--> Abstract: Tectonic Setting of Late Rift Stage Source Rocks of the West African Continental Margin, by G. D. Karner, N. W. Driscoll, and D. H. N. Barker; #90933 (1998).

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Abstract: Tectonic Setting of Late Rift Stage Source Rocks of the West African Continental Margin

Karner, Garry D. - Lamont-Doherty Earth Observatory, Columbia Univ.; Neal W. Driscoll - Woods Hole 0ceanographic Institution; Daniel H. N. Barker - Lamont-Doherty Earth Observatory, Columbia Univ.

The formation of the West African and Brazilian margins was the consequence of rift propagation principally across the region as a function of time. Rifting occurred in three phases: Berriasian to Hauterivian, Hauterivian to mid-Barremian, and late Barremian to early Aptian, recorded by an onlap surface within the basin followed by an overall regressive package representing the subsequent infilling of the basin. Rift phases 1 and 2 resulted in deep, anoxic, lacustrine systems. This spatial partitioning of extension, particularly during rift phase 2, was responsible for the development of two major tectonic hinge zones, the Eastern and Atlantic, that trend sub parallel to the Cabinda, Congo, and southern Gabon margins. In places, highstanding offshore blocks (e.g., Mayumba Spur) formed during rift phase 1 with possible reactivation during subsequent rift phases. Rift phase 2 developed a pronounced horst and graben topography. The associated infill includes a wide range of facies consistent with the varied rift topography. Whereas the onshore Eastern hinge zone demarcates the eastern limit of extension and separates continental margin sediments from Precambrian basement, the Atlantic hinge zone, located basinward beneath the outer shelf/upper slope transition, consists of a series of short, en echelon highstanding blocks. Seaward of the Atlantic hinge, individual rift basins also show an en echelon arrangement but with limited along-strike continuity. Deposition in these basins resulted in the production of a thick sediment wedge (the Outer basin Sediment Wedge; OSW). The wedge can be divided into three seismic facies units, a lower unit that onlaps/downlaps onto either Djeno equivalent or basement, an upper unit located farther to the west, and a relatively thin but regionally distributed rift infill package (Falcão source equivalent). Units 1 & 2 were ostensibly the consequence of the second phase of rifting, being faulted and rotated during the third rifting phase. Unit 3, a post-rift or rift infill package, consists of organic rich shales (Red Cuvo, Falcão, Marnes Noires, and Melania Shale source rocks) and regressive packages of the Argilles Vertes, Dentale and Gray Cuvo formations deposited in a brackish to marginal marine environment. While Unit 3 purportedly has sediment thicknesses of 4000-6000 m in the Kwanza basin, the equivalent sediments on the Congo margin are considerably thinner (600-800 m). Any faulting within these units appears to be associated with compaction and differential loading rather than a tectonic event. The top of the three syn-rift packages is bounded by a regional unconformity. Beneath the modern-day shelf this unconformity clearly truncates dipping reflectors of Unit 2 and presumably locally removed Unit 3. While this unconformity is generally referred to as the rift-drift uncon formity we attribute its formation to a eustatic fall since it post-dates the final tectonic phase. The overlying Gamba and Chela formations are transgressive sequences comprising fluvial sandstones at the base grading up into lagoonal facies, and clearly implying sub-aerial exposure followed by marine transgression.

The lagoonal facies of the upper Gamba and Chela Formations pass conformably into the evaporites of the regionally extensive Ezanga and Loeme formations. The observation of a regional shallow water environment at the end of rifting is critical in constraining the development of the margin and in particular the distribution of lithospheric extension with depth. The possibility of a mid-crustal detachment beneath the west African margin, active during rift phase 3 and controlling the syn-rift and early post-rift subsidence behavior there, has profound implications on the hydrocarbon maturation history of the margin, both between the hinges and in the deep water sub basins seaward of the Atlantic hinge.

AAPG Search and Discovery Article #90933©1998 ABGP/AAPG International Conference and Exhibition, Rio de Janeiro, Brazil