Crostas ferruginosas, geoquímica, evolução, distribuição, mineralogia, Bacia de São Paulo
Abstract
This paper presents the mineralogical, micromorphological and geochemical evolution and as well as geographic distribution of iron crusts in the São Paulo Basin. Two types of crusts were identified, an older one near the surface and a younger type in the subsurface within underlying sediments. Both derive from the underlying sediments through variations in the water table which propitiated iron hydroxides precipitation at the contact between a porous, permeable sediment above, and impermeable sediments below. The ferricrusts are found in at least three leveis within the sediments of the basin, which are from the bottom to the top: 1) about 20 m below the surface with ochre-colored clay covered by sandy silt; 2) 5 m below the surface with purplish clay overlain by sand and, finally, 3) the surface crust where it characteristic features of ferricrust are evident. Geochemical diagrams show that the uppermost crust is enriched in Fe2O3 and poorer in AI2O3 than the other two layers crusts. Micromorphological analysis shows that the surfícial ferricrust are marked by larger grain size than in the deeper crusts, which show relic textural features of the original sediments. This feature is not found in the surface ferricrust.
