Sediment-Routing Reorganization and Associated Sedimentary Responses Across the Es3–Es2 Syn-Rift Stage Transition in the Qinan Sag, Bohai Bay Basin, China

Transitions within the syn-rift stage provide a key window for examining sediment-routing changes and associated sedimentary responses in lacustrine rift basins. In the Bohai Bay Basin, the interval from the third member (Es3) to the second member (Es2) of the Eocene Shahejie Formation records a transition from early strong rifting toward relatively stable rifting. The Qinan Sag, a secondary sag along the Qikou Sag margin, was sensitive to this transition. Using cores, well logs, three-dimensional (3D) seismic data, and heavy-mineral data, this study reconstructs the source configuration, palaeogeomorphology, depositional-system evolution, and Es3–Es2 source-related sediment-dispersal domains. The results show that the supply pattern shifted from coeval supply by a southern regional source and northern and western local sources during Es3 to southern regional-source dominance during Es2. Accordingly, Es3 contains strongly differentiated braided-delta, fan-delta, and subaqueous-fan assemblages. Es2 contains weakly differentiated shallow-water delta and beach-bar assemblages. Three source-related sediment-dispersal domains coexisted during Es3. During Es2, the northern domain was no longer identified, and the western gentle-slope belt evolved into a high-sand-ratio beach-bar belt. This reorganization was mainly controlled by the combined effects of source-configuration changes, geomorphic segmentation, and contrasting slope–A/S conditions (A/S = accommodation/sediment supply). Supply-pattern simplification and weakened geomorphic segmentation shifted sediment routing after basin entry from multiple, dispersed pathways to dominant-source-controlled focused routing. Moderate-to-steep slopes and higher relative A/S proxy values during Es3 favoured discrete, segmented sandy-deposit preservation; gentle slopes and lower relative A/S proxy values during Es2 promoted focused routing and preservation of sandy deposits along the dominant direction, with local shallow-water enrichment. Across the Es3–Es2 syn-rift stage transition, regional-source-related sediment routing showed stronger persistence; local-source-related routing more often weakened or terminated, with corresponding areas tending to show shallow-water redistribution and enrichment signals.