EVIDENCE FOR SUBMARINE DISSOLUTION ALONG A BASIN-WIDE TRUNCATION SURFACE AT THE PERMIAN-TRIASSIC BOUNDARY IN THE NANPANJIANG BASIN, SOUTH CHINA: IMPLICATIONS FOR CAUSES OF THE END-PERMIAN EXTINCTION

The purpose of this project is to evaluate the characteristics of a truncation surface coinciding with the end-Permian mass extinction. Previous studies have reported a widespread, single, sharp truncation surface at the end-Permian extinction horizon in Guizhou Province. Whether the surface resulted from submarine carbonate dissolution connected with ocean acidification at the end of the Permian or from subaerial karsting has been debated.

In this study, we found that the surface occurs in carbonate platforms in Guangxi demonstrating its basin wide extent. In Guangxi the truncation surface is underlain by a chemically altered, micritized zone of Permian packstone that is several millimeters thick. We also observed similarly micritized clasts containing fusulinids that were redeposited in post-extinction strata immediately above the surface. These clasts formed as the result of chemical weakening that happened to the Permian rock along the truncation surface/alteration zone and physical erosion and redeposition. In some areas the alteration zone has been completely removed by physical erosion. Aragonite fans proliferated above subtle topographic highs in the surface while micrite was deposited in the lows. Small clots and mm-thick lenses of volcanic ash trapped within the aragonite fans and micrite above the surface reflect distant volcanic eruptions.

The thin micritized alteration zone is most compatible with genesis by submarine dissolution. We found no diagenetic evidence for subaerial exposure. Earlier reports of possible vadose cements beneath the surface are suspect because there is no facies evidence for shallowing below the exposure surface and because we find no preferential geopetal orientation of anisopachous cements. Furthermore, the anisopachous cements previously suggested to represent subaerial exposure significantly predated the development of the truncation surface and micritized alteration zone at the PTB.

Another discovery we made is that organic matter in the post-extinction microbialites fluoresces under ultraviolet light. This indicates that the organic material has not been thermally matured past the oil window during burial diagenesis.