New age and lake chemistry constraints on the Aptian pre-salt carbonates of the central South Atlantic

Abstract

The Cretaceous lacustrine carbonates of the offshore Brazilian and West African pre-salt basins represent some of the most extensive non-marine carbonates discovered in the geologic record. Despite being intensively studied over the past decade, the age of these carbonates and the overlying regional salt sequences is highly controversial. Similarly, the conditions under which these carbonates were deposited remains poorly understood. Here, we provide the first integrated geochronology-thermometry study of these carbonates to develop an improved understanding of when and under what conditions they formed. We utilize carbonate clumped isotope and ⁸⁷Sr/⁸⁶Sr geochemistry alongside traditional petrographic techniques to identify samples minimally altered from burial diagenesis that may yield reliable age and lake chemistry constraints. Carbonate clumped isotope apparent temperatures for the studied carbonates range from 36 ºC to 91 °C, which we infer to represent a range in sample preservation from minimally altered depositional temperatures through to those that have been overprinted by burial diagenesis. ⁸⁷Sr/⁸⁶Sr values of our samples are consistent with those of previous studies for Cretaceous pre-salt carbonates that have not experienced significant alteration from hydrothermal fluids. Through this approach, we measured the first high resolution isotope dilution U-Pb age constraint of 115.83 ± 1.56 Ma (2σ) on a well preserved carbonate. Combined with overlapping lower resolution laser ablation U-Pb ages for time-equivalent stratigraphy on two separate carbonate platforms of 114.46 ± 4.72 Ma and 109.73 ± 9.26 Ma, these ages provide the first robust direct age calibration for pre-salt carbonates deposited on either side of the South Atlantic during the final stages of the break-up of Gondwana in the Early Cretaceous. These ages also provide the first calibration for a combined ⁸⁷Sr/⁸⁶Sr-facies-log based relative age framework within the Santos Basin, offshore Brazil. We further utilize δ¹⁸O constraints on samples that yield depositional clumped isotope apparent temperatures to constrain the δ¹⁸O of the water in these ancient lakes to between 1.9 and 4.9‰_(Vienna standard mean ocean water). Such heavy values reveal a picture of a hot and arid environment. This is consistent with prior biostratigraphic studies of the carbonates that show a decrease in faunal diversity in these lakes prior to marine ingress and the development of open marine conditions in the South Atlantic Ocean.

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