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Jurassic–Early Cretaceous pelagic succession of the eastern Southern Alps (NW Slovenia) and its correlation with the Central Dinarides (Bosnia and Herzegovina, Montenegro)

The eastern Southern Alps, including the Julian Alps in NW Slovenia, are a structurally complex area where the South Alpine and the Dinaric structures overlap. The Julian Alps preserve remnants of several paleotopographic units (basins and swells), of which the Bled Basin (Cousin, 1981) had the most distal position on the Adriatic continental margin facing the Neotethys. This paleogeographic location is assumed because the flysch-type sedimentation in the Bled Basin started in the Early Cretaceous, significantly earlier than the Campanian to Maastrichtian start in more proximal basins of the Julian Alps. Continental-margin successions of comparable stratigraphy/paleogeography occur in the Central Dinarides. These successions are structurally sandwiched between the External Dinarides and the Western Vardar ophiolites that were obducted onto the Adriatic margin during the Late Jurassic. The latest Triassic to Early Cretaceous stratigraphic evolution of the Bled Basin was restored from several partial sections in vicinity of Bohinj in the Julian Alps. Three additional sections were studied in the Central Dinarides: Kalinovik in Bosnia and Herzegovina, and Kaludarska rijeka and Čehotina in Montenegro. These sections were previously investigated by Cadet (1978) and Rampnoux (1974), respectively. Our stratigraphic research is primarily based on radiolarian dating. During the Late Triassic and early 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 were deposited during this period in the Central Dinarides as well and are well exposed in all three sections studied. At Kaludarska rijeka section these limestones contain resedimented ooids in addition to echinoderms. In the Pliensbachian the 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. A similar breccia is also found above pelagic limestone at Čehotina in Montenegro. A subsidence pulse in the Bajocian was responsible for a stratigraphic gap in the Bled Basin. This gap is documented with uppermost Bajocian-lower Bathonian (UAZ 5 of Baumgartner et al., 1995) radiolarian cherts, which directly overlie the Ribnica Breccia. At Kaludarska rijeka the oldest radiolarian sample yielded late Aalenian age (UAZ 2) within a shale/chert interval. Higher in the Kaludarska rijeka section, radiolarian cherts were also dated as latest Bajocian-early Bathonian (UAZ 5). 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 (UAZ 13) and was followed by carbonate gravity-flow deposits. These Berriasian gravity-flow deposits, named the Bohinj Formation (Kukoč et al., 2012), 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 (Kukoč et al., 2012), which formed on top of an 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 (Cadet, 1978) and containing clasts of shallow-water carbonates and basalt were found at Kalinovik in Bosnia and Herzegovina directly above radiolarites. At Kaludarska rijeka, only pelagic limestone comparable to the Biancone limestone is exposed. 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 carbonate clasts are also common. The lower part of the formation is dated as late Berriasian-early Valanginian (UAZ 13-16). The Studor Formation correlates to the similar Lower Cretaceous formations containing ophiolitic debris in the Northern Calcareous Alps and Dinarides (Rossfeld Formation and Vranduk Formation, respectively). Sandstones with clasts of mafic and metamorphic rocks were found at the topmost part of the Kaludarska rijeka section. Mixed carbonate-siliciclastic turbidites with progressively increasing proportion of siliciclastics are a time-transgressive unit of the entire orogen. Berriasian turbidites at the top of the Kalinovik and Kaludarska rijeka sections in the Central Dinarides are also assignable to this unit. For detailed reconstruction of the Early Cretaceous foreland basin(s) a more systematic biostratigraphic research of this unit is needed. The newly obtained data are in accordance with previously proposed geodynamic scenario, which included opening and an extensional phase within the Neotethys in the Triassic and possibly Early Jurassic, intraoceanic subduction in the Middle Jurassic and progressive closure from the Late Jurassic 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 in the Early Cretaceous as part of the Dinaric orogen.

Južne Alpe; Dinaridi; jura; kreda

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