New stratigraphic proposal of a Paleoproterozoic siliciclastic succession: Implications for the evolution of the Carajás Basin, Amazonian craton, Brazil

Abstract

In this study, we propose a new stratigraphic framework for a well-preserved Paleoproterozoic succession of the Carajás Basin (Amazonian craton) in northern Brazil. A core-based facies analysis is coupled with a critical review of previous data on this succession. We are proposing that the studied succession, which was previously considered as a single lithostratigraphic unit called the Águas Claras Formation, consists of two different formations of Paleoproterozoic age (~2.37–2.06 Ga). The lower formation, which is composed of ~250-m-thick mudstone strata locally enriched with manganese, is formally proposed here and is designated as the Azul Formation, referring to the Azul mine, in which the type-section is described. The overlying Águas Claras Formation is redefined as a stratigraphic unit composed exclusively of an ~800-m-thick succession of sandstone and conglomerate strata. The contact between these two formations is an easily recognizable surface; thus, these formations can be accurately mapped and distinguished within the basin. We suggest that the Azul and Águas Claras formations are the stratigraphic record associated with a transgressive‒regressive sequence, in which these formations are limited one from the other by a maximum flooding zone. The Azul Formation was deposited during a marine transgression related to the latest incursion of the Azul sea towards the Carajás protocontinent. On the other hand, the overlying Águas Claras Formation was deposited in a fluvial system that developed during a period of marine regression. Whereas the marine transgression may have been influenced by the deglaciation occurred aftermath the Siderian–Rhyacian Serra Sul glaciation, the subsequent marine regression was triggered, at least in part, by uplifting related to the Transamazonian cycle (~2.0 Ga). In addition to our results shed new light on the Carajás Basin evolution, they support the hypothesis that the Azul and Águas Claras formations can be correlated with other Paleoproterozoic successions worldwide, mainly those registered in ancient landmasses that amalgamated and later formed the Columbia supercontinent during the Paleo-to Mesoproterozoic.