PALEOHYDROLOGIC EVOLUTION OF LAKE SYSTEMS IN THE WASHAKIE AND PICEANCE BASINS, WYOMING AND COLORADO

Although paleoenvironmental reconstruction models clearly demonstrate that the Greater Green River basin and the Uinta basin were at times both connected as open lake systems during the Eocene, there is a lack of knowledge about the evolution of the basins to individual closed basins at ~50 Ma. The Eocene Green River Formation was deposited between ~53 and 48.5 Ma. Lake Gosiute was located in what is today known as the Greater Green River Basin, which is composed of four sub-basins: the Bridger Basin, the Great Divide Basin, the Washakie Basin and the Sand Wash Basin. Lake Uinta was located in the Uinta Basin and the Piceance Basin lies adjacent to the Uinta Basin. Lacustrine carbonate samples were taken from the Laney Member of the Green River Fm. at Kinney Rim (Washakie Basin) and the Parachute Creek Member of the Green River Fm Hay Gulch (Piceance Basin) focused around a fill-to-spill boundary. Here, we use carbon and oxygen isotopic to determine whether the lakes were internally or externally drained during the Middle Eocene. The statistical correlation of δ¹³C and δ¹⁸O from lacustrine carbonates varies as a function of the residence time of the lake water and provides empirical evidence for the transition from an externally drained to internally drained system. There is a high correlation coefficient both above and below the analcite layer in the Washakie with both being .9 indicating a closed system. Samples with low covariance indicates periods of open lake periods, which is at the beginning and end of our sampling. Samples in the Piceance Basin demonstrated a high correlation coefficient in all samples at about .9 indicating a closed lake system. The δ¹⁸O values of the Washakie Basin are ~3 ‰ more positive than δ¹⁸O values in the Piceance Basin. The isotopic data from the Piceance Basin indicates that it was a closed basin around 49.5 Ma in the “Mahogany Zone” while samples in the Washakie Basin show that the Lake Gosuite went from balance fill to underfilled in the “Buff Layer” and back to balance fill. These results suggest that the lake basins demonstrate distinct phases of closed lake conditions. The different geochemistries indicate that outflow from the Greater Green River Basin was no longer present and resulted in the closure of the Piceance Basin.