Uinta Lake

The sediments deposited in this possible oil shale lake began being deposited about the same time as the earliest deposition in Wyoming and in Colorado, elastics from Uinta Mountain activity apparently destroyed the Utah lake, but oil shale deposition returned to Utah when Colorado s Lake Uinta expanded during the time of deposition of the Mahogany zone (8). 

Figure 8 illustrates the nature of the chemically stratified lake. The upper (low density) layer, called the mixolimnion, is separated at a level called the chemocline from the high density lower layer called the monimolimnion. The chemocline forms a real barrier. The two layers circulate independently mixing very little. Smith and Lee demonstrated the definite stability of such stratification in ancient Lake Uinta (4). 

In ancient Lake Uinta the mixolimnion was exposed to the atmosphere, was oxygenated, and supported life. All fresh water entering the lake (rain, streams, runoff, etc.) joined the mixolimnion. The bottom layer of the lake was isolated from the atmosphere. Notice in Figure 8 that the lower layer (monimolimnion) does not reach the water s edge. Near shore the lake consisted only of the upper layer, forming a normal lake capable of producing shallow water expressions like algal heads, shells, and plant fossils. 

Figure 8. Stratified Lake Uinta. (Reproduced with permission from Ref 4. Copyright 1983, Colorado School of Mines.)...

Smith and Robb (2) describe the formation of dolomite, calcite, and aragonite in Lake Uinta. Calcite was formed in the lake s lower layer from the calcium released by chemical digestion of silicates and aluminosilicates. Calcium released to a calcite-saturated sodium carbonate solution immediately formed calcite. 

In northwest Colorado, the Parachute Creek Member of the lacustrine Green River Formation (Eocene) contains thick sequences of rich oil shale that were deposited in ancient lake Uinta. The richest sequence and the richest oil-shale bed within the Parachute Creek Member are called the Mahogany zone... 

The suite of minerals found in the Uinta Sandstone collected from Parachute Creek reflects the unique geochemical environment provided by Lake Uinta. The dominant minerals in the fresh sandstone are quartz, sodium plagioclase feldspars, and potassic feldspars as indicated by X-ray diffraction analysis (Ij. Important accessory minerals are caldte, nahcolite, dolomite-ankerite, and talc. Tlace minerals include iron-enriched magnesite in the siderite-magnesite series, pyrite, pyrrhotite, and hematite. Chemical properties of representative samples of the Uinta San tone are summarized in Thble I. 

GlLSONITE. Tlie mineral Gilsonite. named for S.H. Gilson of Salt Lake City, is a variety of asphaltum that occurs in Uinta County. Utah. It is found in black lustrous masses that ignite easily. A less frequently used name for it is uinlaite. 

The Green River Formation with its energy-rich oil shales and its remarkable collection of huge amounts of novel minerals is the product of a gigantic chemical process system which established itself in ancient lakes surrounding Utah s Uinta Mountains. Once established the chemical process system stabilized and maintained itself, repeating the same chemical processes continuously for millions of years on the raw materials entering the lake. 

The two (or three) lakes appear to have developed the same characteristics at about the same time. The one major factor they have in common is proximity to the Uinta Mountains. Perhaps the materials exposed by the mountain activity at that particular time led to the development of the unusual chemistry of the Green River lakes. 

Lacustrine sediments of the Green River formation were deposited in two large lakes that occupied 65,000 km in several sedimentary/structural basins in Colorado, Wyoming, and Utah during early through middle Eocene time. The Uinta Mountain uplift and its eastward extension, the Axial Basin anticline, separate these basins. The Green River lake system... 

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