Seismic Stratigraphy of the Central South China Sea Basin and Implications for Neotectonics

Wednesday, 17 December 2014
Chun-Feng Li1, Weiwei Ding2, Dieter Franke3, Yongjian Yao4, Xiong Pang5, Hesheng Shi6, Jiabiao Li7 and Ying Cao1, (1)Tongji University, Shanghai, China, (2)Second Institute of Oceanography, State Oceanographic Administration of China, Hangzhou, China, (3)BGR Federal Institute for Geosciences and Natural Resources, Hannover, Germany, (4)Guangzhou Marine Geological Survey, Guangzhou, China, (5)China National Offshore Oil Company Ltd.-Shenzhen Branch, Shenzhen, China, (6)CNOOC Ltd.-Shenzhen Branch, Shenzhen, China, (7)Second Institute of Oceanograp, Hangzhou, China
International Ocean Discovery Program (IODP) Expedition 349 drilled five sites in the central South China Sea Basin. Three sites (U1431 in the East Subbasin, and U1433 and U1434 in the Southwest Subbasin) reached basalt interpreted to be igneous basement of the oceanic crust. Site U1435 on the northern continent-ocean boundary recovered pre-Oligocene sedimentary rocks deposited prior to the opening of the South China Sea. Furthermore, a full suite of geophysical logging was also carried out at Sites U1431 and U1433. These coring and logging data and physical property measurements are integrated with, and correlated to, regional reflection seismic data to map regional sequence stratigraphic boundaries and seismic facies of the central basin and the continent-ocean transition zone. With our carefully selected seismic profiles, stratigraphic correlation is possible even between the continent-ocean transition zone and the central basin, circumventing preexisting basement highs near the continent-ocean boundary that often prevent direct correlation. We interpret four sequence boundaries, which are Oligocene/Miocene, middle Miocene/late Miocene, Miocene/Pliocene, and Pliocene/Pleistocene boundaries. Seismic facies between sequence boundaries are often characteristic and distinctive, allowing relatively straightforward regional correlation. For example, massive Miocene carbonate deposits, if well consolidated, are readily distinguishable by strong seismic reflectivities, caused by their relatively higher density and velocity in contrast to those of interbedded turbidite clastic sediments. However, we found abrupt seismic facies changes both temporally and spatially. In particular, the fossil spreading ridge and the Zhongnan ridge between the East and Southwest Subbasins acted as major sedimentary barriers, across which seismic facies changes sharply and cannot be easily correlated. The well-constrained seismic sequence boundaries also allow us to estimate the timing of post-spreading volcanism, faulting and vertical movement in the central basin.