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Stratigraphic Studies on the Precambrian in Finland

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Stratigraphic Studies on the Precambrian in Finland STRATIGRAPHIC STUDIES ON THE PRECAMBRIAN IN FINLAND by Ahti Simonen Simonen, A. 1986, Stratigraphie studies on the Preeambrian in Finland. Geological Survey 01 Finland, Bulletin 336, 21-37.3 figures, one table, one appendix. The main historieal outlines of the stratigraphie study of the Preeambrian in Finland are presented. Detailed field work and the applieation of new methods of research have steadily inereased our stratigraphie knowledge, and many reinterpretations of earlier eonelusions have therefore been neeessary. Key words: stratigraphy, basement , Svecokarelides, Preeambrian. research, history , glossary, Finland Prof. Ahti Simonen, Otakllja 3 D 46, 02/50 Espoo /5, Finland CONTENTS Introduction ." , .. " .... "" .. "" .......... , ... ",., ... ",... ... 22 Main historical outlines .",."" .. .... , ... .. , ... ",." ... ... ""... 22 Pre-Svecokarelidic base me nt complex 27 Svecokarelides 28 Karelidic belt 29 Svecofennidic belt ,., ........... ..... , ... "' . ..... , ,, .. .. .. .. , 32 Post-Svecokarelian rocks ... .. , .. ,." .. ", ... ........ .. """, . ", 34 ConcIuding Remarks .... .... .. ............ ..... .. ... ......... .. , 35 References ...... , . ...... .. , ...... , ..... ,......... .. .... .... 35 Appendix: Explanation of the terms (by Ahti Simonen and Carola Eklundh) Geological Survey of Finland, Bulletin 336 22 Ahti Simonen INTRODUCTION The study of the sueeession of events and Finland. Furthermore, the methods suitable the stratigraphie cIassifieation of formations for stratigraphie interpretation of metamorphie have persistently been the essential problems Preeambrian formations are briefly eonsid­ in the geologieal study of the Preeambrian in ered. Finland. Many stratigraphie sehemes, with As an appendix to this paper, a revised modifieations and reinterpretations, new version is given of the stratigraphie lexieon of suggestions, diseussions and polemies have the Finnish Preeambrian (Simonen 1955) been presented, marking the tortuous path published by the International Geologieal followed by the stratigraphie study of the Congress in the work »Lexique Stratigraphi­ Finnish Preeambrian. que International. » Carola Eklundh has The purpose of this paper is to deseribe the taken an aetive part in the preparation of this main historical outlines of the stratigraphie appendix. The explanations of the terms are cIassifieation of the Preeambrian bedroek in deseriptive, not normative. MAIN HISTORICAL OUTLINES The fundamental basis for the geologieal origin was explained aeeording to the neptu­ study of Finland was laid at the end of the nistie eoneeptions of Werner prevailing at that 19th eentury. The systematie geologieal time. mapping of the eountry was started and the The cIassical investigations of Sederholm first geologieal map sheet on the seale of (1891 and 1897) proved the aetualistie or uni­ 1 : 200 000 published in 1879. Important formitarian method to be the right approach events in the progress of geologieal research to the study of metamorphie Preeambrian in Finland were: formations. The Preeambrian erystalline sehists show as relies many struetures and - The organization of the study of geology textures similar to reeent sedimentary and on the University level. The first oeeupant volcanie rocks, and they were primarily de­ of the ehair of geology at Helsinki Universi­ posited in adefinite sueeession. Sederholm ty was appointed in 1876. stressed that geology is a seienee of time and - The establishment of the Geologieal Com­ the study of a sueeession of events. Therefore, mission (now the Geologieal Survey of the stratigraphie cIassifieation of the Pre­ Finland) in 1885 for the geologieal study eambrian runs like a red thread through his and mapping of the eountry. - The founding of the Geologieal Soeiety of whole work. Sederholm developed and modified his ideas in many papers and en­ Finland in 1886 as a forum for seientifie larged his stratigraphie cIassifieation to eom­ diseussions. prise the whole Baltie or Fennoseandian When the geologieal mapping was started, Shield. His last views were presented in 1932 the granites and gneisses, whieh eompose the in the explanatory text aeeompanying the main part of the Finnish bedroek, were re­ geologieal map of Fennoseandia. The main garded as »primitive formations» and their points of this stratigraphie schema eoneerning Geologieal Survey of Finland, Bulletin 336 Stratigraphie studies on the Preeambrian in Finland 23 the Finnish Preeambrian are presented in represented by the rapakivi granites, whieh Table 1. are not assoeiated with the orogenie move­ The essential points of Sederholm's strati­ ments. Sederholm also drew special attention graphie classifieation were based on the to the eonglomerates eontaining granite peb­ ehronologieal relation of the sehists and bles, for he believed that they marked deep gneisses to the granites of different age groups. erosion and uneonformities separating dif­ Sederholm classified the Preeambrian gran­ ferent eycles of sedimentation. Besides, in ites of Finland into four groups on the assump­ some eases, the separation of the age groups tion that every eycle of deposition of the was erroneously based solelyon a differenee sedimentary rocks was assoeiated with the in the grade of metamorphism, beeause it intrusion of a eertain granite group (see Table was believed that the older rocks show a 1). The granites of the first, se co nd and third higher grade of metamorphism than younger groups were eonneeted with orogenie eycles, on es do. whereas the granites of the fourth group were Sederholm' s stratigraphie schema presented Table I. Sederholm's (1932) stratigraphie schema of the Preeambrian in Finland. Jotnian Olivine diabases Sands tones of Pori IVeyele (Late Preeambrian) Uneonformity Hoglandian Rapakivi granites Ossipites and diabases of Jaala Great uneonformity Lapland granites 1II eycle Post-Jatulian epoeh of Granites of Oulu river, Kajaani , ete. (Karelidie) diastrophism Granites of Onas, Bodom, Ava, ete., in southern Finland Kalevian and Jatulian Conglomerates, quartzites and sehists in eastern and northern Finland Great uneonformity Post-Bothnian epoeh of Granites of eentral area in Finland diastrophism 11 eyele (Younger Archean) Bothnian Bothnian meta basalts and sedimentary sehists in southwestern Finland Ladogian Ladogian sehists NW of Lake Ladoga Lapponian Lapponian iron-bearing formations and greenstones in Finnish Lapland Uneonformity I eyele Post-Svionian epoeh of Gneissose granites of the Sveeofennian ( Katarchean) diastrophism range and eastern Finland Svionian Sehists and gneis ses of the leptite formation of southern Finland Substratum unknown Geological Survey of Finland, Bulletin 336 24 Ahri Sirnonen in Table I and his map of Fennoseandia (1932) The first eomprehensive study on the radio­ mainly revealed his ideas about the different aetive ages of Finnish rocks and minerals epoehs of diastrophism and represented the was published by Kouvo (1958). Most of the folded areas of different ages. His first, seeond age determinations have so far been done on and third eyeles eontained many different the zireons of magmatie rocks. Thus the mappable units (formations) of metamorphie igneous rocks and their eontaet relations have rocks, but their respeetive stratigraphie posi­ risen to a new and quite important role in the tions inside the eycle were very poorly known. age c1assifieation of the Finnish Preeambrian. Sederholm's studies were the first modern New geologieal studies have brought about approach to the task of building up a general many radieal ehanges in Sederholm's strati­ pieture of the evolution of the Finnish Pre­ graphie schema (see Table 1). The first reinter­ eambrian. Sinee Sederholm's time, more pretations eoneerned the independent strati­ detailed geologieal maps and the applieation graphie position of the eomplexes belonging of the methods of struetural and sedimentary to the seeond eyele in Sederhol m' s elassifiea­ geology as weil as radioaetive age determina­ tion. tio s have augmented our knowledge of the Diseussion of the uneonformity between the sueeession of geologieal events. Svionian and Bothnian eomplexes led to the Modern methods of tectonics were first interpretation that both of them belong to the applied by Wegmann (1928 and 1931) to the Sveeofennian eomplex, whieh was folded study of the metamorphie eomplexes in Finland. and metamorphosed in the Sveeofennidie This research had a marked influenee on the orogeny. On the other hand, the Ladogian separation of the struetural units as weil as and Lapponian eomplexes of Sederholm's on the development of the magmateetonieal seeond eyele were regarded, together with c1assifieation of orogenie plutonie rocks. the Jatulian and Kalevian formations, as Sederholm (1932) assumed that every orogenie Karelian deposits , whieh were folded and eyele was assoeiated with the intrusion of a metamorphosed in the Karelidie orogeny. granite group; but at present the granitoid Furthermore, the granites of Sederholm's first rocks, emplaeed during an orogeny, are and seeond eyeles were eonsidered to be elassified into three main groups, the so­ granites belonging to the same Sveeofennidie ealled synorogenie, late orogenie and post­ orogenie eycle. Coneerning Sederholm's orogenie grani tes. stratigraphie schema (Table 1), it ean be stated Many investigations using the aspeets of that the independent stratigraphie position of sedimentary geology have been made, espe­ his seeond eycle was eliminated, beeause its eially sinee World War II,
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