Abstract

The Upper Permian–Lower Triassic Quinn River Formation in northwestern Nevada was previously thought to represent an incomplete Permian–Triassic boundary sequence, owing to an inferred disconformable relationship between Permian radiolarian- and spicule-rich chert and overlying Triassic siltstone. Petrographic and geochemical studies demonstrate that the “siltstone” is in fact a radiolarian-bearing early authi-genic dolomicrite, with both the chert and dolomicrite deposited conformably in deep water. Chert production declined or ceased in the Late Permian and reappeared in the Spathian, forming a widespread “chert gap” in Permian–Triassic sequences. Given the conformable lithofacies relationships, deep-water depositional setting, new radiolarian data extending ranges of key taxa, and the presence of the global chert gap, sedimentation in the Quinn River Formation was apparently continuous across the Permian–Triassic boundary. This represents the first Permian–Triassic boundary section in the United States portion of the North American Cordillera, and one of the few deep-water sections worldwide. Organic carbon isotope stratigraphy of the Quinn River Formation displays multiple excursions through sediments of Wuchiapingian–Anisian age, with a negative excursion 1.54 m above the chert-shale transition likely representing the Permian–Triassic boundary.

The multiple excursions in the organic carbon record verify studies of the carbonate carbon record in China that suggest instability in the isotopic record throughout the Early Triassic, and demonstrate that the Permian–Triassic boundary isotope excursion was not an isolated event. Stratigraphic variation in redox-sensitive trace metals indicates that seawater became less oxic slightly before the chert-shale transition, in turn impacting siliceous sponge communities and creating the widespread chert gap. The distinctive dolomicrites in the Quinn River Formation represent a widespread lithofacies deposited in many localities during the Late Permian–Early Triassic and express early authigenic formation of dolomite via microbial sulfate reduction in organic-rich, low-oxygen environments.

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