engleski

Facies characteristics of the Permian-Triassic boundary interval in the Dinarides (Croatia and Slovenia)

Sedimentary rocks that represent Permian-Triassic boundary interval have been investigated at several localites in the External Dinarides - localities Plavno and Brusane in Croatia and locality Lukac in Slovenia. Lukac locality is positioned at the north western end of the Dinarides, Plavno at the south eastern part and Brusane section in the middle part of the Dinarides remote ca. 300 km from the farthest points. In the investigated section at Lukac the Permian-Triassic boundary has been determined biostratigraphicaly for the first time in the Dinarides by the first appearance of condont Hindeodus parvus (Kolar-Jurkovsek et al. 2011). The continuos section from Permian Bellerophon limestones through Permian evaporites to the Transitional Beds seems to be a key section for the whole Dinaride area. The samples from the transitional PT interval were classified in four different lithotypes: laminated mudstone, laminated micritic/biomicritic limestone with abundant ostracodes (rare gastropods), and parallel- or cross-laminated oolitic grainstones. All rock types are intensively altered by dolomitization, silicification and dedolomitization. Facies characteristics in the transitional interval suggest a deposition in very shallow marine environment. Micritic microfacies indicates a suspension settling of fines, while cross-bedded lamination in grainstones points to a migration of ooid detritus, due to tidal- and/or wave currents. At the Plavno and Brusane localities in Croatia a PT boundary had not been determined but similar facies occurred in the oldest Triassic strata compared to the Lukac section in Slovenia. At Plavno locality, the oldest Triassic rocks have been determined by the presence of conodont Isarcicella isarcica (Aljinovic et al. 2011). Facies characteristics indicate that in the oldest part of the section intensively dolomitized and dedolomitized laminated micrites, biomicrites containing ostracods and ooid grainstones occurred. At Brusane locality (Velebit Mts.) almost the same facies has been distinguished. The presence of micritic and ooid facies suggests a deposition in shallow marine environments with sedimentation of predominantly micritic limestones in low energy conditions that alternate with high energy conditions with a deposition of ooid grainstones. In all investigated sections ostracod fauna has very specific appearance: a several ostracod carapaces “joined” together indicate a hypersaline namely very shallow depositional conditions (M. Schreiber, personal communication). Deposition in the extensive shallow marine and hypersaline conditions was a prerequisite for the dolomitization processes. This is in accordance with a significant drop of sea level at the end of Permian that influenced sedimentation in the broad Dinaric region and possibly also in the neighbouring areas (Hungary). Dedolomitization processes are possibly related to the dissolving of underlaying evaporate complex present in the both Croatian and Slovenian parts of the Dinarides.

PT boundary; Dinarids; conodonts

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