The sedimentary basins of the northern shelf show a two-layer structure, with the lower section characterized by half-grabens formed during Paleogene rifting and filled with nonmarine sequences. The upper section is typified by a wide range of terrigenous and marine sediments deposited during the Neogene subsidence of the margin (Fig. F6) (Ru et al., 1994). However, reworked Paleocene and Eocene marine microfossils in the Neogene deposits from the northern shelf indicate that marine intervals existed earlier in this area. Paleocene deltaic and Eocene marine sediments have also been found in the southern part of the SCS, such as the Liyue Bank (Reed Bank) Basin, where carbonate deposition began in the mid-Oligocene (ASCOPE, 1981; Jin, 1989). In the northern SCS, the Pearl River Mouth Basin (PRMB) has been extensively studied and is close to our core sites. More than 150 wells have been drilled on the shelf, and a detailed stratigraphy has been established for the marine sequence from the uppermost Oligocene to Pleistocene on the basis of planktonic microfossils (e.g., Huang, 1997). The composite stratigraphy of the PRMB documents a marine sequence from the upper Oligocene (NP23/24) to Holocene (NN20) (Jiang et al., 1994; Wu, 1994; Huang, 1997). In the PRMB, a number of seismic reflectors have been correlated with sequence boundaries and their associated depositional hiatuses. Unfortunately, the wells and seismic sections used to define the sequence boundaries and reflectors are in the northern part of the PRMB, which has mostly nonmarine sediments. The nonmarine intercalations decrease in volume and thin southward toward the slope, where our sites are located. Hence, the type sections for reflector identification are not well correlated with our sites. In addition, the reflector terminology differs between various industrial and academic groups (Wang, 1996; L. Huang, pers. comm., 1998; Ludmann and Wong, 1999). We have adopted the sequence/reflector scheme of Ludmann and Wong (table 1), which enumerates seven reflectors from the lower Oligocene to the Pleistocene (see "Seismic Systems and Data" in the "Seismic Stratigraphy" chapter, for discussion of the reflectors). Note that this reflector sequence and terminology is similar but not identical to the sequence of Jiang et al., 1994, which is illustrated in Figure F6.
The modern sediments in the SCS consist mainly of terrigenous material, biogenic carbonate and opal, and a small portion of volcanic material. Clastic sediments are discharged mainly from the Mekong, Red, and Pearl Rivers. However, during the past glacial intervals, the paleo-Sunda River system may have contributed large amounts of sediment to the SCS. Recent sediment trap studies in the northern SCS (Jennerjahn et al., 1992; Wiesner et al., 1996) have shown that the highest particle fluxes occurred during the winter monsoon and are correlated with high wind speed rather than riverine-transported sediments. These data indicate that suspended matter from Taiwan and outside the Bashi Strait might be major sediment sources for the northern SCS. The combination of high terrigenous input and the depth (3500 m) of the modern carbonate compensation depth (CCD) blankets the extensive continental slopes of the SCS with hemipelagic sediments, whereas the deep-sea basin is covered by abyssal clay (Su and Wang, 1994). Biogenic carbonates are found around coral reef islands, especially in the southern areas. The accumulation of carbonates in the SCS exhibits two patterns during the late Quaternary. The "Atlantic" pattern, found above the lysocline, has high carbonate during interglacials and low carbonate in glacial phases and is primarily driven by terrigenous dilution. The "Pacific" pattern, found below the lysocline, has low carbonate during interglacials and high carbonate during glacials and is driven by carbonate dissolution on the seafloor (Bian et al., 1992; Thunell et al., 1992; Zheng et al., 1993; Miao et al., 1994; Wang et al., 1995).