Estudo sedimentológico dos sedimentos Barreiras, Ipixuna e Itapecuru no nordeste do Pará e noroeste do Maranhão

Abstract

Barreiras, Itapecuru and Ipixuna are exposed in large regions of Pará and Maranhão State. The Barreiras sedimente are divided in conglomeratic, sandy clay and sandy lithofacies. Textural imaturity and abundant mud-s.upported clastics in particular, suggest depositions mainly by debris flows under semiarid conditions. The three lithofacies were probably derived from Precambrian schists of Santa Luzia Formation and preexisting sedimente. The Itapecuru sedimenta consist of crossbedded sandstones, locally with minor conglomerates and mudstones, deposited in fluvial environment, probably tending to a semiarid conditions. The Ipixuna facies consiste of crossbedded kaolinitic sandstones, minor mudstones (lithology A) and laminated mudstonefine sandstone units including thick kaoline layers (lithology B). The fine sandstones are texturally and mineralogically mature. Lacustrine-fluvial origin is proposed for Ipixuna facies. The impoverished heavy-mineral assemblage and thick kaoline-layers suggest humid hot climate that probably prevailed before (during?) the Ipixuna sedimentation. Textural and structural characteristics of the sedimente studied show that there is a olear difference between Barreiras Group and Ipixuna facies and that the latter is correlated with the Itapecuru Formation. Bauxitization of lower Tertiary age affected only the Ipixuna and Itapecuru sedimentary rocks.