Origin of Lower Triassic microbialites in mixed carbonate-siliciclastic successions: Ichnology, applied stratigraphy, and the end-Permian mass extinction

Studies aimed at understanding the recovery from the end-Permian mass extinction have focused on the distribution of faunas, trace fossils, and microbialites across depositional environments, but few have examined how these individual ecologic components relate to one another, especially the latter relationship between microbialites and trace fossils, which appear to be mutually exclusive. Microbialites occur throughout Lower Triassic strata primarily in two different forms depending on the depositional system in which they were preserved in. In carbonate-dominated settings, microbialites can commonly take the form of large stromatolitic patch reefs, while in siliciclastic settings microbialites are found primarily as wrinkle structures. This study focuses on the palaeoenvironmental distribution of microbial structures and trace fossils from an uppermost Lower Triassic deposit, the Virgin Limestone Member of the Triassic Moenkopi Formation, which is comprised of mixed carbonate-siliciclastic sedimentary rocks, and is known to contain both stromatolites and wrinkle structures, in addition to trace fossils.

Results show that the highest trace fossil diversities are found in lower shoreface environments, while offshore environments contain the lowest diversity. Strata deposited in the offshore transition, separating lower shoreface and offshore environments, contain moderate diversity trace fossil assemblages. Microbialites, in the form of either stromatolites or wrinkle structures, appear to emerge only following transgression, and are commonly found across and following marine flooding surfaces. Wrinkle structures appear to have formed as low-oxygen waters encroached upon nearshore settings, suppressing bioturbation and allowing for microbial mat development. Stromatolites likely formed due to the upwelling of anoxic alkaline waters during transgression, which may have generated a firm substrate for colonization, as well as fostering cementation of the microbialite. While most studies examining the aftermath of the end-Permian mass extinction focus on either carbonate- or siliciclastic-dominated settings, this study reports the environmental distribution of microbialites in both carbonate and siliciclastic facies, and unites them into a single depositional model that accounts for stratigraphic distribution, relation to trace fossils, and the unique environmental conditions present during the aftermath of the end-Permian mass extinction.