Seismic geomorphology, architecture and genesis of unusual confined and semiconfined sedimentary units on the northern slope of the Bonan uplift, Bohai Bay Basin, China

Studying the fundamental influence of basin-floor topography on the growth and morphology of sedimentary systems has largely focused on submarine fans; however, in lacustrine settings, only a limited number of studies are primarily concerned with confined sublacustrine fans, although such fans can form high-quality hydrocarbon reservoirs. By using recently acquired three-dimensional seismic data, a series of unusual confined and semiconfined sedimentary units with seismic reflection patterns different from those of previous studies have been identified. The identified sedimentary units are developed in the Paleogene succession on the northern slope of the Bonan uplift, Bohai Bay Basin, China, and are characterized by either vermicular-like chaotic progradational reflections or coupled vermicular-like and progradational seismic reflections in the dip direction and channel-shaped bidirectional onlap reflections along strike directions. Through restored paleogeomorphology, six elongate incised valleys (V-1 to V-6) with diverse scales on the uplift slope are clearly documented. The identified sedimentary units were deposited within these incised valleys. The units identified in this study are considered “confined” or “semiconfined” based on the following several aspects, they: 1) were deposited within elongate depocenters along dip directions; 2) are clearly separated from each other by topographic highs along strike directions; 3) have belt-shaped sediment dispersal patterns in planar view; and 4) have the thickest deposits primarily distributed in the middle and distal parts. Through comparing sedimentary landforms, feeder systems, seismic reflection characteristics and spatial distribution patterns between sedimentary systems, the confined and semiconfined systems identified in this study display both similarities to and profound differences from previously studied confined sublacustrine fans and are here considered a new unusual type. We anticipate this work may serve as a useful reference for confined sedimentary system research in other types of sedimentary basins.