THE PERMIAN-TRIASSIC BOUNDARY AT QUINN RIVER CROSSING, NORTHWESTERN NEVADA AND IMPLICATIONS FOR THE CAUSE OF THE EARLY TRIASSIC CHERT GAP ON THE WESTERN PANGEAN MARGIN

The upper Permian-lower Triassic sediments of the Quinn River Formation (QRF) in northwestern Nevada were previously thought to represent an incomplete Permian-Triassic (P-Tr) boundary sequence due to an inferred disconformable facies relationship between bedded chert and an overlying siltstone. Petrographic and geochemical studies show that the “siltstone” is in fact a deep-water radiolarian-bearing dolomicrite, and that this facies appears several times throughout the section. Worldwide chert production generally declined or ceased in the late Permian, and reappeared in the Spathian. This ‘chert gap' is reflected at Quinn River in the unfossiliferous 30 meters of shale and deep-water carbonates where the contact between cherts of late Permian and early Triassic age occurs. Given the (1) conformable facies relationships, (2) deep-water setting of the section, (3) new radiolarian biostratigraphic data indicating longer ranges for some key species, and (4) presence of the worldwide chert gap, it is suggested that sedimentation in the QRF may have been continuous across the P-Tr boundary. Organic carbon isotope stratigraphy of the QRF shows multiple excursions throughout the Changxingian-Anisian part of the section. The negative excursion occurring 1.54 meters above the chert-to-dolomicrite/shale transition has been selected as the Permian-Triassic boundary at Quinn River. Proxies for paleoredox conditions (authigenic U and V/Cr ratio) show the water column became less-oxic across the chert-dolomicrite/shale boundary, with anoxia developing within ~5 m of the lithologic change. Abundant radiolarian ghosts can still be seen in the dolomicrites and shales, suggesting that the chert gap was caused by the spread of dysoxic-to-anoxic bottom waters that had a detrimental effect on siliceous sponge communities. The dolomicrites in the QRF represent a widespread facies along the western Pangean margin during the early Triassic, and are analogous to dolomites characterizing low-oxygen environments in the Miocene Monterey Formation of California and Recent sediments offshore Peru. These distinctive rocks represent early authigenic formation of dolomite via microbial sulfate reduction in low-oxygen, organic-rich sediments.