Abstract:
Recent sedimentation in the eastern Angola Basin includes calcareous oozes in the north and south (Guinea Rise and Walvis Ridge) and hemipelagic lutites and terrigenous turbidites on the Congo Cone and on the Angola rise and abyssal plain. Slumped and ponded sediments are dominant within the
Angola diapir field. Illite and montmorillonite are abundant in the southern
part of the basin, reflecting the source in soils of South West Africa
and northward transport in the Benguela Current system. Kaolinite dominates
the clay-mineral assemblage in the north-central part of the basin, reflecting
a source in the tropical-humid Congo Basin and transport to the deep-sea
through the Congo River and canyon systems.
Piston cores from the continental rise revealed major fluctuations in
the surface oceanographic conditions, primary productivity, and near-bottom
depositional environment during the late Quaternary. Sediments deposited
during glacial intervals contain markedly lower carbonate, higher levels of
organic carbon, and more abundant siliceous biogenic components, fecal pellets,
and pyrite. Sedimentation rates during the past 200-300 x 103 years remained
relatively constant on the rise, averaging 3-5 cm/103 years.
Oceanographic changes from interglacial to glacial periods, based on
sediment composition and geochemistry, include:
(1) northward extension and intensification of the Benguela Current and
associated high primary productivity off southern Angola;
(2) onset of upwelling and high surface productivity off northern Angola,
Congo, and Gabon; and
(3) major influx of bottom water into the Angola and Guinea Basins.
These conditions resulted in higher benthic productivity, a shallower
lysocline, and a more reducing near-bottom environment, as bottom water in
the Angola Basin, produced during glacial maxima, became isolated. This
"climax" bottom water was eventually mixed with the overlying water by geothermal heating.
