1. INTRODUCTION The Miocene complex of the Poæeπka Mt. (Figs. 1 and 2) represents a part of the SW marginal zone of the Pannonian Basin (Fig. 3), as part of Central Paratethys. Formation of the Pannonian Basin commenced in the Early and Middle Miocene, due to the collision of the African (Apulian) and European plates. The basin is located along the concave side of the A-type subduction arc, and is surrounded by the Alps, Carpathians and Dinarides (HORVÁTH & ROYDEN, 1981; HORVÁ- TH, 1984, 1993; ROYDEN et al., 1982, 1983a, b; ROYDEN, 1988; CSONTOS et al., 1992). It is super- imposed on the older Palaeogene basinal complex as the result of the back-arc extension, and during the Miocene - Pliocene period it existed as one of many Lower Miocene Alluvial Deposits of the Poæeπka Mt. (Pannonian Basin, Northern Croatia): Cycles, Megacycles and Tectonic Implications Davor PAVELI∆ and Marijan KOVA»I∆ Mediterranean back-arc basins (STEGENA et al., 1975; HORVÁTH & ROYDEN, 1981; ROYDEN et al., 1982; TARI et al., 1992a, b). According to these aforemen- tioned authors, deposition in the entire Pannonian Basin was strongly influenced by extensional tectonics. On the basis of its depositional history, PAVELI∆ (1998) attributed Early Miocene extensional processes in the area between the Sava and Drava rivers, to the forma- tion of a rift-type basin generated by passive continental rifting (Early Miocene North Croatian Basin). Up to the present there have been no published data on the sedimentology of the Lower Miocene (?Ottnan- gian) fresh-water deposits of Poæeπka Mt. Some charac- teristics of these deposits can be found in former region- al works (©PARICA et al., 1980; ©PARICA & BUZA- LJKO, 1984), where they are divided into lower and upper part, and their origin attributed to disintegration of the basement caused by intense tectonic activity. In the lower part of the succession coarse-grained clastics are common: frequent alternations of conglomerates, breccia and sandstone, as well as gravel and sand. This level is characterized by a lack of fossils, and the deposits are interpreted as “prolluvial” in origin. The upper part of these sections is composed of intercala- tions of marl, gravel, sand, clay and coal. This level is characterised by a fossil fauna typical for fresh-water environments, which was interpreted as lacustrine by KOCHANSKY-DEVIDÉ (1978) and KOCHANSKY- DEVIDÉ & SLI©KOVI∆ (1978). Fresh-water deposits are probably conformably overlain by Karpatian marls, sands and gravel of marine origin (©PARICA et al., 1979), as a consequence of the transgression that affect- ed vast areas of the Central Paratethys (RÖGL & STEININGER, 1983). The entire Lower Miocene sedi- mentary complex of Poæeπka Mt. is situated near the Sava fault, which was determined as a dextral strike-slip fault (HORVÁTH & ROYDEN, 1981; JAMI»I∆, 1988, 1995; ROYDEN, 1988; RUMPLER & HORVÁTH, 1988; BERGERAT, 1989; HORVÁTH, 1993 - Fig. 3). Considerable lithological variability of the alluvial deposits, their genetic relationship to tectonic activity, as well as their proximity to the Sava fault, made it nec- cessary to undertake facies analysis of these deposits. Results indicate the depositional style which enables recognition of the vertical trends and allows interpreta- tion of autocyclic and allocyclic control on sedimenta- tion. G EOL . CROAT. 52/1 67 - 76 7 Figs. 1 Tab. ZAGREB 1999 Key words: Braided alluvial fan, Synsedimentary tec- tonics, Sava fault, Early Miocene, Pannonian Basin, Croatia. Institute of Geology, Sachsova 2, P.O.Box 268, HR-10000 Zagreb, Croatia. Abstract In the area of the present Poæeπka Mt. braided alluvial fans were formed during the Early Miocene above the subsiding Cretaceous- Palaeogene basement. Due to autocyclic processes, i.e. lateral migra- tion of flows due to vertical aggradation of longitudinal bars, or migration of the main trench, small-scale fining-upward cycles were formed. The complete succession of the alluvial deposits is composed of two fining-upward megacycles, which are the consequence of allo- cyclic influences, i.e. the pulsating character of synsedimentary tec- tonics. Megacycles were developed parallel to backstepping of the front of the fault scarp towards the mountain massif, caused by nor- mal faulting along the active margin of an extensional basin. This kind of depositional style indicates that the Sava fault operated as a normal fault at the beginning of its life during the Early Miocene, probably the Ottnangian. 68 Geologia Croatica 52/1 2. GEOLOGICAL SETTING AND STRATIGRAPHY Lower Miocene fresh-water deposits cover vast are- as in northern Croatia, both on the surface and in the subsurface (PAVELI∆, 1998). On the surface they are most widespread in the central and NW part of Poæeπka Mt. (Fig. 2). The age of these deposits is not precisely defined by chronostratigraphic methods. In their upper part, KOC- HANSKY-DEVIDÉ (1978) determined endemic speci- es of a lacustrine macrofauna. Based on superposition and correlation with some localities in northern Croatia and northern Bosnia, these deposits are thought to be Ottnangian to Karpatian in age. ©PARICA et al. (1980) and ©PARICA & BUZALJKO (1984) considered these deposits as “Lower Helvetian”, because of their gradual transition into the clastic sediments, the age of which is determined by foraminiferal assemblages as being of “Upper Helvetian”, i.e. marine Karpatian. Later, ©PAR- ICA & PAMI∆ (1986) attributed an Ottnangian age to the entire fresh-water complex, but without new strati- graphic data. The Lower Miocene sedimentary complex of Poæe- πka Mt. (Fig. 2) unconformably overlies the pre-Mioce- ne basement (©PARICA et al., 1979; HALAMI∆ et al., 1990), which in a geotectonic sense represents a part of the Supradinaric (HERAK, 1986), i.e. northern margin of the Inner Dinarides (HERAK et al., 1990; ©IKI∆, 1995). The Miocene basement is composed of Upper Cretaceous (©PARICA et al., 1979, 1980) and probably Palaeogene rocks (HALAMI∆ et al., 1993). The Upper Cretaceous deposits comprise limestones and clastic rocks (©PARICA et al., 1979, 1980), although a part of the clastic sequence could be Oligocene in age (HALA- MI∆ et al., 1993). According to PAMI∆ & ©PARICA (1983), ©PARICA & PAMI∆ (1986) and PAMI∆ et al. (1990) the volcano-sedimentary complex is also of Late Cretaceous age, while BELAK et al. (1998) indicate the possibility of a continuation of the volcanic activity into the Palaeogene. Basement rocks were intensely folded during the Laramian phase of the Alpine orogeny, as well as during the post-Cretaceous tectonic activity (JAMI»I∆ et al., 1985). After the Early Miocene, deposition continued into the Middle Miocene. During the Badenian, marine clas- tics and carbonates were deposited, while Sarmatian clastics and carbonates were deposited in brackish envi- ronments (©PARICA et al., 1980; ©PARICA & BUZA- LJKO, 1984). The transition from the Early to Middle Miocene is gradual, although Pannonian deposits spo- radically transgressively overlie older rocks (©PARICA et al., 1980; ©PARICA & BUZALJKO, 1984). Accord- ing to these authors Pannonian carbonates and marls were deposited in brackish environments, while occur- rences of clastic rocks are infrequent. During the Pon- tian, marls and sands were also deposited in brackish, and later in fresh-water environments. They are confor- mably overlain by fresh-water clastic deposits of Plio- cene age (©PARICA et al., 1980; ©PARICA & BUZA- LJKO, 1984). 3. FACIES Characteristics of the Lower Miocene alluvial depo- sits are well illustrated in two geological sections (Fig. 4 and Table 1), measured in the central part of Poæeπka Mt. (Fig. 2). Their superposition is clearly defined by detailed geological mapping. The depositional sequence measured at Poæeπka Gora I (PG I) is 109 m thick, and at the Poæeπka Gora II (PG II) its thickness is 50 m (Fig. 4). The main characteristics of these deposits are as follows: a domination of clastics over carbonates and Fig. 1 Location map of the study area. Fig. 2 Geological sketch-map of the part of Poæeπka Mt. (from ©PARICA et al., 1979) with locations of sections PG-I and PG- II. Legend: 1) Quaternary; 2) Pontian; 3) Sarmatian, Pannonian; 4) Badenian; 5) Karpatian; 6) Ottnangian; 7) Upper Cretaceous; 8) normal boundary; 9) transgressive boundary; 10) fault. coarse-grained over fine-grained deposits; conspicuous variations in grain size; very rapid lateral and vertical facies alternations; occurrences of red beds; changes in composition; and lack of fauna. During geological map- ping it has been determined that the investigated deposits pass upward into massive unfossiliferous silts, with rare lenses of gravel exhibiting low-angle cross- bedding and massive fabric. These deposits are overlain by lacustrine fossiliferous silts and marls with interbeds of gravel, sand, clay and coal lenses (©PARICA et al., 1980). Deposits in the measured sections were grouped into nine facies units: 1) massive matrix-supported con- glomerates (Gmu - plastic debris flow), 2) massive 69PaveliÊ & KovaËiÊ: Lower Miocene Alluvial Deposits of the Poæeπka Mt. ... Fig. 3 Index map of the Pannonian Basin and surrounding mountain systems. The position of the Sava and Drava faults has been added (simplified after HOR- VÁTH, 1993). Code Facies Position Sediment structure and Interpretation in sections main characteristics Gmu massive conglomerates, PG-I: 14-86 m no grading, reddish colour, plastic debris flow matrix-supported PG-II: 14-17 m fragments up to 10 cm long massive conglomerates, inverse to normal grading, Gmi matrix-supported, rare PG-I: 23-94 m reddish colour, fragments pseudoplastic debris flow clast-supported up to 40 cm long Bcn massive breccia, PG-I: 0-5 m and crude normal grading, rock-fall clast-supported 99-109 m fragments up to 70 cm long crude normal grading, imbrication Gc massive conglomerates, PG-I: 23-95 m a(t) b(i), low-angle cross-bedding; main fan-trench; clast-supported PG-II: 0-50 m no grading; fragments up to longitudinal bar 200 cm long Ge pebble to granule PG-I: 13-97 m lenses stream flows; conglomerates PG-II: 2-49 m crevasse channels Sh fine- to coarse-grained PG-I: 15. and 99. m horizontal lamination upper flow regime sandstones PG-II: 1-49 m F1 silt PG-I: 18-96 m massive, brown colour flood plain; abandoned channel F2 silt, scattered granules, PG-I: 11-99 m indications of a soil-forming palaeosol rare calcrete PG-II: 7-49 m processes, reddish colour P calcrete PG-I: 14. m and horizontal lamination palaeosol 96-98 m Table 1 Facies, their main characteristics and interpretation. Codes are modified after MIALL (1978a) and WARESBACK & TURBEVILLE (1990). 70 Geologia Croatica 52/1 matrix- to clast-supported conglomerates (Gmi - pseu- doplastic debris flow), 3) massive clast-supported brec- cia (Bcn - rock-fall), 4) massive clast-supported conglo- merates (Gc - main fan-trench, longitudinal bar, chan- nels), 5) lenses of pebble to granule conglomerates (Ge - stream flows), 6) horizontally laminated sandstones (Sh - upper flow regime), 7) massive silts (F1 - flood plain), 8) modified silts (F2 - palaeosol), and 9) calcrete (P - palaeosol). These facies were described in detail and interpreted by PAVELI∆ (1998). Their main litho- logical characteristics, position in the sequence and interpretation are presented in Table 1. The vertical succession of the deposits is dominant- ly composed of small fining-upward cycles (Fig. 4). These cycles are recognised within a general, gradual decrease in grain-size upwards, which is followed by a thinning of beds composed of coarse-grained sediments and corresponding thickening of beds composed of fine-grained sediments. Generally, fining-upward cyc- les begin with deposits of the river bar (Gc), commonly formed on the channel lag, followed by deposits of the bar cover (Sh), and are capped by sediments deposited on the flood-plain (F1, F2 and P), shallow channels on the bar or crevasse channels (Ge) (Fig. 4). Conglomer- Fig. 4 Measured sections PG-I and PG-II with typical fin- ing-upward cycles and mega- cycles. For explanation of the facies codes see Table 1. 71PaveliÊ & KovaËiÊ: Lower Miocene Alluvial Deposits of the Poæeπka Mt. ... ates deposited by gravitational mechanisms (Gmu and Gmi) occur in various parts of the cycles, from the basal and middle to the upper part, irrespective of the trend. Cycle thicknesses range from 0.5-7.8 m, most frequently from 2-4 m. In some places cycles are not developed, e.g. in a 14.5 m thick complex unit of con- glomerates of facies Gc (Fig. 4, PG-I section from 24- 38.5 m), which comprises the largest fragments in the entire succession. In addition, fining-upward cycles were not found in the basal part of the PG-I section, and at some other places. 4. PALAEOTRANSPORT Flow directions are reconstructed by measurements of the dip direction of imbricated platy fragments in coarse-grained facies (Gc, Ge and Bcn). A total of 676 fragment orientations were measured - 438 in the section PG-I, and 238 in the section PG-II (Fig. 5). In the entire succession a very wide dispersion of flow orientation data was recorded. The results indi- cate a general palaeotransport direction towards the north, with a weak NE tendency in deposits of the PG- II section (Fig. 5b). 5. FACIES ASSOCIATIONS AND INTERPRETATION Two facies associations were separated in the inves- tigated deposits. The facies association A is composed of facies Bcn, Gc, Ge, F1, F2 and P. This facies associ- ation occurs in the basal and upper part of section PG-I (0-40 m and 99-109 m - Fig. 4). The facies association B is composed of facies Gmu, Gmi, Gc, Ge, Sh, F1, F2 and P. This association comprises the middle part of section PG-I (40-99 m) and the entire succession of the PG-II section (Fig. 4). The association of sediments deposited by debris flows mechanisms (facies Gmu, Gmi and Bcn) with deposits of traction currents (facies Gc, Ge and Sh) and sediments of flood-plains (F1, F2 and P) indicates deposition in alluvial environments, relatively close to the source area, probably in the alluvial fan (BULL, 1972; RUST, 1978; HEWARD, 1978; MIALL, 1978a, 1990, 1996; NEMEC & POSTMA, 1993; STANIST- REET & McCARTHY, 1993). Previously these depo- sits were interpreted as “prolluvial” (©PARICA et al., 1980). According to the subdivision of alluvial fans, which was on the basis of predominant depositional mecha- nisms as proposed by STANISTREET & McCARTHY (1993), this fan could be assigned to the mixed form between the debris flow dominated alluvial fan and the braided alluvial fan model. Considering the predomi- nance of sediments deposited by traction in the succes- sion, the Poæeπka Mt. fans seem closer to the braided alluvial fan. The fan evolved in a semi-arid climate, as indicated by the associated occurrence of calcretes, common gravitational flows, red pigmentation and lack of coal (for discussion of climate related characteristics of alluvial fans see MIALL, 1996). Sediments of the facies association A were deposit- ed in the proximal part of the braided alluvial fan. This interpretation is testified by the occurrence of rock-fall breccia (Bcn), and very large fragments, up to 200 cm in size, in the complex facies unit Gc (Fig. 4, PG-I sec- tion from 24-38.5 m). Sediments of the facies associa- tion B were deposited in the medial part of the braided alluvial fan, as indicated by the predominance of sedi- ments deposited by traction currents (facies Gc and Ge), over debris flows deposits (facies Gmi and Gmu), as well as relatively thicker units composed of fine- grained sediments, which were exposed to pedogenic processes. The erosional base of sediments comprising the small fining-upward cycles suggests an abrupt incision by strong currents into the flood-plain deposits. Sedi- ments of the Gc facies in the basal part of the cycle indicate formation on longitudinal bars. The remainder of the depositional sequence (excluding deposits of Gmu and Gmi facies), which is composed of fine- grained deposits of the F1, F2 or P facies in the top of the cycle, can be interpreted as a result of vertical bar aggradation, and subsequent formation of the flood- plain. Vertical aggradation could have been stopped when the bar rose (due to the large input of the materi- al) above the highest water level. In the investigated deposits there are no traces of the lateral accretion of the macroforms, thus meandering, with gradual filling and abandonment of channels can be excluded. There- fore, fining-upward cycles could indicate lateral shifts of flows caused by the formation of bars. However, flow shifts may also be caused by lateral shift of the main trench located in the proximal part of the fan (cf. MASSARI et al., 1993). The occurrence of sediments deposited by gravitational flows (Gmu and Gmi) within these cycles can be explained by individual events within the fan (sensu HEWARD, 1978). They were ini- tiated locally, from the fan top or associated slopes, per- haps during rain as a consequence of a sudden destabi- Fig. 5 Rose diagram measured as dip of imbrication shows palaeo- transport from the south to the north in deposits of the PG-I sec- tion (a), and from the south-west to the north-east in deposits of the PG-II section (b). A B 72 Geologia Croatica 52/1 lization of the undercut channel walls, or by a different cause of destabilization of previously deposited, uncon- solidated material, similar to the examples presented by LARSEN & STEEL (1978) and NEMEC & POSTMA (1993). This type of cycle is typical for braided alluvial fans as a consequence of the autocyclic process on the sur- face (HEWARD, 1978; MIALL, 1996). It occurs in the second phase of fan development, when backfilling of the main fan-trench and flow dispersion commence (DeCELLES et al., 1991) (Fig. 6). A 14 m thick con- glomerate unit of facies Gc indicates very strong flows and long-lasting vertical aggradation. Conglomerates of this facies are located between the rock-fall breccia (Bcn) and deposits constituting fining-upward cycles formed in the phase of flow dispersion. Therefore con- glomerates of the aforementioned unit could belong to the main trench, formed in the first phase of fan develo- pment (“entrenchment”, after DeCELLES et al., 1991). 6. MEGACYCLES AND TECTONICS The measured succession can be divided into two fining-upward megacycles (Fig. 4). The beginning of the megacycle is marked by facies of normally graded breccia (Bcn - Fig. 4, PG-I section, basal and top part). The fining-upward trend in the first megacycle (Fig. 4, PG-I section except the top) is expressed by a general decrease in the fragment size in coarse-grained deposits and the occurrence of thick calcretes near the top of the cycle (Fig. 4, PG-I section, 96.5-98 m), followed by gradual decreasing of the conglomerate unit thickness. In the second megacycle this trend is indicated by more common occurrences and thickening of facies unit F2 in respect to facies Gc and Ge towards the upper part of the succession (Fig. 4, top of the PG-I section and PG- II section). Fining-upward megacycles indicate the gradual migration of the source area away from the environ- ment of deposition. This succession could be attributed to the backstepping of the fault scarp towards the source mountain massif (cf. HEWARD, 1978), or grad- ual lowering of the relief by normal faulting along the active margin of the extensional basin (cf. MIALL, 1978b). The initial deposition of rock-fall breccias in the base of megacycles indicate the very high intensity of tectonic activity, and trapping of coarse-grained clas - tics near the steep slopes in the proximity of the source area (sensu BLAIR, 1987; BLAIR & BILODEAU, 1988; HELLER & PAOLA, 1992). A relative decrease in tectonic activity is shown by the development of flows in the mountain massif, and the formation of the alluvial fan at the margin of the basin (sensu BLAIR, 1987; BLAIR & BILODEAU, 1988; HELLER & PAO- LA, 1992). These processes are in congruence with the change in sediment composition recorded in the PG-II section with respect to the PG-I section. Namely, the main characteristics of the sediment composition sug- gest that the source area of deposits observed in the PG- I section was predominantly composed of metamorphic rocks, while deposits found in the PG-II section mostly originated by erosion of sedimentary and volcanic rocks. According to the available data (©PARICA et al., 1980; HALAMI∆ et al., 1990; HALAMI∆ et al., 1993; PAMI∆ et al., 1990; BELAK et al., 1998), the base- ment of Poæeπka Mt. below the Miocene deposits is composed of sedimentary and volcanic rocks of Late Cretaceous and Palaeogene age. Therefore, it may be concluded that a source area for deposits of the PG-I section, according to the reconstructed palaeotransport directions, was probably located south of the present Poæeπka Mt. The change in sediment composition in the PG-II section indicates a shift of the source area, which may have been caused by tectonic events along the Fig. 6 A model of the Poæeπka Mt. braided alluvial fan in the phase of dispersed flow. Only facies of coarse-grained deposits are presented: Gmu and Gmi facies are locally initiated debris-flows, Bcn facies represents rock-fall breccias, and Gc facies indicates channels and longitudinal bars. 73PaveliÊ & KovaËiÊ: Lower Miocene Alluvial Deposits of the Poæeπka Mt. ... basin margin (cf. LAWTON, 1986). Normal faulting in the period of enhanced tectonic activity, resulted in gradual subsidence of the block which represented the source area for formation of deposits recorded in the PG-I section (Fig. 7). In this way the other block, com- posed of different rocks, became the highest. In the period of weaker tectonic activity between two megacy- cles a newly exposed block was eroded, which resulted in rapid progradation. Reactivation of tectonics resulted in intensified normal faulting and the successive lower- ing of the relief, and therefore sediments deposited in the fan had the composition reflecting a new eroded block (Fig. 7). The boundary between these megacycles could be compared to a boundary surface of the 5th order, and represents a consequence of allocyclic processes (sensu MIALL, 1996). Composition of the alluvial deposits of the Poæeπka Mt. and reconstructed palaeotransport directions could indicate the now uplift- ed core of the Motajica Mt. as the source area (Fig. 1), which is composed of granites, metamorphic and clastic rocks (PAMI∆ & PROHI∆, 1987). If the geological position of the alluvial deposits of Poæeπka Mt. is related to the defined zone of the Sava fault, which is characterised as a dextral strike-slip fault (HORVÁTH & ROYDEN, 1981; JAMI»I∆, 1988, 1995; ROYDEN, 1988; RUMPLER & HORVÁTH, 1988; BERGERAT, 1989; HORVÁTH, 1993 - Fig. 3), development of the fan would primarily depend on its lateral shift along the fault, as well as the systematic progradation of the fan itself (STEEL, 1988). In the example presented by STEEL (1988), contemporaneous lateral shift and progradation of the fan will result in the likely possibility of the formation of predominantly coarsening-upward or coarsening-fining-upward cycles and megacycles. In the case of the Poæeπka Mt. alluvial deposits, these types of cycles and megacycles are not developed - deposits are composed of fining-upward cycles and megacycles (Fig. 4). Formation of the fin- ing-upward megacycles is interpreted as the result of the normal faulting along the active margin of the sub- siding basin (Fig. 7), indicating that the Sava fault oper- ated as a normal fault in the Early Miocene. This type of faulting in extensional geotectonic units is character- istic for the rift-type basins (BLAIR & BILODEAU, 1988; MIALL, 1990; FROSTICK & STEEL, 1993; LEEDER, 1995). 7. CONCLUSION 1) In the Early Miocene (?Ottnangian), a braided allu- vial fan developed in the area of Poæeπka Mt., as represented by deposits of main trench, braided channels, longitudinal bars and the flood-plain. The fan evolved in a semi-arid climate, and flows were oriented generally towards the north. 2) Autocyclic processes, such as the lateral migration of flows due to the vertical aggradation of longitudi- nal gravely bars or a shift of the main trench of the fan, produced small-scale fining-upward cycles. 3) The entire measured succession of deposits is com- posed of two fining-upward megacycles, which are the consequence of allocyclic control, i.e. the pul- satory character of the synsedimentary tectonics. Megacycles were formed by backstepping of the fault scarp towards the source mountain massif, caused by normal faulting along the active margin of the extensional basin. 4) The determined depositional style suggests that the Sava fault operated as a normal fault at the begin- ning of its formation during the Early Miocene (?Ottnangian). Acknowledgements This paper represents a part of the PhD Thesis of Davor PaveliÊ, which was undertaken under the men- torship of Professor Joæica ZUPANI» (Zagreb), to which the authors are very grateful for very useful com- ments and suggestions. We would like to thank the reviewers, Professor Josip T I © L J A R (Zagreb) and Pro- fessor Orsolya S Z T A N O (Budapest) for their careful revision of the manuscript. These investigations represent a part of the project: Geological Map of the Republic of Croatia, Scale 1:50.000, financed by the Ministry of Science and Technology of the Republic of Croatia. 8. 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