Dobras atectônicas em crostas ferruginosas de Pirapora do Bom Jesus-SP e Bacia Terciária de São Paulo

Abstract

This paper discusses the genesis of folds in iron crust developed in Tertiary sediments of the São Paulo Basin (São Paulo-SP) and sediments in Proterozoic metasediments host rocks of the São Roque Group in Southeastern Brazil. Both iron crusts made up basically of iron hydroxides and present textural organization, similar chemical and mineralogical composition, although they are located in different rocks and geologic settings. They are constituted basically by iron hydroxides. Diverse diagram link the evolution of both sedimentary host-rocks and iron crusts composition, to a geochemical origin. These iron crusts were formed by remobilisation and concentration of iron oxides and hydroxides, originated by the descending meteoric water and groundwater level oscillation, related to seasonal climatic variations and uplift relief ascending movements. No correlation was observed between structural data for iron crust fold and tectonic fold or regional deformation patterns in host-rocks. This indicates that tectonic processes of folding where not directly involved in the origin of this folded iron crust. In addition, fractures in the Pirapora outcrop formed by neotectonic reactivation of regional lineaments, were the conduits for water flow, allowing iron oxides and hydroxides are to be precipitated. Oxides and hydroxides are precipitated obliquely to the fractures in the dransdown cone formed by the groundwater level down. The group formed by the deposition of this oxides and hydroxides in the dransdown cone show structures like folds delineates the pattern and feature of folded iron crusts observed in this outcrop.