Sediments on the continental shelf off Portuguese Guinea, Guinea, and Sierra Leone are mainly terrigenous, and have been stream derived. Except for a lobe of terrigenous material extending seaward from the Orango Delta, most sediments on the outer shelf and upper slope are rich in shallow-water biogenic carbonate and weathered glauconite, suggesting severe restriction in the offshore transport of detrital sediments, both at present and during the last low stand of sea level. In general the surface sediments are medium to fine sands, moderately to moderately well sorted and negatively skewed. For the most part these sands accumulated during the Pleistocene. However, the Holocene transgression and accompanying environmental processes have modified these sediments so that distinctive grain size parameters that characterized their original depositional environments can no longer be detected. Silts and clays on the Bissagos Delta front, derived from the Cacheo, and Geba Rivers and probably the Casamance River, are the only sediments presently accumulating on the middle or outer shelf in this area. Coastal and tidal currents may have moved some of this sediment toward the south, where it has been funneled locally into the deep sea.

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