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

Abstract

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.

Full text

Estonian Journal of Earth Sciences, 2012, 61, 4, 295–306 doi: 10.3176/earth.2012.4.08
295
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
Tarmo Kiipli, Toivo Kallaste and Viiu Nestor
Institute of Geology at Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia; tarmo.kiipli@gi.ee, toivo.kallaste@gi.ee,
viiu.nestor@gi.ee
Received 5 February 2012, accepted 12 June 2012
Abstract. 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.
Key words: correlation, bentonites, K-bentonites, sea currents, Silurian, East Baltic, Gotland.
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, 2008c, 2008d, 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
Fig. 1. Location of the studied sections.

Estonian Journal of Earth Sciences, 2012, 61, 4, 295–306
296
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. 2010a, 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 –
Al2O3 > 26% and TiO2 > 1.2%; Zr > 510, Nb > 37,
Th > 40, Sr > 190, Ba > 320, La > 48, Ce > 140
and Y > 50 ppm; low concentrations of elements –
Al2O3 < 20% and TiO2 < 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.
RESULTS
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 signi-
ficantly 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 TiO2 concentrations of ca 1%.
Aeronian (Llandovery) bentonites
Snäll (1977) recorded nine thin bentonites in the
När core within the interval 363.0–371.8 m 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 Nä
r
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 use
d
for bentonites. For the correct position of bentonites relative to samples studied by Loydell et al. (2003) see Kiipli & Kallaste
(2006).

T. Kiipli et al.: Correlation of upper Llandovery–lower Wenlock bentonites
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T. Kiipli et al.: Correlation of upper Llandovery–lower Wenlock bentonites
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Estonian Journal of Earth Sciences, 2012, 61, 4, 295–306
300
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, 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)AlSi3O8
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)AlSi3O8 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
TiO2–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 charac-
terized 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
_____________________________________________________________________________________
Fig. 3. Sanidine composition (left column) and TiO2/Nb ratio (right column) in the studied bentonites compared with the
bentonites from Estonia (oval contours). ID numbers of Estonian bentonites are in bold font. Ve – Ventspils-D3, Ve-838.8-755 – core-
depth (m)-ID number of the bentonite. A – murchisoni Biozone interval, B – spiralis–lapworthi Biozone interval, C – sartorius
–
crenulata Biozone interval, D – turriculatus–crispus Biozone interval. Broken lines indicate correlations.

T. Kiipli et al.: Correlation of upper Llandovery–lower Wenlock bentonites
301
0.0
0.1
0.2
0.3
20 30 40 50
0
10
20
30
40
50
60
70
0,0 0,5 1,0 1,5 2,0 2,5 3,0
127
150
210
311
När-324.5-127
När-328.1-150
När-333.4-311
Ve-789.2-127
Ve-792.75-210
Nb, ppm
TiO %
2,
0
10
20
30
40
50
60
70
0,0 0,5 1,0 1,5 2,0 2,5 3,0
494
475
480
457 568
488
518-521
När-352.5-488?
När-342.8
När-351.0-weak sanidine
När-348.3-weak sanidine Ve-808.1-480
Ve-811.6-494
När-347.8
När-358.1-568?
0
10
20
30
40
50
60
70
0.0 0.5 1.0 1.5 2.0 2.5 3.0
696
731
744
772
776
755
Ve-837.7
Ve-838.4
-744
Ve-836.4
Ve-837.2
-731
Ve-838.8-755
Ve-840.5
Ve-840.65
0
10
20
30
40
50
60
70
0.0 0.5 1.0 1.5 2.0 2.5 3.0
TiO %
2,
TiO %
2,
TiO %
2,
Nb, ppm
Nb, ppm
Nb, ppm
851
823
860-900
788
777
843
795
818
Ve-842.1-new
Ve-848.5-890
The Geniai Tuff
Ve-848.2-880?
Ve-845.6-851-grey
Ve-845.6-851-red
The O” Tuff”
Ve-843.7-818
Ve-844.7-843
Ve-842.0-795
0.0
0.1
0.2
0.3
20 30 40 50
127
150
210
311
När-324.5-127
När-328.1-150
När-333.4-311
Ve-789.2-127
Ve-792.7-new
Ve-792.8-210
0.0
0.1
0.2
0.3
20 30 40 50
494 475
480
När-348.3-weak
När-367.8-weak
Ve-809.6-488?
Ve-813.3-518?
Ve-808.1-480
Ve-811.6-494
0.0
0.1
0.2
0.3
20 30 40 50
696
731
755
772
744
776
774+775
Ve-831.8-682
Ve-838.8-755
Ve-837.7-new
Ve-838.4-744
Ve-836.4-new
Ve-837.2-731
Ve-840.5-773-new
Ve-840.6-774
Ve-840.7-775
851
795
823
788
841
Ve-843.7-818
Ve-844.7-843
Ve-842.1-800-new
Ve-842.0-795
Ve-845.6-851
Na+Ca component in sanidine, mol%
Na+Ca component in sanidine, mol%
Na+Ca component in sanidine, mol%
Na+Ca component in sanidine, mol %
Widt of the sanidine XRD reflection, 2 thetah Widt of the sanidine XRD reflection, 2 thetah Widt of the sanidine XRD reflection, 2 thetah Widt of the sanidine XRD reflection, 2 thetah
A
A
BB
CC
DD
,
%
%

Estonian Journal of Earth Sciences, 2012, 61, 4, 295–306
302
0.3
0.4
0.5
0.6
0.7
0.29 0.31 0.33
Al/(Al+Si) atom ratio
Mg/(Mg+Fe) atom ratio
Biotite
composition
Ireviken Bentonite
Batchelor (2003)
Ireviken Bentonite
in När corethe
Lusklint Bentonite
Batchelor (2003)
Lusklint Bentonite
in När corethe
När 351.3
När 352.5
När 333.4
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.00 0.20 0.40 0.60 0.80 1.00
0.00
0.01
0.02
0.03
0.00 0.20 0.40 0.60 0.80 1.00
Na, index in the formula
Na, index in the formula
Ca, index in formulathe
Ba, index in formulathe
Sanidine
composition
Sanidine
composition
När 324.5 Ireviken Bentonite
När 328.1 Lusklint entoniteB
När 351.3
albite
När 352.5
anorthoclase
När 324.5 Ireviken Bentonite
När 328.1 Lusklint entoniteB
När 352.5
Fig. 4. Composition of biotite and sanidine phenocrysts
according to the EDS microanalyses.
et al. (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 micro-
analysis. 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)4O10(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-351.3 m) contains Fe-rich biotite.
The same two groups of biotite were found in the
Telychian of the Viirelaid core (Estonia), although

T. Kiipli et al.: Correlation of upper Llandovery–lower Wenlock bentonites
303
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)4O8. 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, pre-
dominantly 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 CaAl2Si2O8 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 BaAl2Si2O8. The Sr, Ti, Mn and Fe con-
centrations 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. (2008b, 2008c, 2008d, 2010b) for ca 20 bentonites
Table 2. Composition of phenocrysts in ash beds of the När core
Ireviken B.
När 324.5
Lusklint B.
När 328.1
Aizpute B.
När 333.4
När 351.3 När 352.5
Quartz, % 63 54 17 67 7
Sanidine, % 13 35 56 19 76
Biotite, % 24 10 28 14 17
Other minerals Kaolinite, pyrite Kaolinite,
apatite in
sanidine
Kaolinite,
barite, illite
Magnetite, Ti-oxide,
pyrite, strontianite,
goyazite-florencite,
albite, kaolinite, illite
Anorthoclase,
barite, illite
Composition of biotite, atoms per chemical formula
Na 0.08 0.11 0.10 0.13 0.13
K 0.92 0.89 0.90 0.87 0.87
Mg 1.65 1.79 1.79 0.99 1.87
Fe 1.00 0.81 0.83 1.65 0.75
Ti 0.29 0.31 0.34 0.30 0.30
Mn 0.016 0.026 0.019 0.019 0.016
Ca 0.007 0.002 0.005 0.019 0.011
Sr 0.005 0.004 0.002 0.006 0.011
Ba 0.030 0.055 0.022 0.019 0.042
Al 1.20 1.24 1.19 1.33 1.24
Si 2.80 2.76 2.81 2.66 2.75
P 0.001 0.000 0.000 0.011 0.003
Composition of sanidine, atoms per chemical formula
Na 0.26 0.21 0.14 0.09 0.06
K 0.68 0.74 0.83 0.90 0.91
Ca 0.029 0.029 0.015 0.006 0.022
Sr 0.002 0.005 0.002 0.001 0.002
Ba 0.027 0.011 0.005 0.006 0.003
Al 1.00 0.93 1.03 1.00 1.01
Si 2.93 3.05 2.94 2.99 2.96
Ti 0.011 0.005 0.021 0.003 0.006
Mn 0.000 0.002 0.001 0.001 0.001
Fe 0.065 0.008 0.014 0.004 0.023

Estonian Journal of Earth Sciences, 2012, 61, 4, 295–306
304
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. 2008b, 2008c, 2008d, 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. 2011, 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 Telychian–
early 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 Estoni
a
and names and numbers of all drill cores can be found in Kiipli et al. (2008b, 2008d, 2010b).

T. Kiipli et al.: Correlation of upper Llandovery–lower Wenlock bentonites
305
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 to Kiipli et al. (2009). Arrows indicating coastal
contracurrent direction are from Worsley et al. (2011).
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 bio-
zonation 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.
Acknowledgements. The authors thank A. Murnieks and
R. Pomeranceva (Latvian Agency of Environment, Meteo-
rology and Geology) for assistance with the study of the
Ventspils-D3 drill core, L. M. Wickström for her help in study
of the När core and S. Peetermann for correcting the English.
This study is a contribution to IGCP591, Estonian Research
Council grant 8963 and project SF0140016s09. Creation and
maintenance of the attached database was supported by the
Estonian Research Council project KESTA.
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Ülem-Llandovery – Alam-Wenlocki bentoniitide korrelatsioon Näri (Gotland, Rootsi) ja
Ventspilsi (Läti) puursüdamike vahel: vulkaanilise tuha pilvede ning merehoovuste
roll bentoniidi levikuala tekkel
Tarmo Kiipli, Toivo Kallaste ja Viiu Nestor
Biostratigraafia, sanidiini koostis ja immobiilsed elemendid bentoniitides võimaldasid kindlaks teha mitu kindlat
ning mõned esialgsed korrelatsioonid Gotlandi ja Ida-Baltikumi vahel. Fenokristallide mikroanalüüs näitas, et
bentoniitides esineb Mg- või Fe-rikast biotiiti. Sanidiini fenokristallid sisaldavad peale K- ja Na-komponendi ka
mõni protsent Ca- ning Ba-komponenti. Uued korrelatsioonid võimaldasid laiendada bentoniitide leviku skeeme
Gotlandini. Bentoniitide levikualad jagunevad Põhja-Lätis – Lõuna-Eestis kaheks, osutades hoovusele Balti Siluri
šelfimeres, mis jaotas vulkaanilise tuha osakesi ringi.