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

S Radzevičius,T Kallaste,T Kiipli

doi:10.3176/earth.2014.02

Abstract

Integrated correlation of bentonites (altered volcanic ashes) and graptolite biozonation is presented. Detailed study of two Lithuanian drill core sections extended previous knowledge of the occurrence and composition of bentonites to the south. Identification of graptolite species allowed bentonites to be assigned their proper stratigraphical position. Silurian bentonites in Lithuania are mostly characterized by wide and very wide XRD 20 1 reflections of the main component of sanidine phenocrysts. Only fourteen of the 69 samples studied contained sanidine with a sharp reflection, which gave the best correlation potential. In the Lithuanian sections one bentonite was found in the Rhuddanian, five bentonites were recognized in the Aeronian, 17 bentonites in the Telychian, 26 in the Sheinwoodian, 10 in the Homerian and six in the Ludlow. All bentonites found in Lithuania are characterized by the main component of sanidine. A large number of Lithuanian bentonites are not known in Latvia and Estonia, indicating that volcanic ashes reached the East Baltic area from two source regions - the Central European and Norwegian Caledonides.

Highlights

Volcanic ashes occurring in palaeontologically wellcharacterized sedimentary sections offer a unique possibility of detailed correlations between different environments (Kiipli & Kallaste 2002; Kiipli et al 2006, 2008a, 2009, 2010b, 2011, 2012a, 2012b)
Wide sanidine reflections in Telychian bentonites probably indicate other volcanic sources than for ash beds of that age in Estonia and Latvia, many of which are characterized by sharp sanidine X-ray diffractometry (XRD) reflections
Central European sources from the collision zone of Avalonia and Baltica can be supposed. This result confirms that Telychian volcanic ashes, forming a number of the Estonian–Latvian bentonites which are not found in Lithuania, reached the East Baltic area from the northwestern direction – from the margins of the Iapetus Ocean (Kiipli et al 2008b, 2008c, 2010b, 2012a, 2013a)

Summary

Introduction

Volcanic ashes occurring in palaeontologically wellcharacterized sedimentary sections offer a unique possibility of detailed correlations between different environments (Kiipli & Kallaste 2002; Kiipli et al 2006, 2008a, 2009, 2010b, 2011, 2012a, 2012b). The good precision of that method, up to ±1% in favourable cases (Kiipli et al 2011), enables discrimination of bentonites of quite similar composition This is an important property, because successive eruptions from the same volcanic source can be characterized by very similar geochemical and mineralogical signatures (Kiipli et al 2010a). Sanidine may be absent in source magma To avoid these shortcomings, researchers have used the composition of apatite phenocrysts (Batchelor 2009; Carey et al 2009; Ray et al 2011) or trace elements for proving correlations (Huff et al 1998; Kiipli et al 2008b, 2013a); Inanli et al 2009; Hetherington et al 2011.

Objectives

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Discussion

Conclusion