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1 Dpto. de Geología, Universidad de Salamanca, Spain
2 Insto. de Recursos Naturales y Agrobiología, CSIC, Salamanca, Spain
On the southwest border of the Duero Basin the Hercynian basement of the Iberian Peninsula is fossilized by a siderolithic sedimentary cover whose thickness increases progressively eastward. Two different palaeoweathering mantles have developed in this zone. From west to east, these mantles are progressively separated in the vertical sense. Whereas the lower mantle affects the Hercynian basement to a depth of > 18–20 m, being fossilized eastward by the siderolithic cover, the upper mantle affects both the Hercynian basement to the west and the siderolithic cover to the east to a depth of c. 3–5 m.
Mineralogical, petrographic and geochemical techniques have shown that the lower mantle shows an upward destruction of the parent minerals with the development of new 2 : 1 phyllosilicates (smectite-like minerals) in the middle levels of weathering and an enrichment in 1:1 phyllosilicates in the upper levels. By contrast, the higher weathering mantle shows important palaeosol features, an enrichment in CT opal, oxyhydroxides mobilizations and occasional concentrations of alunite dated at 58–67 Ma.
A planation surface appears related to the uppermost of these mantles, its remnants now uneven owing to the Alpine tectonic phases. Eastward, the Tertiary sediments of the Duero Basin unconformably fossilize the remnants of this palaeosurface.
