Geochemical characteristics of the lower Silurian Longmaxi Formation on the Yangtze Platform, South China: Implications for depositional environment and accumulation of organic matters

The organic-rich lower Silurian Longmaxi Formation is now regarded as the favorable target for shale gas exploration and production. However, the mechanism of organic matter accumulation in these deposits remains highly controversial. In this study, we investigate the total organic carbon (TOC) content and the major and trace element abundances of a newly recovered core from the Yangtze Platform to reconstruct the paleo-environment and to elucidate the factors that control organic matter accumulation. Our results suggest that the greenhouse conditions during the early Silurian were punctuated by several cooling events. Bottom water redox conditions improve gradually upsection with a transition from anoxic water conditions in the lower Longmaxi Formation to suboxic-oxic water conditions in the upper Longmaxi Formation. The primary productivity on the Yangtze Platform appears to have been high during deposition of the Longmaxi Formation, especially in its lower part, due to an enhanced phosphorus recycling. The strong correlations between TOC and copper and nickel imply that the productivity was especially critical to the organic matter accumulation in the lower Longmaxi Formation. Furthermore, the rapid consumption of dissolved oxygen in bottom water favored the establishment of anoxic water conditions, which facilitated organic matter preservation. The poor correlations between TOC and barium and phosphorus in the lower Longmaxi Formation likely reflect intense recycling of nutrients. The evolution of water redox conditions and accumulation of organic matters in the Longmaxi Formation can be related to global fluctuations in sea level and regional tectonic uplift of the Yangtze Platform.