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Multiply By To obtain

inches (in.) 2.54 centimeters (cm)

feet (ft) 0.3048 meters (m)

miles (mi) 1.609 kilometers (km)

Any use of trade names in this publication is for

descriptive purposes only and does not imply

endorsement by the U.S. Government.

For sale by U.S. Geological Survey Information Services

Box 25286, Federal Center, Denver, CO 80225

U.S. DEPARTMENT OF THE INTERIOR

U.S. GEOLOGICAL SURVEY

GEOLOGIC INVESTIGATIONS SERIES

I–2647

1000 1000 0 2000 3000 4000 5000 6000 7000 FEET

1 .5 0 1 KILOMETER

SCALE 1:24 000

1 1/ 2 0 1 MILE

CONTOUR INTERVAL 40 FEET

DOTTED LINES REPRESENT 20-FOOT CONTOURS

NATIONAL GEODETIC VERTICAL DATUM OF 1929

15o

APPROXIMATE MEAN

DECLINATION, 1962

MAGNETIC NORTH

TRUE NORTH

Base from U.S. Geological Survey, 1962.

Polyconic projection. 1927 North American datum.

10,000-foot grid based on Colorado coordinate

 system, north zone

1,000-meter Universal Transverse Mercator grid

ticks, zone 12

GEOLOGIC MAP OF THE SKULL CREEK QUADRANGLE, MOFFAT COUNTY, COLORADO

By

R.E. Van Loenen and W. Anthony Bryant

1999

COLORADO

MAP AREA

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Holocene and

Pleistocene QUATERNARY

Oligocene TERTIARY

Upper

Cretaceous

CRETACEOUS

Lower

Cretaceous

Upper

Jurassic

Middle

Jurassic

Lower

Jurassic

Upper

Triassic

Middle(?) and

Lower Triassic

Unconformity

Unconformity

Unconformity

Unconformity

Unconformity

Unconformity

Unconformity

Unconformity

Unconformity

Unconformity

Unconformity

Lower

Permian

Upper

Pennsylvanian

Middle

Pennsylvanian

JURASSIC

TRIASSIC

PERMIAN

PENNSYLVANIAN

CORRELATION OF MAP UNITS

DESCRIPTION OF MAP UNITS

Alluvium and colluvium (Holocene and Pleistocene)—Unconsolidated, poorly sorted clay, silt, sand, gravel, and rock fragments 

deposited in and along present stream channels and as slope-wash 

deposits at base of slopes and cliffs. Older alluvial deposits of 

mostly red clay, silt, and sand occur in and adjacent to present 

stream channels, as thick as 80 ft , and are dissected by present 

drainage system. Contact transitional with other surficial deposits

Landslide deposits (Holocene and Pleistocene)—Poorly sorted 

debris derived from Chinle Formation and overlying Glen Canyon 

Sandstone east of Gunsight Gap. Springs and prominent stands of 

chokecherry are common on landslide deposits

Terrace deposits (Holocene and Pleistocene)— 

Unconsolidated, poorly sorted sand and gravel, containing pebbles 

and cobbles of mainly quartzite that overlie Mancos Group in 

southern part of quadrangle. Thickness ranges from a few feet to 

nearly 100 ft

Bishop Conglomerate (Oligocene)—Poorly consolidated, light-gray, 

tan, and white, thin-bedded, poorly sorted pebble, cobble, and 

boulder conglomerate. Conglomerate matrix is locally tuffaceous. 

Clasts consist mostly of red quartzitic sandstone derived from 

Uinta Mountain Group (Middle Proterozoic) and light- to dark-gray 

limestone derived from Paleozoic formations. Highly dissected 

outcrops occur above Wolf Creek drainage in northern part of 

quadrangle. Maximum thickness about 50 ft

Mancos Group (Upper Cretaceous)

Main body—Brownish-gray, mostly noncalcareous, marine shale 

interbedded with minor siltstone, very fine grained sandstone, and 

thin bentonite beds. Lower part is poorly exposed at base of 

hogback formed by Frontier Sandstone and the flats in southern 

part of quadrangle. Maximum thickness is about 4,400 ft (Cullins, 

1969) a few miles to the southwest

Frontier Sandstone—Upper part is resistant, light-olive-gray, finegrained, calcareous and fossiliferous sandstone. Lower part is 

brown and pale-blue, fissile shale and siltstone. Forms first 

hogback north of U.S. Highway 40 along southern limb of Skull 

Creek Monocline. Thickness ranges from 180 to 260 ft

Mowry Shale—Blue-gray to pale-blue, fissile, siliceous shale 

containing fish scales and thin bentonite beds; locally contains 

gypsum. Weathers to distinctive hard chips. Well exposed along 

Frontier Sandstone hogback. About 90 ft thick near Skull Creek, 

Colo.

Dakota Sandstone (Lower Cretaceous)—Resistant, yellowish-brown 

to light-gray, medium- to thick-bedded, medium- to coarse-grained 

sandstone interbedded with conglomerate and minor thin shale. 

Weathers rusty and forms hogback along much of southern limb 

of monocline in southern part of quadrangle. About 100 ft thick 

near Massadona, Colo., in southeast corner of quadrangle, but 

only about 40 ft thick 12 mi to the west near Plug Hat (Rowley 

and Hansen, 1979a)

Cedar Mountain Formation (Lower Cretaceous)—Comprises two 

members: an upper unnamed shale member and lower Buckhorn 

Conglomerate Member. Upper member consists of slope-forming, 

pale-olive and purple shale and mudstone units interbedded with 

thin, fine-grained sandstone and resistant, light-brownish-gray 

limestone and red jasper near base. Buckhorn Conglomerate 

Member is ledge-forming, thick-bedded, medium-gray to light-gray 

conglomerate and fine- to coarse-grained sandstone; conglomerate 

contains distinctive black chert pebbles. Thickness about 110 ft 

thick near Massadona, Colo., but as thick as 200 ft to the west 

(Rowley and Hansen, 1979a). Buckhorn Member ranges from 0 

to 20 ft thick. Unconformable contact with Morrison Formation is 

placed below chert-pebble conglomerate of Buckhorn or, where 

Buckhorn is not present, below red-jasper limestone of upper 

member

Morrison Formation (Upper Jurassic)—Slope-forming, variegated 

pale-olive, dark-red, and reddish-brown siltstone, claystone, and 

shale interbedded with light-gray to olive-gray, fine- to mediumgrained sandstone, conglomeratic sandstone, minor glauconitic 

sandstone, and argillaceous and nodular limestone. Conglomeratic sandstone bed (unnamed member), about 30 ft thick, 

occurs near middle of the Morrison and locally contains fossil 

dinosaur bones and teeth that weather out on dip slopes. 

Lowermost fine- to medium-grained sandstone is strongly 

crossbedded. Unconformable contact with Redwater Member of 

Stump Formation at base of crossbedded Morrison sandstone. 

Thickness ranges from 560 to 650 ft

Stump Formation (Upper and Middle Jurassic)

Redwater Member (Upper Jurassic)—Light-gray and pale-olive 

glauconitic shale interlayered with siltstone, sandy limestone, 

coquina, and thin sandstone beds. Weathers to a greenish cast. 

Fossils include brachiopods, pelecypods, cephalopods (belemnites 

very common), and ammonites. Contact with Curtis Member 

distinct; located between soft green beds of the Redwater and hard 

light-gray beds of the Curtis. Thickness ranges from 85 to 120 ft; 

usually well exposed at base of Curtis flatirons

Curtis Member (Middle Jurassic)—Very resistant, ledge-forming, 

light-gray to greenish-gray, fine- to coarse-grained, thin- to 

medium-crossbedded sandstone. Glauconite and fossils are common; “trash” beds (as much as several feet thick and locally 

containing uranium and copper) of petrified wood and other plant 

debris are common at base. Unconformable contact with Entrada 

Sandstone is sharp. Thickness variable, 0–30 ft

Entrada Sandstone (Middle Jurassic)—Resistant, light-gray, fine- to 

medium-grained, thick-bedded, massive sandstone; glauconitic in 

upper part. Exposed along southern flank of Skull Creek 

Monocline. Where underlying Carmel Formation (unit Jca) is 

absent (generally east of Skull Creek) the contact between Entrada 

and Glen Canyon Sandstones is inferred and mapped along a 

topographic low between the two similar units. Thickness ranges 

from 90 to 150 ft

Carmel Formation (Middle Jurassic)—Slope-forming, reddishbrown, fine-grained sandstone, siltstone, and mudstone. Redbeds 

of the Carmel display striking contrast to eolian sandstone beds in 

the overlying Entrada Sandstone (unit Je) and the underlying Glen 

Canyon Sandstone (unit Jg). The Carmel is present between the 

Entrada and Glen Canyon Sandstones, along southern flank of 

Skull Creek monocline, but is too thin to map separately. Contact 

between the Carmel and underlying Glen Canyon is mapped at a 

color change; however, an unconformity occurs about 10 ft below 

this color change within lithologies similar to Glen Canyon 

Sandstone (Pipiringos and O’Sullivan, 1975). Unconformity is 

marked by a very thin (less than 1 in. thick) layer of chert pebbles. 

The Carmel is 60 ft thick at Plug Hat Rock (12 mi west of 

quadrangle) (Rowley and Hansen, 1979a) and thins eastward 

across quadrangle from a few feet to only a few inches about 1 mi 

west of village of Skull Creek. The Carmel is mostly absent east of 

Skull Creek

Glen Canyon Sandstone (Lower Jurassic)—Massive, thick-bedded, 

well-sorted, light-gray to grayish-pink, medium- to fine-grained 

sandstone. Sandstone contains medium-scale to very large scale 

sand-dune crossbedding, is relatively resistant, and weathers to 

rounded buttes. The Glen Canyon is exposed extensively around 

Skull Creek Monocline and forms the caprock of Skull Creek Rim. 

Thickness ranges from 560 to 760 ft

Chinle Formation (Upper Triassic)—Includes main body and Gartra 

Member (not mapped separately) at base. Main body is slopeforming, moderate-red to moderate-pink siltstone interbedded with 

very fine grained sandstone beds of similar color. Gartra Member 

is ledge-forming, light-gray, coarse-grained, conglomeratic 

sandstone; usually crossbedded; contains streaks of heavy 

minerals, angular clasts as large as 1 ft, and local petrified wood 

locally; forms a conspicuous brush-covered bench, 10-30 ft thick, 

in otherwise relatively soft red cliffs around Skull Creek Dome. 

Unconformable contact with Moenkopi Formation is sharp. Total 

thickness of Chinle ranges from 240 to 280 ft

Moenkopi Formation (Middle? and Lower Triassic)—Moderatereddish-brown, yellowish-gray, and pale-olive micaceous siltstone 

interbedded with minor amounts of mudstone and very fine 

grained sandstone. Gypsum is common. Unconformable contact 

with Park City Formation, covered in most places, was mapped 

below lowermost reddish-brown Moenkopi siltstone bed; subtle 

color change occurs from Moenkopi redbeds to orangish Park 

City. Thickness ranges from 450 to 600 ft

Park City Formation (Lower Permian)—Alternating slope-forming, 

pale-orange and pale-olive mudstone and resistant, ledge-forming, 

yellowish-brown siltstone; contains minor fine-grained, lenticular 

sandstone beds. All lithologies are calcareous. Thought to be 

correlative with Franson Member of Park City (Hansen and 

Rowley, 1980). Poorly exposed around Skull Creek Monocline. 

Possible unconformable contact with Weber Sandstone is marked 

locally by a 2-ft-thick conglomerate containing coarse quartz 

pebbles and angular rock fragments as large as 1 in. Thickness is 

estimated at 125 ft

Weber Sandstone (Lower Permian and Upper and Middle 

Pennsylvanian)—Upper and lower parts exposed in structurally 

separate areas. Upper part is mainly light-gray, massive, thickbedded, fine-grained sandstone except near top, where the 

sandstone is pale yellowish brown and slightly calcareous; largescale crossbedding is common. Upper part is exposed at center of 

Skull Creek Monocline, where several canyons locally expose 500-

600 ft of sandstone. Lower part is highly indurated, interbedded 

light-gray (predominant) to pale-yellowish-orange, fine-grained, 

mostly thin bedded, commonly crossbedded, calcareous sandstone. 

Lower part also contains light-gray, fine-grained, mostly thinbedded, commonly cherty, somewhat crossbedded, fossiliferous, 

locally sandy limestone. Lower part is exposed in steeply dipping 

beds north of Wolf Creek Fault. Contact with Morgan Formation 

mapped at top of uppermost Morgan redbed. Reported to be 

about 1,000 ft thick where mapped about 6 mi north of 

quadrangle (Hansen and others, 1980)

Morgan Formation (Middle Pennsylvanian)

Upper member—Ledge-forming, interbedded light- to medium-red, 

fine-grained, crossbedded sandstone, gray to pale-lavender cherty 

fossiliferous limestone, and purplish-red siltstone. Pink to red chert 

nodules are conspicuous. Beds range from 2 to 10 ft thick. 

Breccia of red siltstone and limestone fragments occurs as float in 

upper part. Contact with lower member placed at break in slope 

at base of ledges formed by relatively resistant limestone and 

sandstone units of upper member. A complete section is not 

present in map area, but Hansen and others (1980) reported the 

member to be about 690 ft thick a few miles north

Lower member—Slope-forming, interbedded varicolored gray, red, 

and lavender shale and siltstone and minor grayish-pink to palelavender limestone and calcareous sandstone. Base not exposed in 

quadrangle but reported as about 280 ft thick nearby (Hansen and 

others, 1980)

Contact—Dashed where inferred

Normal fault—Bar and ball on downthrown side; dashed where 

inferred, dotted where concealed

Thrust fault—Sawteeth on upper plate; dotted where concealed

Monocline—Showing trace of axial surface and direction of plunge

Joint

Strike and dip of inclined beds

Mine or prospect

INTRODUCTION

The Skull Creek quadrangle is located about 15 mi north of Rangely, Colo., 

along the southeastern edge of the Uinta Mountains. The eastern part of Dinosaur 

National Monument is about 5 mi north of the quadrangle. Hogbacks and 

picturesque cliffs of the Skull Creek Rim can be seen from U.S. Highway 40, which 

passes along the southern boundary of the quadrangle. The central part of the 

quadrangle, the Skull Creek Basin, contains an ancient piñon and juniper forest. A 

gravel road links U.S. Highway 40 near Skull Creek to a county road that runs eastwest across the northern part of the quadrangle. Large tracts of private land are in 

the southern part of the quadrangle, and the rest of the land is administered by the 

U.S. Bureau of Land Management.

The Skull Creek quadrangle includes the eastern one-third of the Willow 

Creek and Skull Creek Wilderness Study Areas, located in Skull Creek Basin just 

below the prominent Skull Creek Rim. The geology and mineral resources of the 

study areas and vicinity were mapped and assessed by Van Loenen and others 

(1990). Geologic mapping of the study areas and the remaining parts of the Skull 

Creek quadrangle was done on color aerial photographs at a scale close to 

1:24,000. The geology was transferred from the photographs to the topographic 

base map using a photogrammetric plotter.

GEOLOGIC HISTORY

The geologic history of rocks exposed in the Skull Creek quadrangle began 

about 300 million years ago when sediment of the Morgan Formation (Middle 

Pennsylvanian) was deposited nearshore on older marine limestones and just 

offshore under shallow ocean conditions (Hansen and others, 1980). Deposition of 

sediment was interrupted from time to time, ending about 30 million years ago 

(during the Oligocene) when the area was covered by the Bishop Conglomerate.

During Middle Pennsylvanian to earliest Permian time, the Weber Sandstone 

was deposited over the Morgan Formation. The Weber is chiefly shallow marine, 

intertidal, and eolian sandstone that was deposited in a trough or basin in the 

ancestral Rocky Mountains. The Front Range and Uncompahgre Uplifts of the 

ancestral Rocky Mountains provided sediment to this region during late Paleozoic 

and part of Mesozoic time. Deposition of the Weber was episodic as the sea level 

rose and in response to Pennsylvanian glaciation in the southern hemisphere.

Thick deposits of the Lower Permian Park City Formation accumulated in 

marine and shelf environments of the Oquirrh Basin west of the quadrangle, but 

deposits of the Park City in the Skull Creek quadrangle accumulated in very shallow 

water, and some may include fluvial deposits. Only the upper part of the Park City 

Formation is present in this region (Rowley and Hansen, 1979b).

Clastic redbeds of the Lower and Middle(?) Triassic rocks of the Moenkopi 

Formation were deposited on fluvial flood plains under arid conditions and by 

shallow marine seas. The basal conglomerate of the Upper Triassic Chinle 

Formation was deposited on the eroded surface of the Moenkopi. This coarse 

sediment was deposited by high-energy streams and was in turn covered by the 

main body of the Chinle, a clastic redbed sequence of silts and sands deposited on 

alluvial plains. The Chinle Formation was covered by dune sands of the Glen 

Canyon Sandstone during Early Jurassic time. The spectacular scenery along the 

Skull Creek Rim results from erosion of the soft Moenkopi and Chinle redbeds 

when they are left unprotected by the relatively hard overlying Glen Canyon 

Sandstone. The Glen Canyon forms the high plateau around the Skull Creek 

Monocline. The return of shallow marine conditions over much of the region during 

the Middle Jurassic brought deposition of thin mud and silt of the Carmel 

Formation. The Carmel is thin to absent in the Skull Creek quadrangle, but traces 

of reddish-brown Carmel mudstone impart a distinctive stain to the uppermost part 

of the Glen Canyon and the overlying Entrada Sandstone in many places of the 

quadrangle. Dune sands of the Entrada Sandstone (Middle Jurassic) covered the 

region before the sea again covered the area and deposited the Curtis Member of 

the Stump Formation. The Curtis forms flatirons along the southern part of the 

Skull Creek Monocline.

Deep-sea marine conditions existed during the Late Jurassic, and these 

deposits are represented by the Redwater Member of the Stump Formation. The 

sea again retreated, exposing a land surface that was covered with continental 

deposits of the Morrison Formation (Upper Jurassic). The Morrison includes fluvial, 

eolian, and possible lacustrine deposits. Following deposition of the Morrison and 

periods of erosion, the Cedar Mountain Formation and the overlying Dakota 

Sandstone represent the first deposits of sediments in the initial development of the 

Western Interior Cretaceous Sea. These are largely fluvial deposits in a near-shore 

continental environment that were later covered by thick marine deposits of the 

Upper Cretaceous Mancos Group.

Tectonic deformation that began near the end of Cretaceous time is largely 

responsible for the geologic setting as it appears today. The large Uinta Anticline 

was formed during the Laramide orogeny. During Oligocene time (30 million years 

ago), fluvial deposits containing very coarse material covered parts of the 

quadrangle. Remnants of these deposits (Bishop Conglomerate) are preserved in 

the northern part. Since at least Bishop time, the area of the Skull Creek 

quadrangle has remained above sea level and exposed to erosion.

With the exception of the Park City Formation, all bedrock formations are 

very well exposed throughout the quadrangle. Much of the Park City Formation is 

covered by alluvium. Deep canyons cut in the Skull Creek Monocline expose 

several hundred feet of Pennsylvanian and Lower Permian rocks. Older 

Pennsylvanian rocks are exposed only on the high plateaus in the northern parts of 

the quadrangle. Triassic rocks are exposed nearly uninterrupted around the Skull 

Creek Rim. Excellent exposures of Jurassic through Cretaceous rocks form in the 

steeply dipping hogbacks in the southern part of the Skull Creek quadrangle.

STRUCTURE

The predominant structural features in the Skull Creek quadrangle are the 

Skull Creek and Wolf Creek Monoclines. Asymmetrical folds or monoclines are 

common in the eastern Uinta Mountains (Rowley and Hansen, 1979a) and are 

considered to be formed by movement along thrust faults as older rocks were 

displaced over younger rock.

The Skull Creek Monocline is an east-west-trending structure that extends for 

about 30 mi from near the Utah-Colorado border to the southeast corner of the 

quadrangle. The northern limb of the monocline dips very gently north at 2o–5o, 

and the southern limb dips steeply south at 25o–50o. The Skull Creek Monocline 

forms an ellipsoidal dome that has a large reentrant along the southern border. The 

dome, herein referred to as the “Skull Creek dome,” extends into the adjoining 

quadrangle to the west and is approximately 5 by 10 mi in dimension. The Weber 

Sandstone is exposed in the core of the dome, and younger rock is exposed in the 

surrounding Skull Creek Rim. The Skull Creek Monocline formed as a result of 

movement along the Willow Creek fault, a thrust fault deep in the subsurface that 

trends east-west just south of the quadrangle. Movement along the fault is from 

north to south. The Willow Creek Fault was encountered in a drill hole at a depth 

of 8,000 ft near Dinosaur National Monument headquarters, about 12 mi to the 

west (Powers, 1986). Where encountered in the drill hole, the fault placed 

Precambrian rock over Paleozoic rock. The reentrant in the south side of the Skull 

Creek Monocline at Miller and Skull Creeks is a conspicuous structural feature that 

may be related to stress that developed along the Willow Creek Fault. A basement 

high could cause such stress.

The Wolf Creek Monocline extends east-west across the northern part of the 

Skull Creek quadrangle. North of the monocline axis, beds dip very gently south; 

south of the axis, beds dip very steeply south to vertical in places along its southern 

limb. Unlike the deep Willow Creek Fault, the thrust fault that caused the folding 

here breaches the surface. It is called the Wolf Creek Fault and has, for the most 

part, juxtaposed Weber and Moenkopi rock at the surface. Near the eastern edge 

of the quadrangle, the fault cuts the Park City and Moenkopi Formations.

Normal faulting is rare within the Skull Creek quadrangle. One such fault does 

displace Jurassic rocks near The Sloughs in the northeastern part of the 

quadrangle. Displacement along this fault increases from the west to east into the 

MF Mountain quadrangle. Northeasterly trending joints have developed in the 

Weber and Glen Canyon Sandstones. Most of the drainage, which is from north to 

south, does not follow the joints or faults but intersects the fractures at acute angles.

MINERAL AND ENERGY RESOURCES

Little, if any, potential exists within the Skull Creek quadrangle for mineral or 

energy resources. Formations within the quadrangle and nearby that contain traces 

of metals (discovered by early-day prospectors) include the Chinle and Stump 

Formations. The Weber Sandstone is a reservoir for oil and gas south of the 

quadrangle (Van Loenen and others, 1990).

During the early uranium “boom” of the 1950’s, small deposits of uranium 

and related minerals were discovered in several localities within the Skull Creek 

quadrangle. Favorable depositional conditions apparently existed in sandstones, 

where fossil plant remains served to localize ore.

In the Skull Creek Rim, in the eastern part of the quadrangle (near center of 

sec. 20), a small amount of uranium-bearing petrified wood and clay was mined. 

This occurrence is in the Gartra Member (the coarse basal conglomerate) of the 

Chinle Formation. Another mine, near Skull Creek, produced a small amount of 

uranium, vanadium, and copper from the Curtis Member of the Stump Formation. 

The deposit contains several relatively rare secondary oxide minerals containing 

arsenic, copper, lead, selenium, uranium, and vanadium. “Trash” beds, composed 

mainly of carbonized plant remains in mudstone and sandstone may have served as 

a reductant for primary minerals. Similar, but smaller deposits occur at this 

stratigraphic interval in several places in the quadrangle, and most have been 

explored in prospect pits. Evidence of mineralization is readily recognized as a 

distinctive copper bloom (blue azurite). Petrified wood fragments, dinosaur bones, 

and some sandstone beds in the Morrison Formation are slightly radioactive; 

however, these levels of radioactivity are not unusual for the Morrison.

Just outside the northeast corner of the quadrangle, near Peterson Post Flat 

Reservior, uranium was mined from the Weber Formation. Mineralized rock was 

probably confined to the Wolf Creek fault zone. Several exploratory dozer cuts 

were made across the same fault zone in the Skull Creek quadrangle, but 

apparently no uranium ore was found.

The Moenkopi Formation is typically a redbed sequence throughout the 

Colorado Plateau; however, some Moenkopi beds in the Skull Creek Rim are 

altered to a conspicuous greenish-gray. This unusual alteration is accompanied by 

slight metal enrichment along the interface between the red and green beds 

(Cadigan, 1972). Apparently, reaction occurred at the contact between the red 

rocks (which represent an oxidized environment) and the green rocks (which 

represent an invading reducing environment). Due to the low permeability of the 

Moenkopi mudstones, the flow of fluids and sites for mineral deposition were 

greatly limited.

This region was explored for oil and gas because of a geologic setting similar 

to the nearby Rangely oil field. Oil and gas were discovered in 1933 in the Rangely 

field about 15 mi south of the Skull Creek quadrangle. The principal reservoir rock 

at the field is the Middle Pennsylvanian to Lower Permian Weber Sandstone. Oil is 

trapped in a large monocline very similar to the Skull Creek Monocline. Although 

the Weber is present in a structural setting similar to that of the Rangely field, it has 

very little potential for oil and gas within the Skull Creek quadrangle. Erosion has 

removed all of the Mesozoic and Cenozoic rock from the Skull Creek Monocline, 

thus exposing the Weber Sandstone and allowing any oil and gas accumulations to 

escape. Several wells drilled within the Skull Creek quadrangle and nearby, to test 

the Weber and older Paleozoic rock, met with little success (Van Loenen and 

others, 1990).

Other reservoirs for oil and gas in this region are foreland subthrust traps. The 

Willow Creek Fault, which lies beneath the southern boundary, has been an 

exploration target for this kind of trap. Powers (1986) analyzed the play from 

seismic data and from a drill hole located about 10 mi to the west of the Skull 

Creek quadrangle. Oil was found, but the amount of closure beneath the subthrust 

is too restricted in extent and the reservoir rock qualities are poor (Powers, 1986). 

This play has not been tested by drilling within the Skull Creek quadrangle.

Gravel is abundant in terrace and alluvial deposits in the southern part of the 

quadrangle. Limestone from the Cedar Mountain Formation and shale from the 

Mowry Shale have been quarried a few miles to the west in the Lazy Y quadrangle 

for use as road metal. Both are equally abundant in the Skull Creek quadrangle.

REFERENCES CITED

Cadigan, R.A., 1972, Geochemical anomalies and alteration in the Moenkopi Formation, 

Skull Creek, Moffat County, Colorado: U.S. Geological Survey Professional Paper 761, 

21 p.

Cullins, H.L., 1969, Geologic map of the Mellen Hill quadrangle, Rio Blanco and Moffat 

Counties, Colorado: U.S. Geological Survey Geologic Quadrangle Map GQ–835, scale 

1:24,000.

Hansen, W.R., Carrara, P.E., and Rowley, P.D., 1980, Geologic map of the Haystack Rock 

quadrangle, Moffat County, Colorado: U.S. Geological Survey Geologic Quadrangle 

Map GQ–1535, scale 1:24,000.

Hansen, W.R., and Rowley, P.D., 1980, Geologic map of the Hells Canyon quadrangle, 

Moffat County, Colorado: U.S. Geological Survey Geologic Quadrangle Map GQ–1536, 

scale 1:24,000.

Pipiringos, G.N., and O’Sullivan, R.B., 1975, Chert pebble unconformity at the top of the 

Navajo Sandstone in northeastern Utah: Four Corners Geological Society Guidebook, 

8th Field Conference, Canyonlands 1975, p. 149–156.

Powers, R.B., 1986, The Willow Creek fault, eastern Uinta Mountains-Geological analysis of 

a foreland subthrust play, in Stone, D.S., ed., New interpretations of northwest Colorado 

geology: Denver, Colo., Rocky Mountain Association of Geologists, p. 183–190.

Rowley, P.D., and Hansen, W.R., 1979a, Geologic map of the Plug Hat Rock quadrangle, 

Moffat County, Colorado: U.S. Geological Survey Geologic Quadrangle Map GQ–1514, 

scale 1:24,000.

———1979b, Geologic map of the Split Mountain quadrangle, Uintah County, Utah: U.S. 

Geological Survey Geologic Quadrangle Map GQ–1515, scale 1:24,000.

Van Loenen, R.E., Folger, H.W., Bryant, W.A., and Korzeb, S.L., 1990, Mineral resources 

of the Willow Creek and Skull Creek Wilderness Study Areas, Moffat County, Colorado: 

U.S. Geological Survey Bulletin 1717–D, 20 p.

Van Loenen, R.E., Selner, Gary, and Bryant, W. Anthony, 1999, Geologic map of the Lazy 

Y Point quadrangle, Moffat County, Colorado: U.S. Geological Survey Geologic 

Investigations Series I–2646, scale 1:24,000.

This map was produced on request, directly from 

digital files, on an electronic plotter. It is also available

as a PDF file at http://greenwood.cr.usgs.gov

Geology mapped 1988-89

Edited by Diane E. Lane

Color design by Virginia Scott

Digital cartography by R.E. Van Loenen, 

Gary Selner, and Diane E. Lane

Manuscript approved for publication

June 3, 1998

S K ULL C REE K

M O N O CLI N E

W O L F C R E E K F A U L T

W O L F C REE K

M O N O C L I N E

INDEX MAP SHOWING LOCATION OF THE SKULL CREEK QUADRANGLE

Creek

Vermillion River

Green

River

Yampa

River

Green

Cub Creek

Creek

Creek

Brush

Big

Ashley Creek

Cliff Creek

Little

Brush

Pot Creek

40

40

45

149

45

64

Vernal

Jensen

Dinosaur

Monument

Headquarters

Blue Mountain

Skull

Creek

Elk Springs

Greystone

DINOSAUR

NATIONAL

MONUMENT

UTAH

COLORADO

HAYSTACK

ROCK

GQ-1535

INDIAN

WATER

CANYON

GQ-1516

TANKS

PEAK

GQ-1534

HELLS

CANYON

GQ-1536

STUNTZ

RESERVIOR

GQ-1530

SPLIT

MOUNTAIN

GQ-1515

DINOSAUR

QUARRY

GQ-1513

JONES HOLE GQ-1401

CANYON OF

LODORE

SOUTH

GQ-1403 ZENOBIA

PEAK

GQ-1408

SKULL

CREEK

I-2647

LAZY Y

POINT

I-2646

PLUG HAT

ROCK

GQ-1514

Conway

Draw Douglas

Draw

DOUGLAS

Jones Hole

MOUNTAIN

Gulch

Diamond

Split Mtn

BLUE MOUNTAIN

40o30'

109o30' 109o 108o30'

WOLF CREEK

WOLF CREEK MONOCLINE FAULT

SKULL CREEK

 RIM

SKULL CREEK MONOCLINE

FEET

4000

5000

6000

7000

8000

4000

5000

6000

7000

8000

Phw

hml

hmu

dc

dm

Ppc

Jg

hmu Kcm

Jsr Jm

Jg Je

Jca

Ppc

Kmm Kmf Kd Kmy

Jsc

hml

Phw Phw Older Paleozoic and

Precambrian rocks

Older Paleozoic and Precambrian rocks

dm dc

A A'

FEET

Surficial deposits not shown