Wind-blown materials and W African soils: an explanation of the ‘ferrallitic soil over loose sandy sediments’ profile

Abstract

It is suggested that incorporation of dust from the hinterland during episodes in the Pleistocene when the Sahara encroached far S of its present limits accounts for much of the clay and Fe₂O₃ content of soils of southern Nigeria. The amount of pelagic deposition of clay reported for DSDP Site 366 on the Sierra Leone Rise in the last 1–3 million years is shown to correspond to the anomalous accumulation of clay in deep uniform subsoils of the ‘red ferrallitic soils over loose sandy sediments’. Particle-size analyses of soils at seven sites (showing 10% to 50% clay in the subsoil) are given, with determinations of Fe₂O₃ in the clay fraction (5% to 9%). Silt-size quartz in the dust is thought to have been lost through solution, and feldspars and clay minerals have weathered to kaolin. Wind systems displaced 600–800 km to the S compared with the situation today would have deposited dust similar to present-day Harmattan fall-out in the N of Nigeria, in the latitude of southern Nigeria and of the Sierra Leone Rise. Creep, mass-movement, faunal activity and eluviation of clay and Fe₂O₃ have affected the incorporation of dust, in addition to mixing coarser and finer layers of sediments together.