Four different major sedimentary facies regimes were encountered during Ocean Drilling Program Leg 175. At the northern end of the north–south transect, off the mouth of the Congo River, sediments have a large terrigenous component. This is also true for sediments drilled off Angola, in sites close to the shelf edge, where very high sedimentation rates prevail. On and near Walvis Ridge, sediments have a strong pelagic aspect, although organic matter contents remain high. South of the ridge, in Walvis Bay and near Lüderitz Bay, sediments are unusually rich in opal and organic matter, reflecting the high coastal ocean productivity. At the southern end of the transect, in the Southern Cape Basin, sediments are dominated by pelagic carbonate deposition. Generally, organic carbon contents were quite high at all sites, ranging from a few percent to as much as 20%. Sedimentation rates ranged from 30 to 600 m/m.y. and were most commonly between 50 and 100 m/m.y., roughly three to four times the values typical for deep-sea carbonates.
The high supply of organic matter results in intense diagenetic activity. Parallel to the intensive chemical reactions directly associated with organic carbon, there are other reactions involving the dissolution and precipitation of minerals, which are an integral part of the intense diagenetic activity ultimately driven by the high productivity of overlying waters. These reactions include dissolution of biogenic carbonates and formation of calcite and dolomite, as well as other minerals such as glauconite, pyrite, and phosphate.
One of the more conspicuous discoveries during Leg 175 was the presence of several decimeter-thick dolomite layers, which were found in some cores, that could be located precisely within the holes using various logging tools. If dolomites are widespread, reinterpretation of seismic profiles, among other things, will be required. Also, the presence of dolomite layers in sediments <1 m.y. old indicates rapid development of such layers under favorable conditions.
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31 August 2007