Seismic Stratigraphy around Continent-Ocean Boundary in the NW Sub-basin, Northern South China Sea

The northern continental margin of the South China Sea (SCS) is a passive margin, and have experienced rifting and seafloor spreading. From continental shelf to deep sea basin, the nature of the crust gradually changes from continental crust to oceanic crust. Many studies use a term "continent-ocean boundary (COB)", to describe the boundary between these two kinds of crust. This boundary is roughly located along the boundary between slope and deep sea basin, and roughly parallels to the 3000 m isobath in the northern SCS. This study analyzes many multiple channel seismic reflection profiles in the northern continental margin of the SCS, and has identified an angular unconformity between the slope and deep sea basin deposits at the COB in the NW sub-basin area. This unconformity extends from continental slope to NW sub-basin near the COB. Strata beneath the unconformity downlap to the basement. Furthermore, there is a strong reflector beneath the unconformity, and this strong reflector, called "Tom", represents a hiatus which was recognized by biostratigraphic analyses from Site 1148 of ODP Leg 184. The strata between the unconformity surface and reflector Tom pinch toward deep sea basin, and then the unconformity and reflector Tom merge into one reflector. Therefore, the unconformity might be an erosional surface. The strata covered the unconformity onlap the unconformity. Some portions of the COB have intrusions forming basement high, and the unconformity and strata are coherent with basement relief. The unconformity has been interpreted as a normal fault in some previous studies, but our study distinguishes it as an erosional surface. Besides, some previous studies consider the bottom of the sedimentary layers in the NW sub-basin as the onset of depositing deep sea environment, and suggested that it is related to the onset of seafloor spreading. However, we interpret the angular unconformity as the onset of deep sea environment, and it differs from the onset of seafloor spreading.