ANACHRONISTIC FACIES AFTER MASS EXTINCTION: TETHYAN BASAL TRIASSIC CALCIMICROBIAL CAP ROCKS

Following the end-Permian mass extinction, one of the largest extinction in the history of life, a major crisis occurred in Phanerozoic carbonate systems. A non-skeletal carbonate factory abruptly replaced the prolific upper Paleozoic skeletal carbonate factory. Microbial communities emerged from stressed paleoenvironments to recolonize normal marine shelves. In addition, enhanced abiotic carbonate precipitation (seafloor fans) occurred in these and deeper paleoenvironments.

At least four large microbial episodes spread throughout the giant carbonate platforms of central and eastern Tethys up to South China during the prolonged Early Triassic recovery interval. In Panthalassa, seamounts record microbial and abiotic carbonate precipitation and, close to the end, the western Pangean margin of North America in western Nevada and eastern California.

The major biotic crisis in carbonate systems is expressed by the onset of the first microbial episode at the dawn of the Triassic time. Recently described in S China (Sichuan and Guizou) this episode is well recorded along the carbonate ramp from shallow proximal to deeper open marine distal part of the western and central Tethys (Taurus, S Turkey, Transcaucasia, E Elburz, Central Iran). It consists here of a variety of stromatolite and thrombolite facies including large domal stromatolites and thrombolites that measure up to 2 m high, of oncoids deposits and large oolitical shoals. In some areas, abiotic crystal fans precipitated on the seafloor in association with the other microbial facies. This distinct cliff-forming unit, named "calcimicrobial cap rock", is well dated from post-extinction latest Permian to earliest Triassic. Late Induan terrigenous input, with the muli-colored Claraia shales, marks the end of this first calcimicrobial episode.

Strong similarities exist between this anachronistic basal Triassic calcimicrobial cap rock and the Neoproterozoic cap carbonate in terms of facies (seafloor fans) and strongly negative carbon isotope excursions.