What continued after the mass extinction: insights from carbonate microfacies and biological evolution around the Permian-Triassic boundary in the middle Upper Yangtze Platform, SW China

At the end of the Palaeozoic Era, most species on Earth disappeared completely and the global sedimentary environment and biology changed dramatically. The Permian-Triassic boundary (PTB) was studied in three sections of the middle Upper Yangtze Platform, SW China: Xingwen well and Zhijin and Shangsi sections. These sections are characterized by carbonate-platform, toe-of-slope and basin facies, respectively. Detailed analysis of 100 closely spaced thin-sections revealed a total of 24 microfacies and 11 microfacies associations based on the dominant carbonate grain size and the skeletal material (type and proportion). Six bioassemblages are documented for the first time, spanning c. 1 Ma across the boundary succession, including normal, resurrected and miniaturized Permian biota, and cyanobacteria-dominated, survival post-crisis and neonatal Triassic biota. The Xingwen well indicated sedimentary evolution from a rimmed carbonate platform to a homoclinal carbonate ramp, as well as a sharp fall in sea level just prior to the PTB in the study area. The Zhijin section revealed a slope setting, in which toe-of-slope and middle-ramp microfacies are identified. The Shangsi section showed a complete evolution of basin and outer-ramp microfacies and bioassemblages. Fossil evidence showed that the Permian biota (trilobites, gastropods and phylloid algae) occurred in the uppermost Changhsingian stage and was overlain by biomicrites with miniaturized ostracods (< 0.2 mm in size). This indicated that the major extinction horizon is located up to 14 cm below the PTB, which lies in the middle-lower segments of the miniaturized ostracod layer.