ALBERT

Description: Eolian-sand turbidites form a clearly distinguishable "eolomarine" sediment facies. They are almost devoid of gradation, fine fraction, and mica, and have comparatively coarse sand medians. The sands consist predominantly of quartz grains, a significant portion of which show yellowish-red stains and frosted surfaces, both characteristic of desert sands in subtropical latitudes. The turbidite beds frequently reach thicknesses of 100 to 〉600 cm. A persistent fraction of calcareous shallow-water particles indicates a shoreline derivation. Eolian-sand turbidites occur off passive continental margins where active desert dunes have migrated seaward by dominantly offshore winds during (glacial) periods of low sea level. Accordingly, they indicate and date past environmental zones of minimum precipitation that migrate rapidly over the land and leave few datable traces. For example, during Pleistocene Weichselian time turbidites in the Gulf of Guinea resulted from a northwestward shift of dunes of the Kalahari desert across the Congo River. Similarly, an Early Miocene precursor of the Sahara is suggested for the region of the Spanish Sahara which then lay at 15-16 degrees N. This appears to be a Miocene analogue of the Pleistocene Weichselian desert. A model for the initiation of eolian-sand turbidity currents can start with the Recent situation of high sea level in northwest Africa where desert-dune supplied sand is trapped in a sand wedge at the shoreline, slowly prograding over the shelf platform. However, during a glacial low-sea-level stage, this wedge was located at, or below, the shelfbreak, on gradients of which 37% exceed 10 degrees at the ubiquitous slope-incision heads. A series of structural, petrographical and environmental features then generated repeated failures and slumps of gigantic quantities of sand that became turbidity currents. As they moved downslope, they may have cut or kept open numerous small slope incisions. On the continental rise they formed highly porous deposits of up to hundreds of cubic kilometers with a high potential storage capacity for hydrocarbons.