Sedimentary Process And Controlling Factors Of The Northwest And Southwest Sub-basins,the South China Sea

As one of the largest marginal sea basins in the West Pacific,the South China Sea(SCS)plays a significant role in mass transport and accumulation since the initial seafloor spreading in the Early Oligocene,and becomes a key place for understanding the sedimentary process in the West Pacific continental margins.Calculation of the sedimentary budget in different geological time is an important part in quantifying the sedimentary process in the SCS,which will not only construct the relationship between the sedimentary process of the SCS and the regional tectonic events,climate change,eustatic sea level change,etc,but to improve our understanding on the "source-to-link"process of the West Pacific region.Based on multichannel seismic profiles across the Northwest Sub-basin(NWSB)and the Southwest Sub-basin(SWSB),combined with drilling results from IODP Expeditions 349,367 and 368,the sedimentary budget of the NWSB and SWSB have been calculated,and the controlling factors in different geological times have been discussed in this paper to reconstruct the Cenozoic sedimentary history.Efforts have been also made on the sedimentary provenance analysis(1)The sedimentary budget of the NWSB and the SWSB showed distinct characteristics in different geological times.?During the Oligocene to the Middle Miocene,the SCS was in the spreading stage,so the sedimentary budget was low.The sedimentary budget of the NWSB was 0.65 ± 0.07× 103 km3/my during the Oligocene to the Early Miocene.During the Middle Miocene the sedimentary budget increased to 1.59± 0.16× 103 km3/my;For the SWSB,the sedimentary budget was 5.00 ± 1.00 × 103 km3/my during the Early to the Middle Miocene.? Since the Late Miocene,the SCS started thermal subsidence.The sedimentary budget of the NWSB reached its peak in the Late Miocene,and the value was 2.68 ± 0.27× 103 km3/my;and the sedimentary budget of the SWSB reduced to 4.00 ± 0.80 × 103 km3/my.?During the Pliocene,the sedimentary budget of the NWSB reduced to 1.28 ± 0.13×103 km3/my;And the sedimentary budget of the SWSB increased to 7.50 ± 1.50× 103 km3/my.?Since the Pleistocene,the sedimentary budget of the NWSB increased to 2.25 ± 0.23 × 103 km3/my;And the sedimentary budget of the SWSB reached its peak by 15.80 ± 3.10 × 103 km3/my(2)Sedimentation is generally dominated by regional tectonic events(such as the uplift of the Tibet Plateau),monsoon,continental fluvial systems and sea-level changes,but will be complicated by local tectonic events and geographic position.? The seafloor spreading of the NWSB started at the Late Oligocene,while the SWSB opened during the Miocene.Thus the NWSB had the Oligocene sediment while the SWSB did not.?During the Early Miocene,the SCS was in the spreading stage,thus the sedimentary budget of the NWSB and the SWSB were relatively low.? During the Middle Miocene,both the sedimentary budget of the NWSB and the SWSB increased rapidly,which corresponded to the uplift of the Tibetan Plateau and the strengthened Asian summer monsoon.? During the Late Miocene,the sedimentary budget of the NWSB increased while the sedimentary budget of the SWSB decreased at the same time due to the winter monsoon and weakened chemical weathering.In this stage,the enhanced winter monsoon resulted in the weakened onshore denudation chemical weathering,thus the sedimentary budget of the SWSB reduced.But the uplift event near the Dongsha Islands commenced in the northern SCS during the Late Miocene.Large amounts of eroded sediments would finally have been transported into the NSWB via canyons and channels,leading to the increase of the sedimentary of the NWSB.? During the Pliocene,the sedimentary budget of the NWSB declined while the sedimentary budget of the SWSB increased at the same time.In this stage,the strengthened East Asian summer monsoon resulted that more terrestrial sediment were transported into the SCS.However,the width of the northern continental margin of the NWSB reaches 280 km with well developed marginal basins.During the high stand,terrestrial sediments deposited in the continental shelf first,which resulted in the response lag of the sedimentary budget of the NWSB to regional events,such as the uplift of the Tibet Plateau and the Asian monsoon The width of southwestern continental margin of the SWSB is much less(70 km),which means less accommodation for the terrestrial sediments.The sedimentary budget of SWSB thus had a quick response to the regional events since the sediments could enter the abyssal basin quickly.? During the Pleistocene,both of sedimentary budget of the NWSB and the SWSB increased significantly,suggesting that continental erosion and sediment transport to the abyssal basin were enhanced by an intensified Asian summer monsoon and glacial-interglacial climate fluctuations(3)The sediment provenance of the NWSB and the SWSB changed by stages.?During the Oligocene,the NWSB started seafloor spreading.The terrestrial sediment from the South China were transported into the NWSB via Paleo-Pearl River.And the sediment from the Hainan Island deposited in the Xisha Trough.?During the Early Miocene,the sediment provenance of the NWSB remained the same as the Oligocene;The SWSB opened in the stage,and the probable sediment source included Indochina Peninsula,the Nansha area,and the Palawan.?During the Middle Miocene,the seafloor spreading of the whole SCS ceased.The sediment supply of the NWSB and the SWSB remained unchanged.? During the Late Miocene,the main sediment source of the NWSB continued to be Pearl River.Dongsha local uplifts and Red River also contributed into the NWSB;The Mekong River developed into modern system and became the main sediment source of the SWSB.Small river systems in the Indochina Peninsula,Sunda Islands and Malay Peninsula were also the sediment source for the SWSB.? During the Pliocene,the Hainan uplift event made it become the sediment supply of the NWSB.Thus the sediment source included Pearl River,Red River and Hainan Island;Due to high stand of sea level,the main sediment provenance of the SWSB were Mekong River and river systems of the Indochina Peninsula.?During the Pleistocene,the probable sediment provenance included Pearl River,Hainan Island and Red River;the sediment source of the SWSB were Mekong River,small river systems of the Indochina Peninsula,Sunda Islands and Malay Peninsula.