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Upper Katian (Ordovician) bentonites in the East Baltic, Scandinavia and Scotland: geochemical correlation and volcanic source interpretation

Published online by Cambridge University Press:  07 October 2014

TARMO KIIPLI ,
PETER DAHLQVIST ,
TOIVO KALLASTE ,
ENLI KIIPLI  and
JAAK NÕLVAK
TARMO KIIPLI*
Affiliation:
Institute of Geology, Tallinn University of Technology, Ehitajate 5, 19086 Tallinn, Estonia
PETER DAHLQVIST
Affiliation:
Geological Survey of Sweden, Killiansgatan 10, 223 50 Lund, Sweden
TOIVO KALLASTE
Affiliation:
Institute of Geology, Tallinn University of Technology, Ehitajate 5, 19086 Tallinn, Estonia
ENLI KIIPLI
Affiliation:
Institute of Geology, Tallinn University of Technology, Ehitajate 5, 19086 Tallinn, Estonia
JAAK NÕLVAK
Affiliation:
Institute of Geology, Tallinn University of Technology, Ehitajate 5, 19086 Tallinn, Estonia
*
†Author for correspondence: tarmo.kiipli@ttu.ee
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Abstract

Altered volcanic ash interbeds (bentonites) in the upper Katian of Baltoscandia indicate significant volcanic activity in neighbouring tectonically active areas. Katian bentonites in the East Baltic can be reliably correlated using sanidine phenocryst composition. Ratios of immobile trace elements TiO2, Nb, Zr and Th to Al2O3 enable extension of the correlations to Scandinavia, where late diagenetic alterations could have caused recrystallization of sanidine phenocrysts. At least seven volcanic eruptions were recognized in Baltoscandian sections. Several bentonites found in deep-sea sediments are absent in shallow-sea sediments, indicating extensive breaks in sedimentation and erosion during late Katian and Hirnantian times. The areal distribution pattern of Katian bentonites in Baltoscandia indicates a volcanic source from the north or northwest (present-day orientation) from the margins of the Iapetus Palaeo-Ocean. Signatures of ultra-high-pressure metamorphism in the Seve Nappe (Central Sweden) and intrusions in the Helgeland Nappe Complex in Central Norway have been proposed as potential sources of the magmas that generated the volcanic ashes deposited in the East Baltic in Katian times. Geochemical similarities between Baltoscandian and Dob's Linn bentonites from southern Scotland suggest a common volcanic source in Katian times.

Keywords

K-bentonitesMg-bentonitesKatianOrdovicianvolcanismIapetusBaltica
Type
Original Articles
Information
Geological Magazine , Volume 152 , Issue 4 , July 2015 , pp. 589 - 602
Copyright
Copyright © Cambridge University Press 2014 

1. Introduction

Bentonites (altered volcanic ashes) in sedimentary sections carry information about tectonomagmatic processes in volcanic source areas (Huff et al. Reference Huff, Merriman, Morgan and Roberts1993; Batchelor & Evans Reference Batchelor and Evans2000; Kiipli, Soesoo & Kallaste, Reference Kiipli, Soesoo, Kallaste, Corfu, Gasser and Chew2014), directions to volcanic sources (Bergström et al. Reference Bergström, Huff, Kolata and Bauert1995; Kiipli et al. Reference Kiipli, Kallaste, Kiipli and Radzevičius2013), the diagenetic environments of sedimentary rocks (Hints et al. Reference Hints, Kirsimäe, Somelar, Kallaste and Kiipli2006, Reference Hints, Kirsimäe, Somelar, Kallaste and Kiipli2008; Somelar et al. Reference Somelar, Kirsimäe, Hints and Kirs2010; Williams et al. Reference Williams, Srodon, Huff, Clauer and Hervig2013) and the isotopic ages of rocks (Bergström et al. Reference Bergström, Toprak, Huff and Mundil2008; Cramer et al. Reference Cramer, Condon, Söderlund, Marshall, Worton, Thomas, Calner, Ray, Perrier, Boomer, Patchett and Jeppsson2012; Sell, Ainsaar & Leslie, Reference Sell, Ainsaar and Leslie2013). Recognizing the distinct chemical signatures of eruption layers can lead to exceptionally precise correlations of sections (Emerson et al. Reference Emerson, Simo, Byers and Fournelle2004; Inanli, Huff & Bergström, Reference Inanli, Huff and Bergström2009; Kiipli, Kallaste & Nestor, Reference Kiipli, Kallaste and Nestor2010, Reference Kiipli, Kallaste and Nestor2012; Kiipli et al. Reference Kiipli, Kallaste, Nestor and Loydell2010, Reference Kiipli, Einasto, Kallaste, Nestor, Perens and Siir2011; Ray et al. Reference Ray, Collings, Worton and Jones2011; Kiipli, Radzevicius & Kallaste, Reference Kiipli, Radzevicius and Kallaste2014).

While Ordovician Sandbian and Silurian bentonites in the Baltoscandian region have been described in several publications, Ordovician Katian bentonites have been less well studied. Some correlations in the East Baltic sections based on sanidine phenocryst compositions were described in the WOGOGOB conference abstract book (Kiipli, Kallaste & Kiipli, Reference Kiipli, Kallaste, Kiipli, Hints and Ainsaar2004). The unusual authigenic mineralogy of Katian bentonites represented by the frequent dominance of chlorite–smectite was discussed in Hints et al. (Reference Hints, Kirsimäe, Somelar, Kallaste and Kiipli2006) and aspects of the source magma geochemistry in Kiipli, Soesoo & Kallaste (Reference Kiipli, Radzevicius and Kallaste2014). Geochemical evidence indicates vast environmental changes in Late Ordovician time and palaeontological studies have shown strong extinction events (Brenchley, Carden & Marshall, Reference Brenchley, Carden and Marshall1995; Harper, Hammarlund & Rasmussen, Reference Harper, Hammarlund and Rasmussen2013; Bergström et al. Reference Bergström, Eriksson, Young, Ahlberg and Schmitz2014). Therefore, intensive research during recent decades has been dedicated to this time interval, and data on volcanism and uniquely precise correlations enabled by volcanic ash beds are useful additions to these studies.

Herein, we update correlations in the East Baltic with new finds together with new Scandinavian material. In addition to the sanidine composition, immobile trace elements are applied to proving the correlations. The relationship of Katian bentonites to the magmatic activity in the Iapetus Palaeo-Ocean is also discussed.

2. Material and methods

Samples were collected from 21 drill core sections from Estonia, the Aizpute-41 core from Latvia, the När core from Gotland (Sweden), the Röstånga-1 core from southern Sweden and from natural exposures from the Jämtland and Västerbotten regions (Central Sweden) (Fig. 1). In the cores, all interbeds distinguished by an unusual appearance (e.g. pure clay or shaly interbeds in limestones and marls) were sampled. In the northernmost sections in Estonia (Soonlepa, Käbi and Rabivere), interbeds of hard feldspathic tuffs were found. In total, around 60 samples were studied by X-ray diffractometry (XRD), and the presence of volcanic material was established in 45 samples.

Figure 1. Location of sections studied and discussed in text. (a) Sections sampled from Estonia; (b) sections sampled or discussed from Sweden, Norway, Denmark, Latvia and Lithuania. Broken lines separate palaeoenvironmental zones and confacies belts.

To identify major minerals in the sampled interbeds of supposed volcanic origin, bulk samples were analysed by XRD. An association of illite–smectite and chlorite–smectite as major minerals has been considered as a provisional indication of a volcanic origin for the interbeds. Pure end-member authigenic (Kastner, Reference Kastner1971) K-feldspar forms a major portion of the feldspathic tuffs in North Estonia.

Magmatic sanidine ((K,Na,Ca)AlSi3O8) phenocrysts were analysed by XRD from coarse fractions (0.04–0.1 mm) separated from 2 g of bentonite applying the calibration established in the experimental study by Orville (Reference Orville1967). A shift in the 20ī reflection enables the discrimination of small cations (Na + Ca) from large (K + Ba) cations. The method was described in detail in Kiipli, Kallaste & Nestor (Reference Kiipli, Kallaste and Nestor2010) and Kiipli et al. (Reference Kiipli, Einasto, Kallaste, Nestor, Perens and Siir2011). All measured XRD spectra of sanidine in the upper Katian bentonites are available in the collections database of the Institute of Geology at Tallinn University of Technology at http://geokogud.info/reference/3841.

Major components and trace elements were analysed in 21 samples of pressed powders, where sufficient quantity (8 g) of material was available, by the X-ray fluorescence (XRF) method using an S-4 (Bruker AXS) analyser. The concentrations were calculated automatically by the manufacturer's software and corrected on the basis of reference materials (Govindaraju, Reference Govindaraju1995; Kiipli et al. Reference Kiipli, Batchelor, Bernal, Cowing, Hagel-Brunnstrom, Ingham, Johnson, Kivisilla, Knaack, Kump, Lozano, Michiels, Orlova, Pirrus, Rousseau, Ruzicka, Sandstrom and Willis2000) and proficiency test samples distributed by the International Association of Geoanalysts (http://www.geoanalyst.org).

3. Geological background

3.a. Lithology and facies

Jaanusson (Reference Jaanusson1995) distinguished the following facies areas in the Ordovician of Baltoscandia: North Estonian Confacies, Central Baltoscandian Confacies, Lithuanian Confacies and Scanian Confacies (Fig. 1). In Estonia, an additional transition zone between shallow- and deep-shelf palaeoenvironments was recognized by Põlma (Reference Põlma1967). In the upper Katian Pirgu Stage the transition zone is especially notable with its specific facies, greater thicknesses and frequent breaks in sedimentation (Fig. 2). Rocks of the Pirgu Stage in the East Baltic were described in Hints, Oraspõld & Nõlvak (Reference Hints, Oraspõld and Nõlvak2005) and herein we briefly refer to this content.

Figure 2. Correlation of Katian and Hirnantian sections in Estonia. For legend see Figure 3. B0, BI, BII, BIII and BIV are indexes of bentonites. Among the chitinozoans, only ranges of zonal forms are shown.

In the North Estonian Confacies (shallow shelf sea in terms of palaeoenvironment), the rocks of the Pirgu Stage include two formations: (1) the Moe Formation (lower Pirgu): relatively pure coarsely nodular limestones, typically with interbeds of brownish organic-rich marl and frequent occurrences of the tubular algae Dasyporella; and (2) the Adila Formation (upper Pirgu): relatively argillaceous thin to medium nodular limestones with thin grey marl interbeds. Among the studied sections, a typical development is reflected in the Soonlepa core from Hiiumaa Island.

While the Pirgu Stage in the North Estonian Confacies (shallow shelf) and Central Baltoscandian Confacies (deep shelf) is characterized by a relatively simple and uniform lithology, the transition zone between these areas shows variable types of rocks, and this natural diversity has caused numerous discussions on correlations and the differentiation of stratigraphic units. Among the studied sections the Varbla section is considered to be the most representative, and the following formations (from the base) can be distinguished (Fig. 2): (1) the Jonstorp Formation: argillaceous red-coloured limestone; (2) the Tootsi Formation: finely nodular grey strongly argillaceous limestone with violet patches; (3) the Halliku Formation: marlstone with or without carbonate nodules and argillaceous limestone in the upper part; (4) the Oostriku Formation: relatively pure nodular limestone; and (5) the Kabala Formation: alternating thick (20–50 cm) limestone and marl layers of contrasting composition. In other sections transitional varieties often occur and thicknesses vary greatly.

In the Central Baltoscandian Confacies in Latvia and South Estonia, the Jonstorp Formation represented by red-coloured argillaceous limestones and marls composes the lower thicker part of the Pirgu Stage; upwards follow the Paroveja Formation (relatively pure nodular limestones) and the Kuili Beds (red-coloured marls and argillaceous limestones). Among the studied sections the Aizpute-41 core in Figure 3 is typical. In Central Sweden, Jaanusson (Reference Jaanusson1963) distinguished the Lower Jonstorp Formation (grey-coloured finely nodular limestone and marl), the Upper Jonstorp Formation (red-coloured finely nodular limestone and marl) and between them the Öglunda Limestone (e.g. the När section in Fig. 3).

Figure 3. Correlation of Katian and Hirnantian sections of Sweden, Latvia and Estonia. Among the chitinozoans, only ranges of zonal forms are shown.

In the Scanian Confacies Belt the Pirgu Stage is represented by the Lindegård Mudstone, a section of grey marlstones (Fig. 3) (Glimberg, Reference Glimberg1961; Bergström et al. Reference Bergström, Huff, Koren, Larsson, Ahlberg and Kolata1999, Reference Bergström, Eriksson, Young, Ahlberg and Schmitz2014).

The sample from Jämtland is from the lower allochthonous unit the Kogsta Siltstone at the Högåsen locality (for more information see Dahlqvist, Reference Dahlqvist2005). Thin greenish/grey laminae are seen in the otherwise dark shale/siltstone in the uppermost metres of the unit. The origin of these conspicuous beds has been discussed previously, and Cherns & Karis (Reference Cherns and Karis1995) and Karis & Strömberg (Reference Karis and Strömberg1998) suggested they are of volcanic origin but no analysis has been made until this study. The Västerbotten sample was taken from an island shore section in Lake Björkvattnet. The sample is most probably from the uppermost part of the Vojtja Quartzite, close to the onset of carbonate production reflected in the development of the Slätdal Limestone (Kulling, Reference Kulling1933). The sample is from the Upper Allochthon (Köli Nappe), which is believed to have occupied an intra-Iapetus island arc setting during Ordovician time (Stephens, Reference Stephens1977; Stephens & Gee, Reference Stephens, Gee, Gee and Sturt1985).

3.b. Chitinozoans

An obvious feature is the progressive decrease in chitinozoan diversity long before the Hirnantian glaciation. This low diversity and density of the main groups of microfossils is well visible in the diversity dynamics curves for the Pirgu Age, the prelude to the terminal Ordovician mass extinction (Kaljo, Nõlvak & Uutela, Reference Kaljo, Nõlvak and Uutela1996; Brenchley et al. Reference Brenchley, Carden, Hints, Kaljo, Marshall, Martma, Meidla and Nõlvak2003; Kaljo et al. Reference Kaljo, Hints, Hints, Männik, Martma and Nõlvak2011). However, this study reports ages based on chitinozoans from limited sections. Bentonite layers BII, BIII and BIV (Fig. 2) of the upper Halliku and Adila formations span the Conochitina rugata chitinozoan zone (Nõlvak & Grahn, Reference Nõlvak and Grahn1993; Nõlvak, Hints & Männik, Reference Nõlvak, Hints and Männik2006) and a little above indicating an age of late Pirgu (latest Katian). The lower bentonite beds (BO, BI; Fig. 3) are distributed mainly in the red-beds of the Jonstorp Formation, which are totally barren of all groups of acid-resistant organic-walled microfossils, and the timing of bentonite deposition in these sections remains unclear. In the North Estonian Confacies these two lower bentonites occur in the Moe Formation belonging to the Tanuchitina bergstroemi Zone (Hints, Oraspõld & Nõlvak, Reference Hints, Kirsimäe, Somelar, Kallaste and Kiipli2005).

3.c. Graptolites, brachiopods, carbon isotopes and isotopic age

In the Lindegård section the graptolites D. complanatus, O. gracilis and D. anceps were found in the lower part of the Lindegård Mudstone (Glimberg, Reference Glimberg1961). In the Röstånga-1 section N. persculptus was identified in the upper part of the Hirnantian (Bergström et al. Reference Bergström, Huff, Koren, Larsson, Ahlberg and Kolata1999). A carbon isotope positive excursion in the lower Hirnantian, established in many sections, in particular also in Röstånga-1, is correlated with the N. extraordinarius graptolite Biozone (Bergström et al. Reference Bergström, Eriksson, Young, Ahlberg and Schmitz2014), and it helps to establish the Katian/Hirnantian boundary. Below the Lindegård Mudstone, in the Fjäcka Shale, D. clingani has been recovered (Glimberg, Reference Glimberg1961). Thus, the Pirgu Stage is correlated with graptolite biozones complanatus and anceps (Hints et al. Reference Hints, Hints, Kaljo, Kiipli, Männik, Nõlvak and Pärnaste2010). According to the current time scale (Gradstein, Ogg & Hilgen, Reference Gradstein, Ogg and Hilgen2012), the Katian (including the Pirgu Stage in the upper part) corresponds to the time interval 445.2–453.0 Ma.

In Jämtland the uppermost Kogsta Siltstone is believed to be of latest Katian to earliest Hirnantian age (Dahlqvist, Harper & Wickström, Reference Dahlqvist, Harper and Wickström2010). The age determination of the sample from the Vojtja Quartzite from Västerbotten is unclear, but, close to the sampled interval, the overlying Slätdal Limestone has yielded the brachiopod Holorynchus giganteus (Kulling, Reference Kulling1933). Holorynchus giganteus is restricted in time to the upper Katian and to beds just below the Hirnantian carbon isotope event (Brenchley et al. Reference Brenchley, Marshall, Hints and Nõlvak1997).

4. Results

4.a. Composition of the bentonites

According to XRD measurements, the Estonian bentonites of the Pirgu Stage contain variable amounts of three authigenic minerals: chlorite–smectite, illite–smectite and K-feldspar. According to Kastner (Reference Kastner1971), pure end-member K-feldspars are of authigenic origin. In the shallow palaeoshelf area authigenic K-feldspar dominates, forming hard feldspathic tuffs; in the transition zone chlorite–smectite and illite–smectite dominate being equally abundant; and in the deep shelf area illite–smectite is the most abundant authigenic mineral. A similar distribution pattern (except the dominance of K-feldspar in the shallow shelf area) was revealed when studying the clay fraction of the bentonites (Hints et al. Reference Hints, Kirsimäe, Somelar, Kallaste and Kiipli2006). The major chemical components (Table 1; Fig. 4) display a continuous range of compositions from high-K2O-containing feldspathic tuffs through K-bentonites of intermediate compositions to Mg-bentonites with high contents of MgO.

Table 1. Results of XRF analyses of upper Katian bentonites

Figure 4. Composition of altered volcanic ashes of the Pirgu Stage.

All three samples from the Röstånga-1 core represent K-bentonites, as the main silicate component in all is highly illitic illite–smectite, quartz is absent as a major component, and the Al2O3 content ranges between 24 and 27 % and K2O between 6.9 and 7.8 % in the silicate part. In the grain fractions, rare (frequent in lower sample) hexagonal biotite phenocrysts occur. All the bentonites contain calcite with a very high content of manganese. This is reflected in a shift of the calcite XRD 104 reflection and also in the high CaO and MnO content in the XRF results (Table 1). An especially high content of Mn-calcite is present in the upper bentonite sample from Röstånga.

According to the chemical composition, the samples from Jämtland (Högåsen) and Västerbotten (Björkvattnet) containing Al2O3 over 20 % and K2O over 7 % are defined as K-bentonites (Table 1; Fig. 4). The high content of TiO2, which is 1.5 times higher than is normal in terrigenous rocks (Table 1), is additional evidence for the volcanic origin of these layers. A few more samples from the Jämtland Kogsta Siltstone contained around 70 % SiO2 and correspondingly less Al2O3 and K2O, and have been assigned to metamorphosed terrigenous siltstones. As rocks in Jämtland and Västerbotten are metamorphosed, K-bentonites consist mostly of muscovite with a smaller addition of quartz and chlorite in the Högåsen sample.

In the East Baltic and in the När core of Gotland, volcanic sanidine was also found in several shaly interbeds not forming distinct bentonitic layers. Separation of the phenocryst material from the shaly interbeds was possible owing to the larger grain size of the phenocrysts (0.04–0.1 mm) compared with the terrigenous material (dominantly less than 0.04 mm). In some cases, separation of the volcanic material from the shaly interbeds was even easier than from the bentonites where an abundance of authigenic K-feldspar aggregates of a similar size often hampers this procedure.

4.b. Correlations based on sanidine phenocryst composition

Bentonites from five volcanic eruptions were distinguished in the East Baltic, all of them containing characteristic sanidine (K,Na,Ca)AlSi3O8 phenocrysts, the compositions of which enable trustworthy identification (Table 2; Fig. 5) and correlation (Figs 2, 3). Biotite contents are mostly low, exceeding ten flakes in the separated grain fraction frequently only in the two lower layers. The stratigraphic indexes BI, BII and BIII for the bentonites described in Kiipli, Kallaste & Kiipli (Reference Kiipli, Kallaste, Kiipli, Hints and Ainsaar2004) were assigned by Hints, Oraspõld & Nõlvak (Reference Hints, Oraspõld and Nõlvak2005) and are adopted here. Additional indexes B0 and BIV are used here for the first time. The characterization of sanidine in the bentonites (from the base) is as follows:

Table 2. Composition of sanidine phenocrysts and occurrence of biotite in studied samples

0 – no biotite in grain fraction; + – biotite is rare: 1 to 10 flakes in separated grain fraction; ++ – biotite is frequent: 11 to 100 flakes in separated grain fraction; +++ – biotite is abundant: more than 100 flakes in separated grain fraction;? – biotite content was not estimated.

Figure 5. Composition of sanidine phenocrysts in the bentonites of the Pirgu Stage from the East Baltic according to XRD analysis. Frames embrace correlated bentonites shown by different symbols.

  1. (1) B0 is characterized by weak and wide sanidine XRD reflection. Commonly, sanidine cannot be characterized numerically in this bed. Only from the shaly interbed of the Ruhnu core (628.1 m) did we extract sufficient sanidine for XRD analysis and the result shows 25 mol % of the Na + Ca component. This composition is significantly more potassic than in other layers of volcanic origin in the Pirgu Stage. In contrast to other bentonites in the Pirgu Stage, B0 contains biotite in a notable amount.

  2. (2) BI is characterized by sharp (less than 0.2 deg on the 2-theta scale) XRD reflection and 37.2–38.7 mol % of the Na + Ca component in the sanidine. Stratigraphically close, bentonites B0 and BI occur together in the Käbi core section (Fig. 2) indicating that B0 is older. Sanidine corresponding to the composition typical of BI was also established in the När core (Gotland, Sweden) in a shaly interbed at the depth of 395.05 m in the Upper Jonstorp Formation (Fig. 3). Commonly, slightly more than ten flakes of biotite occur among separated grains.

  3. (3) BII is the thickest bentonite in the Pirgu Stage reaching 30 cm and is characterized by sharp sanidine reflection and 43.8–44.3 mol % of the Na + Ca component in the sanidine. Typically biotite is absent.

  4. (4) BIII is characterized by sharp sanidine reflection and 34.3–36.2 mol % of the Na + Ca component in the sanidine phenocrysts.

  5. (5) BIV is characterized by sanidine having the most sodium-rich composition (average 47–48 mol % of the Na + Ca component) among the bentonites of the Pirgu Stage. XRD reflection is wide and sometimes, like in the Kuressaare section at 302.25 m, two components of sanidine can be recognized. BIV is clearly higher than BII, but its relationship with BIII is not proven as BIII and BIV were not found together in any of the studied sections.

Bentonites from the Röstånga section did not reveal a sanidine XRD reflection. The most probable reason is that Ordovician rocks at Röstånga, although not strongly metamorphosed, were deformed (Bergström et al. Reference Bergström, Huff, Koren, Larsson, Ahlberg and Kolata1999) and tectonic deformation was likely accompanied by some elevated temperatures accelerating recrystallization of sanidine. Similarly, we did not find sanidine in Silurian bentonites from the Garntangen section in the Oslo region. Rocks in the Garntangen section are deformed too, but not strongly metamorphosed (Bergström, Reference Bergström1980; Batchelor, Weir & Spjeldnaes, Reference Batchelor, Weir and Spjeldnaes1995). Separation of magmatic phenocrysts from the metamorphosed bentonites of the Jämtland Region was not attempted.

4.c. Correlations based on immobile trace elements

Following the approach of Kiipli et al. (Reference Kiipli, Kallaste, Kiipli and Radzevičius2013) applied to Silurian bentonites, in Figure 6 TiO2 (%), Nb (ppm), Zr (ppm) and Th (ppm) ratios to Al2O3 (%) are shown. These five elements have been proven to be immobile during the conversion of volcanic ash to authigenic silicates (Kiipli et al. Reference Kiipli, Soesoo, Kallaste and Kiipli2008). To bring all the ratios numerically to the same scale, TiO2/Al2O3 was multiplied by 50 and Zr/Al2O3 by 0.1. After arranging the samples according to previous correlations based on the sanidine compositions we can see the similarity of the trace element spectra between the samples from the same volcanic eruptions (Fig. 6). B0 is characterized by higher Th contents than the other layers, BI by a rise in element ratios from Ti to Th, BII by low Ti and high ratios of other elements, BIII by equally high Ti, Nb and Zr ratios and even higher Th, and BIV by high ratios of all elements, especially Nb and Th. The immobile trace elements perfectly confirm the correlations established by the sanidine phenocryst compositions.

Figure 6. Ratios of immobile trace elements to Al2O3 in upper Katian bentonites. Bars from left to right: black – TiO2(%)/Al2O3 (%) × 50; white – Nb(ppm)/Al2O3 (%); grey – Zr(ppm)/Al2O3 (%) × 0.1; banded – Th(ppm)/Al2O3 (%).

Trace element spectra from Röstånga, where sanidine was likely recrystallized, allow a correlation with bentonites from Estonia. The middle bentonite from Röstånga shows a similar trace element spectrum to BIII and the lower sample to BI (Fig. 6). The upper sample from Röstånga appears to originate from a different volcanic eruption not found in Estonia.

Trace element spectra of bentonites from Jämtland and Västerbotten differ significantly from the Estonian and Röstånga bentonites by having higher TiO2/Al2O3 ratios, but are quite similar to each other based on their trace element ratios (Fig. 6). In the trace element spectra some similarities with bentonites from Dob's Linn in Scotland can be observed (see Section 5).

5. Discussion

5.a. Bentonites and correlation of lithostratigraphic units in the East Baltic and Sweden

Despite the restricted amount of data about chitinozoan distribution from the described sections, it does not contradict the age interpretation of the geochemically studied bentonite layers. Chemical fingerprints of the studied bentonite beds give an additional time control and precision to the correlation, especially in the East Baltic sections with limited biostratigraphic data (Fig. 7). Correlation of the B0 and BI bentonites from the middle of the Moe Formation in shallow-sea sections of North Estonia with bentonites in the Tootsi and Jonstorp formations in the transition zone and in the lower part of the Jonstorp Formation in deep shelf sections in South Estonia and Latvia confirms the correlation proposed by Oraspõld (Reference Oraspõld1982) and Hints, Oraspõld & Nõlvak (Reference Hints, Oraspõld and Nõlvak2005). Finding of the BI bentonite in the När core above the Öglunda Limestone indicates correlation of the lower Pirgu Stage with the lower part of the Upper Jonstorp Formation as defined by Jaanusson (Reference Jaanusson1963). The Lower Jonstorp Formation may correlate with the lower part of the Pirgu Stage or with the Vormsi Stage in the East Baltic. This new version of correlation must be considered preliminary and it needs confirmation by studying bentonites in more sections from Sweden.

Figure 7. Summary stratigraphic chart of the upper Katian Pirgu Stage in the East Baltic and Scåne, Sweden.

Correlations of the BII and BIII bentonites from the middle and upper parts of the Halliku Formation in the transition zone with the upper part of the Jelgava Formation in the Valga section and the upper part of the Jonstorp Formation in the Taagepera and Aizpute sections and BIII from the Adila Formation of the Viki core from the southern part of the shallow palaeoshelf area indicate that these most-argillaceous parts of the middle of the Pirgu Stage are coeval.

Limestones of the Oostriku Formation as they occur in the Varbla and Kuressaare sections (Fig. 2) clearly occur higher than the BII bentonite and correlate with limestones of the Paroveja Formation in Latvia. The Oostriku Formation contains bentonite BIV. In the Põltsamaa, Pärnu and Laeva sections, in the transition zone, the Oostriku Formation probably correlates with the gap indicated by the pyrite-rich discontinuity surface between the Halliku and Kabala formations.

An absence of bentonites higher than BI in North Estonian sections may indicate a gap caused by late Katian to Hirnantian sea level fall(s) due to the glaciation in Gondwana and erosion of sediments (Bergström et al. Reference Bergström, Eriksson, Young, Ahlberg and Schmitz2014).

It is not surprising that bentonites which occur in lithologically variable sections of the East Baltic have only been sporadically preserved in drill cores. In general, this time interval is characterized by a regression cycle, temporary sea level low-stand periods and gaps in sedimentation, which are reflected in very complicated lithofacies schemes (Harris et al. Reference Harris, Sheehan, Ainsaar, Hints, Männik, Nõlvak and Rubel2004; Hints, Oraspõld & Nõlvak, Reference Hints, Oraspõld and Nõlvak2005; Fig. 2), considerably differing from the situation in the sections of the uppermost Sandbian in the East Baltic area below and above the Kinnekulle Bentonite (Bergström et al. Reference Bergström, Huff, Kolata and Bauert1995). Such a difficult and complicated depositional situation is well revealed also in the distribution of bentonite layers in the sections included in this study (Figs 2, 3).

5.b. Estimation of source magma composition

The source magmas of the Katian bentonites from the East Baltic have been interpreted as high-Th rhyolites (Kiipli, Soesoo & Kallaste, Reference Kiipli, Soesoo, Kallaste, Corfu, Gasser and Chew2014). Moderate Nb contents indicate subalkaline source magmas (Table 1). TiO2 contents below 0.5 % and moderately high Zr contents between 150 and 300 ppm in the East Baltic and Röstånga bentonites (Fig. 8) confirm that source magmas were evolved rhyolites. Lower Zr/TiO2 ratios in the Jämtland and Västerbotten bentonites compared with those from the East Baltic and Röstanga (Fig. 8) indicate less-evolved dacitic or andesitic source magmas. Elevated Ga in the Jämtland and Västerbotten bentonites may hint at a slightly alkaline trachyandesitic composition. Higher Al2O3 contents (22–25 %) in the Jämtland, Västerbotten and Scottish bentonites compared with normal concentrations in an evolved source (12–16 %) indicate significant enrichment in Al and immobile trace elements compared with the source magma due to the alteration process which removed much of the SiO2. Removal of SiO2 is proven by frequent chert in host rocks near the bentonites (e.g. Laufeld & Jeppsson, Reference Laufeld and Jeppsson1976). This enrichment is reflected by a shift of the Jämtland, Västerbotten and Scottish points on Figure 8 to the right and up compared with the East Baltic bentonites.

Figure 8. Estimation of source magma composition of the upper Katian bentonites.

5.c. Interpretation of volcanic source area

In the East Baltic area, bentonites in the Pirgu Stage occur in recognizable thicknesses (up to 30 cm) in the northernmost area in Estonia. In Latvian and South Estonian sections, volcanic material has been extracted from shaly interbeds of terrigenous origin. In Latvian sections studied by Hints, Oraspõld & Nõlvak (Reference Hints, Oraspõld and Nõlvak2005) the thicknesses are not reported, and some bentonites from their article, not studied here, are marked with a plus sign in Figure 9 showing the distribution of the thickest BII bentonite. In the southernmost sections of Lithuania bentonites are not recorded (Hints, Oraspõld & Nõlvak, Reference Hints, Oraspõld and Nõlvak2005). This distribution pattern suggests the arrival of a volcanic ash cloud from the north or northwest. Data from Sweden and Denmark suggest a similar distribution. Bentonites of Pirgu age are not recorded in Bornholm (Schovsbo et al. Reference Schovsbo, Nielsen, Klitten, Mathiesen and Rasmussen2011); three bentonites with thicknesses around 1 cm occur in the Röstånga core (Bergström et al. Reference Bergström, Huff, Koren, Larsson, Ahlberg and Kolata1999), and northwards, in the Lindegård core five bentonites less than 5 cm in thickness are known (Glimberg, Reference Glimberg1961). Further to the north, in the Kinnekulle section in the Upper Jonstorp Formation, two bentonites have been found (Jaanusson, Reference Jaanusson1963).

Figure 9. Distribution and thickness (cm) of the BII bentonite, plus marks showing where volcanic material was extracted from shaly interbeds or the thickness was not recorded.

Volcanic and plutonic rocks of Katian age are known in the Helgeland Nappe Complex in the Scandinavian Caledonides (Barnes et al. Reference Barnes, Yoshinobu, Prestvik, Nordgulen, Karlsson and Sundvoll2002, Reference Barnes, Frost, Yoshinobu, McArthur, Barnes, Allen, Nordgulen and Prestvik2007). In the Seve Nappe in Jämtland, metamorphic minerals originated in ultra-high-pressure conditions with an age of 450 Ma, which indicates subduction of crustal rocks (Brueckner, Roermund & Pearson, Reference Brueckner, Roermund and Pearson2004; Klonowska et al. Reference Klonowska, Majka, Janák, Gee, Ladenberger, Corfu, Gasser and Chew2014; Majka et al. Reference Majka, Janák, Andersson, Klonowska, Gee, Rosen, Kosminska, Corfu, Gasser and Chew2014). We propose that intrusions in the Helgeland Nappe Complex (Kiipli, Soesoo & Kallaste, Reference Kiipli, Soesoo, Kallaste, Corfu, Gasser and Chew2014) or the subduction process (Seve Nappe) accompanied by presently unknown magmatism could have supplied the Pirgu sediments in Baltoscandia with volcanic ash (Fig. 9). In reconstructing the volcanic source area it is important to mention that Caledonian rocks have been displaced up to 400 km eastward from their original location (Gee et al. Reference Gee, Juhlin, Pascal and Robinson2010). Considering that the distance from Jämtland to Central Estonia is 800 km, we see that the volcanic eruptions during Pirgu time were very large, distributing ash to more than 1200 km from the source.

5.d. Comparison with bentonites from Dob's Linn, Scotland

Bentonites from the complanatus and anceps graptolite zones are known from the Dob's Linn section of southern Scotland (Batchelor & Weir, Reference Batchelor and Weir1988, samples prefixed by DL; Merriman & Roberts, Reference Merriman and Roberts1990; Huff et al. Reference Huff, Merriman, Morgan and Roberts1993, samples prefixed by BRC). Geochemical comparison of the Dob's Linn and Baltoscandian bentonites reveals remarkable similarities. The bentonites from Dob's Linn have high contents of Th reaching 60 ppm. DL4 has a quite similar immobile trace element spectrum to B0 in the East Baltic, BRC-291 and DL8 to BI and BIII, DL6 to Röstånga-3, and BRC-24 to the Jämtland and Västerbotten bentonites (Fig. 6), suggesting that some of these bentonites may correlate. On the TiO2–Zr chart (Fig. 8) DL4, DL6, DL8 and BRC-291 plot close to the East Baltic and Röstånga bentonites. Higher Al2O3 concentrations (26–29 %) in Dob's Linn bentonites indicate greater residual enrichment with immobile elements than in the East Baltic and Röstånga bentonites, and accordingly, the original concentrations in the source magmas must have been very close in all these localities.

According to the palaeotectonic reconstructions (Leslie, Smith & Soper, Reference Leslie, Smith, Soper, Higgins, Gilotti and Smith2008), the Southern Uplands of Scotland were located north of the Iapetus suture on the Laurentia side of the Iapetus Palaeo-Ocean. This interpretation suggests that in Late Ordovician time the Dob's Linn section was far away from the probable volcanic source of the Baltoscandian bentonites. But considering that the rocks from Dob's Linn are oceanic sediments containing abundant graptolites (Underwood et al. Reference Underwood, Crowley, Marshall and Brenchley1997; Grieg et al. Reference Grieg, Goodlet, Lumsden and Tulloch2005) it is possible that in Late Ordovician time the Southern Uplands of Scotland was located somewhere in the middle of the Iapetus Palaeo-Ocean and was attached to the Laurentian continent later, during the Scandian collision of Laurentia with Baltica. The geochemical similarity of the volcanic ashes in Baltoscandia and Dob's Linn, suggesting the same source, supports this interpretation. The same volcano could distribute ash in different directions: to Baltica or to Dob's Linn depending on weather conditions during the eruption. Considering that the ashes had been transported to the East Baltic from a distance of 1200 km, other eruptions of similar power could also reach more than 1000 km to the southwest where the sediments of the Southern Uplands formed at that time. Geochemical similarities between the bentonites in southern Scotland and Baltica for the Lower Silurian Osmundsberg Bentonite were noted by Inanli, Huff & Bergström (Reference Inanli, Huff and Bergström2009) and for the Ordovician Sandbian bentonites by Batchelor (in press).

6. Conclusions

  1. (1) Katian bentonites in the East Baltic can be reliably correlated using the XRD analysis of the sanidine phenocryst composition. Volcanic phenocrysts can often be extracted, analysed and identified even from visually common terrigenous shaly interbeds. Bentonites from five volcanic eruptions have been established in the East Baltic.

  2. (2) The absence of several bentonites in shallow-sea sediments found in a deep palaeo-sea area indicates extensive breaks in sedimentation and erosion during late Katian and Hirnantian sea level falls.

  3. (3) Ratios of immobile trace elements TiO2, Nb, Zr and Th to Al2O3 have enabled correlations to be extended to Scandinavia, where likely late diagenetic alterations have caused recrystallization of sanidine phenocrysts.

  4. (4) The areal distribution pattern of Katian bentonites in Baltoscandia indicates a volcanic source from the north or northwest (present-day orientation). Signatures of ultra-high-pressure metamorphism in the Seve Nappe (Central Sweden) and intrusions in the Helgeland Nappe Complex in Central Norway have been proposed as potential sources for volcanic ashes deposited in the East Baltic during Katian times.

  5. (5) The geochemical similarity between Baltoscandian and Dob's Linn bentonites from Scotland of Katian age suggests the same volcanic source. This hypothesis needs to reinterpret the location of the Southern Uplands of Scotland as being not close to Laurentia, but more likely in the middle of the Iapetus Palaeo-Ocean in Katian times.

Acknowledgements

This study is a contribution to IGCP591, Estonian Science Foundation project no 8963, the Estonian Research Council project no SF0140016s09 and Estonian Science Agency project PUT378 and PUT759. Dahlqvist was supported by the VR-project SCANDEX, No. 2010–3855. Many thanks to Saima Peetermann and two anonymous reviewers for helpful suggestions and for improving language.

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Figure 0

Figure 1. Location of sections studied and discussed in text. (a) Sections sampled from Estonia; (b) sections sampled or discussed from Sweden, Norway, Denmark, Latvia and Lithuania. Broken lines separate palaeoenvironmental zones and confacies belts.

Figure 1

Figure 2. Correlation of Katian and Hirnantian sections in Estonia. For legend see Figure 3. B0, BI, BII, BIII and BIV are indexes of bentonites. Among the chitinozoans, only ranges of zonal forms are shown.

Figure 2

Figure 3. Correlation of Katian and Hirnantian sections of Sweden, Latvia and Estonia. Among the chitinozoans, only ranges of zonal forms are shown.

Figure 3

Table 1. Results of XRF analyses of upper Katian bentonites

Figure 4

Figure 4. Composition of altered volcanic ashes of the Pirgu Stage.

Figure 5

Table 2. Composition of sanidine phenocrysts and occurrence of biotite in studied samples

Figure 6

Figure 5. Composition of sanidine phenocrysts in the bentonites of the Pirgu Stage from the East Baltic according to XRD analysis. Frames embrace correlated bentonites shown by different symbols.

Figure 7

Figure 6. Ratios of immobile trace elements to Al2O3 in upper Katian bentonites. Bars from left to right: black – TiO2(%)/Al2O3 (%) × 50; white – Nb(ppm)/Al2O3 (%); grey – Zr(ppm)/Al2O3 (%) × 0.1; banded – Th(ppm)/Al2O3 (%).

Figure 8

Figure 7. Summary stratigraphic chart of the upper Katian Pirgu Stage in the East Baltic and Scåne, Sweden.

Figure 9

Figure 8. Estimation of source magma composition of the upper Katian bentonites.

Figure 10

Figure 9. Distribution and thickness (cm) of the BII bentonite, plus marks showing where volcanic material was extracted from shaly interbeds or the thickness was not recorded.