Abstract

During deposition of the Green River Formation, ancient Lake Gosiute started as a fresh-water lake, then evolved to a saline, alkaline lake and finally ended as a fresh-water lake. This evolution reflects the change from a closed-basin hydrologic regime to an open-basin hydrologic regime. As a result, sedimentation in the Lake Gosiute system was strongly influenced by the relationship between evaporation and inflow of water into the basin. In the Green River Formation, stratification sequences, sedimentary structures, and mineralogy of lithofacies provide important insights into the evolution of the system and into the competing factors that determined the type of sediment accumulated in the lake and fringing environments. Carbonate sedimentation was strongly influenced by lacustrine transgressions and regressions across a very low topographic gradient. Terrigenous rocks reflect progradation of beach and deltaic shorelines during wetter climatic intervals when detritus that had been produced and stored in upland areas during preceding drier intervals was transported to the lake.

Hydrochemistry of Lake Gosiute during the deposition of Wilkins Peak Member was controlled by ground water discharge, whereas during the deposition of the Tipton and Laney Members it was controlled largely by surface water. Calcite precipitated in the lake as a result of mixing calcium-rich inflow and saline-alkaline lake waters. Dolomite formed as a result of periodic flooding and drying of the playa fringe (carbonate mud flat). where carbonate muds were saturated with saline-alkaline lake waters and underwent evaporative pumping. Some surface waters reaching the playa-lake were preconcentrated by dissolution of efflorescent crusts generated by capillary draw of connate waters in alluvial plain and mud-flat sediments. Trona and halite precipitated from brine pools as the lake shrank during periods of intense aridity. During more humid periods the lake expanded and oil shale was deposited.

The stratigraphic framework of the Green River Formation reflects the dynamic nature of Eocene Lake Gosiute. All members of the formation are characterized by a diversity of lithofacies and repetitive stratification sequences.