The Permian-Triassic boundary in the Great Basin, western United States (Nevada)

The cause of the Permian-Triassic (PT) extinction is still in question despite decades of research. Although many hypotheses have been introduced, no single theory stands out as the obvious killer. Most PT workers are studying foreign sections, as it has been believed for decades that the N. American continent is missing sediments from this interval, most likely due to subaerial exposure and erosion. Review of the literature suggests that there is little physical evidence of erosion, just an absence of important Upper Permian index fossils. To test the theory that there had been subaerial exposure and erosion during the PT interval, and to determine the ages of the sediments at the PT contact, six sections from three different depositional environments were studied. Measured sections were studied petrographically, to better understand the environments of deposition, and many samples were processed for conodonts in hopes of better constraining sediment age. A cathodoluminescence study was performed on several of the sections to look for evidence of subaerial exposure and/or erosion.; The two volcanic arc sections, Quinn River Crossing and Columbus, are primarily composed of deeper water basinal deposits, and most likely contain relatively complete boundary sections. The discovery of Middle Permian conodonts and sponge spicules within the "Triassic" Candelaria Formation at Columbus indicates that the base of the formation is much older than previously thought. The Palomino Ridge and Currie Canyon sections are from a carbonate ramp depositional environment, and contain a conglomerate at the PT contact that was cited as evidence of subaerial exposure and erosion. This study indicates that the conglomerate is more likely to be a submarine deposit. The Blue Diamond and N. Muddy Mts. sections were deposited in a coastal plain environment, and contain shallow marine sediments with little fauna present. The absence of Upper Permian fossils may be attributed to a lack of accommodation space along the tectonically quiescent coastal margin, on which very little sediment could be accumulated. The proposed monsoonal weather patterns of this region probably generated large influxes of fresh water along the coastal margin, which would also have driven marine life away from shore.