Geological and structural framework

The town of Velestino is located at the base of the eastern slope of the Central Hills, also referred to as Revenia (Sivignon 1974), which represent one of the major structural highs of Thessaly (Caputo 1990) bounded by the Larissa Basin to the east, and the Karditsa Basin to the west. This hilly area is mainly characterized by Pliocene to Upper Pleistocene weakly deformed subhorizontal sediments. In the broader study area, these deposits uncon-formably cover different lithologies of the Subpela-gonian Zone, among which are rocks of the ophiolitic suite, limestones and different terrigenous materials. A low grade metamorphism generally affects all these rocks.

The tectonic setting of the area is complex because it is characterized by some overthrusting units with a southward vergence, as inferred from the regional dips towards NNW (Fig. 2). In particular, within the study area, the principal tectonic unit

Fig. 1. (a) Location map of the Yperia Krini spring (star) and of wells W-1 and W-2 (squares). The box indicates the location of Figure 2a. (b) A representation of the spring in the eighteenth century.

consists, from base to top, of mafic rocks, limestones with rudists, a volcanoclastic sequence grading upwards into siliciclastic deposits.

Due to differential erosion, the Cretaceous limestones generate an almost continuous ridge from Mount Malouka, near Velestino, to the Chalkodonio Massif. The thickness of this carbonate stratigraphic unit is highly variable, being more than 200 m at Mount Malouka, but decreasing westwards to only a few metres (Fig. 3). Further to the west, the huge carbonate body, which generates the Chalkodonio Massif, represents a different and higher tectonic unit. It is noteworthy to say that, in this area, the two carbonate units are in direct (i.e. tectonic) contact and this observation will be a key point in the discussion about the hydrogeolo-gical system connected to the Yperia Krini spring.

During Pliocene times, when most of Thessaly was a lacustrine area, these carbonate massifs represented small islands that were never completely

Fig. 2. (a) Simplified geological map of the investigated area (see Fig. 1a for location). Key: 1, Yperia Krini spring; 2, normal faults; 3, thrusts; 4, Holocene alluvial deposits; 5, Upper Pleistocene alluvial deposits (Red Beds); 6, Pliocene fluvio-lacustrine deposits; 7, Cretaceous terrigenous sediments; 8, Cretaceous limestones; 9, ophiolitic rocks. (b) Geological sections across the area (locations shown in (a)).

Fig. 2. (a) Simplified geological map of the investigated area (see Fig. 1a for location). Key: 1, Yperia Krini spring; 2, normal faults; 3, thrusts; 4, Holocene alluvial deposits; 5, Upper Pleistocene alluvial deposits (Red Beds); 6, Pliocene fluvio-lacustrine deposits; 7, Cretaceous terrigenous sediments; 8, Cretaceous limestones; 9, ophiolitic rocks. (b) Geological sections across the area (locations shown in (a)).

submerged nor covered by younger sediments as occurred all around there. The onlap geometry of the lacustrine deposits can be clearly observed near the carbonate massif of Kalo Nero, representing a very similar geological setting (Caputo 1990). From Late Pliocene and during the Quaternary, two major deformational events affected the region and caused the evacuation of

Fig. 3. The morphological ridge of Mount Malouka consisting of limestones with rudists characterized by highly variable thickness and representing an important 'bottleneck' within the hydrogeological system (Lm, Cretaceous terrigenous sediments; Fy, Cretaceous limestones; Oph, ophiolitic rocks).

waters from the lake, the consequent growth of a hydrographic pattern and the uplift of the Central Hills. This latter phenomenon caused the deep entrenchment of the former sediments and the deposition of thick but localized alluvial bodies along the borders of the structural highs. The town of Velestino is located along such a morphological escarpment and the Upper Pleistocene Red Beds are largely represented (Fig. 1). The continuous tectonism affecting the region caused further uplift of the area and the consequent entrenching of the youngest sediments, as can be observed immediately south of the town of Velestino (Fig. 2).

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