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Jurassic and Cretaceous radiolarian biostratigraphy of the Bled Basin (NW Slovenia) and stratigraphic correlations across the Internal Dinarides

The thesis includes Jurassic – Lower Cretaceous stratigraphic correlation of four areas: 1) Bohinj area in the Julian Alps (NW Slovenia) ; 2) East Bosnian – Durmitor trust sheet in Bosnia and Herzegovina and Montenegro ; 3) Krivaja-Konjug ophiolite complex in Bosnia and Herzegovina ; 4) Guevgueli Ophiolite Complex near Demir Kapija in Republic of Macedonia. The research is focused on radiolarian biostratigraphy. The investigated area in the Julian Alps in NW Slovenia evolved during the Mesozoic as deep-water Bled Basin, which occupied relatively distal part on the Adriatic continental margin. Evolution of the Bled Basin is reconstructed from the Late Triassic to the Early Cretaceous. Biostratigraphy is based largely on radiolarians. The succession of the Bled Basin is closely similar to the three successions (Kalinovik, Čehotina and Kaludarska rijeka) of the distal continental margin from the East Bosnian – Durmitor thrust sheet of the central Dinarides in Montenegro and Bosnia and Herzegovina. Biostratigraphic data from the ophiolitic mélange of the Krivaja-Konjuh Ophiolite Complex in Bosnia and Herzegovina and cherts overlying basalts of the Guevgueli backarc basin at Demir Kapija in Republic of Macedonia are also based on radiolarians. All areas are discussed in the context of the evolution of the Neotethys. During the Late Triassic and the Early Jurassic the Bled Basin was characterized by deposition of relatively thick-bedded limestones with chert of the Zatrnik Formation. In the uppermost part of the formation calcarenites with abundant resedimented echinoderm fragments prevail. Pelagic limestones with chert are deposited during this period in the East Bosnian – Durmitor zone as well. At Kaludarska rijeka section, limestones contain resedimented ooids and echinoderms. In the Pliensbachian coarse-grained Ribnica Breccia was deposited above the Zatrnik Formation in the Bled Basin as a result of tectonic activity that affected the continental margin in this period. Similar breccia is also found above pelagic limestone at Čehotina section in Montenegro. A subsidence pulse in the Bajocian created a stratigraphic gap in the Bled Basin documented with uppermost Bajocian-lower Bathonian radiolarian cherts, which directly overlie the Ribnica Breccia. At Kaludarska rijeka section the oldest radiolarian sample yielded late Aalenian age within a shale/chert interval. Higher in the Kaludarska Rijeka section, radiolarian cherts were also dated as uppermost Bajocian-lower Bathonian. Siliceous sedimentation prevailed in all sections from then onward. The uppermost part of the chert succession was dated only in the Bled Basin as early Tithonian. Deposition of the pelagic Biancone limestone started in the Bled Basin in the late Tithonian, followed by carbonate gravity-flow deposits. These Berriasian gravity-flow deposits, named the Bohinj Formation, contain clasts of shallow-water origin as well as clasts of basalt. They prove the existence of a carbonate platform, named the Bohinj Carbonate Platform, which formed on top of ophiolite nappe stack following the obduction on the continental margin. This platform is not preserved today. Regional correlations suggest that the Bohinj Carbonate Platform may have existed from the Early Kimmeridgian to the Late Berriasian. Similar gravity-flow deposits, previously dated as Berriasian and containing clasts of shallow-water carbonates and basalt were found at Kalinovik section in Bosnia and Herzegovina directly above radiolarites. At Kaludarska rijeka section only pelagic limestone comparable to the Biancone limestone was found. The Bohinj Formation is overlain by mixed carbonate-siliciclastic turbidites in which the proportion of siliciclastic component progressively increases higher in the succession. The Studor Formation in the uppermost part of the Bled Basin succession is composed predominantly of sandstones, whose composition indicates ophiolitic origin, but also have significant amount of carbonate component. The lower part of the formation is dated as late Berriasian-early Valanginian. The Studor Formation correlates to the similar Lower Cretaceous formations containing ophiolitic debris from the Northern Calcareous Alps and the Dinarides (i.e. Rossfeld Fm. and Vranduk Fm.). Sandstone containing clasts of mafic and metamorphic rocks was found at the topmost part of Kaludarska rijeka section. Latest Bajocian-early Bathonian age was obtained from a chert-shale slide block in the mélange of the Krivaja-Konjuh ophiolite complex assigned to the Western Vardar Ophiolitic Unit. The new datation links the Krivaja-Konjuh ophiolite complex with neighboring ophiolite complexes in Croatia and Serbia and provides additional evidence for the Middle Jurassic intraoceanic subduction and mélange formation. The Guevgueli Ophiolite Complex near Demir Kapija, assigned to the Eastern Vardar Ophiolitic Unit was studied for age and facies of the overlying sediments. Cherts in direct contact with basalts are dated to Late Bathonian – Early Callovian with radiolarians. The overstep sequence, informally named the Demir Kapija group, is composed of polymictic conglomerate and a more than 350 m thick carbonate succession. The carbonate succession consists of hemipelagic, slope and platform-margin facies, and contains algae and benthic foraminifers indicative of the Tithonian age. These new data support the previously proposed palaeogeographical connection between the Guevgueli and South Apuseni ophiolite complexes. The newly obtained data are in accordance with previously proposed geodynamic scenario, which included intraoceanic subduction within the Neotethys in the Middle Jurassic and progressive closure through the Early Cretaceous. The data from the Bohinj area confirm that the Bled Basin, whose remnants are now preserved in the eastern Southern Alps, evolved until the Early Cretaceous as a part of the Dinaric orogen.

Bled Basin; Southern Alps; Internal Dinarides; Ophiolites; Radiolaria; Stratigraphy

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