Northwest African Shelf Sediments: Influence of Climate and Sedimentary Processes

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Abstract
Shelf sediments off NW Africa are latitudinally zoned, with carbonate-rich sands predominant off Spanish Sahara, and more muddy and terrigenous sediments off Morocco. This zonation cannot be interpreted simplistically in terms of the present distribution of climatic belts, or zones of relative humidity, as previously thought, but results from the interaction of climatic factors (runoff, upwelling, swell development, wind direction), with tectonic factors (geology and physiography of the source and depositional areas), throughout the late Quaternary. Carbonate sands are widespread off the desert coast of the Spanish Sahara not only because the arid hinterland precludes fluvial terrigenous sediment supply, but also because the coast-parallel wind system prevents offshore aeolian transport of terrigenous detritus, the Holocene transgression was rapid, leading to the development of a widespread sand cover, and the seabed appears to be disturbed regularly by currents and wave action capable of preventing the settling of any fine terrigenous detritus which occasionally might be blown offshore. Where the winds blow consistently offshore, off the southernmost Spanish Sahara, probably aeolian terrigenous muds are quite common. Upwelling is intensified in the same region, resulting in organic enrichment of the sediments of this southern shelf and slope. Muddy and terrigenous sediments are widespread on the Moroccan shelf not only because rainfall is highest there, but also apparently because the physiography permits bypassing of the coastal zone by fine-grained terrigenous detritus, probably mainly during the flood season. Latitudinal zonations in the mineralogy of the sediments (e.g., a southward increase in montmorillonite), and the existence of relict muds off fossil rivers in the N Sahara, are the results of more pluvial hinterland climates during the last glaciation. Generalizations relating shelf sediments to climate should be made with caution, and this should be borne in mind also when examining the depositional environments of ancient shelf sediments. [Pinna ramulosa, Miniacina miniacea, Tellina, Turitella and Halimeda are mentioned.].

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