Multiproxy Geochemical Reconstruction of Paleoenvironment and Its Control on Fine-Grained Sedimentation in the Da’anzhai Member of the Jurassic System, Sichuan Basin

The origin of fine-grained sedimentary rocks in the Da’anzhai Member of the Lower Jurassic Ziliujing Formation in the Sichuan Basin, as well as their paleoenvironmental implications, remains a subject of ongoing debate. Studies on the sedimentary patterns of fine-grained components and quantitative analysis of paleoclimate remain limited. To effectively address this issue, in this work, the fine-grained sedimentary rocks of the Da’anzhai Member were considered as the research object, and petrological and multi-index geochemical investigations were conducted. From our analysis, the paleoenvironment during the sedimentary period was systematically reconstructed and its control mechanism on fine-grained sedimentary rocks was revealed. The results showed that the Da’anzhai Member has a complete sedimentary sequence of “lake transgression-maximum lake flooding-lake regression,” and the climate simultaneously presents a cycle of “dry and cold salinization (CIA avg. 43.77)-warm and humid desalinization (CIA avg. 68.01)-dry and cold salinization (CIA avg. 22.34).” The warm and humid climate (CIA > 55), deep water (avg. 33.90 m), and anoxic environment (V/(V + Ni) avg. 0.75) jointly contributed to the enrichment of high-quality organic matter in the Da.2 submember, with an average TOC of 1.46%. The thickness and continuity of fine-grained sedimentary rocks were jointly determined by paleoclimate (CIA), paleowater depth (Co–La), and paleosalinity (Sr/Ba). A CIA value greater than 55 ensures the supply of fine-grained materials, a paleowater depth greater than 20 m is conducive to the preservation of sediments, while a Sr/Ba ratio less than 0.6 promotes the diffusion of fine-grained materials toward the center of the lake basin. This control process is closely related to the Toarcian Oceanic Anoxic Event of the Early Jurassic: the warming and anoxia triggered by the T-OAE favored the enrichment of organic matter, and when combined with the synergistic effect of the supply transport-preservation three factors, the influence of the paleoenvironmental cycle on fine-grained sedimentation was intensified. The semisaline and freshwater environment, characterized by a warm and humid climate and deep water anoxia, was found to be conducive to the enrichment of fine-grained sedimentary rocks rich in organic matter, providing key geological evidence for the exploration of shale oil in the Jurassic of the Sichuan Basin. At the same time, a representative case for investigating the response of continental sedimentation to climate change during the Toarcian period globally was provided by our work.