GEOBIOLOGY OF MASS EXTINCTION RECOVERY INTERVAL ANACHRONISTIC FACIES: MICROBIAL REEFS IN THE EARLY TRIASSIC

Previous studies have demonstrated that the Early Triassic recovery interval from the end-Permian mass extinction event is a period of time with an intriguing sedimentary rock record. The Early Triassic represents a "coal gap" (Retallack and Veveers, 1996) because it is the only time since the evolution of coal-forming floras that no coals are known from the sedimentary record. Similarly, a "chert gap" is also reported from this time due to the scarcity of chert deposits from the Early Triassic. Others have reported "anachronistic" sedimentary features in carbonates including flat-pebble conglomerates (Wignall and Twitchett, 1999), ribbon-rock (Lehrmann, 1999) and normal marine stromatolites from the Western United States (Schubert and Bottjer, 1991). Thus, the Early Triassic is not only a time of low diversity recovery faunas, but is also characterized by an unusual anachronistic sedimentary rock record.

Microbial reefs represent one such anachronistic facies from the Early Triassic. For example, microbial patch reefs have been described from Spathian strata in South China (Lehrmann, 1999) and the Western United States (Pruss and Bottjer, 2001), illustrating a global phenomenon of microbial build-ups in the Early Triassic. These occurrences indicate that unusual oceanic conditions which fostered development of these build-ups must have persisted throughout the Early Triassic prolonged recovery.

While much emphasis has been placed on determining the absence or presence of taxa after the end-Permian extinction, significant information can be gleaned by closely studying this unusual sedimentary record. The abundance and global distribution of such features as flat-pebble conglomerates, ribbon-rock and normal marine microbial build-ups in the Early Triassic indicate extraordinary conditions must have existed during this 5-10 million year long recovery interval. Study of these anachronistic facies occurrences may provide a unique way of assessing the causal mechanisms involved with this extinction event and recovery.