Petter Bryn

The Storegga Slide Complex; Repeated Large Scale Sliding in Response to Climatic Cyclicity

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.

Seismic analyses of Cenozoic contourite drift development in the Northern Norwegian Sea

Four drift accumulations have been identified on the continental margin of northern Norway; the Lofoten Drift, the Vesterålen Drift, the Nyk Drift and the Sklinnadjupet Drift. Based on seismic character these drifts were found to belong to two main groups; (1) mounded, elongated, upslope accretion drifts (Lofoten Drift, Vesterålen Drift and Nyk Drift), and (2) infilling drifts (Sklinnadjupet Drift). The drifts are located on the continental slope. Mainly surface and intermediate water circulation, contrary to many North Atlantic and Antarctic drifts that are related to bottom water circulation, and sediment availability have controlled their growth. Sediments were derived both from winnowing of the shelf and upper slope and from ice sheets when present on the shelf. The main source area was the Vøring margin. This explains the high maximum average sedimentation rate of the nearby Nyk (1.2 m/ka) and Sklinnadjupet (0.5 m/ka) Drifts compared with the distal Lofoten (0.036 m/ka) and Vesterålen (0.060 m/ka) Drifts. The high sedimentation rate of the Nyk Drift, deposited during the period between the late Saalian and the late Weichselian is of the same order of magnitude as previously reported for glacigenic slope sediments deposited during glacial maximum periods only. The Sklinnadjupet Drift is infilling a paleo-slide scar. The development of the infilling drift was possible due to the available accommodation space, a slide scar acting as a sediment trap. Based on the formation of diapirs originating from the Sklinnadjupet Drift sediments we infer these sediments to have a muddy composition with relatively high water content and low density, more easily liquefied and mobilised compared with the glacigenic diamictons.

Preconditions Leading to the Holocene Trænadjupet Slide Offshore Norway

The Traenadjupet Slide (14,100 km2) remobilised an up to 180 m thick package comprising late Weichselian glacigenic sediments and an underlying late Saalian — late Weichselian contourite drift. Rapid burial of the contourites and the presence of gas, is inferred to have caused development of excess pore pressure of the contourites which probably were the “weak layer” that initially failed. During triaxial compressional tests the contourite sediments show contractive behaviour and shear band development. Shear band development due to porewater pressure increase and liquefaction of contractive sediments is therefore regarded a possible mechanism for initial failure and sediment mobilisation of the Traenadjupet Slide.

A Weak Layer Feature on the Northern Storegga Slide Escarpment

The northern Storegga Slide esearpment and the area affected by the slide has been an area of extensive geological and geotechnical studies during recent years, mainly focusing on sediment transport processes and slope failure mechanisms. The well-preserved stratigraphy established north of the Storegga Slide, also partly exposed along the northern escarpment, is considered to be the closest parallel to the geological setting of the Storegga Slide prior to the slope failure (Fig. 1, 2). The stratigraphic setting is characterised by units consisting dominantly of diamicton and/or ice-proximal upper margin deposits, and hemipelagic and/or glaciomarine seismically well-layered deposits on mid and lower margin settings (Fig. 2). The boundary of these units is regional and usually characterised by a strong seismic reflector. Occasionally, the internal layering of these units also displays a pronounced seismic character, but they have a less distinctive regional appearance. Open image in new window Fig.1. Outline of the Storegga Slide area on the mid-Norwegian margin, with the TOBI sidescan survey area at the northern Storegga Slide escarpment. The location of the airgun seismic profiles presented in Fig. 2 is shown

Gas hydrate drilling conducted on the European Margin

Since 1996, the Norwegian government has licensed hydrocarbon exploration in seven deep water areas on the continental slope north of the Norwegian Trough. Data acquired in this region, which is of interest to both scientists and the oil industry, provide an opportunity to improve understanding of the geology and development of the area through Quaternary times. Gas hydrates, slope stability, and geohazards are especially important topics for research near the Norwegian Trough.

Dynamics, Velocity and Run-Out of the Giant Storegga Slide

A huge slide (volume of 2400 km3 and run-out 450 km) was released in the Storegga area off the western coast of Norway during early Holocene, followed by numerous smaller debris flows. We perform numerical simulations of the giant slide using a Bingham model for the clay material. Agreement with present deposit distribution and run-out is found by assuming that the shear resistance between the debris flow and the seabed decreases during the flow, and we suggest sediment remolding or hydroplaning as possible explanations. Debris velocities are predicted and possible applications to the associated tsunami event are investigated.

Issues in the Assessment of Gravity Mass Flow Hazard in the Storegga Area Off the Western Norwegian Coast

Statistical analysis of the lobes of the Storegga slide reveals a power-law dependence of the runout distance on the release volume. For small to moderate volumes, visco-plastic models with a (remoulded) yield strength of about 10 kPa reproduce this dependence quite well, in contrast to granular-friction models. However, either progressive wetting of the bottom shear layer or hydroplaning has to be invoked to explain the extreme runout distance and the sediment distribution of the largest slide phase. Preliminary es timates of the turbidite volume put severe constraints on the formation rate and density of the turbidity currents accompanying the slide.

Occurrence and implications of large Lophelia-reefs offshore Mid Norway

Large, up to 45 m high and at least 8600 year old coral reefs composed of the reef-building, stony coral Lophelia pertusa (L.) occur off Mid Norway. Their locations have been documented by side-scan sonar, multi-beam echosounder, and ROV (remotely operated vehicle) surveys carried out by the hydrocarbon industry and by authorities ( Foss and Mortensen, 1998 ; Hovland and Mortensen, 1999 ; Jung et al., 2001 ; Freiwald, et al., 2002 ). So far, the following large, continental shelf- and continental slope-based reefs have been found and mapped off Mid Norway: the Sula Ridge reef (including the Haltenpipe reef cluster), the Horse-Shoe Ridge reefs (Hesteskoen), the Tr˦na Deep reefs, the Rost Bank reefs, and the Storegga escarpment reefs (Fig. 1). There are also numerous smaller coral structures of 1–3 m height, which include colonies of Lophelia pertusa . These are scattered on the general seafloor and tend to be located on bathymetric highs such as moraine ridges, on glacial flutes and on iceberg plough marks. Both the Sula Ridge reef and the Tr˦na Deep reefs are located on top of sub-cropping Mesozoic sedimentary rocks that dip towards the west. The Horse-Shoe reefs are located on top of a moraine ridge, which is located over dipping Mesozoic strata. The Storegga escarpment reefs are located on top of Quantenary marine and glacimarinc sediments. Thus, all these large reefs seem to have at least two conditions in common: They are located on top of a firm sea floor, and are on local heights. Because of their importance as feeding and breeding grounds for some of the fish species, and also because of their obvious importance as biological resources, the large coral reefs, off Mid Norway must be carefully respected by the hydrocarbon industry. Large reefs are also known to occur along the coast and in some of the fjords of Mid Norway ( Mortensen and Foss, 2001 ). This means that the existence of reefs will have to be considered in all aspects of hydrocarbon exploration and exploitation, off Mid and Northern Norway, i.e., prior to and during exploration drilling, field development, and hydrocarbon transportation.

Submarine slides on the Mid-Norwegian Continental Margin—A challenge to the oil industry

Two large submarine slides, The Storegga and the Tr˦nadjupet Slides, occurred on the Mid-Norwegian margin during the Holocene. The Ormen Lange gas field is located within the scar of the Storegga Slide. This gigantic submarine slide occurred about 8200 years ago, and caused large waves (tsunamis) that reached the coasts of Norway, Scotland, Shetland and the Faroc Islands. The objectives of this chapter are to present the challenges and the slide risk assessment related to the development the Ormen Lange gas field. The risk evaluation is based on a qualitative approach for large natural slides, and a quantitative approach for new small slides in the vicinity of the development area. The work programme includes extensive, regional multi-disciplinary studies, carried out jointly by academia, industry and research institutions. The database includes an extensive grid of seismic data, detailed sea-floor morphology and sediment properties from a number of ‘geoborings’ (combined geological and geotechnical borings to sub-bottom depths of 200–400 m). Stability of the steepest slopes in the vicinity of the development area is calculated. Effects of excess pore pressures, earthquakes, reservoir compaction during depletion and underground gas blowouts into possible permeable layers have all been included in the stability calculations. To understand the recent slide history in the area and to find the frequency of the sliding, extensive sea-floor mapping and coring to date slide events are also included. A geological model for the Plio-Pleistocene of the area explains the large-scale sliding as a response to climatic variability. Over long periods, marine deposition prevailed with focused deposition due to current effects in the locations of the Storegga and the Tr˦nadjupet Slides. During short intervals of peak glacial conditions, till and glacial debris flow sediments were deposited at high rates directly on the continental slope. This created excess pore pressures in the thick marinc deposits. The most likely triggering mechanism of the slides is a strong earthquake following the onshore uplift after the glaciation. This explains why the slides take place after a glacial period. Since all the soft unstable clays were removed from the Storegga Margin during the last slide, it is concluded that a new cycle with sedimentation of soft clays and deposition of glacial sediments in the upper slopes are needed, to create a new unstable situation in the Storegga area. At present, the slopes in the Ormen Lange area have high safety factors, and the likelihood of new slides, both local and regional, is considered very low.