Tectono-Sedimentary Evolution of the Early Cretaceous South Atlantic Rift and Sag Basin: From Carbonate Reservoir to Stratified Evaporite Seal

Szatmari, Peter ¹
(1)Research Center (Cenpes), Petrobras, Rio de Janeiro, Brazil.

The discovery of giant pre-salt oil fields in the Santos Basin, on the Brazilian side of the South Atlantic, requires a new tectono-sedimentary synthesis. South Atlantic rifting from earliest Cretaceous times led to the formation of fresh-water lakes which became deeper and more isolated as faulting progressed. As the climate became drier, the lakes became saline. Rift-related subsidence exceeded siliciclastic sediment inflow, except in the proximity of the border faults.

Ultimately an almost continuous alkali lake over 1000 km developed whose fluctuating surface lay >500 m deep below sea level. Inland groundwater rich in Ca drained into the lake, as well as hot springs from the still active Paraná Continental Flood Basalt Province. Ca in the inflowing water reacted with the Na-carbonate-rich water of the lakes, causing rapid deposition of Ca-carbonates, aided by microbial activity.

After lacustrine carbonate deposition, sea water entered the basin intermittently form two directions. From the Central Atlantic, it flowed over the Equatorial land connection between South America and Africa; from the southern South Atlantic, between Argentina and southern Africa, it percolated through and flowed over dike-related volcanoes of the Paraná Continental Flood Basalt Province. The entering sea water evaporated under the arid climate, depositing salts in cycles of anhydrite (relatively scarce), halite, carnallite and tachyhydrite (aqueous K-Mg and Ca-Mg salts, respectively). The South Atlantic evaporites are today the largest mass of evaporites in the world.

CO2 emanated into the carbonates from the Paraná volcanic complex and underlying mantle, it was retained beneath the evaporite seal leaching the carbonates already structured by stratification and upward growth, increased their permeability and created excellent reservoirs.

Throughout rifting and evaporite deposition, South America rotated clockwise about the Amazonian Craton as both the Central Atlantic Ocean and the South Atlantic Rift continued to widen. Eventually, the Equatorial land connection between South America and Africa was broken up by the huge extensional transforms of the St. Paul, Romanche and Chain Fracture Zones, permitting unhindered access of seawater from the Equatorial Atlantic. Thus, at the end of Aptian times, evaporite deposition ceased and carbonate deposition recommenced, this time in a marine environment.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.