The Lower Triassic platy limestone in the Jajce area (Bosnia and Herzegovina)

The study presents palaeontological and sedimentological data of the Lower Triassic strata of the Jajce area in Bosnia and Herzegovina. Four microfacies types were differentiated: coarse grained bioclastic packstones, fine grained bioclastic packstones, laminated mudstone and laminated calcisiltites. Sedimentary characteristics of the depositional environment indicate the distal part of a wide, shallow shelf/ramp, in keeping with some other locations spanning the Dinaride chain. Characteristic ichnofossils and macrofauna are present, including ammonoids, bivalves, and gastropods, dominated by Natiria costata (Münster). Conodont elements provide a biostratigraphic framework. The older part of the section is characterized by the conodont Triassospathodus hungaricus (Kozur & Mostler) and the foraminiferans Nodosaria ex gr. skyphica Efimova, Ammodiscus ex gr. minutus Efimova, Glomospirella triphonensis Baud, Zaninetti & Broennimann and Glomospirella shengi Ho. A younger fauna yields T. triangularis (Bender), Meandrospira cheni (Ho) and M. pusilla (Ho). The older fauna belongs to the T. hungaricus conodont Zone based on presence of Spathian conodont elements, while the younger fauna belongs to the T. triangularis conodont Zone.


Introduction
The Jajce old town and the well known mills at Pliva River are often paved by Lower Triassic platy limestones, exposed in a narrow valley west of Bravnice village, southeast of Jajce (Fig. 1).
Bravnice is located in the northeastern part of the Klju~-Radu{a Nappe, near the town of Jajce (Fig. 2) in the central Dinarides of Bosnia and Herzegovina. The Klju~-Radu{a Nappe can be traced along the NW-SE strike for ca. 150 km. Its frontal parts override the Glamo~-Drežnica-Gacko Nappe, and it is underlain along its northeastern margin by the Bosnia Flysch Nappe. The Klju~-Radu{a Nappe is composed largely of Triassic carbonates, subordinate coeval clastic and igneous rocks, and sparse Permian sediments. In its frontal parts, Middle/Late Permian evaporites are exposed which may have lubricated the basal thrust plane. The northern part of the Klju~-Radu{a Nappe adjoins the Paleozoic complex of the Mid-Bosnian Schist Mts. tectonic block (Hrvatovi], 2006).
The Bravnice section lies in the Otomalj-Bukovica-Oborci tectonic unit. This unit is mainly composed of sandstone, siltstone and platy limestone of Early Triassic age. In the southeast part of this unit are sediments of Late Permian and Permian-Triassic age from the edge of the Palaeozoic Mid-Bosnian Schist Mts. (vujnovi], 1980, 1981).
The uppermost part of the Early Triassic strata of the Jajce area were discussed in numerous older works (Katzer, 1921;Puizina et al., 1969;vujnovi], 1981), in which the section has been divided into older »Seis« beds and younger »Campil« beds. The upper part of the Lower Triassic limestone (»Campil« beds) can be easily recognized by presence of ammonoids, bivalves, gastropods and ichnofossils. The basic petrographic composition of the samples from the Bravnice section does not differ significantly from the younger part of the Lower Triassic in the Dinarides and the wider Tethys area (Fig. 3), sharing dominance of micritic carbonate, occasional biodetrital accumulation of shells of molluscs, gastropods and echinoderms, as well as rare presence of siliciclastic detritus in thin layers or individual laminae. The aim of this work is to document biota and microfacies of the strata near Jajce, calibrate their age within the Olenekian based on microfossil associations, and determine the environment of deposition.

Material and methods
The material examined for this study was collected in the course of field work carried out in 2011-12 in the Jajce area in central Bosnia and Herzegovina. The Lower Triassic section, 222 m thick, was measured and sampled (Fig.4). Altogether, 18 samples (BR 1 -18) were collected for conodont processing. A minimum 2 kg of rock were prepared for conodont study using standard laboratory techniques. One to four thin sections from each level of conodont sampling were prepared to study foraminifera and carbonate petrography. The locations of the collected samples is shown in Fig. 4. The laboratory work was carried out at Geological Survey of Slovenia / Geolo{ki zavod Slovenije where the studied material is stored under catalogue numbers 5079 -5087, 5113 -5122 and abbreviated GeoZS. The conodont elements were illustrated using the JEOL JSM 6490LV Scanning Electron Microscope at the Geological Survey of Slovenia. Some macrofossil specimens are stored in the Jajce town museum.

Lithology
Four microfacies were differentiated in the Bravnice section (Fig. 5 (Fig. 5A), mostly calcitic apart from sample BR 17 which is partly dolomitized. Extremely fine lamination is due to alternation of silty siliciclast-rich and carbonate mud-rich laminae. Siliciclastic component grains are mostly quartz and partly feldspar grains. Sometimes pyrite crystals accumulate along laminae. Sample BR 12B is an exception, where the carbonate mud is not laminated, but has been disintegrated in irregular fragments due to bioturbation. Rare crinoidal elements are present. With increase in siliciclastic detritus, a graded transition occurs to laminated calcisiltites.
2) Laminated calcisiltites -samples: BR 12A, BR 15 ( Fig. 5B). In this microfacies type, thin siliciclast-rich laminae or thin beds rich in quartz and feldspar alternate with laminae of carbonate mud or siltsized bioclasts. Siliciclastic particles are poorly rounded, irregular or long and prismatic. Some laminae are extremely rich in pyrite, or in finely preserved foraminifera tests (Glomospirella, Meandrospira).

3) Fine-grained bioclastic packstones (biomicrites)
-samples: BR 6, BR 7, BR 14. This microfacies type consists dominantly of fine-grained biodetritus in a micritic matrix. Alternation of bioclastic-with matrix-rich laminae can be observed in some samples, while in others, reworking by organisms results in a chaotic distribution of fossils and carbonate mud. Among the bioclastic debris, well sorted plates of echinoderms prevail (Fig. 5C). Fairly well preserved foraminifera tests (Glomospirella, Meandrospira) are rare. Some sporadic laminae with well sorted peloids or fine grained siliciclastic detritus are present. Sample BR 13 shows lamination due to alternation of coarse-and finegrained biodetritus. 4) Coarse-grained bioclastic packstones (biomicrites) -samples: BR 1, BR 3, BR 5, BR 15. The lithotype consists of coarse-grained bioclasts consisting of bivalves, gastropods, echinoderms and ostracods within dense micrite (Fig. 5D). Molluscan fragments exceed 2 mm in size and are usually preferentially oriented parallel to bedding. Plates of echinoids and ostracod carapaces are smaller than 2 mm. Size grading from coarser bioclastic detritus to carbonate mud can be observed in sample BR 1. Bioturbation was observed in sample BR 3. Bioclasts are occasionally silicified. Euhedral pyrite crystals are often present.

Depositional environment
A common characteristic of Early Triassic sedimentary facies in the Dinarides is dominance of carbonate mud with sporadic accumulation of coarser bioclastic detritus (bivalves, gastropods and echinoderms), and rarely, thin bedded calcisiltites. The composition and petrographic features of the Bravnice section do not differ significantly from this pattern (aljinovi], 1995, Kolar-jurKovšeK et al., 2013).
The data may be explained as a background deposition of fine carbonate mud in a low energy environment (laminated mudstone facies) signifies slow deposition in where the vast quantities of finest carbonate particles can be accumulated. Pyrite indicates partly anaerobic or disaerobic episodes within the low energy environment. Settling of carbonate mud of the laminated limestone facies in a quiet-water environment alternates with sporadic influx of terrigenous quartz or feldspar grains transported by traction currents. The evidence implies deposition in deeper/distal parts of a ramp, sometimes poorly aerated, and is in line with other late Early Triassic deposits elsewhere in the Dinarides (e.g. Kolar-jurKov{eK et al., 2013).  The laminated calcisiltite microfacies formed by transport of siliciclastic or fine-grained bioclastic material to the distal parts of a ramp by weak currents. Such resedimentation of terrigenous material from the shallow ramp into the laminated mudstones may be attributed to distal storminduced currents. Infaunal bioturbation took place in the undisturbed intervals between stormy periods (sample BR 12 B).
Storm activity also explains the finer-or coarsergrained bioclastic detritus. Fine-grained bioclastic packstones with faint lamination and foraminifera tests along laminae were formed by distal transport by weak currents of molluscan bioclasts, echinoid plates and foraminifera from the shallower part of ramp. Storm transport is supported by the high degree of sorting of bioclastic material and by presence of foraminifera tht usually inhabit the shallow proximal ramp. The coarse-grained bioclastic packstones (biomicrites) represent stronger influence of short-term storm events into the outer ramp where carbonate mud was deposited from suspension. Peak storm currents caused bottom-shear conditions that concentrated shells of living and dead organisms on the sea floor, by exhuming buried shells, ripping up weakly consolidated sediments forming lags, and followed by burial by the influx of storm suspended particles. In this way coarse grained bioclastic detritus including large fragments and sometimes unbroken fossils were preferentially preserved by storm burial, protecting them from normal destructive processes. Preferential orientation of valves parallel to bedding plane also suggests deposition under short-term high energy conditions such as storms. Grading of bioclasts as observed in sample BR 1 takes place in the course of slow settling of the sediment storm cloud. Presence of pyrite indicates low-energy and partly anaerobic/ disaerobic conditions in between storms.
Biostratigraphy Macrofauna (Fig. 6) The rich gastropod, bivalve ( Fig. 6. 3-4), and ammonoid molluscan fauna of the Bravnice section ( Fig. 6. 5), as well as numerous ichnofossils The Lower Triassic platy limestone in the Jajce area (Bosnia and Herzegovina) ( Fig. 6. 1, 2, 6) can be seen on bedding planes of the limestone pavements of the Old Jajce town, as well as at the Pliva mills. The gastropod species Natiria costata (Fig. 6. 3) is most common, whereas genus Turbo appears rarely. Poor preservation of ammonoids and bivalves does not permit their determination. Some well-preserved specimens from the Bravnice section are housed in the Jajce town museum / Zavi~ajni etno muzej u Jajcu. Natiria costata is the most typical gastropod of the Early Triassic Werfen Formation in Europe, suggesting equivalence of the outcrops with Fig. 6. Ichnofossils and macrofossils from the Early Triassic strata in the Jajce area. 1 Bioturbated limestone (plate in a pavement of Jajce old town). 2 Asteroid imprint (plate in a pavement to the entrance of the Jajce fortress). 3 Natiria costata (Münster) and Turbo sp. (Museum of Jajce town / Zavi~ajni etno muzej u Jajcu). 4 Poorly preserved bivalves (plate in a pavement of Jajce old town). 5 Ammonoids (plate in a stairway to the tower in Jajce old town). 6 Ichnofossil (plate in a pavement of the mill at Pliva River).
Recently, poorly preserved Natiria cf. costata has also been reported from Spathian of Utah, usa (Hofmann et al., 2013) and Slovenia (Kolar-jurKovšeK et al., 2013). This gastropod is present also in the equivalent strata near Mu} (Dalmatia, Croatia), that has been proposed as a candidate for the European stratotype section of the late Early Triassic Prlj-Šimi], 2006).

Microfauna
The microfossil material recovered from the Bravnice section includes foraminifera and conodonts, with occurrence and abundance of taxa summarized in Fig. 7. The Conodont Colour Alteration index CAI value sensu rejebian et al. (1987) is 5.5.

Foraminifera faunas
The genera Glomospirella and Meandrospira dominate the taxonomic composition of the foraminifera assemblages (Fig. 8)   The Lower Triassic platy limestone in the Jajce area (Bosnia and Herzegovina)

Taxonomic remarks
Meandrospira cheni (Ho) and M. pusilla (Ho) are very similar, and zaninetti (1976) combined these two taxa in M. pusilla (Ho). On the other hand, later studies (Salaj et al., 1983;trifonova, 1993;rettori, 1995) considered the two forms as separate species. Both of these species can be distinguished in the Bravnice section.

A. minutus was first described in the Lower
Olenekian of the Western Caucasus; efimova (1974) wrote that some Ammodiscus incertus d'Orbigny identified by some micropalaeontologists from the Lower Triassic of the Western Europe were in her opinion A. minutus.
All species from the Bravnice section are known from the Lower Triassic and Anisian, except M. cheni, which is typical for the Lower Triassic only.

Comparison
The generic composition of the foraminiferan assemblages from the Bravnice is similar to Early Triassic assemblages from different parts of the Tethys, from the Alps to China. There are some species in common with foraminiferan assemblages from the Lower Triassic of China (Ho, 1959;He, 1993), the Olenekian of the Caucasus area (efimova, 1991), the Lower Triassic of Bulgaria (trifonova, 1992;1993), Hungary (oravecz-ScHeffer, 1987, Alps (zaninetti, 1976; broglio loriga et al., 1990; rettori, 1995), British Columbia (Sano et al., 2012).  (Salaj et al, 1983;trifonova, 1993) or in the Spathian without lowermost part (rettori, 1995). Therefore it is reasonable to conclude that both foraminiferan assemblages from the Bravnice section in Bosnia and Herzegovina is within the Olenekian. (Fig. 9) Conodont faunas of Bravnice section are predominantly ramiform elements and some can be attributed to Ellisoniidae. The fragmentation of conodont elements does not permit apparatus reconstruction. All collected P elements belong to Triassospathodus Kozur. In a contrast to Neospathodus Mosher (moSHer, 1968), elements of Triassospathodus can be easily distinguished due to downcurved posterior end of the lower margin of the basal cavity (Kozur et al., 1998).
The samples BR 11 and 15 yield rare segminate elements that are relatively short with subtriangular basal cavity developed in the posterior half. Denticles, eight or nine in number are slightly increasing in height and reveal posterior inclination. One smaller denticle is developed behind the cusp. This specimens are attributed to T. triangularis and some forms probably represent an intermediate stage (Fig. 9. 9). This portion of the section is placed into the T. triangularis conodont Zone.

Taxonomic remarks
The classification of the Early Triassic segminate pectiniform elements is still uncertain. Several views on their relationships have been put forward. According to orcHarD (1995) Neospathodus species may belong to any of several Lower Triassic groups depending on their basal profile, development of cusp, or inclination of denticles. In his opinion there exist also some unrelated homeomorphs. This genus evolved from Neogondolella in the Induan as demonstrated by similarities in composition of their apparatuses (orcHarD, 2010). On the other hand, Kozur et al. (1998) placed most Spathian segminate elements in the genus Triassospathodus, and this view is adopted in this paper.

Occurrence
T. hungaricus was first described from the Tirolites beds of Felsöörs in Hungary (Kozur & moStler, 1970). The species has been known in Slovenia so far from the Spathian of the Idrija-Žiri and Kr{ko areas (Kolar-jurKov{eK, unpublished data) and Julian Alps (Kolar-jurKov{eK et al., 2013). Some similar specimens collected in the Thaynes Group of Elko County in Nevada were illustrated and determined as »Neospathodus« cf. hungaricus (lucaS & orcHarD, 2007). The full stratigraphic range of the lowermost Spathian species T. hungaricus is not yet known and its occurrence above the T. hungaricus Zone has not yet been reported.
T. triangularis was first described from the Marmarotrapeza Formation at Chios by benDer (1970). The latest Spathian spathodid faunas that include T. triangularis were hitherto reported from the central Slovenia (Dozet & Kolar-jurKov{eK, 2007) and the Idrija-Žiri area (Kolar-jurKov{eK, unpublished data). In the southeastern continuation of the Dinarides, the species was collected also from the Mu} section in Croatia that was proposed as a standard section for the European Upper Scythian . The fauna with co-occuring elements of T. homeri and T. triangularis was also reported from the Spathian of Krivi potok in NW Serbia (SuDar, 1986a, b).

Discussion
The biostratigraphic scheme of Kozur (2003) of the Late Olenekian is largely based on Triassospathodus (Fig. 10), and the stratigraphic importance of this genus for the Spathian strata was pointed out by Kozur et al. (1998). There are six conodont zones in in the Spathian, and T. hungaricus is the nominal index species of the basal Spathian, that is equivalent to the Tirolites cassianus ammonoid zone. T. triangularis is the marker of the fourth zone (Kozur, 2003). In the shallow western Tethys, the T. hungaricus fauna lies within the lower Spathian, in the absence of the Icriospathodus collinsoni (Solien) fauna (H. Kozur -pers. comm., 2012).

Comparison of conodont faunas:
The genera Triassospathodus and Spathicuspus are dominant in the Olenekian, whereas the longranging late Olenekian species assigned to 'N.' triangularis is an uncommon conodont taxon, that ranges up to the Olenekian/Anisian boundary in the Guandao section (orcHarD et al., 2007b).
In the Deşli Caira section the LAD of characteristic conodont species determined as 'N.' triangularis was recorded in the sample GR7 representing a major faunal change of the succession (Datum 3) that involves also the FADs of Chiosella timorensis nogami and Chiosella n. sp. A (orcHarD et al., 2007a).
According to orcHarD et al. (2007b) species of the T. ex gr. homeri that includes T. symmetricus (Orchard), T. brochus (Orchard), T. sosioensis (Kozur, Krainer & Mostler) are most common and ubiquitous elements of late Spathian faunas in Eurasia and North America. The assemblage of the T. homeri group indicates late Smithian-Spathian interval in the Cache Creek Terrane succession of British Columbia (Sano et al., 2012).
T. triangularis is a widespread late Spathian taxon, present in Deşli Caira in Romania and Guandao in China (orcHarD et al., 2007a, b). It is part of the Fauna 3 that is equivalent to the Prohungarites/Subcolumbites beds within the context of North American ammonoid succession (orcHarD, 2010).

Conclusions
This study presents new sedimentological and palaeontological documentation of the remarkable Lower Triassic limestone of the Jajce area (Bosnia and Herzegovina), a candidate for the UNESCO World Heritage Site. The bioand litho-stratigraphy of the Bravnice section near Jajce has been investigated. The material is rich in ichnofossils and molluscan macrofauna dominated by Natiria costata, a widely distributed and characteristic species for the Early Triassic. Well bedded Spathian limestones of the type exploited by local inhabitants for construction and paving in the old Jajce town, have been investigated from east from the Bravnice village. Four microfacies types were differentiated: 1) laminated mudstonemicrite, 2) laminated calcareous siltstone, 3) finegrained bioclastic packstone -biomicrite and 4) coarse-grained bioclastic packstone -biomicrite. The depositional environment is characteristic of the outer part of a wide shallow shelf/ramp, but subject to distal storm events. Between storms, bioturbated sediments are formed, but may be interrupted by episodes of poor aeration and anoxia. These features have similarly been suggested also for other locations along entire Dinaride area.
The Bravnice section represents an important reference section for the Olenekian of Bosnia and Herzegovina based on Spathian (Olenekian) conodont faunas and the accompanied fossil association. The recovered faunas consist of important correlative elements for comparison with co-eval faunas elsewhere in the Dinaride area, and for the Early Triassic Tethys worldwide.
These Lower Triassic limestones of Bravnice are of great importance for the restoration of old buildings in this area, and particularly for reconstruction of the Jajce town, a candidate for the UNESCO World Heritage List. They can also be used for new architectural projects that should conform to the local style. Such source materials should play a much more significant role in reconstruction, and are much preferred to nonautochthonous magmatic and metamorphic rocks that have unfortunately been used in the past.