Sedimentary geochemical investigation for paleoenvironment of the Lower Cambrian Niutitang Formation shales in the Yangtze Platform

The geochemical data and geochemical proxies of the Niutitang Formation shales in the Yangtze Platform were investigated for deciphering paleodepositional environment and elucidating paleoenvironment conditions responsible for organic carbon accumulation. Ce anomalies, Eu anomalies and Y anomalies of the Niutitang Formation shales is between 0.39 and 0.92, between 0.65 and 1.03, between 0.97 and 1.71, respectively. The REE patterns of the Niutitang Formation shales show enrichment in HREE relative to LREE with negative Ce anomalies, positive Y anomalies and negative Eu anomalies, indicating of a greater influence of seawater compared to hydrothermal fluids. U, V and Mo are enriched in the shales, and Mo/Al exhibits strong covariation with TOC contents, whereas U/Al and V/Al show weak covariation with TOC contents, indicating that H2S associated with hydrothermal vents is present. Compare with upper Niutitang Formation shales, higher V/(V + Ni), Ni/Co and V/Cr ratios in lower Niutitang Formation shales imply that the lower Niutitang Formation shales are deposited in anoxic conditions. Higher Mo/Al and Ni/Al ratios exist in the lower Niutitang Formation shales relative to the upper Niutitang Formation shales, indicating higher primary productivity during deposition of the lower Niutitang Formation shales. Thus, the variations of primary productivity and preservation of organic matter associated with redox conditions of bottom water lead to greater TOC contents in lower Niutitang Formation shales relative to upper Niutitang Formation shales.