Continental Margins of the Equatorial South Atlantic

Fainstein, Roberto; Ueipass Mohriak, Webster

The Equatorial Margins of Eastern South America and Western Africa are predominantly offset margins the result of shear motions that marked the Equatorial Atlantic since its early rifting. These offset margins suggest a remarkable shift that have affected the mid-ocean ridge formation during the rifting process. It would appear that this entire shear rift system followed closely the morphology of older continental structural trends, mainly of high grade metamorphic rocks.

The equatorial Atlantic Fracture Zones are amongst the largest in the world's ocean-floor. Offset between ridge crests in these fracture zones are of the order of 2,000 km, about the same distance of the West Africa coast re-entrance into the Gulf of Guinea. Similar distance is also seen along the trace of volcanic extrusions of the North Brazilian Ridge, suggesting that the main motion of these sheared margins occurred in the continent prior to the final separation between Brazil and Africa. The regional geophysical appraisal of the Northern Brazil and Guyana's upper continental margins are here investigated and compared with the margins offshore Ivory Coast and Liberia to establish some of the important concepts concerning the nature of these of sheared margins. As such magnetic maps from Liberia and Amazon display trends with similar wavelengths, and these are oriented normal to the respective coast lines, therefore representing magnetic sources that have remained intact during the Pan-African, Eburnean and Liberian thermo-tectonic events. We suggest that these anomalies represent Precambrian basement fabrics offset by the rifting of South America and Africa.

The Equatorial Margin of Eastern South America extends from the Guyanas to the Brazilian Bulge whereas the opposite Western Africa Margin extends from Ivory Coast to Nigeria. Vertical movements along the margins constructed the architecture of the corresponding sedimentary basins. Additionally, other factors may account for the thermal subsidence of these basins, such as volcanic cooling and sediment loading. Large rivers such as the Niger and the Amazon accounts for the great Tertiary sediment accumulation off Nigeria and the Amazon continental margins.

In the last decade, geophysical interpretation of modern seismic data sets have led to remarkable oil and gas discoveries in the deepwater realm of the Atlantic Equatorial Margin, offshore Nigeria and Ghana in Western Africa and off the Guyanas, Amazon and Barreirinhas basins offshore South America. This paper reviews the petroleum geology of these basins and also presents modern seismic imaging results from recently acquired deepwater datasets. These were integrated with ancillary geophysical profiles of magnetic and gravity for the purpose of understanding the nature of the deeper continental crust and mapping of pods with hydrocarbon source rocks yielding for new hydrocarbon prospects in the post-breakup sedimentary sequence.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013