Microfacies, sedimentary environment and sea-level changes of the Permian-Triassic boundary succession in the Jiangya section, Cili County, Hunan Province.

This paper focuses on the microfacies features of the Permian-Triassic boundary succession ( including the uppermost 9. 4m of the Dalong Formation and the lowermost 7. 5m of the Daye Formation) in the Jiangya section, Cili County, Hunan Province in order to understand the depositional environments and relative sea-level changes during this critical time interval. Based on the detailed microfacies analysis in combination with sedimentary features observed on outcrops and polished slabs, the uppermost part ( 9. 4m thick) of the Dalong Formation was deposited in gradually deepening settings from basin margin to the center of the basin. At the top surface of the Dalong Formation, relative sea level dramatically dropped, and the depositional environment shifted abruptly from a deep-water basin to a shallow-water platform. From the base of the Daye Formation above, relative sea level rose slowly, causing changes in depositional environments from an isolated basin, to a semi-isolated basin, and to a circulated basin. At the horizon of about 7m above the base of the Daye Formation, relative sea level began to fall again, and the climate became extremely arid, which resulted in an evaporative basin in the Jiangya area during the Earliest Triassic. It is worth noticing that the great regression across the boundary of the Dalong and Daye formations exactly coincided with the End-Permian mass extinction horizon, which may indicate that the rapid regression might have been one of the major causes to the End-Permian mass extinction.