Northwestern African continental shelf sediments

Abstract

An investigation of the gravel, sand, silt, and clay components and carbonate content of the bottom surface sediments on the northwestern African shelf and upper slope reveals that sand, composed of biogenic material, quartz, and glauconite, is the most common sediment type. This sand is primarily of biogenic origin between Ifni and Cape Blanc (21°N) and immediately south of Cape Verde (15°N) whereas the wide Guinea shelf is covered with low carbonate quartz sand. Glauconite is common on the outer shelf and upper slope. Modern mid-shelf silt facies occur immediately south of the Strait of Gibraltar, on the south side of Cape Ghir (30°N), between the Gambia and Geba rivers, and along the narrow Sierra Leone-Liberian shelf. Generally, illite, kaolinite, and montmorillonite are present in these facies with kaolinite predominant in the tropical areas. Modern mid-shelf facies are found where the shelf is narrow and the sediment supply is ample. Apparently bottom turbulence and current action are not sufficient to keep the outer shelf free of fine sediment accumulation. Where the shelf is wide, modern fine sediments are being deposited in estuaries and on the inner shelf. Also, a calcareous sediment type is found where the shelf has been tectonically stable and the sediment supply inadequate. Finally, carbonate values, less than 50%, occur where a modern shelf facies is being deposited or where clastic sediments were distributed during Pleistocene low sea levels.