Tea Kolar-Jurkovšek

Stratigraphic evolution of Triassic arc-backarc system in northwestern Croatia

Middle Triassic arc-related extensional tectonics in the western Tethys generated a complex pattern of intra-and backarc basins. We studied volcano-sedimentary successions of subsided continental-margin blocks (Mts. Zumberak and Ivanscica) and of dismembered incomplete ophiolite sequences interpreted as remnants of a backarc basin (Mts. Medvednica and Kalnik) in northwestern Croatia. We dated the successions with radiolarians, conodonts, foraminifers, algae, and sponges. The continental margin experienced a phase of accelerated subsidence in the late Anisian that was approximately coincident with the onset of intermediate and acidic volcanism; pelagic sediments with volcaniclastics accumulated atop subsided carbonate platforms. These relatively shallow basins were later infilled completely by prograding platforms in the late Ladinian-Carnian. In the backarc basin, sea-floor spreading initiated near the Anisian-Ladinian boundary and continued into the late Carnian. Pillow basalts were erupted and interlayered with radiolarian cherts and shales. The studied area was a part of a larger Triassic arc-backarc system preserved in the southern Alps, Alpine-Carpathian Belt, Dinarides, and Hellenides. Volcano-sedimentary successions of Mts. Medvednica and Kalnik are relics of the Meliata-Maliak backarc basin. In comparison to other previously dated oceanic remnants of this system, the longest continuous sea-floor spreading is now documented in one restricted tectonic unit.

A Middle Jurassic Radiolarite-Clastic Succession from the Medvednica Mt. (NW Croatia)

On the NW part of Medvednica Mt. radiolarites with carbonate olistoliths, shales and siltites, matrix-supported conglomerates and basic volcanic rocks were investigated. This facies association is informally named the Poljanica unit. Major element geochemical data indicate deposition of radiolarites in the vicinity of the middle oceanic ridge, while sedimentological data indicate deposition in an area closer to the continent. Shales and siltites, as well as matrix-supported conglomerates, were deposited in short periods characterised by increased input of terrigenous material. Matrix-supported polymict conglomerates are composed of silicified shales, lithic graywackes, cherts and metabasalts, and were deposited by debris flow mechanisms as a consequence of synsedimentary tectonic activity. Carbonate olistoliths are composed of biomicrosparite, and jointly with deformed radiolarian cherts compose an olistostrome. Basic volcanic rocks represent high-Ti tholeiitic basalts formed in the MORB realm. Micropalaeontological investigation of radiolarite samples proved the Middle Jurassic (latest Bajocian - early Bathonian to late Bathonian - early Callovian) age of the Poljanica unit. Additionally, a new radiolarian species Theocapsomma medvednicensis n.sp. has been described. Conodont analyses from carbonate olistoliths in radiolarites proved their Triassic age. The investigated radiolarite-clastic succession is the result of subduction processes. Further continuation of this process caused incorporation of these deposits into the accretionary prism, where they were brought in direct contact with Triassic volcanic rocks and radiolarites (in the form of a tectonic melange). Based on the lithological similarities with the Middle Jurassic turbidite-olistostrome successions in the Western Carpathians and Northern Calcareous Alps, the study area is considered to be part of the Meliata-Hallstatt Ocean.

THE LOWER TRIASSIC SHALLOW MARINE SUCCESSION IN GORSKI KOTAR REGION (EXTERNAL DINARIDES, CROATIA): LITHOFACIES AND CONODONT DATING

The paper aims to present Lower Triassic lithofacies definition and first conodont fauna of Gorski Kotar region, Croatia. The depositional environment is envisaged as shallow marine realm of a passive continental margin. Sedimentary complex differentiates in predominantly carbonate sedimentation that characterises the beginning of deposition with upward increasing trend of terrigeneous influx. Lithofacial units have been defined as oolitic bar facies, lagoonal facies, shoreface-offshore facies, ooid-sandy shoal facies, restricted bay facies and flat-pebble conglomerate facies.The following conodont taxa were collected: Ellisonia sp., Foliella gardenae, Hadrodontina sp., Hindeodus parvus, Hindeodus sp., Pachycladina obliqua, ?Parachirognathus sp., Platyvillosus costatus and Pl. hamadai. The oldest strata yield Hindeodus parvus marking lowermost Triassic. The biostratigraphical data enable recognition of the parvus-isarcicella zones, obliqua Zone and Platyvillosus Subzone. The finds of Hindeodus parvus, Platyvillosus costatus and Pl. hamadai represent their first records in the External Dinarides and enable correlation of the Early Triassic conodont faunas of the Western Tethyan realm.

Middle Triassic carbonate-platform break-up and formation of small-scale half-grabens (Julian and Kamnik–Savinja Alps, Slovenia)

In the Julian Alps (Mt. Prisojnik, NW Slovenia) and in the Kamnik–Savinja Alps (Mt. Križevnik, N Slovenia), both of which form part of the eastern Southern Alps, several meters of Upper Anisian pelagic red nodular, radiolarian-rich limestone (Loibl Formation) were deposited on the drowned platform carbonates of the Contrin Formation. The time of the platform drowning is dated with radiolarians and conodonts to the Illyrian, more precisely to the upper part of the Paraceratites trinodosus Ammonoid Zone. The red limestone is overlain by pyroclastics and volcanics (rhyolites) or carbonate (mega)breccia (Uggowitz Formation). The following unit consists of thin-bedded limestone, grainstone and subordinate marl (Buchenstein Formation) deposited during the final filling of the basin from the adjacent prograding carbonate platform (Schlern Formation) in the Ladinian. Map-scale geometry, neptunian dykes, the onset of volcanism, the presence of (mega)breccia and related paleo-escarpments, the lateral variations in thickness and the wedge-shaped geometry of the lithological units provide evidence of syn-sedimentary block faulting and the formation of small-scale, relatively shallow half-grabens within the previously uniform Slovenian Carbonate Platform. This analysis indicates a clear tectonic control over the development of the Middle Triassic stratigraphy. The described extensional event is well correlated and genetically connected with the syn-rift formation of the neighboring Slovenian Basin and other Southern Alpine basins that formed in connection with the opening of the Meliata-Maliac branch of the Neotethys Ocean.

Geology of the Classical Karst Region (SW Slovenia–NE Italy)

ABSTRACT The paper aims to present the geology of the western part of the Classical Karst (NW Dinarides), located at the border between Slovenia and Italy. The work is based on archive, published and new data collected by Slovenian and Italian researchers within several scientific national and Cross Border Cooperation projects. The map, produced at a scale of 1:50,000, summarizes the lithological and structural setting and is supplemented by three geological cross-sections of the study area.

A fossil feather from the Upper Cretaceous of Kras (Slovenia)

Abstract A fossil feather preserved as a carbonised trace is described from the Tomaj Limestone at Križ, in the Kras region of southwestern Slovenia. The Tomaj Limestone is a platy and laminated limestone with cherts, which occurs within a well-bedded rudist limestone of the Santonian–Campanian Lipica Formation. It was deposited in a lagoon environment and has yielded a diverse fossil assemblage. Whether this feather belonged to a bird or to a dinosaur is unclear, but it is an addition to the scanty record of Late Cretaceous feathers, from a palaeobiogeographically interesting area.

CONODONT DATING OF THE LOWER TRIASSIC SEDIMENTARY ROCKS IN THE EXTERNAL DINARIDES (CROATIA AND BOSNIA AND HERZEGOVINA)

Two Lower Triassic sedimentary successions have been dated by means of conodonts in the External Dinarides: Plavno section near Knin, Croatia and Bosansko Grahovo section in Bosnia and Herzegovina. Deposition in both sections shows similar characteristics, differentiated in three continuously deposited facies. The Siliciclastic facies was previously considered Seis beds and assigned to the lower Lower Triassic, the Mudstone facies, and the Siltstone-mudstone facies (occurring in the upper part of the succession) were formerly considered as Campil beds of the upper Lower Triassic. Vertical succession of Siliciclastic, Mudstone, and Siltstone-mudstone facies of both investigated sequences was interpreted as deepening of the environment envisaged as a transgressive trend in a shallow shelf environment. Facies successions at Plavno (690 m thick) and Bosansko Grahovo (229 m thick) differentiate for the presence of Dolostone facies in the lowest part of the Plavno succession. Conodont fauna of Dolostone facies at Plavno section is represented by isarcicellids, Isarcicella staeschei and I. isarcica (sample 3) that marks the Griesbachian isarcica Zone. The Siliciclastic facies of Plavno and Bosansko Grahovo sections is characterized by shallow-water euryhaline taxa attributed to the Smithian, part of the late Dinerian-Smithian obliqua Zone. This fauna is prevailed by Hadrodontina anceps and Pachycladina obliqua with co-occurrence of Smithian Parachirognathus ethingtoni and very rare presence of Foliella sp. or ?Furnishius sp. Discerned conodont taxa enable us to establish conodont zonation which gives new insight to the range of the so-called Siusi and Campil beds.

Triassic Pelagic Limestones in Pillow Lavas in the Oresje Quarry near Gornja Bistra, Medvednica Mt. (Northwest Croatia)

Investigated outcrops in the Oresje quarry near Gornja Bistra on Medvednica Mt. contain pillow lavas and massive metabasalts with fragments of carbonate rocks between them. The effusive rocks were determined as high-Ti tholeiitic metabasalts, corresponding to those which originated in the MOR area, which today represent obducted, allochthonous parts of Triassic oceanic crust. Carbonate rock fragments are characterised by micrite, biomicrosparite, fossiliferous microsparite, biosparite and sparite types, all of which are more or less recrystallised. From their appearance between pillows, together with hyaloclasts they were determined as peperites. Micropalaeontological analysis of conodonts from the limestone samples indicated a Middle Triassic age, which is also the age of the effusive rocks discovered in the quarry.

Rollmarks of Soft Parts and a Possible Crop Content of Late Cretaceous Ammonites from the Slovenian Karst

Rollmarks associated with Placenticeratidae of the Lower Campanian milleri-bidorsatum group are interpreted as prints of their soft parts, possibly of the hyponome, while a cluster of minute brachiopods is interpreted as the crop content of the decaying ammonite.

Paleogeographic significance of Upper Triassic basinal succession of the Tamar Valley, northern Julian Alps (Slovenia)

Abstract The Julian Alps (western Slovenia) structurally belong to the eastern Southern Alps. The Upper Triassic succession mostly consists of shallow water platform carbonates of the Dolomia Principale-Dachstein Limestone system and a deep water succession of the Slovenian Basin outcropping in the southern foothills of the Julian Alps. In addition to the Slovenian Basin, a few other intraplatform basins were present, but they remain poorly researched and virtually ignored in the existing paleogeographic reconstructions of the eastern Southern Alps. Herein, we describe a deepening-upward succession from the Tamar Valley (north-western Slovenia), belonging to the Upper Triassic Tarvisio Basin. The lower, Julian-Tuvalian part of the section comprises peritidal to shallow subtidal carbonates (Conzen Dolomite and Portella Dolomite), and an intermediate carbonate-siliciclastic unit, reflecting increased terrigenous input and storm-influenced deposition (Julian-lowermost Tuvalian shallow-water marlstone and marly limestone of the Tor Formation). Above the drowning unconformity at the top of the Portella Dolomite, Tuvalian well-bedded dolomite with claystone intercalations follows (Carnitza Formation). The latter gradually passes into the uppermost Tuvalian–lowermost Rhaetian bedded dolomite with chert and slump breccias, deposited on a slope and/or at the toe-of-slope (Bača Dolomite). Finally, basinal thin-bedded bituminous limestone and marlstone of Rhaetian age follow (Frauenkogel Formation). The upper part of the Frauenkogel Formation contains meter-scale platform-derived limestone blocks, which are signs of platform progradation. The Tarvisio Basin may have extended as far as the present Santo Stefano di Cadore area, representing a notable paleogeographic unit at the western Neotethys margin.