Publication: Geología de la Unidad Salina de la Cuenca de Madrid. Recursos minerales, riesgos geológicos y mineralogénesis salina
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2022
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Real Sociedad Española de Historia Natural
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Abstract
La Unidad Salina de la Cuenca Cenozoica de Madrid, se conoce de forma indirecta desde la Prehistoria por sus manantiales salinos, objeto de aprovechamiento, para la obtención de sales y usos medicinales de sus salmueras. A principios del s. XX, se continuaron antiguas labores de investigación, para la extracción masiva de sulfato sódico para la industria química. Los reconocimientos geológicos, y más fundamentalmente las técnicas de análisis de cuencas a partir de sondeos, el estudio de perfiles geofísicos, y las técnicas modernas mineralógicas, en especial la difracción de rayos X, han permitido definir con precisión la naturaleza petrográfica y mineralógica de la Unidad y mejorar el conocimiento de su posición en el marco estructural de la Cuenca de Madrid, así como su comparación con las sales de otras cuencas cenozoicas ibéricas. Los datos sedimentológicos disponibles de las cuencas actuales y los diagramas de equilibrio del sistema SO42- - Ca2+ - Na1+ apoyan la hipótesis del origen por lixiviación de antiguos depósitos de evaporitas, y su deposición en un sistema lacustre abierto con un exceso de iones de calcio durante el periodo de precipitación de grandes espesores de sedimentos ricos en glauberita. La meteorización de la Unidad genera un residuo insoluble rico en yeso, con espesores superiores a 3-4 decenas de metros, y con texturas pseudomórficas de glauberita, anhidrita e incluso de halita, que muestran claramente el carácter secundario de esta formación secundaria de yesos. La génesis de estos “yesos secundarios” de la Unidad Salina, provoca colapsos que se rellenan por los materiales suprayacentes, y que pueden afectar a las cimentaciones e infraestructuras (túneles y galerías de servicios). La meteorización puede generar zonas enriquecidas en mirabilita, que coexisten con salmueras sulfatadas sódicas que se manifiestan en un sinfín de eflorescencias, y manantiales salinos, cuyo aspecto varía estacionalmente por efecto de la influencia de la temperatura, sobre el grado de saturación de las salmueras en sulfato sódico.
The Saline Unit of the Cenozoic Basin of Madrid recognized since prehistoric times for its saline springs, which were object of exploitation to obtain salts and to use the medicinal properties of brines. At the beginning of the 20th century, the old mine works was continued, aimed at mining developments for the massive extraction of sodium sulphate for the chemical industry. The petrographic and mineralogical distribution and position of Saline Unit through the basin supported on geological and geophysical surveys, and also basin analysis and especially X-ray diffraction salts mineralogy. The distribution across the basin let us define the petrographic and mineralogical nature of the Unit and improve knowledge of its position in the structural framework of the Cenozoic Madrid Basin, and its comparison with salts from other Iberian Cenozoic basins as Ebro, Calatayud, Cerezo de Rio Tirón... The available sedimentological data from present-day basins and the equilibrium diagrams of the SO4 2- - Ca2+ - Na1+ system, supported the hypothesis of leaching of ancient evaporite deposits, an open lake system with a surplus of calcium ions over the course of the precipitation of large thicknesses of glauberite rich sediments. The weathering of the Unit generates an insoluble residue rich in gypsum, with thicknesses of more than 3-4 tens of metres, and with pseudomorphic textures of glauberite, anhydrite... and even halite, which clearly show the secondary character of this secondary gypsum formation. The genesis of these “secondary gypsum layers” of the Saline Unit causes deformations in certain stratified levels, and produced locally sink holes filled by the overlying materials. These deformations may affect foundations and infrastructures (tunnels, service galleries, etc.). Weathering can generate areas enriched in mirabilite, while coexisting with sodium sulphate brines that manifest themselves in many and seasonally varied efflorescence types, and saline springs. Their appearance varies seasonally due to the influence of temperature on the degree of saturation of the brines in sodium sulphate.
The Saline Unit of the Cenozoic Basin of Madrid recognized since prehistoric times for its saline springs, which were object of exploitation to obtain salts and to use the medicinal properties of brines. At the beginning of the 20th century, the old mine works was continued, aimed at mining developments for the massive extraction of sodium sulphate for the chemical industry. The petrographic and mineralogical distribution and position of Saline Unit through the basin supported on geological and geophysical surveys, and also basin analysis and especially X-ray diffraction salts mineralogy. The distribution across the basin let us define the petrographic and mineralogical nature of the Unit and improve knowledge of its position in the structural framework of the Cenozoic Madrid Basin, and its comparison with salts from other Iberian Cenozoic basins as Ebro, Calatayud, Cerezo de Rio Tirón... The available sedimentological data from present-day basins and the equilibrium diagrams of the SO4 2- - Ca2+ - Na1+ system, supported the hypothesis of leaching of ancient evaporite deposits, an open lake system with a surplus of calcium ions over the course of the precipitation of large thicknesses of glauberite rich sediments. The weathering of the Unit generates an insoluble residue rich in gypsum, with thicknesses of more than 3-4 tens of metres, and with pseudomorphic textures of glauberite, anhydrite... and even halite, which clearly show the secondary character of this secondary gypsum formation. The genesis of these “secondary gypsum layers” of the Saline Unit causes deformations in certain stratified levels, and produced locally sink holes filled by the overlying materials. These deformations may affect foundations and infrastructures (tunnels, service galleries, etc.). Weathering can generate areas enriched in mirabilite, while coexisting with sodium sulphate brines that manifest themselves in many and seasonally varied efflorescence types, and saline springs. Their appearance varies seasonally due to the influence of temperature on the degree of saturation of the brines in sodium sulphate.