A sedimentological description of the Middle Triassic vertebrate-bearing limestone from Velika planina, the Kamnik-Savinja Alps, Slovenia

Abstract

In the Kamnik-Savinja Alps (Slovenia), the Lower Serla Dolomite laterally passes into a succession of thin- to medium-bedded bituminous limestones of the Velika planina member. The finely laminated lower part of this member contains well-preserved actinopterygian fish and sauropterygian remains. The research aimed to determine the sedimentological and palaeoenvironmental characteristics of the depositional basin on the basis of three detailed sedimentological sections logged atop the Velika planina plateau. The Velika planina member is underlain by a whitish to light grey, thick bedded to massive dolomite with oncoids, stromatolites, and lumachellas deposited under peritidal to shallow subtidal conditions. The lower part of the Velika planina member consists of thin, often platy, finely laminated beds of bituminous mudstone. The Chondrites ichnofossil is very common; however, in some beds numerous lingulid brachiopods, bivalves, and crinoids were observed. Fossil vertebrates and crustaceans are relatively rare and confined to a few levels. Ammonoids are very rare. Subordinate beds of intraclastic-peloid wackestone to packstone, intraclastic-bioclastic grainstone, and bivalve floatstone occur. Slumps are common. Upwards, bedding gradually becomes thicker and bioturbation more common. Finally, stromatolites, birdseye fenestrae, and oncoids reappear. The entire succession is confined to the early to middle Anisian by the foraminifer Citaella dinarica (KOCHANSKY-DEVIDÉ & PANTIĆ). The absence of breccias at the base of the Velika planina member, the gradual transition upwards into shallow marine carbonates, as well as the presence of sauropterygians of the order Nothosauroidea suggest deposition in a relatively shallow basin. The finely laminated facies of the lower part of the member indicates a stratified water column, with oxygenated near-surface waters and hypoxic to anoxic conditions near the sea floor.