engleski

Radiolarian dating of Lower Cretaceous carbonate gravity-flow deposits from Bohinj area (NW Slovenia) : significance for reconstruction of a lost carbonate platform in the Internal Dinarides

Carbonate gravity-flow deposits at Srednja vas near Bohinj have been studied for radiolarian dating and composition of resedimented limestones. Paleogeographically, the area was part of the Bled Trough, which had a relativelly distal position on the Adriatic continental margin and was rather far from the stable Dinaric Carbonate Platform. The Jurassic to Lower Cretaceous succession of the Bled Trough consists of: Lower Jurassic limestone with echinoderms (Hierlatz facies), Middle and Upper Jurassic bedded radiolarian cherts and shales, Biancone limestone, alternation of marls and shales, and finally siliciclastics with limestone olistoliths and ophiolite debris. This research is focused on the Biancone limestone, an approximately 40 m thick series of well bedded pelagic limestone, which includes intercalations of resedimented carbonates. The lower part of the studied section consists of radiolarian packstone with chert nodules and layers, and in places contains thin interlayers of marl. Radiolarian assemblages from these beds indicate an Early Cretaceous age (Berriasian – early Valanginian). The age assignment is based on the following species: Archaeodictyomitra apiarium (Rüst), Cinguloturris cylindra Kemkin & Rudenko, Dicerosaturnalis dicranacanthos (Squinabol), Hiscocapsa pseudouterculus (Aita), Mirifusus minor Baumgartner, Pantanellium squinaboli (Tan), Pseudodictyomitra carpatica (Lozyniak), Ristola cretacea (Baumgartner), and Tethysetta boesii (Parona). The upper part consists mainly of carbonate breccias. Microfacies analyses showed angular to subangular shallow-water grainstone lithoclasts, ooid grains, fragments of calcareous algae, miliolid foraminifera and various other skeletal fragments of different size. Clasts of pelagic limestone are also present and contain Calpionella alpina Lorenz whose range is late Tithonian to earliest Valanginian. The matrix of breccias is radiolarian-rich lime mudstone. The age of the platform carbonates is determined on the basis of calcareous alga Clypeina jurassica Favre which is characteristic of late Kimmeridgian to earliest Berriasian. In the uppermost part, carbonate breccias alternate with calcarenites. Slump folds are common. Extraclasts and calcareous debris undoubtedly prove a platform origin of resedimented limestone. The Biancone limestone of the Tolmin Trough, which occupied an intermediate position between the Dinaric Carbonate Platform and the Bled Trough, consists of micrite without resedimented limestones. Therefore we conclude that the Dinaric Carbonate Platform could not be the source of shallow-water carbonates in the Bled Trough. We believe that these breccias represent evidence of a carbonate platform which must have been located more internally but is now not preserved. This inferred platform may have developed on top of a nappe stack, which formed during the early emplacement of the internal Dinaric units onto the continental margin. Genetically similar isolated carbonate platforms of more or less the same age are known from several localities in the Alpine – Dinaride – Carpathian mountain belt. The Oxfordian/Kimmeridgian to Berriasian Plassen Carbonate Platform of the Northern Calcareous Alps in Austria and the Kimmeridgian? – Tithonian Kurbnesh Carbonate Platform from the Mirdita Ophiolite Zone in Albania have so far been the best documented. Other examples include Upper Jurassic reef limestones unconformably overlying the Vardar ophiolites in Serbia and the South Apuseni ophiolites in Romania.

Biancone limestone; Calcareous breccia; Radiolarians; Berriasian; Bled Basin; Bohinj Formation

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