This is an arcuate coastal basin, the onshore parts of which underlie the coastal plains of southwest Nigeria, Benin and Togo. A faulted basement high, the Okitipupa Basement Ridge separated the Dahomey Embayment from the Benue Trough until the late cretaceous subsidence and marine transgression united both basins. From available subsurface stratigraphic information, it is apparent that some of the basement blocks underlie the Dahomey Embayment are displaced towards the NNE-SSW basin axis as well as towards the offshore.
At the base of the sedimentary succession is the bitumen- bearing sand of enormous economic potentials. Shallow boreholes have penetrated continuous Late Cretaceous marine shales which are correlat- ed with the Nkporo Shale Formation. Nearer the coast and offshore, the marine beds are older. Lower Tertiary marine units (Palaeocene Ewekoro Limestone Formation and the Eocene phosphatic Oshosun Formation) are exposed in quarries at Shagamu and Ewekoro in Ogun State, and at Onigbolo and Tabligbo in neighbouring Benin Republic. Although the younger Tertiary strata exposed along the coast are non-marine, marine Miocene deposits (ljebu Formation, Afowo Beds) are known in the offshore subsurface.
In its sedimentary succession, the Niger Delta comprises a lower marine unit, the Akata Group; a middle coastal unit, the Agbada Group; and an upper continental sequence, the Benin Group. Each of these units represents an enormous geologic age span, because of the advancement of the Niger Delta oceanward. Ancient marine shales in the Niger Delta provide excellent source rocks for petroleum, whereas the sands are the reservoirs where oil and gas accu- mulation took place; trapping is mostly along faults.
Geologically, Nigeria comprises crystalline basement rocks and sedimentary basins, almost in equal proportions. The ages of the basement rocks span Precambrian to early Palaeozoic, having been consolidated during the Pan-African. The sedimen- tary basins are of Cretaceous to Recent age. While road construction and maintenance are compara- tively cheaper on basement terrain, the sedimenta- ry basins present enormous difficulties and lead to high construction costs, the situation being more challenging in the Niger Delta. However, the geol- ogy of Nigeria provides a unique source of petrole- um and mineral wealth, and unparalleled opportuni- ties for rapid economic development.