Fine-Grained Sedimentation Linked to Jurassic Stratigraphy and Global Events: A Case Study of the Da'anzhai Member

Fine-grained sedimentary rocks (shales and mudstones) are key archives of paleoenvironmental changes and basin evolution, and they provide critical constraints for refining regional stratigraphic frameworks. The Da’anzhai Member of the Lower Jurassic Ziliujing Formation in the Sichuan Basin is a major lacustrine succession bearing widespread organic-rich fine-grained deposits, with great significance for both Jurassic stratigraphic correlation and shale oil exploration. However, integrated stratigraphic–paleoenvironmental studies linking sedimentary processes to global Jurassic events remain insufficient. This study uses a multi-proxy geochemical approach (petrology, major and trace elements, REE, TOC) based on 114 core samples from three wells in the central, northern, and eastern Sichuan Basin to reconstruct paleoclimate (CIA), paleosalinity (Sr/Ba), paleowater depth (Co-La), redox conditions (V/(V+Ni)), and paleoproductivity (TOC, Ba/Al). The aim is to clarify Jurassic stratigraphic evolution and its controls on fine-grained sedimentation. Results show that the Da’anzhai Member records a complete Jurassic lacustrine cycle: lake transgression–maximum flooding–regression, corresponding to a paleoenvironmental shift of “dry-cold salinization →warm-humid desalinization → dry-cold salinization”. The Da.2 submember, corresponding to the maximum flooding interval of the Jurassic lake, is characterized by warm-humid climate (CIA > 55), deep water (~34 m), reducing conditions (V/(V+Ni) = 0.75), and continuous organic-rich fine-grained deposits (average TOC = 1.46%). In contrast, Da.1 and Da.3 were deposited under relatively cold-dry climate, shallow water, and oxygenated conditions, with lower TOC (0.62%–0.72%) and frequent carbonate interbeds. Petrological results show that Da.2 is dominated by montmorillonite and illite, indicating low-energy reducing environments favorable for fine-grained preservation, whereas illite-kaolinite assemblages in Da.1 and Da.3 reflect higher-energy oxic conditions. Statistical analyses indicate that vertical and spatial variations in fine-grained sediments are jointly controlled by paleoclimate, paleosalinity, water depth, and redox conditions. Warm-humid conditions enhanced chemical weathering and fine-grained sediment supply; low salinity promoted clay transport toward the basin center; and deep anoxic bottom waters favored organic matter preservation. These factors controlled sediment thickness, continuity, and organic enrichment, forming a reliable model for interpreting Jurassic lacustrine fine-grained sedimentation. The Early Jurassic Toarcian Oceanic Anoxic Event (T-OAE) provides a crucial global context for the Da’anzhai Member. The warm-humid and reducing conditions of Da.2 coincide with the T-OAE’s global warming and anoxic event, demonstrating that global Jurassic climatic–environmental perturbations were recorded in continental lacustrine successions. Unlike marine basins, continental lacustrine systems require coupled sediment supply, transport, and preservation conditions for organic accumulation. The Da.2 submember represents the optimal Jurassic interval for organic-rich fine-grained deposition, while Da.1 and Da.3 reflect relatively restricted depositional conditions. This study improves the stratigraphic subdivision and correlation of the Lower Jurassic Da’anzhai Member in the Sichuan Basin and establishes a link between regional Jurassic stratigraphic evolution and global Toarcian climatic events. The multi-proxy reconstruction provides a methodological reference for Jurassic continental stratigraphic studies worldwide. The identification of the Da.2 submember as a high-quality organic-rich interval supports target optimization for Jurassic shale oil exploration. In conclusion, this study reveals that: (1) Stratigraphic heterogeneity in the Jurassic Da’anzhai Member is driven by paleoenvironmental cycles; (2) Paleoclimate, paleosalinity, and redox conditions collectively controlled sediment supply, transport, and preservation during Jurassic deposition; (3) The T-OAE provided a first-order control on regional Jurassic lacustrine stratigraphic development; (4) The Da.2 submember is a key interval for both Jurassic stratigraphic research and energy exploration. This work advances our understanding of Mesozoic lacustrine systems and provides a practical template for global Jurassic stratigraphic correlation and paleoenvironmental analysis.