Sedimentological and Geochemical Characteristics of the Lower Cambrian Maidiping and Qiongzhusi Shales in the Northwestern Margin of the Sichuan Basin, SW China

This study integrates outcrop and geochemical data in order to compare the paleoclimates, terrigenous inputs, redox conditions, paleoproductivity and upwelling intensity of the lower Cambrian Maidiping and Qiongzhusi formations. The organic matter enrichment in both formations was jointly controlled by preservation, productivity and terrigenous supply; however, the relative importance of these factors differed significantly. The Maidiping Formation formed in a warm, humid climate with intense chemical weathering, high terrigenous input and persistently anoxic bottom waters. Its organic matter enrichment was primarily driven by upwelling-enhanced productivity and markedly modified by early diagenesis, as indicated by a strong positive correlation between TOC and Co × Mn (r = 0.75), as well as a decoupling from conventional redox proxies. In contrast, the Qiongzhusi Formation formed in a relatively arid climate with weaker weathering, and in dominantly dysoxic to suboxic waters that fluctuated in oxygen content. Here, organic carbon preservation was primarily controlled by basin-scale anoxia, as shown by positive correlations of TOC with U/Th, V/(V + Ni) and V/Cr. From Maidiping to Qiongzhusi, reducing conditions weakened and productivity declined, progressively deteriorating the conditions necessary for organic matter enrichment. This study clarifies the shift from productivity-driven to redox-preservation-driven enrichment, helping to reconcile disagreements regarding the primary controls on high-quality early Cambrian source rocks, while also providing reliable geochemical criteria for shale gas exploration.