Site 1143 is located at 9o21.72´N, 113o17.10´E, at a water depth of 2772 m (Fig. F2 in the "Leg 184 Summary" chapter; Fig. F1). Seismic data provided by the Guangzhou Marine Geological Survey was used to locate Site 1143 near the intersection of Lines NSL95-160 at CDP 1812 and NS95-240 at CDP 3617 (Figs. F1, F2). The basement structure and sediment cover near Site 1143 are complex with basement faulting on a scale of a few kilometers, and sediment thicknesses range from 700 to 1400 m (Fig. F2). A series of seismic reflectors, mapped by the GMGS, indicated that the proposed penetration of Site 1143 (400 meters below seafloor [mbsf]) should reach midway between Reflectors T1 (top of the Miocene) and T2 (top of the middle Miocene). Site 1143 is located in a slight depression within a fault basin that contains ~1.25 s (~1250 m) of sediment.
The JOIDES Resolution 3.5-kHz profiles on the approach to Site 1143 were highly parabolic, indicating an irregular surface on a scale of tens to hundreds of meters (Fig. F3). The 3.5-kHz profiles show that the site lies near the base of a slope in hummocky terrane that has a slightly diffuse echo character and sub-bottom reflectors at ~40 and 80 mbsf (Fig. F3B). These echo characteristics are indicative of downslope transport (Damuth, 1980a, 1980b).
The GMGS seismic reflection lines reveal major reflectors at ~0.27, ~0.55, ~0.67, and ~0.95 s TWT. The equivalent depths and ages for the upper two reflectors are ~210 (~5.2 Ma) and ~460 mbsf (~9.1 Ma) (Figs. F2, F4; Table T2). Hence, the upper reflector is consistent with T1, but the second reflector at 0.55 TWT is somewhat younger than T2. The impedance variations for the logged section (137 to 375 mbsf) of Hole 1143A show a rapid increase in impedance culminating ~0.27 s TWT or ~210-220 mbsf. The variability of impedance also decreases over this same interval. Following a brief reversal centered on ~260 mbsf, the impedance increases to ~340 mbsf (Fig. F4). The T1 reflector appears to be coincident with the rapid increase in acoustical impedance between 200 and 240 mbsf. Although the logged section does not reach the next major reflector at 0.55 s, downward continuation of velocity data indicates that it lies at ~460 mbsf and was presumably penetrated in Hole 1143C, which was cored to 500 mbsf. Although the multisensor track and moisture and density measurements reach their maximum values between 400 and 500 mbsf, other properties do not give any indication of a shift that might cause the impedance peak. Downward continuation of velocity and age structure estimates that the reflector at 0.67 TWT lies at ~560 mbsf (~11.3 Ma) and the strong basal reflector at 0.95 TWT lies at ~820 mbsf (~17 Ma). Hence, the reflectors at 0.55 and 0.67 s seem to span the age (10.2 Ma) of the middle/upper Miocene Reflector T2. These levels do fall within a sequence of rapidly accumulating and somewhat reworked sediments so that they may record local depositional processes associated with rapid filling of the basin.
In summary, the reflector sequence at Site 1143 does not conform well to the sequence derived from the northern SCS. Although the reflector at 0.27 roughly fits the T1 age, the younger and older reflectors are not clearly identified at the drill site.
Site 1144 (SCS-1) is located at 20o03.18´N, 117o25.14´E, at a water depth of ~2037 m, which lies slightly above the sill depth of the Bashi Strait (2600 m) (see Fig. F9 in the "Leg 184 Summary" chapter). The site is located on a thick sediment drift at the intersection of seismic Lines SONNE95-20 (CDP 3482) and SONNE95-10 (CDP 9600) (Figs. F5, F6). The drift nature of the site is indicated by its morphology and internal structure as well as by its high sedimentation rates. The sediment thickness above a prominent basinward dipping reflector is between ~0.75 and ~0.80 s, or ~610 to ~650 mbsf (Fig. F6). The seismic structure of the sediments is characterized by an upper reflector series (0-0.5 s) that has distinct, closely spaced reflectors and a lower, more diffuse zone (0.5-0.7 s) with less distinct reflectors. These diffuse seismic zones are often characteristic of slump or debris flow sediments (Damuth, 1980a, 1980b). The northwest-southeast dip line (SONNE95-10) reveals a wedge of sediment ranging from ~1725 to ~2400 m water depth. Internal reflectors indicate that the beds thin and pinch out in deeper water. The cross line (SONNE95-20) shows wavy reflectors, indicating possible drift or dunelike transport and redeposition of bottom sediments. Site 1144 is located near the center of the deposit, where reflectors are relatively uniform and evenly structured on both lines. Given the sedimentation rates at this location, the base of Site 1144 (450 mbsf) lies well above the expected depth of the Pliocene/Pleistocene Reflector TN).
The JOIDES Resolution 3.5-kHz and PARASOUND (Sarnthein et al., 1994) profiles across and on the approach to Site 1144 show thick sediments heavily draped over relief that locally is tens to hundreds of meters (Fig. F7). The well-defined, conformable sub-bottom reflectors indicate uniform sediment accumulation with systematic variations in sediment characteristics.
A summary of the depth/age relations for Site 1144 reflectors is given in Table T2. At Site 1144, acoustical impedance was calculated from Hole 1144A over the interval 90-450 mbsf (see "Wireline Logging" in the "Site 1144" chapter). The resulting plot of smoothed acoustical impedance as a function of TWT for Site 1144 is shown in Figure F8 along with the SONNE95-10 seismic section. Although the match is far from unique, several of the reflectors at Site 1144 coincide with rapid increases or decreases of acoustical impedance. The major reflector at ~0.47 s TWT (~370 mbsf) that divides the upper reflector zone from the more diffuse zone correlates with the major increase in acoustical impedance associated with increases in both velocity and density and is generally coincident with the boundary between lithologic Subunits 1C and 1B. This reflector also marks the top of the interval with high magnetic susceptibility (see "Physical Properties" in the "Site 1144" chapter). The diffuse seismic zone is characterized by high density, high magnetic susceptibility, and a distinctly older age than the above reflector sequence. The age of this reflector is ~0.7 Ma and may reveal a hiatus or discontinuity in the section. Overall, the reflector sequence at Site 1144 displays the local depositional history within the Pleistocene.
Site 1145 is located at 19o35.04´N, 117o37.86´E, at a water depth of 3175 m. The site is at shotpoint 4704 on the seismic Line SONNE95-10 (Figs. F5, F9) on the lower of a series of terraces that form the continental slope off southern China. The northwest-southeast dip line (SO95-10) reveals sediments that are generally smooth and slightly onlapping onto the continental slope from the adjacent deep basin. The thick, conformable sediments extend almost 1 s below the seafloor (sbsf) to a strong wavy reflector, presumed to be basement (Fig. F9). Using the interval velocities provided by the Department of Marine Geology and Geophysics, Tongji University, this two-way traveltime is equivalent to ~760 m of sediment cover. The 200-mbsf penetration at Site 1145 reached only 0.28 sbsf. The seismic structure within the upper 200 mbsf contains an upper zone of multiple (about six) strong reflectors (0-0.12 sbsf) and a lower, more diffuse zone (0.12-0.28 sbsf) that is divided by a prominent reflector at ~0.21 sbsf (Fig. F9). A summary of the depth/age relations for Site 1145 reflectors is given in Table T2. The JOIDES Resolution 3.5-kHz data (Fig. F10) and SONNE95 PARASOUND data (Sarnthein et al., 1994) reveal relatively smooth, parallel conformable sediment cover with no distinct surface features.
Site 1145 was not wireline logged, so we do not have in situ P-wave velocity and density data. We used the velocity and density data from core measurements and the age model for Site 1145 to estimate the depth and age of the reflectors. The base of the surface reflector zone (0.12 s TWT) is estimated to be at a depth of ~85 mbsf with an age of ~0.6 Ma. The strong reflector at 0.21 s TWT is judged to be at ~160 mbsf with an age of ~1.87 Ma. Hence, this reflector is consistent with the age of the regional TN reflector.
Site 1146 is located at 19o27.40´N, 116o16.37´E, at a water depth of 2092 m (see Fig. F9 in the "Leg 184 Summary" chapter). The site was initially located on seismic Line SONNE95-5 at CDP 1049 within the thick sediment fill of a down-dropped block (Fig. F12). The JOIDES Resolution minisurvey obtained two crossing lines of the proposed Site SCS-4 to provide the PPSP with additional information to approve the final site location (Fig. F5). The PPSP moved Site 1146 to JOIDES Resolution Leg 184, Line 3, at shotpoint 3241 (Fig. F11B).Total sediment thickness at the site is >1.4 s, with a prominent double reflector at ~0.77 s (Figs. F11A, F11B, F12). Several additional reflectors have been identified at Site 1146 and are thought to be regional in nature. A reflector at ~0.2 s was thought to be T1, which lies near the Miocene/Pliocene boundary (~5.2 Ma); a double reflector at ~0.5 s was believed to be T4, near the lower-middle Miocene boundary (15-16 Ma) (Ludmann and Wong, 1999). However, initial reflector assignments in the Leg 184 prospectus were uncertain, at best. The JOIDES Resolution 3.5-kHz data (Fig. F13) and SONNE95 PARASOUND data (Sarnthein et al., 1994) reveal relatively smooth sediment cover with indistinct overlapping hyperbole and parallel conformable sub-bottom reflectors.
At Site 1146A, acoustical impedance was calculated over the downhole logged section (273 to 593 mbsf). Unfortunately, hole conditions necessitated lowering the drill string for the sonic log; therefore, in situ P-wave velocity data are not available for the upper section. We adjusted the physical properties core-log data for bulk density and P-wave velocity to give a continuous gradient the from core top to the logged interval (Fig. F14).
A summary of the depth/age relationships of the Site 1146 reflectors is given in Table T2. Reflector T1 lies at ~0.2 s TWT at the base of a zone of strong reflectors extending from the surface to 0.2 s. This level is equivalent to ~150 mbsf and has an age of ~1.4 Ma. Hence, this reflector is more closely related to Reflector T0 and is definitely not T1. The double reflector thought to be T4 coincides with a distinct minima in the acoustical impedance at ~0.50 s TWT. This level is equivalent to a depth of ~430 mbsf and an age of ~9.8 Ma. Thus, this reflector is equivalent to the middle/upper Miocene Reflector T2 in the regional reflector stratigraphy (Ludmann and Wong, 1999). Given the velocity and age structure of Hole 1146A, if Reflector T4 is 14 Ma, it would be expected to occur at ~0.59-0.60 s TWT at ~520-530 mbsf. This level lies near a minimum in the acoustical impedance, which increases downhole to the base of the section (Fig. F14). At ~530 mbsf, we observe a number of physical changes, including a decrease in linear sedimentation rates (LSR), a decrease in magnetic susceptibility, and an increase in natural gamma radiation (Fig. F17 in the "Leg 184 Summary" chapter). However, a distinct reflector is not observed in the JOIDES Resolution seismic data. This level seems to fall within a layered sequence bounded by the more distinct reflectors at 0.5 and 0.7 s TWT. Perhaps additional postcruise processing and analysis will better resolve the T4 reflector at Site 1146. A deeper series of reflectors, ranging from ~0.77 to ~0.97 TWT (Fig. F11), are estimated to be from 22 to 29 Ma based on downward continuation of velocities and LSRs at the base of the cored section.
The character of these reflectors suggests rapid infilling of a down-dropped block, which may have occurred in the early part of the SCS drift phase, which began at ~32 Ma. Even deeper reflectors at ~1.25 s TWT may be associated with the initiation of the drift phase and thus coincide with the T7 reflector of Ludmann and Wong (1999). Unfortunately, these deeper reflectors are outside our recovered section, and we cannot make reliable estimates of their exact depth and age.
Site 1147 is located at 18o50.11´N, 116o.28´E, at a water depth of 3246 m. Site 1148 is located at 18o50.17´N, 116o.94´E, at a water depth of 3294 m. Site 1148 was initially designated as SCS-5C, near the base of the continental slope on seismic Line SONNE95-5 at CDP 7500 (Figs. F5, F15). The JOIDES Resolution acquired crossing seismic lines as part of the survey required for final site approval. The PPSP evaluation of the new seismic data moved the final Site 1148 location to JOIDES Resolution 184, Line 1, at shotpoint 1980 ~1.05 nmi east of the proposed site SCS-5C (see Fig. F18 in the "Leg 184 Summary" chapter; Figs. F5, F16B). Site 1148 was cored to 853 mbsf, which was thought to penetrate the strong reflector sequence at ~0.85-~0.90 s TWT. Initially, this reflector was described as acoustical basement and was believed to represent the prespreading (and possibly the prerifting) surface. Initial interpretation of the SONNE95 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 (Fig. F15). These same reflectors are clear in the JOIDES Resolution seismic lines (Fig. F16A, F16B), but their age assignments are somewhat in doubt.
The JOIDES Resolution 3.5-kHz data (Fig. F17) and SONNE95 PARASOUND data (Sarnthein et al., 1994) revealed relatively smooth draped sediment cover with parallel conformable sub-bottom reflectors. The 3.5-kHz data disclosed a surface slump scar at the proposed location of Site 1148 (Fig. F17A, F17B). As a result of these data, Site 1147 was located upslope from the slump to obtain a complete upper sediment section.
The wireline logs of Site 1148 enabled construction of acoustical impedance over the interval from ~200 to ~698 mbsf. We used the physical properties core-log data for bulk density and P-wave velocity and adjusted them to calculate the TWT of the top of the logged interval. Below the logged interval (698 to 850 mbsf), we used the physical properties and downward continuation of the velocity model to estimate the TWT depth. On the basis of in situ velocity logs from Site 1148, the 700 (Hole 1148A) and 859 mbsf (Hole 1148B) penetration is equivalent to ~0.78 and ~0.91 s TWT, respectively.
The acoustical impedance log shows a continuous increase in impedance from the top of the logged interval to 450 mbsf, where it first increases rapidly and then decreases abruptly. This strong impedance contrast coincides with the strong reflector at 0.52 s (Fig. F18) and has an age of ~22.6 Ma. Hence, this major reflector is younger than the lower Oligocene Reflector T7 (32 Ma) but is older than the widely observed lower/middle Miocene Reflector T4 (14 Ma). The age of this feature seems most consistent with the T5 boundary of Chen et al. (1987) but is still somewhat younger than their designation of 26.2 Ma. Following this reflector, acoustical impedance continuously increases downhole with a small reversal at the base of the section. The "basement" reflector at ~0.9 s was projected to occur at ~800 mbsf. Although drilling slowed at this level, no other evidence of a change in composition or properties occurred at this depth. The age of the interval from 800 to 850 mbsf is ~31.5 to ~31.8 Ma. It is therefore close in age to the lower Oligocene Reflector T7, which is thought to mark the breakup and transition from the rifting to the spreading phase. In well records on the continental shelf near the Pearl River Mouth Basin, this basement reflector was believed to mark the termination of Paleogene syn-rift sedimentation in midslope half-grabens (Ru et al., 1994). 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; Ludmann and Wong, 1999). Although Site 1148 penetrated the section that should contain Reflectors T1, T2, and T4, the latter two do not seem to be well developed in this deep-water section. Perhaps further processing of the seismic lines will help define these Neogene reflectors.
The sequence of reflectors observed in the seismics and cored during Leg 184 was highly variable. Although some of the features observed in the Pearl River Mouth Basin (Ludmann and Wong, 1999) were observed at our offshore sites, their manifestation varied greatly between sites (Table T2). Both of the eastern high-accumulation sites (1144 and 1145) seem to record a mid-Pleistocene event, which is not readily observed at the western sites (1146 and 1148). However, within the preliminary dating, a Pliocene/Pleistocene reflector seems common to the northern sites, except for Site 1148. Although Sites 1143 and 1146 share a nominal T2 reflector, the older features are more variable and have possible multiple g reflectors. The extrapolation of depth/age at Site 1146 indicates that the reflectors at 0.77 to 0.80 s are consistent with (but somewhat younger than) the age of the T5 Oligocene/Miocene reflector and are the same age as the well-dated reflector at 0.52 s at Site 1148. The basal reflectors at Site 1148 seem to correlate roughly with the T7 reflector, which is thought to mark the initiation of seafloor spreading in the SCS. Although the identification of the Pearl River Mouth Basin reflector sequences in the South China Sea deep-sea sediments is tantalizing and intriguing, much detailed work is necessary to refine the velocity and age models to more rigorously test the temporal and spatial connections between the shelf and the continental margin sediments of the SCS.