03 - Mrinjek.indd 1. INTRODUCTION Clastic sediments in the vicinity of Virovitica, previous- ly described as Late Pliocene–Early Pleistocene in age (GALOVIĆ et al., 1981; MARKOVIĆ, 1986), uncon- formably overlie the Pliocene “Rhomboidea beds”, and are unconformably overlain by Pleistocene loess, lacus- trine–marsh silts and clays. These sediments are well exposed in gravel/sand quarries (Cabuna, Rezovac, Bistrica) south of the Drava river (Fig. 1), and their thickness varies reaching a max- imum of 50 m. They are tectonically deformed and, in most cases, dip towards the North or Northeast. The basal part of the clastic complex, known as the “Belvedere beds”, is alluvial to lacustrine in origin. BABIĆ et al. (1978) suggested the Late Pliocene age, while PRELOGOVIĆ & VELIĆ (1992) presumed a Lower Pleistocene age for these sediments. Quartz grav- els predominate, interbedded with thin beds of arkose sandstones, clays and marls. According to GALOVIĆ et al. (1981) high-grade metamorphic rocks were the source of the clastic material. The real thickness of the gravels can not be measured, because the basal part of these sediments is not exposed at the surface. Pliocene Alluvial Sediments in the Drava Depression of the Virovitica–Slatina Area, Northern Croatia Ervin MRINJEK¹, Jasenka SREMAC¹ and Josipa VELIĆ² The upper portion of the clastic complex consists of arkose sandstones with very thin clay intercalations. At surface outcrops sands directly overlie the basal “Belvedere beds”, but in deep hydrological bore-holes, clay with peat intercalations occurs at the base of these sediments. An Early Pleistocene age was proposed by BABIĆ et al. (1978). A lack of index fossils in these sediments precludes a precise biostratigraphic analysis. 2. LITHOLOGY The investigated sediments are in most cases horizontal or subhorizontal, tectonically undisturbed, except in the Cabuna quarry, where they thin towards the Northeast. Gravel is predominant in the 5–20 m thick lower por- tion of the clastic complex, while sand dominates in the upper 10–40 m of the investigated sections. Gravel consists almost completely of quartzite clasts (95%), with scarce clasts of sandstones, metamorphic rocks and dolomites. Clasts are subrounded to rounded, varying from granule to cobble size. Small pebbles (4– 32 mm in diameter) predominate. The largest cobbles are 256 mm wide. Clasts are supported with a fine– coarse-grained sand matrix. Some units exhibit open- framework packing. Clast sorting varies from moderate to good. Sand is extremely rich in quartzite grains, associated with a stable muscovite and labile, predominantly feld- spar component, together with lithoclasts of carbonate and metamorphic rocks. They vary from fine- to coarse- grained types, but medium-grained type is dominant. Grains are subrounded to rounded, with generally good sorting. Bedding is clearly visible. Trough-cross to tabu- lar-cross stratified sets are up to 0.4 m thick. Most of the sandy portions exhibit synsedimentary deformations. The cross-stratification measurements suggest eastern and northeastern directions of the palaeostreams. Fine-grained sediments are silts, clays and marls composed of clay minerals and silt-grade quartz. X-ray analysis has shown that these sediments contain calcite, dolomite, quartz, muscovite–illite, chlorite and smectite (Table 1). Carbonate minerals (calcite and/or dolomite) could be of detrital or authigenic origin. Smectites (pre- dominantly montmorillonites) are possible products of volcanic ash. Illite is probably of the same origin Geologia Croatica 59/1 65–84 26 Figs. 4 Tabs. ZAGREB 2006 Key words: Sheet flow, Alluvial fan, Sandy braided river system, Fossil flora, Drava river depression, Northern Croatia, Pliocene. ¹ Department of Geology and Palaeontology, Faculty of Science, University of Zagreb, Zvonimirova 8, HR-10000, Zagreb, Croatia. ² Faculty of Mining, Geology and Petroleum Engineering, University of Zagreb, Pierottijeva 6, HR-10000, Zagreb, Croatia. Abstract Clastic sediments in the vicinity of Virovitica (Northern Croatia) con- sist of gravels, sands, silt clays and marls. Gravels with discontinuous sheet-like geometry are typical for an alluvial fan system. The most abundant sediment, quartz rich sand, originates from a sandy braided river system. Silt, clays, and marls were deposited in the flood plain. They contain fossil macroflora indicative of moderate climate condi- tions, including maidenhair leaves (Ginkgo), the presence of which suggests that these sediments were deposited before the Pleistocene glaciation. 66 Geologia Croatica 59/1 (KELLER, 1970), although it can also be the result of diagenetic processes if the temperature is below 50°C, and potassium ion concentration between 0.2 and 0.3% (BAILEY, 1987), which is not very likely in this case. Quartz and chlorite are also probably of detrital origin. Fine-grained sediments alternate with thin intervals (0.2–2 m) of horizontally and cross-stratified sands. 3. METHODS The architectural-element and bounding surface analysis methods developed by MIALL (1985, 1988a, b) follow- ing earlier work by BROOKFIELD (1977) and ALLEN (1983) were used in examination of lateral and vertical facies changes and environmental interpretation. For this purpose, laterally extensive outcrops in the gravel/ sand quarries located between Virovitica and Slatina (Figs. 1 & 2) were selected for study. Their sedimen- tological analysis consists of sketching primary sedi- mentological features, measuring grain size, and pal- aeocurrent directions and making photomosaics of the entire length of each exposure. In order to utilize pho- tomosaics properly, maximum resolution and minimum geometric distortion of the features is achieved. Charac- teristic details were noted by close-up photographs. A Fig. 1 (A) Location map. (B) Position of lateral profiles and logs: a) Cabuna quarry, b) Rezovac quarry, c) Bistrica quarry. sample 12 13 14 15 17 CII9 SK phase Calcite - + + - - +g +g Dolomite + +g +g + +g - - Quartz +g +g +g + + + +g Muscovite-illite +g + + + + + + Chlorite +g + +g +g +g + + Smectite + +g + +g +g - - Table 1 X-ray analysis of fine-grained sediments. Legend: g) main mineral in sample; +) accessory mineral in sample; -) no mineral in sample. 67Mrinjek, Sremac & Velić: Pliocene Alluvial Sediments in the Drava Depression... study of lithological and sedimentary features enabled a six-fold bounding surface hierarchy to be established (Table 2) and eight architectural elements (Table 3) and 13 lithofacies (Fig. 5) were distinguished. 4. LATERAL PROFILE ANALYSIS The contact between lower (gravelly) and upper (san- dy) portions of the investigated profiles is sharp, partly marked by an erosion surface with well developed pal- aeorelief, up to 1 m high (Figs. 7a, b, 17a, b). Along the B-profile in Cabuna quarry palaeosols were detected at the base of sands. 4.1. Gravel complex In vertical sections the lower part is generally well sor- ted, with clearly visible gravel bodies in the form of sheets (element GS) (Figs. 3 and 4). The thickness of gravel sheets varies between 10–40 cm, and their lateral extent is from 10 to >20 m. Clasts in gravel sheets vary from granules, through pebble, to cobble size. Sheets with smaller clasts are more extensive than those with coarser grains. In cobble-sized sheets imbrications of the b-axis are visible, but poorly developed. Well developed flat bedding (Facies Gh) up to the massive, non-stratified portions (Facies Gm) can be dis- tinguished. Sheet base varies from erosionally irregular, to planar, or amalgamated. Upper sheet surfaces can be smooth, or also amalgamated. Sheets are in most cases composed of a combination of flat-bedded pebble-sized gravels, granule gravels and laminated coarse grained granule sands (Facies Gh, Sh). At some places elements of sheets surround planar cross-stratified sets of sand gravels with pebble-sized clasts, 30–50 cm thick (Facies Gp) (Fig. 6a, b). Incli- nation of foresets is towards the south and southwest. This facies is very scarce in comparison with Gh and Gm facies. The upper part of the gravel complex is 1–2 m thick, in the Cabuna and Rezovac quarries it is characterized by horizontal- and cross-stratified, 10–30 cm thick gravel sets with pebble or granule-sized clasts (Facies Gh and Gp). Set bases are usually erosional and they Fig. 2 Detailed sedimentological analysis of profile B was done using mountaineering equipment. underlying Table 2 Bounding surface hierarchy in the Pliocene alluvial sedi- ments. 68 Geologia Croatica 59/1 extend laterally for up to 5 m. Cross-stratified sets are wedge-shaped, often with reactivated surfaces. In most cases they are separated from each other by strati- fied gravel sets. Gravel sets are often interbedded with 5–15 cm thick horizontally stratified granular sands, of >3m lateral extent (Facies Sh) (Figs. 4, 8 and 9). Some stratified sands are normally graded or massive, or show low angle planar cross stratification, but, in most cases, Table 3 Summary of architectural elements in the Pliocene alluvial sediments. Fig. 3 Well developed flat bedding gravels (Facies Gh). Stick is 1 m long. Profile B, Cabuna quarry. Fig. 4 Gravel sheets (GS) composed of facies Gh. The top of the gravel complex is composed of facies Gh, Sl and Sh. Stick is 1 m long. Profile B, Cabuna quarry. 69Mrinjek, Sremac & Velić: Pliocene Alluvial Sediments in the Drava Depression... they are horizontally stratified (Facies Sh). Their bases are flat to irregular, scarcely concave or indistinct, con- tinuously deposited over the gravels. The median part of the gravel complex at the A- profile in Cabuna (Fig. 6a, b) is characterized by a well developed erosional basal surface (5th order bounding surface, Table 3) with erosional relief of >3 m, incised into a 4 m thick sandy interval. The erosion becomes invisible at the right-side of the profile, due to the com- plete erosion of the sandy interval (amalgamated grav- els). Near the erosion surface, gravels are massive or indistinctly stratified, and they contain <0.4 m diameter angular intraclasts of marls and clays (Facies Ge). The sandy interval is built up of planar to low angle cross- stratified sand units, which show synsedimentary defor- mation. 4.2. Sand complex The sandy part of the profiles generally show a fining upward feature. In the Cabuna quarries the upper parts of the profiles are represented by ca. 10 m thick pack- ages of fine-grained sediments (Profile B) (Fig. 7a, b). Description of sedimentary structures and architectural elements in the Cabuna and Bistrica sand-quarries is difficult because of synsedimentary deformation (Figs. 7a, b, 10a, b, 11a, b, 17a, b). Two types of sedimenta- tion can be distinguished: channel and floodplain sedi- mentation (Table 4). 4.2.1. Channel elements Sand profiles C1 and C2 (Figs. 10a, b, 11a, b) and san- dy portions of profiles B in the Cabuna quarry (Fig. 7a, b), profile D in Rezovac quarry (Figs. 13 and 14) and profile E in the Bistrica quarry (FigS. 17a, b) are char- acterized by vertically superimposed sedimentary units in the form of sheets. Sandy sediments consist of 1–8 m thick sandy channel sequences, over 50 m in lateral extent. A com- plete sequence, with a distinct fining upward trend in the upper portions, can only be observed in profiles B and D. Sandy gravels gradually pass into very coarse- grained sands, then fine-grained, cross- to horizontally stratified sands overlain by fine grained floodplain sedi- ments (Figs. 7a, b, 13 and 14). In other cases sequences are incomplete – the uppermost parts are missing due to erosion. The base of the channel fill (5th order bound- ing surface, Table 3) is irregularly erosional, ca. 1 m incised into the base sediment, and it is built up by mas- Table 3 (Continued). 70 Geologia Croatica 59/1 sive to indistinctly cross-stratified coarse-grained sands and intraclasts (channel lag) of fine grained overbank sediments (Facies Ge and Se) (Figs. 11a, b, and 15). Besides the described basal facies, several other sandy facies were observed. Planar cross-stratified sets (Facies Sp) are rather frequent at all localities. These sets extend up to 5 m in lateral view. The thickness of sets varies from 10–40 cm (Figs. 16 and 19), usually increasing in a downstream direction, and their basal surface changes from non-erosive to erosive. Contact of the foresets and lower boundary surface is angular or asymptotic. Looking downstream, modification of fore- sets can be observed, from angular (with gradient 20– 25°) to asymptotic (gradient 10–15°), or even sigmoid. Reactivated surfaces (3rd order bounding surface, Table 3) can often be observed in sections parallel to the incli- nation of foresets. Reactivated surfaces are usually less inclined than foresets (gradient 10–20°), and can extend through one or several sets. These surfaces are usually overlain with coarse grained sand, with scattered gran- Fig. 5 Legend for profiles and logs and lithofacies description. 71Mrinjek, Sremac & Velić: Pliocene Alluvial Sediments in the Drava Depression... ules. In most cases, the inclination of foresets below and above the reactivated surface is similar. Trough cross-stratified sand units (facies St) are less frequent in the studied profiles. They occur as isolated sets or cosets (Fig. 18). Trough sets are 10–30 cm thick, with 40–80 cm wide troughs. Their foresets are asymp- totic in the base, which is either irregular, or partly con- cave, due to erosion. They consist of medium to coarse grained sand. Inclination of foresets is from 10–20°, and the position of the trough axis is towards the north Fig. 6 (A) Photomosaics of Profile A. Cabuna quarry. (B) Detailed sketch of Profile A. Legend is shown in Fig. 5. Lithofacies and element description and interpretation are shown in Fig. 5 and Tables 2 and 3. A B 72 Geologia Croatica 59/1 or northeast. At the base of a trough, sands are in most cases coarse grained, with a high percentage of granules and even small pebbles. Within the trough cross-strati- fied sets it is hard to distinguish bounding surfaces from reactivation surfaces. Sets usually overlie erosional sur- faces (5th order bounding surface), and partly alternate with planar cross-stratified and horizontally stratified sand sets. Horizontally stratified sands (facies Sh) are pres- ent in relatively thin intervals (0.1–2 m; Fig. 19). They overlie the irregular erosional surface, or alternate with planar cross-stratified sands. In the former they con- tain coarse-grained sand with scattered granules, and are 0.5–2 m thick. Horizontally stratified sands extend laterally up to 10 m. In some cases, downstream, they become low-angle planar cross-stratified sets (Facies Sl). At profiles E and B a concave depression (5th order bounding surface) can be observed (Figs. 7a, b, 17a, b and 20). It is symmetrically infilled with facies Si and Sl, and therefore resembles the infill of small channels. It is impossible to reconstruct the 3D geometry, but it is obvious that the axis dips in an upstream direction. Therefore it is presumed that the depression is trough- like rather than an elongate cylinder in shape. Its dimen- sions are 3.6x12.5 m. 4.2.2. Flood plain elements The upper part of profile B and the uppermost por- tion of profile D are composed of silt clays and marls, alternating with medium- to fine grained sands and silts (Figs. 7a, b, 12–14). Fine grained beds are 0.1–1.2 m thick, massive or horizontally stratified. Locally, these sediments contain numerous remnants of fossil macroflora (Table 4; Figs. 21–23). Sand and silt intervals take the form of sheets or lenses, with irregular erosive to concave bases (Figs. 7a, b, 12 and 14). They are 0.2–2 m thick, extending lat- erally from 5 to >20 m. They consist of low angle cross- stratified to horizontal stratified units, 0.05–0.3 m thick. Thin ripple sets can be observed in the upper portions of these bodies. 5. INTERPRETATION 5.1. Gravel complex The vertical and lateral characteristics of gravel sheets (element GS) indicate that these sediments were depos- ited from catastrophic unconfined sheet flows that expand as they move down the steep slope of an alluvial fan, probably when leaving the channel at the top of the fan. Such sediments can be clearly distinguished from other gravitational flows due to their different hydraulic features (high Froude number, abrupt decrease of flow intensity, rapid deposition of material). Flow conditions were in most cases supercritical, due to the steep slope. Alternation of transportation and depositional phases of the coarse- and fine-grained gravel was caused by changes in hydraulic conditions during the expansion of flow and the decrease in slope inclination, as well as the autocyclic variations of depth and velocity of the super- critical flow. A large number of gravel sheets could have been deposited from a single catastrophic flow (BLAIR, 1987). Facies Gp represents deposition from antidunes (Fig. 6a, b), which were moving together with the larg- est gravel clasts in the period of maximum velocity and PLANT GROUP SPECIES GINKGOES Ginkgo adianthoides (UNGER) HEER FLOWERING PLANTS A. HAMAMELIDIDAE Hamamelidales Liquidambar europaea A.BRONGNIART Fagales Fagus haidingeri KOVATS “Quercus” ex.gr. mediterranea UNGER “Q.” cf. lonchitis UNGER “Q.” kamischinensis GÖPPERT Betula alba LINNÉ Alnus julianaeformis (STERNBERG) KVACEK & HOLLY Carpinus betulus LINNÉ Urticales Ulmus laevis PALLAS Myricales Myrica lignitum (UNGER) SAPORTA Myrica sp. B. ROSIDAE Fabales (=Leguminosae) Leguminosae gen. et sp. indet. Sapindales Acer platanoides LINNÉ Table 4 Flora found in facies Fm and Fl. Profile B. Cabuna quarry. 73Mrinjek, Sremac & Velić: Pliocene Alluvial Sediments in the Drava Depression... height of the flow (BLAIR, 1987). Smaller, pebble and granule sized clasts, as well as sands, could have been suspended-load. Massive gravel overlying the erosional surface in Profile A (Fig. 6a, b) was probably deposited from grav- ity flows (currents) in the incised fan channel (BLAIR & McPHERSON, 1994). According to the lithology, facies associations and depositional processes, it can be concluded that the gravelly part of the section probably represents depos- its of alluvial fans, with dominant gravel sheet facies. BLAIR & McPHERSON (1994) concluded that the majority of recent alluvial fans were deposited from cohesive debrite-flows (Type I alluvial fan), or from rapid streams in the form of sheets (Type II alluvial fan), which depend on the lithological conditions, as A B Fig. 7 (A) Photomosaics of Profile B. Cabuna quarry. (B) Detailed sketch of Profile B. Legend is shown in Fig. 5. Lithofacies and elements description and interpretation are shown in Fig. 5 and Tables 2 and 3. Fig. 8 Alternation of facies Gh, Gp and Sh on the top of the gravel complex. Stick is 1 m long. Profile B, Cabuna quarry. 74 Geologia Croatica 59/1 Fig. 9 Facies Gh and Gm with lens-like facies Sh and Sl on the top of the gravel complex. Scale is 10 cm long. Profile D, Rezovac quarry. well as on weathering processes in the fan drainage area. Investigated gravels originate from a hinterland com- posed of quartz conglomerates and sands and/or rapidly uplifted and eroded granites and gneisses. North and north–east palaeotransport directions indicate a possible source area – the crystalline massifs of the Psunj and Bilogora Mts. Hard rocks of different lithologies must have been poorly chemically, but strongly mechanically weathered. This can be concluded from the dominant sheet elements and their lithological composition, with the absence of clays. Facies types in the upper part of the gravel complex could represent deposits from a very shallow braided river system, with low longitudinal bars deposited dur- ing the decrease of the catastrophic flow, or between the two catastrophic flows, during the phase of erosion processes and modification of fan morphology. Such secondary processes are particularly common in Type II alluvial fans, due to the erosive capacity of their sur- faces (BLAIR & McPHERSON, 1994). 5.2. Sandy complex 5.2.1. Channel elements The main feature of the sandy portions of the analyzed profiles is the presence of sandy channels (element CH), which were in most cases deposited by torrents or floods. Rapidly deposited channel complexes consist of dunes (element DU), lateral or downstream elements (element DLA), linguoid bedforms (LB) and laminated sand sheets (LS). Down-stream and lateral accretion macroforms (DLA), with radial distribution of migration direc- tions, and overlain by DU and/or LS elements, can be compared with cross-channel bars (CANT & WALK- ER, 1978) or with mid-channel or side bars (MIALL, 1988b). The thickness of DLA elements varies from 0.5–3 m, depending on the channel depth and growth direction (parallel, oblique or perpendicular to the main direction of the palaeostreams). Though it is impossible to determine their exact length and width in the pro- files, it is obvious that they were longer than 15 m. In most cases these units decrease in thickness and grain size upwards, due to the decline of the stream and long term deposition by moderate to slow streams (MIALL, 1991). Laterally broad (0.5–2 m wide; >30 m long) LS ele- ments are common within the channel complex. Thin, but prominent scour fills (SF), parting lineation and numerous granules within the Sh facies indicate rapid deposition during the upper flow regime, and turbulent conditions, with a velocity probably in excess of 1 m s-1 (HARMS et al., 1975; ALLEN, 1983, 1984). Analysis of the geometry of the sedimentary body and the facies has shown the presence of long and narrow macro- forms, possibly “plane-bedded simple bars” (PBSB) or compound bars described by ALLEN (1983) or “plane- bedded macroforms” (PBM) as described by MIALL (1988). It can be concluded that they were the result of seasonal changes in permanent, long-lasting flows. LB elements are relatively scarce on DLA and LS elements (Profiles C1, C2 and D). They were deposited during decreases in the flow intensity. Each of the elements described above, can be partly interpreted as a segment of a complex, large and long- lasting macroform, probably a sand flat sensu CANT & WALKER (1978). Under such conditions, the upper parts of DLA elements, which would be exposed during reductions in the flow rate, could represent a core, from which complex sand flats were deposited, through verti- cal aggradation and down-stream accretion. Dune elements (DU) migrated down the deeper parts of a channel and/or covered sand bodies or sand flats (DLA, LS, LB), during periods of decreased flow rate or during the low-intensity flows. In most cases palaeo- current directions are similar. Locally, dunes could have migrated obliquely or transversely to the main palaeo- current direction (e.g. on the flanks of sand flats). The origin of depressions (HF element) can be attri- buted to processes producing deep incisions where channels converged. Such processes were investigated both in the field and under laboratory conditions (MOS- LEY, 1974; BEST, 1988). These studies have shown that deep erosion is typical for channel junctions, which 75Mrinjek, Sremac & Velić: Pliocene Alluvial Sediments in the Drava Depression... Fig. 10 (A) Photomosaics of Profile C1. Cabuna quarry. (B) Detailed sketch of Profile C1. Legend is shown in Fig. 5. Lithofacies and ele- ments description and interpretation are shown in Fig. 5 and Tables 2 and 3. A B are common in braided river systems (BEST, 1988). Comparing different localities, current directions are variable, despite the low meandering rate of the river system. Therefore, significant dispersion of palaeocur- rent directions can be observed. Lateral migration of the main channels was a continuous process, resulting in destruction or modification of sand flats. Avulsions, as well as aggradations and channel interruptions were 76 Geologia Croatica 59/1 A B Fig. 11 (A) Photomosaics of Profile C2. Cabuna quarry. (B) Detailed sketch of Profile C2. Legend is shown in Fig. 5. Lithofacies and ele- ments description and interpretation are shown in Fig. 5 and Tables 2 and 3. common. Main channels were shallow (1–8 m, usually 1–3 m deep), very wide (>50 m), low grade sinusoi- dal (not highly meandering), with a general direction towards the north–east and east. According to the geometry of the sediments, lack of palaeosols and palaeobotanical data, it can be con- cluded that the climate was humid, which enabled per- manent streams, with periodic (seasonal?) flooding. The sandy parts of the profiles can be interpreted as rapidly deposited sediments in wide, low-meandering and com- plex channel environments. Presumed channel activi- ties were similar to those in recent sandy braided riv- ers (SMITH, 1970; MIALL, 1977; CANT & WALKER, 1978). 5.2.2. Flood plain elements The upper part of Profile B represents a relatively thick (10 m) sequence of flood plain (lacustrine?) depos- its, laterally extending for more than 60 m. Sheet-like overbank fines (element OF) deposited after infilling of the channels by torrents are dominant, with several interstratified crevasse splays (element CS), deposited in shallow stagnant water on a flood plain after the pen- etration of levees. They are composed of rapidly depos- ited sands (Facies Sh and Sl), which are, in some places convoluted and overlain by facies Sr, deposited during the declining flow. Sheet geometry and fine grain size indicate that these sediments were deposited distally from the main channel, although precise distances can 77Mrinjek, Sremac & Velić: Pliocene Alluvial Sediments in the Drava Depression... Fig. 12 Logs B and C. Legend is shown in Fig. 5. Cabuna quarry. not be determined on the profile. Dominant CS ele- ments and a lack of palaeosols indicate frequent flood- ing episodes. Well preserved plant fossils were discovered in the fine-grained sediments. Flowering plants (angiosperms) are more common than gymnosperms (ginkgoes only) (Table 4). Maple (Acer) and hornbeam (Carpinus) rem- nants predominate (Figs. 22 and 23). Liquidambar, Myrica and Mediterranean oaks are relics of the Mio- cene–Middle Pliocene warm period. Warming may have been related to changes in ocean circulation patterns, possibly combined with the greenhouse effect (CHAN- B' Fm <;0 51 51 <;0 Fm <;0 OVERBANK 5, 45 51 CHANNEL 20 51 "'[.. FILL FI <;0 , " " Fm <;0 CHANNEL " FILL Fm <;0 " 5, --5, Sh , FI 40 " { Sh 5, 15 5, Sh CREVASSE 51 SPLAY 51 CHANNEL ,1 I. FILL 51 Sh /. CHANNEL " FILL 35 Sh, Ge,Se 10 51 ! ,! 5, '1, 'r, 51 CHANNEL " FILL " ..-p /, 51 CHANNEL CHANNEL FILL "& FILL , , 30 Sh " 5, 5 Gh 5 Sh Gh ,/,/ ALUVIAL , CHANNEL FILL Sh FAN Gm ALUVIAL FAN Sh Gh 25 mO m MEAN GRAIN SIZE INTERPRETATION INTERPRETATION 78 Geologia Croatica 59/1 Fig. 13 Numerous sets of planar cross-stratified sands in the upper part of log D. Part of log D is about 10 m long. Rezovac quarry. See legend on Fig. 5. Fig. 14 Log D. Rezovac quarry. Legend is shown in Fig. 5. Litho- facies and element description and interpretation are shown in Fig. 5 and Tables 2 and 3. DLER, 1997). The presence of betulaceans (beech, alder) indicates cooling of the climate, typical for the end of the Pliocene. Maples and hornbeams are known as the first colonizers of the unconsolidated soils. Gink- go adianthoides (Fig. 21) similar to its recent conge- neric, G. biloba LINNÉ, is an ecologically conservative genus. Throughout the late Cretaceous and Cenozoic, it was largely confined to disturbed streamside and levee environments (ROYER et al., 2003), and it can not be found at European and American localities after the Pliocene (see www.ucmp.berkeley.edu/seedplants/ ginkgoales/gingkofr.html). Thin, fine grained sediments (OF) in the uppermost part of profile D (Rezovac) were probably deposited on sand flats during a low water stand, when most of these sand macroforms acted as islands. OF elements were emergent and partly eroded during the periodic floods. 6. DEFORMATION STRUCTURES Synsedimentary deformation of loose sands are well developed at the investigated localities. LS and DU elements (Table 4) in Profiles B, C1 and C2 (Figs. 7a, b, 10a, b, 14a, b) are partly or completely convoluted. Convolution can be described as a series of steep, high (0.5–1.5 m) antiforms divided by 1–3 m wide synforms. In most cases convolution was single phased, and it did not include previously deformed, cross-stratified sands. Large scale, multiphase deformation was observed exclusively within LS elements in profiles B and C1. Such deformation includes several sets, and is abrupt- ly interrupted in the base by a synsedimentary reverse fault (Fig. 24). Small blocks of massive sand indicate synsedimentary slumping. This deformation is also marked with antiforms, slightly overturned in the pal- aeocurrents direction. Oversteepening and overturning can also be obser- ved within isolated sets of HF, DLA and LB elements (Fig. 25). Deformations vary from foresets inclined more than the angle of rest to overturned foresets which resemble the overturned folds. Deformation increases in the upper part of the profiles. Convolute deformations are partly of intrinsic ori- gin, produced by synsedimentary processes (LEEDER, 1987). ALLEN (1984) interpreted these processes as 79Mrinjek, Sremac & Velić: Pliocene Alluvial Sediments in the Drava Depression... Fig. 15 Base of sand channel (5th bounding surface) overlain by facies Ge and Se. Hammer is 30 cm long. Profile C2. Cabuna quarry. Fig. 16 Sets of planar cross-stratified sands (facies Sp) and hori- zontally stratified sands (facies Sh). Transition from planar cross-stratified sands to low-angle cross-stratified sands (facies Sl) and reactivation surfaces can be observed. Stick is 1 m long. Profile C2. Cabuna quarry. Fig. 17 (A) Photomosaic of Profile E. Bistrica quarry. (B) Detailed sketch of Profile E. Legend is shown in Fig. 5. Lithofacies and elements description and interpretation are shown in Fig. 5 and Tables 2 and 3. A B 80 Geologia Croatica 59/1 Fig. 18 Sets of trough cross-stratified sands (facies St, element DU) overlain by sets of low-angle planar cross-stratified and horizontally stratified sands (facies Sl and Sh, element LS). Stick is 1 m long. Profile C1. Cabuna quarry. Fig. 19 Sets of planar cross-stratified sands (facies Sp) and horizontally stratified sands (facies Sh). Transi- tion from horizontally stratified sands (facies Sh) to low-angle cross-stratified sands (facies Sl). Stick is 1 m long. Profile C2. Cabuna quarry. meta- and synsedimentary events, which took place during rapid deposition, or immediately after sedimen- tation. Liquefaction processes were triggered by the unstable density gradient. Intensive, polyphase, laterally extensive deforma- tions in Profiles B and C1 influencing whole set pack- ages, suggest extrinsic origin. Seismic shocks were the probable cause of these events, as indicated by the fol- lowing features: (1) size and extent of deformational structures; (2) deformed/undeformed sand body ratio; (3) synsedimentary faults; (4) proximity of the main transcurrent fault. Rapid subsidence enabled the successful burial of sands and their saturation for liquefaction caused by seismic activity. An earthquake epicenter could not be precisely detected, but was probably situated within 20 km, considering the fact that a magnitude of at least 5 is necessary for the liquification of loose, saturated, fine- to medium-grained sands. In case of magnitude 7, the epicenter could have been within 40 km (IDRISS, 1985). The Southern marginal fault of the Drava depres- sion is within these distances, and was the most prob- able location for the epicenter. 7. TECTONIC SETTING The investigated area is situated along the southern mar- gin of the Drava depression (according to the petrole- um-geological division), within the southwestern part of the Pannonian basin system, which territorially belongs to the Republic of Croatia. This is an area of extreme- ly strong tectonic processes, clearly divided into three periods, resulting in the formation of corresponding tectonic structures (PRELOGOVIĆ et al., 1995, 1998; LUČIĆ et al., 2001; SAFTIĆ et al., 2003): (1) Initial structural changes during the Oligocene and Early Miocene. The onset of extensional tectonics, subsidence and sedimentation began as a result of the first syn-rift extensional tectonic phase; 81Mrinjek, Sremac & Velić: Pliocene Alluvial Sediments in the Drava Depression... Fig. 20 Concave depression (ele- ment HF) symmetrically infilled with facies Si and Sl. The right side of the depression shows synsedimentary deformation. Depression is 1.5 m deep. Pro- file E. Bistrica quarry. Fig. 21 Ginkgo adianthoides (UNGER) HEER. Fig. 22 Carpinus betulus LINNÉ. Fig. 23 Acer platanoides LINNÉ. (2) Major extensional processes during the Early and Middle Miocene; (3) New reshaping processes (prevailing transpression) during the Pliocene, which is still present today. This model is dominated by wrench-faults. According to the structural composition, three main zones can be distinguished, comparable with the west- ern, southern and central marginal portions of the Pan- nonian basin system. They are outlined by the dominant regional faults: the Periadriatic–Drava fault, the Med- vednica fault zone and the southern marginal fault of the Pannonian basin. The explored outcrops of the Pliocene clastic sedi- ments are situated south of the Virovitica–Slatina line, on the northern slopes of the Bilogora and Papuk Mts. Their genesis was strongly influenced by tectonic pro- cesses, in relation to palaeogeographic conditions and climatic changes. This area is situated at the boundary between two large structures – the uplifted Bilogora and Papuk Mts. and subsided Drava depression. (Fig. 26). This boundary is identical to the NW striking dextral Periadriatic–Drava wrench fault. According to seismotectonic analysis (PRELOGOVIĆ et al., 1998), this fault actually represents a 5 km wide zone. After the first 7 kilometres of depth, which are characterized by a reverse movement of the hanging wall, this fault becomes almost vertical. Its position, width of the fault zone and dimensions of displacement were presented by SAFTIĆ et al. (2003). One of the indicators of the presence of a fault is the sediment thickness, deposited due to its influence. 82 Geologia Croatica 59/1 Fig. 24 Synsedimentary deformation expressed by overturned antiforms abruptly interrupted in their base by a synsedimentary reverse fault. Overs- teepening and overturning can be also observed within isolated sets in the upper part of the photograph. The antiforms are about 1 m high. Profile C1. Cabuna quarry. Fig. 25 Synsedimentry deformation expressed by oversteeping and overturn- ing in the middle part of photograph. Oversteepened and overturned interval is 1 m thick. Profile C1. Cabuna quarry. In the vicinity of Virovitica, the extreme depth (some- times exceeding 6000 m) of the sediment filled Neo- gene–Holocene basin was observed (VELIĆ et al., 2002; SAFTIĆ et al., 2003). Extreme values were par- ticularly measured for the sediments of the Pliocene, Pleistocene and Holocene, which are of great interest here. Northeast of Virovitica and Slatina, more than 1,500 m thickness was measured in these sediments. These data clearly emphasize the Pliocene–Holocene synsedimentary tectonic activity, which enabled the for- mation of such a prominent depositional centre. The investigated clastic sediments belong to the 3rd megacycle, formed during Pliocene to Holocene basin inversion resulting in subsidence in the deepest zones, associated with uplift and – particularly important – ero- sion of the most elevated blocks (VELIĆ et al., 2002; SAFTIĆ et al., 2003). Rapidly uplifted blocks, the Sla- vonian Mts., Bilogora and Papuk Mts. represented the source area for the redeposited clastic material (well rounded pebbles of quartz, quartzite and other materi- als), combined with the influence of the moderately warm climate and intense precipitation (CHANDLER, 1997). 8. CONCLUSIONS (1) The detailed analysis of lateral profiles exposed in sand/gravel quarries along the southern edge of the Drava depression, between Virovitica and Slatina, show the existence of a complex and variable allu- vial system during the Late Pliocene. (2) Gravels in the lower portion of Profiles A, B, D and E were deposited in alluvial fans. Gravel sheets deposited during catastrophic floods predominate. The composition and lithological features of the gravels indicate a tectonically uplifted and disturbed hinterland, composed of conglomerates and sands, with predominant mechanical weathering. Granites and gneisses are less probable as the source of the material, because well rounded quartz clasts suggest multiple phases of redeposition. (3) North and northeastern directions of palaeostreams, derived from clast imbrications, are more or less perpendicular to the direction of the main Drava fault, indicating the Psunj and Bilogora areas as a possible source for clasts. (4) Preservation of alluvial fans in the investigated area suggests deposition in an extensive or transtensive 83Mrinjek, Sremac & Velić: Pliocene Alluvial Sediments in the Drava Depression... Fig. 26 Tectonic setting of the study area (from LUČIĆ et al., 2001). land environment, surrounded by steep faults of ver- tical and/or horizontal character, which confirms the previously published data on tectonics in the wider area. (5) Sandy facies of the braided river, which erosively overlie the alluvial fans, indicates the decline of tec- tonic activity. Climatic changes into a stable, moder- ately humid climate, could also be a reason for dep- ositional changes thus forming fining upward fluvial cycles (Fig. 12). (6) Deformation structures indicate the vicinity of earth- quake epicenters, and extensional and transtensional faults. (7) Source of the clastic material was probably relative- ly close, ca. 10 km south or southwest of the investi- gated area. (8) Although the dominant flow direction of the main channel was subparallel to the palaeodirections on the fans, and very near the active faults, the water source could have been tens of kilometers or more from the source area of the clastic material in the alluvial fans. Acknowledgement The authors gratefully appreciate and acknowledge the reviews by Györgyi JUHÁSZ and Josip TIŠLJAR whose comments significantly improved this paper. 9. REFERENCES ALLEN, J.R.L. (1983): Studies in fluviatile sedimentation: bars, bar-complexes and sandstone sheets (low-sinuosity braided streams) in the Brownstones (L. Devonian), Welsh borders.– Sediment. Geol., 33, 237–293. ALLEN, J.R.L. (1984): Sedimentary structures, their charac- ter and physical basis.– Development in Sedimentology, 30, 663 p. BABIĆ, Ž., ČAKARUN, I., SOKAČ, A. & MRAZ, V. (1978): O geologiji kvartarnih naslaga porečja rijeke Drave [Geol- ogy of the Quaternary deposits in the Drava river basin – in Croatian].– Geol. vjesnik, 30/1, 43–61. BAILEY, S.W. (1987): Micas.– Reviews in Mineralogy, 13, Mineralogical Society of America, XII+584 p., Washing- ton. BEST, J.L. (1988): Sediment transport and bed morphology at river channel confluences.– Sedimentology, 35, 481–498. BLAIR, T.C. (1987): Sedimentary processes, vertical stratifi- cation sequences, and geomorphology of Roaring River alluvial fan. Rocky Mountain National Park, Colorado.– Jour. Sed. Petrol., 57, 1–18. BLAIR, T.C. & McPHERSON, J.G. (1994): Alluvial fans and their natural distinction from rivers based on morphology, hydraulic processes, sedimentary processes, and facies assemblages.– Jour. Sed. Petrol., A64/3, 450–489. BROOKFIELD, M.E. (1977): The origin of bounding sur- faces in ancient aeolian sandstones.– Sedimentology, 24, 303–332. 84 Geologia Croatica 59/1 CANT, D.J. & WALKER, R.G. (1978): Fluvial processes and facies sequences in the sandy braided, South Saskatche- wan River, Canada.– Sedimentology, 25, 625–648. CHANDLER, M.A. (1997): The Climate of the Pliocene: Simulating Earth’s Last Great Warm Period.– http://giss. nasa.gov/research/features/pliocene. GALOVIĆ, I., MARKOVIĆ, S. & MAGDALENIĆ, Z. (1981): Osnovna geološka karta SFRJ 1:100000. Tumač za list Virovitica, L33–83 (Basic Geological Map of SFRY 1:100,000. Geology of Virovitica Sheet). – Inst. geol. istraž. Zagreb (1976), Savezni geol. zavod, Beograd, 44 p. HARMS, J.C., SOUTHARD, J.B., SPEARING, D.R. & WALKER, R.G. (1975): Depositional environments as interpreted from primary sedimentary structures and strat- ification sequences.– SEPM, Short course 2, 161 p. IDRISS, I.M. (1985): Evaluating seismic risk in engineer- ing practice.– In: Proceedings of 11th International Con- ference on Soil Mechanics and Foundation Engineering, A.A. Balkema Publishers, Rotterdam, 255–320. KELLER, W.D. (1970): Symposium papers on enviromental aspects of clay minerals.– Jour. Sed. Petrol., 40/3, 788– 813. LEEDER, M.R. (1987): Sediment deformation structures and palaeotectonic analysis of sedimentary basins with case study from Carboniferous of northern England.– In: JONES, E.M. & PRESTON, R.M.F. (eds.): Deformation of Sediments and Sedimentary Rocks. Geological Society, London, 29, 137–146. LUČIĆ, D., SAFTIĆ, B., KRIZMANIĆ, K., PRELOGOVIĆ, E., BRITVIĆ, V., MESIĆ, I. & TADEJ, J. (2001): The Neogene evolution and hydrocarbon potential of the Pan- nonian Basin in Croatia.– Marine and Petroleum Geology, 18, 133–147. MARKOVIĆ, S. (1986): Osnovna geološka karta SFRJ 1:100.000. Tumač za list Podravska Slatina, L33–84 (Basic Geological Map of SFRY 1:100,000. Geology of Podravska Slatina Sheet).– Inst. geol. istraž., Zagreb (1984), Savezni geol. zavod, Beograd, 43 p. MIALL, A.D. (1977): A review of the braided-river deposi- tional environment.– Earth Science Reviews, 13, 1–62. MIALL, A.D. (1985): Architectural-element analysis: a new method of facies analysis applied to fluvial deposits.– Earth Sci. Rev., 22, 261–308. MIALL, A.D. (1988a): Architectural elements and boundary surface in fluvial deposits: anatomy of the Kayenta forma- tion (Lower Jurassic), Southwest Colorado.– Sedimentary Geology, 55, 233–262. MIALL, A.D. (1988b): Facies architecture in clastic sedimen- tary basins.– In: KLEINSPEHN, K. & PAOLA, C. (eds.): New Perspectives in Basin Analysis. Springer-Verlag, New York, Heidelberg, Berlin, 67–81. MIALL, A.D. (1991): Hierarchies of architectural units in ter- rigenous clastic rocks, and their relationship to sedimen- tation rate.– In: MIALL, A.D. & TYLER, N. (eds.): The Three-Dimensional Facies Architecture of Terrigenous clastic Sediments and Its Implication to Hydrocarbon Dis- covery and Recovery. SEPM (Society for Sedimentary Geology). Concepts in Sedimentology and Palaeontology, 3, 6–12. MOSLEY, M.P. (1974): An experimental study of channel confluences.– Journal of Geology, 84, 535–562. PRELOGOVIĆ, E. & VELIĆ, J. (1992): Correlation of Qua- ternary sediments and tectonic activity of the eastern part of the Drava river depression.– Geol. Croat. 45, 151–162. PRELOGOVIĆ, E., JAMIČIĆ, D., ALJINOVIĆ, B., VELIĆ, J., SAFTIĆ, B. & DRAGAŠ, M. (1995): Dinamika nas- tanka struktura južnog dijela Panonskog bazena [Structur- al dynamics in southern part of the Pannonian Basin – in Croatian].– 1 . Hrv. geol. kongr., Opatija, Zbornik radova, 2, 397–398, Zagreb. PRELOGOVIĆ, E., SAFTIĆ, B., KUK, V., VELIĆ, J., DRAGAŠ, M. & LUČIĆ, D. (1998): Tectonic activity in the Croatian part of the Pannonian basin.– Tectonophys- ics, 297, 283–293. ROYER, D.L., HICKEY, L.J. & WING, S.L. (2003): Ecologi- cal conservativism in the “living fossil” Ginkgo.– Paleobi- ology, 29/11, 84–104. SAFTIĆ, B., VELIĆ, J., SZTANÓ, O., JUHÁSZ, G. & IVKOVIĆ, Ž. (2003): Tertiary subsurface facies, source rocks and hydrocarbon reservoirs in the SW part of the Pannonian Basin (Northern Croatia and south-western Hungary).– Geol. Croatica, 56/1, 101–122. SMITH, N.D. (1970): The braided stream depositional envi- ronment: comparison of the Platte River with some Siluri- an clastics rocks, north-central Appalachians.– Bull. Geol. Soc. Am., 81, 2992–3014. VELIĆ, J., WEISSER, M., SAFTIĆ, B., VRBANAC, B. & IVKOVIĆ, Ž. (2002): Petroleum-geological character- istics and exploration level of the three Neogene deposi- tional magacycles in the Croatian part of the Pannonian basin.– Nafta, 53/6–7, 239–249, Zagreb. WIZEVICH, M.C. (1991): Photomosaics of outcrops: useful photographic techniques.– In: MIALL, A.D. & TYLER, N. (eds.): The Three-Dimensional Facies Architecture of Terrigenous Clastic Sediments and Its Implication to Hydrocarbon Discovery and Recovery. SEPM (Society for Sedimentary Geology). Concepts in Sedimentology and Paleontology, 3, 22–24. Manuscript received December 12, 2004. Revised manuscript accepted June 9, 2006.