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Lukač section – a key for a definition of the Permian-Triassic boundary in the Dinarides

The Lukač section situated in NW Slovenia is a part of the External Dinarides geotectonic unit. 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 when it was split to the Julian Carbonate Platform in the north, and the Dinaric Carbonate Platform in the south. The Permian-Triassic interval of the Lukač section in western Slovenia was studied sedimentologically and micropaleontologically by using conodonts and foraminifers (Kolar-Jurkovšek et al., 2011a, b). The analyzed section is composed of the Bellerophon Formation (Bellerophon Limestone and Evaporite-dolomite Members) and the Lukač Formation (Transitional Beds, Streaky Limestone Member and Carbonate-clastic Member). The overall sedimentary characteristics reflect shallow marine conditions that began with the Permian Bellerophon Member where micritic microfacies prevail. A deposition of micrites/biomicrites in a lagoonal, possibly back reef conditions, continuously change to hypersaline very shallow, possibly supratidal conditions depicted as Evaporite-dolomite Member. The conformable boundary with the Transitional Beds suggests maintaining of a shallow marine condition during the depositon of the Transitional Beds. Deposition of laminated mudstone and laminated micrites/biomicrites (with abundant ostracod detritus) suggests a restricted marine conditions, but the presence of grainstone implies periodically established higher energy condition and deposition of oolitic detritus by oscillatory and/or tidal currents. Streaky limestone beds were deposited in strictly subtidal conditions by oscillatory and/or storm currents in a shallow sea. Predominantly subtidal deposition implies deepening of the environment associated with increased terrigenous influx at the beginning of the Early Triassic. Sedimentary rocks of the Carbonate-clastic Member are also characterised by shallow marine deposition of ooid or bioclastic detritus by oscillatory currents. A prevailing deposition of carbonate was periodically punctuated by terrigenous input of silty siliciclastic material. A deposition of the Lukač Formation implies a general deepening of the environment associated with the increased terrigenous influx during Early Triassic time. The importance of the Lukač section for the definition of the Permian-Triassic Boundary in Slovenia is the recovery of conodonts in the boundary interval beds. 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 following conodont biozones have been recognized: the latest Changhsingian (uppermost Permian) praeparvus Zone, and the Griesbachian (lowermost Triassic) parvus, lobata, staeschei-isarcica, postparvus and anceps Zones. The first appearance of Hindeodus parvus in the sample L1 in the Transitional beds marks the systemic boundary. A foraminiferal study resulted description of four new species: Multidiscus zhiriensis, M. dinaridicus, and Globivalvulina lukachiensis from the Upper Permian and Lingulonodosaria slovenica from the lowermost Triassic. The Late Permian assemblage is represented by 43 species of 22 genera, including 2 genera of fusulinaceans: Reichelina and Nankinella that consists of species characteristic for the Changhsingian of many regions in the Tethys (Nestell et al., 2011). Younger foraminifers also co-occur with conodonts and were recovered from the 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 5m above the boundary. The conodont biozonation of the Lukač section is the first introduced for the PTB interval in Slovenia as well as in the entire Dinaride region and thus it represents a key section for definition of the PTB in the Dinarides. The recovered conodont faunas can be well correlated with the Global Stratotype Section and Point (GSSP) at the Meishan section and the Southern Alps, as well as with other coeval sections of the adjacent areas in neighbouring areas of Austria, Hungary and Croatia.

External Dinarides; Permian-Triassic boundary; transitional beds

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