A POSSIBLE PERMIAN-TRIASSIC BOUNDARY SECTION AT QUINN RIVER CROSSING, NW NEVADA

Basic questions surrounding the cause of the Permian-Triassic (P-T) extinction remain unanswered in part due to the lack of complete boundary sections. Ongoing research including thin section petrography and revised ages of Triassic radiolarians indicate that the Permian-Triassic Quinn River Formation exposed at Quinn River Crossing (QRC) in NW Nevada is more complete than previously believed and may contain a record of P-T boundary events in a deep-water setting. The QRC section has been interpreted as incomplete because of incongruous facies relationships between upper Permian radiolarian cherts and overlying lower Triassic siltstones and shales. This ~45 m thick unfossiliferous siltstone and shale package is located between chert units of known P and Tr age. However, petrographic studies demonstrate that (1) the “siltstone” overlying the chert is in fact a recyrstallized micritic limestone containing radiolarian “ghosts”, and is indicative of deep-water deposition, (2) this lithology is present at several horizons in the section, and (3) contacts between lithologic units are generally gradational throughout the QRC section. In addition, published studies of Triassic rocks in New Zealand and China reveal that radiolarian taxa originally used to date the QRC Permian chert as pre-Changxingian actually range into the Triassic. Given the apparent lack of uppermost Permian chert from western Pangea with which to constrain biostratigraphic zonations, it may be that the Permian cherts at QRC are slightly younger than previously suggested. Global chert production is known to have declined or ceased during the late Permian and reappeared in the Spathian. This “chert-gap” is apparently expressed in the QRC section as 45 m of deep-water limestone and shale separating the upper P and lower Tr strata. A series of excursions discovered during preliminary organic carbon isotope stratigraphy indicates the boundary is most likely located in the lower part of this limestone and shale package. Given the apparently conformable lithofacies relationships in the QRC section, the deep-water setting of the sediments, the revised ranges of key radiolarian taxa, and the presence of the global “chert-gap” strata, it is suggested that sedimentation may have been continuous across the P-T boundary at Quinn River Crossing.