EARLY PERMIAN-EARLY TRIASSIC BENTHIC COMMUNITY STRUCTURE OF THE BILK CREEK MOUNTAINS, HUMBOLDT COUNTY, NORTHWESTERN NEVADA

The Bilk Creek Mountains of northwestern Nevada contains a marine record deposited in a back-arc terrain environment starting in the lower Permian Bilk Creek Formation to the middle Triassic Quinn River Formation. Field work through these units reveals changes in the marine benthic fauna through this interval, including across the P-T boundary. Preliminary data collected from the Bilk Creek Formation reveals a diverse benthic marine fauna with brachiopods being the most abundant; fossil preservation and sedimentology indicates debris flow packages throughout the unit. The Permian Volcaniclastic Unit, thought previously to be unfossiliferous, does contain small quantities of articulate brachiopods. Approximately 54 meters of limestones, dolomites, shales, cherts, and volcanic ash within the Quinn River Formation record deposition in mostly deep marine conditions prior to the Permian-Triassic mass extinction event. Literature review plus preliminary field observations and lab data reveal the presence of brachiopods, sponges, and tabulate corals that then disappear with the onset of accumulations of cherts at the end-Guadalupian. This impoverished community structure continues up through the late Permian into the very early Triassic, until approximately 1 meter above the Permian-Triassic boundary, at which point inarticulate brachiopods and bivalves reappear. The next 14 meters of the Quinn River Formation record have very little fossil data, which might indicate the slow nature of recovery of the marine benthic community after the extinction event; the only fossils found within this interval are a few small horizontal burrows and siliceous sponges. Approximately 15 meters above the P-T boundary, conditions return to a more normal marine setting, as indicated by the occurrence of abundant ammonoid fossils.