Mineralogy and geochemistry of the clay fraction of sediments from the Namibian continental margin and the adjacent hinterland

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The continental shelf off Namibia can be divided into two sectors at latitude 18°40′S based on the distinctive morphologies of the two regions, and on differences in composition of their surficial sediments. The Kunene Shelf is narrow, shallow and covered mainly by terrigenous sediment whereas the Walvis Shelf is wide, deep and supports predominantly biogenic sediment.The Walvis Shelf also possesses a number of distinguishing features. Its outer region has a concave profile, which is attributed to upbuilding of the inner shelf during periods of lowered sea level. Its inner region is covered by an extensive belt of diatomaceous mud parallel to the coast. The seaward and landward flanks of this deposit differ in lithology due mainly to the advanced state of dissolution of opal in deep water, and to the influx of aeolian terrigenous detritus, especially illite, close to shore.Long belts of terrigenous mud extend southwards from both the Kunene and Orange Rivers. The currents responsible for transporting this sediment are poleward-directed tongues of the Angola-Benguela Front in the north, and mixed, poleward-moving South Atlantic Central Water in the south. The Kunene River carries more smectite to the sea than the much larger Orange.The distribution of clay minerals on the continental shelf follows that of the adjacent hinterland, with smectite, and to a lesser extent kaolinite, being significant in the north, and illite predominating in the south. Along the upper slope, the concentrations of smectite and kaolinite increase due to their importation from the Kunene River by poleward-directed Antarctic Intermediate Water.The geochemical character of riverine clays reflects the rock types exposed in each of the drainage basins. Compared with marine sediments, these clays are enriched in eight of the fourteen elements investigated, namely Al, Fe, Mg, Co, Mn, Pb, Rb and Zn. In contrast, the offshore element-distribution patterns show little resemblance to the patterns of individual drainage basins. Clays in the marine sediments are enriched, relative to the river sediments, in Ca, K, Cd, Ni and Sr.Five lithofacies recognized in the unconsolidated marine sediments are based on the relative proportions of opal, calcium carbonate, phosphorite, glauconite and terrigenous components. Average element concentrations of individual lithofacies are enriched, relative to the river clays, in Cd (diatomaceous facies); Ca, Cd, Ni and Sr (calcareous and facies); K and Ni, slightly, (glauconitic facies); Cd and Sr (terrigenous facies).Regionally, the clay fractions of unconsolidated marine sediments contain high concentrations of Al, Fe, Mg, K, Mn and Rb on the Kunene Margin due to aluminosilicates being imported by the Kunene River; Ca and Sr reach high values on the Walvis Outer Margin in response to upwelling along the coast; maximum concentrations of Cd are associated with diatomaceous mud on the Walvis Inner Shelf; Cu, Ni and Zn reach high levels at a few “spot locations” where organic matter (Morg) has suffered little dilution from other components, especially opal; and Pb is concentrated with glauconitic sediment on the Kunene Shelf.Semi-consolidated sediments are exposed intermittently on the outer margin. They provide evidence that the palaeoclimate became much drier in post-Miocene times, but in the north this picture is complicated by the introduction of sediment from the Kunene River during the Late Tertiary. Palaeo-oceanographic evidence suggests that the northern part of the study area experienced a concomitant decrease in productivity while in the southern part productivity increased.

Publication Title:

Marine Geology

Item Type:
Journal Article

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