U-Pb ages of pre- to post-salt carbonates, Santos and Campos basins, SE Brazil: Implications for the evolution of the South Atlantic

Abstract

The exploration campaigns and independent studies conducted in the central South Atlantic basins after the discovery of giant oil accumulations in the Lower Cretaceous lacustrine carbonates sealed by the salt layer resulted in a new panorama of the tectonic framework, stratigraphic architecture, and petroleum systems of the investigated basins and passive margins in general. Despite these remarkable advances, the ages of the carbonate reservoirs and the sealing evaporites remain uncertain. Aiming to fill this gap, we present here the results of 30 U-Pb LA-ICP-MS calcite and dolomite age determinations representative of the depositional/eo-diagenetic processes and subsequent phases of carbonate cementation recorded by pre-to post-salt units of the Santos and Campos basins of southeastern Brazil. Microcrystalline calcite of a mudstone of the Itapema Formation (Santos Basin) yielded a minimum age of 126.8 ± 4.7 Ma. 19 depositional/eo-diagenetic ages obtained in mudstones, grainstones, shrubstones, spherulitestones, and travertines of the correlative Barra Velha and Macabu pre-salt lacustrine carbonates cluster around 124.1 ± 2.2 and 122.1 ± 2.3 Ma. Considering the errors, these results limit the deposition/eo-diagenesis of these units to the 126.2–120.0 Ma interval. Two determinations carried out in calcite laminae of a nodular anhydrite sample extracted from the lower portion of the Retiro Evaporite (Campos Basin) returned the ages of 119.2 ± 1.6 Ma and 119.18 ± 0.79 Ma, which we interpreted as the best approximation of the timing of the salt deposition. Analyses performed in 3 samples from the post-salt Quissamã carbonates (Campos Basin) provide a minimum age of 114.2 ± 2.8 Ma yielded by interparticle dolomite cement. Other phases of calcite and dolomite cements present in the investigated units were dated at ca. 119.4, 116.5, and 110.7 Ma. The stratigraphically coherent spectrum of ages obtained differs significantly from most available data, providing a new perspective for the ongoing discussions on the tectonic evolution of the South Atlantic and probably impacting the current views of the burial history and petroleum system of the Santos and Campos basins.