Jurassic and Cretaceous radiolarian stratigraphy of the Southern Alps in northwestern Slovenia: age constraints on basin dynamics at the edge of the Neotethys

The study area belongs to the eastern Southern Alps where the Southalpine and the Dinaric structures now overlap. In the Mesozoic times, this area belonged to the eastern Adriatic continental margin facing the Meliata-Maliac Ocean, which opened in the Middle Triassic as an embayment of the Neotethys. On its western side, the Adriatic plate was bordered by the Alpine Tethys, which opened in the Middle Jurassic and was part of the Atlantic system. Precise stratigraphic research in this area can thus provide clues for better understanding of interrelation between rifting histories of the two oceanic domains and geodynamic evolution of the ensuing Alpine-Dinaride-Carpathian orogenic system.

Four paleogeographic units are differentiated in the study area: the Bovec and Bled troughs that formed near the end of the Early Jurassic, the Julian High (a pelagic plateau since the Middle Jurassic), and the Tolmin Trough, which existed since the earliest Jurassic and was bordered by a large stable carbonate platform. Among these basins, the Bled Trough occupied the most distal position on the continental margin. This position is inferred from the age of flysch-type deposits that started to accumulate in the Early Cretaceous in the Bled Trough but much later, in the Campanian-Maastrichtian, in the Tolmin Trough.

Complete Lower Jurassic to mid-Cretaceous successions of these paleogeographic units are well exposed. For each unit we have selected reference sections, in which the sedimentary record and its fossil content will be examined in detail. The emphasis will be given to those parts of the stratigraphic record that are relevant for the reconstruction of basin dynamics. Precise age constraints will be primarily obtained through paleontological studies, mostly on radiolarians and, in the Cretaceous, also on nannoplankton. The successions will be correlated with corresponding successions in the Dinarides (from Croatia to Greece), Northern Calcareous Alps and Carpathians (Austria, Slovakia to Romania), and the Southern Alps (northern Italy).

The aim of the proposed project is 1) to study in detail the key sections, for which the existing data are poor or out-of-date, and 2) to compile relevant stratigraphic data from different localities into an integral correlation, which will synthesise discernible tectono-stratigraphic events and their timing. The correlation will focus on the following events:

• Subsidence phases in the Early and Middle Jurassic (probably one in the Pliensbachian and another one in the Bajocian).
• Early emplacement of ophiolite nappes in the most internal units and development of carbonate platforms on top of emplaced ophiolites (possibly Late Jurassic to earliest Cretaceous).
• Formation of initial foreland basins (Early Cretaceous).
• Renewed subsidence (associated with an erosional event) in the Tolmin Trough, possibly coinciding with formation of large neptunian dykes on the margins of the Julian High (mid-Cretaceous).

The relevance of the expected results can be summarized as follows:

• 1) Precise age constraints will elucidate the Jurassic and Cretaceous basin dynamics in the study area and will contribute to further development of palinspastic models on a regional scale.




• 2) Accurate dating of different formations will, in addition, enable global correlations and interpretation of basin fill in terms of climate change and eustatic sea-level fluctuations.




• 3) On a more local scale, the refined stratigraphy will serve as a basis to improve tectonic reconstructions of the Julian Alps.




• 4) The hitherto poorly dated and unnamed lithostratigraphic units will be formally described. These results will improve the local stratigraphic nomenclature.




• 5) An important objective of the project is to promote Slovenian natural heritage. The sites to investigate are located in the Triglav National Park and will be evaluated in terms of conservation, protection and educational value.


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