HYDROGEOLOGICAL DATA FROM SE AS A PART OF MARGINAL AREA OF THE GREAT HUNGARIAN AND BASIN

DE. M. KASSAI*-Â. LOBBBBEB**—L. R6NAKI***-DB. T. SZEDERKÉNYI* * Hungarian Geological Institute, Regional Geological Service, Pecs, . ** Research Institute for Water Resources Development, , Hungary. *** Ore Mining Company, Pecs, Hungary.

Geological review of SE Transdanubia

The area of SB Transdanubia is geologically divided into four parts according to the structure of the pre-Tertiary basement and thickness of covering Neogene deposits. On the pre-Tertiary basement contour map (Fig. 1) —which can be regarded as a Neogene isopach map—the following distinct parts are recorded : a) The Neogene Depression of Drava Basin. Thickness of the Neogene sequence increases from 1000 m up to 3500 m showed a fairly uniform SW trend. The basement of this area is generally composed of Precambrian meta- morphic rocks. This depression—as a —is the subject of our detailed investigation. b) The area of Mecsek — Villâny Anticline. Thickness of the Neogene de­ posits on the highland areas goes from 0 m to 500 m but on the southeastern and northwestern flanks of the anticline they enlarge from 500 to 1000 m thickness. This situation of the anticline is motivated by the trench of the "Mecsekalja Tectonic Belt" filled up by more than 500 m thick Neogene deposits. The core of the anticline consists of Precambrian metamorphic rocks, granites and Late Paleozoic sedimentary formations. The bottom part of the flanks consists of Mesozoic cavernous carbonate rocks (see Fig. 2). c) Neogene trench-like area between - and Basin. Thickness of the Neogene sequences of both basins surpasses at least 1000 m. The bottom part is generally composed of karstic carbonate rocks. This Neogene tectonic zone is located to the area of so-called "Villâny — Paleozoic Deep Fracture Zone" and it is regarded as a Neogene renewal of the Late Paleozoic movements. This area is isolated from the great basin, therefore further details are omitted. d) The area located to the east from the Villâny—Szalatnak Paleozoic Deep Fracture Zone where some strips of the rocks can be found with particular geological structures, bordered by fractures striking NE —SW direction. This area is also separated from the great basin. Concluded from the mean-thickness distribution of the Neogene deposits

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\> MÈ2 404 as well as tectonic inference, the main tectonic zones of the investigated are the following ones : 1. NW —SE striking fracture zone at the eastern border of Drava basin, which is a notable water-flow zone too. This gives a possibility for fundamental inferences related to the investigation of great basins. 2. Tectonic zones with NE — SW trend represented in the area of Mecsek — Villâny anticline (Villâny Mts. and northern side of Mecsek Mts) as [well as trench-depression of "Mecsekalja Tectonic Belt". 3. NW—SE striking Neogene tectonic zone coinciding with "Villâny — Szalatnak Paleozoic Deep Fracture Zone" having some tectonic elements which are deeply wedged into pre-Tertiary basement. 4. Tectonic zones having a NE—SW trend which are located to the eastern part of the area.

Hydrodynamie investigations of the deposits j)f Drava "great basin"

Neogene deposits of the young depressions of the examined area are rather uniform. The basement is overlying by predominantly impermeable Middle Miocene clayey-marls, clayey-calcareous sandstones, tuffs and lavas, as well as a Lower Pannonian sequence; and all these at several localities have some separated sandstone lenses with concentrated salty-water content. The oil-prospecting boreholes explored Badenian "Leitha limestone" lenses above the uplifted basement frequently containing waters of extremely high pressure. In loose and porous sandstones Of the Upper Pannonian complexes there are utilizable thermal waters as reservoir fluids with low salinity. In the investigated area such an aquifer is drained by the well of Pettend, village supplying artesian water of 39.5 °C temperature. Similar water quality has the thermal water of , with 48 °C temperature. According to many signs near to the surface, the Upper Pliocene and Pleistocene formation waters and phreatic ones form a single, continuous porous water-bearing formation. Salt content of the Upper Pannonian layers is the same as the sea-water, however, in the lower part of the Upper Pannonian stage brackish-water can be found, during the upflow it changes gradually into fresh-water (B. KLEB 1973). According to the geoelectric soundings carried out on the area, the "regionally impermeable floor" of the coarse-grained complexes near the surface is more or less on the level of the lignite beds of Upper Pannonian complex, which at the same time is the boundary between salty and fresh-water. (See on the hydrogeological cross-section Pig. 3. designat­ ed with "A—B", marked by a resultant line.) Relationship among aquifers occurring in different depths was examined for the first time by method of continual approaches after E. ALMÂSSY (1962). It was established that static pressure of the Quaternary, Levantian and several Upper Pannonian beds of the area shows regular alternations which have relationship with morphology of the "impermeable floor" of the porous complex. Above the uplifted basement the static water levels of the deeper wells are lower-, and on the depression they are higher than the static water- levels of the shallow wells. The static level setting in at different points of the 405 aquifer complex, or formation pressure-values are belonging to the hydrostatic pressure state. The vector sum determined is the so-called "piezometric gradi­ ent", analogically with the hydraulic gradient, derived from the potential .theory of the seepage. When the formation pressure is measured the vertical component of the piezometric gradient is 10-Ap y, [grad uL ' y-Az yQ where Ap is the measured pressure difference in [atm], z is the difference in drainage elevation [m], v is the mean specific gravity of the water head [p/cm3], 3 y0 is equal 1.0 [p/cm ] of the specific gravity of the water on 4 °C temperature. When the specific gravity of the water is unit, then this value is directly determinable from the piezometric surfaces : Ah [grad u]2 % [grad] hz=—^ .

If in some chosen horizontal planes (in the so called "drainage horizons") the values of the piezometric surfaces reduced to the water having an unit specific gravity are available (see in Fig. 4), then it can be obtained average values of the vertical components of the piezometric gradient regarding certain depth intervals as a differential quotiens of the contour lines of the suitable piezometric surface maps. Values of the horizontal components can be directly measured from the individual water level contour maps, and average values referring to the intermediate depth parts can be determined by arithmetical mean of two adjacent hjx or hjy gradient maps (I. LORBERER-SZENTES and A. LORBERER 1976). The Fig. 5. shows the differential quotiens of contour maps, namely the average values of the vertical components of the piezometric gradients between + 0 and + 50 m above Adriatic sea. On the highlands and hilly elevated areas, forming the main reproduction bases of the ground waters, the values of the gradients are negative, in the depressions they are positive, referring to the decisive horizontal flow and descendent as well as ascendent water movement. If the surface or near surface derived colder waters are descending, then the rocks which are connecting with them will be cooling down and due to the heat-extraction they will be converted into thermal waters. If ascendent water flow occurs then a heat emission will take place. Consequently, a vertical water movement is followed by local negative as well as positive geothermal anomalies due to the convective heat transport. On the map of the apparent geothermal gradients (Fig. 6) the areas of negative heat anomaly (larger than 20 m/°C value patches) generally coincide with areas of negative vertical piezometric gradients. On the area of positive heat anomalies with 10 m/°C or less values, the vertical components of the piezometric gradients are also positive. The most conspicuous common feature of the piezometric gradient and geothermal as well as water quality maps (Figs. 5—7) is the definite structural orientation. The piezometric pressure profiles (Fig. 3a and 3b) represent not only potential distribution of the investigated area, but they also give data regarding universal characteristics of the events which had taken place within the basin 406

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412 deposits. Since the multiannual average value of the natural subsurface flow conditions can be regarded constant, the potential distribution represented by water level contour maps and piezometric pressure profiles, corresponds to the permanent seepage condition. With respect to the flow conditions, most decisive proofs are provided by local geothermal anomaly which can be exclu­ sively explained by convective heat-transport. That is ought to observe a nega­ tive heat anomaly in the depression having a larger thick deposit mass, on the basis of the heat-flow conductivity, — as it was indicated by heat-flow measure­ ments made in the deeper horizons by T. BOLDIZSAR (1964) and L. STEGENA (1973). We have not sufficient data available to determine the seepage velocity on the basis of the change of the heat-flux (I. LORBEREE-SZENTBS —A. LOBBERER 1976) ; — in this respect the investigated area is too complicated.

Connection of the ground water resources of the basin deposits with the waters of the deeper reservoirs

According to the investigations, along several main tectonic lines the local pressure-, temperature and hydrochemical anomalies can only be explained by hydrodynamic communication between basin deposits and deeper ther­ mal water reservoirs. It is well known that in a flow system the effect of the local draining and the surface relief have less and less scale (M. ERDÉLYI —GY. KO vies et al. 1972). Since a considerable part of the water-mass flows in subsurface layers having a better transmissibility, therefore approached to the "impermeable floor", the absolute values of the piezometric gradients also decrease. On the other hand, under descending conditions (e.g. Szigetvâr region), the positive values of the vertical components of the piezometric gradients become higher and parallelly with this phenomenon, the geothermal gradients which can be calculated from deeper wells are decreasing; thus the heating effect does not decrease but it parallelly increases with the depth. It was conspicuous that change of the Cl~ ion concentration in several wells (along with 120 — 350 mg values) drilled on Pannonian layers, which proportionally was heightening by the increase of drawdown. These wells are located in a small zone of the southern part of the town about 1.5 km distant from the thermal well of Szigetvâr (which is situated in the northern part of the town) could not exert influence on the latter, because they were constructed earlier. Therefore it is acceptable that the Albian limestone—which comprises the basement below Szigetvâr—with the connecting deeper porous thermal water reservoir, also under natural condition provides a supply basis for the basin deposits (L. RÔNAKI—T. SZEDERKÉNYI 1966). Fig. 3a shows this upflow along the fracture, which is not too considerable under natural condition, but during the intensive water production from the subsurface layers, destruc­ tive changes can be taken place in the water quality of the "productive layers" because of the vertical water flow (A. LOEBERER 1975). The extreme positive heat anomaly is restricted on the buried carbonatio mass of Harkâny. Based on the current system of the karstio waters of the Villâny Mesozoic zone, it can be supposed that the waters derived from Villâny Mts. were descended on a nine-times larger area as a free infiltration one. The concentrât- 413 ed upflow of the heated karstic waters proves a fundemental hydrogeological role of the Villâny —Szalatnak Paleozoic Deep Fracture Zone (M. KASSAI 1972). It is proved by above mentioned reasons that on the marginal area of the and Drava Basin the hydrodynamic connections be­ tween basin deposits and deep thermal water reservoirs are linked to the main marginal structural lines of great basins, and further research trends are determined by this fact.

Conclusions

1. On the area of the great Neogene depressions of Drava Basin, the piezometric gradient maps and water-quality maps drawn to different drainage horizons, as well as the areal distribution of the geothermal gradients show an appreciable tectonic orientation which first of all proves a decisive effect of the neotectonic fracturing on the flow-relations of the ground waters. The large tectonic zone striking NW—SE direction, appearing at the eastern wedging boundary of the basin deposits can generally be characterized by ascendent waters and positive geothermal anomalies (Szigetvâr, Harkâny). 2. It can be directly proved the connection between basin deposits and deeper reservoirs which is realized by marginal fractures of the basin. The proofs are sudden changes in pressure, temperature and water quality. 3. According to the results of these investigations, thermal water yield may be most successful and economical on the marginal parts of the great basin. 4. In generalizing the above-mentioned statements it can be said that in the great basins thermal water research possibilities are connected with margi­ nal fracture zones of the basins, if the basement is a suitable thermal water reservoir. •5. The research referring to the area of SE Transdanubia as a part of marginal area of the Great Hungarian Plain and Drava Basin proves a feasi­ bility of the methods involving applied hydrogeology and regional hydrogeolo­ gical mapping, if we have suitable geological maps and aspect.

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