Sedimentología de los complejos lacustres miocenos de la Cuenca de Madrid

Abstract

La Cuenca de Madrid presenta un registro de sistemas lacustres bien desarrollados a 10 largo del Mioceno. Los depósitos correspondientes a estos sistemas lacustres quedan integrados en tres unidades tectosedimentarias mayores reconocibles a 10 largo de la mayor parte de la cuenca. La Unidad Inferior o Salina esta constituidapor depósitos caracteristicos de sistemas lacustres evaporiticos con salmueras de naturaleza sulfatada-clorurada sódica. Dichos sistemas lacustres cerrados tuvieron un carácter perenne, en conjunto muy somero, con un episodio Último que favoreció el depósito de thenardita. La Unidad Intermedia contiene depósitos carbonatados y yesiferos correspondientes a sistemas lacustres salinos más diluidos, presentando una distribución de facies netamente controlada por la heterogeneidad litológica de 10s bordes de la cuenca. Dicha unidad culmina con un extens0 desarrollo de sistemas lacustres, reflejando un episodio de endulzamiento de 10s cuerpos de agua lacustres, posiblemente controlado por factores paleoclimáticos. La Unidad Superior presenta un mosaic0 heterogéneo de sistemas fluvio-lacustres con amplio desarrollo de facies tobáceas y acumulaciones de oncoides, reflejando en conjunto un periodo de fitoproducción intensa.El control paleomorfológico sobre el desarrollo de estas facies aparece ligado a un contexto exorreic0 que marca una diferenciación evolutiva clara en relación con las unidades miocenas previas. Este episodio Último culmina la evolución desde sistemas lacustres netamente evaporiticos a sistemas predominantemente carbonatados. Dicha evolución es discutida en términos de agotamiento en el aporte de solutos, variación en las condiciones climáticas y funcionamiento tectosedimentario.The sedimentological analysis of lake deposits of the Madrid Basin in central Spain provides evidence of outstanding changes in the sedimentary lacustrine record throughout the Miocene. The Madrid Basin is one of the three most important Tertiary continental depressions in the interior of the Iberian Peninsula. It was filled by up to 3,500 m of tertiary sedimentary successions, from wich near one third corresponds to miocene deposits. The Miocene record is made up of three main tectono-sedimentary units each of them containing well developed lacustrine formations. The Lower or Saline Unit is characterized by saline deposits where anhydrite, magnesite, halite, fine clastics as well as significant deposits of glauberite and thenardite forn the commonly observedfacies associations. The sequential arrangement of these facies may be interpreted as characteristic of highly saline, perennial lakes that were developed in a closed basin. The evaporites deposited in these lakes are thought to be mostly derived from Paleogene and Mesozoic formations extended to the East of the Basin, through recycling of evaporite units occurring in these formations. The Intermediate Unit is made up of a rather monotonous sequence of carbonate and gypsum deposits, the latter having been more widely developed in central and easterly areas of the basin. A complex mosaic of marginal lacustrine facies can be recognized in response both to varied depositional regimes of the associated alluvial systems and source areas. Economic sepiolite and ccbentoniten deposits, micaceous sandstones, carbonates (dolostones andlimestones), detrital gypsum, and chert form the characteristic facies of the lake margins. Central lake facies mainly consist of massive beds of chemically precipitated as well as detrital gypsum which are usually interbedded with carbonates and thin mudstone beds. In general, the lacustrine facies of the Intermediate Unit were deposited in a shallow, perennial, moderately diluted lake under semiarid climaticediment conditions. This pattern changed towards the top of the Unit (Lower Vallesian), where a shifting towards wetter and colder conditions can be recorded. Lacustrine deposits developed in this terminal stage are dominated by fresh-water carbonates as well as slightly deeper lake facies in most of the region. The Upper Unit of the Miocene is composed of clastic and freshwater carbonate deposits that overlie a complex paleokarst surface in northern and central parts of the basin. The analysis of the stratigraphic relationship between sediments belonging to the Upper Unit and underlying formations leads to conclude that paleomorphological heterogenities within the basin strongly controlled the sedimentation in this period. Travertines and oncoid limestones werelargely formed from dissolution of underlying karstified carbonates and gypsum, their deposition taking place in river-related marshes, shallo,w lakes and springs. The evolution of the lacustrine systems in the Madrid Basin throughout the Miocene is clearly indicative of the exhaustion of a prima1.y evaporite basin through more and more dilute stages unti1 a net fresh-water episode at the top of the Tertiary sequence. The process was, in general, governed by the tectonic behaviour of the basin margins, which controlled the recycling of source evaporite formalions, though climate could also influence episodically this pattern. It is suggested that the basin underment a dramatic change from closed geomorphic and hidrologic conditions to exorheism since the upper Vallesian