Publication: Transición de abanicos aluviales a evaporitas en el Mioceno del borde oriental de la Cuenca de Madrid (Sector Barajas de Melo-Illana)
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1991
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Sociedad Geológica de España
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Abstract
La sedimentación de las Unidades Inferior e Intermedia del Mioceno de la Cuenca de Madrid en el Sector Barajas de Melo-Illana, tuvo lugar mediante sistemas de abanicos aluviales áridos o semiáridos que gradaban desde la Sierra de Altomira al Oeste hacia un ambiente de lago salino. Los sistemas de abanicos aluviales tienen un escaso desarrollo en sentido axial, menos de 3 km, presentando en la Unidad Inferior una transición proximaldistal clara, desde depósitos de flujo en masa, con cantos de yeso, a sedimentos de flujos acuosos canalizados, y de estos últimos a niveles tabulares, originados por mantos de arroyada. Las facies lacustres de la Unidad Inferior constan de sales solubles en profundidad, que en afloramiento se muestran transformadas a yesos secundarios. Por el contrario, los sedimentos lacustres salinos de la Unidad Intermedia son yesos primarios de
precipitación química bajo lámina de agua y yesos detríticos. Entre ambas unidades, Inferior e Intermedia,
se puede definir una ruptura sedimentaria correlacionable con las definidas a nivel cuenca.
Las facies detríticas aluviales de borde de la Unidad Inferior presentan una cementación yesífera no dular,
originálmente anhidrítica, generada a partir de flujos subterráneos procedentes de la Sierra de Altomira. El
sulfato cálcico cementante proviene del lixiviado de las formaciones evaporíticas cretácicas y de edad paleó gen a
de dicha sierra y, en menor medida, de la disolución de clastos y matriz yesífera deposicional de las propias facies detríticas. Asimismo estos flujos han aportado grandes cantidades de solutos a la cuenca evaporítica lacustre.
The sediments that form the Lower and Intermediate Units of the Miocene in the Barajas de Melo-Illana region were deposited in arid to semi-arid alluvial-fan systems that graded axially into a saline lake environmento The fanheads attached to the Altomira Range. Areal extent of the alluvial fans of the Lower Unit is short (less than 3 km in length) and the distribution of facies within the fans shows a typical zonation from proximal to distal areas. The proximal facies are characterized by mass flow deposits that contain abundant clasts of gypsum; distally, these deposits evolve to chanelised gravel and sandstone units and furtherly to tabular-bedded sandstones and siltstones originated from sheet floods. The lacustrine facies of the Lower Unit consist of evaporites that occur as gypsum deposits in outcrops but contain abundant soluble salts in subsurface. Contrarily, the lacustrine facies of the Intermediate Unit consist of primary gypsum formed by chemical precipitation as well as detrital gypsum deposits. The detrital facies that form the marginal deposits of the Lower Unit close to Altomira Range are highly cemented by nodular gypsum. The gypsum was initially anhydrite and was precipitated from groundwater flows coming from Altomira Range. The calcium sulphate that form the cement was derived by leaching of Cretaceous and Paleo gene evaporite formations set in the Altomira Range. Dissolution of both gypsum and matrix of the primary detrital facies could also account for the supply of calcium sulphate. It is also assumed that those flows yielded a large amount of solutes that fed the saline lake in the basin.
The sediments that form the Lower and Intermediate Units of the Miocene in the Barajas de Melo-Illana region were deposited in arid to semi-arid alluvial-fan systems that graded axially into a saline lake environmento The fanheads attached to the Altomira Range. Areal extent of the alluvial fans of the Lower Unit is short (less than 3 km in length) and the distribution of facies within the fans shows a typical zonation from proximal to distal areas. The proximal facies are characterized by mass flow deposits that contain abundant clasts of gypsum; distally, these deposits evolve to chanelised gravel and sandstone units and furtherly to tabular-bedded sandstones and siltstones originated from sheet floods. The lacustrine facies of the Lower Unit consist of evaporites that occur as gypsum deposits in outcrops but contain abundant soluble salts in subsurface. Contrarily, the lacustrine facies of the Intermediate Unit consist of primary gypsum formed by chemical precipitation as well as detrital gypsum deposits. The detrital facies that form the marginal deposits of the Lower Unit close to Altomira Range are highly cemented by nodular gypsum. The gypsum was initially anhydrite and was precipitated from groundwater flows coming from Altomira Range. The calcium sulphate that form the cement was derived by leaching of Cretaceous and Paleo gene evaporite formations set in the Altomira Range. Dissolution of both gypsum and matrix of the primary detrital facies could also account for the supply of calcium sulphate. It is also assumed that those flows yielded a large amount of solutes that fed the saline lake in the basin.