Sedimentation in the deep-sea areas adjacent to the Canary and Cape Verde islands

Abstract

Bottom sediments between the eastern Canary Islands and Morocco, as revealed in piston cores, are principally of continental-shelf derivation with little material from the islands. The sediment is in general Globigerina ooze with silt-size quartz and much clay. Clay minerals are iron-rich chlorite, illite and locally abundant smectite. Glauconite, both authigenic within foraminifera tests, and detrital, is common. Carbonates are mainly from marine organisms but dolomite is entirely detrital. Sedimentation rates as determined by 14C dating are about 2–3 cm/1,000 years. In the inner Cape Verde deep-sea area terrigenous material is scarce. Sediments are composed of autochthonous marine organisms, mostly foraminifera, and/or material derived from the shallow-marine areas around the islands where biogenic carbonate is formed and reworked locally with volcanics. Layers of the latter material can be traced within the basin; their 14C ages correspond to low sea-level stages in the Atlantic within the past 35,000 years according to Milliman and Emery (1968). The mineralogy of the Inner Cape Verde Basin sediments studied is different from the Canary Islands samples. Quartz, as well as dolomite, is uncommon. Glauconite was found only within foraminifera chambers. The clay minerals comprise rare chlorite, almost no illite, and some smectite. Normally, the clay fraction consists of allophanic material. Amorphous silica is rather abundant and probably originated from radiolarians and sponge spicules. Carbonates (calcite, high-Mg calcite and aragonite) are biogenic. Volcanic sand mixed with the shallow-water carbonates is composed of abundant basaltic glass with magnetite inclusions, clinopyroxene and scarce olivine and feldspar. This particular sediment distribution within the Inner Cape Verde Basin reflects a combination of open marine conditions and a chain of volcanic islands forming a barrier against terrigenous sedimentation from the African continent. 14C ages indicate average sedimentation rates between 2 and 3 cm/1,000 years. In the Canaries the principal constituents are terrigenous. In both areas, but particularly in the Cape Verdes, sedimentation is discontinuous, because the steady accumulation of the normal marine material is interrupted by a sudden influx of reworked shallow-marine sand. Contrary to general opinion, little volcanic material is present in the sediments investigated of both the Canary and the Cape Verde islands.