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.

巴西国家石油公司在巴西离岸的南大西洋桑托斯和坎波斯盆地发现并生产了超过500亿桶的预盐油，自2007年以来就引起了全世界的关注，因为它的千米厚的白垩纪盐封层。这些盆地中的预盐储层使连续取心甚至是盐的大量采伐的成本高昂。桑托斯和坎波斯盆地的盐封形成了白垩纪南大西洋盐业巨人的一部分，后者是世界上最大的盐业巨人，现在分为巴西和西南非洲。尽管我们的岩石学研究集中在桑托斯和坎波斯盆地以北，但我们仍然讨论了整个巴西盐巨人的年龄，相，构造古地理控制和演化。巴西近海，盐分延伸2。从东北的Sergipe盆地到西南的Santos盆地200公里。Sergipe盆地在其东北端显示出完整的蒸发岩循环，涵盖碳酸盐，硬石膏和卤化物到高度溶解的水合氯化镁镁钠盐，Bischoffite和Tachyhydrite，以及盐巨人西南部的Santos盆地。Mg-氯化物的沉积在Sergipe终止，原因是盐碱岩/银矿接触处的盐内不整合，日期为110.64±0.34 Ma。在圣埃斯皮里图托盆地和坎波斯盆地之间，尚未发现这些高可溶性盐。陆上的Sergipe和EspíritoSanto盆地的整个盐层序列都集中在几口井中，包括高度溶解的Mg–K–Ca氯化物。Sergipe盆地在其东北端显示出完整的蒸发岩循环，涵盖碳酸盐，硬石膏和卤化物到高度溶解的水合氯化镁镁钠盐，Bischoffite和Tachyhydrite，以及盐巨人西南部的Santos盆地。Mg-氯化物的沉积在Sergipe终止，其原因是盐碱岩/银矿接触处的盐内不整合，日期为110.64±0.34 Ma。在圣埃斯皮里图托盆地和坎波斯盆地之间，尚未发现这些高可溶性盐。陆上的Sergipe和EspíritoSanto盆地的整个盐层序列都集中在几口井中，包括高度溶解的Mg–K–Ca氯化物。Sergipe盆地在其东北端显示出完整的蒸发岩循环，涵盖碳酸盐，硬石膏和卤化物到高度溶解的水合氯化镁镁钠盐，Bischoffite和Tachyhydrite，以及盐巨人西南部的Santos盆地。Mg-氯化物的沉积在Sergipe终止，其原因是盐碱岩/银矿接触处的盐内不整合，日期为110.64±0.34 Ma。在圣埃斯皮里图托盆地和坎波斯盆地之间，尚未发现这些高可溶性盐。陆上的Sergipe和EspíritoSanto盆地的整个盐层序列都集中在几口井中，包括高度溶解的Mg–K–Ca氯化物。盐巨人西南部的桑托斯盆地也是如此。Mg-氯化物的沉积在Sergipe终止，其原因是盐碱岩/银矿接触处的盐内不整合，日期为110.64±0.34 Ma。在圣埃斯皮里图托盆地和坎波斯盆地之间，尚未发现这些高可溶性盐。陆上的Sergipe和EspíritoSanto盆地的整个盐层序列都集中在几口井中，包括高度溶解的Mg–K–Ca氯化物。盐巨人西南部的桑托斯盆地也是如此。Mg-氯化物的沉积在Sergipe终止，其原因是盐碱岩/银矿接触处的盐内不整合，日期为110.64±0.34 Ma。在圣埃斯皮里图托盆地和坎波斯盆地之间，尚未发现这些高可溶性盐。陆上Sergipe和EspíritoSanto盆地的整个盐层序列都集中在几口井中，包括高度溶解的Mg–K–Ca氯化物。

在这里，我们分析了Sergipe和EspíritoSanto岩心的岩石学和化学性质。我们证明了在Sergipe和Santos盆地的食盐巨人的两端都存在水苏石床，但至少暂时不在两个盆地之间。通过将巴西的水苏石床与泰国的相似年龄的相似蒸发岩序列进行比较，我们认为白垩纪海水中高的Ca / Mg和Ca / SO ₄比是这两个地区水苏石沉积的事实上原因。Ca / Mg和Ca / SO ₄通过热液活动，在新的中洋海脊和安高原等大洋高原的阿普提安地区，以异常高的速度生产的玄武岩提高了全球海水的比例。由这种异常高的火成活动引起的热损失可能有助于跨越芯幔边界的热化学条件的变化，该条件使白垩纪正时超稳定稳定了将近40 Ma，从123.4至121.2 Ma（2σ）到83.07±0.15马（2σ）马。南大西洋的Aptian火山活动形成了里奥格兰德上升-沃尔维斯山脊，这是盐盆地的南部屏障，熔岩流在圣保罗高原上，玄武岩沿发展中的南大西洋海脊延伸。当浓缩沉积的盐水时，蒸发岩相反映了所有尺度上的周期性变化。因此，我们分析了在多个沉积循环中盐水浓度增加对盐晶体形成的影响。在巴西盐盆地的后期，中大西洋沿赤道拉断裂谷盆地的海水流入量增加，使现有的咸水湖扩大并加深，而盐度，尤其是钙和镁含量下降。这种浓度较低的盐水淹没了不整合面，从先前沉积的角砾岩和水苏石中浸出了Mg和Ca，并用次氯酸钙石代替了它们。我们的结果可以应用于坎波斯和桑托斯盆地基本无核的盐序列，其中火成岩和热液活动提供了额外的钙来源。