Multiple controls on the organic matter accumulation in early Cambrian marine black shales, middle Yangtze Block, South China

Abstract

This study aims to investigate paleoenvironments, hydrothermal activity, and seawater restriction of the lower Cambrian Shuijingtuo Formation in the Three Gorges area and clarify the dominating factors on organic matter (OM) accumulation. In the studied SL well in western Hubei Province, the Shuijingtuo Formation was divided into three intervals based on geological and geochemical characteristics. Total organic carbon (TOC) contents were variable, averaging 0.69% in Unit 1, 5.40% in Unit 2, and 1.12% in Unit 3. Enrichment factors (EF) of Cu and Ni indicated higher paleoproductivity in Unit 2. Unit 1 and the lower part of Unit 2 were affected by hydrothermal activity, whereas hydrothermal events receded in Unit 3. Hydrothermal activities could provide nutrients and may also facilitate the reducing water condition. Redox proxies (Corg/P, EF(Mo)-EF(U)) suggested that Unit 1, Unit 2, and Unit 3 were deposited in oxic-suboxic, euxinic, and suboxic environments, respectively. Upwelling/restriction proxies (EF(Co)*EF(Mn), Co (ppm)*Mn (%), Mo/TOC, and Cd/Mo) implied that Unit 1, Unit 2, and Unit 3 were deposited in upwelling, semi-restricted, strong restricted conditions, respectively. In summary, an anoxic environment, semi-restricted water conditions, and hydrothermal events are beneficial for OM enrichment during the early Cambrian.