Triassic deep-water sedimentation in the Bled Basin, eastern Julian Alps, Slovenia

The Bled Basin was a Middle Triassic–Early Cretaceous basin whose remnants are preserved in the eastern Southern Alps in western Slovenia. The early evolution of the basin is recorded in the Upper Ladinian to Lower Jurassic Zatrnik cherty limestone formation, which in the Pokljuka Nappe overlies Middle Triassic volcanics, volcaniclastics and hemipelagic limestones. The Zatrnik Limestone is poorly documented and biostratigraphically not well constrained. The base of the Zatrnik Limestone was logged in four sections in the eastern part of the Pokljuka plateau. An Upper Ladinian Muelleritortis cochleata Radiolarian Zone was recognised in the lowermost part, whereas conodont data indicate Julian to latest Tuvalian/early Norian age for the rest of the logged sections. Microfacies analysis indicates hemipelagic deposition on a basin plain and/or distal slope, which is often interrupted by distal calciturbidites.


Introduction
Stratigraphically continuous successions of Upper Triassic hemipelagic, pelagic and gravity-flow deposits up to several hundred meters thick preserved in north-western Slovenia testify to the existence of at least three long-lived Mesozoic marine basins located near the western margin of the Neotethys Ocean. Continuous successions of Ladinian to Upper Cretaceous deeper-marine strata are mostly located in the Tolmin Nappe of the eastern Southern Alps (Buser, 1987). This basin is usually referred to as the Slovenian Basin s. str. (Cousin, 1970(Cousin, , 1973Buser, 1989Buser, , 1996 or the Tolmin Basin (Cousin, 1981;Rožič, 2009) and is relatively well studied (e.g., Rožič et al., 2009;Gale, 2010;Gale et al., 2012;Rožič et al., 2017, with references). The second basin has been identified on the basis of Upper Triassic to Lower Jurassic open-marine facies exposed in the northern Julian Alps (Lieberman, 1978;De Zanche et al., 2000;Gianolla et al., 2003;Gale et al., 2015), the Southern Kara-vanke Mountains (Krystyn et al., 1994;Lein et al., 1995;Schlaf, 1996), and in several outcrops west of this area (Geyer, 1900;Gianolla et al., 1998Gianolla et al., , 2010Caggiati, 2014). Gianolla et al. (2010) and Gale et al. (2015) refer to this paleogeographic unit as the Tarvisio Basin. Finally, the third area with Upper Ladinian to Lower Cretaceous deeper marine successions, paleogeographically belonging to the Bled Basin, extends between the lakes Bohinj and Bled (Fig. 1).
The stratigraphic succession of the Bled Basin starts with Upper Anisian -Ladinian carbonate breccias and volcaniclastic rocks deposited on top of massive Anisian dolomite (Buser, 1980). The overlying formation, which constitutes the focus of this paper, is composed of bedded limestone with chert some hundreds of meters thick (Cousin, 1981;Buser, 1987;Goričan et al., 2018). This formation was first noted by Diener (1884), Härtel (1920), and Budkovič (1978). Cousin (1981) referred to it as the Zatrnik Limestone. He considered the upper part of the Zatrnik Limestone to be Rhaetian to Early Jurassic in age, while the base of the formation was dubiously placed in the Carnian. The name Zatrnik Limestone was later abandoned and the informal term Pokljuka Limestone or Pokljuka Formation was more commonly used instead (Dozet & Buser, 2009;Buser, 2010). Fossil bivalves (Buser, 1980) and conodonts (Kolar-Jurkovšek et al., 1983;Ramovš, 1986Ramovš, , 1998, confirmed ages from Late Ladinian to Late Norian, but no continuous sections have  Placer (1999), Buser (2010), Gale et al. (2015), Goričan et al. (2018). The spatial distribution of basinal deposits is based on the map in Rožič (2016). been logged. The recent reambulation of the area stratigraphically places the Zatrnik Limestone between the Upper Anisian to Ladinian volcanics and volcaniclastics, and the Pliensbachian carbonate Ribnica Breccia. The succession continues with various pelagic and gravity deposits (see Fig. 2 and Goričan et al., 2018).
In this paper we present three detailed sections and one schematic section from the eastern part of the Pokljuka plateau encompassing the lower part of the Zatrnik Limestone. The sedimentological characteristics of the Zatrnik Limestone are described for the first time in order to identify the depositional environment and the type of sedimentation there. Conodonts, radiolarians, benthic foraminifera and microproblematica were determined from residues and thin sections.

Geological setting
Recent paleogeographic reconstructions place the Bled Basin on the oceanward side of the Julian Carbonate Platform ( Fig. 3; Kukoč et al., 2012;Goričan et al., 2018). The sedimentary succession of the Bled Basin is situated today within the Julian Alps that belong to the eastern Southern Alps (Fig. 4;Placer, 1999). Two episodes of thrusting were recognised in the eastern Southern Alps: the latest Cretaceous-Eocene NW-SE striking and SW verging thrusts are overlapped by younger (Oligocene?-Miocene) E-W striking, S-verging thrusts (Doglioni & Siorpaes, 1990;Poli & Zanferarri, 1995;Placer & Čar, 1998). Goričan et al. (2018) called all nappes of the eastern Julian Alps the Julian Nappes. From the bottom to the top these comprise the Krn Nappe, the Slatna Klippe and the Pokljuka Nappe (Fig. 5). Primary thrust contacts are only preserved around the Slatna Klippe. Elsewhere, they were cut and displaced along NE-SW and NW-SE striking faults. Both, the Krn Nappe and the Pokljuka Nappe in  the lower part consist of Anisian shallow marine carbonates and Upper Anisian-Ladinian pelagic limestone and volcaniclastics. In the Krn Nappe and the Slatna Klippe these are followed by massive Carnian limestone and dolomite, while in the Pokljuka Nappe the succession continues with the deeper-marine Zatrnik Limestone (Goričan et al., 2018, with references). Towards the south, the Julian Nappes are in thrust or steep reverse-fault contact with the Tolmin Nappe, preserving successions of the Tolmin Basin (Placer, 1999). The investigated sections within the Zatrnik Limestone were logged in relatively tectonically undisturbed blocks of the Pokljuka Nappe with gently to moderately steep dipping strata, generally in the W and NW directions.

Material and methods
The lowermost part of the Zatrnik Limestone was logged in four sections (Fig. 6). Microfacies analysis is based on 174 thin sections 47×28 mm in size. Thin sections were studied in transmitted light with an optical microscope. Dunham's (1962) classification was used for the description of textures. Point-counting was performed on microphotographs taken from selected thin sections. We counted 300 points on a random grid using JMicroVision v1.2.7 (Copyright 2002(Copyright -2008 Nicolas Roduit) software. Composite conodont samples were taken over intervals of 1.5 to 10 m, depending on the uniformity of the lithology. Carbonate rock samples were treated in acetic acid (cca. 8 %) followed by heavy liquid separation. The recovered faunas are kept in the micropaleontological collection at the Geological Survey of Slovenia (samples GeoZS 5793-5798, 5924-5928, 6001-6019). The SEM photos of conodont specimens were taken at the GeoZS (JEOL JSM  6490LV Scanning Electron Microscope). Conodont biostratigraphy follows Rigo et al. (2018), and Kolar-Jurkovšek and Jurkovšek (2019). It should be noted, however, that Chen et al. (2015) have a slightly different view on the species and their ranges, so that stratigraphic boundaries could also be assigned somewhat differently. At the Zatrnik location, two radiolarian samples were taken after radiolarians were detected in the field with a hand lens. These samples were processed in the same manner as the conodont samples. The illustrated radiolarian specimens are stored at ZRC SAZU, Ljubljana (samples RA 5843, RA 5844).

Description of sections
In the lower part the Zatrnik Limestone is represented by thin-and medium-bedded cherty micritic limestone and calcarenites. Medium-thick bedded micritic limestone with chert nodules is the dominant lithofacies in the upper part of the formation (Fig. 7). The logged sections comprise only the lower, Upper Ladinian to Lower Norian part of the formation (Fig. 8), as a good exposure of the Norian and Rhaetian part of the formation has yet to be found. A detailed description of microfacies (MF) types from the logged sections (Figs. 9-12) is given in Table 1 and which are illustrated in Figures 13-14, whereas the determined fossils are listed in Table 2 (Kozur & Mostler, 1994. Marlstone is absent in the section higher up. Filament-radiolarian-peloidal packstone (MF 8; Fig. 13c) sporadically intercalates with loose wackestone. Parallel lamination is rarely present. Moving upwards, the proportion of packstone gradually increases. Slumping and amalgamations are locally visible. Conodonts Gladigondolella malayensis Nogami, (Kozur) and Paragondolella praelindae Kozur suggest that beds above the radiolarian fauna and up to the 24 th meter of the section are (middle-late) Julian in age. Between the 26 th and 46 th meter, the conodont assemblage comprises P. praelindae, Quadralella polygnathiformis (Budurov & Stefanov) and rare Q. aff. noah (Hayashi), indicating early-middle Tuvalian age (Rigo et al., 2018;Kolar-Jurkovšek & Jurkovšek, 2019). From the 57 th meter up, peloidal packstone (MF 4), peloidal grainstone (MF 9), pebbly intraclastic grainstone (MF 10), bioclastic-intraclastic floatstone (MF 12), and closely-fitted intraclastic rudstone (MF 13) also occur among microfacies types. Normal grading is present in some beds. Approximately 60 m from the base of the section, P. praelindae is accompanied by Paragondolella inclinata (Kovacs), and later by Paragondolella tadpole (Hayashi). The association indicates late Julian to early Tuvalian age. When we also take into consideration the low number of specimens, which makes determination of assemblages difficult, we suggest the sequence between 11 th and 24 th m of the section is (undivided) Julian in age, while the upper part of the section is early Tuvalian. The Blejski vintgar section (bottom: 46°23'49''N, 14°5'19''E; top: 46°23'54''N, 14°5'24''E) was logged in shorter subsections and separated by minor faults (Fig. 10). In contrast to the Zatrnik section, this part of the formation is dominated by pebbly intraclastic grainstone (MF 10). Peloidal grainstone (MF 9) and peloidal-intraclastic packstone (MF 7) are also common, while other microfacies types (MF 2 -loose bioclastic wackestone, MF 4 -peloidal packstone, MF 5bioclastic wackestone to packstone, MF 6 -dense mollusc wackestone, MF 8 -filament-radiolarian-peloidal packstone) are subordinate. Scour structures, normal and inverse grading are often observed in micritic limestone, but rarely parallel Fig. 9. Sedimentary log of the Zatrnik section with ranges of conodont species. Microfacies types (see Table 1) are indicated on the right side of the logs.  Table 1) are indicated on the right side of the logs. lamination and amalgamation. Slumps up to approx. 6 m thick are common. Conodont samples from the lower 12 m of the subsection A were negative. Paragondolella foliata Budurov was recovered from the upper 2 m. Subsection B comprises Paragondolella foliata Budurov, Q. polygnathiformis, G. malayensis and G. tethydis (Julian). Subsection D contains Q. polygnathiformis and Q. aff. noah, suggesting early Tuvalian age.
At Mt. Galetovec (bottom: 46°19'38''N, 14°2'28''E, top: 46°19'36''N, 14°2'23''E), the section was measured schematically along the sides of a vertical wall. Micritic limestone with chert nodules predominates (Fig. 11), and the beds tend to thicken towards the top. Amalgamation is commonly present, and some massive beds probably represent slumps. Quadralella tuvalica (Mazza & Rigo) was found at the base of the cliff wall, indicating early to middle Tuvalian age (Rigo et al., 2018). In the samples from higher up in the section Epigondolella quadrata Orchard, Epigondolella rigoi Noyan & Kozur and Epigondolella vialovi (Buryi) were collected. The stratigraphic range of all three species originates from the late Tuvalian to the Lacian (Rigo et al., 2018). The upper part of the section also contains some beds of pebbly intraclastic grainstone (MF 10), but in contrast to the same MF type in the Zatrnik and Blejski vintgar sections, the amount of microbialites, microbial-sponge boundstone clasts and microproblematica is clearly lower on account of mollusc fragments. The difference in foraminiferal assemblages is also very obvious: whereas foraminifera are rare, but diverse in the two stratigraphically lower sections, samples from the top of the Mt.Galetovec section commonly contain involutinids Aulotortus sinuosus Weynschenk and Parvalamella friedli (Kristan-Tollmann), which can be present in relative abundance.
The fourth section was logged at Poljane (46°23'46''N, 14°4›32››E; Fig. 12). Medium, and rarely thick beds of micritic limestone (MF 2loose bioclastic wackestone) and fine-grained bioclastic wackestone to packstone (MF 5) are present in the lower part of the section. Amalgamation and chert nodules are locally present. Just as at the top of the Mt. Galetovec section, skeletal carbonate is relatively abundant, and the foraminiferal assemblage is dominated by aulotortids. Thus, we presume these beds are late Tuvalian or younger in age. The section ends at the foot of the massive limestone unit, which is several tens of meters thick. Indistinct, slightly curved and sloping bedding planes can be seen in the lowermost part of this massive limestone  Table 1) are indicated on the right side of the logs.  Table 1) are indicated on the right side of the logs.

Microfacies (MF) Description Figures
Dark brown marlstone is finely laminated and partly silicified. It is present only in the lowermost part of the Zatrnik section in beds up to 15 cm thick that interchange with limestone. Lamination is caused by different amounts of organic matter (as opaque bands), orientation of elongated particles and small differences in grain size. Besides very fine-grained matrix, there are a few grains large enough to be identified in thin sections: angular quartz grains and chlorite, small peloids and micritic intraclasts (the largest 0.65 mm in diameter), ostracod shells, echinoderm plates, thin-shelled bivalves and radiolarians. Due to variations in clast composition, at least three subtypes were recognised. All have a high amount of matrix in common (77-84 %). In subtype MF 2A, 6 % of the matrix has been washed away. Clasts comprise peloids (12%), sparitic fragments (3 %) and echinoderm plates (1.5 %). Nodosariid foraminifers and radiolarians are very rare. In subtype MF 2B, peloids represent 5.5 % of the area; more common are filaments (4.5 %), and possible radiolarians (2 %), whereas sparitic fragments and undetermined bioclasts occupy 1% of the area, and microproblematica (Plexoramea gracilis), foraminifers and echinoderm plates each 0.5 %. In MF 2C, there are no vugs and very few peloids. Clasts are sparitic fragments (5.5-12 %), echinoderms (2.5-6 %), and filaments (3.5-5 %).
13b-d radiolarian packstone (MF 3) Radiolarians are in rock-forming abundance, packed closely together. Very rare are thin-shelled bivalves ("filaments"). Radiolarian tests are filled with calcite or are silicified. In most samples, intergranular space is filled with micritic matrix. In fewer samples, most of the mud has been washed away and the space filled by calcitic cement. Grains represent more than 50 % of the thin section area. Average grain size is from 0.1 to 0.2 mm, with the largest grains up to 2 mm. Sorting ranges from moderately to well sorted. The majority of grains are peloids and dark intraclasts, together representing from 20 to 55 % of the thin section area. The latter may be mud chips or fragments of microbialites (distinction is rarely possible at smaller magnifications). Other grains are mostly microproblematica (2.5-4.5 %), echinoderm plates (3-8 %), locally thin-shelled bivalves (up to 11 %), very rare nodosariid foraminifers and brachiopods. Differentiation from MF 4 is by virtue of smaller grain size and better sorting of the latter. Dolomitization is locally present.
14c pebbly intraclastic grainstone (MF 10) Grading is commonly present. The largest clasts measure up to 1 cm in size, while average grain size is 0.3 to 0.5 mm (size changes with grading). Grains are angular and in point contacts. They comprise between 45-75 % of the area (on average 67 %). The most common grains are intraclasts (49.5 %). Microproblematica is more common in Ladinian and Carnian samples (7.5 % compared to 0.5 % in the Norian samples; note that this is a very conservative estimate, as point counting was performed mainly at small magnifications, which often do not allow for differentiation from micritic intraclasts). Sparitic fragments (5.9 %), echinoderms (3.4 %), and foraminifers (1 %) appear in approximately the same abundance, irrespective of the samples' age.
In the upper Carnian -lower Norian samples, mudstone, mudstone with filaments and spicules and calcimicrobes are present among intraclasts, whereas smaller grains contain peloids (small micritic intraclasts), echinoderms, foraminifers, spherulites, sparitic fragments (cortoids), agglutinated worm tubes, and brachiopod fragments. Besides fewer (or even the absence of) microbialites, microbial-sponge boundstones and microproblematica, Carnian and Norian samples differ notably in foraminiferal assemblage. Cement is represented by drusy mosaic calcite. Fibrous rim cement occurs locally. In some samples, coarse euhedral dolomite fills the intergranular space. Only one example of silicified valves was found.
14d-e intraclastic-mollusc packstone (MF 11) Grains represent approximately 70 % of the area. The intergranular space is filled with micritic matrix (27 % of the area) and drusy mosaic calcite cement (4 %). Grains are mostly in point contacts. Sorting is medium to poor, with the average size of grains around 0.35 mm and the largest grains measuring 2 mm in size.
14h   Table 2. List of determined fossils. Distribution of conodont taxa (from composite samples) is shown next to sedimentary logs in Figures 9-11. See Figure 9 for position of the radiolarian samples.

Discussion
Conodont and radiolarian data obtained in this study confirm the Longobardian age of the base of the Zatrnik Limestone. The logged succession reaches up to the Lower Norian, but basinal sedimentation clearly continued (cf. Cousin, 1981;Goričan et al., 2018). Micritic limestone with thin-shelled bivalves and radiolarians (e.g., MF 2, 3), dominating in the Zatrnik, Mt. Galetovec and Poljane sections, indicates hemipelagic sedimentation in an open marine environment. Resedimented carbonates are subordinate in these sections, but predominate in the Blejski vintgar section. Normal grading and parallel lamination suggest deposition via turbidites. Sparite-dominated varieties (MF 9, 10) are proximal turbidites, whereas micrite-rich microfacies types (MF 2,(4)(5)(6)(7)8,11) are interpreted as distal turbidites (see Maurer et al., 2003). Mud-supported bioclastic-intraclastic floatstone (MF 12) and intraclastic rudstone (MF 13) might be debris-flow deposits (Mullins & Cook, 1986;Eberli, 1991). The predomination of calcarenite in the Blejski vintgar section suggests deposition in a more proximal part of the basin. Slump structures are also more common in the Blejski vintgar section. The observed transition to massive limestone in the Poljane section remains unexplained, and is also poorly dated by fossils. One possibility is that the massive bed represents a large slump, but no internal deformations were recognised and the underlying beds do not display any deformations. In view of this last argument, we rule out the possibility that the massive bed is a large block of a platform that slid into the basin. A third explanation might suggest that the section records the transition between the basin and the slope/ margin of a platform. This alternative demands further explanation as to why the Zatrnik Limestone continued to deposit until the lowermost Jurassic, after which it is followed by gravity deposits. Perhaps the extent of progradation was limited, or a relative rise in sea levels led to the retreat of the platform. The answer may lie in the younger parts of the Zatrnik Limestone, which remain to be logged.
The predominance of micritic particles in resediments is common for the Middle Triassic -Early Carnian period, when carbonate platforms from slopes to tops were dominated by microbialites (Keim & Schlager, 1999;Russo, 2005;Schlager & Reijmer, 2009). Most grains (boundstone intraclasts, microproblematica) could thus have originated from the margin, slope or top of a carbonate platform (see Marangon et al., 2011). Rare fragments of green algae on the other hand surely derive from shallower parts of the adjacent platform. An increase in resedimented skeletal material is noted in late Tuvalian and/or early Norian (Mt. Galetovec and Poljane) sections. The Latest-Carnian increase in skeletal material is consistent with the conclusions of Martindale et al. (2017), who suggests that the shift towards the skeletal boundstones of the Dachstein-type reefs was gradual rather than sudden, and that the switch in dominant reef ecologies occurred during the late Carnian through early Norian interval.
The lack of siliciclastic input into the Bled Basin during the Carnian stands in contrast to the relatively closely situated Tolmin and Tarvisio Basins. Within the Tolmin Basin, clay-rich "Amphiclina beds" several tens (probably hundreds) of meters thick deposited during the Carnian. This formation is dominated by black shale alternating with lithic sandstone and hemipelagic limestone (Čar et al., 1981;Turnšek et al., 1982Turnšek et al., , 1984Buser, 1986), whereas the uppermost part (dated as Tuvalian) consists of hemipelagic limestone alternating with black shale (Kolar-Jurk- ovšek, 1982). In the Tarvisio Basin, the increase in the siliciclastics is reflected in the deposition of marlstone and marly limestone of the Tor Formation Gianolla et al., 2003;Gale et al., 2015). This is succeeded by a shallow-marine carbonate bank (the Portella Dolomite), followed by upper Tuvalian thin-bedded hemipelagic limestone and dolomite of the Carnitza Formation (Gianolla et al., 2003;Gale et al., 2015). The absence of clay input in the Bled Basin could perhaps be related to its slightly different palaeogeographic position (e.g. a position far from some river outlet), sea currents, slope steepness, etc. Alternatively, the clay-rich part of the formation could be thin and/or covered by vegetation and has thus simply not yet been recorded.

Conclusions
The Zatrnik Limestone marks the Bled Basin as a distinct paleogeographic unit. The following conclusions can be drawn from this study: -The base of the Zatrnik Limestone is Longobardian in age.
-The Zatrnik Limestone was deposited on a basin plain and/or distal slope of the basin. The logged succession is dominated by hemipelagic limestone and distal calciturbidites. The latter are more common in the upper Julian? to lower Tuvalian. No siliciclastic-rich interval is currently recorded for the Carnian part of the formation.
-An increase in resedimented skeletal material in calciturbidites is noted in the upper Tuvalian and/or lower Norian, marking the shift from microbe-dominated to skeletal-dominated carbonate production on the platform.