CLIMATIC CONTROL ON FLUVIAL-DELTAIC SYSTEMS OF THE LACUSTRINE GREEN RIVER FORMATION, UINTA BASIN, UTAH

The Eocene Green River Formation consists of continental, marginal lacustrine strata deposited in Laramide ponded basins in Utah, Colorado, and Wyoming. This study (1) documents fluvial, deltaic, and lacustrine strata from the Douglas Creek and Parachute Creek Members of the Green River Formation, southeastern Uinta Basin, Utah, and (2) uses new interpretations of the link between climate and fluvial-deltaic sedimentary expression to interpret the terrestrial evolution of early Eocene climate. The stratigraphy was analyzed using laterally extensive exposures of 1) updip fluvial systems at Main Canyon on the Tavaputs Plateau, central Uinta Basin and 2) downdip mouthbar complexes at Texas Creek, eastern Uinta Basin. At Main Canyon, two discrete stratigraphic intervals record distinct fluvial regimes. The lower interval is dominated by amalgamated sandstone channels that contain 70-100% upper flow regime sedimentary structures. The channels are interpreted to represent fluvial deposits controlled by a highly seasonal climate, where most deposition was limited to seasonal flooding events. Stratigraphically equivalent intervals of mouthbar complexes at Texas Creek contain sandstone bodies that average 9 m in thickness and 120 m in length and are lenticular to tabular with downstream accretion sets and sharp, weakly erosive bases. Internal structures of the mouth bars indicate rapid deposition and include climbing ripples, convolute bedding, and transitional to upper flow regime bedforms. Seasonal flooding generated high-density sediment flows capable of forming hyperpycnal currents that deposited mouth bars tens of kilometers from the shoreline. The highly seasonal climate regime was caused by rapid warming events that characterized the early Eocene. Correlation between these outcrop locations associated with early Eocene warming events indicates that there was a basin-wide fluvial response to these climate changes.