The changes in Eocene Lakes Gosiute and Uinta are defined by patterns of clastic facies; chemistry, mineralogy, and biology of lacustrine sediments; and stratigraphic distribution of rock types. The variations in the dispersal of sand, distribution of rich oil shale, and mineralogic variations in evaporite facies correspond to major changes in hydrographic limits of the lake systems. The hydrographic limits of the basins controlled the input of terrigenous debris, the supply and nature of solutes, water depth, and organic productivity of the lake systems.

Expansion of the Lake Uinta hydrographic basin to

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include waters from the Lake Gosiute basin, late in the history of Lake Gosiute, led to a significant increase in water supplies to Lake Uinta. Initially, the sediment load of this water was trapped in the greater Green River basin during the final stages of Lake Gosiute, so that the water from the north was devoid of clastic debris. The influx of this water, nearly devoid of terrigenous material, resulted in high biologic productivity and the deposition of rich oil shales (Mahogany Bed) in the Piceance basin.

Facies patterns in the Green River Formation in the Piceance Creek and Uinta basins suggest that these two basins were separate lakes until the addition of water from the Lake Gosiute hydrographic basin. Prior to the merging of the lakes in the Piceance and Uinta basins, the brine evolution and, hence, the saline mineralogy were different in the two basins. In the Piceance Creek basin the evaporite minerals were sodium carbonates and chlorides, whereas, in the Uinta basin, the evaporite minerals were sulfate-rich. After merging of the lakes into greater Lake Uinta, the evaporite facies were characterized by sodium carbonate minerals.

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