Foraminiferal assemblages as an indicator of foreland basin evolution (Carpathian Foredeep, Czech Republic)

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1 Bulletin of Geosciences, Vol. 79, No. 4, , 2004 Czech Geological Survey, ISSN Foraminiferal assemblages as an indicator of foreland basin evolution (Carpathian Foredeep, Czech Republic) Pavla Petrová Czech Geological Survey, Brno Branch, Leitnerova 22, CZ Brno. Abstract. This paper presents a model of depositional history in the Karpatian stage, Lower Miocene of the Carpathian Foredeep based on the study of foraminifers. The paleoecological interpretations, and the data on bioevents and biofacies of the Karpatian deposits were obtained through the statistical evaluation of microfauna. The occurrence of rich faunas alternating with impoverished ones indicates unstable conditions in the depositional area. Euryoxybiont foraminifers such as Bolivina div. sp., Bulimina div. sp., Praeglobobulimina div. sp., and Uvigerina div. sp. dominate these faunas. Two biostratigraphical levels containing Uvigerina and an overlying one with Pappina breviformis (Papp and Turn.) were identified. Two stages of the development of a transgression can be distinguished. The first stage spread to the eastern part of the Carpathian Foredeep, where the sediments contain agglutinated foraminifers. The second stage reached the present location of the Karpatian deposits. Later shallow-water sediments in the western part of the Carpathian Foredeep have been eroded away. Key words: Carpathian Foredeep, Moravia, Lower Miocene, foraminifers, paleoecology, biostratigraphy Introduction The Carpathian Foredeep is a peripheral foreland basin of the Western Carpathian arc, and is filled with molasse deposits. In the Czech Republic its depositional history began in the Egerian-Eggenburgian (Oligo-Miocene), and proceeded up to the Lower Badenian (Middle Miocene). Paleogeographically, the Carpathian Foredeep is included into the epicontinental basins of the Central Paratethys, with specific paleogeographical and paleobiotic characteristics. Foraminifers are considered to be the most important fossils of the Karpatian deposits in the Carpathian Foredeep. They have been used as a reliable tool in biostratigraphical and paleoecological research in this depositional area. The paleoecology and biostratigraphy of the southern part of the Carpathian Foredeep have been studied in publications by Brzobohatý (1980, 1992), Cicha (1995, 2000, 2001), Cicha and Zapletalová (1967, 1974), Jiříček (1983, 1990, 1995), Molčíková (1966), Nehyba and Petrová (2000), and Petrová (1998, 1999, 2003). The conclusions of these previous authors were based on qualitative data, while the present article presents the results of a Ph. D. dissertation based also on quantitative data. Material and methods Samples of the pelitic-aleuritic (locally psammitic) sediments in the Lower Karpatian and typical schliers (laminated, calcareous, light grey, silty mudstones with silt or very fine sand laminae) of the Upper Karpatian were analysed. Foraminifer assemblages were obtained from boreholes in the area to the south of the Nesvačilka trough. The location of the area and of the boreholes is presented in Fig. 1. Various kinds of input material were studied, including rocks, the washed residues, and microfossil assemblages (see Tab. 1 for a summary). The type of data influenced the evaluation of results, and it was not possible to correlate all of the values obtained. Data only from the following boreholes were compared: Nosislav-3 (Petrová 1998; Petrová 2003), HV-boreholes (Nehyba and Petrová 2000, Petrová 2003), Cf-4, Znojmo-12, and Pouzdřany-1 (Petrová, 2003). The rock samples were soaked in sodium carbonate solution for several hours for disaggregation, and then washed under running water through 63 µm mesh sieves. Foraminifers were collected, identified, and counted using a WILD binocular microscope. Some of the samples were barren of fossils, while others contained impoverished or abundant foraminiferal microfauna. 300 specimens of foraminifers were analysed using statistical methods for comparable results (Petrová 1998; Nehyba and Petrová 2000). Impoverished microfaunas, however, give only qualitative data. A summary of the studied borehole sections is shown in Fig. 1. In interpreting our results, bioevents and biofacies (Fig. 5) were plotted on the schematic profiles of that boreholes that were amenable to correlation. The example on section by boreholes Zn-12, HV-304, Hy-1, NP-1, NP-2 and Mik-1 borehole is presented in Fig. 2. The evolution of the Carpathian Foredeep in the Karpatian was reconstructed by the present author based on the spatial and stratigraphical distribution of foraminifers. A summary of the statistical methods and data are presented in the author s unpublished Ph. D. dissertation (Petrová 2003). Results The foraminifer assemblages considered in this study can be generally characterized as impoverished, with low di- 231

2 Pavla Petrová Figure 1. Schematic map of the southern part of the Carpathian Foredeep, showing the location of the studied boreholes and the borehole sections. Břez Březí, HV and Hy Hrušovany nad Jevišovkou, Mik Mikulov, Muš Mušov, No Novosedly, Nos Nosislav, NP Nový Přerov, Poh Pohořelice, Pou Pouzdřany, Zn Znojmo versity, and tests of reduced size and ornamentation. Foraminifers and other microfauna are relatively well preserved, though partly fragmented, while the foraminifer tests are locally rather small. Planktonic foraminifers have small apertures. The assemblages are comprised of foraminifers, local fragments of sponge spicules, fish bone and scale fragments (mainly in the lower part of the sections), fragments of echinoid spines, ostracods, molluscs, bryozoans, radiolarians, and commonly pyritized diatoms. The frequency with which benthic genera occur in these assemblages is usually between 15 25%. A total of 94 benthic genera were identified in the borehole samples. The consistency of several foraminiferal genera is high: specimens of the genera Globigerina, Bolivina, Bulimina, Siphonodosaria, Hanzawaia, Heterolepa, Hansenisca, Lenticulina, Praeglobobulimina, Elphidium, and Uvigerina occur in % of the samples. The number of agglutinated foraminifers reaches up to 2 3%. Occurrences of agglutinated foraminifers coincides with those of euryoxybiont foraminifers (calculations and results presents Petrová 2003). The following microfaunal taphocoenoses were recognized: 1. fish bones exclusively, 2. impoverished assemblages of foraminifers (often with pyritized tests), abundant and low-diversity foraminiferal assemblages, 4. abundant and diversified foraminiferal assemblages (Petrová 2003). The succession of the assemblages often varies. Impoverished foraminiferal assemblages alternate with rich ones, and assemblages with predominantly planktonic genera replace benthic ones. This also indicates an unstable environment. The assemblages indicate a suboxic or dysoxic environment, and seldom an oxic one (mostly in the higher parts of the sections) (see Gebhardt 1999 and Kaiho 1994). Oxygen concentrations at the sediment-water interface control the distribution of benthic foraminifers (Kaiho 1994). It influences the size, wall thickness, porosity, and composition of the assemblages. Kaiho (1994) defined dysoxic, suboxic, and oxic assemblages. Based on his studies, the assemblages of the Carpathian Foredeep can be characterised as follows: Anoxic assemblages: without foraminifers, comprised only of fish bones, teeth, otoliths, and scales, locally with pyritized centric diatoms. Dysoxic assemblages: thin-walled, elongate, flattened, high porosity, usually small tests. Examples include the genera Bolivina, Bulimina, Cassidulina, Chilostomella, Dentalina, Stilostomella, and Praeglobobulimina. They dwell as infauna. The tests are often pyritized. The local occurrence of only small pyritized globigerinas is also indicative of a dysoxic environment. Suboxic assemblages: small tests of rounded planispiral, flat ovoid, and spherical forms, such as the genera Bulimina, Cassidulina, Fissurina, Hansenisca, Hoeglundina, Lagena, Lenticulina, Melonis, Nonion, Oridorsalis, Pullenia, Uvigerina, Valvulineria, and Dentalina. Oxic assemblages: tests are usually bigger, thickwalled, and of planoconvex, biconvex, rounded trochospiral, and spherical forms. These include the genera Cibicidoides, Quinqueloculina, and Triloculina. They dwell as epifauna. All studied foraminiferal assemblages came from the Laa Formation (Rögl 1969), including fauna with rare Globigerinoides bisphericus Tod in the Upper Karpatian. This latter fauna was documented in the uppermost part of the Pohořelice-3 borehole. Rögl (1994) correlated the foraminiferal assemblages of the Laa Formation with the M4a Foraminifera Zone (Berggren et al. 1995) and the N7 Zone (Blow 1969). Assemblages with rare Globigerinoides bisphericus Tod belong to the M4b Foraminifera Zone (after Berggren et al. 1995).

3 Foraminiferal assemblages as an indicator of foreland basin evolution (Carpathian Foredeep, Czech Republic) Figure 2. The correlation of boreholes Zn-12, HV-304, Hy-1, NP-1, NP-2, and Mik-1 (Karpatian, Carpathian Foredeep). One level in the studied area (Fig. 2) contains an assemblage that experienced an abnormally sudden increase in the number of foraminifers of specific genera, and this term is relative and applied for local situation. The levels are comprised of representatives of the genus Stilostomella, Siphonodosaria, Neugeborina, Laevidentalina, Bolivina, Bulimina, Uvigerina, Pappina, Ammonia, Valvulineria, Heterolepa, Melonis, Globigerina, and agglutinated foraminifers (Bathysiphon sp., Cibrostomoides sp., Haplophragmoides sp.). They are plotted as bioevents in Fig. 5, and can be used for correlating the sections (Fig. 2). In the lower parts of the boreholes, containing the Lower Karpatian sediments, assemblages comprised mainly of fish bone fragments (also with scales, teeth, and otoliths) though without foraminifers are observed. Areas comprised purely of the small tests of Globigerina div. sp. appear locally. These assemblages become replaced by ones with relatively numerous microfauna. Within the surroundings of the town of Mikulov the tests of agglutinated foraminifers were found (mainly Haplophragmoides vasiceki Cicha and Zapl., Bathysiphon taurinensis Sacco and Cribrostomoides columbiensis Cicha and Zapl.), distinguishing the first developmental stage of the Karpatian transgression (Figs 3, 4A). Higher in the overlying anoxic sediments of the second developmental stage of the transgression, the following succession of foraminiferal genera is observed: 1. Siphonodosaria and Stilostomella, 2.Neugeborina, 3.Bolivina. This succession is especially apparent in boreholes HV-304, HV-305, HV-306, Znojmo-12, and Nosislav-3 (Fig. 5). A more diversified microfauna with the typical Karpatian species Uvigerina div. sp. and Pappina div. sp. appears in the younger sediments. In several boreholes we identified two positions containing Uvigerina sp. (three positions in Nosislav-3), and a higher level with Pappina breviformis (Papp and Turn.) and P. primiformis (Papp and 233

4 Pavla Petrová Figure 3. Schematic map showing the area of the first stage of the transgression, and the area in which shallow-water sediments were eroded (Karpatian, Carpathian Foredeep). 234 Paleoecological interpretation Turn.). A barren region containing no fauna occurs between the areas containing Uvigerina div. sp. and Pappina div. sp. (see Molčíková 1966). The succession of species of the genus Uvigerina was observed mainly in borehole Nosislav-3: a lower zone containing Uvigerina graciliformis Papp and Turn., U. peregrina Cush., U. multicostata Le Roy, and a higher one with Uvigerina acuminata Hos., U. pygmoides Papp and Turn. and U. semiornata d Orb. A model of the evolution of the basin during the Karpatian is presented in Fig. 4. The zones with anoxic characteristics that develop into dysoxic and suboxic assemblages document both stages of the transgression. Agglutinated assemblages indicate the different development of the eastern part of the Carpathian Foredeep. The two developmental stages were distinguished through sedimentological studies (Nehyba, pers. comm.) and paleontological evidence (Petrová 2003). Figure 5 shows the succession of the foraminiferal taxa (bioevents) that characterizes these biofacies, and the generalized lithology of the individual facies modified by Šikula and Nehyba (2003). Biofacies-ecostratigraphy characterizes the developmental stages of the transgression based on oxygen concentration and paleobathymetry: TA transgressive, anoxic. Fish bones predominate, pyritized diatoms appear, foraminifer tests are rare (small globigerinas in Mikulov-1 borehole). TD transgressive, dysoxic. Agglutinated foraminifers occur in the eastern part of the Carpathian Foredeep. Euryoxybiont taxa Bolivina div. sp., Bulimina div. sp., Praeglobobulimina div. sp., Uvigerina div. sp. predominate. Some levels contain only globigerinas. Oxygen supply increases. Impoverished and diversified assemblages alternate. T B D/TS transgressive (bathyal) dysoxic suboxic. Deeper-water assemblages with agglutinated foraminifers were found. Numerous levels with Siphonodosaria div. sp. and Valvulineria complanata (d Orb.) were observed. S suboxic. Numerous diversified assemblages with Bulimina div. sp., Uvigerina div. sp., Pappina div. sp., Globocassidulina div. sp. The percentage of planktonic foraminifers decreases. MS/MO shallow-water suboxic oxic. Numerous and diversified microfauna including Ammonia vienensis (d Orb.), Elphidium div. sp., Hanzawaia bouena (d Orb.), Heterolepa dutemplei (d Orb.), Hansenisca soldanii (d Orb.) occur. Bioevents document the occurrence of foraminiferal positions and the first appearance of stratigraphically important species such as Uvigerina graciliformis Papp and Turn. and Globigerinoides bisphericus Tod. The model is generally consistent with the quality of the input data. The transgression of the Karpatian sea entered the southern part of the Carpathian Foredeep from the Mediterranean area (Rögl and Steininger 1984). The first stage of the transgression expanded to the current area of Mikulov, where the NP-boreholes and the Pohořelice-3 borehole were drilled (Figs 3, 4A). These sediments are typical of the maximum thickness of Karpatian sediments that bear on the Savian movements (Nehyba and Petrová 2000). They are usually barren (biofacies TA see Fig. 5). Positions with small pyritized globigerinas appear in the anoxic sediments. The sediments locally contain microfauna with euryoxybiont siphonodosarias, stilostomelas, and bolivinas developed from the anoxic and dysoxic positions in higher segments. Assemblages with agglutinated foraminifers indicate that an environment ranging from deeper littoral to upper bathyal existed in this area. These were cold water assemblages (Murray 1991; biofacies TD see Plate I, fig. 1). During the second stage, the transgression proceeded to the western part of the Carpathian Foredeep. There occurred an onlap onto the Eggenburgian and the Ottnangian sediments (Fig. 4), just as in the eastern part. Depressions in the southwest and western part of the Carpathian Foredeep were filled partly as separate basins under anoxic conditions (biofacies TA). These basins gradually became a shallow-water dysoxic sea (biofacies TD). These deposits are formed by muds and mudstones, with the exception of some sediments

5 Foraminiferal assemblages as an indicator of foreland basin evolution (Carpathian Foredeep, Czech Republic) Table 1. Summary of studied material Borehole in borehole Znojmo-12. This borehole contained sands with plant debris, indicating the proximity of the shore (Zdražílková in Pálenský et al unpublished report). Microfauna with fish bone fragments, locally pyritized foraminifer tests, and with levels comprised only of Globigerina sp. (biofacies TD see Plate I, fig. 2) are replaced by more diversified fauna. This indicates a paleoenvironmental change from anoxic to dysoxic, documenting the improvement of environmental conditions and the stability of the oxygen supply. Higher successions of deeper water deposits with taxa the Siphonodosaria, Neugeborina,and Bolivina are also present (biofacies TD see Plate I, fig. 4). Euryoxybiont taxa such as Bolivina div. sp., Bulimina div. sp., Praeglobobulimina div. sp., and Uvigerina div sp. predominate (biofacies S see Plate I, fig. 3). They are followed by numerous planktonic, often pyritized specimens, such as Globigerina sp. and the pyritized cores of centric diatoms. Local lithological indications of the anoxic environment are also observed (as lamination, and the occurrence of the organic matter). Muds and mudstones with sandy siltstones and sandy mudstones (and silty, muddy sands and sandstones in the HV-boreholes) dominate during the second stage of the transgression. The number of horizons with impoverished fauna decreases upwards, and euryoxybiont foraminifers alternate with numerous planktonic specimens. Numerous sudden alternations of impoverished and diverse levels are typical of the Nosislav-3 borehole. Positions with numerous fish bone fragments, pyritized diatoms and foraminifer tests (Globigerina div. sp.) indicate the brief deterioration of the environment in the Upper Karpatian, mainly in boreholes Nosislav-3 and Cf-4 Židlochovice. The foraminiferal fauna is impoverished, and shallow-water foraminifers predominate in the upper parts of most borehole sections. The sudden alternation of impoverished zones with the occurrence of the euryoxybiont foraminifers can be explained by the upwelling and oscillation of the sea level. Lithological considerations also support this interpretation (laminated muds and silts of reduced facies are typical of seasonal changes, such as in Nosislav-3). The depositional conditions became those of a shallow-water environment (infralittoral, biofacies MS/MO see Plate 1, figs 5, 6, Fig. 4C) in the uppermost parts of the borehole sections. The oxygen concentration is higher. Euryoxybiont taxa show the following succession: 1. Pappina; 2. Bulimina; 3. Bolivina (comp. Holcová-Šutovská 1996). The activity of a normal fault was connected with Abbreviation Styrian movements that occurred during and mostly after the shallow-water sedimentation of the Karpatian deposits in the southern part of the Carpathian Foredeep (Čtyroký 1992). The Karpatian sediments in the eastern part of the Carpathian Foredeep dipped. These sediments could have been affected by thrust faulting connected with the movement of nappes. In the western part the shallow-water sediments of the Upper Karpatian became eroded, as evinced in boreholes HV-301, HV-305, and Znojmo-12 (Figs 3, 4D). This is further documented by the absence of shallow-water assemblages in the higher sections of the boreholes (Fig. 2). They were subsequently overlain by the Lower Badenian and Quaternary sediments. Discussion and conclusions Type of borehole Number of sample Type of sample Nosislav-3 N-3 CC 80 WR Cf-4 Židlochovice Cf-4 CC 21 WR HV-301 Čejkovice HV-301 NCC 5 rock HV-303 Božice HV-303 NCC 2 rock HV-304 Hrušovany nad Jevišovkou HV-304 NCC 18 rock HV-305 Slup HV-305 NCC 10 rock HV-306 Hevlín HV-306 NCC 13 rock Znojmo-12 Zn-12 NCC 16 rock Nový Přerov-1 NP-1 NCC 5 rock Nový Přerov-2 NP-2 NCC 2 rock Nový Přerov-3 NP-3 NCC 2 rock Nový Přerov-4 NP-4 NCC 3 rock Nový Přerov-5 NP-5 NCC 2 rock Březí-1 Břez-1 NCC 7 rock Březí-2 Břez-2 NCC 8 rock Hrušovany-1 Hy-1 NCC 1 rock Novosedly-1 No-1 NCC 3 rock Brod-1 Brod-1 NCC 4 rock Mušov-1 Muš-1 NCC 2 rock Mušov-2 Muš-2 NCC 2 rock Pouzdřany-1 Pou-1 NCC 7 rock Ivaň-1 Ivaň-1 NCC 3 rock Pohořelice-1 Poh-1 NCC 6 rock Pohořelice-3 Poh-3 NCC 4 rock Mikulov-1 Mik-1 NCC 31 MA Legend: CC continuous core; NCC non continuous core; WR washed residues; MA microfossil assemblages (revision Holzknecht 1965 unpublished data). At the beginning of the Karpatian, in the southern part of the Carpathian Foredeep, deposition began under anoxic conditions (Brzobohatý 1992). Impoverished and more abundant microfauna later began to alternate. Euryoxybiont taxa predominate and represent a sublittoral or locally upper bathyal environment (eastern part of the Carpathian Foredeep). The oxygen supply gradually increased. These 235

6 Pavla Petrová Figure 4. Model of the development of the southern part of the Carpathian Foredeep based on the occurrence of foraminifers (Karpatian). data are in agreement with the conclusions reached by the sedimentological studies of Adamová et al. (1992), and Nehyba and Petrová (2000), all of whom found evidence of intense circulation in the depositional area. Three stratigraphically important levels containing Uvigerina div. sp. (lower with U. graciliformis Papp and Turn., U. peregrina Cush., U. multicostata LeRoy, and upper with U. acuminata Hos., U. pygmoides Papp and Turn. and U. semiornata d Orb.) have been identified. Higher levels with Pappina breviformis (Papp and Turn.) correspond to the Upper Karpatian. Shallow-water assemblages are observed in the uppermost part of most sections. The model of basin evolution (Fig. 4) in the southern part of the Carpathian Foredeep is based mainly on foraminiferal studies. The present author distinguishes two developmental stages of the transgression based on micro- 236

7 Foraminiferal assemblages as an indicator of foreland basin evolution (Carpathian Foredeep, Czech Republic) faunal data: 1. the first stage extended to the eastern part of the Carpathian Foredeep, where deposits containing agglutinated foraminifers occur, which we consider to be the paleoecological equivalents of the agglutinated assemblages from the initial transgressive Karpatian sediments in the Bánov fold and the Blatnice depression of the Danube Basin (Kováč et al. 1999); 2. the second stage reached present extent of the Karpatian deposits on the eastern margins of the Bohemian Massif. Later, after the Lower Badenian, shallow-water Karpatian sediments in the western part of the Carpathian Foredeep were eroded (Fig. 3). Lithostratigraphy, lithofacies, lithology, biofacies, ecostratigraphy, and bioevents are summarised in Fig. 5. Our model shows some similarities and differences between our study area and other partial Western Carpathian basins. The beginning of marine sedimentation with rapid subsidence and usually anoxic and relatively deepwater conditions is a characteristic feature of all Western Carpathian basins. The final shallowing during the last stage of sedimentation is concordant between the part of the Carpathian Foredeep considered here and the entire Carpathian-Pannonian area. The Ottnangian-Karpatian cycle culminated with a global slowing of subsidence (Kováč 2000). Similarities in the development of the southern part of the Carpathian Foredeep in Moravia and the northern part of the Lower Austrian molasse were identified. The maximum water depth along the front of flysch nappes fluctuated between neritic and upper bathyal. Sediments with high quantities of organic matter were deposited in suboxic and dysoxic environments. Marginal sediments show fluctuating salinity. Sediments in the higher parts of the borehole sections were deposited in more oxygen-rich environments (Spezaferri and Coric 2001). The lithology of all the studied sediments can be assigned to the Laa Formation (Rögl 1969). We also found similarities with the development under the Flysch nappes of the Western Carpathians in the surroundings of the town of Zlín. These sediments contain rich organic matter, though foraminifer tests are rare. The deposits originated in a lagoonal environment with low oxygen supply, as indicated by the occurrence of euryoxybiont species. Oxygen concentrations gradually increased, just as in the southern part of the Carpathian Foredeep. Karpatian sediments are correlated with the first stage of the transgression, whereas the sediments of the second stage of the transgression were not identified (Thonová et al. 1987). According to the model presented here, the absence of these latter deposits is a primary feature. Accordingly, the first stage of the Karpatian transgression was terminated by the thrusting of the Flysch nappes over the Zlín area. The tectonic rabotage of the upper sections of the Karpatian sediments in the area around Mikulov should also be considered (Čtyroká et al. 1989). The changes caused by coastal onlap in the southern part of the Carpathian Foredeep correspond to the coastal onlap curve by Kováč and Hudáčková (1997). This curve was constructed for the part of the Vienna Basin that exists in Slovakia, and differs from the coastal onlap curve of Haqa et al. (1988) by low shallowing of the Upper Karpatian. Assemblages containing rare Globigerinoides bisphericus Todd indicate the long continuation of sedimentation in the Carpathian Foredeep on the Karpatian-Badenian boundary. These assemblages are missing in the East Slovakian Basin (Kováč and Zlinská 1998), Danube basin, Novohrad Basin, and in the Slovakian part of the Vienna Basin (Kováč and Hudáčková 1997). They are, however, documented in the Styrian Basin, and the western part of the Vienna Basin in connection with molasse and Lower Austrian molasse in surroundings of Laa an der Thaya (Rögl et al. 2002). Model complements the general view of partial Western Carpathian basins (Kováč 2000) in the Karpatian. It demonstrates that the southern part of the Carpathian Foredeep shares generally identical paleoecological features with the Vienna, northern Danube, Novohrad, and East Slovak basins. During the Karpatian the circular system of the Central Paratethys was of an estuarine type (Brzobohatý 1987, Kováč 2000). The basins differed mainly during the initial and final stages of Karpatian sedimentation. Acknowledgements. Author is grateful to her Doctoral supervisor Rostislav Brzobohatý (Faculty of Science Masaryk University Brno) for important consultations, reading her thesis manuscript, and helpful comments and suggestions. 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(1995): Stratigraphy and geology of the Lower Miocene sediments of the Carpathian Foredeep in South Moravia and adjacent part of Lower Austria. Knihovnička ZPN 16, Kaiho K. (1994): Benthic foraminiferal dissolved-oxygen index and dissolved-oxygen levels in the modern ocean. Geology 22, Kováč M. (2000): Geodynamic, palaeogeographical and structural development of Carpathian-Pannonian region in the Miocene: New view on the Slovak Neogene Basins. Veda, SAV. Bratislava. Kováč M., Holcová K., Nagymarosy A. (1999): Paleogeography, paleobathymetry and relative sea-level changes in the Danube Basin and adjacent areas. Geol. Carpath. 50, 4, Kováč M., Hudáčková N. (1997): Changes of paleoenvironment as a result of interaction of tectonic events with sea level changes in the northeastern margin of the Vienna Basin. Zbl. Geol. Paläont. Teil I, 5/6, Kováč M., Zlinská A. (1998): Changes of paleoenvironmental as a result of interaction of tectonic events and sea level oscillation in the East Slovakian Basin. Przegl. Geol. 46, 5, Molčíková V. (1966): Biostratigraphic and facial evaluation of Karpatian in the Carpathian Foredeep in Moravia. Zpr. geol. Výzk. na Moravě v Roce 1964, Murray J. W. (1991): Ecology and Palaeoecology of Benthic Foraminifera. Longman Scientific and Technical. New York. Nehyba S., Petrová P. (2000): Karpatian sandy deposits in the southern part of the Carpathian Foredeep in Moravia. Bull. Czech Geol. Surv. 75, 1, Nehyba S., Petrová P., Šikula J. (2001): Lithofacies analysis of the Carpathian Foredeep based on subsurface data. Report Czech Geol. Surv. Brno. Pálenský P., Adamová M., Brzobohatý R., Bucha V., Cicha I., Čtyroká J., Čtyroký P., Eliáš M., Franců J., Hamršmíd B., Hladilová Š., Hladíková J., Horáček J., Horák J., Krejčí O., Kropáček V., Malkovský M., Michalíček M., Müller P., Nehyba S., Novák Z., Pazderková A., Pírková D., Procházková V., Řeháková Z., Říha J., Šmíd B., Švrčinová M., Tížek R., Urdihová O., Zdražílková N., Zelenka J. (1991): Final report on Nosislav-3 Borehole. Report Czech Geol. Surv. Praha. Petrová P. (1998): Application of trend analyses in the reconstruction of the palaeoecological conditions in the borehole Nosislav-3 (Karpatian). Scripta Fac. Sci. Nat. Univ. Masaryk. Brun. Geol. 25, 3 8. Petrová P. (1999): Palaeoecological and biostratigraphic evaluation of foraminiferal fauna in southern part of the Carpathian Foredeep (Karpatian, Czech Republic). Biuletyn Państw. Inst. geol. 387, Petrová P. (2003): Palaeoecological evaluation of the Karpatian sediments in the southern part of the Carpathian Foredeep using trend analysis. Ph. D. thesis. Masaryk University. Brno. Rögl F. (1969): Die miozäne Foraminiferenfauna von Laa an der Thaya in der Molassezone von Niederösterreich. Mitt. Geol. Gessel. 61, Rögl F. (1994): Globigerina ciperoensis (Foraminiferida) in the Oligocene and Miocene of the Central Paratethys. Ann. Naturhist. Mus. Wien 96 A, Rögl F., Spezzaferri S., Coric, S. (2002): Micropaleontology and biostratigraphy of the Karpatian-Badenian transition (Early-Middle Miocene boundary) in Austria (Central Paratethys). Cour. Forsch.-Inst. Senckenberg 237, Rögl F., Steininger F. F. (1984): Neogene Paratethys, Mediterranean and Indo-pacific Seaways. In: Brenchley P. (ed.) Fossils and Climate, Wiley. Spezaferri S., Coric S. (2001): Ecology of Karpatian (Early Miocene) foraminifers and calcareous nannoplankton from Laa an der Thaya, Lower Austria: A statistical approach. Geol. Carpath. 52, 6, Šikula J., Nehyba S. (2003): Lithofacies analysis of Miocene sediments in the southern part of the Carpathian Foredeep on basis subsurface data. Geol. výzk. Mor. Slez. v Roce 2002, Šikula J., Nehyba S. (2004): Lithofacies analysis of Miocene sediments in the southern part of Carpathian Foredeep, besed on the re-interpretation of drill logging data. Bull. Geosci. 79, 3, Thonová H., Holzknecht M., Krystek I., Brzobohatý R. (1987): New views on development of the Karpatian under Flysch Nappes in the segment STŘED. Zem. Plyn Nafta 32, 3, Figure 5. Summary of the lithostratigraphy, lithofacies, lithology, biofacies, ecostratigraphy and bioevents (Nehyba et al. 2001; internal report for Czech Geological Survey). 239

10 Pavla Petrová Plate I Examples of several paleoecologically significant assemblages. 1 NP-4 ( m) assemblage with numerous fragments of teleostei and agglutinated foraminifers; 2 Nosislav-3 ( m) assemblage with pyritized tests of globigerinas; 3 Nosislav-3 (232.2 m) assemblage with uvigerinas; 4 HV-306 ( m) assemblage with siphonodosarias and stillostomelas; 5 Mikulov-1 ( m) shallow-water assemblage with Elphidium div. sp.; 6 Cf-4 Židlochovice ( m) shallow-water assemblage with Elphidium div. sp.

11 Foraminiferal assemblages as an indicator of foreland basin evolution (Carpathian Foredeep, Czech Republic) Plate II 1 Bathysiphon taurinensis Sacco, Nosislav-3 borehole, m (90 ). 2 Textularia sp., Nosislav-3 borehole, 71.4 m (160 ). 3 Sigmoilinita tenuis (Czjzek), Nosislav-3 borehole, m (120 ). 4 Neugeborina longiscata (d Orbigny), Znojmo-12 borehole, 25.5 m (45 ). 5 Lenticulina inornata (d Orbigny), Cf-4 Židlochovice borehole, m (48 ). 6 Favulina hexagona (Williamson), Nosislav-3 borehole, m (260 ). 7 Lenticulina aff. melvilli (Cushman & Renz), Cf-4 Židlochovice borehole, m (110 ). 8 Globigerina ottnangiensis Rögl, Pohořelice-1 borehole, m (240 ). 9, 10 Globigerina bulloides d Orbigny, Nosislav-3 borehole, m (200 ). 11 Globigerinoides trilobus (Reuss), Hrušovany-1 borehole, m (150 ). 12 Globigerinoides bisphericus Todd, Pohořelice-1 borehole, m (100 ). 13 Orbulina suturalis Brönnimann, Troskotovice locality, sample F (90 ). 14, 15 Globorotalia sp., Troskotovice locality, sample H (170 ). 16 Bolivina fastigia Cushman, Nosislav-3 borehole, m (100 ). 17 Bolivina hebes Macfadyen, Nosislav-3 borehole, 83.1 m (150 ). 18 Bolivina plicatella Cushman, Nosislav-3 borehole, 83.1 m (150 ). 19 Bolivina dilatata Reuss, Nosislav locality, sample B (120 ). 20 Bolivina antiqua d Orbigny, HV-304 borehole, m (120 ). 241

12 Pavla Petrová Plate III 1 Bulimina schischinskayae Samoylova, Nosislav-3 borehole, 83.1 m (160 ). 2 Bulimina elongata d Orbigny, Nosislav-3 borehole, m (90 ). 3 Praeglobobulimina pupoides (d Orbigny), Pouzdřany-1 borehole, m (100 ). 4 Pappina breviformis (Papp & Turnovsky), Nosislav locality, sample B (100 ). 5 Uvigerina graciliformis Papp & Turnovsky, Cf-4 Židlochovice borehole, m (100 ). 6 Uvigerina acuminata Hosius, Cf-4 Židlochovice borehole, m (110 ). 7 Siphonodosaria cf. consobrina (d Orbigny), HV-304 borehole, m (45 ). 8 Siphonodosaria scabra (Reuss), Nosislav-3 borehole, 82.6 m (120 ). 9 Nonion commune (d Orbigny), Cf-4 Židlochovice borehole, m (160 ). 10 Hansenisca soldanii (d Orbigny ), Nosislav-3 borehole, 80.1 m (200 ). 11 Pararotalia canui (Cushman), Nosislav-3 borehole, 67.6 m (150 ). 12, 13 Heterolepa dutemplei (d Orbigny ), Nosislav-3 borehole; 80.1 m (200 ). 14 Pullenia bulloides (d Orbigny), Cf-4 Židlochovice borehole, m (190 ). 15, 16 Ammonia vienensis (d Orbigny), Brod-1 borehole, m (120 ). 17 Elphidium fichtelianum (d Orbigny), Cf-4 Židlochovice borehole m (110 ). 242

13 Bulletin of Geosciences, Vol. 79, No. 4, , 2004 Czech Geological Survey, ISSN The contact between the Variscan and Cadomian blocks in the Svratka Dome (Bohemian Massif, Czech Republic) Petr Batík Czech Geological Survey, Klárov 131/3, CZ Praha. Abstract: The present author recently presented an alternative scenario for the tectonic development of the SE margin of the Bohemian Massif based on field studies of the Thaya Dome (Batík 1999). In the present paper, the tectonic evolution of the Svratka Dome is described and F. E. Suess s classic concept (1912) of Moldanubian nappe displacement over the Moravicum unit is discussed. The weaknesses of Suess s concept are shown, which pertain mainly to the character of the detachment planes and the amount of time assumed for all the supposed processes. An alternative tectonic scenario is that the Svratka Dome, consisting of Proterozoic granitoid and its overlying mantle, had already formed during the Cadomian orogeny. After Early Paleozoic erosion, its exposed nucleus was covered by Early Paleozoic basal siliciclastic sediments, which in turn are overlain by Devonian clastic and carbonate deposits and Early Carboniferous shales and greywackes. During the Variscan orogeny, which occurred from the Late Carboniferous to the Permian, a freshly consolidated and metamorphosed tectonic block was thrust along a steeply dipping plane over the Brunovistulicum. The Variscan phase continued by flat shears with eastern vergency. The last tectonic deformation events with western vergency affected the Svratka Dome during the Alpine orogeny. Key words: Bohemian Massif, Moldanubicum, Moravicum, Brunovistulicum, Svratka Dome, Thaya Dome, Cadomian orogeny, Variscan orogeny, nappe structure Introduction The Moravicum, also called the Moravian unit, defined by F. E. Suess (1903), crops out along the SE margin of the Bohemian Massif in the tectonic window of the Svratka Dome and the larger Thaya Dome. The Moravicum, a tectonically specific unit, is of Precambrian age and is separated from the Moldanubicum, which was metamorphosed during the Variscan orogeny. Starting from the tectonic base and going upwards, Suess (1912) defined the Inner Phyllite and the Bíteš Gneiss units of the Moravicum. The Inner Phyllites were correlated with the Lukov Group in the Thaya Dome by Batík (1984) and with the Bílý Potok Group in the Svratka Dome by Jaroš and Mísař (1976). The Outer Phyllites can be correlated with the Vranov-Olešnice Group in the Thaya Dome (Dudek 1962), and with the Olešnice Group in the Svratka Dome (Jaroš and Mísař 1976). Suess (1912) considered these Outer Phyllites and the tectonically overlying mica-schists and mica-schist gneisses of the Mica-schist Zone as products of retrograde metamorphism of the Moldanubian rocks affected by the displacement of a Moldanubian nappe. According to his hypothesis, during the Variscan orogeny the Inner Phyllites and the Bíteš Gneiss were displaced over the autochthonous granitoid nucleus of the domes which was covered by Devonian sediments, and that this entire complex later became covered by the Moldanubian nappe. Although some authors have expressed partial or fundamental objections to Suess s concept even during his lifetime (see detailed discussion in Dudek 1958), some geologists, mainly in Austria, still adhere to it. By contrast, recent investigation in the Czech Republic has resulted in the modification of some older tectonic concepts pertaining to the positions and contacts of the Moravicum and Moldanubicum (Jaroš and Mísař 1974, Mísař 1994, Štipská et al. 2000, Schulmann et al. 1991). Many questions persist, such as how to define the Moravicum, how and when the nappe displacement took place, which rocks are allochthonous, the location of the nappe s base and shear planes, the direction in which the nappes moved, and how to explain the difference in metamorphic grade between the Moravicum and Moldanubicum. These problems hamper the construction of geological maps along the Czech-Austrian border, and our efforts to link Czech- and Austrian-made maps (e.g. Roetzel et al. 1999). The main questionable points in Suess s (1912) concept are the following: the detachment plane of the Moldanubian nappe, the nappe s direction of movement and distance moved, specification of the allochthon, and the time interval during which all the endogenous and exogenous processes are supposed to have occured. Each of these points is briefly discussed below. 1. The nappe detachment plane: According to Suess (1912) the Moldanubian displacement should have occurred at the level of the Mica-schist Zone, which he considered to be part of the Moldanubicum overlying the Bíteš orthogneiss, and formed by retrograde metamorphism of the displaced masses of the Moldanubian nappe. The weakness of this concept is that a similar phenomenon has not been observed even in young orogenic zones where the nappes have been studied in detail. In most cases detachment planes cut several tectonostratigraphic horizons. In this case, however, the detachment plane, according Suess (1912), follows only the Mica-schist Zone. Moreover, his displacement mechanism should thus fully conform to the former Cadomian structure, which is also highly improbable from the tectonic viewpoint. 2. The direction and distance of nappe movement: 243

14 Petr Batík Figure 1. Detailed geological map of the Thaya Dome. Paleozoic 1 Permo-Carboniferous clastic fill of the Boskovice Graben, 2 Lower Carboniferous greywackes in the Boskovice Graben, 3 limestone of Frasnian-Givetian age and basal siliciclastics of the Early Paleozoic, 4 crystalline rocks of the Moldanubicum, 5 granitoids of the Brunovistulicum, 6 crystalline rocks at the towns of Miroslav and Krhovice. Moravicum: 7 Šafov Group, 8 Vranov Group, 9 Lukov Group (upper part), 11 Lukov Group (lower part), 12 boundaries between rocks, 13 faults, 14 overthrusts and overfaults, 15 mylonite zone, 16 tectonic breccia. According to Suess (1912) the Variscan Moldanubian nappe moved eastwards in the Thaya Dome, and covered the Moravicum and the Thaya Massif. Suess (1912) considered the Krhovice crystalline area and some of the crystalline rocks at Miroslav to be its relic. Thus the nappe front is supposed to have moved at least 40 km. Several authors, such as Mísař (1994), presumed that this main nappe movement was accompanied by partial Variscan movements in the Moravicum, with the detachment plane at the base of the Bíteš and Weitersfeld orthogneisses. The distances that such partial subnappes (or tectonic slices) have moved is difficult to estimate because their eastern margins have not been preserved. In the Svratka Dome, however, the proposed displacement distances and even the movement of the masses is disputable. The Moravicum nappe in this Dome has been considered as the main nappe (Jaroš and Mísař 1974). These authors suggested that it was thrust along Dřínov plane and was displaced over the Svratka Granitoid and over relics of the Devonian basal siliciclastics and carbonates. The estimated distance of eastward movement should be 3 10 km. In the alternative case, if such movement is supposed to have been parallel with the identified northward strike of lineation (Hanžl et al. 2001), this Moravicum nappe could have covered the entire Svratka Dome and moved more than 50 km. The eastward displacement distance of the front of the Moldanubian nappe can be estimated as 8 23 km, provided that the mica-schists at the Klucanina locality represent relics of the eastern nappe margin, and that the nappe movement is measured from the Bíteš and Svojanov faults. 3. The problem of time: The amount of time necessary for the erosion of the masses of nappe rocks also casts doubt upon the existence of the Variscan nappe structure. Jaroš and Mísař (1974) estimated the thickness of the Moravicum nappe at 3 4 km. The thickness of the Moldanubian nappe has not been conclusively determined, though it could be several kilometres. The amount of time available for the displacement of all the nappes (3 50 km), including the necessary time gap between the movements of the Moravicum and Moldanubicum nappes, and also for the considerable erosion down to their autochthonous nuclei, could not have been more than 20 million years (stratigraphic interval Frasnian Viséan). It is reasonable to relate this time period to known nappe displacements and erosional rates. It has been calculated that nappe displacement 244

15 The contact between the Variscan and Cadomian blocks in the Svratka Dome (Bohemian Massif, Czech Republic) proceeds at an annual rate of 1 14 mm in younger orogenic belts (Kukal 1990), though it is unclear whether such figures are applicable to older, Variscan orogens. The rate of erosion can be very different for soft and hard rock massifs. Nonetheless, we can apply many examples from younger orogenic belts, such as the Outer Western Carpathians. Their nappes have been displaced during a time interval of 20 million years (from the Middle Eocene up to Lower Miocene, see Stráník 2002), after which only negligible fluvial erosion occurred during the subsequent 17 million years. 4. Differences in the metamorphic intensity of the nappe complexes: Suess (1912) believed that the Moldabicum was deeply metamorphosed in the katazone, and that the Moravicum was less intensively metamorphosed in the epizone. From Suess s hypothesis (1912), and from the concepts of his followers (e.g., Jaroš and Mísař 1974), it could be deduced that the Moravicum nappe was displaced before the main Variscan tectonometamorphic phase. It would follow that its degree of metamorphism would be more intense, which is not the case. In summary, the above-mentioned considerations demonstrate the need for reinterpretation of the tectonic processes in the zone under question. It can be objected that our presumptions are quite theoretical, though the lack of time for the displacement of all the nappes and their subsequent erosion seems rather concrete. The Thaya Dome Research on the Czech part of the Thaya Dome has brought recent results, much of which has been published. The following information is of interest here. Whole rock samples of the Thaya Granitoid analysed by the Rb/Sr method have given ages around 551 ±6million years (Scharbert and Batík 1980). This age represents the formation, intrusion, and cooling of the magma during the Cadomian orogenic cycle. The Krhovice Crystalline, which was taken for the relic of the eastern margin of the Moldanubian nappe by Suess (1912), was later studied by Dudek (1962). The latter called Seuss s interpretation into question and supposed that the unit occurs in an autochthonic position. This conclusion was supported by the subsequent discovery of a 200 m thick carbonate complex found to the West of the Diendorfer Fault (Batík and Skoček 1981), beneath Early Miocene sediments. Within this carbonate complex, in the Tasovice drill hole, Zukalová (in Čtyroký and Batík 1983) found a preserved foraminifera assemblage of Frasnian-Givetian age. In the Žerotice drill hole (in the same complex), Batík and Skoček (1981) identified the clay mineral montmorillonite. The presence of this unstable mineral and well preserved fossils indicate that this carbonate complex was probably not buried beneath the several kilometre thick Moldanubian nappe. This carbonate complex overlies basal Devonian deposits, and perhaps also older siliciclastics which in the Tasovice quarry cover the nonrecrystallized eastern margin of the Thaya Massif. Another small erosional relic of the siliciclastics is situated on the blastomylonitic zone near the western margin of the Thaya Massif (at the village of Únanov), which should evidently be a product of pre-variscan deformation. Although this last occurrence might be of Tertiary age based on its heavy mineral associations (Otava 1997), we believe that the Únanov occurrence can be correlated with that of Tasovice. To the SE of Lukov, in the lower part of the Lukov Group, many granitoids were found within 200 metres of the contact with the Thaya massif. These granitoids follow the foliation, were found, which belong to the Thaya Massive. Thus it can be deduced that the Lukov Group represents the mantle of the Thaya Massif, although the two are now tectonically detached from each other. In the Thaya Valley, however, the detachment distance is far smaller than in the NE closure of the Thaya Dome. Upwards in the Moravicum rock complex, west of Lukov, the Bíteš Orthogneiss is also tectonically detached from its basement. However, granitoid apophyses of the Bíteš Orthogneiss in the topmost part of the Lukov Group indicate that even here the detachment cannot be complete (Fig. 3). The Variscan structure of the Thaya Dome was completed by flat shears with eastern vergency. They were accompanied by a slip several tens of metres long. This is visible on the eastern part of the Thaya Massif in the Tešetice quarry (Čtyroký and Batík 1983) and was also found in the Žerotice drill hole (Batík and Skoček 1981). It is supposed that these shear movements also influenced the tilting of the basal siliciclastics in the Tasovice quarry and Early Carboniferous sediments near the Hostěradice locality. The Thaya Dome a short retrospective Three main geological units are defined at the SE margin of the Bohemian Massif: the Moldanubicum, the Moravicum (both of which were defined by F. E. Suess, 1903), and the Brunovistulicum (defined by Dudek 1980). More recently, the view has been expressed that all three units have had the same or very similar tectonometamorphic development up to the end of the Neoproterozoic (Batík 1999, 2003). The area started to become differentiated as late as during the Variscan orogeny. That part, defined as the Moldanubicum by Suess (1903), was completely reworked by metamorphic processes. By contrast, notable metamorphic changes are not observed in the Brunovistulicum. The Moravicum tectonostratigraphic unit is quite different from the surrounding units, and it appeared only after the termination of the Variscan tectogenesis. It should therefore be emphasized that the Moravicum as an independent unit began to exist only since the Variscan orogeny. The Moravicum is separated from the neighbouring units mainly by steeply sloping overthrusts (marginal overthrust after Zapletal, 1926). Such overthrusts are associated with mylonites in some places, and also by smaller faults and 245