Estudio geoquímico de perfiles de silcretas en sedimentos arcillosos miocenos de borde de lago (Vicálvaro, Madrid)
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1994
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Sociedad Geológica de España
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Abstract
El estudio mineralógico y geoquímico de las secuencias rítmicas silico-arcillosas presentes en sedimentos miocenos de borde de lago, permite conocer los procesos edáficos y diagenéticos primarios que originaron dichas secuencias. El análisis petrológico de los diferentes materiales determina
la existencia de una única secuencia tipo (término A -facies de transición - término B) que se repite en la columna. El término A (base) corresponde a niveles de arcillas esmectíticas formadas por sedimentación rápida bajo lámina de agua. El término B es un encostramiento silíceo (ópalo CT y sepiolita fundamentalmente) que se emplaza sobre los niveles arcillosos ya transformados y que se termina
de formar en ambiente vadoso. Entre ambos términos aparecen facies de transición de composición mixta. Los contenidos en elementos mayores, menores, traza y tierras raras caracterizan los diferentes términos de la secuencia. La situación de los niveles opalinos en los diagramas SiO,-Fe,O,TiO, y SiO,-Al,O,-TiO" en las zonas correspondientes a las silcretas de clima árido, define su ambiente de formación. La composición en elementos traza y tierras raras indica la existencia de cambios diagenéticos en los niveles de arcillas esmectíticas. Las secuencias son analizadas determinando la variación en la proporciones de los elementos móviles frente a los inmóviles (Al, Ti Y Zr), definiéndose el factor de cambio de masas. Este análisis indica que las silcretas resultantes se enriquecieron en SiO" MgO, y, en menor cantidad, Sr y Ba. Dichas soluciones causaron la génesis de las silcretas en un ambiente fisicoquímico que podría considerarse, de acuerdo con las anomalías en Eu y las relaciones La/Y, como oxidante y moderadamente ácido
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 1evels (B Units) in the diagrams SiOz-Fe,o,-TiOz y SiOz-Al,o,-TiO, 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, ("mas s 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 SiO" 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
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 1evels (B Units) in the diagrams SiOz-Fe,o,-TiOz y SiOz-Al,o,-TiO, 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, ("mas s 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 SiO" 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
Description
Este trabajo constituye una aportación al proyecto
317 del "International Geological Correlation Program (LG.C.P.)"