Depositional age of the Bananeiras Formation (Sergipe-Alagoas Basin): an attempt to apply the Rb-Sr radiometric method to sedimentary rocks

Abstract

The Rb-Sr whole rock isochron methodology has been successfully applied to estimate the age of argillaceous, fine grained sedimentary rocks. In many cases age values obtained by this methodology seem to have geological significance, because this method-ology provides ages in fair agreement with fossil content or with biostratigraphic interpretations. In spite of these promising results, some concerns regarding the extension and interpretation of the data still remain. This is due to the fact that sedimentary rocks result from the mixing of associated detrital fragments with different origins. Nevertheless, reasonable absolute ages of the depositional event can be obtained when criteria such as sampling, granulation, depositional environment and mineralogical composition of the samples are observed. In order to improve the methodology application to sedimentary rocks in the search for answers to some questions samples were collected from the Bananeiras Formation - the pre-rift shale section of the Sergipe-Alagoas Basin. The use of the Rb-Sr whole rock isochron methodology suggests that 227.1 ± 2.2 Ma (Late Triassic) is the depositional age of the unit. This unprecedented result has important implications to the paleogeographic schemes   previously   established, consider-ing that this pre-rift unit has been traditionally considered Late Jurassic in age.