Correlation of upper Llandovery lower Wenlock bentonites in the När ( Gotland , Sweden ) and Ventspils ( Latvia ) drill cores : role of volcanic ash clouds and shelf sea currents in determining areal distribution of bentonite

Study of volcanic ash beds using biostratigraphy, sanidine composition and immobile elements within bentonites has manifested several well-established and some provisional correlations between Gotland and East Baltic sections. Energy dispersive X-ray fluorescence microanalysis of phenocrysts has revealed bentonites containing Mg-rich or Fe-rich biotite. Sanidine phenocrysts contain, in addition to a major Na and K component, often a few per cent of Ca and Ba. On the basis of new correlations the mapping of the distribution areas of bentonites has been extended from the East Baltic to Gotland. The bentonite distribution can be separated into two parts in North LatviaSouth Estonia, indicating the existence of shelf sea currents in the Baltic Silurian Basin.


INTRODUCTION
Volcanic ash beds in sedimentary sections have been used as time markers for refining stratigraphy (Rubel et al. 2007;Kiipli et al. 2011).Sometimes large eruptions have caused significant environmental changes followed by extinction of marine biota (Hints et al. 2003).Thin altered volcanic ashes (bentonites, K-bentonites), providing evidence for volcanism at nearby plate margins, have been detected in Baltoscandia, from the lower Silurian (e.g.Bergström et al. 1992;Batchelor et al. 1995;Batchelor & Jeppsson 1999;Hetherington et al. 2011) to the lower part of the upper Silurian (Snäll 1977).Many Silurian bentonites have been recorded also in England (e.g.Ray 2007;Ray et al. 2011) and Scotland (e.g.Batchelor & Weir 1988;Batchelor 2009).In the East Baltic area ash beds from a total of 51 volcanic eruptions have been identified in the Telychian (Kiipli et al. 2008b(Kiipli et al. , 2008c(Kiipli et al. , 2008d(Kiipli et al. , 2010b) ) and 55 in the Wenlock (Kiipli et al. 2010a).Several ash beds are known in the lower Ludlow (Kiipli et al. 2011), but these are very rare in younger sedimentary rocks.
With the aim of extending the correlation of volcanic ash layers, we have studied the Llandovery-lower Wenlock of the Ventspils-D3 core section of Latvia and the När core section of Gotland (Sweden).New correlations are used for interpreting ash clouds and shelf sea currents in the Silurian Baltic Basin.

MATERIAL AND METHODS
Thirty-one bentonite samples were taken from the Llandovery and lower Wenlock of the Ventspils-D3 and När cores (Fig. 1).The thickness of the ash beds varies from 1 mm to 4 cm, which is generally less than in Estonia, where ash beds frequently reach a thickness of 5-10 cm.An additional complication with the När core was that as a result of previous studies only half of the core was preserved and we were able to find only some of the ash beds recorded by Snäll (1977) in the fresh core.The stratigraphical position of the ash beds was established using chitinozoan and graptolite biozonation (Gailite et al. 1987;Nestor 1994;Grahn 1995;Loydell & Nestor 2005; Fig. 2).The composition of sanidine phenocrysts in bentonites was used for correlation with the biostratigraphically well studied Aizpute-41 section (Loydell et al. 2003).The samples were analysed by X-ray diffractometry (XRD) for identifying major minerals and determination of magmatic sanidine phenocryst composition.The XRD spectra of sanidine in Telychian and lower Sheinwoodian bentonites are available online at http://sarv.gi.ee/reference.php?id=2533.The authors have applied the same methods in their previous works (Kiipli & Kallaste 2002, 2006;Kiipli et al. 2010aKiipli et al. , 2011)).The composition of volcanic phenocrysts in the bentonites of the När section was studied also by energy dispersive X-ray fluorescence (EDS) microanalysis in five samples.Samples of sufficient size (at least 2 g) were subjected to standard X-ray fluorescence (XRF) analysis for major and trace elements.The results are available in the database at http://geokogud.info/git/reference.php?id=1586 (Kiipli et al. 2011).Table 1 describes the concentrations of immobile and other useful elements for chemical fingerprinting as follows: high concentration of elements -Al 2 O 3 > 26% and TiO 2 > 1.2%; Zr > 510, Nb > 37, Th > 40, Sr > 190, Ba > 320, La > 48, Ce > 140 and Y > 50 ppm; low concentrations of elements -Al 2 O 3 < 20% and TiO 2 < 0.66%; Zr < 320, Nb < 22, Th < 29, Sr < 110, Ba < 190, La < 18, Ce < 35 and Y < 27 ppm.These values were derived by dividing all available analyses of bentonites from the Baltic Silurian deep shelf area into three equal groups -high, average and low concentrations for each element.Geochemical analyses were carried out in the Institute of Geology at Tallinn University of Technology.

Major minerals in altered volcanic ashes
Bentonites in Estonia are composed mainly of highly illitic illite-smectite and authigenic potassium feldspar (Kiipli et al. 2008b), with kaolinite present only in the sections near the southern border.In Latvia and Gotland, however, kaolinite is a common major component in addition to illite-smectite, while K-feldspar is relatively rare and occurs in lower concentrations (Table 1).This areal difference has been studied in the Ordovician Kinnekulle ash bed (Kiipli et al. 2007) and probably originates from differences in sedimentary facies.During the Early Palaeozoic Era, Latvia and Gotland were located on the deep shelf and Estonia mainly in the shallow shelf area.By contrast, Hints et al. (2008) proposed a late diagenetic origin for potassium feldspar in bentonites.Kaolinite-rich ash beds have lost much more silica and other major components during the conversion of ash to clay than illite-smectite-and feldspar-rich bentonites in Estonia (Kiipli et al. 2006).Consequently, the expected concentrations of immobile elements used for chemical fingerprinting are significantly higher in Latvia and Gotland than in the correlative beds in Estonia (Kiipli et al. 2008d).Ratios of immobile elements can still be used for chemical identification of eruption layers.Authigenic pyrite and its weathering products (gypsum and jarosite) in drill core boxes are frequent in bentonite beds.Pyroclastic quartz occurs in almost all bentonites in a concentration of ca 1%.Reflections of anatase appear on XRD patterns, starting from TiO 2 concentrations of ca 1%.Snäll (1977) recorded nine thin bentonites in the När core within the interval 363.0-371.8m containing Conochitina iklaensis, which Grahn (1995) assigned to the lower Aeronian Coronograptus gregarius graptolite Biozone (= Demirastrites triangulatus-Pribylograptus leptotheca biozones).Due to the poor state of the core, we found material from only two bentonites.The attempts to analyse sanidine composition revealed no XRD reflection (369.8 m) or only a weak reflection (367.8 m).Wide sanidine reflections were established in two bentonites from the Dobele Formation of the Aizpute-41 section.No ash bed correlations can be suggested on the basis of the data available at present.

Bentonites in the Spirograptus turriculatus-Streptograptus crispus Biozone interval (Telychian, Llandovery)
In terms of chitinozoan biozonation, this interval belongs to the lower part of the Eisenackitina dolioliformis Biozone.Eight bentonites were found in the Ventspils _____________________________________________________________________________________ Fig. 2. Correlation of the Llandovery-lower Wenlock sections between Gotland (Sweden) and Latvia.Biostratigraphy of the När core is from Grahn (1995), Aizpute from Loydell et al. (2003) and Ventspils from Nestor (1994), Gailite et al. (1987) and Loydell & Nestor (2005).Streptograptus wimani in the Ventspils core indicates the lower part of the lapworthi Biozone.The interpretations of the depth interval ca 920-935 m of the Aizpute core by Loydell et al. (2003) differ ca 2 m from the depths used for bentonites.For the correct position of bentonites relative to samples studied by Loydell et al. (2003) see Kiipli & Kallaste (2006).core in this interval, four of which can be correlated with bentonites from the Aizpute section on the basis of sanidine composition and Ti and Nb (Figs 2 and 3).Among these, a bed at 845.6 m is correlative with the well-known and widespread Osmundsberg Bentonite (ID851) (Kiipli et al. 2006(Kiipli et al. , 2008b;;Inanli et al. 2009).In Ventspils the Osmundsberg Bentonite consists of a grey and a red part, both revealing identical sanidine composition, but the concentrations of immobile elements are significantly lower in the red part than in the grey part (Fig. 3).A thin bed at 842.1 m in the Ventspils core reveals very high Zr, Th, Nb and Ti concentrations (Fig. 3), excluding (together with its position in the section) correlation with ash beds known in Estonia and the Aizpute-41 section in Latvia.A unique 1 cm thick hard tuff layer (the Geniai Tuff) occurs at 848.5 m containing La, Ce, Nd, Sr and P at the level of major components.We have suggested a source magma of carbonatite composition for this bed (Kiipli et al. 2012).In the När core this interval is in a stratigraphical gap, which, according to Grahn (1995), encompasses much of the Aeronian and the Telychian up to the Oktavites spiralis graptolite Biozone.

Bentonites in the Streptograptus sartorius-Monoclimacis crenulata Biozone interval (Telychian, Llandovery)
In terms of chitinozoan biozonation, this interval belongs to the upper part of the Eisenackitina dolioliformis Biozone.In the Ventspils core ten bentonites have been found in this interval, two of which (at depths 840.65 m and 838.4 m) can be firmly correlated on the basis of their sanidine composition with the Aizpute section and sections in Estonia (Figs 3, 4, Table 1).The Nurme Bentonite (ID 731), which is in the Monoclimacis crenulata Biozone in the Aizpute core (Fig. 2), has two correlation possibilities in the Ventspils section differing by only 0.8 m in depth (836.4 m and 837.2 m).An identical composition of sanidine, immobile trace elements and close stratigraphical position in the section permit of both correlations.At the same time these compositional signatures confidently distinguish these ash beds from others in this interval.Evidently both ash layers originate from the same volcanic source and the short time span between the eruptions did not allow noticeable evolution of the magma composition.Provisionally, the Nurme Bentonite has been correlated with the bentonite at 837.2 m depth, and the bentonite at 836.4 m depth is considered as new.The bentonite at 837.7 m depth is also new, having no counterparts in previously studied sections.This bed is characterized by a low concentration of all immobile elements and potassium-rich sanidine similar to the Mustjala Bentonite occurring 2.3 m lower in the core.In the När core this interval is still within the stratigraphical gap (Fig. 2).

Bentonites in the Oktavites spiralis-Cyrtograptus lapworthi Biozone interval (Telychian, Llandovery)
In terms of chitinozoan biozonation, this interval belongs to the Angochitina longicollis and Conochitina proboscifera biozones.A bentonite in the Ventspils core at 808.1 m depth correlates, on the basis of its distinctive sanidine composition (42.6 mol% of (Na + Ca)AlSi 3 O 8 in the modal component) and high content of Nb and Zr, with a bentonite at 938.6 m depth in the Aizpute core and with the Kaugatuma Bentonite (ID 480) in Estonia.Similarly, a bentonite in the Ventspils core at 811.6 m depth correlates perfectly (45.8 mol% of the (Na + Ca)AlSi 3 O 8 in the modal component and sharp XRD reflection) with the Ruhnu Bentonite (ID 494) in Estonia.In the När core, according to Snäll (1977), 19 ash beds occur in the 334-363 m interval.From 11 of these we found some material for laboratory study.These ash beds are mostly characterized by wide or weak sanidine XRD reflections and a high content of P, Ti, Sr, Ba, La and Ce.In Estonia six ash beds have a similar geochemical type.The wide sanidine reflection does not enable unequivocal correlations.Some provisional correlations can be proposed based on the TiO 2 -Nb chart (Fig. 3).One variant of these provisional correlations is expressed in Fig. 2, but clearly this is not the only one possible.According to this correlation variant, at least nine ash beds of this type occur in Latvia and Gotland and two of the Estonian ash beds (ID 520 and 521) do not extend to these areas.Two ash beds in the När section at depths of 348.3 m and 351.0 m belong to another geochemical type characterized by a high content of Nb, Zr and Th (Fig. 3).By contrast with Nb-, Zr-and Th-rich ashes in Estonia, these beds do not exhibit a strong and sharp sanidine reflection and consequently most probably are the product of other eruptions.The ash beds of these two geochemical types were first distinguished by Batchelor  (1995) in the Garntangen section in Norway correlating with this level (Kiipli et al. 2001).In total, probably seven studied ash beds within this interval in the När section are new (at depths 358.1, 357.4, 351.3, 351.0, 348.9, 348.3 and 347.8 m), i.e. not known from previously studied sections.

Bentonites in the Cyrtograptus murchisoni Biozone interval (Sheinwoodian, Wenlock)
In terms of chitinozoan biozonation, this interval belongs to the Margachitina margaritana Biozone (Fig. 2).The Aizpute Bentonite is distinct from others in this interval in its sharp sanidine reflection with 37 mol% of Na + Ca component and high Nb, Zr and Th contents.In the När core the Lusklint and Ireviken bentonites, in the Ventspils core the Ohesaare and Ireviken bentonites are found as well (Figs 2 and 3).The Ireviken Bentonite marks Ireviken Event Datum 2 (Jeppsson & Männik 1993).Kiipli et al. (2008a) showed that this level correlates to a level within the upper part of the C. murchisoni graptolite Biozone.The occurrence of the Ireviken Bentonite in the Ventspils section (Fig. 2) also within the C. murchisoni Biozone supports that correlation.
A new ash bed occurs in the Ventspils core at a depth of 792.7 m, having a sharp sanidine reflection not found in other sections at this stratigraphical level.

Composition of volcanic phenocrysts according to EDS microanalysis
Five samples from the När core where enough grain material was available were subjected to EDS microanalysis.In each sample 30-70 grains were measured.
Reference glass material BBM-1 distributed by the International Association of Geoanalysts was analysed together with the samples under study and corrections to the results of a few per cent of the concentration were derived from the reference sample analysis.The results are presented in Table 2 and Fig. 4.Among the phenocrysts, quartz dominates in the Ireviken, Lusklint and När-351.3 m bentonites and sanidine in the Aizpute and När-352.5 m bentonites.Biotite is present in all studied samples in variable concentrations.A few grains of authigenic kaolinite, pyrite, barite and illite are found as well.An especially rich assemblage of minerals was detected in the När-351.3 m bentonite, represented by several magmatic (magnetite, Ti-oxide, albite) and authigenic (pyrite, strontianite, goyazite-florencite, kaolinite and illite) grains.
Biotite composition was calculated according to the idealized half-cell chemical formula with eight cations: (K,Na) 1 (Mg,Fe,Ti,Mn,Ca,Sr,Ba) 3 (Al,Si,P) 4 O 10 (OH,F) 2 .The average of 5-9 grains is represented in Table 2 and in Fig. 4. Four of the studied ashes contain Mg-rich biotites and one (När-351.3m) contains Fe-rich biotite.The same two groups of biotite were found in the Telychian of the Viirelaid core (Estonia), although Fe-rich biotites occurred stratigraphically at a somewhat lower level in the Telychian crispus-crenulata graptolite biozones (Kiipli et al. 2008d).Comparison with biotite analyses from Batchelor (2003) shows similar results for the Ireviken and Lusklint bentonites from outcrops on Gotland and from the När core (present study), confirming the correlation based on sanidine composition.
Sanidine composition was calculated according to the idealized chemical formula with five cations: (K,Na,Ca,Ba,Sr) 1 (Al,Si,Ti,Mn,Fe) 4 O 8 .An average of 6-31 grains is represented in Table 2 and Fig. 4. The average content of the Na + Ca component is generally lower according to EDS analysis than the modal component according to XRD analysis.The reason is the presence of other components than modal, predominantly with a lower content of the Na + Ca component.The difference may arise in part also from the smaller number of grains, and consequently the less reliable result analysed by EDS (6-31 grains) compared to XRD (simultaneous average of ca 2000 grains).An interesting result from EDS analysis, not accessible by XRD, is the concentration of other cations than sodium and potassium in sanidine.Relatively high (2-3 mol%) content of the CaAl 2 Si 2 O 8 component in sanidine was established in the Ireviken, Lusklint and När-352.5 m bentonites.Sanidine in the Ireviken and Lusklint bentonites is remarkable for the high content (1-2.5 mol%) of BaAl 2 Si 2 O 8 .The Sr, Ti, Mn and Fe concentrations in sanidine were too low for reliable calculation of the index in the chemical formula (Table 2).

Areal distribution of bentonites
The areal distribution of bentonites depends on the ancient volcanic ash clouds and the distribution maps can reveal the directions from which the ashes were transported.The distribution areas composed by Kiipli et al. (2008bKiipli et al. ( , 2008cKiipli et al. ( , 2008dKiipli et al. ( , 2010b) ) for ca 20 bentonites in the eastern Baltic indicate ash transport from the northwest in terms of the present-day orientation of Europe.Correlation with the När section enables extension of the bentonite distribution areas for the Ireviken, Lusklint, Ohesaare and Aizpute bentonites (Fig. 5).These areas confirm ash transport from the Iapetus Ocean, closing between Baltica and Laurentia.
Judging from these maps, the bentonites in Jämtland and Dalarna in Sweden should be significantly thicker than in Estonia and Latvia.This conclusion is confirmed by the occurrence of the 1 m thick Osmundsberg Bentonite in Dalarna (Inanli et al. 2009).In southern Sweden and in the Oslo region we can expect these volcanic ashes to have similar thicknesses as in the eastern Baltic, e.g. in the range of milli-and centimetres.
Sea currents, too, can modify the areal extent of individual bentonites, redistributing and sorting the material by grain size.This has happened with an ash bed in recent sediments near the coast of Chile (Fisher & Schmincke 1984), where an oceanward current separates the volcanic ash area into two parts.Examining the distribution of bentonites in the Baltic Silurian (Fig. 5; Kiipli et al. 2008bKiipli et al. , 2008cKiipli et al. , 2008dKiipli et al. , 2010b)), we notice that the bentonite distribution areas are often similarly separated into two parts.There are two possible explanations: the influence of sea currents or a change in wind direction during a long-lasting eruption.As the separation occurs in a specific part of the Silurian basin, in present-day North Latvia and South Estonia, a sea current is a more likely reason.In the same area gaps or condensed sequences often occur in Silurian sections (Fig. 2; Loydell et al. 2003;Kiipli et al. 2011Kiipli et al. , 2012)).In terms of basin depth, this area belongs to the transition between shallow shelf and deep shelf regions.An oceanic current coming from the southeast was interpreted from the kaolinite admixture in deep shelf sedimentary rocks by Kiipli et al. (2009) for Telychian time.Separation of the distribution areas of Silurian bentonites into two provides further support for the existence of this current.Figure 6 displays the shelf sea branch of the oceanic current in accordance with the bentonite distribution areas.It is not clear how closed the ocean within the Caledonian collision zone was during the Telychianearly Wenlock: possibly the shelf sea branch of the current returned to the Rheic Ocean (Fig. 6), but alternatively there could also have been a passage to a remnant of the Iapetus Ocean.Worsley et al. (2011) measured the direction of ripple and tool marks on bedding planes in the Telychian of the Oslo Region and established the dominant direction of currents from northwest to southeast, i.e. opposite to the direction we have proposed.A reasonable explanation could be that a Fig. 5. Distribution maps of the four bentonites from the Sheinwoodian (lower Wenlock).Details of the distribution in Estonia and names and numbers of all drill cores can be found in Kiipli et al. (2008bKiipli et al. ( , 2008dKiipli et al. ( , 2010b)).shelf sea current flowing from the southeast and striking the coast formed by the rising Caledonian mountains rebounded as a contra-current along the sea bottom.

CONCLUSIONS
Correlation of volcanic ash beds using biostratigraphy, phenocryst composition and immobile elements within bentonites has revealed several well-established and a number of provisional correlations between the Gotland and East Baltic sections.The occurrence of the Aizpute Bentonite in the subsurface of Gotland together with the Lusklint and Ireviken bentonites increases the confidence that the correlation of exposed sections on Gotland, based on volcanic ash beds, with the graptolite biozonation at the level of the Ireviken Event (Kiipli et al. 2008a) is reliable.Approximately twelve new ash beds were discovered and geochemically characterized.These characterizations could be used for demonstrating correlations in future studies.New correlations enabled extension of the mapping of the distribution areas of bentonites from the East Baltic to Gotland.These extended maps point more reliably than previous studies to the volcanic source areas in the closing Iapetus Ocean.Separation of the bentonite distribution areas into two parts in North Latvia-South Estonia indicates the existence of a shelf sea current in the Baltic Silurian Basin coming from the southeast in terms of the present-day orientation.

Fig. 6 .
Fig.6.Silurian shelf sea in Baltoscandia with interpreted shelf sea currents (arrows with broken lines).Arrows with solid lines indicate currents which separate the volcanic ash distribution areas into two parts.The ID numbers of the bentonite are near the arrows.The oceanic current is shown according toKiipli et al. (2009).Arrows indicating coastal contracurrent direction are fromWorsley et al. (2011).

Table 2 .
Composition of phenocrysts in ash beds of the När core