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Pruss, Sara Brady1, David J. Bottjer1 
(1) University of Southern California, Los Angeles, CA

ABSTRACT: The Global Occurrence of 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 that may have lasted as long as 4-7 million years. Due to the devastation of colonial metazoans during this extinction, the Early Triassic has often been deemed a "reef gap" because no metazoan reefs are found at this time. Whereas colonial metazoan reefs are absent, in Lower Triassic strata of south China, southern Turkey and the western United States, microbial build-ups have been found. 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 represent patch reef systems, and formed without any in situ metazoans. 
The western United States microbial patch reef system consists of an agglomeration of mounds that range in thickness from 2-2.5 m. In places, the mounds coalesce to form 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. Thin-section analysis confirmed that these mounds occurred without any in situ metazoans. The global occurrence of microbial reefs that formed without any in situ metazoans in the Early Triassic suggests that conditions favoring microbial growth must have persisted throughout the prolonged recovery interval from the end-Permian mass extinction.

 

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.