Silurian bentonites in Lithuania : correlations based on sanidine phenocryst composition and graptolite biozonation interpretation of volcanic source regions

Integrated correlation of bentonites (altered volcanic ashes) and graptolite biozonation is presented. Detailed study of two Lithuanian drill core sections extended previous knowledge of the occurrence and composition of bentonites to the south. Identification of graptolite species allowed bentonites to be assigned their proper stratigraphical position. Silurian bentonites in Lithuania are mostly characterized by wide and very wide XRD 201 reflections of the main component of sanidine phenocrysts. Only fourteen of the 69 samples studied contained sanidine with a sharp reflection, which gave the best correlation potential. In the Lithuanian sections one bentonite was found in the Rhuddanian, five bentonites were recognized in the Aeronian, 17 bentonites in the Telychian, 26 in the Sheinwoodian, 10 in the Homerian and six in the Ludlow. All bentonites found in Lithuania are characterized by the main component of sanidine. A large number of Lithuanian bentonites are not known in Latvia and Estonia, indicating that volcanic ashes reached the East Baltic area from two source regions  the Central European and Norwegian Caledonides.


INTRODUCTION
Volcanic ashes occurring in palaeontologically wellcharacterized sedimentary sections offer a unique possibility of detailed correlations between different environments (Kiipli & Kallaste 2002;Kiipli et al. 2006Kiipli et al. , 2008aKiipli et al. , 2009Kiipli et al. , 2010bKiipli et al. , 2011Kiipli et al. , 2012aKiipli et al. , 2012b)).The mentioned studies used magmatic sanidine composition of bentonites (altered volcanic ashes) analysed by X-ray diffractometry as a correlation criterion.The good precision of that method, up to ± 1% in favourable cases (Kiipli et al. 2011), enables discrimination of bentonites of quite similar composition.This is an important property, because successive eruptions from the same volcanic source can be characterized by very similar geochemical and mineralogical signatures (Kiipli et al. 2010a).Other methods, e.g.palaeontological, used in combination with sanidine composition of bentonites, provide the most reliable correlation.A negative aspect of using sanidine composition for fingerprinting bentonites is that magmatic sanidine is unstable at Earth's surface temperatures.Elevated diagenetic or metamorphic temperatures accelerate recrystallization significantly and Early Palaeozoic sanidine is often not preserved in some other regions.Sanidine may also be absent in source magma.To avoid these shortcomings, researchers have used the composition of apatite phenocrysts (Batchelor 2009;Carey et al. 2009;Ray et al. 2011) or trace elements for proving correlations (Huff et al. 1998;Kiipli et al. 2008bKiipli et al. , 2013a)); Inanli et al. 2009;Hetherington et al. 2011.Up to now we have studied mostly Estonian and Latvian sections (Kiipli & Kallaste 2006;Kiipli et al. 2010aKiipli et al. , 2011Kiipli et al. , 2012a)).Some Silurian bentonites of Lithuania have been studied by Kiipli et al. (2008c) using the same methods.The occurrence of bentonites in the Silurian of Lithuania is also described in Lapinskas (1965Lapinskas ( , 2000) ) and Motuza et al. (2002).The areal distribution schemes of ca 20 bentonites from Estonia, Latvia and Scandinavia indicate volcanic sources from the Iapetus Palaeo-Ocean in the Telychian and Lower Sheinwoodian (Kiipli et al. 2008b(Kiipli et al. , 2008d(Kiipli et al. , 2010b(Kiipli et al. , 2012a(Kiipli et al. , 2013a)).Several other researchers discuss a possible Central European source for ash beds (Batchelor & Jeppsson 1999;Böhnke & Katzung 2001;Hetherington et al. 2011).Thick bentonites and massive volcanic rocks of Silurian age are known from Belgium (Andre et al. 1986) and Poland (Timmerman 2008).So, according to the geological data, two source regions, the Central European and Norwegian Caledonides, were possible and Lithuania is in a key position for discriminating between these sources.The distribution of graptolites in Lithuanian sections has been studied by Paškevičius (1982Paškevičius ( , 1997)), Radzevičius (2006), Radzevičius & Paškevičius (2000, 2005) and Radzevičius et al. (2008).The correlation of graptolite biozones applied in different regions of the world is given in Loydell (2012).We studied in detail two Lithuanian drill core sections.Our aim was to extend bentonite correlations to the south and better recognize volcanic sources from the margins of the Iapetus and Rheic Palaeo-Oceans.

MATERIAL AND METHODS
Forty-six bentonite samples were collected from the Šiupyliai-69 and Kurtuvėnai-166 drill cores (Fig. 1).Twenty-three bentonites from other Lithuanian cores, seven of which have been published in Kiipli et al. (2008c), are also included in the present report (Table 1).Bentonites in predominantly grey Silurian shales were recognized by their yellowish or bluish colour.They are very thin, mostly only a few millimetres and rarely up to a few centimetres thick.
Biostratigraphy is based on over 200 rock samples taken from the Llandovery-Ludlow interval of the Viduklė-61 (Kiipli et al. 2008c), Šiupyliai-69 and Kurtuvėnai-161 (Radzevičius & Paškevičius 2000, 2005;Motuza et al. 2002;Kaminskas et al. 2006) drill cores for the study of graptolites.Sampling density increased near the ash layers.Rhabdosomes were extracted from samples using HCl or HF acids.Graptolites were examined under a light microscope in Vilnius University.
To identify major minerals in the sampled interbeds of supposed volcanic origin, bulk samples were analysed by X-ray diffractometry (XRD).An association of illitesmectite and kaolinite as major minerals has been considered to indicate the volcanic origin of the interbeds (Kiipli & Kallaste 2002;Hints et al. 2008).Authigenic K-feldspar forms a significant portion of some bentonites.Host shales are composed of a different association of terrigenous minerals, including illite, quartz, chlorite and minor K-feldspar.
Magmatic sanidine phenocrysts (K,Na,Ca)AlSi 3 O 8 were analysed in coarse fractions (0.04-0.1 mm) separated from 2 g of bentonite using the method described in detail in Kiipli et al. (2011).Various examples of sanidine XRD spectra are published in Kiipli & Kallaste (2006) and Kiipli et al. (2006Kiipli et al. ( , 2010a)).All measured XRD spectra of sanidine in Lithuanian bentonites are available in the collections database of the Institute of Geology at Tallinn University of Technology at http://sarv.gi.ee/reference.php?id=2544.

Bulk bentonite mineralogy
Kaolinite is a common major component besides illitesmectite, whereas K-feldspar is relatively rare and occurs in lower concentrations.Pyroclastic quartz is present in almost all bentonites in ca 1% concentration.Reflections of anatase appear on XRD patterns starting from TiO 2 concentrations of ca 1%.Authigenic pyrite and its weathering products gypsum and jarosite are frequent in bentonites.Complete weathering of pyrite in bentonites and exceptional abundance of gypsum and jarosite is remarkable in the Šiupyliai-69 section.This feature facilitates separation of the pyroclastic material, as the grain fraction does not require treatment with nitric acid for removal of pyrite.

Sanidine composition
Silurian bentonites of Lithuanian cores are mostly characterized by wide and very wide XRD 20 1 reflections of the main sanidine component (Table 1, Fig. 2).Only 14 of the 59 samples studied contain sanidine with sharp reflection having the best correlation value.Besides the sanidine main component, also other less abundant components are present in samples.These are mostly more potassic than the main component, but sometimes also more sodic like in the Y-bentonite known from Gotland (Kiipli et al. 2008a).The combination of the main and less frequent components often forms a unique shape of the 20 1 reflection, which can be used for identification of the eruption layers.

Graptolites
Graptolite studies started in Lithuania in the middle of the 20th century (Paškevičius & Radzevičius 2006).Earlier overall results of graptolite biozonation are available in Paškevičius (1982Paškevičius ( , 1997)).Later research has focused on certain intervals, e.g. the Wenlock-Ludlow boundary (Radzevičius 2006;Radzevičius et al. 2008).New data allowed revision of the Silurian graptolite biostratigraphy of Lithuania (see Fig. 3).However, the graptolite strati- Another problematic interval, which should further be investigated in Lithuania, is the upper part of the lapworthi Biozone in the upper Llandovery.According to Loydell (2012), the Cyrtograptus insectus Biozone occurs between the lapworthi and centrifugus biozones.The insectus Biozone has not been defined in Lithuania, because no findings of Cyrtograptus insectus Bouček are known in that region and therefore this biozone is tentatively included into the lapworthi Biozone.The insectus Biozone is distinguished in the Kaliningrad district (Suyarkova 2012).
The third problematic interval is near the Gorstian-Ludfordian boundary.The interval from the upper boundary of the scanicus Biozone to the lower boundary of the balticus Biozone has been called the Pseudomonoclimacis tauragensis Biozone (Paškevičius 1997).The base of the tauragensis Biozone is marked by the appearance of P. tauragensis (Paškevičius).The identification of this biozone is not very reliable, because (1) Pseudomonoclimacis tauragensis has long biostratigraphical range, (2) P. tauragensis is hardly distinguishable from P. haupti (Kühne), (3) it is impossible to trace the Gorstian and Ludfordian boundary using P. tauragensis, (4) there exist better biozonal index species such as Saetograptus leintwardinensis, S. linearis, Bohemograptus bohemicus tenuis and others with short biostratigraphical ranges.So, this interval as well needs detailed study in future.

Stratigraphical distribution and correlation of bentonites
Only one bentonite has been found in the Rhuddanian of Lithuania and another is known from the Ohesaare core in Estonia (Kiipli & Kallaste 1996).The Aeronian, however, shows signs of remarkable volcanic activity.Five bentonites were established in the Aeronian part of the Dobele Formation in Lithuania.Most of these bentonites are characterized by a wide or weak sanidine reflection (Figs 2, 4, Table 1), while in some bentonites sanidine is absent at all.This type of sanidine does not allow well-proved correlations by the XRD method.The ca 1 cm thick Geniai Tuff, characterized by an unusually high content of REE elements up to 3%, occurs near the Aeronian-Telychian boundary in the Geniai-1 core (Kiipli et al. 2012b).
Seventeen bentonites are found in the Telychian in Lithuanian sections.This number is significantly less than in Latvia and Estonia where bentonites from ca 50 eruptions have been described and correlated (Kiipli et al. 2010b).Two bentonites from the Kurtuvėnai-166 core (depths 971.2 and 971.7 m) may also belong to the upper Telychian.However, as graptolite finds from this level are not sufficiently complete and geochemical data allow correlations with lower Sheinwoodian bentonites, we attributed these layers provisionally to the Sheinwoodian.On the basis of the characteristic XRD reflections of the sanidine and graptolite zonation, three Lithuanian bentonites can be correlated with Estonian and Lithuanian ones (Figs 2, 4, Table 1).These include the ash bed at a depth of 1466.7 m in the Kurtuvėnai-166 core correlating with the Osmundsberg Bentonite.The bentonite at 992.5 m in the Kurtuvėnai-161 core interestingly reveals two sharp sanidine reflections, hinting at the mixture of two eruptions.These eruptions can be correlated with 842.0 and 842.1 m bentonites in the Ventspils-D3 core.Sanidine in other bentonites is characterized by wide or weak XRD reflections or shows no reflection at all.
Twenty-six bentonites, representing 22 eruption layers, were found in the Sheinwoodian part of the section in Lithuania.Six of these bentonites were already described in Kiipli et al. (2008c), but are included also in Table 1.The volcanic record starts with three bentonites (Virbalis-5, 1122.4 m; Kurtuvėnai-166, 971.7 m; Kurtuvėnai-166, 971.2 m), which can be provisionally correlated with the Lusklint, Storbrut and Ireviken bentonites in Gotland, Latvia and Estonia.Finds of graptolites are not conclusive at this level, therefore these correlations are not finally proved.
The riccartonensis to belophorus graptolite biozones include mostly bentonites with wide and very wide sanidine reflections and correlations are therefore often provisional (Fig. 5).Still, very wide reflections can be differentiated from wide reflections with certainty and used for correlations.Some of Lithuanian bentonites contain a specific type of sanidine giving a wide reflection with the modal value of the Na + Ca component between 26 and 30 mol%.Some bentonites in Lithuania (e.g. at 944.5 and 946.7 m in the Kurtuvėnai-166 core) revealed a sharp sanidine reflection and can be correlated with bentonites in Latvia and Estonia (Fig. 5).The Šiupyliai-69 1107.0 m bentonite in the antennularius Biozone showed a sharp reflection with 28 mol% of the Na + Ca component and can be correlated with the Vattenfallet Bentonite from the exposure in the town of Visby on Gotland (Kiipli et al. 2008a).
Only four bentonites were found in the upper part of the Sheinwoodian in Lithuania.Among these the Šiupyliai-69 1076.0 m bentonite can be correlated with Latvian sections according to the sharp sanidine reflection and Na + Ca component of 30.4 mol%.The Ventspils-D3 and Vidale-263 sections of Latvia contain a large number of bentonites at this level (Kiipli et al. 2010a), while in Estonian sections bentonites are again relatively rare in this interval.Y-bentonite from the Slite Formation of Gotland (Batchelor & Jeppsson 1999) can be correlated by sanidine of specific shape (Kiipli et al. 2008a(Kiipli et al. , 2013a) ) with bentonites in Estonian (Ohesaare, 275.3 m) and Latvian (Ventspils-D3, 736.4 m) sections (Fig. 5).
A 0.1 cm thick bentonite with a very strong and sharp sanidine reflection and the Na + Ca component 23.4 mol% was found in the lower part of the lundgreni Biozone at the beginning of the Homerian Stage at a depth of 1052.0 m in the Šiupyliai-69 section.This very distinctive sanidine composition was analysed also in the Kurtuvėnai-166 924.8 m (0.5 cm thick), Ohesaare 215.7 m (6.0 cm) and Ruhnu 337.5 m (0.2 cm) bentonites, indicating correlation close to the lower boundary of the lundgreni Biozone.In total, nine eruption layers were discovered in the lundgreni Biozone in Lithuania (Fig. 5).Nine bentonites have also been recognized in Latvia, partly correlating with the Lithuanian ash beds, and six bentonites are known from the Ohesaare core in Estonia.Considering correlations, up to 18 eruption layers can be counted in the lundgreni graptolite Biozone.Sanidines in these bentonites form two clusters according to the XRD reflections: (1) with sharp reflections and the Na + Ca component between 31 and 36 mol% and (2) with wide reflections with the Na + Ca component 43-56 mol%.Several well-proved correlations occur between Lithuanian and Latvian sections (Fig. 5).Higher in the Homerian, in the interval from parvus to ludensis graptolite biozones, volcanic ashes were not found in the Kurtuvėnai-166 and Šiupyliai-69 sections.
The Ludlow part has not been studied in detail in Lithuanian sections.We collected five bentonite samples, one of which, from 707.7 m in the Pilviškiai-142 section, belongs to the upper scanicus Biozone, possibly correlating with the Ventspils-D3 603.0 m bentonite.

DISCUSSION
The number of Aeronian bentonites increases to the southwest from the East Baltic area.For example, Bergström et al. (1999) recorded ten bentonites in the Röstanga core, southern Sweden, and Bjerreskov (1975) found eight bentonites in Bornholm.This refers to volcanic sources in the Central European Caledonides from the collision zone of the Avalonia and Baltica plates (Kiipli et al. 2013a(Kiipli et al. , 2013b)).A large number of Rhuddanian and Aeronian bentonites are known from the Southern Uplands of Scotland (Batchelor & Weir 1988).This area was in the Laurentian side of the Iapetus Ocean, indicating subduction of the ocean floor and volcanism near the Laurentian margin.Wide sanidine reflections in Telychian bentonites probably indicate other volcanic sources than for ash beds of that age in Estonia and Latvia, many of which are characterized by sharp sanidine XRD reflections.Central European sources from the collision zone of Avalonia and Baltica can be supposed.This result confirms that Telychian volcanic ashes, forming a number of the Estonian-Latvian bentonites which are not found in Lithuania, reached the East Baltic area from the northwestern direction (present-day orientation) -from the margins of the Iapetus Ocean (Kiipli et al. 2008b(Kiipli et al. , 2008c(Kiipli et al. , 2010b(Kiipli et al. , 2012a(Kiipli et al. , 2013a)).Scarcity of bentonites in the upper Homerian and Lower Ludlow of Lithuania and their frequent occurrence in Latvian and Estonian sections indicate a source from the northwest in terms of presentday orientation.
The Šiupyliai-69 1052.0 m bentonite with very distinctive sanidine composition can be used as a marker horizon for tracing the lower boundary of the lundgreni Biozone.In the Ohesaare section this bentonite belongs to the upper part of the Jamaja Formation and in the Ruhnu section to the lower part of the Sõrve Formation (Kiipli & Kallaste 2006) in the middle of the Conochitina subcyatha chitinozoan Biozone (Kiipli et al. 2010a).This result confirms the correlation by Nestor (1994) where the lower boundary of the testis graptolite Biozone (roughly corresponding to the lundgreni graptolite Biozone) was correlated with the middle of the C. subcyatha chitinozoan Biozone.We propose the stratigraphic name 'the Šiupyliai Bentonite' for this marker horizon.This bentonite has not been found in the Ventspils-D3 and Vidale-263 cores, possibly because the sea current (Kiipli et al. 2012a(Kiipli et al. , 2013a) ) has washed small ash falls away.
The Grötlingbo Bentonite occurs within the Mulde Event in the Homerian Stage (Calner et al. 2006;Dahlquist et al. 2012).Previously we studied a 7 cm thick bentonite in the parvus Biozone from the Viduklė-61 drill core (depth 1308 m) and correlated it with the Grötlingbo Bentonite (Kiipli et al. 2008c;Dahlquist et al. 2012).Now doubts have risen about that correlation.This bentonite occurs closely above the lithologically easily recognizable Ančia Member (finely laminated limestone), e.g. in the parvus graptolite Zone and within the lower carbon isotope positive excursion.The only unexplained fact is that the bentonite at 1308.0 m in the Viduklė-61 core did not contain biotite, which is typically abundant in the Grötlingbo Bentonite in other locations (Dahlquist et al. 2012).This correlation was followed by Cramer et al. (2012).Later Kiipli et al. (2010c) assigned the 10 cm thick bentonite, occurring 10 m above the Ančia Beds in the Priekule core, within the second carbon isotope positive excursion in the nassa graptolite Zone, to the Grötlingbo Bentonite.The reasons were significant thickness of the ash bed and abundance of biotite mentioned in core description.Kiipli et al. (2011) provisionally correlated the Grötlingbo Bentonite with the biotite-rich bentonite in the Ventspils-D3 core, 8 m higher than the Ančia Beds in the nassa Biozone.Thus, there are two ways of correlating the Grötlingbo Bentonite with graptolite zonation.

SUMMARY AND CONCLUSIONS
Study of Silurian bentonites in Lithuanian sections enabled us to extend correlations to the south.A wider study area allows better discrimination of volcanic sources from the Laurentia−Baltica and Avalonia−Baltica collision zones.Volcanic sources from the Avalonia−Baltica collision were significant in the Aeronian.In the Telychian this zone yielded ashes with a wide sanidine reflection to Lithuania, but the northern and central Baltic area was reached by a number of ashes from the Laurentia−Baltica collision area.Both sources were active also in the Sheinwoodian, but in the early Homerian the main source direction shifted to the west or northwest (in present-day orientation).Only a northwestern source can be recognized in the late Homerian and Early Ludlow.Volcanic activity almost ended in the Late Ludlow.Only a few ash beds are known in the latest Silurian of Lithuania, indicating again a possible southern volcanic source.
Correlation of the 1107.0m bentonite of the Šiupyliai-69 core in the antennularius Biozone with the Vattenfallet Bentonite on Gotland shows correlation of the Högklint Formation on Gotland with the antennularius Zone.At the lower boundary of the Homerian Stage (lower lundgreni Zone) a new marker layer, the Šiupyliai Bentonite, indicates correlation with the Jamaja (Ohesaare core) and Sõrve (Ruhnu core) formations in Estonia.This integrated bio-and chemostratigraphical study can serve as reference in future correlation works in the region.

Fig. 3 .
Fig. 3. List of Lithuanian graptolite biozones mentioned in text and in figures.