Facies (2004) 50:19–33DOI 10.1007/s10347-003-0001-6

O R I G I N A L A R T I C L E

Marijan Kov�cic · Jozica Zupanic · Ljubomir Babic ·Davor Vrsaljko · Mirjana Miknic · Koraljka Bakrac ·Ivan Hecimovic · Radovan Avanic · Mato Brkic

Lacustrine basin to delta evolution in the Zagorje Basin,a Pannonian sub-basin (Late Miocene: Pontian, NW Croatia)

Received: 10 February 2003 / Accepted: 26 November 2003 / Published online: 26 February 2004� Springer-Verlag 2004

Abstract Pontian deposits of the Zagorje Basin consti-tute a coarsening-upward succession, which reflects theinfilling style of this Pannonian sub-basin, i.e. theprogradation of clastic systems into the brackish lake.Six facies differentiated correspond to lake floor, chan-nelled slope, distal and proximal pro-delta, distal andproximal mouth bars, and lagoon/bay to swamp andalluvial environments. The deposition in the mouth-bararea was dominated by frictional forces. The upper-stageplane-bed sands are the main mouth-bar facies, which isuncommon in the geological record. The main factors forthe origin of such bars include an abundant sand supplyby the high-energy fluvial system(s), low-energy of thereceiving basin and shallow depositional depth. Pro-longed, catastrophic floods generated sustained hyperpy-cnal flows, which bypassed the mouth-bar area and fedthe slope/pro-delta. The high ratio between sedimentsupply and subsidence rate resulted in a fast moving‘progradational wave’, which involved the entire SWPannonian Basin, including the Zagorje Basin.

Keywords Lacustrine delta · Friction-dominatedeffluent · Mouth bars · Pannonian Basin · Zagorje Basin

Introduction

The Pannonian Basin comprises a number of sub-basinsor depressions (e.g. Royden 1988; Fig. 1a). They evolvedin response to several compressive, extensive and rota-tional events accompanied by important movements alongnumerous strike-slip faults (Royden et al. 1982; Kov�cet al. 1993; Peresson and Decker 1996; Csontos andNagymarosy 1998; Kov�c and Marton 1998; Lucic et al.2001; Pavelic 2001; Tomljenovic and Csontos 2001). Thisevolution has been related to the convergence processesbetween the European plate and the Adriatic plate in theTertiary (Decker and Peresson 1996; Kov�c and Marton1998). At the end of the Middle Miocene, the PannonianBasin was isolated from other parts of the formerlymarine Paratethys, which resulted in the formation of thebrackish Pannonian Lake (R�gl and Steininger 1983;R�gl 1998). During the Late Miocene, the PannonianLake experienced a large-scale reduction in size andrelevant widening of terrestrial areas (R�gl and Steininger1983; Magyar et al. 1999a). Subsurface data from severalparts of the Pannonian Basin have been used to recon-struct deep basin, pro-delta, delta front, delta plain andalluvial facies of the Late Miocene–Pliocene, whichreflect the infilling and final disappearance of the Pan-nonian Lake (Berczi and Phillips 1985; Pog�cs�s et al.1988; Mattick et al. 1988; Juh�sz 1991; Juh�sz andMagyar 1992; Vakarcs et al. 1994; Ivkovic et al. 2000).

This work deals with the sedimentary evolution of oneof the Pannonian sub-basins, the Zagorje Basin, duringthe Pontian Age of the Late Miocene (Figs. 1, 2 and 3).The Late Miocene evolution of the SW Pannonian Basin,to the south of the Drava River, including the ZagorjeBasin, is only partly known. A general trend from 'deep,brackish basin to freshwater, terrestrial settings by infill-ing of the lake basin has already been described fromdifferent parts of SW Pannonian Basin (Jenko 1944;Ozegovic 1944; Kranjec et al. 1973; Kranjec andBlaskovic 1976; Simunic et al. 1981). However, thechar-acter and dynamics of the depositional systemsresponsible for the construction of the basin fill during the

M. Kov�cic ()) · D. Vrsaljko · M. Miknic · K. Bakrac ·I. Hecimovic · R. Avanic · M. BrkicInstitute of Geology, Sachsova 2, 10.000 Zagreb, Croatiae-mail: makovacic@igi.hrTel.: +385-1-6160762Fax: +385-1-6144718

J. ZupanicInstitute of Mineralogy and Petrography, Faculty of Science,University of Zagreb, Horvatovac bb, 10.000 Zagreb, Croatia

L. BabicInstitute of Geology and Paleontology, Faculty of Science,University of Zagreb, Kralja Zvonimira 8, 10.000 Zagreb, Croatia

Late Miocene are only partially known. In the westernpart of the Sava Basin subsurface, Ivkovic (1998) andIvkovic et al. (2000) identified a Late Miocene basin todelta plain progradation, whereas Vrbanac (1996) de-scribed depositional dynamics of deep-water marls andturbidites during the Late Pannonian. Additionally, anoutcrop section in the Zagorje Basin has been interpretedto reflect the progradation of a Late Pontian delta (Baschet al. 1995).

This work is based on the outcrop study, in contrast tomost studies dealing with the Late Miocene basin

Fig. 1 a Tectonic sketch of the Pannonian Basin and its surround-ings, with location of Zagorje Basin (asterisk). Largest and deepestsub-basins are shown: W Vienna; Da Danube; G Graz; Z Zala; DrDrava; S Sava; Tc Transcarpathian; Ts Transylvanian; GHP GreatHungarian Plain. Other abbreviations: P.A.L. Peri-Adriatic Line;M.M. Hungarian Mid Mountains. After Royden (1988). Framed isthe area shown in b. b Zagorje Basin shows smaller thicknessescompared to neighbouring Sava, Drava and Zala basins, andappears to comprise two compartments. After Horvath (1988).Framed is the area shown in Fig. 2a

Fig. 2 Geological maps of the main part of Zagorje Basin includingthe study area. Pontian sediments are shadowed. a Simplified afterAnicic and Jurisa (1984), Basch (1981), Sikic et al. (1977) andSimunic et al. (1982). Framed is the area shown in b. b Simplifiedafter Basch (1995). 1–7 Locations of measured sections

Fig. 3 The position of the Pontian in the stratigraphic classificationof the Central Paratethys and a tentative correlation with Mediter-ranean stratigraphic units. After R�gl (1996) and Steininger (1999)

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evolution of the Pannonian areas, which are based onsubsurface data. We provide detailed description of sixlacustrine to deltaic facies, interpret depositional process-es, and propose the explanation of the dynamics andinterrelationship of basinal lacustrine to deltaic deposi-tional systems. It is shown that the sediments studiedrecorded the late-stage evolution of a Pannonian lacus-trine sub-basin located in the SW Pannonian realm. Ourapproach also utilises palaeoecological implications ofmolluscs, ostracodes, and palynomorphs occurring in thestudied sediments. Specific features of lacustrine todeltaic to fluvial systems of the Zagorje Basin describedhere are also discussed in terms of general variability ofsuch settings concerning the processes and controllingfactors. Outcrop-based studies of the Pannonian Basin fillcan provide the insight into several aspects of sedimentsnot available from the subsurface data. Hence, theoutcrop-related methods can help improve the under-standing of subsurface data concerning the type, organi-sation, and evolution of depositional systems, which arerelevant for the reservoir characterisation and prediction.

Outline geology of the study area

The study area is located in the Zagorje Basin, whichcomprises two main compartments, where Neogene toQuaternary sediments may attain a thickness of more than2 km (Fig. 1b). The Pontian sediments described here areunderlain by the sedimentary succession reflecting thechange from marine to brackish water conditions (Fig. 4).The uppermost purely marine strata are Badenian deep-water to coastal sediments, the overlying Sarmatian hasbeen considered to reflect conditions of the marine tobrackish water transition, and subsequent Pannonian andPontian sediments reflect the evolution of brackishlacustrine environments (Kranjec et al. 1973; Sikic et al.1979; Simunic et al. 1981; Simunic and Simunic 1987).Until the Pannonian, marginal basin areas may show gapsin sedimentary successions due to tectonic activity andwater-level changes (Fig. 4).

The base of the Pontian in the study area is defined byendemic mollusc associations (Kranjec et al.1973), fol-lowing the propositions by Jenko (1944) and Ozegovic(1944) for other parts of the SW Pannonian Basin. Hence,an accurate correlation with Mediterranean stage bound-aries is not yet possible. The subdivision of Pontiansuccessions into Abichi and Rhomboidea units tradition-ally used in the SW Pannonian Basin is also based onendemic mollusc assemblages (Jenko 1944; Ozegovic1944). As these faunas migrated across large areas of thePannonian Basin in response to the migration of relevanthost environments (Korp�s-H�di et al. 1992; Juh�sz andMagyar 1992; Magyar 1995), the boundary betweenAbichi and Rhomboidea units is time-trangressive. TheAbichi Unit is characterised by lacustrine marls contain-ing more or less sand intercalations, while the Rhom-boidea Unit consists of sands, silts, and subordinategravels, clays and coal seams indicating an intense fluvial

supply to lake and marsh environments (Kranjec et al.1973; Simunic et al. 1981; Basch et al. 1995). The top ofthe Pontian is lacking in the study area.

Pontian sediments, as well as older units, are uncon-formably overlain by Late Pliocene(?)–Pleistocene allu-vial to lacustrine gravels, sands and clays. They bearwitness to a post-Pontian tectonic deformation, erosionalincision of Pontian strata, and erosion of neighbouring,uplifted terrains (Simunic and Simunic 1987). Othersediments in the Zagorje Basin are Pleistocene loess-typedeposits and Holocene alluvium.

The main tectonic features of the Zagorje Basin areW–E-striking folds (Simunic et al. 1981; Simunic andSimunic 1987). The outcrops studied are parts of thesouthern limb of the Konjscina Syncline, the largesttectonic feature of the Zagorje Basin.

Facies description and interpretation

The Pontian deposits are well exposed only in isolatedoutcrops where segments of vertical successions havebeen studied and logged. The position of these segmentswithin overall Pontian stratigraphy (Fig. 5) has beenreconstructed based on the data derived from the fieldmapping and logging, and on the occurrence of specificmollusc assemblages. The combinations of specificlithologies, sedimentary features and fossils, are used todifferentiate six facies (F1–F6), which are describedbelow. Facies F1 and F2 belong to the lower, Abichi Unit,whereas facies F3–F6 belong to the upper, Rhomboidea

Fig. 4 Badenian to Quaternary succession of the Zagorje Basinincluding environmental interpretation and relevant stratigraphicsubdivision. Compiled from various sources, mainly Kranjec et al.(1973), Simunic et al. (1981), Simunic and Simunic (1987) and ourown observations

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Unit. The entire succession studied amounts to 220 m andshows a general coarsening-upward trend.

Composition of sands

Sands of six facies differentiated here are characterised byalmost identical composition. The particle composition isdominated by quartz and lithic fragments, while feldsparand muscovite are minor components. Heavy mineralassociations are dominated by garnet and epidote.

Facies F1: Marls

Description

Based on the data from the measured sections, as well ason deductions from the mapping data, this facies occursboth directly above well documented strata of thePannonian stage, and at different levels within the Abichiunit (Figs. 2 and 5).

Marls represent about 90% of this facies. They aremassive, commonly bioturbated and contain up to 43%CaCO3. Molluscs in lower F1 marls are more commonand diversified, and better preserved compared to upperF1 marls. Mollusc fauna from lower marls consists ofDidacna (Pontalmyra) otiophora (Brusina), Dreisseno-mya (Carinatocongeria) digitifera (Andrusov), Paradac-na abichi (R. H�rnes), P. lenzi (R. H�rnes) and Valen-ciennius sp. The ostracode associations are characterisedby a number of species, including Amplocypris reticulata(Zal�nyi) Krstic, Candona (Caspiolla) lobata (Zal�nyi),Candona (Pontoinella) paracuminata (Zal�nyi) Krstic,Candona (Bacunella)cf. balcanica Krstic, smooth Hemi-cytheria marginata Sokac, Candona ex gr. macrostigmaKollmann, Candona obesa (Reuss), Candona cf. salopekiSokac, and number of individuals, both adult and larval.Specific features include ‘nodose’ specimens of the genusCyprideis. The palynofacies in the lower F1 marlsconsists of numerous, brackish, organic-walled dino-flagellate cysts: Spiniferites bentori (membranous form),Spiniferites bentori ‘coniunctus’ S�t�-Szentai, Spinifer-ites virgulaeformis S�t�-Szentai, Spiniferites paradoxus(Cookson and Eisenack) Sarjeant, Spiniferites ‘balcani-cus’ S�t�-Szentai, and includes conifer pollen and otherterrestrial phytoclasts. Upper F1 marls are less bioturbat-ed, contain less carbonate and slightly more silt, andcommonly show very fine horizontal lamination (Fig. 5,lower part of logs 4 and 5). Among different molluscs,Congeria zagrabiensis (Brusina) and C. croatica (Brusi-na) appear for the first time. The ostracode associationscontain a smaller number of both species and individualsand show a lower proportion of adults in comparison tothe lower F1 marls. At the same time, the dinocystassociation is less numerous and diverse, containing moreorganic-walled cysts such as Impagidinium spongianumS�t�-Szentai (with thinner walls). This change is accom-panied by an increased amount of conifer pollen.

Rare, sharply based, fine-grained sand and silt beds,ranging in thickness from 5 to 130 cm, are intercalated inmarls. Small, shallow erosional scours are occasionallyobserved at basal sand contacts. Most common are sandsshowing either horizontal lamination with a gradualupward transition to thin sandy silt and overlaying marl,or vertical grading followed by gradual transition to marl.

Fig. 5 Logs of measured sections showing main sedimentaryfeatures and vertical arrangement of facies F1–F6. The correlationof sections is based on deductions from the mapping of isolatedoutcrops, roughly defined Pannonian/Pontian boundary and a time-transgressive character of the boundary between Abichi andRhomboidea units. For details see text. For location see Fig. 2b

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Less represented are current-ripple laminated sands fol-lowed by horizontally laminated silt.

Interpretation

The lower F1 marls containing mollusc assemblagestypical of quiet brackish-lacustrine environment andwater deeper than 50 m (Magyar 1995) suggest a quiet,uniform suspension settling of fine-grained material to thelake bottom, where burrowing organisms were active. Thefeatures of ostracode associations also reflect a quietsetting, where numerous and diverse population wasdwelling, and the carapaces of both larval and adult stagesmay have been preserved. ‘Nodose’ Cyprideis specimensare good indicators of low salinity (below 5–6) and anabundance of organic matter (Carbonel et al. 1988).Numerous and diverse, autochthonous, brackish associa-tions of dinoflagellate cysts support a distal settinginterpretation (S�t�-Szentai 1988). An increased propor-tion of silt characterising the upper F1 marls reflects theapproach of a river mouth supplying fine-grained terrige-nous detritus, which is almost absent in the lower F1marls. These silty marls may have been deposited in lessdeep water compared with the lower part, and probably ina pro-delta setting (see below). This is in accordance withthe appearance of Congeria zagrabiensis in these marls,which indicates less deep water (Magyar 1995). Animpoverished ostracode fauna compared with the lowermarls is also related to terrigenous influence by anapproaching river mouth. An increased terrestrial influ-ence may also be deduced from the decreased number anddiversity of dinocysts (Nehring 1997), the dominance ofphytoclasts over the amorphous organic matter (Tyson1987) and an increased proportion of terrestrial pollen.

Occasional intercalations of sands and minor siltsshowing a sharp to erosional base, overall grading andvertical sequences of structures are comparable to Tbde,Tae, Tde and Tcde turbiditic sequences of Bouma (1962).

They record a distal, infrequent sediment delivery to therelevant part of the lake bottom.

Facies F2: Thick sand units and associated sediments

Description

These sediments are underlain by deeper F1 marls andoverlain by shallower F1 marls. The studied successionconsists of three thick sandy units separated by alternatingsands and marls (location 3 in Figs. 2 and 5; Fig. 6).

– (a) The basal surface of the lower sand unit (a inFig. 6) contains well-developed flute casts directedtowards the azimuths between 85 and 115� (aver-age=95�). This unit is 10 m thick, consisting of fineand medium sand. Its main portion is predominantlycharacterised by crude horizontal lamination showingalternation of thin and thick laminae (Fig. 7A). Large-scale convolutions and gently inclined laminae occurin the central and upper parts. Two conspicuouserosional surfaces with relief up to 8 cm have beenobserved within the sand body. Marl and laminated siltclasts up to 30 cm long are scattered within the body.Some of them are rounded, others may be folded andsome clasts appear to line an internal erosional surface.Degraded, brown plant remains may occur at laminaesurfaces. The top surface of this sand body is irregularwith a relief up to 30 cm deep, except for the morethan 20-m-wide depression in the western portion ofthe outcrop (Fig. 6), described below.

– (b) This unit is about 1–2 m thick (b in Fig. 6). In theeastern part of the outcrop section, the unit showsseveral shear plains almost parallel to bedding. Theunit consists of gently deformed, locally disrupted thinbeds of sands, silty marls and marls. They may showeither horizontal laminae overlain by current-ripplelamination in sands followed by silt and marl, similarsequences lacking a horizontally laminated lower

Fig. 6 Sketch of the outcrop displaying a vertical section ofsediments of the facies F2, and a small portion of the underlyingfacies F1 in the lower right corner. The overlaying F1 marls are notrepresented. The orientation of the outcrop section is 65–245�,

which is approximately diagonal to the palaeo-transport directionsshown by flutes (average=95�) at the base of the sand body (a), andcurrent ripples (SE) in thin turbidites of the unit (c). Drawn fromphotographs. Locality 3 in Fig. 2b

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portion, or silt plus marl sequences. The sands maycontain marl intraclasts. Some beds pinch out east-wards, and the whole unit (b) is getting thinner in thisdirection. In the western part of the exposure, the unitoccupies a more than 20-m-wide depression showing adiffuse basal contact with highly deformed sands ofunit (a). Unit (b) is here represented by massive sandcontaining scattered, angular clasts of thin marl and siltbeds, which may be deformed (Fig. 7B).

– (c) The unit fills a depression, which is up to 2.6 mdeep and more than 25 m wide in the outcrop section(c in Fig. 6). The infill of the depression consists of analternation of thin sands and marls (Fig. 7C), whichshow sedimentary structures identical to those in theunderlying unit (b). Some sands contain marl clasts.Current-ripple lamination in sands is directed towardsthe SE.

– (d) Most of this unit is not accessible for closeobservation. The unit consists of up to 3.5-m-thick,fine-grained sand, which changes its thickness laterally(d in Fig. 6). The erosional base of the unit truncatesunits (b) and (c), and includes a scour, which is 1.2 mdeep and 5 m wide, in the western portion of theoutcrop (Fig. 6, Fig. 7D). This scour is filled withcoarse sand containing pebble- to cobble-sized intra-clasts of fine-grained sediment, as well as molluscdebris. The sediment clasts may be laminated, mayshow plastic deformation, and some of them arerounded. Other parts of the basal surface are floored byintraclasts of fine-grained sediment and such intra-clasts also appear in several strikes within the sandbody. To the east, the unit onlaps a gently inclinedbasal surface. Locally identifiable ripple-drift lamina-

tion, widespread convolute laminae, and a few largefragments of thin beds of probable sand are alsoobserved in sand, which otherwise appear to bemassive.

The units (e) and (f) are not accessible for closeinspection. Unit (e) comprises alternating thin-beddedsands and marls, which are locally sheared. The unit (f) isrepresented by sand, also showing deformation features.

Interpretation

– (a) The deposition of this unit probably started at adepth greater than 50 m, as implied by molluscs in theunderlying F1 marls described above. Basal flute castsand crude horizontal lamination, together with thepresence of marl and silt intraclasts, reflect erosion byturbulent flow and deposition under upper-stage planebed conditions. The great thickness of this sandsuggests deposition by gradual aggradation from aquasi-steady turbidity flow sustained by a prolongedriver flood-stage (Kneller and Branney 1995). Thisalso points to a high constant discharge and the dom-inance of inertial forces at the river mouth (Wright1977). As the horizontal lamination occurs throughoutthe thick sandy unit (later partly convoluted), thedeposition should have occurred at a comparativelylow rate of sediment fallout (Arnott and Hand 1989).The vertical alternation of thick and thin laminae mayindicate temporal velocity variations of a long-lastingflow. Internal erosional surfaces indicate larger scaleflow fluctuation or closely succeeding flows. The sizeand pattern of deformed (convoluted) lamination wereprobably due to a combined effect of liquefaction andsliding. The emplacement of this unit was probablyperformed at the base of slope, or possibly on thelowermost slope, as suggested by both the sedimentaryfeatures and the position above basin floor F1 marls.

– (b) The succession of alternating marls and thin sands(eastern portion of the exposure, Fig. 6) reflects thealternation of fine-grained background deposition anddeposition from low-density turbidity currents. Theywere smaller and short-lived in comparison to thelarge, sustained flows discussed above. Gentle defor-mation, disrupted beds and shear planes in the easternpart, and laterally occurring massive sand containingintraclasts of thin beds of the unit exposed in thewestern part, reflect sliding and sand flow, whichinvolved both a part of the thin-bedded succession anda part of the underlying sand unit (a).

– (c) Internal structures in sands, the alternation ofgraded sands and marls, as well as the verticalassociation of these sediments with other gravity-flowdeposits suggest an alternation of turbidity-currentdeposition and slow, fine-grained lacustrine deposi-tion. As the lateral wall of the depression filled by (c)sediments lacks erosional truncation, it could representa part of a slide scar. The depression was probably

Fig. 7 Specific outcrop features of lacustrine to deltaic facies in theZagorje Basin. A Horizontally laminated sand in the lower unit (a)of the facies F2. Locality 3 in Fig. 2b. The precise location of thephotograph in the outcrop is shown in Fig. 6. Coin is 2 cm indiameter. B Detail of the outcrop of the facies F2 shown in Fig. 6.Locality 3 in Fig. 2b. Partly deformed sand of the unit (a) in thelower right is overlain by unit (b) consisting of thin-beddedturbidites including hemipelagic marls (upper right). In the lowerleft, the massive sand contains disrupted beds of silty marl and thin-bedded turbidites. The truncation surface (arrows) representing aslump scar (or erosional surface) is overlain by alternating thin-bedded turbidites and marls of unit (c). For details see the text. Hoefor scale is 1 m long. C Detail of the outcrop of the facies F2 shownin Fig. 6. Locality 3 in Fig. 2b. Alternation of sands (dark) andmarls (light) of the unit (c). Internal structures represent BoumaTbcde sequences including hemipelagic marl. Current ripplesmigrated towards the SE. Lens cap is 5 cm in diameter. D Detailof the outcrop of the facies F2 shown in Fig. 6. Locality 3 inFig. 2b. Erosionally truncated alternation of sands and thin marls ofthe unit (c) in the lower part, and overlaying basal portion of theunit (d) showing intraclasts in sand matrix. Hammer for scale is31 cm long. E Thin-bedded turbidites of the facies F3. Locality 5 inFigs. 2b and 5. Individual beds consist of horizontally laminatedsand grading upwards into laminated silt and marl. The beds show athickening-upwards trend. Lens cap is 5 cm in diameter. F Scour-and-fill structure in laminated sands of the facies F5. Locality LepaVes (Fig. 2b). Lens cap is 5 cm in diameter. G Horizontallylaminated fine-grained sand with combined current-wave ripples ontop of bed, facies F5. Scale in mm. Locality 7 in Figs. 2b and 5

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acting as a channel for basinward sediment transport,and was subsequently filled by thin-bedded turbidites.The SE direction of ripple migration is similar to thedirection of flutes in the unit (a).

– (d) The erosional base, intraclasts indicating erosion inthe upslope area, and locally observed laminationprobably reflect an origin of the unit similar to that ofunit (a), i.e. the supply and deposition of sand broughtin by sustained underflow(s) related to prolonged riverfloods. The ripple-drift lamination should have orig-inated during slower flow intervals related to thefluctuation of flood intensity. As for unit (a), a deltaslope setting is also envisaged for unit (d).

The origin of units (e) and (f) is probably similar tocomparable units occurring in the lower part of theexposure. Hence, thin-bedded unit (e) probably originatedby alternation of fine-grained deposition and depositionby low-density turbidity currents, and was involved insubsequent sliding. The origin of the sand unit (f) isprobably related to gravity flow(s) and subsequent sliding.

The underlying basin-bottom marls and overlying F1silty marls inferred to represent slope/pro-delta deposits,together with features discussed above, are consistentwith base-of-slope to slope interpretation for the entire a–fsuccession.

The overall depositional setting of facies F2

The F2 sediments reflect a succession of various pro-cesses. The origin of part of the sediments is related tofast, sustained, hyperpycnal flows, which scoured thebottom upslope. The features of sediments also indicatethe deformation and sliding of different sediment types,liquefaction and flows of remobilised sand, as well asclosely related scouring and channelling. These processeswere closely succeeding, thus indicating an unstable slopesetting connected to the mouth of a river, very rich insand. These processes and the related morphology re-semble chute-and-ridge morphology and depositionaldynamics described from recent submarine delta slopes(Prior et al. 1981; Kostaschuk and McCann 1987; Priorand Bornhold 1989, 1990). The packages of alternatingthin turbidites and fine-grained deposits may have beenlaid down either on interchute areas as the overbanksediment or within chutes and slide scars. They have alsobeen involved in sliding. The slope was facing E–SEbased on directions of flute casts and current ripples.Sliding and deformation/liquefaction events were proba-bly induced by seismic shocks. Seismicity is otherwisededuced from intense tectonic activity during the Pontian(Kranjec and Prelogovic 1974; Velic 1985).

Facies F3: Alternating sands and marls

Description

These sediments develop above upper F1 marls, andunderlay F4 silts described below (Fig. 5). F3 consists ofalternating marls, silty marls, silts and fine-grained sands.The thickness of sands varies from lamina-thick units to100 cm. They are sharply based beds commonly showingerosional basal contact. Internal structures include hori-zontal laminae (Fig. 7E), horizontal laminae plus current-ripple laminae, or current-ripple laminae only, which maybe represented by isolated current ripples. Sands gradeupwards into thin laminated silt, which in turn isgradually overlain by silty marl and marl. Rare sand bedsshow a massive lower part, graded middle portion and aneven-laminated silty-sandy top. Some thicker sand bedsshow liquefaction features.

The marls are massive or finely laminated, maycontain mollusc species Congeria zagrabiensis (Brusina)and C. croatica (Brusina), as well as debris of ostracodesand molluscs.

The alternating lithologies described above formupwards-thickening cycles (Fig. 7E), which in their turnshow an overall thickening-upwards trend (Fig. 5).

Interpretation

Mollusc fauna in marls indicates a low-energy environ-ment and a depth between some 5 and 50 m (Magyar1995). This depth estimate is consistent with the gradualdevelopment of this facies from underlying F1 silty marlsinferred to represent pro-delta deposits. The combinationof a sharp to erosional base, overall grading and specificsequences of structures, as well as the alternation of thesesands and silts with marls, suggest that F3 represents analternation of Bouma turbidites Tbcde, Tcde, Tbde, Tdeand lacustrine ‘hemipelagites’, all of them deposited in aproximal pro-delta environment.

Individual upward-thickening cycles may reflectprogradation of specific channel and mouth complexes,while superimposed cycles may have been produced bylateral shifting of outlets (see below). The overallthickening in F3 probably represents a segment ofprogradation of the clastic system.

Facies F4: Silts

Description

Facies F4 overlays facies F3 and alternates with facies F5described below (Fig. 5). F4 consists of 2- to 40-m-thick,strongly bioturbated, massive silt to sandy silt, locallycomprising relics of horizontal lamination and intercala-tions of a few to 60-cm-thick graded beds. Some gradedbeds start with coarse silt grading upwards into finer siltand bioturbated/massive silt. Other graded beds addition-

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ally have a thin sandy base showing horizontal and/orripple-cross lamination. The silts commonly containbrackish molluscs Congeria rhomboidea rhomboideaH�rnes, C. zagrabiensis (Brusina), C. markovici Brusina,Lymnocardium (Arpadicardium) mayeri H�rnes, L. (Za-grabicardium) riegeli H�rnes, Dreissena auricularis(Fuchs), palynomorphs such as Galeacysta etrusca Cor-radini and Biffi, ‘Pontiadinium inequicornutum’ (Baltes)Stover and Evitt, Spiniferites ‘balcanicus’ (Baltes) S�t�-Szentai, Impagidinium spongianum S�t�-Szentai (thickerwall) and Chytroeisphaeridia tuberosa S�t�-Szentai, aswell as ostracodes.

Interpretation

Graded silt intercalations in bioturbated silts, as well assand-based graded silt layers, are comparable to turbiditicbeds Tde, Tcde, Tbcde and Tbde of Bouma (1962).Strongly bioturbated silts, in the presence of relic laminaeand graded and laminated intercalations, are considered tohave resulted from organic reworking of the primary,graded and laminated structures mentioned above. Finegrain-size and strong bioturbation suggest certain distanceto active channel mouths.

The mollusc association indicates a lacustrine envi-ronment, approximately 5 to 50 m deep (Magyar 1995).The dinocyst assemblage originated from the mixing ofthick-walled forms such as Impagidinium spongianumS�t�-Szentai, and Chytroeisphaeridia tuberosa S�t�-Szentai deriving from proximal settings (Tyson 1987;S�t�-Szentai 1988), and membranous forms such asSpiniferites ‘balcanicus’ (Baltes) S�t�-Szentai, derivingfrom relatively distal environments (S�t�-Szentai 1988).Hence, the dinocysts also indicate a certain distance fromriver mouths.

F4 is inferred to be the result of predominant depo-sition from underflows. However, part of the silts mayalso have been deposited from interflows or overflows.All these flows should have been related to high riverstages or floods. All these data reflect a depositionalsetting in front of a river mouth. The superposition of thisfacies above F3, inferred to reflect a proximal pro-deltasetting, and its alternation with F5 sands, inferred torepresent proximal mouth-bars (see below), are consid-ered to indicate a distal mouth-bar environment for F4.

Facies F5: Laminated sands

Description

F5 alternates with F4 and is overlain by F6 (Fig. 5). Thisfacies appears in 1- to 17-m-thick units. The sands arepredominantly fine-grained, and may be either horizon-tally laminated, cross -bedded, current-ripple laminatedor, rarely, current-wave ripple laminated. The last twostructural types are commonly found above and betweenhorizontally laminated sand units. In the upper part of the

Fig. 8 Logs A and B show typical features of facies F5. Locality 7in Figs. 2b and 5. For details see the text

27

Rhomboidea Unit, there are also medium-grained sandsand even small-pebble gravels (e.g. log 7 in Fig. 5;Fig. 8). Mollusc fauna is rare and includes Dreissenaauricularis (Fuchs), Dreissenomya (Dreissenomya) inter-media Fuchs, Lymnocardium (Hungarocardium) inflatuminflatum (Gorjanovic-Kramberger) and Congeria bala-tonica Partsch.

Horizontally laminated sand beds are mostly 30 to100 cm thick. When stacking one above the other oralternating with cross-bedded sand units, they are bound-ed by erosional surfaces (Fig. 8), which are occasionallymarked by small chert pebbles, mollusc debris and mudintraclasts. These surfaces may be horizontal or inclined.The later can be traced for more than several metres, andthe observed depth of erosion amounts to 30 cm. Biotur-bation is of low intensity or absent.

Poor exposure conditions did not permit an accurateobservation of structures in cross-bedded sands. Theyconsist of gently and steeply inclined laminae sets withlocally observed trough shapes. The sets may be stackedone above the other or alternate with horizontally lam-inated sands. In both cases they are based by erosionalsurfaces, which may be very gently inclined or up to 30�steep, and up to 55 cm deep (Fig. 7F). In places, they arefloored by small chert pebbles. Bioturbation is absent.The orientation of several trough axes is approximatelyE–W and NW–SE.

Ripple-laminated sands occur in up to 40-cm-thickunits associated with horizontally laminated sands. Theyare commonly bioturbated so intensively that only relicsof primary structures have been left. They mostly orig-inated by the migration of current-ripples towards SE toSW. Climbing ripples of type A and sinusoidal ripplelamination (Jopling and Walker 1968) have been locallyobserved. Horizontally laminated sand beds may becapped by symmetrical to slightly asymmetrical ripples(Fig. 7G), with their crests oriented W–E to NW–SE.They show features of current and wave action and arecomparable to longitudinal, combined current-wave rip-ples (Reineck and Singh 1980). One example of single-setcross -lamination directed towards 290� has been ob-served covering horizontally laminated sand (localityLepa Ves in Fig. 2b).

Interpretation

Erosional scouring at the base of horizontally laminatedsands reflects the high power of flows. The intraclastscovering some erosional surfaces point to erosion inupcurrent areas. The subsequent deposition of horizon-tally laminated sand has been performed during upper-stage plane bed regime, by sand-rich flows.

Scouring and channelling at the base of cross-beddedsand units reflect erosion by strong flows and depositionin erosional depressions. Part of the cross-beds could havebeen deposited at the transition to upper flow regimeconditions, based on the combination of scoured base,gentle inclination, and irregular trough shapes, which

resemble the sandstone facies mostly filling erosionaldepressions, described by Cowan (1991) from a high-energy, sandy braided system. Another part of the cross-beds may have originated from migrating 3-D dunes.

Most ripple-laminated sands covering horizontallylaminated sands have been deposited under lower flowregime, partly by waned currents of prolonged river floodsand partly during lower discharges. A part of ripple-laminated sands should have originated from reworkingprocesses involving horizontally laminated sands. Theyinclude waves, weak currents directed WNW (290�), i.e.opposite to the direction of flows in F2, and probably alsoweak SW-directed currents.

Intense bioturbation of ripple-laminated sands corre-sponds to quiet periods of a low supply of sand or the lackof it.

As no signs of subaerial exposure have been found, theprocesses mentioned above are considered to have mostlyoperated subaqueously, less than 5 m deep as indicated bythe type of mollusc fauna (Magyar 1995). Fast, sand-richflows responsible for the deposition of comparativelythick, horizontally laminated sand units must have invad-ed the shallow lake during prolonged river floods. Thesand was brought in by effluents dominated by frictionalforces at distributary outlets, that caused lateral spreadingand nearby deposition. The deposition of much sand onmouth-bars related to prolonged floods and frictionalforces is otherwise well understood (e.g. Wright andColeman 1974; Wright 1977). However, the mouth barsconsisting of the upper-stage plane bed regime sands asthe main component represent a peculiar feature, which isdiscussed below (see the section The Zagorje Basinduring the Pontian). Widespread, commonly stackedhorizontally laminated sand units reflect closely succeed-ing floods, as well as a high sand supply. These featuresimply humid climate conditions. Scours and channels,later mostly filled with cross-bedded sands, have likelybeen situated between and above bars. The channelsthemselves may have been the conduits for flows,depositing bars growing adjacently basinwards. On theother hand, they may have served as conduits for theflows, bringing sediment directly to the basin, includingthe base-of-slope and slope/pro-delta settings (F2). Somechannels possibly represented distributary channels in-cised in mouth bars.

Depositional records of periods of low sand supply byslower flows and periods of reworking by physicalprocesses and strong bioturbation could have easily beenremoved by subsequent eroding flows, and only thickerunits recording quieter conditions may have been pre-served.

Facies F6: Clastics with coal

Description

This facies is very poorly exposed. It overlies F5 andconsists of alternating fine-grained sediments, sands,

28

gravels and coal (Figs. 5 and 8). Silts are dominantlymassive, bioturbated, and may contain carbonised plantremains and molluscs, which may be densely packed.Plant material, including leaves and stems, may becommon. The molluscs are brackish in character andinclude Congeria balatonica Partsch, C. triangularisPartsch, and Limnocardium (Pannonicardium) penslii(Fuchs). Silts may alternate with thin, horizontally lam-inated sands. A 50-cm-thick cross-stratified gravel hasbeen observed, representing the infill of a small channelcut in silt (Fig. 8). Coarse sands occur locally. The coal,appearing in a few-centimetres-thick layers associatedwith centimetre-thick clayey sand, has rarely been ob-served.

Besides data from the exposures described above,several lignite coal seams up to 12 m thick, associatedwith sands and gravels, have been reported from smallcoal mines (Kranjec et al. 1973; Simunic et al. 1981),which are now almost completely hidden, covered by soil.Fresh-water molluscs such as Planorbis, Helix and Uniohave also been reported from the coal and associated fine-grained sediments (Kranjec et al. 1973). Based on datafrom the coal mine areas (Kranjec et al. 1973; Simunicand Simunic 1987) and our recent mapping, the thicknessof F6 could measure 100 to 200 m.

Interpretation

The association of strong bioturbation, fine-grained sed-iments and mollusc coquina reflect a rather quiet envi-ronment and a reduced depositional rate. The brackishmollusc association would indicate well aerated, agitatedwater and a sandy substratum, about 0 to 5 m deep, basedon studies in the eastern part of the Pannonian Basin(Magyar 1995). However, as these bivalves have beenobserved in silts and silty marls, they are believed to haveinhabited silty bottoms, at least in this part of the basin. Ingeneral, the observed F6 silts probably originated in avery shallow brackish bay to brackish wetland. The sandbeds observed to alternate with the silts discussed abovemay have resulted from flood flows, which were enteringthe shallow lagoon or bay from time to time, interruptingthe slow deposition and bioturbation of fine-grainedsediments. A small, gravel-filled channel recorded one ofthe rare events of coarse sediment supply.

Coal seams and associated fine-grained sedimentscontaining fresh-water molluscs reported by previousresearchers have been deposited in fresh-water marshesand ponds in an upper delta plain or alluvial plain(Kranjec et al. 1973). In the presence of fresh-watersettings, associated thick sands reported by Kranjec et al.(1973) would have been related to fluvial activity in analluvial plain.

The Zagorje Basin during the Pontian:evolution and comparisons

The overall Pontian vertical succession of the ZagorjeBasin shows a coarsening-upward trend ranging frommarl-dominated to sand-dominated deposits. The relevantarrangement of facies F1 to F6, ranging from basin floor,trough slope/pro-delta, mouth bar and delta plain deposits,to sandy alluvium has been produced by the prograda-tional migration of laterally connected depositional envi-ronments (Fig. 9).

The lower part of the studied sediments (lower F1marls) originated on a brackish lake bottom, more than50 m deep. The main sedimentary products were uniformmarls, while the supply of detritus by turbidity currentswas minor and infrequent. These conditions closelyresemble those occurring during the previous, PannonianAge, when widespread, brackish-lacustrine environmentswere dominated by the deposition of marls (Kranjec et al.1973; Simunic et al. 1981; Simunic and Simunic 1987).

Subsequently, the F1 marls became more silty (upperF1 marls), reflecting the shallowing and progradation ofclastics, with resulting distal pro-delta conditions. In somebasin areas, the deposition of basin floor marls wassucceeded by alternating deposition from sustained tur-bidity currents generated by catastrophic, prolonged floodflows of rivers rich in sand, channelling, erosionalscouring, sliding, sand flowage, as well as depositionfrom low-magnitude, short-lived turbidity currents (F2).These processes occurred partly at slope base and mostlyin a slope/pro-delta marked by chutes/channels and inter-channel areas. The chutes/channels were conduits forthose flows that by-passed the slope and were delivering

Fig. 9 Lateral distribution of Pontian depositional environmentsand facies F1–F6 based on the study of the Zagorje Basin(idealised, not to scale). The facies prograded towards E–SE andhave been emplaced one above the other. See also Fig. 5. Fordetails see the text

29

sediments to the deeper lake, where alternating turbiditesand marls were deposited (F1 pro parte).

The Zagorje Basin possibly contains somewhat moresediments of the facies F2, known at present at only onelocality (see also Fig. 9), and also other sand accumula-tions in pro-delta and/or basin-floor areas produced eitherby sustained hyperpycnal flows or short-lived turbiditycurrents. Otherwise, sustained hyperpycnal flows gener-ated by river floods may deposit sandy lobes on lacustrinepro-deltas (Gustavson et al. 1975; Martinsen 1990) andsandy lobes and fans on lake basin floor (e.g. Buatois andMangano 1995, with references). Namely, the low-salin-ity lake water favours the generation of hyperpycnalflows. Additionally, Pontian pro-delta and basin-floorsandy fans have been inferred to occur in the SEPannonian Basin subsurface, which include importanthydrocarbon reservoirs (Berczi and Phillips 1985; Matticket al. 1988; Vakarcs et al. 1994). On the other hand, ourfield data suggest that similar sand accumulations are notwidespread in the Pontian of the Zagorje Basin.

Large quantities of sand and silt were laid down just infront of the Pontian river mouths of the Zagorje Basin,and formed sub-aqueous bars. Proximal mouth bars weredeposited by fast flows during prolonged river floodingdue to dominant frictional processes that involved theeffluents (F5) (Fig. 9). The bars were affected by scouringand channelling during the deposition of bars growingbasinwards. The top of proximal bars experienced minorreworking by waves and currents, hence there was aconspicuous fluvial dominance on river mouth processes,partly due to large quantities of delivered sands and partlyto the low-energy character of the lake basin. Proximalbar sands continued distally and laterally to distal bar siltsand sandy silts, which were affected by strong bioturba-tion. Short-term advancing, retreating and lateral shiftingof river channel outlets and related mouth bars resulted invertical alternation of proximal and distal mouth-bardeposits. Sands and silts here interpreted as mouth-bardeposits form a laterally continuous package across theentire Zagorje Basin, as derived from the literature andour mapping data. This reflects the uniformity of mouth-bar and delta progradational dynamics across a wide area,and suggests the presence of multiple distributaries andpossibly several fluvial feeding systems (Fig. 9), as wellas the lateral shifting of both of them. The deposition ofsands in the mouth bars due to frictional processes hasbeen explained by Wright (1977) and Wright and Cole-man (1974), and inferred from ancient deltas in numerousstudies; however, these sands predominantly originatedby migration of current-ripples and dunes. In contrast,studies reporting on horizontally laminated sands inmouth bars deposited by river-born flows are few innumber and describe such sands as a minor bar compo-nent (e.g. Orton 1988; Wood and Ethridge 1988). Incontrast, the mouth bars described here are dominated bythese sands interpreted here as upper-stage plane beddeposits. It is suggested that the main factors for theorigin and preservation of these peculiar mouth barsconsist in abundant sand supply by the high-energy fluvial

feeder system(s), low-energy character of the receivingbasin and a shallow depth of the depositional environ-ment. This specific combination appears to be rarelyencountered in mouth bar environments of the geologicalrecord, although various combinations of the main factorscontrolling the variability of the delta facies are wellknown and widely discussed (e.g. Wright 1977; Orton andReading 1993, with references).

The channels in the mouth-bar sands may have alsobeen conduits for flows generated by catastrophic floods,which continued basinwards as sustained, hyperpycnalflows bringing sand to base-of-slope and slope/pro-deltaareas, and possibly further into the basin. The incision ofthe distributary mouth-bar by a large channel duringcatastrophic flood and related deposition of sand lobes onthe pro-delta by issuing sustained hyperpycnal flows hasbeen proposed to be the dominant process in the Namuri-an Scar House delta by Martinsen (1990). In the ZagorjeBasin, however, similar flows were not common, whereasfrictional forces were dominant.

Short-lived river floods may have been responsible forthe supply of sand deposited basinwards, in front ofmouth bars, in proximal pro-delta. The resultant upwardsthickening sequences of sands and marls (F3) reflect theshifting and progradation of the channel mouths, whiletheir overall thickening trend reflects the overall deltaprogradation. Distal pro-delta silty marls were depositedeven farther basinwards (upper F1 marls mentionedabove).

Parts of the delta plain were occupied by brackishbays and vegetated interdistributary zones and wetlands(brackish F6; Fig. 9). Further landwards, there werealluvial plains characterised by various fluvial environ-ments dominated by sand deposition and also includingfresh-water marshes and ponds (fresh -water F6; Fig. 9).

Our interpretation of the mouth-bars is similar to thatprovided by Basch et al. (1995) for an outcrop section ofcoeval sediments exposed a few kilometres NW from thestudy area, in emphasising the fluvial dominance overriver-mouth processes. The relevant section mainly showshorizontally laminated sands of the mouth-bars, whichexhibit partial reworking by lacustrine processes, and anintercalation of cross-bedded sands interpreted to repres-ent a distributary channel fill. On the other hand, theseauthors assumed a meandering river to be the feedingsystem. However, very high sediment supply and depo-sitional rate, and the sandy character of the related fluvialsystem, as well as the dominance of fast flows and upper-stage plane bed conditions in sandy mouth-bar settings,better fit high-energy, braided streams.

The general evolutionary trend of the Zagorje Basinduring the Pontian, described here by clastic progradationand the infilling of a lacustrine basin (Fig. 9), is otherwisetypical for lakes with a humid climate, where muchclastic material is produced by weathering and deliveredto the lake (e.g. Talbot and Allen 1996). The highsediment supply was also related to the high relief ofsource areas and relevant high-gradient streams in upperreaches of the alluvial systems. The humid climate

30

provided much water, available for efficient sedimenttransport. These conditions must have been coupled witha decreased subsidence rate so as to enable the high rateof progradation inferred for the clastic systems in theZagorje Basin. The resultant stratigraphy of the basin fill,therefore includes almost ‘contemporary’ vertical changesin facies across the area of the present-day Zagorje Basin.

The place of the Zagorje Basinwithin the Pannonian Basin realm during the Pontian

The dominance of garnet and epidote in the heavy mineralassociations from the Pontian sands of the Zagorje Basinindicates a provenance of detrital material from theAlpine region (Simunic and Simunic 1987; Kov�cic1999). This Alpine-derived detritus, as well as relevantclastic systems were moving E–SE, based on palaeocur-rents presented above. Alpine sources have also beeninferred for Pontian sands in the Sava and Drava basins,based on heavy minerals and particle composition(Scavnicar 1979; Simunic and Simunic 1987). Theprogradational geometries of basin through deltaic toalluvial facies revealed in the subsurface of the Zalaand Drava basins show SE directions (Pog�cs�s et al.1988). Furthermore, the subsurface data from the SavaBasin have also shown a dominant SE-directed progra-dation of Pontian lacustrine to alluvial systems (Ivkovic1998; Ivkovic et al. 2000).

Taken together (Fig. 10), these data depict a generalE–SE-directed progradational ‘wave’ advancing at a highrate across the SW Pannonian Basin. The high speed ofthe progradational ‘wave’ was related to intense weath-ering in high-relief Alpine source areas, high-energystreams, much water available for sediment transport dueto a humid climate, as well as a decrease in subsidencerate in the basin areas, as was already proposed for theZagorje Basin alone. A decrease in subsidence rate hasalready been proposed to have involved the PannonianBasin in the Late Miocene as a consequence of passivecooling of the lithosphere toward thermal equilibrium,which followed the Middle Miocene (Badenian) activeprocesses in the crust and lithosphere (Sclater et al. 1980;Royden et al. 1982). The passive cooling resulted in aslower subsidence of greater extent and related flat lyingsediments, in contrast to the intense synsedimentaryfaulting, fast subsidence and rotation of bedding duringthe previous stage. The fast progradation of clasticsystems in expense of the Pannonian Lake, thereforeinvolved the entire Pannonian Basin and is amplydocumented from the subsurface of several sub-basinsand especially the Great Hungarian Plain (e.g. Pog�cs�s etal. 1988; Mattick et al. 1988; Vakarcs et al. 1994; Magyar1995; Magyar et al. 1999a, 1999b).

Summary and conclusion

The overall evolution of the Zagorje Basin, a sub-basin ofthe vast Pannonian Basin, was characterised by shallow-ing-upwards, as well as coarsening-upwards trends duringthe Pontian. These trends reflect the infilling of a weaklysaline lake setting by progradation of clastic systems. Thespectrum of the lacustrine sedimentary infill includeslake-bottom marls and turbidites, slope scouring, chan-nelling, sliding and related deposits, pro-delta muds andminor sands, distal and proximal mouth bars, and finally,brackish bay or wetland muds and silts, fluvial sands andfresh-water lacustrine to marsh muds and coal of thedelta-plain and alluvial plain settings. These settingsmigrated together towards E–SE across the basin area.The detritus derived from intense weathering of theAlpine region.

The features of the fluvial feeder system(s), such asprolonged river floods, high sediment supply and fastflows, were highly influential in shaping different faciesof the basin fill. These factors were mainly relevant to thedeposition of sands in the slope/base-of-slope area byquasi-steady turbidity flows sustained by prolonged riverfloods, as well as deposition of an important proportion ofupper-stage plane bed sands in the mouth bars, alsorelated to the river floods. This bar type is not common inthe geological record, and the main factors for the originof this peculiar facies consist in abundant sand supply bya high-energy fluvial feeder system, low-energy character

Fig. 10 Dominant directions of progradation of Pontian lacustrineclastic systems in SW Pannonian Basin: Za Zagorje Basin(presented here); S Sava Basin (Ivkovic et al. 2000); Dr and Z,Drava and Zala basins, respectively (Pog�cs�s et al. 1988). Otherabbreviations: E.A. Eastern Alps; O.D. Outer Dinarides; M.M.Hungarian Mid Mountains. Thicknesses of Neogene to Quaternarysediments after Horvath (1988)

31

of the receiving basin and the shallow depositionalenvironment. The climate was humid.

The data on the Zagorje Basin evolution together withthe data from neighbouring areas show that the SWPannonian Basin experienced the advance of a large-scale, fast moving, E–SE-directed progradational ‘wave’.

Knowledge of the Pontian systems and basin evolutionderived from outcrop studies can help to improve under-standing of the sand accumulations, which representhydrocarbon reservoirs in the neighbouring Sava andDrava basins, as well as in other parts of the PannonianBasin.

Acknowledgements This work was funded by the Ministry ofScience and Technology of Croatia through projects nos. 0181001and 0119403. The senior author is grateful to Dr Davor Pavelic foradvice and discussions. Mr R. Koscal and Mr N. Kurtanjekproduced the drawings. Two anonymous reviewers providedcomments, which helped in improving the manuscript.

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