EARLY TRIASSIC PALEOCEANOGRAPHY ALONG WESTERN NORTH AMERICA: AN ANALYSIS OF THE MIDDLE MEMBER OF THE UNION WASH FORMATION, EAST-CENTRAL CALIFORNIA

Biotic recovery following the Permian-Triassic mass extinction was hampered by environmental stresses present in Early Triassic environments. Deep water anoxia in the global ocean is speculated to be a major contributor to delayed recovery rates of marine fauna following the extinction, as oxygen-poor deep waters periodically transgressed onto continental shelves, and added to other difficulties affecting shallow water settings, including hypercapnic stresses and elevated sea surface temperatures that were potentially lethal. The middle member of the Union Wash Formation of eastern-central California provides a means to reconstruct environmental conditions present in Early Triassic deep water settings along the western margin of North America and test the hypothesis of Woods (1998) that the unit was deposited under anoxic bottom waters. The middle member of the Union Wash Formation was examined at the Darwin Hills, CA locality in order to determine the nature and extent of anoxia off the coast of western North America during the Early Triassic (Olenekian) recovery interval. Initial interpretations of field observations, thin sections and ichnofabric indices point to much of the middle member being deposited under anoxic conditions, based on extensive laminated units and a complete lack of benthic fossils. A pelagic fauna is found within the unit, and consists of scattered ammonoids and radiolarians that were living above anoxic bottom waters; calcispheres are also found in thin section. Simple trace fossils (Planolites?) have been observed from scattered intervals throughout the middle member, and indicate occasional, transient oxygenation of the seafloor. Furthermore, needle-like crystal pseudomorphs, interpreted to be former aragonite crystals, are common within gray, cm-scale layers of micritic limestone that are interbedded with laminated tan dolomite, and are hypothesized to have precipitated within the soupy sediment as the result of the unusual carbonate chemistry of the oceans at the time. Overall, this study hopes to better constrain the environmental conditions present along the western coast of North America during the Early Triassic and determine the degree to which paleoenvironmental stresses affect recovery from extinction events.