Sedimentary Evolution of the Cambrian Qiongzhusi Formation, Northwestern Sichuan Basin: Insights from Quantitative Clinoform Analysis

Clinoforms are characterized by basinward-dipping chronostratigraphic surfaces. They are ubiquitous sedimentary slope architectures in continental-margin settings worldwide. The growth of clinoforms records the influence of sea-level change, sediment supply, and basin subsidence. Quantifying clinoform geometry provides a basis for reconstructing these controlling processes through geological time. We use high-resolution 3D seismic data covering approximately 6,000 km² in northwestern Sichuan Basin, South China. In the study area, the Ediacaran to Cambrian succession records three major shifts in sedimentary environments, from mixed siliciclastic-carbonate shelf system (Doushantuo Formation), to carbonate-dominated platform system (Dengying Formation), to clastic-dominated deepwater system (Maidiping and Qiongzhusi Formation), and ultimately to carbonate-dominated shallow-water system (Longwangmiao Formation). Here, we focus on the clastic-dominated succession of the Qiongzhusi Formation to reconstruct sediment provenance, constrain sedimentary evolution, and evaluate the influence of regional tectonics. Seismic interpretation reveals the lower section of the Qiongzhusi Formation contains submarine fans, turbidite channels, and mass-transport complexes, indicating gravity-driven depositional processes in a relatively deepwater environment. Within the upper section, we interpret at least seven clinoform sets (C1~C7). These clinoform sets show oblique stacking, recording a progradational, basinward migrating shelf-edge trajectory, which is consistent with limited accommodation and sea-level fluctuations during the evolution. To infer progradation rates and total sediment flux during clinoform evolution, we calculate the progradation factor (Pf) and volumes (Ve) of the clinoform sets. Based on the results, we demonstrate that the clinoforms developed in multiple phases, corresponding to periods of intensified and weakened orogenic loads caused by tectonic activity in the northwestern region. In the first stage (Start to C1-C2), the orogenic loading intensified and resulted in a rapid increase in sediment influx and accelerated progradation, as reflected by elevated Pf and Ve values. In the second stage (C1-C2 to C3-C4), the orogenic loading was subdued and led to slower progradation and reduced sediment flux, as indicated by decreased Pf and Ve values. In the third stage (C3-C4 to C6), the orogenic loading intensified again. In the final stage (C6 to End), the orogenic loading neared termination, as evidenced by an abrupt decrease in Pf and Ve values. Thus, we propose that the increases and decreases in sediment flux were driven by the orogenic activity (i.e. Motianling Orogeny) in the northwestern region of Sichuan Basin. In conclusion, the alternating stages of intensified and subdued orogenic activity, and the associated increases and decreases in sediment flux, ultimately controlled the infill processes. We also suggest that quantitative analysis of clinoform sets can effectively reveal the detailed infill processes of sedimentary basins through time and can be readily applied to basin analyses in other similar settings.