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Chitinozoan biostratigraphy of the Pridoli series of the East Baltic./ Pridoli kitiinikute biostratigraafia Ida-Balti puuraukudes.

INTRODUCTION

The most important results of palaeontological and lithostratigraphical investigations of the Silurian in Estonia are summarized in the monograph The Silurian of Estonia, edited by Kaljo (1970). This book was followed by studies on different faunal groups including those from the Ventspils, Pavilosta and Dubovskoye cores (Kaljo & Sarv 1976; Viira 1982, 1999; Marss 1986, 1997; Gailite et al. 1987). Much palaeontological, mineralogical and facies information on the Ohesaare, Ventspils and Pavilosta cores may be found in the volume edited by Kaljo and Klaamann (1982). The currently used stratigraphical scheme, which is followed in the present paper, was published in the comprehensive overview Geology and Mineral Resources of Estonia (see H. Nestor 1997).

Graptolites are missing in the carbonate sediments of the studied Pridoli sections. The lithological logs in this paper were composed after Gailite et al. (1987) and using unpublished descriptions by R. Einasto and H. Nestor. The Dubovskoye core section is divided into formations according to Koren et al. (2009). The boundary between the Kaugatuma and Ohesaare stages in the Ventspils and Dubovskoye cores has been distinguished by the appearance of the fossil fish Nostolepis alta (Marss 1986).

The first records of Pridoli chitinozoans in Estonia, in the Ohesaare core, were presented by Mannil (1970) and Eisenack (1970), who studied some samples from the Kaugatuma and Ohesaare cliffs on Saaremaa Island. Both authors identified Eisenackitina lagenomorpha and E. filifera and noted a good correspondence of higher strata on Saaremaa (Osel) with the Beyrichia Limestone in the erratics of the South Baltic coast. The first attempt to divide the upper Silurian of the Ohesaare core section on the basis of chitinozoan assemblages was made by Nestor (1976). The successions of chitinozoan species in the Silurian of the Ventspils and Pavilosta cores were published more than two decades ago (Nestor in Gailite et al. 1987). Later also the successions of Pridoli species in the Ohesaare (Nestor 1990) and Ruhnu (Nestor 2003) cores were published. Two specimens of Urochitina were identified (Nestor 1990) in the samples from the Ohesaare cliff, but later these identifications appeared to be mistakes resulting from deformation of specimens.

The biostratigraphy of the Ludlow chitinozoans in the East Baltic drill cores has been treated earlier by Nestor (2009). The present paper is sequential to that work, as here the chitinozoan biostratigraphy of the Pridoli Series is examined from the same Ohesaare, Ventspils D-3, Pavilosta and Dubovskoye (Northern-Gusevskaya 2) cores, situated from north to south in Estonia, Latvia and Kaliningrad district, Russia.

Most of the studied samples were collected with colleagues in the Skrunda, Riga and Gussev depositories between 1980 and 1986. The best studied drill core is still Ohesaare, with a large volume of published data concerning all fossil groups.

The distribution of Pridoli chitinozoans in different localities worldwide is discussed in numerous publications, for example Eisenack (1955, 1972), Wrona (1980), Paris (1981), Paris et al. (1981), Paris & Kriz (1984), Jaglin (1986), Kriz et al. (1986), Verniers et al. (1995), Paris & Grahn (1996), Miller et al. (1997), Boumendjel (2002), Jaglin & Paris (2002), and Grahn (2005, 2006). Part of the species described in the above-named papers have been established also in the East Baltic cores, studied for this paper.

The diversity of Silurian chitinozoans is very high. In total more than one hundred chitinozoan species have been identified globally from the Pridoli Series (see Grahn & Paris 2011). Knowledge of the distribution of Pridoli chitinozoans in the East Baltic sections is still quite incomplete. The aim of the present paper is to fill this gap and to correlate more precisely the East Baltic chitinozoan biozones with global biozones (Verniers et al. 1995) and find the stratigraphically most useful taxa for inter-regional correlation.

[FIGURE 1 OMITTED]

A total of 242 samples (with a weight of 100-300 g) were processed; all but six yielded chitinozoans. The studied material is deposited in the Institute of Geology at Tallinn University of Technology (collection GIT 607).

GEOLOGICAL SETTING

During the regression of the Palaeobaltic sea in the Pridoli Epoch the facies belts in the East Baltic area migrated southwestwards. The studied drill cores represent mostly carbonate deposits of the open shelf, except from the Dubovskoye core which contains mostly argillaceous sedimentary rocks of the deeper-water transitional facies belt (Fig. 1). In the East Baltic area the Pridoli Series is represented by the Kaugatuma and Ohesaare regional stages and is subdivided into several lithostratigraphical units, changing laterally from region to region (Fig. 2). The Aigu Beds, forming the lower part of the Kaugatuma Formation in Estonia, are characterized by alternating argillaceous and micritic limestones and argillaceous dolomitic marlstones with numerous hardgrounds. The upper, Loo Beds consist of argillaceous dolomitic marlstones intercalated with argillaceous limestones (Figs 2, 3). The Ohesaare Formation of the regional stage with the same name consists of bioclastic limestones and marlstones. More information about the lithology and facies of the upper Silurian sequence of Estonia is available in H. Nestor (1997). The Minija Formation in Latvia is characterized mostly by argillaceous marlstones in the Ventspils core (Fig. 4) and argillaceous dolomitic marlstones in the Pavilosta core (Fig. 5). The lower part of the Targale Formation in the Ventspils core is represented by intercalations of calcareous and argillaceous marlstone with limestone interbeds. The upper part of the formation contains dolomitic marlstones with dolomite interbeds. The Jura Formation is represented only by its lower part in the Pavilosta core, consisting mostly of argillaceous dolomitic marlstones with limestone interbeds in the lower half and with argillaceous dolomite interbeds in the upper half.

[FIGURE 2 OMITTED]

The Kandievski and Okunevski formations of the Dubovskoye core are characterized by dolomitic mudstones (Fig. 6). These formations overlying the Uljanov Formation (Koren & Suyarkova 2007) were recently described by Koren et al. (2009) (see Fig. 2). It is worth mentioning that earlier Kaljo & Sarv (1976) described this section and identified the Kaugatuma Stage, composed of the Aigu and Loo beds, with the boundary between them at 1050 m, on the basis of the succession of ostracode species.

THE LUDLOW-PRIDOLI BOUNDARY

In graptolitic sections the lower boundary of the Pridoli Series is placed at the base of the ultimus-parultimus Biozone. In the East Baltic drill cores, however, graptolites are absent, except in some Lithuanian sections, where characteristic species of the lowermost graptolite biozone have been identified (Paskevicius 1979).

Besides graptolites, the disappearance of the conodont Ozarkodina crispa (Walliser) is one of the most reliable indicators of the base of the Pridoli Series in the Pridoli type area in Bohemia (Kriz et al. 1986). In the East Baltic cores this species disappears at the beginning (Ohesaare core) or in the middle part (Kolka core) of the underlying Kuressaare Formation. It ranges up to 407.6 m in the Minija Formation in the Ventspils core, but has not been found in the Pavilosta core (Viira 1999). Ozarkodina remscheidensis eosteinhornensis (Walliser) appears just above the boundary of the Kaugatuma Formation in the Ohesaare core, about 20 m higher than the Pridoli boundary in the Ventspils core and deep in the Ludlow in the Pavilosta core (Viira 1999). Thus, according to the conodont distribution in the East Baltic drill cores, it is difficult to find reliable criteria for identification of the exact level of the Ludlow-Pridoli boundary.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

Vertebrates have been studied from the same sections (Marss 1986, 1997). Thelodus sculptilis Gross disappears below or close to the Ludlow-Pridoli boundary, except in the Dubovskoye core, where vertebrate remains have not been found from the boundary interval (1131.8-1257 m).

[FIGURE 5 OMITTED]

The Ludlow ostracodes and graptolites and Pridoli ostracodes of the Dubovskoye core have been studied by Kaljo & Sarv (1976). They determined the boundary of the Kuressaare and Kaugatuma stages at a depth of 1230 m, which has also been followed by Nestor (2009). In comparison with the Dubovskoye core, the Pridoli stratotype sections of Bohemia (Kriz et al. 1986) contain a different ostracode assemblage at the Ludlow-Pridoli boundary.

Recently, Koren et al. (2009) distinguished, on the grounds of the appearance of the brachiopod Isorthis ovalis Paskevicius in the uppermost Silurian of the Severo-Gusevskaya 2 (= Dubovskoye) core, the Kandievski and Okunevski formations, with the Ludlow-Pridoli boundary between them, at a depth of 1168 m. From the Okunevski Formation Oulodus elegans Walliser was found, which was treated as the Pridoli index species, but in the Ohesaare, Ventspils and Pavilosta cores this species appears stratigraphically much lower, in the middle Ludlow (Viira 1999).

[FIGURE 6 OMITTED]

Chitinozoans belong to a fossil group with a planktic mode of life and are thus less dependent on facies conditions than the benthic or nekto-benthic groups. Nevertheless, lateral variation in assemblages can be pronounced. In the stratotype area of Bohemia, at the base of the Pridoli, dated by the first appearance of Monograptus parultimus, there appear also chitinozoans Fungochitina kosovensis Paris & Kriz, Linochitina cf. klonkensis Paris & Laufeld and Urnochitina urna (Eisenack) (Paris in Kriz et al. 1986). In the Dubovskoye core the first two chitinozoan species make their appearance respectively at depths of 1184 and 1181 m, in the upper part of the Kandievski Formation. In addition, in both sections at about the same level the characteristic species of the upper Ludlow, Eisenackitina barrandei Paris (see Kriz et al. 1986), disappears. Thus, the middle part of the Kandievski Formation, in the interval 1187-1226 m in the Dubovskoye core (Fig. 6) belongs to the Ludlow Series, the uppermost part in the interval 1184-1168 m to the Pridoli Series.

CHITINOZOAN BIOSTRATIGRAPHY

Attempts to subdivide the Pridoli Series using chitinozoans have been made by different authors in several regions: Taugourdeau & de Jekhowsky (1960), Sahara; Cramer & Diez (1978), Iberian Peninsula; Paris (1981), southwestern Europe; Schweineberg (1987), Palencia; Nestor (1990), Estonia and Latvia; Paris & Grahn (1996), Podolia; Geng et al. (1997), Yangtze region, China; Jaglin & Paris (2002), Libya; Grahn (2005), Amazonas Basin, Brazil.

Verniers et al. (1995) compiled a global chitinozoan biozonation scheme. According to this scheme three biozones are distinguished in the Pridoli Series: Fungochitina kosovensis, Margachitina elegans and Anthochitina superba. This 'model' works only partly in the East Baltic drill cores. Fungochitina kosovensis Paris & Kriz occurs in the lowermost Pridoli only in the Dubovskoye core, Margachitina elegans (Taugourdeau & de Jekhowsky) is completely lacking in the East Baltic cores and Anthochitina superba Eisenack has been found only in the Ventspils core and only in the middle part of the Pridoli. The most important and widely distributed species for identification of the lower Pridoli boundary, Urnochitina urna (Eisenack), has been identified only in the Dubovskoye core, but about 40 m above the base of the series. Thus, the northern East Baltic chitinozoan biozonal succession is rather different from those of the other regions. Differences exist also between the Dubovskoye section and the more northern sections.

Similar to the global chitinozoan biozones (Verniers et al. 1995) we define the East Baltic Pridoli biozones as interval zones. Their bases are defined by the first occurrence of the index species.

The uppermost Ludlow chitinozoan assemblage in the Dubovskoye core (Fig. 6) corresponds well with the assemblage in the Ohesaare and Pavilosta cores, treated as the Eisenackitina barrandei Biozone (Nestor 2009). It includes Eisenackitina intermedia (Eisenack) (Fig. 7A), Eisenackitina cf. kerria Miller, Sutherland & Dorning (Fig. 7B), Eisenackitina barrandei Paris & Kriz (Fig. 7C), Linochitina sp. (Fig. 7D), Belonechitina? sp. (Fig. 7E), Eisenackitina lagenomorpha (Eisenack) (Fig. 7F), Angochitina cf. paucispinosa Miller, Sutherland & Dorning (Fig. 7H), Rhabdochitina sp. 2 (Fig. 7I), Angochitina sp. 1 (Fig. 7K), Rhabdochitina sp. 1 (Fig. 7R), Cingulochitina hedei Laufeld (Figs 7T, 8I), Pterochitina perivelata (Eisenack) (Fig. 8A), Eisenackitina oviformis Eisenack (Fig. 8B), Angochitina echinata Eisenack (Fig. 8G, H), Eisenackitina elongata Eisenack (Fig. 8F) and Sphaerochitina sphaerocephala (Eisenack) (Fig. 8P). Most of these species occur also in the Pridoli.

[FIGURE 7 OMITTED]

The Fungochitina kosovensis global and Eisenackitina kerria--Ancyrochitina tomentosa regional biozones

Fungochitina kosovensis is widely distributed in the Pridoli sections of the Gondwana palaeoplate, but is also found on the Baltica palaeoplate, in Podolia (Paris & Grahn 1996) and now in the Dubovskoye core in the Kaliningrad district (Fig. 6). It is the index species of the lowermost global biozone of the Pridoli Series (Verniers et al. 1995).

In addition to Fungochitina kosovensis (Fig. 7M), Linochitina klonkensis (Fig. 7L), Bursachitina concava Eisenack (Fig. 8M), Urnochitina urna (Fig. 8N, O), Plectochitina pachyderma (Laufeld) (Fig. 7J), Ancyrochitina cf. libyensis Jaglin (Fig. 7O), Ramochitina sp. 1 (Fig. 7Q) and Ancyrochitina aff. ansarviensis Laufeld (Fig. 7Y) appear in the Dubovskoye core in the lower part of the Pridoli. These species are unknown in the other studied Pridoli sections. Thus, the assemblage of the lower Pridoli chitinozoans in the Dubovskoye core contains several Gondwana-type faunal elements (Fungochitina kosovensis, Linochitina klonkensis, Bursachitina concava Eisenack, Urnochitina urna), which are totally lacking in the northern East Baltic drill cores. They allow us to place this part of the section in the global Fungochitina kosovensis Biozone.

However, in the northern drill cores (Ohesaare, Ventspils, Pavilosta, Figs 3-5), F. kosovensis has not been found in the lowermost Pridoli. Its position has been taken by Eisenackitina kerria Miller, Sutherland & Dorning (Fig. 7G) and Ancyrochitina tomentosa Taugourdeau & de Jekhowsky (Fig. 7W), which have partly overlapping ranges in the interval that probably corresponds to the F. kosovensis Biozone in the Dubovskoye core. Both biozones are characterized by the disappearance of Eisenackitina barrandei in all studied sections, within some metres above the base of the Pridoli (Figs 3-6), as in the Pozary and Hvizdalka sections of the Pridoli stratotype area (Kriz et al. 1986). Angochitina filosa Eisenack (Fig. 8C), Angochitina ceratophora Eisenack (Fig. 8L) and Ancyrochitina fragilis Eisenack (Fig. 8D, E) occur in a short interval just above the base of the E. kerria-A. tomentosa Biozone, whereas Ancyrochitina pedavis Laufeld (Fig. 7U), tiny (< 70 [micro]m) Bursachitina sp. (Fig. 7S), Eisenackitina sp. (ovoid) (Fig. 8K) and Ramochitina sp. 2 (Fig. 8R) have been found only in some sections. Rare finds of Fungochitina pistilliformis Eisenack (Fig. 7N), Eisenackitina cupellata Wrona (Fig. 7X), tiny (<70 [micro]m) Eisenackitina sp. (Fig. 7V) and Angochitina aff. chlupaci Paris & Laufeld (Fig. 8J) come from the middle or upper parts of the biozone.

[FIGURE 8 OMITTED]

The Salopochitina filifera regional Biozone

This biozone is represented in all studied drill cores. According to Eisenack (1968), Salopochitina filifera (Eisenack 1931, Fig. 8X-AB) was previously found only in a few locations, in the Beyrichia Limestone of the south Baltic erratics, Klinta (Sweden) and Ohesaare cliff (Estonia). In the studied East Baltic drill cores this species is represented abundantly in the upper part of the Aigu and in the Loo beds, as well as in the Targale, Jura and Okunevski formations (Figs 3-6), respectively in the Ohesaare, Ventspils, Pavilosta and Dubovskoye cores. Eisenackitina cf. invenusta (Wrona) (Fig. 8T) is one of the common species, appearing somewhat lower than the biozonal boundary. More newcomers occur in the Dubovskoye core, where Ramochitina sp. 2 (Fig. 8R), Angochitina sp. 2 (Fig. 8S) and Bursachitina bursa (Taugourdeau & de Jekhowsky) (Fig. 8U) appear in the lower part and Calpichitina gregaria Paris & Kriz (Fig. 9E, F) in the uppermost part of the biozone. Besides the index species, Angochitina cf. lebaica Eisenack (Fig. 8Q) has been found in the Ventspils core.

Some new species appear in the middle part of the Loo Beds (Ohesaare core) and in the Jura Formation (Pavilosta core) (Figs 3, 5). Angochitina sp. 3 (Fig. 9S), Angochitina sp. 4 (Fig. 9U) and Bulbochitina? sp. (Fig. 9O, P) occur in both drill cores. Ramochitina sp. 3 (Fig. 9Q) is found in the Ohesaare and Eisenackitina sphaerica (Eisenack) (Fig. 9N) in the Pavilosta core.

In the global Silurian chitinozoan range chart of index and characteristic species the appearance level of Salopochitina filifera (Eisenack) coincides with that of Margachitina elegans Taugourdeau & de Jekhowsky (Verniers et al. 1995). The latter species is known in several Gondwana sections and is also found in the Pridoli of Podolia (Paris & Grahn 1996), but does not occur in the East Baltic.

The Anthochitina superba regional Biozone

The index species was described by Eisenack (1971) from the Beyrichia Limestone of South Baltic erratics. According to Verniers et al. (1995), the A. superba Biozone is the highest chitinozoan biozone in the Silurian, established in Algeria, Sweden, Poland and Ukraine. In the East Baltic sections Anthochitina superba (Fig. 9A-D) has been identified only in a short interval (334-336.20 m) of the Ventspils drill core (Fig. 4). The interval 298-330.80 m in the middle of the Targale Formation, lacking the biozonal species, but below the first appearance of Ancyrochitina lemniscata is also provisionally included into the biozone. Besides A. superba, a few other species appear in this zone in the Ventspils core: Eisenackitina clunensis Miller, Sutherland & Dorning (Fig. 9H, I) in the lower part and Fungochitina kosovensis (Fig. 9J, K) in its upper part. The occurrence of the latter species so high in the Ventspils section is quite surprising as commonly this species has been treated as indicative of the base of the lower Pridoli.

In the Ohesaare and Pavilosta drill cores A. superba is lacking, probably due to pre-Devonian erosion of the corresponding strata. This species has not been found in the Dubovskoye core either, where its position is seemingly occupied by Margachitina sp. (Fig. 8W) and Plectochitina sp. (Fig. 8V), appearing at a depth of 1075.2 m (Fig. 6). The latter species occurs in the Ventspils core in the A. superba Biozone. The correspondence of the beds with Plectochitina sp. in the Dubovskoye core to the A. superba Biozone in the Ventspils section is also supported by the disappearance of Fungochitina pistilliformis in both sections, as well as by a similar position of these strata between the under- and overlying biozones. However, these are all indirect evidences, so really we do not know where the exact boundary level might be. Due to the absence of the index species in the Dubovskoye section, the name of the biozone has been put in brackets (see Fig. 6). In the middle and upper parts of this subdivision in the Dubovskoye core Calpichitina velata (Wrona) (Fig. 9G) occurs. Angochitina chlupaci Paris & Laufeld (Fig. 9R), known as a characteristic species of the Silurian-Devonian boundary in the stratotype sections of Bohemia (Paris et al. 1981), appears at a depth of 1008.60 m.

[FIGURE 9 OMITTED]

The Ancyrochitina lemniscata regional Biozone

Ancyrochitina lemniscata Wrona (Fig. 9L, M) has previously been described from the uppermost Pridoli in drill cores from Poland (Wrona 1980); its biozone is erected herein. Besides the index species, occurring in the Dubovskoye and Ventspils cores (Figs 4, 6), Angochitina sp. 3 (Fig. 9S), Angochitina sp. 4 (Fig. 9U), Bulbochitina? sp. (Fig. 9O, P) and Eisenackitina sphaerica (Fig. 9N) have also been identified in the Ventspils core. These species were found in the Ohesaare and Pavilosta cores in lower layers (see Figs 3, 5). In the Dubovskoye core Ancyrochitina sp. (Fig. 9T) is additionally present.

[FIGURE 10 OMITTED]

Pridoli conodonts (Viira 1999) and vertebrates (Marss 1997) have also been studied from the Ventspils core, allowing correlation of different biozones, best represented in deep shelf facies (Fig. 10). A good correspondence exists between the bases of the Salopochitina filifera, Ozarkodina remscheidensis canadensis and Nostolepis gracilis biozones in the middle of the Minija Formation.

CONCLUSIONS

1. The studied East Baltic sections contain an abundant association of the typical Pridoli chitinozoan species, including some species described previously from the Lower Devonian: Calpichitina velata (Wrona), Eisenackitina cupellata Wrona and E. invenusta (Wrona) from Poland and Angochitina chlupaci Paris & Laufeld from Bohemia. This indicates the completeness of the Pridoli succession in the East Baltic drill cores.

2. The new regional Eisenackitina kerria-Ancyrochitina tomentosa Biozone, corresponding to the Fungochitina kosovensis Biozone in the Dubovskoye drill core, is distinguished in the northern East Baltic Ohesaare, Ventspils and Pavilosta sections in the lower part of the Pridoli Series.

3. The Pridoli chitinozoan assemblage of the East Baltic shows an increasing presence of Gondwana elements (Fungochitina kosovensis, Linochitina klonkensis, Urnochitina urna, Bursachitina concava), particularly in the southern, deeper-water areas (Dubovskoye section). This may result from the approaching of palaeoplates in Pridoli time.

4. The Anthochitina superba Biozone is lacking in the Ohesaare and Pavilosta cores probably due to pre-Devonian erosion of the corresponding strata.

5. The Ancyrochitina lemniscata Biozone is erected above the Anthochitina superba Biozone in the East Baltic uppermost Silurian.

6. Correlation of the chitinozoan, conodont and vertebrate biozones in the Ventspils core demonstrates a good correspondence of some biozonal boundaries.

doi: 10.3176/earth.2011.4.01

Acknowledgements. This study was supported by the Estonian Ministry of Education and Research (targeted funding project SF0140020s08). I thank H. Nestor and R. Einasto for the opportunity to use their unpublished lithological descriptions, F. Paris and D. Kaljo for valuable suggestions, and O. Hints and G. Baranov for technical help with the SEM study and photographs. Special thanks go to referees Prof. D. K. Loydell and Prof. J. Verniers for constructive comments on the manuscript.

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Viiu Nestor

Institute of Geology at Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia; Viiu.Nestor@gi.ee

Received 8 April 2011, accepted 11 August 2011
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Author:Nestor, Viiu
Publication:Estonian Journal of Earth Sciences
Article Type:Report
Geographic Code:4EXES
Date:Dec 1, 2011
Words:4539
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