CONCLUSIONS

The highlights of the sedimentological shipboard results may be summarized as follows:

  1. Four different major sedimentary facies were encountered. At the northern end of the north–south transect, sediments have a large terrigenous component. On and near the Walvis Ridge, sediments have a strong pelagic aspect. South of the Walvis Ridge, sediments are unusually rich in opal and organic matter, and at the southern end of the transect, in the Southern Cape Basin, sediments are dominated by pelagic components.
  2. Sedimentation rates are unusually high, ranging between 30 and 600 m/m.y. They are usually near 100 m/m.y. On the whole, the hemipelagic sites north of the Walvis Ridge have somewhat higher rates than those south of the ridge. Within the Quaternary record, hemipelagic sediments show a tendency toward increased sedimentation rates with time.
  3. Organic carbon values are between 1 and 5 wt% north of the Walvis Ridge. South of the ridge, values are typically <5 wt% but can reach well over 10 wt% at sites near the coastal upwelling areas. Black layers extremely rich in organic carbon (as much as 20 wt%) are common at the Lüderitz site (1084). Noteworthy is the overall decrease in downhole percentages. Continued diagenetic destruction of organic matter and an increase in upwelling and productivity within the last 10 m.y. are thought to be responsible for this decrease.
  4. There is pervasive evidence for high productivity leading to intense chemical activity within the sediments, to the production of gas (methane and CO2), and to the formation of new minerals (dolomite, phosphorite, glauconite, and authigenic iron sulfides), especially at sites close to the Walvis Ridge and basin and Lüderitz Bay.
  5. Layers of hard rock (made of dolomite and calcite, or dolomite-cemented and calcite-cemented clay) form within soft organic-rich sediments <1 m.y. old. Logging proved extremely useful in determining the positions and thicknesses of these layers. Because of their high resistivity, they are readily identified by the FMS tool.
  6. Because the region off the Congo and Angola is known to be rich in hydrocarbons, we assumed that clathrates (methane-rich water ice) are common in the sediments. A typical BSR has been observed within restricted intervals. BSRs are commonly interpreted as denoting the lower boundary of clathrate stability. We were surprised, however, not to find any indication of the presence of clathrates at any of the sites drilled during Leg 175.

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