U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2012–5041

The Systematic Geologic Mapping Program and 

a Quadrangle-by-Quadrangle Analysis of TimeStratigraphic Relations within Oil Shale-Bearing 

Rocks of the Piceance Basin, Western Colorado

The Systematic Geologic Mapping Program 

and a Quadrangle-by-Quadrangle Analysis 

of Time-Stratigraphic Relations within 

Oil Shale-Bearing Rocks of the Piceance 

Basin, Western Colorado

By Ronald C. Johnson

Scientific Investigations Report 2012–5041

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2012

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Suggested citation:

Johnson, R.C., 2012, The systematic geologic mapping program and a quadrangle-by-quadrangle analysis of timestratigraphic relations within oil shale-bearing rocks of the Piceance Basin, western Colorado: U.S. Geological 

Survey Scientific Investigations Report 2012–5041, 28 p., 2 pls., CD-ROM, also available at http://pubs.usgs.gov/

sir/2012/5041/.
iii

Contents

Abstract...........................................................................................................................................................1

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

Geology of the Green River Formation .......................................................................................................5

Definition of Stratigraphic Units..................................................................................................................7

Development of Rich and Lean Zone Stratigraphy ..................................................................................9

Constructing the Stratigraphic Charts .....................................................................................................11

Discussion.....................................................................................................................................................20

References Cited..........................................................................................................................................22

Figures

1. Map showing extent of Uinta, Piceance, and Greater Green River Basins, and 

approximate extent of oil shale in the Green River Formation..............................................2

2. Generalized geologic map of the Piceance Basin, northwest Colorado, showing

Upper Cretaceous and lower Tertiary stratigraphic units .....................................................4

3. Maps showing: (A), total in-place oil shale in barrels per acre (BPA); and (B), 

total in-place nahcolite in tons per acre, for all oil shale zones in the Piceance

Basin ...............................................................................................................................................6

4. Index map showing locations of sixty-nine 71⁄2-minute quadrangles and one

15-minute quadrangle (Citadel Plateau) for which geologic maps have been

published for the oil shale area, Piceance Basin and easternmost Uinta Basin...............8

5. North-south cross section A-A’ through the Piceance Basin, northwestern 

Colorado, showing member subdivisions, stages of Lake Uinta, and some of 

the rich and lean zones..............................................................................................................10

6. East-west cross section B-B’ through the Piceance Basin, the Douglas Creek 

arch, and the eastern part of the Uinta Basin showing member subdivisions, 

stages of Lake Uinta, and some of the rich and lean zones ................................................12

7. Oblique aerial photograph of the Piceance Basin showing prominent relief 

along the west, south, and east margins of the Piceance Basin .......................................14

8. Photograph of the Green River Formation exposed on Anvil Points in the 

southeastern part of the Piceance Basin...............................................................................15

9. Photograph of the Green River Formation and upper part of the underlying 

Wasatch Formation along Cow Ridge in the southwestern part of the 

Piceance Basin ...........................................................................................................................16

10. Correlation of individual oil shale beds and groups of beds forming named 

tongues of the Green River Formation (indicated by stratigraphic unit symbols) 

in the interval above A-groove .................................................................................................17

11. North-south cross section C-C’ showing intertonguing of the Uinta Formation 

with the upper part of the Green River Formation in the Piceance Basin, Colorado......18

12. Cross section showing oil-yield histograms, members of the Eocene Green 

River Formation, correlation of rich and lean oil shale zones, and stages in the 

evolution of Lake Uinta ..............................................................................................................19
iv

Plates

1. Time-stratigraphic (A) and thickness-stratigraphic (B) columns for oil 

shale area of the Piceance and easternmost Uinta Basins, western Colorado........... link

2. South–north cross section of the Piceance Basin showing variations in oil and 

water yields from Fischer assay and major water-bearing mineral phases

in Green River Formation........................................................................................................ link 
Abstract

During the 1960s, 1970s, and 1980s, the U.S. Geological 

Survey mapped the entire area underlain by oil shale of the 

Eocene Green River Formation in the Piceance Basin of western Colorado. The Piceance Basin contains the largest known 

oil shale deposit in the world, with an estimated 1.53 trillion 

barrels of oil in place and as much as 400,000 barrels of oil 

per acre. This report places the sixty-nine 71⁄2-minute geologic 

quadrangle maps and one 15-minute quadrangle map published 

during this period into a comprehensive time-stratigraphic 

framework based on the alternating rich and lean oil shale 

zones. The quadrangles are placed in their respective regional 

positions on one large stratigraphic chart so that tracking the 

various stratigraphic unit names that have been applied can be 

followed between adjacent quadrangles. Members of the Green 

River Formation were defined prior to the detailed mapping, 

and many inconsistencies and correlation problems had to be 

addressed as mapping progressed. As a result, some of the geologic units that were defined prior to mapping were modified or 

discarded. The extensive body of geologic data provided by the 

detailed quadrangle maps contributes to a better understanding 

of the distribution and characteristics of the oil shale-bearing 

rocks across the Piceance Basin.

Introduction

The Eocene Green River Formation in the Piceance Basin 

in Colorado, the Uinta Basin in Utah and Colorado, and the 

Greater Green River Basin in Wyoming, Colorado, and Utah 

(fig. 1) contains one of the largest known oil shale deposits in 

the world. Resources are estimated at 1.53 trillion barrels of 

oil in place in the Piceance Basin (Johnson and others, 2010a), 

1.32 trillion barrels in place in the Uinta Basin (Johnson and 

others, 2010b), and 1.44 trillion barrels in place in the Greater 

Green River Basin (Johnson and others, 2011). The oil shale 

was deposited in two large long-lived lakes that covered much 

of these basins in Eocene time, Lake Uinta in the Uinta and 

Piceance Basins and Lake Gosiute in the Greater Green River 

Basin (fig. 1; see Geology of the Green River Formation section). The oil shale deposit in the Piceance Basin (fig. 2), the 

focus of this report, is probably the world’s most concentrated 

oil shale resource with an estimated 400,000 barrels of oil in 

place per acre in the oil shale depocenter in the north-central 

part of the basin (fig. 3A). Interest in developing this oil shale 

deposit increased during periods of high petroleum prices 

due to high demand including (1) just before World War I and 

extending into the 1920s, (2) during and after World War II, 

(3) from 1973 at the start of the Arab oil embargo into the mid1980s, and (4) the first part of this century. For the first three 

high-demand periods, oil prices and demand declined before a 

viable oil shale industry could be established. Oil prices have 

also declined considerably since peaking in 2007. In view of 

these fluctuations in economic conditions, the future of an oil 

shale industry remains unclear.

A considerable body of work concerning the oil shale 

deposit of the Green River Formation was published by the 

U.S. Geological Survey (USGS), most notably during the 

1960s, 1970s, and 1980s, and most of this work was scanned 

and made available on the USGS oil shale web page at the 

beginning of the recent interest in oil shale (http://energy.

cr.usgs.gov/other/oil_shale/). Included in this work are sixtynine detailed 71⁄2-minute, 1:24,000-scale geologic quadrangle 

maps and the Citadel Plateau 15-minute, 1:62,500-scale 

quadrangle map that cover nearly the entire area underlain by 

oil shale in the Piceance Basin and that small part of the Uinta 

Basin that is in Colorado (fig. 4). These maps, which now 

represent the most detailed geologic mapping available for the 

oil shale area in Colorado, (1) largely supersede earlier, less 

detailed 1:63,360-scale mapping around the west, south, and 

east margins of the basin published in the late 1940s and early 

1950s (Duncan and Denson, 1949; Duncan and Belser, 1950; 

Waldron and others, 1951; Donnell and others, 1953) and (2) 

extend the larger scale, detailed mapping into the previously 

unmapped central and northern parts of the basin. 

The stratigraphy of the Green River Formation and 

related units is complex, with high variability, and may be 

confusing to someone unfamiliar with the area. One of the 

The Systematic Geologic Mapping Program and a 

Quadrangle-by-Quadrangle Analysis of Time-Stratigraphic 

Relations within Oil Shale-Bearing Rocks of the Piceance 

Basin, Western Colorado

By Ronald C. Johnson
2 Systematic Geologic Mapping Program and Analysis of Time-Stratigraphic Relations of the Piceance Basin, Colorado

Axial arch

arch

Basin

Uinta uplift

Wyoming thrust belt

Wind

River uplift

Rawlins

uplift

Madre

Sierra

Uncompahgre uplift

Sawatch

uplift

Sevier orogenic

belt

Park

Range

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Glenwood

Springs

Castledale

Grand Aspen

Junction

Kemmerer

Moab Gunnison

Montrose

Pinedale

Soda

Springs

Delta

Lander

Casper

Paris

Vernal

Preston

Logan

Rawlins

Green

River

Ogden

Evanston

Coalville

Manila

Price

Provo

Craig

Steamboat

Springs

Duchesne

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–112° –111° –110° –109° –108° –107°

43°

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Rangely

U.S. Geological Survey

 Uinta-Piceance Province boundary 

Oil shale-bearing rocks deposited

 in Eocene Lake Gosiute 

EXPLANATION

Oil shale-bearing rocks deposited

 in Eocene Lake Uinta 

U.S. Geological Survey

 Southwest Wyoming Province boundary 

0 25 50 75 100 KILOMETERS

0 25 50 75 100 MILES

Green River

Colorado River

White River

Yampa River

Green River

Utah

Lake

Great

Salt

Lake

Colorado

River

WYOMING

IDAHO

UTAH

COLORADO

Green River Basin

Hoback Basin

Washakie Basin

Piceance

Uinta 

Sand Wash Basin

Great Divide Basin

SOUTHWESTERN WYOMING

PROVINCE BOUNDARY

(GREATER GREEN RIVER BASIN)

UINTA-PICEANCE

PROVINCE BOUNDARY

Douglas Creek arch

Greater

Green

River

Basin

Rock

Springs uplift

Wampsutter 

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arch

White River

uplift

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Introduction 3

main purposes of this report is to help new users better understand this large, complex body of work. For that purpose, two 

stratigraphic diagrams for each of the 70 quadrangle maps are 

presented (pl. 1 A and B). The first shows relative time represented by all the units that contain significant oil shale in each 

quadrangle and the second shows relative thicknesses. The 

stratigraphic diagrams are arranged on two large charts with 

each quadrangle plotted in its proper position with respect 

to adjacent quadrangles. This allows for the easy tracking of 

changes in mapped stratigraphic units from quadrangle to 

quadrangle. The diagrams include all mapped quadrangles to 

the Colorado-Utah border, as well as those in the easternmost 

part of the Uinta Basin (fig. 4).

A comprehensive time-stratigraphic system based on 

alternating rich and lean oil shale zones has been developed 

for the oil shale sequences in the Piceance and Uinta Basins. 

The system assumes that rich and lean oil shale units were 

deposited during alternating periods of high and low organic 

productivity, respectively, that occurred simultaneously across 

the entire area of Lake Uinta. Air fall tuffs interbedded with 

the oil shale, which are everywhere in the same stratigraphic 

positions relative to these rich and lean zones, appear to confirm this assumption. Here, all mapped stratigraphic units in 

the basin are placed into this time-stratigraphic system, so that 

the relative time represented by these units can be compared 

basin wide.

The focus of this study is the oil shale-bearing interval of 

the Green River Formation and equivalent marginal lacustrine 

and fluvial rocks. Older rocks exposed and mapped in many 

of these quadrangles are not included in the charts and are not 

discussed further. A discussion of the overlying volcaniclastic 

Uinta Formation is included, however, because it intertongues 

with oil shale-bearing rocks in the upper part of the Green 

River Formation. The oldest unit included in the charts is the 

Long Point Bed of the Green River Formation and its equivalent, the basal transgressive bed of Lake Uinta and the base 

of the stratigraphically lowest significant oil shale interval 

in both the Piceance and Uinta Basins (figs. 2, 5, and 6). The 

Long Point Bed marks the onset of a major transgression that 

resulted in the merger of two earlier freshwater lakes, one in 

the Uinta Basin and one in the Piceance Basin, to form one 

lake that extended between the two basins across the intervening Douglas Creek arch (figs. 2, 5, and 6) (Johnson, 1985a). 

These earlier freshwater-lake deposits compose the Cow Ridge 

Member of the Green River Formation and contain some oil 

shale of low grade. The Cow Ridge Member is not included in 

this study; however, all of the Paleocene and Eocene units that 

were deposited in the Piceance Basin prior to the development 

of Lake Uinta, including the Cow Ridge, are discussed by 

Johnson and Flores (2003).

The Green River Formation crops out in its entirety 

around much of the margins of the Piceance Basin (fig. 2) 

where it is typically exposed on towering cliffs with a total 

vertical relief of as much as 3,600 ft (figs. 7–9). Topographic 

relief throughout the central part of the basin, in contrast, is 

generally in the range of a few hundred feet, with only the 

uppermost part of the Green River Formation and overlying 

Uinta Formation typically exposed (fig. 7). At first glance, 

it would seem that the tremendous relief and near complete 

exposures around the margins of the basin would be a great 

advantage to mapping geologic units. However, it is nearly 

impossible to walk along the treacherously steep slopes to map 

contacts and track the rapid lateral facies changes that are in 

these basin-margin areas. In addition, much of the Green River 

Formation tends to weather white due to its high carbonate 

content, making it difficult to distinguish different lithologies 

from a distance. As a result, mapping in these areas, including 

the 1:62,500-scale mapping published in the late 1940s and 

early 1950s (Duncan and Denson, 1949; Duncan and Belser, 

1950; Donnell and others, 1953) and the more recent 1:24,000-

scale mapping, relied heavily on stratigraphic sections typically measured and described along the end of ridges, where 

access is relatively less difficult. However, measured sections, 

some of the more detailed of which were published along with 

the geologic maps, had to be as closely spaced as possible to 

pick up the subtle lateral facies changes. Once contacts were 

Figure 1 (previous page). Map showing extent of Uinta, 

Piceance, and Greater Green River Basins, and approximate 

extent of oil shale in the Green River Formation. Subbasins in 

the Greater Green River Basin labeled in blue, major uplifts are 

labeled in black, and minor structural arches are labeled in 

red. Extent of the Uinta and Piceance Basins (dark blue) is the 

same as the Uinta-Piceance Province boundary (U.S. Geological 

Survey Uinta-Piceance Assessment Team, 2003). Extent of the 

Greater Green River Basin is the same as the Southwest Wyoming 

Province boundary (U.S. Geological Survey Southwest Wyoming 

Province Assessment Team, 2005). For the extent of oil shale in 

the Piceance Basin, the base of the Parachute Creek Member 

of the Green River Formation as mapped by Tweto (1979) was 

used for all but the northwest part of the basin where the base of 

the lower member of the Green River Formation is used. For the 

extent of oil shale in the eastern part of the Uinta Basin, the base 

of the Parachute Creek Member as mapped by Cashion (1973) 

and Rowley and others (1985) was used. In the western part of 

the Uinta Basin, the top of the Mahogany bed of the Green River 

Formation as mapped by Witkind (1995) was used. In the northern 

part of the Uinta Basin, only the area where oil shale is at a depth 

of 6,000 ft or less is shown; this area was outlined by using a 

structure contour map of the top of the Mahogany oil shale bed 

compiled by Johnson and Roberts (2003). For the Sand Wash, 

Washakie, and Great Divide Basins, and southeastern part of the 

Greater Green River Basin, the base of the Tipton Shale Member 

of the Green River Formation as mapped by Tweto (1979) and Love 

and Christiansen (1985) was used to show extent of oil shale. For 

the western part of the Greater Green River Basin, the base of the 

Wilkins Peak Member of the Green River Formation, and for the 

northern part of the Greater Green River Basin, the base of the 

Laney Shale Member of the Green River Formation as mapped by 

Love and Christiansen (1985) were used.
4 Systematic Geologic Mapping Program and Analysis of Time-Stratigraphic Relations of the Piceance Basin, Colorado

CHAFFEE

DELTA

EAGLE

GARFIELD

GRAND

GUNNISON

HINSDALE

JACKSON

LAKE

LARIMER

MESA

MOFFAT

MONTROSE

OURAY

PARK

PITKIN

RIO BLANCO

ROUTT

SAGUACHE

SAN MIGUEL

40

40

40

285

70

70

50

50

Grand 

Junction

Gunnison

Salida

Montrose

Craig

Glenwood

Springs

39°

38°

41°

40°

109° 108° 107° 106°

Colorado River

Green

River

Lake

Granby

Meeker

Rangely

Steamboat

Springs

64

13

13

139

Rifle

City, town

Tertiary extrusive and 

 intrusive volcanic rocks

Mesaverde Formation and Mesaverde

 Group coal-bearing rocks

Undivided Quaternary, Tertiary,

 Mesozoic, and Paleozoic

 sediments and rocks

EXPLANATION

Tertiary

Cretaceous

Uinta Formation and undivided County line

 Tertiary rocks

Green River Formation

Wasatch Formation and undivided

 lower Tertiary rocks

COLORADO

Index map

Oil shale core hole

Surface section

Uinta-Piceance Province boundary

x

A

A’

B

B’

C

C’

Douglas Creek Member

 of Bradley (1931)

Garden Gulch and 

Parachute Cr. Members

of Bradley (1931)

x

x

x

x

x

x

x

x

x x

x

x

x

Lower Piceance

Creek section

Mouth of

Yellow Creek

Pipeline

section

Rio Blanco

section

Anvil 

Points

Cow Ridge

0 50 KILOMETERS

0 25 50 MILES
Geology of the Green River Formation 5

defined in the detailed sections, they were typically surveyed 

by using a plane table and alidade from high points and by 

tracing on aerial photographs. If aerial photographs were used, 

elevations of contacts were then obtained using an appropriate 

plotter. Most contacts were structure contoured from available 

data, and these structure contour maps were used to help place 

the contacts on topographic base maps. 

Geology of the Green River Formation

The Eocene Green River Formation was deposited in two 

large saline lakes, Lake Uinta in the Piceance Basin of western 

Colorado and the Uinta Basin of eastern Utah and westernmost Colorado and Lake Gosiute in the Greater Green River 

Basin of southwest Wyoming, northwest Colorado, and northeastern Utah (fig. 1). These structural and sedimentary basins 

formed during the Laramide orogeny from Late Cretaceous 

through Eocene. Lake Uinta formed in Eocene time when 

two much smaller freshwater lakes expanded during the Long 

Point transgression and connected across the Douglas Creek 

arch to form one large lake. The Long Point Bed of the Green 

River Formation was deposited across large areas during this 

transgression (Johnson, 1984). Lake Uinta became increasingly saline through time (Johnson, 1985a). Freshwater mollusks, common in lacustrine rocks deposited during the earlier 

freshwater phase, are extremely rare above the Long Point Bed 

(Johnson, 1984), indicating a change to waters too saline for 

them to survive. Stromatolites appear to be completely absent 

from the sediments deposited in the freshwater lakes that 

preceded Lake Uinta, but become common in marginal lacustrine sediments deposited immediately after the Long Point 

transgression (Johnson and others, 1988). Stromatolites in the 

modern environment are known to survive only where grazers 

such as gastropods and algae-eating fish and burrowers are 

rare (Garrett, 1970)—the microbial mats that are the essence 

of stromatolites are consumed by the grazers and are disrupted 

and destroyed by the digging of burrowers. 

Lake Uinta appears to have undergone a relatively steady 

increase in salinity throughout its early history (Johnson, 

1985a), ultimately leading to the precipitation of large quantities of halite and the bicarbonate mineral nahcolite in the oil 

shale and saline mineral depocenter (fig. 3). The water column 

was probably stratified with an anoxic, highly alkaline, and 

highly reducing lower level that prohibited organic matter 

from being oxidized. For more information on the geochemical 

evolution of Lake Uinta see Bradley (1929, 1931), Smith 

(1974), Dyni (1981), and Tuttle and Goldhaber (1991). Johnson 

(1985a) subdivided the history of Lake Uinta into five timestratigraphic intervals or stages (figs. 5 and 6). The first two 

stages are characterized by the deposition of illitic oil shales in 

the offshore areas, whereas carbonate-rich, mainly dolomitic, 

oil shales were deposited in offshore areas during the remaining three stages. The beginning of stage 1 is marked by the formation of Lake Uinta during the Long Point transgression. The 

illitic-rich oil shales from this stage, deposited in the offshore 

areas of the lake, are of low grade and generally average less 

than 15 gallons per ton (GPT) oil. Average oil yield increases 

abruptly at the beginning of the second stage, to a maximum 

of about 27 GPT, averaged for the entire interval in the central 

part of the Piceance Basin, with some thin beds averaging 

nearly 60 GPT. The interval deposited during stage 2 thickens 

markedly toward the margins of the basin (figs. 5 and 6).

A gradual shift from illitic-rich to carbonate-rich oil 

shales occurred at the beginning of the third stage, and for the 

first time the sodium bicarbonate mineral nahcolite was deposited near the center of the lake. Robb and Smith (1974), in 

their study of the Colorado No. 1 core hole from the oil shale 

depocenter, reported that the strata below this transition consist 

of primarily illite and quartz with minor dolomite and feldspar. 

After this transition, illite content drops from about 65 percent 

to less than 20 percent of the rock, quartz decreases, and dolomite and feldspar of probable authigenic origin (Desborough 

and Pitman, 1974; Smith, 1974) increase sharply. All oil shales 

deposited throughout the remaining history of Lake Uinta in 

the Piceance Basin are carbonate-rich with dolomite being the 

dominant carbonate (Robb and Smith, 1974). Ultimately, large 

quantities of nahcolite and halite were deposited in the saline 

depocenter of the lake in the north-central part of the Piceance 

Basin during stages 3 and 4 of the lake (fig. 3). Brownfield and 

others (2010) estimated that about 43.3 billion short tons of 

nahcolite remain in the saline depocenter, and much nahcolite 

and halite has been leached out by relatively recent invasion of 

groundwater into the nahcolite-bearing strata. Nahcolite in oil 

shale can be (1) disseminated, (2) aggregates, and (3) discrete 

beds (Dyni, 1974, 1981; Brownfield and others, 2010). Bedded 

nahcolite commonly grades laterally into halite beds toward 

the middle of the saline depocenter (Dyni, 1974, 1981). The 

interval deposited during stage 3, in contrast to the previous stage, thins considerably toward the margins of the basin 

(figs. 4 and 5). Lake margin areas may have been periodically 

exposed during this stage creating hiatuses (Bartov and others, 

2007; Sarg and others, 2008).

The fourth and fifth stages in the history of Lake Uinta 

are marked by expansions of Lake Uinta. The fourth stage 

began with a relatively minor transgression (figs. 5 and 6) that 

may have been caused by the onset of outflow from Lake Gosiute in the Greater Green River Basin to the north (Johnson, 

1985a, 2007). The transgression at the start of the fifth stage 

was almost certainly related to increased outflow from Lake 

Gosiute, as volcaniclastic sediments thought to be derived 

from the Absaroka volcanic field in northwestern Wyoming, 

Figure 2 (previous page). Generalized geologic map of the 

Piceance Basin, northwest Colorado, showing Upper Cretaceous 

and lower Tertiary stratigraphic units. Locations of cross sections 

A-A’, B-B’, and C-C’ (figures 5, 6, and 11 respectively) and surface 

sections mentioned in text are shown.
R 96 W 

R 101 W R 100 W R 99 W R 98 W R 97 W 

R 101 W R 100 W R 99 W R 98 W R 97 W 

R 96 W R 9

T2N 18208 455 507 578 866 940833 933 897 9 114 346 62 14 2 53 6 431 127351 269 13 44427 239 701 768 423 107 491 612 2419 16 51 24 190 132 461503 325 311549 108°30' 

EXPLANATION Nahcolite content—In tons per acre 0 to 100,000 100,000.1 to 250,000 250,000.1 to 500,000 500,000.1 to 750,000 > 750,000 EXPLANATION BPA, All zones except Bed 44 3,000,001 to 4,000,000 2,000,001 to 3,000,000 1,000,001 to 2,000,000 COLORADO <= 1,000,000 Index map 0 3 6 KILOMETERS 0 2 6 MILES Top 

Bas

Cor

208 Ext

T7S Juhan 4-1 core hole Juhan 4-1 core hole Arco Sorghum Gulch 1 core hole Arco Sorghum Gulch 1 core hole U.S. Bureau of Land Management Station Shale 2 core hole x x GARDEN GULCH AND PARACHUTE CREEK MEMBERS OF BRADLEY (1931) GAR

PAROF B

A B 108°30' 108°00' 40°00' R 96 W R 95 W R 101 W R 100 W R 99 W R 98 W R 97 W R 94 W T8ST7S R 93 W T6S T5ST4ST3S T9S R 94 W R 101 W R 100 W R 99 W R 98 W R 97 W T2S T1ST1N R 96 W R 95 W T2N 39°30'
Definition of Stratigraphic Units 7

reached the north margin of Lake Uinta shortly after maximum 

transgression (Surdam and Stanley, 1980). The Mahogany oil 

shale zone (Mahogany oil shale ledge where exposed) of the 

Green River Formation (figs. 5 and 6) was deposited during 

the early part of the fifth stage. These volcaniclastic sediments 

could only have reached Lake Uinta once Lake Gosiute was 

largely filled in, allowing the detritus to be transported farther 

south to gradually fill in the Piceance Basin part of Lake 

Uinta, thus ending oil shale deposition in that part of the lake 

(fig. 5) (Johnson, 1985a).

Definition of Stratigraphic Units

The four principal members of the Green River Formation are Garden Gulch, Parachute Creek, Douglas Creek, and 

Anvil Points Members (figs. 5 and 6). The Garden Gulch, 

Parachute Creek, and Douglas Creek Members were first 

named and defined by Bradley (1931); Donnell (1953) was the 

first to use the name Anvil Points Member. Locally, informal members, such as the unnamed member (Tgu) (Roehler, 

1972a,b, 1973a,b) and intertonguing member (Tgi) (Scott 

and Pantea, 1985) were used when the authors felt that their 

units did not match the lithology of one of the four principal 

members (pls. 1 and 2). In general, Garden Gulch Member is 

applied to the illitic-rich oil shales deposited in offshore areas 

early in the history of Lake Uinta (stages 1 and 2 of Johnson, 

1985a). Parachute Creek Member (figs. 5 and 6) is applied to 

the carbonate-rich oil shale deposited in offshore areas later in 

the history of Lake Uinta (Bradley, 1931) (stages 3–5 of Johnson, 1985a). There are many exceptions to this that will be 

discussed later. The name Douglas Creek Member is applied 

to marginal lacustrine rocks along the west and southwest margins of the Piceance Basin (Bradley, 1931). The name Anvil 

Points Member is applied to marginal lacustrine rocks along 

the east and southeast margins of the basin (Donnell, 1953, 

1961). All of Garden Gulch Member and most of Parachute 

Creek Member grade into Douglas Creek and Anvil Points 

Members toward the basin margins. Douglas Creek and Anvil 

Points Members reach stratigraphically as high as the lower 

part of the Mahogany oil shale ledge along the west margin 

and the Mahogany oil shale bed along the east margin (figs. 

5 and 6, pl. 1). These largely marginal lacustrine members 

consist of carbonate-rich and clay-rich mudstones, ostracodal, 

oolitic, and stromatolitic limestones, siltstones, and sandstones 

with some oil shale. The Anvil Points Member is generally 

sandier than the Douglas Creek Member, and the Douglas 

Creek Member contains more limestone. 

Bradley (1931) originally described the Parachute Creek 

and Garden Gulch Members along Parachute Creek in the 

south-central part of the Piceance Basin (fig. 2). The sections 

are not ideally located, as they are considerably south of the 

oil shale and saline mineral depocenter (fig. 3) in an area 

where both members include some marginal lacustrine rocks. 

Problems with these sections will be addressed more fully in 

the discussion section. Bradley (1931, p. 10) described the 

Garden Gulch Member as “Paper shale with discrete laminae 

less than 1 millimeter in thickness, many to be measured in 

a few hundredths of a millimeter, is the characteristic feature 

of this second member of the Green River formation....Many 

of these paper-thin layers are low-grade oil shale....Some of 

the paper shale will yield more than 15 gallons of oil to the 

ton....Units of paper shale are intercalated with somewhat 

thicker units of flaky shale....The greater part of the Garden Gulch member consists of flaky shale and more thickly 

laminated marlstone, some of which is shaly.” Clearly, flaky 

or papery shale is emphasized by Bradley in his description of 

the Garden Gulch, but the section contains other lithologies 

including calcareous mudstone and oolitic and algal limestone 

more typical of the Anvil Points and Douglas Creek Members. 

The Garden Gulch where Bradley originally described it along 

Parachute Creek is about 700 ft thick and overlies about 500 

ft of Douglas Creek Member that consists of sandstone, algal 

and oolitic limestone, and flaky shale. The Douglas Creek 

there was later mapped as Anvil Points Member (Hail, 1982; 

O’Sullivan and Hail, 1987) as it has an eastern source.

Bradley (1931) considered the overlying Parachute 

Creek Member to be “...the oil-shale facies of the Green River 

Formation....” because it contained most of the oil shale in the 

formation. Bradley subdivided the Parachute Creek Member, 

in ascending order, into a lower oil shale group, transitional 

beds, and an upper oil shale group and traced these units as far 

to the west as the eastern part of the Uinta Basin. The lower 

oil shale group, where originally described by Bradley along 

Parachute Creek, is about 170 ft thick and includes massive oil 

shale and hard platy marlstone, characteristic of the Parachute 

Creek Member, as well as flaky oil shale more characteristic 

of the underlying Garden Gulch Member. The transitional 

beds are about 100 ft thick and consist mainly of hard platy 

marlstone and flaky shale largely barren of organic matter. The 

upper oil shale group is about 730 ft thick and contains most 

of the rich oil shale in the Green River Formation, including 

the Mahogany oil shale ledge.

Bradley (1931) originally described the Douglas Creek 

Member near the head of Trail Creek along the west margin 

of the basin (fig. 2). Here, the Douglas Creek is about 800 ft 

thick and consists of sandstone, mudstone, and oolitic and 

algal limestone. Johnson (1984) re-described this section 

of the Douglas Creek Member reporting that the basal part 

included (1) a thin freshwater lacustrine interval deposited in 

the freshwater lake that predated the formation of saline Lake 

Uinta and (2) a thin interval of the fluvial Wasatch Formation. 

Johnson (1984) separated the thin freshwater lacustrine interval that had previously been mapped as part of the Douglas 

Figure 3 (prevous page). Maps showing (A) total in-place oil 

shale in barrels per acre (BPA), and (B) total in-place nahcolite in 

tons per acre, for all oil shale zones in the Piceance Basin. Total 

oil shale map is from Mercier and others (2010) and total nahcolite 

map is from Brownfield and others (2010).
8 Systematic Geologic Mapping Program and Analysis of Time-Stratigraphic Relations of the Piceance Basin, Colorado

R 96 W R 95 W

R 101 W R 100 W R 99 W R 98 W R 97 W R 94 W

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R 93 W

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S

T

4

S

T

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R 101 W R 100 W R 99 W R 98 W R 97 W

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T

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R 96 W R 95 W

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Indian

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Smizer

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Gulch

Citadel Plateau

15 minute quadrangle

Divide

Creek

Mellen

Hill

Mesa Molina Collbran

South

Mamm

Peak

Hawxhurst

Creek

Housetop

Mountain De

Beque

Wagon

Track

Ridge

Winter

Flats

North

Mamm

Peak

Grand Rulison

Valley

Red

Pinnacle

Long

Point The

Saddle

Anvil

Points

Forked

Gulch

Circle

Dot

Gulch

Mount

Blaine

Desert

Gulch

Rio

Blanco

McCarthy

Gulch

Cutoff

Gulch

Bull

Fork

Figure

Four

Spring

Thirteenmile

Creek No Name

Ridge

Jessup

Gulch

Rock

School

Yankee

Gulch

Segar

Mountain Greasewood

Gulch

Square

S Ranch

Wolf

Ridge

Buckskin

Point

White

River

City

Barcus

Creek SE

Barcus

Creek

Middle

Dry

Fork

Garvey

Canyon

Henderson

Ridge

Calf

Canyon

Douglas

Pass

Baxter

Pass

Razorback

Brushy Ridge

Point

Big

Foundation

Creek

East

Evacuation Davis Creek Canyon

Black

Cabin

Gulch

White

Coyote

Draw

Texas

Mountain

Texas

Creek Dragon

Sagebrush

Hill

Philadelphia

Creek Banta

Weaver Ridge

Ridge

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340

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GARFIELD

65

65

109°00' 108°30' 108°00'

40°30'

40°00'

39°30'

39°00'

0 10 20 MILES Map area

MESA

COLORADO

0 10 20 KILOMETERS

EXPLANATION

Mahogany ledge outcrop

Base of Parachute Creek Member
Development of Rich and Lean Zone Stratigraphy 9

Creek Member (Cashion, 1969; Roehler, 1972a,b, 1973a) and 

included it in his newly defined Cow Ridge Member of the 

Green River Formation that predated the formation of saline 

Lake Uinta (figs. 5 and 6). This member is as much as 650 ft 

thick where exposed along the west margin of the basin (Johnson and others, 1988); it is not included in the stratigraphic 

charts presented here (pl. 1).

Donnell (1953, 1961) applied the name Anvil Points 

Member to marginal lacustrine strata along the east margin 

of the basin that had previously been called the lower sandy 

member by Duncan and Denson (1949). The type section of 

the Anvil Points Member, according to Donnell (1961, p. 851), 

“...is an extremely heterogeneous unit. At the type locality it 

contains approximately 30 percent gray shale, 25 percent gray 

shale and interbedded thin-bedded brown and gray sandstone, 

20 percent massive brown and gray sandstone beds, and 

slightly less than 10 percent light-brown marlstone containing little or no oil. The remainder of the member consists of 

siltstone, algal limestone, and oolitic limestone.” The base 

of the Anvil Points Member drops stratigraphically approximately 600 ft about 1.5 miles southwest of the type section 

(see O’Sullivan, 1986) where the upper part of the underlying 

Wasatch Formation grades laterally into the Green River 

Formation. The type section for the Anvil Points (Donnell, 

1961, p. 851 and his pl. 49) does not include this lower 600-ft 

interval, but it is included in a detailed section measured 

and described at Anvil Points (Self and others, 2010, their 

pl. 7). This Wasatch interval consists mainly of red and green 

mudstone, and sandstone of predominantly fluvial origin, but 

ostracodes and pelecypods are locally present indicating minor 

lacustrine influence.

The Uinta Formation is a sequence of sandstones and 

siltstones containing abundant volcanic debris that complexly 

intertongues with the upper part of the Green River Formation (figs. 5 and 6) (Cashion and Donnell, 1974). The Uinta 

Formation represents that stage of Lake Uinta in the Piceance 

Basin when the lake was gradually filled in from north to 

south largely by volcanic debris originating in northwestern 

Wyoming. The oldest volcaniclastic sandstone appears to be 

near the top of the Mahogany zone; the questionable stratigraphic position of this sandstone is discussed later. Tongues 

of lacustrine rocks that interfinger with volcaniclastic rocks 

of the Uinta Formation have been mapped throughout much 

of the basin (figs. 5 and 6, pl. 1). Although these tongues are 

lithologically identical to the Parachute Creek Member and 

merge with the Parachute Creek toward the south in the basin 

(fig. 3), the North American Stratigraphic Code (1983) does 

not recognize tongues of a member, because the units have the 

same stratigraphic rank. As a result these tongues have been 

mapped as tongues of the Green River Formation, separate 

from the Parachute Creek Member.

The intertonguing between the Uinta Formation and 

upper part of the Green River Formation is one of the most 

stratigraphically complex sequences in the basin because of 

the rapid lateral pinchouts and thickness changes. Thirteen 

major Green River Formation tongues have been named (figs. 

10 and 11). Many minor, unnamed tongues are also present, 

and two or more Green River tongues commonly merge to 

form composite tongues such as in the Figure Four Spring 

quadrangle where the marlstone at Barnes Ridge and Stuart 

Gulch Tongue combine locally to form a new tongue (pl. 1B). 

Some Uinta tongues wedge out in an east-west direction, but 

all ultimately wedge out to the south. In the southernmost 

part of the basin, all the Green River tongues have joined the 

main body of the Green River Formation (figs. 5 and 11). 

Donnell (2008) determined approximately where most major 

Green River and Uinta tongues fall stratigraphically in the 

continuous oil shale section preserved in the southern part of 

the basin (figs. 10 and 11). Interestingly, Uinta tongues are 

represented by relatively thin intervals in this continuous oil 

shale section to the south, indicating that they were deposited 

much more rapidly and represent far less time than the Green 

River tongues.

The name Evacuation Creek Member of the Green 

River Formation was originally applied to this intertonguing 

sequence (pl. 1) in the Piceance Basin by Bradley (1931), and 

the name was used for this interval on some of the earliest 

71⁄2-minute quadrangles published (Cashion, 1969; Cullins, 

1969; Roehler, 1972a,b, 1973a,b). Cashion and Donnell (1974) 

noted that the Evacuation Creek Member at its type locality in 

the eastern Uinta Basin is lithologically and laterally equivalent to the oil shale interval in the upper part of the Parachute 

Creek Member above the Mahogany zone in the Piceance 

Basin. Furthermore, the Evacuation Creek Member as used 

in the Piceance Basin is lithologically equivalent to the sandy 

beds of the Uinta Formation in the Uinta Basin. This miscorrelation resulted in Cashion and Donnell (1974) abandoning the 

name Evacuation Creek Member, and instead applied the name 

Uinta Formation to the sandy interval that intertongues with the 

upper part of the Green River Formation in the Piceance Basin. 

The name Uinta Formation was used for this interval on all 

71⁄2-minute quadrangles published after 1974. 

Development of Rich and Lean Zone 

Stratigraphy

Individual rich oil shale beds and intervals in the Uinta 

and Piceance Basins can be traced for long distances. Bradley 

(1931) was one of the first to demonstrate this when he traced 

the Mahogany ledge (figs. 5, 6, and 9) along outcrop throughout most of the Piceance Basin and into the eastern part of 

the Uinta Basin. Curry (1964) and Trudell and others (1970) 

demonstrated that individual oil shale beds could be traced for 

Figure 4 (previous page). Index map showing locations of 

sixty-nine 71⁄2-minute quadrangles and one 15-minute quadrangle 

(Citadel Plateau) for which geologic maps have been published for 

the oil shale area, Piceance Basin and easternmost Uinta Basin.
Top of Mahogany oil shale zone 

Top of Cretaceous SOUTH NORTH R0 and L0 zones R2, L2, R3, L3 zones R4, L4, R5 zones L5 zone R6 zone and B-groove Mahogany zone Uinta Formation Uinta Formation Wasatch Formation Fort Union Formation Wasatch Formation Wasatch Formation Wasatch Formation Fort Union Formation Cow Ridge Member, Green River Formation Parachute Creek Member Douglas Creek Member Garden Gulch Member Top B-marker Top D-marker F-marker I-marker 

Para

C

Me

Parachute Creek Member Top of Bed 44 of Donnell (2008) Stage 1 Stage 2 Stage 3 Stage 4 Stage 5 A A' Oil shale and saline mineral depocenter Intersection with east-west cross section (fig. 6) R1 zone R1 zone L1 zone Top C-maker VERTICAL EXAGGERATION ABOUT X 40 Gray shale, carbonaceous shale, and mudstone with molluscs and thin coal beds deposited in fresh-water lacustrine and paludal environments Dolomitic lacustrine oil shale Variegated mudstone, sandstone, and siltstone Kerogen-lean clayey lacustrine oil-shale Kerogen-rich clayey lacustrine oil-shale Carbonate-rich marginal lacustrine mudstone with ostracodal, oolitic, and stromatolitic limestones Volcanoclastic lacustrine rocks Disseminated nahcolite in oil shale Nahcolite and halite EXPLANATION 0 5 10 MILES 10 KILOMETERS 0 500 1,000 1,500 400 200 FEET METERS
Constructing the Stratigraphic Charts 11

considerable distances between core holes drilled in the central 

part of the Piceance Basin. The persistence of some oil shale 

zones in the subsurface in the central part of the Piceance 

Basin was noted prior to 1955 when V.E. Peterson (referenced 

in Trudell and others, 1970, p. 4) traced distinctive electric log markers between gas wells in the central part of the 

basin employing a personal system that included (1) a “black 

marker” corresponding to B-groove, (2) a “blue marker” corresponding to the increase in resistivity at the contact between 

the Parachute Creek and Garden Gulch Members (base of 

the R-2 zone), and (3) an “orange marker” corresponding to 

the top of a distinctive, slight increase in resistivity near the 

top of the R-0 zone due to the presence of minor carbonate 

(Trudell and others, 1970). The blue marker, or base of the R-2 

zone, corresponds to the shift from illitic-rich oil shale below 

to dolomitic-rich oil shale above, as discussed previously. 

The orange marker is equivalent to the carbonate marker 

in the Uinta Basin to the west (Johnson, 1985a). The term 

orange marker is still used today (Johnson and others, 2010a), 

whereas the terms black marker and blue marker are generally 

no longer used.

Cashion and Donnell (1972), in a publication that provided the groundwork for most subsequent assessment work 

in the Piceance and Uinta Basins, recognized that the entire oil 

shale interval in the Piceance and Uinta Basins could be subdivided into a series of oil-rich zones (R-1 through R-6 zones) 

and oil-lean zones (L-1 through L-5 zones) (fig. 12). Later, 

the terms R-0 and L-0 were applied to the lowest oil shale 

zones in the Green River Formation that were deposited just 

after the Long Point transgression (see, for example, Johnson 

and others, 1988). Previously named oil shale units above the 

R-6 zone are, (in ascending order) B-groove, Mahogany zone, 

and A-groove. The oil shale interval above A-groove was not 

named by Cashion and Donnell (1972), but they traced several 

oil shale marker beds through that interval. Donnell (2008) 

correlated 44 individual oil shale beds in the interval above 

A-groove across much of the Piceance Basin and the eastern 

part of the Uinta Basin (fig. 7). All of these oil shale beds 

and zones appear to closely represent time-stratigraphic units 

that reflect changing rates of organic matter production and 

preservation that occurred simultaneously throughout Lake 

Uinta. The differentiation of rich and lean zones is used for 

correlation purposes on the stratigraphic charts compiled for 

this report.

The lower zones, from the R-0 zone through the L-1 

zone, compose the illitic-rich oil shale interval of the Garden 

Gulch Member of the Green River Formation (Smith and 

others, 1968; Trudell and others, 1970); these zones also 

represent stages 1 and 2 of Lake Uinta (Johnson, 1985a) 

(figs. 5, 6, and 12). However, the Garden Gulch Member, 

as mapped around the basin margins, rarely corresponds to 

the R-0 through L-1 zones, and possible reasons for this are 

presented in the Discussion section of this report. The base of 

the R-2 zone marks a shift from illitic-rich to dolomitic-rich 

oil shale discussed previously and corresponds to the base of 

the Parachute Creek Member (Trudell and others, 1970, 1974; 

Dyni, 1974) as well as to the beginning of stage 3 of Johnson 

(1985a) (figs. 5,6, and 12). The R-2 zone consists of interbedded illitic-rich and carbonate-rich oil shale (Dyni, 1974; 

Robb and Smith, 1974), whereas all oil shale deposited after 

the R-2 zone is predominantly carbonate-rich. As previously 

discussed, the transition from clay-rich to carbonate-rich oil 

shale is easily traced on electric logs throughout the central 

part of the Piceance Basin, as the carbonate-rich oil shales are 

much more resistive than the underlying clay-rich oil shale. 

The base of the R-4 zone marks the minor transgression at the 

beginning of stage 4 of Johnson (1985a), and the base of the 

Mahogany zone marks the major transgression at the beginning of stage 5 of Johnson (1985a) (figs. 5 and 6). 

Tracing these rich and lean oil shale zones from the oil 

shale depocenter into marginal lacustrine equivalents in the 

Douglas Creek and Anvil Points Members that crop out around 

the margins of the basin is difficult and as yet incomplete. 

Brobst and Tucker (1973) identified all oil shale zones from the 

Mahogany ledge down to the base of the R-4 zone in outcrop 

along lower Piceance Creek and from the Mahogany ledge 

down to the L-3 zone at their Pipeline section along Cathedral 

Bluffs. Johnson and others (1988) traced some of the rich and 

lean zones into their marginal lacustrine equivalents in the 

Douglas Creek Member along the west basin margin of the 

Piceance Basin. This effort was expanded by Johnson and 

others (2010a,b), and Self and others (2010) and is expanded 

further here by placing all mapped contacts in all the quadrangle areas into the R-zone stratigraphic context (pl. 1). The 

reliability of this attempt varies considerably from quadrangle 

to quadrangle and between different stratigraphic units.

Constructing the Stratigraphic Charts

The stratigraphic sequence of rich and lean zones originally defined by Cashion and Donnell (1972) and later refined 

by other workers (see for example, Pitman and others, 1989), 

forms the basis of both the time-stratigraphic and thickness 

charts (pl. 1). To make the charts more user-friendly, the zones 

are grouped into three intervals shown in different colors in the 

background between the individual stratigraphic diagrams: (1) 

illitic phase or Garden Gulch Member (R-0 through L-1 zones) 

(gray), (2) carbonate phase or Parachute Creek Member prior 

to the arrival of the first volcaniclastics from Wyoming (R-2 

through top of the Mahogany bed) (tan), and (3) the infilling 

stage in which the upper part of the Green River Formation 

Figure 5 (previous page). North-south cross section A-A’ (fig. 2) 

through the Piceance Basin, northwestern Colorado, showing 

member subdivisions, stages of Lake Uinta as defined by Johnson 

(1985a), and some of the rich and lean zones defined by Cashion 

and Donnell (1972). The B-marker through I-marker beds are from 

Chancellor and others (1974).
WEST

B

Cow Ridge

Member of

Green River

Formation

Unconformity at

top of Cretaceous

Unconformity at

top of Cretaceous

Unconformity at top of Cretaceous

Generalized

land surface

Variegated mudstone, sandstone, and siltstone

Kerogen-lean clayey lacustrine oil shale 

Mostly marginal lacustrine sandstone and mudstone

Carbonate-rich marginal lacustrine mudstone with 

ostracodal, oolitic, and stromatolitic limestones

Gray shale, carbonaceous shale, and mudstone with  molluscs and thin coal beds deposited in fresh-water 

 lacustrine and paludal environments 

Volcanoclastic lacustrine rocks

Kerogen-rich clayey lacustrine oil shale

Dolomitic lacustrine oil shale

EXPLANATION

B-marker

C-marker

D-marker

F-marker

I-marker

Douglas Creek

Member

Wasatch

Formation

Green River

Formation

Black Sulfur Tongue

of Green River Formation

Tongue of

Uinta Formation

Base of

R-4 rich

oil-shale

zone

Parachute Creek

Member of Green

River Formation

Garden Gulch Member

Tongue of

Uinta Formation

Tongue of

Uinta Formation

Thirteenmile Creek

Tongue of Green

River Formation

Black Sulfof Green R

Tongue oFormatio

Top of Moil-shal

Base oil-sh

AM

Wasatch

Formation

Fort Union

Formation

FortUnionFormatio

WasatchFort Union

Formation

contact

UTAH COLORADO

Douglas Creek arch

Piceance Basin

Uinta Basin

43

Stage 1

Stage 2

Stage 3

Stage 4

Stage 5

Intersection with

north-south cross

section (figure 5)

EAST

B'

0 5 10 MILES

10 KILOMETERS

0

500

1,000

1,500

400

200

FEET METERS

VERTICAL EXAGGERATION ABOUT X 40
intertongues with the Uinta Formation (top of Mahogany bed to 

top of marlstone at Skinner Ridge—the highest named tongue 

of the Green River Formation) (pink). In a limited area along 

the south and west margins of the study area, significant thicknesses of Green River Formation are found above the marlstone at Skinner Ridge. About 150 ft of the Parachute Creek 

Member lies above the stratigraphic position of the marlstone 

at Skinner Ridge in the Middle Dry Fork quadrangle and more 

than 200 ft of Parachute Creek Member lies above the base 

of a tuffaceous interval (t) that is thought to be equivalent to 

the top of the marlstone at Skinner Ridge in the Walsh Knolls 

quadrangle on the Colorado-Utah border (pl. 1). Lake Uinta 

persisted longer in the southwestern part of the Piceance Basin 

and eastern part of the Uinta Basin than it did throughout the 

rest of the Piceance Basin.

Ideally, the vertical scale on time-stratigraphic charts 

should be proportional to time. At present, however, the time 

duration represented by each rich and lean zone is unknown, 

and it is unlikely that an adequate dating of these intervals will 

be available soon. Dateable tuffs in the Green River Formation are sparse in the Piceance Basin being confined largely to 

the Mahogany zone and overlying beds. These tuffs yielded 

40Ar/39Ar dates between about 49 and 47 Ma (Smith and others, 2008). Only two tuffs of note are in the interval below the 

Mahogany zone—the yellow tuff of Pipiringos and Johnson 

(1975, 1976), which is in the middle part of the R-5 zone (pl. 

1), and the Kimball Mountain Tuff Bed, which is in the lower 

part of the R-1 zone (pl. 1). An 40Ar/39Ar date of 51.39 ± 1.70 

Ma was obtained from the yellow tuff (Smith and others, 

2008), but the Kimball Mountain Tuff Bed has yet to be dated. 

Instead of a framework based on absolute dating, the 

thicknesses of the rich and lean zones in a “representative” 

part of the basin are used here to approximate the relative 

time represented by each zone. An area was selected near the 

central part of the lake and away from marginal areas where a 

fluctuating water level might cause periods of non-deposition 

or erosion. Here, continuous or nearly continuous deposition 

would have occurred throughout the history of Lake Uinta. 

Even here, however, rates of sedimentation would have varied 

through time due to different rates of organic productivity and carbonate precipitation, meaning thicknesses are not 

directly related to time. In addition, the depocenter of the lake 

contains thick deposits of nahcolite and halite that may have 

precipitated comparatively rapidly out of the water column 

and within the unconsolidated lake deposits. Because of the 

saline problem, an area was selected that was on the margin of 

the saline depocenter where saline deposits are present but do 

not appear to have greatly altered the thicknesses of the rich 

and lean zones. All of the above-mentioned criteria are met 

in the Juhan 4-1 core hole (fig. 3) in sec. 4, T. 2. S., R. 98 W. 

This core hole, on the southern margin of the lake’s depocenter, contains some nahcolite, but it does not appear to have 

greatly altered the thicknesses of the affected zones. Zones L-2 

through L-4 contain disseminated nahcolite, with most of it 

concentrated in the L-2, R-3, and L-4 zones. These zones are 

only slightly thicker than the same zones in the Arco Sorghum 

Gulch 09 core hole in sec. 11, T. 3 S., R. 97 W., which is in the 

nahcolite-free area just south of the saline depocenter (Brownfield and others, 2010, their pl. 2). 

The upper intertonguing interval between the Green 

River and Uinta Formations is only partially represented in the 

Juhan 4-1 core hole. The core hole is in the north-central part 

of the basin that was filled with volcaniclastic sediments considerably before the southernmost part of the lake was filled 

with sediments. A core hole in the southern part of the basin, 

the U.S. Bureau of Land Management Triangulation Station 

Shale 2 core hole in sec. 24, T. 7 S., R. 97 W. (fig. 3A), was 

chosen to represent thicknesses in this interval as it includes 

one of the most complete lacustrine sections in the basin and 

was used by Donnell (2008, his fig. 4) in his study of this 

intertonguing interval.

It is difficult to represent all the lateral stratigraphic 

changes taking place within a quadrangle area on two-dimensional diagrams, so compromises were made. On both the 

time-stratigraphic and thickness-stratigraphic diagrams (pl. 1), 

changes in a westerly direction are shown on the left side of 

the charts and changes in an easterly direction are on the right 

side. Changes that are more or less north-south are problematical and must be translated down to either the left or right side 

of the charts at the discretion of the author. A single “representative” thickness is used for each rich and lean zone for 

each quadrangle on the thickness stratigraphic chart (pl. 1B). It 

was felt that this was sufficient to characterize strata deposited prior to the infilling stage, as thicknesses in this interval 

change rather gradually across any given quadrangle. These 

thicknesses were measured from available measured sections 

or from drill holes near the center of each quadrangle. Regionally, however, the thicknesses of individual rich and lean zones 

do change markedly. For example, thicknesses overall increase 

toward the east due to an increase in the rate of subsidence in 

that direction, and this trend is easily seen on plate 1B. 

Thicknesses of stratigraphic units deposited during the 

infilling stage, in contrast, can change greatly across individual 

quadrangles (pl. 1B). A Uinta Formation tongue hundreds of 

feet thick along the north margin of a quadrangle, for example, 

might pinch out before reaching the quadrangle’s south border. 

Although it is difficult to portray diagrammatically all of the 

thickness relations within a given quadrangle, the ranges in 

thickness for each unit shown in the diagrams are intended 

to closely follow the ranges of those listed by the authors for 

their mapped quadrangles.

Figure 6 (previous page). East-west cross section B-B’ (fig. 2) 

through the Piceance Basin, the Douglas Creek arch, and the 

eastern part of the Uinta Basin showing member subdivisions, 

stages 1–5 of Lake Uinta as defined by Johnson (1985a), and some 

of the rich and lean zones defined by Cashion and Donnell (1972). 

The B-marker through I-marker beds are from Chancellor and 

others (1974).

Constructing the Stratigraphic Charts 13
Cathedral Bluffs Roan Creek Parachute Creek Piceance Creek Anvil Points Cow Ridge 70 Mesa Moffat Colorado River Figure 7. Oblique aerial photograph of the Piceance Basin showing prominent relief along the west, south, and east margins of the Piceance Basin. 

of Mahogany oil shale ledge. Location of Anvil Points (labeled on fig. 2) is shown.
Constructing the Stratigraphic Charts 15

West Anvil Point

East Anvil Point

800 FEET

Figure 8. Photograph of the Green River Formation exposed on Anvil Points in the southeastern part of the Piceance Basin.
1,800 FEET Figure 9. Photograph of the Green River Formation (shown with yellow line) and upper part of the underlying Wasatch Formation alon

in the southwestern part of the Piceance Basin. Note that the entire Green River Formation weathers white. The Mahogany oil shale zoconspicuous ledge shown with an arrow near the top of the outcrop. The base of the Green River Formation is about the base of the stLocation of Cow Ridge shown on figure 2.
Constructing the Stratigraphic Charts 17

0

50

100

150

200

250

300

350

400

150

200

250

300

350

400

450

500

550

600

0 20 40 60

0 20 40 60

74

72

70

68

66

64

62

60

58

56

54

52

50

48

46

44

42

40

38

36

34

32

30

28

26

24

22

25

18

16

14

12

108642

Tgsk

Tgsl, Tgj

Tgbr

Tgs

Tgc

Tgb

Tgt

Tgd

Tgy

Tgm

Tgtc

Tggc

Tu 13

Tu12

Tu11

Tu10

Tu9

Tu8

Tu7

Tu6

Tu5

Tu3

“A” Tu2

76

74

72

70

68

67

66

64

62

60

58

56

54

52

50

48

46

44

42

40

39

38

37

34

32

30

28

26

25

24

22

20

18

16

14

12

108642

36

20

Oil yield in gallons per ton

Oil yield in gallons per ton

67

37

35

21

39

35

21

DEPTH IN FEET

Tu4

 U31

Skyline Oil Company

Watson 1

Sec. 5, T. 11 S., R. 25 E.

Uintah County, Utah

C37

U.S. Bureau of Land Management

Triangulation Station Shale 2

Sec. 24, T. 7 S., R. 97 W.

Garfield County, Colo.

Figure 10. Correlation of 

individual oil shale beds 

and groups of beds forming 

formally and informally 

named tongues of the Green 

River Formation (indicated 

by stratigraphic-unit symbol 

beginning with Tg) in the 

interval above A-groove. 

Abbreviations used: “A”, 

base of A-groove; Green 

River Formation units: Tggc, 

marlstone at Greasewood 

Creek; Tgtc, marlstone 

at Trail Canyon; Tgm, 

marlstone at Mare Canyon; 

Tgy, Yellow Creek Tongue; 

Tgd, Dry Fork Tongue; Tgt, 

Thirteenmile Creek Tongue; 

Tgb, Black Sulphur Tongue; 

Tgc, Coughs Creek Tongue; 

Tgs, Stewart Gulch Tongue; 

Tgbr, marlstone at Barnes 

Ridge; Tgsl, marlstone at 

Sleepy Ridge; Tgj, marlstone 

at Jack Rabbit Ridge; Tgsk, 

marlstone at Skinner Ridge. 

Uinta Formation units are 

numbered Tu2 through Tu13. 

Units evenly numbered 2 

through 76 are persistent 

oil shale beds defined by 

Donnell (2008).
350 400 450 500 550 600 650 700 750 800 850 900 950 1000 1050 1100 

Oil yield (gallons per ton) 

1050 1000 950 900 850 800 750 700 650 600 550 500 450 400 350 300 800 850 900 950 1000 1050 1100 1150 1200 1250 1300 1300 1250 1150 1000 950 900 850 800 750 700 650 600 550 500 60 585452 444038322622 1812AMB C0035 U.S. Bureau Of Mines- Atomic Energy Commission, Colorado 2 Sec. 14, T. 1 S., R. 99 W. C0254 Arco (C-b Project), Sorghum Gulch 11 Sec. 7, T. 3 S., R. 96 W. C0333 Getty Oil Co., 32-27 Sec. 27, T. 5 S., R. 97 W. 

U

C0034 U.S. Bureau Of Mines- Atomic Energy Commission, Colorado 1 Sec. 13, T. 1 N., R. 98 W. 0 20 40 60 80 0 20 40 60 80 0 20 40 60 80 0 20 40 60 80 

? ?? ? 1050 1100 1200 66 ? 64 ? 

NORTH 

Depth (ft) C 

Oil yield Oil yield Oil yield 

? Figure 11. North-south cross section C-C’ showing intertonguing of the Uinta Formation (yellow) with the upper part of the Green River Formation (b

Colorado (from Donnell, 2008, his fig. 3). Abbreviations used: MB, Mahogany bed; A, base of A-groove; Tggc, marlstone at Greasewood Creek; Tgtc, mTgm, marlstone at Mare Canyon; Tgy, Yellow Creek Tongue; Tgd, Dry Fork Tongue; Tgt, Thirteenmile Creek Tongue; Tgb, Black Sulphur Tongue; Tgc, CStewart Gulch Tongue; Tgbr, marlstone at Barnes Ridge; Tgsl, marlstone at Sleepy Ridge; Tgj, marlstone at Jackrabbit Ridge; Tgsk, marlstone at Skinnthrough 66 are persistent oil shale beds defined by Donnell (2008).
Constructing the Stratigraphic Charts 19

Top Bed 44–

top A-groove

A-groove

Mahogany zone

B-groove

R-6 zone

L-5 zone

R-5 zone

L-4 zone

R-4 zone

L-3 zone

R-3 zone

L-2 zone

R-2 zone

L-1 zone

R-1 zone

L-0 zone

R-0 zone

Stratigraphic nomenclature for 

oil shale zones from Donnell and Blair 

(1970), Cashion and Donnell (1972), 

Donnell (2008); stages from Johnson (1985a) Garden Gulch Member Parachute Creek Member Green River Formation

Oil yield (gallons per ton)

Oil yield (gallons per ton)

Oil yield (gallons per ton)

 U-53

Skyline Oil Co., Watson 1A

 Sec. 5, T. 11 S., R. 25 E.

Base porcupine

tuff–top Bed 44

of Donnell (2008)

porcupine tuff

Top bed 76 of

Donnell (2008)–

top porcupine tuff

Stage 1 Stage 2 Stage 3 Stage 4 Stage 5

DEPTH IN FEET

0 20 40 60 80 100

CO195

Arco-Mobil-Equity,

Figure Four 31-1 

Sec 31, T. 3S, R. 98 W 

500

1,000

1,500

2,000

2,500

500

1,500

0

1,000

0 20 40 60 80 100

0 20 40 60 80 100

C0172

Shell Oil Co., 23X-10,

Sec 10, T. 7 S., R. 97 W.

0 20 40 60 80 100

100

0 20 40 60 80 100

0 20 40 60 80

500

1,000

1,500

2,000

0

Mahogany bed

USGS ID no.

Figure 12. Cross section showing oil-yield histograms, members of the Eocene Green River Formation, correlation of rich and lean

oil shale zones of Cashion and Donnell (1972), and stages in the evolution of Lake Uinta (Johnson, 1985a). Core hole U-53 is in the 

northeast corner of the Uinta Basin and core holes CO-172 and CO-195 are in the southeastern and central parts of the Piceance 

Basin, respectively.
20 Systematic Geologic Mapping Program and Analysis of Time-Stratigraphic Relations of the Piceance Basin, Colorado

Discussion

There are clearly problems with the definitions of the four 

members of the Green River Formation—Evacuation Creek, 

Garden Gulch, Parachute Creek, and Douglas Creek Members—originally defined by Bradley (1931). These problems 

led to inconsistencies in how these names have been applied 

throughout the basin, owing primarily to the lack of both subsurface and surface information at the time the members were 

defined. Such problems did not become apparent until the 

extensive coring and mapping programs in the 1960s, 1970s, 

and 1980s.

The name Evacuation Creek Member was abandoned 

when it was recognized as being equivalent to the Uinta 

Formation as used in the Piceance Basin by Cashion and 

Donnell (1974). However, the name was used on five of 

the earliest quadrangles mapped in the basin (Cashion, 

1969; Cullins, 1968, 1969; Roehler, 1972a,b, 1973a,b). The 

Douglas Creek Member as originally defined by Bradley 

was later determined to include an interval of the underlying 

Wasatch Formation and rocks deposited in the freshwater 

lake that predated the formation of Lake Uinta. These two 

sequences were removed from the definition of the Douglas 

Creek Member, with the freshwater lake deposits given a 

new name, the Cow Ridge Member (Johnson, 1984), but not 

before the name Douglas Creek Member had been applied 

to these deposits on some of the early maps (Cashion, 1969; 

Cullins, 1968, 1969; Roehler, 1972a,b, 1973a,b).

A particularly large stratigraphic problem involves the 

original definition of the Garden Gulch Member by Bradley 

(1931). As previously discussed, the term Garden Gulch 

Member in the oil shale depocenter of the Piceance Basin is 

now applied to the illitic-rich oil shale interval that extends 

from the base of the R-0 zone to the base of the R-2 zone 

(fig. 12). However, Bradley (1931) defined the member 

for outcrops near the basin margin before drill data were 

available to study the subsurface stratigraphy. He was thus 

unaware of the relatively sharp transition between illitic oil 

shale below and dolomitic oil shale above that is at the base 

of the R-2 zone in the depocenter. By 1970, a significant 

number of core holes had penetrated the entire oil shale 

section in the basin interior, and the illitic and dolomitic 

zones were fairly well defined. The name Garden Gulch was 

applied to the illitic oil shale zone at the base of the section 

(R-0 through L-1 zones) (Trudell and others, 1970; Dyni, 

1974; Ziemba, 1974), and the name Parachute Creek Member was applied to the overlying dolomitic oil shale zone 

(R-2 and above; fig. 12) based on Bradley’s descriptions of 

these two members.

As detailed mapping progressed and correlations 

between the depocenter and the margins of the basin 

improved, inconsistencies between the Garden Gulch as 

defined in outcrop by Bradley and the Garden Gulch as 

defined by the R-zone stratigraphy became apparent. Mapping around the margins of the basin shows that the lower 

contact of the Garden Gulch Member varies from the base 

of the R-0 to about the middle of the R-1 zone, whereas the 

upper contact varies from the top of the R-0 zone to as high 

as the top of the R-5 zone (pl. 1). 

The lower contact of the Garden Gulch Member progressively climbs stratigraphically toward the east margin 

of the basin as the lower part of the Garden Gulch grades 

into the sandy Anvil Points Member (pl. 1). Along Parachute 

Creek, where the Garden Gulch Member was originally 

defined by Bradley (1931), the R-0 zone has graded into 

the sandy Anvil Points Member, and about 3 miles to the 

southeast, the remaining Garden Gulch grades into Anvil 

Points (O’Sullivan and Hail, 1987) (pl. 1). Toward the west, 

in contrast, the lowest part of the Garden Gulch Member 

(about the R-0 zone) persists farther marginward than any 

other part of the Garden Gulch (pl. 1). Thus, marginal 

lacustrine rocks of the Anvil Points underlie the Garden 

Gulch Member toward the east margin of the basin, whereas 

marginal lacustrine rocks of the Douglas Creek Member 

overlie the Garden Gulch Member to the west. Remnants of 

the R-0 zone extend farther toward the southern margins of 

the Uinta Basin to the west than any oil shale zone below 

the R-4 zone (Johnson and others, 2010b).

The stratigraphic variability in the mapped position of 

the upper contact of the Garden Gulch with respect to the 

R-zone stratigraphy is complex. In Bradley’s original Garden 

Gulch section along Parachute Creek, the contact with the 

overlying Parachute Creek Member is placed at about the 

base of the R-4 zone, which is much higher stratigraphically 

than the base of the R-2 zone in the oil shale depocenter. 

The base of the R-4 zone represents a period of expansion 

of Lake Uinta during which the area of oil shale deposition expanded to cover much of the area that had previously 

been marginal lacustrine (Johnson, 1986). Bradley (1931) 

considered the Parachute Creek Member to be “the oil-shale 

facies of the Green River Formation” stating (p. 11) that the 

Parachute Creek Member “contains all the large groups of 

rich beds, most of the individual rich beds, and a very large 

proportion of the low-grade oil shale.” Although Bradley’s 

Garden Gulch Member is characterized by the presence of 

papery fissile oil shale, he does not place the contact with the 

overlying Parachute Creek Member at the top of the highest interval of paper shale but rather about 60 ft above, at 

the base of the lowest major oil shale interval, or about the 

base of the R-4 zone (Bradley, 1931, his pl. 7). Thus, the 

Garden Gulch-Parachute Creek contact, where originally 

described, is primarily defined by the presence or absence of 

significant oil shale and only secondarily by the presence of 

papery shale. As previously discussed, the Garden Gulch as 

defined by Bradley (1931) also includes calcareous mudstone 

and oolitic and algal limestone more typical of the marginal 

lacustrine Anvil Points and Douglas Creek Members and 

thus occupies a position that is transitional between offshore 

lacustrine and marginal lacustrine. 

The base of the R-4 zone is mapped as the base of the 

Parachute Creek Member in 21 of the quadrangles in the 
Discussion 21

basin. Of these, the Garden Gulch Member is mapped below 

the Parachute Creek in 8 quadrangles, the Douglas Creek or 

Anvil Points Member is mapped below the Parachute Creek 

in 10, and in the remaining 3 quadrangles, the Garden Gulch 

Member is mapped below the Parachute Creek/base R-4 zone 

in parts of the quadrangles and Douglas Creek or Anvil Points 

in other parts. 

The assumption that a high degree of fissility on outcrop 

indicates high illite/clay content and low dolomite content 

may not be entirely true. Although the Garden Gulch Member 

in the depocenter is predominantly illitic, it contains some 

dolomite as well (Robb and Smith, 1974). Similarly, the Parachute Creek Member also contains some illite in the depocenter. Green River Formation exposures along the lower part of 

Piceance Creek in the north-central part of the basin probably 

represent the most basinward oil shale facies cropping out 

anywhere in the basin, and thus represent the best place to 

study rocks similar to those in the depocenter (figs. 2 and 3). 

Nahcolite vugs are present in some stratigraphic intervals, and 

there is abundant nahcolite in the subsurface a short distance 

to the south. The rich and lean zones are better defined here 

than in any other outcrops, and the approximate positions of 

all of the rich and lean zones have been identified (Self and 

others, 2010, their pl. 13). The stratigraphically highest fissile 

oil shale interval, the top of which was used by Pipiringos and 

Johnson (1976) as the Garden Gulch-Parachute Creek contact, 

is near the base of the R-5 zone or much higher than the base 

of the R-2 zone. 

Fissility as high as at least the R-5 zone could be the 

result of one or more of the following: (1) dolomitic oil shale 

contains enough clay to cause fissility, (2) the finely laminated nature of much of the dolomitic oil shale causes fissility, and (3) the mineralogy of the dolomitic interval changes 

toward the basin margins, due to changes in water chemistry, 

and these mineralogical changes promote the development 

of fissility in outcrop. Lake Uinta is believed to have been a 

stratified lake with two largely non-mixing layers (Bradley, 

1929; Smith, 1974). The change from illitic-rich oil shale to 

dolomitic oil shale that takes place in the oil shale depocenter near the base of the R-2 zone was the result of changes in 

the chemistry and pH of the lower highly saline water layer 

(Smith, 1974). The mineralogical changes that are evident 

through the oil shale column have been well documented in 

the oil shale depocenter, but much less mineralogic information is available for lake-margin areas where the Garden Gulch 

and Parachute Creek Members were mapped. Mappers thus 

had little mineralogical data to help with defining their units. 

A north-south cross section through the depocenter of the 

basin (pl. 2), constructed by Johnson and others (in press) in 

their study of water-bearing mineral phases in the Green River 

Formation, uses some of the limited mineralogic data for basin 

margin areas. Only water-bearing phases are shown on the 

cross section, but mineralogical changes from the center of the 

depocenter outward are apparent, and these almost certainly 

reflect variations in water chemistry. The early illitic phase 

is shown extending across the entire basin at the base of the 

cross section. In the dolomitic zone above, the water-bearing 

phases grade outward from nahcolite and dawsonite in the 

middle of the depocenter, to just dawsonite, and finally to 

analcime and clay. The section exposed along lower Piceance 

Creek was remeasured for this study and is included in the 

cross section along with mineralogical data for the lower 

Piceance Creek section from Brobst and Tucker (1973). In 

this section, dawsonite-bearing oil shale extends from near the 

base of the R-3 zone to above the R-4 zone, with analcite and 

clay-bearing rocks both above and below (pl. 2).

The Uinta Formation represents a classic deltaic system 

that prograded from north to south into relatively deep Lake 

Uinta (Johnson, 1981a) (fig. 3). In an east-west direction, 

Uinta tongues are wedge-shaped as a result of a constantly 

shifting locus of clastic deposition (pl. 1) (Donnell, 2008, 

his figs. 10–16). The resulting complex pattern of the merging and splitting of sandstone and shale units is difficult to 

map, and only the most prominent Green River and Uinta 

Formations tongues have been named and mapped. Additionally, the lowest volcaniclastic unit of the Uinta Formation, 

which is present near the mouth of Yellow Creek (fig. 2), 

has been placed at two different stratigraphic positions by 

different workers: (1) at the Mahogany marker, a tuffaceous 

bed within the Mahogany zone just above the Mahogany 

bed (Johnson, 1981a; Donnell, 2008; Self and others, 2010, 

their pl. 13) or (2) somewhat higher, just above the top of 

the Mahogany oil shale zone (Hail, 1973, 1988). It may not 

be possible to resolve this issue, as the Mahogany zone (or 

Mahogany ledge on outcrop) grades into a silty and sandy 

interval in the vicinity of Yellow Creek (fig. 2, pl. 1, Rough 

Gulch and Smizer Gulch quadrangles) and cannot be recognized. Core-hole information, however, shows a marked 

thickening of the silty and sandy tuffaceous Mahogany 

marker toward the mouth of Yellow Creek, suggesting that 

the lowest Uinta tongue may be at that stratigraphic position. 

The lowest Uinta tongue is thus placed at the stratigraphic 

position of the Mahogany marker in this report (pl. 1). This 

placement, however, requires that the top of the Mahogany 

ledge as mapped by Hail (1973, 1974a, 1978) in the vicinity 

of Yellow Creek is too low, as these maps show the lowest 

Uinta tongue as being above the top of the Mahogany zone. 

More work is needed to resolve this problem. 

In summary, an understanding of the geology of the oil 

shale strata in the Piceance Basin improved as the systematic 

geologic quadrangle program progressed. As a result, some of 

the geologic units that were defined prior to this detailed mapping were modified or discarded, and many of these inconsistencies that became evident after completion of the earliest 

maps were subsequently corrected on later regional compilations of the basin (Hail and Pipiringos, 1990; Hail and Smith, 

1994, 1997). The extensive body of geologic data provided 

by the detailed quadrangle 7 1⁄2-minute quadrangle maps, 

however, still represent the most detailed mapping available 

for this important area, and it is hoped that this investigation 

has contributed to a better understanding of this important 

body of work.
22 Systematic Geologic Mapping Program and Analysis of Time-Stratigraphic Relations of the Piceance Basin, Colorado

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Publishing support provided by: 

Denver Publishing Service Center

For more information concerning this publication, contact:

Center Director, USGS Central Energy Resources Science Center

Box 25046, Mail Stop 939

Denver, CO 80225

(303) 236-1647

Or visit the Central Energy Resources Science Center Web site at:

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Johnson—Systematic Geologic Mapping Program and Analysis of Time-Stratigraphic Relations of the Piceance Basin, Colorado—SIR 2012–5041