A PALEOENVIRONMENTAL ANALYSIS OF THE UPPER MEMBER OF THE UNION WASH FORMATION, DARWIN, CA: ENVIRONMENTAL CONDITIONS DURING THE LATE POST-EXTINCTION RECOVERY PERIOD

The Union Wash Formation, located in east-central California, is a Lower Triassic marine deposit formed along the outer continental margin of Pangea during the recovery period following the end-Permian mass extinction. Elevated global temperatures repressed post-extinction recovery and contributed to widespread ocean anoxia, which squeezed marine life into narrow habitable zones. Portions of the Union Wash Formation lie within such a zone; biotic recovery within the region was repeatedly set back as environments shifted laterally with changes in sea level. This study aims to test the hypothesis that the uppermost portion of the upper member of the Union Wash Formation was deposited under improving environmental conditions near the end of the post-extinction recovery interval (latest Spathian or earliest Anisian). The Union Wash Formation is divided into three members, with the upper member deposited as sea levels retreated near the end of the Early Triassic. The uppermost portion of the upper member at the Darwin Hills, CA locality has not been previously examined in detail, and consists of 2 distinct subunits: a lower interval made up of tan siltstone with interbeds of micritic limestone near the base and top of the interval, and an upper portion that consists of micritic limestone. Initial results indicate shifting environmental conditions as the result of sea level change. Siltstones that comprise the lower unit were deposited due to increased detrital input, possibly as the result of subduction-related uplift and erosion due to the initiation of subduction along the western margin of North America, while the presence of broken gastropod shells and fossil hash in limestone interbeds indicates a shallow subtidal setting. The overlying micritic limestone subunit contains paleokarst breccias near the base, which are overlain by limestones deposited between fair-weather and storm wave base, as indicated by the presence of flat pebble conglomerates and tempestites. Bioturbation throughout the upper subunit suggests an oxygenated setting, and the presence of Planolites trace fossils may signal low level recovery during deposition. Future work will further define the depositional setting of the unit and the level of biotic recovery during deposition.