Depositional environment and organic matter accumulation of the Lower Cambrian Shuijingtuo Formation in the middle Yangtze area, China

Abstract

The organic-rich shale of the Lower Cambrian Shuijingtuo Formation in the Yangtze area is considered a high-quality source rock in South China. Geochemical proxies are used to reveal the formation and preservation conditions of the Shuijingtuo shale in Yichang in the middle Yangtze area. Geochemical proxies for paleoredox (U/Th, V/Cr, Ni/Co, and UEF versus MoEF), paleoproductivity (Ba/Al, Cu/Al, and Ni/Al), and terrigenous influx (Al and Ti) have been employed to interpret the mechanism of organic matter (OM) accumulation. The total organic carbon (TOC) contents of the Shuijingtuo shale is between 0.19 wt% and 4.77 wt% (with an average of 1.38 wt%). The shale can be divided into two intervals based on TOC content, the lower interval with high TOC (TOC >2 wt%) and the upper interval with low TOC (TOC <2 wt%). The results of the geochemical proxies show that the lower interval shale was formed in an anoxic setting with fair paleoproductivity and relatively low terrigenous influx. The upper interval shale was deposited mainly under oxic conditions, with gradually enhanced oxidation, decreased productivity, and relatively high terrigenous influx compared to the lower interval shale. The water mass inferred by the Mo/TOC ratios was a restricted environment with episodic upwelling in the lower interval. The TOC contents display a positive correlation with both the paleoredox and paleoproductivity proxies, indicating that anoxic/dysoxic and high productivity favored OM accumulation. The Co × Mn versus Cd/Mo relationship also suggests that the OM accumulation is chiefly determined by preservation in a restricted environment. These data indicate that the redox condition acts as the dominant factor for OM accumulation. A depositional model is proposed that emphasizes the importance of preservation for OM accumulation of this shale.