Estudio geoquímico de perfiles de silcretas en sedimentos arcillosos miocenos de borde de lago (Vicálvaro, Madrid)

Résumé

The geochemical and mineralogical study of siliceous-argillaceous rhythmic sequences found in Miocene lake shore sediments, allow us to discern the edafic and early diagenetic process which originated these sequences. The petrological analysis of the different rocks determines the existence of only one sequence type (A Unit - Transitional facies - B Unit) which is repeated in the column. The A Units (bases) are mainly mudstones (smectites) formed by rapid sedimentation under a thin layer of water. The B Units (tops) are siliceous duricrust (mainly opal-CT and sepiolite) formed in the mudstones in a vadose environment. Between the A and B Units there are transitional facies of a mixed composition. The composition of major, minor, trace and REE characterizes the different units of the sequence. The composition of the opaline levels (B Units) in the diagrams SiO2-Fe2O2-TiO2 y SiO2-Al2O3-TiO2 show that they are silcretes formed in an arid environment. The composition of the trace and REE indicates that the smectites already display chemical diagenetic changes. The sequences are analysed using the inmobile element (Al, Ti, Zr) methods, ("mass changes") determining the amount of each mobile element added to or removed from a precursor. The studies of changes of mass suggest that the silcretes are enriched with SiO2, MgO and in lesser amounts with Sr and Ba in relation to the previous rocks. The anomalies of Eu and the La/Y ratios in silcretes indicate that the silicification took place in an oxidizing and moderately acid environment.