Depositional ages of Paleozoic and Mesozoic pre-rift supersequences of the Reconcavo Basin in northeastern Brazil: A Rb-Sr radiometric study of sedimentary rocks

摘要

The relative dating of sedimentary rocks can be carried out either from fossils or by stratigraphic correlation. In the absence of fossil content with unequivocal age significance, or in cases in which correlations are neither possible nor conclusive, the task of estimating the depositional age of a sedimentary rock is inviable. Rb-Sr whole-rock isochron method, combined with granulometric and X-ray diffraction studies, has been successfully used to date the deposition of fine-grained sedimentary rocks. In the present study, samples of argillites and siltstones of the Paleozoic and pre-rift sections of the Reconcavo Basin, in the northeastern coast of Brazil, have been investigated. Samples from the Afligidos (Cazumba Member), Alianca (Capianga Member) and Itaparica formations, yielded ages of 290 ± 21 Ma (Permian), 224 ± 32 Ma, and 226 ± 6 Ma (Neotriassic), respectively, which were interpreted as their original depositional ages. The age results for the units of the pre-rift section (Alianca and Itaparica formations) present significant implications to the paleogeographic model previously established for the Reconcavo Basin and other correlated basins, since these sedimentary units were traditionally considered to be Neojurassic.