The Mid-Cape Basin study area at 29°S was chosen between the main working areas of the Southern Cape Basin (SCB) at 31°S and the Northern Cape Basin at 25°S on the northern side of the Orange Fan. This area originally was not included in the drilling proposal. In the structural map of Dingle et al. (1987), the area appeared at the southern rim of a slump area. From the survey, it was expected (1) to find a less disturbed upper sedimentary sequence than in the SCB and (2) to collect data between 25° and 31°S to study the temporal and spatial evolution of upwelling systems along the coast by north–south correlation and comparison of seismostratigraphic units. The location was also chosen based on previous Parasound surveys during Meteor Cruises M20/2 and M23/1, Polarstern Cruise ANT XI/5 and Sonne Cruise SO 86, where surface sediments appeared to be mostly undisturbed.
A small survey (Fig. 1) was carried out during Meteor Cruise M34/1 (Bleil et al., 1996), which provided an additional location for drilling (MCB-A) near the crossing point between seismic Lines GeoB/AWI 96-009 and 96-011. This location was added to the proposal and received approval for drilling.
Three seismic Lines GeoB/AWI 96-009 to 96-011 with a total length of 350 km were shot (Fig. 2). Both the Hydrosweep swath sounder survey (Bleil et al., 1996) and seismic data from Line GeoB/AWI 96-009 (Fig. 1) document several scarps (common depth points [CDPs] 1100, 2300, 4400, and 5800) probably related to slump events where major portions of the surface sediment (as much as 100 m) were removed. The sedimentary sequences seem to be intact only on the upper slope above 2000 m water depth. Data quality was partially affected by bad weather conditions, but the integrity of the sediment column within the drilling range of 600 m could generally be confirmed. These observations are in agreement with the structural information published by Dingle et al. (1987).
Site 1085 is located in 1713 m water depth on seismic Line GeoB/AWI 96-009 (CDP 6163; Fig. 1). Deposition appears to be pelagic or hemipelagic with only minor lateral and depth variations in layer thickness. In general, the seismic character near the site is similar to the SCB working area, although several differences exist. A prominent feature is a band of high-amplitude reflectors between 800 and 1000 ms sub-bottom two-way traveltime (TWT). In the SCB area (see "Site Geophysics" section, "Site 1086" chapter, this volume) it marks the base of seismic Unit 2, where we assign it to the well-known Reflector D (L) of Cretaceous–Paleocene age (Emery et al., 1975; Bolli, Ryan et al., 1978). The thickness of the sediment package of 1000 ms TWT above Reflector D is slightly higher than the 850 ms TWT in the SCB area, which may be attributed to the absence of hiatuses caused by listric faults. Internal fracturing by small faults is also absent, and lateral variations in reflection strength are minor.
Figure 3 shows a 10-km-long seismic section of Line GeoB/AWI 96-009 across Site 1085. Low seismic amplitudes within the upper 450 ms TWT (seismic Unit 1) indicate a homogenous lithology. Stronger reflectors paralleling the seafloor are probably caused by ghost signals of a deep-lying streamer. Drilling also penetrates the upper part of seismic Unit 2, which is characterized by some higher amplitude reflectors. A gradual increase of seismic energy is observed between 450 and 550 ms TWT. An intercalated unit of 10- to 20-m thickness at 580 ms TWT sub-bottom depth, probably of slump origin, thins out downslope and is associated with a stronger reflector. The unit beneath appears mostly transparent. At its base at 700 ms TWT, reflection amplitudes generally increase.
Figure 4 shows a close-up of the seismic section, plotted against TWT, for a 1-km-long interval near the drill site. Seismic reflectors are compared with the resistivity log (see "Downhole Logging" section, this chapter), which was plotted against the TWT derived from the sound velocity log. The logging data confirm the homogeneity of the section down to 450 ms TWT. Few changes indicate gradually varying lithology, which may not be sharp enough to be imaged with the seismic data. The increase in seismic energy is associated with a higher scatter in the resistivity log, which appears even more pronounced in the sound velocity log (see "Downhole Logging" section, this chapter). Some selected reflectors can be tentatively attributed to major changes in the resistivity log, but a unique assignment will only be possible based on the calculation of synthetic seismograms. The base of Site 1085 shows a distinct increase in resistivity and density, which marks the top of an interval of stronger reflection amplitudes and probably of more lithified sediments with pronounced changes in physical properties (e.g., carbonate content).