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STRATIGRAPHY OF THE NEOGENE AND QUATERNARY SEQUENCES OF THE TOLNA HEGYHÁT HILLS László Koloszár and István Marsi Geological Institute of Hungary, H -1143 Budapest, Stefánia út 14. Keywords: fluvial sedimentation, lithostratigraphy, loess, Pleistocene, Pliocene, residual clays, sediments, Southeastern Transdanubia Geological investigations carried out between 1995-96 in the Hegyhát hills produced a number of new results in the stratigraphy of the area. The two new boreholes identified unambiguously the Tengelic Formation previously unknown in the region. It has also been proved that loess sequences older than Würm play a considerable role in making up the Quaternary sequence of the Hegyhát hills. On the basis of field data collected we succeeded in distinguishing 3 formations of different ages in the fluvial sequences. We can also draw up a more precise picture on the composition and position of Lower and Middle Pleistocene fluvial sequences and older loess sequences. The data available is still insufficient to provide the detailed characteristics of the Quaternary history of evolution and tec­ tonic events. Improvements in this field can be only made by further detailed investigations. 1. Introduction Laboratory analyses of the samples from this year’s boreholes have not yet been completed. Therefore this As field and mapping geologists, we had the opportuni­ paper summarizes our results based essentially on field ty to participate in the survey aimed at siting the disposal information. We have the impression that there are only a facility for radioactive waste in the area of the Hegyhát very limited amount of key and correct data published hills from the very beginning. Regarding geology, the from the surveyed area. We think it thus advisable to Hegyhát hills pertains to the poorly mapped areas of describe the main geological units and outcrops in detail. Hungary. The majority of systematic mapping projects Figure 1 represents the borehole and exposures used as avoided this region. Mapping, structural or hydrocarbon reference in the paper. exploratory boreholes were drilled on a very limited scale there. Field investigations of the last two years and this year’s two exploratory boreholes drilled in the Hegyhát 2. Pannonian sediments hills produced a large amount of data to widen our knowl­ edge about the stratigraphy of the area and the young In the Tolna Hegyhát hills area on the surface there are Cenozoic sequence. only Upper Pannonian sediments whereas boreholes cut The framework of geological knowledge of the area is Lower Pannonian sediments as well. Lower Pannonian provided by some geological and geomorphic papers, as sediments are arranged into the Peremarton Supergroup, well as the profiles and logs of hydrogeological exploratory and the Upper Pannonian sequences, into the Tihany For­ boreholes. The main features of the superficial and near­ mation. In accordance with our ideas the drilled sediments surface formations of the Tolna Hegyhát hills were previ­ in lower position are discussed first, then the outcropping ously described by T oborffy ( 1925) who covered the area sequences will be concerned. between 1921-23 by detailed geological mapping. The geological setting of the Hegyhát hills is represent­ 2.1. Peremarton Supergroup ed on the Pécs and Székesfehérvár sheets of the 1:200,000 and Tihany Formation in boreholes geological map series of Hungary making use of the so- called SüMEGHY-maps prepared in 1950-52 (F orgó et al. The Tolna Hegyhát hills can be regarded as a poorly 1966, Rónai et al. 1972). surveyed segment of the country. Merely 6 boreholes were The most extensive study of the geological-geomorphic drilled there with continuous core sampling (Table 1 ). setting of the area to date was furnished by Á dám ( 1969) Upper Pannonian formations were completely drilled summarizing the results of his research between by just one borehole, the Tolnanémedi-2 structural ex­ 1957-1963. Among the formations of the young Cenozoic ploratory borehole located on the gravitational maximum sequence he covered the following ones in detail: in the northern part of the Hegyhát hills. The borehole Pannonian deposits, clastic horizons, freshwater lime­ was stopped on reaching the Mecsek-type Liassic sequence stones, fossil red clays, fluvial sequences, loess sequences underlying Upper Pannonian. Older Neogene sediments and sediments and flood-plain deposits. Most of his con­ were thus missing. clusions on the geological setting still apply today. Other core boreholes for lignite exploration and siting ^ v | Vámszölőhegy Halastavak Tolnanémei Kisszékely Nagyszékely rszentlőrinc a h \ Keszőhidegkúé ■opparv Diósberény Szaké ly a,; Varsád 1. Significant boreholes and outcrops in the Tolna Hegyhát hills 1 - borehole and its code, 2 — outcrop and its number, 3 — profile of Figures 2 and 5 1. ábra. A tolnai Hegyhát fontosabb fúrásai és feltárásai 1 — fúrás és jele, 2 — felszíni feltárás és sorszám a, 3 — a 2. és 5. ábra szelvényének nyomvonala Exploratory boreholes in the Tolna Hegyhát hills with continuous core sampling T a b le 1 Village, borehole Date of completion Qp Pl-Qp, Pa2 Bottom Pa2 a.s.l Belecska-1 1978 14.0 - 200.0 200.0 127 Diósberény-1A 1996 59.8 63.4 150.1 150.1 138 Nagyszékely-1 1977 50.2 - 200.0 200.0 77 Tolnanémedi-2 1980 27.2 204.2 1200.1 83 Udvari - 1 1978 33.8 - 250.0 250.0 137 Udvari-2A 1996 97.0 150.3 170.4 170.4 28 the repository of radioactive waste have not yielded data 2.2. Tihany Formation on the surface on older Neogene formations either, since they stopped in Upper Pannonian sequences. The same applies to the The superficial occurrences of the Upper Pannonian deepest hydrogeological exploratory borehole (Gyönk-B4) Tihany Formation are associated with three sites having drilled in the central part of the area. Its base at the depth very similar morphological position. All of them are recov­ of 400 m did not hit older Neogene either. ered in the southwestern side of the 3 main SSW-NNE The Hőgyész-K8 hydrogeological exploratory bore­ oriented river and stream valleys, formed by the Kapós hole drilled to 750 m depth in the flood plain of the Kapós river and Donát and Hidas streams and representing the river in the western margin of the Hegyhát hills yielded the principal morphological features of the landscape. only information on old Neogene sediments (A n d ó 1990). Stratigraphic interpretation of this thermal-water ex­ 2.2.1. The surroundings of Pincehely ploratory well was based on the evaluation of the borehole log. Upper Pannonian sediments can be followed down to One of the sites can be found in the northwestern part a depth of 400 m underlain by around 230 m thick Lower of the Hegyhát hills, near Pincehely. Hills in this area Pannonian clay marl, underlain, in turn by Upper Sarmat- extending southeast of the Kapós river are made up of Pan­ ian formations down to 750 m. This data set corresponds nonian sediments (Figure 1, 1). In the Hegyhát hills this is to the contour lines of the map representing the Pannonian actually the site of the widest superficial extent of (s.l.) formations of Hungary (C siky et al. 1987). Pannonian deposits. The sequence can be studied in a Supported by these rather poor data, the thickness of number of gullies and road cuts. Springs rise from several the Upper Pannonian sequence in the Tolna Hegyhát hills horizons. There are only a limited number of outcrops cov­ can be calculated to range between 400-500 m, exhibiting ering a continuous section, and it is mostly covered by a tendency of getting thicker towards the east. In the main slope deposits. The sequence can be tracked from 110 to part of the survey area the sequence is underlain by clay 180 m a.s.l., its in situ overburden has not yet been ob­ marls and silts of the Lower Pannonian Peremarton served. Supergroup. A different picture can be drawn, up in the Its thickest development amounting to 5 m has been northern part of the Hegyhát hills where they were not found in an artificial wall, cut under a sm'all chapel in the deposited, or are missing, and the basement is of Mesozoic southern edge of Pincehely. Caves have been cut into the age. comparatively stable formation (Figure 1, 2). The sequence The main features of the Upper Pannonian sequence is made up of grayish-brown, medium-grained, well-sorted, were described essentially on the basis of the above core mica-rich and poorly or bench-like stratified quartz sand. boreholes. In the late 1950s 9 shallow mapping boreholes Some 10-20 cm large, well-rounded sandstone nodules were deepened in the Gyönk area recovering the top 5-20 and lenses of irregular shape can be found randomly dis­ m thick zone of Pannonian. Together with the borehole tributed in the outcrop. The upper part of the exposure is Udvari-2A their profiles can only be used for determining composed of alternations of silty, clayey and sandy layers the position of the Upper Pannonian layers. and laminae. Parallel, horizontal bedding is characteristic. Within the framework of the lignite reconnaissance in The Pannonian sequence is covered by slope deposits Central Transdanubia in the late 1970s 3 boreholes including eroded soil and calcareous nodules. (Udvari-1, Belecska-1, Nagyszékely-1) were drilled in the Basically the same sort of sandy series is found in gul­ Hegyhát hills. This survey did not bring the desired result, lies around Pincehely bearing occasionally 10-40 cm as the boreholes aimed at recovering thick lignite horizons thick, mostly tilted sandstone benches with wavy bedding of the Torony Formation hit only a maximum 0.5 m thick planes. Yellowish-gray, well-sorted, exfoliated clay of slip­ lignite intercalation.

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