Sedimentary Evolution and Controlling Mechanisms of Tide‐Dominated Transitional Systems in the Eocene Pinghu Formation, Xihu Sag, East China Sea Shelf Basin

ABSTRACT

Coastal‐to‐marine transitional zones, shaped by the complex interactions among fluvial, wave, and tidal processes, display pronounced spatiotemporal heterogeneity in sedimentary facies and reservoir architectures. Focusing on the Eocene Pinghu Formation on the Pingbei slope of the Xihu Sag (East China Sea Shelf Basin), this study integrates continuous cores, wireline logs, and 3D seismic data to conduct a coupled sedimentological and ichnological investigation, thereby establishing a tide‐influenced depositional evolution model within a transgressive–regressive (T–R) sequence‐stratigraphic framework. Sedimentary evidence, including landward‐diminishing fluvial structures, diagnostic ichnofabric suites with variable diversity and bioturbation intensity, rhythmic tidal bedding, and widespread coal‐bearing intervals, collectively highlights the dominant influence of tidal hydrodynamics and terrestrial organic accumulation on sedimentary evolution. On the basis of an integrated lithofacies–ichnofacies analysis, seven sedimentary subfacies are identified and classified into two major depositional dominated, mud‐rich coastal transitional domain containing tidal‐flat, marsh‐related, interdistributary, and shallow‐subaqueous muddy deposits. The tide‐dominated deltaic domain is characterized by tide‐influenced to tide‐dominated channel sand bodies and tidal bars, whereas the mud‐rich coastal transitional domain records sediment‐starved and low‐energy depositional environments with frequent mudstone–coal interbeds. The Pinghu Formation records a complete T–R cycle. During the transgressive stage, increasing accommodation space and reduced sediment supply promoted the accumulation of mud‐rich coastal transitional deposits, whereas during the regressive stage, enhanced sediment influx and reduced accommodation favored the progradation of tide‐dominated deltaic systems, characterized by channelized sand bodies and tidal‐bar complexes. The spatial and temporal evolution of these depositional elements was jointly controlled by tectonic paleotopography, relative sea‐level variations, sediment supply, and the accommodation‐to‐sediment supply ratio. This study establishes a refined evolutionary model for tide‐influenced coastal transitional systems and demonstrates that reservoir heterogeneity is governed not only by the distribution of sand‐rich deltaic elements but also by the architecture and origin of associated mud‐rich deposits. The results provide new insights into reservoir prediction and depositional model reconstruction in tectonically active coastal basins.