Transición de abanicos aluviales a evaporitas en el Mioceno del borde oriental de la cuenca de Madrid (sector Barajas de Melo-Illana)

Abstract

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 environment. 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 Paleogene 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. lt is also assumed that those flows yielded a large amount of solutes that fed the saline lake in the basin.