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Jurassic – Early Cretaceous radiolarians of the Danubian Nappes (eastern Serbia)

The Danubian Nappes of eastern Serbia belong to a much larger Dacia Mega-Unit. The Dacia Mega-Unit actually comprises different allochthonous smaller units in East and South Carpathians that are considered to be parts of the European continent and were detached from it during Jurassic rifting in the domain of the Alpine Tethys (Schmid et al., 2008). Some of these units were separated from Europe by oceanic lithosphere of the Ceahlau-Severin oceanic domain, while other units were just later scraped off the European margin due to strong coupling of the orogenic wedge and the foreland (Ziegler et al., 1995). The Danubian Nappes belong to the latter group. According to Schmid et al. (2008), the Danubian Nappes were detached from the Moesian foreland. Together with more internal units located underneath the Transilvanian Basin and outcropping in the North Apuseni Mountains (Tisza Mega-Unit ; Haas & Pero, 2004), they invaded the Carpathian embayment in Cenozoic times (i.e. Royden, 1988 ; Csontos & Vörös, 2004 ; Horváth et al., 2006) and finally docked with the European foreland during the Miocene. Along the western rim of the Moesian platform, the units of the South Carpathians are followed further southward into eastern Serbia and western Bulgaria where these units are referred to as Carpatho-Balkanides and Serbian-Macedonian Massif (Dimitrijević, 1997). Carpatho-Balkanides in Serbia consist of several zones. The boundaries between these zones are clear, marked by distinct longitudinal faults, but their mutual relationships have been interpreted in different ways. The Danubian units are locally also known as Miroč Unit (west of the Timok fault) and Vrška Čuka Unit (east of the Timok fault) (Dimitrijević 1997), Vrška Čuka-Miroč terrane (Karamata 2006), or as Danubicum (Danubian parachton) (Grubić et al., 1997). Jurassic–Lower Cretaceous system in the Danubian Nappes is developed in the area of Pesača - Greben and Miroč Mt. Jurassic sediments of these areas lie discordantly over the Permian red siliciclastic and volcaniclastic rocks. In both areas, sedimentation started with clastic deposits (quartz sandstone and conglomerate) during the Liassic and lasted until the middle of the Dogger. After the differentiation of the marginal shelf, a deep-water environment with condensed sedimentation of mostly limestone and rarely of siliciclastic rock, was formed in the Pesača – Greben area (Vasić et al., 1997). Flaggy cherty limestone, which bears Malm ammonites and tintinnids, gradually transits upwards into Berriasian to Hauterivian limestone, marlstone and shale, then into Barremian–Aptian marlstone and shale intercalated with marly limestone which contains ammonites. The Mesozoic succession ends with Albian shales, marlstone and marly limestone (Dimitrijević, 1997). Despite the available data, our knowledge on the Mesozoic microfauna from the Danubian Nappes is still insufficient. Particularly scarce are published data on Jurassic – Early Cretaceous microfauna on which this study concentrates. More microfossils are known from equivalent units in Romania. A moderately-preserved Late Jurassic and some excellently preserved Early Cretaceous radiolarian assemblages have been reported from Sviniţa (Dumitrica, 1995), which is located in close vicinity to our sections in Serbia. The information on Jurassic – Early Cretaceous Radiolaria from NE Serbia presented here comes from a few sections which crop out along the road Dobra – Boljetinsko brdo (Tunel 21 at 21°59.5’E longitude, 44°37.0’N latitude ; Tunel 19 at 22°00.1’E longitude, 44°36.1’N latitude ; Tunel 18 at 22°00.1’E longitude, 44°36.1’N latitude ; Tunel 10 at 22°01.0’E longitude, 44°34.1’N latitude ; Boljetinska reka 22°01.5’E longitude, 44°32.1’N latitude ; Boljetinsko brdo at 22°02.2’E longitude, 44°31.5’N latitude). Two considerably different successions were measured above the Lower Jurassic quartz sandstone and conglomerate. The first succession, exposed between tunnels no 17 and 21, is over 150 m thick and typical of a deep-water basin. The following units occur in stratigraphic order: red shale with intercalations of thin beds of nodular limestone ; white marly limestone passing upwards in well-bedded limestone with subordinate marly interlayers ; highly siliceous thin-bedded green and upsection red limestone ; indistinctly bedded grey marly limestone ; well-bedded reddish limestone with chert nodules and interlayers of dark red shale. Several breccia and calcarenite beds are interstratified in the last unit. The second succession (exposed at tunnel no 10) indicates deposition on a pelagic plateau. This section is much more condensed, not exceeding 20 m in total thickness. The predominant facies is red nodular limestone of Rosso Ammonitico type. Rare chert nodules and layers exist only in the middle part of the section. Slumped beds and intraformational conglomerates occur in the upper half. Both sections continue with a thick succession of light grey micrite with chert nodules that closely resembles the Maiolica limestone of the Southern Alps. Up to several meters thick slumped levels are common in this Lower Cretaceous limestone. In addition to these two sections, some short sections in the upper part of the Maiolica limestone and in the overlying marls and marly limestones were sampled for radiolarians. Jurassic radiolarian assemblages from the samples taken at localities Tunel 18 (T 18) and Tunel 19 (T 19) were characterized by scarce and poorly preserved radiolarians. Early Cretaceous radiolarian associations were identified from samples taken at the localities Tunel 21 (T 21), Tunel 10 (T 10) and Boljetinska reka (BK). Three positive samples (T21/1, T21/4a, T21/4b) were obtained from gray fractured cherty micrite with marlstone interlayers at the locality Tunel 21. The radiolarian association in sample T21/1 is abundant and diverse. The following species were identified: Cecrops septemporatus (Parona), Hemicryptocapsa capita (Tan), Hiscocapsa grutterinki (Tan), Pseudodictyomitra lilyae (Tan), Thanarla pulchra (Squinabol), Acaeniotyle dentata Baumgartner, Acaeniotyle diaphorogona Foreman, Xitus clava (Parona), Cryptamphorella clivosa (Aliev), Holocryptocanium barbui Dumitrica, Pantanellium lanceola (Parona), Praeconosphaera sp., Crucella sp., Cyclastrum sp. The radiolarian association indicates UAZ 18 (latest Valanginian to earliest Hauterivian) of Baumgartner et al. (1995). Three positive radiolarian samples were obtained from marly limestone with chert nodules at the locality Tunel 10 (T10-22.90, T10-41.60 and T10-50.00). The fauna is characterized by moderate to extremely poor radiolarian preservation. Interesting feature is a high nassellarian/spumellarian ratio. This data seem to be useful indicator of palaeobathymetry (nassellarians dominate in deep-water faunas). From a horizon sampled at 0.10 m from the bottom of the Boljetinska reka section (BK-0.10), a relatively diverse and moderately preserved radiolarian fauna of Early Cretaceous age was identified. Two positive samples (BB-1 and BB-2) were obtained from yellowish marlstone in the locality Boljetinsko Brdo. Preservation within the marlstone is very poor. Common characteristic of all samples in the section is a relative abundance of indeterminable pyritized nassellarians. The interest of precise stratigraphic studies is based on the fact that, according to the recent geotectonic interpretations (Schmid et al., 2008) the study area actually represents the easternmost part of the Alpine Tethys. Radiolarian dating has been undertaken primarily to elucidate the rifting history in this domain. Postrift sedimentary successions of a deep basin and a pelagic plateau have been distinguished. These successions indicate a typical horst-and-graben topography, well known from other domains of the Alpine Tethys. The topographic difference was apparently diminished by the Early Cretaceous when Maiolica type limestone became ubiquitous as is now confirmed also with radiolarians. Unfortunately, we have not found any datable radiolarians from the Jurassic successions and an exact timing of Jurassic events is yet to be determined. A more detailed radiolarian sampling along with other dating methods (e.g. chemostratigraphy) is planned for near future. REFERENCES Baumgartner, P. O., Bartolini, A., Carter, E. S., Conti, M., Cortese, G., Danelian, T., De Wever, P., Dumitrica, P., Dumitrica-Jud, R., Goričan, Š., Guex, J., Hull, D., Kito, N., Marcucci, M., Matsuoka, A., Murchey, B., O'Dogherty, L., Savary, J., Vishnevskaya, V., Widz, D., Yao, A. (1995). Middle Jurassic to Early Cretaceous radiolarian biochronology of Tethys based on Unitary Associations. 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Radiolaria; Jurassic; Early Cretaceous; Danubian Nappes; Eastern Serbia

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