STABILIZATION OF EOCENE CLIMATE RECORDED IN FLUVIAL DEPOSITS OF THE DOUGLAS CREEK MEMBER IN THE GREEN RIVER FORMATION, MAIN CANYON, UINTA BASIN, UTAH

The Eocene Green River Fm represents fluvial-lacustrine deposition across economically significant basins in Utah, Colorado, and Wyoming. This study defines the stratigraphic architecture and paleoclimate record of the Douglas Creek Mbr in Main Canyon in the southeastern Uinta Basin of Utah, which is stratigraphically equivalent to the better known Sunnyside Delta Interval of the western Uinta Basin. Six measured sections, gigapan analyses, and isotope records collected from a 10 km transect in Main Canyon spanning 180 m of vertical section reveal prominent changes in depositional environments that are divided into three distinct stratigraphic intervals: (1) The lowermost interval consists of five 8-15 m thick fluvial packages containing weakly-channelized, amalgamated sandstone bodies that are interbedded with deltaic, lacustrine, and floodplain deposits. Amalgamated fluvial bodies are dominated by upper flow regime structures, are downstream accreting, and may be up to 12 m thick and laterally extensive for hundreds of meters. (2) The middle interval contains minor isolated fluvial channels within five 11-15 m thick littoral lacustrine sequences that alternate between siliciclastic and carbonate facies, respectively indicating local pulses of siliciclastic input followed by sediment starvation. (3) The uppermost interval contains fine-grained siliciclastic and carbonate deposits interbedded with erosionally-based, discontinuous fluvial sandstone bodies up to 10 m thick and laterally extensive for 30-200 m that are dominated by trough-cross bedding and are obliquely accreting.

The drastically different architecture of the lower and upper fluvial intervals is interpreted as the result of a fundamental shift in Eocene climate. The lower fluvial stories exemplify a highly seasonal climate with deposition under peak flow conditions that are likely tied to early Eocene hyperthermal events, whereas the upper fluvial interval indicates stabilization of climate in the waning phases of the Early Eocene Climatic Optimum that resulted in deposits akin to typical temperate fluvial facies models. Together these intervals record Lake Uinta evolution with Eocene climate and iterate the importance of multiple fluvial facies models to interpret climatic controls on fluvial-lacustrine deposition.