Testing Alternative Paleogeographies Using Comparative Geochemistry and Stratigraphy of Eocene Lake Deposits, Southwest Wyoming

During the early Eocene, large lakes inundated basins among the Rocky Mountain region. Within Fossil and Bridger basins, sediment accumulated, forming the Green River Formation. Despite their proximity, similar stratigraphy, and fossils, the paleogeographic relationship between the basins remains a topic of active discussion. East-directed contraction of the Sevier fold-thrust belt produced a ridge between the two lacustrine basins, but its timing is poorly constrained. Geochemical data from both basins may be able to test whether the ridge pre- or post-dates the lakes. Four stratigraphic sections in the Green River Formation within the two basins were measured and sampled for elemental and isotope geochemistry. In Fossil Basin, increasing Sr/Ca ratios and increasing variability upsection indicate a shift from freshwater lacustrine to evaporative lithofacies that record a transition from an open to closed basin, possibly in response to thrust fault reactivation later than previously thought. Strontium isotope ratios (87Sr/86Sr) within Fossil and Bridger basins have similar values between 0.712–0.714, suggesting either a shared water source or mixing of lake water between Fossil and Bridger basins because these values are higher than Sevier thrust-belt values (~0.709) and are lower than basement-cored uplift values (~0.732–0.843).