Unidades estratigráficas en el registro sedimentario neógeno del sector occidental de la Cuenca de Madrid

Abstract

The western part of the Madrid Basin, central Spain, shows widespread siliciclastic deposits consisting mainly of arkosic sandstone and mudstone. The age of the deposits is mostly Neogene. Both mapping and detailed stratigraphic analysis of the sedimentary record present in several topographic sheets (scale 1:50,000) in the western part of the basin allow recognition of five major stratigraphic units, which have been named according to places where the units are better exposed. From bottom to top, the units are: 1) Arkoses and mudstones of El Carpio, 2) Gravelly-sandstones and mudstones of the Castrejón Reservoir, 3) Arkoses and mudstones of Los Vergales, 4) Gravelly-arkosic sandstones of Batres, and 5) White arkoses of Carranque-Griñón. The boundaries between the units are outlined by upward grain-size changes of the deposits as well as facies variation in vertical. Most of the sediments were deposited in alluvial fan systems, where some changes in the character of the distributary streams, i.e. permanent to ephemeral, is observed. Local deposits representative of shallow-lake to palustrine environments, including carbonate paleosols, are recognised especially towards the top of the stratigraphic units. The age of the units spans Early to Middle Miocene although a Paleogene age is not ruled out for the lower part of the Arkoses and mudstones of El Carpio Unit. Similarly, the White arkoses of Carranque-Griñón Unit could reach the lower part of the Late Miocene (Vallesian).