Nahcolite and Halite Deposition through Time During the 

Saline Mineral Phase of Eocene Lake Uinta, Piceance 

Basin, Western Colorado

By Ronald C. Johnson and Michael E. Brownfield

Open-File Report 2013–1114

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: 2013 

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Suggested citation:

Johnson, R.C., and Brownfield, M.E., 2013, Nahcolite and Halite Deposition through Time During 

the Saline Mineral Phase of Eocene Lake Uinta, Piceance Basin, Western Colorado: 

U.S. Geological Survey Open-File Report 2013–1114, 71 p., http://pubs.usgs.gov/of/2013/1114/. 

Any use of trade, firm, or product names is for descriptive purposes only and does not imply 

endorsement by the U.S. Government. 

Although this information product, for the most part, is in the public domain, it also may contain

copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be 

secured from the copyright owner. 
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The isopach map on the left shows a total-in-place oil shale in all seventeen oil shale 

zones. The isopach map on the right shows the total amount of nahcolite in place in the 

eight oil shale zones that contain nahcolite. The richest oil-shale area in the basin and 

the area with the most nahcolite approximately coincide. Thus, in order to exploit the 

richest oil-shale area, nahcolite must also be produced. 

3
Lake Uinta formed when two smaller fresh-water lakes, one in the Piceance 

Basin and one in the Uinta Basin to the west, expanded during the Long Point 

transgression (represented by the Long Point Bed) and coalesced to form a single large 

lake. Salinity in Lake Uinta progressively increased, first killing off the fresh-water 

mollusks that were ubiquitous during the earlier fresh-water phase and ultimately 

resulting in the deposition of dolomite-rich oil shale, nahcolite, and halite (Johnson, 

1985). An illitic phase of Lake Uinta (labeled with a 1) preceded the carbonate and 

saline-mineral phase. Marginal infilling during the illitic phase produced broad 

marginal shelves surrounding a deeper central lake area where a saline brine layer in 

Lake Uinta later accumulated. The symbol > means greater than, and the symbol < 

means less than. 

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The symbol > means greater than, and the symbol < means less than. 

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The symbol > means greater than, and the symbol < means less than. 

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This map illustrates the present-day extent of nahcolite and the area where nahcolite 

was once present but has been leached. The symbol > means greater than, and the 

symbol < means less than. 

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Photograph of Shell’s freeze wall test, Piceance Basin. 

11
On this map, we’ve put the present-day extent and former extent of nahcolite 

and halite (leached zone) into a geologic context by adding two marginal lacustrine 

facies, sandstone rich and carbonate rich, and different colors for average oil shale 

grades greater or less than 10 gallons per ton ( GPT). The symbol > means greater than, 

and the symbol < means less than. 

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Photo shows a core containing thick nahcolite and halite beds that are present in the 

saline depocenter in the Piceance Basin. Maximum thickness of a single nahcolite and 

halite bed is more than 60 ft. 

22
Collapse breccias are produced when groundwater leaches out thick nahcolite and 

halite beds. These collapse breccias have very high porosities and permeabilities. 

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East-west structural cross section showing variations in elevation of the rich and 

lean oil shale zones in the Piceance Basin. Structural cross sections such as this are 

more useful to visualize groundwater movement than stratigraphic cross sections where 

the datum is a stratigraphic horizon. 

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The blue arrows represent groundwater moving from elevated recharge areas around 

the basin margin downward through the central part of the basin, leaching nahcolite 

and halite, and exiting the basin though saline springs where strata are upturned in the 

northern part of the basin. 

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The blue arrows represent groundwater penetrating the saline interval from elevated 

recharge areas at the basin margins. Groundwater encounters the saline interval and 

leaches some of the nahcolite and halite. Ultimately, the saline groundwater exits the 

basin through saline springs near the mouth of Piceance Creek. 

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All of the control points used in this study are included in this map. 

33
The extent of the present-day nahcolite and the leached zone where nahcolite 

was once present are placed into a geologic context by adding two marginal lacustrine 

facies and areas showing where the oil shale grade is greater or less than 10 GPT. The 

symbol > means greater than, and the symbol < means less than. 

34
Note the thickening of the illitic phase of Lake Uinta toward the marginal 

shelves. At the end of the illitic phase, marginal shelves surrounded a deeper central 

lake area. The symbol > means greater than, and the symbol < means less than. 

35
Note the thickening of the illitic phase of Lake Uinta toward the marginal 

shelves. At the end of the illitic phase, marginal shelves surrounded a deeper central 

lake area. The symbol > means greater than, and the symbol < means less than. 

36
This isopach map of the illitic phase of Lake Uinta thins toward the central part of Lake 

Uinta. 

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The symbol > means greater than, and the symbol < means less than. 

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In contrast to the earlier illitic phase that thins toward the central part of Lake 

Uinta, the early saline phase thins toward the lake margins. The symbol > means 

greater than, and the symbol < means less than. 

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The symbol > means greater than, and the symbol < means less than. 

42
This schematic model attempts to place the development of Lake Uinta into the 

overall Lower Tertiary subsidence history of the Piceance Basin. It extends from west 

to east from the Douglas Creek arch on the west, across the central part of the Piceance 

Basin, to the White River uplift on the east. It shows that, prior to the development of 

Lake Uinta, the arch could be thought of as a hinge-line with subsidence rates 

increasing eastward from the crest of the arch toward the White River uplift. 

The model depicts the beginning of the early saline phase and shows the 

marginal shelves that developed during the earlier illitic phase of Lake Uinta. It is 

proposed here that highly saline brines and salt crusts formed due to intense 

evaporation on these broad, shallow shelves, and that these brines and salt crusts were 

washed into the central deep part of Lake Uinta during storm events (pink arrows) 

forming a deep highly saline brine layer in the lake. 

43
Zones R-2 and L-2 of the early saline mineral phase. Marginal shelves in some places 

were more than 25 miles wide during this phase. Highly saline brines and salt crusts 

evolved due to intense evaporation on these shelves and were washed into the central 

deep part of Lake Uinta during storm events (pink arrows) forming a deep highly saline 

brine layer in the lake. Note that the 10 GPT and greater area contracts during 

deposition of L-2 zone. The symbol > means greater than, and the symbol < means less 

than. 

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The symbol > means greater than, and the symbol < means less than. 

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The middle saline mineral phase is outlined in black and labeled with a 3. This 

phase extends from the base of the R-4 oil shale zone to the top of the Mahogany zone. 

It thickens toward the central part of Lake Uinta due to oil shale and saline mineral 

deposition. Topographic relief may have diminished compared to the previous phase. 

Oil-shale deposition progressively encroached on the outer parts of the marginal 

shelves. By the end of the middle saline mineral phase, rich oil shale deposition was 

occurring over the full extent of the marginal shelves, or at least over that part which is 

still preserved. Relatively recent erosion around the margins of the Piceance Basin has 

removed the marginal lacustrine facies that must have existed during Mahogany time. 

The expansion of Lake Uinta during this phase was probably due in part to inflow from 

Lake Gosiute to the north. The sandy marginal lacustrine area at the north end of the 

cross section (blue arrow) is probably where this inflow occurred. The symbol > means 

greater than, and the symbol < means less than. 

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The middle saline mineral phase is outlined in black in this west-east cross section that 

extends from the easternmost part of the Uinta Basin, eastward across the Douglas 

Creek arch to the east margin of the Piceance Basin. Again, note the thickening of this 

interval toward the center of the lake and the gradual expansion of rich oil shale 

deposition across the marginal shelves. The symbol > means greater than, and the 

symbol < means less than. 

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Lake Gosiute to the north was gradually filled by volcanic sediment from the north 

during the middle saline mineral phase of Lake Uinta, progressively pushing Lake 

Gosiute southward and ultimately causing it to flow into Lake Uinta. This increased 

water supply may have caused Lake Uinta to gradually expand. 

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Sediment deposited during the early saline mineral phase and early part of the middle 

saline mineral phase have been added to the schematic presented earlier. The central 

part of Lake Uinta was gradually filled with oil shale and saline minerals during the 

early and middle saline mineral phases, and the relief between the shelves and deep 

lake area was probably diminishing. Deeper water conditions began to expand across 

the outer parts of the marginal shelves during the middle saline mineral phase. 

Although brines were still probably evolving on the marginal shelves of the Piceance 

Basin, the progressive drowning of the shelves by relatively deep-water conditions may 

have limited the volume of brines produced. Broad marginal shelves still existed in the 

Uinta Basin part of the Lake Uinta, and brines may still have been evolving there. 

50
Progressive drowning of the outer parts of the marginal shelves occurred during 

deposition of the R-4 and L-4 oil shale zones (shelf edge at end of early saline phase 

shown as green dashed line). The original extent of nahcolite deposition, as defined by 

the leached zone, continued to expand, while the present-day extent of nahcolite 

progressively contracts. Note also the sandy marginal lacustrine deposit that appears 

along the north margin of the Piceance Basin during deposition of the L-4 where inflow 

from Lake Gosiute from the north was hypothesized to have occurred. This is the oldest 

possible evidence of inflow from Lake Gosiute. The symbol > means greater than, and 

the symbol < means less than. 

51
Bedded halite (black dashed line) occurs for the first time in the R-5 zone. The original 

extent of nahcolite and halite deposition continues to expand, while the present-day 

extent of these two saline minerals markedly contracts due to later groundwater 

invasion. Note several isolated occurrences of nahcolite preserved in the otherwise 

leached areas. Note again the sandy marginal lacustrine facies along the north margin 

of the basin where flow from Lake Gosiute to the north is hypothesized to have 

occurred. The symbol > means greater than, and the symbol < means less than. 

52
Leaching of nahcolite and halite beds that were 60 feet or thicker caused collapse and 

marked thinning of the R-5 interval from 375 feet to 225 feet. The symbol > means 

greater than, and the symbol < means less than. 

53
Collapsing extended into the L-5 zone which thins from over 200 ft to less than 100 ft

where nahcolite and halite have been leached out. The symbol > means greater than, 

and the symbol < means less than. 

54
With the exception of a few isolated occurrences, all nahcolite and halite have been 

leached out of the R-6 zone. The original extent of nahcolite and halite deposition in 

the R-6 zone and overlying B-groove, as defined by the leached zone, extended to near 

the position of the shelf break at the end of the early saline mineral phase (dashed green 

line), suggesting that there was still a topographic break at this position that confined 

the deep saline brine layer. Some isopach contours from the isopach maps of the R-6 

zone and B-groove are included where these two zones thicken markedly. B-groove is 

a thin lean interval that varies from about 10-30 ft thick throughout most of the basin. 

Note, however, that it thickens to more than 80 ft along the north margin of the basin 

where inflow from Lake Gosiute was ongoing and to more than 80 ft in the southeast 

corner of the basin where another major drainage entered Lake Uinta. Note also the 

lack of evidence of shallow water deposition in most marginal areas in the B-groove 

interval. The symbol > means greater than, and the symbol < means less than. 

55
West-east schematic cross section of Lake Uinta late in the middle saline mineral 

phase, just prior to deposition of the Mahogany zone. The deep central part of the 

lake has been largely filled by oil shale and saline minerals, but some topographic 

remnants of the earlier shelf break were probably present as the deep saline brines 

seem to have been constrained by that break. Deepwater rich oil shale deposition 

occurred over almost the entire preserved area of the former marginal shelves. 

Recent extensive erosion around the margins of the Piceance Basin has removed 

almost all evidence of marginal lacustrine deposition, and thus, only the offshore 

facies from this period can be studied. 

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The original extent of saline deposition during Mahogany time extended to near the 

shelf break at the end of the illitic phase (green dashed line). Offshore oil shale 

deposition extended to the present-day limits of the Green River Formation in the 

Piceance Basin; thus, the marginal lacustrine facies from this period cannot be studied. 

A slight decrease in the original extent of saline mineral deposition occurred during 

deposition of A-groove, another thin organically lean interval similar to B-groove. As 

with B-groove, note the general lack of marginal lacustrine facies. 

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Once Lake Gosiute to the north was completely filled, it no longer acted as a sediment 

sink, and volcaniclastic debris entered the Piceance Basin. This resulted in a 

progressive north-to-south infilling of the Piceance Basin part of Lake Uinta. This 

prograding deltaic complex is as much as 1,500 ft thick, leading to the conclusion that 

the central part of Lake Uinta may have been as much as 1,000 ft deep at this time 

(Johnson, 1981). Only that part of the late saline mineral phase extending from the top 

of A-groove to the top of bed 44 of Donnell (outlined in heavy black) was studied in 

detail here. The symbol > means greater than, and the symbol < means less than. 

59
The late saline mineral phase is outlined on this east-west cross section. Note that some 

of the volcaniclastic tongues of the Uinta Formation are lenticular in an east-west 

direction. The symbol > means greater than, and the symbol < means less than. 

60
Lake Uinta in the Piceance Basin was actively being filled in during both late 

saline phase intervals represented by these maps. The isopach map shown at the north 

margin of the basin in the map on the left is the thickness of the stratigraphically lowest 

tongue of the volcaniclastic Uinta Formation, which probably occurs just above the top 

of the A-groove. Note that it is thickest where outflow from Lake Gosiute was 

indicated on earlier maps. Infilling is pushing rich oil shale deposition (greater than 10 

gallons per ton) to the southern part of the basin. Saline mineral deposition, as defined 

by the extent of the leached zone, is still largely basinward of the shelf-slope break at 

the end of the early saline mineral phase. 

On the map on the right, the isopached interval is from the top of A-groove to 

the top of bed 44 interval. This isopach map thickens markedly to the north from less 

than 300 ft to over 1,500 ft due to volcaniclastic wedges of the Uinta Formation. It 

appears that by the beginning of the bed 44 to top of Green River Formation interval, 

the deep oil shale and saline depocenter in the north-central part of the basin had been 

largely filled. This pushed the saline brines, as defined by the leached zone, into the 

southern part of the basin. The symbol > means greater than, and the symbol < means 

less than. 

61
For the first time, evidence of nahcolite deposition in the form of leached nahcolite 

vugs is found in the eastern part of the Uinta Basin, indicating that the deep saline brine 

layer was shifting to the eastern Uinta Basin as the Piceance saline depocenter was 

filled in. 

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Numerous individual oil shale beds in this interval can be easily correlated between the 

eastern part of the Uinta Basin and the southern part of the Piceance Basin, suggesting 

that the deep brine pools in both basins were connected. The area where these two brine 

pools would have connected, however, has been eroded. This cross section correlates 

individual oil shale beds (numbered 2-76) in the upper part of the Eocene Green River 

Formation between the eastern Uinta Basin and southern part of the Piceance Basin. 

Groups of beds form named tongues (indicated by stratigraphic unit symbols) in the 

Piceance Basin. These tongues have not been identified in the Uinta Basin. Modified 

from Donnell (2008). Abbreviations used: MB, Mahogany bed; A, base of A-groove; 

Tggc, Marlstone at Greasewood Creek; Tgtc, Marlstone at Trail Canyon; Tgm, Marsltone 

at Mare Canyon; Tgy, Yellow Creek Tongue; Tgd, Dry Fork Tongue; Tgt, Thirteenmile 

Creek Tongue; Tgb, Black Sulfur Tongue; Tgc, Cough’s Creek Tongue; Tgs, Stewart Gulch 

Tongue; Tgsl, Tgi, Marlstone at Barnes Ridge and Marder bed at Bull Fork; Tgsk, 

Marlstone at Skinner Ridge. 

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