1. INTRODUCTION The eastern part of the Drava depression in the north and part of the Slavonia-Srijem depression in the south, divided by the central –akovo-Vinkovci plateau together with the Vukovar plateau, are deeply subsided structural depressions which were formed along region- al faults characterised by the horizontal displacement and movement of coherent tectonic blocks to the north- east, which influenced the formation of large extension zones. The plateaux have the morphology and structure Quaternary Deposits as the Hydrogeological System of Eastern Slavonia Andrea BA»ANI1, Marko ©PARICA 2 and Josipa VELI∆ 1 of complex horst structures, that are separated from the neighbouring depressions by systems of deep faults (Figs. 1 and 2). With respect to the surface water and groundwater, two drainage areas are defined - the Drava drainage area in the north and the Sava drainage basin in the south. The surface and subsurface watersheds between these two areas are located on the –akovo- Vinkovci and the Vukovar plateaux. For construction of the geological map shown in Fig. 1, the following sheets of the Basic Geological Map of SFRJ Scale 1:100,000 were used: Donji Miho- ljac (HE∆IMOVI∆, 1985), Naπice (KOROLIJA & JA- MI»I∆, 1989a), Osijek (MAGA©, 1987a), Odæaci (TRIFUNOVI∆ & STOJADINOVI∆, 1985), Slavonski Brod (©PARICA, 1987), Vinkovci (BRKI∆ et al., 1989), BaËka Palanka (»I»ULI∆-TRIFUNOVI∆ & GEOL. CROAT. 52/2 141 - 152 7 Figs. ZAGREB 1999 Key words: Hydrogeological system, Watershed, Lithological continuity, Fault zones, Transmissivity, Plateau, Eastern Slavonia, Croatia. KljuËne rijeËi: hidrogeoloπki sustav, razvodnica, lito- loπki kontinuitet, rasjedne zone, transmisivnost, rav- njak, istoËna Slavonija, Hrvatska. 1 Faculty of Mining, Geology and Petroleum Engineering, University of Zagreb, Pierottijeva 6, HR-10000 Zagreb, Croatia. 1 Institute of Geology, Sachsova 2, HR-10000 Zagreb, Croatia. Abstract The area of eastern Slavonia, situated between the Drava and Sava rivers, comprises three geotectonic units: the eastern part of the Drava depression in the north, part of the Slavonia-Srijem depression in the south and the central –akovo-Vinkovci plateau together with the Vukovar plateau. These units are separated by deep faults that reach the base of the Tertiary sediments. The first 200 m of Quater- nary deposits are saturated with fresh water. The aim of this study was to find out whether the faults form impermeable boundaries sepa- rating the waterbearing deposits into independent hydraulic systems, or if a singular hydraulic entity exists. Results of the analysis indicate that lithological continuity of the aquifers exists along the fault zones on the margins of the –akovo-Vinkovci and the Vukovar plateaux, which means that there is no impermeable hydraulic boundary on the watershed between the Sava and Drava river valley. The part of east- ern Slavonia between the Sava and Drava rivers is one hydraulic sys- tem consisting of zones with different transmissivity values. In the zones of reduced transmissivity, the hydraulic connections are weak- ened, but not broken. Such zones exist not only along the fault zones of the –akovo-Vinkovci plateau and the Vukovar plateau, but also within the Sava and Drava depressions. The terrain morphology influ- enced formation of both the surface and the underground watershed, parallel to the extension of the –akovo-Vinkovci and Vukovar plateau. Therefore, within this single hydraulic entity, when the draw- down reaches the watershed due to excessive pumping, the watershed will be displaced from its natural position. Saæetak U prostoru istoËne Slavonije, izmeu Save i Drave nalaze se tri geotektonske jedinice i to na sjeveru istoËni dio Dravske potoline, na jugu dio Slavonsko-srijemske potoline, a izmeu njih –akovaËko- vinkovaËki i Vukovarski ravnjak. Ravnjaci su u morfoloπkom i struk- turno-tektonskom pogledu sloæene timorske strukture, koje su od sus- jednih potolina odvojene sistemima dubokih rasjeda koji zadiru u podlogu tercijarnih sedimenata. Zbog morfologije terena se duæ pruæanja –akovaËko-vinkovaËkog i Vukovarskog ravnjaka proteæe povrπinska i podzemna razvodnica koja istraæivani prostor dijeli u dva sliva: Dravski na sjeveru i Savski na jugu. Cilj ovog rada bio je istraæiti da li rasjedne zone izmeu ravnjaka i potolina predstavljaju nepropusnu granicu koja naslage dijeli u dva odvojena hidrauliËka sustava ili one Ëine jednu hidrauliËku cjelinu. Analizom su obuhvaÊe- ne kvartarne naslage koje sadræe vodu pogodnu za vodoopskrbu, a koje zalijeæu do dubine od 200 m. Rezultati analize pokazali su da litoloπki kontinuitet vodonosnih slojeva duæ rasjednih zona –ako- vaËko-vinkovaËkog i Vukovarskog ravnjaka nije prekinut, te da na razvodnici savskog i dravskog sliva ne postoji nepropusna hidrauliËka granica. Prostor istoËne Slavonije izmeu Drave i Save smatra se jed- nim hidrauliËkim sustavom unutar kojeg postoje zone s razliËitim vri- jednostima transmisivnosti. Duæ zona smanjene transmisivnosti hidrauliËke veze su oslabljene, ali nisu prekinute. Takve zone nisu prisutne samo duæ rasjednih zona –akovaËko-vinkovaËkog i Vuko- varskog ravnjaka nego i unutar pojedinih depresija. U sluËaju da sniæenje uzrokovano crpljenjem dosegne razvodnicu zapoËinje dotok vode iz podruËja s druge strane razvodnice, tj. razvodnica se premi- jeπta u odnosu na prirodni poloæaj udaljavajuÊi se od mjesta crpljenja. 142 Geologia Croatica 52/2 Fig. 1 Geological map. • .:!t-.' - .--------.. -..... --~ / --__ ~M f ' . ',­-.- • • •• -------~ " .-<>- . • GALOVI∆, 1985a), BrËko (BUZALJKO et al., 1987) and Bijeljina (VRHOV»I∆ et al., 1986). The structures and other tectonic data were compiled after the works of HERNITZ (1983) and PRELOGOVI∆ et al. (1995). The aim of the study was to determine whether the fault zones between the plateaux and depressions con- stitute an impermeable zone that separates the Sava and Drava drainage area into two hydraulic systems, or whether a singular hydraulic entity exists. The shallow- est 200 metres of sediments favourable for water-sup- ply were explored. 2. DATA-SETS AND METHODS The work is largely based on the analysis of geolog- ical and hydrogeological data that were published in scientific and professional papers and books, as well as on the numerous reports in the archives of several com- panies and scientific institutions. These data were inter- preted together with fieldwork and laboratory investiga- tions of the sediments drilled in 518 wells. The most important data obtained from the wells were the total depth, depth-intervals of permeable layers and the ratio of the thickness of permeable and impermeable layers. The following hydrogeological parameters of the wells were calculated: average hydraulic conductivity, trans- missivity, storage coefficient and leakage coefficient. The drilling data and the well-logs (primarily the E-logs and the radioactive logs) were correlated in a north - south direction in order to depict the lateral relation- ships of the plateau sediments to the depressions. Due to the fact that there is a limited number of wells in the marginal zones, the data set was complemented with interpretation of geoelectric measurements. The results obtained by utilisation of the mathematical models that covered the marginal zone were also taken into account. 3. RESULTS The results of comprehensive analyses of all the data that were at our disposal, originating from the sources mentioned above, can be grouped in accor- dance to the scientific disciplines - geological (strati- graphic, tectonic and palaeogeographic), hydrogeologi- cal and hydraulical data. Their synthesis solved the problem of the continuity of the aquifer layers in the studied area of a relatively complex geological struc- ture. 3.1. QUATERNARY GEOLOGICAL EVOLUTION In view of the water-supply possibilities, the most interesting sediments were deposited during the Middle and Upper Pleistocene. This is the reason why this overview of the geological history only covers the Qua- ternary period. 143BaËani, ©parica & VeliÊ: Quaternary Deposits as the Hydrogeological System of Eastern Slavonia Fig. 2 Geological cross-sections A-A’ and B-B’. 144 Geologia Croatica 52/2 In northern Croatia, there is a continuation of like depositional environments between the Pliocene (Uppermost Tertiary) and the Quaternary, the Pliocene being characterised by large aquatic areas with sedi- ments of significant thickness, especially in the subsid- ing depressions. It also means that the Pliocene - P l e i s- tocene boundary has not yet been defined with confi- dence. Subsequent to the strong tectonic events bet- ween the Pliocene and Quaternary, and to the break- through of the Danube in the –erdap canyon, the lakes and swamps gradually dried up, followed by formation of the drainage pattern. There are only scarce data on the Lower Pleistocene of the explored area. There are no outcrops. The age was proved by fossil findings in a small number of wells - in Otok (SOKA», 1976), Vin- kovci (SOKA» & HARTEN, 1978), Trpinja (MAGA©, 1987b; MUTI∆, 1989), in Vladislavci and Osijek (MA- GA©, 1987b), in Erdut and Dalj (SOKA» et al., 1982) and also in Valpovo (KOROLIJA & JAMI»I∆, 1989b). The Lower Pleistocene sediments are mostly clay, silt and some sand. Due to the fact that their contact with the Pliocene is not defined, it is also impossible to tell the thickness of these sediments. The Lower Pleisto- c e ne - Middle Pleistocene boundary is defined on the conditional E-log marker Q’ (Fig. 2). Above the Q’ marker horizon, there is a domination of permeable sed- iments, sand with some gravel, while underlying sedi- ments are of low permeability - silt and clay (URUMO- VI∆ et al., 1976, 1978). In the uplifts of the –akovo- Vinkovci and the Vukovar plateaux, the Q’ horizon lies at depth of approximately 150 m, while in depressions it is mostly below 200 m, sometimes even 300 m (HERNITZ, 1983). The Middle Pleistocene sediments are also buried, apart from a small outcrop on the northern slope foot- hills of the Erdut hill (Fig. 1, TRIFUNOVI∆, 1985). Nevertheless, these sediments are well studied due to the large number of water exploration wells, some of them drawn on the geologic cross-sections (Fig. 2). It was during the Middle Pleistocene that the main mass of the hydrogeologically interesting permeable sedi- ments were deposited. The eastern Slavonia area was then covered by shallow lakes and swamps and the neighbouring Bosnian and Slavonian mountains were land areas exposed to strong denudation. The transport and deposition of clastic material, respectfully several cycles of redeposition, was also very dependent on the palaeoclimate. The most significant palaeoclimate char - acteristic was the alternation of the warm and humid periods with cold and very dry periods. During the warmer interglacial periods, the high waters of surface streams transported a lot of the coarse-grained clastic material and deposited it in the swamps and lakes. At the same time, the level of stagnant waters rose, so the swamps and lakes grew and covered a bigger area, until eventually they became interconnected. In contrast, the glacial periods are characterised by a draw-back of the surface streams into their river-beds, a reduction of their transport capabilities, and by the shallowing of the depositional area. The wind forces took the dominant role in the transport of sedimentary material, while silt and clay were deposited in the remaining lakes and swamps. In some places, the aquatic environments totally dried up which enabled the formation of palaeo- sols. The related palaeogeomorphological circumstan- ces are explored, documented and explained in the works of VELI∆ & DURN (1993) and VELI∆ & SAF- TI∆ (1996). The aforementioned depositional environments are confirmed by fossil findings. The ostracods significant for the shallow stagnant waters were determined by SOKA» (1976), SOKA» & HARTEN (1978), SOKA» et al. (1982) and URUMOVI∆ & SOKA» (1974). The characteristic malacological community of the swamp biotope was interpreted by TRIFUNOVI∆ (1985), and the important biotope for the fluvial-paludal environ- ment was discovered by »I»ULI∆-TRIFUNOVI∆ & GALOVI∆ (1985b). Terrestrial species were deter- mined by POJE et al. (1995) and by MALEZ (1970, 1971, 1978). The majority of the clastic material was transported in the Slavonia-Srijem depression by the rivers and streams from the Bosnian mountains and from Mt. Dilj. This is confirmed by the mineral composition of fluvial sediments which originated from the serpentine zone in Bosnia (MUTI∆, 1993). The main direction of transport from south to north correlates with reducing propor- tions of the coarse-grained clasts and grain size in the same direction (URUMOVI∆ et al., 1978). In this way, in the southern part, along the Sava river, gravel was predominantly deposited, and in the northern sections sand (MILETI∆ et al., 1986a). The Drava river and the streams flowing down the slopes of Mt. Krndija are the main distributors of clastic material in the Drava depression. The Drava river trans- ports the material eroded in the area of the Central Alps, on the Pohorje and Kozjak massifs and from the Slavonian mountains (BABI∆ et al., 1978; MUTI∆, 1989). In eastern Slavonia the river mostly deposits sand. The more frequently occurring gravel along the western margin of the study area was brought by the Mt. Krndija streams (URUMOVI∆, 1982). Apart from the rivers and streams, MAGA© (1987b) discovered the traces of lakes by the rivers and deep pools. In the Erdut area, large shallow marshes existed where rivers deposited sand in the high-water periods (VELI∆ et al., 1985). In the Pliocene, the –akovo-Vinkovci and the Vu- kovar plateaux were mostly submerged uplifts charac- terised by the deposition of mixed sedimentary materi- al. One of components was transported from the north, respectfully the north-west, and another from the south or the south-west. In the Vukovar plateau area, the flu- vial, lacustrine and marsh sediments were determined (»I»ULI∆-TRIFUNOVI∆ & GALOVI∆, 1985b). Apart from the palaeoclimate conditions, sedimenta- tion was influenced by tectonic movements which are 145BaËani, ©parica & VeliÊ: Quaternary Deposits as the Hydrogeological System of Eastern Slavonia still active in eastern Slavonia. The most important tec- tonic displacement is on the main faults that follow the northern and southern margin of the plateau and on the secondary parallel faults of the WNW-ESE system. There is also a marked displacement on the faults of the NNE-SSW system (HERNITZ, 1983). This system has an especially pronounced horizontal character of dis- placement (PRELOGOVI∆ & VELI∆, 1992). In the beginning of the Upper Pleistocene s i m i l a r sedimentary conditions were preserved. The Riss- Würm interglacial was still characterised by an influx of coarse-grained clastic material, which is proved by palaeontological investigation of the topmost part of the gravely aquifer close to the Sava river (MUTI∆, 1993). It seems that the extensive transport of the coarse- grained clastic material by surface waters stopped with the beginning of the last ice age - the Würm glacial. The most remarkable Würm facies are the loess deposits that also cover a significant part of the study area. Transported by strong winds, the loess grains were deposited everywhere - in the lakes, pools and shallower marshes as well as on the mild morphologic land uplifts. In respect to that, the lake-marsh, marsh, marsh-continental and continental loess is differentiated (Figs. 1 and 2). Different types of loess at the same time depict the contemporary distribution of the land and water, which is of special importance for the recon- struction of the evolution of the plateaux. The typical continental loess is found only in the area of the Vukovar plateau, Erdut uplift and in the restricted area near the Otok and Komletinci. This means that land existed only in these areas at the time. The loess on the –akovo-Vinkovci plateau was contem- poraneously deposited in an aqueous environment. The following tectonic movements led to the uplift of the Vinkovci area so that the loess that was in the begin- ning deposited in water was gradually covered by conti- nental loess (GALOVI∆ & MUTI∆, 1984; MUTI∆, 1990). The contact between the aqueous and continen- tal loess can be clearly observed in the quarry of the Vinkovci brick-factory. During the entire Upper Pleis- tocene, in the western part of the plateau (the –akovo part) the wind-blown grains were deposited in the shal- low water environment. Due to neotectonic activity, this part of the plateau became land only in the Holocene (MUTI∆, 1990). In addition to the loess, the Upper Pleistocene sedi- ments in the Drava depression are represented by river palaeoterraces and alluvial sands. The terraces are iden- tified on the surface along the Drava flow between the Osijek and Sarvaπ and along the Danube from Dalj to Vukovar, and the alluvial sand deposits lie in the mar- ginal north-western part of the explored area (Figs. 1 and 2). A large part of the Sava depression area is covered by marshy-lacustrine sediments of Upper Pleistocene age, deposition of which continued into the Holocene. Frequently, they are complicated to differentiate in the field from the marsh loess (©PARICA et al., 1987). Due to the lack of organic matter, it is interpreted that these sediments were deposited in relatively clean water. The analyses of malacologic assemblages resulted in inter- pretation of the water environment with land nearby and marked oscillation of water level which produced repeated floods and drying-up periods (GALOVI∆ et al., 1989). Warming at the Pleistocene - Holocene boundary caused a change in composition of vegetation cover. The steppe was gradually replaced by woods (POJE, 1986). Changes in wildlife are also documented. In the early Holocene there are still grazing herds of North- European moose (Alces Alces) , gigantic deer (M e g a c e - ros) and European bison (Bison priscus), and during the Boreal there were common deer (Cervus elaphus), wild oxen (Bos primigenius), bear (Ursus arctos ), wolf (C a - nis lupus), lynx (Felis) etc. Domestication of some ani- mals started in the Boreal. It was firstly the dog, then the goat, sheep, oxen and some other animals. In the Neolithic and the Bronze ages, it was the horse, don- key, hen etc. (MALEZ, 1979). The climatic changes at the end of the Würm and the beginning of the Holocene caused the melting of ice in the alpine and perialpine area and inflow of large masses of water in and around the study area. Deposi- tion of fluvial sediments started, followed by formation of river terraces. There are four genetic types of Holocene sediments: river and creek alluvium, proluvial deposits, aeolian sands and marsh sediments (Fig. 1). The river and creek sediments are found in the river channels of the Sava, Drava and Danube, and of their larger tributaries. Proluvial deposits cover the southern slopes of the Erdut hill, as well as the northern margin of the –ako- vo-Vinkovci plateau. They were formed by the surface erosion of the continental loess and accumulation of the eroded material in the area of contact with the aquatic loess. The proluvial deposits are thin and made of loose silt containing limestone concretions. Aeolian sands are determined in the surroundings of Valpovo, in the marginal north-western part of the stud- ied area. Actually, the sediments of this genetic type are very common in the Drava valley west of the studied area, and it is only the tract with a maximal eastward reach that is encountered here. Although less than 2 metres thick, t h e s w a m p d e p o s i t s cover large areas in the Brodsko Posavlje region and in eastern Posavina. They represent the final phase of deposition of the lacustrine-marsh genetic type of sediments. In the Drava area, these sediments infilled the larger and smaller forms of negative relief, that were due to the stagnant water transformed into the swamps or swamp forests. In fluvial seasons, some of the dried parts were again filled with water, and the depositional process renewed. This is ongoing today. The swamp deposits consist of the humus-peat enriched silt and silt with some sand, as well as of the silty clay. 146 Geologia Croatica 52/2 About a century ago, when the struggle for reclama- tion of this swampy land began, the lowest parts of the Sava and Drava depression were still covered by pools and swamps. There are works of PILAR (1876) on the swamps south-east of Osijek, and of BÖSENDORFER (1952) on the pools and swamps in the areas of Karaπi- ca, Vuka and VuËica rivers, in the Bi-Bosut depres- sion, and in the areas of Berava, SpaËva and Studva. They all had their maximal extent during the high waters of the Drava and Sava rivers. There are also the archaeological findings that are worth mentioning, because they indirectly depict the distribution of aquatic surfaces. The oldest settlements at present that were found in eastern Slavonia are from the Younger Stone Age - the Neolithic (6,000 - 3 , 5 0 0 B.C.) and pertain to the cultural layers named StarËevac and Sopot. These are the oldest settlements of farmers that were located in groups on the more elevated dry terrain along the rivers and lakes, but out of the reach of floods (MINICHREITER, 1992a, b). The remains of these cultures are found near the Stari Perkovci, Vrpol- je, –akovo, Vinkovci, Vukovar, Otok, Erdut hill and near Osijek. 3.2. HYDROGEOLOGICAL CHARACTERISTICS The permeable sediments saturated with drinking water were deposited during the Middle Pleistocene and part of the Upper Pleistocene, under conditions of cyclic sedimentation followed by tectonic movements. These depositional conditions resulted in pronounced lithological heterogeneity in both the horizontal and vertical directions. According to core determination of the shallowest 200 m of sediments in the stratigraphic column, five to seventeen permeable layers can be discerned in the Drava depression. The number of layers depends on location, while some of them reach the maximal thick- ness of 40 m (Figs. 3, 4 and 5). These permeable layers are mostly composed of fine- to medium-grained sands with some gravel occasionally, (usually encountered in the basal parts of thicker sandstone layers). Hydraulic conductivity is commonly in the range of 10-20 m/day, with the exception of some of the wells in Osijek, where over 40 m/day was determined, and, in contrast wells in Ostrovo, Korog and Borovo which have less than 10 m/day. The storativity is in the range of 4 x 10-5 to 3 x 10-3 while the transmissivity depends on the depth of drilling and is in a wide range of 70 -1000 m2/ d a y . The distribution of the net thickness of permeable lay- ers in the entire studied area is shown in Fig. 6, where it clearly depicts the depositional conditions as described above. The sediments of the Sava depression can be divid- ed into two units with essentially different hydrogeo- logical properties. The first unit that lies in the south, along the Sava, consists of a single gravel aquifer (Fig. 2) with a maximal thickness exceeding 100 m. The lithological composition is dominated by fine- to medi- um-grained gravel with 10-50 wt. % of sand. This layer has the most favourable hydrogeological characteristics in the triangle between the Velika Kopanica, Gundinci and Kruπevica. In this area, the gravel rarely contains more than 20 wt. % of sand, and the thinner beds of fin- er material appear only sporadically. The more pro- nounced lithological inhomogeneity is found in the area Fig. 3 Hydrogeological cross-section I-I’. 147BaËani, ©parica & VeliÊ: Quaternary Deposits as the Hydrogeological System of Eastern Slavonia between the Babina Greda and ©titar and further to the east, where the gravel contains frequent interbeds of silty sand and clay. The mentioned aquifer has a hydraulic conductivity in the range of 30-≥200 m/day, the transmissivity is between 423 - 7344 m2/day, and storativity between 1.7 x 10-4 and 8 x 10-3. To the north and east of the gravel aquifer, lies the second hydrogeological unit of the Sava depression. It consists of a system of sand aquifers that can number from 2 to 11 in the shallowest 200 m of sediments. The thickness of individual layers rarely exceeds 30 m. Lay- ers are mostly composed of fine- to medium-grained sand and occasionally of coarse-grained sand. As in the Drava depression, the thicker layers of sand contain some gravel in their basal parts, especially in the east- ern section, in the localities of Vranjevo, Otok and Slakovci. The hydraulic conductivity of these layers is in the range of 5.5 -37 m/day, the transmissivity is Fig. 4 Hydrogeological cross-section II-II’. Fig. 5 Hydrogeological cross-section III-III’. 148 Geologia Croatica 52/2 Fig. 6 Isolith map of permeable layers in the Quaternary aquifer system. • -- ., • " , -• " • -. - " • • ; ~"'-~ .-,. ~ _