Pleistocene forced regressions and tidal sand ridges in the East China Sea

Tidal sand ridges are common features on modern shelves but only few examples of such preserved sand bodies are described in Pleistocene deposits. In the stratigraphic record, some sand bodies encased in shales, previously interpreted as sand ridges, have been reinterpreted as shoreface deposits. More than 5000 km of high-resolution seismic data from the East China Sea, correlated to geotechnical boreholes and shallow cores, demonstrate the potential of sand ridge preservation and allow reconstruction of the depositional history of Pleistocene fourth order (100-kyr) depositional sequences. A high subsidence rate of about 300 m/Myr allows the preservation of three elementary sedimentary facies, constitutive of a ‘motif’ which was repeated during glacio-eustatic cycles. They consist of (1) regressive marine prodeltaic prograding wedges, (2) estuarine and continental (deltaic) facies, and (3) transgressive sand ridges, similar in shape and orientation to modern sand ridges. Major discontinuities, traceable over the entire outer continental shelf along distances of hundreds of kilometers, are transgressive and regressive surfaces of marine erosion, whereas sequence boundaries formed by fluvial erosion are difficult to identify on this low-gradient shelf. Because of the asymmetry of the Pleistocene glacio-eustatic cycles, most of the preserved sedimentary record (with the exception of sand ridges) corresponds to forced regressive deposits (deposits that formed during a seaward shift of the shoreline due to relative sea-level lowering).