Publication: Petrología y geoquímica isotópica de los depósitos de magnesita de la cuenca de Calatayud (provincia de Zaragoza). Interpretación paleoambiental
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2002
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Sociedad Geológica de España
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Abstract
Los sedimentos miocenos de la Cuenca de Calatayud, en particular los correspondientes a las unidades Inferior e Intermedia, contienen nllmerosos niveles de magnesita, que aparecen comúnmente asociados con depósitos lacustres sulfatados (anhidrita, yeso, glauberita), así como con
margas y arcillas. L¡t magnesita de la Cuenca de Calatayud es de tipocriptocristalino, presentándose en capas masivas de color blanquecino, de espesor variable (mm a dm), entre arcillas, margas, niveles de yeso laminado y yeso nodular. Se han distinguido cuatro asociaciones de facies (MLH, Mys, Myp, MIN) con presencia de magnesita. Estas asociaciones de facies se ordenan básicamente en dos tipos de secuencias sedimentarias principales que reflejan, en un caso, la evolución de subambientes lacustres de alta salinidad y, en otro, de salinidad moderada a alta. La textura de la magnesita, observada mediante M.E.B. en muestras con alta pureza en este mineral, se caracteriza por presentar mosaicos densos y homogéneos de cristales romboédricos euhedrales a subeuhedrales, con tamaño alrededor
de 1 m.Los valores de isótopos estables (0180, 013C) de la magnesita (n=34) muestran una ciertadispersión, con valores de 0180 de -4,8 a +4,9 %0 PDB y valores negativos de 013C, de -9,3 a +0,4 %0 PDB, más bajos en los niveles de magnesita correspondientes a las asociaciones de facies MLH y Mys.
Las características estratigráficas, texturales e isotópicas sugieren que la magnesita se formó en condiciones evaporíticas, posiblemente por precipitación primaria a partir de soluciones concentradas con mezcla de aguas meteóricas, ricas en CO2 biogénico, aunque los valores negativos de o\3C pueden interpretarse también como reflejo de la actividad bacteriana en la precipitación de la magnesita. El análisis de las asociaciones de facies en que aparece la magnesita y su evolución secuencial permiten
concluir que los depósitos de magnesita de la Cuenca de Calatayud se formaron tanto en condiciones lacustres hipersalinas como de salinidad moderada a alta.
Miocene sediments from the Lower and Intermediate units of the Calatayud Basin show abundant magnesite beds, commonly associated to sulphate lake deposits (anhydrite, gypsum, glauberite), marls and clays. The magnesite is cryptocrystalline and occurs as mm- to dm-thick, massive, whitish deposits interbedded with claystone, marlstone, and laminated and nodular gypsum. Four lithofacies associations containing magnesite beds (MLH, Mys, Myp, M¡N) have been distinguished. These lithofacies associations occur in two main sedimentary sequences which reflect the evolution of high-salinity lake and moderate- to high-salinity lake sub-environments, respectively. The texture of magnesite, as observed from magnesite-rich samples. under S.E.M., is mainly formed of a dense, homogeneous aggregate of subhedral to euhedral rhomb-like crystals showing about 1 micron in average size. Isotope values (0 180, 013C) from 34 magnesite samples are relatively spread, with 0180 values from -4,8 to +4,9 %0 PDB y negative values for 013C (-9,3 to +0,4 %0 PDB), being lower in the magnesite beds corresponding to MLH and Mys lithofacies associations. The stratigraphic, textural and isotopic features suggest that the magnesite formed primarily under evaporitic conditions, probably as a result of mixing of concentrated and meteoric waters enriched in biogenic CO2• Morerover, the negative o\3C values determined in many magnesite samples could reflect a bacterial mediation in the precipitation of the magnesite. Sedimentological analysis of the lithofacies associations containing magnesite beds and their sequential arrangement indicates that this carbonate formed in both hypersaline and moderate- to high-saline lake subenvironments
Miocene sediments from the Lower and Intermediate units of the Calatayud Basin show abundant magnesite beds, commonly associated to sulphate lake deposits (anhydrite, gypsum, glauberite), marls and clays. The magnesite is cryptocrystalline and occurs as mm- to dm-thick, massive, whitish deposits interbedded with claystone, marlstone, and laminated and nodular gypsum. Four lithofacies associations containing magnesite beds (MLH, Mys, Myp, M¡N) have been distinguished. These lithofacies associations occur in two main sedimentary sequences which reflect the evolution of high-salinity lake and moderate- to high-salinity lake sub-environments, respectively. The texture of magnesite, as observed from magnesite-rich samples. under S.E.M., is mainly formed of a dense, homogeneous aggregate of subhedral to euhedral rhomb-like crystals showing about 1 micron in average size. Isotope values (0 180, 013C) from 34 magnesite samples are relatively spread, with 0180 values from -4,8 to +4,9 %0 PDB y negative values for 013C (-9,3 to +0,4 %0 PDB), being lower in the magnesite beds corresponding to MLH and Mys lithofacies associations. The stratigraphic, textural and isotopic features suggest that the magnesite formed primarily under evaporitic conditions, probably as a result of mixing of concentrated and meteoric waters enriched in biogenic CO2• Morerover, the negative o\3C values determined in many magnesite samples could reflect a bacterial mediation in the precipitation of the magnesite. Sedimentological analysis of the lithofacies associations containing magnesite beds and their sequential arrangement indicates that this carbonate formed in both hypersaline and moderate- to high-saline lake subenvironments