Herkunft und Verteilung von Oberflächensedimenten des westafrikanischen Schelfs und Kontinentalhanges

Samples from surface sediments of the shelf, continental slope, and adjacent deep sea regions off West Africa between 27° N and 15° N were investigated with respect to grain size distribution of the total samples, sand contents of the acid insoluble residues, carbonate content of the total samples, and the separate grain size fractions, specific surfaces, colours and mineralogical composition of the clay fractions. The grain size distributions of the total samples of the sediments of the shelf and the continental slope off Spanish-Sahara are controlled mainly by biogenic components. The supply of terrigeneous material in this area is very low. At deeper parts of the continental slope and in the deep sea areas, the relative amounts of carbonate minerals in the sediments are considerably reduced. The prevailing sand contents of the upper slope changes into clay dominance. On the shelf off Mauritania - represented by profiles extending down to 200 m of water depth - the grain size is also controlled mainly by biogenic carbonates. Nevertheless, the admixture of fossil silicate components is important, too. The southern parts of the area investigated are located in a region influenced by sediments of the Senegal River, which especially control the contents of silt and clay. The silicate sands, predominatcly of quartz, are fossil and form a mixed sediment with younger deposits. The carbonate contents of the different grain size fractions are formed either by sedimentation of carbonate and silicate particles of the respective grain sizes or by autochtonous disintegration of coarser sediment particles, as shown by the occurrence of Mg-rich calcite and especially aragonite in the clay sized fractions. In the northern parts of the area investigated, which have very minute terrigeneous supply, the latter mechanism is the dominant factor, controlling the carbonate contents of the fine grain sized fractions. In the vicinity of the mouth of the Senegal the carbonate contents are influenced by extremely high dilution with terrigeneous silicates. Mg-rich calcite and aragonite are produced preferentially in shallow slope and shelf areas up to 500 m of watet depth. The spccific surfaces of the carbonate-frce clay fractions indicate that the clay fractions of the shelf areas with little terrigeneous supply consist of relatively coarse particles. Very fine particles are removed and transported towards the deep sea. Lateral differentiation of this kind was not observed in the area off Senegal. The high surface areas, characterizing the clay fractions of this region, are thought to be due to high montmorillonite contents as was also found for deep sea sediments. The mineralogical composition of the clay fractions from the southern parts of the area is characterized by high kaolinite and montmorillonite contents, while in the north illite is predominating. At least two types of montmorillonites are present: in areas influenced by the Senegal mostly one type was found, which could swell to 17 A; on the shelves and slopes of the other regions the montmorillonitegroup is represented by a montmorillonite-micatype mixed-layer mineral. A "glauconite", found in the sand fractions, which bad very similar properties to those of the montmorillonite-mica mixed-layer, is believed to be the source of this mixed-layer-type mineral. Palygorskite is present in all samples out of range of the Senegal supply. lt may be an indicator of eolian transported material. The occurrence of rich palygorskit deposits in the arid hinterlands emphasizes the terrestrial origin