How Did Sediments Disperse and Accumulate in the Oceanic Basin, South China Sea 

Abstract

As one of the largest marginal sea basins in the Western Pacific, the South China Sea (SCS) is an important part on the “source-to-sink” process in East Asia. The SCS oceanic basin was not only the crucial pathway for transporting terrigenous materials from Eurasia to the Western Pacific Ocean since the Oligocene, but also the dominant accumulation place because of limited material exchange between the semi-closed oceanic basin and the open ocean since the Middle Miocene. This semi-closed basin can help establishing a systematic connection between the infilling process and tectonic events, climate change, sea level changes, etc. Based on 32 multichannel seismic profiles covering the whole oceanic basin and the drilling results from the IODP Expeditions, we got the sedimentary budget on a million-year geological scale. Sediment isopach maps in different geological times were reconstructed to understand the regional and local controlling factors dominating the Cenozoic sedimentary infilling process, as well as the possible provenances. Results showed that the sedimentary budgets of the SCS oceanic basin and the East Subbasin (ESB) increased gradually with time, especially since the Pleistocene, the whole sedimentary budget increased significantly (increnment 39.32×103 km3/my). Regional factors superimposed on local factors jointly controlled the sedimentation process. In the Middle Miocene, the uplift and denudation of the Tibetan Plateau and the intensification of the summer monsoon led to the increased terrigenous sediments entering the SCS through the widely developed canyon systems. Submarine channel/canyon systems developed in the continental margin acted as the major conduits for terrestrial sediments discharging into the oceanic basin. During this period, the sedimentary budgets in most of the marginal basins and oceanic basin increased significantly. The three sedimentary centers on the northern flank were located at the mouth of the canyon systems. Sediment isopach maps indicate that the sediment accumulation was dominantly in the northern flank of the SCS oceanic basin before the Late Miocene, with the depocenters generally distributed at the mouth of the channel/canyon systems and slope foot area near basement highs; the main depocenters gradually migrated to the southern flank of the basin since the Pliocene. This is likely due to an increase sediment from the Mekong River and the high sea level. An exception is the development of a new depocenter in the northeast related with the Taiwan orogeny. The primary sediment provenances include the South China, Tibet Plateau, Indo-China Peninsula, and Taiwan Island, with minor contributions from the Hainan Island, Palawan, Luzon Islands, as well as local basement highs developed on the continental margin.