| Abstract (english) | Middle and Upper Jurassic carbonates comprise the first large-scale sequence deposits in Istria, ending with the deposition of the Oxfordian to lowermost Kimmeridgian Muča unit. This succession is overlain by the second large-scale sequence (Upper Tithonian‒Lower/Upper Aptian) which begins with the deposition of the upper Tithonian Kirmenjak unit. These two large-scale sequences are separated by important discontinuity characterized by stratigraphic hiatus of different duration, ref lecting a compressional tectonic event caused by ophiolite obduction along the NE Adria Microplate margin. The beginning of this subaerial exposure phase is marked by the Rovinj breccias formed during the regression that preceded the emergence. These breccias display a gradual transition from Muča and Lim units and are composed of fragments belonging to both units. This regionally recognized subaerial exposure phase is locally marked by bauxites that filled paleo-depressions within the Lim and Muča unit limestones and the Rovinj breccias. In other places subaerial exposure is mostly recorded by palaeosols, and one of the best examples can be found at Zlatni rt locality in Rovinj. The palaeosol marks the contact between Muča and Kirmenjak units. The Muča unit consists of two lithofacies (LF) types, representing high to moderate energy marine environments: LF 1 – bioclastic-peloidal grainstone to rudstone characterized by common to abundant peloids and benthic foraminifera, rare ooids, algae, fragments of echinoderms, hydrozoans, coated fragments of Cladocoropsis, stromatoporids, bivalves, Rivulariacean-like (Cayeuxia) cyanobacteria, corals and Lithocodium; and LF 2 – bioclastic- peloidal packstone with common to abundant peloids, benthic foraminifera and fragments of echinoderms. The beginning of regression was marked by Rovinj breccias, which form lenses atop of Muča unit. The palaeosol forms a decimetre-thick horizon of grey clay, which is also present as infills in karstified channels and fissures of the Muča unit and Rovinj breccia. Among other minerals, the palaeosol contains glauconite that formed during the incipient flooding of the carbonate terrain. As the transgression progressed, the palaeosol was covered by a decimeter-thick layer of transgressive breccia, containing fragments of Muča unit and upper Tithonian black pebbles. The formation of black pebbles under vadose conditions is evidenced by the presence of Microcodium-like structure, alveolar-septal fabric (in voids of fenestral origin?) and other features indicating subaerial exposure. In the area of Zlatni rt the Kirmenjak unit, which covers the palaeosol and the breccia, begins with a bioclastic (algal)-peloidal grainstone to rudstone abundant in bioclasts of Campbelliella striata, Clypeina sulcata, Salpingoporella annulata and Favreina faecal pellets. The rest of the Kirmenjak unit is composed of a cyclic alternation of mudstones, peloidal to bioclastic packstones to grainstones and black pebble breccias with subaerial exposure surfaces. A very well preserved palaeosol level at the Zlatni rt locality documents one of unique terrestrial palaeoenvironments that existed during the early Kimmeridgian to late Tithonian in the northern part of the Adriatic Carbonate platform. |
