CALCRETES, PALYCRETES AND SILCRETES IN THE PALEOGENE DETRITAL SEDIMENTS OF THE DUERO AND TAJO BASINS, CENTRAL SPAIN

Three duricrust types (calcretes. palvcretes and silcretes) have been distinguished in the Lower Paleogene arkosic materials from the margin s of the Duero and Tajo basins (central Spain). In the calcretes the c ements are composed of calcite plus palygorskite. whereas palygorskite is the only cement in the palvcretes. In the silcretes. the cement co nsists chiefly or opal and chalcedony. with minor quartz. The importan t duricrust development in the Paleogene detrital materials can be cor related with a drastic climatic change towards colder and more arid co nditions during this lime, as inferred from the sedimentological featu res and the clay mineral evolution. The origin of calcretes and palycr etes is related to the movement of alkaline phreatic waters generated from dolomitic rocks of Cretaceous age cropping out in the near surrou ndings, Silcretes originated from the vertical transfer of silica from supersaturated groundwaters. from the Hercynian granitic rocks, up to the surface. Silcretes are developed either on unconsolidated arkosic materials or replace previous calcrete or palycrete levels.