PESGB/HGS 3rd International Joint Meeting
Africa : the continent of challenge and opportunity, London, 7-8th September 2004
Extended Abstract

Early Opening of the South Atlantic: Berriasian Rifting to Aptian Salt Deposition

Ian Davison, Earthmoves Ltd. Camberley, UK; Ray Bate, Global Exploration Services Ltd. West Sussex, UK; and Colin Reeves, Earthworks, Delft, Netherlands.
corresponding e-mail: i.davison@earthmoves.co.uk

Rifting Period

South Atlantic rifting (north of the Walvis Ridge) began during the Berriasian-Valanginian (127-142 Ma) period with the opening of the Benue Trough and the NE Brazilian rifts (Potiguar, Recôncavo, and Sergipe-Alagoas). Strata from this period are exclusively non-marine. Rifting of the rest of the South Atlantic margin is poorly dated, but believed to have occurred during the Barremian (127-121 Ma). No sediment older than Aptian has been found along the Equatorial continental margins of Brazil and Africa.

Barremian Age Marine Connection with Tethys

A late Barremian marine ostracod (genus Orthonotacythere) has been recorded from Congo (Grosdidier, 1967) whilst a second species of Orthonotacythere has been recorded from the early Aptian of the Kwanza Basin (Bate 1998). Both species are Tethyan in their origin, and they are considered to have migrated by means of a marine connection. Tethyan faunas are believed to have been introduced by ocean influx down the Benue-Termit-Tenere Rift system in Nigeria and Chad, or though the Parnaiba-Araripe-and Sergipe-Alagoas connection of NE Brazil (Fig. 1). The main rift system reaches up to 10 km deep in Chad, but only the upper non-marine section of late Aptian strata have been penetrated by drilling. The first marine incursion from the Central Atlantic Ocean (with Tethyan fauna) into the central Equatorial margin is dated as Mid Albian in Benin and the Potiguar Basin. Stemless crinoids originating in Tethys, and present also in Texas, occur in the early Albian of the Santos Basin, Brazil. They also occur in the late Albian of the Congo Basin. Southern Atlantic ammonite faunas are only considered to have migrated through into the Central Atlantic during late Albian times (Kennedy & Cooper, 1975).

The Pernambuco Plateau was the last topographic barrier between the South and Central Atlantic. Both the Falkland Plateau and the Walvis Ridge could have acted as barriers to marine incursion from the south during the Berriasian to Early Albian, although the first marine transgression took place across the South Africa and Argentina margins during the Valanginian (Reyment, 1980).

Aptian Salt Deposition

A Barremian age salt (500 m thick) is present in the Maculungo-1 well in the Kwanza Basin, but whether it is truly marine is not known, as lacustrine sediments occur both above and below the salt. Syn-rift Barremian strata were eroded and covered by Aptian age transitional sediments containing organic rich shales (Falcao) and transgressive sandstones (Gamba) The base salt horizon exhibits fault offsets with up to 2 km of relief in the main African salt basin and Santos-Campos basins. These may be residual fault scarps preserved at the end of the main rifting phase. No large fault offsets (> 1 km) are visible at the Top Salt horizon, except in the Sergipe-Alagoas Basin. The salt was deposited in a series of separate salt basins which were divided by a sub-aerial Mid-Ocean Ridge spreading centre, topographic basement highs (Pernambuco Plateau, NE Brazil) and deep basins that never dried up (e.g. Jacuipe Basin, NE Brazil) (Fig. 2). The Brazilian margin has four separate salt basins: a) Santos-Campos-Espirito Santo; b) South Bahia; c) Sergipe-Alagoas; and d) Ceara. The African Aptian salt basin is separated into three: a) Kwanza -Gabon-Congo-Angola, b) Rio Muni, and Douala. Rio Muni was separated from the main salt basin by the Ascension Fracture Zone. Lacustrine sediments are present above the Rio Muni salt, whereas Albian marine carbonates directly overlie the Aptian salt in the main salt basin. The salt is believed to be of the same late Aptian age throughout the South Atlantic, except for the earlier local Barremian salt in the Kwanza Basin.

References

Gosdidier E., 1967. Quelques ostracods nouveaux de la serie Ante-salifere (Wealdienne) des Bassins Cotiers du Gabon et du Congo. Revue de Micropaleontologie, 10, 107-118.

Bate, R.H. 1998. Report on the Pre-Salt Lacustrine Sediments of the Kwanza Basin, Angola. Unpublished Confidential Report.

Reyment R A. 1980. Paleo-oceanology and Paleobiogeography of the Cretaceous South Atlantic. Oceanologica Acta. 3: 127-133.

Kennedy W J & Cooper M. 1975. Cretaceous ammonite distributions and the opening of the South Atlantic. Journal.Geol.Soc.Lond. 131: 283-288.