THE STOREGGA SLIDE COMPLEX;
REPEATED LARGE SCALE SLIDING IN RESPONSE TO CLIMATIC
CYCLICITY.
P. BRYN1, A. SOLHEIM2, K. BERG1, R. LIEN1, C. F. FORSBERG2,
H. HAFLIDASON3, D. OTTESEN4, L. RISE4
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 8.200 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.
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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).
The Storegga Slide Complex
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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.
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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
Water content
Unit weight
Sensitivity
Plasticity
Behaviour
30-40%
10-20%
20-22 kN/m3
Low
15-25
Dilatant
50-60%
25-35%
18-19 kN/m3
Higher
30-35
Contractant
30-55%
70-90%
c. 15 kN/m3
High
70-80
Contractant
3. The Storegga Slide Complex
Seven slides predating the Storegga Slide with lateral extents exceeding 2000km2 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 90000 km2, of which the slide scar is 27000 km2.
The back wall runs along the margin for nearly 300km, and the estimated slide volume
is around 3500 km3. 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 27100 km2. 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.
The Storegga Slide Complex
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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
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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.
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.
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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.
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