Geochemical features of the Middle–Upper Permian cherts and implications for origin, depositional environment in the Sichuan Basin, SW China

Abstract

Permian cherts are widely developed in the Sichuan Basin and carry significant information about the evolution of the sedimentary environment. To understand the origins and depositional environments of the Middle–Upper Permian cherts in the Sichuan Basin, we studied the lithological and geochemical characteristics of the cherts. Based on field observations and thin section analysis, Permian cherts occur mainly in the form of nodular and bedded cherts. Nodular cherts present irregular lumps or tuberculous patterns and are composed of mostly cryptocrystalline and microcrystalline quartz with small amounts of carbonate mineral. Bedded cherts contain organic matter, and radiolarians can be found in Late Permian bedded cherts. The geochemical analysis of Permian cherts shows that the chert sources are complex; nodular Middle Permian cherts show hydrothermal and upwelling influences, with less terrestrial influence. Nodular chert is the product of diagenetic metasomatism, formed when silica‐rich fluids entered carbonate that was partially dissolved by H+ released from the oxidation of H2S. Bedded cherts of the Maokou Formation have the same source as nodular cherts but were deposited in a slope or depression of the platform, which was below the CaCO3 compensation depth (CCD) interface, so that bedded chert could be deposited directly. Bedded cherts of the Wujiaping Formation show influence from hydrothermal, biogenetic, and terrestrial sources and were deposited in a deeper water environment, such as basin or the bottom of slope. The Permian cherts in the Sichuan Basin are the response to the global Permian Chert Event (PCE), which was closely related to the Late Palaeozoic Ice Age (LPIA) and the global active volcanism during the Permian.