Fragipans and Duripans are frequently found in soils developed from sediments of the Barreiras Formation, in the low coastal plateaus of northeastern Brazil. Previous studies in arid regions have attributed Duripan cohesion to the amorphous silica. However, the Duripans and Fragipans in northeastern Brazil are located in semi-arid and high annual precipitation level zones (1.500 to 2.000 mm year-1). This study, based on a detailed characterization of soils from four depressions with different sizes, aimed to propose an evolution scheme of these horizons. Toposequences were studied by boring and excavating the trench (2-70 m in size) from the center to the border of four depressions. The morphology and the organization of the horizons were described in detail and samples were characterized for granulometry, mineralogy, geochemistry and micromorphology. The results indicated that the formation of these hardened horizons was related to the evolution of the depressions, hydromorphic conditions and particle translocation. At the first stage, the hydromorphic conditions caused the formation of the Fragipans and the iron bands. Their evolution and the increased depression size caused Duripan formation and podzolization, with the formation of the albic horizon.
hardened horizons; water infiltration; podzolization; Spodosols; Barreiras Group