A PALEOENVIRONMENTAL RE-INVESTIGATION OF THE TERRESTRIAL PERMIAN–TRIASSIC TRANSITION IN SOUTH CHINA

Numerous studies have been conducted in marine Permian–Triassic boundary (PTB) successions allowing a detailed description of the biotic changes associated to the end-Permian extinction event (EPE). Despite these efforts, the PTB interval remains poorly resolved in the terrestrial realm. Here we present new data from the re-investigation of four terrestrial PTB sections from western Guizhou and eastern Yunnan. Paleoenvironment study of the PTB transition was done using sedimentological analysis, paleobotany, palynology and XRF.

Our results show major differences in the depositional settings between the Xuanwei and Kayitou formations. Changes from terrestrial fluvial-lacustrine to coastal marine setting are observed, with a clear segregation supported by our combined dataset. Exact stratigraphic placement of the PTB remains unclear, although current data indicates that the PTB is likely occurring at the top of the Xuanwei Fm. Recent radiometric dates bracketing the PTB from the Meishan GSSP are within range of bed 68 at Chahe. The disappearance of the Gigantopteris flora cannot be used as a marker for the EPE as this plant group persist into the Triassic. However, plant fossils reveal two distinct assemblages: A Late Permian (Changhsingian) Gigantonoclea guizhouensis – Annularia pingloensis assemblage in the upper Xuanwei Formation, and an Early Triassic (Induan) Annalepis – Peltaspermum assemblage associated with relicts of Permian gigantopterids in the uppermost Xuanwei and basal Kayitou formations. The Early Triassic age of the Kayitou Formation is further supported by the occurrence of typical Triassic marine fauna. Around 15% of the 116 collected and analyzed palynological samples yielded an extremely low recovery of palynomorphs, the remaining were barren. Palynological assemblages are almost exclusively composed of spores, while three pollen grain were also identified. A ~1 meter zone with fungal spores including Reduviasporonites has been identified near the top of the Xuanwei Fm. This interval may correspond to the EPE "fungal spike" conventionally associated to ecosystem collapse. It is yet to demonstrate whether diversity changes are related to paleoenvironmental change coeval the transition between the Xuanwei and Kayitou formations, or rather linked to the EPE.