9. Site 1148¹

Shipboard Scientific Party²

BACKGROUND AND OBJECTIVES

The objectives of Site 1148 (SCS-5C) were to (1) recover a continuous sequence of hemipelagic sediments to reconstruct the early paleoclimate history (Oligocene to Miocene) of the South China Sea (SCS); (2) identify the onset of monsoonal variability in the South China Sea and to establish its evolution in the Oligocene-Miocene interval; and (3) establish whether the Oligocene-Miocene pattern of accumulation rates is consistent with models of the SCS continental margin evolution and with changes in Himalayan-Tibetan uplift, monsoon intensification, and sea-level change.

Site SCS-5C was initially located near the base of the continental slope on seismic Line SO95-5 at common depth point 7500 (Figs. F5, F15, both in the "Seismic Stratigraphy" chapter). Pollution Prevention and Safety Panel evaluation of the JOIDES Resolution seismic Line JR184-1 moved the final Site 1148 location ~1.05 nmi east to 18°50.169´N, 116°33.939´E, at shotpoint 1980 at a water depth of ~3294 m (Fig. F9 in the "Leg 184 Summary" chapter; Figs. F5, F16B, both in the "Seismic Stratigraphy" chapter). This placed Site 1148 within the scar of a slump feature. Site 1147, ~0.45 nmi upslope from Site 1148, was cored to recover surface sediments missing at Site 1148 (see "Background and Objectives" in the "Site 1147" chapter). The approved penetration depth of Site 1148 was 700 meters below seafloor (mbsf), and the sediment thickness above acoustical basement was ~0.85 s two-way traveltime (TWT), or ~790 mbsf.

Site 1148 (SCS-5C) is located on the lower continental slope of China near the continent/ocean crust boundary. Drilling at the site was planned to reach Oligocene-age sediments and possibly recover sediments coincident with the major postrifting subsidence of the basin. Therefore, we expected to recover mostly hemipelagic muds and silts and possibly some shallow-water facies near the base of the hole. On the basis of the Sonne seismic records (Fig. F15 in the "Seismic Stratigraphy" chapter) and their correlation with Chinese reflector stratigraphy (H.-K. Wong, pers. comm., 1998), Site 1148 was targeted for 700 m penetration. Initial interpretation of the seismic stratigraphy identified Reflector T₁ (top of Miocene, 5.2 Ma) at ~0.2 s subsurface and Reflector T₇ (mid-Oligocene, ~30 Ma) at ~0.52 s subsurface. These same reflectors are clear in the JOIDES Resolution seismic lines (Fig. F16A, F16B in the "Seismic Stratigraphy" chapter), but their age assignments are somewhat in doubt. On the basis of in situ downhole velocity logs from Site 1146, the 700 mbsf penetration at Site 1148 is equivalent to ~0.78 s subsurface. This revised TWT indicates that Site 1148 should penetrate both T₁ and T₇ reflectors and come within ~100 m of the basement reflector (T_g).

We hoped that Site 1148 would shed some light on the opening of the SCS and its relation to the Cenozoic deformation of Asia. The Central Basin of the SCS is underlain by oceanic crust that was actively spreading from ~32 to 16 Ma (Briais et al., 1993). During and after the seafloor spreading phase, the continental margin of China experienced crustal extension, subsidence, and rapid deposition of terrigenous and marine sediments. On the continental shelf near the Pearl River Mouth Basin (close to the Leg 184 sites), the sedimentary basins have a two-layer structure. The lower section is characterized by half-grabens formed during Paleogene rifting and filled with syn-rift sediments, whereas the upper section is characterized by a wider distribution of deposits formed during the broad subsidence during the Neogene (Ru et al., 1994). The Cenozoic stratigraphy of the Pearl River Mouth Basin shows nonmarine intercalations in the northern part of the basin that thin and decrease in proportion southward toward the deeper part of the slope (Jiang et al., 1994; Wu, 1994; Huang, 1997). Major depositional hiatuses have been observed on the shelf in the lower part of the lower Miocene, near the end of the middle Miocene, and around the Pliocene/Pleistocene boundary (L. Huang, pers. comm., 1998). The lower accumulation rates and greater penetration at Site 1148 should recover a record that will establish stratigraphic ties between the SCS marine record and the terrestrial record of China. In particular, the recovery of sediments associated with the T₁ and T₇ reflectors should help to correlate the terrestrial and marine records and thereby constrain the timing of basin subsidence, changes in denudation/accumulation, and monsoon intensification.

The original drilling plan for Site 1148 included three advanced hydraulic piston corer (APC) holes to refusal, one extended core barrel (XCB) hole to refusal, and a rotary core barrel (RCB) hole to the target depth. Hole 1148A was to be APC/XCB cored to refusal or to the target depth of 700 mbsf. Depending on the depth of XCB refusal, Hole 1148B was designed to be APC/XCB cored to a target depth that would ensure time to use the RCB for the deeper section. For Hole 1148C, we planned to change to the RCB, wash down to the depth of XCB refusal, and RCB core to the 700 mbsf target depth. We planned to log the deepest and best-conditioned hole with a standard program, including the triple combination tool, Formation MicroScanner (FMS)-sonic tool suite, and geological high-resolution magnetic tool (GHMT). Hole 1148A was scheduled to be logged if it reached the target depth of 700 mbsf and hole conditions were adequate. Alternatively, we planned to log Hole 1148C, which would be the deepest RCB-cored section. Hole 1148A reached the depth of 704 mbsf and was successfully logged. Permission to deepen Hole 1148B to basement, or 850 mbsf, was received. To achieve the target depth, intervals between 145 and 440 mbsf and between 646 and 700 mbsf were drilled ahead. Given the leg objectives and the available time, we ultimately modified the drilling plan to include two deep holes in which to recover the Oligocene-Miocene section.