For resolving question about the age of its upper boundary, the Zatrnik formation was recorded at the Zajamniki mountain pasture in Pokljuka in the total length of 74.4 m.. Sedimentological and micropaleontological analysis was performed on thin sections. Conodont samples were all negative.
Twelve microfacies were recognized. Zatrnik formation comprise seven microfacies: peloidal packstone, intraclastic packstone, intraclastic peloidal grainstone, intraclastic ooid pelletal packstone, intraclastic ooid bioclastic grainstone, pelletal filament packstone and pelletal bioclastic packstone. Ribnica breccia comprise five microfacies: litoclastic bioclastic packstone to grainstone (locally wackestone), litoclastic rudstone, litoclastic bioclastic wackestone, bioclastic wackestone to packstone and filament rudstone.
Foraminifera comprise Earlandia sp., ?Epistominidae, Lagenida indet, Lenticulina sp., Meandrovoluta asiagoensis, Miliolida, Nodosariidae, Siphovalvulina sp., Spirillina sp., Textulariida and Valvulinidae. Less common are microproblematica Thaumatoporella, Baccinela irregularia and Tubiphytes.
The Zatrnik formation was deposited on a deep shelf, which was under the influence of turbidites and debris flows and/or the slope or toe-of-slope of the sea basin. It's a hemipelagic sedimentation interrupted with turbidity currents. The Early Jurassic age for the upper boundary of the Zatrnik formation was based on the presence of foraminifers Meandrovoluta asiagoensis and Siphovalvulina sp.
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