Research on Late Permian Conodont Biostratigraphy at the Erlongkou Section Fuling, Chongqing City, South China

The research of paleoenvironmental changes and mass extinction during the Permian and Triassic(P/T) transition is a global hot point in geological field. It has been proposed that massive regression was one of the potential drivers of this biotic crisis. Previously, a weathering crust has been reported at top Permian in Erlongkou section Fuling, Chongqing City, South China, and linked to this event. Nevertheless, its precise age and relationship to the extinction remain unresolved. To determine the age of this weathering crust, we conducted high resolution conodont biostratigraphic research on Erlongkou section. In total, we collected 33 samples and obtained 750 conodont specimens. Based on these materials, 3 conodont Zones had been established. In ascending order, the newly established conodont Zones are as follows: Clarkina Changxingensis Zone (beds 1–4), Clarkina yini Zone (beds 4.1-5.2), Clarkina meishanensis Zone (beds 6 and above). Clarkina changxingensis Zone (beds 1-4): Lower limit: First occurrence of C. changxingensis; Upper limit: First occurrence of C. yini. Associated taxa include：C.wangi, C.deflecta, C.postwangi, Hindeodus praeparvusand H. typicalis. Clarkina yini Zone (beds 4.15.2): Lower limit: First occurrence of C. yini; Upper limit: The bottom of bed 6. Associated taxa include：C.changxingensis, C.deflecta, C.postwangi,H. typicalis,H. latidentatus,H.pisaiand H. eurypyge.Clarkina meishanensis Zone (beds 6 and above): Lower limit: The lowest stratigraphic limit is placed at bottom of bed 6. The first occurrence (FO) of Clarkina meishanensis is in bed 9, which represents the lowest fossiliferous horizon of the Zone; Upper limit: undefined. The base of this Zone, corresponding to a significant lithological transition in the latest Changhsingian. The Zone includes beds 6 - 9 and overlying strata. Beds 6 (weathering crust), 7 (claystone), and 8 (mudstone) are conodont-barren beds, which is interpreted as a result of dramatic environmental changes, weathering, and unfavorable taphonomic conditions. Clarkina meishanensis first occurs at bed 9. Associated taxa include：C.changxingensis, C.yini and Hindeodus typicalis.The results suggest that weathering crust (bed 6) is located in upper C. yini Zone to lower C. meishanensisconodont Zone. It indicates the specific age of this weathering crust is upper part of the upper Permian, rather than the top of the upper Permian as previously considered. Based on previous studies and our newly measured sections, we briefly summarize the end-Permian regression records from key sections in South China, and carry out stratigraphic correlation with several sections in South China. Through investigations of multiple sections in South China, it suggests there was a massive regression in upper part of the upper Permian in South China, and the regression which leading to habit loss may be one of causes for P/T mass extinction.