Calcrete-silcrete duricrusts from distal-alluvial fan deposits (Madrid Basin, Torrijos area, Toledo, Spain) Encostramientos calcreta-silcreta en depósitos distales de abanicos aluviales (Cuenca de Madrid, zona de Torrijos, Toledo, España)

Three duricrust profiles, developed on fine clastic Miocene deposits of the Southwestern area of the Madrid Basin, are studied. Different horizons (3 or 4, depending on the studied sections) are described defining their structure, mineralogy and petrology. They are interpreted as calcrete and silcrete (silicified calcrete and silicified mudstone) profiles formed in distal facies of arkosic fans. These profiles are quite different from those of typical calcretes because they lack the nodular horizon and most of the carbonate appears as cm-thick laminae, mostly horizontal, but also vertical. The carbonate in the profiles formed in relation to root systems and associated microorganisms. All the profiles have been subject to silicification at various intensities and many characteristic textures and structures of both calcretes and mudstones are preserved, suggesting a groundwater origin for the silicification. The levels of silcretes have an extensive lateral continuity and therefore seem to be related to important palaeosurfaces.