GEOCHEMICAL, PETROGRAPHIC AND SPATIAL ANALYSES OF MICROBIALITES FROM THE VIRGIN LIMESTONE, NEVADA: INSIGHTS INTO EARLY TRIASSIC BIOTIC RECOVERY FROM THE END-PERMIAN MASS EXTINCTION

The protracted period preceding the biotic recovery following the End Permian mass extinction may have resulted from widespread anoxia and other environmental factors (e.g., Hallam and Wignall, 1997). Lower Triassic microbialites, such as the stromatolites of the Virgin Limestone Member of the Moenkopi Formation at Lost Cabin Springs (LCS) have been interpreted to have been formed under anoxic, alkaline conditions (Pruss and Bottjer, 2004). The LCS microbialites were first reported as stromatolite mounds from two distinct units (Schubert and Bottjer, 1992). The upper of these two units actually consists of sponge-microbe buildups (Griffin et al., 2010; Marenco et al., in review). Our work uses field observations, petrography, and geochemistry to expand on other studies investigating the depositional environment of the microbialites from both units (e.g., Mata and Bottjer 2010).

Field mapping of microbialite units shows a pattern of meter-scale spaces between mounds and, where exposure is good, northwest-southeast elongation of mounds. Petrographic analysis reveals fossil debris and discrete burrows within stromatolitic laminations. Although rare framboidal pyrite is found within the microbialites in thin section, the entire stratigraphic section is characterized by low percent total sulfur (assumed to be pyrite) and low percent organic carbon. The organic carbon and total sulfur results are notable because limestones deposited in anoxic waters would be expected to have high values of organic carbon and total sulfur due to the effects of anaerobic sulfur reduction.

Mound spacing and elongation is possibly suggestive of a spur-and-groove reef morphology, found today in environments with constant wave and/or current action. Skeletal debris within stromatolitic laminations indicates that the microbialites exhibited trapping and binding similar to modern stromatolites. The burrow-disrupted laminations imply that grazing organisms coexisted with the microbialites. Our combined results suggest that anoxia was not required for the formation of the LCS stromatolites. The microbialite structures are reefs that resisted water energy and acted as habitat for a variety of organisms, and may indicate that the transition to biotic recovery was underway during the Spathian on the western margin of Pangea.