An early Eocene subaqueous fan system in the steep slope of lacustrine rift basins, Dongying Depression, Bohai Bay Basin, China: Depositional character, evolution and geomorphology

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The Lower Eocene siliciclastics in the northern steep slope of the Dongying Depression, Bohai Bay Basin, are described and interpreted as subaqueous fans associated with a thick succession of lacustrine deposits. As a whole, the complicated interplay between the depositional process of these lacustrine fans and slope topography is underrepresented. An integrated study of cores, well-logs, seismic sections and seismic attribute maps have enabled the identification of three types of subaqueous fans at the root of slope, including channelized near-shore subaqueous fans, non-channelized near-shore subaqueous fans and slump turbidity fans. Channelized near-shore subaqueous fans, characterized by straight or slightly curved multi-distributary channels in N-S direction, are mainly filled with conglomerates or pebbly sandstones interbedded with mudstones. In contrast, non-channelized near-shore subaqueous fans are lobate in plane-view, lacking major channels. Slump turbidity fan typically shows an elongate shape with a single, straight-to-weakly meandering channel fronted by terminal splay. Spatial distribution and the types of these subaqueous fans varied through time along the strike of steep slope during the Eocene Es4s depositional time. Slope geomorphology from which the sediments were derived has been reconstructed. The steep slope, evolved from the joining points of extensional fault scarps, preserves numerous gullies incised downdip the whole slope. These gullies, acting as conduits for transporting sediments through the steep slope into the lacustrine basin, are downslope - oriented, ca 4 km long, ca 1 km wide and <300 m deep. Differences in the types and distribution of subaqueous fans are not only in response with the intensity of fault movement and climatic change, but also controlled by the heterogeneity in slope topography and source supply. Understanding the spatial distribution of subaqueous fans and morphological control on the depositional process of lacustrine fans is important to the new exploration strategies for the Dongying Depression, Bohai Bay Basin.