Génesis y evolución de los caliches miocenos del sureste de la depresión del Duero

The aim of this paper is to describe and to study the genesis of the fossil calcretes (caliches) interbedded in Miocene alluvial-fan sediments bordering the pre-Tertiary basament in the North of Segovia Province. Four types of crusts have been distinguished: brechic, massive, vertically laminated and horizontally laminated, the two later types being differentiated by their position relative to the bedding surfaces. Massive crusts show a variety of discrete carbonate components (pisoids, ooids and peloids) embedded in a fine-grained matrix. Erosion of crusts and incorporation of the newly-formed fragments into a different horizon undergoing caliche-forming processes are common. "In situ" rosette like colonies and disengaged prisms of MICROCODIUM have been found. Many features, including basal elongation of the accretion laminae of pisoids and existence vadose cements, suggest that calcretes formed in vadose zones of calcimorph soil profiles under a warm climate with well-marked dry season, X-ray analysis show the existence of diagenetic pali,gorskite that, presumibly, formed before the intense precipitation of calcite in the soil profile that resulted in mature crusts, Afterwards, partial silicification as well as dolomitization to pseudosparite took place.