Sedimentary environment and model for lacustrine organic matter enrichment: lacustrine shale of the Early Jurassic Da'anzhai Formation, central Sichuan Basin, China

Abstract

Based on the analysis of element geochemistry and total organic carbon (TOC), this study investigates the main factors controlling organic matter (OM) enrichment, reconstructs the evolution process of the sedimentary environment, and proposes a dynamic OM enrichment model of the Jurassic Da'anzhai (D) Formation, Sichuan Basin. The results indicate that the Sichuan Basin was generally dominated by a warm and oxidizing sedimentary environment, but with some peculiarities, such as a hotter climate in the D1 member and more anoxic lake water in the D2a member. The sedimentary evolution of the Da'anzhai Formation can be divided into a fluctuating sedimentary stage, a stable sedimentary stage and a reef-building stage. The D2a member showed the strongest hypoxia, the weakest weathering, the largest amount of terrestrial inputs, and the highest TOC content. The TOC is positively correlated with reducing conditions and terrestrial inputs, negatively correlated with weathering. Based on these findings, it is suggested that the global climate in the Early Jurassic period had a complex regional effect and the global oceanic anoxic events of the Toarcian did not spread to the Sichuan Basin. Thus, the anoxic deep water, high terrestrial inputs, and weak weathering were conducive to rapid deposition and preservation of lacustrine OM.