Publication: Análisis de los minerales de la arcilla y sus procesos genéticos en las formaciones arcillosas de la Cuenca del Tajo.
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Publication Date
1997
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Consejo Superior de Investigaciones Científicas, Museo Nacional de Ciencias Naturales
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Abstract
Se han estudiado seis sondeos, realizados en la cuenca neógena del Tajo, con el fin de conocer los procesos genéticos de los materiales arcillosos de la cuenca. Estos sondeos cortan facies correspondientes a distintos ambientes deposicionales. La evolución observada puede relacionarse con dos etapas distintas de sedimentación. La primera corresponde a la sedimentación de la Unidad de Yesos; la asociación de minerales de arcilla en esta unidad está constituida principalmente por illita, menores contenidos de esmectita, interestratificados I/Sm, Cl/Sm, Glauconita/Sm, palygorskita y clorita. El origen de esta asociación es en parte detrítica y en parte neoformada; los aportes detríticos corresponden a illita y esmectita, mientras que los minerales neoformados son principalmente illita e interestratificados, clorita y palygorskita. Esta etapa se caracteriza por la existencia de procesos de illitización. En la segunda etapa de sedimentación (Unidad de Arcillas Verdes), se observa un incremento de los aportes detríticos y una evolución hacia condiciones de menor salinidad. El ambiente de formación de esta unidad se encuentra enriquecido en magnesio y los minerales de la arcilla más característicos de las facies que componen dicha unidad son las esmectitas magnésicas (saponita y estevensita) y, en menor proporción, los interestratificados Cl/Sm. Esta asociación mineralógica se origina por la reacción entre las partículas detríticas arcillosas y los cationes del medio.
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.
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.