Fan-delta and related turbiditic deposits on the steep slope of Dongying depression, Bohai Bay basin

The fan-delta in the fourth member of Shahejie Formation in steep slope of Dongying depression belongs to prograded type. The proximal part of the deposit is dominated by the successive deposition of sandstone aprons with a few channel plug sandbodies produced through rapid sedimentation from channelized sediment-laden debris flows. The middle part, on the other hand, is mainly represented by the stacked channel-rills with alternate sandstone aprons. Following the deposition of bedded sandstones, as the slope of the depositional surface reduced, debris flows with relatively less grain concentration extended towards the ambient waterbody through channels incised into the preexisting unconsolidated sediments. The thick stacks of channel-fill sandbodies were developed as a consequence of vertical channel aggradation under continued hangingwall subsidence, and the upward declining trend in vertical aggradation suggests gradual fall in the rate of this subsidence. Distal fan-delta deposits represents beds characterized by a repetition of sandstone and mudstone layers. Each stratum is thin-to medium-bedded and forms a succession with a thickness range of decimetres to several metres. The present study indicates that gravity channel and slumps formed in the prodelta region of the fan delta.