Northwest African Shelf Sediments: Influence of Climate and Sedimentary Processes
Shelf sediments off northwest 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 relativ...
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Veröffentlicht in: | The Journal of geology 1976-05, Vol.84 (3), p.277-300 |
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Zusammenfassung: | Shelf sediments off northwest 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. For example, 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 (i) the coast-parallel wind system prevents offshore aeolian transport of terrigenous detritus, (ii) the Holocene transgression was rapid, leading to the development of a widespread sand cover, and (iii) 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 quiet common. Upwelling is intensified in the same region, resulting in organic enrichment of the sediments of this southern shelf and slope. As another example, 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 (for example a southward increase in montmorillonite), and the existence of relict muds off fossil rivers in the northern Sahara, are the results of more pluvial hinterland climates during the last glaciation. Our data reemphasize that 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. |
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ISSN: | 0022-1376 1537-5269 |
DOI: | 10.1086/628196 |