Facies (2013) 59:425–449

DOI 10.1007/s10347-012-0300-x

ORIGINAL ARTICLE

Environmental changes close to the Lower–Middle Devonian boundary; the Basal Choteb Event in the Prague Basin (Czech Republic)

S. Vodráqková · J. Frýda · T. J. Suttner · L. Koptíková · P. Tonarová

Received: 10 March 2011 / Accepted: 6 March 2012 / Published online: 12 June 2012© Springer-Verlag 2012

Abstract The Basal Choteb or jugleri Event, close abovethe Lower–Middle Devonian boundary, has been regardedas a minor but important eustatic transgressive event, whichis characterized by signiWcant environmental changes, fau-nal extinction, appearance of new forms, and maximumradiation, particularly among the goniatites. This study con-tributes to a more precise stratigraphic allocation of theevent, and provides a reconstruction of paleoenvironmentalsettings in the type area of the event, the Prague Basin(Czech Republic). The beginning of a transgression isrecorded already in the Tlebotov Limestone (partitus Zone,Eifelian, early Middle Devonian). The basin-wide changein the sedimentation pattern (onset of peloidal and crinoidalgrainstones (calciturbidites) of the Choteb Formation) cor-responding to the uppermost partitus and costatus conodontzones, base of Nowakia (Dmitriella) sulcata sulcata

dacryoconarid Zone, and Pinacites jugleri goniatite Zone isinterpreted here to be linked to a maximum Xooding of thebasin. A hypothesis of enhanced nutrient load during sedi-mentation of the Choteb Formation is suggested here as atriggering mechanism for intense micritization and peloidformation and prasinophyte blooms, which could be, alongwith a greater depositional depth, responsible for oxygendeWciency and consequent reduction of diversity and habi-tat tracking among benthic invertebrates.

Keywords Basal Choteb Event · Lower–Middle Devonian · Prague Basin · Microfacies analysis · Carbon isotope geochemistry · Environmental changes

Introduction

The Basal Choteb Event (Middle Devonian, Eifelian) wasWrst recognized and studied in the Prague Basin (CzechRepublic) and designated as a “rather important event” dem-onstrated by distinctive facies and faunal changes (Chlupáb

and Kukal 1986, 1988). It was introduced as the “jugleriEvent” by Walliser (1985) referring to the spread of thegoniatite species Pinacites jugleri Roemer. According toChlupáb and Kukal (1986, 1988), this event in deeper-watersettings of the Prague Basin is “clearly anoxic, possibly bestexplainable by a quick rise of sea level”. According to theabove-mentioned authors, a profound, although gradual,faunal change took place among almost all faunal groups.Faunal and especially facies changes, close above theLower–Middle Devonian boundary, were also reported else-where; however, the crucial question of global correlationsremains in many cases a matter of debate.

As shown in Table 1 and Fig. 1a, the facies change hasbeen reported from many areas of the world in the time

S. Vodráqková (&) · J. Frýda · P. TonarováCzech Geological Survey, P.O.B. 85, 118 21 Prague 1, Czech Republice-mail: stana.vodrazkova@seznam.cz

S. VodráqkováGeoZentrum Nordbayern, Fachgruppe Paläoumwelt, Friedrich-Alexander-Universität Erlangen-Nürnberg, Loewenichstr. 28, Erlangen, Germany

J. FrýdaFaculty of Environmental Sciences, CULS, 165 21 Prague 6, Czech Republic

T. J. SuttnerCPSA, Austrian Academy of Sciences c/o Institute for Earth Science, University of Graz, Heinrichstrasse 26, 8010 Graz, Austria

L. KoptíkováInstitute of Geology, Academy of Sciences of the Czech Republic, v.v.i., Rozvojová 269, 16500 Prague 6, Czech Republic

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Table 1 Facies changes linked to the Basal Choteb Event recorded in various regions of the world

a partitus Zone not deWned at that timeb P. jugleri recorded in Man Member of Gustalapiedra Fm., above La Loma beds (Jahnke et al. 1983; Montesinos 1987)c Correlation supported by Wndings of representatives of genus Foordites WEDEKIND (House 1962)

Area Biozones Lithostratigraphy Lithology References

Czech Republic N. (D.) sulcata sulcata,P. jugleri, costatus

Choteb Limestone Onset of dark, crinoidal grainstones, alternating with dark-grey lime-mudstone and wackestone

Chlupáb and Kukal (1986, 1988)

Morocco N. (D.) sulcata sulcata, P. jugleri,

Uppermost partitus-costatus

“Pinacites Limestone” Dark-black crinoidal limestones, and dark marls, with styliolinids

Alberti (1979, 1980, 1981), Becker and House (1994), Kaufmann (1998), Bultynck and Walliser (2000), Klug et al. (2000), Klug (2002a, b), Ebbighausen et al. (2004); Kröger et al. (2005), Wendt et al. (2006), Kröger (2008)

Eifel Hills N. (D.) sulcata, sulcata costatus

Dorsel Member of Lauch Formation

Change of sedimentation from crinoidal limestones, to dark marls

Alberti (1982), Struve (1982, 1990), Weddige (1982, 1988)

Rhenische Schiefergebirge (Rhenisch Slate Mts.)

Base of N. (D.) sulcata sulcata, base of P. jugleri, patulusa/costatus Zone transition

Top of Ballersbacher Lm. and base of Günteröder Lm. darkening within Rupbach Shales, upper Wissenbacher Shales

Transition from thicker-bedded, gray limestones, into coarser, darker limestones, and thin-bedded, dark-colored, Wne-grained limestones (Ballersbacher and Günteröder Lm). Darkening within Rupbach Shales

Carls et al. (1972), Alberti (1977, 1978,1980), Requadt and Weddige (1978), Schubert (1996), Avlar and May (1997)

Armorican Massif costatus Saint Fiacre Fm. Alternation of shales, sandstones, and clayey limestones of Bolast Fm, overlain by shaly Saint Fiacre Fm.

Morzadec (1983)

Cantabrian Mts. costatus, P. juglerib, N. (D.) sulcata sulcata

Huergas, Naranco, and Gustalapiedra Fms. (La Loma Beds)

Dark shaly sediments with marlstones and siltstones (Huergas or Naranco fms.) and dark micritic limestones of La Loma Beds (lowermost Gustalapiedra Fm.)

Jahnke et al. (1983), Henn (1985), García-Alcalde (1998), Garcia-Alcalde et al. (2002), Ellwood et al. (2006)

Eastern Iberian Chains

costatus Moyuella Fm. Shaly sedimentation at the base and muddy, nodular limestones, and dacryoconarid shales passing into black shales without benthos

Carls (1979)

Portugal costatus Odivelas Lm. Transition from cherty tuYtes to hemipelagic grey-pink limestones

Machado et al. (2010)

Siberia costatus Malaya Salairka Beds of Mamontovo Fm.

Transgressive unit with siliciclastic sediments with plant remains at the base and black limestones at the top

Yolkin et al. (1997), Yolkin et al. (2000), Yolkin et al. (2005)

Ural costatus, P. jugleri

Afonin horizon Dark bituminus shales and dark-grey limestones with styliolinids

Sapelnikov and Mizens (1980) Artyuszkova and Maslov (2008)

Appalachian Basin

costatusc Nedrow Member of Onondaga Fm.

Dark shaly sediments passing upwards to more massive Wne-grained limestones

House (1962), Klapper (1971), Ver Straeten (2007), Brett et al. (2009)

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Fig. 1 a Early Devonian paleogeography after Scotese (1997) showingpositions of areas where facies changes linked to the Basal Choteb Eventwere recorded (see also Table 1). 1 Czech Republic (Prague Basin),2 Germany (Eifel area and Rheinisches Schiefergebirge), 3 Spain

(Cantabrian Mts. and eastern Iberian chains), 4 France (ArmoricanMassif), 5 Morocco, 6 Siberia (Altai), 7 Ural, 8 Appalachian Basin.b Stratigraphic distribution of taxa crucial for the Basal Choteb Eventcorrelations (basal Choteb Limestone) as recorded in the Prague Basin

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interval corresponding to the upper partitus to the basalcostatus conodont zones, and/or Nowakia (Dmitriella) sul-cata sulcata dacryoconarid tentaculite Zone, and/or Pina-cites jugleri goniatite Zone and therefore points to eustaticsea-level Xuctuations.

In the Prague Basin, where the Basal Choteb Event wasWrst described and discussed, the lithological change fromlight skeletal wackestones of the Tlebotov Limestone todark calciturbidites of the Choteb Limestone has beenregarded as the event interval (Chlupáb and Kukal 1986,1988). As implied by Fig. 1b, the base of the Choteb Lime-stone has been correlated roughly with the base of the bioz-ones mentioned above.

Aims of the study

Here, we focus on sedimentological and geochemical inves-tigations of the Tlebotov and Choteb limestones in order toshed more light on the Basal Choteb event in its type area,the Prague Basin. The crucial question arises whether theonset of environmental changes is really linked to the onsetof sedimentation of the Choteb Limestone, thus to the majorlithologic change, or whether the perturbations took placeearlier. The aim of this paper is therefore to reveal the natureof environmental changes linked to the Basal Choteb Eventby means of sedimentological and geochemical analyses andto describe the stratigraphy of the critical interval in moreprecise terms and thus to allow a better global correlation ofthis event. This study presents carbon isotope curves fromsix sections from the Prague Basin along with the conodontzonation (Klapper et al. 1978; Zusková 1991; Berkyová2009). Carbon isotope analysis has been widely and suc-cessfully used in paleooceanography and geology as a proxyfor changes in primary productivity, preservation, and burialof organic matter, and also for paleoclimatic situationchanges, due to the linkage between the global carbon bud-get and Xuctuations in atmospheric pCO2 (e.g., Kump andArthur 1999). In order to understand oceanographic and sed-imentological factors aVecting accumulations and preserva-tion of organic matter, TOC and Rock-Eval pyrolysis ofselected samples has been carried out.

Furthermore, in order to reconstruct the depositionalenvironment, we focus on general facies distribution pat-terns, sea-level history, and detailed petrographic investiga-tions.

Geological setting

The study area is situated in the central part of the BohemianMassif, the Teplá-Barrandian tectonic unit (Bohemicum),which is widely accepted as a Gondwana-derived crustal block

(Armorican Terrane Assemblage) characterized by Neoprote-rozoic (Cadomian) basement unconformably overlain by lowerPaleozoic sequences (see e.g., Franke 1989, 2000; Chlupáb

et al. 1998; Kachlík 1999). The Early Ordovician crustalextension at the northern Gondwana margin, linked with theopening of the Rheic Ocean, resulted in the formation of thePrague Basin (Havlíbek 1981; Kraft et al. 2004).

During the Devonian, the area was located in southernsubtropical latitudes (Krs et al. 1986; Tait 1999). Thepaleogeographic position, along with a generally warmerclimate established after Ordovician and Silurian glacia-tions (e.g., Caputo 1998), resulted in widespread carbonatesedimentation, which persisted from early Ludlow(Silurian) to Givetian times (Middle Devonian, see sum-mary in Chlupáb et al. 1998). Devonian deposits in the Pra-gue Basin are characterized by diversiWcation of facies(Fig. 2), especially in the Pragian (Lower Devonian), whenbasinal as well as reefal facies were established. During thelate Early to Middle Devonian (late Emsian–Eifelian),when facies pattern show less diversiWcation, relativelyshallow-marine skeletal-rich carbonates predominatedtowards the southwestern and northwestern part of thebasin, with deeper-water oVshore carbonates dominatingtowards the southeast and northeast.

Carbonate sedimentation in the Prague Basin terminatedin late Eifelian–early Givetian when black shales and silici-clastic Xysch-like sequences of the Srbsko Formationreplaced carbonate sedimentation (Petránek 1950; Kukaland Jäger 1988) due to the initiation of the collision withadjacent terranes. The main episode of tectonic deformationcoincides with the mid- to late Carboniferous (Cháb et al.2008), preceded by Upper Devonian thrusting, folding andburial (e.g., Glasmacher et al. 2002). The long lasting post-Variscan erosion of lower Paleozoic deposits resulted inalmost complete obliteration of the original distribution ofDevonian facies (Chlupáb et al. 1998).

Materials and methods

The following sections were studied in the Prague Basin(Fig. 3): Barrandov road-cut (50°02�08�N, 14°23�33�E), Pra-stav quarry (50°02�0.56�N, 14°21�14.9�E), Jelínek Millquarry (49°59�50.6�N, 14°16�13.8�E), Na Kkrábku quarry(49°59�20�N, 14°16�45�E), Hostím section (49°57�41.9�N,14°07�48�E), U N5mc4 section (Karlntejn, 49°56�37.207�N,E 14°11�0.96�E), and Bervený quarry (Suchomasty,49°54�37.54�N, 14°04�38.35�E). For the stratigraphy of theabove-mentioned sections see Berkyová (2009) and refer-ences therein. Correlations between individual sections havebeen mostly based on conodonts (Klapper 1977; Klapperet al. 1978; Berkyová 2009) with regard to carbon isotopetrends measured.

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Microfacies analysis

Sampling for microfacies analysis was carried out bed bybed with regard to facies changes observed in the Weld.More than 240 thin-sections of the formats 3 £ 4, 5 £ 5and 5 £ 10 cm were studied. The carbonate classiWcationsof Dunham (1962) and Embry and Klovan (1972) wereused.

Stable isotope geochemistry: �13Ccarbonate

In an attempt to produce higher resolution �13C curves,bulk-rock samples were analyzed at the sections mentionedabove. Micritic matrix was preferably sampled from a freshrock surface; calcite cements were avoided by choosing themost appropriate samples under a binocular microscope.Whole-rock carbonate samples were powdered and reacted

Fig. 2 Facies distribution of Lower and Middle Devonian strata of the Prague Basin (modiWed after Chlupáb et al. 1998)

Fig. 3 Schematic map showing distribution of Daleje-Tlebotov (upper Emsian–Eifelian) and Choteb formations (Eifelian) within the PragueBasin, with positions of the studied sections. Slightly modiWed after Berkyová (2009)

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with phosphoric acid (McCrea 1950). The evolved CO2

gases were measured for carbon isotope ratios using a Finn-igan MAT 251 mass spectrometer (Czech Geological Sur-vey, Prague). Some samples from the U N5mc4 sectionwere analyzed at the University of Erlangen using a Ther-moFinnigan 252 mass spectrometer (26 samples from theChoteb Limestone). The values are reported in relative toVienna Pee Dee Belemnite using the conventional notation.The accuracy was controlled by replicate measurementsof laboratory standards, with the reproducibility being§0.15 ‰ (1�).

Organic carbon geochemistry: Rock-Eval pyrolysis

The analysis was completely conducted by the laboratoryof organic geochemistry of the Czech Geological Survey(Brno branch). Mineral and organic carbon content andpyrolytic parameters have been measured using the Rock-Eval 6 pyrolyser (Lafarge et al. 1998). The rocks wereextracted using dichloromethane and methanol mixture(93:7). Saturated and aromatic hydrocarbons in the rockextracts were analyzed by gas chromatograph AT 6890with Xame ionization detector (GC–FID and GC–MS). Tensamples of basal beds of the Choteb Limestone at NaKkrábku quarry were analyzed.

Results

Microfacies analysis and depositional environments

Tlebotov and Suchomasty Limestone (Daleje–Tlebotov Formation)

The study of the outer ramp and basinal deposits repre-sented by the Suchomasty and the Tlebotov limestonesresulted in the discrimination of Wve microfacies, which aredescribed and discussed below and in Tables 2 and 3.

Facies 1 Light-grey, skeletal wackestone with highdegree of bioturbation, rare isolated Wlaments of Girvanellaand rare micritized calcispheres (Table 2; Figs. 4c, 5, 6, 7c,9b, c).

The lower part of the Tlebotov Limestone (serotinus—lower partitus zones) at the Barrandov road-cut, JelínekMill quarry, Na Kkrábku quarry, and Prastav quarry is com-posed of facies 1, consisting of intensively bioturbated,medium-bedded, reddish-light-grey micritic limestones.

Depositional environment The presence of micriticmatrix, higher amount of planktonic (dacryoconaridtentaculites) and nektonic (nautiloids and goniatites)organisms, a benthic fauna typical of muddy bottom envi-

ronments, and absence of sedimentological features indic-ative of current activity suggest calm, low-energy,relatively deep settings rich in dissolved oxygen (inferredfrom intense bioturbation and the presence of abundantbenthic faunas). Biological activity (“the biological mill”)at the sea bottom was probably very high as inferred fromfragmentary preserved macrofossils and intense bioturba-tion. The sedimentary environment is interpreted here asproximal oVshore, below storm wave-base. Conclusionsabout the presence of rare isolated Wlaments of Girvanellaand micritized calcispheres cannot be made because oftheir scarcity. The absence of peloids (micritized bio-clasts) can be explained either by absence of microborersor by only short exposure of bioclasts on the sea bottom(due to intense bottom reworking).

Facies 2 Grey, peloidal packstone with rare isolated Wla-ments of Girvanella, rare micritized calcispheres and micri-tized crinoidal ossicles (Table 2; Fig. 4d, e).

This lithofacies consists of grey, medium-beddedmicritic limestones as recorded in the lower part of theTlebotov Limestone at the Hostím section (serotinusZone).

Depositional environment Sedimentological features aswell as a general scarcity of benthic and also planktonic andnektonic faunal components suggests deeper marine set-tings, most probably indicating distal gravity Xow deposits.Other fossils such as calcispheres and isolated Wlaments ofGirvanella might represent allochthonous components.

Facies 3 Bioturbated reddish wackestone (Table 2,Fig. 4f, g).

The Tlebotov Limestone at the U N5mc4 section (seroti-nus to partitus zones) corresponds to this type of lithofa-cies. There, gradually increasing amounts of red, thin-bedded limestone mark the transition from the underlyingDaleje Shales into the Tlebotov Limestone. It is succeededby thin-bedded limestones (beds 10 cm thick on average)with shale intercalations (5–7 cm), which are typical for thelower part of the unit.

Depositional environment Based on the lack of benthicfauna, the presence only of a few specimens of tentaculi-toids and ostracods, and the absence of sedimentologicalfeatures indicative of any current activity, leads to an inter-pretation of a deep subtidal environment, well below stormwave-base.

Facies 4 Grey peloidal wackestone with horizontally ori-ented vugs Wlled with calcite (Table 2; Fig. 4h).

This lithofacies occurs 6 m below the base of the Choteb

Limestone at the U N5mc4 section (serotinus Zone).

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Depositional environment Based on the co-occurrence ofhorizontally oriented vugs, Wlaments of Girvanella and agreater accumulation of peloids, a higher microbial activitywith formation of microbial mats is assumed. Overall, thebasic characteristic of this facies are thin-bedded lime-stones with shaly intercalations. Under- and overlyingstrata of this facies type consist of facies type 3 (wacke-stones with only a subordinate faunal component). From alevel 2 m below the base of the Choteb Limestone (4 mabove the bed with horizontally oriented vugs Wlled withcalcite) to the top of the Tlebotov Limestone, shaly interca-

lations decrease and a slightly higher amount of bioclasts ispresent (besides dacryoconarid tentaculites and Wne shelldebris also fragments of nautiloid shells and crinoidal ossi-cles, altogether comprising approximately 20 % of thelimestone). Thus a relatively deep, poorly oxygenated (lackof benthic organisms) subtidal environment with microbialmats bounding the sediments is interpreted here.

The contemporary scarcity of open marine planktonicorganisms (dacryoconarid tentaculites), which are otherwisethe most common faunal constituent of coeval strata else-where, is not easily explained. There are several possibilities

Table 2 Facies types of the Tlebotov and Suchomasty Limestone in the Prague Basin

Name/facies Short description Grain types and remarks Interpretation

Skeletal wackestone/Fl Light-grey bioturbated skeletal wackestone with rare isolated Wlaments of Girvanella and rare micritized calcispheres

Fossil content 35–40 %: dacryoconarid tentaculites (50 %, styliolinids prevail), ostracods (10–15 %), nautiloids and goniatites (5 %), echinoderms (5 %, crinoid ossicles), trilobites (5 %), brachiopods (3 %), unidentiWed small-sized shell debris. Macrofossils fragmentary preserved and partially bioeroded. Rare micritized calcispheres (1–3 per thin-section) and Girvanella Wlaments

Proximal oVshore, below storm wave-base, environment rich in dissolved oxygen

Peloidal packstone/F2 Grey, peloidal packstone with rare isolated Wlaments of Girvanella, rare micritized calcispheres and micritized crinoidal ossicles

Well-sorted crinoidal ossicles (100–200 �m), micritized calcispheres (80–120 �m), unidentiWed small-sized shell debris, Girvanella Wlaments (10–15 per thin-section), peloids (20 %, uniform in size, reaching 100 �m). Two preservation types of calcispheres recorded (calcispheres with micritic walls and totally micritized calcispheres = peloids with fuzzy boundaries

Deep subtidal, gravity Xow deposits

Wackestone/F3 Bioturbated reddish wackestone Fossils scarce, bioclasts (10–15 %): dacryoconarid tentaculites (styliolinids), ostracods and unidentiWed small-sized shell debris. Rare Girvanella Wlaments and micritized calcispheres

Deep subtidal

Peloidal wackestone/F4 Grey peloidal wackestone with horizontally oriented vugs Wlled with calcite

Accumulations of Girvanella Wlaments and peloids concentrated in one layer with horizontally oriented vugs Wlled with calcite. Similar to F2, two kinds of calcispheres were observed

Poorly oxygenated subtidal with microbial mats

Skeletal wackestone and echinoderm pack/grainstone/F5

Reddish and grey stromatactis-bearing skeletal-rich wackestone with abundant echinoderm remains and echinoderm pack-grainstone

Amount of skeletal components varies between 40 and 60 %, represented echinoderms (crinoid ossicles in particular), ostracods, brachiopods, trilobites, dacryoconarid tentaculites, rugose and tabulate corals, bryozoans, gastropods and unidentiWed small-sized shell debris. Alternation of low-energy skeletal wackestones (rich in dacryoconarids, ostracods and crinoids) with higher-energy, poorly sorted coarser-grained crinoidal packstone with transition to grainstone facies. Stromatactis structure present (see Kukal 1972; Dieken 1996; Hladil 2005). Rare Girvanella Wlaments and lumps. Micritized calcispheres recorded only in the uppermost part of the succession

Shallow, sheltered environment bellow storm wave-base

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for this, e.g., deposition in a part of the basin that wassomehow restricted from the open ocean. In this respect itis necessary to mention that we do not have any other data(paleontological, geochemical), which would speak infavor of environmental restriction. The paleoecologicalrequirements of planktonic tentaculites should be consid-ered. In this respect, the greater distality and/or greaterdepth could have played a role in dacryoconarid distribu-tion. Implications might be inferred from recent zooplank-ton, where changes in horizontal, i.e. onshore–oVshoreand vertical distribution have been described (e.g., Her-man and Rosenberg 1969). However, considering the factthat dacryoconarid tentaculites are ubiquitous in theDevonian deposits of the Prague Basin, present in bothshallow-water carbonates and deeper-water shales, limita-tions do not seem to be probable, but cannot be ruled out.The higher abundance of calcimicrobes and calcispheresdoes not contradict the greater depth and/or restrictioninterpretation.

Facies 5 Reddish and grey stromatactis-bearing skeletal-rich wackestone with abundant echinoderm remains andechinoderm packstone-grainstone (Table 2; Fig. 7a, b).

The succession of reddish and grey Suchomasty Lime-stone in the Bervený quarry corresponds to this type offacies (serotinus–partitus zones) deposited in theKon5prusy area.

Depositional environment The alternation of skeletalwackestones with coarser grained packstones-grainstonespoints to episodic higher energy events, which took placeduring deposition of the Suchomasty Limestone. The sedi-ment underwent a relatively short-distance transport fromnearby crinoidal meadows. The depositional environmentcan be interpreted as shallow, but sheltered, probably below

Table 3 Facies types of the Choteb and Acanthopyge Limestone in the Prague Basin

Name/facies Short description Grain types and remarks Interpretation

Peloidal and crinoidal grainstone/F6

Graded peloidal grainstone with crinoid ossicles and graded crinoidal grainstone with peloids

Crinoidal ossicles reach 250–500 �m (depending on the locality) at the base and higher in the Choteb Lmst. Peloids in identical level reach 100–180 �m). Abundant micritized calcispheres present. Rare Girvanella Wlaments, ostracod, brachiopod and trilobite fragments form subordinate components of the grainstones. Phycomata of prasinophytacean algae occur in certain levels of the Choteb Lm. (200–250 �m, most commonly at the base of the formation at Na Skrabku quarry reaching the size of 500 �m). Larger crinoid ossicles (0.8–3 mm, depending on the locality) along with various types of larger peloids (180–250 �m) observed at the following levels above the base of the Choteb Lmst.: 1 m, Na Skrabku quarry (costatus Zone), ca. 6 m, U Nemcu section (costatus Zone), 2.6 m, Jelinkuv Mill quarry (costatus Zone), 3 m, Cerveny quarry (costatus Zone)

Calciturbiditic deposits, outer ramp settings

Wackestone and lime-mudstone/F7

Bioturbated dark-grey wackestone and laminated lime-mudstone

Faunal content (15–25 %): dacryoconarid tentaculites (styliolinids), nautiloid remains, phycomata of prasinophytacean algae, and unidentiWable small-sized skeletal debris. Girvanella Wlaments and micritized calcispheres recorded, however, not as abundant as in allochtonous deposits. Micritic matrix with “clotted” appearance

Radiolarian wackestone/F8

Dark laminated and/or bioturbated radiolarian wackestone with chert lenses

Radiolarians, occasional sponge spicules Deep subtidal environment

Fig. 4 a Barrandov road-cut section with position of the Tlebotov(T) and Choteb (C) Limestone boundary. b Na Kkrábku quarry (typelocality of the Choteb Limestone) with position of the Tlebotov andChoteb Limestone boundary. c Bioturbated dacryconarid wackestone,Tlebotov Limestone, 8.8 m below the base of the Choteb Limestone,Lower Devonian, late Emsian, serotinus Zone, Na Kkrábku quarry,facies type 1. d Peloidal packstone with micritized calcispheres, andcrinoidal osscicle, 2.5–3 m above the base of the Tlebotov Limestone,Lower Devonian, late Emsian, serotinus Zone, Hostím section, faciestype 2. e Peloidal packstone with micritized calcispheres (white arrow)and Wlaments of Girvanella (black arrow), 2.5–3 m above the base ofthe Tlebotov Limestone, Lower Devonian, late Emsian, serotinusZone, Hostím section, facies type 2. f, g Bioturbated skeletal wacke-stone with dacryoconarid tentaculites, and unidentiWed shell debris,facies type 3. f Tlebotov Limestone, 11.9 m below the base of the Cho-teb Limestone, Lower Devonian, late Emsian, serotinus Zone, UN5mc4 section. g Tlebotov Limestone, 10.2 m below the base of theChoteb Limestone, Lower Devonian, late Emsian, serotinus Zone, UN5mc4 section. h Peloidal wackestone with horizontally oriented vugsWlled with calcite, Tlebotov Limestone, 6 m below the base of the Cho-teb Limestone, Lower Devonian, late Emsian, serotinus Zone, UN5mc4 section, facies type 4

�

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Fig. 5 Type locality of the Choteb Limestone, Na Kkrábku quarry, with semi-quantitative abundances of calcispheres, prasinophytes, and peloidscorresponding also to positions of microfacies samples. Conodont zonation based on Berkyová (2009)

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storm-wave base judging from the lack of current generatedsedimentary structures and poorly washed limestones.

Choteb and Acanthopyge Limestone (Choteb Formation)

Three microfacies are distinguished in the Choteb Forma-tion.

Facies 6 Graded peloidal grainstone with crinoid ossiclesand graded crinoidal grainstone with peloids (Table 3;Figs. 7c–h, 8b–g).

This facies occurs at the base of the Choteb and theAcanthopyge limestones and was observed within all sec-tions studied. The grainstones are well to moderately well-sorted.

Interpretation of facies type 6 follows description offacies type 7.

Facies type 7 Bioturbated dark-grey wackestone and lam-inated lime-mudstone (Table 3; Figs. 8h, 9a, d).

The background sediment of the Choteb Limestone, asrecorded in all of the sections, is composed of facies 7,which consists of medium-bedded dark-grey to graymicritic limestones. Although thin-bedded lime-mudstonesalso form the uppermost part of the Tlebotov Limestone atthe Barrandov road-cut section, dark wackestones are char-acteristic especially of the Choteb Limestone at the UN5mc4 section, where prasinophytes and small peloids arecommon.

Depositional environment of facies types 6 and 7 The maincomponents of the allochthonous beds (facies type 6),which are crucial for our interpretations, are: crinoidal ossi-cles with varying degree of micritization, various types ofpeloids, prasinophytes, calcispheres, and the presence ofGirvanella.

The peloids of the Choteb Formation were discussed indetail by Berkyová and Munnecke (2010). The latterauthors recognized several kinds of peloids in the Choteb

Formation and, based on the several stages of grain degra-dation recorded, interpreted the origin of peloids as (1)totally micritized calcispheres (e.g., Fig. 8b), (2) crinoidand/or calcimicrobial remains (Fig. 8f) and (3) abradedGirvanella clusters (e.g., Fig. 8d). The authors interpretedthe peloids as a result of the activity of microborers.

It is noteworthy that echinoderms, due to their highlyporous stereom (Macurda et al. 1978; Smith 1980), areregarded as less prone to grain degradation via micritization

Fig. 6 Barrandov road-cut section with semi-quantitative abundancesof calcispheres, prasinophytes, and peloids corresponding also to posi-tions of microfacies samples. Conodont zonation based on Berkyová(2009) and Zusková (1991)

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(Perry 1998). Thus, the high intensity of micritization isthought to reXect the high activity of bioeroders.

The wackestones and lime-mudstones of facies 7 andgrainstones of facies 6 are interpreted as calciturbidites

deposited in the outer ramp settings, with basal and higherparts of the succession of the Choteb Limestone represent-ing more distal sediments associated with sea-level rise.The background sediments, represented by dark-gray to

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light-gray lime-mudstones (laminated at some levels) andwackestones, show certain similarities to facies 1 of theTlebotov Limestone, but the oxygen deWciency of theformer is obvious (darker color, impoverished benthicfauna, presence of prasinophytes). MonospeciWc assem-blage of Chondrites isp. and mixing of variously alteredbioclasts, recycled fragments of cements, and Wrm-/hard-grounds beds have been reported in Koptíková (2011).

The triggering mechanisms of turbidity currents in thissetting are not known. Frequent storms and/or tectonicactivity (ongoing Variscan deformation) and/or sea-levelXuctuations are plausible. Transport took place from theshallow water, probably from the photic zone (the shallow-water origin of the grainstone beds was suggested alreadyby Chlupáb 1959). As suggested by Berkyová and Mun-necke (2010), the source area (which is not preserved) wasprobably somehow sheltered, probably by a crinoidal bar,and poorly agitated so that micritization and peloid forma-tion could have taken place. An origin within the photiczone is probable, considering the co-occurrence of Girva-nella (which was found with other material transporteddown-slope) and the activity of bioeroders. However, itshould be kept in mind that not all bioeroders causing micr-itization are necessarily photoautotrophs. In addition, thelight requirements of Girvanella species are also not astraightforward issue.

Facies type 8 Dark, laminated and/or bioturbated radio-larian wackestone with chert lenses (Table 3; Fig. 9e).

This type of facies is typical of the upper part of theChoteb Formation within the upper costatus to kockelianus

zones as recorded at the Na Kkrábku quarry, Jelínek Millsection, and Hostím section. The strata included in thisfacies consist of medium- to thin-bedded, dark, laminatedand/or bioturbated radiolarian wackestones with chertlenses. Other components apart from radiolarians are veryrare; occasional sponge spicules are present.

Depositional environment Inferring from the presence ofradiolarians, the lack of benthic fauna, sedimentologicalfeatures (dark color, medium-thin bedding, lamination orburrowing) this facies is interpreted here as being depositedin an open, deep-water marine environment, reXecting pro-gressive deepening of the basin.

�13Ccarb trends

Hostím section

An trend of increasing �13C values was recorded close tothe base of the partitus Zone to slightly above the base ofthe kockelianus Zone, which was followed by a decreasehigher in the kockelianus Zone (Fig. 10).

U N5mc4 section

A general trend of increasing �13C values was recorded inthe Tlebotov and Choteb limestones, with the data beingmore scattered in the latter. A positive shift starts in thelower Tlebotov Limestone to the upper Choteb Limestone(Fig. 10).

Prastav quarry

The general trend in �13C is marked here by a minordecrease in the lower Tlebotov Limestone, serotinus Zone,to the lowest values one meter above the Lower–MiddleDevonian boundary (one meter above FAD of P. costatuspartitus), followed by an increase in the lower strata of theChoteb Limestone, in the basal costatus zone (Fig. 10).

Barrandov road-cut section

�13C values increase in the basal parts of the TlebotovLimestone, serotinus Zone (Fig. 10). A decrease of �13Cvalues took place close to the base of the partitus Zone fol-lowed by a rise of values to ca. 2 ‰ in the lower part of theChoteb Limestone, in the basal costatus Zone. A decrease isrecorded at the base of the australis Zone, which is drawnon the occurrence of Polygnathus pseudofoliatus Witte-kindt. The relatively wide scattering of �13C values in theuppermost Tlebotov and Choteb limestones is most likelycaused by the presence of carbonates known as “whitebeds”, which originated during intense Tertiary weathering,

Fig. 7 a Skeletal wacke/packstone with echinoderms, SuchomastyLimestone, 1.5 m below the Acanthopyge Limestone, Middle Devo-nian, Eifelian, upper partitus Zone, Bervený quarry, facies type 5.b Crinoid packstone with fragment of rugose coral (arrowed), Su-chomasty Limestone, 0.5 m below the base of the Acanthopyge Lime-stone, Middle Devonian, Eifelian, upper partitus Zone, Bervenýquarry, facies type 5. c Skeletal wackestone with dacryoconarid tentac-ulites and trilobite exoskeleton overlain by peloidal grainstone withcrinoidal ossicles and brachiopod shell fragment, contact of the Tleb-otov and Choteb Limestone, Middle Devonian, Eifelian, uppermostpartitus Zone, Jelínek Mill quarry, facies type 1 (lower part of the thin-section) and 6 (upper part of the thin-section). d Peloidal grainstonewith crinoid ossicles and micritized calcispheres, basal beds of theChoteb Limestone, Middle Devonian, Eifelian, uppermost partitusZone, Jelínek Mill quarry, facies type 6. e, g, h Fine-grained peloidalgrainstone with micritized calcispheres (white arrows), phycomata ofprasinophytacean algae (black arrows) and crinoids (gray arrows),0.2 m above the base of the Choteb Limestone, Middle Devonian, Eif-elian, costatus Zone (zonal identiWcation based on the occurrence ofPolygnathus sp. aV. P. trigonicus), Nowakia (D.) sulcata sulcataZone), Na Kkrábku quarry, facies type 6. f Crinoidal grainstone withpeloids, syntaxial rim cements around crinoidal osscicles (lower partof the thin-section) and partly micritized brachiopod shell fragment(micritic envelope, upper right corner), 2.4 m above the base of theChoteb Limestone, Middle Devonian, Eifelian, costatus Zone. JelínekMill quarry, facies type 6

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probably by ground water solutions along Wssures of tec-tonic origin (Petr et al. 1997; Plusquelec and Hladil 2001).The occurrences of above-mentioned weathered carbonatesare limited to the uppermost Tlebotov and the basal Choteb

limestones.

Jelínek Mill quarry

The �13C values are characterized by a slight increase andsubsequent decrease in the lower Tlebotov Limestoneserotinus Zone to the lowest values in the partitus Zone,

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succeeded by an increase in the lower part of the costatusZone, Choteb Limestone (Fig. 10). The succeeding ca. 5 mof the section are covered. A slight increase of values isrecorded through the costatus to kockelianus zones.

Na Kkrábku quarry

There is a slight decrease of �13C values with minor distur-bance in the lower part of the Tlebotov Limestone (seroti-nus and lower patulus zones) followed by an increasebeginning in the mid-patulus Zone and a subsequentdecrease in the australis Zone (the base of the australisZone is drawn based on the joint occurrences of P. pseudo-foliatus Wittekindt and P. cf. trigonicus BischoV & Ziegler(Fig. 10).

Rock-Eval pyrolysis (Table 4)

Depositional environment and sea-level Xuctuations in the serotinus–kockelianus zones in the Prague Basin

The Tlebotov and Choteb limestones (and their shallow-water equivalents) represent inner to outer ramp environ-ments. The rather patchy occurrences of the Devonian sec-tions and the tectonic setting (Klíq 1991) of the PragueBasin complicate the interpretation of depositional environ-ments using Weld and microfacies data. The situation is alsocomplicated by the fact that the Devonian outcrops repre-sent only an erosional relict of an originally broader andmore diverse association of facies (Chlupáb et al. 1998).

The Prague Basin is thus referred to by many authors asPrague Synform (e.g., Melichar 2004).

Serotinus to patulus zones (upper Emsian, Lower Devonian)

Skeletal wackestones, rich especially in planktonic andnektonic faunas, were deposited within the time intervalcorresponding to the serotinus to patulus zones in thenortheastern and central parts of the basin (TlebotovLimestone at the Barrandov road-cut section, Prastavquarry, Jelínek Mill section, and Na Kkrábku section). Thefacies pattern is rather similar throughout the area and sta-ble for the time interval discussed at the sections studied,and corresponds to facies 1. The environmental interpreta-tion is in agreement with that of previous authors (see sum-mary in Chlupáb et al. 1998) i.e., an open marine, relativelydeep but well oxygenated oVshore setting. A diVerent set-ting is recorded in the southwestern (Karlntejn vicinity andKon5prusy area) and northwestern parts of the basin(Hostím vicinity). In the SW part (U N5mc4 section,Karlntejn), thin-bedded micritic limestones with shale inter-calations, corresponding to facies 3 and 4, were depositedin this time interval.

In the Kon5prusy area (SW part of the basin), theSuchomasty Limestone was deposited, represented mainlyby reddish and grey crinoidal packstone with commonstromatactis structures.

When judging diVerences between the depositional envi-ronments in the Hostím and Karlntejn areas, the tectonic settingof the basin should be considered. Both areas belong to diVer-ent tectonic segments (Klíq 1991), and local diVerences in tec-tonic activity may have inXuenced sedimentation (due togeographic variations in re-activation and deformation duringthe ongoing Variscan Orogeny). The relatively stable develop-ment of carbonate deposits is not suggestive of any environ-mental perturbation and sea-level Xuctuations, thus a stablerelative sea-level is proposed for the serotinus to patulus zones.

Partitus Zone (Eifelian, early Middle Devonian)

Berkyová and Munnecke (2010) report an increase in calci-sphere abundances (see Figs. 5, 6 for quantitative expressionof the change) from the basal partitus Zone onwards andinterpreted this as a Wrst indicator of environmental changes,which culminated in the costatus Zone. We therefore regardthe basal partitus Zone as the beginning of transgression.

Upper partitus to basal costatus zones (Eifelian, Middle Devonian)

Judging from the increased abundances of micritized calci-spheres and Girvanella Wlaments, darker color and thinner

Fig. 8 a Micritized calcispheres in peloidal grainstone with variousdegrees of grain diminution, 0.15 m above the base of the Choteb

Limestone, Middle Devonian, Eifelian, uppermost partitus Zone, Pra-stav quarry, facies type 6. b Heavily micritized calcispheres in thepeloidal grainstone, 0.5 m above the base of the Choteb Limestone,Middle Devonian, Eifelian, costatus Zone, Barrandov road-cut, faciestype 6. c Moderately sorted crinoidal grainstone with peloids, approx-imately 4.5 m above the base of the Choteb Limestone, Middle Devo-nian, Eifelian, undetermined zone. Hostím section, facies type 6.d Calcimicrobe Girvanella peloid (on the right) and partly micritizedcrinoid ossicle (centripetal micritization, on the left) in crinoidal grain-stone with peloids, 3.6 m above the base of the Choteb Limestone,Middle Devonian, Eifelian, ?costatus Zone, U N5mc4 section, faciestype 6. e Peloids interpreted as of calcisphere origin in peloidal grain-stone, base of the Choteb Limestone, Middle Devonian, Eifelian, co-status Zone, Jelínek Mill section, facies type 6. f Two stages of grainalteration (micritization); crinoid osscicle in the stage of micritic enve-lope on the right, more advanced grain alteration of (probably) crinoidosscicle on the left (=peloid), crinoidal grainstone with peloids, base ofthe Choteb Limestone, Middle Devonian, Eifelian, costatus Zone, NaKkrábku quarry, facies type 6. g Girvanella Wlaments, basal beds of theChoteb Limestone, Middle Devonian, Eifelian, uppermost partitusZone, Jelínek Mill quarry, facies type 6. h Laminated lime-mudstonewith phycomata of prasinophytacean algae (dark dots), approximately4.3 m above the base of the Choteb Limestone, Middle Devonian,Eifelian, undetermined zones. U N5mc4 section, facies type 7

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bedding, accumulations of prasinophytes, occurrences ofhardgrounds and higher accumulation of skeletal material(Barrandov road-cut section) it is suggested that progres-sive deepening of the basin took place in the interval corre-sponding to the upper partitus and basal costatus zones (topof the Tlebotov Limestone–lowermost Choteb Limestone).

In addition, changes in faunal composition in the upperpartitus to basal costatus zones were described by Chlupáb

(1977), who reported higher diversity of proetid trilobitesand also changes in phacopid trilobite assemblages. Smalland juvenile forms of Pygmaeocrinidae and Ramacrinidae(Crinoidea) in the upper Tlebotov and the lower Choteb

limestones point to unfavorable conditions with reducedoxygen (Rudolf J. Prokop, personal communication).

A basin-wide change in sedimentation patterns occurredin the basal costatus Zone, reXected by the onset of darkpeloidal grainstone sedimentation (Choteb Limestone,facies 6), which marks the maximal deepening of the basin(interval of the maximum Xooding). A sea-level lowstandwas recorded a few meters above the base of the Choteb

Limestone in all the sections studied, with thicker and

coarser beds of crinoidal grainstones deposited (proximalgrainstones), representing probably the end of this trans-gressive–regressive cycle. The shoreline proximity wouldalso explain accumulation of land Xora recorded anddescribed from this level in the Choteb Limestone (Obrhel1958; Chlupáb 1959). After this shallowing event withinthe costatus Zone, deepening was recorded again withdeposition of thin-bedded micritic limestones with cherts,representing the T–R cycle Id sensu Johnson et al. (1985).

Base of kockelianus Zone

A second shallowing pulse (the end of T–R cycle Id) wasrecorded in the kockelianus Zone, when peloidal limestonesand crinoidal grainstones (tempestites) with hummockycross-stratiWcation, rare sole marks, and signs of redeposi-tion (common intraclasts at the base of course crinoidalbeds) were deposited (Hostím area). This was followed bydeepening (T–R cycle Ie, Johnson et al. 1985), which cul-minated in the deposition of dark Kabák Shales of the Srb-sko Formation (Kabák Event).

Fig. 9 a Bioturbated skeletal wackestone with dacryoconarid tentacu-lites, nautiloid shell (upper left corner) and echinoderm fragment (onthe right), 0.5 m above the base of the Choteb Limestone, MiddleDevonian, Eifelian, upper partitus Zone, Prastav quarry, facies type 7.b, c Skeletal wackestone with rare calcispheres (black arrow), Girva-nella Wlaments (white arrow) and ostracod shell (below the black ar-row of b). Tlebotov Limestone, 4.5 m below the base of the Choteb

Limestone. Lower Devonian, late Emsian, top of the patulus Zone. Na

Kkrábku quarry, facies type 1. d Bioturbated skeletal wackestone withdacryoconarid tentaculites and gastropod shell fragment (upper leftcorner), the uppermost Tlebotov Limestone, 0.2 m below the base ofthe Choteb Limestone, Middle Devonian, Eifelian, partitus Zone, UN5mc4 section, facies type 7. e Laminated radiolarian wackestone,approximately 8–10 m (part of the Jelínek Mill section is covered)above the base of the Choteb Limestone, Middle Devonian, Eifelian,costatus Zone, Jelínek Mill quarry, facies type 8

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Discussion and interpretation

�13C analysis results

Only minor Xuctuations are recorded in the �13C trend inupper Emsian–Eifelian sections in the Prague Basin. How-ever, these may be correlated by constraining the ages ofthe Xuctuations using conodonts. The small, however sig-niWcant, negative “peak” close to the Lower–Middle Devo-nian boundary could be of a greater importance as it mayhave a global correlation potential. A correlation betweenthe recorded negative peak of �13C values and partitusZone (proximity of the Lower–Middle Devonian boundary)is suggested herein (Fig. 10). In the U N5mc4 section at

Karlntejn and at Na Kkrábku quarry, the position of the neg-ative peak corresponds to the patulus Zone. However, con-sidering the position of the peak at other sections, it seemsthat this lower position of the negative peak could be theresult of sampling bias. Conodonts from U N5mc4 sectionat Karlntejn are rare, so it is very probable that the Wrstrecorded occurrence of the index taxon of the partitus Zone(Polygnathus costatus partitus) does not represent its realWrst appearance.

�13C values in the Choteb Limestone show considerablescatter, however, a relationship between certain lithologictypes and �13C value in individual samples was not found.As noted above, the Choteb Limestone is composed of cal-citurbidites (peloidal and crinoidal grainstones), which alter-

Fig. 10 �13Ccarbonate of inorganic carbon of Lower–Middle Devonian(upper Emsian–Eifelian) sections studied in the Prague Basin. Cono-dont zonation based on Berkyová (2009), Klapper et al. (1978), and

Zusková (1991). The dashed line stands for suggested correlationbased on the correlative geochemical signal, i.e. slight negative excur-sion of �13Ccarb close to the partitus Zone

Table 4 Results of Rock-Eval pyrolysis analysis of the samples from the Na Kkrábku section (Choteb Limestone), where accumulations of largestprasinophytes were recorded

Centimeter above the base of the Choteb Lmst.

Tmax (°C)

S1 (mgHC/g rock)

S2 (mgHC/g rock)

S3 (mgCO2/g rock)

TOC(%)

HI (mgHC/g TOC)

OI (mgCO2/g TOC)

Total TIC (%)

910 357 0.01 0.02 0.15 0.05 40 300 10.74

1,035 437 0.01 0.05 0.27 0.10 50 270 11.29

1,042 443 0.02 0.09 0.17 0.09 100 189 11.90

1,061 445 0.02 0.13 0.10 0.15 87 67 11.91

1,083 454 0.01 0.03 0.16 0.08 38 200 11.37

1,235 471 0.01 0.05 0.12 0.08 63 150 11.50

1,645 436 0.04 0.18 0.23 0.13 138 177 11.40

1,745 444 0.02 0.34 0.38 0.19 179 200 11.02

1,845 448 0.02 0.10 0.16 0.14 71 114 11.71

2,075 446 0.02 0.09 0.17 0.11 82 155 11.62

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nate with background sediments (lime-mudstones andskeletal wackestones). Moreover, the dark color and thepresence of prasinophyte accumulations are characteristic ofcertain levels in the Choteb Limestone, especially near thebase of this unit (Fig. 11). However, a clear correlationbetween periods of high primary productivity (and/or peri-ods of better preservation) and speciWc carbon isotope trendhas not been observed. Furthermore, it seems that the scatterof �13C values in the Choteb Limestone is not due to chang-ing environmental conditions (diVerent lithology and/orhigher amount of organic matter), but might rather representdiagenetic overprint of primary �13C values. However thelong-term gradual shift of �13C values suggest progressiveburial of organic carbon from the partitus Zone onwards—ashift from ca. 1 ‰ in the partitus Zone to ca. 2 ‰ in thecostatus Zone was recorded at all of the sections studied.This low-magnitude gradual change in carbon isotope com-position correlates well with the step-wise sea-level riserecorded. The �13C reaches about 1.6-1.7 ‰ at the Tlebo-tov/Choteb Limestone boundary interval at every section.Comparable �13C values from coeval strata have also beenrecorded elsewhere in Europe and North Africa (Buggischand Mann 2004; Buggisch and Joachimski 2006; Van Geld-ern et al. 2006) and also from the Onondaga Limestone inNew York (Fay and Saltzman 2006). This signal, as long asit can be regarded as global, points to a relatively steadystate carbon cycling, with processes of organic carbon burialand remineralization being balanced by unrestricted oceancirculation, but the long-term trend of increased organic car-bon burial from the partitus Zone is obvious.

Environmental control on prasinophytes, calcispheres, peloids and Girvanella occurrences

Four features are characteristic of the Choteb Formation:accumulations of prasinophytes, intensive activity ofmicroborers resulting in micritization and peloid formation,the presence of cyanobacteria, and calcisphere blooms.

Micritized calcispheres

Calcispheres occur in the Tlebotov Limestone, but massiveaccumulations of calcispheres are only recorded in the cal-citurbidite beds of the Choteb Limestone. It is worth men-tioning that an increase in the number of micritizedcalcispheres from the basal partitus Zone (early Eifelian,Middle Devonian) is recorded in all sections of the Tlebo-tov Limestone (Berkyová and Munnecke 2010). Althoughthe recorded abundances are not comparable to the massiveaccumulations recorded in the Choteb Formation, it is anobvious change when compared with the underlying TlebotovLimestone (serotinus and patulus zones) (see also Figs. 5,6). It is important to mention that in the Suchomasty Lime-stone (shallow-water equivalent of the Tlebotov Lime-stone) calcispheres are missing, except for its uppermostpart. Thus the facies where micritization of calcispherestook place and from which the calcispheres were winnowedto deeper-water environments is probably not preserved.The micritized grains were interpreted by Berkyová andMunnecke (2010) as a result of microbial activity, thereforethe lack of both bioeroders and calcispheres at the time ofsedimentation of the Tlebotov and the Suchomasty Lime-stone should be taken into account as well. Then the mas-sive occurrences of calcispheres in the Choteb limestonewould be suggestive of an increase in bioerosion and calci-sphere populations as a response to environmental pertur-bations linked to the Basal Choteb Event, which could haveresulted from higher nutrient load as suggested by theabove mentioned authors.

Prasinophytes

Variable abundances of prasinophytes were recorded, beingmost common in the basal Choteb Limestone at the NaKkrábku quarry (=basal costatus Zone) (Fig. 11) and in sev-eral intervals of the U N5mc4 section. The most character-istic feature of the prasinophytes is their size—the largest

Fig. 11 Grainstone sample (on the left) etched by 6 % acetic acid showing episodic accumu-lations of large phycomata of prasinophytacean algae and SEM of the prasinophytes recov-ered from this level (on the right); scale 100 �m. Basal beds of the Choteb Limestone (0.2 m above its base), Na Kkrábku quarry (Middle Devonian, Eifelian, costatus Zone, N. (D.) sulcata sulcata Zone)

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Facies (2013) 59:425–449 443

size, nearly 500 �m, was recorded in the basal Choteb

Limestone beds from the Na Kkrábku quarry; the averagesize is 200–250 �m. In addition to their large size, also agreater thickness of the phycomata walls was recorded.Therefore, the ecological conditions (light, nutrient avail-ability, temperature) in the water column must have beenfavorable for prasinophyte growth.

A positive peak in �18Oapatite values, corresponding to acooling episode (Elrick et al. 2009), was recorded 0.1 mabove the level with a higher prasinophyte concentration atthe topmost Tlebotov Limestone at the Barrandov road-cut(basal costatus Zone). Prasinophytes are regarded as rela-tively cool water algae (e.g., Tyson 1995; Wrenn et al.1998; Prauss 2007). Thus it appears that higher nutrientload and cooler temperatures could be the triggering mech-anisms for the onset of prasinophyte blooms.

The cooler water temperatures and especially highernutrient availability were probably not the only controllingmechanisms of prasinophyte accumulations. The petro-graphic characteristics of the host rocks must be taken intoconsideration as well, along with the question of availabil-ity of dissolved oxygen. Concerning the latter, there areseveral paleo-oxygenation indicators, among which, lami-nation and absence of macro and megafauna, together withgeochemical proxies (total organic carbon TOC, hydrogenindex, uranium enrichment etc.), are the most important(see e.g., Allison et al. 1995; Kemp 1996).

Rock-Eval pyrolysis analysis from samples of the NaKkrábku section (Choteb Limestone, Table 4) revealed verylow TOC values, not exceeding 0.19 % (other parameters aretherefore not discussed here). The low TOC values can bemost probably attributed to poor preservation of organicmatter due to the benthic activity at the bottom as inferredfrom the bioturbation or degradation during burial. Laminatedlime-mudstones occur only subordinately. In addition, thepreservation of organic matter in coarse crinoidal and peloidalgrainstones, where diagenesis may have taken place in anopen system, is not likely (e.g., Tyson 1995). The highestTOC values recorded (0.15–0.19 %) came from the crinoidaland peloidal grainstones. It was probably the high sedimenta-tion rate of the grainstones, which facilitated organic matterpreservation. Other geochemical proxies have not been exam-ined so far (except gamma-ray and magnetic susceptibilitymeasurements and REE distribution from the Prastav, NaKkrábku and Bervený quarries; Koptíková 2011), but the dataavailable i.e., rare laminated fabric and impoverished benthicfauna, suggest oxygen depletion, but not anoxic conditions.

It seems, therefore, that conditions for organic matterpreservation in the Choteb Limestone were not good. Thatis in agreement with Koptíková (2011), who also proposeda suboxic environment in the Basal Choteb Event intervalprior to anoxic conditions. However, this does not implythat anoxic conditions did not occur during sedimentation

of some parts of the Choteb Limestone (inferring from theoccasional presence of dark, laminated lime-mudstonefacies). Despite unfavorable conditions for organic matterpreservation in the Choteb Limestone, large concentrationsof prasinophytes were recorded within this unit, especiallyat its base (base of the costatus Zone). It seems thereforethat the higher primary productivity due to favorable eco-logical conditions (e.g., higher nutrient load) played a sig-niWcant role in prasinophyte concentrations as well.Therefore it cannot be ruled out, that the oxygen deWciency,which is obvious, was not only the result of greater deposi-tional depth but also a consequence of a higher rate oforganic matter accumulation and its subsequent degrada-tion.

Triggering mechanisms for the activity of microborers and calcispheres and prasinophytes blooms

As shown e.g., by Hallock and Schlager (1986), Hallock(1988), Chazottes et al. (1995), Holmes et al. (2000), Peter-hänsel and Pratt (2001), Carreiro-Silva et al. (2005), andCarreiro-Silva et al. (2009), there is a relationship betweenintensive bioerosion and increased dissolved nutrient sup-ply in modern and ancient aquatic environments (but seeKoop et al. 2000; Vogel et al. 2000 for diVerent view).Therefore, nutrient availability acting as a triggering factorfor microborer activity seems to be a plausible scenario inthis case. Moreover, the presence of calcispheres and pra-sinophytes supports this idea, as increased abundance ofthese groups is usually linked to eutrophication events (e.g.,Kasmierczak 1975, 1976; Tyson 1995). However, the taxo-nomic uncertainty of calcispheres should be kept in mind.

In addition, increased barium input was reported closeabove the base of the Choteb Limestone (Koptíková 2011).According to this author, the Ba-enrichment level corre-sponds to the uranium enrichment recorded in the sectionsstudied (both the shallow-water and deeper-water sectionsand thus independent of facies change). It was interpretedas indicating maximum transgression.

If the intensive activity of microborers and calcispheres,together with prasinophyte blooming and Ba-enrichment,can be regarded as a proxy for higher nutrient availabilityas suggested in this case, the question of nutrient sourcearises. The changes in the terrestrial biosphere described byAlgeo and Scheckler (1998) (evolution of trees and seedplants, enhanced chemical weathering, accelerated pedo-genesis and chemical weathering and subsequent higherriverine nutrient Xuxes) had not taken place yet. The earli-est forests known are from the Givetian (Retallack 1997).Interestingly, changes in the distribution of the REE andchanges in magnetic susceptibility led Koptíková (2011) tothe conclusion that increased aeolian input might havetaken place during the event interval.

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It has been postulated that in the present ocean, in addi-tion to light and grazing (Miller et al. 1991; Mitchell et al.1991) it is iron concentration, supplied to the ocean mainlyvia atmospheric dust deposition, which is the limiting fac-tor for phytoplankton growth (e.g., Martin and Fitzwater1988). Thus it can be assumed that higher atmospheric dustXuxes, which have been postulated by Koptíková (2011) forthe event interval (the base of the costatus Zone), couldhave fertilized the ocean. However, the community struc-ture of the phytoplankton must also be taken into account,as diVerent phytoplankton groups may have diVerent nutri-ent requirements. As mentioned above, calcispheres andprasinophytes were recorded so far. Due to the uncertainsystematic position of calcispheres, this group will not bediscussed in this context. However, recent experimentswith iron fertilization of certain parts of the ocean showedthat iron is not a biolimiting nutrient for prasinophytes(e.g., Hutchins et al. 2002; Takeda and Tsuda 2005). Prauss(2007) suggested that the prasinophytes were successful inusing a reduced form of nitrogen, i.e. ammonia, andexplained this way the linkage of prasinophyte blooms toanoxic black shale deposits. However, the environment ofthe Choteb Limestone cannot be assigned to anoxic condi-tions. Oxygen depletion is obvious (dark color, impover-ished benthic fauna), but true anoxic conditions probablywere not established. Thus the interplay of nutrient sourcese.g., nutrient recycling due to sea-level Xuctuations and fer-tilization via atmospheric dust deposition seems to be aplausible scenario but requires further research.

Sea-level Xuctuations in the serotinus–kockelianus zones in the Prague Basin and the Devonian eustatic sea-level curve

Johnson et al. (1985) described and discussed three trans-gressive–regressive (T–R) cycles in the interval corre-sponding to the serotinus–kockelianus zones, i.e. T–R cycleIc (beginning at the top of the serotinus Zone), Id (begin-ning in the mid-costatus–australis zones) and Ie (beginningin the kockelianus Zone).

The sea-level curve reconstruction for the Prague Basin(Fig. 12) does not match the T–R cycles recognized byJohnson et al. (1985) for the serotinus to basal costatuszones. Neither facies nor faunal changes in the PragueBasin correspond to the sea-level rise in the uppermost partof the serotinus Zone i.e., T–R cycle Ic sensu Johnson et al.(1985). Conversely, the point of maximum transgressionclose to the base of the costatus Zone recorded in the Pra-gue Basin (and elsewhere, see Table 1; Fig. 1a, b) i.e., theBasal Choteb Event, corresponds to the “steady state” in theeustatic curve of Johnson et al. (1985). It does, however,match the sea-level curve of Ver Straeten (2007) recon-structed for the Appalachian Basin, who reWned the curve

of Johnson et al. (1985). The sea-level rise linked to theBasal Choteb Event would correspond to transgression inhis depositional sequence Eif-1 T–R cycle with the point ofmaximum transgression corresponding to the base of thecostatus Zone. The eVect of eustatic sea-level Xuctuationson the sedimentary pattern of the Prague Basin is obvious(see also Chlupáb and Kukal 1988; Chlupáb 2000). How-ever, the regional tectonic situation, which could aVectlocal sea-level changes and thus be superimposed on theeustatic signal, should always be considered. Nevertheless,the early Eifelian sea-level rise beginning in the partitusZone and culminating at the base of the costatus Zoneseems to be global and thus eustatic in origin.

Conclusions

• Evolution of the carbonate depositional system in thePrague Basin in the serotinus to kockelianus zones andresponses of the system to environmental changes andsea-level Xuctuations have been described on the basis ofmicrofacies analysis and facies distribution pattern.

• The occurrences of various peloids types, calcispheres,and prasinophyte blooms are interpreted as a result ofhigher nutrient availability. Nutrient recycling duringsea-level rise and/or iron donation via atmospheric dustdue to climatic perturbation is proposed as the plausiblenutrient source (Fig. 13).

• Oxygen consumption due to degradation of higheramounts of organic matter (as a consequence of highernutrient availability) could be, along with greater deposi-tional depth, regarded as a cause for the extinction ofbenthic taxa and migrations recorded by previousauthors.

Fig. 12 Euroamerican transgressive–regressive cycles of Johnsonet al. (1985) and sea-level curve for Emsian and Eifelian stages of thePrague Basin interpreted herein

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• Results of stable isotope analysis (�13Ccarb), which corre-late well with those from other regions show rathersteady carbon cycling, with organic matter accumulationhaving no signiWcant impact on the carbon budget. Thebalance between organic and inorganic carbon Xuxespoints to normal marine, unrestricted circulation.

• A small but distinct negative “peak” of �13C values closeto the base of the partitus Zone (i.e. Lower–MiddleDevonian boundary), was recorded. This could be ofsome signiWcance as it may have correlative potential.

• The shift to positive values from the basal partitus Zoneonwards, along with higher concentrations of calci-spheres, and thinner bedding, and darker rock color asrecorded in the upper Tlebotov Limestone (upper parti-tus Zone), marks the beginning of environmentalchanges (deepening) linked to the Basal Choteb Event.

• The sea-level Xuctuations in the critical interval can beinterpreted as follows: transgression started in the basalpartitus Zone (Tlebotov Limestone) with the onset ofsedimentation of the basal Choteb Limestone represent-ing maximum Xooding of the basin (base of the costatusZone). A shallowing pulse was recorded a few metersabove the base of the Choteb Limestone (coarser-grainedand thicker-bedded crinoidal grainstones within thecostatus Zone) followed by sea-level rise, which is char-acteristic for nearly the entire succession of the Choteb

Limestone (T–R Id sensu Johnson et al. 1985). A furthershallowing pulse, probably the end of T–R Id, in partenhanced by regional tectonic activity, coincides with thekockelianus Zone and is characterized in the shallow-water area of Hostím by tempestite sedimentation andcommon reworking. It represents the last shallowingpulse before the sea-level rise during which the KabákShale of the Srbsko Formation was deposited (KabákEvent, T–R cycle Ie).

Acknowledgments This work was supported by the Czech-Ameri-can Cooperation Program (Kontakt ME08011), grant from the GrantAgency of the Czech Republic (210/10/2351) and partly P210/12/2018, projects of the Czech Geological Survey (332500, 333300,334000), and the NAP0001 (subproject of IGCP 497). The Wrst authoralso gratefully acknowledges the Palaeontological Association for theSepkoski Grant award, which allowed the study to proceed. MayaElrick (University of New Mexico) and an anonymous reviewer arethanked for their meticulous reviews, which resulted in substantialimprovement of the manuscript. Franz T. Fürsich (Friedrich-Alex-ander-Universität Erlangen-Nürnberg, Germany) is to be thanked forthe editorial work and valuable comments. Charles VerStraeten (NewYork State Museum) is deeply appreciated for his comments and sug-gestions on the earlier version of the text. The linguistic help of GilbertKlapper (Northwestern University Evanstone, Iowa) is gratefullyacknowledged. Axel Munnecke and the Stable Isotope Laboratory ofthe Friedrich-Alexander-Universität Erlangen-Nürnberg (Germany)provided part of the �13C analysis from the U N5mc4 section, which isdeeply appreciated. Radek Vodráqka (Czech Geological Survey,Czech Republic) is to be thanked for his help during Weldwork and also

Fig. 13 Graphic illustration of the proposed model of environmentalchanges linked with the Basal Choteb Event. Climatic perturbationscould have led to higher nutrient load and subsequently to calcisphereand prasinophyte blooms and intensive micritization and peloid forma-

tion. Oxygen consumption due to the degradation of higher amounts oforganic matter deposited (as a consequence of higher nutrient avail-ability) could be, together with greater depositional depth, regarded ascausal “killing” factor of many benthic taxa

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for useful remarks on the manuscript. Mr. N5mec (Karlntejn) is grate-fully acknowledged for access to his as well as for his kind help duringsampling. Nad5qda Hrdlibková (former employee of the Czech Geo-logical Survey, Prague) is to be thanked for her help during laboratoryprocessing of conodont samples. Although numerous colleagues arethanked for their help, responsibility for possible errors remains entire-ly with the authors. This is a contribution to IGCP 596.

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