TM-D- 1-18
U. S. ATOMIC ENERGY COMMISSION
DIVISION OF RAW MATERIALS
DENVER AREA OFFICE
PRELIMINARY REPORT ON THE
GEOLOGY OF URANIUM DEPOSITS IN THE
BROWNS PARK FORMATION IN MOFFAT COUNTY,
COLORADO, AND CARBON COUNTY, WYOMING
By
Allen Ormond
"This report was prepared as an account of work sponsored by the United States Government. Neither the United States nor the United States Atomic 3aergy Ocmission, nor amy of their employees, nor any of their contractors, subcontractors, or their iaployees, make any warrantyr, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness or usefulness of ay infor- nation apparatus, product or process disclosed, or represents that its use would not infringe privately- owned rights."
Casper, Wyoming June 1957
metadc1 393321 CONTENTS
ABSTRACT...... 1
INTRODUCTION...... 1
Location and Accessibility...... 1 Climate, Topography, and Drainage . .. . . 3 History of Uranium Operations...... 3 Land Ownership...... 3 Previous Investigations...... 4 Map Coverage...... 4 Purpose and Scope...... 4 Acknowledgments...... 5
GENERAL GEOLOGY...... 5
Geologic History ...... 5 Stratigraphy ...... 5 Precambrian ...... 5 Paleozoic...... 6 Mesozoic...... 6 Tertiary ...... 6 Structure...... 8
URANIUM DEPOSITS OF THE MAYBELL AREA . 10
Marge and Gertrude Depos:its...... 10 Location ...... 10 Extent ...... 10 Mine Workings . . .. 10 Structure...... 11 Host Rock...... 11 Mineralogy...... 13 Equilibrium Factors .. 18 Sugar Loaf Mine . . .. 18 Three Sisters Prospect . 18 White Star Prospect . .. 21 Buffalo Head Prospect .. 21 Cedars Prospect...... 21 Leon Prospect...... 21 Agnes Prospect...... 21 September Morn Prospect 22
URANIUM DEPOSITS OF THE POISON BASIN AREA
Cedar Hills Deposit...... 22 Location ...... 22 Extent ...... 22 Page
Mine Workings...... 22 Structure...... 22 Host Rock...... 22 Mineralogy...... 23 Poison Basin Deposit ...... 23 Location...... 23 Extent...... 23 Mine Workings...... 23 Structure...... 23 Host Rock...... 23 Mineralogy...... 27 Teton Prospect ...... 27 Section 36 Prospect...... 27
PARAGENESIS...... 27
ORIGIN OF THE URANIUM...... 28
SUMMARY...... 29
REFERENCES ...... 30
- ii - ILLUSTRATIONS
Plate Pg
1. Geologic map showing the distribution of the Browns Park Formation in northwest Colorado and south-central Wyoming...... 31
Figure
1. Index map of south-central Wyoming and northwest Colorado...... 2
2. Geological map of the Maybell re, Moffat County, Colorado...... 9
3. Geological map of the Baggs area, Carbon County, Wyoming...... 9
4. Geological map of the open pit at the Marge mine as of March, 1957, Moffat County, Colorado . . . . 12
5. Generalized geologic section A-A' Marge open pit, Marge No. 1 and Maybell No. 1 claims, Moffat County, Colorado...... 14
6. Generalized geologic section B-B' Marge open pit Marge No. 1 and Maybell No. 1 claims, Moffat County, Colorado...... 15
7. Association of uranium, limonite and calcium carbonate in Browns Park sandstone in the Marge mine , Marge claim No. 1, section 24, T . 7 N. , R. 95 W., Moffat County, Colorado...... 16
8. Concentric rings of visible uranium mineral in Browns Park sandstone in the Gertrude mine, Gertrude No. 6 claim, section 17, T. 7 N., R. 94 W., Moffat County, Colorado...... 17
9. Geological cross section C-C' Marge open pit Marge No. 1 claim, section 24, T. 7 N., R. 95 W. , Moffat County, Colorado...... 19
10. Geological cross section D-D' in the Marge mine, Maybell No. 1 claim, sec. 19, T. 7 N., R. 94 W., Moffat County , Colorado...... 20
- iii - 11. Geological cross section A-A' No. 1 open pit Cedar Hills No. 6 claim, section 32, T . 13 N. , R . 92 W ., Carbon County Wyoming...... 24
12. Geological cross section A-A' No. 2 open pit Cedar Hills No. 10 claim, section 32, T . 13 N. , R . 92 W ., Carbon County Wyoming...... 25
13. Geological cross section B-B' No. 2 open pit Cedar Hills No. 10 claim, section 32, T . 13 N. , R . 92 W ., Carbon County , Wyoming...... 26
- iv - TM-D- 1-18
GEOLOGY OF URANIUM DEPOSITS IN THE BROWNS PARK FORMATION IN MOFFAT COUNTY, COLORADO, AND CARBON COUNTY, WYOMING
ABSTRACT
Uranium was first discovered in the Browns Park Formation in 1951 in the Miller Hill area of south-central Wyoming . Since that time economical- ly important deposits in this formation have been discovered and developed in the Poison Basin of south-central Wyoming and in the Maybell area of northwest Colorado.
The Browns Park is the youngest formation (Miocene) in the region and overlies older rocks with angular unconformity . The formation consists of a basal conglomerate, fluviatile, lacustrine, and eolian sandstones, and locally a few thin beds of clay, tuff, and algal limestone. The sandstones are predominantly fine- to medium-grained and consist of quartz grains, scattered black chert grains, and interstitial clay.
The uranium deposits are of the sandstone-impregnation type and are not confined to specific stratigraphic horizons. The important ore minerals are autunite and uranophane in oxidized sandstones, and uraninite and coffinite in unoxidized sandstones. Uranium is often associated with limonite and calcium carbonate in concretionary forms. Woody material, thought to play an important part in the deposition of uranium in many sandstone-type deposits, is not present in the deposits of the Browns Park Formation. However, organic carbon in the form of petroleum and petroleum residues has been observed in association with uranium in both the Poison Basin and the Maybell areas.
INTRODUCTION
Location and Accessibility
The Maybell area is in the east-central part of Moffat County, in the northwest corner of Colorado (fig. 1). The area is served by U. S. Highway 40, which crosses the county in an east-west direction. All parts of the area are accessible by state secondary roads or improved trails . Rail service is available from Craig, Colorado, to Denver and Grand Junction, Colorado.
The Poison Basin area is in the southwest corner of Carbon County, in the south-central part of Wyoming (fig. 1). The area is accessible from state highway 789 by six miles of dirt road. The closest rail he ads are Craig , 45 miles to the south , and Rawlins , Wyoming , 80 miles to the northeast.
- 1- Medicine Bow
S W E E T W A T E RRawlins1 - RBA0LN I.A.N.Y C A
-- 130 tAL B A Y
4789 /ooi
POISON B ASIN A A3
______Bogs _ __WYOMING 1 COLORADO UTAH JMAYBELL AREA
0 20 40
------0MJL E S
TM- 0-1-18
Figure I, Index map of south- central Wyoming andad nrhetnorthwest ColoradoClao Climate, Topography, and Drainage
The climate is semiarid, the average precipitation being about 16 inches annually , principally as snow . The approximate temperature range is - 400 F to 1000 F.
The topography of the region is characterized by rolling hills and a few prominent mountains and ridges. Altitudes range from about 6,000 to 8,000 feet and average about 6,300 feet. The Maybell area is drained by the Yampa River, a westward-flowing perennial stream. The Poison Basin area is drained by the Little Snake River, a southwestward-flowing perennial tributary which joins the Yampa west of Maybell, Colorado. Most of the tributary streams are dry during the summer months. Wells and reservoirs , the only other sources of water , are private, and water from them is usually not available for use in mining or drilling .
History of Uranium Operations
Uranium was first discovered in the Browns Park Formation in the Miller Hill area (fig. 1) of south-central Wyoming in October 1952. Discoveries were made in the Poison Basin area in October 1953, and in the Maybell area in March 1954. After the initial discoveries, exploration drilling and mining was begun. Economically important deposits were developed in the Poison Basin area by the Trace Elements Corporation, now wholly-owned subsidiary of the Union Carbide Nuclear Corporation, the King Oil Company , which now operates the Trace Elements Corporation property, and the Shawano Development Corporation. Economically important deposits were developed in the Maybell area by the Trace Elements Corporation, and further development was done by the Union Carbide Nuclear Corporation. Extensive exploration was also carried out by many other companies, including Utah Construction Company, Teton Exploration Drilling and Development Company, James Eskridge and Associates, Thunderbird Development Company, and American Leduc Corporation.
In the Maybell area, Trace Elements Corporation began regular shipments of ore to the processing mill at Rifle, Colorado, from the Marge mine, in October 1955, and from the Gertrude mine in May 1956 . In the Poison Basin area, the Cedar Hills mine , operated by the King Oil Company, began ore shipments in May 1956. A uranium processing plant near Maybell, Colorado, operated by the Trace Elements Corporation, treats present production from the areas; the first shipment of ore was received at the Maybell mill in December 1957.
Land Ownership
Most of the surface and mineral rights are part of the public domain. Sections 16 and 36 of every township are designated as school sections and are controlled by the state .
- 3- Pr evious Investig ations
Earliest geological studies were conducted by the U. S. Geological Survey and were of a regional nature, covering northwestern Colo- rado and south-central Wyoming . Sears (1924) investigated the geology and oil and gas prospects of part of Moffat County, Colorado, and southern Sweetwater County, Wyoming. Bradley (1936 and 1945) studied the geomorphology of the north flank of the Uinta Mountains and the geology of the Washakie Basin. In 1954, J. D. Vine and G. E. Pritchard, U. S. Geological Survey, reported on uranium in the Poison Basin area. M. J. Bergin (1959) of the U. S. Geological Survey has prepared a geologic map of the Maybell area, with accompanying text.
The U. S. Atomic Energy Commission conducted airborne scintillation surveys for uranium in the Poison Basin area (Magleby and Mallory, 1954), in both the Maybell and Poison Basin areas in 1954, and in the Maybell area in 1955. In 1955, a uranium exploration drilling program was completed by the Atomic Energy Commission in the Maybell area.
Ma Coverage
Geological maps covering northwestern Colorado include the "Geologic Map of Colorado" issued by the U. S. Geological Survey in 1935; "Geological Map of Part of Moffat County, Colorado, and Sweetwater County, Wyoming," by Sears (1924); and the "Generalized Area Geological Map, Northwest Colorado and Adj acent Southern Wyoming," in the Guidebook to the Geology of Northwest Colorado (1955) issued jointly by the Intermountain Association of Petroleum Geologists and the Rocky Mountain Association of Geologists. South-central Wyoming is covered by the "Geologic Map of Wyoming" issued by the U. S. Geological Survey in 1955 and "Geological Map of the Washakie Basin, Sweetwater and Carbon Counties, Wyoming, and Moffat County, Colorado," by Bradley (1954). The U. S. Geological Survey has made available a detailed large-scale geologic and topogra- phic map of the Maybell area (Bergin, 1959). Complete aerial photo coverage for both areas is available from the U. S. Department of Agriculture.
Purpose and Scope
The U. S. Atomic Energy Commission undertook a program of geological and engineering studies of uranium deposits in the Browns Park Formation in northwestern Colorado and south-central Wyoming to determine the uranium potential of the area and to consider the genesis of deposits and ore guides. This report includes the results of areal geological investigations and mine mapping done during 1955 and 1956 in the Maybell and the Poison Basin areas.
- 4- Acknowledgments
The writer is indebted to personnel of the Trace Elements Corporation and King Oil Company for their cooperation during this project. Thanks are extended to M. J. Bergin and G. E. Pritchard of the U. S. Geological Survey for the use of data from their geologic maps. Dr. John W. Gruner, Department of Geology, University of Minnesota, made the mineral identifications under contract to the U. S. Atomic Energy Commission.
GENERAL GEOLOGY
Geologic History
The evidence of earliest structural deformation in northwest Colorado is in the Steamboat Arch, an east-west trending uplift which was formed in post-Cambrian time and persisted well into the Pennsylvanian (Crowley, 1955). Many thousands of feet of sediments were deposited in northwest Colorado and south-central Wyoming during Paleozoic time , and, except for a period of minor movements during the Pennsylvanian, marine and continental deposition was continuous throughout the Paleozoic and Mesozoic to the close of the Cretaceous period. There is no evidence of major orogenic movements in the region from the Pennsylvanian to the beginning of the Laramide orogeny.
In early Tertiary time, the Uinta and Rocky Mountains were uplifted. Faulting and folding took place and were followed by a period of quiescence in early Eocene time, when the mountains were peneplaned. In late Eocene further orogeny took place and peneplanation truncated folded Eocene strata and older rocks (Crowley, 1955).
The second peneplanation was followed, in Miocene time, by a period of differential uplift and collapse, during which time the Browns Park Formation was deposited (Crowley, 1955). The Browns Park Formation, which is marked by a basal conglomerate, rests upon all older formations with angular unconformity . Because of widespread volcanism during Miocene time, the Browns Park Formation contains large amounts of tuffaceous material.
A period of vulcanism occurred in late Miocene time. Browns Park sandstones are capped by lava flows in the Elkhead Mountains and at Cedar Mountain, northwest of Craig, Colorado.
Stratigraphy
The following summary of stratigraphy includes formations exposed in northwestern Colorado and south-central Wyoming (p1. I):
Precambrian
Precambrian rocks of the Uinta Mountain Group, consisting of conglom- erate sandstone, quartzite, and red sandy shale, are exposed in Juniper and Cross Mountains in the Maybell area.
- 5- Paleozoic
The only Paleozoic rocks in the Maybell-Poison Basin area crop out in the Juniper and Cross Mountains near Maybell and consist primarily of sandstone and quartzite of the Cambrian Sawatch Quartzite and of limestone, dolomite and shale of the Mississi- pian Madison Limestone and the Pennsylvanian Morgan Formation and Weber Sandstone.
Mesozoic
Triassic and Jurassic formations , exposed only on Juniper and Cross Mountains , have been mapped as undifferentiated sandstones, siltstones, and shales. Most of the Browns Park Formation in the Maybell area is underlain by Upper Cretaceous strata. Cretaceous formations, conformable with each other, are as follows:
Mancos Shale: The Mancos Shale, lowermost of the Upper Cretaceous formations, consists chiefly of gray marine shale, calcareous in the lower part and sandy in the upper part. Massive sandstone beds occur near the top of the formation.
Mesaverde Group: The Iles and Williams Fork Formations of the Mesaverde Group consist of interbedded marine shales, marine and continental sandstones, and coal beds .
Lewis Shale: The Lewis Shale is a gray marine shale, calcareous in places, which contains small, lenticular masses of limestone and ferruginous concretions .
Lance Formation: The Lance consists of continental gray to yellow shales, thin coal beds, and massive discontinuous cross-bedded sandstones . Thick massive sandstone marks the top of the formation in many places.
Tertiary
Tertiary formations, ranging in age from Paleocene to Miocene, are unconformable with each other and with underlying Cretaceous strata. They cover a large part of the map area (p1. I).
Fort Union Formation: The Paleocene Fort Union Formation consists chiefly of sandstone, shale, carbonaceous shale, lignite, and coal beds . A conglomerate is present at the base of the formation. The Fort Union underlies the Browns Park Formation in some parts of the Maybell area and east of the Poison Basin area.
Was atch Formation: The Was atch Formation, Eocene in age, consists of sandstone and variegated shale in this region. In the Poison Basin area the relatively thin upper member, the Cathedral Bluffs Tongue, is separated from the main body of the Was atch Formation by the Tipton Tongue of the Green River Formation. The Wasatch Formation covers most of the region between the Poison Basin and Maybell are as . - 6- Green River Formation: This Eocene formation, immediately overlying the Wasatch Formation, covers large areas in northwestern Colorado and south-central Wyoming, west of the Maybell and Poison Basin areas. It consists predominantly of lacustrine marlstone, oil shale, and carbonaceous shale. The Tipton Tongue, which is separated from the main body of the formation by the Cathedral Bluffs Tongue of the Was atch, consists of soft brown papery shales.
Bridger Formation: The Bridger Formation (Eocene) consists of gray-green claystone and shale containing thin beds of limestone. This formation also covers large areas west of the Poison Basin area.
Browns Park Formation: The Browns Park Formation (Miocene) of continental origin, consists of fluvial, lacustrine, and eolian sandstones. It is everywhere marked by a basal conglomerate. Theobald and Chew (1955) recognize three major areas in the Browns Park Formation. Each area is characterized by a different lithology.
The area west of the Little Snake River in Colorado is characterized by fluvial sandstones containing some tuffaceous material. In the Browns Park Formation east of Baggs, Wyoming, lacustrine deposits consisting of clayey sandstones and siltstones and more abundant tuffaceous material are predominant. Separating the eastern and western areas is a belt of eolian sandstone interbedded with both fluvial and lacustrine sandstones containing some tuffaceous material.
The Browns Park Formation once covered extensive regions in southern Wyoming and northwest Colorado from which it has since been eroded away.
In the Maybell area the formation reaches a maximum thickness of 800 feet. The basal conglomerate, which has a maximum thickness of 150 feet, consists of pebbles and cobbles of schist, gneiss, granite, quartzite, vein quartz, and volcanic rocks, in a finer grained matrix of similar materials . A few fine- to coarse-grained thin sandstone beds are intercalated with the conglomerate. The upper part of the formation consists of fine-grained soft sandstones with varying amounts of interstitial clay; medium-grained soft friable cross-bedded sandstones with sparse amounts of interstitial clay; calcareous sandstones; silicified sandstones; tuffaceous sandstones; tuff beds; and clay beds .
In the Poison Basin area the formation is between 300 and 400 feet thick. The basal conglomerate averages about 75 feet thick and consists of quartzite pebbles and cobbles (Bradley, 1945). Above the conglomerate are medium-grained sandstones with interstitial clay , and calcareous and silicified s andstones .
In the Maybell and Poison Basin areas the primary constituents of the Browns Park sandstones are subrounded to rounded quartz
- 7- grains and scattered grains of black chert. Interstitial clay is present in most Browns Park sandstones. The amount varies consid- erably from bed to bed but is fairly uniform within individual beds. The amount of interstitial clay is small in the medium-grained cross-bedded sandstones and increases with a decrease in grain size and degree of cross-bedding . The relatively clay-free sandstones are commonly light gray to buff; sandstones with a higher clay content are commonly dark buff, greenish buff, or light brown.
The calcareous sandstones are lenticular . The lenses are for the most part only a few feet thick, although some are several hundred feet in lateral extent.
Oxidized and unoxidized zones in the Browns Park Formation can be easily differentiated. The buried, unoxidized sandstones are blue and gray because of an abundance of very finely divided and well distributed pyrite (Gruner, 1956). The sandstones on and near the surface are light gray to buff or brown because of limonite and j arosite resulting from the oxidiation of the pyrite. The contact between oxidized and unoxidized zones ranges in depth from 20 to 300 feet.
Structure
The dominant structural feature of the Maybell area is the Axial Basin anticline, which is the eastward extension of the Uinta Mountain arch. West- and northwest-trending faults and folds are characteristic of both the Maybell and Poison Basin areas.
The Browns Park Formation in the Maybell area unconformably overlies truncated Precambrian, Paleozoic, Mesozoic, and early Cenozoic strata forming the Axial Basin anticline. A shallow west- trending syncline characterizes the Browns Park outcrop. Dips in the Browns Park Formation range from 10 to 30 degrees on the northern and southern margins of the syncline. The basal conglom- erate is observable only in these marginal areas. The basal contact dips steeply to the south in the northern part of T. 7 N., R. 94 W., as shown by the fact that holes drilled to a depth of over 500 feet, less than a mile from the north margin of the formation, did not reach the bottom of the Browns Park Formation.
Most of the faults in the Browns Park Formation in the Maybell area strike between N. 50 E. and west (fig. 2). The U. S. Geologi- cal Survey established, by seismograph studies, the presence of many northwest-striking faults that were not evident in surface outcrops. Several of these faults have inferred displacements of approximately 250 feet.
In the Maybell area the most prominent joint system in the Browns Park Formation strikes N. 60 W. Minor joint systems strike between N. 100 W. and N. 300 W.
- 8- TbTb
\hiSu Sr ?
e TbsCedars prospect
MILES~ eon PropeciXg______Fiue2.Googclma fth Tabel araTbfaponyClrd
EXPLANATIO
Ts* Strike B dip of bedding 42
TM -D-l-i8
bp6 Tp Poison Tbp
Tw mTw
0 I / 2 3
MILES
Figure 3. Geological map of the Baggs area, Carbon County, Wyoming
-9- In the Poison Basin area (fig . 3) a zone of east-trending faults and folds extends from near Baggs westward for about 50 miles along the southern margin of the Washakie Basin. The folding is pre-Browns Park in age (Bradley , 1945), but most of the faults displace the Browns Park Formation. In the Poison Basin area, the Browns Park Formation overlies the gas-bearing Baggs anticline in the Green River and Wasatch Formations .
URANIUM DEPOSITS OF THE MAYBELL AREA
The significant uranium deposits in the Maybell area are near north and south highway U .5. 40 between the towns of Maybell and Lay , Colorado. A few minor deposits occur as far east as Craig, Colorado.
Marge and Gertrude Deposits
Location
The Marge deposit is in secs. 18 and 19, T. 7 N., R. 94 W., and secs. 13 and 24, T. 7 N., R. 95 W., Moffat County, Colorado (fig. 2). The Gertrude deposit is in sec. 17, T. 7 N., R. 94 W., Moffat County, Colorado. Both deposits are owned by Trace Elements Corpor ation . Extent
The deposits are made up of many individual ore bodies. The bodies are blanket-like, with lateral dimensions greater than the thicknesses. They are roughly parallel to the bedding and highly irregular in outline. Some of them are elongated along faults. The ore bodies occur at several stratigraphic positions and range from a few feet to more than 250 feet in depth.
The Marge deposit comprises at least four separate ore bodies. The long axis of the deposit trends about N. 150 W. Individual tabular ore bodies range in depth from 10 to 150 feet and are from 2 to 18 feet in thickness. The Gertrude deposit is elongate east- west and is composed of several individual ore bodies in different stratigraphic positions . The thickness ranges from about 2 to over 20 feet.
Mine Workings
Exploratory shafts were sunk in the Marge and Gertrude deposits in 1955. The Marge shaft was sunk to intersect an ore body at a depth of 60 feet, where sampling drifts were driven. The shaft in the Gertrude deposit was sunk to a depth of 140 feet and intersected three separate ore horizons, at depths of 31, 97, and 123 feet, from which short drifts were driven to obtain samples for milling tests.
- 10 - Excavation of an open pit was started in October 1955, south of the Marge shaft, to mine the ore body penetrated by the shaft. Depth to ore ranged from 10 to 60 feet. In March 1957, the pit was about 500 feet long and about 60 feet deep (fig . 4) . Mining in the future will greatly enlarge this pit .
Excavation of the Gertrude open pit was begun in the spring of 1956. In March 1957, the pit was about 500 feet long, 250 feet wide, and 40 feet deep.
Structure
The Marge and Gertrude deposits are on the flanks of a large shallow syncline in the Browns Park Formation and between one-half and two miles south of the exposed contact with Cretaceous and Tertiary formations . The Browns Park strikes N. 75O-85O E . and dips 30 to 60 NW at the Marge mine. At the Gertrude mine the formation strikes N. 450-90 E. and dips 60 to 100 SE.
At the Marge mine, seven small faults are exposed ~fig. 0 4). Four of these are high-angle normal faults striking N. 55 -70 W. and having displacements ranging from 2 to 30 inches. The other three faults strike N. 30-200 W. Two of these are normal with slight displace- ments and the third, the Marge fault, strikes N. 30 W. and dips 750 to 850 SW . Displacement along this fault is not known with certainty . The contact between oxidized and unoxidized zones exposed along the Marge fault is at different elevations on either side, and if this difference can be used as a measure of displacement, the east side, or foot wall, is down relative to the west side approximately 30 feet. This indicates reverse movement. The fault zone contains from one to three feet of sandstone gouge and can be traced in mine openings for 650 feet along the strike . No evidence of this fault was found in outcrops of older resistant formations two miles to the north. Therefore, movement may have been limited to the Browns Park Formation.
Twenty faults are exposed at the Gertrude mine, sixteen of which strike N. 650-900 W. The remaining four strike N. 350-50o W. Most of them are high-angle normal faults with less than two feet of displacement.
The most persistentgoint system in both the Marge and Gertrude mines strikes N. 60 -704 W. A minor joint system strikes N. 10O- 300 W.
Host Rock
The Browns Park Formation exposed in the Marge and Gertrude mines consists of fine- to medium-grained, light gray to buff, cross- bedded, friable sandstone with sparse amounts of interstitial clay; very fine- to fine-grained, dark buff or greenish-buff to light
- 11 T bp7
TbpMA RGE CLAIM NO.1I MAYBELL CL AIM NO.1I
Tbp
Tb 0-
78
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Tbp Tbp Browns Park formation
r-,--r---r Top of pit wall
Toe of pit wall
*80 Fault showing dip
Inferred fault
.. rike 0 20 40 60 80 L- St and dip of bedding I I A FEET T M-D-I- 18 Figure 4. Geological rmap of the open pit at the Marge mine as of March , 1957, Moffat County, Colorado
- I2- br-own , soft sandstones with abundant interstitial clay; calcareous sandstones; sandy clay beds; and clay beds (figs. 5 and 6).
Individual beds of sandstone have a maximum thickness of about 20 feet and are remarkably consistent in lithology except for scattered calcareous concretions . Concentrations of limonite and jarosite staining are common in the oxidized sandstone.
The calcareous sandstone beds are less than a foot to several feet in thickness and commonly consist of a series of hard calcareous lenses in a softer sandstone matrix rather than a continuous calcar- eous sandstone bed.
The clay beds are as much as 12 inches in thickness. They are discontinuous but are commonly found in about the same stratigraphic positions over considerable areas. These zones can be traced for several hundreds of feet in the mines.
Mineralogy
The dominant ore minerals are meta-autunite and uranophane in the oxidized zone, and uraninite and coffinite in the unoxidized zone. Of the oxidized minerals, greenish-yellow meta-autunite predominates . Other uranium minerals are phosphuranylite, liebigite, tyuyamunite, meta-tyuyamunite, becquerelite-schoepite, and two unidentified minerals. Gangue minerals are quartz, calcite, gypsum, limonite, j arosite, montmorillonite, and pyrite.
Uranium minerals occur as microscopic and megascopic impregnations of sandstones, coatings on fracture surfaces in sandstones, and specks in clayey beds . They are commonly distributed as halos or rings concentric with heavy limonite staining (fig . 7) and as bands in cross-bedded sandstones (fig. 8).
In many places visible yellow uranium minerals are associated with concentrations of limonite in halos surrounding calcareous concretions (fig . 7). The longest axes of concretions are usually parallel to the bedding. They range from 3 to 24 inches in length and from 3 to 6 inches in thickness . Bands of limonite staining as much as 2 inches wide commonly surround concretions in oxidized sandstones, and the concretions are also stained with limonite. Bands of yellow uranium minerals are peripheral to both the limonite and calcium carbonate. Bands of limonite have been observed connecting as many as three calcareous concretions .
Figure 8 illustrates an apparently concretionary structure composed of concentric rings of autunite. This structure does not contain calcium carbonate or limonite as do other concretions that contain uranium. The rings occur in a light buff cross-bedded friable sandstone .
- 13 - - --. -- 625 - .- -- -
2 ------. ------
6240- . ------
6 01 ------.. - - . ... -. ...-...------.
., Fine to medium grain, cross bedded sandstone with sparse Claeu adtn. Bf inoidzd.T71 . e n - i- interstitial clay. Light gray to buff in oxidized zone and I.T1 gra inunxdie- zn me dium g ray in uno xidiz ed zo ne. ----Very fine to fine grain sandstone with abundant interstitial ~~.+ clay. Dark buff to greenish buff in oxidized zone and E--- Contact of oxidized and unoxidized zones medium gray in unoxidized zone.
Light olive green in zone and dark 0 ---. Clay bed. .. oxidized SOre lens. 0.15+ /a U 0 Gray to black in unoxidized zone. 3 8
.T~ Sandy clay bed. Light olive green in cxidized zone 9 49 89 ~j and dark gray to blck in unoxidized zone. FEET TM-l- 1-18 Figure 5. Generalized geologic section A-A' Marge open pit, Marge No.1 and Maybell No. I claims. Moffat County, Colorado B B
620 ____Alluvium -______Profile of pit
6250 -
6250 ------
[71Fine to medium grained, light gray to buff, crossbedded sandstone with sparse -inteterstitial clay.
---- Very fine to fine groined, dark buff to greenish buff, sandstone with abundant interstitial clay.
- Fine to medium grained, light gray to buff, crossbedded sandstone interbedded with veryy fine to fine grained, dark buff to greenish buff sandstone.
--- Light alive green clay bed
-E-I- Sandy clay bed
EIII Calcareous sandstone
SOre lense 0.15 +0/o U3o8
0 40 80
FEET
TM-D-l- 8
Figure 6. Generalized geologic section B-' Marge open pit Marge No.1 and Maybe II No. I claims Moffat County, Colorado
-15-- E X PLANAT ION
- Very fieto fine grind dark bufto greihbuff
sandstone with abundant interstitial clay. F~.] Brown colcoreous sandstone with -5.i.: abundant limonite stain.
-!!![.: Heavy limonite band. Intensity of concentrotion indicated by shading.
LIIPI nd non fluorescent minerals present.
.e .. n.....e ...... e
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section 17, T. 7 N., R. 94 W., Moffat County, Colorado
,|7- Most of the uranium is not associated with concretions or concentra- tions of limonite but is disseminated interstitially .
Becquerelite-schoepite, an oxidation product of uraninite, is found in some oxidized ores near the contacts with the unoxidized ores. Liebigite is found in fracture veinlets in calcareous sandstones.
Subrounded to rounded quartz grains are the dominant constituent of the sandstones. Calcite is present as cement in some sandstones.
Montmorillonite is an important constituent of the interstitial clays and clay beds and is probably an alteration product of tuffs . Gypsum in thin partings is locally abundant in the sandstone, and crystals of gypsum as much as one inch in length occur in fault gouge . Limonite is abundant as a staining on sand grains and in interstitial clay; it imparts a buff color to the sandstones . Limo- nite concentrations in the form of streaks, concretions, and concentric rings are common (figs. 9 and 10). Jarosite is abundant in the oxidized deposits as staining and cement. Pyrite, abundant in the unoxidized ores, is finely divided and uniformly disseminated.
Equilibrium Factors
A comparison of 38 chemical and radiometric assays of drill-hole samples from the Gertrude deposit shows that the average ratio of the chemical assay to the radiometric assay is 1 to 0.86. For those samples which assay greater than 0..15% U30, the ratio is 1 to 0.57. ~8
Sugar Loaf Mine
The Sugar Loaf mine (fig. 2), owned by the Sugar Loaf Venture Group, is in sec. 27, T. 7 N., R. 94 W. The Browns Park Forma- tion here dips 5O-10O north. The ore occurs in a fault zone about three to five feet wide. The fault zone dips steeply and strikes about due north. The host sandstone is very fine- to medium-grained and somewhat calcareous; it is silicified in places along the fault. The uranium occurs as a uranophane-like mineral. Uranium also occurs in asphaltite nodules and streaks in the fault zone.
Several short adits were driven in and along the fault in 1954 and 1955. Drilling indicated that mineralization was confined to the fault and extended to a depth of 20 feet from the surface.
Three Sisters Prospect
The Three Sisters prospect, owned by the Thunderbird Development Company, is in sec. 30, T. 5 N., R. 93 W., southeast of the map area of figure 2. The prospect is in an erosional remnant of the Browns Park Formation, which is here about 50 feet thick and nearly flat-lying. The ore is low grade and is in the basal conglom-
- 18 - Suface C CI
6250' Alluvium- 6250'
i240 6240------\- - -
3230'
--- Oxid zc-d - .
Unoxidzed------6220' Bottom of bench
FEE T
EXPLANA TION
Very fine to fine grai~ned, buff sandstone Fault with some interstitial clay, buff in oxidized Fault . - zone and gray in unoxidized zone.
4-'~ /Top Of Fine to medium grained, gray sandstone.
IL Old 'IWO Clay bed
surface 'of Heavy limonite stain. Q bench Ore lens. o 40 80
FEET I ~TM-D-l- 18 Figure 9. Geological cross section c- C' Marge open pit Marge No.1 claim. section 24, T. 7 N., R 95 W., Moffat County, Colorado
-19------.--- - - .- h.ZA.-- -. - 7- W -6240'
. -. . - --.. ...- -.. . .- ;...... - - - .-..- : .. ; . . ...- . ..- - - --.-...... -. -- - .. -
.2av -
0 20 of pit FEET EX PLANATION
L-I:I Fine groined, gray , crossbedded sandstone. [------Medium grained, gray sandstone, -- interbedded with fine grained, buff - sandstone with abundant interstitial clay.
Ged Calcareous sondstone- I ~slope To. o Y/ pit woll K- - Cloy bed.
Heavy limonite stain. E13wuVisible uranium mineral. Ore lens.
PLAN VIEW 0. 40 60
PEET ______LTM- D- 1-18 Figure 10. Geological cross section D-D' in the Marge mine, May bell- No. I c laim, se c. 19, T. 7 N., R. 94 W, Moffat Co unt y, Colo rado
-20- erate of the formation. Yellow uranium minerals occur as coatings on pebbles and as interstitial fillings in the sandstone beds.
The prospect has been explored by drilling and developed by a small open pit.
White Star Prospect
The White Star prospect (fig. 2) is in sec. 13, T. 7 N., R. 95 W ., and is owned by the Front Range Mining Company . Uranium occurs at a depth of 90 feet in the basal conglomerate of the Browns Park Formation, which dips 200 south. The prospect is about 1,000 feet south of the outcrop of the contact of the Browns Park Formation with older beds . The uranium was discovered by wildcat drilling .
Buffalo Head Prospect
The Buffalo Head prospect (fig. 2), in sec. 29, T. 7 N., R. 94 W ., is owned by the Buffalo Head Mining Company . Uranium occurs in northerly dipping sandstone beds that are displaced by high-angle northwest-trending faults. The prospect has been drilled, and a small open pit has been excavated.
Cedars Prospect
The Cedars prospect (fig. 2), in sec. 9, T. 6 N., R. 94 W., is controlled by the Cedars Mining Company . Visible yellow uranium minerals occur in flat-lying beds in the basal conglomerate of the Browns Park Formation. The uranium minerals occur as coatings on pebbles and as interstitial fillings in the sandstone. The area is crossed by a northwest-trending steeply dipping fault of unknown displacement. Some exploratory trenching has been done.
Leon Prospect
The Leon prospect (fig. 2) is in sec. 17, T. 6 N., R. 94 W., in nearly horizontal sandstone beds in the basal conglomerate of the Browns Park Formation. Yellow uranium minerals are visible as coatings on pebbles and as interstitial filling in the sandstone. Meta-tyuyamunite has been identified in a sample from this location.
Agnes Prospect
The Agnes propsect (fig. 2) in sec. 35, T. 7 N., R. 94 W., is owned by the Trace Elements Corporation. Uranium occurs in interbedded conglomerate and sandstone beds of the Browns Park Formation near the base of the formation. The area is crossed by a northwest-trending steeply dipping fault which contains asphaltic nodules. A small amount of drilling has been done.
- 21 - September Morn Prospect
The September Morn prospect is in sec. 3, T. 5 N., R. 96 W., southwest of the map area of figure 2. Slightly anomalous radioactivity is associated with a west-trending fault zone in the Browns Park Formation. The fault zone contains asphaltic material. The host formation is here a fine-grained limonite-stained sandstone .
URANIUM DEPOSITS OF THE POISON BASIN AREA
The deposits in the Poison Basin area are in an erosional remnant of the Browns Park Formation that is eight miles long and two miles wide (fig . 3) . Many smaller occurrences of uranium are found in this formation east and northeast of Poison Basin as far as Saratoga, Wyoming, which is the eastward limit of Browns Park outcrop.
Cedar Hills Deposit
Location
The Cedar Hills deposit (fig. 3) is in sec. 32, T. 13 N., R. 92 W., and is operated by the King Oil Company . Extent
The deposit is irregular and composed of several ore bodies. Like those in the Maybell area it is blanket-like and essentially parallel to the bedding . However, the ore bodies are smaller and not as continuous as those in the Maybell area. The over-all north- erly trending zone of mineralized rock is approximately 800 feet long . The maximum thickness of ore is about 12 feet. Uranium has been found from the surface to a depth of over 60 feet. Produc- tion has been almost entirely from the oxidized zone.
Mine Workings
Ore has been mined from three open pits, which become interconnected as mining progressed. At present the mine workings are irregular in shape and about 860 feet long .
Structure
The beds of the Browns Park Formation are gently dipping and strike northward. Two high-angle faults exposed in the pits strike N. 200 W., and N. 600 W., respectively, and contain from one to four inches of gouge. The most persistent joint system strikes N. 500-85O W., and a minor system strikes N. 5 -35 W.
Host Rock
The sandstones of the Browns Park Formation in the open pits are fine- to coarse-grained, light gray to buff, cross-bedded sandstones
- 22 - that contain sparse amounts of interstitial clay; and calcareous sandstones (figs . 11, 12, & 13). Individual sandstone beds are as thick as 15 feet and are remarkably uniform in lithology vertically . Isolated hard calcareous lenses are common, and oxidized sandstones contain abundant limonite and jarosite .
Mineralogy
The dominant ore minerals are meta-autu~nite and uranophane in the oxidized sandstones, and coffinite and uraninite in the unoxidized sandstones. Other minerals are carnotite, liebigite, and rutherfordine. Accessory minerals are calcite, gypsum, limonite, jarosite, and pyrite.
The unoxidized uranium minerals occur as microscopic disseminations and megascopic specks intimately associated with very finely divided pyrite . Oxidized uranium minerals are visible as coatings on fracture surfaces and as cementing material in the higher grade ore.
Poison Basin Deposit
Location
The Poison Basin deposit (fig. 3) is in sec. 4, T. 12 N., R. 92 W ., Carbon County , Wyoming , approximately one mile southeast of the Cedar Hills deposit. It is operated by the Shawano Development Corporation. Extent
Ore bodies in this deposit are similar to those of the Cedar Hills deposit in shape and distribution. Uranium, widespread areally, is found to depths of over 100 feet, well into the unoxidized s andstones .
Mine Workings
Ore has been developed in two open pits and an inclined adit. The open pits extend over a mineralized zone about 700 feet in length.
Structure
The beds of the Browns Park Formation are gently dipping in most of the mine area. One fault, exposed in the mine workings, strikes N. 600 W., and dips 750 NE. The fault contains as much as two inches of gouge, in which there is some ggrpsum. Two prominent joint systems strike N. 600 W., and N. 40 -800 E., respectively.
Host Rock
The rocks in the Poison Basin deposit are very fine- to medium- grained, buff, cross-bedded tuffaceous sandstone with some intersti- tial clay; medium-grained gray sandstones; and calcareous sandstones.
- 23 - 980 --- - ~-- . ' - - -- - ' -- 9 '
9 0 ---. '- -. '970'
Fine grain lens with abundant Bottom of pitSadtn beo liec tis jarosite stain. adtn eo iecnan more I imonite as a general 0 20 40 Feeti colo ration than sandstone S I a I above line.
N. _ E XPL ANAT ION
Fine to coarse rained, ight gray to buff, Bat tomn of p i Zn- crossbedded sandstone wit sparse interstitial clay.
Heavy limonite stain FSurface~/ " Visible uranium mineral
Calcareous sandstone
Ore lens
o 40 80 PLAN VIEW I I I I ' No.1 OPEN PIT FEET j TM-D-l-18 Figure II. Geological cross section A-A' No.1I open pit Cedar Hills No. 6 claim, section 32, T.13 N., R.92 W., Carbon Count y, Wyoming.
-24- 970
960
.- ~- 950
-940
N C" N~ /Toe ofI EX PLANATION 0 I Top of c~ /t/ 3 J pit wall ---- Very fine to medium grained, light gray to buff crossbedded ..JL // of pit ------sandstone with sporse to some interstitial cloy.
~ Heavy limonite stain. -- PlAE Visible uranium minerals. o40 ao; ' N.2 PENPU IEE-0 ~ Ore lens.
TM-D-l-IS Figure [2. Geological cross sec tion A -A' No. 2 open pit Cedar Hills No. 10 claim, sec tion 32, T. 13 N., R. 92 W. Carbon County, Wyoming B
-9-0-95
Sandstone below line contains more limonite Fe as a general coloration than sandstone above line. Bottom of pit
N EX PLANATION N Toe of II S slope TpIr f o opaf ------Fine to coarse groined, light groy to buff, crossbedded ----- sandstone with sparse interstitial cloy. 'Bottom J section. ~I Colcoreous sandstone.
S\ Heavy limonite stain.
'33 .. /11 ~ Ore lens
0 40 6aO PLAN VIEW - - 'No.2 OPEN PIT FEET TM-fl-i-IS Figure 13. Geological c ross se ction B -B' No. 2 o pen pit Ce dar H ills N o. 10 cla im, se ction 32 , T. 13 N., R. 9 2 W. , Car bo n County , Wyoming Mineralogy
The dominant uranium minerals appear to be meta-autunite and uranophane in the oxidized ore and uraninite and/or coffinite in the unoxidized ore. Sabugalite has been identified in a drill- hole sample. The secondary uranium minerals are disseminated interstitially and cement sand grains in high-grade ore. Accessory minerals are calcium cabnt, limonite, j arosite, and gypsum.
An unusually high content of selenium is present in some parts of the deposit, samples assaying greater than 1.0 percent having been collected from the oxidized sandstone. The occurrence of ilsemannite, as a blue "bloom" on pit walls indicates the presence of molybdenum in the ores .
Teton Prospect
The Teton prospect (fig. 3) in sec. 31, T. 13 N., R. 92 W., is controlled by the Teton Exploration Drilling and Development Company. Oxidized uranium minerals occur in medium-grained sandstone of the Browns Park Formation at depths ranging from the surface to about 50 feet.
Section 36 Prospect
Uranium is present in medium-grained sandstones of the Browns Park Formation in sec. 36, T. 13 N., R. 93 W. (fig. 3). The property is controlled by the Teton Exploration Drilling and Develop- ment Company.
PARAGENESIS
The sandstones in the unoxidized zone of the Browns Park Formation consist of quartz grains, finely divided pyrite, which imparts a gray color to the sandstones, irregularly distributed interstitial bentonitic clay, and calcium carbonate, which is restricted to thin beds, lenses, and concretions.
Uranium, carried in solution in ground water , is thought to have been precipitated as uraninite and coffinite under reducing conditions in the unoxidized zone. As the erosion surface moved downward in the process of hundreds and perhaps thousands of feet of sediments that once constituted the Browns Park Formation, the oxidizing zone moved lower into previously unoxidized rock. The top of the present unoxidized zone ranges in depth from a few feet below the surface to 200 feet or more, depending on the permeability of the sandstone. With the introduction of oxidizing agents the pyrite oxidizes to limonite and jarosite, and the uraninite and coffi- nite to becquerelite-schoepite. The other oxidized uranium minerals are produced in subsequent oxidation processes.
- 27 - The oxidation of the pyrite to limonite and jarosite effects a change in color of the sandstones from gray to buff.
The limonite and yellow uranium minerals, being more soluble than their counterparts in the unoxidized zone, are subject to solution, movement, and redeposition by moving ground water . The horizontal orientation of concentrations of limonite, calcium carbonate, and yellow uranium minerals, even in steeply cross- bedded sandstones, suggests that they were deposited at or near a fluctuating water table.
As the erosion surface and the zone of oxidation moved downward, the uranium and associated interstitial minerals were leached and dispersed or perhaps redeposited in the unoxidized zone.
ORIGIN OF THE URANIUM
A theory of origin of uranium in Tertiary sediments based on the leaching of tuffs known to be abnormally radioactive has been advanced by J. D. Love (1952). He postulates that uranium in the Wasatch Formation of the Pumpkin Buttes area of Wyoming may have been derived from tuffaceous beds in the overlying White River Formation, being leached from the tuffaceous beds by percolating meteoric waters and redeposited in the Was atch Formation.
Tuff beds are common in the Maybell area, and tuffaceous sandstones are found in the Browns Park Formation of both the Maybell and Poison Basin areas (Theobold and Chew, 1955, and Bradley, 1936). Interstitial clay and bentonite beds in the formation are believed to be the result of devitrification of the contained tuffaceous material. Uranium may have been leached from these tuffs by migrating ground water and been deposited where reducing conditions, favorable for deposition, were encountered.
The origin of the uranium may be related to some of the non-tuffaceous constituents of the Browns Park sandstone units . The Browns Park Formation includes lacustrine, fluviatile, and eolian sands. These sands may have been derived from granitic masses, although the absence of extreme range in grain size and arkosic constituents seems to indicate a nearby source. Much of the material in the Browns Park Formation was derived from older sedimentary rocks in the region, which range in age from Paleozoic to Eocene. Both granites and the older sediments probably contained sufficient amounts of uranium for the formation of the deposits in the Browns Park Formation.
A hypogene origin cannot be ruled out, but evidence for it has not been found. Drilling on faults below the deposits indicates that the faults are not mineralized at depth and therefore have probably not served as conduits for upward-moving solutions.
- 28 - SUMMARY
Favorable host rocks for the deposition of uranium in the Browns Park Formation are found in the interbedded fluvial, lacustrine, and eolian sandstones east of the confluence of the Yampa and Little Snake Rivers in Colorado and west of the Miller Hill area in Wyoming .
Most of the formation in this region has been removed by erosion, leaving a few small remnants on the Colorado-Wyoming state line, of which the Poison Basin is one, and a relatively large remnant farther south, in which the Maybell area is located.
The Browns Park Formation in both the Poison Basin and Maybell areas overlies breached anticlines that are proven petroleum reservoirs. Petroleum has migrated upward into the Browns Park sandstones and is found as solid residues associated with uranium in faults in the Maybell area and as natural gas pockets near the surface in uranium deposits in the Poison Basin. In most of the uranium deposits faults have been found; some of them have several hundreds of feet of vertical displacement. There are serveral occurrences of uranium and solid petroleum residues in the faults .
Yellow uranium minerals are associated with concentrations of limonite and calcium carbonate in the oxidized zone of the formation.
In some cases faults have acted as channels for petroleum moving up from reservoir rocks below .
- 29 - REFERENCES
Bergin, M. J., 1959, Preliminary geologic map of the Maybell-Lay area, Moffat County, Colorado: U. S. Geol. Survey, open file map.
Br adley , W . H ., 1936 , Geomorphology of the north flank of the Uinta Mountains: U. S. Geol. Survey Prof. Paper 185-I, p. 163-204.
Bradley, W. H., 1945, Geology of the Washakie Basin, Sweetwater and Carbon Counties, Wyoming, and Moffat County, Colorado: U. S. Geol. Survey, Oil and Gas Inv. Prelim. Map GM 32.
Crowley, A. J., 1955, A structural history of northwest Colorado and parts of northeastern Utah, in Intermountain Assoc. Petrol. Geologists and Rocky Mountain Assoc. Geologists: Guidebook to the Geology of Northwest Colorado, 6th Annual Field Conf. , p . 53-55.
Gruner, J. W., 1956, Concentration of uranium in sediments by multiple migration-accretion: Econ. Geology, v. 51, no. 6, p. 495-517.
Love, J. D., 1952, Preliminary report on uranium deposits in the Pumpkin Buttes area, Powder River Basin, Wyoming: U. S. Geol. Survey Circular 176.
Magleby, D. N., and Mallory, N. 5., 1954, Airborne radiometric survey of the Browns Park Formation, Carbon County, Wyoming: U. S. Atomic Energy Comm. RME 1055 (rev.), Technical Information Service, Oak Ridge, Tenn.
Sears, J. D., 1924, Geology and oil and gas prospects of part of Moffat County, Colorado, and southern Sweetwater County, Wyoming: U. S. Geol. Survey Bull. 751-G, p. 269-319.
Theobald, P. K., and Chew, R. T., 1955, written communication.
Vine, J. D., and Pritchard, G. E., 1954, Uranium in the Poison Basin area, Carbon County, Wyoming: U. S. Geol. Survey Circular 344.
- 30 - EXPL ANAT ION I
A Rawlins SEDIMENTARY ROCKS 89 88 87 86 as I L F F i -- ~.z - Aluvial deposits QUATE RNARY {Qal -I KIe 20 N Terrace deposits K 94 93 92 -C9 I Kmv Tn North Park Formation
59 Browns Park Formation 7) I N. A A Tbi Bishop Conglomerate Twh Km 18 Tbr |Bridger Formation L Qal T! Morrow Cr. and Laney Member Tri of Green River Formation Kd Saratoga Tfu K! Tgr Green River Formation Tg Km a 57. TERTIARY 103W 502 101 100 99 98 97 9595 Kie Twcb Cathedral Bluffs Member of Wasatch Formation 7- Tgt Tipton Tongue of Green River Formation Tm! jTwcb Krr 56 Tw Wasatch Formation AT n f a~-~*~-.-~- 4. 4 Twh Hiawatha Member of Wasatch Formation Tw h Tgt Tbr L Tfu Fort Union Formation 5 II Lance Formation Lewis Shale CR ETAC EQUS Kmv | Mesaverde Group
Km | Mancos Shale - Steele Shale Kd | Dakota Sandstone TLi___ 1Tcb JURASSIC- -_____| Jurassic-Triassic (undivided) TTI TRIASS IC Tweb PENNSYLVANIAN | P| Permian and Pennsylvanian (undivided) Tgt Tgt Twebw Twh MISSiSSIPPIAN Mississippian (undivided) F 9 6 DEVO IAN Devonian - Cambrian (undivided) pC K D-C | Tg g K I0 PRE- CAM BRIAN Pre-Cambrian (undivided) Tg
Qig o 9 IGNEOUS ROCKS Km -H [jW~ QUATER NARY Qtv | Quat rnary - Tertiary Tbr Twcb S -- I Tvr | Terti ory volcanic rubble MAYBELL Twh Tgry AREA Tu Tf u OKc ~ Th~ K TERTIARY i Tertiary extrusives 7[ {Te / K M V Ti | Tenticry intrusives MaybelI Craig Kd-C IP 8 0 8 l6 N KIe ~ KIe .1 a 6 I.. .1 Knv KeKm Kmv Qt SCALE OF MILES I- M-n h U- KI Kmv Km \ Kmv 5N Geologic base map from USGS Geologic map of Wyoming, 1955 Kd Tw Kmv and Geologic map of Colorado, 1935, with modification from 2 Km Km v L .~ I KI SKm Generalized Area Geologic Map, Northwest Colorado and Adjacent Southern Wyoming by A. E. Owen, 1955, Sinclair Oil and Gas Company.
Plate 1. Geologic map showing the distribution of the Browns Park and Formation in northwest Colorado south -central Wyomninc g