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Rifting, Subsidence and Evolution of Benguela Basin, Angola

Author(s): Alex Bump Fixme, Fixme
Bryan Gill Fixme
Chris Pearse Fixme
Teresa Sabato-Ceraldi Fixme
Nicki Adams Fixme

The Benguela basin lies on the rifted Atlantic margin of southern Angola, between the narrow Namibe basin and the broader Kwanza basin. Discovery of hydrocarbons in the conjugate Brazilian Santos and Campos basins has focused exploration on the pre-salt sections of Benguela and Kwanza and led to the acquisition of modern, high quality seismic surveys. In Benguela, these data show a crustal structure characterized by a narrow proximal margin and necking zone and very broad region of hyper-extended crust, averaging 2-3 km in thickness with apparent local windows where tilted growth strata appear to rest directly on mantle. These data also show evidence of at least 2 generations of faulting, with early, detached faults rotated to low angle and cross-cut by younger, high-angle faults that cut much or all of the crust. Close to the margin, these faults tip out well below salt, while farther outboard they reach and even offset base salt. The resulting tilted fault blocks are covered by variable thicknesses of pre-salt sediments, interpreted as Hauterivian extrusive volcanics, Barremian and Aptian clastics and Aptian lacustrine carbonates. The pre-salt sedimentary section is thickest close to the margin and thins toward paleo outer highs where salt rests directly on basement. Fossil and geochemical evidence indicates a progression in depositional environment from sub-aerial to increasingly saline waters, culminating in the deposition of a thick evaporate section before the establishment of normal marine conditions.

Integration of these observations with subsidence modelling and regional tectonic interpretations yields a model for the rifting and deposition of the prospective section: Rifting began with the peneplaned remnants of the Pan-African thrust belt and a topographic surface above sea level. The onset of extension is marked by the outpouring of Parana-Entendeka flood basalts at 132Ma. Extension initiated over a relatively wide and progressively focussed toward the line of eventual break-up. Initial deposits were sub-aerial and shallow-water clastics, deposited above sea level in an externally-drained lake. As rifting progressed and the crust subsided, lake level sank and drainage degraded, resulting brackish conditions and local deposition of coquinas. The volcanic Walvis Ridge separated the lake from marine waters to the south while continued rifting and subsidence caused lake level to sink below global sea level. Development of internal drainage resulted in increasingly saline waters and a mass-off, marked by an unconformity and condensed section. Establishment of hyper-saline microbes then led to deposition of carbonates within the photic zone, even as lake level subsided further. By the late Aptian, when marine waters began to overtop the Walvis Ridge, subsidence modelling suggests that lake level was perhaps 400-600 m below global sea level. Repeated marine incursions and evaporation led to the rapid deposition of 1-2 km of evaporates.

This model links observations of rift history and interpretations of subsidence to water chemistry and stratigraphic observations, creating a framework within which to understand exploration results and perhaps the development of other rifted margins.

Rifting, Subsidence and Evolution of Benguela Basin, Angola
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