engleski

Permian-Triassic boundary interval of Slovenia: Conodont and foraminiferal biostratigraphy

Conodonts and foraminifers have been studied from Permian-Triassic boundary (PTB) strata of the Lukac section (Slovenia). This section is located in the area belonging to the External Dinarides and is composed of the Permian Bellerophon Formation (Bellerophon Limestone and Evaporite-dolomite Members) and the Triassic Lukac Formation (Transitional Beds, Streaky Limestone and Carbonate-clastic Members). The main objective of the study was to document the conodont biozonation of this boundary section. The conodont fauna is dominated by shallow water elements of Hindeodus and Isarcicella. Hindeodus parvus, the diagnostic species and globally recognized marker defining the systemic boundary according to internationally accepted criteria, first appears in sample L1 in the Transitional Beds. Six conodont biozones were recognized: the latest Changhsingian praeparvus Zone, and the Griesbachian parvus, lobata, staeschei-isarcica, postparvus and anceps Zones (Kolar-Jurkovsek et al. 2011). The foraminiferal assemblages were studied in detail and four new species were recognized, Multidiscus zhiriensis, M. dinaridicus, and Globivalvulina lukachiensis from the Upper Permian, and Lingulonodosaria slovenica from the lowermost Triassic. The abundantly represented Late Permian assemblage also includes two forms of small fusulinaceans (Reichelina, Nankinella) and yields characteristic species of the Changhsingian. The assemblage of Early Triassic age is very poor. In the Permian-Triassic Transitional Beds, the species “Cornuspira” mahajeri, and “Earlandia” spp. that mark the lower boundary of the Triassic worldwide based on foraminifers appear below the first appearance of Hindeodus parvus and they are considered to be ecological species as they are also found above the PTB. The appearance of these two species coincides with the onset of a shallow restricted environment, and they should not be used as stratigraphic markers. The first interval with nodosariid foraminifers appears at 2 m and a second one is found approximately 5 m above the PTB (Nestell et al. 2011). The overall sedimentary characteristics investigated in this continuous section reflect shallow marine conditions that began with the Permian Bellerophon Member where micrite, or biomicrite of lagoonal, possibly back reef facies prevailed. The deposition of these lithofacies had changed to a hypersaline very shallow environment as found in the Evaporite-dolomite Member. Shallow marine conditions were maintained during the depositon of the Transitional Beds where the PTB can be defined. Sedimentary rocks of the Streaky Limestone and Carbonateclastic Members were deposited in predominantly subtidal conditions and there has been documented a general deepening trend during Early Triassic time. The Permian-Triassic Lukac section of Slovenia is so far the only section in the Dinarid area with a recognized conodont biozonation and is therefore of significant importance for correlation in the entire Tethyan Realm and to the GSSP at Meishan.

PT boundary; biostratigraphy

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