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Mesozoic deep-water basins of the eastern Southern Alps (NW Slovenia)

The field-trip deals with the eastern Southern Alps where the Southalpine and the Dinaric structures now overlap. In the Mesozoic times, this area belonged to the northeastern Adriatic continental margin facing the Meliata-Maliac-Vardar Ocean but was, on the other side, also close to the Alpine Tethys. Several paleogeographic units are differentiated in the area. The Bled Basin occupied the most distal position on the continental margin. This basin was characterized by deep-water sedimentation from the Norian onwards and by a relatively early onset of flysch-type sedimentation in the Valanginian-Hauterivian. The Julian High was a pelagic plateau that formed when the Triassic to Early Jurassic carbonate platform (the Julian Carbonate Platform) was dissected into blocks, subsided and drowned. The Bovec Basin evolved during the Pliensbachian to the Bajocian on a more deeply subsided block of the same platform. The Tolmin Basin existed between the Julian High and the stable Dinaric Carbonate Platform from the Triassic to the end of the Cretaceous. Based on the stratigraphic record, the Norian to Albian evolution of these sedimentary basins is divided in the following stages: 1) In the Norian and Rhaetian, the lagoonal Dachstein limestone and marginal coral reefs were deposited on the Julian Carbonate Platform. Limestones and dolomites with chert accumulated in the Bled and Tolmin basins. These deeper-water deposits include fault-derived coarse-grained breccias. Until the Pliensbachian, both basins were characterized by resedimented carbonates shed from the coeval Julian Carbonate Platform. 2) The main subsidence phase occurred in the Pliensbachian and is documented by a shift to distal-shelf environment and the first generation of neptunian dikes on the subsided platform, coarse-grained breccia in the Bled Basin, and a change in composition of resedimented limestones (crinoidal limestones start to prevail) in the Tolmin Basin. The Toarcian was a tectonically quiet period. Clay-rich deposits including Early Toarcian black shales are preserved in the Bovec and Tolmin basins. 3) The subsequent subsidence phase in the Bajocian caused further deepening of the entire area. Until the late Early Tithonian condensed Rosso Ammonitico type limestones accumulated on the Julian High, whereas the surrounding basins were characterized by radiolarian cherts and shales. Carbonate gravity-flow deposits, this time sourced from the Dinaric Carbonate Platform, were common in the Tolmin and Bovec basins, but absent in the Bled Basin. 4) In the Late Tithonian, the Biancone limestone started to accumulate in all paleogeographic units of the area. The Berriasian limestone breccias in the Bled Basin provide evidence of an isolated carbonate platform that, after the ophiolite emplacement and uplift, formed in more internal Dinaric units and was later eroded. The overlying Valanginian-Hauterivian turbidites with clastic admixtures document the evolution of the Bled Basin into a typical foreland basin. 5) Around the Aptian-Albian boundary, the Early Cretaceous orogeny was intensified and caused a renewed subsidence (associated with an erosional event) in more external paleogeographic units. Coarse-grained polymictic breccias accumulated in the Tolmin Basin. The second generation of deep breccia-filled neptunian dikes on the Julian High was most probably also related to this tectonic event.

Mesozoic; Deep-water basins; Julian Alps

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