CONTROLS ON THE DISTRIBUTION OF BENTHIC MICROBIAL MATS DURING THE EARLY TRIASSIC AFTERMATH OF THE END-PERMIAN MASS EXTINCTION: EXAMPLES FROM THE SOUTHWESTERN UNITED STATES

The expansion of microbial mats into marine subtidal settings has become a hallmark of the Lower Triassic sedimentary record and has greatly influenced the understanding of the unusual benthic and oceanographic conditions that persisted throughout the duration of Early Triassic time. It has been shown by many workers that there are numerous significant pulses of microbial carbonate in the form of stromatolitic and thrombolitic patch reefs that can be found in marine subtidal settings in the aftermath of the end-Permian mass extinction, yet knowledge of the existence of benthic microbial mats in siliciclastic settings during this interval was lacking until only recently. The discovery of wrinkle structures—microbial features comprised of mm-scale ridges, pits, and troughs that commonly exhibit a patchy distribution and an irregularly meandering morphology—by Pruss et al. (2004) in Lower Triassic storm-dominated shelf strata showed that the microbialite record for this time was not restricted solely to carbonate environments.

It is now known that wrinkle structures occur in storm-dominated strata from the southwestern United States, northern Italy, northern Pakistan, and northern Iraq. At each locality wrinkle structures are associated with subtidal storm deposits that often display hummocky or swaley cross-stratification. In the southwestern United States, wrinkle structures are found within the Lower Triassic Thaynes Formation and the Virgin Limestone Member of the Moenkopi Formation and are found predominantly across marine flooding surfaces—stratal surfaces in which there is an abrupt increase in water depth upsection—separating underlying nearshore sandstones from overlying deeper-water siltstone and shale. Wrinkle structures provide profound insight into the nature of the Early Triassic seafloor and suggest, by the presence of these benthic microbial mats, that during their formation a surface mixed-layer, which is a characteristic feature of most post-Cambrian seafloors, was commonly non-existent during this unusual interval in Earth’s history.