Stromatolitic bioherm from the Lower Triassic Virgin Limestone at Blue Diamond, NV: Paleoenvironmental and paleoecological significance.

摘要

The Permian-Triassic mass extinction was the most devastating biotic crisis in Earth history; recovery of Early Triassic communities varied temporally and spatially. Study of a stromatolitic bioherm found in the Lower Triassic Virgin Limestone at Blue Diamond, NV, was conducted in order to test the hypothesis that detrimental environmental conditions played a role in the formation of widespread microbialites in the aftermath of the Permian-Triassic mass extinction. A 49-meter stratigraphic section was measured to characterize the depositional setting of the bioherm and associated facies; as a part of this analysis, redox-sensitive trace metals were measured in shales and siltstones. Although geochemical results indicate that the environment was not anoxic over the interval, two stratigraphic units are enriched in vanadium, and are indicative of periods of reduced benthic oxygenation. Of particular interest is the second spike in vanadium enrichment, which coincides with the occurrence of the bioherm. In terms of the bioherm, four distinct stromatolite morphologies are observed over a three-km transect, including meter-thick groups of high-relief hemispherical domes with well-developed laminae at the southernmost extent of the transect, and lozenge-shaped aggregates of intergrown columnar stromatolites with dimpled top surfaces. Northwards, stromatolites are sparser and lower in relief (usually <7cm), and occur as isolated hemispheroids or meandering ridges of intergrown domes. Petrographic thin sections of the stromatolites indicate that thick accumulations with well-defined laminae generally have a homogenous micritic matrix and are likely to include horizons of encrusting paper pectens. Low-relief, irregularly shaped masses have clotted fabrics, and are associated with a greater degree of bioturbation. In all four morphologies, skeletal grains and peloids are incorporated into the stromatolitic matrices, indicating that metazoans were active periodically during accretion of the microbial mats. The results of this study suggest that suggest that deleterious environmental conditions in the wake of the extinction were intermittent, and that local environmental factors, including ocean chemistry, water depth, and makeup of communities, influenced the timing and extent of reef recovery and that these factors contribute to a more complex model of recovery than simple succession in the rebound of shallow marine communities following the Permian-Triassic mass extinction.