Publication: Estudio petrológico de las facies Saxoniense y Buntsandstein
de la zona de El Pobo de Dueñas (Cordillera Ibérica)
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1983
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Instituto Geológico y Minero de España (Madrid)
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Abstract
Las areniscas rojas de facies Saxoniense y Buntsandstein estudiadas son cuarzoarenitas y sublitoarenitas, que hacia el tramo medio del Buntsandstein se hacen progresivamente más ricas en feldespatos, llegando al techo de la serie a ser arcósicas. Utilizando los distintos tipos de granos de cuarzo, así como la variación de minerales pesados, se ha podido deducir la naturaleza de sus áreas fuentes en esta zona de la Cordillera Ibérica. En el caso del Saxoniense encontramos una procedencia metamórfica de bajo y medio grado con una influencia ígnea subordinada. Asimismo hemos encontrado que las áreas fuentes de los sedimentos del Buntsandstein son de naturaleza metamórfica de bajo, medio y alto grado (incluidos los neises). La influencia neisica crece progresivamente hacia el techo de las series, donde es casi exclusiva. Entre los procesos diagenéticos más importantes que han afectado a las areniscas estudiadas destacan la cementación silícea y caolinítica que suelen afectar a los tramos de areniscas de grano grueso de la base y a algunos niveles del techo. En menor proporción aparece cementación ferruginosa, dominante en el Saxoniense, y feldespática que afecta sobre todo al tramo superior del Buntsandstein. Otros fenómenos diagenéticos encontrados han sido: La alteración de los feldespatos y fragmentos de roca volcánica a ilita y caolinita, constituyendo una fuente importante de matriz diagenética (epimatriz), y la deformación y rotura de los fragmentos de roca metamórfica, dando lugar a una fuerte compactación mecánica y consiguiente pseudomatriz. Asimismo, existen moderados fenómenos de disolución por presión (contactos cóncavo-convexos dominantes). De la intensidad de los fenómenos diagenéticos, hemos deducido que nos encontramos en la etapa de mesodiagénesis madura «B» de SCHMIDT y McDONALD (1979), corroborándose estos datos con los parámetros cristaloquímicos de las ilitas de los niveles de lutitas intercalados y con las medidas de la reflectividad de la vitrinita en areniscas de áreas muy próximas a la estudiada. Asociados a lentejones de areniscas, intercaladas con los conglomerados basales, aparecen mineralizaciones de uranio en forma de óxidos (torbernita), que cementan y reemplazan fundamentalmente a la matriz de estas areniscas.
This research is part of a project concerned with the provenance and diagenesis of Triassic and Permian sandstones of the Iberian Range. The red sandstones of Saxonian and Buntsandstein facies are quartzarenites and sublitharenites which in the middle of Buntsandstein progresively increase in feldespars and finally they are arkoses. By using the different types of quartz grains and the composition of heavy minerals, we haYe found the provenance of these sandstones. In the Saxonian sandstones the provenance is fundamentally from low and middle rank, metamorphic rocks with subordinated igneous influence. In the Buntsandstein sandstones the mineral association shows that the provenance is more complex in this area of the lberian Range: low, middle and high rank metamorphic rocks (with gneisses included). The gneisses influence increasses progressively to the top. The most characteristic diagenetic aspects are: silica and kaolinite as cement, being most abundant in the bottom and the top of the Buntsandstein facies. Secondly we have found ferric-oxide as cement in the Saxonian and K-feldespar in the top of Buntsandstein. Other frequent diagenetic processes are the alteration of K-feldespar and volcanic rock fragments to illite and kaolinite (epimatrix) and breakage of metamorphic rocks with strongly mechanical compaction (pseudomatrix). The pression-solution phenomena are moderated (concave-convex contact). The principal observation concerning the burial diagenesis show that we are in the Mesodiagenesis mature «B» of ScHMIDT and McDONALD (1979) which is corroborated by the crystallinity of illites in the intercalated lutites and by the vitrinite Ro in sorne sandstones near to the study area. Associated with channels of sandstones intercalated in the basal conglomerates, we have found Uranium - bearing sandstones. The Uranium minerals, principillyoxides, are as pore filling and replacing the grains and the matrix of these sandstones.
This research is part of a project concerned with the provenance and diagenesis of Triassic and Permian sandstones of the Iberian Range. The red sandstones of Saxonian and Buntsandstein facies are quartzarenites and sublitharenites which in the middle of Buntsandstein progresively increase in feldespars and finally they are arkoses. By using the different types of quartz grains and the composition of heavy minerals, we haYe found the provenance of these sandstones. In the Saxonian sandstones the provenance is fundamentally from low and middle rank, metamorphic rocks with subordinated igneous influence. In the Buntsandstein sandstones the mineral association shows that the provenance is more complex in this area of the lberian Range: low, middle and high rank metamorphic rocks (with gneisses included). The gneisses influence increasses progressively to the top. The most characteristic diagenetic aspects are: silica and kaolinite as cement, being most abundant in the bottom and the top of the Buntsandstein facies. Secondly we have found ferric-oxide as cement in the Saxonian and K-feldespar in the top of Buntsandstein. Other frequent diagenetic processes are the alteration of K-feldespar and volcanic rock fragments to illite and kaolinite (epimatrix) and breakage of metamorphic rocks with strongly mechanical compaction (pseudomatrix). The pression-solution phenomena are moderated (concave-convex contact). The principal observation concerning the burial diagenesis show that we are in the Mesodiagenesis mature «B» of ScHMIDT and McDONALD (1979) which is corroborated by the crystallinity of illites in the intercalated lutites and by the vitrinite Ro in sorne sandstones near to the study area. Associated with channels of sandstones intercalated in the basal conglomerates, we have found Uranium - bearing sandstones. The Uranium minerals, principillyoxides, are as pore filling and replacing the grains and the matrix of these sandstones.