Análisis de los minerales de la arcilla y sus procesos genéticos en las formaciones arcillosas de la cuenca del Tajo

Abstract

The Tagus Basin is an intramontanous basin that had an important sedimentary activity during the Neogene. Six drill boreholes have been studied; these boreholes pierce through facies corresponding to different depositional environments. Such sedimentary environments correspond to different sedimentation stages. During each stage the basin shows areas with different geochemical gradients which rule the neoformation and transformation processes of the detrital particles that reach it.
A first stage corresponds to the sedimentation of the Gypsum Unit. Clay minerals during this stage are mainly illites, with minor contents of smectite, I/Sm, Cl/Sm, Glauconite/ Sm, palygorskite and chlorite. The origin of such association is partly detrital and partly neoformational. Detrital contribution is composed by illite and smectite. Neoformed minerals are mainly illite and mixed-layer I/Sm, chlorite and palygorskite. This stage is characterised by illitization processes.
The second sedimentation stage corresponds to the Green Clays Unit which shows an increase in detrital supply, the environment is enriched in magnesium and silica. The most characteristic clay minerals of these facies are magnesium smectites (saponites and stevensites) and, to a lesser extent, mixed-layer Cl/Sm. The mineralogical assemblage of this unit must have been originated by reaction between detrital clay particles and cations in the sedimentary environment.