Early Cretaceous ferruginous and its control on the lacustrine organic matter accumulation: Constrained by multiple proxies from the Bayingebi Formation in the Bayingebi Basin, inner Mongolia, NW China

Abstract The greenhouse climate in the Cretaceous period promoted widespread organic-rich marine/lacustrine black shales or mudrocks. Organic matter (OM) accumulation is generally affected by preservation conditions and surface water primary productivity. The detail of organic-rich mudrock formation in lacustrine system still remains enigmatic. The widespread OM-rich mudrocks in the Cretaceous period as petroleum source rocks in many large terrestrial petroliferous basins in China offer an opportunity to explore the mechanism of lacustrine OM accumulation. Fe speciation, C S Fe system, pyrite morphology, pyrite-sulfur isotopic composition, elements, and biomarkers in the Bayingebi Formation from a drill core in the Bayingebi Basin, Inner Mongolia, NW China are used to constrain redox conditions, OM source and primary productivity in a lacustrine system. Our results show that the dark-gray mudrocks in the Bayingebi Formation were deposited in ferruginous and stratified water-column in the Bayingebi Basin. The biomarkers suggest a mixed organic matter source of algae, bacteria and high plant with a low algal contribution. The surface-water primary productivity during the deposition of the Bayingebi Formation was low and shows a weak correlation with total organic carbon (TOC) content. The OM accumulation in early Cretaceous lacustrine system was mainly controlled by redox conditions. Ferruginous anoxic conditions in lacustrine system may have high potential for hydrocarbon source rocks owing to the reduction of oxygen decomposition.