engleski

Sedimentology of Istrian flysch Megabeds

The Istrian Foreland basin was formed during the Eocene period in collisional processes between the Adria microplate and the Eurasian plate as a foredeep depozone in front of the migrating Dinaridic wedge (KORBAR, 2009). Its sedimentary fill consists of Foraminiferal limestones, Transitional beds (Globigerina marls) and Istrian flysch which represent the lower, middle and upper unit of the underfilled peripheral foreland basin, respectively (SINCLAIR, 1997 ; OTONIČAR, 2007). The distinct packages within the Istrian flysch are interpreted as deposits of deep sea environments, either distal fan or base of the slope. Istrian flysch is characterized by the alternation of hemipelagic marls and gravity-flow deposits (BERGANT et al., 2003). The gravity-flow deposits are 5–40 cm thick turbidites, developed mostly as plane-parallel laminated and cross-laminated sandstone beds (shale-sandstone/siltstone couplets without a Bouma Ta division). Complete Ta-e Bouma sequences are rare. Sandstones are of mixed carbonate-siliciclastic composition. The monotonous succession of marls and mixed carbonate-siliciclastic sandstones is intercalated with several thick carbonate beds composed of breccias, conglomerates, calcarenites/calcsiltites and marls. They show significant thickness, occasionally more than 10 m (BERGANT et al., 2003). These carbonate beds are interpreted as complex single event deposits, composed of rockfall and avalanche deposits, debrites and turbidites and are labeled as Megabeds. Some of them become thinner and pinch out laterally, but thicker ones are continuous over distances of more than 10 km. Here, we describe Istrian flysch megabeds and present their variations, mechanisms and environments of their deposition. For this purpose several sedimentological sections in Central Istria and Labin area have been documented and are presented here. Most megabeds have normally graded, clast supported breccias/conglomerates in the lower part, with up to boulder-size clasts. This part of megabed was deposited by a rockfall or a debris flow. The composition of lower part clasts are: Cretaceous limestones, different lithofacies types of Foraminiferal limestones, bioclasts, skeletal fragments of large benthic foraminifera (nummulites and orthophragminides), calcarenites and marl clasts (rip-up clasts). This features imply a short transport and base of the slope environment. The upper part of the megabed is a turbidite, suggesting flow transformation into high-density turbidity current (HAUGHTON et al., 2009). The thickness of these two parts can vary greatly. Clast composition of the upper, turbidite part is similar, but clasts are smaller and mainly composed of bioclasts and skeletal fragments of large benthic foraminifera (Nummulitic breccia). The turbidite parts show normal grading into calcarenites. Furthermore, some megabeds have only the turbidite part which in some cases can be thick up to 5 m. The provenance of megabed material is from Istrian foreland ramp. The research has been supported by Basic geological map of the Republic of Croatia 1:50, 000 project (HGI) and the Croatian Science Foundation project IP-2016-06-1854, GEOSEKVA.

Turbidity current ; Concentrated density flow ; Rockfall ; Composite bed ; Foreland basin ; Dinaride

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