Abstract
This paper gives a first description of the Lower Quaternary Macacu.Formation, in a contribution to a better understanding of the geologic significance of the Barreiras Group in Southeastern Brazil. The Macacu Formation has been mapped in the surroundings of the Guanabara bay and lies unconformably. over the Precambrian bedrock and the arkosic sands and montmorillonite clays ofthe Pre-Macacu (Tertiary) beds. It consists on a sequence oftabular beds and lenses ofpoorly sorted sandy clayey, clayey sandy and silty clayey incoherent, afossiliferous and seldom stratified materials. The post-depositional weathering seems to be partially responsible for the high mineralogical maturity of the sediments. The sediment characteristics indicate water flows and debris flows as the main processes involved in deposition, in a piedmont, type alluvial fan environment.
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