SEDIMENTOLOGY AND STRATIGRAPHY OF THE DOUGLAS CREEK MEMBER OF THE GREEN RIVER FORMATION, EASTERN UINTA BASIN, UTAH

The lacustrine deposits of the Green River Formation reflect the tectonic and climatic environment of the early to late Eocene across several basins in Utah, Colorado and Wyoming. Previous studies have documented the Sunnyside Delta interval of the Green River Formation in the western and central parts of the Uinta Basin. Time-equivalent to the Sunnyside Delta interval is the less studied Douglas Creek Member outcropping near the Utah-Colorado border on the eastern-most edge of the Uinta Basin. This study utilizes these lower Green River Formation outcrops to examine the sedimentology and stratigraphy of the Douglas Creek Member through detailed measured sections and analysis of facies architecture. The vertical facies changes observed through the lower Green River document the tectonic and climatic changes of Lake Uinta. The lowermost deposits are dominated by fine-grained siliciclastic siltstones and littoral to sublittoral carbonate mudstones. The introduction of coarsening upwards packages of tabular shoreface sandstone bodies record Lake Uinta’s transition to deltaic deposition of the Douglas Creek Member. The peak of deltaic deposition is marked by cycles of 5-10 m thick, multistoried, weakly channelized sandstone bodies. These sandstone complexes extend laterally for several kilometers and mark pulses in delta growth. The transition out of the Douglas Creek interval is marked by a decrease in clastic deposits and an increase in deposition of upward shoaling cycles of calcareous mudstone and siltstone capped by ooid grainstones and microbialites. The coarsening upward stacking patterns observed throughout the lower Green River reflect fluctuations in lake level. Whether each cycle is dominated by siliciclastic or carbonate facies is dependent on fluvial sediment input driven by tectonics or climate. Specifically, the early Eocene hyperthermal events may have influenced the alternations observed between carbonate and siliciclastic facies by driving changes in sediment availability. The patterns observed in the lower Green River facies of the Douglas Creek Member document the early stages of Lake Uinta when deltaic deposition dominated in both the eastern and western parts of the basin. This study offers insight into basin-scale paleogeographic changes in a major lacustrine delta system.