Community replacement sequences and paleoenvironmental changes in reef areas of South China from Late Permian to Early Triassic exemplified by Panlongdong section in northeastern Sichuan Basin

The well-known Permian Changhsingian calcisponge reef located at Panlongdong section, Xuanhan county, northeastern Si- chuan Basin has attracted wide attention. Due to severe dolomitization and poor quality of the fossils, the P-T boundary in this section is difficult to determine. This study, for the first time, recognized six communities in the Upper Permian Changhsingian Changxing Formation through the Lower Triassic Induan Feixianguan Formation of the Panlongdong section. They are Bryo- zoan-Archaeolithoporella-calcisponge Community, Calcareous green algae-foraminifer-crinoid Community, Microgastro- pod-foraminifer Community, Ostracod-microgastropod-cystic microbe Community, Ostracod-small brachiopod Community, and Non-calcified cyanobacteria Community. By using community replacement and palaeoenvironmental analysis, for the first time, we set the P-T boundary of the Panlongdong section at the middle of the calcimicrobialite containing cystic microbes. The community replacement sequence in the Panlongdong section is similar to that in other contemporaneous sections in reef areas of South China, indicating universal palaeoenvironmental changes during the Permian-Triassic transition. The results show that： （1） Changhsingian calcisponge reefs in South China generally vanished before the mass extinction and may be re- lated to the large regression in the Late Permian. （2） The Calcareous green algae-foraminifer-crinoid Community replaced reef community and continued till the mass extinction. The mass extinction was probably related to the global sea-level drop. （3） The first aftermath community was dominated by specialized microgastropods, followed by the microbes, and then by the spe- cialized microgastropods and small brachiopods. The succession of the three communities reflected the change in environmen- tal conditions from dysoxic to anoxic and again to dysoxic. （4） In the Early Triassic, the relict community in shallow sea had very low diversity and low abundance, and was dominated by crinoids, gastropods, and bivalves.