THE PROLONGED END-PERMIAN MASS EXTINCTION RECOVERY: ANACHRONISTIC FACIES AND ANOMALOUS ENVIRONMENTAL CONDITIONS

The prolonged recovery from the end-Permian extinctions spanned the entirety of the Early Triassic (~8 m.y. compared to <1 m. y. for the post-K/T recovery interval). In addition to depauperate faunas, an atypical sedimentary rock record also characterizes Lower Triassic carbonate strata worldwide. In the southwestern United States, unusual cm-scale formerly aragonite seafloor precipitates are found in the uppermost Lower Triassic slope-basin Union Wash Formation, eastern California; seafloor precipitates are a significant component of this formation and are found recurrently throughout 150 m of the first precipitate-bearing limestone. Microbial reef mounds and other microbialites occur in the broadly coeval shallow shelf Virgin Limestone Member (southern Nevada) of the Moenkopi Formation in a variety of paleoenvironments. Ribbon rock and flat-pebble conglomerates are also found in various beds. Aragonite seafloor precipitates, microbial reef mounds, ribbon rock and flat-pebble conglomerates are highly unusual, or ‘anachronistic’, in Phanerozoic time; these facies are more common in Archean/Proterozoic sediments and are rare in post-Cambrian strata. The seafloor precipitates indicate unusually elevated alkalinity in the slope to basin environment. Sequence stratigraphic analysis demonstrates that the microbialites occur in association with flooding events. Therefore, the periodic incursion of deep basin water with unusual ocean chemistry may have enhanced microbialite formation in the shallower environments. The anomalous ocean chemistry, as evidenced by the occurrence of anachronistic facies as well as associated geochemical data, may have contributed to the slow recovery of metazoans throughout the Early Triassic in the aftermath of the end-Permian extinctions.