Organic matter enrichment due to high primary productivity in the deep-water shelf: Insights from the lower Cambrian Qiongzhusi shales of the central Sichuan Basin, SW China

Abstract

• Cambrian shales in the Sichuan Basin were formed under humid anoxic conditions with low-moderate productivity. • High primary productivity was responsible for organic matter enrichment. • Much higher primary productivity occurred in the deep-water shelf than in the shallow-water shelf. • Hydrothermal upwelling can increase primary productivity. The formation and enrichment mechanisms of organic-rich shales under different marine environments remain unclear. Recently, Cambrian Qiongzhusi (QZS) shales with greater thickness and total organic carbon (TOC) content were found in a deep-water shelf (DWS) than in the adjacent shallow-water shelf (SWS) of the central Sichuan Basin, and these locations provide ideal conditions for determining the mechanisms underlying differences in organic matter (OM) enrichment. In this study, we investigated the TOC, pyrolysis, organic carbon isotopes (δ 13 C org ), biomarkers, and elemental geochemistry of the QZS shales. Based on a detailed comparison between shales from the DWS to the SWS, an OM enrichment model of different shelf conditions was established and the reasons for the higher TOC content in the DWS were clarified. The deposition of QZS shales was accompanied by a large-scale regional transgression. Against the background of an overall humid climate, the water mass at the bottom of the DWS was more restricted, resulting in limited external material exchange and supply. The water mass of the DWS represented by the bottom of the QZS was anoxic and showed the development of euxinic conditions. The biogenic Ba (Ba bio ) content of the samples in the DWS under deep basinal conditions was significantly greater than that in the SWS region, which reflects higher primary productivity and is associated with a large accommodating space and upwelling flows caused by the hydrothermal activities in the DWS. Under conditions in which OM enrichment is dominated by productivity, the DWS is a more favorable location for the formation of high-quality shales.