ORIGIN OF LOWER TRIASSIC MICROBIALITES IN MIXED CARBONATE-SILICICLASTIC SUCCESSIONS: APPLIED STRATIGRAPHY AND THE AFTERMATH OF THE END-PERMIAN MASS EXTINCTION

The occurrence of microbialites in Lower Triassic successions has become a widely documented phenomenon and appears global in extent. Microbialites are known to 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 level-bottom stromatolites or can occur as large stromatolitic patch reefs that likely had significant synoptic relief above the seafloor. In siliciclastic settings microbialites are found primarily as wrinkle structures, which occur in storm-dominated shallow marine successions.

In strata of the Lower Triassic (Olenekian) Virgin Limestone Member of the Moenkopi Formation both types of microbialites occur in mixed carbonate-siliciclastic strata, with stromatolitic patch reefs preserved in carbonate facies and wrinkle structures preserved in siliciclastic facies. Stratigraphic analysis shows that each type of microbialite within these facies appears to occur across or immediately following a marine flooding surface—a stratigraphic surface in which there is an abrupt increase in water depth upsection. These stratal surfaces are significant because they afford two circumstances that may have favored the development of microbial mats during the aftermath of the end-Permian mass extinction: 1) the Early Triassic is associated with offshore anoxic and dysoxic conditions that likely encroached upon shallow marine environments during sea level rise, and the incursion of these stresses during transgression may have allowed for the suppression of bioturbation, a necessary requirement for mat development; and 2) transgression typically leads to sediment starvation on the shelf and would allow microbial mats to propagate upward outpacing sedimentation.

These microbialite-associated marine flooding surfaces represent intervals of deepening, which may have brought low-oxygen conditions into shallow marine settings, suppressing bioturbation, and allowing for the development of macroscopic microbial mats during an interval of low sedimentation. The stratigraphic occurrences of these microbialites may serve as a model for interpreting the context of other Lower Triassic microbialites found globally in carbonate, siliciclastic, or mixed settings.