Print this pageABSTRACT: Sedimentary Basins on the Conjugate Margins of South America and Africa
George T. Moore
An Early Cretaceous spreading system formed the South Atlantic by separating South America from Africa along two subparallel major transform fault systems. The distribution of major sedimentary depocenters is controlled by the complex interplay of two factors: the late Mesozoic-Cenozoic cycle of sea-floor spreading and the legacy of a Precambrian collage of ancient cores that comprised western Gondwana.
Three spreading modes created this configuration: rift, transform, and subduction. Each produces a different geometry and tectonic framework for the accumulation of sediment. Rifted margins (60%) contain basins that are elongate, form with their depocenter axes inboard of the ocean-continent transition, and rest on a tectonically complex, foundered basement. Transform margins have abrupt ocean-continent transitions. Such margins (30%) may be sediment starved or contain a thick sedimentary section controlled by the volcanic ridges of transform faults. Off Tierra del Fuego, Burdwood Bank is bounded on the north by a fossil (aseismic) subduction zone. The associated basin is an elongate, deformed accretionary prism of sediments on a gently dipping, faulted oceanic plate.
The South Atlantic margins are divisible into 68 basins or segments that collectively contain over 33 × 106 km3 of syn- and postbreakup sediments. The South American margin contains 22 × 106 km3 in 46 basins, and the African margin, 11 × 106 km3 in 22 basins. The syn- and postrift sediment distribution forms two parallel bands that are elliptical in cross section and irregular in width.
Over 65% of the basins have a sediment column greater than 5 km with some depocenters that locally exceed 10 km. The source rock quality and character vary along both margins. The top of the oil generation window averages about 3.3 km; however, due to differing thermal histories, individual basins can depart significantly from this average. Six African and four South American basins each contain reserves in excess of 0.5 × 109 bbl of oil. A large volume of reserves in these major oil-producing basins is in settings dominated by delta-fan complexes.
AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990