2. Niger Delta setting
The Niger Delta is situated in the Gulf of Guinea on the passive Atlantic margin of West Africa and consists of a tripartite sedimentary sequence (Figure 2). During the Palaeogene and Neogene, it built out onto the Atlantic Ocean at the mouth of the Niger-Benue River system, a catchment area that encompasses more than one million square kilometres of predominantly savannah-covered lowlands (Morley, 1995; Adeonikpekun and Sowunmi, 2019). A recent comprehensive analysis by George et al. (2019) indicates that the extent of the Niger Delta is 70,000 km2, contrary to earlier publications that documented 75,000 km2 (Figure 2). The delta has also been remapped and classified into mega- sedimentary environments as follows: upper deltaic plain, lower deltaic plain, and delta front comprising 69%, 25%, and 6% of the total extent, respectively (Doust and Omatsola, 1990; Omuije et al., 2015; George et al., 2019). The regressive wedge of clastic sediments which comprises the delta is estimated to reach a maximum thickness of about 12 km (Okpoli and Arogunyo, 2020).
The palaeoenvironmental and sedimentological evolution of the Niger Delta has experienced dramatic changes over the Quaternary period (Doust and Omatsola, 1990; Reijers, 2011). The accumulation of marine sediments in the rifted basin undoubtedly commenced during the Early Cretaceous period (Albian time), following the opening of the South Atlantic Ocean as a result of rifting between the African and South American plates (Bonne, 2014; George et al., 2019). However, the current delta development only began in the late Palaeocene-Eocene when sediments began to bulge outside the syncline between the basement horst blocks at the northern flank of the recent delta setting (Figure 2). Since then, the delta plain has advanced southward onto the oceanic crust, possibly assuming a lobate morphology across the shallow offshore shoreline (Reijers, 2011). The proprietary concern of the petroleum companies prospecting in the basin and lack of published data have complicated the interpretations about the evolution of the delta for a long time.
The Niger Delta is normally categorised as tidally and wave-influenced in the classic terminology of Galloway (1975). The Cenozoic stratigraphic sequence comprises clastic sediments (Agbada and Benin formations) advancing over ductile marine shales (Akata Formation) (George et al., 2019; Figure 2). Previous studies indicate that the sedimentation pattern is composed of five depobelts (Figure 1) controlled by gravity-driven tectonism due to the presence of a mobile ductile shale at the base of the sediment fill (e.g., Corredor et al., 2005; George et al., 2019). The deltaic configuration has experienced broad progradation (circa 300 km) since the Palaeocene, with a maximum sediment thickness of approximately 12 km, and a surface area of 140,000 km2 (Doust and Omatsola, 1990; Stacher, 1995; Reijers, 2011; George et al., 2019; Figure 2). In addition, past investigations of marine core sediments obtained from the Niger Delta suggest that changes in the riverine input during the Late Quaternary were linked to global glacial-interglacial cycles (e.g., Riboulot et al., 2012; Adojoh et al., 2017, 2019; Adeonikpekun and Sowunmi, 2019). Periods of warm temperature over Greenland were found to be associated with enhanced river discharges in West Equatorial Africa and conversely, cold conditions were related to weak river input (Shannahan et al., 2007; Weldeab et al., 2007; Miller and Gosling, 2013; Adojoh et al., 2017, Adeonikpekun and Sowunmi, 2019).