The origin of the Nigerian continental shelf is linked to the origin of the Gulf of Guinea basin of the South Atlantic Ocean which began with the separation of South America from Africa about 180 million years ago. Continental separation of the two continental land masses resulted in the creation of recognizable continental margins and deep ocean basin which is marked by numerous fracture zones like Romanche, Charcot and Chain fracture zones.
The Nigerian continental shelf area is a narrow strip of land bordered by the gulf of Guinea of the Central Eastern Atlantic in the South. The zone lies within the Atlantic Ocean with its continental shelf, the Exclusive Economic Zone and the coastal fresh water and brackish wet lands ramified by an atomizing network of rivers and creeks. These water bodies are characterized by periodic tidal variations and ranges along water channels and the differences depend on the hydrological properties and slopes of the various channels (CEDA 1997).
The Nigerian coastal zone sprawls a total of nine coastal states (out of the thirty-six state of the Federation) namely; Akwa Ibom, Bayelsa, Cross River, Delta, Edo, Lagos, Ogun, Ondo and Rivers. The coastal states are estimated to account for 25% of the national population. The coastal areas stretch inland for a distance of about 15km in Lagos in the West to about 150km in the Niger Delta and about 25km East of the Niger Delta (fig. 8). The coastline stretches for about 853km comprising inshore waters, coastal lagoons, estuaries and mangrove especially in the Niger Delta.
The Nigerian continental shelf is grooved by three major canyons namely: Avon canyon just east of Lagos, Mahin canyon off the mahin mud coast and Calabar canyon off Calabar (Awosika et al 2001) (fig. 8). These canyons have been reported to serve as conduits for channelling of sand into offshore sub-marine fans on the slope. The overall nature of the Nigerian continental shelf is mainly depositional as compared to adjacent parts of the continental shelf along the Gulf of Guinea where Basement complex rocks appear close to the coast (CEDA 1997).
Figure 8: Generalized geological section of the Nigerian offshore (Allen, 1964, 1965).
The Nigerian coast and marine areas are under the influence of moderate oceanographic dynamics consisting of semi diurnal tides with tidal ranges varying from 1meter in the west to 3 meters in the east. The prevailing south-westerly waves vary from spilling to plunging waves. Long shore currents are prevalent in the near shore while the West East Guineas currents constitute the major ocean currents. The surface water off the Nigerian coast is basically warm with temperature generally greater than 24°c.
The Nigerian coastal environment is divided into four main physiographic zones, viz.
1. The Barrier Lagoon Coast which lies between Badagry and Ajumo east of Lekki town (fig 9).
2. The mahin mud coast lying between Ajumo and Benin river estuary in the North Western flank of the Niger Delta (fig 9).
3. The Niger delta lying between Benin River in the West and Imo River in the east (fig 9) and
4. The strand coastline lying between Imo River and the Nigerian/Cameroon border in the east with the Cross River inclusive (Awosika 2001) (fig 9).
Figure 9: The main morphological features of the coastal and adjacent land areas (Allen, 1965).