U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2013–5176

Development, Evolution, and Destruction of the Saline 

Mineral Area of Eocene Lake Uinta, Piceance Basin, 

Western Colorado
COVER. View of the Green River Formation at Anvil Points, west of Rifle, Colorado. Photo by 

Ronald C. Johnson, U.S. Geological Survey.
Development, Evolution, and Destruction 

of the Saline Mineral Area of Eocene Lake 

Uinta, Piceance Basin, Western Colorado

By Ronald C. Johnson and Michael E. Brownfield

Scientific Investigations Report 2013–5176

U.S. Department of the Interior

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

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2015

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Suggested citation:

Johnson, R.C., and Brownfield, M.E., 2015, Development, evolution, and destruction of the saline mineral 

area of Eocene Lake Uinta, Piceance Basin, western Colorado: U.S. Geological Survey Scientific Investigations 

Report 2013–5176, 76 p., 2 pl., http://dx.doi.org/10.3133/sir20135176.

ISSN 2328-0328 (online)
iii

Contents

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

Introduction.....................................................................................................................................................2

The Development of Eocene Lake Uinta and Contemporary Lake Gosiute .........................................2

Subdividing the Green River Formation into Members, Rich and Lean Zones, and Stages..............4

Nahcolite and Halite in the Piceance Basin .............................................................................................7

Development of the Leached Zone in the Piceance Basin...................................................................15

Methods Used ..............................................................................................................................................16

Results of Data Analyses............................................................................................................................21

Present-Day Elevation of the Leached Zone..................................................................................21

Saline Mineral Deposition and Extent of Leaching for Each Rich and Lean Zone ..................26

Illitic Phase...........................................................................................................................................26

Early Saline Mineral Phase...............................................................................................................30

Middle Saline Mineral Phase ...........................................................................................................31

Late Saline Mineral Phase ................................................................................................................52

Discussion and Conclusions......................................................................................................................53

Acknowledgments.......................................................................................................................................71

References Cited..........................................................................................................................................71

Plates .............................................................................................................................................................76

Figures

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

approximate extent of oil shale in the Green River Formation..............................................3

2. Maps showing total in-place oil in barrels per acre and total in-place nahcolite 

in tons per acre for all oil shale zones in the Piceance Basin..............................................5

3. Cross section A-A' showing oil-yield histograms, members of the Eocene Green 

River Formation, correlation of rich and lean oil shale zones of Cashion and 

Donnell, stages in the evolution of Lake Uinta, and the informal phases of Lake 

Uinta used for convenience here...............................................................................................8

4. East-west cross section B-B' through the Piceance Basin, the Douglas Creek 

arch, and the eastern part of the Uinta Basin showing members, stages of Lake 

Uinta as defined by Johnson, and some of the rich and lean zones defined by 

Cashion and Donnell ..................................................................................................................10

5. North-south cross section C-C' through the Piceance Basin, northwest Colorado, 

showing members, stages of Lake Uinta as defined by Johnson, and some of the 

rich and lean zones defined by Cashion and Donnell...........................................................12

Plates

1. Detailed west to east structural cross section across the Piceance Basin ................. link

2. Detailed north to south structural cross section across the Piceance Basin .............. link
iv

6. Drill core containing white, coarse crystals of nahcolite, small nodules of 

nahcolite, and dark-brown nahcolite aggregates of bladed crystals in oil shale ...........14

7. Drill core containing alternating beds of light-brown nahcolite and oil shale.................14

8. Drill core containing alternating beds of deeply dissolved halite and brown 

resistant wafer-like nahcolite...................................................................................................14

9. Geohydrologic cross section D-D' through the Piceance Basin showing major 

aquifers as defined by Weeks and others..............................................................................17

10. Map showing locations of coreholes and surface sections used in this 

investigation ................................................................................................................................18

11. Map showing elevations of the highest occurrence of nahcolite and the lowest 

evidence of leaching of the saline interval ............................................................................22

12. Structure contour map based on the lowest evidence of leaching and structure 

contour map based on the top of the Mahogany oil shale zone.........................................24

13. West-to-east schematic cross section across the Piceance Basin for three time 

periods..........................................................................................................................................27

14. Map of the Piceance Basin at the end of the illitic phase showing extent of 

offshore and marginal lacustrine facies at the end of the freshwater phase that 

pre-dated the illitic phase of Lake Uinta, extent of marginal lacustrine shelves 

at the end of the illitic phase, thickness of the illitic phase, from the base of the 

Long Point Bed to the top of the L-1 zone, limits of saline mineral deposition during 

each of the four oil shale zones that compose the early saline mineral phase, and 

approximate line of section for schematic cross section shown in figure 13..................28

15. Photographs of slabbed core and mine rock.........................................................................31

16. Photograph of oil shale with clasts of laminated marlstone...............................................31

17. Paleogeographic map of the Uinta and Piceance Basins at the beginning of the 

saline phase showing where nahcolite was deposited, where oil yields are greater 

than and less than 10 gallons per ton, and extent of marginal lacustrine and fluvial 

deposition.....................................................................................................................................32

18. Map of the Piceance Basin at the end of the illitic phase showing extent of 

marginal lacustrine facies at the end of the freshwater phase that pre-dated the 

illitic phase of Lake Uinta, thickness of the illitic phase, and where the R-2, L-2, 

R-3, and L-3 oil shale zones are the thickest..........................................................................34

19. Map of the R-2 oil shale zone showing where nahcolite was deposited and total 

in-place nahcolite, where oil yields are greater than and less than 10 gallons per t

on, and extent of marginal lacustrine deposition..................................................................36

20. Map of the L-2 oil shale zone showing where nahcolite is present and total inplace nahcolite, where nahcolite has been leached or partially leached out by 

groundwater, where oil yields are greater than and less than 10 gallons per ton, 

and extent of marginal lacustrine deposition ........................................................................38

21. Map of the R-3 oil shale zone showing where nahcolite is present and total inplace nahcolite, where nahcolite has been leached or partially leached out by 

groundwater, where oil yields are greater than and less than 10 gallons per ton, 

and extent of marginal lacustrine deposition ........................................................................40

22. Map of the L-3 oil shale zone showing where nahcolite is present and total inplace nahcolite, where nahcolite has been leached or partially leached out by 

groundwater, where oil yields are greater than and less than 10 gallons per ton, 

and extent of marginal lacustrine deposition ........................................................................42

23. West-to-east schematic cross section across the Piceance Basin for two time 

periods..........................................................................................................................................44
v

24. Map of the R-4 oil shale zone showing where nahcolite is present and total inplace nahcolite, where nahcolite has been leached or partially leached out by 

groundwater, where oil yields are greater than and less than 10 gallons per ton, 

extent of marginal lacustrine deposition, and limit of marginal lacustrine facies at 

the end of the illitic phase .........................................................................................................46

25. Map of the L-4 oil shale zone showing where nahcolite is present and total inplace nahcolite, where nahcolite has been leached or partially leached out by 

groundwater, where oil yields are greater than and less than 10 gallons per ton, 

extent of marginal lacustrine deposition, and limit of marginal lacustrine facies 

at the end of the illitic phase.....................................................................................................48

26. Map of the R-5 oil shale zone showing where nahcolite is present and total inplace nahcolite, where nahcolite has been leached or partially leached out by 

groundwater, where oil yields are greater than and less than 10 gallons per ton, 

extent of marginal lacustrine deposition, limit of marginal lacustrine facies at the 

end of the illitic phase, and approximate area where halite is present............................50

27. Map of the L-5 oil shale zone showing where nahcolite is present and total inplace nahcolite, where nahcolite has been leached or partially leached out by 

groundwater, where oil yields are greater than and less than 10 gallons per ton, 

extent of marginal lacustrine deposition, limit of marginal lacustrine facies at the

end of the illitic phase, and approximate area where halite is present............................54

28. Map of the R-6 oil shale zone showing where nahcolite has been leached out by 

groundwater, where oil yields are greater than and less than 10 gallons per ton, 

extent of marginal lacustrine deposition (modified from Johnson, 1985), and limit 

of marginal lacustrine facies at the end of the illitic phase ................................................56

29. Map of B-groove oil shale zone showing where nahcolite has been leached out 

by groundwater, where oil yields are greater than and less than 10 gallons per ton, 

and limit of marginal lacustrine facies at the end of the illitic phase................................58

30. Map of the Mahogany oil shale zone showing where nahcolite has been leached 

out by groundwater, where oil yields are greater than and less than 10 gallons per 

ton, total oil yield for the Mahogany zone in barrels per acre, and limit of marginal 

lacustrine facies at the end of the illitic phase......................................................................60

31. Map of A-groove oil shale zone showing where nahcolite has been leached out 

by groundwater, where oil yields are greater than and less than 10 gallons per 

ton, limited area in the northern part of the basin where marginal lacustrine 

rocks were identified, and limit of marginal lacustrine facies at the end of the 

illitic phase...................................................................................................................................62

32. Map from the beginning of deposition of the A-groove to the top of bed 44 of 

Donnell showing where nahcolite has been leached out by groundwater, where 

oil yields are greater than and less than 10 gallons per ton, thickness of the Agroove to top of bed 44 interval, and limit of marginal lacustrine facies at the end 

of the illitic phase........................................................................................................................64

33. Map from the top of bed 44 to the top of the oil shale interval showing where 

nahcolite vugs occur above bed 44, extent of marginal lacustrine deposition at 

top of bed 44, total oil in place in the B-groove to bed 44 interval, and limit of 

marginal lacustrine facies at the end of the illitic phase.....................................................66

34. Cross section showing correlation of individual oil shale beds between two 

coreholes for the interval above A-groove ............................................................................68

35. Paleogeographic map of the Uinta and Piceance Basins at the end of the Agroove to bed 44 interval showing where nahcolite vugs occur in both basins, 

where oil yields are greater than and less than 10 gallons per ton, and areas 

of marginal lacustrine deposition ............................................................................................70
vi

Tables

1. List of coreholes used in this study .........................................................................................19

2. List of surface sections used in this study .............................................................................21
vii

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

 Length

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

 Area

acre 0.4047 hectare (ha)

 Volume

barrel (bbl), (petroleum, 

1 barrel=42 gal)

 0.1590 cubic meter (m3) 

gallon (gal) 3.785 liter (L) 

 Mass

ton, short (2,000 lb) 0.9072 megagram (Mg) 

SI to Inch/Pound

Multiply By To obtain

 Length

millimeter (mm) 0.03937 inch (in.)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32
 
Development, Evolution, and Destruction of the Saline 

Mineral Area of Eocene Lake Uinta, Piceance Basin, 

Western Colorado

By Ronald C. Johnson and Michael E. Brownfield

Abstract

Halite and the sodium bicarbonate mineral nahcolite were 

deposited in Eocene-age saline Lake Uinta in the Piceance 

Basin, northwestern Colorado. Variations in the areal extent 

of saline mineral deposition through time were studied using 

descriptions of core and outcrop. Saline minerals have been 

extensively leached by groundwater, and the original extent 

of saline deposition was determined from the distribution of 

empty vugs and collapse breccias. Because vugs and breccias 

strongly influence groundwater movement, determining where 

leaching has occurred is an important consideration for in-situ 

oil shale extraction methods currently being developed.

Lake Uinta formed when two much smaller freshwater 

lakes, one in the Uinta Basin and the other in the Piceance 

Basin, expanded and coalesced across the Douglas Creek 

arch. Early Lake Uinta inherited much of the topography in 

the freshwater lake and surrounding alluvial plains that preceded it, and the deep central lake area of Lake Uinta developed largely over the offshore area of the freshwater lake. A 

prolonged period of infilling followed the formation of Lake 

Uinta, creating broad lake-margin shelves prior to the onset 

of saline mineral deposition. These shelves almost certainly 

played a critical role in the evolution of the brine layer that 

accumulated in the deep central lake area. Broad marginal 

shelves formed as well around the Uinta Basin part of Lake 

Uinta. Brines that formed on those shelves also probably 

migrated to the deep central lake area in the Piceance Basin, as 

no saline minerals were deposited in the Uinta Basin until late 

in the history of Lake Uinta.

Oil shale in the deep central lake consists of interbedded

laminate oil shale beds that originated within the deep lake 

area, and blebby and streaked oil shale beds that were transported into the deep lake area by sediment gravity flows. 

Blebby and streaked oil shale beds contain carbonate clasts 

and siliciclastic clasts similar to lithologies found on the 

marginal shelves. It is possible that these clasts and the highly 

saline brines that evolved on the marginal shelves were incorporated into the same gravity flows.

Saline mineral deposition is informally subdivided into 

early, middle, and late phases. During the early phase, nahcolite and rich oil shale were deposited in the deep central lake 

area, and carbonate-rich sandstone, siltstone, and mudstone 

and ostracodal, oolitic, algal limestone were deposited on the 

marginal shelves. During the middle saline mineral phase, 

oil shale deposition gradually expanded across the marginal 

shelves, and by the end of that phase, oil shale deposition covered almost all of the former shelf areas. An increasing flow of 

water from Lake Gosiute to the north, as it was gradually filled 

in by volcaniclastics, may have caused this expansion. Saline 

mineral deposition also expanded during the middle saline 

mineral phase, reaching to near the former shelf break by the 

end of that phase. This suggests that the former shelf break 

remained a topographic feature that confined the deep saline 

brine layer throughout the middle phase.

By the beginning of the late saline mineral phase, Lake 

Gosiute had been completely filled in and volcaniclastic debris 

reached the northern shore of Lake Uinta. This initiated a 

north-to-south infilling of the Piceance Basin part of Lake 

Uinta that progressively pushed the saline mineral area southward and ultimately onto the former marginal shelf areas in 

the southern part of the basin. A saline mineral area formed for 

the first time in the eastern part of the Uinta Basin during this 

infilling and, for a time, saline minerals were deposited in both 

basins. By the end of the late saline phase, the Piceance Basin 

part of Lake Uinta was filled in and saline mineral deposition 

shifted entirely into the Uinta Basin.

Leaching of saline minerals began sometime after the 

Green River Formation was lithified enough to allow collapse 

breccias to form. Leaching is ongoing today, indicated by the 

discharge of highly saline water from a series of springs in the 

northern part of the basin. Groundwater invasion and saline 

mineral dissolution is commonly incomplete in areas that lack 

fractures, leaving behind pockets of unleached saline minerals 

in otherwise leached intervals. Today, the base of the leached 

zone slopes toward the north and toward the area where the 

brines are being discharged.
2 Development, Evolution, and Destruction of Saline Mineral Area of Eocene Lake Uinta

Introduction

The Eocene Green River Formation in Colorado, Wyoming, and Utah was deposited in two large lakes: (1) Lake 

Uinta in the Piceance Basin of western Colorado and the 

Uinta Basin of eastern Utah and westernmost Colorado, 

and (2) Lake Gosiute in the Greater Green River Basin in 

southwest Wyoming and northwest Colorado (fig. 1). The 

Green River Formation contains the largest in-place oil shale 

resource in the world, recently estimated at 1.53 trillion 

barrels of oil for the Piceance Basin (fig. 2) (Johnson and 

others, 2010a), 1.32 trillion barrels of oil for the Uinta Basin 

(Johnson and others, 2010b), and 1.44 trillion barrels of oil for 

the Greater Green River Basin (Johnson and others, 2011). It 

must be emphasized that only a small fraction of this in-place 

resource is of a thickness, grade, and depth favorable for 

extraction.

During the long history of Lake Uinta, as much as 

8 million years in the Uinta Basin where the lake persisted 

the longest (Smith and others, 2008), the lake remained 

saline and did not have an outlet. Lake Gosiute, in contrast, 

evolved from a highly saline lake to a freshwater lake late 

in its history once drainage southward into Lake Uinta was 

established. Saline minerals were deposited in both lakes, with 

trona (Na3H(CO3)2•2H2O) and halite (NaCl) deposited in Lake 

Gosiute and nahcolite (NaHCO3) and halite deposited in Lake 

Uinta. Nahcolite resources in the Piceance Basin were recently 

estimated at 43.3 billion short tons (fig. 2) (Brownfield and 

others, 2010a). Several companies currently mine underground 

beds of trona in Wyoming, and one company solution mines 

a thick bed of nahcolite in Colorado. Saline minerals in 

the Uinta Basin were recently studied by Brownfield and 

others (2010b).

Nahcolite and halite deposits in the Green River Formation of the Piceance Basin were once much more extensive, 

both vertically and laterally, than they are today. The progressive penetration of groundwater into the Green River Formation has increasingly encroached upon the saline interval, 

leaving behind a leached zone estimated by Brownfield and 

others (2010a) to be as much as 580 feet (ft) thick. Evidence 

for leaching includes cavities, collapse breccias, and open 

fractures that form the main source of secondary porosity 

and permeability in the oil shale interval (Weeks and others, 

1974). Groundwater moves freely though this highly porous 

and permeable zone, whereas groundwater has not yet reached 

intervals that still contain large amounts of intact nahcolite 

and halite. Understanding groundwater movement is critical 

to the in-situ methods for the extraction of oil from oil shale 

currently being developed. Some methods are designed to 

work only in oil shale intervals where groundwater movement is negligible (Day, 2011; Janicek, 2011, 2012), whereas 

others are designed to work within groundwater-bearing 

zones (Deeg and others, 2007, 2009). Nahcolite (NaHCO3) 

decomposes to natrite (Na2CO3) or soda ash, carbon dioxide, 

and water at temperatures near 100 °C or significantly below 

retort temperatures; thus, large quantities of carbon dioxide, 

a greenhouse gas, would be released by any nahcolite in the 

retorted interval. Extracting nahcolite prior to in-situ retorting 

is thus critical (Janicek, 2012).

The oil shale section in the Piceance Basin is subdivided 

into 17 rich and lean oil shale zones (fig. 3) representing alternating periods of high and low organic productivity and (or) 

preservation and thus closely representing time-stratigraphic 

intervals (Donnell and Blair, 1970; Cashion and Donnell, 1972; 

Donnell, 2008). The recently completed assessment of inplace oil shale in the Piceance and Uinta Basins (Johnson and 

others, 2010 a, b) assessed each of these 17 zones separately. 

Thirteen of these zones in the Piceance Basin contain saline 

minerals or evidence that saline minerals were once present. 

A separate map was generated for each zone displaying where 

saline minerals are present and where there is evidence that 

saline minerals were once present. Three maps were generated 

for the uppermost oil shale zone, which is unusually thick and 

complex. The advantage of using the time-stratigraphic rich 

and lean zone stratigraphy is that it allows for the tracking of 

overall depositional changes in Lake Uinta through time. A disadvantage is that individual zones can be quite thick and thus 

represent a considerable period of time. Thus, important events 

that occurred within an individual zone, such as a short-term 

expansion or contraction of the lake, are not captured, although 

they may be very important to understanding the evolution of 

the saline zone. In addition, two detailed structural cross sections that use sea level as a datum were constructed to better 

track the penetration of groundwater into the oil shale interval. 

The focus of this paper is thus twofold: (1) to describe the 

development and evolution of the saline mineral facies in the 

Piceance Basin through time, and (2) to delineate the extent of 

the leaching of this saline interval in order to better understand 

groundwater movement in the basin.

The Development of Eocene Lake Uinta 

and Contemporary Lake Gosiute

The Eocene Green River Formation of the Piceance Basin 

was deposited in lower to middle Eocene time in Lake Uinta, 

a large saline lake that extended across the Piceance Basin, the 

Uinta Basin to the west, and the intervening Douglas Creek 

arch. The Uinta and Piceance Basins are structural and sedimentary basins formed during the Laramide orogeny, a major 

tectonic event that affected the central Rocky Mountain region 

from Late Cretaceous through Eocene time. The Douglas Creek 

arch was an area with relatively low subsidence rates throughout the Paleocene and Eocene prior to the development of Lake 

Uinta, as pre–Lake Uinta lower Tertiary strata thin and largely 

wedge out on both flanks of the arch. Lake Uinta strata, in 

contrast, extend across the arch unbroken (fig. 4). To the north, 

Lake Gosiute occupied much of the Greater Green River Basin 

of southwest Wyoming and part of northwest Colorado in 

lower to middle Eocene time (fig. 1). The two lakes may have 

been connected across the intervening Axial arch (fig. 1) for a 
The Development of Eocene Lake Uinta and Contemporary Lake Gosiute 3

Oil shale–bearing rocks deposited in Eocene Lake Gosiute

EXPLANATION

Oil shale–bearing rocks deposited in Eocene Lake Uinta 

U.S. Geological Survey Uinta-Piceance Province boundary

U.S. Geological Survey Southwesten Wyoming Province boundary Figure 1. (Caption on following page.)

0 25 50 100 KILOMETERS

0 25 50 75 100 MILES

75

Green River

Colorado River

White River

Yampa 

Green 

 River

Utah

Lake

Great

Salt

Lake

Colorado

River

River

WYOMING

IDAHO

UTAH

COLORADO

Green River Basin

Hoback Basin

Washakie Basin

P i c e a n c e 

U i n t a B a s i n 

Sand Wash Basin

Great Divide Basin G r e a t e r

G r e e n

R i v e r

Basin

Rock

Springs uplift

Wamsutter 

Cherokee Ridge

Sandy Bend

arch

White River

uplift

San

Rafael

Swell

Uinta uplift

Wyoming thrust belt

Wind

River

uplift

Sierra

Madre

Uncompahgre

uplift

Sawatch

uplift

Sevier

orogenic

belt

Park

Range

Axial arch

arch

Rawlins

uplift

82

789

70

6

50

70

40

30

26 20

89

25

80

191

6

30

50

89

40

15

191

15

15

84

80

30

191

287

189

91

189

189

112° 111° 110° 109° 108° 107°

43°

42°

41°

40°

39°

Eagle

Glenwood

Springs

Castledale

Aspen

Grand

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

Nephi

Meeker

Manti

Green River

Rock

Springs

Rangely

Salt Lake City

Location of

figure 4

Douglas Creek arch

A A’

B a s i n

UV139
4 Development, Evolution, and Destruction of Saline Mineral Area of Eocene Lake Uinta

relatively brief time shortly after they formed (Johnson, 1985, 

p. 272; Roehler, 1993, figs. 50, 52), but this is uncertain. There 

is, however, considerable evidence that Lake Gosiute drained 

southward into Lake Uinta late in its history (Roehler, 1974; 

Surdam and Stanley, 1979, 1980; Johnson, 1985, 2007; Smith 

and others, 2008). This southward drainage had a profound 

impact on Lake Uinta.

Lake Uinta formed when two much smaller freshwater 

lakes, one each in the Uinta and Piceance Basins, expanded 

and coalesced across the intervening Douglas Creek arch 

(figs. 1, 4) (Johnson, 1984, 1985). Freshwater mollusks, found 

throughout strata deposited during the earlier freshwater 

lacustrine phase, are abundant in the Long Point Bed of the 

Green River Formation (fig. 4), the basal transgressive bed of 

Lake Uinta. Mollusks, however, are absent or nearly absent 

in strata above, suggesting that salinity began to increase in 

Lake Uinta shortly after maximum transgression was reached. 

Stromatolites appear for the first time immediately above the 

Long Point Bed (Johnson and others, 1988, measured sections 

6 and 9). Stromatolites will generally not form in the presence 

of mollusks, as they graze on the algal mats that create them. 

The abundance of freshwater mollusks in the Long Point Bed 

suggests a mass mortality event, possibly triggered by a slight 

increase in salinity. Freshwater mollusks never returned to 

Lake Uinta, and stromatolites remained common in marginal 

facies throughout the history of the lake, indicating that Lake 

Uinta remained internally drained and saline. Freshwater 

mollusks, however, do reappear late in the history of Lake 

Gosiute, indicating that it evolved from an internally drained 

lake to an externally drained lake as southward drainage into 

Lake Uinta was established.

The creation of a large internally drained or closed saline 

lake from much smaller, externally draining freshwater lakes 

is unusual and is not fully understood. Recent examples of 

freshwater lakes evolving into saline lakes, such as Pleistocene 

Lake Bonneville, did so after a period of contraction caused 

the lake level to drop below the spill point, thus eliminating 

its outlet. Johnson (1985, p. 271–272) suggested there was 

one spill point for the freshwater lakes in the Piceance and 

Uinta Basins and that this spill point gradually rose, possibly because of tectonic uplift, causing the lakes to expand 

and coalesce. The expansion was ultimately halted by the 

increased evaporation from the newly enlarged surface area 

of the lake. The outlet of the lake could have subsequently 

been lost if the spill point continued to rise once maximum 

expansion was reached. This hypothesis could also explain 

how a thick interval of Lake Uinta sediments could have been 

deposited over the Douglas Creek arch (fig. 4), a hinge line 

that had previously accumulated very little sediment during 

early Tertiary time, as subsidence would no longer be required 

to produce accommodation space. Lake level could rise freely 

in the basin provided a spill point was not reached.

Subdividing the Green River Formation 

into Members, Rich and Lean Zones, 

and Stages

The Green River Formation has been divided variously 

into (1) members, based on lithology; (2) stages, based on the 

evolution of the lake; and (3) “rich” and “lean” oil shale zones, 

representing approximately time-stratigraphic intervals of 

alternating high and low organic productivity and (or) preservation (figs. 3–5). These various schemes will be discussed 

only briefly here. For more complete summaries see Johnson 

and others (2010 a, b), Johnson (2012), and Mercier and 

Johnson (2012).

Five major members of the Green River Formation are 

now widely used (figs. 4, 5): (1) Cow Ridge Member (Johnson,

1984), applied to the freshwater lacustrine interval that 

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. Location of cross section A-A‘ (see fig. 3) is shown in red. Subbasins in the Greater Green River Basin 

are 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 (USGS Uinta-Piceance Assessment Team, 2003). Extent of 

the Greater Green River Basin is the same as the Southwestern Wyoming Province boundary (USGS Southwestern 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 was 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 basin, the top of the 

Mahogany bed 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 feet or less is shown; this area was outlined 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 the southeastern part of the 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 Green River Basin, the base of the Wilkins Peak Member of the Green 

River Formation was used, and for the northern part of the Green River Basin, the base of the Laney Shale Member of the Green River 

Formation was used, both as mapped by Love and Christiansen (1985).
Subdividing the Green River Formation into Members, Rich and Lean Zones, and Stages 5

Figure 2. Maps showing (A) total in-place oil in barrels per acre (from Johnson, 2012) and (B) total 

in-place nahcolite in tons per acre (modified from Brownfield and others, 2010a) for all oil shale 

zones in the Piceance Basin. Location of highly saline springs (red circles) and zone of faulting and 

fracturing (blue area) in the northern part of the basin are also shown (modified from Weeks and 

others, 1974; Robson and Saulnier, 1981). Location of cross section D-D‘ (see fig. 9) is shown in black. 

(Continued on next page.)

108°30' 108°0'

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

<= 1,000,000

A B

Top of Mahogany zone

T.

2

N.

0 3 6 MILES

0 3 6 KILOMETERS

3,000,001 to 4,000,000

2,000,001 to 3,000,000

1,000,001 to 2,000,000

≤1,000,000

Base of Parachute Creek Member

EXPLANATION

In-place oil, in barrels per acre, 

for all zones except bed 44

40°00'

108°30' 108°00'

39°30'

Study area

A

R. 96 W. R. 95 W.

R. 101 W. R. 100 W. R. 99 W. R. 98 W. R. 97 W. R. 94 W. R. 93. W

T.

1

 N.

T.

1

 S.

T.

2

 S.

T.

3

 S.

T.

4

 S.

T.

5

 S.

T.

6

 S.

T.

7

 S.

T.

8

 S.

T.

9

 S.
6 Development, Evolution, and Destruction of Saline Mineral Area of Eocene Lake Uinta

Top of Mahogany zone

Base of Parachute Creek Member

Extent of nahcolite

oo0

T.

2

N.

0 3 6 MILES

0 3 6 KILOMETERS

EXPLANATION

0 to 100,000

100,000.1 to 250,000

250,000.1 to 500,000

500,000.1 to 750,000

> 750,000

Nahcolite content, in tons per acre

Study area

D’

D

Piceance

Creek

Creek

Creek

Creek

Yellow

Para chute

Roan

Colorado River

White River

B 108°30’ 108°00’

40°00’

39°30’

R. 96 W. R. 95 W.

R. 101 W. R. 100 W. R. 99 W. R. 98 W. R. 97 W. R. 94 W. R. 93. W

T.

1

 N.

T.

1

 S.

T.

2

 S.

T.

3

 S.

T.

4

 S.

T.

5

 S.

T.

6

 S.

T.

7

 S.

T.

8

 S.

T.

9

 S.

Figure 2—Continued. Maps showing (A) total in-place oil in barrels per acre 

(from Johnson, 2012) and (B) total in-place nahcolite in tons per acre (modified from 

Brownfield and others, 2010a) for all oil shale zones in the Piceance Basin. Location 

of highly saline springs (red circles) and zone of faulting and fracturing (blue area) in 

the northern part of the basin are also shown (modified from Weeks and others, 1974; 

Robson and Saulnier, 1981). Location of cross section D-D‘ (see fig. 9) is shown in black.
Nahcolite and Halite in the Piceance Basin 7

predates the Long Point transgression and the formation of 

saline Lake Uinta; (2) Garden Gulch Member (Bradley, 1931), 

applied to the illite-rich oil shale interval deposited early 

in the history of Lake Uinta and prior to the onset of saline 

mineral deposition; (3) Parachute Creek Member (Bradley, 

1931), applied to the dolomite-rich oil shale interval deposited 

during saline mineral deposition; (4) Douglas Creek Member 

(Bradley, 1931), applied to marginal lacustrine rocks around 

the west margins of Lake Uinta; and (5) Anvil Points Member 

(Donnell, 1953), applied to marginal lacustrine rocks around 

the east margin. The Douglas Creek and Anvil Points Members are the marginal equivalents of both the Garden Gulch 

and Parachute Creek Members. The name Uinta Formation is 

applied to a sequence of sandstones and siltstones containing 

abundant volcanic clasts that interfingers with the upper part 

of the Green River Formation (Dane, 1954; Cashion and 

Donnell, 1974). The Uinta Formation represents the infilling 

stage of that part of Lake Uinta that occupied the Piceance 

Basin (figs. 4, 5).

Johnson (1985) subdivided the history of Lake Uinta 

into five roughly time-stratigraphic periods or stages with the 

change from one stage to the next corresponding to significant changes in conditions in the lake (figs. 3–5). Stages 1 

and 2 are equivalent to the illitic Garden Gulch Member, and 

stages 3–5 are generally equivalent to the dolomitic Parachute 

Creek Member (fig. 3, pl. 1). Johnson (1985) cited evidence of 

increasing salinity during the first two stages, culminating in 

the deposition of large quantities of nahcolite and halite during 

stages 3 and 4. Stage 5 of Johnson (1985) begins with a major 

transgression, probably caused by an increase in outflow from 

Lake Gosiute to the north followed fairly closely by an influx 

of large quantities of volcaniclastic sediments from volcanic 

centers in northwest Wyoming and possibly Idaho (Carroll and 

others, 2008).

Recently, Tanavsuu-Milkeviciene and Sarg (2012) used 

somewhat different criteria to subdivide the Green River Formation in the Piceance Basin into six stages: (1) Stage 1 (R-0 

through R-1 zones), which they considered to be freshwater; 

(2) Stage 2 (L-1 through L-3 zones) or transitional lake, when 

the lake became highly saline; (3) Stage 3 (R-4 through L-5 

zones), a period when lake levels were highly fluctuating; 

(4) Stage 4 (R-6 and B-groove) or rising lake stage; (5) Stage 5 

(Mahogany zone) or high lake stage; and (6) Stage 6 (above 

Mahogany zone) or closing lake stage.

The oil shale interval in the Piceance Basin has been 

additionally subdivided into eight oil-rich zones in ascending 

order, R-0 through R-6 and the Mahogany zone (Mahogany 

ledge in outcrop), and seven oil-lean zones in ascending order, 

L-0 through L-5, B-groove, and A-groove (fig. 3). These 

zones represent alternating periods of high and low organic 

productivity and (or) preservation in Lake Uinta, representing 

roughly time-stratigraphic intervals. The rich and lean zone 

architecture formed the basis for recent resource assessments 

of both the Piceance and Uinta Basins, with each rich and lean 

zone being assessed separately (Johnson and others, 2010 a, 

b). These zones also form the basis of this study, in which the 

extent of saline minerals and leaching was determined for each 

rich and lean zone within the saline mineral interval. Cashion 

and Donnell (1972) originally defined the rich and lean zones 

in the Piceance Basin and demonstrated that zones from R-4 to 

the top of the oil shale interval could be traced into the eastern 

part of the Uinta Basin as well. Recently, Johnson and others 

(2010b) were able to trace all the rich and lean zones below 

the R-4 zone into their equivalent intervals in at least the 

easternmost part of the Uinta Basin. A significant interval of 

moderate to rich oil shale also occurs above A-groove, in 

the interval that intertongues with southward-prograding 

volcaniclastic wedges of the Uinta Formation (figs. 3–5). 

Donnell (2008) recently traced a large number of individual oil 

shale beds in this interval throughout the offshore lacustrine 

area in the Piceance Basin and eastern Uinta Basin. The lower 

part of this interval, from the top of A-groove to the top of bed 

44 of Donnell (2008) (fig. 3), was assessed in the Piceance 

Basin by Johnson and others (2010a).

Nahcolite and Halite in the Piceance 

Basin

At present, nahcolite in the Piceance Basin is confined to 

a relatively restricted area in the north-central part of the basin 

(fig. 2) in the stratigraphic interval from the uppermost part 

of the R-2 zone through the lowermost part of the L-5 zone. 

Halite is confined to the R-5 and L-5 zones in an even more 

limited area in the north-central part of the basin (Smith and 

Young, 1969; Young and Smith, 1970; Dyni and others, 1971 

a, b; Smith and others, 1972; Beard and others, 1974; Dyni, 

1974 a, b, c; Brownfield and others, 2010a). Nahcolite occurs 

as (1) crystalline aggregates, (2) laterally continuous units 

of fine-grained disseminated crystals in oil shale, (3) brown 

microcrystalline beds, and (4) white coarse-grained beds 

(figs. 6–8). Individual nahcolite nodules range from less than 

1 inch (in.) to several feet in diameter, but most nodules range 

in diameter from 1 to 3 in. (Beard and others, 1974; Dyni, 

1974 b, c). Although nodular nahcolite is not bedded, nodules do appear to be concentrated in discrete zones parallel 

to bedding (Dyni, 1974c, fig. 10). Nodules formed while the 

encasing oil shale was still soft, as lamina in the oil shale beds 

bend around them. Dyni (1981, p. 123–124) suggested that 

the nodules began to form in the upper meter or so of sediment during the initial stages of dewatering in areas where 

overlying lake waters were not quite concentrated enough to 

precipitate nahcolite directly. Dyni (1974c) also believed that 

disseminated nahcolite precipitated at or near the sedimentwater interface when lake waters were at or near saturation, 

with precipitation keeping pace with oil shale deposition. 

Disseminated nahcolite intervals have sharp to gradational 

contacts with adjacent oil shale beds and range in thickness 

from a few feet to 23 ft (Dyni, 1974c, p. 116). Beds of microcrystalline brown nahcolite have a texture and appearance of 

brown sugar, get their color from small amounts of organic 
0 20 40 60 80 100

C0195

Arco-Mobil-Equity,

Figure Four 31–1 

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

500

1,000

1,5002,000

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

0 20 40 60 80 100

0 20 40 60 80

500

1,000

1,5002,000

0

Mahogany bed

0 20 40 60 80 100

0 20 40 60 80 100

C0158

Shell Oil Co.

Greeno 1–4

sec. 4, T. 3 S., R. 97 W.

500

1,0001,5002,0002,500

3,000

 U–53

Skyline Oil Co., Watson 1A

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

100

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

Garden Gulch Member Parachute Creek Member

Green River Formation

Oil yield (gallons per ton)

Oil yield (gallons per ton)

Oil yield (gallons per ton)

Oil yield (gallons per ton)

Base Porcupine

Creek Tufftop Bed 44

Porcupine Cr. tuff

Top bed 76-top

Porcupine Creek

tuff

Stage 1 Stage 2 Stage 3 Stage 4 Stage 5 

DATUM

Lon

A

WEST
Nahcolite and Halite in the Piceance Basin 9

Figure 3 (previous page). Cross section A-A‘ showing (1) oil-yield histograms (red graphs), (2) members of the Eocene Green River 

Formation, (3) correlation of rich and lean oil shale zones of Cashion and Donnell (1972), (4) stages in the evolution of Lake Uinta, and 

(5) the informal phases of Lake Uinta used for convenience here. Corehole U-53 is in the northeastern part of the Uinta Basin, corehole 

C0172 is in the southern part of the Piceance Basin, and coreholes C0195 and C0158 are in the central part of the Piceance Basin. 

See figure 1 for location of cross section.

Volcaniclastic rocks 

Mostly oil shale with some volcaniclastic

material

Carbonate-rich oil shale (>10 gallons per ton) 

Carbonate-rich oil shale (<10 gallons per ton) 

Clay-rich oil shale (>10 gallons per ton) 

Clay-rich oil shale (<10 gallons per ton) 

Sandstone, siltstone and mudstone

EXPLANATION
 Land surface Stage 2 Stage 1 Stage 3 Stage 4 Stage 4 R-0 and L-0 zones 

FEET METERS 2,000 1,000

0 0

600400200

Douglas Creek

Member

Wasatch

Formation

Green River

Formation

Black Sulfur Tongue

of Green River Formation

Tongue of

Uinta Formation

Intersection with

cross section C-C’

Base of

R-4 rich

oil-shale

zone

Parachute Creek

Member of Green

River Formation

Garden Gulch

Member

Cow Ridge

Member of

Green River

Formation

Tongue of

Uinta Formation

Tongue of

Uinta Formation

Thirteenmile Creek

Tongue of Green

River Formation

W

F

Fort Union

Formation

Wa

Fort 

For

co

Douglas Creek arch

Piceance Basin

Uinta Basin

00 10 20 30 KILOMETERS

10 20 30 MILES

West

B

Mahogany zone

Cretaceous rocks

Cretaceous rocks

R-1 and L-1 zones

R-2 through L-3 zones

B-marker

LOCATION MAP

1

2

3

4

B

B'

VERTICAL EXAGGERATION X 40
Nahcolite and Halite in the Piceance Basin 11

Volcaniclastic rocks 

Mostly oil shale with some 

volcaniclastic material

Carbonate-rich oil shale (>10 gallons per ton) 

Carbonate-rich oil shale (<10 gallons per ton) 

Clay-rich oil shale (>10 gallons per ton) 

Clay-rich oil shale (<10 gallons per ton) 

Freshwater lacustrine oil shale

Sandstone-rich marginal lacustrine

Carbonate-rich marginal lacustrine

Marginal freshwater lacustrine

Paludal

Fluvial and alluvial

EXPLANATION

Figure 4 (previous page). East-west cross section B-B‘ through the Piceance Basin, the Douglas Creek arch, and the eastern 

part of the Uinta Basin showing members, stages of Lake Uinta as defined by Johnson (1985), and some of the rich and lean zones

defined by Cashion and Donnell (1972). The illitic phase (labeled 1) and the early, middle, and late saline mineral phases (labeled 

2–4) are outlined with heavy black lines.
R-2 through L-3 zones R-1 and L-1 zones R-0 and L-0 zones L-5 zone Mahogany zone R-5 and L-4 zones R-5 and L-4 zones R-4 zone Top of Mahogany oil shale zone Top of Cretaceous R0 and L0 

R1 zo

L1 zone R4, L4, R5 zones L5 zone R6 zone and R6 zone and B-groove B-groove Mahogany zone Uinta Formation U

Form

 WasatFormat

 Wasatch Formation Wasatch F

 Fort UnionFormation

Cow Ridge Member,Green River Formation

Parachute

Creek 

Member

DouglasCreek Member

Garden Gulch Member

ParachuteCreek Mbr.

Top of bed 44of Donnell (2008)

Intersection with cross section B-B’ Thirteenmile Creek Tongue

of Green River Formation

0 20,000 60,000 FEET

10 KILOMETERS

0

500

1,0001,500

400200

FEET METERS

VERTICAL EXAGGERATION X 40

LOCATION MAP

R-4 zone

C

SOUTH

B-marker

C-marker

D-marker

I-marker

2

C

C’

1

34
Nahcolite and Halite in the Piceance Basin 13

Volcaniclastic rocks 

Mostly oil shale with some 

volcaniclastic material

EXPLANATION

Carbonate-rich oil shale (>10 gallons per ton) 

Carbonate-rich oil shale (<10 gallons per ton) 

Clay-rich oil shale (>10 gallons per ton) 

Clay-rich oil shale (<10 gallons per ton) 

Freshwater lacustrine oil shale

Sandstone-rich marginal lacustrine

Carbonate-rich marginal lacustrine

Marginal freshwater lacustrine

Paludal

Major nahcolite and halite zones

Fluvial and alluvial

Figure 5 (previous page). North-south cross section C-C‘ through the Piceance Basin, northwest Colorado, showing members, 

stages of Lake Uinta as defined by Johnson (1985), and some of the rich and lean zones defined by Cashion and Donnell (1972). 

The illitic phase (labeled 1) and the early, middle, and late saline mineral phases (labeled 2–4) are outlined with heavy black lines.
14 Development, Evolution, and Destruction of Saline Mineral Area of Eocene Lake Uinta

Figure 8. Drill core containing alternating beds of deeply 

dissolved halite and brown resistant wafer-like nahcolite. 

Recovered core from the U.S. Bureau of Mines 01A (C0334) 

borehole (1,712.4 to 1,719.4 feet, R-5 oil shale zone). Core is stored 

at the U.S. Geological Survey’s Core Research Center, Denver, 

Colorado.

Figure 6. Drill core containing white, coarse crystals of 

nahcolite, small nodules of nahcolite, and dark-brown nahcolite 

aggregates of bladed crystals in oil shale. Some nahcolite 

aggregates are rimmed with pyrite. Recovered core from the U.S. 

Bureau of Mines 01A (C0334) borehole (1,578.4 to 1,585.2 feet, R-5 

oil shale zone). Core is stored at the U.S. Geological Survey’s Core 

Research Center, Denver, Colorado.

Figure 7. Drill core containing alternating beds of light-brown 

nahcolite and oil shale. White crystalline aggregate of nahcolite 

is located in right part of upper piece of core. Recovered core 

from the U.S. Bureau of Mines 01A (C0334) borehole (1,566.5 to 

1,574 feet, R-5 oil shale zone). Core is stored at the U.S. Geological 

Survey’s Core Research Center, Denver, Colorado.

impurities, and form laterally persistent beds from 0.5 to 12 ft 

thick. White, coarse-grained nahcolite beds have crystals from 

1 to 6 millimeters across and are relatively free of kerogen. Bed 

thicknesses generally range from less than 1 ft to as great as 

10 ft, but one nahcolite and halite bed is more than 60 ft thick. 

Six of the seven white nahcolite beds grade into mixed halite 

and nahcolite toward the center of the saline mineral area 

(Dyni, 1981, p. 42). Nahcolite aggregates formed diagenetically in soft sediments as the lamina in adjacent oil shale beds 

bend around the aggregates. Dyni (1981, p. 99) suggested that 

the saline mineral area was topographically complex, consisting of the main area in the eastern half of T. 1 S., R. 98 W. and 

two smaller areas, one to the north in T. 1 N., Rs. 97 and 98 W., 

and the other to the south in Tps. 2 and 3 S., R. 97 W. (fig. 2).

The evolution of the highly saline brines in Lake Uinta is 

controversial and not entirely understood, in large part because 

there is no modern analog (Smith, 1974, p. 71; Eugster and 

Hardie, 1978, p. 250). For discussions on the evolution of 

saline brines in Lake Uinta and Lake Gosiute see Bradley 

(1963), Smith (1974), Eugster and Hardie (1978), and Dyni 

(1974c, 1981). The importance of evaporation on lake-margin 

mud flats in producing concentrated brines has been stressed 

by many authors (Smith, 1974, p. 77; Ryder and others, 1976; 

Eugster and Hardie, 1978; Surdam and Stanley, 1979; Johnson, 1985, p. 272; Remy and Ferrell, 1989). Johnson (1985) 

demonstrated that these shallow lake-margin areas were tens 

of miles wide throughout much of the history of Lake Uinta. 

Remy and Ferrell (1989) proposed that evaporative pumping on exposed mud flats concentrated the moderately saline 

and alkaline lake waters, thereby producing Na-rich brines 

below the surface of the mud flats. According to Eugster and 

Hardie (1978, p. 245–246), complete evaporation on mud flats 

such as these produces effervescent crusts containing all the 

solutes previously present in the interstitial waters. Rainwater 

and dilute runoff would re-dissolve the most soluble salts, 

while the least soluble minerals, such as calcium-magnesium 

carbonates, would remain in the crusts. The carbonates could 

later be washed into the lake by flood waters. Other authors 

have argued that most of the carbonate found in the Parachute 

Creek Member precipitated directly from the lake waters 

(Dyni, 1974c, 1981; Smith, 1974).
Development of the Leached Zone in the Piceance Basin 15

Dyni (1981) suggested that the presence of nahcolite 

rather than trona is evidence that Lake Uinta was a deep, permanently stratified lake with a highly saline lower layer and 

a fresher upper layer as suggested by Bradley (1930, 1931, 

1936, 1948, 1963, 1964) rather than a shallow playa-lake 

complex suggested by Eugster and Hardie (1978). Bradley 

and Eugster (1969) indicate that sodium carbonate brines in 

contact with the atmosphere generally have bicarbonate quotients of about 0.15 and precipitate trona, as in Lake Gosiute 

to the north during deposition of the Wilkins Peak Member of 

the Green River Formation. The precipitation of nahcolite, in 

contrast, would require a much higher bicarbonate quotient of 

about 0.45, leading Dyni (1981, p. 108–109) to suggest that 

the brines in the Piceance Basin were not in contact with 

the atmosphere, thus supporting the permanently stratified 

lake model.

Development of the Leached Zone in 

the Piceance Basin

The “leached zone” is defined as that part of the Parachute Creek Member of the Green River Formation in which 

nahcolite and halite have been leached out by groundwater 

movement, leaving behind solution cavities of varying sizes, 

collapse breccias, and open fractures that can greatly increase 

porosity, permeability, and groundwater storage capacity (Hite 

and Dyni, 1967; Dyni and others, 1970; Beard and others, 

1974; Dyni, 1974c). Bradley (1931, pl. 5B) may have been 

the first to recognize the existence of the leached zone in the 

Piceance Basin when he noted large cavities that were once 

filled with radial aggregates of an unknown saline mineral 

in the Parachute Creek Member along lower Piceance Creek 

(fig. 2). Much later, after nahcolite had been discovered in the 

subsurface of the basin, Brobst and Tucker (1973, p. 6) speculated that the cavities along lower Piceance Creek probably 

once contained nahcolite. They (Brobst and Tucker, 1973) 

noted that the crystal cavities were in two intervals: the lower 

part of the Mahogany ledge and about 240 ft stratigraphically 

above the top of the ledge. Pipiringos and Johnson (1976) 

also noted solution cavities in the R-6 zone and B-groove in 

their measured section along lower Piceance Creek. Donnell 

(1961, p. 867) traced a persistent zone of solution cavities in 

the upper part of the Parachute Creek Member in measured 

sections along the Colorado River and its tributaries (fig. 1) in 

the southern part of the basin. Donnell (1961) noted that the 

solution cavities were from 280 to 345 ft above the Mahogany 

marker, a persistent tuff bed in the Mahogany ledge (fig. 3).

In general, the leached zone has been much less studied 

than the interval that still contains nahcolite and halite, 

largely because nahcolite is a leasable mineral. Dyni (1974c, 

p. 119), indicated that the extent of the leached interval had 

not been defined at the time of his study, but that solution 

breccias and cavities were present at least as high as A-groove 

(fig. 3). Trudell and others (1970) show vugs and collapse 

breccias extending stratigraphically upwards to near the top 

of the Parachute Creek Member in some of their plotted core 

descriptions from the central part of the basin. Day and others 

(2010, p. 6) noted that the permeability of the leached zone 

decreases towards the basin margins where the nahcolite facies 

“thins out or was not deposited.”

The leached zone is a major water-bearing zone in the 

Piceance Basin, with reported transmissivities as high as 

20,000 gallons of water per foot per day (Coffin and others, 

1970). Several thick collapse breccias have been traced into 

thick unleached beds of halite and nahcolite (Dyni, 1974c, 

1981). The base of the leached zone does not correspond to 

any one stratigraphic horizon (Dyni, 1974c, 1981; Brownfield 

and others, 2010a). In addition, small amounts of nahcolite 

have been noted above the lowest evidence of leaching in 

some areas (Beard and others, 1974, p. 102).

It is unclear when the dissolution began, but Dyni (1974c, 

p. 119) indicated that on the basis of the angularity of the collapse breccias and the preservation of open cavities, it must 

have started after the rocks were well lithified. Weeks and 

others (1974) and Robson and Saulnier (1981) presented considerable evidence that nahcolite and halite are still actively 

being dissolved by groundwater that is being recharged from 

exposures of the Green River and Uinta Formations around 

the margins of the basin. Saline groundwater is ultimately 

discharged from saline springs along lower Piceance Creek 

and lower Yellow Creek (fig. 2).

Weeks and others (1974) defined three aquifers in the 

basin: (1) alluvial aquifers, which are generally less than 

140 ft thick and confined to alluvial valleys; (2) the upper 

aquifer, which includes the Uinta Formation and Parachute 

Creek Member of the Green River Formation above the top of 

the Mahogany zone; and (3) the lower aquifer, which includes 

that part of the Parachute Creek Member below the Mahogany 

zone (fig. 3). They (Weeks and others, 1974) believed that 

the Mahogany zone acted as a leaky confining layer and 

constructed a hydrologic model for the basin reproduced on 

figure 9. The high resistivity zone of Weeks and others (1974) 

is the interval that still contains intact saline minerals. Robson 

and Saulnier (1981, p. 7) further subdivided the upper aquifer 

into layer 1, Uinta Formation, and layer 2, Parachute Creek 

Member of the Green River Formation above the Mahogany 

zone. They apparently considered the contact between the 

two units to be a fairly persistent stratigraphic horizon. The 

complex intertonguing between the upper part of the Parachute 

Creek Member and the Uinta Formation (figs. 3, 4) had not 

yet been worked out at the time of their publication. Robson 

and Saulnier (1981) also subdivided the lower aquifer into 

layer 3, or Mahogany zone; layer 4, consisting of B-groove 

and the R-6 zone; and layer 5, consisting of that interval below 

the R-6 zone. Recently, Shell Exploration and Production Co. 

published the results of their extensive hydrologic studies in 

the northwestern part of the Piceance Basin near their oil shale 

holdings (Day and others, 2010). They (Day and others, 2010) 

identified a previously unidentified major seal, the R-5 seal in 

the uppermost part of the R-5 oil shale zone (fig. 3).
16 Development, Evolution, and Destruction of Saline Mineral Area of Eocene Lake Uinta

Water in the lower aquifer of Weeks and others (1974) is 

sodium bicarbonate–rich because of the ongoing dissolution 

of nahcolite and halite. Total dissolved solid concentrations of 

groundwater in the lower aquifer increases as intact nahcolite and halite are encountered, and the composition switches 

from a mixed cation bicarbonate type to a sodium bicarbonate 

type (Weeks and others, 1974; Robson and Saulnier, 1981). 

Water then moves upward from the lower aquifer, through the 

Mahogany zone, and into the upper aquifer, ultimately being 

discharged in saline springs along Piceance Creek and Yellow 

Creek in the northern part of the basin (figs. 2, 9). This upward 

movement is aided by fracture systems connecting the lower 

and upper aquifers (figs. 2, 9) near where the highly saline 

springs occur (Robson and Saulnier, 1981, p. 26). Springs near 

these fractures can have concentrations of dissolved solids as 

high as 22,100 milligrams per liter (Weeks and others, 1974, 

p. 19) and be accompanied by travertine deposits, indicating 

that calcite is near or at saturation (Robson and Saulnier, 

1981, p. 29).

The Uinta Formation is hydrologically connected to the 

upper part of the Parachute Creek Member but is generally 

less permeable except where locally fractured (Robson and 

Saulnier, 1981, p. 6). Day and others (2010, p. 16) cite evidence of significant vertical permeability through joints in the 

Uinta Formation, particularly in the upper part.

Methods Used 

The U.S. Geological Survey has collected a considerable amount of information on the Green River Formation, 

including core and cuttings stored at the U.S. Geological 

Survey Core Research Center in Lakewood, Colo; oil yield 

analyses from Fischer assays, recently compiled into databases 

and published (Mercier and others, 2010 a, b, 2011); and a 

large number of geophysical logs and core descriptions for 

oil shale coreholes in the Piceance Basin that were recently 

made available at the USGS Oil Shale Research homepage: 

http://energy.usgs.gov/OilGas/UnconventionalOilGas/

OilShale/OilShaleDataDownload.aspx. These core descriptions were made by geologists at (1) the USGS, (2) the former 

U.S. Bureau of Mines, and (3) private companies that conducted oil shale research prior to the end of the last oil shale 

boom in the early 1980s. The U.S. Bureau of Mines data were 

acquired by the USGS when that agency was disbanded in 

1996; the data from private companies were donated to the 

USGS. These unpublished core descriptions form the basis of 

this investigation.

The quality of these descriptions is highly variable, and 

many were considered unsuitable for the purposes of this 

investigation. A total of 57 core descriptions were considered 

detailed enough (fig. 10, table 1). In general, the descriptions 

used noted the presence of the vugs and collapse breccias 

formed from the dissolution of nahcolite and halite. Many 

of the core descriptions, in particular those made by the U.S. 

Bureau of Mines, included x-ray diffraction analyses that confirmed the presence or absence of nahcolite. In some instances, 

nahcolite-bearing intervals were identified from Fisher assay 

analyses, as nahcolite releases large amounts of water and 

carbon dioxide gas during decomposition. Both water and gas 

quantities are measured with Fischer assay (for a description 

of the method, see Brownfield and others, 2010a). It is not 

possible, however, to determine how much nahcolite was once 

present in a leached interval. Core descriptions rarely give 

quantitative estimates of the void space present, and there is 

no way to reliably estimate the original thickness of a nahcolite bed from a collapse breccia. In general, however, there 

appears to be fewer vugs and collapse breccias toward the 

outer margins of the leached zone as Day and others (2010) 

have suggested.

In addition, detailed descriptions of 28 surface sections 

were used in our analyses (fig. 10, table 2). Unfortunately, 

only the most conspicuous evidence for leaching is apparent 

in surface exposures. Small vugs, created by the leaching of 

individual nahcolite crystals or small aggregates, would likely 

be interpreted as differential weathering and not be noted. 

Breccias, due to dissolution and collapse, were not found in 

any of the measured sections.

A separate map was generated for each of the rich and 

lean oil shale zones within the saline interval (fig. 3). Each 

map shows the approximate areas were nahcolite is present 

today and where evidence of leaching indicates that nahcolite 

was once present. Extent of present-day nahcolite for each 

zone is generally from Brownfield and others (2010a) but was 

modified when necessary to reflect occurrences of nahcolite 

identified in this study that are outside the previously defined 

nahcolite resource areas. Extent of the leached zone and modifications to the digital maps of Brownfield and others (2010a) 

were done by hand. Each corehole on the maps is represented 

by a symbol indicating one of the following: (1) nahcolite is 

present and unleached, (2) nahcolite was present but is entirely 

leached, (3) nahcolite is only partially leached from the oil 

shale zone, or (4) there is no evidence that nahcolite was ever 

present. If only very minor nahcolite remains in a zone, it is 

plotted in the leached area. If it was deemed that significant 

nahcolite was still present in the zone, it is plotted in the 

nahcolite area.

In order to better understand the evolution of saline mineral deposition, additional information is included on all of the 

maps: (1) total in-place nahcolite in tons per acre (modified 

from Brownfield and others, 2010a), (2) extent of the marginal 

lacustrine facies (modified from Johnson, 1985), and 

(3) approximate area where oil shale exceeds 10 gallons per 

ton (GPT) (modified from Johnson and others, 2010a). In 

addition, some maps include (1) total in-place oil in barrels 

per acre (modified from Johnson and others, 2010a) and 

(2) isopach maps or partial isopach maps (modified from Johnson and others, 2010a). In some instances, a 10-GPT resource 

line was not present on the resource maps of Johnson and others (2010a) and one had to be extrapolated. The broad areas of 

marginal lacustrine deposition—more than 20 miles (mi) wide 
Figure 9. Geohydrologic cross section D-D’ through the Piceance Basin showing major aquifers as defined by Weeks and others (1974, fig. location of cross section.

BEND IN

SECTION

Direction of

Garden Gulch Member Douglas Creek Member

water movement

Parachute Creek Member

Mahogany zone

High-resistivity zone

Upper aquifer

Lower aquifer

UINTA FORMATION

GREEN RIVER FORMATION

3,0004,0005,0006,0007,000

8,000

9,000

FEET

Piceance Creek

Black Sulphur Creek

00

2

2

4

4

6 MILES

6 KILOMETERS

VERTICAL EXAGGERATION X 21DATUM IS MEAN SEA LEVEL

D 
18 Development, Evolution, and Destruction of Saline Mineral Area of Eocene Lake Uinta

T.

8

 S.

108°30' 108°00'

40°00'

39°30'

C196

C34 C212

C1

Lower Piceance

Creek

C329

Fletcher

Gulch

C315 

C200

C3

C35

C2 C332

C5

C6 

C155

C171 

C529 

C154

C156

C299

C533 

C161

C151 

C177

Hay Gulch

C179

C5

C199 

C213

C273

C272

C466S

C116

C271

C162

C175

C249

C163 C195

C117

C231

C854 C855

C957

C951

C136

C766

C767

C172 C525

C535

C204

C434

C399

C350

C469

C422

C360

C505

C153

Mount

Callahan

Anvil 

Points

Cottonwood

Point Allen

Point

Bowdish 

Gulch

Glover

Point

Long

Point

South

Shale Ridge (2)

South

Shale Ridge (1) Lefthand

Draw

Cow 

Ridge (1)

Cow 

Ridge (2)

Middle Dry Fork

Eby

Point

Horsethief

Mountain

Munger

Creek

Smith 

Gulch

Yellow

Creek

Spring Creek

Tommy’s Draw

Philadelphia Creek

Cathedral Creek

Little Burma

Road

Douglas

Pass

Corehole

Measured section

Mahogany ledge outcrop

Base of Parachute Creek Member

EXPLANATION

Rio Blanco

0 3 6

0 3 6

9 12 MILES

9 12 KILOMETERS

Study area

R. 96 W. R. 95 W.

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

R. 94 W. R. 93. W

T.

2

 N.

T.

1

 N.

T.

1

 S.

T.

2

 S.

T.

3

 S.

T.

4

 S.

T.

5

 S.

T.

6

 S.

T.

7

 S.

T.

9

 S.

Figure 10. Map showing locations of coreholes and surface sections used in this investigation. Names of coreholes are listed in 

table 1, and names and sources of measured sections are listed in table 2.
Table 1. List of coreholes used in this study.—Continued[All columns ending in “J” originated from J. Dyni’s original tops file. ID, software-calculated identifier; FEL, from east line; FWL, from west line; FSL, from south line; FNL, 

USGSIDJ

Unique ID 

assigned by

staff geologist

CMPNYPROJJ

Name of the company 

or agency that drilled

the borehole

NAMEJ

Name of the borehole 

assigned by the company 

or agency that drilled it

EASTWESTJ

Distance in feet 

measured east or 

west from section line

NORSOUJ

Distance in feet 

measured north or 

south from section line

QQJ

Quarterquarter 

section

SECJ

Secion

TWPJ

Township

RANGEJ

Range

LA

Latitude 

degree

origin

C0001 Kaiser Alum. & Chem. Co. Nielsen 20-1 369 FEL 979 FSL SE SE 20 01 S 097 W 39

C0002 Wolf Ridge Minerals Corp. Dunn 20-1 1172 FEL 1197 FNL NE NE 20 01 S 098 W

C0003 Sinclair Oil and Gas Co. Federal 8024 2640 FEL 2640 FSL C 20 01 N 098 W 40

C0005 Juhan- Joe T. Juhan 4-1 1947 FEL 1859 FNL SW NE 4 02 S 098 W 39

C0006A Sinclair Oil and Gas Co. Federal strat 1 & 1-B 643 FWL 820 FSL SW SW 6 01 S 097 W

C0034 U.S. Bureau of Mines/AEC Colorado 1 1284 FEL 2331 FSL NE SE 13 01 N 098 W 40

C0035 U.S. Bureau of Mines/AEC Colorado 2 2005 FEL 710 FNL NW NE 14 01 S 099 W 39

C0116 Equity Oil Co. Boies Corehole 1 1770 FWL 930 FSL SE SW 19 02 S 097 W 39

C0117 Equity Oil Co. BX-1 425 FEL 1870 FSL NE SE 6 03 S 098 W 39

C0136 Getty Oil Co. 02-30 2258 FWL 767 FSL SE SW 30 05 S 097 W

C0151 Marathon Oil Co. Square S No. 1 589 FWL 58 FNL NW NW 4 02 S 097 W 39

C0153 Industrial Resources Inc. Colorado Minerals 28-1 1360 FWL 400 FNL NE NW 28 01 S 098 W

C0154 Wolf Ridge Minerals Corp. Savage 24-1 332 FEL 343 FNL NE NE 24 01 S 098 W 39

C0155 Wolf Ridge Minerals Corp. Colorado Minerals 14-1 1699 FEL 641 FNL NW NE 14 01 S 098 W

C0156 Kaiser Alum. & Chem. Co. Nielsen 17-1 2566 FEL 905 FNL NW NE 17 01 S 097 W 39

C0161 General Petroleum Corp. 11-24 500 FWL 500 FNL NW NW 24 02 S 096 W 39

C0162 Equity Oil Co. Oldland 3 450 FEL 425 FNL NE NE 10 03 S 096 W 39

C0163 Equity-Exxon Ebler 1/1-A 2035 FEL 2175 FNL SW NE 30 03 S 097 W 39

C0171 U.S. Bureau of Mines/AEC Colorado 3 210 FWL 640 FSL SW SW 14 01 N 098 W 40

C0172 Shell Oil Co. 23X-10 1869 FWL 1781 FSL NE SW 10 07 S 097 W 39

C0175 Atlantic Richfield Co. Federal 2-B 1715 FEL 2261 FSL NW SE 10 03 S 096 W 39

C0177 Shell Oil Co. 22X-1 2610 FEL 2260 FNL SW NE 1 02 S 098 W 39

C0179A Shell Oil Co. 23X-2 1580 FWL 2325 FSL NE SW 2 02 S 098 W 39

C0195 Arco-Mobil-Equity Figure Four 31-1 2060 FEL 2660 FSL SW NE 31 03 S 098 W 39

C0196 Superior Oil Co. Core hole 10 355 FWL 1735 FNL SW NW 6 01 N 097 W 40

C0199 Occidental Petroleum Co. Stake Springs Draw 1 1650 FEL 600 FNL NW NE 12 02 S 099 W 39

C0200 Mintech Corp. Portland 1 1775 FEL 1475 FSL SW SE 19 01 S 099 W 39

C0204 Tosco Corp. TG-71-4 125 FWL 2200 FSL NW SW 6 04 S 096 W 39

C0212 Superior Oil Co. Core hole 15 250 FWL 150 FSL SW SW 7 01 N 097 W 40

C0213 Cameron Engineers CE-708 975 FEL 1250 FNL NE NE 5 02 S 099 W 39

C0231 Humble Oil & Refining Co. Jumps Cabin 1 550 FWL 925 FSL SW SW 15 04 S 098 W 39

C0249 Arco (C-b project) Sorghum Gulch 01 1946 FEL 1314 FSL SE 2 03 S 097 W 39

C0271 Carter Oil Co. Upper Piceance Creek 2 1950 FWL 1250 FSL SE SW 9 03 S 095 W 39
Table 1. List of coreholes used in this study.—Continued [All columns ending in “J” originated from J. Dyni’s original tops file. ID, software-calculated identifier; FEL, from east line; FWL, from west line; FSL, from south line; FNL, 

USGSIDJ Unique ID assigned by staff geologist CMPNYPROJJ Name of the company or agency that drilled the borehole NAMEJ Name of the borehole assigned by the company or agency that drilled it EASTWESTJ Distance in feet measured east or west from section line NORSOUJ Distance in feet measured north or south from section line QQJ Quarter- quarter section SECJ Secion TWPJ Township RANGEJ Range LA

Latitude 

degree

origin

C0272 Rio Blanco Oil Shale Co. C-14 810 FWL 3800 FSL SW NW 10 02 S 099 W 39

C0273 Rio Blanco Oil Shale Co. C-15 825 FEL 400 FSL SE SE 10 02 S 099 W 39C0299 U.S. Bureau of Mines CH 02-A 1087 FWL 2387 FSL NW SW 29 01 S 097 W 39C0315 Exxon Co. Pinto Gulch 1 1800 FEL 1800 FNL SW NE 32 01 N 098 W 40C0329 Carter Oil Co. Opportunity 2 2150 FEL 850 FNL NW NE 28 01 N 096 W 40C0332 Carter Oil Co. Yellow Creek 2 300 FEL 1450 FNL SE NE 9 01 S 098 W 39C0350 U.S. Navy Core hole 15/16 (NOSR 1) 1330 FWL 3109 FNL NW SW 1 05 S 095 W 29C0360 U.S. Navy Core hole 20 (NOSR 1) 610 FWL 1251 FSL SW SW 14 05 S 095 W 39C0399 Union Oil Co. Fay 6-75 2275 FWL 1600 FSL NE SW 30 04 S 095 W C0422A Union Oil Co. LR 4-75 2535 FEL 1685 FNL SW NE 20 05 S 095 W 39C0434 Union Oil Co. Mary Ann 32/32B 390 FNL 2450 FEL NW NE 22 04 S 096 W 39C0469 Exxon Co. CD-18 1650 FEL 2850 FNL NW SE 12 05 S 096 W C0505A Cities Service Oil Co. Core hole 3 2320 FEL 2940 FNL NW SE 3 06 S 097 W 39C0525 Cities Service Oil Co. Core hole 5 1295 FEL 615 FSL SE SE 22 06 S 097 W 39C0529 Exxon Co. Yellow Creek 3 1850 FEL 1450 FSL NW SE 24 01 N 098 W 40C0533 Exxon Co. Cole Gulch 1A 300 FWL 350 FSL 300 11 01 S 097 W 39C0535 Exxon Co. Ten Section 1 SE SW 32 04 S 097 W C0766 Shell Oil Co. DH 5 (Pacific Property) 2510 FEL 2590 FSL NW SE 25 06 S 098 W C0767 Shell Oil Co. DH 6 (Pacific Property) 4560 FEL 2250 FNL SW NW 36 06 S 098 W C0854 Arco-Mobil-Equity Figure Four 09-1 2734 FWL 2165 FNL SW NE 9 04 S 099 W C0855 Arco-Mobil-Equity Figure Four 14-1 4653 FWL 2406 FSL NE SE 14 04 S 099 W C0951 Chevron Oil Co. CC-20 1997 FWL 418 FSL SE SW 9 05 S 098 W C0957 Chevron Oil Co. CC-26 1878 FEL 71 FNL NW NE 2 05 S 099 W
Results of Data Analyses 21

in the southern part of the Piceance Basin and about 45 mi 

wide in the eastern Uinta Basin (figs. 4, 5)—will be referred 

to here as marginal shelves, as a distinct break in slope at the 

outer edge of these marginal areas can be documented using 

isopach maps. The limit of the marginal lacustrine facies 

shown was not extended beyond present day outcrop, thus the 

widths of marginal lacustrine facies stated here are minimum 

widths.

Two detailed cross sections, constructed from coreholes 

and measured sections, are also presented (pls. 1, 2). Datum 

is sea level, so the relationship between leaching of saline 

minerals and the structure of the basin can be studied. The 

cross sections were constructed by combining several cross 

sections recently published by Self and others (2010). In some 

instances, coreholes with detailed lithologic descriptions were 

added and some without detailed descriptions were removed. 

Detailed lithologic descriptions are included for the measured 

sections shown on the cross sections (pls. 1, 2). Oil-yield 

histograms are shown for each corehole, and intervals that 

contain nahcolite and halite and evidence of leaching are 

identified. Water content from Fischer assay analyses were 

used to determine the presence or absence of nahcolite in some 

of the coreholes (Brownfield and others, 2010a). Many of the 

coreholes were drilled in the bottom of drainages with significant thicknesses of strata exposed on nearby valley walls. 

This stratigraphic information (as much as 820 ft of additional 

strata) was added using available 71⁄2-minute geologic quadrangle maps. For a recent summary of geologic quadrangle 

mapping in the Piceance Basin, see Johnson (2012).

Results of Data Analyses

Most of the information collected for this study is 

summarized on Figure 11, which shows for each corehole 

examined (1) the elevation of highest preserved nahcolite 

and (or) halite, in feet, and the R-zone in which it occurs; and 

(2) the elevation of the top of continuous nahcolite and (or) 

halite (lowest evidence of leaching), in feet, and the R-zone in 

which it occurs. In addition, different colored boxes are used 

to highlight whether leaching reached the base of the saline 

interval. Permeability of oil shale is typically very low, and 

fractures are needed for groundwater penetration. Occurrences 

of nahcolite above the lowest evidence of leaching probably 

represent intervals where fractures are not present. In some 

core descriptions it was noted that nahcolite was present but 

eroded. It is assumed here that this erosion occurred as a 

result of contact with drilling fluids. In some instances, where 

nahcolite is generally sparse in the corehole, there can be a 

significant vertical distance between the lowest vugs and the 

highest nahcolite. The lowest evidence of leaching represents 

the maximum depth of penetration of groundwater into the 

saline interval.

Present-Day Elevation of the Leached Zone

A structure contour map on the base of the leached zone, 

or the lowest evidence of leaching, is included on figures 11 

and 12; additionally, figure 12 includes a structure contour 

map of the top of the Mahogany zone. Elevation of the base of 

leaching presented here is similar to the elevation on the base 

of the lower aquifer of Robson and Saulnier (1981, fig. 9). 

The base of leaching has reached the base of the saline zone 

beyond the dashed red line. Elevation of the base of leaching 

varies from more than 5,250 ft in the southwest, near recharge 

areas along the west margin of the basin, to less than 4,200 ft 

near discharge areas along lower Piceance Creek and Lower 

Yellow Creek in the northern part. Elevation of the base of 

leaching generally increases over the Piceance Creek dome 

(fig. 12), similar to that shown by Robson and Saulnier (1981, 

Table 2. List of surface sections used in this study.

 Section Name Source

Yellow Creek Self and others (2010)

Lower Piceance Creek Johnson (2012)

Smith Gulch Johnson (unpub. section)

Hay Gulch Johnson (unpub. section)

Rio Blanco Johnson (unpub. section)

Anvil Points Self and others (2010)

Cottonwood Point Donnell (1961)

Allen Point Donnell (1961)

Glover Point Donnell (1961)

Mount Callahan Donnell (1961)

Horsethief Mountain Johnson and May (1978)

Bowdish Gulch Donnell (1961)

Long Point Johnson (1975)

South Shale Ridge (1) Johnson and May (1978)

South Shale Ridge (2) Johnson and May (1978)

Cow Ridge (1) Johnson (1975)

Cow Ridge (2) Johnson (1977)

Eby Point Johnson (1977)

Lefthand Draw Johnson and May (1978)

Middle Dry Fork Johnson and others (1988)

Munger Creek Johnson and others (1988)

Douglas Pass Johnson and others (1988)

Little Burma Road Johnson and others (1988)

Cathedral Creek Roehler (1972)

Tommy’s Draw Johnson and others (1988)

Philadelphia Creek Johnson and others (1988)

Fletcher Gulch Johnson and others (1988)

Spring Creek Self and others (2010)
22 Development, Evolution, and Destruction of Saline Mineral Area of Eocene Lake Uinta

R. 96 W. R. 95 W.

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

R. 94 W. R. 93. W

108°30' 108°00'

40°00'

39°30'

4,250

4,500 4,750

5,000

5,000

5,250

4,981.3 R-5

4,981.3 R-5

4,730 R-5

4,703 R-5

4,649 R-4

4,704 L-4

6,908.7 L-5

6,992.1 L-5

4,739 R-5

4,739 R-5

5,050 R-5

4,830 L-4

4,489.4 R-6

4,319 L.5

4,792 R-5

4,909.9 R-4

5,185 R-5

4,925 R-4

5,254 R-4

5,297 L-4

4,964 R-5

4,739.8 L-4

4,585 R-4

4,545 R-4

5,400 R-6

5,618 B-groove

5,430 R-4

5,801.5 R-5

5,476.6 R-4

5,397.1 L-3

4,869.2 R-6

4,093.5 L-4

7,960 Bed 70

7,846 Bed 58

4,989.6 R-5

4,989.6 R-5

5,060.3 R-5

4,982.8 R-5

4,952 L-5

4,960.8 L-5

9,483.4 Above ML

6,282.9 R-3

5,000.3 R-6

4,448.4 R-4

5,492.7 R-5

5,094.4 R-3

6,190.8 R-3

5,898.7 R-5

5,757.2 R-5

4,241.8 L-5

4,257 L-5

6,022.2 R-3

5,469.5 L-5

5,470.5 L-5

6,107.8 L-5

5,631 L-2

5,707.7 R-4

5,454 L-2

4,833 R-5

4,887 L-5

5,121.8 R-5

4,959.7 L-4 6,428.3 R-6

5,396 B

5,065 R-6

7,219 R-6

7,224.9 B-groove

7,207 R-6

7,065 R-6

6,810.7 R-6

6,378 L-4

6,711.4 L-5

6,308.7 R-5

6,308.1 R-5

6,049 L-5

6,079 L-5

7,455.2 R-6

7,354.6 R-6

7,727.4 ML

7,629 B-groove

4,497.1 L-5

4,426 R-5

4,584.3 L-5

4,373.5 L-4

16.2

551.9

170.4

775.7 71.1

162.1

27 0

117.9 208.8

54

0 331 0 8.8 224.2

218

371.5

77.5 1.0 55

260

40 43

476 230

253.7

398.3

79.5

141.5

0.6

432.7

5.9

142

100.6

98.4

114

0 3 6

0 3 6

9 12 MILES

9 12 KILOMETERS

Study area

T.

2

 N.

T.

1

 N.

T.

1

 S.

T.

2

 S.

T.

3

 S.

T.

4

 S.

T.

5

 S.

T.

6

 S.

T.

7

 S.

T.

8

 S.

T.

9

 S.
Results of Data Analyses 23

 Leaching has not reached the base of the saline interal.

7,207 R-6

7,065 R-6

6,308.7 R-5

6,308.1 R-5

6,308.7 R-5

6,308.1 R-5

Elevation of highest preserved leaching, in feet, and oil shale zone in which it occurs. 

Leaching has reached the base of the saline interval.

Elevation of base of leached zone.

Contour interval 250 feet.

Elevation of lowest evidence of leaching, in feet, and oil shale zone in which it occurs.

If box not colored then corehole was not drilled to base of saline interval.

EXPLANATION 

Mahogany ledge outcrop

Base of Parachute Creek Member

Figure 11 (previous page). Map showing elevations of the highest occurrence of nahcolite and the lowest evidence of leaching of the 

saline interval. Structure contour map is based on elevation of the lowest evidence of leaching. Beyond the dashed contour line, the 

base of leaching has reached the base of the saline zone.
24 Development, Evolution, and Destruction of Saline Mineral Area of Eocene Lake Uinta

40°00'

39°30'

108°30' 108°00'

4,500 4,750

5,000

Piceance Creek

dome Piceance

Creek

Creek

Creek

Creek

Yellow

Parac hute

Roan

Colorado River

White River

5,250

5,000

4,250

0 6

0 6

12 MILES

12 KILOMETERS

R. 96 W. R. 95 W.

R. 101 W. R. 100 W. R. 99 W. R. 98 W. R. 97 W. R. 94 W. R. 93. W

T.

2

 N.

T.

1

 N.

T.

1

 S.

T.

2

 S.

T.

3

 S.

T.

4

 S.

T.

6

 S.

T.

7

 S.

T.

9

 S.

T.

5

 S.

T.

8

S.

Study area
Results of Data Analyses 25

Figure 12 (previous page). Structure contour map based on the lowest evidence of leaching (heavy red lines) and structure contour 

map based on the top of the Mahogany oil shale zone (modified from Mercier, 2010). Locations of highly saline springs and the zone of 

faulting and fracturing in the north part of basin are from Weeks and others (1974) and Robson and Saulnier (1981).

EXPLANATION

Mahogany ledge outcrop

Base of Parachute Creek 

Member

Faults

> 9,000 

8,001–9,000 

7,001–8,000 

6,001–7,000 

5,001–6,000

<5,000

Elevation of top of Mahogany 

zone. Contour interval 100 feet.

Elevation of base of leached 

zone. Contour interval 250 feet.
26 Development, Evolution, and Destruction of Saline Mineral Area of Eocene Lake Uinta

fig. 9). The base of leaching clearly cuts across stratigraphic 

units as Dyni (1974c) indicated. This cross-cutting relationship 

can also be seen on the two detailed cross sections constructed 

for this study (pls. 1, 2).

Saline Mineral Deposition and Extent of 

Leaching for Each Rich and Lean Zone

For convenience in discussing the evolution of the saline 

mineral deposition, the “history” of the evolution of Lake 

Uinta is informally subdivided into four phases: (1) illitic 

phase that begins with the Long Point transgression and 

extends to the base of the R-2 zone; (2) early saline mineral 

phase, extending from the base of the R-2 zone to the top of 

the L-3 zone; (3) middle saline mineral phase, extending from 

the base of the R-4 zone to the top of A-groove; and (4) late 

saline mineral phase that includes all lacustrine strata above 

A-groove (figs. 3–5). Although the illitic phase predates the 

onset of saline mineral deposition, lake-margin shelf progradation during this period influenced many aspects of the saline 

mineral and oil shale deposition throughout the saline phases 

of Lake Uinta that followed. It is thus important to briefly 

discuss the illitic phase here.

Figure 13 presents schematic cross sections of the 

Piceance Basin for three times: (1) at the end of the freshwater 

lake (Lake Cow Ridge) period that predated the development 

of Lake Uinta; (2) at maximum transgression after the Long 

Point transgression formed Lake Uinta, or beginning of the 

illitic phase; and (3) at the end of the illitic phase. The cross 

sections extend from the crest of the Douglas Creek arch on 

the west to the White River uplift on the east. Prior to the 

development of Lake Uinta, the Douglas Creek arch acted as 

a sort of hingeline, with rates of subsidence increasing to the 

east toward the White River uplift. Only a thin interval of pre–

Lake Uinta, early Tertiary strata is preserved on the crest of 

the arch (fig. 4, 13A). Although a fairly thick interval of Lake 

Uinta sediments were deposited over the crest of the arch 

(fig. 4), the interval from the base of Long Point Bed to the 

top of Mahogany oil shale zone thickens markedly to the 

east in the Piceance Basin (Johnson and Finn, 1986, fig. 12), 

indicating that overall rates of subsidence increased to the 

east throughout much if not all the history of Lake Uinta. The 

schematic cross section shown in figure 13 and subsequent 

schematic cross sections presented in this report assume that 

subsidence rates increased toward the White River uplift 

throughout the history of Lake Uinta.

A freshwater lake, Lake Cow Ridge, surrounded by alluvial plains, occupied the Piceance Basin immediately prior 

to the Long Point transgression (figs. 4, 5, 13A). Both the 

depth of Lake Cow Ridge and the slope of the alluvial plains 

that surrounded it are unknown, but a combination of the 

two controlled the topography and depth of early Lake Uinta 

(fig. 13B), and thus had some impact on the saline mineral 

area that developed later. About 600–700 ft of nearly continuous oil shale was deposited in the center of Lake Cow Ridge 

prior to the development of Lake Uinta. Only Fischer assay 

data from cuttings samples taken every 10 ft are available for 

this oil shale sequence, but on the basis of this limited dataset, 

the oil shale averaged about 5–10 GPT (R.C. Johnson, U.S. 

Geological Survey, unpub. data, 1977). The depth of Lake 

Cow Ridge prior to the Long Point transgression is unknown, 

but it may not have been shallow, as it lasted as long as 3 million years, depositing near continuous low-grade oil shale for 

about the last half of its existence.

Illitic Phase

The illitic phase of Lake Uinta began with the Long Point 

transgression, when two freshwater lakes, one in the Piceance 

Basin and one in the Uinta Basin, expanded and coalesced 

across the Douglas Creek arch to form Lake Uinta (fig. 4). 

During the Long Point transgression, the lake expanded across 

the surrounding alluvial plain about 10 to 12 mi to the east 

(fig. 13B) and nearly 40 mi to the south before maximum 

transgression was reached (Johnson, 1985, figs. 7, 8). Again, 

the slope of the alluvial plain is unknown, but the difference 

in the distance of transgression suggests that the slope to the 

south was much gentler than that of the slope to the east. 

The R-0 oil shale zone is directly above the Long Point Bed 

and represents the first and one of the most widespread oil 

shale zones deposited in Lake Uinta. It was deposited over 

large areas of both the Piceance and Uinta Basins (figs. 4, 5) 

(Johnson, 1985, fig. 8; Mercier and Johnson, 2012, figs. 6–8). 

The salinity of Lake Uinta began to increase very early on, as 

freshwater mollusks do not occur above the basal transgressive 

bed, the Long Point Bed (Johnson, 1985).

Lake-margin shelves began to prograde into Lake Uinta 

after maximum transgression, reaching widths in the Piceance 

Basin of more than 20 mi to the southeast, south, and southwest, and 2–5 mi to the east by the end of the illitic phase 

(figs. 4, 5, 13). This is a minimum width, as a significant part 

of the marginal facies has been removed by relatively recent 

downcutting on the perimeter of the basin by the Colorado 

River and White River drainage systems (fig. 1). The illitic 

interval is thickest on the marginal shelf areas and thinnest 

in the deep lake area (fig. 14). The deep lake area defined by 

marginal shelf progradation during the illitic phase developed 

almost directly over the offshore area of the preceding Lake 

Cow Ridge (fig. 14). Thus, the depth of the earlier freshwater

lake contributed directly to the depth of Lake Uinta at the 

end of the illitic phase, as shown in figure 13. Marginal 

lacustrine rocks from the illitic phase of Lake Uinta include 

stromatolites, ostracodal and oolitic limestone, sandstone, and 

carbonate-rich mudstone (Johnson, 1985; Johnson and others, 

1988), whereas offshore rocks consist almost exclusively of 

dark-gray to black illitic oil shale. The amount of sandstone in 

the marginal lacustrine facies increases in the southeastern part 

of the basin, where a major drainage entered Lake Uinta.

Water depth on the marginal shelves undoubtedly fluctuated because of variations in water supply and evaporation in 

the lake that was now internally drained, but conditions were 
Results of Data Analyses 27

Figure 13. West-to-east schematic cross section across the Piceance Basin for three time 

periods. A, End of the freshwater period. B, After the Long Point transgression. C, End of the 

illitic phase. Approximate line of section shown on figure 14.

Section thickens to east

because of increased subsidence

toward the White River uplift.White River uplift

Douglas

Creek arch

Broad marginal shelves developed. Depth of the central lake area increased. Saline brines

evolved on these shelves and ultimately made their way into the deep lake. Organic matter

that accumulated on the shelves also washed into the deep lake, producing “blebby oil shale.”

C. End of illitic phase

B. After Long Point transgression

A. End of freshwater period

Variegated mudstone, sandstone, and siltstone

Carbonate-rich marginal lacustrine mudstone 

with ostracodal, oolitic, and stromatolitic 

limestones (carbonate mudflats)

Sandstone-rich marginal lacustrine mudstone 

with some ostracodal, oolitic, and stromatolitic 

limestones (sandy mudflats)

Rich illitic oil shale

Saline brines

Organic matter and 

carbonate ripups

Pre–Lake Uinta Eocene rocks

Pre–Lake Uinta Eocene rocks

White River uplift

Douglas

Creek arch

This freshwater lake expanded across the Douglas Creek arch and coalesced with a similar 

freshwater lake in the Uinta Basin to form Lake Uinta. Lake Uinta inherited topography from

both the earlier freshwater lake and the surrounding alluvial plains.

The Douglas Creek arch was a sort of hingeline, 

with subsidence increasing in a regular fashion 

from the crest of the arch to the White River uplift.

White River uplift

50 MILES

Douglas

Creek arch

A long-lived Eocene freshwater lake, Lake Cow Ridge, 

occupied the Piceance Basin from near the end of the 

Paleocene to the formation of Lake Uinta, a period of 

over 3 million years. Freshwater oil shale was 

deposited in the central part of this lake.

Paleocene rocks

Freshwater lacustrine oil shale

Marginal freshwater lacustrine and paludal rocks 

with molluscs and thin coal beds 

Variegated mudstone, sandstone, and siltstone

Paleocene rocks

Paleocene rocks

Upper Cretaceous rocks

Upper Cretaceous rocks

EXPLANATION

EXPLANATION

Freshwater influx on sandy

shelves may have inhibited

brine formation.

? ?

?

?

Areas where intense evaporation could 

produce brines. Queried where freshwater 

input may have inhibited brine formation.

0

0 50 KILOMETERS
28 Development, Evolution, and Destruction of Saline Mineral Area of Eocene Lake Uinta

T.

8

 S.

0 6

0 6

12 MILES

12 KILOMETERS

108°30' 108°00'

40°00'

39°30'

R. 96 W. R. 95 W.

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

R. 94 W. R. 93. W

T.

2

 N.

T.

1

 N.

T.

1

 S.

T.

2

 S.

T.

3

 S.

T.

4

 S.

T.

6

 S.

T.

7

 S.

T.

9

 S.

T.

5

 S.

During the early saline mineral

phase, saline minerals were

deposited in the deep central

part of the lake formed by 

marginal shelf progradation 

during the preceeding illitic

phase. This deep lake area 

was in turn superimposed on 

a freshwater lake of 

unknown depth.

Study area
Results of Data Analyses 29

Figure 14 (previous page). Map of the Piceance Basin at the end of the illitic phase showing (1) extent of 

offshore and marginal lacustrine facies at the end of the freshwater phase that pre-dated the illitic phase 

of Lake Uinta (modified from Johnson, 1985, fig. 7), (2) extent of marginal lacustrine shelves at the end of the 

illitic phase (modified from Johnson, 1985, fig. 10), (3) thickness of the illitic phase, from the base of the Long 

Point Bed to the top of the L-1 zone (new to this report), (4) limits of saline mineral deposition during each of 

the four oil shale zones that compose the early saline mineral phase (modified from Brownfield and others, 

2010a), and (5) approximate line of section for schematic cross section shown in figure 13 (heavy black line).

300–400

400–500

500–600

700–800

600–700

800–900

Thickness from R-2 zone 

to base of Long Point Bed.

Countour interval 100 feet.

EXPLANATION

Extent of marginal

lacustrine facies just 

prior to Long Point

Transgression

Extent of offshore

lacustrine facies just 

prior to Long Point

Transgression

R-3 saline 

mineral area

L-2 saline 

mineral area

L-3 saline 

mineral area

R-2 saline 

mineral area

Location of cross

section in Fig. 13

Marginal lacustrine

facies

Sandstone-rich 

marginal lacustrine

Carbonate-rich

marginal lacustrine
30 Development, Evolution, and Destruction of Saline Mineral Area of Eocene Lake Uinta

probably suitable for the formation of brines and salt crusts 

during periods when large areas of the shelves were exposed. 

It is proposed here that highly saline brines evolved on these 

marginal shelves and that these brines ultimately made their 

way into the deep central lake area, helping to create a deep 

brine layer in a permanently stratified Lake Uinta as envisioned by Bradley (1963) (fig. 13C).

Organic matter and mineral matter that accumulated on 

the marginal shelves may also have been transported into the 

deep central lake area. Most of the oil shale found in outcrop 

around the margins of the basin consists of the classic Green 

River laminated oil shale interpreted as varved by Bradley 

(1930, 1931), who noted, however, that a few oil shale beds 

around the basin margins possessed a “blebby and streaked” 

fabric that he believed was formed from subaerial exposure 

and desiccation (figs. 15, 16). Dyni and Hawkins (1981), in 

a subsurface study of the central part of the basin, indicated 

that blebby oil shale can compose more than 50 percent of the 

strata there, with the remainder being well laminated. They 

(Dyni and Hawkins, 1981) noted the lack of shallow-water 

indicators, such as mudcracks, algal structures, and oxidized 

zones, and suggested instead that blebby oil shale was a deep 

water deposit that had been laterally transported, in part by 

turbidity currents down sloping surfaces on the floor of Lake 

Uinta. In general, blebby oil shale consists of clasts that vary 

in size from near-microscopic to several inches across in a 

very fine kerogen-rich matrix (figs. 15, 16). Clasts vary from 

massive to well laminated and are most commonly dolomitic 

marlstone (Dyni and Hawkins, 1981). Dolomitic marlstone 

is one of the most abundant lithologies found in the marginal 

lacustrine facies (see, for example, Johnson and others, 1988). 

Tanavsuu-Milkeviciene and Sarge (2012) recently published 

cross sections based on detailed core descriptions that document the increase in blebby oil shale toward the center of the 

deep lake area. It is proposed here that the blebby oil shale 

largely originated on the marginal shelves and was transported 

into the deep central lake area by mass-movement processes. 

As we are also proposing that brines were transported to the 

deep central lake area from the marginal shelves, it is possible 

that both brines and blebby oil shale were transported together. 

This hypothesis requires further study.

A paleogeographic map at the end of the illitic phase 

that includes both the Piceance and Uinta Basins is shown 

in figure 17. Maximum preserved width of marginal shelves 

in the Uinta Basin at the end of the illitic phase is about 

45 mi in the southern part of the basin, where a major drainage 

entered the lake from the south. Offshore oil shale deposition 

in the Uinta Basin was confined to a narrow east-west trending trough in the northern part of the basin south of the Uinta 

uplift (fig. 1). As there are no known saline mineral deposits in 

the Uinta Basin for the early and middle saline mineral phases, 

it is likely that any brines that formed on the broad marginal 

shelves in the Uinta Basin were transported into the deep lake 

area in the north-central part of the Piceance Basin, as shown 

on figure 17. The movement of brines from the Uinta Basin 

into the Piceance Basin was previously proposed by Dyni 

(1987). Brine formation is queried in areas were major rivers 

entered Lake Uinta to produce sandy shelf deposits, as freshwater input may have inhibited brine formation in these areas.

Early Saline Mineral Phase

The early saline phase consists of the R-2 through L-3 

oil shale zones. The saline mineral area during this phase 

was confined to a limited area in the north-central part of the 

basin, where marginal shelf progradation during the earlier 

illitic phase produced a deep lake area (fig. 14). Figure 18 

shows where each of the four zones that compose the early 

saline mineral phase is thickest. The R-2, L-2, and R-3 oil 

shale zones are thickest along the southern margin of the deep 

central lake area, with the thick area for each succeeding zone 

slightly farther to the north than that of the preceding zone. 

This is consistent with sediment being transported from the 

shelf, down the slope, and accumulating at the break in slope 

along the southern margin of the deep central lake area. There 

appears to have been minimal change in the position of the 

shelf edge during deposition of the early saline mineral phase, 

and the shelf edge is shown at the same position for all four 

zones that compose this phase (figs. 19–22).

The first occurrence of nahcolite in the Piceance Basin 

is in the form of scattered disseminated crystals and crystalline aggregates in the upper part of the R-2 zone in a limited 

area in the north-central part of the basin (Dyni, 1974c; Dyni, 

1981, pl. 1) (fig. 19). A shift from illitic oil shale to dolomitic 

oil shale also occurred in the R-2 zone. No evidence of saline 

leaching from the R-2 zone has been reported, and thus the 

present-day extent of saline minerals is the same as the maximum extent. Total in-place nahcolite in the R-2 zone is quite 

small, with a maximum of 13,248 tons per acre (Brownfield 

and others, 2010a, fig. 18).

The saline mineral area expanded significantly during 

L-2 time, covering an irregularly shaped area extending from 

the northern part of T. 2 S., R. 98 W. to the southern part of 

T. 1 N., R. 97 W. (fig. 20). Nahcolite is confined largely to an 

interval about 10 ft thick in the lowest part of the L-2 zone 

and consists of nahcolite beds, bands, and aggregates (Dyni, 

1974c, 1981). Total nahcolite in place has increased markedly compared to the R-2 zone, to a maximum of more than 

75,000 tons per acre (Brownfield and others, 2010a). Evidence 

of leaching, in the form of empty vugs, occurs in two coreholes in sec. 10, T. 2 S., R. 99 W. (fig. 20, pl. 1), west of the 

main nahcolite area in T. 2 S., R. 99 W. The area of 10-GPTor-greater oil yield has contracted when compared with the 

underlying R-2 zone (figs. 19, 20).

The area of nahcolite deposition increased again during 

R-3 time, covering much of the deep central lake area (fig. 21).

 Nahcolite occurs as disseminated crystals, aggregates, and 

discrete beds throughout the R-3 zone, with maximum inplace values of as much as 135,000 tons per acre (Brownfield 

and others, 2010a). For the first time, there is abundant evidence for widespread leaching of nahcolite along the western 

margin of the saline mineral area and downdip from outcrops 
Results of Data Analyses 31

A B

C

D

1 CM

1 CM

1 CM

3

2

1

Figure 15, Photographs of slabbed core and mine rock. A, Streaked 

oil shale containing large flat clasts of laminated marlstone. B, Blebby 

oil shale displaying graded bedding. C, Blebby oil shale containing 

unsorted clasts of nonlaminated marlstone. D, Blebby marlstone cut 

parallel to bedding. From Dyni (1981, fig. 12).

Figure 16, Photograph of oil shale with clasts of laminated 

marlstone. Photo was taken near the center of sec. 16, T. 5 S., 

R. 98 W. at an elevation of about 6,970 feet. The outcrop is in the 

middle of the R-6 oil shale zone about 220 feet below the top of the 

Mahogany ledge.

along the western margin of the basin (fig. 21). Leached and 

unleached intervals alternate in the R-3 zone in two coreholes 

in T. 2 S., R. 99 W. The 10-GPT-or-greater oil shale area 

expanded during deposition of the R-3 zone.

During deposition of the L-3 zone, the area of nahcolite 

deposition remained largely unchanged from the previous 

R-3 zone (fig. 22). Maximum in-place nahcolite is only about 

35,000 tons per acre (Brownfield and others, 2010a), largely 

because of thinning of the L-3 zone to about one-third to onequarter the thickness of the underlying R-3 zone (Johnson and 

others, 2010a, figs. 65, 69). Nahcolite occurs mainly as thin 

beds and aggregates. As in the underlying R-3 zone, saline 

minerals have been leached out from the western part of the 

saline area. The area of 10-GPT-or-greater oil yield has contracted significantly from the previous zone, almost matching 

the area of nahcolite deposition. By the end of the early saline 

mineral phase, about 300–400 ft of blebby and laminated oil 

shale and nahcolite had been deposited in the deep central lake 

area, whereas a somewhat thinner interval of marginal lacustrine lithologies had been deposited on the marginal shelves 

(figs. 4, 5, 23A).

Middle Saline Mineral Phase

The middle saline mineral phase of deposition in Lake 

Uinta begins with the R-4 zone and ends with deposition of 

A-groove (figs. 3–5). During the middle saline phase, offshore 

oil shale deposition gradually expanded over the marginal 

lacustrine shelves, covering nearly all the shelf areas by the 

beginning of deposition of the Mahogany oil shale zone 

(figs. 4, 5, 23B, 24–31). Despite the ultimate drowning of the 

marginal shelf, the approximate position of the shelf edge at 

the end of the illitic phase seems to have exerted some control 

over both oil shale and saline mineral deposition throughout 

the middle saline phase, suggesting that a topographic break 

was still present near there. The position of this shelf break 

at the end of the illitic phase is shown on each of the middle 

saline phase maps presented here. Schematic cross sections 

are shown in figure 23 for (1) the end of the early saline phase 

and the beginning of the middle saline phase, and (2) the 

period just before deposition of the Mahogany oil shale zone. 

As in the previous schematic cross sections, these assume an 

increase in the rate of subsidence from the crest of the Douglas 

Creek arch eastward toward the White River uplift. Climate 

change may have played a role in the expansion of Lake 

Uinta during the middle saline mineral phase (TanavsuuMilkeviciene and Sarg, 2012), but increasing outflow from 

Lake Gosiute to the north, as it was progressively filled in 

by volcaniclastic debris, was probably the dominant factor 

(Surdam and Stanley, 1980; Johnson, 1985; Roehler, 1993). 

Sandy deltaic deposits, possibly from outflow from Lake 

Gosiute, appear for the first time in the northern part of the 

basin near the base of the R-4 zone (figs. 4, 25) (TanavsuuMilkeviciene and Sarg, 2012, fig. 10). Sandy deltaic deposits 

continue in this area throughout the middle saline zone. Later, 

at the beginning of the late saline mineral phase, volcaniclastic 

debris from Wyoming entered Lake Uinta near this location.
RIO BLANCO GARFIELD MOFFAT SUMMIT UINTAH UTAH CARBON WASATCH DUCHESNE MESA

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111° 110° 109° 

40°'

39°

0 10 20 MILES

0 10 20 KILOMETERS

UTAH

COLORADO

Sandstone-rich marginal lacustrineCarbonate-rich marginal lacustrine Fluvial and alluvial variegated

mudstone, sandstone, and siltstone

Oil shale >10 gallons per tonOil shale <10 gallons per ton

Top of Mahogany Bed outcropBase of Parachute Creek Member

<25,000

Nahcolite (short tons/acre)Interval 2,500 short tons/acre.

Generalized present-daylimit of R-2 zone of theGreen River Formation.

A major drainage entered the Uinta Basin from the south,producing a sandy shelf or delta. Influx of freshwater may have inhibited brine formation here.

Poorly defined area oflow-grade oil shale deposition.

Multiple small drainages ofthe Uinta Uplift produced a narrow sandy shelf. Influxof freshwater may have inhibited brine formation here.

Brines from UintaBasin enterted Piceance Basinnear here.

Marginal lacustrine faciesconnects between Uinta and Piceance Basins.

Areas where intenseevaporation could produce brines.Queried where freshwater input may have inhibited brine formation. Brine movement

?

? ? ?

? ?

?

?

? ?

?

? ?

?

? ? ?

?

? ?

?

? ?

?

?

?

?

?

? ? ?

?

?

?

? ?

EXPLANATION

Figure 17. Paleogeographic map of the Uinta and Piceance Basins at the beginning of the saline phase showing (1) where nahcol(2) where oil yields are greater than and less than 10 gallons per ton (GPT) (modified from Johnson and others, 2010a), and (3) extenand fluvial deposition (modified from Johnson, 1985, fig. 10).
Results of Data Analyses 33

During R-4 time, low-grade (<10 GPT) oil shale deposition, expanding to cover outer areas of the marginal shelves 

and the area of 10 GPT or greater oil shale, expanded to near 

the shelf edge at the end of the illitic phase (fig. 24). Expansion of the two oil shale areas was greatest to the south, 

where marginal shelves and slopes, as defined during previous phases, were the widest. The area of nahcolite deposition 

prior to leaching has also expanded when compared to the 

underlying L-3 zone. Leaching has occurred along a 4- to 

8-mi-wide zone on the western margin of the saline mineral 

area, and for the first time leaching has occurred along a 3- 

to 6-mi-wide area on the southern margin of the area. Saline 

minerals are partially leached from the R-4 zone in four 

coreholes (fig. 24). Nahcolite is confined largely to the upper 

two-thirds of the R-4 zone, with maximum in-place nahcolite of about 115,000 tons per acre (Brownfield and others, 

2010a). Both nahcolite aggregates and disseminated nahcolite 

are present.

The L-4 zone marks a slight expansion of the area of 

original nahcolite deposition to the southeast (fig. 25). The 

leached zone has expanded while the area with preserved 

saline minerals has contracted in comparison to the 

underlying R-4 zone. One isolated occurrence where nahcolite is preserved in the middle part of the L-4 zone was 

identified in the otherwise leached area in the southeastern 

part of the saline area (T. 3 S., R. 96 W.) (fig. 25). The area 

of 10-GPT-or-greater oil yield and the area occupied by 

marginal lacustrine rocks are similar to that of the underlying 

R-4 zone. Total in-place nahcolite is significantly greater than 

in the underlying R-4 zone, with maximum values of more 

than 152,000 tons per acre. For the first time, nahcolite beds 

are present along with disseminated nahcolite and nahcolite 

aggregates. A thick sandstone unit is present in the L-4 zone 

near Yellow Creek in the northern part of the basin (figs. 5, 25, 

pl. 2), where Lake Gosiute is hypothesized to have flowed into 

Lake Uinta.

The original extent of saline mineral deposition expanded 

significantly, mainly to the south and east during deposition 

of the R-5 zone (fig. 26). The area where saline minerals are 

preserved has diminished, producing a broad area of leaching 

around the entire saline mineral area. Four isolated occurrences of nahcolite interspersed with leached vugs and breccia 

in the R-5 zone were identified in the southeast part of the 

leached area, with three of them possibly aligning along a 

linear southeast trend. Several nahcolite and halite beds, as 

much as 40 ft thick, are present in the upper and lower parts 

of the R-5 zone (Dyni, 1974c, 1981). The area where halite is 

present is outlined on figure 26. The halite and nahcolite beds 

are leached around the margins of the saline mineral area, 

thus the original extent of these beds is unknown. Maximum 

nahcolite in the R-5 zone is more than 365,000 tons per acre 

(fig. 26). The area of low-grade (<10 GPT) oil shale expanded 

slightly when compared to the underlying L-4 zone. The R-5 

zone grades into a sandy interval near the mouth of Yellow 

Creek in the northern part of the basin (fig. 26, pl. 2).

The L-5 zone is the highest oil shale zone where saline 

minerals are still widely preserved (fig. 27). It includes two 

thick beds (up to about 60 ft thick) of nahcolite and halite 

in its lower part in a comparatively small area in the northcentral part of the basin. The original extent of these beds is 

unknown as they have been extensively leached, but Dyni 

(1974c, 1981) traced breccias that are laterally equivalent to 

these two beds for a considerable distance into the leached 

zone. Dyni (1974c, 1981) also traced a laterally continuous 

breccia stratigraphically higher in the L-5 zone, indicating that 

another thick nahcolite-halite bed was once present. The area 

of saline deposition, including the leached area, is somewhat 

larger during L-5 time than for the previous R-5 zone. Isolated 

occurrences of preserved nahcolite aggregates in the leached 

area were identified near the northern, northeastern, southeastern and western margins of the saline area. The marginal 

lacustrine areas expanded slightly, and the area where oil shale 

is greater than 10 GPT shrank markedly during deposition of 

the L-5 zone when compared to the R-5 zone.

Deposition of the R-6 zone is characterized by a continued overall expansion of the area of oil shale deposition onto 

the former marginal lacustrine shelves. The area of 10-GPTor-greater oil shale expanded to its maximum extent thus far 

(fig. 28). It is uncertain how much nahcolite and possibly 

halite was originally present in the R-6 zone because, as previously mentioned, core descriptions rarely note what percentage of the total volume of the core is represented by vugs and 

breccias. However, vugs and breccia zones have been identified throughout the R-6 zone in many coreholes, indicating 

it once contained large quantities of saline minerals. Today, 

only a few scattered occurrences of preserved nahcolite are 

present in the R-6 zone (fig. 28). The R-6 zone thickens near 

the mouth of Yellow Creek (fig. 18) because of the influx of 

sandstone and siltstone from the north (fig. 28, pl. 2).

B-groove represents the first of two unique, relatively 

thin, very lean intervals that bracket the rich Mahogany zone 

(fig. 3). B-groove is less than 40 ft thick throughout most of 

the basin, but it thickens markedly in two areas where rivers 

entered Lake Uinta: (1) in the southeastern corner of the basin 

and (2) in the vicinity of the mouth of Yellow Creek along 

the northern margin of the basin (fig. 29). B-groove exceeds 

10 GPT oil in only two restricted areas in the central part of 

the basin (fig. 29). The saline mineral area for B-groove prior 

to leaching is only slightly smaller than that for the underlying R-6 zone. B-groove tends to be more silty and sandy than 

the overlying and underlying strata, but it is similar in that it 

is mostly even-bedded to laminated. Interestingly, there are no 

reported occurrences of stromatolites, a shallow-water indicator, in B-groove anywhere in the basin (fig. 29).
34 Development, Evolution, and Destruction of Saline Mineral Area of Eocene Lake Uinta

40°00'

39°30'

EXPLANATION

R-2 Base to Long Point Base

300 - 400

400 - 500

500 - 600

600 - 700

700 - 800

800 - 906

R-2

thickest here

R-3

thickest here

90’

L-2

thickest here

50’

L-3 

thickest here

90’

45’

108°30' 108°00'

0 6

0 6

12 MILES

12 KILOMETERS

R. 96 W. R. 95 W.

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

R. 94 W. R. 93. W

T.

2

 N.

T.

1

 N.

T.

1

 S.

T.

2

 S.

T.

3

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T.

4

 S.

T.

5

 S.

T.

6

 S.

T.

7

 S.

T.

9

 S.

T.

8

 S.

Study area
Results of Data Analyses 35

Figure 18 (previous page). Map of the Piceance Basin at the end of the illitic phase showing (1) extent of marginal lacustrine 

facies at the end of the freshwater phase that pre-dated the illitic phase of Lake Uinta, (2) thickness of the illitic phase, and 

(3) where the R-2, L-2, R-3, and L-3 oil shale zones are the thickest (modified from Johnson and others, 2010a). (ft, feet)

Maximum thickness L-3 zone: 70.4 ft

300–400

400–500

500–600

700–800

600–700

800–900

Thickness from R-2 zone 

to base of Long Point Bed.

Countour interval 100 feet.

EXPLANATION

Marginal lacustrine

facies

Sandstone-rich 

marginal lacustrine

Carbonate-rich

marginal lacustrine

Top of Mahogany 

Bed outcrop

Base of Parachute

Creek Member

Maximum thickness R-2 zone: 155 ft

Maximum thickness L-2 zone: 115 ft

Maximum thickness R-3 zone: 153 ft
36 Development, Evolution, and Destruction of Saline Mineral Area of Eocene Lake Uinta

Study area

10 GPT

T.

8

 S.

T

5

S

T

2

S

2,500

5,000

7,500

10,000

R. 96 W. R. 95 W.

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

R. 94 W. R. 93. W

T.

2

 N.

T.

1

 N.

T.

1

 S.

T.

2

 S.

T.

3

 S.

T.

4

 S.

T.

5

 S.

T.

6

 S.

T.

7

 S.

T.

9

 S.

First evidence of

nahcolite deposition

in north-cental part 

of basin.

Marginal shelf

progradation during

previous illitic period

produces broad shelves

and well-defined 

central deep lake area.

Intense evaporation

on marginal shelves

produced brines and 

salt crusts that 

eventually made it to

the deep central part

of the lake.

Brines were probably

entering near here

from Uinta Basin to west.

Generalized presentday limit of Green

River Formation.

?

?

?

?

?

?

40°00'

0 3 6 12 MILES 0 6

0 6

12 MILES

12 KILOMETERS

108°30' 108°00'

40°00'

39°30'
Results of Data Analyses 37

Figure 19 (previous page). Map of the R-2 oil shale zone showing (1) where nahcolite was deposited and total in-place nahcolite 

(modified from Brownfield and others, 2010a), (2) where oil yields are greater than and less than 10 gallons per ton (GPT) (modified 

from Johnson and others, 2010a), and (3) extent of marginal lacustrine deposition (modified from Johnson, 1985, fig. 10). Locations of 

coreholes used are shown.

Sandstone-rich marginal lacustrine

Carbonate-rich marginal lacustrine

Oil shale >10 gallons per ton

Oil shale <10 gallons per ton Top of Mahogany Bed outcrop

Base of Parachute Creek Member

<25,000

Nahcolite (short tons/acre)

Interval 2,500 short tons/acre.

Nahcolite

No vugs

No samples

Coreholes

Movement of organic 

matter and carbonate ripups

Areas where intense

evaporation could 

produce brines. 

Queried where freshwater input may have 

inhibited brine formation.

Brine movement

?

EXPLANATION
38 Development, Evolution, and Destruction of Saline Mineral Area of Eocene Lake Uinta

Study area

10 GPT

10 GPT

10 GPT

75,000

25,000

50,000

?

?

?

?

?

?

?

108°30' 108°00'

39°30'

40°00'

0 6

0 6

12 MILES

12 KILOMETERS

Evidence of leached

nahcolite found in 

two coreholes along

west margin of deposit.

Little change in

position of 

marginal shelves.

Area of 10 gallons 

per ton oil shale or

greater contracts.

Nahcolite 

area expands.

Brines were probably

entering near here

from Uinta Basin to west.

Intense evaporation

on marginal shelves

produced brines and 

salt crusts that 

eventually made it to

the deep central part

of the lake.

Generalized presentday limit of Green

River Formation.

R. 96 W. R. 95 W.

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

R. 94 W. R. 93. W

T.

2

 N.

T.

1

 N.

T.

1

 S.

T.

2

 S.

T.

3

 S.

T.

4

 S.

T.

5

 S.

T.

6

 S.

T.

7

 S.

T.

9

 S.

T.

8

 S.
Results of Data Analyses 39

Figure 20 (previous page). Map of the L-2 oil shale zone showing (1) where nahcolite is present and total in-place nahcolite (modified 

from Brownfield and others, 2010a), (2) where nahcolite has been leached or partially leached out by groundwater, (3) where oil yields 

are greater than and less than 10 gallons per ton (GPT) (modified from Johnson and others, 2010a), and (4) extent of marginal lacustrine 

deposition (modified from Johnson, 1985). Locations of coreholes used are shown.

Movement of organic 

matter and carbonate ripups

Sandstone-rich marginal lacustrine

Carbonate-rich marginal lacustrine

Oil shale >10 gallons per ton

Oil shale <10 gallons per ton

Nahcolite

Vugs and breccias

No vugs

No samples

Top of Mahogany Bed outcrop

Base of Parachute Creek Member

<25,000

Nahcolite leached

Nahcolite (short tons/acre)

Interval 25,000 short tons/acre.

25,000–100,000

Coreholes and surface sections Areas where intense

evaporation could 

produce brines. 

Queried where freshwater input may have 

inhibited brine formation.

Brine movement

?

EXPLANATION
40 Development, Evolution, and Destruction of Saline Mineral Area of Eocene Lake Uinta

10 GPT

T.

8

 S.

40°00'

39°30'

0 3 6 12 MILES

39°30'

40°00'

25,000

25,000

50,000

75,000

100,000

125,000

First evidence of 

widespread leaching

of nahcolite by 

groundwater. 

Little change in

position of 

marginal shelves.

Brines were probably

entering near here

from Uinta Basin to 

west.

Generalized presentday limit of Green

River Formation.

Intense evaporation

on marginal shelves

produced brines and 

salt crusts that 

eventually made it to

the deep central part

of the lake.

?

? ?

? ?

?

0 6

0 6

12 MILES

12 KILOMETERS

108°30' 108°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. R. 93. W

T.

2

 N.

T.

1

 N.

T.

1

 S.

T.

2

 S.

T.

3

 S.

T.

4

 S.

T.

5

 S.

T.

6

 S.

T.

7

 S.

T.

9

 S.

Study area
Results of Data Analyses 41

Figure 21 (previous page). Map of the R-3 oil shale zone showing (1) where nahcolite is present and total in-place nahcolite (modified 

from Brownfield and others, 2010a), (2) where nahcolite has been leached or partially leached out by groundwater, (3) where oil yields 

are greater than and less than 10 gallons per ton (GPT) (modified from Johnson and others, 2010a), and (4) extent of marginal lacustrine 

deposition (modified from Johnson, 1985). Locations of coreholes used are shown.

Movement of organic 

matter and carbonate 

ripups

Sandstone-rich marginal lacustrine

Carbonate-rich marginal lacustrine

Oil shale >10 gallons per ton

Oil shale <10 gallons per ton

Areas where intense

evaporation could 

produce brines. 

Queried where freshwater input may have 

inhibited brine formation.

Brine movement

Nahcolite

Nahcolite vugs

No vugs

Isolated occurrence of nahcolite

in the leached zone and outside

the area assessed by Brownfield

and others (2010a)

Both nahcolite and vugs

No samples

Top of Mahogany Bed outcrop

Base of Parachute Creek Member

<25,000

Nahcolite leached

Nahcolite (short tons/acre)

Interval 25,000 short tons/acre.

25,000–100,000

100,000–200,000

Coreholes and surface sections

?

EXPLANATION
42 Development, Evolution, and Destruction of Saline Mineral Area of Eocene Lake Uinta

R 99 W

0 3 6 12 MILES

7,500 15,000

22,500

30,000

10 GPT

? ?

?

? ?

?

?

0 6

0 6

12 MILES

12 KILOMETERS

Study area

108°30' 108°00'

40°00'

39°30'

R. 96 W. R. 95 W.

R. 101 W. R. 100 W. R. 98 W. R. 97 W.

R. 94 W. R. 93. W

T.

2

 N.

T.

1

 N.

T.

1

 S.

T.

2

 S.

T.

3

 S.

T.

4

 S.

T.

5

 S.

T.

6

 S.

T.

7

 S.

T.

9

 S.

T.

8

 S.

Area of 10 gallons

per ton or greater

oil shale contracts.

Little change in

position of 

marginal shelves.

Brines were probably

entering near here

from Uinta Basin to 

west.

Generalized presentday limit of Green

River Formation.
Results of Data Analyses 43

Figure 22 (previous page). Map of the L-3 oil shale zone showing (1) where nahcolite is present and total in-place nahcolite (modified from 

Brownfield and others, 2010a), (2) where nahcolite has been leached or partially leached out by groundwater, (3) where oil yields are greater 

than and less than 10 gallons per ton (GPT) (modified from Johnson and others, 2010a), and (4) extent of marginal lacustrine deposition 

(modified from Johnson, 1985). Locations of coreholes used are shown.

Movement of organic 

matter and carbonate 

ripups

Sandstone-rich marginal lacustrine

Carbonate-rich marginal lacustrine

Oil shale >10 gallons per ton

Oil shale <10 gallons per ton

Areas where intense evaporation 

could produce brines. Queried 

where freshwater input may 

have inhibited brine formation.

Brine movement

Nahcolite

Nahcolite vugs

No vugs

No samples

<25,000

Nahcolite leached

Nahcolite (short tons/acre)

Interval 7,500 short tons/acre.

25,000–100,000

Coreholes and surface sections

?

EXPLANATION

Top of Mahogany Bed outcrop

Base of Parachute Creek Member
44 Development, Evolution, and Destruction of Saline Mineral Area of Eocene Lake Uinta

Douglas

Creek arch

Laminated oil shale. Laminated oil shale

Lake Uinta gradually expands throughout most of the middle saline mineral phase except 

during deposition of the L-5 zone when the lake contracts. Deepwater oil shale deposition 

expands across former marginal shelf areas. The deep brine layer expands to nearshelf edge 

at end of the illitic phase. Marginal shelves, where brines could evolve, have been greatly 

reduced. Brines may still be entering the Piceance Basin from the Uinta Basin to the west. 

Mainly laminated oil shale is deposited on former shelf areas.

L-5 zone

White River uplift

Douglas

Creek arch

Oil shale and saline mineral depocenter begins to

fill in. Offshore lacustrine conditions expand across

marginal flats.

A. End of early saline mineral phase

B. Just prior to Mahogany zone deposition

Laminated and blebby oil

shale, and nahcolite.

Laminated and blebby oil

shale, nahcolite and halite.

Section thickens to east

because of increased subsidence

toward the White River uplift.

Section thickens to east

because of increased subsidence

toward the White River uplift.

White River uplift

? ? ? ?

Organic matter, carbonate 

ripups, and brines.
Results of Data Analyses 45

Figure 23 (previous page). West-to-east schematic cross section across the Piceance Basin for two time periods. A, End of early 

saline mineral phase. B, Just prior to deposition of the Mahogany oil shale zone. Figure 13 shows this cross section in three earlier time 

periods. Approximate line of section shown on figure 14.

Variegated mudstone, sandstone, and siltstone

Carbonate-rich marginal lacustrine mudstone with 

ostracodal, oolitic, and stromatolitic limestones

(carbonate mudflats)

Sandstone-rich marginal lacustrine mudstone with 

some ostracodal, oolitic, and stromatolitic limestones

(sandy mudflats)

Rich illitic oil shale

Lean dolomitic oil shale 

Nahcolite

Rich dolomitic oil shale with nahcolite and halite

Saline brines

Organic matter and 

carbonate ripups

Areas where intense evaporation could 

produce brines. Queried where freshwater 

input may have inhibited brine formation.

EXPLANATION

?
46 Development, Evolution, and Destruction of Saline Mineral Area of Eocene Lake Uinta

10GPT

25,000

50,000

75,000

100,000

T.

8

 S.

0 3 6 12 MILES

Groundwater is

now leaching 

nahcolite from both

west and south.

Nahcolite area

prior to leaching

expands.

Brines were probably

entering near here

from Uinta Basin to 

west.

Intense evaporation

on marginal shelves

produced brines and 

salt crusts that 

eventually made it to

the deep central part

of the lake.

Generalized presentday limit of Green

River Formation. ?

? ?

? ?

?

?

0 6

0 6

12 MILES

12 KILOMETERS

108°30' 108°00'

40°00'

39°30'

R. 96 W. R. 95 W.

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

R. 94 W. R. 93. W

T.

2

 N.

T.

1

 N.

T.

1

 S.

T.

2

 S.

T.

3

 S.

T.

4

 S.

T.

5

 S.

T.

6

 S.

T.

7

 S.

T.

9

 S.

Oil Shale deposition

expands to cover

outer parts of

marginal shelves.

Study area
Results of Data Analyses 47

Figure 24 (previous page). Map of the R-4 oil shale zone showing (1) where nahcolite is present and total in-place nahcolite (modified 

from Brownfield and others, 2010a), (2) where nahcolite has been leached or partially leached out by groundwater, (3) where oil yields 

are greater than and less than 10 gallons per ton (GPT) (modified from Johnson and others, 2010a), (4) extent of marginal lacustrine 

deposition (modified from Johnson, 1985), and (5) limit of marginal lacustrine facies at the end of the illitic phase. Locations of coreholes 

used are shown.

Movement of organic 

matter and carbonate 

ripups

Sandstone-rich marginal lacustrine

Carbonate-rich marginal lacustrine

Oil shale >10 gallons per ton

Oil shale <10 gallons per ton

Areas where intense

evaporation could 

produce brines. 

Queried where freshwater input may have 

inhibited brine formation.

Brine movement

Nahcolite

Nahcolite vugs

No vugs

Both nahcolite and vugs

No samples

Top of Mahogany Bed outcrop

Base of Parachute Creek Member

<25,000

Nahcolite leached

Nahcolite (short tons/acre)

Interval 25,000 short tons/acre

25,000–100,000

100,000–200,000

Coreholes and surface sections

?

EXPLANATION

Limit of marginal lacustrine

facies at end of illite phase
48 Development, Evolution, and Destruction of Saline Mineral Area of Eocene Lake Uinta

COLORADO

Index map

10 GPT

30,000

60,000

90,000

120,000

150,000

T.

8

 S.

?

?

?

?

? ?

?

?

0 6

0 6

12 MILES

12 KILOMETERS

108°30' 108°00'

40°00'

39°30'

R. 96 W. R. 95 W.

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

R. 94 W. R. 93. W

R. 94 W.

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

R. 96 W. R. 95 W.

T.

2

 N.

T.

1

 N.

T.

1

 S.

T.

2

 S.

T.

3

 S.

T.

4

 S.

T.

5

 S.

T.

6

 S.

T.

7

 S.

T.

9

 S.

Area of nahcolite

deposition begins

to expand to 

southeast.

Area of intact

nahcolite contracts

slightly from R-4 zone.

Little change in

position of 

marginal shelves

from previous R-4

zone.

Sandy area along

north margin of basin

possibly indicates

inflow from Lake

Gosiute.

Brines were probably

entering near here

from Uinta Basin to 

west.

Intense evaporation

on marginal shelves

produced brines and 

salt crusts that 

eventually made it to

the deep central part

of the lake.

Generalized presentday limit of Green

River Formation.
Results of Data Analyses 49

Figure 25 (previous page). Map of the L-4 oil shale zone showing (1) where nahcolite is present and total in-place nahcolite (modified 

from Brownfield and others, 2010a), (2) where nahcolite has been leached or partially leached out by groundwater, (3) where oil yields 

are greater than and less than 10 gallons per ton (GPT) (modified from Johnson and others, 2010a), (4) extent of marginal lacustrine 

deposition (modified from Johnson, 1985), and (5) limit of marginal lacustrine facies at the end of the illitic phase. Locations of coreholes 

used are shown. An isolated occurrence of preserved nahcolite in the southeastern part of the leached area was identified.

Isolated occurrence of nahcolite

in the leached zone and outside

the area assessed by Brownfield

and others (2010a)

Movement of organic 

matter and carbonate 

ripups

Sandstone-rich marginal lacustrine

Carbonate-rich marginal lacustrine

Oil shale >10 gallons per ton

Oil shale <10 gallons per ton

Areas where intense

evaporation could 

produce brines. 

Queried where freshwater input may have 

inhibited brine formation.

Brine movement

Nahcolite

Nahcolite vugs

No vugs

Both nahcolite and vugs

No samples

Top of Mahogany Bed outcrop

Base of Parachute Creek Member

<30,000

Nahcolite leached

Nahcolite (short tons/acre)

Interval 30,000 short tons/acre

30,000–90,000

90,000–180,000

Coreholes and surface sections

?

EXPLANATION

Limit of marginal lacustrine facies

at end illite phase
50 Development, Evolution, and Destruction of Saline Mineral Area of Eocene Lake Uinta

Study area

10GPT

300,000

200,000

100,000

10,000

R. 97 W.

0 3 6 12

Area of nahcolite

deposition continues

to expand to southeast.

Halite beds to 64 ft

thick are deposited

during this time.

Oil shale deposition

continues to expand

over areas that were

previously marginal

shelves.

Several isolated

occurrences of 

intact nahcolite

identified in southeast part of saline

area.

Sandy R-5 zone along

north margin of basin

possibly indicates

outflow from Lake

Gosiute.

Brines were probably

entering near here

from Uinta Basin to 

west.

Generalized presentday limit of Green

River Formation.

Saline minerals have

been leached from

upper part of R-5 zone

throughout much of 

remaining saline area.

Marginal shelf areas

where brines can

evolve continue to

shrink in size.

?

?

?

?

? ?

?

?

0 6

0 6

12 MILES

12 KILOMETERS

39°30'

108°30' 108°00'

40°00'

T.

2

 N.

T.

1

 N.

T.

1

 S.

T.

2

 S.

T.

3

 S.

T.

4

 S.

T.

5

 S.

T.

6

 S.

T.

7

 S.

T.

9

 S.

T.

8.

S.

R. 96 W. R. 95 W.

R. 101 W. R. 100 W. R. 99 W. R. 98 W. R. 97 W. R. 94 W. R. 93. W
Results of Data Analyses 51

Figure 26 (previous page). Map of the R-5 oil shale zone showing (1) where nahcolite is present and total in-place nahcolite (modified 

from Brownfield and others, 2010a), (2) where nahcolite has been leached or partially leached out by groundwater, (3) where oil yields 

are greater than and less than 10 gallons per ton (GPT) (modified from Johnson and others, 2010a), (4) extent of marginal lacustrine 

deposition (modified from Johnson, 1985), (5) limit of marginal lacustrine facies at the end of the illitic phase, and (6) approximate area 

where halite is present. Locations of coreholes used are shown. Four isolated occurrences of preserved nahcolite in the southeast part 

of the leached area were identified.

Isolated occurrence of nahcolite

in the leached zone and outside

the area assessed by Brownfield

and others (2010a)

Movement of organic 

matter and carbonate 

ripups

Sandstone-rich marginal lacustrine

Carbonate-rich marginal lacustrine

Oil shale >10 gallons per ton

Oil shale <10 gallons per ton

Areas where intense

evaporation could 

produce brines. 

Queried where freshwater input may have 

inhibited brine formation.

Brine movement

Nahcolite and halite

Vugs and breccias

No vugs or breccias

Saline minerals partially leached

No samples

Top of Mahogany Bed outcrop

Base of Parachute Creek Member

<10,000

Nahcolite leached

Nahcolite (short tons/acre)

Interval 90,000-100,000 short tons/acre

10,000–100,000

100,000–200,000

Coreholes and surface sections

?

EXPLANATION

200,000–300,000

300,000–400,000

Limit of marginal lacustrine facies

at end illite phase

Area of halite deposition
52 Development, Evolution, and Destruction of Saline Mineral Area of Eocene Lake Uinta

The Mahogany zone was deposited during the most 

expansive period of Lake Uinta, when the lake may have 

extended to near the flanks of the surrounding uplifts (Johnson, 1985). It is one of the richest oil shale zones in the basin, 

with nearly 192 billion barrels of oil in place, and is typically 

the richest oil shale zone in outcrop around the margins of the 

basin (Johnson and others, 2010a). As such, it has been the 

primary target of nearly all, if not all, of the oil shale surface 

mining operations in the Piceance Basin, both currently and in 

the past. Oil shale in the Mahogany zone everywhere exceeds 

10 GPT except (1) in the extreme northeastern corner of the 

basin; (2) near the mouth of Yellow Creek along the northern 

margin, where the Mahogany zone grades into sandstone, 

siltstone, and marlstone and cannot be recognized (fig. 30); 

and (3) in the southeastern part of the basin, where it contains 

thin sandstones (not shown on map). Total oil in place in the 

Mahogany zone increases from less than 200,000 barrels per 

acre (BPA) to more than 300,000 BPA toward the central part 

of the saline mineral area (fig. 30).

The original extent of saline mineral deposition in 

the Mahogany zone is slightly larger than in the R-6 zone 

or B-groove, with minor amounts of nahcolite preserved 

in five isolated occurrences (fig. 30). The total amount of 

saline minerals in the Mahogany zone prior to leaching 

appears to have been relatively low when compared to the 

R-6 zone, as only scattered vugs and breccias have been 

reported in that zone. These occurrences are most abundant 

in the lower part of the Mahogany zone, suggesting an overall 

decline in saline mineral deposition during deposition of the 

Mahogany zone.

A-groove, the other thin lean zone that brackets the 

Mahogany zone, varies from about 5 to 25 ft thick throughout most of the basin, thickening to more than 30 ft along the 

extreme southeastern margin, where a major river entered 

Lake Uinta (fig. 31). A-groove cannot be distinguished from 

underlying and overlying strata in the vicinity of Yellow Creek 

along the northern margin of the basin, where another major 

river entered Lake Uinta (figs. 5, 31, pl. 2). It occurs, however, 

within an overall sandy marginal lacustrine interval and is 

shown as such on figure 31. Nahcolite vugs occur in A-groove 

throughout an area in the central part of the basin that is significantly smaller than the area with vugs and breccias in the 

underlying Mahogany zone (fig. 31). Intact nahcolite nodules 

were reported in A-groove in two coreholes in the northern 

part of the leached zone (fig. 31). Three coreholes within the 

leached area had no reported vugs. Oil shale in A-groove 

averages less than 10 GPT throughout most of the basin 

except for a few isolated areas in the central part of the basin. 

A-groove is generally less silty and sandy than B-groove. 

Similar to the underlying Mahogany zone and B-groove, there 

are no stromatolites reported from A-groove anywhere in 

the basin.

Late Saline Mineral Phase

The interval above A-groove in the Piceance Basin, considered here to be the late saline mineral phase, was deposited 

during the infilling of the Piceance Basin part of Lake Uinta 

with volcaniclastic sediments from the north. The interval is 

a generally southward-prograding deltaic complex consisting 

of volcaniclastic wedges of the Uinta Formation interfingering with oil shale and marlstone of the upper part of the Green 

River Formation (fig. 5, pl. 2). This study focuses on the 

interval from the top of A-groove to bed 44 of Donnell (2008) 

(figs. 3, 5, pl. 2), as it is preserved over most of the basin. The 

interval above bed 44 is preserved only in limited areas in the 

southern part of the basin. Bed 44 is within the upper part of 

the Parachute Creek Member of the Green River Formation 

in the southern part of the basin and within the Thirteenmile 

Creek Tongue of the Green River Formation in the northern 

part, where it is overlain and underlain by tongues of the Uinta 

Formation (fig. 5). The Thirteenmile Creek Tongue consists 

mainly of marlstone and oil shale but contains stromatolite and charophyte remains in the northernmost part of the 

basin (Johnson, 1981), where deepwater conditions had been 

replaced by a shallow lake-margin shelf by that time.

The late saline mineral phase is represented by four figures 

(figs. 32–35). Figures 32 and 33 are paleogeographic reconstructions for the earliest and latest part of the A-groove to bed 

44 interval, respectively. Figure 32 includes an isopach map of 

the entire A-groove to bed 44 interval and the area where the 

entire interval exceeds 10 GPT (Johnson and others,

2010a, fig. 107). Figure 33 includes estimates of total oil in 

place for the entire A-groove to bed 44 interval (Johnson and 

others, 2010a, fig. 108). Figure 34, modified from Donnell 

(2008, fig. 4), shows the detailed correlation of individual oil 

shale beds in the interval above A-groove for two coreholes, one 

in the eastern Uinta Basin and the other in the southern part of 

the Piceance Basin. Both coreholes are in areas where the upper 

part of the Green River Formation does not include tongues 

of the Uinta Formation. Donnell (2008) was able to trace a 

large number of individual oil shale beds between the Uinta 

and Piceance Basins and to correlate most of the Green River 

tongues into their equivalent stratigraphic positions in the upper 

part of the Parachute Creek Member. The stratigraphic positions 

of these tongues are shown on the right side of figure 34.

Red bars to the right of the Piceance Basin corehole in 

figure 34 roughly show the stratigraphic intervals of saline 

mineral deposition for five different time periods, with the 

bars on the left representing the northernmost part of the basin 

and the bar on the right the southernmost. The approximate 

areal extent of each of these five time intervals is shown on 

figures 32 and 33. Only a limited amount of core and surface 

control was available to define these five areas, but they 

nonetheless demonstrate the southward migration of the saline 

mineral area as Lake Uinta was progressively filled in from 

the north. This southward stratigraphic rise is also apparent 
Discussion and Conclusions 53

on the detailed north to south cross section (pl. 2). The shelf 

edge at the end of the illitic phase roughly defines the extent 

of the saline mineral deposition during time periods 1 and 2, 

as it does for underlying oil shale zones. As Lake Uinta was 

progressively filled in, the saline mineral area was pushed onto 

the former marginal shelf area and ultimately into the southernmost part of that area (figs. 32–34, time periods 3–5).

As the Piceance Basin part of Lake Uinta was gradually 

filled in, nahcolite vugs appeared for the first time in the eastern part of the Uinta Basin, indicating that the deep brine layer 

was shifting from the Piceance Basin into the Uinta Basin. 

First appearance of nahcolite vugs in the eastern part of the 

Uinta Basin occur at about the stratigraphic level of bed 58 of 

Donnell (2008), or about 65–90 ft stratigraphically above bed 

44 (fig. 34) (Vanden Berg, 2010). Figure 35 shows the area 

in the eastern Uinta Basin where nahcolite was deposited for 

the first time. The interval that contains nahcolite vugs in the 

eastern part of the Uinta Basin is shown as a red bar to the left 

of the oil-yield histogram for the Uinta Basin on figure 34.

Saline mineral deposition in both basins overlapped for a 

period of time.

Discussion and Conclusions

Lake Uinta formed during the Eocene when two much 

smaller freshwater lakes, one in the Uinta Basin and the other 

in the Piceance Basin, expanded and coalesced across the 

Douglas Creek arch during the Long Point transgression. After 

maximum transgression was reached, Lake Uinta gradually 

increased in salinity, ultimately depositing large quantities of 

nahcolite and halite. Early Lake Uinta inherited the preexisting topography of the transgressed alluvial plains as well as 

that within the preceding freshwater lake, Lake Cow Ridge. 

Although the depth of Lake Cow Ridge prior to the Long 

Point transgression is unknown, it is unlikely that the lake 

was shallow, as shallow freshwater lakes tend to be short 

lived—filling in, for instance, with even a modest increase 

in sediment supply. Lake Cow Ridge existed continuously in 

the Piceance Basin for as much as 3 million years prior to the 

development of Lake Uinta, depositing thick, low-grade oil 

shale intervals in the central part of the lake in the northcentral part of the Piceance Basin. The center of subsequent 

saline mineral deposition in Lake Uinta occupied approximately this same area.

Lake-margin shelves began to prograde into Lake Uinta 

shortly after maximum transgression, and by the onset of 

saline mineral precipitation, these shelves had reached a width 

of at least 20 mi in the southern part of the Piceance Basin 

and more than 45 mi in the southern part of the Uinta Basin. 

Marginal shelf progradation was towards the deepest part of 

the lake in the north-central part of the basin, where oil shale 

deposition had occurred during the earlier freshwater lake 

interval. These shallow shelves were exposed to intense evaporation , with brines and salt crusts forming in areas where 

near complete evaporation of the water column occurred. 

Brines and salt crusts were likely washed into the deep central 

lake area once water level rose again, contributing to the deep 

brine layer that was accumulating there, as previously suggested by many authors (Smith, 1974, p. 77; Ryder and others, 

1976; Eugster and Hardie, 1978; Surdam and Stanley, 1979; 

Johnson, 1985, p. 272; Remy and Ferrell, 1989). As there are 

no known saline mineral deposits in the Uinta Basin part of 

Lake Uinta until much later, after the deep central lake area 

of the Piceance Basin had been filled in, any brines and salt 

crusts that formed on the broad marginal shelves in the Uinta 

Basin must have been washed into the deep central lake area 

in the Piceance Basin as well (Dyni, 1981). Large amounts of 

mineral and organic matter that accumulated on the marginal 

shelves also appears to have washed into the deep central lake 

area, creating the “blebby oil shale beds” that are common 

there, and it is possible that the brines, mineral matter, and 

organic matter were transported together in density currents 

that sometimes resembled turbidity currents, as envisioned by 

Dyni and Hawkins (1981).

Saline mineral deposition throughout the early saline 

mineral phase (R-2 through L-2 oil shale zones) was confined 

to the deep central lake area. Nahcolite aggregates and disseminated nahcolite crystals in the upper part of the R-2 oil 

shale zone in the north-central part of the Piceance Basin mark 

the onset of saline mineral deposition in Lake Uinta. Nahcolite aggregates formed in the soft sediments during the initial 

stages of dewatering, whereas disseminated nahcolite was 

deposited along with kerogen-rich marl near the sedimentwater interface (Dyni, 1974c, 1981). Nahcolite beds, precipitated directly from the deep brine layer, appear for the first 

time in the L-4 oil shale zone (Dyni, 1981).

Isopach maps of the R-2, L-2, and L-3 zones display 

maximum thicknesses in generally east-west trending areas 

along the southern margin of this topographic low with the 

thick area for each progressively younger zone occurring 

somewhat to the north of the preceding one (fig. 23). The R-3 

zone thickens toward the saline mineral area. These thick 

areas are significantly basinward from the marginal shelf areas 

during the R-2 through L-3 period (figs. 7–10), suggesting 

that these sediments were deposited at the base of the slope 

between the marginal shelves and deep-basin floor.

Oil shale deposition gradually expanded over the marginal shelf areas during the middle saline mineral phase as the 

water level of Lake Uinta rose, probably because of increasing 

inflow from Lake Gosiute to the north. The saline mineral area 

also expanded throughout much of the middle saline mineral 

phase, reaching approximately to the shelf edge at the end of 

the illitic phase by the end of the middle saline mineral phase. 

This suggests that the shelf break persisted as a topographic 

feature throughout the middle saline mineral phase.
54 Development, Evolution, and Destruction of Saline Mineral Area of Eocene Lake Uinta

35,000

70,000

105,000

10 GPT

10 GPT

?

?

?

?

? ?

?

?

0 6

0 6

12 MILES

12 KILOMETERS

108°30' 108°00'

39°30'

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. R. 93. W

T.

2

 N.

T.

1

 N.

T.

1

 S.

T.

2

 S.

T.

3

 S.

T.

4

 S.

T.

5

 S.

T.

6

 S.

T.

7

 S.

T.

9

 S.

T.

8

 S.

Study area

Area of nahcolite

deposition continues

to expand to 

southeast.

Several isolated

occurrences of 

intact nahcolite

identified around 

margins of saline

area.

For the first time

nahcolite zone 

extends into lowgrade (<10 GPT)

oil shale area.

Highest oil shale

zone where nahcolite

and halite widely

preserved.

Sandy L-5 zone along

north margin of basin

possibly indicates

outflow from Lake

Gosiute.

Brines were probably

entering near here

from Uinta Basin to 

west.

Generalized presentday limit of Green

River Formation.

Marginal shelf areas 

expand slightly from

underlying R-5 zone.
Discussion and Conclusions 55

Figure 27 (previous page). Map of the L-5 oil shale zone showing (1) where nahcolite is present and total in-place nahcolite (modified 

from Brownfield and others, 2010a), (2) where nahcolite has been leached or partially leached out by groundwater, (3) where oil yields are 

greater than and less than 10 gallons per ton (GPT) (modified from Johnson and others, 2010a), (4) extent of marginal lacustrine deposition 

(modified from Johnson, 1985), (5) limit of marginal lacustrine facies at the end of the illitic phase, and (6) approximate area where halite 

is present. Locations of coreholes used are shown. Several isolated occurrences of preserved nahcolite in the leached area were 

identified.

Areas where intense

evaporation could 

produce brines. 

Queried where freshwater input may have 

inhibited brine formation.

Brine movement

Nahcolite and halite

Vugs and breccias

No vugs or breccias

Saline minerals partially leached

No samples

35,000–105,000

105,000–175,000

Coreholes and surface sections

?

EXPLANATION

Isolated occurrence of nahcolite

in the leached zone and outside

the area assessed by Brownfield

and others (2010a)

Sandstone-rich marginal lacustrine

Carbonate-rich marginal lacustrine

Oil shale >10 gallons per ton

Oil shale <10 gallons per ton

Nahcolite (short tons/acre)

Interval 35,000 short tons/acre

Limit of marginal lacustrine facies

at end illite phase

Area of halite deposition

Top of Mahogany Bed outcrop

Base of Parachute Creek Member

<35,000

Nahcolite leached
56 Development, Evolution, and Destruction of Saline Mineral Area of Eocene Lake Uinta

10 GPT

10 GPT

0 3 6 12 MILES

250 ft

400 ft

500 ft

?

? ?

? ?

?

?

0 6

0 6

12 MILES

12 KILOMETERS

108°30' 108°00'

40°00'

39°30'

R. 96 W. R. 95 W.

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

R. 94 W. R. 93. W

T.

2

 N.

T.

1

 N.

T.

1

 S.

T.

2

 S.

T.

3

 S.

T.

4

 S.

T.

5

 S.

T.

6

 S.

T.

7

 S.

T.

9

 S.

T.

8

 S.

Study area

Area of saline mineral

deposition continues

to expand to shelf

edge at end of R-3

time but not onto shelf.

Several isolated

occurrences of 

intact nahcolite

identified throughout

saline area.

Area of oil shale

deposition continues 

to expand over former 

marginal shelves.

R-6 zone thickens

markedly toward

point source along

north margin of 

basin.

Brines were probably

entering near here

from Uinta Basin to 

west.

Generalized presentday limit of Green

River Formation.
Discussion and Conclusions 57

Figure 28 (previous page). Map of the R-6 oil shale zone showing (1) where nahcolite has been leached out by groundwater, 

(2) where oil yields are greater than and less than 10 gallons per ton (GPT) (modified from Johnson and others, 2010a), (3) extent of 

marginal lacustrine deposition (modified from Johnson, 1985), and (4) limit of marginal lacustrine facies at the end of the illitic phase. 

Locations of coreholes used are shown. Nahcolite is mostly leached from the R-6 zone, but several isolated occurrences of preserved 

nahcolite were identified. Isopach lines in the northern part of the basin show thickness of the R-6 zone (modified from Johnson and 

others, 2010a).

Sandstone-rich marginal lacustrine

Carbonate-rich marginal lacustrine

Oil shale >10 gallons per ton

Oil shale <10 gallons per ton

Areas where intense

evaporation could 

produce brines. 

Queried where freshwater input may have 

inhibited brine formation.

Brine movement

 Nahcolite vugs

No vugs

Both nahcolite and vugs

No samples

Top of Mahogany Bed outcrop

Base of Parachute Creek Member

Nahcolite leached

Coreholes and surface sections ?

EXPLANATION

Limit of marginal lacustrine facies

at end illite phase

Partial Isopach map of R-6 zone. 

Contour interval variable, in feet.

Nahcolite (short tons/acre)

Interval 35,000 short tons/acre

Isolated occurrence of nahcolite

in the leached zone and outside

the area assessed by Brownfield

and others (2010a)

<35,000
58 Development, Evolution, and Destruction of Saline Mineral Area of Eocene Lake Uinta

10 GPT

10 GPT

60 ft

80 ft

80 ft

80 ft

60 ft

Stromatolites, a shallow

water indicator, are not

reported in B-groove.

Saline area of B-groove

is similar to that for the 

underlying R-6 zone.

A sandy shelf was

present in a limited

area of the southeast

part of the basin.

B-groove is a thin, very

lean interval between

two very rich zones. B-groove only locally

exceeds 10 gallons 

per ton.

Marginal shelves were

reducing in size in Uinta 

Basin to the west, thus 

brine evolution from 

evaporation may have

largely ended, and brine 

movement from the Uinta

Basin may have ceased.

Generalized presentday limit of Green

River Formation.

?

?

0 6

0 6

12 MILES

12 KILOMETERS

108°30' 108°00'

40°00'

39°30'

R. 96 W. R. 95 W.

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

R. 94 W. R. 93. W

T.

2

 N.

T.

1

 N.

T.

1

 S.

T.

2

 S.

T.

3

 S.

T.

4

 S.

T.

5

 S.

T.

6

 S.

T.

7

 S.

T.

9

 S.

T.

8

 S.

Study area
Discussion and Conclusions 59

Figure 29 (previous page). Map of B-groove oil shale zone showing (1) where nahcolite has been leached out by groundwater, (2) where 

oil yields are greater than and less than 10 gallons per ton (GPT) (modified from Johnson and others, 2010a), and (3) limit of marginal 

lacustrine facies at the end of the illitic phase. Locations of coreholes used are shown. Sandy marginal lacustrine rocks are limited to the 

southeastern part of the basin and to an isolated area in the northern part of the basin. Three isolated occurrences of preserved nahcolite 

were identified. Isopach map in the northern part and southeast part of the basin is thickness of B-groove (modified from Johnson and 

others, 2010a).

Sandstone-rich marginal lacustrine

Oil shale >10 gallons per ton

Oil shale <10 gallons per ton

Areas where intense

evaporation could 

produce brines. 

Queried where freshwater input may have 

inhibited brine formation.

Brine movement

 Vugs and breccias

No vugs or breccias

No samples

Top of Mahogany Bed outcrop

Base of Parachute Creek Member

Coreholes and surface sections

?

EXPLANATION

Limit of marginal lacustrine facies

at end illite phase Nahcolite leached

Nahcolite (short tons/acre)

Partial Isopach map of 

B-groove in feet. Contour 

interval variable
60 Development, Evolution, and Destruction of Saline Mineral Area of Eocene Lake Uinta

Study area

0 3 6 12 MILES

10GPT

10 GPT

Marginal shelves largely

if not totally drowned 

by deep water greatly

reducing brine evolution.

Several isolated

occurrences of 

intact nahcolite were

identified, mainly near

the margins of the 

saline area.

Limit of saline area

is similar to position

of shelf edge at end

of R-3 time.

The Mahogany zone is the 

most extensive rich oil 

shale zone in the basin.

Marginal shelves greatly

diminish in size in Uinta 

Basin, to the west. Thus, 

brine evolution from 

evaporation may have

largely ended, and brine 

movement from the Uinta

Basin may have ceased.

Total oil in place in barrels

per acre for the Mahogany 

zone increases toward the 

central part of the saline 

mineral area.

Generalized presentday limit of Green

River Formation.

200,000 BPA

300,000 BPA

0 6

0 6

12 MILES

12 KILOMETERS

108°30' 108°00'

39°30'

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. R. 93. W

T.

2

 N.

T.

1

 N.

T.

1

 S.

T.

2

 S.

T.

3

 S.

T.

4

 S.

T.

5

 S.

T.

6

 S.

T.

7

 S.

T.

9

 S.

T.

8

 S.
Discussion and Conclusions 61

Figure 30 (previous page). Map of the Mahogany oil shale zone showing (1) where nahcolite has been leached out by groundwater, 

(2) where oil yields are greater than and less than 10 gallons per ton (GPT) (modified from Johnson and others, 2010a), (3) total oil yield 

for the Mahogany zone in barrels per acre (BPA) (modified from Johnson and others 2010a), and (4) limit of marginal lacustrine facies at 

the end of the illitic phase. Locations of coreholes used are shown. Sandy marginal lacustrine rocks are present in a limited area in the 

northern part of the basin. Several isolated occurrences of preserved nahcolite were identified.

Sandstone-rich marginal lacustrine

Oil shale >10 gallons per ton

Oil shale <10 gallons per ton Vugs and breccias

No vugs or breccias

Saline minerals partially leached

No samples

<25,000

Nahcolite leached

Nahcolite (short tons/acre)

Interval 25,000 short tons/acre)

Coreholes and surface sections

EXPLANATION

Top of Mahogany Bed outcrop

Base of Parachute Creek Member

Limit of marginal lacustrine facies

at end illite phase

Barrels of oil per acre in the 

Mahogany zone. Interval 100,000 

barrels per acre (BPA).

Isolated occurrence of nahcolite

in the leached zone and outside

the area assessed by Brownfield

and others (2010a)
62 Development, Evolution, and Destruction of Saline Mineral Area of Eocene Lake Uinta

10 GPT

10 GPT

10 GPT

10 GPT

10 GPT

10 GPT

10 GPT

10 GPT

0 3 6 12 MILES

0 6

0 6

12 MILES

12 KILOMETERS

108°30' 108°00'

40°00'

39°30'

R. 96 W. R. 95 W.

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

R. 94 W. R. 93. W

R. 96 W.

T.

2

 N.

T.

1

 N.

T.

1

 S.

T.

2

 S.

T.

3

 S.

T.

4

 S.

T.

5

 S.

T.

6

 S.

T.

7

 S.

T.

9

 S.

T.

8

 S.

Study area

Two isolated

occurrences of 

intact nahcolite were

identified, in northern

part of saline area.

A-groove only locally

exceeds 10 gallons per ton.

Stromatolites, a shallow 

water indicator are not

reported in A-groove.

A-groove is a thin, very

lean interval between

two rich zones. 

A-groove becomes sandy in

a local area along the north

margin of the basin.

Area of saline mineral

deposition contracts

somewhat.

Brines may have ceased

entering the Piceance

Basin from the Uinta 

Basin to the west.

Generalized presentday limit of Green

River Formation.
Discussion and Conclusions 63

Figure 31 (previous page). Map of A-groove oil shale zone showing (1) where nahcolite has been leached out by groundwater, 

(2) where oil yields are greater than and less than 10 gallons per ton (GPT) (modified from Johnson and others, 2010a), (3) limited area 

in the northern part of the basin where marginal lacustrine rocks were identified, and (4) limit of marginal lacustrine facies at the end 

of the illitic phase. Locations of coreholes used are shown. Two isolated occurrences of preserved nahcolite were identified near the 

northeastern margin of the leached zone.

Sandstone-rich marginal lacustrine

Oil shale >10 gallons per ton

Oil shale <10 gallons per ton Vugs and breccias

No vugs

Saline minerals partially leached

No samples

Top of Mahogany Bed outcrop

Base of Parachute Creek Member

<25,000

Nahcolite leached

Nahcolite (short tons/acre)

Interval 25,000 short tons/acre)

Coreholes and surface sections

EXPLANATION

Limit of marginal lacustrine facies

at end illite phase
64 Development, Evolution, and Destruction of Saline Mineral Area of Eocene Lake Uinta

0 3 6 12 MILES

300

600

900

1,200

1,500

10 GPT

Area of intertonguing 

between the upper 

part of Green River Fm.

and Uinta Fm.

Early in the deposition of

the A-groove to bed 44

interval, the leached zone

is confined to the northern

part of the basin.

Generalized presentday limit of Green

River Formation.

The upper part of the 

bed 44 interval is 

eroded from the northwestern part of basin.

Volcaniclastic wedges of the Uinta 

Formation increase in thickness and

abundance to the north causing the 

A-groove to bed 44 interval to thicken

in that direction.

(a) Vugs only found in 

the lowermost 50 feet

of the A-grrove to

bed 44 interval.

(b) Vugs found throughout 

most of the A-groove to

bed 44 interval.

Oil shale grade decreases 

to north due to influx of 

volcaniclastics from that 

direction. Total oil in place,

in contrast, increases toward

the central part of the basin.

0 6

0 6

12 MILES

12 KILOMETERS

108°00'

40°00'

108°30'

39°30'

Study area
Discussion and Conclusions 65

Figure 32 (previous page). Map from the beginning of deposition of the A-groove to the top of bed 44 of Donnell (2008) showing 

(1) where nahcolite has been leached out by groundwater, (2) where oil yields are greater than and less than 10 gallons per ton (GPT) 

(modified from Johnson and others, 2010a), (3) thickness of the A-groove to top of bed 44 interval (modified from Johnson and others, 

2010a), and (4) limit of marginal lacustrine facies at the end of the illitic phase. Locations of coreholes used are shown. The leached area 

is subdivided into (a) where vugs are found only in the lowermost 50 feet of the A-groove to bed 44 interval (dotted area), and (b) where 

vugs are found throughout most of the A-groove to bed 44 interval.

Limit of marginal lacustrine facies

at end illite phase

Isopach map of the interval from 

top A-groove to top Bed 44 interval. 

Contour interval 300 feet.

EXPLANATION

Both nahcolite and nahcolite

vugs below bed 44

Nahcolite vugs below bed 44

Nahcolite vugs above bed 44

No vugs

No samples

Top of Mahogany Bed outcrop

Nahcolite leached Base of Parachute Creek Member

Nahcolite vugs both above

and below bed 44

Oil shale >10 gallons per ton Coreholes
66 Development, Evolution, and Destruction of Saline Mineral Area of Eocene Lake Uinta

0 3 6 12 MILES

400,000 BPA

300,000 BPA

0 6

0 6

12 MILES

12 KILOMETERS

108°30' 108°00'

40°00'

39°30'

R. 96 W. R. 95 W.

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

R. 94 W. R. 93. W

T.

2

 N.

T.

1

 N.

T.

1

 S.

T.

2

 S.

T.

3

 S.

T.

4

 S.

T.

5

 S.

T.

6

 S.

T.

7

 S.

T.

9

 S.

T.

8

 S.

(a) Nahcolite vugs occur 

both above and below 

bed 44 interval here.

Stromatolites, a shallow water 

indicator, are not reported in

this interval.

Area of intertonguing 

between the upper 

part of Green River Fm.

and Uinta Fm.

Generalized presentday limit of Green

River Formation.

The upper part of the 

bed 44 interval is 

eroded from the northwest part of basin.

(c) Nahcolite vugs 

occur only in upper 

part of interval

interval above bed 44.

(b) Nahcolite vugs 

occur throughout

interval above 

bed 44.

Total oil in place is greatest

in the central part of the 

basin where Uinta tongues

are present.

Study area
Discussion and Conclusions 67

Figure 33 (previous page). Map from the top of bed 44 to the top of the oil shale interval showing (1) where nahcolite vugs occur above 

bed 44, (2) extent of marginal lacustrine deposition at top of bed 44, (3) total oil in place in the B-groove to bed 44 interval, and (4) limit of 

marginal lacustrine facies at the end of the illitic phase. Locations of coreholes used are shown. The leached area is subdivided into 

(a) where vugs occur both above and below bed 44 (dotted area), (b) where vugs occur throughout interval above bed 44, and (c) where 

nahcolite vugs occur only in uppermost part of interval above bed 44 (dotted area).

Limit of marginal lacustrine facies

at end illite phase

Total oil in place. Interval 

100,000 barrels per acre (BPA).

Arrow showing direction of 

transport of volcaniclastic 

material interbedded 

with oil shale

No samples

EXPLANATION

Mostly oil shale with some 

volcaniclastic material

Nahcolite vugs below bed 44

Nahcolite vugs above bed 44

No vugs

Top of Mahogany Bed outcrop

Base of Parachute Creek Member

Nahcolite leached

Nahcolite vugs both above

and below bed 44

Carbonate-rich marginal lacustrine Coreholes

Both nahcolite and nahcolite

vugs below bed 44
68 Development, Evolution, and Destruction of Saline Mineral Area of Eocene Lake Uinta

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

10

8

6

4

2

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

3837

34

32

30

28

26

25

24

22

20

18

16

14

12

10

8

6

4

2

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.

4 5

3

2

1

A B
Discussion and Conclusions 69

Figure 34 (previous page). Cross section showing correlation of individual oil shale beds between two coreholes for the interval 

above A-groove (modified from Donnell, 2008, fig. 4). A, Eastern part of the Uinta Basin. The red bar for Uinta Basin corehole shows 

stratigraphic interval where nahcolite vugs are found in the eastern part of the Uinta Basin. Area where nahcolite vugs are found in the 

eastern part of the Uinta Basin shown on figure 35. B, Southern part of the Piceance Basin. Five red bars show stratigraphic interval 

where nahcolite vugs are found for five different areas in the Piceance Basin with area (1) the farthest north and area (5) the farthest 

south. Areas where vugs are found in these five intervals are shown on figures 32 and 33. Locations of coreholes are shown on figure 

35. Uinta Formation units (Tu) are numbered 2 through 13. Numbers 2 through 76 are persistent oil shale beds defined by Donnell (2008). 

(“A”, base of A-groove; 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 Sulfur 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.)
Figure 35. Paleogeographic map of the Uinta and Piceance Basins at the end of the A-groove to bed 44 interval showing (1) whoccur in both basins, (2) where oil yields are greater than and less than 10 gallons per ton (GPT) (modified from Johnson and othe(3) areas of marginal lacustrine deposition (modified from Johnson, 1985).

RIO BLANCO

GARFIELD

MOFFAT

SUMMIT

UINTAH

UTAH

CARBON

WASATCH

DUCHESNE

MESA

M

EMERY

GRAND

§ ̈¦7

§ ̈¦70

£¤191

£¤6

£¤40

£¤

£¤191

£¤40 £¤87

£¤6

£¤191

Grand Junction

Fruita

Vernal

Roosevelt

Price

MMooaabb

111° 110° 109° 

40°

39°'

10 GPT

10 GPT

Brines and salt crustsmay still have been produced on much reduced marginal shelves.

Nahcolite vugs appear forthe first time in the easternUinta basin, indicating that saline brines were shiftingfrom the Piceance Basininto the Uinta Basin.

Saline mineraldepocenters possibly connected here.

Sandstone-rich marginal lacustrineOil shale >10 gallons per tonOil shale <10 gallons per ton

Top of Mahogany Bed outcrop

Base of Parachute Creek Member

Nahcolite leached

General direction of volcaniclastic sandstone and siltstone transport Mostly oil shale with some volcaniclastic material

?

?

Areas where intenseevaporation could produce brines. Quered where freshwater input may have inhibited brine formation.

Hypothesized directionof brine movement.Queried because brinesmay not have formedat this time.

Isopach map if the interval from top of A-groove to topbed 44 interval. Contourinterval: 300 feet

0 10 20 MILES

0 10 20 KILOMETERS

UTAH COLORADO

EXPLANATION

?

? ?

?

?

?

?

?

?

Nahcolite 

Nahcolite vugs

300

600

9001,200

1,500

U31
References Cited 71

In the late saline mineral phase, the saline mineral area 

was progressively pushed southward as the Piceance Basin 

portion of Lake Uinta was filled in by volcaniclastic sediments, ultimately reaching the former shelf areas in the southern part of the basin. Saline minerals appear for the first time 

in the eastern part of the Uinta Basin during this period, indicating that the brine layer was being pushed into that basin, 

and for a time, saline minerals were deposited in both basins.

Leaching of the saline interval is complex, and a transition zone is commonly present in which saline minerals 

have been only partially removed. Permeability of oil shale 

is low, and a scarcity of fractures may explain the incomplete groundwater penetration in these areas. The base of the 

transition zone appears to be a fairly well-defined surface that 

generally slopes downward from recharge areas around the 

southern and western margins of the basin to discharge points 

just south of the White River in the northern part of the basin. 

It is not entirely clear what controls the position of this surface 

or even if it is a relatively smooth surface, as the limited 

data used here suggest. The surface appears to rise over the 

Piceance Creek dome, suggesting that basin structure plays 

some role, at least locally. The base of leaching also slopes 

toward the discharge area, suggesting that this may play a role. 

The base of the surface is about 1,600 ft lower than the saline 

springs in the discharge area. Robson and Saulnier (1981) 

presented compelling evidence that faults and fractures allow 

highly saline waters to reach the surface in the discharge area, 

but basin structure may also play a role. The basin sharply 

upturns, with dips to as much as 14o (Pipiringos and Johnson, 

1976) (fig. 21), in the discharge area, possibly allowing saline 

groundwater to move updip along bedding planes. In addition, 

groundwater movement may have been aided by the presence 

of permeable lithologies such as marginal lacustrine sandstones and carbonates in this area, particularly in the vicinity 

of the mouth of Yellow Creek (pl. 2).

Acknowledgments

We thank Thomas Finn and Peter McMahon for reviewing this manuscript and adding many useful comments. We 

would also like to thank Jennifer Eoff, who reviewed the 

Power Point version of this manuscript and suggested a major 

reorganization, which was adopted in this published version 

as well. We would also like to thank the many oil companies 

that released their oil shale data to the U.S. Geological Survey 

making this research possible. Funding was provided under 

the New Petroleum Processes Research Project headed by 

Michael D. Lewan.

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76 Development, Evolution, and Destruction of Saline Mineral Area of Eocene Lake Uinta

Plates

Detailed structural cross sections through the Eocene Green River Formation across the Piceance Basin, northwestern Colorado, 

constructed from oil-yield histograms and core descriptions from coreholes drilled to evaluate oil shale resources and from detailed 

surface sections. Correlation of individual oil shale zones, variations in lithologies, and intervals where saline minerals are present or 

where there is evidence that saline minerals were leached out are shown. Datum is sea level.

1. West to east cross section across the Piceance Basin ...........................................................................................................Link

2. North to south cross section across the Piceance Basin .......................................................................................................Link

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http://energy.usgs.gov/
Johnson and Brownfield—Development, Evolution, and Destruction of Saline Mineral Area of Eocene Lake Uinta—Scientific Investigations Report 2013–5176

ISSN 2328-0328 (online)

http://dx.doi.org/10.3133/sir20135176