Kjell Berg

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.

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

Sub-Glacial Features Interpreted from 3D-Seismic

In the Norwegian Channel offshore Western Norway (Fig.1) lineated surfaces are observed on 3D-seismic (Figs. 2, 4 and 5) within the Pleistocene sediments. The lineated pattern is interpreted to be sub-glacial features, reflecting icemovement.

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.