Baltic Sea (Elsevier oceanography series ; 30)

THE BALTIC SEA

FURTHER TITLES I N THIS SERIES 1 J.L.MER0 THE MINERAL RESOURCES OF THE SEA 2 L.M. FOMIN THE DYNAMIC METHOD I N OCEANOGRAPHY 3 E.J.F. WOOD MICROBIOLOGY OF OCEANS A N D ESTUARIES 4 G.NEUMANN OCEAN CURRENTS 5 N.G.JERLOV OPTICAL OCEANOGRAPHY 6 V.VACQUIER GEOMAGNETISM IN MARINE GEOLOGY 7 W.J. WALLACE THE DEVELOPMENT OF THE CHLORINITY/SALINITY CONCEPT I N OCEANOGRAPHY 8 E. L l S l T Z l N SEA-LEVEL CHANGES 9 R.H.PARKER THE STUDY OF BENTHIC COMMUNITIES 10 J.C.J. NIHOUL (Editor) MODELLING OF MARINE SYSTEMS 11 0.1. MAMAYEV TEMPERATURE-SALINITY ANALYSIS OF WORLD OCEAN WATERS 12 E.J. FERGUSON WOOD and R.E. JOHANNES (Editors) TROPICAL MARINE POLLUTION 13 E. STEEMANN NIELSEN MAR1N E PHOTOSYNTH ESlS 14 N.G. JERLOV MARINE OPTICS 15 G.P. GLASBY (Editor) MARINE MANGANESE DEPOSITS 16 V.M. KAMENKOVICH FUNDAMENTAL OF OCEAN DYNAMICS 17 R.A. GEYER (Editor) SUBMERSIBLES A N D THEIR USE IN OCEANOGRAPHY AN D OCEAN ENGINEERING 18 J.W. CARUTHERS FUNDAMENTALS OF MARINE ACOUSTICS 19 J.C.J. NIHOUL (Editor) BOTTOM TURBULENCE 2 0 P.H. LEBLOND and L.A. MYSAK WAVES I N THE OCEAN 21 C.C. VON DER BORCH (Editor) SYNTHESIS OF DEEP-SEA DRILLING RESULTS IN THE IN D IAN OCEAN 22 P. DEHLINGER MARINE GRAVITY 23 J k J . NIHOUL (Editor) HYDRODYNAMICS OF ESTUARIES A N D FJORDS 24 F.T. BANNER, M.B. COLLINS and K.S. MASSIE (Editors) THE NORTH-WEST EUROPEAN SHELF SEAS: THE SEA BED AN D THE SEA I N MOTION 25 J.C.J. NIHOUL (Editor) MARINE FORECASTING 26 H.-G. RAMMING and Z. KOWALIK NUMERICAL MODELLING OF MARINE HYDRODYNAMICS 27 R.A. GEYER (Editor) MARINE ENVIRONMENTAL POLLUTION 28 J.C.J. NIHOUL (Editor) MARINE TURBULENCE 29 M. WALDICHUK, G.B. KULLENBERG and M.J. ORREN (Editors) MARINE POLLUTANT TRANSFER PROCESSES

Elsevier Oceanography Series, 30

THE BALTIC SEA edited by

AARNO VOlPlO Institute of Marine Research, Helsinki, Finland

ELSEVIER SCIENTIFIC PUBLISHING COMPANY Amsterdam

-

Oxford

-

New York

1981

ELSEVIER SCIENTIFIC PUBLISHING COMPANY

1, Molenwerf 1014 AG Amsterdam P.O. Box 21 1, lo00 AE Amsterdam, The Netherlands

Distributors for the United States and Canada: ELSEVIERINORTH-HOLLAND INC. 52, Vanderbilt Avenue New York, N.Y. 10017

Library of Congress Calsloging in Publicalion Data

Main e n t r y under t i t l e : The B a l t i c Sea. ( E l s e v i e r oceanography s e r i e s ; 30) Includes b i b l i o g r a p h i e s end index. 1. Oceenopaphy--Baltic Sea. 2. Marine biology-Baltic Sea. 3. Fisheries-Baltic Sea. 4. Marine pollution--Baltic Sea. I. Voipio, Aarno, 1926-

CC571.B24 551.46'134 0-444-41864-9

80-17385

ISBN

ISBN 044441884-9(Vol. 30) ISBN 0444416234 (Series)

0 Elsevier Scientific Publishing Company, 1981 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic mechanical, photocopying, recording or otherwise, without the prior written permission of the publication, Elsevier Scientific Publishing Company, P.O. Box 330,1000 AH Amsterdam, The Netherlands

Printed in The Netherlands

V

PREFACE

The Baltic Sea is neither an ocean nor a lake, but a large brackish-water basin with very pronounced density stratification prevailing the whole year round. In addition to this, its recent geological history has been very complicated, resulting in profound changes in the hydrographic conditions and subsequently also in the biological features of this sea during the last ten thousand years. The above special features make it rather difficult for a deep-sea oceanographer to find the marked differences in physical properties between the shallow Baltic Sea and, for instance, the shelf seas with a similar mean depth. A limnologist, on the other hand, seems to have equal problems in remembering that in the Baltic Sea the thermal convection never extends to the bottom in basins whose depth is greather than the mean. The surprisingly great difficulties encountered in discussing the Baltic Sea conditions which colleagues representing either deep-sea oceanography or limnology gave me an incentive to accept the kind invitation of the Elsevier Scientific Publishing Company to edit a volume on the Baltic Sea for inclusion in their Oceanography Series. The publication of the present volume has also made it possible to present a large body of unpublished material available in Finland and Sweden, using it to expand the summary of the rather fragmentary earlier studies on the geology of this sea. I was also encouraged by the fact that most of the persons from whom I requested contributions on the other and perhaps better known areas of Baltic marine sciences agreed without hesitation to participate in this project. The only major disadvantage in having several authors has been the difficulty of synchronizing the delivery of the manuscripts. As the editor, I wish to express my sympathy with those contributors who were able to send in their manuscripts within the agreed time, I also hope that the reader will understand that it has been impossible to update those articles which were completed before the original deadlines. It is my pleasant duty t o record my gratitude to the late Professor Ilmo Hela and to Professor Kalervo Rankama, who persuaded me to accept the task of editor. Professor Rankama has always been ready to advise me on the work and has checked the English language of the manuscripts. Ms. Mirja

VI

Ristola and Ms. Terttu Someroja have given me indispensable help in the editorial work. Finally, I wish t o extend my warm thanks t o my wife, Raija, for her constant understanding and encouragement during this work. January 1980 AARNO VOIPIO

VII

CONTENTS

Preface . . . . . . List of contributors .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

V xi11

Chapter 1. GEOLOGY OF THE BALTIC SEA B. Winterhalter. T . FlodCn. H . Ignatius. S. Axberg and L . Niemisto A . Pre-Quaternary geology of the Baltic Sea (T. FlodCn and B . Winterhalter) Introduction . . . . . . . . . . . . . . . . . . . . . 1 Bothnian Bay . . . . . . . . . . . . . . . . . . . . . 23 25 Bothnian Sea . . . . . . . . . . . . . . . . . . . . . h a n d Sea . . . . . . . . . . . . . . . . . . . . . . 28 Gulf of Finland . . . . . . . . . . . . . . . . . . . . 31 Baltic Proper . . . . . . . . . . . . . . . . . . . . . 33 B. Quaternary geology of the Baltic Sea (H. Ignatius. S. Axberg. L. Niemisto and B. Winterhalter) Introduction . . . . . . . . . . . . . . . . . . . . . 54 Evolution of the Baltic Sea . . . . . . . . . . . . . . . 58 Stratigraphy of the clay sediments . . . . . . . . . . . . . 6 3 Geomorphology of the Baltic Sea floor . . . . . . . . . . . 69 Quaternary sediments of the Baltic Sea . . . . . . . . . . . 86 C. Natural resources (B. Winterhalter) 105 Hydrocarbons . . . . . . . . . . . . . . . . . . . . Ferromanganese concretions . . . . . . . . . . . . . . . 107 Amber, phosphorite and glauconite . . . . . . . . . . . . 114 110 Sandandgravel . . . . . . . . . . . . . . . . . . . . Placer deposits . . . . . . . . . . . . . . . . . . . . 116 References . . . . . . . . . . . . . . . . . . . . . . . . 117 Chapter 2 . HYDROLOGY OF THE BALTIC SEA U . Ehlin Hydromorphology . . . . . . . . . . . River inflow . . . . . . . . . . . . . . Precipitation and evaporation . . . . . . Water transport through the Danish Sounds .

. . . .

. . . .

. . . .

. . . .

. . . .

. . . . . 123 . . . . . 125 . . . . . . 126 . . . . . . 129

VIII Water storage and water exchange References . . . . . . . . .

. . . . . . . . . . . . . . .133 . . . . . . . . . . . . . . . 133

Chapter 3 . PHYSICAL OCEANOGRAPHY G . Kullenberg Introduction . . . . . . . . . . . . . . . . . . . . . . . 135 Salinity and temperature distribution . . . . . . . . . . . . . . 135 Salinity . . . . . . . . . . . . . . . . . . . . . . . . . 135 Temperature . . . . . . . . . . . . . . . . . . . . . . 139 Long-term variations . . . . . . . . . . . . . . . . . . . 143 Causes of long-term variations . . . . . . . . . . . . . . . . 147 Density stratification and its variability . . . . . . . . . . . . . 149 Circulation . . . . . . . . . . . . . . . . . . . . . . . . 150 Mean circulation . . . . . . . . . . . . . . . . . . . . . 150 Time-dependent motion . . . . . . . . . . . . . . . . . .151 Coastal boundary layer . . . . . . . . . . . . . . . . . . 155 Theoretical considerations . . . . . . . . . . . . . . . . .157 Mixing conditions . . . . . . . . . . . . . . . . . . . . . Small-scale motion . . . . . . . . . . . . . . . . . . . . 160 Vertical and horizontal motion . . . . . . . . . . . . . . . 162 Optical properties. heat balance and ice conditions . . . . . . . . . 167 Optical properties . . . . . . . . . . . . . . . . . . . . Heat balance . . . . . . . . . . . . . . . . . . . . . . 170 Ice conditions . . . . . . . . . . . . . . . . . . . . . . 174 References . . . . . . . . . . . . . . . . . . . . . . . . 175

,

Chapter 4 . CHEMICAL OCEANOGRAPHY K . Grasshoff and A . Voipio Anomalies in the composition of the Baltic Sea water . . . Distribution of dissolved oxygen . . . . . . . . . . . Nutrients . . . . . . . . . . . . . . . . . . . . . . Trace metals . . . . . . . . . . . . . . . . . . . . Sediment -water interactions . . . . . . . . . . . . Dissolved organic matter . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . .

. 183 . 188 194 203 . 205 . 211 213

Chapter 5. BIOLOGICAL OCEANOGRAPHY G . Hallfors. Niemi. H. Ackefors. J . Lassig and E . Leppakoski A . Introduction (G. Hallfors and A . Niemi) . . . . B . Vegetation and primary production (G. Hallfors and

. . . . . . .219 A. Niemi) . . . 220

IX Phytoplankton. general . . . . . . . . . . . . . . . . Phytoplankton production. succession and regulating factors . Phytoplankton in near-shore areas . . . . . . . . . . . . Benthic vegetation. general aspects . . . . . . . . . . . Zonation of the benthic vegetation . . . . . . . . . . . Littoral primary production . . . . . . . . . . . . . . Trophic status of the Baltic Sea . . . . . . . . . . . . . C. Zooplankton (H. Ackefors) . . . . . . . . . . . . . . . Introduction . . . . . . . . . . . . . . . . . . . . . Sampling technique . . . . . . . . . . . . . . . . . Composition of the fauna . . . . . . . . . . . . . . . Environment and fauna . . . . . . . . . . . . . . . . Vertical distribution and die1 migration . . . . . . . . . . Food web . . . . . . . . . . . . . . . . . . . . . . Dynamics of the plankton community . . . . . . . . . . D. Benthic fauna of the Baltic Sea (J. Lassig and E . Lappakoski) . . Origin of the benthic fauna . . . . . . . . . . . . . . Littoral zone . . . . . . . . . . . . . . . . . . . . . Sublittoral zone . . . . . . . . . . . . . . . . . . . . Production and utilization of zoobenthos . . . . . . . . . Benthos research . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . .

. 220 . 221 . 229 . 229 . 232 . 236 . 237 . 238 238

. 239 . 242 . 245 . 249 251

. 251 . 254 .254 256 257 . 262 .264 265

Chapter 6. FISHES AND FISHERIES E .Ojaveer. A. Lindroth. 0. Bagge. H . Lehtonen and J . Toivonen A . Fish fauna of the Baltic Sea (E. Ojaveer) . . . . B . Marine pelagic fishes (E . Ojaveer) . . . . . . . Geographical distributions and groups . . . . Seasonal distribution patternsandmigrations . Spawning. larval and adolescent phase . . . . Fecundity . . . . . . . . . . . . . . . . Feeding . . . . . . . . . . . . . . . . . Growth and age . . . . . . . . . . . . . . Y ear-class abundance . . . . . . . . . . . Catches and mortality . . . . . . . . . . Other fishes . . . . . . . . . . . . . . . C . Anadromous and catadromous fishes (A . Lindroth) Salmon . . . . . . . . . . . . . . . . . Sea-running brown trout . . . . . . . . . Grayling . . . . . . . . . . . . . . . . . Whitefish . . . . . . . . . . . . . . . . Vimba . . . . . . . . . . . . . . . . .

. . . . . . . 275

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. 276 . 276 . 279 . 281

284 285 287 . 288 . 290 291 . 292 293 . 303 306 306 307

X Lamprey . . . . . . . . . . . . . . . . European eel . . . . . . . . . . . . . . . D. Demersal fishes (0. Bagge) . . . . . . . . . . Cod . . . . . . . . . . . . . . . . . . . Flounder . . . . . . . . . . . . . . . . Plaice . . . . . . . . . . . . . . . . . . Turbot . . . . . . . . . . . . . . . . . Brill . . . . . . . . . . . . . . . . . . . Dab. . . . . . . . . . . . . . . . . . . Sandeel . . . . . . . . . . . . . . . . . Greater sandeel . . . . . . . . . . . . . . Snakeblenny . . . . . . . . . . . . . . . Four-bearded rockling . . . . . . . . . . Father lasher . . . . . . . . . . . . . . . Sea scorpion . . . . . . . . . . . . . . . Four-horned cottus . . . . . . . . . . . Eelpout . . . . . . . . . . . . . . . . . Black goby . . . . . . . . . . . . . . . . Sandgoby . . . . . . . . . . . . . . . . Lumpsucker . . . . . . . . . . . . . . . Sea snail. . . . . . . . . . . . . . . . . Gold sinny . . . . . . . . . . . . . . . . Butterfish . . . . . . . . . . . . . . . . E . Fresh-water fishes (H. Lehtonen and J . Toivonen) . Species composition and distribution . . . . . Migrations . . . . . . . . . . . . . . . . Spawning grounds and reproduction . . . . . Growth . . . . . . . . . . . . . . . . . Fishery and catches . . . . . . . . . . . References . . . . . . . . . . . . . . . . . .

. . . . . . 308 . . . . . . 309 . . . . . . . 311 . . . . . . 312 . . . . . . 320 . . . . . . 323 . . . . . . 325 . . . . . . 327 . . . . . . 327 . . . . . . 328 . . . . . . 328 . . . . . . 329 . . . . . . .329 . . . . . . 330 . . . . . . 330 . . . . . . . 330 . . . . . . 331 . . . . . . 331 . . . . . . 331 . . . . . . 332 . . . . . . 332 . . . . . . 333 . . . . . . 333 . . . . . . . 333 . . . . . . . 333 . . . . . . 338 . . . . . . . 338 . . . . . . 339 . . . . . . . 340 . . . . . . 341

Chapter 7 . POLLUTION B.I. Dybern and S.H. Fonselius Introduction . . . . . . . . . . . . . . . . . . . . . . . . Global aspects of pollution . . . . . . . . . . . . . . Spreading and accumulation of pollutants in the Baltic Sea . . Sensitivity of the Baltic ecosystems . . . . . . . . . . . Eutrophication . . . . . . . . . . . . . . . . . . . . . . . Waste discharge from communities and industries . . . . . . Oxygen utilization . . . . . . . . . . . . . . . . . . Effects on the ecosystems . . . . . . . . . . . . . . . Toxic matter. . . . . . . . . . . . . . . . . . . . . . . . Organochlorine compounds . . . . . . . . . . . . . .

351

. 351

. 352

. 353 353

. 353 . 354 .358 362

. 362

XI Metals. . . . . . . . . . . . . . Oil pollution . . . . . . . . . . . . . . Radioactive pollution . . . . . . . . . . Physical pollution . . . . . . . . . . . . Warm-water effects . . . . . . . . Solid waste . . . . . . . . . . . . Extraction of sand and gravel . . . . Other kinds of physical pollution . . References . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . 365 . . . . . . 369 . . . . . . 371 . . . . . . 372 . . . . . . .372 . . . . . . 373 . . . . . . .374 . . . . . . . 375 . . . . . . 376

Chapter 8 . INTERNATIONAL MANAGEMENT AND COOPERATION V . Sjoblom and A . Voipio A. International management of the Baltic Sea fisheries (V . Sjoblom) . 383 B . International cooperation as the basis of the protection of the marine environment of the Baltic Sea area (A . Voipio) . . . . . . . 386 Author index Subject index

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

391 405

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XI11

LIST OF CONTRIBUTORS

H. ACKEFORS S. AXBERG

0. BAGGE B.I. DYBERN

U. EHLIN T. FLODEN S.H. FONSELIUS

K. GRASSHOFF G. HALLFORS

H. IGNATIUS G. KULLENBERG J. LASSIG

H. LEHTONEN E. LEPPAKOS KI A. LINDROTH

University of Stockholm, Department of Zoology, Box 6801, S-113 86 Stockholm, Sweden University of Stockholm, Department of Geology, Box 6801, S-113 86 Stockholm, Sweden Danmarks Fiskeri- og Havundersrbgelser, Charlottenlund Slot, DK-2920 Charlottenlund, Denmark National Board of Fisheries, Institute of Marine Research, Biological Department, S-453 00 Lysekil, Sweden Swedish Meteorological and Hydrological Institute, Fack, S-601 01 Norrkoping, Sweden University of Stockholm, Department, of Geology, Box 6801, S113 86 Stockholm, Sweden National Board of Fisheries, Institute of Marine Research, Hydrographic Department, Box 2566 S-403 1 7 Goteborg, Sweden Institut fur Meereskunde an der Universitat Kiel, 2 3 Kiel, Dusternbrooker Weg 20, Federal Republic of Germany University of Helsinki, Tviirminne Zoological Station, SF-10850 Tviirminne, Finland Geological Survey of Finland, SF-02150 Espoo 15, Finland University of Copenhagen, Institute of Physical Oceanography, Haraldsgade 6, DK-2200 Copenhagen N, Denmark Institute of Marine Research, P.O. Box 166, SF-00141 Helsinki 14, Finland Institute of Finnish Game and Fisheries Research, Fisheries Division, Box 193, SF-00131 Helsinki 13, Finland Abo Akademi, Porthansgatan 3-5, SF-20500 Abo 50, Finland Rattviksvagen 29, S-161 42 Bromma, Sweden (formerly: University of UmeQ, Institute of EcoIogical Zoology, UmeQ, Sweden)

XIV

A. NIEMI L. NIEMISTO E, OJAVEER

v. SJOBLOM J. TOIVONEN A. VOIPJO

B. WINTERHALTER

University of Helsinki, Department of Botany, Ecological laboratory, Apollonkatu 5 B 45, SF-00100 Helsinki 10, Finland Institute of Marine Research, P.O. Box 166, SF-00141 Helsinki 14, Finland Tallinn Department of the Baltic Fishery Research Institute, Apteegi 1-2, 200001 Tallinn, Esthonian SSR, USSR University of Helsinki, Department of Limnology, Viikki, SF-00710 Helsinki 71, Finland Institute of Finnish Game and Fisheries Research, Fisheries Division, P.O. Box 193, SF-00131 Helsinki 13, Finland Institute of Marine Research, P.O. Box 166, SF-00141 Helsinki 14, Finland Geological Survey of Finland, SF-02150 Espoo 15, Finland

Chapter 1 GEOLOGY OF THE BALTIC SEA BORIS WINTERHALTER, TOM FLODfiN, HEIKKI IGNATIUS, STEFAN AXBERG and LAURI NIEMISTO

A. PR,E-QUATERNARY GEOLOGY OF THE BALTIC SEA*

Introduction The Baltic Sea including its adjoining gulfs (Fig. 1.l), as we know it today, fills a complex depression within the East European platform and its southwestern border zone. The Precambrian crystalline basement of the Baltic Shield is exposed along the major part of the western, northern and eastern coasts of the Baltic Sea (Fig. 1.2). Only the southeastern and southern part of the present-day marine area exhibits a coastline consisting of sedimentary rocks being part of the East European sedimentary complex (Fig. 1.3a, b). The crystalline basement of the Baltic region is represented by various metamorphic and igneous rocks referable to Svecokarelian, and Gothian orogenies. Anorogenic rapakivi-granite intrusions are common in several localities both as supramarine (Vorma, 1976) and submarine outcrops (e.g., Winterhalter, 1967). They were formed during Middle Proterozoic (1.65 Ga) postorogenic activities. For a comprehensive presentation of the Precambrian the reader is referred to Rankama (1963). Prior to the evolution of the sub-Cambrian peneplane (cf. pp. 31 and 36) a more or less uniform deposition of sandstones, generally reddish arkose and siltstones occurred in Late Proterozoic. Some of the sandstones are known to exhibit dyke intrusions of diabase. The sedimentary deposits are often referred t o as Jotnian. Their age is approximately 1.3 Ga (Middle Riphean) according t o Simonen (1971). Today the erosional remnants of these unmetamorphosed sedimentary rocks are found exposed both on the sea floor and on the adjacent land areas (Winterhalter, 1972; Flodhn, 1973). The intense postdepositionary erosion leading to the formation of the sub-Cambrian peneplane explains the rather thin and patchy distribution of the Proterozoic sedimentary rocks. These rocks have only been preserved in tectonic depressions, where they often attain considerable thicknesses, e.g. the Satakunta and Gavle sandstones on land, and the submarine deposits in the h a n d Sea (p. 28) and the Landsort Deep (p. 36).

*

By Tom Flod6n and Boris Winterhalter.

2

f“

BOTHNIAN SEA

Fig. 1.1. Index map of the Baltic Sea region.

A “blue clay” sequence found outcropping NW of Leningrad in the eastern part of the Gulf of Finland was later observed in several drillings in, e.g., Esthonia and Latvia. This sequence was’formerly confused with the Lower Cambrian Blue Clay, but now it is known that it is of Late Precambrian age constituting part of the arenaceou‘sargillaceous Vendian sedimentary rocks. The Muhos-Formation in Finland and its submarine extension in the Bothnian Bay was considered by Veltheim (1969) t o be of Jotnian (Middle Riphean) age. Current investigations of the sediments cored on the Hailuoto Island indicate an upper Riphean or even Vendian age (R. Tynni, pers. commun., 1979.) possibly comparable with the Vendian of Estonia.

3

Fig. 1.2. Baltic Shield and the northwestern part of the East European Platform bounded in the west by the Caledonides and in the southwest by the Tornquist Line.

The Early Paleozoic seas exhibiting a multitude of depositional and erosional phases covered the major part of the present-day Baltic Sea region. The exact maximum extent of Paleozoic sedimentation in the north is unknown. However, Cambrian and Ordovician sedimentary rocks occur in situ in the Bothnian Sea, and Cambrian sandstone erratics have been found along

4

the coast of the Bothnian Bay. The sedimentary rock sequence in the Bothnian Bay may in addition to the Muhos-Formation contain in the north central part of the Bay beds of Lower Cambrian sedimentary rocks. A new occurrence of Lower Cambrian sedimentary rocks has been found in the coastal area of the NE Bothnian Sea, just south of the town of Vaasa (Laurhn et al., 1978). The total thickness of the deposit has been estimated at 300-400 m. Slightly over 100 m was penetrated by drilling. The recovered sedimentary sequence is comparable with the Lontova and Liikati beds of Esthonia. In the Baltic Sea, the total thickness of the Paleozoic sequence increases rapidly towards the southeast measuring over 3 km off the Polish coast. Mesozoic and Tertiary sedimentary rocks are known only from the southern part of the Baltic Sea. Within a large part of the Baltic Sea the sedimentary strata are more or less horizontal suggesting very little postdepositional deformation. Numerous shallow block faults and fractures have, however, been revealed by continuous seismic profiling. In contrast, strong block faulting has occurred most dramatically along the “Tornquist Line” (Tornquist, 1913), the major fracture zone that denotes the southwest border of the East European Platform (see Fig. 1.2). Along the Tornquist Line, from Scania in the h W to the Polish coast in the SE, the maximum vertical displacement has been estimated to exceed 7000 m. The major tectonic features: faults, fractures, and lineaments are shown in the maps, Fig. 1.4a, b. The figures are based on data gathered both from literature (e.g., Harme, 1961; Tuominen et al., 1973), bathymetric maps and available seismic reflection and refraction profiles. The detection of faults and fractures in seismic and echosounding profiles is seldom difficult (Fig. 1.12). However, the rather limited number of available profiles,and the fact that the profiles often transect possible lineaments at small angles makes it obvious that only the most persistent features can be reliably evaluated and have been included in the maps. Whenever possible the downthrown side is also noted. No attempt has been made t o interpret the ages of the various fractures and faults noted on the map, It is clear, however, that many of the major lineaments denote tectonic zones, that have been activated during several geological events since the Archean. Some of the tectonic events can be related to the various orogenies of northern and central Europe. Epeirogenic movements have also obviously been very active, not forgetting the Late Pleistocene and Holocene crustal uplift still active within the Baltic region (Fig. 1.5). It has a maximum annual rate of almost 10 mm in the central part of the Bothnian Bay. The distribution of earthquake epicentres is shown in Fig. 1.6. Large-scale neotectonic faulting seems, however, to be very scarce. Vertical displacements of some meters have been observed in acoustic profiles only rarely (Fig. 1.7).

Fig. 1.3a. Simplified map of the bedrock of the northern part of the Baltic Sea based on the interpretation of continuous seismic reflection profiles and refraction data collected by the authors. The bedrock boundaries in the SE Gulf of Finland are based on an interpretation of the seafloor morphology from Finnish Nautical Charts and on available Soviet geological data from the adjacent land area. The line encircling the Aland islands denotes the assumed extent of the Rapakivi granites (Proterozoic) in the crystalline basement complex.

Fig. 1.3h. Simplified map of Lhe bedrock of the southern part of the Baltic Sea based on work done at the Marine Geological Department of the University of Stockholm and on available information from the coastal zone (see text).

Fig. 1.4a.

Fault lines and tectonic lineaments in the northern part of the Baltic Sea.

Fig. 1.4b. Fault lines and tectonic lineaments in the southern part of the Baltic Sea.

21

Fig. 1.5. Present-day relative crustal uplift (and submergence) in the Baltic Sea region. The isobases (mm a - ' ) are based on precise levellings, and tide-gauge data from a number of papers (e.g., Boulanger et al., 1975; Kukkamtiki,'1975; Lillienberg et al., 1975; Liszkowski, 1975; Bergqvist, 1977; Morner, 1977).

The following, more detailed, description of the pre-Quaternary geology of the Baltic Sea and its adjoining gulfs is based on the results of marine geological research conducted by the Marine Geological Department of the University of Stockholm and the Geological Survey of Finland. Further information has been acquired from published papers and unpublished

22

Fig. 1.6. Seismicity of the Baltic Sea and its adjoining land areas based on earthquake epicenter data from Panasenko (1977) and Penttila (1978). Norwegian and offshore Norwegian earthquakes are omitted.

23

Fig. 1.7. Tectonic elements in an area south of Oland. The upper figure is an echo sounding profile and the lower one is a reflection profile. A = lineament in the sedimentary bedrock; B and C = features in Quaternary strata that may be associated with neotonic movements.

manuscripts updated by personal communications with many colleagues working within the Baltic Sea.

Bothnian Bay The crystalline complex forming the outcropping bedrock in the adjacent land areas does not exhibit any fundamental differences between the two sides of the Bothnian Bay. Svecokarelian granites and gnekses dominate, except for the northern shore of the Bay, where the bedrock is characterized by Karelian schists. The crystalline complex can be assumed to continue under the Bothnian Bay without any significant variations. The existence of erosional remnants of unmetamorphosed sedimentary rocks on the bottom of the Bothnian Bay, placewise covering the crystalline basement, has been anticipated for a long time due to the numerous finds of erratics along the shores of the Bay. Likewise, the Proterozoic Muhos-Forma-

tion (Tynni, 1978) found both on the Finnish mainland and on the Hailuoto Island in the NE part of the Bay was assumed t o have a submarine continuation (Veltheim, 1969). Subsequent seismic reflection profiling and refraction shooting have verified this assumption but, in addition, they have in fact shown that a major part of the bottom of the Bothnian Bay is covered by sedimentary rocks. Sound-velocity measurements indicate considerable lithological variations. These are comparable both with the transitions between sandy and silty beds observed in the drill cores from the Muhos-Formation on the Hailuoto Island and those found in the Bothnian Sea. The total thickness of the sedimentary rock strata seems to exhibit considerable variations in different parts of the marine area, varying from depressions with several hundred meters of sediments t o thin erosional remains as outliers smoothening out the otherwise rather irregular relief of the crystalline basement. Due t o the rather limited geological data so far available from the Bothnian Bay, the extent of the sedimentary bedrock denoted on the map in Fig. 1.3a should not be taken literally but more as an indication of the relation between the sedimentary bedrock and the crystalline basement. The complexity of the Pleistocene deposits covering the sedimentary rocks makes interpretation of the available reflection profiles very difficult (Fig. 1.8). Except for some erratics of evidently Cambrian Sandstone discovered along

Fig, 1.8. Part of a reflection profile showing the complicated nature of the sedimentary deposits (and bedrock) in the Bothnian Bay. G = glacial drift; P = pre-Weichselian sediments; M = Middle o r Late Proterozoic sedimentary rock (see text); w.i. = water line, i.e., sea surface. Length of profile is approximately 20 km.

25 the eastern shore of the Bothnian Bay indicating the probable existence of Lower Cambrian deposits within the submarine area, no younger sedimentary rocks have as yet been ascertained. The available reflection profiles do point towards the existence of Cambrian sedimentary rocks in the northcentral part of the Bay.

Bothnian Sea In contrast t o the Bothnian Bay, the geology of the Bothnian Sea has been studied in considerable detail (Veltheim, 1962; Winterhalter, 1972; Thorslund and Axberg, 1979). Thus the various bedrock boundaries given in the map (Fig. 1.3a) are based on rather reliable data. In fact, the only difficulties encountered in the evaluafion of the available marine seismic data were t o distinguish between the crystalline basement and the Late Proterozoic arkose (Jotnian sandstone), especially when the latter occurs as thin erosional remnants within a rather rugged basement topography. Late Proterozoic sedimentary rocks, commonly referred to as Jotnian (Riphean) sandstones are known from several localities in Finland and Sweden. Within the coastal part of the Bothnian Sea such Late Proterozoic sedimentary rocks have been described from two major areas, the SatakuntaFormation in Finland (Simonen and KOUVO, 1955) and the Gavle-Formation in Sweden (Gorbatschev, 1967). In both cases, the age of the rocks has been estimated t o be Middle Riphean. The maximum thickness of the sequence has in both localities been estimated to be approximately 1000 m. The strata have been preserved through downfaulting. The extremely high seismic velocity of the sandstone (close t o 5000 ms-’ ) makes it difficult t o detect the depth of the basement since the basement exhibits a similar or only slightly higher velocity. Sonobuoy refraction data indicate that the sandstone may be present below at least part of the Paleozoic rocks in the central parts of the Bothnian Sea. One verified exception is the Finngrunden Shoals in the Gavle Bay, where drillings have revealed a weathered crystalline basement directly below the Paleozoic (Thorslund, 1970). The shoal areas have been studied in detail by Thorslund and Axberg (1979). They noted that the shoals coincide with an uplifted part of the crystalline basement, and that the Paleozoic sequence is locally reduced in thickness within the shoal areas. As demonstrated in the map in Fig. 1.3 a substantial part of the bottom of the Bothnian Sea consists of Cambrian and Ordovician sedimentary rocks. A cross-section of the sea is given in Fig. 1.9. In the Gavle Bay, the Paleozoic strata consist of about 40 m of Cambrian clays with subordinate layers of sandstone followed by some 50 m of Ordovician limestone. Fig. 1.10 shows that both the Cambrian and the Ordovician sequences increase generally in thickness towards the NW-central part of the formation (Winterhalter, 1972). In the northern part of the Paleozoic area, the Cambrian strata have attained

W

E

@Ordavlcion

~ i C a m o r l a n

latnian

Precambrmon cryslalline basement

__ Acoust~c reflectors ond

louits

Fig. 1.9. Schematic geological profile (W-E) across the southern part of the Bothnian Sea from Soderhamn in Sweden to Rauma in Finland. The faulting off the Swedish coast is in reality considerably more complex than shown in the figure.

A

0.

0

01

F

o,

I

I

I

Fig. 1.10. Interpreted reflection profiles from the central and eastern parts of the Bothnian Sea showing the gentle sloping of the sub-Cambrianpeneplane towards the west. Profiles G and Hare shown in Figs. 1.11 and 1.12, respectively.

28 a thickness of about 200 m. Although the Ordovician sedimentary sequence also exhibits a general increase in thickness towards the north, the maximum total thickness of these beds (200-350 m) is to be found in the central part of the outcrop area. This is probably caused by more intense postdepositional erosion of the northern part of the formation. The youngest stratigraphic level so far dated within the Bothnian Sea is the Tretaspis - stage of the lowest part of the Upper Ordovician represented by a hard calcilutitic limestone. This reddish or greyish limestone is commonly known as ‘Baltic limestone’ (Thorslund, 1960). The well-developed peneplane surface separating the Cambrian sedimentary rocks and the Jotnian sandstone is evident in the profiles in Fig. 1.10. This well-preserved peneplane surface is typical of the eastern part of the formation. The westem extent is governed by a set of strong faults along the Swedish coastline (see Figs. 1.4a and 1.11).Minor faulting exists in the central parts of the Paleozoic formation (Fig. 1.12). For a more comprehensive description of the geology of the Bothnian Sea the reader is referred to Winterhalter (1972).

A land Sea The k a n d Sea encompasses a local tectonic depression of considerable vertical dimensions evidently genetically closely related to the previously mentioned grabenlike occurrences of Jotnian sandstones in the vicinity of Gavle, in the Satakunta area, and the Muhos-Formation in the Bothnian Bay. The main fault line runs south from the Bothnian Sea between the islands, Miirket and Understen, close to the Swedish coast, first south and southeast -=”*=.--. *_-I-._.. I

-

-.I_ .... .. . __, ..-_...----. _. _ __. -.---.--...-- ~..._

I _

~

--_-

._-._I___ ,.-.-C_,_I__I

__

-. _.-_ ,~ ” - .- .

I ~

_I___

_.

W.I.

-. -. . .-

I

~

-.--

-__--_I_

Fig. 1.11. Detail of a reflection profile recorded near Sundsvall in Sweden showing the complicated (faulted) contact between the Precambrian basement ( B ) and the sedimentary rocks off the Swedish coast: C = Cambrian; 0 = Ordovician; D = diabase intrusion. For location of profile see Fig. 1.10, line G .

6Z Fig. 1.12. Block faulting in the southern part of t he main Paleozoic formation in the Bothnian Sea. F o r location of profile see line H in Fig. 1.10.

and then turning due east (Fig. 1.4a). This fault forms the southwestern and southern limit of the Jotnian sandstone formation. In the north and northcoast, the sandstone is bounded by the Middle Proterozoic rapakivi massif of Aland (Winterhalter, 1967). The boundoary itself is located within the erosional trough forming the Deep of the Aland Sea (Fig. 1.13). A substantial

Fig. 1.13. Reflection profile across th z southern part of t h e trough-like depression, the h a n d Deep, trending NW-SE in the Aland Sea (Fig. 1.14). Although acoustic penetration in t h e Jotnian sandstone, in the left part of t h e profile, is very limited, the difference in the bedrock characteristics o n both sides of t he trough is obvious. The rugged relief in the right pa r t of the profile is typical of t h e Middle Proterozoic h a n d m a s s i f . Vertical scale lines are 25 ms apart.

30 part of the h a n d Sea probably consists of arkosic sandstone. The maximum thickness of this sequence is estimated to exceed 700 m. The map in Fig. 1.14 shows that the sandstone occupies two basins, a larger northern basin and a smaller southern one, separated by a fault line trending ENE-WSW. West of Mariehamn on k a n d the sub-Jotnian peneplane, in part detectable’ in seismic reflection profiles, evidently dips approximately 2.5” towards the southwest. If the dip and strike of the Jotnian sandstone beds are assumed t o coincide with those of the underlying peneplane, it follows that the NE limit of the sedimentary formation is erosionally induced and that the Jotnian sandttone of much the same thickness, viz., 700 m, has o