Datación de silcretas a través de ópalos con uranio. Un estudio preliminar en el Mioceno, área de Paracuellos de Jarama (Cuenca de Madrid)

Abstract

U-Pb-series analyses of two U-rich opal samples have been used for dating silcretes included in Miocene rocks in the Paracuellos zone (Madrid Basin.) A sensitive high-resolution ion microprobe (SHRIMP) was used. The silcretes are composed mainly of opal CT and have variable amount of quartz and locally minor amounts ofmoganite, dolomite, and relics of clays and detrital minerals. The silcretes formed bysilicification of sepiolites (precipitated in small ponds) and smectites (formed in distal alluvial fans). The initial sediments are Aragonian in age. Green fluorescence of two silcrete samples, observed under short-wave UV light, was used to establish the opaline zones to analyze. The estimated U content varies from 76 to 168 ppm. The 206pb-23SU and 207pb-235U ages obtained varies from 10.5 ± 0.54 m.a. to 8.78 ± 0.54 m.a. These values can be interpreted as: 1) Indicative ofa silicification latter (Vallesian) than expected from previous studies; 2) Corresponding to a local and latter formation U-rich opal zones inside the initial silcrete; 3) The opaline zones analyzed, formed by replacement ofclays, are too heterogeneous to be used for dating