Publication: 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)
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Publication Date
1997
Authors
Tortosa, A.
Fernández García, Paloma
Palomares Herranz, Marta
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Sociedad Geológica de España
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Abstract
La caracterización composicionaI y textural de los depósitos arenosos de terrazas de los
ríos Adaja, Voltoya y Duero ha permitido resolver la procedencia de otras secuencias de terrazas,
relacionadas con estos ríos pero hasta ahora de difícil asignación genética, como son las situadas al SE
de Pozáldez y en el área de Olmedo. La secuencia de terrazas del área de Pozáldez se ha dividido en
dos grupos: el primero, correspondiente a las terrazas más antiguas, está relacionado con el
retrabajamiento de las terrazas del Duero por el río Voltoya durante su encajamiento. El segundo
grupo, formado por las terrazas más modernas, se relaciona con los depósitos del río Adaja, marcando
el límite de ambos grupos el momento de la captura de este último por el Voltoya. Por otra parte, la
secuencia de terrazas del área de Olmedo refleja una gran similitud con las arenas del sistema del
Voltoya, pudiendo ser considerado como parte del mismo. La integración de los datos geomorfológicos
y petrológicos ha permitido elaborar un modelo de evolución de la red fluvial basado en capturas
sucesivas entre los ríos Adaja, Voltoya y Eresma.
[ABSTRACT]
The analysis of the composition and texlures 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 aboye.
TheAdaja terraces are constituted by arkoses with crystalline rock fragments and a light dominance of
k-feIdspar over plagioclase grains, representing the more immature deposits analysed. The Voltoya
terraces al so 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 lhe Volloya 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 VoItoya
terraces, so that its origin is related to this terrace system. The integration of geomorphologic and
petrologic data permits lhe reconstruction of the evolution of the fluvial network, based on successive
channel piracy. Firslly, lhe 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 lhe
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.