Seismic geomorphology of multiple fluvial patterns and response of climate-induced sequence architecture: a case study from the Miocene Shijiutuo area, Bohai Bay Basin (China)

Investigation of the evolutionary characteristics and controlling factors of fluvial seismic geomorphology is critical for interpreting sedimentary evolution and predicting the distribution of hydrocarbon and groundwater resources. Since the middle to late Neogene, the Shijiutuo Uplift in the Bohai Bay Basin has deposited a variety of fluvial systems with diverse morphologies. This study integrates core data with previous research results, 3D seismic data and well logs, applying seismic sedimentology and sequence stratigraphy theory to systematically characterise the fluvial systems in the lower member of the Minghuazhen Formation in the Shijiutuo Uplift. Comprehensive analysis indicates that the Neogene lower Minghuazhen Formation in the Bozhong Sag of the Bohai Bay Basin can be subdivided into three third-order sequences from bottom to top. In low-accommodation systems tracts, channel–point-bar complexes are widely developed, exhibiting relatively wide and low-sinuosity planar geometries, with channel widths typically ranging from 130 to 180 m and sinuosity values around 1.05. High-accommodation systems tracts are dominated by narrow and highly sinuous fluvial channels, with measured widths and sinuosity ranging from 80–140 m and 1.15–1.30, respectively. Paleoclimate fluctuation serves as another critical driver for morphological variation of fluvial systems in the study interval. Under alternating arid and humid climatic conditions, channel–point-bar assemblages constitute the primary fluvial landform, whereas narrow sinuous channels only occur in localised zones. In contrast, humid-prevailing stages witness a proliferation of narrow sinuous channel systems, while arid-dominated environments are characterised by widespread sheet-fluvial sand deposits. Statistical comparisons further reveal systematic variations in channel geometric parameters across different paleoclimatic backgrounds. The average channel width decreases from 192 m in alternating dry–wet conditions to 100 m in humid-dominated periods and rebounds to 140 m during arid-dominated stages. Correspondingly, channel sinuosity displays a continuous increasing trend, rising from 1.07 under alternating climates to 1.18 in humid settings and ultimately reaching 1.31 in arid-dominated conditions. Such distinct morphological differences demonstrate strong climatic control on the architecture and morphometry of fluvial sand bodies. Thus, the findings of this study provide insights into the evolution and controls of fluvial seismic geomorphology and offer guidance for hydrocarbon exploration in analogous basins.