Impact of Delta-Front Sedimentation on Submarine Geomorphology: A Case Study of the Neogene Hanjiang Formation, Northern Enping Sag, South China Sea

Deltaic sedimentation plays a crucial role in shaping submarine geomorphology. During the deposition of the Neogene Hanjiang Formation, the Enping Sag was situated at the front of the ancient Pearl River delta. Influenced by the interplay of fluvial, wave, and tidal hydrodynamics, it developed a complex submarine geomorphological pattern. Investigating the genesis and distribution of sand bodies at the Neogene delta front in this area is essential for understanding the spatial arrangement of various sedimentary bodies and their geomorphological effects. Based on a high-frequency sequence stratigraphic framework, this study integrates core, thin-section, well-log, and seismic data. Utilizing seismic sedimentology, thin-layer seismic inversion, and 3D geological modeling, combined with fifth-order relative sea-level curves, we systematically characterize the three-dimensional distribution of sedimentary microfacies within sub-layers of the Hanjiang Formation and their impact on submarine geomorphology. Results indicate that sedimentation during this period was predominantly controlled by fluvial-wave interactions, forming delta-front deposits comprising five microfacies. Subaqueous distributary channels, trending approximately north–south, consist mainly of medium-grained sandstone with muddy intraclasts and scour-fill structures, forming a distinct submarine channel system. Channel-adjacent deposits (e.g., crevasse splays and natural levees) form belt-like gentle slopes or levee microtopography along channel margins, characterized by poor sorting. Wave influence promoted the development of shore-parallel coastal sand bars and extensive inter-bar sheet sands. Coastal bars are composed of medium- to fine-grained sand, exhibit segmented grain-size curves and bioturbation, and form elongated uplifted geomorphological units. Sheet sands are dominated by silty fine sand containing bioclasts, forming broad, flat submarine plains. Relative sea-level fluctuations significantly influenced submarine geomorphological evolution: during lowstands, subaqueous distributary channels and adjacent sands dominated, producing an incised and aggradational channel–levee landscape; during highstands, coastal bars and sheet sands were widespread, shaping a shore-parallel bar–sheet sand geomorphology.