The southwest African margin, which formed the target area for Leg 175, is represented by three major geologic provinces: (1) the northern margin, where salt tectonics is important; (2) the Walvis Ridge intersection, where hot-spot-derived basalt abuts the continent; and (3) the southern margin, which is a "normal" passive margin, without major salt deposits, large fan deposits, or volcanism.
The fact that salt deposits are present north of the Walvis Ridge greatly affected the choice of targets for Leg 175. Salt accumulated in the early rifting stage of the South Atlantic in Aptian–Albian times when the Walvis Ridge was part of a barrier preventing free exchange of a proto-Brazil-Angola Basin with the open ocean (see Deep Sea Drilling Project [DSDP] Leg 40 report; Bolli, Ryan, et al., 1978). Throughout the late Cretaceous and to the present, salt tectonism has helped shape the morphology of both seafloor and coastal regions and has dominated regional sedimentation patterns. Salt domes and ridges provide convenient barriers for the development of sedimentary basins (which aids in preserving undisturbed sequences). However, they also provide conditions favorable for hydrocarbon accumulation, given organic-rich sediments of Cretaceous age, at the bottom of a thick sediment sequence. In recent years, the offshore petroleum potential of salt-dome structures has attracted much attention. This potential limits exploration for scientific purposes when using riserless drilling.
The Walvis Ridge continues to the present to act as a major barrier to deep-water flow, thus influencing deep-sea sedimentation in the Angola Basin throughout the Cenozoic. This influence may be of some significance in the deepest sites occupied north of the Walvis Ridge (Sites 1075 and 1080). The main difference between the northern and southern provinces results from the fact that the Benguela Current turns northwest above the Walvis Ridge to feed the South Equatorial Current (Fig. 2, Fig. 4). Thus, the large-scale dynamics of production of organic matter and opal are fundamentally different north and south of the Walvis Ridge. We refer to the region on and south of the ridge as part of the "Benguela Current system" in contrast to the northern area, which belongs to the "Angola regime." The two are separated by the Angola-Benguela Front (ABF; Fig. 4), a frontal zone lying between 16° and 17°S, where the poleward-flowing Angola Current meets the equatorward-flowing Benguela Current (Shannon, 1985).
As previously mentioned, the 13 sites occupied during Leg 175 are readily grouped into transects exploring the record of different environments off the coast of southwestern Africa. These groupings are (1) The Congo transect (Sites 1075, 1076, and 1077); (2) the Angola transect (Sites 1078, 1079, and 1080); (3) the Walvis transect (Sites 1081, 1082, and 1083), which includes DSDP Site 532; (4) the Lüderitz Site 1084, which is unique in its position next to an intense upwelling cell; and (5) the Cape transect (Sites 1085, 1086, and 1087). The Walvis, Lüderitz, and Cape of Good Hope sites make up the Benguela set, which contains the history of the Benguela Current and the associated coastal upwelling regime.
An important result of the expedition is the fact that so many of the sections recovered show continuous sedimentation at high rates of accumulation (see Wefer et al., Chap. 16, this volume). Several conditions are responsible for this finding: high rates of supply from high productivity and terrigenous contributions, favorable sites of deposition through salt tectonics (north of the Walvis Ridge), and a tectonically quiet setting (in the Cape Basin). This is not to imply that sediments are undisturbed, as a rule, along this margin. They are not. However, it was possible to find stratigraphically useful sections based on well-prepared site selection.
The major environmental provinces and their sedimentary records may be characterized as follows.
The regional environment in the Congo Fan region is dominated by three major influences: (1) the freshwater input from the Congo River (the second largest river in the world), (2) seasonal coastal upwelling and associated filaments and eddies moving offshore, and (3) incursions of open-ocean waters, especially from the South Equatorial Countercurrent. According to Jansen (1985), river-induced phytoplankton activity extends ~160 km beyond the shelf edge, which would affect all three sites drilled. Much of the regionally enhanced productivity is river related. However, divergence and doming and glacial/interglacial changes in the dynamics of the South Equatorial Countercurrent and the Benguela Current may be just as important. In the site closest to shore, the effects of seasonal coastal upwelling should be superimposed on the riverine effects. Late Quaternary Congo Fan sediments have exceptionally high opal content (Müller and Schneider, 1993), ~10 times higher than in the slope sediments recovered from anywhere else off southwestern Africa. Fluctuations in the opal content are controlled by large-scale climatic patterns, as is evident from the presence of Milankovitch cycles (cryosphere cycles of 41-k.y. and 100-k.y. periodicity and wind-system cycles of 23-k.y. and 19-k.y. periodicity; see Schneider et al., 1997). The Congo transect provides a window into the climatic history of western tropical Africa.
The hemipelagic sediments in this province are dominated by diatomaceous, partially carbonate-bearing clays. Sedimentation rates are typically between 15 and 20 cm/k.y. for the late Quaternary and are close to 10 cm/k.y. for the early Quaternary. Presumably, terrigenous sediment supply increased with the onset of the large-amplitude 100-k.y. climate- and sea-level cycles 650 k.y. ago.
The mid-Angola region has surprisingly low productivity compared with adjacent upwelling areas to the north and south (Wefer et al., 1988). Upwelling is seasonal and comparatively weak; opal accumulation is extremely modest for this coastal environment. The two shallow-water sites off Lobito (Sites 1078 and 1079,) have silty clays accumulating at extremely high sedimentation rates, with maximum values of 60 cm/k.y. at Site 1078 and nearly 40 cm/k.y. at Site 1079. Sediments are supplied by an actively eroding coast that is presumably being uplifted by salt tectonics. Site 1080 was drilled off the Kunene River (Angola). It is located near the northernmost coastal upwelling cell along the southwestern margin. Sediments consist of diatom-bearing and diatom-rich silty clays that are accumulating at a rate near 10 cm/k.y. However, the late Quaternary section is greatly attenuated. Drilling was terminated after a dolomite layer was hit because of slow progress.
The Walvis Ridge provides a large shallow area favorable for the preservation of carbonate. Also, it is situated at a crucial latitude where the Benguela Current turns westward into the open sea (Fig. 4). Thus, the ridge is the major monitoring region for the history of the Benguela Current. It was the target for drilling during two earlier legs, whose sites are part of the transect (Leg 40, Site 362; Bolli, Ryan, et al., 1978; and Leg 75, Site 532; Shipboard Scientific Party, 1984). The DSDP sites are well off the coast but contain an upwelling signal which has been transported outward from its coastal zone of origin by the eddies and filaments of the Benguela Current. The DSDP sites produced evidence for cyclic sedimentation from varying carbonate dissolution, productivity, and terrigenous sediment supply. Site 1081 extends this transect toward the coast where upwelling is strong. Sediments recovered (from Sites 1081, 1082, and 1083) consist of calcareous clays, which are especially rich in diatoms in the upper Pliocene and lower Quaternary sections (Matuyama Opal Maximum [MOM]). Sedimentation rates are typically between 5 and 10 cm/k.y., with an apparent minimum in the middle portion of the Pliocene section.
Lüderitz Bay, on the Namibian shelf, is in the center of the Benguela upwelling region, which is characterized by a series of upwelling cells between the Cape of Good Hope and the Kunene River mouth. The Lüderitz cell is the largest and most active of these (Duncombe Rae et al., 1992). It vigorously sheds eddies and filaments into the coastal Benguela Current. Poleward subsurface flow brings nutrient-rich waters southward to this location from the Walvis Bay area, whereas upwelling is induced by winds and the northward-flowing Benguela Current, which approaches close to the coast thanks to a narrow shelf (Fig. 4). Drilling at Site 1084, off Lüderitz, recovered sediments with the highest organic carbon contents of any site occupied (as much as 20%). Diagenetic activity, as reflected in the presence of methane and carbon dioxide and in the rapid reduction of sulfate within interstitial waters, is extremely intense; alkalinity values were the second highest ever measured in sediments recovered by DSDP or ODP. (The highest were found off Peru, during Leg 112; Suess, von Huene, et al., 1988, 1990.) The central portion of the sediment column recovered (upper Pliocene to lower Pleistocene) consists of diatom-bearing clay to diatomaceous clay and diatom ooze sandwiched between nannofossil clay and ooze of early Pliocene and late Pleistocene age. Black organic-rich layers are prominent throughout the section. Sedimentation rates are typically between 10 and 20 cm/k.y., with the younger sediments having the higher rates of accumulation.
The southernmost sites of Leg 175 (1085, 1086, and 1087), near the Cape of Good Hope, have pelagic calcareous oozes, mainly nannofossil oozes, but with high proportions of foraminifers in some areas (especially at Site 1086). Diatoms, radiolarians, and other opaline fossils generally are rare or absent, except in sediments corresponding to the MOM where abundances are distinctly higher. These sites are excellently situated to monitor the fluctuating influence of Southern Ocean water, including intermediate water, and the import of warm water from the Indian Ocean. Sedimentation rates typically vary between 2 and 7 cm/k.y. at all three sites.
The Walvis transect, Lüderitz site, and Cape of Good Hope transect form one coherent megatransect for the Benguela Current system. Indications are that the important trends and events in the history of this system are recorded simultaneously along the entire transect. A reconstruction of latitudinal migrations of elements of the Benguela upwelling system (and the associated South Atlantic high-pressure cell) will be of special interest.