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/SO 4 ratios in Cretaceous seawater was the de facto cause...