A sedimentological description of the Middle Triassic vertebrate-bearing limestone from Velika planina, the Kamnik-Savinja Alps, Slovenia

Abstract

In the Kamnik-Savinja Alps (Slovenia), the Lower Serla Dolomite laterally passes into a succession of thin- to medium-bedded bituminous limestones of the Velika planina member. The finely laminated lower part of this member contains well-preserved actinopterygian fish and sauropterygian remains. The research aimed to determine the sedimentological and palaeoenvironmental characteristics of the depositional basin on the basis of three detailed sedimentological sections logged atop the Velika planina plateau. The Velika planina member is underlain by a whitish to light grey, thick bedded to massive dolomite with oncoids, stromatolites, and lumachellas deposited under peritidal to shallow subtidal conditions. The lower part of the Velika planina member consists of thin, often platy, finely laminated beds of bituminous mudstone. The Chondrites ichnofossil is very common; however, in some beds numerous lingulid brachiopods, bivalves, and crinoids were observed. Fossil vertebrates and crustaceans are relatively rare and confined to a few levels. Ammonoids are very rare. Subordinate beds of intraclastic-peloid wackestone to packstone, intraclastic-bioclastic grainstone, and bivalve floatstone occur. Slumps are common. Upwards, bedding gradually becomes thicker and bioturbation more common. Finally, stromatolites, birdseye fenestrae, and oncoids reappear. The entire succession is confined to the early to middle Anisian by the foraminifer Citaella dinarica (KOCHANSKY-DEVIDÉ & PANTIĆ). The absence of breccias at the base of the Velika planina member, the gradual transition upwards into shallow marine carbonates, as well as the presence of sauropterygians of the order Nothosauroidea suggest deposition in a relatively shallow basin. The finely laminated facies of the lower part of the member indicates a stratified water column, with oxygenated near-surface waters and hypoxic to anoxic conditions near the sea floor.

Headnote

Article history:

Manuscript received June 22, 2021

Revised manuscript accepted October 19, 2021

Available online January 31,2022

Keywords: Southern Alps, intraplatform basin, Anisian, sedimentology, vertebrates, Sauropterygia

Abstract

In the Kamnik-Savinja Alps (Slovenia), the Lower Serla Dolomite laterally passes into a succession of thin- to medium-bedded bituminous limestones of the Velika planina member. The finely laminated lower part of this member contains well-preserved actinopterygian fish and sauropterygian remains. The research aimed to determine the sedimentological and palaeoenvironmental characteristics of the depositional basin on the basis of three detailed sedimentological sections logged atop the Velika planina plateau. The Velika planina member is underlain by a whitish to light grey, thick bedded to massive dolomite with oncoids, stromatolites, and lumachellas deposited under peritidal to shallow subtidal conditions. The lower part of the Velika planina member consists of thin, often platy, finely laminated beds of bituminous mudstone. The Chondrites ichnofossil is very common; however, in some beds numerous lingulid brachiopods, bivalves, and crinoids were observed. Fossil vertebrates and crustaceans are relatively rare and confined to a few levels. Ammonoids are very rare. Subordinate beds of intraclastic-peloid wackestone to packstone, intraclastic-bioclastic grainstone, and bivalve floatstone occur. Slumps are common. Upwards, bedding gradually becomes thicker and bioturbation more common. Finally, stromatolites, birdseye fenestrae, and oncoids reappear. The entire succession is confined to the early to middle Anisian by the foraminifer Citaella dinarica (KOCHANSKY-DEVIDÉ & PANTIĆ). The absence of breccias at the base of the Velika planina member, the gradual transition upwards into shallow marine carbonates, as well as the presence of sauropterygians of the order Nothosauroidea suggest deposition in a relatively shallow basin. The finely laminated facies of the lower part of the member indicates a stratified water column, with oxygenated near-surface waters and hypoxic to anoxic conditions near the sea floor.

1.INTRODUCTION

The Triassic was an important period for the recovery of life on Earth after the end-Permian extinction. Not only were new ecosystems established, but new animal lineages came to play key roles in the emerging Modern Evolutionary Fauna (PAYNE & VAN DE SCHOOTBRUGGE, 2007; BENTON, 2016; BUATOIS et al., 2016). Among such groups were also the actinopterygian fishes (TINTORI et al., 2014a, b) and marine reptiles (MOTANI, 2009; STUBBS & BENTON, 2016; BRAYARD et al., 2017). The most famous finds of the latter come from the classic deposits of the Germanic Basin (RIEPPEL, 1999; HAGDORN & RIEPPEL, 1999; DIEDRICH, 2012), and the Southern Alps of the Monte San Giorgio area (FURRER, 1995a; RÖHL et al., 2001; STOCKAR, 2010; HUGI et al., 2011; FURRER & VANDELLI, 2014; RENESTO et al., 2014; BEARDMORE & FURRER, 2016, 2019), as well as from more recently discovered sites in southern China (LIU et al., 2011; BENTON et al., 2013).

As recently as 20 years ago, reports of sauropterygians from the Triassic beds of Slovenia were scarce. DEECKE (1886) mentioned remains of a marine reptile collected from the scree below Mt. Storžič in the Kamnik-Savinja Alps. The fossil was later described as a pachypleurosaur of the Serpianosaurus - Neusticosaurus clade (RIEPPEL, 1997). More than two centuries later, new discoveries started to emerge: a placodont dentary bone from the upper Ladinian or lower Carnian beds of Toško Čelo near Ljubljana (BUFFETAUT & NOVAK, 2008), isolated bones and teeth of sauropterygians from the Carnian shallow marine deposits in the Poljane Valley (KRIŽNAR, 2009), pachypleurosaur, placodont, and ichthyosaur bones from the Anisian Strelovec Formation (HITIJ & RENESTO, 2010), and sauropterygian bones from the Ladinian Buchenstein Formation in the Kamnik-Savinja Alps and the Southern Karavanke Mountains (HITIJ et al., 2010b). However, most of the new finds of sauropterygians came from the Anisian beds of the Velika planina member on the Velika planina mountain plateau (Fig. 1), where they occur together with a number of actinopterygian fish genera of the so-called "Early Anisian Fish Fauna" (HITIJ et al., 2010a; TINTORI et al., 2014a). Despite the importance of these finds and the large potential for future palaeontological research, the stratigraphic position of the Velika planina member remains poorly constrained, with no information on its lower and upper boundaries. Moreover, the sedimentological composition of the member as a whole and its microfacies association have not been previously described.

The aim of this paper is to fill this gap by presenting several key sections of the Velika planina member. We provide direct evidence for the early to middle Anisian age of the member on the basis of determinations of foraminifers. Based on the detailed description of microfacies associations and currently known fossils from this member, we try to provide some answers about the relative depth of the depositional area, the type of sedimentation, and the palaeoenvironmental conditions on the sea floor.

2.STRUCTURAL SETTING OF THE KAMNIKSAVINJA ALPS

Velika planina is a karstified mountain plateau with an average altitude of 1500 m, located in the southern part of the KamnikSavinja Alps (Fig. 1). In terms of regional tectonic structure, the Kamnik-Savinja Alps belong to the eastern Southern Alps, shaped during the Cretaceous to Cenozoic Alpine orogeny (PLACER, 1999; SCHMID et al., 2008). Together with the Southern Karavanke Mountains, they represent a mega shear lens between two major dextral strike-slip faults in the Periadriatic Fault Zone: the Periadriatic Fault to the north and the Sava Fault to the south. While the main, northern branch of the Periadriatic Fault formed during the Oligocene and was later reactivated several times, the main movements along the Sava Fault took place after the latest Miocene (VRABEC & FODOR, 2006). The two dextral strikeslip faults are connected by left strike-slip faults running in NESW directions. Palaeomagnetic directions from Oligocene to Neogene sediments indicate domino-like block rotations along these faults (FODOR et al., 1998; VRABEC & FODOR, 2006; VRABEC et al., 2006). Prior to the formation of the shear zone, the area experienced two major phases of thrusting: the NE - SW directed Eocene - Oligocene thrusts preceded the N - S directed Miocene - recent thrusts (PLACER, 1999; PONTON, 2014). In the central part of the Kamnik-Savinja Alps, MIOČ et al. (1983) distinguished the so-called Savinja Thrust overlying the Southern Karavanke Thrust. CELARC (2004) later found no evidence of a thrust in the northern part of the Kamnik- Savinja Alps, which called into question the existence of the Savinja Thrust in that area. DOLŽAN (2017) recently confirmed the existence of a detachment surface (named the Kočna Detachment Surface, KDS) in the central Kamnik-Savinja Alps, dipping 8° to the E. However, no evidence for thrusting was found, and the KDS was interpreted as a low-angle normal fault, that is towards the NW successively lowered along normal faults until it is no longer exposed.

3.TRIASSIC STRATIGRAPHY OF THE KAMNIKSAVINJA ALPS

Systematic geological research of the Kamnik-Savinja Alps started in the second half of the 19th century (LIPOLD, 1856; ROLLE, 1857; TELLER, 1885, 1898 a, b). Modern geological maps were produced by BUSER and CAJHEN (1977), MIOČ et al. (1983), PREMRU (1983a), and CELARC (2004). Accompanying explanatory books were written by BUSER (1980), MIOČ (1983), and PREMRU (1983b). Stratigraphic research, focused on Triassic deposits, was carried out by RAMOVŠ (1973), JURKOVŠEK (1984), GORIČAN & BUSER (1990), RAMOVŠ & JAMNIK (1991), CELARC (2004), CELARC & GORIČAN (2007), CELARC et al. (2013, 2014). The Triassic succession preserved in the Kamnik-Savinja Alps first reflects the existence of a mixed siliciclastic - carbonate ramp in the Early Triassic and then the gradual establishment of a flat and uniform carbonate platform, the formation of various smaller intraplatform basins with accompanying volcanism from the Anisian until the Late Ladinian, regional drowning in the Late Carnian, and progradation of the platform in the Norian (BUSER, 1989, 1996). The development took place at the western passive margin of equatorial Pangea, close to the Palaeotethys and from the Middle Triassic onwards also next to the Neotethys Ocean (HAAS et al., 1995; STAMPFLI & BOREL, 2002; STAMPFLI & KOZUR, 2006). In the Kamnik-Savinja Alps the Lower Triassic is represented by sandstones, marly limestone, and oolitic limestone belonging to the Werfen Formation (Fig. 2). This gradually passes into the lower Anisian dolomite (or limestone), equivalent to the Lower Serla Dolomite in the central Southern Alps (GIANOLLA et al., 1998). The Lower Serla Dolomite laterally and vertically passes into a succession of thin- to medium-bedded, locally finely laminated bituminous limestone several tens of metres thick, here referred to as the Velika planina member. The unit locally contains isolated bones and partial to complete vertebrate skeletons. On the Velika planina plateau, the Velika planina member gradually passes upwards into medium-bedded and then massive limestone and dolomite still belonging to the Lower Serla Dolomite. On the Velika planina plateau, this dolomite is in a faulted contact with the limestone and dolomite of the Schlern Formation. Elsewhere in the Kamnik-Savinja Alps (as well as in parts of the Julian Alps), where the contact is not disturbed by younger faults, the Lower Serla Dolomite passes upwards into a unit of marlstone, mudstone, thin- to medium-thick bedded limestone and dolomite of the middle to upper Anisian (Pelsonian - early Illyrian) Strelovec Formation which is up to 60 m thick (CELARC et al., 2013; MIKLAVC et al., 2016; PRIMOŽIČ, 2020). The Strelovec Formation is concordantly overlain by the upper Anisian Contrin Formation, comprising shallow marine bedded and massive limestone, or locally dolomite (CELARC et al., 2013). The Contrin Formation is locally dissected by palaeofaults. Small half-grabens are filled with red radiolarian-bearing limestones of the Illyrian Loibl (Ljubelj) Formation, volcanic and volcaniclastic rocks, polymict breccias, and conglomerates (equivalent to the Ugovizza/Uggowitz Breccia), marls, and hemipelagic limestones (equivalent to the Buchenstein/Livinallongo Formation). Upwards follow several hundred metres of massive limestone of the Ladinian Schlern Formation (CELARC et al., 2013). The growth of the Schlern platform was interrupted during the late Ladinian (Longobardian). Volcaniclastics, platy limestone with chert, and calcarenites of the Korošica Formation were locally deposited (JURKOVŠEK, 1984; CELARC, 2004; ŽALOHAR & CELARC, 2010). Elsewhere, the top of the Schlern Formation is marked by breccias, indicating a stratigraphic gap between the Schlern Formation and the lower Carnian peritidal Razor Limestone. During the late Carnian, approximately 25 m of hemipelagic Martuljek Platy Limestone was deposited. In distal positions relative to the prograding Dachstein platform approximately 150 m of lower Norian limestone with chert follows. Finally, after a narrow prograding reef margin, a thick succession of peritidal limestones of the Dachstein Formation deposited over the wider area (CELARC et al., 2013).

4.GEOLOGICAL MAP AND SETTING OF THE VELIKA PLANINA PLATEAU

Geological maps of the Velika planina mountain plateau were produced by TELLER (1898a, b), and PREMRU (1983a, b). Reambulation of the geological map covering the research area was performed by one of the authors of the paper (B.V) in 2014. The southern part of the Velika planina plateau is dominated by Middle Triassic carbonates, i.e. the Anisian Lower Serla Dolomite and the Velika planina member (Fig. 3). To the north, these two Anisian units are in faulted contact with the younger shallow marine dolomites and subordinate limestones of the Ladinian Schlern Formation. To the south, the Lower Serla Dolomite is in normal stratigraphic contact with the Lower Triassic Werfen Formation. The latter is thrusted over the Middle Triassic dolomite. Lineation on the thrust plane indicates initial thrusting to the SW, overprinted by younger thrusting to the S. The area is additionally transected by steep faults of SSW-NNE, NE-SW, NEE-SWW, and NW-SE strikes, respectively. We were unable to determine whether some of these are reactivated synsedimentary faults.

5.MATERIALS AND METHODS

The Velika planina member was logged and sampled in three sections on top of the Velika planina plateau (Figs. 3-4). In addition, samples from the Lower Serla Dolomite below the Velika planina member were collected from outcrops at the "Jarški dom" hut (samples Jar 1-17 on Fig. 4), and an additional sample from a massive limestone overlying the Velika planina member was taken at "Dovja raven" (sample DR-K). Altogether, 71 thin-sections 28 x 47 mm in size were made. Selected thin-sections were stained with Alizarin Red. Textures were defined according to classifications by DUNHAM (1962), and EMBRY & KLOVAN (1971). In determining floatstone, we follow the criteria of WRIGHT (1992), considering matrix as mud to silt-sized grains. Textures were also defined based on a comparison with examples in FLÜGEL (2010). When adding the constituents to the name of the carbonate, we follow the reverse ranking order, putting the dominant component first (e.g. packstone dominated by bioclasts and with subordinate intraclasts was named bio-intraclastic packstone), as suggested by WRIGHT (1992). The fossil material was collected from several outcrops of the Velika planina member. All specimens are housed within the Hitij & Žalohar Palaeontological Collection, curated, and registered according to Slovenian legislation in the Slovenian Museum of Natural History, Ljubljana, Slovenia.

6.LITHOLOGICAL COMPOSITION OF THE STUDIED SECTIONS

Lower Serla Dolomite

The Velika planina member is underlain by whitish to light grey thick bedded to massive dolomite some 100 m thick, sampled at the locality "Jar" (see Fig. 3). The dolomite macroscopically contains numerous oncoids, stromatolites, bivalve and gastropod lumachellas. Several foraminifers, including Citaella dinarica (Kochansky-Devidé & Pantie) have been found in several microfacies types. Microfacies types comprise mudstone, partly washed bioclastic wackestone, partly washed intraclastic-bioclastic wackestone to packstone, dolomitized dasycladacean grainstone, microbial intraclastic rudstone, dolomitized bivalve rudstone, microbialite boundstone, and microbialite wackestone or bafflestone (Table 1; Fig. 5).

Velika planina member - section VP1

The transition from light grey Lower Serla Dolomite into darker, thinner, and finely laminated limestone beds of the Velika planina member appears to be gradual. The basal part of the Velika planina member was logged in the 26 m long section VP1 (Fig. 6). Platy to thin-bedded dark bituminous and finely laminated micritic limestone predominates. Slumps are common. Subordinate are beds of calcarenite up to 15 cm thick, which are lighter in colour and without lamination. Seven microfacies types are determined, namely mudstone, finely laminated mudstone, Earlandia mudstone, peloid wackestone with ostracods, partly washed intraclastic-peloid wackestone to packstone, intraclastic-bioclastic grainstone, and bivalve floatstone (Table 2; Fig. 7). Small-scale bioturbations are locally present.

Velika planina member - section VP2

Section VP2 comprises dark brown to almost black micritic limestone 10.5 m thick in total. Most beds are thicker than in section VP1 from the lowermost part of the member, measuring between 15 cm and 1 m in thickness. Lamination is commonly present, but the laminae are also much thicker (approximately 1 cm). Subordinate are platy to thin-bedded beds with finer lamination, which no longer form thicker bundles. Slumping of the sediment is evident, and intradastic rudstone ("flat-pebble breccia") is locally present. Mudstone and mudstone with filaments and sessile foraminifers (Table 3) are the only microfacies types in this section.

Velika planina member - section VP3

The uppermost part of the Velika planina member was logged in section VP3. The total length of the section is approximately 72 m, but large parts of it are poorly exposed. The bedding, however, remains constant. Platy to thin-bedded dark brown mudstone predominates in the first 16 m of the section. Subordinate are densely bioturbated beds of micritic limestone 70 - 100 cm thick. Above, thin to medium-thick beds of light grey limestone follow. Possible neptunian dykes were observed in one bed. Above the 30-metre mark of the section the limestone becomes light grey in colour. Wavy lamination and birdseye fenestrae are commonly present. Calcarenite is present along with micritic limestone, and oncoids are present in the uppermost part of the section. Microfacies types from the VP3 section are bioturbated mudstone with sparse bioclasts, microbial laminated mudstone (stromatolite), microbially-bound grainstone with fenestral fabric (stromatolite), partly washed intraclastic-peloid wackestone to packstone, peloid wackestone, bioclastic-peloid grainstone, oncoid floatstone, peloid-bioclastic grainstone, and mud-supported breccia (Table 4; Fig. 8). Citaella dinarica (KOCHAN SKY-DEVIDÉ & PANTIĆ) was found approximately 20 m higher in a seemingly massive limestone (sample DR-K in Fig. 4).

Macrofossils of the Velika planina member

Macrofossils can be found throughout the entire Velika planina member; however, they are most frequent in its lower part (section VP1). The trace fossil Chondrites is relatively commonly present in the laminated bituminous limestone of the Velika planina member (Fig. 9c,d). In the lower part of the member there are several beds with accumulated imprints of the bivalves of the genus Modiolus. The shells are only rarely preserved. Bakevellia costata (Fig. 9a) is rarer, as well as some well-preserved crinoid specimens (Fig. 9b). The latter belong to the genera Dadocrinus and Holocrinus. One complete, fully articulated crinoid specimen probably belongs to a new yet undescribed crinoid family. Small ammonoids are extremely rare. The organisms with predominantly chitinous shells are generally well preserved. In some beds we find numerous lingulid inarticulate brachiopods. The inarticulate brachiopods of the genus IDiscinisca are rarer and occur only as single specimens. Other brachiopods are also present, but are not sufficiently preserved for a determination. Crustacean fossils are very rare (Fig. 9d), e.g. mass mortality beds with lobsters in their larval stage. Vertebrate fossils are more common in the lower part of the member. Some specimens are completely articulated and can even bear impressions of soft tissues. Others show partial disarticulation (Fig. 9f). Several remains of sauropterygian reptiles of the order Nothosauroidea were discovered, among them also a complete, fully articulated specimen more than 1 m long. Several genera of actinopterygian fish are also present (see TINTORI et al., 2014a). These include at least two different species of the genus Saurichthys, together with Eosemionotus, Placopleurus, Furo (Fig. 9e), at least one other neopterygian fish, probably closely related to the basal semionotiform Sangiorgioichthys, and the isolated remains of a coelacanth. Coprolites are quite abundant (Fig. 9g).

7.DISCUSSION ON CHARACTERISTICS AND EVOLUTION OF THE BASIN

Triassic rocks, which today form the majority of the KamnikSavinja Alps, were deposited on the continental shelf offshore from the eastern equatorial Pangea (HAAS et al., 1995; STAMPFLI & BOREL, 2002; STAMPFLI & KOZUR, 2006). No signs of tectonic activity during the Early Triassic have been reported from this region so far, and a shallow marine mixed carbonatesiliciclastic shelf developed. With the rising sea level, carbonate sedimentation prevailed, giving rise to the Anisian carbonate platforms (BUSER, 1989, 1996). The Velika planina member, underand overlain by light-coloured peritidal carbonates, testifies to a relative deepening of part of the Anisian platform, probably due to local tectonic subsidence. The finding of the foraminifer Citaella dinarica, both in the underlying rocks and in the transitional interval to the overlying peritidal limestones, confines the deposition of the Velika planina member to the early to middle Anisian (RETTORI, 1995; UENO et al., 2018). This age estimation is consistent with the previously estimated pre-Pelsonian age of the Velika planina member based on the biostratigraphy of fossil fishes (TINTORI et al., 2014a).

The diversity of bioclasts, which include echinoderms and dasycladacean algae, and the light grey colour of the underlying massive or poorly bedded Lower Serla Dolomite suggests initial deposition in an oxygenated shallow marine environment of a carbonate platform. The platform was microbially-dominated, as evidenced by the abundance of microbialites. Bivalve and gastropod lumachellas represent local accumulations of shells.

The absence of breccia at the base of the Velika planina member and a gradual shift from light grey intertidal dolomites to dark bituminous limestone suggest a gradual deepening and the subsequent establishment of a shallow restricted basin. Due to the limited areal extent of the basin, the deepening was probably caused by subsidence along normal faults. Slumping is especially common in the lower and middle part of the Velika planina member (sections VP1, VP2), but was not observed in the uppermost part (section VP3). This could suggest the basin was deeper in its early evolution, or (more likely) initially a more active tectonic subsidence, with a quiescence of tectonic activity later on. The relatively shallow nature of the basin, or at least the lack of connection with an open sea, is supported by the nearabsence or even lack of open-marine plankton and nekton, such as radiolarians and ammonoids, within the Velika planina member and a subsequent gradual transition into oxygenated shallow marine facies within section VP3.

The lower part of the Velika planina member (section VP1) is recognised by the predominance of finely laminated mudstone. The facies association here is similar to the lower Ladinian Meride Limestone of Monte San Giorgio in southern Switzerland/ northern Italy (FURRER, 1995a; STOCKAR, 2010: "laminate lithofacies"), the upper Anisian to lower Ladinian Posanto Formation in eastern Switzerland (BÜRGIN et al., 1995; FURRER, 1995b, 2019), and the Cretaceous Komen limestone in Slovenia (PALCI et al., 2008). At this stage, the water column was likely stratified, with oxygenated near-surface layers and hypoxic to anoxic conditions at the bottom. This is suggested by the vation of as well as articulated vertebrate skeletons. Mudstone, finely laminated mudstone, and Earlandia mudstone all represent background sedimentation. The darker laminae are richer in organic matter and could represent microbial mats on the basin floor. The general absence of larger benthos, the bituminous nature of the limestone, and the fine lamination of the sediment suggest poorly ventilated bottom conditions. This is supported by the profuse presence of Chondrites and the near exclusion of all other trace fossils (BROMLEY & EKDALE, 1984). However, the local presence of bioturbations, small and rare bivalves, rare crinoids, and even the presence of small benthic foraminifers, such as Earlandia and thin-shelled lagenids, suggest an occasional presence of free oxygen in the uppermost sediment. Earlandia is considered a benthic opportunist (KRAINER & VACHARD, 2009), and thin-shelled lagenids were tolerant of oxygen-poor conditions (STOCKAR, 2010). Partly washed intraclastic-peloid wackestone to packstone and intraclastic-bioclastic grainstone, on the other hand, indicate deposition in a more energetic setting. Size-sorting of particles indicates transport, and at least some of the bioclasts (e.g., foraminifers Glomospira sp., Endothyracea, and mollusc fragments) probably originated from a better aerated shallow marine environment. We suggest deposition from diluted turbidity currents. Parautochthonous or allochthonous origin is also suggested for peloid wackestone with ostracods, and bivalve floatstone.

Higher up in the succession (sections VP2, VP3), beds are thicker and lamination is less pronounced. More sediment was perhaps shed from the surrounding shallow carbonate platform, or perhaps the conditions on the sea floor became more stable and less restricted. In the last of the logged sections (VP3), the intraplatform basin's deposits gradually give way to the well oxygenated shallow platform carbonates of the platform top. Bioturbation is common in the lower part of the VP3 section, and from the 30-metre mark upwards there are several indicators of peritidal conditions: wavy stromatolites, fenestrae, even a foraminiferal assemblage, dominated by glomospiral forms. Grainstones with various bioclasts, especially fragments of molluscs, again suggests an environment densely populated by benthic invertebrates. Finally, oncoids from the top of the section indicate the presence of waves and/or currents.

Even though the Velika planina member was initially considered to represent a deeper marine basin (HITIJ et al., 2010a), the evidence presented above tends to support the idea of a rather shallow intraplatform basin. This is also supported by the vertebrate remains found in the lower part of the Velika planina member. Among the groups of marine reptiles, only nothosauroids have been identified so far, and these were limited to shallow intraplatform basins and shallow epicontinental seas (RIEPPEL, 1999; ČERNANSKY et al., 2018).

8.CONCLUSIONS

The Velika planina member was deposited within a relatively shallow and restricted intraplatform basin during the early to middle Anisian. The basal part of the member is dominated by finely laminated mudstone deposited in a stratified basin with mainly hypoxic to anoxic bottom conditions. Actinopterygian fishes and marine reptiles are well preserved in this facies due to limited bioturbation and scavenging on the sea floor. Moving up - wards, restricted facies gradually gives way to well-oxygenated shallow platform carbonates of the Lower Serla Dolomite.

ACKNOWLEDGEMENT

This research was financially supported by the Slovenian Research Agency (research core funding No. P1-0011 to L.G. and B.C., core funding No. P1-0195 to M.V. and J.Ž., research project 1555-007-P1-0195 to J.Ž.), and Quantectum AG (funding to J.Ž. and B.V.). We thank the technical staff of the Geological Survey of Slovenia for the preparation of the thin sections. Finally, we are very grateful to Heinz FURRER and an anonymous reviewer for their positive attitudes and approaches, as well as for their concrete and constructive remarks.

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