Geomorfología, estratigrafía, paleontología y procedencia de los depósitos arcósicos cuaternarios de la depresión Prados-Guaten (SW Madrid)

Abstract

RESUMEN. El presente trabajo analiza el registro geomorfológico, estratigráfico y paleontológico ligado al nivel de terraza compuesto (+15-40m) que aflora en la Depresión Prados-Guatén. Los depósitos de naturaleza arcósica, y origen fluvial, presentan restos de Mammuthus meridionalis y Equus sp. que permite asignarlos al Pleistoceno inferior-medio. El depósito se encuentra constituido por, al menos, cuatro unidades fluviales superpuestas y solapadas con un importante evento interno de tipo «cut & fill», con una potencia superior a los 17m. Todas estas unidades se encuentran fosilizadas por un glacis de cobertera que enmascara su morfología aterrazada. Tanto las facies de llanura de inundación de los depósitos fluviales, como los depósitos de piedemonte previos al desarrollo del glacis que los fosiliza registran un importante proceso de carbonatación, dando lugar al desarrollo incipiente de calcretas. El análisis textural y composicional de dichos depósitos y los del Valle inferior del Manzanares revelan similar procedencia, indicando que fueron depositados por un mismo sistema fluvial: un antiguo sistema Manzanares-Guaten (Pleistoceno inferior-medio).

ABSTRACT. In this study the geomorphological, stratigraphical and paleontological record of the Quaternary fluvial arkoses of the Prados-Guatén Depression is analysed. These arkosic deposits, of fluvial origin, are recorded in a composite fluvial terrace (+15-40m). This terrace level consists of four main overlapping and/or offlapping fluvial units, showing an accumulated thickness up to 17m and displaying a main dissectional event which is recorded by an important cut & fill episode linked to the sedimentation of the younger fluvial unit. Mammals remains localised at the base of this last unit (Mammuthus meridionalis and Equus sp.) indicate a Lower-Middle Pleistocene age. All these fluvial units are capped by an upper glacis level, which masks the former staircase topography of the ancient fluvial valley. An important process of carbonate accumulation affects both the more surficial fluvial plain facies (gredas) and the ancient piedmont deposits of the zone. In the latter, carbonate accumulation gave place to incipient calcrete development. Provenance analyses (texture and composition) of the Quaternary fluvial arkoses compared with fluvial arkoses deposited in the lower Manzanares valley indicate that both deposits had a similar source area (60-90% granites). These data suggest that both arkosic materials were deposited by the same fluvial system, an ancient Manzanares-Guatén system which during the Lower Pleistocene and the early Middle Pleistocene drained the Depression flowing directly into the Tajo River. This ancient fluvial system was captured by the Jarama River downstream of Madrid City. The capture process was triggered by the middle Pleistocene tectonic crisis linked to the development of the large gypsum scarpments bounding the main fluvial valleys in this sector of the Madrid Basin. The eventual process of carbonate accumulation can be interpreted as the environmental signature of valley abandonment in response to the expected abrupt change in the hydrological behaviour of the Prados-Guatén Depression.