engleski

The significance of the Middle Triassic cephalopod bearing strata in the vicinity of Kunovac Vrelo, southern Lika, External Dinarides

In the area of Kunovac Vrelo, near Gračac in the southern part of Lika (Croatia), a significant cephalopod bearing strata were formerly recognized by many researches (KOCH, 1911 ; SALOPEK, 1913 ; SALOPEK, 1914 ; ŠUŠNJAR et al., 1974 ; ŠUŠNJAR & BUKOVAC, 1978 ; GRIMANI et al., 1975). They recorded a rich cephalopod assemblage in the Middle Triassic strata of Ladinian age. SALOPEK (1914) identified Ptychites, Ceratites, Arcestes, Pinacoceras and Gymnites ammonoid genera, some of which belong to the formerly known Ceratites trinodosus ammonoid zone of the Illyrian age, previously dated as of Ladinian age ; while other species suggested the transition towards the Ladinian. Two section were recorded: “Mali Kunovac Vrelo” section (20, 8 m thick) and “Kunovac Vrelo” section (3, 6 m thick). Two recorded section are considered to be continuous, although a covered interval is present between them. “Mali Kunovac Vrelo” represents the older strata, while the “Kunovac Vrelo” section represents younger strata of the same depositional sequence. In both sections (“Mali Kunovac Vrelo” and “Kunovac Vrelo”), two types of lithofacies have been determined: Limestone facies and Pyroclastic facies. The Limestone facies is present at the beginning of the “Mali Kunovac Vrelo” section and it begins with massive, grey bioclastic limestone beds, containing predominantly shallow-marine biota (algae, echinoids and foraminifera), subordinately pelagic organisms (crinoid fragments), and various amount of lime mud. Limestones with volcaniclastic material conformably overly grey bioclastic limestones. They contain hyaline rock fragments, plagioclase and quartz crystaloclasts, deposited together with carbonate detritus of crinoids and radiolarians, cemented with sparry calcitic cement. The top of the Limestone facies in “Mali Kunovac Vrelo” section is marked by the presence of the limestones with predominantly pelagic organisms (radiolarians, thin-shelled bivalves and crinoids), incorporated within a lime mud. Pyroclastic facies consists of two lithologies: crystaloclastic and vitriclastic tuffs, is present at the very top of “Mali Kunovac Vrelo” section. The deposition at “Kunovac Vrelo” section began with limestones containing poorly sorted resedimented limestone bioclasts, together with shallow-marine fossils. Limestones are overlain by pelagic limestones with radiolarians, thin- shelled bivalves, sponge spicules and crinoid fragments. All limestone types have lime mud in intergranular space and scarce volcaniclastic material. The very top of the section is marked again by the presence of Pyroclastic facies, containing crystaloclastic and vitriclastic tuffs. The majority of cephalopod fauna is collected from the interval of limestones with volcaniclastic material (Limestone Facies) at “Mali Kunovac Vrelo” section. Ptychites, Arcestes, Gymnites genera were determined and, for the first time in the External Dinarides, Reitziites reitzi species is found, marking the Reitziites reitzi ammonoid zone of the Middle Illyrian. The investigated sections imply that in the Upper Illyrian time External Dinarides were severely influenced by volcanism and tectonic activity, causing abrupt changes in sedimentary environments.

Middle Triassic, ammonoids, biostratigraphy, External Dinarides, Croatia

nije evidentirano