MICROBIAL REEFS IN THE EARLY TRIASSIC

The end-Permian mass extinction was the largest extinction in the Phanerozoic, with a prolonged ensuing recovery interval in the Early Triassic. Due to the devastation of marine metazoans during this extinction, the Early Triassic has often been deemed a "reef gap" because no metazoan reefs are found at this time. While colonial metazoan reefs are absent, in Lower Triassic strata of South China and the Western United States microbial build-ups have been found. The biostromes found in South China have been determined to represent patch reefs. Research in the western United States to reinvestigate the previously reported stromatolites of the Virgin Limestone Member of the Moenkopi Formation (southern Nevada) has shown that these microbial build-ups also represent patch reef systems, and formed without any in situ metazoans.

The Virgin microbial reef system consists of an agglomeration of mounds that range in thickness from 2-2.5 m. In places, the mounds coalesce to form tabular biostromes, but they also occur as individual mounds. These mounds were measured and sampled in the field, and were later thin-sectioned to study their internal constituents. Field measurements show that these mounds attained significant topographic relief off of the sea floor (>2 m), and thin-section analysis shows that these microbial build-ups consist of a clotted network consistent with thrombolitic microfabrics, and were devoid of in situ metazoans. Results from this study illustrate that these Early Triassic microbial reefs are inherently different from most reef systems of the post-Ordovician, reiterating that the recovery interval from the end-Permian mass extinction is an anomalous time period. The global occurrence of only microbial reefs in the Early Triassic suggests that conditions favoring microbial growth must have existed throughout this prolonged recovery interval from the end-Permian mass extinction.