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87Sr/86Sr Isotope Ratios as a Tool for Stratigraphic Correlations in Pre-Salt Carbonates, Santos Basin, Offshore Brazil

Abstract

The pre-salt lacustrine carbonate sequences of the Santos Basin, Brazil, lack reliable time markers for field and basin-scale chronostratigraphic correlations. This is particularly evident for the closed lake deposits of the Cretaceous Barra Velha (BVE) Formation. Except for ostracods, the succession is largely devoid of body fossils that might facilitate stratigraphic correlation. Marker beds suitable for high-resolution age dating have not been identified to-date and magneto-stratigraphic approaches are not applicable. As a result, the basin evolution of pre-salt lakes is largely inferred from log and lithostratigraphic correlations. However, the substantial thickness variation of the BVE Formation across the basin accompanied by drastic lateral lithofacies variations might render lithostratigraphic correlations questionable. The potential of 87Sr/86Sr ratios to aid high-resolution chronostratigraphic correlation of lake deposits has recently been documented in Eocene and Quaternary lacustrine strata. We demonstrate the significance of Sr-isotope ratios for stratigraphic correlations and interpretation for Cretaceous pre-salt lacustrine sequences through integration of Sr-isotopes ratios from six wells with well (cores, SWC, BHI dip/azimuth) and seismic data. Measured 87Sr/86Sr ratios range from 0.71073 to 0.71373, but they show consistent and statistically significant differences in isotopic ratios between major stratigraphic units. The 87Sr/86Sr ratios also show a clear shift to more radiogenic values (0.708-0.714) across the top SK38 surface (i.e. the pre-Alagoas unconformity). The 87Sr/86Sr data from the overlying evaporites (SK50) indicate a trend to more marine values. Well correlations anchored to 87Sr/86Sr data and integrated with seismic data reveals that thick BVE units (i.e. SK44) are preserved in present-day structural lows and are thin or absent on the present-day structural highs. The missing strata on the highs is marked by jumps in the Sr isotope signal and is often substantiated by changes in BHI dip/azimuth data and lithofacies. Our results suggest that 87Sr/86Sr ratios can serve as a powerful tool to aid chronostratigraphic correlations of Cretaceous pre-salt lacustrine deposits to improve the understanding of the geological evolution of the lacustrine system.