Características y evolución de los sedimentos lacustres miocenos de la región de Tordesillas («Facies de las Cuestas» en el sector central de la Cuenca del Duero)

Abstract

In the centrai sector of the Duero Basin, the region situated to the north of the town of Tordesillas (National Topographic Map Sheet N? 371, Spain) exhibits the presence of detritic and carbonated materials of Miocene age represented by four major lithostratigraphic Units (Figs. 1 and 2).The present work focuses on the stratigraphie and sedimentological aspects of one such unit known as the «Facies de las Cuestas» which has a clear lacustrian character. The correlation of the different uphfted sections (Figs. 3, 4, 5, 6 and 7) by guide levels characterized by accumulations of Gasteropods, points to the existence of three lesser Lithostratigraphic Units (III A, III ? and III C) vahd for the region and neighbouring sectors, corresponding to a further three positive megasequences (Fig. 10). The basal part starts in the infradjacent fluvial sediments.The facies of these Units define several lacustrian subenvironments: marginal lacustrian (litoral and sublitoral) and open lacustrian, which have been sporadically subjected to process of palustrian and pedogenetic diagenesis.The facies (Fig. 8), geometry of the lithofacies (Fig. 9), associations and sequences (Figs. 10, 11, 12, 13 and 14) are suggestive of a lacustrian sedimentation model and of its evolution as time progressed from freshwater to salt water conditions (Fig. 15).