Los arrasamientos Plioceno y Plio Pleistoceno en el sector Sur-Oriental de la Cuenca del Duero

Abstract

in the SE. border of the Duero Basin (central Spain) a set of planation surfaces has been found. These surfaces are the remainds of two pediments developed on Mesozoic and Tertiary series and represent the more important morphological feature of the region (Fig. 1). The oldest pediment is located between 1170-1040 m. height and, owing to its position within the geological and morphological evolution of the region, a Pliocene age is attributed. It is characterized by displaying a karst more than 5 m. deep (Fig. 2) which holes appear filled up by red clays, traditionally considered as «Terra Rossa», composed by kaolinites and illites-micas (Fig. 4). The contact between the limestone and the «Terra Rossa» shows reaction borders and recrystalhzation of calcite within the red clays. This first surface is eroded by the alluvial fans of the «Raña» formation within which is possible to distinguish two levels. The base of the lower level is the beginning of the new pediment at about 960 m. height close to the sierra of Honrubia Pradales (Fig. 1). This new pediment is located between 960-850 m. height and cronologically represents the PUo-Pleistocene transition. It is characterized by displaying a new karst no more than 3 m. deep, their holes being filled up by a new and more sandy material in which clay illuviation processes are very important (Fig. 3). The clay fraction is composed of smectite, kaohnite and ilhte-micas (Fig. 4) and there is not reaction borders with the Hmestones.