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The southwesterly prevailing wind of the Gulf of Guinea strikes symmetrically on the nose of the Niger delta, causing divergent longshore drifts which meet opposing drifts near Lagos and Fernando Poo. Submarine canyons channel about 1 million cu m of sand a year from each pair of opposing drifts to feed submarine fans on either side of the delta foot. In times of low sea level axial distributaries of the Niger feed a third, now inactive, submarine fan in front of the delta.
At the time of the last low sea level numerous submarine canyons formed on the front of the Niger delta, and their heads cut back into the Benin Formation continental sands. As the sea rose, these canyon heads formed wide estuaries which have since been filled. All Quaternary canyons except three currently scoured by resedimented longshore drift material have been filled.
Because the Niger delta has prograded toward the southwest throughout the Tertiary and because the prevailing wind has blown persistently from the southwest, the longshore drift pattern long has been as it is now, and the two corners of the delta have been areas of high concentrations of submarine canyon formation. There may have been a third area of high concentration of submarine canyons between the Cross River and Niger deltas when these were separate.
Recognition of the submarine fan environment leads to a new symmetrical five-layer interpretation of the structure of the Niger delta: (5) top continental sand unit (Benin Formation); (4) transitional sand/shale unit (Agbada Formation); (3) marine shale unit (Akata Formation); (2) transitional shale-sand unit (newly distinguished); (1) bottom submarine fan sand unit (newly distinguished). Other deltas feeding into waters of oceanic depths may have a comparable structure.
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