Holocene Coral Banks and Subsidence in the Niger Delta

The oldest deposits exposed on the Nigerian continental shelf are beach sands formed during the Holocene marine transgression into the hinterland. Three groups of narrow linear coral thickets lie on the top surface of these sands. Two groups extend with only one major break for 300 miles along the shelf. Near Lagos the banks occur at depths of approximately 26, 42, and 48 fathoms but gradually deepen by about 6 fathoms to the east. The corals are non-reef forms and are dead. They include species of Madracis and Den rophyllia, these being predominant, as well as Cladocora, Cyathoceras, Paracyathus, and Desmophyllum. Living faunas indicate growth temperatures colder than 20° C. and probably nearer 15° C. or 16° C. The thickets are reminiscent of deep-water coral banks off Morocco; a chart is included to show the distribution of comparable coral structures in the eastern Atlantic. The present thermal structure of the Nigerian seas suggests that growth of the thickets was localized by a horizontal thermocline layer, and that each group of banks represents brief stillstand of sea level during progressive relative deepening. Radiocarbon dates point to stillstands in this area terminating about 3,900 years B.P. and 2,900 years B.P. The present variation in depth of the coral banks suggests that differential subsidence of the continental margin accompanied the relative deepening, itself largely the result of down-warping. The thickets are shallowest just east of Lagos, and an axis of relative stability is inferred to cross the shelf in this area. Deposits of the Niger Delta are largely concentrated in a downwarped region bounded on the west and north by this axis; on the east by the physiographic-structural barrier of the Cameroon Mountains, Fernando Póo, and São Tomé.