Effects of Emeishan Large Igneous Province on organic matter accumulation, Sichuan Basin, China

Abstract

Volcanic activity is often associated with the formation of organic-rich sedimentary deposits. However, the relationships between volcanism and depositional events of hydrocarbon source rock during geological history remain unclear. To fill the knowledge gap, we investigated the relationships based on a case study of the Emeishan Large Igneous Province (ELIP) during the late Guadalupian. We carried out a detailed petrological and geochemical investigation of the upper Maokou Formation (i.e., the Mao III Member) in the Shangsi section, Sichuan Province, China. The Mao III Member has a relatively high abundance of organic matter (OM), good OM type, and appropriate thermal maturity that is conducive to hydrocarbon generation. Paleoenvironmental reconstructions based on multiple geochemical proxies indicate that the Mao III Member records relatively high primary productivity, enhanced anoxic conditions, intense upwelling but weak water mass restriction, decreased detrital input and sedimentation rate, and fluctuations in volcanic and hydrothermal activity. The marked positive correlations between primary productivity, redox conditions, and OM abundance suggest that the former two factors controlled OM accumulation. In addition, the detrital input has a slight dilution effect on the OM abundance. No direct relationship was observed between hydrothermal activity and OM abundance. In contrast, volcanic activity related to ELIP had a major role in enhancing the primary productivity and more reducing conditions, which is confirmed by the marked correlations between Hg/TOC, Cuauth, and Vauth values. This suggests that ELIP volcanism had a significant positive effect on OM enrichment in the Mao III Member, which may be a common feature associated with large volcanic events at other times in Earth's history.