Knowledge of the relative motion between South America and Africa since their separation in the Cretaceous and the uniform relation of depth to age for almost all active mid-ocean ridges permits us to reconstruct the paleobathymetry of the South Atlantic. The match between the theoretical and present topography is good except for the aseismic Rio Grande and Walvis Rises. If the history of these two rises is known from drilling, realistic charts of the paleobathymetry of the South Atlantic can be constructed for 35 and 75 m.y. B.P. These charts show that between the initial break and 75 m.y. B.P., the South Atlantic formed four basins at depths below 4,000 m (Brazil, Guinea, Argentine, and Cape) separated by the mid-ocean ridge and the Walvis and Rio Grande Rises and closed to the north and probably the south. Between 75 m.y. B.P. (Late Cretaceous) and 35 m.y. B.P. (late Eocene), the Brazil and Guinea basins became open to the north and a deep-water channel appeared between the Argentine and Brazil basins. The bathymetry of the South Atlantic, including such features as the Mid-Atlantic Ridge and the Walvis and Rio Grande Rises, controlled all the deep-water current systems and probably much of the deep-sea sedimentation. Accurate paleobathymetric charts present a major constraint on possible flow patterns of Atlantic bottom water. They and sediments recovered from selected JOIDES deep-sea drilling sites may enable the quantitative determination of the past history of this current system in the South Atlantic.

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