Petrography, geochemistry and origin of South Atlantic evaporites: The Brazilian side

Abstract

The discovery and production, by Petrobras, of over 50 billion barrels in place of pre-salt oil in Brazil's offshore South Atlantic Santos and Campos basins has drawn worldwide attention to its km-thick Cretaceous salt seal since 2007. However, the depth of the pre-salt reservoir in these basins make prohibitive the costs of continuous coring or even extensive logging of the salt. The salt seal of the Santos and Campos basins forms part of the Cretaceous South Atlantic salt giant, the largest in the world, now divided between Brazil and southwestern Africa. Although our petrographic study is concentrated north of the Santos and Campos basins, we nevertheless discuss the age, facies, tectonic-paleogeographic controls and evolution of the entire Brazilian salt giant. Offshore Brazil, salt extends for 2.200 km from the Sergipe Basin in the northeast to the Santos Basin in the southwest. The Sergipe Basin at its NE end displays the full spectrum of evaporite cycles, spanning from carbonates, anhydrites and halites to the highly soluble hydrated Mg-chlorides carnallite, bischoffite and tachyhydrite, as does the Santos Basin in the SW part of the salt giant. The deposition of Mg-chlorides was terminated in Sergipe by an intra-salt unconformity at the carnallite/sylvinite contact, dated as 110.64 ± 0.34 Ma. In the intervening Espírito Santo and Campos basins, these highly soluble salts have not yet been found. Onshore Sergipe and Espírito Santo basins the entire salt sequence has been cored in several wells, including the highly soluble Mg–K–Ca chlorides.Here, we analyze the petrography and chemistry of cores in Sergipe and Espírito Santo. We prove the presence of tachyhydrite beds at both ends of the salt giant, in the Sergipe and Santos basins, but, at least for the time being, not in between the two basins. By comparing the presence of tachyhydrite beds in Brazil with similar evaporite sequences of similar age in Thailand, we defend that the high Ca/Mg and Ca/SO4 ratios in Cretaceous seawater was the de facto cause for tachyhydrite deposition in both regions. Ca/Mg and Ca/SO4 ratios of global seawater were raised by hydrothermal activity over basalts produced at exceptionally high rates in the Aptian along new mid-oceanic ridges and in oceanic plateaus such as Ontong-Java. The heat loss caused by this exceptionally high igneous activity may have been instrumental to the change of the thermochemical conditions across the core-mantle boundary that stabilized the Cretaceous Normal Superchron for nearly 40 Ma, from 123.4 to 121.2 Ma (2σ) to 83.07 ± 0.15 Ma (2σ) Ma. Aptian volcanic activity in the South Atlantic formed the Rio Grande Rise - Walvis Ridge that was the southern barrier of the salt basin, lava flows on the São Paulo Plateau, and basalts along the developing South Atlantic Ridge. Evaporite facies reflect cyclic changes on all scales when concentrating the depositing brines. We therefore analyze the effect of increases in brine concentration on the formation of salt crystals throughout multiple depositional cycles. In the later stages of the Brazilian salt basins, increased inflow of seawater from the Central Atlantic Ocean along the Equatorial pull-apart rift basins enlarged and deepened the existent brine lake while its salinity, and especially its Ca and Mg contents, dropped. Flooding by this less concentrated brine created an unconformity, leaching Mg and Ca from the carnallite and tachyhydrite previously deposited, and replacing them with secondary sylvinite. Our results can be applied to the essentially uncored salt sequence of the Campos and Santos basins, where igneous and hydrothermal activity provided additional sources of calcium. The Ca excess may have been increased still further by serpentinization of lithospheric mantle beneath hyperextended crust and by percolation of seawater through mafic rocks of the proto-Rio Grande Rise – Walvis Ridge that formed the southern barrier of the salt basin.