LITHOSTRATIGRAPHY OF UPPER GUADALUPIAN (MIDDLE PERMIAN) ROCKS AT CHAOTIAN IN SICHUAN, SOUTH CHINA: SECULAR CHANGE IN SEA LEVEL AND REDOX CONDITION OF THE SEDIMENTARY ENVIRONMENT

The age of the Middle–Late Permian (Guadalupian–Lopingian; G-L) boundary related to mass extinction is close to the onset timing of the global prolonged anoxic event in deep-sea (superanoxia). The environmental fluctuation of relatively deep shelf is likely important, although possible causal relationship between the shallow marine extinction and the deep-sea anoxia is still unknown. We examined in detail the lithostratigraphy and lithofacies of Middle–Upper Permian shallow-marine limestone (150 m thick) at Chaotian in northern Sichuan, South China, in order to clarify temporal changes in the sedimentary environment. The main part of the upper Middle Permian Maokou Fm and the lowermost Upper Permian Wujiaping Fm consist of bioclastic (fusuline, algae, and coral) limestone that was deposited in the euphotic zone on a continental shelf. In contrast, the uppermost Maokou Fm (11 m thick) is composed of thinly interbedded black shale/chert with abundant radiolarians, which was deposited in the dysphotic zone on a continental slope/basin. The Wordian to lower Capitanian age of the Maokou Fm is indicated by fusulines (Neoschwagerina, Lepidolina), conodonts (Jinogondolella), and ammonoids (Altudoceras, Paraceltites). The stratigraphic change in lithofacies at Chaotian indicates that the sea level rose during the latest Guadalupian, but then fell rapidly across the G–L boundary. The redox condition of the sedimentary environment also shows a remarkable change in accordance with sea-level fluctuations. The transgression was probably caused by a local tectonic event that involved basin subsidence in western South China, while the following regression reflects a global sea-level fall across the G–L boundary. The emergence of mid-depth anoxia associated with transgression at Chaotian was likely related to the shallow-sea mass extinction and also to the onset of superanoxia in deep-sea immediately before the G-L boundary.