Mechanism for the enrichment of organic matter in the Liushagang Formation of the Weixinan Sag, Beibuwan Basin, China

Eocene lacustrine organic-rich shales, mudstones and shales in the middle member of Liushagang Formation (designated as LS2) are the most important source rocks in the Beibuwan Basin located in the northwest of the South China Sea. In this study, we explored the mechanisms responsible for the accumulation of organic matter in source rocks from LS2 of Weixinan Sag by investigating the paleoclimate, paleoredox, paleoproductivity and lithological characteristics of the parent rocks using biomarkers and elemental geochemistry data. Based on our analytical data, the deposition of the source rocks from LS2 occurred in a warm, humid paleoclimate with moderate chemical weathering, as indicated by CIA indexes and Sr/Cu ratios. The Sr/Ba ratios and gammacerane indexes indicated that the source rocks from LS2 were deposited in a freshwater paleolake. The Corg/P ratios and Redox Sensitive Elements (RSEs) -TOC bivariate diagrams demonstrated that the organic-rich shales and mudstones/shales deposited under anoxic and oxic-suboxic conditions, respectively. The levels of TOC, copper (Cu), molybdenum (Mo) and C30 4-methylsterance index in the organic-rich shales were higher than those of the mudstones and shales, indicating greater algae bloom and paleoproductivity during the deposition of the organic-rich shales compared to during the sedimentation of the mudstones and shales. Combined with La/Yb-∑REE, SiO2–TiO2, Al2O3/TiO2, Nb/Ta and Cr/Co ratios, it is considered that the parent rocks of the organic-rich shales were granite of relatively intermediate lithological compositions, with greater levels of nutrients such as phosphorus (P), Mo, Cu, compared to those of the mudstones and shales. Large-scale decomposition of dead algae led to oxygen depletion, which created favorable conditions for the preservation of organic matter and the release of P from the sediments to water. The increased content of P in water subsequently aggravated algal blooms. Such a positive feedback loop was conducive to the formation of organic-rich shales. The inter-relationships of algal blooms (controlled by P), paleoproductivity and paleoredox conditions play an important role in the accumulation of organic matter in organic-rich shales from LS2 of Weixinan Sag, Beibuwan Basin.