Late Triassic radiolarians from the Ov~ar-Kablar Gorge (SW Serbia)

Detailed micropalaeontological research of Triassic siliceous rocks was carried out at a locality in the Ov~ar–Kablar gorge, NE of Ov~ar Banja. According to the determined radiolarian associations, the investigated chert and radiolarite are of Late Carnian–Early Norian age.


Introduction
Siliceous sedimentary rocks, such as cherts and radiolarites, are widespread in southwestern Serbia.Noteworthy is that they originated in different geological settings and during different geological intervals (DIMI-TRIJEVI] 1997, KARAMATA 2006).They mostly occur in the internal parts of the Dinarides, in the vicinity of ophiolitic units and/or within ophiolitic mélange formations, less frequently in other geological environments.The maximum accumulation of radiolarian cherts occurred in the Middle to Late Triassic, as well as in the Middle to Late Jurassic, occasionally extending into the Cretaceous.
The territory of Serbia is rather interesting for studies of Mesozoic Radiolaria.Within the Serbian part of the internal Dinarides, Triassic and Jurassic radiolarians were described for the first time by [. GORI^AN (personal communication 1988, 1990) as well as by OBRA-DOVI] & GORI^AN (1988) and OBRADOVI] et al. (1986, 1987/1988).Recent works reflect the increase of radiolarian studies in Serbia and their importance for strati-graphic, palaeogeographic, tectonic and palaeotectonic implications (GORI^AN et al. 1999, KARAMATA et al. 2004

Geological setting
The studied area is located in SW Serbia, about 2 km NE of Ov~ar Banja (7434755, 4861714).This region belongs to the Vardar Zone Western Belt (Fig. 1), according to KARAMATA et al. (2000).On the territory of western Serbia, there are two belts of ophiolitic mélange overlain by large ultramafic massifs.The more external belt is known as the Dinaridic Ophiolites or Dinaridic Ophiolite Belt (PAMI] et al. 2002, KARAMATA 2006) or as the Central Dinaridic Ophiolite Belt (LU-GOVI] et al. 1991).The more internal belt is referred to as the Vardar Zone Western Belt by KARAMATA (2006) The Western oceanic basin of the Vardar Ocean, existing from the Late Triassic, became a wide oceanic basin during the Jurassic-Early Cretaceous and then closed by the latest Cretaceous; its suture is the Vardar Zone Western Belt (VZWB).
According to KARAMATA (2006), within the Western Basin of the Vardar Ocean, representing the precursor of the Vardar Zone Western Belt (VZWB), deep-water cherts and shales were deposited over basalts of the ophiolitic association from the Late Triassic to the Kimmeridgian.Large masses of trench deposits, represented by olistostrome mélange and gravity slides from the oceanic crust and the continental margin, accumulated within this basin from the Mid-Jurassic time.
According to SCHMID et al. (2008), there is a single Jurassic ophiolite sheet in the Dinarides, namely the Western Vardar Ophiolitic Unit that was obducted onto the passive margin of Adria during the latest Jurassic times.The ophiolitic mélange, which occurs below the metamorphic sole flooring the obducted ophiolites, typically contains a mixture of (1) rock types derived from the lower plate, mechanically scraped off and accreted to the upper plate, and (2) gravitationally emplaced olistoliths derived from the upper plate.The blocks derived from the lower plate consist, amongst other lithologies, of Triassic ultramafics and mafics (MORB-type ophiolitic blocks up to several km in diameter) that were derived from the Meliata-Maliac-Vardar Ocean, the age of which was inferred from preserved stratigraphic contacts with Triassic radiolarites (SCHMID et al. 2008) sonal comm. 2007), this particular part of the mélange formation does not contain ophiolites but Triassic sediments, which probably derived from the adjacent passive margin.These Triassic sediments consist not only of radiolarites but also of cherty or micritic limestones of presumably Triassic age.These sediments were also sheared off during mélange formation, but they are tectonically underneath (and further W) of the ophiolitic blocks described above.These large (km-size) slices of Triassic sediments are analoguous to similar occurrences in Greece ("Maliac" nappes).The mélange formation underlies W-Vardar ophiolites, i.e. the Maljen ophiolite (S.SCHMID personal communication 2007).

Material and Methods
The described radiolarian assemblages originate from one single section at the Ov~ar-Kablar Gorge (Fig. 1).Three samples were collected from the radiolarian cherts.The chert samples were only treated with dilute 5-7 % hydrofluoric acid, following the method of PES-SAGNO & NEWPORT (1972).In all samples, spumellarians were much more abundant than nassellarians.The residues of the acid treatment, which yielded well preserved faunas, were studied for biostratigraphic purposes.In order to establish the age of the radiolarian assemblages, the zonation schemes proposed by TEKIN (1999) were used.An SEM microscope ISI-160 in GIN RAN (Moscow) was utilized for the precise identification and illustration of the radiolarians.These are illustrated in Plates 1 and 2. The micropalaeontology material is housed at the Faculty of Mining and Geology in Belgrade (registration numbers NDJ 100 to NDJ 103).

Section description and biostratigraphy
Characteristic layered chert with siliceous claystone and subordinate limestone occurs on a 70 m wide section along the road ^a~ak-Po`ega (Fig. 1), about 2 km NE of Ov~ar Banja (7434755, 4861714).
The lowermost part of the section is composed of stratified, gently folded chert of dark red color (Fig. 2), which are from 10 to 50 cm thick, in alternation with thin-bedded green siliceous claystone, tuffite and light silicified and recrystallised limestone.The chert is intersected by fractures, which are mostly filled with calcite, rarely with secondary quartz.Microlamination due to a higher concentration of ferruginous material is locally visible in the chert.The approximate average thickness of the layers composed of siliceous claystone is 5 cm, while the limestone layers reach a thickness of about 10 cm.The whole sequence is about 20 m thick (Fig. 2).
Four samples were taken from the chert at the locality Ov~ar Banja, three of which gave positive results.

Comparison
The majority of the species extracted from the samples taken from the section at the locality Ov~ar Banja are widely known, primarily from the Triassic beds of the Mediterranean Region.Almost all of them occur in the Upper Triassic (Upper Carnian-Lower Norian) of southern Turkey (DE WEVER et al. 1979, TEKIN 1999) and southern Cyprus (BRAGIN 2007).The late Carnian to Early Norian radiolarians at the locality Ov~ar Banja can be compared to the radiolarians in the volcano-sedimentary formation of the Rubik area in Albania (CHI-ARI et al. 1996, BORTOLOTTI et al. 2006), Greece and Sicily (DE WEVER et al. 1979), as well as to the Early Norian fauna of Slovakia (KOZUR & MOSTLER 1981).Some species, for example Capnodoce anapetes, Capnodoce crystallina, Capnuchosphera theloides, Xiphothecella longa and Xiphothecella rugosa were recorded in the Late Carnian of Transcaucasia (KNIPPER et al. 1997), the Upper Carnian-Lower Norian of Oman (OTSUOKA et al. 1992), the Upper Carnian-Lower Norian of the Far East of Russia (BRAGIN 1991) and Japan (SUGIYAMA 1997) and the Upper Carnian-Middle Norian of Mexico and Oregon (PESSAGNO et al. 1979, BLOME 1984).

Final remarks
The siliceous deposits from the Ov~ar Banja locality consist of radiolarian cherts with clay, tuff and limestone interlayers.On the basis of the radiolarians, the analyzed cherts were deposited between the Late Carnian and the Early Norian.These sediments were also sheared off during the formation of mélange; they are, however, tectonically underneath the ophiolitic blocks described above.These large (km-size) slices of Triassic sediments are analoguous to similar occurrences in Greece ("Maliac" nappes).The late Carnian to Early Norian radiolarians at the locality Ov~ar Banja can be compared to the radiolarians in the volcano-sedimentary formation of the Rubik area in Albania (CHIARI et al. 1996, BORTOLOTTI et al. 2006), Greece and Sicily (DE WEVER et al. 1979), as well as to the Early Norian fauna of Slovakia (KOZUR & MOSTLER 1981) and Cyprus (BRAGIN 2007).
, DJERI] & VISHNEVSKAYA 2005, 2006; VISHNEV-SKAYA & DJERI] 2006a, b; DJERI] et al. 2007a, b; GAW-LICK et al. 2007a).Despite this considerable progress, knowledge of the Mesozoic Radiolaria from Serbia is still insufficient.Actually, there are only a few published data on Middle and Late Triassic radiolarians, as well as on Middle and Late Jurassic radiolarians.The main aim of this study was to obtain information on the late Triassic Radiolaria from SW Serbia.