How did sediments disperse and accumulate in the oceanic basin, South China Sea

Abstract

Quantification of the sedimentary accumulation and dispersion process of the South China Sea (SCS) not only establishes an important part on the “source-to-sink” process in Southeast Asia, but also sheds lights on the submarine slope failure and associated submarine canyon/channels formation. Based on geological interpretations of more than 30 multichannel seismic profiles covering the whole oceanic basin with constraints from IODP Expedition drilling results, we calculated the sediment budget of the oceanic basin since the Oligocene on a million-year geological scale. Sediment isopach maps in different geological time were reconstructed to understand the controlling factors dominating the Cenozoic sedimentary accumulation and dispersion process, as well as the possible sedimentary provenances. Results show that the sediment budget of the whole oceanic basin increased gradually with time, although the rate has not been consistent over time. It was complicated by regional tectonics, including the spreading of the SCS, the uplift of the Tibet Plateau and Taiwan, changes in the Asian monsoon, rerouting and organization of the river systems, and sea level changes. Submarine channel/canyon systems developed in the continental margin acted as the major conduits for terrestrial sediments discharging into the oceanic basin. We observe two decrease trends in sediment accumulation rates occurred in the Southwest Sub-basin during the Late Miocene, and in the Northwest Sub-basin during the Pliocene. The former might be related with the local rifting event in the Mekong Shelf, which trapped most of the terrestrial input. While the later was due to the retreat of the Central Canyon and Pearl River Canyon in the Late Pliocene. 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 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.