GFF Upper Silurian stratigraphy of Podolia revisited ...

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Upper Silurian stratigraphy of Podolia revisited: carbon isotopes, bentonites and biostratigraphy a

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a

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Dimitri Kaljo , Volodymyr Grytsenko , Toivo Kallaste , Tarmo Kiipli & Tõnu Martma

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Institute of Geology, Tallinn University of Technology, 5 Ehitajate tee, 19086 Tallinn, Estonia; , b

Geological Museum of the State Natural History Museum, Ukraine National Academy of Sciences, 15 B. Khmelnitsky Str., 01030 Kiyv, Ukraine; Published online: 08 Jan 2014.

To cite this article: Dimitri Kaljo, Volodymyr Grytsenko, Toivo Kallaste, Tarmo Kiipli & Tõnu Martma (2014) Upper Silurian stratigraphy of Podolia revisited: carbon isotopes, bentonites and biostratigraphy, GFF, 136:1, 136-141, DOI: 10.1080/11035897.2013.862850 To link to this article: http://dx.doi.org/10.1080/11035897.2013.862850

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GFF, 2014 Vol. 136, No. 1, 136–141, http://dx.doi.org/10.1080/11035897.2013.862850

Article

Upper Silurian stratigraphy of Podolia revisited: carbon isotopes, bentonites and biostratigraphy DIMITRI KALJO1, VOLODYMYR GRYTSENKO2, TOIVO KALLASTE1, TARMO KIIPLI1 and ˜ NU MARTMA1 TO

Downloaded by [Tallinn University of Technology] at 23:29 30 September 2014

Kaljo, D., Grytsenko, V., Kallaste, T., Kiipli, T. & Martma, T., 2014: Upper Silurian stratigraphy of Podolia revisited: carbon isotopes, bentonites and biostratigraphy. GFF, Vol. 136 (Pt. 1, March), pp. 136–141. q Geologiska Fo¨reningen. doi: http://dx.doi.org/10.1080/11035897.2013.862850.

Abstract: Nowadays carbon isotopes and bentonites have become new authorities in stratigraphy besides traditional biostratigraphy. Three positive carbon isotope excursions have been identified in the Upper Silurian of Podolia: the mid-Ludfordian excursion, the late Prˇ´ıdolı´ excursion at the Trubchyn/ Dzvenygorod junction and the SIDE at the Silurian–Devonian boundary. Fourteen bentonite beds are known from this upper Ludlow– Prˇ´ıdolı´ interval. Some of these are better studied and are helpful for correlation of sections. Using the stage slices as a framework, we show that the Prˇ´ıdolı´ can be subdivided into “natural” stage rank units based on the integrated bio-, litho- and chemostratigraphy. Keywords: stratigraphy; bentonites; carbon isotopes; chitinozoans; Upper Silurian; Podolia. 1

Institute of Geology, Tallinn University of Technology, 5 Ehitajate tee, 19086 Tallinn, Estonia; [email protected], [email protected], [email protected], [email protected] 2 Geological Museum of the State Natural History Museum, Ukraine National Academy of Sciences, 15 B. Khmelnitsky Str., 01030 Kiyv, Ukraine; [email protected] Manuscript received 30 May 2013. Revised manuscript accepted 3 November 2013.

Introduction Besides traditional biostratigraphy, carbon isotopes and bentonitic clay interbeds have become new authorities in stratigraphy. When applied in the Ordovician and Silurian carbonate rock areas, biostratigraphy is more oriented on microfossils, chitinozoans and conodonts in particular. The popularity of these trends derives from successful studies performed, but sometimes possibilities are overestimated. The best results are achieved when all three methods are used together. Proceeding from the above, we discuss the Upper Silurian stratigraphy of Podolia based on new material from the Dniester outcrop area and the Kotuzhiny-25 drill core (see Fig. 1 for location). Details of the geological setting and a stratigraphical framework (Fig. 2) of the study area arise from Tsegelnyuk et al. (1983) and include later modifications by Kaljo et al. (2012). Good overviews of the geological setting and some problems are available in several recent papers by a Polish –Ukrainian team (Skompski et al. 2008; Malkowski et al. 2009; Racki et al. 2012). Cramer et al. (2011) reminded the Silurian community that the Prˇ´ıdolı´ is still the only series in global stratigraphy without stage rank subdivisions. At the same time, they made a step forward in the right direction, introducing stage slices of Bergstro¨m type into the Silurian stratigraphy (Fig. 2). Inspired by this approach, we elaborate here the idea about the stage subdivision of the series raised earlier (Kaljo et al. 2012). Using the stage slices as a framework, we try to show that the Prˇ´ıdolı´ can be subdivided based on detailed integrated bio- and chemostratigraphy and

those subdivisions do have a “natural” or, more precisely, an environmental background. For sake of brevity, the results and discussion chapters of this paper are combined under topical items of the text. Table 1 presents carbon and oxygen isotope data and Table 2 presents results of X-ray fluorescence analyses of bentonites and bentonite-like marl interbeds.

Carbon isotopes Three positive carbon isotope excursions (CIEs) have been identified in the Upper Silurian of Podolia (Kaljo et al. 2007, 2012; Malkowski et al. 2009). The first occurs in the upper Ludlow [Isakivtsy and Prygorodok Formations (Fms)]. Upwards, the next excursion follows in the upper Prˇ´ıdolı´ (at the boundary of the Trubchyn and Dzvenygorod Fms) and the SIDE excursion at the Silurian – Devonian boundary (junction of the Dzvenygorod and Khudykivtsy Fms). The first and the third excursions are well known also elsewhere (Cramer et al. 2011). The second excursion is very conspicuous in the Kotuzhiny-25 core, but not in Dniester outcrops and Lithuanian cores. A negative excursion of d13C values (the Post-Prygorodok low) has been noted in the lowermost Prˇ´ıdolı´ (Varnytsya Fm), having a counterpart in the East Baltic, called the Sˇilale low. New data obtained support our earlier observations (Kaljo et al. 2012). Here we publish a new set of analyses (Table 1) that was made in order to confirm a detailed trend of d13C values through the

Zbruch

B

Kaljo et al.: Upper Silurian stratigraphy

POLAND

GFF 136 (2014)

BELARUS RUSSIA KYIV

Lviv

Kotuzhiny-25

Podolia

UKRAINA

L.

MO ROMANIA

A

Ustje-7

Black Sea

k chi

van

Zh

Dnistrove-63 Okopy-46

Prygorodok-107 Trubchyn-65 KHOTIN

r ste


Zhvanets 39

ie Dn

Dzveny gorod West

Ataki-117

Fig. 1. Location and numbering of the studied and mentioned sections according to Tsegelnyuk et al. (1983), except Dzvenygorod West [here subdivided to A (original place) and B (slightly downstream)] and Kotuzhiny-25 defined by Kaljo et al. (2012).

Silurian – Devonian boundary interval in a quarry called by Kaljo et al. (2012, see also GPS data there) Dzvenygorod West on the Dniester River. The new series of samples came from the eastern part (B) of the quarry some 0.5 km downstream. The sampled interval embraces the upper 16.5 m of the Dzvenygorod Fm including bentonite bed C11 and 10 m of the Devonian Khudykivtsy Fm (Fig. 3). The d13C curve is very similar to the

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one known from earlier data (Kaljo et al. 2012). After a negative and variable (between 2 0.05‰ and 2 0.55‰) beginning of the curve, the d13C values turn to a steady rise, forming the SIDE CIE with a peak (not finished) of 3.57‰ at 8 m above the boundary. The smooth rising limb of the excursion testifies to continuous sedimentation through the Silurian – Devonian boundary interval in the open shelf environment of the study area. The facies situation and the corresponding d13C trend in the Kotuzhiny borehole area (Kaljo et al. 2012) are different and indicative of much shallower conditions with possible gaps.

Bentonites Fourteen bentonite beds are known from our study interval (three in the Isakivtsy Fm, six in the Prygorodok Fm, two in the Varnytsya Fm, two in the Trubchyn Fm, one in the Dzvenygorod Fm; Tsegelnyuk et al. 1983). Some of them are better studied (Huff et al. 2000; Kiipli et al. 2000) and help chemostratigraphic correlation of sections. Wishing to check (Kaljo et al. 2012, p. 173) the position of the Post-Prygorodok low at the bottom of the Prˇ´ıdolı´, we analysed bentonite beds C5 and C6 in the topmost Prygorodok Fm (the latter at the boundary with the Varnytsya Fm), C7 in the lowermost Varnytsya Fm and the three uppermost ones by the X-ray fluorescence method. The results of the analyses are presented in Table 2 and correlated with earlier data in Fig. 4. In general, according to the K2O (6.7 – 7.8%) and Al2O3 (18.5 –24.0%) contents, the samples analysed can predominantly be classified as K-bentonites. As an exception, the samples from 200.0 m depth of the Kotuzhiny-25

Fig. 2. A general stratigraphical scheme of the Prˇ´ıdolı´ and neighbouring rocks of Podolia and Estonia with biozonations needed for discussion. Graptolite Bzs for Baltica simplified (the first and the last zone of an interval shown) from Loydell (2012). Chitinozoan zones according to Nestor (2012), conodont and vertebrate ones according to Ma¨rss & Ma¨nnik (2013). L. Dev., Lower Devonian; S.S., stage slices; Lochk., Lochkovian; “M”. uniformis, “Monograptus” uniformis; I. transgrediens, Istrograptus transgrediens; I. samsonowiczi, Istrograptus samsonowiczi; N. lochkovensis, Neocolonograptus lochkovensis; N. ultimus, Neocolonograptus ultimus; N. parultimus, Neocolonograptus parultimus; U. spineus, Uncinatograptus spineus; P. latilobus, Pseudomonoclimacis latilobus; N. kozlowskii, Neocucullograptus kozlowskii; B. praecornutus, Bohemograptus praecornutus; S. leintwardinensis, Saetograptus leintwardinensis; A. lemniscata, Ancyrochitina lemniscata; A. superba, Anthochitina superba; S. lifera, Salopochitina lifera; F. kosovensis, Fungochitina kosovensis; E. barrandei, Eisenackitina barrandei; E. lagenomorpha, Eisenackitina lagenomorpha; A. elongata; O. elegans detorta, Oulodus elegans detorta; O. r. remscheidensis, Ozarkodina remscheidensis remscheidensis; O. r. canadensis, Ozarkodina remscheidensis canadensis; O. r. eosteinhornensis, Ozarkodina remscheidensis eosteinhornensis; O. r. baccata, Ozarkodina remscheidensis baccata; O. s. parasnajdri, Ozarkodina snajdri parasnajdri; O. crispa, Ozarkodina crispa; T. timanica, Trimerolepis timanica; P. punctatus, Poracanthodes punctatus; N. gracilis, Nostolepis gracilis; T. admirabilis, Thelodus admirabilis; T. sculptilis; Thelodus sculptilis; A. hedei, Andreolepis hedei.

138


GFF 136 (2014)


Table 1. Carbon and oxygen isotope data from the Dzvenygorod west B locality. Sample no.

Fms

1 2 3 4 5 6 7 8 9 10 11 12 12A 13 13A 14 15 16 17 18 19 20 21 22 23 24 25 26

Dzvenygorod Dzvenygorod Dzvenygorod Dzvenygorod Dzvenygorod Dzvenygorod Dzvenygorod Dzvenygorod Dzvenygorod Dzvenygorod Dzvenygorod Dzvenygorod Dzvenygorod Dzvenygorod Dzvenygorod Dzvenygorod Dzvenygorod Dzvenygorod Khudykivtsy Khudykivtsy Khudykivtsy Khudykivtsy Khudykivtsy Khudykivtsy Khudykivtsy Khudykivtsy Khudykivtsy Khudykivtsy

Distance (m) Bottom 0 1 3 4 5 5.5 6.5 7.5 8.5 9.5 10.5 11.5 12 13 13.5 14.5 15.5 16.5 17.5 18.5 19.5 20.5 21.5 22.5 23.5 25.5 26.5 27.5

13

18

d C (‰)

d O (‰)

20.27 20.05 20.53 20.55 20.27 20.21 20.29 20.39 20.54 20.24 20.13 0.57 20.02 0.00 0.03 0.44 0.65 1.22 1.67 1.51 1.70 2.13 2.07 2.99 2.63 3.57 2.61 2.81

25.41 25.66 25.93 26.45 26.68 26.24 26.24 25.29 26.47 25.77 26.66 24.62 26.52 27.09 26.72 25.64 26.09 25.93 24.61 25.12 24.99 23.73 23.12 25.27 25.33 24.81 26.00 25.52

Note: Sample numbers range from the bottom upwards.

core and from the Okopy-46 outcrop have a significant content of dolomite and relatively high contents of TiO2 and Cr in their terrigenous parts, thus representing terrigenous interbeds, not bentonites. A small portion of volcanogenic component cannot be excluded in these beds but is not indicated by the available analytical data of bulk samples. For geochemical check of bentonite correlations, the ratios of immobile elements (data in Table 2) TiO2 (%) and Nb, Zr, Th (all in ppm) with Al2O3 (%) were calculated (Fig. 4). In order to bring the ratios numerically close for better visual comparison, the Ti and Zr ratios were modified by coefficients TiO2/Al2O3 £ 50, Zr/Al2O3 £ 0.1, following the approach used in Kiipli et al. (2013). The analyses (Table 2) show that beds C5 and C6 are clearly identifiable, which makes them good markers for the top of the Prygorodok Fm. Despite some differences, their similarity can be ascertained in the Dniester outcrops and the Korneshty core located in Moldova (Huff et al. 2000; Kiipli et al. 2000). A bed, earlier thought to be bentonite C7 in the Kotuzhiny-25 core and Okopy-46 outcrop, is actually a terrigenous marlstone and cannot be correlated with any of the bentonites. The lower sample from a depth of 200 m of the Kotuzhiny core contains 1.7% Sr, referring to a possible occurrence of celestine associating with gypsumbearing rocks. In summary, we may conclude that the bentonite evidence available about the Prygorodok –Varnytsya boundary in the Kotuzhiny core is not reliable, but it does not contradict to the boundary position suggested by Kaljo et al. (2012) either. The composition of beds C9 and C10 in the Trubchyn Fm is rather stable in the Dniester outcrops but variable in some other sections studied. For example, Tsegelnyuk et al. (1983) identified C9 in the Trubchyn Fm, but geochemically it is more likely C10 (Fig. 4). Bed C11 of the Dzvenygorod Fm is well identifiable and can serve as a marker bed. Published and new analytical data indicate a necessity to revise some indexes of bentonites and their correlation in sections outside the Dniester area.

Biostratigraphy The Silurian biostratigraphical data-set of Podolia is rich, especially in the part of macrofossils (Gritsenko et al. 1999). Microfossil data need certain modernisation, but a general framework and several adjustments as shown in papers by Kaljo et al. (2007, 2012), Malkowski et al. (2009) and Racki et al. (2012) seem to reflect a nowadays state of knowledge. In this note, we include only two brief comments on biostratigraphy, which seem most topical. Firstly, Paris & Grahn (1996) identified Eisenackitina barrandei Paris et Krˇ´ızˇ from two localities on the Dniester River banks – one from the Isakivtsy Fm at Zhvanets (locality no. 83 ¼ 39 according to Tsegelnyuk et al. 1983; Fig. 1) and the other from the uppermost Dzvenygorod Fm at Volkovtsy ( ¼ Dnistrove-63 here). This chitinozoan species dates in the Baltic and elsewhere the very end of the Ludlow, forming there an eponymous globally accepted biozone (Bz; Verniers et al. 1995; Nestor 2012). The first occurrence is in good agreement with current age interpretation of the Isakivtsy Fm (Kaljo et al. 2012) but the latter one (repeated also by Racki et al. 2012) strongly contradicts it. These two occurrences are separated in the section by more than 100 m of rocks of the Prygorodok, Varnytsya and Trubchyn Fms and the main part of the Dzvenygorod Fm containing at some levels rich assemblages of biostratigraphically valuable fossils. The corresponding lists of species names are easily available (e.g. Tsegelnyuk et al. 1983; Abushik et al. 1985; Koren et al. 1989; Gritsenko et al. 1999), as well as data on carbon isotopes and bentonites mentioned earlier in the text. The geological realities already mentioned, which are supported by the Prˇ´ıdolı´ carbon isotope curve and well correlated with those of Lithuania (Kaljo et al. 2012), place these beds into the Prˇ´ıdolı´, hinting at another possible explanation for the second occurrence level of E. barrandei in Podolia. Data by Kaljo et al. (2007) indicate that the first appearance datum (FAD) of E. barrandei in the Isakivtsy Fm is located within the rising limb of the Mid-Ludfordian CIE. This observation is important from two aspects: first, it linked the FAD and CIE and second, correlation to Estonia (Fig. 2) refers to free space in the uppermost Ludlow for the upper part of the Prygorodok Fm. The Mid-Ludfordian CIE continues into the lower Prygorodok Fm (in the Zhvanets 39 exposure a peak d13C value 6.6‰ was measured) but before bentonite beds C2 (at Ataki-117 þ Braga-49) or C3 (at Isakivtsy-45, Kaljo et al. 2012) return to the baseline values. This means that the higher beds of the Prygorodok Fm, usually nearly barren of fossils, could remain within the Ludlow and correlate with the E. barrandei Bz in Estonia (Fig. 2), occurring mostly in the Kuressaare Fm above the Mid-Ludfordian CIE. The end of the CIE is slightly differing in some sections (Ataki, Isakivtsy); still, as a rule, CIEs are synchronous but peak values may be a little shifted due to local reasons. Another aspect is that we did not check the indexes used here (C2 and C3). So this correlation seems trustworthy (Fig. 2); however, without fossils a doubt remains. The age discussion concerning the Ludlow –Prˇ´ıdolı´ boundary can also involve some microfossil information available in the Baltic area. Despite differences in taxonomic nomenclature used by Polish –Ukrainian and Baltic specialists (species-level taxa are easily recognisable), conodont data are most expressive but allow different interpretations. Based on studies by Drygant (1984), reporting occurrences of Ozarkodina crispa in the lower

,5 181 ,20 47 16 7 30 9 20 8 78 ,5 88 9 49 15 31 97

,5 2044 ,20 15 13 ,5 23 2 6 5

As Ba Ce Cr Cu Ga La Nb Ni Pb Rb Sc Sr Th V Y Zn Zr

Note: Comments in the text and in Fig. 4.

11

16

,5 17789

28.7 26.9 0.389 8.2 2.61 0.072 12.71 18.27 0.04 2.63 0.05 0.02 0.09 0.10

C-7? Terrigeneous

37.7 12.1 0.089 2.1 1.02 0.071 16.19 25.79 0.03 0.72 0.02 0.04 0.71 0.04

C-7? Terrigeneous

Kotuzhiny-25, depth 200.0 m (upper)

LOI SiO2 TiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O P2O5 Cl S F

Index Lithology

Kotuzhiny-25, depth 200.0 m (lower)

16 91 42 11 7 16 37 32 9 38 134 ,5 198 51 18 23 31 246

7.2 53.5 0.338 22.3 2.08 0.007 4.31 1.45 0.06 7.28 0.05 0.01 0.43 0.37

C-9 Bentonite

Kotuzhiny-25, depth 135.8 m Dnistrove-63

C-9/10? C-9? Bentonite Bentonite Major components in % 5.8 10.5 53.9 48.2 0.170 0.351 18.7 22.2 1.52 4.46 0.002 0.009 6.04 3.79 0.52 6.73 0.01 0.03 7.76 6.82 0.03 0.05 0.01 0.00 0.00 0.01 0.61 0.29 Trace elements in ppm ,5 63 77 82 ,20 25 12 21 4 20 28 20 45 25 20 18 2 21 6 78 240 191 ,5 10 29 69 18 39 8 22 22 22 11 33 165 350

Trubchin-65

Table 2. Results of X-ray fluorescence analyses of bentonites and bentonite-like marl interbeds.

6 95 ,20 11 20 19 ,20 24 9 7 202 5 28 44 30 22 15 253

6.5 51.6 0.396 24.0 2.29 0.010 3.83 1.86 0.03 7.65 0.09 0.00 0.03 0.26

C-11 Bentonite

Dzvenigorod West A

7 121 ,20 16 28 18 ,20 23 10 13 194 5 44 41 26 25 17 246

7.9 50.6 0.393 23.6 2.57 0.014 3.89 3.12 0.03 7.35 0.09 0.01 0.01 0.25

C-11 Bentonite

Dzvenigorod West B


,5 738 32 74 8 11 ,20 10 20 23 99 ,5 307 6 49 22 33 103

24.6 31.0 0.455 9.3 2.24 0.028 6.51 21.73 0.24 2.90 0.10 0.02 0.05 0.13

C-7? Terrigeneous

Okopy-46

8 469 ,20 ,10 22 20 21 19 11 22 190 6 71 30 22 25 15 266

8.1 49.9 0.382 18.5 2.12 0.006 4.60 4.28 0.03 7.76 0.09 0.01 0.02 0.40

C-6 Bentonite

Ataki-117

,5 129 ,20 ,10 20 20 ,20 27 10 7 206 ,5 69 17 20 26 10 165

8.7 51.0 0.193 21.3 1.33 0.003 4.80 3.96 0.01 6.67 0.01 0.00 0.01 0.42

C-5 Bentonite

Ataki-117

GFF 136 (2014) Kaljo et al.: Upper Silurian stratigraphy 139

140


LOCHKOVIAN

KHUDYKIVTSY

PRÍDOLÍ

DZVENYGOROD

SILURIAN


DEVONIAN

DZVENYGORODWEST B –1

13

δ 0

1

GFF 136 (2014)

C, ‰ 2

3

4

SIDE exc.

10m C11

0

bentonite bed marl with lime ston nodules

limestone marl

Fig. 3. Carbon isotope curve from the Dzvenygorod West B locality. Data for d13C and d18O are presented in Table 1.

part of the Varnytsya Fm, Skompski et al. (2008) and Racki et al. (2012) assigned these beds to the Ludlow, but the upper part of the Varnytsya Fm and the Trubchyn and Dzvenygorod Fms, all containing Parazieglerodina eosteinhornensis (see also Koren et al. 1989) to the Prˇ´ıdolı´. We are not discussing here the Silurian – Devonian boundary, marked in Podolia by the appearance of Ozarkodina remscheidensis remscheidensis (Racki et al. 2012) that occurs in the East Baltic (Fig. 2) together with Oulodus elegans detortus in the uppermost Prˇ´ıdolı´ (Ohesaare Stage; Viira 2000; Kaljo et al. 2012). Ozarkodina remscheidensis eosteinhornensis is mostly linked to the lower part of the Prˇ´ıdolı´ (Kaugatuma Stage) but has rarely been reported also from the uppermost Ludlow, as well as O. crispa from the lowermost Prˇ´ıdolı´ (Viira 1999). Those differences in ranges seem to be caused, at least partly, by environmental conditions and therefore a many-sided interpretation of biostratigraphical data should be preferred. Secondly, commenting on an undivided series, the senior author should confess that he disliked such a unit from the very beginning. At the time of commission discussions on the Silurian subdivisions, there was only one conodont Bz in the whole Prˇ´ıdolı´ – the O. remscheidensis eosteinhornensis Bz as though supporting the homogeneity of the series (Aldridge & Scho¨nlaub 1989). This was the situation 30 years ago at the geological congress in Moscow where the Silurian subcommission’s (SSS) decisions were ratified, but rather soon diverging views got published, e.g. by Viira (1999). She showed that in Baltic sections, the eosteinhornensis Bz could be subdivided into four subzones using the following Ozarkodina remscheidensis subspecies: eosteinhornensis and canadensis in the Kaugatuma

Fig. 4. Bentonite correlation based on similarities of four element ratios with aluminium oxide (see text for explanation and Table 2 for data). Data for Ustje-7 and Prygorodok-107 are taken from Kiipli et al. (2000) and for C6 Ataki-117 (on the left) from Huff et al. (2000).

Stage and remscheidensis and O. elegans detortus in the Ohesaare Stage (Fig. 2). Several authors today treat those as independent species (Racki et al. 2012). More than one Bz unit in the Prˇ´ıdolı´ has been used also for graptolites (cf. Table 1 by Urbanek & Teller 1997) and chitinozoans (Verniers et al. 1995; Nestor 2012), suggesting different possibilities for subdivision of the series (Fig. 2) and showing some linking points with stage slices by Cramer et al. (2011). Two or three Bz-type units have been suggested for ostracodes and vertebrates, partly rising from the so-called Beyrichien-Kalk faunas described originally from erratics on the southern coasts of the Baltic Sea (north of Poland and Germany, Martinsson 1977). Later studies (e.g. Siveter 1989; Ma¨rss 1997) have confirmed a great potential of these and some other groups for biostratigraphy of the uppermost Silurian rocks, especially

GFF 136 (2014) when different groups are treated in an integrated manner (Ma¨rss & Ma¨nnik 2013) and sections of different facies origin should be correlated. We see here a real geological content for two stage slices of the Prˇ´ıdolı´ suggested by Cramer et al. (2011) and even more – for two normal (global) stages, of course, after a detailed study of different aspects of the geosystem discussed. Western margins of the Baltica palaeocontinent show great potential for search of a stratotype and GSSP section. As candidates from different facies realms, we can mention graptolitic sections in Poland (an easy level to fix Pr1/Pr2) with good possibilities of correlations to shelly faunal sections in the East Baltic and Podolia. For the last two areas, a level close to the bottom of the Ohesaare and Dzvenygorod Fms seems promising even if different details are still unclear.


Conclusions Our combined bio- and chemostratigraphical data show that the Isakivtsy and Prygorodok Fms are of latest Ludlow age. The Post-Prygorodok bentonite located below bentonite bed C7, belongs to the lowest Prˇ´ıdolı´ but in some sections might embrace some parts of the Ludlow. The upper Prˇ´ıdolı´ d13C excursion and bentonite C11 predate the occurrences of E. barrandei in the uppermost Dzvenygorod Fm. These seem to represent a curious case of Lazarus-type occurrence in the Dnistrove outcrop or a simple error. Recent developments in bio- and chemostratigraphy of the Prˇ´ıdolı´ in the Baltic and Podolia show that all needed qualities do exist in these areas in order to fulfil a gap in the global classification of the Silurian System. Acknowledgements – The authors are grateful to both reviewers E. Jarochowska and M. Melchin for useful comments that considerably aided to improve the manuscript. We thank G. Baranov for help with computer graphics and A. Noor for linguistic corrections. The study was partly supported by the Estonian Research Council (projects SF0140020s08, SF140016s09 and grant ETF8182) and the Ukrainian National Academy of Sciences (project 0112U001459), connected with an Agreement about co-operation of the Ukrainian and Estonian academies of sciences. This report is a contribution to the IGCP project 591.

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