The Niger delta is situated on the Gulf of Guinea on the west coast of central Africa (Figure 1). During the Tertiary it built out into the Atlantic Ocean at the mouth of the Niger-Benue river system, an area of catchment that encompasses more than a million square kilometers of predominantly savannah-covered lowlands. The delta is one of the world’s largest, with the subaerial portion covering about 75,000 km2 and extending more than 300 km from apex to mouth (Figure 1). The regressive wedge of clastic sediments which it comprises is thought to reach a maximum thickness of about 12 km.
Accumulation of marine sediments in the basin probably commenced in Albian time, after the opening of the South Atlantic Ocean between the African and South American continents. True delta development, however, started only in the late Paleocene/Eocene, when sediments began to build out beyond troughs between basement horst blocks at the northern flank of the present delta area. Since then, the delta plain has prograded southward onto oceanic crust, gradually assuming a convex-to-the-sea morphology.
Throughout the geological history of the delta, its structure and stratigraphy have been controlled by the interplay between rates of sediment supply and subsidence. Important influences on sedimentation rate have been eustatic sea-level changes and climatic variations in the hinterland. Subsidence has been controlled largely by initial basement morphology and differential sediment loading on unstable shale. The delta sequence is extensively affected by synsedimentary and postsedimentary normal faults, the most important of which
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Divergent/Passive Margin Basins
All basins in the Divergent Margin class occur along continental margins near a midplate position, where they overlie and straddle rigid continental and oceanic crust. These basins are now essentially aseismic and non-volcanic. All extant divergent margins are post-Traissic and are related to the sequential breakup of Pangea. They represent the opening ocean part of the Wilson cycle of earth history ? as Pangea split, continental plates moved away from the African center. The inboard roots of divergent margin basins lie on attenuated, subsided continental crust beneath the coastal plain, continental shelf, and continental slope. Outboard, they lie on subsided oceanic crust beneath the continental slope and rise. All phases of basin development are dominated by gravity-driven extension tectonics. Hence, they are called passive, pull-apart, Atlantic-type, or divergent continental margins. This publication was initiated by the AAPG Publications Committee in 1985 and contributors were invited to write. AAPG designed their ?World Petroleum Basins? series and sought to publish the definitive volume on each of several basin types. In this volume, “Divergent/Passive Margin Basins,” a detailed overview was written about the Campos Basin (Brazil) as representative of divergent margin basins. The key paper was followed by less detailed reviews of three other selected passive margin basins: 1. Northwest Shelf of Australia 2. Gabon basin and 3. Niger Delta area. Contributors undertook to provide useful geologic information on the regional setting, stratigraphy, structure, tectonics, basin evolution, and oil and gas systems of these four basins. The goal was to develop a better understanding of the basin-forming, basin-filling, and basin-modifying processes that control hydrocarbon plays and resultant oil and gas fields in this class of basin. The volumes in the AAPG Basin Series include: 1. Divergent/Passive Margin Basins (AAPG Memoir 48) 2. Interior/Cratonic Basins (AAPG Memoir 51) 3. Active Margin Basins (AAPG Memoir 52) 4. Foreland Basins and Foldbelts (AAPG Memoir 55) 5. Interior Rift Basins (AAPG Memoir 59).