engleski

Early Triassic shelf sedimentation of External Dinarides (Bosnia and Herzegovina) and conodont based biostratigraphic revision

Recently a Lower Triassic investigation started in the central part of the External Dinarides in the Bosnia and Herzegovina. The area is tectonically characterized by series of overthrusted faults which formation was influenced by diapirism of Permian evaporates (Chorowitz, 1977) The 239 m thick Lower Triassic sedimentary succession were investigated in the vicinity of Bosansko Grahovo. According to sedimentological study three facies have been differentiated in a conformable succession: The Siliciclastic facies present at the beginning of the succession, Mudstone facies conformably overlying Siliciclastic facies and Siltstone-mudstone facies comprising the upper part of the succession. Siliciclastic facies, characterised by red mixed siliciclastic-carbonate deposits (shales, sandstones, oolithic and/or bioclastic grainstones) ; represent deposition of a shallow well aerated storm influenced inner shelf. Red dominantly clastic deposits (Siliciclastic facies in this case) have been defined as Siusi beds (Grimani et al, 1975) and like elsewhere in the External Dinarides assigned to the Early Lower Triassic. Grey micritic limestones, marls and calcareous siltstones that occur in the middle (Mudstone facies) and upper part of the succession (Siltstone-mudstone facies) represent deposition of a deeper inner shelf, partly also of an outer shelf. Grey calcareous deposits of middle and upper part of the succession were correlated with Campil deposits of the Late Lower Triassic (Grimani et al, 1975) here and elsewhere in the Dinarides. Biostratigraphic dating by means of conodonts proved that the lithostratigraphic correlation with Siusi and Campil Members sensu stricto is not adequate in this particular case. Conodont content of the collected samples in the Bosansko Grahovo section is marked by presence of two conodont taxa Hadrodontina anceps and Pachycladina obliqua. Both taxa co-occur in its lower part, whereas Pachycladina obliqua is present throughout the entire section and has been therefore attributed to the Smithian obliqua Zone. Preservation of specimens is mostly fragmented and their abundance is relatively low. Conodont biofacies is marked by shallow-water euryhaline taxa. The Spathian age of the upper part of the section is not excluded but it is debated due to absence of Neospathodus triangularis that marks the youngest Lower Triassic (Spathian) zone. New conodont data and sedimentary characteristic of vertically superposed facies allows following interpretation: After hiatus, that lasted from Permian throughout Griesbachian and Dienerian, in Smithian a marine transgression drowned evaporite complexes, and marine shelf conditions have been established. The differences among facies were interpreted as deepening of the environment envisaged as a transgressive trend in a shallow shelf environment. This paper is the fist report on the Lower Triassic conodonts of Bosnia and Herzegovina.

Conodont biostratigraphy; Lower Triassic; Smithian; obliqua Zone; Siusi Member; Campil Member; External Dinarides; Bosnia and Herzegovina

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