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Sediments in the Angola Basin (Site 364) can be divided into five petrographic units. Units 1 (Cores 1-7) and 2 (Cores 8-21) are deficient in carbonates, with predominant clays while Units 3 (Cores 22-27) and 4 (Cores 28-43) are dominated by calcite interbedded with shales carbon-rich in Unit 4. Unit 5 (Cores 44-46) is characterized by the predominant occurrence of dolomite interbedded with carbon-rich sepropelic shales. Sediments on and near the Walvis Ridge (Sites 362 and 363) are highly calcareous, except for the Pleistocene sediments at Site 362 and some upper Cretaceous mudstones at Site 363. Characteristic differences between the Angola Basin and Walvis Ridge sediments are the occurrences of gypsum and barite in the former and opal in the latter. Relative abundances of clay minerals show marked changes throughout Site 364. Five clay mineral zones are recognized as follows: Zone I (Cores 1-6); mica-kaolinite-smectite-chlorite; Zone II (Cores 7-21); mica-smectite-kaolinite-(chlorite); Zone III (Cores 22-24); smectite-mica; Zone IV (Cores 25-38); mica-smectite(kaolinite); and Zone V (Cores 39-45); kaolinite-mica-chlorite. The boundaries between the clay zones are approximately similar to those of the petrographic units. Some of the mica in Zones IV and V was probably transformed from smectite during diagenesis. Kaolinite-bearing sediments occur chiefly in the Aptian (?) and the Oligocene to Miocene in the Angola Basin and the Walvis Ridge. A common provenance is considered for these sediments, because of their similarity. The upper sapropel in the Coniacian to Albian of Site 364 is chiefly composed of smectite, suggesting that active volcanism occurred somewhere in the proximity in the ocean or on the continent. Two types of dolomite are recognized on the basis of chemical compositions and occurrences. One is dolostone, which is found exclusively in Unit 5 of the Angola Basin and is characterized by a high content of sodium, averaging 0.1 wt.% as Na2θ, and a scarcity of iron (type A dolomite). The other is found both in the Angola Basin and Walvis Ridge. It occurs as argillaceous dolomitic limestone and contains abundant iron, averaging 5 mol % as FeCθ3 (type B dolomite). Type A dolomite is considered to have formed from solutions with high salinity and high Mg:Ca ratio, relating to the underlying evaporite formation. Type B dolomite is assumed to have formed in a reducing environment with excess carbonate ions derived from decomposition of the organic materials in the sediments.

Sedimentary Petrology of DSDP Cores from Sites 362 and 363, the Walvis Ridge, and Site 364, the Angola Basin, Drilled on Leg 40