On the landscape of the Eastern Plateau, north of the State of São Paulo, Brazil, there are ferricretes in different elevations. Although these features are relatively frequent over the cretaceous sandstone (Adamantina Formation, Bauru Group), little investigation has been carried out on their genesis. A morphological study at different levels of observation was conducted in order to elucidate the genesis of ferricretes (plinthite and petroplinthite), occurring at the toeslope of a landscape where soils with argillic horizon predominate, at the Pindorama Experimental Station of the Agronomic Institute (IAC). Two soil profiles were selected for detailed morphological description and samples taken for micromorphological observation using a binocular (reflected light) and a petrographic microscope. The macro and micromorphological evidence showed that ferricrete genesis is associated with the removal of ferrous iron from the upper slope position by throughflow and re-precipitation in the vadose zone. These ferricretes showed different internal fabrics, most having structures of degradation, giving evidence that the ironstone layer is currently being broken down. The fabric differentations are related to the skeleton and plasma distribution. Some ferricretes have well selected skeleton, with predominance of very fine sand, others have predominance of medium-size sand and yet others have poorly selected skeletons, resembling the matrix-s interglaebules. Most of the glaebules show some volume occupied by plasma only. It is shown that pedo-lito-biological factors are involved in the diversity of ferricretes of one soil horizon. Thus, "in situ" glaebules well differenciated in their fabric may be mistakenly interpreted as having been originated at different landscape positions and deposited at the lower positions during ferricrete formation.
ferricretes formation; micromorphology; petroplinthite; laterite; soil-landscape relationships