THE STOREGGA SLIDE COMPLEX; REPEATED LARGE SCALE SLIDING IN RESPONSE TO CLIMATIC CYCLICITY.

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1 THE STOREGGA SLIDE COMPLEX; REPEATED LARGE SCALE SLIDING IN RESPONSE TO CLIMATIC CYCLICITY. P. BRYN 1, A. SOLHEIM 2, K. BERG 1, R. LIEN 1, C. F. FORSBERG 2, H. HAFLIDASON 3, D. OTTESEN 4, L. RISE 4 1 Norsk Hydro, N-0246 Oslo, Norway 2 NGI POB 3930 Ullevaal Stadion, N-0806 Oslo, Norway 3 Department of Geology, University of Bergen, N-5007 Bergen, Norway 4 NGU, N-7491 Trondheim, Norway Abstract The Holocene Storegga Slide is the last of a series of slides occurring in the same area during the last 500ky. The objectives of the present paper are to present the current understanding of the trigger mechanisms and development of the Storegga Slide, and to show the link between the sliding and Pleistocene climatic fluctuations in the area. Instability is created by the rapid loading of fine-grained hemipelagic deposits and oozes by rapid glacial deposition during peak glaciations. Postglacial earthquake activity was the most likely trigger. Although slide development is complicated and involves a number of slide mechanisms and processes, the overall development is retrogressive, starting at the mid- to lower slope. Sliding stops when the headwall reaches the flat lying, overconsolidated glacial deposits of the shelf. Keywords: NE Atlantic, Storegga Slide, glaciation, submarine slide, glacial deposition. 1. Introduction The gigantic submarine Storegga Slide (Fig. 1) occurred about calendar years ago, and caused a tsunami that reached the surrounding coasts. The Ormen Lange gas field is located within the scar of the Storegga Slide and extensive studies have been undertaken to explain the slide, and to evaluate the present stability conditions in the vicinity of the Ormen Lange gas field. The data base includes a regional grid of high-resolution 2D seismic lines as well as exploration 2D and 3D seismic data, detailed swath bathymetric data, sub-bottom profiler data, oceanographic data, and geotechnical and geological data from boreholes. The latter encompasses samples, wire-line logs and in-situ pore pressure measurements and monitoring. The two main objectives of this paper are: To present an updated interpretation of slide triggering and development on the low slopes of the Mid-Norwegian continental slope. To show the close link between the Pleistocene climatic fluctuations and the slides on this glaciated continental margin. 215

2 216 Bryn et al. Figure 1. Location of the Holocene Storegga and Trænadjupet slides on the Norwegian continental margin. Location of the Ormen Lange gas field and the Solsikke dome structure, as well as the Skjoldryggen and North Sea Fan Pleistocene depocenters are shown. 2. Geological setting Thick units of biogenic ooze, belonging to the Eocene to Late Pliocene Brygge and Kai formations, were deposited in the basins and deeper parts of the margin prior to the main Late Pliocene continental uplift and progradation of a clastic sediment wedge. The first glacial advance to the shelf break off Mid-Norway occurred at about 1.1 Ma (Haflidason et al., 1991). After a period of more limited ice extent repeated advances to the shelf break have occurred since about 0.5Ma (Sejrup et al., 2000). After the onset of continental shelf glaciations, the main periods of sediment transfer have been the relatively short periods of peak glaciation. During these periods, fast flowing ice streams (Alley et al., 1989) developed in transverse shelf troughs (Fig. 2) (Ottesen et al., 2000). Glacial extent to the shelf break, which limits the lifetime of fast flowing ice streams, may be as short as 10-20% of a glacial interglacial cycle (Elverhøi et al., 1995, Dowdeswell & Elverhøi, 2002). Dahlgren and Vorren (in press) show ice sheet presence at the shelf break off Mid-Norway for 7 Ky during the Late Weichselian, which is the maximum situation of the Weichselian. The glacial sediments, brought out through layers of deformable till under ice-streams, were rapidly deposited on the outer margin, and subsequently transported further down slope as glacial debris flows to form extensive glacial fan systems (Solheim et al., 1998, Dimakis et al., 2000). The two most important depocenters for glacial sediments are in the North Sea Fan and Skjoldryggen areas. The Storegga Slide is located in the depression between these main glacial depocenters (Fig. 2).

3 The Storegga Slide Complex 217 Figure 2. The main depocenters of glacial clays (green) seen in relation to past ice stream locations. Yellow fields mark contouritic drift deposits (Fig.3), overlain by the glacial fan deposits. (NSF=North Sea Fan). Figure 3. Location of the main contouritic drifts (yellow), controlled by the seabed topography, current direction and the position of the thermocline. Sediment supply to the Storegga area is from erosion of the North Sea Fan and areas further south. During the considerably longer periods of more reduced ice cover, including interglacial periods, normal marine and/or distal glacial marine deposition prevailed on the slope and outer shelf. The oceanic current system was relatively sluggish during peak glaciations, due to reduced inflow of Atlantic water (Lassen et al., 2000). The current activity had a profound influence during the periods of reduced or non-existent ice cover, however. Seismic sections show stratified, mounded deposits, partly containing upslope climbing structures, that are interpreted to be contouritic sediment drifts deposited during these intervals of reduced ice cover, and may reach thicknesses exceeding 100 m, partly filling in sea floor depressions such as old slide scars. Similar infill was reported by Laberg et al. (2001) for an area to the north of the present study area.

4 218 Bryn et al. The three main sediment types in the Storegga area are pre-glacial biogenic oozes, glacial tills and debris flows, and distal glacial marine to normal marine fine-grained deposits. These three sediment types have distinctly different lithology and geotechnical properties (Table 1). Whereas the marine clays and oozes have only very small sand and silt contents, the glacial sediments are unsorted diamictons. With regards to geotechnical behaviour, however, they all classify as clays. Table 1. Physical properties of the main sediment types in the Storegga area. The contractant behaviour implies that distinct shear planes are developed during triaxial testing. Oozes are mainly of the Oligocene to Late Pliocene Brygge and Kai Formations. Property Glacial clay Marine clay Ooze Clay content 30-40% 50-60% 30-55% Water content 10-20% 25-35% 70-90% Unit weight kn/m kn/m 3 c. 15 kn/m 3 Sensitivity Low Higher High Plasticity Behaviour Dilatant Contractant Contractant 3. The Storegga Slide Complex Seven slides predating the Storegga Slide with lateral extents exceeding 2000km 2 have been mapped in this part of the margin (Fig. 4). Five of these partly underlie the Holocene Storegga Slide. The Storegga Slide covers an area of km 2, of which the slide scar is km 2. The back wall runs along the margin for nearly 300km, and the estimated slide volume is around 3500 km 3. In the Ormen Lange area, three main slip planes have been found. All occur in the fine-grained marine clay units. The pre-storegga slides vary in lateral extents from 2400 to km 2. With the exception of the Sklinnadjupet and Vigrid Slides (Fig. 4), all slides are concentrated in the Storegga area and the eastern part of the North Sea Fan. Hence, this area seems to be particularly slide prone. The Storegga Slide and the older slides display several similar features, indicating similar slide mechanisms, slide development and relationship to the regional geology. The main common characteristics include: The layer-parallel slip surfaces are found in seismically stratified marine clay units. The slip surfaces jump between different stratigraphic levels, in particular when approaching the headwall of the slides. Rotated blocks with relatively modest total displacement and little internal remoulding are common near the headwalls. The headwalls seem to have been stable during the time needed to fill in the slide scar.

5 The Storegga Slide Complex 219 Figure 4. Map showing the outline of pre Holocene slides in the Storegga region. The Storegga Slide is outlined in red, and the Ormen Lange gas field is shown. Tampen, Møre, Slide S, etc. are informal names given to each of the paleoslides. Green filled circle shows the location of the inset data example. This shows features of Slide R, including the headwall, rotated blocks near the headwall, an intact but faulted slide block (thick yellow line), infill of slide debris, and glide plane in stratified marine deposits. Note also the incipient failure below the glide plane (stippled green Despite relatively sparse age control, the slides appear to have occurred on a semiregular basis during the last 0.5 My, in good agreement with the main continental shelf glaciations that commenced at about the same time and occurred repeatedly thereafter. This points at a close link between cycles of glacial deposition and the large scale sliding in this area. A similar connection is envisaged for the younger Trænadjupet Slide further north (Laberg et al., 2002). 4. Slide mechanisms Instability was most likely created by excess pore pressure and the reduced the effective strength of a sedimentary unit. Excess pressures of 20% have been measured in marine clay units outside the main slide scar. When fine grained, water rich marine clays are rapidly overlain by glacial debris flows on the continental slope during peak glaciations, excess pore pressures may build up. Permeability is a key factor, however, both in trapping excess pressure, and also in transferring excess pore pressure laterally to potentially more unstable areas, e.g. with less overburden and steeper slopes. Sliding is unlikely to occur unless an external trigger is applied. The most likely trigger is earthquake activity, which was at its highest in Scandinavia between 10 and 7 ka as a response to glacioisostatic rebound. Recent modelling of earthquakes generated from faults in the area show that the time of ground shaking can be up to 1-2minutes due to

6 220 Bryn et al. the effect of the deep Møre sedimentary basin (NORSAR, 2002). The seismic activity offshore cluster in and around Plio- Pleistocene depocenters, such as the North Sea Fan (NORSAR & IKU, 1998). The combination of locally steeper slopes, near-exposure of pre-glacial ooze, and distance to earthquake centres favours an initial slide in the Solsikke Dome area (Figs. 1&5). The largest sediment output during the Late Weichselian glaciation was in the central North Sea Fan, causing compaction of the underlying oozes of the Eocene to Late Pliocene Brygge and Kai formations. Permeability measurements of the Brygge Fm. ooze, indicate 100 times higher permeability than in the clays of the Naust Fm. Hence, transfer of excess pore pressure caused by glacial fan loading, through the oozes to the Solsikke Dome is likely (Fig. 5). This situation is likely to have occurred during older glacial-interglacial cycles, and is therefore believed to be important also for other slides of the Storegga Slide Complex. Figure 5. Schematic section across the lower part of the Storegga Slide. The sediment load from the North Sea Fan (left arrow) caused compaction and pressure transfer through the Brygge Fm. oozes to the Solsikke Dome (SOL), which may have been the site for initial sliding. Red line in inset map shows the line location. See figure 1 for location of the profile and the Solsikke Dome. Further slide development involved several slide processes, as seen from the complicated morphology of the slide scar, including crosscutting and overlying lobes, shear zones and slide impact compression zones. However, the main overall slide development is interpreted and modelled to happen retrogressively, i.e. upslope headwall retreat. When the headwall reached the near horizontal and overconsolidated (ice loaded) strata on the continental shelf, the sliding stopped. The last sediment blocks to be released from the headwall show slight rotation but minor lateral movement. The sensitivity of the marine clays is higher than for the glacial clay at the same consolidation stress and makes these layers the preferred lateral slip planes (Fig. 6) (Kvalstad et. al. in press). The present day headwall consists of stable over-consolidated glacial clays. A new cycle with current controlled infill of soft marine clays in the slide scar has started. However, a prolonged period of marine deposition, followed by a new glacial advance to the shelf edge and rapid deposition of glacial clays seem to be required in order to create a new unstable situation in the area.

7 The Storegga Slide Complex 221 Figure 6. Illustration of the cyclic deposition and slide processes in the Storegga area. Arrows in the lower panel indicate marine, hemipelagic deposition, in which the preferred glide planes are found. (Green = glacial clays. Red = slide deposits. Blue = marine clays) (From NGU, 2002). 5. Conclusions The Storegga Slide occurred 8200 years ago and is the last of a series of slides, which have taken place at semi-regular intervals during the last 500 Ky. Preconditions appear to have been similar for all the slides. A sedimentary succession in response to glacial interglacial variability, consisting of fine grained marine clays, rapidly overlain by dense, less sorted glacial clays creates excess pore pressures and cause instability. Glide planes are developed in the marine clays, which are geotechnically weaker than the glacial deposits and show contractant behaviour upon loading. Initial failure of the Storegga Slide may have been facilitated by the loading of biogenic oozes in the North Sea Fan during the Late Weichselian glaciation The most likely trigger is a strong earthquake or a series of earthquakes associated with glacioisostatic rebound. The slides probably developed retrogressively, with an initial slide occurring on the mid- to lower slope, and retardation to a final halt when the flat-lying overconsolidated glacial shelf deposits are reached. Headwalls of older slides have remained stable until the slide scars have been filled by subsequent deposition. The present understanding requires a new continental shelf glaciation for regional instability to re-occur.

8 222 Bryn et al. 6. Acknowledgements This study is based on the efforts of a number of institutions and individuals in addition to the authors. A sincere thanks is given to all these, as well as to the partner companies (AS Norske Shell, BP, EXXON-Mobil, Petoro and Statoil) in the Ormen Lange license. Anders Elverhøi and Jan Sverre Laberg are acknowledged for reviewing the manuscript. 7. References Alley, R. B., Blankenship, D. D., Rooney, S. T., and Bentley, C. R. 1989: Sedimentation beneath ice shelves - The view from Ice Stream B. Marine Geology, 85, Bryn, P., Berg, K., Lien, R., Solheim, A., Ottesen, D. & Rise, L., in press. The Storegga Geomodel and its use in slide risk evaluation. International Conference Offshore Site Investigations and Geotechnics, Sustainability through Diversity, London, Nov., Dahlgren, T, & Vorren, T.O., in press. Sedimentary environment and glacial history during the last 40 ka of the Vøring continental margin, mid-norway. Marine Geology. Dimakis, P., Elverhøi, A., Høeg, K., Solheim, A., Harbitz, C.B., Laberg, J.S., Vorren, T.O. & Marr, J., Submarine slope stability on high-latitude glaciated Svalbard-Barents Sea margin. Marine Geology, 162: Dowdeswell, J.A. & Elverhøi, A., The timing and initiation of fast-flowing ice streams during a glacial cycle inferred from glacimarine sedimentation. Marine Geology, 188, Elverhøi, A., Svendsen, J.I., Solheim, A., Andersen, E.S., Milliman, J.D., Mangerud, J. & Hooke, R.L.B., Late Quaternary sediment yield from the high Arctic Svalbard area. J. Geology, 103, Haflidason, H., Aarseth, I., Haugen, J.E., Sejrup, H.P., Løvlie, R, & Reither, E Quaternary stratigraphy of the Draugen area, Mid-Norwegian Shelf. Marine Geology, 101, Kvalstad,T.J., Gauer,P., Kaynia, A.M., Nadim, F. & Bryn, P., in press. Slope Stability at Ormen Lange. International Conference Offshore Site Investigation and Geotechnics, Sustainability through Diversity, November 2002 Laberg, J.S., Dahlgren, T., Vorren, T.O., Haflidason, H. & Bryn, P., Seismic analyses of Cenozoic contourite drift development in the northern Norwegian Sea. Marine Geophysical Researches, 22, Laberg, J.S., Vorren, T.O., Mienert, J., Haflidason, H., Bryn, P. & Lien, R., Preconditions leading to the Holocene Trænadjupet Slide offshore Norway. This volume. Lassen, S., Jansen, E., Knudsen, K.L., Kristensen, M. & Christensen, K., Paleoceanographic changes and foraminiferal response in the Faeroe region, NE Atlantic, during the last years. ENAM Final Scientific Report: NGU, Large scale development of the mid Norwegian shelf and margin with emphasis on the last 3 million years. NGU Report , 190pp. NORSAR and IKU, Earthquake hazard update and slope stability evaluation; Active faulting and earthquake loading. Report to Norsk Hydro. 141pp. NORSAR, Site specific hazard for the Ormen Lange field and postglacial earthquakes in the larger Møre region. Report to Norsk Hydro. Ottesen, D., Olsen, L. & Thorsnes, T., Ice sheet dynamics on the mid-norwegian continental shelf based on regional and detailed bathymetric and seismic data. NGU Report , 52pp. Sejrup, H.P., Larsen, E., Landvik, J., King, E.L., Haflidason, H. & Nesje, A., Quaternary glaciations in southern Fennoscandia: evidence from southwestern Norway and the northern North Sea region. Quaternary Science Reviews 19 (2000) Solheim, A., Faleide, J. I., Andersen, E. S., Elverhøi, A., Forsberg, C. F., Vanneste, K., Uenzelmann-Neben, G., and Channell, J. E. T. 1998: Late Cenozoic seismic stratigraphy and glacial geological development of the East Greenland and Svalbard-Barents Sea continental margins. Quaternary Science Reviews, 17,