HIGH-RESOLUTION PETROGRAPHIC AND CARBON ISOTOPE ANALYSES OF THE END-TRIASSIC MASS EXTINCTION HORIZON, LOWERMOST MULLER CANYON MEMBER (GABBS FORMATION), FERGUSON HILL, NEVADA

Detailed petrographic and carbon isotope analyses of the lowermost Muller Canyon Member (MCM) of the Gabbs Formation reveal insight into the geochemical conditions of Panthalassa at the beginning of the end-Triassic mass extinction. The base of the MCM is associated with a major carbonate collapse observed in numerous sites across the globe. In this study, high-resolution rock samples were collected from the lower three meters of the MCM, emphasizing the base of the extinction interval. The lowermost meter of the extinction interval 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 a 1 m thick interval ranging from 1.2 to 2.2 m in our stratigraphic section. The studied section is predominantly unfossiliferous and is composed of thin to massive bedded fine-grained calcareous siltstone. Microfacies analysis reveals the presence of goethite framboids and very early diagenetic pore-filling carbonate cement. Framboids occur at 1 m and are present through the remainder of the section. The goethite framboids are likely pseudomorphs after pyrite that would suggests anoxic conditions within the sediment and potentially within the water column, supported by the depauperate benthic community and absence of bioturbation. The carbonate cement precipitated before compaction and filling available pore space suggests an unusual carbonate system that fostered very early cementation, likely near the sediment water interface. A negative excursion in d¹³C_inorg is recorded within the EH, ranging from 1.4‰ to -0.7 ‰ with a continuous decrease in isotope values through 2.5 m. This geochemical and petrographic signature shows the first evidence of a major environmental disturbance in Panthalassa spanning the EH bed that may have ultimately resulted in the high magnitude of the end-Triassic mass extinction.