COBISS: 1.01 UPPER CRETACEOUS TO PALEOGENE FORBULGE UNCONFORMITy ASSOCIATED wITH FORELAND BASIN EVOLUTION (KRAS, MATARSKO PODOLJE AND ISTRIA; Sw SLOVENIA AND Nw CROATIA) ZAKRASELA PERIFERNA IZBOKLINA POVEZANA Z RAZVOJEM ZGORNJEKREDNO-PALEOGENSKEGA PREDGORSKEGA BAZENA; KRAS, MATARSKO PODOLJE IN ISTRA (JZ SLOVENIJA IN SZ HRVAŠKA) Bojan OTONIČAR1 Abstract UDC 551.44.551.7(497.4-14) 552.541.551.7(497.4-14) Bojan Otoničar: Upper Cretaceous to Paleogene forbulge un-conformity associated with foreland basin evolution (Kras, Matarsko Podolje and Istria; SW Slovenia and NW Croatia) A regional unconformity separates the Cretaceous passive margin shallow-marine carbonate sequence of Adriatic Carbonate Platform from the Upper Cretaceous and/or Paleogene shal-low-marine sequences of synorogenic carbonate platform in southwestern Slovenia and Istria (a part of southwestern Slove-nia and northwestern Croatia). Te unconformity is expressed by irregular paleokarstic surface, locally marked by bauxite de-posits. Distinctive subsurface paleokarstic features occur below the surface (e.g. flled phreatic caves, spongework horizons…). Te age of the limestones that immediately underlie the un-conformity and the extent of the chronostratigraphic gap in southwestern Slovenia and Istria systematically increase from northeast towards southwest, while the age of the overlying limestones decreases in this direction. Similarly, the deposits of synorogenic carbonate platform, pelagic marls and fysch (i.e. underflled trinity), deposits typical of underflled peripheral foreland basin, are also diachronous over the area and had been advancing from northeast towards southwest from Campan-ian to Eocene. Systematic trends of isochrones of the carbonate rocks that immediately under- and overlie the paleokarstic sur-face, and consequently, of the extent of the chronostratigraphic gap can be explained mainly by the evolution and topography of peripheral foreland bulge (the forebulge). Te advancing fexural foreland profle was the result of vertical loading of the foreland lithospheric plate (Adria microplate) by the evolving orogenic wedge. Because of syn- and post-orogenic tectonic processes, and time discrepancy between adjacent foreland basin deposits and tectonic (“orogenic”) phases it is difcult to defne the exact tectonic phase responsible for the evolution of the foreland complex. According to position and migration of the subaerially exposed forebulge, distribution of the foreland Izvleček UDK 551.44.551.7(497.4-14) 552.541.551.7(497.4-14) Bojan Otoničar: Zakrasela periferna izboklina povezana z razvojem zgornjekredno-paleogenskega predgorskega bazena; Kras, Matarsko podolje in Istra (JZ Slovenija in SZ Hrvaška) V jugozahodni Sloveniji in Istri so kredna karbonatna zaporedja Jadranske karbonatne platforme pasivnega obrobja Jadranske mikroplošče ločena z regionalno diskordanco od zgornjekrednih in paleogenskih karbonatnih zaporedij sino-rogene karbonatne platforme. Razgibano paleokraško površje, ki diskordanco označuje, je lokalno prekrito z boksitom. Pod površjem se pojavljajo različne podpovršinske paleokraške oblike, med drugim večje zapolnjene freatične jame in diskretni horizonti drobnih prepletajočih se kanalčov. Starost apnencev neposredno pod paleokraškim površjem in obseg stratigrafske vrzeli v jugozahodni Sloveniji in Istri sistematično naraščata od severovzhoda proti jugozahodu, nasprotno pa starost apnencev, ki paleokraško površje pokrivajo v tej smeri upada. Preko obravnavanega območja so med campanijem in eocenom od severovzhoda proti jugozahodu napredovala tudi sedimentna zaporedja sinorogenih karbonatnih platform (karbonatne kamnine Kraške grupe) ter pelagičnih laporjev in fiša, ki predstavljajo sedimente podhranjenega predgorskega bazena. Sistematične trende izohron karbonatnih kamnin, ki ležijo neposredno pod in nad paleokraškim površjem in posledično razpona stratigrafske vrzeli lahko v veliki meri razložimo z evolucijo in topografjo periferne predgorske izbokline. Napredujoči feksurni predgorski profl je nastal zaradi vertikalne obremenitve predgorske litosferske plošče (Jadranske mikroplošče) z nastajajočim orogenim klinom. Zaradi sočasnih in postorogenih tektonskih procesov ter časovnega neskladja med sedimenti sosednjih predgorskih bazenov in med različnimi tektonskimi (»orogenimi«) fazami tega dela zahodne Tetide v kredi in paleogenu, je opredelitev tektonske faze, ki je neposredno odgovorna za evolucijo obravnavanega predgorja otežena. Glede na položaj in migracijo periferne iz- 1 Karst Research Institute ZRC SAZU, Titov trg 2, Si-6230 Postojna, Slovenia, e-mail: otonicar@zrc-sazu.si Received/Prejeto: 01.02.2006 TIME in KARST, POSTOJNA 2007, 101–120 BOJAN OTONIČAR related macrofacies and orientation of tectonic structures, espe-cially of Dinaric nappes, and Dinaric mountain chain I suggest that the foreland basin complex in western Slovenia and Istria was formed during mesoalpine (“Dinaric”) tectonic phase due to oblique collision between Austroalpine terrane/Tisia micro-plate and Adria microplate when probably also a segmentation of the foreland plate (Adria microplate) occurred. Key words: forebulge unconformity, paleokarst, chronostrati-graphic gap, fysch, Adriatic Carbonate Platform, synorogenic carbonate platform, foreland basin, Adria microplate, Dinaric orogene, Cretaceous, Paleogene, Sw Slovenia, Istria. bokline, razporeditev makrofaciesov podhranjenega predgor-skega bazena ter usmerjenost tektonskih struktur, predvsem Dinarskih pokrovov, in Dinarskega gorstva v celoti domnevam, da je nastal predgorski sistem v zahodni Sloveniji in Istri med mezoalpidsko (»Dinarsko«) tektonsko fazo, kot posledica bočne kolizije med Avstroalpidskim terranom in/ali Tisa mikroploščo ter Jadransko mikroploščo, pri čemer je verjetno prišlo tudi do segmentacije Jadranske mikroplošče. Ključne besede: diskordanca, paleokras, kronostratigrafska vrzel, fiš, Jadranska karbonatna platforma, periferna predgorska izboklina, sinorogena karbonatna platforma, predgorski bazen, Jadranska mikroplošča, Dinarski orogen, kreda, paleogen, jugozahodna Slovenija, Istra. INTRODUCTION Plate tectonics theory had a crucial impact on our un-derstanding of sedimentary basins, and consequently, of carbonate sedimentary systems. Plate tectonics de-termines not only the gross architecture (dimension and shape) and lithological/structural characteristics of carbonate platforms (Bosellini, 1989), but also their evolution and the longevity. Tose characteristics are largely defned by specifc geotectonic setting in which certain carbonate platform begin to grow. Carbon-ate platform(s), which colonize certain area through longer or shorter period of geologic history, constantly change its/their position in relation to the equator and plate boundaries and pass through diferent phases of the wilson cycle. Te sedimentary and diagenetic char-acter of the carbonate platform(s) constantly change(s) during this journey and at a stretch, the platform evolution may be stoped. In this case, the area formerly inhabited by the carbonate platform may fall under conditions which are not favourable for considerable carbonate production. In one scenario it may imme-diately afer the deposition or later in the geologic his-tory be uplifed, subaerially exposed and karstifed. Similarly as the plate tectonics governs the sedimen-tary evolution of the carbonate platforms, it may also determins their diagenetic evolution, including karsti-fcation. Te gross architecture, lithological/structural caharacteristics, and the evolution and the longevity of the uplifed area with subaerially exposed carbonate platform are mainly dependent on its geotectonic position regard to plate boundaries, former geodynamics and consequently topography of the area, especially of the carbonate platform. Although important for the ap-pearance of the karstic landscape, the efects of other variables, such as climate and ground water level, may be just superimposed on the geotectonically predis-posed framework. 102 TIME in KARST – 2007 Each karstic landscape carries its specifc geotecton-ic signature which can be read from and explained with specifc evolution of karstic features and a karst system as a whole. In addition, studies of sedimentary succes-sions of rocks that under- and overlie the (paleo-) karstic surface and that of the adjacent sedimentary basins as well as the general geologic conditions of the region may signifcantly improve our knowledge on geodynamics of the uplifed area. Te paper documents an example of paleokarst that occurred during the uplif of the Adriatic Carbon-ate Platform (sensu Vlahović et al., 2005) in the distant foreland region of the evolving collision related orogenic belt between the Adria microplate (sensu Stampfi et al., 1998) and the Austroalpine terrane and/or Tisia micro-plate (sensu Neugebauer et al., 2001) in the Late Creta-ceous and the Early Paleogene. Te study is based on 36 geological profles from the karstic regions of southwestern Slovenia, both Slo-venian and Croatian part of Istria peninsula and the area between Trieste bay and Italian-Slovenian border in northeastern Italy (Figs. 1, 2). To get a whole picture of conditions that dominated the region during the emersion period, I expend the area of interest to syno-rogenic carbonate platform that onlap the paleokarstic surface and to siliciclastic fysch regions of afore men-tioned areas and the adjacent regions of western Slove-nia and northeastern Italy (along the border between Italy and Slovenia). Te aim of this work is to show the causes of the uplif and subaerial exposure of the northwestern part of the Cretaceous Adriatic Carbonate Platform in Late Cretaceous and Early Paleogene. Te data presented here were provided mainly from the studies of paleogeograph-ic and topographic extent of the emersion, stratigraphy of the carbonate successions that immediately under- and UPPER CRETACEOUS TO PALEOGENE FORBULGE UNCONFORMITy ASSOCIATED wITH FORELAND BASIN EVOLUTION Fig. 1: Geographical position and simplifed geological map of the western Slovenia and Istria showing major structural elements (modifed from Placer, 1999). overlain the paleokarstic surface, stratigraphy and sedi- time of the uplif is correlated with events on the adjacent mentology of the onlapping synorogenic carbonate plat- plate boundaries of the western Tethian domain (tradi-form and adjacent deeper marine basin as well as from tional “orogenic phases”) and global eustatic curve. regional geotectonic and general geologic situation. Te GEOLOGy OF THE AREA Te geology of southwestern Slovenia and Istria has been carbonate successions of diferent Cretaceous formations studied from the late 19th century on. Since that time also from shallow-marine limestones of the Upper Creta-a regional unconformity which separates shallow-marine ceous/Lower Paleogene Liburnia Formation or Eocene TIME in KARST – 2007 103 BOJAN OTONIČAR Fig. 2: Simplifed lithostratigraphic columns of Cretaceous to Eocene successions in southwestern Slovenia and Istria (NW Croatia and SW Slovenia), with one column from NW Italy. Authors of original geological columns are listed below: 1) šribar (1995); Rižnar (1997), 2) drobne (1977, 1979), 3) drobne et al. (1988, 1996); šribar (1995); jurkovšek et al. (1996), 4) jurkovšek et al. (1996), 5) drobne (1981); jurkovšek et al. (1996), 6) jurkovšek et al. (1997), 7) hamrla (1959); drobne (1977, 1979); Pavlovec et al. (1991), 8) hamrla (1959, 1960); jurkovšek et al. (1996), 9) brazzatti et al. (1996), 10) hamrla (1960); drobne et al. (1991); jurkovšek et al. (1996), 11) hamrla (1959); buser & Lukacs (1979); delvalle & buser (1990); jurkovšek et al. (1997); this study, 12) drobne (1977); delvalle & buser (1990), 13) delvalle & buser (1990); šribar (1995); buser & Radoičić (1987), 14) šikić et al. (1972); drobne (1977), 15) drobne (1977); this study, 16) drobne (1977, 1981); hottinger & drobne (1980); drobne & Pavlovec (1979); drobne et al. (1991); turnšek & drobne (1998); this study, 17) drobne (1977), 18) šikič et al. (1972); drobne (1977), 19) biondić et al. (1995), 20) šikić et al. (1972); drobne (1977), 21) šikić et al. (1968); drobne (1977), 22) Pleničar et al. (1969); drobne (1977); Gabrić et al. (1995), 23) Pleničar et al. (1969); drobne (1977), 24) hamrla (1959); Pleničar et al. (1973); drobne (1977); velić & vlahović (1994); matičec et al. (1996), 25) šikić et al. (1968); drobne (1977); hottinger & drobne (1980); drobne et al. (1991), 26) matičec et al. (1996), 27) tarlao et al. (1995), 28) buser & Lukacs (1972); drobne (1977); hottinger & drobne (1980); matičec et al. (1996), 29) Polšak & šikić (1973); drobne (1977), 30) drobne et al. (1991), 31) - 34) matičec et al. (1996), 35) šikić et al. (1968); magaš (1973); šikić et al. (1973); šikić & Polšak (1973); höttinger & drobne (1980); Otoničar et al. (2003), 36) Polšak (1970); drobne (1977); matičec et al. (1996). 104 TIME in KARST – 2007 UPPER CRETACEOUS TO PALEOGENE FORBULGE UNCONFORMITy ASSOCIATED wITH FORELAND BASIN EVOLUTION Alveolina-Nummulites Limestone has been known. Te Liburnia Formation, Alveolina-Nummulites Limestone and intermediate Trstelj Beds represent the Kras Group (Košir, 2003) (Fig. 3), which corresponds to the lower unit of the underflled peripheral foreland basin stratigraphy (i.e. the lower unit of the “underflled trinity” of Sinclair, 1997). Tus the unconformity represents a megasequence boundary and typically separates the underlying passive margin carbonate succession from the overlying deposits of the synorogenic carbonate platform at periphery of the foreland basin (Košir & Otoničar, 2001). Te synorogen-ic carbonate platform was fnally buried by prograding hemipelagic marls (i.e. the middle unit of the “underflled trinity” of Sinclair, 1997) and deep-water clastics (fysch) (i.e. the upper unit of the “underflled trinity” of Sinclair, 1997) (Fig. 3). Because the name of the carbonatre platform that overlie the unconformity has not been defned yet, I will use in this paper only the general geodynamic term – i.e. the synorogenic carbonate platform. Fig. 3: Generalized stratigraphic column of Upper Cretaceous-Eocene succession in the Kras (Karst) and matarsko podolje regions, SW Slovenia, showing major lithostratigraphic units (modifed from Košir, 2004). Te unconformity is expressed by an irregular paleokarstic surface, locally marked by bauxite depos- its. Although the unconformity has been repeatedly mentioned, no systematic study of paleokrast has been performed. Relatively numerous papers on biostratig-raphy, especially on the carbonate successions of the Kras Group, have been published (see list of references attached to Fig. 2), yet not more than few attempts on explanation of the sedimentology of the paleokarstic deposits and onlapping beds have been done (Otoničar, 1997; Debeljak et al., 1999; Durn et al., 2003). Only oc-casionally, the geotectonic conditions under which the paleokarst (uplif) evolved have been briefy mentioned (Košir & Otoničar, 2001; Otoničar & Košir, 2001; Durn et al., 2003). Tectonically, the discussed area corresponds to three macrotectonic units, the Southern Alps, the Ex- Fig. 4: Illustrative geological map showing distribution of fysch deposits and major structural elements in western Slovenia. Te map is based mainly on data from basic geological maps of yugoslavia, 1:100.000, sheets beljak & Ponteba (jurkovšek, 1986), Udine-tolmin & videm (Udine) (buser, 1986), Kranj (Grad & Ferjančič, 1974), Gorica (buser et al., 1968), Postojna (buser et al., 1967), trst (Pleničar et al., 1969) and Ilirska bistrica (šikić et al., 1972). Copyright: Geološki zavod Slovenije (Geological survey of Slovenia), 2002 – All rights reserved. TIME in KARST – 2007 105 BOJAN OTONIČAR Fig. 5: A) Paleogeographical map showing major geotectonic units at Santonian-Campanian boundary in western tethys and central Atlantic (modifed from Neugebauer et al., 2001). b) Geotectonic and paleogeographic units of Adria microplate and adjacent areas. ternal Dinarides and the Dinaric foreland (Placer, 1999) (Fig. 1). while the fysch-related sediments can be fol-lowed across the all three units (Fig. 4), the unconfor-mity and the overlying carbonate successions of the Kras Group correspond to the most external thrust unit of the Dinaric fold and thrust belt – the northwestern External Dinarides in southwestern Slovenia, Italian part of the Kras plateau and northeastern Istria, and to more stable foreland domain of the Dinaric mountain belt in other parts of Istria (Figs. 1, 2). Te nappe structure of northwestern part of the Ex-ternal Dinarides comprises fve successively lower and younger thrust units from northeast to southwest: Trnovo Nappe, Hrušica Nappe, Snežnik Trust Sheet, Komen Trust Sheet and Kras Trust Edge (Placer, 1981, 1999, 2002) (Fig. 1). Te External Dinarides and the Dinaric foreland correspond to the northwestern part of the Cretaceous Adriatic Carbonate Platform and the Upper Cretaceous-Eocene synorogenic carbonate platform which occupied northeastern part of the Adria microplate s.s. (Fig. 5). In the Cretaceous the area of present day Southern Alps 106 TIME in KARST – 2007 was a part of deeper marine realm which comprised the Slovenian Basin formed in the Middle Triassic (Cousin, 1981; Buser, 1989) and the area of former Julian Carbon-ate Platform which was drowned in the Lower and Mid-dle Jurassic (Cousin, 1981; Buser, 1989). Te geologic and paleogeographic situation started to change severely in the Late Cretaceous (see below). It is important to note, that the described region is re-cently confned from the north side by the Periadriatic fault zone, from the west by the deposits of the Southern Alpine Molasse Basin and from the south and southwest by the Adriatic Sea and its sediments (Fig. 1). To understand the mechanisms that governed the uplif and emersion, regional geotectonic conditions of the wider area of the Late Cretaceous-Early Paleogene western Tethys were taken into consideration. During the Mesozoic, the area between Eurasia and Gondwana or the western part of the extensive Tethys bay of the Pangea was occupied by more or less uniform Adria microplate surrounded by smaller tectonic units or terranes (Fig. 5). with regard to major geotectonic events, the extent and shape of Adria microplate was UPPER CRETACEOUS TO PALEOGENE FORBULGE UNCONFORMITy ASSOCIATED wITH FORELAND BASIN EVOLUTION changing constantly through the geologic history. Te re-sults of these events (e.g. tecto-sedimentary successions or cycles) could be correlated between geographically and geologically distant parts of the Adria microplate. Afer substantial Permian to Middle Triassic and Triassic/Jurassic extensional tectonics, the Adria domain became encircled by oceanic bays and dissected by nu-merous deepwater basins and drowned carbonate plat-forms (Fig. 5). It is considered that since Early Jurassic the Adriatic Carbonate Platform had been isolated by deeper marine realms (Vlahović et al., 2005). At the Middle/Late Jurassic boundary compression-al tectonic regime prevailed over the peri-Adriatic region. It was caused by the beginning of closure (subduction) of the adjacent oceanic bays of the western Tethys. During the Late Jurassic and Cretaceous gentle broad-scale positive and negative lithospheric defections periodically oc-curred on the Adriatic Carbonate Platform. Te defec-tions were expressed by coexistence of karstic areas and somewhat deeper marine intra-platform basins (Tišljar et al., 1995; 1998; Vlahović et al., 2005). Distinctive de-fections correspond to period of ophiolite emplace-ment [e.g. the Late Jurassic/Early Cretaceous obduction of ophiolite suite of the Dinaric Tethys on the E margin of the Adria microplate (Pamić et al., 1998; 2000)] and distant collisions [e.g. the mid-Cretaceous Eoalpine oro-genesis in the Pelso/Austroalpine/Tisia domain (Faupl & wagreich, 2000; Neugebauer et al., 2001)]. Topographic disunity over the platform gave rise to irregular facies distribution and thickness of carbonate successions of diferent parts of the platform. Signifcant interruptions of carbonate successions are also related to global eustatic oscillations and/or oce-anic anoxic events, but they are mainly superimposed on tectonically induced changes of relative sea-level. Tus before the beginning of the uplif of northern part of the Adriatic Carbonate Platform in the Late Cre-taceous and the synchronous onset of fysch sedimentation in the area north and north-eastern of the platform, the whole region was already topographically distinctly heterogeneous. Flysch started to deposit in deeper marine basin with partly inherited bathymetry from former deeper marine domain of Slovenian Basin and drowned Julian Carbonate Platform (Fig. 5). Deeper marine realms with more or less uninterrupted sedimentation had still encircled the carbonate platform from its western and southwestern side (Vlahović et al., 2005) (darker grey area on Fig. 5). Later tectonic activity which shortened the area and displaced diferent parts of the region, prevent more ac-curate interpretation of geotectonic conditions at those time. Namely, except the substantial shortening of the region due to diferent “thrusting” phases of Alpine oro-geney, the area north from the Periadriatic Fault Zone was displaced for at least 100 km eastward during the Miocene (Ratschbacher et al., 1991; Frisch et al., 1998; Vrabec & Fodor, 2006), in some estimates up to 500 km (Haas et al., 1995). It should be noted that western Istria (i.e. Dinaric Foreland on Figure 1) experienced signif-cant counterclockwise rotation most likely between the end of Miocene and the earliest Pliocene (Márton et al., 1995; Márton, 2006). PALEOKARST In the investigated area both surface and subsurface pale-okarstic features occur. In places the paleokarstic surface is denoted by surface karst forms like karrens, dolines and depressions of decimetric amplitude (Fig. 6a). Pedogenic features and enlarged root-related channels characterize the upper part of the vadose zone, the epikarst. Vadose channels, shafs and pits penetrate up to a few tens of meters bellow the paleokarstic surface, where they may merge with originally horizontally oriented phreatic cav-ities. Te latter comprise characteristics of caves forming in fresh/brackish water lenses. At least some of them may be defned as fank margin caves (Fig. 6b, 6c). In extensive outcrops, the remains of such caves can be followed as much as few hundreds of meters along strike. In one case a breccia body which was defned as paleokrastic cave re-lated deposit (Otoničar et al., 2003), is so extensive that was used even as mappable unit for Basic geologic map of yugoslavia 1:100.000 (see Magaš, 1965). Te cavities are usually irregular and elongated in shape, and could be up to few tens of meters long and up few meters high (Fig. 6b). Depending on locality, the phreatic cavities were found in diferent positions regarding to the paleokrastic surface, the lowest one some 75 meters below it. Te cavi-ties had been subsequently partly reshaped and entirely flled with sediments and fowstones in the upper part of the phreatic, epiphreatic and vadose zones (Figs. 6b, 6c). Similarly, the vadose channels and voids are also flled by sediments and fowstones, but they usually difer from these of phreatic cavities in higher content of noncar-bonate material, lower ?13C values of carbonate material and more distinctive pedogenic modifcation. Te denudation had frequently exposed flled paleokarstic subsur-face cavities on the paleokarstic surface, where they may be identifed only by the remains of their fll (Otoničar TIME in KARST – 2007 107 BOJAN OTONIČAR et al., 2003) (Fig. 6c). Te internal sediments and fow-stones ofen occur as grains in deposits that cover the paleokarstic surface or fll subsurface paleokarstic cavi-ties of diferent generations. Paleokarstic surface with its depressions as well as subsurface channels and voids are ofen covered and flled by bauxite deposits which were locally exploited (Fig. 6d) (Gabrić et al., 1995). Certain limestone lithofacies of immediate cover of the unconformity are commonly locally confned, sug-108 TIME in KARST – 2007 Fig. 6: A) Paleokarstic surface is locally denoted by small scale depressions (motorway road-cut at Kozina village, SW Slovenia). Note colour contrast between Upper Cretaceous shallow marine limestone of the Lipica Formation and dark grey palustrine limestone of the Liburnia Formation. hammer for scale is about 30 cm high. b) horizontally oriented cave of irregular shape largely flled with reddish-stained calcareous mudstone/siltstone (Podgrad, matarsko Podolje, SW Slovenia). Te maximal height of the cave is approximately 4 meters. Te cave deposits are artifcially marked by reddish transparent colour on the photograph. C) breccia body represents a part of flled roofess paleokarstic phreatic cave at Koromačno in Istria, NW Croatia. (1,8 m tall geologist for scale in the upper right corner) d) Excavated paleokarstic cavity (vadose shaf?) originally flled with bauxite (minjera, Istria, NW Croatia). UPPER CRETACEOUS TO PALEOGENE FORBULGE UNCONFORMITy ASSOCIATED wITH FORELAND BASIN EVOLUTION gesting highly irregular topography of the karstic surface before the beginning of transgression. In places it is clear that the incipient transgression involved gradual increase of groundwater table and, eventually, ponds or “blue holes” were formed in karstic depressions (Durn et al., 2003). In the Kozina site (southwestern Slovenia) during the “blue hole” stage of the transgression, a paleokarstic pit was flled by coarse grained breccia with vertebrate remains, mainly dinosaurian and crocodilian bone fragments and teeth (Debeljak et al., 1999, 2002). General-ly, the cover sequence (i.e. the Liburnian Formation of Maastrichtian and early Paleogene age) is characterized by restricted, marginal marine and palustrine lithofacies, which frequently show pedogenic modifcations. EVOLUTION OF THE PERIPHERAL BULGE (THE FOREBULGE) Besides the research on paleokarst related phenomena, the study of sedimentary successions of the host rock in which the paleokarstic features occur and those that overlie the paleokarstic surface is of crucial importance to understand the uplif of substantial part of the Adriatic Carbonate Platform above the sea-level in the Late Cre-taceous and Paleogene. To explain the mechanisms that govern the uplif, regional and global geotectonic and eu-static conditions were taken into consideration, too. STRATIGRAPHy Te age of the limestones that immediately underlie the unconformity and the extent of the chronostratigraphic gap in southwestern Slovenia and Istria systematically increase from northeast towards southwest (Figs. 2, 7a, 7b), while the age of the overlying limestones decrease in this direction (Figs. 2, 7c). In western part of Istria the orientation of the isochrones is slightly diferent and VN. »im a 1 If ^^^s*Jf// / / t I h Fig. 7: A) Isochrones of carbonate rocks that immediately underlie the unconformity. b) Isochrones of the extent of the chronostratigraphic gap. C) Isochrones of carbonate rocks that immediately overlie the unconformity. Isochrones in all fgures are in ma. major structural elements of the area (see Fig. 1) and positions of the geological profles used in the research (see Fig. 2) are also shown in the fgures. TIME in KARST – 2007 109 BOJAN OTONIČAR shows a dome-like topography of the forebulge. Te isochrones represent a statistic result acquired by kriging in Surfer programme version 8.00 (© Golden Sofware, Inc.). Te data were provided from 36 geological profles from the karstic regions of southwestern Slovenia, both Slovenian and Croatian part of Istria peninsula and the area between Trieste bay and Italian-Slovenian border in northeastern Italy (Figure 2 and red dots on Figures 7a, 7b, 7c). Te youngest rocks below the unconformity be-long to mid-Campanian and occur in the central and northeastern part of the Kras (Karst) plateau (the Komen thrust sheet) (Fig. 1) (Jurkovšek et al., 1996) and close to Postojna (the Snežnik thrust sheet) (Fig. 1) (Šribar, 1995; Rižnar, 1997) in southeastern Slovenia, while the oldest one, Valanginian and Hauterivian in age, crop out in the western part of Istria (Matičec et al., 1996) (Figs. 2, 7a). Te beds that cover the unconformity correspond to diferent ages, litofacies, members and formations. As mentioned afore, the age trend of the immediate cover is opposite to that of the footwall. In this case the oldest rocks occur in southwestern Slovenia and belong to the youngest stage of the Late Cretaceous - the Maastrich-tian. Towards southwest, progressively younger deposits onlap the paleokarstic surface (Figs. 2, 7c). However, the youngest strata that onlap the unconformity don’t ft ex-actly with the oldest one immediately below it. with regard to described situation, the chronostratigraphic gap increases considerably from few Ma on the Kras plateau (southwestern Slovenia) to more than 80 Ma in western Istria (Figs. 2, 7b). Te lithofacies of the lower part of the cover sequence (Te Liburnian formation) frequently show features typi-cal of subaerial exposure surfaces, including calcrete, pseudomicrokarst, brecciated horizons and karstic sur-faces. Locally, the lowermost subaerial exposure surface of the Liburnija Formation, which shows karstic topog-raphy of decimetric amplitude, and the main paleokarstic surface form a composite unconformity. Sporadically, thin coal beds and seams occur in the lower part of the sequence. Although the stratigraphy of the Kras Group, “Transitional Beds” and Flysch (Fig. 3) shows overall deepening of the basin, prominent subaerial exposure surfaces also occur in carbonate successions of Trstelj Beds and Alveolina-Nummulites Limestone (Košir & Otoničar, 1997; Košir, 2003). Much thicker successions of paralic sediments with more frequent unconformities and marsh related sediments occur in southwestern Slo-venia and northeastern Istria in comparison with other parts of Istria, yet local variation can be signifcant (Figs. 2, 8). In western Istria, where the chronostratigraphic gap is the most extensive, the foraminiferal limestones fre-quently lie directly on the paleokarstic surface (Matičec at al., 1996). Te thickness of the Kras Group generally decreases from northeast toward southwest, although also in this case signifcant deviations may occur (Figs. 2, 8). Te point where the unconformity pinch-out to-wards the foreland basin occurs somewhere between the northeastern part of the Kras plateau on the Komen Trust Sheet and some 10 km (approximately 25 km in original position – see Placer, 1999) distant Mt. Nanos on the Hrušica nappe (Fig. 1). From this point on towards the foreland basin, the uplif of the forebulge didn’t take place because the area was so close to the orogene that experienced only a subsidence. Here, the sedimentary succession of the Adriatic Carbonate Platform gradually passes into progressively deeper-marine carbonate suc-cession of synorogenic carbonate platform. Namely, on the Mt. Nanos at Campanian-Maastrichtian boundary, the deepening of the shallow marine carbonate platform without any evidence of preceding emersion is docu-mented (Šribar, 1995). Further towards the northeast, in the Julian Alps (the eastern part of the Southern Calcareous Alps) and in the most northern part of recent Dinaric mountain belt in western Slovenia and northeastern Italy (the Trnovo Nappe), the turbiditic siliciclastic sediments (fysch) started to deposit in Campanian and Maastrichtian over the rocks of diferent lithology, age and origin (Pavšič, 1994). Flysch ofen overlies deeper marine pelagic marls of “scaglia” type and alodapic carbonates, which were re-ceiving the material from Adriatic Carbonate Platform. It is important to note that in this part of western Slovenia deep-marine basin existed before fysch or above men-tioned deeper marine pelagic marls started to deposit. However, the oldest pelagic marls (pre-fysch deposits) which overlie the Upper Cretaceous shallow marine car-bonates of the northeastern margin of the Adriatic Car-bonate Platform also belong to Maastrichtian. Similar as I stated for chronostratigraphic gap, the pelagic marls and fysch deposits are also diachronous over the area. From northeast toward southwest, successively younger strata onlap the pre-foreland basin deposits (Fig. 4). Te successions of pelagic marls and especially si-liciclastic fysch were periodically interrupted by deposition of calcarenitic and calcruditic beds/megabeds, locally even of olistostrome character. Tose beds were supplied by turbiditic currents from the fault-related es-carpments of distorted and seismically active marginal areas of former Adriatic Carbonate Platform (Skaberne, 1987; Tunis & Venturini, 1987) and later also from outer parts of synorogenic carbonate platforms (distally steep-ened ramps?) (Fig. 9). Te synorogenic carbonate and siliciclastic deposits of other parts of External Dinarides (e.g. Dalmatia) are 110 TIME in KARST – 2007 UPPER CRETACEOUS TO PALEOGENE FORBULGE UNCONFORMITy ASSOCIATED wITH FORELAND BASIN EVOLUTION Fig. 8: Lithostratigraphic columns for three adjacent sites in matarsko Podolje and mt. Slavnik (SW Slovenia). Note signifcant variations in thickness of lithostratigraphic units and in time span of stratigraphic gap. younger than these described here. Tey started to de-posit not before Eocene (Marjanac & Ćosović, 2000) and probably represent deposits of diferent foreland system or at least of diferent segment of the one described here. DISCUSSION Systematic trends expressed by isochrones showing the age of the carbonate rocks that immediately under- and overlie the paleokarstic surface (Figs. 7a, 7c), and conse-quently, the extent of the chronostratigraphic gap (Fig. 7b), can be explained mainly by the evolution and topography of peripheral foreland bulge (the forebulge) (Fig. 9). when the foreland continental lithospheric plate is vertically loaded by the fold and thrust belt, it responds with fexure. In front of the evolving orogen an asym-metric foreland basin is formed; the deepest part of the basin (the foredeep) is located adjacent to the orogenic wedge (Fig. 9). Because of the isostatic rebound on ver-tical loading of the lithosphere, the opposite side of the basin (opposite to the orogenic wedge) is instantaneously upwarped and the bulge with subtle relief is formed, the peripheral bulge or the forebulge. Te bulge is especially well expressed in early, fysch stage of the foreland basin evolution (Crampton & Allen, 1995). while the wave-length of the defection is approximately the same for both, foreland basin and peripheral bulge, the amplitude of the basin subsidence is typically much greater as the uplif of the bulge (Crampton & Allen, 1995; Miall, 1995). If the conditions are suitable, synorogenic car-bonate platforms with distinctive ramp topography may colonise the gentle slope of the forebulge toward the fore-deep (Dorobek, 1995). Signifcantly, as the whole complex of the orogenic wedge advances forelandward, the fexural profle pro-duced by the orogenic wedge advances with it. Topog-raphy of the forebulge is controlled by numerous factors, among which the rigidity of the foreland lithospheric plate and the rate of emplacement of the load are the most important (Allen & Allen, 1992; Dorobek, 1995; Miall, 1995). An expected maximal height of the forebulge above the sea level (if the foreland plate is at or close to sea-level prior to fexural loading) would be in the range of up to a few tens to few hundreds meters (Crampton & TIME in KARST – 2007 111 BOJAN OTONIČAR Fig. 9: Schematic block diagram of foreland basin complex showing the position of the orogenic wedge, foredeep and forebulge with distribution of macrofacies belts before plate convergence ended (modifed from bradley & Kidd, 1991). Allen, 1995; Miall, 1995). According to topography of the forebulge, the rate of erosion (see white, 2000) and the style of migration of the orogenic wedge, the area of maximal denudation should occur in the central part of the region, which is over-passed by the bulge (Crampton & Allen, 1995). In addition, non-fexural deformations (e.g. reactivation of pre-existing heterogeneities, enhanced defections because of horizontal in-plane stresses…) and inherited topography may signifcantly infuence the evolution and topography of the forebulge (Allen & Allen, 1992; Dorobek, 1995; Miall, 1995; Crampton & Allen, 1995). On Mt. Nanos (Hrušica Nappe; Fig. 1) shallow water rudist limestone of the Adriatic Carbonate Platform gradually passes over limestone with orbitoidiform larger foraminifera into pelagic marls without any emersion at the base of the deepening sequence – the erosional gap reduces to conformity. Te age span of this transition falls within a period of the shortest documented chronostrati-graphic gap between the northeastern part of former Adriatic Carbonate Platform and the overlying synogen-ic carbonate platform (Fig. 2), which extends from mid-Campanian to Late Maastrichtian. Maastrichtian in age are also the oldest pelagic marls which in places directly overlie the Upper Cretaceous shallow water carbonates of former northeastern margin of the Adriatic Carbon-ate Platform. Although the oldest turbiditic siliciclastic fysch was deposited in a basin with inherited deeper marine bathymetry (former Slovenian Basin) its Campan-ian and Maastrichtian age could be correlated with other incipient foreland related deposits and phenomena. with regard to these criteria and trends of unconformity re-lated isochrones elsewhere (Figs. 2, 7a, 7b, 7c), I suggest that northern part of the Adriatic Carbonate Platform had thrived more or less prosperously till the end of Campanian, when an initial uplif of the forebulge occurred. Te carbonate sediments that had origi-nally been deposited till that time, and are now missing in carbonate successions immediately below the unconformity, had been erased dur-ing the paleokarstic period by the karstic denudation processes. According to topography of the forebulge and advancing nature of the foreland geodynamic complex as a whole, the most extensive denudation is expected in the central area over which the forebulge migrates. Te western part of Istria, where the chronostratigraphic gap is the larg-est and the beds immediately below the unconformity are the oldest (Fifs. 2, 7a, 7b), most probably corresponds to this zone. However, in an ideal conceptual/mathematical model of the forebulge unconformity, the amount of erosion should remain more or less constant over vast area in the central part of the region over-passed by the bulge, and decreases on its distal slope towards back-bulge basin (Crampton & Allen, 1995). Instead, in western Istria the isochrones of the beds underlying the unconformity show distinctive condensation compared to situation in northeastern Istria and southwestern Slovenia (Fig. 7a). I suggest that this is not the result of rapid increase of the amount of footwall eroded but rather of denudation of primarily much thinner Cretaceous carbonate succes-sions in western Istria. Namely, in this part of Istria the carbonate successions are relatively thin (Matičec et al., 1996), partly because of repeating emersions throughout the Cretaceous (Velić et al., 1989) and partly because of reduced accommodation space of Cretaceous shallow marine environments. Evidence of considerable Late Ju-rassic and Cretaceous land areas in the vicinity of western Istria (probably ofshore form its recent west coast), came also from dinosaur record (footprints and bones) (Della Vecchia et al., 2000; Mauko & Florjančič, 2003; Mezga et al., 2003) and distribution of sedimentary fa-cies of the adjacent peritidal to deeper marine environments of intraplatform basins (Tišljar et al., 1995; 1998). why was the area of western Istria beeing preferentially uplifed during the Cretaceous is still questionable, but the reasons for defections should be searched at adjacent plate boundaries where their reorganisation and difer-ent collision-related events and processes (see Faupl & wagreich, 2000; Neugebauer et al., 2001) produced hori- 112 TIME in KARST – 2007 UPPER CRETACEOUS TO PALEOGENE FORBULGE UNCONFORMITy ASSOCIATED wITH FORELAND BASIN EVOLUTION zontal in-plane stresses that may be transmitted many hundreds of kilometers inboard of actual collision (Zei-gler et al., 1995). It is also possible that the central zone of the foreb-ulge and the slope towards back-bulge basin in their fnal position occurred ofshore of recent Istrian west coast. However, we should be aware that the Late Cretaceous Adriatic Carbonate Platform was surrounded from the western side by deeper marine interplatform basins (Vlahović et al., 2005) what might considerably afected the appearance of the forebulge and the back-bulge area. Although the “abnormal thickness” of denuded stratigraphy in western Istria is mainly the result of previous sedimentary history, some uncertainties may also arise from diferential uplif/subsidence of certain parts of the forebulge. Evidence for diferential subsid-ence along reactivated ancient tectonic structures is for example well documented in carbonate successions of the Kras Group, where the thickness of chrono- and lithostratigraphic units may vary considerably over short distances (Figs. 2, 8). In conclusion I suggest that the denudation exposed the oldest carbonate rocks in the western Istria partly be-cause of specifc evolution (migration) and topography of the forebulge and partly because of primarily thinner carbonate successions in this part of Istria compared to more northeastern parts of the investigated area. Te rate of transgression over the paleokarstic sur-face is expressed by the isochrones of the strata that onlap the unconformity (Fig. 7c). while the large scale diachro-nism of the onlapping strata shown in Figure 7c is the result of specifc large-scale topography and migration of the forebulge as a whole, local smaller scale spatial dif-ferences in the onlap pattern (not observable in Figure 7c) are due to shorter oscillations of relative-sea level and deposition over topographically irregular paleokarstic surface (e.g. dolines, shafs… – a “blue hole phase” of the transgression). Te pattern of the isochrones shown in Figure 7c suggests that the transgression during its earlier stages (southwestern Slovenia and northeastern Istria) was slower compared to its later stages (western Istria). Although subsequent tectonic deformations, such as tectonic shortening, faulting and rotation, substantially afected the area, the rate of the onlap in southwestern Slovenia and northeastern Istria is estimated to about 2-3 km/Ma while in southwestern Istria to about 4-5 km/Ma. we should be aware that some apparent anomalies, espe-cially at terminations of the isochrones may be the result not only of later tectonic deformations of the area but also of limited number of data points which are not uniformly distributed, spatially confned area of the investigation along the strike of the forebulge and defectiveness of sta-tistic method (kriging) used. Slightly diferent orienta- tion of the isochrones in western part of Istria compared to those in southwestern Slovenia and northeastern Istria (Figs. 7a, 7b, 7c) may also be the result of diferent syde-positional or synorogenic orientation of the prevailing stresses (see Marinčić & Matičec, 1991; Matičec et al., 1996) during the Cretaceous and Paleogene and later counterclockwise rotation of the area (see Márton et al., 1995 amd Márton, 2006). In spite of all that, the reasons for diferent stratigraphic pinch-out rate are many sided and may arise from diferential rheologic and structural characteristics of the foreland plate itself, events at colli-sion zone and adjacent plate boundaries, sublithospheric processes and external reasons like eustatic sea-level os-cillations and climate changes. In our case it is difcult to determine the exact reason for the increasing rate of the onlap in Lower Eocene, not only because diferent pro-cesses may lead to the same result, but also because they can act simultaneously. Long term sea-level fall (i.e. second-order cycle of Haq et al., 1987) may for example slow-down the onlap rate and vice-versa long term sea-level rise may increase the onlap rate. If we observe the eustatic curve for the Cretaceous and Paleogene (Haq et al., 1988) we can notice that the rate of the onlap is in relatively good agreement with mid-Campanian to Late Paleocene second-or-der fall and Early Eocene rise of the sea-level. However, the foreland basin should progressively widen and pinch-out migration rate would increase also if, for example, the orogenic wedge loaded a progressively stronger elastic lithosphere (Allen & Allen, 1992). Although not all local variations of relative sea-level oscillations and so the onlap rate could be identifed from isochrones in the Figure 7c, they could be observed in the feld. Namely, the subaerial exposure surfaces that periodically interrupt the carbonate sedimentation of the Liburnia Formation refect relative sea-level falls. Short term falls (i.e. third-order cycles of Haq et al., 1987), which were documented in Late Maastrichtian, Late Pa-leocene and Early Eocene (Haq et al., 1988), could cause these unconformities. On the other hand, a few other processes may infu-ence the rate of the onlap. Te forebulge should increase in height and migrate toward the orogenic wedge over time if the foreland lithosphere behaves viscoelastical-ly even when the load is unchanging (Tankard, 1986). However, estimations for time constants of the viscous relaxation of stresses are longer than actual amount of time available for the forebulge migration (Allen & Allen, 1992; Dorobek, 1995). Variation in onlap rate may refect also changes in sediment supply, or within the orogenic wedge, such as the formation of a new thrust complex (Crampton & Allen, 1995) or transition from passive to active thrusting phase. An increase in com- TIME in KARST – 2007 113 BOJAN OTONIČAR pressive in-plane stress produced during convergence also might enhance uplif of the forebulge and causing shoreline regression along its fank (Allen & Allen, 1992; Dorobek, 1995). Evidence of short term sea-level oscillations could also be recognized from the specifc evolution of the pa-leokarst, especially phreatic caves. If the majority of len-ticular caves with irregular walls and discrete horizons of spongework or swisse-cheese like vugs on young carbon-ate islands originated at/in fresh/brackish water lenses (see Mylroie & Carew, 1995), then in our case the major part of the cavities had been emplaced in the vadose zone prior to submergence and burial. Namely, the caves are frequently completely flled with deposits originated in vadose zone, like fowstone and bauxite, or they had been opened to the paleokarstic surface by complete denudation of the roof (i.e. roofess caves of Mihevc, 2001). If the water-level is stagnant and the forebulge migrates, than in the conceptual sense only those phreatic cavi-ties developed below that forebulge fank that facing to-wards back-bulge basin should be uplifed in the vadose zone before subsidence. On the contrary, phreatic caves developed below the fank facing the foreland basin and the advancing orogenic wedge should sufer nothing but subsidence and subsequent burial. Teoretically it is possible that because of the advancing character of the forebulge, caves formed in diferent sides of the forebulge may occur in the same karstic profle. Phreatic cavities developed below the fank facing towards the back-bulge region should be uplifed and modifed in the vadose zone. Subsequently, afer the crest of the forebulge mi-grates over the back-bulge fank, the “back-bulge” phre-atic caves should re-immerge into phreatic zone, but this time below the fank facing towards the foreland basin. It is important to note that frequently observed multiphase modifcations of originally phreatic caves could also be the result of the same causes of relative sea-level oscil-lations that govern the onlap character of the beds that overlie the unconformity (e.g. relaxation of the viscoelas-tic bulge, formation of a new thrust complex, increase of horizontal in-plane stress, eustatic sea-level fall…). Te carbonate platform was subsequently re-estab-lished and fnally buried by prograding deeper-marine clastics (pelagic marls and fysch) of the migrating fore-land basin (Fig. 9). As it has been already discussed, shal-low-water carbonate successions that cover the uncon-formity may yield a considerable amount of information about relative sea-level oscillations and geodynamics of the forebulge. Paralic/shallow-marine successions with frequent unconformities and palustrine deposits of the Liburnia Formation (Fig. 3) are usually much thicker in south-western Slovenia and northeastern Istria than in central and western Istria (Fig. 2). Tere the paleokarstic surface is frequently directly overlain by foraminiferal limestones (Matičec at al., 1996). Te general trend of thickness and the rate of transition from shallow to deep marine environments (drowning) (Fig. 2) are in good agreement with the rate of the onlap (Fig. 7c) and should be the result of the same processes that caused the diferentiations in the onlap pattern. I suggest that the anomalies in thickness and facies distribution that could be in places quite dis-tinctive may arise from reactivation of inherited geologi-cal structures due to the approaching orogenic wedge. It has been discussed already, that the orogenic phas-es could be recognised from structural and stratigraphic data even in areas that are located at some distance from the source of tectonic activity at plate boundaries (e.g. collision and orogenesis). Because of later tectonic defor-mations it is sometimes difcult to defne the exact tec-tonic phase which afects the area and the actual source of tectonic activity. In our case, the structural and stratigraphic data in-dicate the evolution of migrating synorogenic foreland basin complex, which should be the result of collision processes and the evolution of the advancing orogenic wedge (see e.g. Allen & Allen, 1992; Crampton & Allen, 1995; Miall, 1995). At frst sight it seems normal to link the foreland complex to tectonic phase that generated structures by mainly NE-Sw compression (mesoalpine phase of some authors; see Doglioni & Bosellini, 1987) and gave rise to Dinaric mountain belt during its fnal stages. However, the Dinaric orogenic belt of which fnal uplif occurred during the Oligocene-Miocene (Vlahović et al., 2005) is supposed to be the result of collision be-tween Tisia and Adria microplates with onset of colli-sion during the Eocene (Pamić et al., 1998; Pamić, 2002), what is also the age of the oldest synorogenic deposits of the “coastal” part of the External Dinarides (Marjanac & Ćosović, 2000). On the contrary, although the nappe structures of western Slovenia and Late Cretaceous – Pa-leogene compressional deformations of northeastern Italy indicate NE-Sw or ENE-wSw compression, and so “Dinaric” orientation of prevailing regional stress, the oldest foreland basin deposits in these regions are much older than those of other parts of the External Dinarides and belong to the latest stages of Late Cretaceous (Pavšič, 1994; Doglioni, 1987; Doglioni & Bosellini, 1987). As it is shown on Figure 4 the age distribution of fysch de-posits indicates the advancing nature of foreland basin from northeast towards southwest what is in accordance with “Dinaric” orientation of the prevailing regional stress. while south of Zagreb-Zemplen fault line, the remnants of oceanic lithosphere (i.e. ophiolite melange) as well as subduction and collision related rocks of In-ternal Dinarides (i.e. the Sava-Vardar zone by Pamić et 114 TIME in KARST – 2007 UPPER CRETACEOUS TO PALEOGENE FORBULGE UNCONFORMITy ASSOCIATED wITH FORELAND BASIN EVOLUTION al, 1998), which could be linked to closing processes of the Vardar Ocean and collision between Tisia and Adria (Pamić, 2000) are widespread, north of Zagreb-Zemplen line no such rock has been found so far. It seems possible that in central Slovenia, in prolongation of the Sava-Var-dar zone, such rocks have been buried by Tertiary sediments and Southern Alpine nappes. In addition, on the NNE side the nappe structure of western Slovenia was cut from its “root zone” by Periadriatic fault. Te “root zone” should be displaced for at least 100 km eastward during the Miocene (Ratschbacher et al., 1991; Frisch et al., 1998; Vrabec & Fodor, 2006). Although, the structural and sedimentary features of eoalpine tectonic phase which culminated in mid-Cretaceous orogeny in the Austroalpine domain (Faupl & wagreich, 2000) and also afected the central and western part of the Italian Southern Alps (Doglioni, 1987; Doglioni & Bosellini, 1987) mostly pre-date the foreland related features and sediments described here, it should be noted that in Istria Tertiary tectonic cycle (from Eo-cene on) display distinctively diferent orientation of the prevailing stress than Mesozoic one (Marinčić & Matičec, 1991; Matičec et al., 1996). In conclusion, the foreland basin complex in western Slovenia and Istria was probably formed during me-soalpine (“Dinaric”) tectonic phase, although some infu-ences of eoalpine tectonic phase could be important in earlier stages of its evolution. Te time discrepancy and also the exact orientation of prevailing regional stress are probably the result of oblique collision between Adria and Tisia microplates (and/or Austroalpine terrane?) and/or segmentation of the foreland plate (see Ricci-Luc-chi, 1986; Allen & Allen, 1992). Oligocene to recent tectonic events especially in Dinarides and Apennines, and conter-clockwise rotation of Adria importantly modifed the area formerly occupied by the forebulge, but this is already beyond the scope of this paper. CONCLUSIONS In spite of all structural and depositional heterogenei-ties and subsequent tectonic deformation of the area the paleokarstic unconformity marked by distinctive surface and subsurface paleokarstic features exhibits characteris-tics typical of a forebulge unconformity: 1) From northeast towards southwest the uncon-formity cuts progressively older units which are onlapped by progressively younger shallow water carbonates; the chronostratigraphic gap progressively increases. 2) Deepening upward sequences of synorogenic ramp-like carbonate systems overlie the unconformity. In marginal parts of the former Adriatic Carbonate Platform towards the foreland basin, a deepening upward sequence is documented also without intermediate un-conformity – here the sequence is conformable because the orogenic wedge was so close that the area experi-enced only subsidence and forbulge uplif had no taken place. 3) Te foreland basin with siliciclastic turbiditic fysch deposits was developing synchronously with the forebulge and synorogenic carbonate platforms. It was also advancing synchronously in the same direction as they were forebulge and synorogenic carbonate plat-forms. Te stratigraphy overlying the unconformity (i.e. underflled trinity) representing subsidence in under-flled peripheral foreland basin. 4) Evidence of contemporary seismic activity arises from periodic carbonate resediments (megabeds, olistostromes) fnd in siliciclastic fysch successions. Tey were supplied by turbiditic currents from the fault related escarpments of the forebulge slope (reactivated ancient faults). Besides fexural upwarping because of the isostatic rebound on vertical loading of the foreland lithosphere, other smaller scale fexural and non-fexural deformations signifcantly infuenced the evolution and appearance of the forebulge (incuding its diagenesis and karstifcation), lithofacies distribution and thickness of the carbonate successions above the unconformity. At least some infuence of eustatic sea-level oscillations can-not be excluded. 5) Te subaerially exposed area and the facies belts of progressive forelandward advancing shallow-marine, pelagic, and turbiditic depositional environments ahead of the orogenic front are roughly parallel to the Dinaric mountain chain. However, the Dinaric fore-land-related system supposedly began to evolve during the Eocene when Tisia and Adria microplates began to collide what is much later comparing to Late Cretaceous onset of foreland basin evolution and forebulge uplif in western Slovenia and Istria. In Istria the orientation of the prevailing regional stress during Cretaceous tectonic cycle difers signifcantly from Eocene one. I suggest that the foreland basin complex in western Slovenia and Istria was probably formed during mesoalpine (“Dinaric”) tectonic phase, due to oblique collision of Adria and Ti-sia microplates (and/or Austroalpine terrane?) and seg-mentation of the foreland plate. TIME in KARST – 2007 115 BOJAN OTONIČAR ACKNOwLEDGMENTS Most of this work was performed as part of doctoral dissertation at University of Ljubljana. I am grateful to David Culver for English correction of an earlier version of the manuscript and Adrijan Košir for constructive reviews. 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