Author(s): John Robert Laurence Allen
The Late Quaternary Niger delta in the Gulf of Guinea is a large, arcuate "classical" delta associated with marginal estuary and barrier island-lagoon complexes. The Late Quaternary deltaic lens (minimum volume 900 cubic kilometers) is the latest accretion within the Nigerian Coastal Plain geosyncline. Sediment originating in a vast and geologically complex hinterland is dispersed through the delta by river, tidal, wave, and ocean currents. Delta growth began during the Late Wisconsin lowstand of the sea when the rivers entrenched the continental shelf to reach mouths above submarine canyons at the shelf edge. The oldest stratigraphical unit of the Late Quaternary deltaic pile is a strand plain sand (Older Sands) representing a marine transgression into the hinterland. Later, in the Holocene, when sea-level became relatively stable, regressive advance across the sands of concentrically arranged delta environments gave rise to the Younger Suite, the uppermost stratigraphical formation in the deltaic lens. Lithofacies of this suite grade upward from open shelf clays, through pro-delta slope layered clays, silts, and sands, to well-bedded sands formed on the delta-front platform, river mouth bars, and beaches. Behind beach idge barrier islands fringing the visible part of the delta occur tidal mangrove swamps in which organic-rich sands and silts are being deposited. Cross-stratified river bar sands are accumulating in association with top-stratum silts and clays in the delta floodplain environment. The sedimentary framework of the Late Quaternary Niger delta is based on essentially concentric facies elements, as in many other "classical" deltas, rather than on radial elements as in the Mississippi birdfoot delta.