The Mjølnir Impact Event and its Consequences (eBook)

Preface 5
Acknowledgements 7
Contents 8
Contributors 10
1 Introduction 12
1.1 Background 12
1.2 Barents Sea Geology 15
1.3 Mjlnir Impact at Volgian/Ryazanian Boundary 21
1.4 The Investigation History of Mjlnir 23
1.5 The Search for Oil and Gas in the Barents Sea 29
1.6 Future Mjlnir Studies 30
1.7 Etymology 33
2 Geological Framework 34
2.1 Plate Tectonic Evolution of the Arctic 34
2.2 Mesozoic Stratigraphy and Depositional Environments of the Arctic 35
2.2.1 Geological and Palaeogeographical Setting 37
2.2.1.1 Cretaceous Palaeogeographic Setting 37
2.2.1.2 The Barents Sea in Time and Space 38
2.2.2 Svalbard 39
2.2.3 Barents Sea 45
2.2.4 Greenland 47
2.2.5 Siberia 51
2.2.6 Late Jurassic and Early Cretaceous Depositional Configuration 53
3 Impact Structure and Morphology 57
3.1 Seismic Reflection Database 57
3.2 Shallow Structure 62
3.2.1 Main Features 62
3.2.2 Detailed Seismic Correlation to Nearby Shallow Boreholes 66
3.2.2.1 Borehole 7430/10-U-01 66
3.2.2.2 Borehole 7329/03-U-01 71
3.2.2.3 Impact Timing as Revealed from Seismic Correlation 73
3.2.3 Impact-Induced Deformation 74
3.2.4 Near-Field Erosional Features 76
3.2.4.1 Resurge Gullies 76
3.2.4.2 Crater Rim 78
3.3 Deep Structure 79
3.3.1 Impact-Induced Disturbance 79
3.3.1.1 Seismic Reflectivity Patterns 79
3.3.1.2 Shape and Dimensions 82
4 Impact Geophysics and Modelling 84
4.1 Features Related to the Cratering Process 84
4.1.1 Excavated Crater and Breccia 84
4.1.2 Impact Melts 90
4.1.3 Gravitational Collapse 91
4.1.4 Structural Uplift 92
4.2 Impact into a Marine Sedimentary Basin 94
4.3 Impact Crater Modelling 98
4.3.1 Potential Field Data 98
4.3.2 Marine Gravity Anomalies and Modelling 100
4.3.3 Marine Magnetic Anomalies and Modelling 106
4.3.4 Traveltime/Velocity Anomalies and Modelling 108
4.4 Modelled Porosity Anomalies 110
4.4.1 Density-Derived Porosity Anomaly 111
4.4.2 Velocity-Derived Porosity Anomaly 113
4.4.3 Postimpact Deformation-Derived Porosity Anomaly 113
4.4.4 Porosity Anomaly and Pore Space Volume 115
4.4.5 Porosity Anomaly and Hydrocarbon Potential 116
4.5 Potential Non-impact Origin 119
4.5.1 Clay Diapir 120
4.5.2 Salt Diapir 120
4.5.3 Igneous Feature 123
4.6 Alternative Interpretation of Mjlnir Crater Dimensions Based on Regional Gravity and Aero-magnetic Profiles and Modelling 123
4.6.1 The Mjølnir Aero-magnetic Anomaly 124
4.6.2 The Mjølnir Regional Free-Air Gravity Anomaly 126
4.6.3 Alternative Interpretation of Mjølnir Crater Dimensions 127
4.7 Impact-Induced Changes in Physical Properties 132
4.8 Mjlnir as an Oblique Impact Event 134
4.8.1 Elongated Crater Diameter 134
4.8.2 Seismic Disturbance Asymmetry 135
4.8.3 Peak-Ring Character 137
4.8.4 Offsets in Brecciation and Structural Uplift 138
4.8.5 Impact Direction and Angle 139
4.8.6 Mjølnir Impact Obliquity Constrains Models for Near-Field Perturbations 142
4.8.6.1 Nature and Distribution of Proximal Ejecta 143
4.8.6.2 Tsunami-Wave Distribution 144
5 Impact Cratering and Post-impact Sedimentation 147
5.1 Introduction 147
5.2 The Mjlnir Crater Core (7329/03-U-01) 148
5.2.1 The Ragnarok Formation 148
5.2.2 Ragnarok Formation, Unit I 153
5.2.3 Ragnarok Formation, Unit II 155
5.2.4 Hekkingen Formation 164
5.2.5 Klippfisk Formation 165
5.2.6 Spectral Gamma Results 167
5.2.7 Paleontology of the Ragnarok Formation 167
5.2.8 Paleontology of the Hekkingen Formation 169
5.2.9 Magnetic Properties and Densities of the Mjølnir Crater Core (7329/03-U-01) 170
5.3 The Mjlnir Impact Event in a Sequence Stratigraphical Framework 170
5.4 The Evidence for Impact Crater Formation 175
5.4.1 The Crater: Its Structure and Shape 175
5.4.2 Fracturing and Conglomerates 176
5.4.3 Mineralogical Evidence of Impact Cratering 176
5.4.4 Geochemistry 178
5.4.5 Paleontological Evidence of Impact Cratering 181
6 Ejecta Geology 183
6.1 The Identification of Ejecta Beds 183
6.1.1 Introduction 183
6.1.2 The Ragnarok Formation and Sindre Bed 184
6.1.3 The Discoveries of Large Amounts of Soot in Mjølnir Related Sediments 190
6.2 The Stratigraphical Distribution of the Ejecta Beds 198
6.2.1 Borehole 7430/10-U-01 199
6.2.2 Borehole 7018/05-U-01 201
6.2.3 Janusfjellet, Central Spitsbergen 201
6.2.4 Nordvik Peninsula, North-Western Siberia 202
6.2.5 The Mjølnir Ejecta as a Regional Stratigraphic Marker 202
7 The Impact Dynamics 203
7.1 Introduction 203
7.2 Numerical Model 204
7.3 Cratering Process 205
7.4 Ejecta Formation and Distribution 210
7.5 Resurge Flow and Tsunami Generation 215
7.6 Conclusions 217
8 Structural Analysis of Deformed Central Peak Sediments 218
8.1 Structural Position of the Mjlnir Impact Crater 218
8.2 Structural Geological Analysis 219
8.2.1 Type A Structures: Early Extensional Micro-faults and Fissures 220
8.2.2 Type B-Structures: Fragmentation of Semi-consolidated or Consolidated Beds 224
8.2.3 Type C-Structures: Liquefaction and Shearing 225
8.2.4 Type D-Structures: Folds, Rotated Strata and Shear Bands 227
8.2.5 Type E-Structures: Intensely Sheared Sequences 228
8.2.6 Type F-Structures: Late Brittle Fractures and Microfaults 229
8.3 Deformation History of the Ragnarok Formation 231
9 Postimpact Deformation Due to Sediment Loading: The Mjlnir Paradigm 236
9.1 Postimpact Burial 236
9.2 Mjlnir Crater 238
9.2.1 Postimpact Infilling 239
9.2.2 Faulting and Differential Vertical Movements 241
9.3 Other Craters: Chesapeake Bay, Chicxulub, Bosumtwi, and Montagnais 242
9.4 Original Crater Relief Reconstruction 246
9.4.1 Mjølnir 248
9.4.2 Chicxulub 252
9.4.3 Bosumtwi 256
9.4.4 Chesapeake Bay 257
9.5 Correction of Crater Morphological and Structural Parameters 259
9.5.1 Parameters Prone to Postimpact Burial Modification 259
9.5.2 Postimpact Modification Correction Factor 262
10 The Mjlnir Tsunami 264
10.1 Introduction 264
10.2 Tsunami Models 265
10.3 Tsunami Generation 267
10.3.1 Near Field Evolution 268
10.3.2 Far Field Propagation 273
10.3.2.1 Estimates of Far-Field Behaviour 273
10.3.2.2 Computations of Far-Field Behaviour 275
10.4 Discussion 277
References 280
Index 301