COMPARATIVE GEOCHEMISTRY AND STRATIGRAPHY OF EOCENE LAKE DEPOSITS, SOUTHWEST WYOMING

During the early Eocene, large lakes occupied the landscape in southwest Wyoming. Within two of these lakes, Fossil Lake in Fossil Basin and Lake Gosiute in the Green River Basin, sediment accumulated and lithified into the sedimentary rocks of the Green River Formation. These rocks record the chemical evolution of the lakes through time. Thrust faults attributed to contractile deformation produced by the subduction of the Farallon Slab formed a ridge between the two lake basins. Despite their proximity, similar stratigraphy, and fossils, the relationship between the two lakes remains a topic of active discussion. Newly gathered geochemical data from Fossil Basin provides insight into the lakes’ geochemical evolution and connectivity.

Four stratigraphic sections were measured spanning the Tipton and Wilkins Peak Members in the Green River Basin and the Fossil Butte and Angelo Members in Fossil Basin. A total of 175 samples from these sections formed the basis for stable and radiogenic isotope analysis. Preliminary results include ratios between stable carbon isotopes, which correspond to shifts in biological productivity and lake stratification. Ratios between stable oxygen isotopes reflect variations in evaporation and weathering. Strontium isotopes in carbonate samples track watershed provenance.

Distinct shifts in lithofacies across both basins indicate changes in the depositional environments through time. Lower strata within the Tipton and Fossil Butte Members exhibit laminated and massive micrite and marlstone. Lower strata also include oil shale beds and finely laminated muds. Several beds contain fish, coprolite, burrow, ostracod, and gastropod fossils. Lower strata are believed to reflect a freshwater, lacustrine environment. The upper strata, represented by the Wilkins Peak and Angelo Members, are characterized by bedded evaporites and oolitic and stromatolitic limestone facies. Some upper beds contain meter-scale spherical stromatolites, tufa encased wood impressions, and wood fragments. Upper strata are interpreted to reflect a transition to a shallower, saline environment. This initial data reveals a story of an evolving lacustrine environment in southwest Wyoming.