A Middle Devonian radiolarian fauna from the Chotec Limestone (Eifelian) of the Prague Basin (Barrandian, Czech Republic) Andreas BRAUN Institute of Paleontology, University of Bonn, Nussallee 8, 53115 Bonn (Germany) [email protected] Petr BUDIL Cesky geologicky ustav, Klarov 3, 118 21 Praha 1 (Czech Republic) [email protected] Braun A. & Budil P. 1999. — A Middle Devonian radiolarian fauna from the Chotec Limestone (Eifelian) of the Prague Basin (Barrandian, Czech Republic), in De Wever P. & Caulet J.-P. (eds), InterRad VIII, Parls/Biervllle 8-13 septembre 1997, Geodiversitas 21 (4): 581-592. ABSTRACT The occurrence of radiolarian faunas in the upper part of the Chotec Limestone is discussed in terms of faunal composirion, systematics and geo­ logical implications. The most common entactinid species are rreated syste­ matically. The occurrence of radiolarians in large numbers in the rock KEYWORDS succession began approximately 2 meters below rhe onset of the black shale Radiolarians, Middle Devonian, sedimentation (Kacak Member). The abrupt sedimentological change, com­ upper Eifelian, monly viewed as an event therefore does not coincide with the faunal turno­ Barrandian, ver, leading to a radiolarian dominance well before the onset of black shale Chotec Limestone, Kacak event. deposition. RÉSUMÉ Une faune de radiolaires du Calcaire De Chotec (Devonien moyen, Eifelien supérieur) du bassin de Prague (Barrandien, République tchèque). La présence de faunes de radiolaires dans la partie supérieure du Calcaire de Chotec esr discutée en tenant compte de la composition faunique, de la sys­ tématique et des implications géologiques. Les espèces d'entactinés les plus communes sont traitées systématiquement. Les radiolaires sont présenrs en MOTS CLES grand nombre dans la succession sédimentaire environ 2 mètres au-dessous Radiolaires, Dévonien moyen, des premiers schisres noirs (Kacak Formation). Le brusque changement de Eifelien supérieur, sédimentation ne correspond donc pas comme habituellement à un change­ Barrandien, ment faunique, comme l'indique cette dominance des radiolaires bien avant Calcaire de Chotec, Kacak Formation. le début de l'accumulation des schistes noirs. • 1999 • 21 (4) 581 Braun A. & Budil P. INTRODUCTION The Paleozoic area around Prague is a famous and classical region for many investigations in CZECH REPUBLIC paleontology and biostratigraphy. The occurren­ ce of radiolarians in certain rocks especially of Silurian age has been commonly mentioned (Rodic 1925; Prantl 1949) and some very detai­ Vltava River» Praha-Hlubocepy^ 1 led petrographic studies have been carried out Î BARRANDIAN (Rodic 1931). Although typical radiolarian rocks Ï (radiolarian cherts, lydites) are not very common especially in the Devonian sequence it has been known for some decades, that some lithologies M Karlstejn-Knezi hora Hill may bear radiolarian skeletons in significant amounts (Fabian 1933). The fact, that some of 7 Berounka River _ the sedimentary rocks in the Barrandian area 0 10 km bear well preserved radiolarians, is not well known, although in cases where preservation is FIG. 1. — Distribution of the Kacak Member (in black) in the good such occurrences might be of special Barrandian area and geographic situation of the discussed loca­ importance for our knowledge of stratigraphy lities. and development of radiolarian faunas during the Silurian and Devonian. The fauna that is figured and partly described in this paper has dings of radiolarian faunas come from rocks 4 m been considered in two earlier papers (Cejchan below the Kacak sequence (Budil 1995). 1987; Budil 1995). The present paper intends to 2. Knezi hora hill near Karlstejn. This locality, supply some addional information, observations although offering a good sequence through the and a systematic treatment of the common uppermost Chotec Limestone and the base of the entactinid species in these faunas. The special Kacak Member in its typical development, had lithologic framework in which these faunas occur not been studied in detail until quite recently may be worth noting in the context of the "bioe- (Budil 1995: 4-8). Radiolarians are even better vent character" of the Kacak sequence (and the preserved here than those from the Hlupocepy "otomari-event", cf.Walliser 1984), immediately railway cut. Our sample comes from two limes­ overlying the source rocks of our faunas. tone beds approx. 2 m resp. 1 m below the Kacak base (beds -10/-12 and -20). LOCALITIES (cf. Figs 1, 2) LITHOLOGY Radiolarian bearing rock samples come from two The radiolarians come from the uppermost part localities situated near Prague: of the Chotec Limestone (biomicritic limestones 1. Railway cut in Praha 5-Hlupocepy. This expo­ with intercalations of pelbiodetritic limestones) sure has been figured and described several times underlying the predominantly siliceous black (e.g., in Budil 1995: 2-4; Chlupac 1960, 1993). sequence, the latter representing the so called It exposes a sucession from the higher part of the Kacak event (Chlupac 1989). The limestone Chotec Limestone until the Roblin beds, over­ occurs in beds of thicknesses between 5 and lying the Kacak Member. The Radiolarians have 30 cm. It is dark grey in colour and bituminous. been extracted from a 15 cm thick bed of Chotec The cherts of the Kacak sequence, restricted to Limestone 50 cm below the base of the predomi­ the area around Prague, have long been known nantly black siliceous Kacak sequence. Rare fin- to contain radiolarians (Fabian 1933; see 582 GEODIVERSITAS • 1999 • 21 (4) Czech Middle Devonian radiolarians Chlupac 1960: 176) for changes in terminology of the sequences). Radiolarian preservation seems to be more favorable in the limestone beds com­ pared to the Kacak cherts. AGE CONTROL According to faunal data, the Kacak succession 2 m above the upper Chotec Limestone was ini­ tially considered to be basal Givetian (Chlupac 1960: 177), and later regarded uppermost Eifelian ( Chlupac & Kukal 1986, 1988; Chlupac 1993). Kalvoda (1992) discussed the stratigraphic implications of conodonts in beds near the base of the Kacak sequence which in terms of the conodont chronology indicate an upper Tortodus kockelianus zone age (near the base of the Polygnathus ensensis zone). In terms of modern conodont chronologies (Sandberg & Ziegler 1996; Weddige 1996) this zone is well below the actual Eifelian/Givetian boundary. As our sample horizon is located a maximum of 2 m below the Kacak base our radiolarian fauna pre­ sumably also belongs to the upper Tortodus koc­ kelianus zone. Budil (1995) discusses the considered interval (Chotec-Kacak boundary interval) in terms of the uppermost Eifelian. Because of the scarcity of well preserved cono­ donts and other faunas, the question of where the actual Eifelian/Givetian boundary is situated within the sequence has not been resolved. It is clear however, that on the basis of recent cono­ dont zonation schemes (Sandberg & Ziegler 1996) the radiolarian faunas discussed herein Praha-Hlubocepy Karlstein-Knezi stratigraphically belong to the upper Eifelian. railway cut hora Hill TECHNIQUES OF STUDY The limestone samples were acid treated as in any processing for conodonts, in this case with acetic acid. Isolated radiolarians from the residue FIG. 2. — Lithologie succession of the sequences exposed at Praha-Hlubocepy railway cut and Karlstejn-Knezi hora hill. Note have been either picked and mounted on SEM the different lithologie composition of the Kacak Member at the stubs or embedded in Caedax for light microsco­ two localities. 1, massive biomicrithc limestones; 2, thin bedded biomicritic limestones; 3, fine biodetrital limestones; 4, coarse py. The latter technique serves well in the inves­ biodetrital limestones; 5, lithoclastic breccia (not present in the tigation of internal structures, which are of sequence shown); 6, dark calcareous claystones and clay limes­ importance in generic and suprageneric taxono­ tones; 7, thin bedded cherts; 8, calcareous siltstones and biode- tritic limestones with silt. my of spherical radiolarians. GEODIVERSITAS • 1999 • 21 (4) 583 Braun A. & Budil I». SYSTEMATIC PALEONTOLOGY mon faunal constituents in the investigated fau­ nas. The radiolarian fauna in general is dominated by Entactiniidae in terms of absolute numbers and Genus Stigmosphaerostylus Rüst, 1892 species diversity. Ceratoikiscids and Palaeo- sceniids are rarer. All radiolarians are exceptional­ For synonymy with Entactinia Foreman, 1963 and ly well-preserved; in many of the entactinids the remarks see Aitchison & Stratford 1997. internal structures are present. Whereas in the Ceratoikiscids several clearly different morpho­ TYPE SPECIES — Stigmosphaerostylus notabilis Rüst, 1982. types are distinguishable, the genus Palaeo- scenidium is present only with one species. As the Stigmosphaerostylus herculeus (Foreman, 1963) ceratoikiscids and palaeoscenids have been syste­ (Fig. 4B) matically treated in the paper of Cejchan (1987) no detailed taxonomic discussions of these will DESCRIPTION be made here. A more detailed taxonomic treat­ Spherical skeleton bearing six three-bladed ment is given for the most common species of spines, the length of which is approximately entactinids. equal to the diameter of the skeletal sphere. Material figured and mentioned in the text (SEM Spines may be disposed at 90° angles to one ano­ stubs, light microscope slides) are housed at the ther or placed slightly asymmetrical. No secon­ collection of the Institute of Paleontology, dary spines have been observed, only small