ENVIRONMENTAL STRESS AND BIOTIC RECOVERY FOLLOWING THE END-PERMIAN MASS EXTINCTION: EVIDENCE FROM THE UPPER MEMBER OF THE UNION WASH FORMATION, UNION WASH, CA

The Lower Triassic (Smithian to Spathian) Union Wash Formation (UWF), located in east-central California, was deposited within the paleotropics, along the outer continental shelf edge of western Pangaea. Deposition of the UWF took place concurrently with the biotic recovery following the End-Permian mass extinction and allows us to examine the interaction between biotic recovery and environmental conditions. The recovery was complex in the region, taking place throughout the entire Early Triassic, and demonstrates at least three distinct recovery events interspaced with periods of increased temperatures and intrusion of anoxic and/or hypercapnic waters leading to setbacks in biotic recovery (Woods et al., 2019). The UWF at Union Wash, CA was deposited in a deep, basinal setting (Stone et al., 1991), and is divided into three informal members, the uppermost member is the focus of this study. The upper member consists of a lower Subunit 1, comprised of micritic limestone and shale, and an upper Subunit 2, made up of laminated siltstone. Hand samples were taken from the UWF every five meters and in areas of lithological change; samples were cut and polished and thin sections were made for each sample. The base of Subunit 1 is occupied by the Parapopanoceras bed, which consists of tightly packed ammonoid phragmacones with minor amounts of micritic matrix that has been attributed to a period of decreased background sedimentation (Woods and Bottjer, 2000). The remainder of Subunit 1 is predominantly massive micritic limestone with laminated intervals, and a few instances of burrows and scattered bivalve shells. Subunit 2 is comprised of a lower pink, laminated siltstone that lacks fossils, and an upper dark grey siltstone containing scattered ammonoids. Subunit 2 is overlain unconformably by volcanic conglomerate. These findings indicate deposition under persistent suboxic to anoxic conditions, with burrows and massive intervals providing evidence of periods of higher oxygenation. Overall, the results of this study indicate anoxic conditions that were persistently hostile to recovery with periodic intervals of improvement over time.