Geochemistry of Shallow Water Shales from the Virgin Limestone, NV, USA: Evidence for Periodic Shallow-Water Anoxia Following the Permian-Triassic Mass Extinction

Sullivan, Shawn and Woods, Adam D.
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The end of the Permian (~252 My) was marked by a mass extinction that proved greater than any mass extinction of the past 600 million years, with an estimated loss of 85% of marine genera. Reconstructing environmental conditions after a mass extinction such as the Permian – Triassic event is key to understanding how and when recovery occurs following major biotic crises. The the Spathian (uppermost Lower Triassic) Virgin Limestone at Ute, NV is comprised of interbedded red and green shales and limestone beds (primarily packstones and grainstones) that are fossiliferous, often oolitic, and commonly contain anachronistic facies (i.e., fabrics and features that were common in the Precambrian). The limestone units were previously examined in detail (Woods, in press); the purpose of this study was to determine paleoenvironmental conditions in shallow water environments during the Early Triassic via geochemical analysis of interbedded shales. Twenty-five samples were analyzed using a Perkin-Elmer 7300 DV ICP-OES to measure major, minor and trace elements to reconstruct environmental conditions and the overall sedimentology of the shales. Vanadium, molybdenum, cobolt, chromium, and manganese values indicate that oxygen levels within the shale beds range from oxic to dysoxic, while low barium, copper, nickel, phosphorus, and zinc values suggest the efficient breakdown of organic matter in shallow environments. Overall, results suggest that in near shore environments, the water was mostly oxygenated but was punctuated by occasional anoxic events that were likely short-lived based on the lack of preservation of organic matter. The close juxtaposition of dysoxic to anoxic shales and anachronistic facies supports previous hypotheses that suggest unusual Lower Triassic facies and fabrics are the result of the impingement of anoxic, alkaline deep waters onto the continental shelves (Woods, in press) that likely resulted in the long, slow biotic recovery from the Permian-Triassic mass extinction that has been previously documented in the southwestern United States (e.g., Schubert and Bottjer, 1992).

AAPG Search and Discovery Article #90162©2013 Pacific Section AAPG, SPE and SEPM Joint Technical Conference, Monterey, California, April 19-25, 2013