Stewart Gulch Tongue A New 

Tongue of the Eocene Green River 

Formation, Piceance Creek Basin, 

Colorado

GEOLOGICAL SURVEY BULLETIN 1422-E

Stewart Gulch Tongue A New 

Tongue of the Eocene Green River 

Formation, Piceance Creek Basin, 

Colorado

By WILLIAM J. HAIL, JR

CONTRIBUTIONS TO STRATIGRAPHY

GEOLOGICAL SURVEY BULLETIN 1422-E

Definition and description of a useful 

stratigraphic marker that tongues into 

the Eocene Uinta Formation

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1977
UNITED STATES DEPARTMENT OF THE INTERIOR 

CECIL D. ANDRUS, Secretary

GEOLOGICAL SURVEY 

V. E. McKelvey, Director

Library of Congress Cataloging in Publication Data

Hail, William James, 1922-

Stewart Gulch Tongue, a new tongue of the Eocene Green River formation, Piceance Creek

basin, Colorado.

(Contributions to stratigraphy) (Geological Survey Bulletin 1422-E) 

Bibliography: p.

1. Geology, Stratigraphic Eocene. 2. Geology-Colorado Piceance Creek watershed. 

I. Title. II. Series. III. Series: United States Geological Survey Bulletin 1422-E. 

QE75.B9 no. 1422-E [QE692.2] 557.3;08s [551.7*8] 77-2162

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, D.C. 20402 

Stock Number 024-001-02961-8
CONTENTS

Page

Abstract ................................................................. El

Introduction.............................................................. 1

Stewart Gulch Tongue of Green River Formation ............................... 2

Definition and type section ............................................. 2

Stratigraphic relations .................................................. 2

Distribution, lithology, and thickness ..................................... 3

References cited .......................................................... 7

ILLUSTRATIONS

FIGURE 1. Index map of part of the Piceance Creek basin .................... E4

2. Generalized Stratigraphic relations of the Stewart Gulch Tongue to other

parts of the Green River Formation and to the Uinta Formation ... 5

3. Measured sections showing position of Stewart Gulch Tongue and associated units ................................................ 6

COMMON MEASURES AND THEIR METRIC EQUIVALENTS

English unit___________________________Metric equivalent__________

foot (ft) = 0.305 meter (m)

mile = 1.609 kilometers (km)

gallon (gal) = 3.785 liters (1)

ton____________________=____________.908 metric ton (t)_________

III

CONTRIBUTIONS TO STRATIGRAPHY

STEWART GULCH TONGUE A NEW

TONGUE OF THE EOCENE GREEN RIVER

FORMATION, PICEANCE CREEK BASIN,

COLORADO

By WILLIAMJ. HAIL JR

ABSTRACT

Mapping in the northern and central parts of the Piceance Creek basin of northwestern Colorado shows that several relatively thin dominantly marlstone units tongue 

northward into the Eocene Uinta Formation and merge southward with the Eocene 

Parachute Creek Member of the Green River Formation. Five of these tongues, useful 

in mapping, have been previously named. A sixth is here named the Stewart Gulch 

Tongue of the Green River Formation. The Stewart Gulch Tongue ranges in thickness 

from about 10 to 55 feet (3-17 m). It has been identified and locally mapped in the 

south-central part of the Piceance Creek basin in the area of the Rio Blanco-Garfield 

County line.

INTRODUCTION

A continuing program of geologic quandrangle mapping in the Piceance Creek basin during the past several years has demonstrated 

the usefulness of certain marlstone units as stratigraphic markers. 

Five of these units have been previously named, and a sixth is here 

named and described.

The Uinta Formation of Eocene age forms most of the surface rocks 

of the Piceance Creek basin. The Uinta is as much as 1,400 feet (430 

m) thick and consists largely of a complex sequence of sandstone and 

siltstone, with lesser mudstone, shale, and marlstone. The main body 

of the Uinta overlies the main body of the Green River Formation. 

Both formations were deposited in or adjacent to the Eocene Lake 

Uinta (Donnell, 1961, p. 862-863). Rocks of the Green River are dominantly lacustrine; rocks of the Uinta are dominantly alluvial and 

near-shore lacustrine, deposited during the waning stages of Lake 

Uinta. The two formations are complexly intertongued. The 

widespread marlstone tongues of Green River in the Uinta were 

deposited during short-lived transgressions of the lake during its waning stages. E1
E2 CONTRIBUTIONS TO STRATIGRAPHY

Mapping and other stratigraphic studies thus far suggest a general 

stratigraphic rise in the intertonguing Green River-Uinta contact 

from north to south. Tongues of the Uinta point generally south; 

marlstone tongues of the Green River point generally north. (See 

O'Sullivan, 1974; O'Sullivan, 1975, fig. 2; Duncan and others, 1974, fig. 

4.) Thus a northerly source is indicated for most of the Uinta Formation clastic sediments at least as far south as the rim of the Piceance 

Creek drainage basin, and probably beyond (fig. 1).

Duncan and others (1974) named four marlstone tongues that they 

found useful in mapping the northern and central parts of the Piceance Creek basin. These are, in ascending order, the Yellow Creek, 

Dry Fork, Thirteenmile Creek, and Black Sulphur Tongues of the 

Green River Formation. A higher tongue, the Coughs Creek Tongue of 

the Green River, was named and described by O'Sullivan (1975), as a 

result of investigations generally south of Piceance Creek. A still higher marlstone tongue of the Green River Formation is here named the 

Stewart Gulch Tongue of the Green River Formation.

STEWART GULCH TONGUE OF GREEN RIVER FORMATION

DEFINITION AND TYPE SECTION

A light-gray dominantly marlstone sequence of Eocene age is here 

named the Stewart Gulch Tongue of the Green River Formation for 

exposures along the Middle Fork of Stewart Gulch, a tributary of Piceance Creek (fig. 1) in Rio Blanco County, Colo. The type section was 

measured in the SW X A NE */4 NW l/4 sec. 3, T. 4 S., R. 96 W., about 

2,400 feet (730 m) east of the road along the Middle Fork, and about 

500 feet (150 m) above the valley bottom. The locality lies about 7*/2 

miles (12 km) south of Piceance Creek via the dirt road which follows 

the Middle Fork.

STRATIGRAPHIC RELATIONS

Figure 2 shows the stratigraphic position of the Stewart Gulch 

Tongue in the Uinta-Parachute Creek sequence. All the named 

tongues merge with the main body of the Parachute Creek Member of 

the Green River Formation. The two unnamed Green River units 

above the Stewart Gulch Tongue, shown in figure 2, were mapped as 

informal units in parts of the Cutoff Gulch quadrangle (Hail, 1975) 

and the Bull Fork quadrangle (Hail, 1977). The lower of these two unnamed units was informally designated "marlstone at Barnes Ridge" 

in these quadrangles. The upper unit was informally designated 

"marker bed at Bull Fork" in the Bull Fork quadrangle. Figure 3 

shows the correlation of the Stewart Gulch Tongue and associated 

units in two measured sections. It also shows the heterogeneous 

lithology of the enclosing Uinta Formation.
STEWART GULCH TONGUE, PICEANCE CREEK BASIN, COLO. E3

Type section of the Stewart Gulch Tongue of Green River Formation 

[SW V4 NE V4 NW lA Sec. 3, T. 4 S., R. 96 W., mo Blanco County, Colo.]

Thickness 

(feet) (m)

Uinta Formation (part):

Sandstone, medium-brown, fine-grained, impure; contains

some light-gray lenses, variably resistant; poorly

exposed; higher beds not measured ____________ 40+ 12+ 

Shale, light-brownish-gray, marly, silty, sub fissile; grades

to sandstone above _____________________ 11 3.4 

Marlstone, light-gray, slightly silty _____________ 5 1.5 

Sandstone, light-brownish-gray, fine- to medium-grained,

moderately resistant; forms ledge _________ 4 1.2 

Siltstone, medium-brown, marly to argillaceous; contains

carbon trash; becomes very sandy in upper few feet and

grades to overlying unit; weathers out to thin irregular

chips _____________________________ 16 4.9 

Limestone, dark-brownish-gray, finely crystalline;

weathers out to thin rusty-brown slabs __________ 0.2 0.06

Stewart Gulch Tongue of Green River Formation:

Marlstone, light-brownish-gray to light-gray, silty ____ 9 2.7 

Shale, light-brown, marly; gradational from underlying

unit ______________________________ 1 0.3 

Marlstone, light-gray, thin-bedded; locally weathers out to

thin flat chips ________________________ 13 4 

Sandstone, light-brown, fine- to medium-grained; exhibits

some even bedding; forms rounded ledges ________ 10 3 

Siltstone, greenish-gray, marly; contains sparse carbon

trash; weathers out to subfissile chips _______ 13 4 

Marlstone, light-brown, silty, even-bedded; weathers light

gray; weathers out to thin subfissile chips ________ _4__ 1.2 

Total, Stewart Gulch Tongue _____________ 50_ 15.2

Uinta Formation (part):

Sandstone, light-grayish-brown; forms obscure ledge ___ 1 0.3 

Siltstone, light-brown, marly ________________ 4 1.2 

Sandstone, light-brown, medium-grained, poorly sorted,

impure, massive, resistant; contains some massive

spheroidal concretions; forms ledges and major bench to

west; lower beds not measured _______ ___ _ 15+ 4.6+

DISTRIBUTION, LITHOLOGY, AND THICKNESS

The Stewart Gulch Tongue was mapped throughout the Cutoff 

Gulch quadrangle (Hail, 1975) where it was designated "marlstone at 

Stewart Gulch," and throughout the Bull Fork quadrangle (Hail, 

1977). The unit has also been recognized in parts of all the adjacent 

quadrangles. Geologic mapping in the drainage of Stewart Gulch and 

Willow Creek indicates that the Stewart Gulch Tongue extends about 

6 to 7 miles (10-11 km) north of the Rio Blanco-Garfield County line
E4

108° 30'

CONTRIBUTIONS TO STRATIGRAPHY

15' 108° 00' 107° 45'

10 KILOMETERS

FIGURE 1. Index map of part of the Piceance Creek basin. Star shows locality of the 

type section of the Stewart Gulch Tongue. Cross section through localities 1 4 is 

shown in Figure 2; correlation between localities 3 and 5 is shown in figure 3.

as a useful map unit. Subsurface drill-hole studies indicate that the 

Stewart Gulch Tongue probably joins the main body of the Parachute 

Creek Member of the Green River about 11 to 12 miles (18-19 km) 

south of the county line.
Yellow Creek

UINTA FORMATION STEW

TON

'GREE

FOR

.COUGHSCREE 

". TONGUE OF 

-GREEN RIVER 

/..-^FORMATION

PARACHUTE CREEK MEMBER OF GR

0 5 

10 KILOMET

TOP OF MAHOGANY LEDGE

FIGURE 2. Generalized stratigraphic relations of the Stewart Gulch Tongue to other parts 

mation. In part from Duncan and others (1974, fig. 4, p. 12). See also O'Sullivan (1975, fi

mation stippled. Datum is top of Mahogany ledge. Vertical exaggeration about x 70.
E6 CONTRIBUTIONS TO STRATIGRAPHY

EXPLANATION

Sandstone

Concretionary 

sandstone

Channeled 

sandstone

Siltstone

Silty 

marlstone

Marlstone

Shale

FEET METERS

200-1^-60

150-

-40

100 - - 30

-20 

50-

PARACHUTE CFTEEK MEMBER OF 

GREEN RIVER FORMATION

FIGURE 3. Measured sections showing position of Stewart Gulch Tongue and associated units. Section at locality 5 was measured in the S lh sec. 18, and the NVz sec. 

19, T. 4 S., R. 97 W. Section at locality 3 was measured in the SWV4 sec. 9, NWV4 sec. 

16, and the NE'A sec. 17, T. 4 S.( R. 96 W. Position of Uinta-Parachute Creek contact 

was determined from nearby drill holes.
STEWART GULCH TONGUE, PICEANCE CREEK BASIN, COLO. E7

The Stewart Gulch Tongue locally forms prominent outcrops. In 

areas of good exposure, its light-gray color contrasts with the brown 

color of the Uinta Formation. In many places, the Stewart Gulch 

Tongue caps benches or crops out at the break between benches of 

Uinta Formation sandstones.

Where examined thus far, the Stewart Gulch ranges in thickness 

from about 10 to 55 feet (3-17 m), probably averaging about 35 feet 

(11 m). It is generally thinnest near its northern limit.

Throughout most of its known outcrop area, the Stewart Gulch 

Tongue is dominantly marlstone. In the southern part of its outcrop 

area, near its merger point with the Parachute Creek Member, the 

marlstone is largely low grade oil shale with values ranging mostly 

between 5 and 10 gallons per ton (about 20-401/t), but locally as much 

as 15 gallons per ton (about 60 1/t). Farther north, oil-shale values in 

the Stewart Gulch become generally sparse, silt content increases, 

and a few beds of siltstone are present. In the northern part of its outcrop area, oil-shale values are generally lacking, the proportion of silty 

marlstone and siltstone increases, and sand becomes a prominent constituent. At its northern edge, the Stewart Gulch is marked locally by 

greenish-gray clayey siltstone; it becomes very silty and sandy and 

loses its identity as a distinctive map unit.

The wedge of dominantly clastic Uinta Formation sediments 

separating the Stewart Gulch Tongue from the underlying Coughs 

Creek Tongue varies in thickness but in general thins markedly from 

north to south. The maximum known thickness is near the Rio Blanco-Garfield County line, where it ranges from about 250 to 400 feet 

(75-120 m) in thickness (figs. 2, 3). To the south, along the West Fork 

of Parachute Creek, the Uinta wedge is only about 50 to 150 feet 

(15-45 m) thick, diminishing to the vanishing point still farther 

south. The wedge of Uinta separating the Stewart Gulch Tongue from 

the overlying marlstone at Barnes Ridge is also highly variable in 

thickness but thins generally to the south. The Uinta wedge ranges 

from about 150 to 400 feet (46-120 m) in thickness in the drainages of 

Stewart Gulch and Willow Creek, thinning south westward to about 50 

feet (15 m) in upper Camp Gulch. Presumably, it too thins to the 

vanishing point farther south.

REFERENCES CITED

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. Geol.

Survey Bull. 1082-L, p. 835-891. 

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. Geol. Survey Bull. 1394-F, 13 p.
E8 CONTRIBUTIONS TO STRATIGRAPHY

Hail, W. J., Jr., 1975, Preliminary geologic map of the Cutoff Gulch quadrangle, Rio 

Blanco and Garfield Counties, Colorado: U.S. Geol. Survey Misc. Field Studies Map 

MF-691.

 1977, Preliminary geologic map of the Bull Fork quadrangle, Garfield and Rio 

Blanco Counties, Colorado: U.S. Geol. Survey Misc. Field Studies Map MF-830. (In 

press.)

O'Sullivan, R. B., 1974, Chart showing correlation of selected restored.stratigraphic 

diagram units of the Eocene Uinta and Green River Formations, east-central Piceance Creek basin, northwestern Colorado: U.S. Geol. Survey Oil and Gas Inv. 

Chart OC-67.

 1975, Coughs Creek Tongue A new tongue of the Eocene Green River Formation, Piceance Creek basin, Colorado: U.S. Geol. Survey Bull. 1395-G, 7 p.

<rU.S. GOVERNMENT PRINTING OFFICE 1977-777-034/37