Palynology, microfacies and biostratigraphy across the Daleje Event (Lower Devonian, lower to upper Emsian): new insights from the offshore facies of the Prague Basin, Czech Republic
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The Zlíchovian/Dalejan boundary interval (Emsian, Lower Devonian) of the Pekárek Mill section was studied employing biostratigraphy (dacryoconarid tentaculites, conodonts) and palynology (chitinozoans, prasinophytes, scolecodonts) and microfacies analysis in order to shed more light on the timing and characteristics of the Daleje Event. The results of our study stress the great importance of the base of the Nowakia elegans Zone for the substage level division of the Emsian. Onset of the Daleje transgression is linked with higher terrigenous input, and coinciding changes in the chitinozoan assemblages were recorded at this level. The transgression at the base of the N. elegans Zone preceded the main transgression taking place in the N. cancellata Zone; it can be correlated with the Upper Zlíchov Event. For the first time, Emsian chitinozoans and a jawed polychaete fauna are described in detail from the Prague Basin and can be correlated with other northern Gondwanan regions. The family-level composition of scolecodont assemblage confirms the dominance of paulinitids in the peri-Gondwanan realm.
KeywordsDaleje Event Prague Basin Biostratigraphy Chitinozoans Scolecodonts Microfacies N. elegans Zone
Elucidation of the timing, duration and nature of the lower to upper Emsian (Lower Devonian) transgressive-regressive cycles and decisions on the Emsian substages are among the key issues of the Subcomission on Devonian Stratigraphy (see Becker 2007). Two events linked with the lower and upper Emsian transgressions were identified by García-Alcalde (1997) in the Iberian Peninsula and adjoining areas, specifically the Upper Zlíchov Event (UZE) and the Daleje-Cancellata Event. The latter event was named as the Daleje Event by House (1985), who associated the level with the extinction of some goniatite groups (De Baets et al. 2012; Korn et al. 2015) and pointed out the coincidence of the observed bioevent and facies deepening seen in the base of the Daleje Shales of the Prague Basin. Chlupáč and Kukal (1986, 1988) described gradual faunal and lithologic changes and interpreted them as the result of a progressive sea-level rise that took place in the Nowakiabarrandei-Nowakia elegans and Nowakia cancellata biozones. The main deepening associated with this event is correlated with the base of the N. cancellata and laticostatus zones, thus with the onset of fully argillaceous sedimentation (Walliser 1985).
According to García-Alcalde (1997), the UZE corresponds to a transgression in the uppermost N. barrandei Zone and N. elegans and lower N. cancellata Zone, within the gronbergi Zone. It thus overlaps with the stratigraphic range given for the Daleje Event by Chlupáč and Kukal (1988).
Ferrová et al. (2012), who studied facies changes and biostratigraphy of the Daleje Event interval in the carbonate-dominated environment of the Prague Basin, recorded a shallowing in the upper N. barrandei Zone, followed by deepening at the base of the N. elegans Zone, which was correlated by Aboussalam et al. (2015) with the UZE. This is in contrast to Chlupáč and Kukal (1988), who recorded a single transgressive episode (gradual deepening) in this interval.
The Prague Basin is situated between the cities of Prague and Pilsen (Fig. 1) and represents the central part of the Teplá-Barrandian unit of the Bohemian Massif. It is interpreted as part of a rift-system basin formed as a result of the Ordovician extension at the northern Gondwana margin, linked with the opening of the Rheic Ocean (Kraft et al. 2004). The basin infill is comprised of non-metamorphosed marine volcanosedimentary successions of Early Ordovician to Middle Devonian (Givetian) age, and it disconformably overlies the Neoproterozoic basement and partly the middle Cambrian sediments (summary in Chlupáč et al. 1998).
The Teplá-Barrandian unit, either as a separate microcontinent within the “Armorican terrane assemblage” (Perunica concept of Havlíček et al. 1994) or rather remaining closely attached to Gondwana (Drost 2008; Žák et al. 2013), drifted towards lower latitudes in the early Palaeozoic, reaching the peri-equatorial zone in the Devonian (Krs et al. 1986). This is also manifested by the takeover of carbonate sedimentation, which persisted in the Prague Basin from the Silurian (Ludlow) to the Middle Devonian (Givetian), when sedimentation in the Prague Basin terminated (Chlupáč et al. 1998).
The Zlíchov Formation is formed by the Zlíchov Limestone and its only member, the Chýnice Limestone, which is developed in the uppermost part of the Zlíchov Formation and occurs only in the NW part of the Prague Basin (Svoboda and Prantl 1948). In the SE part of the Prague Basin, the upper Zlíchov Limestone is developed as a grey, mostly micritic thin-bedded nodular limestones with intercalations of grey and grey-green calcareous shales with common traces of Chondrites. The upper Emsian Daleje Shale is represented by dark calcareous shales with sparse intercalation of thin micritic limestone beds or lenses in its lowermost part. The upper part of the Zlíchov Limestone was assigned by Chlupáč (1983a) to a deeper subtidal, quiet sedimentary environment as proven by the presence of thin-walled bivalves, articulated trilobite exoskeletons, ubiquitous Chondrites burrows and rich planktic and nektic faunas. Chlupáč (1983a) interpreted the sedimentary environment of the Daleje Shale similarly, with the main difference being the increased influx of clastic material in his conception.
Pekárek Mill section
The section is located SW of Prague, in the Švarcava valley (GPS 49° 57′ 53.26″ N, 14° 16′ 35.35″ E; Fig. 1) near the village of Solopisky. The outcrop provides a great opportunity for high-resolution stratigraphical research through the middle and upper parts of the Zlíchov Limestone to the lower part of the Daleje Shale in one uninterrupted section, with a gradual transition between these two units. The section has been studied for its rich fossiliferous content since the pioneer studies of Joachim Barrande, who used the name “Pekarkowitz” for that section (Chlupáč 1983b). The abundant well-preserved dacryoconarids were studied by Alberti (1971) and Lukeš (in Chlupáč et al. 1979; Chlupáč and Lukeš 1999). Conodonts were studied by Zikmundová (in Chlupáč et al. 1979). The occurrence of early goniatites is described in Chlupáč and Turek (1983). In contrast to the intense study of biostratigraphically important groups, microfossils have been far less studied. The first more detailed data on palynomorphs, specifically spores, from this outcrop were provided by McGregor (1979), who also briefly summarised the previous data on this stratigraphic level from the Barrandian area. This author also mentioned the presence of other potentially important microfossils, such as acritarchs, chitinozoans and scolecodonts. However, these fossils have not been studied in detail since then.
Materials and methods
The middle and upper parts of the Zlíchov Limestone (Zlíchov Formation) as well as the lower Daleje Shale (37 m in total) were sampled for the purposes of biostratigraphy and palynological studies, microfacies analyses, as well as stable isotope geochemistry (δ13Ccarb).
For purposes of conodont biostratigraphy, 25 samples of limestone (ca. 4 kg each) were collected through the section. Clean and crushed samples (ca. 4 × 5 cm) were placed in sieve cloths and hung on a plastic container with a lid. Acetic acid (6%) was used for the maceration; pH was regularly checked following the method of Jeppsson et al. (1999). Phosphatic microfossils were separated using Na-polytungstate and hand-picked under a stereomicroscope.
Samples for dacryoconarids were obtained from the Zlíchov Limestone-Daleje Shale transitional interval (26.6 to 27 m, beds CH16 to CH18). In the case of limestone beds, the rock was mechanically crushed into smaller pieces that were checked under a stereomicroscope for small shells of dacryoconarids. Rewoquat surfactant was used for disintegration of calcareous shale interbeds (the methodology is described in Jarochowska et al. 2013). The latter residues were checked for conodonts as well.
Microfacies analysis was carried out on 63 thin sections of the format 45 × 27, 45 × 55 and 75 × 100 mm, using both Carl Zeiss Stemi 2000C and Nikon Eclipse E600 microscopes; photos were taken with a Nikon DS-Fi1 camera mounted onto the latter. Both the carbonates and shales intercalated were examined. The Baccelle and Bosellini (1965) chart was used for estimations of dacryoconarid and radiolarian frequencies (other counting methods were not employed due to fragmentation of the bioclasts).
For the purpose of stable isotope analysis (δ13Ccarb, δ18Ocarb), 23 bulk-rock samples were extracted from the carbonate facies. Micritic matrix was preferably sampled from a fresh rock surface; the most appropriate samples were chosen under a binocular microscope, powdered and treated with phosphoric acid. The evolving CO2 gases were measured for carbon isotope ratios using a Finnigan MAT 251 mass spectrometer (Czech Geological Survey, Prague). The values are reported relative to Vienna Pee Dee Belemnite using the conventional notation. The accuracy was controlled by replicate measurements of laboratory standards, with the reproducibility being 0.15‰.
For imaging, the Hitachi S-3700N scanning electron microscope housed in the National Museum in Prague and Mira3 Tescan electron microscope housed in the Czech Geological Survey were used. The descriptive terminology for scolecodonts follows Kielan-Jaworowska (1966). The microfossil collection is housed in the Czech Geological Survey (under collection numbers PT40 to PT81 and SB 44–46).
Carbon isotope record
Carbon and oxygen isotope composition of carbonates of the Zlíchov Limestone and Daleje Shale from the Pekárek Mill section
Results of the whole-rock (semiquantitative) XRD analyses (in weight %) of the Zlíchov Limestone and Daleje Shale from the Pekárek Mill section
The studied section is characterised by the succession of thin- to medium-bedded limestones, irregularly intercalated by mostly thin-bedded calcareous shales. Sedimentary textures and fossil content are similarly developed in both the carbonates and the intercalated shales. Silicification (chert nodules) is common, especially in the lower part of the section.
Interval 0–23.6 m, Zlíchov Limestone, beds with Criteriognathus steinhornensis (Ziegler, 1956)
The relatively low rate of sediment reworking here is ascribed to low epifaunal and infaunal activity. Skeletal remains of benthic organisms are rare, not exceeding 3 individuals per thin section. The scattering of larger skeletal fragments (e.g. trilobite exoskeletons) points to a low horizontal redistribution and is evidence of a low-energy sedimentary environment. The minimum of bioerosion observed here is also interpreted as a result of the deficiency of bioeroders rather than of insufficient exposure time on the seafloor. The sedimentary environment here is not assumed to represent a favourable environment for benthic colonisation. The only proliferating groups were zooplankton (radiolarians and dacryoconarids), nektobenthic ostracods, polychaete worms (evidenced by scolecodonts) and Chondrites. Taken together, the environment is interpreted as deep, calm, non-agitated and oxygen-stressed.
Interval 24.5–26.5 m, Zlíchov Limestone, upper N. barrandei Zone, beds with Polygnathus perbonus (Philip, 1966) and C. steinhornensis
Bioturbated tentaculite wacke- to packstones with a high component content are the characteristic microfacies type (MF 3, Fig. 5d–f). Dacryoconarids, crinoid ossicles, ostracods and trilobite exoskeletons represent the most common bioclast types; these are followed by bivalves, brachiopods, gastropods, nautiloids, ammonoids and rare rugose corals. Larger shells are preserved as fragments. Only very few bored shell fragments were observed. No micritisation was recorded. The telescoping of tentaculite shells is common. Locally, a grainstone texture is developed within this microfacies. Burrows of Chondrites are still present, although a homogenous ichnofabric prevails. On the top of this unit, a graded crinoidal grainstone with ostracods, trilobite exoskeletons, ammonoids and nautiloids was deposited.
The diverse benthic, nektonic and planktic communities that are present; the packing of the components; as well as the intensity of sediment reworking (telescoping of tentaculite shells, local development of grainstone texture, homogenous ichnofabric) are suggestive of improved conditions on the seafloor in an oxygenated and more agitated environment (also see the decrease in organic walled microfossil abundance, Fig. 4). The allochthonous crinoidal grainstone on top represents calciturbidite deposited at the end of the shallowing cycle.
Interval 26.8 –37 m, uppermost Zlíchov Limestone and lower part of the Daleje Shale, N. elegans Zone
Bioturbated (Chondrites ichnofabric), recrystallised skeletal wackestones, locally developed as lime-mudstones, contain only sporadically dacryoconarids, trilobites, ostracods, radiolarians and thin-walled bivalves (MF 4, Fig. 5h). Most bioclasts are preserved as fragments. The matrix is microsparitic. Bioturbation is distinctive; the degree of reworking is lower than in the underlying beds. A higher clastic content in both the carbonate and the intercalated pelitic facies was recorded from the bed 27 m onwards, which was also confirmed by XRD analysis (see above).
Facies development points at a more distal depositional environment. The progressive deepening of the basin is associated with a greater terrigenous input.
In general, there are only a handful of papers dealing with Lower Devonian scolecodonts. The pioneering study was published by Lange (1949), who studied Emsian scolecodonts from the Ponta Grossa Formation of the Paraná Basin, Brazil (Malvinokaffric cold water realm). The assemblage reported by Lange (1949) is monospecific, with a single paulinitid species, Paulinites paranaensis Lange, 1947. Lange’s materials were later restudied by Eriksson et al. (2011) who resampled the localities using modern microfossil extraction methods, ending up with similar results, and reporting a single species in the collection. Following Lange’s work, the Lower Devonian scolecodonts were reported by Taugourdeau (1968), who studied Silurian to Carboniferous scolecodonts from the Sahara, and Ye (1994), who described Silurian to Devonian scolecodonts from the Qinling Mountains; however, both of these authors used an outdated taxonomical approach. Suttner and Hints (2010) described a small and poorly preserved collection of imprecisely dated Devonian scolecodonts from the Tyrnaueralm, Graz Palaeozoic, Austria. More recently, Szaniawski and Drygant (2014) published a well-preserved collection from the Lochkovian of Podolia, Ukraine, and Tonarová et al. (2016) studied a rich Eifelian scolecodont collection from the Eifel region, Germany. The latter two papers provided information on taxa that are comparable with specimens from the Pekárek Mill section. There is only a single brief study on Devonian scolecodonts from the Prague Basin by Šnajdr (1951), who described findings of two species (Kettnerites langei Šnajdr, 1951, and Staurocephalites sp. A) from the Zlíchov Limestone coming from the Zabitá rokle locality near Řeporyje.
The most abundant family, Paulinitidae, is represented by two genera. The first one, Kettnerites Žebera, 1935 seems to be represented by at least two species Kettnerites sp. 1 (Fig. 6a–k) and Kettnerites sp. 2 (Fig. 6u, v). Kettnerites sp. 1 looks similar to “Kettnerites” huberti Bergman, 1987, documented from the Lochkovian of Podolia, Ukraine (Szaniawski and Drygant 2014), and K. aff. huberti from the Eifelian of the Eifel region (Tonarová et al. 2016). The Devonian species are almost indistinguishable from the Silurian K. huberti, described from the Wenlock to Ludlow of Gotland, Sweden (Bergman 1989). The latter species was described as an eurytopic long-ranging taxon, found especially in deep water marls and argillaceous limestone (Bergman 1989, p. 25). There is also a resemblance to the species Pa. paranaensis, but the first maxillae denticulation pattern of the latter species is very different. Kettnerites sp. 2 has more slender first maxillae than Kettnerites sp. 1, with dense, thin denticles. The maxillae of K. langei have a slightly similar appearance, but it was described based only on badly preserved rock surface material that does not allow for a definitive comparison. The second paulinitid genus is Hindenites Bergman, 1987, and the specimens are assigned to Hindenites sp. (Fig. 6l–t). This species may be similar to Hindenites sp. 1 from the Eifelian of the Eifel region (Tonarová et al. 2016). Other paulinitid maxillae (Fig. 6aa–ac) in the collection could not be determined due to insufficient numbers of specimens and/or their poor preservation.
The second most common family, Polychaetaspidae, is represented by several species belonging to the genus Oenonites Hinde, 1879; however, distinguishing them is complicated, and at this stage of our study, they remain identified only as Oenonites spp. (Fig. 6ae–ap) until more material is gathered.
The maxillae of the family Mochtyellidae are commonly broken, probably because of their more fragile constitution. One maxilla showing a striking resemblance to the posterior jaw of the genus Pistoprion Kielan-Jaworowska, 1966, common in the Ordovician and Silurian, was assigned to Pistoprion? sp. (Fig. 7i). This would mean that, up to now, it is the youngest occurrence of this genus. Other maxillae were identified as mochtyellids (Fig. 7a, c) or as Mochtyella sp. (Fig. 7e, f), even though the assignment is questionable because of missing secondary ridges that are typical for the compound maxillae of this genus.
Especially in the Zlíchov limestone, the majority of samples also contain representatives of the family Skalenoprionidae, Skalenoprion sp. (Fig. 7m), although in low numbers. The jaws are small (around 100 μm), and the number of denticles was only 3–4, which is lower than in the older strata (compare with Kielan-Jaworowska 1966) but similar to Skalenoprion sp. 1 from the Eifelian (Tonarová et al. 2016). Therefore, it could be an evolutionary trend. On the other hand, an atypical denticulation also occurs on the fang of the first maxillae, which has never been observed before.
Several specimens were assigned to the family Tetraprionidae (Fig. 7b, j, o), but their further identification was not possible. Three specimens belong to Lunoprionella sp. (Fig. 7k), which is a genus of unknown family affinity. This genus ranges from the Ordovician to the Devonian, with the first documented Devonian record from the Pekárek Mill section. Some maxillae reflect transitional characteristics between the families Paulinitidae and Kielanoprionidae (Fig. 6w–z). The indisputable record of the latter family is known from the Eifelian (Middle Devonian; Tonarová et al. 2016).
The abundance of scolecodonts is quite stable throughout the section (approximately 1 posterior maxilla per gram of rock) but lower than the abundance of chitinozoans (Fig. 4). On average, 4 species were found per sample (Fig. 3), and the total number of species identified in the section is at least 11. There are no obvious changes in either abundance or diversity that could be connected with the Daleje Event, which might confirm the opportunistic behaviour of jawed polychaetes (Tonarová et al. 2014). Nevertheless, the quantitative data may be biased due to a relatively small sample size when compared to other studies. The majority of scolecodonts are isolated maxillae, but five partly fused apparatuses were also recovered (Fig. 7a–d). The length of the maxillae generally varies from 100 to 300 μm, only occasionally reaching 500 to 600 μm (in samples PM0 and PM19.9 even to 1 mm). Based on a large collection of paulinitids from Gotland, Bergman (1989) suggested that a low diversity fauna dominated by small specimens and often coupled with low abundances indicates a relatively deeper water environment.
Conodonts are extremely rare in both abundance and diversity. Only 14 specimens of C. steinhornensis (Ziegler, 1956) (Fig. 7w), 51 specimens of Pseudooneotodus beckmanni (Bischoff and Sannemann, 1958) (Fig. 7y), a few fragments of Belodella sp. and two Pa elements of Po. perbonus (Philip, 1966) (Fig. 7x) were recorded. Most of the conodonts were recovered from sample 25.15, thus from the shallowest facies. No conodonts were extracted from calcareous shales. The occurrences of C. steinhornensis and Po. perbonus may be regarded as a confirmation of the gronbergi conodont Zone due to the association of the latter with Polygnathus gronbergi in the Prague Basin as reported by Klapper et al. (1978) as well as in other regions (e.g. Klapper and Johnson 1980). No conodonts were recorded from the N. elegans tentaculite Zone in this study, which hampers any allocation of the environmental changes recorded along with the conodont zonation.
Research on dacryoconarid tentaculites has been focused on a very short interval from 26.6 to 27 m (corresponding to CH16 to CH18 sensu Chlupáč and Lukeš 1999) and its shale interbeds (i.e. an interval about 0.5 m thick). The range of this interval comes out of the previous work done by Pavel Lukeš (Chlupáč and Lukeš 1999), who recorded the disappearance of Nowakia (N.) barrandei Bouček and Prantl, 1959 (Fig. 7z(1, 2)) and the appearance of N. (N.) elegans Barrande, 1867 (Fig. 7aa(1, 2)) within this level. His work was, however, constrained to only the material from limestone beds and lenses within this level. Introduction of a new method using the surfactant Rewoquat (Jarochowska et al. 2013) allowed obtaining new data from the shale interbeds of this narrow interval.
The occurrence of N. (N.) barrandei in the limestone bed at 26.6 m (CH16) comprises dozens of excellently preserved specimens of this taxon in approximately 1 kg of material. The last occurrence of this taxon was recorded in the shale layer at 26.7 m, with more than 20 easily ascertainable specimens per 0.5 kg of the rock.
Three kilograms of rock of the subsequent limestone bed at 26.8 m (CH17) yielded only one specimen N. cf. elegans without any other determinable specimens of the subgenus Nowakia (Nowakia).
The first appearance of N. (N.) elegans falls into the shaly interval, which lies just above the limestone bed at 26.8 m (CH17). Ten easily determinable fragments of the shells of N. (N.) elegans were obtained from 0.5 kg of shale. The presence of a limestone bulge on the top of CH17 with a well-preserved specimen of N. (N.) elegans showed that the first appearance is not only restricted to shaly layers. The presence of N. (N.) elegans is accompanied by a large number (more than 100) of fragments of proximal shell parts, which clearly belong to the subgenus Nowakia (Nowakia); however, due to their fragmentation and poor preservation, it is impossible to assign them either to N. (N.) elegans or to N. (N.) barrandei, nor to treat them as a new species. More samples from different localities are needed to resolve this problem.
Ferrová et al. (2012) documented co-occurrences of adjacent index tentaculite biozones (e.g. N. (N.) barrandei + N. (N.) elegans; N. (N.) elegans + N. (N.) cancellata) in the Daleje Event interval. However, previously published tentaculite biozonal charts were used as the taxon-range zones. To solve this problem, Ferrová et al. (2012) revised the tentacutite biostratigraphy of the Daleje Event interval and emended all of the above-mentioned biozones (i.e. the N. (N.) barrandei, N. (N.) elegans and N. (N.) cancellata biozones) as interval zones (in accordance with the International Stratigraphic Guide, Chapter 7, Section 3; Salvador 2013). In addition, Ferrová et al. (2012) established a new barrandei-elegans Subzone within the N. (N.) elegans Interval Zone. The lower boundary of the barrandei-elegans Subzone was defined by the first occurrence of N. (N.) elegans, and its upper boundary by the last occurrence of N. (N.) barrandei. Therefore, the Subzone represents a typical concurrent-range biozone (in accordance with the International Stratigraphic Guide, Chapter 7, Section D2b; Salvador 2013). The common occurrence of N. (N.) barrandei and N. (N.) elegans was recognised not only in the Prague Basin (Chlupáč et al. 1979; Ferrová et al. 2012) but also at South Tien Shan (Kim et al. 1978; Kim 2011; Frýda, unpublished data) as well as in Spain (Montesinos and Truyols-Massoni 1987; García-Alcalde et al. 1988). Ferrová et al. (2012) noted that the lack of densely sampled sections within the Daleje Event interval and the low number of tentaculite specialists are probably the main reasons for the missing records of the co-occurrences of N. (N.) elegans and N. (N.) barrandei from other areas around the world. Nevertheless, at present, the barrandei-elegans Subzone, representing a rather short time interval (see Ferrová et al. 2012), is a more precise biostratigraphic tool within the Daleje Event interval, not only within the Prague Basin but also among some palaeocontinents.
However, the presence of this subzone was not confirmed at the Pekárek Mill section studied herein. Therefore, there might be a short stratigraphical gap or period with extremely low sediment accumulation rates concerning at least the barrandei-elegans Subzone on the Pekárek Mill section. This gap might be placed on the boundary between the shale interval at 26.7 m and the limestone bed at 26.8 m (CH17). However, two problems have to be taken into account. The first is the absence of data from the CH17 layer and the second is the presence of numerous proximal parts of shells certainly belonging to the subgenera Nowakia (Nowakia) from the shale interval at 26.9 m. An evaluation of this new material consisting of proximal parts would best be accomplished with a comparative study of the material from other localities.
Prasinophytes (Fig. 7p–v) are abundant, especially in the transition interval from the Zlíchov Limestone to the Daleje Shale and in the lower part of the Daleje Shale (sample 24.5 m, interval 28–29 m, and above 35 m; Fig. 4). Their abundance in the most productive samples reached over 200 specimens per gram of rock. On the other hand, other microfossils, such as chitinozoans and scolecodonts, are very rare in these samples. The diameters of the prasinophytes also increase towards the Zlíchov Limestone/Daleje Shale boundary (reaching over 1 mm in sample PM24.5). These phenomena could be associated with changes in the oceanic circulation resulting from a transgression and eutrophication of the basin due to increased terrigenous input, which was herein also recorded close to the Zlíchov Limestone/Daleje Shale boundary (for comparison, see Vodrážková et al. 2013). Moreover, Tappan (1980, 1986) already regarded prasinophytes as a “disaster species” because they appear to prefer specific environmental conditions and are generally more abundant when other phytoplankton is absent.
The assemblage of chitinozoans is not very diverse with only one to seven species per sample (Fig. 3). The abundance of chitinozoans is variable throughout the section (Fig. 4). It is higher in the calcareous shale intercalations than in the limestone. The well-known higher abundance of chitinozoans in low-energy siliciclastic environments (e.g. Paris 1981a, b) was also confirmed by the Lower Devonian associations of eastern Canada (Achab et al. 1997). The differential compaction of the sediments could also have had an influence on the relative abundance of the species in the different levels. Thus, the shale beds contain a higher number of chitinozoans than in limestone (Paris 1981a, b). The approximate abundance value is lower than 5 chitinozoans per gram of rock, although especially in samples with Bursachitina Taugourdeau, 1966, it increases to over 40 specimens per gram. The latter values may be less precise because of problematic counting. Similarly, the abundance values are only approximations in samples with prevailing prasinophytes, as they could have covered some of the vesicles of the chitinozoans. The preservation state of chitinozoans is, as in other microfossils, moderate to well, with the best preservation in the limestone layers. Ornamented specimens such as Ancyrochitina spp. are commonly laterally compressed and appendices are broken off, which hampers their determination (Fig. 4). On the other hand, Bursachitina riclonensis (Paris, 1981b), a simple chitinozoan, is easier to identify even when the preservation of the specimens is poor. Therefore, the difficulty in the identification of these species can have an influence upon the determination of their diversity (Paris 1981a, b).
R. marettensis was described from the upper Emsian of the Armorican Massif, France (Paris 1981a, b), and was also recorded from the Emsian of eastern Canada (Achab et al. 1997). Bursachitina sp. A was recorded from the Emsian (Paris et al. 2000) and Bur. riclonensis has a long record from the lower Emsian to the Eifelian (Paris et al. 2000). These Bursachitina species and Ang. devonica are common components of the global Bursachitina bursa Biozone, mostly recorded from northern Gondwana, and tentatively assigned to the lower Emsian (Paris et al. 2000). Bur. bursa (Taugourdeau and de Jekhowsky, 1960) (Fig. 8r, v), originally described from the Lower Devonian of Algeria (Taugourdeau and de Jekhowsky, 1960), occurs near the Zlíchovian/Dalejan boundary and disappears at the boundary.
Starting with the Daleje Shale, the number of representatives of the genus Ancyrochitina Eisenack, 1955 is found to be increasing (in some samples, this genus represents the prevailing component of the assemblage; e.g. sample PM29.1). Ancyrochitina parisi Volkheimer et al., 1986 (Fig. 8aj, ak) appears in this unit. It was recorded from the upper Emsian of southwestern Gondwana (Argentina, Bolivia and Brazil) (Volkheimer et al. 1986; Grahn 2005; Grahn et al. 2013). Ramochitina magnifica? (Fig. 8at, au) and Bulbochitina bulbosa Paris, 1981a (Fig. 8ae) are present in the uppermost studied level of the Daleje Shale. The species R. magnifica Lange, 1967 is indicative of the Ramochitina magnifica Biozone of the Pragian of southwestern Gondwana (Grahn 2005; Grahn et al. 2013), but the documented stratigraphic range is until the upper Emsian (Paris et al. 2000); therefore, our finding probably does not represent the FAD of the species in the region. Bul. bulbosa is a Pragian-basal Emsian species, which indicates the upper Pragian Bulbochitina bulbosa Biozone in northern and eastern Gondwana as well as in Laurussia (Paris et al. 2000). Its presence also needs to be confirmed from other sections before a conclusion on its longer stratigraphical range can be made. There is no evidence for reworked material in the Daleje Shale.
The high predominance of Ancyrochitina spp., observed in some levels of Emsian carbonate deposits of the Armorican Massif, has been suggested as being related to the presence of superficial currents, with the input of a sudden and ephemeral influx of plankton, or of modifications in the marine biochemistry (Paris 1981a, b). In the Armorican section, ornamented species such as Ancyrochitina spp. are the only component in some beds, whilst Bur. riclonensis, a species without long processes, shows minor dependence to the lithology throughout the section. This observation was in agreement with the planktic nature of the chitinozoans (Paris 1981a, b). In the Lower Devonian assemblages of eastern Canada, species adapted to carbonate-dominated sedimentation were susceptible to facies variations with the preservation of large and bulbous forms in the shallower lithofacies (Achab et al. 1997). The varied distribution of the Bursachitina and Ancyrochitina throughout the Pekárek Mill section can respond to some of these facies changes, considering abundance zones of 99% of the assemblage for Bursachitina in the upper part of the Zlíchov Limestone unit, from PM21.9 to PM27, and for Ancyrochitina in the uppermost part of the Zlíchov Limestone (shale beds) and the Daleje Shale units, from PM27.8 to PM29.1 (Fig. 3).
In the studied section, the Zlíchov Limestone displays a rather uniform microfacies (except for the uppermost part), being mostly skeletal wackestones with radiolarians and dacryoconarids; only rarely, they contain the remains of benthos and ubiquitous Chondrites ichnofabric. The abundance of dacryoconarids and radiolarians varies throughout the section; starting from the interval around 24 m (upper N. barrandei Zone), tentaculites prevail and radiolarians are either absent or occur in small abundances. At the same time, current activity is increasing as indicated by the presence of skeletal packstones with a locally developed grainstone fabric; the telescoping of shells; and diverse planktic, nektic and benthic assemblages. Also, abundant conodonts (mainly Ps. beckmanni) were recovered from this interval, which is in contrast to the underlying beds where conodonts are very rare. Based on these findings, we interpret it as a regressive phase in the upper Zlíchov Limestone, culminating in the uppermost N. barrandei Zone with deposition of well-sorted crinoidal graded grainstones (calciturbidites) at 26.5 m. Wackestones, locally developed as lime-mudstones, with only rare occurrences of skeletal remains, were deposited at the base of the N. elegans Zone, which is interpreted herein as a deepening phase of the sea level. Higher amounts of silt were recorded during petrographical observations and proven by XRD analysis in both carbonate and pelitic facies in this interval. Interestingly, large and abundant prasinophytes were recovered, and the predominance of Ancyrochitina spp. was recorded, which might be connected to a higher nutrient load due to increased terrigenous input.
The interplay of bathymetric changes due to a sea-level rise at the base of the N. elegans Zone, together with increasing terrigenous input, supposedly connected with a higher nutrient load, is interpreted as the main cause of the changes in the chitinozoan assemblages recorded herein. These changes also coincide with faunal changes summarised by Chlupáč and Kukal (1988). Siliciclastic input also took place during the sedimentation of the Zlíchov Limestone (irregular alternation of limestone and argillaceous beds); it increased in the Zlíchov Limestone-Daleje Shale transition and took over in the Daleje Shale. The cyclic nature of the Zlíchov Limestone and Daleje-Třebotov Formation was interpreted by Chlupáč (2000) as a result of “climatic variations, which periodically affected the regime of water masses, oxygenation, production of CaCO3, clay supply…” (Chlupáč 2000, p. 120). Nevertheless, the character of terrigenous input and the source area is not specified herein; this is the aim of an ongoing study.
The transgressive-regressive phases recorded herein correlate well with the findings of Ferrová et al. (2012), who studied the uppermost Zlíchov Limestone and Chýnice Limestone and the lowest part of the Daleje Shale in the carbonate-dominated environment near Bubovice (located approximately 6 km to the west of the Pekárek Mill section). These authors recorded shallowing in the uppermost N. barrandei Zone and deepening at the base of the N. elegans Zone. The sea-level changes recorded herein and by Ferrová et al. (2012) differ from the interpretation of Chlupáč and Kukal (1988), who considered a gradual deepening from the Zlíchov Limestone onwards. On the other hand, it is in agreement with the results of García-Alcalde (1997) and Aboussalam et al. (2015) from Spain and Morocco, respectively (Upper Zlíchov Event). It is highly probable that a transgression at the base of the N. elegans Zone also took place elsewhere, but was lumped into a single event with a younger transgressive episode in the N. cancellata Zone (Daleje Event sensu stricto) due to problematic biostratigraphic correlations: the taxonomy of N. (N.) barrandei Bouček and Prantl, 1959 and N. (N.) elegans Barrande, 1867 was settled only recently (Ferrová et al. 2012) and conodonts from shaly facies are mostly absent.
We recorded a regressive trend in the N. barrandei Zone and in the gronbergi conodont Zone, culminating in the uppermost N. barrandei Zone, with onset of the transgression from the base of the N. elegans Zone, representing the initial transgressive phase of the main flooding taking place in the N. cancellata Zone (Daleje Event sensu stricto). Our interpretation of sea-level change confirms the findings of Ferrová et al. (2012) from a shallow water area. It can be correlated with the “Upper Zlíchov Event” of García-Alcalde (1997) and Aboussalam et al. (2015).
The report presented here for the first time describes in greater detail Emsian chitinozoans and scolecodonts from the Prague Basin.
Scolecodonts show both a low abundance and diversity of jawed polychaetes throughout the section, without significant quantitative or qualitative changes across the event interval, thus confirming their reputation as opportunistic taxa. The low abundance and diversity, together with the small size of the scolecodonts, likely indicates a deeper water environment. The family-level composition of scolecodont assemblage confirms the dominance of paulinitids in the peri-Gondwanan realm.
The reported chitinozoan species can be correlated with, e.g. the Lower Devonian of the Armorican Massif, Libya, or other northern Gondwanan regions.
The results of our study suggest the greater importance of the base of the N. elegans Zone, marking the onset of the transgression linked with higher terrigenous input, changes in chitinozoan assemblages (extinction of Bur. bursa followed by a predominance of Anc. parisi in the lower N. elegans Zone) and changes in the size and abundances of prasinophytes. Similarly, the base of the N. elegans Zone seems to be at the level of distinct fauna turnover (end of Zlíchovian fauna; see Ferrová et al. 2012 for details) in the carbonate-dominated environment of the Prague Basin.
Taking all the above data into account, we follow Ferrová et al. (2012) and regard the base of N. elegans as a suitable candidate for the definition of the Emsian substage boundary.
This research was supported by the Czech Science Foundation through the Project No. P210/12/2018. This study is a contribution to IGCP 596 (Climate change and biodiversity patterns in the Mid-Palaeozoic – Early Devonian to Early Carboniferous). One of the authors (SV) wishes to acknowledge the support of Alexander von Humboldt Foundation as part of the study was carried out during her AvH Research Fellowship in Universität Erlangen-Nürnberg. The authors gratefully acknowledge the thorough reviews by C. Klug (Paläontologisches Institut und Museum, Universität Zürich), L. Slavík (Institute of Geology AS CR, Prague) and an anonymous reviewer.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
- Alberti, G. K. B. (1971). Tentaculiten (Nowakiidae) aus dem Grenzbereich Zlichovium/ Eifelium und Bemerkungen zur Unter-/Mittel-Devon-Grenze nach Nowakiidae. Senckenbergiana lethaea, 52(1), 93–113.Google Scholar
- Baccelle, L., & Bosellini, A. (1965). Diagrammi per la stima visiva della composizione percentuale nelle rocce sedimentarie. Annali della Università di Ferrara, Sezione IX, Science Geologiche e Paleontologiche, 1, 59–62.Google Scholar
- Becker, R. T. (2007). Emsian substages and the Daleje event—a consideration of conodont, dacryoconarid, ammonoid and sea-level data. Subcommission on Devonian Stratigraphy Newsletter, 22, 29–32.Google Scholar
- Bergman, C. F. (1989). Silurian paulinitid polychaetes from Gotland. Fossils and Strata, 25, 1–128.Google Scholar
- Buggisch, W., & Mann, U. (2004). Carbon isotope stratigraphy of Lochkovian to Eifelian limestones from the Devonian of central and southern Europe. International Journal of Earth Sciences, 93, 521–541.Google Scholar
- Chlupáč, I. (1983a). Trilobite assemblages in the Devonian of the Barrandian area and their relations to palaeoenvironments. Geologica et Palaeontologica, 17, 45–73.Google Scholar
- Chlupáč, I. (1983b). Stratigraphical position of Barrande’s paleontological localities in the Devonian of Central Bohemia. Časopis pro mineralogii a geologii, 28, 261–276.Google Scholar
- Chlupáč, I. (2000). Cyclicity and duration of Lower Devonian stages: observations from the Barrandian area, Czech Republic. Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen, 215(1), 97–124.Google Scholar
- Chlupáč, I., & Kukal, Z. (1988). Possible global events and the stratigraphy of the Palaeozoic of the Barrandian (Cambrian-Middle Devonian, Czechoslovakia). Sbornik Geologických Věd (Geologie), 43, 83–146.Google Scholar
- Chlupáč, I., & Turek, V. (1983). Devonian goniatites from the Barrandian area. Rozpravy Ústředního ústavu geologického, 46(1), 1–159.Google Scholar
- Chlupáč, I., Lukeš, P., & Zikmundová, J. (1979). The Lower/Middle Devonian boundary beds in the Barrandian area, Czechoslovakia. Geologica et Palaeontologica, 13, 125–156.Google Scholar
- Chlupáč, I., Havlíček, V., & Kříž, J. (1998). Palaeozoic of the Barrandian. Prague: Czech Geological Survey. 183 pp.Google Scholar
- Drost, K. (2008). Sources and geotectonic setting of Late Neoproterozoic - Early Palaeozoic volcanosedimentary successions of the Teplá-Barrandian unit (Bohemian Massif): evidence from petrographical, geochemical, and isotope analyses. Geologica Saxonica, 54, 1–168.Google Scholar
- Eriksson, M. E., Grahn, Y., Bosetti, E. P., & Vega, C. S. (2011). Malvinokaffric realm polychaetes from the Devonian Ponta Grossa Formation, Parana Basin (Southern Brazil), with a discussion and re-evaluation of the species described by Lange. In E. P. Bosetti, Y. Grahn, & J. H. G. Melo (Eds.), Essays in honour of Frederico Waldemar Lange (pp. 118–150). Rio de Janeiro: Editoria Interciencia.Google Scholar
- Eriksson, M. E., Hints, O., Paxton, H., & Tonarová, P. (2013). Ordovician and Silurian polychaete diversity and biogeography. In D. A. T. Harper, & T. Servais (Eds.), Early Palaeozoic biogeography and palaeogeography (pp. 257–264). Geological Society, London, Memoirs, 38.Google Scholar
- García-Alcalde, J. L. (1997). North Gondwanan Emsian events. Episodes, 20, 241–246.Google Scholar
- García-Alcalde, J. L., Montesionos, J. R., Truyols-Massoni, M., García-Lopez, S., Arbizu, M. A., & Soto, F. (1988). The Silurian and Devonian of the Palentian domain (NW Spain). Revista de la Sociedad Geológica de España, 1(1–2), 7–13.Google Scholar
- Grahn, Y. (2005). Devonian chitinozoan biozones of Western Gondwana. Acta Geologica Polonica, 55, 211–227.Google Scholar
- Grahn, Y., Mendlowicz Mauller, P., Bergamaschi, S., & Bosetti, E. P. (2013). Palynology and sequence stratigraphy of three Devonian rock units in the Apucarana Sub-basin (Paraná Basin, south Brazil): additional data and correlation. Review of Palaeobotany and Palynology, 198, 27–44.CrossRefGoogle Scholar
- Havlíček, V., & Vaněk, J. (1996). Brachiopods and trilobites in the Chýnice Limestone (Emsian) at Bubovice (Čeřinka hillside; Prague Basin). Palaeontologia Bohemiae, 2, 1–16.Google Scholar
- Havlíček, V., Vaněk, J., & Fatka, O. (1994). Perunica microcontinent in the Ordovician (its position within the Mediterranean Province, series division, benthic and pelagic associations). Sborník geologických věd, odd. Geologie, 46, 23–56.Google Scholar
- Jarochowska, E., Tonarová, P., Munnecke, A., Ferrová, L., Sklenář, J., & Vodrážková, S. (2013). An acid-free method of microfossil extraction from clay-rich lithologies using the surfactant Rewoquat. Palaeontologia Electronica, 16(3), 1–16.Google Scholar
- Kielan-Jaworowska, Z. (1966). Polychaete jaw apparatuses from the Ordovician and Silurian of Poland and a comparison with modern forms. Palaeontologia Polonica, 16, 1–152.Google Scholar
- Kim, A. I. (2011). Devonian tentaculites from the Kitab State Geological Reserve (Zeravshan-Gissar mountainous area, Uzbekistan). News of paleontology and stratigraphy, 15, 65–82 [in Russian].Google Scholar
- Kim, A. T., Yolkin, E. A., Erina, M. V. & Gratsianova, R. T. (1978). Type section of the Lower and Middle Devonian boundary beds in the middle Asia. Field session of the International Subcomission of Devonian system. A guide to field excursions. Tashkent, 54 pp.Google Scholar
- Klapper, G., & Johnson, J. G. (1980). Endemism and dispersal of Devonian conodonts. Journal of Palaeontology, 54(2), 400–455.Google Scholar
- Klapper, G., Ziegler, W., & Mashkova, T. V. (1978). Conodonts and correlation of Lower–Middle Devonian boundary beds in the Barrandian area of Czechoslovakia. Geologica et Palaeontologica, 12, 103–116.Google Scholar
- Korn, D., Klug, C., & Walton, S. A. (2015). Taxonomic diversity and morphological disparity of Paleozoic ammonoids. In C. Klug, D. Korn, K. De Baets, I. Kruta, & R. H. Mapes (Eds.), Ammonoid paleobiology, volume II: from macroevolution to paleogeography. Topics in geobiology, 44 (pp. 431–464). Dordrecht: Springer.CrossRefGoogle Scholar
- Kraft, P., Lehnert, O., & Frýda, J. (2004). Evolution of the Prague Basin reflecting the lifecycle of the Rheic Ocean. In P. Kraft, U. Linnemann & S. Mazur (Eds.), Gondwanan margin of the Rheic Ocean in the Bohemian Massif. Excursion guidebooks and abstracts, opening meeting of the IGCP project No. 497. (p. 101). Prague.Google Scholar
- Krs, M., Krsová, P., Pruner, P., & Havlíček, V. (1986). Paleomagnetism, palaeogeography and multi-component analysis of magnetisation of Ordovician rocks of the Barrandian in the Bohemian Massif. Sborník geologických věd, Užitá Geofyzika, 22, 9–48.Google Scholar
- Lange, F. W. (1949). Polychaete annelids from the Devonian of Parana, Brazil. Bulletins of American Paleontology, 33, 5–103.Google Scholar
- McGregor, D. C. (1979). Devonian spores from the Barrandian region of Czechoslovakia and their significance for interfacies correlation. Geological Survey of Canada Paper, 79, 189–197.Google Scholar
- Montesinos, J. R., & Truyols-Massoni, M. (1987). La Fauna de Anetoceras y el límite Zlichoviense-Dalejiense en el Dominio Paleontino (NO. de España). Cuadernos do Laboratorio Xeoloxico de Laxe, 11, 191–208.Google Scholar
- Paris, F. (1981a). Les chitinozoaries dans le paléozoïque du sud-ouest de l’Europe. Mémoires de la Société géologique et minéralogique de Bretagne, 26(412), 1–496.Google Scholar
- Paris, F. (1981b). Les chitinozoaires. In P. Morzadec, F. Paris & P. Racheboeuf (Eds.), La tranchée de la Lezais Emsien Supérieur du Massif Armoricain. Mémoires de la Société géologique et minéralogique de Bretagne, 24(313), 55–75.Google Scholar
- Paris, F., Winchester-Seeto, T., Boumendjel, K., & Grahn, Y. (2000). Toward a global biozonation of Devonian chitinozoans. Courier Forschungsinstitut Senckenberg, 220, 39–55.Google Scholar
- Salvador, A. (2013). International stratigraphic guide. Second edition. Boulder: Geological Society of America, 214 pp.Google Scholar
- Šnajdr, M. (1951). O errantních Polychaetech z českého spodního paleozoika (translated title: On errant polychaetes from the Czech Lower Palaeozoic). Sborník Ústředního ústavu geologického, 18, 241–292.Google Scholar
- Suttner, T. J., & Hints, O. (2010). Devonian scolecodonts from the Tyrnaueralm, Graz Palaeozoic, Austria. Memoirs of the Association of Australasian Palaeontologists, 39, 139–145.Google Scholar
- Svoboda, J., & Prantl, F. (1948). O stratigrafii a tektonice staršího paleozoika v okolí Chýnice (translated title: The stratigraphy and tectonics of the Early Palaeozoic in the vicinity of Chýnice). Sborník Státního geologického ústavu, 15, 1–40.Google Scholar
- Szaniawski, H., & Drygant, D. (2014). Early Devonian scolecodonts from Podolia, Ukraine. Acta Palaeontologica Polonica, 59(4), 967–983.Google Scholar
- Tappan, H. (1980). The paleobiology of plant protists. San Francisco: W. H. Freeman. 1028 pp.Google Scholar
- Taugourdeau, P. (1968). Les scolecodontes du Siluro-Dévonien et du Carbonifére de sondages sahariens; Stratigraphie-systematique (translated title: Scolecodonts from the Siluro-Devonian and Carboniferous of Saharan borings; systematics and stratigraphy). Revue de l’Institut Français du Pétrole et Annales des Combustibles liquids, 23, 1219–1271.Google Scholar
- Taugourdeau, P. & Jekhowsky, B. de (1960). Répartition et description des chitinozoaires Siluro-Dévoniens de quelques sondages de la C. R. E. P. S., de la C. F. P. A. et de la S. N. Repal au Sahara. Revue de l'Institut Français du Pétrole et Annales des Combustibles liquides, 15(9), 1199–1260.Google Scholar
- Tonarová, P., Hints, O., & Eriksson, M. E. (2014). Polychaetes and the end-Ordovician mass extinction: new data from the basal Silurian Varbola formation of Estonia. In H. Bauert, O. Hints, T. Meidla, & P. Männik (Eds.), 4th Annual meeting of IGCP 591, Estonia, 10 –19 June 2014. Abstracts and field guide (p. 93). Tartu: University of Tartu.Google Scholar
- Urban, J. B. (1972). A reexamination of Chitinozoa from the Cedar Valley. Formation of Iowa with observations on their morphology and distribution. Bulletin of American Paleontologist, 275, 1–44.Google Scholar
- Volkheimer, W., Melendi, D. L., & Salas, A. (1986). Devonian chitinozoans from northwestern Argentina. Neues Jahrbuch für Geologie und Paläontologie, Abhandlungen, 173, 229–251.Google Scholar
- Walliser, O. H. (1985). Natural boundaries and Commission boundaries in the Devonian. Courier Forschungsinstitut Senckenberg, 75, 401–408.Google Scholar
- Ye, X. (1994). Upper Silurian to Devonian scolecodont fossils from west Qinling mountains. Acta Micropaleontologica Sinica, 11, 479–501.Google Scholar