Špela Goričan

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.

Triassic Radiolarites from Mts. Kalnik and Medvednica (Northwestern Croatia)

Bedded radiolarites of Mt. Kalnik are of Carnian-Norian age, while similar rocks of Mt. Medvednica are of Late Ladinian - Carnian age, as determined on the basis of radiolarian assemblages. Radiolarites from Mt. Kalnik concordantly rest upon the pillow-lavas, or are interbedded with them. Their footwall at Mt. Medvednica is not known, due to tectonic reduction. The sedimentary rocks described were deposited from low-density turbidity currents, therefore representing fine-grained turbidites. They were deposited closely below, and partially above the CCD. Results of petrological, sedimentological and geochemical investigations indicate sedimentation under terrigenous influence with contemporaneous hydrothermal activity. The studied sections are correlated with the nearest outcrops of Triassic radiolarites associated with basic effusive rocks, i.e. with the Vardar Zone and the Meliata 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.

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.

Middle Jurassic age of basalts and the post-obduction sedimentary sequence in the Guevgueli Ophiolite Complex (Republic of Macedonia)

The Guevgueli Ophiolite Complex near Demir Kapija (Eastern Vardar Ophiolitic Unit) was studied for the age and facies of the overlying sediments. Cherts in direct contact with basalts are dated to late Bathonian–early Callovian with radiolarians. The post-obduction sequence, here informally named the Demir Kapija group, is composed of polymictic conglomerate, probably Kimmeridgian in age, and a more than 350-m thick carbonate succession. The carbonate succession consists of hemipelagic, slope and platform margin facies and contains algae and benthic foraminifers indicative of the Tithonian age. These new data support the previously proposed palaeogeographical connection between the Guevgueli and South Apuseni ophiolite complexes.

Erosion of a Jurassic ophiolitic nappe-stack as indicated by exotic components in the Lower Cretaceous Rossfeld Formation of the Northern Calcareous Alps (Austria)

Abstract The microfacies and biostratigraphy of components in mass-flow deposits from the Lower Cretaceous Rossfeld Formation of the Northern Calcareous Alps in Austria were analysed. The pebbles are classified into six groups: 1) Triassic carbonates (uppermost Werfen to basal Gutenstein Formations), 2) Upper Jurassic to lowermost Cretaceous carbonates (Oberalm Formation and Barmstein Limestone), 3) contemporaneous carbonate bioclasts (?Valanginian to ?Hauterivian), 4) siliceous pebbles (radiolarites, ophicalcites, siliceous deep-sea clays, cherts), 5) volcanic and ophiolitic rock fragments and 6) siliciclastics such as quartz-sandstones and siltstones. The radiolarites show three age groups: Ladinian to Early Carnian, Late Carnian/Norian and Late Bajocian to Callovian. The Middle Triassic radiolarites are interpreted as derived from the Meliata facies zone or from the Neotethys ocean floor, whereas the Late Triassic radiolarites give evidence of the sedimentary cover of the Neotethys ocean floor. During late Early to early Late Jurassic, the Triassic to Early/Middle Jurassic passive margin of the Neotethys attained a lower plate position and became obducted by the accreted ocean floor of the Neotethys Ocean. The accreted ocean floor was contemporaneously eroded and resedimented in different deep-water basins in front of the nappe-stack. These basin fills were subsequently incorporated in the orogen forming mélanges in this complex ophiolitic nappe-stack. The Middle Jurassic radiolarites are interpreted as the matrix of these mélanges. Together with the volcanic and ophiolitic material the siliceous rocks were eroded from this ophiolitic nappe-stack in Early Cretaceous times and brought by a fluvial system to the Rossfeld Basin within the Tirolic realm of the Northern Calcareous Alps. The different fining-upward sequences in the succession of the Lower Cretaceous Rossfeld Formation can be best explained by sea-level fluctuations and decreasing tectonic activity in the Jurassic orogen