THE LITTLE HUNGARIAN PLAIN (KISALFÖLD)

by Scharek, P. and Tullner, T. 1994

Geographical setting

Kisalföld is situated in the NW part of the Pannonian Basin. This geologically forms a post Miocene depression with unified Pliocene (Pannonian) and Quaternary sediments, politically is divided among Slovakia, Austria and Hungary .

Geological setting

Description of Pannonian deposits and the character of the basin's basement

Pannonian sequences attain their greatest thickness approaching 6000 metres at the belt of junction of the two basement complexes of different age (Rába-line). Upon data provided by superficial geophysical measurements, the presence of a 3000-m-thick Miocene complex can be assumed thereby as well. The lack in sediments in the lower segment of Miocene deposits leads us to support the idea of deposition on a preformed basement, but the action of subsequent tectonic effects should also be taken into account. The latter is justified by the series of faults penetrating down to the upper horizon of Miocene beds observed on the K-1 seismic profile and by the partial absence of Sarmatian sequences in the area concerned. Inside the basin, the uppermost level of Pannonian deposits is constituted by the series of the Hanság Formation described by scanty reserves of fauna, the appearance of cross-bedded sands and an outspoken oxidising effect. Presumably, the upper part of the formation bears some volcano-sedimentary complexes including tuffs, tuffites and bentonite assigned as an independent unit to the Tapolca Basalt Formation. At the same time, the Torony Formation underlying the above complex is suggested to be the thickest member of the Pannonian sequence in the basin. In the SE segment of Kisalföld the Tertiary basement is made up of Mesozoic, carbonate rocks containing essentially Upper Triassic dolomites typical in the Transdanubian Central Range. The drillings recovered mudstone, siliceous schist and aleurolite. At the same time, NE from Rába-line the basement is composed of Palaeozoic metamorphic rocks, primarily garnet-muscovite-sericite schist, and mica schist, whereas the drilling Mosonszolnok-1 revealed Lower Triassic - Upper Permian sandstone. It is the Rába-line considered as a principal structural lineament that makes up the junction between the two basement units, along which geophysical measurements located some displacements discernible even in Upper Pannonian formations

Thickness and description of Quaternary deposits

Pannonian deposits are recovered by the lateral cutting of the Danube at Gönyü–. Westward and northward from the line of the Rába and Moson-Danube rivers a sharp increase in thickness of Quaternary sediments can be experienced achieving its highest value in the surroundings of the villages of Sérfenyõsziget and Püski exceeding 700 m. To our present knowledge it can be suggested that the Moson depression discovered during previous surveying is well divided into two parts. In order to elucidate stratification and lithological properties of Quaternary deposits, two structural exploratory drillings have been carried out in the region (at the villages of Arak, Arak-1; and of Tárnokréti‚, Trt-1). Up to the present day the overall characteristic lack in fauna of coarse clastic sediments renders virtually impossible the lithological and chronostratigraphic classification of the thick Pleistocene-Holocene, principally also coarse clastic fluvial sedimentary sequence of Kisalföld. Although the comparative abundance in hydrogeological exploratory drillings furnishes some information on structural aspects of those sequences, core sampling bound to a limited number of intervals and deficiency in data provided by superficial geophysical measurements inhibit a precise stratigraphic interpretation. The structural exploratory drilling at Arak deepened within the framework of the Kisalföld Project with continuous core sampling reached the base of Quaternary deposits in the depth of 358.0 m. It was stopped in Upper Pannonian formations, 400.0 m below the surface. This drilling brought some important results from stratigraphic point of view, it revealed, namely the existence of fine grained horizons with irrelevant thicknesses as compared to that of the whole profile but enabled us to subdivide the coarse, clastic sequence indicated so far as a homogeneous complex by geophysical methods. These thin horizons shelter the rests of some vertebrates and molluscs preserved from mechanical deformation. They were deposited in a low-velocity, rather shallow fluvial environment dividing into 10 levels the coarse clastic sequence. The substantial lithological contrast allowed to determine with high precision the thickness of coarse-grained sediments by geophysical methods even before the drilling at Arak. In conjunction with the information administered by this drilling, the related lithological transition can simultaneously be interpreted as the Pleistocene-Pannonian boundary in intrabasinal areas. It provides a reliable guide horizon for superficial geophysical methods heading from the basin's periphery toward its interior. The discovery that contrarily to previously admitted assumptions, the bulk of the sequence concerned accumulated during the Early Pleistocene period can be interpreted as an outstanding result considering stratigraphy . Additionally, upon data of this drilling, the presence of thicker Holocene sediments in Kisalföld can also be justified. Tracing layers of conglomerates and sandstone recovered on certain levels of this drilling proves to be difficult by geophysical methods, for they have not yet been recovered by any structural exploratory drilling, they cannot thus been counted on for the time being for the lithological classification of Pleistocene complexes. We assume that they occur only locally and they are pinched out fairly rapidly, they cannot thus presumably been used for the subdivision of these sequences.

State of environment - potential pollution sources

In compliance with the objective of this study, susceptibility of ground-water to pollution has been given priority among the number of factors affecting environmental-geological conditions, for progressively intensified pollution of superficial and subsurface waters is one of the not much expected but firmly manifesting side-effects of the current economic-industrial development. The damage caused by this process can substantially be boosted by the eventual improper implantation of a water barrage or other establishments on the most elevated point of the region. In Kisalföld ground-water and other subsurface aquifers, all of them being in intense communication are stored in the Quaternary, principally coarse clastic sedimentary sequence. Differences in the permeability of superficial deposits enable us to depict comparatively well protected areas from pollution described, for instance, by ground-water stored in fine-grained flood-plain deposits, the so-called overburden According to our drilling data, the SE periphery of Kisalföld can be considered as an area relatively securely protected by the existence of an overburden. On the other hand, its central part and the proximity of recent streams do not have any natural barrier that could stop pollution infiltrating from the surface or from superficial water bodies. Setting up waste depositories in this area as well as the effect of stagnant water bodies gradually filled up and eventually becoming enriched in heavy metals would equally provoke processes in the upper part of the alluvial fan whose damage for the environment cannot actually even be appreciated. Apart from its hydrochemical composition, the position of ground-water represents an equally crucial environmental-geological factor in Kisalföld. The rapid and prolonged sinking of ground-water would, namely bring about an ecological catastrophe affecting first of all every vegetation species whose habitat is linked to areas with permanently high ground-water level that would certainly provoke a biological chain reaction with unpredictable consequences.

State of geological knowledge

As elsewhere in our country, detailed geological survey of Kisalföld and its surroundings started in the sixties of the last century. At that time 1:28 800-scale maps were prepared, serving as a basis for the compilation of the 1:144 000 manuscript geological map of Transdanubia in the Geological Institute of Hungary that provided a detailed description even of lowland areas.. The end of this century marks the beginning of pedological and agrogeological mapping in Hungary led by H. Horusitzky, G. László and I. Timkó. Upon results furnished by the Ist. Agrogeological Congress held in 1909, P. Treitz compiled in 1918 the pedological map of Hungary based on the conception of climatic zones and published with explanatory text following Word War I. Between the two World Wars the geological research of the Kisalföld and adjacent territories also received an impetus. Upon data of artesian wells already at hand, H. Horusitzky made around 1929 some still up-to-date assumptions concerning the make-up of Quaternary and Pliocene formations of the basin at Csorna. Investigation of coarse clastic sediments has been launched by E. Szádeczky-Kardoss (1938, 1941) creating a tradition with his methodology. L. Kreybig initiated pedological mapping in this area that has not been, however, finished before the fifties of this century on the scale of 1:25 000. The 1:200 000 pedological map of Hungary compiled by P. Stefanovits and L. Szücs has been accomplished by 1955. The preparation in 1959 of the 1:500 000 map illustrating genetic types of soils is due to the same authors. The initiation and completion of detailed geological mapping of lowland areas is the merit of J. Sümeghy. These 1:25 000-scale maps prepared by 1952 cover the major part of the area and they provided the only unified geological data collection up to the start of our remapping. In the end of the 1950s morphological and hydrological surveying received an impetus. In this respect, the morphological research of S. Láng (1950), L. Kárpáti (1955), M. Pécsi (1959, 1962), L. Góczán (1962) as well as the hydrogeological activity of K. Ubell and A. Rónai should be emphasised. Results of geophysical surveying have been reported by J. Lányi (1960) and V. Scheffer (1965). Within the framework of the 1:200 000 mapping of the whole country, remapping of the Gyõr sheet finished in 1966 followed by its publication in print (F. Franyó et al 1971). Several comprehensive studies have been prepared in association with this latter project. Quaternary deposits and tectonic features have been described by F. Franyó (1967) and Gy. Wein (1971), respectively. Deep-seated geological structures have firstly been summarised by L. Kõrõssy (1963, 1965). In the 1980s, within the framework of the compilation of the 1:500 000 geological map series of Hungary, maps illustrating geology of deep structures have been prepared by Geological Institute of Hungary based on data available in OKGT (Hungarian Oil and Gas Trust). Their publication is underway providing invaluable information on geology of deep horizons in Kisalföld.

Hydrogeological investigations carried out recently in the Kisalföld have been summarised by M. Erdélyi (1971, 1979) and K. Korim (1973) supplemented by studying ecological problems (M. Erdélyi 1991, 1994).

Within the framework of the map series representing geographical land units of Hungary, L. Ádám and S. Marosi published a geographical summary followed by the morphological study of I. Göcsei (1979) in Kisalföld.

With regard to geophysical surveying in Kisalföld, the Geophysical Research Company (GKV) of OKGT carried out regular seismic measurements for hydrocarbon exploration completed by recordings of the Geophysical Institute of Hungary (ELGI).

Recognising the lack of proper geological and hydrogeological surveying in association with the water barrage system Bõs-Nagymaros (GNV) the Geological Institute of Hungary initiated the complex geological mapping of the concerned region as well as the whole Kisalföld prompted by the resumption of the construction in 1977.

The Little Hungarian Plain (Kisalföld) Project

Our geological field-work carried out between 1982 and 1994 included the sinking of more than 4000 mapping boreholes marked on 1:25 000-scale topographic maps.(Scharek P. et al) It was complemented by field checks on areas covered by "preliminary" geological maps and by the description and sampling of exposures. Boreholes have been deepened partly using our hydromotor-driven, hydraulic Rotary drilling machines mounted on UAZ chassis and partly by subcontractors. These machines allowed to carry out 10-to-15-m-deep drillings of 120 mm diameter with sampling at every 20 cm of the profile. This drilling network has been supplemented by engineering-geophysical sounding accomplished by ELGI. Some 6 samples have been taken from the uppermost 2 m of the boreholes followed by collecting 1 sample from every metre up to 10 metres. The so-called registers including the documentation of drillings can immediately be integrated into the national catalogue of drillings and together with data of related laboratory analyses they also support computer-aided compilation of some map variants. Ground-water entries and hydrostatic levels have also been recorded in boreholes recovering it, accompanied by ground-water sampling for hydrochemical analyses.

A substantial change occurred two years ago in the way of compiling and publishing our maps. This change was due to the simultaneous intervention of several factors. First of all, it has to be stressed that the budget of the Geological Institute of Hungary was severely cut preventing us from going on with the time-consuming and expensive traditional preparation of maps for printing. The alternative solution was offered by the afore-mentioned acquisition of the Intergraph MGE product allowing us to capture, process, analyze and output our data in GIS environment. These procedures resulted in the production of the first digital geological atlas in Hungary displaying the sixth (Sopron-Kõszeg) quadrangle in the row of the project. Starting from data capture up to the printing, they have fully been produced using computer-assisted methods. Their output took place on HP plotter. The geological map of the related area was prepared on the scale of 1:100 000, whereas the other ten derived thematic maps are of 1:200 000 scale.

A brief summary of the agrogeological research in Kisalföld

Agrogeological maps making up a division of the Kisalföld's complex geological mapping project serve first of all as a basis for regional agricultural planning, elaboration of regional and local amelioration schemes and environmental management on a highly organized level. The applicability of agrogeological data and information reaches beyond the limits represented by agriculture, they can support projects requiring complex environmental analysis. In a narrow sense, agrogeological division incorporates the following maps:

Together with other divisions of this mapping, agrogeological variants constitute a uniform system based on closely interrelated thematic constituents. Geological, hydrogeological and engineering-geological studies provided fundamental data for agrogeological mapping. They have been complemented by the results of related laboratory analyses, by field mapping information as well as remote sensing data. Laboratory tests of highest priority included the examination of grain-size distribution, lime content, pH and hydrochemical composition. The overwhelming majority of disturbed samples taken from shallow boreholes were subjected to analysis of grain-size composition and lime content accompanied by 4-6 pH examinations of the uppermost 2m section of the profile.

Organic matter content was tested chiefly in two horizons of the shallow boreholes grid. Distribution of samples undergoing detailed pedological testing shows considerably lower density. The chief goal of detailed pedological analyses is to give more comprehensive and accurate description about agrogeological features of wide-ranging, major formations considered as typical in the area. The related data bank based on the analyses of intact cores incorporates results of basic pedological tests as well as pF values and coefficient of permeability deriving from 2 or 3 cores taken from 20-30 shallow boreholes in each quadrangle . The simultaneous interpretation of field mapping and remote sensing data can be considered as an invaluable tool in identifying and qualifying different soil types and factors reducing the soils' fertility as well as in delimiting formation boundaries on the map. This methodology not only contributes considerably to alleviate the project's budget but yields more reliable and accurate picture about agrogeological features of the area.

Genetic types of soils

This map depicts merely genetic types of soils, for the rest of pedological features traditionally outlined on a similar variant make part of some other map versions of the Atlas. The legend of the map adopts essentially the classification of soils applied by the Institute for Pedological and Agrochemical Research of the Hungarian Academy of Sciences (MTA TAKI) on a scale of 1:100 000.

Soil-fertility reducing factors

This map represents, however, a more detailed version. It offers fundamental clues concerning most of the factors reducing the soil's fertility with major emphasis on those interpretable by geological methods. Thanks to its topographic accuracy this map can also be applied for the planning of particular projects and other practical purposes. Key information for this map were provided by traditional field description, in the first place of natural exposures, by remote sensing imagery and laboratory tests together with specific subject data of geological, pedological, hydrogeological and engineering-geological maps at hand.

Erosion and accumulation as agents of soil destruction

The process of sheet erosion has been evaluated on unconsolidated parent rocks, without conducting either a classification or a precise percentage assessment. A vital clue to denudation assessment was provided by tone differences perceptible on aerial photographs. Upon experience gained in the course of the execution of our mapping project, we considered soil to have been eroded off when most of its profile proved to be worn away. Surface wash representing one type of areal erosion is particularly extended throughout the area. Quite similarly to surface wash, linear erosion affects equally the whole land unit. Delineation of areas prone to denudation or accumulation was managed through the evaluation of aerial imagery supported by field checking.

Unfavorable mechanical composition

Soil-textural attributes affecting soil fertility have also been evaluated on aerial photographs and confirmed by laboratory tests. The ratio of sand particles in the soil sample, degree of sorting as well as the multitude of constituents of the parent material have been interpreted as key criteria for specifying harmful sand content. The more sand particles are contained in the soil and the more uniform its constituents (mainly quartz) are the less fertile the land is. In a quite similar way, adverse effects generated by high clay content are neither as much related to the percentage ratio of clay particles as to some other properties characteristic of argillaceous soils including difficult cultivation because of compactness, presence of mud etc. In most cases high clay content of the soil profile exerts its adverse influence on soil fertility and deteriorates conditions of cultivation practices solely when it is coupled with a lower position of the respective terrain as compared to its surroundings. Consequently, they are chiefly related to alluvial flood-plains as well as to areas filled by lacustrine and paludal sediments. Gravel-bearing soils of disadvantageous feature in the area are bound in the first place to older alluvial fans, the periphery of ranges, gravel members of Tertiary deposits in upland areas and to upper flood-plain surfaces. As having been badly cultivable, their unfavorable water budget with concomitant drying out qualify gravely soils as harmful to agriculture. The overwhelming majority of them occurring in the former area represent formations of Lower and Upper Pleistocene terraces elevated to higher morphological position. Fossil red clay evolving on the one-time Pleistocene relic surface presents, to a certain extent, an additional feature deteriorating pedological features of this area by sealing superficial strata and forming a strongly adhesive layer known as clay-coated gravel. Additionally, the clayey horizon developing under recent, humid climatic conditions in the soil further increases the compactness of its profile. This layer inhibits infiltration leading to the creation of pseudogley soils with extreme chemical properties in a stagnant-water environment bound to comparatively small districts without an outlet. Strongly cemented, old metamorphic rocks breaking or approaching the surface have a considerable bearing on pedological features. These areas cannot, however, be utilized for agricultural purposes. Adverse effects caused by abundant water In the area of study two types of water abundance unfavorable to soil fertility have been distinguished, namely swamps and permanent or seasonal inundation by inland water. During evaluation we made use of two key sources, namely aerial imagery interpretation and hydrogeological map variants. Development of swamps is associated with recent fluvial-paludal and lacustrine-paludal terrain, while permanent or seasonal inundation by inland water takes place in immediately adjacent zones and in those of young, alluvial sediments.

Extreme chemical reaction

Three types of disadvantageous features reducing the soil's fertility have been outlined to which special attention should be paid in any agricultural practice:

Among a number of varieties of extreme chemical properties, soils identified as acid or liable to harmful acidification are widely extended in the area. The notion of critical acidification has been applied uniformly to all regions whose pH tests revealed high acidity or which need soil amelioration for preventing further progress of the process. The evaluation of areas exposed to imminent soil acidification was supported by pH tests of the topsoil and by data of the soil map.

Distribution of lime content

Charts illustrating the lime content pattern of soils are not appropriate in their present form to help undertaking precise evaluation of lime profiles required by pedological practice for purposes of wide-ranging standard analyses. Collation of the two charts and laboratory tests provide some opportunity to outline the distribution of lime content along the soil profile. The cartographic representations (1:200 000) of lime content interpretation are based on data of lime content and pH referring to the ploughed horizon and adopted without modification from the map series issued by the Institute for Pedological and Agrochemical Research of the Hungarian Academy of Sciences (MTA TAKI) (I. Szabolcs-Gy. Várallyay, 1978). Accordingly, it serves as a basis of reference. It was complemented with lime-content tests carried out within the framework of the Kisalföld Project. Charts demonstrate the distribution of lime in two key levels, namely 0.2-0.4m (topsoil) and 1.4-1.6m (subsoil). Available data support the detailed examination of 6 and the general description of another 8 (in all 14) horizons, i. e. the whole uppermost 10m of the profile. The reason for selecting, at the first sight arbitrarily, the above two levels (topsoil and subsoil) for the compilation of the maps is that they were equally subjected to a variety of additional examinations including pF, trace element analyses, organic matter content etc. omitted for other horizons and facilitating thus collating detailed analyses with the standard pedological ones.

Distribution of pH values

These maps illustrate the distribution of pH values in soils. Tests with aqueous extract have been conducted on 4-6 samples of the upper part of boreholes sections. From the 6 horizons that can be represented on map at all, these two charts correspond to the levels illustrated on lime-content maps. They are based entirely on laboratory tests conducted within the framework of the Project adopting limits in pH values as approved in pedology. Because of the comparatively large spacing of sampling intervals these charts demonstrating pH distribution do not reflect the same diversity as expected upon the great variability of soil types occurring in the area.

Environmental studies in Kisalföld

Geological map for environmental protection

This map depicting the susceptibility to pollution of superficial formations is based on the simultaneous evaluation of interrelations between the permeability of rocks, the thickness of the overburden and the highest ground-water level. Accordingly, the classification of the principal parameters into 4 categories each, facilitated a more subtle subdivision of the area according to pollution susceptibility. When evaluating boreholes from the angle of the overburden, the average value of the coefficient of permeability ("k") deduced from granulometric curves of rocks and of the results of laboratory permeability tests, respectively, has been taken into account. While selecting depth intervals, geological setting, properties of the overburden and ground-water level were our guiding principles. As for permeability, superficial deposits overlying gravels and showing coefficients of permeability <10-6 cm/sec were attributable to overburden. The limiting value of 2m of the depth scale corresponds to environmental law regulations. The possible division of the threefold variations corresponding to different grades of pollution susceptibility are assign able to 6 categories denoted on the map by digits, various colour shades and by their variation as follows:

While offering the most appropriate sites for planning depositories for hazardous wastes, it requires no protective measures for the storage of communal wastes. Obviously, when one faces a particular problem, the combined effect of the above considered key criteria may substantially be modified by a number of other factors including mass-movement susceptibility, slope-steepness characteristics, earthquake hazard and tectonic features, which bring about in most cases a shift towards higher susceptibility. This map depicts merely general susceptibility trends trying to lend some guide-lines for planning regional as well as local waste depositories. Consequently, it cannot substitute for the detailed survey of a specific site.

Other applications in Kisalföld:

The Szigetköz project

This project is strongly associated with environmental protection. The Gabcikovo-Nagymaros water barrage that has been the subject of frequently overheated debates in scientific as well as in political circles raised the need for a geoecological information system from the concerned area. As the principal owner of related data in Hungary, our Institute was charged by the Ministries for Water Management and Environmental Protection to provide baseline information about the environmental aspects of the region. One of the basic factors defining the sensibility of the area for superficial pollution is the composition of geological formations, in other words the proportion of gravel, sand, silt or clay and so on in their constitution. In this context, digital single-theme base maps displaying the composition of the formations at depth levels of 2m, 5m, and 10m have been generated. They were based on the data of ca. 300 10-m-deep boreholes. The combination of these 3 maps results in a complex map illustrating prevailing patterns of the rocks' composition in the area as related to the uppermost 10m of the profile. If needed, this picture can be precised by including data available from any of the other 11 depth levels recorded. The resulting map demonstrates clearly the predominance of gravels in the uppermost 10 m of the profile making up the Danube river's alluvial fan. This geological setting brings about favorable conditions for the rapid extension of superficial pollution into deeper horizons that implies to the danger due to the eventual upsetting of the fragile natural equilibrium in this nature conservation area.

A kind of the application of GIS methods:

Site selection for waste depositories

The growing amount of communal, industrial and radioactive wastes imposes an ever growing burden on regional and local authorities to find sites suitable for their deposition. Given the large amount of related data available in the Geological Institute of Hungary as well as the simultaneous intervention of a number of different thematic aspects that have to be taken into account, we have set up a GIS project in the Kisalföld for handling this issue. Our chief objective is to be given a contract for the preliminary selection of the most appropriate sites for the eventual disposal of wastes deriving from the power plans and fabrics.

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