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3D Seismic Analysis of Pleistocene Tunnel Valleys in the Central North Sea

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3D Seismic Analysis of Pleistocene Tunnel Valleys in the Central North Sea 3D Seismic Analysis of Pleistocene Tunnel Valleys in the Central North Sea Margaret A. Stewart Earth Science and Engineering Imperial College London June 2009 This thesis is submitted to the University of London for the degree of Doctor of Philosophy (Ph. D) and the Diploma of Imperial College Declaration The work presented in this thesis is the candidates own. No portion of the work referred to in this thesis has been submitted in support of an application for another degree or qualification in this or any other university or institute of learning. Margaret A. Stewart 2 Abstract Dense networks of glacial tunnel valleys were imaged at a regional scale beneath the central North Sea using commercial 3D seismic reflection datasets. Buried and seabed tunnel valleys were investigated with the aim of making progress in resolving their method of formation and relationship to the Pleistocene glaciations of the region. The 3D seismic reflection datasets reveal that the buried tunnel valleys are characterised by cross-cutting relationships which provide the basis for the separation of generations of tunnel valleys. Up to eight cross-cutting generations of buried tunnel valleys were defined across an area of 100 x 100 km and provide conclusive proof that the tunnel valleys of the central North Sea are preserved as palimpsest landforms. A single tunnel valley comprises the oldest generation of tunnel valleys identified in this work, and is found to predate the Bruhnes-Matuyama magnetic reversal at 0.78 Ma. This tunnel valley provides the first piece of direct evidence for a pre-MIS 19 glaciation in the central North Sea and associated with a cold period indicated in the marine isotope record at MIS 22. The seven younger buried generations of tunnel valleys correspond to seven peaks in the marine isotope record which indicate glacial episodes at MIS 18, 16, 12, 10, 8 and 6. This work finds that the tunnel valleys of the central North Sea record a much more complex glacial history for the North Sea than the current three-stage model and that significant ice sheets were present in the area much earlier in the Mid-Pleistocene than is currently accepted. Analysis of the morphology and distribution of the buried tunnel valleys suggested their formation some distance from ice margins, possibly central to an ice mass controlled by multiple ice domes which provided hydrostatic head for tunnel valley initiation. Comparison between buried and seabed tunnel valleys reveal similarities in form, but differences in direction and distribution suggest that the configuration of the ice sheet at last glacial maximum (responsible for the formation of the seabed tunnel valleys) was significantly different to those responsible for the formation of the older, buried tunnel valleys. 3 Table of Contents Acknowledgements 9 List of Figures 10 List of Tables 16 Chapter 1: Introduction and Rationale 17 1.1 Introduction and Aims 17 1.2 Thesis Structure 18 Chapter Two: Tunnel Valleys — Background 20 2.1 Location of Referenced Works 20 2.2 Theories of Tunnel Valley Formation 22 2.2.1 History of the Tunnel Valley Concept in Europe 23 2.2.2 Development of North American Theory 29 2.2.3 Current Arguments: Steady State vs. Catastrophic 33 2.3 Tunnel Valley Morphometries and Character 40 2.3.1 Depths, Widths, and Lengths 40 2.3.2 Planform Geometry 43 2.3.3 Associated Landforms 45 2.4 Tunnel Valley Infill 47 2.4.1 Pleistocene Tunnel Valleys 47 2.4.2 Pre-Quaternary Tunnel Valleys 50 2.5 Conclusion and Rationale 51 Chapter 2 - Figures 53 Chapter 3: Regional Background 64 3.1 Introduction and Study Area 64 3.2 The Stratigraphy of the North Sea 66 3.2.1 BGS Stratigraphy 67 3.2.2 A New Seismostratigraphic Framework for the North Sea 71 4 3.3 Chronology of the central North Sea Glacial Record 74 3.3.1 Pre-tunnel valley incision: Unit I 74 3.3.2 Unit II — The Buried Tunnel Valleys 75 3.3.3 Seabed tunnel valleys 77 3.4 Summary 77 Chapter 3 - Figures 79 Chapter 4: Methodology 87 4.1 Seismic Datasets 87 4.1.1 Seismic Reflection Theory 87 4.1.2 Seismic Stratigraphy. 90 4.1.3 Study Datasets 90 4.1.4 Uncertainty and Errors 92 4.2 Interpretation and analysis of central North Sea Tunnel Valleys 95 4.2.1 Workflow and Tools 95 4.2.2 Tunnel Valley Measurements 96 4.3 Summary 100 Chapter 4 - Figures 101 Chapter 5: Results — Generations of Buried Tunnel Valleys 111 5.1 Concept and Overview 111 5.2 Tunnel Valley Generations — Local Scale 113 5.2.1 Dataset A — Illustration of Method 114 5.2.2 Dataset B 116 5.2.3 Dataset C 118 5.2.4 Dataset D 119 5.2.5 Dataset E 120 5.2.6 Dataset F 120 5.2.7 Dataset G 120 5.2.8 Dataset H 121 5.2.9 Dataset I 122 5.2.10 Dataset J 122 5 5.2.11 Dataset M 123 5.3 Tunnel Valley Generations — Regional Scale 123 5.3.1 Generation 1 125 5.3.2 Generation 2 125 5.3.3 Generation 3 125 5.3.4 Generation 3.5 — re-occupied generation 126 5.3.5 Generation 4 126 5.3.6 Generation 5 127 5.3.7 Generation 6 127 5.3.8 Generation 7 128 5.4 Implications of Multiple, Cross-cutting Tunnel Valley Generations 128 5.4.1 Morphology and Form 129 5.4.2 Glaciations in the North Sea 129 5.4.3 Mechanism of Formation 132 5.5 Conclusions 132 5.6 Chapter 5 — Figures 135 Chapter 6: Morphology of Buried Tunnel Valleys 176 6.1 Northern Study Area 176 6.1.1 Analysis of Morphology 176 6.2 Southern Study Area 189 6.2.1 Results - Dataset M 189 6.3 Discussion 191 6.3.1 Width, Length, Depth and Planform 191 6.3.2 Orientation 194 6.3.3 Morphology and Formation 196 6.4 Conclusions 200 Chapter 6 - Figures 203 Chapter 7: Fill and Stratigraphy 221 7.1 Tunnel Valley Fill 221 6 7.1.1 Background 221 7.1.2 Fill Observations 223 7.1.3 Tunnel valley fill and generations 226 7.1.4 Morphological Features within tunnel valley fill 228 7.2 Regional Stratigraphy 229 7.2.1 Boreholes and Buried tunnel valleys 229 7.2.2 Tunnel valley tvla 230 7.2.3 Pre-glacial Features 231 7.3 Conclusions 231 Chapter 7 — Figures 233 Chapter 8: Seabed Tunnel Valleys 249 8.1 Location and Morphology of Seabed Channels 250 8.2 Interpretation of Olex Seabed Channels 256 8.2.1 Moray Firth and Montrose Groups 256 8.2.2 Fladden and Devil's Hole Groups 260 8.2.3 Fladden Tunnel Valley Feature 262 8.3 Comparison with buried tunnel valleys 265 8.3.1 Data Comparison 265 8.3.2 Morphology and Orientation 266 8.4 Conclusions 268 Chapter 8 — Figures 271 Chapter 9: Synthesis and Conclusions 284 9.1 Summary of Results 284 9.2 Tunnel Valley generations and the glacial history of the central North Sea 285 9.3 Relationship between tunnel valleys and Pleistocene ice sheets 286 9.3.1 Ice-Marginal Scenarios 287 9.3.2 Alternative hypothesis: tunnel valley formation distal from ice margin289 9.3.3 Conclusions on ice sheet configuration 290 7 9.4 Future Work 291 9.5 Conclusion 292 Chapter 9 — Figures 294 References 297 Appendix A. 315 8 Acknowledgements I would like to thank all of those who provided support and encouragement during this project, particularly my supervisors Lidia Lonergan and Gary Hampson who have been helpful, dedicated, and enthusiastic throughout the life of this work. This project was carried out with the support of a Janet Watson Scholarship from Imperial College London. I am also grateful for financial assistance received from the Hilary Bauerman Trust, the Dudley Stamp Memorial Fund, the British Sedimentological Research Group and the Quaternary Research Association. I would like to thank the companies which provided the data and software used in this work, particularly PGS, Conoco, Chevron, Landmark, Halliburton and Texaco. Tom Bradwell and Christian Wilson at the British Geological Survey made available the Olex bathymetric dataset used in this work, and I am grateful for their assistance and enthusiasm. I would also like to thank Chris Jackson and Karla Kane from the lab at Imperial for their assistance in matters of 3D seismic reflection data, and also the support staff in the department, particularly John Dennis and Sam Delamaine. I am grateful to my colleagues Ulrike, Kerry, Veronica, Caroline, Ali, Rachel and Andrew for their friendship and support during our time at Imperial, and to Jen Milne who was always willing to put me up in London and listen to conversations about rocks. To Cath, your love and support are what has made this work possible, and I'm so glad to be in the team with you! Finally, I would like to thank my parents who have given their love and unwavering support throughout this project, and who have always had faith in me. This work is dedicated to my grandfathers: Glenn R. Conrad, 1932 - 2003 Archibald A. Stewart, 1927 - 1988 9 List of Figures Figure 2.1 Distribution of Quaternary tunnel valleys in northwest Europe. Figure 2.2 Distribution of tunnel valley studies in North America. Figure 2.3 Distribution of tunnel valley studies in north Africa, the Middle East and Australia. Figure 2.4 Longitudinal section down the Brett Valley from Boswell (1913). Figure 2.5 Model of tunnel valley formation as applied by Attig et al.
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