Geology and Petrology of the Ute Mountains Area Colorado

GEOLOGICAL SURVEY PROFESSIONAL PAPER 481

Prepared on behalf of the U.S. Atomic Energy Commission

Geology and Petrology of the Ute Mountains Area Colorado

By E. B. EKREN and F. N. HOUSER

GEOLOGICAL SURVEY PROFESSIONAL PAPER 481

Prepared on behalf of the U.S. Atomic Energy Commission

The petrography^ structure^ and petrology of a laccolithic mountain range on the Colorado Plateau

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1965 UNITED STATES DEPARTMENT OF THE INTERIOR STEWART L. UDALL, Secretary

GEOLOGICAL SURVEY Thomas B. Nolan, Director

The U.S. Geological Survey Library catalog card for this publication appears after index.

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C., 20402 CONTENTS

Abstract. --______-_-_-__--_-______..__-.______Igneous rocks Continued Introduction. __------___-______Petrology. ______------35 Geographic setting and accessibility. ______Chemical composition of the igneous rocks. _ _ _ _ 35 Culture __ ----______Variation diagrams _------37 Topography. ______Minor elements_-______--_------38 Climate and vegetation______Variation in the igneous rock series______--_- 38 Purpose and scope of investigation. ______Variation of minor elements with rock Previous work______alteration.. ______-___------40 Fieldwork and acknowledgments. ______Variations in igneous rocks. ______40 Geologic setting. ______Variations in sedimentary rocks adja­ Stratigraphy of the sedimentary rocks______cent to igneous intrusives.------43 Pre-Navajo rocks. ______Origin of the igneous rocks and hornblende in­ Triassic(?) and Jurassic...... ______clusions ___ _-___-_------_------45 Glen Canyon Group...... ______Structural geology and forms of the igneous bodies ___ 50 Navajo Sandstone. ______General features______50 Jurassic System. ______Ute dome______----_ 50 San Rafael Group______McElmo dome______-_--- 51 Entrada Sandstone. ______Faults _ ___-_-_---_------52 Summerville Formation______10 Intrusive bodies.______52 Junction Creek Sandstone ______12 Stocks____---_------53 Morrison Formation. ______13 Black Mountain stock. 53 Salt Wash Sandstone Member______14 "The Knees" stock. _ _ 53 Recapture Shale Member___-_-______14 Ute Peak. 54 Westwater Canyon Sandstone Member- 15 Bysmaliths______-_-----_------55 Brushy Basin Shale Member. ______15 Sentinel Peak.______55 Cretaceous System ______18 Mable Mountain. _-______-_ 55 Burro Canyon Formation...... ______18 The "West Toe" -- - 56 Dakota Sandstone. ______20 Laccoliths. _-__-_------_--- 56 Mancos Shale______23 North Ute Peak laccolith 56 Mesaverde Group______25 Horse Mountain laccolith____ 56 Point Lookout Sandstone. ______26 East Horse laccolith______56 Quaternary System. ______26 Sundance cluster of laccoliths. 56 Igneous rocks ______27 Razorback laccolith. 57 General features and evidence for age of igneous Last Spring laccolith ______--__-_--- 57 activity. ______27 Flat laccolith___------57 Relative ages of the intrusive rocks ______27 Three Forks laccolith___-____-__------57 Occurrence and petrography. ______28 Banded laccolith ______----_------58 General features. ______28 The "West Toe" cluster of intrusive bodies. 58 Microgabbro ______28 Mushroom laccolith ------58 Diorite porphyry ______28 Pack Trail laccolith . 58 Normal diorite porphyry. ______30 Tongue laccolith . ____-_--__------58 Leucocratic diorite porphyry ______30 Yucca cluster of laccoliths ______---_---_- 59 Grandiorite porphyry ______30 "The Buttes"laccolith__---_-_------59 Quartz monzonite porphyry, ______31 North Black Mountain laccolith ______59 Lamprophyre ______32 Sills and dikes ------59 33 Breccia pipes______------^9 Alteration. ______35 Summary of geologic events..- ______------60 Deuteric alteration.... ______35 Economic geology..------Hydrothermal alteration at Mable Moun­ Metallic mineral deposits______-_------61 tain..... ______35 Uranium deposits.- ______-_-_------ft9°^ Pyritization in the vicinity of igneous Fault-controlled deposits ______-_-__-_-- 62 contacts. ______35 Cliff House group of claims ______-_-_ 62 Hydrothermal (hot-spring) alteration asso­ Three States Natural Gas Co. pros­ ciated with breccia pipes______35 pect....------63 m IV CONTENTS

Page Page Economic geology Continued Economic geology Continued Metallic mineral deposits Continued Metallic mineral deposits Continued Uranium deposits Continued Uranium potential of the Ute Mountains area_ _ _ _ 67 Bedded uranium deposits.___--___-__--.-______64 Northern Ute Mountains area______---___ 67 Karla Kay mine______64 Southern Ute Mountains area.______----__ 68 Nonmetallic mineral deposits__-_--____------68 Coffin's prospect______66 Petroleum, natural gas, and carbon dioxide. _____ 68 Copper deposits.______66 Known occurrences______-____------_- 68 Battle Rock mine____-__----______66 Oil and gas possibilities of the Ute Moun­ Ute Creek dike prospects. ______66 tains area___------69 Ute dome______--____-__------69 Little Maude mine______66 Coal.-__---_------69 Relation of the metallio mineral deposits to the Literature cited_____--_-----__------69 igneous rocks__-_-______67 Index.____----_----_------71

ILLUSTRATIONS

Page PLATE 1. Geologic map of the Ute Mountains area______-______-______------_-_---_-_------In pocket FIGUKE 1. Index map of southwestern Colorado showing quadrangles in the Ute Mountains area______3 2. Navajo Sandstone in McElmo Canyon______--____-__-______----_------_- 9 3. Upper unit of the Junction Creek Sandstone underlying the Salt Wash Member of the Morrison Formation _ 12 4. Salt Wash Sandstone Member, Recapture Shale Member, and Westwater Canyon Sandstone Member of the Morrison Formation near Karla Kay mine______-______-__--_------_------15 5. Lower part of the Dakota Sandstone and sandstone of the Burro Canyon Formation in Western McElmo Canyon. 22 6. "The Mound." The Juana Lopez Member in the Mancos Shale____.______-______-----_------_--__ 25 7. Contact of quartz monzonite porphyry with diorite porphyry.__-_------27 8. Specimens of the four main types of igneous rock from the Ute Mountains-______-_-____------_ 29 9. Specimen of hornblende granodiorite porphyry from Ute Peak______-___------_--_------31 10. Inclusions in intrusive igneous rocks of the Ute Mountains..----______----_--_--_------34 11. Variation diagrams for igneous rocks of the Ute Mountains.______-______-_-_-_------_-_------37 12. Oxide weight percentages of rocks from the Ute Mountains plotted against the sum of normative quartz, albite, and orthoclase (the differentiation index)______--______-______----__---_--__---_------39 13. Frequency distribution histograms of semiquantitative spectrographic analyses of minor elements common to the igneous rocks of the Ute Mountains.______-______-______-_-__----__------_------41 14. Diagram showing variation of five trace elements with respect to increasing silica and total pyrible content- _ _ 46 15. Graphical solution of Harker diagram to determine possible materials added or subtracted to produce quartz monzonite from microgabbro______-______-______-----__------47 16. Approximate limits of composition of amphibole derived from various rocks showing plot of four samples of hornblende from the Ute Mountains.______-______----_-_-_-_------48 17. Sketch showing inferred structural relations near the contact of the Sentinel Peak intrusive with a structural dome thought to be the result of igneous intrusion at depth______-___------__------55 18. Sketch showing inferred stratigraphic and structural relations of laccoliths in the Sundance cluster and mechanism of "trapdoor" structure______-______-_____-_-______-__-______-_-----_------57 19. Two sills in Mancos Shale along Pine Creek north of Black Mountain.-______-__------60 20. Boulder of brecciated lamprophyre cemented with travertine.-______-_-____------_----.------61 21. Fractured and brecciated sill of lamprophyre______-____-__-----_------61 22. Map showing location of mines and prospects.______--__---_-_------62 23. Sketch showing stratigraphic and structural relations at the Cliff House prospect, McElmo Canyon, Montezuma County, Colo______63 24. Cliff House No. 4 area, McElmo Canyon, Montezuma County, Colo-_------_------64 25. Map and section of the Karla Kay mine, McElmo Canyon, Montezuma County, Colo______-_-__-_-_----- 65 26. Sketch showing stratigraphic relations at the Karla Kay mine.______-____--_------66 CONTENTS V TABLES

Page TABLE 1. Average rainfall and temperature at Mancos, Colo., 1898-1919-___-______-_------_____--__---__---_-__ 4 2. Sedimentary rocks exposed in the Ute Mountains area, Colo.------6 3. Chemical analyses and norms of igneous rocks from the Ute Mountains____-_-_-______----__--__-__-__ 36 4. Semiquantitative spectrographic and chemical analyses, of igneous rocks from the Ute Mountains-.------40 5. Semiquantitative spectrographic analyses of igneous rocks from the Ute Mountains______42 6. Semiquantitative spectrographic and radiometric analyses of altered and unaltered igneous and sedimentary rocks from the Ute Mountains.______44 7. Comparison of the chemical composition of microgabbro phenocrysts having the chemical composition of material that might be subtracted from microgabbro to produce quartz monzonite______47 8. Chemical analyses of hornblende from the Ute Mountains.______-___-___-______-_----____--_ 48 9. Atomic ratios of elements in hornblende on basis 24 (O, OH, F)____-______--____-_---_-_---_-__---- 48 10. Optical properties of hornblende from the Ute Mountains______48 11. Quantitative spectrographic analyses of hornblende from the Ute Mountains_--______-_-----______---- 48 12. Semiquantitative spectrographic analyses of hornblende from the Ute Mountains______--__-___--_ 49 13. Radiometric and chemical analyses of samples from radioactive prospects, McElmo Canyon, Montezuma County, Colo__------__-_------___.-_------_--- 63 14. Semiquantitative spectrographic and radiometric analyses of samples from mineral deposits in the Ute Moun­ tains area______In pocket

GEOLOGY AND PETROLOGY OF THE UTE MOUNTAINS AREA, COLORADO

By E. B. EKREN and F. N. HOTJSER

ABSTRACT to include an overlying light-gray glauconitic coarse-grained sandstone. Overlying the Mancos Shale on a few high places The Ute Mountains area is on the Colorado Plateau in the in the Ute Mountains are thin flat-stratified persistent beds of southwest corner of Colorado, in Montezuma County, about 20 buff and yellowish-gray sandstone interstratified with dark-gray miles northeast of the Four Corners where Colorado, Utah, mudstone similar to the Mancos. This sequence is probably the Arizona, and New Mexico join. The mountains are a laccolithic basal part of the Point Lookout Sandstone of the Mesaverde group that rises to an altitude of nearly 10,000 feet and stands Group. 2,500 to 4,000 feet above the surrounding area. Sedimentary rocks exposed in the Ute Mountains area ag­ The igneous rocks in the Ute Mountains intrude beds of the gregate about 3,800 feet in thickness and are of Triassic(?), Point Lookout Sandstone of Late Cretaceous age. This evi­ Jurassic, Cretaceous, and Quaternary age. The Triassic( ?) and dence is the only direct indication available of the age of the intrusive rocks in the Utes. The lack of coarse detritus in Jurassic rocks in the area include the Navajo Sandstone, the San Rafael Group, and the Morrison Formation. Cretaceous Upper Cretaceous rocks that crop out a few miles east of the rocks include the Burro Canyon Formation, the Dakota Sand­ Ute Mountains, and that are younger than the Point Lookout stone, the Mancos Shale, and basal beds of the Point Lookout Sandstone but older than the McDermott Member of the Animas Sandstone of the Mesaverde Group. Tertiary and Cretaceous Formation of late Cretaceous and Paleocene age indicates that rocks younger than the lowermost beds1 of the Point Lookout igneous activity and accompanying uplift did not occur prior Sandstone have been removed by erosion. to the deposition of the McDermott. The McDermott Member contains abundant andesite boulders probably derived from the The physical characteristics of the Navajo Sandstone (oldest La Plata Mountains, a laccolithic group lying 35 miles east of formation exposed) and formations of the San Rafael Group are similar and indicate alternating subaerial and subaqueous the Ute Mountains and presumably of the same age as the in­ deposition. These formations are predominantly cross-stratified trusive rocks of the Ute Mountains. The occurrence of andesite sandstone that was deposited in an eolian environment, but in­ boulders in the McDermott may indicate a precise age for the clude beds of flat-stratified sandstone, siltstone, and mudstone laccolithic mountains on the Colorado Plateau. that apparently were deposited in a marine or lacustrine en­ The igneous rocks of the Ute Mountains form a differentiated vironment. The Morrison Formation in the Ute Mountains calc-alkaline series that ranges from microgabbro through area is composed of fluvial and flood-plain deposits, and com­ quartz monzonite. Field mapping indicates that the earliest prises four members: the Salt Wash Sandstone Member, Re­ intrusive rock was microgabbro, followed by diorite, grano- capture Shale Member, Westwater Canyon Sandstone Member, dlorite, and finally quartz monzonite. All the igneous bodies and Brushy Basin Shale Member. The lower two members, were intruded forcibly into sedimentary rock. No evidence of Salt Wash and Recapture, and the upper two members, West- wallrock replacement or assimilation was found. The textures water Canyon and Brushy Basin, intertongue and intergrade of the igneous rocks vary with rock type and from one intrusive to a considerable extent. The Burro Canyon Formation con­ body to another. In general, the microgabbros are slightly sists of discontinuous fluvial conglomeratic sandstone lenses porphyritic, the diorites moderately porphyritic, and the grano- and interbedded mudstone and claystone. In places only mud- diorites and quartz monzonites conspicuously porphyritic. The stone is present. The mudstone weathers hackly or fissile and groundmasses of the different types of rock make up from 40 can be easily distinguished from that of the Brushy Basin Mem­ to 60 percent of the volume and, in general, are dense; grains ber of the Morrison Formation, which weathers to a hard and range in size from less than 0.01 mm. to about 0.1 mm. frothy-appearing material because of abundant bentonite. The Chemical analyses of 16 samples of igneous rocks from the Dakota Sandstone consists predominantly of fluvial sandstone Ute Mountains reveal that silica ranges from about 50 percent and interbedded carbonaceous shale. Although the Dakota in the microgabbro to almost 69 percent in the quartz monzonite; Sandstone is separated from the Burro Canyon Formation by a alumina content is nearly constant (about 17 percent for all disconformity, the two formations are similar and, in places, the rocks in the series) ; total iron, magnesia, and lime decline their distinction is difficult. The Mancos Shale in the Ute Moun­ from high values in the microgabbro to low values in the quartz tains area consists of approximately 1,900 feet of dark-gray monzonite; Na2O and K2O show distinct increases from rela­ gypsiferous mudstone. The formation contains abundant ma­ tively low values in the microgabbro to higher values in the rine fossils and two conspicuous lithologic marker units. The quartz monzonite. Greenhorn Limestone Member is about 75 feet from the base, Semiquantitative spectrographic analyses indicate that sev­ and consists of 15 to 35 feet of dense gray limestone and inter­ eral minor elements show distinct trends. The systematic vari­ bedded gray mudstone. The Juana Lopez Member is about 475 ation of the major and minor elements suggests that the igneous feet above the base of the Mancos; it consists of 3 to 50 feet of rock series in the Ute Mountains resulted from differentiation thin-bedded tan sandy fossiliferous limestone and was mapped of a single-source magma. 1 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO

Hornblende-rich inclusions are abundant in the igneous rocks, by ascending solutions that also carried copper. The ubiquity particularly in the diorite and granodiorite. The hornblende of barite in both the radioactive and nonradioactive deposits in the inclusions is not distinguishable optically or chemically suggests that the deposits are related to the Ute igneous rocks, from the hornblende of the phenocrysts. Chemical, optical, and which are relatively rich in barium. No evidence was found spectrographic analyses of hornblende from two inclusions and that directly related the uranium deposits in beds of conglom­ phenocrysts from two porphyries are so similar that a common erate and sandstone to the fault-controlled deposits. The minor origin is inferred. Two such origins seem most probable: the elements of the two types of uranium deposits and the nonradio­ inclusions are either fragments of early differentiates, or frag­ active copper deposits are similar. The possibility exists, there­ ments derived from a deep hornblendic substratum from which fore, that the bedded deposits of the McElmo Canyon area are the Ute magma itself was derived. related to the Ute igneous activity. The largest structural features in the Ute Mountains area The oil and gas possibilities of the Ute Mountains area have are Ute and McElmo domes. Ute dome is nearly circular in been enhanced by the discovery of several major oil fields plan view and averages about 10 miles in diameter, although in southeastern Utah. Both McElmo dome and Ute dome are the western margin is poorly denned. The relief on the dome potentially excellent structural traps for oil. McElmo dome, ranges from only about 400 feet on the north, where it is in however, by 1958 had been tested by six wells drilled on or near contact with McElmo dome, to more than 2,000 feet on the south. the crest of the structure, only one of which yielded hydro­ The top of the dome appears to be fiat, but is complicated in carbons. This well produces natural gas from the Shinarump detail by the occurrence of numerous laccoliths and three Member of the Chinle Formation and had an initial potential stocks. Ute dome is thought to be entirely the result of forceful of 500,000 cubic feet of gas per day. Carbon dioxide was found injection of magma, most of the doming being caused by the in three other wells in Mississippian and Lower Cambrian rocks, injections of three stocks. McElmo dome is also nearly cir­ and two of these were completed as carbon dioxide wells. The cular in plan view, but the flanks of the dome pass into a series paucity of oil and flammable gas in McElmo dome and the abund­ of radiating anticlines. The total area affected by McElmo ance of carbon dioxide have discouraged exploration. The dome and its satellitic anticlines measures about 20 miles east origin of carbon dioxide in the McElmo structure is unknown, to west and 10 miles north to south. The dome has about 500 but probably is related to igneous intrusion either directly be­ feet of closure. The origin of McElmo dome and its relation neath McElmo dome or in the Ute Mountains. Carbon dioxide to Ute dome is uncertain, but the proximity of the two suggests probably occurs in the Ute dome and may completely fill the that they are related genetically. The general lack of igneous potential reservoir rocks. rock at depth in McElmo dome, however, suggests that most of The Dakota Sandstone in the Ute Mountains area contains the structural relief may be a result of basement uplift related many thin beds of coal, which, with few exceptions, are lentic­ to Late Cretaceous or early Tertiary folding recognized else­ ular and have no commercial value at present. where on the Colorado Plateau. Steeply dipping normal faults of small displacement occur INTRODUCTION on the north, west, and south flanks of Ute dome, and on the southwest flank and in the central part of McElmo dome. No This report describes the geology of the Ute lacco- postintrusive faults were noted on the steeply dipping east lithic mountain group on the Colorado Plateau and par­ flank nor in the central part of Ute dome, but preintrusive frac­ ticularly the stratigraphy of the sedimentary rpcks ture zones appear to have controlled emplacement of some ig­ neous bodies. The possibility exists, therefore, that many of the exposed in the area surrounding the mountains, the pe­ faults and fractures are earlier than Ute dome and reflect a trography and petrology of the igneous rocks, the struc­ zone of weakness in the basement that localized the igneous ture of the sedimentary and igneous rocks, and the eco­ activity in the Ute Mountains. nomic geology. Intrusive rocks in the Ute Mountains form laccoliths, bys- maliths, sills, and dikes, in addition to stocks. In general, the GEOGRAPHIC SETTING AND ACCESSIBILITY least siliceous rocks occur as sills or flat-topped laccoliths; and the more siliceous intrusive rocks occur as mushroom-shaped The Ute Mountains area (fig. 1) is in the southwest laccoliths and bysmaliths. Dikes occur mainly adjacent to the corner /of Colorado, in Montezuma County, about 20 stocks. Most of the intrusive rocks (excluding dikes) probably miles northeast of the Four Corners where Colorado, were fed laterally from the three stocks. Deposits of uranium, vanadium, and copper occur in McElmo Utah, Arizona, and New Mexico join. The mountains Canyon. Although these deposits are small and currently have extend about 8 miles from north to south and 7 miles at no commercial value, they are of considerable geologic interest the widest part from east to west. McElmo Canyon lies Uranium and vanadium occur in the Karla Kay Conglomerate just north of the Ute Mountains. The west-trending Member of the Burro Canyon Formation and in the Entrada canyon cuts into the broad McElmo dome, which is Sandstone. Uranium, traces of copper, and abundant barite occur in faults northwest of the Ute Mountains. Copper, barite, about 8 miles in diameter. and traces of lead, zinc, and silver occur in nonradioactive From the Ute Mountains area, the La Plata Moun­ deposits in faults that lie between the intensely altered Mable tains are visible to the east, the Aba jo Mountains to the Mountain bysmalith and the uranium deposits. The Mable northwest, the Carrizio Mountains to the south, and Mountain bysmalith is cut by a fracture zone and has been in­ Ship Rock, the famous volcanic neck in northwestern tensely mineralized with iron sulfides. The copper deposits and the fault-controlled uranium deposits are spatially related to the New Mexico, is visible to the south-southeast. Mable Mountain fracture zone and may be genetically related to The Ute Mountains area is accessible from Cortez, the Ute igneous activity. The uranium was probably deposited Colo., by U.S. Highway 666 to the Ute Reservation INTRODUCTION

109 C 45' 108°

10 20 MILES ! I

FIGUBE 1. Index map of southwestern Colorado showing quadrangles mapped in the Ute Mountains area.

745-807 O 65 2 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO

Road to Towaoc, Colo., and by State Highway 32 Various ridges and peaks of the range are known through McElmo Canyon. A road passes south of the locally by the parts of the "Sleeping Ute" that they mountains to Aneth, Utah. Roads have been con­ represent. Thus, Mable Mountain represents the head, structed through parts of the eastern half of the moun­ Ute Creek dike the headdress, Ute Peak the arms folded tains by the Ute Mountain Tribal Council since the over the chest, and Hermano Peaks the knees. Sentinel conclusion of the field study. Many of these roads open Peak in the southeast part of the mountains and a topo­ areas that during mapping were reached on foot. graphically similar peak in the southwest are the large Cortez is serviced by bus, truck, and airlines. The toes, termed, respectively, East Toe and West Toe. nearest railroad is at Durango, Colo., 48 miles east of The east, south, and west sides of the mountains blend Cortez, and can be reached by U.S. Highway 160. gently with the plains or mesas of the Colorado Plateau, at altitudes between 5,000 and 5,500 feet above sea level. CULTURE On the north side, McElmo Creek has cut a deep west- The Ute Mountains are the sit© of the Ute Mountain trending canyon through the southern flank of McElmo Indian Reservation, which covers most of the area dome. mapped except McElmo Canyon and its tributaries west CLIMATE AND VEGETATION of the mountains. Most residents of the Ute Mountain The climate of the area is semiarid. McElmo Creek Reservation make their permanent homes at Towaoc, is the only perennial stream, but springs are abundant Colo., which is the headquarters of the Ute Mountain in the mountains, and some of them flow throughout the tribe and the location of the local agency of the U.S. year. Bureau of Indian Affairs. A store, restaurant, filling Rainfall and temperature recorded (table 1) at Man- station, and school are available at Towaoc. During cos, Colo., 25 miles to the east, indicate approximate the summer months, many Indian families move into rainfall and temperatures in the Ute Mountains, inas­ the mountains where they hunt and herd sheep. In­ much as the altitude of 7,035 feet at Mancos is close to come from land royalties during recent years has en­ the mean altitude of the Ute Mountains. Thunder­ abled the Ute Indians to raise their standard of living storms are common over the mountains in summer and and to make significant capital investments in a water early fall months. Prevailing winds are from the system and in livestock. southwest. Alluvium in Montezuma Valley and McElmo Can­ yon (pi. 1) supports small but productive fruit, small TABLE 1. Average rainfall and temperature at Mancos, Colo., 1898-1919 grain, and cattle ranches. The alluvium occurs at low [After U.S. Department of Agriculture (1936, sec. 22, p. 16 and 23)] elevations and irrigation is necessary. Dry farming Precipitation Temperature of pinto beans and wheat is profitable on higher mesas Period (inches) (°F; 20 record on the north flank of McElmo dome and on the plains days) north of the city of Cortez. January 1.36 25.5 1.46 29.1 2.02 36.8 TOPOGBAHY ApriL - .-_-____.__ _.__ ...... 1.76 44.4 1.26 51.5 .Tune .81 61.2 The Ute Mountains rise 2,500 to 4,000 feet above the July..-...... 1.91 66.2 Aug'ist 2.01 65.0 surrounding area, which ranges in altitude from 4,800 1.55 57.6 1.55 47.3 feet in McElmo Canyon to 6,000 feet on mesas north of 1.14 37.9 1.21 26.5 McElmo Canyon. The altitude of Ute Peak is 9,977 18.04 45.8 feet, of Black Mountain 9,405 feet, and of Hermano Peaks ("The Knees") in the south-central part of the Sagebrush, juniper, and pinon grow on the plains mountains 8,960 feet. Although moderately sharp north of McElmo Canyon, and sparse grass, juniper, ridges and intervening valleys make up most of the and pinon grow on the plains west of the mountains. range, a few topographic benches occur on flat-topped The plains south and east of the Ute Mountains have laccoliths. only sparse grass, sage, and thistle. Lower slopes sur­ From the east, the view of the mountain range is rounding the mountains are covered moderately with striking. It has led many to visualize a man lying on juniper, pinon, and shrubs. Most of the middle-level his back, with his head to the north, his feet to the south, slopes in the mountains proper are covered with oak his arms folded upon his chest, and his face turned brush, juniper, and pinon. Pine and fir grow on the toward the setting sun. This configuration has led to upper slopes and ridges, and small clumps of aspen the popular name, Sleeping Ute or Sleeping Ute grow on the north slopes of Horse Mountain, Black Mountain. Mountain, and Ute Peak. INTRODUCTION

The vegetation in the Ute Mountains is similar to source for the vanadium and uranium found in some of that in the Henry Mountains of Utah described in con­ the faults and sedimentary 'beds of the McElmo Canyon siderable detail by Hunt, Averitt, and Miller (1953, p. area. 27-35). FIELDWORK AND ACKNOWLEDGMENTS

PURPOSE AND SCOPE OF INVESTIGATION The Ute Mountains and the adjacent McElmo Canyon area were mapped during the spring and sum­ The Ute Mountains of southwestern Colorado were mer of 1955 and the summer of 1956. The geology was mapped by the U.S. Geological Survey as part of the mapped on aerial photographs furnished by the U.S. program of uranium investigations undertaken by the Geological Survey and by the Soil Conservation Serv­ Survey on behalf of the Division of Raw Materials of ice of the Department of Agriculture. Base maps for the U.S. Atomic Energy Commission. The objectives the area were prepared from aerial photographs 'by the of the work were: to determine the uranium resources use of the Kelsh plotter and a radial planimetric plotter and potentialities of the area; to provide detailed in­ and slotted templates. Geology and planimetry were formation on the relations of uranium ore deposits in transferred simultaneously from the photographs to the the sedimentary rocks of the area to the igneous rocks base maps at a scale of 1:20,000. Horizontal and verti­ of the Ute Mountains; and to fill a gap in the modern cal control was established by setting up a triangulation geologic map of the Colorado Plateau. net tied to the primary triangulation station "Ute" and PREVIOUS WORK to Oast and Geodetic Survey leveling lines in McElmo Canyon and Montezuma Valley. Approximately six The Ute Mountains were first described by Holmes, stations were placed in each of the five 7%-minute of the Hayden Survey (1877, p. 236-237), who referred quadrangles mapped. Clyde Duren, Jr., and Lloyd L. to them as the El Late Mountains. Cross (1894, p. 211- Smith, formerly of the U.S. Geological Survey, estab­ 214) reviewed Holmes' work and described the igneous lished many of the stations, and their assistance and rocks sampled by Holmes. Two of these samples were advice are gratefully acknowledged. analyzed chemically and the analyses are reproduced Geologic maps of the five 7^2-minute quadrangles on in this report. Coffin (1920) mapped the structure and planimetric bases have been published in the U.S. Geo­ stratigraphy of McElmo dome. P. H. Metzger (written logical Survey Mineral Investigations Field Studies commun., 1944) studied the geology of the McElmo Map series (Ekren and Houser, 1957,1959b, c, d; Houser Canyon area, with emphasis on the Morrison Forma­ and Ekren, 1959). Topographic maps of the Ute Moun­ tion. Cadigan studied the Junction Creek Sandstone tains area were published by the Geological Survey late in McElmo Canyon in 1949.1 In the spring of 1952, in 1957. The geologic map accompanying this report diamond-drill exploration of radioactive areas along was prepared by reducing the original planimetric- faults in McElmo Canyon was undertaken by the geologic maps to 1:48,000 scale and transferring the Atomic Energy Commission. Geologic1 logs of the dia­ geology to the new topographic base. mond-drill core are on file in the office of the Atomic Energy Commission in Grand Junction, Colo. GEOLOGIC SETTING The Ute Mountains and the McElmo Canyon area The Ute Mountains are in the western Colorado mar­ were mapped in reconnaissance by E. M. Shoemaker gin of the Canyon Lands section of the Colorado and W. L. Newman during a succession of brief visits Plateau (Fenneman, 1931, p. 307). The mountains rise in 1951, 1952, and 1953. Shoemaker and Newman above the Great Sage Plain, a broad expanse cut by (1953) noted approximately 30 major laccoliths, and deep, steep-sided canyons that extends northward from they concluded that the laccoliths had been fed later­ the Ute Mountains to the La Sal Mountains. The plain ally from two stocks Black Mountain and Hermano is formed mainly on gently dipping resistant sand­ Peaks ("The Knees"). They noted that most of the stone of the Upper Cretaceous Dakota Sandstone. large dikes and the two stocks lie in a north-south The broad Mesa Verde (Fenneman, 1931, p. 308-309) zone, and suggested that the orientation indicated con­ begins 6 miles east of the Ute Mountains and extends trol of intrusion by a deep-seated structure in the base­ 20 miles eastward beyond the Mancos River. West and ment complex. Geochemical studies by Shoemaker and south of the Ute Mountains is the rugged canyon coun­ Newman (written commun., 1955) suggested that the try of the San Juan River drainage. igneous rocks in the Ute Mountains may have been a The Ute Mountains area is on a structural terrace or bench between Blanding basin to the west-northwest and 1 Cadigan, R. A. 1952, The correlation of the Jurassic, Bluff, and Junction Creek sandstones in southeastern Utah and southwestern San Juan basin to the southeast. The laccolithic moun­ Colorado : Pennsylvania State College master's thesis. tains lie on a structural dome formed by injection of 6 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO magma and are just south of McElmo dome, a large star- (1956, p. 162), on the basis of stratigraphic evidence shaped structure that probably originated as a result of discussed on page 27, concluded that the intrusions basement uplift, igneous intrusion, and, possibly, move­ took place near the end of the Cretaceous Period. ment of salt. To the southwest is the Defiance uplift and to the northeast, the San Juan uplift (Luedke and STRATIGRAPHY OF THE SEDIMENTARY ROCKS Shoemaker, 1952; Kelley, 1955). The sedimentary rocks exposed in the Ute Mountains Within a radius of 150 miles from the Ute Mountains area are listed in table 2. Except for the surficial are seven other laccolithic mountain groups: the La deposits, these rocks range in age from Triassic( ?) and Plata and Rico Mountains to the east, the Carrizo Moun­ Jurassic (Navajo Sandstone) to Late Cretaceous (Point tains to the south, the Henry and the Abajo Mountains Lookout Sandstone). Tertiary and Cretaceous rocks to the northwest, the La Sal Mountains to the north- younger than the lowermost beds of the Point Lookout northwest, and the West Elk Mountains to the north­ Sandstone have been removed by erosion. east. The igneous rocks in these laccolithic mountains The thickness of Upper Cretaceous and Tertiary sedi­ are quite similar. Geologists who have studied the mentary rocks that once overlay the Ute Mountains Colorado Plateau generally agree that the rocks are re­ area cannot be accurately determined. The lithology lated and that most of the laccolithic groups were formed of the thick post-Point Lookout, pre-Animas sedimen­ during the same geologic period. On the basis of geo- tary rocks of Late Cretaceous age that crop out a few morphic evidence, the laccolithic intrusive rocks and re­ miles southeast of the Ute Mountains indicates that the lated structures are believed by Hunt (1956, p. 45) to Ute Mountains area was not a topographic high during be of late Miocene or early Pliocene age. Shoemaker those periods of deposition. These sediments reflect

TABLE 2. Sedimentary rocks exposed in the Ute Mountains area, Colorado

S ystem and Group, formation, and member Character Thickness Economic value Series (feet)

Alluvium, 3 to about 75 ft thick; composed of interbedded mud, Fanglomerate gravel in McElmo sand and gravel in valley fill and stream deposits; windblown Canyon and pediment gravel on Quaternary sand and silt; talus; landslide deposits; block rubble; stream 0-75 eastern slopes of Ute Mountains terrace gravel; pediment gravel; fanglomerate. are used in highway construction. Unnnnformitv Mesaverde Group Point Lookout Sandstone Thin-bedded yellowish-gray and buff fine-grained sandstone 200 No recognized economic value in interbedded with gray sandy shale. (incomplete) Ute Mountains area.

Mancos Shale Predominantly gray to black shaly mudstone and claystone. Upper The Juana Lopez Member is cuesta-forming sandy fossilif- 1, 800-1, 900 Some of the claystone has ceramic Cretaceous \ Juana Lopez erous limestone and shale 25 to 100 ft thick, 475 ft above base value for making brick and tile. \ Member of formation. Yellowish lenticular sandstone and conglomeratic sandstone Dakota Sandstone interbedded with carbonaceous shale and coal. Coal beds are 110-140 Coal, ground water. extremely lenticular and usually contain considerable amounts of mud and silt. IJurro Canyon Formation White, gray, and brownish sandstone and conglomerate mter- Karla Kay Conglomerate Member Lower bedded with nonbentonitic green and red mudstone. Includes 30-200 is uranium bearing in McElmo Cretaceous Karla Kay Conglomerate the Karla Kay Conglomerate Member in McElmo Canyon. Canyon area. Member Brushy Basin Shale Member Varicolored bentonitic mudstone and a few conglomeratic sand­ 150-300 stone lenses. FormationMorrison Westwater Canyon and Salt Wash Westwater Canyon Sandstone Pale yellow -brown fine- to medium-grained sandstone inter­ 75-200 Members are uranium bearing in Member bedded with green bentonitic mudstone. many parts of the Colorado Plateau. No uranium deposits Recapture Shale Member Tan and reddish-gray fine- to medium-grained sandstone inter­ 0-200 are known in these members in bedded with red mudstone. the Ute Mountains area. Salt Wash Sandstone Member Pale-brown fine- to medium-grained sandstone interbedded with 0-200 predominantly red-brown mudstone. Predominantly pink fine- to coarse-grained, poorly sorted sand­ Upper Junction Creek Sandstone stone, cross-stratified at high angle; weathers to a "slick rim." 250-300 Ground water. Jurassic Correlates with the Bluff Sandstone of Utah and Arizona. § 2 Flat- and thin-bedded argillaceous sandstone and siltston'e, brick Upper sandstone unit is uranium o Summerville Formation red. Includes a 20-ft fine-grained well-sorted ledge-forming 120-130 bearing in McElmo Canyon. 8 sandstone at top. Except for this unit, the formation is slope No mineral value 1958. 03 forming. tf Slick Rock Member, 70 to 80 ft thick, is sandstone that is pale brown and light pink at top and that grades to white and CQ Entrada Sandstone finally to orange red in lower part, weathers to a "slick rim." 100-115 Ground water. Dewey Bridge Member is very fine grained brick-red argil­ laceous sandstone, 25 to 35 ft thick.

Jurassic and Navajo Sandstone Predominantly orange fine-grained eolian sandstone. 300 Ground water. Triassic(?) Jfi STRATIGRAPHY OF THE SEDIMENTARY ROCKS marine or marginal marine conditions, show no lateral The Navajo Sandstone is the only formation in the Glen change in the proximity of the Ute Mountains, and un­ Canyon Group that is exposed in the Ute Mountain doubtedly were once continuous over the Ute Mountains. area. The lower part of the Animas Formation (McDermott The Navajo Sandstone (Gregory, 1917, p. 57-59) Member), however, contains abundant andesite detritus, crops out along the south flank of McElmo dome in Mc­ and the Ute Mountains may have been uplifted first dur­ Elmo Canyon. A thickness of about 300 feet is exposed ing McDermott time (Shoemaker, 1956, p. 162). The along Sand Creek, a tributary of McElmo Creek, in sec. post-Animas (early Tertiary) sediments that crop out 26, T. 36 N., R. 18 W. The base of the formation is not a few tens of miles southeast of the Ute Mountains are exposed in Sand Creek, but a gamma-ray log of the largely fluviatile in origin. Present knowledge of these National Oil and Gas Co.'s West 1, located a few miles early Tertiary sediments is inadequate to determine east of Sand Creek, indicates a total thickness of 300 whether the Ute Mountains area was supplying or re­ feet for the Navajo Sandstone. This thickness suggests ceiving sediments. that the Navajo-Kayenta contact can be no more than a PRE-NAVAJO ROCKS few feet below the bed of Sand Creek. Throughout the McElmo Canyon area the Navajo In the Ute Mountains area, the sedimentary rocks Sandstone is predominantly orange and fine grained, that underlie the Navajo Sandstone according to Zabel and is composed of subangular clear quartz grains. (1955, p. 132-135) are in descending order: the Upper "Berries" of medium-grained well-rounded quartz are Triassic(?) Kayenta Formation, Wingate Sandstone, common. In general, the Navajo is thick bedded and the Upper Triassic Chinle Formation (includes Shina- is alternately cross-stratified and flat stratified. The rump Member), the Lower and Middle(?) Triassic cross-stratified beds are thickest and are separated from Moenkopi Formation, the Permian Cutler Formation, flat-stratified beds by planar erosion surfaces that form the Pennsylvanian and Permian (?) Kico Formation, the distinct horizontal partings in the weathered outcrop Middle Pennsylvanian Pinkerton Trail Formation of (fig. 2). The sequence of cross-stratified beds horizon­ Wengerd and Strikland (1954) and the Hermosa For­ tally truncated at many intervals suggests eolian depo­ mation (includes Paradox Member), the Mississippian Leadville Limestone, the Upper Devonian Ouray Lime­ sition alternating with subaqueous deposition. stone and Elbert Formation, and equivalents of the That water played a role in the deposition of the Middle Cambrian Ophir Formation and Lower Cam­ Navajo Sandstone is also indicated by the occurrence of brian Tintic Quartzite of northern Utah. a bed of dense gray limestone about 4 feet thick near Several of these formations are of economic interest the top of the formation that crops out in the northern in the Ute Mountains area. The Paradox Member of part of McElmo Canyon. The limestone is nonfossilif- the Hermosa Formation produces oil and gas in the erous and was probably deposited in a fresh-water lake. nearby Aneth field of Utah and is a potential producer The following section was measured on the east side in the Ute Mountains area; the Leadville in the McElmo of Sand Creek Canyon in sec. 26, T. 36 N., R. 18 W., by dome produces carbon dioxide gas, and in McElmo V. L. Freeman and L. C. Craig of the U.S. Geological Canyon the Shinarump Member produces flammable Survey; minor changes have been made in this section gas. as recorded to conform with the nomenclature used in The nearest exposures of Triassic and uppermost this report. In this report, rock color terms that are Paleozoic rocks are in the Dolores Biver Canyon near followed by color symbols are from the National Re­ Dolores, Colo., in the La Plata Mountains, Colo., and search Council "Rock-color Chart" (Goddard and along the San Juan Kiver in Utah. The rocks in the others, 1948). La Plata Mountains were described in detail by Eckel 1. Lower McElmo Canyon section (pi. 1) (1949, p. 8-24). Rocks older than the Hermosa For­ Entrada Sandstone. mation of Pennsylvanian age crop out north of Du- Navajo Sandstone: Feet rango, Colo., along the Animas River valley in the Sandstone, very pale orange; some pale reddish- vicinity of the Needle Mountains (Cross and Hole, brown areas; fine-grained; large-scale cross lami­ 1910). nations ; forms a bench .. 80.2 TRIASSIC (?) AND JURASSIC Sandstone, red to yellow, very fine grained, very an­ gular ; contains some disseminated coarse grains GLEN CANYON GROUP ("berries") ______. - 1- 2 NAVAJO SANDSTONE Sandstone, pinkish-gray, medium fine-grained; some larger angular grains ; cross laminated 7. 6 The Glen Canyon Group includes the Wingate Sand­ Sandstone, pinkish-gray, medium fine-grained; con­ stone, Kayenta Formation, and Navajo Sandstone. tains some larger angular grains ; cross laminated- 1. 9 8 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO

1. Lower McElmo Canyon section (pi. 1) Continued The contact of the Navajo Sandstone with the over­ Navajo Sandstone Continued Feet lying Entrada Sandstone is sharp in the McElmo Can­ Sandstone, pink and white, fine-grained, subangular, yon area. thin-bedded, cross laminated______15. 7 JURASSIC SYSTEM Sandstone, silty, yellowish-gray; some dark-red patches; very fine grained ; cross laminated___ 23. 8 SAN RAFAEL GROUP Sandstone; pale reddish orange to very pale orange at top; very fine grained, subangular; massive cross- ENTRADA SANDSTONE laminated unit______44. 2 The San Eafael Group in McElmo Canyon includes Sandstone; pale red at base to very pale orange at top; fine-grained, subrounded; massive cross-lami­ the Entrada Sandstone, Summerville Formation, and nated unit______21.9 the Junction Creek Sandstone. These formations are Sandstone; light-brown, fine-grained, subrounded; of Late Jurassic age and are about 1,000 feet thick. contains some black accessory mineral grains; cal­ The local nature of the exposures of the San Eafael careous ; wedging cross lamination______2. 8 Group in the Ute Mountains area precludes a discussion Sandstone, pale reddish-brown (10.R 6/4), fine­ grained, subangular ; contains very minor accessory of sedimentary history other than the brief statements minerals; calcareous ; wedging cross lamination 18. 5 included with the lithologic discussion. A synthesis Sandstone, pale reddish-brown (1012 6/4), fine to very based on a regional study was made by Harshbarger fine grained, subangular, calcareous______38. 3 and others (1957, p. 23-51). Sandstone, light-brown, fine-grained, subrounded, calcareous, cross-laminated______8.5 The Entrada Sandstone (Gilluly and Eeeside, 1928, Sandstone, moderate reddish-orange to pale reddish- p. 76) consists of two units in McElmo Canyon and its brown, fine-grained; subangular at base to very fine tributaries. The two units have been named the Dewey grained at top; contains scattered black accessory Bridge Member and the Slick Kock Member by Wright, mineral grains; calcareous; massive with broad Shawe, and Lohman (1962, p. 2057). The type locality cross lamination; base of exposure______43.0 of the Dewey Bridge Member is at Dewey Bridge, Grand County, Utah, and the type locality of the Slick Total (incomplete?) Navajo Sandstone_____ 307.6 Kock Member is at Slick Eock, San Miguel County, Baker, Dane, and Eeeside (1936, p. 44) regarded the Colo., about 30 miles north of the Ute Mountains area. Navajo Sandstone as a large northeastward-thinning The Dewey Bridge Member (lower unit) is red hoodoo- wedge of eolian sand. The Ute Mountains area appar­ weathering sandstone that forms a bench over the re­ ently is very close to the eastern edge of the wedge, for sistant Navajo Sandstone. The Dewey Bridge Member the Navajo pinches out between McElmo Canyon and formerly was thought to correlate with the Carmel the La Plata Mountains. Formation. The upper unit of the Entrada, the Slick No fossils were found in the Navajo Sandstone in the Eock Member, is white to orange clean sandstone that Ute Mountains area. A fossil vertebrate found in the weathers to a nearly vertical rounded cliff or "slickrim." Navajo Sandstone of northeastern Arizona was de­ The two sandstone units are conspicuous in the Mc­ scribed by Camp and Vanderhoof (1935, p. 385) as Elmo Canyon area. (See fig. 2.) The Dewey Bridge "a small dinosaur about the size of a turkey." Camp Member, which is 25 to 35 feet thick, is composed of (1936, p. 39) named \tSegltaurus halli and stated (1936, brick-red argillaceous and silty, very fine grained sand­ p. 52) : "It represents a single member of an unknown stone. The Slick Eock Member is 70 to 80 feet thick; upland fauna and despite its primitive characters it it is orange to light pink in the lower part and grades could be placed in either the Triassic or Jurassic." to white and pale brown in the upper part. It is fine Harshbarger and others (1957, p. 22-31) described grained, is composed of subangular clear quartz and findings of other organic remains from the Navajo abundant well-rounded medium to medium-coarse Sandstone, but none of these are diagnostic of age. The quartz "berries," and is cross-stratified at medium assignment of Triassic (?) and Jurassic age for the angles. The cross-strata commonly are truncated by Navajo Sandstone is based on a variety of evidence planar surfaces of erosion that underlie horizontal lami­ summarized by Lewis and others (1961) who point out nae. The Slick Eock Member includes a 6-foot thick that the age of the Navajo must be based upon: (a) ledge at the top that may correspond to the lower part the age of the Kayenta Formation, which intertongues of the Bilk Creek Sandstone Member of the Wanakah with the lower part, of the Navajo, and (b) the age of Formation. the fossiliferous Carmel Formation, which intertongues The following section was measured in Sand Creek with the upper part of the Navajo. Canyon in sec. 26, T. 36 N., E. 18 W., by Ekren. STRATIGRAPHY OF THE SEDIMENTARY ROCKS

FIGURE 2. Navajo Sandstone (J"fin) in McElmo Canyon. Note horizontal partings in upper 200 feet. Bench above Navajo is formed in silty sand­ stone facies of the Dewey Bridge Member of the Entrada. Slick Rock Member of the Entrada (Je) is above bench. The Summerville Formation Us), the Junction Creek Sandstone (Ji), the Morrison (Jm), and the Burro Canyon and Dakota (Kbd) occupy slope in the background.

2A. Sand Creek section (pi. 1) 2A. Sand Creek section (pi. 1) Continued Entrada Sandstone Continued Feet Summerville Formation. Slick Bock Member Continued Entrada Sandstone: Sandstone, light-tan, buff; weathering composed Slick Rock Member: feet of very fine quartz grains and a few medium- Sandstone, pale-brown to light yellow-brown, coarse grains of well-rounded clear quartz fine-grained, poorly sorted; contains abundant "berries"; the very fine grains subangular to medium to medium-coarse clear quartz grains subrounded; unit resistant (abundant calcar­ ("Entrada berries") ; sandstone is limonite- eous cement), does not weather "slick" as unit flecked and mottled; unit forms ledge ("non- above does; base sharp______2 slick-rim" weathering) ______6 Dewey Bridge Member: Sandstone; light-pink at base grading to white Sandstone, argillaceous; very fine to silt-sized and to pale-brown at top; weathers to very grains, brick red; hoodoo weathering. Upper pale brown and white; very fine to fine 10 ft well exposed, lower part (about 15 ft) grained; well-rounded quartz grains or "ber­ poorly exposed and mantles bench formed on ries" fairly common in light-pink zone, rare top of resistant Navajo Sandstone______30 in white zone, common in pale-brown zone near top. Light-pink zone is abundantly freckled with limonite; entire unit is "slick- Total Entrada Sandstone______106 rim" weathering and is cross-stratified at The thickness and lithology of the Entrada Sand­ medium angle. Cross strata are bounded by stone show little variation throughout the McElmo horizontal partings______48 Canyon area. The following section was measured in Sandstone, orange-red; weathering orange to Goodman Canyon approximately 3 miles east of Sand red; very fine grained, grains angular to sub- angular ; bedding obscure, massive; "slick-rim" Creek Canyon, by L. C. Craig, V. L. Freeman, and J. D. weathering; calcite cement______20 Strobell, Jr. 10 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO

3. Upper McElmo Canyon section (composite) (pi. 1) 3. Upper McElmo Canyon section (composite) (pi. 1) Con. Entrada Sandstone Continued Summerville Formation. Dewey Bridge Member: Feet Entrada Sandstone: Top contact gently undulant with sharp lithic Slick Rock Member: Feet change-______Sandstone; very pale orange (IOYR 8/2) in tip­ Sandstone, pale reddish-brown (WR 5/4) and per half, pinkish cast in lower half; fine moderate reddish-orange (WR 6/6), very fine grained; contains disseminated medium-fine, to fine-grained; composed of subangular clear rounded to well-rounded grains or "berries'"; quartz and minor accessory minerals; color composed of subangular clear quartz and col­ and weathering controlled by degree of irregu­ ored minor accessory minerals; structureless ; lar discontinuous dark clayey lamination; lower half forms reentrant- ______5.6 weathers shaly to earthy and locally forms Section A. Measured on north side of McElmo rounded biscuit-shaped cliffs-______18.1 Canyon just east of old oil rig and west of Sandstone, moderate reddish-orange (WR 6/6) mouth of Goodman Canyon. Sec. 30, T. 36 N., to moderate orange-pink (1QR 7/4), fine­ R. 17 W. grained; contains subangular clear quartz, Sandstone, white, pale yellow weathering; pre­ pink and black accessory minerals; contains dominantly fine grained; rare medium-sized numerous layers with concentrations of grains; composition same as unit below; unit medium-grained well-rounded frosted quartz forms massive rounded top to' "slick rim"; "berries"; platy bedded______2.3 poorly formed irregular horizontal bedding- 8. 6 Sandstone, white, predominantly fine grained; Total Entrada Sandstone______102.8 composition same as unit below; noncalcar- Stratification in the Dewey Bridge Member (red silty eous; unit predominantly horizontally lami­ nated but a few beds show fine-scale cross facies) in the McElmo Canyon area is inconspicuous lamination______.______12.1 Most of the visible stratification is flat. The sparseness Sandstone, white, fine- to medium coarse-grained ; of cross-stratification of either eolian or fluviatile type composed of subangular to well-rounded clear together with fair sorting of the grains suggests quartz and very minor black, gray, and pink deposition in a lacustrine or marine environment. accessory minerals; cross laminated; medium The most conspicuous feature of the stratification of scale (3 to 4 ft) wedge-bedding______15.4 the Slick Rock Member of the Entrada Sandstone of Sandstone, white to pale-yellow, very fine grained; composition same as unit below; hori­ the McElmo Canyon area is the large number of hori­ zontally laminated; unit forms a reentrant_ 4. 0 zontal (planar) surfaces of erosion that are continuous Sandstone, white to pale-yellow, fine- to medium- over long distances. These surfaces commonly separate grained; composed of subangular to rounded wedge-stratified beds from horizontal laminae. The clear quartz and minor green and black acces­ alternating stratification suggests subaqueous and sory minerals; highly cross laminated; wedge subaerial deposition. bedded at a fine to medium scale. Top con­ Several alcoves have been weathered into the Slick tact sharp and beveled; forms spring line__ 4.2 Rock Member of the Entrada Sandstone in the McElmo Sandstone; moderate reddish orange in lower half and white to pale yellow in upper half; Canyon area. These alcoves were used by ancient In­ very fine to fine grained; composed of sub- dians as building sites for cliff houses. The cliff houses angular to subrounded clear quartz and black, are most numerous and are best preserved in the Sand gray, green, and pink accessory minerals; Creek area. slightly wavy horizontal laminations; weathers The contact of the Slick Rock Member with the over­ massive and rounded; color contact ranges lying siltstones and mudstones of the Summerville For­ from sharp to vague and irregular; orange sandstone slightly calcareous, white is highly mation is sharp in McElmo Canyon, but beds of sand­ calcareous______9. 2 stone similar to those in the Entrada occur in the lower Sandstone, moderate reddish-orange (IOR 6/6), part of the Summerville Formation, and the two fine to very fine grained; composed of subangu­ formations may intertongue. lar clear quartz and disseminated medium- coarse "berries" ; massive and structureless in SUMMERVILLE FORMATION lower 15 ft; discontinuous clayey laminae in The Summerville Formation of Late Jurassic age was upper 4 ft indicate horizontal bedding_____ 18. 8 named by Gilluly and Reeside (1928, p. 80) from ex­ Sandstone, grayish-orange-pink (1QR 8/2) to moderate reddish-orange, very fine to medium posures on Summerville Point in the San Rafael Swell, fine-grained, poorly sorted; contains subangular Utah. In the type locality the Summerville overlies to rounded clear quartz, medium-coarse "ber­ the Curtis Formation. ries" of well-rounded frosted quartz, and white In the McElmo Canyon area the Summerville For­ to light- and dark-gray chert; cross laminated, mation overlies the Entrada Sandstone and underlies wedge bedded at a fine scale (6 in to 2 ft) ____ 4. 5 the Junction Creek Sandstone. The formation is domi- STRATIGRAPHY OF THE SEDIMENTARY ROCKS 11 nantly brick red to red brown and is composed of al­ The resistant sandstone near the top of the Summer­ ternating and gradational beds of very fine to fine­ ville is continuous throughout the McElmo Canyon area. grained well-sorted sandstone and silty mudstone. This sandstone is generally reddish brown or pink, but Stratification is generally flat. The formation is from near igneous intrusive rocks in the Ute Mountains and 125 to 150 feet thick and, with the exception of a resist­ near some faults in McElmo Canyon it is yellow brown. ant sandstone bed 10 feet below the top, is bench form­ It is radioactive near faults in the vicinity of Eock Creek ing. The lower 10 to 15 feet of the formation contains in sees. 22 and 23, T. 36 N., E. 18 W. thin beds of white to pale-brown sandstone that resem­ L. C. Craig, J. D. Strobell, Jr., and V. L. Freeman ble the Entrada Sandstone. measured the Summerville Formation on the west side The following section was measured by Ekren in of Goodman Canyon about 1^4 miles north of the fruit Sand Creek Canyon, sec. 24, T. 36 N., E. 18 W. farm at the mouth of the canyon in sec. 29, T. 39 N., E. 2B. Sand Creek section (pl.l) 17 W. 3. Upper McElmo Canyon section (pi. 1) Junction Greek Sandstone. Summerville Formation: Feet Junction Creek Sandstone. Mudstone!, red-brown______5 Summerville Formation: Feet Sandstone, pale-brown, fine-grained______1 Claystone and sandstone, grayish-red (10.R 4/2). Sandstone, red-brown, very soft, fine-grained; con­ Claystone is fine grained to medium grained, sandy, tains abundant red-brown hematite or limonite__ 3 shaly to earthy weathering. Sandstone is white, Mudstone, red-brown______1 medium fine to fine grained, noncalcareous; acces­ Sandstone, white to pale-green, light-green-weather­ sory minerals as in unit below; forms several beds ing, fine-grained, well-cemented; protects softer as much as 1 ft thick___ 16. 2 sandstone units below______4 Sandstone, grayish orange-pink (10.R 8/2) to white, Sandstone, red-brown to chocolate-brown, fine­ medium fine- to fine-grained; composed of rounded grained, soft and friable; abundant hematite___ 4 clear quartz and minor amber, pink, and black ac­ Sandstone, light-red to light-gray, soft, limonite- cessory minerals; forms fairly resistant ledge- 4. 3 fiecked, flat-bedded, massive-weathering______9 Sandstone, clayey, dark reddish-brown (10R 3/4), Siltstone, red, hard, spheroidal-weathering.______3 very fine grained; composed of subangular clear Sandstone, light-red, very fine grained; this unit, and quartz; earthy to shaly weathering; forms small the overlying sandstone and siltstone units form bench______.. 3.1 a ledge______10 Sandstone, grayish orange-pink (WR 8/2) to white, Sandstone, light-red to white, very fine to fine­ medium fine- to fine-grained; composed of rounded grained, very soft; bedding obscure; bench form­ clear quartz and minor amber, pink, and black ac­ ing ______.______14 cessory minerals; predominantly wavy lamination; Siltstone, argillaceous, gray-green and light-red___ 2 forms prominent ledge_ 15. 8 Sandstone, paTe-yellow to pink; fine to very fine Sandstone, light grayish-red (10J? 5/2) to white, non- quartz grains; limonite flecked and blotched with clayey; weathers rounded; structureless or show­ unknown black mineral; contains sparse black ing wavy lamination and thin shaly partings _ 9.8 opaque minerals; fiat-bedded______8 Sandstone, moderate orange-pink (KM? 7/4) to white, Mudstone, red______2 finely mottled, medium fine- to fine-grained; some Sandstone, tight, hard, flat-bedded; contains very medium-sized grains; coarser grains well-rounded, fine grained quartz______2 finer grains subangular; rock slightly calcareous; Sandstone and mudstone, light-red and brick-red, weathers, soft, friable; indistinct bedding, 60 per­ alternating and gradational, flat-bedded; sand­ cent cross-laminated at fine to medium scale, 40 stone is very fine grained______8 percent horizontally laminated; several prominent Mudstone, brick-red; a few thin layers of very fine horizontal bedding planes______24.4 grained sandstone usually less than 1 ft thick that Sandstone, clayey, dark-red, mottled light-green, are light red to pink, dark red weathering; unit is very fine grained, noncalcareous; weathers rounded fiat bedded______37 to shaly and earthy______2.2 Sandstone, white buff-weathering; very fine grained Sandstone, white, dark-brown weathering; predomi­ quartz "berries"; rock is tight and hard, calcareous nantly fine grained; clear quartz and red, pink, cemented; unit splotched and flecked with limonite- 2 green, yellow, and black accessory minerals; wavy Siltstone and mudstone, brick-red; forms brick-red laminated and cross laminated on a fine scale; soil; bench forming. Layer of fine-grained sand­ forms prominent ledge; sand typical of Summer­ stone 2.5 ft thick crops out about 6 ft above base ville______2.6 of unit; this sandstone is light brown to pale Sandstone and claystone interbedded. Sandstone is brown, hard with calcareous cement, spotted with pale reddish brown (WR 5/4), silt sized to medium limonite, and together with unit above probably fine-grained sand size; weathers to shaly irregular corresponds to the Bilk Creek Member (Goldman to rounded ledges. Claystone is dark red, very and Spencer, 1941) ______12 sandy, earthy weathering; contains disseminated sand grains. Two ledges of white brown-weather­ Total Summerville Formation. 127 ing, very fine grained to fine-grained resistant non- 745-807 O 65 3 12 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO 3. Upper MoElmo Ccmyon section (pi. 1) Continued reddish sandstone, commonly mottled or blotched green Summerville Formation Continued and gray. This unit has obscure flat stratification and calcareous sandstone between 29 and 34 ft above weathers to hoodoos (fig. 3). base of unit______59.1 Sandstone; mottled very pale orange and pale-red; predominantly fine grained; composed of suban- gular clear quartz and colored accessory minerals; contains disseminated rounded medium fine­ grained "berries"; forms prominent ledge____ 1.9 Sandstone and claystone. Sandstone is pale red (5i# 6/2), very fine to medium fine grained; forms sev­ eral ledges as much as 1 ft thick. Claystone is dark red, very sandy, nonoresistant, earthy weathering 9. 7 Total Summerville Formation______149.1 The flat stratification and good sorting of grains in the Summerville Formation indicate deposition in clear water. The gradual increase in sand content up­ ward in the formation suggests deposition in a receding sea. The contact of the Summerville Formation with the overlying Junction Creek Sandstone is gradational FIGURE 3. Upper unit (Jij) of the Junction Creek Sandstone underlying the Salt Wash Member of the Morrison Formation (Jms). Unit throughout the McElmo Canyon area. Sandstone and weathers to hoodoos. Ute Peak, Mable Mountain, and North Black mudstone of the Summerville Formation are inter- Mountain appear in the background. Photograph courtesy of R. A. bedded with sandstone similar to that of the overlying Cadigan. Junction Creek in many places. The contact was chosen Subangular red and orange chert grains are common where rocks lithologically similar to the Junction Creek in the lower and middle units. The chert commonly is predominate above and rocks lithologically similar to concentrated in single beds within a set of strata. The the Summerville predominate below. angular chert grains contrast with the subrounded to JUNCTION CREEK SANDSTONE rounded quartz grains. The Junction Creek Sandstone was named by Gold­ The following section was measured by Ekren on the man and Spencer (1941, p. 1750-1751) from an ex­ east side of Sand Creek in sec. 24, T. 36 N., R. 18 W. posure between Junction Creek and the Animas River 2B. Sand Creek section (pi. 1) a few miles north of Durango, Colo. -It correlates with Salt Wash Member of the Morrison Formation. the Bluff Sandstone of Utah, Arizona, and New Mexico. Junction Creek Sandstone: Feet In the McElmo Canyon area the Junction Creek Upper unit: Sandstone forms a conspicuous cliff, characteristically Sandstone, argillaceous, light-red; weathers to a "slick rim," and its outcrops closely resemble those of red with pink, green, and gray blotching; obscure fiat bedding; weathers to massive the Entrada Sandstone. Generally, however, the Junc­ hoodoos; contains very fine grained quartz__ 30 tion Creek Sandstone is light red, reddish orange, or Middle unit: pink, in contrast with the white to orange Entrada Sandstone, pink in variegated hues; weathers Sandstone. In the vicinity of igneous intrusions, the darker pink to red brown; "slick-rim" weath­ Junction Creek Sandstone is light brown. The forma­ ering; contains fine- to medium fine-grained pink-coated quartz grains, minor fine-grained tion averages about 280 feet in thickness and is divisible variegated chert, cross-stratified (strata near into three gradational units based on the type of bed­ top dip 30°) ; joints or fractures very sparse; ding. The lower unit, 30 to 50 feet thick, is composed rock poorly cemented; base gradational____ 85 of alternating beds of flat-stratified and cross-stratified Sandstone, pink to orange; weathers darker pink sandstone. The cross-strata dip at low angles and are to reddish orange; medium fine to medium grained, poorly sorted; calcareous cement; con­ truncated by planar surfaces of erosion. A few of the tains clear subrounded quartz and sparse horizontal surfaces of erosion may be continuous across medium- to coarse-grained subangular red, all the exposures in McElmo Canyon. The middle unit, gray, and orange chert; cross-stratified at 150 to 250 feet thick, is composed of thick-bedded fine- medium angles; "slick-rim" weathering; joint­ to coarse-grained sandstone that is cross-stratified at ing conspicuous in upper part; joints "veined" as described in unit below. A conspicuous high angles (nearly 30° in several exposures). The up­ horizontal parting surface separates this unit per unit, 20 to 50 feet thick, is argillaceous fine-grained from unit below______135 STRATIGRAPHY OF THE SEDIMENTARY ROCKS 13 2B. Sand Greek section (pi, 1) Continued near Denver, Colo., where the formation is about 200 Junction Creek Sandstone Continued Feet feet thick and is composed of green or gray fresh-water Lower unit: marls and thin beds of limestone and sandstone. Sandstone, light-red to orange-pink; weathers dark coral; fine to medium grained at base Throughout the Colorado Plateau the Morrison For­ becoming fine grained at top; alternating fiat- mation is composed of stream and flood-plain deposits. and cross-stratified beds; cross-strata trun­ Sandstone and conglomeratic sandstone occur in ancient cated by horizontal surfaces of erosion; sand­ stream channels, and mudstone on contiguous flood stone is soft and friable. Near the base of this plains. Fluviatile deposition prevailed during early unit the sandstone contains more cement (cal­ Morrison time and gradually gave way to predominant careous?) and is greatly fractured; joints are vertical, horizontal, and at low angles, and are flood-plain deposition during late Morrison time. filled with sandstone that is well cemented with The Morrison of the Ute Mountains area has been silica; filled joints protrude as "veins" from divided into four members. These are, in ascending weathered sandstone surfaces; joints die out order: the Salt Wash Sandstone Member, Eecapture or become obscure in upper part of the unit__ 40 Shale Member, Westwater Canyon Sandstone Member, Total Junction Creek Sandstone______290 and Brushy Basin Shale Member (Gilluly and Eeeside, 1928; Gregory, 1938; and Lupton, 1914). A discussion The Junction Creek Sandstone of southwestern Colo­ of the regional aspects of the Morrison is given by Craig rado has been correlated with the Bluff Sandstone of and others (1955, p. 134r-159). southeastern Utah by Cadigan (in Craig and Cadigan, The lower three members the Salt Wash, Recapture, 1958, p. 182-185). Cadigan divided the Junction Creek and Westwater Canyon are composed predominantly into four members on the basis of sedimentary struc­ of lenticular light-tan and light-gray sandstone which tures. His basal member, "A," corresponds to the writ­ is interstratified with thin red or green siltstone and ers' lower unit; his "B" and "C" members correspond claystone. The Brushy Basin, a bentonitic mudstone, is to the writers' middle unit; his "D" member to the upper mainly green, but is subordinately red, purple, and gray. unit. Because the upper two members, and also the lower The lithology of the lower unit of the Junction Creek two, intertongue and intergrade to a considerable extent Sandstone indicates alternating subaqueous and sub- field division is arbitrary in many places. The West- aerial deposition, which in turn suggests an oscillating water Canyon Member intertongues with the Brushy sea at the end of Summerville time. The middle unit Basin. The Westwater Canyon and the Eecapture reflects predominantly eolian deposition. Sand dunes Members do not intertongue, but their distinction is 50 feet high were common during this period. The flat stratification of the upper unit may indicate a return of extremely difficult and is based largely on the predom­ subaqueous conditions or deposition on a low flood plain inance of red mudstone in the Kecapture in contrast to near sea level. predominantly green mudstone in the Westwater Can­ The contact with the overlying Salt Wash Member of yon. Where the Eecapture is absent in eastern McElmo the Morrison Formation is locally disconformaJble. In Canyon, the Westwater Canyon possibly intertongues many places lenses of fluviatile sandstone of the Salt with the upper lenses of the Salt Wash. The Salt Wash Wash Member have been deposited in channels scoured Member intertongues and intergrades with the Eecap­ in the Junction Creek Sandstone by streams that re­ ture to a considerable degree in most places. moved much, and in some places all, of the upper unit The intertonguing and intergrading of members of of the Junction Creek Sandstone. In other places the the Morrison Formation in the Ute Mountains area argillaceous sandstone of the upper unit grades into are believed to reflect alternating deposition from two mudstone and siltstone typical of the Salt Wash. different source areas. Sediments forming the Eecap­ The irregular Junction Creek-Salt Wash contact sug­ ture and Westwater Canyon Members were derived from gests that the earliest streams of Salt Wash time flowed an area south of Gallup, N. Mex. (Craig and others, with sufficient velocity over a Junction Creek surface of 1955, p. 150). Sediments forming the Salt Wash and low relief to cause downcutting through the underlying Brushy Basin probably were derived from a major soft muddy sands. In places, stream deposition of sand source area in west-central Arizona and southeastern alternated with subaqueous (flood-plain?) deposition of California. The lower three members of the Morrison argillaceous sand, mud, and silt. in the Ute Mountains area show considerable blending of sediments from the two sources. MORRISON FORMATION Despite the complexities of deposition during Morri­ The Morrison Formation was named by Emmons, son time and considerable variation in the thicknesses of Cross, and Eldridge (1896) from the town of Morrison, the individual members, the total thickness of the Mor- 14 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO rison is consistently between 500 and 650 feet. Where it is approximately 150 feet thick, and the Recapture one member is thin, generally, another is correspond­ Member is absent. Still farther east, at Trail Canyon, ingly thick. The Brushy Basin Member and overlying the Salt Wash is 200 to 250 feet thick and the overlying Burro Canyon Formation have a similar nearly con­ Westwater Canyon Member is thin. stant total thickness. This consistency in total thick­ The basal sandstone of the Salt Wash Member com­ ness is of value in determining structure below the monly is separated from the underlying Junction Creek Morrison from data obtained on formations above the Sandstone by red-brown mudstone 3 to 50 feet thick. Burro Canyon. Because this mudstone resembles the mudstone inter- The assignment of a Late Jurassic age to the Morri­ layered higher in the Salt Wash, it is included with the son in the Ute Mountains area is based on lithologic member. The contact of the Salt Wash with the over­ correlation with the Morrison of other parts of the lying Recapture Member, and locally with the West- Colorado Plateau where a Late Jurassic age seems in­ water Canyon Member, is difficult to determine. In dicated by vertebrate remains (Stokes, 1944). contrast to the Salt Wash, the sandstone beds of the Recapture and Westwater Canyon Members contain SALT WASH SANDSTONE MEMBER very sparse detrital grains of feldspar and generally The name Salt Wash Sandstone Member was first are interbedded with red and green mudstones, respec­ proposed by Lupton (1914, p. 125-127) for a member of tively. The pink tone of the Recapture Shale Member the McElmo Formation. The name McElmo Formation is distinctive and contrasts with the colors of the Salt was later abandoned, and these rocks were divided into Wash and Westwater Canyon Members. In much of the Morrison, Summerville, Entrada, and Carmel for­ western McElmo Canyon the contact between the Salt mations; and the base of the Morrison was defined as Wash and the Recapture is marked by a narrow bench the base of the Salt Wash (Baker and others, 1927; Gil- formed on the top of the uppermost part of the Salt luly and Reeside, 1928). Wash Sandstone Member. (See fig. 4.) In the Ute Mountains area, the Salt Wash Member consists of interbedded sandstone and predominantly RECAPTURE SHALE MEMBER red mudstone. It lies upon massive crossbedded sand­ The Recapture Shale Member was first named and stone of the Junction Creek and is overlain by the Re­ described by Gregory (1938, p. 58) from Recapture capture, Westwater Canyon, or Brushy Basin Members. Creek, southeastern Utah. In the Ute Mountains, it The sandstone lenses of Salt Wash range from white or is composed of interbedded sandstone and mudstone. light gray to pale yellow or very pale brown. They are The sandstone beds are light gray, pinkish gray, or light trough cross-stratified and are composed of fine- to brown on fresh surfaces and weather to pinkish gray medium fine-grained clear subangular to subrounded or brownish gray. The quartz grains are fine grained, quartz accompanied by sparse to common accessory poorly sorted, and subangular to subrounded. Black, grains of black, white, red, or pink color. The sandstone orange, pink, and white accessory grains are common. lenses are interbedded with dark-red, dark-purple, and Detrital grains of feldspar are extremely sparse. The red-brown mudstone. Gray or green-gray mudstone is mudstone beds are pale red, reddish green, and pale extremely rare. green. The Salt Wash Member is exposed continuously for a McElmo Canyon marks the approximate northern distance of about 11 miles along the slopes of McElmo limit of the Recapture Member in southwest Colorado Canyon. It is absent between the Junction Creek Sand­ (Craig and others, 1955, p. 137-140). It is present stone and the Recapture Member on the structural dome west of a north-south line through the junction of Sand just south of Sentinel Peak in the Sentinel Peak NE Creek with McElmo Creek. In the lower McElmo Can­ quadrangle, although some of the sandstone there yon section measured by L. C. Craig (written commun., mapped as the Westwater Canyon Member is lithologic- 1949), the Recapture is 117 feet thick. It is more than ally similar to the Salt Wash of McElmo Canyon and 260 feet thick south of the Ute Mountains where ex­ may be a bed of the Salt Wash. The Salt Wash Mem­ posed in a breached dome south of Sentinel Peak. In ber is absent also in northeastern Arizona, a few miles this exposure the member contains more siltstone and northwest of the Carrizo Mountains (L. C. Craig, oral claystone than in McElmo Canyon. commun., 1956). The intertonguing and intergrading relations of the The thickness of the Salt Wash Member varies con­ Recapture with the overlying and underlying members siderably in McElmo Canyon. The member is 90 to have been discussed. The Recapture is recognized 110 feet thick near the west end of the canyon, where regionally in an oval-shaped area stretching from west it is overlain by the Recapture Member. Farther east, of Santa Fe, N. Mex., into southeastern Utah. In that STRATIGRAPHY OF THE SEDIMENTARY BOCKS 15

FIGDRB 4. Salt Wash Sandstone Member (Jms), Recapture Shale Member (Jmr), and Westwater Canyon Sandstone Member (Jmw) of the Mor­ rison Formation viewed northward from State Highway 32 near the Karla Kay mine. Upper hoodoo-weathering unit of the Junction Creek Sandstone (Jj) is barely visible in lower part of photograph. Note the topographic break at the top of the Salt Wash Sandstone Member. area, it has been divided into three facies (Craig and Canyon. The Westwater Canyon Sandstone Member others, 1955, p. 137-142): a conglomeratic sandstone is exposed in the dome south of Sentinel Peak and in facies, a sandstone facies, and a claystone and sandstone Towaoc dome. facies. The conglomeratic sandstone facies and the sandstone facies occupy the south-central part of the BBTJSHT BASIN SHALE MEMBER oval area. The Ute Mountains area is on the north­ The uppermost member of the Morrison Formation, eastern edge of the claystone and sandstone facies. the Brushy Basin Member, was first described by Gregory (1938, p. 59) from the region around Brushy WESTWATEB CANYON SANDSTONE MEMBER Basin, Utah. It is composed of bentonitic varicolored The Westwater Canyon Member (Gregory, 1938, p, mudstone that forms slopes and becomes hard and 59) ranges from 40 to more than 200 feet in thickness frothy appearing where weathered. The frothy appear­ in the Ute Mountains area and averages about 100 feet. ance is a result of swelling and subsequent drying of Slightly more than 240 feet of the member was meas­ bentonite in the mudstone. Swelling muds are not ured at the south end of the Ute Mountains. The sand­ known in the lower members of the Morrison nor in the stone beds in the Westwater Canyon are pale yellow overlying Burro Canyon Formation. Moderately per­ gray to light brown on fresh surfaces and weather to sistent interbeds of thin resistant, very fine grained light yellow gray or yellow. They are composed of silicified sandstone and siltstone are common in the fine- to medium-grained subangular to subrounded Brushy Basin Member. Conglomerate and conglom­ quartz and numerous white, red, or pink accessory min­ eratic sandstone are uncommon, although in many erals. Detrital feldspar is sparse. The sandstone lenses places green friable conglomeratic sandstone about 20 are cross-stratified channel-fills. They form rounded feet thick occurs near the middle of the member. Local­ and irregular cliffs. ly, where the Brushy Basin is thin and the Burro Can­ The Westwater Canyon Member intertongues and yon is thick, the conglomerate is near the top of the intergrades with the mudstone beds of the Brushy member. The conglomerate contains many fragments Basin. It is conformable with the Recapture and may of igneous rock. possibly intertongue with the Salt Wash. It is present The lower part of the Brushy Basin intertongues with throughout McElmo Canyon and to the north in Trail the Westwater Canyon Sandstone Member hi places, but 16 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO in others the contact appears to be a nearly flat surface 4. Lower McElmo Canyon section C of Craig (pi. 1) Con. for long distances. The base is placed at the top of the Morrison Formation Continued Brushy Basin Member Continued highest sandstone of Westwater Canyon lithology. Claystone, very pale orange to cream, silty to The upper part of the Brushy Basin intertongues in slightly sandy, bentonitic; bright orange-red many places with the Burro Canyon Formation (Ekren spots suggest incipient chert development; sam­ and Houser, 1959a). Localities where intertonguing is ple of siltstone from this unit reported by W. most pronounced are generalized on the geologic map D. Keller to be very fine grained silica, a cherty rock which may be a secondary product from (pLl). hydrolysis of volcanic ash______52. 0 In many areas the top of the Brushy Basin is iden­ Sandstone, very pale orange (10F.R 8/2) to pale- tified by a change from bentonitic to nonbentonitic mud- green (5G 7/2) ; weathers very pale green to stone. Locally, the two mudstones are separated by a light brown; fine to medium fine grained; con­ ledge of silicified siltstone about 1 foot thick, partic­ tains subangular to rounded clear quartz and numerous pink, orange, black, green, and red ularly in exposures southwest of the Ute Mountains. accessory minerals; forms several slabby In other areas, bentonitic mudstone typical of the ledges______6.0 Brushy Basin is overlain directly by conglomeratic Claystone, very pale orange to grayish-orange- sandstone or conglomerate typical of the Burro Canyon. pink, silty to medium fine-grained, sandy, In still other areas, intertonguing is extensive. Most bentonitic; considerable slumping ___ _ 27. 0 commonly, sandstone and conglomerate typical of the Claystone, very pale orange (1QYR 8/2) to gray­ ish orange-pink (5YR 7/2), silty to medium Burro Canyon are interbedded with mudstone typical fine-grained, sandy, bentonitic. In top 2 to 3 of the Brushy Basin, but interbedding of bentonitic and ft of unit are slabby ledges of fine- to medium nonbentonitic mudstone has been observed in western fine-grained sandstone; contains numerous McElmo Canyon. pink and red accessory minerals and a few The Brushy Basin forms slopes throughout McElmo chlorite flakes______32.4 Canyon and its tributary canyons in the Moqui SW Total Brushy Basin Member______197.9 quadrangle west of the Ute Mountains. It ranges in thickness from 150 to 300 feet. Westwater Canyon Member: The following stratigraphic sections are representa­ Sandstone, white to very light gray; weathers light brown; fine to medium fine grained; con­ tive of the Morrison Formation in the Ute Mountains tains subangular clear quartz and minor pink area. The section in lower McElmo Canyon was meas­ and black accessory minerals; channeled, cross ured by L. C. Craig near Tozer Gulch (pi. 1) where the laminated; ledge lenses out laterally from Recapture Member is present in sec. 32, T. 36 N., R. section______8.5 18 W. The Trail Canyon section was measured by Claystone, pale-green and greenish-gray, very Houser and Ekren where the Recapture Member is ab­ sandy; float indicates; bed of bright-green hard slabby calcareous siltstone near top. sent in sees. 34 and 27, T. 36 N., R. 17 W. Interval poorly exposed 17.9 4. Lower McElmo Canyon section C of Craig (pi. 1) Sandstone, pinkish-gray (5YR 8/1) to yellowish- gray (5F 8/1) ; weathers yellow brown to light Morrison Formation. brown; fine to medium fine grained; contains Brushy Basin Member: Feet subangular to subrounded clear quartz and Top contact not well exposed. numerous pink, red, and white accessory min­ Interval covered by debris from above except for erals ; channeled, cross laminated _ 12.6 2 ft of grayish-red claystone at base; float in­ Claystone and sandstone; claystone, light green­ dicates of white to cream claystone above. ish gray (50 8/1) and light greenish gray W. D. Keller reports partially devitrifled ash (&GY 8/1) with vague mottling of pale red; shards from a sample a few feet from top of silty to sandy, grading to nonresistant very fine unit ______30.9 to medium fine-grained clayey sandstone; this Claystone, grayish yellow-green (5GY 7/2) to unit is the lowest predominantly green unit- 30. 9 dusky yellow-green, slightly silty to very fine Sandstone, white to light greenish-gray; grained, sandy, hard and dense; forms two weathers pale to light brown; fine to very fine beds separated by bright-green clay parting_ 2. 0 grained; contains clear subangular quartz, Claystone, very pale orange, white, and very abundant black, pink, and red and green acces­ pale green, silty to fine-grained sandy, sory minerals, granules of light chert, and a bentonitic ______32.4 few feldspar grains; channeled, cross lami­ Claystone, same as in unit below; contains sev­ nated; ledge thickens 300 ft to east to form eral beds of dusky yellow-green siliceous clay- cliff______9. 1 stone, as much as 1 ft thick, consisting of dense noncalcareous matrix and disseminated Total Westwater Canyon Member 79.0 fine-grained sand______15.2 STRATIGRAPHY OF THE SEDIMENTARY ROCKS 17

4. Lower McElmo Canyon section C of Craig (pi. 1)> Con. 4. Lower McElmo Canyon section C of Craig (pi. 1) Con. Morrison Formation Continued Morrison Formation Continued Recapture Member: Feet Salt Wash Member Continued Feet Claystone, pale-red (5R 6/2) mottled to pale- rounded clear quartz and commonly black, green, silty to medium fine-grained, sandy. white, red, and pink accessory minerals; chan­ Interval very poorly exposed 23.1 neled, cross laminated; contains sandy to silty Sandstone, pale-yellow; weathers light brown; red claystone parting as much as 6 ft thick off medium fine to fine grained; composed of sub- line of section______37.6 rounded to well-rounded clear quartz and Interval covered except for upper 7 ft which is numerous red and pink accessory minerals; interbedded claystone and sandstone; clay- forms rounded structureless ledge 2. 8 stone dark red and green, sandy to silty; sand­ Claystone, variably sandy, pale- to grayish-red, stone yellowish gray, brown-weathering very mottled pale-green, bentonitic. Sand is sub- fine to fine grained; contains clear quartz and rounded and reaches medium-fine size 4.4 pink, black, and green, accessory minerals; ir­ Sandstone, very light gray to white and pale-yel­ regular but continuous beds; float of claystone low, very fine to medium fine-grained, poorly covers lower part of unit _ 29. 2 sorted; contains clear subangular to sub- Claystone and minor sandstone. Claystone rounded quartz and abundant black and minor sandy, dark red below to dark purple above. pink, orange, and white accessory minerals; Sandstone yellowish gray (5Y 7/2), very fine in irregular structureless beds with worm bur­ grained; contains clear subangular quartz and rows______.______6.6 numerous colored accessory minerals; sand­ Olaystone, pale-red (10JR 6/2) to grayish-red stone forms massive, structureless ledge 1.3 ft (10JR 4/2), silty to somewhat sandy, slightly or more thick at top of unit-_ _ 13.1 bentonitic______8.4 Sandstone, white to pale-yellow, light-brown Sandstone, white to pale-yellow, very fine to fine­ weathering, very fine to fine grained; con­ grained ; contains clear quartz and minor pink, tains clear subangular quartz and abundant black, and orange accessory minerals; chan­ black, dark-gray, and orange to pink accessory neled, faintly cross laminated______5.9 minerals; channeled, cross-laminated; bed ap­ Olaystone (70 percent) and sandstone. Olay­ pears to lens out laterally from section .. 17. 2 stone is silty to sandy, grayish red (10JR 4/2) Claystone and sandstone, interbedded. Clay- to pale green; sandstone weathers pale red to stone sandy to silty, dark red to purplish red, greenish brown. Unit very fine to fine grained; calcareous; sandstone greenish gray to light forms even, continuous beds; lower two-thirds purplish gray, very fine to medium fine grained, poorly exposed______31. Q finely banded. Unit forms shaly nonresistant Sandstone, pinkish-gray, light-brown weathering, reentrant; bottom contact is gradational, top fine- to medium fine-grained; commonly con­ contact sharp. Formation assignment de­ tains pink, black, and white accessory min­ batable______3.8 erals; weathers to rounded ledge of irregular beds______2. 8 Total Salt Wash Member______100.9 Sandstone, clayey, pale-red to dark-red to green, Total Morrison Formation.______494. nonresistant; interval very poorly exposed_ 3.4 Sandstone, pinkish-gray (5YR 8/1), fine-grained; composed of clear subangular quartz and, com­ 5. Trail Canyon section (pi. 1) monly, pink, black, and white accessory min­ Morrison Formation: erals; channeled, cross laminated and ripple Brushy Basin Member (incomplete) : laminated ; some worm burrows. Unit thickens Mudstone, green. Total incomplete Brushy laterally from section and fills scours that cut Basin Member ______22 through underlying unit______21.4 Claystone and sandstone. Claystone grayish red Westwater Canyon ( ?) Member : (10JR 4/2), slightly silty, flaky earthy weather­ Sandstone, pale yellow-gray to light-yellow, fine­ ing. Sandstone very light gray to pale yellow, grained ; composed of subrounded quartz, very fine to fine grained; contains subangular opaque white clay(?) grains (5 percent); clear quartz and many colored accessory min­ soft, friable; upper 2 ft is quartzite; forms erals; horizontally laminated, platy bedded; ledge; joints strike N. 40° W., dip 80° SW_ 34 forms 2 ft bed in middle of unit______6. 7 Mudstone, green, mostly covered______19

Total Recapture Member______117.1 Total Westwater Canyon(?) Member-

Salt Wash Member: Salt Wash Member: Sandstone, white to pale-yellow, with limonite Sandstone, light-gray, fine-grained, friable, speckling; weathers pale brown; fine to me­ quartzose; calcareous cement; abundant fine dium fine grained; contains subangular to sub- limonite spots _ _ 7 18 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO

5. Trail G any on section (pi. 1) Continued CRETACEOUS SYSTEM

Morrison Formation Continued BURRO CANYON FORMATION Salt Wash Member Continued feet Sandstone, light-gray, yellow-gray weathering; In the Ute Mountains area, the Burro Canyon Forma­ fine-grained; calcareous cement; quartzite in tion (Stokes and Phoenix, 1948) ranges from. 30 to 200 basal 1.5-2.0 ft grades upward to soft, friable feet in thickness and consists of green, predominantly sandstone; contains abundant limonite and nonbentonitic mudstone interbedded with lenses of con­ traces of very fine opaque white grains; 0.2-ft green flat-bedded mudstone at top______11 glomerate and conglomeratic sandstone that vary con­ Sandstone; grayish tan, fine grained, and thinly siderably in number and thickness. Red mudstone oc­ fiat bedded at base; grades to light yellow curs sporadically in the lower part of the formation. gray, fine to medium fine grained at top; firmly The nonbentonitic mudstone weathers hackly or fissile. cemented at base to weakly cemented at top; The conglomerate and conglomeratic sandstone are limonitic, very sparse biotite(?) fragments at commonly white or light gray. They contain pebbles base; very sparse pale-orange grains present throughout; thin green mudstone at top____ 12 and granules of colored chert, silicified limestone, and Covered; believed predominantly siltstone, siltstone in a matrix of quartz sand. Fragments of clayey; contains a few thin layers of hard petrified wood and silicified dinosaur bone are common silicified sandstone______6 in the lowermost lenses. Carbonized plant remains are Sandstone, iight-gray, limonite-spotted, pale- extremely rare. The conglomerate lenses in the upper brown-weathering; contains fine-grained part of the formation are more numerous, finer grained, quartz, opaque white clay grains, sparse, very and blanket larger areas than those in the lower part. fine rounded pale-orange grains, interstitial black MnO2 ( ?) flakes, very sparse black opaque The lowest unit in the Burro Canyon Formation, a grains, angular opaque white to pale-green system of shoestring lenses of conglomerate and con­ clay grains, and green and pale-green glomeratic sandstone, has been named the Karla Kay grains; thick bedded______12 Conglomerate Member from an exposure at the Karla Sandstone, light-gray to white, limonite-stained, Kay mine in McElmo Canyon (Ekren and Houser, fine- to medium fine-grained; contains sub- 1959a). The Karla Kay is highly resistant to weather­ rounded clear quartz; commonly contains, white opaque fine angular clay grains, sparse ing, and forms vertical cliffs and dark-brown knobby to common very fine rounded pale-orange outcrops. The shoestring lenses or channel fillings are grains, green mud pellets and clay galls, very rarely more than 2,000 feet wide or more than 65 feet sparse fine angular green grains, sparse pink thick; commonly they are 500 to 800 feet wide. The quartz grains, interstitial light-gray to white channel fillings have been undercut in many areas, and clay, numerous green mudstone partings, lim­ onite flakes, and calcareous cement; joints are their former locations are marked by huge, resistant, N. 55° W. 70° SW______48 dark blocks many as large as a house that have Sandstone, light-gray to white, pink-weathering, slumped down the slopes of soft underlying mudstone. very fine to fine-grained; contains sparse, very Where the Karla Kay Conglomerate Member is absent, fine orange grains; weathers to rounded nobu- the base of the Burro Canyon is marked either by hack­ lar hoodoos; thin (1-2 ft) green mudstone at top______15 ly-weathering mudstone or by fine-pebble and granule Siltstone and sandstone, very fine grained, conglomerate or sandstone. These rocks, although ba­ partly covered______16 sal, are stratigraphically younger than the Karla Kay. Sandstone, light-gray, fine- to medium fine­ Stratigraphic sections of the Burro Canyon follow. grained; contains subrounded quartz grains Section 6 was measured by Ekren in sec. 2, T. 35 N., R. and sparse subrounded orange and pink grains; 19 W., and sec. 35, T. 36 N., R. 19 W., about half a mile calcareous cement; friable; has interstitial north of Colorado Highway 32. Section 7 was measured mudstone ; calcite-filled joint trends N. 50° W__ 16 by Houser and Ekren in sec. 30, T. 36 1ST., R. 18 W., on Sandstone, light-gray, fine-grained; contains subrounded to subangular quartz grains and the northwest side of Tozer Gulch, about three-quarters sparse very fine orange grains (feldspars?), of a mile west-northwest of the Karla Kay mine. calcareous cement; current lineations N. 15° E.; crossbedded, lenticular; fills channel in 6. Wood Chuck section (pi. 1) mudstone of unit 1______16 Dakota Sandstone. Mudstone, red, purple, reddish-gray, sandy___ 42 Disconformity. Burro Canyon Formation: Total Salt Wash Member- 201 Sandstone, conglomeratic, white, weathering Base of Morrison Formation. white; contains granules and pebbles of white Top of Junction Creek Sandstone. chert, sparse red chert; grades to very fine to STRATIGRAPHY OF THE SEDIMENTARY ROCKS 19 6. Wood Chuck section (pL 1) Continued Sentinel Peak (the "East Toe" of the Ute Mountains), Burro Canyon Formation Continued Feet about 3 miles southwest of Towaoc, Colo., the Burro medium grained sandstone at top; massive Canyon consists of approximately 30 feet of hackly weathering; forms a cliff.______70.0 Mudstone, green-gray; weathers hackly and fissile weathering mudstone. Farther south, in the Carrizo without significant swelling______50.0 Mountains of New Mexico and Arizona, the upper part Sandstone, conglomeratic, pale-brown to white; of the Morrison Formation as mapped by Strobell weathers buff and brown; cross-stratified (fes­ (1956) contains lenses of conglomeratic sandstone that toon type) ; pebbles and granules mostly col­ may be equivalent to those of the Burro Canyon of ored chert as much as % in. in diameter; white chert common______20.0 southwestern Colorado, although no mudstone typical of the Burro Canyon is present. Total Burro Canyon Formation______140.0 The top of the Burro Canyon Formation is an ero- sional disconformity observed throughout the Ute 7. Tozer section (pi. 1., section 7) Mountains area. The relief on this surface ranges from Dakota Sandstone (not measured). a few inches to more than 40 feet within a distance of Erosional disconformity, relief of about 2 feet. 300 feet. White quartzite, sandstone, or less commonly, Burro Canyon Formation: mudstone of the Burro Canyon Formation are found at Sandstone, white, fine-grained; carbonate ce­ this erosion surface in different parts of the area. ment ; contains subangular quartz______2.0 Mudstone, green; weathers hackly, without sig­ The erosional disconformity has been observed in nificant swelling______15. 5 other areas of the Colorado Plateau (Carter, 1957), but Sandstone and conglomerate; sandstone light tan in places in the Slick Eock district in southwestern Colo­ gray, medium- to coarse-grained, interbedded rado approximately 30 miles northeast of the Ute Moun­ with conglomerate (granules to pebbles as tains the Dakota Sandstone rests conformably upon the much as 1^ in. in diameter) ; pebbles consist mostly of chert and some quartzite and silicified Burro Canyon (Simmons, 1957). limestone; unit forms rounded, blocky, verti­ The change in the Burro Canyon southward within cal cliff ______32. 0 the Ute Mountains area suggests that this area may Mudstone, pale-green; weathers hackly, with­ have been near the southern or southeastern edge of the out significant swelling______46.0 area of deposition of coarse clastic material during Mudstone, pale-green and red, well-silicified; forms resistant ledge ______2. 0 Burro Canyon time. In the Four Corners area of New Mudstone and sandstone, interbedded; sandstone, Mexico and Arizona, thin discontinuous lenses of con­ very light gray, fine-grained; forms thin ledges, glomeratic sandstone typical of the Burro Canyon predominates in lower one-third and decreases Formation are interbedded with mudstone typical of upward, poorly exposed; mudstone, very light the Brushy Basin, and within a few feet of the Dakota gray, silty; weathers hackly, without signifi­ cant swelling______44. 5 Sandstone (stratigraphic section 8). The thinness or Karla Kay Conglomerate Member: absence of the Burro Canyon in this region is probably Conglomerate, very pale tan-gray to pale grayish- due primarily to pre-Dakota erosion but may be due, green ; weathers dark brown and black; firmly in part, to a facies change southward from the Ute cemented; subrounded pebbles range from l/2 to Mountains area. Farther south the Dakota lies on suc­ 2l/2 in. in diameter, averaging about 1 in., peb­ bles consist mostly of brown, gray, blue-gray, cessively older formations (Craig and others, 1955, p. green-gray, and red-brown chert; matrix of 161). coarse-grained clayey sandstone; conglomerate Brown (1950), Katich (1951), Stokes (1952), and and sandstone silicified somewhat differen­ Simmons (1957) have presented evidence that the Burro tially but generally along bedding; unit cross- Canyon, or its equivalent in central Utah, the Cedar stratified (festoon type), with troughs plung­ ing eastward; forms massive cliff with very Mountain Formation (Stokes, 1944, 1952), is of Early knobby surface due to pebbles; dinosaur bones Cretaceous (Aptian) age. Simmons (1957) reports that and silicified trees common. Unit occurs as invertebrate fossils in upper beds of the Burro Canyon nonpersistent lenses about 600 ft wide. Total in the Slick Rock mining district were identified as Karla Kay Conglomerate Member______64. 0 Early Cretaceous. The fossils correlate with an Early Total Burro Canyon Formation______206.0 Cretaceous fauna in Wyoming or Montana which in turn contains fossils found in the Cedar Mountain Forma­ The amount of conglomerate and sandstone in the tion of Utah. Burro Canyon Formation decreases southward within Little, if any, break in deposition occurred between the map area, and in many exposures in the south the the Morrison and Burro Canyon Formations in the Burro Canyon is entirely mudstone. Just south of Ute Mountains area. The complete lack of fossils

745-807 O 65 4 20 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO in the vicinity of the contact prevents precise deter­ 8. Yucca section (pi. 1) Continued mination of the boundary between the Jurassic and Dakota Sandstone Continued Veet Cretaceous Systems. Sandstone, dark-gray, yellowish-gray weathering; very fine grained; grades to siltstone; well-sorted; DAKOTA SANDSTONE composed of subangular quartz and argillaceous The Dakota Sandstone of Late Cretaceous age ranges material; firm calcareous cement; very thin bedded; weathers rounded, forms a ledge; contains in thickness from 95 to 140 feet but averages 125 feet in sparse limonite and abundant plant fragments; base most of the Ute Mountains area. It is composed of sharp ______3.5 yellow-gray and tan carbonaceous sandstone of fluvial Mudstone, light- to medium-gray, thinly to very thinly origin in the lower part and yellow-brown and yellow- laminated; forms a regular slope; unit is car­ bonaceous ; upper 3 ft very impure coal containing gray sandstone of fluvial and beach origin in the upper limonite particles - 17. 0 part. These are interbedded with gray to dark-gray Ironstone and autochthonous chert zone, dark-red, carbonaceous mudstone and coal that were probably de­ sandy; base gradational___ 1.0 posited in a swamp or tidal-flat environment. The coal Sandstone, yellowish-gray, yellowish-brown; weather­ beds are not persistent and range from a few inches to ing very fine grained, fairly well sorted; grades to siltstone; composed of frosted quartz; poorly ce­ 3 feet in thickness. Sandstone cemented with iron mented, limonitic; thinly cross laminated (rib and oxide is common in the upper part of the formation. furrow) ; forms a ledge; contains abundant cal­ Two complete stratigraphic sections of the Dakota, careous films and limonite stains, spots, and measured southwest of the mountains, follow. Section streaks; base gradational _ 3.5 8 was measured in sec. 6, T. 34 N., R. 18 W.; section 9 in Mudstone, silty, dark-gray; indistinct bedding; weathers hackly; forms a slope; base gradational 3.0 sec. 10, T. 33% N., R. 19 W. Both sections were meas­ Coal, impure__ 2.5 ured by J. H. Irwin, Houser, and Ekren. Mudstone, silty, dark-gray; indistinct bedding 3.0 Sandstone, light-gray; weathers pale yellowish gray; 8. fucca section (pi. 1) medium- to fine-grained, fairly well sorted; con­ Block rubble. tains subrounded frosted quartz, some minor argil­ Unconformity. laceous material, firm calcareous cement; thin bed­ Dakota Sandstone: Feet ded planar crossbedding; weathers smooth to Mudstone and coal, gray to black, very thin bedded. slabby, forms a ledge; contains some powdery cal­ Unit forms a smooth, covered slope; it is carbona­ cite and limonite flecks on fresh fracture___ 3.0 ceous mudstone at base and grades to coal at top; Sandstone, light-gray, medium- to fine-grained; con­ base sharp______3. 5 tains subrounded quartz, firm calcareous cement; Sandstone, light-gray, yellowish-gray-weathering, thin bedded; weathers knobby, forms a slope; con­ very fine grained; contains subangular quartz, rare tains common iron stains and abundant carbonace­ black accessory minerals, firm calcareous cement; ous material; 1-ft-thick dark-gray carbonaceous grades to siltstone; fiat, thin bedded; weathers mudstone at top; base sharp 6.0 slabby, forms a ledge; contains carbonized plant Carbonaceous mudstone, dark-gray, thinly laminated; fragments, and ironstone concretion zones which weathers fissile, forms irregular slope; base grada­ are parallel to the bedding planes; base sharp___ 1.0 tional______1. 5 Sandstone, pale yellow-gray to yellowish-brown, fine- Siltstone (contains some clay stone), gray, poorly to very fine-grained, well-sorted; contains sub- sorted, thin-bedded; weathers blocky, forms a rounded to subangular frosted quartz, abundant slope; contains abundant carbonaceous material argillaceous material (accessories obscured by iron and sparse limonite; base gradational 1.0 stains), firm calcareous and ferruginous cement; Mudstone (silty with minor sand grains), dark- to wedge-plane crossbedding; weathers blocky, forms medium-gray; fissile bedding; forms an irregular a ledge; bedding contorted; contains sparse plant slope; unit very carbonaceous at base, becomes fragments and a few ironstone concretions which sparsely carbonaceous toward top; base grada­ weather out as spheroidal nodules; base sharp_ 5. 0 tional______.______15.0 Sandstone, pale brownish-gray; weathers yellowish Sandstone, medium-gray; weathers pale yellowish brown and yellowish gray; medium to fine grained; gray; medium- to very fine-grained; contains some contains quartz and, at intervals, abundant argil­ siltstone, poorly sorted and subrounded frosted laceous material (accessories obscured by iron quartz, rare black and orange accessory minerals; stain) ; firmly cemented, calcareous and limonitic; argillaceous material common; fiat, thin bedded; flat, thin-bedded; weathers flaggy, forms a ledge; weathers smooth, forms a ledge. This unit is dis­ contains abundant films of carbonaceous material continuous ; base sharp______1. 0 and epigenetic seams of calcite along horizontal Mudstone, very sandy; grades to muddy sandstone; fractures; base sharp______2. 5 light gray, weathers mottled gray and yellow; Mudstone, very dark gray to black, fissile to thin- medium to very fine grained to silt size, poorly bedded ; forms a regular slope; unit is very carbo­ sorted, weakly cemented, probably calcareous; flat, naceous; contains ironstone concretions at the top thin to very thin bedded; forms a slope; contains in a zone as much as 8 in. thick; base sharp___ 10.0 limonite stains and some clay-size matrix material /STRATIGRAPHY OF THE SEDIMENTARY ROCKS 21

8. ^ucca section (pi. 1) Continued 9. Kiva section (pi. 1) Continued Dakota Sandstone Continued Feet Dakota Sandstone Continued Feet which resembles Brushy Basin Member of Mor- Sandstone, light yellowish-gray, buff-weathering; rison. This unit grades to sandstone horizontally medium grained, well sorted; contains rounded, in both directions of exposure; base flat- 13.0 frosted quartz, black accessory minerals; weakly Sandstone, white to very light gray, pale-yellow- cemented by limonite (?); unit is a coset contain­ weathering; very fine grained to silt size, fairly ing fiat thin beds, and wedge-planar crossbeds, and well sorted; contains frosted quartz, rare to com­ small-scale crossbeds; weathers smooth to blocky, mon black, orange, and green accessory minerals forms a ledge; contains masses of rounded medium- and argillaceous material; fiat, thin bedded; grained sandstone, firmly cemented with calcite; weathers smooth, forms a ledge; contains medium- base gradational_ ___ 28.0 grained quartz "berries" and a few limonite specks; Conglomerate, pale yellowish-gray. Matrix isi fine­ base sharp _ _ _ _ 5.0 grained sandstone composed of subrounded to sub- angular frosted quartz, firmly cemented. Pebbles Total Dakota Sandstone- 101.0 are quartzite,.red chert. Unit is a coset containing Top of Burro Canyon Formation. flat thin beds, wedge-planar-type crossbeds and low-angle small-scale concave crossbeds; weathers 9. Kiva section (pi. 1) smooth, forms a ledge; unit is short channel or Mancos Shale (not measured) : lens, thickness reaches 6 in. a short distance on Sandstone, yellow, soft, friable. either side of the point measured; coarser material Dakota Sandstone: concentrated along the bounding surfaces of cross- Sandstone, pale yellowish-gray, tan-weathering; fine bedded sets; base irregular and gradational 4.0 to very fine grained, sugary, well sorted; contains Sandstone, pale-yellow, medium-grained, well-sorted; subrounded frosted quartz, black accessory mineral contains subrounded frosted quartz, quartz over­ grains, weak limonite cement; unit is a coset com­ growths; weakly cemented (calcareous and limo- posed mainly of flat beds, some wedge-planar-type nitic) ; wedge- and tabular-planar crossbeds and crossbeds, and some concave low-angle crossbeds; concave low-angle small-scale crossbeds; weathers weathers knobby to slabby, forms a ledge; contains blocky to knobby, forms an irregular ledge; thick­ limonite stains; base sharp_____ 4.5 ness varies considerably laterally; base grada­ Mudstone and impure coal, medium-gray to black, tional______-______7.0 flatbedded; weathers hackly, forms an irregular Sandstone, gray to brownish-gray; weathers to dark slope; contains limonite pockets; base sharp 4. 0 brownish-gray; medium grained, fairly well to Sandstone, pale yellowish-gray, tan-weathering; fine poorly sorted; contains rounded quartz, red, orange, grained, sugary, well sorted; composed of sub- and sparse green chert, firm cement (calcareous), rounded quartz; weakly cemented (iron) ; unit is a flat bedding; weathers blocky, forms an irregular coset composed of flat thin to thick beds, wedge- ledge; contains calcite overgrowth; unit grades up­ planar-type crossbeds, and some concave low-angle ward into overlying unit; base sharp _ 3. 0 crossbeds; weathers pitty and slabby, forms a Mudstone (mostly covered), carbonaceous, dark-gray, ledge; contains rib and furrow and secondary silica flat, thinly laminated; weathers fissile, forms a overgrowths; base sharp______8.0 slope; base concealed ______4.0 Mudstone, dark-gray, flat-bedded; weathers fissile; mostly a covered interval; contains abundant car­ Sandstone and rare short conglomeratic beds, white, bonaceous material and some limonite; base light-gray-weathering, medium-grained, well- sharp______5. 5 sorted; contains subangular sugary quartz, rare Sandstone, gray, weathers lightgray; abundant yel­ black accessory minerals, weak cement; wedge- and low limonite staining; carbonaceous, fine to very tabular-planar crossbeds and low-angle small-scale fine grained, well sorted; contains subrounded to crossbeds; weathers rounded to pitty to knobby, subangular quartz; thick bedd'ed; weathers mas­ forms an irregular ledge; contains abundant limo­ sive and blocky, forms an irregular ledge; contains nite at base; flashes in sunlight owing to quartz pitted surface marks, abundant carbonized plant overgrowths; base gradational______10.0 remains, and, at top of the unit, a 3-ft bed of impure Sandstone, light-gray, pale yellowish-gray-weather­ coaly material and hackly-weathering carbona­ ing ; medium to fine grained, well sorted; contains ceous mudstone; contains pale greenish-yellow rounded frosted quartz, rare black accessory min­ material; base sharp______13. 0 erals, firm cement (calcareous?) ; fiat, thin beds Mudstone, medium-gray to black to yellowish-gray, (rib and furrow) ; weathers blocky, forms a ledge; carbonaceous; weathers hackly, forms a slope; con­ contains abundant limonite spots and stains; base tains limonite stains and beds of impure coal; base sharp ______3. 0 sharp______11. 0 Mudstone, carbonaceous, dark-gray, flat-bedded; Ironstone, dark-red, flat-bedded, cherty; weathers weathers fissile, forms a slope; contains a 6-in. blocky to knobby, forms an irregular ledge; con­ flaggy gray beige-weathering sandstone about 1 ft tains abundant hematite and limonite; base from base; composed of angular quartz, abundant sharp______1.0 limonite cement; contains a few iron concretions; Mudstone, same as mudstone unit above______15. 5 base sharp______4.5 22 GEOLOGY, PETROLOGY, UTE MOmsrTATNTS AREA, COLORADO 9. Kiva section (pi. 1) Continued The fluviatile sandstone beds in the lower part of the Dakota Sandstone Continued Feet Dakota are much like the sandstone in the upper part Sandstone, very pale yellow, light grayish-yellow, of the Burro Canyon Formation; where the discon- weathering, fine-grained, sugary, well-sorted; con­ tains subangular, frosted quartz, weak cement (cal­ formity between the two formations is not apparent, careous) ; unit is a coset composed of flat thick- distinction is difficult. Several criteria were used to bedded sets, wedge-planar crossbedded sets, and distinguish the two formations, and these gave reliable low-angle crossbeds; weathers smooth to blocky, results in mapping: forms a ledge; desert varnish on resistant bedding surfaces; contains limonite spots and streaks; Burro Canyon Formation Dakota Sandstone quartz flashes in sunlight owing to secondary quartz overgrowths______6.0 Carbonaceous material absent Carbonaceous material com­ Sandstone and short conglomeratic units, light-gray, or scarce. mon in all types of strata. coarse- to medium-grained, poorly sorted; contains Sandstone beds are commonly Sandstone contains very little rounded to subrounded frosted quartz, pebbles of conglomeratic and white. conglomeratic material ex­ quartzite, red chert, and clay; firmly cemented cept in lowermost beds and (calcareous?) ; wedge-planar type crossbedding are yellow-brown or yellow- (concave, low- to medium-angle, medium-scale gray. crossbeds) ; weathers blocky, forms a ledge; con­ Mudstone beds are mostly Interbedded mudstone com­ tains limonite stains and some argillaceous mate­ green and red; a few are monly is medium gray to rial ; base gradational______7.0 very light gray to white. black, and, very rarely, light gray. Conglomerate, mottled gray and red; weathers light Mudstone pebbles in conglom­ Mudstone pebbles in basal gray. Matrix is coarse-grained quartz sandstone. eratic material are red or beds are mostly gray or Pebbles in unit comprise quartzite, clay fragments, green. black, but some are green, chert, and siliceous limestone(?). Unit weathers white, and red. blocky, forms a ledge; contains a 2-in. sandstone layer at the base which is gray, medium grained, In figure 5 the contrasting colors of the Dakota Sand­ and well sorted and contains abundant carbon and limonite; base sharp______1.0 stone and the sandstone beds' of the Burro Canyon Formation are apparent. Total Dakota Sandstone- 140.0 The basal conglomerate of the Dakota has a maxi­ Top of Burro Canyon Formation. mum thickness of about 5 feet and is absent locally.

FIGUEB 5. Lower part of Dakota Sandstone (Kd) and sandstone of Burro Canyon Formation (Kb) display contrasting colors in western McElmo Canyon. Rounded slopes in middle ground are on Brushy Basin Member (JmbJ of Morrison Formation. Sandstone outcrops in fore­ ground are in Westwater Canyon Member (Jmw) of Morrison Formation. Viewed from the west. STRATIGRAPHY OF THE SEDIMENTARY ROCKS 23 Most of it is granule conglomerate but some is pebble 10. Mound section (pi. 1) Continued conglomerate. In areas of relatively high pre-Dakota Mancos Shale (incomplete) Continued Feet relief, slabs, chips, and, rarely, cobbles and boulders of Sandstone, very light gray; weathers buff; very coarse to coarse grained, poorly sorted; con­ quartzite are present in the conglomerate. Most of the tains subrounded clear and frosted quartz, granules and pebbles are chert and quartzite, although abundant glauconite, and rounded grains of pebbles of mudstone and sandstone are common locally. quartzite (?) and chert (?); medium to thin The sandstone beds in the lower part of the Dakota beds, small-scale low-angle crossbedding; are cross-stratified at high angles. These beds are weathers slabby; forms an irregular ledge; interbedded with fissile dark-gray mudstone; highly lenticular and have little value for structural contains shark teeth; base sharp______9 interpretation. The uppermost sandstone bed in the Juana Lopez Member: Dakota is fairly continuous throughout the Ute Moun­ Limestone and mudstone, interbedded, dark gray; tains area and is a useful horizon marker. The top of fissile bedding. Limestone thin-bedded (0-2 the Dakota was mapped at the top of this sandstone in. thick, average */2 in.), pale yellow gray, bed. The bed contains fossil pelecypods and is prob­ finely crystalline, very fossiliferous; contains pelecypods (Inoceramus perpleseus Whitfield ably a reworked beach deposit. The pelecypods are the and Ostrea lugubris Conrad) and skate tooth same as those occurring a few feet higher in the Mancos (Ptychodus whipplei Marcou). Six ft above Shale, identified by W. A. Cobban (written commun., base of unit is fine-grained limy sandstone lens 1956) as Gryphaean. sp. (6 in. thick) that contains frosted quartz, The assignment of an age to the Dakota Sandstone abundant black accessory minerals, and argil­ laceous material; weathers rounded to blocky, is based on its lithologic similarity to the Dakota else­ forms a ledge; base of sandstone lens sharp. where on the Colorado Plateau where it overlies the Limestone-mudstone unit forms an irregular Burro Canyon Formation and where a Late Cretaceous cliff; base gradational. age has been established (Brown, 1950). Katich (1951, Total Juana Lopez Member______42 p. 2094) mentioned, however, that faunal evidence in­ Mudstone, dark-gray to black; weathers dark dicates that in parts of central Utah the lithologic unit gray; very thinly laminated; weathers hackly, forms a regular slope; contains carbonaceous called Dakota Sandstone is Early Cretaceous in age. material and limestone concretions similar to Early Cretaceous fossils also have been found in a lower concretionary units below; base gradational__ 62 sandstone of the Dakota in the Carrizo Mountain area Limestone concretionary zone in shale unit; con­ (J. D. Strobell, oral commun., 1957). In the latter cretions are dark gray, weather tan to buff and areas the Burro Canyon Formation has not been recog­ rounded to blocky; a 3- to 6-in. zone at base nized. Where the Burro Canyon Formation is recog­ contains same crystalline cone-in-cone struc­ nized, the Dakota Sandstone is therefore apparently of ture noted in concretionary unit below; in some places this structure occurs at top of limestone Late Cretaceous age, and it is assigned this age in the concretions; limestone not composing this Ute Mountains area. crystalline structure is partly fossiliferous (Lingula-like pelecypods) ; some of upper sur­ MANCOS SHALE faces of the limestone are rounded and nodu­ Cross and Purington (1899) named and described the lar ; contains vugs lined with calcite crystals; Mancos Shale exposed along the Mancos River in east­ basal 3- to 6-in. zone contains some banding; ern Montezuma County, Colo., where they estimated a base sharp_ _ 2 thickness of 1,200 feet. The Mancos Shale in the Ute Mudstone, dark-gray to black; weathers dark Mountains region consists of 1,900 feet of beds com­ gray; lacks limestone concretions 33 posed almost entirely of gray to dark-gray gypsiferous Limestone concretionary zone in shale unit, dark- gray ; weathers brown; concretions are 1 to 5 shale. The following section was measured by Houser, ft in diameter, bounded by 4- to 6-in. zone of J. H. Irwin, and Ekren south of the Ute Mountains in radiating crystalline cone-in-cone structure 2 sees. 24 and 25, T. 33i/2 N., R. 19 W., on the north slope Mudstone, gray to dark-gray; fissile bedding, cal­ of "The Mound" (pi. 1). careous ; forms a regular slope; interval is mostly covered; contains fossils and in places 10. Mound- section (pi. 1) abundant gypsum; base gradational- 266 Top of local exposure. Greenhorn Limestone Member: Pediment deposits. Limestone, dense, gray; weathers light gray to Unconformity. white; slightly sandy, flat, thin bedded, dis­ Mancos Shale (incomplete) : *"e«* continuous lenticular; intertongues with mud- Mudstone, pale yellow-gray, fissile-bedded; stone above and below; weathers to fiat cobbles, weathers hackly, forms a regular slope; base forms a thick ledge; has conchoidal fracture. gradational ______18 Total Greenhorn Limestone Member____ 20-33 24 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO 10. Mound section (pi. 1) Continued shells of Cfryphaea neivberryi Stanton (W. A. Cobban, Mancos Shale (incomplete) Continued Feet written commun., 1956). About a foot above this bed Mudstone, gray, silty; contains a few black ac­ is a 6- to 12-inch very light gray bed of bentonite stained cessory minerals; weathers hackly; forms a regular slope; zone, about 1 ft thick, 2 to 3 ft by limonite. The Greenhorn Limestone Member is a below the top of the unit, which contains conspicuous marker on flanks of the Ute Mountains. It abundant shells of Gryphaea newberryi______40 is not shown separately from the Mancos on the map Sandstone, poorly cemented except for a 1- to 2-ft (pl.l). bed 14 ft below top of unit; yellow, limonite The Juana Lopez Member is composed of brown to stained, medium grained; contains subrounded quartz, rare green accessory minerals; base tan sandy f ossilif erous limestone interbedded with dark- gradational______20 gray shale. It was originally defined by Eankin in Sandstone, same as unit below except gray and 1944 (p. 12,19-20) in a section measured 6 miles north­ contains no limonite; zone 4 ft from base is west of Cerillos, Santa Fe County, N. Mex., as a very firmly cemented (calcareous)______10 calcareous thin-bedded, very fossiliferous sandstone 10 Sandstone, yellow-gray, fine-grained; contains subangular frosted quartz, abundant green feet thick that occurs near the top of the Carlile Shale accessory minerals; poorly cemented (calcare­ in northern New Mexico and that contains Prionocyclus ous and ferruginous) ; indistinct bedding; wyomingensis Meek and Inoceramus Idbiatus Schlot- weathers to soil; forms irregular slope; base heim. Eankin, in fact, considered the top of the Juana sharp ______7 Lopez Sandstone Member as the top of the Carlile Shale. Total incomplete Mancos Shale______544 In the Ute Mountains area, the Juana Lopez consists of 42 feet (Mound section) of very fossiliferous, finely Dakota Sandstone (incomplete) : crystalline pale yellow-gray limestone and interbedded Sandstone, pale yellow-gray, weathering yellow dark-gray fissile mudstone. The member contains Ino- gray, medium- to fine-grained, well-sorted; con­ tains subrounded frosted quartz, common black cemmus perplexus Whitfield, Ostrea lugubris Conrad, and red accessory minerals!; firmly cemented Ptychodus whipplei Marcou, Prionocyclus wyormngen- (calcareous) thick beds; wedge-planar low- sis Meek, and Inocercvrrws dimidius White (W. A. Cob- angle small-scale crossbedding; forms rounded ban and J. B. Eeeside, Jr., 1956, written commun.). ledge; contains abundant calcareous material, This assemblage suggests a Carlile age for the Juana and limonite stains, streaks, and spots, and a 2-in. coal bed about 10 ft below top. Total Lopez Member in the Ute Mountains. incomplete Dakota Sandstone______15 The basal contact of the Juana Lopez Member is gradational and is placed at the base of the lowermost The transition from swamp, tidal flat, and beach dep­ prominent limestone bed. The top of the member is osition during late Dakota time to marine deposition placed at the top of uppermost limestone; however, as in Mancos time was gradual. In the southern and south­ mapped in the mountains, and as far south as "The western parts of the area, the Mancos sea reworked Mound" (pi. 1; fig. 6), the Juana Lopez Member in­ sands of Dakota age and deposited a pale-yellow to cludes at its top a set of light-gray glauconitic cross- light-tan, weakly consolidated, structureless clayey stratified coarse-grained quartz sandstone beds a few sandstone about 35 feet thick. Fossils from this sand­ inches to as much as 9 feet thick. South of "The stone were identified by W. A. Cobban (written com­ Mound," however, two beds of this sandstone are sepa­ mun., 1956) as Grryphaea n. sp. rated by about 40 feet of gray shale. Two conspicuous lithologic marker units are present In this area the combined thicknesses of the Juana in the lower part of the Mancos: the Greenhorn Lime­ Lopez Member and the overlying interval as defined stone Member of the Mancos Shale (see Dane, 1957, and by these two sandstones is nearly 100 feet thick, whereas Wanek, 1959, p. 681), about 75 feet above the base, and at "The Mound" it is 50 feet thick, and northward in the the Juana Lopez Member of the Mancos Shale, about mountains proper it is less than 25 feet thick. Since the 475 feet above the base of the Mancos. completion of the present study, C. H. Dane (1960) re­ The Greenhorn Limestone Member is remarkably per­ ported that the glauconitic sand overlying the Juana sistent throughout the map area although it ranges in Lopez Member at "The Mound" is of Niobrara age, con­ thickness from 15 to about 35 feet. It consists of thin taining Inoceramus defornds. According to Dane beds of dense, locally almost lithographic, gray lime­ (1960, p. 53) the glauconitic sandstone cuts sharply stone interbedded with shale. The limestone weathers downward across underlying beds northward from the to light gray or white and forms low ledges. Three to San Juan River. Dane reported (p. 53): "At a locality six feet below the base of the limestone is a persistent 3 miles south of the Colorado State line coarse-grained bed, 3 inches to 3 feet thick, composed almost entirely of glauconitic sandstone rests directly on shales of Carlile 25

FIGURE 6. "The Mound." The Juana Lopez Member (Kmsj) in the Mancos Shale (Km). age within 60 feet of Rankin's Juana Lopez sandstone Lopez Member a few hundred yards east of Towaoc is member. West of Towaoc, Colo. (loc. 8), south of Ute so deficient in limestone that it has no topographic Mountains, coarse-grained glauconitic quartz sandstone expression. containing Inaceranws deformis of Mobrara age rests The lower part of the Mancos Shale in the vicinity of on only 15 feet of shale above the Juana Lopez sand­ Point Lookout, 25 miles east of the Ute Mountains, is stone member. It is of interest to note that the basal of Colorado age and the upper part is of early Montana few inches of the glauconitic sand contains a few shells age (Pike, 1947, p. 20-24). (For a discussion of the of Ostrea lugubris, doubtless reworked from the under­ terms Colorado Group and Montana Group and of the lying shales in \vhich it is also present." Dane con­ faunal evidence for these divisions, see Pike 1947, p. cluded (p. 53) that beds of the Inoceramus deformis 20.) The Mancos in the Ute Mountains is similar to zone cut down unconformably northward and that 300 that near Point Lookout and almost certainly correla­ feet or more of beds present south of San Juan River tive with it. are missing just south of Ute Mountains about 30 miles MESA VERDE GROUP to the north. The Juana Lopez Member is sporadically exposed in The Mesaverde Group of Late Cretaceous age is pre­ cuestas along the lower flanks of the Ute Mountains served in many places in western Colorado. It was first from "The Mound" to Towaoc, Colo., and forms a low described by Holmes (1877, p. 245, 248). Later, the ridge across the southern part of the Cortez SW quad­ major subdivisions were named by Collier (1919, p. rangle. North of the Yucca cluster of laccoliths, along 296) from bottom to top: the Point Lookout Sandstone, the Western slopes of the mountains, the member is hid­ 120 feet thick, the Menefee Formation, 800 to 900 feet den by talus and debris and its location is not certain. thick, and the Cliff House Sandstone, 190 feet thick. The member is probably continuous throughout the Ute In the vicinity of the San Juan River in southwestern Mountains, but it could not be traced in the higher parts Colorado and northwestern New Mexico the group owing to heavy cover of vegetation and soil, the non- ranges from 1,200 to 1,500 feet in thickness. resistant nature of the limestone, and general thinning Only the Point Lookout Sandstone is present in the of the member, especially toward the north. The Juana Ute Mountains. 26 GEOLOGY, PETROLOGY, TTTE MOUNTAINS AREA, COLORADO POINT LOOKOUT SANDSTONE higher parts of the area. The windblown material Overlying the Mancos Shale on a few high places in was probably derived mainly from arid areas to the the Ute Mountains are thin flat persistent beds of buff southwest. The bulk of it probably accumulated dur­ and yellowish-gray sandstone interstratified with dark- ing the Pleistocene Epoch, but some is still accumulat­ gray mudstone that resembles that of the Mancos. ing. This sequence is believed to be the lower part of the Fan-shaped deposits of coarse gravel occur on the Point Lookout Sandstone of the Mesaverde Group be­ south side of McElmo Canyon and in Montezuma Val­ cause of its lithdlogic similarity to that sandstone at ley near the base of the Point Lookout Sandstone. the type locality. The maximum preserved thickness These gravel deposits, classed as fanglomerate, were is about 100 feet. The beds are silicated where in­ spread over rather even, steeply sloping pediment sur­ truded by diorite and granodiorite northwest of the faces. Those of McElmo Canyon are divided into two Black Mountain stock. In the vicinity of "The Knees" groups on the basis of relative age. The older deposits stock, the Point Lookout rocks are slightly recrystallized are lithified; the younger deposits are lithified only and contain abundant crystals of pyroxene, epidote, locally and lie at the level of streams that drain the sericite, and garnet. northern parts of the mountains (pi. 1). The cobbles and boulders in the fanglomerate of McElmo Canyon QUATERNARY SYSTEM are mostly of igneous rock; those in the deposits in Alluvium, landslide debris, windblown material, and Montezuma Valley are mainly of Point Lookout Sand­ talus of Quaternary age obscure the bedrock in many stone. parts of the Ute Mountains area. These materials were Thinner deposits of coarse gravel mantle gently slop­ mapped only where they form large bodies or where ing pediment surfaces that extend radially away from they obscure bedrock contacts. The deposits were not the mountains. These deposits were mapped as pedi­ studied closely, and no systematic attempt was made ment gravel and no sharp line of distinction exists be­ to determine their relative ages; the sequence of presen­ tween them and the thicker fan-shaped deposits. In tation, therefore, does not reflect age. places, the gravel deposits are well cemented with In the valley of McElmo Creek, thick deposits of al­ caliche, but in others they are loose. Many of the luvium (pi. 1) border the creek and provide soil that gravel-covered pediments between creek beds serve as is intensively cultivated. The alluvial deposits are natural roads for the Ute Indians, and the gravels in mostly of silt but include beds of sand and gravel. Al­ the larger deposits are used extensively in highway luvium also occurs in smaller stream valleys of the Ute construction. Mountains area, and although it is similar to the al­ Although, several pediment surfaces are in evidence, luvium along McElmo Creek, it generally contains more they were not differentiated, and the deposits were gravel. In the western part of the Ute Mountains mapped as one unit. much sheet-wash gravel was mapped as alluvium. Al­ Deposits of fine material, cobbles, boulders, and great luvium composed of reworked Mancos Shale and wind­ slabs of rock that have moved downward through thei blown silt is found in Montezuma Valley east of the combined action of gravity, water, and, in some in­ Ute Mountains. It forms a good soil and is extensively stances, snow, were mapped as landslides (pi. 1). The farmed. In places, especially in McElmo Canyon, the largest of such deposits are on the north-facing slopes alluvium is being rapidly removed by stream down- of the Ute Mountains. A large landslide deposit that cutting. lies between Black Mountain and Ute Peak (pi. 1) is Stream terrace gravel (pi. 1) occurs in McElmo and of interest because in places it contains abundant mate­ in East McElmo Canyons and vicinity. The gravel rial derived from the Point Lookout Sandstone, a unit rests on stream-leveled bedrock and consists of pebbles, that now exists only in thin remnants in the Ute Moun­ cobbles, and boulders of both igneous rock and well- tains. A large landslide deposit on the north slope of cemented sandstone. The gravel was deposited during Mable Mountain consists mostly of Mancos Shale and a period of aggradation that preceded the cycle of downcutting and aggrading that gave rise to the valley- boulders and cobbles of porphyry. fill alluvium. Bodies of talus (pi. 1) surround many of the igneous Light-brown windblown silt and very fine grained intrusive bodies. The talus is thick in the vicinity of sand (pi. 1) mantle most of the mesas and pediment Ute Peak, and on the west side of the peak it has surfaces in the Ute Mountains area. They form an streamed downward to form a rock glacier (pi. 1). On excellent porous soil capable of holding water for long the north, east, and south sides of the peak, talus grades periods of time and are utilized in dry farming in the imperceptibly into block rubble. IGNEOUS ROCKS 27 In places along the flanks of the mountains, blocks of latest Cretaceous age. In contrast, Hunt, Averitt, and rock principally of porphyry have accumulated in Miller (1953, p. 212) outlined geomorphic evidence that bodies 5 to 40 feet thick, which are classed as block suggests a middle Tertiary age for the Henry Mountains rubble. The rubble has a hummocky form, and in some RELATIVE AGES OF THE INTRUSIVE ROCKS areas may include the slumped remnants of sills or laccoliths. Water and possibly snow played major roles and thus, inferentially, for other laccolithic groups. in the movement of the blocks. Bodies of the rubble Evidence that the microgabbro was the earliest intru­ commonly lie at considerable distances from possible sive rock and that diorite porphyry, granodiorite por­ source rocks. They differ from landslide deposits phyry, and quartz monzonite porphyry are successively younger is found in both the northern and southern mainly in that they rest on gentler slopes and consist parts of the mountains. In the vicinity of North Black mostly of porphyry. Block rubble, landslide, and talus Mountain in the northern Ute Mountains, sills of micro­ grade from one to another, and the contacts between gabbro are domed upward over the underlying North them are drawn arbitrarily in many places. Black Mountain laccolith of diorite porphyry (pi. 1). IGNEOUS ROCKS On the east side of Black Mountain, quartz monzonite porphyry has a chilled border where it is in contact GENERAL FEATURES AND EVIDENCE FOR AGE OF with microgabbro. In the vicinity of the Horse Moun­ IGNEOUS ACTIVITY tain laccolith a discordant intrusive of quartz monzonite The igneous rocks of the Ute Mountains are members has a chilled border where in contact with diorite por­ of a calc-alkalic series that ranges in composition from phyry (fig. 7). microgabbro through quartz monzonite. Field map­ ping indicates that the succession of intrusive rocks from earliest to latest was microgabbro, diorite, grano- diorite, and quartz monzonite. All the igneous bodies were intruded forcibly through or between layers of sedimentary rock. No evidence was found that the igneous rocks replaced or assimilated their wall rocks during intrusion. The igneous rocks occur in stocks, laccoliths, bysma- liths, sills, and dikes. The dikes are radially distrib­ uted about the Black Mountain stock in the north, "The FIGURE 7. Contact of quartz monzonite porphyry (qmp) with diorite Knees" stock in the south, and an inferred concealed porphyry (dp). Note that flow llneatlon In the quartz monzonite stock at Ute Peak. porphyry parallels the contact zone. The similarity of the intrusive forms, the geologic Southeast of "The Knees" stock in the southern Ute structure, and the types of igneous rock of each of the Mountains, sills of microgabbro and diorite porphyry laccolithic mountain groups on the Colorado Plateau are domed upward by a large underlying intrusive body strongly suggests that all the laccolithic mountains of granodiorite porphyry. formed at approximately the same time. Some of the evidence for the relative ages of the The igneous rocks in the Ute Mountains intrude sedi­ intrusive rocks, such as the doming of microgabbro by mentary rocks as young as the Point Lookout Sand­ an underlying, more silicic intrusive mass, is indirect. stone of Late Cretaceous age. This intrusion is the only The microgabbro might possibly have been intruded direct evidence of the age of the intrusive rocks avail­ into beds that had already been domed; however, this able in the Utes. Other lines of evidence (see p. 6 to possibility is not considered likely because (a) the 7) indicate that igneous activity in the Ute Mountains microgabbro is generally more fractured than the un­ did not occur prior to the deposition of the McDermott derlying diorite porphyry and this fact suggests doming Member of the Animas Formation of latest Cretaceous after solidification, and ('b) there is evidence in some age. Shoemaker (1956, p. 162) pointed out that the exposures that the microgabbro was altered slightly by only known stratigraphic evidence that may permit pre­ underlying, more silicic rocks. A thin section of micro­ cise assignment of age to the laccolithic mountains is gabbro from an intrusive body that overlies granodiorite the occurrence of boulders of andesite in the McDermott. porphyry southeast of "The Knees" contains signifi­ He concluded that at least some of the coarse debris in cantly more quartz and calcite than a sample of the McDermott was probably derived from the La Plata microgabbro taken from an exposure several hundred Mountains, a laccolithic group, and that some of the in­ yards from the granodiorite. Some of the calcite and trusive bodies in the La Plata Mountains are probably of quartz may have been introduced by hot ground water 745-807 O 65 5 28 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO when the enclosing Mancos Shale was heated by the bytownite or calcic labradorite and outer zones of large underlying granodiorite laccolith. andesine. These crystals are about An65 in average composition, and the overall plagioclase composition OCCURRENCE AND PETROGRAPHY is estimated to be about An55. GENERAL FEATURES Apatite is abundant in the microgabbro, and some In the descriptions that follow, modal analyses of crystals are as large as 1 mm in cross section. several samples are given for each rock. Analyses that Both varieties of microgabbro have been considerably include modes of the groundmasses are close approxi­ altered. Original augite has been altered in part or mations only, owing to the dense nature of the ground- completely to chlorite and calcite in all the microgabbro masses. The term "dense" as used in this paper implies examined in thin section. Plagioclase and hornblende that the grain size of the groundmass crystals averages are fresh to moderately altered. The altered plagio­ less than 0.03 mm; "extremely dense" implies that the clase commonly contains calcite, kaolin, and chlorite; groundmass crystals are less than about 0.01 mm in av­ the altered hornblende contains calcite and chlorite. erage diameter. Locally, pyrite is abundant. Vesicles filled with quartz and calcite are fairly common in the microgabbro. The texture of the rocks varies with rock type and from one intrusive body to another. In general, the Modes of three samples of microgabbro are listed below. microgabbro is slightly porphyritic; the diorite is mod­ Modes of microgablwo erately porphyritic; and the granodiorite and quartz [Sample 56-E-77 is porphyritic; 56-E-94 and 56-E-97 have seriate texture. For monzonite are conspicuously porphyritic. The ground- chemical analysis of 56-E-77, see table 3, sample 2] masses of the various rocks make up from 40 to 60 per­ Field sample cent of the volume and, in general, are less dense in Mineral the mafic rocks. 56-E-77 56-E-94 56-E-97 4 3 3 MICROGABBRO 59 53 60 10 8 6 3 13 12 The microgabbro is an aphanitic black rock closely 3 1 4 17 resembling basalt (fig. 8A). It occurs as sills in the Calcite 27 7 6 Pyrite 1 northern part of the mountains and as small tongue- Biotite <1 shaped laccoliths and thin sills in the central part of Chlorite- _ __ _ 14 7 9 the mountains. Two varieties are recognized. One is Total. 100 100 100 porphyritic and contains about 10 percent hornblende 1 Relicts are now filled with chlorlte, limonite, and unknown green to white opaque mineral. and 3 to 10 percent augite in phenocrysts that are 2 to 2 Formed largely at the expense of pyroxene. 10 mm in length. The texture of this rock varies from Locality descriptions: 56-E-77: Tongue laccoliths, northeast of "The Knees," central Ute Mountains. diabasic to trachytic. Major constituents of the ground- 56-E-94: Sill in Pine Creek, northern Ute Mountains. mass are plagioclase and chlorite. The plagioclase oc­ 56-E-97: Sill on east flank of Black Mountain, northern Ute Mountains. curs as twinned microlites that average about 0.04 by DIORITE PORPHYRY 0.1 mm and as nearly equant zoned crystals as much as Diorite porphyry is the most abundant igneous rock 2 mm in length. Chlorite fills interstices between the in the Ute Mountains. It occurs as laccoliths, as thin plagioclase microlites, and occurs with calcite as an sills ranging from a few feet to 100 feet in thickness, alteration product of augite, hornblende, and the larger and in a few dikes. Although mapped as a single unit, crystals of plagioclase. Biotite is common as tiny books the diorite porphyry is divided for purposes of descrip­ as much as 0.2 mm in length. Quartz is ubiquitous and tion into two varieties: normal diorite porphyry and averages about 4 percent; magnetite and apatite are leucocratic diorite porphyry. The two varieties gen­ the most common accessory minerals. erally are distinguishable by differences in color caused The other variety of microgabbro has seriate tex­ primarily by a greater abundance of mafic minerals in ture and contains very few crystals that exceed 1 mm the groundmass of the normal diorite porphyry, al­ in length. The rock contains about 15 percent augite though they are petrographically gradational. Only and 8 percent hornblende that form the largest crystals one variety occurs in any individual intrusive body. and range in size from about 0.1 mm to slightly more The normal diorite porphyry is thought to be the than 3 mm. Plagioclase occurs as tabular zoned crys­ older. The two varieties of diorite were observed to tals ranging from less than 0.1 mm to about 0.6 mm, and be in contact in only one place, on the northeast side of as tiny microlites. Razorback laccolith, where leucocratic diorite porphyry The larger crystals of plagioclase in both varieties of of the Razorback laccolith has intruded normal diorite microgabbro are conspicuously zoned with centers of porphyry of the East Horse laccolith. Structural re- 29

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FIGURE 8. Specimens of the four main types of igneous rock from tbe Ute Mountains. A, Microgabbro, nearly seriate texture. B, Diorite porphyry (leucocratic variety), phenocrysts are hornblende and plagioclase; note the hornblendic inclusion. C, Granodiorite porphyry; southern type; phenocrysts are plagioclase and pseudomorphs after hornblende, now filled with sericite, clilorite, and calcite. JO, Quartz monzonite porphyry; phenocrysts are bipyramids of quartz, plagioclase, and pseudomorphs after hornblende. lations elsewhere suggest indirectly that laccoliths of phyry occurs mainly as flat-topped laccoliths 200 to 500 normal diorite porphyry have influenced the location feet thick. The leucocratic diorite porphyry occurs pre­ and extent of intrusion of leucocratic diorite porphyry. dominantly as thick (500 to 1,200 feet), steep-sided Such relationships are discussed in detail in the section laccoliths that have higher ratios of vertical dimension describing the structure of the individual intrusive to areal extent than laccoliths of normal diorite por­ masses. phyry. Differences in viscosities of the two diorite por­ The shape of laccoliths bears a general relationship to phyry magmas probably caused differences in the forms the type of diorite porphyry. The normal diorite por­ of the laccoliths. 30 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO

NORMAL DIOKITE PORPHYRY LEUCOCRATIC DIORITE PORPHYRY The fresh rock is mostly greenish gray; some is yel­ The leucocratic diorite porphyry crystallized from a lowish gray. It weathers to a moderate yellowish magma slightly richer in silica and water than the brown and medium brown, and on many outcrops is magma of the normal diorite porphyry. The rock con­ stained medium to dark brown by iron. Some stain of tains no pyroxene but contains more hornblende than black manganese oxide coats the older exposures. the normal diorite porphyry. Phenocrysts constitute The distinction between phenocrysts and groundmass an average of 50 percent of the leucocratic diorite is not microscopically sharp inasmuch as many of the porphyry. These are zoned crystals of labradorite or rocks have a nearly seriate texture. The phenocrysts calcic andesine that constitute somewhat more than 35 constitute about 50 percent of the volume of the rock. percent of the volume and horn'blende that constitutes Of these, plagioclase is the most abundant, comprising 10 to 20 percent. Quartz phenocrysts are rare and aver­ 25 to 45 percent. The plagioclase phenocrysts are age less than 3 'percent. The groundmass is dense or twinned and zoned and average about 0.7 by 1.0 by 2.0 extremely dense and consists of sodic plagioclase, mm in size. The composition ranges from calcic labra- potassium feldspar, and quartz. dorite in the cores to oligoclase in the outer zones. The The leucocratic diorite porphyry is megascopically maximum difference in the anorthite content in the very similar to the granodiorite porphyry of Ute Peak zones of one crystal is about 4:5 percent An. The pheno­ but differs from the rock at Ute Peak in its lower con­ crysts are anhedral to euhedral in outline. tent of potassium feldspar and its lack of pyroxene. A Phenocrysts of hornblende and augite constitute 12 specimen of the leucocratic diorite porphyry is shown to 17 percent of the rock and average about 14 percent. on figure 8B. Most crystals are euhedral. The hornblende crystals Modes of diorite porphyry average about 0.6 by 2.5 mm but generally vary consid­ [See table 3 for chemical analyses of 56-E-42 (sample 5), 56-E-33 (sample 6)] erably from these dimensions. The crystals are of the Field sample common green variety of hornblende, and in most sam­ Constituent Normal diorite porphyry Leucocratic ples have been moderately to considerably altered to diorite porphyry magnetite, sericite, and chlorite. (See table 6.) The augite phenocrysts average about 0.3 by 1.0 mm. 55-E-47 56-E-42 56-E-37 56-E-33 H-539

Accessory minerals magnetite, apatite, and sphene Tr. 1.6 4.2 0.4 2.6 4.8 3.0 rarely exceed 3 percent of the rock. Eare crystals of 38.1 60.4 68.6 28.2 25.6 4.3 7.0 7.2 17.0 19.2 clinozoisite near the margin of one intrusive body of 8.5 "9.4 5.0 Epidote. - __ . . _ . diorite porphyry are thought to have originated during 4.0 2.2 .4 .4 1.7 .3 Tr. .2 .6 the late magmatic stage. Alteration products: Chlorite 12.4 12.8 8.6 9.4 1.0 .3 A medium-gray to dark greenish-gray groundmass Calcite __ . __ ___ .. 1.8 2.6 1.2 Kaolinite __ . . . . .2 forms about half the volume of normal diorite porphyry Sericite ______- .2 .6 and contains grains ranging in size from less than 0.01 Groundmass 2 32.4 -f - 44.0 47.6 Total 100.0 100.0 100.0 100.0 100.0 mm to as much as 0.1 mm. Somewhat more than 70 percent of the groundmass is sodic plagioclase, anhedral 1 Augite occurs as fine-grained prisms. 2 Groundmass consists mainly of sodic plagioclase, quartz, and potassium feldspar. Although the groundmasses of 56-E-42 and of 56-E-37 were counted, the modes as quartz, and potassium feldspar. Some of the plagio­ shown are considered to be approximations only. clase is euhedral and a little of the potassium feldspar is Locality descriptions: 55-E-47: South side Yucca laccolith. subhedral. The composition of plagioclase in the 56-E-42: Unnamed laccolith in Yucca cluster of laccoliths, SEJi sec. 33, T. 35 N.f R. 18 W. groundmass can seldom be determined accurately. 56-E-37: Southwest part of unnamed laccolith in the West Toe cluster, SEJi unsurveyed sec. 4, T. 34 N., R. 18 W. Small amounts of hornblende or augite normally occur 5&-E-33: West side Pack Trail laccolith. H-539: West end Horse laccolith. in the groundmass, and chlorite is abundant. Diorite porphyry having a composition near microgabbro con­ GRANODIORITE PORPHYRY For mapping purposes, three distinctly different tains as much as 10 percent augite. Quartz is mainly in­ rocks have been grouped together as granodiorite por­ terstitial and formed during a late magmatic stage. phyry. Because the three rocks are localized geo­ Eare rounded and embayed phenocrysts of quartz are graphically within the Ute Mountains, they will be interpreted as having formed under conditions of fa­ referred to as the southern, central, and northern types. vorable pressure and temperature at considerable depth, The southern type (fig. 1367) is a light-gray rock that although they may be partially assimilated crystals de­ forms laccoliths, bysmaliths, dikes, and a stock in the rived from a preexisting silicic rock. southern and south-central parts of the mountains. It IGNEOUS ROCKS 31 is characterized by phenocrysts of zoned plagioclase in equant grains as much as 10 mm across, and by pseudo- morphic aggregates of secondary minerals that indicate the former presence of phenocrysts of hornblende. The plagioclase is estimated to average about An40. The pseudomorphs after hornblende are filled with sericite, chlorite, and calcite. Quartz phenocrysts are rare. An extremely dense groundmass constitutes 50 percent of the volume of this rock. Although the groundmass can­ not be effectively examined microscopically, staining by sodium cobaltinitrite reveals the presence of abundant potassium feldspar, and norms have as much as 16 per­ cent potassium feldspar. (See p. 36 to 37.) Sodic FIGURE 9. Specimen of hornblende granodiorite porphyry from Ute Peak. plagioclase and quartz are the other principal constit­ uents of the groundmass. curs as equant crystals (0.5-2.5 mm) that are conspicu­ The central type of grandodiorite porphyry is a light- ously zoned from calcic labradorite in the cores to sodic gray rock that occurs only in the Mushroom laccolith. andesine and oligoclase in the outer zones. The aver­ It contains abundant fine-grained crystals of quartz as age plagioclase (phenocrysts and groundmass) is much as 0.5 mm in diameter. This rock was mapped estimated to be about An35. as tonalite porphyry in the field but was later placed In contrast to the generally altered character of the with the granodiorite on the basis of total K2O and granodiorite porphyry in the central and southern parts Na2O. This rock is similar to the southern type of of the mountains, the hornblende granodiorite porphyry granodiorite in that andesine (An40) and pseudomorphs of Ute Peak and Black Mountain is relatively fresh. after hornblende are the most common phenocrysts, al­ Many of the plagioclase crystals are clear, and some though the andesine crystals are much smaller in the of the hornblende is fresh except for slight alteration central type and rarely exceed 4 mm in diameter. The to pyroxene and magnetite along crystal borders. hornblende pseudomorphs are filled with chlorite, sericite, calcite, and iron ore. Modes of the three types of granodiorite porphyry are shown below. Most of the plagioclase in the central and southern types of granodiorite has been altered in part or com­ Modes, in percent, of granodiorite porphyry pletely to sericite, calcite, and chlorite. In the most in­ [See table 3 for chemical analysis of 56-E-82 (sample 13)] tensely altered rocks, pseudomorphs after plagioclase contain fine-grained quartz or chalcedony in addition to Field sample the minerals listed above, and the hornblende pseudo­ Northern type Southern type Central type (hornblende morphs contain partly opaque material that is prob­ Constituent granodiorite) ably a mixture of clay, sericite, chlorite, and calcite. 56-E-140 56-E-82 55-E-16 Pyrite is locally abundant. Epidote and garnet are common in granodiorite of the southern type in the vi­ 1.4 7.2 4.2 34.8 37.2 65.6 cinity of "The Knees" stock (south-central mountains). 12.8 . 18.6 J9.8 8.2 The northern type of granodiorite porphyry, called Tr. 2.0 .8 1.6 3.6 hornblende granodiorite porphyry, occurs in and adja­ Tr. Tr. .2 Tr. cent to the Black Mountain stock and in the Ute Peak .2 Chlorite 1.8 bysmalith. A specimen of this rock is shown on figure Pointc 1.4 2.8 9. The rock closely resembles the diorite porphyry in 54.4 41.4 hand specimen but contains more fine-grained potas­ Total. 100.0 100.0 100.0 sium feldspar. The potassium feldspar, sanidine, is 1 Pseudomorphs after hornblende are filled with chlorite, sericite, and calcite. Locality descriptions: confined to the groundmass, which is coarser than in the 56-E-140: Dike north of "The Knees" stock. 56-E-82: Mushroom laccolith. other types of granodiorite porphyry and can be ana­ 55-E-16: Approximately 1,500 ft. below top of Ute Peak, east side. lyzed fairly accurately with the microscope. In addi­ tion to sanidine, the groundmass consists mainly of QUARTZ MONZONITE PORPHYRY sodic plagioclase and quartz. Phenocrysts in this rock The name quartz monzonite porphyry is applied to include plagioclase, hornblende, augite, magnetite, and rocks of two types in the Ute Mountains. One type is sparse rounded crystals of zircon. The plagioclase oc- characterized by pyrogenic biotite phenocrysts and 32 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO occurs only in the northern mountains in the vicinity pseudomorphs after hornblende about 3 percent. Al­ of Black Mountain. The other type contains mica though quartz phenocrysts comprise a relatively small only as secondary crystals in pseudomorphs after horn­ fraction of the rock, they are the most conspicuous blende. This type occurs only in the southern part of crystals owing to the fact that they are never altered the mountains. The two types will be referred to as the and are highly resistant to erosion. In nearly all ex­ northern and southern types of quartz monzonite posures the quartz crystals (as much as 5 mm in diam­ porphyry. eter) protrude from the surface of the rock (fig. 8Z>). The northern type of quartz monzonite porphyry is Plagioclase phenocrysts are commonly 3 by 4 by 5 mm light blue gray where unaltered and pale green to green­ (rarely as much as 10 mm in length) and are con­ ish white where altered. It contains phenocrysts of spicuously zoned from cores of labradorite or andesine quartz, plagioclase, and biotite that make up about 50 to outer layers of oligoclase or albite. They are com­ percent of the volume of the rock. Quartz phenocrysts monly altered to calcite, kaolinite, chlorite, albite, and average about 9 percent occurring as rounded bipyra- sericite. Pseudomorphs after hornblende are filled with mids that range in diameter from about 0.1 to 1 mm sericite, chlorite, and calcite. and average about 0.4 mm. Tabular or equant euhedral Tiny phenocrysts of epidote and apatite are common crystals of plagioclase that reach 2 mm in length make in the southern type of quartz monzonite. Strongly up from 30 to 35 percent of the rock, and biotite oc­ zoned yellow-green, distinctly pleochroic crystals of curring as books makes up 3 to 9 percent. The plagio­ epidote are as much as 0.6 mm in length. Apatite crys­ clase crystals are conspicuously zoned with cores of tals as much as 0.5 mm in length are of euhedral tabu­ labradorite or andesine and outer zones of oligoclase lar habit, and are commonly clear to slightly brownish or albite. The books of biotite range from about 0.1 in thin section. The brown rarely penetrates the entire to 1.5 mm and average about 0.4 mm. Magnetite crystal, but is an ubiquitous feature of the apatite in usually exceeds 1 percent of the volume and occurs as both the quartz monzonite and granodiorite of the subhedral grains that average about 0.2 mm in diameter. southern type. Indices of refraction of this brownish The groundmass of the northern type of quartz mon­ apatite range from 1.616 to 1.621. Magnetite is ex­ zonite is extremely dense; most of the grains average tremely rare in the southern type of quartz monzonite. about 0.01 mm in diameter. Although the amount of This scarcity is probably due to alteration because, potassium feldspar in the groundmass cannot be ac­ where magnetite is present, it is always partly altered to curately determined optically, staining of thin sections limonite or hematite or both. with sodium cobaltinitrite reveals that potassium feld­ The groundmass of the southern type of quartz mon­ spar constitutes more than 10 percent of the rock. The zonite, as with the northern type, is extremely dense. anorthite content of the fine-grained plagioclase in the Staining of thin sections with sodium cobaltinitrite re­ groundmass could not be determined with the micro­ vealed abundant potassium feldspar in the groundmass, scope, but chemical analyses of the rock indicate a high and a comparison of modal analyses of the phenocrysts content of Na2O, and the plagioclase in the groundmass with normative minerals of the rock suggests that the is probably albite. groundmass is also rich in albite and quartz. The rock is generally quite altered; biotite crystals LAMPROPHYRB are wholly or partly altered to chlorite and plagioclase crystals wholly or partly altered to calcite, kaolinite, A lamprophyre, spessartite, is found in the extreme and chlorite. Epidote is common (as much as 1 per­ western part of the Ute Mountains in the vicinity of cent) as subhedral grains 0.15 mm in length. Yucca laccolith, and in the northern part at Black The southern type of quartz monzonite is light blue Mountain. The rock forms sills and short dikes that gray where unaltered and light tan where altered. It intrude Mancos Shale in the western exposures. At differs from the northern type of quartz monzonite in Black Mountain the spessartite appears to intrude grain size and texture, and is petrographically grada- granodiorite porphyry, although the relations are ob­ tional with the southern type of granodiorite porphyry, scure because the contacts between the rocks are not which it resembles. In the field, it is distinguished from exposed. A planar structure or rock cleavage in the the southern type of granodiorite by the abundance of spessartite dips 22° E., in contrast with the steeply quartz phenocrysts. Quartz, plagioclase, and pseudo­ dipping and plunging granodiorite porphyry. morphs after hornblende are the most abundant pheno­ The rock exposed in the western part of the moun­ crysts in the quartz monzonite, and make up about 45 tains is dark brownish gray and is conspicuously percent of the volume. Quartz phenocrysts average porphyritic; it contains large phenocrysts of pyroxene about 4 percent, plagioclase about 35 percent, and and hornblende that commonly exceed 10 mm in length. IGNEOUS ROCKS 33

Modes, in percent, of quartz monsonite porphyry sium feldspar. Augite is abundant as euhedral crystals [See table 3 for chemical analysis of samples 56-E-02 (sample 15), H-465 (sample 16)] about 2 mm in length, most of which are rimmed with Field sample actinolite. Chlorite with actinolite occurs in sheaves and radiating fibrous groups 0.1 to 0.3 mm in diameter Northern type Southern type Constituent and formed largely from fine-grained augite. Biotite 56-E-92 56-E-101 H-465 56-E-13 occurs as books that range in size from 0.1 mm to more (altered) than 4 mm. A few crystals are partly altered to chlo­ 7.8 0.4 3.8 3.5 rite. Magnetite and pyrite are common accessory Potassium feldspar __ . ______131.8 30.6 35.6 31.8 minerals and two zeolites, thomsonite and chabazite( ?), Biotite. __ __ . ___ ..... 1.8 28.8 Sericite (after hornblende) __ . __ _ .9 coat fractures in the rock. Epidote __ __ . ______. .8 .1 .4 1.6 In the field the spessartite can easily be mistaken for Tr. Tr. .1 .1 Alteration products: microgabbro, but the absence of quartz in the spessar­ Chlorite (after hornblende or biotite) . 1.2 4.6 2.2 Calcite _ . ______2.6 tite and the large crystals of hornblende or augite are Kaolinite ______. .2 2.3 2.4 .1 distinctive. Two modes of spessartite follow. 54.6 48.8 55.6 56.6 Total.. . 100.0 100.0 100.0 100.0 Modes, in percent, of spessartite [See table 3 for chemical analysis of sample 65-E-46 (sample 1)] 1 Largely altered to calcite and chlorite. 2 Partly altered to chlorite. Field sample Locality descriptions: Mineral 56-E-92: Cottonwood Creek vicinity, east side Black Mountain. 56-E-101: Black Mountain (central part). 55-E-46 56-E-104 H-465: West side Banded laccolith. 56-E-13: Sentinel Peak dome. MS 36 17 9 A zeolite (thomsonite) fills cavities as much as 20 mm 3 10 1 1 in diameter and adds to the porphyritic appearance of Biotite... ______.- _ ___ 5 6 2 3 the rock. The pyroxene was determined to be common 19 Chlorite __ ...... 24 16 augite with a 2V of 54° ±1° and N, of 1.70. nnl/»ifrp 2 22 The groundmass of the rock is almost entirely plagi- Thomsonite. ~ 1 oclase and chlorite but contains sparse amounts of bio­ Total... _ _ . 100 100 tite, potassium feldspar, and zeolites. The average 1 Largely altered to kaolin and analcime. composition of the plagioclase is andesine which occurs 2 This value does not include analcime replacing parts of plagioclase crystals. Locality descriptions: as tabular crystals that range from 0.2 to 1 mm in 65-E-46: Yucca laccolith. length. Biotite is common as small books as much as 56-E-104: Black Mountain. 1 mm in length and as tiny shreds. Potassium feldspar occurs as small anhedral crystals that average less than Inclusions are common in the igneous rocks of the 1 mm in diameter and as an alteration product of plagi­ Ute Mountains. More than 90 percent are rich in horn­ oclase. Analcime is abundant in small cavities and as blende, although a few contain none. Xenoliths de­ a replacement product of plagioclase. The analcime in rived mostly from Mesozoic sedimentary rocks are ubiq­ the cavities shows very low birefringence and pseudo- uitous but rare. They are mostly hornfels of Mancos polysynthetic twinning. Other zeolites are heulandite, Shale and silicated sandstones derived from the Dakota stilbite, natrolite, and mesolite. Magnetite is abundant, and Burro Canyon. Most of the xenoliths are less making up from 2 to 4 percent of the rock, and apatite than a foot in diameter although a few are large as a is a ubiquitous accessory. house. Fragments of granophyric granite are fairly The rock exposed at Black Mountain has nearly seri­ common as inclusions in the northern mountains, es­ ate texture except for large hornblende crystals as much pecially in the Pack Trail laccolith (pi. 1). as 30 mm in length. The rock is dark brownish gray, The granitic inclusions superficially resemble the Pre- and, in addition to hornblende, contains plagioclase, cambrian granite observed in the Gulf Oil Co. Fulks 1 augite, actinolite, and chlorite. Plagioclase occurs as drilled in the NW%NW% sec. 27, T. 37 N., R. 17 W., tabular subhedral crystals that range from about 0.2 to 10.5 miles north-northeast of Ute Peak but differ from 1.5 mm across. The plagioclase is relatively unaltered, it in several important ways. A characteristic feature and extinction angles indicate an average composition of the inclusions is the occurrence of large grains of of sodic labradorite for the larger crystals and calcic orthoclase intergrown with crystals of quartz in typical andesine for the smaller crystals; the average composi­ granophyric texture. The texture of the granite in the tion is probably calcic andesine. The central parts of Gulf well, on the other hand, is hypautomorphic-granu- a few crystals of plagioclase have been altered to potas­ lar. Most of the inclusions are nearly free of mafic 34 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO

minerals, whereas the granite in the Gulf well is rich Diorite porphyry in mafic minerals. A few inclusions contain biotite and (or) hornblende, but these show a strong gneissic fabric. According to Zabel (1955, p. 136) and Phillip Katich (oral commun., 1958), structureless granite has been the only Precambrian rock penetrated by the drill in the Ute Mountains region. Thus the granitic in­ clusions in the Ute Mountains evidently were not of local origin but were, perhaps, brought to the surface from considerable depth. A few inclusions contain both granite and hornblende-rich layers (fig. 104), and Argillite the possibility exists that both the granite and the hornblende-rich inclusions were derived from the same substratum. The hornblende-rich inclusions are found in all the igneous rocks in the Ute Mountains, but are most abun­ dant in the diorite, where in some outcrops they com­ prise as much as 5 percent of the volume of the rock. They range in diameter from less than an inch to more than a foot. Although most of the inclusions have sharp contacts (fig. 10) with the enclosing igneous host Hornblende gneiss ;! rock, a few have gradational contacts that is, the hornblende becomes less abundant outward from the central part of the inclusion. Some of the hornblendic inclusions are nearly monomineralic and appear struc­ A tureless; others have a distinct gneissic or schistose texture and contain other minerals in large proportions. Modes of three hornblende-rich inclusions and a granite inclusion are shown below.

Modes, in percent, of inclusions [Chemical analyses are shown in table 3]

Field sample Mineral 56-E-223 EMS-64-52 56-E-105 EMS-101-52 (granite)

Hornblende.. . - . 90.2 39.5 44.2 Augite . . - _- _ 32.5 Plagioclase _ - - . - .. . 124.2 4.4 227.8 Potassium feldspar _ . . 3.7 30.6 Quartz _ __ . . 2.8 1.3 40.0 Biotite _ . ___ , _ .... _ 33.4 38.0 .6 Apatite.. . _ . - . .2 1.5 .6 4.8 1.5 9.7 .2 Sphene __ ...... 8 Pyrite.... _ . _ . __ _ ...... Tr. Calcite...- __ . __ . .. .2 3.5 Epidote _ ... 5.7 Chtarite -.- _ - 1.2 6.5 7.3 Hematite. _ ... Tr. 2.3 Garnet _ .8 FIGUBE 10. Inclusions in intrusive igneous rocks of the Ute Mountains. A, Metasediment composed of argillite and hornblende gneiss in­ Total...... _. _ 100.0 100.0 100.0 100.0 jected with granite. B, Hornblende-rich inclusion composed almost entirely of hornblende. tGranodiorite is the enclosing host rock. . 2 Anio. 3 Includes yellow-brown chlorite. Locality descriptions: phous after hornblende. Both samples contain biotite, 56-E-223: Sentinel Peak bysmalith. but neither they nor the host rock contains pyroxene. EMS-64-52: Sentinel Peak bysmalith. Collected by E. M. Shoemaker. 56-E-105: Black Mountain stock. The hornblende in sample 56-E-223 has been partly EMS-101-52: Black Mountain stock. Collected by E. M. Shoemaker. altered to yellow-brown chlorite and green chlorite Samples 56-E-223 and EMS-64-52 are inclusions near the contact with the granodiorite porphyry. This from the Sentinel Peak bysmalith, which consists of alteration probably occurred at the same time as that granodiorite whose mafic .minerals are all pseudomor- of the phenocrysts of hornblende in the host rock. IGNEOUS ROCKS 35

Magnetite is abundant in this inclusion, and some grains Magnetite is abundant in this inclusion, and some grains contain cores of pyrite. contain cores of pyrite. Sample EMS-64-52 consists of large crystals of horn­ In addition to being pyritized, the rocks at Mable blende and epidote in a matrix of plagioclase, potas­ Mountain have been slightly argillized and slightly sium feldspar, biotite, and quartz. The plagioclase is sericitized. Thin sections of altered diorite porphyry more calcic than that of the host rock, averaging about reveal that plagioclase phenocrysts have been altered to An50 in composition, and is not zoned. The epidote clay, sericite, and calcite, and phenocrysts of hornblende crystals are conspicuously zoned and are commonly to chlorite, calcite, pyrite, and magnetite. engulfed by single large crystals of plagioclase. PYRITIZATION IN THE VICINITY OF IGNEOUS CONTACTS Sample 56-E-105 has distinct gneissic structure ex­ Exposures of contacts of diorite porphyry and Man- pressed by alternating layers of light and dark minerals. cos Shale commonly reveal disseminated crystals and Augite is nearly as abundant as hornblende in the in­ irregular masses of pyrite in both the porphyry and the clusion. Within the dark layers, laminae consisting shale as much as 20 feet from the contacts. Because the almost entirely of augite alternate with laminae com­ shale is gypsiferous, it may have furnished most of the posed almost entirely of hornblende. Apatite is com­ sulfur for the pyrite. mon in this inclusion and is much more abundant near the contact with the host rock. HYDROTHERMAL (HOT-SPRING) ALTERATION ASSOCIATED WITH BRECCIA PIPES ALTERATION Kocks in and above nearly circular breccia pipes in the Four types of alteration other than baking effects near Mancos Shale in the extreme western part of the moun­ contacts have been observed in or near intrusive bodies. tains (in or adjacent to sec. 31, T. 35 N., R. 18 W.) have They are: (a) deuteric alteration of the central parts been altered by hydrothermal solutions of the hot-spring of laccoliths, (b) hydrothermal alteration at Mable type. Above the pipes, a laccolith composed of diorite Mountain, (c) pyritization in the vicinity of igneous porphyry and a sill of lamprophyre have been intensely contacts with Mancos Shale, and (d) hydrothermal or fractured and impregnated with travertine. The traver­ hot-spring alteration associated with the formation of tine was apparently deposited by calcium carbonate- breccia pipes in the Yucca cluster of intrusives. bearing hot water that rose along the pipes. Pieces of DEUTERIC ALTERATION diorite porphyry fill the central parts of some of the pipes and in places are argillized and nearly white. The central parts of many laccoliths have been in­ Mancos Shale within the pipes is light gray green to tensely altered by deuteric action. The alteration is a light tan in contrast to its normal dark-gray to black direct result of the partial sealing in of residual liquids color. The breccia pipes are discussed further on pages in the interior of an intrusive mass. 59 to 60. In the altered rock, phenocrysts of calcic plagioclase PETROLOGY have been partly replaced by more sodic plagioclase and sericite, calcite, and minor chlorite and kaolinite. CHEMICAL. COMPOSITION OF THE IGNEOUS BOCKS Hornblende has been altered to sericite, chlorite, and Chemical analyses of 16 igneous rocks and of 2 in­ magnetite, and pyroxene to chlorite. The groundmass clusions are shown in table 3. All the analyses, with in the altered rock generally contains more chlorite, the exception of two from Cross (1894), were made by sericite, kaolinite, and quartz than in the fresh rock. methods described by Shapiro and Brannock (1956). The Sundance laccolith, the Three Forks laccolith, The norms were computed by using the direct method and several laccoliths in the Yucca cluster (pi. 1) have described by Wahlstrom (1947, p. 235-237). In this altered central parts that are well exposed. method, diopside is not computed and excess CaO is allotted to wollastonite. MgO and FeO are allotted to HYDROTHERMAL ALTERATION AT MABLE MOUNTAIN enstatite, ferrosilite (iron hypersthene), and magne­ The rocks at Mable Mountain are fractured along a tite. northeast-trending zone that cuts through the central Sample 1 in table 3 is of the porphyritic spessartite part of the mountain and appears to be an upward ex­ lamprophyre. This rock contains the least silica and tension of the zone along which the Ute Creek dike was alumina of any rock in the Ute Mountains and is com­ intruded (pi. 1). Pyrite occurs in veins in the fracture pletely without quartz. The norm indicates that the or shear zone and as discrete crystals in the rocks for rock is undersaturated in silica. Olivine (forsterite) a considerable distance on either side of the fracture is computed in the norm, although it is not actually zone. At the surface the pyrite is altered to red limonite found in the rock. The principal mafic minerals,

745-807 O-65 6 36 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO

TABLE 3. Chemical analyses and norms of igneous rocks from the Ute Mountains, Colo. [Analyses, in percent, by P. L. D. Elmore, S. D. Botts, and K. E. White (samples 1, 2, 5, 6, 13, 15); Elmore, Botts, and M. D. Mack (samples 3, 4, 12); H. F. Phillips, Elmore, and White (samples 7,11, 14, 17, 18); W. F. Hillebrand (samples 8, 10); Elmore and Botts (samples 9,16)]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Field No 56-E- 56-E-77 55-E- 55-E- 56-E-42 56-E-33 EMS- H-515 EMS- 56-E-40 56-E-82 EMS- 56-E-92 H465-1 EMS- EMS- 46A 48A 57A 96-52 65-52 94-52 101-52 64-52

Laboratory No 148755 148758 150490 150489 148757 148756 81653 150061 77081 150492 148759 81651 148760 150060 81654 77080

Analyses

S1O2 ...... 48.8 50.0 55.3 55.9 K.A Q 58.6 57.4 59.42 fti i 62.65 62.6 64.6 65.2 68. 4 68.7 68.9 76.0 40.8 AbOj. -. . _ 15.5 17.3 16.7 17.2 16.7 17.4 1ft "7 16.79 16 9 16.68 17.6 16.5 16.8 16.7 17.1 17.4 14.0 14.4 F62O3------4.8 2.9 5.0 4.8 3.1 2.6 g 3 0O 3 0 2.35 1.9 1.3 2.3 1.3 1.4 1.3 .5 5.3 FeO ...... 5.7 5.8 1.8 2.8 3.8 4.4 5.5 3 on 2 q 9 fia 2.1 1.8 1.1 .78 .64 .43 ' .29 10.0 MgO.... 5.3 3.4 9 15 3.1 3 1 3 A 9 9 2 94. 2 1 1 A.H 1.1 1.0 1.0 .36 .78 .54 .07 8.1 CaO.... 8.5 9.0 6.0 6.4 6.5 4.8 5.8 5.57 4.7 4.96 4.3 3.7 3.7 1.8 1.7 2.3 1.3 11.3 . O . O 4.0 O. S7 5.1 . / 4.0 4.0 5.4 K2O...... 1.9 1.1 1.9 2.0 9 1 9 1 2 1 2 go 2 9 2 7K 9 n 2.7 2.4 2.8 3.6 3.0 4.7 1.1 TiOs.. . 1.2 .85 .72 .78 .72 .68 .60 .68 .66 49 .35 .30 .32 .17 .20 .18 .04 1.4 PiO...... 42 .49 .36 .37 90 «m 34 OK 01 28 19 .10 .16 .10 .11 .08 .00 .76 MnO-._ - .17 .16 .15 .16 .12 10 .14 19. .09 .16 .07 .12 .08 .06 .06 .05 .02 .23 H2O ...... 3.7 2.8 4.1 2.2 2.6 1.6 4.0 1.06 1.4 0<4 1.5 1.9 1.6 1.4 2.0 1.0 .20 2.0 CO2.._ ...... 46 2.8 .61 .20 .58 05 Oft 44 .10 1.3 1.5 1.2 .34 .12 .13 .05 1.1 Total- .... 100. 25 99.90 00 14 QQ 81 100. 11 100. 05 100. 08 100. 17 99 69 100. 11 QQ 99 99.86 100. 21 100.41 100. 71 100. 77 98.79

Norms

o 0.0 7.6 11.2 9.7 11.1 9.2 8.0 11.0 15.0 14.1 17.7 26.7 27.0 19.7 26.9 21.0 11.2 6.5 11.2 11.9 12.4 12.4 12.4 16.7 17.1 16.3 11.8 16.0 14.2 16.6 21.3 17.7 ab ...... - 32.1 27.9 33.8 33.0 31.3 37.2 35.5 35.1 33.8 37.6 43.1 31.3 33.8 50.7 33.8 45.7 an ______19.9 24.4 21.9 23.4 22.4 20.9 20.4 18.7 19.4 17.2 11.9 8.5 10.0 6.0 7.5 10.0 C...... 0 1.7 0 0 0 .2 0 0 0 0 2.6 4.3 3.9 1.5 3.9 1.6 ol...... 9.2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 8.5 6.3 7.8 7.8 7.5 5.5 5.5 5.3 3.6 2.8 2.5 2.5 .9 2.0 1.4 ts...... 4.9 7.3 0 .2 3.5 5.1 8.4 2.5 0 2.5 1.8 2.0 0 .2 0 0 6.9 4.2 4.5 7.0 4.5 3.8 1.3 4.7 5.5 3.4 2.8 1.9 3.3 1.9 2.0 1.9 0 0 .5 0 0 0 0 0 0 0 0 0 0 0 0 0 il--._-.._ ._ 2.2 1.6 1.4 1.5 1.4 1.3 1.1 1.3 1.2 .8 .7 .6 .6 .3 .4 .3 1.0 6.4 1.4 .5 1.3 0 .5 1.0 .2 0 3.0 3.4 2.7 .8 .3 .3 .9 1.1 .8 .8 .6 .7 .7 .8 .7 .6 .4 .2 .4 .2 .2 .2 9.3 0 .8 2.1 .6 0 2.1 2.2 .6 2.9 0 0 0 0 0 0

Sample material Locality Sample m aterial Locality Sa mple material Locality 1. Spessartite ______Vicinity of Yucca cluster. 9. Grant diorite por- Ute Peak. 15. Quartz monzonite por- V icinity of Black Moun­ 2. Microgabbro. - The Tongue laccolith. phyi y. phyry. tain, 3. Diorite porph yry Yucca cluster. 10. D( 'The Knees" (Cross, 1894). 16. Do - - B anded laccolith, 4. Do Do. 11. D( Sentinel Peak (vicinity of 17. Granophyre inclusion. Vicinity of Black Moun­ 5. Do Do. "The Knees"). tain, 6. Do ... Pack Trail laccolith. 12. D( ) (altered 1 Mushroom laccolith. 18. Hornblendite inclu- Se ntinel Peak. 7. Do... ______Flat laccolith. 13. D< Do. sion. 8. Granodiorite por­ Ute Peak (Cross, 1894). 14. Quart z monzonite Vicinity of Sentinel Peak. phyry. porp hyry. chlorite, augite, and hornblende presumably contain all plagioclase than, in the microgabbro. The K2O content the magnesium in the rock. Only about 1 percent potas­ in the five samples of diorite porphyry analyzed is sium feldspar was observed in thin section whereas nearly constant at about 2 percent. This content is there is 11 percent of normative potassium feldspar. nearly twice the K2O in the microgabbro, and the differ­ The difference is in part due to abundant modal biotite ence is expressed in thin section by abundant small (not calculated in the norm), and probably in part to crystals of potassium feldspar in the groundmasses of KAlSi3O8 in solid solution in the plagioclase. The rock all the diorite porphyry samples. is undersaturated with respect to alumina and, as a Six samples, 8-13, are of granodiorite porphyry and consequence, abundant wollastonite appears in the include the southern type (10,11), the central type (12, norm. 13), and the northern type (8,9). Sample 2 is of the porphyritic microgabbro. The The granodiorite is richer in SiO2, K2O, and Na2O rock is moderately altered and contains about 7 percent than the diorite and has slightly less MgO, CaO, and calcite. The microgabbro is quite low in magnesium total Fe. The K2O is fairly uniform except in sample relative to gabbro from other parts of the United States 11, which apparently contains much albite and an ab­ (Clarke, 1924, p. 465). This low value reflects the lack of olivine and the paucity of pyroxene in the porphyritic normally small amount of potassium feldspar. variety of microgabbro. The central type of granodiorite porphyry is the most Samples 3-7 are of diorite porphyry. In general, the silicic of the granodiorite and is the poorest in total iron analyses show more Na2O than inferred from the mode and magnesia. This type is intermediate between and indicate a considerably higher content of sodic granodiorite and quartz monzonite. IGNEOUS ROCKS 37

The northern type of granodiorite (hornblende SAMPLE granodiorite of Ute Peak and Black Mountain) has the coarsest groundmass. An approximate mode agrees well with the norm except that there is 11 to 15 percent of normative quartz and only 4 to 5 percent of modal quartz. This difference is probably due entirely to errors in identifying the minerals of the groundmass. Samples 14-16 are of quartz monzonite porphyry. These rocks are all characterized by conspicuous pheno- crysts of quartz and by an extremely dense groundmass. The extremely fine grain of the groundmass and the high content of Na2O in these rocks create a problem as to rock name. The norms indicate a greater abundance of albite than in the granodiorite but, in general, about the same amount of orthoclase. The rocks could have been included with the granodiorite, but their quartz phenocrysts and low color indices make them separable in the field, and their high silica content, total alkali metal content, and low calcium, iron, and magnesium content make them easily separable chemically from the granodiorite. The name quartz monzonite is believed justified on the basis of total alkali feldspar (albite+ potassium feldspar). The chemical analyses indicate an average of about 68 percent silica, which is approxi­ mately 5 percent more than the average silica for gran­ odiorite. The samples of quartz monzonite porphyry contain more Na2O plus K2O than the granodiorite, less calcium, and markedly less total iron and magnesium. The K2O content averages about 3 percent.

VARIATION DIAGRAMS The weight percentages of six major oxides in the analyses were plotted against the percentage of silica. (See fig. 11.) Ferric iron was recomputed to ferrous iron for each analysis and is shown with the reported ferrous iron. The resulting total iron eliminates the erratic though somewhat mutually compensating varia­ 10 tions of the two iron oxides. The diagrams clearly illustrate the nearly constant alumina content of the rocks (excluding the lampro- phyre), the steep decline of total iron, magnesia, and lime from high values in the microgabbro to low values in the quartz monzonite, and the gentle rise of Na2O + K2O with increasing silica. The curve for the sum of the two alkalies shows a pronounced reciprocal relation­ ship with lime. This relation is a reflection of the in­ creased amount of sodic plagioclase and potassium feld­ spar in the more silicic rocks. The alkali-lime index (Peacock, 1931) provides a means of comparing igneous rocks throughout a region 50 55 60 65 70 WEIGHT PERCENT Si0 2 or district. The alkali-lime index for the igneous rocks of the Ute Mountains is 59 (fig. 17). According to FIGURE 11. Variation diagrams for igneous rocks of the Ute Mountains. All analyses in weight percent recalculated (minus H2O, SOg, and Peacock's classification, these rocks are within the calc- C02) to 100 percent. Analyses are shown in table 3. 38 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO alkalic field. Significantly, the alkali-lime index of about 63 to 75. The quartz monzonite falls in the field the Ute Mountains rocks is similar to that of other lac- of felsic differentiates having indices between 82 and colithic groups in the Colorado Plateau. Indices for 87. porphyritic rocks of the La Plata Mountains from anal­ yses published by Cross and Purington (1899, p. 6-7), MINOR ELEMENTS for rocks of the Carrizo Mountains from analyses made VARIATION IN THE IGNEOUS ROCK SERIES available by E. M. Shoemaker and J. D. Strobell, and for the Abajo Mountains from analyses furnished by The minor elements of the igneous rocks are shown I. J. Witkind and E. M. Shoemaker, were computed as in tables 4 and 5. All amounts, except for those of follows : titanium and manganese, were determined by semi- La Plata Mountains______«=*57 quantitative spectrographic analysis. In this proce­ Carrizo Mountains______=»58 dure a weighed amount of the sample mixture is burned Abajo Mountains______60. 5 in a controlled d-c arc and the spectrum recorded on a photographic plate. Selected lines on the resulting The alkali-lime indices are not listed for the rocks in plate are visually compared with those of standard the Henry and La Sal Mountains laccolithic groups spectra prepared in a manner similar to that for the because they are too uncertain. Many of these rocks unknowns. The approximate visual detection limits for are high in alkalies and low in silica (Hunt and others, the elements determined by this semiquantitative spec­ 1953, p. 154; Hunt, 1958, p. 324-333). Graphs of their trographic method are shown in table 4A. The as­ oxides show a wide dispersion of points that do not de­ signed group for semiquantitative results will include fine single curves for either lime or total alkalies. the quantitative value about 60 percent of the time More than one igneous series may be represented in (Myers, Havens, and Dunton, 1961). The chemical these laccolithic groups. Analyses of only the diorite analyses for titanium and manganese were by methods porphyry and the monzonite porphyry in the Henry described by Shapiro and Brannock (1956). and La Sal Mountains suggest an alkali-lime index be­ tween 59 and 62. Frequency distribution of minor elements in each type Thornton and Tuttle (1956) suggested a diagram to of rock is shown by histograms in figure 13. Some of illustrate trends from mafic to felsic rocks without re­ the histograms depart considerably from a normal dis­ gard to the processes contributing to the formation of tribution curve, as, for example, cobalt in the granodio­ the rocks. In this diagram, oxide weight percentages rite. The spread in values is probably due partly to are plotted against the sum of normative quartz, albite, variation between samples and partly to the analytical orthoclase, nepheline, leucite, and kaliophilite. This method. Although the mean content of cobalt cannot sum is called differentiation index because of the be determined accurately from these values, the trend tendency for all magmas to change in composition of cobalt values from about 17 ppm (parts per million) toward the system quartz-nepheline-kaliophilite. In in the diorite to a trace in the quartz monzonite is figure 12 the oxide weight percentages of rocks from the significant. Ute Mountains are plotted against the sum of normative In determining the geometric mean of an element for quartz, albite, and orthoclase, there being no normative which one or more values were reported as zero (amount nepheline, leucite, or kaliophilite. Figure 12 is sub­ is below the visual detection limit), it was assumed that divided according to the scheme of Sukheswala and because the majority of the samples showed significant Poldervaart (1958, p. 1489). Except for Fe2O3 and quantities, that element is present in the remaining FeO, the scattering of points is very minor, and straight samples, but in amounts slightly less than the sensitiv­ lines fit the points very well. The scattering of Fe2O3 ity. For example, in the diorite, 4 samples were ana­ and FeO is thought to be due to mutually compensating lyzed as having less than sensitivity values for nickel variations of the two individual iron oxides. SiO2 and 19 samples contained determinable amounts. In shows a gradual increase throughout the series; A12O3 determining a mean value for the 23 analyses, the 4 is nearly constant throughout; TiO2, MgO, and CaO samples were assumed to contain 0.00015 percent nickel show steady decreases. Na2O and K20 increase (one-half the visual detection limit of 0.0003 percent throughout the series. The microgabbro has a differ­ for nickel). Values reported as trace were considered to entiation index of 40, and it plots in the field of late- be threshold amounts when computing the mean for an stage basalt, The diorite falls in the field of inter­ element. mediate differentiates; the granodiorite falls along the In comparing the amounts of trace elements in the boundary between the intermediate differentiates and igneous rocks of the Ute Mountains to averages in other the felsic differentiates having indices ranging from parts of the world (table 4), it is apparent that the IGNEOUS ROCKS 39

Early and Ultramafic Late-stage Intermediate Felsic middle stage differentiates basalts differentiates differentiates basalts

70

60

j* 10 o

10

15

10

20

I 10

10

20 40 60 80 100

FIGURE 12. Oxide weight percentages of rocks from the Ute Mountains plotted against the sum of normative quartz, albite, and orthoclase (the differentiation index). (After Thornton and Tuttle, 1956, and Sukhes- wala and Poldervaart, 1958.) 40 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO

TABLE 4. Semiquantitative spectrographic and chemical analyses, in parts per million, of igneous rocks from the Ute Mountains {All values, except italic, are geometric means (antilogs of mean logs) determined from semiquantitative spectrographic analyses (table 5; See Miesch and Blley, 1961). Italic values were determined from chemical analyses (values for microgalsbro is based on a single chemical analysis; for diorite porphyry on 5 analyses; for granodiorite on 6 analyses; for quartz monzonite on 4 analyses). Semiquantitative spectrographic determinations made by the rapid visual-comparison method. Comparisons of similar data with those obtained by quantitative methods show that the assigned semiquantitative-class interval includes the quantitative value in about 60 percent of the de­ terminations. Tr., near threshold amount of element. Analysts: Spectrographic, N. M. Conklin, R. G. Havens, J. C. Hamilton, P. J. Dunton; chemical, P. L. D. El- inore, S. D. Botts, M. D. Mack, K. E. White]

Average abundance Microgabbro Diorite Granodiorite Quartz mon­ Lamprophyre (world-wide distribution) Element (3 analyses) porphyry porphyry zonite por­ (3 analyses) (23 analyses) (9 analyses) phyry (6 analyses) Gabbro Diorite

Barium ______410 660 740 880 630 60 1230 Beryllium______(2) (2) Tr. Tr. 82 <3 (2> 68 (2) Tr. «3 Cobalt...... __ « .7 11 Tr. S» 79 «32 88 6 6 3 60 340 «68 160 30 16 10 116 149 '38 Gallium...... _-___ .-.-....-._...... _ 30 18 17 19 18 12.8 »14.5 (2) Tr. «30 Tr. (2) 9 «26 Manganese. ______...... 1.Z40 1,080 850 460 1,$40 1,160 "696 Molybdenum.... ______<10 Tr. 0) 4 (2> T, (2) 103 '2.6 Nickel...... 40 6 16 168 640 Tr. Tr. (2) Tr. 19 "3.6 Lead. ____ .. ______0 T, 0 T, 12 16 6 1230 Scandium 60 17 13 «6 32 20 a 4. 6 Strontium 680 1,060 1,040 880 680 170 i»260 Titanium.... ______..... 5,100 4,eoo 8,700 1,100 7,100 14 4 400 Vanadium 316 150 74 26 246 u 150 Yttrium. . ... - . . _ 41 23 32 «13 30 "7.9 Ytterbium. ... ______.. ______6 3 3 »l.fi 4 15 1 70 116 126 100 70 140 "280

lEngelhardt (1936). Otto (1936). 1 Indicates the amount is less than the sensitivity. 10 Hevesy and Hobble (1933). »Goldschmidt (1954). 11 Rankama and Sahama (1950). «Sandell(1952). u Hevesy, Hobbie, and Holmes (1931). Goldschmidt and Peters (1932c). « Noll (1934). « Goldschmidt (1937a). i« Goldschmidt (1937b) (average for igneous rocks). ? Sandell and Goldich (1943). " Tongeren (1938) (average for igneous rocks). 8Sandell(1947). i« Hevesy and Wiirstlin (1934).

Ute Mountains rocks are relatively high in barium, appreciable quantities only in the granodiorite strontium, scandium, and low in chromium, nickel, and porphyry. Lanthanide elements are admitted by zirconium. capture into the crystal lattices of compounds of quadri­ Barium exhibits a marked increase from about 400 valent elements of similar size, such as zirconium ppm in the microgabbro to more than 800 ppm in the (Goldschmidt, 1954, p. 315). The lanthanum in the quartz monzonite porphyry. Most of the barium is granodiorite may be related to the greater abundance probably contained in the potassium feldspar and bio- of zircon in these rocks. Yttrium and ytterbium do not tite. Strontium is most abundant in the diorite and follow the variation in lanthanum, and they show a granodiorite, where it is probably present in both the slight decline as silica increases in the rock series. plagioclase and potassium feldspars. Beryllium was Traces of molybdenum were detected only in the micro­ detected only in the granodiorite and quartz monzonite, gabbro and in the diorite porphyry. Niobium in and boron only in the quartz monzonite. amounts near the visual detection limit was observed The amounts of cobalt, chromium, copper, manganese, only in the granodiorite and quartz monzonite. Lead nickel, scandium, titanium, and vanadium are relatively gradually increases from barely detectable amounts in high in the microgabbro and low in the quartz mon­ the microgabbro and diorite to approximately 15 ppm zonite porphyry. These elements are nearly all con­ in the quartz monzonite. fined to the mafic minerals. In figure 14, five elements VARIATION OF MINOR ELEMENTS WITH ROCK ALTERATION (vanadium, copper, cobalt, chromium, and nickel) are plotted against total silica and pyribole. In general, VARIATIONS IN IGNEOUS BOOKS the five elements decrease gradually in amount from As previously mentioned in (p. 35) the intrusive rocks microgabbro to quartz monzonite. However, two of the Ute Mountains are partly or wholly altered. samples of diorite porphyry, 56-E-42 and 56-E-33, are Most of the alteration is due to the action of deuteric unusually high in mafic minerals for diorite in the Ute solutions; however, the Mable Mountain bysmalith and Mountains. Sample 56-E-42 contains approximately 9 associated dike were altered by sulfide-bearing solutions percent pyroxene and 7 percent hornblende, and al­ which were introduced after the intrusive masses had though sample 56-E-33 contains no pyroxene, it has 17 completely solidified. percent hornblende. Paired samples or suites of samples of altered and Gallium is markedly more abundant in the microgab­ unaltered rocks were taken from several intrusive bro than in the other rocks. Lanthanum was found in bodies. These samples were analyzed spectrograph- Barium Cobalt Chromium Copper Gallium Nickel Scandium Strontium Vanadium Yttrium Ytterbium Zirconium 100 Microgabbro 3 analyses

50

0 100 Diorite 23 analyses

50

1 -z. 100 LJJ o Granodiorite 9 analyses

m S _ ;: S ',. _ I 50 - L i p 1 11 1 r- li-, Wi pP P« P 11 g a :;;;v| k;:x a i pi m m m m m iJlil 100 Quartz monzonite 6 analyses Fl m Fl 50 -

:-;;A^v ;f .;.;.;.] PVXyA PIfa$ Fl\\-\\-] Sfea t-.v.-.-.-.. ' -'.-'. i'.-'.\-:-:-:-: o:':::v| f::!v::;:;:::;:::::^:-:->>>:?x-X| t:::':-:::':::::-:::\-:;;;;:.:.v,v:v:-;-;j fvTrX'r-:^ IS |Si .: :.: tv vX-X "A l'.'.'.'.'.'.',:'.'"'" '/ '// " ] [ ' "_ X-XvXvX'*-"- vX X] K':'-': ' 0 li J - . :) t>:-:-.v: ' - li | 100 Lamprophyre 3 analyses

50

r~- in i o o .-H m r~- ooo.-i m r^m _ S_ o_ ^i roi r~- oo I-H rorx rt n r^ in m r~- in 0000 OOOOOO^H I OOC o o o o o o ooo oo r^ m o o o .-i n o o o .-i m P P I~l § q q qq o ooo o oo oo o ooo l O O( o o o o o o OOO 00 S O-H o o o oo q q q q q q q odd d d d d d o ooo ooo ooo ooo oooooo oo ooo o o o o o o o o o o o

CONCENTRATION, IN PERCENT

FIGURE 13. Frequency distribution histograms of semiquantitative spectrographic analyses of minor elements common to the igneous rocks of the Ute Mountains. The vertical line in each group is the approximate plot of the geometric mean. Asterisk (*) indicates the amount is less than the sensitivity; Tr, near threshold amount of element. The nickel histogram for quartz monzonite includes one reporting of trace graphed at 0.0003. 42 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO

TABLE 5. Semiquantitative spectrographic analyses Percent Code XX 5 -10 x+ 2 - 5 X 1 - 2 X- .5 -1 . 2 - .5. .1 - .2. -...... X- .05- .1. .OX+ [Sample numbers with prefix H were collected by F. N. Houser; prefix EMS, E. M. Shoemaker. Analysts: 1. N. M. Conklin; 2, R. Q. Havens; 3, J. O. Hamilton. .. .. looked for but not detected. Approximate visual detection limits for the elements determined by the Semiquantitative spectrographic methods at the Denver laboratory,

Sample No. Elements Rock type Labora­ Si Al Fe Mg Ca Na K Ti p Mn B Ba Be Field tory (0.002) (0.001) (0.0008) (0.0005) (0.005) (0.05) (0.7) (0.0002) (0.2) (0.0002) (0.002) (0.0002) (0.0001) 1 (i 0.0005) (i 0.002)

Microgabbro 56-E-76 244798 1 XX XX XX X XX X X o.x o.x- O.OX 56-E-77 244799 1 XX XX XX X XX X X .X .X .ox Diorite 56-E-94 244713 1 XX XX x+ x+ x+ X X .X .X- .OX+ Tr. 55-E-44A 238259 2 XX XX X- X X X .X ...... OX+ ...... OX+ 55-E-47B 238260 2 XX XX X x+ X X .X .X- .OX+ 55-E-48B 238261 2 XX XX X X- X X X .X .X- .OX+ 55-E-49 238262 2 XX XX X X- X X X .X .OX+ .OX+ 55-E-50A 238263 2 XX XX X X- X x+ X .X .OX+ .OX+ """"""Tr"" 56-E-33 244757 1 XX XX x+ X x+ X .X .X- .OX+ 56-E-37 244760 1 XX XX X x+ X .X .X- .OX+ Tr. Altered diorite porphyry (mate to 56-E-37) .. . 56-E-38 244761 1 XX XX X x+ x+ X .X .X- .OX+ Tr. Diorite porphyry. _ 56-E-42 244764 1 XX XX X X X X .X .X- .ox+ 56-E-78 244800 1 XX x+ X X X X X .X- .X- .ox 56-E-175 253492 3 XX XX X X X X X .X .X- .OX+ 56-E-189-4 251988 1 XX XX X- Xf X X .X- .OX+ H 387-2 251993 1 XX XX X- x+ X X !x .X- .ox+ H 398-1 251994 1 XX XX X X- x+ X X .X .X- .OX+ H 408-1 251995 1 XX XX X X X X X .X .X- ...... OX+ ----- H 411-2 251996 1 XX XX X X- X x+ X .X .OX+ H 414-1 251997 1 XX XX X X- X X X .X- .ox+ H 517-1 252026 1 XX XX X X- X X X .X- .ox+ H 533-2 252031 1 XX XX X X- x+ X X .X------.ox+ H 534-2 252032 1 XX XX x+ X- x+ X X .X- .OX+ Altered diorite por­ phyry (mate to H 387-2)...... H 387-1 251992 1 XX XX X X- x+ x+ X X- .OX+ Tr. Diorite porphyry (mate to H 387-1)- H 387-2 251993 1 XX XX X X- x+ X X .X .X- .OX+ Tr. Diorite porphyry .. . . EMS-96-52 D-81653 XX XX X X- X X X- .X- .X- .ox Granodiorite 56-E-51A 244772 1 XX XX X X- X X X .X .OX+ Tr. 56-E-82 244804 1 XX XX X X- X X X .X- .X- .OX+ O.OOOX- 56-E-S8 244708 1 XX XX X x+ X X .X .X- .ox+ .ooox H 270-1 244834 1 XX XX X- X x+ X .X .OX+ .OX+ Tr. H 373-1 251991 1 XX XX X X- X X X .X .OX+ ...... X- H 515-2 252024 1 XX XX X X- x+ X X .X .OX+ .OX+ ;;;;;; .X- H 516-2 252025 1 XX XX x+ X- x+ X X .OX+ 56-E-40 244762 1 XX XX X X- X X X X .X- .X- .ooox- EM8-«5-52 D-77081 4 XX X X X X X .X !x .ox .ox Quartz monzonite 56-E-13 244743 1 XX XX X- X x+ X .X- .OX+ .oox .X- .ooox- 56-E-92 244812 1 XX XX X X X X .X- ...... OX+ .OX+ .ooox- H 465-1 252004 1 XX XX X X- x+ X .X- .OX+ .OX+ Tr. H 526 252030 1 XX XX X !x X- X x+ .X- .OX+ .X- EMS-94-52 D-81651 2 XX XX X- .X X- X X- .OX+ .OX+ .OX+ EMS-95-52 D-81652 2 XX XX X- .X X- X X- .OX+ .OX+ .OX+ Lamporphyre...... 55-E-46A 253490 3 XX XX X X .X------.ox 55-E-40C 238258 2 XX XX x+ X .OX+ .ox+ 55-E-51 238264 2 XX XX x+ X X X X !x .X- .OX+ .... ----

i A different exposure is required for the lower detectabilities shown. ically, and the results of the analyses are shown in from the Mable Mountain bysma'lith and con­ table 6. tains copper deposits. Nickel and chromium decrease Apparently, there are few consistent variations. For slightly in the altered rocks of several of the suites. example, the data from suite 7, taken from the Three Although these variations are small and generally Forks intrusive mass, a laccolith with an intensely al­ within a single order of magnitude, they appear to be tered center, show that vanadium was removed during significant because sedimentary rocks adjacent to the alteration of diorite porphyry,' but samples from other igneous intrusive rocks have corresponding increases intrusive bodies show virtually no variation in va­ in these elements. Variations in equivalent uranium nadium. are slight as determined by radiometric analyses (a Cobalt decreases in the altered rock of three suites, 6, maximum of 2 ppm for one suite). Chemical deter­ 7, and 9, and increases in altered rock in suite 8. Suite minations of uranium for one suite show virtually no 9 is sheared rock from Ute Creek dike whicn extends variation. IGNEOUS ROCKS 43 of igneous rocks from the Ute Mountains, Colo. Percent Code 0.02 -0.05 O.OX .01 - .02 .OX- .005 - . 01 .OOX+ .002 - .OOi005 ------__....______. .OOX .001 - .00!002 ...._____.---.-___--_ .OOX- .0005- 0001. __ .OOOX+ .0002- 0005. - .. _- .OOOX .0001- .0002...... OOOX- U.S. Geological Survey (revised August 1956,) are shown in parentheses. Some combinations of elements affect the detectabilities. Approximate values are given. In unusually favorable materials, concentrations somewhat lower than the values given may be detected. In unfavorable materials, the given detectabilities may not be obtained for some of the elements]

Elements Continued

Co Cr Cu Ga La Mo Nb Ni Pb Sc Sr V Y Yb Zr (0.0005) (0.0001) (0.0001) (0.0002) (0.002) (0.0005) (0.001) (0.0003) (0.001) (0.0005) (0.0002) (0.001) (0.001) (0.0005) (0.001)

O.OOX+ O.OOX+ O.OX- O.OOX Tr. <0.001 O.OOX Tr. O.OOX O.OX+ O.OX O.OOX+ O.OOOX+ O.OOX+ .OOX+ .OOX+ .ox- .OOX <.001 .OOX .OOX+ .OX+ .ox .oox .OOOX+ .OOX+ .OOX+ .ox- .ox- .OOX <.001 .OOX+ Tr. .OOX+ .OX+ .ox .OOX .ooox .OOX+ .oox- .OOOX+ .oox .oox- .OOOX+ Tr. .oox .OX+ .ox- .OOX .OOOX .ox- .oox- .OOOX+ .OOX .oox- .OOOX+ Tr. .OOX .X- .ox- .oox- .OOOX .OOX+ .oox- .OOOX+ .OOX .oox- .OOOX+ Tr. .oox- .OX+ .ox- .oox- .ooox- .OOX+ .OOX+ .ooox .OOX .oox- .OOOX+ o.oox- .oox- .OX+ .ox- .oox- .ooox- .OOX+ .oox- .OOOX+ .OOX .oox- .OOOX+ .oox- .oox- .X- .ox- .oox .ooox .ox- .oox .oox .OOX+ .oox O.OOX .oox- Tr. .oox .OX+ .ox .OOX .OOOX .ox- .OOX .OOOX+ .oox .OOX Tr. .OOOX+ Tr. .OOX .X- .ox- .OOX .OOOX .ox-

.OOX .OOOX+ .OOX .OOX .OOX .OOX- .oox- .X- .ox- .OOX .OOOX .ox- .OOX .oox- .OOX+ .OOX .OOOX+ .OOX- .oox- .OX+ .ox- .oox- .OOOX .ox- .OOX .OOOX+ .OOX+ .OOX .OOOX+ .oox- .OX+ .ox- .oox- .OOOX .OOX+ .oox- .OOOX+ .oox .OOX .OOX .oox- .OX+ .ox- .oox .OOOX .OOX+ .oox .oox- .OOX .oox- .oox- Tr. .oox .X- .ox- .OOX .OOOX .ox- .oox- -OOOX+ .OOX .oox- .OOX .OOOX+ .OOX- .oox- .X- .ox- .OOX .OOOX .ox- .oox- .ooox .OOX .oox- .OOX .OOOX+ Tr. .oox- .X- .ox- .OOX .OOOX .ox- .oox- .ooox- .oox- .oox- .OOX- .oox- .OX+ .OOX+ .oox- .OOOX .ox- .OOOX+ .ooox- .OOOX+ .oox- .OOX Tr. .oox- .X- .ox- .oox .OOOX .ox- .oox- .ooox .oox .oox- .OOX .ooox Tr. .oox- X .ox- .oox- .OOOX .ox- .oox- .OOOX .oox- .oox- .OOX Tr. .OOOX .OOX- .oox- .X- .ox- .oox .OOOX .ox- .oox- .OOOX+ .oox .oox- .OOOX+ Tr. .oox- .OX+ .ox- .oox- .OOOX .ox- .oox- .oox .OOX nnTT ftftftTT-L Tr. .oox- .X- .ox- .oox- .OOOX .OOX+

.oox- .OOOX+ .OOX .OOX .OOX .OOOX+ .OOX- .oox .X- .oox .ooox .ox- .ox- .oox- .OOOX+ .OOX .OOX .OOX .OOOX+ .OOX- .OOX+ .X- .ox- .OOX .ooox .ox- .OOOX+ .OOOX+ .OOX .oox- .OOOX+ .OOOX+ .OX+ .OOX+ .ooox- .ooox- .OOX+ .oox- .OOOX+ .OOOX+ .oox .OOX Tr. .OOX- .oox .X- .OOX+ .oox .ooox .ox .oox- .OOOX+ .oox .oox- .OOX o.oox- .OOOX+ .OOX- .OOOX+ .OX+ .OOX+ .OOX .OOOX .ox- .oox .OOOX+ .OOX .oox .OOX+ .OOOX+ Tr. .oox .ox- .ox- .OOX+ .OOOX+ .OOX+ .oox- .OOOX+ .OOOX+ .OOX .oox- .OOOX+ .OOX .oox- .X- .OOX+ .oox- .ooox- .ox- .ooox .OOOX+ .oox- .oox .OOX- .OOOX+ .X- .oox .oox .ooox .ox- .oox- .OOOX+ .oox .oox- .OOX .OOOX+ .OOX- .oox- .X- .ox- .OOX .OOOX .OOX+ .oox- .ooox .oox- .oox- .OOX .ooox .oox- .X- .ox- .OOX .OOOX .ox- .oox- .OOOX+ .oox- .oox .OOX+ .oox- .OOOX+ .oox- .oox .OX+ .OOX+ .OOX .OOOX .ox .ooox .ooox .oox .OOOX .ooox .OOOX .ox .oox .OOX .OOOX .OOX Tr. .OOOX+ .OOOX+ .OOX .oox- Tr. .oox .000X4- .OX+ .OOX+ .oox- .ooox- .ox- Tr. .ooox- .oox- .OOX .oox nmr _ .oox- .OOOX+ .OX+ .oox .oox .ooox .ox- .ooox .ooox .oox- nmr _ .oox- .OOOX+ .X- .OOX Tr. Tr. .ox- .oox- .oox- .oox- .OOOX+ .X- .oox- .oox- .OOOX .ox- .OOOX .OOOX .oox- .oox- .OOOX+ .OX+ .oox- .oox- .OOX+ .OOOX .oox- .oox- .OOOX+ .OX+ .oox- .oox .OOX .OOX+ .ox- .oox .oox -.oox .X- .ox .oox .OOOX+ .OOX+ .oox- .oox .OOX+ .oox- .OOOX+ .0015 .OOX .ox .ox- .OOX .ooox .OOX+ .oox .OOX+ .ox- .oox- .oox- .OOX .OX+ .ox .OOX .OOOX .OOX+

VARIATIONS IN SEDIMENTARY ROCKS ADJACENT TO IGNEOUS (table 6) indicate that cobalt, chromium, gallium, INTRUSIVES nickel, strontium, vanadium, yttrium, and ytterbium are more abundant in the altered than in the fresh rock. The Junction Creek Sandstone exhibits color changes Samples of Dakota Sandstone (table 6, suite 10) yielded wherever exposed in the vicinity of intrusive rocks in generally similar results. The altered Dakota Sand­ the Ute Mountains area. Away from intrusive rock stone was taken about 20 yards from a contact with the sandstone is predominantly orange or red. Ad­ granodiorite porphyry, and the unaltered specimen was jacent to intrusive rock, the color is light brown or yel­ collected from the same sandstone bed about a mile from low brown, probably because of the introduction of the nearest exposure of igneous rock. The altered Da­ iron oxide. kota Sandstone contains slightly greater amounts of Spectrographic analyses of paired samples of red chromium, copper, gallium, nickel, and ytterbium than unaltered sandstone and yellow-brown altered sandstone the unaltered Dakota. GEOLOGY, PETROLOGY, UTE MBBSTJUBfS AREA, COLORADO

TABLE 6. Semiquantitotive spectrog/rapMe analyses and radiometrie analyses of [For code see table £. VaJaesfiktt_liefara-te-^n^ iffi_rfi_^ SamptenHmberswtth|»efixEMSwCTecoIlectedbyE.M.Shoeinaker.

Spectrographic analyses Labora­ Pair or Sample tory Roektype suite No. Minor elements 1 Ba Co Cr Gu Qa Mo Ni Pb So

Igneons

Deuteric atiaatifm f55-E-47B 238260 Diorite porphyry . . _ . .... 1 O.OX+ O.OOX- O.OOOX+ O.OOX O.OOX- O.OOOX+ Tr. O.OOX I.... _ (55 E-48B 238261 Altered diorite porphyry.. _ . 1 .OX+ .oox- .OOOX+ .oox .oox- .OOOX+ Tr. .oox- /56-E-37 244760 Diorite porphyry. ______? .OX+ .oox .OOOX+ .oox .oox .OOOX+ Tr. .oox 2... .. \56-E-38 244761 fl ftV-U .oox .OOOX+ .oox .oox O.OOX- .oox- f56-E-77 244799 ? nv -OOX+ .OOX+ mr _ .oox <0.001 .oox .OOX+ 3______J56-E-76 244798 ? ftV nmr_u. .OOX+ .ox- .oox <.001 .oox Tr. .oox ry- -387-2 251903 Diorite porphyry ______2 .OX+ .oox- .OOOX+ .oox .oox- .OOOX+ .oox- .oox- 4...... -387-1 251992 Altered diorite porphyry _____ 2 .OX+ .oox- .OOOX+ .oox .oox .OOOX+ .oox- .oox -516-2 5.... __ 252024 Diorite porphyry______? .OX+ .oox- .OOOX+ .oox .oox .OOOX+ .oox- .oox- -516-2 252025 ? nv_i- nf\V _ .ooox .oox- .oox .ooox .oox- §fEMS-63-52 D77079 .OOOX+ f¥_1VJ- nnv .ooox .ooox .ooox EMS-65-52 D77081 3 nv .OOOX+ nmr .OOX .ooox .ooox .ooox 6...... EMS-66-52 D77082 nv .ooox nnA'V _l_ OOX nnnv .ooox .ooox ry. EMS-70-52 D77086 3 nv nnnY-L. Tr. .oox .OOOX .ooox .ooox 7... __ . EMS-71-52 TW7_"-Q'7 nV OOX numr oox nnmr .ooox nmr nnnv EMS-72-52 D77088 3 .OX .oox- .ooox .oox .ooox .ooox .OOX .OOOX /EMS-73-52 D77089 Altered diorite porphyry _____ 3 .OX .oox- .ooox .oox .ooox .ooox .ooox IEMS-74-52 D77090 3 .OX f_~1Y_U nnnv .oox .ooox .ooox .ooox Hydrothermal /EMS-12-53 D88148 Altered diorite porphyry.. ____ 4 .ox- .OOOX+ .OOX+ .oox- .oox .OOOX+ .OOOX+ g \EMS-14-53 D88150 Diorite porphyry .--.... _ 4 .OOX+ .oox- .oox- .OX+ .oox- .oox- .oox- Tr. .OOOX+

Sedimentary

56-E-20 244747 O.OX- o.oox- O.OOX- Tr. O.OOOX+ Sandstone. 10...... 56-E-224 253493 5 nnY ftAAV t flfln'V'-i. Sandstone. 56-E-15 244745 nv Tr. nnv _ nnnvj- O.OOOX- .OOOX+ Tr. 11.. ___ stone. 56-E-16 244746 2 nv AAftV 1 .OOOX+ Tr. Tr. Sandstone. 56-E-51C 244774 ?, fly_u. Q.QQX- .OOX+ .oox .OOX .OOX+ Tr. O.OOX- H-368-1 251990 _.___do______...._..______?, .OX n/iy _ .OOX+ .oox .OOX- .oox O.OOX+ .OOX J--& _-_-,_ .032 ftftnTT-U 004 flOV" _-|_ fiflV" 0.001 .oox- .OOX- .OOX- (average of three samples) . 56-E-25 244751 ?, nnnY-i. flnfiV" .oox- Tr. 13_.__... Mancos Shale. 55-E-70 OK34Q1 5 nv ftonir .OOOX+ cos Shale. *

' Chemical uranium determination. The "variations in minor metallic elements noted above cos Shale, which shows little variation in grain size, for the Junction Creek and the Dakota Sandstones are supplies corroborative evidence. Two samples of baked very slight and the quantities involved are near the shale contain more cobalt, chromium, nickel, strontium, limits of detection for those metals. Both sandstones yttrium, and ytterbium than the unaltered shale (table are rather poorly sorted and differences in grain size 6, suit 12). Suite 13, baked and unbaked sandy lime­ may exist between the individual samples of a pair, stone from the Mancos Shale, shows little variation even though such differences are not apparent with the except a loss of barium and a slight gain in copper, hand lens. Other workers have found that the amount yttrium, and ytterbium in the baked sample. of a minor element varries considerably with grain size within a single stratum (V. C. Kennedy, oral commun., The authors conclude that the altered and baked sedi­ 1957). This relation may explain the variation noted ments adjacent to the Ute intrusive rocks gained small in both the Junction Creek and Dakota Sandstones, but amounts of cobalt, chromium, gallium, nickel, yttrium, it seems more likely that the variations are related to ytterbium, and probably vanadium. These elements the Ute instrusive rocks, because the effects of alteration apparently were transported by solutions from adjacent are similar in the two sandstone formations. The Man- intrusive bodies. IGNEOUS ROCKS altered and unaltered igneous and sedimentary rocks from the Ute Mountains, Cote. Analysts spectrographlc: 1, E. Q. Havens; 2, N. M. Conklin; 3, P. J. Dunton; 4, G. W. Boyes, Jr.; 5, J. C, Hamilton; radtometrie: lt C. CL Aogefo; 2; J. P. Sstee&J

Speetrographic analyses-^Cbntinned analyses

Minor elements Continued Major elements Equivalent (percent) Sr V Y Yb Zr SI AI Fe Mg Ca Na K TI Mn | rocks

O.X- O.OX- O.OOX- 0.090X O.OOX+ XX XX X X X-f- X X OuX O.X- <0.«tt 1 .OX+ .ox- .oox- .ooox- .OOX-f- XX XX X X X X X .X .X < .001 1 .X- .ox- .oox .ooox .ox- XX XX x+ X x+ X X .X .X .001 1 .X- .ox- .oox .ooox .ox- XX XX x+ X x-t- i+ X .X .X- < .001 1 .OX+ .ox .oox .OOOX+ .OOX+ XX XX XX X XX X X .X .X < .001 1 .OX+ .ox .OOX+ .OOOX+ .OOX+ XX XX XX X XX X X .X .X- < .001 I .X- .ox- .oox .ooox .ox- XX XX X X- x+ X X .X .X- < .001 1 .X- .ox- .oox .ooox .ox- XX XX X X- x+ x+ X -x+ .X < .001 1 .X- .ox- .oox .ooox .OOX+ XX XX X X- x+ X X .X .OX+ < .001 1 .X- .ox- .oox .ooox .ox- XX XX x+ X- x+ X X -x+ .X- .003 1 .X .oox .oox . IAAJ.A.nnnY .oox .A.YY -A. X X X X X X .X .ox _ .ox .oox .oox .ooox .oox XX X X X X X .X .X .ox _

.ox .oox .oox . IRRr-A.noftTT .oox .d~&YY X .X X X .X .X .ox .ox .oox .oox .OOOX .oox XX X X X X X X .X .X

.ox . lA/^XftflY . nnvIRAA. .ooox .oox .A.YY -A. X X X X .X .X .X _ .ox .ox .oox . IAJU.A.nomr .oox XX X X X X X .X .X .X .ox .oox .oox .OOOX .oox XX X X X X X X .X .X _ .ox .oox .oox .ooox .oox XX X X X X X .X .X .X _

.ox .ox- Tr. .ox- XX XX X .OX+ .ox+ .OX+ .x+ .X .ox- i .0003(U) 2 .ox .OOX+ .oox- .OOX-f- XX XX x+ X X. - X. - X.- .X .X- i .0004(U) 2 rocks

O.OX- O.OOX- O.OOX- O.OOOX- O.OOX+ XX O.X- O.X+ o.x XX O.X O.OX O.X-

s Data from W. L. Newman and E. M. Shoemaker (written commun., 1952; average of three samples). ORIGIN OP THE IGNEOUS ROCKS AND microgabbro to obtain quartz monzonite is determined HORNBLENDE INCLUSIONS by projecting the line connecting the points for K2O to The igneous rocks of the Ute Mountains are a group 0.5 percent, which is considered to be a realistic value of intergrading hypabyssal intrusive rocks that changed for a calc-alkalic magma. Other values are based on in composition from gabbro to quartz monzonite as in­ the silica value thus found (points on vertical line at trusion proceeded. This progressive change with time left in figure 15A); they are compared to the chemical of intrusion suggests a derivation from a common an- composition of microgabbro phenocrysts in table 7. cester magma by some process of differentiation. The chemical composition of the phenocrysts of the micro- Augite, hornblende, and labradorite are the earliest gabbro is very similar to the calculated chemical com­ mineral components of the rocks; it seems reasonable position of material that might have been subtracted that large quantities of these crystals were separated from microgabbro to form quartz monzonite. In figure from the magma by settling or by being pressed from 15 the major oxides are plotted against silica for micro­ the liquid during movement of the magma, thus chang­ gabbro and quartz monzonite; the silica content of the ing the composition of the rest of the magma. If this most mafic material that might be subtracted from the process took place, it occurred before the magma rose 46 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO

i T TT

g 20

10

0.03

0.015

0.007

0.003 r

0.0015

0.0007

0.0003

0.00015 - Microgabbro and lamprophyre porphyry

48 50 52 54 56 60 62 64 66 68 70 SILICA, IN PERCENT

FIGDEB 14. Diagram showing variation of five trace elements with respect to increasing silica and total pyribole content. The pyribole includes pyroxene and amphibole. Pyribole in the granodiorite (excluding H-515) and quartz monzonite has been replaced by calcite, chlorite, and sericite. Sample numbers shown at bottom of chart. Percentage of trace element determined by semiquantitative spectrographic analysis. IGNEOUS ROCKS 47

20 A. C. Waters and C. B. Hunt (in Hunt, 1958, p. 348- 354) described hornblendic inclusions in the La Sal Mountains and suggested that the inclusions in the 15 igneous rocks are either "* * * fragments of diverse wall rocks brought to equilibrium with a magma at great depths, or else they are the leftover unfused frag­ o 10 ments of the substratum from which the magma itself was derived or perhaps both." Waters and Hunt (Hunt, 1958, p. 348-355) pointed out that the evidence in the La Sal Mountains does not support the evolution of the igneous suite of rocks from diorite to syenite by crystallization differentiation of a primary basalt parent. They suggested that the igne­ 45 50 55 60 65 70 ous rocks in the La Sal Mountains were derived from Si02 , IN PERCENT a magma formed by the partial fusion of amphibolite or FIGURE 15. Graphical solution of Barker diagram to determine possible related metamorphic rocks. This anatectic theory sat­ materials added or subtracted to produce quartz monzonite from microgabbro. isfactorily explains: (a) the lack of basalt or its plu- tonic equivalents in the La Sal suite of rocks, (b) the TABLE 7. Comparison of the chemical composition of micro­ gabbro phenocrysts with the chemical composition of material lack of a progressive increase in silica in the later formed that might be subtracted from microgabbro to produce quartz rocks, and (c) the similarity of the hornblende in the monzonite inclusions with the hornblende in the rock. The theory [Mode of microgabbro phenocrysts was determined from thin section of field sample 56-E-77, in percent: Augite, 21.6; hornblende, 36.2; plagioclase, 40.5; magnetite, does not explain the absence of the mafic alkalic rocks 1.7. The chemical composition of hornblende used in the calculation is an average of four chemical analyses of hornblende from the Ute Mountains. The chemical which should result from fractionation of hornblende composition of augite is that given by Wahlstrom (1947, p. 238). Only plagioclase phenocrysts larger than about 0.3 mm were counted in the mode. These crystals (Bowen, 1928, p. 269-273). However, Waters and are estimated to average about 60 percent anorthite in composition] Hunt (Hunt, 1958, p. 353) suggested that "filtration Constituent Microgabbro Subtracted phenocrysts material effects" took place that decreased the amount of mafic material carried upward by the rising magma. They SiO» .-...- _. . - _ Aft Q 47.2 AhOs-- - _ _ .-...... 17.5 18.7 did not attempt to extend their theory to explain the FeO .. _- ...- -.-. _-_. 10.7 11.6 CaO...... -...... 13.5 12.5 origin of all the laccolithic magmas of the Colorado MgO _ _ .-.-. .. 7.3 5.0 NajO...... 2.5 2.9 Plateau, although the theory does have plateau-wide KjO ...... -.-...... 3 .5 appeal because it could explain the hornblende inclu­ sions characteristic of all the laccolithic mountain to the level of the intrusive bodies now visible, for these groups. Furthermore, if a hornblendic substratum as bodies are petrographically uniform throughout. broad as the plateau is assumed to exist, the theory The hornblendic inclusions found widely in the explains the similarity of the dioritic intrusive rocks in igneous bodies of the Ute Mountains are also charac­ the laccolithic groups. teristic of other laccolithic mountain groups on the To provide further data bearing on the origin of the Colorado Plateau. The origin of these inclusions is hornblendic inclusions and possibly on the magma of probably closely related to the origin of the magma or the Ute Mountains, two separates of hornblende pheno­ magmas that gave rise to the laccolithic mountain crysts from diorite porphyry and two of hornblende groups. from inclusions were analyzed chemically, optically, In the Henry Mountains, hornblendic inclusions com­ and spectographically. (See tables 8, 9, 10, 11, 12.) prise 95 percent of the total of all inclusions (Hunt and The data derived from these analyses indicate that the others, 1953, p. 164). Hunt noted that the hornblende two types of hornblende are very similar. The chemi­ phenocrysts in the porphyries are not sharply distin­ cal similarity is especially apparent when plotted on a guishable from aggregates of hornblende in the inclu­ diagram designed by Hallimond (1943). In a study of sions. He concluded: hornblende from North America, Europe, Asia, and It seems necessary to infer that the hornblende inclusions Africa, Hallimond found that a generalized relation were derived at great depths and have had a very different his­ exists between parent rock and the composition of the tory than the obvious xenoliths. The inclusions may be altered contained hornblende. He plotted the compositions of fragments of diverse wall rock floated from great depths, or hornblende from nearly 200 localities on a triangular rock fragments from early differentiates in the magma reser­ voir, or fragments of marginal unfused layers of the substratum diagram in terms of two variables: (a) the number of from which the magma was derived. Si atoms, and (b) the number of atoms alloted to the 48 PETROLOGY, UTE MOUNTAINS AKEA, COLOBABO

fall within the diorite and amphibolite fields of Hallimond. TABLE 9. Atotnio rofio* of elements; m. h&rriblertge ott basis £| (O, OH, F) ETbe meifeod used to recast the ehemkal analyses Is that oattineti by Graves (1851, p. 306-320) who used the general formate far manoelinie ampMbates: (OH, F)j (Na* Ca, K)M (Mg, Few, FJe«, Tf. Mn, Al)« [(Sf, Al)» OsQ

Ratio in field sample Element EMS-74-52 56-E-37 56-E-105 56-E-223

Si __ __ 6.36 6.40 6.52 6.42 AV 1.64 1.59 1.48 1.5S Al,.. .66 .64 .53 .71 Ti_ .17 .13 .09 .12 Fe+» . - - .34 .36 .54 .55 Fe+» 1.34 1.34 1.44 1.29 Mg. .. . 2.45 2.60 2.51 2.41 Mn _____ ... __ .04 .03 .03 .02 Na._ __ . ... - . .59 .58 .49 .57 Ca. 1.80 1.81 1.83 1.73 K...... 15 .14 .15 .19 OH 1.74 1.71 1.57 1.60 F.. .17 .12 .07 .07 5.02 5.09 5.15 5.10 Mg+Aly: Y. - .62 .64 .59 .61 Mg+Aly: Fe+H-Fe+a--. 1.85 1.91 1.02 1.70 Fe+»: Fe+»... __ . _ .25 .27 .38 .43 Na+Ca+K 2.54 2.53 2.47 2.49 EXPLANATION 1 Ala, aluminum used to make up the number of metal atoms in the chain to eight; x Aly, aluminum remaining which is assumed to replace Mg in the Y group. Hornblende inclusion 8 Y group includes Ti, Fe+3, Fe+*, Mg, and Mn. TABLE 10. Optical properties of hornblende from the Ute Hornblende phenocryst Mountains [Determinations of indices of refraction by Marie Lindberg who found a considerable FIGURE 16. Approximate limits of composition of amphibole derived range in the indices of individual grains, especially for samples EMS-74-52 and from various rocks (Hallimond, 1943, p. 75) showing plot of four 56-E-37. This lack of homogeneity may be due to zoning, a characteristic feature samples of hornblende from the Ute Mountains. of hornblende from the Ute Mountains. All other optical determinations were made by E. B. Ekren. The extinction angles were measured on a four-axis univer­ sal stage using the method described by Turner (1942). Optic angles were meas­ ured directly on the universal stage. The accuracy of these measurement is believed vacant space in the amphibole unit cell. Figure 16 is a to about ±1°. Abbreviations used in color descriptions: gn, green; gy, gray; reproduction of Hallimond's diagram with the com­ It, light; med, medium; p, pale; y, yellow; o, olive] position points of two separates of hornblende pheno- Field sample Optical property crysts and two separates of hornblende from inclusions 56-E-37 56-E-105 56-E-223 of the Ute Mountains superimposed on it. All four EMS-74-52 Min, 1.660; Min, 1.65.. Min, 1.659.... Min, 1.653. avg, 1.664. TABLE 8. Chemical analyses of hornblende from the Ute 0...... Min, 1.668; Max, 1.67- Range, 1.676- Range, 1.665- Mountains, Colo. avg, 1.675- 1.69; avg, 1.680; avg 1.675. 1.685. 1.673-1.678. in low [Analyses, in percent, by S. M. Berthold. Sample EMS-74-52 (collected by E M range. Shoemaker) is from the Three Fork laccolith; 56-E-37 is from a laccolith in the Max, 1.71; Avg, 1.687- Max, 1.687.... Max, 1.685. Yucca cluster (pi. 1) about 2,000 ft. southwest of Mushroom laccolith. Both samples avg, 1.690- 1.692. are of hornblende phenocrysts from diorite porphyry. Samples 56-E-105 and 1.695. 56-E-223 are from hornblendic inclusions in granodiorite porphyry of Black Moun­ 7-a(calculated) ... 0.016-0.021. .... 0.037.. 0.028.. 0.032. tain and of Sentinel Peak, respectively] 7-a(Berek com­ 0.019...... 0.019.. 0.021.. 0.018. pensator). 16°±1°. Field Sample EMS-74-52 f7 A C* 17°±1° 18°±1° - 15°±1° 2V...... 79°±1° - 83°±1° - 73°±1° ~ 75°±1°. Laboratory No. . .. . 151063 Pleochroism: 151064 151065 X pgn-y...... py-gn ... y-gn.- ... Pgn-y. o-gn ...... med gn. Y.. Ito-gn...... y-gy-gn. SiOj...... 43 19 4.Q J.1 Z...... gy-gn ...... gy-gn...... med to dk gn.. A12O3. . . i Q on 11 48 13.10 FejOa . o 09 o on 4 80 i 80 FeO...... 10.88 10.82 10.42 MgO...... 11.14 n oo U OJ. 10.90 CaO ...... U OQ 11.42 11 46 10.92 TABLE 11. Quantitative speotroyraphic analyses of hornblende Na20...... 2 nft 9 HA 1.70 K80...... 77 .72 Of from the Ute Mountains Ti02 _ __ . . . i *& 1.15 PiOt. .-___ ...... 10 .20 .05 10 [Analyses, in percent, by K. V. Hazel] H|0+...... 1.77 1.74 HjO...... 53 .36 43 56-E-37 56-E-105 56-E-223 MnO. . OQ .28 .27 .17 Field sample... EMS-74-52 F...... 07 OR .14 .14 151063 151064 151065 151066 Total...... 100.55 100.26 100.07 100.07 -0=F._ fiA .16 .10 0.005 0.006 0.007 0.006 Total... .0007 .002 .002 .002 100.39 100.16 100.01 100.01 .0007 .0015 .0006 .0007 .0021 .0023 Specific gravity...... 0 10 19 fl7 .002 .0018 3 3.24 3 .04 .04 .05 .06 FeO:MgO... .55 .51 54 IGNEOUS ROCKS

TABLE 12. Semiquantitative spectrographie analyses of of the fact that the four samples were taken from four hornblende from the Ute Mountains chemically and mineralogieally different rocks. The [Analyses Jn percent, by K. V. Hazel. Looked for but not detected: P, Ag, As, Au, Bi, Cd, Ce, Cs, Dy, Er, Eu, Gd, Ge, Ht, Hg, Ho, Ir, La, Lu, Nb, Nd, Os, Pd, Pr, hornblende phenoerysts (table 8y samples 1 and 2), are Pt, Bb, Re, Rh, Ru, Sb, Sm, Ta, Tb, Te, Th, Tl, Tm, U, W] from different laccoliths composed of diorite porphyry. EMS-74-52 56-E-37 56-E-105 56-E-223 Sample 1 is from hornblende-rich diorite porphyry that contains no pyroxene; sample 2, on the other hand, is 150163 150164 150165 150166 from diorite porphyry that contains about 5 percent 0.003. 0.003 0.003 0.003 augite as phenocrysts, as well as about 7 percent horn­ .GOT .0)7 .007 .003 .00007 .00007 blende as phenocrysts. Sample 3 is hornblende from Cobalt...... -..-.. . .007 .007 .007 .007 .0007 .0015 .0015 .0015 an inclusion that has a strong gneissie structure and that .00015 .00007 .00007 .00015 ftaiiluTn .0015 .0015 .0015 .003 contains nearly as much augite as hornblende. This Tn

BYSMALITHS The most conspicuous part of the Sentinel Peak SENTINEL PEAK bysmalith is Sentinel Peak itself, a vertical spine that The Sentinel Peak intrusive mass is composed of rises 500 feet above the rest of the bysmalith. The granodiorite porphyry of the southern type and is spine is part of a dikelike mass that apparently in­ approximately iy2 miles south of "The Knees" stock. truded the original roof of the bysmalith, probably It is a bysmalith nearly 2 miles long, three-fourths of along a fracture that was part of the fracture system a mile wide, and, excluding Sentinel Peak proper, extending from "The Knees" stock south-southeast­ about 800 feet thick. The bysmalith may actually ward beyond the Sentinel Peak dome (pi. 1). The comprise two separate intrusive masses that have co­ spine has many vertical ridges formed by differential alesced. (See pi. 1.) weathering of alternating zones of moderately and in­ Almost all the north side of the bysmalith is in fault tensely altered rock, and probably served as a conduit contact with Mancos Shale; the south side probably or neck that fed magma from the Sentinel Peak bysma­ had a similar fault contact, but most of the shale has lith into overlying sedimentary rocks. been removed by erosion. On the east and west sides, the bysmalith is in intrusive contact with older diorite MABLE MOUNTAIN porphyry. At two places along the north side near Mable Mountain, one of the largest single intrusives the top, blocks of Burro Canyon and Dakota strata in the Ute Mountains, is approximately 1 mile north­ crop out. These blocks appear to be uplifted parts of west of Ute Peak. The mountain is composed of diorite the sides and roof. porphyry and has a nearly mushroom shape. It is about The base of the intrusive is extremely irregular, 1,500 feet thick in its thickest part and probably over­ being located stratigraphically at various horizons be­ lies the Ute Creek dike. tween the base of the Burro Canyon Formation and Beds of Mancos Shale and thin sills of diorite por­ the Juana Lopez Member of the Mancos Shale. At phyry form the southwest side and part of the top of its southern edge, the base is in the Mancos Shale, Mable Mountain. The base of the intrusive body is not probably at a level between the Juana Lopez Member exposed on the north side, where contacts with the and the Greenhorn Limestone Member. The shale beds Mancos Shale are buried by thick talus. The base of are only slightly baked. The blocks of Burro Canyon the body is only a few tens of feet above the top of the and Dakota strata that crop out along the north side Dakota near the north side of Mable Mountain, but is of the intrusive may mark the locations of two feeders several hundred feet above this level a few hundred feet that extended from "The Knees" stock beneath Burro southward along Pine Creek. Canyon and Dakota strata and broke upward into On its northeast and north sides, the bysmalith is Mancos Shale at the bysmalith. Except near these probably in fault contact with Mancos Shale, for the conduits, the bysmalith is probably mostly in the shale is greatly brecciated and the sides of the intrusive Mancos. mass are very steep. The south side is probably also Exposures along the southeast side of the Sentinel faulted, although no faults were seen during mapping. Peak bysmalith suggest that the igneous rock was in­ The base of the Point Lookout Sandstone crops out at jected in a nearly horizontal southward direction with about the 8,400-foot contour (pi. 1) near the highest sufficient force to thrust beds of Mancos Shale over part of Mable Mountain. Only 1,500 feet to the north, beds of the Brushy Basin Member of the Morrison that igneous rock crops out at or near the same elevation, had been domed upward by an earlier intrusion. The and a little farther north, roof rocks of the intrusive inferred structural relations are shown in figure 17. include beds of the Greenhorn Member in the basal part of the Mancos Shale. A fault must lie between the FKg Point Lookout Sandstone and the igneous mass, prob­ SENTINEL PEAK ably at the contact of shale and diorite porphyry as BYSMALITH shown on plate 1. The Ute Creek dike probably was the feeder for the Mable Mountain intrusive rock and may be connected to the Black Mountain stock at depth. FIGURE 17. Sketch showing inferred structural relations thought to be Much of the diorite porphyry of Mable Mountain has the result of igneous intrusion at depth near the contact of the Sen­ been intensely altered and mineralized with pyrite, tinel Peak intrusive with the Sentinel Peak dome. Not drawn to scale. TKg, granodiorite porphyry; TKd(?), diorite porphyry; Km, especially in the western part of the body. Keddish iron Mancos Shale; Kdb, Dakota Sandstone and Burro Canyon Formation; stain produced by the oxidization of pyrite attracted Jmb, Brushy Basin Member, and Jmwr, Westwater Canyon and Recapture Members, all of the Morrison Formation ; Jj. Junction Creek Sandstone. many prospectors to the area during the early 1900's. 56 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO THE "WEST TOE" This lineation, together with the proximity to the Black The "West Toe" body of quartz monzonite porphyry Mountain stock, the elongation of the laccolith toward is a small discordant body intruded through gently the east-southeast, and the structural setting at its west dipping beds of the Dakota Sandstone and Mancos end, suggests that the laccolith was fed laterally from Shale. It is called a bysmalith because the discordant the Black Mountain stock. side contacts suggest that the intrusive lifted its roof, EAST HORSE LACCOLITH but nothing is known regarding the floor. The body is structurally very similar to that of Sentinel Peak (the East Horse laccolith is composed of normal diorite "East Toe"), and probably was a feeder for igneous rock porphyry and is east of Horse Mountain. In plan view intrusive into overlying sedimentary beds. it is oval, elongated N". 80° E., and is about l^j miles long and 1 mile wide. It is at least 350 feet thick in LACCOLITHS the center and tapers gradually to gently rounded sides. NORTH UTE PEAK LACCOLITH It forms a low topographic dome that is dissected by The North Ute Peak laccolith is nearly continuous two east-trending canyons, neither of which has pene­ with the north side of the Ute Peak bysmalith and ex­ trated underlying sedimentary strata. The laccolith tends half a mile north from Ute Peak. It is about 600 is in Mancos Shale. feet thick and a little more than a mile wide. It con­ Many readings of primary flow structure as defined sists of leucocratic diorite porphyry that is generally by hornblende phenocrysts were taken in East Horse similar to porphyry in Ute Peak bysmalith but contains laccolith. Two consistent trends were noted: in the less potassium feldspar. northern part of the laccolith, lineations plunge about North Ute Peak laccolith was intruded into the Man- 15° toward the east-southeast, and in the southern part cos Shale near the base of the Juana Lopez Member, or they plunge gently north or south, or are horizontal. about 475 feet stratigraphically above the base of the These trends suggest that two separate intrusions Mancos. It is believed to have been intruded from the occurred. same source as Ute Peak and to be concordant. Much The source appears to have been under the Ute Peak of the laccolith lies astride the southeastward-trending bysmalith (section A-A'', pi. 1), although the laccolith syncline between the Ute Mountain mass and McElmo may have been fed from the Black Mountain stock, or dome (pi. 1). from both stocks. Flow structure suggests thai the feeder was almost flat lying. HORSE MOUNTAIN LACCOLITH The Razorbaek laccolith intruded and uplifted part Horse Mountain laccolith is east of Cbttonwood Creek of the south edge of East Horse laccolith, and thus is and about 1^4 miles directly south of Ute Peak. It is the younger of the two. The straight, north side of the composed of leucocratic diorite porphyry which is Razorback laccolith clearly reflects the influence and petrographically similar to the granodiorite porphyry control of the earlier East Horse laccolith to the north. of Ute Peak except for a lack of abundant potassium feldspar. SUNBA1TCE CLUSTER OF LACCOLITHS The laccolith is elongated east-southeast and is about Laccoliths of the Snndance cluster are all composed 1 mile long and not more than half a mile wide. The of the normal type of diorite porphyry, and all were in­ roof has been removed and only the sides of the. laccolith truded into the Dakota Sandstone or the lowermost part are preserved, but the top of Horse Mountain is thought of the Ma\neos Shale at about the same time. The cluster to be near the original top of the laccolith. The lacco­ includes the Sundance, East Sundance, Trapdoor, and lith is at least 1,100 feet thick and, because its horizontal Irwin laccoliths southeast of Black Mountain. The dimensions are small, it has very steep sides, parts of Trapdoor laccolith is west of Sundanee laccolith, and which may be faulted. Irwin laccolith (not shown on pi, 1) is east of the East Two alined vertical dikes a few hundred feet sooth Snndance laccolith. All four laccoliths probably are of Horse Mountain are probably upward offshoots from offshoots of the Black Mountain stock, through con­ the concealed wedge-edge of the laccolith, as probably cordant conduits in the strata stratigraphically lower are dikelike bodies on the northeast side. than the laccoliths. All members of the group have Linear flow structure is well displayed in most out­ flat tops and, with the possible exception of the Irwin crops. As in other laccoliths composed of hornblende- laccolith, the base of which is not exposed, average about bearing1 diorite porphyry,, the flow structure is exhibited 350 feet in thickness. by alined phenocrysts of hornblende. Except in places The floors of the four laccoliths probably are at or at the ends of the laccolith, the lineation is oriented to near the Dakota-Mancos contact. With the exception the east-northeast and suggests flew in that direction. of the Irwin laccolith, the roofs of the laccoliths are STRUCTURAL GEOLOGY AND FORMS OF THE IGNEOUS BODIES 57 Dakota Sandstone in some places and Mancos Shale much as 1 mile wide, and is about 750 feet thick in its in others (pi. 1). Large blocks of Dakota Sandstone central part. apparently were tilted upward by the rising magma in The roof of the laccolith is composed of Dakota a manner analogous to the tilting of a trapdoor, and Sandstone and Mancos Shale. The fact that beds of magma then spread into the overlying Mancos Shale the Dakota are exposed at the top of the laccolith (fig. 18). and along the west side suggests a trapdoor effect sim­ BAZOBBACK LACCOLITH ilar to that described for the Sundance cluster of lac­ The Razorback laccolith is in part contiguous with coliths. The southwest end of the laccolith is in con­ the south edge of the East Horse laccolith. It is oval tact with "The Knees" stock. This relation, together in plan view, elongated N. 80° W., and measures 1 mile with the orientation of the body, suggests that the in length by a maximum of 0.4 mile in width. The laccolith was fed laterally from the stock. abrupt western end has a minimum thickness of 300 The Last Spring laccolith consists of granodiorite feet. The greatest original thickness of the laccolith porphyry of the southern type, but contains more was. probably 850 feet in the central part. The roof phenocrysts of quartz and crystals of epidote than most of Mancos Shale is missing over all the crestal part of this rock. A petrographically similar rock crops of the laccolith but can be seen on the north side. out a few hundred feet east of the Last Spring lacco­ The southern part of the laccolith was intruded into lith, on slopes west of Towaoc, Colo. This rock forms the Mancos Shale about 100 feet above the Greenhorn a flat-topped laccolith in mudstone beds of the Brushy Limestone Member. Along its north side the laccolith Basin Member, and probably came from the conduit is somewhat discordant, and is in contact with strati- that supplied the Last Spring laccolith. graphically lower 'beds from west to east. In its west­ FLAT LACCOLITH ern third, the base of the laccolith lies as much as 100 Flat laccolith is in contact with the south side of the feet above the Greenhorn Limestone Member of the Last Spring laccolith, which cut and uplifted parts of Mancos. On the northeast the base of the laccolith lies it. To the south, Banded laccolith has also cut and below the top of the Dakota (pi. 1). uplifted parts of Flat laccolith. The linear flow structure defined by alined pheno- Flat laccolith is l1/^ miles in length by a maximum of crysts of hornblende indicates a general southeastward one-third mile in width and is estimated to be 250 to movement of the magma in the western part of the 300 feet thick; its long dimension trends southeast. Razorback body, an eastward flow in the central part, The laccolith is flat-topped and has tapered sides. It and an east-northeastward flow in the eastern part. was intruded into the lower few hundred feet of the This laccolith was probably fed from the Black Moun­ Mancos Shale throughout its length, but is stratigraphi- tain stock by a conduit that was structurally and strati- cally lower toward the southeast. Wherever exposures graphically lower than and directly under, or just to are good, the contact appears concordant and the nature the south of, the Horse Mountain laccolith. The con­ of the downward crosscutting is not known. duit probably lies about 100 feet above the base of the Flat laccolith was fed from "The Knees" stock to the Mancos. northwest, as indicated by the northwest trend of linear LAST SPRING LACCOLITH flow structure. (See pi. 1.) The Last Spring laccolith is exposed over a broad area east of "The Knees" stock and south of the Sun- THREE FORKS LACCOLITH dance cluster of laccoliths. The long dimension of this Three Forks laccolith is composed of normal diorite laccolith trends northeastward. The laccolith is as porphyry and is approximately half a mile southeast

1000 FEET

FIGUBH 18. Sketch showing inferred stratigraphic and structural relations of laccoliths in the Sundance cluster, and mechanism of "trapdoor" structure. 58 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO of "The Knees" stock. It is bounded on the southwest laccoliths composed of normal diorite porphyry that and southeast by younger intrusive masses of granodio- were intruded into Mancos Shale a few tens of feet rite and quartz monzonite. Alined hornblende crystals above the top of the Dakota Sandstone. The laccoliths in this hornblende-rich porphyry define a general south­ were probably intruded laterally from "The Knees" east trending lineation which strongly suggests that the stock. Numerous sills crop out in the area between the magma came from "The Knees" stock. laccoliths and "The Knees" stock and are stratigraphi- A conspicuous feature of Three Forks laccolith is the cally above the laccoliths. There are also many dikes pronounced alteration of its central part. Pheno- of granodiorite porphyry, several of which cut sills and crysts of hornblende and plagioclase have been almost laccoliths of diorite porphyry. completely altered to secondary minerals, whereas those MUSHROOM LACCOLITH near the margins are clear and fresh. The alteration Mushroom laccolith is near the center of the Ute was evidently caused by solutions and vapors that ac­ Mountains midway between "The Knees" stock and cumulated in the central part of the igneous mass, the Black Mountain stock and is composed of the cen­ probably immediately following consolidation. The tral type of granodiorite porphyry. It is 2^4 miles alteration thus is considered to be deuteric and is the long and almost 1 mile wide. A thickness of about 500 best known example of this type of alteration in the feet is exposed but, because the base is not exposed, the Ute Mountains. total thickness is not known. The laccolith lies in the BANDED LACCOLITH Mancos Shale. It may have been fed laterally from Banded laccolith, immediately southeast of Three either Black Mountain or "The Knees," or it may have Forks laccolith, is composed of quartz monzonite por­ had a nearly vertical conduit connected to a stock at phyry. It is half a mile wide and originally was a considerable depth. mile long, but the southeastern third has been separated Many dikes radiate outward from the approximate from the main body by erosion. The central part of center of the Mushroom laccolith. The dike rocks con­ the laccolith is 550 feet thick; considering the lateral tain crystals of andesine as much as 10 mm in length dimensions, this is thicker than most laccoliths com­ and resemble the southern type of granodiorite por­ posed of diorite porphyry. Most of the northwestern phyry more closely than the central type. As shown part of the laccolith was intruded into a zone ranging by thin sections, the dikes contain less quartz than the from 100 feet below the Dakota-Mancos contact to just granodiorite of the laccolith. above the contact. The southeastern third intruded the Mancos near the level of the Greenhorn Limestone PACK TRAIL LACCOLITH Member. Pack Trail laccolith, composed of diorite porphyry, The bare, steep-faced outcrops of the quartz monzo­ is between Mushroom laccolith and Black Mountain nite porphyry display impressive vertical banding. in sees. 34 and 35, T. 35 N., R. l£ W. The laccolith The banding is due to weathering of unevenly altered was intruded into Mancos Shale below the Juana vertical layers that are interpreted as flow structure. Lopez Member. Sedimentary rocks have been almost In the strongly altered layers, hornblende is replaced completely removed from the flanks of the laccolith; by chlorite and magnetite, and plagioclase by kaolinite only a few remnants remain. These remnants dip and abundant calcite. The alteration is probably deu­ steeply, but the fact that the top surface of the in­ teric. Unaltered rock occurs within short distances of trusive body is conformable with them suggests a the contacts with the sedimentary rocks. laccolith rather than a bysmalith. The base of the lac­ As shown on plate 1, the flow banding in the north­ colith is exposed for a few feet in a deep valley cut western part of the laccolith strikes generally northwest into it by a tributary of Cottonwood Creek in N% and dips slightly southwest or as much as 45° NE. In sec. 35, T. 35 K, R, 18 W. The laccolith is 1*4 miles detail, the attitude of the banding is much more diverse long, about 1 mile wide, and at least 700 feet thick. than the map indicates, and is more erratic in the south­ It was probably fed from the Black Mountain stock, eastern part of the laccolith than in the northwestern which is contiguous on the north. part. TONGUE LACCOLITH THE "WEST TOE" CLUSTER OF INTRUSIVE BODIES Tongue laccolith is exposed a few hundred feet east The "West Toe" cluster includes several intrusive of Mushroom laccolith and about half a mile north­ bodies between "West Toe" bysmalith, "The Knees" east of "The Knees" stock. It is composed of micro- stock, and the western part of Mushroom laccolith. The gabbro porphyry and is tongue-shaped in plan. It major intrusive bodies in this area are tongue-shaped probably was intruded laterally from "The Knees" STRUCTURAL GEOLOGY AND FORMS OF THE IGNEOUS BODIES 59

stock. It is about 200 feet thick, half a mile wide, and the fact that the Mancos dips as much as 30° away of unknown length. from North Black Mountain suggests that the lacco­ The laccolith has been intruded by a younger and lith bulges as it approaches Black Mountain stock. smaller laccolith of diorite porphyry, and has been SILLS AND DIKES domed upward along its southern extremity by grano- Near the head of Pine Creek between Black and diorite porphyry of the Last Spring laccolith. It is also cut by a dike of granodiorite porphyry. Mable Mountains are several sills composed of micro- gabbro and diorite porphyry rich in mafic minerals; YUCCA CLUSTER OF LACCOLITHS two of these sills are shown on figure 19. Field rela­ The Yucca cluster of laccoliths lies west and south­ tions indicate that the sills were domed locally by the west of Black Mountain. Laccoliths of this cluster are emplacement beneath them of the North Black Moun­ composed of diorite porphyry of the normal type. tain laccolith and Mable Mountain bysmalith, which They have distinct northeast-trending lineation, ex­ consist of more silicic rocks. tend almost to Black Mountain, and probably were Many dikes occur in the Ute Mountains, especially intruded laterally from the Black Mountain stock. in the vicinity of "The Knees" stock and the Black With the exception of a small laccolith in the south­ Mountain stock. Most of the dikes are composed of ern part of sec. 33, T. 35 N., K. 18 W., the laccoliths granodiorite porphyry, and several of these cut sills were intruded into Mancos Shale above the Juana or laccoliths composed of diorite porphyry. The long­ Lopez Member. The laccoliths average less than 300 est dike in the mountains is the Ute Creek dike, which feet in thickness. extends northeastward from the base of Mable Moun­ tain to McElmo Creek (pi. 1). This dike was probably "THE BUTTES" LACCOLITH the feeder of the Mable Mountain bysmalith. "The Buttes" laccolith is about 2 miles northwest of the top of Black Mountain. It has the shape of a BRECCIA PIPES crude equilateral triangle and is about a mile long. Several breccia pipes cut the Mancos Shale in the It is asymmetric in cross section, and is thickest (about extreme western part of the Ute Mountains, in sees. 31 500 feet) in the northwestern part. and 36, T. 35 N., Ks. 19 and 18 W. The pipes crop out The southern part of the laccolith was intruded some a few feet below the projected base of the westernmost 200 feet above the base of the Mancos, the northern and largest laccolith in the Yucca cluster. They range part within 100 feet of the base, and the western part in diameter from about 10 to 150 feet and are nearly at or below the top of the Dakota. Inasmuch as the circular, although the largest is elliptical and about 50 laccolith was probably fed from the south and south­ feet wide and 150 feet long. Relations between the east through a sill-like conduit from Black Mountain, pipes, the surrounding Mancos Shale, and the base of the magma evidently cut downward stratigraphically the laccolith are well exposed in draws leading north­ in its advance toward the northwest. west along the northern flank of the laccolith in the NEi/4 sec. 31, T. 35 N., R. 18 W. The regional dip of NORTH BLACK MOUNTAIN LACCOLITH the Mancos Shale in and around the northern flank of North Black Mountain laccolith, composed of nor­ the laccolith is about 4° W., but near the pipes the shale mal diorite porphyry, is southwest of Mable Mountain dips 18° to 30° NE. (pl.l). and northwest of Black Mountain stock. It is elongate The individual pipes consist of Mancos Shale and to the north and about 2 miles long; the north half is diorite porphyry in brecciated blocks and fragments about three-fourths mile wide but the south half is cemented with travertine. The shale is principally in narrower. The laccolith has a maximum thickness of the outer parts of the pipes. It is so altered and about 700 feet, in the south-central part. hardened that it resembles mudstone, and is light gray On the southwest, the thin edge of the laccolith lies green to pale brown, in contrast with the dark-gray to several hundred feet above the base of the Mancos black fissile shale surrounding the pipes. The brec­ Shale, but farther north along the west side, it lies in ciated diorite porphyry is principally in the central the upper 100 feet of the Dakota Sandstone. Because parts of the pipes. In places, the porphyry fragments of poor exposures, the nature of the northward change have been argillized, but most of them show no altera­ in stratigraphic level is not known. tion effects other than the cementation by travertine. The steeply dipping east side of the laccolith is well The Mancos Shale below the laccolith is brecciated exposed. There, Mancos strata dip 30° away from the into fragments that average only a few inches in diame­ laccolith and can be traced upward into a nearly flat ter, and the diorite porphyry for several feet above the roof (pi. 1). On the south, toward Black Mountain, base of the laccolith is broken into angular fragments 60 GEOLOGY, PETROLOGY, TJTE MOUNTAINS AREA, COLORADO

FIGURE 19. Two sills In Mancos Shale (Km) along Pine Creek, north of Black Mountain. Upper sill Is microgabbro (TKm) about 200 feet thick; lower sill is diorite porphyry (TKd). Viewed from the east. that range from a few inches to several feet in diame­ have been caused by faulting and quaking prior to the ter. Both the brecciated shale and porphyry are hydrothermal activity, but the possibility exists that cemented with travertine for a distance of about 15 feet this brecciation was also due to the explosive action of from the contact. water which gave rise to the breccia pipes themselves. A sill of lamprophyre crops out in a draw south of The open nature of many of the fractures and the occur­ the locality described above, in sec. 6 of unsurveyed rence of travertine suggest that the fracturing took T. 34 N., E. 18 W. Although no travertine-bearing place when the rocks were at shallow depth or were pipes are exposed in the immediate vicinity of the sill, exposed at the surface. The fracturing and hydrother­ the sill is fractured, brecciated, and cemented with trav­ mal (hot-spring) activity are therefore believed to be ertine. Figure 20 shows a boulder of brecciated lam­ much later than the igneous activity that gave rise to prophyre that has slumped from the sill. The bounder the Ute Mountains. is composed entirely of angular fragments of lampro­ phyre in a matrix of travertine. Figure 21 is a photo­ SUMMARY OF GEOLOGIC EVENTS graph of the sill. Some of the fractures are open and The structural and stratigraphic history of the Ute only partly filled with travertine. The solutions carry­ Mountains area is an integral part of the geologic his­ ing calcium carbonate evidently moved laterally rather tory of the Colorado Plateau. Cater (1955) has out­ than vertically along the fractures in the sill, for the lined the structural history of the area of salt anticlines shale beneath the sill is "tight" and unfractured. on the plateau north of the Ute Mountains, and Stro- The pipes were probably formed by the explosive re­ bell (1956) has summarized the geologic history of the lease of steam and hot water. The brecciation and frac­ Carrizo Mountains area, located approximately 25 miles turing of the sill and the basal part of the laccolith may south of the Ute Mountains. The important geologic ECONOMIC GEOLOGY 61

7. Pronounced uplift and widespread erosion south of the Ute Mountains area. 8. Deposition of fluvial, swamp, and littoral rocks of the Dakota Sandstone during Late Cretaceous time. 9. Marine invasion and deposition of the Mancos Shale of Late Cretaceous age. 10. Deposition of marine sandstone and shale of the Point Lookout Sandstone in an oscillating Late Cretaceous sea. The events that followed the deposition of the Point Lookout Sandstone in the Ute Mountains area and the age of the igneous rocks of the mountains can be in­ ferred only from rocks exposed southeast of the Ute Mountains. 11. Deposition of the marine and continental shale and sandstone of pre-Animas, post-Point Lookout FIGDBE 20. Boulder of brecclated lamprophyre cemented with travertine., age (Menefee Formation, Cliff House Sandstone, Lewis Shale, Pictured Cliffs Sandstone, Fruit- land Formation, and Kirtland Shale). 12. Intrusion of igneous rocks: a. Microgabbro. b. Diorite porphyry, c. Granodiorite porphyry, d. Quartz monzonite porphyry. 13. Erosion in the Ute and La Plata Mountains and deposition of coarse elastics (in the McDermott Member of the Animas Formation of Late Cretaceous age) to the south and east. ECONOMIC GEOLOGY METALLIC MINERAL DEPOSITS Small deposits of uranium, vanadium, and copper along the northern edge of the Ute Mountains and in the vicinity of McElmo Canyon have been extensively prospected, but thus far have not proved to be of FIGUEB 21. Fractured and brecciated sill of lamprophyre. Some of much commercial value. Except for the generally the fractures are open and only partly filled with travertine. barren iron-stained pyritic alteration zones in some events as recorded by the rocks exposed in the Ute of the intrusive bodies, no metallic mineral deposits Mountains area are: are known in the Ute Mountains proper. 1. Predominantly subaerial deposition of the Navajo The uranium deposits include (a) fault-controlled Sandstone from sources to the west in Late Tri- deposits that contain abundant pyrite and sparse cop­ assic( ?) and Jurassic time. per minerals, and (b) flat-lying deposits containing 2. Deposition of the Entrada Sandstone under alter­ vanadium minerals in beds at considerable distances nating subaqueous and subaerial conditions. from known faults. The copper deposits are all as­ 3. Deposition of the marine Summerville Formation. sociated with faults or shear zones. 4. Deposition of the Junction Creek Sandstone, partly Commercial deposits of uranium and vanadium occur subaqueous and partly subaerial. in the Salt Wash Member of the Morrison Formation 5. Deposition of the Morrison Formation, partly flu­ in Montezuma Canyon, Utah, about 10 miles north­ vial and partly flood plain. west of the northwest corner of the Moqui SW quad­ 6. Deposition of coarse fluvial elastics of the Burro rangle, and in the Carrizo Mountains of Arizona, about Canyon Formation of Early Cretaceous age from 25 miles southwest of the Ute Mountains. Very large sources west or southwest of the Ute Mountains deposits of uranium and vanadium occur in the Jack- area. pile sandstone (of local usage) in the Morrison Forma- 62 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO tion far south of the Ute Mountains in the vicinity group are about 1% miles north of McElmo Creek (fig. of Grants, N. Mex. Because of the widespread occur­ 22) .and can be reached by an access road, built by the rence of uranium deposits in the Salt Wash Member Atomic Energy Commission, that leaves Colorado on the Colorado Plateau, the Salt Wash was consid­ Highway 32 just north of Battle Kock. The Cliff House ered to have the greatest potential in the Ute Moun­ group was owned in 1957 by the Four Corners Uranium tains area and was studied in considerable detail dur­ Co., Denver, Colo. ing the present work. Cliff House prospect. The Cliff House prospect (fig. 22) is along a northeast-trending fault along which TJKANIUM DEPOSITS beds of the Summerville Formation are displaced 180 FAULT-CONTROLLED DEPOSITS feet down to the northwest and brought against the up­ CLIFF HOTJ8E GROUP OF CLAIMS per part of the Navajo Sandstone (fig. 23). The radio­ The Cliff House group of claims lies in sees. 22, 23, active area is about 100 feet long and is limited to a and 27, T. 36 N., R. 18 W. All the prospects in this zone where mudstone and sandstone of the Summerville

R. 18 W. R. 17 W.

T. 36 N.

T. 35 N.

EXPLANATION

,Cu Contact Prospect pit or small mine i U, uranium Cu, copper Fault Bar and ball on downthrown side FIGCEB 22. Map showing location of mines and prospects. TKd, diorlte porphyry; Jms, Salt Wash Member of the Mor- rison Formation (stippled). ECONOMIC GEOLOGY 63 brown by limonite near the prospect, but farther away the sandstone changes gradually to the pink color char­ Entrada Sandstone! acteristic of this stratum in the McElmo Canyon area. The sandstone is probably the same bed that is radio­ Iff Maximum radioactivity occurs at base * / of limestone active in the Cliff House No. 4 prospect. Summerville Formation Cliff House No. 4 prospect. The Cliff House No. 4 workings consist of two very short drifts and several small pits dug into or near two east-trending faults Navajo Sandstone (fig. 24). The northern fault has about 80 feet of dis­ placement with the south side down. The beds are displaced 15 feet down on the north side of the south­ ern fault; consequently there is a small graben in the

200 FEET prospect area. The southern fault dips about 80° to the north and probably intercepts the northern fault FIGURE 23. Sketch showing stratigraphic and structural relations at at depth (section A-A ', fig. 24). The highest radio­ the Cliff House prospect, McElmo Canyon, Montezuma County, Colo. activity occurs near the southern fault; a chip sample Formation are in contact with a limestone bed about (55-E-10, table 13) taken in this zone assayed 0.053 4 feet thick in the Navajo Sandstone. Rocks in the pros­ percent eU3O8, 0.028 percent U3O8, and less than 0.1 pect area are intensely stained with limonite, and much percent V2O5. The radioactive material has not been carbonate has been leached from the limestone bed. identified; it is closely associated with limonite, hema­ Scintillation-counter measurements indicate that most tite, barite, asphaltite, and sparse copper carbonates of the radioactive material is concentrated in sandstone concentrated in a flat-lying upper sandstone bed of the at the base of the leached limestone. No minerals of Summerville and in fault breccia adjacent to this bed. uranium or vanadium were identified. Sample 55-E-12 The radioactivity diminishes southward from the (table 13), chipped from sandstone in contact with the faulted area and, in general, where it diminishes the leached limestone, assayed 0.038 percent eU3O8, 0.040 content of limonite decreases. The upper bed of the percent U3O8, and 0.16 percent V2O5. Summerville between the faults, and north of the north A radiometric survey was made of the fault line for fault, is not radioactive. In holes drilled by the U.S. several hundred yards east and west of the prospect Atomic Energy Commission (AEC) along the north area; no radioactivity above background was noted. and south faults and in the intervening graben there Cliff House No. 2 prospect -The Cliff House No. 2 is no increase of radioactivity at depth. Pyrite and prospect is about half a mile northeast of the Cliff marcasite were identified in the AEC drill cores by House prospect (fig. 22) along the same northeast- Coleman and Delevaux (1957, p. 513). trending fault. In the Cliff House No. 2 area, down­ ward displacement on the northwest side of the fault is THREE STATES NATURAL GAS CO. PROSPECT only about 100 feet, and the Entrada Sandstone is in Uranium prospects in sec. 24, T. 36 N., R. 18 W., are contact with uppermost beds of the Summerville. on an oil and gas lease owned by the Three States Radioactivity is only slightly above background. The Natural Gas Co. and on a mineral claim located by sandstone in the Summerville is strongly stained yellow- Ben Archibeque, Mancos, Colo. The area is about 200 TABLE 13. Radiometric and chemical analyses, in percent, of samples from radioactive prospects, McElmo Canyon, Montezuma County Colo. [Analysts, 1, D. L. Schafer, H. H. Lipp, J. E. Wilson; 2, C. G. Angelo; 3, C. G. Angelo, J. P. Schuch, J. E. Wilson] Sample No. Prospect or mine sampled Radiometric Chemical analysis analysis Analysts Labora­ Equivalent Uranium Vanadium Field tory Name Material uranium (U808) pentoxide (eU308)

55-E-5...... 229457 1 Three States Natural Gas Co ..... 0.049 0.015 <0.1 55-E-6__..__ ...... 229458 1 _____ do...... _.._-- _..._ - __do...._ _._ .... .028 .019 <.l 55-E-7-. _._._._ 229459 1 _____do__.. -.....- ...... -...... do...... 021 .006 <.l 55-E-8 . __ ...... 229460 1 Coffin's- ______.....do .... _ - ______.004 .52 55-E-9__...... 229461 1 .....do ...... do...... 008 .93 55-E-10.... _ ...... 229462 1 Cliff House No. 4...... 053 .028 <.l 55-E-12____.._ _._ 229464 1 Cliff House.. ______-.do...... 038 .040 .16 55-E-26B...... 238257 2 .....do ...... Leached limestone. ______. __ ...... 002 55-E-20_-...... 238266 3 Karla Kay.. ______...... Green mudstone, Brushy Basin Member...... _. <.001 55-E-21-. ______238267 3 Sandstone from Karla Kay Conglomerate Member. .004 55-E-22... ._.______238268 3 _____do.... ___ . _ Mudstone pebble conglomerate, Karla Kay .016 .019 .32 Conglomerate Member. 55-E-24A. .___.__.__ 238269 3 ..do.... __ .007 .012 <.l 64 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO EXPLANATION

Contact

______u______D Fault U, upthrown side; D, downthrown side X Prospect pit

Radioactive material 0- AEC angle diamond-drill hole

O Four Corners Uranium Co. Vertical solid-bit-drill hole

FIGURE 24. Cliff House No. 4 area, McElmo Canyon, Montezuma County, Colo. Jj, Junction Creek Sandstone: Jsu, upper part, Jsr, radio­ active sandstone bed, and Jsl, lower part, all of the Summerville Formation, and Je, Bntrada Sandstone. feet south of the Schmidt 1 carbon dioxide well and Veinlets of asphaltic material were noted in fault can be reached by a 2-mile access road from Colorado breccia in the prospect area. The asphaltic material Highway 32. has been "dried"; it is black, has a pitchy luster, is Two prospect pits about 60 feet apart are dug into brittle, and can be ground to a brown powder between the Entrada Sandstone along a northeast-trending the fingers. fault that displaced beds of the Summerville Forma­ BEDDED URANIUM DEPOSITS tion 10 feet down to the southeast, against the Entrada KABI.A KAY MINE Sandstone. The highest radioactivity is in the east The Karla Kay mine is near the NW cor. sec. 32, pit, wl;ich is dug into sandstone on the north side of T. 36 N., E. 18 W. The mine area is claimed by the the fault. Of three samples taken in this pit during Four Corners Uranium Co. and can be reached by a the present work, the most radioactive (sample 55-E-5, 1^-mile access road that leaves Colorado Highway 32 table 13) assayed 0.049 percent eU3O8, 0.015 percent at the house site of the Fowler Farm in McElmo U3O8, and no V2O5. Canyon. ECONOMIC GEOLOGY 65

The mine has 70 feet of workings divided among Abundant carbon trash in back three drifts to the north, east, and south (fig. 25). The Carbonized tree frag­ mine was started in radioactive talus and the workings ments (most radio­ active material in entered coarse clastic rocks of the lowermost part of mine) the Burro Canyon Formation. Three sets of joints break the rocks in the mine one set strikes consistently northeast, another north­ west, and a poorly defined third set, about east. The ground is badly broken by these fractures and by col­ lapse due to removal by underground water of mud- stone beneath the sandstone of the Burro Canyon. Most of the radioactive material in and around the Joints are coated with gypsum, mine is in a lentil of mudstone conglomerate (fig. 26) limonite, and pyrolusite that is part of the Karla Kay Conglomerate Member (Ekren and Houser, 1959a) of the Burro Canyon Formation. The conglomerate contains pebbles and slabs of green mudstone in a matrix of coarse conglom­ eratic and cherty yellow-brown sandstone. It con­ tains abundant tree molds, but few carbonized tree remnants. The conglomerate lentil occupies a channel Carnotite, weakly disseminated cut in bentonitic mudstone of the Brushy Basin Member in sandstone of the Morrison Formation and underlies a fine- to medium-grained, poorly sorted quartz sandstone that contains very sparse pebbles of variegated chert. This sandstone bed underlies the main lens of the system of Carnotite coating shoestring conglomerates of the Karla Kay. Both the mudstone pebbles mudstone conglomerate and the overlying coarse sand­ stone were eroded prior to the deposition of the main lens of the Karla Kay east and west of the mine. The highest radioactivity is associated with carbon­ ized plant debris in the back of the north drift at the top of the mudstone conglomerate lentil (fig. 25). This Drift to north material is estimated to have about 0.05 percent U3O8. Carnotite is weakly disseminated in the conglomerate and forms a coating on pebbles of mudstone. Kadio- activity is not concentrated along any of the fractures in the mine, many of which contain gypsum, limonite, and dendrites of pyrolusite. Kadiometric traversing to the east, north, and west of the mine did not disclose 0 10 FEET significant radioactivity. I______i______I The radioactive material in the Karla Kay mine has not proved to be of commercial value (table 13), and no ore has been shipped from the mine. Several small EXPLANATION deposits of uranium have been found associated with the shoestring conglomerates of the Karla Kay west of Contact Dashed where approximately located

FIGURE 25. Map and section of the Karla Kay mine, McElmo Canyon, Montezuma County, Colo. Qt, Quaternary talus deposits; Kbk2, sand­ stone, and Kbk1( mudstone, of Karla Kay Conglomerate Member of the Strike and dip of joints Burro Canyon Formation; Jmb, mudstone of Brushy Basin Shale Member of the Morrison Formation. Level of plan view is waist height. Strike and dip of vertical joints 66 GEOLOGY, PETROLOGY, XITE MOU1STAINS AREA, COLORADO

sw NE Burro Canyon Formation

Karla Kay Conglomerate Member

KARLA KAY MINE

Brushy Basin Shale Member of Morrison Formation (Mudstone)

100 FEET

FIGURE 26. Sketch showing- stratigraphic relations at the Karla Kay mine. Collapsed beds have been restored to horizontal position. the Ute Mountains area, however, and some of this COPPER DEPOSITS material has been mined. BATTLE ROCK MINE The Battle Kock workings are in a mineralized shear COFFIN'S PROSPECT zone that extends the full length of Battle Kock, a nat­ Coffin's prospect is on the John Meadows Kanch a ural monolith in McElmo Canyon in sec. 34, T. 36 N., few feet north of Colorado Highway 32, in sec. 34, K. 18 W. The shear zone trends N. 75° W. and is from T. 36 N., R. 18 W. This prospect was described by 10 to 15 feet wide. The introduction of silica, barite, Coffin (1921). limonite, and carbonates along the shear zone indurated The mineralized material is in flat-bedded sandstone the normally friable Junction Creek Sandstone and near the middle of the Entrada Sandstone. The pros­ altered its color from reddish orange to brown. The pect was opened in 1913 and the pit has since been filled indurated rock in and adjacent to the shear zone weath­ with alluvium and windblown silt. The mineralized ers in relief, and thus forms the monolith. rock is exposed in an outcrop about 130 feet long by The workings are only partly accessible; only the 40 feet wide that extends south under the highway. It main drift was examined. Sparse copper carbonates is eroded away north and east of this exposure and is in a barite gangue, and abundant limonite or hematite, covered by alluvium to the south and west. Holes dug or both, were found there. Kock on the mine dump for fenceposts and power poles along the road show has sparse chalcopyrite. No ore has been shipped from the mineralized zone to be about 6 inches thick. The the mine. vanadium-uranium minerals and traces of malachite UTE CREEK DIKE PROSPECTS are concentrated along bedding planes and do not seem Several prospect pits have been dug into baked sedi­ to be influenced or controlled by fractures. A 1-inch mentary rocks and sheared diorite porphyry along the seam within the mineralized zone is richer than the sides of the Ute Creek dike (fig. 22). Most of these rest and contains a dark-gray mineral, probably vana­ pits are devoid of visible ore materials, although a few dium mica. The secondary minerals pintadoite and show sparse copper minerals distributed along frac­ carnotite occur as coatings on fresh fractures. tures. A shallow shaft that exposes copper minerals No ore has been shipped from this prospect. Assays in sheared diorite was studied by E. M. Shoemaker and of chip samples taken along the most intensely mineral­ W. L. Newman, who reported (written commun., 1953) ized seam during the present study are shown in table 13. that: "The sheared diorite is intensely kaolinized lo­ A sample taken by Coffin (1921) assayed more than cally. Copper minerals, chiefly malachite, azurite, and 1.0 percent V2O5 and 0.09 percent U3O8. The deposit copper pitch are distributed along fractures, in small at the prospect differs from others in the Ute Mountains cavities, and replace altered feldspar and hornblende area in that vanadium is much more abundant than phenocrysts." uranium. The assays in table 13 indicate approxi­ LITTLE MAUDE MINE mately a 100 :1 ratio, and the assay reported by Coffin The Little Maude mine, in sec. 14, T. 35 N., K. 18 W. shows a 10 :1 ratio. (fig. 22), is a single drift approximately 50 feet long ECONOMIC GEOLOGY 67 in baked and sheared limy mudstone of the Mancos Fourteen samples from the fault-controlled and the Shale. The back of the drift is the base of a thin bedded mineral deposits were analyzed spectrographi- sill that is the featheredge of the Mable Mountain cally in an attempt to find clues that might indicate a bysmalith. The drift follows a shear zone that con­ source for the metals (table 14). Analyses of barren tains much pyrite and seems to be part of the frac­ rock from the formations that contain these deposits ture system along which the Ute Creek dike was are also shown in the table for comparative purposes. intruded. The spectrographic data indicate that iron, barium, RELATION OF THE METALLIC MINERAL DEPOSITS TO THE cobalt, chromium, copper, molybdenum, nickel, lead, IGNEOUS ROCKS strontium, vanadium, yttrium, and ytterbium occur in greater than background quantities in both the fault- The occurrence of uranium and other metals in faults controlled and the bedded deposits. On the other hand, in the Ute Mountains area is of geologic interest be­ silver, arsenic, antimony, niobium, zinc, and manganese cause of the controversy regarding the origin of the occur in greater than background values only in the uranium deposits on the Colorado Plateau. The fun­ fault-controlled deposits. All the elements listed above, damental questions regarding the uranium in the with the exception of silver, antimony, and niobium, McElmo Canyon area are these: Were the uranium also occur in greater than background values in the and associated metals derived from hypogene solutions ? nonradioactive copper deposits in Battle Rock. If so, were the solutions related to the igneous rocks of the Ute Mountains? Vanadium is found only in one fault-controlled radio­ The field data indicate that the copper deposits of active deposit, the Cliff House prospect. Mineralized limestone in the Navajo Sandstone at this prospect con­ Battle Rock and the Ute Creek dike were almost cer­ tainly derived from hydrothermal solutions related to tains about 300 ppm of vanadium (sample EMS-9-53, the igneous activity of the Ute Mountains. These de­ table 14), and limonitic sandstone contains 900 ppm of posits occur in sheared or faulted zones that are radial vanadium (sample 55-E-12, table 13), more than 50 to the intrusive bodies (pi. 1) and, specifically, to the times the background for Navajo Sandstone. The Mable Mountain bysmalith that probably overlies a vanadium may be related to dried asphaltic material fracture zone. that is common as veinlets in the fractures of this A large part of the Mable Mountain intrusive body vicinity. has been pyritized and altered. Intensely altered areas It is concluded that most of the metals, including partly surrounded by vein deposits of metallic minerals uranium, were introduced into the fault-controlled de­ (especially base metals) are a well-known feature of posits by ascending (hypogene) solutions related to the mineral districts of hydrothermal origin in southwest­ igneous activity in the Ute Mountains. The minor ern Colorado. The pyritization at Mable Mountain elements of the bedded uranium deposits are similar to was probably hydrothermal and related to the intro­ those in the fault-controlled deposits and they may duction of copper minerals in the veinlike deposits therefore be of the same origin. It is unlikely, however, of Battle Rock and along the Ute Creek dike. that the Ute magmas were the source of uranium and The introduction of uranium and sparse copper in other metals in the large bedded deposits in the Morri- the faults of the Cliff House No. 4 and Three States son Formation of nearby areas in southwestern Colo­ Natural Gas Co. prospects probably was accomplished rado, because the hydrothermal activity associated with during the same hydrothermal stage as the pyritization the igneous intrusions of the Ute Mountains was feeble at Mable Mountain. The abundance of pyrite and and therefore probably not capable of supplying the marcasite at depth in the Cliff House No. 4 veins and necessary volume of metals. In contrast with the stocks the occurrence of asphaltite in both this prospect and of the Henry Mountains (Hunt and others, 1953, p. that of the Three States Natural Gas Co. indicate that 217-220) and the Abajo Mountains (I. J. Witkind, the introduced material in the arcuate faults was largely oral commun., 1957), the stocks of the Ute Mountains derived from ascending solutions. The asphaltite prob­ are unmineralized. ably came from petroliferous beds that underlie the URANIUM POTENTIAL OF THE UTE MOUNTAINS AREA uranium deposits, because, in the McElmo Canyon area, petroleum is unknown in rocks younger than those that NORTHERN UTE MOUNTAINS AREA carry the deposits. The uranium potential of the area that includes the The ubiquity of barite in fractures along McElmo northern fringes of the mountains and McElmo Canyon Canyon is significant because the igneous rocks of the is believed to be extremely small. The known deposits, Ute Mountains are abnormally rich in barium. (See described above, have thus far proved to be too poor to table 4.) be of commercial value. Intense prospecting in this 68 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO area of dissected and well-exposed beds of the Morrison missivity (Jobin, 1962) within the beds of the Salt Formation between 1950 and 1958 failed to disclose com­ Wash exist between the southern Ute Mountains area mercial deposits. Nevertheless, because the lower and McElmo Canyon. Uranium deposits may occur in members of the Morrison Formation contain uranium or near these zones. The most economical area to ex­ deposits in adjacent areas, they were studied in consider­ plore for such deposits is the western flank of the Ute able detail during geologic mapping. dome. Many drilling sites are readily accessible in the The Westwater Canyon Member of the Morrison canyons and on the tops of mesas and pediments lying Formation averages less than 100 feet in thickness in west of the igneous terrane. The approximate depth McElmo Canyon. The Salt Wash Member averages to the base of the Salt Wash in such places ranges from about 200 feet in thickness in the eastern part of the 400 feet in areas where the Brushy Basin Member of canyon, where the Recapture Member is absent, but it the Morrison is at the surface to 800 feet where the top thins westward to about 100 feet where the Eecapture of the Dakota is at the surface. Although the Morri­ Member is present. Throughout McElmo Canyon the son Formation is present beneath many of the igneous Salt Wash consists of continuous lenses or rims of sand­ bodies, exploration beneath these bodies would be ex­ stone separated by lenses of reddish-brown mudstone. pensive because of the depth of drilling and the structure In some areas, there are four or more distinct lenses of and hardness of the rock. mudstone, but in others mudstone is in extremely thin Criteria for recognizing favorable ground in areas units and the Salt Wash forms a single thick sandstone containing the Westwater Canyon Sandstone Member unit. Very little green or altered mudstone is present have never been completely outlined. Thaden and below or above the individual lenses of sandstone and, Santos (1956) reported that ore has not been found in where present, the altered mudstone extends for only a the Westwater Canyon Member where the sandstone few tens of feet and is only a few inches thick. Pebbles is split into many tongues or where it is less than about and seams of mudstone within the sandstone lenses, 100 feet thick. The Westwater Canyon is thicker than however, are commonly green. No accumulations of 100 feet in the southern part of the Ute Mountains area, "carbonaceous trash" are known, but fine particles of but the possibility of its being a host for large deposits carbon are fairly abundant along bedding planes. is probably remote inasmuch as no deposits have been The lack of extensive greenish or altered mudstone, found in this member where it is widely exposed in adja­ the absence of "carbonaceous trash", and the nonlenticu- cent areas of Utah, Arizona, and northwestern New lar character of the sandstone of the Salt Wash in this Mexico. area as compared to uranium-producing districts sug­ gest that the area is generally unfavorable for uranium NONMETALLIC MINERAL DEPOSITS deposits (Weir, 1952). The environment in which the PETROLEUM, NATURAL GAS, AN1> CARBON DIOXIDE sandstone beds were deposited apparently did not per­ mit large accumulations of carbon, nor the development KNOWN OCCURRENCES of zones of contrasting transmissivity. Only minor amounts of oil and gas had been pro­ duced from the Ute Mountains area before 1958, and SOUTHERN TTTE MOUNTAINS AREA this production was confined to McElmo dome. The The lower beds of the Morrison are exposed in only structure and oil and gas possibilities of McElmo dome one place in the southern Ute Mountains area. This ex­ have been summarized by Zabel (1955), who stated that posure is approximately a quarter of a mile south of the of six wells drilled on or near the crest of the dome, "East Toe" or Sentinel Peak. Here, approximately 250 only one, the Macintosh 1 (pi. 1), produced hydrocar­ feet of the Westwater Canyon Sandstone Member over­ bons. Initial potential of the well was 500,000 cu ft lies 250 feet of the Recapture Shale Member of the 'flammable gas per day. The gas from this well is used Morrison Formation. The Salt Wash Sandstone Mem­ as fuel by a dry-ice plant of the Colorado Carbonics ber either is absent or is in a mudstone facies mapped Co. in McElmo Canyon. Two other wells also supply as Recapture Member. this plant with carbon dioxide gas. The A. H. The abrupt change in lithology of the lower beds of Schmidt 1 yields carbon dioxide from the Leadville the Morrison may be a favorable feature. The rich limestone and had an initial potential of 20,000 Mcf uranium deposits of the Uravan district, Montrose (thousand cubic feet) gas per day. The Dudley 1 County, Colo., are localized in sandstone that pinches yields from the same rocks and had an initial potential out completely or thins abruptly within short distances of Y,000 Mcf gas per day. from productive areas (Boardman and others, 1956). The Fulks 1 well, drilled in the NW*4 sec. 27, T. 37 N., It seems quite certain that zones of contrasting trans­ R. 17 W. (not in the mapped area), yielded some car- .LITERATURE CITED 69 bon dioxide from the Mississippian and Lower Cam­ potential reservoir rocks in the structure can be deter­ brian rocks. Granite was reached at a depth of 8,637 mined only by drilling. feet, and the hole was abandoned at 8,787 feet. COAL, The West 1 well (pi. 1) found strong shows of oil in the upper part of the Hermosa Formation at an approxi­ The Dakota Sandstone in the Ute Mountains area mate depth of 3,300 feet. contains abundant thin beds of coal, but with few exceptions these beds are extremely lenticular and have OIL AND GAS POSSIBILITIES OF THE UTE MOUNTAINS AREA little commercial value. One bed with reportedly large The oil and gas possibilities of the Ute Mountains reserves is exposed a few miles east of Cortez, Colo. area have recently been enhanced by the discovery of M. A. Pishel (written commun., 1911) studied the several major oil fields in southeastern Utah, the largest coals in the Dakota of southwestern Colorado and of which is the spectacular Aneth field. Production in stated, "The coal is low-grade bituminous in character, these fields is from two zones in the Hermosa Forma­ containing relatively large amounts of finely dissemi­ tion designated the Desert Creek and the Bluff zones nated siliceous matter which in the analysis shows up by Herman and Barkell (1957). The reservoir at as ash." One of the several samples of coal from the Aneth, Utah, is described as a carbonate buildup in Dakota that Pishel had analyzed was from the McElmo the Desert Creek zone and is considered by some geol­ Canyon area. This analysis follows. ogists to be a reef. The reef apparently has a north­ Analysis of coal sample from the Dakota Sandstone, SEty sec. west trend that parallels the southwest flank of the 3, T. 35 N., R. 18 W., MoElmo Canyon area Paradox structural basin. [Analyst unknown, sample fairly fresh]

UTE DOME Loss of moisture on air-drying percent- 3.20 Analysis of air-dried sample: The possibility has been discussed by Hunt (1942) Moisture ______do 4. 69 that the domes formed by laccolithic mountains may Volatile matter ___ do 21. 68 be oil bearing. Hunt pointed out that oil geologists Fixed carbon______do 60. 62 have had little interest in laccolithic mountains because Ash______.______do__ 13. 00 Sulfur______do . 48 the laccoliths have been generally interpreted as mush­ Calories _____-____ 6,232 room-shaped bodies overlying undisturbed strata. The Btu ______11,218 knowledge that laccoliths were injected laterally from stocks that occupy the centers of structural domes Coal beds more than 2 feet thick were observed in greatly enhances the oil and gas possibilities. several exposures during the geologic mapping, prin­ Hunt's (1942) conclusions are believed to be applica­ cipally in the Moqui SW quadrangle in T. 35 N., Ks. 18 ble to the Ute Mountains. The Ute dome could 'be and 19 W. Local ranchers have mined small amounts an excellent structural trap, for it has large closure of the coal in this area for use as fuel. A coal bed ap­ and contains formations that are productive in nearby proximately 8 feet thick was observed in the Sentinel areas. Peak NE. quadrangle, about half a mile south of Senti­ The paucity of oil and flammable gas in McElmo nel Peak. This bed consists of a lower seam of good dome and the abundance of carbon dioxide may be con­ coal about 4 feet thick, impure coal about 1 foot thick, sidered negative indications as to the possibilities of oil and two upper 1-foot seams separated by impure coal and gas in the Ute dome. The origin of the carbon and carbonaceous shale. The exposure is limited and dioxide gas in the McElmo structure is unknown. Car­ the extent of the bed could not be determined. bon dioxide in other areas is thought to have originated LITERATURE CITED from the metamorphism of hydrocarbons by contact Baker, A. A., Dane, C. H., and Reeside, J. B., Jr., 1936, Corre­ with hot mineralized waters, or from the metamorphic lation of the Jurassic formations of parts of Utah, Arizona, action of magmas and hot solutions intruding limestone New Mexico, and Colorado: U.S. Geol. Survey Prof. Paper (Dobbin, 1935, p. 1068-1069). Whatever the origin, 183, 66 p. the carbon dioxide in the McElmo dome apparently is Baker, A. A., Dobbin, C. E., McKnight, E. T., and Reeside, J. B., Jr., 1927, Notes on the stratigraphy of the Moab region, related to igneous activity either directly below Utah: Am. Assoc. Petroleum Geologists Bull., v. 11, no. 8, McElmo dome or in the Ute Mountains, and it seems p. 785-808. probable that carbon dioxide will also be found in car­ Barnes, Harley, Baltz, E. H., Jr., and Hayes, P. T., 1954, Geology and fuel resources of the Red Mesa area, La Plata bonate-bearing rocks in parts of the Ute dome. and Montezuma Counties, Colorado: U.S. Geol. Survey Oil Whether this gas, if it exists at all, completely fills the and Gas Inv. Map OM-149. 70 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO

Boardman, R. L., Ekren, E. B., and Bowers, H. E., 1956, Sedi­ Eckel, E. B., 1949, Geology and ore deposits of the La Plata mentary features of upper sandstone lenses of the Salt Wash district, Colorado: U.S. Geol. Survey Prof. Paper 219, 179 p. member and their relation to uranium-vanadium deposits in Ekren, E. B., and Houser, F. N., 1957, Preliminary geologic map the Uravan district, Montrose County, Colorado, in Page, of the Sentinel Peak NW quadrangle, Montezuma County, L. R., Stocking, H. E., and Smith, H. B., compilers, Contribu­ Colorado: U.S. Geol. Survey Mineral Inv. Map MF-132. tions to the geology of uranium and thorium by the United 1959a, Relations of Lower Cretaceous and Upper Jurassic States Geological Survey and Atomic Energy Commission rocks in the Four Corners area, Colorado: Am. Assoc. Petro­ for the United Nations International Conference on Peaceful leum Geologists Bull., v. 43, no. 1, p. 190-201. Uses of Atomic Energy, Geneva, Switzerland, 1955: U.S. Geol. 1959b, Preliminary geologic map of the Cortez SW quad­ Survey Prof. Paper 300, p. 221-226. rangle, Montezuma County, Colorado: U.S. Geol. Survey Min­ Bowen, N. L., 1928, The evolution of the igneous rocks: Prince- eral Inv. Map MF-217. ton Univ. Press, 334 p. 1959c, Preliminary geologic map of the Moqui SE quad­ Brown, R. W., 1950, Cretaceous plants from southwestern Colo­ rangle, Montezuma County, Colorado: U.S. Geol. Survey Min­ rado: U.S. Geol. Survey Prof. Paper 221-D, p. 45-66. eral Inv. Map MF-221. Camp, C. L., 1936, A new type of small bipedal dinosaur from 1959d, Preliminary geologic map of the Sentinel Peak NE the Navajo sandstone of Arizona: California Univ. Dept. quadrangle, Montezuma County, Colorado: U.S. Geol. Survey Geol. Sci. Bull., v. 24, p. 39-55. Mineral Inv. Map MF-224. Camp, C. L., and Vanderhoof, V. L., 1935, Small bipedal dino­ Emmons, S. F., Cross, W. C., and Eldridge, G. H., 1896, Geology saur from the Jurassic of northern Arizona [abs.]: Geol. Soc. of the Denver Basin in Colorado: U.S. Geol. Survey Mon. 27, America Proc. 1934, p. 384-385. 556 p. Carter, W. D., 1957, Disconformity between Lower and Upper Engel, C. G., 1959, Igneous rocks and constituent hornblendes Cretaceous in western Colorado and eastern Utah: Geol. Soc. of the Henry Mountains, Utah : Geol. Soc. America Bull., v. 70, America Bull., v. 68, no. 3, p. 307-314. no. 8, p. 951-980. Cater, F. W., Jr., 1955, The salt anticlines of southwestern Engelhardt, Wolf von. 1936, Die Geochemie des Barium : Chemie Colorado and southeastern Utah, in Four Corners Geol. Soc. der Erde, v. 10, no. 2, p. 187-246; or see table LVII, in Gold- Guidebook, Field Conf. 1955; p. 125-131. schmidt, V. M., 1954, Geochemistry: London, Oxford Univ. Clarke, F. W., 1924, The data of geochemistry: 5th ed., U.S. Press, p. 252. Geol. Survey Bull. 770, 841 p. Fenneman, N. M., 1931, Physiography of western United States: Coffin, R. C., 1920, An anticline in Montezuma County, Colorado: New York, McGraw-Hill Book Co., Inc., 534 p. Colorado Geol. Survey Bull. 24, p. 47-59. Gilluly, James, and Reeside, J. B., Jr., 1928, Sedimentary rocks 1921, Radium, uranium, and vanadium deposits of south­ of the San Rafael Swell and some adjacent areas in eastern western Colorado: Colorado Geol. Survey Bull. 16, 231 p. Coleman, R. G., and Delevaux, Maryse, 1957, Occurrence of sele­ Utah: U.S. Geol. Survey Prof. Paper 150-D, p. 61-110. nium in sulfides from some sedimentary rocks of the western Goddard, E. N., chm., and others, 1948, Rock-color chart: Wash­ United States: Econ. Geology, v. 52, p. 499-527. ington, D.C., Natl. Research Council; repr. by Geol. Soc. Collier, A. J., 1919, Coal south of Mancos, Montezuma County, America, 1951. Colorado: U.S. Geol. Survey Bull. 691-K, p. 293-310. Goldman, M. I., and Spencer, A. C., 1941, Correlation of Cross' Craig, L. C., and others, 1955, Stratigraphy of the Morrison and La Plata sandstone, southwestern Colorado: Am. Assoc. Petro­ related formations, Colorado Plateau region, a preliminary leum Geologists Bull., v. 25, no. 9, p. 1745-1767. report: U.S. Geol. Survey Bull. 1009-E, p. 125-168. Goldschmidt, V. M., 1937a, Principles of distribution of chemi­ Craig, L. C., and Cadigan, R. A., 1958, The Morrison and ad­ cal elements in minerals and rocks: Jour. Chem. Soc., p. 655- jacent formations in the Four Corners area, in Intermountain 673; or see tables 18.5, 28.2, and 34.2, in Rankama, Kalervo, Assoc. Petroleum Geologists Guidebook, Field Conf. 1958; p. and Sahama, Th. G., 1950, Geochemistry: Chicago Univ. Press, 182-192. p. 528, 621, 682. Cross, C. W., 1894, The laccolitic mountain groups of Colorado, 1937b, Geochemische Verteilungsgesetze der Elemente. Utah, and Arizona: U.S. Geol. Survey 14th Ann. Rept, pt. 2, IX. Die Mengenverhaltnisse der Elemente und der Atom- p. 157-241. Arten: Skrifter Norske Videnskaps Akademi Oslo, I. Mat.- Cross, C. W., and Hole, A. D., 1910, Description of the Engineer naturv. Klasse, No. 4, p. 1-148; or see tables 21.1, and 25.1 in Mountain quadrangle [Colorado]: U.S. Geol. Survey Geol. Rankama, Kalervo, and Sahama, Th. G., 1950, Geochemistry: Atlas, Folio 171. Chicago, Univ. Chicago Press, p. 558, 594. Cross, C. W., and Purington, C. W., 1899, Description of the Tel- 1954, Geochemistry: London, Oxford Univ. Press, 730 p. luride quadrangle [Colorado]: U.S. Geol. Survey Geol. Atlas. Goldschmidt, V. M., and Peters, C., 1932c, Zur Geochemie des Folio 57, 19 p. Bors: Nachr. Gesell. Wiss. Gottingen, Math-phys. Klasse, Dane, C. H., 1957, Geology and oil possibilities of the eastern III, p. 402; or see table p. 281, in Goldschmidt, V. M., 1954, side of San Juan Basin, Rio Arriba County, New Mexico: U.S. Geochemistry : London, Oxford Univ. Press, p. 281. Geol. Survey Oil and Gas Inv. Prelim. Map OM-78. Gregory, H. E., 1917, Geology of the Navajo country a recon­ 1960, The boundary between rocks of Carlile and Nio- naissance of parts of Arizona, New Mexico, and Utah: U.S. >brara age in San Juan basin, New Mexico and Colorado Geol. Survey Prof. Paper 93,161 p. (Bradley volume258-A) : Am. Jour. Sci., p. 46-56. Gregory, H. E., 1938, The San Juan country, a geographic and Dobbin, C. E., 1935, Geology of natural gases rich in helium, geologic reconnaissance of southeastern Utah: U.S. Geol. nitrogen, carbon dioxide, and hydrogen sulfide, in Ley, H. A., iSurvey Prof. Paper 188,123 p. ed., Geology of natural gas: Am. Assoc. Petroleum Geologists, Groves, A. W., 1951, Silicate analysis: London, George Alien p. 1053-1072. and Unwin, Ltd., 336 p. LITERATURE CITED 71

Hallimond, A. F., 1943, On the graphical representation of the Luedke, R. G., and Shoemaker, E. M., 1952, Tectonic map of calciferous amphiboles: Am. Mineralogist, v. 28, no. 2, p. the Colorado Plateau: U.S. Geol. Survey TEM-301, issued by 65-89. U.S. Atomic Energy Comm. Tech. Inf. Service, Oak Ridge, Harshbarger, J. W., Repenning, C. A., and Irwin, J. H., 1957, Tenn. Stratigraphy of the uppermost Triassic and the Jurassic rocks Lupton, C. T., 1914, Oil and gas near Green River, Grand County, of the Navajo country: U.S. Geol. Survey Prof. Paper 291, Utah: U.S. Geol. Survey Bull. 541-D, p. 115-133. 74 p. Miesch, A. T., and Riley, L. B., 1961, Basic statistical measures 1958, Stratigraphy of the uppermost Triassic and the used in geochemical investigations of Colorado Plateau Jurassic rocks of the Navajo country, in New Mexico Geol. uranium deposits: Am. Inst. Mining Metall. Engineering Soc. Guidebook, 9th Field Conf., Black Mesa Basin, northeast Trans. (Mining), v. 220, p. 247-251. Arizona, 1958: p. 98-114. Myers, A. T., Havens, R. G., and Dunton, P. J., 1961, A spectro- Herman, George, and Barkell, O. A,, 1957, Pennsylvanian stra­ chemical method for the semiquantitative analysis of rocks, tigraphy and productive zones, Paradox salt basin [Colorado minerals, and ores: U.S. Geol. Survey Bull. 1084-1, p. 207-229. Plateau] : Am. Assoc. Petroleum Geologists Bull., v. 41, no. Noll, W. von, 1934, Geochemie des Strontiums. Mit Bemerkungen 5, p. 861-881. zur Geochemie des Bariums: Chemie der Erde, v. 8, no. 4, p. 507-600; or see table 15.3, in Rankama, Kalervo, and Sahama, Hevesy, G., and Hobbie, R., 1933, Die Ermittlung des Molyban Th. G., 1950, Geochemistry: Chicago Univ. Press, p. 476. und Wolframgehaltes von Gesteinen: Zeitschr. anorg. allgem. Otto, Helmut, 1936, Die Rolle des Mangans in den Mineralien: Chem. no. 212, p. 134-144; or see table 29.2, in Rankama, Mineralog. und petrog. Mitt., no. 47, p. 89-137; or see table Kalervo, and Sahama, Th. G., 1950, Geochemistry: Chicago 31.2, in Rankama, Kalervo, and Sahama, Th. G., 1950, Geo­ Univ. Press, p. 626. chemistry : Chicago Univ. Press, p. 643. Hevesy, G., Hobbie, R., and Holmes, Arthur, 1931, Lead content Peacock, M. A., 1931, Classification of igneous rock series: of rocks: Nature, v. 128, p. 1038-1040; or see table 41.4, in Jour. Geology, v. 39, no. 1, p. 54-67. Rankama, Kalervo, and Sahama, Th. G., 1950, Geochemistry: Pike, W. S., Jr., 1947, Intertonguing marine and nonmarine Chicago Univ. Press, p. 733. Upper Cretaceous deposits of New Mexico, Arizona, and Hevesy, G., and Wiirstlin, K., 1934, Die Haufigkeit des Zirko- southwestern Colorado: Geol. Soc. America Mem. 24, 103 p. niums, Zeitschr, anorg. allgem, Chem. no. 216, p. 305-311: or Rankama, Kalervo, and Sahama, Th. G., 1950, Geochemistry: see table 21.3, in Rankama, Kalervo, and Sahama, Th. G., Chicago Univ. Press, 911 p. 1950, Geochemistry : Chicago Univ. Press, p. 566. Rankin, C. H., Jr., 1944, Stratigraphy of the Colorado group, Holmes, W. H., 1877, Report of the San Juan district, Colorado: Upper Cretaceous in northern New Mexico: New Mexico U.S. Geol. and Geog. Survey Terr. (Hayden), 9th ann. rept., p. 237-276. (School Mines Bull. 20, 27 p. Sandell, E. B., 1947, Determination of gallium in silicate rocks: Houser, F. N., and Ekren, E. B., 1959, Preliminary geologic map Anal. Chem. v. 19, no. 1, p. 63-65; or see table 40.3, in Ran­ of the Moqui SW quadrangle, Montezuma County, Colorado: kama, Kalervo, and Sahama, Th. G., 1950, Geochemistry: U.S. Geol. Survey Mineral Inv. Map MF-216. Chicago Univ. Press, p. 725. Hunt, C. B., 1942, New interpretation of some laccolithic moun­ 1952, The beryllium content of igneous rocks: Geochim. tains and its possible bearing on structural traps1 for oil and et Cosmochim. Acta, v. 2, p. 211-216. gas: Am. Assoc. Petroleum Geologists Bull., v. 26, no. 2, p. 197-203. Sandell, E. B., and Goldich, S. S., 1943, The rarer metallic con­ stituents of some American igneous rocks: Jour. Geology, v. 1956, Cenozoic geology of the Colorado Plateau: U.S. 51, p. 99-115; or see tables in Rankama, Kalervo, and Sa­ Geol. Survey Prof. Paper 279, 99 p. hama, Th. G., 1950, Geochemistry: Chicago Univ. Press, p. 1958, Structural and igneous geology of the La Sal Moun­ 626, 697. tains, Utah: U.S. Geol. Survey Prof. Paper 294-1, p. 305-360. Shapiro, Leonard, and Brannock, W. W., 1956, Rapid analysis Hunt, C. B., Averitt, Paul, and Miller, R. L., 1953, Geology and of silicate rocks: U.S. Geol. Survey Bull. 1036-C, p. 19-56. geography of the Henry Mountains region, Utah: U.S. Geol. Shoemaker, E. M., 1956, Structural features of the central Survey Prof. Paper 228, 234 p. [1954]. Colorado Plateau and their relation to uranium deposits, in Jobin, D. A., 1962, Transmissive character and distribution Page, L. R., Stocking, H. E., and Smith, H. B., compilers, of uranium deposits in the exposed sedimentary rocks of the Contributions to the geology of uranium and thorium by the Colorado Plateau: U.S. Geol. Survey Bull. 1124, 151 p. United States Geological Survey and Atomic Energy Com­ Katich, P. J., Jr., 1951, Recent evidence for Lower Cretaceous mission for the United Nations International Conference deposits in Colorado Plateau: Am. Assoc. Petroleum Geol­ on Peaceful Uses of Atomic Energy; Geneva, Switzerland, ogists Bull., v. 35, p. 2093-2094. 1955: U.S. Geol. Survey Prof. Paper 300, p. 155^170. Kelley, V. C., 1955, Tectonic map of the Colorado Plateau show­ Shoemaker, E. M., and Newman, W. L., 1953, Ute Mountains, ing uranium deposits, in Regional tectonics of the Colorado a laccolithic feature in southwestern Colorado [abs.]: Geol. Plateau and relationship to the origin and distribution of Soc. America Bull., v. 64, no. 12, p. 1555. uranium: New Mexico Univ. Pubs. Geology 5, fig. 2. Simmons, G. C., 1957, Contact of Burro Canyon formation with Larsen, E. S., and Bennan, Harry, 1934, The microscopic deter­ Dakota sandstone, Slick Rock district, Colorado, and cor­ mination of the nonopaque minerals: 2d ed., U.S. Geol. Sur­ relation of Burro Canyon formation: Am. Assoc. Petroleum vey Bull. 848, 266 p. Geologists Bull., v. 41, no. 11, p. 2519-29. Lewis, G. E., Irwin, J. H., and Wilson, R. F., 1961, Age of the Stokes, W. L., 1944, Morrison formation and related deposits Glen Canyon group (Triassic and Jurassic) on the Colorado in and adjacent to the Colorado Plateau: Geol. Soc. America Plateau: Geol. Soc. America Bull., v. 72, p. 1437-1440. Bull., v. 55, p. 951-992. 72 GEOLOGY, PETROLOGY, UTE MOUNTAINS AREA, COLORADO

Stokes, W. L., 1952, Lower Cretaceous in Colorado Plateau: U.S. Department of Agriculture, 1936, Climatic summary of the Am. Assoc. Petroleum Geologists Bull., v. 36, no. 9, p. 1766- United States, Martin, R. J., and Corbin, E., eds.: Section 21, 1776. Eastern Utah, p. 21-1 to 21-23; section 22, Western Colo., Stokes, W. L., and Phoenix, D. A., 1948, Geology of the Egnar- p. 22-1 to 22-33; section 25, Northern Ariz., p. 25-1 to 25-26; Gypsum Valley area, San Miguel and Montrose Counties, section 27, Northwestern N. Mex., p. 27-1 to 27-26. Colorado: U.S. Geol. Survey Oil and Gas Inv. Map OM-93. Wager, L. R., and Mitchell, R. L., 1943, Preliminary observa­ Strobell, J. D., Jr., 1956, Geology of the Carrizo Mountains area tions on the distribution of trace elements in the rocks of the in northeastern Arizona and northwestern New Mexico: U.S. Skaergaard intrusion: Greenland Mining Mag., v. 26, no. 180, Geol. Survey Oil and Gas Inv. Map OM-160. p. 283-296. Sukheswala, R. N., and Poldervaart, Arie, 1958, Deccan basalts Wahlstrom, E. E., 1947, Igneous minerals and rocks: New York, of the Bombay area, India: Geol. Soc. America Bull., v. 69, John Wiley & Sons, Inc., 367 p. no. 12, p. 1475-1494. Wanek, A. A., 1959, Geology and fuel resources of the Mesa Thaden, R. E., and Santos, E. S., 1956, Grants, New Mexico, Verde area, Montezuma and La Plata Counties, Colorado: Project, in Geologic investigations of radioactive deposits, U.S. Geol. Survey Bull. 1072-M, p. 667-721 [I960]. Semiannual progress report, June 1 to Nov. 30, 1956: U.S. Weir, D. B., 1952, Geologic guides to prospecting for carnotite Geol. Survey TEI-640, p. 73-76, issued by U.S. Atomic Energy deposits on Colorado Plateau: U.S. Geol. Survey Bull. 988-B, Comm. Tech. Inf. Service, Oak Ridge, Tenn. p. 15-27. Thornton, C. P., and Tuttle, O. F., 1956, Applications of the Wengerd, S. A., and Strickland, J. W., 1954, Pennsylvanian differentiation index to petrologic problems tabs.]: Geol. Soc. stratigraphy of Paradox salt basin, Four Corners region, America Bull., v. 67, no. 12, pt. 2, p. 1738-1739. Colorado and Utah: Am. Assoc. Petroleum Geologists Bull., Tongeren, W. van, 1938, Contributions to the knowledge of the chemical composition of the earth's crust in the East Indian v. 38, no. 10, p. 2157-2199. Archipelago. II. On the occurrence of rarer elements in the Wright, J. C., Shawe, D. R., and Lohman, S. W., 1962, Defini­ Netherlands East Indies: Amsterdam, D. B. Centen's Uit- tion of members of the Entrada Sandstone in east-central gevers-Maatschappij, 181 p. or see table 18.4, in Rankama, Utah and west-central Colorado: Am. Assoc. Petroleum Geolo­ Kalervo, and Sahama, Th. G., 1950, Geochemistry: Chicago gists Bull., v. 46, p. 2057-2070. Univ. Press, p. 527. Zabel, V. H., 1955, Geology of McElmo dome, Montezuma County, Turner, F. J., 1942, Determination of extinction angles in mono- Colorado, in Four Corners Geol. Soc. Guidebook Field Conf., clinic pyroxenes and amphiboles: Am. Jour. Sci., v. 240, no. 8, Geology of parts of Paradox, Black Mesa, and San Juan p.571-583. Basins, 1955: p. 132-136. INDEX

[Italic page numbers indicate major references]

Page Page Accessibility of area. . ______2 Climate. ______4 Hydrothermal hot-springs alteration, breccia Acknowledgments,. __ 5 Coal--. . 69 pipes 35 Agriculture 4 Cobalt.... . 38,40,42,44,67 Alkali-lime indexi ...... 37 Coffin's prospect.... ______66 Alluvium __ __ 26 Copper 40,42,44,66,67 Igneous activity, age..... _..._ 27 Alteration, general...... SB Cretaceous System.... ______18 Igneous rocks, chemical composition_____ 35 variation of minor elements . 40 Culture.. ______4 description. ..- - . 27 Altitude...... 4 inclusions..-.____ ._...___ 33 Alumina. __ 37 D occurrence_------_ .- _ _..- 28 Analcime...... -.-...... 33 Dakota Sandstone ______20,43 origin___ . 45 Analyses, coal. 69 Dense, defined.-.. ______28 relation of the metallic mineral deposits_ 67 hornblende. 48 Deuteric alteration.. ______35 Igneous rock series, variation.....__.__ 38 igneous rocks 35,40,42 Dewey Bridge Member, Entrada Sandstone. . 8 Inclusions...-______.__-___ 33 samples from radioactive prospects... 63 Dikes-.-... .. 50 Inoceramusdeformis-...... 24.25 Andesine . 31,33,58 Diopside-. __ . __ .. _ ------______dimidius...... 24 Antimony. __ . _ .. 67 Diorite porphyry ______labiatus.------_ 24 Apatite 28,30,32,35 Disconformity, top of the Burro Canyon perplezus...... - 24 Arsenic _ 67 Formation ______19 Intrusive bodies--_ _ . . 52 Augite...... 28,30,31,33,35,36,45,49 Intrusive rocks, relative ages.-. - - 27 E Iron- 37,67 East Horse laccolith 56 Irwin laccolith....__.__...... 56 Banded laccolith..... _ . -_ 58 East Sundance laccolith. 56 Barite 66,67 Economic geology. . ..-. ___ . 61 Barium______.___ 40,67 Engel, C. G., quoted 49 Battle Rock mine. _ __ 66 Enstatite _ . __ ... _ . . __ 35 Juana Lopez Member, Mancos Shale ____ 24 Berthold, S. M., analyst______-___ 48 Entrada Sandstone -. 8 Junction Creek Sandstone______-__ 12,43 Beryllium 40 Epidote...... 32,35 Jurassic rocks______-._ 7 Bilk Creek Sandstone Member, Wanakah Jurassic System. _ 8 Formation______8 See also Entrada Sandstone. Fanglomerate....______26 Biotite... .. 28,30,31,32,34,35 Faults______50,52 Kaolinite______32 Black Mountain, altitude..______4 Ferrosilite...... 35 KarlaKaymine..------18,64 doming... .. 50 Fieldwork.... 5 Kiva section___.. -- . 21 Black Mountain stock______53 Flat laccolith. 57 Knees stock.__ . . 53 Block rubble_ ...... 27 Folds.. . 50 Blufl Sandstone- ...... 12 Forsterite--..______.____.___ 35 See also Junction Creek Sandstone. Fossils- -. . -_...... 8,23,24 Labradorite ______-_-____-. 45 Boron___..___..______40 Laccoliths_-_.-___--. ...------... 56 Breccia pipes, description_.. 59 G Lamprophyre..------32 hydrothermal hot-springs alteration.. __ 35 Gallium.---.-.---.-.-.---___.____._ 40 Landslide deposits_... - 26 Brushy Basin Shale Member, Morrison For­ Geography.______2 Lanthanum-.__ __------_ ...: 40 mation.______13, IB Geologic history ...-. .. __.__ 60 La Plata Mountains, alkali-lime index._... 38 Burro Canyon Formation, comparison with Geologic setting.--...______.__ _ 5 La Sal Mountains, igneous rocks.-..__ . 47 Dakota Sandstone______'... 22 Glen Canyon Group.______7 Last Spring laccolith...... 57 description._ __ __ 18 Granodiorite porphyry.______30 Lead._-_-_ . . . 40,67 Buttes laccolith___.______-...__ 69 Gravel, terrace.. ______..._ 26 Leucocratic diorite porphyry. ______, 30 Bysmaliths______55 Great Sage Plain______5 Lime______37 Greenhorn Limestone Member, Mancos Shale. 24 Limonite______35, 65,66 Gryphaea...... 23,24 Lithology, Burro Canyon Formation. . ... 18 Calcite... - -- 31,32 Gryphaea newberryi...... 24 Dakota Sandstone.. .._ . 20 Camp, C. L., quoted.... . - 8 Gypsum...---...____.... _ ... 65 Entrada Sandstone.-. ------8 Carbon dioxide------..-..-.------68 Junction Creek Sandstone ------__ -. 12 Carnotite. - - 65 II Mancos Shale_... . - 23 Carrizo Mountains, alkali-lime index. ... 38 Hazell, K. V., analyst.. - 48 Morrison Formation___ _.._.. 13 Chemical composition, igneous rocks...___ 35 Hematite__.______35 Navajo Sandstone...... 7 Chlorite -- - - 28,30,31,32,36 Henry Mountains, doming______. 51 Summerville Formation_._... _.. 10 Chromium.. 40,42,44,49,67 hornblendic inclusions______47 Little Maude mine.... ______...... 66 Clifl House No. 2 prospect. .. 63 Hermano Peaks, altitude.---.------4 No. 4 prospect______63 Hornblende--- 28,30,31,34,35,36,45,58 M Clifl House prospect..______62 Hornblende inclusions, origin_ - _____ 45 Clifl House Sandstone______25 Horse Mountain laccolith....____..._. 56 Mable Mountain, description...______. 55 Clifl houses, Entrada Sandstone, Sand Creek Hunt, C. B., quoted _-... 47 hydrothermal alteration...._.._... 35 area.______.___ 10 Hydrothermal alteration, Mable Mountain... 35 Mable Mountain bysmalith...... __. . 40 73 74 INDEX

Page Page Page McElmo Canyon, lower section______. 7,16 Pre-Navajo rocks 7 Sundance cluster of laccoliths. 56 upper section______10,11 Previous work ______. 5 Sundance laccolith...... 56 MeElmodome, description..______50, 61 Prinocydus wyomingensis...... 24 oil 69 Ptychodwswhipplei...... 24 Magnesia______37 Purpose and scope of investigation 5 Talus______-___ 26 Magnesium______36 Pyrite 31,35,67 Temperature- __-- _ . 4 Magnetite...... 28,30,31,32,35 Pyritization, igneous contacts.. __ . 35 Three Forks laccolith_..__ ...... 57 Mancos Shale...______.______... 23,44 Mable Mountain ______67 Three States Natural Gas Co. prospect. 63 Manganese______38,40,67 Pyrolusite- ______. ______65 Thomsonite..____-.___--__ 33 Mapping...... ____....._ 5 Pyroxene... ______36,49 Titanium.__.__... ___ __...... 38,40 Marcasite______67 Tongue laccolith....--..---___ 58 Menefee Formation______.. 25 Topography______.. 4 Q Mesaverde Group....____.______25 Towaoc dome..-.--_-----__ _._... 50 Metalh'c mineral deposits. ______61 Quartz.-.- . - . -- - 28,31,32,35 Tozer Gulch, section...--.-__-___. 19 Metamorphism, Black Mountain stock____ 53 Quartz monzonite porphyry. .__ __ --. ___ 31 Trail Canyon, section. _ . ______17 The Knees stock....______54 Quaternary System ______. 26 Trapdoor laccolith.. ______56 Ute Peak. - . . . 54 Triassic rocks______-----_.... 7 Microgabbro____.______28,47 R Minor elements______38 RainfaU...... -. 4 U Modes, diorite porphyry______... 30 Razorback laccolith..... ______28,57 Uranium. -- 42,61,62,67 granodiorite porphyry______31 Recapture Shale Member, Morrison Forma­ Uranium potential.______-_____ 67 inclusions...._._.__.___ _... 34 tion.... - ... . 13, 14 Ute Creek dike.. 55 quartz monzonite porphyry...... -.... 33 Rock glacier ____ ... ______26 Ute Creek dike prospects______. 66 spessartite______._ 33 Ute dome, description..''.. __ -____ 60 Molybdenum. . . ..-...... 67 oil. . - 69 Morrison Formation..______..._ 13 relation to McElmo dome. Mound section..__.____.__.____ 23 Salt Wash Sandstone Member, Morrison For­ Ute Peak, altitude...... _.. Mushroom laccolith...... __._._...... 58 mation.. .-- ...... 13, 14 description..__...__ Sand Creek, section...______9,11,12 N Sandidine.. .. --. 31 Natural gas._.__..__...... 68 San Rafael Group. ______8 Vanadium.... 40,44,61,67 Navajo Sandstone______.___ 7 Scandium..______40 Variation diagrams____. __ 37 Newman, W. L., quoted.. _ 66 Sedimentary rocks, stratigraphy._ 6 Vegetation....._._..____.. .__.. 4 Nickel... . 38,40,42,44,49,67 variations in adjacent intrusives 43 Niobium.... 40,67 Segisaurus halli...,. ______.. .._. 8 W Nonmetallic mineral deposits______.... 68 Sentinel Peak.______55 Wells, oil and gas.------___ . .. 51,68 Normal diorite porphyry. . . 30 Sentinel Peak dome.-.___-______50 West Toe, bysmalith______.____. 66 Norms, igneous rocks...___....._ . - 36 Sericite. . - 32,35 West Toe cluster of intrusive bodies_. 58 North Black Mountain Laccolith ______69 Sills. - . .- 55,58,55 Westwater Canyon Sandstone Member, North Ute Peak laccolith-...... 66 Silver...... - ...... 67 Morrison Formation______13, 16 Slick Rock Member, Entrada Sandstone__ 8 Windblown silt andsand...._ .... 26 O Spessartite-.._____-______32 Wollastonite... - 35,36 Olivine...... 35,36 Sphene______30 Wood Chuck section. ___..._____._ 18 Origin, igneous rocks and hornblende Stocks -..-__...______.... ._-. 63 inclusions....__..-.._ . 45 Stratigraphic section, Burro Canyon Forma­ Ostrea lugubris...... 24,25 tion....-. __.__...__-__ 18 Xenoliths. 33 Dakota Sandstone.____.______20 Entrada Sandstone______9 Pack Trail laccolith.._..__.. ___ 58 Junction Creek Sandstone.. ______12 Ytterbium.... - . 40,44,67 Paradox Formation_.______51 Mancos Shale.--.___--____-_- 23 Yttrium 40,44,67 Petrography, igneous rocks______... t8 Morrison Formation.______16 Yucca cluster of laccoliths.------___.. 69 Petroleum.______.__ 68 Navajo Sandstone______.____ 7 Yucca section.. .__ __...... __.. 20 Petrology ..-.. _ S6 Summerville Formation______11 Pishel, M. A., quoted-,______69 Stratigraphy, sedimentary rocks__.-__-.- 6 Plagioclase...... -- . 28,30,31,32,35,58 Strontium. .. 40,44,67 Zeolite- - 33 Point Lookout Sandstone...__.__..__ 25,06 Structure___-..___-____-_..-..___ 60 Zinc------67 Potassium feldspar..______- 35 Summerville Formation______.__ 10 Zircon. - 31,40,49 O The U.S. Geological Survey Library has cataloged this publication as follows: Ekren, Einar Bartlett, 1923- Geology and petrology of the Ute Mountains area, Colo­ rado, by E. B. Ekren and F. N. Houser. Washington, U.S. Govt. Print. Off., 1965. v, 74 p. illus., maps (1 fold. col. in pocket) 30 cm. (U.S. Geological Survey. Professional paper 481) Prepared on behalf of the U.S. Atomic Energy Commission. Bibliography: p. 69-72. (Continued on next card)

Ekren, Einar Bartlett, 1923- Geology and petrology of the Ute Mountains area, Colo­ rado. 1965. (Card 2) 1. Geology Colorado Montezuma Co. 2. Petrology Colorado Montezuma Co. 3. Rocks Analysis. 4. Mines and mineral re­ sources Colorado Montezuma Co. I. Houser, Frederick Northrop, 1924- joint author. II. Title. III. Title: Ute Mountains area, Colorado. (Series)