Sedimentological and structural evolution of the Arabian continental margin in the Musandam Mountains and Dibba zone, United Arab Emirates

M. P. SEARLE ] N. P. JAMES \ Department of Earth Sciences, Memorial University of Newfoundland, St. John's, Newfoundland, A1B 3X5 Canada T. J. CALON ) J. D. SMEWING Earth Sciences and Resources Institute, University of South Carolina, Columbia, South Carolina 29208

ABSTRACT gressively removed all of the Lower Creta- first subjected to stratigraphie and structur- ceous. al studies by Iraq Petroleum Company The northernmost extremity of the Oman The Tethyan basinal and ophiolitic rocks geologists (Hudson and others, 1954b; Mountains, the Musandam Peninsula, is of the Dibba zone were emplaced from the Hudson and Chatton, 1959; Hudson, 1960) composed of an allochthonous sequence of east-southeast during the Turonian to lower and later mapped by Shell International Permian to middle Cretaceous shelf carbo- Maastrichtian. After emplacement of these Petroleum Company geologists (Glennie nates. These are separated from ophiolitic allochthonous units, compressional defor- and others, 1974) and in more detail by Elf rocks to the south by a northeast-south- mation in the mid-Tertiary resulted in large- Oil Company (Biehler and others, 1975; west-trending belt known as the Dibba scale, open, "whaleback" folds with wave- Ricateau and Riche, 1980). zone. This structurally complex belt is lengths as much as 15 km, generally with The Dibba zone is a northeast-southwest- composed of allochthonous slope- and north-south axes. In places, these folds, trending belt of tectonic complexity that basin-facies sediments, Haybi volcanic which affect the complete shelf and alloch- separates the Musandam platform sequence rocks, "Oman Exotic" limestones, sub- thonous sequences, are overturned toward to the north from the Late Cretaceous ophiolitic metamorphic rocks, and ultra- the west, and thrusting has caused pre- Semail ophiolite rocks of the Oman Moun- mafic slices. viously lower tectonic units of the Late Cre- tains to the south (Fig. 1). It consists of Lithofacies correlation confirms that taceous stacking order to be thrust over allochthonous sheets of Tethyan slope- fragmentation and rifting of a vast east- previously higher tectonic units, thus revers- facies carbonates (Sumeini Group), pelagic facing carbonate platform occurred in Mid- ing the Late Cretaceous tectonostratig- basinal sediments (Hawasina Complex), dle to Late Triassic time, resulting in the raphy. The maximum amount of transla- alkaline and tholeiitic basalts, "Oman Ex- establishment of a shelf edge and small tion on the later thrusts is in excess of 5 km otic" limestones, subophiolitic metamorphic ocean basin. This margin was typified from on the Hagab thrust, where the complete rocks, and mélanges, all constituting the northwest to southeast by ooid-skeletal lime shelf carbonate sequence of the Musandam Haybi Complex (Searle and Malpas, 1980). sand shoals or small bioherms on the shelf Mountains has been thrust west-northwest These rocks tectonically overlie the shelf edge, a bypass foreslope of well-laminated over the previously higher Hawasina and carbonate sequence, underlie the 10- to 14- periplatform ooze, and basin-margin ac- Haybi thrust sheets. The Tertiary folding km-thick Semail ophiolite sequence, and cumulations of carbonate turbidites or and thrusting can be correlated in time and were emplaced from the east and east- debris flows. The shelf edge now coincides space with the Zagros fold belt of south- southeast during the Late Cretaceous. The with the northern boundary of the Dibba western Iran. only previous specific studies of the Dibba zone and appears to have remained station- zone were made by Alleman and Peters ary from Middle Triassic to Late Jurassic INTRODUCTION (1972), Searle (1980), and Lippard and oth- time. Periods of arrested carbonate sedi- ers (1982). mentation on the platform either because of The Musandam Peninsula forms the This study was undertaken to unravel the exposure (Late Triassic and middle Cre- northern extremity of the 600-km-long stratigraphie and structural history of the taceous) or because of drowning (Late arcuate Oman Mountain belt (Fig. 1) and Musandam and Dibba zones. We first pre- Jurassic) are represented in the basin by continues southeastward from the Tethyan sent an outline of the stratigraphie evolution starved sedimentation and deposition of Zagros crush zone at a major orogenic syn- of the Arabian continental margin, incorpo- radiolarian cherts. A spectacular platform taxis. This area of approximately 3,000 km2 rating new data gained from work in the margin collapse occurred in the middle and is a mid-Permian to Late Cretaceous car- western Ruus al Jibal (Ras al Khaimah) and Late Cretaceous, as represented by massive bonate platform, overlapped on the west by the southern margin of the Musandam (AI conglomerates, with clasts as young as Tertiary shallow-marine and Holocene flu- Fujairah). This is followed by structural Albian, above an unconformity that pro- viatile and eolian deposits. The area was analysis of the emplacement and post

Geological Society of America Bulletin, v. 94, p. 1381-1400, 15 figs., December 1983.

1381

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emplacement folding and thrusting of all stratigraphy similar to that of the autoch- beneath the Jebel Qamar South Exotic in units of the Musandam and Dibba zones. thonous carbonates to the south, but in the Dibba zone (see Fig. 8). Ordovician- Musandam they pass southward along Silurian quartzose sandstones containing PALEOGEOGRAPHIC AND strike into shelf-edge or slope deposits. The Cruziana trilobite trails (Rann Formation STRATIGRAPHIC FRAMEWORK platform rim that coincides with the north- of Hudson and others, 1954a) pass upward ern boundary of the Dibba zone appears to to thin phosphatic bone beds and ferrugi- From late Paleozoic to late Mesozoic have remained stationary from at least nous limestones containing abundant ortho- time, the Arabian Shield lay along the Middle Triassic to Late Jurassic time. cone cephalopods, ostracods, and bryozoa southern margin of the southern Tethys A geologic map of the Musandam Moun- of possible Devonian age. The unconform- Ocean. The outer parts of the continental tains and Dibba zone is shown in Figure 1, ably overlying Permian-Triassic massive shelf and adjacent Tethyan deep-sea floor and a stratigraphic correlation table is given limestone of the Jebel Qamar is the only are now exposed as a series of thrust sheets in Figure 2. For a detailed description of all exotic in the Oman Mountains that shows a emplaced southwestward onto a passive units, the reader is referred to Glennie and Paleozoic continental clastic substrate and continental margin during the Late Cre- others (1974). Owing to the tectonized and has been interpreted as a carbonate buildup taceous (Alleman and Peters, 1972; Glennie detached nature of the deep-water slope on a horst of basement subsequently rifted and others, 1973; Searle and Malpas, 1980). (Sumeini Group) and basin (Hawasina away from the continental platform (Searle Similar allochthonous Tethyan rocks are Complex) sediments, Glennie and others and Graham, 1982). The whole exotic lime- known from a belt stretching at least from (1974) assigned separate formation names stone and its basement now form an Cyprus through the Zagros Mountains of to contemporaneous strata in different tec- enormous slab in a mélange that is tenta- Iran to Baluchistan and Makran and prob- tonic slices. Consequently, a unique strati- tively correlated with the Aruma Group. ably continuing eastward along the Indus- graphic sequence cannot be applied to Tsangpo suture zone north of the Hima- allochthonous units; each major thrust slice Permian-Triassic layas. This belt is thought to represent part has a different stratigraphic sequence, ac- of a larger, Mesozoic, southern (Neo-) cording to its pretectonic position within The late Paleozoic and early Mesozoic in Tethys Ocean that rifted during Permian- the basin. This has resulted in some confu- this region were times of prolonged aridity, Triassic time and closed during the Late sion when mapping, and we have found it followed in the Late Triassic by rifting and Cretaceous and Tertiary (Smith, 1971). more useful to use lithofacies criteria for plate fragmentation. A vast, shallow plat- The Tethyan zone in the Middle East is distinguishing sedimentary packages. A form existed across Arabia, with the impli- bounded to the north by the stable, seismi- sedimentary-profile reconstruction of shelf, cation of a west-to-east shoreline to outer cally mainly inactive block of central Iran slope, and basin facies with the formations shelf transition, illustrated by the passage along the Zagros crush zone (Stocklin, of Glennie and others (1974) in their from arid, alluvial-plain, terrigenous clastic 1974). Seismic evidence from the Gulf of approximate paleotectonic positions is sediments to shallow-water evaporite to Oman (White and Klitgord, 1976; White, sketched in Figure 3. This has been facili- carbonate sediments (Murris, 1980). The 1977) indicates that northward underthrust- tated by the detailed paleontological dating carbonates in the Musandam Peninsula are ing of oceanic crust beneath Makran has made by Glennie and others (1974), by the dolomites of the Bih, Hagil, and Ghail resulted in a large accretionary prism off the presence of coarse conglomerates that can Formations (Fig. 3), which are similar to Makran coast, which includes much of the be traced from one tectonostratigraphic contemporaneous shelf strata in other parts coastal ranges of Makran (Farhoudi and formation to another (Graham, 1980a), and of the western Tethys (Fischer, 1964; Bosel- Karig, 1977). The Gulf of Oman, therefore, by the basin-wide occurrence of distinctive lini and Rossi, 1974). Innumerable shoal- must be a relict part of the Mesozoic Tethys chert units. These cherts, furthermore, re- ing-upward cycles formed on or near not yet affected by continent-continent col- cord times of starved sedimentation in the muddy carbonate tidal flats much like those lision that has occurred in Iran and the basin and can be tied to times of interrupted of offshore Abu Dhabi today (Kendall and Himalayas. carbonate sedimentation on the platform. Skip with, 1969; Evans and others, 1969). In the following sections, important re- South of the Tethyan zone, the Arabian Equivalent Saiq and Mahil dolomites in gional correlations between shelf, slope, and platform consists of Precambrian basement Oman are similar, but relic textures suggest basin strata are discussed and related to the overlain by a conformable sedimentary somewhat higher-energy facies of ooid and evolution of the Arabian continental sequence from Cambrian to Pliocene in age peloid grainstones, as well as rare bioherms, margin. and approximately 5.5 km thick (Brown, implying a position somewhat closer to the 1972; Murris, 1980). Deposition of the shelf margin. Mesozoic shelf sequence along its north- Pre-Permian Basement The first break in this long period of arid- eastern margin was interrupted during the ity is documented in the Musandam Moun- Late Cretaceous by emplacement of Teth- The predominantly Mesozoic shelf car- tains by 200 m of Late Triassic limestones in yan thrust sheets from the northeast. Shal- bonate rocks of the Musandam Peninsula the Milaha Formation. In Oman, equiva- low-water carbonate sedimentation re- are interpreted to have been deposited on lent strata are dolomite. The Milaha grades sumed in the Maastrichtian and Tertiary. deformed Paleozoic strata, but as the base is from shales, tidal-flat dolomites, muddy In the Oman Mountains, pre-Permian a tectonic contact with strata other than open-shelf bioclastic Megalodon-rich lime- basement and Mesozoic shelf carbonate Paleozoic in age wherever exposed, this stones (Fig. 4a), and mollusc-rich bio- rocks are exposed in a number of windows cannot be proven. The interpretation is stromes in the north and central Musandam through the tectonic pile. Carbonate slope- based on analogy with central and southern to outer-shelf skeletal and peloidal lime facies rocks (Sumeini Group) crop out only Oman, where mid-Permian Saiq Forma- sand shoals and coral-algal reefs in the in detached thrust slices at the base of the tion shelf carbonate rocks unconformably south, along the edge of the Dibba zone. allochthon. The platform carbonates in the overlie Paleozoic rocks. This facies pattern heralds a major change Musandam Mountains show an internal A Paleozoic clastic sequence is exposed in the geological evolution of the region and

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Figure 1. Geologic map of the Musandam Peninsula and Dibba zone, United Arab Emirates.

the establishment of a carbonate platform, of deposition of the Sumeini (slope) and Permian-Triassic shallow-water dolomites the margin of which remained in more or Hawasina (basin) rocks, indicating that by and some limestones similar to those of the less the same place until Early Cretaceous this time continental rifting had created a Mahil Formation, but these are abruptly time. small ocean basin with a shelf edge and overlain by Late Triassic to Early Jurassic The formation of the Tethyan basin in slope. The Maqam Formation of the Su- deep-water turbidites, conglomerates, and Triassic time is indicated by the beginning meini Group shows a 500-m section of pelagic limestones (Glennie and others,

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Stage

PLIOCENE

MIOCENE

OLIGOCENE

EOCENE TERTIAR Y PALAEOCENE

MAASTRICHTIAN

CAMPANIAN U SANTONIAN CO CONIACIAN D n TURONIAN LU CENOMANIAN <<-> M h- ALBIAIM LU DC APTIAN a BARREMIAN L HAUTERIVIAIM VALANGINIAN BERRIASIAN

TITHONIAN

U KIMMERIDGIAN OXFORDIAN co CALLOVIAN co < M BATHONIAN ZD BAJOCIAIM —>

PLEINSBACHIAN L SINEMURIAN HETTANGIAN

RHAETIAN

U NORIAN

CO CARIMIAN co LADINIAN E M 1- AMIS IAN

L SCYTHIAN

TATARIAN U KAZANIAN z < f M KUNGURIAN Figure 2. Stratigraphic chart for the Oman Mountains in the United Arab UJ ARTINSKIAN Emirates and Oman, compiled mainly from L Ordovician SAKMAR1AN Glennie and others (1974). basement J. Qamar

1974). Coarser-grained beds contain many sponges, encrusting algae, and foraminifers isolated blocks of Permian and Late Trias- blocks of shelf-margin reef facies (Jebel that are similar to the well-documented sic carbonate rocks are preserved in the Wasa Formation). We interpret this to be Upper Triassic reef facies of the Alps highest thrust slices (Haybi Complex) be- the result of extensional tectonics and (Zankl, 1971; Flügel, 1981). neath the Semail ophiolite. On the basis of foundering of parts of the preexisting plat- structural position, the "Oman Exotics" Oman Exotics form coincident with formation of an ocean (Lees, 1928; H. H. Wilson, 1969) were, dur- basin. The Jebel Wasa Formation shows Although the bulk of the carbonate rocks is ing the Mesozoic, sited away from the main true reef rocks composed of coral-stroma- clearly part of the continental margin, in the shelf carbonates (Glennie and others, 1973, toporoid framestones with abundant calci- Dibba zone and Oman Mountains, large, 1974).

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OMAN MUSANDAM MOUNTAINS PENINSULA Figure 3. Sedimentary profile re- 3500 construction of the Arabian conti- nental margin during the Mesozoic, showing shelf, slope (Sumeini Group), and basin (Hawasina Complex) for- 2500 - mations of Glennie and others (1974) J, M SAHTAN in their approximate original paleo-

2000 geographic positions.

1000

R — SAIO

SUMEINI HAWASINA GROUP GROUP

AUTOCHTHONOUS a -ALLOCHTHONOUS- PARA-AUTOCHTHONOUS I Section to scale

In Oman, many "exotics" have a sub- Stratigraphic similarities imply that these of a chert unit near the base of the Hamrat strate of Triassic alkaline and transitional "exotics" were part of the carbonate plat- Duru Group (Zulla Formation) and the alkaline to tholeiitic volcanic rocks (Searle form until at least the Late Triassic but were progressively more distal Dibba, Dhera, and others, 1980) and are interpreted to rifted away at that time and subsequently and Haifa Formations (Fig. 3). The subse- have developed as buildups on off-axis vol- foundered. This provides additional sup- quent period of sandstone progradation canic islands (Searle and Graham, 1982). port for the concept of fragmentation of an across the shelf is represented in proximal Although "exotics" of Permian age are older platform in mid-Late Triassic and basin facies of the Hamrat Duru by the small (< 200 m thick) and consist of lagoon, formation of a small ocean basin to the east. sandstone and ooid turbidites that make up reef, and forereef rocks, those of Late Trias- the Guwayza Formation. This complex sic age are as much as 1 km thick and are Triassic-Jurassic Boundary unit, which was studied in detail by Glennie mainly reef and back-reef facies, containing and others (1974), varies considerably along abundant Megalodon bivalves similar to A major eustatic drop in sea level at the strike with no sandstone in some areas, all those of the shelf sequences. Adjacent deep- end of the Triassic, (Vail and others, 1977) sandstone in other places, and all conglom- water rocks of the A1 Aridh and Ibra For- is reflected in the Middle East by wide- erate in still other localities. We think that mations contain volcanic and limestone spread progradation of fluvial and coastline this is an excellent example of reciprocal debris and are interpreted partly as a talus sandstones eastward across the shelf (Mur- sedimentation (J. L. Wilson, 1975) and deposit around the "exotic" seamounts. In a ris, 1980). These sands not only blanketed reflects sediment being moved across the few examples where overlying strata are the platform margin but bypassed it and shelf margin via gullies and canyons into seen in contact, the "exotics" are overlain by accumulated in the adjacent basin. In local deep-water submarine-fan accumula- pelagic carbonates of Jurassic age, suggest- Oman, this event is preserved as an ero- tions (Walker, 1976). ing that the carbonate-covered volcanic sional unconformity with ferruginous zones islands became guyots at the end of the Tri- at the top of the Triassic Mahil Formation Jurassic issic as they subsided and became covered together with thin sandstones, ferruginous >y deep-water sediments. oolite, and conglomerates at the base of the The style of Jurassic carbonate sedimen- The two large Jebel Qamar Exotics in the Jurassic Sahtan Group (Glennie and others, tation throughout the Tethys is character- Dibba zone are different in that they con- 1974). The effect is more dramatic in the ized by deposition of shallow-water sedi- ain both Permian (Asfar and Qamar For- Musandam area where the Late Triassic- ments on wide, rapidly sinking shelves and nations) and Triassic (Ummaili Formation) Early Jurassic Ghalilah Formation is com- isolated Bahama-type platforms, with adja- itrata (Hudson and others, 1954a) and at posed of two distinct orange-red sandstones cent belts of turbidites and distal zones of he southeastern corner (see Fig. 8) show a separated by a limestone rich in bivalves, starved sedimentation (Bernoulli and Jen- 'aleozoic substrate. The basal Permian gastropods, brachiopods, and ostracods. At kyns, 1974; Winterer and Bosellini, 1981). ^sfar Formation marine sandstones are the shelf edge, along the northern margin of In eastern Arabia, it is no different, and dur- imilar to those at the base of the Permian the Dibba zone, the Ghalilah thins to virtu- ing the Jurassic, distinctive platform, slope, ilatform carbonates known from southern ally nothing, and there is abundant evidence and basin facies are better developed than at )man subsurface, whereas the quartzose of subaerial exposure. any other time. ands in the Upper Permian and Upper Tri- The subaerial exposure and cessation of The bulk of the relatively thin (~300 m) ssic can be correlated with platform units shallow-water deposition on the platform Sahtan Group in the south is an open plat- s well (for example, Ghalilah Formation). are reflected in the basin by the deposition form deposit of muddy skeletal limestones

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Figure 4. a. Gigantism in Megalodon sp. lamellibranch. Upper Triassic Milaha Formation from the Musandam plateau platform margin, b. Numerous Lithiotis sp. bivalves in Lower Jurassic back-reef facies adjacent to the platform margin in Wadi Ausaq along the northern boundary of the Dibba zone. c. Deep-water radiolarian lime mudstones of the Musandam 4 carbonate exposed at Wad Wid, north of the Dibba zone. The angular unconformity is one of many intraformational truncation surfaces in these slope limestones, d. A chaotic conglomerate-breccia overlying the major unconformity along the northern margin of the Dibba zone that cuts through Lower Cretaceous to Jurassic strata. The accumulation contains clasts as young as Aptian and records failure and collapse of the margin during middle Cretaceous time.

that contain lithiotids near the base and are of the Dibba zone, and the facies consist of erate and breccia, and mudstone and occasionally dolomitized. The upper parts cross-bedded ooid to peloid grainstones and grainstone that give the deposit a vague are commonly cyclic, occurring as numer- coral boundstones. These shelf-edge shoals bedding. Other clasts are made up of cross- ous mudstone to grainstone cycles. The and reefs pass quickly leeward into more bedded oolite that can be recognized in equivalent Musandam units 1, 2, and 3 muddy facies and in the Lias are bordered nearby platform facies. Although some (Glennie and others, 1974) are similar in by lagoons populated with large lithiotid large masses are certainly clasts, others have composition but span a much greater thick- bivalves (Fig. 4b). a biohermal shape suggesting that they may ness, as much as 1,100 m. Muddy, tidal-flat, These well-bedded platform margin stra- be downslope mounds (J. L. Wilson, 1975). shoaling-upward sequences are numerous, ta pass abruptly southward into what can These coarse-grained deposits abruptly and the subtidal facies contains a diverse best be described as a "chaotic zone" of grade laterally into slope mudstones of the molluscan fauna, including Lithiotis. The periplatform talus similar to modern exam- Mayhah Formation. very top of the Jurassic contains a variety of ples of carbonate talus slopes (Mcllreath The Jurassic-Early Cretaceous proximal corals, stromatoporoids, and large gastro- and James, 1978; James and Ginsburg, basin-slope facies (Mayhah Formation, pods that are more numerous southward 1979). This narrow, 1-km-wide zone has a Sumeini Group) is a classic foreslope de- toward the Dibba zone. vague bedding, estimated to average 10° posit as in the model of J. L. Wilson (1975), As in the underlying Triassic facies, relative to horizontal platform strata. The composed of thin-bedded argillaceous lime changes take place from north to south on chaotic zone is composed of boulders and mudstones or periplatform ooze and similar the Musandam Peninsula. The platform rim blocks as large as 30 m across with conspic- to the Holocene example of the Bahamas is partly exposed along the northern margin uous corals and lithiotids, pods of conglom- (Schlager and James, 1978) and interbedded

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resedimented breccias, conglomerates, and Everywhere in the Dibba zone, the Juras- The more distal facies represented by the grainstones (Schlager and Ginsburg, 1981). sic is directly overlain by a porcelaneous Nayid Formation of the Hamrat Duru In the Dibba zone, the Jurassic part of the pelagic lime mudstone. This is buried by Group and the Dhera Formation, although Mayhah is dark gray to black, nonbur- several hundred metres of deep-water argil- still turbidites as in the Jurassic, are now rowed, relatively unfossiliferous lime mud- laceous lime mudstone. The uppermost 200 lithoclastic and peloidal rather than oolitic, stones, the high organic content of which to 300 m is a shallojv-water muddy succes- reflecting the changing nature of the plat- suggests deposition within a euxinic, oxy- sion with numerous beds and biostromes form margin. The Nayid locally contains gen-minimum zone that impinged on the rich in bryozoans, gastropods, rudists, oc- two thick debris flows that Graham (1980b) slope (compare Arthur and Schlanger, casional stromatoporQids, solitary corals, correlates with those in the Mayhah. The 1979). There is abundant evidence of insta- and bivalves. Dhera Formation contains few coarse con- bility, notably conglomerates composed of Along the northern margin of the Dibba glomerate layers and is chiefly turbidites resedimented Mayhah blocks in slope mud- zone, these facies not only are deep-water and interbedded mudstones with chert nod- stones, and spectacular slope unconformi- but they exhibit abundant evidence of slope ules and thin chert beds. The most distal ties (compare J. L. Wilson, 1969; Davies, deposition as well. The lower half of the Wahrah is thinnest but still contains a few 1977) that are particularly numerous in succession is well-bedded porcelaneous turbidites. upper-slope sections adjacent to the peri- limestone grading up into radiolarian lime In this region, the Early Cretaceous was a platform talus (Fig. 4c). mudstones and red to pink marls that are time of platform-margin drowning, but car- The Hawasina basin facies are ooid well burrowed and often illustrate such bonate sediment production soon caught up grainstone turbidites that illustrate all the bathyal ichnofauna as Zoophycus. Punc- to subsidence, and the margin, now skeletal characteristic Bouma subdivisions and that tuating the lower part of this succession are sand shoals and coral-stromatoporoid-rud- are capped by radiolarian cherts (Hamrat conglomerates composed of deep-water ist banks, prograded basinward. Duru Group, Dibba and Dhera Forma- clasts in a similar muddy matrix (the Ashab tions). Occasional conglomerates, like those Limestone of Hudson and Chatton, 1959) Middle Cretaceous in the Mayhah, contain clasts of Triassic as well as deformed and slumped beds. and Jurassic platform-margin and slope Submarine erosion associated with these A disconformity between Musandam and facies. The most distal facies of the Haifa resedimented accumulations has in places Wasia Groups near the base of the Albian and Shamal Formations are mainly red removed some of the lowest Cretaceous so (Fig. 2) in the shelf sequence indicates a cherts and micropeloidal limestone. that pelagic limestones and conglomerates brief period of emergence and nondeposi- As a whole, the Jurassic continental mar- of Valanginian age now lie on Upper Juras- tion. A chert interval during the Aptian- gin was a carbonate platform rimmed by sic shallow- or deep-water strata and con- Albian in the Dibba and Wahrah Forma- ooid lime sand shoals and scattered small tain clasts of Tithonian, Berriasian, and tions in the basin indicates that the emer- bioherms. A chaotic zone of periplatform Valanginian age (Alleman and Peters, 1972). gence and the source area of carbonates on talus lay at the foot of the sand shoals and In the upper part of this sequence, mixed the shelf was again cut off (Fig. 3). graded downward into the fine-grained sed- siliciclastic-carbonate turbidites are com- Shallow-water deposition during the sub- iments of the slope over which passed the mon together with impressive intraforma- sequent middle Cretaceous is recorded in sediment gravity flows before they were tional unconformities similar to those in the two formations of the Wasia Group, the deposited as turbidites and debris flows in Mayhah (Fig. 4c). Nahr Umr Formation and the overlying the adjacent basin. The margin at this time The basinal facies of Early Cretaceous Mauddud Formation. The Nahr Umr was clearly a declivity and not a carbonate age exposed in the Sumeini and Hawasina represents deposition in the Albian sea as it ramp in the sense of J. L. Wilson (1975). and corresponding to the prograding meg- flooded across the eroded surface. It is the asequence are somewhat different from fine-grained and calcareous equivalent of Early Cretaceous those in the underlying Jurassic. The upper the Burgan sandstone to the northwest, two members of the proximal Mayhah which is made up of clastics eroded from the The widespread deposition of red chert in Formation exposed in the Oman Moun- Arabian Shield (Murris, 1980). The Nahr basin facies at the Jurassic-Cretaceous tains are represented by two massive con- Umr becomes progressively more carbon- boundary records not exposure but inunda- glomerate units made up of boulders from ate-rich eastward and in the Oman Moun- tion of the Arabian platform (Murris, all lower Sumeini Group strata, including tains and Musandam is a thick (~ 100 m), 1980). In central Arabia, the oil-bearing some as old as Permian-Triassic Jebel argillaceous, muddy carbonate distinguish- Arab carbonates and Hith evaporite seal Wasa reef facies and others as young as the ed by abundant benthic foraminifers, par- (Upper Jurassic) were covered by shallow- Orbitolina grainstones. The upper 45 to 50 ticularly Orbitolina. marine carbonates; in eastern Arabia, this is m is generally lithoclast grainstone with The Natih (late Albian to Cenomanian) is reflected by the abrupt upward transition abundant Orbitolina and rudist fragments. a sequence of terrigenous to carbonate, from platform-margin facies to pelagic The upper part of the Dibba Formation, shallowing-upward cycles with local radio- limestones. The Early Cretaceous is repre- which is probably Early Cretaceous in age, litid rudist biostromes. West of the moun- sented by the Kahmah Group in the Oman is similar in composition to the upper May- tains in the subsurface, the Mauddud Mountains, Musandam unit 4 in the Mu- hah and upper Hamrat Duru (Nayid For- Formation shows extensive skeletal-peloi- Sandam Mountains, and the Thamama mation), containing massive debris flows dal mudstones. Group in the subsurface to the west (Mor- with clasts as old as Permian-Triassic. In In the Hagil Window (see Fig. 11), Hud- ton, 1959; Tschopp, 1967; H. H. Wilson, the Dibba zone, a few 5- to 15-m-thick sills son and others (1954b) recognized a 50-m- 11969; Glennie and others, 1974). of alkali basalt intrude this sequence. thick sequence of gray limestone conglom-

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erates, breccias, and marls (Lakshaifa For- (Searle, 1980; Graham, 1980a) correspond formably overlie Aruma Group flysch. mation) immediately beneath the Hagab in age and lithology to the Aruma Group, Normal sedimentation resumed at this time thrust. These conglomerates contain abun- and a correlation between the two is pro- after the catastrophic emplacement events dant Orbitolina foraminifers, in both posed. in the Coniacian to lower Maastrichtian. boulders and matrix, of Aptian-Albian age. Along the shelf edge, this period appears The limestone conglomerates are similar to to have been characterized by great instabil- Tertiary upper-slope facies rocks of the Sumeini ity. Exposed along the northern margin of Group, and although the Lakshaifa beds in the Dibba zone is a major unconformity Tertiary rocks have received detailed Wadi Hagil are everywhere thrust-bounded, that progressively cuts down through older study in only part of the Oman Mountain we consider a correlation with the Sumeini Lower Cretaceous strata southward until, at area (Montenat and Blondeau, 1977). Group to be possible (Fig. 2). Batha Mahani, all 600 m or more of the Shallow-marine, highly fossiliferous lime- The middle Cretaceous is characterized Lower Cretaceous have been removed. The stones form the main lithology throughout by extensive progradation over unstable fill above this erosion surface is a chaotic the Paleocene-Eocene and contain abundant slope sediments, followed by the failure of conglomerate containing clasts as much as large coiled gastropods, lamellibranchs, the continental margin on a massive scale 5 m in diameter of all Early Cretaceous echinoids, Nummulites sp., and Assilina sp. during the Late Cretaceous. facies described previously, including the Rudists, which occur commonly in the Aptian grainstones (Fig. 4d). This appears upper Maastrichtian limestones unconfor- Late Cretaceous to record failure of the continental slope on mably on top of the allochthon, died out in a massive scale in Late Cretaceous time. the massive faunal extinctions at the end of Deposition in the shelf, slope, and basin In Wadi Ausaq, a thick sequence of prox- the Cretaceous. The Pabdeh Formation sequences all ended abruptly in the Ceno- imal conglomerates of Campanian age (the defined in Iran (G. A. James and Wynd, manian, when orogenic activity began. A Ausaq conglomerates of Alleman and 1965) and recognized in the United Arab major regional emergence and period of Peters, 1972) is lithologically similar to the Emirates subsurface (Biehler and others, nondeposition occurred in the Turonian Muti Formation, and a correlation between 1975) is the Paleocene to lower Oligocene throughout the Middle East (Murris, 1980), the two is proposed. planktonic marls facies. The shallow carbo- known in the Oman Mountains as the The youngest basinal sediments in the nate facies of the same age used in the "Wasia-Aruma break" (Glennie and others, Dibba zone are sequences of red radio larian Oman Mountains is the Jahrum Formation. 1974). After the "Wasia-Aruma emer- cherts containing Cenomanian radiolaria The overlying Oligocene to lower Miocene gence," a Late Cretaceous foredeep (the (C. Blome, personal commun.). In the Hagil (Asmari and Lower Fars equivalent in Iran) Aruma basin) developed on the continental Window, these cherts are interbanded with is an evaporitic sequence throughout the margin by downwarping of the shelf car- alkaline tuffs containing blocks as much as Gulf area and corresponds to the beginnings bonate sequence. This is indicated by 1 m in diameter of platform carbonates. of uplift in the Oman Mountains and devel- opment of the Zagros-Oman foreland > 1,300-m thickness of Santonian to late A 100-m succession of ankaramitic vol- folded belt. It also coincides with initiation Campanian Fiqa shales in oil-well sections canic rocks containing abundant alkaline of rifting in the Red Sea and the Gulf of west of the mountains (Glennie and others, plutonic xenoliths and trachytic dikes crops Aden (Gass and Gibson, 1969) at the west- 1974). The Fiqa Formation grades laterally out north of Jebel Qamar North. Biotite ern end of the Arabian plate (Fig. 5). toward the mountain front (northeastward) phenocryst separates from the ankaramites into 300 m or more or Muti conglomerates yield a K-Ar age of 96 ± 4 m.y. (D. Rex, Folding and thrusting of the second stage and turbidites dated as Coniacian to Cam- personal commun.), indicating Cenomanian in the Musandam Peninsula and Oman panian. Nearly 3,000 m of Juweiza Forma- alkaline volcanism off the continental mar- Mountains are thought to be late Oligocene tion marls and shales (late Campanian- gin. In Oman, alkaline volcanic or plutonic to Miocene. In that no Tertiary rocks are Maastrichtian) overlie the Fiqa and Muti rocks of Late Triassic, Jurassic, and Cre- exposed in the Dibba zone, this cannot be Formations west of Musandam. The Ju- taceous ages occur in the Haybi Complex proved here, but the same event is seen to weiza contains blocks of Hawasina sedi- (K-Ar dating by D. Rex, personal com- affect Eocene rocks along the western edge ments and Semail ophiolite lithologies, mun.), and in the Cenomanian, volcanism of Musandam and at several localities in indicating that the thrust sheets were em- was associated with a deepening of the mar- Oman, for example, Jebel Sumeini (Glennie placed and available for erosion by that gin, as evidenced by widespread radiolarian and others, 1974); Jebel Awaynah, north of time. The total thickness of the Aruma chert deposition. Ibri (Ricateau and Riche, 1980), around Group from subsurface data is approxi- The subsequent destruction and telescop- Muscat; and lower Oligocene rocks in the mately 4,300 m, and time of accumulation is ing of the Oman continental margin Sur area, southeastern Oman. Seismic data 12 to 15 m.y., giving average subsidence spanned ~20 m.y. from the Turonian to around the Musandam Peninsula (Biehler rates of approximately 350 m/m.y. lower Maastrichtian, when neoautochtho- and others, 1975; Ricateau and Riche, 1980) In that the only localities for the Fiqa and nous, shallow-marine limestones were de- also show east-dipping thrusts cutting up Juweiza Formations are described from oil- posited unconformably on all allochthonous into the Eocene Pabdeh Formation, young- well sections, correlation with any similar rocks. Throughout the Oman Mountains, er Tertiary sediments (upper Miocene and rocks in the mountains is difficult. How- upper Maastrichtian and Paleocene-Eocene younger) being draped over the thrust ever, it would appear logical that the Late shallow-marine limestones, the former con- fronts. Cretaceous sedimentary mélanges in the taining abundant rudists and gastropods, In the Zagros-Oman foreland, this fold- Haybi Complex recognized throughout the unconformably overlie all allochthonous ing episode was responsible for the great Oman Mountains and the Dibba zone rocks, and west of the mountains uncon- "whaleback" folds seen in southwestern

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FORMATIONS REGIONAL EVENTS

IRAN UA.E

Figure 5. Tertiary stratigraphic chart for Iran, United Arab Emirates, and the Oman Mountains, with a correlation of regional tectonic events. Iranian stratigraphy is based on James and Wynd (196S).

Iran, United Arab Emirates, and Oman that south parallel to the Dibba zone, down- phases of crustal shortening involving fold- now form many of the oil traps. Infra- throwing to the east. Whereas most of the ing and thrusting, (1) spanning the Turo- Cambrian Hormuz salt plugs intrude Oman Mountains appear to be rising at nian to lower Maastrichtian period (Late through these large-scale folds in the Zagros present with numerous raised beaches and Cretaceous) and (2) post-Eocene, probably Mountains and central Oman (Kent, 1970). stranded wadi terraces, anomalous areas Oligocene-Miocene. After Miocene time, vertical movements around Muscat and the north and east The first phase ended 160 m.y. of rela- of compensation were dominant with the Musandam coast show drowned fjord- or tively stable sedimentary conditions on the uplift of Jebel Akhdar, Saih Hatat, and the ria-type coastlines, indicating recent tilting shelf, shelf edge, and basin. It involved the Musandam Peninsula. Wells drilled off- toward the northeast and north. closing of a marginal basin and long- shore east of Dibba and seismic data (Rica- distance westward thrusting of nappes dur- teau and Riche, 1980) indicate a thickness STRUCTURE ing the destruction of the continental mar- of at least 3,000 m of Tertiary in the Gulf of gin. Following these events, stable sedimen- Oman and the presence of large-scale nor- The tectonic development of the Oman tation resumed for about 25 m.y. during mal faults aligned approximately north- Mountains belt was controlled by two main upper Maastrichtian to Eocene time. The

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stacking order of thrust sheets (Fig. 6A) has Neo-autochthorious (mainly Tertiary) limestone sequence been mapped in the tectonic windows of Aruma Group: syn-orogenlc flysch Sumeini, Asjudi, Haybi, and Hawasina, wherever the base of the Semail ophiolite is Semail Ophiolite Complex: peridotite, gabbros, dykes, volcanics well exposed (Searle, 1980; Searle and Mal- Haybi Complex: Volcanic-Exotic limestones, melanges, metamorphic sheet. pas, 1980). Each major thrust sheet is bounded by a thrust fault that truncates all Hawasina Complex: abyssal slope-basin sediments imbricate thrusts and folds with associated structural elements. During the last stages Sumeini Group: shelf slope carbonates of emplacement, the ophiolite split into a Hajar Super Group: shelf carbonate sequence number of fault-bounded blocks (Smewing, 1981) and a tectonic mélange (the Batinah Pre-Mid-Permian basement: granites, gneiss. Paleozoic sediments Mélange) developed around and on top of each block. A few large thrust sheets of Companion emplacement siliceous and calcareous sediments were thrust over this Batinah Mélange (Wood- cock and Robertson, 1982) before the whole zone was unconformably covered by upper 2 Maastrichtian and Paleocene shallow-water limestones. The sequence of events leading up to and including the emplacement of the allochthon is discussed in detail by Glennie and others (1974) and by Searle and Malpas B post-Campanian folding of thrust sheets (1980; see their Fig. 3, p. 216). After this first tectonic event, normal shallow-marine carbonate sedimentation resumed unconformably on top of all allochthonous units as the continental mar- gin became stable again. Subaerial exposure was breached in the southeastern Oman Mountains, where a thin laterite horizon C Tertiary re-imbrication - thrusting event — refolding and rethrusting of stacked Campanian thrust sheets. rests on the allochthon (Hopson and others, 1981).

Tertiary Folding and Thrusting

The second phase of crustal shortening D Hagab thrust phase — MusandamPeninsula and interior Oman. occurred in the Cenozoic after the Late Cre- taceous emplacement was completed and affects the nappe complex as well as much of the upper Maastrichtian-Eocene neoau- tochthonous cover. Exact timing of this event is difficult to determine at present Figure 6. Diagrammatic representation of original Late Cretaceous stacking order of because of the lack of detailed mapping of thrust sheets in the Oman Mountains (A), with later folding of thrust sheets (B) and second the Tertiary sequence and the lack of sub- thrusting event (C and D). surface structural control in the desert foreland. Tertiary "whaleback" folds, similar in second phase occurred after the Eocene trichtian period is well established (Alleman style to those of the Zagros fold belt, are when renewed compression caused large- and Peters, 1972; Glennie and others, 1973; well exposed at Jebel Faiyah and Jebel scale (Zagros-style) folding accompanied by Searle and Malpas, 1980) and is summa- Hafit and form major oil traps at Fahud short-distance thrusting of the whole au- rized in Figure 6A. This phase of orogenic and Natih in Oman (Glennie and others, tochthon plus allochthon package. The activity involved formation of a foredeep on 1974). They have wavelengths of 5 to 15 km timing of the second phase is not precise in the continental margin and its rapid infilling and affect the complete Mesozoic shelf the Dibba zone, as no Tertiary rocks are with flysch (Aruma Group). The translation sequence as well as the Aruma Group and exposed here, but the same event affected of the Hawasina, Haybi, and Semail thrust the Tertiary cover. In the foreland, these Eocene rocks elsewhere in the region. sheets was in excess of 150 km, and the folds affect a wholly autochthonous se- ophiolite, being the uppermost thrust sheet, quence; nearer the mountain front, it ap- Late Cretaceous Stacking Order was the farthest traveled and the first to pears that simple fold geometries become dislocate. more complex at depth, where the space The stacking order of the thrust sheets Along the western edge of the Oman problems are accommodated by reverse emplaced during the Turonian to late Maas- Mountains, a regular Late Cretaceous faulting and thrusting.

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Figure 7. a. Folding of Late Cretaceous thrust sheets illus- trated at Jebel Qamar South Exotic, looking south from Jebel Qamar North, b. Inversion of Late Cretaceous stacking order north of Jebel Qamar North. Folded Hawasina sediments thrust over Cenomanian ankaramitic lavas and basal serpen- tinite. c. Sheared serpentinite interleaved with limestone mylo- nite along a thrust contact at base of Jebel Qamar North Exotic, d. Frontal fold of the Jebel Agah (Mayhah Formation; Sumeini Group) dome. e. Inverted east-dipping Juweiza For- mation flysch on the outer frontal fold above the Hagab thrust, which is subsurface at this locality, north of Ghalilah. In back- ground, Jurassic Musandam units 1, 2, and 3 overlie Triassic Ghalilah Formation above AI Jeer village.

Deep seismic soundings in the United which affect the entire tectonostratigraphy, extreme mechanical problems, and the Arab Emirates foreland and off the west are due to westward-directed thrusting on processes involved are not well understood coast of Musandam have revealed extensive the overturned western limbs of large folds. (Elliott, 1976; Ramberg, 1977). The Dibba post-Cretaceous tectonism in the form of They cannot, therefore, be attributed en- zone provides an excellent picture of com- asymmetric folds and westward-directed tirely to paleogeographic irregularities in plex thin-skinned thrust tectonics involving thrusting (Ricateau and Riche, 1980). Map- the Late Cretaceous. the Mayhah Formation (Sumeini Group ping in the Musandam Mountains and slope carbonates), Hawasina Complex, Dibba zone has shown that major reversals DIBBA ZONE: THIN-SKINNED Haybi Complex, and Oman "exotic" lime- of the Late Cretaceous stacking order occur THRUST TECTONICS stones, as well as the metamorphic sheet (Fig. 7b). The sequence of folding and and harzburgite slices (Fig. 8). Tectonic ser- thrusting events that followed the main Late The emplacement of thin nappes of sedi- pentinite mélanges frequently bound thrust Cretaceous emplacement is illustrated dia- mentary or crystalline rocks onto continen- slices. Cross sections are given showing a grammatically in Figure 6. These reversals, tal margins in a coherent fashion poses transect across the western part of the

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Semail Ophiolite Complex peridotite

Basal serpentinite 3 serpentinite melange

—k— Semoit thrust

Amphibolites Metamorphic Greenschists_ Sheet Sedimentary melange Exotic limestone ! P,TR) Rann Fm.(Ord" ? Dev) Haybi tholeiitic Volcanics (TR)

alkalic " (mK"96+4my) Haybi thrust

Hawasina Complex

Hawasina thrust Mayhah Fm. (Sumeini Group) —4— Sumeini thrust

Figure 8. Geologic sketch map of the Jebel Qamar area in the Dibba zone, United Arab Emirates. See Figure 1 for location.

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Sumeini Gp. Upper Hawasina N.W. Mayhah Fm. duplex

S.E.

Jebel Qamar North W. Ayim Jebel Qamar South 11... Ex

500-1 Haybi duplex

STP Semail thrust plane S.E. HyTP Haybi thrust plane

Km- HTP Hawasina thrust plane

Figure 9. Structural cross sections through the Dibba zone, United Arab Emirates, from the Musandam shelf carbonates in the northwest to the base of the Semail ophiolite in the southeast. See Figures 1 and 8 for location.

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Dibba zone from the Musandam shelf car- been thrust directly upon a thin, imbricate red and white cherts interbedded with shales bonates to the base of the Semail ophiolite slice of Hawasina rocks, which in turn over- and grainstones. Unit I is made up of 50 m (Figs. 9, 10). lie with a marked thrust contact Cenoman- of dominantly mafic volcanics and tuffs ian ankaramites and alkali basalts (Fig. 7b). with interbedded limestone conglomerates, Jebel Qamar Transect The thrust sheets all are rooted in a serpen- overlain by 100 m of green and red shales. tinite mélange. Injection of serpentinite The units are everywhere confined to sepa- The peridotite and underlying metamor- from the sole thrust into the imbricates rate thrust slices, and original stratigraphie phic sheet, together with a thin unit of must have facilitated thrusting (Fig. 7c). relationships are uncertain. These units Haybi volcanic rocks, form the uppermost Translation of this Hawasina thrust slice share elements of the Dhera and Dibba structural slices, exposed in the southeast (Fig. 7b) is estimated to be 5 km at the min- Formations described by Glennie and oth- (Fig. 8). They are folded in a series of late, imum. The thrust stack occupies the core of ers, (1974). open to tight synforms and antiforms a late, large synform, the axial trace of The Hawasina contains two thrust com- (Fig. 7a). The two Jebel Qamar limestone which is parallel to the line of cross section plexes showing a repetition of the tectono- massifs, together with their Paleozoic base- (Fig. 8). stratigraphy described above. Individual ment where exposed, form enormous broken- The main body of Hawasina rocks under- thrust slices are thin and rather discontinu- up slabs in a mélange that is tentatively lies a thin, discontinuous sheet of Haybi ous (maximum exposed surface area is correlated with the Aruma Group (Fig. 9). volcanic rocks in the northwestern part of 5 km2). Most sheets extend in subsurface, At the northwestern corner of Jebel the transect. A threefold tectonostratigra- but a few examples of trailing edges can be Qamar North, the "exotic" limestones have phic subdivision of the Hawasina is shown identified (Fig. 9). Individual slices often on Figures 9 and 10. Unit III is composed of contain large, asymmetric, tight to isoclinal, approximately 200 m of limestone turbi- reclined folds (Fj) related to the first stage dites, and Unit II of approximately 300 m of of thrusting. Décollement surfaces are a)

N.W. W. Bat ha Mahani Jebel Ag ah S.E.

Mayhah Fm

N Sumeini Gp

Transported shelf carbonates, late, steep IP mainly Valanginian faults (b es Reimbricated slope carbonates, axial plane Mayha Formation traces, rel. age 8 facing m 3 limestone turbidite units white/red cherts, 1st turbidites HAWASINA red/green shales 8 cherts, coarse Complex limestone conglomerates, volcanics

\/\ brecciated hinge zone Hawasina î- younging direction (C

Figure 10. a. Structural cross section through the Jebel Agah structure in the Dibba zone, United Arab Emirates. See Figures 1 and 8 for location, b. Equal- area, lower-hemisphere, stereographic projection of poles to slaty cleavage (open circles; 9 measurements) and poles to bedding planes (contours at 1.5%, 4.5%, 10.0%, and 15.0% per 1% area; 70 measurements) in rocks of the Mayhah For- mation. c. Equal-area, lower-hemisphere, stereographic projection of poles to bedding planes (contours at 0.7%, 1.5%, 3.0%, and 5.0% per 1% area; 140 measurements) and fold axes (crosses; 29 measurements) in Hawasina rocks to the northwest of the Jebel Agah structure.

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markedly curved, cutting out the over- tion rocks on top of previously higher tion, but in most cases the mylonite is turned short limbs of the folds and cutting Hawasina rocks. derived from rocks of the hanging wall. The upsection toward the trailing edges in the In the Hawasina rocks to the north and mylonite shows a well-developed foliation long normal limbs. The formation of the south of Jebel Agah, the displacement of outlined by the preferred dimensional orien- large-scale Hawasina duplex1 is most likely the Mayhah Formation is accommodated tation of the calcite. The foliation is accen- related to the first thrusting event. After by strike-slip movement on vertical faults tuated by stylolitic surfaces, suggesting that initial emplacement, the thrust slices were trending west-northwest-east-southeast. pressure solution was an important defor- refolded along roughly northeast-south- The Hawasina rocks exposed to the north- mation mechanism. west-trending axes into large, closed to west are presently rooted below the trans- The stereograms (Fig. 12) indicate that tight, inclined folds (F2). They are strongly ported slope carbonates but may have there is a marked difference in structural asymmetric, verging toward the northwest. directly overlain the latter unit in the origi- trend of the units overlying and underlying Small displacements along décollement nal stacking order. They represent the lower the thrust. The folding mechanisms in the planes cutting through the short, over- unit of the two thrust complexes exposed in shelf carbonate sequence vary from flexural turned limbs are common. the northwest part of the Jebel Qamar tran- slip in the monotonous, thick-bedded, com- sect (Fig. 9). The orientation data for the petent sequences to flexural flow in the mul- Jebel Agah Transect Hawasina differ from the simple pattern tilayered sequences. In all cases, the bulk displayed by the Mayhah Formation, re- strain of the cover sequence outside the This transect parallels the northwestern flecting the more complicated deformation thrust zone itself is extremely low. part of the Jebel Qamar transect (Fig. 9) but history of the Hawasina. It is clear, how- The geometry of the frontal fold of the runs across the Jebel Agah dome farther to ever, that the F2 folds in the Hawasina can cover sequence (Fig. 12) suggests a west- the northeast (Fig. 10). Jebel Agah is the be correlated on the basis of style and orien- ward direction of transport. At Al Jeer, prominent window of Mayhah Formation tation with the folds affecting the Mayhah inverted Juweiza Formation flysch contain- (Sumeini Group) slope carbonates sur- Formation. ing clasts of Semail and Hawasina origin rounded by allochthonous Hawasina sedi- crops out on the frontal fold, dipping ments on all sides (Fig. 1). It contains two MUSANDAM MOUNTAINS: steeply eastward (Fig. 7e). The fold axial large, asymmetrical, northwest-verging GEOMETRY AND KINEMATICS directions of the underlying Hawasina rocks folds. The central fold has a hinge zone OF THE HAGAB THRUST are markedly oblique to this trend (Fig. 12). showing extensive brecciation. The frontal It is interesting to note that on either limb of fold (Fig. 7d) has an overturned short limb, The Musandam carbonate platform is the frontal fold in the cover sequence the which to the northwest also contains over- allochthonous, having been transported to sense of asymmetry of minor folds is con- turned beds of Hawasina unit II. Along its present position along the Hagab thrust. sistently westward-oriented. This suggests strike, away from the line of cross section, The thrust nature of the Ruus al Jibal shelf that the minor folds developed prior to the these beds overlie with a thrust contact carbonate sequence was first demonstrated large-scale frontal fold structure, probably rocks of Hawasina unit II in normal strati- by Hudson and others (1954b). The Hagab in response to a predominantly simple graphic position. Poles to bedding of the thrust, well exposed in the Hagil Window, shear-type of deformation. Mayhah Formation define a broad, partial, Ras al Khaimah (Fig. 11), is defined as the Finally, the question remains—How is great-circle girdle reflecting the gentle do- contact between the overlying Permian to the westward displacement of the Musan- mal structure of the unit; the dominant fold Cenomanian shelf carbonates and the un- dam shelf carbonates on the Hagab thrust axial trend is north-northeast-south-south- derlying allochthonous rocks of the Hawa- accommodated farther south in the alloch- west (Fig. 10). sina and Haybi Complexes. The strati- thon? Previous explanations have assumed Strain in the Mayhah Formation is graphic thickness of the shelf sequence the "Dibba line" to represent a transform extremely low and pressure solution effects transported on the Hagab thrust is approx- fault (the "Oman Line" of Falcon, 1967; are rare. Shortening was accommodated by imately 3,800 m. The minimum amount of Stocklin, 1968; Gansser, 1974), in which flexural slip folding, as indicated by the translation of the thrust sheet is 5 km (the case, the inferred left-lateral displacement ubiquitous presence on bedding planes of distance between the westernmost over- along the Dibba line could equal the dis- slickensides orientated perpendicular to the thrust unit and the easternmost Hawasina placement along the Hagab thrust. How- fold axis. The transport direction is inferred unit), but the true distance could be consid- ever, detailed mapping reveals no evidence to be toward the west-northwest, in view of erably more. No pre-Permian basement of transform strike-slip tectonics in the the vergence of the folds. The shortening rocks are exposed in the Musandam Penin- Dibba zone. Rather, from sedimentary calculated for the maximum outcrop width sula, and therefore it is not known to what facies reconstructions, it can be demon- of the unit is thought not to exceed 1 km extent basement is involved in the thrusting. strated that the shelf edge faced southeast, (30% of outcrop width), in view of the fold In Figure 12, a cross section of the frontal and structural evidence indicates that the style. This figure may represent a minimum fold and thrust of the Hagil duplex is shown nappes of the Dibba zone were emplaced amount for displacement on a basal décol- on an east-west traverse from just south of from the east-southeast onto this margin. In lement surface. The displacement is thought Rams across to the northern part of the other words, the "Oman Line" may exist in to be a post-Cretaceous rethrusting event Hagil Window (Fig. 11). In all exposures of Iran, but it does not occur in Oman-United placing previously lower Mayhah Forma- the thrust contact, a marked calc-mylonite Arab Emirates. A more feasible solution is developed, reaching a thickness of 10 m in suggests that the Tertiary thrusting or reim- the northwest corner of the Hagil Window. brication event seen in Musandam is con- 'A "duplex" is a thrust sheet (mass) that is tinuous southward at greater depth beneath bounded by a "floor thrust" and a "roof thrust" Hudson and others (1954b) mapped part of McCIay and Price, 1981, p. 8). this mylonite zone as the Lakshaifa Forma- the desert foreland and that these thrusts

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loides rohri, Globigerina boweri, and Glo- borotalia centralis (A.T.S. Ramsay, 1981, personal commun.). They are, therefore, of middle Eocene age and are correlated with the Pabdeh Formation.

Jebel Sumeini (Oman)

This locality is in Oman territory south of the area of study (Glennie and others, 1974). On the western side of the jebel, Upper Tri- assic reefal beds of the Jebel Wasa Forma- tion are exposed in a westward-facing anticline. These beds have been thrust westward over Eocene nummulitic lime- stones along a tectonic contact that dips eastward at 30° to 40°. Taking evidence from all of these areas into consideration, it seems most likely that the second major deformation is post- middle Eocene and may correspond to a pronounced regional uplift of the Arabian platform when evaporites were deposited (Lower Fars Formation) (Fig. 5) in the Oligocene-Miocene.

STRUCTURAL AND TECTONIC EVOLUTION

The structural evolution of the area can be broadly related to the destruction of a passive Mesozoic continental margin be- ginning in Cenomanian time, shortly after the Semail ophiolite complex was formed. Coleman (1981) and Hopson and others (1981), working in the southeastern Oman Mountains, believed that the Semail ophio- lite crustal sequence was formed at a spread- ing center, whereas Pearce and others (1981), working in the northern Oman Mountains, stated that it formed by back- arc spreading accompanied by eruption of basalts and andesites from discrete volcanic centers above a short-lived subduction zone. Subophiolite metamorphic rocks of greenschist and amphibolite facies were formed during the Cenomanian to Campan- Figure 11. Geological sketch map of the Hagil Window area, Ras al Khaimah, United ian by underthrusting of Tethyan basin sed- Arab Emirates, showing line of section of Figure 12. See Figure 1 for location. iments and volcanics beneath the ophiolite. This zone of underthrusting must have carried all the overlying units to the east in a Dhayah (United Arab Emirates) penetrated to a depth of at least 9 km below "piggyback" fashion. the petrologic Moho into the mantle to A section across the western edge of the account for such a thickness of peridotite AGE OF TERTIARY FOLDING platform sequence is shown (Fig. 13), 1 km now present in the Oman Mountains (Hop- AND THRUSTING east-southeast of the old fort at Dhayah. son and others, 1981). The metamorphic Thick-bedded lime mudstones of Musan- rocks, strongly imbricated, outcrop in a None of the transects reveals the absolute dam unit IV are exposed in a northwest- wide area of the Dibba zone (Fig. 8), as well age of the second deformation, because facing inclined anticline. These beds have as in tectonic windows beneath the ophiolite none exposes strata younger than Late Cre- been thrust northwestward over Wasia in Oman. taceous. Two localities were studied where Group limestones and a steeply dipping Geothermometry of coexisting mineral Tertiary rocks are involved in this later sequence of yellow marls. These marls con- phases (garnet-clinopyroxene) indicates a event. tain Globorotalia spinulosa, Truncorota- temperature range of 755 to 865 "Cat pres-

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lail

Kilometre

b) Figure 12. a. Structural cross sections through the Hagil duplex and frontal fold of the Hagab thrust sheet, Hagil Window, Ras al Khaimah, United Arab Emirates. See Figure 11 for location, b. Equal-area, lower-hemisphere, stereo- graphic projection of poles to bedding planes in the Hagil duplex (contours at 0.75%, 1.5%, 3.0%, 4.5%, and 6.0% per 1% area; 130 measurements), c. Equal- area, lower-hemisphere, stereographic projection of poles to bedding planes (solid circles; 77 measurements) and fold axes (crosses; 21 measurements) in Ha- wasina rocks below the Hagab thrust.

Hagil duplex - Shelf carbonates

sures of 5 kbar (Ghent and Stout, 1981) or does not imply the absence of a subduction Haybi Complexes were formed. A model close to 900 °C at 4 kbar, according to zone. 40 Ar-39 Ar dates on hornblendes from for the emplacement of the thrust sheets is Searle and Malpas (1982), for formation of amphibolites have a mean total fusion age drawn in Figure 15. the high-grade amphibolites. Such narrow of 90 ± 3 m.y. (Lanphere, 1981). Formation In the southeastern Oman Mountains, zones of high-grade dynamothermal meta- of the amphibolite beneath the ophiolite however, Coleman (1981) argued that sub- morphic rocks, formed at these tempera- occurred only a very short time (less than duction did not begin until post-Eocene tures and pressures, can only have been 3-7 m.y.) after crystallization of plagiogran- time beneath Makran. The Cenomanian- produced by shearing along a mantle- ite in the ophiolite that gives a mean iso- Turonian subduction zone, thought by tapping thrust system that Searle and Mal- topic U-Pb age on zircons of 95 m.y. (Tilton Searle and Malpas (1980, 1982) to be respon- pas (1980, 1982) argued must have been a and others, 1981). A Late Cretaceous recon- sible for obduction of the Oman ophiolite, short-lived subduction zone. The fact that struction of the Arabian continental margin would have been a separate and earlier true blueschist-(glaucophane + lawsonite) in the north Oman Mountains is shown in event, unrelated to the late Tertiary-Holo- facies rocks do not occur in Oman is due Figure 14. Subsequent emplacement of cene subduction zone in the Gulf of Oman. simply to the fact that temperatures were thrust sheets spanned the Late Cretaceous It is indeed difficult to envisage how the too high for their stability, and their absence when major duplexes in the Hawasina and Semail ophiolite, which includes a complete

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N W SE fragmentation occurred in Middle to Late Triassic time and that the new carbonate shelf margin remained in approximately the Main Hagab-type same position (now along the northern thrusts margin of the Dibba zone) throughout Musandam4 (Aptian) much of the Mesozoic. - grey lime mudstones 2. The new (Jurassic-Cretaceous) plat- form margin was a series of ooid-skeletal, lime sand shoals that graded seaward into a bypass foreslope and basin-margin carbon- Ma uddud Fm.- Grey Orbitolina » (U Albion) packstones ate turbidites and debris flows. Periods of - (Eocene) 7 ~ . arrested carbonate sedimentation on the ~ - __ yellow marls.glauconitic at base platform, either because of exposure (Late Nahr Umr Fm. - - - - ____ Mauddud Fm. Triassic and middle Cretaceous) or because (L. Al bian)-Buff Orbitolina packstones of drowning (latest Jurassic), are repre- sented in the basin by starved sedimentation Figure 13. Schematic cross section through footwall imbrications of main Hagab thrust, and deposition of radiolarian cherts. showing thrusting affecting Eocene Pabdeh Formation; 1 km east-southeast of the old fort 3. Failure and collapse of the platform at Dhayah. margin coincident with exposure of the platform interior (Wasia-Aruma break) are crustal sequence and 9 to 12 km of mantle Oligocene-Miocene time caused large-scale documented along the northern boundary material, could have been obducted without (Zagros-type) folding of all units associated of the Dibba zone by a major unconformity a mantle-tapping zone of subduction, as with short-distance thrusting (Hagab thrust- that progressively removed all of the Lower Coleman (1981) postulated. The fact that type), as recorded in Musandam. Late- Cretaceous and is overlain by a massive and only a narrow zone of tectonically repeated stage thrusts affect the complete Mesozoic chaotic breccia-conglomerate composed of Sumeini, Hawasina, and Haybi rocks and lower Tertiary sequence and probably clasts as young as Albian-Aptian. separates the shelf edge along the northern extend down into the pre-Permian base- 4. The Dibba zone is thought to represent margin of the Dibba zone from the ophio- ment. the original position at which the shelf edge lite suggests that the Cretaceous shelf edge collided with the subophiolite thrust sheets lay adjacent to the northeast-dipping sub- CONCLUSIONS during Late Cretaceous emplacement. duction zone that was responsible for the 5. Tertiary deformation caused major tec- original displacement of the ophiolite. 1. Lithofacies correlation in the Musan- tonic reversals by rethrusting of the Upper After emplacement of all thrust sheets in dam Mountains confirms that platform Cretaceous thrust stack known from farther the Oman Mountains, there was a 25-m.y. period of tectonic stability during which fossiliferous shallow-marine limestones of upper Maastrichtian to Eocene age were deposited. There is no record of subduction N during this period either in Oman or Iran, TETHYS where northward subduction and active arc A volcanism began in Makran and Baluchi- stan after the Eocene (Coleman, 1981). In Oman, renewed crustal compression during

Late Cretoceous olistostromes

v v Mid Cretaceous alkaline volcanics in Dibba zone v v V and alkaline pyroxenite sills in Central Oman Mountains

Permian- Triassic Oman Exotics

Hawasina basin sediments Central Oman Mountains Subduction zone Figure 14. Late Cretaceous reconstruction of the Arabian

continental margin impinging on an arcuate subduction zone Shelf- slope break Trench to the east and northeast. Hawasina basin shown in horizontal Line of Exotic Limestone dashed lines and oceanic crust on the overthrust plate (Semail Seamounts ophiolite) in flecked pattern. Plate Motion

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N W S E bonate sequence westward a minimum of 5 Platform margin km over allochthonous Hawasina sedi- collapse ments. This structure is continuous south- ward at increasing depth and affects rocks of middle Eocene age on the outer frontal fold. 8. The "Dibba line" or "Oman Line," in previous work interpreted as a transform fault terminating the Zagros crush zone, is not continuous south into Oman or the Uni- dynamotherma I ted Arab Emirates. No evidence of large- Westward - migrating metamorphism scale transform tectonics is found in the beneath Aruma foredeep Dibba zone, and it has been demonstrated ophioiite here that the Tethyan shelf edge and alloch- thon swing around through a 90° syntaxis to face southeast in the Dibba zone and that translation direction was perpendicular to -+- -t- -+• the original orientation of the shelf edge. 9. The United Arab Emirates-Oman fore- B Coniacian-Santonian land folded belt can be correlated in time and space with the Zagros fold belt of SimsimaFm. Limestones southwestern Iran, to which it is connected Juweiza Fm.+ debris Oahlah Fm. eroded off allochthon along a major orogenic syntaxis in the Straits of Hormuz and the Musandam Semail ophioiite Drowned Peninsula. Diapirs of infra-Cambrian shelf Haybi Complex Hawasina Complex Hormuz salt and Tertiary folds now form some of the major oil traps of the Middle East. Campanian-Maastrichtian ACKNOWLEDGMENTS — Musandom This work was funded by Amoco Petro-

Te r t i a r y leum Company (International) through an operating grant awarded to W. H. Kanes of - Tertiary Hagab thrust the Earth Sciences Resources Institute, cuts up section towards University of South Carolina. We particu- leading edge larly thank Wolfgang Schollnberger, Ian T Tertiary thrusts Poyntz, and Terry O'Brien for fruitful dis- Oligocene-Miocene uk Late Cretaceous thrusts cussions, and Mike Hughes-Clarke for comments on the manuscript. Ian Gass pro- Figure 15. Model of the Late Cretaceous deformation of the Musandam Peninsula- vided support for M. P. Searle and J. D. Dibba zone continental margin and subsequent Tertiary deformation. A. Emergence of the Smewing during previous seasons in Oman. shelf during the Wasia-Aruma break (Turonian). Crosses, continental basement; bricks, We also thank Wilf Marsh for the photo- platform carbonate sequence; horizontal dashed lines, Hawasina sediments; cross hachures, graphic work and Winston Howell for car- transitional crust; flecked pattern, oceanic crust. B. Deepening of shelf to form the Aruma tographic assistance. basin, and development of the Hawasina and Haybi duplexes. C. Westward migration of Aruma foredeep. Blocks derived from Hawasina, Haybi, and Semail allochthons in the Juweiza flysch. D. Tertiary (Hagab-type) thrusting affecting the complete stratigraphy REFERENCES CITED from basement to middle Miocene along northeastern edge of the Arabian plate. Alleman, F., and Peters, T., 1972, The ophiolite-radiolarite bell of the north Oman Mountains: Eclogae Geologicae Helvetiae, v. 65, p. 657-697. Arthur, M. A., and Schlanger, S. O., 1979, Cretaceous -oceanic anoxic events" as causal factors in development of reef-reservoired giant south in Oman. The Tertiary thrusting that it was more likely an Oligocene- oil fields: American Association of Petroleum Geologists Bul- letin, v. 63, p. 870-886. event resulted from large-scale "whaleback" Miocene event. Whereas translation of Late Bernoulli, D., and Jenkyns, H. C., 1974, Alpine, Mediterranean and central Atlantic Mesozoic facies in relation to the early evolution folding with steep or overturned, short, Cretaceous thrust sheets may have involved of Tethys, in Dott, R. K., Jr., and Shover, R. H., eds.. Modern west-facing limbs and wavelengths of as and ancient geosynclinal sedimentation: Society of Economic hundreds of kilometres, translation of Ter- Paleontologists and Mineralogists Special Publication 19, much as 15 km. tiary thrust planes was 1 to 25 km and usu- p. 129-160. Biehler, J., Chevalier, C-, and Ricateau, R„ 1975, Geological map of the 6. It is possible that rethrusting in the ally less than 5 to 10 km. Musandam Peninsula, Sultanate of Oman: Directorate General of Petroleum and Minerals, Sultanate of Oman. Dibba zone was the latest stage of Late Cre- 7. The Hagab thrust shows a reversal of Bosellini, A., and Rossi, D., 1974, Triassic carbonate buildups of the taceous emplacement tectonics, but correla- Late Cretaceous tectonostratigraphic order Dolomites, northern Italy, in Laporte, L., ed., Reefs in time and space: Society of Economic Paleontologists and Mineralogists tion with the whole mountain belt suggests by thrusting the Musandam platform car- Special Publication 18, p. 209-233.

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