The Formation of the South Atlantic Between Brazil and West Africa: Did It Un-Zip South to North or North to South?

Iain Scotchman¹, Gil Gilchrist², Paul Brockbank¹, Nick J. Kusznir³, and Alan Roberts^4
1Statoil (UK) Ltd, London, United Kingdom
²Consultant, London, United Kingdom
³University of Liverpool, Liverpool, United Kingdom
⁴Badley Geoscience, Spilsby, United Kingdom

Exploration drilling on the Sao Paulo Plateau (SPP) area of the deepwater Brazilian Santos Basin has revealed the presence of a thick (>2km) sequence of bedded lacustrine evaporites, including magnesium and potassium salts, above a mobile halite layer. These evaporites are of Aptian to at least early Albian age or possibly younger, on the basis of seismic interpretation. They appear in part to be contemporaneous with the shallow in-board marine Albian carbonate platform, present in the Santos Basin to the north. They are overlain by mid-late Cretaceous-aged turbidites which cover the carbonate platform elsewhere in the basin. The persistence of lacustrine / sabka conditions into the early Albian poses a problem for the current break-up model which predicts that this area lay under deep ocean by this time.

Current crustal reconstruction models for the South Atlantic Ocean between Brazil and West Africa during the Aptian envisage break-up moving systematically northwards from the Florianopolis Fracture Zone (FFZ) to the north-east tip of Brazil. However, this simple picture is difficult to reconcile with several key geological observations: the development of earlier rift basins further north such as Reconcavo and Tucano; evidence of a failed attempt at sea floor spreading in the centre of the Santos Basin north of the FFZ; the breaching of the Walvis - Rio Grande Ridge system only in the Cenomanian - Turonian; and biostratigraphic evidence for marine influence in pre-break up faunas in West Africa.

These key observations, combined with the thick, bedded evaporites observed on the SPP, suggest that the margin between the SPP and Angola was last section of the rift to break up. Consequently, the South Atlantic break-up may have been more of a ‘broken zip’ or may even have even un-zipped from north to south between Brazil and West Africa north of the FFZ.

AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery