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Antonio Barbosa, Universidade Federal de Pernambuco (Brazil)
Alcides Sial, Universidade Federal de Pernambuco (Brazil)
Virgínio Neumann, Universidade Federal de Pernambuco (Brazil)
Valderez Ferreira, Universidade Federal de Pernambuco (Brazil)
Marcio Pimentel, Universidade Federal de Brasília (Brazil)
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The two main Late Cretaceous coastal basins in northeastern Brazil, namely Paraí ba Basin and Natal Platform, possess carbonate successions and are separated by a basement high. A Maastrichtian age for Gramame Formation in the Paraiba Basin is supported by its fossil record, while the age of correlative carbonates of the Natal Platform is poorly constrained. While the Paraí ba Basin carbonates are composed of limestones and marls unaffected by late-stage diagenesis, the deposits of the Natal Platform represent sandy limestones, shales and calciferous sandstones, with features of late diagenesis as dissolution, dolomitization and strong recrystallization.
Based on XRF (Mg/Ca, SiO2, Al2O3, Mn/Sr, Sr, Rb) and CL analyses, six apparently pure carbonate samples from the Gramame and four from the Natal Platform have been analyzed for 87Sr/86Sr and results compared to the statistical LOWESS fit to the marine 87Sr/86Sr record Version 3 (2001) for age estimate. The five samples from Gramame Formation yielded values between 0.7080 and 0.7082, slightly above the range for the Late Campanian-Maastrichtian LOWESS curve (0.7076 to 0.7078).
This small 87Sr/86Sr enrichment is likely due to presence of clay minerals in tiny amounts or, perhaps, to inaccuracy of the LOWESS curve in this age interval. Likewise, three samples from the Natal Platform carbonate succession yielded values surprisingly much higher (0.7086 to 0.7088) than the range expected for Maastrichtian carbonates. As the age of these carbonates is poorly constrained, this deviation could result either from clay impurities, slight recrystallization or inaccuracy of the LOWESS curve.
However, one sample from each of the studied carbonate successions yielded a value of 0.7077, which fits into the expected range for the Maastrichtian interval. This study has confirmed limitations of this method since it requires extremely pure carbonates, entirely devoid of late diagenesis, alteration or recrystallization, something that may escape to even well-trained eyes.
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