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Reservoirs

Taibilla reservoir

Territorial framework.

Geology and seismology.

Regional Geology.

The reservoir is located within the Zona Prebética, in the northernmost part of the Cordilleras Béticas.It constitutes a basically autochthonous body.

The Zona Prebética can be considered from two aspects: the sedimentary and the structural, and within this we can distinguish two tecto-sedimentary areas: the Prebético Externo (outer) and the Prebético Interno (inner) (the inner running seawards and the outer running towards the Meseta. Within the Prebético Interno we can distinguish the southern Prebético. It is here that the natural downstream boundary for the storage area of the dam would be established.

The Prebético Interno has in general a Cretaceous covering although this is broken at times by the Jurassic, allowing the appearance of great aquifers, such as the Hydrogeological Units of Taibilla and Revolcadores-Serrata, which are affected by the dam in its south-western area. Elsewhere, the Cretaceous layer is not dismantled, although its limy content allows the creation of aquifers, as is the case of the hydrogeological unit of the Sierra Segura-Cazorla.

Within the Prebético there are four subzones each associated with their own tectonic styles- Subzone One would be the Prebético Interno with its huge folds and fault folds. This coincides with the tecto-sedimentary domain with its wide Mesozoic and Tertiary covering. The area of the natural downstream boundary for the storage area of the dam is situated on this layer, which breaks the surface in the Cretaceous outcrop below the Tertiary layer immediately downstream from the dam. The storage area upstream of the dam is situated on the lower lip of the fault and within the Tertiary layer formed by limestones and loams from the Upper Oligocene.

Regional Geological Outline
Regional Geological Outline

Geology and Soils of the Natural Downstream Boundary.

The Prebético Interno where the dam is located constitutes a subzone of the Zona Prebética, whose general style is characterized by the existence of large folds and in some cases fault folds.

Although an autochthonous body, it has undergone large internal shifts on account of the large cleft fault lines. Specifically, the area is located between the great fault lines of Socovos (or the “Linea Eléctrica”) to the North, and the Falla de Tíscar (Tíscar Fault), in the SW area of the Sierra de Cazorla. These cleft fault lines shifted some tens of kilometres, following the movement of the base line. In tectonic terms, the different fault lips are more submerged the further south one goes. These upliftings may be hidden under the Tertiary layers mentioned in the section above.

The area to the south of Sierra de Zacatín, where the reservoir and its basin are located, is sunken from the fault line which crosses the Estrecho del Aire from west to east. The Sierra has countless relief fault lines and folds running from south-west to north-east (locally west to east), and hence the raised blocks are to be found to the north-east and they descend towards the south, although the lesser erosion of the Sierra (formed by competent calcareous materials) means that topographically it is higher than the Taibilla reservoir and valley, in general the synclinal axial areas have sunk more and the anticlinal ones have been raised more. The Sierra de Zacatín belongs to the south-western edge of the “Anticlinorio de Socovos-Sierra de la Muela”.The several inverted fault lines and overfolds which affect the marine Miocene materials may be due to surface compressions caused by reductions in the Cretaceous base level coming from deep separating fault lines.

There are also neotectonic indices to be found in the area, such as the existence of two terraces in the Segura (Quaternary activity), the existence of normal fault lines in the alluvial materials from the Upper Miocene and Pliocene conglomerates.

In the area of the natural downstream boundary itself, we find the confluence of a synclinal axis with a normal fault line. The normal fault line clearly follows the direction of the fault lines and folds of the “Anticlinorio de Socovos-Sierra”, which may indicate that their formation is prior to the synclinal folding, whose north-south direction points to a more recent formation. This folding does not fit in chronologically with the remaining SW – NE and may be the result of a fold in the covering due to the falling of inner plates which did not affect the raised lip of the fault line, as no continuation of the fold in question is detected.

Geology and Soils of the Natural Downstream Boundary
Geology and Soils of the Natural Downstream Boundary.

At certain points in the area there appears an intra-Miocene fault line which separates the northern part (immediately downstream from the natural downstream boundary) from the southern part (natural downstream boundary and storage area upstream), which has a completely different stratigraphy. The northern part, made up of Eocene limestones, is more fractured. Hence, at the point where the fault crosses the river, there is another fault plane which is angled northwards at almost 90º. There is also a series of fault lines and lesser fractures around the main fractures which are of the same origin.

The southern area, with its more ductile and less competent Tertiary materials, is subject to the same deformations as the northern one, although here they are reflected in a series of folds and deformations of the covering, in turn reflected in the scarce continuity of the strata in the geotechnical profiles taken in the natural downstream boundary.

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