BIOTURBATION AND THE END-TRIASSIC MASS EXTINCTION AT NEW YORK CANYON, NEVADA

The Muller Canyon Member spans the Triassic-Jurassic boundary (TJB) in the New York Canyon area of the Gabbs Valley Range, Nevada. Previous studies of the record of bioturbation in this interval of latest Jurassic-earliest Triassic age attribute low trace fossil diversity (relative to the underlying Mount Hyatt Member and overlying Ferguson Hill Member) to facies change (Twitchett and Barras, 2004). These earlier studies also reported burrow-mottled fabrics which at times are thoroughly mottled (Hallam and Wignall, 2000; Twitchett and Barras, 2004). In this study Muller Canyon Member trace fossils, ichnofabric and primary sedimentary structures were examined. Here, we confirm the reduction in ichnodiversity that has been reported for the Muller Canyon Member. Many of the studied samples show an ichnofabric index (ii) of 6, with bioturbation having occurred wholely in the surface mixed layer. Other samples with ii6 also show preservation of distinct to indistinct trace fossils indicating their emplacement at greater depths below the seafloor within the transition layer. This indicates low levels of oxygenation in seafloor sediments and water overlying the seafloor during deposition of much of this interval, although evidence for anoxic conditions in the form of laminations is also found. This reduced record of bioturbation from strata deposited during the latest Rhaetian towards the TJB is associated with significant changes in geochemistry. Following the TJB, as previously reported, the record of bioturbation returns to more typical marine levels. We attribute this decrease and subsequent recovery to changes in environmental conditions related to emplacement and eruption of the Central Atlantic Magmatic Province (CAMP).