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).