Depositional model for lacustrine nearshore subaqueous fans in a rift basin: The Eocene Shahejie Formation, Dongying Sag, Bohai Bay Basin, China

The origin of the fourth member of the Eocene Shahejie Formation in the northern steep slopes of the Minfeng Sub-sag, Dongying Sag, China, was investigated by integrating core studies and flume tank depositional simulations. A non-channelized depositional model is proposed in this paper for nearshore subaqueous fans in steep fault-controlled slopes of lacustrine rift basins. The deposits of nearshore subaqueous fans along the base of steep border-fault slopes of rift basins are typically composed of deep-water coarse-grained sediment gravity-flow deposits directly sourced from adjacent footwalls. Sedimentation processes of nearshore subaqueous fans respond to tectonic activities of boundary faults and to seasonal rainfall. During tectonically active stages, subaqueous debris flows triggered by episodic movements of border-faults dominate the sedimentation. During tectonically quiescent stages, hyperpycnal flows generated by seasonal rainfall-generated floods, normal discharges of mountain-derived rivers and deep-lacustrine suspension sedimentation are commonly present. The results of a series of flume tank depositional simulations show that the sediments deposited by subaqueous debris flows are wedge-shaped and non-channelized, whereas the sediments deposited by hyperpycnal flows generated by sporadic floods from seasonal rainfall are characterized by non-channelized, coarse-grained lobate depositional bodies which switch laterally because of compensation sedimentation of hyperpycanal flows. The hyperpycnal-flow-deposited non-channelized lobate depositional bodies can be divided into a main body and lateral edges. The main body can be further subdivided into a proximal part, middle part and frontal part. Normal mountain-derived river-discharge-deposited sediments are characterized by thin-bedded, fine-grained sandstones and siltstones with a limited distribution range. Normal mountain-derived river-discharge-deposited sediments and deep-lacustrine mudstones are commonly eroded in the area close to boundary faults. A nearshore subaqueous fan can be divided into three segments: inner fan, middle fan and outer fan. The inner fan is composed of debrites and the proximal part of the main body. The middle fan consists of the middle part of the main body and lateral edges, normal mountain-derived river-discharge-deposited fine-grained sediments and deep-lacustrine mudstones. The outer fan comprises the frontal part of the main body, lateral edges, and deep-lacustrine mudstones. Based on the non-channelized depositional model for nearshore subaqueous fans, criteria for stratigraphic subdivision and correlation are discussed and applied.