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A System of Nomenclature for Rocks in Olkiluoto

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A System of Nomenclature for Rocks in Olkiluoto Working Report 2006-32 A System of Nomenclature for Rocks in Olkiluoto Jussi Mattila June 2006 POSIVA OY FI-27160 OLKILUOTO, FINLAND Tel +358-2-8372 31 Fax +358-2-8372 3709 Working Report 2006-32 A System of Nomenclature for Rocks in Olkiluoto Jussi Mattila Posiva Oy June 2006 Working Reports contain information on work in progress or pending completion. A SYSTEM OF NOMENCLATURE FOR ROCKS IN OLKILUOTO ABSTRACT Due to international interest in the Finnish deep repository project at Olkiluoto (SW Finland) and the need for collaboration between scientists involved in site investigations for the disposal of spent nuclear fuel in other countries, a well-documented system of rock nomenclature is required, based on existing classification schemes and international recommendations. The BGS (British Geological Survey) rock classification scheme is the most comprehensive rock classification scheme and the basic principles behind it are utilised for the system of nomenclature for rocks in Olkiluoto. The BGS classification system is based on the use of descriptive names and a clear hierarchy, making it possible to classify rocks at different levels depending on the specific goals of the study, the level of available information, and the expertise of the user. Each rock type is assigned a root name, which is based on structural and textural characteristics or modal compositions of the rock and the root names are refined with qualifier terms as prefixes. Qualifier terms refer to the structure or modal composition of the rock. The bedrock at the Olkiluoto site consists of metamorphic and igneous rocks. The metamorphic rocks consist of migmatitic gneisses and (non-migmatitic) gneisses, which are further divided according to their structural characteristics and modal compositions, the former into stromatic, veined, diatexitic gneisses, the latter into mica, quartz, mafic and TGG gneisses. Igneous rocks consist of pegmatitic granites, K-feldspar porphyry and diabases. Keywords: Rock classification, rock nomenclature, metamorphic rocks, igneous rocks, Olkiluoto, Finland, disposal of spent nuclear fuel OLKILUODON KIVIEN NIMEÄMISKÄYTÄNTÖ TIIVISTELMÄ Ydinjätteiden loppusijoitukseen liittyvän kansainvälisen kiinnostuksen ja yhteistyön vuoksi on kivien luokittelussa ja nimeämisessä tarpeellista noudattaa olemassa olevia luokitussysteemejä sekä kansainvälisiä suosituksia. Britannian geologian tutkimuskeskuksen (BGS) kivien luokittelusysteemi on ainoa olemassa oleva perusteellinen kivien luokittelusysteemi ja sen perusperiaatteita on myös käytetty Olkiluodon kivien nimeämisessä. BGS:n luokittelusysteemi perustuu kuvailevien termien sekä hierarkian käyttöön, joka mahdollistaa kivien luokittelun eri tasoilla riippuen tehtävän tutkimuksen tarkoituksesta, käytettävissä olevan tiedon määrästä sekä käyttäjän asiantuntemuksesta. Luokittelusysteemissä jokaiselle kivilajille määritellään perusnimi, joka pohjautuu kiven rakenteeseen, tekstuuriin tai modaaliseen koostumukseen ja lisäksi perusnimeä voidaan täydentää ylimääräisillä määritteillä, jotka perustuvat kiven rakenteeseen, tekstuuriin tai mineralogiaan. Olkiluodon metamorfiset kivet koostuvat migmatiittisista sekä (ei-migmatiittisista) gneisseistä, jotka voidaan edelleen jakaa alaluokkiin kiven rakenteen ja mineralogian perusteella. Alaluokkia ovat stromaattiset, suoni-, diateksiittiset, kiille-, kvartsi-, mafiset ja TGG-gneissit. Syväkivet muodostuvat puolestaan pegmatiittisistä graniiteista, K- maasälpäporfyyreistä sekä diabaaseista. Asiasanat: Kivien luokitus, kivien nimeäminen, metamorfiset kivet, syväkivet, Olkiluoto, Suomi, ydinjätteiden loppusijoitus 1 TABLE OF CONTENTS ABSTRACT TIIVISTELMÄ 1 INTRODUCTION .................................................................................................... 2 2 BACKGROUND FOR THE NOMENCLATURE ...................................................... 3 3 NOMENCLATURE FOR ROCKS IN OLKILUOTO ................................................. 4 3.1 BASIC CONCEPTS AND SUBDIVISION OF THE ROCK TYPES ................................... 4 3.2 ROOT NAMES AND QUALIFIER TERMS ................................................................. 5 3.2.1 Metamorphic rocks.................................................................................. 6 3.2.2 Igneous rocks.......................................................................................... 9 4 USING DIFFERENT LEVELS OF HIERARCHY IN THE NOMENCLATURE OF ROCKS ................................................................................................................. 10 5 FUTURE MODIFICATIONS TO THE NOMENCLATURE .................................... 12 6 CORRELATION TO OLD ROCK NAMES ............................................................ 13 SUMMARY ................................................................................................................... 14 REFERENCES ............................................................................................................. 15 APPENDIX 1................................................................................................................. 16 2 1 INTRODUCTION Due to international interest in the Finnish deep repository project at Olkiluoto (SW Finland) and the need for collaboration between scientists involved in site investigations for the disposal of spent nuclear fuel in other countries, a well-documented system of rock nomenclature is required, based on existing classification schemes and international recommendations. Any good and practical system of rock classification (or any other classification system) should fulfil the following requirements: x The scheme should be based on descriptive terms, which can be determined unambiguously based on features directly observable in the field. Apart from the primary classification of rocks into metamorphic, igneous and sedimentary, any genetic terms should be disfavoured, since understanding of the genetic nature of rocks may change within time, whereas the textural, structural and modal properties of rocks do not. x The scheme should have a hierarchical structure, which is a prerequisite for being simple and practical in application, proceeding in a series of levels and giving the possibility of classifying rocks in different levels according to the level of information available and the expertise of the user. x Well-established names should be used as far as possible, yet changes should be allowed if necessary. Changes may be necessary, for example, in cases where the existing terminology does not fully describe the whole scale of rock characteristics and does not serve the goal of a specific study. Careful documentation of such changes is important in any proposed classification scheme. This report introduces a system of nomenclature, which is applicable to Olkiluoto during the presently ongoing site confirmation studies and establishes a link to existing classification schemes and international recommendations. 3 2 BACKGROUND FOR THE NOMENCLATURE IUGS (International Union of Geological Sciences) and its subcommissions are currently working towards the establishment of unified terminology and classification of rock types. However, the work is far from ready and so far the IUGS has only produced general principles and recommendations on the use of terms. The process of introducing a comprehensive classification system is still some time in the future. The BGS (British Geological Survey) rock classification scheme (Robertson 1999, Gillespie & Styles 1999) is at present the most comprehensive classification system for rocks. It adopts the general principles of the current IUGS recommendations, yet, as stated by Gillespie & Styles (1999), it “... contains many refinements and changes to the IUGS recommendations, where it was considered necessary and appropriate, and the resulting scheme is more logical, consistent, systematic and clearly defined”. The basic principle of the BGS scheme is the use of a relatively small amount of accepted root names with or without qualifier terms, based, as far as possible, on descriptive attributes. The use of genetic terms is discouraged, as opinions on the genetic factors of rocks are likely to change during time whereas textural and structural characteristics and modal compositions do not. These basic principles have also been utilised in the nomenclature of rocks in Olkiluoto 4 3 NOMENCLATURE FOR ROCKS IN OLKILUOTO 3.1 Basic concepts and subdivision of the rock types The bedrock at the Olkiluoto site consists of two main types of rock, metamorphic rocks and igneous rocks (treated separately in the BGS system in the reports of Robertson 1999 and Gillespie & Styles 1999, respectively). The metamorphic rocks in Olkiluoto are gneisses, based on their texture and grain size, and can be further divided into migmatitic gneisses and (non-migmatitic)1 gneisses. Migmatitic gneisses can be classified according to the migmatitic structure of the rock into stromatic, veined and diatexitic gneisses. (Non-migmatitic) gneisses consist of four subgroups, which are named according to their modal composition as mica, quartz, mafic and TGG gneisses. Igneous rocks are subdivided into pegmatitic granites, K-feldspar porphyry and diabase. Igneous rocks are named according to their modal composition and texture, with the exception of diabase, which is named according to its field association, i.e. diabase is a basaltic rock occurring as dikes and sills. The subdivision of the rocks in Olkiluoto is shown in Figure 1. 1 The term non-migmatitic is a trivial qualifier
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