The unusual sedimentary rock record of the Early Triassic: A case study from the southwestern United States
Abstract Sedimentary systems, as well as the earth's biota, changed dramatically following the end-Permian mass extinction. This global sedimentological transformation is reflected in the widespread occurrence of flat-pebble conglomerates, subtidal wrinkle structures, microbialites, and carbonate seafloor fans in various Lower Triassic sections. As a case study, a facies analysis of the Lower Triassic Virgin Limestone Member of the Moenkopi Formation and the Middle and Upper Members of the Union Wash Formation, southwestern United States was conducted and all unusual facies were documented within a palaeoenvironmental framework. The occurrence of the flat-pebble conglomerates and wrinkle structures is significant because these reflect a long-term reduction in vertical bioturbation. In the case of the subtidal microbialites and carbonate seafloor fans, their formation is linked to an increase in alkalinity. The presence of these facies in the uppermost Lower Triassic strata of the southwestern United States indicates that reduced levels of infaunal bioturbation as well as (and perhaps a consequence of) unusual ocean chemistry extended for 5–6 million years after the end-Permian mass extinction, suggesting that depositional environments as well as the biota were significantly affected by the extinction event.