Análisis petrológico de depósitos de terrazas aplicado al estudio de los procesos de captura en los ríos Adaja, Voltoya y Eresma (provincias de Segovia y Valladolid)

Laburpena

The analysis of the composition and textures of the sandy deposits of terraces of Adaja, Voltoya and Duero rivers permits to solve the provenance of other terrace sequences, as those appearing at the SE of Pozáldez and near Olmedo, without a clear parentage with the rivers mentioned above. The Adaja terraces are constituted by arkoses with crystalline rock fragments and a light dominance of k-feldspar over plagioclase grains, representing the more immature deposits analysed. The Voltoya terraces also consist of arkoses with a greater dominance of k-feldspar over plagioclase and with the occurrence of metamorphic rock fragments. The Duero terraces are formed of subarkoses with low content in rock fragments, showing a great dispersion of the contents in feldspars and rock fragments typologies. Based on these data, the upper terraces appearing near Pozáldez are constituted by the reworking product of Duero terraces by the Voltoya river during the incision of its channel. The lower terraces are constituted by recent terraces whose origin is related to the Adaja river. Thus the limit between these two groups of terraces represents the capture of the Adaja by the Voltoya river. On the other hand, the terrace sequence near Olmedo shows a great similarity with the deposits of the Voltoya terraces, so that its origin is related to this terrace system. The integration of geomorphologic and petrologic data permits the reconstruction of the evolution of the fluvial network, based on successive channel piracy. Firstly, the Adaja river was captured by the Voltoya river, and subsequently both the Adaja and Voltoya rivers were captured by the Eresma river. The origin of this set of captures is the general trend to the NE of the groundwater flow, caused by tectonic tilting, and improving the hydraulic efficiency of the basin. Finally, the composition of the terrace deposits was greatly influenced by the piracy processes that produced a new input of sediments with different provenance. Incision of the channels also affected composition by reworking of intrabasinal sediments. Both processes acted by mixing sediments producing a homogenisation of its composition. As a consequence, an important loose of detailed information about the source area of the different feeders occurs.