Development characteristics and genetic mechanisms of a debris flow-dominated submarine fan in the Upper Permian Maomaolong Formation, Hezuo region, Western Qinling Orogenic Belt

Debris flow-dominated submarine fans are a critical type of deep-water gravity-flow deposits and constitute a key research target for deep-water hydrocarbon exploration and submarine geohazard assessment. The Upper Permian Maomaolong Formation in the Hezuo region of the Western Qinling Orogenic belt contains well-preserved outcrops of typical debris flow-dominated submarine fan deposits, which record the Late Permian deep-marine sedimentation events. This study focused on the Laoloudi and Cegou sections. Integrating high-resolution three-dimensional (3D) digital outcrop models, field sedimentological investigation, and petrographic microscope analysis in laboratory, we reveal the development characteristics and genetic mechanisms of submarine fans. As indicated by the research results, the Upper Permian Maomaolong Formation in the Hezuo region of the Western Qinling Orogenic belt was divided into seven lithofacies, including two conglomerate lithofacies, four sandstone lithofacies, and one mudstone lithofacies. The gravity-flow deposits include cohesive debris-flow deposits, coarse-grained hybrid event beds, and high-density turbidites. The cohesive debris-flow deposits, among others, were further subdivided into moderate-to low-strength and high-strength cohesive deposits, whereas the coarse-grained hybrid event beds represented the products of fluid transformation from moderate-to low-strength cohesive debris flows. However, the gravity-flow fan bodies in the Maomaolong Formation exhibit sheet-like or tabular geometries and were dominated by lobe deposits, with channels being poorly developed or absent. Based on event-bed thickness, sandstone ratios, and lithofacies associations, the lobe system was further classified into two architectural units: proximal and distal lobes. Furthermore, the debris flow-dominated submarine fan in the Hezuo region is developed in a paleogeomorphic framework of “narrow shelf-steep slope-deep basin”, while controlled by a composite transport mechanism involving slope instability and shelf-current reworking. This submarine fan type is characterized by the dominance of multi-type debris-flow deposits and the coexistence of composite flow-transformation products, exhibiting rapid lateral facies transitions and pronounced heterogeneity. These findings provide important insights for the prediction and architectural characterization of deep-water gravity-flow hydrocarbon reservoirs.