SEDIMENTOLOGIC AND PALEONTOLOGIC ANALYSIS OF THE LOWER MEMBER OF THE EARLY TRIASSIC UNION WASH FORMATION: PALEOCEANOGRAPHIC CONDITIONS ALONG THE WESTERN COAST OF NORTH AMERICA FOLLOWING THE PERMIAN-TRIASSIC MASS EXTINCTION

The recovery interval following the Permian-Triassic mass extinction is a time characterized by unusual oceanic conditions unlike any other seen throughout the Phanerozoic. There is a collapse of marine communities worldwide as a result of the extinction, and many ecosystems do not appear to recover for several million years after the crisis. Low levels of oxygen in the oceans sporadically appeared in some areas and prevented marine ecosystems from recovering and reestablishing themselves until those stresses retreated. The Lower Member of the Lower Triassic Union Wash Formation (Dienerian(?)-Smithian) near Fossil Hill (east of Independence, California) records a marine transgression that occurred following the Permian-Triassic mass extinction, and may provide further insight into the unusual oceanic conditions inhibiting the recovery of marine organisms during the Early Triassic. The Lower Member of the Union Wash Formation has been interpreted to have been deposited in a shallow water, nearshore environment, and is comprised of silty and sandy limestone (Stone et al., 1991), and exhibits a sparse and impoverished fauna composed primarily of small, ~3mm, microgastropods, which occur as molds. The Lower Member has not previously been studied in detail, especially with regards to paleoenvironmental conditions recorded by the unit. In addition, the Lower Member may be older than previously thought, with the implication that the unit might record conditions from earlier in the Triassic than previously realized. Paleoenvironmental analysis of these rocks will make it possible to gain further insight into the unusual oceanic conditions that persisted along the western coast of North America following the extinction and attain a better understanding of how those conditions affected the recovery of life from the crisis.