Depositional Characteristics and Sedimentary Models of Ramp‐Type Fine‐Grained Rocks in the Lower Cambrian Qiongzhusi Formation, Southern Sichuan Basin

ABSTRACT The Lower Cambrian Qiongzhusi Formation in the Southern Sichuan Basin represents a newly recognized fine‐grained sedimentary succession with significant potential for shale gas exploration and development. The formation comprises shallow‐ and deep‐water ramp subfacies deposited within a rift‐related marine trough system; however, the depositional models of these ramp‐type fine‐grained rocks remain poorly constrained. This study integrates total organic carbon (TOC) analyses, major and trace element geochemistry, and well‐log data from the Jianyang and Ziyang areas to characterize sedimentary features, reconstruct depositional environments, and establish depositional models for ramp‐type fine‐grained rocks of the Qiongzhusi Formation. The results indicate that deposition was primarily controlled by two sediment transport mechanisms: gravity‐driven flows and bottom‐current reworking. In the shallow‐water ramp subfacies, the combined influences of these processes produced distinctive sedimentary structures, including flame structures and convolute lamination. In contrast, deposition in the deep‐water ramp subfacies was dominated by suspension settling, resulting in the development of laminated and massive shales. Geochemical proxies indicate that shallow‐water ramp deposits were strongly influenced by terrigenous influx ((La/Yb) N ≈ 0.99), which diluted organic matter in water column, reduced oxygen demand during organic matter degradation, and, together with enhanced hydrodynamic disturbance, promoted oxic to dysoxic redox conditions (U/Th ≈ 0.34; V/Cr ≈ 1.53). By comparison, deep‐water ramp deposits reflect higher primary productivity (Ba bio = 1146.45–1523.12 µg/g) under strongly restricted dysoxic to anoxic conditions (U/Th ≈ 1.26; V/Cr ≈ 2.54). On the basis of these results, two depositional models are proposed: (1) an organic‐lean, gravity‐flow‐ and bottom‐currents‐modified model for the shallow‐water ramp subfacies, and (2) an organic‐rich, weakly reworked, and relatively quiescent model for the deep‐water ramp subfacies. This study refines the depositional framework of the Qiongzhusi Formation, elucidates the spatial heterogeneity of organic matter enrichment within ramp systems, and provides a robust geological basis for shale gas exploration in analogous fine‐grained successions across the Sichuan Basin and comparable tectono‐sedimentary setting worldwide.