PETROGRAPHY OF A PERMIAN-TRIASSIC EROSION SURFACE AND MICROBIALITES OF THE GREAT BANK OF GUIZHOU, SOUTH CHINA: IMPLICATIONS FOR CAUSES OF THE END-PERMIAN MASS EXTINCTION

This project evaluates the origin of an erosion surface that coincides with the end Permian mass extinction. Two hypotheses have been proposed for the origin of the surface: 1) it formed as the result of submarine dissolution from high ocean CO2 connected with the extinction and Siberian Traps volcanism, or 2) formation as a karst surface (subaerial diagenesis) during sea level fall.

The extinction horizon coincides with a sharp and irregular erosional contact between the underlying Upper Permian skeletal packstone and an overlying microbialite. Conodont biostratigraphy places the Permian-Triassic boundary slightly above the base of the microbialite. Petrography was used to evaluate the genesis of the erosional surface. The surface is sharp and irregular and truncates both skeletal grains and cement. The surface has > 4 cm of relief and contains cavities that penetrate the underlying Permian limestone up to 20 cm beneath the surface. Microbial framestone was deposited above the erosion surface, and fills in cavities beneath the surface. It is composed of dendritic to globular calcified cyanobacteria, and contains thin gastropods, bivalves, carbonate mud, and fans of marine aragonite cement.

The Upper Permian skeletal packstone-grainstone was deposited in a shallow, open marine environment. It contains isopachous and anisopacuous cements. The thickened portions of the anisopachous cement are random in distribution, disproving the interpretation that these are pendant (or vadose dripstone) cements. No meniscus cements or other vadose fabrics were observed in thin sections. These observations support a submarine origin for the truncation surface. White tubular structures, <1 mm in diameter immediately beneath the truncation surface were examined to determine whether they represented ichnofossils or skeletal grains. The structures have micritic walls, are fragmented and are embedded in the sediment indicating they are skeletal grains. They are distributed well below the surface and exhibit greater micritization near the surface. We hypothesize that chemical alteration produced the micritization and was associated with chemical dissolution that formed the truncation surface.

Our results favor genesis of the erosion surface as a submarine dissolution surface resulting from ocean acidification.