Evaporitic carbonates in the pre-salt of Santos Basin – Genesis and tectonic implications

Abstract

Abstract Since the first discoveries of oil accumulations in the carbonate reservoirs of the Lower Cretaceous Barra Velha Formation of the Santos Basin, their genesis has been the subject of intense discussion. Different depositional models have been proposed, with some authors advocating a microbial origin, while others an abiotic origin as travertines or travertine-like rocks. This search for a modern carbonate analogue for these unusual rocks may have disregarded an intriguing alternative: an evaporative model supported by our petrographical and geochemical data. Evaporitic sediment patterns in endorheic basins, in a depositional framework of brine drawdown, are dominated by ephemeral salt pan hydrology. Bedded salt crusts form a stack of subaqueous aligned crystal beds at the sediment-water interface; salt grows diagenetically within saline and mud layers beneath the dry pan surface; finally laminated sediments are deposited. We observe a similar cyclic pattern in the pre-salt core we described: beds of shrub-like crystals and diagenetic spherulites growing in magnesium clay matrix are followed by laminites. The model we propose attempts to contribute solving the puzzle of Pre-Salt carbonate deposition by integrating sedimentology and geochemistry with basin tectonics. Our observations and basin tectonics point to the exciting possibility that the sag phase carbonates of the Santos-Campos basin were deposited as the initial/transitional stage of a closed sub-sea-level evaporite basin whose rising salinities culminated in the deposition of halite and bittern salts (carnallite, sylvite, tachyhydrite). Basinwide evaporites are singular events in the geological record. They form when a giant sub-sea-level basin becomes a sump of continental, marine and hydrothermal fluids, producing unexpected brine compositions in unusual depositional settings present in ancient evaporitic basins but absent from most modern ones. The novelty in this paper is that we apply to pre-salt carbonate deposition the sedimentology, hydrology and geochemistry of ancient evaporitic basins. With this concept in mind, we propose a geochemical model for the unusual pre-salt carbonate precipitation of the Santos Basin. Regarding the Walvis-Rio Grande volcanic high as a topographic barrier during the Aptian, we suggest that the main sources of calcium to the Barra Velha paleolake were hydrothermal brines, mostly from infiltrating seawater that reacted at depth with the basalts of the volcanic barrier. Hyperextension of the lithosphere, with a shallow Moho, increased geothermal gradients to levels favorable for the generation of hydrothermal CaCl2 brines, a process facilitated by the already elevated Ca++ ion and low sulfate concentration of the Cretaceous ocean. These CaCl2-rich fluids would pass through the volcanic barrier into a sub-sea-level paleolake in the rift basin. The high salinity paleolake would become saturated in calcium bicarbonate/carbonate as internally drained NaHCO3-bearing alkaline waters, formed by physical/chemical weathering of penecontemporaneous basaltic rocks, mix with the CaCl2-bearing hot brines percolating through the barrier, producing a hybrid brine (2NaHCO3 + CaCl2 = Ca(HCO3)2 + 2NaCl). Such a depositional system, integrating the geochemical-depositional controls of sag phase carbonates with those of the South Atlantic evaporites, has important implications for unravelling the genesis of these exotic deposits.