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Conodont dating of the Lower Triassic - Scythian sedimentary rocks in the External Dinarides (Croatia and Bosnia and Herzegovina)

Two Lower Triassic - Scythian sediment successions have been dated by means of conodonts in the External Dinarides: Plavno section in the vicinity of Knin (Croatia) and Bosansko Grahovo section in neighboring Bosnia and Herzegovina. Deposition at both sections shows similar characteristics differentiated in three continuously deposited facies: The Siliciclastic facies (previously considered Siusi beds and assigned to the Early Scythian), Mudstone facies and Siltstone-mudstone facies (occured in the upper part of the Scythian succession that was formerly considered Campil beds of the Late Scythian). Vertical succession of Siliciclastic, Mudstone and Siltstone-mudstone facies were interpreted as deepening of the environment envisaged as a transgressive trend in a shallow shelf environment. Facies sucessions at Plavno (ca 700 m thick) and Bosansko Grahovo (ca 130 m thick) differentiate in the appearance of Dolomitic facies that occurred only at the beginning of Plavno sucession. Conodont fauna of Dolomitic facies at Plavno section is represented by isarcicellids, Isarcicella staeschei and I. isarcica that mark the Griesbachian isarcica Zone. The Siliciclastic facies of Plavno and Bosansko Grahovo section is characterized by shallow-water euryhaline taxa attributed to the Smithian obliqua Zone. This fauna is prevailed by Hadrodontina anceps and Pachycladina obliqua with co-ocurrence of Parachirognathus ethingtoni and very rare presence of Furnishius sp. Discerned conodont taxa enable us to establish conodont zonation which gave a new insight to the range of so-called Siusi and Campil beds. Bipartite dismembering of the Lower Triassic beds that is still used in the Dinarides is not actual any more, because also in the Southern Alps it has been changed and further defined. As seen in the examples from the Southern Alps and Slovenia lithology varies from one section to the other. It is particularly true for the shallow shelf environment where slight changes in palaeomorphology influence important differences. Thus, there is no need to compare Lower Triassic developments of the Dinarides strictly with the type localities, considering only lithology. The important comparison can be made based on established zoning especially using very precise conodont zonation.

Lower Triassic; Dinarides; conodont zonation; Croatia; Bosnia and Herzegovina

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