Effects of volcanic activities in Ordovician Wufeng–Silurian Longmaxi period on organic-rich shale in the Upper Yangtze area, South China

Abstract Based on the corresponding relationship between the paleoproductivity, redox conditions and volcanism within a chronostratigraphic framework, the effects of volcanic events in the Wufeng–Longmaxi period on organic abundance of shale were examined. Bentonite layers were mostly developed in the transgressive systems tract 1 (TST1, Wufeng Formation) and transgressive systems tract 2 (TST2, Longmaxi Formation), and the two systems tracts corresponded to favorite shale lithofacies with high silica and total organic carbon (TOC) contents. According to the stratigraphic characteristics of bentonite rich interval, TST1 is classified as the interval with dense bentonite layers with the frequency of bentonite layer (bentonite layers/time) of more than 1.5 layers/Ma and the cumulative thickness ratio of bentonite layers (thickness of bentonite layers/thickness of shale) of more than 1%; TST2 is classified as the interval with sparse bentonite layers (frequency