MICROFACIES OF PERMIAN–TRIASSIC BOUNDARY LAYERS IN THE SHANGSI SECTION, SOUTHWEST CHINA: DETAILS OF SEDIMENTARY ENVIRONMENT CHANGE

Accompanying the Late Permian mass extinction (LPME), the sedimentary environment underwent significant changes in both marine and non-marine settings. However, researchers have not yet fully understood these changes. As an ever-present candidate for the Global Stratotype Section and Point (GSSP) for the Permian–Triassic boundary (PTB), the Shangsi section has preserved trails of geological events and environmental information from the eastern Paleo-Tethys Ocean. In this study, we describe the microfacies of PTB layers at Shangsi through micropetrological and geochemical analyses. Calcite replacement in siliceous radiolarians is observed across the layers between Beds 24 and 26, suggesting that the d13Ccarb composition of siliceous limestones in the upper Permian (Clarkina yini conodont zone) was strongly affected by diagenesis. After deeply excavating samples in the Bed 27b, ferruginous microspheres were still found, indicating they are not industrial fly ash. The foraminifer fauna in the lower part of Bed 28 was likely the result of the Lilliput effect, indicating deteriorated environmental conditions following the mass extinction. A dramatic increase in terrestrial material input in Bed 29 probably represents an increase in the weathering rate during the earliest Triassic. Multiple volcanic ash beds in the Shangsi section coincide with the deterioration of the deep-water environment. These observations suggest that microfacies may help elucidate the detailed pattern of high-resolution isotope changes across the PTB in the Shangsi section.