Abstract Davison Bate & Reeves. 2004

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Abstract Davison Bate & Reeves. 2004 2012-11-27T23:03:35+00:00

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.