Thesis Type:

PhD Thesis

Abstract:

This thesis describes an effort to combine various specialties of earth sciences (applied geophysics, hydrogeology, tectonics, stratigraphy, geomorphology), with the final objective to determine the subsurfacial geological conditions of the basin located on the plateau, north of the city of Tripolis and the assessment of its hydrogeological conditions. The wider region of the Tripolis plateau faces shortage problems for fresh water supply and irrigation. The small number of water consuming infrastructures and natural springs, that lie around the margins of this plateau with relatively small supplies, cannot satisfy the existing irrigation and watering needs of the wider region and particularly those of the city of Tripolis. For the successful exploitation of the water consuming infrastructures, the investigation of the geological and hydrogeological conditions of the Tripolis plateau was highly required. The geological and borehole data for this area were limited, mostly based on projects that had been accomplished in the past, but no systematic study on the determination the subsurface geological and hydrogeological conditions of area was carried out. Given that homogeneity in the distribution of measurement points is required for the best coverage of the research area, the (geophysical) geoelectrical soundings, using the Schlumberger array, proved to be the most suitable for this application. Seventy two (72) in situ electric resistivity measurements were scheduled on the geological formations outcropping throughout the area, ten (10) in situ electric resistivity measurements were scheduled next to already drilled boreholes and two hundred thirty five (235) resistivity soundings (VES), were carried out for investigating the deeper structures.
After the interpretation of the geological and geophysical data, ƒ the lithology, the thickness and the hydrogeological behaviour of the postalpine sediments were investigated, ƒ the shape of the alpine basement was clarified, and ƒ the tectonic structure of the alpine basement was mapped.
In particular, the shape and the thickness of the post-alpine sediments were determined. Two different environments of deposition were detected and two individual sub basins were located (a) the northernmost sub basin of NestaniMantineia and (b) the southernmost sub basin of Tripolis. The shape and the depth of the alpine basement were defined in detail for the majority of the measurement sites, with the exception of an area around the Tripolis Industrial Region (VI.PE.), where the thickness of the recent sediments is estimated to be exceeding 250 metres. By applying this methodology, the investigation of several structural zones (faults, thrusts) and the delimitation of reversed structures (Nestani, Merkovouni) of the Pindos unit formations, were made possible. The collected information and data were organised and imported in a Geographic Information System for future enhancement, acting as a dynamic digital geodatabase for the whole area. From this process new information was derived concerning ƒ the assessment of hydrogeological conditions, regarding the subsurface water reservoirs, and ƒ the investigation of the sinkhole system growth and operation throughout the basin. Typical optimum conditions for water reservoir existence were located at the regions where -due to the reversed structure of Pindos unit formations- the permeable limestone overlie the semi-permeable marly layers or the impermeable flysch. Such regime was detected in the region of eastern Mantineia. Similar regime was detected at the Merkovouni reversed structure, with the difference of the limited volume of the limestone and therefore the control of the regulative reserves should be very careful. Favourable conditions for water reservoirs can also exist in regions where the alpine basement consists exclusively of carbonate formations of either Pindos unit or Tripolis unit. The water existence is controlled by the purity of carbonate formations regarding the argillaceous or siliceous admixture, the degree and level of karstification and the orientation of the main tectonic structures (faults, thrusts). Regarding the Pindos unit carbonate formations, the geophysical measurements showed that the limestone with resistivity values less than 350 Ohm.m, are compatible with water existence, while resistivity values larger than 450 Ohm.m, imply the predominance of compact and arid Pindos unit limestone. Water reservoirs inside limestone beds were detected in the central parts of the Saga-Nestani sub basin, in the central parts of the Milia sub basin and at the Tripolis industrial region. On the contrary, the regions where the alpine basement consists of either the semipermeable marly limestones or the impermeable flysch, are characterised by the lack of water presence and consequently these areas are not ideal for new water consuming infrastructures.
As far as the growth of sinkhole systems, the results of this thesis made clear that the development of the karst network is connected exclusively with the presence of the pure carbonate formations of Tripolis unit. It is remarkable that in the cases of Nestani and Kanata sinkhole systems, developed within the Pindos unit formations, the geophysical research revealed that they tectonically overlie the Tripolis limestones, without the interference of the Tripolis flysch, at a relatively small depth, thus facilitating the development of karst networks in these regions.