Geology of the Lower Yellow Creek Area, 

Northwestern Colorado

U.S. GEOLOGICAL SURVEY BULLETIN 1787-O

 O. .' -' '.'- ". ' ."- '. -' ' ' ,."- "."-:

 - ' ' -' "- ". .! -...
AVAILABILITY OF BOOKS AND MAPS OF THE U.S. GEOLOGICAL SURVEY

Instructions on ordering publications of the U.S. Geological Survey, along with prices of the last offerings, are given in the current-year issues of the monthly catalog "New Publications of the U.S. Geological Survey." Prices of available U.S. Geological Survey publications released prior to the current year are listed in the most recent annual "Price and Availability List." Publications 

that are listed in various U.S. Geological Survey catalogs (see back inside cover) but not listed in the most recent annual "Price and 

Availability List" are no longer available.

Prices of reports released to the open files are given in the listing "U.S. Geological Survey Open-File Reports," updated monthly, which is for sale in microfiche from the U.S. Geological Survey, Books and Open-File Reports Section, Federal Center, Box 

25425, Denver, CO 80225. Reports released through the NTIS may be obtained by writing to the National Technical Information 

Service, U.S. Department of Commerce, Springfield, VA 22161; please include NTIS report number with inquiry.

Order U.S. Geological Survey publications by mail or over the counter from the offices given below.

BY MAIL 

Books

Professional Papers, Bulletins, Water-Supply Papers, Techniques 

of Water-Resources Investigations, Circulars, publications of general interest (such as leaflets, pamphlets, booklets), single copies of Earthquakes 

& Volcanoes, Preliminary Determination of Epicenters, and some miscellaneous reports, including some of the foregoing series that have gone 

out of print at the Superintendent of Documents, are obtainable by mail 

from

U.S. Geological Survey, Books and Open-File Reports

Federal Center, Box 25425

Denver, CO 80225

Subscriptions to periodicals (Earthquakes & Volcanoes and 

Preliminary Determination of Epicenters) can be obtained ONLY from 

the

Superintendent of Documents

Government Printing Office

Washington, D.C. 20402

(Check or money order must be payable to Superintendent of Documents.)

Maps

For maps, address mail orders to

U.S. Geological Survey, Map Distribution

Federal Center, Box 25286

Denver, CO 80225

Residents of Alaska may order maps from

Alaska Distribution Section, U.S. Geological Survey,

New Federal Building - Box 12 

101 Twelfth Ave., Fairbanks, AK 99701

OVER THE COUNTER 

Books

Books of the U.S. Geological Survey are available over the 

counter at the following Geological Survey Public Inquiries Offices, all 

of which are authorized agents of the Superintendent of Documents:

WASHINGTON, D.C.-Main Interior Bldg., 2600 corridor,

18th and CSts.,NW.

DENVER, Colorado-Federal Bldg., Rm. 169, 1961 Stout St.

LOS ANGELES, California-Federal Eldg., Rm. 7638,300 N.

Los Angeles St.

MENLO PARK, Californla-Bldg. 3 (Stop 533), Rm. 3128,

345 Middlefield Rd.

RESTON, Virginia-503 National Center, Rm. 1C402,12201

Sunrise Valley Dr.

SALT LAKE CITY, Utah-Federal Bldg., Rm. 8105,125

South State St.

SAN FRANCISCO, California-Customhouse, Rm. 504,555

Battery St.

SPOKANE, Washington-U.S. Courthouse, Rm. 678, West

920 Riverside Ave..

ANCHORAGE, Alaska~Rm. 101, 4230 University Dr.

ANCHORAGE, Alaska-Federal Bldg, Rm. E-146, 701 C St.

Maps

Maps may be purchased over the counter at the U.S. Geological Survey offices where books are sold (all addresses in above list) and 

at the following Geological Survey offices:

 ROLLA, Missouri-1400 Independence Rd.

 DENVER, Colorado-Map Distribution, Bldg. 810, Federal 

Center

 FAIRBANKS, Alaska-New Federal Bldg., 101 Twelfth Ave.
Chapter O

Geology of the Lower Yellow Creek Area, 

Northwestern Colorado

By WILLIAM J. HAIL, JR.

A multidisciplinary approach to research studies of 

sedimentary rocks and their constituents and the 

evolution of sedimentary basins, both ancient and modern

U.S. GEOLOGICAL SURVEY BULLETIN 1787

EVOLUTION OF SEDIMENTARY BASINS UINTA AND PICEANCE BASINS
DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Any use of trade, product, or firm names in this publication is for descriptive 

purposes only and does not imply endorsement by the U.S. Government.

UNITED STATES GOVERNMENT PRINTING OFFICE: 1990

For sale by the

Books and Open-File Reports Section

U.S. Geological Survey

Federal Center

Box 25425

Denver, CO 80225

Library of Congress Cataloging-in-Publication Data

Hail, William James, 1922-

Geology of the lower Yellow Creek area, northwestern Colorado / by 

William J. Hail, Jr.

p. cm. (U.S. Geological Survey bulletin ; 1787) 

(Evolution of sedimentary basins Uinta and Piceance basins)

Includes bibliographical references.

Supt. of Docs, no.: I 19.3:1787-0

1. Geology Colorado Yellow Creek Watershed (Rio Blanco County) 

2. Geology, Stratigraphic. I. Title. II. Series. III. Series: U.S. Geological 

Survey bulletin ; 1787-O. 

QE75.B9 no. 1787-O 

[QE92.Y4] 

557.3 s-dc20

[557 .881'5 89-600310

CIP
CONTENTS

Abstract Ol 

Introduction O2 

Stratigraphy O3

Subsurface rocks O3 

Cretaceous rocks O3 

Mancos Shale O3 

Main body O4 

Buck Tongue OS 

Fossils O8 

Mesaverde Group O8 

Castlegate Sandstone O8 

lies Formation O9 

Main body O9

Trout Creek Sandstone Member O9 

Williams Fork Formation O9 

Tertiary rocks O10

Fort Union Formation O10

Correlation and history of nomenclature O10 

Lower member Oil 

Upper member O13 

Wasatch Formation O15 

Main body O15 

Dark shale facies O15 

Fossils and correlation O18 

Green River Formation O18

Anvil Points Member O18 

Garden Gulch Member O19 

Parachute Creek Member O20

Uinta Formation and tongues of the Green River Formation O23 

Evacuation Creek Member of the Green River Formation-Uinta

Formation nomenclature change O25 

Tongues of the Uinta Formation O26

Lower group of tongues of the Uinta Formation O26 

Upper group of tongues of the Uinta Formation O27 

Tongues of the Green River Formation O30

Lower group of tongues of the Green River Formation O31 

Upper group of tongues of the Green River Formation O32 

Quaternary deposits O33 

Structure O34

Midland anticline, graben at Pinyon Ridge, and Crooked Wash

syncline O34

Folds and faults in the vicinity of the White River O35 

Red Wash syncline and central graben zone O35 

Yellow Creek anticlinal nose O36

Contents III
Economic geology O36

Oil shale and associated minerals O36 

Coal O37

lies Formation O38

Williams Fork Formation O38

Resources O38 

Oil and gas O38

White River Gas Field O39 

Uranium O42 

Gravel O43 

References cited O43

FIGURES

1. Map showing location of the lower Yellow Creek area, Piceance Creek

basin, northwestern Colorado O2 

2-8. Measured sections of :

2. Mancos Shale showing stratigraphic position of fossil 

collections O7

3. Lower member of Fort Union Formation O12

4. Upper member of Fort Union Formation O14

5. Lower part of main body of Wasatch Formation O16

6. Part of Wasatch Formation and Anvil Points Member of Green 

River Formation O17

7. Garden Gulch Member of Green River Formation O21

8. Parachute Creek Member of Green River Formation O24

9. Chart showing correlation of outcropping mapped units of the

intertonguing sequence of Uinta and Green River Formations in the 

Barcus Creek, Barcus Creek SE, Calamity Ridge and White River City 

quadrangles O28

10. Composite measured section of intertonguing sequence of Uinta and

Green River Formations O29 

11-12. Maps showing:

11. Faults and fold axes in the lower Yellow Creek area O34

12. Area of outcrop of upper member of Fort Union Formation and 

locations of uranium-enriched rocks, lower Yellow Creek 

area O39

TABLES

1. Summary description of exposed bedrock units, lower Yellow Creek 

area O4

2. Description of subsurface rocks penetrated by drilling, lower Yellow Creek 

area O6

3. Correlation of map units of the Uinta and Green River Formations between 

the Rough Gulch and Barcus Creek quadrangles and between the Smizer 

Gulch and Barcus Creek SE quadrangles O25

IV Contents
4. Oil-shale resources in the Barcus Creek quadrangle O37

5. Oil-shale resources in the Barcus Creek SE quadrangle O37

6. Oil and gas drill holes, lower Yellow Creek area O40

7. Description of samples from uranium-enriched localities, lower Yellow Creek 

area O42

CONVERSION FACTORS FOR SOME SI METRIC AND U.S. UNITS OF 

MEASURE________________________________________ 

To convert from_________________To_________________Multiply by 

Feet (ft) Meters (m) 0.3048 

Miles (mi) Kilometers (km) 1.609 

Acres Hectometers2 0.405 

Miles Kilometers2 2.590 

Pounds (Ib) Kilograms (kg) 0.4536 

Gallons Liters (L) 3.785 

Barrels Meters3 0.159 

Tons Metric tons 0.907 

Gallons per ton Liters per metric ton 4.171 

Barrels per acre Meters3 per hectometer2 0.393 

Tons per mile2 Metric tons per kilometer2 0.350

Contents

EVOLUTION OF SEDIMENTARY BASINS-UINTA AND PICEANCE BASINS

Geology of the Lower Yellow Creek Area, 

Northwestern Colorado

By William J. Hail, Jr.

Abstract

The lower Yellow Creek area comprises four 7.5-minute 

quadrangles, an area of 229 mi2 (593 km2), at the northwestern margin of the Piceance Creek basin in northwestern 

Colorado, about midway between the towns of Meeker and 

Rangely. Subsurface sedimentary rocks not penetrated by 

drill holes, but probably present in the area, range in age 

from Cambrian to Pennsylvanian and may be as thick as 

2,500 ft (760 m). Subsurface rocks penetrated by drill holes 

aggregate as much as 6,000 ft (1,830 m) in thickness and 

include the Morgan Formation and Weber Sandstone of 

Pennsylvanian age, the Park City Formation of Permian age, 

the Moenkopi and Chinle Formations of Triassic age, the 

Glen Canyon Sandstone of Triassic and Jurassic age, the 

Entrada Sandstone and Stump and Morrison Formations of 

Jurassic age, and the Cedar Mountain Formation, Dakota 

Sandstone, and lower part of the Mancos Shale of Cretaceous age. Exposed rocks include the Mancos Shale, 

Castlegate Sandstone, lies Formation, and Williams Fork 

Formation of the Mesaverde Group of Late Cretaceous age 

and the Fort Union, Wasatch, Green River, and Uinta 

Formations of Tertiary age. Surficial deposits of Quaternary 

age (Pleistocene and Holocene) include terrace gravels, 

alluvium, and landslides.

Exposed rocks of the Mancos Shale consist mostly of 

gray to brown clay shale of offshore marine origin. The 

Castlegate Sandstone of marine origin separates the main 

body of the Mancos from the overlying Buck Tongue of the 

Mancos. The Buck Tongue includes the Loyd Sandstone 

Bed. The lies Formation includes a marine sandstone at its 

base but is mostly claystone, mudstone, and sandstone and 

some carbonaceous shale and coal of fluvial and paludal 

origin. The Trout Creek Sandstone Member, probably of 

nonmarine origin in this area, lies at the top of the lies. The 

overlying Williams Fork Formation consists of sandstone, 

claystone, and mudstone and lesser carbonaceous shale

Manuscript approved for publication, September 6, 1989.

and coal of fluvial and paludal origin. A regional unconformity 

separates the Williams Fork Formation from overlying Tertiary rocks. The Williams Fork Formation thins as much as 

600 ft (183 m) from east to west across the area, owing to 

truncation of beds beneath the unconformable surface.

The lowermost Tertiary formation is the Fort Union 

Formation of early or middle to late Paleocene age. It is 

divided into two members a lower member consisting of 

fluvial sandstone, claystone, and mudstone, and an upper 

member consisting mostly of paludal shale, sandstone and 

carbonaceous shale and minor coal and siltstone. The 

Wasatch Formation of latest Paleocene and Eocene age is 

mostly varicolored alluvial claystone and lesser mudstone 

and fluvial channel sandstone. It includes a dark shale facies 

consisting of alluvial claystone and channel sandstone, 

lacustrine sandstone, shale and limestone, and paludal carbonaceous shale and claystone. The Eocene Green River 

Formation consists almost entirely of rocks of lacustrine 

origin and includes three members. The eastward-thinning 

basal Anvil Points Member consists mostly of nearshore 

lacustrine sandstone and lesser limestone, siltstone, claystone, and shale. The Garden Gulch Member is mostly 

freshwater lacustrine gray or brown clay shale and includes 

some oil shale and minor sandstone and limestone. The 

Parachute Creek Member consists mostly of dolomitic marlstone, much of which is oil shale, dolomitic shale, and clay 

shale, minor siltstone and sandstone, and several very thin 

altered tuff beds. The youngest Tertiary formation is the Uinta 

Formation of Eocene age, a heterogeneous mass of clastic 

sediments deposited in a generally southward prograding, 

fluvial-deltaic complex that ultimately filled Eocene Lake 

Uinta. The Uinta Formation is complexly intertongued with 

the Green River Formation.

Fold axes and faults in the area trend northwesterly. 

The major structural elements are (from north to south) the 

Crooked Wash syncline, Midland anticline, Red Wash syncline, and Yellow Creek anticlinal nose. A graben zone of 

small stratigraphic displacement closely parallels much of 

the axis of the Red Wash syncline, which forms the deepest 

part of the Piceance Creek basin. Maximum structural relief

Geology of the Lower Yellow Creek Area, Northwestern Colorado 01
on the Precambrian surface in the area is about 15,000 ft 

(4,600 m).

The southern part of the area contains major oil-shale 

resources, estimated to be about 111 billion barrels of oil for 

all the oil-shale zones. Most of the oil shale is in the Parachute 

Creek Member of the Green River Formation. The sodium 

minerals nahcolite and dawsonite are abundant in the area. 

Resources of nahcolite, a potential source of soda ash and a 

gas scrubbing agent, are estimated to be about 8.9 billion 

tons. Resources of alumina in dawsonite are estimated to be 

about 6.1 billion tons.

Coal-bearing zones in the upper, middle, and lower 

parts of the Williams Fork Formation and near the top and 

base of the lies Formation contain considerable coal. The 

coal-resource potential is limited, however, by the nonpersistence of the thicker coal beds.

The White River Gas Field in the northeastern part of the 

area has yielded small amounts of gas, about 7.5 million 

cubic feet as of January 1987, all of it from shallow, lenticular 

Tertiary sandstones. Several deep oil and gas test holes in 

the area have failed to find significant amounts of oil.

Large, but very low grade uranium resources are 

present in carbonaceous shale and claystone beds of the 

upper member of the Fort Union Formation.

Abundant gravel is available from Quaternary terrace 

deposits mostly along the White River.

INTRODUCTION

The lower Yellow Creek area is located in Rio 

Blanco and Moffat Counties of northwestern Colorado, 

about midway between the towns of Rangely and Meeker 

(fig. 1). It includes the Rough Gulch, Smizer Gulch, 

Barcus Creek, and Barcus Creek SE 7.5-minute quadrangles, an area of 229 mi2 (593 km2 ). Paved highways, 

Colorado 64 along the White River and Rio Blanco 

County 5 along Piceance Creek, and a network of dirt 

roads give fair access to most of the area. The climate is 

semiarid, and natural vegetation is largely sagebrush, 

pinyon pine, and juniper. Hay, the principal crop of the 

area, is grown along the irrigated valleys of the White 

River and Piceance Creek. The area is dissected into a 

topography of ridges and valleys of relatively moderate 

relief. Dominant topographic features are the wide 

valleys of the White River, Piceance Creek, and Yellow 

Creek, the high ridges of Pinyon Ridge in the northwest, 

and the ridge forming the basin rim south of the White 

River. The ridges are 1,000-1,200 ft (300-370 m) above 

the valleys below. About 85 percent of the land is owned 

by the Federal Government and 6 percent by the State of 

Colorado; 9 percent is privately owned.

Geologic mapping of the area of this report was 

begun in 1969 as a part of a larger program of detailed 

geologic mapping on 7.5-minute (1:24,000 scale) topographic base maps, undertaken chiefly because of a 

renewed interest in oil-shale resources of the Piceance 

Creek basin. The study area is in the northwestern part of

109°00' 

40°30T

108°30' 108°00' 107°30'

40°00

39°30

River

MOFFAT COUNTY

^^1^ Map area 

COLORADO

 RIO BLANCO COUNTY 

__ -n_ __ __ __ j GARFIELD COUNTY

30 40 MILES

0 10 20 30 40 50 60 KILOMETERS

Figure 1. Location of the lower Yellow Creek area 

(shaded), Piceance Creek basin, northwestern Colorado. 

Quadrangles: A, Rough Gulch; B, Smizer Gulch; C, Barcus 

Creek; D, Barcus Creek SE.

the Piceance Creek basin, a very deep structural and 

sedimentary basin that formed during the Laramide 

orogeny. Potentially important resources in the area are 

oil shale and related minerals, oil and gas, coal, and 

uranium. All four quadrangles of the study area (fig. 1) 

have been published individually, and references in this 

report are made to these published maps. A preliminary 

map of the Barcus Creek SE quadrangle was published 

as MF-347 (Hail, 1972) and is superseded by a revised 

map, GQ-1613 (Hail, 1988). The map of the Smizer 

Gulch quadrangle was published as GQ-1131 (Hail, 

1973) and the map of the Rough Gulch quadrangle as 

GQ-1195 (Hail, 1974a). The map of the Barcus Creek 

quadrangle was published as GQ-1578 (Hail, 1984) and 

supersedes preliminary map MF-619 (Hail, 1974b). 

Some changes and modifications to the various stratigraphic units shown on the individual maps were made 

during the compilation of a l:50,000-scale map of these 

and four other quadrangles to the east, and a modified 

stratigraphic terminology is used in this report.

Prior geologic studies of the area have been parts 

of broader, mostly reconnaissance, studies of the 

Piceance Creek basin and do not include any detailed 

large-scale geologic mapping of the area. Most of the 

older work has been described by Bradley (1931, p. 2-3) 

and Donnell (1961, p. 840). A reconnaissance geologic 

map at a scale of 1:125,000 was published by Gale (1910) 

and shows coal-bearing Cretaceous rocks in the lower 

White River Coal Field, including parts of the Rough 

Gulch and Smizer Gulch quadrangles. A report on the 

Tertiary geology and oil shale of the Piceance Creek 

basin by Donnell (1961) includes a planimetric geologic

02 Evolution of Sedimentary Basins Uinta and Piceance Basins
map at a scale of 1:125,000. A detailed geologic map of 

the Elk Springs quadrangle (scale 1:48,000), which 

immediately adjoins the Rough Gulch and Smizer Gulch 

quadrangles on the north, was published by Dyni (1968). 

Since about 1961, many studies touching on the geology 

of the lower Yellow Creek area have been published and 

most deal with oil shale.

STRATIGRAPHY

Rocks exposed at the surface in the lower Yellow 

Creek area range from Late Cretaceous to late Eocene in 

age. Surficial deposits are Pleistocene and Holocene in 

age. Table 1 summarizes the bedrock stratigraphic units 

exposed in the study area. Cretaceous formations, 

aggregating about 6,600 ft (2,000 m) in thickness, include 

the Mancos Shale, the Castlegate Sandstone, the lies 

Formation, and the Williams Fork Formation. The 

Castlegate, lies, and Williams Fork compose the Mesaverde Group. The Castlegate Sandstone merges into the 

Mesaverde Group west of the study area and it separates 

the main body of the Mancos Shale from the overlying 

Buck Tongue of the Mancos Shale. Tertiary formations, 

aggregating as much as 7,000 ft (2,100 m) in thickness, 

include the Fort Union Formation of Paleocene age, the 

Wasatch Formation of Paleocene and Eocene age, the 

Green River Formation of Eocene age, and the Uinta 

Formation of Eocene age.

A major unconformity separates Cretaceous from 

Tertiary rocks in the study area and throughout the entire 

Piceance Creek basin. Structural discordance, however, 

is only moderate. An unknown thickness of latest 

Cretaceous rocks was removed by erosion during the 

initial stages of the Laramide orogeny. The oldest 

Tertiary rocks lying on the erosion surface in the study 

area are early to middle Paleocene in age. Within the 

Tertiary sequence, only minor and local erosional breaks 

are present.

The Cretaceous formations were deposited along 

the western margin of a north-trending epicontinental 

sea. The Tertiary formations were deposited in a variety 

of terrestrial environments including fluvial, paludal, 

lacustrine, and lacustrine-deltaic.

Subsurface Rocks

Subsurface strata in the lower Yellow Creek area 

above the Precambrian basement range from probable 

Cambrian to Cretaceous in age. Three Shell Oil Company deep drill holes within the area penetrated rocks of 

Pennsylvanian age: Government 31-10, sec 10, T. 2 N., 

R. 98 W.; Colorow 1, sec. 17, T. 2 N., R. 97 W.; and 

Government 22X-17, sec. 17, T. 2 N., R. 97 W. The rocks

penetrated in these drill holes are exposed in nearby 

areas. Table 2 is an interpretive summary of the 

subsurface units based on drill-hole logs and on published descriptions in U.S. Geological Survey maps 

including the Mellen Hill quadrangle (Cullins, 1969) and 

the Plug Hat Rock quadrangle (Rowley and Hansen, 

1979), both about 20 mi (32 km) west of the lower 

Yellow Creek area; the Elk Springs quadrangle (Dyni, 

1968), 14-18 mi (22-29 km) north of the lower Yellow 

Creek area; and the Dinosaur National Monument area 

(Hansen and others, 1983), 8-18 mi (13-29 km) northwest of the lower Yellow Creek area.

Little is known about the rocks older than those 

shown on table 2, but based on descriptions of rocks that 

crop out in nearby areas, the following formations may be 

present: the Lodore Formation of Late Cambrian age, 

the Madison Limestone of Early Mississippian age, the 

Humbug Formation and Doughnut Shale of Late Mississippian age, the Round Valley Limestone of Early 

Pennsylvanian age, and the lower part of the Morgan 

Formation of Middle Pennsylvanian age. The following 

descriptions of the rocks are from Dyni (1968), Hansen 

and others (1983), and Rowley and Hansen (1979). The 

Lodore Formation is a largely marine unit, about 

250-540 ft (76-160 m) thick, consisting of gray to green 

glauconitic sandstone and quartzite, conglomeratic sandstone, and varicolored glauconitic shale and siltstone. 

The Madison Limestone, about 500-600 ft (150-180 m) 

thick, is gray, cherty, locally dolomitic marine limestone. 

The Humbug Formation is largely marine, gray, yellow, 

and red sandstone, gray limestone, and red and black 

shale. The Doughnut Shale is dark-gray and red, mostly 

marine shale. The combined thickness of the Humbug 

and Doughnut is about 100-230 ft (30-70 m). The 

Round Valley Limestone, about 400 ft (120 m) thick, is 

gray, locally cherty marine limestone and lesser gray 

shale. The Morgan Formation, about 500-800 ft 

(150-240 m) thick, is red to brown fine-grained sandstone, gray fossiliferous cherty marine limestone, and 

lesser varicolored shale, siltstone, and sandstone.

Cretaceous rocks

Mancos Shale

The Mancos Shale in the lower Yellow Creek area 

includes the main body and the overlying Buck Tongue. 

The two units are separated by the Castlegate Sandstone 

of the Mesaverde Group. The main body includes a 

mapped sandstone, the B sandstone of Dyni (1968), and 

the Buck Tongue includes a mapped sandstone, the Loyd 

Sandstone Bed. Fossils from about the upper 1,500 ft 

(460 m) of the 2,700 ft (820 m) of exposed rocks of the 

Mancos are Campanian (Late Cretaceous) in age, and it 

is likely that all the exposed rocks are Campanian.

Geology of the Lower Yellow Creek Area, Northwestern Colorado O3
Table 1 . Summary description of exposed bedrock units, lower Yellow Creek area 

[Thickness in feet and in meters (shown in parentheses)]

Age Unit Thickness Description

Eocene Uinta Formation and tongues of Green River 1,400 

Formation (430)

Eocene Parachute Creek Member of Green River Formation 900-1,600

(275-490)

Eocene

Eocene

Eocene and 

Paleocene

Eocene

Garden Gulch Member of Green River Formation 190-700

(58-214)

Anvil Points Member of Green River Formation 40-300

(12-92)

Upper and lower parts (main body) of Wasatch 

Formation

Dark shale facies of Wasatch Formation

1,000-2,100 

(305-640)

0-425 

(0-130)

Intertongued sequence of deltaic clastic rocks 

of Uinta Formation and open-water lacustrine 

rocks of Green River Formation. Tongues of 

Uinta Formation are brown-weathering 

sandstone and siltstone and lesser marlstone, 

mudstone or claystone, conglomerate, and 

conglomeratic sandstone. Tongues of Green 

River Formation are mostly variably silty, 

light-gray marlstone and a considerable 

proportion of limestone and sandstone; very 

minor oil shale.

Dolomitic marlstone; lesser limy marlstone, 

clay shale, dolomitic shale, and siltstone. 

Several beds of siltstone and sandstone and 

numerous very thin beds of analcimized tuff. 

Much of the marlstone and some clay shale is 

rich oil shale that locally contains deposits of 

nahcolite and dawsonite. Lacustrine.

Gray to brown, mostly fissile clay shale; some 

variably ostracodal claystone; minor siltstone 

and ostracodal limestone. Oil shale in upper 

part. Lacustrine.

Brown-weathering ostracodal or oolitic 

sandstone; lesser ostracodal or oolitic 

limestone, claystone, shale, and siltstone. 

Lacustrine.

Gray and varicolored claystone; lesser 

mudstone and lenticular channel sandstone, 

locally conglomeratic; minor carbonaceous 

shale and siltstone. Fluvial.

Gray and varicolored claystone; lenticular 

sandstone; dark-brown or gray ostracodal 

shale, sandstone, and limestone; carbonaceous 

shale and claystone. Fluvial, lacustrine, and 

paludal.

Main body

The main body of the Mancos Shale is mostly 

brown to gray marine shale; it also contains a few 

nonresistant sandstone and siltstone beds and sparse 

yellow- to orange-weathering concretions. The Mancos is 

soft and nonresistant and is mostly very poorly exposed 

except on the steep slopes below cliffs formed by resistant 

sandstones of the Castlegate Sandstone and lies Formation. The Mancos forms the low barren ground in the 

northwest part of the Rough Gulch quadrangle in the 

valley of Wolf Creek where exposures commonly weather 

yellowish brown. The total thickness of the main body of 

the Mancos, including that in the subsurface, is about 

4,500-4,700 ft (1370-1430 m). The thickness of exposed 

rocks of the main body is about 2,200 ft (670 m).

Exposed rocks of the main body of the Mancos 

Shale below the B sandstone of Dyni (1968) lie in the 

fossil zones of Baculites mcleami, Baculites obtusus, and 

Baculites sp. (weak flank ribs) (fig. 2, U.S. Geological 

Survey (USGS) fossil localities D7598, D7612, D7611, 

D7610, D7609, D7608, and D8789).

A thin persistent marine sandstone bed lies about 

150-200 ft (46-60 m) below the top of the main body of 

the Mancos Shale. The top of the bed is shown on the 

Rough Gulch quadrangle map (Hail, 1974a). The unit 

was informally named the B sandstone and mapped by 

Dyni (1968) in the Elk Springs quadrangle to the north. 

The B sandstone is light brown to light gray, rusty brown 

weathering, poorly sorted, fine to medium grained, and 

massive to crudely even bedded. It is moderately resistant

O4 Evolution of Sedimentary Basins Uinta and Piceance Basins
Table 1. Continued

Age

Paleocene

Unit

Upper member of Fort Union Formation

Thickness

300-530 

(92-162)

Description

Gray shale; thin, persistent sandstone beds, 

carbonaceous shale; minor coal and siltstone. 

Paludal.

Paleocene Lower member of Fort Union Formation

Late Cretaceous Williams Fork Formation of Mesaverde Group

Late Cretaceous Trout Creek Sandstone Member of lies Formation

Late Cretaceous Buck Tongue of Mancos Shale

100-1,800 

(30-550)

2,000-2,600 

(610-790)

Late Cretaceous Main body of lies Formation of Mesaverde Group 600-750

(180-230)

0-110 

(0-34)

440-560 

(130-170)

Gray to brown channel sandstone; olive-green 

to gray claystone and mudstone; minor 

siltstone, carbonaceous shale, and limestone. 

Fluvial.

Lenticular, gray sandstone; gray to grayishgreen claystone and mudstone; lesser 

carbonaceous shale and coal. Fluvial and 

paludal. Regional unconformity at top.

Brown sandstone at base (marine), 35-65 ft 

(11-20 m) thick. Higher beds of gray 

claystone, mudstone, and lenticular brown 

sandstone; minor carbonaceous shale and 

coal. Fluvial and paludal.

Light-gray sandstone. Nonmarine.

Brown to gray shale; some siltstone and 

sandstone. Includes Loyd Sandstone Bed. 

Marine.

Late Cretaceous Castlegate Sandstone of Mesaverde Group 

Late Cretaceous Main body of Mancos Shale

40-60 

(12-18)

4,500-4,700 

(1,370-1,430)

Brown to gray sandstone. Marine.

Brown to gray shale; minor sandstone and 

siltstone. Marine. Thickness includes 

subsurface.

relative to the enclosing soft shale, forms a ledge on 

steeper slopes, and caps some low knobs on lower 

ground. It is 10-35 ft (3-11 m) thick. It was mapped to 

the west in the Cactus Reservoir quadrangle (Barnum 

and Garrigues, 1980) and Rangely NE quadrangles (Garrigues and Barnum, 1980) and is apparently correlative 

with the lower unit of the Castlegate Sandstone as 

designated by Cullins (1969, 1971) in the Rangely area. 

Fossil evidence suggests, however, that the B sandstone is 

older than the lower unit of the Castlegate in the Rangely 

area (Gill and Hail, 1975). The B sandstone in the lower 

Yellow Creek area lies in the zone of Baculites mcleami 

(fig. 2, USGS fossil locality D7599).

Buck Tongue

The Buck Tongue of the Mancos Shale is mostly 

brown to gray marine shale and lesser siltstone and 

sandstone. That part of the Buck Tongue below the Loyd 

Sandstone Bed, about 300 ft (90 m) thick, is gray to 

brown, brown-weathering, locally silty, soft clay shale. It

contains relatively abundant yellow-orange septarian concretions. That part of the Buck Tongue above the Loyd 

Sandstone Bed, about 170 ft (52 m) thick, consists of 

dark-gray to brown shale in the lower part; it becomes 

increasingly silty and sandy in the upper part and contains 

several soft siltstone and sandstone beds.

In the lower Yellow Creek area, the Loyd Sandstone Bed has a maximum thickness of about 70 ft 

(21 m). It consists of massive to thin-bedded sandstone 

and lesser sandy shale. Locally abundant trace fossil 

burrows suggest shallow-water deposition. The Loyd is 

relatively resistant and locally forms cliffs and ledges, but 

it is nonpersistent and locally phases out laterally into 

shale marked only by a zone of orange-weathering 

concretions. In the Elk Springs quadrangle to the north, 

the Loyd contains large limy concretions containing 

abundant invertebrate fossils (Dyni, 1968).

Most, if not all, of the Buck Tongue in the area lies 

in the fossil zone of Baculites perplexiis Cobban (fig. 2, 

USGS fossil localities D7607, D7606, D7596, D7595, 

and D7605). In the Winter Valley area of the Elk Springs

Geology of the Lower Yellow Creek Area, Northwestern Colorado 05
m

w

(D 

O.

(D

3

(0 

I

q 5"

Table 2. Description of subsurface rocks penetrated by drilling, lower Yellow Creek area 

[Thickness in feet and in meters (shown in parentheses)]

Age Unit Thickness Descripti

Late Cretaceous Main body of Mancos Shale

Late Cretaceous Frontier Sandstone Member of Mancos Shale

Early Cretaceous Mowry Shale Member of Mancos Shale

Early Cretaceous Dakota Sandstone

Early Cretaceous Cedar Mountain Formation

Late Jurassic Morrison Formation

Middle Jurassic Stump Formation 

Middle Jurassic Entrada Sandstone

Early Jurassic and Glen Canyon Sandstone 

Late Triassic

Late Triassic Chinle Formation

Early Triassic Moenkopi Formation

Early Permian Park City Formation and related rocks

Middle Weber Sandstone

Pennsylvanian 

Middle Morgan Formation

Pennsylvanian_____________

4,500-4,700 

(1,370-1,430)

200

(60)

100 

(30)

95 

(29)

70-100 

(21-30)

460-510 

(140-155)

100-200 

(30-60) 

150-200 

(46-60)

320-380 

(98-116)

255 

(78)

490-600 

(150-180)

130 

(40)

800-850 

(240-260) 

240 

(73)

Brown to gray shale; minor sandstone and siltsto

rocks. 

Brown to gray, fossiliferous, calcareous, fine-gr

and siltstone. Marine.

Dark-gray siliceous shale; some bentonite beds, 

Light-gray, fine- to coarse-grained quartzose san

some dark-gray shale; local thin coal beds. Fl

Varicolored claystone and siltstone; some nodul

conglomerate. Fluvial. Unconformity at top.

Varicolored claystone and siltstone and gray, fin

pebble conglomeratic sandstone and limestone

top.

Light-gray to greenish-gray, locally glauconitic, 

Marine. Unconformity at top. 

Grayish-orange, very fine to fine grained, crossb

at top.

Grayish-orange, very fine to fine grained, crossb

Varicolored, predominantly red, siltstone, shale, 

sandstone. Fluvial, paludal, lacustrine. Unco

Varicolored, mostly red, siltstone and shale. Sh

top.

Light-gray, calcareous siltstone; limestone, dolo

Marine.

Light-gray, fine to very fine grained, crossbedde

Red to brown, fine-grained sandstone. Lesser li
lies Formation of 

Mesaverde Group

Buck Tongue of 

Mancos Shale

Loyd Sandstone Bed 

of Buck Tongue

Buck Tongue of 

Mancos Shale

Castlegate Sandstone of 

Mesaverde Group

Main Body of 

Mancos Shale

B sandstone of MancosN 

Shale of Dyni /

Main Body of 

Mancos Shale

D7607 

'D7606

D7596 

D7595

 D7605

Silty or sandy shale, 

siltstone, thin interbedded sandstone 

and shale

Shale, locally sandy. 

May contain minor 

siltstone or sandstone

Concretion, locally 

fossiliferous

D7599 

D7598

METERS FEET 

r-400

100-

D7612 

D7611

-300

-200

-100

Figure 2. Measured section of Mancos Shale showing stratigraphic position of fossil 

collections in the report area. Collection numbers are USGS Denver catalog numbers. 

Adapted from Pinyon Ridge section of Gill and Hail (1975).

Geology of the Lower Yellow Creek Area, Northwestern Colorado 07
quadrangle, adjoining the study area to the north, Baculites perplexus is present in the Buck Tongue above, 

within, and below the Loyd Sandstone Bed (Gill and 

Hail, 1975).

Fossils

Figure 2 shows the approximate stratigraphic 

position of fossil collections taken from the Mancos Shale 

in the study area. Collections, except as noted, were made 

by James R. Gill, and identifications were made by W.A. 

Cobban.

USGS D7607. SWtfNEVi sec. 19, T. 3 N., R. 98 W. 

Inoceramus subcompressus Meek and Hayden, 

Baculites perplexus Cobban.

USGS D7606. SWViSWVi sec. 18, T. 3 N., R. 98W. 

Pyriporoid bryozoan (attached to baculite), Inoceramus subcompressus Meek and Hayden, 

Cymella montanensis (Henderson), Baculites perplexus Cobban, Placenticeras aff. P. planum 

Hyatt.

USGS D7596. SEV4NWV4 sec. 25, T. 3 N., R. 99 W. 

Inoceramus subcompressus Meek and Hayden, 

Clisocolus sp., Baculites perplexus Cobban.

USGS D7595. SEtfNWtf sec. 25, T. 3N., R. 99 W. 

Inoceramus subcompressus Meek and Hayden, 

Ostrea sp., Anomial sp., Pholadomya sp., Baculites perplexus Cobban, Hoploscaphites n. sp., 

Placenticeras aff. P. planum Hyatt.

USGS D7605. NWViSWVi sec. 19, T. 3 N., R. 98 W. 

Pyriporoid bryozoan, Baculites perplexus Cobban.

USGS D7599. NWV4SEV4 sec. 12, T. 3 N., R. 99 W. 

Baculites mclearni Landes? (juvenile), lobster.

USGS D7598. SEWNWtf sec. 24, T. 3 N., R. 99 W. 

Inoceramus sp., Baculites mcleami Landes? 

(juvenile).

USGS D7612. NWV4 sec. 24, T. 3 N., R. 99 W. 

Inoceramus sp., Pteria sp., Baculites obtusus 

Meek.

USGS D7611. NW'/4 sec. 24, T. 3 N., R. 99 W. 

Baculites obtusus Meek.

USGS D7610. NWK sec. 24, T. 3 N., R. 99 W. 

Baculites sp. (weak flank ribs).

USGS D7609. NWV4 sec. 24, T. 3 N., R. 99 W. 

Cymbophora sp., Baculites sp. (weak flank ribs).

USGS D7608. NWV4 sec. 24, T. 3 N., R. 99 W. 

Baculites sp. (weak flank ribs).

USGS D8789. SWJ/4SW 1A sec. 12, T. 3 N., R. 99 W. 

Inoceramus balticus Boehm, Cymbophora canonensis (Meek), Baculites sp. (probably the weakly 

ribbed species), Hoplitoplacenticeras aff. H. van 

(Schluter). Collected by W.J. Hail.

Mesaverde Group

The term Mesaverde Group in northwestern 

Colorado and northeastern Utah has been used generally 

to include virtually all of the Cretaceous marine sandstones and nonmarine rocks in the complex intertonguing 

sequence in which these rocks are separated by tongues 

of the marine Mancos Shale (Gill and Hail, 1975). In 

this area, the Castlegate Sandstone, the lies Formation, 

and the Williams Fork Formation are regarded as formations of the Mesaverde Group. The Castlegate is a 

southeastward-thinning formation of the Mesaverde 

Group separating the Buck Tongue of the Mancos Shale 

from the main body of the Mancos. Although the Castlegate merges in eastern Utah with rocks of the Mesaverde 

Group owing to the pinchout of the Buck Tongue of the 

Mancos, it has not been consistently regarded as a part of 

the Mesaverde Group. Detailed maps in the Rangely, 

Colo., area by Cullins (1969, 1971), Barnum and Garrigues (1980), and Garrigues and Barnum (1980) do not 

include the Castlegate as a part of the Mesaverde Group 

but include only those Cretaceous rocks above the Buck 

Tongue of the Mancos Shale.

Separation of the Mesaverde Group above the 

Buck Tongue of the Mancos into the lies and Williams 

Fork Formations follows the usage of Dyni (1968) in the 

Elk Springs quadrangle. These names are used to the 

east at the northeast margin of the Piceance Creek basin 

and in the Sand Wash basin farther north and east. The 

two formations are separated on the basis of the presence 

of the Trout Creek Sandstone Member at the top of the 

lies. The Trout Creek pinches out within the lower 

Yellow Creek area, but the names lies and Williams Fork 

are retained where the Trout Creek is absent and the 

contact between the two is placed at the base of the coaly 

carbonaceous zone that elsewhere immediately overlies 

the Trout Creek.

Castlegate Sandstone

The Castlegate Sandstone is a light-brown to 

yellowish-gray, brown-weathering, thin-bedded to massive, very fine to fine grained sandstone. It contains some 

interbedded dark shale, mostly in the lower part. 

Although no fossils were found in the Castlegate in the 

study area, the entire unit is believed to be marine. The 

Castlegate is very hard and resistant and forms persistent 

cliffs and dip slopes. It is the unit mapped by Dyni (1968) 

as sandstone unit C in the Mancos Shale in the Elk 

Springs quadrangle to the north. It has been mapped to 

the west in the Rangely area (Cullins, 1971; Barnum and 

Garrigues, 1980; Garrigues and Barnum, 1980). In the 

study area the Castlegate is about 40-60 ft (12-18 m) 

thick.

O8 Evolution of Sedimentary Basins Uinta and Piceance Basins
lies Formation Trout Creek Sandstone Member

Main Body

The lies Formation consists of a basal marine 

sandstone overlain by a sequence of nonmarine fluvial 

and paludal rocks of which the uppermost unit is, in part 

of the study area, the Trout Creek Sandstone Member.

A persistent cliff-forming marine sandstone lies at 

the base of the lies. This unit, 35-65 ft (11-20 m) thick, 

consists generally of a lower, cliff-forming, brownweathering, fine-grained sandstone and an upper, lightgray-weathering, fine- to medium-grained sandstone. 

Sparse trace-fossil burrows, probably Ophiomorpha, suggest a shallow-water marine origin. This unit is the 

youngest known marine unit in the area. It is probably 

equivalent to the Sego Sandstone as mapped to the west 

in the Cactus Reservoir quadrangle (Barnum and Garrigues, 1980), also the youngest marine unit. Rocks of 

marine origin, if present in younger beds of the lies, have 

not been identified.

The lower part of the main body of the lies, above 

the basal sandstone, is mostly gray claystone and mudstone, relatively thin beds of fine- to medium-grained 

sandstone, and abundant carbonaceous shale. The sandstones are variably resistant, forming alternating soft 

slopes and some resistant ledges. A persistent coaly 

carbonaceous shale zone lies just above the basal marine 

sandstone. Coal beds in this carbonaceous zone are 

nonpersistent but locally are as thick as 3.4 ft (1 m). 

These beds have been locally removed by scour and the 

interval is now occupied by sandstone beds. Other carbonaceous shale zones in the lower part of the lies 

contain only a few thin shaly coal beds. This part of the 

lies reflects deposition mostly in a coastal-plain fluvial 

and paludal environment.

The upper part of the main body of the lies 

con-sists of thick, lenticular, massive, brown-weathering 

sandstone that forms resistant ledges and cliffs and lightgray-weathering mudstone and claystone. Carbonaceous shale is rare except near the top of the unit where 

a few thin coal beds are present. These rocks repre 

sent deposition in a dominantly fluvial, flood-plain 

environment.

The main body of the lies above the basal marine 

sandstone (Sego equivalent) is correlative to the west 

with the lower unit of the Mesaverde Group and that part 

of the coal unit of the Mesaverde Group below the base 

of the main coal zone as mapped in the Cactus Reservoir 

quadrangle (Barnum and Garrigues, 1980) and in the 

Rangely NE quadrangle (Garrigues and Barnum, 1980). 

The main body of the lies above the basal marine 

sandstone is about 600-750 ft (180-230 m) thick.

A conspicuous light-gray, fine-grained, crossbedded nonmarine sandstone was mapped by Dyni 

(1968) in the Elk Springs quadrangle as the Trout Creek 

Sandstone Member. The sandstone was mapped southward into the study area to a point in sec. 17, T. 3 N., 

R. 98 W., where it pinches out and cannot be traced 

farther south as a continuous unit. In the Meeker area 

and farther to the east, the Trout Creek is a nearshore 

marine unit. The unit crops out about 12 mi (19 km) 

northeast of the study area in the Citadel Plateau 

quadrangle, where it locally contains the marine trace 

fossil Ophiomorpha (Izett and others, 1984). In the lower 

Yellow Creek area, the Trout Creek is a very light gray to 

light-brown, massive to crossbedded, fine- to mediumgrained sandstone and is entirely nonmarine. The 

maximum thickness of the Trout Creek is about 110 ft 

(34 m) near the northern boundary of the area.

Williams Fork Formation

The Williams Fork Formation of Late Cretaceous 

age is entirely of nonmarine origin in the lower Yellow 

Creek area. It consists of approximately equal proportions of channel and other fluvial sandstones and 

fine-grained rocks, mostly claystone, shale, and mudstone. Coal beds are present in the upper and lower parts 

of the formation. The formation ranges in thickness from 

about 2,000 ft (610 m) at the west edge of the study area 

to about 2,600 ft (790 m) near Crooked Wash in the 

eastern part of its outcrop area. The rocks were 

deposited in environments ranging from fluvial flood 

plain to coastal plain.

The sandstone is mostly nonpersistent, lenticular, 

light to medium gray, crossbedded to massive, and very 

fine to medium grained; many sandstone beds are thin, 

even bedded, and tabular. Limy concretions are abundant in the thicker sandstones, many of which are conglomeratic and contain claystone or other soft-rock 

clasts. Most of the sandstone beds are relatively resistant 

and form strong cliffs or ledges. Claystone, shale, and 

mudstone are gray to grayish green and commonly 

contain orange-weathering limy septarian concretions, 

especially in the upper part of the formation. Most of the 

claystone and mudstone beds are soft and nonresistant 

and form slopes. Brown carbonaceous shale and claystone are relatively abundant.

Two major coal-bearing carbonaceous zones are 

present in the Williams Fork. Coal beds occur in 

discontinuous carbonaceous shale zones, and individual 

coal beds are lenticular and nonpersistent. Coal beds are 

locally scoured and replaced by overlying channel 

sandstones. The lower coal-bearing carbonaceous zone 

lies chiefly within the lower 300 ft (92 m) of the 

formation and persists throughout the area and

Geology of the Lower Yellow Creek Area, Northwestern Colorado 09
continues westward into the Rangely area. The upper 

coal-bearing zone lies mostly within the upper third of 

the formation near Crooked Wash, where the formation 

reaches its maximum thickness of about 2,600 ft (790 m). 

The upper zone does not persist into the western part of 

the area, and erosional truncation from east to west may 

account in part for the absence of the upper coal-bearing 

zone in the western part of the area.

The Cretaceous-Tertiary boundary in the study 

area and throughout the Piceance Creek basin is an 

unconformity below which a considerable thickness of 

upper Cretaceous beds was removed. In the study area, 

the Williams Fork thins as much as 600 ft (180 m) from 

east to west below the unconformity. Much of this 

thinning is probably due to pre-Tertiary erosion, 

presumably the result of early Laramide uplift along the 

Douglas Creek arch to the west.

Geologic maps of the Rough Gulch and Smizer 

Gulch quadrangles (Hail, 1974a, 1973) show a stratigraphic unit designated Ohio Creek(?) Formation of 

Paleocene age. It was mapped as a thin, nonpersistent 

unit at the Cretaceous-Tertiary boundary and consists of 

very light brown to white sandstone that locally contains 

very sparse chert or quartzite fragments. A study by 

Johnson and May (1980) shows that the Ohio Creek at its 

type locality and elsewhere in northwestern Colorado is, 

for the most part, a weathered zone beneath the preTertiary erosion surface. Weathered sandstone in this 

zone is commonly white or very light gray, owing to 

kaolinization of feldspars in the sandstone (Johnson and 

May, 1980, p. 11-12). Furthermore, the uppermost Cretaceous beds are locally pebbly, as are certain Tertiary 

beds above the unconformity (Johnson and May, 1980, 

fig. 2, p. 6, 14, 15, and 23). Thus, the Ohio Creek, as 

commonly used in maps and other studies, does not 

constitute a lithostratigraphic unit. In the lower Yellow 

Creek area, most of the rocks previously designated as 

Ohio Creek(?) Formation are herein tentatively included 

in the Williams Fork Formation.

Three localities in the Williams Fork Formation 

yielded identifiable fossils. W.A. Cobban identified the 

following mollusks from USGS fossil locality D7058: 

Tulotomops thompsoni (White), and Lioplacoides cf. L. 

nebrascensis (Meek and Hayden); the gastropod T. 

thompsoni is restricted to the Lance Formation and 

equivalent formations. The locality lies about 250 ft 

(76 m) below the top of the Williams Fork in the 

SWtfSEVi sec. 31, T. 3 N., R. 98 W. Robert H. Tschudy 

(USGS) identified fossil pollen from USGS paleobotany 

localities D4773 and D4777. Locality D4773 yielded 

Aquilapollenites, Proteacidites, Araucariacites, and Ephedra (small), all of which indicate a Late Cretaceous age 

no older than Campanian. This locality lies about 520 ft 

(160 m) below the top of the Williams Fork Formation in 

the SWWNWK sec. 11, T. 3 N., R. 98 W. Locality D4777

yielded Proteacidites, Aquilapollenites, Liliacidites cornplexus, Eucommiidites, and Gunnera, which also indicate 

a Late Cretaceous age no older than Campanian. This 

locality lies about 80 ft (24 m) below the top of the 

Williams Fork in the SEtfNWtt sec. 30, T. 3 N., R. 97 W. 

The Williams Fork Formation is correlative to the 

east with the Williams Fork Formation on the eastern 

margin of the Piceance Creek basin, although the 

Williams Fork in that area is considerably thicker below 

the pre-Tertiary unconformity and probably includes 

younger rocks. Near Meeker, about 20 mi (32 km) 

southeast of the study area, the Williams Fork is about 

5,000 ft (1,520 m) thick (Hancock and Eby, 1930, p. 197). 

Northwest of Meeker along Strawberry Creek, about 

17 mi (27 km) east of the study area, the Williams Fork 

is about 4,000 ft (1,220 m) thick (Pipiringos and Rosenlund, 1977b). In the Citadel Plateau quadrangle, about 

10 mi (16 km) northeast of the area, the Williams Fork is 

about 3,800 ft (1,160 m) thick (Izett and others, 1984). 

To the west, in the Rangely area, the Williams Fork is 

correlative with the upper unit of the Mesaverde Group 

and that part of the coal unit of the Mesaverde Group 

above the base of the main coal zone (Barnum and 

Garrigues, 1980; Garrigues and Barnum, 1980).

Tertiary Rocks

Fort Union Formation

A lithologically distinct sequence of Paleocene 

rocks comprises the basal Tertiary section throughout 

the northern part of the Piceance Creek basin and much 

of the basin. In the lower Yellow Creek area, these rocks 

have been designated Fort Union Formation. The 

formation consists of a lower member of fluvial channel 

sandstone and claystone and an upper member of mostly 

paludal shale and thin sandstone.

Correlation and History of Nomenclature

A distinctive sequence of Paleocene rocks has long 

been recognized in both the Sand Wash and Piceance 

Creek basins of Colorado. Early studies, along with 

geologic mapping in the Sand Wash basin, recognized the 

distinctive character of the Paleocene beds. These strata 

were termed "post-Laramie" beds in most of the early 

studies, at a time when the now geographically restricted 

term Laramie was applied to the latest Cretaceous rocks 

in the Sand Wash basin. These post-Laramie rocks were 

recognized as Paleocene in age, based on fossil leaves 

and their stratigraphic position beneath the largely 

Eocene Wasatch Formation. Bass and others (1955, 

p. 161) used the name Fort Union Formation for these 

rocks in the Sand Wash basin, and the name has been

O10 Evolution of Sedimentary Basins Uinta and Piceance Basins
used in most subsequent reports. Correlative rocks in the 

Piceance Creek basin, however, generally have been 

included with the Wasatch Formation.

In his description of the western Yampa Coal Field, 

Gale (1910, p. 223, and pi. 16) suggested the possibility of 

using the term Fort Union for the "post-Laramie" rocks 

in the Sand Wash basin north of Lay and Craig, Colo. 

Gale (1910, p. 44) described these rocks as a basal 

conglomeratic sandstone and massive, white or lightcolored sandstone and shale that contain valuable coal 

beds. He retained the term "post-Laramie," however, 

because of the uncertainty of a Tertiary age determination. He tentatively correlated basal Tertiary beds in 

the northern Piceance Creek basin with the Fort Union 

but did not map a unit separate from the Wasatch. "On 

the geologic maps no attempt has been made to separate 

Fort Union from Wasatch in the Grand and White River 

region, as it is not yet clear that two such formations can 

be differentiated or at least are to be distinguished by any 

recognizable features in this region" (Gale, 1910, p. 80, 

81). His map unit Tw for the Piceance Creek basin map 

is designated "Wasatch formation (including Fort 

Union?)" (Gale, 1910, pis. 10, 16,18, and 19).

Sears (1925, p. 291-292), in his description of rocks 

in the Sand Wash and Piceance Creek basins, followed 

Gale's usage and showed "post-Laramie" rocks in the 

Sand Wash basin but included probably correlative rocks 

in the Piceance Creek basin in the Wasatch. Hancock and 

Eby (1930, p. 209), in their report on the Meeker 

quadrangle on the northeastern margin of the Piceance 

Creek basin, also included all pre-Green River Formation Tertiary rocks in the Wasatch Formation. They 

described the Wasatch as comprising two units: a lower 

unit, as thick as 4,180 ft (1,270 m), of sandstone, sandy 

shale and conglomerate, and an upper unit of varicolored 

clay and shale. They described the lower unit: "In lithology and stratigraphic occurrence these beds bear a 

striking resemblance to the post-Laramie beds of the 

Yampa field, which are most likely of Fort Union age. "

Donnell (1961, p. 844-846) described an unnamed 

Paleocene unit that is probably correlative with the Fort 

Union and probably underlies much of the Piceance 

Creek basin between the Colorado and White Rivers. 

This unit is as thick as 500 ft (150 m) along the Grand 

Hogback at the eastern margin of the basin and consists 

of massive brown and gray sandstone beds, gray and 

brown clay and shale beds, and a few thin coal beds. The 

unit was not mapped separately but was included with the 

Wasatch Formation. In a later report, Donnell (1969, 

p. 1-11, 13-14) assigned the Paleocene beds in the 

southern part of the Piceance Creek basin to the Atwell 

Gulch Member of the Wasatch Formation.

In the White Rock quadrangle, 9-12 mi (14- 

19 km) east of the lower Yellow Creek area at the 

northeastern margin of the Piceance Creek basin, Pipiringos and Rosenlund (1977a) mapped a sandstone 

member of the Fort Union Formation that is about 

1,200-1,400 ft (370-430 m) thick. They included in the 

overlying Wasatch Formation a sequence of Paleoceneage beds at least 735 ft (224 m) thick and described these 

beds as drab-gray and white claystone, a few ferruginous 

zones, and at least one brown carbonaceous shale bed. 

Izett and others (1984) extended the Fort Union northwestward in the Citadel Plateau quadrangle, where it is 

about 1,500-2,300 ft (460-700 m) thick. In the Indian 

Valley quadrangle, to the west of the White Rock quadrangle, Pipiringos and Rosenlund (1977b) determined 

from an analysis of drill-hole information that Paleocene 

rocks may range in thickness from about 1,500 to 2,200 ft 

(460-670 m).

In the Elk Springs quadrangle, immediately north 

of the lower Yellow Creek area at the northwestern edge 

of the Piceance Creek basin, Dyni (1968) designated all 

pre-Green River Tertiary beds, including undifferentiated rocks of Paleocene age at the base, as "Wasatch 

Formation and associated rocks." Both members of the 

Fort Union of the lower Yellow Creek area are present in 

the basal part of the Tertiary in the Elk Springs 

quadrangle (Hail, 1981), and in a drill hole at the eastern 

margin of the quadrangle the Fort Union is as thick as 

2,200 ft (670 m).

The Fort Union extends westward and persists 

around the northwestern margin of the Piceance Creek 

basin and thence southward into the Cathedral Bluffs 

area. In the Cactus Reservoir quadrangle, about 8 mi 

(13 km) west of the lower Yellow Creek area, Barnum 

and Garrigues (1980) included these beds in the lower 

member of the Wasatch. In the Calamity Ridge quadrangle, about 8 mi (13 km) southwest of the study area, 

Donnell and Hail (1984) also included these beds in the 

basal part of the main body of the Wasatch Formation.

Lower Member

Throughout most of the study area, the lower 

member of the Fort Union Formation consists of 

approximately equal proportions of light-gray to lightbrown, fluvial-channel sandstone and olive-green to gray, 

fluvial-plain claystone and mudstone. Minor constituents 

are siltstone, carbonaceous shale, and limestone. In the 

outcrop area north and east of the White River, the 

sandstones form resistant, conspicuous cliffs and ridges, 

but the claystones and mudstones are relatively nonresistant. The lower member is early or middle Paleocene to late Paleocene in age.

Sandstone generally is present at the base of the 

lower member and commonly forms the thickest 

sandstone sequence in the member (fig. 3). The basal 

contact with the Williams Fork Formation is sharp and 

unconformable. The basal sandstone commonly lies on

Geology of the Lower Yellow Creek Area, Northwestern Colorado 011
O^

roS1

o_

J*

o o

fi>3a

-Q 

o'

(D

3 

O 

(D

ff

^03

to CD « 

CD aw

GO Sr _ 

CD m ^

- 3 W

Z < ®

- "I °-

-n *^ W 

^CO ® CD® a

00 P O

«r- _. 3

5s 3

CO CD

< O

» 3 

3 0) 5:5-

n

I 

t !

»5* |a

1 

s.

Q n w

^ 3- £ 

! § "

HHHo iv?*-:;-i -^

f P c3

I t i

§ § g-

^ s s- 8. 8.

" S

i I

' H;vS '

0) Q) 

CD -i

"1

0) Er
eroded and locally bleached carbonaceous and other 

beds of the Williams Fork. Beds of the upper coalbearing zone of the Williams Fork are truncated at the 

contact. Overlying beds of the lower member of the Fort 

Union contain only a few very thin carbonaceous beds. 

The basal sandstone is mostly massive but is locally 

crossbedded and contains limy spheroidal concretions. A 

few lenses of soft-pebble conglomerate are present 

locally, at or near the base. Most of the sand is medium 

grained, but grain size varies widely. Stratigraphically 

higher sandstone beds in the lower member also are 

mostly nonpersistent channel deposits. They are massive 

to crossbedded, mostly fine to medium grained and 

rarely coarse grained, and mostly calcareous. The more 

resistant beds are firmly cemented with calcite. A few 

conglomeratic lenses are present, mostly near the bases 

of the sandstone beds. These lenses consist of claystone 

clasts and lesser siltstone or sandstone clasts in a poorly 

sorted calcareous matrix of medium- to coarse-grained 

quartz sand and abundant dark rock and mineral 

fragments.

Claystone and mudstone of fluvial origin in the 

lower member are mostly olive green to greenish gray. 

The greenish color is a conspicuous characteristic of the 

member in the outcrop area north of the White River. A 

few red or drab-purplish beds or weathered zones are 

present locally. Most claystone and mudstone beds are 

poorly exposed and nonresistant and form valleys and 

covered slopes between the resistant sandstone beds. 

Some resistant cliff-forming mudstone beds occur 

locally. The claystone beds are variably silty and locally 

contain beds of clayey siltstone. Wherever silt or sand is 

abundant, the beds are termed mudstone. Bedding is 

mostly absent, although some shaly structure is present 

in the rare carbonaceous shale or dark-gray shale beds.

The lower member thins markedly to the west 

within the area. At the eastern edge of its outcrop area it 

is about 1,800 ft (550 m) thick, based on subsurface 

information. Near the mouth of Crooked Wash, 5 mi 

(8 km) to the west, it is 1,140 ft (350 m) thick (fig. 4). At 

the east end of Stadtman Mesa, 4 mi (6.5 km) farther 

west, it is only about 400 ft (120 m) thick, and at the west 

end of Stadtman Mesa, 3.5 mi (5.5 km) farther west, it is 

only about 150 ft (45 m) thick. This westward thinning of 

about 1,650 ft (500 m) in a distance of less than 13 mi 

(21 km) is believed to be the result of depositional onlap 

on the rising Douglas Creek arch during Paleocene time 

(Johnson and Keighin, 1981, p. 203). The Douglas Creek 

arch apparently began to rise in Late Cretaceous time 

and continued rising until early to middle Eocene time 

(Tweto, 1975, p. 34; Johnson and Keighin, 1981, p. 203). 

Westward thinning of the lower member of the Fort 

Union by onlap on the Douglas Creek arch coincides 

with westward erosional thinning of the underlying

Cretaceous rocks. Total thinning in the Williams Fork 

and Fort Union Formations is as much as 2,250 ft 

(690 m) within the Yellow Creek area. At the western 

margin of the area, where the lower member of the Fort 

Union is thinnest, sandstone beds are mostly absent and 

the member consists mostly of claystone. Beds of middle 

Paleocene age are present at least 4 mi (6.5 km) west of 

the area in the Divide Creek quadrangle (R.H. Tschudy, 

written commun., 1975).

Two localities in the lower member of the Fort 

Union Formation yielded identifiable fossils. R.H. 

Tschudy (written commun., 1971) identified fossil pollen 

from USGS fossil localities D4629 and D4633. Locality 

D4629 "yielded a middle to early Paleocene assemblage 

of plant microfossils. The presence of Maceopolipollenites in the sample suggests, however, that it is not 

representative of the earliest part of the Paleocene." This 

locality lies about 300 ft (90 m) above the base of the 

member in the SWtfNWtf sec. 29, T. 3 N., R. 97 W. 

Locality D4633 "yielded a good middle to late Paleocene 

suite of plant microfossils." This locality lies about 250 ft 

(76 m) below the top of the member in the SEViSEVi sec. 

31, T. 3 N., R. 97 W. Several collections of freshwater 

mollusks, mostly gastropods, from the lower member 

were identified by W.A. Cobban (oral commun., 1970) as 

being of Tertiary age.

Upper Member

The upper member of the Fort Union Formation 

consists mostly of paludal and possibly some lacustrine 

deposits: gray shale, thin persistent sandstone beds, carbonaceous shale and claystone and some coaly carbonaceous shale and coal, and minor siltstone. Olive-green 

claystone is present in the lower part of the upper 

member in the transition zone between the fluvial claystones of the lower member and the overlying paludal 

beds of the upper member. The basal contact is arbitrarily placed within this transition zone and may not 

everywhere be at the same stratigraphic horizon. In the 

area near the mouth of Colorow Gulch (fig. 4), the 

contact is placed at the base of the lowest of several 

conspicuous carbonaceous shale beds typical of higher 

carbonaceous beds throughout the upper member. The 

distinctive darker color of the upper member is in 

contrast to the lighter colors of the underlying lower 

member and the overlying Wasatch Formation. Resistant 

sandstone beds locally form cliffs and ledges, but the 

more common shale beds are nonresistant and form a 

subdued topography of poorly exposed rocks. The upper 

member is late Paleocene in age.

The dominant lithology of the upper member is 

gray clay shale. This share locally grades laterally to 

brown carbonaceous shale and claystone with a corresponding increase in sapropelic material; some thin carGeology of the Lower Yellow Creek Area, Northwestern Colorado 013
B

Wasatoh _____ 

Formation .-=.-=--=L-=L\

 ~- . - .-?r*a

I

Lower 

member

Wasatch 

Formation - = 

I

& 

.2

D

EXPLANATION

Shale and carbonaceous 

shale (heavy bars)

Claystone

Siltstone

Sandstone

Conglomeratic sandstone

| I | Poorly exposed interval

i

Lower 

member

I

METERS 

60 I

40 

20 

FEET 

-200

 100

Figure 4. Measured sections of upper member of Fort Union Formation. A, Near mouth of 

Yellow Creek in SE!/4 sec. 4, T. 2 N., R. 98 W. B, Near mouth of Colorow Gulch in NW!/4 

sec. 1, T. 2 N., R. 98 W. Township-rangemap shown in figure 12.

bonaceous shale grades laterally into coal. Coal beds are 

sparse and very thin, generally only a few inches thick. 

Most of the sandstone is gray to dark brown, fine grained 

and calcareous, and contains much non-quartz mineral 

or rock constituents. The sandstone beds are thin, 

commonly even bedded, and relatively persistent. Most 

are firmly calcite cemented, hard, and resistant. A few 

crossbedded sandstones are present, and from near 

Yellow Creek westward the upper member contains 

several apparent channel sandstones, some containing 

soft-pebble conglomeratic lenses.

The upper member is about 330-530 ft (100- 

160 m) thick and exhibits no consistent thinning trend. 

Differences in thickness may be due in part to the 

indefinite lower boundary

Three localities yielded identifiable fossils in the 

upper member of the Fort Union. R.H. Tschudy (written 

commun. 1971) identified palynomorphs from USGS 

fossil localities D4631, D4632, and D4637. Locality 

D4631 yielded Carya, Abietineaepollenites, Maceopolipollenites tenuipolis, Pediastrum, Botryococcus, and Pistillipollenites, a late Paleocene assemblage. This locality is

014 Evolution of Sedimentary Basins Uinta and Piceance Basins
65 ft (20 m) above the base of the upper member in the 

SWtfNWH sec. 1, T. 2 N., R. 98 W. Locality D4632 

yielded Carya, Ulmus-Zelkova,Inaperturopollenites,Abietineaepollenites, Pistillipollenites, and Erdtmanipollis, a 

definitive late Paleocene palynomorph assemblage. This 

locality is about 50 ft (15 m) below the top of the upper 

member in the NEtfSWtf sec. 1, T. 2 N., R. 98 W. 

Locality D4637 yielded the same forms as D4632, as well 

asAlnus, Pediastrum, and Botryococcus, a late Paleocene 

pollen and spore assemblage. Locality D4637 is about 

150 ft (46 m) below the top of the upper member in the 

sec. 4, T. 2 R, R. 98 W.

Wasatch Formation

The Wasatch Formation in the lower Yellow Creek 

area consists mostly of alluvial claystone and sandstone. 

Throughout much of the study area, a dark shale and 

claystone facies is present in the upper part of the 

Wasatch and consists of rocks of alluvial, paludal, and 

lacustrine origin. The Wasatch lies conformably on the 

upper member of the Paleocene Fort Union Formation. 

The boundary marks the change across a narrow interval 

from paludal rocks of the upper member of the Fort 

Union to alluvial rocks of the Wasatch. The contact 

commonly is placed at the base of the lowermost beds of 

red or purple claystone or of fluvial sandstone of the 

Wasatch above the dark-gray clay shale or brown carbonaceous beds of the Fort Union. The lowermost beds of 

the Wasatch are late Paleocene in age and do not exceed 

a few tens of feet in thickness. All higher beds of the 

Wasatch are Eocene in age.

The diverse rock types of the Wasatch form a 

varied topography. In general, the lower part of the main 

body of the Wasatch contains thicker and more resistant 

sandstone beds than does the upper part and forms 

prominent ledges and ridges. The higher ground of the 

divide between Smizer and Colorow Gulches is formed 

mostly on these beds. The claystones are relatively 

nonresistant and form slopes, valleys, or badland 

topography. The strike valley of Smizer Gulch is cut into 

the nonresistant claystones above the more resistant 

sandstone sequence. Resistant thin sandstones of the 

dark shale facies form strong ledges and dip slopes that 

dominate the topography of the outcrop area just east of 

the White River. In the southeastern part of the area, in 

the drainages of Kissinger, Short, and Tom Little 

Gulches, a rough topography of gullies, benches, and 

badlands is cut on the main body of the Wasatch.

Main Body

The following description applies to all alluvial 

beds of the Wasatch, above, below, and lateral to the dark

shale facies. The dominant rock type in the main body is 

gray, greenish-gray, brown, and varicolored claystone. 

The varicolored rocks are green, purple, red, and 

yellowish brown and generally are more abundant in the 

upper part of the main body than in the basal few 

hundred feet. The brighter colored, red or purple varicolored rocks are the most conspicuous feature of the 

main body of the Wasatch. Some of the claystone 

especially near the base, locally has shaly bedding. For 

the most part, however, the claystone lacks shaly bedding 

and weathers out into blocky fragments. It is locally silty 

or sandy and grades into mudstone. Abundant fluvial 

channel sandstone beds are mostly fine grained, but some 

of the thicker beds are medium to coarse grained. A few 

contain soft-pebble conglomerate beds. Concentrations 

of freshwater mollusks are locally abundant in some of 

the sandstone beds. Most of the sandstone beds are 

lenticular and nonpersistent and show little or no discernable bedding; some, however, are locally crossbedded or even bedded. The sandstone beds are mostly light 

gray to light brown, locally concretionary, and highly 

calcareous and form resistant ledges where firmly calcite 

cemented. The thickest sandstone beds generally are in 

the lower part of the main body and are as thick as 50 ft 

(15 m). The lithology of the Wasatch, as exposed along 

the east side of Yellow Creek, is shown in figures 5 and 6.

Other rock types in the main body of the Wasatch 

are sparse. They include a few beds of brown carbonaceous shale and siltstone, very thin beds of ostracodal 

sandstone or limestone, and beds of dark-gray shale of 

probable lacustrine origin.

The rocks of the main body of the Wasatch were 

probably deposited on an arid fluvial plain that contained 

a few small ponds and swamps. The Wasatch, including 

the dark shale facies, ranges in thickness from about 

1,000 ft (300 m) at the western edge of the area to about 

2,100 ft (640 m) in the northeastern part of the area; it 

apparently continues to thicken east of the area. 

Equivalent strata in the eastern parts of the Indian Valley 

and White River City quadrangles, just east of the lower 

Yellow Creek area, are about 3,400 ft (1,040 m) thick 

(Pipiringos and Johnson, 1976; Pipiringos and Rosenlund, 1977b).

Dark Shale Facies

On the geologic maps of the Rough Gulch and 

Smizer Gulch quadrangles (Hail, 1973, 1974a), a 

separate map unit (Twl) of the Wasatch was described as 

containing some probable lacustrine beds. The unit was 

mapped separately only west of the White River.

Geology of the Lower Yellow Creek Area, Northwestern Colorado 015
£

o

Dark 

shale 

facies

.S

03

S

 s

ml ==^^=^3 O0

I

&

n

X X

« «^_ M_M *J

Fort Union 

Formation

METERS FEET 

60-T-200

40 

20-

 100

EXPLANATION

Shale, gray, minor purple 

Qaystone, gray and brown

Sandstone 

Conglomeratic sandstone

Claystone, varicolored I I | Poorly exposed interval

Siltstone Ostracodes

Figure 5. Measured section of lower part of main body of Wasatch Formation. East side 

of Yellow Creek in SE1/4 sec. 4, SW1/4 sec. 3, NWV4 sec. 10, and NE!/4 sec. 9, T. 2 N., 

R. 98 W. Township-range map shown in figure 12.

Subsequent mapping (Hail and Pipiringos, in press) 

extends the unit for several miles east of the White River, 

almost to the east boundary of the area, insofar as the 

unit can be separated from the main body of the Wasatch 

The dark shale facies comprises a wide variety of 

rock types including green or gray, locally varicolored, 

alluvial claystone; fluvial channel sandstone; lacustrine

ostracodal sandstone, shale, and limestone; and paludal 

carbonaceous shale and claystone. The unit lacks the 

prominent light-colored and varicolored claystones of the 

main body of the Wasatch and generally presents a 

relatively dark aspect. The even-bedded shale and thin, 

persistent lacustrine sandstone and limestone give the 

unit a layered appearance.

O16 Evolution of Sedimentary Basins Uinta and Piceance Basins
Anvil Points 

Member of Green 

River Formation

tion

Uppe

fac Dark shale

Lower 

part

METERS 

60 1

40-

20 

FEET 

-200

 100

g 8

O

EXPLANATION

Shale

Qaystone, gray and brown III

Conglomeratic sandstone 

Limestone

Claystone, varicolored I T { Poorly exposed interval 

Siltstone O Ostracodes 

Sandstone

Figure 6. Measured section of part of Wasatch Formation and Anvil Points Member of Green River 

Formation. East side of Yellow Creek in NE1/4 sec. 9 and NW1/4 sec. 10, T. 2 N., R. 98 W. Township-range map 

shown in figure 12.

Johnson (1984, p. 13-19, pi. 1) correlated the dark 

shale facies with the newly named Cow Ridge Member of 

the Green River Formation and designated the exposure 

on the east side of Yellow Creek (Rough Gulch 

quadrangle) as a reference section of the Cow Ridge. The 

Cow Ridge is the oldest member of the Green River

Formation and represents an early Eocene to early 

middle Eocene freshwater stage of Lake Uinta (Johnson, 

1984, p. 4).

At Yellow Creek, the dark shale facies includes 

ostracodal, probably lacustrine, brown to gray shale in 

the lower part and near the top. Thin ostracodal

Geology of the Lower Yellow Creek Area, Northwestern Colorado 017
limestone and limy sparsely ostracodal sandstone are 

present throughout the unit and form resistant ridges and 

dip slopes. Nonostracodal, gray to greenish-gray claystone beds comprise almost half of the unit and appear to 

be typical Wasatch alluvial claystone.

In the outcrop area east and north of the White 

River, the dark shale facies contains abundant lacustrine 

ostracodal sandstone, limestone, and shale as at Yellow 

Creek. It contains a greater proportion of paludal carbonaceous shale and dark, locally purple weathering, 

alluvial claystone than at Yellow Creek. Several massive 

to crossbedded channel sandstone beds also are present.

The dark shale facies is about 425 ft (130 m) thick 

at Yellow Creek but thins abruptly to the west. It was 

traced as a map unit to a point about 3 mi (5 km) west of 

Yellow Creek in sec. 1, T. 2 N., R. 99 W., where it is only 

a few feet thick. In the area north and east of the White 

River, it reaches a maximum thickness of about 250 ft 

(76 m) and, because the base is not exposed, may be 

thicker. Near the eastern edge of the mapped exposures, 

where top and base are both present, the unit is only 

about 100 ft (30 m) thick. Presumed outliers of the unit 

are present as far east as the eastern boundary of the 

area.

Pipiringos and Rosenlund (1977b) mapped a 

possible tongue of the Green River Formation in the 

extreme northern part of the Indian Valley quadrangle 

and suggested its possible equivalence to the unit 

mapped as the dark shale facies in this study; however, 

similar beds were not observed elsewhere in the Indian 

Valley quadrangle and the correlation remains doubtful.

The dark shale facies appears to terminate rather 

abruptly in a southwesterly direction parallel with and 

just northeast of the White River in sees. 7, 17, 18, and 

20, T. 2 N., R. 97 W. Firm evidence of post-Wasatch 

faulting was not found, and the abrupt change from dark 

shale facies to the main body of the Wasatch near the 

eastern edge of section 18 is probably depositional, 

possibly owing to relief on a local Eocene erosion surface 

or on an Eocene eroded fault scarp.

Fossils and Correlation

Three localities yielded identifiable fossils in the 

Wasatch Formation. R.H. Tschudy (written commun., 

1971, 1972) identified fossil palynomorphs from USGS 

fossil localities D4636, D4775, and D4626. Locality 

D4636 yielded Carya, Abietineaepollenites,Alnus, UlmusZelkova, Pediastrum, Botryococcus, and Pistillipollenites, 

a good late Paleocene assemblage. This locality lies just 

above the base of the Wasatch east of Yellow Creek in 

the SEViSE'/i sec. 4, T. 2 R, R. 98 W. Locality D4775 

yielded Araucariacites, Juglanspollenites, Ulmipollenites, 

Chenopodipollis, Alnus, Carya, Abietineaepollenites, 

Classopollis, Ephedra, Erdtmanipollis, Momipites, Zlivisporis, Pofypodiumsporites, Onagraceae. "It is probably of 

latest Paleocene age. The presence of Chenopodipollis in 

abundance and Onagraceae pollen suggests an Eocene 

age, but other forms heretofore characteristic of the 

Eocene were not observed." This locality lies about 80 ft 

above the base of the Wasatch in the SEKNWVi sec. 5, 

T. 2 N., R. 97 W. Locality D4626 yielded Platyccaya, 

Carya, Ulmus-Zelkova, Inaperturopollenites, Tilia, Abietineaepollenites, and Gleicheniidites. "The presence of 

Platycarya, and a questionable grain of Tilia identifies this 

sample as definitely Eocene." This locality lies about 60 

ft below the top of the dark shale facies in the SWtfNWV* 

sec. 7, T. 2 N., R. 97 W.

The Wasatch Formation of the study area is 

correlative to the east and northeast with the mostly 

Eocene part of the Wasatch, as mapped by Pipiringos and 

Rosenlund (1977a) in the White Rock quadrangle and by 

Izett and others (1984) in the Citadel Plateau 

quadrangle. It is equivalent to the upper part of the 

Wasatch of the Elk Springs quadrangle (Dyni, 1968) to 

the north. It is correlative westward generally with the 

Eocene part of the Wasatch, as mapped by Barnum and 

Garrigues (1980) in the Cactus Reservoir quadrangle 

and by Garrigues and Barnum (1980) in the Rangely NE 

quadrangle.

Green River Formation

The Green River Formation of Eocene age consists 

almost entirely of rocks of lacustrine origin deposited in 

the long-lived Eocene Lake Uinta. The Green River 

conformably overlies and intertongues with the Wasatch 

Formation and is conformably overlain by and intertongues with the Eocene Uinta Formation. An early 

phase of lacustrine deposition is represented by some 

rocks of the previously described dark shale facies of the 

Wasatch Formation that Johnson (1984, p. 13-19, pi. 1) 

correlated with the Cow Ridge Member of the Green 

River. The main body of the Green River in the lower 

Yellow Creek area is early to middle Eocene in age and 

includes (in ascending order) the Anvil Points, Garden 

Gulch, and Parachute Creek Members. Rocks of the 

Anvil Points Member are laterally equivalent to the 

lower part of the Garden Gulch Member. The Parachute 

Creek Member is extensively intertongued with the Uinta 

Formation.

Anvil Points Member

The Anvil Points Member of the Green River 

Formation is present in the eastern and southeastern 

parts of the Piceance Creek basin and represents a 

westward-thinning wedge of nearshore lacustrine rocks 

including abundant sandstone beds and other sandy and

O18 Evolution of Sedimentary Basins Uinta and Piceance Basins
silty units. The coarser clastic rocks of the member have 

an easterly or southeasterly source, and to the west rocks 

of the Anvil Points intertongue with and grade into 

deeper water lacustrine rocks of laterally equivalent 

units. On the geologic maps of the Smizer Gulch and 

Rough Gulch quadrangles (Hail, 1973,1974a), the Anvil 

Points Member was designated basal sandstone member 

of the Green River Formation.

In the lower Yellow Creek area, the Anvil Points is 

made up mostly of sandstone and lesser amounts of 

limestone, siltstone, claystone, and shale. It is gradational, across a narrow interval, with the underlying 

Wasatch Formation. Cliff-forming sandstone beds of the 

Anvil Points lie on relatively nonresistant claystone beds 

of the Wasatch.

Sandstone beds of the Anvil Points are mostly 

brown weathering, fine to medium grained, massive to 

even bedded, and calcareous to firmly lime cemented; 

they commonly form conspicuous timber-covered cliffs, 

benches, or ledges. The prominent butte in sec. 27, 

T. 2 N., R. 97 W., just north of the White River, is capped 

by resistant cliff-forming beds of the Anvil Points. Some 

sandstone beds are as thick as 30 ft (9 m). The sandstones 

are variably ostracodal or oolitic, and many are abundantly ostracodal at the top, such that they are essentially 

ostracodal limestones. Concentrations of mollusks, 

mostly gastropods, are present locally. The Anvil Points 

also contains light-brown-weathering ostracodal or 

oolitic limestone beds interbedded with the sandstones. 

Many such beds are grainstone or calcarenite composed 

almost entirely of ostracodes or oolites. These also are 

resistant and form ledges and cliffs. They are thinner than 

the sandstone beds, seldom thicker than 2-3 ft (0.5-1 m). 

Claystone, shale, and clayey siltstone beds are variably 

ostracodal, soft, and nonresistant and generally are 

poorly exposed between the dominant sandstone and 

limestone beds.

The Anvil Points thins to the northwest along its 

outcrop by pinchout of sandstone beds in the upper part. 

Sandstone beds in the lower part persist farther westward 

than do sandstone beds in the upper part. The member is 

arbitrarily terminated at Yellow Creek, west of which 

lacustrine sandstone beds are not clearly identifiable or 

are too thin to map. Interbedded ostracodal limestone 

beds and a few sandstone beds persist far to the west, 

however, within and beyond the study area, and are 

included in the lower part of the Garden Gulch Member. 

The Anvil Points pinches out southwestward in the 

subsurface within the study area. The member ranges in 

thickness from about 280 ft (85 m) near the east edge of 

the Barcus Creek SE quadrangle to about 40 ft (12 m) at 

Yellow Creek in the northwest.

The Anvil Points contains a distinctive black 

coquina in the outcrop area in the southwestern part of 

the Smizer Gulch quadrangle. In this area the member is

as thin as 40 ft (12 m). The coquina comprises the upper 

part of a limy, cliff-forming, coarse-grained, sparsely 

ostracodal sandstone bed. It consists mostly of broken 

fragments of mollusks in a black, sandy, bituminous 

matrix that contains much recrystallized calcite.

The Anvil Points Member is correlative eastward 

with the lower part of the Anvil Points as mapped in the 

White River City quadrangle (Pipiringos and Johnson, 

1976) adjacent to the east. In the White River City 

quadrangle, the lower part (map unit Tgal) is one of two 

westward-thinning units of Anvil Points and the upper 

part (map unit Tgau) is a tongue that pinches out several 

miles east of the lower Yellow Creek area.

The Anvil Points Member occupies a stratigraphic 

position analogous to that of the Douglas Creek Member 

of the Green River Formation in the western and 

southern parts of the Piceance Creek basin. Both reflect 

an early nearshore phase of Green River lacustrine 

deposition. The abundant sandstone and siltstone of the 

Anvil Points were derived mostly from bordering 

highlands to the east and southeast, however, and are 

lacking in the largely shallow-water shelf deposits of the 

Douglas Creek Member.

Garden Gulch Member

In his original definition of the members of the 

Green River Formation in the Uinta and Piceance Creek 

basins, Bradley (1931, p. 9-15, pis. 3, 7, 8) established 

four members (in ascending order), the Douglas Creek, 

Garden Gulch, Parachute Creek, and Evacuation Creek. 

Bradley (1931, p. 9) stated, "These members persist 

throughout most of this area, though they vary in 

thickness from place to place and lose their identity 

where they merge into a shore fades." Geologic mapping 

since 1931 does not substantiate the basinwide extent of 

these members as proposed by Bradley but instead shows 

complex lateral facies changes that have resulted in a 

confusing application of map-unit names, especially for 

those strata below the Mahogany ledge of the Parachute 

Creek Member. This problem is demonstrated by a 

comparison of maps showing the geology in the southern 

part of the Piceance Creek basin in the upper drainages 

of Parachute Creek and Roan Creek (Waldron and 

others, 1951; Donnell, 1961; Cashion, 1973; Johnson, 

1977, 1981a; Hail, 1978, 1982). This area includes the 

type locality of the Garden Gulch Member.

The type section of the Garden Gulch was measured by Bradley (1931, p. 10) in sec. 8, T. 6 S., R. 96 W., 

near the mouth of Garden Gulch along Parachute Creek, 

and he stated, "The greater part of the Garden Gulch 

consists of flaky shale and more thinly laminated marlGeology of the Lower Yellow Creek Area, Northwestern Colorado 019
stone." He showed a considerable proportion of nonoil-shale marlstone in the upper part of the type Garden 

Gulch and apparently established its contact with the 

overlying Parachute Creek Member at the base of the 

lowest group of conspicuous oil-shale beds, although the 

type Garden Gulch does contain a few papery oil-shale 

beds. Elsewhere in the Piceance Creek basin, including 

the lower Yellow Creek area, the Garden Gulch 

generally is defined on the basis of its dominant lithology 

of fissile to papery or thin-bedded, dark clay shale. It 

lacks the indicators of nearshore lacustrine deposits that 

characterize laterally equivalent parts of the Douglas 

Creek Member such as numerous algal beds, abundant 

siltstone and sandstone, nonbedded claystone and mudstone, abundant ostracodal shale, and a relatively low 

content of organic material. It generally does not include 

the barren marlstone of the upper part of the type 

section; this lithology is included in the overlying 

Parachute Creek Member. The shale of the Garden 

Gulch was deposited in relatively deeper, quieter water 

of Lake Uinta during its early freshwater phase.

In the study area, the Garden Gulch Member 

consists mostly of dark-gray or brown clay shale and 

some oil shale, minor siltstone, sandstone and limestone, 

and a few analcimic tuff beds. It is a nonresistant 

slope-forming unit between overlying resistant steep 

cliffs of the carbonate marlstone of the Parachute Creek 

Member and underlying resistant cliff- and benchforming sandstone and limestone of the Anvil Points 

Member. Figure 7 shows a section of the Garden Gulch 

Member as measured near Yellow Creek. West of 

Yellow Creek, the Anvil Points is absent and the Garden 

Gulch lies directly on alluvial rocks of the Wasatch 

Formation.

On the original 7.5-minute quadrangle maps of the 

area the Barcus Creek SE, Smizer Gulch, and Rough 

Gulch quadrangles (Hail, 1972, 1973, 1974a), the 

Garden Gulch-Parachute Creek contact was placed at 

the top of the highest sequence of dark fissile clay shale, 

a sequence overlain by carbonate marlstone of the 

Parachute Creek Member. As then mapped, the Garden 

Gulch included a sequence of light-gray-weathering 

dolomitic shale and siltstone. This dolomitic shale is 

probably the earliest indication of deposition from an 

alkaline-water environment in Lake Uinta. In general, 

shales of the Garden Gulch were deposited during the 

early freshwater phase, and carbonate-rich rocks of the 

Parachute Creek Member were deposited during the late 

alkaline and saline phases. An interpretation of USGS 

oil-shale test core hole 78-5 drilled in sec. 32, T. 2 N., 

R. 97 W. (Smith and O'Sullivan, 1982, p. 68-81) shows 

that the base of the R-2 oil-shale zone was penetrated at 

232 ft. This horizon, the top of which is indicated by an 

electric-log marker commonly known as the Blue 

marker, generally is regarded as the break between

freshwater and alkaline deposition and saline deposition 

in the northern part of the Piceance Creek basin. This 

break generally is selected as the boundary between the 

Garden Gulch and Parachute Creek Members in this 

area (Trudell and others, 1970, p. 4, 8, figs. 4, 5, 6; Dyni, 

1974, fig. 3; Donnell and Hail, 1984). For this reason, the 

Garden Gulch-Parachute Creek boundary is herein 

moved downward to the base of the dolomitic shale and 

all overlying beds are included in the Parachute Creek. 

The dolomitic shale unit can be easily mapped and crops 

out as a relatively resistant, locally timber-covered band 

between smooth slopes of nonresistant shale.

The dominant dark clay shale of the Garden Gulch 

Member is mostly fissile to papery; locally it contains 

some nonbedded claystone that weathers blocky. The 

claystone beds are resistant relative to the fissile shale 

and locally form ledges. Claystone beds are variably 

ostracodal. The shale is dark gray to dark brown and 

weathers medium to light gray. Siltstone beds in the 

lower part of the member locally contain abundant 

ostracodes. Limestone beds, mostly ostracodal grainstone 

or calcarenite, are present only in the basal part of the 

Garden Gulch. East of Yellow Creek, these beds 

commonly are thin, generally no thicker than 1-2 ft 

(30-60 cm), but are resistant and form ledges. West of 

Yellow Creek, the basal part of the Garden Gulch 

contains several ostracodal limestone beds that are 

included in the Anvil Points Member to the east. These 

are as thick as 2-3 ft (60-90 cm) and form ledges. They 

are interbedded with thin, discontinuous sandstone beds 

that are also laterally equivalent eastward to some of the 

sandstones in the basal part of the Anvil Points Member.

The Garden Gulch Member, as now defined for 

the lower Yellow Creek area, is correlative eastward with 

about the lower three-fourths of the Garden Gulch as 

defined in the White River City quadrangle (Pipiringos 

and Johnson, 1976). The Garden Gulch continues 

westward and then southward along the west rim of the 

Piceance Creek basin, with essentially the same lithology 

and upper and lower boundaries (B.E. Barnum, written 

commun., 1980; Donnell and Hail, 1984).

The Garden Gulch Member ranges in thickness on 

the outcrop from about 190 ft (58 m) near the eastern 

edge of the area to about 700 ft (210 m) near the western 

edge. This westward thickening is largely at the expense 

of the westward-thinning Anvil Points Member. The 

Garden Gulch is about 400 ft (120 m) thick in the U.S. 

Bureau of Mines AEC-1 drill hole in the SEVi sec. 13, 

T. 1 N., R. 98 W.

Parachute Creek Member

The Parachute Creek Member is the youngest 

member of the Green River Formation and is present 

throughout most of the Piceance Creek basin. It contains

O20 Evolution of Sedimentary Basins Uinta and Piceance Basins
c

 1

I

o

Parachute

Creek

Member

£

I

us

3 

O

I

O

EXPLANATION

Clay shale, gray and brown. 

Contains a few very thin 

limestone beds

.~z_~n_~ Claystone

Sandstone

I Poorly exposed interval

Ostracodes

METERS FEET 

60 f~ 200

40 

 100

20 

Anvil

Points

Member

Figure 7. Measured section of Garden Gulch Member of Green River 

Formation. West side of Yellow Creek in NW!/4 sec. 9, T. 2 N., 

R. 98 W. Township-range map shown in figure 12.

Geology of the Lower Yellow Creek Area, Northwestern Colorado 021
most of the potentially valuable oil-shale deposits of the 

Green River and in the study area also contains large 

deposits of nahcolite and dawsonite. The Parachute 

Creek consists mostly of dolomitic marlstone and lesser 

amounts of limy marlstone; in its lower part it includes a 

considerable proportion of clay shale and dolomitic shale 

and siltstone. It also contains several beds of siltstone, 

sparse sandstone, and numerous very thin beds of analcimic tuff. It conformably overlies the Garden Gulch 

Member and is complexly intertongued with the 

overlying Uinta Formation. It includes the rich oil-shale 

zones R-2, R-3, R-4, R-5, R-6, and the Mahogany zone.

The Parachute Creek Member was named by 

Bradley (1931, p. 11-14, 17) for exposures along Parachute Creek in the southeastern part of the Piceance 

Creek basin. Bradley defined the member chiefly on its 

content of oil-shale beds. At the type locality, he placed 

the base of the Parachute Creek at the base of the lower 

oil-shale group and the top of the member at the top of 

the upper oil-shale group. Subsequent studies have 

shown that visual estimation of oil content of outcropping 

beds is not a reliable basis for stratigraphic correlation. 

The use of drill-hole logs, especially Fischer assay logs, 

shows that rocks low in oil-shale values and possibly 

perceived to be barren in outcrop exposures can be 

correlated with considerable precision with high-yield oil 

shale elsewhere. This problem of correlation was 

discussed by Cashion and Donnell (1974) in their 

revision of correlations and nomenclature for the upper 

part of the Green River Formation. They showed by 

means of assay logs of nearby drill holes that the upper 

part of the type Parachute Creek is laterally equivalent to 

part of the type Evacuation Creek Member of the Green 

River (Cashion and Donnell, 1974, p. 4, 5). Most 

mapping and other stratigraphic studies since Bradley's 

work have based stratigraphic correlation involving the 

Parachute Creek on lithologic studies and oil-shale assay 

and other drill-hole logs. In the vicinity of the mouth of 

Yellow Creek, the Parachute Creek Member seems 

largely barren of oil shale. On lower Piceance Creek, a 

considerable thickness of rocks that Bradley (1931, 

pi. 4A) assigned to the Garden Gulch is herein included 

in the Parachute Creek.

The lithologic terminology used by Bradley (1931, 

p. 6-8, 22) for rocks of the Green River Formation 

generally has been followed in subsequent studies, but 

the terms marlstone and oil shale have been used with 

little consistency among various writers.

Bradley (1931, p. 7) adopted the term marlstone 

from the European terminology of H. Rosenbusch to 

refer to a hard variety of marl consisting of an intimate 

mixture of calcite (occasionally dolomite) and clay. The 

term marlstone has been widely applied to carbonate 

rocks of the Parachute Creek Member in which dolomite 

is generally more abundant than calcite. In much of the

Parachute Creek marlstone, however, the clay content 

generally is low; quartz, feldspar, analcime, and saline 

minerals, where present, are significant constituents 

(Brobst and Tucker, 1973, p. 28-34).

The term oil shale has been used both as a lithologic term and as an economic resource term, and both 

kerogen-rich marlstone and kerogen-rich clay shale have 

been termed oil shale. Bradley (1931, p. 7) defined oil 

shale as "***a fine-grained sedimentary rock containing 

organic matter which was derived chiefly from aquatic 

organisms or waxy spores and pollen grains, which is only 

slightly soluble in ordinary petroleum solvents, and of 

which a large proportion is distillable into artificial 

petroleum." Bradley pointed out that most rich oil-shale 

beds are not shale but rather are more or less magnesian 

marlstones rich in organic matter. He set no lower limit 

on oil yield but set 10 gallons per ton as the boundary 

between low-grade and high-grade oil shale. Many 

workers, however, have set a lower limit on oil yield 

below which the rock is not termed an oil shale; this limit 

varies widely, generally from 3 to 15 gallons per ton. 

Visual estimates of oil yield in the field are based 

primarily on the pervasive brown color imparted by the 

organic material. Color seldom is seen in outcropping 

marlstone containing less than about 3 gallons per ton. 

The limits of 10-15 gallons per ton are probably based on 

a concept of foreseeable economic value. Fischer and 

other assays can detect very low oil-yield values, 

approaching zero, values useful in stratigraphic correlation but meaningless in terms of economic potential. In 

this report, the term oil shale refers to marlstone or clay 

shale that has an estimated or assayed oil yield of no less 

than 3 gallons per ton. The amount of organic material in 

rocks termed oil shale in the study area varies from about 

1 to 18 percent.

The basal contact of the Parachute Creek Member 

on the Garden Gulch Member is placed at or near the 

base of the R-2 oil-shale zone and marks the first 

significant change upward from the early freshwater 

stage of Eocene Lake Uinta, in which dark clay shale was 

deposited, to the increasingly alkaline and saline late 

stage of the lake, in which carbonate-rich rocks were 

deposited. Carbonate deposition was intermittent, however, and temporary returns to freshwater conditions are 

reflected by interbedded clay shale and carbonate-rich 

rocks above the lowermost carbonate rocks of the 

Parachute Creek.

In the eastern part of the study area, the lowermost 

rocks of the Parachute Creek, constituting about the 

lower one-fifth of the member, are gray-weathering 

dolomitic shale interbedded with brown-weathering clay 

shale. In a northwesterly direction along the outcrop, 

dolomitic beds of the lower part become increasingly silty 

and, in the vicinity of Yellow Creek, include thin-bedded 

to shaly, brown-weathering calcareous or dolomitic siltO22 Evolution of Sedimentary Basins Uinta and Piceance Basins
stone that locally forms resistant ledges and is also 

interbedded with brown, gray-weathering, slightly calcareous or dolomitic shale that locally contains carbonized plant fragments and fish scales. In the vicinity of 

Yellow Creek, these rocks constitute approximately the 

lower one-fourth of the Parachute Creek where the 

member is about 1,300 ft thick (fig. 8).

Rocks in the upper three-fourths of the Parachute 

Creek Member in the southeastern part of the study area 

are dominantly marlstone, and most of the marlstones 

are oil shale. Marlstones of the Parachute Creek are 

mostly dense, hard, resistant rocks and generally form 

steep cliffs and strong ledges. In this area, as elsewhere 

throughout the Piceance Creek basin, they form the high, 

commonly precipitous cliffs that rim much of the basin. 

The oil-shale marlstones are medium to dark brown on 

fresh surfaces, and most weather light gray. Rich oil 

shale, exceeding 30 gallons per ton, weathers to a distinctive bluish-gray color. Barren marlstone is gray to 

light grayish brown and also weathers light gray. An 

X-ray mineralogy study of the Parachute Creek Member 

in a measured section near the eastern boundary of the 

study area on lower Piceance Creek shows that most of 

the mineral fraction of the Parachute Creek consists of 

mixtures of dolomite, calcite, quartz, feldspar, analcime, 

albite, and pyrite (Brobst and Tucker, 1973, p. 12-19, 

24-28, 30-32, 36-37, 38). The saline minerals nahcolite 

and dawsonite are locally abundant. This same study 

(table 5) shows that in the lower Piceance Creek area 

marlstones of that part of the Parachute Creek below the 

Mahogany ledge are chiefly dolomite; they contain little 

calcite but substantial amounts of analcime, quartz, 

feldspar, and minor illite. Within the Mahogany ledge, 

the amount of calcite increases considerably, although 

dolomite is still dominant. The amount of calcite continues to increases in rocks above the Mahogany ledge 

until the amounts of calcite and dolomite are approximately equal. In the upper three-fourths to four-fifths 

of the Parachute Creek, rock types other than marlstone 

are minor. The marlstones are variably silty, and thin 

marly siltstone beds become more numerous to the 

northwest along the outcrop. From near Yellow Creek 

westward, the Parachute Creek contains several thin 

sandstone beds. Numerous very thin and several thick 

analcimic tuff beds are present in the Parachute Creek, 

and several of these tuff beds have been correlated widely 

throughout the Piceance Creek basin (Donnell, 1961, 

p. 865).

The upper boundary of the Parachute Creek 

Member is placed at the base of the lowest overlying 

tongue of the Uinta Formation. The Green River and 

Uinta Formations are complexly intertongued. Tongues 

of the Green River are marlstones, most of which merge 

laterally into rocks of the Parachute Creek Member, but 

owing to constraints of stratigraphic nomenclature, the

marlstone tongues are designated tongues of the Green 

River Formation rather than tongues of the Parachute 

Creek Member. The intertonguing Green River and 

Uinta units are described elsewhere.

The Parachute Creek Member is correlative 

eastward with the Parachute Creek Member as mapped 

by Pipiringos and Johnson (1976) in the adjoining White 

River City quadrangle, except that rocks above the lowest 

mapped tongue of the Uinta are termed tongues of 

Green River rather than including them in the Parachute 

Creek and the basal contact is lower than the contact as 

mapped to the east. The member continues westward 

and then southward along the west rim of the Piceance 

Creek basin.

The Parachute Creek Member varies widely in 

thickness within the study area. On the outcrop, it is 

about 900 ft (270 m) thick at the eastern boundary of the 

area near Piceance Creek. It thickens northwestward to 

about 1,300 ft (400 m) in the vicinity of Yellow Creek and 

then thins northwestward to about 920 ft (280 m) at the 

western boundary of the study area. Measured surface 

sections by Brobst and Tucker (1973, p. 12-19) and 

Pipiringos and Johnson (1976) indicate a thickness 

greater than 900 ft (270 m) at the eastern edge of the 

study area, but the upper part of the Parachute Creek as 

defined by them includes marlstones herein designated 

as tongues of the Green River Formation. The greatest 

change in thickness for the Parachute Creek is in the 

subsurface toward the structural axis of the Red Wash 

syncline, which in the study area coincides with the 

depositional center for strata of the Garden Gulch and 

Parachute Creek Members. In the U.S. Bureau of Mines 

AEC-1 drill hole in the SEVi sec. 13, T. 1 N., R. 98 W., 

the Parachute Creek is about 1,650 ft (500 m) thick. In 

the General Petroleum Yellow Creek 64-8 drill hole in 

the NEVi sec. 8, T. 1 N., R. 98 W., it may be more than 

2,100 ft (640 m) thick, but this figure includes several 

hundred feet of strata above the Mahogany zone that 

may contain tongues of the Uinta Formation.

Uinta Formation and Tongues of the 

Green River Formation

The Uinta Formation of Eocene age is a heterogeneous mass of clastic sedimentary rocks, mostly sandstone, siltstone, mudstone, and variably silty to sandy 

marlstone. These rocks comprise the complex, generally 

southward prograding, fluvial-deltaic sequence that 

ultimately filled Eocene Lake Uinta. Deposition was 

localized, shifting, and intermittent and was interrupted 

by episodes of open-water lacustrine deposition during 

which marlstones of the Green River Formation were

Geology of the Lower Yellow Creek Area, Northwestern Colorado 023
Uinta 

Formation

I

es

£

o

OS C

I

^

u

a

s

IAH

OSc

osc

OSC

os[

-T

 jj ^^11^ ^^ -1- I ~VI

__T1

T

 T

I 

«

0

1

H

3

 s

OH

Garden 

Gulch

Er.v .v.=r.v33

..__ I _ _i_ 

. '

EXPLANATION

day shale 

Dolomitic shale

Interbedded marlstone and 

dolomitic siltstone

Tuff bed

METERS 

60-i

40

20 

FEET 

-200

 100

Figure 8. Measured section of Parachute Creek Member of Green River Formation. West side of Yellow Creek 

in SW1/4 sec. 9, T. 2 N., R. 98 W. Township-range map shown in figure 12.

024 Evolution of Sedimentary Basins Uinta and Piceance Basins
deposited. This depositional pattern resulted in complex 

intertonguing between the Uinta and Green River 

Formations. The southward-prograding deltaic nature of 

the Uinta Formation has been described by Johnson 

(1981b, p. 56-58). In general, successively younger 

tongues of the Uinta extend farther south beyond the 

pinchout point of older tongues, such that in north to 

south cross section the sequence consists of southwardthinning tongues of Uinta between northward-thinning 

tongues of Green River. This pattern of intertonguing 

has been shown by Trudell and others (1970, fig. 4), 

based on subsurface studies, and in subsequent studies 

by Duncan and others (1974, fig. 4), O'Sullivan (1975, 

fig. 2), Hail (1977, fig. 2), and Johnson (1981b, fig. 1), 

based on detailed surface mapping and subsurface 

correlations. Many of the detailed mapping and subsurface studies throughout the Piceance Creek basin 

show that most of the marlstone tongues of the Green 

River Formation merge laterally, generally southward, 

with other Green River tongues or with the main body of 

the Parachute Creek Member. Intertonguing of the 

Uinta and Green River units occurs throughout the 

entire sequence. Not all marlstone tongues of the Green 

River have been mapped in the study area or elsewhere 

in the Piceance Creek basin; only certain widespread, or 

conspicuous, or otherwise selected units were mapped. 

Those marlstone units not mapped separately are 

included in the Uinta Formation. Within the intertonguing sequence, the Uinta Formation dominates and the 

marlstone tongues of the Green River are relatively thin. 

Figure 9 is a generalized diagram showing the correlation 

of mapped Uinta and Green River tongues, from west to 

east, in the southern part of the lower Yellow Creek area 

including the Barcus Creek and Barcus Creek SE 

quadrangles (Hail, 1984, 1988), the adjoining Calamity 

Ridge quadrangle (Donnell and Hail, 1984), and the 

adjoining White River City quadrangle (Pipiringos and 

Johnson, 1976). Table 3 shows correlations of Uinta and 

Green River map units for the Rough Gulch and Barcus 

Creek quadrangles (Hail, 1974a, 1984) and for the 

Smizer Gulch and Barcus Creek SE quadrangles (Hail, 

1973,1988). Figure 10 shows a representative composite 

section of the Uinta and Green River intertonguing 

sequence in the northwestern part of the Barcus Creek 

SE quadrangle.

The Uinta and Green River intertonguing 

sequence forms by far the largest area of exposed Tertiary rocks in the Piceance Creek basin and is the surface 

rock for most of the basin within the drainage area of 

Piceance and Yellow Creeks. (See geologic map of 

Donnell, 1961, pi. 48.) Mapping of the thin marlstone 

tongues of the Green River Formation throughout the 

basin has provided the basis for recognizing and subdividing the complex stratigraphy of the sequence and for 

deducing its depositional history.

Table 3. Correlation of map units of the Uinta and Green 

River Formations between the Rough Gulch and Barcus 

Creek quadrangles and between the Smizer Gulch and 

Barcus Creek SE quadrangles

[All units shown except Tgp (Parachute Creek Member of the Green 

River Formation) are tongues of the Uinta Formation (Tu) or tongues of 

the Green River Formation (Tg). References: Rough Gulch (Hail, 

1974a); Barcus Creek (Hail, 1984); Smizer Gulch (Hail, 1973); Barcus 

Creek SE (Hail, 1988)]

Rough Gulch 

quadrangle 

GQ-1195

Barcus Creek 

quadrangle 

GQ-1 578

Tug

Tgt

Tu

Tgpt

Tut

Tgp

Smizer Gulch 

quadrangle 

GQ-1131

Tgt

Tue

Tgy 

Tud 

Tgm 

Tuc 

Tgtc 

Tub

Tgua

Tua

Tgp

Barcus Creek SE 

quadrangle 

GQ-1 61 3

Tu2 and higher units 

(see fig. 9)

Tgpa

Tgea*

Tgp

Tgl

Tul

Tgp

*Part of Evacuation Creek Member of Green River Formation; now 

Uinta Formation

Evacuation Creek Member of the Green River Formation 

Uinta Formation Nomenclature Change

Prior to 1974, rocks herein termed Uinta Formation were regarded as the uppermost member of the 

Green River Formation. Bradley (1931, p. 14-15, pi. 8) 

named the Evacuation Creek Member of the Green 

River Formation and designated its type as a section 

measured in sec. 27, T. 9 S., R. 25 E., Uintah County, 

Utah, where it consists mostly of oil shale and marlstone. 

He applied the name to a sequence of rocks above the 

Parachute Creek Member of the Green River Formation 

in the Piceance Creek basin that includes both barren 

marlstone and sandy beds. Cashion and Donnell (1974, 

figs. 2, 3) showed by correlations of oil-shale histograms 

of nearby drill holes that Bradley's type Evacuation 

Creek is stratigraphically equivalent to much of that part 

of his type Parachute Creek above the Mahogany bed 

and has essentially the same lithology. They (p. 3-9, 

fig. 3) also concluded that rocks designated by Bradley as 

Bridger Formation in the eastern Uinta basin are

Geology of the Lower Yellow Creek Area, Northwestern Colorado 025
correlative with rocks designated by him as Evacuation 

Creek in the Piceance Creek basin. The rocks termed 

Bridger by Bradley are now regarded as Uinta Formation 

in the Uinta basin; these rocks, dominantly sandstones, 

had in the past commonly been designated as Evacuation 

Creek Member in the Piceance Creek basin. On the basis 

of these conclusions, Cashion and Donnell (p. 8, 9) 

made the following changes in nomenclature. They 

(1) abandoned the name Evacuation Creek, (2) placed 

the upper boundary of the Parachute Creek Member at 

the base of the Uinta Formation, (3) applied the name 

Uinta Formation in the Piceance Creek basin to those 

rocks previously assigned to the Evacuation Creek 

Member, and (4) abandoned the name Bridger Formation for the Piceance Creek basin.

Tongues of the Uinta Formation

The tongues of the Uinta Formation include a wide 

range of rock types. Brown-weathering sandstone and 

siltstone predominate, but variably silty gray-weathering 

marlstone, mudstone or claystone, conglomerate, and 

conglomeratic sandstone also are abundant. Thin beds of 

Green River-type light-gray marlstone are also included 

in some tongues. Altered tuffaceous material is abundant 

and forms the dominant mineral constituent in many 

beds. Thickness of the mapped tongues varies greatly. 

Several terminate laterally in or near the study area as a 

result of either pinchout or abrupt facies change. Basal 

contacts of the tongues of the Uinta range from gradational to sharply disconformable. They commonly are 

sharp and show no evident structural discordance, or they 

are gradational across a narrow interval from white 

marlstone of the underlying tongues of the Green River 

to brown-weathering siltstone or sandstone of the tongue 

of the Uinta. Locally, especially where medium- to 

coarse-grained sandstone forms the basal beds, contacts 

appear to be disconformable, and locally the underlying 

beds may be scoured. In many such places, the bases of 

broken sandstone masses are downwarped or slumped 

into the underlying marlstone beds. Slumping was penecontemporaneous and occurred after some degree of 

consolidation of the sandstones, which were deposited on 

underlying soft, nonresistant, or poorly indurated marly 

sediments. The underlying marlstones also commonly 

show considerable distortion including differential compaction, plastic flow, and minor local faulting.

Most of the Uinta sedimentary rocks are finegrained, marly or argillaceous siltstone or very fine 

grained sandstone that grades to mudstone or silty marlstone. These rocks were probably deposited in a deltafront lacustrine environment, but almost all the tongues 

contain at least some alluvial-type channel sandstones. 

Many of the channel sandstones have sharply scoured 

basal contacts and are locally crossbedded and conglomeratic. Conglomerate beds generally contain locally 

derived soft-pebble clasts. Where firmly cemented, the 

siltstones and sandstones form cliffs, ledges, and benches, 

resulting in the typical ridge and valley topography of the 

basin interior.

For purpose of discussion, the tongues of the Uinta 

are divided into a lower group and an upper group; the 

boundary between the two groups is a tongue of the 

Green River, the marlstone at Mare Canyon and its 

equivalents.

Lower Group of Tongues of the Uinta Formation

The stratigraphically lowest mapped tongue of the 

Uinta Formation (map units Tua and Tul, fig. 9) lies 

within or just above the Mahogany ledge of the 

Parachute Creek Member of the Green River 

Formation. Its relationship to the Mahogany ledge is not 

clear because it crops out mostly in an area where the 

Mahogany is not well identified owing to very low oilshale values. The maximum outcrop thickness of the 

tongue is about 300 ft (90 m) on lower Yellow Creek 

between Barcus and Greasewood Creeks. The tongue 

thins abruptly in both directions along the outcrop from 

this point and pinches out entirely about 4.5 mi (7 km) to 

the southeast. It was mapped as far west as the western 

boundary of the study area, where it is only a few feet 

thick, and it pinches out or phases out entirely in the 

extreme northeastern corner of the Calamity Ridge 

quadrangle (Donnell and Hail, 1984), adjacent to the 

western part of the study area. It apparently is absent to 

the southeast, but isolated unmapped pods and lenses 

along the North and Middle Forks of Greasewood Creek 

may represent outliers of the tongue. It apparently is 

present in the subsurface in U.S. Bureau of Mines drill 

holes AEC-1 and AEC-3 (sees. 13, 14, T. 1 N., R. 98 

W.), where it is about 120-140 ft (37-43 m) thick, but 

pinches out southward inasmuch as it apparently is not 

present in the Humble Oil Co. 1 Yellow Creek drill hole 

4 mi to the south in sec. 2, T. 1 S., R. 98 W. In the vicinity 

of Yellow Creek, its basal contact appears disconformable and locally lies on eroded beds of the underlying 

Parachute Creek Member of the Green River Formation. The stratigraphic extent of the disconformity is 

not known. The tongue consists of brown to gray, mostly 

brown weathering, fine- to medium-grained sandstone; 

lesser brown-weathering, locally marly siltstone; and 

some variably silty marlstone. The sandstone is mostly 

massive to even bedded and is locally concretionary. 

Channel deposits are relatively abundant and have sharp 

basal contacts and coarse soft-pebble basal conglomeratic beds. Limonite, biotite, and other rock and mineral 

fragments are abundant in the coarser grained sandstones. Near the mouth of Barcus Creek, the unit is 

mostly siltstone and is very firmly quartz cemented, hard, 

and resistant.

026 Evolution of Sedimentary Basins Uinta and Piceance Basins
The next higher mapped tongue of the Uinta (map 

units Tub and Tu2, fig. 9) is present across the entire 

study area and basinward in the subsurface and underlies 

the marlstone at Trail Canyon (map unit Tgtc) or its 

lateral equivalents. Although the tongue was not mapped 

separately in the Rough Gulch quadrangle, it constitutes 

the basal part of the undifferentiated Uinta (map unit Tu, 

table 3) in that area. The tongue thins to the southeast 

and extends beyond the pinchout point of the lower 

tongue into the adjacent White River City quadrangle 

(Pipiringos and Johnson, 1976), where it is no thicker 

than 40 ft (12 m) and is included in the main body of the 

Parachute Creek Member of the Green River Formation. It is relatively thin in outcrops in the northwestern part of the Barcus Creek quadrangle, possibly as 

thin as 40 ft (12 m), but abruptly thickens southward to as 

much as 300 ft (90 m) along the Middle Fork of Greasewood Creek. It extends westward into the adjacent 

Calamity Ridge quadrangle (Donnell and Hail, 1984) 

and pinches out in the south-central part of that quadrangle. The tongue is tentatively identified in the 

subsurface and may be as thick as 270 ft (82 m) in the 

Sundance Oil and Gas Company Federal 8024 drill hole 

in sec. 2, T. 1 N., R. 98 W. Subsurface studies suggest that 

the lower part of the unit phases to the east and west into 

rocks of the Parachute Creek Member of the Green 

River Formation. The tongue consists mostly of brownweathering sandstone, lesser amounts of variably marly 

siltstone, and minor marly siltstone or mudstone. 

Channel deposits are fairly abundant. The sandstone is 

mostly light to medium brown, very fine to medium 

grained, and locally coarse grained to sparsely conglomeratic in channel units. The bedding is massive or even to 

crossbedded, and some spherical concretionary masses 

are present. Paleoslump blocks are locally present at the 

base. The more resistant beds form cliffs along some 

valley walls. In an area in the lower part of the forks of 

Greasewood Creek in the northern part of the Barcus 

Creek quadrangle, the overlying marlstone at Trail 

Canyon of the Green River Formation is absent owing to 

the presence of a local deltaic lobe that results in a 

vertically continuous clastic sequence including both this 

and the overlying tongue of the Uinta (map units Tuc and 

Tu3, fig. 9). This sequence is designated map unit Tucb. 

(See fig. 9 and Barcus Creek geologic map, Hail, 1984.)

The next higher Uinta tongue of the lower group 

(map units Tuc and Tu3, fig. 9) is present only in the 

western part of the area and pinches out eastward in 

surface exposures about a mile east of Yellow Creek. It 

was not mapped separately in the Rough Gulch quadrangle but was included in the main body of the Uinta 

(map unit Tu, table 3). The unit is relatively thin in the 

northwestern part of the area, about 50 ft (15 m) thick in 

the drainage of Greasewood Creek, but thickens 

southward in the upper drainage of Barcus Creek to as

much as 220 ft (67 m). The tongue consists of light-gray 

to brown, very fine to medium grained sandstone, siltstone, and considerable mudstone and variably siltymarlstone. It contains a few ostracodal limestone and 

sandstone beds in the northwestern part of the Barcus 

Creek quadrangle. The tongue also contains several 

channel deposits. The basal contact is locally sharp, 

scoured, and disconformable.

The highest tongue of the lower group of Uinta 

tongues (map unit Tu4, fig. 9) crops out only in the 

Barcus Creek SE quadrangle. It pinches out westward at 

Yellow Creek and thins eastward to about 30 ft (9 m) at 

Piceance Creek. Its maximum thickness is about 120 ft 

(37 m) on the divide between Piceance and Yellow 

Creeks. This; tongue was tentatively identified in the 

subsurface in the U.S. Bureau of Mines AEC-1 drill hole 

in sec. 13, T. 1 N., R. 98 W., and in Sundance Oil 

Company Federal 8005 drill hole in sec. 29, T. 1 N., 

R. 97 W., where it is about 70-80 ft (21-24 m) thick. The 

tongue consists mostly of light- to medium-brown, fineto coarse-grained sandstone and includes several conglomeratic channel deposits in the western two-thirds of its 

outcrop area. Eastward, near Piceance Creek, the tongue 

grades laterally into thin tuffaceous sandstone interbedded with marly siltstone and silty marlstone. It is not 

present as a mappable unit east of Piceance Creek.

Upper Group of Tongues of the Uinta Formation

The stratigraphically lowest mapped tongue of the 

Uinta Formation (map unit Tud and Tu5, fig. 9) of the 

upper group of Uinta tongues underlies the widespread 

Yellow Creek Tongue of the Green River Formation. It 

is one of the most widespread and persistent of the Uinta 

tongues in the study area and has been mapped eastward 

as far as the eastern boundary of the White River City 

quadrangle (Pipiringos and Johnson, 1976), where it is 

the unnamed unit of the Uinta Formation lying above the 

Parachute Creek Member of the Green River Formation 

and below the base of the Yellow Creek Member. The 

tongue is present to the west in the southern part of the 

Calamity Ridge quadrangle (Donnell and Hail, 1984) 

and to the south in the Wolf Ridge quadrangle (map unit 

Tu3; Duncan, 1976a). It was not mapped separately in 

the Rough Gulch quadrangle but is included in map unit 

Tu of that quadrangle (table 3). The tongue varies greatly 

in thickness. It is generally no thicker than 100 ft (30 m) 

in the northwestern part of the Barcus Creek quadrangle. 

It is only about 80 ft (24 m) thick on lower Yellow Creek 

near the mouth of Barcus Creek (fig. 10) but thickens 

southeastward to about 450 ft (140 m) just west of 

Piceance Creek and southwestward to about 350 ft 

(110 m) in the uppermost drainage of Barcus Creek. It is 

about 400 ft (120 m) thick in several drill holes in the 

southern part of the Barcus Creek SE quadrangle. The

Geology of the Lower Yellow Creek Area, Northwestern Colorado 027
WEST

BARCUS CREEK BARCUS CREEK SE

QUADRANGLE QUADRANGLE

(GQ-1S78) (GQ-1613)

EAST

WHITE RIVER CITY 

QUADRANGLE

(MF-736)

CALAMITY RIDGE 

QUADRANGLE

(MF-1690)

Tud

Tgm

Tuc

1 Tgtc

\ Tub

Tgp | Tgua

< Tua

Tug

Tgt

Tuf

N"4

rgubj

Tg

Tud

Tgm

Tuc |

Tgtc JTucb

Tub f

Tgua

Tua

Tgp

Tue

y

Tuc

Tgtc

Tub

Tu8

Tgt

| Tu7

i/Tgd

Tu6

Tgy

TuS

Tg4

Tgm^ Tu4

Tg3j

Tu3 N Tg2

Tgtc j

Tu2

Tgl I

Tul y Tgp

Tu

Tgt

Tu

Tgd

Tu

Tgy >

Tu

Figure 9. Correlation of outcropping mapped units of the 

intertonguing sequence of Uinta and Green River Formations 

in the Barcus Creek (Hail, 1984), Barcus Creek SE (Hail, 

1988), Calamity Ridge (Donnell and Hail, 1984), and White 

River City (Pipiringos and Johnson, 1976) quadrangles. All 

units shown are tongues of the Green River (Tg) or Uinta (Tu) 

Formations except Tgp (Parachute Creek Member of the 

Green River Formation). Formally named tongues of the 

Green River Formation are Thirteenmile Creek Tongue (Tgt), 

Dry Fork Tongue (Tgd), and Yellow Creek Tongue (Tgy). 

Vertical dashed lines are lateral boundaries between map 

units.

tongue is mostly brown and gray sandstone and siltstone, 

but considerable claystone, mudstone, and marlstone 

including several thin, widespread, beds of Green Rivertype marlstone are also present. The sandstone is fine to 

coarse grained, variably tuffaceous, limonitic, and locally 

concretionary. Channel deposits are commonly conglomeratic sandstones in sharp contact with underlying sandstone or siltstone beds. Clasts in the conglomeratic beds 

are soft-rock fragments. The basal contact of the tongue 

is locally sharp and disconformable on the underlying 

marlstone tongue of the Green River but elsewhere is 

gradational. The tongue forms conspicuous cliffs in much 

of its outcrop area. Among these are the brown cliff 

on lower Piceance Creek, which form a conspicuous 

contrast to the white-weathering marlstone of the 

underlying Parachute Creek Member of the Green River 

Formation.

The next higher tongue (map units Tuf, Tue and 

Tu6, fig. 9) in the upper group of Uinta tongues is 

underlain by the Yellow Creek Tongue of the Green 

River Formation or its lateral equivalents and overlain by 

the Dry Fork Tongue or by the Thirteenmile Creek 

Tongue of the Green River Formation beyond the pinchFigure 10 (facing page). Composite measured section of 

intertonguing sequence of Uinta (Tu) and Green River (Tg) 

Formations. Measured between Barcus and Yellow Creeks in 

W1/2 sec. 26, NW1/4 sec. 35, and E!/2 sec. 34, T. 2 N., R. 98 W., 

Barcus Creek SE quadrangle. Explanation of symbols: Tu8, 

unnamed tongue 8 of Uinta Formation; Tgt, Thirteenmile 

Creek Tongue of Green River Formation; Tu6, unnamed 

tongue 6 of Uinta Formation; Tgy, Yellow Creek Tongue of 

Green River Formation; TuS, unnamed tongue 5 of Uinta 

Formation; Tgm, marlstone at Mare Canyon of Green River 

Formation; Tu3, unnamed tongue 3 of Uinta Formation; Tgtc, 

marlstone at Trail Canyon of Green River Formation; Tu2, 

unnamed tongue 2 of Uinta Formation; Tg1, unnamed 

tongue 1 of Green River Formation; Tu1, unnamed tongue 1 

of Uinta Formation; Tgp, Parachute Creek Member of Green 

River Formation.

out of the Dry Fork Tongue. The unit is widespread and 

persistent and forms the surface rock of much of the 

southern part of the study area. It was mapped west of 

the area in the Calamity Ridge quadrangle (Donnell and 

Hail, 1984), to the east in the White River City quadrangle (Pipiringos and Johnson, 1976), where it is an 

unnamed tongue of the Uinta between the Thirteenmile 

Creek and Dry Fork Tongues of the Green River 

Formation, and to the south in the Wolf Ridge 

quadrangle (Duncan, 1976a), where it is designated Tu4, 

and in the Square S Ranch quadrangle (Hail and Duncan, 1984), where it is map unit TuS. It was not mapped 

separately in the Rough Gulch quadrangle but is 

included in map unit Tu (table 3). In those areas where 

the tongue is directly underlain by the Yellow Creek 

Tongue of the Green River Formation and directly 

overlain by the Thirteenmile Creek Tongue, it is 250- 

350 ft (76-106 m) thick and thickens from northwest to 

southeast. An even greater southeastward thickening for 

the sequence between the Yellow Creek and Thirteenmile Creek Tongues would be indicated if the laterally 

equivalent tongue (map unit Tu7, fig. 9) were included. 

In the southwestern part of the study area, lower beds of 

the tongue interfinger with and grade laterally into 

marlstones of the Green River (map units Tgub and 

Tguc, fig. 9); this gradation results in a thinning of the 

tongue in that area. Rocks of the tongue are mostly 

brown weathering sandstone and siltstone and silty marlstone. The unit also contains considerable mudstone, 

claystone, and sparse limestone, and as many as three 

thin, but laterally persistent Green River-type marlstone 

beds are present in the southern part of the area. The 

sandstone is light to medium brown or gray, fine to 

medium grained and locally coarse grained, massive to 

even bedded, and locally crossbedded. Channel deposits 

are numerous, especially in the southeastern part of the 

area. Many of these contain soft-pebble clasts in the

028 Evolution of Sedimentary Basins Uinta and Piceance Basins
Tu8

Tu6

Tgy

Tu5

Tgm

Tu3

Tgtc

Tu2 ^iiir

Tu2

Tgl

Tul

Tgp

m

EXPLANATION

Qaystone 

Marlstone 

Limestone 

Siltstone

Literbedded sandstone 

and siltstone

Sandstone

Crossbedded sandstone 

Channel sandstone 

Conglomeratic sandstone

I Poorly exposed interval

O Ostracodes

METERS 

60 

40 

20 

FEET 

 200

 100

Geology of the Lower Yellow Creek Area, Northwestern Colorado 029
conglomeratic basal beds. In the adjacent White River 

City quadrangle, Pipiringos and Johnson (1976) noted 

fossil wood just below the top of the unit where it is 

overlain by the Dry Fork Tongue of the Green River 

Formation. The siltstone, most of which weathers brown, 

is tuffaceous and marly and locally argillaceous and 

grade to mudstones. A distinctive grayish-green claystone or mudstone is present at or near the top of the 

tongue, where it is overlain by the Thirteenmile Creek 

Tongue of the Green River. The unit contains an 

increasingly large proportion of silty marlstone in the 

southern part of the study area, especially west of the 

divide between Yellow and Piceance Creeks, and 

becomes more sandy and silty eastward and less silty 

westward. The basal contact of the tongue generally is 

disconformable, sharp, and scoured but is locally gradational. Penecontemporaneous slump blocks are 

common at the basal contact. The more firmly cemented 

sandstone beds locally form cliffs, benches, and ledges.

Wherever the Dry Fork Tongue of the Green River 

Formation is present, overlying rocks of the Uinta 

Formation below the Thirteenmile Creek Tongue of the 

Green River are designated as a separate tongue (map 

unit Tu7, fig. 9) although they are equivalent to the upper 

part of map unit Tu6 into which they merge. The tongue 

of the Uinta is present in the southeastern part of the 

area and was mapped east of the area, in the White River 

City quadrangle (Pipiringos and Johnson, 1976), as the 

unnamed tongue of the Uinta between the Dry Fork and 

Thirteenmile Creek Members and south of the area, in 

the Square S Ranch quadrangle (Hail and Duncan, 

1984), as unit Tu4. The tongue is about 80-140 ft (24- 

43 m) thick. It is mostly variably marly sandstone, siltstone, and claystone or mudstone and includes sparse 

marlstone and minor limestone. The sandstone is light to 

medium brown and fine to medium grained. Most 

sandstone beds are poorly cemented and nonresistant but 

locally form ledges wherever firmly calcite cemented. 

The tongue includes several channel deposits that locally 

contain sparse clay-pebble clasts. The siltstone is medium 

brown to greenish brown and mostly argillaceous grading 

to mudstone. The sandstone and siltstone are increasingly marly in the southern part of the area. Claystone in 

the upper part of the tongue commonly is greenish brown 

to grayish green and grade into mudstones with an 

increase in silt and sand content. Green claystone commonly is present near the top of the tongue.

The uppermost tongue of the Uinta (map units 

Tug and Tu8, fig. 9) is the highest stratigraphic unit in the 

study area. It overlies the Thirteenmile Creek Tongue of 

the Green River Formation. Only a few small isolated 

patches of this tongue are present in the study area, and 

their maximum thickness does not exceed 30 ft (9 m). 

Higher beds have been removed by erosion. The unit is 

unique among the Uinta tongues in that it contains

chert-pebble conglomeratic beds. South of the area, in 

the Wolf Ridge (Duncan, 1976a) and Square S Ranch 

(Duncan, 1976b; Hail and Duncan, 1984) quadrangles, a 

thick tongue of the Uinta was mapped above the Thirteenmile Creek Member as map unit Tu5. The tongue 

locally contains chert-pebble conglomerate beds. The 

outcrop of the tongue in the study area east of Yellow 

Creek consists of gray to greenish-gray and brown 

calcareous sandstone and lenses of chert-bearing conglomerate and conglomeratic sandstone. The chert 

fragments range from grit size to about 0.5-in. (1 cm) 

diameter pebbles. In the outcrop outliers west of Yellow 

Creek, the unit includes some marlstone and limestone, 

as well as conglomeratic sandstone. The conglomeratic 

sandstone contains mostly clasts of soft rocks siltstone 

and claystone as long as 1.25 in. and considerable chert 

and quartzite pebbles as long as 0.75 in.; enclosing 

sandstone is moderately clean, quartzitic, and calcareous. 

The conglomeratic sandstone beds locally are overlain by 

thin beds of hard granular limestone and marlstone more 

typical of the underlying Thirteenmile Creek Tongue. 

The conglomeratic sandstone beds are soft and nonresistant in contrast to the hard, bench-forming marlstones at 

the top of the underlying Thirteenmile Creek Tongue of 

the Green River Formation.

Tongues of the Green River Formation

Intertonguing beds of the Green River and Uinta 

Formations were first shown on a preliminary map of the 

Barcus Creek SE quadrangle (Hail, 1972). In the lower 

part of the intertonguing sequence, two tongues of Green 

River (map units Tgpa and Tgpb) were mapped. In 

higher strata, only the top of a marlstone unit designated 

Yellow Creek marker bed and the base of a unit 

designated Barcus Creek marker bed were shown on the 

map; these were included with the Uinta Formation 

(then Evacuation Creek Member of the Green River 

Formation). The Yellow Creek marker bed later was 

formally named the Yellow Creek Tongue of the Green 

River Formation and the Barcus Creek marker bed was 

included in the Thirteenmile Creek Tongue of the Green 

River Formation (Duncan and others, 1974, p. 4, 9). As 

a result of detailed quadrangle mapping in the northern 

part of the Piceance Creek basin, both the nature and 

lateral extent of the marlstone units were recognized, and 

Duncan and others (1974) formally named four tongues 

of the Green River. These are (in ascending order) the 

Yellow Creek, Dry Fork, Thirteenmile Creek, and Black 

Sulphur Tongues. Subsequent published maps show 

these formally named tongues as well as certain other 

unnamed tongues. Other marlstone units not mapped 

separately were included with the Uinta Formation. In 

revising the preliminary maps of the Barcus Creek and 

Barcus Creek SE quadrangles for publication as USGS

030 Evolution of Sedimentary Basins Uinta and Piceance Basins
Geologic Quadrangle maps, the formally named Green 

River tongues (Yellow Creek, Dry Fork, and Thirteenmile Creek) and several other informally designated 

tongues were shown. These tongues are shown diagrammatically on figure 9 and table 3.

The tongues of the Green River are chiefly marlstone; they also include a fairly large proportion of 

limestone and numerous sandstone beds. Much of the 

marlstone is silty, ranging to marly siltstone. Significant 

oil-shale beds generally are absent in the Green River 

tongues, but a few thin, low-grade oil-shale beds are 

present locally. Brobst and Tucker (1974, p. 28, table 5) 

noted the dominance of calcite over dolomite in those 

marlstones of the Parachute Creek Member above the 

Mahogany ledge in the lower Piceance Creek area, a 

dominance consistent with the abundant limestone of the 

Green River tongues that merge with the upper part of 

the Parachute Creek Member. The tongues are mostly 

very light gray or very light brownish gray. Sandstone in 

the tongues is light gray to light grayish brown. The light 

gray of the marlstone tongues is in conspicuous contrast 

to the pervasive brown of the enclosing Uinta tongues. 

For purposes of discussion, the Green River tongues are 

divided into a lower group and an upper group, the 

boundary between the two being the tongue of the Uinta 

identified as map unit Tud and Tu5 (fig. 9).

Lower Group of Tongues of the Green River Formation

The lowermost tongue of the Green River Formation (map units Tgpt, Tgua, and Tgl; fig. 9, table 3) 

merges with the Parachute Creek Member of the Green 

River Formation in sec. 32, T. 2 N., R. 97 W., and just 

west of the study area in the northeastern part of the 

Calamity Ridge quadrangle (Donnell and Hail, 1984). In 

the subsurface of the study area, it merges southward 

with the Parachute Creek Member between the U.S. 

Bureau of Mines AEC-1 drill hole in sec. 13, T. 1 N., 

R. 98 W., and the Humble Oil Co. 1 Yellow Creek drill 

hole about 4 mi to the south in sec. 2, T. 1 S., R. 98 W. 

The tongue is about 40-200 ft (12-60 m) thick. It was 

mapped as a separate unit in all four quadrangles of the 

study area. It consists mostly of light-gray dolomitic 

marlstone and considerable calcareous marlstone and 

several beds of limestone. The dolomitic marlstone is 

variably silty and grades to marly siltstone. It is mostly 

thin and flat bedded and locally weathers to hard platy 

slabs. The unit also contains numerous very thin, clean, 

quartzose beds of light-gray sandstone and siltstone. 

West of Yellow Creek, it includes a thin (3 ft (1 m) thick) 

bed of resistant granular limestone near the top of the 

tongue.

The next higher tongue of the Green River is an 

informally named unit, the marlstone at Trail Canyon 

(Hail, 1984), shown as map unit Tgtc on the Barcus

Creek quadrangle. It merges eastward in surface exposures with another tongue of the Green River, map unit 

Tg2 (fig. 9) about a mile (1.5 km) east of Yellow Creek. 

West of the study area, in the southern part of the 

Calamity Ridge quadrangle, it merges with the Parachute 

Creek Member of the Green River (Donnell and Hail, 

1984). It is locally absent in the northern part of the 

Barcus Creek quadrangle owing to the presence of an 

apparent lobe of Uinta Formation clastic rocks. It is 

present but was not separately mapped in the Rough 

Gulch quadrangle (Hail, 1974a) where it is included with 

the Uinta Formation (map unit Tu, table 3). It is 

evidently present throughout most of the subsurface of 

the study area. It ranges in thickness from about 60 to 

250 ft (18-76 m). It consists mostly of light-gray to 

light-brown, variably silty marlstone, most of which is flat 

bedded and locally weathers to platy slabs or chips. The 

unit also contains some marly siltstone and minor thin 

beds of very' fine grained, light-gray to light-brown 

sandstone. In the northwestern part of the Barcus Creek 

quadrangle, the unit contains some thin beds of ostracodal limestone and sandstone. In the extreme southwestern part of the study area, the tongue contains two 

thin ledges of very low grade oil shale (4 gallons per ton 

or less).

The next higher tongue of the Green River River in 

the area west of Yellow Creek is the informally named 

marlstone at Mare Canyon shown as map unit Tgm 

(fig. 9, table 3). It is a generally thin, but widespread unit. 

It splits eastward at Yellow Creek to form two tongues 

(map units Tg3 and Tg4, fig. 9) separated by a tongue of 

the Uinta (map unit Tu4). It is present but not mapped 

separately in the Rough Gulch quadrangle, where it is 

included with the Uinta Formation (map unit Tu, 

table 3). It is present west of the study area in the 

southern part of the Calamity Ridge quadrangle (Donnell and Hail, 1984). The tongue consists of light-gray to 

light-grayish-brown, variably silty marlstone and some 

marly siltstone. In the north-central part of the Barcus 

Creek quadrangle it contains some greenish-gray marly 

claystone. In the southwestern part of the Barcus Creek 

quadrangle il: contains a very thin bed of papery, rich oil 

shale (20-25 gallons per ton). The tongue is about about 

5-60 ft (1.5-18 m) thick.

In the area east of Yellow Creek, the marlstone at 

Trail Canyon and the lower part of the marlstone at Mare 

Canyon (map unit Tg3) merge laterally eastward into one 

tongue of the Green River (map unit Tg2). This tongue 

is similar in lithology to the marlstone at Trail Canyon; it 

consists of light-gray to light-brown, variably silty, locally 

platy weathering marlstone. It contains a few thin, lightgray sandstone beds, and near Piceance Creek it contains 

several beds of very low grade oil shale and several very

Geology of the Lower Yellow Creek Area, Northwestern Colorado 031
thin tuff beds. It ranges in thickness from about 150 to 

350 ft (46-107 m). The tongue designated map unit Tg3 

occupies only a small outcrop area east of Piceance 

Creek between the pinchout of the two enclosing Uinta 

tongues. The tongue consists of light-gray silty marlstone 

and several beds of light-brown sandstone including a few 

channel deposits. It ranges in thickness from about 60 to 

90 ft (18-27 m). The tongue designated Tg4 is chiefly the 

eastward lateral equivalent of the marlstone at Mare 

Canyon and is similar in lithology. It is mostly light brown 

to light gray, locally platy weathering, variably silty marlstone. It contains a few thin, very low grade oil-shale beds 

in the eastern part of its outcrop area and a few very thin 

tuff beds. It ranges in thickness from about 45 to 90 ft.

Upper Group of Tongues of the Green River Formation

The upper group of tongues of the Green River 

Formation comprises mostly three formally named 

tongues (in ascending order), the Yellow Creek, Dry 

Fork, and Thirteenmile Creek, and, in the extreme 

southwestern part of the area, two unnamed tongues 

(map units Tgub and Tguc, fig. 9).

The Yellow Creek Tongue of the Green River 

Formation is widespread through the north-central part 

of the Piceance Creek basin. It crops out in the Barcus 

Creek, Barcus Creek SE, and Rough Gulch quadrangles. 

It was not shown separately on the map of the Rough 

Gulch quadrangle but was included in the Uinta Formation (map unit Tu, table 3). In the southwestern part 

of the Barcus Creek quadrangle, it merges with a higher 

marlstone tongue (map unit Tguc, fig. 9) and the merged 

unit is designated Tgub. The Yellow Creek Tongue was 

mapped eastward into the White River City quadrangle 

(Pipiringos and Johnson, 1976). In the study area, it is 

10-70 ft (3-21 m) thick. Local thinning is due to scouring 

and channel filling by the overlying tongue of the Uinta 

Formation. The Yellow Creek Tongue consists of lightgray to light-brownish-gray, variably silty marlstone; 

lesser light-gray siltstone, much of which is marly; and 

several beds of light-gray to light-brown siltstone and 

very fine grained sandstone. The marlstone beds are flat 

and even bedded and weather to thin chips or platy slabs. 

In the western part of the study area, the marlstone is 

more resistant and forms ledges and benches. It also is 

more limy, and in this area the unit contains one or more 

granular ostracodal limestone beds, which also are 

resistant and form benches. In the upper drainages of 

Barcus Creek and Greasewood Creek, these rocks cap 

buttes on interstream divides. Near the mouth of Barcus 

Creek, the tongue contains a thin bed of very low grade 

oil shale.

The tongue of the Green River designated map 

unit Tguc (fig. 9) occupies only a small area in the 

southwestern part of the Barcus Creek quadrangle. Its

eastern boundary is an arbitrarily designated line that 

marks a relatively abrupt facies change westward from 

the dominantly sandy tongue of the Uinta to the east. The 

tongue consists mostly of light-gray, variably silty and 

sandy marlstone and several sandstone beds. Where it 

merges with the Yellow Creek Tongue it is mostly 

marlstone. Its maximum thickness is about 90 ft (27 m).

The tongue of the Green River designated map 

unit Tgub represents the merged Yellow Creek Tongue 

and overlying tongue and occupies only a small area in 

the southwestern part of the Barcus Creek quadrangle. It 

is mostly very light gray, variably silty and sandy marlstone and includes minor light-brown siltstone and sandstone. Its maximum thickness is about 80 ft (24 m). This 

unit was mapped to the south in the Wolf Ridge 

quadrangle (Duncan, 1976a), where it was designated 

map unit Tgu.

The Dry Fork Tongue of the Green River Formation crops out in the southeastern part of the study area 

between Yellow and Piceance Creeks. In the study area, 

the Dry Fork Tongue wedges out into the Uinta 

Formation approximately along the axis of the Red Wash 

syncline. The tongue was mapped to the east in the White 

River City quadrangle (Pipiringos and Johnson, 1976), to 

the southeast in the Greasewood Gulch quadrangle 

(Duncan, 1976c), and to the south in the Square S Ranch 

quadrangle (Hail and Duncan, 1984). The Dry Fork 

Tongue is light-gray to light-brown, variably silty marlstone to marly siltstone; it also includes one or two 

sandstone beds and locally some argillaceous marlstone 

in the upper part. The marlstone beds are mostly soft and 

nonresistant but in the southern part of the area locally 

are resistant enough to form benches on interstream 

divides. The Dry Fork Tongue thins to the northwest, 

becoming more silty and sandy, and finally ceases to be a 

mappable unit. In the Square S Ranch quadrangle to the 

south, it thins to the west and apparently approaches its 

western wedge edge near the mouth of Duck Creek. The 

Dry Fork Tongue reaches its maximum thickness of 

about 60 ft near the southern boundary of the study area.

The Thirteenmile Creek Tongue of the Green 

River Formation is the stratigraphically highest tongue of 

the Green River in the study area. It crops out as outliers 

mostly along the trough and southern flank of the Red 

Wash syncline. It is one of the most widespread of the 

formally named Green River tongues in the northern 

part of the Piceance Creek basin and crops out almost as 

far south as upper Piceance Creek in T. 2 S., Rs. 94, 95, 

and 96 W. (O'Sullivan, 1974a; Duncan, 1976d). It was 

mapped in adjacent quadrangles to the east, south, and 

southwest. It merges westward with other Green River 

tongues about a mile (1.5 km) south of the southwest

032 Evolution of Sedimentary Basins Uinta and Piceance Basins
corner of the study area, in the Wolf Ridge quadrangle 

(Duncan, 1976a). The thickness of the tongue in the 

study area is not well known inasmuch as most of the 

outcrops are stripped of overlying beds. The maximum 

remaining thickness is about 50-80 ft (15-24 m). The 

Thirteenmile Creek Tongue in the study area comprises 

mostly light gray carbonate rocks including dolomitic, 

limy, and argillaceous marlstone and limestone. Thin 

beds of light-brown siltstone and very fine grained 

sandstone and grayish-green claystone commonly are 

present but are less abundant or sparse. The marlstone 

beds in the lower part of the unit are hard and resistant 

and generally form stripped bench surfaces that cap 

interstream divides. These marlstones commonly are 

even bedded and weather to platy slabs or chips. Some 

marlstones higher in the unit, sparsely preserved from 

erosion, are argillaceous, soft, nonresistant and form 

slopes. The relatively abundant limestone beds generally 

are dense to coarsely crystalline; some are granular. Most 

are variably ostracodal or oolitic, and algal structures are 

common. These limestones probably reflect shallow 

open-water lacustrine deposition. The Thirteenmile 

Creek Tongue contains a larger proportion of ostracodal 

and algal limestone beds than any other tongue of the 

Green River. Oil-shale beds are absent, but several miles 

to the south, in the southern part of the Square S Ranch 

quadrangle (Duncan, 1976b), the Thirteenmile Creek 

Tongue contains three very thin, low-grade oil-shale 

beds. These beds are correlative in part with the so-called 

Big Three oil-shale beds (Duncan and others, 1974, p. 8; 

O'Sullivan, 1974b), widely recognized in oil-shale assay 

logs throughout much of the central and southern part of 

the Piceance Creek basin (Cashion and Donnell, 1972). 

In the Sagebrush Hill quadrangle to the southwest of the 

study area, Donnell (1982) reported oil-shale beds, 

including the Big Three beds, in a marlstone tongue 

(marlstone at Duck Creek) that is correlative in part with 

the Thirteenmile Creek Tongue.

Quaternary Deposits

Surficial deposits of Holocene and Pleistocene age 

include terrace gravels, alluvium, and landslides. Streamgravel terrace deposits are present along the White River 

and several of its tributaries. These deposits are 40-400 ft 

(12-120 m) above the river and represent former 

positions of the valley floor. All of these deposits along 

the White River have similar lithologies. The constituent 

material ranges from sand size to as much as 1 ft (30 cm) 

in longest dimension. The larger clasts are mostly 

silicified sandstone, siltstone, and quartzite but also 

include abundant basalt or similar volcanic flow rocks 

and sparse granitic or other crystalline or metamorphic 

rocks. The larger clasts are well rounded. Many of the 

terrace gravels along the White River are covered or

mostly obscured by hillwash or other alluvial material. A 

few patches of terrace gravel along the White River are 

firmly calcite cemented and form resistant conglomerates. The few small patches of terrace gravel along 

Piceance Creek are about 60-140 ft (18-43 m) above the 

present valley floor and consist mostly of silicified 

sandstone and siltstone and minor limestone. Basaltic 

and granitic rocks are absent. Scattered patches of 

stream gravel also occur along other tributaries of the 

White River including Colorow Gulch, Deep Channel 

Creek, Crooked Wash, Coal Creek, and Wolf Creek. 

These patches of gravel are 20-180 ft (6-55 m) above the 

valley bottoms. The rock constituents of these gravels 

include quartzitic to nonquartzitic sandstone and siltstone, locally limestone, and chert. Clasts of basaltic and 

granitic or other crystalline rocks are absent. The maximum thickness of the stream-gravel terrace deposits is 

about 20 ft (6 m).

The term alluvium is applied to all mapped alluvial 

and colluvial deposits in the area except the terrace 

gravels, and it includes the valley-bottom alluvium of the 

White River and its tributary streams, alluvial fans at the 

junctions of branch streams with the White River and 

Piceance Creek, fans at the base of cliffs along the White 

River, and some hillwash and colluvial cover. These units 

are designated on the original four 7.5-minute maps of 

the area as map units Qal (alluvium), Qaw (alluvium in 

the valley of the White River), and Qac (alluvial and 

colluvial deposits undifferentiated). The alluvium is a 

mixture of rounded gravel derived from distant sources 

to the east and locally derived mud, silt, sand, and gravel. 

It also includes fans at the mouths of intermittent branch 

streams where they join trunk streams. Such fans build 

periodically following heavy rainstorms. Torrential fans 

spread out from the bases of steep cliffs onto the lower 

ground flanking the cliffs and cover appreciable areas of 

terrace gravel deposits along the White River.

Landslide deposits are relatively sparse in the area. 

Most consist of coherent to chaotic rocks masses or 

slumped ground and originate in or on nonresistant clay 

or shale of the Mancos Shale, Wasatch Formation, and 

Garden Gulch Member of the Green River Formation. 

The largest landslide mass lies in sec. 27, T. 1 N., 

R. 97 W., on the northeast side of the conspicuous butte 

just north of the White River. It is composed of jumbled 

masses mostly of sandstones of the Anvil Points Member 

of the Green River Formation and some units of the 

underlying Wasatch. A small area of landslide in sec. 11, 

T. 1 N., R. 97 W., near the east boundary of the area, 

involves burned or clinkered beds probably of the 

Mahogany ledge oil-shale zone. There are no major areas 

of active landsliding. Minor slumping occurs as a result of 

heavy rainstorms in gullied areas of the Wasatch 

Formation.

Geology of the Lower Yellow Creek Area, Northwestern Colorado O33
STRUCTURE

The lower Yellow Creek area is in the northwestern part of the Piceance Creek basin, the deepest 

structural basin in Colorado (MacLachlan and 

Kleinkopf, 1969, pi. 1) A segment of the structural 

trough line of the Piceance Creek basin trends northwestward through the lower Yellow Creek area, where it 

is designated the Red Wash syncline (fig. 11). The 

deepest part of the basin, about 18,000 ft (5,500 m) 

below sea level to Precambrian basement, is a few miles 

southeast of the study area, but the deepest occurrence of 

Tertiary rocks is within the study area in Tps. 1 and 2 N., 

R. 98 W. The maximum structural relief on the Precambrian surface within the lower Yellow Creek area is 

about 15,000 ft (4,600 m).

Structural trends in the study area generally are 

characteristic of those throughout the entire Piceance 

Creek basin. Fold axes and faults generally trend 

northwesterly. The area contains several low-ampitude 

folds and some faults having relatively small stratigraphic 

displacement.

1 Da'301 R99W. 

40e15

H97W. 108"! 5'

40eOO'

T.3N.

T.1S.

EXPLANATION

T r Anticlinal axis Showing direction of plunge p ) 2 p 4 MILES

{ «. Synclinal axis-Showing direction of plunge ° 2 * B KILOMETERS 

 i Fault Bar and ball on downthrown side

Figure 11. Faults and fold axes in the lower Yellow Creek 

area. Some small faults are not shown.

Midland Anticline, Graben at Pinyon 

Ridge, and Crooked Wash Syncline

Both the Midland anticline and the Crooked Wash 

syncline generally trend east-west and extend eastward 

from off the Skull Creek anticline, a major uplift a few 

miles west of the study area. Axial traces of these folds as 

shown on figure 11 are based on structure contours 

drawn on Cretaceous horizons at or below the top of the 

Mancos Shale.

The Midland anticline is a major structural 

element that extends across the northern part of the 

study area. The anticline controls the outcrop pattern of 

formations in the northern part of the area and marks the 

abrupt change in strike demonstrated by the southstriking rocks that form Pinyon Ridge and the eaststriking rocks that form Coal Ridge. The anticline forms 

the structural boundary of the northwestern Piceance 

Creek basin, west of the Gray Hills segment of the basin. 

It plunges eastward and has steep dips on its south flank 

and gentle dips on its north flank. In the Stadtman Mesa 

area (Rough Gulch quadrangle), dips on the south flank 

are as much as 3,500-4,000 ft/mi (660-760 m/km); 

farther to the east, dips generally are less than 2,500 ft/mi 

(470 m/km). Dips on the north flank generally are 

500-1,000 ft/mi (95-190 m/km). Nine oil and gas test 

holes were drilled along or near the anticlinal axis. All 

were dry except Mobil Oil Federal 15-29 and 1-30, 

which yielded gas and minor oil.

A graben about 2 mi (3.2 km) long is present in the 

northwestern part of the study area (Rough Gulch 

quadrangle). Exposed rocks in the faulted area include 

the upper part of the Cretaceous Mancos Shale and the 

lower part of the Williams Fork Formation. The faults 

are vertical to steeply dipping normal faults. Some of the 

apparent sinuosity of the fault traces is due to topography. The graben is about 250-2,000 ft (76-610 km) 

wide. The maximum stratigraphic displacement is about 

350 ft (106 m) on its northern fault and about 200 ft 

(60 m) on its southern fault. The fault zone is just south 

of the axis of the Midland anticline at the position of 

maximum change in outcrop trend direction that marks 

the geographic boundary between the generally south 

trending Pinyon Ridge and the east-trending Coal Ridge. 

This position coincides with the marked structural 

steepening of the south flank of the asymmetrical 

Midland anticline.

A small segment of the Crooked Wash syncline is 

present in the extreme northern part of the study area. 

The fold is somewhat asymmetric. Dips on the north 

flank are about 1,500 ft/mi (280 m/km), whereas dips on 

the south flank are about 750 ft/mi (140 m/km). The 

syncline plunges eastward throughout its extent, both 

within and north of the study area.

O34 Evolution of Sedimentary Basins Uinta and Piceance Basins
Folds and Faults in the 

Vicinity of the White River

Several northwest-striking folds and normal faults 

are present along or just south of the White River in the 

east-central part of the area (fig. 11).

Fold axes were determined on the basis of dips of 

mapped formation contacts in the area; control points 

are sparse and the placement of the fold axes is not well 

established. The existence of the syncline drawn along 

the alluvium of the White River (Barcus Creek SE 

quadrangle) is chiefly speculative but accounts for dip 

reversals of the Wasatch-Green River contact on the 

north and south sides of the river. The axis of this 

syncline apparently extends only a short distance east of 

the study area. The anticlinal axis, about 0.5 mi (0.75 km) 

southwest of the syncline, extends to the east boundary of 

the study area where it aligns with a major southeasttrending fault (Pipiringos and Johnson, 1976). Analysis 

of drill-hole logs in sec. 1, T. 2 N., R. 97 W., does not 

confirm the presence of faulting, and it is assumed that 

the fault merges to the northwest into the anticlinal fold. 

A separate short anticlinal segment is also present just 

north of the larger anticline.

Extending northwesterly beyond the fold axes are 

three faults that locally offset contacts of the Wasatch 

Formation and the Anvil Points and Garden Gulch 

Members of the Green River Formation. The offsets are 

relatively small, from 10 to 50 ft (3-15 m), but at the 

northwest corner of sec. 29, T. 2 N., R. 97 W. (Barcus 

Creek SE quadrangle), the displacement is as much as 

100 ft (30 m). The two larger faults are downthrown on 

the northeast, whereas the shortest of the three faults is 

downthrown on the southwest, forming a small graben. 

The largest fault is aligned roughly with, and extends 

northwest from, the larger anticline. This alignment 

suggests differing structural adjustments to a common 

structural trend.

Two very short structural segments are shown on 

figure 11 at the eastern boundary of the study area in 

T. 2 N., R. 97 W. The anticlinal axis is the northwest end 

of the so-called White River dome, a structural feature 

present mostly east of the study area. The normal fault, 

downthrown to the southwest, is the northwest end of a 

major fault that extends southeastward more than 6 mi 

(10 km) (Pipiringos and Johnson, 1976).

Red Wash Syncline and 

Central Graben Zone

The Red Wash syncline is the dominant fold in the 

area. The axial trace of the syncline as shown on figure 11 

is based on structure contours drawn on the top of the 

Mahogany oil-shale zone in the Parachute Creek

Member of the Green River Formation. A structure 

contour map by Johnson (1983) drawn on the top of the 

Cretaceous lies Formation and a structure contour map 

by Dyni (1969, pi. 1) drawn on the orange marker in the 

upper part of the Garden Gulch Member of the Green 

River Formation both show a similar axial trend for the 

syncline in the older rocks. The northwestward extension 

of the Red Wash syncline forms the structural boundary 

between the Rangely anticline and the Skull Creek 

anticline to the west of the area. The syncline extends 

southeastward for about 11 mi (18 km), with two 

reversals in plunge direction, and dies out before 

reaching the eastern margin of the Piceance Creek basin 

(Dyni, 1969, pi. 1; Pitman and Johnson, 1978, sheet 2). 

Within the study area the syncline is doubly plunging 

(fig. 11) toward the structural low just southeast of 

Barcus Creek near the township boundary between Tps. 

1 and 2 N. (see Barcus Creek and Barcus Creek SE 

geologic maps). The maximum structural closure within 

the study area is about 400 ft (120 m).

The Red Wash syncline is asymmetric; it has a 

steep north flank and a gentle south flank. North flank 

dips are about 1,000 ft/mi (190 m/km) and south flank 

dips are about 200 ft/mi (38 m/km). Maps by Pitman and 

Johnson (1978), Pitman (1979), and Pitman and others 

(1989) show that isopach trends of the R-5, R-6, and 

Mahogany oil-shale zones roughly coincide with the 

structural configuration along the Red Wash syncline in 

the study area. These oil-shale zones are thickest in the 

structural low, and structural deepening probably 

occurred during deposition of the upper part of the 

Parachute Creek Member of the Green River Formation. Lower oil-shale zones in the Parachute Creek 

Member show no such deepening.

The central graben zone (fig. 11) is closely parallel 

with the axis of the Red Wash syncline for much of its 

extent in the study area. The fault zone extends 

southeasterly from sec. 19, T. 2 N., R. 98 W., in the 

Rough Gulch quadrangle, for an overall distance of 

about 13 mi (21 km), to sec. 35, T. 1 N., R. 97 W., in the 

Barcus Creek SE quadrangle. Faulting is continuous, 

however, only along the northwestern 6.5 mi (10.5 km) of 

the zone. A few intermittent faults mark the zone to the 

southeast and displacements are small. Stratigraphic 

displacements are greatest generally along the southwestern fault; maximum displacement is about 100 ft 

(30 m), but most displacements do not exceed 50-60 ft 

(15-18 m). Along the northeastern fault, the maximum 

displacement is about 80 ft (24 m), but most displacements do not exceed 10-20 ft (3-6 m). The faults 

are locally and very sparsely mineralized by calcite. 

Zones of fault breccia are sparse but locally present. The

Geology of the Lower Yellow Creek Area, Northwestern Colorado O35
nature of the faults at depth is unknown owing to the lack 

of subsurface information, but the system is presumed to 

be deep seated. Surface exposures indicate that the faults 

are essentially vertical.

The proximity and parallel trend of the fault zone 

and the axis of the Red Wash syncline suggest a genetic 

relationship. The alignment of normal faults along welldefined fold axes, both synclinal and anticlinal, is common in the study area and elsewhere in the northern 

Piceance Creek basin. In addition to the fault zones 

described in the study area, all of which are clearly 

associated with fold axes, at least two others are 

noteworthy. The Dudley Bluffs graben, about 8 mi 

(13 km) south and southeast of the study area, generally 

is south of and parallel with the axial trace of the 

Piceance Creek dome throughout much of its length of 

about 12 mi (19 km) (see maps by Dyni, 1969, pi. 1; 

Duncan, 1976b, c, d; Pitman and Johnson, 1978, sheet 2). 

The epicenter of possibly the largest earthquake in 

Colorado in historic time, November 7, 1882, may have 

been the Dudley Bluffs graben (McGuire and others, 

1982). If so, the Dudley Bluffs graben is likely a deepseated fault system. The Sulphur Creek anticlinal nose 

(Dyni, 1969, pi. 1), about 10 mi (16 km) south and 

southwest of the study area, is marked by numerous 

normal faults throughout most of its length of about 12 

mi (19 km) (see maps by Duncan, 1976a, e, f; Donnell, 

1982; Pitman and Johnson, 1978, sheet 2). The faults are 

probably deep seated and developed along the trends of 

the Laramide-age folds.

Yellow Creek Anticlinal Nose

The Yellow Creek anticlinal nose is present near 

the southern boundary of the study area (fig. 1). The axial 

trace of the fold is based on structure contours drawn on 

the top of the Mahogany oil-shale zone in the Parachute 

Creek Member of the Green River Formation. The fold 

represents structural steepening of the Rangely anticline 

to the west of the area and was designated Rangely 

anticline on structure contour maps by Dyni (1969, pi. 1) 

and Hail (1974). Redrawing of structure contour lines 

for the revised Barcus Creek quadrangle geologic map 

(Hail, 1984) did not demonstrate, however, a definitive 

fold axis for the Rangely anticline near the western 

boundary, and the use of the name Rangely anticline is 

not regarded as appropriate for the eastward extension of 

the clearly evident structure now termed the Yellow 

Creek anticlinal nose. The structure extends from the 

boundary between Rs. 98 and 99 W., where obvious 

nosing begins, southeastward about 8 mi (13 km) to the 

southern boundary of the study area, and terminates 

about 3 mi (5 km) farther southeast in a structural saddle 

near Piceance Creek. It is relatively symmetrical and

plunges southeastward about 40-110 ft/mi (8-21 m/km). 

There is no structural closure.

ECONOMIC GEOLOGY

Oil Shale and Associated Minerals

Rich oil-shale deposits underlie the southern part 

of the study area, mostly in the Barcus Creek and Barcus 

Creek SE quadrangles. Oil shale is present in the 

Parachute Creek and Garden Gulch Members of the 

Green River Formation, and all the named oil-shale 

zones are exposed. The Garden Gulch Member includes 

rich oil-shale zone R-l and lean zone L-l, as well as 

some oil shale below zone R-l. The Parachute Creek 

Member includes all oil-shale zones above zone L-l. 

Fischer assay logs showing oil yield and oil-shale zones 

for six core holes in and near the study area are shown on 

the geologic maps of the Barcus Creek and Barcus Creek 

SE quadrangles (Hail, 1984, 1988).

Tables 4 and 5 show oil-shale resources for the 

Barcus Creek and Barcus Creek SE quadrangles, 

respectively. The information in these tables is derived 

from published resource maps by Pitman (1979), Pitman 

and Johnson (1978), and Pitman and others (1989). Oil 

shale is present in the Rough Gulch quadrangle, but 

subsurface information is sparse and outcrops of the 

oil-shale-bearing part of the Green River Formation in 

the quadrangle are mostly barren of significant oil shale. 

Trends of oil-shale values depicted in the published 

resource maps suggest that none of the oil-shale zones 

in the Rough Gulch quadrangle contains significant 

resources. Within the Barcus Creek and Barcus Creek 

SE quadrangles, oil-shale values and thicknesses for all 

the zones generally decrease northward from the 

southern parts of the quadrangles. Oil-shale resources 

for oil-shale beds above the Mahogany zone and below 

the R-l zone are not included in the resource tables. 

Oil-shale resources above the Mahogany zone are 

probably insignificant in the area. These oil-shale beds 

generally are of low grade and lie within the Uinta and 

Green River intertonguing sequence. Pitman and Donnell (1973) showed, however, that large resources of oil 

shale are present above the Mahogany farther south in 

the Piceance Creek basin. The total oil-shale resources in 

the study area, as shown in tables 4 and 5, are about 111 

billion barrels of oil.

Potentially valuable deposits of the sodium 

minerals nahcolite and dawsonite are closely associated 

with the oil shale. These minerals presumably would be 

produced commercially only as byproducts of oil-shale 

production. Nahcolite (NaHCO3 ) is a potential source of 

industrial soda ash and a gas-scrubbing agent. Dawsonite

O36 Evolution of Sedimentary Basins Uinta and Piceance Basins
Table 4. Oil-shale resources in the Barcus Creek quadrangle

[No subsurface resource information is available for area north of township line between T. 1 N. and T. 2 N., and, although oil shale is present in the 

area, no resources are estimated. No resources are estimated for individual oil-shale zones below the following barrels-per-acre isovalue lines: R-1, 

200,000; L-l, 10,000; R-2,50,000; L-2,20,000; R-3, 50,000; L-3, 20,000; R-4,100,000; L-4,50,000; R-5,100,000; L-5,25,000; R-6,50,000; 

Mahogany, 50,000]

Zone

Mahogany

R-6

L-5

R-5

L^

R-4

L-3

R-3

L-2

R-2

L-l

R-1

Thickness 

(approx. range in feet)

170-240

<200-250

<140-180

<200-320

100->160

<120-150

<40-65

<60-120

30-43

<60-70

<30-30

130-220

Gallons per ton 

(approx. ran Re)

<10-24

<5-23

<5-12

<15-30

<10-22

<25-37

<10-20

<20-32

<10-23

<25-37

<10-13

<20-21

Barrels per acre Approx. total resource 

(approx. range) (billions of barrels)

50,000-340,000

<50,000->350,000

<25,000-145,000

<100,000-500,000

<50,000-245,000

<100,000-355,000

<20,000-60,000

<50,000-240,000

<20,000-75,000

<50,000-55,000

<10,000-25,000

<200,000-320,000

7.2

8.2

2.6

9.7

4.1

7.2

1.2

4.4

1.1

3.4

0.3

5.0

Table 5. Oil-shale resources in the Barcus Creek SE quadrangle

Zone

Mahogany

R-6

L-5

R-5

L-4

R-4

L-3

R-3

L-2

R-2

L-l

R-1*

Thickness 

(approx. range in feet)

180-240

<200->260

<100-195

140-310

<80-160

60-150

25-40

<40-115

20-55

45-70

20-40

135-220

Gallons per ton 

(approx. range)

10-26

<5-23

<5-15

<!5-28

<10-22

<20-37

<10-20

<20-32

10-21

15-37

5-14

15-26

Barrels per acre 

(approx. range)

50,000-400,000

<50,000->350,000

<25,000-165,000

<100,000-535,000

<50,000-240,000

<100,000-365,000

<20,000-70,000

<50,000-225,000

<20,000-75,000

<50,000-150,000

5,000-35,000

<200,000-3 15,000

Approx. total resource 

(billions of barrels)

8.6

8.1

3.1

10.0

4.2

6.6

1.3

4.0

1.4

3.4

0.6

5.3

*Figures shown are for southern half of quadrangle only because of lack of subsurface information in northern half of quadrangle.

(NaAl(OH)CO3 ) is a potential source of aluminum. 

Both minerals occur in the saline facies of the Parachute 

Creek Member of the Green River Formation (Kite and 

Dyni, 1967; Beard and others, 1974; Dyni, 1974). The 

saline facies occupies the deeper parts of the northern 

Piceance Creek basin. Most of the Parachute Creek 

Member in the southern part of the study area is within 

the saline facies. Stratigraphically, the nahcolite and most 

of the dawsonite are below the base of a widespread 

leached zone (approximately the L-5 or R-5 oil-shale 

zone level) from which the readily soluble nahcolite has 

been dissolved and removed and above approximately 

the L-2 or R-2 oil-shale zone (Trudell and others, 1970; 

Beard and others, 1974; Dyni, 1974). Published resource 

data for the study area are sparse, and definitive 

estimates are not possible, but some information is 

available. Resource maps by Beard and others (1974)

show that nahcolite resources range from about 300 

million tons per square mile at the southern boundary of 

the area to zero in the central part; dawsonite resources 

range from about 120 million tons per square mile at the 

southern boundary to zero in the central part. Based on 

these maps, with some modifications suggested by J.R. 

Dyni (written commun., 1985), the total nahcolite 

resources of the study area are estimated to be about 8.9 

billion tons and the total alumina resources in dawsonite 

are estimated to be about 6.1 billion tons.

Coal

Coal-bearing rocks of the Cretaceous lies and 

Williams Fork Formations occupy the southeastern part 

of the Lower White River Coal Field, as defined by Gale

Geology of the Lower Yellow Creek Area, Northwestern Colorado 037
(1910, p. 179, pi. 18). They are present in the northwestern part of the study area in the Rough Gulch and 

Smizer Gulch quadrangles.

Coal beds are present in the upper, middle, and 

lower parts of the Williams Fork Formation and near the 

top and base of the lies Formation. A few very thin coal 

beds of no economic value are present in the upper 

member of the Fort Union Formation. The coal beds 

occur in carbonaceous shale zones, some of which persist 

for several miles along the outcrop, but most of the 

individual coal beds are lenticular and nonpersistent. On 

the geologic maps of the Rough Gulch and Smizer Gulch 

quadrangles, measured sections of 160 coal-bearing 

localities are shown. Coal beds less than 1.2 ft (37 cm) 

thick generally were not measured. The maximum coalbed thickness is 7.2 ft (2.2 m) in the Rough Gulch 

quadrangle and 6.7 ft (2.0 m) in the Smizer Gulch 

quadrangle. Most of the beds are less than 4 ft (1.2 m) 

thick. Most of the available information on coal in the 

study area is based on examination of surface exposures. 

No coal has been produced in the area except for local 

use by ranches from a few small, long-abandoned mines.

Analyses are not available for coals in the area. The 

U.S. Bureau of Mines (Aresco and others, 1966, p. 6) 

reported analyses of three samples from the StaleyGordon mine about 11 mi (17.5 km) west of the area, 

from probably correlative strata. Heat values range from 

10,830 to 11,210 Btu/lb (moist basis), and sulfur content 

from 0.4 to 0.5 percent. The coal is ranked as highvolatile C bituminous.

lies Formation

A persistent coal-bearing carbonaceous shale zone 

overlies the basal sandstone of the lies Formation north 

of the White River, but coal beds in the zone are 

discontinuous. The zone is covered by alluvium of the 

White River in R. 99 W., except at the west edge of the 

area where coal beds are apparently absent. Ten coal 

sections were measured in this zone and are numbered 

64 through 73 on the Rough Gulch geologic map. The 

thickest measured bed in the lower lies coal zone is 3.4 ft 

(1 m) thick. Coal is mostly absent in higher beds of the 

lies except for a single bed south of the White River (coal 

section 85 on the Rough Gulch geologic map), which is 

2.4 ft (73 cm) thick and lies about 80 ft (24 m) below the 

top of the formation.

Williams Fork Formation

Most of the coal in the study area is in the Williams 

Fork Formation, and the two principal coal-bearing 

zones of the area are near the base and in the upper part 

of the formation. The lower zone, present only in the 

Rough Gulch quadrangle, actually comprises as many as

three closely spaced coal-bearing carbonaceous zones, 

the lowest of which is the most persistent. All generally 

are within about 300 ft (92 m) above the base of the 

formation. Sixty-three coal sections (numbers 11-63 and 

74-83 on the Rough Gulch geologic map) were measured in the lower zone. About one-third of the sections 

contain more than one coal bed. The thickest measured 

coal bed in the lower zone is 7.2 ft (2.2 m) thick. 

Coal-bearing rocks are widespread in this stratigraphic 

interval throughout the Piceance Creek basin. They are 

equivalent to the main coal zone of the Mesaverde Group 

in the Rangely area to the west (Barnum and Garrigues, 

1980; Garrigues and Barnum, 1980), to the Fairfield coal 

group in the Meeker area (Hancock and Eby, 1930, 

p. 24), and to the Cameo-Fairfield coal zone in the 

southern part of the Piceance Creek basin (Johnson, 

1983).

The upper Williams Fork coal zone comprises 

several closely spaced coal-bearing carbonaceous zones, 

all within about 950 ft (290 m) below the top of the 

formation. The upper coal zone is limited mostly to the 

Smizer Gulch quadrangle and is absent from most of the 

Rough Gulch quadrangle owing to truncation on the 

Cretaceous-Tertiary unconformity. Seventy-six coal sections (numbers 1-75 on the Smizer Gulch geologic map; 

number 1 on the Rough Gulch geologic map) were 

measured in the upper zone. About one-third of these 

sections contain more than one coal bed. The thickest 

measured coal bed in the upper zone is 6.7 ft (2.0 m) 

thick.

Coal beds between the upper and lower coal zones 

are mostly thin and discontinuous and do not occur in 

persistent carbonaceous zones. Ten coal sections of these 

beds were measured (numbers 1-10 on the Rough Gulch 

geologic map), and the thickest coal bed is 3.4 ft (1 m) 

thick.

Resources

U.S. Geological Survey reports on coal resources 

for the Rough Gulch and Smizer Gulch quadrangles 

(AAA Engineering and Drafting, 1979 a, b) contain no 

resource estimates except for isolated localities, owing to 

the absence of laterally persistent beds more than 5 ft 

(1.5 m) thick. The economic potential of coal in the area 

is limited by the small lateral extent of the individual coal 

beds, the apparent absence of thick individual coal beds, 

and relatively steep structural dips, which preclude 

possible strip mining.

Oil and Gas

Table 6 lists oil and gas drill holes in the study area 

for which records are available. The area includes the

O38 Evolution of Sedimentary Basins Uinta and Piceance Basins
108°30' 

40°15*

ROUGH GULCH QUADRANGLE SMIZER GULCH QUADRANGLE

T. 3 N.

T. 2 N.

R. 99 W. R. 98 W.

0123456 KILOMETERS I ii I i I i

01234 MILES

R. 97 W.

Figure 12. Area of outcrop (shaded) of upper member of Fort Union Formation, lower Yellow Creek area. Numbered circles 

show locations of uranium-enriched rocks (table 7).

western part of the White River Gas Field from which gas 

is currently being produced.

Sixteen of the drill holes tested pre-Tertiary rocks 

for oil and gas, but only one, the Mobil Oil Co. Federal 

1-30 in the NEJ/4 sec. 30, T. 3 N., R. 97 W., yielded 

commercial amounts of oil and gas. This well produces 

gas and a small amount of oil from the Shinarump 

Member of the Chinle Formation of Triassic age. The 

well was drilled on the crest of the Midland anticline in an 

area where the fold is an anticlinal nose having no 

structural closure. In the Rangely area, west of the study 

area, oil is produced mostly from the Pennsylvanian 

Weber Sandstone; a smaller amount is produced from 

fractured shale of the Upper Cretaceous Mancos Shale. 

Oil fields along the crest of the Axial Basin uplift at the 

northeastern margin of the Piceance Creek basin, have 

yielded substantial amounts of oil from the Jurassic Entrada Sandstone and Morrison Formation, the Lower 

Cretaceous Dakota Sandstone, and fractured shale in the 

Mancos Shale (Dunn, 1974, p. 218-220). In the Elk 

Springs Oil Field northwest of the study area, oil is 

produced from the Weber Sandstone.

White River Gas Field

Gas was first discovered in 1890 in the White River 

Gas Field (Dunn, 1974, p. 221), but most of the production from the field has been in recent years because of 

the construction of pipeline facilities. Total Tertiary 

production has been relatively small, about 7.5 million 

cubic feet as of January 1987, and about two-thirds of this 

production has been from wells in the study area 

(Colorado Oil and Gas Commission, 1987, sec. V, 

p. 325). The eastern part of the field lies east of the study 

area, in the White River City and Indian Valley 

quadrangles. In that area, most of the gas, as well as some 

oil, is produced from Upper Cretaceous nonmarine 

rocks. Within the study area, however, all gas production 

is from Tertiary rocks. The reservoirs are lenticular 

sandstone beds at various stratigraphic levels in the 

Wasatch and Fort Union Formations. The entrapment 

apparently is stratigraphic rather than structural, and 

there is no evident relationship to the so-called White 

River dome to the east. The total cumulative Tertiary gas 

production in the study area to January 1, 1987, is

Geology of the Lower Yellow Creek Area, Northwestern Colorado O39
O40 Evolution of

SPai'

(D

S

»<

00 

0)5"(0

1

c

R? (a

to3a.

 ao'

(D0)

3O (D

D) 0) (05'

w

Table 6. Oil and gas drill holes, lower Yellow Creek area 

[Cumulative production (cum. prod.) as of January 1987]

Total depth

Name

General Petroleum Yellow Creek 64-8

Teton Energy Colorado Wildlife 24-1

General Petroleum Government 46-27Pacific Transmission Federal 22-12General Petroleum Yellow Creek 26-25General Petroleum Yellow Creek 88-26Union Oil Colorow Government 1

American Resources Government 17-2

American Resources Government 17-1

Shell Oil Government 22X-17

Frontier Refining Colorow 2

Frontier Refining-Shell Oil Colorow 1

American Resources Government 17-3American Resources Government 18-1-5

American Resources Government 20-2

Trend Exploration Parker 1

Location

NE1/4 sec. 8, T. 1 N., R. 98 W.NE1/4 sec. 24, T. 1N..R.98W.

SW1/4 sec. 27, T. 1 N., R. 98 W.

NW1/4 sec. 12, T. 1 N., R. 99 W.

SW1/4 sec. 25, T. 1 N., R. 99 W.SE1/4 sec. 26, T. 1 N., R. 99 W.SW1/4 sec. 16, T. 2 N., R. 97 W.

NE1/4 sec. 17, T. 2 N., R. 97 W.

NW1/4 sec. 17, T. 2 N., R. 97 W.

NW1/4 sec. 17, T. 2 N., R. 97 W.

SW1/4 sec. 17, T. 2 N., R. 97 W.SEl/4sec. 17.T.2N..R.97W.

SEl/4sec. 17.T.2N..R.97W.SW1/4 sec. 18, T. 2 N., R. 97 W.

NEl/4sec.20,T.2N.,R.97W.

SW1/4 sec. 20, T. 2 N., R. 97 W.

(in feet)

3,8599,102

3,223

15,686

7,624

1,691

3,3482,183

2,179

15,85711,51314,617

2,1502,144

2,644

250

Formation at surface

Tongue of UintaTongue of UintaTongue of Green RiverTongue of Green RiverTongue of UintaTongue of UintaWasatch

WasatchWasatch

Wasatch

Wasatch

Wasatch

Wasatch

Green RiverWasatchWasatch

Formation at

total depth

WasatchDes(?)Wasatch

Morrison

Williams Fork

Green River

Fort Union

Fort Union

Fort Union

Morgan(?)

Mancos

Weber

Fort UnionFort Union

Fort Union

Wasatch

Year

drilled Comments

19521980195219791955

1955 Completed as water well1952

1977 Gas in Wasatch1977 Gas in Wasatch

1967195919581977 Gas in Wasatch1977

19791973

Status

AbandonedAbandonedAbandoned

Abandoned

Abandoned

AbandonedAbandoned

Shut-in

Shut-in

Abandoned

AbandonedAbandoned

Shut-in

AbandonedAbandonedAbandoned

American Resources Caldwell 1-20

Trend Exploration Parker 1A-20 

American Resources Oldland 2-28

SEl/4sec.20,T.2N.,R.97W. 3,081 WasatchSE1/4 sec. 20, T. 2 N., R. 97 W. 15,118 Wasatch

NWl/4sec.28,T.2N.,R.97W. 2,939 Wasatch

American Resources Oldland-Fritzlan 1 SE1/4 sec. 28, T. 2 N., R. 97 W. 3,100 Wasatch

Fort Union

Morgan(?) 

Fort Union

Fort Union

1972

1974

1972

1970

Gas in Wasatch; cum. prod. 

1,041,373 MCF

Gas in Wasatch; cum. prod.

777,287 MCF 

Gas in Wasatch; cum. prod.

44,982 MCF

American Resources Ivory-Fritzlan 1 

American Resources Government 1 American Resources Government 2-34 American Resources Ivory 1

NE1/4 sec. 34, T. 2 N., R. 97 W. 3,040 WasatchNW1/4 sec. 34, T. 2 N., R. 97 W. 1,273 Wasatch

SW1/4 sec. 34, T. 2 N., R. 97 W. 3,001 Wasatch

SE1/4 sec. 34, T. 2 N., R. 97 W. 3,701 Wasatch

Gas well

Abandoned 

Gas well

Shut-in

Abandoned Fort Union 1970 Gas in Fort Union; cum.

prod. 84,291 MCF 

Wasatch 1970 Gas in Wasatch; cum. prod. Gas well

191,441 MCF 

Fort Union 1972 Gas in Wasatch; cum. prod. Gas well

1,197,178 MCF 

Williams Fork 1957 Gas in Wasatch; cum. prod. Gas well

1,709,184
Continued Table 6.

in

£3 "rein1

E

O

U

S 7=

rt j*

||

o o

UHI

in

rt

c

.0

^3

E

£

f~

"O <1J

"rt .c

c

_o

§_l

HI

E

TO

13 13 T3

<L> <L> <L>

C C C

|||-s

C3 C3 C3 ^

*O ,£) ,O S

< < < CO

i

(ij

8°

P, J3

|f 0

13 4« ,H

fe w

 ^ °*3 §

^ o S

S UH S

CO

oo vo r^ oo vo i/"i i/"i r^-

ON ON ON ON

i 1 » 4 i-H i-H

C w w

a o o >H w) o o a?

o 53 S u

g e o o

*^ *^ 'f-3 *f3

 g ^3 D P

^ 23 f t-1 ca w b ts

O l~^ */^ CN

CO *^ O ^

rj- r-^ I-H_ o

«n vo" r-~ of

i-H i-H

^^ON ON ON ON

O^OH OH OJ

>r£ >^ >*>*

CN ^ ^ ^

H H HH

5 t^-" ON ON 1-1 CN CN CS

« 0 0 Q

" 55 «> o

W pj yj pj

^ CO CO CO

0

1 1

Government 31OilShell 

Trident 1-ACoyote Trident Coyoe 2 

15-29 Mobil OilFederal 

13

£H * ""*

I 1 

ll

~*

«2 °.

§|

O {T)^

1 |

|| t

1^ '

w O '^

O

VO fON ON

i t l-H

O m O fl>

§ u

O

u- a

w 0

g 3

II

0 0

CO l-H

CO* ^H"

r^ rON ON

H-H.

 -£ ^

CO CO

H H 0*0" 

CO CO

o o

CA in

g§

Federal California Oil1Federal 1-30 Mobil Oil

13 TJ

f"j cj Abando Abando

u

a <

 H

OO ON 

ON ON

i-H i-H

Vi

VH O 

O 0

S O

PU tin

en co .§ .i

II

O CN

VO i-H

ON oo

««ON QS,

P^ qi

*5 12

CO or.

H H

rfof

8 *-*

^^

CO Z

i-H

CN

24X Houston OilFederal 

Trident Coyoe 3

TJ T3 T3

O O O

c c f-3 8-8-8

TO TO C3

CO VO O

vo in oo ON ON ON l-H » 4 l-H

C

e/5 O 

O U "5

1^1

g

PH

CO

§ 8 8

III

VO CO O

>D Tf VO

O ON (N

cs <* rtOO *~£ ON ON ON ON

oi f^oi

z< <^-< 'z

CO ^ CO

H H H

r-Tv^ co" 

(N ^ (N

6 < > o

fl^ W (^

^ "* ^

tQ ^ PL] 5? co Jz;

CO (L>

- i?

i ll go S

Trident Goven Stockmar Oil Massadona Seaboard Fed MobilOil-G Corp 

1-23

Geology of the Lower Yellow Creek Area, Northwestern Colorado 041
Table 7. Description of samples from known uranium-enriched localities, lower Yellow Creek area 

[Localities shown in figure 12]

Sample Radiometric count Uranium 

Locality number (times background) (in ppm)

1 MDW219

MDW218

MDW217

MDW216

2 MDW206 

MDW205 

MDW204 

MDW202 

MDW203

MDW278

3 MDW211

MDW210

MDW209 

MDW208 

MDW207

4 MDW270

MDW271

MDW272

MDW273

MDW279

MDW280

5 MDW274 

MDW275

6 MDW214

MDW215

7 MDW283 

MDW282 

MDW281

4

3

3

2

3 

3 

2 

2 

3

3.5

3

3

2 

3 

2

5

5

5

3

2.5

3.5

2.5 

2.5

3

3

2 

3 

3

340

251

216

154

23 

169 

25 

48 

76

109

152

110

19 

73 

66

215

248

110

78

125

168

114 

89

84

153

100 

257 

84

Lithology

Carbonaceous shale

Carbonaceous shale

Carbonaceous shale

Coal

Coaly carbonaceous shale 

Coaly carbonaceous shale 

Coaly carbonaceous shale 

Coaly carbonaceous shale 

Coal

Shaly coal

Carbonaceous shale

Carbonaceous shale

Carbonaceous claystone 

Carbonaceous claystone 

Carbonaceous claystone

Carbonaceous shale

Carbonaceous shale

Carbonaceous shale

Carbonaceous shaleCarbonaceous shale

Carbonaceous shale

Shaly coal 

Coaly carbonaceous shale

Carbonaceous shale

Carbonaceous shale

Coaly carbonaceous shale 

Coaly carbonaceous shale 

Carbonaceous claystone

Thickness of 

sampled bed 

(in feet)

1.1

1.2

0.3

0.5-2.5

2.5 

1.0 

2.5 

0.5-1.0 

0.5-1.0

0.5-1.0

2.0-2.5

1.2

6.0 

0.5-1.0 

0.5-1.0

0.8-1.2

0.8-1.2

0.8-1.2

0.8-1.2

0.8-1.2

0.8-1.2

1.0-2.5 

1.0-2.5

1.2-2.5

1.2-2.5

1.2 

0.4 

0.8

Approximate 

stratigraphic 

position

50

115

126

260

184 

187 

190 

206 

206

206

138

428

444 

462 

464

245

245

245

245

245

245

316 

316

402

402

187 

188 

189

5,045,736 thousand cubic feet (Colorado Oil and Gas 

Commission, 1987, sec. V, p. 325).

Uranium

Uranium occurs at several localities in thin carbonaceous beds of the upper member of the Fort Union 

Formation. Evaluation of drill-hole gamma-ray logs 

indicates that uranium-bearing beds are also present in 

the subsurface. Although the deposits are of too low a 

grade to be of potential economic value at the present 

time, the tonnages of uranium-bearing rock are large. No 

other uranium deposits are known in the study area. The 

deposits were described in the National Uranium 

Resource Evaluation (NURE) of the Vernal I°x2° 

quadrangle (Craig and others, 1982), in which areas 

favorable for uranium deposits are defined as those

containing at least 100 tons of U3O8 and having an 

average grade of 0.01 percent U3O8 at depths of less than 

5,000 ft (1,525 m). A more detailed report on the Fort 

Union occurrences is included in a report by Hail 

(1981).

Figure 12 shows the area of outcrop of the upper 

member of the Fort Union Formation in the study area 

and the seven localities from which samples of radioactive carbonaceous shale or claystone were collected. 

Table 7 shows uranium values of samples from these 

localities. Uranium localities 1-3 (fig. 12) are in generally 

well exposed ground, and radiometric examination of 

these exposures is therefore believed to have revealed 

most, if not all, of the anomalously radioactive beds. 

Localities 4-7, however, are in poorly exposed ground, 

and radiometric examination may have missed radioactive beds at these localities.

O42 Evolution of Sedimentary Basins Uinta and Piceance Basins
The seven known uranium localities span a distance along the outcrop of about 13 mi (21 km). 

Uranium-bearing beds are also present north of the 

study area in the Elk Springs quadrangle (Hail, 1981, 

pi. 2). Reconnaissance along the Fort Union outcrop 

west of locality 1 (fig. 12) indicates no evident anomalous 

radioactivity. All the uranium localities are clearly stratiform, although they occur at various stratigraphic 

positions within the upper member. Most of the 

uranium-bearing or radioactive beds are very thin; those 

beds containing at least 100 ppm U are a few inches to 

about 5 ft (1.5) thick, but most are less than 2 ft (60 cm) 

thick. Anomalous concentrations of uranium and anomalous radioactivity are limited to the more carbonaceous 

beds including coal, carbonaceous shale, and carbonaceous claystone. Commonly, anomalous radioactivity 

persists within a relatively thin bed along its strike for a 

considerable distance. Known distances range from a few 

feet to 1,100 ft (336 m), and these distances generally are 

limited to the dimensions of the surface exposures. 

Uranium minerals are unknown, and it is assumed that 

the uranium is in organic compounds.

Table 7 is a summary of information on samples 

collected from the seven numbered localities shown on 

figure 12. Several of the samples contain less than 0.01 

percent U3 O8 (83.6 ppm U), and these would not be used 

to calculate resources.

Gravel

Gravel is readily available from terrace deposits 

along the White River and lower Wolf Creek (map unit 

Qg; Barcus Creek SE, Smizer Gulch, and Rough Gulch 

quadrangles). Several pits have yielded gravel for use in 

local road construction and maintenance. Terrace gravel 

probably underlies part of the alluvial and colluvial 

deposits (map unit Qac; Smizer Gulch and Rough Gulch 

quadrangles).

REFERENCES CITED

AAA Engineering and Drafting, 1979a, Coal resource 

occurrence maps of the Rough Gulch quadrangle, Rio 

Blanco and Moffat Counties, Colorado: U.S. Geological 

Survey Open-File Report 79-194, 15 p., 5 pi.

____ 19795, Coal resource occurrence maps of the Smizer 

Gulch quadrangle, Rio Blanco and Moffat Counties, 

Colorado: U.S. Geological Survey Open-File Report 

79-195, 14 p., 5 pi.

Aresco, S.J., Janus, J.B., and Walker, F.E., 1966, Analyses of 

tipple and delivered samples of coal collected during 

fiscal year 1965: U.S. Bureau of Mines Report of Investigations 6792, 49 p.

Barnum, B.E., and Garrigues, R.S., 1980, Geologic map and 

coal sections of the Cactus Reservoir quadrangle, Rio

Blanco and Moffat Counties, Colorado: U.S. Geological 

Survey Miscellaneous Field Studies Map MF-1179, scale 

1:24,000.

Bass, N.W., Eby, J.B., and Campbell, M.R., 1955, Geology and 

mineral fuels of parts of Routt and Moffat Counties, 

Colorado: U.S. Geological Survey Bulletin 1027-D, 

p. 143-250.

Beard, T.N., Tail, D.B., and Smith, J.W., 1974, Nahcolite and 

dawsonite resources in the Piceance Creek basin, 

Colorado, in Guidebook to the energy resources of the 

Piceance Creek basin, Colorado: Rocky Mountain 

Association of Geologists, Annual Field Conference, 

25th, p. 101-109.

Bradley, W.H., 1931, Origin and microfossils of the oil shale of 

the Green River Formation of Colorado and Utah: U.S. 

Geological Survey Professional Paper 168, 58 p.

Brobst, D.A., and Tucker, J.D., 1973, X-ray mineralogy of the 

Parachute Creek Member, Green River Formation in the 

northern Piceance Creek basin, Colorado: U.S. Geological Survey Professional Paper 803, 53 p.

Cashion, W.B., 1973, Geologic and structure map of the Grand 

Junction quadrangle, Colorado and Utah: U.S. Geological Survey Miscellaneous Investigations Map 1-736, 

scale 1:250,000.

Cashion, W.B., and Donnell, J.R., 1972, Chart showing 

correlation of selected key units in the organic-rich 

sequence of Green River Formation, Piceance Creek 

basin, Colorado, and Uinta Basin, Utah: U.S. Geological 

Survey Oil and Gas Investigations Chart OC-65.

____ 1974, Revision of nomenclature of the upper part of the 

Green River Formation, Piceance Creek basin, Colorado, and eastern Uinta Basin, Utah: U.S. Geological 

Survey Bulletin 1394-G, p. G1-G9.

Colorado Oil and Gas Conservation Commission, 1987, Oil and 

gas statistics, 1986: State of Colorado Department of 

Natural Resources.

Craig, L.C., Hail, W.J., Jr., and Luft, S.J., 1982, U.S. Geological 

Survey, National uranium resource evaluation, Vernal 

quadrangle, Colorado and Utah: U.S. Department of 

Energy Open-File Report PGF/F-026 (82), 109 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.

____ 1971, Geologic map of the Rangely quadrangle, Rio 

Blanco County, Colorado: U.S. Geological Survey 

Geologic Quadrangle Map GQ-903, scale 1:24,000.

Donnell, J.R., 1961, Tertiary geology and oil-shale resources of 

the Piceance Creek basin between the Colorado and 

White Rivers, northwestern Colorado: U.S. Geological 

Survey Bulletin 1082-L, p. 835-891.

____ 1969, Paleocene and lower Eocene units in the southern 

part of the Piceance Creek basin, Colorado: U.S. Geological Survey Bulletin 1274-M, 18 p.

____ 1982, Preliminary geologic map of the Sagebrush Hill 

quadrangle, Rio Blanco County, Colorado: U.S. 

Geological Survey Miscellaneous Field Studies Map 

MF-1398, scale 1:24,000.

Geology of the Lower Yellow Creek Area, Northwestern Colorado 043
Donnell, J.R., and Hail, W.J., Jr., 1984, Preliminary geologic 

map of the Calamity Ridge quadrangle, Rio Blanco 

County, Colorado: U.S. Geological Survey Miscellaneous 

Field Studies Map MF-1690, scale 1:24,000.

Duncan, D.C., 1976a, Preliminary geologic map of Wolf Ridge 

quadrangle, Rio Blanco County, Colorado: U.S. 

Geological Survey Miscellaneous Field Studies Map 

MF-753, scale 1:24,000.

____ 19765, Preliminary geologic map of Square S Ranch 

quadrangle, Rio Blanco County, Colorado: U.S. 

Geological Survey Miscellaneous Field Studies Map 

MF-754, scale 1:24,000.

____ 1976c, Preliminary geologic map of Greasewood Gulch 

quadrangle, Rio Blanco County, Colorado: U.S. 

Geological Survey Miscellaneous Field Studies Map 

MF-755, scale 1:24,000.

____ 1976d, Preliminary geologic map of Jessup Gulch 

quadrangle, Rio Blanco County, Colorado: U.S. 

Geological Survey Miscellaneous Field Studies Map 

MF-756, scale 1:24,000.

____ 976e, Preliminary geologic map of Rock School 

quadrangle, Rio Blanco County, Colorado: U.S. Geological Survey Miscellaneous Field Studies Map MF-757, 

scale 1:24,000. 

1976f, Preliminary geologic map of Yankee Gulch

quadrangle, Rio Blanco County, Colorado: U.S. Geological Survey Miscellaneous Field Studies Map MF-758, 

scale 1:24,000.

Duncan, D.C., Hail, W.J., Jr., O'Sullivan, R.B., and Pipiringos, 

G.N., 1974, Four newly named tongues of Eocene Green 

River Formation, northern Piceance Creek basin, 

Colorado: U.S. Geological Survey Bulletin 1394-F, 

p. F1-F13.

Dunn, H.L., 1974, Geology of petroleum in the Piceance Creek 

basin, northwestern Colorado, in Guidebook to the 

energy resources of the Piceance Creek basin, Colorado: 

Rocky Mountain Association of Geologists, Annual 

Field Conference, 25th, p. 217-224.

Dyni, J.R., 1968, Geologic map of the Elk Springs quadrangle, 

Moffat County, Colorado: U.S. Geological Survey 

Geologic Quadrangle Map GQ-702, scale 1:62,500.

____ 1969, Structure of the Green River Formation, 

northern part of Piceance Creek basin, Colorado: The 

Mountain Geologist, v. 6, no. 2, p. 57-66.

____ 1974, Stratigraphy and nahcolite resources of the saline 

facies of the Green River Formation in northwest 

Colorado, in Guidebook to the energy resources of the 

Piceance Creek basin, Colorado: Rocky Mountain 

Association of Geologists, Annual Field Conference, 

25th, p. 111-112.

Gale, H.S., 1910, Coal fields of northwestern Colorado and 

northeastern Utah: U.S. Geological Survey Bulletin 415, 

265 p.

Garrigues, R.S., and Barnum, B.E., 1980, Geologic map and 

coal sections of the Rangely NE quadrangle, Rio Blanco 

and Moffat Counties, Colorado: U.S. Geological Survey 

Open-File Report 80-274, scale 1:24,000.

Gill, J.R., and Hail, W.J., Jr., 1975, Stratigraphic sections across 

upper Cretaceous Mancos Shale-Mesaverde boundary, 

eastern Utah and western Colorado: U.S. Geological 

Survey Oil and Gas Investigations Chart OC-68.

Hail, W.J., Jr., 1972, Preliminary geologic map of the Barcus 

Creek SE quadrangle, Rio Blanco County, Colorado: 

U.S. Geological Survey Miscellaneous Field Studies 

Map MF-347, scale 1:24,000.

____ 1973, Geologic map of the Smizer Gulch quadrangle, 

Rio Blanco and Moffat Counties, Colorado: U.S. 

Geological Survey Geologic Quadrangle Map GQ-1131, 

scale 1:24,000.

___ 1974a, Geologic map of the Rough Gulch quadrangle, 

Rio Blanco and Moffat Counties, Colorado: U.S. 

Geological Survey Geologic Quadrangle Map GQ-1195, 

scale 1:24,000.

____ 1974b, Preliminary geologic map of the Barcus Creek 

quadrangle, Rio Blanco County, Colorado: U.S. Geological Survey Miscellaneous Field Studies Map MF-619, 

scale 1:24,000.

___ 1977, Stewart Gulch Tongue A new tongue of the 

Eocene Green River Formation, Piceance Creek basin, 

Colorado: U.S. Geological Survey Bulletin 1422-E, 8 p.

____ 1978, Preliminary geologic map of the Mount Blaine 

quadrangle, Garfield County, Colorado: U.S. Geological 

Survey Miscellaneous Field Studies Map MF-984, 

scale 1:24,000.

____ 1981, Low-grade uranium in the Fort Union Formation, 

Crooked Wash area, northwestern Colorado: U.S. 

Geological Survey Open-File Report 81-586.

____ 1982, Preliminary geologic map of the Circle Dot Gulch 

quadrangle, Garfield County, Colorado: U.S. Geological 

Survey Miscellaneous Field Studies Map MF-1293, 

scale 1:24,000.

____ 1984, Geologic map of the Barcus Creek quadrangle, 

Rio Blanco County, Colorado: U.S. Geological Survey 

Geologic Quadrangle Map GQ-1578, scale 1:24,000. 

1988, Geologic map of the Barcus Creek SE quadrangle, Rio Blanco County, Colorado: U.S. Geological 

Survey Quadrangle Map GQ-1613, scale 1:24,000.

Hail, W.J., Jr., and Duncan, D.C., 1984, Revised preliminary 

geologic map of the northern half of the Square S Ranch 

quadrangle, Rio Blanco County, Colorado: U.S. Geological Survey Open-File Report 84-202, scale 1:24,000.

Hail, W.J., Jr., and Pipiringos, G.N., in press, Geologic map of 

the lower Piceance Creek area, northwestern Colorado: 

U.S. Geological Survey Miscellaneous Investigations 

Series Map 1-1936, scale 1:50,000.

Hancock, E.T., and Eby, J.B., 1930, Geology and coal resources 

of the Meeker quadrangle, Moffat and Rio Blanco 

Counties, Colorado: U.S. Geological Survey Bulletin 

812-C, p. 191-242.

Hansen, W.R., Rowley, P.D., and Carrara, P.E., 1983, Geologic 

map of Dinosaur National Monument and vicinity, Utah 

and Colorado: U.S. Geological Survey Miscellaneous 

Investigations Series Map 1-1407, scale 1:50,000.

Kite, R.J., and Dyni, J.R., 1967, Potential resources of dawsonite and nahcolite in the Piceance Creek basin, 

northwest Colorado: Colorado School of Mines 

Quarterly, v. 62, no. 3, p. 25-38.

O44 Evolution of Sedimentary Basins Uinta and Piceance Basins
Izett, G.A., Honey, J.G., and Brownfield, M.E., 1984, Geologic 

map of the Citadel Plateau quadrangle, Moffat County, 

Colorado: U.S. Geological Survey Miscellaneous Investigations Series Map 1-1532, scale 1:48,000.

Johnson, R.C., 1977, Preliminary geologic map of The Saddle 

quadrangle, Garfield County, Colorado: U.S. Geological 

Survey Miscellaneous Field Studies Map MF-829, 

scale 1:24,000.

____ 1981a, Preliminary geologic map of the Desert Gulch 

quadrangle, Garfield County, Colorado: U.S. Geological 

Survey Miscellaneous Field Studies Map MF-1328 

scale 1:24,000.

____ 19815, Stratigraphic evidence for a deep Eocene Lake 

Uinta, Piceance Creek Basin, Colorado: Geology, v. 9, 

no. 2, p. 55-62.

____ 1983, Structure contour map of the top of the Rollins

Sandstone Member of the Mesaverde Formation and 

Trout Creek Sandstone Member of the lies Formation, 

Piceance Creek basin, Colorado: U.S. Geological Survey 

Miscellaneous Field Studies Map MF-1667.

____ 1984, New names for units in the lower part of the 

Green River Formation, Piceance Creek basin, Colorado: U.S. Geological Survey Bulletin 1529-1, 20 p.

Johnson, R.C., and Keighin, C.W., 1981, Cretaceous and 

Tertiary history and resources of the Piceance Creek 

basin, western Colorado: New Mexico Geological Society 

Field Conference, 32nd, Western Slope, Colo., 1981, 

p. 199-210.

Johnson, R.C., and May, F.E., 1980, A study of the CretaceousTertiary unconformity in the Piceance Creek basin, 

Colorado; the underlying Ohio Creek Formation (Upper 

Cretaceous) redefined as a member of the Hunter 

Canyon or Mesaverde Formation: U.S. Geological 

Survey Bulletin 1482-B, 27 p.

MacLachlan, J.C., and Kleinkopf, M.D., eds., 1969, Configuration of the Precambrian surface of Colorado: The 

Mountain Geologist, v. 6, no. 4, map.

McGuire, R.K., Krusi, Alan, and Oaks, S.D., 1982, The 

Colorado earthquake of November 7,1882: size, epicentral location, intensities, and possible causative fault: The 

Mountain Geologist, v. 19, no. 1, p. 11-23.

O'Sullivan, R.B., 1974a, Preliminary geologic map of the Segar 

Mountain quadrangle, Rio Blanco County, Colorado: 

U.S. Geological Survey Miscellaneous Field Studies 

Map MF-570, scale 1:24,000.

____ 1974b, Chart showing correlation of selected units of 

the Eocene Uinta and Green River Formations, eastcentral Piceance Creek basin, northwestern Colorado: 

U.S. Geological Survey Oil and Gas Investigations 

Chart OC-67.

____ 1975, Coughs Creek Tongue A new tongue of the 

Eocene Green River Formation, Piceance Creek basin, 

Colorado: U.S. Geological Survey Bulletin 1395-G, 7 p.

Pipiringos, G.N., and Johnson, R.C., 1976, Preliminary geologic 

map of the White River City quadrangle, Rio Blanco 

County, Colorado: U.S. Geological Survey Miscellaneous 

Field Studies Map MF-736, scale 1:24,000.

Pipiringos, G.N., and Rosenlund, G.C., 1977a, Preliminary 

geologic map of the White Rock quadrangle, Rio Blanco 

and Moffat Counties, Colorado: U.S. Geological Survey 

Miscellaneous Field Studies Map MF-837, scale 

1:24,000.

____ 1977b, Preliminary geologic map of the Indian Valley 

quadrangle, Rio Blanco and Moffat Counties, Colorado: 

U.S. Geological Survey Miscellaneous Field Studies 

Map MF-836, scale 1:24,000.

Pitman, J.K., 1979, Isopach, structure contour, and resource 

maps of the R-6 oil-shale zone, Green River Formation, 

Piceance Creek basin, Colorado: U.S. Geological Survey 

Miscellaneous Field Studies Map MF-1069, scale 

1:126,720.

Pitman, J.K., and Donnell, J.R., 1973, Potential shale-oil 

resources of a Stratigraphic sequence above the 

Mahogany zone, Green River formation, Piceance Creek 

basin, Colorado: U.S. Geological Survey Journal of 

Research, v. 1, no. 4, p. 467-473.

Pitman, J.K., and Johnson, R.C., 1978, Isopach, structure 

contour, and resource maps of the Mahogany oil-shale 

zone, Piceance Creek basin, Colorado: U.S. Geological 

Survey Miscellaneous Field Studies Map MF-958, 

scale 1:126,720.

Pitman, J.K., Pierce, F.W., and Grundy, W.D., 1989, Thickness, 

oil-yield, and kriged resource estimates for the Eocene 

Green River Formation, Piceance Creek basin, Colorado: U.S. Geological Survey Oil and Gas Investigations 

Chart OC-132, 6 sheets.

Rowley, P.D., and Hansen, W.R., 1979, Geologic map of the 

Plug Hat Rock quadrangle, Moffat County, Colorado: 

U.S. Geological Survey Geologic Quadrangle Map 

GQ-1514, scale 1:24,000.

Sears, J.D., 1925, Geology and oil and gas prospects of part of 

Moffat County, Colorado, and southern Sweetwater 

County, Wyoming: U.S. Geological Survey Bulletin 

751-G, p. 269-319.

Smith, M.C., and O'Sullivan, R.B., 1982, Results of core drilling 

for oil shale in 1978, in the Eocene Green River 

Formation, Piceance Creek area of western Colorado: 

U.S. Geological Survey Open-File Report 82-476,190 p.

Trudell, L.G., Beard, T.N., and Smith, J.W., 1970, Green River 

Formation lithology and oil-shale correlations in the 

Piceance Creek basin, Colorado: U.S. Bureau of Mines 

Report of Investigations 7357, 14 p.

Tweto, Ogden, 1975, Laramide (Late Cretaceous-Early 

Tertiary) orogeny in the Southern Rocky Mountains: 

Geological Society of America Memoir 144, 44 p.

Waldron, F.R., Donnell, J.R., and Wright, J.C., 1951, Geology 

of the DeBeque oil-shale area, Garfield and Mesa 

Counties, Colorado: U.S. Geological Survey Oil and Gas 

Investigations Map OM-114.

Geology of the Lower Yellow Creek Area, Northwestern Colorado 045

U.S. GOVERNMENT PRINTING OFFICE: 1990 773-047/06,038 REGION NO. 8

SELECTED SERIES OF U.S. GEOLOGICAL SURVEY PUBLICATIONS

Periodicals

Earthquakes & Volcanoes (issued bimonthly). 

Preliminary Determination of Epicenters (issued monthly).

Technical Books and Reports

Professional Papers are mainly comprehensive scientific reports of 

wide and lasting interest and importance to professional scientists and engineers. Included are reports on the results of resource studies and of 

topographic, hydrologic, and geologic investigations. They also include 

collections of related papers addressing different aspects of a single scientific topic.

Bulletins contain significant data and interpretations that are of lasting scientific interest but are generally more limited in scope or 

geographic coverage than Professional Papers. They include the results 

of resource studies and of geologic and topographic investigations; as well 

as collections of short papers related to a specific topic.

Water-Supply Papers are comprehensive reports that present significant interpretive results of hydrologic investigations of wide interest 

to professional geologists, hydrologists, and engineers. The series covers 

investigations in all phases of hydrology, including hydrogeology, 

availability of water, quality of water, and use of water.

Circulars present administrative information or important scientific 

information of wide popular interest in a format designed for distribution 

at no cost to the public. Information is usually of short-term interest.

Water-Resources Investigations Reports are papers of an interpretive nature made available to the public outside the formal USGS publications series. Copies are reproduced on request unlike formal USGS 

publications, and they are also available for public inspection at 

depositories indicated in USGS catalogs.

Open-File Reports include unpublished manuscript reports, maps, 

and other material that are made available for public consultation at 

depositories. They are a nonpermanent form of publication that may be 

cited in other publications as sources of information.

Maps

Geologic Quadrangle Maps are multicolor geologic maps on 

topographic bases in 71/2- or 15-minute quadrangle formats (scales mainly 1:24,000 or 1:62,500) showing bedrock, surficial, or engineering geology. Maps generally include brief texts; some maps include structure 

and columnar sections only.

Geophysical Investigations Maps are on topographic or planimetric 

bases at various scales; they show results of surveys using geophysical 

techniques, such as gravity, magnetic, seismic, or radioactivity, which 

reflect subsurface structures that are of economic or geologic significance. 

Many maps include correlations with the geology.

Miscellaneous Investigations Series Maps are on planimetric or 

topographic bases of regular and irregular areas at various scales; they 

present a wide variety of format and subject matter. The series also includes 71/2-minute quadrangle photogeologic maps on planimetric bases 

which show geology as interpreted from aerial photographs. Series also 

includes maps of Mars and the Moon.

Coal Investigations Maps are geologic maps on topographic or 

planimetric bases at various scales showing bedrock or surficial geology, stratigraphy, and structural relations in certain coal-resource areas.

Oil and Gas Investigations Charts show stratigraphic information 

for certain oil and gas fields and other areas having petroleum potential.

Miscellaneous Field Studies Maps are multicolor or black-andwhite maps on topographic or planimetric bases on quadrangle or irregular areas at various scales. Pre-1971 maps show bedrock geology 

in relation to specific mining or mineral-deposit problems; post-1971 

maps are primarily black-and-white maps on various subjects such as 

environmental studies or wilderness mineral investigations.

Hydrologic Investigations Atlases are multicolored or black-andwhite maps on topographic or planimetric bases presenting a wide range 

of geohydrologic data of both regular and irregular areas; principal scale 

is 1:24,000 and regional studies are at 1:250,000 scale or smaller.

Catalogs

Permanent catalogs, as well as some others, giving comprehensive listings of U.S. Geological Survey publications are available under 

the conditions indicated below from the U.S. Geological Survey, Books 

and Open-File Reports Section, Federal Center, Box 25425, Denver, 

CO 80225. (See latest Price and Availability List)

"Publications of the Geological Survey, 1879- 1961" may be purchased by mail and over the counter in paperback book form and as a 

set of microfiche.

"Publications of the Geological Survey, 1962-1970" may be purchased by mail and over the counter in paperback book form and as a 

set of microfiche.

"Publications of the U.S. Geological Survey, 1971-1981" may be 

purchased by mail and over the counter in paperback book form (two 

volumes, publications listing and index) and as a set of microfiche.

Supplements for 1982,1983,1984,1985,1986, and for subsequent 

years since the last permanent catalog may be purchased by mail and 

over the counter in paperback book form.

State catalogs, "List of U.S. Geological Survey Geologic and 

Water-Supply Reports and Maps For (State)," may be purchased by mail 

and over the counter in paperback booklet form only

"Price and Availability List of U.S. Geological Survey Publications," issued annually, is available free of charge in paperback booklet form only.

Selected copies of a monthly catalog "New Publications of the U.S. 

Geological Survey" available free of charge by mail or may be obtained 

over the counter in paperback booklet form only. Those wishing a free 

subscription to the monthly catalog "New Publications of the U.S. 

Geological Survey" should write to the U.S. Geological Survey, 582 

National Center, Reston, VA 22092.

Note. Prices of Government publications listed in older catalogs, 

announcements, and publications may be incorrect Therefore, the 

prices charged may differ from the prices in catalogs, announcements, 

and publications.