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First evidence of the Palaeogene age of the Bosnian Flysch Unit (Dinarides, Bosnia and Herzegovina)

Pre Karst and Bosnian Flysch Unit tectonically belong to the Dinarids, spreading from Croatia in the northwest to Serbia and Montenegro in the southeast. The main part of Bosnia Flysch Unit however is placed in Bosnia and Herzegovina. This tectonic unit is sandwiched between the High Karst Unit of the External Dinaridic Platform in the south and the Dinaride Ophiolite Zone in the north. It forms an about 3000m thick stack of intensely folded rocks, comprising two formations: Ugar Fm. and Vranduk Fm. Investigations of calcareous nannofossils (Mikes et al., 2008) documented a Late Cretaceous age for the Ugar Fm. and Latest Jurassic-Early Cretaceous age for the Vranduk Fm. Sedimentation of the Flysch started during the latest Jurassic and became progressively younger in the more external parts (Schmid et al. 2008). The age of this unit is generally and traditional regarded as the Upper Cretaceous, according to sheets of Geological map (OGK, Osnovna Geološka Karta ; 1:100.000). The investigated area is situated in Western Bosnia, NE of the city of Jajce, locality Donji Bespelj (Fig. 1). During prospection for bauxite in this area, a detailed geological mapping and intensive drill campaign were performed. Stratigraphic investigation of calcareous nannoplankton was performed on 20 samples from the Donji Bespelj - Daljevac flysch section and borehole DB2. Standard methods were used for the preparation of smear slides, which were analysed under the microscope in normal and cross-polarized light at x1000 magnification. All investigated samples yielded generally well preserved calcareous nannofossils assemblages allowing stratigraphic attribution of this part of the Bosnian Flysch succession. Calcareous are abundant in this Upper Cretaceous assmblage, but are moderately preserved nannoplankton. The occurrence of Micula prinsii in the assemblage including Arkhangelskiella cymbiformis, Arkhangelskiella maastrichtiana, Eiffellithus gorkae, Micua decussata, Micula murus, Placozygus fibuliformis, Prediscosphaera cretacea etc., allows attribution to the uppermost Maastrichtian Subzone UC20d (Burnett, 1998). Accumulation of Palaeocene sediments from Danian to Thanetian was documented by the nannoplankton Zones NP1-NP9 (Martini, 1971). Nannoplankton Zone NP1 was documented by the occurrence of Biantholithus sparsus in the assemblage with high percentage of Thoracosphaera spp. The assemblage of Zone NP2 is characterised by a large amount of Coccolithus pelagicus co-occurring together with: Braarudosphaera bigelowii, Cruciplacolithus tenuis, Ericsonia subpertusa, Markalius inversus, Placozygus sigmoides etc. Nannoplankton Zones NP3-NP8 contain zone markers: Chiasmolithus danicus, Ellipsolithus macellus, Fasciculithus tympaniformis, Heliolithus kleinpellii and Heliolithus riedelii. Stratigraphicaly youngest sediments contain calcareous nannoplankton with Discoaster multiradiatus, D. lenticularis, D. delicatus and can be attributed to the NP9 (uppermost Palaeocene ; Thanetian). Beside Palaeocene sediments occurring in the High Karst Unit in North Bosnia (covering Ophiolitic Units), this is the very first documentation of sediments in this part of Dinarides.

Bosnian Flysch; calcareous nannoplankton; Dinarides; Palaeocene; stratigraphy.

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