Lower Permian Through Lower Trassic Paleontology, Stratigraphy, and Chemostratigraphy of the Bilk Creek Mountains of Humboldt County, Nevada
The primary goal of this study was to use paleontological, geochemical (? ^13C), and sedimentological data to determine if a complete Permian-Triassic boundary section is present at the Bilk Creek Mountains of northwestern Nevada. The Bilk Creek Mountains of northwestern Nevada contain a marine record deposited in a back-arc terrane environment, starting in the Lower Permian Bilk Creek Limestone and extending 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 Permian-Triassic boundary. Data collected from the Bilk Creek Limestone reveals a diverse benthic marine fauna, with brachiopods being the most abundant. Within the Bilk Creek Limestone, two different faunal signatures are apparent. The transition and separation of these groups are marked by the appearance and the abundance of Boreal brachiopods such as Spiriferella, Neospirifer, Stenoscisma, Muirwoodia transversa, Neophricodothyris sp.,and Derbyia, replacing mid-latitude to Tethyan-derived brachiopods such as Crurithyris, Dielasma, Squamularia sp.,and Rhynchopora. When the brachiopod faunas of the Bilk Creek were compared statistically with other known Early Permian rocks deposited along northwestern and western Pangea, analysis showed that the Bilk Creek brachiopod fauna was similar to that of the Eastern Klamath and Quesnellia terranes. This faunal transition to a more Boreal assemblage may mark the local onset of the Permian Chert Event (PCE). While previously thought to be strictly Early Permian, the Bilk Creek Limestone does contain Middle Permian brachiopod taxa as well. The overlying 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 overlying Quinn River Formation record deposition in deep marine conditions prior to the Permian-Triassic mass extinction event. Field observations and lab data reveal the presence of brachiopods, sponges, and rugose corals that then disappear with the onset of accumulations of cherts at the end-Guadalupian. The Permian-Triassic boundary was defined stratigraphically with the use of the distinctive Permo-Triassic negative ? ^13C excursion, along with early Triassic fauna such as the brachiopod Lingulaand the bivalve Claraia. With no evidence of unconformities within the Quinn River Formation, this unit appears to represent a continuous Permo-Triassic boundary section.
School:Bowling Green State University
School Location:USA - Ohio
Source Type:Master's Thesis
Keywords:permo triassic boundary quinn river formation bilk creek mountains brachiopods paleoecological analysis
Date of Publication:01/01/2007