Limestones in Middle and Upper Jurassic Succession of the Slovenian Basin Presedimentirani apnenci v srednje in zgornjejurskem zaporedju Slovenskega bazena

The Middle and Upper Jurassic succession of the Slovenian Basin is characterized by pelagic sedimentation of siliceous limestones and radiolarian cherts. In the southern and central part of the basin two packages of resedimented limestones are interbedded within pelagic sediments. The Lower resedimented limestones are lower-middle Bajocian to lower Callovian in age. In the southern part of the basin they form laterally discontinuous sequences composed of limestone breccias, calcarenites and micritic limestone and in the central part of the basin calcarenite intercalations within pelagic beds. They were transported by turbidity currents from highly productive ooidal shoals of the Dinaric Carbonate Platform. The Lower resedimented carbonates correlate with the lower three members of the Travnik Formation in the Bovec Trough and similarly developed but much thicker Vajont Formation in the Belluno Basin. The difference in thickness is interpreted as a consequence of shallow-water and longshore currents on the Dinaric Carbonate Platform that transported platform material towards southwest in the direction of the Belluno Basin. The Upper resedimented limestones are upper Kimmeridgian to lower Tithonian and occur within radiolarian cherts in the upper part of the succession as calcarenite beds that originated by turbidity currents. Onset of resedimentation coincides with the emersion-related demise of barrier reef and following deposition of micritic and rare oolitic limestones on the Dinaric Carbonate Platform. Approximatelly coeval resedimented limestones occur in the fourth member of the Travnik Formation in the Bovec Trough, but are not reported from the Ammonitico Rosso Superiore Formation in the Belluno Basin.


Introduction
The investigated succession of the Slovenian Basin is located in the northwestern Slovenia (Fig. 1). The Slovenian Basin deepened at the end of the Triassic and beginning of the Jurassic (B u s e r, 1989, 1996). The deepening was a consequence of accelerated subsidence caused by rifting-related extensional tectonics characteristic of the southern Tethyan passive continental margin during this period ( (C o u sin, 1970, 1981; R o ' i~, 2003a, b). Therefore three sections (Poljubinj, Zapo{kar and Perbla) were recently studied in detail. The aim of this paper is: 1. to describe the geometry and facies associations of resedimented limestones and to establish a relative distance of these three sections from the source area, 2. to correlate the investigated sections with the successions from the northern margin of the Dinaric Carbonate Platform, the Bovec Trough and the Belluno Basin.

Geological setting
In the aspect of macrotectonic subdivision of Slovenia (P l a c e r, 1999) the successions of the Slovenian Basin structurally belong to the Tolmin Nappe which together with the overlying Julian Nappe with successions of the Julian Carbonate Platform represents eastern continuation of the Southern Alps (Fig. 1). The Tolmin Nappe overlies the Trnovo Nappe of the External Dinarides where successions of the Dinaric Carbonate Platform are placed. In the transitional area between the External Dinarides and the Southern Alps, which includes also the southern parts of the Tolmin Nappe, two thrusting phases were recognized. The NE to SW thrusting of the Dinaric phase ended at the end of the Eocene and was overprinted by middle to late Oligocene Southalpine phase characterized by N to S thrusting (Placer & ^ar, 1998;Placer, 1999). In general, The Tolmin Nappe can be subdivided in three second-order thrust sheets (Buser, 1986(Buser, , 1987: the lowest Podmelec Thrust sheet is overthrust by the middle Rut Thrust sheet and the highest Kobla Thrust sheet. The successions of the Slovenian Basin that are most proximal in aspect to the Dinaric Carbonate Platform are located in the Podmelec Thrust sheet and became more distal in the Rut Thrust sheet and finally the Kobla Thrust sheet (Fig. 2b).
The overall Jurassic succession of the Slovenian Basin begins with the lower to middle Liassic Krikov Formation characterized by resedimented and hemipelagic limestones. It is overlain by the Toarcian Perbla Formation composed of black shales to marls, marly limestones and rare beds of resedimented limestones (Cousin, 1973(Cousin, , 1981. The Perbla Formation passes upwards in the Tolmin formation (at present informal name proposed by Ro'i~, 2006), which is divided in Aalenian to Bajocian lower member characterized by siliceous limestones, and Bajocian to lower Tithonian upper member composed predominantly of radiolarian cherts. Within pelagic deposits of the Tolmin formation resedimented limestones discussed in this paper are locally abundant in the Podmelec Thrust sheet, but appear also as interbeds in the Rut Thrust sheet (Fig. 3). The Jurassic succession ends with the upper Tithonian to Neocomian Biancone Formation (Cousin, 1981;Buser, 1987).

Description of studied sections
Three sections were studied. In the Podmelec Thrust sheet the Zapo{kar and the Poljubinj sections are located. The Zapo{kar section is located three kilometers north of Cerkno in a gorge on the southern slopes of Mt. Porezen near the abandoned Zapo{kar farm (x = 113600, y = 421150). The Poljubinj section is located two kilometers southeast of Tolmin in the western slopes of Mt. Kuk near the Poljubinj village (x = 115550, y = 404500). The Perbla section structurally belongs to the Rut Thrust sheet and is located five kilometers northeast of Tolmin in the Jelov{~ek gorge near the Perbla village (x = 120800, y = 404450).

Background sediments
The base of the sections is represented by the Perbla Formation composed predominantly of marls (Fig. 3). The Perbla Formation gradually passes into the Tolmin formation, which is divided in two members. The lower member consists of dark gray siliceous limestones of wackestone type with calcified radiolarians and sponge spicules. In the Zapo{kar section marls are common within siliceous limestones through the entire member. In the Perbla section the silicification is locally very strong and beds of black cherts are observed in the middle part of the member. Cherts exhibit similar microfacies as siliceous limestones but are composed of microcrystalline quartz. In the Poljubinj section the lower member of the Tolmin formation is missing. The upper member of the formation is present in all sections and is represented by radiolarian cherts that are colored greenish-gray in the lower part and violet-red in the upper part of the member. The composition corresponds to black cherts described in the lower member of the Tolmin formation in the Perbla section. In the upper part of the upper member marls and shales are common. In the Zapo{kar section the upper part of the member is composed almost exclusively of marls. Within the Tolmin formation two packages of resedimented limestones are distinguished (Fig. 3).

Lower resedimented limestones
Lower resedimented limestones in the Zapo{kar and Poljubinj sections are amalgamated sequences (Fig. 4). In the Zapo{kar section the lower resedimented limestones are 25 meters thick and placed between two members of the Tolmin formation. They are composed of bedded limestone breccias, calcarenites and rare beds of micrite. Calcarenites are bedded (10 to 180cm), gray, usually graded and parallel laminated. Microfacies is predominantly grainstone composed of peloids, micritized ooids, shallow-water and basinal intraclasts and fossils, predominantly echinoderms and rarer benthic foraminifers, bivalve and brachiopod shells and bryozoan fragments (Pl. 1, Fig. 1). As suggested by rounded circular shape, a large portion of peloids probably represents strongly micritized ooids. Dark gray, fine-grained, wavy and cross laminated grainstone and packestone occur in the upper parts of graded or thin-bedded (up to 20 cm) calcarenites. Grains are pellets to peloids and bioclasts, predominantly echinoderms, calcified radiolarians and rare small benthic foraminifers (Pl. 1, Fig. 2). Cements in grainstones are mosaic and syntaxial around echinoderms. In the lower and upper part of the package nodules of replacement chert are observed in calcarenites. The finest beds are darkgray, poorly bioturbated, wavy laminated wackestones and mudstones. Among allochemes calcified radiolarians and sponge spicules prevail. In bioturbated borings pellets are common. Breccias occur at the base of thick (up to 4 m) beds. Beds are graded and breccias upward gradually turn into calcarenites. Previously they were described as one thick (17 m) graded breccia bed (R o-'i~, 2003a, b), but detailed work revealed that this package is composed of five smaller amalgamated sequences, where breccias appear only in the lower two. Microfacies of breccias is rudstone and two major clast types (up to 5 cm large) are differentiated. The composition of deep-water intraclasts corresponds to the pelagic limestones described above. Shallow-water clasts are grainstones and rarely packstones composed of peloids, ooids, intraclasts and bioclasts, mostly echinoderms and rare benthic foraminifers. In grainstone clasts rim and mosaic cements are observed. Matrix in breccias is earlier described grainstone. Resedimented limestones in this area laterally wedge out and in the western slopes of Mt. Porezen radiolarian cherts directly overlie siliceous limestones (Ro'i~, 2006).
In the Poljubinj section the lower resedimented limestones are 20 meters thick and overlie with erosional contact the marls of the Perbla Formation (Fig. 4). The package begins with up to 11 meters thick, massive and channeled breccia megabed. The composition of breccia corresponds to that described in the Zapo{kar section (Pl. 1, Fig.  3) but contains also rare chert clasts. It is overlain by two dark gray, coarse-grained packstone beds (7 to 8 cm), each covered by wavy laminated wackestone bed (10cm). Packestone is composed of the same material as grainstone in the Zapo{kar section but clasts are more abundant (Pl. 1, Fig. 4, 4b).
In wackestone thin-shelled bivalves and spicules prevail and are oriented parallel to the bedding. The upper part of the resedimented limestones is composed of bedded (10 to 90 cm), structure-less, coarse-to-medium-grained grainstone. Its composition is similar to the grainstone of the Zapo{kar section, but contains more ooids. A 2,5 meters thick bed with no apparent internal bedding stands out. It is slightly coarser and includes thin chert lenses.
In the Perbla section lower resedimeted limestones appear as isolated, graded, parallel and rarely cross-laminated calcarenite beds (10 to 50 cm) within the pelagic succesion around the boundary between the two members of the Tolmin formation (Fig.  4). Texture and composition of coarse-tomedium-grained grainstone are the same as in the calcarenites of the Zapo{kar section (Pl. 2, Fig. 1,2). Ooids are more abundant and, if larger, also micritized. Fine-grained calcarenites are also packestones composed as those from the Zapo{kar section. Beds are partially replaced by nodular chert.
Lower-middle Bajocian to Lower Callovian age of the lower resedimented limestones was determined with foraminifers and radiolarians. Middle Jurassic foraminiferas were determined by Gu{i} (personal communication, 2006): Protopeneroplis striata (Weynschenk) (Pl. 1, Fig. 4b) in the Zapo{kar and Poljubinj sections, Protopeneroplis sp. in the Zapo{kar and Perbla sections and Mesoendothyra croatica (Gu{i}) (Pl. 2, Fig. 2) in the Perbla section. The oldest resedimented limestones in the Perbla section occur 4 meters below a radiolarian sample, assignable to lower -middle Bajocian (UAZone 3-4 of Baum g a rt n e r et al., 1995;in Gori~an et al., 2006). In the Poljubinj section a radiolarian sample 2 meters above the boundary with resedimented limestones was dated as middle Callovian -lower Oxfordian (UAZone 8 of B a u m g a rt n e r et al., 1995;in Gori~an et al., 2006).
Sedimentary structures in limestones indicate a deposition by gravity flows, mostly by turbidity currents. Massive, channelized breccia megabed in the Poljubinj section was deposited either by a debris flow or a high-density turbidity current. Overlying packstone and wackestone beds are probably genetically related to the megabed and therefore represent a two-pulsed upper part of a two-component gravity flow (M u l l i n s & Co o k, 1986; M u t t i, 1992). Nevertheless, these beds could also originate as separated turbidites. Massive structure in calcarenite beds observed especially in the upper part of the resedimented limestones in the Poljubinj section could indicate deposition by modified grain flows or sandy debris flows (Stow & J o h a n s s o n, 2000; S h a n m u g a m, 2000). But we presume that the deposition by turbidity current is more probable and that the massive structure is a consequence of pre-sorting of the material in a source area. The distribution of facies associations where the thickness, abundance, and grain-size of resedimented limestones decrease from south to north proves a south-lying source area. The composition of resedimented limestones, especially calcarenites indicates that the source area of the resedimented material were ooidal shoals on the northern margin of the Dinaric Carbonate Platform. Breccia clasts indicate an erosion of approximately coeval platform limestones as well as slope or basinal limestones. The lower resedimented limestones are discontinuous in the Podmelec Thrust sheet. In the Zapo{kar area the resedimented limestones thin and finally wedge out towards the west. In the Poljubinj section the resedimented material filled a channel. Since the horizon cuts directly into marls of the Perbla Formation (siliceous limestones of the Tolmin formation are missing) the erosion of older basinal strata is evidenced.

Upper resedimented limestones
Resedimented limestones reappear as calcarenite intercalations within radiolarian cherts in the uppermost part of the Tolmin formation (Fig. 5). In the Zapo{kar section beds are 10 to 140 cm thick, graded and parallel laminated. Thicker beds are gray, coarse-to-medium-grained grainstones composed predominantly of diverse limestone clasts, fossils and very rare ooids. Clasts of shallow-water origin are mostly grainstones composed of peloids, intraclasts and pellets (Pl. 2, Fig. 3). Mudstone clasts are common. Some are well rounded and are better classified as peloids, whereas others contain calcified radiolarians and are therefore pelagic intraclasts. Fossils are diverse but the most abundant are still echinoderms (Pl. 2, Fig.  4,). Others bioclasts are benthic foraminifers, bryozoan fragments, bivalves, brachiopods and rare stromatoporid and dasycladacean fragments. Cements are mosaic and syntaxial around echinoderms. Especially in the lower and upper margins of beds grainstones are replaced by nodular chert. Thinner limestone beds (10 to 20cm) are dark gray, finegrained, rarely graded and parallel or wavy laminated grainstones and packestones to wackestones composed of calcified radiolarians, echinoderms, rare benthic foraminifers, micritic intraclasts and/or pellets (Pl. 2, Fig. 5). This microfacies are replaced by chert nodules as well. Similar rare limestone beds occur also in the lower and middle part of radiolarian cherts. Above the last thick calcarenite bed marls gradually prevail over radiolarian cherts and compose the upper 20 meters of the Tolmin formation. Chert beds and lenses reappear in the uppermost 2 meters of the Tolmin formation.
In the Poljubinj section upper resedimented limestones are represented only by two thin (10 cm), medium-grained grainstone beds of similar composition as those described in the Zapo{kar section (Fig. 5).
In the Perbla section limestone beds (10 to 90 cm) are quite abundant (Fig. 5). They occur as intercalations between radiolarian cherts in the upper 10 meters of the Tolmin formation, but thicker and coarser calcarenite beds are present only in the uppermost 5 meters. Although graded and parallel laminated grainstones prevail, packstones and wackestones are also present in thinner beds. Composition generally corresponds to the microfacies described in the Zapo{kar section (Pl. 2, Fig. 6), but rare aptychi and Saccocoma fragments were also observed in the uppermost beds.
Late Kimmeridgian to Lower Tithonian age of the upper resedimented limestones was determined by foraminifers, dasycladaceas, radiolarians and nannoplankton. From the Upper Jurassic foraminifers stratigraphically important were determined (Gu{i}, personal communication): Mohlerina basiliensis (Mohler), Nautiloculina oolithica (Mohler). (Pl. 2, Fig. 4), and Kurnubia sp. in the Zapo{kar section and Mohlerina basiliensis (Pl. 2, Fig. 6), Nautiloculina sp. and Tubyphyites morronensis (Crescenti) in the Perbla section. In the Zapo{kar section also the Upper Jurassic (Gu{i}, personal communication) dasycladacea Salpingoporella pygmaea (Gümbel) was found. Cessation of resedimented limestones coincides with the boundary between the Tolmin and the Biancone formations which was dated with radiolarians and nannoplankton in the lower/ upper Tithonian (G o r i~a n et al., 2006). The lower boundary is less successfully dated, but is probably of late Kimmeridgian -early Tithonian age, since resedimented limestones occur only in the uppermost part of the Tolmin formation.
Sedimentary structures in resedimented limestones suggest a deposition by turbidity currents. Although no apparent differences of facies associations are observed between the successions in the Podmelec and the Rut Thrust sheets we are still able to determine a southward source area of resedimented limestones, because limestone beds are absent in the Upper Jurassic successions of the northernmost Kobla Thrust sheet (R o ' i~, 2006). The composition of resedimented limestones indicates a shallow-water sedimentary environment exceedingly different from ooidal shoals described as the source area for the lower resedimented limestones. Composition comparison with the lower resedimented limestones elucidates that the late Jurassic Dinaric Carbonate Platform provided smaller amounts of ooids, but more abundant shallow-water intraclasts and more diverse fossils. Simultaneously with the resedimentation of the platform material an increased terrigenous input is observed. Terrigenous deposits are especially abundant in the Zapo{kar section where marl forms the upper 20 meters of the Tolmin formation. It is presumably related with a more warm and humid climate in the Late Jurassic. Such climatic conditions increased the erosion of continental areas and consequently the transport of clay material into basins (e.g. Weissert & Mohr, 1996;Picard et al., 2002).
In the Zapo{kar section resedimented limestones exceptionally occur also in the lower and middle part of radiolarian cherts. Beds are thin and represented predominantly by packstones to wackestones composed of echinoderms, micritic intraclasts and calcified radiolarians. For these beds we propose a slope as a source area of apparently gravity-displaced limestone material.

Correlation with the Dinaric Carbonate Platform, the Bovec Trough and the Belluno Basin
Within the Middle and Upper Jurassic Tolmin formation of the Slovenian Basin two packages of resedimented limestones were recognized (Fig. 6). The composition of the lower resedimented limestones reflects sedimentary conditions on the Dinaric Carbonate Platform, which in the Middle Jurassic was a highly productive open carbonate shelf (Orehek & Ogorelec, 1978, Bosellini et al., 1981, where large masses of ooids and peloids were formed (the formation Oolith des Trnowaner Waldes; Kossmat, 1906). Thickness of this formation is up to 700 meters (Buser, 1986). The exceeding material that originated during maximal carbonate productivity between lower -middle Bajocian to lower Callovian was exported from platform margins to proximal parts of the Slovenian Basin. The redeposition was mainly with turbidity currents that also eroded platform, slope and basinal limestones. The lower horizon is correlated with the upper Bajocian to Bathonian lower three members of the Travnik Formation from the Bovec Trough ([muc, 2005;[muc & Gori~an, 2005) and Vajont Limestone from the Belluno Basin also assigned to late Bajocian and Bathonian (Cobianchi, 2002;Clari & Masetti, 2002) (Fig. 6). The composition of resedimented limestones is generally similar in three correlated paleogeographic domains, but a great difference occurs when thicknesses are compared (Fig. 7). In the Slovenian Basin lower resedimented limestones are only up to 25 meters thick. On the other hand, in the Belluno Basin the Vajont Limestone reaches 600 meters in proximal parts, but decreases The Bovec Trough could therefore represent a link between the Slovenian and the Belluno basins but this is questionable since the exact paleogeographic location of succession from Mt. Mangart area is not satisfactorily resolved. The resedimentation of shallow-water material in the Slovenian Basin ceased during the Callovian and was scarce until late Kimmeridgian. During this period the northern margin of the Dinaric Carbonate Platform was first drowned and thinbedded micritic limestone and calcarenite with chert nodules deposited (Buser, 1978(Buser, , 1986. This correlation is not certain, because limestones with cherts of the Dinaric Carbonate Platform are poorly dated. In the late Oxfordian the Dinaric Carbonate Platform became rimmed by barrier reef (Turn{ek, 1997) (Fig. 6). From this time only in the Za-po{kar section rare resedimented limestones with slope as a source area are known. At the end of the Kimmeridginan large areas of the Dinaric Carbonate Platform became subaerially exposed (Dozet, 1994;Dozet et al., 1996;Ti{ljar et al., 2002) which lead to the demise of barrier reefs (Turn{ek, 1997). The reestablishment of sedimentation followed in the latest Kimmeridgian or early Tithonian, when bedded micritic limestones and rare oolitic limestones with Clypeina jurassica (Favre) started to deposit (Buser, 1986). The described change of sedimentation on the Dinaric Carbonate Platform is recorded with the onset of deposition of upper resedimented limestones in the Slovenian Basin. It is necessary to consider that especially in the Zapo{kar section upper resedimented limestones could be older and marly top of the section would form with the dispersal of terigenous material from emerged areas of the Dinaric Carbonate Platform. But a very small amount of reef-building fossils in platform-derived material in resedimented limestones denies such interpretation. Resedimented limestones are reported also in the upper part of the Callovian to lower Tithonian fourth member of the Travnik Formation in the Bovec Trough ([ m u c, 2005; [ m u c & G o r i~an, 2005) (Fig. 6). Upper Kimmeridgian to lower Tithonian of the Belluno Basin is represented by the Ammonitico Rosso Superiore Formation that is free of resedimented limestones (Fig. 6). But they occur in proximal parts of the Belluno Basin in the Fonzaso Formation that is otherwise characterized by siliceous pelagic sedimentation (C l a r i & M a s e t t i, 2002; C o b i a n c h i, 2002). These resedimented limestones are older and therefore not correlatable with upper resedimented limestones of the Slovenian Basin.

Conclusions
The Middle and Upper Jurassic succession of the Slovenian Basin is characterized by the Tolmin formation that conformably overlies the marls of the Toarcian Perbla Formation. The Tolmin formation is divided in two members. The lower member is composed predominantly of siliceous limestones and rare cherts, whereas the upper member consists almost entirely of radiolarian cherts. In the southern and central part of the basin resedimented limestones are common and are divided in two packages. Lower resedimented limestones are lowermiddle Bajocian to lower Callovian in age. In the Podmelec Thrust sheet, which paleogeographically corresponds to the southern part of the basin, lower resedimented limestones form up to 25 meters thick limestone packages deposited either discomformably over the Perbla Formation or conformably over the lower member of the Tolmin formation. In the Rut Thrust sheet where successions of the central part of the basin are found lower resedimented limestones occur as calcarenite intercalations within pelagic beds around the boundary between two members of the Tolmin formation. Resedimented limestones were deposited mostly by turbidity currents. They are represented predominantly by coarse-to-medium-grained grainestones composed of peloids, micritized ooids, shallow-water and basinal intraclasts and fossils, predominantly echinoderms. Other microfacies are fine-grained grainstone and packestone composed of pellets to peloids and bioclasts, predominantly echinoderms and wackestones to mudstones with calcified radiolarians and sponge spicules. In the southern part of the basin thick-bedded limestone breccias are common. The texture is rudstone with grainstone matrix. Clasts are deep-water intraclasts and eroded coeval shallow-water clasts composed similary as grainstone matrix of the breccias. From the described distribution of facies associations and composition of resedimented limestone we can conclude that during the early-middle Bajocian to early Callovian limestone material that originated in ooidal shoals in the northern margin of the Dinaric Carbonate Platform was redeposited predominantly by turbidity currents into the proximal-southern parts of the Slovenian Basin. Lower resedimented limestones correlate with the lower three members of the Travnik Formation from the Bovec Trough and the Vajont Formation from the Belluno Basin. The overall thickness of resedimented limestones is much smaller in the Slovenian Basin when compared with other two paleogeographic domains. Differences are interpreted as a consequence of southwest-ward transport of the material on the Dinaric Carbonate Platform, induced by shallow-water and longshore currents.
Upper resedimented limestones are late Kimmeridgian to early Tithonian in age and occur as calcarenite intercalations within radiolarian cherts in the upper part of the Tolmin formation. They were sedimented by turbidity currents and are represented predominantly by coarse-to-medium-grained grainstones composed mostly of shallowand deep-water limestone clasts, diverse fossils and very rare ooids. Rarer fine-grained packestones to wackestones also occur. They are composed similarly as grain-size counterparts in the lower resedimented limestones. The upper resedimented limestones deposited after emersion-related demise of barrier reef, which until late Kimmridgian rimmed the Dinaric Carbonate Platform. They are coeval with the following sedimentation of micritic and rare oolitic limestones on the platform. Resedimented limestones are reported also from the approximately coeval fourth member of the Travnik Formation in the Bovec Trough. Resedimented limestones do not occur in the simultaneous Ammonitico Rosso Superiore Formation in the Belluno Basin.