Genesis and mechanisms of organic matter enrichment of the Hongshuizhuang Formation in the Zhoukoudian area of Jingxi sag, North China

Abstract

The Hongshuizhuang Formation is a Meso-Neoproterozoic high-quality source rock in the North China Craton (NCC), comprising abundant organic matter. The present study focuses on the analysis of the Hongshuizhuang Formation in the Zhoukoudian area of the Jingxi sag, discussing its genesis and mechanisms of organic matter enrichment through geological and geochemical methods. The Hongshuizhuang Formation is divided into three members from bottom to top respectively as the lower, middle and upper member. Trace elements analysis indicate that the Hongshuizhuang Formation developed restricted neritic facies in an extensional environment within a continental island arc under a post-orogenic background. The lower and upper members were deposited in a relatively shallow, strongly retained water mass within a suboxic and saline environment influenced by subhumid climate, while the middle member was deposited in an anoxic deep–water environment with relatively low salinity and weak restrictions. The PAAS-normalized rare earth element distributions of the middle and upper members show an enrichment of LREEs and a depletion of HREEs, and a low mean Y/Ho ratio with a positive Eu anomaly, indicating that the regional deposition has been affected by hydrothermal fluids. The negative δ¹³C and δ¹⁸O values and the positive⁸⁷Sr/⁸⁶Sr values confirm that the deposition of the middle and upper members was accompanied by crustal hydrothermal activities. Accumulation of organic matter and enrichment of Ba_xs and P indicate that productivity is the basis of organic matter enrichment in the Hongshuizhuang Formation, where high-quality source rocks are concentrated in the middle member. In addition, reduced water mass controls the preservation of organic matter. Hydrothermal activity, humid climate, and salinity support a higher primary productivity and the formation of reduced water masses. However, due to limitations in depth, the high-quality source rocks in the Jingxi sag are thinner than the Jibei sag.