Next Section | Table of Contents


Geologic Setting

Walvis Ridge is a northeast-southwest–trending aseismic ridge that effectively divides the eastern South Atlantic Ocean into two basins, the Angola Basin to the north and the Cape Basin to the south (Figs. F2, F3). It consists of a series of interconnected crustal blocks that slope gradually toward the northwest and more steeply toward the southeast. Magnetic and gravity anomalies indicate the ridge was formed by hotspot volcanism near the mid-ocean ridge as the basin gradually widened (Rabinowitz and Simpson, 1984). The subsidence history of Walvis Ridge has been determined using simple thermal subsidence models (Moore, Rabinowitz, et al., 1984).

Pelagic sediments drape most of the ridge and generally increase in thickness toward the continental margin (Moore, Rabinowitz, et al., 1984). In the vicinity of the proposed target area on the southeast portion of the ridge, sediment thickness varies from ~300 m on the deeper portions of the ridge to ~500 m near the summit, a pattern that is clearly expressed in acoustic reflectors (Fig. F4). The sediments are primarily calcareous oozes and chalks that range in age from Campanian to Holocene (Fig. F5). The Neogene sequences are dominated by nannofossil and foraminifer-nannofossil oozes with relatively high carbonate contents, in excess of 90%. Turbidites and occasional slumps are present in some areas. The underlying Paleogene sediments are dominated by nannofossil and foraminifer nannofossil chalks. Carbonate contents are also high, in excess of 80% through most of the Paleocene, Eocene, and Oligocene with the exception of several short carbonate-poor intervals at the deeper sites (i.e., Site 527; upper Paleocene, upper Eocene, and early–middle Miocene). Most of these carbonate-poor layers represent episodes of CCD shoaling. A few thin chert layers are present below the upper Paleocene of the shallowest sites (525 and 528) and in the lower Eocene of Site 529. Slump deposits of various scales are present in the upper Paleocene at Site 529 as well. In most sections, calcareous microfossil preservation varies from good to excellent. The natural remanent magnetization of the Walvis Ridge sediments appears to be strong and stable. As a result, the quality of the magnetic polarity records are excellent, particularly over the Upper Cretaceous and lower Paleocene (Chave, 1984).

Stratigraphic Evolution

Sediment accumulation rates on Walvis Ridge are quite variable in both space and time. On average, the highest rates (~8–13 m/m.y.) occur in the Quaternary, Paleocene, and Maastrichtian. Hiatuses are a common feature of the Neogene at most sites. For example, most of the Pleistocene is missing at shallow sites (Sites 525 and 526), whereas the lower and middle Miocene is absent at several of the deeper sites. Most of the Neogene hiatuses appear to be erosional in nature, although dissolution clearly contributed in some cases (i.e., lower–middle Miocene at Site 527). Hiatuses are also present in the upper Eocene and upper Oligocene. In contrast, at middepths (Site 529), most of the Paleogene is present but the middle–upper Miocene is either condensed or absent. In the deepest section (Site 527), the middle–upper Eocene is relatively condensed and the Oligocene and Miocene is highly condensed and/or absent. Nevertheless, it appears that for most of the early Paleogene, deposition was more or less continuous over much of the rise. This continuity in sediment accumulation is reflected to some extent in the parallel trends in the low-resolution carbon isotope stratigraphies for four of the Leg 74 sites (Fig. F6) (Shackleton and Hall, 1984).

Cyclical variations in sedimentation are evident in various lithologic/physical properties indices, particularly in the more expanded Maastrichtian and Paleocene sequences. Spectral analysis of sediment color banding reveals the presence of a strong precessional beat in upper Maastrichtian and lower Paleocene sediments at Sites 525 and 528 (Herbert et al., 1995; Herbert and D'Hondt, 1990). Similar color cycles are present in some cores from the upper Paleocene, lower Eocene, and Oligocene as well, although no attempt has been made to document their frequency because of the poor condition of the cores.

Next Section | Table of Contents