Printer Friendly

New correlations of Telychian (Silurian) bentonites in Estonia/Telychi (Silur) bentoniitide uued korrelatsioonid Eestis.

INTRODUCTION

The use of bentonites in the correlation of geological sections offers a unique possibility for recognition of exactly the same time levels in several outcrop and drill-core sections (e.g. Einasto et al. 1972). Limestones (Rumba Formation) and marlstones (Velise Formation) of the Adavere Stage contain a large number of thin altered volcanic ash beds--bentonites (Jurgenson 1964). These regional stratigraphic units belong to the Telychian Stage of the international stratigraphic scheme (Bergstrom et al. 1998; Nestor & Nestor 2002; Kiipli et al. 2006). The sanidine composition has been studied in twelve drill-cores with an aim to identify the ash beds in the Telychian of Estonia. The results show that the volcanogenic interbeds originate from at least 37 different eruptions (Kiipli et al. 2001; Kiipli & Kallaste 2002), although the greatest number of bentonite interbeds found in one section is only 22.

The aims of bentonite study are precise correlation of sections, mapping of the distribution of bentonites, and restoration of wind directions at the eruption time and locations of source volcanoes. Herein, we are going to report the results obtained through the study of bentonites in six new sections using the same method as in Kiipli & Kallaste (2002). To bring some clarity to this rather difficult and confused subject, identification (ID) codes were assigned to all the identified bentonites. Names were given to more widespread bentonites.

MATERIAL AND METHODS

Seventy-seven bentonite samples from six drill-cores (see Fig. 1) were collected from 0.2-20 cm thick interbeds, which differed from their host marlstones and limestones in their soft clayish consistency and/or in colour. The abundance of biotite flakes was a good criterion for recognizing volcanogenic bentonites in situ. For correlation purposes, the data on bentonites studied earlier in the Ohesaare, Viki, and Ruhnu sections were used (Kiipli & Kallaste 2002, 2003). The lithology and distribution of microfossils of many studied sections are discussed in Einasto et al. (1972), Jeppsson & Mannik (1993), Nestor (1994), and Hints et al. (2006).

[FIGURE 1 OMITTED]

All bulk samples were scanned by X-ray diffractometry (XRD) from 5 to 45 degrees using Co Ka radiation. The occurrence of illite-smectite and/or kaolinite reflections was considered as an indication of volcanogenic material. Some volcanogenic interbeds had a high content of authigenic potassium feldspar. A low content or absence of quartz was typical of bulk volcanogenic bentonite material.

The Na content of K-Na sanidine was studied by XRD (Kiipli & Kallaste 2002). From the separated 0.04-0.1 mm fraction the range from 23.5 to 26.0[degrees]2[theta] was scanned using Co K[alpha] radiation with the step size of 0.01[degrees]2[theta]; the measuring time was 15 s per point. The content of NaAlSi308 in K-Na sanidine (in mol %) was calculated according to Orville (1967), who established that the position of the 20 [bar.1] reflection almost linearly depends on the composition of sanidine solid solution. The precision of the analysis of the K-Na sanidine composition was [+ or -] 1% in favourable cases (low intensity of authigenic feldspar reflection, no kaolinite, high intensity of the reflection of interest) and [+ or -] 2% in less favourable cases. Separate beds were correlated on the basis of the magmatic K-Na sanidine composition (Table 1). As several bentonites may have the same sanidine composition, graphic correlation between sections was applied to improve the probability of correlations.

RESULTS

The NaAl[Si.sub.3][O.sub.8] content of sanidine varied from 21 mol % in the Osmundsberg Bentonite to 45-48 mol % in the Valgu, Ruhnu, and Viki bentonites. The width of the sanidine reflection varied from sharp (0.05-0.15[degrees]2[theta]; indicating homogeneous sanidine composition) to very wide (exceeding 0.35[degrees]2[theta]). Such wide reflections were difficult to characterize in numerical values and were described as wide or very wide (Table 1). A wide sanidine reflection clearly discriminates a particular bentonite from those with sharp reflections, but is useless for discrimination between other bentonites with wide sanidine reflections. Wide reflections probably indicate heterogeneous (maybe zoned) sanidine crystals.

By combining sanidine properties and graphic correlation most of the studied bentonites can be correlated with volcanogenic interbeds established earlier in other drill-cores (Table 2). Many volcanic eruptions have been detected in more than five sections, thus their stratigraphic position among other bentonites is well proven. Compared with the earlier study (Kiipli & Kallaste 2002), six new volcanic eruptions have been established: ID 504, 720, 750, 773, 793, and 794, five of those only in one section. ID 504 was additionally found in the earlier studied Kaugatuma section at a depth of 262.2 m and possibly in the new Kihnu section in the lower part of the 10 cm thick bentonite at a depth of 211.95 m. The total number of Telychian volcanic eruptions recorded in Estonia is 43.

BENTONITE IDENTIFICATION NUMBERS AND NAMES

The former bentonite ID numbers (Kiipli et al. 2001) started from 0 (Osmundsberg Bentonite) and were assigned in stratigraphic order up- and downward from it. However, difficulties arose when new bentonites were found, because no vacant numbers were available between the earlier known and numbered bentonites. Therefore, new bentonite fords were left without an ID number in Kiipli & Kallaste (2002).

In the present study, new stratigraphic ID numbers were assigned to all established bentonites. The ID numbers were derived from the projection of the bentonite stratigraphic position to the Viki drill-core depth scale. The number marks the depth in decimetres in the Viki section. For the sake of shortness, centimetres were discarded from the end of the depth number. Besides, 1000 decimetres were subtracted from the depth, as all bentonites occur between 1000 and 2000 decimetre depth. This combination resulted in a list of three-digit ID codes for bentonites (Tables 1-3). Any new bentonite find can be easily accommodated into this list. The Viki section was selected as a basis for deriving ID numbers because of the great thickness of the Telychian, large number of bentonites, and a well-established conodont biostratigraphy (Jeppsson & Mannik 1993; Kiipli et al. 2001).

Names were assigned to 20 widespread bentonites recognized in more than five sections in Estonia (Table 3). Most of those were named after Estonian drill-cores where they were found. The Valgu Bentonite was named after the Valgu outcrop (Klaamann 1990) in southern Rapla District. The names of Osmundsberg, Lusklint, and Ireviken were applied on the basis of correlation with the described bentonites in the literature (Batchelor & Jeppsson 1994; Bergstrom et al. 1998). The most frequent (found in 10-15 sections) bentonites in Estonia are as follows: Osmundsberg (851), Tehumardi (744), Nurme (731), Virtsu (719), Nassumaa (696), Loetsa (520), Viirelaid (518), Ruhnu (494), Viki (475), and Kirikukula (457). Correlation of these and other named bentonites forms a well-proved framework, where the stratigraphic position of rarely occurring bentonites can be established.

DISCUSSIONS ON SEDIMENTOLOGY IN THE TELYCHIAN

Bentonites were formed from very fine-grained volcanic dust and are therefore rarely preserved in shallow-water sediments from where wave activity transports fine ash material to the deeper and quieter sedimentary environments. The Telychian sediments in Estonia are represented by relatively deep-water marlstones and limestones, containing therefore a large number of bentonites. Despite a presumed quiet sedimentary environment, the record of bentonites in these sediments is still uneven. As a maximum, only 22 out of the total of 43 bentonites were found in one core (Ohesaare). Although the completeness of an established bentonite record depends partly on the quality of drilling and the experience of the sample-collecting researcher, the studied material revealed some regularities in the natural distribution of bentonites (Kiipli & Kallaste 2002 and the present study):

1. The Ireviken and Lusklint bentonites occur only in the sections of south-western Saaremaa. They are lacking in eastern Saaremaa and mainland Estonia. Often even the Ohesaare and Aizpute bentonites are absent there. This gap in the bentonite record was probably caused by a break in sedimentation near the Llandovery--Wenlock boundary, which was also proposed on the basis of biostratigraphical evidence (Nestor & Nestor 2002, 2003). Now this is also confirmed by the distribution of bentonites.

2. In many drill-cores in the eastern part of the study area several bentonites are absent in the lower part of the Velise Formation and the Velise--Rumba transition interval. The most extensive gap in the bentonite records was established in the Ruhnu section, where even the bentonites of the Rumba Formation are entirely absent. In mainland Estonia, the Paatsalu section is the only exception with its almost full record of bentonites in the lower part of the Velise Formation. The best records of bentonites in this interval were established in the southwestern part of Saaremaa Island (Tehumardi, Viki, and Ohesaare sections). This gap in the record of bentonites was possibly caused by a major hiatus in sedimentation near the Rumba--Velise boundary.

3. Correlation of bentonites from the Nurme section is provisional as all bentonites there are very rich in authigenic feldspar, which complicates seriously the analysis of sanidine. The Nurme and Tehumardi bentonites were identified on the basis of sanidine composition, but other bentonites were correlated only graphically.

4. Correlation of bentonites 518, 520, and 521 (embraced by the frame in Table 2) is provisional as these bentonites reveal similar wide sanidine reflections and occur closely in a section. The occurrence of at least three bentonites with similar properties at this level is proved by the Kuressaare section, where all three bentonites were found.

5. In some cases an extremely low rate of sedimentation caused the deposits of two succeeding eruptions to merge. As a result, the sanidine originating from those different eruptions occurs within a single volcanic ash bed. The examples are 731 + 744 (Nurme and Tehumardi bentonites) in the Ruhnu section and 823 (Valgu) + 818 bentonites in the Ohesaare section.

6. The occurrence of a mixed (823 + 818) bentonite in the Ohesaare section at 370.77 m, which in other sections is found in the Rumba--Velise transition interval, indicates the presence of a condensed marlstone section in Ohesaare (370.9-372.6 m) corresponding to the Rumba Formation in shallow-water sections.

7. The mapped thickness of the volcanic ash layer can provide useful information on the direction of wind at the time of eruption and location of the source volcano. Up to now only the Kinnekulle eruption layer (Caradoc) is well mapped over a large area in Baltoscandia (Vingisaar 1972; Bergstrom et al. 1995). A number of the Osmundsberg Bentonite outcrop sites were described by Bergstrom et al. (1998). The thickness map of the Osmundsberg Bentonite in Estonia, presented by Kiipli et al. (2006), indicates ash transport from the northwest. The thickness of three other Telychian bentonites in Estonia shows different distribution patterns (Fig. 2). The Ruhnu Bentonite is characterized by even distribution of thickness (3-5 cm). Possibly the source volcano was located so far that no changes could be observed within the small studied area measuring 150 km x 200 km. The thickness of the Nassumaa and Nurme bentonites decreases rapidly to the southeast, probably perpendicular to the ash cloud axis. If this interpretation is correct, the Nassumaa ash was transported from the southwest and the Nurme ash from the west.

8. Restricted distribution of several bentonites, including all new discoveries, can be explained by patchy sedimentation accompanied by areas of 0-sedimentation, small thickness of many bentonites complicating their identification, and loss of soft bentonite interbeds during drilling.

[FIGURE 2 OMITTED]

CONCLUSIONS

The study of bentonites in new Telychian sections revealed a more complete volcanogenic record in Estonia including bentonites from 43 different volcanic eruptions. The assigned ID numbers and stratigraphic names make it easier to handle the information available on bentonites. New correlations enable us to trace gaps in the sedimentary record. Large hiatuses were confirmed at the transition of the Rumba and Velise formations and the Llandovery--Wenlock boundary. On the basis of the identification of bentonites, we assume that the deep-sea marlstone in the Ohesaare section correlates with the shallow-water Rumba Formation. Thickness distribution patterns of bentonites indicate volcanic ash transport from different directions and, correspondingly, from different sources.

ACKNOWLEDGEMENTS

This study is a contribution to IGCP project 503 and was supported by the Estonian Science Foundation (grants 5921, 6749, and target funding project 0332652s04). We are grateful to the referees D. Kaljo and R. A. Batchelor for useful comments and suggestions.

Received 18 May 2006, in revised form 27 June 2006

REFERENCES

Batchelor, R. A. & Jeppsson, L. 1994. Late Llandovery bentonites from Gotland, Sweden, as chemostratigraphic markers. J. Geol. Soc. London, 151, 741-746.

Bergstrom, S. M., Huff, W. D., Kolata, D. R. & Bauert, H. 1995. Nomenclature, stratigraphy, chemical fingerprinting and areal distribution of some Middle Ordovician K-bentonites in Baltoscandia. GFF, 117,1-13.

Bergstrom, S. M., Huff, W. D. & Kolata, D. R 1998. The Lower Silurian Osmundsberg K-bentonite. Part I: stratigraphic position, distribution, and palaeogeographic significance. Geol. Mag., 135,1-13.

Einasto, R., Nestor, H., Kala, E. & Kajak, K. 1972. Correlation of the Upper Llandoverian sections in West Estonia. Eesti NSV Tead. Akad. Toim. Keemia Geol., 21, 333-343 (in Russian).

Hints, O., Killing, M., Mannik, P. & Nestor, V. 2006. Frequency patterns of chitinozoans, scolecodonts, and conodonts in the upper Llandovery and lower Wenlock of the Paatsalu core, western Estonia. Proc. Estonian Acad. Sci. Geol., 55, 128-155.

Jurgenson, E. 1964. Silurian metabentonites of Estonian SSR. In Litologiya Paleozojskikh odozhenij Estonii, pp. 87-100. Institute of Geology, Tallinn (in Russian).

Jeppsson, L. & Mannik, P. 1993. High resolution correlations between Gotland and Estonia near the base of the Wenlock. Terra Nova, 5, 348-358.

Kiipli, E., Kiipli, T. & Kallaste, T. 2006. Identification of the O-bentonite in the deep shelf sections with implication on stratigraphy and lithofacies, East Baltic Silurian. GFF (submitted).

Kiipli, T. & Kallaste, T. 2002. Correlation of Telychian sections from shallow to deep sea facies in Estonia and Latvia based on the sanidine composition of bentonites. Proc. Estonian Acad. Sci. Geol., 51,143-156.

Kiipli, T. & Kallaste, T. 2003. Altered volcanic ash beds. In Ruhnu (500) Drill Core (Poldvere, A., ed.), Estonian Geol. Sections, 5, 31-33.

Kiipli, T. & Kallaste, T. 2006. Wenlock and uppermost Llandovery bentonites as stratigraphic markers in Estonia, Latvia and Sweden. GFF, 128, 139-146.

Kiipli, T., Mannik, P., Batchelor, R. A., Kiipli, E., Kallaste, T. & Perens, H. 2001. Correlation of Telychian (Silurian) altered volcanic ash beds in Estonia, Sweden and Norway. Norwegian J. Geol., 81, 179-194.

Klaamann, E. 1990. Locality 8:3 Valgu outcrop. In Field Meeting Estonia 1990 (Kaljo, D. & Nestor, H., eds), pp. 181-182. Estonian Academy of Sciences, Tallinn.

Nestor, H. & Nestor, V. 2002. Upper Llandovery to Middle Wenlock (Silurian) lithostratigraphy and chitinozoan biostratigraphy in southwestern Estonia and northernmost Latvia. Proc. Estonian Acad. Sci. Geol., 51, 67-87.

Nestor, H. & Nestor, V. 2003. Adavere lademe vanusest ja piiridest. In Eesti Geoloogide neljas ulemaailmne kokkutulek Eesti geoloogia uue sajandi kunnisel. Konverentsi materjalid ja ekskursioonijuht (Plado, J. & Puura, I., eds), pp. 53-55. EGS, TU Geoloogia Instituut.

Nestor, V. 1994. Early Silurian chitinozoans of Estonia and North Latvia. Academia, 4.

Orville, P. M. 1967. Unit-cell parameters of the microcline-low albite and the sanidine-high albite solid solution series. Amer. Mineral., 52, 55-86.

Vingisaar, P. 1972. On the distribution of the main metabentonite stratum (d, XXII) in the Middle Ordovician of Baltoscandia. Eesti NSV Tead. Akad. Toim. Keemia Geol., 21, 62-70 (in Russian).

Toivo Kallaste and Tarmo Kiipli

Institute of Geology at Tallinn University of Technology, Estonia pst. 7, 10143 Tallinn, Estonia; tarmo.kiipli@lx.egk.ee
Table 1. Sanidine properties of Telychian bentonites

Identification numbers and names Number of Stage and
 Vild Interpo- Bentonite sections formation
 depth lated depth name
 ID in the Vild
 core, m

127 112.70 Ireviken 6 Jaani Stage
150 115.00 Lusklint 7 Mustjala
210 121.00 Ohesaare 6 Formation
311 131.10 Aizpute 6
457 145.75 Kirikukula 10
475 147.50 Viki 12
480 148.00 Kaugatuma 6
488 148.80 Kuressaare 8
494 149.40 Ruhnu 14
504 150.40 2
518 151.80 Viirelaid 10
520 152.00 L6etsa 10
521 152.10 8?
564 156.40 3
568 156.80 10?
569 156.90 1
658 165.80 2
682 168.20 2
693 169.30 2
696 169.60 Nessumaa 13 Adavere Stage
719 171.95 Virtsu 12 Velise
720 172.00 1 Formation
722 172.20 2
731 173.10 Nurme 13
744 174.40 Tehumardi 11
750 175.00 1
755 175.55 Paatsalu 8
772 177.20 Pahapilli 8
773 177.30 1
774 177.40 1
776 177.60 5
777 177.70 2
788 178.80 2
793 179.30 1
794 179.40 1
795 179.50 Mustjala 6
800 180.00 2
805 180.50 2 Adavere Stage
818 181.80 3 Rumba-Velise
823 182.30 Valgu 5 transition
841 184.15 2
843 184.35 3 Adavere Stage
851 185.10 Osmundsberg 15 Rumba
880 188.00 4 Formation

 Pyroclastic K-Na sanidine,
 main component parameters
Number of
sections Width of the Content of
 reflection NaAl[Si.sub.3]
 (degrees) [O.sub.8] in
 and other sanidine,
 notes mol

 6 Much of biotite and quartz, little sanidine
 7 0.19-0.34 35.2-35.8
 6 0.25-0.35 38-40
 6 0.08-0.12 36.2-37.8
 10 Very wide reflection
 12 0.12-0.20 45.2-46.3
 6 0.18-0.27 42.0-42.8
 8 Very wide reflection
 14 0.05-0.09 45.7-46.4
 2 0.08-0.20 45.6-46.6
 10 Very wide reflection
 10 Very wide reflection
 8? Very wide reflection
 3 0.12-0.17 45.0-45.8
 10? Very wide reflection
 1 0.07 29.00
 2 0.09 45.50
 2 Very wide reflection
 2 0.05 22.60
 13 0.04-0.06 22.9-23.3
 12 Much of biotite and quartz, little sanidine
 1 0.085-0.122 26.5-28.0
 2 26.5 + wide reflection
 13 0.10-0.16 38.7-40.3
 11 0.07-0.10 25.8-26.7
 1 Wide reflection
 8 0.25-0.30 25.526
 8 0.30-0.34 20.524
 1 Feldspathic
 1 0.09-0.12 46.2-48.2
 5 0.07-0.08 28.128
 2 0.25-0.30 22.326
 2 0.17-0.19 40.1-40.6
 1 Biotite flakes on bedding plane
 1 0.12 43.70
 6 0.05-0.11 24.525
 2 Feldspathic
 2 Feldspathic
 3 Very wide reflection
 5 0.12-0.17 45.2-47.6
 2 0.19-0.22 35.5-35.8
 3 Very wide reflection
 15 0.05-0.09 20.721
 4 Very wide reflection

Table 2. Correlated Telychian bentonites in the studied core sections
(depth in metres). Small font indicates that sanidine was not
studied--graphic correlation was used. Provisional correlation is
embraced by a frame

Viki depth Viki Kures- Ohe- Ruhnu Kihnu
ID saare saare

127 158.30 340.76
150 115.00 160.00 342.08
210 121.00 345.83
311 131.10 351.72 459.00
457 145.75 359.31 467.60 211.20
475 147.50 361.30 470.80 211.70
480 148.00 361.70 471.80
488 148.80 184.80 362.23 473.10
494 149.40 185.40 362.46 473.70 211.85
504 211.95
518 151.80 187.40 364.76 478.90 212.20
520 188.50
521 152.10 189.50 365.08 478.90 214.50
564 367.39
568 156.80 193.75 367.60 215.70
569 215.72
658 369.12
682 205.20
693 369.72
696 169.60 369.75 488.24 220.70
719 171.95 205.40 488.30 221.70
720 488.30
722 488.40
731 173.10 369.98 489.05 222.30
744 174.40 489.05 222.70
750 209.20
755 175.55 370.09 223.10
772 210.20 370.44
773
774 210.30
776 370.63
777
788 178.80
793
794 212.70
795 212.80
800
805
818 181.80 214.00 370.77
823 182.30 370.77
841 370.99
843 184.35
851 185.10 215.70 228.40
880 223.80

Viki depth Varbla Paat- Virtsu Nurme
ID salu

127
150
210 134.90
311 137.20
457 139.40 68.55
475 139.85 72.50 69.20
480
488
494 140.28 69.50
504
518 73.70
520 74.00 71.30
521 141.50
564
568 143.25
569
658
682
693
696 146.90 79.25
719 148.00 80.80 80.20
720
722
731 148.20 81.05 80.68 17.90
744 148.60 81.09 20.10
750
755 81.50 21.10
772 82.00 81.40 23.45
773 82.10
774
776 82.40
777 82.40
788
793 83.75
794
795 150.40 83.90
800 150.75
805 24.50
818 24.60
823
841
843
851 155.60 88.20
880 158.30

Table 3. Type localities of bentonites

Viki ID Bentomtename
depth (Kiipli
ID et al.
 2001)

127 29 Ireviken
150 28 Lusklint
210 27 Ohesaare
311 Aizpute
457 26 Kirikukula
475 23 Viki
480 21 Kaugatuma
488 22 Kuressaare
494 19 Ruhnu
518 18 Viirelaid
520 17 Loetsa
696 13 Nassumaa
719 12 Virtsu
731 11 Nurme
744 10 Tehumardi
755 9 Paatsalu
772 7 Pahapilli
795 Mustjala
823 3 Valgu
851 0 Osmundsberg

Viki Type locality
depth
ID

127 Ireviken section, Gotland, Sweden
150 Lusklint section, Gotland, Sweden
210 Ohesaare drill-core, depth 345.8 m
311 Aizpute drill-core, depth 931.8 m
457 Kirikukula drill-core, depth 12.59 m
475 Viki drill-core, depth 147.5 m
480 Kaugatuma drill-core, depth 261.1 m
488 Kuressaare drill-core, depth 184.8 m
494 Ruhnu drill-core, depth 473.7 m
518 Viirelaid drill-core, depth 67.75 m
520 Loetsa drill-core, depth 47.2 m
696 Nassumaa drill-core, depth 219.4 m
719 Virtsu drill-core, depth 80.2 m
731 Nurme drill-core, depth 17.9 m
744 Tehumardi drill-core, depth 185.1 m
755 Paatsalu drill-core, depth 81.5 m
772 Pahapilli drill-core, depth 68.5 m
795 Mustjala drill-core, depth 117.8 m
823 Valgu trench, Rapla district
851 Osmundsberget, Central Sweden

Viki Thick- Reference
depth ness,
ID cm

127 10 Batchelor & Jeppsson (1994)
150 5 Batchelor & Jeppsson (1994)
210 2 Kiipli & Kallaste (2006)
311 1 Kiipli & Kallaste (2006)
457
475 5
480 0.5
488 0.5
494 5
518 1
520 3
696
719 1
731 6
744 1.2
755 4
772 5
795
823 3
851 115 Bergstrom et al. (1998)
COPYRIGHT 2006 Estonian Academy Publishers
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 2006 Gale, Cengage Learning. All rights reserved.

Article Details
Printer friendly Cite/link Email Feedback
Author:Kallaste, Toivo; Kiipli, Tarmo
Publication:Proceedings of the Estonian Academy of Sciences: Geology
Date:Sep 1, 2006
Words:3738
Previous Article:Conodonts of the Kivioli Member, Viivikonna Formation (Upper Ordovician) in the Kohtla section, Estonia/Viivikonna kihistu Kivioli kihistiku...
Next Article:An improved gravity anomaly grid and a geoid model for Estonia/Tapsustatud raskusjou anomaaliate vorgustik ja geoidi mudel Eestis/[TEXT NOT...

Terms of use | Privacy policy | Copyright © 2022 Farlex, Inc. | Feedback | For webmasters |