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Environmental stress within Lower Triassic clastic and carbonate deposits (Muć–Ogorje, Central Dalmatia)

Lower Triassic clastic and carbonate deposits are well exposed and known in the area of Muć (HERAK et al., 1983), enabling detailed interpretations concerning the depositional environment and changes throughout a 230 m thick succession exposed along the road Muć‒Ogorje in Central Dalmatia. The succession comprises mainly reddish clastic deposits in its lower part and yellowish-grey carbonate deposits in the upper part. Detailed logging revealed that clastics and carbonates are often interchanging in the upper part of the succession, giving new clues on the depositional conditions. Clastic deposits are commonly micaceous, vary in size from siltstones to medium-grained sandstones and are attributed to deposition in an offshore transition environment, as attested by the occurrence of storm deposits (Fig. A) and slumps (Fig. B). Similar deposits were reported about 50 km away in the vicinity of Knin (ALJINOVIĆ, 1995), thus suggesting that a broad shelf occupied this area during the Early Triassic. Fossil findings within the clastic lower part are rare, mainly internal and external moulds of bivalve shells (Unionites). In the carbonate part of the succession, fossil findings include common gastropod remains (Werfenella) and visible bioturbation, with sporadic occurrence of ammonites (Tirolites) (VUDRAG & SREMAC, 2015 ; this study). Variations in lithology, colour of deposits and fossil content throughout the succession suggest frequent changes in depositional conditions. Siliciclastic deposits in the lower part of the succession comprise only bivalves, indicating modest macrofossil recovery, while carbonates in the upper part contain a more diverse fossil community, including gastropods, rare bivalves and ammonites. Carbonate deposits are thought to reflect tectonically driven episodes of basin deepening, whereby the reduction of siliciclastic input generated favourable conditions for biota. However, siltstone intercalations are barren of any visible fossils, suggesting that siliciclastic input negatively affected present biota leading to enhanced stress in already stressed environment in the aftermath of the end-Permian extinction.

Lower Triassic, Ogorje, clastics, carbonates, storm deposits

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