Calcretes e registros de paleossolos em depósitos continentais neocretáceos

Abstract

The results presented are based on regional research of calcrete and palaeossols records in the Eastern part of the Bauru Basin, in Sao Paulo State and Triângulo Mineiro region. This basin was filled by Upper Cretaceous siliciclastic sandy sequence, in semi-arid climate on the edges, desert in the center. The most significant calcretes occurrences belong to the the Marilia Formation (Bauru Group), whichon a regional scale, are represented by two extreme types, related to the depositional context. The Ponte Alta type was formed in marginal alluvial fan deposits, the Echapora type in inner aeolian sandy deposits, of wide sand sheets and smooth depressions with temporary rivers and lakes. Three genetic contexts are proposed for the Marilia Formation calcretes: pedogenetic peripheral (Pp), pedogenetic interior (Pi) and phreatic (F). In the first two contexts the edaphic vadose and palustrine processes prevailed, as well as soil profiles poorly developed. It was noticed in them a big frequency of the beta microstructure association. They occur in the Triângulo Mineiro (MG), regions of Monte Alto, Agudos and Bauru (SP). In the phreatic context there was intense development of micritic and spar textures, which probably destroyed the original features of the beta association, generated in contexts Pp and Pi. The superimposed phreatic processes determined the predominance of the alpha microstructure association. They were probably more intense and of the last and/or the most important event in the internal calcrete formation of the basin,in Echapora type profiles. They occur in the regions of Echapora and Marilia (SP). The dendritic configuration of the long and narrow plateaux, sustained by calcified sandstones of the Marilia Formation indicates processes of landscape elaboration through the relief invertion .