Stratigraphy of Upper Permian and Lower Triassic Strata of the Žiri Area (Slovenia)

Clanek obravnava stratigrafijo zgornjepermskih in spodnjetriasnih plasti v profilu Lukac na žirovskem ozemlju. To je zaenkrat edini profil v Zunanjih Dinaridih, v katerem je dolocena permsko-traisna meja po mednarodnih kriterijih na osnovi prvega pojava konodontne vrste Hindeodus parvus. Formalizirane so naslednje litostratigrafske enote: Bellerophonski apnenec in Evaporitno-dolomitni clen Belerophonske formacije ter Lukac formacija s tremi cleni: Prehodne plasti, Pasnati apnenec in Karbonatno-klasticni clen. V clanku podajemo tudi rezultate mikropaleontoloskih raziskav na osnovi foraminifer in konodontov ter rezultate petrografskih in sedimentoloskih reziskav. Analiza konodontnih združb je omogocila uvedbo konodontne bioconacije permsko-triasnega intervala reziskanega profila Lukac.


Introduction
At the end of the Permian Period and the beginning of the Triassic the most severe mass extinction in Phanerozoic life history occurred where up to 96% of all existing biota was lost (sePkoski, 1984, hallaM & WiGnall, 1997. This event has stimulated scientists all over the world to study this phenomenon and many hypotheses have been put forward but the extinction cause remains still unknown. An extraterrestrial impact in the Permian-Triassic boundary (PTB) interval appears unlikely (isozaki, 2001). Possible causes of this catastrophic event are connected with large scale environmental changes such as volcanic eruptions, enhanced atmospheric carbon dioxide, rapid climate change with gradual warming of the planet, changing ocean chemi stry (anoxia, salt content, oxygen, carbon, sulphur, strontium isotopes) and changing sea level as well as fungal virulence (BauD et al., 1989;WiGnall & tWichett, 1996;erWin et al., 2002;Bottjer, 2004;visscher et al., 2011).
The Permian-Triassic boundary (PTB) interval beds have long been topic of numerous studies globally that intensified after the establishment of the Permian-Triassic Working Group (PTWG) in 1981.
Important records of the PTB extinction event have been established in the Tethyan realm. The section at Meishan in South China was chosen as the Global Stratotype Section and Point (GSSP) of the PTB that was ratified by IUGS in 2001 (yin et al., 2001). Stratigraphically the most important conodont species across the PTB belong to the genera Hindeodus and Isarcicella. The first appearance datum (FAD) of the species Hindeodus parvus in the middle of Bed 27 (27c) in the Meishan D section in South China marks the base of the Triassic System. The approved proposal clea rly separates the event stratigraphic and biostratigraphic boundaries and is of key importance, not only for defining the GSSP of the basal Triassic boundary, but also to study the Permian-Triassic mass extinction and recovery (WanG, 1999). Hindeodus parvus is an easily recognizable species with wide geographic distribution and it is the first globally distributed species that appears just above the minimum faunal diversity indicated by a minimum in δ 13 C and has no facies restriction (kozur, 1996; kozur et al., 1996).
During the last decade, study of the PTB interval has been intensified in the Dinarides and the presence of hindeodids and isarcicellids was documented.
In Croatia, H. parvus was reported from the Školski Brijeg section of the Gorski Kotar region (aljinovi} et al., 2006), whereas a recovery of Isarcicella is documented in the Plavno section of the Knin area (aljinovi} et al., 2011). The two faunas are assigned to the Griesbachian parvusisarcicel la and isarcica zones.
In Serbia, an extensive biostratigraphic study was carried out in the Komiri} section of the Jadar Block in the Vardar Zone of NW Serbia where elements of Hindeodus typicalis belonging to the Lower praeparvus Zone (Changhsingian) were collected (suDar et al., 2007;nestell et al., 2009;crasquin et al., 2010).
In the Idrija-Žiri area, the Upper Permian is represented by the Bellerophon Formation (also named the Žažar Formation in Slovenia) consisting of limestone and dolomite with a thickness of 60m to 350m (raMov{, 1958;GraD & oGorelec, 1980;Buser et al., 1989;Mlakar & Placer, 2000;skaBerne & oGorelec, 2003). The Žažar Formation is an equivalent of the Bellerophon Formation in the Carnic Alps and the Dolomites of Austria and Italy (FarBeGolli et al., 1986;holser & schöulauB, 1991) and therefore this term is eliminated herein. In the Žiri area, the strata of the lowermost Triassic are known as the "Streaky Limestone Member" of the Werfen Formation with a thickness of up to 40m (Mlakar, 2002). The Permian-Triassic boundary was mostly »hidden« within the dolomitic beds of the Bellerophon Formation (GraD & oGorelec, 1980;Buser et al., 1989;Mlakar & Placer, 2000). In the sections where the dolomite member is not developed, the PTB was traditionally defined lithologically between the dark gray algal limestone of the Bellerophon Formation and light-medium gray thin bedded limestone that is already of Triassic in age (Buser, 1986). In the Masore section near Idrija, west from Žiri, Dolenec et al., (2004) described the disappearance of skeletal algae below the lithological boundary and above it the appearance of ostracodes, echinoderms, foraminifers of the genus "Earlandia", filaments of cyanobacteria and mollusk shells. In the same section, the authors documented a negative shift of organic carbon in an aproximatelly 50 cm thick interval and it coincides more or less with the lithological boundary (Dolenec et al., 2004). An earliest Triassic age of the laminated limestone in the Masore section is based on the appearance of foraminifers Earlandia tintinniformis and the annelid Spirorbis phlyctaena (Buser, 1986), both of which have been known only from Triassic strata in many regions (BrönniMann & zaninetti, 1972;Brönni-Mann et al., 1972). According to the foraminifers, the lower boundary of the Triassic worldwide has been drawn on the basis of the appearance of the species "Cyclogyra" (= "Cornuspira") mahajeri and Rectocornuspira kalhori, especially in sections where conodonts could not be found (Brön-niMann & zaninetti, 1972;BrönniMann et al., 1972;altiner & zaninetti, 1981;köylüoGlu & altiner, 1989;rettori, 1995;Groves et al., 2005Groves et al., , 2007. Recently, one of the sections of the Permian-Triassic interval in the Žiri area, the Luka~ section, was studied biostratigraphically with the documentation of a Hindeodus-Isarcicella conodont population through this interval (kolar-jurkov{ek & jurkov{ek, 2007). The species Hindeodus parvus (Kozur and Pjatakova) was found in sample L1 in the Transitional Beds (kolar-jurkov{ek et al., 2011) permitting an accurate placement of the Permian-Triassic boundary in the Luka~ section. Because the precise position of the lower boundary of the Triassic in the Luka~ section is established based on conodonts, the distribution of the associated foraminifers around this boundary also was examined in detail (nestell et al., 2011). In the Luka~ section, the species "Cornuspira" mahajeri, "Earlandia" gracilis and "E." sp. have been found in the Transitional Beds below the first occurrence of the conodont species Hindeodus parvus. These foraminiferal taxa are considered to be ecological species and should not be used as stratigraphic markers (nestell et al., 2011).
The Permian-Triassic interval of the Luka~ sec tion in western Slovenia was studied sedimentologically and micropaleontologically by using conodonts and foraminifers. The following lithostratigraphic units are formalized herein: the Bellerophon Limestone and Evaporite-dolomite Members of the Bellerophon Formation and the Luka~ Formation with the Transitional Beds, Streaky Limestone and Carbonate-clastic Members in ascending order.

Materials and methods
The study in the Luka~ section started in 2006 and was focused on the PTB interval only (kolar-jurkov{ek & jurkov{ek, 2007). In the next three years additional sampling of the entire section was carried out. The present study is based on conodont collections recovered from 53 samples that produced conodonts out of the 124 processed carbonate samples. The conodont fauna is assigned to 15 species of several genera (kolar-jurkov{ek et al., 2011).
The foraminiferal study is based on the examination of 38 samples collected from the uppermost Permian beds, Permian-Triassic Transitional Beds and lowermost Triassic strata. Foraminifers were found in twenty one samples from which thin sections were made. A few recrystallized free specimens were obtained in the conodont residues. For studying the internal morphology, some thin sections were made from free specimens and the tiny specimens were studied with Cargille Meltmount (nestell et al., 2011).
For the petrographic purposes 117 samples have been studied. They were stained with K-fericyanid and Alizarin Red S aiming to determine dolomite and the dedolomitization processes.
The sampled horizons are shown in Figs. 2, 3.

Geological setting
The Luka~ section is situated in the Žiri area of the north-western part of Slovenia that belongs to the External Dinarides geotectonic unit ( Fig. 1). A wider area is composed of Carboniferous, Permian and Triassic rocks. The Carboniferous is represented by clastic rocks with prevailing black shale, sandstone and conglomerate. These strata are discordantly overlain by Middle Permian beds developed in the continental depositional environments of the Gröden Formation (Val Gardena Formation) in which shale prevails, but sandstone and locally also conglomerate and breccia are present. The Upper Permian and Lower Triassic strata of the External Dinarides in Slovenia were formed on an extensive Slovenian Carbonate Platform which became established during the Late Permian, and it remained stable until Middle Triassic time (late Anisian) when it was split by the Slovenian Basin into the Julian Carbonate Platform in the north, and the Dinaric Carbonate Platform in the south (Buser, 1989(Buser, , 2003Buser et al., 2007Buser et al., , 2008. The investigated section is located 4 km northwest from Žiri (x=5,102,525, y=5,428,000), in a ravine between Mrzli vrh (862 m) and Ledinski gri~ (893 m) ( Fig. 1). In the period of 2006-2009, the section was part of a detailed biostratigraphic study based on conodonts that enabled the definition of the Permian-Triassic boundary according to internationally accepted criteria (kolar-jurkov{ek et al., 2011). The base of the Luka~ section is formed by the reddish-brown clastic rocks of the Gröden Formation that are in a tectonic contact with the Upper Permian and Lower Triassic rocks. The strata of the section are in overturned position and they dip from 60° to 75° north-northeast. The lowermost part of the section starts with the Bellerophon Limestone Member that continuously passes into the Evaporite-dolomite Member of the Bellerophon Formation. Then follows the Luka~ Formation with the Permian-Triassic Transitional Beds in its lowermost part, and these are overlain by the Streaky Limestone Member and Carbonate-clastic Member. The thickness of the entire section is 283 m (Figs. 2, 3).
Six conodont biozones have been established in the section that can be compared with the biozonation of various sections in the Southern Alps and the Meishan section in China, as well   as other sections in the world (kolar-jurkov{ek et al., 2011). The following zones have been recognized: the latest Changhsingian (uppermost Permian) praeparvus Zone, and the Griesbachian (lowermost Triassic) parvus, lobata, staescheiisarcica, postparvus and anceps Zones. The first appearance of Hindeodus parvus in the sample L1 in the Transitional Beds marks the systemic boundary between the Permian and Triassic and the boundary between the Paleozoic and Mesozoic erathems (kolar-jurkov{ek et al., 2011).

Bellerophon Formation
The Bellerophon Limestone Member is represented by black nodular or faintly bedded limestone (Fig. 4) that predominantly consists of arenite or rudite size fossil detritus (algae, corals, echinoderms, foraminifers, gastropods, bivalves and brachiopods) included in the micritic matrix. Carbonate detritus is commonly tightly packed with the small amount of matrix forming dense packstone, only rarely wackestone.

Luka~ Formation
Evaporite-type dolomite continuously passes to the Transitional Beds that consist of light yellow to red colored carbonate beds that vary in thickness from 0.03 -to 0.62 m (Fig. 7). The base of the 3.3 m thick transitional interval consists The predominantly, micritic microfacies imply deposition under low energy conditions, possibly lagoon or back reef.
Well bedded, intensively recrystallized bio micrites are present at the top of the Bellerophon Limestone Mem ber. They pass continuously into a 30 m thick eva porite unit named as Evaporitedolomite Member (Fig. 5). It is represented by tabular, 0.5-3 m thick dolomite beds with dissolved cm-sized molds of primary evaporitic minerals. The dolomite has a unimodal ma crocrystalline planar e-or s-structure with stair-step or rounded molds (Fig. 6).
Dolomite and the stair-step molds present in the Evaporate-dolomite Member suggest deposition under hypersaline conditions where evaporites were possibly deposited as primary mine rals which have been removed by dissolution, possibly in supratidal conditions.   of laminated mudstone, laminated micritic/biomicritic limestone and plane parallel or trough ripple cross-lamina ted grainstone. The laminated mudstone consists dominantly of limy lamina that alternate with the lamina composed of limy mud, clay and/or 5-7% of siliciclastic terrigenous coarse-silt component (Fig. 8). The laminated micritic/biomicritic limestone type consists of prevailingly micritic la minae that alternate with prevailingly bioclastic laminae (very often containing ostracodes) (Fig. 9). The grainstone consists of ooid and bioclastic detritus and sparry-calcitic cement (Fig. 10).
Grey and pail-red dolomite and less dedolomite occur also in the Transitional Beds. They have homogenous micro-to macrocrystalline struc ture and occasionally very often preserved ooid ghosts (Fig. 11).
The Transitional Beds were deposited in shallow marine conditions. The presence of micrite rich microfacies types imply more restricted con-ditions with the predominance of suspension settling of fines. Trough and ripple cross-lamination found in the grainstone suggests migration of ooid detritus due to oscillatory and/or tidal cur rents.
The Transitional Beds with the Permian--Triassic boundary are conformably overlain by the ca 30 m thick Streaky Limestone Member (samples V to 25). This unit consists of a repetitive alternation of very thin bedded light grey or yellow and dark gray bed couplets (Fig. 12). The beds are planar, wavy or irregularly shaped. Pa rallel and wavy-cross la mination can be seen. Siliciclastic sandy, silty and clayey detritus is present in greater amounts then before. Within the streaky limestone, light colored interbeds consist dominantly of siliciclastic or bioclastic material whereas dark interbeds are dominantly of carbonate mud components (Fig. 13). Bioclastic detritus are represented by silicified ostracodes and are rarely of recrystallized ooids. In each bed couplet, the uppermost laminae are usually destroyed due to the activi ty of organisms. Within the thin bedded streaky alternation some 0.3 m thick recrystallized ooid rich beds occasionally occur. A wavy structure observed in the streaky   limestone suggests deposition in a shallow subtidal environment and deposition by oscillatory currents. Irregularly shaped beds were formed due to intensive bioturbation. Predominantly sub tidal deposition implies deepening of the environment associated with increased terrigenous influx during Early Triassic time.
In the upper part of the section until its very end there is the approximately 80 m thick Carbonate-clastic Member of the Luka~ Formation that consists of: a) ooid-grainstone, b) laminated silty micrite/biomicrite and of c) calcareous siltstone. Rarely ooid rich biocalcarenite occurs. In the uppermost 30 m of the succession, ooid grainstone beds disappear and the limy mudstone occurs more often with a nodular appearance. In the upper part of the succession micrite rich limestone containing reworked ooids is also present.
Wave ripple cross lamination was found in the ooid grainstone as well as in the calcareous siltstone. Plane parallel lamination can be seen only in the calcareous siltstone. Reworking by organisms is often present and can be seen as mottling of the siliciclastic and carbonate material. a) The ooid grainstone consists of poorly to fairly sorted fine to medium sand sized ooid detritus. Exceptionally very coarse grained varieties (diameter 1.6 mm) occur. The ooid grainstone can be intensively dolomitized or dedolomitized. Mollusk and gastropod fragments are common in the ooid grainstone (Fig. 14). b) The laminated silty micrite/biomicrite layers consist of an alternation of laminae contai ning silty siliciclastic detritus and micritic/ biomicritic material. The total amount of the silty component is less than 50%. A small amount of clayey component is present as well. Biomicritic lamina consists of ostracod biodetritus and some mollusk and gastropod fragments. The lamination in the silty micrite/biomicrite is mostly of wave origin. Reworking by organi sms is common. c) The calcareous siltstone is composed of more than 50% of silty siliciclastic material and calcareous (usually bioclastic) detritus. The cement is calcitic and/or dolomitic. Biodetritus is represented mainly by ostracodes and subordinarily by mollusk fragments.
The nodular mudstone varieties consist of den se micrite containing some silty siliciclastic gra ins. The mudstone is dolomitized, silicified and contains iron oxide.
Ripple cross-lamination in the ooid grainstone as well as in laminated siltstone exhibits characteristics of oscillatory/storm currents related to waves. Plane parallel lamination can be seen only in the calcareous siltstone and is probably due to su spension settling. Reworking by organisms is often present and can be seen as mottling of siliciclastic and carbonate material. All of these characteristics imply deposition in a marine, shal low environment, possibly between fair and storm weather wave base and an intensive terrigenous input.

Biostratigraphy
The importance of the Luka~ section for the definition of the Permian-Triassic Boundary in Slovenia is the recovery of conodonts in the boundary interval beds. This section is the first found in Slovenia from which conodonts are recorded from the Permian-Triassic interval. The conodont fauna is characterized by a Hindeodus-Isarcicella population that provides a good basis for a very fine biozonation (kolar-jurkov{ek & jurkov{ek, 2007; kolar-jurkov{ek et al., 2011) The succession enables the definition of the PTB based on the first appearance of Hindeodus parvus, the diagnostic species and globally recognized marker defining the systemic boundary (yin, 1993, 1996; yin et al., 2001). Conodont faunas are characterized by shallow water elements with prevailing Hindeodus and Isarcicella that are in the higher part of the section accompanied by representatives of Hadrodontina and ellisonids. The absence of gondolellids is obvious. The identified conodont elements are assigned to 15 species of 3 genera. The following conodont biozones have been recognized based on very detail collecting: -the latest Changhsingian (uppermost Permian) praeparvus Zone, -and the Griesbachian (lowermost Triassic) parvus, lobata, staeschei-isarcica, postparvus and anceps Zones. The recovered conodont faunas can be well correlated with the Global Stratotype Section and Point (GSSP) at the Meishan section (yin et al., 2001) and the Southern Alps (Fig. 15), as well as with other coeval sections of the adjacent areas in neighbouring areas of Austria, Hungary and Croatia.
A study of the foraminiferal fauna was also undertaken and identifications of the faunal elements were based on examination of thin sections and isolated forms. Four new species of foraminifers were described from the Luka~ section: Multidiscus zhiriensis, M. dinaridicus, and Globivalvulina lukachiensis from the Upper Permian and Lingulonodosaria slovenica from the lowermost Triassic (nestell et al., 2011).
Late Permian foraminifers were obtained from the lower and middle part of the Bellerophon Formation as its uppermost part in the Luka~ section is represented by dolomite and does not contain foraminifers. The assemblage is represented by 43 species of 22 genera, including 2 genera of fusulinaceans: Reichelina and Nankinella. Foraminifers were recognized in the samples from B1 through B11. It should be noted that two forms were determined as Hemigordius cf. H. komiricensis as well as Multidiscus cf. M. vlasicus that were just recently introduced from the Komiri} section in the Internal Dinarides (nestell et al., 2009). The Late Permian foraminiferal assemblage of the Luka~ section consists of species characteristic for the Changhsingian of many regions in the Tethys: northwestern Caucasus, Transcaucasia, Turkey, northern Italy, northwestern Serbia, northeastern Hungary, and South China (nestell et al., 2011 with references).
Younger foraminifers also co-occur with conodonts and were recovered from the Permian-Triassic boundary interval represented by carbonate Transitional Beds deposited in shallow restricted marine conditions. The assemblage of Early Triassic foraminifers is very poor and it is represented by seven species of four genera. Among them is important the recognition of the species "Cornuspira" mahajeri and "Earlandia" spp., marking the lower boundary of the Triassic worldwide based on foraminifers. These species are found below the first appearance of the conodont species Hindeodus parvus which officially marks the lower boundary of the Triassic, and they are considered to be ecological species and their appearance coincides with a stressful shallow water environment. The first interval with nodosariid foraminifers appear 2m above the Permian-Triassic boundary and second one is approximately 5 m above the boundary. Both intervals are in the range of the last appearance of the co nodont species H. parvus and within I. staeschei -I. isarcica conodont Range Zone (kolar-jurkov{ek et al., 2011; nestell et al., 2011).

Conclusions
The Permian-Triassic interval of the Luka~ section in western Slovenia was studied sedimentologically and micropaleontologically by using conodonts and foraminifers. The analyzed section is composed of the Bellerophon Formation (Bellerophon Limestone and Evaporite-dolomite Members) and the Luka~ Formation (Transitional Beds, Streaky Limestone Member and Carbonateclastic Member). All described lithostratigraphic units, but the Bellerophon Formation, are here formalized.
The overall sedimentary characteristics reflect shallow marine conditions that began with the Permian Bellerophon Limestone Member where micritic microfacies prevail. A deposition of micrites/biomicrites (Bellerophon Limestone Member) in a lagoonal, possibly back reef conditions, continuously change to hypersaline very shallow conditions depicted as Evaporite-dolomite Member. The conforma ble boundary with the Transitional Beds suggests maintaining of a shallow marine condition du ring the depo siton of the Transitional Beds. Laminated mudstone and laminated micritic/biomicritic limestone suggest deposition in a restricted marine condition, but the presence of grainstone implies periodically established hig her energy condition and deposition of oolitic detritus by oscillatory and/or tidal currents.
A wavy and/or hummocky structure observed in the Streaky Limestone Member that overly Tran sitional Beds suggests deposition in strictly subtidal conditions by oscillatory currents in a shallow sea. Irregularly shaped beds were formed due to intensive reworking by organisms.
Sedimentary rocks of the Carbonate-clastic Member show characteristics of shallow marine deposition of ooid or bioclastic detritus by oscillatory/storm currents. A prevailing deposition of carbo nate was periodically punctuated by terrigenous input of silty siliciclastic material. The deposition reflects predominantly subtidal conditions. A deposition in the Luka~ Formation implies a general deepening of the environment associated with the increased terrigenous influx during Early Triassic time.
The studied PTB interval beds are characterized by a diverse microfauna. The entire section is characterized by a Hindeodus-Isarcicella association and only the highest part of the section is marked by the presence of ellisoniids, predominantly Hadrodontina. The absence of gondolellids is noteworthy. Six conodont zones have been recognized in ascending order, the latest Changhsingian (uppermost Permian) praeparvus Zone, and the Griesbachian (lowermost Triassic) parvus, lobata, staeschei-isarcica, postparvus and anceps zones.
The first occurrence of H. parvus in sample L1 in the Transitional Beds marks the systemic boundary between the Permian and Triassic. A rapid entry of several conodont taxa is observed in the highest level of the Transitional Beds, in the lobata Zone and in the succeeding staescheiisarcica Zone in the lowermost part of the Streaky Limestone Member, and both probably represent a recovery event.
The introduced conodont biozonation for the Luka~ section is the first proposed for the PTB interval in Slovenia as well as in the entire Dinaride region. The recognized conodont fauna of the Luka~ section enables correlation with the similar age sequences in the Southern Alps in Italy, and with the GSSP Meishan D section in South China (Fig. 15).
Foraminifers are found together with conodonts in the Permian Bellerophon Limestone Mem ber of the Bellerophon Formation, Permian-Triassic Transitional Beds and in the lower part of the Streaky Limestone Member of the Luka~ Formation of the lowermost Triassic. Foraminifers of the Bellerophon Limestone Member are cha racteristic for the Changhsingian of various regions of the Tethyan realm. In the Permian-Triassic Transitional Beds, the species "Cornuspira" mahajeri , and "Earlandia" spp. marking the lower boundary of the Triassic worldwide based on foraminifers appear below the first appea rance of Hindeodus parvus and are conside red to be an ecological species as two of determined species are also found above the Permian-Triassic boundary. The appearance of the species "C." mahajeri and "E." spp. coincides probably with a shallow restricted environment, and thus, they cannot be used for biostratigraphic pur po ses as stratigraphic markers of the lower boun dary of the Triassic. The first nodosariids appear at the base of the Streaky Limestone Member, approximately 2 m above the PTB and are represented by the one recently described new species, Lingulonodosaria slovenica not found anywhere else yet. Wang CY, 1999Jiang et al., 2007Perri & Farabegoli, 2003Farabegoli et al., 2007Kolar-Jurkovšek et al. 2011 PERMIAN Changhsingian  Yin et al., 2001 TRIASSIC of Slovenia are gratefully acknowledged. The investigation was partly supported by the Slovenian Research Agency (program number P1-0011 and project number J1-6665), Croatian Ministry of Science (project number 195-0000000-3202) and through the program of bilateral cooperation in science and technology Slovenia-Croatia (2009-2010. This is a contribution to the IGCP-Project 572 (»Recovery of ecosystems after the Permian-Triassic mass extinction«).