PRECONDITIONS LEADING TO THE HOLOCENE TRÆNADJUPET SLIDE
OFFSHORE NORWAY
J. S. LABERG, T. O. VORREN, J. MIENERT
Department of Geology, University of Tromsø, N-9037 Tromsø, Norway
H. HAFLIDASON
Department of Geology, University of Bergen, N-5007 Bergen, Norway
P. BRYN, R. LIEN
Norsk Hydro, N-0246 Oslo, Norway
Abstract
The Trænadjupet 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 Trænadjupet Slide.
Keywords: Submarine slide, contourites, contractive, shear band, liquefaction.
1. Introduction
Understanding the causes of submarine slides is crucial for continental slope stability
evaluation. However, the preconditions leading to failure as well as the triggering of
many of the large submarine slides throughout the world is still not well understood (e.g.
Lee et al., 1991; Hampton et al., 1996; Locat and Lee, 2000; Mienert et al., in press).
This is partly because information on the physical and geotecnical properties of the
failed sediments, including the slip plane sediments, is scarce (Pratson, 2001). In most
cases, only samples from short gravity and piston cores are available while some of the
largest slope failures involve more than 100 m of the subsea-floor strata (e.g. Bugge et
al., 1988; Laberg and Vorren, 2000).
During the Holocene three large submarine slides occurred on the continental slope of
the passive Norwegian continental margin, the Storegga, Trænadjupet and Andøya
Slides (Bugge et al, 1987; Kenyon, 1987; Dowdeswell et al., 1996; Vorren et al., 1998;
Laberg et al. 2002a). Recently, a number of geotechnical borings, penetrating up to 300
m below the sea floor have been conducted for risk assessment studies of the continental
slope as part of deep-water oil and gas field development (Bryn et al., 1998). Based on
sedimentological, chronostratigraphic and geotechnical analyses of sediments from one
of these boreholes, 6606/3-GB1, as well as seismic data, we discuss the preconditions
leading to failure of the Trænadjupet Slide (Fig. 1).
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Figure 1. Bathymetric map (from Perry et al., 1980) including the location of the Trænadjupet Slide and the
Nyk Drift. The location of borehole 6606/3-GB1 and Figures 2 and 3 is also shown.
2. Materials and methods
The geological and chronological analyses from borehole 6606/3-GB1 were performed
by the University of Bergen (Haflidason et al., 1998). All the geotechnical analyses of
the sediments were undertaken by the Norwegian Geotechnical Institute, and both the
offshore and onshore analyses and results are detailed in their reports (Norwegian
Geotechnical Institute, 1998; 2002). The carbonate content was measured by means of a
LECO-carbon analyser at the University of Bergen. Three samples were dated by the 14C
AMS method. The dating was done on handpicked planktic foraminifers of a single
species, Neogloboquadrina pachyderma. All dating results are reported in 14C years BP
and have been corrected for an 800 years reservoir age according to Haflidason et al.
(1995). The high-resolution single-channel seismic data were acquired using an airgun
array of two 0.6 litre sleeve guns with filter setting of the Geopulse Receiver at 100-700
Hz.
Figure 2. Alongslope oriented seismic profile UiTø 01-039 from borehole 6606/3-GB1 across the upper
Trænadjupet Slide scar (see Fig. 1 for location). The Trænadjupet Slide remobilized late Weichselian
glacigenic sediments and part of the underlying Nyk contourite drift. The glacigenic and contouritic
sediments can be followed alongslope to the drill site. See text for further discussion.
Preconditions leading to the Trænadjupet
249
3. The failed sediments: properties, sedimentation rate and origin
The Trænadjupet Slide affected an area of about 14,100 km2 and remobilized an up to
180 m thick sediment package comprising glacigenic sediments (~160 m) and an
underlying contourite (~20 m) (Laberg and Vorren, 2000) (Figs. 2 and 3). A glide plane
developed in the lower part of the contourite sequence (Fig. 2), and indications of
remobilization of this sequence are also seen outside the slide affected area (Fig. 3). The
failed succession was correlated to geotechnical borehole 6606/3-GB1 located at 848 m
water depth about 100 km south-west of the Trænadjupet Slide scar using an along-slope
oriented high-resolution seismic profile (Figs. 2 and 4). The drilling was terminated 105
m below sea floor, with a total recovery of c. 15 %. The drilled succession included a
thin (< 1 m) veneer of Holocene sediments inferred to represent part of the winnowing
lag identified by Holtedahl and Bjerkli (1975), a ~40 m thick unit of late Weichselian
glacigenic sediments, a ~40 m thick contourite drift sequence, and part of the underlying
late Saalian glacigenic sediments (Fig. 4).
Figure 3. Downslope oriented seismic profile UiTø 00-002 located immediately outside the area affected by
the Trænadjupet Slide (see Fig. 1 for location). An irregular upper reflection and a chaotic internal signature
(A) and a discontinuous upper reflection further downslope (B) indicate remobilization of part of the Nyk
contourite drift sediments also in this area outside the Trænadjupet Slide.
The late Weichselian glacigenic sediments are a homogeneous grey diamicton (25 %
clay, 45 % silt, 30 % sand) characterised by an average water content of ~20% and an
average unit weight of about 21 kN/m3 (Tab. 1). These sediments can be classified as
inorganic clays of low to medium plasticity and they have a sensitivity of about 1.5. The
organic carbon and CaCO3 content is uniform and on average 0.5% and 6.8%,
respectively. Triaxial compression tests showed a dilatant behaviour of these sediments
and no shear band development was identified (Tab. 1; Fig. 5). The debris flow
sediments were transported to the shelf break mainly as a subglacial, deformable till and
deposited on the upper slope in front of the Fennoscandian Ice Sheet during the late
Weichselian glacial maximum (Laberg et al., 2002b; Dahlgren and Vorren, in press).
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Figure 4. Recovery, lithology, origin and age of the sediments recovered at borehole 6606/3-GB1. Stars shows
levels were indications of gas have been identified. The Holocene sediments are inferred to represent part of
the winnowing lag identified by Holtedahl and Bjerkli (1975).
14
C AMS dating of the youngest unit immediately overlying the late Weichselian
glacigenic sediments in borehole 6606/3-GB1 gave 15,510 +/- 130 14C years BP
(reservoir corrected) at 2.1 m, while from the base of the glacigenic sediments at 44.3 m
depth an age of 17,870 +/- 135 14C years BP (reservoir corrected) was found (Fig. 4,
Tab. 2). Applying these dating results the average sedimentation rate in the area of the
Trænadjupet Slide was calculated at ~65 m/ka during the late Weichselian glacial
maximum.
The underlying Nyk Contourite Drift sediments (Laberg et al., 2001) comprise more
than 50 % clay and up to 40 % silt. The average water content is ~40 %, the average unit
weight about 18 kN/m3. On the plasticity chart, these sediments mainly plot in the group
of inorganic clays with high plasticity, and they have a sensitivity higher than 2.5 (Tab.
1).
Preconditions leading to the Trænadjupet
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Figure 5. Two samples from borehole 6606/3-GB1 of the late Weichselian glacigenic sediments at 40.2 m
depth (A) and the Nyk contourite drift at 67.5 m depth (B) following triaxial testing. In sample (B) shear band
development is clearly seen, while similar features did not develop in (A).
The average organic carbon and CaCO3 content is 0.4% and 15.8% respectively, with
increasing values downcore, up to 0.6 % and 35 % at 71.5 m depth. During drilling, gas
bubbles and cracking were reported from 45 to 50 and 70 to 75 m depths (Fig. 4). For
these sediments triaxial compression tests showed contractant behaviour (Tab. 1) and
development of shear bands (Fig. 5). The contourite sediments were deposited from an
alongslope flowing ocean current on the northernmost part of the Mid-Norwegian
continental slope during the late Saalian to the late Weichselian (Fig. 4, Tab. 2). The
maximum average sedimentation rate was up to 1.2 m/ka (Laberg et al., 2001).
Table 1. Grain-size distribution, physical and geotechnical properties of the late Weichselian glacigenic
sediments and the Nyk contourite drift sediments. 1) Sieving of fraction > 75µm, fall drop method for the
fraction < 75 µm. N = 7 (glacigenic sediments) and 11 (contourite sediments). 2) Mass of water expressed as
a percentage of the mass of solids. N = 25 and 23. 3) Density is determined by measuring its diameter and
length and then weighing it. Unit weight is density x g. N = 26 and 21. 4) According to standard methods
described in the Norwegian Standards 8002 and 8003 (Norges Byggstandardiseringsråd 1982a and b). N = 6
and 11. 5) Using the fall cone test on an undisturbed and on a remoulded sample. N = 7 and 9. 6) Using an
LECO-carbon analyser. N = 5 and 4. 7) Using an LECO-carbon analyser. N = 5 and 4. 8) Anisotropically
consolidated, undrained triaxial compression tests. Berre (1982) describes the triaxial equipment and method
used. N = 6 and 7.
4. Preconditions leading to failure
Shearing resistance is one important factor controlling the initiation of sliding. Shearing
resistance is the result of resistance to movement at interparticle contacts. Sediment
density (and water content) is a good indicator of shearing resistance, as increased
density generally implies an increase in interparticle contact area and, thus, in shearing
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resistance (Terzaghi et al., 1996). The contourite sediments have markedly lower density
and higher water content than the overlying glacigenic sediments (Table 1) indicating
low shearing resistance within these sediments.
The initial low shearing resistance may have been reduced even more during burial due
to increased excess pore pressure caused by rapid deposition of overlying sediments.
Accumulation of gas may have further contributed to excess pore pressure build-up
(Allen, 1997). Our dating results show that during the late Weichselian the contourite
sediments were rapidly buried by the glacigenic sediments. In the area where the
Trænadjupet Slide occurred the late Weichselian glacigenic sediments were up to 160 m
thick, nearly three times the thickness at borehole 6606/3-GB1 (Fig. 2). This implies that
the shearing resistance of the underlying contourite sediments probably was markedly
lower in the area of sliding than at the drill site, because the more rapid sediment loading
gave a more pronounced excess pore pressure build-up. The glide plane developed in the
lower part of the contourite sediments where gas was identified in borehole 6606/3GB1. Thus the presence of gas may have decreased the shearing resistance even more at
the lower level of the contourite unit.
Laboratory triaxial testing showed contractant behaviour of the contourite sediments. A
contractant behaviour is characteristic of sediments of low pre-shear density. It occurs
after the more tightly packing when porewater can not leave the system immediately
causing excess porewater pressure. This gives decrease in effective stress (Terzaghi et
al., 1996; Maltman, 1994). The triaxial testing also showed that the strain was
concentrated in shear bands (Fig. 5), a phenomenon previously reported from laboratory
experiments and field observations (e.g. Besuelle et al., 2000; Desrues and Chambon, in
press and references therein). From our data it is not possible to identify the way by
which localisation of the shear bands was initiated. However, the fact that they
developed in contractive and not dilative sediment lead us to suggest that the pore
pressure could be important, as suggested by Sabatini and Finno (1996). If this is correct
a possible mechanism for initial failure and sediment mobilisation into flow could be
shear band development due to porewater pressure increase and liquefaction of
contractive sediments. Dilative clay-rich sediments characterised by higher density, as
the late Weichselian glacigenic sediments, do most likely not fail under undrained
conditions because it is difficult to introduce enough water to permit flow (Fleming et
al., 1989; Terzaghi et al., 1996). This indicates that the glacigenic sediments probably
were re-mobilised as a result of initial failure in the underlying contourite sediments.
Table 2. AMS-radiocarbon dates obtained from borehole 6606/3-GB1.
Preconditions leading to the Trænadjupet
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5. Conclusions
1.
2.
3.
4.
The Trænadjupet Slide remobilised an up to 180 m thick package containing late
Weichselian glacigenic sediments and an underlying late Saalian – late Weichselian
contourite drift about 4000 years ago.
The contourites are muddier (~50% clay, ~40% silt, ~10% sand), and characterised
by a markedly lower unit weight (~18 kN/m3) and higher water content (~40%) than
the overlying glacigenic sediments (~25% clay, ~45% silt, ~30% sand; density ~21
kN/m3; water content ~20%).
Rapid burial of the contourites, the overlying glacigenic sediments were deposited
at a rate of about 65 m/ka, 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 contourites show contractive behaviour and
shear band development. Thus in the Trænadjupet Slide area a possible mechanism
for initial failure and sediment mobilisation was shear band development due to
porewater pressure increase and liquefaction of contractive sediments.
6. Acknowledgements
This work is a contribution to the COSTA program. The core material from bore hole
6606/3-GB1 was kindly made available through the SEABED project. Financial support
from EC 5th Framework project EVK-CT-1999-00006 and the Ormen Lange Licence
group (Contract no. 5140865) to the University of Tromsø is gratefully acknowledged.
We would also like to thank Tove Midtun who produced the figures and the crews of
RV Jan Mayen and MV Bucentaur for the data collection. The manuscript benefited
from constructive reviews by Tom Bugge and Anders Solheim.
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