NEW RECORD OF BIOTIC RESPONSE AND OF AN UNUSUAL CARBONATE SYSTEM FOR THE END-TRIASSIC MASS EXTINCTION AT FERGUSON HILL, NEVADA

High-resolution petrographic and carbon isotope analyses of the upper Mount Hyatt Member and lowermost Muller Canyon Member (MCM) of the Gabbs Formation record the geochemical conditions in Panthalassa just prior and at the beginning of the end-Triassic mass extinction. At Ferguson Hill, Nevada, the lowermost second meter of the MCM can be recognized in outcrop by a distinctive pinkish white color (produced by gypsum, generated as an artifact of weathering) referred to here as the EH (extinction horizon). The extinction horizon is predominantly unfossiliferous and is composed of fine laminated to thinly bedded fine-grained calcareous siltstone. Sediment samples were collected every 10 cm above and below the EH and every 5 cm within the EH bed. Point-counting (n=100) in thin sections generated four main categories: bioclastic remains, clastic grains, calcium carbonate cement, and sulfide pseudomorphs (typically goethite after pyrite). Bioclastic remains decrease from 40% to 2% towards the EH bed with subsequent demise right above it. The negative carbon isotope excursion (NIE) is recorded 50 cm above the EH and coincides with the last occurrence of Choristoceras crickmayi. Additionally, petrographic analysis reveals the presence of very early diagenetic pore-filling carbonate cement and an elevated amount of goethite (after pyrite) framboids within the EH. The framboids would suggest anoxic conditions within the sediment and potentially within the water column, supported by the depauperate benthic community and absence of bioturbation. The geochemical, faunal, and petrographic signature of the EH bed is distinctive from the bed below, signifying the first evidence of major environmental disturbance in Panthalassa. Non-metric multidimensional scaling analysis corroborates the distinctive signature between the EH and the beds above and below. If the NIE represents a global signal and biotic change reveals the local response to environmental perturbations with association of an unusual carbonate system, then our evidence suggests the biotic collapse preceded the negative carbon isotope excursion, at least in this part of the Panthalassa.