Nigeria is underlain by seven major sedimenta- ry basins, viz: (from the oldest), the Calabar Flank, the Benue Trough, the Chad Basin, SE lullem- menden (Sokoto) Basin, the Dahomey Basin, and the Niger Delta Basin. Sedimentary succes- sions in these basins are of middle Mesozoic to Recent age. Older sedimentary deposits were not preserved, probably because during the Paleozoic - early Mesozoic, what is now Nigeria was a broad regional basement uplift, with no major basin subsi- dence for sediment accumulation. Nigerian basins are broadly divisible into coastal Calabar Flank, Niger Delta, Dahomey Basin and interior basins (Benue Trough, Chad Basin, Nupe Basin, SE lullemmeden Basin). However, the sedimentary successions in these basins are broadly divisible into:
(1) basal continental sandstones, siltstones, and mudstones;
(2) middle marine shales and limestones interbedded with sandstones and siltstones;
(3) upper sandstone sequence that is conti- nental or paralic.
Although a tripartite subdivision is also applicable to the Niger Delta, this sequence has been growing seaward with lower marine shales grading through thick ancient coastal sandstones and shales, into an upper continental sequence. The middle ancient coastal sediments are oil-and gas-bearing.