LATE EARLY TRIASSIC (SMITHIAN – SPATHIAN) ENVIRONMENTAL CHANGES ALONG THE WESTERN MARGIN OF NORTH AMERICA: AN ANALYSIS OF THE UNION WASH FORMATION AT THE CERRO GORDO, CA LOCALITY

Early Triassic biotic trends were affected by a combination of the lingering effects of the Permian – Triassic mass extinction and severe environmental challenges that developed during the extinction and post – extinction recovery interval. Documented environmental stresses include widespread anoxia in the global oceans, hypercapnic stresses, and elevated sea surface temperatures that were potentially lethal to organisms living in the shallow ocean. The middle member of the Lower Triassic (Smithian – Spathian) Union Wash Formation at the Cerro Gordo, CA locality consists of 238.5 m of yellow shale with interbeds of micritic limestone that was deposited in a basinal setting and offers a means to examine paleoenvironmental conditions during the Smithian – Spathian interval and determine the nature and extent of environmental perturbations. Th/U ratios, along with V enrichment factors are indicative of deposition under anoxic conditions that abruptly shift to oxygenated conditions midway through the middle member that is in the general vicinity of the Smithian – Spathian boundary. This shift corresponds to a decrease in Ni and Cu enrichment factors, and a concomitant increase in Ba enrichments factors, which likely reflects a change in the preservation potential of productivity – related elements, and indicates relatively stable productivity across the study interval. Sea level changes are not likely to have driven the increase in benthic oxygenation as water depths appear to have been increasing during deposition of the middle member. Therefore, improving benthic oxygenation is probably related to a decline in global temperatures around the time of the Smithian – Spathian boundary. Improving benthic oxygenation also corresponds to an increase in the occurrence of synsedimentary seafloor cements, which are present as crusts made up of acicular to bladed crystals oriented perpendicular to the seafloor. The precipitate – bearing interval lacks a benthic fauna, suggesting that the conditions that led to the precipitation of the seafloor cements also prevented organisms from reoccupying the seafloor. Environmental conditions were therefore complex, with multiple stressors acting to prevent the reestablishment of deep benthic communities in the region during the latter part of the Early Triassic.