Four Newly Named Tongues of 

Eocene Green River Formation, 

Northern Piceance Creek Basin, 

Colorado 

GEOLOGICAL SURVEY BULLETIN 1394-F 
Four Newly Named Tongues of 

Eocene Green River Formation, 

Northern Piceance Creek Basin, 

Colorado 

By D. C. DUNCAN, W. J. HAIL, JR., R. B. O'SULLIVAN, 

and G. N. PIPIRINGOS 

CONTRIBUTIONS TO STRATIGRAPHY 

GEOLOGICAL SURVEY BULLETIN 1394-F 

Description of new stratigraphic 

units related to the vast oil-shale 

deposits of northwestern Colorado 

UN I TED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON: 1974 
UNITED STATES DEPARTMENT OF THE INTERIOR 

ROGERS C. B. MORTON, Secretary 

GEOLOGICAL SURVEY 

V. E. McKelvey, Director 

Library of Congress catalog-card No. 73-600339 

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

Washington, D. C. 20402- Price 35 cents (paper cover) 

Stock Number 2401-02457 
CONTENTS 

Page 

Abstract ------------------------------------------------- Fl 

Introduction ---------------------------------------------- 1 

Geologic setting -------------------------------------------- 3 

Type sections of named tongues------------------------.,---------- 3 

Yellow Creek Tongue of the Green River Formation----------------- 3 

Dry Fork Tongue of the Green River Formation ------------------- 5 

Thirteenmile Creek Tongue of the Green River Formation ------------- 6 

Black Sulphur Tongue of the Green River Formation ---------------- lO 

Stratigraphic relations of newly named tongues of the Green River Formation ___ lO 

References cited -------------------------------------------- 13 

ILLUSTRATIONS 

Page 

FIGURE l. Index map of northern Piceance Creek basin, Colorado------------ F2 

2. Sections showing stratigraphic relations in the upper part of the Green 

River Formation and in the lower part of the Uinta Formation in the 

White River City quadrangle-------------------------- 7 

3. Columnar section and description of the type Thirteenmile Creek Tongue 

of Green River Formation and related rocks---------------- 8 

4. Generalized diagram showing stratigraphic relations of newly named 

tongues of Green River Formation ---------------------- 12 

III 
CONTRIBUTIONS TO STRATIGRAPHY 

FOUR NEWLY NAMED TONGUES OF EOCENE 

GREEN RIVER FORMATION, 

NORTHERN PICEANCE CREEK BASIN, 

COLORADO 

By D. c. DUNCAN, w. J. HAIL, JR., R. B. O'SULLIVAN, 

and G. N. PIPIR.INGOS 

ABSTRACT 

Mapping in the northern part of the Piceance Creek basin, Colorado, shows that several 

marlstone beds in the Eocene Uinta Formation merge laterally with the Parachute Creek 

Member of the Eocene Green River Formation. Four of these beds are here formally named 

and designated as tongues of the Green River Formation. These are, in ascending stratigraphic 

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

Green River Formation. 

INTRODUCTION 

Recent geologic mapping in the northern part of the Piceance Creek 

basin (fig.1) shows that several madstone units of the Green River Formation intertongue with the Uinta Formation of Eocene age. These madstone 

units are widespread, and they are readily mappable throughout much of 

the area. Some of the units were used previously by Kramer ( 1939) to map 

the structure of the Piceance Creek anticline. At least 10 marls tone units 

within the Uinta Formation merge laterally with the Parachute Creek 

Member of the Green River Formation. We here name four of these marl~ 

stone units, designating them as tongues of the Green River Formation. 

These are, in ascending stratigraphic order, the Yellow Creek, Dry Fork, 

Thirteenmile Creek, and Black Sulphur Tongues of the Green River Formation. 

Mapping these tongues aids in the interpretation of local geologic structures, serves to divide the thick sequence of rocks assigned to the Uinta Formation, and aids in interpretation of the character and thickness of over"' 

burden above the oil-shale zones of the Parachute Creek Member. 

Formal recognition of other marlstone units may be determined to be appropriate as mapping continues. We have individually mapped other 

tongues of the Uinta and Green River Formations locally but have not inFl 
F2 CONTRIBUTIONS TO STRATIGRAPHY 

40° A. 99 W. A. 98 W. 

15' ROUGH GULCH 

T. 

3 

N. 

T. H 

2 

N. 

7' 

30" 

+ 

T. H 1 

N. 

40° 

oo' 

T. 

1 

s. 

52' 

TYPE SECTIONS 

1. Yellow Creek Tongue 

2. Dry Fork ·Tongue 

3. Thirteenmile Creek Tongue 

4. Black Sulphur Tongue 

Area of fig. 1 

~Piceance Creek 

basin 

COLORADO 

A. 96 W. A. 95 W. 

WHITE RIVER BUCKSKIN POINT 

CITY 

+ 

p 

OS 

+ 

'30"1----------+-----~--+---____..!. 

T. YANKEE GULCH 

2 

S. 

T. 

3 

s. 

39° 

+ 

D 

45'L-~~-----~-------~--------~-------~ 

108°30' 22'30" 1s'oo" 7'3o" 108°00' 

0 

I 

0 

I 

5 

5 

I 

I 

10 MILES I 

10 KILOMETERS 

FIGURE 1. - Index map of northern Piceance Creek basin, Colorado, showing type localities 

for the four tongues of the Green River Formation, quadrangle names, and the initial of the 

writer of this report responsible for mapping the quadrangle: D, Duncan; H, Hail; OS, 

O'Sullivan; and P, Pipiringos. 

troduced formal names for them. O'Sullivan (1973) showed the stratigraphic relations of some marlstone tongues of the Green River Formation 

and the Uinta Formation in the east-central part of Piceance Creek basin. 
NEWLY NAMED TONGUES, GREEN RIVER FORMATION, COLO. F3 

Those parts of this report that describe the Yellow Creek, Dry Fork, 

Thirteenmile Creek, and Black Sulphur Tongues were· written by Hail, 

Pipiringos, O'Sullivan, and Duncan, respectively. However, each author is 

responsible for statements concerning the stratigraphy of the area he has 

mapped wherever such statements may. appear in the report. 

GEOLOGIC SETTING 

All rocks of Eocene age overlying the Eocene Wasatch Formation in the 

Piceance Creek basin were formerly assigned to the Green River Formation. Recently, however, rocks assigned to the Evacuation Creek Member 

at the top of the Green River Formation were reassigned to the Uinta Formation, and the name Evacuation Creek Member was abandoned (Cashion 

and Donnell, 1973). The Parachute Creek Member, which underlies the 

Uinta Formation, now is the top member of the Green River Formation. 

The Parachute Creek Member is at least 1,800 feet thick and consists 

largely of madstone. The member contains the richest and thickest oil-shale 

beds in the Green River Formation. One of these rich oil-shale beds is 

named the Mahogany ledge at surface outcrops. Because the Mahogany 

ledge is so widespread, it is useful for correlation. The Mahogany ledge i's 

bounded at the top by a zone of lean oil shale referred to as the "A groove." 

Another rich oil-shale bed is named the Stillwater zone (Cashion and 

Donnell, 1972); it lies 140-250 feet above the A groove. 

The Uinta Formation, overlying the Green River Formati()n, is as much 

as 1,250 feet thick and consists largely of brown sandstone and siltstone 

with minor beds of mudstone, shale, and light-gray madstone. It forms 

much of the surface rock in -the Piceance Creek basin. 

The Green River Formation was deposited in the Eocene Uinta Lake 

(Donnell, 1961, p. 862-863), and its rocks reflect the widely varying environments of lacustrine deposition. The Uinta Formation was deposited 

during the waning stages of the lake; its rocks reflect a change from a lacustrine to a fluvial depositional environment. This change, however, was 

gradual and fluctuating, resulting in a complex intertonguing relationship 

between the Green River Formation and the Uinta Formation. Madstone, a 

dominant rock type of the Parachute Creek Member of the Green River 

Formation, was deposited in quiet lake water far from shorelines. 

Numerous madstone beds are present in the Uinta Formation. Some of 

these are lenticular and of limited extent, probably having been deposited in 

isolated bodies of lake water not connected to Uinta Lake. Other marlstones, however, are of wide lateral extent and continuity and probably were 

deposited during widespread stages of Uinta Lake. 

TYPE SECTIONS OF NAMED TONGUES 

YELLOW CREEK TONGUE OF THE GREEN RIVER FORMATION 

A light-gray to light-brown dominantly madstone sequence is here 

named the Yellow Creek Tongue of the Green River Formation, for ex-
F4 CONTRIBUTIONS TO STRATIGRAPHY 

posures along Yellow Creek, in Rio Blanco County, Colo. (fig. 1). It was infprmally designated as the Yellow Creek marker bed on the preliminary 

map of the Barcus Creek SE quadrangle (Hail, 1972). The type section of 

the Yellow Creek Tongue was measured by Hail on the east side of Yellow 

Creek in the NEY4SE14 sec. 13, T. 1 N., R 98 W., about 150 feet southeast 

of U.S. Bureau of Mines AEC core hole 1. The locality may be reached by a 

graded road that branches off from the Piceance Creek road in sec. 22, T. 1 

N., R. 97 W., reaching the valley of Yellow Creek in the NEY4 sec. 24, T. 1 

N., R. 98 W., a distance of about 5 miles; from there, north 0.6 mile along a 

dirt road on the east side of Yellow Creek to the site of AEC core hole 1. 

Type section of Yellow Creek Tongue of Green River Formation 

[NE't.SE\4 sec. 13, T. I N., R. 98 W., in the Barcus Creek SE quadrangle, Rio Blanco County, Colo.] 

Thickness 

(ft) 

Uinta Formation (part): 

Marlstone, olive-green, variably silty, nonresistant ------------------- 6+ 

Yellow Creek Tongue of Green River Formation: 

Marlstone, light-brownish-gray, variably silty; contains a few thin beds of marly 

siltstone; gradational from underlying unit; weathers out to thin chips ___ 6 

Siltstone, very light brownish gray; marly with some thin interbedded marlstone 

and silty marlstone; weathers out to thin chips; forms upper part of 

resistant bench ---------------------------------------- 3 

Siltstone, very light gray, calcareous; some wavy bedding and crossbedding; very 

hard; forms lower part of resistant bench ----------------------- 2 

Marlstone, light-gray to light-brownish-gray, variably silty; contains a few thin 

beds of marly siltstone; weathers out to thin chips ----------------- 8 

Siltstone, very light gray; weathers light brown; marly; thin wavy bedding; 

gradational from underlying unit; forms resistant ledge ---,..-----------

Marlstone, very light brownish gray to light-gray; with a few beds of silty 

marlstone; upper part is very silty; weathers out in thin chips ---------- 5 

Sandstone, light-gray, fine- to medium-grained, calcareous; contains abundant 

dark grains, including biotite; forms thin resistant ledge -------------

Siltstone, light-gray, marly; irregular thin bedding -------------------

Marlstone, light- to medium-brown, silty; contains sparse carbonized plant trash; 

thin bedded; nonresistant; forms mostly covered slope; weathers out in thin 

chips; sharp contact with underlying unit ----------------------- 18 

Total thickness of Yellow Creek Tongue---------------------- 45 

Uinta Formation (part): 

Sandstone, very light yellow to brownish-gray; weathers very light brown; medium 

to coarse; contains abundant yellow rusty grains and sparse biotite _____ 2 + 

The Yellow Creek Tongue crops out in the Barcus Creek, Barcus Creek 

SE, White River City, and Greasewood Gulch quadrangles. The Yellow 

Creek Tongue is recognizable in at least the southeastern part of the Rough 

Gulch quadrangle, but becomes increasingly silty to the west and loses its 

identity as a distinct marlstone unit. It ranges from 0 to 77 feet in thickness. 

Variations in thickness are due mostly to changes from marlstone to 

siltstone in both the upper and lower parts. Locally, however, thinning is 

due to channeling prior to or during deposition of the overlying sandstone. 
NEWLY NAMED TONGUES, GREEN RIVER FORMATION, COLO. F5 

The Yellow Creek Tongue merges with a stratigraphically higher marlstone unit in the extreme southwestern part of the Barcus Creek quadrangle. 

The combined unit, which locally contains thin oil-shale beds, was mapped 

separately by Duncan in the Wolf Ridge quadrangle a·s an unnamed tongue 

of the Parachute Creek Member of the Green River Formation. 

In the White River City quadrangle, the Yellow Creek Tongue thins 

eastward from about 65 feet thick along the west margin of the quadrangle 

(fig. 2A) to a feather edge in sec. 27, T. 1 N., R. 96 W. It reappears farther 

to the southeast near the Juhan 1 Government core hole, where the unit is 77 

feet thick, but consists mostly of silty marlstone (fig. 2B). In the southcentral part of the White River City quadrangle (sees. 31, 32, and 33, T. 1 

N., R. 96 W.) the tongue is about 10 feet thick and includes a thin bed of oil 

shale at the top. 

DRY FORK TONGUE OF THE GREEN RIVER FORMATION 

The name Dry Fork Tongue of the Green River Formation is here 

applied to a sequence of light-greenish-gray marlstone, silty marlstone, and 

. calcareous siltstone that is well exposed near the southwest corner of the 

White River City quadrangle. The type section, designated by Pipiringos, is 

in the NE'/.$SE'/.$NW'/.$SE'/.$ sec. 35, T. 1 N., R. 97 W. It is located about 

1,500 feet northeast of the junction of Piceance Creek and the Dry Fork of 

Piceance Creek on the hillside directly north of the dirt road along Dry 

Fork. The base of the type section is about 270 feet, and the top is about 340 

feet, above the level of the road. At this place the Dry Fork. Tongue is 70 

feet thick and contains a light-brown massive sandstone lens, 5-10 feet 

thick, 10-20 feet above the base. 

The lower contact of the Dry Fork Tongue is gradational downward into 

sandstone whose upper few feet, wherever it is exposed, nearly everywhere 

contains brown fossil wood in moderate· abundance. The upper contact 

similarly is gradational upward into sandstone, typical of the Uinta Formation, through an interval of about 5-15 feet. 

The Dry Fork Tongue is exposed along the south margin of the White 

River City quadrangle, on the south side of the Dry Fork and in the 

southeast corner of the quadrangle in adjacent parts of Tps. 1 N. and 1 S., 

R. 96 W. It ranges in thickness from about 70 feet in the southwestern part 

to about 30 feet in the southeastern part of the White River City quadrangle. In the northwestern part of the Greasewood Gulch quadrangle, the 

Dry Fork Tongue is about 60 feet thick. The Dry Fork Tongue is present in 

the southeastern part of the Barcus Creek SE quadrangle but cannot be 

identified with certainty to the north and west. 

The stratigraphic position of the Dry Fork Tongue is shown in figure 2, 

which is a composite of subsurface and surface data. Stratigraphic details 

for column A (fig. 2) from the base of the Yellow Creek Tongue to the top 

of the Mahogany ledge are based on sample studies, geophysical logs, and 

U.S. Bureau of Mines assays of cuttings from the Tin tic Standard 1-36 
F6 CONTRIBUTIONS TO STRATIGRAPHY 

Government well in the NE~ sec. 36, T. lN., 97 W. The well was spudded 

in alluvium at or just above the top of the Yellow Creek Tongue (ground 

level 6,100 ft) and penetrated the top of the Mahogany ledge at a depth of 

740 feet. Stratigraphic details from the base of the Yellow Creek Tongue to 

the top of the Dry Fork Tongue are based on surface measurements made 

directly west of the well site; details above the Dry Fork Tongue are based 

on surface measurements south of the well site. Stratigraphic details shown 

in the lower 200 feet of columnar section B (fig. 2) are from studies of 

geophysical logs, core studies by J. R. Donnell, and U.S. Bureau of Mines 

assays of the Juhan 1 Government core hole. This core hole is located on the 

boundary of the Greasewood Gulch and Segar Mountain quadrangles about 

1,000 feet south of the southeast corner of the White River City quadrangle 

in the SE~NE~NE~SE~ sec. 2, T. 1 S., R. 96 W. The Juhan hole was 

spudded in the Stillwater zone (ground level about 6,320 ft) and penetrated 

the top of the Mahogany ledge at a depth of 178 feet. The upper part of 

columnar section B is based on a surface section measured from the core 

hole to an outlier of the Dry Fork Tongue in the SE~SW~ sec. 35, T. 1 N., 

R. 96 W. 

The interval from the Dry Fork Tongue to the Mahogany ledge (base of 

A groove) increases westward from about 830 feet to. about 1,000 feet 

between the two well sites, owing to an increase in the thickness of sa-ndstone between the base of the Yellow Creek Tongue and the base of the Uinta Formation. The interval between the ,Dry Fork and Yellow Creek 

Tongues is relatively constant across the sohthern part of the White River 

City quadrangle, ranging from about 265 fec!t in the southwest to about 235 

feet in the southeast in a distance of about' 5 miles·. 

THIRTEENMILE CREEK TONGUE OF THE GREEN RIVER FORMATION 

A heterogeneous sequence - consisting mostly of marlstone but with appreciable amounts of sandstone, locally ostracoda! beds, limestone, and rich 

but thin oil-shale beds - is here named the Thirteenmile Creek Tongue of 

the Green River Formation. The name is taken from Dry Thirteenmile 

Creek, which joins Piceance Creek about 10 miles downstream from the 

Post Office at Rio Blanco. The type section is readily accessible along a dirt 

road about 3.5 miles northeast of the paved road along Piceance Creek. A 

columnar section (fig. 3) rather than a written description is presented to illustrate the type section of the Thirteenmile Creek Tongue and to show its 

position with respect to adjacent beds. 

The stratigraphic section shown in figure 3 is a composite of several 

measurements made by O'Sullivan at different places along Dry Thirteenmile Creek in T. 2 S., R. 95 W. The units below the Stillwater zone were 

measured on the south side of Dry Thirteenmile Creek in theSE~ sec. 26. 

The remainder of the various stratigraphic sections was measured on the 

north side of Dry Thirteenmile Creek in the SW~ sec. 26, SE~ sec. 27, and 

NW~ sec. 34. 
NEWLY NAMED TONGUES, GREEN RIVER FORMATION, COLO. F7 

A 

LITHOLOGIC SYMBOLS 

UINTA FORMATION 

THIRTEENMILE CREEK TONGUE OF 

GREEN RIVER FORMATION a 120o··~r-----

Sandstone UINTA FORMATION 

Siltstone 

S ihy marlstone 

D 

Marlstone 

Oil shale 

I~A I 

Fossil wood 

EJ 

Tuff 

800' 

600' 

400' 

200' 

100' 

UINTA 

FORMATION 

B 

1------+--T 

1-----1--T 

1-;-:-:--:-:-:-:-:-:-:--:-;-:-t--'T 

1------t--T 

FIGURE 2.- Stratigraphic relations in the upper part of the Green River Formation and in the 

lower part of the Uinta Formation in the White River City quadrangle. A, The Tin tic Standard l-36 Govt. well, sec. 36, T. l N., R. 97 W., and vicinity. B, The Juhan I Govt. well, sec. 

2, T. l S., R. 96 W., and vicinity. 
F8 CONTRIBUTIONS TO STRATIGRAPHY 

FORMATION, MEMBER, AND 

OTHER SUBDIVISIONS 

I Thickness l. (ft) DESCRIPTION 

z 

0 

j: 

~ 

~ 

a:: 

0 

LL 

~ 

1-

z 

::> 

Estimated oil yield 

60 0 400 + ..--.::.:tO:...t~h:..:.i:..:.n...:::.;..: plla::..:t~es~-------,---.,...-----:-1 

(gallons per ton) ~Marlstone, calcareous, light-gray; weathers 

L....L..J ~Sandstone, light-brown, very fine grained, 

.------------,.~~-:-;/~(~:;.:."::'{"-':::.:~'::':~:~"':::1~:;= .. { -~ Ms~~:::ocna~~~irge:t~;ray; upper part poorly 

Black Sulphur Tongue of ~:::~:::~.:~~::: 51 exposed; 14.5 ft below top of unit is a 

Green River Formation . ::::::::.:::.:::.:::~:::: -f---- light-brown calcareous siltstone bed 3ft 

Thirteenmile 

Creek 

Tongue 

of 

Green River 

Formation 

i~\i?/?f!f(\. 59 ::ir~k~b~:;~ part forms ledge; lower 

:.: ·:.: ·_:.' ·~·-~i ---:----- 1 /Sandstone, light-brown, very fine grained, 

~+.-+z+.::::,."'..:::-r 27 I silty tuffaceous, calcareous; base sharp; 

;:::;·.::;·.::;:.::;:;:::;:.:: l 25 cont~i ns beds of 1 ight-gray calcareous 

Big [. :::::.=:::;:::.=: siltstone and marlstone that weather to 

three l ::::~::~:: ..... ::~:: B5 \..__w_a_v..:...y...:..p_la_t...:..Y_b_e_d_s ________ -t 

Lo.wer :_~~:-:---::~0 TJ' Marlstone and oil shale,.light-gray, brown, 

z"~~~ { ~. -.~.- .:": f-t--- · and blue-gray, variably silty; rich oil-

'---------'-'........._ ::::·-:::::~··:·.~·;-,::;:.:-. 38 shale beds form ledges; 36ft above base 

Unnamed tongue of -.: ... · ... · .. :.:: ~ isatansiltstonebed2.6ftthickthat 

Green River Formation ~ :::-.:::::::.::: . 

25 forms slope. Unit contains four tan 

1---.,L.-::.~-~-~_"--"_"-l._!.:!._:::.._:.._!-_:::!._:..:._!!_~_':!:_':!..!_.:.... ____ :_..Jh:~~:·~Z;:-7!::...-:=;:·-~;_-~:·.:~:1 33 sandy tuffs(?) as thick as 6 in. 

__, .:::::::.::::::.:::~::: Siltstone, tan, very si'ightly calcareous; 

z 

0 

j: 

~ 

~ 

a:: 

0 

LL 

a:: 

w 

> 

a:: 

z 

w 

w 

a:: 

Cl 

Parachute 

Creek 

Member 

-. --"":....~-~-~ T \ .Poorly exposed in slope interrupted by 

Stillwate1'1111 =~-:--_- T t\ thin ledges 

zone ll ::::::;=;) Oil shale and marlstone, light-brown, dark-

- - ..... ~ ..... -_ ..... __ T gray, and blue-gray, partly laminated, 

• _ ~ _ _ T variably silty 

-:-:-:-:-~TTT Siltstone, light-gray; weathers the same; 

calcareous; irregularly bedded; basal 

--:: ~::::.:.::;:.:.::.:.::_.:.:· \ 8ft concealed 

: ..... :..._: ..... :....: 271 

~+.r+z+=+:.: 

Four { -:-:-:-::- T 

senators ,-1 : ..... :. ..... : ..... :.~: T 

Marlstone and oil shale, light-gray, brown, 

and blue-gray; weather to thin chips; 

variably silty 

Sandstone, light-brown, very fine grained; 

as much as 1 ft thick 

Siltstone, light-tan, flat and thinly bedded; 

as much as 4 ft thick 

Tuffs, light-tan, partly biotitic; as much as 

4 in. thick 

FIGURE 3. -Columnar section and description of the type Thirteenmile Creek Tongue of the 

Green River Formation and related rocks at Dry Thirteenmile Creek. Names of rich oilshale beds from Cashion and Donnell ( 1972). T indicates presumed tuff bed. 

The type section of the Thirteenmile Creek Tongue was measured at two 

localities. The lower 85 feet was measured in the SE~NE~SE~ sec. 27 on 

the west side of a reentrant bisected by the common section line of sees. 26 

and 27. The lower rich zone at the base of the Thirteenmile Creek Tongue 

makes a conspicuous finely banded ledge about 100 feet above the floor of 

the reentrant. The middle sandstone and upper marlstone of the Thirteenmile Creek Tongue were measured on the west side of Open Gulch in the 

NWV4SE14SE14 and the NE~SW~SE~ of sec. 27. 

At the type section, the Thirteenmile Creek Tongue i's 137 feet thick and 

contains a sandstone bed 25 feet thick near the top. This sandstone bed 

probably is lenticular and apparently does not join the main body of the 

Uinta Formation. The bed can be identified in the General Petroleum Corp. 
NEWLY NAMED TONGUES, GREEN RIVER FORMATION, COLO. F9 

27-8 test hole drilled in the SW1/.I sec. 8, T. 2 S., R. 95 W., about 4 112 miles 

northwest of the type section, but where the Thirteenmile Creek Tongue reappears at the surface along Dry Fork of Piceance Creek, the bed is absent. 

The outcrop pattern of the Thirteenmile Creek Tongue on the north side 

of Dry Thirteenmile Creek, near the. type section, is complex, and exposures 

of the tongue on the south side are poor. Consequently, a convenient way to 

recover the type section of the Thirteenmile Creek Tongue is to locate the 

Webb Resources, Inc., 22-15 Federal drill hole in the NEI/.ISWI/.ISEl/.1 sec. 

22, T. 2 S., R. 95 W. The test hole, drilled on the west side of Open Gulch 1 

mile north of Dry Thirteenmile Creek was spudded 2 feet below the top of 

the Thirteenmile Creek Tongue. From the well site, the tongue can be 

traced easily down to Dry Thirteenmile Creek. A useful stratigraphic 

marker 130 feet beneath the Thirteenmile Creek Tongue is the conspicuous 

ledge formed by the Stillwater zone. The position of the Stillwater zone is 

clearly indicated by the numerous prospect pits in it on the south side of Dry 

Thirteenmile Creek in sec. 26, T. 2 S., R. 95 W. 

The Thirteenmile Creek Tongue, ranging in thickness from 60 to 200 feet, 

is widespread in the northern part of the Piceance Creek basin. The tongue_ 

has been identified in the No Name Ridge, Segar Mountain, Greasewood 

Gulch, White River City, Jessup Gulch, Rock School, Square S Ranch, 

Barcus Creek, Barcus Creek SE, and Rough Gulch quadrangles. 

The Thirteenmile Creek Tongue has been mapped previously (Hail, 1972) 

as the Barcus Creek marker bed, an informal unit, in the Barcus Cre~k SE 

quadrangle. In the Barcus Creek SE, Barcus Creek, and Rough Gulch 

quadrangles, the Thirteenmile Creek Tongue caps isolated stripped erosion 

surfaces, and the stratigraphically higher beds are absent. 

In the White River City quadrangle, all outcrops but one of the Thirteenmile Creek Tongue are confined to the south side of the Dry Fork of 

Piceance Creek along the margin of the quadrangle. There, the tongue is 

about 50 feet thick and caps the higher ridges that project northward from 

the Greasewood Gulch quadrangle into the White River City quadrangle. 

The one outcrop of Thirteenmile Creek Tongue not in that area is an outlier of marlstone about 10 feet thick near the SV2NW1/.INE1

/.I sec. 3, T. 1 S., 

R. 95 W., that caps a small isolated hill at about the 6,940-foot contour. The 

occurrence of a thin ostracoda! sandstone ledge in this exposure and the interval down to the Dry Fork Tongue (about 70ft) suggests that this is a remnant of the lower unit of the Thirteenmile Creek Tongue, as recognized by 

Duncan in the Greasewood Gulch quadrangle about 1 mile to the south. 

The Thirteenmile Creek Tongue attains a thickness of about 200 feet 

along Dry Fork of Piceance Creek in the northeastern part of the 

Greasewood Gulch quadrangle, sec. 10, T. 1 S., R. 96 W., and consists of 

two parts. The lower part, 75 feet thick, consists of interbedded grayweathering sandstone and marlstone interbedqed with a few thin beds of 

ledge-forming porous algal-ostracoda} limestone. The upper part, 50-125 

feet thick, consists predominantly of white-weathering marlstone which 
FlO CONTRIBUTIONS TO STRATIGRAPHY 

contains a few thin beds of porous algal-ostracodal limestone and locally 

abundant gastropods. 

The lower part lenses out southward in the Greasewood Gulch 

quadrangle, whereas the upper part is a widespread persistent madstone to 

the south. The upper part has been mapped southward to the junction of 

Black Sulphur Creek and Piceance Creek, in the Rock School quadrangle, 

and from there northwestward to the northwestern part of the Wolf Ridge 

quadrangle, where it merges with the main body of the Green River Formation. The composition of the unit changes gradually southward. The algalostracodallimestone beds present in the unit to the north are absent in more 

southerly outcrops, and in their stead are several thin low-grade oil-shale 

beds. The Thirteenmile Creek Tongue merges with the main body of the 

Green River Formation in the subsurface of the central parts of the Rock 

School and Jessup Gulch quadrangles and in the subsurface and outcrop 

areas in the western part of the Piceance Creek basin in Yankee Gulch and 

Wolf Ridge quadrangles. It also merges with the main body of the Green 

River Formation in the southern part of the No Name Ridge quadrangle in 

the eastern part of Piceance Creek basin. 

BLACK SULPHUR TONGUE OF THE GREEN RIVER FORMATION 

A light-gray marlstone sequence is here named the Black Sulphur Tongue 

of the Green River Formation, for exposures along Black Sulphur Creek, a 

tributary to Piceance Creek, in Rio Blanco County, Colo. (fig. 1 ). The type 

section is readily accessible and is located about 43;4 miles west of the junction of the dirt road along Black Sulphur Creek and the paved county road 

along Piceance Creek. The type section, described below, was measured by 

Duncan in the NW~SW~SW~ sec. 23, T. 2 S., R. 98 W. The section starts 

600 feet east of the west line of sec. 23 at the base of a spur on the 6,420-foot 

contour and was measured upslope to the north-northwest. 

The Black Sulphur Tongue ranges from about 20 to 60 feet in thickness 

throughout its outcrop area in most of the central part of the Piceance 

Creek basin. The tongue is thickest in the northern part of the Jessup Gulch 

quadrangle and in the southern part of the Greasewood Gulch quadrangle, 

where a middle sandstone bed as much as 10 feet thick separates upper and 

lower beds of madstone. The tongue gradually changes to a gray siltstone in 

the northern part of the Greasewood Gulch quadrangle, where it becomes 

indistinguishable from the siltstone of the Uinta Formation. In the southern 

part of the Rock School quadrangle, the Black Sulphur Tongue also 

gradually changes to gray siltstone. The tongue merges with the main body 

of the Green River Formation in the No Name Ridge quadrangle. 

STRATIGRAPHIC RELATIONS OF NEWLY NAMED TONGUES OF 

THE GREEN RIVER FORMATION . 

The stratigraphic position of each of the newly named madstone tongues 

at or near their type localities is shown in figure 4. The diagram is 
NEWLY NAMED TONGUES, GREEN RIVER FORMATION, COLO. Fll 

Type section of the Black Sulphur Tongue of •Green River Formation 

(NW~SW~SW~ sec. 23, T. 2 S., R. 98 W., in the Rock School quadrangle, Rio Blanco County, Colo.] 

Thickness 

(ft) 

Uinta Formation (part): 

Sandstone, light-gray and tan, friable, tuffaceous; interbedded with siltstone; unit 

weathers locally to smooth slope, but forms ledges laterally ----------- 91 

Black Sulphur Tongue of Green River Formation: 

Marlstone, gray and light-brown, silty; weathers to very light gray slope; 

gradational to overlying unit ------------------------------- 18 

Siltstone; weathers tan; calcareous ----------------------------- 6 

Marlstone, light-gray, silty; forms bench; lower part gradational to siltstone__ 6 

Total thickness Black Sulphur Tongue ----------------------- 30 

Uinta Formation (part): 

Sandstone, fine-grained, and interbedded siltstone; weathers to tan slopes; base 

concealed by alluvium------------------------------------ 104 

generalized, and lithologic detail is omitted. Detailed stratigraphic changes 

between sections shown on the columns are not precise. It is obvious, 

however, that there are very large changes in stratigraphic intervals from 

place to place. Much of this change is due to abrupt changes in the thickness 

of the dominantly clastic rocks of the Uinta Formation. 

The columnar section at Yellow Creek (fig. 4) is a composite of exposed 

rocks above a horizon near the base of the Yellow Creek Tongue and of subsurface rocks, based on information from studies by Carroll, Coffin, Ege, 

and Welder (1967, fig.2) and by Trudell, Bear, and Smith (1970, table B-1) 

of the U.S. Bureau of Mines AEC core hole 1. The stratigraphic interval 

between the Yellow Creek Tongue and the top of the Mahogany oil-shale 

zone of the Parachute Creek Member ranges from about 720 feet in the 

southwestern part of the Barcus Creek SE quadrangle to about 1,220 feet in 

the east-central part of the Barcus Creek quadrangle. 

The columnar section at Dry Fork of Piceance Creek shows the stratigraphic relations near the mouth of the creek in the White River City 

quadrangle. This column is described in detail under the discussion of the 

Dry Fork Tongue. 

The columnar section at Dry Thirteenmile Creek is a composite of exposed rocks above a horizon about 100 feet above the Mahogany zone and 

of subsurface rocks, based on information from the Webb Resources, Inc., 

22-12 Federal drill hole in the SE ~ sec. 22, T. 2 S., R. 95 W. The interval 

from the base of the Thirteenmile Creek Tongue to the top of the 

Mahogany oil-shale zone of the Parachute Creek Member increases greatly 

from southeast to northwest. The interval is 450 feet at Dry Thirteenmile 

Creek, the type section, and is about 1,475 feet in the east-central part of the 

Barcus Creek quadrangle. The interval is about 380 feet in the central part 

of the Rock School quadrangle near the line of mergence with the main 

body of the Green River Formation. 
1 2 3 4 

~5.5 miles 15.5 miles 17.5 miles FE1,T ME.TERS .. , 

Yell.ow Dry t-ork Piceance Dry Thirteenmile TO Black Sulphur 

Creek Creek Creek 50 Creek 

Top of Mahogony ledge 

PARACHUTE CREEK MEMBER OF 

GREEN RIVER FORMATION 

(Base of A groove) 

FIGURE 4. - Generalized diagram showing stratigraphic relations of newly named tongues of 

the Green River Formation. Green River Formation, unpatterned; Uinta Formation, 

shaded. Datum is top of Mahogany ledge. Vertical exaggeration about X 70. 

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NEWLY NAMED TONGUES, GREEN RIVER FORMATION, COLO. F13 

The columnar section at Black Sulphur Creek is a composite of exposed 

rocks above a horizon that is about 100 feet stratigraphically below the base 

of the Black Sulphur Tongue and of subsurface rocks, based on data derived 

largely· from description of cuttings and the core of Equity Oil Co. Boies 1 

core hole in the SWY<I sec. 19, T. 2 S., R. 97 W. The stratigraphic interval 

between the base of the Black Sulphur Tongue and the Mahogany oil-shale 

zone of the Parachute Creek Member ranges from 570 feet in the southwestern part of the Wolf Ridge quadrangle, to about 1,200 feet in the Segar 

Mountain quadrangle. 

REFERENCES CITED 

Carroll, R. D., Coffin, D. L., Ege, J. R., and Welder, F.A., 1967, Preliminary report on 

Bureau of Mines Yellow Creek core hole no. l, Rio Blanco County, Colorado: U.S. Geol. 

Survey open-file report, TEl-869, 36 p. 

Cashion, W .B., and Donnell, J. R., 1972, Chart showing correlation of selected key units in the 

organic-rich sequence of the Green River Formation, Piceance Creek basin, Colorado, 

and Uinta Basin, Utah: U.S. Geol. Survey Oil and Gas Inv. Chart OC-65. 

__ 1973, Revision of nomenclature of the upper part of the Green River Formation, 

Piceance Creek basin, Colorado, and eastern Uinta Basin, Utah: U.S. Geol. Survey Bull. 

1394-G (in press). · 

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. 

Hail, W. J., Jr., 1972, Preliminary geologic map of the Barcus Creek SE quadrangle, Rio Blanco County, Colorado: U.S. Geol. Survey Misc. Field Studies Map MF-347. 

Kramer, W. B., 1939, Geologic map and section of Piceance Creek dome, Rio Blanco County, 

Colorado: U.S. Geol. Survey map. 

O'Sullivan, R. B., 1973, Chart showing correlation of selected units of the Eocene Uinta and 

Green River Formations in east-central Piceance Creek basin, northwestern Colorado: 

U.S. Geol. Survey Oil and Gas Inv. Chart OC-67 (in press). 

Trudell, L. G., Bear, T. N., and Smith, J. W., 1970, Green River Formation lithology and oilshale correlations in the Piceance Creek basin, Colorado: U.S. Bur. Mines Rept. Inv. 

7357, 14 p. 

*u.s. GOVERNMENT PRINTING OFFICE: 1973 0-543-SB0/:!59 
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