engleski

POSSIBLE OCCASIONAL RECOVERY OF CARBONATE SEDIMENTATION WITHIN ISTRIAN FLYSCH BASIN

During geological mapping of Istria, numerous carbonate beds (megabeds) were recorded within the informal lithostratigraphic unit known as Istrian flysch. Istrian flysch deposits are characterized mostly by the typical alternations of hemipelagical marls and gravity-flow deposits. The monotonous succession of marls and mixed carbonate-siliciclastic sandstones is intercalated with several thick carbonate beds (megabeds) composed of breccias, conglomerates, bioclastic arenites/siltites and marls. The thickness of these megabeds is 0.5 – 5 m, rarely over 10 m. The megabeds are interpreted as a complex sequences of debrites and turbidites and characterize the lower part of the basin fill (Bergant et al. 2003). In eastern Istria, near the town of Pićan, an unusual carbonate layer is mapped, named “Pićan bed“, whose genesis can not be explained by the depositional mechanisms of turbidite currents or debris flows. The aim of this research is to define the depositional environments of the “Pićan bed“. For this purpose a detailed field investigation was performed and a detailed sedimentological section was logged at Pićan (Pić-I). It is estimated that the base of the investigated section is composed of normal, tectonically undisturbed Paleogene succession, which begin with the “Foraminiferal limestone” informal litostratigraphic unit, which gradually transits into “Marls with Crabs” and then into “Globigerina marls”. The interval of massive “Globigerina marls” is 100 m thick and transits into the Istrian flysch unit. Flysch deposits start with the first occurrence of calcarenite beds and are generally covered by vegetation in the selected section but are well documented in the surrounding area. Preliminary results show that the “Globigerina marls” and Flysch belong to the NP17 Calcareous Nannofossil zone according to investigated nannofossil assemblage. The “Pićan bed” occurs in the top part of the section, is about 18 m thick and is homogeneous with no visible internal structure. Its microfacies is described as wackestone-packstone, with bioclasts: numulites, ortophraminids and corallinaceas. According to its petrographical characteristics, “Pićan bed“ is very likely a results of a similar carbonate sedimentation as characteristic for the underlying Foraminiferal limestones. REFERENCES: Bergant, S., Tišljar, J., Šparica, M. 2003: Eocene carbonates and flysch deposits of the Pazin Basin. In: Vlahović , I., Tišljar, J. (eds.): Evolution of Depositional Environments from the Paleozoic to the Quaternary in the Karst Dinarides and the Pannonian Basin, 22nd IAS Meeting of Sedimentology-Opatija 2003, Field trip guidebook, 57–63.

Flysch, Carbonate ramp, Eocene

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