Publication: Estudio petrológico de las series posthercínicas (Carbonífero-Pérmico?) en el borde del Sistema Central (Guadalajara)
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Publication Date
1987
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Abstract
Los depósitos detríticos que rellenan las cuencas de carácter intermontano controladas por la tectónica tardihercínica en el ámbito de la Cordillera Ibérica y borde del Sistema Central, tienen planteados aún una serie de problemas en cuanto a su caracterización, datación y correlaciones. En este trabajo, a partir de los datos correspondientes a series estratigráficas levantadas en el sector Valdesotos-Retiendas (fig. 1), se estudia en detalle la petrología de dichos depósitos para contribuir tanto al conocimiento de sus áreas fuentes (procedencia) como a las características de los medios de sedimentación. Asimismo, se analizan los procesos postsedimentarios sufridos para conocer la incidencia de los ambientes geoquímicos deposicionales y las transformaciones acaecidas durante la diagénesis de enterramiento. Desde el punto de vista petrológico (figs. 2, 3, 4, 5 y 6), los depósitos estudiados están integrados principalmente por: brechas polimícticas, grauvacas feldespáticas («arkosic wackes>), grauvacas líticas y lutitas caolínicas con ilita subordinada. Además existen numerosas intercalaciones de tobas de cristales, cineritas y tonsteins, fundamentalmente en la zona de Valdesotos. Todo ello denota la existencia de áreas fuentes de naturaleza metamórfica (pizarras, esquistos y cuarcitas) y volcánica (riolítica), estas últimas citadas por primera vez en este sector de la Cordillera Ibérica. La intensa caolinitización de los feldespatos y micas, semej ante a la descrita por BOUROZ (1972) para el Estefaniense del Loira, y la presencia de abudante materia orgánica y pirita asociada, apuntan hacia unas condiciones diagenéticas ácidas y anóxicas. La gran abundancia de niveles silicificados en los tramos basales está en relación con la sílice liberada en la caolinitización de los feldespatos (fig. 4) de las tobas y con la desvitrificación de los vidrios volcánicos. La alta cristalinidad de las ilitas, que las sitúan en la anquizona, está por su parte en contradicción con los datos aportados por los carbones intercalados, los cuales, de acuerdo con los criterios de MOLCHANOV (1961), indican temperaturas inferiores a 1000 C y presiones bastante bajas. Dichos límites térmicos están asimismo apoyados por la abundancia de palinomorfos conservados. Por ello, deducimos que la cristalinidad de las ilitas es heredada de la roca madre. Del estudio de los tipos genéticos y texturales de porosidad secundaria, según los criterios de SCHMIDT & Mc DONALD (1979b), se ha constatado la creación y preservación de porosidad secundaria en las grauvacas estudiadas por procesos diagenéticos tales como formación de «pore fillings” y epimatriz de caolinita fundamentalmente, dependientes de la cantidad de feldespatos presentes.
Lithological characterization, age and correlations of detrital deposits that filí the intermountain basins, controlled by the late Hercynian tectonic in the Iberian Range and edge of the Central System, still have many unanswered questions. A petrological study of these deposits has been done in this papa based on several stratigraphic colums (hg. 1). We have tried to finid out both provenance and the features of the sedimentary environnients of these materials. Likewise, we have analyzed the postsedimentary processes to explain the influence of the depositional geochemical environment and the transformations that occur during the burial diagenesis. From a petrological point of view (figs. 2, 3, 4, 5 and 6) the deposits are composed mainly of polimictic breccias, feldspathic graywackes (arkosic -wackes) lithic graywackes anid kaolinitic lutites with subordinated illite. Furthermore, frequent intercalation of crystal tuffs, ashes and levels of tonstein are observed. This proves the influence of the metamorphic and volcanic source areas. This vulcanism is mentioned for first time in this sector of the Iberian Range. The intensive kaolinitization of feldspars and micas, similar to the one described by BOUROZ (1972) in the Stephanian of the Loira region, suggest an acidic and anoxic diagenetic conditions. The great number of silicified leveis anid concretions at the base of the series are associated with available silica from the feldspar kaolinitization processes (fig. 4) in the crystal tuffs and with the devitrification of the volcanic glasses. The high crystallinity of illites, corresponding to the anchizone, does not correspond with the range of the interbedded coals, which indicate temperatures lower than 1000 C, and lower pressures. Moreover, these thermal limits are confirmed by the abundance of palynomorphes. For this reason we have assumed that the illite crystallinity is inherited from its source area. Finally, according to the genetic and textural types of secondary porosity described by SCHMIDT & Mc DONALD (1979b) we have verified that this kind of porosity in the volcanoclastic graywackes is generated and preserved by diagenetic processes like the kaolinite pore filling, epimatrix formation and K-feldspar dissolution, which are in relation with the amount of K-feldspar in the sandstones.
Lithological characterization, age and correlations of detrital deposits that filí the intermountain basins, controlled by the late Hercynian tectonic in the Iberian Range and edge of the Central System, still have many unanswered questions. A petrological study of these deposits has been done in this papa based on several stratigraphic colums (hg. 1). We have tried to finid out both provenance and the features of the sedimentary environnients of these materials. Likewise, we have analyzed the postsedimentary processes to explain the influence of the depositional geochemical environment and the transformations that occur during the burial diagenesis. From a petrological point of view (figs. 2, 3, 4, 5 and 6) the deposits are composed mainly of polimictic breccias, feldspathic graywackes (arkosic -wackes) lithic graywackes anid kaolinitic lutites with subordinated illite. Furthermore, frequent intercalation of crystal tuffs, ashes and levels of tonstein are observed. This proves the influence of the metamorphic and volcanic source areas. This vulcanism is mentioned for first time in this sector of the Iberian Range. The intensive kaolinitization of feldspars and micas, similar to the one described by BOUROZ (1972) in the Stephanian of the Loira region, suggest an acidic and anoxic diagenetic conditions. The great number of silicified leveis anid concretions at the base of the series are associated with available silica from the feldspar kaolinitization processes (fig. 4) in the crystal tuffs and with the devitrification of the volcanic glasses. The high crystallinity of illites, corresponding to the anchizone, does not correspond with the range of the interbedded coals, which indicate temperatures lower than 1000 C, and lower pressures. Moreover, these thermal limits are confirmed by the abundance of palynomorphes. For this reason we have assumed that the illite crystallinity is inherited from its source area. Finally, according to the genetic and textural types of secondary porosity described by SCHMIDT & Mc DONALD (1979b) we have verified that this kind of porosity in the volcanoclastic graywackes is generated and preserved by diagenetic processes like the kaolinite pore filling, epimatrix formation and K-feldspar dissolution, which are in relation with the amount of K-feldspar in the sandstones.