Impact Studies Series Editor: Christian Koeberl Editorial Board Eric Buffetaut (CNRS, Paris, France) lain Gilmour (Open University, Milton Keynes, UK) Boris Ivanov (Russian Academy of Sciences, Moscow, Russia) Wolf Uwe Reimold (University of the Witwatersrand, Johannesburg, South Africa) Virgil L. Sharpton (University of Alaska, Fairbanks, USA) Springer-Verlag Berlin Heidelberg GmbH Juri Plado Lauri J. Pesonen (Eds.) Impacts in Precambrian Shields with 98 Figures and 43 Tables Springer DR. JORI PLADO Institute of Geology University of Tartu Vanemuise 46 51014 Tartu Estonia PROFESSOR LAURI J. PESONEN Division of Geophysics University of Helsinki Gustaf Hăllstromin katu 2 00014 Helsinki Finland ISBN 978-3-642-07803-3 Library of Congress Cataloşing-in-Publication Data Impacts in Precambrian shlelds / JUri Plado; Lauri J. Pesonen (eds.). p.cm. -- (Impact studies) Inc1udes bibliographical references. ISBN 978-3-642-07803-3 ISBN 978-3-662-05010-1 (eBook) DOI 10.1007/978-3-662-05010-1 1. Geology, Stratigraphic-Precambrian. 2. Cratering. 3. Meteorites.1. Plado, Jiiri, 1969- II. Pesonen, Lauri J. 1lI. Series. This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilm or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer-Verlag. 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Kirchner, Heidelberg Printed on acid-free paper SPIN 10868654 32/3130/a5 5432 1 O Preface The present volume is the result of activities within the scientific programme "Response of the Earth System to Impact Processes" (IMP ACT) of the European Science Foundation (ESF). The ESF is an association of 67 national member organisations devoted to scientific research in 24 European countries. The IMPACT programme is aimed at understanding meteorite impact processes and their effects on the Earth System. Launched in 1998 for a duration of 5 years, the programme is now supported by 15 ESF membership countries. The programme deals with all aspects of meteorite impact research and operates through workshops, exchange programs, and short courses. The 4th IMPACT programme workshop "Meteorite Impacts in Precambrian Shields" took place on May 24-28, 2000, in Lappajarvi, western Finland. A total of 84 scientists from 19 countries from Europe, North America, and Africa participated in the workshop. During the workshop, 43 oral, 31 poster, and several video presentations were made. An exhibition of impactite rocks from Finnish craters and two excursions were organised. The excursion to impact melt rock outcrops of the Lappajarvi structure took place during the workshop. The Karikkoselka and Saaksjarvi impact structures in south-central Finland were visited during the post-meeting excursion. Scientifically, the workshop focussed on the characterization of meteorite impact structures of all ages and sizes occurring in the Precambrian shields, stable parts of the continents composed of Precambrian rocks with little or no sediment cover. Precambrian shields, together with platforms (collectively referred to as cratons), represent areas where most of the impact structures on Earth have been found (Fig. 1). Especially rich in impact structures are the mid-latitude shields, and those where intensive research programmes have been carried out. The Precambrian shields provide opportunities for a wide range of scientific studies of impact structures, because of good exposure and a generally homogeneous target. The workshop consisted of nine scientific sessions on various topics of impact cratering, including the global perspective, structural aspects, geophysical and geochemical signatures, dating, shock effects, and impacts into the sea. A special session was devoted to the Popigai impact structure in Siberia. This volume, "Impacts in Precambrian Shields", contains fourteen papers, most of which result from the workshop. All the manuscripts were reviewed by at least two referees and were selected on the basis of originality and scientific value. The volume begins with two introductory contributions, which present an overview of meteorite impact structures in two prominent Precambrian shields: the Fennoscandian Shield and the Canadian Shield. Abels et al. give a detailed overview of thirty confirmed impact structures in Fennoscandia, and discuss possible features that await demonstration of their meteoritic origin. They compare the crater ages of the Canadian and Fennoscandian shields with the global record and show that the peak of early Paleozoic craters is restricted to these regions due to similarities in their geological history. However, an enhanced accretion rate of meteorite impacts for these times cannot be excluded. VI Preface Dence lists data of eleven Canadian impact craters in Precambrian rocks, focusing on the Brent structure, Ordovician of age. This structure is well understood due to several drillcores and extensive studies. Therefore, it provides an excellent example for the study of the main structural elements of simple impact craters. Particular attention is given to mineralogical signatures that reflect the shock pressure attenuation with depth. Also, crater morphometry, structure, geophysical signature, and the distribution of pseudotachylites and allochthonous rocks are discussed in terms of varying crater diameter. A group of papers on the 100 km wide Popigai impact structure in the Anabar Shield of Siberia follows. Masaitis introduces the geological aspects of this Late Eocene (35.7 Ma) crater. Pilkington et al show that geophysically Popigai is characterized by a large negative gravity anomaly (35 mGal) and an extensive magnetic low. Two-dimensional gravity and magnetic models are created by geophysical fields and petrophysical data, collected partly during the International Popigai Expedition in 1999. Armstrong et al. describe their first results on dating of four zircon grains from Popigai impact melt rocks. They conclude that "group B" zircons (isometric grains without any traces of melting along the grain boundaries) have neither crystallized from the impact melt, nor suffered any shock-induced Pb-loss. Naumov describes the general scheme, features and mineral assemblages of impact-generated hydrothermal systems, based mainly on studies of three Russian craters, Popigai, Kara, and Puchezh-Katunki. He concludes that hydrothermal solutions are weakly alkaline and have higher activities of silica creating favourable conditions for formation of Fe-smectites and zeolites. Gurov et al. present geological and petrographical features of the 165 Ma old and extensively eroded Zapadnaya impact structure on the Ukrainian Shield. As many others, this structure hosts high-pressure mineral phases in impactites, such as coesite and impact diamonds. The diamonds are composed of cryptocrystalline aggregates of hexagonal and cubic phases. Recently intensified research activities have resulted in three papers about the Bosumtwi crater in Ghana. This 10.5 km wide, well-preserved structure formed in the Precambrian shield of the West African Craton about 1.07 Ma ago. Wagner et al. analyse the Landsat Thematic Mapper and ERS radar data in order to constrain the surface and subsurface geology. These remote sensing data reveal a detailed picture of the crater rim, including arrays of radial and concentric fracture zones. Two concentric structural features at distances of about 10 - 11 and 17 -19 km from the lake centre are established. Boamah and Koeberl report geochemical features from around Bosumtwi to find signatures of impact in the lateritic soils of the tropical rainforest environment. They compare the geochemical data of soil samples with the geochemical signals (such as the K-content) obtained by airborne radiometric mapping of the structure. Compared to average upper continental crustal composition, the Bosumtwi soils are more siliceous and ferruginous and dominated by quartz, Fe- and Al- oxides, and kaolinite. Concentric areas with a high radiometric signal of potassium reflect possible K mobilization due to the impact event. Artemieva has mathematically modelled a Bosumtwi-sized crater to simulate the formation of impact melt and tektites. Various impact angles and 0 ... <:"Witt?i/1 ~ I .. co 'I ,.., - -, 0 60'N ~en~ • =---- "· I ...,0"{'92 _g VQ f 0 0 0 0 0 I - ,4 ""'~ .-; · ,....., u ~ __ z6~0io I ...... I 30'N 6 I ,_, ~- Equator , . ...... I __ f-- - "' ..---- - Strucures:Im~act 30°S - I0300