LOWER TRIASSIC MICROBIALITES AND THEIR RELATIONSHIP TO LONG-TERM ENVIRONMENTAL STRESS FOLLOWING THE END-PERMIAN MASS EXTINCTION

In studies of Lower Triassic strata exposed in the southwestern United States and southern Turkey, the Early Triassic has been found to represent an anachronistic time period dominated by depositional systems reminiscent of the Neoproterozoic—Early Paleozoic. In particular, planar microbialites, small stromatolites, thrombolites, and microbial reefs developed extensively in normal marine carbonate environments of the Early Triassic. These microbialites have been found in strata deposited immediately after the end-Permian mass extinction (southern Turkey), as well as in sections deposited in the latest Early Triassic (southwestern United States). In the 4-7 million years that followed the end-Permian mass extinction, there existed a metazoan reef gap, a reduction in trace fossil infaunalization, and an abundance of opportunists, which point to an overall long-term ecologic relaxation that set the stage for the proliferation of disaster taxa. Lower Triassic strata also provide evidence for increased oceanic alkalinity in the form of carbonate precipitates and flat-pebble conglomerates, indicating that unusual environmental conditions existed and likely played a role in microbialite formation. The presence of Lower Triassic wrinkle structures, indicators of microbial mats in siliciclastic environments, represents a reduction in infaunal bioturbation, suggesting that ecological changes fostered the resurgence of disaster form microbial structures. It is therefore hypothesized that the environmentally widespread and global development of microbial structures represents an ecological response to a decimation of organisms during the end-Permian mass extinction followed by a prolonged interval of environmental stress that suppressed the recovery of metazoans and fostered the formation of microbial structures in their absence.