Radiolarians from the Devonian of Prague Basin (Barrandian, Bohemia, CZ) were first mentioned by Wähner, 1917 from thin sections of the chert beds belonging to the topmost Choteč Fm. and early Kačák Mb., of the 'Hlubočepy-railway cut' locality. Radiolarians in thin sections of cherts, locally abundant in the upper part of the Choteč Limestone, were later studied by Fabian 1935, and Storm 1935, and mentioned by Prantl 1940a, 1940b, Svoboda & Prantl 1948, Petránek 1946, 1950, 1951. ###????? Chlup. 1959??
Former attempts to get isolated radiolarians by chemical treatment (e.g. Wähner, 1917; Storm 1935) failed. Čejchan 1987 reported a well-preserved, chemically isolated siliceous radiolarian assemblage from the Hlubočepy-Nad tratí Quarry locality (Prague), from the middle and upper part of the Choteč Fm.. Budil 1995 reported an extraordinarilly well-preserved, isolated radiolarian assemblage in the uppermost Choteč Fm., just about 1.65m below the 'Kacak-otomari Event' (Walliser 1984, House 1985), at the locality Kněží Hora Hill near Karlštejn (Budil 1995). The assemblage was described later by Braun & Budil ####
During the field prospection for radiolarians in 2003 - 2005, several hundreds of rock samples of both limestones, and cherts derived from CHO, were taken. Only a small proportion of these, however, yielded moderately, or well-preserved radiolarians after chemical preparation.
The Lower Palaeozoic (Cambrian to Middle Devonian) constitutes the central part of the Barrandian Area, an asymmetrically folded synform thrusted to SSE, situated in the Middle and Western Bohemia, cropping out approximately between the cities of Prague (Praha) and Pilsen (Plzeň). This area was intensively studied since Barrande's 1852 pioneering work, by numerous authors , e.g., Barrande 1878, Krejčí1877, Katzer 1892, Kettner and Kodym 1919 Kodym 1924 1925 Svoboda and Prantl 1950a Chlupáč 1982b 1988 et al 1992, and many others.
Choteč Limestone is the last marine carbonate formation of the Prague Basin, cropping out in several "tectonic belts" in the core of the Barrandian synform. It was studied in detail by Barrande 1865, Barrande 1878, Petránek J 1946, 1950, Storm 1935, Prantl 1940, Svoboda & Prantl 1948,1950a, Klapper, Ziegler & Mashkova 1978, Chlupáč, Lukeš & Zikmundová 1979, Chlupáč 1982b, and Chlupáč & Kukal 1986.
Limestones of the Choteč Fm. vary from grey, fine-grained, well-sorted crinoidal sparites, to light-coloured, sometimes yellowish weathering micrites. Radiolarians were isolated mainly from the micrites; radiolarian content of sparitic limestones is usually either low (also poorly preserved), or none. Shales were not processed for radiolarians (although they might be present there), due to shales' susceptibility to tectonic deformation, that is substantial in the area studied.
Cherts, although macroscopically quite uniform, show considerable variability in calcium carbonate, and organic matter content. Also, etched cherts originally uniform, vary in colour and/or lamination.
Radiolarian abundance varies considerably in both limestones, and cherts, from high-diversity associations to (almost) monospecific consociations. Well preserved radiolarians are bound to often thin, sparsely-spaced layers. Ascertained radiolarian taxa belong to Entactiniidae, Astroentactiniidae, and Ceatoikiscidae. The benthic biota of the Choteč Limestone belong to Community 4-5, episodically up to 6, of Boucot 1975 (Chlupáč et al 1992), reflecting deeper shelf conditions.
Choteč Limestone was introduced by Svoboda and Prantl 1948, redefined by Chlupáč 1957b, 1959, with its stratotype at the Škrábek-Western Quarry near Choteč. They represent the most widespread facies of the Choteč Fm.. The base of Choteč Fm. is marked by a facial change, probably indicating the rise of the sea-level (so called 'Basal Choteč Event, or jugleri-Event, Chlupáč - Kukal 1986, 1988. According to the conodont stratigraphy, Choteč Fm. spans from the upper part of the Polygnathus costatus partitus to the topmost part of the Tortodus kockelianus kockelianus Biozone (Chlupáč, Lukeš & Zikmundová 1977, Chlupáč , Lukeš & Zikmundová 1979, Klapper, Ziegler & Mashkova 1978). If the correlation of the Jirásek (= Jíras) Quarry presenter by Budil 1995 is correct, then, according to findings of P. eiflius, ensensis well below the onset of the presumed 'Kacak-Event', the top of the CHO may well lie within the eiflius, or ensensis Zones (Hladil et al. 1993, Kauffmann 2006). According to the dacryoconarid stratigraphy, Choteč Fm. base is synchronous with the base of Nowakia sulcata Biozone, top is at he base of the Nowakia otomari Biozone (Bouček 1964, Chlupáč, et al 1992).
Two rock types have been processed for radiolarians, namely limestones, and cherts. Limestones were processed using the standard procedure used for extraction of conodonts, i.e., dissolution in 6 to 10% acetic acid. Samples were crushed into pieces about 5 cm in diameter, and dissolved in diluted acetic acid for weeks to months, depending on the type of limestone, and total amount.
Cherts were processed by modified method of Pessagno&Newport (1972###), and Braun 1989, 1990. Samples were crushed into pieces about 1cm in size, and exposed to 3% hydrofluoric acid for 24 hours, then sieved. This procedure was repeated until the acid got exhausted, then it was replaced by a fresh acid until (almost) all rock got dissolved.
After dissolution of rock-matrix, the residue was sieved on a set of 250-100-63 µm set of sieves, individual fractions washed in ethylalcohol, filtered, and dried at 80°C. Isolated radiolarian shells were picked up from the residue using binocular optical microscope at magnification ×40 and mounted on gelatine-coated film glued to SEM-stubs.
Limestones of the Choteč Fm. vary from grey, fine-grained, well-sorted crinoidal sparites, to light-coloured, sometimes yellowish weathering micrites. Radiolarians were isolated mainly from the micrites; radiolarian content of sparitic limestones is usually either low (with bad preservation, too), or none. Shales were not processed for radiolarians, although they might be present there, due to shales' susceptibility to tectonic deformation, that is substantial in the area studied. Usually, the dark cherts with low Ca content, with glossy appearance and flint-like breakage have the highest probability to yield well preserved radiolarians. Radiolarian abundance, as well as preservation state vary considerably in both limestones, and cherts.
Choteč-village
WGS-84 coordinates: 49:59.274N 14:16.792E alt. 315 m a.s.l. A road-cut in the centre of Choteč village, on the eastern side of the road from Ořech to Třebotov. Middle part of the Choteč Fm. is exposed here, formed by undulated platy to nodular limestones, containing chert nodules/layers in distinct horizons. A single sample of dark chert (PAC-SMP-564) yielded well-preserved entactiniacean Radiolaria. Choteč-Škrábek, western quarry [WPT-254]
WGS-84 coordinates: 49:59.327N 14:16.731E alt. 308 m a.s.l. An abandoned Quarry exposing the Třebotov and the base of the Choteč Formations. It is situated at the southern foot of the Škrábek Hill, NW of Choteč village. Local stratotype of Choteč Fm. is situated here. The locality is mentioned in Chlupáč 1959. Almost complete sequence of the Třebotov Fm., and the lowermost part of the Choteč Fm. are exposed in the main quarry wall and, with a small interruption, at the quarry entrance (eastern side). Radiolarians were found in a thin limestone intercalation in brown-grey shales (SMP-580), and in several layers of dark-coloured chert (SMP-561, 579), derived from the lower part of the Choteč Fm., immediatelly above the basal turbidite beds, clost to the 'jugleri-Event' . These radiolarian occurrences are the oldest ones documented from the Eifelian of the studied area. Třebotov-road cut [WPT-277]
WGS-84 coordinates: 49:58.419N 14:17.164E alt. 345 m a.s.l. Road-cut and a small abandoned quarry at the road from Třebotov to Choteč, at the eastern side of the road, close to the village of Třebotov. It is a continuation of the stratotype of the Třebotov Fm. Platy and nodular limestones with thin shale intercalations and subordinate chert nodules of the Choteč Fm (Eifelian). Radiolarians were ascertained in a bulk sample of limestone (PAC-SMP-674) collected bed-by-bed from the whole outcrop. The precise position(s) of radiolarian-bearing bed(s) is still to be resolved. the outcrop represents the lower half of the Chotec Fm. Třebotov-Bambaska [WPT-278]
WGS-84 coordinates: 49:58.504N 14:17.258E alt. 363 m a.s.l. Scree of limestones and cherts of the Choteč Fm. occur in the field on the northern slope of the Bambaska hill (co.### a. s. l.) N of Třebotov village, under the triple-point of the high-voltage electric power line. This locality is a continuation, both geographic and stratigraphic, of the 'Třebotov-road cut' locality. Well-preserved radiolarians were obtained from some chert nodules only, although numerous samples of limestone had been processed, too. Productive cherts belong rather to dark, glossy, amorphous ones, than to the 'unfinished cherts' sensu Petránek J 1951, i.e., imperfectly silicified limestones. Chýnice-Jelínkův mlýn (=Jelínek Mill) [WPT-268]
WGS-84 coordinates: 49:59.852N 14:16.174E alt. 310 m a.s.l. An abandoned quarry, and a pathway-cut in its continuation exposing almost complete Třebotov Fm., and a fair portion of the Choteč Fm. Radiolarians were isolated from a single limestone bed (PAC-SMP-618) near a small cave, about 10m above the base of the Choteč Fm. Trněný Újezd [WPT-95]
WGS-84 coordinates: 49:58.416N 14:12.941E alt. 396 m a.s.l. The railway cut N of the village, W of the road to ###. Probably middle part of the Choteč Fm. Fm., heavily folded and fractured, is here in tectonic contact with the Roblín Mb of the overlying Srbsko fm there. The Choteč Fm. Fm is formed by grey bedded limestone with thin intercalations of grey-brown shale. Cherts are present locally as nodules, or bands in limestone, or within the shale. Samples ## taken from chert, and ## taken from limestone were productive. Bubovice S of Branžovy Quarry [WPT-180 to 191]
WGS-84 coordinates: 49:58.813N 14:10.827E alt 411 m a.s.l. Limestones and cherts of the Choteč Fm. crop out as a scree in the field about 100 m S of the southern rim of the active quarry "Na Branžovech", between Loděnice and Bubovice villages. Multiple samples taken from chert were productive. Bubovice S of Výšinka [WPT-192]
WGS-84 coordinates: 49:58.757N 14:10.500E alt 407# m a.s.l. This locality is a continuation of the Choteč Fm. limestone belt from the previous one to the W behind the transversal fault that displaces the belt a bit to the N. Samples ## taken from chert were productive. Dolní Roblín - Tůmův mlýn (= Tůma Mill) - old quarries [WPT-274, 275]
WGS-84 coordinates: 49:57.890N 14:15.870E alt. 321 m a.s.l. Small abandoned quarries below the road from Dolní Roblín to Třebotov, N of the Tůmův mlýn Mill. The middle and upper parts of the Choteč Fm., forming a flat anticline, are exposed in these quarries. Radiolarians have been isolated from two thin beds of micritic limestone only, in the section sampled bed-by-bed. However, poorly preserved inner casts of radiolarian shells occur frequently. Chert samples cololected from the outcrop yielded only rough, white spheres, about 100µm in diameter, interpreted as inner casts of radiolarian shells. Dolní Roblín - Tůmův mlýn (= Tůma Mill) - forest [WPT-273]
WGS-84 coordinates: 49:57.882N 14:16.005E alt. 321 m a.s.l. Scattered small outcrops and scree in the forest right above (northward) the Tůma Mill settlement. Radiolarians have been isolated from a single sample of micritic limestone only: SMP-656 from a natural otcrop; laminated limestones fro scree (SMP-657) were barren of radiolarians. Lužce [WPT-119, 216, 217]
WGS-84 coordinates: 49:59.281N 14.11.684E alt. 415 m a.s.l. Limestones and cherts of the Choteč Fm. crop out as a scree in the fields W of Lužce village (near the cemetery at the crossroads to Loděnice), and south of the road to Vysoký Újezd. Several chert nodules collected here (samples ###)yielded isolated, 3D-preserved radiolarians.
1. Ten new radiolarian-yielding localities are reported from the Choteč Fm. of the Prague Basin (Barrandian, CZ), besides the original two reported before (Čejchan1986, Budil###, Braun and Budil ####).
2. Contrary to the previous reports, these new occurrences are not limited to the upper, or uppermost part of the Choteč Fm., but reach fairly deep into Eifelian Stage, immediately following the basal turbidite sequence (e.g., at Choteč-Škrábek, sample SMP-561). Although not yet proved by conodonts, this occurrence probably belongs to the upper part of partitus Zone, by comparison with localities in Klapper, Ziegler & Mashkova 1978, and Chlupáč, Lukeš & Zikmundová 1979.
3. 3D-preserved radiolarians were for the first time extracted also from cherts, using a modified HF dissolution technique of Pessagno & Newport.
The work has been supported by GAČR grant #205/03/1124, and by #####. Thanks are due to A. Langrová, Z. Korbelová, and V. Böhmová for SEM photagraphy.