Dual tectonic-climatic controls on salt giant deposition in the Santos Basin, offshore Brazil

The stratigraphic evolution of ancient salt giants is controversial, mainly due to the absence of modern analogues that are of comparable scale and thickness and that occur in similar tectonic and hydrological settings. Furthermore, investigating the original stratigraphy of salt giants is often made difficult by postdepositional flow and dissolution. Layered evaporites of the Ariri Formation in the Santos Basin (offshore Brazil), deposited during opening of the South Atlantic Ocean, form part of one such salt giant. Despite being well imaged in seismic data and being penetrated by more than 50 boreholes, little work has explored the stratigraphic architecture of this unit and what this may tell us about the syndepositional tectonics, basin physiography, and variations in climate and sea level. Here we integrate three-dimensional seismic and borehole data from the SAo Paulo Plateau, deep-water Santos Basin, to document the intrasalt stratigraphy of the Ariri Formation. Our analysis suggests a combination of an arid paleoclimate, low-amplitude local sea-level variations, and basin physiography controlled the deposition of this thick (2.5 km) salt sequence during a short time span (< 530 k.y.). The Ariri Formation records at least 12 cycles of basin desiccation and filling, resulting in the deposition of four key units (A1-A4) that have a distinct composition and therefore seismic expression; i.e., low-frequency, transparent, chaotic seismic facies represent relatively halite-rich (> 85%) units (A1 and A3), whereas high-frequency, highly reflective seismic facies represent still relatively halite-rich (65%-85% halite) units, but contain relatively high proportions (15%-35%) of anhydrite and bittern salts (i.e., K- and Mg-rich salts; A2 and A4 units). Our findings suggest that during salt deposition the Santos Basin was characterized by a series of subbasins of varying water depth; as a result the thickness and composition of these units vary laterally and are spatially related to structural domains. Overall, thinner salt (similar to 1.8 km) and higher anhydrite net thickness (similar to 350 m) occur toward the structurally high Sugar Loaf domain, compared to flanking, structurally lower domains where the mean salt thickness is > 2.2 km and anhydrite net thickness are less (similar to 180 m). In addition, stratigraphic variations in the basin suggest that seawater incursions came from the south, through the Sao Paulo and Walvis Ridges; consequently, more anhydrite was deposited closer to the ridge, whereas more bittern salts were deposited in more distal and restricted locations. The results of our study, although based on an analysis of Aptian salts preserved offshore Brazil, offer valuable insights into the sedimentology and stratigraphic architecture and evolution of other ancient salt giants.