The H ungarian G eological Survey has been taking part in the geological research in the environs o f the Paks Nuclear Pow er Plant since 1985. M ost o f the results in the area W o f the Danube w ere supplied by geological investigations to support exam inations concerning the N PP’s seism ic safety. In the surroundings o f the N PP geological works for the planned extension and for the support o f seism ological exam inations, carried out by numerous institutions and experts were the most productive. A ctivities o f the Transdanubian D ivision o f the Geological Institute o f Hungary (M AFI), o f the experts from different divisions o f the H ungarian Geophysical Institute (ELGI), from the G eodetical and G eophysical Research Institute (Sopron) (GGKI), from the D epartm ent o f Geophysics Eotvos Lorand U niversity (B udapest) (ELTE), from the G eophysical E xploration Com pany (Budapest) (GKV), and o f A. F. G racsov and his colleagues from the G eophysical Institute o f M oscow should be mentioned, w ho took part in the assessm ent o f seism ic safety o f the area between 1985 and 1991. In cooperation with the m easuring team o f ELG I and considering the recom m endations o f the Scientific Coordination C om m it
tee, Hungarian G eological Survey issued a sum m ary in 1992 based on the elaboration o f earlier data. In 1994 an other im portant w ork was published sum m arizing the geological conditions in the area, evaluating tectonical conditions and adapting the results o f the accom plished projects to GIS. The present study sum m arizes the results and m ethods o f the geological studies in the surroundings o f the N PP (Figure I) pointing out inform ation about the surface and near-surface geological form ations. It provides an outline o f the main characteristics o f installing and using the GIS database w hich contains the results o f the geological investigations carried out in the area in a unified system.
The m ain purpose o f the studies ordered by different authorities during the ten years w ere to collect the basic geological data needed for the assessm ent o f seism ic hazard in the surroundings o f the NPP, to supply certain m issing data and to accom plish the geological synthesis o f them. W e also used the results o f earlier geological- geophysical studies for the final synthesis (Chikan et al. 1994a). A part o f the considered data originates from areas more rem ote from the N PP; the clarification o f the distribution, developm ent and interrelationship o f certain form ations m ade this outlook necessary. The m ost im portant aims set eventually and reached w ere the possibly most exact cognition o f the geological conditions in the region, the construction o f a geological m ap o f high accuracy and the specification o f the main characteristics o f the geological form ations influencing earthquake liability. An opportunity o f the edition o f a geological map o f increased accuracy presented itself by using the information gained continuously during these ten years o f research. O ur m ap which was edited as a result o f the geological synthesis is the base o f num erous m ap versions arranged in the GIS (T urczi 1994). A sim plified version o f this m ap o f 1:25.000 scale is show n by F igure 2.
The results of the completed geological research are grouped below by topics. The surface geological mapping, the elaborated borehole surveys, the establishment of artifi
cial exposures, the synthesis of the results of our survey are to be dealt with separately; the works performed in relatiori with the structural conditions are covered to the extent they contributed to the better understanding of the geological conditions. Their tectonic evalu
ation can be found in the next essay o f the present volume (Balia et al. 1997). The introduction of the GIS database is placed in a separate chapter.
2. S urface geological m apping
The basis of the geological research is the geological map, which describes the geological formations, and their relation to each other within the region to be studied.
Explanatory notes are attached to this map, introducing the m ost important features of the formations. The region around the Paks NPP belongs to those areas of the country, where no geological mapping had been performed before (not considering the plain mapping of the 1950’s), and as a consequence an adequate detailed geological map essential to the seismic safety evaluations was not available. To remedy the lack of this map geological survey was accomplished in the region three times. In the first step in 1985, as a result of which a geological map o f 1:50.000 scale was completed for the closer environs (Chikan et al. 1985), then in 1992, when a 1:50.000 scale geological map o f a 30 km radius circle around the NPP was constructed (Chikan 1992) (further on this area is referred to as
’’region”), and at last such surface geological mapping was carried out in 1994, when the 1:25.000 map of a 10 km area around the NPP (further we refer to this as the ’’closer region”
of the NPP) was compiled (Chikan et al. 1994a, b Figure 2). In relation to these works, the Survey carried out structural-geological examinations and analyses (Dudko 1992, Chikan and Dudko 1992, Gerner 1993, Dudko and Maros 1994) and these geological studies contributed also to evaluation of seismic hazard (Balia et al. 1993.). Naturally, besides the results of the surface mapping we also used the results o f other surveys done partly by MAFI, partly by other researchers, consequently the reliability o f our maps (parallel to the enlargement o f the map scale) has risen continuously. The results of the surface mapping, and the geological conditions can be summarized as follows.
The area investigated is located in the middle part of the Pannonian basin. In its geological setting, three m ajor sequences can be distinguished which are as follows:
A Paleo-M esozoic basement, Neogene formations,
Quaternary overlying deposits.
The amount of information available concerning the Paleo-M esozoic basement is highly limited and sporadic. Stemming from the com plicated structure of the mountains areas, it is obvious that the small number of boreholes reaching the basin basement (8 in a circle of 30 km radius: M iske-1,2,3, Nem etker-1, Szekszard B - 17, Tengelic-1, Tolna B-47 and Vajta-3) have only given an informative picture concerning the actual geological and
tectonic setting of the basement. The rocks it consists of are partly metamorphic, partly sedimentary and have a development which is conform to that of rocks o f similar age in the Mecsek Mountains. No data suitable for clarifying the relation between the metamorp
hic and sedimentary developments has been found in the area.
The Neogene rocks filling the basin were reached, or penetrated by several boreho
les. The data obtained from these boreholes are inhomogeneous in several aspects: very few boreholes reached the older part of the Neogene and also the basement (12 in a circle of 30 km radius), while upwards in chronology the amount o f data shows an increasing tendency; the Upper Pannonian rocks were reached by 900 boreholes. Nevertheless, the major part of data from the boreholes have been obtained from hydrological exploratory boreholes using intermittent coring, and only a few boreholes using continuous coring studied and documented in details are found in the area. The Upper Pannonian sediments cannot be studied in surface exposures over the area. The Miocene rocks are of sedimentary and volcanic origin, and the Upper Pannonian beds are known as an average basin facies.
The data concerning these formations gained as new during the survey are reported at the chapter reviewing borehole research.
The Quaternary sedimentary cover is the most studied sequence subjected to most detailed examinations in the area. This volume o f our report is devoted, first of all, to the geological setting o f this sequence. As far as the western and lastern parts o f the region in concern regarded the facies features of the deposits are rather diverse in which the role the fluvial, eolian and slope deposits play is o f the greatest importance. During the study of the Quaternary sequence both geographic and geological considerations were included, thus, we have sets of data and examinations suitable for a versatile interpretation. These deposits were formed during the past 2.5 million years. In our study the lower boundary of the Pleistocene is represented by the 2.43 million years old Gauss-M atuyama boundary, whereas the border between Early and Middle Pleistocene is the 700,000 years old ' M atuyama-Brunhes boundary. The boundary between the Middle and Late Pleistocene is represented by the 125,000 years old Riss-Wiirm thermal peak. The beginning of the Hoiocene is dated to 12,000 years. For the appearance, facies, and the most typical mineralogical-lithological features and regional extent of each sediment, see the comment of the geological map (Chikan et al. 1994) and the expert report about the area (Kokai
1988). The data available concerning the interrelationships between the deposits have allowed to outline the present picture created by the geological development o f the area, and to reconstruct the relevant processes. The evaluation of data sets enabled a better understanding of the chronological and facies conditions of the sedimentation, and to get to know horizontal and vertical relations. However, even today there are some problems concerning correlation.
At the boundary between the Upper Pannonian and the Pleistocene, a considerable erosional activity took place in the area which caused a large amount of Upper Pannonian deposits to be removed from the area. Only borehole Paks-4b has provided data concerning the presence of the Lower Pleistocene, (or in another opinion Pliocene (see in our present volume Balia et al.) Tengelic Red Clay Formation described in boreholes Tengelic-2 is found W of the area. However, no evidence to the age of this formation, supported by
examinations exceeding the bedding and macroscopical features is available, therefore this occurrence was represented as part of the loess sequence in our log Dszgy-1 also crossing the borehole concerned. This solution is even more justified because during our present examinations it is the loess sequence for which, concerning its classification, dating and chronological subdivision, the most data could be collected.
Based on the comprehensive studies of the key profile at the Paks brickyard, carried out over a period of some years, an approximately precise picture of age and facies conditions of the series exposed here is available to us. Being a profile exposing the largest, best studied and thickest Pleistocene sequence in the area, It provides an excellent basis for an attempt to classify the Pleistocene deposits found in the area chronologically, by relying on comparisons with this sequence.
According to the present classification (Pecsi 1993), the lowermost beds in the Paks loess profile are dated to be about 1 million years old, thus they are as old as Early Pleistocene. According to the original study o f the Paks key profile (Krivan 1955), deposits older than that and lying between the Upper Pannonian beds and the lowermost beds of the brickyard can also be found S o f the brickyard. With regard to its age, this sequence would correspond to the Tengelic Formation. However, no data from the area concerning such an old Pleistocene deposit is available to us, although it can also be imagined that the deposits penetrated between the Pleistocene fluvial deposits and the Upper Pannonian sediments in borehole Paks-4b can also be assigned here.
*
In the loess sequence, a further classification and correlation are made possible, on one hand, by the analysis of paleosols, and on the other hand, by the macrofauna that occurs at some sites. The lithological features o f the sequence are not too suitable for correlation purposes (particularly, in the case of minor exposures), although some signs indicate that the consolidation status, and thus the textural feature of the deposits exhibit a certain consistent change upwards the profile. The correlation of the different levels in the loess sequence were tried to be solved by establishing artificial exposures dealt with later.
Within the loess sequence, the interrelationship between the sediment types is so occasional, and the loess formations o f different age perform so similar properties, that it is impossible to map each level, and, in the case of m inor exposures the lithological features are insufficient for dating. That is why loess sequence was represented in a uniform manner as a single element on the map. However, in the profiles, on the basis o f data and results obtained from the evaluation o f exposures, each loess presumed to be older than Wiirm was distinguished using a marking 18R, and the loess likely to be of Wiirm age was distinguished with a marking 18W (Figure 3). A formation that is in a special relation to loess is the Pleistocene drift sand found in the area. This deposit type was formed at the same time when the loess sequence. As a function o f changes in climatic conditions, loess always contains a certain amount of sand, but marked drift sand levels were only formed at some places. However, this drift sand generally has a more limited areal extent than the regional scale, and in m ost cases it wedges out horizontally. As a result, it rarely can be determined whether two drift sand occurrences distant from each other in space belong to the same bed, or not.
14
Upwards along the Paks key profile, the number of sand beds shows a decrease.
During the investigations, several concepts concerning the development of sand beds found here were formed: some researchers (Adam et al. 1954) deemed that the sand beds were of fluvial origin. Krivan (1955) considered sand horizons of eolian origin. Pecsi (1993) supposed that the most typical sand beds in the Paks exposure were formed as derasional valley fillings. On the basis of their facies the Pleistocene wind blown sands found W of Paks both in surface exposures and in boreholes cannot be clearly correlated with the sand layers found in the key profile. Had we taken into consideration, however, the climatic conditions that led to the development of drift sands, and the sedimentary conditions that had been able to supply sufficient amounts o f sediments for drift sand development, it could be made probable that the best conditions for the forming of this drift sand started to prevail near the Riss-Wiirm boundary, or in the beginning of the Wurm.
O f geological profiles compiled on the basis o f surface exposures, boreholes penetrating each sediment, and the geoelectrical logs, it is log Phsz-2 attached as an annex to the GIS, and profiles 5 and 6 (Figures 4 and 5) that show which setting position can be assumed for these rocks on the basis of data available at present. It should be noted that for a part of drift sand occurrences, only the thickness and properties of soils formed on them provide opportunities for dating.
The bedding conditions of the Pleistocene slope deposits are only rarely doubtful:
their development is likely to have taken place during the entire Pleistocene, however, in most o f the cases, as loess sequence they are represented in the form of reworked soil levels, delies, valley infillings. Only the younger talus deposits likely to have been formed during the final stage o f the Wurm were represented in the map with a separate symbol. Their position matches with the present terrain conditions and they were accumulated in a larger amount particularly at the bottom o f the terrain elements having already formed during the Pleistocene and along the valley sides. Their altitude with respect to sea level does not have relevance since it is the current base level that plays an important role in the evolution of the valleys, and its alterations also influence the emergence of slope deposits.
The topics concerning the dating and stratigraphic position of Pleistocene fluvial deposits including their relationship with deposits of other facies should considered to be problematic. Both in the exposures containing fluvial deposits and in the boreholes, the development conditions of the sediments could be relatively well assessed. However, no exposure at all could be found in the region that would allow for a precise determination of the relationship between the fluvial deposits and the loess sequence. As far as the chronology is concerned, there exist different opinions. The sporadic biostratigraphic data (Jasko and Krolopp 1991) refer, at some sites, to an Early Pleistocene origin whereas at other sites to a Late Pleistocene one. Consequently, it can be stated that in a part of the area fluvial sedimentation also took place simultaneously with loess formation during the Pleistocene. However, the lateral contact and relationship between the two facies are unknown to us. This also means that although in some cases the age of the sequence can be dated in vertical profiles, its correlation with deposits o f a different facies is difficult.
This represents a significant problem not only concerning this particular geographical area but also within the Quaternary studies in Hungary.
Based on data concerning chronology and bedding, two types o f Pleistocene fluvial deposits are distinguished. The age and classification of deposits of the Danube valley can be evaluated on a sedimentary or on a biostratigraphic basis (Jasko and Krolopp 1991).
Beds with an age and facies similar to the part assigned to the Upper Pleistocene can be found NW of Paks, in a valley stretching towards Cseresnyes and in a sequence in the row o f hills between Tengelic, Szolohegy and Pusztahencse. Beneath the latter, the Lower Pleistocene levels can also be found W of the area, in boreholes found in the vicinity of the Training Center at Tengelic.
Based on the aforesaid, the solution selected in geological profiles was to represent the boundary between fluvial deposits and loess as an erosional boundary and the sedimen
tary terrains of fluvial deposits were represented as erosional troughs that partly existed earlier (Figure 6).
On the surface of the Pleistocene fluvial sand, the conditions of drift sand develop
ment were created at several places due to a change in the climatic factors. At those parts of the area where Pleistocene drift sand overlies the fluvial sand, generally the two facies cannot be separated sharply. In many cases, sand dunes of material blown out from a relatively small distance are found in the area. As a function o f changes in the wind intensity and the paleomorphological conditions, at some places blow-outs are found, whereas at other sites larger dunes were built by the wind. This situation also allowed small lakes to form at the end of the Pleistocene. The relation between Pleistocene fluvio-eolian sand and Pleistocene drift sand*is still not clear. They have a similar m ineralogical composition, thus the blow-out terrain level and the place of origin o f the original deposit may be the same for both sequences, however, their regional extents show some differences. It is not excluded that the two deposits may be of the same age but there is no direct evidence to it, and no deposits overlying the fluvio-eolian sand and indicating new inundations by the river have been found either.
In the whole area, there is an unconformity between the Pleistocene and Holocene deposits. Even in the areas where the sedimentary conditions were similar during the two periods (such as near the Danube), there is a hiatus between the facies, thus there is no continuous transition between the Pleistocene fluvial deposits and Holocene fluvial depo
sits.
Among the Holocene deposits, the fluvial deposits are considered to be the oldest.
In the area they occur at several levels, in the form of channel and flood plain deposits alike. Their interrelationships and the changes in granulometric composition are determi
ned mainly by phenomena associated with water streaming and water level fluctuation.
The only usable order as to the age o f deposits of different grain size can be set up in that bed deposits in the area W of river Danube are older than the flood plain deposits also
The only usable order as to the age o f deposits of different grain size can be set up in that bed deposits in the area W of river Danube are older than the flood plain deposits also