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Biostratigraphy of the Ludlow chitinozoans from East Baltic drill cores/ Ludlow' kitiinikute biostratigraafia Ida-Balti puuraukudes.
The distribution of chitinozoans in the East Baltic Ludlow sequences was studied in the Ohesaare, Ventspils, Pavilosta, and Dubovskoye drill core sections. The Angochitina elongata, Eisenackitina lagenomorpha, and Eisenackitina barrandei biozones were described and correlated with the regional stratigraphical units, as well as with conodont and vertebrate biozones in the Ohesaare core.

Key words: chitinozoans, biostratigraphy, Ludlow, Silurian, East Baltic.

Kitiiniku liikide levik ja biotsoonid on kindlaks tehtud neljas Ida-Balti puuraugu Ludlow' vanusega kivimites. Lisaks varem kirjeldatud Alam-Ludlow' biotsoonidele (Nestor 2007) on selles artiklis eristatud kitiinikute Angochitina elongata, Eisenackitina lagenomorpha ja E. barrandei biotsoonid, neist viimane esmakordselt kogu Balti regioonis. On roobistatud kitiinikute, konodontide ja vertebraattde biotsoonid Ohesaare puuraugus.

Geology, Stratigraphic (History)
Chitons (History)
Nestor, Viiu
Pub Date:
Name: Estonian Journal of Earth Sciences Publisher: Estonian Academy Publishers Audience: Academic Format: Magazine/Journal Subject: Earth sciences; Science and technology Copyright: COPYRIGHT 2009 Estonian Academy Publishers ISSN: 1736-4728
Date: Sept, 2009 Source Volume: 58 Source Issue: 3
Geographic Scope: England Geographic Name: Ludlow, England Geographic Code: 4EUUE England
Accession Number:
Full Text:

The distribution of chitinozoans in the Wenlock-Ludlow boundary beds in five East Baltic drill cores (Ohesaare, Kolka, Ventspils, Pavilosta, Gussev-1) has been described in Nestor (2007). Up to now only a brief review of chitinozoan distribution in the Ludlow sections of Estonia, based on the Ohesaare (V. Nestor 1990, 1997) and Ruhnu cores (Nestor 2003), is available. The first, lowest stratigraphical level identified by chitinozoans is at the base of the Torga Formation of the Paadla Regional Stage, where Angochitina elongata appears. Chitinozoans are absent below this level in the Soeginina Beds, recently attributed to the Ludlow by Viira & Einasto (2003). Angochitina elongata is a zonal species of the global biozone, widely distributed around the world in the lower part of the Ludlow (Verniers et al. 1995). This biozone forms a base for the discussion in the present paper.

Graptolite data are very scarce above the Lobograptus scanicus Biozone in the East Baltic Ludlow. R Ulst (in Gailite et al. 1987) has identified all the graptolites collected from the Pavilosta and Ventspils drill cores, but these were only rare findings, which did not allow exact identification of the biozonal boundaries. This is also true for the graptolite records from the Dubovskoye core (Kaljo & Sarv 1976). Therefore, the boundary between the Gorstian and Ludfordian stages in the studied drill cores has been drawn tentatively.

In the present paper the distribution of chitinozoans in the Ohesaare, Ventspils, Pavilosta, and Dubovskoye (Northern-Gusevskaya 2) core sections is considered. As Nestor (2007) discussed also the distribution of chitinozoans in the lowermost Ludlow, this interval is not dealt with in the present article. Chitinozoan data from the Kolka core are also omitted due to their great similarity to those from the Ohesaare core and the much greater number of samples in Kolka, empty of chitinozoans. The data of the Gussev-1 core are replaced by the chitinozoan succession of the Dubovskoye core, where the series of samples is more representative, as chitinozoan samples are missing from the uppermost 23 m of the Ludlow in the former core. Besides, all chitinozoan data from the omitted drill cores confirm the information presented below.

The main aim of this paper is to present more biostratigraphical information concerning the Ludlow of the East Baltic region and to improve the chitinozoan distribution data in general. Most of the studied samples have been collected together with colleagues in the Skrunda, Riga, and Gussev depositories between 1980 and 1986. Several papers have been published on these materials, concerning assemblages of trilobites (Mdnnil 1982), ostracodes (Sarv 1982), conodonts (Viira 1982), and vertebrates (Mdrss 1986), which enabled us to compare chitinozoan assemblages with some other faunas (see below).

The studied specimens are housed at the Institute of Geology at Tallinn University of Technology (collection GIT 576).


The studied drill cores represent open shelf, transitional facies, and shelf depression deposits (Fig. 1). The Torga Formation and the upper part of the Kuressaare Formation of the Ohesaare core are predominantly characterized by argillaceous nodular limestones; the lower part of the Kuressaare Formation is represented by calcareous and dolomitic argillaceous marlstones. Ludlow deposits in the Ventspils, Pavilosta, and Dubovskoye cores are of deeper water origin. The Dubysa Formation is characterized by argillaceous marlstones with rare thin limestone interbeds, the Engure Formation by different marlstones with limestone nodules, and the Mituva and Ventspils formations consist of nodular limestones with a number of hardgrounds. The mainly dolomitic mudstones in the upper Ludlow succession of the Dubovskoye core belong to the Uljanov Formation (Koren & Suyarkova 2007) (Fig. 2).


Conodont-based correlations between Saaremaa (Estonia) and Gotland (Sweden) have revealed a major gap in the middle part of the Ludlow sequence of Estonia, between the Paadla and Kuressaare regional stages (Jeppsson et al. 1994). The extent of this gap in the Ohesaare core is supposed to be smaller than in the outcrop area (Viira & Aldridge 1998). A gap in the lowermost Ludlow of Estonia has been observed earlier, based on the chitinozoan succession (missing biozones) (Nestor 1982). In the other East Baltic sections (Ventspils and Pavilosta), the Ludlow deposits are represented more or less in full extent. In the Dubovskoye core we have studied only the upper part of the Ludlow, i.e. the Ludfordian Stage.

A model of oceanic and climatic cyclicity, linked to the sedimentological and biotic changes in the Ludlow, was proposed by Jeppsson & Aldridge (2000). They determined two levels of intense faunal extinctions, the Linde and Lau events in the mid-Ludlow. According to Kaljo et al. (1998), carbon isotope cycling is one feature of environmental cyclicity and the most positive excursions occur after important faunal extinctions, usually tied to regression and cooling of climate. Carbon isotopes have been studied and [[delta].sup.13]C curves compiled for the Ohesaare and Ventspils cores (Kaljo et al. 1998; Kaljo & Martma 2006). The [[delta].sup.13]C curve of the late Gorstian Linde Event (Jeppsson & Aldridge 2000) is hardly recognizable in the East Baltic cores (Kaljo & Martma 2006). The peak values of the mid-Ludfordian [[delta].sup.13]C excursion occur within the limits of the Lau Event, defmed by conodonts (Jeppsson & Aldridge 2000). Among the studied sections the Lau Event is expressively reflected in the Ventspils core, approximately within the depth interval 460-470 m, in the upper part of the Mituva Formation (Kaljo et al. 1998; Kaljo & Martma 2006). In the chitinozoan succession this event is characterized by the extinction of 14 species. High values of the carbon isotope curve have been reported from various localities around the world, giving evidence of the global dimension of this event (Martma et al. 2005; Calner & Eriksson 2006). In the Ohesaare core the peak level of the carbon isotope excursion is represented by a gap (Kaljo et al. 1998).



According to Kaljo & Paskevicius (1993), the P. tumescens and S. leintwardinensis graptolite biozones have not been established in the East Baltic drill cores and the boundary between the Gorstian and Ludfordian stages falls somewhere within the middle of the regional P. tauragensis Zone. Graptolites are scarce, but chitinozoans occur abundantly in the East Baltic upper Silurian sections, as well as in the type Ludlow area, studied by Sutherland (1994). According to the last paper, only minor changes occur in the chitinozoan succession at the boundary of the Gorstian and Ludfordian stages. Most of the species which appear in the Upper Elton or Bringewood formations range over this boundary. However, there are some species (Sutherland 1994, text-fig. 6) with overlapping ranges at the stages boundary, e.g. Belonechitina mortimerensis disappearing below the boundary and E. lagenomorpha? appearing a bit lower. After being absent in the Upper Elton Formation, Angochitina elongata re-appears in the lowermost part of the Ludfordian Stage. A similar situation occurs in the Ventspils core in the interval 512-516 m and in the Pavilosta core in 744.3-745.3 m, though A. elongata is not found above this interval in the latter section. These distribution data were not sufficient for recognizing this boundary in the Ohesaare core (Fig. 3) but allowed us to establish the provisional position of the stages boundary in the Ventspils and Pavilosta cores (Figs 4, 5). In the Dubovskoye core only sediments of the Ludfordian Stage are represented (Fig. 6).


The Ludlow chitinozoans from Baltic glacial erratics were first described by Eisenack (1931), from Gotland by Eisenack (1964), and from Estonia (Saaremaa = Osel) by Eisenack (1970). Laufeld (1974) identified among other species, many Ludlow taxa from the Gotland sequence, but without establishing biozonal units. V. Nestor (1990) distinguished six preliminary biozones from the East Baltic Ludlow cores. The first short account of chitinozoan biostratigraphy in the Ludlow of Shropshire was published by Doming (1981). A complete review of chitinozoans from the Ludlow type area was provided by Sutherland (1994), who established 12 numerical biozones in the interval from the Lower Elton Formation to the Lower Whitcliffe Formation. A year later Vemiers et al. (1995) published a global biozonation of Silurian chitinozoans with three biozones and two undefined intervals in the zonal succession of the Ludlow Series. Nestor (2007) defined and described two new regional biozones (Conochitina postarmillata and Ancyrochitina desmea), which filled the previous lower Ludlow gap in the chitinozoan zonal succession (Fig. 7). The younger biozones from the Ludlow of East Baltic drill cores are described in ascending order.

The Angochitina elongata Biozone

Eisenack (1931) described the holotype of Angochitina elongata from an erratic boulder and defined a neotype from the Hemse Marl of Gotland (Eisenack 1964). In the type Ludlow area this species appears 24 m below the top of the Middle Elton Formation, Gorstian Stage, correlated with the uppermost part of the Lobograptus scanicus graptolite Biozone (Sutherland 1994). In the Pavilosta core R Ulst identified the highest L. cf. scanicus at 806.50 m and Saetograptus chimaera at 811.30 m, but the lowest Pristiograptus tumescens at 784.10 m. In the Ventspils core L. ex. gr. scanicus was established at 609.40 m, S. chimaera at 600 m, and P. tumescens (single fording) at 574.30 m (Gailite et al. 1987). Thus, the East Baltic chitinozoan data confirm the opinion of Sutherland (1994) that the lower boundary of the A. elongata Biozone corresponds to the uppermost part of the scanicus graptolite Zone.

In the East Baltic sequences the A. elongata Biozone coincides with the lower and middle parts of the Torga Formation, the upper part of the Dubysa Formation, and the lower part of the Engure Formation, which all are characterized by a diverse and abundant assemblage of chitinozoans. Many species from the uppermost Wenlock range into the Ludlow of the Ventspils and Pavilosta cores: Conochitina pachycephala Eisenack (Fig. 8M), C. claviformis Eisenack (Fig. 8N), C. tuba Eisenack (Fig. 80), C. rudda Sutherland (Fig. 8L), Cingulochitina convexa (Laufeld) (Fig. 8T), Linochitina erratica Eisenack (Fig. 8V), and Cingulochitina gorstyensis Sutherland (Fig. 8U). According to Sutherland (1994, p. 39), the lowest occurrence of the last species is in the midGorstian in the type Ludlow area (see also Nestor 2007, p. 123). Belonechitina latifrons (Eisenack) (Fig. 8P), B. lauensis (Laufeld) (Fig. 8R), Ancyrochitina diabolus Eisenack (Fig. 8I), A. desmea Eisenack (Fig. 8H), and A. gogginensis Sutherland (Fig. 8J) range from the basal Ludlow upwards. According to Sutherland (1994, p. 39), the last species appears already in the upper part of the Much Wenlock Limestone.

The zonal species, Angochitina elongata Eisenack (Fig. 8A), appears together with Angochitina echinata Eisenack (Fig. 8F) in the Ohesaare and Ventspils cores (Figs 3, 4), Palencia (North Spain) (Schweineberg 1987), Skane (Sweden) (Grahn 1996), Amazonas Basin (North Brazil) (Grahn 2005), and probably also in the type Ludlow area (Angochitina sp. aff. echinata in Sutherland 1994). Angochitina ambrosi Schweineberg (Fig. 8B, C) is previously only known from Gondwana, Palencia, and is for the first time identified in Baltic sections. Sphaerochitina impia Laufeld (Fig. 8E) and Ancyrochitina brevispinosa Eisenack (Fig. 8D) were identified only in the Ohesaare core. Ramochitina spinipes (Eisenack 1964) (Fig. 8G), a characteristic species of the lower part of the Hemse Beds, was recognized only in the Pavilosta core. Eisenackitina intermedia (Eisenack) (Fig. 8Y) is widely distributed, but the appearance level of this taxon is variable. In the outcrop area of the Brabant Massif, in the Ronquieres Formation (Verniers et al. 2002) and in the Prague Basin of the Barrandian area (Moravek 2007) it appears in the lowermost Ludlow, in the type Ludlow area (Sutherland 1994) and in the Ventspils and Pavilosta cores it appears in the middle of the Gorstian Stage, in the lower part of the Angochitina elongata Biozone. In Gotland (Laufeld 1974), Skane (Gratin 1996), and in the Ohesaare core its first occurrence is higher, within the Ludfordian Stage. It is not excluded that these differences in the species appearances might have been caused by local facies differences. Ramochitina swifti Sutherland (Fig. 8W), Eisenackitina toddingensis Sutherland (Fig. 8Z), E. kerria Miller, Sutherland & Doming (Fig. 8X), and E. clunensis Miller, Sutherland & Doming (Fig. 9A) are previously known from the Welsh Basin, and were for the first time identified in the East Baltic Ludlow. The first two species occur in the upper Gorstian of the type Ludlow area (Sutherland 1994) and two others in the uppermost Ludfordian of the Clun area (Miller et al. 1997). Belonechitina mortimerensis Sutherland (Fig. 8S) has been identified in the upper Gorstian of the type Ludlow area (Sutherland 1994) and at the same stratigraphic level in the Pavilosta and Ventspils cores. The fording of Ancyrochitina brevis Taugourdeau & Jekhowsky (Fig. 8K) in the upper part of the Gorstian Stage in the Pavilosta core is also notable, as this species is known from the Pridolian strata (Jaglin & Paris 2002).





A number of chitinozoan species of the studied cores are left in open nomenclature, as the material is insufficiently or badly preserved and the ornamentation is hardly visible. Some of these forms are represented in Figs 9 and 10. The Angochitina elongata Biozone includes Eisenackitina sp. 1 (Fig. 9C), Eisenackitina sp. 2 (Fig. 9D), and Sphaerochitina sp. (Fig. 9L) in the Pavilosta core and Ramochitina? sp. 1 (Fig. 9N) and Ramochitina sp. 2 (Fig. 9T) in the Ventspils core.


The chitinozoan succession in the Ohesaare core (Fig. 3) is interrupted by many barren intervals, where chitinozoans are missing, referring to unfavourable conditions for these microfossils (regression?) or their preservation. In the Ventspils core (Fig. 4) two intervals of chitinozoan extinctions can be recognized in the Angochitina elongata Biozone, one in the upper part of the Dubysa Formation (564-568 m) and the other step by step disappearance of species in the lower-middle part of the Engure Formation (527-546 m). Six species disappear in the uppermost Dubysa Formation in the Pavilosta core (742-752 m) (Fig. 5).

The Eisenackitina lagenomorpha Biozone

Vemiers et al. (1995) distinguished the global E. philipi Biozone in the middle-upper Ludlow. In the present paper, as E. philipi is rare in the East Baltic sections, we used the regional E. lagenomorpha Biozone instead of the E. lagenomorpha E. philipi Biozone, which has been published earlier by V. Nestor (1990, 1997).

In the Pavilosta core Eisenackitina philipi Laufeld (Fig. 9H) ranges for only about 10 m in the lowermost Ludfordian, i.e., in the upper part of the Dubysa Formation. It is rare in about 50 m at the boundary interval of the Engure and Mituva formations in the Ventspils core and is missing from the Dubovskoye core. Eisenackitina lagenomorpha (Eisenack) (Fig. 9G) is well represented in all studied sections, except for the upper part of this zone in the Dubovskoye core. According to Doming (1981), E. lagenomorpha and E. philipi appear together in the Lower Leintwardine Formation in the Welsh Basin. In the sections of the Ludlow area E. lagenomorpha? was identified in the uppermost part of the Bringewood Formation (Sutherland 1994). Actually this taxon is rather similar to E. lagenomorpha in the East Baltic cores. In the Pavilosta and Dubovskoye cores different Cingulochitina species occur abundantly throughout the biozone: Cingulochitina wronai Paris & Kriz (Fig. 9M), C. hedei Laufeld (Fig. 9V), Cingulochitina sp. 1 (Fig. 9S) as well as C. gorstyensis and C. convexa. The facies control on Cingulochitina species is obvious, as they are completely missing in the Ohesaare core and occur only sparsely in the Ventspils core. The chitinozoan assemblage of this biozone is better represented in the Pavilosta and Dubovskoye cores. Ancyrochitina pedavis Laufeld (Fig. 9U), Ramochitina villosa Laufeld (Fig. 9P), and Pterochitina perivelata Eisenack (Fig. I OE) appear in the lowermost part of the Pagegiai Formation in the Pavilosta core. The same species are present in the Hamra Formation on Gotland, also including Eisenackitina oviformis Eisenack (Fig. IOD), which appears in a somewhat higher level in Pavilosta. Angochitina paucispinosa Miller, Sutherland & Doming (Fig. IOB, C) is known from the Ludfordian of the Clun area, Shropshire (Miller et al. 1997). It occurs in the Ohesaare, Dubovskoye, and Pavilosta cores within the overlying Eisenackitina barrandei Biozone. Angochitina crassispina Eisenack (Fig. l0A) is found only sporadically. Atypical specimens of Eisenackitina elongata Eisenack (Fig. 9R) are represented in the Pavilosta and Dubovskoye cores, Calpichitina squamosa (Laufeld) (Fig. 9E) and Angochitina aff. echinata Eisenack (which has spines with wide bases) (Fig. 91) have been identified in the Dubovskoye core, and Sphaerochitina acanthifera Eisenack (Fig. 9F) is found only in the Pavilosta core. The last two species are characteristic of the Eke Formation of Gotland.


In addition, some species kept in open nomenclature are also identified from the E. lagenomorpha Biozone: Belonechifna sp. 1 (Fig. 9J), Sphaerochitina sp. (Fig. 9K), Belonechitina sp. 2 (Fig. 90), Ramochitina sp. (Fig. 9W), and Plectochitina sp. (Fig. 10F).

The Eisenackitina barrandei Biozone

The index species, Eisenackitina barrandei Paris & Kriz (Fig. 10G) has been described from the uppermost Ludlow of the Prague Basin (Paris & Kriz 1984). In the East Baltic this species is best represented in deep-water silt- and mudstones of the Pavilosta and Dubovskoye cores, in the upper parts of the Pagegiai and Uljanov formations, respectively.

The assemblage of chitinozoans in the E. barrandei Biozone in its type area (Khz et al. 1986) is similar to the corresponding assemblage in the East Baltic upper Ludlow sections. Angochitina echinata occurs in all studied drill cores, Ancyrochitina pedavis in Pavilosta. Sphaerochitina sphaerocephala (Eisenack) (Fig. 10M) is best represented in the upper part of the biozone in the Pavilosta and Ohesaare cores. Eisenackitina intermedia, E. oviformis (Eisenack) (Fig. 10D), and E. lagenomorpha occur in all studied sections, but more abundantly in the Pavilosta core. These species are also found in the Hamra and Sundre formations of Gotland (Laufeld 1974). Calpichitina gregaria Paris & Kriz (Fig. 10P) has been identified in the Dubovskoye core. Atypical specimens of Ancyrochitina ansarviensis Laufeld (Fig. 10J) are recorded in the Pavilosta core and Belonechitina? cf. granosa (Laufeld) (Fig. 10L) is present in all studied cores, except the Dubovskoye core. Some taxa with open nomenclature are also identified in the E. barrandei Biozone, mainly in the Pavilosta core: Plectochitina sp. (Fig. 10H), Ramochitina sp. 2 (Fig. 10I), Eisenackitina sp. 3 (Fig. 10R), Eisenackitina sp. 4 (Fig. 100), and Belonechitina sp. 3 (Fig. 10N) in the Ventspils core.

The lower boundary of the E. barrandei Biozone is fixed by the appearance of the index species. V. Nestor (1990) and Grahn (1996) distinguished the Sphaerochitina sphaerocephala Biozone in the topmost Ludlow. This zone can be traced in the Ohesaare and Pavilosta cores, but not in the other studied sections.

In the Ventspils core the Lau Event, related to intense extinction of chitinozoans, has been established in the topmost Mituva Formation (Kaljo et al. 1998), roughly in the depth interval 460-470 m (Fig. 4). It coincides with the lowermost part of the E. barrandei Biozone and is characterized by the disappearance of 14 chitinozoan species. Above the layers barren of chitinozoans at the boundary of the Mituva and Ventspils formations, only three species reappeared. In general, chitinozoans are scarce in the whole of the Ventspils Formation. In the Ohesaare core a stratigraphic gap corresponds to the Lau Event (Kaljo et al. 1997).


Vertebrate microfossils (scales), conodonts, and chitinozoans have been studied in the same East Baltic cores, permitting comparison of their ranges. The distribution of all three microfossil groups is facies controlled, as was proved by the integrated study of the Ohesaare, Ventspils, and Pavilosta cores (Kaljo et al. 1986). Although many samples lack chitinozoans in the Ludlow strata of the Ohesaare core, this section still offers the best opportunity for comparison of the zonal successions of vertebrates (Marss 1986; Marss & Miller 2004), conodonts (Viira 1982; Viira & Aldridge 1998), and chitinozoans (Fig. 11).


According to Marss (1986, fig. 41), the lowermost Ludlow strata (113.40-124.4 m) are barren or contain only scarce fragments of vertebrates. Upwards the zonal species appear successively: Phlebolepis ornata, P. elegans, Andreolepis hedei, Thelodus sculptilis, and T. admirabilis. In the Ohesaare core two zonal boundaries of vertebrates coincide with those of chitinozoans: (1) Andreolepis hedei, appearing at 99.50 m, correlates with the Eisenackitina lagenomorpha Biozone boundary and (2) T. admirabilis, appearing at 83.10 m, correlates with the Eisenackitina barrandei Biozone boundary. The richest assemblage of vertebrates occurs in the lower part of the Kuressaare Formation, whereas the chitinozoan assemblages are the most diverse and abundant in the lower part of the Torga Formation.

The succession of conodonts in the Ohesaare core was published by Viira (1982). Later, Viira & Aldridge (1998, figs 5, 6) provided more precise distribution of conodont species in the Ludlow of the Ohesaare core. In the Paadla Stage they established the Ozarkodina snajdri snajdri and O. crispa biozones, and the O. snajdri parasnajdri Biozone in the Kuressaare Stage. No direct criteria for correlation of chitinozoan and conodont biostratigraphical units have been observed. The zonal species of conodonts and vertebrates appear more closely tied in the Ohesaare core: O. s. parasnajdri (at 93.40 m) and Thelodus sculptilis (95.15 m), as well as Phlebolepis ornata (112.25 m) and O. s. snajdri (113.35 m).




1. Chitinozoans are well represented in the studied core sections with the highest diversity in the deepest-water Pavilosta core (57 species).

2. In most sections the Wenlock species range into the Angochitina elongata Biozone, except in the Ohesaare core, where a sedimentation gap exists in the early Ludlow.

3. Many chitinozoan species, previously known only from Avalonia (Welsh Basin) and Gondwana (Prague Basin, Palencia), were for the first time identified in Baltic sections.

4. Pterochitina perivelata has its first appearance in the E. lagenomorpha Biozone earlier than previously supposed (Verniers et al. 1995).

5. The Lau Event, recognized in the uppermost part of the Mituva Formation in the Ventspils core (Kaljo et al. 1998), coincides with the lowermost part of the E. barrandei Biozone, characterized by the disappearance of 14 chitinozoan species.

6. Several chitinozoan and vertebrate zonal boundaries coincide in the Ludlow of the Ohesaare core.

Acknowledgements. This study was supported by the target funding project SF 014002008. I am grateful to H. Nestor and R. Einasto for the opportunity to use their unpublished lithological colums and to D. Kaljo and H. Nestor for valuable suggestions on the manuscript. Special thanks to F. Paris and an anonymous reviewer for thorough reviews. I thank also O. Hints and G. Baranov for technical help with the SEM study and photographs.

doi: 10.317/earth.2009.3.02

Received 17 December 2008, accepted 3 March 2009


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