2. Mid-Angola Basin (MAB)
The MAB sites, off the bight of Angola near 12°S, were chosen to provide
information on "most nearly normal" margin sedimentation, being
influenced neither by riverine input, nor by sustained year-round
upwelling. Upwelling is greatly influenced by variations in the Angola
Thermal Dome. It is seasonal, and productivity is relatively weak
compared to that of adjacent regions (Schneider, 1991). This setting
allows maximum expression of a pelagic signal in the regional high
productivity record. Proposed drill sites are located on seismic Line GeoB
93-015 (Fig. 16), because the southerly profile Line GeoB 93-017 is
significantly influenced by slumping of shelf sediments and turbidity
currents. Of the various sites proposed originally, the two shallow sites
were determined to be feasible for drilling: Site MAB-1 for a maximum of
200 m, and Site MAB-2 for 120 m.
3. Southern Angola Basin (SAB)
The Southern Angola Basin sites are positioned to sample the northern end
of the Angola/Namibia upwelling region. The transect should nicely
complement previous results obtained from Walvis Ridge. This transect is
important not only for the history of the Benguela Current and coastal
upwelling migration, but also for its contribution to the climatic history
of southern Africa. The Kunene River, reaching the coast at ~17°S, is at
the climatological barrier between an illite zone in arid areas to the south
and a kaolinite zone from tropical weathering areas to the north
(Bornhold, 1973). The proposed sites are situated on a climatic boundary,
and should sensitively reflect changes in the position of continental
climatic zones. Suitable drill sites were identified from seismic lines in
water depths between 2200 and 3000 m. The bathymetric survey
confirmed the complex nature of the depositional environment. Although
the survey was not sufficient to analyze all structures in detail, it is
clear from the combined HYDROSWEEP and PARASOUND echosounder data
set that few potential drill sites may be found in the area. Seismic Line
GeoB 93 030 lies across proposed Site SAB-2 (Fig. 17). Stratigraphic data
from two gravity cores (GeoB 1023-5, 17°09.4'5, 11°00.7'E, water depth
1918 m; GeoB 1024-2, 17°09.8'E, water depth 2799 m) show high
Pleistocene sedimentation rates (10-50 cm/1000 yr) (Wefer et al., 1988;
Schneider et al., 1992). We will attempt to drill to 600 m at Site SAB-2.
Time permitting, Site SAB-1 will be drilled for double APC coring.
4. Walvis Basin (WB)
Sites WB-B and WB-C, together with DSDP Sites 532 and 362 at water
depths of 1331 m and 1325 m, form a transect that is central to the
reconstruction of the history of the Benguela Current (Fig. 18). The DSDP
sites are seaward of the upwelling center, but contain an upwelling signal
that was transported to this location by the Benguela Current and its
filaments. At the other end of the transect, proposed Site WB-B will give
a better record of the upwelling itself. Cores spanning the late Quaternary
from nearby areas show a sedimentation rate of 4-7 cm/1000 yr (Schulz
et al., 1992). We will attempt to drill to 600 m at Site WB-B. Time
permitting, Sites WB-C and/or WB A will be drilled for double APC coring.
5. Northern Cape Basin (NCB)
The NCB site will help document the northward migration of the Benguela
Current system from the Miocene to the Quaternary, as well as the
shoreward/seaward migration of the upwelling center. This site will also
provide a record of maximum productivity in the system. Previous work in
this area (for a summary see Dingle et al., 1987) has documented
anaerobic, in part varved, sedimentation in the upper margin regions.
Phosphatic deposits also are abundant (Calvert and Price, 1983). Seismic
lines and the proposed drill site are shown in Figure 19.
The results of Emery et al. (1975) and Austin and Uchupi (1982) show a thick hemipelagic wedge sitting on "rifted continental crust." Slumps would not seem to pose a major problem, although hiatuses are anticipated. Noteworthy is the confirmation of a thick sequence below the shelf region. A close tie-in between pelagic and terrigenous sedimentation is expected to be present within the slope record. During the SONNE Cruise SO-86, vertical profiles were shot over multichannel seismic (MCS) Line AM-1, which were collected by the University of Texas to obtain detailed data for the planned site. A first stratigraphy on an 11-m-long core taken in the high-production upwelling area off Namibia (GeoB 1711-4, 23°18.9'S, 12°22.6'E) from a depth of 1967 m indicated a sedimentation rate of 11 cm/1000 yr (Schulz et al., 1992). We will attempt to drill to 600 m at Site NCB-2B.
6. Mid- and Southern Cape Basin (MCB, SCB)
These sites are located in the southernmost area of the Cape Basin (Figs.
1, 20, 21). The sedimentary records will help in exploring the early
history of the Benguela Current in the southern Cape Basin and in
detecting possible Agulhas Current influences. The sites are located close
to the continent to detect upwelling signals and signals from continental
climates (pollen, clay minerals, and coarser terrigenous matter), as well
as sea-level changes. South of the proposed transect, the margin becomes
too steeply sloped to support undisturbed sediments. Site MCB-A is
located along GeoB/AWI Line 96-009 (Fig. 20), Site SCB-1 is located along
MSC Line AM-54, collected by the University of Texas and along profile
GeoB/AWI 96-003 (Fig. 21). A sedimentation rate of about 5 cm/1000 yr
was determined (Schulz et al. 1992) for an 11-m-long core collected from
near the same water depth (GeoB 1719-7, 28°55.6'S, 14°10.7'E, water
depth 1010 m), but ~150 miles to the north. Time permitting, Site MCB-A
will be drilled for double APC coring, on the way to SCB. Site SCB-1 will
be attempted to a depth of 600 m.
Site SCB-1, as the last site, could be deepened if time is available at the end of the leg. If it appears that time is indeed available, permission to deepen this hole beyond the 400 m now allowed should be sought promptly, to take account of recommendations by the Ocean History Panel (OHP). Alternatively, second priority sites (SCB-A, SCB-B, SCB-C) will be drilled for APC coring.
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175 Table of Contents