Geometry of a Major Slump Structure in the Storegga Slide Region Offshore Western Norway

NORWEGIAN JOURNAL OF GEOLOGY Geometry of a major slump structure in the Storegga

Geometry of a major slump structure in the Storegga slide region offshore western Norway

Roald B. Færseth & Bjørn Helge Sætersmoen

Færseth, R.B. & Sætersmoen, B.H. 2008: Geometry of a major slump structure in the Storegga slide region offshore western Norway. Norwegian Journal of Geology vol 88, pp 1-11. Trondheim 2008. ISSN 029-196X.

A gravitational collapse structure, 155 km long with a maximum width of 35 km, has been released as a single slope failure in the Storegga region off western Norway. The released volume of sediment, which we interpret as a slump, is a part of and of the same age as the Storegga Slide. This means that the slump was released approximately 8200 years ago. Unlike the main Storegga Slide, in which slide sediments have been totally disintegrated by debris flows, the slump was subject to relatively modest down-slope displacement and is an intact body located along the southern margin of the Storegga Slide scar. Rotational movement of the slump sediments released horizontal stress at the head of the slump and at the same time, horizontal stress intensified down-slope where contractional structures such as folds, reverse faults and thrusts developed.

Roald B. Færseth, Hydro ASA, Research Centre, Box 7190, N-5020 Bergen, Norway. Present address: Ryenbergveien 75A, N-0677 Oslo, Norway (e-mail: [email protected]); Bjørn Helge Sætersmoen, Petroleum Geo-Services, P.O. Box 290, N-1326 Lysaker, Norway (e-mail: Bjorn.Helge. [email protected])

Introduction an approximately 300 km long backwall along the present-day shelf edge. A volume of some 3000 km3 of sedi- Gravitational collapse structures, often termed landslides, ment was removed leaving the slide scar (Haflidason et possess two essential features: a rupture surface (failure al. 2004; Bryn et al. 2005a). The run-out distance was surface, glide surface, detachment, décollement) and a 450 km for the debris flows and 800 km for the associ- displaced mass of sediment (Hampton et al. 1996). The ated turbidity currents (Haflidason et al. 2005). The slide displaced mass is the material that travelled down-slope. affected an area of 90.000 km2 and was released in an It commonly rests partly on the rupture surface, but it area where the average slope inclination was of the order might have moved completely beyond it. Moreover, the of 0.6-0.7°. Unstable sediments in the area were removed displaced mass may remain intact, be slightly to highly during one main slide event at approximately 8200 years deformed, or break up into distinct slide blocks. In some BP (Haflidason et al. 2004; Bryn et al. 2005a). cases, all or part of the mass completely disintegrates, producing a flow. Gravitational collapse structures are The present study is concerned with the morphology common in many settings around the world, both sub- and internal structure of a specific gravitational col- aerially and subaqueously. Such structures are typically lapse structure in the Storegga region, which has not associated with areas of instability where some sort of been described previously. Unlike the Storegga Slide triggering mechanism resulted in tectonic failure (e.g. proper where slide sediments have, to a large extent dis- Hampton et al. 1996). integrated, the allochthonous mass of sediment we now describe was subject to relatively modest down-slope The Storegga region off western Norway has been the displacement and is an intact body located along the site of several giant submarine slides (Evans et al. 1996; southern margin of the Storegga Slide scar (Fig. 2). High- Bryn et al. 2003; Haflidason et al. 2004; Sejrup et al. 2004; resolution 2D and 3D seismic data and Multibeam Echo Nygård et al. 2005; Solheim et al. 2005). The Storegga Sounder data from the seabed excellently displays the Slide (Fig. 1) of Holocene age is one of the largest known slump-like shape of the body as well as its internal defor- submarine slides and has been known since the 1970s mation structures and their spatial distribution. (Bugge 1983; Jansen et al. 1987). Over the last decade, a large number of publications have described the Storegga Slide. One main reason for this interest in the area is Gravitational collapse structures related to the Ormen Lange gas field, discovered in 1997, which is located within the scar of the Storegga Slide Gravitational collapse structures may range in length (Fig. 2). This situation raises concerns about the hazard from centimetres to hundreds of kilometres and affect to which the production installations would be exposed. both loose sediments and consolidated rocks under The Storegga Slide scar defines a depression bounded by subaerial as well as subaqueous conditions (Martinsen R. B. Færseth & B. H. Sætersmoen NORWEGIAN JOURNAL OF GEOLOGY

Fig. 1. Bathymetrical map of the southern Norwegian Sea and the Norwegian margin area with outline of the Storegga Slide.

Fig. 2. Bathymetric image of the Storegga Slide scar with outline of the slump in its present location along the southern margin of the slide scar (stippled blue line). See Fig. 1 for location of this figure. Late Pliocene-Pleistocene sediments of the North Sea Fan represent the southern boundary of the slump on the sea floor, whereas inliers of the older Tampen Slide (T) are located along the northern boundary. The Ormen Lange gas field (continuous blue line) is located within the Storegga Slide scar. NORWEGIAN JOURNAL OF GEOLOGY Geometry of a major slump structure in the Storegga

Fig. 3. Schematic illustration showing characteristic geometries associated with gravitational collapse structures (after Echel 1958). This idealized model shows the displaced unit to have a well-defined upper extensional zone (head) and a down-slope contractional zone (toe). This is because movement of the mass of sediment releases horizontal stress at the head and at the same time, horizontal stress increases towards the toe.

1994; Hesthammer & Fossen 1999; Canals et al. 2004 and the rupture surface is layer-parallel (Ramsay & Huber references therein). There is, however, a significant dif- 1987). ference in size between subaerial and subaqueous landslides (e.g. Hampton et al. 1996). The largest subaerial Terminology slides displace a few tens of cubic kilometres of material. The terms “slide” and “slump” are referred to in the lit- By comparison the largest submarine slide known, the erature as features produced by various mass gravita- post-Pliocene Agulhas slide off South Africa has a vol- tional processes, but the use of the terms has often been ume of 20.000 km3 and is interpreted to have occurred imprecise. Most classification schemes make a distinc- as a single slope failure event (Dingle 1977). Submarine tion between slide and slump mechanisms although landslides can originate in nearly flat areas. Analyses have different definitions are proposed (e.g. Hampton 1972; shown that the force of gravity alone is not sufficient to Coates 1977; Woodcock 1979; Allen 1985; Stow 1986; be the sole cause of failure (e.g. Ross 1971; Hampton et Nemec 1990; Martinsen 1994; Hesthammer & Fossen al. 1978). Gravity failure is generally due to a triggering 1999; Lucente and Pini 2003, Canals et al. 2004). Slide mechanism provided by seismic shocks, rapid sedimen- masses are in general said to move on planar rupture tation, over-steepening of slopes, changes in pore pres- surfaces (translational movements) and show no or little sure, or extensional deformation (Morton 1993). internal deformation, whereas slump masses move on an upwards-concave rupture surface (rotational move- Despite the variation in scale as well as variation in lith- ments) and show a wide range of internal deformation ification of sediments affected, there is a striking similar- mechanisms. Following this definition the term slump ity in the overall geometry of many gravity-related fail- should be used for the displaced mass of sediment ures (Fig. 3). The area affected is commonly described as described in this paper. spoon-shaped or amphitheatre-like (e.g. Eckel 1958; Var- nes 1958; Lewis 1971; Brundsen 1973; Brundsen & Jones Models of gravitational collapse structures 1976; Martinsen 1994; Hesthammer & Fossen 1999, Idealized models of gravitational collapse structures (e.g. Canals et al. 2004). In cross section, the rupture surface Eckel 1958; Lewis 1971; Farrell 1984) show the displaced typically displays a concave upwards geometry. The sur- unit to have a well-defined upper extensional zone (head) face may form a flat that is bedding-parallel in its cen- and a down-slope contractional zone (toe) (Fig. 3). This tral part but cuts across bedding planes at the head and is because movement of the mass of sediment releases in some cases also at the toe to define ramps (e.g. Varnes horizontal stress at the head and at the same time, hori- 1958; Levis 1971; Schwarz 1982). The bedding-parallel zontal stress increases towards the toe. Rupture surfaces part of a rupture surface is typically confined to strati- below slumps may display substantial curvature or sharp graphic horizons that appear to be inherently weak either bends. As the slump moves down and away from the because of low sediment strength, high fluid pressure, or scarp past irregularities along the substrate, there must both. The term décollement is generally used only where be deformation in the sediments above the detachment R. B. Færseth & B. H. Sætersmoen NORWEGIAN JOURNAL OF GEOLOGY

(Ramsay & Huber 1987). For this reason, folds may sediments of the upper part of the Naust Formation. develop within the advancing slump, formed by bend- Sediments in the Storegga region are severely dissected ing of the sediments as they slip over non-planar rupture by slides, and repeated sliding during the last 1.7 mil- surfaces. lion years is well documented (Evans 1996; Bryn et al. 2003; Haflidason et al. 2004; Sejrup et al. 2004; Nygård et al. 2005; Solheim et al. 2005). Five slides predating the Geometry of a large, intact slump in the Storegga Slide with lateral extent from 7000 to 27.000 Storegga region km2 have been mapped (Solheim et al. 2005), and they partly underlie the area where the Storegga Slide was Geological setting: sediments and slides in the Storegga later released (Bryn et al. 2003). The Storegga Slide and region the older slides display several common features regard- Thick units of biogenic ooze of the Eocene-Oligocene ing slide mechanisms, slide development and their rela- Brygge Formation and Miocene-Early Pliocene Kai tionship to the regional geology (Solheim et al. 2005). A Formation (Dalland et al. 1988) were deposited in the common characteristic is the layer-parallel slide surfaces deeper parts of the margin offshore Mid-Norway fol- in stratified marine clay units (Bryn et al. 2005a; Kvalstad lowing break-up in the North Atlantic. This was followed et al. 2005). The most likely triggering mechanism caus- by deposition of the Mid-Late Pliocene Naust Forma- ing the slides was strong earthquake activity following tion, which is a thick prograding wedge with interbed- onshore uplift after glaciations. This explains why the ded claystone, siltstone and sand, occasionally with very slides take place after a glacial period (Bryn et al. 2005b). coarse clastics in its upper part (Riis 1996; Bryn et al. Earthquake activity was at its highest in Scandinavia 2003). The Naust Formation has been subdivided into five between 10 and 7 ka as a response to glacial rebound. hemipelagic/glacigenic sequences (Berg et al. 2005; Bryn et al. 2005a, their Fig. 4). An intra Naust bedding-parallel Slump morphology reflector represents the most pronounced failure surface In map view, the gravitational slump described here associated with the main Storegga Slide (Haflidason et al. has an overall lens-shape (Fig. 2) that covers an area of 2003; Bryn et al. 2005a). The sediments involved in this approximately 3700 km2. it is 155 km long with a maxi- slide were mainly deposited during the last glacial-inter- mum width of 35 km, and is bounded to the north by glacial cycle, and their only consolidation took place in three huge bodies of rocks that are circular to elliptical in the shelf edge area where they were in close contact with map view (Figs. 2 and 4). These are remnants (inliers) of shelf glaciers (Haflidason et al. 2004). The displaced mass the older (130-150 Ka, Solheim et al. 2005), more com- or slump we describe here is located along the southern petent and once continuous Tampen Slide, which is now margin of the Storegga Slide scar (Fig. 2), and comprises preserved south of the Storegga Slide scar. The concave

Fig. 4. A shaded relief map from multibeam bathymetry shows the mid segment of the slump on the seabed. Irregularities at the upper surface of the slump reflect reverse as well as normal faults. See Fig. 2 for location this figure. The faults strike consistently N-S, i.e. orthogonal to the transport direction of the slump. Inliers of the older Tampen Slide, circular to elliptical in map view, are located along the northern boundary of the slump. Small and large rectangle outline insets in figs 7 and 10, respectively. NORWEGIAN JOURNAL OF GEOLOGY Geometry of a major slump structure in the Storegga

Fig. 5. Schematic profile of the slump. See Fig. 2 for location of the profile. The slump involves Pliocene sediments of the Naust Formation (dark brown) as well as interlayered, older and more competent sediments of the Tampen Slide (green). The slump is a composite feature characterized by different structural associations that occur in three well-defined segments (proximal, mid and distal). Shortening in the western part of the slump is evident due to folding, reverse faulting and thrusting, which compensated for extension in the proximal part. BF – Breakaway fault; FR – Frontal ramp. The profile is not to scale.

upwards rupture surface below the slump is confined to thickness ranges from 250 to 600 m, whereas the distal an intra Naust bedding plane horizon below the mid seg- segment of the slump immediately west of the ramp has ment and the western part of the proximal segment (Fig. a thickness of 300 m and thins gradually further west. 5). The intra Naust bedding plane horizon dips gently (0.1°) to the west. To the west, the rupture surface cuts The heave associated with the breakaway fault at the head 250 m of the sequence to form a ramp, the distal part of the slump involves approximately 10 km of lateral of the slump being transported beyond the ramp along movement away from the footwall (Fig. 5). Hence, the a base Tampen Slide detachment (Fig. 5). On a regional slump has moved a minimum distance of 10 km down- scale and in a west-east cross-section, the upper surface slope in its proximal part. However, thinning of the of the slump exhibits a convex outline on the seabed, and slump in the eastern part of the proximal segment indi- the maximum thickness of 700 m is associated with the cates that total extension may be significantly more than mid segment of the slump. In the proximal segment, the 10 km. To the west, shortening is evident due to a gradual

Fig. 6. W-E oriented seismic section across the mid segment of the slump showing the style of deformation.. The segment is characterized by conjugate reverse faulting and folding east of the frontal ramp, followed by normal faults in the easternmost part of this segment. The fold pattern displayed by Tampen Slide sediments in the easternmost part of the section attests to contraction and shortening in the western part of the proximal segment. Reflections: Seabed (blue); Top Tampen Slide (yellow); Base Tampen Slide (green). The detachment east of the ramp is confined to an intra Naust bedding plane horizon (light green). At the frontal ramp, the detachment climbs 250 m across the lithological layers to the base of the older Tampen Slide that is interlayered with sediments of the Naust Formation in the footwall. R. B. Færseth & B. H. Sætersmoen NORWEGIAN JOURNAL OF GEOLOGY thickening of the slump as well as folding, thrusting and mass movement (e.g. Schwartz 1982; Martinsen 1994). reverse faulting, features which compensated for exten- This is in contrast to the mid segment of the slump we sion in the proximal part. describe, which exhibits considerable deformation.

The slump we describe is a composite feature where dif- The segment is approximately 60 km long and is bounded ferent structural associations occur in three well-defined to the west by a ramp where the detachment cuts segments. The three segments and their characteristic obliquely up-section in the direction of tectonic trans- structures are described below. The structures and their port (Fig. 5). The mid segment, which is the thickest part spatial distribution throughout the displaced mass of of the slump, has a maximum thickness of 700 m. The sediment are schematically shown in Figure 5. segment has been shortened horizontally and extended vertically due to lateral compaction, folding and reverse The proximal segment of the slump faulting. Within the mid segment, the upper surface of the slump is irregular due to faults that are contractional From the head of the slump and to the west, the proximal as well as extensional in origin. The faults strike consis- segment is approximately 80 km long (Fig. 5). The thick- tently N-S, i.e. orthogonal to the transport direction to ness of this segment increases from 250 m at the head to form transverse features at the seabed level (Fig. 4). The 600 m in its westernmost part. The seismic resolution is faults are planar and most of them terminate downwards generally poor in this area as a result of internal defor- at the detachment surface, and hence in general transect mation, but also due to the presence of relatively thick the total thickness of the slump (Figs. 5 and 6). The faults post-slump sediments within the Storegga Slide scar. The typically occur in “families”, which means that a number rupture surface, dips approximately 4° immediately west of faults with similar appearance are characteristic for a of the fault scarp but flattens down-slope to become sub- specific volume of the mid segment. East of the ramp, horizontal in the westernmost part of the segment. In the regularly spaced reverse faults were generated. The con- western part of the proximal segment, over a distance of jugate set of reverse faults created uplifted ridges 450-900 some 30 km, folding is evident (Figs. 5 and 6). The fold m wide over the upper surface of the slump (Figs. 5 and pattern is typical of ptygmatic folds (Ramsay and Huber 6). These faults have maximum vertical offsets from 40 to 1987), and folding is displayed by the most competent 70 m, and exhibit dips in the range 50° to 60°. layer, the Tampen Slide, enclosed in the less competent sediments of the Naust Formation. The disharmonic style Bedding is well preserved within the mid segment, which of ptygmatic folds usually implies layer-parallel contrac- allows exact calculation of vertical offset as well as hori- tion, and the lack of folding at some distance away from zontal overlap associated with individual reverse faults. the competent layer into the over- and underlying rocks Based on measurements of the bed length of the folded implies that the contraction is taken up as homogeneous Tampen Slide sediments as well as horizontal overlap for strain in the less competent sediments (Ramsay and a total of some 45 reverse faults, the shortening is esti- Huber 1987). The western part of the proximal segment mated to be 3.5 km. Hence, reverse faulting and folding has been subject to significant shortening, a process that account for 11% horizontal shortening over a part of the reduced the length of the slump and increased its thick- mid segment where the present length is 28 km. Apart ness. An estimate of the shortening can be obtained by from folding and reverse faulting, shortening and thick- comparing the bed length of the folded Tampen Slide ening of the mid segment of the slump are also results sediments with the present length of the folded segment. of lateral compaction. Assuming that the original thick- This indicates that the western part of the proximal seg- ness of the slump in the mid segment was the same as the ment has been shortened from an original length of 44 original thickness in the westernmost part of the proxi- km to its present length of 30 km, i.e. a shortening of mal segment, which we calculate to be 4.5 km, the mid 32%. We interpret shortening and folding in this part of segment has then been shortened from an initial length the slump to result from the rapid down-slope motion in of some 80 km to its present length of 60 km. Hence, the the east when the slump slipped away from the backwall total shortening of the mid segment is approximately and moved down a relatively steep rupture surface. As the 25%, and this may be subdivided into a component of movement of the slump had to adjust to a near horizon- 11% due to folding and reverse faulting and a compo- tal rupture surface in a westerly direction, the velocity of nent of 14% due to lateral compaction. the advancing slump slowed down and the most competent sediments above the rupture surface, i.e. the Tampen A number of fault blocks, all tilted to the west, occupy the Slide, started to buckle. eastern part of the mid segment (Figs. 5 and 6). The fault blocks are typically 1200-1400 m wide, and are bound by The mid segment of the slump normal faults which dip 45-50° east, with fault throws in The rupture surface is sub-horizontal below the mid seg- the range of 100 to 200 m. This sideway collapse of fault ment of the slump, indicating translational movement. blocks to give a domino-style geometry probably resulted For slumps and slides where the mid section shows evi- from a change in the velocity of dislocation. The normal dence of translational movement, the displaced mass of faults represent an adjustment to the space created in a part sediment commonly remains nearly undisturbed after of the advancing slump undergoing localized extension. NORWEGIAN JOURNAL OF GEOLOGY Geometry of a major slump structure in the Storegga

Fig. 7. W-E oriented seismic section showing fault-bend folding associated with the frontal ramp and associated reverse faults east of the ramp. Reflections: Seabed (blue); Top Tampen Slide (yellow); Base Tampen Slide (green); Base slump (light green).

The distal segment of the slump In the westerly, distal part of the slump, the detachment climbs 250 m, from an intra Naust level and crosses the lithological layers to the base of the older Tampen Slide that is interlayered with sediments of the Naust Forma- tion in the footwall (Figs. 5 and 7). The ramp dips 30° east (Fig. 8) and is classified as a low angle reverse or thrust fault (Ramsay & Huber 1987). The ramp is perpendicular to the main direction of slump movement and accordingly is a frontal ramp that allowed the detached mass of sediment to override younger sediments in the footwall. The compatibility constraints that arise when the slump (hanging wall) moves relative to the in situ footwall are resolved by allowing the hanging wall to adjust its shape to conform with the morphology of the underlying footwall and, as a result, to undergo fault-bend folding. Such folds form only in the hanging wall as it is forced over the underlying staircase-like step (Fig. 8).

The deformation associated with the fault-bend folding resulted in uplift of the slump and local vertical duplica- tion of horizontal strata. The uplifted/overthrust masses at sea bottom level were partly removed by succeeding Fig. 8. The progressive development of hanging wall fault-bend folds erosion (Figs. 7 and 8). Fault-bend folds are typically where the slump moves across the frontal ramp. Horizontal and associated with foreland fold-and-thrust belts on land vertical scale is equal. Stage 1: Mid-Late Pliocene Naust Formation (e.g. Suppe 1985). However, although most geometric (brown colours), and the interlayered older Tampen Slide prior to analyses of folds associated with ramp-flat situations have the release of the slump. Stage 2: The slump moves down-slope along been undertaken in studies of thrust faulting on land, a detachment, which is an intra Naust bedding-parallel horizon. The stepping up of the detachment to form a frontal ramp, results such features are also associated with large-scale gravita- in fault-bend folding as the hanging wall adjust its shape to the form tional collapse structures elsewhere. The folding associ- of the underlying fault. As the slip on the detachment continues, the ated with the frontal ramp of the slump we describe has anticline grows in amplitude (Stage 3). The uplifted/overthrust mass the ideal kink-like geometry of fault-bend folds related of sediment at seabed has been partly removed by post-slump ero- to a single step in the rupture surface, following Suppe sion. The distance x´ – x (1.5 km), equals the amount of overthrus- (1985). This geometry conserves cross-sectional area and ting in this position. also rock volume in three dimensions (Ramsay & Huber R. B. Færseth & B. H. Sætersmoen NORWEGIAN JOURNAL OF GEOLOGY

Fig. 9. A composite seismic section that illustrates deformation within the distal segment of the slump, west of the frontal ramp. In this area, the detachment below the slump follows the base of the Tampen Slide. Contraction resulted in relatively small asymmetrical folds, thrusts and high angle reverse faults that bifurcate from the underlying detachment level. Folds and thrusts decrease in frequency and size to the west. Reflections: Top Tampen Slide (yellow); Base Tampen Slide (green).

Fig.10. N-S oriented seismic section across the southern lateral margin of the slump. In this area the margin is inclined at a relatively low angle to the transport direction of the slump to cause a left-lateral shear sense of transpression. Along this part of the slump margin the detachment climbs from an intra Naust bedding plane horizon (light green) below the slump, across lithological layers to the top of the Tampen Slide in the footwall to the south, to form a steep lateral ramp. Reflections: Seabed (blue); Top Tam- pen Slide (yellow); Base Tampen Slide (green).

1987). The characteristic shape of single-step fault-bend atively small asymmetrical folds, thrusts and high angle folds is a steeper front dip and a more gentle back dip reverse faults that bifurcate from the underlying detach- that is equal to the angle of fault dip (Figs. 7 and 8). As ment level (Figs. 5 and 9). Asymmetrical folds typically slip on the detachment continues, the anticline grows in have a westerly vergence and reverse faults dip to the east amplitude and the fold reaches its maximum amplitude to give an imbricate structure. The faults cut up through when it equals the height of the step in the detachment, the stratigraphy and some of them may even reach the i.e. 250 meters. The distance x´ – x (Fig. 8), equals 1.5 km, seafloor. Folds and thrusts decrease in frequency and size and represents the amount of overthrusting in this posi- to the west as the effects of contraction deteriorate when tion and accordingly the amount of shortening associ- approaching the toe of the slump. ated with the frontal ramp. Deformation along lateral margins of the slump West of the ramp, where the detachment follows the base The slump has a sharp lateral margin to the south (Fig. of the Tampen Slide, contraction is evident over a dis- 2). Lateral margins of slumps in general are likely areas tance of approximately 15 km. Contraction results in rel- of strike-slip or oblique-slip motion, depending on the NORWEGIAN JOURNAL OF GEOLOGY Geometry of a major slump structure in the Storegga width variability of the slump scar and also whether they slump and increased its thickness. We have attempted to maintained parallelism to the movement direction or are calculate the amount of shortening in various parts of the skewed. When the slump described here was released, the slump associated with lateral compaction as well as fold- overall direction of transport was to the west (Fig. 2). In ing, reverse faulting and thrusting. Applying the present map view, the lateral margin south of the mid segment length and thickness of a slump segment and assuming is oriented at a small angle to the transport direction of a constant cross sectional area before and after deforma- the slump (Fig. 4), and may cause a left-lateral sense of tion, the original length of the deformed segment can movement (transpression) to take place. Along this part be obtained. A limitation in applying this area balance of the slump margin, the detachment is typically a steep method is that the original thickness of the deformed ramp against in situ sediments (Fig. 10). The detachment segment must be known. As knowledge of this is not climbs from the intra Naust level below the slump across available, it introduces some uncertainties in our calcula- lithological layering to the top of the Tampen Slide to the tions. However, based on these calculations as well as a south. The amount of thrusting and shortening of the comparison of bed lengths before and after deformation slump along the lateral/oblique ramp is subordinate to in parts of the slump, the slump as a whole is interpreted that associated with the frontal ramp (Fig. 7). The north- as having been shortened some 30-35 km in the area ern slump margin is not well displayed due to post-slump represented by the mid segment and the western part of erosion. However, with a westerly directed transport of the proximal segment. We interpret shortening to have the slump this lateral margin was most likely subject to resulted from the obstacle represented by the ramp at the overall transtension. rupture surface. Following the initial acceleration at the head of the slump, the velocity came to a halt across the Discussion ramp, and the mid segment as well as the western part of the proximal segment became subject to overall con- The seabed morphology and seismic facies of the traction. Farell (1984) suggested that if slumps halt first Storegga Slide indicate that the main slide developed at their down-slope area, a contractional strain wave is by a headwall retreating upslope with unloading at likely to propagate up-slope through the slump to form the toe as the main driver (regressive slide) (Bryn et al contractional structures such as folds and reverse faults, a 2005a; Haflidason et al. 2005; Kvalstad et al. 2005). The model that is consistent with our observations. slide left a deep scar to the east with headwall slope gra- dients in the range 20-45° (Haflidason et al. 2004). The We interpret the slump failure as a single event. How- slide scar associated with the main Storegga Slide is the ever, the speed at which the mass movement took place scar that has eroded deepest into the shelf area (Figs. 1 is uncertain. The velocity achieved during gravitational and 2). The slump we describe here is located along the mass movements, which is an important factor with southern margin of the main Storegga slide scar, and respect to deformation within the displaced mass of sedi- the intra Naust rupture surface below it can be traced ment, can be influenced by factors such as the volume of to the north where it coincides with the deepest-seated sediment dislodged, the dip of the detachment, the cur- failure surfaces associated with debris lobe phases of the vature of the detachment and associated obstacles as well Storegga Slide. Haflidason et al. (2004) reconstructed the as the rheological properties of the sediments involved. continuous retrogressive development of the Storegga Schwartz (1982) concluded that subaqueous slope fail- Slide. They mapped slide debris lobe phases associated ures move down-slope with a velocity mostly less than 5 with this c. 8200 years BP slide event, as well as distinc- m/sec, i.e. 18 km/h. By applying this velocity, the slump tive provinces defined by morphological textures on the we describe was displaced in less than one hour. As a seabed.. Five major lobe phases were identified by Hafli- comparison, modelling has indicated maximum front dason et al. (2004) and interpreted to have taken place velocity of the Storegga Slide to be in the range 10-20 within a very short time interval. The slump we describe m/sec (Bryn. et al. 2005a; Kvalstad et al. 2005). De Bla- was termed lobe 2, identified as Province Type IV (com- sio et al. (2003), modelled slide velocities of 25-30 m/sec pression zone), and interpreted to be a part of and of the based on run-out studies of the Storegga Slide. Bondevik same age as the main Storegga Slide. et al. (2005), compared deposits from the Storegga tsunami with numerical simulations for a retrogressive slide We have observed increases and decreases in length development, and obtained a good agreement applying within the slump, and associated deformation features velocities of 25-30 m/sec. For the slump we describe, a are both brittle and ductile. The slump shows a gradual lower velocity (< 5 m/sec) seems more likely. thickening from the breakaway fault in the east towards Following the initial acceleration at the head of the a frontal ramp to the west (Fig. 5). There is no evidence slump, the velocity slowed down above the sub-hori- suggesting that the observed changes in bed thicknesses zontal part of the rupture surface and across the frontal are primary. We interpret the thickening as a result of lat- ramp, to come to a halt some 15 km kilometres west of eral compaction processes taking place in high porosity the ramp. A relatively low velocity may also explain why sediments, which moved down an inclined rupture sur- the slump we describe has remained intact, in contrast face. Hence, the displaced mass of sediment was subject to the Storegga Slide proper, in which the sediments have to layer-parallel shortening that reduced the length of the been totally disintegrated. 10 R. B. Færseth & B. H. Sætersmoen NORWEGIAN JOURNAL OF GEOLOGY

Acknowledgements: – We are grateful to Ole Martinsen for suggestions Conclusions and comments on an earlier version of the manuscript. We also want This study is concerned with the morphology and inter- to thank Reidulf Bøe and Ebbe Hartz for constructive reviews of the article. nal structure of a gravitational collapse structure of some magnitude in the Storegga region, which has not been described previously. The displaced mass of sediment is References located along the southern margin of the main Storegga Slide scar and displays internal structures that are consid- Allen, J.R.L. 1985: Principles of Physical Sedimentology. Allen & Unwin, ered to be diagnostic for submarine slumps. The slump London, 272 pp. was transported to the west without loss of cohesion. Berg, K., Solheim, A. & Bryn, P. 2005: The Pleistocene to recent geolo- High-resolution 2D and 3D seismic data and Multibeam gical development of the Ormen Lange area. 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