Here, we summarize the major sediment patterns (Fig. 3, Fig. 4, Fig. 5). Rates of accumulation are typically near 10 cm/k.y. (100 m/m.y.), and vary by about a factor of 2 between sites (Fig. 3). On the whole, the hemipelagic sites to the north of the Walvis Ridge have somewhat higher rates than those south of the ridge. In hemipelagic sediments within the Quaternary record, there is a tendency for increased rates with time, with maximum values in the latest Quaternary. For the pelagic sediments on the ridge and south of it, a minimum in sedimentation rate may be indicated within the early Pliocene (although this is not seen at Site 1085).
The concentration of organic carbon (Fig. 4) records a relatively modest range of variation in the sites north of the Walvis Ridge (Sites 1075–1079). Most commonly, values are between 1 and 3 wt%, with some as much as 5 wt%. There is no obvious relationship with sedimentation rates, although this has not been tested statistically. At sites south of the Walvis Ridge (Sites 1081–1085), values also are typically below 5 wt%, but reach well over 10 wt% at some sites (1082 and 1084). Noteworthy is the overall decrease in downhole percentages. Presumably, two factors are at work: (1) continued destruction of organic matter by diagenesis and (2) an increase in upwelling and productivity within the last 10 m.y. (Siesser, 1980).
The hemipelagic vs. pelagic character of sediments recovered north and south of the Walvis Ridge, respectively, is well reflected in the carbonate values (Fig. 5). At the northern sites, values tend to be low and rarely exceed 10 wt%, except at the very shallowest sites (1078 and 1079). In the southern sites, carbonate values commonly vary between 30 and 80 wt%, with the highest values in the fully pelagic sites off the Cape of Good Hope (the last two of which are not shown in Fig. 3, Fig. 4, Fig. 5). At the northern sites, a maximum carbonate event may be indicated, centered near 0.8 to 0.9 Ma. In the southern sites, a carbonate minimum appears near 1.9 Ma, at the beginning of the Quaternary record.
The patterns may be grouped into (1) Congo Fan sites (1075–1077), (2) Angola (Lobito) sites (1078 and 1079), (3) Walvis sites ([Walvis Ridge and Walvis Basin] 362, 363, and 530–532, and 1081–1083), Namibia (Lüderitz) site (1084), and South African sites (360, 361, and 1085–1087), based on carbonate and organic matter contents and sediment composition, as well as other components (opal and terrigenous contents; Fig. 6)
The Congo Fan sites (1075, 1076, and 1077) feature hemipelagic muds, typically classified as greenish gray and olive-gray diatomaceous clay, which is nannofossil bearing in places. Carbonate contents are low, especially in sediments older than 1 Ma. Sedimentation rates are close to 100 m/m.y. and typically vary by a factor of 2 around that value. Organic carbon content varies mostly between 2 and 3 wt%. Distance from the coast is reflected in the amount of reworked material and in sedimentation rates, with Site 1076 being closest to the coast and Site 1075 being the most distal. Surprisingly, we found no turbidites intercalated into the sequences. Presumably, coarse sediments moving within the deeply incised Congo Canyon are carried far into the deep Angola Basin (Jansen et al., 1984). At the sites drilled, only fine materials not confined to the canyon are delivered, perhaps from riverine plumes, through sedimentation by aggregates and in fecal pellets.
The Lobito sites (1078 and 1079), on the upper continental slope off Angola, are strongly influenced by the influx of terrigenous material. They feature hemipelagic muds typically classified as olive-gray silty clay with varying amounts of nannofossils and foraminifers. Rapid uplift of the coast and canyon cutting into the uplifted terraces provide much of the terrigenous supply, which drives up sedimentation rates (Fig. 3). Considering the dilution effect of the terrigenous silt and clay, the organic content is surprisingly high, with values between 2 and 5% (Fig. 4). High nutrient supply associated with the Angola Dome upwelling, as well as coastal mixing, combine to provide for high productivity in this area.
Site 1080, off the Kunene River, was drilled only to 52 meters below seafloor (mbsf) when drilling was halted by hard dolomite. The sediment is diatomaceous hemipelagic mud, classified as greenish gray and olive-gray diatom-bearing and diatom-rich silty clay. Sedimentation rates are near 100 m/m.y., but the upper Quaternary record is missing.
The Walvis sites ([Walvis Ridge and Walvis Basin] 1081, 1082, and 1083) show a strong influence from high productivity in the coastal ocean. Sediments are intermediate between hemipelagic mud and pelagic ooze and are typically classified as olive-gray nannofossil-rich clay or ooze with or without a strong admixture of diatoms. In sections with very high carbonate content (e.g., Site 1083), the color is pale yellow or pale olive. Alternations of sediment types are common and produce color cycles. The opaline component is especially high in the upper Pliocene and lower Pleistocene sediments.
The Lüderitz site (1084), located between Lüderitz Bay and Walvis Bay, represents the extreme among all sites regarding upwelling and productivity. Sediments are intermediate between hemipelagic mud and pelagic ooze and are typically classified as nannofossil-rich diatomaceous clay, diatomaceous clay, and nannofossil clay or ooze. On the whole, sedimentation rates well exceed 100 m/m.y., but not so much from unusually high delivery of terrigenous mud as from the high supply of nannofossils, diatoms, and organic matter. Black layers extremely rich in organic carbon are common at this site beginning in the upper Pliocene, and they increase in abundance toward the present. As at the Walvis sites, diatom abundance reaches a maximum in upper Pliocene and lower Pleistocene sediments.
The South African sites (1085, 1086, and 1087) show, on the whole, pelagic sedimentation. Sediments are typically classified as foraminifer-rich nannofossil ooze, with variants on this theme, depending on clay, diatom, and foraminifer content. Carbonate content is high, whereas organic carbon content is modest (although higher than average for pelagic carbonates). Sedimentation rates vary mostly between 20 and 60 m/m.y., which is about twice the rate for pelagic deep-sea calcareous ooze. At these sites, drilled to a maximum depth of 604 m in places, the oldest sediments were recovered. At Site 1087, a continuous record (apparently) dates back to the middle Mio-cene, whereas older sediments (early Miocene, Oligocene, and Eocene) are present but separated by hiatuses.