Tectono-stratigraphic evolution of the offshore Benin Basin, SW Nigeria
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This thesis presents an evaluation of the tectonic and stratigraphic evolution of the offshore Benin Basin based on the analysis of 2D and 3D seismic data, in addition to exploration well data of four closely-spaced wells. To date, little has been published on this part of the African Equatorial Atlantic margin because of the lack of publicly available datasets, and this is the first academic study to be provided with 3D seismic data from this basin. The aims of the study were to establish a tectono-sequence stratigraphic framework for the basin, and to establish the timing, distribution, and nature of tectonic and gravity-driven deformation events. This tectono-stratigraphic evolution was then compared to adjacent and formerly adjacent basins to better understand their significance in relation to the opening of the Equatorial Atlantic and its post-drift history. Five megasequences have been identified which relate to key tectonic phases in the basin history, and/or major changes in its genetic stratigraphic behaviour. These sequences have been subdivided into seismic sequences. The megasequences are MS1- pre-rift (Precambrian - Barremian); MS2 - syn-rift (Barremian - late Aptian); MS3 - Cretaceous post-rift (late Aptian - latest Maastrichtian), MS4 - transgressive (latest Maastrichtian - middle Miocene) and MS5 - regressive (Middle Miocene - Holocene). Rifting by N-S orientated orthogonal extension took place in the Barremian to late Aptian. This led to the formation of two synchronous asymmetrical half-grabens named the northern and southern half-grabens. The rifting ceased in the offshore Benin Basin in the late Aptian (MSB3). This syn-rift megasequence is subdivided into three sequences which can be interpreted to represent rift onset, and rift climax phases, probably associated with the growth, linkage, and abandonment of normal fault systems. There is no evidence in these sequences for transform-related tectonics. Several short-lived episodes of contractional deformation probably occurred from late Aptian/early Albian to Cenozoic, with the Cretaceous events studied in detail in this thesis. The localised late Aptian/early Albian deformation is restricted to the northern half-graben, and immediately postdates the syn-rift phase. The late Aptian/early Albian deformation formed four structures: Elo thrust, Oga fold, Iro transfer and Ore thrust structures. The E-W striking basin-bounding normal faults (F1 and F2) were not reactivated. However, a NE-SW striking rifts transfer fault was probably reactivated by NW-SE shortening. The late Aptian/early Albian event immediately post-dated rifting and suggests a shift in regional stresses, which may be related to a change from N-S extension to the NE-SW shearing that led to the final separation of plates along the Equatorial Atlantic margin. The second deformation event occurred in the Santonian (SB3F), and led to the development of the Eji anticline. Despite their age differences, their structures strike in the same NE-SW direction. The late Aptian/early Albian contraction resulted in mild inversion causing intense buckling of the roll-over anticlines associated with the basin-bounding normal fault (F1). Deformation was accommodated by thrusting and folding on the hanging-wall of the basin-bounding normal fault that remains in net extension after the deformation. Contractional deformation propagated to the south in the Santonian (SB3F) reactivating new thrusts and folds. The Santonian event is basement-involved (thick-skinned). Both deformations were probably caused by the reactivation of pre-existing zones of lithospheric weakness due to change in plate motion. A change in motion of the Nubian block from originally N-S to NE-SW direction towards the European Plate has been suggested as the cause of these structures. Both extensional and contractional structures in the study area can serve as potential traps for petroleum. The data presented in this dissertation are consistent with a two-stage opening model (as proposed by Fairhead et al., 2013) rather than an oblique model (e.g. Heine and Brune, 2014) for the Equatorial Atlantic. This involved an initial phase of N-S orthogonal extension (Barremian-Aptian) which extended into the Central African Rift System and Portiguar Basin (Brazil) and a later NE-SW tranpressional movement, accommodated along evolving transform faults, in the post-Aptian.
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