Refletions on the Recôncavo Series, Brazil

Abstract

Chronostratigraphic units are of prime importance in geology. Their limits provide the basis for reliable chrono-correlations, both locally and globally. Of the several criteria used nowadays for the definition and characterization of chronostratigraphic units, fossil events are particularly important. This article presents a brief review of chronostratigraphic units and their applicability, with special emphasis on the Recôncavo Series and the current correlation of its stages with those of the Upper Jurassic-Lower Cretaceous. Some lines of research are suggested, including Rb/Sr radiometric datings by the total rock method. Limited evidence based on the latter challenges the classic correlation of the Recôncavo Series with the Upper Jurassic-Lower Cretaceous. This is the case of the Dom João and the base of Rio da Serra stages, dated radiometrically as Triassic. Lastly, it is suggested that the base of the Albian Stage (i.e, the top of Alagoas Stage) coincides with the base of a distinctive, thick salt layer (Ibura Event) which is widely represented in basins Santos through Sergipe-Alagoas. This interpretation contrasts with the new tendency noted in some recent Brazilian papers, which place the Aptian-Albian boundary within post-evaporite marine (carbonatic shelf) sediments of those same basins, on the basis of microfossils, particularly planktonic foraminifera. This apparent contradiction could simply reflect the diachronous LADs of some index species. The magnitude of the evaporitic event in the South Atlantic proto-ocean (originated from the Tethys Sea to the north) could have triggered major paleoenvironmental disturbances in oceanic surface waters over much of the planet. A possible consequence of such changes was the faunal turnover noted in foraminiferal successions across the Aptian-Albian boundary, in its type section and elsewhere in the world.

Keywords

• Chronostratigraphy
• Geochronology
• Recôncavo Series