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Jurassic and Cretaceous siliceous deposits of the distal Adriatic continental margin (Bled Basin, Julian Alps, NW Slovenia)

The Julian Alps in NW Slovenia preserve remnants of several paleogeographic units of the northeastern Adriatic continental margin. Among these units, the Bled Basin had the most distal position on the continental margin. This paleogeographic location is assumed because the flysch-type sedimentation in the Bled Basin started in the Early Cretaceous but significantly later, in the Campanian to Maastrichtian, in more proximal basins of the Julian Alps. Here we present preliminary results on sedimentology and biostratigraphy of the Bled Basin. The Lower Jurassic succession, overlying Norian-Rhaetian limestones with chert, is characterized by bedded, often cherty limestones with echinoderms that laterally and vertically pass into massive oolitic limestone. The uppermost part consists of several beds of coarse-grained carbonate breccia that contain large limestone and chert clasts, and also irregularly shaped folded layers of chert. Based on the foraminifer Involutina liassica (Jones) found in breccia clasts and regional stratigraphic correlations, the Pliensbachian is the most probable age of this breccia. The overlying lithostratigraphic unit comprises radiolarian cherts and shales. The lower part consists of several meters thick slumped chert beds with almost no shale. The content of shale significantly increases higher up in the succession. Chert beds are generally laminated. In places, normal grading and channel structures clearly indicate that the chert beds were deposited as low-density turbidites. Radiolarian assemblages from cherts give age constraints from the middle Bathonian-early Callovian to the late Kimmeridgian-early Tithonian, suggesting a significant stratigraphic gap between this unit and the underlying breccia. In the upper part, cherts and shales alternate with marly limestone and are overlain by laminated limestone, which passes into typical Biancone limestone. Radiolarians from the uppermost beds of the Biancone limestone indicate a latest Tithonian to earliest Berriasian age. The Biancone limestone is overlain by gravity-flow deposits composed mostly of shallow-water limestone clasts and isolated bioclasts (e.g. Clypeina jurassica Favre) derived from a penecontemporaneous carbonate platform. The overlying succession starts with siliceous limestone with a significant proportion of marl and then passes into mixed siliciclastic-carbonate turbidites. These turbidites represent the oldest synorogenic deposits in the area. The first mixed turbidites are dated with radiolarians as Berriasian to early Valanginian. Higher up in the succession Valanginian-Hauterivian calcareous nannofossils were found. During the extensional phases in the Late Triassic and Early Jurassic the Bled Basin was a relatively deep basin as evidenced by bedded limestones containing chert nodules and layers. This tectonic activity culminated in the Pliensbachian subsidence pulse, which was responsible for the deposition of the coarse-grained breccia. During the next major subsidence phase in the Bajocian, the Bled Basin evolved into a true deep-water basin with radiolarian cherts as back-ground sediments. Since the Bled Basin was facing the Meliata-Maliac-Vardar Ocean, which was already in a convergent regime in the Middle Jurassic, tectonic down-warping is the likely explanation for this subsidence phase. The Berriasian limestone breccia in the Bled Basin provides 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 turbidites with clastic admixtures document the evolution of the Bled Basin into a typical foreland basin.

Jurassic; Cretaceous; Siliceous deposits; Radiolarians; Bled Basin

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