--> Mesozoic Tectonostratigraphy of the Guinea Plateau, West Africa, Records Successive Rift Phases of the Central and Equatorial Atlantic

AAPG ACE 2018

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Mesozoic Tectonostratigraphy of the Guinea Plateau, West Africa, Records Successive Rift Phases of the Central and Equatorial Atlantic

Abstract

The Guinea Plateau, offshore Guinea and Guinea-Bissau, is a 100,000 km2 submarine plateau whose tectonostratigraphy records west-northwest rifting of the Central Atlantic during the Triassic and early Jurassic and east-west rifting of the Equatorial Atlantic. Interpretation of 24,000 km of 2D seismic profiles tied with 8 wells indicates the presence of three unconformity-bounded sequences of Late Triassic – Lower Jurassic, Aptian-Albian and Maastrichtian age. The oldest sequence of northwestward-dipping Seaward Dipping Reflectors (SDRs) likely corresponds to continental and shallow marine rocks interbedded with volcanics. This sequence is interpreted as syn-rift or very early post-rift. The overlying, 6-km average thickness Upper Jurassic – Lower Cretaceous sequence is a carbonate platform characterized by parallel, high-to-low amplitude reflectors. Wells drilled into this section confirm the presence of micritic limestone and dolomite interbedded with minor shale and sandstone. The carbonate platform extends 4,000 km along the northwestern margin of Africa and it is correlative with a carbonate margin of the same age on its conjugate margin in Eastern North America. On the Guinea Plateau, the top of this platform is marked by an angular unconformity with Cenomanian-Turonian rocks overlying Aptian rocks. An isopach shows that a 200-km-wide depocenter during the Upper Jurassic and Lower Cretaceous followed a north-south trend that we propose reflects an underlying rift formed during the Triassic – Jurassic rifting of the Central Atlantic Ocean. The overlying 2-km-thick Upper Cretaceous sequence includes siliciclastic facies with two interpreted deltaic lobes to the west and southwest. The top of this sequence is marked by an erosional angular unconformity that puts in contact Upper Cretaceous and Cenozoic rocks. Another Isopach indicates that a 400-km-wide depocenter during the Upper Cretaceous, followed a northwest trend that we propose reflects an underlying rift formed during the Aptian-Albian, oblique rifting of the Equatorial Atlantic Ocean. The continuous, steady sedimentation at the Guinea Plateau during the Mesozoic indicates a period of post-rift subsidence. Source rocks in the area include Lower Jurassic, Aptian and Turonian-Cenomanian. Only Aptian and Turonian-Cenomanian source rocks are proven. Known reservoirs in recent large discoveries along the Senegal - Mauritania coast include sandstones of Albian age.