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The geology of the Namibe Basin, Angola

Author(s): Roger Swart BlackGold Geosciences, Namibia
Cai Puigdefabregas University of Barcelona/NTNU, Spain
Erik Holtar Global Exploration, Statoil, Oslo, Norway
Now at: Det Norske Oljeselskap ASA, Oslo, Norway

The onshore Namibe Basin provides a unique insight into the development of South Atlantic rift systems, It is the southernmost of the Angolan Atlantic margin basins, bordered to the south by the Walvis Ridge and to the north by the transform faulted Benguela Basin. The Namibe Basin is an oblique rifted margin - essentially the conjugate margin to the Santos Basin and Sao Paulo Plateau province of Brazil. The outcrop belt is a narrow 3-20 km wide and 220 km long coastal strip. Mapping of the basin was undertaken from 2009-2011 and a revised stratigraphy and map for the basin have been compiled.

The oldest rift related deposits are a sequence of basalts, acid volcanics/volcaniclastics and intrusives with aeolian-fluvial sediments (Bero Volcanic Complex). The complex is an extension of the 132 Ma Parana-Etendeka province, confined to the southern half of the area and resting with an angular discordance on the Precambrian basement.

After eruption erosion resulted in a topography that was later infilled by clastics (Tumbalunda Fm), carbonates and travertines (Cangulo Fm). Age constraints are poor, but these units are overlain by Aptian-aged evaporites (Bambata Fm), an equivalent of the Loeme Fm further north. The evaporites comprise a 40-?0m thick package of gypsum-anhydrite. In large parts of the basin this unit has been completely removed by later dissolution.

Relatively thin (10-50m) carbonates with marine and non-marine clastics (Gaia Fm) overlie the evaporites. These are age equivalent of the Pinda Group carbonates and post-salt carbonate systems of the Brazilian margin.

The Gaia Fm is overlain by a thick (locally 1500m) Albian to Early Cenomanian succession of clastics (Giraul Fm). Alluvial fans and braided

rivers were shed over large areas at this time. In the northern part, west of Lucira, there are carbonates and marine sandstones, time equivalent to the Giraul Fm, which represent the transition to a marine environment (Santa Marta Fm). Near the Inamagando River, a progradational mixed carbonate-clastic succession is exposed (lnamagando Mbr). This has an upper contact with alluvial-aeolian clastics.

Onlapping and overstepping the tilted Albian and older units is a succession of fossiliferous marine to non-marine conglomerates, sandstones and carbonates of Latest Cenomanian to Turonian/Early Coniacian age (Salinas Fm). The sediments of the Salinas Fm are intruded and overlain by basanite rocks (Bentiaba Fm), dated as 88Ma in age (Coniacian). Locally subaerial to lacustrine carbonates include travertines, spring mounds, fissure ridges and silicified trees which grade laterally into lacustrine carbonates and mudstones (Mariquita Member).

The Bentiaba Fm. is onlapped and overstepped by a transgressive wedge of fossiliferous Santonian to Campanian age sandstones (Piambo Fm). This unit forms an unfaulted, transgressive to regressive sequence of shelf margin, shoreface to lower delta plain sediments. A younger Maastrichtian transgression has a greater landward extent and consists of a simple transgressive-regressive cycle. At Chapeu Armada there is an undated nephelinite complex, which appears slightly younger than the Bentiaba Fm (Chapeu Armada Volcanic Complex).

The post Cretaceous stratigraphic record is represented by Eocene and younger shoreface to shelfal wedges and both marine and river terrace deposits.

The geology of the Namibe Basin, Angola
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