Tectonic and stratigraphic development of the West African and eastern Brazilian Margins: insights from quantitative basin modelling
Authors: Karner Garry D. , Driscoll Neal W. 
Published: April 2008

By applying an integrated basin modelling approach that combines quantitative kinematic and isostatic basin modelling with the principles of sequence stratigraphy the tectonic significance of the major syn- and post-rift stratigraphic packages that comprise the West African and eastern Brazilian Margins have been determined. Rifting occurred in three phases: Berriasian-Hauterivian, Hauterivian-late Barremian and late Barremian-early Aptian, with each phase being recorded by an onlap surface followed by an overall regressive package representing the subsequent infilling of the basin. Each rift phase resulted in the formation of deep, anoxic, lacustrine systems. The spatial partitioning of extension was responsible for the development of two major tectonic hinge zones, an inner-onshore and outer-offshore hinge zone subparallel to the margin. Seaward of the offshore hinge, individual rift basins tend to show an en-echelon arrangement. Deposition in these basins resulted in the production of thick sediment wedges. Using the modelling results from the West African Margin, together with palaeontological and stratigraphic correlations, the tectonic history of the rift basins comprising the eastern Brazilian Margin (i.e. Jequitinhonha to Sergipe-Alagoas) can be inferred. For example, the unconformities bounding the rift packages within the Camamu-Almada Basins (late Berriasian, Valanginian-Hauterivian and late Barremian-early Aptian) are identical to those on the West African Margin. Further, the rift architecture of the eastern Brazilian Margin is controlled by an onshore and an offshore hinge zone, similar to that observed on the West African Margin.

The late Barremian-early Aptian depositional packages of the Congo margin are bounded by a number of regional unconformities. The unconformity separating the Marnes Noires and the Djeno formations registers the onset of the third phase of the rifting. The top of the third rift phase sediment package is bounded by two unconformities; a sequence boundary termed the pre-Chela unconformity associated with an early Aptian sea-level fall, and the slightly younger Chela unconformity produced during the ensuing flooding of the margin. The resulting Gamba and Chela formations are transgressive sequences comprising fluvial sandstones at the base grading up into lagoonal facies, and imply 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. Significant post-rift accommodation occurs in the same region characterized by only minor brittle, syn-rift deformation of the upper crust and negligible erosional truncation of earlier syn-rift units. This subsidence form implies that extension was partitioned as a function of depth, with plastic deformation thinning the lower crust with brittle deformation of the upper crust occurring to the west near the ocean-continent boundary. Lower crustal extension beneath an intracrustal detachment maintains the region near sea level during the final stages of the rifting process (i.e. as break-up is approached). These shallow water regions are extremely sensitive to small-scale eustatic fluctuations. Repeated marine incursion and desiccation cycles could feasibly explain the thick salts of the Ezanga and Loeme formations. The evaporites are thus late syn-rift deposits, placing the timing of continental break-up as early Aptian. The existence of a mid-crustal detachment beneath the west African margin, active during the late stages of continental rifting, has profound implications on the hydrocarbon maturation history of the margin, both between the hinges and in the deep water rift basins seaward of the Atlantic hinge.

Although the post-rift development of the West African and eastern Brazilian Margins was dominated by carbonate deposition, major rivers delivered clastics at select points along the margins. Given that local tectonics controls sediment delivery to the evolving rift basins, it is the development and interaction of continental drainage systems that ultimately determines the history, volume and distribution of clastic delivery to the post-rift development of the margin. In particular, the process of river capture in which the headwaters of one drainage system are captured from another is undoubtedly a common, but poorly documented, process controlling landscape evolution and margin development. We suggest that: (1) the antecedent Congo River System, which once flowed in the valley now occupied by the Ogooué River, migrated southward as a function of time, as evidenced by the age of the major unconformities and turbidite deposits developed along the margin. This southward migration from Gabon in the Senonian to Congo in the Oligocene Period predicts that reservoir formation along the margin is linked to the migration of the Congo River; (2) the Parnaíba River of eastern Brazil, being the major source of clastics for the northeast Brazilian Margin (Mundaú Basin), was captured by the São Francisco River in the mid-Eocene Period, abruptly reducing clastic delivery and the frequency of turbidites to the Mundaú Basin margin while accentuating clastic delivery to the Sergipe Basin Margin; (3) the southwest headward retreat of the Paraíba River, a subsequent northeast flowing river of southeast Brazil, progressively captured the consequent south and southeasterly flowing streams delivering clastics to the Santos Basin, which decreased the sediment supply to the Santos Basin while increasing clastic delivery to the Campos Basin during the Tertiary Period.