Sea-level changes control organic matter accumulation in the Longmaxi shales of southeastern Chongqing, China

Abstract

Fine-grained sediments in the Ordovcian–Silurian (O-S) Longmaxi Formation on the Yangtze Platform represent a significant shale gas source and reservoir in South China. The formation of the organic-rich black rock has been attributed to high primary productivity, suboxic/anoxic environments, and/or low detrital dilution. However, these claims remain to be debated. Here, we investigated total organic carbon (TOC) contents, iron speciation, major and trace elements of the O–S Longmaxi shale in the inner-shelf Qianqian #1 drillcore (southeastern Chongqing, South China). Our results show that the Longmaxi Formation is composed of black shale with high TOC values (4.7% ± 0.6%) in the lower member (LM: 0–6 m) and silty shale with low TOC values (1.3% ± 0.5%) in the upper member (UM: 6–77 m). The data from the LM suggests high primary productivity, ferruginous conditions punctuated with episodes of intermittent euxinia, and low terrigenous fluxes. In contrast, data for the UM indicate reduced primary productivity, ferruginous conditions, and higher terrigenous fluxes leading to dilution of the organic matter. A good coupling of local redox conditions with primary productivity and P/Al ratios, rather than water mass restriction, suggests that water-column anoxia was mainly driven by high primary productivity, and had a minor effect on primary productivity through marine P cycling. Changes in primary productivity might result from variable upwelling intensities associated with the fluctuations of sea levels. Variations in local redox conditions and terrigenous fluxes are mainly controlled by sea-level fluctuations. This study thus suggests that sea-level fluctuations and related environmental effects play key roles in organic matter accumulation through primary productivity, redox state, and detrital dilution. These findings highlight that multiple factors are responsible for organic matter accumulation at this location during the O-S transition in South China.