Sedimentary microfacies and sedimentary responses to the biotic extinction events within the Penglaitan section at the Guadalupian-Lepingian (Permian) boundary in Laibin, Guangxi

A global biotic extinction event once took place at the end of the Middle Permian (Guadalupian), and directly led to the disappearance of approximately 34% of marine invertebrates. The marine regression also occurred on a global scale during the periods. This event is responsible for the initiation of the Dongwu movement in China, which resulted in the widespread unconformities at the Guadalupian-Lepingian (Permian) boundary in most parts of China. Meanwhile, the Middle-Late Permian marine deposits were developed in the Laibin region located on the southern margin of the Yangtze platform. The emphasis in this study is placed on sedimentary microfacies and organic fossils in the Penglaitan section, a GSSP section in the eastern part of the Laibin geosyncline. Five types of sedimentary microfacies associations are recognized in the 1-7 beds of the section, including reef base, reef core, reef cap, shoal and deep-water slope microfacies associations. The abundance and diversity of fossil fragments suddenly decreased in Bed 7a, which agrees with the observations of negative carbon isotope excursion and large mercury content anomaly in the Penglaitan section. However, only a few of conodont and ammonoid species appear in this horizon. The authors contend that the sharp decrease of fossil fragments represent the sedimentary responses to sea-level changes rather than the biotic extinction horizon at the Guadalupian-Lepingian (Permian) boundary.