Survey data were collected for Site 1179 in August 1996, during cruise KH96-3-1 of the Hakuho Maru from the University of Tokyo Ocean Research Institute (Fig. F6). The ship was navigated by GPS satellite and collected standard underway geophysical data, including magnetic field measurements and 3.5-kHz echo-sounder profiles. The magnetic data were used to define magnetic anomalies around the site, and the echo-sounder profiles were used to show shallow sediment layers. To define the complete sedimentary section, a 1.2-km-long 24-channel multichannel seismic (MCS) streamer was used to collect reflection seismic data over the site. The MCS data were stacked, filtered, and migrated for final interpretation. Because Site 1179 is situated at the intersection of two GPS-navigated MCS lines, it was decided that additional seismic data collection by the JOIDES Resolution was unlikely to provide significantly better characterization of the sedimentary sequence so no additional seismic data were collected over the site during Leg 191.
Most knowledge of sediment thickness and character in the northwest Pacific basin has come from single-channel seismic profiles collected by various expeditions over the past several decades. These profiles show that sediments in this region are generally thin, typically <500 m from seafloor to acoustic basement, which is often presumed to be the top of igneous ocean crust (Ludwig and Houtz, 1979). Ewing et al. (1968) characterized the appearance of single-channel seismic lines in the region, dividing the sediment section into as many as four layers: an upper transparent layer (weakly reflective), an upper opaque layer (well stratified and more reflective or reverberant), a lower transparent layer, and acoustic basement (a strong reflector below which no coherent reflections are seen). In older single-channel seismic profiles, the lower opaque layer is often highly reflective owing to the bubble pulse of the seismic source reverberating in an interval of multiple strong impedance contrasts. As a result, the lower opaque layer may mask the top of the igneous crust and acoustic basement may not correspond to the sediment/basalt contact. Flat-lying acoustic basement is often an indicator that the reverberant layer masks true igneous basement.
In the northwest Pacific near Site 1179, seismic sections typically show only one transparent and one reverberant layer. DSDP and ODP drilling in the region (Legs 6, 20, 32, 86, and 185) showed that the transparent layer consists of Late Cretaceous and late Cenozoic siliceous oozes and clay, frequently with ash layers. The reverberant layer appears to correspond to pervasive chert layers (Fig. F4) (Fischer et al., 1971; Heezen, MacGregor, et al., 1973; Larson, Moberly, et al., 1975; Heath, Burckle, et al., 1985; Plank, Ludden, Escutia, et al., 2000). In the cherty interval, host sediments are frequently poorly indurated and washed away during coring; however, the few samples that have been recovered suggest the layers containing chert range from siliceous and calcareous oozes to chalk and marl (e.g., Larson, Moberly, et al., 1975; Plank, Ludden, Escutia, et al., 2000). In much of the northwest Pacific, the lower layer of calcareous sediments indicates a period when the lithosphere was young and shallow above the CCD, whereas a highly condensed barren clay interval and siliceous oozes above were formed after the lithosphere subsided beneath the CCD. In the midgyre region of the north Pacific, Neogene sediments are thin. Beneath the western boundary currents, however, Miocene to Pleistocene sediments can be >200 m thick (Heath, Burckle, et al., 1985; Plank, Ludden, Escutia, et al., 2000).
Site 1179 seismic profiles display a seismic stratigraphy similar to that outlined by Ewing et al. (1968), with a transparent layer ~0.32 s thick and a reverberant layer 0.13 s deep. Acoustic basement is a high-amplitude reflector at a depth of 0.44 s two-way traveltime below the seafloor (Fig. F7). This reflector displays relief on the order of 0.1 s, including diffractions and offsets, and little coherent seismic energy at greater times. Such characteristics are typical of the top of oceanic crust, which is how this reflector is interpreted at Site 1179. On one seismic line, the site appears to be located at the bottom of a flat-floored graben, ~1 km across, with a relief of 0.05 s (~50 m). On the perpendicular seismic profile (Fig. F8), this graben is not evident but the basement reflector shows downward deflection and discontinuous reflectors 0.05 s higher in the section. This appearance may be the result of being near the edge of the graben, with diffracted energy and imperfect migration causing the shallower reflectors.
Seismic lines show sedimentary layers draping conformably on layers below, so seafloor undulations reflect basement topography (Figs. F7, F8, F9). This character is also seen in greater detail in 3.5-kHz profiles showing the uppermost sediments (Fig. F9). These observations imply that sedimentation has been laterally uniform and likely originates from pelagic sources. In seismic profiles, the sedimentary section consists of an upper transparent layer from the seafloor to a depth of ~0.30 s two-way traveltime. Weak, discontinuous reflectors characterize this layer. The lower sedimentary section contains two strong reflectors at depths of 0.30 and 0.38 s two-way traveltime. Beneath these horizons is another transparent layer, ~0.06 s two-way traveltime in thickness, overlying acoustic basement (Figs. F7, F8).
Figure F10 shows the correlation between one of the seismic lines over Site 1179, the Site 1179 lithostratigraphic column developed from core data, and predicted depths of unit boundaries, using a velocity-depth relation derived from ocean drilling velocity data (Carlson et al., 1986) (See "Physical Properties"). Using this velocity-depth relationship, the predicted depths of the sediment/basalt contact and the tops of Units III and IV all correspond to strong reflectors. The basalt/sediment contact correlates to the reflector, supposed to be acoustic basement at 0.44 s two-way traveltime below seafloor. The top of the chert section (at the boundary between Units III and IV) and the boundary between the red-brown pelagic clays (Unit III) and the diatom-bearing radiolarian ooze (Unit II) line up with the two parallel reflectors at depths of 0.38 and 0.30 s two-way traveltime, respectively. Unlike the other unit boundaries, the boundary between Units II and I does not align with a prominent reflector but this boundary is not sharp, nor does it have a large change in physical properties. The weak, discontinuous reflectors in the upper transparent layer appear to correlate with groups of ash layers.