LINKING BIOTIC RECOVERY AND ENVIRONMENTAL STRESS: A CASE STUDY FROM THE LOWER TRIASSIC VIRGIN LIMESTONE, NV, USA

The Permian-Triassic mass extinction was the most devastating crisis in the history of the Earth. Recovery from the extinction was complex and lengthy in many regions and appears to have been strongly controlled by environmental conditions. The Virgin Limestone is an upper Lower Triassic (Spathian) mixed carbonate-clastic sequence from the southwestern United States (primarily southern Nevada) deposited across a broad, gently-sloping platform (Moenkopi Platform) that passed laterally into deeper-water facies of the Union Wash Formation of east-central California. The Union Wash Formation was largely deposited under anoxic conditions in outer shelf to slope settings, and goal of this study was to determine the extent to which anoxic waters impinged into shallow water environments of the Virgin Limestone. The Virgin Limestone consists of interbedded fossiliferous limestone and green shale, referred to as “pine needle shale” by Larson (1968). While the limestone units have been the focus of extensive study, mostly related to recovery from the mass extinction, the green shale has not been examined closely due to poor exposure and a lack of fossils. Green shale interbeds were sampled for trace metal analysis from 3 localities across southern Nevada: Lost Cabin Springs, Blue Diamond, and Ute. Results indicate that the green shale was deposited under reduced benthic oxygenation ranging from anoxic to suboxic. Overall, anoxic conditions appear to have been more prevalent in the lower parts of the section, with oxygenation gradually improving over time, however, brief pulses of anoxic conditions are also found higher in the section. Repeated incursions of oxygen-deficient waters had the long-term effect of slowing recovery from the Permian-Triassic mass extinction in the region, while elevated sea surface temperatures may have dampened recovery in shallower settings. The results of this study provide further confirmation of the importance of environmental conditions in controlling the timing and direction of biotic recovery from mass extinction events, and point the complex nature of recovery, which can vary greatly between geographic areas.