Sedimentary environments offshore Norway--Palaeozoic to Recent: proceedings of the Norwegian Petroleum Society Conference, 3-5 May 1999, Bergen, Norway

Norwegian Petroleum Society (NPF), Special Publication No. 10 Sedimentary Environments Offshore Norway Palaeozoic to R...

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Norwegian Petroleum Society (NPF), Special Publication No. 10

Sedimentary Environments Offshore Norway Palaeozoic to Recent Proceedings of the Norwegian Petroleum Society Conference, 3-5 May 1999, Bergen, Norway

Further titles in the series:

1. R.M. Larsen, H. Brekke, B.T. Larsen and E. Talleraas (Editors) STRUCTURAL AND TECTONIC MODELLING AND ITS APPLICATION TO PETROLEUM GEOLOGY- Proceedings of Norwegian Petroleum Society Workshop, 18-20 October 1989, Stavanger, Norway 2. T.O. Vorren, E. Bergsager, Q.A. DahI-Stamnes, E. Holter, B. Johansen, E. Lie and T.B. Lund (Editors) ARCTIC GEOLOGY AND PETROLEUM POTENTIAL- Proceedings of the Norwegian Petroleum Society Conference, 15-17 August 1990, Tromso, Norway 3. A.G. Dore et al. (Editors) BASIN MODELLING" ADVANCES AND APPLICATIONS- Proceedings of the Norwegian Petroleum Society Conference, 13-15 March 1991, Stavanger, Norway 4. S. Hanslien (Editor) PETROLEUM EXPLORATION AND EXPLOITATION IN NORWAYProceedings of the Norwegian Petroleum Society Conference, 9-11 December 1991, Stavanger, Norway

5. R.J. Steel, V.L. Felt, E.P. Johannesson and C. Mathieu (Editors) SEQUENCE STRATIGRAPHY ON THE NORTHWEST EUROPEAN MARGIN Proceedings of the Norwegian Petroleum Society Conference, 1-3 February, 1993, Stavanger, Norway 6. A.G. Dore and R. Sinding-Larsen (Editors) QUANTIFICATION AND PREDICTION OF HYDROCARBON RESOURCESProceedings of the Norwegian Petroleum Society Conference, 6-8 December 1993, Stavanger, Norway 7. P. Moller-Pedersen and A.G. Koestler (Editors) HYDROCARBON SEALS- Importance for Exploration and Production

8. F.M. Gradstein, K.O. Sandvik and N.J. Milton (Editors) SEQUENCE STRATIGRAPHY- Concepts and Applications Proceedings of the Norwegian Petroleum Society Conference, 6-8 September 1995, Stavanger, Norway 9. K. Ofstad, J.E. Kittilsen and P. Alexander-Marrack (Editors) IMPROVING THE EXPLORATION PROCESS BY LEARNING FROM THE PAST Proceedings of the Norwegian Petroleum Society Conference, September 1998, Haugesund, Norway

Norwegian Petroleum Society (NPF), Special Publication No. 10

Sedimentary nvironments Offshore Norway P a l a e o z o i c to R e c e n t Proceedings of the Norwegian Petroleum Society Conference, 3-5 May 1999, Bergen, Norway

Edited

by

Ole J. Martinsen Norsk Hydro Research Centre, R O. Box 7190, N-5020 Bergen, Norway

and

Tom

Dreyer

Norsk Hydro Research Centre, R O. Box 7190, N-5020 Bergen, Norway

2001 ELSEVIER

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9 2001 Elsevier Science B.V. All rights reserved. This work and the individual contributions contained in it are protected under copyright by Elsevier Science B.V., and the following terms and conditions apply to its use: Photocopying Single photocopies of single chapters may be made for personal use as allowed by national copyright laws. Permission of the Publisher and payment of a fee is required for all other photocopying, including multiple or systematic copying, copying for advertising or promotional purposes, resale, and all forms of document delivery. Special rates are available for educational institutions that wish to make photocopies for non-profit educational classroom use. Permissions may be sought directly from Elsevier Science Global Rights Department, PO Box 800, Oxford OX5 1DX, UK; phone: (+44) 1865 843830, fax: (+44) 1865 853333, e-mail: [email protected]. You may also contact Global Rights directly through Elsevier's home page (http://www.elsevier.nl), by selecting 'Obtaining Permissions'. In the USA, users may clear permissions and make payments through the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, USA; phone: (978) 7508400, fax: (978) 7504744, and in the UK through the Copyright Licensing Agency Rapid Clearance Service (CLARCS), 90 Tottenham Court Road, London W1P 0LP, UK; phone: (+44) 171 631 5555, fax: (+44) 171 631 5500. Other countries may have a local reprographic rights agency for payments. Derivative Works Tables of contents may be reproduced for internal circulation, but permission of Elsevier Science is required for resale or distribution of such material. Permission of the Publisher is required for all other derivative works, including compilations and translations. Electronic Storage or Usage Permission of the Publisher is required to store or use electronically any material contained in this work, including any chapter or part of a chapter. Except as outlined above, no part of this work may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without prior written permission of the Publisher. Address permissions requests to: Elsevier Science Global Rights Department, at the mail, fax and e-mail addresses noted above. Notice No responsibility is assumed by the Publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. Because of rapid advances in the medical sciences, in particular, independent verification of diagnoses and drugs dosages should be made. First edition 2001

Library of Congress Cataloging-in-Publication Data Sedimentary environments offshore N o r w a y w Palaeozoic to Recent/edited by Ole J. Martinsen, Tom D r e y e r - 1st ed. p. cm - (Norwegian Petroleum Society (NPF) Special Publication; no. 10) Includes bibliographical references and index. I S B N 0-444-50241-6 1. Petroleum-Prospecting-Congresses. I. Martinsen, Ole J. II. Dreyer, Tom. III. Series TN271.P4 I47 2001 622'. 1828-dc21

00-021616

ISBN: 0-444-50241-6 @ The paper used in this publication meets the requirements of A N S I / N I S O Z39.48-1992 (Permanence of Paper). Printed in The Netherlands

Dedication to Arne Dalland (1945-1998) Arne Dalland, who was a member of the organising committee for the Norwegian Petroleum Society Conference "Sedimentary Environments Offshore Norway Palaeozoic to Recent", died in 1998. Arne was a pioneer in Norwegian onshore and offshore sedimentology, and he was the first sedimentology graduate from the University of Bergen. He worked under the supervision of professor Anders Kvale, carrying out work on the Mesozoic section at AndOya, northern Norway (Dalland, 1975). Later on, he became a senior lecturer at the University of Bergen and worked extensively in Spitsbergen, before joining Statoil in 1983, where he worked until 1998. During Arne's spell at the University of Bergen, both the editors of this volume had the privilege of taking G 103-"Historical Geology" where Arne Dalland lectured on the offshore sedimentology. His lectures, given with Arne's well-known quiet and calm expression but clear and in-depth knowledge, inspired us in our careers as sedimentologists. There is a close tie between the theme of the conference and the conference proceedings published in this volume, and Arne Dalland's extensive work. Arne had a strong focus on detailed sedimentological and stratigraphic work for understanding palaeogeography. He was a leader and forerunner in formalising offshore stratigraphy leading to the publication of Dalland et al. (1988) on offshore Mid- and North Norway stratigraphy. The innovative use of names (such as boat and fish names) that relate to the all-important fishing industry, rather than traditional place names, in this part of Norway is remarkable. Ame Dalland was also a forerunner in using new methodology on offshore data. He initiated studies on using Sm/Nd isotope stratigraphy for reservoir and provenance studies. Arne Dalland was a highly respected and well-known geologist. He had his heart in the science, always being concerned with acquiring the highest quality analyses and results. We find it appropriate to dedicate this volume to Arne for his inspirational ideas and work on the sedimentary environments offshore Norway. References Dalland, A., 1975. The Mesozoic rocks of AndCya, Northern Norway. Norg. Geol. Unders. Skr., 316: 271-287. Dalland, A., Worsley, D. and Ofstad, K., 1988. A lithostratigraphic scheme for the Mesozoic and Cenozoic succession offshore mid- and northern Norway. Norw. Pet. Dir. Bull., 4:65 pp.

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VII

Preface and Acknowledgements The aim of the Norwegian Petroleum Society Conference "Sedimentary Environments Offshore Norway - - Palaeozoic to Recent" held in Bergen from May 3-5, 1999, was to show a representative selection of case studies which illustrated how the sedimentary environments in presently offshore areas had changed through time. Whether we as the organising committee and editors have been successful in this, can only be judged by the readers of the book. Naturally, it is impossible to produce an all-inclusive book with examples from all stratigraphic levels and various offshore basins. However, we believe that this volume contains a selection of papers that give examples of the offshore development over time particularly in the Mid-Norway and North Sea areas. Furthermore, several analogue outcrop examples to the offshore cases are included. Moreover, the papers in this volume record the issues and stratigraphy currently in focus. We are particularly happy to include several papers related to the Holocene development of the Norwegian margin, because understanding this part of the succession is important for hazard evaluation and burial and uplift history of the offshore areas. Readers of the volume should combine these proceedings with the abstract volume (Martinsen and Dreyer, 1999), where extended abstracts are available of all presentations made at the conference. Neither the conference organisation nor the proceedings volume could have been a success without the inspirational help of an effective organising committee and a highly competent group of referees. We would like to thank the other members of the organising committee for providing innumerable ideas and suggestions: Tom Bugge Arne Dalland (deceased) Lars Magnus F~ilt

Roy Gabrielsen Karin Haugna~ss (secretary) William Helland-Hansen

Johan Petter Nystuen Rodmar Ravnfis

We aimed to have an international group of referees judge the quality of the submitted papers in order to secure an international standard of the volume. The referees were very effective in providing comments and feedback and in returning manuscripts on time. Thus, we extend our thanks to the referees for helping us significantly in the preparation of this volume: Morten Bergan Arnold Bouma Tom Bugge Reidulv B0e Mike Charnock Ed Clifton John Collinson Steve Corfield Bob Dalrymple Tony Dor6 Lars Magnus F~ilt Atle Folkestad Roald Fa~rseth Ashton Embry Steve Flint

Bill Galloway Mike Gardner Rob Gawthorpe John Gjelberg William Helland-Hansen Erik Johannessen Ragnar Knarud Dale Leckie Trond Lien Finn Livbjerg Gunn Mangerud Tor Nedkvitne Wojtek Nemec Johan Petter Nystuen Snorre Olaussen

Torben Olsen Cai Puigdefabregas Rodmar Ravnfis Phillip Ringrose Alf Ryseth Ian Sharp Ron Steel Jim Steidtmann Finn Surlyk Kristian S0egaard Mike Talbot Tore Vorren Roger Walker Brian Zaitlin

Most importantly, we would like to thank the authors for being very co-operative in delivering and returning manuscripts on time. Without effective authors, there would be no publications.

VIII

Preface and Acknowledgements

Finally, we thank the Norwegian Petroleum Society for willingness to organise the conference originally based on a hand-written note. Especially, the effortless work by Karin Haugna~ss is deeply appreciated. We would also like to thank Norsk Hydro for giving us time to complete these proceedings. Ole J. Martinsen Tom Dreyer Bergen, October 2000

Reference Martinsen, O.J. and Dreyer, T. (Editors), 1999. Sedimentary Environments Offshore N o r w a y - Palaeozoic to Recent. Extended Abstracts, Norwegian Petroleum Society/NPF Conference, Bergen, May 3-5, 1999, 258 pp.

IX

List of Contributors

J. A N D S B J E R G

Geological Survey of Denmark and Greenland (GEUS), Thoravej 8, DK-2400 Copenhagen NV, Denmark

S. BACKSTROM

Applied Biostratigraphy, Blekksoppgrenda 41, N-1352 KolsCts, Norway

M.O. BADESCU

Delft University of Technology, Faculty of Applied Earth Sciences, P.O. Box 5028, 2600 GA Delft, The Netherlands

H. B R E K K E

Norwegian Petroleum Directorate, P.O. Box 600, N-4003 Stavanger, Norway

T. BUGGE

Norsk Hydro ASA, N-9480 Harstad, Norway

M. CECCHI

Enterprise Oil Norge Ltd., Lekkeveien 193b, N-4002 Stavanger, Norway

M.A. CHARNOCK

Norsk Hydro Research Centre, P.O. Box 7190, N-5020 Bergen, Norway

S. CORFIELD

Department of Earth Sciences, University of Manchester, Oxford Road, Manchester M13 9PL, UK

T. D R E Y E R


K. DYBKJ}ER


T. ENOKSEN


J.I. FALEIDE

Department of Geology, University of Oslo, N-0316 Oslo, Norway

J.EG. FENTON

Robertson Research International Ltd., Llandudno, North Wales LL30 1SA, UK

A. FOLKESTAD

Statoil, Research and Technology, Department of Reservoir Characterisation, N-4035 Stavanger, Norway

R.H. GABRIELSEN

Geological Institute, University of Bergen, N-5007 Bergen, Norway

G.K. GILLMORE

University College Northampton, School of Environmental Science, Northampton NN2 7AL, UK

J. GJELBERG


C. GUARGENA

Enterprise Oil Norge Ltd., Lekkeveien 193b, N-4002 Stavanger, Norway

K.-O. HAGER


L. HANSEN

Enterprise Oil Norge Ltd., LOkkeveien 193b, N-4002 Stavanger, Norway

G. HELGESEN

Statoil Stavanger, Forusbeen 50, Stavanger, Norway

EN. JOHANNESSEN


M.D. JACKSON

Centre for Petroleum Studies, T.H. Huxley School of Environment, Earth Sciences and Engineering, Imperial College, Prince Consort Road, London SW7 2BP, UK

X


H.D. JOHNSON


I. KAAS

Statoil Bergen, Sandslihaugen 30, N-5020 Bergen, Norway

J.M. KJiEREFJORD

Statoil Bergen, Sandslihaugen 30, N-5020 Bergen, Norway

E KJiERNES

Norsk Hydro Exploration, N-0246 Oslo, Norway

T. KJENNERUD

SINTEF Petroleum Research, N-7465 Trondheim, Norway

M. KREINER-MOLLER

Geological Institute, University of Copenhagen, r DK-1350 Copenhagen K, Denmark

I.L. KRISTIANSEN


R. KYRKJEBO

Geological Institute, University of Bergen, N-5007 Bergen, Norway

J.S. LABERG

Department of Geology, University of TromsO, N-9037 TromsO, Norway

M. LARSEN


D.A. LEITH

Statoil StjOrdal, Strandveien 4, N-7501 StjOrdal, Norway

S.J. LIPPARD

Department of Geology and Mineral Resource Engineering, NTNU, N-7465 Trondheim, Norway

H. LOSETH

Statoil Research Centre, N-7005 Trondheim, Norway

C. MAGNUS


G. MANGERUD


A.W. MARTINIUS

Statoil Research Centre, Arkitekt Ebbellsveg 10, N-7005 Trondheim, Norway Present address: c/o Statoil Venezuela - Sincor Project, N-4035 Stavanger, Norway

O.J. MARTINSEN


A. MORK


M.B.E. M{0RK


A.H. MUGGERIDGE


A. NAESS

Statoil Research Centre, Arkitekt Ebbellsveg 10, N-7005 Trondheim, Norway Present address: Statoil StjOrdal, Strandveien 4, N-7501 StjOrdal, Norway

T. NEDKVITNE


L.H. NIELSEN


N. NOE-NYGAARD

Geological Institute, University of Copenhagen, Oster Voldgade 10, DK-1350 Copenhagen K, Denmark

S. OLAUSSEN


D. OTTESEN

Geological Survey of Norway, N-7491 Trondheim, Norway

D. RHODES


Voldgade 10,


XI

L. RISE


A. ROBERTS


E. ROE


K. ROKOENGEN

Norwegian University of Science and Technology, N-7034 Trondheim, Norway

A. RYSETH

Norsk Hydro Research Centre, P.O. Box 7190, N-5020 Bergen, Norway. Present address: Norsk Hydro Harstad, Storakern 11, Kanebogen, N-9401 Harstad, Norway

J. S/ETTEM

SINTEF Petroleum Research, N-7465 Trondheim, Norway Present address: Sauherad Kommune, N-3812 Akkerhaugen, Norway

I. SHARP


H.I. SJULSTAD


M. SMELROR


R.J. STEEL

University of Wyoming, Department of Geology and Geophysics, Laramie, WY 82071, USA

L. STEMMERIK


F. SURLYK

Geological Institute, University of Copenhagen, Oster Voldgade 10, DK-1350 Copenhagen K, Denmark

K.E. SVELA

Norske Conoco A/S, Tangen 7, N-4070 Randaberg, Norway

B. TVEITEN

Norsk Hydro ASA, E & P International, N-0246 Oslo, Norway

J. UNDERHILL

Department of Geology and Geophysics, University of Edinburgh, Edinburgh EH9 3JW, UK

E. VAGNES


T.O. VORREN

Department of Geology, University of TromsO, N-9037 TromsO, Norway

H.M. WEISS


R.W. WILLIAMS


S. YOSHIDA

Centre for Petroleum Studies, T.H. Huxley School of Environment, Earth Sciences and Engineering, Imperial College, Prince Consort Road, London SW7 2BP, UK Present address: Surface Processes and Modern Environments Research Group, Department of Geology, Royal Holloway, University of London, Egham, Surrey TW20 OEX, UK


XIII

Contents Dedication to Arne Dalland (1945-1998) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Preface and Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . List of Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

V VII IX

I. Introductory Papers Sedimentary environments offshore Norway O.J. Martinsen and T. Dreyer

Palaeozoic to Recent: an introduction . . . . . .

1

Sedimentary environments offshore Norway ~ an overview . . . . . . . . . . . . . . . . . . . . . . . . H. Brekke, H.I. Sjulstad, C. Magnus and R.W. Williams

II. Palaeozoic The alluvial cyclicity in Hornelen Basin (Devonian western Norway) revisited: a multiparameter sedimentary analysis and stratigraphic implications . . . . . . . . . . . . . . . . . . . . . . A. Folkestad and R.J. Steel

39

Upper Permian lowstand fans of the Bredehorn Member, Schuchert Dal Formation, East Greenland . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . M. Kreiner-MOller and L. Stemmerik

51

III. Mesozoic Sedimentology and palaeogeography of the Statfjord Formation (Rhaetian-Sinemurian), North Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Ryseth

67

Sedimentary facies in the fluvial-dominated Are Formation as seen in the Are 1 member in the Heidrun Field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . K.E. Svela

87

Sedimentology of the heterolithic and tide-dominated Tilje Formation (Early Jurassic, Halten Terrace, offshore mid-Norway) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A.W. Martinius, I. Kaas, A. Na~ss, G. Helgesen, J.M. Kja~refjord and D.A. Leith

103

Sequence stratigraphy of the Lower Jurassic Dunlin Group, northern North Sea . . . . . . . . . . . M.A. Charnock, I.L. Kristiansen, A. Ryseth and J.EG. Fenton

145

Divergent development of two neighbouring basins following the Jurassic North Sea Doming event: the Danish Central Graben and the Norwegian-Danish Basin . . . . . . . . . . . . . . . J. Andsbjerg, L.H. Nielsen, EN. Johannessen and K. Dybkjaer

175

An integrated study of the Garn and Melke formations (Middle to Upper Jurassic) of the SmOrbukk area, Halten Terrace, mid-Norway . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . S. Corfield, I. Sharp, K.-O. H~iger, T. Dreyer and J. Underhill

199

Middle Jurassic-Lower Cretaceous transgressive-regressive sequences and facies distribution off northern Nordland and Troms, Norway . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . M. Smelror, A. MOrk, M.B.E. MOrk, H.M. Weiss and H. LOseth

211

XIV

Contents

Outcrop studies of tidal sandstones for reservoir characterization (Lower Cretaceous Vectis Formation, Isle of Wight, southern England) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . S. Yoshida, M.D. Jackson, H.D. Johnson, A.H. Muggeridge and A.W. Martinius

233

Lower Cretaceous (Barremian-Albian) deltaic and shallow marine sandstones in North-East Greenland ~ sedimentology, sequence stratigraphy and regional implications . . . . . . . . M. Larsen, T. Nedkvitne and S. Olaussen

259

The depositional history of the Cretaceous in the northeastern North Sea . . . . . . . . . . . . . . . . T. Bugge, B. Tveiten and S. B~ickstr6m Cretaceous faulting and associated coarse-grained marine gravity flow sedimentation, Traill O, East Greenland . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . F. Surlyk and N. Noe-Nygaard

279

293

IV. Mesozoic-Cenozoic Transition Cretaceous-Tertiary palaeo-bathymetry in the northern North Sea; integration of palaeo-water depth estimates obtained by structural restoration and micropalaeontological analysis... R. KyrkjebO, T. Kjennerud, G.K. Gillmore, J.I. Faleide and R.H. Gabrielsen Structural restoration of Cretaceous-Cenozoic (post-rift) palaeobathymetry in the northern North Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . T. Kjennerud, J.I. Faleide, R.H. Gabrielsen, G.K. Gillmore, R. KyrkjebO, S.J. Lippard and H. LOseth The reconstruction and analysis of palaeowater depths: a new approach and test of micropalaeontological approaches in the post-rift (Cretaceous to Quaternary) interval of the northern North Sea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . G.K. Gillmore, T. Kjennerud and R. Kyrkjebr

321

347

365

V. Cenozoic Outcrop-based classification of thick-bedded, deep-marine sandstones . . . . . . . . . . . . . . . . . M.O. Badescu

383

Use of integrated 3D seismic technology and sedimentology core analysis to resolve the sedimentary architecture of the Paleocene succession of the North Sea . . . . . . . . . . . . . M. Cecchi, C. Guargena, L. Hansen, D. Rhodes and A. Roberts

407

The Maastrichtian and Danian depositional setting, along the eastern margin of the MOre Basin (Mid-Norwegian Shelf): implications for reservoir development of the Ormen Lange Field . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . J.G. Gjelberg, T. Enoksen, R Kj~emes, G. Mangerud, O.J. Martinsen, E. Roe and E. V~gnes

421

Glacial processes and large-scale morphology on the mid-Norwegian continental shelf . . . . . . D. Ottesen, L. Rise, K. Rokoengen and J. Sa~ttem Late Quaternary sedihaentary processes and environment on the Norwegian-Greenland Sea continental margins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . T.O. Vorren and J.S. Laberg Accretionary, forced regressive shoreface sands of the Holocene-Recent Skagen Odde spit complex, Denmark ~ a possible outcrop analogue to fault-attached shoreface sandstone reservoirs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . L.H. Nielsen and EN. Johannessen

441

451

457

References index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

473

Subject index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

483

Sedimentary environments offshore Norway--- Palaeozoic to R e c e n t : an i n t r o d u c t i o n Ole J. Martinsen and Tom Dreyer

Sedimentary environments offshore Norway have evolved through time as a response to changing climatic conditions, basin physiography and tectonic setting. This volume includes examples from various time periods and offshore basins, including some adjacent onshore and offshore analogues from Greenland, Denmark and Great Britain. These examples illustrate characteristics of the sedimentary environments of various time periods, from the Devonian of the onshore Hornelen Basin to the Holocene of the Mid-Norway area, including continental, shallow-water and deep-water depositional settings. Cases range from detailed facies analysis of highly prolific, hydrocarbon-bearing Jurassic reservoir rocks, through to Recent, giant submarine slides, showing the changes in processes and setting that the Norwegian offshore areas have experienced. The examples from analogues in East Greenland are particularly important both to understand the downdip evolution of environments in the MOre and VOring Basins, but also because the onshore areas on the Norwegian mainland largely lack a post-Devonian sedimentary record. This volume presents an account of the sedimentary development of the Norwegian basins offshore Mid-Norway and in the North Sea region, and thus complements earlier volumes dealing with the Arctic areas.

Introduction From May 3-5, 1999, more than 200 geologists assembled in Grieghallen, Bergen to discuss the sedimentary development of the Norwegian offshore areas and their analogues. The conference was organised by the Norwegian Petroleum Society. The participants came from universities, government agencies and oil and consulting companies in Norway, Denmark, Great Britain, Holland, France, Spain, Canada and USA. The presentations included 25 talks, 10 core examples and 37 posters. Particular emphasis was on the posters and the core examples, recognising that it was within these presentations that most of the data were presented. Three keynote addresses were given: by Robert Dalrymple on non- and marginalmarine environments, by H. Edward Clifton on shallow-marine environments, and by Arnold Bouma on deep-water environments. For various reasons, papers from these keynote addresses were not included in the volume, but their abstracts are included in the abstracts volume (Martinsen and Dreyer, 1999).

Sedimentary environments in Norway through time This conference volume includes 23 papers, from both the poster and the oral presentations. They range from detailed analysis of single stratigraphic units to

overview articles. Previous volumes treating the Norwegian offshore geology have dealt with correlation methodology (Collinson, 1988), sequence stratigraphy (Steel et al., 1995; Gradstein et al., 1998) or concentrated on Arctic geology (Vorren et al., 1993). Since the present volume has papers mainly from the Mid-Norway and North Sea Basins, it complements the volume edited by Vorren et al. (1993) on the Arctic areas. Together, these two volumes give a comprehensive account of how both the northern and southern basins evolved through time. Both publications are required reading for geologists working in the offshore areas. In the following, we give a brief review of the papers contained in the present volume and put the information into a time-stratigraphic context and provide some comparison with onshore data from Norway. Palaeozoic

As is the case in the Arctic and in the Barents Sea region (cf. Vorren et al., 1993), Palaeozoic sedimentary successions are poorly known from offshore Norway, although they most likely exist in considerable thickness and may link up with the onshore successions (e.g. Fa~rseth et al., 1995). Onshore, Precambrian strata in Finnmark (e.g. Siedlecka, 1975) and Cambrian-Carboniferous sedimentary successions in the Oslo region (e.g. Olaussen et al., 1994) are well

Sedimentary Environments Offshore Norway - Palaeozoic to Recent edited by O.J. Martinsen and T. Dreyer. NPF Special Publication 10, pp. 1-5, Published by Elsevier Science B.V., Amsterdam. 9 Norwegian Petroleum Society (NPF), 2001.

2

known. Furthermore, The onshore Devonian sedimentary record from western Norway (Andersen, 1998; Folkestad and Steel, 2001) is a widely known and most impressive record of basin formation and sedimentation by anyone's standard. However, the existence of pre-Devonian sedimentary record in western Norway is less well known (e.g. Brekke and Solberg, 1987; RavnSs and Fumes, 1995 and references therein). Although the Devonian and older rocks are unlikely to form reservoir rocks offshore Norway, their onshore existence and character are valuable for implications of the offshore development. The overview given by Brekke et al. (2001) on pre-Mesozoic offshore sedimentation is valuable although the database is sparse. Carboniferous reservoir successions are well known from the British offshore areas (e.g. Collinson et al., 1993). The Carboniferous succession offshore Norway is perhaps a speculative reservoir target because of large burial depths in most basins, but as of the present day it is untested and still a possibility where burial depth is not excessive. Permian sedimentary rocks are well known in the Barents Sea (Vorren et al., 1993), but further south, there is little information from offshore areas. In the Oslo Graben, there is a Permian sequence (Olaussen et al., 1994). In fact, the Permian may be the least well-known time period in from Norwegian offshore and onshore areas. Therefore, the data and interpretations given by Kreiner-Mr and Stemmerik (2001) from well-exposed Permian deep-water deposits in East Greenland are valuable, because they provide important ideas for the sedimentary evolution of this time period in adjacent areas offshore Norway. Mesozoic The Mesozoic Era is obviously the most important time period offshore Norway for hydrocarbon resources and implicitly for the occurrence of reservoir and source rocks. Especially, there was a major change of sedimentary environments from continental in the Triassic through mainly coastal plain and nearshore marine to deep water in the Cretaceous. Brekke et al.'s (2001) overview paper covers several aspects of these changing sedimentary environments. The Triassic period was dominated by arid, continental deposition (e.g. Steel and Ryseth, 1990; Steel, 1993), while the latest Triassic to earliest Jurassic saw a progressive change into a wetter climate with the initiation of shallow-marine sedimentation. Ryseth (2001) documents the important changes that took place during the deposition of the Statfjord Formation, the major reservoir-bearing sandstone formation that is important both on the eastern and western margin of the Viking Graben in major fields such as

O.J. Martinsen and T. Dreyer

Snorre, Statfjord and Oseberg. In Mid-Norway, equivalent Lower Jurassic sandstones are also highly important as hydrocarbon reservoirs and Svela (2001) describe the Are Formation in the Heidrun Field. Another important stratigraphic unit in the Mid-Norway area is the Tilje Formation, which forms both a primary and secondary reservoir unit in several Mid-Norway fields. The tidally dominated Tilje Formation marks the change from mainly fluvial deposition in the ,~re Formation to shallow-marine, tidally dominated sedimentation. Martinius et al. (2001) document the facies and sedimentary patterns within the Tilje. Early Jurassic sedimentation in the North Sea Basin was dominated by shallow marine sedimentation recorded by the Dunlin Group. The Dunlin Group has played a subordinate role as a reservoir unit, but Charnock et al. (2001) document the regional depositional patterns and sequence stratigraphy. The Dunlin Group may have an unexplored reservoir potential, and the hydrocarbon resources could be underestimated. The Brent Group, the most prolific oil reservoir unit on the Norwegian shelf, lies on top of the Dunlin Group. It marks a change from margin fed shallow marine systems in Dunlin Group (see Charnock et al., 2001), to an axial system which prograded and retrograded largely along a N-S axis. The Brent Group has been amply documented in earlier publications (see for instance the papers in Steel et al., 1995). While the Early and Middle Jurassic periods were dominated by relative tectonic quiescence, tectonism becomes very important in the Late Jurassic. Extensional faulting controls depositional patterns, and sedimentary environments vary considerably on a local scale. This situation is important both offshore Mid-Norway, in the North Sea and in adjacent areas, and Corfield et al. (2001), Andsbjerg et al. (2001) and Smelror et al. (2001) show examples of the relationships between tectonism and sedimentation during this period. One particularly important part of the Jurassic reservoirs is the reservoir behaviour and characterization. Analogue work is important for this aspect and Yoshida et al. (2001) show an example from tidal sandstones in southern England that compares with the tidally dominated Tilje Formation. Other field analogues for the Jurassic and the lower Cretaceous stratigraphy offshore Norway are found in East Greenland, and both Larsen et al. (2001) and Surlyk and Noe-Nygaard (2001) show important examples. Tectonism and rifting in the Late Jurassic and earliest Cretaceous mark the change from shallow-marine sedimentation to deep-water sedimentation. The example by Larsen et al. (2001) from Greenland (see above) shows this change. In the Norwegian offshore

Sedimentary environments offshore N o r w a y - Palaeozoic to Recent: an introduction

areas, Cretaceous reservoirs have been difficult to locate as the passage from the Jurassic to Cretaceous time also records a change from a sand-rich to a mudrich depositional setting. Bugge et al. (2001) describe the depositional history in the northern North Sea, an area with some Cretaceous discoveries, such as in Agat, but where the volumes of hydrocarbons so far have been too small for commercial exploitation. Understanding the basin development through the Cretaceous-Cenozoic period is challenging, and severe effort has been put into solving this important question for making predictive models. The three papers by KyrkjebO et al. (2001), Kjennerud et al. (2001) and Gillmore et al. (2001) show different approaches of restoring palaeobathymetry. Such modelling is important because they provide ideas on how basins filled and thus where source and reservoir rocks may be located. Sand-rich deep-water turbidite systems are present in the western Voring Basin (Brekke et al., 2001). These systems relate to supply from East Greenland, showing the importance of the western basin margin of the offshore Mid-Norway area for supply of sands (Larsen et al., 2001; Surlyk and Noe-Nygaard, 2001). The western margin of the VOring and MOre Basins still has a high potential for hydrocarbon exploration. Cenozoic

The Palaeocene of the North Sea and the MidNorway area has proven to be a prolific, oil- and gas-bearing reservoir succession. Sedimentation took place in deep-water fans and related depositional systems (Badescu, 2001; Cecchi et al., 2001; Gjelberg et al., 2001). The change from a relatively mud-rich Cretaceous period to more sandy Palaeocene systems relate to basin margin uplift and tectonism in concert with incipient rifting in the North Atlantic (Martinsen et al., 1999; Brekke et al., 2001). This volume lacks papers dealing with the Eocene-Pliocene periods, and thus we give a short review to complete this introduction. The EoceneMiocene period saw a change from deep-water to shallow-marine conditions in the North Sea (Dalland et al., 1988; Isaksen and Tonstad, 1989; Martinsen et al., 1999). This period is in general poorly documented on the Norwegian shelf in terms of sedimentary history. Many wells, drilled for deeper targets, have no data from much of this stratigraphic succession. The petroleum potential of the EoceneMiocene is highly questionable because of shallow burial depths. Nevertheless, the depositional history has more than academic interest, because the accumulation of these stratigraphic successions, and the overlying Pliocene, caused underlying packages to

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reach burial depths where hydrocarbons could form, migrate and accumulate in reservoir-bearing successions of older age. The Pliocene is also relatively poorly documented in general, despite its great importance for burial of older units, and the fact that it records the highest sedimentation rate on the Norwegian shelf (e.g. Jordt et al., 1995; Rokoengen et al., 1995; Henriksen and Weimer, 1996). The thick Pliocene wedges probably relate to glaciation, high erosional rates and consequent high sedimentation rates (cf. Rokoengen et al., 1995 and references therein). The Pleistocene and Holocene periods are dominated by glacial sedimentation and erosion in the offshore area. Ottesen et al. (2001) show how glacial erosion has created large-scale erosional features and depositional products on the Mid-Norwegian continental shelf. In a time-stratigraphic sense, erosion is probably more important than sedimentation in many areas (e.g. Sejrup et al., 1996; Ottesen et al., 2001). A major component of the Pleistocene and Holocene history is the occurrence of giant submarine slides. Vorren and Laberg (2001) describe several examples and their occurrence. Bugge et al. (1987) and Haflidason et al. (1999) described the Storegga slide, the largest known submarine slide (see also the illustration on the front cover of the book). The sedimentary environments and patterns during this recent period have major implications for installation of hydrocarbon-producing equipment on or above the sea floor, and thus the importance of understanding the Quaternary development cannot be underestimated. In addition, studies of the Recent Skagen Odde complex in northern Denmark show a valuable modern analogue for Jurassic, fault block-related shoreface sands (Nielsen and Johannessen, 2001), which shows the importance of studying the present to understand the ancient. Conclusions

This volume covers many important aspects of sedimentation and sedimentary environments offshore Norway from the Palaeozoic to the Recent. The sedimentary geological setting has changed significantly over this time period as a response to changing tectonic setting, basin physiography and morphology. Further knowledge will be attained through an increasing database with new wells, and in particular a growing seismic database where 3-D data play a vital role. Our prediction is that the most important progress in understanding the development of sedimentary environments through time offshore Norway will be made from studying seismic data. There is a general knowledge on how sedimentary environments changed from core data, and naturally these data have

4

to be supplemented by new core data. However, a fully three-dimensional understanding of plan view morphology and cross-sectional architecture can only be attained from high-resolution seismic data used with a sedimentologist's eye. References Andersen, T., 1998. Extensional tectonics in the Caledonides of southern Norway, an overview. Tectonophysics, 285:333-351. Andsbjerg, J., Nielsen, L.H., Johannessen, EN. and Dybkja~r, K., 2001. Divergent development of two neighbouring basins following the Jurassic North Sea doming event: the Danish Central Graben and the Norwegian-Danish Basin. In: O.J. Martinsen and T. Dreyer (Editors), Sedimentary Environments Offshore Norway Palaeozoic to Recent. Norwegian Petroleum Society (NPF), Special Publication 10. Elsevier, Amsterdam, pp. 175-197 (this volume). Badescu, M.O., 2001. Outcrop-based classification of thick-bedded, deep marine sandstones. In: O.J. Martinsen and T. Dreyer (Editors), Sedimentary Environments Offshore N o r w a y - Palaeozoic to Recent. Norwegian Petroleum Society (NPF), Special Publication 10. Elsevier, Amsterdam, pp. 383-405 (this volume). Brekke, H. and Solberg, EO., 1987. The geology of Atl0y, Sunnfjord, Western Norway. Nor. Geol. Unders. Bull., 410: 677-690. Brekke, H., Sjulstad, H.I., Magnus, C. and Williams, R., 2001. Sedimentary environments offshore N o r w a y - an overview. In: O.J. Martinsen and T. Dreyer (Editors), Sedimentary Environments Offshore Norway Palaeozoic to Recent. Norwegian Petroleum Society (NPF), Special Publication 10. Elsevier, Amsterdam, pp. 7-37 (this volume). Bugge, T., Befring, S., Belderson, R.H., Eidvin, T., Jansen, E., Kenyon, N.H., Holtedahl, H., Sejrup, H.E, 1987. A giant threestage submarine slide off Norway. Geo-Mar. Lett., 7: 191-198. Bugge, T., Tveiten, B. and B~ickstr0m, S., 2001. The depositional history of the Cretaceous in the northeastern North Sea. In: O.J. Martinsen and T. Dreyer (Editors), Sedimentary Environments Offshore Norway - - Palaeozoic to Recent. Norwegian Petroleum Society (NPF), Special Publication 10. Elsevier, Amsterdam, pp. 279-291 (this volume). Cecchi, M., Guargena, C., Hansen, L., Rhodes D. and Roberts, A., 2001. Use of integrated 3D seismic technology and sedimentology core analysis to resolve the sedimentary architecture of the Palaeocene succession of the North Sea. In: O.J. Martinsen and T. Dreyer (Editors), Sedimentary Environments Offshore Norway Palaeozoic to Recent. Norwegian Petroleum Society (NPF), Special Publication 10. Elsevier, Amsterdam, pp. 407-419 (this volume). Charnock, M.A., Kristiansen, I.L., Ryseth, A. and Fenton, LEG., 2001. Sequence Stratigraphy of the Lower Jurassic Dunlin Group, northern North Sea. In: O.J. Martinsen and T. Dreyer (Editors), Sedimentary Environments Offshore Norway - - Palaeozoic to Recent. Norwegian Petroleum Society (NPF), Special Publication 10. Elsevier, Amsterdam, pp. 145-174 (this volume). Collinson, J.D. (Editor), 1988. Correlation in Hydrocarbon Exploration. Graham and Trotman, London, 381 pp. Collinson, J.D., Jones, C.M., Blackbourn, G.A., Besly, B.M., Archard, G.M. and McMahon, A.H., 1993. Carboniferous depositional systems of the southern North Sea. In: J.R. Parker (Editor), Petroleum Geology of Northwest Europe: Proceedings of 4th Conference. Geological Society, London, pp. 677-687. Corfield, S., Sharp, I., H~iger, K.-O., Dreyer, T. and Underhill, J., 2001. An integrated study of the Garn and Melke Formations (Middle to Upper Jurassic) of the Sm0rbukk area, Halten Terrace, mid-Norway. In: O.J. Martinsen and T. Dreyer (Editors), Sedimentary Environments Offshore Norway - - Palaeozoic to Recent. Norwegian Petroleum Society (NPF), Special Publication 10. Elsevier, Amsterdam, pp. 199-210 (this volume).

O.J. Martinsen and T. Dreyer Dalland, A., Worsley, D. and Ofstad, K., 1988. A lithostratigraphic scheme for the Mesozoic and Cenozoic succession offshore midand northern Norway. Norw. Pet. Dir. Bull., 4:65 pp. Fa~rseth, R., Gabrielsen, R.H. and Hurich, C.A., 1995. Influence on basement in structuring of the North Sea Basin. Nor. Geol. Tidsskr., 75: 105-119. Folkestad, A. and Steel, R.J., 2001. The alluvial cyclicity in Hornelen Basin (Devonian western Norway) revisited: a multiparameter sedimentary analysis and stratigraphic implications. In: O.J. Martinsen and T. Dreyer (Editors), Sedimentary Environments Offshore Norway - - Palaeozoic to Recent. Norwegian Petroleum Society (NPF), Special Publication 10. Elsevier, Amsterdam, pp. 39-50 (this volume). Gillmore, G.K., Kjennerud, T., Kyrkjeb0, R., 2001. The reconstruction and analysis of palaeowater depths: a new approach and test of micropalaeontological approaches in the post-rift (Cretaceous to Quaternary) interval of the northern North Sea. In: O.J. Martinsen and T. Dreyer (Editors), Sedimentary Environments Offshore Norway - - Palaeozoic to Recent. Norwegian Petroleum Society (NPF), Special Publication 10. Elsevier, Amsterdam, pp. 365-381 (this volume). Gjelberg, J.G., Enoksen, T., Kj~ernes, E, Mangerud, G. et al., 2001. The Maastrichtian and Danian depositional setting, along the eastern margin of the MOre Basin (Mid-Norwegian Shelf): implications for reservoir development of the Ormen Lange Field. In: O.J. Martinsen and T. Dreyer (Editors), Sedimentary Environments Offshore Norway - - Palaeozoic to Recent. Norwegian Petroleum Society (NPF), Special Publication 10. Elsevier, Amsterdam, pp. 421-440 (this volume). Gradstein, F., Sandvik, K.O. and Milton, N.J. (Editors), 1998. Sequence Stratigraphy: Concepts and Applications. Norwegian Petroleum Society (NPF), Special Publication 8. Elsevier, Amsterdam, 437 pp. Haflidason, H., Gravdal, A., Sejrup, H.P., Bryn, E, Lien, R. and Mienert, J., 1999. TOBI iamgery side-scan sonar and seismic data of the northern escarpment of the Storegga Slide off Mid-Norway: evidence for long-term instability. In: O.J. Martinsen and T. Dreyer, (Editors) 1999. Sedimentary Environments Offshore Norway-Palaeozoic to Recent. Extended Abstracts, Norwegian Petroleum Society/NPF Conference, Bergen, May 3-5, 1999, pp. 205-207. Henriksen, S. and Weimer, E, 1996. High-frequency depositional sequences and stratal stacking patterns in lower Pliocene coastal deltas, Mid-Norwegian continental shelf. Bull. Am. Assoc. Pet. Geol., 80:1867-1895. Isaksen, D. and Tonstad, K., 1989. A revised Cretaceous and Ter tiary lithostratigraphic nomenclature for the Norwegian North Sea. Norw. Pet. Dir. Bull., 5:59 pp. Jordt, H., Faleide, J.I., Bj0rlykke, K. and Ibrahim, M.T., 1995. Cenozoic sequence stratigraphy of the central and northern North Sea Basin: tectonic development, sediment distribution and provenance areas. Mar. Pet. Geol., 12: 845-879. Kjennerud, T., Faleide, J.I., Gabrielsen, R.H., Gillmore, G.K., Kyrkjeb0, R., Lippard, S.J. and L0seth, H., 2001. Structural restoration of Cretaceous-Cenozoic (post-rift) palaeobathymetry in the northern North Sea. In: O.J. Martinsen and T. Dreyer (Editors), Sedimentary Environments Offshore Norway Palaeozoic to Recent. Norwegian Petroleum Society (NPF), Special Publication 10. Elsevier, Amsterdam, pp. 347-364 (this volume). Kreiner-M011er, M. and Stemmerik, L., 2001. Upper Permian lowstand fans of the Bredehorn Member, Schuchert Dal Formation, East Greenland. In: O.J. Martinsen and T. Dreyer (Editors), Sedimentary Environments Offshore Norway - - Palaeozoic to Recent. Norwegian Petroleum Society (NPF), Special Publication 10. Elsevier, Amsterdam, pp. 51-65 (this volume). Kyrkjeb0, R., Kjennerud, T., Gillmore, G.K., Faleide, J.I. and Gabrielsen, R.H., 2001. Cretaceous-Tertiary palaeo-bathymetry in the northern North Sea; integration of palaeo-water depth estimates obtained by structural restoration and micropalaeontological

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analysis. In: O.J. Martinsen and T. Dreyer (Editors), Sedimentary Environments Offshore Norway - - Palaeozoic to Recent. Norwegian Petroleum Society (NPF), Special Publication 10. Elsevier, Amsterdam, pp. 321-345 (this volume). Larsen, M., Nedkvitne, T. and Olaussen, S., 2001. Lower Cretaceous (Barremian-Albian) deltaic and shallow marine sandstones in North-East Greenland - - sedimentology, sequence stratigraphy and regional implications. In: O.J. Martinsen and T. Dreyer (Editors), Sedimentary Environments Offshore Norway - - Palaeozoic to Recent. Norwegian Petroleum Society (NPF), Special Publication 10. Elsevier, Amsterdam, pp. 259-278 (this volume). Martinius, A.W., Kaas, I., Na~ss, A., Helgesen, G., Kj~eref]ord, J.M. and Leith, D.A., 2001. Sedimentology of the heterolithic and tide-dominated Tilje Formation (Early Jurassic, Halten Terrace, offshore Mid-Norway). In: O.J. Martinsen and T. Dreyer (Editors), Sedimentary Environments Offshore Norway Palaeozoic to Recent. Norwegian Petroleum Society (NPF), Special Publication 10. Elsevier, Amsterdam, pp. 103-144 (this volume). Martinsen, O.J. and Dreyer, T. (Editors), 1999. Sedimentary environments offshore Norway Palaeozoic to Recent. Extended Abstracts, Norwegian Petroleum Society (NPF) Conference, Bergen, May 3-5 1999, 258 pp. Martinsen, O.J., Belen, F., Charnock, M., Mangerud, G. and Nottvedt, A., 1999. Cenozoic development of the Norwegian margin 60-64~ sequences and sedimentary response to variable basin physiography and tectonic setting. In: A.J. Fleet and S.A.R. Boldy (Editors), Petroleum Geology of Northwest Europe: Proceedings of the 5th ConfErence. Geological Society, London, pp. 293-3O4. Nielsen, L.H. and Johannessen, RN., 2001. Accretionary, forced regressive shoreface sands of the Holocene-Recent Skagen Odde spit complex, Denmark a possible outcrop analogue to faultattached shoreface Sandstone reservoirs. In: O.J. Martinsen and T. Dreyer (Editors), Sedimentary Environments Offshore Norway Palaeozoic to Recent. Norwegian Petroleum Society (NPF), Special Publication 10. Elsevier, Amsterdam, pp. 457-472 (this volume). Olaussen, S., Larsen, B.T. and Steel, R., 1994. The Upper Carboniferous-Permian Oslo Rift: basin fill in relation to tectonic development. In: A.F. Embry, B. Beauchamp and D.J. Glass (Editors), Pangea-Global Environment and Resources. Can. Soc. Pet. Geol. Mem., 17:175-197. Ottesen, D., Rise, L., Rokoengen, K. and Saettem, J., 2001. Glacial processes and large-scale morphology on the mid-Norwegian continental shelf. In: O.J. Martinsen and T. Dreyer (Editors), Sedimentary Environments Offshore Norway - - Palaeozoic to Recent. Norwegian Petroleum Society (NPF), Special Publication 10. Elsevier, Amsterdam, pp. 441-449 (this volume). Ravnfis, R. and Fumes, H., 1995. The use of geochemical data in determining the provenance and tectonic setting of ancient sedimentary successions: the Kalv~g Melange, western Norwegian Caledonides. In: A.G. Plint (Editor), Sedimentary Facies Analysis A Tribute to the Reseach and Teaching of Harold G. Reading. Int. Assoc. Sediment. Spec. PUN., 22: 237-264. Rokoengen, K., Rise, L., Bryn, R, Frengstad, B., Gustavsen, B., Nygaard, E. and Saettem, J., 1995. Upper Cenozoic stratigraphy on the Mid-Norwegian continental shelf. Nor. Geol. Tidsskr., 75: 88-104. Ryseth, A., 2001. Sedimentology and palaeogeography of the Statfjord Formation (Rhaetian-Sinemurian), North Sea. In: O.J. Martinsen and T. Dreyer (Editors), Sedimentary Environments Off-

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shore Norway Palaeozoic to Recent. Norwegian Petroleum Society (NPF), Special Publication 10. Elsevier, Amsterdam, pp. 67-85 (this volume). Sejrup, H.R, King, E.L., Aarseth, I., Haflidason, H. and Elverhr A., 1996. Quaternary erosion and depositional processes, western Norwegian fjords, Norwegian channel and North Sea Fan. In: M. De Batist and R Jacobs (Editors), Geology of Siliciclastic Shelf Seas. Geol. Soc. London, Spec. Publ., 117: 187-202. Siedlecka, A., 1975. Late Precambrian stratigraphy and structure of the northeastern margin of the Fennoscandian Shield (East Finnmark-Timan region). Nor. Geol. Unders., 316:313-348. Smelror, M., M0rk, A., M0rk, M.B.E., Weiss, H.M. and L0seth, H., 2001. Middle Jurassic-Lower Cretaceous transgressive-regressive sequences and facies distribution off Northern Nordland and Troms, Norway. In: O.J. Martinsen and T. Dreyer (Editors), Sedimentary Environments Offshore Norway Palaeozoic to Recent. Norwegian Petroleum Society (NPF), Special Publication 10. Elsevier, Amsterdam, pp. 211-232 (this volume). Steel, R., 1993. Triassic-Jurassic megasequence stratigraphy in the Northern North Sea: rift to post-rift evolution. In: J.R. Parker (Editor), Petroleum Geology of Northwest Europe: Proceedings of the 4 th Conference. Geological Society, London, pp. 299-315. Steel, R., Felt, V.L., Johannessen, E. and Mathieu, C. (Editors), 1995. Sequence Stratigraphy on the Northwest European Margin. Norwegian Petroleum Society (NPF), Special Publication 5. Elsevier, Amsterdam, 608 pp. Steel, R. and Ryseth, A., 1990. The Triassic-Early Jurassic succession in the Northern North Sea: megasequence stratigraphy and intra-Triassic tectonics. In: R.F.R Hardman and J. Brooks (Editors), Tectonic Events Responsible for Britain's Oil and Gas Reserves. Geol. Soc. London, Spec. Publ., 55: 139-168. Surlyk, F. and Noe-Nygaard, N., 2001. Cretaceous faulting and associated coarse-grained marine gravity flow sedimentation, Traill O, East Greenland. In: O.J. Martinsen and T. Dreyer (Editors), Sedimentary Environments Offshore Norway - - Palaeozoic to Recent. Norwegian Petroleum Society (NPF), Special Publication 10. Elsevier, Amsterdam, pp. 293-319 (this volume). Svela, K.E., 2001. Sedimentary facies in the fluvial-dominated ,~re Formation as seen in the .~re 1 member in the Heidrun Field. In: O.J. Martinsen and T. Dreyer (Editors), Sedimentary Environments Offshore Norway Palaeozoic to Recent. Norwegian Petroleum Society (NPF), Special Publication 10. Elsevier, Amsterdam, pp. 87-102 (this volume). Vorren, T.O. and Laberg, J.S., 2001. Late Quaternary sedimentary processes and environment on the Norwegian-Greenland Sea continental margins. In: O.J. Martinsen and T. Dreyer (Editors), Sedimentary Environments Offshore Norway Palaeozoic to Recent. Norwegian Petroleum Society (NPF), Special Publication 10. Elsevier, Amsterdam, pp. 451-456 (this volume). Vorren, T., Bergsager, E., Dahl-Stamnes, O.A., Holter, E., Johansen, B., Lie, E. and Lund, T.B. (Editors), 1993. Arctic Geology and Petroleum Potential. Norwegian Petroleum Society (NPF), Special Publication 2. Elsevier, Amsterdam. 751 pp. Yoshida, S., Jackson, M.D., Johnson, H.D., Muggeridge, A.H. and Martinius, A.W., 2001. Outcrop studies of tidal sandstones for reservoir characterization (Lower Cretaceous Vectis Formation, Isle of Wight, southern England). In: O.J. Martinsen and T. Dreyer (Editors), Sedimentary Environments Offshore Norway Palaeozoic to Recent. Norwegian Petroleum Society (NPF), Special Publication 10. Elsevier, Amsterdam, pp. 233-257 (this volume).

Norsk Hydro Research Centre, P.O. Box 7190, N-5020 Bergen, Norway Norsk Hydro Research Centre, P.O. Box 7190, N-5020 Bergen, Norway


Sedimentary environments offshore Norway--- an overview Harald Brekke, Hans Ivar Sjulstad, Christian Magnus and Robert W. Williams

The evolution of sedimentary environments in the Norwegian continental margin since the Early Carboniferous is directly linked with the evolution of the tectonic framework of the broader region of the northern North Atlantic. After the end of the continent-building Variscan and Uralian Orogenies in the Permo-Carboniferous, the tectono-magmatic history is that of a 200 million year period of general extension and rifting of the continent, ending with a final continental rupture and opening of the northern North Atlantic by seafloor spreading in Early Eocene times. The sedimentary environment of the Norwegian continental margin therefore is a record of an evolution from waning orogenic forelands and incipient rifts in an equatorial climate, through a stage of dominantly continental rifting while drifting from equatorial to temperate climates, to the present stage of a passive subsiding continental margin in a temperate to arctic climate. In earliest Carboniferous times the general lateral distribution of sedimentary environments reflects the existence of a northern ocean (the Boreal Ocean) and a southern ocean (the Proto-Tethys) separated by an area of emergent land with a central zone of continental rifting. In the south, the Carboniferous was generally a period of marginal marine, fluvial deltaic and alluvial deposits progressively filling up the central and southern North Sea. Purely continental alluvial, fluvial and lacustrine environments prevailed in the Norwegian Sea and East Greenland. In the Barents Sea, alluvial and fluvial deltaic environments were transgressed at an early stage by marine carbonates and evaporites. The Early Permian period of the North Sea, East Greenland and the Norwegian Sea was a time of continental environments including an early episode of widespread magmatism in the south. In the Barents Sea marine carbonate and evaporite environments prevailed. Middle Permian time was characterised by uplifts and large erosional breaks. Although most prominent in the southern and central areas, such an erosional hiatus is also recorded across much of the Barents Sea. At the end of the Permian period the sea transgressed the low-lying parts of the entire region - - recorded by coarse clastics and evaporites in the south and central area and fine-grained clastics in the Barents Sea. Characteristic of the Triassic period were the numerous marine transgressions and regressions both in the north and south of the region. In the south, the evaporitic environment of the Permian continued but with an increased input of clastics. The northern North Sea, the Norwegian Sea and East Greenland were characterised by marine deposits in the Lower Triassic, followed by continental fluvial and alluvial systems interbedded with marine incursion cycles. A main feature of Triassic times was the shallowing of the Barents Sea by input of large volumes of clastic sediments. A relative sea-level rise, that started in latest Triassic times, caused the Lower and Middle Jurassic of the whole region to become uniformly dominated by shallow marine clastic shelf environments and approximately simultaneous delta oscillations. Early to Middle Jurassic domes and uplifts on regional and semi-regional scales caused a complex pattern of hinterlands, depo-centres and seaways. In the latest Middle Jurassic and through Late Jurassic times, a major sea-level rise considerably deepened the northern and southern seas and finally drowned the central area (between East Greenland and Norway). This caused the widespread accumulation of marine shale with intervals of very rich source rock. Following a period of marked oscillations of the sea level prior to the Aptian, the sea level continued to rise through the Cretaceous period and reached its peak in Late Cretaceous times. Lower Cretaceous deep-water shales and marls accumulated in the basins and rifts of the southern and central parts of the region, while shallow marine and coastal plain deposits dominate on the flanking platforms and in the vast platform of the Barents Sea. The facies pattern of the Lower Cretaceous continues unchanged into the Upper Cretaceous in the central province, while the high sea level gave rise to pelagic limestones in the southern. The central Barents Sea was transgressed with the development of a condensed Upper Cretaceous marine sequence of clastics and carbonate. The volcanism, tectonism and regional uplift preceding the earliest Tertiary continental break-up and subsequent seafloor spreading between Greenland and Norway, effectively ended the carbonate environments in the south, and the whole region became dominated by marine clastic deposits. In the Neogene the stratigraphy is a record of oscillating glaciations. The glaciations and regional uplifts caused deep erosion of the surrounding mainland areas and the Barents Sea shelf in the latest Neogene and the progradation of a huge sediment apron onto the margins of the Norwegian-Greenland Sea.

Introduction

The Norwegian continental shelf is an integral part of the North Atlantic continental margin, and extends

from the central North Sea and well into the Arctic Ocean north of Svalbard. Prior to the opening and seafloor spreading of the Norwegian-Greenland Sea, the present surrounding continental margins were


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joined in one continent, and their sedimentary environments and tectonic history were closely related. The authors have chosen the approach to discuss the sedimentary environments and associated tectonic evolution in a palaeogeographic framework. In this respect, the present study falls in the family of regional compilations like the ones of Ziegler (1987, 1988, 1990), Dor6 (1991, 1992), Dor6 et al. (1999) and Roberts et al. (1999). The scope of the present work is by no means an attempt to make a full revision of the work of Ziegler and others. The aim is to update the areas that are of most relevance to the Norwegian continental margin, i.e. the North Sea, the Norwegian-Greenland Sea, and the western and northwestern Barents Sea, by including some new data and own ideas. Some extra attention is paid to the Carboniferous stratigraphy and tectonic framework, and to the possible configuration of sedimentary basins and emergent land areas at all times in the area of the present Norwegian-Greenland Sea. In the major parts of the area, the Carboniferous is the transition period between convergent plate tectonics and the subsequent long history of intraplate extension and rifting tectonics. Due to overprinting by later events, the complex Carboniferous tectonic framework is subject to much interpretation and assumptions. Some extra details and new information are therefore provided to illustrate the diversity of sedimentary environments of that period. The tectonic framework of the Carboniferous is assumed to have influenced the subsequent tectonic development, including the hinterland/basin configurations through time. Tectonic and stratigraphic framework During the time from the Early Devonian to the Eocene, the region under study developed from a phase of plate conversion and continent growth, through a period of rifting, until subsequent continental rupture (e.g. Anderton et al., 1979; Ziegler, 1988; Glennie and Underhill, 1998; Dor6 et al., 1999). Subsequent to the onset of seafloor spreading, the continental margin has been subject to compression. Through earliest Carboniferous to Late Permian times the region of the present North Atlantic and Barents Sea was a part of the Pangean supercontinent characterised by orogenic accretion around its fringes associated with the Inuitian, Variscan and Uralian Orogenies (Fig. 1). Still, the interior of the region was subject to rifting from the present southern North Sea, through the Norwegian Sea into the central and western Barents Sea in latest Devonian to Middle Carboniferous times. By the Early Triassic onwards, the orogenic events had ended and the supercontinent

H. Brekke et al.

was progressively broken up by successive rifting events along very much the same general trend as in Early Carboniferous time. A compilation of the main tectonic events during the same period shows that, on a broad scale, Carboniferous and Permian times represent a period of very active tectonics all over the area (Fig. 1). The Middle Triassic to late-Early Jurassic seems to have been mainly a period of thermal relaxation. Then intermittent tectonic activity is again seen through Middle and Late Jurassic, Cretaceous and Tertiary times. The timing of events is remarkably coincident across the region, except that the central and easternmost Barents Sea areas decoupled from the tectonic systems to the south and west in Late Cretaceous times (Fig. 1). During Carboniferous to Jurassic times the general lateral distribution of sedimentary environments reflects the existence of a northern ocean (the Boreal Ocean) and a southern ocean (the Proto-Tethys and Tethys) separated by an area with a central zone (the present Norwegian-Greenland Sea) of continental rifting and intermittent shallow seaways. The continuing long history of persistent extension and rifting turned the central zone into a permanent seaway between the northern and southern oceans. A stratigraphic compilation for the whole of the Norwegian continental shelf and adjacent areas from the end of Devonian to Pleistocene times reflects this picture (Fig. 2). At the same time, the northward continental drift caused the climate to change from equatorial through temperate to partly arctic climates. This tectonic and climatic development is reflected by a change in the marine deposits from Palaeozoic carbonates and evaporitic deposits to Mesozoic and Tertiary clastic shelf and basin deposits (Fig. 2). Plate tectonic reconstruction To illustrate the geological development of the region, a set of palaeogeographic maps have been constructed. The palaeogeography is plotted onto appropriate base maps of lithospheric plate tectonic reconstructed configurations. A number of simplifications were introduced in order to arrive at these plate tectonic reconstructions. Firstly, the basic assumption behind the plate tectonic reconstructions is that it is possible to view the tectonic development in terms of three main rifting episodes: the first comprising all rifting from Early Carboniferous to Middle Triassic times, the second comprising all rifting from Middle Jurassic to Late Cretaceous times, and the third the rifting in latest Cretaceous/early Tertiary times (see Fig. 1). Considering the scope of the study, the scale of the maps

Sedimentary environments offshore N o r w a y - - an overview

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Fig. 1. Summary compilation of the tectonic and magmatic evolution of the northern North Atlantic and surrounding mainlands. Blue patches indicate the timing and duration of tectonic events, red 'v's indicate timing of significant magmatic events. Arrows indicate sense of tectonic movements: diverging arrows, extension; converging arrows, compression; lateral arrows, strike-slip movements.

and the lack of detailed knowledge of the rifts of Carboniferous, Permian, Triassic and early Tertiary times, it is deemed sufficient to construct only four different base maps of plate tectonic configurations: at 300 Ma, at 150 Ma, at 70 Ma, and at 53 Ma. These maps illustrate the plate configurations prior to rifting in Carboniferous to Triassic times, prior to rifting in Late Jurassic/Early Cretaceous times, prior to rifting in early Tertiary times, and at the continental break-up in the Early Eocene, respectively. Secondly, it is assumed that the general extension direction between Greenland/North America and northern Europe/Baltic Shield through the whole time span was mainly parallel to the old N W - S E structural grain of the basement (as discussed in Gabrielsen et al., 1999). This is the trend of present prominent lineaments like the Jan Mayen Lineament

and Senja Fracture Zone, documented by several workers to be a controlling factor at least from Late Jurassic times (e.g. Brekke and Riis, 1987; Torske and Prestvik, 1991; Blystad et al., 1995; Dor6 and Lundin, 1996; Dor6 et al., 1999; Brekke, 2000). Here it is assumed that this was the main direction of extension between the regions of present Greenland and Europe also in pre-Jurassic times. The same is assumed for the late Palaeozoic rifting in the Barents Sea shelf (Gudlaugsson et al., 1998). In the rifts of the North Sea, however, detailed models indicate that the direction of rifting changed from NE-SW-directed escape tectonics in Permo-Carboniferous times (Coward, 1993; Corfield et al., 1996; Besly, 1998) to an E - W and/or N W - S E direction in Jurassic-Early Cretaceous times (Faerseth, 1996; Dot6 et al., 1999; Errat et al., 1999). This may imply that the Shetland-

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Sedimentary environments offshore Norway

an overview

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British Isles region at times may have moved as an independent block between Europe/Scandinavia and Greenland. Determining the amount of extension for each of the three major rifting episodes involved consideration of a great variety of published models and estimates (e.g. Lippard and Liu, 1992; Skogseid, 1994; Roberts et al., 1995; F~erseth, 1996; Gudlaugsson et al., 1998; Dor6 et al., 1999; Errat et al., 1999; Gabrielsen et al., 1999; Odinsen et al., 2000; Christiansson et al., 2000; Reemst and Cloetingh, 2000). In view of the literature, it was deemed more fruitful to focus on estimating a reasonable order of total extension and to try to decide the relative importance of the three rifting episodes rather than attempting to arrive at exact details. Based on a subjective set of Beta-factors from the literature and distances over which they were to be applied, the authors propose a semi-quantitative model for the total rifting (Fig. 3). The model concentrates on the continental margins of the Norwegian-Greenland Sea, since this occupies the crucial area for the rifting and separation between Scandinavia and Greenland. The model assumes that the total extension of the Carboniferous-Triassic episode was greater than the following Middle Jurassic/earliest Cretaceous episode, as the crustal thinning was probably greater on average and was uniformly distributed over a larger area (e.g. Odinsen et al., 2000; Gabrielsen et al., 1999). The latest Cretaceous/Paleocene rifting episode is suggested to have contributed least to the total crustal extension between Norway and Greenland. This is true if extension estimates are based only on the observed Tertiary subsidence derived from sediment thickness in the outer parts of the continental margin (i.e. western parts of the Voring and MOre Basins) (e.g. Skogseid et al., 1992; Skogseid, 1994). However, large Beta-factors are estimated for this episode assuming underplated magmatic bodies of significant thickness emplaced at the base of the crust at that time (e.g. Skogseid et al., 1992, 2000; Skogseid, 1994). Velocity analysis of the crust of the outer continental margin (beneath the MOre and VOring Marginal Highs) support the existence of high-velocity bodies at the base of the crust that fit with such an underplating mechanism (e.g. Olafsson et al., 1992; Mjelde et al., 2001). However, these deep-seated bodies are not dated, and if they have originated at a different time and/or by a different process, we are left with only the observed subsidence as the key to

extension estimates. The crystalline continental crust underneath the eastern parts of the V0ring Marginal High is almost as thick as that underneath the Tr0ndelag Platform close to mainland Norway (Mjelde et al., 2001). This is not in concert with a wide zone of a high degree of crustal attenuation underneath the V0ring Marginal High, which would have to be the site of the main latest Cretaceous/Paleocene rift. The data of Mjelde et al. (2001) indicate an abrupt western termination of the continental crust, implying a very narrow zone for the actual rifting and final continental break-up. If so, even a locally very high Beta-factor for the latest Cretaceous/Paleocene rifting would not add up to a total extension comparable to that of the two preceding rifting episodes. On the assumption that the general extension direction was N W - S E and that the total extension in each rifting episode did not vary greatly from north to south (no relative rotations), the model gave estimates for the amount of extension in the NorwegianGreenland Sea between Scandinavia and Greenland as follows: For the latest Cretaceous/Paleocene rifting: 45km For the Middle Jurassic/earliest Cretaceous rifting: 80 km For the Carboniferous-Triassic rifting: 135 km For the sake of simplification, the same total distances of extension for each rifting episode are used in the plate tectonic reconstructions for the whole study area south of the Barents Sea (Fig. 3). This implies that the estimated extensions in the North Sea are compensated by less extension west of the British Isles (Fig. 3). In the Barents Sea, the post-Triassic extension is considered as insignificant relative to the Carboniferous-Triassic extension (Fig. 3). Thirdly, the plate tectonic reconstructions were made by progressively subtracting the amount of extension of the three main episodes of extension from the Ypresian plate reconstruction of Srivastava and Tapscott (1986). Because the exact locations of the rifts of Carboniferous-Triassic and latest Cretaceousearly Tertiary times are not known, this exercise was done in a rather schematic manner (see Fig. 3). This allowed for the moving of the coastlines and the known rifts and other tectonic structures into their relative past positions. In this picture, the exact locations of the different Palaeozoic and Cenozoic grabens and rifts were not regarded as critical. The plate reconstruction presented here is an attempt to reflect the

Fig. 2. Summary compilation of the lithostratigraphic evolution of the northern North Atlantic and surrounding mainlands. The compilation is built from a selection of four central stratigraphic columns for each of the three sub-regions, the North Sea, the Norwegian-Greenland Sea, and the Barents Sea. The colour legend is given in Fig. 5b.

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Fig. 3. Method for progressive removal of the effect of stretching during major post-Devonian rifting episodes. The base map is the 24 anomaly time (53 Ma) plate reconstruction of Srivastava and Tapscott (1986). Coloured corridors indicate to scale the amount of lateral extension resulting from each of the three major rifting episodes as estimated in the continental margin of the Norwegian Sea (see inset legend). The plate-reconstructed base maps of 70 Ma, 150 Ma and 300 Ma for plotting of palaeogeography was derived by closing the light orange, the blue, and the brown corridors, respectively, and in that sequence. The red discontinuous lines indicate major tectonic transfer zones parallel to the direction of extension. See text for comments.

overall picture (Fig. 3), indicating that at the beginning of Carboniferous times the distance between Norway and Greenland was in the order of 200-300 km shorter than at the beginning of Eocene times. It should be noted that the position of Ellesmere Island relative to Greenland and Svalbard at all times is based on the plate tectonic reconstructions of Rowley and Lottes (1988). Furthermore, the palaeogeography includes the notion of "Crokerland" which was a substantial land area north of Ellesmere Island, that served as an important sediment source from the Middle Carboniferous to the end of the Middle Jurassic (Embry, 1993) (see Fig. 4). A possible model for the tectonic framework of the region at the beginning of Carboniferous times has been set up based on the general plate tectonic reconstruction (Fig. 4). At the beginning of Carboniferous times the Caledonian orogenic compression had ended, the Ellesmerian Orogeny in the north was in its waning stages (Embry, 1993), and the Variscan orogenic cycle was at work in the south. The central area between the Variscan and Ellesmerian orogenic fronts was evidently subject to a complicated system of extension of unknown details throughout Carbonif-

H. Brekke et al.

Fig. 4. Tectonic framework of Early Carboniferous times, plotted on 300 Ma plate reconstruction. See text for comments. JL -- Jameson Land; JML -- Jan Mayen Lineament; SBL = Scoresby-Bergen lineament; SL -- Scoresby Sound; OFC = •ygarden Fault Complex.

erous times. It seems likely that Spitsbergen was situated in a strike-slip regime between the Ellesmerian front and the central rift system (Fig. 4). This is supported by the Carboniferous sedimentary facies of Svalbard (Steel and Worsley, 1984). Based on field relationships onshore East Greenland (e.g. Surlyk, 1990; Stemmerik et al., 1993; Escher and Pulvertaft, 1995) and seismic mapping on the Norwegian continental margin (e.g. Blystad et al., 1995; Gudlaugsson et al., 1998) it seems likely that the Carboniferous central rift system between Greenland and Norway was dominated by N-S- to NE-SW-trending normal faults and NW-SE-trending transfer faults including the prominent lineaments like the Jan Mayen Lineament (see discussion in Gabrielsen et al., 1999). In the overall structural picture, there are several large-scale features that have led the present authors to speculate on the existence of yet another prominent N W - S E lineament, here informally termed the Scoresby-Bergen lineament. This runs to the south of, and parallel to, the Jan Mayen Lineament, between the B lossville Coast area on East Greenland and the Bergen area in western Norway (Fig. 4). Onshore East Greenland, in the area between Scoresby Sound and Kangerlussuaq, this lineament fits with the


an overview

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southern termination of the late Palaeozoic to Middle Jurassic/earliest Cretaceous rift graben system (see Surlyk, 1978). Offshore, the lineament fits with a fracture zone running right up to the Blossville Coast (see Larsen, 1984). Like other oceanic fracture zones in the Norwegian Sea, this fracture zone may extend as a tectonic lineament into the adjacent continental crust. In the B lossville Coast area, this would be masked by the extensive cover of the onshore Tertiary lava plateau. On the Carboniferous plate tectonic reconstruction, the eastward extension of the lineament falls along a lineament of N W - S E elongated magnetic anomalies running from the oceanic/continental crust transition north of the Erlend Platform, through approximately 62~ 2~ to the west coast of Norway north of Bergen at approximately 61 ~ 4~ (the "Marflo Lineament" of Smethurst, 2000). There, it links directly with the northern end of the Oygarden Fault Complex, which shows up as very prominent arcuate magnetic structure (Olesen et al., 1997). The lineament clearly truncates the pronounced N E - S W magnetic trend of the MOre Basin and the MOreTrOndelag Fault Complex indicating a crustal involvement. It is clear on the magnetic data that the southeastern end of the Scoresby-Bergen lineament (if it exists) then connects with the northeastern end of the Midland Valley-Ling Depression rift through the arcuate Oygarden Fault Complex (see Fig. 4). Parallel to the arcuate magnetic expression of the Oygarden Fault Complex is the onshore, arcuate fault system of the Bergen-Sunnhordland Arcs that show evidence of reactivations in the Devonian, PermoCarboniferous, Early Triassic and Late Jurassic/Early Cretaceous (Fossen, 1998; Wennberg et al., 1998) (Fig. 4). Similar dates are documented on other major basement faults along the southwest coast and well into mainland Norway (e.g. Andersen et al., 1999). It is possible that the Scoresby-Bergen lineament constitutes the southern limit of the whole of the Permo-Carboniferous central rift system between East Greenland and Norway, and hence acted as a transfer zone between the Greenland rift system and extensional fault system in the North Sea in front of the Variscan Orogen south of the Midland ValleyLing Depression rift line (Fig. 4). Any difference in extension between the North Sea and the Norwegian Sea areas would then very likely be compensated for in the area adjacent to the Oygarden Fault Complex (i.e. in the Stord Basin). A rapid Early Carboniferous cooling event related to erosional unroofing of the mainland basement is documented along the adjacent coast, substantiating that the area was tectonically active at the time (Eide et al., 1999). This also implies that it is likely that Lower Carboniferous sediments are present at depth offshore western Norway.

The overall extension estimate takes into consideration the idea of a central area of stable unrifted basement blocks between Norway and East Greenland, so that pre-Tertiary extension occurred in rift zones on both sides (e.g. Fig. 5a). This is a way to achieve comparable amounts of extension to the north and south of the Scoresby-Bergen lineament in Carboniferous to Triassic times (Fig. 3). The sedimentary record in the Jurassic and Cretaceous of the East Greenland and Norwegian Sea continental margins strongly indicates that such a stable, shallow to emergent central area persisted until the final continental break-up in earliest Tertiary (discussed later). The Carboniferous

The Visean map may be taken as representative for the Early Carboniferous palaeogeography (Fig. 5). Evidence from Greenland, Svalbard and BjornOya indicates that, in this period, the whole area north of Scoresby Sound drained northwards (Steel and Worsley, 1984; Stemmerik et al., 1993), while there is strong evidence from Britain and the North Sea of a substantial source hinterland to the northwest of the Midland Valley-Ling Depression rift line. (Anderton et al., 1979; Bristow, 1988; Cliff et al., 1991) (Figs. 4 and 5a). This configuration indicates a regional watershed situated in the area between the Scoresby-Bergen lineament and the Midland ValleyLing Depression rift line.

The Early Carboniferous In the central parts of the eastern Barents Sea, the Early Carboniferous was a marine carbonate environment which was established already in the Devonian, while the Pechora area in the southeast comprised the transition from emergent land through fluvial, delta plain, shallow marine clastic into carbonate facies (Ziegler, 1987, 1988; Johansen et al., 1993). The Nordkapp Basin and Bjarmeland Platform area further west (see Fig. 6 for location) was a complicated system of emergent land, alluvial, fluvial and deltaic environments (Gudlaugsson et al., 1998) at the end of the northward draining system in the main N E - S W rift system (Fig. 5a). The important Carboniferous outcrops of Svalbard and BjOrnOya found themselves in a strikeslip transfer setting relative to the main N E - S W rift system, with the most important fault lines being the Palaeo-Hornsund, Hornsund, B illefjorden and Lomfjorden-Aghardbukta Faults (Steel and Worsley, 1984; NOttvedt et al., 1993a). Typical of the Tournaisian and Visean was subsidence along narrow, isolate zones accommodating large alluvial fans building

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Fig. 5. (a) The palaeogeography of Early Visean times plotted on the 300 Ma plate reconstruction. Some key references for compilation: Ziegler, 1988; Embry, 1993; Stemmerik et al., 1993; Corfield et al., 1996; Besly, 1998; Glennie and Underhill, 1998 supplemented with in-house studies. See (b) for legend. SUB = Southern Uplands block; WBB = Wales-Brabant Block. (b) Colour legend for all palaeogeographic maps and lithostratigraphic compilation in Fig. 2. Palaeolatitudes for all maps based on Scotese, 1997.

from graben edges into swamps, lakes and minor flood plains on Svalbard, and rivers and floodplains on Bj~rn~ya.

In the Namurian there was a change to a larger and more continuous sediment system dominated by large braided fans of quartz arenites, now building


Fig. 6. Main structural elements of the Norwegian continental margin.

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from the west onto flood basins and floodplains in the southeast (Steel and Worsley, 1984). This change in sediment pattern was probably controlled by uplift west of the Palaeo-Hornsund Fault. At the end of Namurian (early Bashkirian) times the braided fans retreated and were replaced by humid coal-bearing marsh and floodplain environments, probably due to a decreased subsidence rate (see fan retreat on Fig. 2). The Carboniferous sedimentary facies of the central Barents Sea was probably similar to those developed in the Early Carboniferous grabens and basins of BjornOya and Svalbard, but not subject to the same frequent strike-slip inversions (Gudlaugsson et al., 1998). In Greenland the Lower Carboniferous is entirely continental (Fig. 2). In the north it consists of more than 70 cycles of sandstones and shales with minor coalbeds, interpreted to represent a meandering river system (Hfikansson and Stemmerik, 1984). Palaeocurrents indicate transport from the southwest towards the northeast supporting the drainage model (Fig. 5a). In central East Greenland, the Tournaisian and Visean show a development from red fluvial sandstones and siltstones to later yellow fluvial sandstones, grey siltstones and thin coal seams. This is believed to be part of the northward draining alluvial/fluvial system with the provenance area and associated coalescing alluvial fans and braid plains in the west with a major flood plain with northward draining rivers developed laterally to the fans and braid plains (Stemmerik et al., 1993). It is possible that these outcrops represent the western half of a symmetric rift-drainage system, the eastern half being hidden beneath younger strata on the continental shelves of East Greenland and Norway. In the North Sea area, most of the information comes from onshore outcrops in the British Isles and continental Europe, though oil industry offshore well data are now gaining volume. Lower Dinantian (Tournaisian) red beds are widely distributed and indistinguishable from the underlying Devonian Old Red. During Dinantian times the Old Red Continent was broken up by crustal extension (e.g. Leeder, 1988; Coward, 1993). The area was transgressed progressively from the south through Tournaisian times giving a diachronous sequence of shallow marine limestones, clastics and localised evaporites (Anderton et al., 1979). By Early Visean times regionally extensive carbonate facies became established in the southern areas. The crustal extension gave rise to topographic differentiation, resulting in horsts and grabens that directly influenced the distribution of facies: carbonate platforms on the highs and clastic turbidites and shales in the grabens (Grayson and Oldham, 1987). Some of the large highs were emergent land areas (WalesBrabant, Southern Uplands Block) (Fig. 5).

H. Brekke et al.

In the Visean, clastic, mainly deltaic sediments are dominant in northern Britain and the central and northern North Sea (north of 54~ (Besly, 1998) (Fig. 5a). In the Midland Valley, lacustrine oil shales developed (Anderton et al., 1979). The main sediment transport was from the north and west, and coarse clastics spread gradually southwards. However, this infilling of clastics was contested by a gradually rising sea level that is recorded by a gradual increase in coal abundance and the onset of Yoredale cycles at the end of the Visean. These cycles consist of prograding delta clastics capped by abandonment phase shallow marine limestones (Ramsbottom, 1979; Anderton et al., 1979). The Middle Carboniferous

In Namurian times the tectonic activity in the North Sea ceased and was replaced by regional subsidence, drowning even carbonate platforms on relict highs (Collinson, 1988). At the same time, clastic input from the north increased dramatically, perhaps due to a change to more humid conditions (Van der Zwaan et al., 1985). This ended the carbonate deposition and initiated a sequence of major delta advances (Millstone Grit) (Figs. 2 and 7). Deltas in Scotland and northern England experienced a greater marine influence and Yoredale facies continued into the Namurian there; the route of the marine transgression is not known (Besly, 1998). As the carbonate deposition ended in the North Sea, it started its major advance in the Barents Sea (N0ttvedt et al., 1993a; Johansen et al., 1993) (Fig. 2). From its more restricted occurrence in the easternmost Barents Sea in the Early Carboniferous, marine carbonate platform facies had spread all over the Barents Sea by Bashkirian-Moscovian times (Fig. 7). The Bashkirian of Svalbard was a return to the deposition in narrow zones of subsidence along the major fault lines, like in the Tournaisian_Visean. En echelon arrangements of these troughs and local inversions indicate strike-slip movements also in Middle Carboniferous times. This may be explained as a consequence of the geometric position of Svalbard/Bj0rnOya relative to the main NE-SW trend of the Carboniferous rifting in the central Barents Sea (Fig. 7). Otherwise, Bashkirian times saw a gradual change to semiarid terrestrial climate and a general rise in sea level. In the basinal areas, the Gipsdalen Group is marked by the incoming of red, alluvial fan deposits and their laterally equivalent fan-delta deposits, evaporites and carbonates. Three separate major depositional systems are described in the Bashkirian (Steel and Worsley, 1984; NOttvedt et al., 1993b).

Sedimentary environments offshore N o r w a y - an overview

(1) In central and east Spitsbergen (Ebbadalen Formation in B illefjorden) alluvial fans and fan-deltas are building out eastwards from the B illefjorden Fault into restricted marine and sabkha environments (dolomites, limestones, evaporites). These facies display rhythmic intercalations organised in upwardscoarsening cycles, believed by Steel and Worsley (1984) to be the result of an intricate interplay of local fault activity and regional transgression. (2) The St. Jonsfjord Trough is characterised by alluvial fans from both east and west, passing into floodplain, shoreline, and open marine environment northwards. (3) The Hornsund Basin saw alluvial fans building out eastwards. The ongoing transgression produced fan-delta systems and interbedded carbonates and repeatedly submerged coastal plain environments. During the Moscovian all of these systems and adjacent basement were transgressed and overlain by shallow marine carbonate platform deposits. Contemporaneous evaporite development in basihal troughs caused the later pronounced salt tectonics (e.g. in the TromsO and Nordkapp Basins) (Stemmerik and Worsley, 1989; Gudlaugsson et al., 1998). The salt facies of the deep basins were probably fringed by carbonate-evaporite facies on the basin flanks (Fig. 7). In northern Greenland, the Middle-Late Carboniferous consists of three fining-upwards mega-cycles of sandstone-carbonate rhythmics, probably resulting from tectonic activity overprinting the regional transgression (Hftkansson and Stemmerik, 1984). Towards the top of each mega-cycle the sandstones become rare being replaced by thick-bedded carbonates with minor shales and evaporites. The sandstonecarbonate rhythmics are capped by a sequence of thick shallow to deep-water marine shelf carbonates. This may be correlated with the transition from clastics to carbonates on Svalbard and Bj~m~ya (Ebbadalen to Nordskioldbreen Formations and Kapp Hanna to Kapp Duner Formations). Further south in central East Greenland, the northward draining alluvial/fluvial system of the Early Carboniferous persisted (Stemmerik et al., 1993). A typical feature of Middle-Late Carboniferous times are the upper Bashkirian lacustrine black shale facies patchily located along the western tectonic boundary of the Carboniferous alluvial fan/braid plain system (i.e. proximal to the clastic source areas) (Stemmerik et al., 1991). In the absence of other evidence, one may expect similar environments in the Mid-Upper Carboniferous of the Norwegian Sea margin.

17

The Late Carboniferous

During Middle-Late Carboniferous times the North Sea was filled in by clastics (Anderton et al., 1979; Corfield et al., 1996; Besly, 1998) (Figs. 2 and 7). During early-middle Westphalian (late Bashkirian) times, shallow water deltaic environments prevailed on a major scale. Low-lying, near-emergent conditions existed over the whole of the Variscan foreland, and sediment provenance changed from a northerly source to a westerly. The early-middle Westphalian period saw two cycles of flooding and regression. In the flooding periods elongate deltas were infilling shallow water-bodies on a basin-wide scale. Marine bands are common while regionally correlatable coals are few. In the intermittent regressive periods much of the area was emergent with some freshwater lakes being infilled by local deltas. Marine bands are rare and major regional coal beds are developed. In middle-late Westphalian (Moscovian) times the earlier sedimentary environments were modified by the northward advance of the Variscan deformation front. This caused uplift and folding resulting in unconformities and widespread deposition of red beds in the North Sea. The red beds were spreading both from the north and as pulses of molasse from the south (Corfield et al., 1996; Dahlgren and Corfu, 2001). Above continental red beds in the extreme east, in the Oslo Graben, a Moscovian marine limestone is identified in a braid delta sequence prograding into a shallow marine basin (Olaussen et al., 1994) (Fig. 2). But the marine fossils can be correlated with the Russian marine stages and indicate that the regional Bashkirian-Moscovian sea-level rise caused a transgression from the north, opening a seaway across the Baltic Shield from the eastern Barents Sea to the Oslo area at the peak of the Moscovian transgression (Fig. 7). The plant fossils and freshwater fauna of the unconformably overlying red-bedded fluvial delta/lacustrine sequence indicate that by Stephanian times the Oslo area was again comparable to the West European/North American realm. The Carboniferous of the northern North Sea

The Carboniferous of the Norwegian sector of the North Sea is practically unknown. Nearby, however, Visean to Namurian fluvial red beds, alluvial/deltaic coal-bearing sequences and lacustrine facies are recorded in the Outer Moray Firth, and there are indications of sediment transport from the Viking Graben. Westphalian red beds are reported from the South Viking Graben in well UK9/13a-22 (Cameron, 1993).

18

H. Brekke et al.

Fig. 9. Well correlation of pre-Triassic deposits in the Utsira High. See Figs. 8, 10 and 11 for well locations. Fig. 7. The palaeogeography of Bashkirian-Moscovian times plotted on the 150 Ma plate reconstruction. Some key references for compilation: Hfikansson and Stemmerik (1984); Steel and Worsley (1984); Ziegler (1988); Stemmerik et al. (1991, 1993); Embry (1993); Olaussen et al. (1994); Corfield et al. (1996); Besly (1998); supplemented by in-house studies. See Fig. 5b for legend.

Regional seismic data in eastern parts of the northern North Sea, supplemented by some well data, however, strongly indicate the existence of preLate Permian strata of possible Carboniferous age across the Stord Basin and Utsira High (Fig. 8). Data on the Utsira High show a clear pre-Late Permian half-graben development. The lower parts of the wells 25 / 10-2 and 25/10-4 penetrated pre-Zechstein (Upper Permian) clastics (Fig. 9). In both wells, the Zechstein Group marginal facies overlie a polymict, fluvial/alluvial conglomerate, interpreted to be an Upper Rotliegende equivalent. This conglom-

erate rests on an angular unconformity cutting a tilted sequence of massive, medium to coarse, well sorted sandstone to the east (25/10-4). To the west, the conglomerate apparently overlies gneissic basement in the well 25/10-2; this is thought to be juxtaposition by faulting. This fits exactly with the well positions in the geometry of the half-graben revealed by the seismic data (Fig. 10). A strong reflector at what is believed to be the top of the sandstone sequence, is tentatively interpreted to be a coal horizon at the base of a shaly interval (Fig. 10). Alternatively, the strong reflector may be a volcanic horizon or a sill. By lithostratigraphic and seismic correlation, it seems very likely that the red conglomerates and sandstone in well 25/12-1 are equivalents to those of 25/10-2 and-4 (Fig. 9). This implies the existence of the eroded remnants of two parallel half-grabens of possible Carboniferous age within the Utsira High basement (Fig. 11).

Fig. 8. Regional geoseismic profile, based on seismic line CNST-82-08, showing possible distribution of pre-Permian strata in the Stord Basin and local grabens in the Utsira High of the northeastern North Sea. See Fig. 6 for line location.

19


Fig. 10. Detailed geoseismic profile, based on seismic line CNST-82-08, of a Palaeozoic half-graben in western Utsira High (see Fig. 8 for location). Note the unconformable relationship between the sandstone (light yellow) and possible shales and coal-beds (green with black stringers) of possible Carboniferous age, and the overlyingRotliegende conglomerate. The juxtaposition of the Upper Permian strata and the basement, and the unconformable overstepping nature of the Triassic, indicate that the boundary fault moved through the Late Permian, but had stopped by Triassic times (Fig. 10). The angular relationship between the Upper Rotliegende conglomerate and the underlying strata shows that the half-graben fill was tilted prior to Late Permian times. Faerseth (1996) proposes a Devonian age for the lower sandstone unit. However, the good porosities and non-metamorphic state of the sandstone does not compare at all with the lowmetamorphic state of the Devonian of the Norwegian mainland nearby. An Early Permian age also seems unlikely as the sandstones are devoid of any traces of volcanic material, which elsewhere is the trade mark of the Lower Rotliegende equivalents. Hence, the present authors prefer a Carboniferous age for these clastics, although it may also be argued that they may be correlated with the non-metamorphosed Devonian sandstones offshore Scotland (e.g. Downie, 1998). The Permian

Permian tectonics were characterised by the final stages of the Variscan Orogeny in the south and the

evolving Uralian Orogeny in the northeast (Fig. 1) (e.g. Ziegler, 1988; Glennie and Underhill, 1998). In Late Permian/Early Triassic times there was a widespread tectonic phase all across the region, possibly reflecting the reconfiguration of plate movements following the final plate coupling in the Uralian. In the studied region, this is mainly expressed as an extension and marks the onset of the break-up of the Pangean supercontinent.

The Early Permian In Asselian times continued transgression established large platforms of mixed dolomite and clastics in the easternmost areas of the northern region (Figs. 2 and 12). In the central parts of the present Barents Sea the shallow marine carbonate environment persisted with evaporite deposition in basin areas, and marginal evaporite facies and carbonate buildups on previous emergent land (Gudlaugsson et al., 1998). Transgression and carbonate platform facies also dominated Spitsbergen and Bj~rmaya, though the BjornOya area experienced repeated inversions in the Asselian (Steel and Worsley, 1984). The Lower Permian carbonate platform facies also

20

H. Brekke et al.

Fig. 12. The palaeogeography of Asselian times plotted on the 150 Ma plate reconstruction. Some key references for compilation: Pegrum (1984); Hfikansson and Stemmerik (1984); Steel and Worsley (1984); Ziegler (1988); Stemmerik et al. (1993); Embry (1993); Olaussen et al. (1994); Glennie (1998); supplemented by in-house studies. See Fig. 5b for legend.

Fig. 11. Map view of Palaeozoic grabens in the Utsira High.

spread across northern Greenland (Hfikansson and Stemmerik, 1984). Further south in East Greenland the Lower Permian is a continuation of the alluvial and fluvial facies of the Carboniferous. The same is assumed for the areas of the present Norwegian shelf in the Norwegian Sea. The last stages of the Variscan Orogeny in Asselian times induced an E - W extension/transtension regime that opened up a graben system of mainly NW-SE trends by the reactivation of inherited lines

of weakness in the southern region south of the Midland Valley-Ling Depression rift line (Figs. 4 and 12) (Glennie and Underhill, 1998) However, prominent graben structures of NE-SW and N-S trends also developed. This E-W extension was accompanied with a volcanic flare-up of rhyolites, ignimbrites and basaltic flows (e.g. see Glennie, 1998). These Lower Permian volcanics and volcaniclastics, mixed with minor fluvial and lacustrine sediments, are mainly recorded onshore (Oslo area, northern Germany and Poland), but an increasing volume of well data show that there may be major areas of volcanic strata also offshore in the present North Sea, perhaps with a wider distribution in the eastern parts of the North Sea than in the southwestern (Glennie, 1998). In northern Germany it is possible to identify a Lower Permian, regional erosional unconformity that may have been caused by the uplift associated with the extension and magmatism, i.e. the Altmarkian Unconformity (Glennie, 1998). The major Saalian Unconformity that developed all across the southern region in middle to Late Permian times indicating a later phase of uplift, cuts through the Altmarkian Unconformity. This middle Permian uplift and erosion is also recorded as a major peneplanation in East Greenland (Surlyk et al., 1984). An apparently coincident regional unconformity is seen even as far as into the present Barents Sea (Johansen et al., 1993) (Fig. 2).


The Late Permian

Late Permian times brought about dramatic changes in sedimentary environments. It was the end of the carbonate deposition in the Barents Sea region and the onset of widespread fluvial deposition and later transgression in the Norwegian Sea region, and establishment of the two well known southern and northern Permian Basins in the North Sea region (Figs. 2 and 13). Late Kazanian/early Tatarian times saw the onset of a regional transgression and the closing of the ocean in the northeast by the Uralian Orogeny. This transgression was accompanied by (or responsible for) a transition from warm and arid to temperate and humid climate all across the northern region. That climatic change effectively ended the carbonate platform/evaporite environment of the northern ocean. The change to marine clastic environments was accompanied by a regional blooming of sponges that gave rise to spiculitic shales, siltstones and cherts (Figs. 2 and 13). In the central region there is evidence of renewed clastic sedimentation all across the Permian peneplain, possibly caused by the change to a more humid climate. Typically, the Upper Permian (upper Kazanian?) starts with a basal fluvial conglomerate with a remarkably widespread distribution (Figs. 2 and 13). Evidence of marine reworking towards the top of the

Fig. 13. The palaeogeography of late Kazanian-early Tatarian times plotted on the 150 Ma plate reconstruction. Some key references for compilation: Hfikansson and Stemmerik (1984)" Steel and Worsley (1984); Ziegler (1988); Surlyk (1990); Stemmerik et al. (1993); Embry (1993); Olaussen et al. (1994); Gudlaugsson et al. (1998); Glennie (1998); Taylor (1998); supplemented by in-house studies. See Fig. 5b for legend.

21

conglomerate is common and testifies to a southward transgression of the northern ocean (Stemmerik et al., 1993). In the south, the Permian Basins of the present North Sea were the sites of very rapid subsidence and the accumulation of the Upper Rotliegende 2 (Glennie, 1998), consisting of fluvial (wadi), aeolian, sabkha, and lacustrine facies. This reflects a desert climate that was in strong contrast to the newly established temperate humid climate in the northern region. The lacustrine muds and salts of the desert lakes in the Southern Permian Basin was flanked by sabkha facies which in turn passed into wadi deposits and aeolian dune sands. In its thickest parts these deposits presently amount to up to 2500 m which were deposited in the course of 4-8 Ma, giving an extreme rate of subsidence. The mechanism for such a rapid subsidence is not known. By late Tatarian times the continued transgression caused flooding of the Permian Basins of the present North Sea (Fig. 14). This flooding of the low-lying basins gave rise to the vast evaporite deposits of the Zechstein Group. The sea probably entered from the north via the early Viking Graben, Stord Basin, and Central Graben areas and/or the Pennines, and may have dramatically completed the flooding of the basins in the order of 6 years (Glennie, 1998). This

Fig. 14. The palaeogeography of late Tatarian times plotted on the 150 Ma plate reconstruction. Some key references for compilation: H~kansson and Stemmerik (1984); Steel and Worsley (1984); Ziegler (1988); Surlyk (1990); Stemmerik et al. (1993); Embry (1993); Olaussen et al. (1994); Gudlaugsson et al. (1998); Glennie (1998); Taylor (1998); supplemented by in-house studies. See Fig. 5b for legend.

22 also compares to the estimated rate of the catastrophic flooding of the Black Sea 7000 years ago through the Strait of Bosporus (Ryan et al., 1997). A catastrophic flooding event is indicated by the way the initial sapropelic black shales of the Kupferschiefer abruptly drapes delicate topographic features like aeolian sand dunes (Glennie, 1998). The first Zechstein cycle of black shale, marine carbonate, and anhydrites, has been correlated with upper parts of the Late Permian Foldvik Creek Group of similar facies in East Greenland (Stemmerik et al., 1993; Taylor, 1998) (Fig. 2). It is assumed that the Foldvik Creek Group was deposited during the southward transgression. Since the present Norwegian-Greenland Sea margins occupied the intermediate zone between the warm arid North Sea and the temperate humid Barents Sea, and with rather open marine conditions, the postulated shallow stable central block would be a good place for the establishment of a Tatarian shallow marine carbonate platform. Carbonate deposits, probably of this age, are recorded in the fringing areas: in wells on the northwest part of the TrCndelag Platform, in outcrop on And~ya (Dalland, 1981), and on East Greenland (Surlyk et al., 1984; Stemmerik et al., 1993). Such an environment would also include a good chance for source rock at that time. In the present Barents Sea region the marine spiculitic clastic facies, initiated in the Kazanian, persisted. The Triassic

Triassic strata are well known in all parts of the region, in some places amounting to 5-6 km thickness, e.g. the Stord Basin in the North Sea and the South Barents Basin in eastern Barents Sea (Johansen et al., !993; Fa~rseth, 1996) (Figs. 6 and 17). But the details of the Triassic tectonic development seem obscure and difficult to assemble into a complete picture (Fisher and Mudge, 1998; Dor6 et al., 1999; Errat et al., 1999). Except for the widespread Late PermianEarly Triassic crustal extension the Triassic apparently remained a period mainly of thermal relaxation (Fig. 1) (Surlyk, 1990; Johansen et al., 1993; Glennie and Underhill, 1998; Dor6 et al., 1999; Roberts et al., 1999). A characteristic feature of the Early Triassic seems to be that most of the extension and sediment volumes were accommodated by a limited number of major rift boundary faults trending north-northeast to north-northwest (Fig. 15) (Surlyk, 1990; Blystad et al., 1995; Fa~rseth, 1996). Major uplift and erosion of the southern Norwegian mainland seems to be linked to the subsequent thermal subsidence stage of the rift basin in Middle Triassic-Early Jurassic times (van der Beek, 1994; Rohrman et al., 1995; Riis, 1996).

H. Brekke et al.

In the present North Sea the Permian evaporite environments more or less continued into Early Triassic times, but with an increase in clastic input. Below the mid-Scythian Hardegsen Unconformity the Lower Triassic demonstrates a transition from marginal marine to alluvial and fluvial environments, by which the Zechstein Group was covered by prograding fluvial sandy deposits. Above the Hardegsen Unconformity, the Triassic environment of the southern North Sea diverges from that of the rest of the North Sea. Above the Hardegsen Unconformity, the present central and northern North Sea is dominated by continental alluvial and fluvial environments (Figs. 2 and 15). The southern North Sea, on the other hand, is characterised by several evaporite cycles through the rest of the Triassic, giving the R6t, Muschelkalk and Keuper evaporites (Figs. 2, 15 and 16). In the present Norwegian Sea and East Greenland, the Triassic is dominated by continental fluvial and alluvial environments, interrupted by short-lived marine incursions from the north. In the present Norwegian shelf area there is evidence of only one marine transgression, seen as a Ladinian-Carnian evaporite sequence (Jacobsen and van Veen, 1984). In East Greenland there are indications of an earlier, Anisian black shale of some source rock potential (Surlyk, 1990). This may then be the southern limit of the time equivalent widespread Barents Sea source rock of the Bottenheia Formation (Fig. 15). In the Barents Sea, the marine clastic environments of the Permian continued, but with a considerable increase in sandy influx which ended the dominance of the spiculitic facies. Large volumes of clastics came in from the east by the peak of the Uralian Orogeny in Novaya Zemlya (Fig. 1). 7-8 km of Permo-Triassic sediments accumulated in the progressively subsiding South Barents Basin at the foot of the orogen (Johansen et al., 1993) (Fig. 17). Clastics were also derived from the other flanks of the basin, all contributing to the filling in and shallowing of the northern ocean. Recent provenance studies show that in Early Triassic times the Uralian Mountains were the provenance area for the eastern Barents Sea, the Baltic Shield margin and the Caledonides for the Hammerfest Basin, while Svalbard had its provenance area in a palaeo-land area to the northwest (M~rk, 1999). In the central Barents Sea there is a mixture of Uralide and Caledonide provenance. In Middle Triassic times provenance areas became more localised. In the Upper Triassic of the Barents Sea there is a distinct increase in sandstone maturity on a regional scale, possibly due to extensive reworking and/or a change to more favourable climatic conditions for kaolinitisation (Bergan and Knarud, 1993; MCrk, 1999). In the central, deeper parts of the Boreal


23

Fig. 15. The palaeogeography of late Anisian-Ladinian times plotted on the 150 Ma plate reconstruction. Some key references for compilation: Jacobsen and van Veen (1984); H~tkansson and Stemmerik (1984); Steel and Worsley (1984); Ziegler (1988); Surlyk (1990); Van Veen et al. (1993); N0ttvedt et al. (1993a,b); Leith et al. (1993); Stemmerik et al. (1993); Embry (1993); Fisher and Mudge (1998); supplemented by in-house studies. See Fig. 5b for legend.

Fig. 16. The palaeogeography of late Carnian times plotted on the 150 Ma plate reconstruction. Some key references for compilation: Jacobsen and van Veen (1984); Hfikansson and Stemmerik (1984); Steel and Worsley (1984); Ziegler (1988); Surlyk (1990); Van Veen et al. (1993); N0ttvedt et al. (1993a,b); Stemmerik et al. (1993); Embry (1993); supplemented by in-house studies. See Fig. 5b for legend.

Sea and the Sverdrup Basin, Anisian times saw the widespread deposition of black shales of very good source potential (e.g NOttvedt et al., 1993a,b; Leith et al., 1993) (Fig. 15). A number of transgressive-regressive cycles have been identified by MOrk (1994) in the Triassic stratigraphy in the Barents Sea. Only four of these are recognised as "simultaneous" (i.e. truly eustatic). These transgressions/regressions had great impact on the distribution of emergent land and facies in the shallowing Boreal Sea. This is illustrated by comparing the palaeogeography of the Barents Sea under the transgressive event in late Anisian-Ladinian times (Fig. 15) with that of the regressive period of late Carnian (Fig. 16). It is obvious that in regression periods like late Carnian times, there must have been a significant river system to transport clastics out to the distant shoreline to the west.

1999). However, a characteristic feature of Middle Jurassic times is the progradation of clastic wedges from regional and semi-regional areas of erosion in all parts of the region. A widely known example of this is the Mid-Cimmerian Unconformity and the associated building of the Middle Jurassic Brent Delta in the central and northern parts of the North Sea (Underhill and Partington, 1993). Underhill and Partington (op cit.) attributes this to the growth, erosion and deflation of a central thermal dome during the period from the Aalenian to the Oxfordian. It was noted that the doming was accompanied by a marked faunal provinciality between the northern Boreal and southern Tethyan Seas from Aalenian to late Bathonian/Callovian times, suggesting that the central North Sea constituted a significant barrier in that period (Callomon, 1979; Enay and Mangold, 1982; Dor6, 1992; Underhill, 1998)(Fig. 18). Dor6 et al. (1999) suggest that the North Sea dome is only one of a family of such uplifts extending across Northwest Europe. The latter authors base this on references to late-Early to Middle Jurassic unconformities west of Shetland and west of Ireland, and the coincident marine faunal separation between the northern and southern oceans. One may add to this "family" similar uplifted land areas of the same period between the sedimentary basins of East Greenland and the Norwegian continental margin as proposed by several au-

The Jurassic

The stratigraphy reflects a relative sea-level rise in the Early to Middle Jurassic that caused the whole study area to become dominated by shallow clastic shelf environments (Fig. 2). This may partly be attributed to a marine flooding of the passively subsiding old Permo-Triassic rift basins as Pangea started breaking up (Dor6 et al., 1999; Roberts et al.,

24

H. Brekke et al.

Fig. 17. Regional geoseismic east-west profile across the Barents Sea shelf. Note the pronounced role of the South Barents Basin as a sediment sink in Triassic times. See Fig. 6 for line location. Modified from Johansen et al. (1993).

of the main stages of the rifting process from Late Jurassic times onwards. This deflation and collapse may well be the mechanism behind the pronounced sag geometry of the deepest basins from this phase of rifting (e.g. the Vcring and Mere Basins). That would fit with the interpretation of Brekke (2000) that the flanks of the VCring and MOre Basins formed mainly by large-scale monoclinal downflexing of the crust rather than down-to-the basin faulting.

The Early and Middle Jurassic

Fig. 18. The palaeogeography of late Bajocian times plotted on the 150 Ma plate reconstruction. Some key references for compilation: Dalland (1981); Hgtkansson and Stemmerik (1984); Ziegler (1988); Surlyk (1990); Bergan and Knarud (1993); Ncttvedt et al. (1993a); Stemmerik et al. (1993); Embry (1993); Johannessen et al. (1995); Underhill (1998); Brekke et al. (1999); supplemented by in-house studies. See Fig. 5b for legend.

thors (e.g. Dalland, 1981; Larsen, 1987; Dor6, 1992; Brekke, 2000). From this review, treated in more local detail in the following, it seems that the axial areas for the subsequent Middle Jurassic to earliest Cretaceous rifting experienced precursory doming and uplifts all the way from the south of Ireland to the borders of the Barents Sea. The deflation and collapse of these domes and uplifts were then an important integral part

In the North Sea, large portions of the Lower Jurassic stratigraphy are represented by an erosional hiatus, probably due to the thermal updoming of the central parts of the area in late-Early Jurassic times (Whiteman et al., 1975; Leeder, 1983; Underhill and Partington, 1993). However, based on the erosional remnants from different parts of the North Sea, it seems that shallow marine environments had established all over the area in earliest Jurassic times following the transgression that had started in the Late Triassic (Fig. 2). Apparently, increased sediment input in late Aalenian to early Bajocian times forced the northward progradation of the Brent Delta (Rannoch, Etive, and Ness Formations) against the regional sea-level rise (Graue et al., 1987; HellandHansen et al., 1992; Johannessen et al., 1995). This probably reflects the increased erosion following the domal uplift of the central North Sea (Underhill and Partington, 1993, 1994), and the uplift and erosion of the adjacent Shetland area (Dor6 et al., 1999) and mainland Norway (van der Beek, 1994). The semicircular subcrop pattern beneath the Mid-Cimmerian Unconformity on the central North Sea dome shows


that this was a dome of semi-regional scale with its central apex in the triple junction of the Viking Graben, Central Trough and Moray Firth Basins (Underhill, 1998). This observation, together with the reports of separated areas of erosional unconformities further west, implies that, within the widespread uplift on the regional scale suggested above (e.g. Dor6 et al., 1999) there were probably a set of several domes of semi-regional scales. The Early/Middle Jurassic dome-shaped uplift of the south Norwegian mainland (van der Beek, 1994) also fits into this pattern. The Early Jurassic coastal plain/delta plain deposits of the Norwegian Sea (the Are Formation), East Greenland (Kap Stewart Formation) and the Barents Sea (the Tubfien Formation) appear to be time equivalents. In all cases the sedimentation seems to involve the progradation of the coastlines by sediment influx onto the shelf. Evidence of influx of sand from the west onto the Halten and DOnna Terraces and the eastern margin of the MOre Basin during the Early to Middle Jurassic (Gjelberg et al., 1987; Jongepier et al., 1996), is taken to imply that the present deep Cretaceous MOre and V0ring Basins were areas of uplift, sub-aerial exposure and deep erosion in that period (Dor6, 1992; Brekke et al., 1999) (Fig. 18). Brekke (2000) argues that the areas of highest extension in the Middle Jurassic/earliest Cretaceous rifting phase (i.e. the MOre and V0ring Basins) were subject to the highest elevation and the deepest erosion in the Middle Jurassic. Hence, the areas of the MOre and V0ring Basins were the sites of thermal domes in that period and developed an erosional unconformity equivalent to the Mid-Cimmerian of the North Sea. This implies that Lower and Middle Jurassic deposits are missing in the deep MOre and V0ring Basins (Brekke et al., 1999; Brekke, 2000). A western hinterland is supported also by in-house studies in the NPD, which show a transition from proximal sands in the west to distal marine shales towards the northeast on the Halten Terrace and the Tr0ndelag Platform, including the Helgeland Basin, in Early to Middle Jurassic times (Fig. 18). The co-existing Early to Middle Jurassic basin of the Jameson Land area in East Greenland shows evidence of sediment influx from the east and north (Surlyk, 1990). Evidence from And0ya north of the Tr0ndelag Platform strongly indicates a Bajocian/Bathonian delta prograding towards the south being fed by clastic sediments from hinterlands to the west, north and east (Dalland, 1981). Together with the evidence of a western hinterland for the Halten Terrace and TrOndelag Platform, this implies a central landmass exposed for erosion between present Norway and East Greenland (Fig. 18). Such a landmass has been suggested by several authors (Dalland, 1981; Larsen, 1987; Dor6, 1992; Brekke et al., 1999;

25 Brekke, 2000). This hinterland configuration indicates that the seaway between the present Norwegian Sea and Barents Sea was to the east rather than along the overall rift axis between present Norway and East Greenland (Fig. 18). This also fits with the Middle Jurassic erosional hiatus in East Greenland (Fig. 2) (Surlyk, 1991). Recent apatite fission track data substantiate a phase of rapid uplift and erosion in the Middle Jurassic in East Greenland, the first phase of uplift since the Carboniferous of the area (Johnson and Gallagher, 1999). As argued above, the uplift of the central landmass between Norway and East Greenland seems to be part of a more regional uplift of the whole of the Norwegian-Greenland Sea and surrounding area, and linked to the onset of crustal extension and increased heat flow. In the Norwegian-Greenland Sea this extension mainly affected the axial area of postulated, long-lived stable, unrifted basement blocks. Being previously unrifted, and thereby not subjected to "strain hardening" at deep crustal levels during the Triassic thermal relaxation, the crust in this area would be the natural location for the subsequent Middle Jurassic to Early Cretaceous rifting episode. However, the crust of these stable basement blocks did not react uniformly to the extension and rifting. The foci of uplift (doming), extension and attenuation of the crust were along the axes of the MOre and V0ring Basins. The crust beneath the elevated flanks of these new domal areas, i.e. mainland Norway and the Halten Terrace/Tr0ndelag Platform to the east and the MOre and V0ring Marginal Highs to the west, was not attenuated. The thickness of the crystalline continental crust beneath the V0ring Marginal High west of the V0ring Escarpment is still in the order of 15 km, whereas it is reduced to 5 km beneath the V0ring Basin (Mjelde et al., 2001). Thus, the stable basements blocks west of the Faeroe-Shetland and V0ring Escarpments were unaffected by the Jurassic/Early Cretaceous extension and became the elevated flanking platforms to the west when the attenuated crust of the MOre and V0ring Basins started to subside in the Late Jurassic. During Cretaceous times, this western platform area constituted intermittent emerged land areas in the Norwegian Sea between the Norwegian and Greenland mainlands (e.g. Brekke et al., 1999). The continental crust beneath this platform area only became involved in extension at the end of the Cretaceous and was finally ruptured in the continental break-up in Early Eocene times. In the present Barents Sea sediments continued to pour into the basin from the east, keeping up the coastal plain/delta plain development. However, this coastal plain/delta plain environment was gradually transgressed from the west during Middle Jurassic

26

times. It seems probable that the Lower Jurassic Tubfien Formation and the Middle Jurassic Nordmela Formation (Dalland et al., 1988) are parts of the same time transgressive coastal plain/delta plain system, which had its maximum western extent in Toarcian times (Tubfien Formation) and which was finally transgressed in the east (South Barents Basin) in earliest Oxfordian times (Fig. 2). After the long period of erosional denudation of the hinterlands (e.g. Figs. 15 and 16) through Triassic and Early Jurassic times, the whole region was probably dominated by a low-lying peneplain (Riis, 1996). The local and semi-regional Middle Jurassic domes and uplifts therefore had great effects on the distribution of emerged land and sea, and the palaeogeography of Middle Jurassic times was probably complex (Fig. 18). The Late Jurassic

The initial phase of the major extensional tectonic period that caused the break-up of the Central Atlantic, started at the end of Middle Jurassic times in the North Sea and Norwegian-Greenland Sea (e.g. Blystad et al., 1995; F~erseth, 1996), and probably in early-Late Jurassic times in the central and western Barents Sea (Johansen et al., 1993). The major regional tectonic phase, however, started in late Oxfordian/early Kimmeridgian times and continued intermittently into Ryazanian/Valanginian times (e.g. Blystad et al., 1995; Underhill, 1998). In East Greenland the major phase is reported to be as late as middle Volgian to Valanginian times (Surlyk, 1990). During early- to middle-Late Jurassic times the North Sea dome deflated and the elevated areas in the central parts of the present MOre and VOting Basins subsided rapidly (Underhill, 1998; Brekke, 2000). These tectonic events caused a marked rejuvenation of the topography into a complicated system of tectonic highs and basins on a variety of scales. Amongst these was the emergent platform area separating the VCring and MOre Basins from the basins of East Greenland (Dor6, 1992; Brekke et al., 1999; Brekke, 2000). At the same time, from the early Bathonian to early Kimmeridgian, there was a major sea-level rise that flooded this topography. However, it did not succeed to drown all the new highs (Dor6, 1992; Underhill, 1998; Brekke, 2000) (Figs. 2 and 19). This period was entirely dominated by open marine claystone deposition (e.g. Heather, Melke and Fuglen Formations) (Fig. 2). The sea-level rise was followed by a regional sea-level fall in the early to mid-Volgian with a low-stand lasting till the mid-Ryazanian (e.g. Rawson and Riley, 1982; Surlyk, 1991; Dor6, 1992). In combination with the renewed and complicated

H. Brekke et al.

rift topography this caused temporary faunal provinciality (e.g. Dor6, 1991). This fluctuation in sea level under such tectonic circumstances seems to have been very favourable for the widespread accumulation of large volumes of black shales, of which large parts have very good source potential (Fig. 19). Source rock deposition seems to have been most pronounced during the relative sea-level fall and low-stand. Upper Jurassic sandy deposits include syn-rift clastic wedges and shallow marine sheet sands associated with deltas and coastal plains. Deep-water submarine fans of coarse clastics deposited on the hanging walls of major faults are reported from several locations, including the Brae trend of the Southern Viking Graben (Stow et al., 1982) and similar facies in East Greenland (Surlyk, 1990). Fan and bar deposits encapsulated in source rock shales are also typically found on the back of major tilted footwall fault blocks of the North Sea (e.g. Dahl and Solli, 1993; Underhill, 1994). The sands in these cases are believed to be derived from the erosion of the crest of the fault block itself. In the platform areas one may find high-energy shallow marine sheet sands and bar deposits like the Sognefjord Formation on the Horda Platform (Vollset and Dor6, 1984) and the Rogn Formation (Dalland et al., 1988) on the Tr0ndelag Platform, respectively (Fig. 2). These deposits are believed to have been sourced froria clastic shorelines of delta plains/coastal plains. Early Kimmeridgian marine sands are also reported from the Janusfjellet Formation of Svalbard (N0ttvedt et al., 1993a,b). The Cretaceous

The Early Cretaceous

The Ryazanian low-stand was followed by a renewed sea-level rise to an intermediate maximum in the Barremian (Fig. 20). The early Neocomian was still dominated by emergent structural highs and platform areas, and the Ryazanian/Berriasian erosional unconformity on top of the carbonaceous marine shales is observed across the entire study area (e.g. Vollset and Dor6, 1984; Dalland et al., 1988; Surlyk, 1990; Smelror et al., 1998), except in north Greenland (Hfikansson and Stemmerik, 1984). In Ryazanian through Hauterivian times deep basinal areas continued to develop by subsidence along the rift axis of the North Sea, in the Mere and V~ring Basins, Jameson Land, and the Harstad, Troms~ and SCrvestsnaget Basins, and probably their north Greenland conjugate parts. The present onshore outcrops in north Greenland show a laterally diverse development of shallow marine to fluvial sandy deposits (Hfikansson and Stemmerik, 1984), which may be the fringe de-


Fig. 19. The palaeogeography of late Oxfordian-early Kimmeridgian times plotted on the 70 Ma plate reconstruction. Some key references for compilation: Dalland (1981); H&kansson and Stemmerik (1984); Larsen (1984); Steel and Worsley (1984); Ziegler (1988); Surlyk (1990); Leith et al. (1993); Stemmerik et al. (1993); Embry (1993); Underhill (1998); Brekke et al. (1999); supplemented by in-house studies. See Fig. 5b for legend.

posits to deep, fault-bounded basins indicated in the offshore areas to the east by Larsen (1984). Also in the North Sea and in the Norwegian Sea the emergent highs and land areas were fringed by shallow marine sands as transgression progressed (e.g. Oakman and Partington, 1998; Brekke et al., 1999). The V~aring Basin area comprised several sub-basins at the time (Brekke et al., 1999; Brekke, 2000), and the same may have been the case for the Harstad, Troms~a and S~arvestsnaget Basins, and north Greenland. The central platform area of the Norwegian-Greenland Sea, separating the M~are and V~aring Basins from the Jameson Land Basin, seems to have existed throughout Cretaceous times (Brekke et al., 1999; Brekke, 2000). All these deep basin areas accumulated open marine mudstones and shales during the early Neocomian. The platform areas and structural highs were unconformably capped by a condensed sequence of limestone and marl, like the Lyr Formation in the Norwegian Sea (Dalland et al., 1988) and the Klipprisk Formation in the northern Barents Sea (Smelror et al., 1998; Gradstein et al., 1999). The shallow basins within the platform areas (e.g. the Helgeland, Jameson Land, Hammerfest and Nordkapp Basins) accumulated lime-rich open marine mudstones and shales. The increasing sea level that in this way led to widespread shale and marl deposition in platform

27

Fig. 20. The palaeogeography of Barremian times plotted on the 70 Ma plate reconstruction. Some key references for compilation: Dalland (1981); Hfikansson and Stemmerik (1984); Steel and Worsley (1984); Ziegler (1988); Surlyk (1990); Dor6 (1992); N~attvedt et al. (1993a); Johansen et al. (1993); Stemmerik et al. (1993); Oakman and Partington (1998); Larsen et al. (1999); Brekke et al. (1999); supplemented by in-house studies. See Fig. 5b for legend.

areas and in starved distal deeps, was halted by a sudden sea-level drop at the peak of the Barremian high-stand. This gave time to renewed delta progradations from the transgressed land areas (Fig. 20). These include the Wealdon paralics in southern England (e.g. Dor6, 1991), the Nordelva Member on AndCya (Dalland, 1981), and the major delta deposits of the Helvetiafjellet Formation on Svalbard (Steel and Worsley, 1984; Nemec et al., 1988). The Helvetiafjellet Formation may have been a response to a major uplift of the northwestern Barents Sea area associated with the break-up of the Amerasian Basin of the present Arctic Sea (Dor6, 1991; Ne~ttvedt et al., 1993a,b). This event is dated to Barremian times by Rowley and Lottes (1988) and was accompanied by magmatism in the platform areas around Kong Karls Land and Franz Josef Land (Steel and Worsley, 1984; in-house studies) (Fig. 20). A new pulse of regional transgression started in the Aptian and continued into the Late Cretaceous, slowly drowning emergent intrabasinal highs and surrounding land areas throughout the entire study area. In the North Sea area, however, this transgression formed the background to the rejuvenation of older landmasses caused by the Austrian tectonic phase. This rejuvenation caused a new pulse of progradation of shelfal greensands all around the fringes of the North Sea Basin in the Aptian. Progressive deepening

28

during Albian times subsequently re-established the starved deep shelfal and basinal marl facies environment. The Late Cretaceous

By the end of the Albian, the sea had flooded most of the lowlands surrounding the North Sea Basin, effectively cutting off the clastic input to the whole basin area (Oakman and Partington, 1998). This led to the establishment of the early Chalk Sea of Cenomanian to Santonian times as the basinal marl facies changed to pelagic chalky limestones and spread across the whole North Sea Basin (Fig. 2). The global sea level continued to rise until its maximum in the middle of Campanian times. This was the time span of the mature Chalk Sea that also transgressed the crystalline basement of southern Norway and southem Sweden (Riis, 1996) (Fig. 21). The increased area of submergence caused the mature Chalk Sea to produce carbonates of higher purity than the early Chalk Sea. The general deposits of the chalk seas were a combination of bioturbated homogenised chalk ooze and downslope redeposited, better sorted ooze. The shallower shelf area was dominated by deposits from benthic forms as bryozoa, echinoids and crinoids (Oakman and Partington, 1998). A number of hiati are identified within the chalk sequence in the North Sea, of which the intra-Campanian and the base Paleocene are the most prominent (Fritsen et al., 1999) (see Fig. 2). A Late Cretaceous polyphasal tectonic episode is documented in the Norwegian-Greenland Sea area (e.g. Surlyk, 1990; Blystad et al., 1995; Dor6 et al., 1999; Brekke, 2000) (Fig. 1). Brekke (2000) dated the onset of this tectonic episode to end Cenomanian/earliest Turonian times by seismic correlation with shallow water wells on the Norwegian Sea shelf. However, in-house studies on wells 6607/5-1, 6707/10-1 and 6706/11-1 indicate a latest Turonian age for the initial stages of this tectonism. These events are superimposed on the regional transgression and were probably linked to the incipient seafloor spreading in the Labrador Sea. This is also the timing of the onset of rifting between Greenland and the Rockall Plateau according to the models of Rowley and Lottes (1988) and Srivastava and Verhoef (1992). In East Greenland the crystalline basement was transgressed during the Albian (Stemmerik et al., 1993; Larsen et al., 1999), and it is likely that this also has been the case on the northern Norwegian mainland (Riis, 1996). The tectonism was expressed as faulting, accelerated basin subsidence and conjugate uplift, tilting and emergence of the bounding platform areas to the major basins, i.e. the

H. Brekke et al.

MOre, V0ring, Harstad and Troms0 Basins (Brekke and Riis, 1987; Brekke, 2000). Evidence of coincident flank uplift in the East Greenland basins is given by thin conglomerates and tidal deposits of Cenomanian/Turonian age abruptly overlying Albian shales (Stemmerik et al., 1993) (Fig. 2). This topographic rejuvenation gave rise to new basin-bounding platforms that, once again, showed up as axial emergent land areas from the Rockall Platform to And0ya (Fig. 21). The flank uplifts and platform areas were deeply eroded and gave a pronounced Turonian/Coniacian unconformity that was subsequently tilted and partly transgressed (e.g. Brekke and Riis, 1987). The deep Cretaceous basins of the Norwegian-Greenland Sea contain up to 13 km of sediments in their axial parts, of which the Cretaceous succession alone makes up 8-9 km (Skogseid et al., 2000; Brekke, 2000). The background sedimentation is deep marine mudstones, but in local depo-centres of rapid subsidence this may be overprinted by coarse-grained turbidites in stacks of considerable thickness. The deep-water wells 6707/10-1 and 6706/11-1 in the northern V0ring Basin proved a more than 1000-m-thick interval of Coniacian/Campanian sandy turbidites (Kittilsen et al., 1999). The provenance area for these sands may have been the emergent axial platform areas to the west and north. Alternatively, the clastics may have been transported from Greenland itself along regional channels along prominent NW-SE lineaments bisecting the axial highs (Fig. 21). This would fit with the Late Cretaceous shelf break margin model described by Whitham et al. (1999). In this model, fluvial point sources provided fine to coarse clastic material to the narrow shallow marine shelf. In periods of lowstand these clastics were transported more or less directly from the terrestrial source (e.g. a delta), with very little shallow marine reworking, into the deep basin to the east beyond the shelf break as fine- to coarse-grained sediment gravity flows, accumulating as basin floor fan deposits. The outcrops of the Upper Cretaceous of East Greenland are dominated by dark deep-water shales and the evidence of such sandy low-stand fans are scarce. However, the locations for the sediment transport routes for these fan deposits anticipated by Whitham et al. (1999) fit very well with the prominent NW-SE lineaments leading into the VOting Basin (Fig. 21). In the course of Late Cretaceous times, the Barents Sea shelf and Svalbard were finally de-coupled from the areas south of the de Geer Zone which is the broad zone of deformation along the present western continental margin of the Barents Sea, including the Wandel Sea Basin of north Greenland, the Tromsr and Vestbakken Basins, the Palaeo-Homsund Fault and the Senja Fracture Zone (Harland, 1969; Faleide


an overview

Fig. 21. The palaeogeography of early Campanian times plotted on the 70 Ma plate reconstruction. Some key references for compilation: Hfikansson and Stemmerik (1984); Steel and Worsley (1984); Dalland et al. (1988); Ziegler (1988); Surlyk (1990); Stemmerik et al. (1993); Riis (1996); Oakman and Partington (11998); Brekke et al. (1999); Larsen et al. (1999); supplemented by in-house studies. See Fig. 5b for legend.

et al., 1993a,b) (Fig. 6). The whole of the Barents Sea shelf was uplifted while the deep Cretaceous basins of the Norwegian-Greenland Sea to the south continued to subside rapidly (e.g. Breivik et al., 1999). The regional Cretaceous transgression into the Barents Sea therefore only resulted in a shallow shelf leaving a condensed marine sedimentary sequence of calcareous sandstones, sandy and glauconitic mudstones and thin limestones of the Kviting Formation in the central parts of the Barents Sea (Fig. 21) (Dalland et al., 1988; N0ttvedt et al., 1993a,b). The degree of uplift increased northwestwards so that Svalbard and the whole of the northwestern Barents Sea platform areas were eroded during Late Cretaceous times. The timing of the regional uplift in relation to the onset of the tectonic phase in the Norwegian-Greenland Sea and its bearing on the interpretation of the Eurekan Orogeny is uncertain. Data from Svalbard indicate that the first phase of compression and folding of strata in north Greenland and Svalbard took place between Albian and Paleocene times (Steel and Worsley, 1984; Hanisch, 1984). Basin fill in pullapart basins of the Wandel Sea Mobile Belt seems to constrain the dating of the deformation to between middle Turonian and end Maastrichtian times. Further constraint may be inferred by the fact that the transgressive base of the Kviting Formation is a prominent hiatus of Cenomanian to Santonian age,

29

pointing to the initiation of tectonism and associated regional uplift at that time. This coincides with the onset of the Late Cretaceous tectonic episode and accelerated subsidence of the MOre and Vcring Basins, strongly pointing to a link with the rapidly subsiding basins along the Barents Sea margin and the Wandel Sea Basin through the de Geer Zone (as suggested by Brekke and Riis, 1987). Separate from the Turonian rifting and increased basin subsidence, Faleide et al. (1993a,b) argues for the initiation of the Eurekan Orogeny in the late Santonian, and refers to a related phase of uplift and faulting in And0ya in that respect (Dalland, 1981). This is also in agreement with the plate reconstruction models of Rowley and Lottes (1988) which predicts transcurrent movements between north Greenland and Ellesmere Island/Svalbard from anomaly 34 time onwards. In that case, the initiation of transpression on the de Geer Zone may have been coincident with the Campanian compressional phase of the Late Cretaceous tectonic episode of the MOre and V0ring Basins (Brekke and Riis, 1987; Bj0rnseth et al., 1997; Brekke, 2000).

The Tertiary The sedimentary environment of the Tertiary was a response to the Palaeogene transition from the continental rift setting to a drift and passive conti-

Fig. 22. The palaeogeography of Early Eocene times plotted on the 53 Ma plate reconstruction. Some key references for compilation: H~kansson and Stemmerik (1984); Steel and Worsley (1984); Larsen (1984); Ziegler (1988); Surlyk (1990); Livsic (1992); NOttvedt et al. (1993a); Knott et al. (1993); Stemmerik et al. (1993); Bowman (1998); Larsen et al. (1999); Brekke et al. (1999); supplemented by in-house studies. See Fig. 5b for legend.

30 nental margin setting, and the subsequent climatic change giving Neogene glaciations. The widespread Late Cretaceous polyphasal tectonic episode may be viewed as plate tectonic adjustments and rifting in the North Atlantic leading to the final continental break-up in the Norwegian-Greenland Sea area in the Late Paleocene/Early Eocene (e.g. Skogseid and Eldholm, 1989; Roberts et al., 1997; Brekke, 2000). This event was associated with a regional uplift of the whole of the Norwegian-Greenland Sea and its circumference. The cause of the regional uplift is uncertain; the debate involves the Iceland mantle plume (White, 1989; Skogseid, 1994) to intraplate stress (Cloetingh et al., 1990). The axial part of the Norwegian-Greenland Sea was highly uplifted due to increased heat flow along the future spreading axis just prior to break-up (Fig. 22). The general uplift drastically reduced the size of the basins and expanding the hinterland areas. The depositional area of the gross North Sea Basin has been estimated to have been reduced to 70% of that of the late Maastrichtian (Oakman and Partington, 1998). The Paleocene and Eocene

In the southern and central North Sea, chalk deposition continued through the Maastrichtian and Danian, typically filling in a slightly tectonically rejuvenated seafloor topography by the redeposition of pelagic ooze. But as the regional uplift, basin reduction and hinterland expansion initiated in the late Danian/early Thanetian, clastic input increased drastically and effectively ended the carbonate depositional environment (Fig. 2). The clastics that derived from the emergent Shetland Platform in Paleocene to Early Eocene times in the North Sea area may be divided into two sedimentary units (Bowman, 1998). The older unit consists of a sequence of aggradational submarine fan deposits. The younger, latest Thanetian to earliest Ypresian unit consists of a progradational sequence of muds and localised sands. In the late Thanetian the North Sea Basin was cut off from oceanic circulation giving a basin-wide anoxic phase. This was probably at the peak uplift of the Norwegian-Greenland Sea axis just prior to continental rupture. Subsequent to the final break-up, the whole area started to subside and the resulting relative sea-level rise ended the North Sea anoxic phase in the latest Thanetian, at the base of the progradational unit. The rest of the Eocene was characterised by deep-water sandy turbidite pulses as a response to fluctuations in relative sea-level and hinterland rejuvenation. The well-known sandy Frigg Fan is one of the major low-stand fan systems of this period (Bowman, 1998; Martinsen et al., 1999).

H. Brekke et al.

The regional uplift is recorded as a hiatus and erosional break of probable late Danian/early Thanetian age across the western bounding platforms and basin flanks of the MOre and Voring Basins, and also across highs and domes within the V0ring Basin (Brekke et al., 1999; Martinsen et al., 1999). The Danian/Thanetian was therefore a period of dramatic shallowing of the MOre and VCring Basins and the emergence of the surrounding areas. Isolated outcrops in East Greenland show Upper Cretaceous and Lower Paleocene offshore shales and submarine channel turbidites unconformably overlain by upper Danian and Thanetian fluvial conglomerates and sandstones, implying a coincident similar dramatic shallowing there (Larsen et al., 1999). In the Voring Basin, the basin flanks, highs and domes were eroded and sediments were deposited in shallow synclinal, perhaps circulation restricted, areas within the basin. In the deeper MOre Basin and in the northern North Sea, thick Paleocene/earliest Eocene sedimentary wedges prograded into the basin from the platforms on both flanks (Brekke et al., 1999; Martinsen et al., 1999). This symmetrical progradation of low-stand wedges downlapping the base Paleocene hiatus and thinning towards the basin axis, is seen throughout the northern North Sea and the MOre Basin (Martinsen et al., 1999). In the MOre Basin, there is a widespread sand-rich unit near the base of the Paleocene. The progradation from the western platform areas probably led to the eastward advancement of the western shoreline into the MOre Basin (Brekke et al., 1999). The final rupture of the continental crust within the axial platform area between Greenland and Norway was accompanied by the eruption of basaltic lavas (Fig. 22). Large volumes of tholeiitic flood basalts flowed across the whole of the eroded platform area and stopped at the newly established shoreline in the VOring and MOre Basins (Brekke et al., 1999). This shoreline is defined by the limit of the early flows, termed the "inner flows" by Talwani et al. (1983). By the subsequent subsidence and tectonic activity the shoreline retreated westwards to the present FaeroeShetland and VOting Escarpments building a line of lava deltas (Smythe et al., 1983; Planke et al., 1999). Due to recent uplift and erosion, the position of the Paleocene shoreline in East Greenland is not known. Subsequent to the latest Paleocene/earliest Eocene voluminous break-up magmatism the newly established spreading axis and surrounding lava platforms started to subside and eventually became submerged. The main part of the Eocene succession is a slope/basin floor system downlapping and thinning towards the east throughout the northern North Sea and the MOre Basin. This system is interpreted to be a low-stand wedge related to fall and subsequent rise in

31

S e d i m e n t a r y environments offshore N o r w a y - - an o v e r v i e w

relative sea level (Martinsen et al., 1999). The Eocene of the Norwegian Sea area is dominated by marine claystone (Dalland et al., 1988). After the Late Cretaceous uplift and erosion, the western Barents Sea shelf was transgressed in early Thanetian times leaving a Paleocene to Oligocene uniform sequence of outer sublittoral to deep-shelf claystone with minor siltstone, tuffaceous and carbonaceous horizons (Dalland et al., 1988; Faleide et al., 1993a). The Paleocene to Oligocene environments of Svalbard were much more complex because of the involvement in the Spitsbergen Orogeny (Fig. 2). The Late Cretaceous uplift and erosion is testified by Lower Paleocene strata overlying Albian and Aptian strata onshore Spitsbergen (Steel and Worsley, 1984). The Spitsbergen Orogeny was due to dextral movement on the de Geer Zone megashear in which Steel and Worsley (1984) record Paleocene transtension, Eocene transpression and Oligocene oblique separation and rifting. Further south, in the S~rvestsnaget Basin and Troms~ Basin, the timing of transtension and transpression was different (Faleide et al., 1993b). Although changing configuration considerably through time, the Paleocene and Eocene basins onshore Spitsbergen exhibited a complete lateral transition from alluvial fan, delta plain and fan delta into delta front and shallow to distant shallow shelf environments (Steel and Worsley, 1984). The Oligocene and Miocene The Oligocene and Miocene of the Norwegian continental margin reflect the sedimentation on a marine, subsiding passive margin overprinted by intermittent regional phases of tectonic movements and uplift. The two main phases of compression are associated with the formation of intrabasinal domes and arches in the V~ring Basin and around the Faeroe Islands, dated to latest Eocene/earliest Oligocene and Middle Miocene (Boldreel and Andersen, 1993; Blystad et al., 1995; Andersen and Boldreel, 1995; Dor6 and Lundin, 1996; Brekke, 2000). Vfignes et al. (1998) proposes that the compression was continuous from Eocene to Miocene without discrete phases. As documented by Gradstein and B~ickstr6m (1996), however, a Late Eocene/Early Oligocene phase of compression would coincide very well with a regional hiatus on the eastern basin margins across major parts of the North Sea and on the Halten Terrace. Martinsen et al. (1999) prefer an Early Oligocene date for this stratigraphic break. A prograding unit of coastal (deltaic?) deposits on the northeastern part of the Tr~ndelag Platform, dated to the Early Oligocene by Eidvin et al. (1998), constitutes a stratigraphic record of this event (Fig. 2). Seismic and biostratigraphical

evidence points to a regional uplift and erosion of the North Sea and Norwegian Sea region and surrounding mainlands just prior to the late-Middle Miocene phase of compression (Jordt et al., 1995; Gradstein and B~ickstr6m, 1996; Martinsen et al., 1999; Eidvin et al., 2000). This phase of uplift is reported to have lasted for about 7 million years (Anderton et al., 1979; Jordt et al., 1995). In the deep V~ring and M~re Basins the Miocene erosion was entirely submarine and sedimentation remained deep marine mud and siliceous oozes across the hiatus (Eidvin et al., 1998). In the V~ring Basin a considerable seafloor dome topography was filled in during the Late Miocene (Brekke et al., 1999). In the North Sea Basin, more proximal to the mainland, the uplift gave rise to widespread influx of sand (Utsira Formation) resting unconformably on the Lower and Middle Miocene shales (Rundberg et al., 1995; Eidvin et al., 2000). The Pfiocene and Pleistocene The periods of glaciation caused by the climatic deterioration in the Neogene had a significant impact on the sediment supply to the shelf in the Late Pliocene and Pleistocene. The onset of major glaciations at approximately 2.7 Ma led to deep mainland erosion and to the deposition of huge sediment volumes on the adjacent shelf (Riis and Fjeldskaar, 1992; Vfignes et al., 1992; Riis, 1996; Martinsen et al., 1999; Eidvin et al., 2000). The mainland and the Barents Sea area were uplifted tectonically in dome-shaped areas, and in general as an isostatic response to the erosion (Riis, 1996; Dehls et al., 2000). A regional hiatus is recorded in the lower part of the Upper Pliocene (2.7 Ma) along the Norwegian continental margin (Eidvin et al., 2000) (Fig. 2). The hiatus is mainly preserved as a surface of nondeposition downlapped by the sandy muds of the prograding Upper Pliocene sediment apron. In the V~ring Basin, the Upper Pliocene sediments rest on the Middle Miocene unconformity on the summits of large domes and arches and in large areas in the western parts of the basin and the adjacent marginal high (Brekke, 2000). On the mainland and on the shallow parts of the margin, there is an erosional unconformity below the glacigenic sediments. This unconformity was formed in the Pleistocene as a result of large ice sheets which extended to the shelf break. The largest sediment volumes are found in several kilometres thick fan systems adjacent to major submarine channel systems, like just northeast of Kvit~ya in the Arctic Ocean, west of Bj~rn~ya, and in the M~re Basin area at the mouth of the Norwegian Channel (Riis, 1996; Eidvin et al., 2000).

32

Summary and conclusions (1) In Carboniferous times the area, situated at low latitudes, developed a wide range of sedimentary environments through time and space. In that period, the area was situated between a northern and southern ocean. Through the regional drainage pattern sediments were transported into these oceans from a highland situated in the southern part of the Norwegian-Greenland Sea, probably governed by the tectonic framework. (2) Permian times saw both the final plate tectonic assemblage of Pangea and the subsequent onset of rifting of the supercontinent. The period was characterised by an early period of magmatism, followed by widespread erosion, and the subsequent development of shallow marine environments of low latitudes. (3) Triassic and Early Jurassic times were a period of peneplanation of hinterlands and clastic infilling and shallowing of basin areas. (4) Through renewed rifting and relative sea-level rise in the Middle to Late Jurassic, the seaway between the northern and southern oceans became permanent and shallow marine; clastic environments were established throughout the region, now at middle latitudes. Subsequent to sea-level oscillations during Late Jurassic tectonic activity, clastic-starved environments were established as large basin areas subsided and the sea level rose through the Cretaceous. (5) Through hinterland rejuvenation by early Tertiary rifting and subsequent continental separation and seafloor spreading, clastic marine environments were re-established, this time at high latitudes and cold waters. In the Neogene, major parts of the clastic input to the marine environments originated from glacial erosion. (6) An extension model has been proposed in which the crustal extension and rifting activity is grouped into three broad rifting episodes, the Carboniferous/Triassic, the Middle Jurassic/earliest Cretaceous, and the latest Cretaceous/Paleocene episodes. These episodes of extension were separated by two periods of thermal relaxation, from the end of the Early Triassic to the middle of the Early Jurassic, and from the earliest to latest Cretaceous. It is argued that the Carboniferous/Triassic episode resulted in a significantly larger share of the total accumulated extension than the Middle Jurassic/earliest Cretaceous episode. The latest Cretaceous/Paleocene rifting episode is considered to have contributed least to the total extension of the continental crust between Scandinavia and Greenland. (7) It seems that the Carboniferous/Triassic rifting was distributed across a broad area. For each of the two subsequent rifting episodes, the central axes of

H. Brekke et al.

rifting shifted towards the axial parts of the continental margins of the present Norwegian-Greenland Sea, thereby progressively narrowing the zone of actual rifting. (8) In the rift model proposed, the axial area of the continental margins of the present NorwegianGreenland Sea was underlain by long-lived, stable basement blocks. The contention is that these were established already in Early Carboniferous times as the Carboniferous to Triassic rifting activity took place symmetrically along both flanks, i.e. the inner parts of the present continental margins of mid-Norway and East Greenland, respectively, leaving the axial area non-rifted and non-attenuated. In such a setting, the progressive shift of the rift axes of the subsequent extension episodes towards the central parts of the Norwegian-Greenland Sea area is believed to reflect the preference for non-rifted crust in place of previously rifted and attenuated crust. The explanation may be that crust that has been through the cycle of rifting and attenuation, and subsequent thermal relaxation and subsidence becomes physically more resistant to further extension. By the time of the latest Cretaceous/Paleocene extension episode, the area of crust still unaffected by previous rifting was considerably reduced, so that the rifting activity was restricted to a narrow zone, along which the final continental break-up occurred. Such a narrow zone of crust available for extension would then not contribute much to the total extension accumulated through time, even with a high Beta-factor locally. The lack of a broad area of easily extendable crust, may also explain the apparently clean-cut, vertical rupture of the crust in the process of continental break-up, in which the extension was compensated by magmatism instead of attenuation of the crust., (9) Rejuvenation and subsequent denudation of hinterlands played an important role in the evolution of sedimentary environments and facies through time. In the Carboniferous and Permian, the uplifted hinterlands were of two categories by origin. The hinterlands of one category were caused by uplift by orogenic activity (Variscan and Uralian Orogenies). Uplifts of the other category were probably associated with the rifting between northern Europe and Greenland, including the flanking mainlands of Norway and Greenland and the elevation of the suggested central watershed area of Visean times. The Early to Middle Jurassic and the Paleocene were periods of prominent hinterland rejuvenation. Both were associated with increased heat flow in the initial stages of the major episodes of rifting and extension. In both cases, the whole area of the regional rift zones and their flanks experienced regional uplift causing widespread erosion and concomitant development of progradational


systems of clastic sediments. Typical of the Early and Middle Jurassic was the emergence of a number of semi-regional domes within the regionally elevated area from the North Sea and British Isles to the borders of the Barents Sea. This caused a complex configuration of emerged and submerged areas in that period.

Acknowledgements We want to thank our colleagues Fritjof Riis and Paul Grogan for their valuable input to the present study. We are also grateful to the referees, Roy H. Gabrielsen and Snorre Olaussen, for their very critical review of the first draft of the manuscript. We are indebted to the editors, for their patience.

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H.I. SJULsTAD C. MAGNUS R.W. WILLIAMS

Norwegian Norwegian Norwegian Norwegian

Petroleum Petroleum Petroleum Petroleum

Directorate, Directorate, Directorate, Directorate,

P.O. Box P.O. Box P.O. Box P.O. Box

37 Vollset, J. and Dor6, A.G. 1984. A revised Triassic and Jurassic lithostratigraphic nomenclature for the Norwegian North Sea. Norwegian Petroleum Directorate Bulletin, 3, The Norwegian Petroleum Directorate, Stavanger, 65 pp. Wennberg, O.R, Milnes, A.G. and Winsvold, I. 1998. The northern Bergen Arc Shear Zone - an oblique-lateral ramp in the Devonian extensional detachment system of western Norway. Norsk Geol. Tidssk., 78:169-184. White, R.S., 1989. Initiation of the Iceland plume and opening of the North Atlantic. In: A.J. Tankard and H.R. Balkwill (Editors), Extensional Tectonics and Stratigraphy of the North Atlantic Margins. Am. Assoc. Pet. Geol., Mere., 46: 149-154. Whitham, A.G., Price, S.R, Koraini, A.M. and Kelly, S.R.A., 1999. Cretaceous (post-Valanginian) sedimentation and rift events in NE Greenland (71-77~ In: A.J. Fleet and S.A.R. Boldy (Editors), Petroleum Geology of Northwest Europe: Proceedings of the 5th Conference. Geological Society, London, pp. 325-336. Whiteman, A., Naylor, D., Pegrum, R. and Rees, G., 1975. North Sea troughs and plate tectonics. Tectonophysics, 26: 39-54. Ziegler, RA., 1987. Evolution of the Arctic-North Atlantic borderlands. In: J. Brooks and K.W. Glennie (Editors), Petroleum Geology of North West Europe. Graham and Trotman, London, pp. 1201-1204. Ziegler, RA., 1988. Evolution of the Arctic-North Atlantic and the western Tethys. Am. Assoc. Pet. Geol., Mere., 43. Ziegler, RA., 1990. Geological Atlas of Western and Central Europe. 2nd ed., Shell International Petroleum Maatschappij B.V., Geological Society, Bath (distributors).

600, N-4003 600, N-4003 600, N-4003 600, N-4003

Stavanger, Stavanger, Stavanger, Stawmger,

Norway Norway Norway Norway


39

The alluvial cyclicity in Hornelen Basin (Devonian western Norway) revisited: a mu lti parameter

sedimentary analysis and stratigraphic implications Atle Folkestad and Ronald J. Steel

Accommodation space and sediment supply are the main factors controlling the spatial and stratigraphic pattern of the infill of sedimentary basins. The interaction of these factors, over periods of time, can be identified in the basin-fill succession by the changes of, for example, grain size, bed thicknesses and erosion-surface frequency, which are parameters easily measurable in outcrops as well as in cores and on image logs of subsurface successions. This study demonstrates how this approach can be used to recognise changes in the accommodation-space/sediment-supply ratio, based on an analysis of the alluvial succession in the Devonian Hornelen Basin of western Norway. Four of the basin-fill cyclothems have been logged (a total of 525 m of the basin-fill) with a systematic quantification of such parameters as grain size, bed thicknesses, erosion-surface density, occurrence of intraformational clasts and extraformational clasts, clast position within beds, and the degree of soft-sediment deformation. The analysis leads to a new understanding of the cyclicity and style of sedimentation in the Hornelen Basin. The deposits of the alluvial succession can be divided into three facies associations: (1) fluvial channels; (2) channel-mouth splays; and (3) distal floodbasin deposits. The dynamics of basin infilling can be expressed as an interplay of stratigraphic accommodation-space creation (A) and sediment supply (S), commonly expressed as the A/S ratio. In broad terms, an increasing A/S ratio implies increasing preservation potential of sediment infill, whereas a decreasing A/S ratio signifies a decrease of preservation potential and an increasing probability of sediment bypass or erosion. Because of the critical importance of erosion in the A/S ratio concept, cycles in the A/S ratio for the succession can be identified by using peaks in the frequency of occurrence of erosion surfaces (quantified as "erosion-surface frequency") to pick A/S ratio minima, and minima of erosion surfaces to pick A/S ratio maxima. One of the more interesting results from the study shows that peaks in grain size occur somewhat after A/S ratio minima, the offset being caused by continued high levels of sediment supply and flow competence despite a relative increase in A where the offset represents a time-lag. Bed thicknesses show low values close to the A/S ratio maxima and minima, and peak where A approaches S. The effect can be compared with the depositional pattern along the length of a clinoform with a low-angle trajectory. In a proximal position the clinothem, after a certain time period, is thin due to low A/S ratio conditions, in the distal part it is thin due to a high A/S ratio conditions, whereas the greatest thicknesses are recorded in between these two extremes. The soft-sediment deformation parameter follows the pattern of the bed thickness parameter and is thus interpreted as being linked to the bed thickness. The clast parameters (intra-, extra-formational clasts and clast position within the beds) follow the pattern of grain-size and erosion-surface frequency parameters where increasing clast occurrence reflects lower A/S ratio and decreasing clast occurrence indicates higher A/S ratio. The approach described here can be applied easily to subsurface successions. In cored intervals, parameters such as grain size, erosion surfaces and bed thicknesses can be extracted. The same approach has been used on a Formation Micro Image Log from a well in the North Sea where bioturbation, erosion-surface density, set density and angle of lamination were quantified and cross-analysed in terms of shallowing and deepening trends.

Introduction

The dynamics of a basinal stratigraphic system can be described in terms of the changing ratio of the rate of accommodation-space development and the rate of sediment supply (referred to below as the A/S ratio) as done by Shanley and McCabe (1994). Bars, hydraulic bedforms and other geomorphic elements of a depositional system are likely to be better preserved when the A/S ratio is high, but poorer preserved when this ratio is low. Erosion and sediment bypass may prevail in the latter case.

In an alluvial basin such as the Old Red Hornelen Basin in Norway, where the sediment accumulation rates are estimated to have been as high as 2 m/ka (Steel et al., 1977), it is appropriate to consider the accumulation/preservation potential of sediments in terms of A / S ratio changes. The conceptual A/S ratio defines the dynamic state of the sedimentary system, whether it evolves towards a higher degree of preservation (maximum A/S) or a greater degree of erosional destruction (minimum A/S) of the deposits (Fig. 1) (see also Shanley and McCabe, 1994). Consequently, it can be expected, as a working hypothesis,

Sedimentary Environments Offshore Norway - Palaeozoic to Recent edited by O.J. Martinsen and T. Dreyer. NPF Special Publication 10, pp. 39-50, Published by Elsevier Science B.V., Amsterdam. 9 Norwegian Petroleum Society (NPF), 2001. "

40

A. Folkestad and R.J. Steel

'"~

Bypass surface"-'~1

Accommodationspace creation -" decrease

Starvation

Bypass and | erosion I Quadrant L/1 Deposition i..i" /

~= -

Starvationsurface

Accommodation space creation increase

Erosion Starvation Quadrant Quadrant Sediment supply decrease

Fig. 1. Conceptual relationship ~etween the changes in the rates of accommodation-space creation (A) and sediment supply (S). The A/S ratio defines conditions of sediment erosion, deposition and bypass (non-deposition). The negative axis of the accommodation-space creation represents erosion, whereas the negative axis of sediment supply is included to account for the mass-balance of the system; when deposition occurs in one area, erosion must occur elsewhere. The equilibrium line 1:1 in the diagram represents equal rates of accommodation-space creation and sediment supply (A -- S). The area of bypass in the positive quadrant of the diagram (A > 0, S < 0), is meant to indicate the condition of non-deposition in a situation where the rate of sediment supply is disproportional higher than the rate of accommodation-space creation. Similarly, the area of starvation in the same quadrant indicates conditions where the rate of accommodation-space creation grossly exceeds the rate of sediment supply.

that some measurable sedimentary parameters reflecting variation in the degree of sediment preservation may indicate changes in the A / S ratio in a stratigraphic succession (see also Gardner, 1995). If this is the case, this crucial aspect of a depositional system's dynamics might then be deciphered from the sedimentary succession. The aim of the present study is to evaluate this hypothesis through an analytic approach in identifying time-trends in the A / S ratio and their turn-around points (i.e. changes from a decreasing to an increasing trend, or vice versa) in a thick, representative portion of the alluvial succession in the Hornelen Basin. The basin is probably ideal for testing such ideas because of its high but variable rates of sediment accumulation and its very thick stratigraphic succession. The quantitative method used for this purpose was originally developed by Folkestad (1995) and is presented here in a refined form.

Tectonic and stratigraphic setting The Hornelen Basin is a small (.,

O9

Pliensbachian (.9 .m

Danish Subbasin/ Inner Moray Hebrides Basin NorwegianFirth Danish Basin Offshore S sw NE Onshore

Fjerritslev Formation

Pabba Shale Formation

~ ~..L---'~ Orrin Fm /

81

Viking Graben (this study)

Dunlin Group

Lady's Walk Shale

(t)

'- . . . . .

Sinemurian

W~iTe-

DC

Sandstone Member

DPC

Member

>., m i..

LU

~

Hettangian

~9

rmation

Rhaetian

._~

Blue Lias

Statfjord Formation

Fm.

Gassum O

Broadferd Beds Fm.

-'

Lossiemouth Formation

_Vinding "~,~__? Formation / ~ "-

Penarth / Group /

__5

Raude ~'Mb E-E

New Red Sandstone

Lunde Formation

Norian

Sk: Skagerrak Formation

DC: Dunrobin Castle Member DPC: Dunrobin Pier Conglomerate

ff/

Non-deposition/erosion

--'~--

Uncertain correlation of boundaries

Fig. 11. Late Triassic and Early Jurassic lithostratigraphy of the Danish Sub-basin/Norwegian-Danish Basin, Moray Firth Basin and Hebrides Basin, compared to the Viking Graben area.

In the inner Moray Firth Basin Early Jurassic (Hettangian-Pliensbachian) strata, resting unconformably upon Triassic deposits, have been described from both onshore and offshore areas (Batten et al., 1986; Stephen et al., 1993). Onshore, Hettangian-Sinemurian deposits comprise proximal, alluvial fan deposits (Dunrobin Pier Conglomerate, Dunrobin Castle Member; Fig. 11) that can be correlated with offshore packages of lacustrine mudrocks, coals and fluvial/estuarine channel deposits (Varicoloured and White Members; Fig. 11) of a more distal fluvial to marginal marine character. These deposits are buried below offshore mudrocks of late Sinemurian to Pliensbachian age (Lady's Walk Shale). Stephen et al. (1993) noted that the Hettangian-Sinemurian deposits form a conformable progression from lacustrine to marginally marine fluvial/deltaic and eventually offshore environments that can be related to relative sea level rise. Stephen et al. (1993) also used biostratigraphic data to define a late Sinemurian transgressive surface separating the fluvially influenced deposits from overlying marine mudrocks, equivalent to the Dunlin Group (Charnock et al., 2001).

To the west of the British Isles, Late Triassic and Early Jurassic (Rhaetian-Pliensbachian) sediments include fluvial deposits (New Red Sandstone) of Rhaetian-Hettangian age which are conformably overlain by Rhaetian-Hettangian offshore mudrocks (Penanth Group, Blue Lias Formation; Fig. 11) and Hettangian-Sinemurian nearshore limestones and sandstones (Broadford Beds Formation; Fig. 11; see Morton et al., 1987; Morton, 1989). Offshore mudrocks of the Pabba Shale (late SinemurianPliensbachian) rest unconformably upon these deposits in the Hebrides Basin. Morton et al. (1987) noted a strong diachroneity in the Late Triassic to Early Jurassic marine transgression. Rhaetian deposits are marine inthe southern part of the Hebrides Basin, whereas the first marine influence is dated as Hettangian further north, thus indicating marine incursion from the south. This was also the case during the Sinemurian, as the base of the Pabba Shale Formation is younging to the north. The combined evidence for a possible northerly source area for the Statfjord Formation, and the Late Triassic marine incursion to the south (Danish Sub-

Fig. 10. Cored sections of the shallow marine Nansen Member. The deposits on the Tampen Spur and Horda Platform are related to the development of fluvial-dominated mouth-bar and distributary channels within wave-agitated embayments. Further to the south (well 30/11-4), the l~resence of low-angle cross-stratification (hummocky and swaley types?) is related to deposition in a shoreface environment. For legend, see Fig. 5.

82 basin and Hebrides Basin) followed by subsequent migration of marine environments to the north during the Hettangian and Sinemurian stages serve to demonstrate that the contemporary fluvial environment of the Statfjord Formation may have drained into coastal and marine settings to the south.

Synthesis and discussion The sedimentological interpretation of the Statfjord Formation presented above accords with previous studies from the Tampen Spur area, which focus on braided fluvial systems as the principal depositional environment for the sandstones (Nystuen et al., 1989; Ryseth and Ramm, 1996). Also, the similarity seen in the lithofacies composition and grain size distribution in sandstone bodies from the Tampen Spur and Horda Platform areas suggest that fluvial systems here were rather similar in terms of river discharge, competence and morphology, although floodplain deposits from the Horda Platform area contain significantly higher proportions of non-pedogenic material. Fluvial sandstones from the Utsira High are finer grained, and the associated floodplain deposits contain higher proportions of non-pedogenic deposits. These observations suggest that the highest depositional gradients existed on the Tampen Spur to the north, and the lowest on the Utsira High to the south. The lithofacies composition of fluvial sandstones on the Utsira High is seemingly more compatible with meandering than braided systems. The palaeogeographic implication of the comparative facies study is that the main fluvial drainage system of the Statfjord Formation was directed to the south, with a strong transverse supply. Additional observations from the marine deposits capping the Statfjord Formation fit with a depositional model involving marine incursion from the south through the Viking Graben. Fully marine conditions were established in the Inner Moray Firth in the late S inemurian, and in the earliest Pliensbachian in the Viking Graben. This diachroneity is compatible with a gradual marine flooding from the south onto a southerly dipping palaeoslope existing throughout the Viking Graben and into the Norwegian-Danish Basin further south. By the palaeogeographic scheme presented in Fig. 12, Early Jurassic deposits of the northern and central North Sea can be linked to a basin-wide depositional system. However, the palaeogeographic model implies that Early and Middle Jurassic palaeoslopes in the Viking Graben were oppositely directed, as the evidence for northward progradation of the Middle Jurassic Brent delta system is indisputable (Graue et al., 1987; Helland-Hansen et al., 1992; Johannessen et al., 1995).

A. Ryseth

The postulated late-Early Jurassic reorganization of drainage patterns can be explained due to the rise of a thermal dome at the structural junction between the Moray Firth Basin, Viking Graben and Central Graben. This feature has clearly influenced the drainage pattern of the Middle Jurassic Brent delta, but its actual effect during the Early Jurassic is much more uncertain. Underhill and Partington (1993) found that the initial rise of the dome and the first phase of related shallowing occurred during the late Toarcian, or approximately 16 million years after the Pliensbachian transgression and termination of alluvial deposition in the Statfjord Formation. Probably, the dome did not exist during the Hettangian and Sinemurian stages. The southerly drainage direction may also apply for the older, Triassic alluvial system of the North Sea. Steel and Ryseth (1990) indicated that Triassic alluvial deposits of the Skagerrak Formation (Anisian-Norian) in the southern part of the Viking Graben/Horda Platform and NorwegianDanish Basin are replaced by fine-grained lacustrine deposits further to the south. Goldsmith et al. (1995) also indicated that continental Triassic deposits (Scythian-Norian) in the Central Graben correlates with marginal marine succession further to the south. These stratigraphic relationships are indicative of a southerly directed Triassic fluvial drainage system in the Viking Graben, prior to the deposition of the Statfjord Formation.

Conclusions Comparison of lithofacies compositions of the continental deposits of the Statfjord Formation suggests that the steepest depositional slope gradients possibly existed on the Tampen Spur to the north, and the lowest on the Utsira High to the south, with the Horda Platform representing an area of medial slope gradient. The palaeogeographic implication of these interpretations is that a southerly dipping palaeoslope existed in the Viking Graben during deposition of the Statfjord Formation, and that the continental environment of the Statfjord Formation was terminated by a marine inundation from the south during the late Sinemurian. By the earliest Pliensbachian, fully marine conditions were established in the Viking Graben. Data from the Danish Sub-basin, Norwegian-Danish Basin and the Moray Firth Basin, and from the Hebrides-West Shetland Basin to the west, confirms that Rhaetian-Sinemurian marine sediments show a successive younging to the north, thus supporting the interpretation of southerly dipping palaeoslopes at these stages. The palaeogeography may have been controlled by uplift to the north of the Viking Graben during the

Sedimentology and palaeogeography of the Statfjord Formation (Rhaetian-Sinemurian), North Sea

!iiiiiii_i_~] Continental, fluvial

CG DSB FFZ

l"-.':.i.~.ii

Marginal marine

- Central Graben Danish Subbasin

-

- Fjerritslev Fault Zone

FWSB - Faroe-West Shetland Basin HE

- Hebrides Basin

HP

- Horda Platform

MB

- Mere Basin

MFB Sediment transport direction

83

NDB

- Moray Firth Basin -

Norwegian-Danish Basin

MNSH - Mid North Sea High Direction/timing of marine inundation

RFH

- Ringkeping-Fyn-High

SI

- Shetland Islands

SP

- Shetland Platform

TS

- Tampen Spur

UH

- Utsira High

VG

- Viking Graben

Fig. 12. Palaeogeographic map illustrating the proposed Early Jurassic (Hettangian-Sinemurian) sediment transport pattern. The Viking Graben may have acted as the alluvial conduit for clastic sediment shed off the Fennoscandian hinterland and source areas to the west and northwest of the Tampen Spur, linking up with shallow marine environments to the south (map based on Dord and Gage, 1987; Morton et al., 1987; Mearns et al., 1989; Ziegler, 1990).

Early Jurassic due to thermal expansion along the North Atlantic Rift system. The proposed palaeogeographic model implies that a reorganization of the drainage pattern in the Viking Graben occurred during the late-Early Jurassic, preceding the northward progradation of the Middle Jurassic Brent Group. This reorganization can be related to the rise of a thermal dome to the south of the Viking Graben during the late Toarcian.

The proposed palaeogeographic model links the Early Jurassic continental deposits of the Viking Graben to time-equivalent marginal marine sandstones (Gassum Formation) and offshore mudrocks (Fjerritslev Formation) in the Norwegian-Danish Basin to the south. It also indicates that the contemporary shoreline of the Early Jurassic fluvial drainage system was located about 300-350 km to the south of the Tampen Spur and Horda Platform areas. A further

84

implication of this is that the main area separating southerly and northerly drainage provinces lay not as far south as previously believed, but rather to the north, in the North Atlantic rift of the Norwegian Sea.

Acknowledgements The present paper is based on the author's Dr. Scient. dissertation at the Bergen University. Norsk Hydro ASA is thanked for financial support. I also wish to thank Ron Steel and Wojtek Nemec for their supervision and encouragement throughout the thesis work. Reviewers John Collinson and Ragnar Knarud provided many useful comments to the original manuscript. Jan Andsbjerg kindly supplied me with publications from the Geological Survey of Denmark and Greenland (GEUS).

References Ainsworth, N.R., O'Neill, M. and Rutherford, M.M., 1989. Jurassic and upper Triassic biostratigraphy of the North Celtic Sea and Fastnet Basins. In: D.J. Batten and M.C. Keen (Editors), Northwest European Micropalaeontology and Palynology. British Micropalaeontological Society Series, pp. 1-44. Badley, M.E., Price, J.D., Rambech Dahl, C. and Agdestein, T., 1988. The structural evolution of the northern Viking Graben, and its bearing upon extensional modes of basin formation. J. Geol. Soc., London, 145: 455-472. Batten, D.J., Trewin, N.H. and Tudhope, A.W., 1986. The TriassicJurassic junction at Golspie, inner Moray Firth Basin. Scott. J. Geol., 22: 85-98. Bertelsen, F., 1978. The Upper Triassic-Lower Jurassic Vinding and Gassum Formations of the Norwegian-Danish Basin. Geol. Surv. Denm. Ser., B3: 1-26. Besly, B.M. and Fielding, C.R., 1989. Palaeosols in Westphalian coal-bearing and red-bed sequences, central and northern England. Palaeogeogr., Palaeoclimatol., Palaeoecol., 70: 303-330. Blodgett, R.H., 1988. Calcareous paleosols in the Triassic Dolores Formation, southwestern Colorado. In: J. Reinhardt and W.R. Sigleo (Editors), Paleosols and Weathering Through Geologic Time: Principles and Applications. Geol. Soc. Am. Spec. Pap., 216: 103-121. Brekke, H., Dahlgren, S., Nyland, B. and Magnus, C., 1999. The prospectivity of the Vcring and MOre basins on the Norwegian continental margin. In: A.J. Fleet and S.A.R. Boldy (Editors), Petroleum Geology of Northwest Europe: Proceedings of the 5th Conference. Geological Society, London, pp. 117-131. Bridge, J.S., 1985. Paleochannel patterns inferred from alluvial deposits: a critical evaluation. J. Sediment. Petrol., 55: 579-589. Brierley, G.J. and Hickin, E.J., 1991. Channel planform as a non-controlling factor in fluvial sedimentology: the case of the Squamish River floodplain, British Columbia. Sediment. Geol., 75: 67-83. Cant, D.J. and Walker, R.G., 1978. Fluvial processes and facies sequences in the sandy braided South Saskatchewan River, Canada. Sedimentology, 25: 625-648. Charnock, M.A., Kristiansen, I.L., Ryseth, A. and Fenton, LEG., 2001. Sequence stratigraphy of the Lower Jurassic Dunlin Group, northern North Sea. In: O.J. Martinsen and T. Dreyer (Editors), Sedimentary Environments of the Norwegian Continental Shelf: Palaeozoic to Recent. Norwegian Petroleum Society (NPF), Special Publication 10. Elsevier, Amsterdam, pp. 145-174 (this volume).

A. Ryseth Coleman, J.M., 1969. Brahmaputra River: channel processes and sedimentation. Sediment. Geol., 3: 129-239. Dalland, A., Mearns, E.W. and McBride, J.J., 1995. The application of Samarium-Neodymium provenance ages to correlation of biostratigraphically barren data: a case study of the Statfjord Formation in the Gullfaks oilfield, Norwegian North Sea. In: R.E. Dunay and E.A. Hailwood (Editors), Non-biostratigraphical Methods of Dating and Correlation. Geol. Soc., London, Spec. Publ., 89:201-222. Dor6, A.G. and Gage, M.S., 1987. Crustal alignments and sedimentary domains in the evolution of the North Sea, Northeast Atlantic Margin and Barents Shelf. In: J. Brooks and K. Glennie (Editors), Petroleum Geology of North West Europe. Graham and Trotman, London, pp. 1131-1148. Dybkjam K., 1991. Palynological zonation and palynofacies investigation of the Fjerritslev Formation (Lower Jurassic-basal Middle Jurassic) in the Danish Sub-basin. Geol. Surv. Denm. Ser., A30: 1-150. Eide, F., 1989. Biostratigraphic correlation within the Triassic Lunde Formation in the Snorre area. In: J.D. Collinson (Editor), Correlation in Hydrocarbon Exploration. Norwegian Petroleum Society (NPF). Graham and Trotman, London, pp. 291-297. Ettensohn, F.R., Dever, G.R. Jr. and Grow, J.S., 1988. A palaeosol interpretation for profiles exhibiting subaerial exposure "crusts" from the Mississippian of the Appalachian Basin. In: J. Reinhardt and W.R. Sigleo (Editors), Paleosols and Weathering Through Geologic Time: Principles and Applications. Geol. Soc. Am., Spec. Pap., 216: 49-79. Fa~rseth, R.B., 1996. Interaction of Permo-Triassic and Jurassic extensional fault blocks during the development of the northern North Sea. J. Geol. Soc., London, 153:931-944. Goldsmith, RJ., Rich, B. and Standring, J., 1995. Triassic correlation and stratigraphy in the South Central Graben, UK North Sea. In: S.A.R. Boldy (Editor), Permian and Triassic Rifting in North West Europe. Geol. Soc., London, Spec. Publ., 91: 123-143. Graham, J.R., 1983. Analysis of the Upper Devonian Munster Basin, an example of a fluvial distributary system. In: J.D. Collinson and J. Lewin (Editors), Modern and Ancient Fluvial Systems. Int. Assoc. Sedimentol. Spec. Publ., 6: 473-483. Graue, E., Helland-Hansen, W., Johnsen, J.R., L~amo, L., Ncttvedt, A., Re~nning, K., Ryseth, A. and Steel, R., 1987. Advance and retreat of the Brent Delta system, Norwegian North Sea. In: J. Brooks and K. Glennie (Editors), Petroleum Geology of North West Europe. Geological Society, London, pp. 915-937. Hallam, A., 1988. A reevaluation of Jurassic eustasy in the light of new data and the revised Exxon curve. In: C.K. Wilgus, B.S. Hastings, C.G.St.C. Kendall, H.W. Posamentier, C.A. Ross and J.C. Van Wagoner (Editors), Sea-Level Changes: An Integrated Approach. Soc. Econ. Paleontol. Mineral., Spec. Publ., 42: 261273. Helland-Hansen, W., Ashton, M., L~amo, L. and Steel, R., 1992. Advance and retreat of the Brent delta: recent contributions to the depositional model. In: A.C. Morton, R.S. Haszeldine, M.R. Giles and S. Broom (Editors), Geology of the Brent Group. Geol. Soc., London, Spec. Publ., 61: 109-127. Johannessen, E.R, Mj~as, R., Renshaw, Dalland, A. and Jacobsen, T., 1995. Northern limit of the 'Brent delta' at the Tampen Spur - - a sequence stratigraphic approach for sandstone prediction. In: R.J. Steel, V. Felt, E.R Johannessen and C. Mathieu (Editors), Sequence Stratigraphy of the Northwest European Margin. Norw. Pet. Soc. Spec. Publ., 5: 213-256. Johnson, S.Y., 1984. Cyclic fluvial deposition in a rapidly subsiding basin, Northwest Washington. Sediment. Geol., 38: 361-391. Jones, B.G. and Rust, B.R., 1983. Massive sandstone facies in the Hawkesbury sandstone, a Triassic fluvial deposit near Sydney, Australia. J. Sediment. Petrol., 53: 1249-1259. Kirk, M., 1983. Bar development in a fluvial sandstone (Westphalian "A"), Scotland. Sedimentology, 30: 727-742. Knott, S.D., Burchell, M.T., Jolley, E.J. and Fraser, A.J., 1993. Mesozoic to Cenozoic plate reconstructions of the North Atlantic

Sedimentology and palaeogeography of the Statfjord Formation (Rhaetian-Sinemurian), North Sea and hydrocarbon plays of the Atlantic margin. In: J.R. Parker (Editor), Petroleum Geology of North West Europe: Proceedings of the 4th Conference. Geological Society, London, pp. 953-974. Lervik, K.S., Spencer, A.M. and Warrington, G., 1989. Outline of Triassic stratigraphy and structure in the central and northern North Sea. In: J.D. Collinson (Editor), Correlation in Hydrocarbon Exploration. Norwegian Petroleum Society (NPF). Graham and Trotman, London, pp. 173-189. MacCarthy, I.A.J., 1990. Alluvial sedimentation patterns in the Munster Basin, Ireland. Sedimentology, 37: 685-712. MacDonald, A. and Halland, E.K., 1993. Sedimentology and shale modelling of a sandstone-rich fluvial reservoir: upper Statfjord Formation, Statfjord Field, northern North Sea. Am. Assoc. Pet. Geol. Bull., 77: 1016-1040. Masson, A.G. and Rust, B.R., 1990. Alluvial plain sedimentation in the Pennsylvanian Sydney Mines Formation, eastern Sydney Basin, Nova Scotia. Bull. Can. Pet. Geol., 38: 89-105. McCabe, EJ., 1984. Depositional environments of coal and coalbearing strata. In: R.A. Rahmani and R.M. Flores (Editors), Sedimentology of Coal and Coal-bearing Sequences. Int. Assoc. Sedimentol. Spec. Publ., 7: 13-42. Mearns, E.W., Knarud, R., Ra~stad, N., Stanley, K.O. and Stockbridge, C.R, 1989. Samarium-Neodymium isotope stratigraphy of the Lunde and Statfjord Formations of Snorre Oil Field, northern North Sea. J. Geol. Soc., London, 146:217-228. Miall, A.D., 1992. Alluvial sediments. In: R.G. Walker and N.R James (Editors), Facies Models: Response to Sea Level Change. Geological Association of Canada, Toronto, pp. 119-142. Michelsen, O., 1989. Log-sequence analysis and environmental aspects of the Lower Jurassic Fjerritslev Formation in the Danish Sub-basin. Geol. Surv. Denm. Set., A25: 1-23. Morton, N., 1989. Jurassic sequence stratigraphy in the Hebrides Basin, NW Scotland. Mar. Pet. Geol., 6: 243-260. Morton, N., Smith, R.M., Golden, M. and James, A.V., 1987. Comparative stratigraphic study of Triassic-Jurassic sedimentation and basin evolution in the northern North Sea and northwest of the British Isles. In: J. Brooks and K. Glennie (Editors), Petroleum Geology of North West Europe. Geological Society, London, pp. 697-709. Morton, A.C., Claoue-Long, J. and Berge, C., 1996. SHRIMP constraints on sediment provenance and transport history in the Mesozoic Statfjord Formation, North Sea. J. Geol. Soc., London, 153: 915-929. Mossop, G.D. and Flach, RD., 1983. Deep channel sedimentation in the Lower Cretaceous McMurray Formation, Athabasca Oil Sands, Alberta. Sedimentology, 30: 493-509. Nielsen, L.H., Larsen, F. and Frandsen, N., 1989. Upper TriassicLower Jurassic tidal deposits of the Gassum Formation on SjaJland, Denmark. Geol. Surv. Denm. Set., A23: 1-30. Nystuen, J.E, Knarud, R., Jorde, K. and Stanley, K.O., 1989. Correlation of Triassic to lower Jurassic sequences, Snorre Field and adjacent areas, northern North Sea. In: J.D. Collinson (Editor),

A. RYSETH

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Correlation in Hydrocarbon Exploration. Norwegian Petroleum Society (NPF). Graham and Trotman, London, pp. 273-289. Partington, M.A., Copestake, E, Mitchener, B.C. and Underhill, J.R., 1993. Biostratigraphic calibration of genetic sequences in the Jurassic-lowermost Cretaceous (Hettangian-Ryazanian) of the North Sea and adjacent areas. In: J.R. Parker (Editor), Petroleum Geology of North West Europe: Proceedings of the 4th Conference. Geological Society, London, pp. 371-386. Raunsgaard Pedersen, K. and Lund, J.J., 1980. Palynology of the plant-bearing Rhaetian to Hettangian Kap Stewart Formation, Scoresby Sund, East Greenland. Rev. Palaeobot. Palynol., 31: 1-69.

R~e, S.-L. and Steel, R., 1985. Sedimentation, sea-level rise and tectonics at the Triassic-Jurassic boundary (Statfjord Formation), Tampen Spur, northern North Sea. J. Pet. Geol., 8: 163-186. Rust, B.R., 1978. Depositional models for braided alluvium. In: A.D. Miall (Editor), Fluvial Sedimentology. Can. Soc. Pet. Geol., Mem., 5: 605-625. Ryseth, A. and Ramm, M., 1996. Alluvial architecture and differential subsidence in the Statfjord Formation, North Sea: prediction of reservoir potential. Pet. Geosci., 2:271-287. Steel, R.J., 1993. Triassic-Jurassic megasequence stratigraphy in the Northern North Sea: rift to post-rift evolution. In: J.R. Parker (Editor), Petroleum Geology of North West Europe: Proceedings of the 4th Conference. Geological Society, London, pp. 299-315. Steel, R. and Ryseth, A., 1990. The Triassic-Early Jurassic succession in the northern North Sea: megasequence stratigraphy and intra-Triassic tectonics. In: R.F.E Hardman and J. Brooks (Editors), Tectonic Events Responsible for Britain's Oil and Gas Reserves. Geol. Soc., London, Spec. Publ., 55: 139-168. Stephen, EJ., Underhill, J.R., Partington, M.A. and Hedley, R.J., 1993. The genetic sequence stratigraphy of the Hettangian to Oxfordian succession, Inner Moray Firth. In: J.R. Parker (Editor), Petroleum Geology of North West Europe: Proceedings of the 4th Conference. Geological Society, London, pp. 485-505. Stewart, D.J., 1981. A meander-belt sandstone of the Lower Cretaceous of Southern England. Sedimentology, 28: 1-20. Underhill, J.R. and Partington, M.A., 1993. Jurassic thermal doming and deflation in the North Sea: Implication of the sequence stratigraphic evidence. In: J.R. Parker (Editor), Petroleum Geology of North West Europe: Proceedings of the 4th Conference. Geological Society, London, pp. 337-345. Vollset, J. and Dote, A.G., 1984. A revised Triassic and Jurassic lithostratigraphic nomenclature for the Norwegian North Sea. Norw. Pet. Direct. Bull., 3: 1-53. Yielding, G., Badley, M.E. and Roberts, G., 1992. The structural evolution of the Brent Province. In: A.C. Morton, R.S. Haszeldine, M.R. Giles and S. Brown (Editors), Geology of the Brent Group. Geol. Soc., London, Spec. Publ., 61: 27-43. Ziegler, EA., 1990. Geological Atlas of Western and Central Europe. 2nd ed., Shell Internationale Petroleum Maatschappij B.V., The Hague, 239 pp.

Norsk Hydro Research Centre, P.O. Box 7190, N-5020 Bergen, Norway Present address: Norsk Hydro Harstad, Storakern 11, Kanebogen, N-9401 Harstad, Norway


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Sedimentary facies in the fluvial-dominated Are Formation as seen in the Are 1 member in the Heidrun Field Knud Egil Svela

The ,~re 1 member in the Heidrun Field was deposited in a fluvial to deltaic setting where variations in accommodation space and sediment supply controlled the different sedimentary facies and reservoir properties. It shows an overall transgressive sequence from fluvial plain in the lower part to lower delta plain in the upper part. Fluvial channels and thick coal-bearing floodplain deposits dominate the lower part of the Are 1. Stacked (multi-storey) channels were deposited as a response to relative fall in sea level. These sands have large lateral continuity compared to the meandering (single-storey) channel sands that are more a function of autocyclic processes and are not correlatable between wells. The upper part of the Are 1 member is dominated by stacked bay/lake fill sequences and thin, but laterally continuous coals. These represent deposition in a lower delta plain setting where the dominant depositional agents were crevasse channels and splays (crevasse deltas) with subsequent wave reworking in places. The reservoir properties of the Are 1 member are strongly controlled by depositional processes and the sequence stratigraphic framework. Incised valley fill deposits (LST) have large lateral continuity and the internal reservoir properties are very good. The TST deposits are more dominated by thick, coal-bearing, floodplain deposits with no reservoir quality, thin crevasse splays with restricted lateral continuity and limited reservoir properties and single-storey channel sands with very good internal reservoir properties but limited lateral continuity. The individual parasequences in the stacked bay fill sequences (HST) have good lateral continuity but reservoir properties are limited, and the bases of individual bay fill sequences are vertical permeability barriers, at least on a local scale.

Introduction

The Heidrun Field is located offshore Mid-Norway (Fig. 1), and was discovered by Conoco in 1985. The field is now operated by Statoil and the current estimate of total STOIIP is 2733 mmbo, of which 465.5 mmbo is within the Are 1 member. Estimated total recoverable oil reserves are 1132 mmbo, of which 125.7 mmbo comes from the Are 1. This gives an estimated recovery factor of 27% from the Are 1 member. To date, there are no production data from the Are 1 member. The Are Formation overlies the Triassic Grey Beds and comprises a succession of sandstones, mudstones and coals of Rhaetian to early Pliensbachian age. In the Heidrun Field, the top Are Fro. (base Tilje Fro.) is defined by the first full marine flooding as seen from biostratigraphy and is clearly reflected by a gamma-ray peak on wireline logs. This definition is slightly different from the standard lithostratigraphic definition as described by Dalland et al. (1988), with the implication that the upper part of the Are Fm. in Heidrun is time-equivalent with the lower part of the Tilje Fm. elsewhere on the Halten Terrace. The Are Fro. has been informally subdivided into two members in the Heidrun Field; Are 1 (base) and

Are 2 (top) (Fig. 2). The top of coal-bearing strata approximately corresponds to the top of the Are 1 member. B iostratigraphic data show that the entire Are 1 member was deposited in a non-marine setting. The Are 1 member has a maximum observed vertical thickness of 486 m in well 6507/7-2, which is the only well in the Heidrun Field that has penetrated a complete Are Formation. Only two wells, 6507/7-6 and 6507/7-A38, have been cored in the ,~re 1 member in the Heidrun Field. Of these, well 6507/7-A-38 has the best coverage, and a summary of the core description is presented in Fig. 3. Detailed examples of the main facies associations are presented in Fig. 4. The main focus of this paper is a description and sedimentological interpretation of the Are 1 member as seen in the Heidrun Field. A sequence stratigraphic interpretation based on Exxon terminology (Van Wagoner et al., 1990) is also proposed. Due to the limited study area, it is difficult to conclude firmly on basinal effects on the sequence stratigraphic framework. Gjelberg et al. (1987) presented a regional interpretation of the Are Fro. (then the Hitra Fro.) and concluded that the Are Fro. had an overall transgressive nature. This study agrees with this interpretation of the overall vertical change in depositional environment, but a different interpretation of

Sedimentary Environments Offshore N o r w a y - Palaeozoic to Recent edited by O.J. Martinsen and T. Dreyer. NPF Special Publication 10, pp. 87-102, Published by Elsevier Science B.V., Amsterdam. 9 Norwegian Petroleum Society (NPF), 2001.

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Fig. 1. Heidrun Field location map. Hydrocarbon area, i.e. oil and gas distribution is composite for all reservoir formations and not specific for the Are Fm. (After Statoil).

the fluvial systems is presented. Finally, a short discussion of reservoir properties in the different facies associations is presented.

facies, can occur in more than one association (e.g. crevassing).

Stacked (multi-storey) fluvial channels Sedimentary facies associations In this study, individual lithofacies have been grouped together in facies associations that represent depositional environments. It is also attempted to define facies associations that represent reservoir (or seal) units that are of sufficient thickness that they can be recognized on wireline logs. This means that individual sedimentary processes, and thereby litho-

Description This facies association has an erosive lower boundary, and comprises thick units of dominantly cross-bedded, medium-grained sand (Figs. 4 and 5). The thicknesses of these units range from 7 to 18 m. Except for just above the erosive base, where beds with granules are seen in a few occasions (Fig. 5), there is little vertical variation in grain size. The

Sedimentary facies in the fluvial-dominated fire Formation as seen in the fire 1 member in the Heidrun Field

89

Fig. 2. Wireline logs from well 6507/7-A-38 that show the boundary between the Tilje Fm. and Are Fm. as defined in the Heidrun Field.

lower parts of units often contain large coal clasts and smaller coal fragments are seen on foresets. The sands generally display sets of large-scale tabular cross-stratification, both tangential and angular. In places, small current ripples on toe-sets can be seen. Trough cross-bedding and current ripples are present in places, generally without any preferred vertical arrangement. Individual cross-sets are normally 20 to 50 cm thick, but are found up to 90 cm thick and occur in up to 4 m thick cosets. As far as it is possible to determine from a non-oriented core, palaeocurrent direction seems fairly constant. Set boundaries are normally erosional within cosets. The upper boundary to overlying rooted horizons and coal is commonly abrupt.

Interpretation The erosive lower boundaries, associated coal intraclasts and pebbles together with the internal vertical

arrangement of facies suggest that this facies association represents the in-channel or coarse member of fluvial channels. Tidal channels can also develop sequences which resemble those of fluvial channels (Oomkens, 1974; Barwis, 1978; Weimer et al., 1982). However, the lack of clay drapes and other tidal indicators make such an origin unlikely in the Are 1. The biostratigraphic data together with the overall setting with erosional surfaces into fluvial plain/delta plain associations and the abundance of coal intraclasts also suggest a fluvial origin. In this facies association, the dominance of largescale cross-bedding and the uniformity of grain size throughout the units make it likely that deposition took place by vertical accretion rather than lateral accretion (point bars) in meandering channels. The significant increase in grain size compared to the sands in the other facies shows a considerable increase in hydraulic regime. Together with the fact

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Fig. 3. Summary of core description and interpretation from well 6507/7-A-38.

that these channels can be correlated between wells, they have been interpreted to represent low-sinuosity, braided channels instead of ribbon-like, anastomosing channels (Collinson, 1986). The presence of coal intraclasts shows that part of the eroded topstratum included swamp deposits (peat). However, the fact that the channel sands often sit on top of coal layers also shows the resistive nature of peat to erosion. In exposures, like the Breathitt Group in Kentucky, channels are also often seen to erode down to coal layers without cutting through them. Because peat has such a strong resistive nature, it often controls the degree of incision such that erosion tends to take place laterally, a process that extends the lateral dimensions of these channel fill sequences. As it is difficult to observe a correlatable interfluvial sequence from the limited well data in the Heidrun Field, and because it has not been possible to see channel geometries on 3D seismic, it is difficult to use the term "incised valley fill". However, the indications of braided type deposition together with the correlatable nature of these sands in the Heidrun Field and the abrupt change from the underlying fine-grained floodplain deposits make it likely

that this facies association represents deposition following a relative fall in sea level. Such lowstand channel deposits generally have large lateral extent. Similar channels in the Breathitt Group in Kentucky, USA, are seen to have kilometres of lateral continuity (Aitken and Flint, 1995). This is also proposed to be the case for these channels in the Heidrun area, where some units are correlatable in all wells drilled to date (Fig. 6). Similar multi-storey fluvial channels have also been described in many studies of the Carboniferous in the UK and Germany. A summary of their character and recognition is presented by Hampson et al. (1999).

Meandering (single-storey) fluvial channels Description This facies association has an erosive lower boundary and comprises cross-bedded and current-rippled sand (Figs. 4 and 7). The thickness of these units ranges from 6.2 to 9.0 m. They display a clear fining-upward trend from medium- to fine-grained sand at the base, grading upwards into very fine sand and silt in the upper part (Fig. 4). This facies association has a gradual boundary with the overlying sediments,

Sedimentary facies in the fluvial-dominated Are Formation as seen in the Are 1 member in the Heidrun Field

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Fig. 4. Detailed core descriptions of the main facies associations in the Are 1 member from well 6507/7-A-38. The depths for these intervals within the Are Fm. succession are shown on Fig. 11.

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Fig. 5. Core photo from stacked (multi-storey) channels in 6507/7-A-38 showing: (A) erosional channel base; (B) granule size channel lag; (C) tabular cross-bedding; (D) fine-grained sand; (E) coal clasts. See Figs. 4 and 11 for location of photo.

which normally comprises rooted floodplain deposits. Internally, the sands display mostly trough cross-bedding. The upper parts of the sand bodies show current ripples and lenticular bedding. Coal intraclasts are abundant at the bases of units.

Interpretation The erosive lower boundary fining-upward trend and the internal vertical arrangement of facies suggest that this facies association represents deposition by lateral accretion in high-sinuosity, meandering fluvial channels. In contrast to the stacked (multi-storey) fluvial channels, this facies association shows a clear vertical decrease in grain size and change in sedimentary structures upwards to current ripples. This indicates decreasing current strength, something that is seen in meandering channels where lateral accretion develops point bars (Collinson, 1986). The average grain size is also finer than in the stacked (multistorey) fluvial channels. Diagnostic features, such as

lateral accretion surfaces, are however impossible to identify in core. The lateral extent of these channels is much more limited than the stacked (multi-storey) fluvial channels. From wireline logs, these single-storey channels are seldom seen to be correlatable between wells. Similar channels in the Breathitt Group in Kentucky, USA, are seen to have a wide range in lateral continuity, some of them down to 10s of metres (Fig. 8). Heterolithic channel fills with very limited lateral extent, as frequently seen in the Breathitt Group, are however not recognized in any of the cores from the Are 1 member. Gjelberg et al. (1987) interpreted the fluvial channels in the Are Fm. to be anastomosing. The channels seen in cores in the Heidrun Field contradicts this interpretation. Anastomosing channels are characterized by extremely stable channel positions (low lateral mobility) and vertical accretion in areas of very low downstream slopes (Collinson, 1986).

Sedimentary facies in the fluvial-dominated Are Formation as seen in the Are 1 member in the Heidrun Field

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Fig. 6. Stratigraphic correlation of the Are Fm. in the Heidrun Field. Note that the lower part of Are 1A has a high density of meandering channels. This probably reflects lower rate of accommodation space generation, but still within the TST. Porosity plug data show cored intervals. See Fig. 1 for location of wells.

Minor (crevasse) channels Description This facies association has a fining-upward nature with an erosive base and grades upward from fine to very fine sand into silt (Fig. 4). These units have a thickness range from 2.0 to 4.0 m. Sands displays mostly current ripples, often climbing, and smallscale cross-bedding is sometimes seen toward the base of units. Bed boundaries are sometimes sharp, occasionally erosive and minor soft-sediment deformation is seen in places. Mud drapes between sand sets are also frequently seen. The tops of the units are both gradational and sharp.

Interpretation As for the previous facies associations, this facies has been deposited by erosive, channelized processes, although the transition to crevasse splays may be gradual and difficult to determine. The thickness of the units, fining-upward trend and presence of largescale cross-bedding make it reasonable to interpret these features as crevasse channels. On the other hand, the often thin, lateral margin of meandering (single-storey) fluvial channels could be misinterpreted as crevasse channels. Crevasse channels emanate from a relatively prominent break in the levee of the main channel during flood

events. Due to levee development and vertical aggradation of the main channel, the channel often becomes elevated above the surrounding areas (Elliott, 1974). This produces a gradient difference and thereby a tendency for crevasse channels to develop. These channels probably fed crevasse splays and crevasse deltas (Fielding, 1984, 1986). Flow through such a crevasse channel is not always continuous. After the flood, the channel may be abandoned due to lowering of the water level in the main channel (Elliott, 1974). Crevasse channels are often only active during succeeding flood events, thus leading to several reactivation surfaces and fine-sediment drapes. These crevasse channels are often of very limited lateral extent and become less confined and more like crevasse splays downcurrent. The Breathitt Group shows numerous examples where metre-thick crevasse channels have a width of only a few metres. Crevasse channels in the Heidrun Field are found both within thick floodplain deposits and sometimes in the upper part of the bay fill sequences.

Crevasse splay complexes Description This facies association comprises sequences with stacked beds of very fine- to fine-grained sandstone. Individual beds are from a few cm to 60 cm thick.

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Fig. 7. Core photo from fining-upward meandering (single-storey) channel in 6507/7-A-38 showing: (A) large-scale trough cross-bedding; (B) current ripple lamination. See Figs. 4 and 11 for location of photo.

Fig. 8. Photo showing how meandering (single-storey) fluvial channels may have very limited lateral continuity. Note lateral accretion surfaces. Example from the Breathitt Group, Kentucky, USA.

Complexes of stacked beds, showing mainly current ripple lamination, reach thicknesses of up to 9.7 m (Fig. 3). Beds of fine-grained sand showing

cross-bedding are occasionally seen. Climbing ripples and evidence for decreasing current strength (horizontally laminated sandstone that grades into

Sedimentary facies in the fluvial-dominated f~re Formation as seen in the f~re 1 member in the Heidrun Field

current ripples) are frequently seen. Individual beds have sharp contacts, often defined by thin clay and coal fragment layers. The top of these units is often rooted and the facies association is always found within units of rooted floodplain deposits.

Interpretation The sharp-bedded nature of beds together with the dominance of current ripples, frequent climbing ripples and indications of decreasing current strength, indicate that this facies was deposited by crevasse splays. The close association with fine-grained floodplain deposits and the thin individual beds support this. Individual sandstone beds are probably the result of a single crevasse splay episode. These are the product of discrete incursions of sediment-laden waters onto the floodplain after a breach in the levee of the main distributary channel (Elliott, 1974). In the proximal part, close to the main channel, this facies often grades laterally into crevasse channels. Crevasse splays from modern fluvial and delta plains are known to be erosive and channelized in their proximal part, becoming less confined and less erosive as they splay out downcurrent (Fielding, 1984). The stacked nature of beds in this facies association suggests proximity to a nearby distributary, although the non-erosive nature of many of the individual crevasse splay beds indicates some distance from the distributary. Many of the sequences show several crevasse splay sandstones interbedded with thin mud and silts layers deposited by overbank flooding, thus showing that current activity was not continuous, but occurred as several distinct events. This might indicate that the crevasse splays are the product of major flood events, while minor, maybe more "normal" flood events resulted in overbank flooding and deposition of mud and silt over large parts of the floodplain. Aitken and Flint (1996) described a stacked crevasse splay complex in an interfluve setting that clearly correlates with incised valley fill sandstones in the Breathitt Group. This crevasse splay complex is interpreted to have been deposited by overspill of the incised valley margins during major flood events. Such an interpretation is not proposed for the Are Fm. as there are no indicators that the crevasse complexes correlate to incised valley fill sandstones. Thin, single-bed crevasse splays are also found in the floodplain facies association, but here they are thin and often strongly rooted and have no reservoir potential. Crevasse splays and thin crevasse channels also occur in the upper part of bay/lake fill sequences (see below).

95

Floodplain (overbank fines) Description This facies association comprises up to 10 m thick, intensely rooted units of claystone, siltstone, carbonaceous shale and coal (Fig. 9). Individual coals are up to several metres thick and are underlain by rooted horizons. The fine-grained, rooted horizons (palaeosols) are dark grey and often have a high organic content. Only minor horizontally laminated dark grey claystones are preserved in places. Thin, very fine sand and silt beds with sharp boundaries and some sedimentary structures, mostly current ripples, are occasionally seen.

Interpretation The fine-grained nature and strong abundance of strongly rooted sections (palaeosols) and in-situ coals show that this facies association represents deposition mainly by overbank flooding. The most likely environment was an intensely vegetated, swampy inter-channel area with shallow lakes and occasional crevasse splays on a fluvial/delta plain (Coleman and Prior, 1982; Fielding, 1984; Elliott, 1986). The hydromorphic nature (after Besley and Fielding's 1989 classification) of the palaeosols and the presence of numerous and thick coal horizons, shows that deposition took place under reducing conditions, most likely in an area of low topographic relief with poor drainage and a constantly high and rising, reducing water table (Duchaufour, 1982; Retallack, 1983; Besley and Fielding, 1989). Some of the palaeosols with high organic content can be classified as humic gley's after Duchaufour's (1982) classification. The total lack of desiccation cracks and red (oxidized) sediments indicates that the site was submerged, without significant lowering of the water table. Mature soil profiles that show oxidation and concretions suggesting well-drained conditions are often regarded good indicators for interfluves, i.e. areas between fluvial channels during a lowering of base level. However, better-drained palaeosols may have been overprinted by a later rise in ground water level during transgression, thereby giving such soils a hydromorphic imprint (Aitken and Flint, 1996). This can make it difficult to identify interfluves in cores, at least from macroscopic descriptions. More detailed geochemical analysis can potentially detect two-stage palaeosol developments, and thereby interfluvial sequence boundaries like those described by Gardner et al. (1988) in the Breathitt Group (Aitken and Flint, 1996). The thick intervals of organic-rich, rooted sediments imply vertical accretion strata characterized by continuous vegetation. Sediments brought into the vegetated areas of the fluvial/delta plain by over-

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Fig. 9. Core photo from floodplain deposits (overbank fines) in 6507/7-A-38 showing: (A) organic-rich (carbonaceous) shale with rootlets and thin silt layers; (B) thin crevasse splay sands; (C) roots; (D) in-situ coal. See Figs. 4 and 11 for location of photo.

bank flooding were not sufficiently thick to terminate vegetation growth. More than 1 m of sediment from catastrophic flood events will normally terminate vegetation (Retallack, 1983). Normal floods do not deposit more than a few centimetres of sediment by overbank flooding (Collinson, 1986), and these sediments will easily be incorporated into the pre-existing soil (Retallack, 1983). The transition into coals clearly shows that clastic input ceased for longer periods, thus allowing highquality peat to be deposited (McCabe, 1984). This was probably due to removal of the clastic sources (McCabe, 1984). Significant volumes of peat can only be preserved to form coal when the overall increase in accommodation space approximately equals the accumulation rate of peat (Bohacs and Suter, 1997). Intense vegetation could also decelerate the flow during floods, causing the clastic sediments to be deposited at the margins of the swamp, allowing high-quality peat to be deposited in central parts (Staub and Cohen, 1979; Collinson, 1986).

The limited thickness of some of the coal layers makes an origin in low-lying backswamps, relatively close to distributaries, likely (Gersib and McCabe, 1981). This is supported by the fact that several "sheet sandstones", which are thought to be crevasse splays, interrupt the section, thus strongly suggesting the presence of a nearby distributary. The sharp, but non-erosive bases of these crevasse sandstones indicate deposition on distal parts of splay lobes. The presence of thin organic-rich, but still welllaminated mudstones shows that deposition also took place in a shallow anoxic lake environment (Coleman and Prior, 1982; Fielding, 1984). The very finegrained nature of these lake deposits was probably due to intense vegetation surrounding the shallow lake, which protected the lake from coarser clastic input. The lakes were slowly filled by fine-grained sediments, and became the site for re-establishment of vegetation. It is difficult to distinguish upper and lower delta plain from facies alone, but this facies association

Sedimentary facies in the fluvial-dominated Are Formation as seen in the ,3.re 1 member in the Heidrun Field

probably represents deposition in inter-channel areas of the upper to middle delta plain and fluvial plain. In modern deltaic environments the boundary between lower and upper delta plain is defined as the limit of marine influence (Coleman and Prior, 1982). The total lack of marine fauna could thereby indicate upper delta plain and fluvial plain. However, this could not be used as a criterion if the water in the receiving basin was not fully marine. The limited thickness of lake deposits also suggests an upper delta plain environment. Relative thick lake and bay fill sequences are more common on the lower delta plain (Elliott, 1986), while lakes of the upper delta plain are commonly extremely shallow (Coleman and Prior, 1982).

Bay~lake fill Description This facies association shows overall coarsening (shallowing) upward sequences with a lower muddy unit and an upper sandy unit (Figs. 4 and 10). It often

97

has rootlets and coal at the top. Thicknesses range from 1.1 to 5.4 m. Horizontally laminated silty claystone in the lower parts often grades upward into lenticular-laminated sandy siltstone and then to very fineand fine-grained sandstone. Sandstone layers in the uppermost part sometimes have erosional and sharp bed boundaries. Internally, sand beds display current ripples that are sometimes climbing and show indication of decreasing current strength. Coal fragments are abundant in this facies association. Towards the upper part of Are 1, both claystones and sandstones sometimes comprise small sub-vertical and horizontal burrows. Increasing wave reworking and sometimes hummocky cross-bedding is also seen towards the upper part of Are 1 (Fig. 4). Different sequences display great variations in abundance and thickness of individual lithofacies. The number and thickness of sandstone beds in each sequence also vary greatly. Some sequences show several thin beds interbedded with mudstones, while others show only a single, thick sandstone layer. This facies association is generally

Fig. 10. Core photo from a full shoaling (coarsening) upward bay/lake fill sequence in 6507/7-A-38 showing: (A) finely laminated organic-rich (carbonaceous) shale; (B) crevasse splay sands; (C) roots; (D) in-situ coal. See Figs. 4 and 11 for location of photo.

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found in stacked aggradational successions of up to 40 m thick in the upper part of the Are 1 section.

Interpretation This facies association is thought to represent sediments deposited in an inter-distributary bay or lake environment on the lower delta plain. Inter-distributary areas form the major land areas on the lower delta plain (Coleman and Prior, 1982). Sediments are derived mainly from nearby fluvial channels by various flood-generated processes. Some sediments may have been transported into open bays by wave action or littoral drift from the distributary mouth area (Elliott, 1974, 1986). The laminated dark and organic-rich mudstones that are sometimes found at the base of sequences suggest very slow rates of sedimentation. These are interpreted as anoxic bay/lake floor deposits, and represent deposition in an early phase of bay/lake development through drowning of peat swamps due to subsidence or rise in sea level. More silty and laminated grey mudstones, which are often found in lower parts of the sequences, are deposited from overbank flooding in areas beyond the influence of significant traction current activity. No breaching of the channel margin or levee, i.e. crevassing, was necessarily involved in this process (Coleman et al., 1964). The sands generally found in the upper parts of the sequences are interpreted to be the products of crevassing caused by breaching of the levee during flood events. Both crevasse splays and sometimes thin crevasse channels are seen. These sands have often led to complete filling of the bay, resulting in the establishment of intense vegetation as seen from the rooted horizons and overlying coals. The relatively thin palaeosols and coals (compared with those found in the floodplain facies association) indicate little or no sediment aggradation after complete infill of the bay. The overlying laminated black mudstones, seen in some sequences, show that peat production was terminated by drowning of the swamp due to subsidence or rise in sea level. Sequences with more wave ripples and hummocky cross-bedding represent more open bays. This is supported by the fact that these sequences show some bioturbation and a more brackish water fauna. This continues into the overlying Are 2 where tidal influence is also seen. Kj~erefjord (1999) describes these bay fill sequences in more detail.

Sequence stratigraphic development The interpretation of system tracts in this study follows the same principles as used by Aitken and

Flint (1995) in the Breathitt Group, Kentucky, USA, and is based on Exxon terminology (Van Wagoner et al., 1990). Lowstand systems tract (LST) deposits are identified by the presence of erosional surfaces and stacked channel fills. Thick intervals of floodplain deposits (overbank fines) and thick correlatable coals with single-storey channels have been interpreted as transgressive systems tracts (TST). Thick units of stacked bay/lake fill sequences of mainly aggradational nature overlying TST are believed to be parasequences and are interpreted to be highstand systems tracts (HST). It is difficult to define the maximum flooding surface (MFS) in these alluvial deposits, but it could be defined at a change in parasequence stacking pattern from retrogradational to aggradational (Van Wagoner et al., 1990). No attempt has been made to classify sequences according to "third order", "fourth order", etc., but it seems clear that "smaller" sea-level fluctuations are superimposed on "larger" changes in relative sea level. The overall Are Fm. succession grading into the Tilje Fm. is clearly transgressive (large scale), as seen from the vertical change in sedimentary facies and fauna. This interpretation was also presented by Gjelberg et al. (1987). Several relative falls in sea level (reduction in accommodation space) are superimposed on this larger-scale cycle, resulting in the deposition of low-sinuosity (braided) channels. To establish a more precise picture of what kind of sequences this represents, more detailed biostratigraphic control (time control) and more regional data are needed. Although it can be difficult to identify sequence boundaries in predominantly alluvial strata, several sequence boundaries have been interpreted in the Are 1 member. The criteria used in this study follow those summarized by Hampson et al. (1999). Flooding surfaces and maximum flooding surfaces are also difficult to interpret in alluvial deposits. However, the transgressive flooding surfaces probably coincide with the thick, laterally continuous coal beds capping the incised valley fills (Figs. 6 and 11) (Aitken and Flint, 1995; Flint et al., 1995; Hampson et al., 1999). The main criterion used to identify fluvial sequence boundaries is the occurrence of laterally consistent, abrupt vertical change from fine-grained floodplain deposits into channel sands of a low-sinuosity "braided" nature. Such lateral correlatable change in fluvial style and thereby hydraulic character indicates a relative fall in sea level. The stacked, multi-storey channels in the * r e Fm. are not seen to be incised into marine deposits. As such, facies tract dislocations are not observed. Although the stacked multi-storey channel units represent a relative fall in sea level, and sit on top

Sedimentary facies in the fluvial-dominated Are Formation as seen in the ,3,re 1 member in the Heidrun Field

99

Fig. 11. Summary log of well 6507/7-A-38. Note the different vertical change in core plug permeability between the meandering channels and the stacked channels (incised valley fills). The trends are highlighted by blue lines. Also note the aggradational nature and slight upward increase in sandiness in the stacked bay fill sequences. This stacking pattern is the reason for interpreting this section as HST. The wide scatter in permeabilities in this zone clearly reflects the heterolithic nature of this facies association.

of sequence boundaries, they cannot automatically be classified as incised valley fills if lateral correlatable interfluves are not identified. Numerous multi-storey fluvial sandstone bodies have been described in the Upper Carboniferous in the UK and Germany (Hampson et al., 1999). Many of them have been proven to have lateral correlation with an interfluve surface, and therefore represent incised valley fills with widths of 5-25 km (Hampson et al., 1999). Others have much more sheet-like geometry and exceed 35-70 km in

width. In some cases these represent laterally amalgamated incised valley fills. Others may have a non-valley origin, but still occur above sequence boundaries and may represent fluvial braidplains formed as a result of a relative sea-level fall (Hampson et al., 1999). The limited number of well penetrations, and even more limited core coverage, make it difficult to identify correlatable interfluves in the Heidrun Field area. However, within the Are 1A reservoir zone in well

1 O0

6507/7-A-38 (Figs. 6 and 11), a 32 m thick multistorey channel sandstone has all the sedimentological characteristics of an incised valley fill (Fig. 4). A thick coal that probably represents the associated initial flooding surface overlies it. This sandstone unit is not seen to be present in any of the few other wells that have penetrated the Are 1A (Fig. 6). As such, this sandstone unit most likely represents an incised valley fill, and the associated interfluve should be found in the uncored sections in the other wells (Fig. 6). A strong seismic reflector, named Coal Marker 1, is mapped all over the Heidrun Field. This reflector is closely associated with the interval where this initial flooding surface is identified (Fig. 6). A seismic reflector like this probably does not represent reflection from one single coal bed. It is more likely that it corresponds to a unit with several thick coal layers representing the early part of the TST where the rate of peat production balanced the rate of accommodation space generation (Bohacs and Suter, 1997). The overlying TST unit, dominated by fine-grained floodplain deposits with less coal, probably reflects that the rate of accommodation space generation exceeded the rate of peat production. Assuming that this strong, correlatable seismic event represents a time line, it gives good aid to the stratigraphic correlation in these alluvial deposits with otherwise poor biostratigraphic control. The basal sandstone in ,~re 1B reservoir zone is seen in all wells drilled to date in the Heidrun Field (Fig. 6). It also represents a relative fall in sea level, but no potential interfluve has been identified within the limits of the field. However, since this unit is not found in exploration well 6507/8-2 located approximately 5 km to the east (Fig. 1), an incised valley fill interpretation is proposed rather than a multi-storey, multilateral braided system similar to those described by Hampson et al. (1999). On the other hand, the meandering (single-storey) distributary channels, identified both in cores and from wireline logs, are clearly not correlatable between wells (Fig. 6) and deposition is most likely due to autocyclic processes during TST rather than changes in relative sea level. The whole of the .~re 1A zone and in the lower part of the Are 1B zone (subzones .~IB 1-A1B5) are dominated by fluvial channels and thick floodplain deposits with no marine indicators (Figs. 6 and 11). This is interpreted to represent a fluvial plain to upper delta plain setting where relative changes in sea level have produced several sequences containing incised valley fills deposited during LST and thick fine-grained floodplain deposits with thick coals and single-storey channels during TST. The upper part of the .~re 1B zone (subzones ,~IB6-,~IB 11) is dominated by stacked bay/lake fill

K.E. Svela

sequences and thin but laterally continuous coals. This represents a lower delta plain setting where fluctuations in sea level caused the development of several parasequences during a HST. It is difficult to identify a maximum flooding surface in these deposits and thereby to define the exact boundary between the TST and HST of the .3,re 1B unit. However, the boundary has been placed at the top of reservoir zone ,~e 1 B7 (Fig. 11) where there is a change in parasequence stacking pattern from slightly retrogradational to aggradational (Van Wagoner et al., 1990). Reservoir characteristics

Reservoir properties in the Are Fm. are controlled at two levels. The first is the lateral connectivity of the reservoir units and the second is the internal character of the sandstones. In the Heidrun Field, depth of burial is limited and diagenesis has not significantly altered reservoir properties (Olsen et al., 1999). Internal reservoir properties are therefore governed primarily by original permeability, which in turn is a function of pore throat radius. In sedimentological terms, this is basically a function of grain size and sorting. As a consequence, variations in porosity and permeability are mainly a function of depositional processes. In addition, reservoir performance is also influenced by the fact that (1) the sandstones are unconsolidated, (2) the Heidrun Field is normally pressured, (3) the ,~e oil is viscous with an API gravity of 22 ~ and (4) the field is strongly segmented by faults. The multi-storey fluvial channels (incised valley fills) have superb reservoir quality with few internal barriers and little internal variation in permeability (Figs. 11 and 12). These LST sand deposits also have large lateral continuity and where present in the hydrocarbon column, they should be easy to target with wells. Large lateral continuity will also make it easier to provide pressure support and sweep by water injection. The meandering (single-storey) distributary channels deposited during TST also have very good internal reservoir quality, although an upward reduction in permeability reflects the fining-upward trend in grain size (Fig. 11). The lateral extent of these fluvial sand bodies is more restricted (Figs. 6 and 8) and it will be difficult to target specific channels. This is also reflected in Fig. 12, where the individual reservoir zones representing TST deposits have high permeabilities, but the relatively low net/gross shows the limited lateral distribution. Limited lateral extent also limits effective pressure support and sweep. Crevasse splays and crevasse channels have good horizontal permeabilities, but show significantly lower

Sedimentary facies in the fluvial-dominated f~re Formation as seen in the f~re 1 member in the Heidrun Field

101

Fig. 12. Summary of average reservoir properties for all Are 1 wells in the Heidrun Field and their relationships to system tracts. Note that permeability values (plotted on linear scale) are from net sand, and as such largely represent grain size and sorting. Also note that because the number of cores are limited, there is a difference in number of samples between the wireline-log-generated horizontal permeabilities and the vertical permeabilities from core plugs.

vertical permeability due to mud drapings. Individual sands within such complexes are likely to be discontinuous and the bodies themselves will sometimes have limited lateral extent. Bay/lake fill sequences also have good reservoir properties, although not as good as in the channel deposits. Within the Heidrun Field they are thought to have large lateral extent and stacked sequences make up thick units. This is shown in Fig. 12 where deposits in the upper part of Are 1B are seen to have good horizontal permeabilities and very high net/gross. One should bear in mind that the thin mudstones at the base of individual sequences are barriers to vertical flow. Unfortunately, these mudstone intervals are not easily detected on wireline logs. Fig. 12 clearly shows that vertical permeability from core plugs is much lower than horizontal permeability. Finally, one of the main challenges in producing the Are Fro. in the Heidrun Field is not how these reser-

voir facies will perform individually. The question is how they will perform when completed together. Due to the unconsolidated nature of the sands, production wells have to be gravel packed. The contrast in flow properties between different sedimentary facies will then limit economic recovery, especially when gravelpacked wells limit the possibility to selectively isolate zones after water or gas breakthrough.

Acknowledgements I want to express my thanks to Conoco for allowing me to write this paper and to Statoil and Fortum Petroleum for giving permission to publish it. The majority of these data was gathered when I was seconded to Statoil's Heidrun Petek group in Stj0rdal during 1994 and 1995. During my time with Statoil I had many fruitful discussions with Jostein Kja~refjord, Lars-Magnus F~ilt and Arne Dalland. I would

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also like to thank Ed Clifton with whom I worked closely on the sedimentology of the Heidrun Field.

References Aitken, J.E and Flint, S.S., 1995. The application of high-resolution sequence stratigraphy to fluvial systems: a case study from the Upper Carboniferous Breathitt Group, eastern Kentucky, USA. Sedimentology, 42: 3-30. Aitken, J.F. and Flint, S.S., 1996. Variable expressions of interfluvial sequence boundaries in the Breathitt Group (Pennsylvanian), eastern Kentucky, USA. In: J.A. Howell and J.F. Aitken (Editors), High Resolution Sequence Stratigraphy: Innovations and Applications. Geol. Soc. Spec. Publ., 104: 193-206. Barwis, J.H., 1978. Sedimentology of some South Carolina tidalcreek point bars, and a comparison with their fluvial counterparts. In: A.D. Miall (Editor), Fluvial Sedimentology. Can. Soc. Pet. Geol., Mem., 5: 129-160. Besley, B.M. and Fielding, C.R., 1989. Palaeosols in Westphalian coal-bearing and red-bed sequences, central and northern England. Palaeogeogr., Palaeoclimatol., Palaeoecol., 70(4): 303-330. Bohacs, K. and Suter, J., 1997. Sequence stratigraphic distribution of coaly rocks: fundamental controls and paralic examples. Am. Assoc. Pet. Geol. Bull., 81(10): 1612-1639. Coleman, J.M. and Prior, D.B., 1982. Deltaic environments. In: EA. Scholle and D. Spearing (Editors), Sandstone Depositional Environments. Am. Assoc. Pet. Geol., Mem., 31: 139-178. Coleman, J.M., Gagliano, S.M. and Webb, J.E., 1964. Minor sedimentary structures in a prograding distributary. Mar. Geol., 1: 240-258. Collinson, J.D., 1986. Alluvial sediments. In: H.G. Reading (Editor), Sedimentary Environment and Facies. 2nd ed., Blackwell, Oxford, pp. 20-62. Dalland, A., Worsley, D. and Ofstad, K., 1988. A lithostratigraphic scheme for the Mesozoic and Cenozoic succession offshore midand northern Norway. NPD Bulletin 4. Duchaufour, E, 1982. Pedology. Allen and Unwin, London, 448 pp. Elliott, T., 1974. Abandonment facies of high-constructive lobate deltas, with an example from the Yoredale series. Proc. Geol. Assoc., 85: 359-365. Elliott, T., 1986. Deltas. In: H.G. Reading (Editor), Sedimentary Environments and Facies. 2nd ed., Blackwell, Oxford, pp. 113154. Fielding, C.R., 1984. Upper delta plain lacustrine and fluvio-lacustrine facies from the Westphalian of the Durham coalfield, NE England. Sedimentology, 31: 547-567. Fielding, C.R., 1986. Fluvial channels and overbank deposits from the Westphalian of the Durham coalfield, NE England. Sedimentology, 33:119-140. Flint, S., Aitken, J.F. and Hampson, G., 1995. Application of sequence stratigraphy to coal-bearing coastal plain successions: implications for the UK Coal Measures. In: M.K.G. Whateley and D.A. Spears (Editors), European Coal Geology. Geol. Soc. Spec. Publ., 82: 1-16. Gardner, T.W., Williams, E.G. and Holbrook, EW., 1988. Pedogenesis of some Pennsylvanian underclay: ground water, topo-

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Norske Conoco A/S, Tangen 7, N-4070 Randaberg, Norway

graphic and tectonic controls. In: J. Reinhardt and W.R. Sigleo (Editors), Palaeosols and Weathering through Geologic Time: Principles and Applications. Geol. Soc. Am., Spec. Pap., 216: 81-102. Gersib, G.A. and McCabe, EJ., 1981. Continental coal-bearing sediments of the Port Hood Formation (Carboniferous), Cape Linzee, Nova Scotia, Canada. In: F.G. Ethridge and R.M. Flores (Editors), Present and Ancient Nonmarine Depositional Environments: Models for Exploration. Spec. Publ. Soc. Econ. Paleontol. Mineral., 31: 95-108. Gjelberg, J., Dreyer, T., H0ie, A., Tjelland, T. and Lilleng, T., 1987. Late Triassic to Mid-Jurassic sandbody development on the Barents and Mid-Norwegian shelf. In: J. Brooks and K. Glennie (Editors), Petroleum Geology of North West Europe. Graham and Trotman, London, pp. 1105-1129. Hampson, G.J., Davies, S.J., Elliott, T., Flint, S.S. and Stollhofen, H., 1999. Incised valley fill sandstone bodies in Upper Carboniferous fluvio-deltaic strata: recognition and reservoir characterization of Southern North Sea analogues. In: A.J. Fleet and S.A.R. Boldy (Editors), Petroleum Geology of Northwest Europe: Proceedings of the 5th Conference. Geological Society, London, pp. 771-788. Kja~refjord, J.M., 1999. Bayfill successions in the Lower Jurassic ,~re Formation, offshore Norway: sedimentology and heterogeneity based on subsurface data from the Heidrun Field and analog data from the Upper Cretaceous Neslen Formation, eastern Book Cliffs, Utah. In: T.F. Hentz (Editor), Advanced Reservoir Characterization for the 21st Century. Gulf Coast Section, Society of Economic Paleontologists and Mineralogists Foundation 19th Annual Research Conference, Tulsa, OK, pp. 149-158. McCabe, EJ., 1984. Depositional environments of coal and coalbearing strata. In: R.A. Rahmani and R.M. Flores (Editors), Sedimentology of Coal and Coal-Bearing Sequences. Spec. Publ. Int. Assoc. Sedimentol., 7: 13-42. Olsen, T., Rosvoll, K.J., Kj~erefjord, J.M., Arnesen, D.M., Sandsdalen, C., JCrgenvfig, S.H., Langlais, V. and Svela, K.E., 1999. Integrated reservoir characterization and uncertainty analysis, Heidrun Field, Norway. In: A.J. Fleet and S.A.R. Boldy (Editors), Petroleum Geology of Northwest Europe: Proceedings of the 5th Conference. Geological Society, London, pp. 1209-1220. Oomkens, E., 1974. Lithofacies relationships in the late Quaternary Niger Delta Complex. Sedimentology, 21: 195-222. Retallack, G.J., 1983. A palaeopedological approach to the interpretation of terrestrial sedimentary rocks: the Mid-Tertiary fossil soils of Badlands National Park, South Dakota. Bull. Geol. Soc. Am., 94: 823-840. Staub, J.R. and Cohen, A.D., 1979. The Snuggy Swamp of south Carolina: a back-barrier estuarine coal-forming environment. J. Sediment. Petrol., 49: 133-144. Van Wagoner, J.C., Mitchum, R.M., Campion, K.M. and Rahmanian, V.D., 1990. Siliciclastic Sequence Stratigraphy in Well Logs, Cores and Outcrops. Methods in Exploration Series 7, American Association of Petroleum Geologists, Tulsa, OK. Weimer, R.J., Howard, J.D. and Linsay, D.R., 1982. Tidal flats and associated tidal channels. In: EA. Scholle and D. Spearing (Editors), Sandstone Depositional Environments. Am. Assoc. Pet. Geol., Mem., 31: 191-245.

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Sedimentology of the heterolithic and tide-dominated Tilje Formation

(Early Jurassic, Halten Terrace, offshore mid-Norway) Allard W. Martinius, Inge Kaas, Arve Naess, Geir Helgesen, Jostein M. Kjaerefjord and Deborah A. Leith

The Early Jurassic Tilje Formation on the Halten Terrace, offshore mid-Norway, was deposited in a relatively narrow but long seaway connecting the Boreal Ocean in the north and the Tethys Ocean in the south. Sediments of the Tilje Formation in the SmOrbukk and Heidrun Fields have been classified into ten facies associations. Many of the lithofacies are mud-rich and typified by strong grain-size contrasts. In addition, all but two of the facies associations are tidally influenced or dominated. As a result, 80% of the total rock volume consists of very heterolithic sediments characterised by rapidly alternating grain-size changes between mudstone or siltstone and fineto medium-grained sandstone. In the Tilje Formation, the recognition and interpretation of heterolithic facies is crucial to understanding the depositional conditions and stratigraphic architecture. The classification scheme and the associated facies breakdown in cored wells served to define two successive conceptual depositional models that are placed in a sequence stratigraphic framework. The lower part of the Tilje Formation (T1 and most of T2) are envisaged to have formed in response to base-level fall, creating a series of low-relief valleys, and subsequent base-level rise resulting in the formation of a tide- and wave-dominated estuarine system. The upper part of the Tilje Formation (top of T2 to T6) is interpreted to have formed as a tide- and fluvial-dominated delta-like system. These two contrasting depositional styles resulted in different three-dimensional facies architectures, relative facies proportions, and facies stacking patterns, which have implications for reservoir model-building methods.

Introduction Hydrocarbon exploration in the Haltenbanken area, offshore mid-Norway, began in the early 1980s, with the discovery of the Midgard Field in 1981. Since then several major discoveries have been made and the area today is regarded as a fairly mature hydrocarbon province. Production is mainly from siliciclastic sequences deposited in shallow marine environments. These comprise either relatively homogeneous sands, or heterogeneous packages formed by an intercalation of mudstone, siltstone, and sandstone. The Tilje Formation, on which this paper will focus, forms a reservoir interval in several hydrocarbon-producing fields on the Halten Terrace, a smaller section of the Haltenbanken area (Fig. 1). Production from the Tilje Formation is significantly affected by the strongly heterolithic nature of many of the depositional facies and the complicated reservoir architecture. Various bedding styles are found at several scales and approximately 80% of the total rock volume is formed by heterolithic facies. Over the years, the complex sedimentology of the Tilje Formation has resulted in various published palaeoenvironmental interpretations (Karlsson, 1984; Gjelberg et al., 1987;

Pedersen et al., 1989; Ekern, 1990; Dreyer, 1992, 1993; Taylor and Gawthorpe, 1993; Van de Weerd, 1996), commonly tailored to accommodate field-specific observations, although the overall strongly tidal nature has been recognised by most workers. This paper will discuss the Tilje Formation as it is specifically developed in the Heidrun and SmOrbukk Fields (Fig. 1). However, the presented facies classification scheme and interpretations are based on selected observations from most of the hydrocarbon fields on the Halten Terrace where the Tilje Formation is present, and hence represent an overview of subregional character. The aim of the paper is, firstly, to present a conceptual depositional model for the Tilje Formation that is based on a generally applicable facies classification scheme. In that sense, it differs from previous studies. The second aim is to analyse in more detail depositional conditions for the different stratigraphic intervals of the Tilje Formation.

Data base and methodology This study is based on core observations and consistent facies breakdowns in 25 wells from 9 hy-


1 04

A.W. Martinius et al.

Fig. 1. Location of hydrocarbon fields producing from the Tilje Formation on the Halten Terrace.

drocarbon fields on the Halten Terrace (Nome, Heidrun, SmCrbukk, SmCrbukk SCr, Midgard, Trestakk, Tyrihans, Lavrans, and Njord; Fig. 1) with emphasis on the Heidmn and Sm0rbukk Fields. Lithofacies characterisation and interpretation were supported by detailed trace-fossil and ichnofabric analysis. In addition to data obtained from numerous published reports, conventional log data, in-house biostratigraphic data, samarium-neodymium (147Sm/144Nd and 143Nd/144Nd) isotopic data, and dip-meter data were used for correlation purposes, to support facies interpretations and/or identify provenance areas. Biostratigraphic data were obtained from several separate studies by different companies and, as a result, the findings are not always directly comparable. Two other complicating factors are: (1) the recurring dilemma of

whether the same events in different wells are time equivalent, or whether they are simply facies dependent and thus time transgressive; and (2) the fact that preservation of palynological material in deep wells (>4000 m in the SmCrbukk Field) is so poor that not only is the number of observations extremely low but the results may be uncertain. Nevertheless, Sm/Nd isotopic markers fit well with correlations based on biostratigraphic events and sedimentological criteria.

The Tilje Formation Geographical occurrence The Early Jurassic Tilje Formation (Dalland et al., 1988) is geographically confined to a NNE-


105

Fig. 2. Paleogeographic setting of the early Pliensbachian seaway during times of deposition of the Tilje Formation (after Dor6, 1991). Inset shows global configuration of continents during the Pliensbachian (190 Ma; after Smith et al., 1994). The boxed area indicates the area covered by Fig. 3.

SSW-oriented belt that is known to be at least 470 km long and 150 km wide judging by data from hydrocarbon exploration and nearshore shallow drill holes (Figs. 2 and 3). It is inferred that in an eastward direction the Tilje Formation passes into time-equivalent upper delta plain and alluvial fan environments typified by kaolinitic claystones alternating with both mud- and grain-supported conglomerates (Bugge et al., 1984). The formation has an average thickness of 120 m in the Heidrun Field and 150 m in the SmCrbukk Field, but reaches maximum thicknesses of more than 300 m in the central western part of the Halten Terrace (Lavrans Field). It has been traced north as far as 68o02 ' (where it gradually pinches out

on the R0st High; Fig. 3) and to the south as far as 63006 ' (where the facies pass into deeper marine sediments). It is also inferred that the Tilje Formation may extend to the east as far as approximately 09~ ' (eastern part of the Tr0ndelag Platform; Bugge et al., 1984; Fig. 3). The facies similarities between the comparable stratigraphic intervals in the various Halten Terrace oilfields suggest that deposition occurred while the source terrains were relative stable or underwent equal isostatic change. The inferred paleolatitude of the Halten Terrace was between 49 ~ and 53 ~ north (Dor6, 1991; Smith et al., 1994) during deposition of the Tilje Formation. In general, mid-paleolatitudinal temperature and pre-

106

A.W. M a r t i n i u s et al.

Fig. 3. Proposed paleogeographic sketch of the seaway in which the Tilje Formation sediments were deposited.

cipitation data indicate that the Jurassic climate was warm with rather equable global conditions (Hallam, 1985, 1994). Strong seasonal differences in temperature and rainfall may have existed (Hallam, 1994).

Structural setting and provenance The Early Jurassic seaway was approximately 1500 km long and 250 km wide, and during deposition of the Tilje Formation connected the Boreal Ocean in the north to the Tethys Ocean in the south (Fig. 2). Both oceans were bordered by shallow shelves with numerous emergent terrains (Dor6, 1991). Deposition of sediments occurred in association with (a) minor tectonic pulse(s), which resulted

in the development of N-S-oriented growth faults (inhouse data; Ehrenberg et al., 1992). The smaller Tilje seaway was separated from the main Early Jurassic seaway by the Helland-Hansen Arch-Bode High (Fig. 3). Connections with the Early Jurassic seaway existed both in the north, south of the ROst High and the uplifted Ribban Basin structure, as well as in the south (Fig. 3). The structural history of the Halten Terrace is typified by a long period of rifting and subsidence, that commenced in the Triassic, or earlier, and that continued up to early Eocene time. Bukovics et al. (1984) suggested that the late Palaeozoic to mid-Mesozoic subsidence of the Norwegian-Greenland Sea Rift was mainly governed by mechanical stretching of the


crust. Crustal extension accelerated during the Early Triassic with deposition of mostly continental strata on the TrOndelag Platform. Paleogeographical studies suggest that the Tilje Formation was deposited in the Halten-TrOndelag Basin, which encompassed the Halten Terrace and the TrOndelag Platform (Fig. 3). This coast-parallel riftgenerated basin, which became the locus of the Early Jurassic seaway (Gjelberg et al., 1987; Dor6, 1991; Fig. 3), was associated with the Kristiansund-BodO Fault Complex, a NNE-SSW-oriented fault system that included the FrOya High, the Bremstein Fault Complex, and the Nordland Ridge (Fig. 3). In a plate-tectonic perspective, the basin is part of the continental passive margin of the Northern Atlantic Rift Domain (Dor6, 1991). The Halten Terrace (Fig. 3) is located in the centre of the fault complex, between the TrOndelag Platform to the east and the Sklinna Ridge to the west (Bukovics et al., 1984; Schmidt, 1992; Fig. 3). In-house structural data suggest early activity of a domal structure to the west of the basin that became uplifted as part of the Early Jurassic proto-rift phase forming the Helland-Hansen Arch. The fossil rift underlying the basin formed during the Triassic as part of the proto-Atlantic Ocean. The Early Jurassic basin in which the Tilje Formation was deposited can be characterised as a late-stage pre-rift basin (cf. NOttvedt et al., 1995) in which subsidence was caused mainly by thermal sagging and sediment compaction (Fig. 4), and in which syn-sedimentary faults locally played an important role. Application of this model is further supported by: (a) absence of evidence for basin-bounding fault margins; (b) absence of volcanic

107

rocks; (c) relative uniformity of facies; and (d) large size of depositional systems. Provenance age data derived from Sm/Nd isotope analysis are considered totally independent of sample grain size because minerals from rocks in a specific source area and with a certain age weather down to a range of grain sizes (from mud to coarse sand) which maintain their similar provenance age (Dalland et al., 1995). This characteristic allows Sm/Nd isotope data to be used without restriction. Generally, in-house dip-meter and Sm/Nd isotope data suggest three possible source areas for the Tilje Formation. The main source area is considered to be the mainland east of the Tr~ndelag Platform (Fennoscandian Shield; Dalland et al., 1995). Additionally, sediment was shed from the Ribban Basin structure in the north (Fig. 3), which was dominated by erosion during the entire Jurassic. Furthermore, an emergent area in the west, the Helland-Hansen-Bod~ High (Fig. 3) is also inferred to have supplied sediment to the Halten Terrace area.

Stratigraphy The late Pliensbachian to early Toarcian Tilje Formation is one of four formations comprising the Early Jurassic Bfit Group (Fig. 5A). The formation is underlain by the Hettangian-Sinemurian Are Formation, dominated by lower delta plain and bay deposits (including coal seams), and overlain by the Toarcian Ror Formation, composed of marine mudstones. In the northernmost part of the Halten Terrace, the Rot Formation is replaced by the Tofte Formation, which is interpreted as a fan-delta deposit (in-house data). The

Fig. 4. Cross-sectional sketch of the depositional basin at Tilje time showing the position of the Middle Triassic fossil rift and Late Triassic salt layers.

108

A. W. Martinius et al.

A

GENERAL LITHOSTRATIGRAPHY

MID-NORWAY

] HALTENBANKEN INORDLANDRIDGEI CHRONOSTRATIGR. GP,

TR/ENABANKEN

FORMATIONS p

Callovian

LM

~

"

" "

i

J

O m

aa

Bathonian

J

J12 J6

I

T5 (part)

.... Z .....

Johansen 4

R5

.... 30R

h~t.

Drake 3

Drake

MIDDLE i bifrons

/

J20

63 --->

3C

184.06

185 -

~ooF- 1-- ~-st 5--

i

Statfjord 3

!

.....

Statfjord Fm.

2B c~

Fig. 4. Comparison of sequence stratigraphic schemes for the Lower Jurassic Dunlin Group. t~

Sequence stratigraphy of the Lower Jurassic Dunlin Group, northern North Sea

(denoted J12/J14, J16 and J18 using the framework established by Partington et al., 1993; Fig. 4), with the Johansen, Cook and intra-Drake Formation sandstones representing regressive maxima. Dreyer and Wiig (1995) applied the sequence stratigraphic concepts established by Van Wagoner et al. (1987) to the reservoir zonation of the Cook Formation in the Gullfaks Field. They noted the incised nature of the major sandstone units with the transition from Cook-2 to the main reservoir Cook-3 being interpreted as a sequence bounding unconformity (within J16B of this study; see Fig. 4) being associated with a major change in drainage pattern. Finally, Marjanac and Steel (1997) pointed at the incised nature of late Pliensbachian-late Toarcian sandstones in the North Sea, and argued for the existence of four higher-order sequences (denoted Cook-1 to Cook-4; see Fig. 4).

Sedimentary facies and environments A number of 21 cored sections have been studied for sedimentology. Fig. 5 shows that the cores cover almost the entire stratigraphic interval of the Dunlin Group and provides an important foundation for the facies and sequence stratigraphic interpretation. In addition, there is a wide geographic spread of cores from sequences J14 to J16, comprising the Cook Formation, providing a good basis to determine spatial variations in sedimentary facies type at this stratigraphic level. Horda Platform and Sogn area

Figs. 6 and 7 illustrate a number of cored sections from the eastern part of the basin, including the Horda Platform, the Lomre Terrace (well 35/10-1) and the Sogn area. Well 35/9-2 (Fig. 6) in the Sogn area has full core coverage of the Dunlin Group and serves to outline the main temporal changes in facies and depositional environments. Sequences J13 and J14 are characterized by a vertical alternation of coarse-grained to pebbly sandstones and sections dominated by mudrocks and fine-grained sandstones. The main sandstone bodies, corresponding to the Johansen and Cook Formations (Fig. 6) are sharply based, composite fining-upwards units dominated by cross-stratified troughs and current ripple lamination. In this proximal setting, these bodies are related to fluvial and/or estuarine deposition. The intercalated finer-grained intervals comprise a variety of biogenic traces (Chondrites, Helminthopsis) and wave-generated structures including hummocky/swaley types of cross-stratification, and are related to deposition in the offshore and lower shoreface of a wave-dominated shoreline. The alternation of fluvial/estuarine

151

and shelf/lower shoreface deposits in this part of the succession testify to rather significant variations in relative sea level as a major factor controlling the distribution of sediment in the Pliensbachian. The late Pliensbachian-Toarcian part of the succession (sequences J15-J18; Fig. 6) can be related to successively deeper marine depositional environments, reflecting an overall transgressive setting at this stage. The Brent Group rests with marked unconformity on the Dunlin Group in this well (Fig. 6), probably reflecting Aalenian uplift of the basin margin. Additional cores from the Horda Platform and Lomre Terrace are presented in Fig. 7. The basal surface of the Dunlin Group is cored in well 30/3-A-5 (Fig. 7). Notably, the lithological contact with the underlying Statfjord Formation is sharp and marked by a sideritic crust, separating fluvial and shallow marine sandstones from intensively bioturbated very fine-grained deposits of the lowermost Dunlin Group (assigned to the J13 sequence). The lowermost part of the Dunlin Group (below J13 mts in well 30/3-A-5) comprises pervasively burrowed, fossiliferous and sideritic very fine-grained sandstones, that may record reworking and condensation during the initial marine transgression of the Early Jurassic basin. Above J13 mts, the sediment becomes significantly finer grained, with lenticular wave ripples and a relatively diverse burrowing assemblage. These deposits are related to deposition in a low-energy marine shelf environment, showing that relatively deep marine conditions were established early in the Pliensbachian. The cored section in well 31/2-3 (Fig. 7) is also from the J13 sequence, but stratigraphically younger than the section in well 30/3-A5. The core comprises medium- to coarse-grained sandstones that can be correlated to the Johansen Formation (Dor6 et al., 1984). Internally, the sandstone can be divided into a series of sharply based fining-upwards units (storeys) characterized by a basal quartzite pebble lag succeeded by cross-stratified, laminated and massive sandstones, sparsely bioturbated by Skolithos and Ophiomorpha. The burrowing traces and sedimentary structures indicate deposition in a marginal marine environment (estuarine, see discussion below) characterized by strong traction currents. The superposition of this sandstone facies above shelf deposits demonstrates significant shoreline regression within the J 13 sequence. The J14 sequence is fully cored in wells 30/3-4, 30/6-16 and 35/10-1. A sharply based, coarsening-upward sandstone interval is present above J14 mts, terminated by the J15 mts in well 35/10-1 (Fig. 7). Wells 30/3-4 and 30/6-16 (Fig. 7) com-

MY

AMMONITES

PALYNO. ZONES

+

SEQUENCES

KEY BIOEVENTS

31OSTRATIGRAPHY

AGE [Hardenbol et al. 1998)

+

+

~

+

+

~+

+

,

EARLY

MICROPAL. ZONES

(part)

(part)

176.5

LATE

-

177.33

AALENIAN -

180 -

MIDDLE

laeviuscula175.81 PJ discites 4

,~O

O'~

CO

03

03

0'3

Or;

CO

O r)

concavum

177.33 bradford-

murchisonae

ensis 178.11 murchi-

180.1

03

("3

03 i

VIJ 7

3D

sonae

179.29

EARLY

pj

03 .......

I

J24

~OR

176.5

+

+

SEQUENCES

i

BAJOCIAN

I ~

J22

179.29

opalinum 180.1

34R

aalensis180.88

pseudoradiosa

-

181.67

-

LATE

dispansum 182.47

.houarsense 84.06

pj

VIJ 6

3C

,~3

>

J20

J20

184.10,.6

32R ---> 185 -

TOARCIAN

variabilis 185.25

VIlDDLE bifrons

B 186.84

pj 3B

falciferum

89.6

tenuicostatum PJ 189.6 3 A spinatum 19o.38 .ATEE ari-margarimargari- imarg ~91.51 tatus ~ I stokesi davoei

PLIENSBACHIAN EARLY

ibex

INEURIAN

--B

~oR~--~

|

l

I------

,,,

I . . !i. .

"

,,- -

-

-

---B

~,9R, ---> MJ 4

B A

~5R ~sF~ 44R

)

.... ~-

.....

.....

A _,I/L . J14

.

-i

.

i

.

-

-_

.

.,--~L~_--~.~_~

= .

n 11-i

m

-

-- -- =_---

.-

- ----

_I --

-

,

--

~----'-

I-H1 1 -

-

l i

,-"

i

J13

B

i

"aricostatum 196,83

--

9

|=

I

MJ 3 30R

LATE (part)

|

5

PJ 193.77 2 C

195.3

195.3

)

J18

l

----

-

A ":Z_~

. . . . . J14

192.64

jamesoni 195 -

60R

j1

188.04

EARLY

190-

MJ

- -- J12--

-- "

l

i

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

J13

J12

PJ 2B

IR'I~V

oxynotu m 197.61

Wells illustrated on correlation figures + Cores illustrated - - Maximum transgressive surfaces (MTS) Vailian type sequence boundaries (SB)

Fig. 5. Stratigraphic distribution of studied cored intervals within the Lower Jurassic Dunlin Group.

153


35/9-2 KEY SYMBOLS

~;~ F l u v i a l c h a n n e l fill ~" ~~ Q T

e-

.o

Current ripples Wave ripples Lenticular bedding Trough cross-strat. Low angle cross-strat. Tabular cross-strat. Belemnites Shells Nodules Bioturbation

Delta plain

(I= El __

_(

- ~__.~. o ~ _ _ ~ . ~

!

! i

i .....

I

~

Wave-dominated shoreface

HCS

!

e-

-

El

Hummocky cross-strat. Anconichnus Asterosoma Chonddtes Diplocraterion Ophiomorpha Palaeophycus Pelecypodichnus Planolites Polyclodichnus Rosselia Skolithos Teichichnus Terebellina Zoophycos

Foreshore/ upper shoreface ....................

2650 -- ~ _

ABBREVIATIONS

HCS An Ast Ch Dip Oph Pal Pei Piano Poly Ros Sk Teich Ter Zoo

HCS

RS


L_

70R

SB m.

i~

roarc,

c o

63

~--'~

Offshore zone

i

transition

60R 55

..... I~ .

'~

M,

J16B MTS

- ~'z-f-r-f d


45

.cs RS/TS

t,O "a

Estuarine 2700

c

e'~

E ,-

"-~-

g

~

9

~ n

~

---'-v"r"'~

mouth

bar

J 15 MTS

-.-:rT.~.

44R

1

,,,~ .~ .

Estuarine/ f l u v i a l v a l l e y fill

o

4--- 40R

- ~ o o

~

m !

SB

|

Wave-dominated shoreface 4--- B44

e.o

RS/TS ~ 3 4

L O [,t,. Estuarine

e-

g

b a s i n fill ( ? )

SB ,~

E

~ ~ ' ~ Lower shoreface

2750 ~"

"-a m Q., ~,~

,

~=

SB

e--

o "~ "" o.,,,_,,

~g

TS

Estuarine/ f l u v i a l v a l l e y fill ( ? )

o

tat.

G,~

-

o

e-

r

~ - ~ / -

T T T--gQ

,..

.-~ i._ e.m

-

I Oh

'

_xe__~" -

45---o,,

dh o o o

s

Lower shoreface/ offshore transition zone TS

o

Proximal braid plain/ alluvial fan

Fig. 6. Cored section of well 3 5 / 9 - 2 on the M~l~ay Terrace.

prise very fine- to fine-grained sandstones with wave ripples and lenticular lamination, associated with a diverse burrowing assemblage (Chondrites, Palaeophycus, Planolites, Ophiomorpha, Rosselia) as well as shell debris and scattered belemnite tails. The

same section in well 35/10-1 is significantly coarser

grained, comprising bioturbated and cross-stratified fine- to medium-grained sandstones. The coarseningupward grain size distribution of the J14 sandstones is related to shoreline progradation (Fig. 11). Furthermore, the fine-grained nature of the sandstones, the burrowing style and small-scale sedimentary struc-

154

M.A. Charnock et al.


155

Fig. 7. Selective cored sections: facies and sequence interpretation, Horda Platform and Lomre Terrace.

tures in wells 30/3-4, 30/6-16 are related to deposition within the offshore transition to lower shoreface. The coarser grain sizes and cross-stratification seen in well 35/10-1 may reflect a shallower part of the shoreline, and deposition in the middle to upper shoreface is inferred. Notably, a regressive surface of marine erosion (RSME) is inferred at the base of the J 14 sandstones to account for the sharp basal contacts with underlying deeper marine facies (Fig. 7), indicating forced shoreline regression at this stratigraphic level.

Mts J 15 re-established deep, shelfal deposits above the J14 shoreline. However, fine- to medium-grained sandstones of the J15 sequence (wells 30/3-4, 30/6-16, 35/10-1; Fig. 7) rest with a sharp, pebble-strewn contact on fine-grained, bioturbated shelf deposits immediately above J 1 5 rots. These sandstones are characterized by common din-size crossstratification, ripple lamination and planar lamination, with numerous cm-size mudrock partings and drapes throughout. Burrowing is moderate and of low diversity (mainly Planolites and Skolithos). The

156 cross-stratification is generally indicative of sustained traction currents, whereas the mudrock laminae and partings reflect common periods of slack-water deposition. Thus, an estuarine environment is inferred for the J15 sandstones, with the basal surface representing a sequence boundary cut during the preceding relative sea-level lowstand (see Dalrymple et al., 1994). In well 30/6-16 on the western margin of the Horda Platform (Fig. 7), the J15 sandstones are capped by generally dark muddy deposits of sequences J16A, J16B, Jl8 and J20 (basal part). The entire succession is organized into a series of stacked coarsening-upwards units of laminated dark mudrocks in the lower part and bioturbated, occasionally tipple-laminated siltstones and very fine- to fine-grained sandstones in the upper parts. Burrowing is weak to moderate and dominated by Anconichnus. The dark colour and preserved lamination of the mudrocks probably reflect deposition in a slightly oxygen-depleted marine shelf environment, with the coarsening-upwards units reflecting cycles of (distal) shoreline progradation. In contrast to the succession in well 30/6-16, the J16A sequence in well 35/10-1 (Lomre Terrace) contains significant sandstones of inferred shoreface and estuarine origin (Fig. 7). Above J16A rots, wavetippled very fine-grained sandstones burrowed by Helmintoidea are related to deposition in the lower shoreface. These deposits are abruptly succeeded by fine-grained, laminated, cross-stratified and massive sandstones with numerous mudrock partings and laminae, thought to record deposition in an estuarine environment. The emplacement of estuarine sandstones above the initial J16A shoreface indicates a significant base level fall and possible valley incision at this level. The upper part of the J16A sequence shows a regressive to transgressive development. Above the estuarine interval in well 35/10-1, fine-grained sandstones with hummocky cross-stratification and relatively diverse burrowing record deposition in a wave-influenced lower shoreface, whereas coarsergrained sandstones are thought to represent the upper shoreface. Below J16B mts in well 35/10-1, finergrained sandstones with hummocky cross-stratification related to the lower shoreface reflect deepening prior to the flooding of J 16B mts. Similar to well 30/6-16, the J16B and J18 sequences in well 35/10-1 comprise coarsening-upward sequences of mudrocks and sandstones. However, the sections are more bioturbated and contain hummocky cross-stratification as well as belemnite fragments. These deposits are related to deposition in the lower shoreface (J16B) and inner shelf (J18). The facies of sequences J16A to J20 in well 30/6-16 (Fig. 7)


show that a significant deepening occurred in the late Pliensbachian, and that a rather deep marine environment was maintained throughout the early to middle Toarcian. The onset of the J16A sequence is also marked by a significant deepening on the Lomre Terrace (well 35/10-1), but shallower marine conditions seemingly persisted here, particularly during deposition of the J 16A sequence. This may be due to a more proximal setting in this part of the basin. Whereas the J20 mts marks a significant deepening and establishment of anoxic shelf conditions, significant shoreline progradation occurred in the subsequent phase of the J20 sequence. The cored section of sequence J20 in well 30/6-22 contains coarsening-upwards siltstones and sandstones with wave ripples, small-scale hummocky cross-stratification and a diverse trace fossil assemblage (see Fig. 7). Furthermore, the J20 sequence in 30/6-22 is capped by coarse-grained sandstones with cm-size quartzite pebbles. The sandy section is related to deposition in the offshore transition zone to the upper shoreface, reflecting a major seawards shift of the shoreline in the middle to late Toarcian. Notably, the J20 sandstones seen in well 30/6-22 represent the basal parts of the Oseberg Formation as defined by Graue et al. (1987). The cored section above J22 mts in well 30/6-22 shows the main part of the Oseberg Formation and the base of the Rannoch Formation. The Oseberg Formation (above J22 rots) comprises four coarsening-upwards facies sequences of medium- to coarsegrained, pebbly sandstones and associated mudrocks and siltstones. Cross-stratification and crude parallel lamination characterize the otherwise rather massive sandstones, which also are weakly bioturbated (Skolithos, Ophiomorpha). The finer-grained intervals contain wave ripples and are more intensively burrowed. A blackish mudrock interval with abundant Helmithoidea traces is present at the base of the uppermost coarsening-upward sequence (Fig. 7). This mudrock interval is dated as late Toarcian-earliest Aalenian. The primary features of the Oseberg Formation are related to repeated phases of shoreline progradation punctuated by transgressions, demonstrating significant sand emplacement into the basin during late Toarcian times. The blackish mudrock at the base of the last facies sequence is related to deposition in a rather deep shelf environment, and is a candidate for another maximum transgressive surface.

TampenSpur Fig. 8 shows a number of representative core sections from the Tampen Spur. The lower part of the


] 57

Fig. 8. Selective cored sections: facies and sequence interpretation, western margin of the North Viking Graben.

J13 sequence, immediately above the Statfjord Formation, is cored in well 34/8-1. As on the Horda Platform, intensively bioturbated siltstones are related to an offshore environment. The overlying fine- to

medium-grained sandstones rest abruptly on the underlying lithology, and are also well structured with common current ripples and dm-size trough crossstratification. Both the sediment type and structures

O'! Oo

Fig. 9. Sequence stratigraphic interpretation and facies correlation of line 1, Oseberg area.

t~

e~

~,~~

~,.~~ e~

~,~~

Fig. 10. Chronostratigraphic interpretation of line 1, Oseberg area.

0-1


160

indicate deposition from strong, sustained traction currents, and a distributary channel/estuarine environment is inferred. Consequently, the base of the sandstone unit is taken as a potential sequence boundary, reflecting a significant basinward shift of the shoreline. The upper part of sequence J13 is cored in well 34/4-5, comprising intensively bioturbated siltstones with scattered wave ripples and belemnite tails. These deposits are related to an offshore environment, reflecting subsequent re-establishment of deep marine conditions upon the distributary/estuarine deposits. Sequence J14 (Fig. 8, well 34/4-5) comprises two facies associations organized into a coarseningupwards sequence. The lower association commences with laminated silty mudrocks near J14 mts, passing vertically into fine-grained, wave-rippled and bioturbated (Chondrites) sandstones and silty interbeds deposited in the lower shoreface. Potentially, a regressive surface of marine erosion (RSME) is indicated at the base of the J14 shoreface, as a similar event can be picked on the Horda Platform (e.g. wells 30/3-4 and 30/6-16; see Figs. 6 and 9). The upper facies association 014; well 34/4-5) comprises sharply based fine- to coarse-grained, cross-stratified and massive sandstones with scattered ripples and sparse bioturbation (Planolites, Skolithos). Furthermore, numerous muddy drapes and partings occur throughout the sandstone body. An estuarine depositional environment is inferred for this facies association with the sharp lower boundary representing a sequence boundary cut during a preceding lowstand. Sequence J15 is partly cored in well 34/2-4 (Fig. 8). The medium-grained sandstones below J16A rots are characterized by dm-size cross-stratification and scattered trace fossils of a low-diversity assemblage (Skolithos, Planolites). As for the J14 sandstones, an estuarine environment is suggested. Above the J16A mts in wells 34/2-4 and 34/10-9 (Fig. 8), intensively bioturbated fine-grained sandstones and wave-rippled/hummocky cross-stratified sandstone/mudrock interbeds are related to deposition in offshore and lower shoreface environments. As on the Horda Platform, the J16A mts seems to mark a relatively significant deepening of the basin in the late Pliensbachian. Sequence J16B (Fig. 8: well 34/10-9) comprises intensively bioturbated siltstones and fine-grained sandstones of assumed offshore and lower shoreface environments. Potentially, the J16B sequence boundary in well 34/8-1 is represented by a coarse-grained sandstone bed immediately below the J 18 mts. Sequence J18 (34/10-9, Fig. 8) is represented by fine-grained sandstones of assumed offshore and lower shoreface origin and is succeeded by a sharply based medium-grained sandstone unit. The sand-

stones are cross-stratified with mudrock partings, and are related to an estuarine environment (see also Dreyer and Wiig, 1995), with the basal surface representing a sequence boundary (SB). This estuarine facies is not present in well 34/8-1 (upper core, Fig. 8), where the sequence comprises fine-grained sandstones, intensively bioturbated by high-diversity trace fossil assemblages (including Palaeophycus and Zoophycos). These deposits reflect deposition in a marine environment, and are placed in the lower shoreface. The upper part of the sandstone body in well 34/8-1 is regarded as a transgressive sandstone as a possible ravinement surface is indicated within the sandy unit due to the presence of Glossifungites trace fossils. Sequences J20 and J22 are not covered by core data in the Tampen area. However, these sequences are dominated by mudrocks on the Tampen Spur, and a deep offshore environment is inferred. Importantly, shallow marine sandstones of J20 and J22 are seemingly restricted to the eastern part of the basin.

Biostratigraphy The maximum transgressive surfaces identified on wireline logs and sedimentologically in core have been calibrated biostratigraphically using a combination of palynology and micropalaeontology. The biostratigraphic zonation (Fig. 3) has been directly calibrated to a standard ammonite biostratigraphy following an outcrop study on the Yorkshire coast. The geochronological ages of the maximum transgressive surfaces are based on the time scale of Hardenbol et al. (1998), which in some instances differs slightly from that of Partington et al. (1993). The following terminology is used to describe the micropalaeontological and palynological events: LO: LCO: LAO: FAO: FCO: FO:

last occurrence (i.e. species "top" or extinction) last common occurrence last abundant occurrence first abundant occurrence first common occurrence first occurrence (i.e. species "base" or evolutionary inception)

The maximum transgressive surfaces have been defined, wherever possible, using the biostratigraphic criteria established by Partington et al. (1993). It has been possible to directly relate most of the bioevents seen in the North Viking Graben to the ammonitecalibrated Lower Jurassic sequence exposed along the Yorkshire coast (to the level of ammonite subzones) suggesting a provincial similarity that is useful for large-scale correlation. Consequently, this study has

Sequence stratigraphy of the Lower Jurassic Dunlin Group, northern North Sea provided an independent test on the stratigraphic calibration by Partington et al. (op. cir.) and is in effect the first detailed practical application of their framework to be published for the North Viking Graben area.

Sequence stratigraphy: calibration and interpretation The sequence stratigraphic interpretation of the Dunlin Group in the North Viking Graben is illustrated by means of three correlation lines depicting key wells relating most of the principal fields in the area (Fig. 1). Wells in the Oseberg area are shown in correlation line 1 (Figs. 9 and 10), correlation line 2 trends approximately N W - S E from the Tampen Spur to the Lomre Terrace and Horda Platform on the eastern margin of the Viking Graben (Figs. 11 and 12), and correlation line 3 trends approximately in an N-S direction along the western margin from the Tampen Spur southwards to the Gullfaks area (Figs. 13 and 14). For each of these lines there are two figures that show (1) a facies and sequence stratigraphic interpretation, and (2) a chronostratigraphic interpretation (Wheeler diagram).

J12 sequence

Age" late Sinemurian-earliest Pliensbachian. J12 mts calibration: latest Sinemurian, Raricostatum Zone, 196.05 Ma. Primary bioevent: below LO Liasidium variabile (event 30R). Example: 30/6-18, 3141 m. Comment: on a supra-regional scale the J12 mts probably equates to the Boreal third-order sequence Si4 mfs of Jacquin and De Graciansky (1998). Lithostratigraphic associations" Statfjord Formation (upper part) and "lowermost" Amundsen Formation. Regional correlation: upper part of the Statfjord megasequence 3 (Steel, 1993). The base of the Dunlin Group is interpreted as being slightly diachronous (latest Sinemurian-earliest Pliensbachian). It is not possible (within the biostratigraphic framework) to equate the J12 mrs to the inundation of the Statfjord Formation in this area as Partington et al. (1993) indicated for the South Viking Graben. If their biostratigraphic calibration is correct then the flooding of the Statfjord Formation is diachronous and the onset of Dunlin deposition is later in the North Viking Graben than further south and suggests that this boundary is even more diachronous than indicated in this study.

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Interpretation: Regressive part: the regressive part of the sequence was not studied in detail since it is represented by sediments of the "upper" Statfjord Formation. Transgressive part: the J12 sequence is only partly represented by sediments of the Dunlin Group. Basal sediments of the Dunlin Group constitute only the later stage transgressive component of the sequence. In some wells (e.g. 30/3-A-5, Fig. 7), it is represented by a marine shale within the lowermost few metres of the Amundsen Formation directly overlying the Statfjord Formation and/or transgressive marine sandstones of the Nansen Member. A series of minor sandstones with fining-upward cycles and representing transgressive backstepping facies is locally developed, such as in well 30/6-22. Significantly, these represent the first marine transgression within the Dunlin Group and the inundation of the Statfjord Formation. This transgressive event is dated as being earliest Pliensbachian since late Sinemurian markers are present within or below the Nansen Member (e.g. 30/6-18) and therefore post-dates the J12 maximum flooding surface of Partington et al. (1993). A thin marine shale unit separates the major progradational units of the Statfjord and Johansen megasequences (Steel, 1993). This represents a phase of marine transgression that according to Steel (op cir.) penetrated far towards the Norwegian hinterland and to at least block 31/3. In this study it is clearly identifiable as far east as well 31/2-3 on the Horda Platform (Fig. 11).

J13 sequence (new; this study)

Age: early Pliensbachian. J13 mts calibration: earliest Pliensbachian, Jamesoni Zone, 194.89 Ma. Primary bioevent: above LO Liasidium variabile (event 30R). Secondary bioevents: below LCO Ogmoconcha spp., notable LCO O. danica, LCO O. amalthei and LO Ogmoconcha sp. B Apostolescu (event 32); below LCO Cerebropollenites cf. thiergartii (event 34). Examples: 30/6-18, 3120.5 m; 30/3-2, 3227 m (see Fig. 9). 31/2-19S (Fig. 11). Comment: the base of the new sequence J13 is defined above the LO Liasidium variabile (event 30R). Thus this does not correspond to the J 12 mfs of Partington et al. (1993) which is defined below this event and calibrated to the late Sinemurian Raricostatum zone. The LO of L. variabile has in the present study been found at a level within the Nansen Member within the Statfjord Formation (e.g. 30/6-18). This seems to be a general feature in the studied area and therefore it is not possible to equate the J12 rots to the inundation of the Statfjord Formation as

Fig. 11. Sequence stratigraphic interpretation and facies correlation of line 2, Troll/Fram area.

e5

~..~~

~..~~ e5

t...,

Fig. 12. Chronostratigraphic interpretation of line 2, Troll/Fram area.

r

(3)

Fig. 13. Sequence stratigraphic interpretation and facies correlation of line 3, Gullfaks area.

~..~~

Q t...,

e5

r~ ~..~~ e~

~.~~

Fig. 14. Chronostratigraphic interpretation of line 3, Gullfaks area.

~n


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indicated by Partington et al. (op. cit.) for the South Viking Graben area. This suggests that the termination of the Statfjord Formation results from a series of transgressive events rather than one single event as indicated by Partington et al. (op cit.; Fig. 12). On a supra-regional scale the J13 mts correlates with the Boreal second-order transgressive-regressive cycle 5 mts and third-order sequence Si5 mfs of Jacquin and De Graciansky (1998). This transgressive event was also recognised by Van Buchem and Knox (1998) and Hesselbo and Jenkyns (1998) in Yorkshire. The former authors considered it to be a major, i.e. 2nd order, early Pliensbachian transgressive event of worldwide significance. Lithostratigraphie associations: Amundsen and Johansen Formations. Regional correlation" Johansen megasequence 4 of Steel (1993). Interpretation. Regressive part: Steel (1993) correctly noted that the lower boundary of his megasequence 4 (equivalent to sequence J13 of this study; see Fig. 4) is well defined by a marine shale unit (assigned to the Amundsen Formation) that separates the Statfjord Formation from the overlying Johansen Formation over many areas of the Horda Platform. On the Horda Platform and the Lomre Terrace, e.g. wells 31/2-19S, 35/10-1 and 35/11-2 (Fig. 11), the regressive phase is characterised by a series of coarsening-upward units. These form a wedge of marine shoreface-dominated sandstones that represents the initial progradation of the sequence. Marjanac and Steel (1997) attributed these Johansen Formation sandstones to a relative fall of sea level and deposition in a large delta confined within a broad incised valley. Within the sequence are mudstone units that may represent minor transgressive surfaces but these are not readily correlatable within current biostratigraphic resolution. The regressive phase, therefore, is probably characterised by a series of progradational cycles that reflect minor base level changes or variations in sediment supply within the shore zone. Elsewhere, and west of the Horda Platform, the regressive part of the sequence is mud-dominated and deposited in a marine shelf setting. However, on the Tampen Spur area (e.g. 34/2-4) the sequence is represented by a thick, stacked unit of aggradational sands that may have been sourced locally. Transgressive part: within the J13 sequence, sandstones of both estuarine and marine origin have been interpreted, e.g. 31/2-3 and 35/10-1 (Fig. 7). The superposition of estuarine sandstone facies above shelf deposits indicates significant shoreline migration with the J13 sequence. The interpretation of estuarine facies is in contrast to others (e.g. Steel, 1993) and is open to debate since core data are lim-

ited. Steel (op. cit.) noted the possible presence of alluvial sediments and more brackish-water conditions in the Johansen Formation in the most easterly wells on the Horda Platform but he generally considered the succession as representing nearshore and inner shelf deposits following Dor6 et al. (1984). An interval representing a possible estuarine/fluvial valley fill is also interpreted in the cored section of well 35/9-2 (Fig. 6) on the Mhl0y Terrace that may indicate a more easterly source of sediments into the Fram area. In general, the later transgressive component of the J13 sequence is represented by a fining-upward retrogradational cycle of backstepping facies (e.g. well 31/2-19S) which is overlain by a thin but widespread shaly level associated with the next marine transgression and the J14 mts (cf. Marjanac and Steel, 1997). In some wells (e.g. 35/10-1) the transition from aggradational sandstones to transgressive mudstones is relatively abrupt and represents abandonment. A major regionally extensive transgressive event tied to the J 14 mts and approximately coinciding with the early/late Pliensbachian boundary terminates this sequence and Johansen sandstone deposition. J14 sequence

Age: late Pliensbachian. J14 mts calibration: "earliest" late Pliensbachian, basal Margaritatus Zone, 191.5 Ma. Primary bioevents: above LO Gramminacythere ubiquita and LO Ogmoconchella mouhersensis (event 36).

Secondary bioevents: above FO Nannoceratiopsis senex/gracilis (event 40); above LCO Cerebropollenites cf. thiergartii (event 34); below FO Leuhndea spinosa (event B55R); above LCO Ogmoconcha spp. notable LCO O. danica, LCO O. amalthei and LO Ogmoconcha sp. B Apostolescu, 1961 (event 32). Examples: 30/6-9 and 30/3-2 (both with only moderate biostratigraphic constraints). Comment: this maximum transgressive surface corresponds to the J14 mfs of Partington et al. (1993) and the surface lies at a level closely associated with the first downhole appearance of early Pliensbachian marker species and above the inception (FO) of a suite of dinocyst species associated with "late" early Pliensbachian and younger sediments, notably, FO N. gracilis/senex (event 40R). The J14 mfs may coincide with the Boreal third-order sequence P14 mts of Jacquin and De Graciansky (1998) although the stratigraphic calibration is not certain. Lithostratigraphic associations: Burton and Cook (lower part) Formations. Regional correlation: lower part of the Cook mega-

Sequence stratigraphy of the Lower Jurassic Dunlin Group, northern North Sea sequence 5 of Steel (1993) based on direct comparison with well 31/2-5. Interpretation. Regressive part: the base of the Cook Formation in many instances (e.g. well 30/6-11, Fig. 9; 35/10-1, Fig. 7 and 34/4-5, Fig. 8) is represented by a sharply based coarsening upward sandstone. This surface is interpreted as a regressive surface of marine erosion (RSME) (cf. Marjanac and Steel, 1997) or marine downshift surface sensu Dreyer and Wiig (1995) that marks the initiation of relative sea-level fall (falling stage systems tract of their terminology). In proximal parts of the Gullfaks Field, Dreyer and Wiig (1995) noted that this section may be absent due to erosion from the subsequent maximum regressive phase. This is considered likely although it has not been possible to verify this suggestion due to the lack of appropriate well coverage in this study. The sequence is laterally extensive on the Horda Platform and Lomre Terrace and is typically represented by a coarsening upward sandy unit (e.g. 31/2-19S and 35/10-1, Fig. 7) interpreted as representing deposition in a marine middle to upper shoreface setting. Transgressive part: a widespread estuarine/fluvial valley fill unit (e.g. 34/2-4, Fig. 11; 35/4-1, Fig. 11; and 35/9-2, Fig. 6) is interpreted in the northern part of the study area. However, in many instances (e.g. wells 30/3-2 and 31/4-3) the transgressive part of this sequence is either condensed or absent as a result of erosion from the overlying sequence (see discussion of this event in the next sequence).

J15 sequence (new; this study)

Age: late Pliensbachian. J15 mts calibration" late Pliensbachian, intra-Margaritatus Zone, 191 Ma. Primary bioevent: below LCO Botryococcus spp. (event 44R). Secondary biovents: below LO Dentalina terquemi (event 42); above FO Luehndea spinosa (event B55R). Example: 30/6-7, 3022 m (see Fig. 9). Comment: it is uncertain how the J15 mts relates to the supra-regional Boreal third-order sequences of Jacquin and De Graciansky (1998) since they recognise a series of sequences around this stratigraphic level which are poorly defined in terms of palynology or micropalaeontology. The most likely candidate is the P15 mfs (or possibly P16 mfs). Lithostratigraphic associations: Cook Formation (part). Regional correlation" upper part of the Cook megasequence 5 of Steel (1993) based on direct comparison with wells 30/6-9 and 31/2-5 (see his fig. 8); upper

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part of the Cook-1 of Marjanac and Steel (1997) based on comparison with well 30/3-2. Interpretation. Regressive part: in wells 34/2-4 and 35/8-1 (Fig. 11) the regressive part is characterised by a coarsening-upward profile but elsewhere this part of the sequence is poorly represented or possibly truncated (beyond biostratigraphic resolution). Transgressive part: the sequence is dominated by a laterally extensive unit of "blocky" sandstones (e.g. wells 30/6-7 and 35/4-1) that frequently have sharp bases (e.g. well 30/6-7). Evidence from core (e.g. wells 30/3-4, 30/6-16 and 35/10-1, Figs. 6 and 7) suggests that these sandstones are of estuarine origin and the basal surface represents a sequence boundary cut during the preceding relative sea-level lowstand. Marjanac and Steel (1997) came to a similar conclusion based on the analysis of well 30/3-2 (see their fig. 7). These sand units appear to be correlatable over a large area of the North Viking Graben and were deposited onto a surface representing a marked intra-late Pliensbachian lowering of relative sea level. Steel (1993), however, interpreted the sandstones to represent a wave/storm-dominated shelf system and suggested that this unit is characteristic of a lowstand or forced regression. In either case they represent a period in which sandstone deposition extended much further beyond the Horda Platform. A time-equivalent unit is also present on the Tampen Spur (e.g. 34/2-4) which herein is also interpreted as of estuarine origin. but it is not clear whether this has been generated from an easterly source (as suggested by Steel, op. cit.) or westerly source. The late stage transgressive part of this sequence is poorly developed and is represented by either an abrupt termination of sand (e.g. well 30/3-4) or a thin, fining-upward unit (e.g. well 30/3-2, Fig. 9). Steel (op. cit.) also observed the abrupt termination of sandstone deposition and the thinness of the transgressive component. A major, regionally extensive transgressive event culminating in the J16A mts of intra-late Pliensbachian age terminated Cook sandstone deposition on the Horda Platform and most noticeably within the Oseberg area (Fig. 9). In this area significant sandstone deposition was not resumed until middle Toarcian times.

J16 sequence

Age: late Pliensbachian-early Toarcian. Comment: this study follows Partington et al. (1993) who distinguished two sequences within J16 and these are discussed separately below.

J16A sequence

Age: late Pliensbachian


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J16A mts calibration" late Pliensbachian, intra-Margaritatus Zone, 190.5 Ma. Primary bioevent: below LO Ogmoconcha/Ogmoconchella spp. (event 49R). Secondary biovents: below LCO Luehndea spinosa (event 50R); below LO Kraeuselisporites reissingeri (event 45R); above LCO Botryococcus spp. (event 44R); above LO Dentalina terquemi (event 42). Examples: 30/6-7, 2956 m (see Fig. 9), 35/9-2, 2694.5 m (core) (Fig. 6 and 11). Comment: Partington et al. (1993) discussed in detail the problems in distinguishing the J 16A and J 16B mts and stressed the importance of integrating both palynological and micropalaeontological data sets. In this study we suggest that this surface can be recognised in the North Viking Graben based on its relationship below the LO Ogmoconcha/Ogmoconchella spp. (event 49R) and the LCO Luehndea spinosa (50R). Its relationship below the FAO sphaeromorphs as suggested by Partington et al. (op. cit.) is difficult to determine in offshore settings without the use of cores or sidewall cores because of frequent cavings. As a guide we have also identified the surface using its relationship above the inception (FO) of the dinocysts Nannoceratopsis gracilis/senex and FO L. spinosa (B55R). Other events suggested by Partington et al. (op. cit.) namely FO Scriniocassis weberi and FO Maturodinium inornatum are less well documented. Likewise the intra-late Pliensbachian events LO Dentalina matutina and LO Haplophragmoides lincolnensis are known to be important elsewhere in the North Sea but are rarely recorded in this area of the North Viking Graben. The calibration of the J16A mfs to the supra-regional Boreal scheme of Jacquin and De Graciansky (1998) is unclear since they recognise many third-order sequences over this stratigraphic interval which are poorly defined in terms of palynology or micropalaeontology. Based on the available data the most likely candidates are the P16 mfs or P17 mfs although the latter, more pronounced event, is interpreted as early Spinatum Zone. Lithostratigraphic associations: Cook Formation (part). Regional correlation: this is equivalent to the lowest part of the Drake megasequence 6 of Steel (1993) based on direct comparison with wells 30/6-9 and 31/2-5 (see his fig. 8). Note in particular that Steel's Cook megasequence 5 only corresponds to the lower part of the Cook Formation and the J14/J15 sequences of this study. This exemplifies the problem with the broad usage of the term "Cook". This sequence also equates to the upper part of the Cook-2 of Marjanac and Steel (1997) based on comparison with well 30/3-2.

Interpretation. Regressive part: the J16A mts is a regionally significant transgressive event and marks a significant deepening of the basin during the late Pliensbachian which on the Tampen Spur and Horda Platform areas was maintained throughout the early and middle Toarcian. In the Oseberg area significant sandstone deposition terminated at this event, e.g. 30/6-22 (Figs. 7 and 9) which is earlier than in the Gullfaks area where Cook sandstones are commonly found until the J18 sequence. On the Horda Platform, the regressive part of the J16A sequence is represented by the first of a series of stacked coarsening-upward units of offshore, marine sediments reflecting cycles of (distal) shoreline progradation (e.g. 30/6-16, Fig. 7; 30/6-22, Fig. 9). In the Veslefrikk area (e.g. well 30/3-4, Fig. 9), laterally discontinuous sandstones are developed at the top of a much thicker, underlying coarsening-upward progradational unit. Lower shoreface and offshore, fine-grained sandstones and mudstones also dominate the sequence in the Tampen (e.g. 34/2-4, Fig. 11) and Gullfaks areas (e.g. 34/10-9, Fig. 8). Transgressive part: a sandstone-dominated unit in the core of well 35/10-1 (Fig. 7) on the Lomre Terrace is interpreted as representing deposition in an estuarine environment. The emplacement of estuarine sandstones above a shoreface sequence indicates a significant base level fall and possibly valley incision prior to the subsequent transgressive phase (cf. Marjanac and Steel, 1997). It is possible that this area provides the easterly source for sediments in the Gullfaks Field inferred by Dreyer and Wiig (1995). On the Horda Platform the transgressive component is typically represented by a thin fining-upward unit overlying a much thicker coarsening-upward unit (e.g. 30/6-22, Fig. 9). J16B sequence

Age: earliest Toarcian. J16B mts calibration: earliest Toarcian, Tenuicostaturn Zone, 189 Ma. Primary bioevent: above LO Ogmoconcha/Ogmoconchella spp. (event 49R).

Secondary bioevents: below LCO Luehndea spinosa (event 50R); above LO Kraeuselisporites reissingeri (event 45R). Example: 31/4-3, 2511 m (see Figs. 9 and 11); 30/6-9, 2784 m (Fig. 9). Comment: this corresponds to the J 16B MFS of Partington et al. (1993). They noted that palynological data alone are insufficient to distinguish this surface and suggest placing emphasis on the ostracods and

Sequence stratigraphy of the Lower Jurassic Dunlin Group, northern North Sea foraminifera. They defined the surface as being below the LCO Luehndea spinosa (50R) but above LO Ogmoconcha/Ogmoconchella spp. (event 49R) defining the Toarcian/Pliensbachian boundary. In the Yorkshire outcrop data the ostracods become extinct in the youngest ammonite subzone of the Spinatum Zone. We differ from Partington et al. (op. cit.) in not attaching weight to the benthic foraminifer LO Marginulina prima which we consider to range younger than the ostracods into the early Toarcian (Tenuicostatum Zone) and is only rarely recorded in this area. This is supported by onshore ranges (see Copestake and Johnson, 1989). In addition to the events cited above, this study also has used the LO Kraeuselisporites reissingeri (event 45R) to define Pliensbachian sediments and facilitate the positioning of this surface. We consider it necessary to use a (total) suite of bioevents to calibrate the surface rather than one single event. On a supra-regional scale the J 16B mts equates, with a high degree of certainty, to the Boreal third-order P18 mfs of Jacquin and De Graciansky (1998). Lithostratigraphic associations: Cook Formation, lower part of the Drake Formation. Regional correlation: lower part of the Drake megasequence 6 of Steel (1993) based on direct comparison with wells 30/6-9 and 31/2-5 (see his fig. 8 and the discussion under the J 16A sequence). It is also equivalent to the upper part of the Cook 2 sequence of Dreyer and Wiig (1995) (see Fig. 4). Note that the Cook-3 unit of Marjanac and Steel (1997) is considered to equate to the J18 sequence and not J16B sequence as indicated on their fig. 2 based on calibration with well 30/3-2. Interpretation. Regressive part: sandstones associated with the J16B sequence are rarely developed on the Horda Platform and Lomre Terrace and the J 16B mts reflects a significant deepening event within the early Toarcian. Its effect is most marked on the Lomre Terrace (e.g. 35/10-1; Figs. 7 and 11) and MfilOy Terrace (e.g. 35/9-2, Fig. 6) where more marine conditions were established. In the Oseberg area the sequence is locally condensed and mud-dominated as a result of either sediment by-pass or starvation (e.g. 30/6-16, Fig. 7). Dreyer and Wiig (1995) indicated that the boundary between Cook reservoir units 2 and 3 in the Gullfaks area corresponds to a Vailian-type sequence boundary and is associated with a major change in drainage pattern. This corresponds to a level within the J 16B sequence of this study based on calibration with well 34/10-B-4 (equates with well B in fig. 12 of Dreyer and Wiig, op. cit.). Transgressive part: the transgressive part of the J 16B sequence is generally represented by a thin, condensed, fining-upward sequence (e.g. 30/3-4, Fig. 9) that culminated in the regionally identifiable J 18 rots.

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In the Gullfaks area, the lack of a well-defined transgressive component may be explained by erosion from the overlying J 18 regressive phase (e.g. 34/11-1) where the interval is represented by a series of stacked coarsening-upward sandstones. J18 sequence

Age: early-middle Toarcian. J18 mts calibration: early Toarcian, Falciferum Zone, 187.75 Ma.

Primary bioevent: below LCO sphaeromorph acritarchs (event 60R).

Secondary bioevents: below LO Luehndea spinosa (event 55R); above LCO Luehndea spinosa (event 50R); above LO Kraeuselisporites reissingeri (event 45R). Examples: 30/6-9, 2766.5 m (see Fig. 9), 35/9-2 2682.5 m (core) (Figs. 6 and 11). Comment: details of the age calibration are discussed by Partington et al. (1993) who argue that the record of LO Luehndea spinosa above the surface suggests a calibration to at least the early Toarcian Tenuicostaturn Ammonite Zone based on the onshore borehole data of Riding (1987). Partington et al. (1993) indicated that the surface could be calibrated to the Exaratum Ammonite Subzone by extending the range of L. spinosa for unspecified reasons. Our study of the Yorkshire section provides positive support for this interpretation since the extinction of this species is, in fact, within the Exaratum Subzone. This correlates the transgressive surface to the global anoxic event of Jenkyns (1988). On a supra-regional scale that the J 18 mts coincides (on an ammonite zonal level) to the peak transgression of the Ligurian cycle of Jacquin and De Graciansky (1998, chart 6) and cycle 6 of De Graciansky et al. (1998, not fig. 2), the first of two transgressive-regressive second-order cycles that characterise the Jurassic of Europe. However, on a subzonal ammonite scale, i.e. Exaratum Subzone, the J18 mts appears to correlate to the Boreal third-order sequence Toal mfs rather than the more significant Toa2 mfs of Jacquin and De Graciansky (1998). Lithostratigraphic associations: Cook and Drake Formations. Regional correlation: lower part of the Drake megasequence 6 of Steel (1993) based on direct comparison with well 31/4-3 (see his fig. 9); Cook-3 of Marjanac and Steel (1997) based on comparison with well 30/3-2. The sequence is widespread in the northern North Sea, although sandstones are less common north of the Gullfaks area. Sandstones in the Oseberg, Troll and Fram areas (e.g. 35 / 10-1) have been lithostratigraphically assigned as units of the Drake Formation.

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Interpretation. Regressive part: the J18 mts is connected with the regional "mid"-early Toarcian Falciferum Zone anoxic event established over large areas of northwest Europe. On the Horda Platform (e.g. 30/6-22, Fig. 9) and Lomre Terrace (eg. 31/2-19S, Fig. 11) the regressive part of this sequence consists of a series of coarsening-upward sandstones. In the Gullfaks area (e.g. 34/11-1, Fig. 13) a lowstand wedge of deltaic to marginal marine deposits (Cook-3B/C) has been interpreted by Dreyer and Wiig (1995). Transgressive part: a series of backstepping transgressive sandstones are present on the Horda Platform and Lomre Terrace (e.g. 31/2-19S, Fig. 11). In the Tampen area a transgressive sandstone has also been identified in core in well 34/8-1 (Fig. 8). In the Gullfaks area sandstone deposition was terminated abruptly (e.g. 34/10-9) and the late stage transgressive part is poorly represented or condensed above an inferred estuarine sandstone unit (e.g. 34/10-9, Fig. 8). J20 sequence (new; this study)

Age: middle-late Toarcian. J20 mts calibration: middle Toarcian, Variabilis Zone, 184.86 Ma.

Primary bioevent: below LAO Chasmatosporites spp. (event 63R).

Secondary biovents: below LO Camptocythere toarciana (event 62R); above LCO sphaeromorph acritarchs (event 60R); above LO Luehndea spinosa (event 55R).

Example: 30/3-4, 2984 m (see Fig. 9). Comment: the ostracod Camptocythere toarciana provides a means of age calibration since this species which occurs above the rots (e.g. 34/8-1, 3007 m) ranges no younger than the middle Toarcian Variabilis Zone onshore (Lord, 1978 and Yorkshire outcrop data). On a supra-regional scale the J20 mts coincides with the Toa4 mfs of Jacquin and De Graciansky (1998). Lithostratigraphic associations: Drake Formation. This sequence includes the informally named "Drake sand unit" and the basal part of the Oseberg Formation as defined by Graue et al. (1987). Regional correlation" part of the Drake megasequence 6 of Steel (1993) based on direct comparison with well 31/4-3 (see his fig. 9); Cook-4 of Marjanac and Steel (1997) based on comparison with well 30/3-2; J18 cycle (part) of Parkinson and Hines (1995) based on direct comparison with well 34/8-1. There is potential to subdivide this unit into two separate sequences although this is currently beyond stratigraphic resolution. In some instances (e.g. 30/6-11, 3590 m) there is a potentially correlatable

M.A. Charnock et al. log signature that may represent a maximum transgressive surface but this requires further study. Interpretation. Regressive part: the lower part of this sequence is dominated by fine-grained sediments and consists of one or sometimes two (e.g. 30/3-2, Fig. 9) coarsening-upward units. In the Veslefrikk and Brage areas the regressive part of this sequence is represented by a progradational wedge of marine, lower and upper shoreface sandstones (e.g. 30/3-2 and 30/6-22, Fig. 7). The base of these sandstones may be represented by a marked change in log character interpreted as possibly representing an erosional hiatus and a basinward shift in deposition that Steel (1993) related to the tectonic uplift of the Horda Platform. This interpretation is considered likely since these sandstones are locally restricted to this area. In the Tampen area the sequence is dominated by mudrocks that were deposited in a relatively deep marine environment. Transgressive part: this part of the sequence maybe represented by a fining-upward mudstone-dominated interval (e.g. 30/6-7 and 31/4-3, Fig. 9) although in many wells on the Horda Platform (e.g. 30/6-11) it is truncated by later progradational phases of the Oseberg Formation.

J22 sequence

Age: late Toarcian-Aalenian. J22 mts calibration: late Toarcian, Levesquei Zone, 182 Ma.

Primary bioevent: below LCO Parvocysta/Phallocysta spp. (event 64R).

Secondary biovents: below LCO Haplophragmoides spp. (event 65R); above FCO Parvocysta/Phallocysta spp. (event B64R); above LAO Chasmatosporites spp. (event 63). Example: 31/4-3, 2385 m (Figs. 9 and 11). Comment: this transgressive event is defined using the same criteria as Partington et al. (1993). The age calibration of this surface is primarily based on its relationship within the total range of Parvocysta nasuta which ranges from the "earliest" Aalenian Opalinum Zone to the "latest" middle Toarcian Variabilis Ammonite Zone. In the context of the supra-regional Boreal third-order sequences of Jacquin and De Graciansky (1998) there is apparently no equivalent surface. Lithostratigraphic associations: Drake Formation and part of the Oseberg Formation as defined by Graue et al. (1987). Regional correlation: upper part of the Drake megasequence 6 of Steel (1993) based on direct comparison with well 31/4-3 (see his fig. 9).


Interpretation.

Regressive part: in some wells (e.g. 31/4-3, Fig. 9) a coarsening-upward unit of sandstones is present but more often there is an abrupt change in facies that corresponds to the boundary between the Dunlin and Brent Groups and in many cases, the base of the Oseberg Formation which is interpreted as representing a marked basinward shift in deposition (e.g. 30/6-7; compare with Rider, 1996, fig. 15.3). Note, however, that the initial deposition of sandstones conventionally assigned to the Oseberg Formation started within the previous J20 sequence in the Veslefrikk and Brage areas and that this represents a later phase of progradation. In the cored section of well 30/6-22 (Fig. 7) the section comprises four coarsening-upward, sandstone-dominated, lower to upper shoreface sequences that represent a complicated history of repeated shoreline progradation and subsequent transgression in an overall regressive regime. Transgressive part: the transgressive part of the sequence is outside the scope of the present study being represented by deposition within the Brent Group.

Reservoir potential In the North Viking Graben area the Cook Formation has proven hydrocarbons and is a secondary reservoir in the Statfjord, Gullfaks (Dreyer and Wiig, 1995), Oseberg and Veslefrikk Fields. The Drake Formation is also known to be gas-prone. The discussion of depositional environments and stratigraphic sequences have significant bearings on the reservoir prediction in the Dunlin Group. The sedimentological study suggests that the main reservoir sandstones were deposited within estuarine environments. Particularly, the vertical and lateral alternation between these sandy systems and deposits of shelfal and lower shoreface environments (Figs. 9, 11 and 13) indicates that sand emplacement throughout the Pliensbachian was controlled by major falls in relative sea level. These major sea-level falls brought about major basinwards translations of the contemporary shoreline, with pronounced valley incision. This suggests that forced regression is the main mechanism by which reservoir quality sand is distributed into the basinal areas. One of the problems of reservoir potential within the Dunlin Group is maintaining high porosity and permeability values at deep burial depth. Ehrenberg (1993) documented the preservation of anomalously high porosities in some late Pliensbachian sandstones in the Veslefrikk Field, due to the presence of graincoating chlorite. The presence of grain-coating chlorite is an important factor in preserving good reservoir

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quality at great burial depths although its occurrence alone does not guarantee these factors. The development of this mineral phase may be favoured by deposition in marine environments with a significant supply of fresh water, and can lead to the preservation of reservoir properties at burial depths below 4 km. Consequently, the estuarine setting is ideal for the formation of this mineral phase. A predictive model to explain the occurrence of chlorite in terms of the depositional environment and its position within a sequence stratigraphic framework may be highly valuable for ongoing exploration in the area. Chlorite coatings have been widely observed in both the northern and southern parts of the study area in sands restricted to sequences J 14 and J 15. These sequences are interpreted as representing an intra-late Pliensbachian period that was strongly affected by relative sea-level lowstands and the widespread development of estuarine systems. The forced regressive mechanism for sandstone emplacement in the Dunlin Group may also have led to deposition of reservoir lithologies outside the area of economic deposits in the Brent Group. For instance, the estuarine facies identified in well 34/2-4 (J15, Fig. 8) occurs well outside the northern pinchout of the Brent Group (see Graue et al., 1987; Johannessen et al., 1995). Similar observations have also been made in the Sogn area, where coarsegrained estuarine sandstones of sequences J14 and J15 (well 35/4-1, Fig. 11) occur below non-reservoir lithologies age-equivalent to the Brent Group.

Conclusions The main purpose of this study has been to define a sequence stratigraphic framework of the Lower Jurassic Dunlin Group within Norwegian Quadrants 30, 31, 34 and 35 of the North Viking Graben. It is intended that this regional study provides a framework in which more focused field and block studies can be viewed in a wider context and that the sequence stratigraphic methods employed provide a better understanding of sediment distribution patterns than conventional interpretations based on the lithostratigraphic scheme. Nine sequences are defined on the basis of their depositional systems and bounding surfaces utilising well-log correlations, detailed core descriptions and extensive biostratigraphic information. The boundaries represent maximum transgressive surfaces (mts) and mark the points between a series of regionally identifiable regressive-transgressive episodes in the Early Jurassic (latest Sinemurian to Toarcian) section of the North Viking Graben. These surfaces are calibrated to standard schemes established elsewhere

1 72

within the North Sea Basin and the ammonite-dated Lower Jurassic section exposed along the Yorkshire coast. More specifically, the final conclusions of the present study are: (1) The sequences have thicknesses of between 10 m and 100 m and a duration of approximately 1-4 million years. They consist of facies representing deposition in a variety of shelf, shoreline and estuarine systems. The significance of estuarine facies within this interval has been generally underestimated. (2) The J13 rots represents the terminating transgression of the Statfjord Formation. Sandstones of the Johansen Formation and deposited during the J13 sequence are locally developed on the Horda Platform. (3) The J13 sequence was terminated by a regionally identifiable transgressive event (J14 mts) which is related to the deposition of the Burton Formation. (4) The Cook Formation was deposited in five separate sequences (J14 to J18) which are characterised by thick, geographically widespread, estuarine and marine shoreface sandstones deposited during periods of lowstand incision, progradation and transgression. (5) The J16A mts (intra-late Pliensbachian) was a significant transgressive event that terminated Cook sandstone deposition in the Oseberg-Veslefrikk area although sandstone deposition persisted longer, into sequence J 18 (early Toarcian) in the Gullfaks field. (6) Two early Toarcian transgressions, namely J16B mts and J18 mts profoundly affected the later depositional styles since later sequences are dominated by marine sediments and more locally restricted sandstones deposited in positions centred on the Lomre Terrace and the Horda Platform on the eastern basin margin. (7) The relationship between the Dunlin Group and overlying Brent Group is more complex than previously envisaged. The J20 sequence includes sandstones that conventionally have been assigned to the Oseberg Formation. It can be demonstrated that, in the Veslefrikk and Brage areas, sandstone deposition began within the late Toarcian and that these sandstones represent the first of a series of progradational episodes on the Horda Platform and which continued across the Early-Middle Jurassic boundary.

Acknowledgements The authors would like to thank Norsk Hydro and Robertson Research International Ltd for permission to publish this study. We are also indebted to the partners of PL194 (Elf, Saga and Statoil) for permission to publish the results from this licence area. We would particularly like to thank Roger Davey (Robertsons) and the two reviewers Lars-Magnus Ffilt (Statoil) and Ron Steel (University of Wyoming) for


constructive criticisms and suggestions on improving the manuscript. This study was carried out within the research project on Deep Structures under the guidance of Harald Flesche and Mogens Ramm. They are gratefully acknowledged for help and encouragement. Within the context of the Deep Structures project we would especially like to thank Ruth Elin Midtbr for discussions on the reservoir potential of the sequences. Thanks are also directed to Gry Arnesen and Tom Thorstensen in the Norsk Hydro drafting department for support in the preparation of this paper.

References Badley, M.E., Egeberg, T. and Nipen, O., 1984. Development of rift basins illustrated by the structural evolution of the Oseberg feature, Block 30/6, offshore Norway. J. Geol. Soc., London, 141: 639-649. Cannon, S.J.C., Giles, M.R., Whitaker, M.E, Please, EM. and Martin, S., 1992. A regional reassessment of the Brent Group, U.K. Sector, North Sea. In: A.C. Morton, R.S. Haszeldine, M.R. Giles and S. Brown (Editors), Geology of the Brent Group. Geol. Soc., London, Spec. Publ., 61: 81-107. Copestake, E and Johnson, B., 1989. The Hettangian-Toarcian. In: D.G. Jenkyns and J.W. Murray (Editors), A Stratigraphical Atlas of Fossil Foraminifera. Ellis Horwood, Chichester, pp. 129-188. Dalrymple, R.W., Zaitlin, B.A. and Boyd, R., 1994. Estuarine facies models: conceptual basis and stratigraphic implications. J. Sediment. Petrol., 62:1130-1146. De Graciansky, E-C., Jacquin, T. and Hesselbo, S.P., 1998. The Ligurian cycle: an overview of Lower Jurassic 2nd-order transgressive/regressive facies cycles in western Europe. In: R-C. De Graciansky, J. Hardenbol, T. Jacquin and ER. Vail (Editors), Mesozoic and Cenozoic Sequence Stratigraphy of European Basins. Soc. Econ. Paleontol. Mineral. Spec. Publ., 60: 467-479. Dor6, A.G. and Gage, M.S., 1987. Crustal alignments and sedimentary domains in the evolution of the North Sea, North-east Atlantic Margin and Barents Shelf. In: J. Brooks and K. Glennie (Editors), Petroleum Geology of North West Europe. Graham and Trotman, London, pp. 1131-1148. Dor6, A.G., Hamar, G.E, Lilleng, T., Shaw, N.D., Skarpnes, O. and Vollset, J., 1984. Revised Jurassic lithostratigraphy of the Norwegian North Sea, northern area. In: J. Vollset and A.G. Dor6 (Editors), A Revised Triassic and Jurassic Lithostratigraphic Nomenclature for the Norwegian North Sea. Norwegian Petroleum Society (NPF) Special Publication, 3. Elsevier, Amsterdam, pp. 2-53. Dreyer, T. and Wiig, M., 1995. Reservoir architecture of the Cook Formation on the Gullfaks field based on sequence stratigraphic concepts. In: R.J. Steel, V. Felt, E.E Johannessen and C. Mathieu (Editors), Sequence Stratigraphy of the Northwest European Margin. Norwegian Petroleum Society (NPF), Special Publication 5. Elsevier, Amsterdam, pp. 109-142. Ehrenberg, S.N., 1993. Preservation of anomalously high porosity in deeply buried sandstones by grain-coating chlorite: examples from the Norwegian continental shelf. Am. Assoc. Pet. Geol., Bull., 77: 1260-1286. Eynon, G., 1981. Basin development and sedimentation in the Middle Jurassic of the northern North Sea. In: L.V. Illing and G.D. Hobson (Editors), Petroleum Geology of the Continental Shelf of North-West Europe. Heyden, London, pp. 196-204. Fa~rseth, R.B., 1996. Interaction of Permo-Triassic and Jurassic extensional fault-blocks during the development of the northern North Sea. J. Geol. Soc., London, 153: 931-944. Fjellanger, E., Olsen, T.R. and Rubino, J.L., 1996. Sequence stratigraphy and palaeogeography of the Middle Jurassic Brent and Vestland deltaic systems, Northern North Sea. Nor. Geol. Tidsskr., 76: 75-106.

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Mitchener, B.C., Lawrence, D.A., Partington, M.A., Bowman, M.B.J. and Gluyas, J., 1992. Brent Group sequence stratigraphy and regional implications. In: A.C. Morton, R.S. Haszeldine, M.R. Giles and S. Brown (Editors), Geology of the Brent Group. Geol. Soc., London, Spec. Publ., 61:45-80. Parkinson, D.N. and Hines, F.M., 1995. The Lower Jurassic of the North Viking Graben in the context of western European Lower Jurassic stratigraphy. In: R.J. Steel, V.L. Felt, E.R Johannessen and C. Mathieu (Editors), Sequence Stratigraphy of the Northwest European Margin. Norwegian Petroleum Society (NPF), Special Publication 5. Elsevier, Amsterdam, pp. 97-107. Partington, M.A., Copestake, P., Mitchener, B.C. and Underhill, J.R., 1993. Biostratigraphic calibration of genetic stratigraphic sequences in the Jurassic-lowermost Cretaceous (HettangianRyazanian) of the North Sea and adjacent areas. In: J.R. Parker (Editor), Petroleum Geology of North-West Europe: Proceedings of the 4th Conference. Geological Society, London, pp. 371-386. Rattey, R.P. and Hayward, A.B., 1993. Sequence stratigraphy of a failed rift system: The Middle Jurassic to Early Cretaceous basin evolution of the central and northern North Sea. In: J.R. Parker (Editor), Petroleum Geology of North-West Europe: Proceedings of the 4th Conference. Geological Society, London, pp. 215-249. Rider, M., 1996. Sequence stratigraphy and stratigraphy. The geological interpretation of well logs. Whittles, Caithness, 280 pp. Riding, J.B., 1987. Dinoflagellate cyst stratigraphy of the Nettleton Bottom Borehole (Jurassic: Hettangian to Kimmeridgian), Lincolnshire, England. Proc. Yorkshire Geol. Soc., 46:231-266. Ryseth, A., 2000. Sedimentology and palaeogeography of the Statfjord Formation (Rhaetian-Sinemurian), North Sea. In: O.J. Martinsen and T. Dreyer (Editors), Sedimentary Environments Offshore Norway - - Palaeozoic to Recent. Norwegian Petroleum Society (NPF), Special Publication 10. Elsevier, Amsterdam, pp. 67-85 (this volume). Ryseth, A. and Ramm, M., 1996. Alluvial architecture and differential subsidence in the Statfjord Formation, North Sea: prediction of reservoir potential. Pet. Geosci., 2: 271-287. Steel, R.J., 1993. Triassic-Jurassic megasequence stratigraphy in the Northern North Sea: rift to post rift evolution. In: J.R. Parker (Editor), Petroleum Geology of North-West Europe: Proceedings of the 4th Conference. Geological Society, London, pp. 299-315. Steel, R.J. and Ryseth, A., 1990. The Triassic-Early Jurassic succession in the northern North Sea: megasequence stratigraphy and intra-Triassic tectonics. In: R.F.E Hardman and J. Brooks (Editors), Tectonic Events Responsible for Britain's Oil and Gas Reserves. Geol. Soc., London, Spec. Publ., 55: 139-168. Underhill, J.R. and Partington, M.A., 1993. Jurassic thermal doming and deflation in the North Sea: implications of the sequence stratigraphic evidence. In: J.R. Parker (Editor), Petroleum Geology of North-West Europe: Proceedings of the 4th Conference. Geological Society, London, pp. 337-345. Van Buchem, F.S.E andKnox, W.O'B., 1998. Lower and Middle Liassic depositional sequences of Yorkshire (U.K.). In: E-C. De Graciansky, J. Hardenbol, T. Jacquin and P.R. Vail (Editors), Mesozoic and Cenozoic Sequence Stratigraphy of European Basins. Soc. Econ. Paleontol. Mineral. Spec. Publ., 60: 545-559. Van Wagoner, J.C., Posamentier, H.W., Mitchum, R.M., Vail, ER., Sarg, J.F., Loutit, T.S. and Hardenbol, J., 1987. An overview of the fundamentals of sequence stratigraphy and key definitions. In: C.K. Wilgus, B.S. Hastings, C.G.St.C. Kendall, H.W. Posamentier, C.A. Ross and J.C. Van Wagoner (Editors), Sea-Level Changes: An Integrated Approach. Soc. Econ. Paleontol. Mineral. Spec. Publ., 42: 40-45. Vollset, J. and Dor6, A.G., 1984. A revised Triassic and Jurassic lithostratigraphic nomenclature for the Norwegian North Sea. Nor. Pet. Directorate, Bull., 3, 53 pp. Woolam, R. and Riding, J.B., 1983. Dinoflagellate cyst zonation of the English Jurassic. Institute of Geological Sciences Report 83/2, pp. 1-42. Yielding, G., Badley, M.E. and Roberts, A.M., 1992. The structural

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evolution of the Brent Province. In: A.C. Morton, R.S. Haszeldine, M.R. Giles and S. Brown (Editors), Geology of the Brent Group. Geol. Soc., London, Spec. Publ., 61: 27-43.

M.A. CHARNOCK I.L. KRISTIANSEN A. RYSETH LEG. FENTON

Ziegler, EA., 1990. Geological Atlas of Western and Central Europe (2nd edition). Shell International Petroleum Maatschappij, The Hague, 239 pp.

Norsk Hydro Research Centre, P.O. Box 7190, N-5020 Bergen, Norway Norsk Hydro Exploration, N-0246 Oslo, Norway Norsk Hydro Research Centre, P.O. Box 7190, N-5020 Bergen, Norway Present address: Norsk Hydro Harstad, Storakern 11, Kanebogen, N-9401 Harstad, Norway Robertson Research International Ltd., Llandudno, North Wales LL30 1SA, UK

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Divergent development of two neighbouring basins following the Jurassic North Sea doming event: the Danish Central Graben and the Norwegian-Danish Basin Jan Andsbjerg, Lars Henrik Nielsen, Peter N. Johannessen and Karen Dybkja~r

The two neighbouring basins, the Danish Central Graben and the Norwegian-Danish Basin were both affected by the regional uplift of the North Sea and adjacent areas in the early Middle Jurassic that caused the formation of a regionally extensive unconformity. The uplifted area was not a simple dome structure but of a more irregular shape with an east-west oriented branch that included the Ringk~bing-Fyn High and much of the Norwegian-Danish Basin and the Fennoscandian Border Zone. Late Aalenian-Bajocian deposition was confined to fault-controlled depocentres in both the Danish Central Graben and the Norwegian-Danish Basin. An initial southward slope in the Danish Central Graben changed to a north- to eastward slope before the end of the Middle Jurassic, and the change possibly coincides with the formation of a conspicuous sequence boundary in the Bathonian. The depositional area began to expand in the late Middle Jurassic as a result of a regional sea-level rise. In the Danish Central Graben, accelerating half-graben subsidence during the Callovian-Early Kimmeridgian enhanced the sea-level rise. Several periods of rapid subsidence during the Callovian-Volgian (mainly in the Oxfordian-Early Kimmeridgian and latest Kimmeridgian-Middle Volgian) gave accommodation space to more than four kilometres of marine mud. A break in subsidence in the late Kimmeridgian, probably related to a change of fault directions, resulted in deposition of shallow marine sandstones on platforms and hanging-wall slopes. The Norwegian-Danish Basin was characterised by a small rate of subsidence and continuous expansion of the depositional area throughout the Late Jurassic. The slow subsidence and a large supply of sediment from the Fennoscandian Border Zone caused repeated progradational events from the northeast. Hydrocarbon discoveries are known only from the Danish Central Graben where Middle Jurassic and Upper Jurassic reservoirs have been charged from Upper and to a smaller degree Middle Jurassic source rocks. Within the Norwegian-Danish Basin, reservoir rocks are abundant in the Upper Triassic-lowermost Jurassic, the Middle Jurassic and Upper Jurassic successions. The presence of mature source rocks, however, is the main risk factor as they most likely only occur within Lower Jurassic mudstones deeply buried in rim-synclines and in local grabens.

Introduction

The Danish Central Graben (Fig. l a,b) is a mature hydrocarbon province, and though the principal production comes from Upper Cretaceous-Danian chalk, exploration of the Jurassic rift succession has shown several encouraging discoveries both in the Middle and Upper Jurassic. The Jurassic of the Danish part of the Norwegian-Danish Basin (Fig. l c) likewise contains both reservoirs and potential source rocks, but commercial hydrocarbon accumulation has not been found yet. The two neighbouring basins, the Danish Central Graben and the Norwegian-Danish Basin are located in an area that was affected by extensional movements during the Jurassic. Both the Danish Central Graben and the eastern part of the NorwegianDanish Basin show a relatively complete MiddleUpper Jurassic succession. However, whereas the Middle Jurassic succession in the two basins shows a

remarkable similarity, the Upper Jurassic succession differs strongly in thickness, distribution and type of sedimentary facies, and distribution and quality of reservoir and source rocks (Figs. 2 and 3). The variables that affected these patterns may include tectonic events, climate changes, eustasy and sediment supply. Among the tectonic events that had a pronounced influence on both basins was the formation of the "mid-Cimmerian" unconformity at the base of the Middle Jurassic. This regionally extensive unconformity evolved as a response to regional uplift that has been described by various authors including Eynon (1981) and Ziegler (1982, 1990). Recently, Underhill and Partington (1993) have presented a model for the early Middle Jurassic uplift in the North Sea that has been frequently cited. This model depicts the results of the uplift as a broad domal structure, that within its concentric periphery includes the Danish Central Graben, the Ringk~bing-Fyn High and much of the Norwegian-Danish Basin (Fig. 4). However,


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Fig. 1. (a) The North Sea area. The study area including the Danish Central Graben and the Norwegian-Danish Basin is outlined by a dashed frame. (b) Danish Central Graben with well locations; structures active in the Late Jurassic are shown. (c) Eastern part of the Norwegian-Danish Basin (Danish Basin) with well locations; structural elements in the Jurassic are shown.

both pre-Middle Jurassic subcrop and Middle-Upper Jurassic onlap data presented by Underhill and Partington (1993) differ significantly from the results of the present study (Fig. 5). This paper attempts to compare the two neighbouring basins in order to investigate the relative influence of the variables that determined the depositional pattern and distribution of reservoir and source rocks in the two basins. The comparison demonstrates that the uplift of the North Sea Basin included the large west-northwest-east-southeast trending RingkObingFyn High and the uplifted area formed a large arch rather than a concentric dome as postulated by Underhill and Partington (1993, 1994). Recently, the Danish Central Graben and the eastern part of the Norwegian-Danish Basin have been studied extensively by the Geological Survey of Denmark and Greenland (GEUS) and this work forms the basis for this paper. Based on well logs and palynological data from more than 50 wells, a sequence stratigraphic framework was established for the Jurassic in the Danish Central Graben by Andsbjerg and Dybkja~r (2001). This work was further supported by regional seismic lines and by sedimentological studies of all available Middle Jurassic and Upper Jurassic cores from the area. The core studies are presented in detail by Johannessen (2001), who mainly undertook studies of the Upper Jurassic and by Andsbjerg (1997,

2001) for the Middle Jurassic. For the eastern part of the Norwegian-Danish Basin, Nielsen (1995, 2001) has presented sedimentological and sequence stratigraphic studies based on well logs, palynological data and detailed core studies of more than 40 wells. Other papers relevant to this work include Johannessen and Andsbjerg (1993), Johannessen et al. (1996), Johannessen (1997), Ineson et al. (2001), Michelsen et al. (2001) and MNler and Rasmussen (2001) on the Danish Central Graben, and Michelsen (1989a,b) and Poulsen (1996) on the Norwegian-Danish Basin. The lithostratigraphic subdivision of the two basins is summarised in Fig. 6.

Geological background The Danish Central Graben and the NorwegianDanish Basin were formed as a result of plate reorganisations in Late Carboniferous-Early Permian time, and have both undergone a long complex history of differential subsidence (Ziegler, 1982; Vejb~ek, 1989). In Early Jurassic time, tectonic quiescence prevailed. A marine shelf covered both basins, and thick laterally consistent sequences of homogeneous mudstones were deposited under strong influence of eustatic changes (Michelsen, 1978; Pedersen, 1985; Nielsen, 1995, 2001). These conditions lasted until regional uplift occurred in earliest Middle Jurassic due to crustal pro-

Divergent development of two neighbouring basins following the Jurassic North Sea doming event

Fig. 1 (continued).

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Fig. 2. Chronostratigraphic summary diagram of the Danish Central Graben and the eastern part of the Norwegian-Danish Basin showing distribution of lithologies and interpreted depositional environments (modified from Nielsen, 1995, 2000; Andsbjerg and Dybkjam 2000; Johannessen, 2000). The chronostratigraphic scale is from Gradstein et al. (1995).

cesses related to the updoming of the central North Sea as described by Hallam and Sellwood (1976), Eynon (1981), Ziegler (1982, 1990) and Underhill and Partington (1993, 1994). The regional uplift and subsequent rifting caused development of grabens, which determined the depositional style, and thus the distribution of the Middle and Upper Jurassic sediments. In the Danish Central Graben, the thickest succession of Middle Jurassic deposits occurs in the SCgne

Basin and the Tail End Graben, along the main boundary fault at the western margin of the RingkCbing-Fyn High (Figs. lb, 2 and 3). A thin Middle Jurassic succession occurs across a wider area in the southern and southeastern part of the Danish Central Graben. Progressively younger Upper Jurassic deposits are found updip on the western hanging-wall slope of the Danish Central Graben. Middle Jurassic deposits extend across a large part

179


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of the Norwegian-Danish Basin and the Fennoscandian Border Zone. However, a thick Middle Jurassic succession is found only in the Sorgenfrei-Tornquist Zone, a narrow northwest-southeast faulted part of the Fennoscandian Border Zone that formed the transition from the Baltic Shield to the NorwegianDanish Basin (Figs. l c, 2 and 3). Progressively younger Upper Jurassic sediments occur updip along the southern margin of the basin.

Early-Middle Jurassic uplift The depositional conditions that prevailed during the Early Jurassic changed abruptly in the earliestMiddle Jurassic due to regional uplift. The restricted and oxygen-poor shelf environment established in the Norwegian-Danish Basin during the Early Toarcian continued throughout the Late Toarcian-Early Aalenian as a response to the early phase of the uplift (Michelsen, 1978; Michelsen and Nielsen, 1991; Nielsen, 1995,2001). Due to extensive and continued uplift, a highly erosive regional unconformity formed in the North Sea, in large parts of the NorwegianDanish Basin, on the RingkObing-Fyn High and in the Fennoscandian Border Zone (Fig. 2). In the southern part of the Danish Central Graben the resulting hiatus comprises the upper Pliensbachian, Toarcian

and parts of the Aalenian. The time span of the hiatus increases to the north, where Middle-Upper Jurassic strata onlap Triassic, Permian and Carboniferous rocks (Fig. 5; Johannessen et al., 1996). A few tens of kilometres southwest of the Danish Central Graben, in the Dutch Central Graben, continuous deposition of marine mud took place across the LowerMiddle Jurassic boundary. Deposition in that area was terminated by a later uplift that resulted in the development of a late Bathonian-Callovian unconformity (Van Adrichem Boogaert and Kouwe, 1993). The lowermost part of the Middle Jurassic succession that overlies the unconformity in the Danish Central Graben has been assigned an Aalenian?-Bajocian age based on palynological evidence (Figs. 6 and 7; Andsbjerg, 1997; Andsbjerg and Dybkja~r, 2001). In the fault-bounded Sorgenfrei-Tornquist Zone, slow subsidence occurred while the rest of the area was uplifted (Nielsen, 1995, 2001). In this area the uplift-related unconformity is replaced by a basinward shift in facies from offshore marine mudstones (Fjerritslev Formation) to shoreface sandstones (Haldager Sand Formation) overlying a regressive surface of marine erosion (Fig. 8, Haldager-1 and Vedsted-1 wells). The surface is dated to the Aalenian (top Opalinum Zone; Poulsen, 1996) similar to the age of the facies change that occurs at the marginal

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Fig. 4. "Mid-Cimmerian" unconformity subcrop patterns suggested by Underhill and Partington. Modified from Underhill and Partington (1993, 1994).

zone of the uplifted area in the northern North Sea (Underhill and Partington, 1993). On ramps dipping towards the Sorgenfrei-Tornquist Zone, the surface is developed as a marked erosional unconformity, which shows truncation of progressively older strata toward the RingkCbing-Fyn High (Fig. 5a and Fig. 8). Onlap of the overlying strata shows younging in the same direction (Fig. 5b and Fig. 8). On the shallowest parts of the Ringk0bing-Fyn High, the Lower JurassicTriassic successions were eroded. In southern Sweden, the early-Middle Jurassic uplift was accompanied by faulting, erosion and

volcanism with basalts intruding along northwestsoutheast trending faults and fracture zones (Norling and Bergstr6m, 1987; Erlstr6m et al., 1997). The oldest basalt is palaeomagnetically dated to a Toarcian-Aalenian age, radiometrically dated to the Bajocian (167 Ma), while related tuffites are dated to the Aalenian by palynology (Printzlau and Larsen, 1972; Tralau, 1973; Klingspor, 1976; Norling and Bergstr6m, 1987; Bylund and Halvorsen, 1993). Sedimentation changed from marine muds in the Early Jurassic to continental-paralic deposits in the Middle Jurassic which became confined to fault-bounded


181

Fig. 5. (a) Map showing subcrop to the "base Middle Jurassic unconformity" in the Norwegian-Danish Basin and the Danish Central Graben. (b) Map showing the Upper Jurassic marine onlap to the "base Middle Jurassic unconformity". Note the deepening of the truncation and the younging of the onlap toward the Ringkobing-Fyn High.

areas (Norling and Bergstr6m, 1987; Norling et al., 1993).

Fault-controlled deposition: Aalenian-Bajocian-early Bathonian The Aalenian-Bajocian deposits that overlie the intra-Aalenian unconformity are confined mainly to

narrow fault-controlled depocentres, where they attain a thickness of 150 to 250 m (Figs. 2, 7 and 8). The major depocentres were the SOgne Basin and the Tail End Graben, including its southern extension into the Salt Dome Province in the Danish Central Graben (Fig. l b) and the deep part of the Sorgenfrei-Tornquist Zone in the Norwegian-Danish Basin (Fig. l c). These grabens subsided slowly; sed-

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Fig. 6. Jurassic lithostratigraphy of the Danish Central Graben (left) and Norwegian-Danish Basin (right).

Fig. 7. Log correlation panel of the Middle Jurassic in the Danish Central Graben. The lower boundary of the Bryne Formation corresponds to the base Middle Jurassic unconformity that truncates Triassic rocks in the West Lulu-3 well and Lower Jurassic Fjerritslev Formation mudstones in the two southernmost wells. The base of the Middle Jurassic is not penetrated in the Amalie-1 well.

iment thickness combined with palynological data indicates a subsidence rate of about 10-20 m/m.y. The continuous existence of paralic environments in the grabens suggests that subsidence was balanced

by sediment input. The sediment package thickens towards the main boundary fault of the Danish Central Graben, indicating syn-sedimentary fault activity (Damtoft et al., 1992; Korstg5rd et al., 1993).


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Fig. 8. Log correlation panel of the Middle Jurassic and lower Upper Jurassic in the Norwegian-Danish Basin. In the Sorgenfrei-Tornquist Zone, a relatively continuous Lower-Middle Jurassic succession occurs, and the base Middle Jurassic unconformity corresponds here to an intra-Aalenian marine regressive surface of erosion at the base of the first shoreface sandstones (Vedsted-1 and Haldager-1 wells). On the ramps in Fars~-1 and Frederikshavn-1 wells, a major hiatus separates Toarcian mudstones of the Fjerritslev Formation from Bathonian? fluvial sandstones of the Haldager Sand Formation. Further to the southwest in the Vemb-1 well, Lower Pliensbachian mudstones are overlain by fluvial sandstones presumably of Callovian age.

In the Danish Central Graben, deposition of fluvial channel sands, overbank deposits and extensive lacustrine muds of the Bryne Formation dominated during the late Aalenian?-early Bathonian (Fig. 7). The presence of thin marine mudstones in the lower part of the succession in the southern part of the Danish Central Graben and contemporaneous marine mudstones in the Dutch Central Graben suggests a regional slope towards the south. In the northern part of the Danish Central Graben, the Bryne Formation is dominated by up to 10 m thick packages of channel sandstones that are separated by up to 50 m thick suites of floodplain deposits (Fig. 7; Andsbjerg, 2001). Individual chan-

nel units can be traced between most wells in the area suggesting that they represent laterally migrating channels. In the wells closest to the basin axis, lacustrine mudstones frequently dominate the floodplain deposits in the upper part of the succession. In the southern and southwestern part of the Danish Central Graben, floodplain and lacustrine deposits are more prominent, and channel sandstones are generally thinner and less correlatable. The presence of thin interbedded marine mudstones in this area, and more extensive marine mudstones in the Dutch sector a few tens of kilometres to the south, may suggest that the channel sandstones were deposited in minor distributary channels on a coastal plain.

184 In the eastern part of the Norwegian-Danish Basin, the deposition was confined to the deepest part of the narrow Sorgenfrei-Tornquist Zone bounded by the Fjerritslev and BCrglum Faults and their southeastward continuation in Kattegat, Oresund and SkSne (Figs. 2 and 8; Nielsen, 1995, 2001). The initial deposition of shoreface sands, 18-20 m thick, was succeeded by the deposition of transgressive muds and thin sands during the Aalenian. The marine mudstones were later incised and overlain by the fill of an estuary that was confined to the deepest part of the Sorgenfrei-Tornquist Zone. The estuarine fill is about 45 m thick and comprises fluvial sandstones that are overlain by lagoonal sandstones and mudstones with thin coaly seams and topped by barrier sandstones (Fig. 8). Transgressive marine mudstones and sandstones overlie the valley-fill. Based on a top occurrence of cysts of the dinoflagellate species Nannoceratopsis gracilis, these deposits are not younger than the lower Bajocian (Poulsen and Riding, 2001). After a sea-level fall accompanied by deep incision, rising sea level resumed the generation of accommodation space. Estuarine valley-fills, up to 25 m thick, were deposited consisting of fine-grained, muddy sandstones, thin mudstones and possibly coal seams (Fig. 8). The fills contain a mixed assemblage of marine and freshwater palynomorphs indicating an early Bathonian age. Deposition overstepped the Sorgenfrei-Tornquist Zone as accommodation space was created on the lower part of the basin-ward dipping ramps and braided fluvial channel sandstones were deposited (Fig. 9). The sandstones are sharply topped by lacustrine and lagoonal mudstones reflecting generation of further accommodation space. In general, however, accommodation space was limited during the Aalenian-Callovian as indicated by the absence of highstand and uppermost transgressive systems tracts, which presumably were cannibalised due to erosion during the subsequent sea-level falls (Nielsen, 1995, 2001).

Ramp to basin deposition: Bathonian-Callovian A pronounced and extensive intra-Bathonian sequence boundary occurs in both the Central Graben and the Norwegian-Danish Basin (Figs. 2, 7 and 8). Its formation was associated by changes in depositional patterns and sedimentary environments (Nielsen, 1995, 2001; Andsbjerg, 1997, 2001). Extensive channel deposits initially dominated sedimentation above the sequence boundary. On the ramps dipping towards the deep basins, braided-river deposits have been encountered above the sequence boundary in both the Danish Central Graben and in the Norwegian-Danish Basin (Figs. 9 and 10). In

J. Andsbjerg et al. more basin-ward settings, interbedded estuarine and fluvial deposits in the Danish Central Graben and interbedded estuarine and shallow marine deposits in the Norwegian-Danish Basin were formed. Braided-river gravel beds referred to the Bryne Formation are found above the intra-Bathonian unconformity in the Elly-3 well in the southwestern part of the Danish Central Graben (Figs. l b and 10). They occur as a 7.5 m thick unit of cross-bedded, mainly clast-supported, pebble conglomerate. On the gamma-ray log, this unit exhibits a pronounced blocky pattern, which can also be seen in the U-1 well, 35 km to the southeast (Fig. lb; Andsbjerg, 1997, 2001). In the northern part of the Danish Central Graben, an up to 40 m thick succession of incised valley-fill deposits belonging to the Bryne Formation occurs in several wells (Fig. 7; Andsbjerg, 1997, 2001). The incised valley is bounded at the base and laterally by the intra-Bathonian unconformity. The valley fill consists of cross-bedded sandstones with abundant double mud-drapes, mud-flasers and mudstone- and coal clasts (Fig. 10). Pebble layers, interpreted as basal channel lags, occur at the base of amalgamated sandstone units, and thinner mudstone units separate the sandstone units. Most of the succession was deposited within an estuary channel environment (Andsbjerg, 1997, 2001). In the Danish part of the Norwegian-Danish Basin, biostratigraphic evidence of the Bathonian-Callovian is poor (Michelsen, 1978; Poulsen, 1996). Fluvial erosion may have prevailed for a long period in large parts of the basin, and deep erosion into the Lower Jurassic succession occurs locally southwest of the Sorgenfrei-Tornquist Zone (Fig. 2; Nielsen, 1995, 2001). Also in the deep part of the SorgenfreiTornquist Zone, erosion appears to have been significant as upper transgressive to highstand systems tracts deposits of the underlying Bajocian-Bathonian sequence seem to be absent. On the ramps dipping toward the Sorgenfrei-Tornquist Zone, the intra-Bathonian sequence boundary is overlain by 520 m of mainly cross-bedded, medium-grained sandstones (Fig. 9). The sandstones were deposited by braided rivers as an initial response to the formation of accommodation space presumably in the late Bathonian-Callovian. In the more basin-ward parts of the Sorgenfrei-Tornquist Zone, the initial deposition on the sequence boundary was 5-10 m of transgressive shoreface sands (Figs. 2 and 8; Nielsen, 1995, 2001).

Regional transgression: Callovian-Oxfordian A regional Callovian-Oxfordian transgression caused the formation of interfingering paralic and


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Fig. 9. Cores from the FarsO-1 and Ars-1 wells in the Norwegian-Danish Basin; both wells are located in a ramp setting. The lower cores in both wells show two units of braided-river sandstones separated by lacustrine mudstones. The lower sandstone unit, presumably of Bathonian age is overlying Toarcian offshore mudstones of the Fjerritslev Formation; the boundary corresponds to the "base Middle Jurassic unconformity". The upper sandstone unit is sitting on an intra-Bathonian unconformity. The fluvial sandstones are capped by transgressive lagoonal beach deposits of the Flyvbjerg Formation. The late Callovian-earliest Oxfordian lagoonal transgressive surface is erosive and overlain by claystone clasts (Ars-1). The upper core in Ars-1 represents the extensive lagoonal deposits that developed in the Early Oxfordian as a response to the regional transgression. The lagoonal deposits are overlain by marine mudstones and thin sandstones that constitute the upper part of the Flyvbjerg Formation. For location see Fig. lb. For legend on sedimentary structures see Fig. 14.

shoreface deposits in both the SOgne Basin and the northern part of the Tail End Graben. A series of east- and northward prograding shoreface units of the Callovian Lulu Formation stepped back towards the west and southwest (Michelsen et al., 2001). This took place on the lower part of the hanging-wall ramp of the evolving half-graben (Figs. 2, 7 and 11). The final

transgression of this area was achieved at the end of the Callovian. Further south in the Danish Central Graben an extensive, low-energy coastal plain dominated by lagoons and coastal swamps was transgressed in the earliest Oxfordian (Fig. 7; Andsbjerg, 1997, 2001). In the northern part of the Danish Central Graben, the transgressive succession consists of an up to 4 m

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thick, regionally extensive coal bed overlain by two, approximately 10 m thick, sandstone units. Each unit consists of thin transgressive shelf sandstones overlain by thin offshore mudstones and by up to 8 m thick upward-coarsening shoreface to beach and back-barrier sandstones (Fig. 11). Less than 5 km updip on the hanging-wall slope the sandstone units wedge out to 2 m thick, upward-coarsening sandstones interbedded with the deposits of estuary channels, bay-head and tidal deltas, tidal flats and lagoons (Andsbjerg, 1997,2001). At the top of the succession, a transgressive conglomerate occurs, which is overlain by thoroughly bioturbated shallow marine siltand sandstones that are succeeded by fully marine mudstones of the Oxfordian to early Kimmeridgian Lola Formation. In the southern part of the Danish Central Graben, the transgressive succession is dominated by lagoonal and lacustrine mudstones of the Middle Graben Shale Formation. In this area, abundant coal beds at the base of the succession may locally attain a thickness of 7 m (Andsbjerg, 1997, 2001). The initial sea-level rise that created accommodation space over the intra-Bathonian sequence boundary in the Sorgenfrei-Tornquist Zone and on the lower parts of the flanking ramps continued in the Oxfordian (Fig. 2; Nielsen, 1995, 2001). In the Sorgenfrei-Tornquist Zone, up to 40 m of transgressive marine muds and thin sands belonging to the Flyvbjerg Formation were deposited on the shoreface sandstones that constitute the upper part of the Haldager Sand Formation (Fig. 8). Palynological data suggest that the change from deposition of shoreface sands to offshore muds occurred in the late Callovian (Poulsen, 1992, 1996). On the ramps on both sides of the Sorgenfrei-Tornquist Zone, deposition of braided fluvial sands was succeeded by deposition of lagoonal muds and thin sands occasionally with abundant rootlets (Figs. 2, 8 and 9). The deepening continued and the lagoonal deposits were overlain by transgressive shoreface sands and offshore muds, which show younging toward the northeast (Frederikshavn-2, Fig. 8). To the southwest the deepening is recorded by a few metres of lagoonal or marine mudstones (Vemb-1, Fig. 8). The lower part of the Flyvbjerg Formation thus shows backstepping up the ramps toward the Baltic Shield and toward the southwestern part of the basin testifying that these areas again became part of the depositional basin. Dur-

187

ing this time the Ringkc~bing-Fyn High functioned as a low-relief hinterland, that only supplied minor amounts of sediments to the basin, as indicated by the absence of significant sand at this stratigraphic level (Nielsen, 1995,2001).

Differential subsidence and transgression: Oxfordian-Kimmeridgian A significant difference in the rate of subsidence between the Danish Central Graben and the Norwegian-Danish Basin began in the Oxfordian. During the Oxfordian, a high rate of subsidence along the main boundary fault of the Danish Central Graben initiated the deposition of a 900 m thick succession of marine mudstones of the Lola Formation (Fig. 12). The rate of subsidence decreased and came to a temporary halt before the end of the early Kimmeridgian (Andsbjerg and Dybkja~r, 2001; Mc~ller and Rasmussen, 2001). The marine mudstones onlap the pre-Jurassic and Middle Jurassic on the hanging-wall slopes (Fig. 12; Mc~ller, 1986). Turbidite sands forming only a few metres thick unit, were deposited in the axial parts of the basin, and locally marginal marine sands accumulated on the hanging-wall slope (Andsbjerg, 1997; Andsbjerg and Dybkja~r, 2001). A low rate of subsidence still characterised the Norwegian-Danish Basin. The southwestern part of that basin was finally transgressed in the middle-Late Oxfordian and a thin, up to 15 m thick succession of marine offshore mudstones, that shows thinning toward the Ringk~bing-Fyn High, was deposited. In the latest Oxfordian, a short-lived regressive event of coastal progradation from the Baltic Shield deposited 5-15 m of fluvial sands that continued seaward into to 2-10 m of shell-bearing shoreface sandstones and siltstones constituting the upper part of the Flyvbjerg Formation (Figs. 2 and 8). The regression was followed by a renewed transgression that caused deposition of backstepping thin coastal sands that was succeeded by deposition of more widespread marine mud of the Bc~rglum Formation in the early Kimmeridgian (Figs. 2 and 8; Michelsen, 1978; Nielsen, 1995, 2001). Fully marine conditions with deposition of offshore mud probably dit not reach the Fennoscandian Border Zone before the late Kimmeridgian (Poulsen, 1992, 1996).

Fig. 10. Core and gamma-ray logs from the Middle Jurassic of the Danish Central Graben. In the Elly-3 well, the intra-Bathonian unconformity separates braided-river conglomerates above from a floodplain dominated succession below the unconformity. A thin marine mudstone is present near the base of the succession. In the West Lulu-3 and Amalie-1 wells the unconformity bounds an incised valley with a valley fill dominated by estuarine channel sandstones. For legend on sedimentary structures see Fig. 14.

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189

Fig. 12. Log correlation panel from the Oxfordian-Kimmeridgian in the Danish Central Graben. The asymmetry of the subsiding half-graben is evident from the dip line Elly-3-Nora-1. Onlap of the Heno Plateau by marine mudstones is seen in the north in the Feda Graben (Gert-1, Jeppe-1, Gwen-2 wells) and in the south (Ravn-1, Falk-1, Elly-3).

Pause in subsidence: late Kimmeridgian In the Danish Central Graben, rift-related subsidence ceased or slowed down significantly for a period in late Kimmeridgian time, possibly in relation to a shift in activity from north-south trending to northwest-southeast trending faults (Johannessen et al., 1996; M011er and Rasmussen, 2001). The cessation of rift-related subsidence and the associated decrease in accommodation-space generation, and possibly an increase in sediment supply, caused the progradation of shallow marine sands of the late Kimmeridgian Heno Formation in the northwestern parts of the Danish Central Graben (Fig. 12). Sands sourced from the Mid North Sea High prograded towards the east on the Heno Plateau (Fig. 13), while sands sourced from the Mandal High prograded towards the west on the Gertrud Plateau and in the Feda Graben (Fig. 12; Johannessen and Andsbjerg, 1993; Johannessen et al., 1996; Johannessen, 2001). In the southern part of the Feda Graben, syn-depos-

itional subsidence balanced by a large sediment supply caused the development of an up to 90 m thick succession of aggradational back-barrier sandstones (Johannessen and Andsbjerg, 1993; Johannessen et al., 1996; Johannessen, 2001; Andsbjerg and Dybkjam 2001). The back-barrier sandstones are interbedded with mudstones; strong bioturbation and abundant water-escape structures have destroyed primary sedimentary structures. Thin coals and abundant rootlets also occur. The sediments are typically arranged in 3-8 m thick upward-coarsening to upward-fining units (Fig. 14A). The back-barrier sandstones are separated from an overlying shoreface succession by a ravinement surface. On the Heno Plateau, which formed a part of the hanging-wall slope of the Danish Central Graben, a coarsening- to fining-upward succession of mainly very fine- to fine-grained sandstones was deposited (Fig. 2). The succession is in places more than 100 m thick and contains one or two distinct pebble conglomerate beds. The sandstones, which normally are

Fig. 11. Cores from the middle Callovian-earliest Oxfordian in the SCgne Basin. Lulita-1 well wedge out in the wells further updip, where estuarine deposits are overlain by Oxfordian offshore mudstones of the Lola Formation. The complete transgression of the Danish Central Graben. For legend on sedimentary structures

The upward-coarsening shoreface-dominated succession in the dominant. The Callovian shallow marine-paralic succession is succession represents a stepwise Callovian-earliest Oxfordian see Fig. 14.

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grid dimensions) and small length (< grid resolution) measurements are discarded. Using this methodology, all large length measurements are included, which may explain why the frequency distributions are slightly skewed towards large lengths. If so, the adjusted frequency curves (continuous line) accommodate both large and small lengths beyond the grid resolution and, hence, may be closer to the true length distribution. We also analyzed spatial changes of mud drape length distribution. Fig. 15A to C shows graphs in which the horizontal extent of mud drapes (corresponding to Fig. 14A to C, respectively) are crossplotted against their vertical coordinates. There is

a clear trend of upward-decrease in the length of mud drapes. Such features should be taken into account when constructing geologically realistic reservoir models for flow simulation. Stochastic reservoir models based solely on the length distribution of the mud drapes within the bulk volume of rock (Fig. 14A-C) would be misleading. For example, within tidal bar 1 (Fig. 14A,B), fluid migration may occur preferentially in the upper part of the bar where the horizontal extent of the mud drapes is smaller. Most mud drapes associated with cross-bedding (e.g. Assemblage V4 within tidal bar 1; Fig. 12) are isolated in two dimensions, which implies that their connectivity in three dimensions is extremely low. In certain instances, however, clustering of geologic features occurs, notably (1) lenticular bedding, and (2) thin wavy mud drapes. In both of these cases, as well as in the case of cross-bedding, clustering or high connectivity of objects commonly occurs periodically, and is constrained to several stratigraphic horizons.

Three-dimensional reconstruction of small-scale heterogeneities The three-dimensional architecture and lateral continuity of small-scale tidal sedimentary structures is poorly understood, because the existing models are based either on two-dimensional outcrop data (e.g.

Outcrop studies of tidal sandstones for reservoir characterization

Fig. 14. Frequency analysis for the horizontal extent of mud drapes: (A) along set boundaries (assemblages V3 and V4 in tidal bar 1 (outcrops U-S) on Fig. 8; grid 3.5 m high x 17.2 m wide, see Fig. 13); (B) along the foreset of trough cross-bedding (assemblages V3 and V4 in tidal bar 1 (outcrops V-U); grid 3 m high x 4.9 m wide); (C) associated with the strike section of trough cross-bed ("mega" flaser bedding) within Assemblage V3 in tidal bar 3 (outcrops H-E; grid 0.8 m high x 13.4 m wide).

Fig. 15 (right). Cross-plot between stratigraphic height against the horizontal extent of mud drapes: (A) along set boundaries (assemblages V3 and V4 in tidal bar 1 (outcrops U-S) on Fig. 8; grid 3.5 m high x 17.2 m wide, see Fig. 13); (B) along the foreset of trough cross-bedding (assemblages V3 and V4 in tidal bar 1 (outcrops V-U); grid 3 m high x 4.9 m wide); (C) associated with the strike section of trough cross-bed ("mega" flaser bedding) within Assemblage V3 in tidal bar 3 (outcrops H-E; grid 0.8 m high x 13.4 m wide).

249

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Fig. 16. Visualization of the reconstructed models of the rock specimens. Light colours denote sand; dark colours denote mud. Note that the dark colour on the top face of specimen two is caused by the 3-D perspective shading.

Reineck and Wunderlich, 1968), or on modern analogues in which the preservation potential is uncertain (e.g. Terwindt, 1981). We used serial sectioning techniques to reconstruct the 3-D architecture of smallscale tidal sedimentary structures directly from large rock specimens. The resulting rock models are a close representation of the rock specimens, and are not based upon synthetic data or stochastic modelling techniques. The rock specimens, measuring approximately 60 cm x 60 cm x 20 cm, were taken from Assemblages V5 and V6 (distal tidal bar facies) of the Vectis Formation, and are representative of facies found throughout the Tilje Formation and other heterogeneous tidal sandstone reservoirs. Specimen one predominantly contains lenticular sand lenses connected both horizontally and vertically; subordinate, but more continuous, wavy beds are also present. Specimen two predominantly contains isolated mud flasers embedded in sand. Both specimens were sectioned at ca. 20 mm intervals, and the section faces photographed. Each section photograph was digitized, and the boundaries between sand and mud traced. These boundaries represent the intersections between bedding surfaces and the section faces, and by manually correlating the traced boundaries between each 2-D section, the 3-D architecture of the bedding surfaces was reconstructed.

The reconstructed rock specimen models may be visualized using any standard reservoir modelling package. Fig. 16 shows the reconstructed models visualized using IRAP RMS (Smedvig Technologies, 1998). The advantage of the reconstructed models is that a range of visualization options is available. For example, the models may be sectioned in any orientation. Fig. 17A shows two orthogonal sections through specimen one; note that the lenticular sandbodies often "stack up" and are connected vertically. Fig. 17B shows two orthogonal sections through specimen two; note the variation in the lateral continuity and vertical connectivity of the mud flasers. Alternatively, individual blocks may be sampled from the whole model. Fig. 18 shows several core-plug-sized blocks sampled from specimen one; note that the lateral continuity of the sand and mud layers is likely to be higher at the scale of a core-plug than at the scale of the whole model. Finally, individual sand and mud layers may be viewed independently of the rest of the model. Fig. 19 shows sand and mud layers within specimen one; note that many of the lenticular sandbodies are connected horizontally in 3-D, although they may appear to be isolated in 2-D (Fig. 17A). Note also that the mud layers are not entirely laterally continuous where the sandbodies connect vertically. The rock specimens from which models one and two were reconstructed sample only a small volume


251

Fig. 17. Visualization of the reconstructed models of the rock specimens: (A) two orthogonal sections through specimen one; (B) two orthogonal sections through specimen two. Light colours denote sand; dark colours denote mud.

of heterolithic, small-scale tidal sedimentary structures. It is therefore possible that they are not generally representative of those structures. Nevertheless, the models provide valuable information about the 3-D distribution of sand and mud, which would not be available from 2-D observations. For example, 3-D visualization of specimen two (lenticular-wavy bedding) indicates that lenticular sandbodies may appear isolated in 2-D yet be connected in 3-D. Moreover, mud layers may appear continuous in 2-D, yet be discontinuous in 3-D where sandbodies connect vertically. These observations indicate that a proper understanding of the 3-D architecture of heterolithic, small-scale tidal sedimentary structures has a significant impact on our assessment of their reservoir quality, particularly reservoir connectivity.

The representation of small-scale heterogeneities in reservoir-scale models The representation of small-scale heterogeneities in reservoir-scale models is problematic: as yet, they cannot be represented explicitly, because the resolution of a reservoir-scale model grid is insufficient

(Haldorsen, 1986). Typically, they are grouped within facies types, and their "effect on flow" is represented using "averaged" petrophysical data obtained from core-plugs and well-logs. This approach is valid only if the averaged petrophysical data, measured at the scale of the core-plugs and well-logs, properly represents the effect on flow of the small-scale heterogeneities at the scale of a reservoir model grid-block (Haldorsen, 1986). We used the reconstructed model of specimen one to investigate numerically the effect of sample volume on the measured single-phase permeability of lenticular- wavy-bedded facies. The effect of sample volume was investigated by calculating the effective permeability of the rock model for a range of sample volumes, using the "sealed side" pressure solver of Warren and Price (1961). This is the numerical equivalent of taking cores of different sizes from the rock specimens, and measuring their permeability in the laboratory. The permeability of the sand and mud facies in the fine-grid model (sand (ks) and mud (km)) w e r e assumed to be uniform and isotropic, and permeability values were assigned on a facies basis. For

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Fig. 18. Individual blocks sampled from rock specimen one. Each block measures 8 x 8 x 2 cm. Light colours denote sand; dark colours denote mud, with some variation in the light colour of sand due to 3-D perspective shading.

each sample volume, effective (measured) permeability values (ke) w e r e derived for the x, y (horizontal) and z (vertical) directions, and expressed in terms of a dimensionless (normalized) permeability k, where k for a given direction is given by: ke -- km

k=

ks - km

Expressed in this way, the normalized measured permeability values for a facies are independent of the dimensional permeability values (ks and km) of the sand and mud; rather, they depend only upon the sand/mud permeability ratio: ks X/~ =

km

Fig. 20A, B shows the variation in the (arithmetic) mean measured permeability as a function of sam-

ple volume, for the case X~ = 10, 000. The mean horizontal permeability (k~) generally decreases with increasing volume. In contrast, the mean vertical

permeability (kz)oscillates with increasing sample volume, and no clear trend is present. Note the large contrast between the horizontal (k~) and vertical (kz) mean permeabilities, which is a characteristic feature of these heterolithic facies. For comparison, Fig. 20C, D shows the variation in the modal (most frequently measured) permeability as a function of sample volume. For the horizontal permeabilities (k~ and ky), the modal value oscillates as the sample size increases (Fig. 20C), and no clear trend is present. In contrast, for the vertical permeability (kz), the modal value generally increases with increasing sample size (Fig. 20D), and varies by almost an order of magnitude between the smallest and largest sample volumes used. These results demonstrate that the measured permeability varies with sample volume, and more significantly, that the "averaged" (modal and mean) measured permeability also varies with sample volume. This indicates that permeability distributions obtained


253

Fig. 19. Individual sand and mud layers within rock specimen model one. Images A and B show sand and mud layers towards the top of the model; images C and D show sand and mud layers towards the base of the model. Light colours denote sand; dark colours denote mud.

from numerous core-plug and well-log measurements of complex bedform-scale sedimentary structures do not properly represent their effective permeability in reservoir-scale models. Conclusions Tidal deposits in the Lower Cretaceous of southern England (Vectis Formation) have been used as outcrop analogues for heterolithic tidal sandstone reservoirs. Quantitative data have been collected to constrain numerical flow simulations at a variety of scales. The key results are as follows. (i) The outcrops comprise meso- to low macrotidal estuarine deposits. The main outcrop area is characterized by vertically stacked, coarseningupward units and is interpreted as a composite tidal bar formed within the inner part of a broad, mixedenergy, estuary (incised valley). (ii) Reservoir heterogeneities have been characterized at the following scales: (1) large-scale (external

geometry defined by key stratal surfaces-sequence boundaries and flooding surfaces); (2) intermediatescale (internal geometry defined by sand bar accretion surfaces, bar abandonment surfaces and facies boundaries); and (3) small-scale (internal facies variability covering lenticular-wavy-flaser bedding). (iii) Quantification of small- to intermediate-scale heterogeneities, notably mud drapes along the set boundaries and on cross-bed foresets, has been achieved by placing a grid on small, representative areas of the 2-D outcrop and measuring the coordinates of mud drapes. The length of most mud drape types exhibits a close to log-normal distribution. In addition, mud drape length shows a progressive upwarddecreasing trend within each upward-coarsening unit. (iv) Reservoir characterization of small-scale heterogeneities (lenticular-wavy-flaser bedding) has been achieved through the use of serial sectioning techniques to reconstruct their 3-D architecture directly from large rock specimens. (v) The reconstructed rock specimen models have

S. Yoshida et al.

254

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Fig. 20. The variation in measured (normalized) permeability with sample volume, for specimen one with a sand/mud permeability ratio of Xk -- 10, 000. Plot A shows the horizontal mean measured permeability (kx); plot B shows the vertical mean measured permeability (kz). In both plots, the error bars denote one standard deviation; note that the decrease in size of the error bars with increasing sample volume reflects in part the decrease in the number of samples measured owing to the finite size of the rock specimen model. Plot C shows the horizontal modal measured permeability (kx and ky); plot D shows the vertical modal measured permeability (kz). In both plots, the error bars denote the bin sizes used to obtain the modal permeability.

been used to investigate representative reservoir properties for individual facies types. For lenticularwavy-bedded facies, both the mean and modal measured single-phase permeability varied with sample volume, which indicates that "averaged" well-log and core-piug measurements do not yield representative permeability data for these facies. Consequently,

it may not be valid to assign rock properties to heterolithic facies in tidal reservoir models, using conventional well data. (vi) Quantitative outcrop studies offer one approach to obtain more representative reservoir properties for heterolithic tidal facies.


Acknowledgements This study is part of the FORCE project (Forum of Reservoir Characterization and Reservoir Engineering) on tidal sandstone reservoir characterization, funded by BP-Amoco, Norske Conoco A/S, Fortum Petroleum A/S, Saga Petroleum (now Norsk Hydro), and Statoil. The authors gratefully acknowledge these sponsors, and the Norwegian Petroleum Directorate for helping to coordinate the project. Smedvig Technologies are thanked for providing the IRAP RMS software. We appreciate our discussions with Roland Goldring on the fauna and ichnofacies of the Wealden Group. Robert Dalrymple, Brian Willis, Duna Mellere and Steve Flint, Duna Mellere and Lars-Magnus F~ilt, as well as other geoscientists at Statoil, are thanked for their comments and constructive suggestions. Field and technical assistance was provided by John Dennis, Tom Huang, Zoe Hansen, Francis Longworth, Richard Evans, Susan Nuttall, Innocent Ofoma, Nigel Huggins and Mark Buckley. The paper has greatly benefited from the reviews of Brian Zaitlin and Phillip Ringrose. Nick Lee proofread and improved our final manuscript. We thank the editors Ole Martinsen and Tom Dreyer for helping with preparation of the final manuscript.

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M.D. JACKSON H.D. JOHNSON A.H. MUGGERIDGE A.W. MARTINIUS

257 Yoshida, S., 2000. Sequence stratigraphy and facies architecture of the upper Blackhawk Formation and the Lower Castlegate Sandstone (Upper Cretaceous), Book Cliffs, Utah, USA. Sediment. Geol., 136: 239-276. Yoshida, S., Willis, B.J. and Miall, A.D., 1996. Tectonic control of nested sequence architecture in the Castlegate Sandstone (Upper Cretaceous), Book Cliffs, Utah. J. Sediment. Res., 66: 737-748. Zaitlin, B.A., Dalrymple, R.W. and Boyd, R., 1994. The stratigraphic organization of incised-valley systems associated with relative sea-level change. In: R.W. Dalrymple, R. Boyd and B.A. Zaitlin (Editors), Incised-Valley Systems: Origin and Sedimentary Sequences. Soc. Econ. Paleontol. Mineral. Spec. Publ., 51: 45-60.

Centre for Petroleum Studies, T.H. Huxley School of Environment, Earth Sciences and Engineering, Imperial College, Prince Consort Road, London SW7 2BP, UK Present address: Surface Processes and Modern Environments Research Group, Department of Geology, Royal Holloway, University of London, Egham, Surrey TW20 OEX, UK Centre for Petroleum Studies, T.H. Huxley School of Environment, Earth Sciences and Engineering, Imperial College, Prince Consort Road, London SW7 2BP, UK Centre for Petroleum Studies, T.H. Huxley School of Environment, Earth Sciences and Engineering, Imperial College, Prince Consort Road, London SW7 2BP, UK Centre for Petroleum Studies, T.H. Huxley School of Environment, Earth Sciences and Engineering, Imperial College, Prince Consort Road, London SW7 2BP, UK Statoil Research Centre, Arkitekt Ebbellsveg 10, N-7005 Trondheim, Norway Present address: c/o Statoil Venezuela - Sincor Project, N-4035 Stavanger, Norway


259

Lower Cretaceous (Barremian-Albian) deltaic and shallow marine sandstones in N o r t h - E a s t G r e e n l a n d

-- sedimentology,

sequence stratigraphy and regional implications Michael Larsen, Tor Nedkvitne and Snorre Olaussen

Lower Cretaceous sandstones of the Steensby Bjerg Formation are exposed in northern Hold with Hope where they form an unusual coarse-grained clastic wedge in the otherwise mudstone-dominated 'Cretaceous succession of North-East Greenland. The sandstones were deposited as early post-rift basin fill, in a deltaic and shallow marine environment during overall sea-level rise following intense faulting, block rotation and subaerial erosion in the Late Jurassic-earliest Cretaceous. Seven major facies associations are identified and the succession is divided into unconformity-bounded units. Deposition started with transgressive shoreface sandstones in the early Barremian followed by at least two phases of southwards delta progradation and valley incision during the Barremian-early Aptian. In the late Aptian-early Albian the deltaic system backstepped and progressively deeper-water facies were deposited. The coarse-grained clastic system was drowned in the early Albian and marine mudstones dominated from this time and onwards. A tectonic phase with renewed uplift and submarine erosion in the middle Albian is represented by an angular unconformity between mudstones of the Steensby Bjerg Formation and mudstones with intercalated sandy turbidites of the overlying Home Forland Formation. The position of the Hold with Hope clastic wedge at a relay ramp in the western bounding fault of the Mesozoic rift basins implies that Lower Cretaceous, shallow marine sandstone wedges may be predicted to form a new reservoir play model along steps in older Mesozoic lineaments in the Mesozoic basins offshore Norway and the West Shetland-Faeroe Basin.

Introduction

The Upper Permian-Jurassic sedimentary succession in East Greenland has long been recognised to be an important analogue for the petroliferous basins offshore Norway and in the northern North Sea (e.g. Surlyk et al., 1986; Stemmerik et al., 1993; Price and Whitham, 1997). The recent move of exploration to the western, deep-water areas has, however, changed the interest towards the Cretaceous-Palaeogene succession. The rift basins of North-East Greenland (north of 72~ include well-exposed CretaceousPalaeogene sediments with a pre-drift position approximately 100-150 km northwest of the Gjallar Ridge (Fig. 1). Data from the onshore sections may help to derive new play models for the offshore basins on the western Norwegian Shelf. The Cretaceous succession of North-East Greenland is more than 2 km thick, and consists of siliciclastic, mainly marine sediments deposited following a major rift phase and reorganisation of the basin in the latest Jurassic-earliest Cretaceous (Surlyk, 1978). During the Barremian-Aptian, the rift topography became submerged and offshore mudstones

were deposited in most of the East Greenland Basin (Donovan, 1957) (Fig. 2). Fine-grained marine sediments onlap Triassic and Jurassic sediments on the footwall of rotated fault blocks at Traill 0, Geographical Society 0 and Wollaston Forland. Only locally, coarse-grained deltaic and shallow marine sandstones were deposited in this overall transgressive stage. The best-exposed example of these Lower Cretaceous sandstones is the Steensby Bjerg Formation of northern Hold with Hope (Figs. 2 and 3). The Upper Cretaceous succession in East Greenland consists mainly of offshore mudstones with subordinate turbidite sandstones and fault-scarp derived conglomerates (Donovan, 1957; Stemmerik et al., 1993, 1997; Surlyk and Noe-Nygaard, 2001) (Fig. 2). The palaeocurrent directions changed from mainly coast-parallel (north-south) during the Early Cretaceous to offshore-directed flows (eastwards) in the Late Cretaceous (Stemmerik et al., 1997; Whitham et al., 1999). Koch (1931) and Maync (1949) described the succession at Hold with Hope, but not until recently (Kelly et al., 1998) a formal lithostratigraphy for the Barremian-lower Albian sandstones and mudstones


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Fig. 1. Pre-drift reconstruction of the northern North Atlantic showing Cretaceous outcrops in East Greenland, major faults and the position of the study area relative to the Vcring Basin. Note the relatively short distance between the study area in North-East Greenland and the Gjallar Ridge on the Norwegian Shelf, which is the area of recent drillings (1999) for hydrocarbons. Pre-drift reconstruction by courtesy of SAGA Petroleum.

Fig. 2. Stratigraphic scheme of the Barremian-Maastrichtian succession in North-East Greenland. Based on Donovan (1957), NChr-Hansen (1993), Kelly et al. (1998), Surlyk and Noe-Nygaard (2001) and H. NChr-Hansen (pers. commun., 1999).

Lower Cretaceous (Barremian-Albian) deltaic and shallow marine sandstones in North-East Greenland

261

Fig. 3. Lower Cretaceous (Barremian-lower Albian) sediments (LC) of the Steensby Bjerg Formation are exposed along the north coast of Hold with Hope in North-East Greenland. The sandstone-dominated Lower Cretaceous succession unconformably overlies rotated Lower Triassic (LT) and Middle-Upper Jurassic (MJ) strata. The succession is overlain by a thin horizon of sandstones and conglomerates of Paleocene? age overlain by Paleocene and Eocene flood basalts (P). View towards the east. The Lower Cretaceous succession (LC) is approximately 300 m thick.

of the Steensby Bjerg Formation and the overlying middle Albian-Santonian mudstones of the Home Forland Formation was established (Fig. 4). A general sedimentological interpretation dividing the succession into coarse shallow marine, fine shallow marine and coarse basinal facies associations was presented by Whitham et al. (1999). They furthermore identified six coarsening-upward sequences and two sequence boundaries within the Lower Cretaceous sandstones. In this paper, we present a sedimentological facies analysis and sequence stratigraphic interpretation of Steensby Bjerg Formation based on fieldwork in 1996-1998. The formation is up to 300 m thick and exposed in the coastal cliffs of northern Hold with Hope for a distance of approximately 18 km (Fig. 5). The studied succession is deposited as the result of a sea-level rise followed by at least two episodes of southward delta progradation and valley incision. The sandstone-dominated depositional system was drowned during Aptian marine flooding and the delta stepped back towards the north. From the late-early Albian mudstone deposition prevailed. Reconstruction of the Lower Cretaceous palaeogeography in the North-East Greenland Basin shows that the sand-rich deltaic system on Hold with Hope is located at a major relay zone in the western boundary fault of the Mesozoic rift basins (Whitham et al., 1999). Similarly located Lower Cretaceous, shallow

marine sandstone wedges may be present in similar structures in the older Mesozoic lineaments in the Norwegian Sea and Faeroe-Shetland Basin and may there form important new exploration targets.

Regional setting A series of north-south elongate sedimentary basins of late Palaeozoic-Palaeogene age is exposed along the East Greenland margin due to Neogene uplift. The basins formed due to extensional collapse following the Caledonian orogeny and later episodes of rifting (Surlyk, 1990). The basins are the result of a complex series of tectonic events caused by plate movements and reorganisations and thermal contraction. Mesozoic rifting culminated in North-East Greenland in Late Jurassic-earliest Cretaceous time (Vischer, 1943; Surlyk, 1990). The Lower Cretaceous succession overlies the degraded rift topography of this tectonic phase. The Mesozoic rift basins of East Greenland are mainly bounded by eastward-dipping normal faults (Vischer, 1943; Haller, 1970). Between the Traill O-Geographical Society 0 area and the Wollaston Forland area the major north-south-trending fault zone steps approximately 50 km towards the east forming a major southward-facing relay zone across Hold with Hope (Whitham et al., 1999) (Fig. 1).

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Fig. 4. Litho- and biostratigraphy of the Lower Cretaceous succession in northern Hold with Hope. Lithostratigraphy based on Kelly et al. (1998). Biostratigraphy based on Kelly et al. (1998), NChr-Hansen (1993) and unpublished work by H. Nchr-Hansen (pers. commun., 1999). See Fig. 5 for location.

The western part of Hold with Hope probably represents the southwards extension of the fault block forming a structural high exposing westerly- and southwesterly-dipping crystalline basement in Clavering 0 (Vischer, 1943; Stemmerik et al., 1993). The Clavering 0 high is bounded to the east by an eastward-dipping normal fault, which probably has its southwards extension in Fosdalen at Hold with Hope (Fig. 5). Kelly et al. (1998) suggested that the Fosdalen fault was active in Early Cretaceous time and thereby influenced deposition of the Lower Cretaceous sediments. A number of smaller synthetic faults present in the footwall block of the Fosdalen fault seem also to have been active in the Early Cretaceous. The Lower Cretaceous succession rests with an angular unconformity (3~ ~ on Lower Triassic (Wordie Creek Formation) (Koch, 1931; Nielsen, 1935; Maync, 1949) and Middle-Upper Jurassic strata (Pelion, Payer Dal and Bernbjerg Formations) (Stemmerik et al., 1997; Kelly et al., 1998; Larsen et al., 1998). The unconformity, Ukl of Kelly et al. (1998), was formed following Late Jurassic-earliest Cretaceous rifting associated with block faulting and rotation (Fig. 4). The Lower Triassic and Jurassic strata are, thus, rotated in 0.5 to 2 km wide fault

blocks and dip 5~ 14~ towards the southwest, whereas the overlying Lower Cretaceous strata dip approximately 3~ ~ towards the southwest. In Steensby Bjerg, a number of north-south-striking normal faults of Late Jurassic-Early Cretaceous age can be seen to terminate against the unconformity at the base of the Cretaceous succession (Larsen et al., 1998). Most of the Mesozoic faults in northern Hold with Hope, however, were reactivated during the Palaeogene and a number of new, southwest-northeast-striking faults were formed which cross-cut the Cretaceous sediments and the overlying basalts. The Cretaceous succession is truncated by an erosional unconformity, Ut of Kelly et al. (1998), suggested to be of Paleocene age (Maync, 1949; Upton et al., 1980). In northern Hold with Hope, up to 24 m of sandstones and conglomerates with subordinate mudstones overlie the unconformity. These sediments of possible Paleocene age are overlain by up to 900 m of Paleocene-Eocene flood basalts (Upton et al., 1980).

Stratigraphy The Cretaceous succession in northern Hold with Hope consists of the sandstone-dominated Barremian-


263

Fig. 5. Geological map of the studied area in northern Hold with Hope. Geographical names mentioned in the text and the position of the geological cross-sections shown in Figs. 6, 7 and 13 are indicated.

lower Albian Steensby Bjerg Formation, up to 300 m thick, unconformably overlain by the mudstone-dominated middle Albian-Santonian Home Forland Formation, up to 1000 m thick (Kelly et al., 1998) (Fig. 4). In the studied sections only the lowest part of the Home Forland Formation, the Fosdalen Member, is exposed. The sediments crop out in two areas separated by the wide valley of the river Gulelv (Fig. 5). East of the river at Steensby Bjerg-Diener Bjerg, the Steensby Bjerg Formation is divided into the Diener Bjerg (lower Barremian), the Gulelv (Barremian-Aptian) and the Rc~delv (lower Aptian) Members (Kelly et al., 1998) (Fig. 4). At Diener Bjerg and in the eastern part of Steensby Bjerg the formation is unconformably overlain by the Fosdalen Member (middle Albianupper Turonian). At Stensi6 Plateau to the west of Gulelv, the Steensby Bjerg Formation consists of the Stribedal (lower Barremian), Blfielv (upper Barremian?), Stensi6 Plateau (upper Barremian-lower Aptian), Gulelv (Aptian-lower Albian) and the ROdelv (lower A1bian) Members (Kelly et al., 1998) (Fig. 4). The

litho- and biostratigraphy was thoroughly discussed by Kelly et al. (1998) and ages were assigned to the lithostratigraphic units based on dinoflagellate cysts and ammonites. Our observations roughly confirm the established stratigraphy; however, a detailed palynological study (H. NOhr-Hansen, pets. commun., 1999) suggests, that the age of the Stribedal Member should be earliest Barremian Nelchiopsis kostromiensis Subzone (I1) of N0hr-Hansen (1993) and not the Pseudoceratium anaphrissum Subzone (I2) as stated by Kelly et al. (1998). This shifts the timing of the inundation of the Hold with Hope block down towards the Hauterivian-Barremian boundary. The Cretaceous succession is bounded by two major angular unconformities (Ukl and Ut of Kelly. et al., 1998) as described above. In addition, four unconformities are present within the Cretaceous succession. They are of late Barremian (SB1), early Aptian (SB2), late Aptian (SB3) and late-early A1bian (SB4) age (Fig. 4). SB 1-SB3 are characterised by erosional surfaces, lag deposits and abrupt facies changes across the unconformity surface, whereas

264 an angular unconformity is present across SB4. SB4 separates the Steensby Bjerg and Home Forland Formation and was originally described by Kelly et al. (1998) under the heading Uk2. Across two of the unconformities (SB 1, late-early to late Barremian, and SB4, late-early Albian) a hiatus is present based on the dinoflagellate cysts stratigraphy (H. NOhr-Hansen, pers. commun., 1999) (Fig. 4).

Sedimentary facies associations A number of sedimentary facies are identified based on lithology, sedimentary structures and body and trace fossils. For the purpose of description, the facies are grouped into seven major facies associations, of which each characterises a depositional environment. The facies associations allow a better and slightly different subdivision of the succession than the lithostratigraphic units defined by Kelly et al. (1998).

Shoreface sandstones and conglomerates The shoreface association consists of interbedded pebble conglomerates, fine- to medium-grained sandstones and mica-rich mudstones. The conglomerates can be followed laterally for a few tens of metres and vary from a few cm to 40 cm of thickness over short distances. The bases of the conglomerate beds are erosional, whereas the upper boundary may be either sharp or shows a gradual transition into the overlying sandstones. The conglomerates are mainly clast-supported with well-rounded spherical to discoidal quartz pebbles (grain size around 3 cm) and locally cobbles (up to 25 cm). The conglomerates are commonly structureless, but locally show trough cross-bedding with foresets dipping towards northeast. Rare belemnites are present throughout the association and reworked Middle Jurassic ammonites are present in the conglomerate bed overlying the base Cretaceous unconformity (see also Kelly et al., 1998). The sandstones are generally well-sorted and form beds of 1-2 m thick, with a maximum of up to 5 m thick. In the fine-grained sandstones the dominant structure is wave-ripple cross-lamination, but locally hummocky and swaley cross-stratification are present. Scour-and-fill and trough cross-bedding dominate in the more coarse-grained beds. Foreset dip azimuths are bi-directional, but with a clear dominance towards the northeast (mean 39~ The finegrained sandstone beds are strongly burrowed showing the trace fossil Curvolithos multiplex, whereas Skolithos isp. occur scattered in the medium-grained beds. Ophiomorpha nodosa locally occur in great numbers extending from bedding surfaces at the top

M. Larsen et al.

of the sandstone succession (upper boundary of the Gulelv Member). Laminae and thin beds of mica-rich and carbonaceous mudstones occur throughout the association. They either form lenticular units draping troughs of cross-sets or they form laterally continuous beds, up to a few centimetres thick. Dinoflagellate cysts and disseminated plant fragments are present in the mudstones. The shoreface association is present at the base of the Cretaceous succession. It reaches a maximum thickness of 37 m in the eastern part of Stensi6 Plateau (Stribedal Member) (Fig. 6). At Steensby Bjerg it forms the lower part, up to 24 m thick, of the Gulelv Member. At Diener Bjerg a conglomeratic lag deposit up to 1 m thick forms the base of the Diener Bjerg Member. The shoreface association is furthermore present at the top of the Gulelv Member representing the wave-reworked top of the Lower Cretaceous sandstone succession (Figs. 6 and 7). The sedimentary structures and the strong marine burrowing suggest that the association was deposited in a shoreface environment (see also Whitham et al., 1999). The abundant conglomerates with scoured bases and local occurrence of swaley and hummocky cross-stratification suggest periodic highenergy, wave-dominated conditions during storms.

Cross-bedded delta front sandstones Coarse-grained locally pebbly sandstones dominate sediments of this association. The sandstones are planar and trough cross-stratified with set thicknesses between 0.5 and 2 m, locally up to 5 m, and forming cosets, up to 40 m thick, bounded by coarse-grained lag deposits (Figs. 7 and 8a). Locally, the cross-sets can be seen to form intrasets in large-scale compound cross-beds with low-angle master-surfaces dipping a few degrees towards the southwest. Foreset dip directions indicate that the cross-bedded units prograded towards the south and southwest (mean 222~ In the eastern part of Steensby Bjerg and around Diener Bjerg a single large-scale foreset bed with foresets up to 50 m high occurs (Figs. 7 and 8b). The largescale foresets dip towards the southeast (mean 150~ The sandstones are generally unfossiliferous although Kelly et al. (1998) reported a single ammonite specimen from the basal part of the large-scale cross-bedded unit at Diener Bjerg. Locally, sinuous burrows occur along the bedding planes of the large-scale foresets. The sandstones grade downdip into toesets of laminated silty and sandy mudstones with intercalated thin sandstone beds. The grain-size and the number of sandstone beds increase upward and the toesets form upward-coarsening units up to 5 m thick. The


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i~'~ 800 m) and the North Viking Graben (> 300 m). Elsewhere, condensed units of this age are occasionally present, as proven by drilling. The preserved sediments near the present-day coast are seen as spectacular highs with well-developed erosional edges (Fig. 3). Internal reflection patterns are parallel and often conform to the underlying Upper Jurassic shallow marine strata. The Lower Cretaceous sediments in these highs were drilled in wells 36/7-2 and 36/1-1 and are also interpreted to be of shallow marine origin. There is no structural break across the Base Cretaceous unconformity. Along the axes of the Sogn and North Viking Grabens the Cretaceous reflections are also parallel with the underlying Jurassic strata, while they onlap the Base Cretaceous on the flanks of the basin. The environment in the grabens is assumed to be deep marine, based on palaeogeographic reconstruction from seismic data. It is proposed that sedimentation during the earliest Cretaceous took place in two basin systems: one following the Late Jurassic rift axis in the Viking/Sogn Grabens and the other along the present-day coast of Norway, possibly related to the Oygarden Fault Zone. The Oygarden Fault Zone consists of a series of down-to-the-east faults and is probably still active (F~erseth et al., 1995). The area between the two basin

The depositional history of the Cretaceous in the northeastern North Sea

283

uplift during Hauterivian-Barremian and subsequent erosion is well known from most of the north Atlantic region, e.g. west of Shetland, the mid-Norway area, Barents shelf and Greenland (e.g. Dor6, 1992). Only a few of the drilled wells contain sediments of late Ryazanian/early Valanginian age, i.e. the oldest sediments above "The Base Cretaceous" boundary (Fig. 6). These wells were situated in a basin or slope position, while the wells missing this section seem to be located on structurally higher areas. A greater number of wells record Hauterivian-aged deposits, while most wells have sediments of Barremian age. This is interpreted as reflecting an overall transgressive trend lasting until the late Barremian. This was followed by a drastic lowering of relative sea level during the latest Barremian, which was related to the above described basin flank uplift and structuring. The Sola Formation (Aptian)

Fig. 4. Geological setting during deposition of late RyazanianBarremian (Asgard Fm.) sediments. Shallow marine sediments were deposited in "inner basins" and deep marine sediments in the North Viking Graben and the Sogn Graben. The area between was partly bypassed and partly covered by relatively thin deposits.

systems is interpreted as a region that received little sediment and was mainly a zone of bypass (Fig. 4). It is proposed that strong inversion of the eastern basin occurred during the late Barremian, and by the end of Barremian these uplifted sediments were strongly eroded, as seen in Fig. 5. A similar history with 36/1-1

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The Sola Formation is recognised in well logs as a highly radioactive shale of Aptian age, and is defined as a formation as such (Isaksen and Tonstad, 1989). We interpret the erosional products resulting from the sea level lowering in the latest Barremian to have been deposited in the latest Barremian-earliest Aptian. Such deposits have been drilled in one of the wells in the Selje High area immediately to the north of the Agat area, but have only been observed on seismic data in the Sogn Graben within the study area. Low-amplitude "background" reflections are seen with a few high-amplitude features internally. Mapping reveals the high-amplitude events as having an elongate shape coming out of an interpreted palaeovalley running westwards across the southern part of the Agat area. This high-amplitude seismic facies can be traced across the Sogn Graben before bending northwards when reaching the Marflo Ridge (Fig. 7). The high amplitudes are interpreted as the response of possible turbidite sandstone systems encased in the "background" shales. The Aptian succession is condensed and incomplete in most of the wells drilled. Seismic data can be interpreted to suggest that Aptian sediments exist in the deeper parts of the area, including the North Viking Graben and Sogn Graben. There is 90 m of Aptian sediment in the westernmost of the Agat wells (35/3-1), but it pinches out between wells 35/3-2 and 35/3-4. Seismic onlap and pinchout are observed further south along the basin margin. The dark shales of the Sola Formation are characterised by high gamma radiation and were deposited in a deep marine environment. A high gamma-ray level in the lower Aptian is interpreted to represent maximum flooding and is correlated with deposition of

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Fig. 6. Chronogram of selected wells, based on integration of biostratigraphic zonation data, seismic data and well data. The figure shows that many sequences are incomplete in the drilled wells, particularly in the Lower Cretaceous. It is important to notice that sequences of the same age are deposited and preserved in many of the wells. This probably indicates that the drilled areas have not been subaerially exposed and eroded.

the organic-rich clays of the Fischschiefer in Germany (Kemper, 1973; Riley et al., 1992). Together with the later mentioned earliest Turonian flooding event, this is one of the major transgressions in the Cretaceous. The rapid regression in the late Barremian was thus succeeded by rapid transgression in the earliest Aptian. The lack of younger Aptian sediments in most of the wells is interpreted as the combined result of the topographic relief left by the Barremian structuring, a relative lowering of sea level and a general regressive trend until the end of the Aptian.

The Redby and Agat Formations (Albian) The Albian period was dominated by hemipelagic deposition of clay interrupted by the sandy products

of mass-flow events. Lithologically, the claystone is assigned to the RCdby Formation and the sandstone to the Agat Formation (Isaksen and Tonstad, 1989). Gamma-ray (GR) log patterns show a stePwise reduction in the radioactivity level of the background shales with time (Fig. 8). High radioactivity is taken as an indicator of a restricted environment. The microfauna does not indicate any changes in palaeowater depth during this period. The GR trend is therefore interpreted as a stepwise opening of the basin from the early Albian to a fully open marine environment by the end of the Albian. Sandstones belonging to the Agat Formation occur frequently in the sequence. With the exception of well 35/3-5, most of the sandstones seem to be of late A1bian age. The units appear as anomalous sediments


Fig. 7. Erosional products from the latest Barremian-earliest Aptian erosion were deposited in the Sogn Graben. The lobe is interpreted as deposits from turbidity currents running in a palaeovalley through the Agat area, being deflected northward in the Sogn Graben. The map shows the high-reflectivity lobe superimposed on the Base Cretaceous surface (both as depth in seismic reflection time).

in an environment characterised by hemipelagic deposition of clay and were transported into the system as gravity mass flows. This activity started in the early/middle Albian, but increased dramatically by the middle/late Albian transition and can be traced as such

285

in wells over a wide area. It is worth noticing that biostratigraphic reanalysis indicates that none of the sandstones are of Aptian age. Although Shanmugam et al. (1995) suggested that many of the sandstone units were mass deposits from large-scale slumping, there is general agreement that the apparently massive sands are the result of amalgamation of thin turbidite beds or sandy debris flows (Nystuen, 1999). From well data three possible transgressive/regressive cycles can be recognised in the Albian succession (Fig. 6). These can be correlated with the sequence stratigraphic subdivisions presented for European basins by De Graciansky et al. (1998) and Jacquin et al. (1998), who classified them as secondorder facies cycles. Most of the lower and middle Albian sandstones are interpreted as part of second-order regressive cycles, while most of the upper Albian sandstones are placed within a second-order transgressive cycle (wells 35/3-1, -2, -4, 35/9-3). Because the Agat sandstones are so widespread but restricted to certain time intervals, it could be suggested that they are related to periods of higher tectonic activity and frequency of earthquakes. There is, however, no direct evidence in seismic data of increased tectonic activity such as active faulting or change of basin configuration, tilting, onlap surfaces etc. Except for the Agat wells, most of the other wells contain upper Albian sediments, but lack sediments of early to middle Albian age (Fig. 6). Being located on structural highs or on the upper slope, this suggests that they first were bypassed and then flooded by late Albian time. This could further

Fig. 8. Albian correlation between Agat wells 35/3-1 and -5 and GjOa well 35/9-3. There is a stepwise reduction in maximum gamma-ray level throughout the Albian, which is interpreted to reflect a gradual opening of the basin to more oxygenated conditions. The prevailing hemipelagic deposition of clay was interrupted by episodic deposition of sand by gravity mass flows (Agat Fm.). Most of the Agat sand was deposited in the early part of the late Albian, with the exception of well 35/3-5, which contains both older and younger sands.

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The Svarte Formation (Cenomanian)

Fig. 9. (a-c) In the Agat area sand was probably deposited by debris flows and turbidity currents in a slope setting. The turbidites are typically 10-30 cm thick and were amalgamated to thicknesses of many tens of metres. In a bypass situation on the slope this would hardly occur. Seismic data indicate, however, that slide scars from small-scale slumping and sliding formed accommodation space for the sand to be preserved in large, isolated bodies. This would also explain why there is no pressure communication between the Agat sandstone in wells 35/3-2 and -4, which are only 5.5 km apart in an updip direction.

Deposition of hemipelagic clay seems to have continued into the Cenomanian period, and sand input by gravity mass flows decreased. There are some seismic indications that the Svarte Formation sediments onlap the underlying Lower Cretaceous sediments, thus suggesting a minor tectonic event with basin floor subsidence/flank uplift. The Svarte Formation can be divided into two subunits. The lower unit has been dated to early Cenomanian and seems to infill and smoothen some of the underlying relief. The upper part is of late Cenomanian age and has a more even thickness and more widespread distribution. The two subunits seem to correspond to the facies cycles indicated in Fig. 6. There is an overall thickening trend of the Svarte Formation towards the basin centre in the west and a slight thickening to the north. Palaeobathymetric interpretation indicates a general transgression throughout the Cenomanian, which is coincident with the last phase of the overall Early to mid-Cretaceous transgression.

The Blodeks Formation (latest Cenomanian-early Turonian) indicate that the Agat sands were sourced from sandrich areas that were flooded during a period of rising relative sea level. Gas and condensate were found in the Agat sandstones in some of the wells drilled in the Agat area (35/3-1, 35/3-2, and 35/3-4), but there appears to be no communication between the wells. The sandstones are therefore interpreted to occur as isolated bodies. These wells are located on the northern flank of the earlier mentioned E-W-striking palaeovalley, probably in a palaeogeographic slope setting with the basin floor in the Sogn Graben to the west and in the Mere Basin further out to the north. This part of the Agat area was therefore generally a bypass area for the turbidity currents. The seismic data quality is not the best, but there are indications that local slumping and sliding occurred. This would have created accommodation space for thin turbidites and debris flow deposits to amalgamate to thicknesses of tens of metres, thus creating sand bodies with no internal communication (Fig. 9). Well 35/3-5 is located more in the centre of the palaeovalley and contains thicker sandstone units that tend to follow the strike of the valley. These sandstone units may have more internal communication, but seem to be isolated from the sandstone bodies encountered in the above-mentioned wells on the northern flank of the valley.

The long-term Early to mid-Cretaceous transgression is part of the North Atlantic first-order cycle described by Jacquin et al. (1998). The cycle culminated in the latest Cenomanian/early Turonian with deposition of a condensed section of organic-rich clay referred to as the B lodCks Formation in the area of study. This formation represents the most pronounced transgression in the Cretaceous and is equivalent with a known condensed interval from Svalbard to Italy (De Graciansky et al., 1984). It corresponds to a well-known oceanic anoxic event with starvation and can be correlated with the Plenus Marl Formation further south in the North Sea (Deegan and Scull, 1977). The typical thickness in the study area is only a few metres and is therefore below seismic resolution. The impedance contrast is, however, high and the B lod~ks Formation is recognised as a strong and continuous reflection on seismic data (Fig. 10). It is recognised on well logs by high gamma-ray values, and the drilled wells show that the formation is absent on some of the structural highs and the highest parts of the eastern basinal slope. This could be explained by later erosion or by non-deposition. Because lower Turonian sediments exist where B lodCks sediments are missing, any erosion must have occurred very soon after deposition. Alternatively, B lod~ks Formation sediments bypassed and were never deposited on the structural highs and on the entire eastern slope of the basin.


287

Fig. 10. The BlodCks Fm. (late Cenomanian-early Turonian age) represents the most pronounced transgression in the Cretaceous. It was followed by a significant relative lowering of sea level before the Tryggvason Fm. transgressed and backstepped the Blod0ks Fm. Downlap features in the basin represent the regressive phase, while the backstepping is expressed by onlap on the eastern basin margin. The overlying Kyrre Fm. backstepped the Tryggvason Fm. and comprises two sand lobes (upper Turonian-Coniacian) represented by high-reflection amplitudes. The onlap geometries on top Blod0ks and top Tryggvason are interpreted to reflect tectonic episodes with flank uplift and basin floor subsidence.

The Tryggvason Formation (early-middle Turonian) The Tryggvason Formation can be divided into subunits of high and low internal reflectivity. These can be interpreted by well data trends as regressive and transgressive, respectively. After deposition of the transgressive B lod0ks Formation, there was a significant regression that left vague downlap features in the basin. On the eastern basin margin the Tryggvason Formation shows clear onlap on the underlying BlodOks and Svarte Formations (Fig. 10), with increasingly higher onlap to the east. The onlap is seismically one of the most pronounced features in the Cretaceous succession and could represent a significant tectonic tilting event after deposition of the B lodCks Formation in the early Turonian. The lithology of the Tryggvason Formation is dominated by shale and marly shale, while some of the high reflectivity, regressive units contain some sandstone. Oil shows were encountered in thin sand stringers in the Agat well 36/1-2. Attribute analysis shows a pattern of semi-concentric arcs that are much like the features interpreted as water escape structures or compaction features in for instance the Eocene shales in the North Sea (Cartwright, 1996) or in shales and oozes in the Oligocene/Miocene in

the V0ring Basin (internal data). The features observed in the Tryggvason Formation are asymmetric in form and occur only in the western part of the elsewhere-symmetric polygonal features (Fig. 11). This is interpreted to be the result of gravitational forces acting on the slope where the Tryggvason Formation sediments were deposited. On seismic cross-sections these features are seen as mainly intraformational small-scale faulting. Fig. 11 is an example from the uppermost subunit of the Tryggvason Formation. We suggest that these features are related to clay compaction and water escape and that presence of sand can be postulated where they are absent. This pattern or lack of pattern could thus be used as a lithology indicator.

The Kyrre Formation (late Turonian-Campanian) There are strong onlap geometries observed at the top of the Tryggvason Formation (Figs. 11 and 12), in a similar fashion to those seen at its base (Fig. 10). This indicates that another tectonic tilting event occurred with rapid basin floor subsidence and flank uplift at the transition between the Tryggvason and Kyrre Formations in late Turonian time. Sedimentation rate increased and can be interpreted to have

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Fig. 11. Seismic attribute analysis used for lithology classification. (Upper panel) Attribute analysis of the upper part of the Tryggvason Fm. demonstrates a pattern of largely concentric half-circles similar to water escape structures observed in clay- and ooze-dominated sediments elsewhere. They coincide with intraformational faults seen on seismic data. (Lower panel) Faulting is absent where wells show presence of sand in the upper Tryggvason Fm. Because the polygonal water escape structures are related to compaction of non-sandy sediments, we suggest that presence of such features can be used to distinguish between sand and clay. The asymmetric pattern observed here is due to gravitational forces induced on the westward-dipping slope.


289

Fig. 12. Conceptual diagram of the interpreted sequences. The Asgard Fm. sediments in the east were deposited in shallow marine basins, uplifted in the Hauterivian-Barremian and eroded in the latest Barremian-earliest Aptian. The younger sequences onlap each other or the Barremian unconformity. Onlap surfaces at top Albian, early Turonian, middle/late Turonian and Coniacian probably represent tectonic events with tilting of the basin. Sand was deposited in certain tectonically active periods as a result of gravity mass flows.

reached a maximum around Santonian time. Palaeobathymetric interpretation indicates deep marine environments in the Sogn Graben and North Viking Graben and shallower conditions to the east. Except for local presence of upper Turonian/Coniacian sandstone (see below) the entire Kyrre Formation is dominated by deposition of hemipelagic clay. Despite the pronounced basin floor subsidence, the basins were largely infilled by the end of the Campanian, probably with only a gentle dip remaining towards the basin centres in the west. On seismic data the upper Turonian/Coniacian sandstones are observed as well-defined high-reflectivity units (see Fig. 10). The sandstone-bearing interval can be divided into two subunits, which have a distribution suggesting that they are submarine fan units with a common apex point. The lower fan seems to extend slightly more to the west than the younger one, suggesting a transgressive and backstepping trend. The distal part of this fan was drilled in exploration well 35/9-3 and proved to contain fineto medium-grained turbidite sandstone with gas. In the Selje High area immediately to the north of the study area there are sandstone intervals of the same age, but which also seem to range into the Santonian. Elsewhere in the study area, however, there are no high-amplitude reflections that might indicate the presence of sandstone in the Kyrre Formation. The lower part of the Kyrre Formation (from its base to top of the Coniacian sandstones) shows an overall uniform thickness from east to west, even

where the sand shales out into the basin. There is a clear onlap surface between this part of the succession and the overlying units. This is interpreted to reflect basin floor subsidence and basin flank uplift and is probably related to plate reorganisation and opening of the Labrador Sea in early Campanian (chron 33, Roest and Srivastava, 1989).

The Jorsalfare Formation (Maastrichtian) The Jorsalfare Formation consists of marl in the eastern region of the study area and is shale dominated further west. Limestone dominates on the Horda Platform to the south. Sedimentation rate had decreased from that of the Kyrre Formation. The formation is divided into a lower transgressive part that was followed by deposition of an upper regressive part. Eastward thinning and signs of erosion imply that basin flanks were exposed by the end of Cretaceous.

Discussion and conclusions It can be concluded that the Cretaceous in the northern North Sea was not a period of passive postrift infilling. The effects of the rifting event in the Late Jurassic seem to have continued into the Early Cretaceous, and in addition the rifting preceding the opening of the North Atlantic in the early Tertiary is expressed as a series of precursor events in the Late Cretaceous. We suggest that the strong basin flank uplift and inversion/uplift of some intrabasinal highs

290 that can be observed until the end of the Barremian should be attributed to the last phase of the "Late Jurassic" rifting. The structural setting and topography left at the end of the Barremian should thus be taken as the true situation at the onset of the post-rift phase of basin evolution. Other possible tectonic events are represented by onlap geometries on certain horizons, generally reflecting tilting with basin floor subsidence and flank uplift. They could be minor extensional events or alternatively vertical or lateral movements without any direct link to rifting. They could be related to mantle heating or to a combination of this and effects of plate reorganisation associated with the opening of the southern and middle Atlantic Ocean. Onlap geometries are observed at the following levels (Fig. 12): (1) Top RCdby (transition Early/Late Cretaceous, Albian/Cenomanian); (2) Top Blod0ks (early Turonian); (3) Top Tryggvason (middle/late Turonian); (4) Top Kyrre sandstone (Coniacian). The possible precursor of the Tertiary seafloor spreading could be represented by the rapid basin floor subsidence in the Santonian. From latest Ryazanian to end Barremian times fairly thin sequences of clay and marl were deposited on the structural highs, while thicker sequences were deposited on the basin slopes and nearly continuous sedimentation took place in the basins (Fig. 6). It is worth noticing that lowermost Cretaceous sediments present in the drilled wells represent certain stratigraphic intervals, while sediments from the intervening periods are absent throughout. We suggest that this reflects alternating and regional periods of sediment deposition and non-deposition, and because similarly aged sediments are preserved on intra-basinal structural highs it can be inferred that the structural highs were not subaerially exposed and eroded. Sediment distribution throughout the remaining part of the Cretaceous is characterised by large missing sections on the structural highs in the Aptian/ Albian and first part of the Late Cretaceous. Sedimentation seems to have been continuous in the basin areas and on the slopes. This overall setting continued until all highs were flooded around Santonian time. Hemipelagic deposition of clay and some marl dominated throughout the entire Cretaceous. Deposition of sand is broadly restricted to certain time intervals and probably linked to and following increased tectonic activity and/or changes of relative sea level. The Agat sands are all of Albian age, with a marked increase in sand input at the onset of late Albian. There are no indications in seismic data for tectonic activity directly affecting the area of study, such as change in basin topography (faulting, tilting, subsidence, etc.). The other periods of increased sand

T. Bugge et al. supply may, however, be linked to tectonic tilting events: expressed as the Tryggvason Formation in the early-middle Turonian and the Kyrre sands in the late Turonian-Coniacian. In the late Barremian/early Aptian, sand was probably also deposited in the Sogn Graben as the result of the Barremian basin flank uplift. The Ryazanian-Barremian sand seems to be derived from uplifted and eroded areas within the basin and along the basin flanks, while the Agat, Tryggvason and Kyrre sandstones were probably sourced from the east. The Agat sandstones seem to occur in palaeovalleys, as observed in wells 35/3-5 and 35/9-3 for example, and in the Sogn Graben and its extension to the south. The Kyrre sandstone is even more localised with more or less one single entry point. It can be speculated whether the sand was originally sorted and accumulated in a coastal setting where the basin morphology concentrated sand accumulation into certain depocentres. These could occur at positions where rivers entered the sea and/or where underlying structures formed embayments to accumulate sand fed by the rivers and by longshore transport. The combination of lowered relative sea level and tectonic activity could expose these areas and earthquakes could also contribute to the triggering of mass flows. The gravitational mass flows would be guided by the underlying topography and tend to follow palaeovalleys. It is not known where the coastline was situated during the Cretaceous, but the sand could possibly be transported over long distances. This complicates the reconstruction of possible source areas and leaves us with only conceptual models for the palaeogeography to the east of the area of deposition. The results described above have important implications for hydrocarbon exploration and can be summarised as follows. (1) Compared to a geological model of passive, post-rift infilling by the Cretaceous sediments, the indications of fairly active tectonism will severely influence the hydrocarbon exploration in the northern North Sea as well as other areas. Correct structural reconstruction is not possible without taking this into account. It will also have consequences for modelling of maturation and secondary migration of oil and gas. (2) The majority of the sandstones seem to have been deposited by different mass-flow processes and can probably be linked to certain periods of tectonic activity and/or change in relative sea level. This helps to identify prospective successions and areas. (3) There are almost no structural closures in the prospective Cretaceous intervals, and any potential oil and gas prospect will partly or fully rely on stratigraphic closures. This requires enhanced under-


standing of palaeogeographic setting, sedimentary environment and depositional processes.

Acknowledgements We thank Saga Petroleum who allowed us to publish this paper. We would also like to thank Bj~rn Tore Larsen, Snorre Olaussen and Sarah Prosser for reading the manuscript and giving valuable comments, and other colleagues in Saga Petroleum who have contributed throughout the work. Erik E Johannesen and Mike Talbot gave valuable referee comments.

References Blystad, P., Brekke, H., Fa~rseth, R.B., Larsen, B.T., Skogseid, J. and T~rudbakken, B., 1995. Structural elements of the Norwegian continental shelf. Part II: The Norwegian Sea Region. Norwegian Petroleum Directorate Bulletin, 8, 45 pp. Cartwright, J.A., 1996. Polygonal fault systems: a new type of fault structure revealed from 3-D seismic data from the North Sea Basin. In: E Weimer and T.L. Davies (Editors), Application of 3-D Seismic Data to Exploration and Production. AAPG Studies in Geology 42 and SEG Geophysical Development Series, 5, pp. 225-230. Dalland, A., Worsley, D. and Ofstad, K., 1988. A lithostratigraphic scheme for the Mesozoic and Cenozoic succession offshore midand northern Norway. Norwegian Petroleum Directorate Bulletin, 4, 65 pp. Deegan, C.E. and Scull, B.J. (compilers), 1977. A standard lithostratigraphic nomenclature for the central and northern North Sea. Institute of Geological Sciences Report 77/25, Norwegian Petroleum Directorate Bulletin, 1, 35 pp. Dor6, A.G., 1992. Synoptic palaeogeography of the Northeast Atlantic Seaway: late Permian to Cretaceous. In: J. Parnell (Editor), Basins on the Atlantic Seaboard: Petroleum Geology, Sedimentology and Basin Evolution. Geol. Soc., London, Spec. Publ., 62: 421-446. Fa~rseth, R.B., Gabrielsen, R.H. and Hurich, C.A., 1995. Influence of basement in structuring of the North Sea basin, offshore southwest Norway. Nor. Geol. Tidsskr., 75: 105-119. Gabrielsen, R.H., Odinsen, T. and Grunnaleite, I., 1999. Structuring of the Northern Viking Graben and the M~re Basin; the influence of basement structural grain, and the particular role of the M~reTr~ndelag Fault Complex. Mar. Pet. Geol., 16: 443-465. De Graciansky, E-C., Deroo, G., Herbin, J.-P., Montadert, L., Muller, C., Schaaf, A. and Sigal, J., 1984. Ocean-wide stagnation episode in the Late Cretaceous. Nature, 308: 346-349. De Graciansky, E-C., Hardenbol, J., Jacquin, T. and Vail, E (Editors), 1998. Mesozoic and Cenozoic Sequence Stratigraphy of European Basins. Soc. Econ. Paleontol. Mineral., Spec. PUN., 60, 786 pp. Gulbrandsen, A., 1987. Agat. In: A.M. Spencer, S.O. Johnson, A. MCrk, E. Nysa~ther, E Songstad and A. Spinnangr (Editors), Petroleum Geology of the Norwegian Oil and Gas Fields. Graham and Trotman, London, pp. 363-370. Hesjedal, A. and Hamar, G.E, 1983. Lower Cretaceous stratigraphy and tectonics of the south-southeastern Norwegian offshore. In:

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J.EH. Kaasschieter and T.J.A. Reijers (Editors), Petroleum Geology of the Southeastern North Sea and the Adjacent Onshore Areas. Geol. Mijnbouw, 62: 135-144. H~gseth, K., Vagle, G.B., Bergfjord, E., Granholm, P.G. and Skjervold, R., 1999. The Cretaceous depositional systems of the frontier V~ring Basin evidence from the Nyk High well (6707/10-1) and the Vema Dome well (6706/11-1). In: O.J. Martinsen and T. Dreyer (Editors), Sedimentary Environments Offshore Norway Palaeozoic to Recent. Extended Abstracts, Norwegian Petroleum Society/NPF Conference, Bergen, Norway, May 3-5, 1999, ISBN 82-92032-00-2, pp. 199-200. Isaksen, D. and Tonstad, K., 1989. A revised Cretaceous and Tertiary lithostratigraphic nomenclature for the Norwegian North Sea. Norwegian Petroleum Directorate Bulletin, 5, 24 pp. Jacquin, T. and Thomson, M., 1999. 4-dimensional stratigraphic modeling of the northern North Sea. In: O.J. Martinsen and T. Dreyer (Editors), Sedimentary Environments Offshore Norway Palaeozoic to Recent. Extended Abstracts, Norwegian Petroleum Society/NPF Conference, Bergen, Norway, May 3-5, 1999, ISBN 82-92032-00-2, pp. 49-52. Jacquin, T., Rusciadelli, G., Amedro, F., De Graciansky, E-C. and Magniez-Jannin, F., 1998. The North Atlantic cycle: an overview of 2nd-order transgressive/regressive facies cycles in the Lower Cretaceous of Western Europe. In: E-C. De Graciansky, J. Hardenbol, T. Jacquin and E Vail (Editors), Mesozoic and Cenozoic Sequence Stratigraphy of European Basins. Soc. Econ. Paleontol. Mineral., Spec. Publ., 60: 397-409. Kemper, E., 1973. The Aptian and Albian stages in northwest Germany. In: R. Casey and EF. Rawson (Editors), The Boreal Lower Cretaceous. Geol. J. Spec. Issue, 5: 345-360. Nystuen, J.E, 1999. Submarine sediment gravity flow deposits and associated facies: core examples from the Agat Formation. In: O.J. Martinsen and T. Dreyer (Editors), Sedimentary Environments Offshore Norway Palaeozoic to Recent. Extended Abstracts, Norwegian Petroleum Society/NPF Conference, Bergen, Norway, May 3-5, 1999, ISBN 82-92032-00-2, pp. 211-215. Riley, L.A., Harker, S.D. and Green, S.C.H., 1992. Lower Cretaceous palynology and sandstone distribution in the Scapa Field, U.K. North Sea. J. Pet. Geol., 15:97-110. Roest, W.R. and Srivastava, S.E, 1989. Sea-floor spreading in the Labrador Sea: a new reconstruction. Geology, 17: 1000-1003. Shanmugam, G. and Moiola, R.J., 1995. Reinterpretation of depositional processes in a classic flysch sequence (Pennsylvanian Jackfork Group), Ouachita Mountains, Arkansas and Oklahoma. Am. Assoc. Pet. Geol. Bull., 79: 672-695. Shanmugam, G. and Moiola, R.J., 1997. Reinterpretation of depositional processes in a classic flysch sequence (Pennsylvanian Jackfork Group), Ouachita Mountains, Arkansas and Oklahoma: Reply. Am. Assoc. Pet. Geol. Bull., 81: 476-491. Shanmugam, G., Bloch, R.B., Mitchell, S.M., Beamish, G.W.J., Hodgkinson, R.J., Damuth, J.E., Straume, T., Syvertsen, S.E. and Shields, K.E., 1995. Basin-floor fans in the North Sea: sequence stratigraphic models vs. sedimentary facies. Am. Assoc. Pet. Geol. Bull., 79:477-512. Skibeli, M., Barnes, K., Straume, T., Syvertsen, S.E. and Shanmugam, G., 1995. A sequence stratigraphic study of Lower Cretaceous deposits in the northernmost North Sea. In: R.J. Steel, V. Felt, E.E Johannessen and C. Mathieu (Editors), Sequence Stratigraphy of the Northwest European Margin. Norwegian Petroleum Society (NPF), Special Publication 5. Elsevier, Amsterdam, pp. 389-400.

Norsk Hydro ASA, N-9480 Harstad, Norway; E-maih [email protected] Norsk Hydro ASA, E & P International, N-0246 Oslo, Norway Applied Biostratigraphy, Blekksoppgrenda 41, N-1352 Kols~s, Norway


293

Cretaceous faulting and associated coarse-grained marine gravity flow sedimentation, Traill t21, East Greenland Finn Surlyk and Nanna Noe-Nygaard

The Cretaceous succession of the Traill 0 region in East Greenland is more than 2 km thick and is dominated by mudstones. It occurs in a system of fault blocks, about 5-10 km wide, delimited by roughly parallel, NNE-trending faults. The main faults in the area are from west to east the Post-Devonian Main Fault, the Bordbjerg Fault, the M~nedal Fault and the Mols Bjerge Fault. Cretaceous subsidence was governed by thermal contraction following the protracted Late Bajocian-Valanginian rift phase. The regional subsidence was punctuated by several possibly rift-related fault episodes marked by deposition of breccias, conglomerates and sandstones. Transport was by sediment gravity flows and clasts were derived from the uplifted footwalls and fault scarps of the main basin bounding faults. Four coarse-clastic deep-water units represent different types of depositional systems which are described to illustrate the variety of fault-associated deposits. Three of the coarse-clastic units form the basis for defining the new Rold Bjerge, Mfinedal and Vega Sund Formations. A chaotic breccia of earliest Middle Albian age forms the new Rold Bjerge Formation. It contains blocks, up to 60 m long, and has a clast assemblage characterised by Upper Permian and Lower Triassic carbonates. It is truncated to the west by the Mfinedal Fault, and the Lower Triassic Wordie Creek Formation which is exposed in the footwall, is considerably younger than the dated clasts. The breccia was thus derived from the area further west delimited by the Post-Devonian Main Fault, while the Mfinedal Fault appears not to have been active at the time of deposition. Transport was by submarine, probably hydroplaning, debris avalanches, and the estimated runout distance was about 20 kin. An allochthonous succession comprising mudstones and sandstone turbidites of Late Albian age is also exposed immediately adjacent to the M~nedal Fault. It was formed by extensive downslope sliding and slumping along discrete, densely spaced detachment planes, and the mudstones and sandstones were folded during transport. The age of the slide-slump event is not known, but a Late Turonian-Early Coniacian age is tentatively suggested, by analogy with conglomerates and pebbly sandstones of the Mfinedal Formation which are situated in a similar position along strike. Remobilisation, downslope transport and subsequent redeposition took place during an important phase of footwall uplift. A conglomerate-sandstone package forming the new Mfinedal Formation is limited to the west by the Mfinedal Fault. The age is not precisely known, but macrofossils indicate a Late Turonian-Early Coniacian age. The common presence of boulders composed of Upper Permian carbonates indicates that the Mfinedal Fault was not yet formed or played only a minor role at the time of deposition because the rocks exposed in the footwall are of Early Triassic age. The main source area was again situated in the area west of the Post-Devonian Main Fault, and the conglomerates and pebbly sandstones were transported downslope by turbidity currents and related processes towards the east-southeast at a right angle to the faults. The system shows a crude fining-upward in the top part and is interpreted as a faulted slope apron. A thick succession of planar sandstone turbidites forming the new Vega Sund Formation is exposed 6 km east of the Mfinedal Fault. The age is highly uncertain. A Cenomanian age has been suggested previously but an Early-Middle Coniacian age is very tentatively preferred. Transport direction was again towards east-southeast at a right angle to the faults. The sheet-like bed geometry, lack of channeling and scouring, and gradual fining-upward of the top part suggest deposition on the outer part of a sand-rich basin floor fan. The four systems differ greatly in grain size, sorting, downslope transport processes, geometry and lateral extent. The transverse eastward transport directions and the clast provenances indicate, however, that the Cretaceous fault blocks were wider and not yet fragmented into the narrow present-day blocks. This interpretation contrasts with previous accounts which suggest that the Mfinedal and Bordbjerg Faults were active and exerted a profound control on sedimentation during Cretaceous time. It is, however, possible that an early expression of the Mhnedal Fault was formed in post-middle Albian time. The systems provide evidence for phases of possibly rift-related faulting accommodated by the old faults and associated footwall uplift and erosion in the earliest Middle Albian, and Late Turonian-Early Coniacian. The absence of Hauterivian and in part Barremian strata virtually throughout East Greenland may reflect marginal or probably regional uplift during an earlier fault event; the scarcity of Upper Aptian deposits and the presence of a Lower Cenomanian basal conglomerate may likewise reflect rifting and marginal uplift. The coarse-clastic deep-water deposits of the Traill 0 region were emplaced by a variety of sediment gravity flows and may serve as useful field analogs for the deeply buried correlative strata of the outer Norwegian Shelf. They illustrate the variability and complexity of the depositional systems, and allow identification of several significant Cretaceous possibly rift-associated fault pulses superimposed on the long-term regional subsidence governed by thermal contraction following the protracted Late Bajocian-Valanginian rift phase.


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Introduction The main Mesozoic rift event in East Greenland began in Middle Jurassic time, culminated in the Late Jurassic with formation of strongly tilted fault blocks, and faded out in the earliest Cretaceous (Vischer, 1943; Maync, 1947, 1949; Surlyk et al., 1981; Surlyk, 1990, 2001; Surlyk and Noe-Nygaard, 2000). Deposits and structures associated with this event are well exposed in East Greenland and have to some extent overshadowed the importance of subsequent Cretaceous fault phases. The scattered nature of the Cretaceous outcrops and lack of seismic profiles makes it difficult to evaluate if the faulting is related to true extensional rifting. Demonstration of episodes of footwall uplift, rejuvenation of fault scarps and formation of relatively steep slopes suggest, however, that the faulting may represent actual rift events. The Cretaceous succession of East Greenland is dominated by dark basinal mudstones, and is up to about 2 km thick. Debris avalanche breccias, slide and slump deposits, turbiditic conglomerates and sandstones occur at several stratigraphic levels. Similar Cretaceous successions are at present subject of intense exploration on the Norwegian shelf and elsewhere around the northern North Atlantic Ocean. The aim of the

study is to describe and interpret the wide spectrum of coarse-grained gravity flow deposits of Traill D, to relate them to the tectonic evolution of the area, and to make a comparison with similar Cretaceous deposits known from wells on the Norwegian Shelf.

Geological setting The important Late B ajocian-Valanginian rift phase is well known from East Greenland, the North Sea and the Norwegian Shelf (e.g. Surlyk et al., 1981; Ziegler, 1988; Surlyk, 1990, 1991, 2001; Dor6, 1992; Surlyk and Noe-Nygaard, 2000). Great economic interest is associated with this event because it created the main hydrocarbon play types in the region. Extensive outcrops of correlative successions occur in East Greenland and have received much attention as analogues for the correlative deeply buried subsurface successions (e.g. Surlyk, 1978, 1990, 1991,2001; Stemmerik et al., 1992; Price and Whitham, 1997). Cretaceous faulting events and associated syn-tectonic deposits have, however, also been reported from East Greenland (Donovan, 1953, 1955, 1957; Surlyk et al., 1981; Surlyk, 1990; Whitham et al., 1999). In Jurassic-Cretaceous time a N-S-trending rifted seaway was situated between East Greenland and

Fig. 1. Pre-drift reconstruction showing the structural framework of the mid-Cretaceous seaway between East Greenland and Norway. Based on Stemmerik et al. (1998), Granholm (1999), Larsen et al. (1999), and unpublished data.

Cretaceous faulting and associated coarse-grained marine gravity flow sedimentation, Traill O, East Greenland

295

Fig. 2. The Mesozoic rift basin of East Greenland showing outcrop of Cretaceous deposits and main faults. Based on Haller (1970), Koch and Haller (1971), GEUS maps, and own data.

Norway (Fig. 1). The western margin of the seaway was located up to a few hundred kilometres inland from the present-day coast of central and northern East Greenland. In the study area on the island Traill 0 (O means island in Danish) (Figs. 2 and 3), Middle Jurassic early-rift deposits rest unconformably on Triassic sediments, and consist of a succession of lower(?) Bajocian fluvial pebbly sandstones of the Bristol Elv Formation, marine sandstones and mudstones of the Upper BajocianCallovian Pelion and Fossilbjerget Formations, shallow marine sandstones of the Lower-Middle Oxfor-

dian Olympen Formation, and dark deep-marine Upper Oxfordian-Kimmeridgian shales of the Bernbjerg Formation (Fig. 4). Note that the rank of some units differs from older literature (Surlyk et al., 2001). Rift climax deposits of latest Jurassic-earliest Cretaceous age are not exposed, but late-rift, relatively deepwater, red, calcareous mudstones occur in isolated outliers on eastern Traill 0 (Donovan, 1953). They are included in the Valanginian ROdryggen Member and are characteristic of submerged fault block crests suggesting the presence of a buried rift-climax prism (S urlyk, 1978).

296


Fig. 3. Map of the Traill 0 region showing faults, fault blocks and place names mentioned in the text. Based on Stemmerik et al. (1998). The studied outcrops are marked with a number. Locality *1: submarine debris avalanche, Rold Bjerge Formation. Locality *2: conglomeratesandstone slope apron, M~nedal Formation. Locality *3: submarine slide-slump complex. Locality *4: sandstone-dominated basin floor fan, Vega Sund Formation.

The irregular Jurassic rift topography was partly filled in by a mudstone-dominated Cretaceous succession, up to about 2 km thick. It is not well exposed and has not been lithostratigraphically classified. It represents post-rift deposition during thermal contraction following Middle Jurassic-earliest Cretaceous rifting, punctuated by several possibly rift-related fault events. The age relations of the Cretaceous succession in the Traill 0 region are only broadly known because macrofossils are scarce at most levels and because dinocysts are generally poorly or not preserved due to heating by Cenozoic intrusions (Donovan, 1953, 1955, 1957; NOhr-Hansen, 1993; this study). The Cretaceous mudstones are thus disturbed by numerous dolerite dykes and sills and are overlain by Cenozoic plateau basalts. The presence in the Traill 121region of Lower Aptian, Lower and Upper Albian, and possibly Lower Cenomanian strata was demonstrated on the basis of dinocysts (NChr-Hansen, 1993), whereas Upper Aptian, basal Middle Albian, Upper Turonian-Middle(?)

Coniacian, and Upper Campanian strata are documented on the basis of ammonites, belemnites and inoceramid bivalves (Donovan, 1953, 1955, 1957; this study). The East Greenland margin was uplifted in the order of 1.5-3 km in Cenozoic times (Christiansen et al., 1992), and the results of Mesozoic rifting can now be studied in the mountains along the East Greenland coast. Uplift was not uniform but was associated with faulting, partly by rejuvenation of the older Mesozoic faults, and it is not always possible to reconstruct the Mesozoic component of the fault system nor the amount of throw on the individual faults. The main faults trend NNE and are connected by NW- and NE-trending transfer faults. The faults are identified by mapping on aerial photographs and ground mapping, and the time of their activity is estimated by comparison of successions in the footwall and hanging wall, and by matching clast lithologies in syn-tectonic deposits on the hanging wall with successions exposed in the footwall. In this


297

Fig. 4. Stratigraphic scheme of the Upper Permian-Cretaceous of East Greenland. The Jurassic lithostratigraphic nomenclature is from Surlyk et al. (2001 ).

way it is possible to identify which of the main NNEtrending faults were active during the Cretaceous. Main faults

The fundamental, N-S-trending, Post-Devonian Main Fault (Figs. 1 and 2) was recognised in the

region north of Traill O by Vischer (1943). In the southern Geographical Society O-Traill 0 the orientation changes to NE-SW and this diagonal fault links up with the N-S-trending Stauning Alper Fault which limits the Jameson Land Basin to the west (Figs. 1 and 2). The fault system downthrows Carboniferous-Lower Permian strata to the east against

298 Devonian and older rocks and was the main basin bounding feature in Late Permian-Mesozoic times. Another fault runs parallel to the Post-Devonian Main Fault 4 km to the east (Fig. 3). It downthrows Upper Permian-Lower Triassic against Upper Carboniferous strata and is probably a late feature as it cuts Cenozoic sills. The next fault to the east, the Bordbjerg Fault, downthrows Triassic against Upper Carboniferous strata (Figs. 1 and 3). The fault has several splays, dips 50-60 ~ towards the east, post-dates the earliest Triassic and is mainly of Cenozoic age. The Mgmedal Fault downfaults Cretaceous against Lower Triassic strata, dips 50-60 ~ to the east, and is probably mainly of Cenozoic age (Figs. 1 and 3). Faults A and B are splays at the southern end of the Mfinedal Fault (Fig. 3). Fault A downfaults Middle Jurassic-Cretaceous against Triassic strata, whereas Fault B downfaults Cretaceous against Middle Jurassic-Cretaceous strata. Coarse-clastic deep-water deposits

Cretaceous coarse-clastic, deep-water deposits are exposed in dip sections in several E-W-trending valleys which cut across the N-trending Mfinedal Fault (Fig. 3). An early expression of this fault may have existed in Cretaceous time, but its present appearance is due to Cenozoic faulting. Four different types of coarse-grained deep-water deposits are described below, starting with the most chaotic and ending with the most organised system. Their general setting and age is presented, and the facies are described, followed by interpretation of transport processes and depositional environment. The coarse-clastic deposits are in most cases rather chaotic without showing any pronounced organisation in the form of vertical facies trends. Clasts in the breccias and conglomerates vary in size from pebbles to blocks up to 60 m across, and in composition from carbonate over sandstone to metamorphic quartzite, granite and gneiss. Several of the clast lithologies are sufficiently characteristic to allow determination of provenance, and clasts composed of Upper Permian carbonates and shales, Lower Triassic carbonates and pebbly sandstone and Middle Jurassic sandstones have all been identified with more or less certainty. The clast ages allow determination of the stratigraphic units exposed in the footwall fault scarps and of the minimum height of the scarps at the time of deposition. The provenance of the clasts also indicates which faults were active during deposition. The resedimented units show significant differences and are highly characteristic. Three of the units form the basis for the definition of the new Rold

F. Surlyk and N. Noe-Nygaard

Bjerge, Mfinedal and Vega Sund Formations (see Appendix A). Submarine debris avalanche - - Rold Bjerge Formation

The studied section is located at 600 m altitude in Rold Bjerge in a mountain pass, about 3 km north of the Mfinedal valley (locality *1 in Fig. 3). The pass marks the position of the N-S-trending, eastward-downthrowing, M~nedal Fault which forms the eastern boundary of the 5 km wide Mfinedal Block limited to the west by the Bordbjerg Fault (Fig. 3). The footwall of the Bordbjerg Fault exposes a succession of Carboniferous pebbly sandstones and conglomerates overlain by Upper Permian conglomerates, sandstones, hypersaline carbonates, evaporites and mudstones, topped by sandstones and shales of the Lower Triassic Wordie Creek Formation. The Mgmedal Block exposes a thick succession of shallow marine sandstones of the Wordie Creek Formation (Figs. 3 and 4). Pebbly sandstones and sandstones occurring in or adjacent to the M~nedal Fault separating Triassic and Cretaceous strata were previously interpreted as a sliver of the Middle Jurassic Pelion Formation (Donovan, 1953; Price and Whitham, 1997). They are here considered to represent the Svinhufvuds Bjerge Member of the Wordie Creek Formation (Clemmensen, 1980). If this interpretation is correct, the M~nedal Fault is much more simple than indicated by previous studies. The succession exposed in the hanging wall immediately east of the Mfinedal Fault comprises a chaotic sedimentary breccia, at least 25 m thick (Fig. 5), sharply overlain by dark-grey to black, laminated mudstones with abundant inoceramid bivalves, including Actinoceramus sp. aft. concentricus (Parkinson), indicating an earliest Middle Albian age (K.-A. Tr6ger, written commun., 1997). Dinoflagellate cysts from shales below the breccia indicate an Early A1bian age (N~hr-Hansen, 1993). The breccia is exposed in a series of bluffs extending over a distance of several hundred metres (Fig. 5). The clast size varies from pebbles and cobbles to very large blocks. The largest block observed is 60 m by more than 10 m and is oriented with the long axis parallel to bedding. It consists of brown-weathering, well sorted, fine-grained quartzose sandstone. Another block of the same lithology is 30 x 30 m in size and blocks in the 5-10 m range are common (Fig. 6). Many clasts consist of pebbly sandstone. Angular platy boulders of white or pinkish limestone are very common and may reach a maximum length of several metres. They show irregular banding or crude lamination reminiscent of algal lamination (Fig. 7).

Cretaceous faulting and associated coarse-grained marine gravity flow sedimentation, Traill r East Greenland

299

Fig. 5. Submarine debris avalanche breccia sheet of the Lower or lowermost Middle Albian Rold Bjerge Formation. The breccia is overlain by black basal Middle Albian mudstones and is bounded to the west by the Mfinedal Fault. The main left part of the sheet is one big block of pebbly sandstone, 60 m long. Lower Triassic sandstones of the Wordie Creek Formation are exposed in the footwall west of the fault. Locality * 1 in Fig. 3.

Fig. 6. Large blocks of whitish limestone and sandstone with densely packed angular boulders (hammer encircled for scale). Debris avalanche breccia of the lower or lowermost Middle Albian Rold Bjerge Formation. Locality * 1 in Fig. 3.

300


Fig. 7. Detail of the debris avalanche breccia of the lower or lowermost Middle Albian Rold Bjerge Formation. Note the dense packing of angular blocks composed of pebbly sandstone, sandstones and Upper Permian carbonates showing wavy, possibly algal lamination (above hammer head). The breccia is strongly cemented by several generations of cement. Hammer for scale. Locality * 1 in Fig. 3.

Light-brown, cobble-sized, equidimensional and angular limestone clasts dominate in parts of the unit. The elongate clasts show a crude orientation subparallel to bedding, but the sorting is extremely poor. Grading and clast imbrication have not been identified. Two to three highly irregular bedding planes can, however, be traced through much of the unit. They show a dip of 5-10 ~ to the east. The chaotic nature, lack of organisation of the poorly defined breccia beds, and the content of very large blocks suggest deposition from submarine debris avalanches which may have been hydroplaning (cf. Mohrig et al., 1999). The laminated limestone clasts were undoubtedly derived from the Upper Permian Karstryggen Formation, whereas others are similar to carbonates of the Lower Triassic Odepas Member. The source of the pebbly sandstone clasts is more difficult to identify but they may have been derived from the Lower Triassic S vinhufvuds Bjerge Member. The provenance of clasts shows that the adjacent Mfinedal and Bordbjerg Blocks could not have served as source areas. The Karstryggen Formation and the Svinhufvuds Bjerge and Odepas Members occur in the Mfinedal and Bordbjerg Blocks in undisturbed stratigraphic succession, well below the topographical

level of the breccia. The source thus has to be sought further to the west in the footwall of the Post-Devonian Main Fault. This gives a minimum runout distance of the debris avalanches of about 25 km. The Rold Bjerge breccia thus records earliest Middle Albian rejuvenation of the Post-Devonian Main Fault, footwall uplift, and shedding of a series of submarine debris avalanches down the faulted slope (Fig. 8). The trigger mechanism was probably earthquakes caused by movements on the fault. Submarine slide-slump complex ~ Eastern Svinhufvud Bjerge

This locality is situated in an east-west-oriented valley cutting the B fault splay of the N-S-trending Mfinedal Fault (locality *3 in Fig. 3). The section was mapped using a mosaic of overlapping polaroid photographs (Fig. 9). It exposes an impressive slideslump complex which extends over several hundred metres downdip towards the east-southeast (140~ Yellow sandstones of the Pelion Formation are exposed in the footwall and gently E-dipping, dark-grey mudstones and yellow sandstones in the hanging wall. The mudstones have yielded dinoflagellate cysts indicating a Late Albian age (NOhr-Hansen, 1993).


301

Fig. 8. Depositional model for the Lower to lowermost Middle Albian submarine debris avalanche breccia of the Rold Bjerge Formation. PDMF = Post-Devonian Main Fault; MF -- possible early expression of the M~nedal Fault.

Turbidite sandstone beds, mainly 10-20 cm thick, are interbedded with the mudstones. They are dominated by Bouma Ta divisions and appear relatively sheet-like. The succession is disturbed by numerous low-angle irregular faults which are roughly parallel to overall bedding, and separate packets, up to a few metres thick (Fig. 9). The strata in the packets have orientations ranging from parallel to the faults to vertical and from gently to strongly folded. Isoclinal folds are common as are overturned or subvertical beds which may show S-like folds (Figs. 9 11). Some packets are sandstone-dominated, whereas others consist of mudstone with only a few thin sandstone beds (Figs. 9 and 11). This clearly reflects an original difference in the faulted succession with alternating sandstone- and mudstone-dominated units. Orientations of slump fold axes show a broad scatter from 0 ~ over 90 ~ to 160 ~ averaging 74 ~ and vergences range from south over east to northeast. General direction of transport seems to have been roughly towards the southeast. A succession of depositional events can now be interpreted. A mud-rich turbidite system was formed in the Late Albian. The mainly thin-bedded nature of the sandy turbidites and the low sandstone : mudstone ratio suggest a distal, relatively deep-water environ-

ment, probably a fan fringe. A levee origin cannot be excluded, but is considered less likely on the basis of the apparent sheet-like nature of the turbidites, the alternation between sandstone- and mudstone-dominated units, the dominance of Ta divisions, and the overall absence of climbing ripples in the Tc divisions. At some later time the whole succession was disturbed by slope failure and was transported by sliding and slumping down the palaeoslope resulting in the formation of a thick slide-slump complex (to the left on Fig. 12). Movements were both translational and rotational as shown by the planar to curved rupture surfaces and differences in orientation of strata between the detachment planes. The timing of the slide-slump event is not known and cannot be directly determined from the section. This can only be done if a locality is found where the disturbed succession is overlain by younger, undisturbed deposits. It is suggested that the event was triggered by the same fault episode that caused the formation of the turbiditic conglomerate-sandstone complex of the Mfinedal Formation described below, and a Late Turonian-Early Coniacian age is tentatively suggested for the event. This interpretation is based on the evidence for strong footwall uplift of the Post-Devonian

C.O 0

Fig. 9. Sketch of slide-slump complex, based on field mapping on polaroid photographs. Note the abundant subparallel detachment planes delimiting strongly folded packages of dark-grey mudstones and sandstone turbidites. Locality *3 in Fig. 3.

Cretaceous faulting and associated coarse-grained marine gravity flow sedimentation, Traill 0, East Greenland

303

Fig. 10. Slide-slump complex, eastern Svinhufvud Bjerge. Interbedded dark-grey mudstones and turbidite sandstones have undergone translational down-slope sliding and slumping. The strata were folded and faulted during transport but were not homogenised and transformed into debris flows, probably due to consolidation and early cementation. The shales and sandstones were deposited in the Late Albian (N~hr-Hansen, 1993), but the subsequent slide-slump event cannot be dated. It is tentatively suggested that it was caused by the Late Turonian-Early Coniacian fault episode that resulted in deposition of the Mfinedal Formation conglomerates. Locality *3 in Fig. 3.

Main Fault during the Late Turonian-Early Coniacian event, the possible initiation of the M~nedal Fault, and the similar position of the two systems along strike of the M~nedal Fault.

Conglomerate-sandstone slope apron - - M~nedal Formation

A relatively thick succession of pebble, cobble and boulder conglomerates was described from the low northern bank of the Mfinedal valley by Donovan

304

F. Surlyk and N. Noe-Nygaard

Fig. 11. Details from the slide-slump complex of Fig. 9. Note the sharp detachment surface at the top of the sandstone-dominated lower unit.

Fig. 12. Depositional model for the Late Turonian-Early Coniacian conglomerate-sandstone slope apron of the M~nedal Formation. The submarine slide-slump complex in eastern Svinhufvud Bjerge (locality *3 in Fig. 3) is shown in the foreground to the left. The strata are of Late Albian age and the slump event is tentatively referred to the Late Turonian-Early Coniacian. PDMF = Post-Devonian Main Fault; MF = possible early expression of the M~nedal Fault.

(1953). The succession has yielded poorly preserved ammonites, belemnites and inoceramids which together suggest a Late Turonian-Early Coniacian age

(W.J. Kennedy and W.K. Christensen, written commun., 1999). The rather poor exposure starts 500 km east of the Mfinedal Fault and is about 500 m long


but is interrupted several places by unexposed intervals (locality *2 in Fig. 3). The strata appear to dip between 10 and 20 ~ towards the ESE, and this gives a thickness between 90 and 180 m. Donovan (1953) estimated a thickness of 80-100 m based on similar considerations. Direct measurement of the composite section made up of the numerous small, closely spaced outcrops gives, however, a thickness of only 45 m. This thickness estimate is corroborated by independent measurement by M. Larsen (pers. commun., 1999). It cannot be excluded, however, that parts of the section are cut out by later faulting, whereas repetition of strata seems not to have taken place. A total of nine outcrops were studied down-dip from west to east covering the same stretch as localities 65-62 of Donovan (1953) and mapped on polaroid photographs. Locality 33 farthest to the west and locality 38 close to the eastern end provided the best outcrop. A late fault cuts the eastern end of locality 34, but otherwise only minor faults were observed, and some of these are syn-depositional. The sections in localities 36-39 appear to be in stratigraphic continuity in spite of the poorly exposed nature of the intervening parts of the river bank. The succession is volumetrically dominated by pebble and cobble conglomerates, and pebbly sandstones are prominent at some levels, whereas others are dominated by boulders (Figs. 13-18). Shale zones up to 40 cm thick occur at several levels and form marker beds for correlation between exposures. Five facies groups are identified and are described and interpreted below.

Boulder conglomerates This facies is highly irregular in appearance. It is dominated by large, commonly outsized boulders and several characteristic boulder lithologies occur. The most common type comprises characteristic, muddraped yellow sandstone boulders (Fig. 17). They are commonly about 0.5-1 m long, may reach lengths of almost 2 m, are subrounded, and have striations on the surface. The mud drape is black and quartz and shale pebbles are commonly pressed into the surface of the boulder. The yellow sandstone is mainly well sorted but may contain stringers of quartzite pebbles. The lithology is highly similar to the Middle Jurassic Bristol Ely and Pelion Formations, but a source in the Lower Triassic Svinhufvuds Bjerge Member is also possible. The subrounded shape, surface striations and rather poorly consolidated nature of the sandstone boulders show that the sand was only weakly cemented at the time of erosion in the source area, suggesting only shallow burial before uplift, exposure, erosion and redeposition.

305

Another characteristic, but less common, boulder type consists of grey limestone (Fig. 16A). The boulders are up to 250 cm long and may have tabular to almost spherical shapes. The largest boulder observed measures 250 x 150 x 150 cm and consists of thick-bedded dark-grey wackestone to packstone with abundant brachiopods and bivalves at the base of the original succession and with algal stromatolites at the top. The lithology and fossil content of the limestone boulders are similar to the Upper Permian Wegener HalvO Formation and there is no doubt that this unit formed the source of the boulders. A third boulder lithology consists of black shale, appearing as elongate and platy, angular to subrounded clasts. The lithology is similar to the Upper Permian Ravnefjeld Formation but other black shales may also have formed the provenance. The Upper Jurassic Bernbjerg Formation is thus another possibility but was probably too poorly consolidated to form angular clasts at the time of Late Cretaceous erosion. The boulder-dominated beds pass laterally into pebble and cobble conglomerates with only a few boulders. They can thus be considered as a variant of the pebble-cobble conglomerates described below, but some boulders may be genetically unrelated to the beds in which they occur. In cases where isolated boulders or boulder trains are found at the base of a pebble-cobble conglomerate they were probably deposited by an earlier event and were subsequently embedded in a finer-grained gravity flow deposit. The mechanisms active during transport and deposition of the isolated boulders and boulder-dominated conglomerates are thus difficult to interpret. A debris fall transport mechanism (see Nemec, 1990) is suggested for those boulder accumulations forming clast assemblages which have clearly been overridden by a finer-grained tail. Others were obviously part of a finer-grained sediment gravity flow with sufficient competence to transport outsized clasts, and some were probably emplaced from hydroplaning debris flows (cf. Mohrig et al., 1999).

Pebble-cobble conglomerate This facies is volumetrically dominant (Figs. 1316). Beds are mainly about 0.5-1.5 m thick, but may reach a thickness of 3-4 m. The lower boundary of the beds may be erosional or sharp without showing signs of marked erosion. In some cases beds rest on a pronounced scour surface and the lower boundary is in a few cases developed as a detachment surface (Fig. 18). The upper boundaries are planar or scoured by the base of the overlying bed. The beds are non-graded or show a crude coarse-tail or content grading, while basal inverse

306

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2250m).Calcareous benthonics occur, e.g.Quadrimorphina albertensis. Quao'rimorphina is typically noted in shelf and upper-middle slope depths (1.e. subrnorphogroup CH.43.1, Koutsoukosand Hart, 1990) in the Cretaceous. Stensioeinids were nlso noted (outer shen to upper slope in theTertiary. Gradstein and Backstrom, 1996).Cystammlnids were noted at the base of K3K2 (more common in middle slope depths, Gradstein and Backstrom, 1996) Low tau values suggest shallow conditions.The presence of Uvigerinammina rnoesiana suggests slope depths (Kuhnt el al., 1989).Other typical forms of this slope assemblage are Glomospira cbaroides. G. gaultina, Recurvoides ,Gavelinella 'barremianaformis' [= G. barremiana?),Lentrcu//na kug/er/ and L. muensferi.Gavelinellids noted by Sliter (1972) as an important part of an upper slope assemblage in the late Cretaceous. G. barremma belongs lo submorphogroupCH.-A5 of Koutsoukos and Hart (1990).middle outer neritic (she0 and upper slope depths. Lenticulina is an outer shell to upper slope genus (Murray, 1991).

Fig. 3. Composite generalised well analysis relating ages, seismic units and inferred palaeowater depths to selected microfaunal comments.

I

The reconstruction and analysis of palaeowater depths: a new approach and test of micropalaeontological approaches

out in an area where there was good stratigraphic and palaeogeographic control. (6) The concept of morphogroups or morphofunctional analysis (Gibson, 1988) has been recognised and employed by Jones and Chamock (1985), Corliss and Chen (1988) and Koutsoukos and Hart (1990). This approach facilitates an examination of different microhabitats, because they may be dominated by different shapes, or forms of tests. Koutsoukos and Hart (1990) pointed out that one of the functions of the foraminiferal test is to favour a particular mode of life (e.g. dwelling habits, feeding strategy) in a particular substrate niche. Jones and Charnock (1985) presented an ecological model that related four morphogroups based on feeding strategies and inferred life positions to water depth. Corliss and Chen (1988) proposed that distinct morphological trends in foraminifera and microhabitat patterns both exist and vary with depth in the modem Norwegian Sea. According to Geroch and Kaminski (1992) this approach has become an increasingly popular method of palaeoenvironmental analysis, but care must be taken in inferring feeding strategy groups and water depths. (7) Measures of foraminiferal species diversity have been frequently used in the past (Table 1). The Fisher alpha index (Murray, 1991; Ujetz, 1996) is one widely used measure of species diversity or richness. This index is a reasonable guide to the environment and possibly depth (Murray, 1991). There is a tendency, however, for alpha to increase with sample size for statistical reasons a feature that can complicate interpretation. (8) Multivariate analysis of taxon abundance data has been successfully used in many ecological and palaeoecological studies (Kovach, 1989) and is employed as one strand in this new eclectic study. A number of different multivariate methods have been used here, but they have different mathematical bases and assumptions that must be kept in mind. The aim of such studies is to detect and summarise any underlying trends or patterns. In Kovach's (1989) study, clustering of the Spearman's Correlation Coefficient using UPGMA (unweighted pair group averaging methods) gave distinct clusters of taxa and samples that could be easily interpreted. In the present study, the statistical procedure adopted was the "Constrained Clustering" provided by the MVSP software package (Kovach, 1995). This is the only available software package designed especially for geological applications which contains the Constrained Cluster Analysis technique. This method is used for the classification of sequential data (Vincent, 1995). Clustering proceeds as normal, but the objects to be fused are constrained by having to be adjacent in the data matrix. The calculated dendogram thus has the ob-

375

jects (samples) in the same order as the input matrix (Kovach, 1993). In this study, this feature has enabled sections to be examined whilst the samples are kept in the correct stratigraphic order. It is from the dendogram that the palaeoenvironmental interpretations used here have been derived.

New procedures devised In this study faunal associations were used to define maximum and minimum water depths. Within these faunal associations we looked at key species and suggested a best-fit palaeowater depth curve. No algorithm was used to define the latter. The faunal associations were recognised using a series of techniques previously outlined. Key species recognised were those for which reliable information already existed. By reliable information the authors mean: (1) modem analogues/relatives; and/or (2) published information on fossil assemblages established with reliable models concerning water depths. Data sources

Data for this particular project have been gathered from consultancy well reports for twelve Norwegian North Sea wells placed along or near the key seismic lines: 15/5-3, 17/3-1, 24/9-1, 29/6-1, 30/4-1, 30/10-6, 31/6-3, 33/9-18, 34/7-1, 34/11-1, 35/11-3 and 35/12-1 (Fig. 1). Palaeobathymetric models have been erected for Cenozoic and Cretaceous times in these wells. The palaeowater depth curves here presented are related to the sequences of Skibeli et al. (1995) and Bjerke (1998) for K1 to K6 (Cretaceous sediments), and Jordt et al. (1995) CSS 1-10 (Cenozoic sediments). Fig. 3 shows a composite generalised well analysis, relating ages, seismic units and inferred palaeowater depths to some selected microfaunal comments used in our account. Results

The present study explicitly attempts to improve the coherence and consistency of results likely to be obtained in all such palaeobathymetric and palaeoenvironmental studies, a goal suggested by Peet (1974). All the methods described in the previous section of this paper have been used in this study to assess the palaeoenvironment and possible transgressive and regressive cycles that have taken place in the northern North Sea from the Cretaceous to the Quaternary. The results of this new integrated and eclectic analysis are displayed on Fig. 2 for an example well (30/10-6). This plots and compares the trends of tau, NCA, P:B values and species diversity in a form of

376 a concise graph. In brief, this research demonstrates that it is possible to reconstruct water depths using consultancy well data of variable quality. The nature of ancient water depths in the Norwegian North Sea are explored in more detail below.

Structural and micropalaeontological palaeowater depth comparisons In the following we compare suggested water depths derived from independent structural approaches to the new integrated approach to micropalaeontological data. Palaeowater depth analysis are determined for twelve wells and selective depth curves are presented as representative examples (Fig. 4).

Cretaceous palaeowater depths A shallowing is suggested in the Early Cretaceous by the structural reconstruction (Fig. 4). This shallowing, however was only observed in micropalaeontological data from well 30/4-1, and implied in wells 34/11-1, 35/11-3, 35/12-1 (Fig. 4). Often Early Cretaceous sediments are missing in wells in this study; an unfortunate fact that makes the comparison of evidence impossible in many wells. The Early Cretaceous stratigraphic breaks noted in well 35/11-3 for example (Fig. 4) are suggested by Kyrkjebr et al. (2001) to be a result of subaerial erosion and exposure. A general deepening was noted in the mid-Cretaceous, on the basis of the structural restoration (Fig. 4). This is supported by the present analysis of micropalaeontological data from cores, side-wall cores and ditch cuttings for most wells (15/5-3, 17/3-1, 30/4-1 and 34/11-1, Fig. 4). In well 34/11-1, lower tau values at the base of K3 (Cenomanian) than towards the top of K3 and base of K4 (Cenomanian to Turonian) suggests a deepening up-hole. This is also supported by the microfaunal character. In well 31/6-3, this analysis suggests a shallowing in K3/4 times; but this interesting new event also seems to be fully supported by the structural restoration. In well 30/10-6 microfaunal analysis suggests a slight shallowing in K3/4 and into the base of K5/6 (Fig. 4, Coniacian to Maastrichtian), whilst in well 35/12-1 the new integrated approach shows a brief shallowing in K3/4 followed by a deepening in K5/6. There seems to be a slight delay before deepening events suggested by the structural restoration became noticeable by micropalaeontological evidence. This delay may have been due to the influence of local tectonic activity. The wells concerned may have been on structural highs, so that they affected the crests of these highs rather late (Fig. 1). In general, in the Late Cretaceous, there is a widespread shallowing suggested by the microfaunal evidence from all of

G.K. Gillmore et al.

the wells in the study area. The shallowest part within K6 (Maastrichtian) appears to be just below the K6/CSS1 (Maastrichtian/Paleocene) boundary. Above this point, there is a deepening into CSS1 suggested by structural restoration.

Cenozoic to Quaternary palaeowater depths The trend towards basin deepening suggested by the structural restoration is supported by the new analysis of micropalaeontological evidence. The deepest point was reached towards the top of CSS 1 (Fig. 4). This deepening continued into the lower part of CSS2 (Eocene) according to the structural approach. The analysis suggests a very brief shallowing at the Late Paleocene/Early Eocene boundary (Figs. 3 and 4). This inference is based on the presence of the diatom-rich Balder/Sele Formations with rare agglutinated foraminifera that was followed by the occurrence of a rich and diverse red-stained planktonic foraminiferal assemblage. The estimated tau values in wells 29/6-1, 33/9-18 and 34/11-1 (Fig. 4) suggest deeper water conditions within this planktonic-rich horizon, although a number of the species present suggest a shallowing (that is, the presence of calcareous benthonic forms typically noted in outer shelf to upper slope depths). Either way, there was a distinct faunal change at this time, perhaps as a result of changing oceanic conditions (more oxygenated bottom waters and better water circulation than in the underlying Balder/Sele Formations). The analysis here suggests a deepening within CSS2 (Eocene), with the presence of a rich and diverse agglutinated foraminiferal fauna in well 34/11-1 followed by a distinct shallowing in the Oligocene (CSS3-4). This is evident in most wells (Fig. 4). In well 34/11-1 there was a decrease in P : B % and tau in the Early Oligocene (CSS3), while in the Late Oligocene the abundance of sponge spicules together with a fairly rich and diverse calcareous benthonic fauna suggests shelfal to upper slope conditions. In several wells, this shallowing occurs mainly at the top of CSS4 (Fig. 4). The structural restoration suggests a distinct shallowing beginning at the base of the Oligocene, a conclusion supported by the present analysis of micropalaeontological data in some of the wells in this study. In the seismic interpretation (for example, for well 33/9-18) it was usually not possible to differentiate between the Miocene seismic units CSS5, CSS6 and CSS7 in the northern North Sea (Kyrkjebr et al., 2001). It could also not be decided with any certainty in this study whether subaerial/submarine erosion or non-deposition caused the Middle Miocene (CSS6) hiatus (KyrkjebO et al., 2001) in many wells, although Kyrkjebr et al. (2001) suggest that the missing middle-Late Miocene and latest Miocene to Early

e5

~..~~

r

t...,

~176

e5

t..., ~.~~

o-a e~ ~..~~

Fig. 4. Comparisons of palaeobathymetry from structural restoration and micropalaeontology. The inferred palaeowater depth curves shown have been constructed in the following ways. The solid error bars on the palaeowater depth curves represent our estimate of "most likely" range of palaeowater depths, while the dotted error bars indicate our estimate of the "possible" range of palaeowater depths. The blue palaeowater depth line defines our view of the "most likely" curve suggested by micropalaeontology. The red lines indicate the reconstruction of palaeobathymetry based upon consideration of the geological structure over time. The palaeowater depths for each environment (shelf, slope, etc.) are based on Gradstein and B~ickstr6m (1996). For information, these charts also show the sequences of Skibeli et al. (1995), Jordt et al. (1995) and Bjerke (1998), together with hiati (or stratigraphic breaks) and sedimentation rates.

e5

"-4 "-,1

378

Pliocene section of well 29/6-1 is a result of subaerial erosion (Fig. 4). Jordt et al. (1995) documented a large hiatus in the Miocene in the Norwegian North Sea. Sediments of this age are usually missing in the study area (Fig. 4). Martinsen et al. (1999) note that this Miocene break is characterised by significant erosion of the underlying Oligocene in the MOre Basin. This break varies time-wise depending on the extent of the Utsira Formation in the Central North Sea Basin axis (Martinsen et al., 1999). Martinsen et al. (1999) also suggest that the lower part of the Miocene break is an erosional vacuity with the upper part being a hiatus of 12 m.y. duration. Eidvin et al. (1999) indicate that this hiatus is related to tectonic events in the Middle Miocene. Both our micropalaeontological investigations and structural reconstruction suggest that the Norwegian North Sea Basin in the study area deepened again in the Pliocene (Figs. 3 and 4). Many wells show a deepening in the Pliocene sequence CSS8, a shallowing into the Quaternary CSS9, and a deepening into CSS10 (wells 17/3-1, 30/4-1, 30/10-6, 31/6-3, 33/9-18, 34/11-1, 35/11-3, Fig. 4). The deepening in sequence CSS8 is fully supported by the micropalaeontological evidence (for example, the presence of Oridorsalis u m b o n a t u s in well 29/6-1 suggesting slope depths or deeper; Mackensen et al., 1985; Murray, 1991; higher tau values in well 30/10-6 within the Early Pliocene than in the Late Pliocene) but the deepening in CSS 10 is not clear from this approach. This is apparent in wells 29/6-1 (Fig. 4) and 35/12-1 where micropalaeontology suggests a shallowing from CSS8 to CSS10. A few wells in the study area show an apparent shallowing within the Quaternary (wells 15/5-3, 24/9-1 and 34/7-1), but two of these lack structural results. The structural reconstruction in well 35/12-1 contradicts the general trend by suggesting an overall deepening throughout CSS8 to CSS10, without a shallowing within CSS9. This illustrates the complexity of the Quaternary palaeoenvironmental record. Discussion

A number of key points emerge as a result of these investigations. (1) The two independent methods (structural and micropalaeontological) for deriving past water depths are in substantial agreement. There is, for example, a distinct and rapid deepening in CSS3 into CSS2 shown by the microfaunas and structural restoration in all wells in this study. This event is usually associated with relatively high sedimentation rates (Fig. 4). Nevertheless, some contradictions occur. These centre mainly around the position of wells for micropalaeontological analysis compared to the restored transects.


For example, in well 30/10-6 the structural restoration suggests a deepening in the Cenomanian (K34), while the micropalaeontology shows relatively little change, indeed a shallowing may have occurred locally at the base of K5. The Cenomanian deepening suggested by the structural analysis can be seen in other wells in the study area and is confirmed by micropalaeontology from those wells (e.g. well 34/11-1; Fig. 4). According to the micropalaeontological reconstruction there is a deepening that occurs in well 30/10-6 in the Campanian (Figs. 3 and 4), followed by a shallowing that is indicated by both micropalaeontology and structural work. Well 30/10-6 was located on the flanks of the Cretaceous deep-water basin (Kjennerud et al., 1999, 2001), so palaeowater depths calculated from micropalaeontology will reflect depths at that particular location, i.e. the basin flank. Other contradictions centre around variable data quality available for microfaunal analysis. In well 30/4-1 (Fig. 1) in CSS2 times palaeowater depths suggested by structural analysis were around 800 m; while microfaunal evidence suggests a possible 500 m water depth, the bulk of the microfaunal evidence indicates depths of around 300-400 m. When the contractors report was originally produced for well 30/4-1 in 1979 many deep-water indicator species were not recognised and listed, hence the suggested microfaunal depths are probably too shallow. Kjennerud et al. (1999) show that in the earliest Eocene well 30/4-1 would have been near the centre of the basin in water depths deeper than 500 m. In well 34/11-1 in CSS2 (Figs. 1 and 4) water depths have been suggested by the micropalaeontological approach of around 500 m with deepest depths around 750-800 m in mid-CSS2. The structural analysis in this well over this interval suggests water depths around 800 m throughout. This discrepancy between micropalaeontological and structural results for part of CSS2 in this well is possibly a result of facies control on the occurrence of deep-water agglutinated benthonic foraminifera, such as small cystamminids (Gradstein and B~ickstr/Sm, 1996). At the top of CSS2 sand is occasionally present in this well, with light greenish claystones predominating mid-CSS2 and below. Indeed, such cystamminids occur inconsistently at the top of CSS2, and do not occur consistently until mid-CSS2, Middle Eocene. (2) The new integrated/eclectic method has recognised features and trends not previously signalled by the structural method, but which are likely to be real and not statistical anomalies or inventions. (3) The present authors conclude that their new approach is robust and that it has been tested successfully against independent data derived from a long period of geological time.

The reconstruction and analysis of palaeowater depths: a new approach and test of micropalaeontological approaches

(4) The approach has revealed coherence between the behaviour of the various measures and parameters calculated from existing micropalaeontological data. Therefore this new approach has appropriate internal consistency. As a result of this research, we can now derive with greater confidence palaeowater depth trends based upon interpretations of micropalaeontological data and explore with greater confidence the nature and significance of past variations in sea level in the northern North Sea.

Conclusions This paper has successfully established estimates of palaeowater depth in the Norwegian North Sea through the use of modem foraminiferal species analogues and by calculating a series of surrogate statistical measures derived from existing information on identification and frequencies. These derivative measures have been compared and contrasted to produce a best micropalaeontologically based estimate of palaeowater depth changes from the Cretaceous to Quaternary, although the complexity of change in Quaternary palaeogeography and water depths in this area are shown to be far from fully realised by this analysis. This study has established which micropalaeontological approaches that in combination appear to give the most consistent results. A number of microfaunal methods to assess palaeoenvironment, and hence palaeowater depth, should be used, because according to Peet (1974) there is no sound basis for comparing the richness of a series of communities through using only a single index. A verification of this micropalaeontological method estimating palaeowater depth was obtained by comparing the outcomes of this new eclectic approach with palaeodepth estimates derived independently from a "structural method". The latter method involved section balancing/restoration techniques designed for extensional regimes in combination with interpretations of seismic sequence stratigraphy. This comparison revealed an essential similarity between the outcomes of the micropalaeontological and structural approaches; an outcome that encourages confidence in this new approach. The present study has emphasized the correctness of the comments by Gradstein and B~ickstr6m (1996) who pointed out the importance of palaeobathymetry for establishing the depositional history, subsidence and burial analysis of a region. An overestimation of palaeowater depth may lead to inadequate estimates of uplifts and an underestimation or reduction in subsidence rates. Ignorance of palaeobathymetric trends may therefore lead to an underestima-

379

tion of basin subsidence and incorrectly assigned palaeoslopes (Gradstein and B~ckstr6m, 1996). Palaeobathymetry has also been restored in this study on regional transects, using this new combination of methods for structural restoration and microfaunal analysis. This has enabled restoration of the basin profile in the post-rift interval in the Norwegian North Sea region. A series of shallowing and deepening trends have been recognised in the study area. A shallowing was noted in the Early Cretaceous (based primarily on structural restoration), followed by a deepening in the mid-Cretaceous and a shallowing in the Late Cretaceous. The basin became deeper in the Paleocene, followed by a shallowing in the Oligocene to Miocene. The basin became deeper again in the Pliocene. The complexity of palaeowater depth change in the Quaternary interval is well illustrated here in a manner which has not previously been achieved. This complexity is a result of the interactions between various factors: eustatic sea-level changes, multiple glaciotectonic events, erosion and a lack of precise dating. The Norwegian North Sea Quaternary palaeogeography cannot be understood without a sound understanding of previous water depths and seafloor topography and vice versa. This requires further investigation. Finally, it is evident that a broader understanding of fluctuations in palaeowater depth can help to reveal tectonic events. Evidently significant changes in water depth may be associated with increased sedimentation rates. This is illustrated in wells 29/6-1 and 35/11-3 (Fig. 4) where a significant shallowing in the Oligocene sequence CSS3 is associated the significantly greater sedimentation rates. Tectonic events may have led to uplift and hence increased erosion of the source area.

Acknowledgements Thanks are due to SINTEF Petroleum Research, the Research Council of Norway, University College Northampton, U.K., NTNU, Universities of Bergen and Oslo, and clients (Agip, Amoco, Mobil, Norsk Hydro, Phillips, Saga, and Statoil) of the Tecsed (Tectonic Impact on Sedimentary Processes in the PostRift Phase - Improved Models) project for financing this research and permission to publish.

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UniversiO' College Northampton, School of Environmental Science, Northampton NN2 7AL, UK SINTEF Petroleum Research, N-7034 Trondheim, Norway Geological Institute, UniversiO' of Bergen, N-5007 Bergen, Norway


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Outcrop-based classification of thick-bedded, deep-marine sandstones M.O. Badescu

The outcrop-based classification of thick-bedded deep-marine (TBDM) sandstones from sand-rich and mud/sand-rich deep-water systems is built upon the evaluation of facies associations of the TBDM sandstones, their geometry, lateral extent, and the variation of the sand-to-gross ratio (S/G) of the complexes containing TBDM sandstones. The study evaluates individual basin-fill complexes related to an equivalent stage of sedimentation. Three main generic and geometric types of TBDM sandstones were identified. Type A represents TBDM sandstones with a very good correlation (along and across the palaeocurrent direction). The S/G ratio of complexes comprising this type is low, but uniform (0.5-0.4). Type A TBDM sandstones were deposited in unconfined basins, or basins with little obstruction to flow. Type B represents thick sandstones with a good correlation along and poor correlation across the palaeocurrent direction. Complexes comprising type B TBDM sandstones present a highly variable S/G ratio (0.9-0.3). They represent amalgamated, channelised thick sandstones, deposited either in open or in constricted basins, with the poor lateral correlation being due to the elongated geometry of the channels. Type C represents sandstones with generally poor correlation. Complexes comprising type C TBDM sandstones locally present very high S/G ratios (0.9). Type C TBDM sandstones were deposited in topographically confined basins. The classification can be applied to the North Sea's Paleocene and Eocene reservoirs comprising TBDM sandstones. The Paleocene reservoirs are of large size and very sand-rich, with stacked pay zones. The Eocene reservoirs are more localised. Basin-floor topography defines their geometry and lateral extent. Type A and B TBDM sandstones are mostly recognised in the Paleocene reservoirs and type C TBDM sandstones are commonly recognised in the Eocene reservoirs. The study reveals that the TBDM sandstones present different spatial signatures that influence the reservoir behaviour in the early stages of production.

Introduction

Thick-bedded deep-marine sandstones (TBDM) from sand-rich and mud/sand-rich deep-marine depositional systems have been the subject of ongoing research for many years. The processes of transport and deposition of the TBDM sandstones, and the factors controlling their architecture and their geometry are still relatively poorly understood. The architecture and geometry of the TBDM sandstones have significant implications for reservoir quality (porositypermeability variations); hence an improved understanding of the nature of these deposits is critical for the accurate evaluation of reservoirs containing TBDM sandstones. In the last five years, there has been considerable debate concerning the nature and the mechanisms of deposition of the TBDM sandstones (Shanmugam and Moiola, 1994, 1995; Slatt et al., 1997; Lowe, 1997; Bouma et al., 1997; Shanmugam and Moiola, 1997; Shanmugam, 1999). The debate focused on core studies from the North Sea and on outcrop studies. The main concern is whether the TBDM sandstones are deposited by high-density turbidity

currents or by sandy debris flows. In the high-density turbidity current scenario TBDM sandstones are deposited by turbulent flow and are considered to be continuous, homogeneous and with little variation in the porosity-permeability characteristics. If the sandy debris flow scenario is accepted, the TBDM sandstones are deposited by freezing of laminar flow and are expected to be discontinuous, heterogeneous and with unpredictable geometry and porositypermeability characteristics. In previous work the influence of the basin-floor topography on the geometry and architecture of the TBDM sandstones was often neglected. The present study introduces important recognition criteria and common elements of TBDM sandstones based on an overview of TBDM sandstones in outcrops. The objective is to establish characteristics for thick-bedded sandstone successions in order to differentiate types of TBDM sandstones that could be recognised in the subsurface. This overview suggests that the basin-floor topography plays an important role in the geometry, shape and architecture of the TBDM sandstones. The idea proposed here is based on the evaluation of facies associations of the TBDM


3t34 sandstones, their geometry, lateral extent and the variation of the sand-to-gross ratio (S/G). This study evaluates individual basin-fill complexes related to an equivalent stage of sedimentation. The cases represent a variety of basin shapes and dimensions. The thickness of the complexes that were evaluated ranges from 40 to 215 m and their extent from 1 to more than 100 km. All the information used in this text was gathered from open literature, with the exception of the "Mames B leues" study, which represents own field results. Eighteen case studies were initially evaluated. From this data set only ten cases remained, after careful consideration of the sandstone beds' geometries, and after evaluating the nomenclature used so that those contributions could be compared without bias. This paper comments on more than 50 articles on thick-bedded sandstones and associated facies.

Deep-water sandstone facies characteristics and depositional settings Historically, prominent papers on turbidites and other deep-water deposits have been made by Kuenen (1957), Bouma (1962), Mutti and Ricci Lucchi (1972), Walker (1978), Normark (1970) and Shanmugam and Moiola (1988). TBDM sandstones have received special interest over many years. However, only in the past few years their origin and mechanics of transport and deposition have become the subject of serious debate. Much of our knowledge about turbidite facies and mechanisms of deposition of deep-marine sandstones is attributed to Bouma (1962), whose work was primarily related to the Annot sandstones, SE France, where his now classical Bouma sequence was described. Mutti and Ricci Lucchi (1972) developed a comprehensive fan model based on Apenninic outcrops. TBDM sandstones (facies B of Mutti and Ricci Lucchi, 1972) occur from the slope to the mid-fan deposits. Normark (1970) proposed a suprafan model based on recent turbidites of Californian borderland basins, and Walker (1978) incorporated the suprafan concept within the Mutti and Ricci Lucchi models (1972), resulting in a hybrid model, based on both ancient and recent deep-marine sediments. These models are shown in Fig. 1. Later, Mutti (1979) underlined the importance of the grain size and the volume of sediment that influence the facies characteristics and the architecture of the deep-water deposits and developed a new model of deep-marine clastics: efficient vs. inefficient systems. However, this model was widely criticised by Shanmugam and Moiola (1988), who also proposed a new classification scheme based on tectonic setting.

M. O. Badescu

Regarding mechanisms of deposition, Kuenen and Migliorini (1950) carried out the first experiments. Kuenen also proposed the term "turbidite" for the deposits of a turbidity current (Kuenen, 1957). Further experimental work was carried out by Middleton and Hampton (1973) who recognised four main endmember flow types based on the grain support mechanism (gravity, fluidised, liquefied and grain flows). Lowe (1982) suggested a classification and nomenclature based on rheology of the flow and the particle support mechanism. More recently, Shanmugam and Moiola (1995) claimed that TBDM sandstones could be deposited by sandy debris flows. Their novel, still debatable interpretation was based on a case study of the Jackfork Group and on cores from the North Sea, but not on experiments. Their interpretation was amply criticised by Slatt et al. (1997), Lowe (1997), and Bouma et al. (1997). Another interpretation was suggested by the experiments of Kneller (1995) who proposed that deposition of massive sands occurs due to non-uniformity in prolonged, quasi-steady, highdensity turbidity currents. His work on experiments took into account the flow velocity and its spatial and temporal variations. Shanmugam (1999) challenged this idea. Parallel with this work on outcrop and experimental studies, seismic and sequence stratigraphic concepts were developed (Vail et al., 1977; Mutti, 1985; Posamentier and Vail, 1988). New emphasis was put on temporal relationships between source area and basin fills. Mutti (1985) developed a model for the evolution of fan systems during rises and falls of relative sea level that incorporates the features of high- and low-efficiency fans into a single scheme (Types I, II and III). His model was developed for sand-rich delta-fed systems in tectonically active areas. TBDM sandstones can occur within systems I or II of these models (Fig. 2). More recently, Reading and Richards (1994) have introduced a novel classification scheme of turbidite systems in deep-water margins. The focus was on the grain size and the feeder system. It is not clear how one can distinguish between sand-rich and mud/sand-rich systems and the evolution of the systems in time was not taken into account. TBDM sandstones can belong to the sand-rich fans, mud/sand-rich fans, sand-rich ramps, mud/sand-rich ramps or sand-rich aprons (Fig. 3). These studies have led to the present state of the art, which still contains controversies in many respects. Although our knowledge has much increased, TBDM sandstones are still not satisfactorily understood. This problem is related to the fact that the origin of massive TBDM sandstones is due to a variety of processes involving both steady and unsteady

Outcrop-based classification of thick-bedded, deep-marine sandstones

385

Fig. 1. Historical overview of models for deep-water clastic systems. (A) Mutti and Ricci Lucchi (1972) - single fan model. (B) Normark (1970) point source, fan model observed in recent sediments. (C) Walker (1978) - point source, multiple fan model observed both in ancient and modern sediments. Reprinted by permission of the American Association of Petroleum Geologists, Tulsa, OK, whose permission is required for further use. -

Fig. 2. Application of sequence stratigraphy principles in turbidite systems, Mutti (1985). Reprinted with kind permission from Kluwer Academic Publishers, Dordrecht.

386

M. O. Badescu

Fig. 3. Overview of models for deep-water clastic systems. From Reading and Richards (1994). (A) Mud/sand-rich fan. (B) Sand-rich fan. (C) Mud/sand-rich ramp. (D) Sand-rich ramp. (E) Mud/sand-rich slope apron. (F) Sand-rich slope apron. Reprinted by permission of the American Association of Petroleum Geologists, Tulsa, OK, whose permission is required for further use.

flows. There are currently three scenarios for deposition of TBDM sandstones (Stow et al., 1996): (1) deposition from high-concentration turbidity currents, where grain concentration at the depositional surface is sufficiently high to cause hindered settling; some sands, containing scattered outsize clasts, may originate from this process; (2) deposition from freezing of sandy debris flows; (3) sediment collapse fallout. Another fact that complicates the problem of TBDM sandstones is that they can be deposited in a va-

riety of tectonic settings. Relative sea level, the source area and the basin-floor topography are interacting factors that have a major influence in the deposition of TBDM sandstones. TBDM sandstones are generally deposited during a relative fall in sea level, and the source of the TBDM sandstones must be very sandrich (Stow et al., 1985; Picketing et al., 1989; Stow et al., 1996). The models previously considered are too general and basically take into account only three (major) fac-

Outcrop-based classification of thick-bedded, deep-marine sandstones tors: tectonics, source area and relative sea-level variations. These models do not provide sufficient data for an accurate appraisal of an oil reservoir. More factors should be taken into consideration, if working on a smaller scale (up to 5-10 km). The local factors (basin-floor topography, differential compaction, local tectonics), at this scale will have a critical influence on the geometry and the architecture of these deposits. Information about reservoir aspects was mainly restricted to oil company reports for many years. Lately, these aspects were discussed in the literature (Chapin et al., 1994; Cossey, 1994), but the production behaviour received little attention. This paper describes an approach for predicting reservoir properties around exploration and appraisal wells.

Evaluation of outcrop descriptions The objectives of recognising facies-types characteristics for TBDM sandstones requires outcrop descriptions that give reliable vertical profiles combined with a sufficient spatial distribution of data. In

387

addition, the basin setting, provenance and synsedimentary geological events have to be fairly well known. The selection criteria (see Fig. 4 for details) used in this study are as follows: (1) occurrence of sandstone beds thicker than 1 m; (2) information on the architecture and geometry of the TBDM sandstones; (3) at least two well-described time-equivalent detailed sedimentary logs. From an original collection of twenty-five outcrop studies, initially a batch of eighteen appeared promising (Fig. 5). However, a careful analysis left only ten cases. The reason for omitting the other eight cases were: (1) poorly logged sections unsuitable for quantification; (2) severe post-sedimentary deformation prohibiting reliable correlations; (3) existing controversy on interpretation. For examples see Table 1. Several of these cases would appear interesting for more quantitative investigation, such as Tabernas (Spain). All cases provide important information regarding facies and facies associations, the mechanisms of deposition and their environment. However, the small

LITERATURE

I

4,

q, All sandstones beds thinner than 1 m

Sandstone beds thicker than l m Go on Information about environment (channels, interchannels, lobes) exists Go on

~/

,4, No information about bed continuity

Information about bed continuity

Go on

Well described sedimentary logs (SL)

At least 2 time-equivalent SL exist

Reject: Butano, Numidian Flysch Jackford (too controversial)

q,

~/

Go on

Reject: Capistrano, Oquirhh, Torlesse Venado

No information about environment

Poorly described SL

Reject: Tarcau, Tabernas

q, There are not even 2 time-equivalent SL

Annot Beardmore-Geraldton Cengio (PTB) Guipuzcoa-Jaizkibel Marnes Bleues (VB) Marnoso Arenacea Rock Shale Grit Tourelle Tyee Fig. 4. Criteria used for the selection of the case studies.

Reject: Matilija

M. O. Badescu

388 TABLE 1 Overview of outcrops Formation name

Age

Basin setting

Annot (Les Alpes Maritimes, France)

Late Oligocene-Early Eocene

Foreland

100/35

P

550-650/< 10

HDTC

Beardmore-Geraldton (Ontario, Canada)

Archaean

Fore- arc

80/30

M

0.5

17 (max.) 2.75 (min.) 120 (c) 50 (c) 15 (c) 30 (c) 120 (c) 120 (c) 80 (c) 60 (c) 120 (c) 40 (c) 120 (c) 120 (c) 40 (c) 120 (c) 100 (c) 120 (c) 120 (c) 120 (c)

Continuity perpendicular a (km) 1.4 (min.) 1.12 (min. exposed)

l(e) l(e) 1-1.5 (e)

>0.4

>0.8 >1.2 >1.7 >2.4 >0.075 0.05 (e) 11 (max.) 1.2 (rain.) 16 (c) l0 (c) 16 (c) 7 (c) 16 (c) 16 (c) 14 (c) 10 (c) 16 (c) 16 (c) 16 (c) 8 (c) 16 (c) 16 (c) 8 (c) 16 (c) 8 (c) 16 (c) 2 (max.)

3.85 (min.) 1 (min.)

l(c) 5 (c) --~1-2 (e)

0.8 (max.) l(c) 0.74 (c) 1 (c) 0.2-0.5 0.06-0.17 0.06 (min.) 0.23 (min.) 0.23 (min.) 0.26 (min.) 0.26 (min.) 0.12 (min.) 50 (e)

(e) (e) (e) (e) (e)

394

M. O. Badescu

Fig. 8. Correlative measured sections from outcrops lateral of the axial zones of the basins shown in Fig. 6. MA2, C2, $2, A2, R2, G2, To2, B2, Ty2, Jb2 and MB2 represent the sedimentary sections as they are shown in Fig. 6.

(3) The bedding surfaces are generally sharp, even and parallel, although cut-and-fill features may be abundant in amalgamated packages. These features are regarded to be the product of sheet-flow erosion by a single turbidity current. They may influence the variation in vertical permeability of the beds. (4) They are underlain and overlain by basin-plain fines (muddier turbidites, low-density turbidity currents or hemipelagites) or thin-bedded, Bouma-type turbidites. Amalgamated beds are rare in this type of bed succession. (5) The grain size is commonly medium to fine sand, but variation from fine to coarse sand exists. (6) They form thickening-upward or asymmetrical sequences. (7) They are generally massive, but parallel lamination, subtle grading and dish structures occur. Outsized mudstone clasts may occur at the top of the beds. (8) Amalgamation is common. Detailed data on the nature of amalgamation surfaces are not available. However, such surfaces are considered to be important for the vertical permeability of the sand bodies. The facies characteristics of the non-channelised thick-bedded sandstones indicate that they were deposited in outer fan environments, by waning, sandloaded gravity flows. The poor development of scour marks and the lack of the structures caused by trac-

tion (i.e., flute casts, groove marks, etc.) suggest that these deposits were the results of a transition between liquefied flows and high-density turbidity currents (see for example Marnoso-Arenacea, Mutti, 1985). Examples for this type of facies are the lobe units from Cengio (Tertiary Piedmont Basin) and the non-channelised thick-bedded sandstones from Marnoso-Arenacea. Channelised thick-bedded sandstones

These thick-bedded sandstones present the following characteristics. (1) The beds are channelised, with little lateral extent (individual channel width from 0.4 km to 1.5 km). (2) The bed thickness ranges from 1 to 10 m, and the beds are commonly amalgamated or stacked, forming channel-fill successions. (3) Their base is commonly sharp or channelised while the top is flat and can be channelised. (4) Stacked thick-bedded sandstones are most common and stacked beds are underlain and overlain by thin-bedded turbidites or fines. (5) Grain size varies from fine to coarse sand; normal- or inverse-graded pebbles and cobbles are common in the channel axis. Rip-up clasts occur in the lower part of the beds.

Outcrop-based classification of thick-bedded, deep-marine sandstones (6) Sandstone beds are commonly arranged in fining-upward and thinning-upward sequences. (7) Sedimentary structures are not very common; graded bedding, parallel lamination, groove casts, flute casts, load casts, horizontal lamination, current ripples, dish structures, and trace fossils are occasionally observed. (8) Amalgamated contacts are common. Two or more separate beds can be traced laterally into a single bed of sandstone. The amalgamated junctions may form permeability baffles and are expressed in different ways: (a) sharp, bed-parallel, with small load casts; (b) a row of angular or rounded mud flakes; (c) a change in grain size; (d) no grain size trend discernible. This facies type is interpreted to be deposited by various types of sediment gravity flows, including turbidity currents (mainly), grain flows, and fluidised sediment flows (Walker, 1966a,b; Link and Nilsen, 1980; Hiscott, 1980; Kleverlaan, 1989). Examples are bed units from Annot, Beardmore-Geraldton, Guipuzcoa, Jaizkibel, Matilija, Rock, Shale Grit, Tabernas, and Tourelle.

Slurry sandstones This type was recognised only in the Tourelle Formation (Hiscott, 1980) and is characterised by the following features. (1) They occur together with the channelised sands. (2) The bed thickness varies from 0.5 m to 2 m. (3) The bases are flat and sharp and the tops are graded (into shale). (4) They are underlain and overlain by shales. (5) The grain size is fine to coarse. The sand is dispersed within a mud matrix. (6) No obvious arrangement in sequences is observed. (7) Sedimentary structures include convolute bedding and locally developed pseudonodules. Rip-up clasts up to 1 m long and shale chips with random orientation are frequent. (8) Amalgamation is common. Slurry sandstones are interpreted to be the product of slumping of mud and unconsolidated sand. The absence of scouting at the base of the beds, the absence of the internal structures caused by the traction, the abundance of matrix and large fragments and the random appearance of the shale chips give reason to interpret them as the result of submarine debris flows formed by the slumping of mud and unconsolidated sand on a submarine slope (Hiscott and Middleton, 1979).

395

Thick-bedded sandstones confined by basin-floor topography The following features are characteristic for this type of beds. (1) The sandstones are channelised or non-channelised, but their confined aspect is not due to the channelling but to the basin topography. (2) The bed thickness ranges from 1 to 3 m. (3) The beds have either undulating, erosive bed boundaries or flat, sharp, non-erosive bed boundaries. (4) They form stacked sand-on-sand successions. Intercalated fines are eroded away by subsequent turbidity currents. (5) The grain size varies from very fine to coarse sand. (6) Sandstone beds together form blocky log patterns, rather than fining- or coarsening-upward sequences. (7) Sedimentary structures include dish structures and parallel and ripple lamination, but the general appearance is massive. (8) Amalgamation is characterised by a sudden change in grain size. These sandstone beds were deposited by highdensity turbidity currents in topographically confined basins. Examples of this facies are some lobes or channelised units from the Tertiary Piedmont Basin, units from the Vocontian Basin (Les Marnes Bleues), and Jaizkibel successions from the Guipuzcoa Basin.

Sand-to-gross ratio (S/G) Complexes of TBDM sands and associated facies present different S/G ratios (Fig. 9). S/G ratio varies in three styles. (A) Low (0.4-0.5) but stable S/G ratio (e.g., Marnoso-Arenacea, Cengio). This is likely to be the result of the large extent of the individual beds. (B) Highly variable (0.3-0.9) S/G ratio across the palaeocurrent direction and stable S/G ratio along the palaeotransport direction (e.g., Annot, Rock, Tourelle, Beardmore-Geraldton). This is the result of the presence of channels running more or less in the same direction. (C) Highly variable (0.5-0.9) S/G ratio both across and along the palaeocurrent direction (e.g., Les Marnes Bleues). These are partly associated with the pillow-type sandstones related to confined basins.

Bed continuity and shapes of the bodies Table 2 presents quantification on the continuity of the sandstone beds derived from ten outcrop studies. In some cases (Annot and Shale Grit), the bed by

M. O. Badescu

396

~.4

)-0.3 Fig. 9. Complexes of TBDM sandstones showing little thickness variation on a 10 km scale.

bed quantification was impossible, so that packets of sandstone beds were quantified. Out of these ten outcrop examples, nine are the same as in Fig. 6. The Tyee case was deleted because of lack of correlation data both as bed by bed and as sandstone packets. On the other hand, Tabernas was added because some sandstone beds correlations are available for that site. The absolute dimensions (length and width) of the beds directly depend on the basin size and shape and the direction of the palaeoflow. The more exact shape and lateral continuity of the sand bodies are strongly influenced by the particular tectonic style. Reading and Richards (1994) emphasised the complexity of the factors that control the architecture and the final shape of the deep-marine sandstone beds. A major problem is that many factors (the morphology of the source area, climate, shelf system, shape and type, variation in clastic input, etc.) cannot be recognised directly in the sedimentary facies and the geometry and shape of the beds. A new approach could be to compare the volume of the sandstone beds to the volume of the basin and relate this ratio to the tectonic style, character of the sources of the sand and S/G ratio. The basin-floor topography has a significant control on the geometry and on the shape of the thickbedded sandstones. Ricci Lucchi et al. (1985b) emphasized the influence of the basin-floor topography on the geometry of the thick-bedded sands deposited in small, tectonically controlled basins (lower units of the Cengio fan). Firstly, the basin-floor topogra-

phy influences the turbidity currents flow trajectory and beds with various shapes can be deposited. Secondly, sediments can be trapped in topographically controlled depressions, forming pillow-like, laterally discontinuous beds. It was also recognised in the subsurface that basin-floor topography caused by tectonic half-graben geometry (Ravnas and Steel, 1998) and by salt diapirism (Weimer et al., 1998) significantly influences the final geometry of the sandstone beds. Three basic types of turbidite system geometries were recognised in the outcrops. (1) Sheet-like geometries are characteristic for basin plain deposits. The beds are continuous on a basin scale. Non-channelised, thick-bedded sands exhibit a tabular shape. The thickness variation is insignificant, both across and along the palaeocurrent direction. (2) Ellipsoidal geometries are characteristic for channelised thick-bedded sandstone beds. Good continuity is expected downcurrent and poor continuity across the palaeocurrent direction. The thickness variation is significant across the palaeocurrent direction. The thickness variation is much lower downcurrent (see Table 2 for details on quantifications). Particularly for this type, the dynamics of the distributary sediment flow form an important factor that controls the geometry and the continuity of the sandstone beds. (3) Pillow-like TBDM sandstones, which are the third geometrical type of thick-bedded sandstones, were identified in few cases (some lobes or channelised units in the Piedmontan Tertiary Basin study, Les

Outcrop-based classification of thick-bedded, deep-marine sandstones Marnes Bleues, and Guipuzcoa-Jaizkibel). Here the basin-floor topography plays an important role on the geometry and on the lateral extent of the thick-bedded sandstones. Pillow-like, thick-bedded sandstones are characteristic for TBDM sandstones deposited in confined basins. A significant thickness variation is observed both along and across the palaeocurrent.

Main generic and geometrical types of the TBDM sandstones found in the outcrop studies The above-described geometries and sedimentological features of the TBDM sandstones recognised in the outcrops can be grouped in three main generic and geometrical types (Fig. 10). Type A represents continuous, tabular thick sandstone beds. The minimum distance of continuity is dependent on the basin shape and size and it varies from a few kilometres to tens of kilometres (depending on the basin dimensions). Bed-thickness gradient is low. Type A thick-bedded deep-marine sandstones represent the sandy facies association of non-channelised, thick-bedded sandstones. They were deposited in unconfined basins, or basins with little obstruction to flow, which is the reason why they correlate over long distances. However, examples (Marnoso-Arenacea) show that successive beds tend to compensate the topography created by the deposition of a previous bed.

397

Type B represents thick ellipsoidal sandstone beds. The minimum distance of continuity is dependent on basin shapes and sizes and the palaeocurrent direction. Good continuity on the order of tens of kilometres is expected along the palaeocurrent direction, and poor continuity on the order of few kilometres (depending on the basin size) is expected across the palaeocurrent direction. Bed-thickness variation is low along the palaeocurrent, and high across the palaeocurrent. The type B thick-bedded deep-marine sandstones represent channelised thick sandstones, deposited either in open or in confined basins. The slurry sandstones occur together with the channelised thick-bedded sandstones. Their poor lateral correlation is due to the geometry of the channels. Type C represents thick pillow sandstone beds. The minimum distance of continuity is defined by the basin topography and the basin size and shape. Beds are continuous on a range from hundred metres to a few kilometres, both along and across the palaeocurrent direction. The bed-thickness gradient varies significantly. Type C beds represent thick-bedded sandstones that were deposited in topographically confined basins. This is the reason why they correlate over short distances. However, the dimensions of the depressions that cause confinement can also vary, depending on the causes that generated the highs and lows on the basin topography. Causes of confinement could be: (a) graben/half-graben faulting (e.g., Cengio, Cazzola

Type A TBDM sands non-channelized

-

- tabular

shape

- good continuity

Type C TBDM sandstones channelized or n o t

-

- pillow-like

Type B TBDM sandstones channelized

- poor

shape

continuity in all directions

-

- spoon -

shaped

good continuity parallel to t h e p a l e o c u r r e n t poor continuity l a t e r a l t h e p a l e o c u r r e n t Fig. 10. Main generic and geometrictypes of TBDM sandstonebeds.

398 TABLE 3 Variation of types of thick-bedded sandstones in outcrop

et al., 1981); (b) salt or mud diapirism (e.g., Jaizkibel, Van Vliet, 1982); (c) differential compaction (e.g., Mames Bleues, Fries et al., 1984); (d) synsedimentary folding; (e) batholith anticlines. It is important to underline that more than one type can occur within the same basin (see Table 3). Hybrid types that are gradational between these three main types are the rule rather than the exception. This is mainly because: (1) factors controlling the architecture and the geometry of the TBDM sandstones (tectonics, source area, sea-level variations, and pre-existing topography) are variable and interrelated; (2) their relative importance can change in time and space; (3) TBDM sandstones are deposited from a variety of processes that give specific signature to the deposits. The most common type, recognised in this study, is type B: ellipsoidal, channelised thick-bedded sands. For this type of sandstone it is critical to know the direction of the palaeocurrent and how the basinfloor topography influences deposition, in order to predict the geometry and architecture of these sandstone beds. Good-quality 3-D seismics is probably the best method to delineate the shape of the individual channel-fill sandstones.

Linking prospects to outcrop analogues Although the outcrop studies are the only source of reliable information on the interwell scale, it is very difficult to identify an exact outcrop model for a given reservoir field. At present, there is not yet a clear methodology describing how deep-marine outcrop analogues can be used for the quantification

M. O. Badescu

of reservoir properties. A major problem is that the data from outcrops and subsurface reservoirs are in different formats. Transfer functions are used to convert outcrop data to subsurface data formats. These include synthetic seismic profiles, pseudo-logs and vertical sedimentological diagnostics. At this time, this study addresses only sedimentological diagnostics that can be recognised in cores. The main concern for using outcrop analogues for dimension predictions is that the lateral continuity of TBDM sandstones depends on the tectonic setting, basin shape, and its dimensions. However, a comparison of outcrop data with a number of reservoirs reveals that similarities in shape and facies characteristics occur. The classification of TBDM sands can be applied to the Tertiary of the North Sea. Fig. 11 illustrates the Paleocene-Eocene North Sea reservoirs that will be considered.

The Tertiary of the North Sea - - comparison of the Paleocene and Eocene reservoirs (source: Parker, 1994; Bowman, 1998) During the Tertiary, the North Sea sedimentation was controlled by a complex interplay between tectonic activity, eustasy, and hinterland characteristics. The major control on the siliciclastic supply during the Tertiary was the rifting of the GreenlandEuropean Plate in the Early Paleocene, with rejuvenation of older Mesozoic hinterlands and basin margins (Bowman, 1998). The volume and grain size of the clastic detritus feeding the major submarine fans were not constant in time. During the Paleocene, the volume increased gradually to its peak (midThanetian). The tectonic uplift of the hinterland was the major factor controlling sedimentation during the Paleocene. However, the impact of tectonic activity was not uniform. Differential uplift caused the development of geographically and temporally separate depocentres (Morton et al., 1993). The Paleocene is very sand-rich, involving stacked pay zones, within the distal part of the main submarine fan complexes. Type A and B TBDM sandstones are likely to occur. The Eocene is characterised by reduced rates of clastic input along the newly developed passive margins. Relative sea-level changes were the primary control on deposition. The Eocene is largely mudprone and contains localised submarine fans. The sandstones are typically clean and well sorted, and to some degree different from the more clay-rich, delta-fed systems of the Paleocene. The changes are a consequence of reworking sands in shoreline and upper shelf settings near the submarine canyon heads.

Outcrop-based classification of thick-bedded, deep-marine sandstones

399

Fig. 11. Paleocene and Eocene discoveries of the North Sea (after Bowman, 1998). (A) Paleocene. (B) Eocene. Reprinted by permission of the Geological SocietyPublishing House.

The Tertiary submarine fans of the North Sea are divisible into two main categories, with different characteristics and production behaviour. (1) The Early Paleocene fans of the Central Graben are large-scale basin-fill complexes. In the main depocentres the sand bodies have sheet-like geometries (type A). Outside these areas, the architectural patterns are dominated by channel fills (type B) with overbank deposits. The complexes of TBDM sandstones present higher S/G ratios along the main axis of flow and lower S/G ratios perpendicular to the flow. The sands are more clay-rich, which influences the reservoir permeability. Typically for the Early Paleocene reservoirs, the TBDM sandstones are interbedded with very continuous shales. This has a significant influence on the vertical permeability, compartmentalising the reservoirs. Notably, the direction of the palaeoflow is critical to achieve a good appraisal of these types of reservoirs. (2) The Late Paleocene-Eocene fans tend to be localised and smaller in size. The basin topography is the factor that defines the sediment architecture and geometry. The internal architecture is characterised by pod-like (type C) or elongate bodies. The complexes of TBDM sandstones have steep, abrupt margins and comprise homogeneous masses of clean,

well-sorted sand, affected by slumping and liquefaction. This modifies the primary depositional geometry. In contrast with the Paleocene reservoirs, the Eocene reservoirs contain cleaner sands, with higher vertical permeability. Although the sandstone beds are very thick in some wells they are thinner elsewhere. The geometry of the TBDM sandstones is complex and therefore very difficult to predict. However, good-quality 3-D seismic can help with this problem. The TBDM sandstones bodies contain very few shale intercalations that could act as barrier for fluid flow. Table 4 shows an overview of the Tertiary reservoirs of the North Sea. Type A (sheet-like) and B (channels) sandstones occur interbedded with thick, laterally extensive shales that generally compartmentalise the reservoir. Reservoirs containing type A sandstones, interbedded with laterally continuous shales should receive special attention when planning water injection. Generally, these reservoirs present a very active aquifer and there is no need to inject water in the early stages of production. Examples such as Forties (Carman and Young, 1981; Wills and Peattie, 1990), Frigg (Mure, 1987a; Brewster, 1991), and Heimdal (Mure, 1987b; Grinde et al., 1994) show that water breakthrough

O O TABLE 4 Overview of the Tertiary reservoirs of the North Sea Aquifer e

Sand-body configuration f

References

33/2800

Not available

Topographically confined HDTC (type C)

Newton and Flanagan (1993), Harding et al. (1990)

Post-plateau EW (1971), 8 AW (1971-1976); 5 platforms with 48 PW (1976-1979), production since 1977

29/1500

Efficient, not anticipated, 3 WI (1984-1985)

Massive channels (type B) Levees and lobes (type A)

Mure (1987c), Brewster (1991)

Production (since 1988) EW (1974) + 2 AW (1984) 2 platforms

28.5/1000

Efficient, communication with Frigg

Channelised sediments Deposited by GF and TC (type B TBDM)

Mure (1987d)

Northeast Frigg (VG) Ypresian (T70)

Production (since 1983) EW (1974), 1 platform with 6 PW

28/1000

Efficient, communication with Frigg

GF and TC deposited in a channelised environment (type B TBDM)

Mure (1987c)

Odin (VG)

Ypresian (T70)

Production (since 1984) 2 EW (1973, 1975) Fixed platform with 11 PW

29.5/1000

Limited

Channelised sands ("fluxo-turbidites"); type B and HDTC non-channelised, type A

Nordgard Bolas (1987)

Gannet (CG)

Thanetian (T40) and Ypresian (T70)

Production, no information on field development

38/650-2800

Not available

T40: channelised (type B) T70: salt-confined, channels (type B TBDM) Lobes (extensive) (type A)

Armstrong et al. (1987)

Gryphon (VG)

early Ypresian (T50)

Development drilling EW (1987), 13 AW + ST

not available

Not available

Stacked massive sands,topographically confined (type C TBDM)

Newman et al. (1993), Timbrell (1993)

Guillemot D (CG)

Thanetian (T40 § T50)

Production EW (1969), EW (1988), 2 AW (1988)

not available

Not available

T40: sheet-like sand bodies (A) T50: shoestring channels (B)

Banner et al. (1992)

Balder (VG)

Thanetian (mainly)

Production

Active, no WI

Mounded sand bodies, with rapid pinch-outs (type C)

Hanslien (1987), Jenssen et al. (1993)

Forties (WGG, CG)

Thanetian (T40)

Mature, post-plateau 17EW + AW EW (1970), 4 AW (1971-1972) 4 fixed platforms (1975) + 5th platform (1985) WI (since 1976), GI

Very active, WI too early

HDTC deposits: braided channels, sheet-like geometry, topographically confined (types A and B TBDM)

Wills and Peattie (1990), Carman and Young (1981)

Reservoir a

Age b

Status/field development c

Alba (WGG)

Lutetian (T92-T98)

Early production (?1998) 16 AW + ST

Frigg (VG)

Ypresian (T70)

East Frigg (VG)

Ypresian (T70)

Porosity (%) / permeability (mD) d

33 (32-35) 1000-3000 27 (10-36) 700 (30-4000)

r~

t..., e5

|

TABLE 4 (continued) Reservoir

a

Nelson (WGG, CG)

Age b

Status/field development c

Thanetian (T40)

Early production (1994) 6 EW (196%1987) 6 AW, 1 platform jacket with 24 PW (including 7 WI)

Porosity (%)/ permeability (mD) d 20-25 100-300

Aquifer e

Sand-body configuration f

References

e5

Small, requires early WI

Channelised HDTC (type B)

Whyatt et al. (1991 ), Griffin et al. (1994)

t..., t..~.

Arbroath (CG)

Thanetian (T40)

Early production Satellite platform 6 PW + 4 WI

24 (3-30) 80 (1-2000)

Small

Stacked channelised sands with little lateral extent (type B)

Crawford et al. (1991)

Montrose (CG)

Thanetian (T40)

Post-plateau production 1 platform, 15 PW/6 WI

24 (3-30) 80 (1-2000)

Small

Stacked channelised sands with little lateral extent (type B)

Crawford et al. (1991 )

Cod (CG)

Thanetian (T40)

Production 3 EW + 9 DW (from which 7 are PW), 1 platform

17 (15-24)

Not available

Lenticular, channelised bodies (type B)

D'Heur (1987), Kessler et al. (1980)

Production (since 1986) DW (1975), 9 AW (1979-1983) floating production vessel 13 PW + 6 WI

25 (20-30) 20-3300

Small, early WI

Stacked lobes, sheet-like (type A) and channels (type B)

Tonkin and Fraser (1991 )

Balmoral (WGG)

early Thanetian (T30)

Cyrus (WGG, CG, VG)


13 AW, 2 horizontal wells tied back from vessel

Heimdal (VG)


Post-plateau production (1986) DW (1972), 2 AW (1975, 1981) 1 jacket platform with 10 PW

Sleipner East (VG)

early Thanetian (T20 + T30)

Production (1993, planned) DW (1981), 3 AW, 1 platform

Maureen (WGG)

Danian (T20)

Production (1983)" WI, GI

15 (15-100)

e~

t....,

~20 -~200 24-25 --~1 0 0 0 ~26 200-1500 Not available

Efficient

Sheet-like sandstones with lateral extent beyond the field (type A)

Mound et al. (1991)

Strong

Channelised sands (type B) and sheetlike sands in pressure communication

Mure (1987d), Grinde et al. (1994)

In pressure communication with Heimdal

Massive channels (type B)

Ostvedt (1987)

Small

3 thick, amalgamated lobes, with abrupt lateral pinch-out (type C)

Cutts (1991)

a CG = Central Graben; VG = Viking Graben; WGG - Witch Ground Graben. b T20-T98 = stratigraphic sequences proposed by Bowman (1998). c AW -- appraisal wells; WE = exploration wells; PW = production wells; ST = side tracks; WI - water injectors; GI = gas injectors. d Average/(min.-max.). e WI = water injection. f H D T M = high-density turbidity currents; GF = grain flows; TC = turbidity currents; TBDM = thick-bedded deep-marine sands. 4x O

M. O. Badescu

402

occurred due to too early water injection and pockets with significant volumes of remaining hydrocarbons still exist. Reservoirs containing only type B (channels) TBDM sandstones, such as Nelson (Whyatt et al., 1991; Griffin et al., 1994), Arbroath (Crawford et al., 1991), Montrose (Crawford et al., 1991) and Cod (Kessler et al., 1980; D' Heur, 1987), present a complex geometry, and reservoir correlation is problematical. Their aquifer support is generally reduced and early water injection, eventually combined'with gas lift, is required. Reservoirs containing type C (pillows) TBDM sandstones are very problematical for the prediction of their lateral extent. They are generally small, localised, but present excellent, homogeneous internal porosity-permeability characteristics (for example: Alba, Harding et al., 1990; Newton and Flanagan, 1993; Balder, Hanslien, 1987; Gryphon, Newman et al., 1993; Timbrell, 1993). Type C sandstones are generally very clean, well sorted, with vertical permeabilities higher than 1 D. Their lateral extent is difficult to predict. Data on aquifer properties are not available, but it is likely that the aquifer has little lateral extent (sand bodies are confined by the topography). When economically feasible, water injection and artificial lift are required in the early stages, for maximal production in the first years.

Conclusions TBDM sandstones can be described by subdividing the sandstone beds into three main types: sheets, channels and pillows. Although it is understood that in detail one can distinguish between a large number of sand-body shapes, it is practical to simplify this variety into three main types, because this relates the reservoir characteristics to the expected reservoir behaviour. For the appraisal of an oil reservoir comprising mainly thick-bedded deep-marine sands one should bear in mind that: (1) thick-bedded deep-marine sandstones represent different architectural types, with specific spatial signatures; (2) on a basin scale, more than one type of thick-bedded sandstones can occur; (3) to predict reservoir architecture, it is critical to know the palaeoflow direction; (4) on a small scale, the basin-floor topography is the factor that mostly influences facies architecture of TBDM sandstones; (5) the three types TBDM sandstone beds can be differentiated in cores form their few diagnostic features, such as structures, occurrence and types of

the mudclasts, bed boundaries and facies associations (shale types); (6) for future planning it is necessary to define the palaeoflow direction and the interplay between synsedimentary topography and deposition.

Acknowledgements The work presented in this paper is part of Ph.D. research carried out at the Delft University of Technology, Holland. Thanks are due to K.J. Weber and M.E. Donselaar for continuous support and for valuable comments on earlier versions of the manuscript. I.M. Voiculescu is acknowledged for drawing Fig. 10 and for her warm support. The comments of A.H. Bouma and J.R Nystuen improved the final manuscript and are greatly appreciated. Discussions with C. Puigdefabregas and T. Lien were very fruitful and are acknowledged.

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M. O. Badescu Reading, H.G. and Richards, M., 1994. Turbidite systems in deepwater basin margins classified by grain size and feeder system. Am. Assoc. Pet. Geol. Bull., 78: 792-822. Ricci Lucchi, F., 1975. Depositional cycles in two turbidite formations of the Northern Apennines (Italy). J. Sediment. Petrol., 45: 3-43. Ricci Lucchi, F., 1978. Turbidite dispersal in a Miocene deep-sea plain: the Marnoso-Arenacea of the Northern Apennines. Geol. Mijnbouw, 57: 559-576. Ricci Lucchi, F., 1985a Marnoso-Arenacea turbidite system, Italy. In: A.H. Bouma, W.R. Normark and N.E. Barnes (Editors), Submarine Fans and Related Turbidite Systems. Springer, New York, pp. 209-216. Ricci Lucchi, F., 1985a. Turbidite systems and their relations to depositional sequences. In: G.G. Zuffa (Editor), Provenance of Arenites. NATO ASI Series, 148. Kluwer Academic Publishers, Dordrecht, pp. 65-93. Ricci Lucchi, F. and Valmori, E., 1980. Basin-wide turbidites in a Miocene, over-supplied deep-sea plain: a geometrical analysis. Sedimentology, 27: 241-270. Rosell, RJ., Remacha, E., Zamorano, M. and Gabaldon, V., 1985. Estratigrafia de la cuenca turbiditica terciaria de Guipuzcoa. Comparacion con la cuenca turbiditica prepirenaica central. Bol. Geol. Min., 96: 471-482. Rubino, J.L., 1988. Organisation des s6quences de d6p6t de la plateforme au bassin dans l'Aptien et l'Albien du Bassin Vocontien (SoE de la France). In: S. Ferry and J.L. Rubino (Editors), Eustatisme et S6quences de D6p6t dans le Cr6tac6 du Sud-Est de la France (Livret-guide de l'Excursion du Groupe Franqais du Cr6tac6 en Fosse Vocontienne, 25-27 Mai 1988). Geo Trope, 1: 53-73. Rubino, J.L., 1989. Introductory remarks on the Gargasian/Albian mixed carbonate/siliciclastic system. In: S. Ferry and J.L. Rubino (Editors), Mesozoic Eustasy Record on Western Tethyan Margins, Post-meeting Field Trip in the Vocontian Trough (25th-28th November 1989). Assoc. S6dimentol. Fr. Spec. Publ., 12: 28-45. Sandulescu, M., 1989. Sinteza Carpatilor Orientali. Internal Report, Romanian Geological Survey Archives, 189 pp. Shanmugam, G., 1999. 50 years of the turbidite paradigm (1950s1990s): deep-water processes and facies models a critical perspective. Mar. Pet. Geol., 5: 1-58. Shanmugam, G. and Moiola, R.J., 1988. Submarine fans: characteristics, models, classification and reservoir potential. Earth-Sci. Rev., 24: 383-428. Shanmugam, G. and Moiola, R.J., 1994. An unconventional model for the deep-water sandstones of the Jackfork Group (Pennsylvanian), Ouachita Mountains, Arkansas and Oklahoma. In: R Weimer, A.H. Bouma and B.F. Perkins (Editors), Submarine Fans and Turbidite Systems. Gulf Coast Section, 15th Annual Research Conference, Society of Economic Paleontologists and Mineralogists, Tulsa, OK, pp. 311-326. Shanmugam, G. and Moiola, R.J., 1995. Reinterpretation of depositional processes in a classic flysch sequence (Pennsylvanian Jackfork Group), Ouachita Mountains, Arkansas and Oklahoma. Am. Assoc. Pet. Geol. Bull., 79: 672-695. Shanmugam, G. and Moiola, R.J., 1997. Reinterpretation of depositional processes in a classic flysch sequence (Pennsylvanian Jackfork Group), Ouachita Mountains, Arkansas and Oklahoma: reply. Am. Assoc. Pet. Geol. Bull., 81: 476-491. Sinclair, H.D., 1994. The influence of lateral basinal slopes on turbidite sedimentation in the Annot sandstones of SE France. J. Sediment. Res., 64: 42-54. Slatt, R.M., Jordan, D.W. and Davis, R.J., 1994a. Interpreting formation microscanner log images of Gulf of Mexico Pliocene turbidites by comparison with Pennsylvanian turbidite outcrops, Arkansas. In: R Weimer, A.H. Bouma and B.F. Perkins (Editors), Submarine Fans and Turbidite Systems. Gulf Coast Section, 15th Annual Research Conference, Society of Economic Paleontologists and Mineralogists, Tulsa, OK, pp. 335-348.

Outcrop-based classification of thick-bedded, deep-marine sandstones Slatt, R.M., Jordan, D.W., Stone, C.G. and Wilson, M.S., 1994b. Stratigraphic and structural compartmentalization observed within a "model turbidite reservoir", Pennsylvanian Upper Jackfork Formation, Hollywood Quarry, Arkansas. In: E Weimer, A.H. Bouma and B.E Perkins (Editors), Submarine Fans and Turbidite Systems. Gulf Coast Section, 15th Annual Research Conference, Society of Economic Paleontologists and Mineralogists, TuLsa, OK, pp. 349-356. Slatt, R.M., Weimer, E and Stone, C.G., 1997. Reinterpretation of depositional processes in a classic flysch sequence (Pennsylvanian Jackfork Group), Ouachita Mountains, Arkansas and Oklahoma: discussion. Am. Assoc. Pet. Geol., 81: 449-459. Stanley, D.J., 1975. Submarine Canyon and Slope Sedimentation (Gr6s d'Annot) in the French Maritime Alpes, IX-~me Congr~s International de S6dimentologie, Nice. Stanley, D.J. and Bouma, A.H., 1962. Methodology and paleogeographic interpretation of flysch formations: a summary of studies in the Maritime Alps. In: A.H. Bouma (Editor), Sedimentology of Some Flysch Deposits. Elsevier, Amsterdam, pp. 36-64. Stauffer, EH., 1967. Grain-flow deposits and their implications, Santa Ynez Mountains, California. J. Sediment. Petrol., 37: 487508. Stow, D.A.W., Howell, D.G. and Nelson, C.H., 1985. Sedimentary, tectonic and sea-level controls. In: A.H. Bouma, W.R. Normark and N.E. Barnes (Editors), Submarine Fans and Related Turbidite Systems. Springer, New York, pp. 15-22. Stow, D.A.W., Reading, H.G. and Collinson, J.D., 1996. Deep seas. In: H.G. Reading (Editor), Sedimentary Environments: Processes, Facies and Stratigraphy. 3rd ed., Blackwell, Oxford, pp. 395-453. Timbrell, G., 1993. Sandstone architecture of the Balder Formation depositional system, UK Quadrant 9 and adjacent areas. In: J.R. Parker (Editor), Petroleum Geology of Northwest Europe. The Geological Society, London, pp. 107-121. Tonkin, EC. and Fraser, A.R., 1991. Balmoral oil field, block 16/21 UK North Sea. In: I.L. Abbotts (Editor), United Kingdom Oil and Gas Fields, 25 Years Commemorative Volume. Geol. Soc. Mem.,

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14: 237-243. Vail, ER., Mitchum, R.M., Jr., Todd, R.G., Widmier, J.M., Thompson, S., Sangree, J.B., Bubb, J.N. and Hatleid, W.G., 1977. Seismic stratigraphy and global changes of sea level. In: C.E. Payton (Editor), Seismic Stratigraphy Applications to Hydrocarbon Exploration. Am. Assoc. Pet. Geol. Mem., 26: 49-204. Van Vliet, A., 1978. Early Tertiary deepwater fans of Guipuzcoa, northern Spain. In: D.J. Stanley and G. Kelling (Editors), Sedimentation in Submarine Canyons, Fans and Trenches. Dowden, Hutchinson and Ross, Stroudsburg, PA, pp. 190-209. Van Vliet, A., 1982. Submarine fans and associated deposits in the lower Tertiary of Guipuzcoa (northern Spain). PhD thesis, Landbouwuniversiteit Wageningen, The Netherlands, 45 pp. Walker, R.G., 1966a. Deep channels in turbidite-bearing formations. Am. Assoc. Pet. Geol. Bull., 50:1899-1971. Walker, R.G., 1966b. Shale Grit and Grindslow Shales: transition from turbidite to shallow water sediments in the Upper Carboniferous of northern England. J. Sediment. Petrol., 36:90-114. Walker, R.G., 1978. Deep-water sandstone facies and ancient submarine fans: models for exploration for stratigraphic traps. Am. Assoc. Pet. Geol. Bull., 62: 932-966. Weimer, E, Rowan, M.G., McBride, B.C. and Kligfield, R.M., 1998. Evaluating the petroleum systems of the northern deep Gulf of Mexico through integrated basic analysis: an overview. Am. Assoc. Pet. Geol., Bulletin, 82: 865-877. Wezel, F.C., 1970. Numidian flysch: an Oligocene-Early Miocene continental rise deposit off the African Platform. Nature, 228: 275-276. Whyatt, M., Bowen, J.M. and Rhodes, D.N., 1991. Nelson successful application of a development geoseismic model in North Sea exploration. First Break, 9: 265-280. Wills, J.M. and Peattie, D.K., 1990. The Forties field and the evolution of a reservoir management strategy. In: A.T. Bullet and E. Berg (Editors), North Sea Oil and Gas Reservoir, II. The Norwegian Institute of Technology, Graham and Trotman, London, pp. 1-23.

Delft University of Technology, Faculty of Applied Earth Sciences, P.O. Box 5028, 2600 GA Delft, The Netherlands


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Use of integrated 3D seismic technology and sedimentology core analysis to resolve the sedimentary architecture of the Paleocene s u c c e s s i o n of the North Sea M. Cecchi. C. Guargena, L. Hansen, D. Rhodes and A. Roberts

The integration of extensive 3D seismic data sets with core sedimentology analysis from the Siri and Heimdal areas indicates that the stratigraphic architecture of the Paleocene succession of the North Sea is highly variable and fractal in nature. Moreover, we propose that, at least in some parts of this area, debris flows, slides and strong current reworking were the prevailing depositional processes operating during the Paleocene, in addition to the more commonly cited turbiditic ones. This interpretation helps to explain the occurrence of massive sands associated with slumps and olistostromes. Future wildcat and development projects associated with Paleocene plays must consider this variability of reservoirs and seals.

Introduction

About 30 years have elapsed since hydrocarbons were first discovered in Paleocene sediments of the North Sea, these sediments proving to be billion barrel reservoirs. More recent discoveries in the 1990s (Siri, Bittern, Jotun, Grane) have revitalised the interest for this play and have indicated that the Paleocene of the North Sea still represents an underexplored target. This sluggishness in exploration and exploitation may also be related to the understanding of the geological model, as perhaps also indicated by older Paleocene discoveries that still await to be developed. Traditionally, 2D seismic, wireline logs and core sedimentology have been the classical data sets and tools used to interpret the sedimentary architecture of the Paleocene succession. North Sea Paleocene sands have usually been interpreted to be deposited by deep-water fan turbidity currents (Newman et al., 1993; Jenssen et al., 1993; Newton and Flanagan, 1993; Timbrell, 1993). An alternative interpretation of North Sea Paleocene examples (debris flows and slumps) was given by Shanmugam et al. (1994). The aim of this paper is to firstly show that the sedimentary architecture of the Paleocene of the North Sea is different from area to area and can not be forced into fixed models. Also, for the Paleocene of the North Sea, facies distribution can be described as essentially fractal in nature. Secondly, we present evidence that many Paleocene sands could have been

deposited and reworked by mechanisms other than the more conventional "turbiditic" ones. These two points have a profound impact on our understanding of facies distribution. In this paper we show how the compilation of attribute maps from regional 3D seismic data sets, combined with core observations, may provide a step forward in the interpretation of the stratigraphic and sedimentary architecture of the Paleocene succession of this area. This approach has been used by oil companies during the last years, but, to date, the number of recent published papers on the subject is quite limited. We believe that this approach is important for future exploration and exploitation projects dealing with this sedimentary sequence. Two case histories are presented, taken from a database of fifteen Enterprise North Sea fields and discoveries (Fig. 1). Our interpretation challenges the classical "turbidite deep basin fan" interpretation, which we believe to represent more of a mind-set than an objective interpretation. For reasons of confidentiality, most of the data are reported in a general way. Preliminary results from ongoing petrographic studies are also presented. Siri area

Linear and S-shape features from 3D seismic The isochore of the Lower Paleocene (Top Chalk to Top Lista Seismic Markers) show a striking o c -


408

M. Cecchi et al.

Fig. 1. Study area and Enterprise's database superimposed over a regional isopach of the Paleocene succession. Warm colours indicate a thicker section. The Siri Field in the Central Graben and one South Viking Graben example (Heimdal area) are covered in this paper. The asterisks refer to the location of the Norfolk and Flemish ridges cited in the text.

Fig. 2. The 3D isochore of the Lower Paleocene Lista Formation in the Siri area shows rectilinear, parallel and S-shape geometries. These correspond to narrow mounds on seismic sections (arrows), here interpreted to represent sedimentary ridges resulting from strong current reworking. A fan geometry is visible to the south west.

currence of linear and parallel to S-shape, N E SW-oriented features in most of this area (Fig. 2). Length of these bodies is 30-40 km. These features correspond to narrow but prominent mounds on seismic sections (ca. 40 ms TWT, i.e. ca. 50 m thick). Towards the southeast, the overall pattern changes to a fan-shape geometry. This seismic package is

contained within the so-called "Siri fairway", a depression within the underlying Cretaceous-Paleocene chalk. The origin of the fairway is still a matter of debate, and there is no apparent link to the underlying structural grain of the chalk. Interestingly, the fairway is aligned with the present day Skagerrak to the east.

Use of integrated 3D seismic technology and sedimentology core analysis to resolve the sedimentary architecture

409

TABLE 1 Paleocene sedimentary facies of cores from the Siri area Facies

Sedimentary structures

Interpretation

Relative abundance

1. Fine-grained, massive sands

Sharp lower and upper contacts No/reverse grading Dish structures Planar fabric Rafted/floating clasts Parallel and trough cross-lamination

Sands transported by debris flow mechanisms and reworked by strong submarine (bottom?/tidal?) currents

>80% (up to 50% current reworked)

2. Brecciated sands and shales

Sharp lower and upper contacts Brecciated appearance Slumps Rafted/floating clasts

Sands and shales transported by debris flow mechanisms with higher percentage of fines; probably ignited by submarine slides; associated to slumps; could be classified as olistostromes AA

< 10%

3. Shales with rippled sandy intervals

Truncated ripples, laminated shales

Sands and shales reworked by submarine currents

< 10%

4. Graded sands

Sharp lower contact and gradual upper contact Fining upward, normal grading

Sands transported by turbidite flows and deposited by settling

50-60%

2. Brecciated sands and shales

Sharp lower and upper contacts Brecciated appearance Slumps Rafted/floating clasts "sheared matrix" Boudins Sand injections

Sands and shales transported by debris flow mechanisms with higher percentage of fines; probably triggered by submarine slides; associated to slumps; could be classified as olistostromes AA

3. Graded sands

Sharp lower contact and gradual upper contact Fining upward, normal grading Truncated ripples

Sands transported by turbidite flows, deposited by settling and reworked by submarine currents

4. Shales with rippled sandy intervals

Truncated ripples Laminated shales

Sands and shales reworked by submarine currents

100,000 years) between each sand transporting turbidity current reached the area. Facies E -

427

contorted and broken up into sandstone fragments, associated with sand injections. Another level with deformation and associated sand injections is present at 2770.5 m, where rotated and truncated, laminated sandstone blocks with irregular sand injections occur. This deformation seems to be associated with a minor normal fault.

hemipelagic black mudstones

Stratigraphic development This facies is common in the middle part of the cored succession where it occurs just above the K-T boundary, consisting of black and dark-grey mudstones with millimetre-scale thin laminae of silt and very fine-grained sandstone. It is up to 3 m thick, and occurs over an interval of 8 m, interbedded with three high-density turbidite sandstones. The degree and diversity of bioturbation is generally very low. Planolites is one of the few trace fossils in this facies, which represents a highly different depositional setting compared to the green mudstones below, and it is tentatively suggested that it may represent significant changes in basin physiography. This facies is also interpreted to represent finegrained sediments deposited mainly from suspension in deep water, and again, formed the background sedimentation during the Early Paleocene. The siltstone and very fine-grained sandstone lenses and laminae within this facies probably represent thin very lowdensity turbidites. The considerable thickness of this facies indicates that it was deposited over a long period of time without interruption by turbidity flows. Facies F ~ chalk

This facies consists of chalk and occurs only in the lower part of the cored interval (core 6), as a single bed, 13 cm thick, immediately on top of a highdensity turbidite (2798.7 m). The lower boundary is sharp whereas the upper is transitional. It is strongly bioturbated (degree 5) with both Planolites, Zoophycos and possibly Taenidium satanassi trace fossils. As mentioned above, there are a few other beds within the Maastrichtian succession that also have increased carbonate content due to chalk; however, these beds are strongly mixed with clastic mud. Facies G m deformed s a n d s t o n e / m u d s t o n e with sand injection

Some of the sandstone beds are significantly deformed and differ from the contorted bedding related to water escape structures within turbidites (e.g. Facies B, see above). One such a deformed interval is located at the K-T boundary (Fig. 8), where the top of a relatively thick sandstone bed is strongly

Two formations are represented in the cored interval in well 6305/5-1: the Jorsalfare Formation (or the Springar Formation according to mid-Norwegian Shelf stratigraphy of Dalland et al., 1988) of Maastrichtian age, and the Vfile Formation (or the corresponding Tang Formation) of Danian age. The V~le Formation is present in the upper part of the cored succession and comprises two distinct lithological units: a mudstone-dominated lower part and a sandstone-dominated upper part (Fig. 4). The Jorsalfare Formation (Maastrichtian)

The top of the Jorsalfare Formation is dated, based on the last occurrence (LO) of Palynodinium grallator, at 2779.30 m in well 6305/5-1 (Fig. 4), taken as evidence for a latest Maastrichtian age. P. grallator is recorded in all three wells drilled on the Ormen Lange structure, and its last occurrence corresponds approximately to the top of the deformed sand bed (Facies G). This occurrence also corresponds to a distinct change in the overall fossil assemblages. The assemblages in the Jorsalfare Formation are dominated by relatively rich and diverse marine microplankton and species like Isabelidinium majae, Areoligera "horrida ", Heterosphaeridium ? heteracanthum, Alterbidinium acutulum and Isabelidinium cooksoniae have their last occurrences within this formation. The Cretaceous pollen Aquillapollenites spp. is present occasionally. Microfaunal assemblages are poor, but agglutinated foraminifera, indicative of upper bathyal depths, are present. There is a considerable difference between the Maastrichtian succession in this well compared to the development in other wells in the northern North Sea region: the presence of significant amounts of sand. Most of the Maastrichtian succession has been cored, and it shows a poorly defined sandier upwards development, combined with a thickening upwards of individual sandstone beds (Fig. 4). The lithology of the lower part of the Maastrichtian succession consists mainly of high-density turbidites (Facies A), interbedded with thick beds of green and grey, highly bioturbated mudstones of Facies D (Fig. 5). There are also a few interbedded low-density turbidites

428

Fig. 5. Alternation between thick greenish mudstones and turbidites in the Maastrichtian succession of well 6305/5-1. Note the thin chalk bed at 2798.7 m (arrow) located on top of a classical turbidite. This is the only chalk bed in the cored succession of well 6305/5-1, but may be widely distributed in the area since it most likely correlates with a similar chalk bed present in well 6305/7-1 at the same stratigraphic level.

(Facies B). Carbonate cement is relatively common within the turbidites, and results in a significant reduction of pore volume. A bed of bioturbated chalk, 13 cm thick, (Facies F) is present in the upper middle part of the succession (Fig. 6). It is suggested that the chalk was deposited from suspension in periods favourable for production of coccoliths in the water column. Such sudden changes may occur due to short-lived climatic changes or changes in basin circulation patterns. Fig. 5 shows the typical development of the Maastrichtian succession characterised by alternating greenish, strongly bioturbated mudstones with turbidites. The upper part of the Maastrichtian succession shows more or less the same facies as the lower one, with alternating high-density turbidites and green and dark-grey mudstones (Fig. 4). The main difference is, however, that individual turbidites are thicker, commonly amalgamated or separated by thinner mudstone intervals between the turbidites (Fig. 4). The stratigraphic development of the Jorsalfare Formation indicates slow background sedimentation mainly from suspension fallout in a well oxygenated,

J. G. Gjelberg et al.

Fig. 6. The chalk bed (Facies F) in the middle part of the cored Maastrichtian succession in well 6305/5-1 (see Fig. 5), showing strong bioturbation with high diversity. open marine basin. This was interrupted by several pulses of turbidity current input that entered the basin floor. There seem to be a gradual or slightly episodic change upwards in the fine-grained background sediments in the uppermost part of the Maastrichtian succession from greenish-grey, bioturbated mudstone to dark-grey, less bioturbated mudstone. Turbidites of Facies A, contorted and broken up into sandstone fragments, associated with sand injections occur at the top of the Maastrichtian succession (Facies G, Figs. 7 and 8).

The Egga member, VMe Formation (Danian) The cored section of the Vgtle Formation contains a typical assemblage reported from Danian strata elsewhere, including a number of last occurrences (from oldest to youngest) as last common occurrence (LCO) of Cerodinium diebelii, last abundant occurrence (LAO) of Trithyrodinium fragile, LCO of Spongodinium delitiense, LO of S. delitiense, LAO and LO of Senonisphaera inornata and LO of Spiniferites

The Maastrichtian and Danian depositional setting, along the eastern margin of the MOre Basin

429

Fig. 7. Cored succession across the K-T boundary. The boundary is located at 2779.3 m and occurs at the top of a deformed bed characterised by brecciation and sand injection. Note the dark-grey, virtually non-bioturbated mudstones immediately above the boundary. These mudstones represent fine-grained background sediments that differ considerably from the Maastrichtian greenish, strongly bioturbated background sediments.

"magnifica". The presence of Carpatella cornuta in one of the wells indicates an oldest Danian age and is the first observation of this species in the mid-Norwegian area. It appears in extremely low numbers in association with S. delitiense close to the FO of Xenicodinium lubricum. Similar assemblages are recorded in all three wells appearing just above the LO of Palynodinium grallator. Other first occurrences (FO) include FAO of T. fragile (at 2778.6 m in well 6305/5-1). The top of the Egga member falls between the two regional markers: the LCO of Palaeocystodinium bulliforme and the LO of Alisocysta reticulata (Fig. 4). A. reticulata is regarded to present the top Danian implying that the reservoir is of late Maastrichtian to Danian in age. The V~le Formation consists of two distinct lithological units: a lower mudstone unit, and an upper sandstone unit. The sandstone unit is here referred to as the Egga member. The lower mudstone unit There is a significant change in background sedimentation immediately above the K-T boundary.

Fig. 8. Strongly deformed turbidite at the K-T boundary (2779.3 m) in well 6305/5-1. The width of the core is approximately 10 cm. Sand injection outlined in black.

This change is especially well expressed in well 6305/5-1. Below this level fine-grained material of Facies D (hemipelagic green, greenish-grey and grey mudstones) dominates. Above this level, however, mainly fine-grained sediments of Facies E (hemipelagic black, virtually non-bioturbated mudstones) constitute the background sedimentation, all the way up to the sandstones of the overlying Egga member (Fig. 7). The fine-grained background sediments occur in association with coarse- to very coarse-grained, high-density turbidites at about the same spacing and thickness as below (Fig. 7), and is associated with a pronounced increase of acoustic velocity (downwards) across the K-T boundary. The spacing and thickness of the associated turbidites do not differ significantly from the distribution of simi-

430

Y. G. Gjelberg et al.

lar facies in the Maastrichtian succession below. The low degree and diversity in bioturbation of the finegrained facies, however, suggest a highly different depositional setting compared to the green mudstones below, and it is tentatively suggested that significant changes in basin physiography and/or climate took place.

The Egga (sandstone) member In the overlying sandstone-dominated Egga member (Danian), a different development of the finegrained background sediments occurs. Greenish, strongly bioturbated facies (Facies C and D) again dominate. This change is most significant across the base of the Egga member. However, a gradual transition to greenish bioturbated mudstone is also recorded slightly below the base. The Egga member is sanddominated, consisting mainly of high-density turbidites (Facies A) in alternation with thin, finingupwards sandy mudstones of Facies C and D (Fig. 4). Individual turbidites range in thickness from a few decimetres to more than 3 m, and may be amalgamated into several metres thick successions (up to 9 m), without significant fine-grained sediments between. The high-density turbidites show a surprisingly uniform grain size distribution, ranging mainly between medium- and coarse-grained sandstone. The vertical distribution of the turbidites of the Egga member shows no obvious vertical arrangements, such as well defined fining- or coarsening-upwards trends. There are, however, two intervals that may be defined as thinning-upwards successions, one of which is located in the lower part of the member (from 2764 m to the top of core 2 in well 6305/5-1) and the other one from the base of core 1 to the base of a thick turbidite (2731.25 m) on the top of the cored succession (Fig. 9). These trends are generally poorly defined and do not justify extensive conclusions. It is tentatively suggested that the fining- and thinning-upwards successions in this member may reflect the in-fill of poorly defined distal channels on a submarine fan system or simply lateral switching of lobes. Based on facies interpretation and distribution together with detailed seismic mapping the Egga member in the central area of the Ormen Lange Dome is interpreted to represent a basin floor fan complex. According to the classification scheme of Reading and Richards (1994), it may be classified as a sand-rich, point-sourced submarine fan system. The regional depositional setting of the Egga member will be discussed more in detail below. The second well drilled by Norsk Hydro on the Ormen Lange Dome (well 6305/1-1) is located in the northern part of the structure and shows a heterolithic

Fig. 9. The upper part of the Egga member in the central part of the Ormen Lange structure (well 6305/5-1), showing amalgamated high-density turbidites separated by fine greenish mudstones and siltstones of hemipelagic origin. The turbidites thin upwards towards the uppermost bed of the cored succession which is relative thick. This interval represents the interval of the Egga member where they are thinnest.

development mainly consisting of fine-grained sediments, such as strongly bioturbated siltstones and mudstones in alternation with thin turbiditic sandstones (Fig. 10). This development indicates that the submarine fan terminates northward and develops into fan fringe facies. Seismic interpretation of the Egga member across the Ormen Lange Dome shows a gradual thinning both northwards and westwards. Mineral composition and provenance area

The sandstones of the Egga member in well 6305/5-1 are all classified as subarkoses according to the classification scheme of Pettijohn et al. (1972), with close to 10% feldspar. Mineralogical and element compositions of the fine-grained background sediments in the well indicate that there is a significant change across the K-T boundary. The concentrations of both kaolinite, mica/illite and calcite decrease significantly across the boundary, whereas smectite and quartz increase (Fig. 11). Minerals such as K-feldspar and dolomite seem to be evenly dis-


431

Fig. 10. Typical sedimentary facies of the Egga member in the northern part of the Ormen Lange Dome (well 6305/1-1). Strongly bioturbated (Zoophycus) siltstones and mudstones are the dominant lithologies (A) and represent probably a mixture of thin turbidites and hemipelagic mudstones. There are a few classical sandy turbidites present in the cored interval (B) consisting of fine-grained, relatively well sorted sandstone.

tributed across the K - T boundary. There is a gradual change from green, strongly bioturbated mudstones to less bioturbated dark-grey and black mudstones upwards towards the boundary. However, the major changes both in mineralogy, ichnofabric and chemistry occur at the boundary itself, and may be related to a catastrophic event such as a meteorite impact (see below). Eight samples from the cored reservoir interval of well 6305/5-1 have been analysed for samariumneodymium isotope (Sm-Nd) provenance ages, aiming to get some indications of the provenance area for the Maastrichtian and Paleocene reservoir sands of the Ormen Lange Field. The Sm-Nd technique relies on the natural radioactive decay of 147Sm to 143Nd (+ 4He) with a half-life time of 106 Ga. By determining the 143Nd/144Nd and J47Sm/144Nd isotope

ratios of mantle-derived crustal rock it is possible to calculate a model age for the sample that reflects the time elapsed since the rock first formed from the mantle (Faure, 1986; Dalland et al., 1995). The distribution of the 147Sm/144Nd ratio shows that all samples fall in the range 0.108-0.119, with a relatively even distribution. This indicates that there is no evidence for fractionation of Sm/Nd ratios between the Maastrichtian and upper Danian (Egga member) sandstone complexes of the reservoir. The Nd concentration shows little variation (less than 20 ppm in all samples). There are no significant differences between the two sandstone complexes, although there is a slight tendency of lower concentrations in the lower sandstone complex. The 143Nd/la4Nd ratios plotted against the Sm-Nd provenance age (Fig. 12B) show that all samples fall in the

432


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A g e (Million years) Fig. 12. Plots of Sm-Nd isotope data versus age. (A) Sm-Nd age versus depth. (B) 143Nd/144Nd versus Sm-Nd age. All samples indicate an age close to 1.6 Ga which is the provenance age for the sediments derived from the Norwegian mainland.

same range with a provenance age between 1650 and 1670 Ma, with one exception for the lowermost sample that shows an age of 1760 Ma. The provenance age plotted against depth is shown in Fig. 12A and

shows an even distribution with no major differences above and below the Cretaceous-Tertiary boundary. All the samples fall within the normal North Sea "background" range of 1400-1800 Ma (Dalland et


433

the source for the Cretaceous sandstones at the eastern margin of the MOre Basin was mainly the Western Gneiss region, with no influence from any other major provenance area such as East Greenland.

Regional depositional setting and tectonostratigraphic development

Fig. 13. Sm-Nd isochron plot of samples from the Ormen Lange reservoir, compared with data from modern source areas represented by Norwegian and British river systems (data from Mearns, 1988 and Dalland et al., 1995). The samples both from the Jorsalfare Formation and the Egga member (Vfile Formation) fall in the field of both Norwegian river sediments and British river sediments south and east of the Moine Thrust. The latter is, however, not a possible source for the Ormen Lange reservoir due to the geographical position.

al., 1995). Fig. 13 shows the isotope data from this study together with the range of isotopes from Norwegian rivers, together with British river sediments west and south/east of the Moine Thrust (Mearns, 1988; Dalland et al., 1995). All isotope data show more or less the same range as the Norwegian mainland river sediments but correspond also with the British river sediments south and east of the Moine Thrust. The latter is, however, not considered as a possible source area for the Ormen Lange reservoir sand due to the long transport distance and complex basin configuration. It is therefore concluded that the provenance area for the sands of the Ormen Lange reservoir was the Norwegian mainland domain that exhibits a provenance age of 1.6 Ga, and there are no differences between the Danian and the Maastrichtian sandstone complexes in terms of Sm-Nd isotope composition, indicating the same provenance area for the sands. This is also in agreement with the heavy-mineral composition that is dominated by titanium oxides and garnet. By comparing the different heavy-mineral suites from the Jorsalfare Formation and the Egga member sandstones it is not possible to identify significant differences in composition that can be related to different source areas. So far, there are no published provenance studies from the Palaeogene succession of the mid-Norwegian Shelf. However, based on heavy-mineral constraints Morton and Grant (1998) analysed several intervals within the Cretaceous succession, and concluded that

The Ormen Lange reservoir succession is interpreted to represent a submarine fan extending in time from the Maastrichtian through most of the Danian, and closely related to the depositional system of the widespread sandy Egga member (late Danian) present along the eastern margin of the MOre Basin. The development of the Egga member was strongly associated with the development of the extensive unconformity at the K-T boundary, probably related to uplift and rotation of the Fennoscandian shield.

The Egga member at the eastern margin of the More Basin The Egga member has been known as a prominent sandstone interval in the S10rebotn Subbasin area since 1989 when it first was penetrated in well 6205/3-1 (Fig. 14). During a period from 1989 to 1994 several wells were drilled along the eastern margin of the MOre Basin from the Selje High in the south to the Fr0ya High in the north, and the Egga member is present in all these wells, consisting of thick, amalgamated medium- to coarse-grained turbidite sandstones, with a maximum total thickness close to 150 m (Fig. 14). The base of the Egga member in the S10rebotn Subbasin area and in other areas close to the eastern margin of the MOre Basin is a significant unconformity with the Danian succession overlying strata of Campanian age. In the Halten Terrace and Tr0ndelag Platform the base-Tertiary unconformity is also well developed, and may represent an angular unconformity in the more proximal regions to the east. None of the wells drilled on the Halten Terrace and Tr0ndelag Platform areas have proven sand above the unconformity and differ therefore considerably from the development in the MOre Basin. The K-T unconformity was initiated during Maastrichtian time probably with shelf erosion and sediment bypass. The bypassed coarse clastic sediments were redeposited as turbidites further out in the MOre Basin, and represent the lower reservoir unit in the Ormen Lange Field, consisting of alternating turbidite sandstones and hemipelagic claystones. This development suggests that the generation of turbidity currents occurred occasionally with long periods of quiescence.

4x

Fig. 14. Stratigraphical correlation of the Egga member in wells along the eastern margin of the MOre Basin, showing thickness variation and internal geometries.


Change of background sedimentation at the Cretaceous-Tertiary boundary A significant change in sedimentation took place across the K - T boundary. This change is mainly seen in the fine-grained background sedimentation which changes from greenish-gray, strongly bioturbated claystones and mudstones of Maastrichtian age into dark-grey, low-degree bioturbated mudstones of early Danian age. The Maastrichtian mudstones exhibit a high diversity of trace fossils mainly of the Zoophycus ichnofacies together with Helminthoida and Anconichnus, whereas the Danian mudstones immediately above the boundary show a very low-diversity assemblage (isolated Planolites). The change in background sedimentation is also reflected by the mineral composition that shows a pronounced change across the boundary (see above). The change in the background sedimentation occurs, however, slightly below the boundary, portrayed by a change from greenish, bioturbated mudstones to dark-grey mudstones in the Maastrichtian succession, reflecting a change in basin physiography that seems to occur gradually or episodically towards the K - T boundary. However, the most significant change, both with respect to lithology and ichnofabric, occurs at the K - T boundary itself. The observation in well 6305/5-1 suggests a similar development as that deduced from mass extinction in planktic foraminifera from several other localities, suggesting that the extinction can be interpreted as a catastrophic event that centred at the K - T boundary, and was superimposed on a gradual mass extinction that began in the late Maastrichtian and continued into the early Danian (e.g. Haslett, 1994; Molina et al., 1996). Other studies, on the other hand, conclude that the K-T boundary is not characterised by dramatic changes in nannofossils and that the concept of an instantaneous mass extinction and its proposed causal connection to bolide impact may be challenged (e.g. Macleod and Keller, 1994). The strongly deformed sandstone bed at the K T boundary with brecciation and sand injection is the only deformed bed of that type in the cored succession of well 6305/5-1. A similar deformed bed has also been recorded from well 6305/7-1, also here at the K - T boundary. It is therefore tempting to believe that this deformation was a result of a catastrophic event caused by severe shock that could be related to bolide impact.

Tectonic implications In early Danian time uplift and rotation of the basin margin area to the east (Norwegian mainland) led to extensive erosion and redistribution of sandy sedi-

435

ments. These sediments prograded westwards into the MOre Basin and gave rise to deposition of amalgamated, high-density turbidites of the Egga member, that probably developed at the front of a rapidly prograding delta (the delta itself is not preserved today due to late Tertiary uplift and erosion). The seismic character of the Egga member along the eastern margin of the MOre Basin changes between different localities. In the Fr0ya High area, the internal seismic pattern is parallel or shingled with very low-gradient clinoforms (Fig. 15A), whereas a more complex and chaotic pattern with channel geometries dominates in the S10rebotn Subbasin area (Fig. 15B). The development on the FrOya High may reflect a relative shallow marine setting, whereas the pattern in the SlOrebotn Subbasin reflects a channelised and probably deformed sandy fan complex. It is tentatively suggested that delta front turbidity currents occasionally bypassed the S10rebotn Subbasin and continued into the deeper part of the MOre Basin, and hence contributed to the development of the upper reservoir interval of the Ormen Lange Field. The area west of the rotation axis (related to the Paleocene uplift) probably went through a period with overall relative sea-level rise, whereas the area to the east was uplifted with an accompanied relative sea-level fall. It is tentatively suggested here that the rotation axis was located east of the SlOrebotn Subbasin, not far away from the present-day shoreline, and that the rotation gave rise to a rapid transgression across the base-Tertiary unconformity in the SlOrebotn Subbasin and the Fr0ya High area, accompanied by rapid delta progradation westwards into the MOre Basin. A generalised palaeogeographic reconstruction of the MOre Basin and its eastern margin in late Danian time is shown in Fig. 16. The development of the base-Tertiary unconformity along the eastern area of the Norwegian continental shelf and the responding sedimentation further out in the MOre Basin and S10rebotn Subbasin imply that severe tectonic uplift of the Norwegian mainland started already in Late Cretaceous/Paleocene times. Based on correlation between offshore geology and onshore geomorphological evidence of an enveloping summit level and remnants of deep weathering in the mountain area of Scandinavia, Riis (1996) suggested that an uplift between 600 and 800 m took place during that period, and suggested that it represents a marginal uplift related to the rifting of the North Atlantic. It is tentatively suggested here that during latest Cretaceous and Palaeogene times the location of the Ormen Lange Field was an area of relative rapid subsidence (indicated by increased thickness across the dome), probably with the development of a temporary

436


Fig. 15. The seismic character of the Egga member along the eastern margin of the MOre Basin. On the Fr0ya High area (A) the internal seismic pattern is parallel or shingled with very low angle, whereas a more complex and chaotic pattern dominates in the S10rebotn Subbasin area (B).


437

Fig. 16. Palaeogeographic reconstruction of the MOre Basin and its eastern margin in late Danian time. This reconstruction is based on regional sedimentological and stratigraphical studies together with detailed seismic mapping.

basin floor depression (associated with movements of underlying Upper Jurassic-Cretaceous fault blocks), where coarse-grained gravity-flow-transported sediments were deposited. Inversion and development of the Ormen Lange structure took place in latest Eocene-Early Oligocene time, probably as a result of compression related to ridge push (Vfignes et al., 1998). Fig. 17 gives an outline of the tectonostratigraphic development at the eastern margin of the Me~re Basin from the Maastrichtian to the present day. Conclusions

(1) The cored interval of well 6305/5-1 comprises both the Maastrichtian and Danian. The Maastrichtian succession (Jorsalfare Fm.) consists of grey-green strongly bioturbated, relative thick mudstone in alternation with sandy turbidites. The relative thin (6 Ma) and long time periods were involved between deposition of each turbidite. (2) At the Cretaceous-Tertiary boundary there was a significant change in depositional environment. The degree of bioturbation and diversity of trace fossils decreased considerably, and a black laminated shale was deposited as "background" fine-grained sediments. There is also a significant change in mineral

distribution and composition across the K-T boundary, and it is suggested that significant changes in basin physiography took place. (3) The Danian Egga member is approximately 50 m thick in well 6305/5-1 and consists of amalgamated, high-density turbidites, with very good reservoir properties (more than 30% porosity and more than 1D permeability locally). (4) There is a considerable increase in the occurrence and thickness of turbidites in the Danian compared with the Maastrichtian succession. This may reflect increased tectonic activity with rotational uplift of the Fennoscandian provenance area related to the rifting and opening of the North Atlantic. It is therefore concluded that the development of the Danian basin floor fan at the Ormen Lange area mainly originated from increased erosion due to epeirogenic uplift, combined with slope instability and collapse due to increased basin margin gradient. (5) Seismic data indicate that the Ormen Lange area was a local depocentre during Paleocene and Early Eocene times, probably with the temporary development of a small depression on the sea floor capturing coarse-grained turbidity flows. Inversion of the depocentre and development of the Ormen Lange Dome took place in Late Eocene'Oligocene time. (6) Heavy-mineral composition and Nd-Sm iso-

438


Fig. 17. Outline of the tectonostratigraphic development for the eastern margin of the MOre Basin from the Maastrichtian to the present-day situation. (The Jurassic fault geometry illustrates the strong extensional events taking place in the Late Jurassic with rotational fault movements and large lateral displacement on fault blocks such as the Gossen High.)

tope analyses suggest that both the Jorsalfare Formation and the Egga member sands were derived from the same source area, probably strongly dominated by a metamorphic basement terrain that in provenance age corresponds to the age of the Norwegian mainland (1.6 Ga).

(7) B iostratigraphical data indicate that there has been some reworking of both Cretaceous, Jurassic and Triassic sediments (Robertson Research, unpublished data), and may suggest the former presence of Mesozoic sediments in the provenance area to the east, probably in a position corresponding to the pre-


sent-day metamorphic basement terrain of the M~re and Romsdal areas.

Acknowledgements We thank the license group of PL209 for permission to publish the data. We also want to thank Ian Sharp and Finn Surlyk for a very constructive review of the manuscript. Thanks are due to Inger Holmefjord at Norsk Hydro Research Centre for analyses of the samples for mineralogical composition, Rolf Birger Pedersen, University of Bergen, for Sm/Nd isotope analyses and Gry Arnesen for drafting some of the figures.

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M~re Basin margin, off mid-Norway. Nor. Geol. Tidsskr., 76(4): 199-214. Kneller, B.C. and Branney, M., 1995. Sustained high-density turbidity currents and the development of thick massive sands. Sedimentology, 42:607-616. Lowe, D., 1979. Sediment gravity flows: their classification and some problems of application to natural flows and deposits. In: L.J. Doyle and O.H. Pilkey (Editors), Geology of Continental Slopes. Soc. Econ. Paleontol. Mineral. Spec. Publ., 27: 75-82. Lowe, D., 1982. Sediment gravity flows, II. Depositional models with special reference to the deposits of high-density turbidity currents. J. Sediment. Petrol., 52(1): 279-297. Macleod, N., Keller and G., 1994. Comparative biogeographic analysis of planktic foraminiferal survivorship across the Cretaceous/Tertiary (K/T) boundary. Paleobiology, 20(2): 143-177. Mearns, E.W., 1988. A samarium-neodymium isotopic survey of modern river sediments from northern Britain. Chem. Geol. (Isot. Geosci. Sect.), 73: 1-13. Middleton, G.V., 1967. Experiments on density and turbidity currents, III. Deposition of sediments. Can. J. Earth Sci., 4: 475505. Molina, E., Arenillas, I. and Arz, J.A., 1996. The Cretaceous/Tertiary mass extinction in planktic foraminifera at Agost, Spain. Rev. Micropaleontol., 39(3): 225-243. Morton, A.C. and Grant, S., 1998. Cretaceous depositional systems in the Norwegian Sea: heavy mineral constraints. Am. Assoc. Pet. Geol. Bull., 82(2): 274-290. Mutti, E., 19791 Turbidites et cones sous-margins profonds. In: R Homewood (Editor), S6dimentation D6trique (fluviatile, littorale et marine). Institut de G6ologie, Universit6 de Fribourg, Fribourg, pp. 353-419. Mutti, E. and Ricci Lucchi, F., 1972. Le tobiditi delt Apennino settentrionale: introduzione all'analisi di facies. Mere. Soc. Geol. Ital., 11: 161-199. Nemec, W. and Steel, R.J., 1984. Alluvial and coastal conglomerates: their significant features and some comments on gravelly mass-flow deposits. In: E.H. Kostler and R.J. Steel (Editors), Sedimentology of Gravels and Conglomerates. Can. Soc. Pet. Geol., Mere., 10:1-31. Pettijohn, F.J., Potter, RE. and Siever, R., 1972. Sand and Sandstone. Springer, Berlin, 618 pp. Pickering, K.T., Stow, D.A.V., Watson, M.E and Hiscott, R.N., 1986. Deep water facies, processes and model: a review and classification scheme for modern and ancient sediments. Earth Sci. Rev., 22: 75-174. Picketing, K.T., Hiscott, R.N. and Hein, F.J., 1989. Deep Marine Environments: Clastic Sedimentation and Tectonics. Unwin Hyman, London, 416 pp. Reading, H.G. and Richards, M., 1994. Turbidite systems in deepwater basin margins classified by grain size and feeder system. Am. Assoc. Pet. Geol. Bull., 78(5): 792-822. Riis, F., 1996. Quantification of Cenozoic vertical movements of Scandinavia by correlation of morphological surfaces with offshore data. Global Planet. Changes 12, (1-4): 331-357. Surlyk, F., 1984. Submarine fan conglomerates of the VolgianValanginian Wollaston Foreland Group, East Greenland. In: E.H. Koster and R.J. Steel (Editors), Sedimentology of Gravels and Conglomerates. Can. Soc. Pet. Geol., Mere., 10: 395-382.

Norsk ttydro Research Centre, P.O. Box 7190, N-5020 Bergen, Norway Norsk Hydro Research Centre, P.O. Box 7190, N-5020 Bergen, Norway Norsk Hydro Exploration, N-0246 Oslo, Norway Norsk Hydro Research Centre, P.O. Box 7190, N-5020 Bergen, Norway Norsk Hydro Research Centre, P.O. Box 7190, N-5020 Bergen, Norway Norsk Hydro Exploration, N-0246 Oslo, Norway Norsk Hydro Exploration, N-0246 Oslo, Norway

440 V~gnes, E., Gabrielsen, R.H. and Haremo, R, 1998. Late Cretaceous-Cenozoic intraplate contractional deformation at the Norwegian continental shelf: timing, magnitude and regional implications. Tectonophysics, 300: 29-46. Walker, R.G., 1967. Turbidite sedimentary structures and their relationship to proximal and distal depositional environments. J. Sediment. Petrol., 37(1): 25-43. Walker, R.G., 1978. Deep-water sandstone facies and ancient sub-

J. G. Gjelberg et al. marine fans: models for exploration for stratigraphic trans. Am. Assoc. Pet. Geol. Bull., 62: 932-966. Walker, R.G. and Mutti, E., 1873. Turbidite facie and facies associations. In: G.V. Middleton and A.H. Bouma (Editors), Turbidites and Deep Water Sedimentation. Pacific Section, Short Course Notes, Society of Economic Paleontologists and Mineralogists, Tulsa, OK, pp. 119-157.

441

Glacial processes and large-scale morphology on the mid-Norwegian continental shelf Dag Ottesen, Leif Rise, K&re Rokoengen and Joar Sa~ttem

A regional digital bathymetric data set covering most of the mid-Norwegian continental shelf is presented and gives a unique regional view into glacial processes and ice-sheet dynamics on this part of the continental shelf during the Weichselian, indicating that forms and deposits were created by a highly dynamic ice sheet. At times, ice flow was mainly channelised through ice streams located in bathymetric depressions on the shelf areas. Glacial sedimentary processes are discussed with a focus on the marine-based part of the Scandinavian ice sheet during the last glaciation (the Weichselian). Ice sheets that grounded on the shelf edge are thought to have been responsible for depositing complex prograding sequences on the mid-Norwegian shelf during several glaciations from Late Pliocene time, reaching a maximum thickness of 1500 m on the shelf edge. During interglacials, the shelf areas were sediment-starved with little or no clastic sedimentation. On top of these prograding units, several packages of Quaternary sediments (mainly till of Weichselian age) show a more aggradational pattern. Improved knowledge about the deposition and age of the upper Cenozoic sediment wedge has proved vital for revealing the ice-sheet dynamics and may also be important in understanding the maturation and migration of hydrocarbons on the mid-Norwegian shelf.

Introduction A number of studies during the last 30 years have confirmed that the present morphology of the midNorwegian continental shelf (Fig. 1) is mainly a result of glacial processes (Holtedahl and Sellevold, 1972; Bugge, 1980; Rokoengen, 1980; Gunleiksrud and Rokoengen, 1980; Lien, 1983; Rise and Rokoengen, 1984; Rise et al., 1984; King et al., 1987; Holtedahl, 1993). The stratigraphy and age of the offshore deposits have also shown that glacial processes on the mid-Norwegian continental shelf involved sediment redistribution to a far greater extent and much faster than previously thought (Haflidason et al., 1991; Rokoengen et al., 1995; Henriksen and Vorren, 1996; Sa~ttem et al., 1996; Vorren and Laberg, 1997; Eidvin et al., 1998; Rokoengen and Frengstad, 1999). Improved models of ice-sheet dynamics within areas where the substratum shows changes on a regional scale are very important in order to understand the sediment transport from land to shelf areas, within shelf areas and onto the upper continental slope. The combination of sedimentological, geotechnical and acoustic data from the shelf areas off mid-Norway offer a unique data set to constrain such models both qualitatively and quantitatively. The purpose of the present contribution is to discuss the glacial sedimentary processes and the dynamics of the large ice sheets on the mid-Norwegian continental shelf in the

light of regional bathymetric, seismic and sedimentological data. We will focus on the marine-based part of the Scandinavian ice sheet during the last glaciation (the Weichselian), and especially the behaviour of the ice streams, which are fast moving parts of an ice sheet.

Previous investigations/geological setting In IKU's regional mapping off mid-Norway during the 1970s and early 1980s, the bedrock surface was divided into eleven units informally named I to XI and with ages of the sampled units ranging from Triassic to Pliocene. Due to basinward subsidence and glacial erosion in the inner part of the shelf, the units subcrop more or less parallel to the coast with decreasing ages westwards (Bugge et al., 1984; Rokoengen et al., 1988, 1995; Sigmond, 1992). Bedrock unit IX (Fig. 2) is found about 50 km west of the crystalline basement as a prominent ridge dominated by sand and with greater resistance to later glacial erosion than the presumably more clay-rich sediments below and above. From unit IX and landwards, the Quaternary is fairly thin. The bathymetry, especially between Fr~yabanken and Haltenbanken, reveals varying resistance to erosion of the Mesozoic and Tertiary bedrock units. An interpreted profile on the mid-Norwegian shelf (Fig. 2) illustrates the upper Cenozoic stratigraphy.


442

D. O t t e s e n et al.

Fig. 1. Shaded relief map covering the mid-Norwegian shelf with 50 m depth contours. The data have been collected by the Norwegian Hydrographic Service with single-beam echosounder. LI = Lofoten Islands; VF = Vestfjorden; HB = Haltenbanken; SKD = Sklinnadjupet; TD = Traenadjupet; SD = Suladjupet; FB = Fr0yabanken; FR = FrCyryggen; R F -- Romsdalsfjorden; SF = Storfjorden; LG = Langgrunna; M P = Mgllc~yplatgtet; BG = Buagrunnen; BD = Breisunddjupet; OD = Onadjupet; N T = Norwegian Trench; TB -- Traenabanken; SR = Skjoldryggen.

At the present shelf edge, the extensive and complex wedge reaches a maximum thickness of about 1500 m (Rokoengen et al., 1995). A marked change in depositional pattern is observed at the regional unconformity below unit D: (1) the lower units show complex and strongly prograding sequences; (2) the upper units are subhorizontal, exhibiting both progradation and aggradation. There has been a long debate whether the different units are of glacial or non-glacial origin, especially with age estimates varying from Oligocene to Quaternary (Rokoengen et al., 1995). Eidvin et al. (1998) analysed six exploration wells on the mid-Norwegian shelf and dated the oldest parts of the sedimentary wedge on the outer continental shelf to Late Pliocene.

They correlated this with a pronounced expansion of the north European glaciers dated at about 2.6 Ma by Jansen and SjOholm (1991). Unit IX was assigned an Early Oligocene age (Eidvin et al., 1998), but a younger age can still not be excluded. The succession below the irregular base of unit D (units L-E, Fig. 2) exhibits large-scale clinoforms prograding towards the northwest and gradually building out the shelf edge. In general, the units are sheet-like with erosional boundaries in the inner part. Most of the sediments below the irregular base of unit D (units K-E) seem to have been deposited by glacial processes, e.g. deposition by grounded glaciers as various types of tills, as proximal glaciomarine sediments, or by redeposition by

Glacial processes and large-scale morphology on the mid-Norwegian continental shelf

443

Fig. 2. Composite geoseismic profile showing the upper Cenozoic stratigraphy across the mid-Norwegian shelf. Modified after Rokoengen et al. (1995). The units K-E represent Upper Pliocene/Pleistocene sediments, while the units D, B, A and U above the angular unconformity probably represent the last interglacial/glacial cycle. See Fig. 1 for location.

slumping or debris flows down the continental slope during the Late Pliocene/Pleistocene (Rokoengen et al., 1995; Henriksen and Vorren, 1996; Eidvin et al., 1998), but a younger age can still not be excluded. The sediments above the angular unconformity (units D, B, A and U) represent mainly the last interglacial/glacial cycle (Rokoengen et al., 1995; S~ettem et al., 1996). The typically irregular base of unit D is interpreted to be the result of strong glacial sculpturing, probably both constructional and erosional, in late Saalian time. The acoustically layered unit D consists of marine (Eemian) and glaciomarine sediments (Sa~ttem et al., 1996). The three topmost units (B, A and U) are dominated by unsorted material representing Weichselian tills, possibly from three major glacial advances on the continental shelf. Unit B may represent deposits from the first (early) Weichselian glaciation on the shelf, and unit A deposits from the maximum glaciation. Unit U consists of till from the last glacial advances reaching the shelf edge and glaciomarine clays from the last deglaciation (Rokoengen et al., 1995). Units B, A and U have been subdivided into a number of subunits using the till-tongue model (King et al., 1987). Knowledge about climatic changes and Scandinavian ice-sheet variations during the last glacial period (the Weichselian) have increased significantly during the last 20 years as a result of studies in the deep sea (Veum et al., 1992; Fronval et al., 1995), of Greenland ice cores (Grootes et al., 1993; Taylor et al., 1993), and from investigations of the Quaternary stratigraphy on land (Larsen and Sejrup, 1990; Olsen, 1997). Based on more than 100 ~4C AMS-datings, Olsen (1997) reported the glacial variations during the last 45,000 years, suggesting the existence of extensive ice-free areas in several intervals alternating with rapid ice-growth periods. The last glacial maximum comprises two major glacial expansion phases, dated at 22,000 years BP and 16,000 years BE Both advances probably reached the outer parts of the shelf

and are correlated with the uppermost unit A and unit U, respectively. The two youngest till units near the shelf edge (till tongues 23 and 24 in unit U) were deposited at about 15,000 and 13,500 years BP according to 14C AMSdating of shells (Rokoengen and Frengstad, 1999). Along the mid-Norwegian coast, several ~4C-dates give ages older than 12,000 years BE indicating a very rapid deglaciation of the entire shelf area. How these rapid changes in ice-sheet extent and configuration are expressed on the shelf is so far poorly known, but through the regional bathymetric data set we can better imagine the nature of these processes (Figs. 2 and 4).

Sea-bottom morphology related to ice-sheet dynamics Bathymetric data base The Norwegian Hydrographic Service collected single-beam echosounder data during the years 19651985. The regional bathymetric data set covers a large part of the Norwegian continental shelf south of 68~ with an average line spacing of 500 m. Previous bathymetric charts have also been produced (Bugge, 1975; Bugge et al., 1987). The data were gridded with a cell size of 500 m and plotted as coloured contour maps with a 5 m contour interval, and as shaded relief maps. The data set covers the areas from the outer coastal zone with crystalline bedrock to the shelf edge and parts of the continental slope, in certain areas down to 1000 m water depth. The data were collected by an Atlas Penguin echosounder (100 kHz). The positioning system used was Decca Main Chain with an absolute accuracy commonly better than 100 m, but within some areas not better than 500 m. The relative accuracy (repeatability) is, however, much better and the morphology on maps in scales of 1 : 500,000 or less will not be significantly influenced by the inaccuracy.

444 The major morphological features on the mid-Norwegian shelf between the outlet of the Norwegian Trench at about 62~ and the Lofoten Islands at about 67~ are shown in Figs. 1, 3 and 4. Between the outlet of the Norwegian Trench and northwards to 64~ the shelf is rather narrow (60-100 km wide), in contrast to areas further north. In the Skjoldryggen area (Fig. 1), the shelf reaches its greatest width, about 250 km. The shelf includes bank areas with water depths of 100-300 m north of 63~ (Tra~nabanken, Sklinnabanken, Haltenbanken and FrOyabanken). South of 63~ there are several large bank areas with water depths less than 100 m (Buagrunnen and Langgrunna, Fig. 1), in addition to Griptarane west of Kristiansund where crystalline rocks crop out at the surface. In the north, Tr~enabanken and Haltenbanken represent the largest bank areas (Fig. 1). The banks are separated by east-west-oriented depressions 350-500 m deep north of 64~ and 150300 m deep south of 64~ North of 64~ they are up to 60 km wide, while on the southern half of the shelf, between 62~ and 64~ they are narrower, and reach 10-20 km in width.

The Traenadjupet area Tr~enadjupet is the best expressed ice-stream drainage depression in the northern part of the study area (Fig. 1). It is between 40 km and 60 km wide and generally widens towards the shelf edge. The bathymetry shows linear elements parallel to the trough axis, reflecting the flow direction of ice streams. In the eastern part of Tr~enadjupet, at least two different glacial drainage systems coalesced. Ice flow from the northeast (Vestfjorden) and southeast joined to become one major ice stream along Tra~nadjupet (Fig. 3). The bathymetry indicates that the ice stream from the southeast cuts linear features formed by an ice stream from the northeast, and thus reflects the latest erosive phase. The glacial deposits in the Tr~enabanken area are eroded in the Tr~enadjupet depression (King et al., 1987).

The Sklinnadjupet-Skjoldryggen system The Sklinnadjupet is a symmetric, U-shaped trough approximately 30 km wide, up to 470 m deep and 100 km long, presumably mainly eroded by ice streams flowing south of Tra~nabanken during the last glaciation (Fig. 3). The eastern part of Sklinnadjupet has acted as a confluence basin for drainage of ice from the onshore areas. The trough is oriented approximately east-west. The form and trend of the western part of Sklinnadjupet indicate

D. Ottesen et al.

that during the latest phase, the Sklinnadjupet ice stream was deflected towards the north, and flowed in a northwesterly direction out to the shelf edge. This is probably because the ice sheet east of Skjoldryggen was frozen to the ground or pinned by a large, supposedly mainly ice-pushed ridge, Skjoldryggen (S~ettem et al., 1996). In the eastern part of the Sklinnadjupet, the bathymetry indicates that former confluencing glaciers drained into the depressions (Fig. 3). Evidently, Sklinnadjupet received glacial ice from large inland areas. It appears that the shallow bank areas acted as barriers that partly controlled the dynamics of the ice flowing from inland areas to the continental shelves. In the TromsOflaket bank area off northern Norway, a similar observation and outline of a possible glacial mechanism was discussed by S~ettem (1990). Sklinnadjupet partly parallels another major icestream drainage route east of the Haltenbanken area and southwest of Sklinnabanken (Fig. 3). This depression is oriented N W - S E between Haltenbanken and Skinnabanken and continues westwards towards the Skjoldryggen area. Skjoldryggen (Fig. 3) has for a long time been interpreted as an end-moraine ridge at the outermost shelf edge (for further reference, see Holtedahl, 1993). It is almost 200 km long, up to 200 m high and 10 km wide and is by far the largest end-moraine on the Norwegian continental shelf. The morphology east of the Skjoldryggen is complex, comprising several depressions and ridges (Fig. 3). Sa~ttem et al. (1996) reported glaciotectonic deformation in this area, and the bathymetric data set discussed herein supports this interpretation. It seems that the displaced blocks were either transported to and incorporated into the Skjoldryggen moraine ridge, or existed as individual or complex ridges. Sa~ttem et al. (1996) suggested that, following the advance of the ice margin to Skjoldryggen, the ice lobe which deposited the ridge froze to the ground beneath. This pinned the ice, and led to a build up of stress at the ice lobe base which gave rise to glaciotectonic displacement of blocks of frozen sediments. The glacial stratigraphy in both Haltenbanken and Tra~nabanken outlines thick units of Weichselian sediments (units A, B and U, Fig. 2). King et al. (1987) described three till units, each comprising stacked till tongues with intervening glaciomarine sediments deposited during successive advances and retreats of the ice-sheet grounding line. The units generally occupy the outer and central portions of the shelf, with a thickness of up to 400 m in the Skjoldryggen area, whereas an erosional morphology dominates the central to inner shelf.

Glacial processes and large-scale morphology on the m i d - N o r w e g i a n continental s h e l f

445

Fig. 3. Colour-shaded relief map with 20 m depth contours based on a 500 m grid cell size. LI = Lofoten Islands; V F = Vestfjorden; HB = Haltenbanken; SKD = Sklinnadjupet; TD = Tramadjupet; SD = Suladjupet; FB = Fr0yabanken; FR = Fr0yryggen; TB = Tramabanken; SR = Skjoldryggen.

Ice drainage offshore Trondelag The shallow parts of Haltenbanken have in certain periods probably prevented an active westward flow of the ice sheet. Several SW-NE-trending depressions (including Suladjupet, Fig. 3) indicate that ice mainly drained southwestwards, east of Haltenbanken, turning westwards across FrOyryggen north of FrOyabanken. The Suladjupet depression is more than 500 m deep and eroded 200-300 m below the surrounding sea bottom (Figs. 3 and 4). The depression was formed by glacial erosion, mainly of the dark, Upper Jurassic/Lower Cretaceous claystone of the Spekk Forma-

tion. IKU bedrock unit IX subcrops below a thin Quaternary cover at Fr0yryggen, and it is evident that this sandy unit has been resistant to glacial erosion. Prograding glacial sequences and a wide erosional depression are seen west of Fr0yryggen (Bugge, 1980; Bugge et al., 1987; Rokoengen et al., 1995).

Ice drainage offshore More The continental shelf offshore MOre is very narrow compared to the areas further north (Fig. 4). Several WNW-ESE-trending depressions are separated by shallow bank areas. We believe that these troughs have also been drainage routes for ice streams. Out-

446

D. O t t e s e n et al.

Fig. 4. Colour-shaded relief map of the shelf area off MOre with 20 m depth contours. On Mfil0yplatgtet large curved ridges dominate, while further north, depressions which extend from land to the shelf break separate shallower bank areas. The depressions are interpreted to have been drainage routes for ice streams during maximum expansion of the late Weichselian ice sheet. SD = Suladjupet; FB = Fr0yabanken; FR = Fr0yryggen; R F = Romsdalsfjorden; SF = Storfjorden; LG = Langgrunna; M P = Mfil0yplat~et; BG = Buagrunnen; BD = Breisunddjupet; OD = Onadjupet; N T = Norwegian Trench; K = Kristiansund;/~ = ,~.lesund.

side Smr (Fig. 4), the ice drainage is directed in a southwesterly direction, towards the northern part of the Storegga slide. Another ice stream has passed south of the Griptarane highs towards the northwest, and coalesced with the ice stream off SmNa (Fig. 4). Northwest of Romsdalsfjord, a NW-SE-trending depression (Onadjupet) ends in the Storegga slide area at the shelf break. Northwest of ,~lesund, another depression extends almost to the shelf break where it coalesces with the aforementioned ice-stream eroded channel (Fig. 4). Langgrunna is a large bank area that guided icestream flow both south and north of the bank (Fig. 4). Breisunddjupet forms a narrow, elongated depression, representing the continuation of the deep Storfjord drainage system onto the open shelf, and ends in the eastern part of Langgrunna (Fig. 4). This extended fjord

feature is very uncommon on the shelf and indicates special glacial conditions during its formation; for instance, erosion by very channelised ice flow in an area where the surroundings were covered by frozen based ice could produce such a feature. A possible tectonic origin has also been discussed (Rokoengen, 1980). South of Breisunddjupet, an ice-stream drainage route from the east onto the northern part of M~10yplatfiet can be inferred (Fig. 4). MgdCyplat~et is the southernmost bank area of the mid-Norwegian shelf, located close to the outlet of the Norwegian Trench. Thus, this area probably was influenced by the large Norwegian Trench Ice Stream from time to time (King et al., 1996), as the ice dramage from the mainland either was deflected in a northerly direction and/or partly assimilated by the Norwegian Trench Ice Stream. On MfilCyplatfiet (Fig. 4), arcuate ridges

Glacial

processes

and

large-scale

morphology

on the mid-Norwegian

indicate major halts during deglaciation (Rokoengen, 1980; Rise and Rokoengen, 1984; Rise et al., 1984). Ice-flow model From the present bathymetric data set (Figs. 1, 3 and 4) and earlier investigations on the Norwegian

continental

shelf

447

continental shelf (Rise and Rokoengen, 1984; King et al., 1987; Rokoengen et al., 1995; Sa~ttem et al., 1996; Vorren and Laberg, 1997), we have reconstructed a probable flow pattern of the western part of the Scandinavian ice sheet during the late Weichselian (Fig. 5). In addition we have used investigations from Antarctica as a basis for the model.

Fig. 5. Interpreted ice-flow model during the late Weichselian with ice streams flowing along the main offshore depressions/troughs. V F = Vestfjorden; H B -- Haltenbanken; S K D = Sklinnadjupet; T D - Tramadjupet; S B = Sklinnabanken; S D - Suladjupet; F B = Fr0yabanken; M P = Mfil0yplatfiet;N T - Norwegian Trench; T B - Tr~enabanken;L G - Langgrunna; S K - Skagerrak; T = Trondheim.

448

Extensive research has been carried out in West Antarctica during recent years in order to understand the ice-sheet dynamics of large, marine-based ice sheets (e.g. Shabtaie and Bentley, 1987). The emphasis has been on the large ice streams which drain about 90% of the West Antarctic ice sheet. These ice streams are fast moving parts of ice sheets, normally 300-500 km long, 50-80 km wide and with speeds of 300-700 m/year, whereas the surrounding ice sheet may have a speed of less than 10 m/year (Bindschadler et al., 1996). Generally, the ice streams are located in overdeepened troughs, often eroding several hundred metres below the surrounding seafloor. The glaciological setting of West Antarctica today can partly be compared to the situation on the midNorwegian shelf during late Weichselian time. Studies on the shelf areas in Antarctica have outlined both prograding and aggrading glacial sequences (e.g. Cooper et al., 1991; Latter and Cunningham, 1993), comparable to what we find on the mid-Norwegian shelf. In the Ross Sea, Shipp and Anderson (1997) have described glacial megaflutes and trough forms, both related to palaeo-ice streams across the Ross Sea. The largest ice stream followed the Norwegian Trench along the southern and western coast of Norway (Fig. 4), and ended where the ice calved in the Norwegian Sea west of Mfil~yplatfiet (King et al., 1996). The idea of an immense Skagerrak glacier flowing along the Norwegian coast was introduced by Helland (1885). For some years this theory was generally accepted, but later became more controversial or was even rejected (Andersen, 1964; Holtedahl, 1993). Investigations both in the northern North Sea (Rise and Rokoengen, 1984) and in the Skagerrak (Longva and Thorsnes, 1997), however, have demonstrated the ice movements along the trench and proven the existence of the ice stream. In the Vestfjorden/Tra~nadjupet area another major ice stream has flowed out to the shelf edge several times (Fig. 5). On the mid-Norwegian shelf, the location of the ice-stream drainage routes are mainly located between the shallow bank areas, such as Tra~nabanken, Haltenbanken, FrOyabanken, Buagrunnen and Langgrunna (Figs. 3-5). Grounded ice sheets are thought to have been responsible for depositing the prograding sequences. During the initial advance of the grounded ice, the inner shelf would have been heavily eroded and gently dipping glacial strata were probably deposited on the shelf. Ice streams carved broad depressions across the shelf and carried sediments directly to the continental shelf edge, thereby creating trough-mouth fans (Vorren and Laberg, 1997) and sheet-like prograding

D. Ottesen et al.

sequences (King et al., 1987). During interglacial periods, the shelf areas were starved of sediment and thus received little or no clastic sedimentation. A ckn owled g ements

We gratefully acknowledge the Norwegian Hydrographic Service for the regional bathymetric data set and IKU for access to seismic data. This paper benefited from reviews by Tom Bugge and Tore Vorren. The English language has been improved by David Roberts. References Andersen, B.G., 1964. Har Ja~ren v~ert dekket av en Skagerrakbre? Er "Skagerrakmorenen" en marin leire? Nor. Geol. Unders., 228: 5-11. Bindschadler, R., Vornberger, E, Blankenship, D., Scambos, T. and Jacobel, R., 1996. Surface velocity and mass balance of Ice Streams D and E, West Antarctica. J. Glaciol., 42(142): 461-475. Bugge, T., 1975. Kart med kystkontur og dybdekoter for den norske kontinentalsokkel. Cont. Shelf Inst. (IKU), Publ. 55, 21 pp. Bugge, T., 1980. Ovre lags geologi pfi kontinentalsokkelen utenfor MOre og Tr0ndelag. Cont. Shelf Inst. (IKU), Publ., 104, 44 pp. Bugge, T., Knarud, R. and M0rk, A., 1984. Bedrock geology on the mid-Norwegian continental shelf. In: A.M. Spencer, S.O. Johnsen, A. M0rk, E. Nys~ether, E Songstad and A. Spinnanger (Editors), Petroleum Geology of the North European Margin. Graham and Trotman, London, pp. 271-283. Bugge, T., Rise, L. and Rokoengen, K., 1987. Dybdekart over midtnorsk kontinentalsokkel. Mfilestokk 1" 1,000,000. Cont. Shelf Inst. (IKU), Publ., 115. Cooper, A.K., Barrett, P.J., Hinz, K., Traube, V., Leitchenkov, G. and Stagg, H.M.J., 1991. Cenozoic progradation sequences of the Antarctic continental margin: a record of glacio-eustatic and tectonic events. In: A.W. Meyer, T.A. Davies and S.W. Wise (Editors), Evolution of Mesozoic and Cainozoic Continental Margins. Mar. Geol., 102: 175-213. Eidvin, T., Brekke, H., Riis, F. and Renshaw, D., 1998. Cenozoic stratigraphy of the Norwegian Sea continental shelf, 64~176 Nor. Geol. Tidsskr., 78:125-151. Fronval, T., Jansen, E., Bloemendal, J. and Johnsen, S., 1995. Oceanic evidence for coherent fluctuations in Fennoscandian and Laurentide ice sheets on millennium timescales. Nature, 374: 443-446. Grootes, EM., Stuiver, M., White, J.W.C., Johnsen, S. and Jouzel, J., 1993. Comparison of oxygen-isotope records from the Gisp2 and Grip Greenland ice cores. Nature, 366: 552-554. Gunleiksrud, T. and Rokoengen, K., 1980. Regional geological mapping of the Norwegian continental shelf with examples of engineering applications. In: D.A. Ardus (Editor), Offshore Site Investigations. Graham and Trotman, London, pp. 23-35. Haflidason, H., Aarseth, I., Haugen, J.E., Sejrup, H.E, LOvlie, R. and Reither, E., 1991. Quaternary stratigraphy of the Draugen area, Mid-Norwegian Shelf. Mar. Geol., 101: 125-146. Helland, A., 1885. Om jordens 10se afleiringer. Meddelelse fra Den naturhistoriske Forening i Christiania, pp. 27-42. Henriksen, S. and Vorren, T., 1996. Late Cenozoic sedimentation and uplift history on the mid-Norwegian continental shelf. Global Planet. Change, 12: 171-199. Holtedahl, H., 1993. Marine geology of the Norwegian continental margin. Nor. Geol. Unders. Spec. Publ., 6, 150 pp. Holtedahl, H. and Sellevold, M.A., 1972. Notes on the influence of glaciation on the Norwegian continental shelf bordering on the Norwegian Sea. Ambio Spec. Rep. 2, 31-38.

Glacial processes and large-scale morphology on the mid-Norwegian continental shelf Jansen, E. and Sj0holm, J., 1991. Reconstruction of glaciation over the past 6 My from ice-borne deposits in the Norwegian Sea. Nature, 349: 600-603. King, E.L., Sejrup, H.R, Haflidason, H., Elverh0i, A. and Aarseth, I., 1996. Quaternary seismic stratigraphy of the North Sea Fan: glacially fed gravity aprons, hemipelagic sediments, and large submarine slides. Mar. Geol., 130: 293-315. King, L., Rokoengen, K. and Gunleiksrud, T., 1987. Quaternary seismostratigraphy of the Mid-Norwegian Shelf, 65~176 A till tongue stratigraphy. Cont. Shelf Inst. (IKU), Publ., 114, 58 pp. Larsen, E. and Sejrup, H.R, 1990. Weichselian land-sea interactions: Western Norway-Norwegian Sea. Quat. Sci. Rev., 9: 85-97. Larter, R.D. and Cunningham, A.R, 1993. The depositional pattern and distribution of glacial-interglacial sequences on the Antarctic Peninsula Pacific margin. Mar. Geol., 109: 202-219. Lien, R., 1983. P10yemerker etter isfjell p~ norsk kontinentalsokkel. Cont. Shelf Inst. (IKU), Publ., 109, 147 pp. Longva, O. and Thorsnes, T. (Editors), 1997. Skagerrak in the past and at the present an integrated study of geology, chemistry, hydrography and microfossil ecology. Nor. Geol. Unders. Spec. Publ., 8, 100 pp. Olsen, L., 1997. Rapid shifts in glacial extension characterise a new conceptual model for glacial variations during the Mid and Late Weichselian in Norway. Nor. Geol. Unders. Bull., 433: 54-55. Rise, L. and Rokoengen, K., 1984. Surficial sediments in the Norwegian sector of the North Sea between 60~ and 62~ Mar. Geol., 56:287-317. Rise, L., Rokoengen, K., Skinner, A. and Long, D., 1984. Nordlige Nordsj0. Kvart~ergeologisk kart mellom 60~ og 62~ og 0st for 1~ M 1: 500,000. Continental Shelf Institute (IKU) in cooperation with British Geological Survey (BGS). Rokoengen, K., 1980. De 0vre lags geologi p~ kontinentalsokkelen utenfor MOre og Romsdal. Beskrivelse til kvarta~rgeologisk kart 6203 i mfilestokk l" 250,000. Cont. Shelf Inst. (IKU), PUN., 105, 49 pp. Rokoengen, K. and Frengstad, B., 1999. Radiocarbon and seismic

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evidence of ice-sheet extent and the last deglaciation on the midNorwegian continental shelf. Nor. Geol. Tidsskr., 79: 129-132. Rokoengen, K., Rise, L., Bugge, T. and Sa~ttem, J., 1988. Berggrunnsgeologi pfi midtnorsk kontinentalsokkel. Kart i mfilestokk 1 1,000,000. Cont. Shelf Inst. (IKU), Publ., 118. Rokoengen, K., Rise, L., Bryn, R, Frengstad, B., Gustavsen, B., Nygaard, E. and S~ettem, J., 1995. Upper Cenozoic stratigraphy on the Mid-Norwegian continental shelf. Nor. Geol. Tidsskr., 75: 88-104. Sa~ttem, J., 1990. Glaciotectonic forms and structures on the Norwegian continental shelf: observations, processes and implications. Nor. Geol. Tidsskr., 70: 81-94. Sa~ttem, J., Rise, L., Rokoengen, K. and By, T., 1996. Soil investigations, offshore mid-Norway: a study of glacial influence on geotechnical properties. Global Planet. Change, 12:271-285. Shabtaie, S. and Bentley, C.R., 1987. West Antarctic ice streams draining into the Ross ice shelf: configuration and mass balance. J. Geophys. Res., 92(B2): 1311-1336. Shipp, S. and Anderson, J., 1997. Paleo-ice stream boundaries, Ross Sea, Antarctica. In: T. Davies, T. Bell, A. Cooper, H. Josenhans, L. Polyak, A. Solheim, M. Stoker and J. Stravers (Editors), Glaciated Continental Margins. An Atlas of Acoustic Images. Chapman and Hall, London, pp. 106-109. Sigmond, E.M.O., 1992. Berggrunnskart, Norge med havomrfider. Mfilestokk 1"3 millioner. Norges Geologiske Unders0kelse, Trondheim. Taylor, K.C., Lamorey, G.W., Doyle, G.A., Alley, R.B., Grootes, RM., Mayewski, RA., White, J.W.C. and Barlow, L.K., 1993. The "flickering switch" of Late Pleistocene climate change. Nature, 361: 432-436. Veum, T., Jansen, E., Arnold, M., Beyer, I. and Duplessy, J.C., 1992. Water mass exchange between the North-Atlantic and the Norwegian Sea during the past 28,000 years. Nature, 356: 783785. Vorren, %0. and Laberg, J.S., 1997. Trough mouth fans - - paleoclimate and ice-sheet monitors. Quat. Sci. Rev., 16(8): 865-881.

Geological Survey of Norway, N-7491 Trondheim, Norway Geological Survey of Norway, N-7491 Trondheim, Norway Norwegian University of Science and Technology, N-7034 Trondheim, Norway SINTEF Petroleum Research, N-7465 Trondheim, Norway Present address: Sauherad Kommune, N-3812 Akkerhaugen, Norway


451

Late Quaternary sedimentary processes and environment on the Norwegian-Greenland Sea continental margins Tore O. Vorren and Jan Sverre Laberg

Our aim is to explain the morphology and late Quaternary sedimentary processes and palaeoenvironment of the Norwegian-Greenland Sea continental margin. In particular we concentrate on the North Norwegian and Barents Sea continental margin. Three main groups of morphological features will be discussed: slides, trough mouth fans and channels. Most of the present paper is an extended summary of Vorren et al. (1998). However, some new data concerning the Tra~nadjupet Slide are added.

Slides Several smaller and larger slides have been identified on the Norwegian continental margin (Fig. 1). Table 1 summarises data on dimensions of the largest slides according to our most recent results. The age of the slides varies from more than 200,000 years (Bj~rn~yrenna Slide) to probably less than 5000 years (Traenadjupet Slide). Some slide features are illustrated by the Tra> nadjupet Slide located on the continental slope just northeast of the VOring Plateau (Fig. 2). The area affected by the slide extends from the shelf break to more than 3000 m water depth in the Lofoten Basin, implying a run-out distance of ca. 200 km. The slide headwall is about 150 m high and 20 km long. The slide scar (area of evacuation) can be followed downslope to about 2400 m water depth, and covers

an area of about 5000 km 2. The slide deposits cover an area of about 9100 k m 2 and total slide-affected area was estimated to be about 14,100 km 2 (Table 1). Some of the morphological features like the escarpment, sediment blocks and flow features from the upper slide area are illustrated in Fig. 3. The large slides (Fig. 1) occurred in areas characterised by high sediment input. Thus, relatively high sediment supply leading to unstable sediments may have been important for the initiation of slope failures in these areas. Another important factor may be the presence of shallow gas, suggested by Knutsen et al. (1992) as important for the instability of sediments found on the central part of the Bear Island trough mouth fan. Studies of the present seismicity in the eastern Norwegian-Greenland Sea have shown relatively high activity along older fault systems (Kvamme and

TABLE 1 Dimensions of large slides on the Norwegian continental margin

Continental slope gradient Run-out distance (km) Maximum thickness (m) Height/length ratio b Slide scar area (kin 2) Total slide influenced area (km 2) Volume (km 3)

Traenadjupet Slide

Storegga Slide a

And0ya Slide

Bj0rnOyrenna Slide

1.25 ~ 200 150 0.0125 5000 14,500 900 d

0.6 ~ 850 430 0.004 34,000 112,500 5580

7~ 190 c

0.6 ~

0.0126 3630 c 9700 c

0.0063 6200 23,000 1100

400

a From Bugge (1983) and Bugge et al. (1987). b Height is the elevation difference between the top of the failed mass at the point of initiation and the top of the failed mass in the depositional zone, and length is the distance from the origin (Hampton et al., 1996 and references therein). c Based on Dowdeswell and Kenyon (1995). d Average thickness 100 ms (= 100 m assuming a seismic velocity of 2000 m/s). Sedimentary Environments Offshore Norway - Palaeozoic to Recent edited by O.J. Martinsen and T. Dreyer. NPF Special Publication 10, pp. 451-456, Published by Elsevier Science B.V., Amsterdam. 9 Norwegian Petroleum Society (NPF), 2001.

452

T.O.

Vorren and J.S. Laberg

Fig. 1. Bathymetric map showing location and extent of slides, small slides and large channels on the Norwegian-Greenland Sea continental margin. The figure is compiled from several sources such as Bugge et al. (1987); Mienert et al. (1993); Laberg and Vorren (1993) and Dowdeswell et al. (1996).

Hansen, 1989; Bungum et al., 1991). From other continental margin areas, earthquakes have been recognised as one of the most likely triggering mechanisms for submarine slides (e.g. Hampton et al., 1996). Thus, for the triggering of the large slides along the eastern Norwegian-Greenland Sea continental margin, earthquakes have probably been an important mechanism (Bugge, 1983; Bugge et al., 1987; Kenyon,

1987; Knutsen et al., 1992; Laberg and Vorren, 1993, 1996a; Evans et al., 1996). Decomposition of gas hydrates was also suggested as a possible triggering mechanism for the Storegga Slides (Bugge et al., 1987). Andreassen and Hansen (1995), using a phase boundary diagram for a methane hydrate system, infer that gas hydrates may be present under the Norwegian/Barents Sea continental slope

TABLE 2

Dimensions and slope gradients of trough mouth fans along the Norwegian-Greenland Sea continental margin

Radius (km) Width upper (km) Width lower (kin) Depth upper (kin) Depth lower proximal (kin) Depth lower distal (km) Area (kin 2) Gradient (upper) Gradient (middle) Gradient (lower)

Kongsfjorden

Isfjorden

Bellsund

Storfjorden

Bear Island

North Sea

Scoresby Sund

TMF

TMF

TMF

TMF

TMF

TMF

TMF

55 40 60 0.2

50 45 75 0.25

70 55 85 0.15

2.0 2700

3.0 3700

2.3 6000

3.2 ~

1.8 ~

590 250 550 0.5 3.0 3.2 215,000 0.8 ~ 0.4 ~ 0.2 ~

560 165 300 0.4 2.7 3.5 142,000 0.6 ~ 0.8 ~ 0.3 ~

110 180 240 0.3 1.5 1.5 19,000

1.9 ~

190 130 210 0.4 2.4 2.7 35,000 1.8 ~ 1.0 ~ 0.2 ~

2~

Late Quaternary sedimentary processes and environment on the Norwegian-Greenland Sea continental margins

453

Eig. 2. Map of the Traenadjupet Slide area. The dotted line within the slide separates the upslope slide scar area and the downslope area of slide deposition.

today. Increased bottom water temperature and/or lowering of sea level would cause destabilisation of the hydrate zone (McIver, 1982). To summarise, high sedimentation rates, which in turn may have led to a build-up of excess pore water pressure, and perhaps with additional pressure caused by gas bubbles, probably led to unstable or metastable

sediments within relatively large parts of the eastern Norwegian-Greenland Sea continental margin. Destabilising and triggering may have been prompted by earthquakes or perhaps by decomposition of gas hydrates.

T.O. Vorren and J.S. Laberg

454

Fig. 3. Mosaic of TOBI side-scan sonar data covering the western, upper Tra~nadjupet Slide. Escarpments and sediment blocks are indicated. For location, see Fig. 2.

Trough mouth fans and debris flows

On the continental margins surrounding the Norwegian-Greenland Sea, fan- or delta-like protrusions

occur in front of many of the glacial troughs or channels crossing the continental shelf and ending on the shelf break (Fig. 1). Nansen (1904) noted many of these protrusions. Vogt and Perry (1978) pointed

TABLE 3 Dimensions of individual debris flows on trough mouth fans in the Norwegian-Greenland Sea

Width (kin) Thickness (m) Length (kin) Area (km 2) Volume (km 3)

Isfjorden TMF

Storfjorden TMF

Bear Island TMF

North Sea TMF

Scoresby Sund TMF

2-5 10-30 10-20 < 100 0.5-1

1-5 15 50-100

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