The decrease of terrigenous accumulation rates throughout the Tertiary was related to the aridification of the southern part of the African continent, which led to starved conditions in the Neogene for the deeper part of the Cape Basin and the deposition of pelagic sediments at the continental margin. Neogene sedimentation was accompanied by successive slope failures (e.g., Dingle, 1980; Dingle and Robson, 1992) because of the buoyant behavior of the sediment-loaded margin. During the Pleistocene, a significant portion of the continental slope and rise was affected by further massive slumps, slides, and debris flows, and in many places the Neogene sedimentary column was partially or completely removed. This was observed at the two Deep Sea Drilling Project (DSDP) Sites 360 and 361 off Cape Town, and also in the working area of the Southern Cape Basin (SCB), as identified in the original seismic data of Austin and Uchupi (1982).
However, the SCB area remained a viable target for drilling because it is located at the southern rim of the Orange Fan. The underlying morphology created by the Orange River until the late Oligocene or Miocene modified the shape of the continental margin toward moderate slope angles. The working area in the SCB was chosen to study the onset and the early history of the Benguela Current and to detect possible influences of the Agulhas Current. The proximity to the continent allows the identification of local upwelling signals, sea-level changes, and a study of continental climate.
In the original proposal, three sites were selected from the seismic Line AM-54 by Austin and Uchupi (1982) for a depth transect. After the Meteor M34/1 seismic survey (Bleil et al., 1996), one high-priority site was chosen for APC and extended core barrel (XCB) coring down to 600 m sub-bottom depth, with additional alternate sites at different water depths (Fig. 2).
Two major reflectors can be used for a first chronostratigraphic assignment of seismic units. Seismic Reflector AII was first described by Emery et al. (1975) and later named Reflector P by Gerrard and Smith (1982). It was assigned a late Aptian age by Bolli et al. (1978), and it indicates the "earliest recognizable shelf-slope-rise relationship" (Dingle and Robson, 1992) in the Cape Basin. The second basinwide observable Reflector D (Emery et al., 1975; Reflector L of Gerrard and Smith, 1982) denotes a condensed section and a sharp downhole decrease in the contents of calcium carbonate at the Paleocene/Eocene boundary, according to drilling results of DSDP Leg 40 (Bolli et al., 1978).
Altogether, eight seismic lines with a total length of 854 km were measured during Meteor Cruise M34/1 near the proposed site SCB-1 (Bleil et al., 1996). Bathymetric measurements revealed a significant variability in the morphology, which confirmed the existence of major slump events. A ridge-type structure is observed between the two Lines Geosciences Bremen (GeoB)/AWI 96-001 and 96-003. Slump scarps are identified (e.g., on Line GeoB/AWI 96-001 between 2000 and 2500 m water depth).
Major slump structures are known from earlier studies of Dingle (1980), Dingle et al. (1987), and Dingle and Robson (1992). Seismic Line GeoB/AWI 96-003 (Fig. 3), which runs perpendicular to the continental margin, shows clear evidence for such slumps, particularly in greater water depth. Therefore, the development of a seismo-stratigraphic concept and its application to all seismic lines in the area is not as straightforward as was expected from the earlier, limited-resolution data of Austin and Uchupi (1982). Near site SCB-1, the sediment column appears to be less disturbed than elsewhere in the region. Although normal faulting of larger blocks is observed and the curved shape of the base faults suggests horizontal displacement and a lack of structural integrity, the sedimentary units at Site 1087 seem to represent mostly undisturbed sections separated by one or more hiatuses.
Figure 3 indicates the location of Sites 1086 and 1087, projected on this profile. Site 1086 is offset from Line GeoB/AWI 96-003 by 22.5 km to the south; Site 1086 by 6 km. The uppermost sedimentary unit of 300 to 400 ms two-way travel time (TWT) thickness is characterized by moderate amplitudes, indicating pelagic deposition. At Site 1087, a stronger reflector marks the transition to seismic Unit 2 of 100 to 150 ms TWT thickness, which reveals higher amplitudes. Within this unit, a major unconformity is observed, which can be identified as a gliding plane of a massive slump event. Sediments are disrupted, and the morphology is significantly disturbed. Seismic Unit 3 of 300 to 400 ms TWT thickness represents the remains after the slump event and is mostly undisturbed, except for some block faulting. Reflectors are of high amplitudes and mostly continuous. The seismic character of the base of this unit at a depth of ~900 ms TWT near Site 1087 is similar to that of DSDP Sites 360 and 361 (Bolli et al., 1978) and is tentatively assigned to Reflector D of Paleo-cene–Eocene age. Toward the shelf break, the uppermost unit thins out, and it must be assumed that recent sediments have been eroded or not deposited, possibly because of stronger currents in intermediate water masses.
Four additional sites were proposed at the crossings of the seismic lines. None of them allows drilling of a continuous sedimentary section, but the locations of hiatuses and thicknesses are different and allow adjustment to different drilling targets in time and water depth.
Site 1086 is located in 781 m water depth (Fig. 2) on Line GeoB/AWI 96-006 (common depth point [CDP] 1680), 22.5 km south of the crossing with Line GeoB/AWI 96-008. Figure 4 shows a 10-km-long seismic section of Line GeoB/AWI 96-006 across Site 1086. The site was moved from its originally proposed position into shallower water to record signals of intermediate water-mass influence in the area.
The uppermost 400 ms TWT appear mostly undisturbed, although structures in the upper 20 m remain unresolved because of an apparently strong and long seafloor return echo. Beneath 400 ms TWT, a disturbed zone marks the gliding plane of a slump, mentioned above, with sheared sediment blocks. Drilling was therefore limited to a maximum of 250 m.
Figure 5 shows a close-up of the seismic section, plotted against sub-bottom depth for an average sound velocity of 1600 m/s, for a 1-km-long interval near the drill site. Seismic reflectors are compared with the GRAPE density core log (see "Physical Properties" section, this chapter). Although some stronger reflectors can be tentatively correlated with pronounced peaks in the density core log, the seismic signal is not suitable to resolve these fine-scale changes. Seismic returns are therefore a product of interference from closely spaced reflectors, which have to be compared directly with synthetic seismograms to draw further conclusions. Whether the band of reflectors in the uppermost 20 m is an artifact of seismic data acquisition or results from stronger fluctuations of grain size observed in the cores (see "Lithostratigraphy" section, this chapter) must be studied in detail.
Site 1087 is located in 1372 m water depth in the middle of the survey area (Fig. 2) on Line GeoB/AWI 96-008 (CDP 1227). Figure 6 shows a 10-km-long seismic section of Line GeoB/AWI 96-008 across Site 1087. This seismic section reveals more details about disturbances at different levels in the sedimentary column (e.g., at 150 and 450 ms TWT), which could not be avoided because of the complex nature of the site area. Although the uppermost unit of 150 ms TWT reveals some lateral variation in thickness, the sediment package from 150 to 450 ms TWT represents pelagic deposition causing continuous reflectors of constant thickness. The interval beneath 450 ms TWT is hummocky with some high-amplitude reflectors, which may suggest a shear zone. Reflector D was tentatively identified at the base of another undisturbed sedimentary unit beneath at 950 ms TWT (see above).
Figure 7 shows a close-up of the seismic section, plotted against sub-bottom depth in TWT, for a 1-km-long interval near the drill site. Seismic reflectors are compared to the GRAPE density core log (see "Physical Properties" section, this chapter). The depth scale of the core log was linearly compressed according to the average sound velocity of 1775 m/s, determined from downhole logging between 80 and 488 mbsf (see "Downhole Logging" section, this chapter). Some reflectors are tentatively correlated to changes in sediment density, but more thorough editing and analyses of both seismic data and core data have to be carried out on shore to relate lithologic changes to seismic reflectors. The rough comparison confirms that the higher seismic amplitudes beneath 430 mbsf are associated with the beginning lithification of the sediment, sharp peaks in GRAPE density beyond 2000 kg/m3, and the major Miocene/Eocene hiatus.