Facies change and the end-Permian mass extinction in S.E. Sichuan, China

A detailed facies investigation has been undertaken of two key sections across the Permian-Triassic boundary in SE Sichuan, China, with a view to learning more about the timing and cause of the end-Permian mass extinction. Both high diversity reef and level-bottom faunas survived in carbonate platform environments until almost the end of the last, Changxingian, stage of the Permian. The onset of biotic decline is abrupt at the Laolongdong locality and very rapid in the inter-reef setting at Beifengjing. The extinction at both localities is associated with the development of dysaerobic facies characterised by abundant pyrite, a decline in burrow depth and size and an associated increase in the preservation of fine laminae. Benthic oxygen levels declined further in the Early Triassic and both anaerobic and lowest dysaerobic facies are developed. Comparable facies transitions occur in other Chinese era-boundary sections, which record deeper-water basinal and slope settings. The SE Sichuan sections are particularly important because they record the effects of bottom-water dysoxia in much shallower water, although the oxygen deficiency was apparently less intense. The disappearance of reefs at the end of the Permian was bound up with this oxygen deficiency associated with water deepening and not, as previously claimed, with an episode of emergence and karstification.