The continental margins of the South China Sea (SCS) reflect a complex structural history of deposition related to rifting and subsidence as well as to climate and sea level-related depositional processes. Briefly, the original rifting of the SCS is thought to have begun in the Paleogene and continued until the Oligocene (~32 Ma). The continental crust of the northern continental margin was initially faulted, and then the margin subsided to develop a series of down-dropped blocks that filled with syn-rift sediments. Following the initial rifting and subsidence, active seafloor spreading began in the mid-Oligocene (~32 Ma) and continued until the middle Miocene (~16-17 Ma) (Taylor and Hayes, 1983; Briais et al., 1993). During both rifting and spreading phases, the outer subsiding margin was draped with terrigenous and hemipelagic sediments that were deposited and modified by mass wasting and current-related processes. Several magmatic intrusion events have added to the complexity of the margin. The margin is now characterized by an irregular series of sediment-filled basins and ridges. Several of the sites located in the northern SCS lie in the midslope basins (Sites 1144 and 1146) and on the subsided blocks (Sites 1145 and 1147/1148). In the southern SCS, part of the margin is characterized by a series of blocks, islands, and banks that are relatively uncharted and referred to as the Dangerous Grounds or Nansha Islands area. These features are thought to be underlain by a rifted continental crust that developed carbonate platforms and banks during the Neogene. Site 1143 lies among these features.
Much of the sediment deposition in the South China Sea is related to mass wasting processes (i.e., slumping, debris flows, and turbidites) (Damuth, 1979, 1980a; Wu et al., 1999). Maps of the 3.5-kHz echo characteristics show that Sites 1144-1148 lie within the type IIIB characteristic, which is indicative of mass slumping and downslope transport (Damuth, 1980b). Others (Sarnthein et al., 1994) have suggested that thermohaline contour currents may produce similar echo characteristics and morphology in the SCS.
Given the complex geologic history of the South China Sea, both the selection of coring sites and the interpretation of their regional context and depositional history require establishing the seismic stratigraphy of the sites. The initial selection of Site 1143 was based on the Guangzhou Marine Geological Survey (GMGS), Ministry of Land and Resources of China, and Nansha Survey multichannel lines (NS and NSL) in the southern SCS. Sites 1144-1148 (SCS-1, -2, -4, and -5C) were originally chosen using SONNE95 seismic data (Lines 5, 10, and 20). Unfortunately, cross tracks for Sites 1145, 1146 and 1148 did not exist in the seismic data bank. Therefore, the Pollution Prevention and Safety Panel (PPSP) required Leg 184 scientists to acquire crossing seismic lines as part of the final site approval. The JOIDES Resolution scientific party carried out a single-channel seismic survey of proposed sites SCS-4 (1146), -5C (1148), and alternate -5E (contingency site; not drilled) on 12-13 March 1999. The seismic data were processed aboard the JOIDES Resolution, and critical sections were faxed to the Ocean Drilling Program, Texas A&M University, for final approval of the sites and penetration depths. A summary of all seismic lines and sections used to select and evaluate Leg 184 sites is given in Table T1.
In this chapter, we summarize the characteristics of various seismic reflection systems used to collect the data, review seismic reflection and 3.5-kHz records of each site, and relate seismic stratigraphy to the drilling results through the use of acoustic impedance logs.
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