Origin and evolution of large-scale sublacustrine fans in a lacustrine rift basin: a case study from the Liaozhong Depression (Bohai Bay Basin, E China)

Large-scale sublacustrine fans developed in lacustrine rift basins in China received extensive attention due to their significant implications in both sedimentological studies and petroleum exploration. Taking Member 3 of the Dongying Formation in the central-southern Liaozhong Depression, Liaodong Bay, Bohai Bay Basin as a case study, we conducted a systematic investigation into the depositional characteristics, sedimentary processes, and controlling factors governing the development of large-scale lacustrine fans through integrated analysis of seismic profiles, core samples, and well logging data. The research results indicated that three braid deltas developed in the transgressive systems tract of Member 3 of the Dongying Formation in the western slope belt of the study area. In contrast, the large-scale sublacustrine fan developed in the lake basin environment on the east side of the braid delta system. The sublacustrine fan is primarily composed of grey sandstone and mudstone, with deformed structures and coarse debris. Based on sedimentary characteristics, it can be categorized into two main types: turbidity currents and sandy debris flows. The development of large-scale sublacustrine fans is primarily influenced by four key factors: tectonic activity, paleogeomorphology, sediment supply, and lake-level fluctuations. During the deposition of the Dongying Formation, strike-slip and extension processes created multiple uplifts and depressions in the Liaodong Bay, including the Liaoxi Depression, the Liaoxi Uplift, and the Liaozhong Depression. The Liaoxi Uplift and the Yanshan Fold Belt to the west of the Liaodong Bay are the main sediment sources for the formation of multiple braid deltas on the western slope of the Liaozhong Depression, which in turn provided sediments for the sublacustrine fans. In addition, the sedimentary slope break located between the delta front and the prodelta controls the development and distribution of sublacustrine fans. The rising lake-level during the transgressive systems tract led to frequent fluctuations in water levels, establishing optimal water depth and hydrodynamic conditions conducive to the development of sublacustrine fans. The sublacustrine fan in the lacustrine rift basin was influenced by various factors, with multiple provenance supplies being one of the crucial factors for the development of large-scale sublacustrine fans. This favorable geological setting provided optimal conditions for the development of high-quality reservoirs and facilitated the exploration of substantial hydrocarbon accumulation. This research will serve as a reference for other similar lake basins worldwide.