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 < 1.5 layers/Ma; cumulative thickness ratio < 1%). TST1 (dense interval) witnessed more intense and high-frequency volcanic activities than TST2 (sparse interval), so the TST1 has generally higher TOC than TST2. The intense and frequent volcanic activities had dual effects on organic-rich shale: on one hand, volcanic ash provided a sufficient supply of nutrients, which triggered high marine productivity; on the other hand, the extremely anoxic environment caused by volcanic activity enhanced the burial amount and preservation rate of organic matter. Key words: shale, organic abundance, volcanism, sequence stratigraphy, Ordovician Wufeng Formation, Silurian Longmaxi Formation, Upper Yangtze area, bentonite