The Edinburgh Geologist
Issue no 28

The BGS deep-tow boomer meets the Storegga Slide

by Dan Evans

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The Storegga Slide, located in the North Atlantic off Norway, is recognised as a large bathometric depression with a steep headwall. It is one of the worlds largest submarine slides (Bugge, 1983), and is thought that a total of 5600 cubic kilometres of sediment has been displaced from an area not much smaller than the mainland of Scotland (Figure 1). For such a world-class feature, surprisingly little research work has been carried out into the detail of its configuration and history; publications are confined to a few papers of which Tom Brugge of IKU in Norway is the lead author following his reconnaissance Ph.D study of the feature, and further work by the Institute of Oceanographic Sciences and Bergen University.

Figure 1. Location of the Storegga Slide

Bugge et al. (1987) considered that the displacement occurred during three slide events. Slide 1 was originally dated between 30 000 and 50 000 years ago, but there is now evidence that it took place later than first thought, perhaps nearer 25 000 years ago (T Bugge pers. comm.). Slide 2 occurred about 7 000 years ago, and Slide 3 soon after.

At a number of localities near to the eastern coast of Scotland, a sand deposit up to 5 metres above see level has also been dated at around 7 000 years in age. It has been proposed by Long et al. (1989) that this sand is a tsunami deposit, leading to the suggestion that the tsunami was the result of sediment displacement associated with Slide 2. Similarly dated tsunami deposits are also found in western Norway.

BGS are currently involved in an EC funded Mast II project called ENAM European North Atlantic Margin. This project aims to study sediment processes, pathways and fluxes along the European continental margin. A particularly important part of this study is an investigation into mass movement, so that the Storegga Slide has naturally become the focal point for research. In October-November 1993 a team of BGS staff took part in a studies aboard the German research vessel Meteor in order to collect profiles with the BGS Deep-Tow Boomer (DTB).

The DTB is able to collect profiles with a vertical resolution of 1 metre, with a penetration of up to 200 metres in water depths of up to 1500 metres. As its name suggests, the boomer fish is towed a long way below the sea surface,usually about two thirds of the way down the water column. To enable it to 'fly'at such depths a long length of cable is required, and the fish may be towed more than a kilometre behind the ship. This piece of equipment is ideally suited to studying the sedimentary deposits in and around the Storegga Slide. However, as the Slide extends into water depths in excess of 3 000 metres, only its upper reaches were studied.

The BGS team travelled to Trondheim to join the Meteor, which had on its previous leg sailed from Edinburgh, were the equipment had been mobilised for another project involving the DTB. The ship had sailed via the Barents Sea, where it had experienced particularly severe weather conditions in a hurricane, the ship rolling 40 degrees in both directions. It was therefore with some trepidation that we sailed on the 28 October into the late autumn North Atlantic!

Fortunately the weather was generally remarkably good during the Storegga survey (although we did run for shelter on a couple of occasions). Good profiles were collected from an area on the northern flank of the Slide. The relatively small area covered by the survey only serves to emphasise the size of the slide.

The sediments adjacent to the northern edge of Slide 1 form a sequence of sea-bed parallel reflectors, although more massive debris-flow units were recorded in the shallower depths. Locally, the regular pattern of the reflectors is broken by faults, graben, and a variety of fluid escape structures, which probably indicate the presence of gas in the sediments (Figure 2).

Figure 2. Seismic reflection profile from the Storegga Slide. Disruption of the layering indicating fluid escape structures due to degassing from the sediment.

Profiles across the Slide 1 scarp (Figure 3) show the great change in the seismic character of the disturbed sediments. They become jumbled, massive and form an irregular sea-bed, and produce strongly hyperbolic reflectors. Figure 3 also shows an almost intact slumped block immediately below the scarp. Using the DTB records an the ship's swath-bathometry system, the trace of the scarp has been mapped in detail and shown to be quite sinuous.

Figure 3. Seismic reflection profile of Slide 1 within the Storegga Slide complex showing the jumbled nature of the sediment pile. 

A particularly interesting discovery made using the DTB profiles was that there was evidence for slides that substantially predate Slide 1 of Bugge et al. (1987). Unfortunately we have very little idea of the ages of these older structures, but that they probably date back as far as the Pleistocene, and possibly into the Neogene. As another part of the ENAM project, workers from the University of Bergen have also identified pre-Slide 1 slide structures on the southern flank of the Storegga Slide in the North Sea Fan.

It is clear that the Storegga area has been a major zone of instability for a long time, and is likely to remain so in the future. The triggers for slide propagation were probably earthquakes, for this region is the most seismically active part of the UK-Norway area, and the northern flank of the Storegga Slide lies along the line of the Jan Mayen Fracture Zone. The presence of gas may have facilitated movement, and the high sedimentation rate was probably an important tactor for the slides on the North Sea Fan to the south of the Slide.

Following the Storegga survey, the Meteor sailed north to the East Greenland margin, allowing us a distant view of Jan Mayen. After experiencing our own brief hurricane with a maximum wind speed of 93 knots, we sailed for Kiel and berthed on the 26 November in a Christmas card winter scene to be welcomed with a gluwein party. This was a pleasant end to the successful Storegga DTB survey which provided excellent data and valuable new insights into the nature and history of the slide area.

References

BUGGE, T. 1983. Submarine slides on the Norwegian continental margin, with special emphasis on the Storegga area. Publikas jon Institut for Kontinentalsokke Lundersookelser, 110, 152.

BUGGE, T. and seven others. 1987. A giant three-stage submarine slide off Norway. Geo-rnarine Letters, 7, 191-198.

LONG, D., SMITH, D.E. & DAWSON, A.G. 1989. A Holocene tsunami deposit in eastern Scotland. Journal of Quaternary Science, 47 61-66.

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Author: Dan Evans, Marine Geology and Operations Group, British Geological Survey, Edinburgh.

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