Paleogeografía del Eoceno Superior-Oligoceno en el SO de la Cuenca del Duero (España)

Abstract

Large fluviatile systems related to the erosion of the SE and SW borders (edges) of the Duero Basin flowed during the Late Eocene. The alluvial system attached to the W/NW areas laid down white sandstone, microconglomerate and conglomerate (Figs. 4, 5, D, E) that slow, but continuously, prograded towards the SE and ? changing in the process the channel pattern from sandy braided to proximal conglomerated braided (Figs. 7A y B). The fluvial system draining the metamorphic areas placed towards the SE and ? laid down the reddish median to fine sands referred to as Aldearrubia Sandstone Formation (Fig. 4, 5A). At first the channel pattern of the system was of median to high sinuosity, with large flood plain areas under pedogenic processes, but a change to sandy distal braided took place along with time (Figs. 6 y 7A-B). By the Uppermost Eocene, both systems coalesced around the present day Vallesa de Guareña area, generating a mixed, indented white/reddish succession. Eventually, the unstable balance broke down under the effects of the Pirenaic (?) orogeny, which strongly favoured the enlargement of those systems flowing from the northwestern edges of the basin. Consequently, the Aldearrubia fluvial system was pushed towards the east and, eventually, became inactive. The southwestern areas of the basin also suffered the effect of the unstability and large alluvial systems (Teso de la Flecha Conglomerates) brought down quartzitic sediments eroded away from the Peña de Francia Mountains (Early Ordovician). As a conclusion, it is assumed that the sedimentation during Late Eocene times was of fluvial origin, whereas large alluvial systems prograded in the Oligocène times in responde to tectonisn.