggregate pellet; Foraminifera common..____ 0. 3 D138 GEOLOGY OF THE ARABIAN PENINSULA SECTION 41. Dammam well 7 and Abqaiq well 71 Continued SECTION 41. Dammam well 7 and Abqaiq well 71 Arab Formation Continued Arab Formation Continued D Member (55.0 m thick) Continued *$3£ff D Member (55.0 m thick) Continued Aphanitic limestone, brown, partially dolo Calcarenitic limestone, light- to dark-browr, mitized; some channel porosity filled with moderately to very porous, poorly sortec1, clear crystalline calcite, some stylolites, rare rounded to angular, partially dolomitizec', Foraminifera-______0. 6 dominantly aggregate pellet; abundant stro Calcarenite, dark-brown, tight, moderately to matoporoid fragments, "algal" nodules, an-i strongly cemented, moderately sorted, algal debris at several levels, very rare Foran:- medium- to coarse-grained, aggregate pellet; inifera. Common channel porosity, som? most pore space filled with clear, coarsely intergranular pore shape very irregular- _____ 1. 6 crystalline calcite cement (about 30 percent). Coarse carbonate, light-gray, very porous (in- Middle part strongly altered to deeply sutured tragranular), gravel-sized, stromatoporoicf; mosaic of aggregate pellets in stylolitic contact about 5 percent well-rounded aggregate pellets. with adjoining pellets. Significant amounts Very rare Foraminifera-______0. 1 of algal debris and selective recrystailization Calcarenite, light-brown, highly porous, moder of algal fragments and stromatoporoid grains ately sorted, medium-grained to gravel-sized in lower part. Foraminifera common.______1.2 well-rounded aggregate pellet; well-developei Coarse carbonate, light-gray-brown, low poros drusy coat around each grain, rare Forarr- ity, moderately cemented (about 10 percent), inifera_.______0.2 poorly sorted, medium-grained to gravel-sized, Calcarenitic limestone, dark-brown, highly po "algal" nodule; each grain surrounded by rous, moderately sorted, dominantly fine to drusy coat of acicular calcite crystals generally medium-grained, aggregate pellet; 10 percent followed by calcite cement. Many individual algal debris, rare Foraminifera. "Fecal" particles strongly recrystallized to mosaic pellets occur in lower part. Irregular shape cf calcite. Rare Foraminifera-______0. 2 algal debris plus some channeling makes pore Calcarenite, dark-brown, very low porosity, shape highly irregular-______0. 9 well-sorted and rounded, medium-grained, Calcarenite, brown, very porous, well-sorted strongly cemented (about 25 percent), aggre and rounded, medium- to coarse-grained, gate pellet; about 10 percent medium-grained, weakly cemented, aggregate pellet; rare Fc- well-rounded fragments of stromatoporoid; raminifera______0. 2 Foraminifera common.______0. 5 Coarse carbonate, gray-brown, very porour, Calcarenite, brown, porous, well-sorted, medium- weakly cemented, moderately sorted, coarse to coarse-grained, aggregate pellet; common grained to gravel-sized, algal-aggregate pellet; drusy coat around individual grains. Some individual grains covered by well-developed stromatoporoid fragments, algal debris, and drusy coat. Rare Foraminifera-__.____-_-_ 0. 2 clear calcite cement, Foraminifera very Calcarenitic limestone, brown, tight, partially common.______1. 6 dolomitized, moderately sorted, dominantly Coarse carbonate, light-gray-brown, very porous, fine to medium-grained, aggregate pellet; poorly sorted, dominantly gravel-sized, stro some algal debris near top, rare stromatopo matoporoid-"algal" nodule; small patches of roid, Foraminifera, and echinoderm fragment? aggregate pellets and algal debris, small throughout______-_-_____-___-_--_- 4.0 amount of mud matrix, rare Foraminifera. Dolomite, light-brown, tight to porous, open Pore shape is highly irregular and in part euhedral to strongly sutured anhedral mosaic, solution channel.______0. 4 finely crystalline, (average crystal size about Calcarenitic limestone, dark-brown, porous, 0.12 mm); 10 percent chert (variety micrc- poorly sorted, fine-grained to gravel-sized, crystailine quartz) and authigenic anhydrite ir dominantly aggregate pellet; common "algal" upper part.______1. 0 nodules and algal debris, some molluscan frag Calcarenitic dolomite, dark-brown, finely cry ments and Foraminifera common. From 20 stalline, tightly sutured; small percentage ag to 50 percent original mud matrix.______0. 4 gregate pellets, stromatoporoid fragments, and Coarse carbonate, brown, very porous, gravel- brachiopod debris as well as mud matrir sized, skeletal.___-_-_--_-______-_____ 0. 2 visible in patches, trace authigenic anhydrite. _ 0. 1 Calcarenitic limestone, light-brown, porous, Calcarenitic limestone, light-gray-brown, mod poorly sorted, medium- to coarse-grained, erately porous, tight, poorly sorted, fine aggregate pellet; Foraminifera common. _____ 0. 3 grained to gravel-sized, commonly disrupted, Calcarenite, light-brown, porous, well-sorted, medium- to coarse-grained, rarely angular to angular to well-rounded, partially dolomitized very well-rounded, aggregate pellet; 10 to 20 aggregate pellets; significant amounts of algal percent each of algal debris, Foraminifera, and debris, "algal" nodules, and echinoid frag "algal" nodules, rare stromatoporoid frag- ments occur at various levels, very rare Fo ments______0. 5 raminifera______0. 5 SEDIMENTARY GEOLOGY OF SAUDI ARABIA LN 139 SECTION 41. Dammam well 7 and Abgaiq well 71 Continued SECTION 43. Dahl Hit Continued Arab Formation Continued Hith Anhydrite Continued Thickness D Member (55.0 m thick) Continued (meters) Anhydrite, massive; contains alternating bands of Calcarenite, light-brown, compact, moderately blue and white color. ______9. 5 sorted, angular to well-rounded, aggregate Anhydrite, blue-gray, massive; finely nodular part pellet; pore space is intergranular and clean; ings __..- ______--__-_--_-----__---_---_.._____ 12. 0 however, many grains appear to have been Anhydrite, blue-gray; some small nodules in a matrix soft on deposition and later compacted and of yellow- weathering argillaceous dolomite. ______2. 1 distorted. Rare Foraminifera____ .____._ 0.5 Anhydrite, blue-gray, massive, laminated to platy; common sinuous partings and blue and white lam Total thickness of Arab Formation_ __._. 124, 0 inations and banding. Wavy flow structure prom Jubaila Limestone. inent in parts of interval. ______20. 0 SECTION 42. Wadi Nisah Total thickness of Hith Anhydrite. ._-____.____ 90. 3 Arab Formation. [ Generalized section of the basal Arab Formation at Wadi Nisah; measurements by R. W. Powers and H. A. McClure in 1961] SULAIY FORMATION SECTIONS Carbonate of the middle and upper Arab Formation (Upper Jurassic). SECTION 44. Dahl Hit Basal Arab carbonate: [Sulaiy Formation exposed in Dahl Hit; clifl above and back slope were studied and Calcarenite, gray, tightly cemented, fine- to medium- designated Sulaiy type section by R. A. Bramkamp and T. C. Barger iT 1938. grained, pellet-skeletal; thin bed of gray, tight, The same outcrop, reworked in more detail by S. J. Roach in 1952, is given as aphanitic limestone near middle____-____--___.-_ 3. 5 follows] Solution-collapse breccia, gray-brown, coarse lime Calcarenite of the Yamama Formation (Lower Cretaceous). stone breccia; fragments up to 1.0 m in diameter, usually angular, set in heterogeneous matrix of jum Sulaiy Formation: T£&"' bled carbonate fragments.---_._____-_--___-____ 12. 0 Aphanitic limestone, off-white to tan to gray, thinly Sandstone, yellow, fine-grained, moderately cemented, bedded, commonly nodular-weathering, rarely calcareous, rounded to subrounded, quartzose.. _. 1. 5 sandy; common thin interbeds of pellet-skeletal Aphanitic and calcarenitic limestone, alternating layers calcarenitic limestone and tightly cemented of tan, tight, aphanitic limestone with rare blebs of calcarenitc.______---_-__-_---_----_---_-___- 14. 3 of anhydrite and reddish-brown, tight, pellet-skel Aphanitic limestone, light-gray, cream-colorcd- etal calcarenitic limestone__---__-__.__--____-__ 3. 5 weathering, thinly bedded, moderately porous. ... 7. 0 Calcarenite, yellow-brown, tightly cemented, fine- to Aphanitic limestone, light-gray, cream-colored- medium-grained, sandy, pellet-skeletal___-_-_-_- 1.5 weathering, thinly and nodular-bedded, moderately porous; occasional thin interbeds of yellow-tan, Total thickness of basal Arab carbonate unit- - - _ 22. 0 pellet-skeletal calcarenitic limestone and calcar- Jubaila Limestone. cnitc.------40.0 Aphanitic limestone, light-gray, cream-colored- weathering, thinly bedded, tight to moderately HITH ANHYDRITE SECTION porous; a few thin beds of yellow-tan, pellet- SECTION 43. Dahl Hit skeletal calcarenitic limestone and Calcarenite. Top of interval caps escarpment above Dahl Hit- - 44. 5 [Hith Anhydrite section at Dahl Hit was first described and recognized as the type Calcarenitic limestone, tan to brown, rarely sandy, sequence by R. A. Bramkamp and T. C. Barger in 1938. A detailed remeasure- pellet-skeletal; some interbeds of aphanitic lime ment by S. J. Roach in 1952 is given as follows] stone and skeletal calcarenitc------20. 0 Oolite (Upper Jurassic?) of Sulaiy Formation. Aphanitic limestone, tan, thinly bedded, nodular, Disconformity. Thickness moderately porous; rare shell fragments. ------5. 1 Hith Anhydrite: (meters) Calcarenite, tan, thinly bedded and nodular, tightly Limestone breccia, gray-tan, thin to platy, angular cemented, pellet-skeletal; at places grades to co- blocks recemented in fine-grained marl matrix; thin quinoid calcarenitc with pellet matrix. Small bed of gray-tan aphanitic and calcarenitic lime pebbles of older limestone are common in calcar stone occurs in lower part. May represent slumped enitc beds. A few thin interbeds of tan, moder Sulaiy beds rather than residuum of Hith carbon ately porous aphanitic limestone..__-__-_--_-_-- 12. 6 ates but interval originally contained at least one Aphanitic limestone, gray, tan-weathering, tight to bed of anhydrite..______12. 7 moderately porous, thinly bedded and nodular; Anhydrite, blue-gray, thin, regularly bedded, common rare thin interbeds of calcarenitic limestone. All color banding and lamination; some laminae of yellow beds arc about 0.1 m thick and separated by clay shale in upper part. Highly irregular sinuous con well-defined bedding planes._-_-_-__------14. 7 tact at top with thin beds of anhydrite clearly trun Calcarenitc, tan, tightly cemented, irregularly bedded, cated; knobs and sharp pinnacles of anhydrite pellet-skeletal ______.-_-----_--_-_------12.0 extend up to 0.5 m into the overlying breccia. _ _ _ _. 32. 0 Anhydrite, blue-gray, massive, finely nodular; few Total thickness of Sulaiy Formation. 170.2 irregular lenses of white gypsum.-._.__-_____.._- 2. 0 Hith Anhydrite. 228-776 < D140 GEOLOGY OF THE ARABIAN PENINSULA SECTION 45. Near lat 28°50r N. SECTION 46. Vicinity of lat 24°01' N. Continued [Generalized section of Sulaiy Formation near latitude 23°60/ N., measurements by Yama ma Formation: Continued R. W. Powers and H. A. McClure, 1962] 3. Calcarenite and aphanitic limestone (16.0 m thicM Calcarenite of Yamama Formation (Lower Cretaceous). uontinueaPnnHrmoH Thickness(meter*) Sulaiy Formation: (meter*)* Pellet-foraminifera calcarenite, brown, mod Aphanitic limestone, golden-brown, tight, partially erately well sorted, tightly cemented; 20 per dolomitized.--.------.-.------____.._._..__ 2. 0 cent tabular disarticulated pelecypod valves Pellet calcarenite, golden-brown, moderately sorted, medium- to coarse-grained, tightly cemented; abun showing strong lineation parallel to bedding dant Foraminif era and pelecypod debris. _.-..____ 1.8 and set in matrix of medium- to coar'e- Calcarenitic limestone, light-brown, moderately po grained pellets and Foraminifera all tightly rous, chalky; 20 percent coarse-grained pelecypod cemented with clear secondary calcite. and gastropod shell debris. Original shell material Hill-capping unit______0. 4 altered to clear, coarsely crystalline calcite. Lower Calcareous sandstone, golden-brown, tight, 1.0-m-thick bed is tan aphanitic limestone______3. 2 quartzose; 50 percent fine- to medium- Pellet calcarenite, tan, fine-grained, well-sorted, tightly grained, subrounded to angular quartz sand cemented; abundant Foraminifera ______2. 0 uniformly admixed with lime-mud matrix.. 0. 6 Aphanitic limestone, tan, moderately porous, chalky; Covered______-_____-_----__-_____--__ 1. 0 very rare thin beds of brown, moderately porous, chalky pelecypod calcarenitic limestone______35.5 Pellet calcarenite, golden-brown, well-sortod, Covered interval; thickness unknown______? fine-grained, rounded, tightly cemented; Aphanitic limestone, tan, moderately porous, chalky; abundant calcite-replaced monaxon spor SECTION 46. Vicinity of lot 24°01' N. Continued BtfWAIB FORMATION SECTIONS Yamama Formation Continued SECTION 47. Near Khafs Daghrah 2. Aphanitic and calcarenitic limestone (10.1 m [Buwaib Formation reference section was measured and sampled in 1062 in detail by thick) Continued R. W. Powers and H. A. McClure 14 km northeast of Khafs Daghrah in vic'nity of Calcarenite and limestone; tan, strongly ce lat 24°64' N] mented, well-sorted pellet calcarenite inti Sandstone of the Biyadh Sandstone Lower (Cretaceous) Th'f.knes» mately interbedded with coarse molluscan Buwaib Formation: (meters) calcarenitic limestone as below. Gastropods Coarse pelecypod carbonate, golden-brown, gravel- and Foraminifera uniformly scattered sized, poorly sorted, sandy; almost exclusively pele through both rock types. Weathered bench cypod hash. Most pelecypod shells leached or surface contains abundant, well-preserved replaced by clear crystalline calcite, rare original pelecypods, echinoids, and scarce ammonites. shell material. Angular to subrounded medium- Pygurus cf. P. rostratus common.______0. 5 grained quartz sand common..______0. 1 Coarse molluscan calcarenitic limestone, brown, Dolomite, reddish-brown, rarely sandy, medium- tight, poorly sorted; 50 percent calcite-re- grained crystalline; abundant ghosts of calcite- placed, gravel-sized pelecypod and gastropod replaced pelecypod shells.______1.9 shells randomly oriented in lime-mud matrix Coarse pelecypod carbonate, reddish-brown, poorly and commonly infilled with lime mud. sorted, strongly rersrystallized and partially dolo- Forms brown, biogenic limestone bench____ 2. 5 mitized; original texture partially obliterated by Partly covered; probably off-white, soft, recrystallization; however, still possible to recognize chalky, aphanitic limestone_____-__.__-_ 2. 1 1. Calcarenite and coarse carbonate (19.4 m thick): original textural elements including pelecypod shells, pellets, and secondary calcite cement.______0. 2 Pellet-molluscan calcarenite, golden-brown, Pellet calcarenite, golden-brown, moderately cemented moderately well sorted, fine-grained, strongly very commonly sandy, well-sorted, fine- to medium- laminated, tightly cemented; disarticulated grained______.___-____-__-_---____-_-___ 0. 6 pelecypod valves show definite lineation par allel to laminations. Foraminifera and Aphanitic limestone, brown, tight, partially recrystal- lized; forms soft rubbly bench.______1.0 gastropod debris common. Very thin bed of pelecypod calcarenitic limestone near top. Coarse pelecypod carboiiite, golden-brown, dominantly Upper 0.3-m-thick bed forms prominent dip gravel-sized, moderately cemented; pelecypod shells slope. ______3. 3 replaced by coarsely crystalline calcite or leached; remaining internal molds of lime mud are most com Coarse gastropod carbonate, brown, poorly mon gravel-sized grain______0. 2 sorted, moderately cemented; 60 percent Poorly exposed; probably olive-green shale and impure gastropod remains commonly infilled with aphanitic limestone; 0.1-m-thick beds of brown, cream-colored lime mud. Some gravel-sized rarely sandy, tightly cemented pellet calcarenite, fragments of tan aphanitic limestone appar form weak benches 2.0, 4.0, and 4.4 m above base of ently derived from older limestones. About intcrval.______6. 3 20 percent fine- to medium-grained Foram Sandstone, dark-brown, strongly weathered, quartzose. 1. 0 inifera...______1. 6 Molluscan-foraminiferal calcarenite, golden-brown, Pellet calcarenite, golden- to reddish-brown, poorly sorted, tightly cemented, partially recrystal- generally well-sorted, tightly cemented, very lized; large amounts of indeterminate molluscan? fine to medium-grained; Foraminifera and shell debris, Foraminifera, and pellets.______1.5 molluscan debris common. Occasional thin Dolomite, reddish-brown, medium-grained crystalline, beds of brown, poorly sorted, tightly ce moderately porous; 10 percent original calcitic matrix mented, coarse-grained to gravel-sized, preserved in intercrystallinc areas.______1.5 coarse gastropod carbonate. Single 0.3-m- Pellet calcarenite, golden-brown, very fine grained, thick bed of golden-tan, tight aphanitic lime angular to well-rounded, tightly cemented, well- stone 2.7-3.0 m below top of interval._ _ _.. 9. 5 sorted; angular nature of many of the carbonate Covered__-______----_.______1. 0 grains indicates possible derivation form older Pellet calcarenite, as below, top of unit forms limestone. Top of bed forms major dip slope. _ _ __ 0. 3 extensive dip slope.___._.______.._____.__ 1. 5 Pellet calcarenitic limestone, light-brown, tight, par Covered_.----__-_.___.______... 1. 0 tially recrystallized; recrystallization of mud pellets Pellet calcarenite, brown, poorly sorted, tightly and mud matrix tends to obscure calcarenitic nature cemented; poorly preserved arenaceous Fo of bed..------0.5 raminifera common. About 20 percent of Aphanitic limestone, tan, porous, rarely sandy, chalky; debris is of organic origin, clominantly mol scarce monaxon sponge spiculcs and scattered mol luscan- -_-_-----_--__-.-_.___.____.____ 1. 5 luscan remains and pellets.____--___--_------2. 6 Total thickness of Yamama Formation. _. 45. 5 Total thickness of Buwaib Formation. 17. 7 Sulaiy Formation. Sulaiy Formation. D142 GEOLOGY OF THE ARABIAN PENINSULA SECTION 48. Qo?r Himam SECTION 49. Al Biydd Continued [Generalized section of Buwaib Formation near Qasr Himam; measurements by Biyadh Sandstone Continued R. A. Bramkamp, S. J. Roach and S. B. Henry, 1964] 2. Poorly exposed sandstone (205.2 m thick) Sandstone of the Biyadh Sandstone (Lower Cretaceous). Continued Jg« Buwaib Formation: Covered ______--__--______---____--._ 8. 0 Sandstone, yellowish-brown, silty, fine- to medium- Sandstone, gray, tightly cemented, massive. _ 6. 0 grained, friable; common interbeds of light-gray Poorly exposed; soft shale and sandstone.-.. 43. 0 sandy limestone and some reddish-brown dolomite. 8. 2 Sandstone, black- weathering, conglomeratic. 0. 1 Aphanitic limestone, gray to tan, soft, shelly; common Covered ___ ----- __ ___-______._._-_- 6. 0 thin interbeds of greenish-gray marl and shale and Sandstone, white, massive.------... 0. 5 tan, coarsely skeletal calcarenitic limestone and Covered; probably soft shale and sandstone. 15. 0 tightly cemented calcarenite.-__._____-.__..____ 14. 6 Sandstone, gray, massive... ______1.0 Aphanitic limestone, cream to tan, fossiliferous, platy Poorly exposed; red clay shale. ______19. 0 to rubbly-weathering; occasional thin beds of cal 1. Sandstone (107.2 m thick): carenitic limestone and calcarenite______22. 2 Sandstone, brown, massive, coarse-grained.. 17. 0 Arab Formation. Covered; probably soft shale and sandstone. 34. 0 Ironstone, thin, black- weathering, nodular; BIYADH SANDSTONE SECTION caps low hill_------0. 1 Shale and sandstone; poorly exposed, gr..y SECTION 49. Al Biya$ shale and brown friable sandstone. ______15. 0 Sandstone, brown, platy; impressions or cas*.s [Biyadh Sandstone type section was measured and described by D. A. Holm and N. M. Layne, Jr., in 1949 in Al Biyad about 20 km south of Wadi of tree trunks _ - _____-__------_----_ 4. 0 Sahba'] Sandstone, platy, black- weathering; forms low bench. ______-__-__--_---_- 0. 1 Sandstone of the Wasia Formation (Middle Cretaceous). Covered; probably friable sandstone; fonrs Unconformity (not exposed at type locality). flat plain ______---_-__--__------11. 0 Biyadh Sandstone: Sandstone, black to brown, platy, ferruginous 4. Sandstone (101.8 m thick): nodular. _____-______---_--_-__-_---- 2. 0 Covered______23. 8 Sandstone, brown, iron-stained, coarse Sandstone, black-weathering, massive-bedded; grained. -______-___-_-_------_-__-- 4. 0 forms weak bench-______1.0 Sandstone, brown, massive, coarse-grained. - 20. 0 Sandstone, poorly exposed, coarse-grained to pebbly---_-_-----__--___-__-____---._. 20.0 Total thickness of Biyadh Sandstone- _ _ - 425. 0 Sandstone, gray, nodular-weathering___-_. 1. 0 Buwaib Formation. Sandstone, coarse-grained, in part friable, crossbedded; common quartz pebbles. __-- 35. 0 WASIA FORMATION SECTIONS Sandstone, gray, sugary, hard, crossbedded-_ 7. 0 Covered; probably friable quartz sandstone; SECTION 50. Wasl' some shale at base_____--___-_-_-.___ 14. 0 [Wasia Formation type section was measured and described by R. A. Bramkamp and 3. Shale, limestone, and marl (10.8 m thick): S. J. Roach in 1966 at Wasl'] Limestone, brown to tan, tight, bench- forming; weathers to black chips on dip Dolomite of the Aruma Formation (Upper Cretaceous). slope.-___------_____-_---______0. 5 Disconfonnity. Thickness Shale, green to tan, soft, gypsiferous..-----. 3. 7 Wasia Formation: (meters) Marl, tan to buff; forms weak bench_._-__ 0. 3 Sandstone, brown and yellow-weathering, very fine to Shale, red, sandy______0. 9 fine-grained, in part silty; scattered quartz pebbles. Marl, brown to yellow, sandy; forms several Crossbedding is usually well developed except in weak ledges.______3. 3 uppermost part. Fossil wood is present as fer-u- Shale, red to tan to gray, in part hematitic.. 2. 1 ginous molds.__._------_------19-2 2. Poorly exposed sandstone (205.2 m thick): Shale, red and purple, silty; some green clay shrle, Sandstone, cream-colored to gray, cross- and thin brown to black ironstone partings common. bedded, fine-grained, sugary, porous, A single 0.1-m-thick reddish-brown sandy, siHy friable....------_-_------___---_--. 1.7 dolomite with white-filled vugs at base. Elsewhere Sandstone, coarse-grained, conglomeratic; nearly similar dolomite layers occur at higher levels. forms weak bench, weathers to chips of At one locality just east of Wasl' a local lens of ironstone__-_-___-__---_____.______1. 8 cream-colored, colored, nodular, fine-grained lime Covered______23. 0 stone up to 2 m in thickness, containing rare ammo Sandstone, black-weathering; forms thin nites, extends laterally for about 200 m. Neolobiles ledge___------_.---__--._-----.---_..- 0. 1 vibrayeanus, cidaroid echinoids, and pelecypods Covered______37. 0 8. 1 Sandstone, brown, coarse-grained; forms occur in carbonate lens.------low bench.______2. 0 Sandstone and siltstone; buff and red-banded, fine Covered; sandy plain.______40. 0 grained ; shows small-scale crossbedding ------5.3 Sandstone, gray, massive, tightly cemented. _ 1. 0 Shale, green, brown-weathering, silty sandy.------6.4 SEDIMENTARY GEOLOGY OF SAUDI ARABIA D143 SECTION 50. Wasl' Continued SECTION 52. Wadl al 'Atk Continued Wasia Formation Continued (meters)* Aruma Formation Continued Sandstone and siltstonc; gray, green, and red, rela Disconformity. tively thin-bedded, silty fine-grained sandstone and 3. Dolomite (upper Atj member, 27.6 m thick: coarse siltstonc______3. 0 Limestone, tan, brown-weathering, detrital and oolitic; a few thin beds of sucrose gran Total thickness of Wasia Formation______42. 0 ular dolomite. Orbitoids and large gas Biyadh Sandstone. tropods locally abundant. Upper part of interval is commonly silicified. Strong SECTION 51. Vicinity of Sakakah bench at top.______-_----_--____- 5.0 Dolomite, tan, brown-weathering, sucrose [Generalized section of the Wasia (Sakaka) Formation near Sakakah; measurements granular, in part argillaceous and rubbly- by D. A. Holm and S. D. Bowers, 1952] weathcring; scattered patches of oolite and Limestone of the Aruma Formation (Upper Cretaceous). detrital limestone. Forms strong bench Unconformity. Approximate at top.__------_------_--- 22.6 Wasia (Sakaka) Formation: (meters) 8 2. Limestone (middle Atj member, 41.0 m thick: Siltstonc and sandstone; thin, alternating beds of Limestone, cream-colored, tan-weathering, variegated siltstone and off-white to gray, red- to chalky, granular, sparsely detrital, mas tan-weathering, commonly crossbedded sand sively bedded, mostly rubbly-weathering; stone; some reddish-brown micaceous laminae. alternately forms strong and weak benches. 34.6 Thin layers of ironstone occur at several levels _ _ _ _ 100 Limestone, cream-colored, brown-weathering, Sandstone, off-white to gray-brown, brown- >and massively bedded, soft, chalky and slightly red-weathering, massive, crossbedded, fine- to argillaceous.. __. ______...______---_-_. 3.2 medium-grained, micaceous; occasional lenses of Shale, green, brown-weathering, flaky, fissile; mottled clay shale. ______145 in part calcareous varying to a marl and Sandstone and siltstonc; thinly interbedded red, argillaceous limestone along strike. Abun brown, and maroon, friable, fine-grained, cross- dant corals and Foraminifcra (Loftusia).-. 3.2 bedded micaceous sandstone and siltstone; often 1. Calcarenitic limestone (lower Atj member, 40.0 m lenticular.______40 thick): Molluscan calcarcnitic limestone, tan, brown- Approximate total thickness of Wasia (Sakaka) weathering; consists of abundant rudistids, Formation ______285 oysters, and large gastropods set in matrix Jauf Formation. of chalky limestone. Bedding is irregular, rubbly weathering, and biostromal.._-._- G. 4 Calcarcnitic limestone, cream-colored, tan- ARUMA FORMATION SECTION weathering, chalky, nodular; abundant carbonate material.______--__--__--- 16 0 SECTION 52. Wadl al 'Atk Calcarcnitic limestone, cream-colored, tan- [Aruma Formation type section was measured and described by S. J. Roach in 1952 weathering, chalky, nodular; abundant in area just south of Wadi al 'Atk] clastic carbonate debris. Nodules arc set in a matrix of sandy marl.______.__-_-- 16. 0 Dolomite of the Umm cr Radhuma Formation (Paleo- Dolomite, reddish-brown, dark-red- to brown- cenc and lower Eocene). weathering, granular, sandy; a few small Aruma Formation: pebbles and abundant vugs, many filled 4. Shale and dolomite (Lina member, 32.9m thick: with white coarsely crystalline calcite..--- 1. 6 Shale and dolomite; complexly interbedded yellow-weathering dolomitic shale and Total thickness of Aruma Formation.------141. 5 pink-, blue-, and yellow-mottled, platy, Wasia Formation. impure dolomite______14.4 Dolomite, yellow-, pink-, and blue-mottled, UMM ER RADHUMA FORMATION SECTION thinly bedded, platy, argillaceous....-..- 7.8 Dolomite, blue-gray, sucrose crystalline, ir SECTION 53. Al Bcftin regular rubbly bedded; abundant forami- [Umm er Radhuma Formation reference section was measured and descr'bed by nifcra.-_--__----.___-______3.2 W. II. Reiss and R. D. MacDougall in 1953 in the Al Batin channel] Limestone, tan, gray-weathering, fine Sandy limestone of Miocene and Pliocene age. grained; irregular nodular bedding.______0.5 Unconformity. T^'ekness Marl, greenish-gray, granular, silty. ______2.0 Umm cr Radhuma Formation: (meters) Limestone, cream-colored- to yellow-weather 2. Calcarcnitic limestone, aphanitic limestone, and ing, granular, massively bedded; thin beds dolomite (140.4 m thick): of crystalline, nodular limestone at top Covered.---.-.------2- ° and bottom______1.0 Dolomite, light-brown to gray, medium- Shale, brown-weathering, flaky, calcareous; grained to finely crystalline; upper 1.0 m a few limestone nodules.___-___-____..___ 4.0 shows relic clastic texture.__..--.------4. 5 D144 GEOLOGY OF THE ARABIAN PENINSULA SECTION 53. Al Bfyin Continued SECTION 53. Al Ba\in Continued Umm er Radhuma Formation Continued 2. Calcarenitic limestone, aphanitic limestone, and Umm er Radhuma Formation Continued dolomite (140.4 m thick) Continued 2. Calcarenitic limestone, aphanitic limestone, and dolomite (140.4 m thick) Continued Covered ______4. 2 Dolomite and limestone; brown, medium to Covered ______2. 0 coarsely crystalline dolomite alternating Aphanitic limestone, tan to gray, tight, with thin beds of pellet calcarenitic lime slightly fossiliferous, irregularly bedded, stone and tan soft chalky limestone _ ____ 8. 0 chalky, partially dolomitized; rather rare Aphanitic limestone, light-gray to tan, vuggy, gastropods throughout and some Foram partially dolomized; some megafossil shell inifera near top. ___-----_--__-__-__-_._ 12. 8 debris. Upper bed strongly dolomitized Calcarenite, light-gray, moderately friablo, and weathers to nodular surface _ _ _-__-_ 8. 5 skeletal (coquina)______1. 6 Calcarenitic limestone, gray to tan, irregu Aphanitic limestone, off-white, tight, very larly bedded, partially dolomitized, pellet; thin bedded, fossiliferous; siliceous in upper total interval has brecciated appearance, part becoming chalky in lower; small pele rare Foraminifera. ____------_-____-___. 7. 0 cypods, shell debris, and Foraminifera Aphanitic limestone, gray-brown, tight, par common. ___-_------_-_--__--___---_- 2. 0 tially dolomitized, commonly f ossiliferous ; Calcarenitic limestone, light-gray, soft, po interval capped by thin bed of fine-grained rous, slightly fossiliferous, in part siliceous pellet calcarenitic limestone ______9. 0 pellet..------2. 6 Calcarenitic limestone, dark-gray, irregularly Aphanitic limestone, yellow, porous, fossili*- bedded, fine- to medium-grained, commonly erous, thinly laminated, alternately chalky f ossiliferous, pellet-detrital ; thin bed of and siliceous.... ______2. 2 brown, friable, coarsely crystalline dolomite Calcarenitic limestone, gray, fossiliferou". at top. __---_---_-___-______-_ 7. 5 pellet.-..-- ___ -__-_.._-- ______2. 0 Aphanitic limestone, gray, tight, rarely fos- Aphanitic limestone, gray, in part siliceou^- silif erous ._-__--__-______.__ 2. 8 fossiliferous ______4. 6 Calcarenitic limestone, gray, tight, partially Dolomite, gray, tight, finely crystalline; su~- dolomitized, pellet-f ossiliferous; red- brown f ace weathers to small nodules __-_-_----_ 1.6 to black and cherty on weathered surface. 3. 1 Aphanitic limestone, off-white to gray, tigl t Aphanitic limestone, gray, tight, rather com to slightly porous, rarely fossiliferou^, mon pelecypod shells. ______-______._--_ 0. 5 partially dolomitized; large vugs lined with Calcarenitic limestone, gray, tight, in part coarse calcitc crystals in lower part and partially dolomitized, coquinoid; thin bed common veinlcts of calcite in upper. _ _ . - _ 3. 3 of rubbly-weathering dolomite near middle. 9. 2 Calcarenitic limestone, gray, tight, coquinoid; Aphanitic limestone, gray, finely laminated, upper part with common pellets ______4. 8 tight, hard, siliceous. ______1.2 Aphanitic limestone, gray, tight, common]" Calcarenitic limestone, gray, tight, fossilif - chalky, partially dolomitized, fossilif erou^ erous-detrital; pelecypods rather common. 2. 5 rarely siliceous; beds with concentrators Aphanitic limestone, off- white, brittle to of gastropods common ______7. 3 partly chalky, slightly siliceous; capped by Calcarenitic limestone, gray, tight, coquinoic' . 1. 0 cherty limestone surface ______1.0 1. Aphanitic limestone (102.7 m thick) : Marl, off-white to light-yellow, soft, dull, im Aphanitic limestone, gray, tight, partial!^ pure, fine-grained. ______1.0 dolomitized, chalky, commonly fossilifer Aphanitic limestone, light-gray, tight, sili ous; 4.0 m thick covered interval in lower ceous. ______2. 0 part...... ------22.4 Calcarenitic limestone, gray, tight, skeletal. . 2. 6 Dolomite, blue-gray, tight, fossiliferous, very Dolomite, light-gray, tight, finely crystalline; finely crystalline; common vugs and rare common relic clastic texture. Abundant small gastropods and pelecypods. __._-__. 5. 7 fossil trash in upper part. ______2. 0 Covcred____-__-_-___-_-___-._------3. 2 Calcarenitic limestone, light-brown, tight, Dolomite, gray, tight, finely crystalline; coquinoid- ______0. 5 weathers gray to black______---..- 1.0 Aphanitic limestone, light-gray, tight, thinly Covered ______-______._------1. 8 laminated, chalky, slightly fossiliferous. _ _ 4. 7 Dolomite, gray, mottled black, fossiliferous, Dolomite, brown, medium-grained crystalline; finely crystalline; fossils completely re weathers dark gray______3. 7 placed by dolomite. ______._-_____-_--- 1. 6 Aphanitic limestone, light-gray, chalky, Aphanitic limestone, off-white to light-gray, slightly fossilif erous, partially dolomitized- 1. 5 Covered ______2. 0 thin-bedded, partially dolomitized, chalky, Aphanitic limestone, light-gray, black-mot commonly fossiliferous; poorly exposed in tled, tight, slightly fossilif erous; some lower part__ . ______------_ 18. 2 patches strongly fossiliferous, siliceous, Aphanitic limestone, mostly concealed, prob and partially dolomitized___-______1. 6 ably white, chalky ______3. 2 SEDIMENTARY GEOLOGY OF SAUDI ARABIA D145 SECTION 53. Al Ba\in Continued SECTION 55. Near Dhahran Continued Umm er Radhuma Formation Continued Dammam Formation Continued 1. Aphanitic limestone (102.7 m thick): Khobar Member (9.3 m thick) : Aphanitic limestone, off-white to light-gray, Limestone, light-brown, partially rccrystallizcd, limonite stained in lower part, commonly tight, nummulitic______---_-_____-______3. 0 fossiliferous, chalky, rarely siliceous; gas Limestone, yellow-brown, soft, marly ______1. 2 tropods and pclecypods at several levels, Limestone, off-white, partially rccrystallizcd, rare Foraminifera throughout. Rare vugs. 34. 4 tight, nummulitic____--_---_------_- 3. G Aphanitic limestone, dark-gray, tight, vuggy, Marl, off-white to tan, dolomitic. ______1.5 platy- weathering, strongly dolomitized, in Alvcolina Limestone Member (1.0 m thick) : part siliceous; conchoidal fracture ______3. 0 Limestone, light-tan, partially rccrystallized, tight; Aphanitic limestone, mottled tan and dark- abundant Alveolina eliptica var. flosculina. _ _ _ _ 1.0 gray, rubbly-weathering, tight, partially Saila Shale Member (4.2 m thick) : dolomitized; vermiform weathering surface Shale, dark-brownish-ycllow, subfissilc clay-shale. 3. 6 in lower 2.0 m. Rare, poorly preserved Limestone, gray-buff, tight, Alveolina cf. A Foraminifera- ______7. 2 decipiens common. ______0. 6 Dolomite, dark-gray to yellow and white- Midra Shale Member (3.0 m thick) : mottled, tight, sugary, finely crystalline; Shale, yellow-brown, fissile, very thinly laminated. 3. 0 off-white to tan nodular weathered surface- 1. 0 Total thickness of Dammam Formation. _ _ _ 32. 5 Total thickness of Umm er Radhuma Rus Formation. Formation______243. 1 Aruma Formation. HADRUKH FORMATION SECTION BUS FORMATION SECTION SECTION 56. Jabal al Haydaruk SECTION 54. Umm ar Ru'us [Hadrukh Formation type section was measured and described by Max Ste'nekc [ Rus Formation type section was measured and described by R. A. Bramkamp in and T. W. Koch in 1935 at Jabal al Haydaruk] 1946 at Umm ar Ru'us] Limestone of the Dam Formation (middle? Miocene). Shale (lower Eocene) of Dammam Formation. TT i i i -n i- Thickness Thickness Hadrukh Formation: Rus Formation: (meters) Sandstone, buff, coarse-grained, fossiliferous; abun 3. Limestone, white, soft, chalky, porous; several thin dant calcitc crystals, fish and cchinodcrm remains. _ 3. 8 beds of calcarenite at top______3. 6 Marl, white, conglomeratic _ _ ___-_-.---____------0. 5 2. Marl and limestone; light-colored marl with local Sandstone, buff to yellow-green __-___------_-_---_ 1.5 irregular masses of crystalline gypsum and occa Clay, green ______-_----___------__------2.8 sional thin harder limestone beds; geodal quartz is present at several levels.______31.8 Sandstone, gray-green, medium-grained. ______-_.--- 0. 3 1. Limestone, gray to buff, compact, commonly par Clay, green. ______-____------_--- 1. 8 tially dolomitized; minor beds of soft limestone Gypsum and abundant chert ; becomes marly eastward. 1. 6 made porous by leaching of small organic remains. Sandstone, green, medium-grained ____--_____-__--- 1.8 Rare quartz geodes in lower part and arc typical Clay, green, gypsiferous. ______----_------_------1.5 of uppermost beds_---___-______21. 0 Sandstone, buff, fine-grained, marly ------3. 3 Sandstone, buff and brown, medium-grained- __.-.-- 5. 0 Total thickness of Rus Formation.______56. 4 Marl, white, platy- weathering. ------1.0 Dolomite of the Umm er Radhuma Formation. Clay and sandstone; alternating green clay and gypsif erous sandstone; abundant chert concretions.----- 5.0 DAMMAM FORMATION SECTION Marl, white, clayey. ------0- 3 SECTION 55. Near Dhahran Clay and sandstone; alternating green clay and medium-grained sandstone; abundant chert con [Dammam Formation reference section was measured and described by R. W' cretions. ------3. 0 Powers, M. C. Coffleld, and H. W. Schneider in 1953 in the vicinity of Dhahran] Clay and sandstone; grccu gypsiferous clay and fine Sandy limestone of the Hadrukh Formation (lower Miocene?). grained sandstone ; abundant chert. _ ------1.6 Unconformity. Sandstone, buff and gray, medium-grained, marly _ 1. 6 Dammam Formation: Clay, green; thin bed of green fine-grained friable Alat Member (15.0 m thick): Thickness (meters) gypsiferous sandstone in middle ___--_----_------4. 0 Limestone, cream-colored to tan, chalky, porous, Sandstone, green, hard ------ SECTION 56. Jabal al Haydaruk Continued HOFUF FORMATION SECTION Hadrukh Formation Continued SECTION 58. Near Al Huf-af Marl, gray, very sandy; occasional thin beds of off- [Hofuf Formation type section was measured and described by Max Steineke and white sandy limestone and tan friable sandstone. __ 32. 5 T. W. Koch in 1935 near Al Hufuf] Total thickness of Hadrukh Formation. ------84. 0 Surficial deposits (Quaternary). Dammam Formation. Unconformity. DAM FORMATION SECTION Hofuf Formation: SECTION 57. Jabal al Liddm 4. Conglomerate (9.1 m thick): '$£8%' [Dam Formation type section measured and described by Max Steineke and T. W. Conglomerate, gray, marly, limestone; lime Koch in 1935 at Jabal al Lidam] stone boulders in marly quartz sancl matrix. 9. 1 Clay of the Hofuf Formation (Miocene or Pliocene). 3. Sandstone (48.6 m thick): Dam Formation: (meters)* Sandstone, alternating red and white, argil laceous______---__-__-____._____ 48. 6 Marl, gray, fragmental, fossiliferous-______1.0 2. Limestone (18.2 m thick): Marl, pink, strongly argillaceous.___-___--.-_____-_ 1.0 Marl, white, tough______1.0 Limestone, white, chalky, sandy; weathers to granular calcitic sand.__-_____.____-.____ 16. 2 Clay and sandstone; red and green sandy and silty clay and red sandstone._____--__-_____-__-_____ 8. 0 Limestone, brown, bench-forming. ______2. 0 Marl, buff to white, pebbly; abundant red marl frag 1. Conglomerate (19.1 m thick): ments. Thin bed of conglomerate at top_------1. 7 Conglomerate; red and white limestone boulder Marl, white, chalky.__-______-__.______--____-_ 1. 6 and pebble.______17. 1 Marl, mottled greenish-gray and redd^h- Chalk and sandstone; white chalk and gray cross- bedded sandstone; fossils at base______1.0 brown-._.______-_-__-----.______2. 0 Marl, gray and buff, thin-bedded, fossiliferous-______1. 0 Marl and clay; white marl and green clay___-_----.- 1. 8 Total thickness of Hofuf Formation- _ _ _ _ 95. 0 Marl and clay; yellow fossiliferous marl and green clay. 1. 0 Dam Formation. Sandstone and marl; gray-green sandstone and buff conglomeratic fossiliferous marl.______.--_-__.__ 1.0 BIBLIOGRAPHY Clay, green.______7. 5 Covered.______0. 5 Arkell, W. J., 1952, Jurassic ammonites from Jebel Tuwaiq, Limestone and marl; tan tight sandy limestone and central Arabia: Royal Soc. [London] Philos. Trans., ser. buff fossiliferous marl._____-_---__.___-__----._ 1. 4 B., v. 236, p. 241-313. Marl, limestone, and sandstone; buff and white marl, Basse, filiane, 1937, Les c6phalopodes cretacgs dcr massifs yellow fossiliferous limestone and sandstone.------12. 6 c6tiers syriens: [Syria], Haut-Comm. Rep. Franc. Syric ct Covered.--______-______-_--._____-__--____ 0. 5 Liban, Sec. fitud. Geol., Notes et M6m. v. 2, p. 165-200. Limestone, tan, sandy.______-______-._-_.._- 0. 5 1940, Les cSphalopodes cr6tac6s des massifs c6tiers Marl, greenish-tan, fossiliferous, sandy ______2. 6 syriens: [Syria], Haut-Comm. R6p. Franc. Syric et Liban, Sandstone, greenish-yellow______1.0 Sec. fitud. Geol., Notes ct M6m. v. 3, p. 441-471. Marl, white, chalky.____-______._-__-----_ 0. 2 Bramkamp, R. A., Brown, G. F., Holm, D. A., and Layne, N. Marl and sandstone; white tough foraminiferal marl M., Jr., 1963, Geologic map of the WadI as Sirhan quad and gray oolitic sandstone.___-______.-_--_---- 2. 0 rangle, Kingdom of Saudi Arabia: U.S. Geol. Survey Misc. Marl, white, fossiliferous______8. 8 Geol. Inv. Map 1-200 A. Marl, off-white to yellow, chalky, sandy, fossiliferous; Bramkamp, R. A., Gierhart, R. F., Brown, G. F., and Jackson, Foraminifera common.______4. 8 R. O., 1956, Geology of the southern Tuwayq quadrangle, Limestone, tan, marly, fossiliferous; abundant Foram Kingdom of Saudi Arabia: U.S. Geol. Survey Misc. Geol. inifera______---___-_--__-___-__-_.-_-___--- 3. 6 Inv. Map 1-212 A. Marl, buff and white, tough______1. 0 Bramkamp, R. A., and Powers, R. W., 1958, Classification of Clay, green.______1. 8 Arabian carbonate rocks: Geol. Soc. America Bull., v. 69, Limestone, greenish-tan, fossiliferous, sandy_...__-_- 1. 8 p. 1305-1318. Marl, greenish-gray, fossiliferous, chalky______7. 3 Bellen, R. C. van, Dunnington, H. V., Wetzel, R., and Morton, Marl, abundant echinoids form "Button bed" essen D. M., 1959, Iraq in Dubcrtret, Louis, director, Lexiquc tially an echinoid coquina______1.0 stratigraphique international, volume 3, Asie: Paris, Marl, white, chalky, foraminiferal______3. 5 Internat. Geol. Cong., Comm. Stratig., pt. lOa, 333 p. [I960]. Sandstone, marly______.__.______..___.______-__ 2. 0 Field, Henry C., 1958, Stone implements from the Rub' al Marl, buff, fossiliferous-__--_____---_-_____-_-_-__ 1. 5 Khali, Southern Arabia: Man, v. 58, no. 121, p. 9V94. Sandstone, hard, oolitic; few fossils______1.0 1960, Stone implements from the Rub' al Khali, Southern Sandstone, white, very fossiliferous, marly; few echi- Arabia: M$m, v. 60, no. 30, p. 25-26. noids___-_-_-_-_____---______-______-__-_-_-_- 2. 8 Clay, green______0. 5 Henson, F. R. VS., 1948, Larger imperforatc Foraminifera of Marl and sandstone; white sandy marl and marly sand- South-western Asia: London, Jarrold and Sons, Ltd., 117 stone____-..-_._-_____---__------_--_.-----__. 0. 5 p. [1947]. Hudson, R. G. S., and Chatton, M., 1959, The Musandam Total thickness of Dam Formation_ - ______90. 8 limestone (Jurassic and lower Cretaceous) of Oman, Arabia: Hadrukh Formation. Notes et Mem. Moyen-Orient, v. 7, p. 69-93. SEDIMENTARY GEOLOGY OF SAUDI ARABIA D147 Maync, Wolf, 1959, Biocaracteres et analyse morphome'trique des 1964b, Lituolid Foraminifera from the Jurassic and especes jurassiques du genre Pseudocyclammina (foramini- Cretaceous of Saudi Arabia: Micropaleontology, v. 10, feres); I, Pseudocyclammina lituus (Yokoyama): Rev. de no. 4, p. 405-414, pis. 1-2. Micropale'ontologie, v. 2, no. 3, p. 153-172. 1965, Three new genera of Foraminifera from the Jurassic 1960, Biocaracteres et analyse morphome'trique des of Saudi Arabia: Micropaleontology, v. 11, no. 2, p. 133- especes jurassiques du genre Pseudocyclammina (foramini- 140, pi. 1. feres); II, Pseudocyclammina jaccardi (Schrodt): Rev. de Rezak, Richard, 1959, Permian algae from Saudi Arabia: Jour. Micropale'ontologie, v. 3, no. 2, p. 103-118. Paleontology, v. 33, no. 4, p. 531-539. Miller, Robert P., 1937, Drainage lines in bas-relief: Jour. Sander, N. J., 1962, Apergu pale'ontologique et stratigraphique Geology, v. 45, no. 4, p. 432-438. de palgoge'ne en Arabic Se"oudite orientale: Rev. de Micro Morton, D. M., 1959, The geology of Oman: World Petroleum pale'ontologie, v. 5, no. 1, p. 3-40. Cong., 5th, New York 1959, Proc., sec. 1, p. 277-294. Smout, A. H., 1954, Lower Tertiary Foraminifera of the Qatar Newton, R. B., 1921, On a marine Jurassic fauna from central Peninsula: London, Jarrold and Sons, Ltd., 96 p. Arabia'. Annals and Mag. Nat. History, v. 7, p. 389. Steineke, Max, and Bramkamp, R. A., 1952a, Mesozoic ro^ks of Philby, H. St. J. B., 1922, The heart of Arabia: London, eastern Saudi Arabia [abs.]: Am. Assoc. Petroleum Geol Constable and Co., Ltd., 2 v. ogists Bull., v. 36, no. 5, p. 909. Philby, H. St. J. B., 1928, Arabia of the Wahhabis: London, 1952b, Stratigraphical introduction in Arkell, w. J., Constable and Co., Ltd., 422 p. Jurassic ammonites from Jebel Tuwaiq, central Arabia: 1933, The empty quarter: New York, H. Holt and Co., Royal Soc. [London] Philos., Trans., ser. B., v. 236, p. 241-313. 433 p. Steineke, Max, Bramkamp, R. A., and Sander, N. J., 1958, 1939, Sheba's daughters: London, Methuen and Co., Stratigraphic relations of Arabian Jurassic oil in Habitat Ltd., 485 p. of oil: Am. Assoc. Petroleum Geologists Symposium, Powers, R. W., 1962, Arabian upper Jurassic carbonate reservoir p. 1294-1329. rocks: Am. Assoc. Petroleum Geologists Mem. 1, p. 122-192. Thralls, H. W., and Hasson, R. C., 1956, Geology and oil re Quennell, A. M., 1951, The geology and mineral resources of sources of eastern Saudi Arabia: Internat. Geol. Cong., (former) Trans-Jordon: Colonial Geology and Mineral 20th, Mexico, Symposium Sobre Yacimentos do Petroleo y Resources, v. 2, no. 2, p. 85-115. Gas, v. 2, p. 9-32. Redmond, C. D., 1964a, The foraminiferal family Pfenderinidae U.S. Geological Survey-Arabian-American Oil Company, 1963, in the Jurassic of Saudi Arabia: Micropaleontology, v. 10, Geologic map of the Arabian Peninsula: U.S. Geol. furvey no. 2, p. 251-263, pis. 1-2. Misc. Geol. Inv. Map 1-270 A. U. S. GOVERNMENT PRINTING OFFICE : 1986 O - 22M76