Print this pageA Quantitative Model for Salt Deposition in Actively Spreading Basins
Following the impact of the Tristan Da Cunha Hot Spot on the African-South American lithosphere, about 133 million years ago, the South American plate started to separate from the African plate at an average rate on order of 2 cm/yr. A narrow rift formed along the newborn plate boundary, filled with sediments brought by rivers flowing from either continent. At the beginning of the Aptian (120 Ma), the 1700 km-long northern half of this now marine extensional basin was isolated between the large volcanic swell built by the Hot Spot in the south (Walvis Ridge), and the Vema - Saint Paul Transform/Fracture Zones in the nascent Gulf of Benin to the north. The restricted depositional environment created by these two volcanic/tectonic “flood-gates” persisted for about 9 Ma, before perennial open-marine conditions were established at the beginning of the Albian (111 Ma). During this period, the rapid evaporation of sea-water created massive deposits of layered evaporites and salt, capping both oceanic and attenuated continental crust topped with rift and pre-rift sediments, while the two plates continued to spread apart in a roughly EW direction. The geometrical, kinematic and temporal environment of such tectonically governed salt deposition was strikingly similar to that of the Mid-Late Miocene Red Sea (15-5 Ma).
In most evaporite basin models it is generally assumed that salt deposition occurs in very shallow water, within sags of fixed geometry. Here we explore the possibility of salt deposition in deep marine environments with rapidly changing depths and widths, and the impact this might have on subsequent halokinesis or on the existence of potential plays above oceanic crust. We present a quantitative salt deposition model that takes into account the mass balance for fresh water, seawater, evaporation and salt precipitation, combined with continuous plate motion and the resulting increase of accommodation space. The set of differential equations obtained is solved numerically. The model is applied backward in time to calibrate parameters, based on seismic measurements of evaporite/salt layers thicknesses, basin geometry and plate separation rate. We discuss results concerning the main salt deposits in the Campos and Santos basins offshore Brazil, and in basins offshore Angola and Congo.
AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil