Pedogenetic calcretes from the Itaboraí Basin, Rio de Janeiro State, Brasil

Abstract

The Itaboraí Basin, located in Rio de Janeiro State, Brazil, is considered the first segment from the Southeastern Brazil Continental Rift opened, during the Paleocene. The occurrence of calcretes was previously reported in the literature, although studies dealing specifically with such calcretes and their distribution were not performed. Furthermore, there is limited research involving the petrographic characterization of the sedimentary deposits of the basin. This study developed a characterization on both macro and micro scale of the vadose calcretes that occur along the northern and western margins of the basin and constitute its initial infilling. The compositional, textural and fabric features of 27 thin sections prepared from calcrete samples were described and interpreted. The pedogenetic calcretes were separated in different facies (chalky, nodular, massive, platy, incipient laminar and brecciated), and classified according to morphogenetic stages. The main microscopic features include nodules, contraction cracks, vadose pisoliths, rhizocretions, infiltrated clays, corroded grains, expanded micas and clay aggregates. The pedogenetic calcretes were developed on weathered alluvial deposits during tectonic stability periods. They are typically generated on continental environments under arid to semiarid climates and occur intercalated with travertines (carbonate crusts of hydrothermal origin), which deposition is favored by fault activity during humid climate periods. The results indicate both pedogenetic and hydrothermal processes have been extremely active and have played important role during the early evolution of Itaboraí Basin. Thus, the characterization of such processes is essential to the understanding of the initial evolution of Southeastern Brazil Continental Rift.