mass movements in glaciomarine sediments on the barents sea continental slope

E L S E V I E R Global and Planetary Change 12 (1996) 287-307

GLOBAL AND PLANETARY CHANGE

Mass movements in glaciomarine sediments on the Barents Sea continental slope

Berit Kuvaas *, Yngve Kristoffersen Institute of Solid Earth Physics, University of Bergen, All~gaten, 41, N-5007 Bergen, Norway

Received 16 September 1994; accepted 3 March 1995

Abstract

The Pliocene and Pleistocene sediments in the Bear Island Trough and continental slope of the western Barents Sea constitute a large depositional wedge. This wedge, believed to consist of glaciomarine sediments, is divided into two major sediment packages, separated by the upper regional unconformity (URU). On the shelf, this boundary is observed as a distinct angular unconformity between older, prograding sediments and an upper, horizontally layered unit. On the outer shelf and continental slope, the upper regional unconformity represents the transition between heavily disturbed sediments below, and less disturbed sediments above. We propose that almost all of the glaciomarine sediment units below the upper regional unconformity on the continental slope have been subject to various degrees of mass movements. These features occur as a complex of large sheet and rotational slope failures covering a total area of approximately 25,000 km 2. The mass movements occurred only over short distances and took place in at least five separate events. The distinct contrast in the seismic character below and above the upper regional unconformity is a result of a change in the amount of sediment input, which may correlate with the frequency of glacial advances and their erosional capacity at different times.

1. Introduction

Large slide and slump scars have been observed along the continental margins in many parts of the world (Moore et al., 1970; Bugge, 1983: Barnes and Lewis, 1991) and suggest that mass movement deposits may be important in the stratigraphic record. The largest known along the Norwegian continental margin is the Storegga slide off the coast of Mc~re in Mid-Norway, where the headwall has a total length of 290 km along the shelf break (Bugge, 1983). The slide travelled at least 300 km downslope into the Norway Basin (water depth of 2800-3000 m) and

* Corresponding author.

occurred in three separate events (Bugge, 1983; Bugge et al., 1987; Jansen et al., 1987).

The best documented examples of s lumps and slides as observed from seismic data are described from areas where the mass movement still has some expression in the present morphology, but buried slides have also been described (Moore et al., 1976; Field, 1981; Droz and Beilaiche, 1985).

In this study, we focus on the deposit ional processes during the development of a large submarine fan in the Barents Sea (Fig. 1). We interpret and describe the multistage formation of large scale slumps and slides, all being buried below younger, less disturbed sediments. The present morphology of the fan deposit is smooth except for an indentation in the bathymetric contours at 72°N, described as a

0921-8181/96/$15.00 ~ 1996 Elsevier Science B.V. All fights reserved SSDI 0921-8181(95)00025-9

288 B. Kuvaas, Y. KristofJersen / Global and Planetary Change 12 (1996) 287-307

slide scar by Kristoffersen et al. (1978), Bugge (1983), Vorren et al. (1989) and Laberg and Vorren (1993).

The morphology of the continental shelf in the Barents Sea is dominated by a transverse trough, the Bear Island Trough (Bjornwrenna) which is exca- vated by glacial erosion and reaches water depths of 500 m. Other parts of the shelf have water depths of less than 300 m (Fig. 1). The average slope gradient off the mouth of the Bear Island Trough is 0.9-1.0 °. The bathymetric contours in this area have an ocean- ward convex pattern which outlines a large sediment wedge, termed the Bear Island Trough Mouth Fan (TMF) by Vorren et al. (1989).

Various ages and depositional origins have been suggested for these deposits. Spencer et al. (1984) subdivided the wedge into four main units, ( I - IV) and suggested ages based on microfossil stratigraphy in two wells on the Senja Ridge. In this study, we focus on their two uppermost units ( I I IA/ I I IB and IV, Fig. 2a), which forms a westerly prograding wedge of shelf and slope sediments.

Based on seismic stratigraphy and well informa- tion, Vorren et al. (1991) divided this wedge into three prograding units, named TeC, TeD and TeE, and assigned an age of Mid- to Late Miocene (15.5 Ma) for the base of the lower unit, TeC (Fig. 2a,b). Ricbardsen et al. (1992) and Knutsen et al. (1992) presented a detailed sequence stratigraphic analysis of unit TeC and TeD, respectively, and suggested that these units were deposited prior to the onset of extensive ice-sheet formations on the Barents Shelf. In contrast, Eidvin and Riis (1989), interpreting the prograding sequences in the area and redating the two wells on the Senja Ridge by microfossil analysis and strontiom isotope stratigraphy, and Eidvin et al. (1993), comparing the previous results with a well further to the east suggest that the sediment wedge at the continental margin is of glacial origin and was deposited in the Late Pliocene and the Pleistocene. Eidvin et al. (1993) named the base of the sediment wedge as Reflector 3 and correlated it to the base of unit TeC by Vorren et al. (1991) (Fig. 2a,b). Smttem et al. (1992) defined informal, local seismic units on the outer continental shelf (Fig. 2a) and interpreted their observations of overdeepened incisions in the base unit A0 unconformity as subglacial glacial/glaciofluvial erosion forms, and therefore

considered the entire sedimentary wedge as part of a largely glaciomarine sequence. The base of their lowermost unit A 0 consequently corresponds to base TeC and Reflector 3 (Fig. 2a,b).

Drilling on the northern flank of the Bear Island Trough penetrated upper Pliocene and Pleistocene glaciomarine material above lower Miocene sediments deposited under open marine conditions (Eidvin et al., 1994). This transition correlates with the reflector corresponding to base TeC, base unit A 0 and Reflector 3, verifying the interpretations of Sa~ttem et al. (1992) and Eidvin et al. (1993). More recent work of S~ettem et al. (1994), studying shallow boreholes southwest of Bjcrncya, also support the interpretation of the sedimentary wedge being Late Pliocene-Pleistocene in age and consisting of glaciomarine material. They also found Upper Pliocene sediments below the clastic wedge and suggested a possible glacial influence. Mcrk and Duncan (1993) found volcanoclastic clasts with an age of 2.35 + 0.12 and 2.20 _+ 0.12 Ma in sediments just below the base of the studied wedge, believed to be deposited in sediments of corresponding age. In this study we follow the chronostratigraphy of Saettem et al. (1992) and Eidvin et al. (1993).

The present study, which covers the western Bar- ents Sea continental margin in the Bear Island Trough and the upper slope (Figs. 1 and 3), employs some of the same seismic profiles interpreted by Richardsen et al. (1992) and Knutsen et al. (1992). Richardsen et al. (1992) interpreted unit TeC of the Bear Island Trough Mouth Fan mainly in terms of lowstand fans and lowstand prograding complexes, together with several submarine canyons. Knutsen et al. (1992) studied the overlying unit TeD and outlined a huge slide on the continental slope and rise, covering an area of about 12,000 km 2. The slide was interpreted as representing slumped materials and debris or mud flow deposits occurring within the preglacial sequence in sediments of Late Miocene age. The seismic facies overlying the slide base was subdivided into two sub-facies, both onlapping the slide scar. Their lowermost facies A comprised the sediments derived from the slide/slump event, whereas the overlying facies B was interpreted as turbidites infilling the slide scar. The sequence was otherwise interpreted in terms of the standard seismic stratigraphic techniques (Mitchum et al., 1977; Vail et al., 1984).

B. Kuvaas, Y. Kristoffersen / Global and Planetary Change 12 (1996) 287-307 289

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URU= Upper regional unconformity; Base glaciomarine material (Knutsen et al., (1992); Richardsen et al. (1992).

Base glaciomarine material, Saettem et al. (1992); Eidvin et al. (1993); this s tudy. Sa~ttem et al. (1994) further indicated glaciomarine conditions in pre "base unit II1A/base unit TeCfioase unit A0/reflector 3" time.


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Several seismic surveys have been shot in the Barents Sea by the Norwegian Petroleum Directorate (NPD) and by a number of oil companies. Based on approximately 1500 km of these data (Fig. 3), we present evidence to support an interpretation which differs from Knutsen et al. (1992) and Richardsen et al. (1992) in two aspects: Firstly, we favour a more complicated system of mass movements representing at least five separate phases of slumps and slides. Secondly, we suggest that the observed slope failures all occurred within glaciomarine sediments of Plio- Pleistocene age, below the upper regional unconformity (URU). This implies that glaciers have flowed across the shelf during glacial maxima, overdeepened the shelf and controlled erosional and depositional processes. Therefore, we do not think

that the approach of Knutsen et al. (1992) and Richardsen et al. (1992) to interpret the record in terms of depositional and erosional events controlled by eustatic changes in sea level is appropriate for this depositional environment. Aspects of the seismostratigraphic record on high latitude glaciated margins have been discussed by, among others, Larter and Barker (1989), Cooper et al. (1991) and Kuvaas and Kristoffersen (1992).

2. Seismic interpretation

The prograding shelf sequences are characterized by a complex sigmoid-oblique seismic reflection pattern. Two major regional unconformities are ob-

Fig. 2, a. Schematic line drawing of profile 730075, illustrating the different interpretations of main reflectors and sequences in the studied area. Profile location shown in Fig, 1. b. Summary of main reflectors and seismic sequences with ages, as suggested by earlier studies.

292 B. Kuvaas, Y. Kristoffersen / Global arul Planetary Change 12 (1996) 287-307

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B. Kuoaas, Y. Kristoffersen / Global and Planetary Change 12 (1996) 287-307 293

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served. The upper regional unconformity (URU), (Fig. 4a,b) extends over most of the continental shelf and its glacial origin is documented by several core and drill samples (Vorren et al., 1984; Elverh0i et al., 1989). A deeper erosional unconformity forms the base of the studied sedimentary wedge, here termed reflector BG (base glaciomarine) (Fig. 4a).

Following the stratigraphy suggested by Eidvin et al. (1993,, 1994) and Sa~ttem et al. (1992), reflector BG corresponds to reflector 3 and base unit A 0, respectively (Fig. 2a,b). Despite the possible pre reflector BG glacial influence suggested by Saettem et al. (1994), we consider it likely that the seismic signature of reflector BG represents a change in depositional style, representing the onset of extensive glaciomarine deposition (Figs. 4 and 5a). This implies that the prograding outer 50-60 km of the shelf sequences consist of glaciomarine sediments.

The sequence between reflector BG and URU is characterized by several major stratigraphic breaks,

which we interpret to be a result of extensive slope failures.

On dip-oriented profiles, slope failures can be observed as an abrupt truncation of the prograding sequence by one or more concave upward reflectors (Fig. 4a,b), interpreted as listric faults which sole out into a basal glide plane. The overlying sediment body may maintain its original reflector pattern with only small internal disturbances (Fig. 4a, profile 732230-87, east), or may be totally deformed to a chaotic sequence (Fig. 4a,b). On strike-oriented profiles, slope failures often have the appearance of large channels which cut major parts of the previously deposited sediments (Fig. 5a; 151586A and 5d), or may be observed as chaotic sequences bounded by parallel reflectors at the margins (Fig. 5a,c).

A lack of seismic data on the middle and lower slope complicates the interpretation, since the downslope margins of the presumed slope failures are not

294 B. Kuoaas, Y. Kristoffersen / Global and Planetary Change 12 (1996) 287-307

always seen. Also, since the studied slope failures are buried it is difficult to recognize many of the diagnostic features proposed to identify large-scale submarine slumping (Dingle, 1977). Finally, slumped and slid sediment masses often have their glide planes parallel or subparallel to the bedding of the underlying sediments, particularly in dip sections (Dingle, 1977), and glide planes may therefore be difficult to observe.

However, we do believe that the typical characteristics described above, together with the geometry of the features illustrated in Fig. 6, imply that the sediments can be interpreted as a result of five phases of slope failures. We often observe that the same glide plane appears to have been repeatedly activated during subsequent phases of mass movement (Fig. 4a). It is therefore difficult to interpret the extent of individual glide planes.

We have coded the different generations of mass movements by colours and numbers; orange (O, oldest), red (R), blue (B), green (G) and yellow (Y, youngest) (Figs. 4 and 5a). In the following, we will discuss the different generations in this order.

2.1. Orange slope failure (0)

The glide plane of the oldest, orange slope failure can be traced over a distance of about 20 km along the present shelf edge (profile 151586A, Fig. 5a) and 20 km on profile 143077 (Fig. 5a) located on the upper slope. It occurs at a deep stratigraphic level, and truncates reflector BG, interpreted as the preglacial-glacial boundary. The extent of its glide plane can be traced upslope to the continental shelf and the slide appears to have been initiated very close to the preglacial-glacial boundary. The slide head is best illustrated on profile GBW 88-055, observed as several concave upward reflectors, interpreted as subsidiary glide planes with an average slope gradient of 4 ° (Fig. 7). These reflectors trun- cate the older, prograding sequence. The detached sediments were later eroded by both the red and the blue mass movements (Figs. 4 and 5a). The strata above the slide base have an anticlinal form (Fig. 5a,d (profile 151586A)), which indicates rotation of the original bedding along listric faults, being one of the characteristics of slides (Martinsen, 1994). Our seismic data coverage does not allow us to study the

downslope parts of this slide. The extent of its slide base has, however, been mapped as illustrated in Fig. 6.

2.2. Red slope failure (R)

Fig. 4a illustrates the red slide base as observed perpendicular to its main transport direction. In its upslope part, the slide base truncates the sediments within the orange slide (Fig. 4a, profile GBW 88- 017), whereas, in more downslope parts, it is trun- cated by the overlying blue slide. The slide head can be observed as a concave upward glide plane that truncates the underlying prograding sequence, having a slope gradient of 3 °. We also observe deformed sediments above the slide base (Fig. 5a, profile 151586A). The outline of this slide base suggests that the transport direction was towards the southwest (Fig. 6).

2.3. Blue slope failure (B)

The blue slide base is observed on all seismic profiles and is the most extensive of the slope failures observed in the area. The slide head is defined as that part of the slide where the basal glide plane becomes concave upwards and cuts the underlying prograding sequence with a slope gradient of 2.5 ° (Fig. 4a, profile 732230-87). It is laterally continuous for over 100 km along strike (measured along profile 151586A). We can observe some secondary glide planes above this surface, but any closer study of the proximal slide body is difficult, as the upper parts of the glide plane were apparently reactivated during the later yellow slope failure (Fig. 4a). In a more distal position (Fig. 5a, profile 730075), we see evidence of deformed sediments above the slide base. The westernmost parts show evidences of local contraction in the form of overthrusting in the direction of transport away from the head region (Fig. 8), typical for the toe region of slides and slumps (Dingle, 1977).

The glide plane can be followed over 150 km towards the west, where it extends down the continental slope with an average slope gradient of 1.5 °. At one location (Fig. 5a, profile 730075), the basal glide plane abruptly ramps to a shallower level. This change may possibly be a result of laterally varying

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Fig. 5. a. Line drawings of seismic profiles 143077, 151-' O = Orange, R = Red, B = Blue, G = Green, Y= Ye drawing of profile 143077 (Fig. 5a), illustrating the southeJ green slide. Location shown in Fig. 3. c. Close-up of fram~ slope failure, as well as the northern margin of the green profile 151586A (Fig. 5a), illustrating the orange slope fa; slide base, having an anticlinal form. Location shown in illustrating the distal definition of the green slide as well as in Fig. 3.

/Global and Planetary Change 12 (1996) 287-307

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86A and 730075. Profile locations shown in Fig. 3. BG = Base Glac low, URU = Upper Regional Unconformity. b. Close-up of framed ai n margin of the blue and green slope failures together with the folding, .d area in line drawing of profile 143077 (Fig. 5a), illustrating parts of t slide. Location shown in Fig. 3. d. Close-up of framed area in line d ilure, truncating the preglacial-glaciomarine boundary. Note the strata Fig, 3. e. Close-up of framed area in line drawing of profile 730075 an abrupt shift to a shallower level of the blue basal glide plane. Locati

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mechanical properties of either the detached material or the basin floor sediments.

Sediments within our blue slope failure corresponds to parts of facies A of Knutsen et al. (1992).

2.4. Green slope failure (4)

The fourth generation of slope failure is, on profile 143077 (Fig. 5a,c), observed as a sequence with


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discontinuous and chaotic internal reflectors bounded by parallel reflectors at its margins. The parallel reflectors are not parts of the green slope failure, but onlap the glide plane of the lower blue slope failure and are interpreted as sediments detached during the blue slide event. On a dip-oriented profile we observe that the proximal parts of the green glide plane coincide with the blue glide plane (Fig. 4a,b). The green slide has characteristics typical of a rotational failure; the slide head (slope gradient 10 °) is repre-

sented by a listric fault with rotated strata on the downthrown side, and we observe an upper chaotic unit which occupies the space left by the rollover within the slide head. This chaotic unit is characterized by contorted and discontinuous reflections. Far- ther downslope (Fig. 5a, profile 730075), the regional slope gradient is 0.2 ° and we observe the same two units; here the lower unit consists of deformed sediments. The reflectors are commonly broken, distorted and exhibit reverse dips caused by


rotation during sliding. The upper unit is the downslope continuation of the chaotic unit filling the space left at the slide head. This chaotic unit can be traced for about 35 km downslope from the slide head, whereas the lower unit can be traced to about 60 km from the head (Fig. 5a,e and 6). The internal structure of the green slope failure might suggest two episodes of mass wasting. The lower unit was deformed during an initial episode of transport. This event resulted in steepening of the slope at the slide head, which may have initiated a later episode of mass wasting, represented by the upper, chaotic unit. The internal structure of this unit suggests deposition of debris or mud flows, which often accompany large slide or slump movements on continental margins (Trincardi and Argnani, 1990). Along the southern margin of the green slide (Fig. 5a,b, profile 143077), we observe folding which indicates contraction. This may be a result of transpressional movement in a slide scar that narrows in a downslope direction (Martinsen, 1994).

Subsidiary rotational glide planes are commonly observed within slumped and slid sediments (Dingle, 1977). This is one of the reasons it is difficult to define the green slope failure plane, as it is located within earlier transported sediments of the blue slide event. It should therefore be noted that especially the westernmost definition of the green slide is uncer- tain. However, as observed from the seismic data, the downslope end of the green slope failure should be within 5 -10 km to the west or east of the interpreted distal end of its toe.

The green slope failure is not observed on profile 151586A (Fig. 5a), and therefore must have been initiated in the area between this profile and profile 143077 (Fig. 6). In the interpretation of Knutsen et al. (1992) the sediments above the green slide plane is also included in their facies A. The green slope failure has been eroded by the later yellow slope failure.

2.5. Yellow slope failure (Y)

The youngest generation of mass movement (yellow) is observed on all profiles. This failure has an areal extent roughly similar to the blue slide (Fig. 6) and measures 120 km along profile 151586A (Fig. 5a). In the northern part, we observe a sinuous

upslope margin, interpreted as tributary slump scars (Figs. 4 and 6). They appear as spoon-shaped features with extensional deformation along their margins. In the head region, the glide plane has a slope gradient of 2.1 ° and coincides with the previously formed blue glide plane for approximately 10-20 km, before they separate (Fig. 4a, profile 732230-87). The downslope regional slope gradient is approximately 0.2 ° . On strike-oriented profiles, we can clearly identify the yellow slope failure as a separate feature which shows sediments onlapping the glide plane (Fig. 5a, profile 143077). We are not able to observe any clearly defined toe zone in the downslope direction and the slide may therefore be classi- fied as an "open-ended" slide (Garfunkel, 1984). "Open-ended" slides may be explained as a result of a high degree of lateral compaction so that the strain is taken up by a porosity reduction rather than by thrust zone deformation (Crans et al., 1980). The sediments involved in our yellow slope failure corresponds to facies B of Knutsen et al. (1992), which has been interpreted as turbidites infilling a slide scar. In their interpretation, the turbidites are believed to represent slope fan complexes.

3. Discussion

The interpretation presented above implies that almost all of the glaciomarine sediments below the upper regional unconformity (URU) have been involved in large scale mass movements. Despite the evidence of large-scale mass movements, we cannot discount the possibility that some of the observed reflection patterns have an origin related to local canyon/channel systems or depositional systems as suggested by Richardsen et al. (1992). Indeed, some of our slope failures have the appearance of canyons/channels. However, the combined observation of listric faults which sole into a basal glide plane, rotation along faults, deformed sediments, anticlinal structures within the features, together with their areal extent as illustrated in Fig. 6, strongly point towards mass movements in the form of slumps and slides.

The various slope failures are characterized by little deformation in the slide head area, suggesting only a small distance of travel. Even though gravity

304 B. Kuvaas, Y. Kristoffersen / Global and Planetary Change 12 (1996) 287-307

driven mass movements on continental margins often involve significant downslope translation of sediments, there are also several examples of instances where the displacement is very small, for example the Gela submarine slide off Sicily (Trincardi and Argnani, 1990). Since we have only limited seismic data coverage on the middle and lower slope, we cannot pursue this problem any further at present.

Vorren et al. (1990) found indications of sediment gravity sliding all along the shelf break in the uppermost sediments above URU, and glaciomarine mud- flows were described from SeaMARC II sidescan imagery on the fan proper (Vogt et al., 1993). These features are of much smaller scale (10-20 m thin, 5-20 km wide) and cannot be compared to the underlying slope failures described here. However, at 72°N, a major slide scar has been observed in the present sea-floor morphology (Kristoffersen et al., 1978; Bugge, 1983; Vorren et al., 1989; Laberg and Vorren, 1993). This slide, which lies to the south of our study area, is 40 km wide and situated between the shelf break at 400 m waterdepth and the 1500 m depth contour (Vorren et al., 1989). Still, the differ- ence in seismic signature below and above URU at the mouth of the Bear Island Trough may be interpreted as a change in sediment input and hence stability at different times.

The large dimensions of the slides described in the present paper and the great volume of sediments involved require that the basal detachment surface functioned as an efficient slip plane. Often, the slip surfaces appear as a high-amplitude reflector (e.g. yellow slip surface along the southern part of profile 151586A, (Fig. 5a)) which imply a significant acous- tic impedance contrast and an abrupt change in physical properties at this interface.

Sliding or slumping occurs when excess pore pressures develop in the sedimentary column and the effective shear strength is reduced below the failure limit for a given slope angle (Embley, 1982). Com- mon triggering mechanisms for submarine slumps and slides include: earthquakes (Heezen and Ewing, 1952; Dingle, 1977), high sedimentation rates causing underconsolidation (Prior et al., 1982), cyclic wave loading (Suhayda et al., 1976), slope oversteepening (Martinsen, 1989) and the presence of gas in the sediments (Prior and Coleman, 1978; Carpen- ter, 1981). Sediment instability in high latitude areas

have often been related to high rates of glacial deposition at the seaward end of major ice outlets (Piper and Sparkes, 1987). Studies from Baffin Bay also indicate that sediment gravity flows are important, resulting from high rates of sediment supply along the entire length of the shelf edge during Pleistocene glaciations (Aksu and Hiscott, 1989; His- cott and Aksu, 1994). Knutsen et al. (1992) suggested, on the basis of the detection of shallow gas and gas hydrates in the Barents Shelf areas (Andreassen et al., 1990), that this could have con- tributed to the triggering of slides in the area. Other factors considered by Knutsen et al. (1992) include a high sediment supply together with intensified glaciations in the Barents Sea area combined with a glacioeustatic fall of sea-level.

Several studies confirm that ice sheets have repeatedly advanced to the shelf edge in the western Barents Sea during the time period represented by the sediments above URU (Solheim and Kristof- fersen, 1984; Vorren et al., 1990). Glaciers large enough to reach sea-level in the Norwegian Sea area were present as early as 5.5 Ma (Jansen and Sjoholm, 1991). At 2.56 Ma, IRD-deposition (Ice Rafted De- bris) increased by several orders of magnitude, which signals the first large ice sheets formed in Scandi- navia (Jansen et al., 1990). The period 2.5-1 Ma has been suggested to reflect almost continuous glacial sedimentation, but without ice sheets growing as large as during the last 1 Ma when a new intensifica- tion of glaciation took place (Jansen and Sj0holm, 1991). During the interval 1.2-0.7 Ma, there was a gradual change from relatively low-amplitude Mi- lankovitch variations dominated by 41 kyr cyclicity to higher amplitude 100 kyr cycles in addition to the 41 and 23 kyr cyclicity (Ruddiman et al., 1989).

There are different opinions about the age of the sediments overlying URU (Spencer et al., 1984; Vorren et al., 1990; S~ettem et al., 1992). Here, we follow the stratigraphy by S~ettem et al. (1992) which suggests that the sediments lying above URU are younger than 440 kyr.

Following the oxygen isotope interpretations described above together with the stratigraphy of S~ettem et al. (1992), the sediments above URU should represent the period characterized by the largest (highest ~ ~80 amplitudes) but less frequent glaciations. The observation of many large scale

B. Kuvaas, Y. Kristoffersen / GIobal and Planetary Change 12 (1996)287-307 305

slope failures below URU has been interpreted to be a result of a period with a much higher sediment input in the lowermost sequence. Thus, although the glacial advances represented by the seismic sequences below URU were smaller in magnitude, they probably occurred more frequently, and might have supplied large amounts of sediments to the area.

When the grounding line was located at the shelf edge, the sedimentation rate might have been high enough to promote underconsolidation, and hence instability. Also, the ice sheet could have introduced horizontal forces as well as an additional vertical load triggering the slides, as has been suggested for the triggering of the Storegga Slide (Bugge et al., 1987). This interpretation is supported by the results of S~ettem et al. (1992), who suggested that glaciers extended to a paleo-coastline in the position of the present outer Bear Island Trough as early as the Late Pliocene. Eidvin et al. (1993) also presumed that ice covered large parts of the Barents Sea in a number of glaciations earlier than the last glacial maximum. It is possible that the triggering of some of the late slope failures was related to the morphology and local increased slope angles created by earlier slides, as they all lie in more or less the same area. Multiple events are quite common and have been described from several areas, e.g. the Storegga submarine slides on the Norwegian continental margin (Bugge, 1983; Bugge et al., 1987).

Tertiary uplift of the western Barents margin has been postulated by numerous authors (e.g. Gabrielsen et al., 1990). Riis and Fjeldskaar (1992) pointed out that the timing of the uplift in the Barents Sea was most likely related to Paleogene thermal effects during rifting of the North Atlantic and to isostatic adjustments due to Pliocene/Pleistocene glaciation and erosion. They further estimated approximately 800-1000 m of Late Pliocene-Pleistocene erosion in the central parts of the Southern Barents Sea, on the basis of sediment volumes in the wedge off the Bear Island Trough. S~ettem et al. (1992) and Eidvin et al. (1993) found indications for approximately 1 km of glacial erosion in the area. Loseth et al. (1992) also suggested submarine glacial erosion in the Svalis Dome area of approximately 1 km, but noted further that a total late Cenozoic glacial erosion of the order of 1.5-2 km or more in parts of the Barents Sea should not be excluded. Saettem et al. (1994) noted a

Late Pliocene uplift phase associated with volcanism and possibly glaciations. The uplift processes operat- ing before and possibly after the initiation of glacial loading of the shelf may thus have enhanced erosion and deposition in the sedimentary wedge at the mouth of the Bear Island Trough. The dating of volcanoclastic material also led M¢rk and Duncan (1993) to suggest a Late Pliocene volcanic phase and volcanotectonic activity as a triggering mechanism for mass movements.

In summary, uplift of the Barents Shelf, possibly associated with volcanic activity was an important factor in the erosional and depositional history of the area. We interpret slope oversteepening and glacial loading of the outer shelf to be the main triggering mechanisms of the observed slope failures. Frequent and smaller glacial advances probably supplied more sediments to the fan area than larger but less frequent glacial advances.

4. Conclusions

Several major stratigraphic breaks have been observed within the glaciomarine sediment wedge on the continental slope of the western Barents Sea. These have been interpreted in terms of multiphase slope failures which took place in five stages and involved glaciomarine strata below the upper regional unconformity (URU).

The glaciomarine sediments above this boundary are less disturbed. The main phases of mass movements are considered to be older than 500 kyr. The slope failures occur as a complex of large sheet and rotational slides covering a total area of approximately 25,000 km 2. They are characterized by ten- sional listric faults that sole out on basal slide surfaces which can be traced along slope for tens of kilometers, dipping 0.2-1.5 ° seaward. One of the slides also shows indications of contraction in the toe zone. It is likely that the slope failures were triggered by frequent ice sheet advances with uplift of the Barents Shelf as an important factor. The ice sheet advances deposited large amounts of sediments at the shelf edge and upper slope, causing oversteepening and underconsolidation.

306 B. Kuvaas, Y. Kristoffersen / Global and Planetary Change 12 (1996)287-307

Acknowledgements

This paper was prepared with support provided by VISTA, a research cooperation between the Norwe- gian Academy of Science and Den Norske Stats Oljeselskap a.s. (Statoil). The seismic lines and shot- point maps were placed at our disposal by the Nor- wegian Petroleum Directory and Statoil. Joar Sa~ttem (IKU Petroleum Research), Ole J. Martinsen (Norsk Hydro a.s.), and Stig-Morten Knutsen (Norsk Hydro a.s.) kindly read the manuscript and helped to im- prove it. To these institutions and persons we offer our sincere thanks.

References

Aksu, A.E. and Hiscott, R.N., 1989. Slides and debris flows on the high-latitude continental slopes of Baffin Bay. Geology, 19: 885-888.

Andreassen, K., Hogstad, K. and Berteussen, K.A., 1990. Gas hydrate in the southern Barents Sea, indicated by a shallow seismic anomaly. First Break, 8: 235-245.

Barnes, P.M. and Lewis, K.B., 1991. Sheet slides and rotational failures on a convergent margin: the Kidnappers Slide, New Zealand. Sedimentology, 38: 205-221.

Bugge, T., 1983. Submarine slides on the Norwegian continental margin, with special emphasis on the Storegga area. Cont. Shelf Inst. Norway Publ., 110, 152 pp.

Bugge, T., Befring, S., Belderson, R.H., Eidvin, T., Jansen, E., Kenyon, N.H., Holtedahl, H. and Sejrnp, H.P., 1987. A giant three-Stage submarine slide off Norway. Geo-Mar. Lett., 7: 191-198.

Carpenter, G., 1981. Coincident sediment slump/clathrate complexes on the U.S. Atlantic continental slope. Geo-Mar. Lett., 1: 29-32.

Cooper, A.K., Barrett, P.J., Hinz, K., Traube, V., Leitchenkov, G. and Stagg, H.M.J., 1991. Cenozoic prograding sequences of the Antarctic continental margin: A record of glacio-eustatic and tectonic events. Mar. Geol., 102: 175-213.

Crans, W., Mandl, G. and Haremboure, J., 1980. On the theory of growth-faulting--a geomechanical delta model based on gravity sliding. J. Pet. Geol., 2: 265-307.

Dingle, R.V., 1977. The anatomy of a large submarine slump on a sheared continental margin (SE Africa). J. Geol. Soc. London, 134: 293-310.

Droz, L. and Bellaiche, G., 1985. Rhone Deep-Sea Fan: Mor- phostrncture and Growth Pattern. Am. Assoc. Pet. Geol. Bull., 69(1): 460-479.

Eidvin, T., Goll, R.M., Grogan, P., Smelror, M. and Ulleberg, K., 1994. En stratigrafisk undersCkelse av Cvre d e l a v brCnn 7316/5-1 (Bj¢rn~ya Vest). Unders¢kt intervall: 567-1600 m (pleistocen-midtre eocen). Oljedir., Stavanger, NPD Contrib., 38, 81 pp.

Eidvin, T., Jansen, E. and Riis, F., 1993. Chronology of Tertiary Fan Deposits off the western Barents Sea: implications for the uplift and erosion history of the Barents Shelf. Mar. Geol., 112: 109-131.

Eidvin, T. and Riis, F., 1989. Nye dateringer av de tre vestligste borehullene i Barentshavet. Resultater og konsekvenser for den Terti~ere hevingen. Oljedir., Stavanger, NPD Contrib., 27, 44 pp.

Elverh~i, A., Pfirman, S.L., Solheim, A. and Larssen, B.B., 1989. Glaciomarine sedimentation in epicontinental sea exemplified by the northern Barents Sea. Mar. Geol., 85: 225-250.

Embley, R.W., 1982. Anatomy of some Atlantic margin sediment slides and some comments on ages and mechanisms. In: S. Saxov and J.K. Nieuwenhuis (Editors), Marine Slides and Other Mass Movements, Plenum, New York, pp. 189-213.

Field, M.E., 1981. Sediment mass-transport in basins: controls and patterns. In: R.G. Douglas, I.P. Colbum and D.S. Gorsline (Editors), Depositional Systems of Active Continental Margin Basins. SEPM Short Course Notes, pp. 61-83.

Gabrielsen, R.H., Fa.~rseth, R.B., Jensen, L.N., Kalheim, J.E. and Riis, F., 1990. Structural elements of the Norwegian Continen- tal Shelf. Part 1, The Barents Sea Region. NPD Bull., 6.

Garfunkel, Z., 1984. Large-scale submarine rotational slumps and growth faults in the eastern Mediterranean. Mar. Geol., 55: 305-324.

Heezen, B.C. and Ewing, M., 1952. Turbidity currents and submarine slumps and the 1929 Grand Banks Earthquake. Am. J. Sci., 250: 849-873.

Hiscott, R.N. and Aksu, A.E., 1994. Submarine debris flows and continental slope evolution in front of Quaternary ice sheets, Baffin Bay, Canadian Arctic. Am. Assoc. Pet. Bull., 78: 445-460.

Jansen, E., Befring, S., Bugge, T., Eidvin, T., Holtedahl, H. and Sejrup, H.P., 1987. Large submarine slides on the Norwegian continental margin: Sediments, transport and timing. Mar. Geol., 78: 77-107.

Jansen, E. and Sj¢holm, J.l., 1991. Reconstruction of glaciation over the past 6 Myr from ice-borne deposits in the Norwegian Sea. Nature, 349: 600-603.

Jansen, E., Sj~holm, J.l., Bleil, U. and Erichsen, J.A., 1990. Neogene and Pleistocene glaciations in the Northern Hemi- sphere and late Miocene-Pliocene global ice volume fluctua- tions: Evidence from the Norwegian Sea. In: U. Bleil and J. Thiede (Editors), Geologic History of the Polar Oceans: Arctic versus Antarctic. Kluwer, Dordrecht, pp. 677-705.

Knutsen, S.-M., Richarden, G. and Vorren, T.O., 1992. Late Miocene-Pleistocene sequence stratigraphy and mass-movements on the western Barents Sea margin. In: T.O. Vorren, E. Bergsaker, O.A. Dahl-Stamnes, E. Holter, E. Lie, B. Johansen and T.B. Lund (Editors), Arctic Geology and Petroleum Poten- tial (N.P.F. Spec. Publ., 2). Elsevier, Amsterdam, pp. 573-607.

Kristoffersen, Y., Elverhoi, A. and Vinje, T., 1978. Bar- entshavsprosjektet. Matin geofysikk, geologi og havis. Nor. Polar Inst. Rep., 81 pp.

Kuvaas, B. and Kristoffersen, Y., 1992. Seismostratigraphic evidence of multiple glacial advances on the Weddell Sea margin, Antarctica. In: B. Kuvaas, Glaciomarine Sedimentation in


Polar Regions, with Emphasis on the Antarctic Continental Margin. Thesis. Univ. Bergen.

Laberg, J.S. and Vorren, T.O., 1993. A Late Pleistocene submarine slide on the Bear Island Trough Mouth Fan. Geo-Mar. Lett., 13: 227-234.

Larter, R.D. and Barker, P.F., 1989. Seismic stratigraphy of the Antarctic Peninsula Pacific margin: A record of Pliocene- Pleistocene ice volume and paleoclimate. Geology, 17: 731- 734.

Lcseth, H., Lippard, S.J., Sa~ttem, J., Fanavoll, S., Fjerdingstad, V., Leith, L.T., Ritter, U., Smelror, M. and Sylta, O., 1992. Cenozoic uplift and erosion of the Barents Sea-Evidence from the Svalis Dome area. In: T.O. Vorren, E., Bergsaker, O.A. DahI-Stamnes, E. Holter, E. Lie, B. Johansen and T.B. Lund (Editors), Arctic Geology and Petroleum Potential (N.P.F. Spec. Publ., 2). Elsevier, Amsterdam, pp. 643-664.

Martinsen, O.J., 1989. Styles of soft-deformation on a Namurian (Carboniferous) delta slope, Western Irish Namurian Basin, Ireland. In: M.K.G. Whateley and K.T. Picketing (Editors), Deltas: Sites and Traps for Fossil Fuels. Geol. Soc. London Spec. Publ., 41: 167-177.

Martinsen, O.J., 1994. Mass movements. In: A. Maltman (Editor), Geological Deformation of Sediments. Chapman and Hall, London, pp. 127-165.

Mitchum, R.M., Vail, P.R. and Thompson, S., 1977. Seismic stratigraphy and global changes of sea level, Part 2: The depositional sequence as a basic unit for stratigraphic analysis. In: C.E. Payton (Editor), Seismic Stratigraphy--Applications to Hydrocarbon Exploration. Am. Assoc. Pet. Geol. Mem., 26: 53-62.

Moore, D.G., Curray, J.R. and Emmel, F.J., 1976. Large submarine slide (Olistostrome) associated with Sunda Arc subduc- tion zone, Northeast Indian Ocean. Mar. Geol., 21: 211-226.

Moore Jr., T.C., Van Andel, T.J.H., Blow, W.H. and Heath, G.R., 1970. Large submarine slide off northeastern continental margin of Brazil. Am. Assoc. Pet. Bull., 54(1): 125-128.

Mork, M.B.E. and Duncan, R.A., 1993. Late Pliocene basaltic volcanism on the Western Barents Shelf margin: implications from petrology and 4°Ar-39Ar dating of volcaniclastic debris from a shallow drill core. Nor. Geol. Tidsskr., 73: 209-225.

Piper, D.J.W. and Sparkes, R., 1987. Proglacial sediment instability features on the Scotian slope at 63°W. Mar. Geol., 76: 15-31.

Prior, D.B., Bomhold, B.D., Coleman, J.M. and Bryant, W.R., 1982. Morphology of a submarine slide, Kitimat Arm, British Columbia. Geology, 10: 588-592.

Prior, D.B. and Coleman, J.M., 1978. Submarine landslides on the Mississippi River delta-front slope. Geosci. Man, 18: 41-53.

Richardsen, G., Knutsen, S.-M., Vail, P.R. and Vorren, T.O., 1992. Mid-late Miocene sedimentation on the southwestern Barents shelf margin. In: T.O. Vorren, E. Bergsaker, O.,~,. Dahl-Stamnes, E. Lie, B. Johansen and T.T. Lund (Editors), Arctic Geology and Petroleum Potential (N.P.F. Spec. Publ., 2). Elsevier, Amsterdam, pp. 539-572.

Riis, F. and Fjeldskaar, W., 1992. On the magnitude of the Late

Tertiary and Quaternary erosion and its significance for the uplift of Scandinavia and the Barents Sea. In: R.M. Larsen and B. Larsen (Editors), Structural and Tectonic Modelling and its Application to Petroleum Geology. Nor. Pet. Soc., Graham and Trotman, London, pp. 163-185.

Ruddiman, W.F., Raymo, M.E., Martinson, D.G., Clement, B.M. and Backrnan, J., 1989. Pleistocene evolution: northern hemi- sphere ice sheets and north Atlantic Ocean. Paleoceanography, 4(4): 353-412.

Solheim, A. and Kristoffersen, Y., 1984. Sediments above the Upper Regional Unconformity: Thickness, seismic stratigraphy and outline of the glacial history. Nor. Polarinst. Skr., 179B, 26 pp.

Spencer, A.M., Home, P.C. and Berglund, L.T., 1984. Tertiary structural development of the western Barents Shelf: Troms to Svalbard. In: A.M. Spenceret al. (Editors), Petroleum Geology of the north European Margin. Nor. Pet. Soc., Graham and Trotman, London, pp. 199-209.

Suhayda, J.N., Whelan III, T., Coleman, J.M., Booth, J.S. and Garrison, L.E., 1976. Marine sediment instability: interaction of hydrodynamic forces and bottom sediments. In: Proc. Off- shore Technol. Conf., 1: 29-40.

S~ettem, J., Bugge, T., Fanavoll, S., Goll, R.M., Mork, A., M~rk, M.B.E., Smelror, M. and Verdenius, J.G., 1994. Cenozoic margin development and erosion of the Barents Sea: Core evidence from southwest of Bjcrncya. Mar. Geol., 118: 257- 281.

Saettem, J., Poole, D.A.R., Ellingsen, K.L. and Sejrup, H.P., 1992. Glacial geology of outer Bjorn0yrenna, southwestern Barents Sea. Mar. Geol., 103: 15-51.

Trincardi, F. and Argnani, A., 1990. Gela submarine slide: A major basin-wide event in the Plio-Quaternary foredeep of Sicily. Geo-Mar. Lett., 10: 13-21.

Vail, P.R., Hardenbol, J. and Todd, R.G., 1984. Jurassic unconformities, chronostratigraphy and sea-level changes from seismic stratigraphy and biostratigraphy. In: J.S. Schlee (Editor), Interregional unconformities and hydrocarbon accumulation. Am. Assoc. Pet. Geol. Mem., 36: 129-144.

Vogt, P.R., Crane, K. and Sundvor, E., 1993. Glaeigenic mud- flows on the Bear Island submarine fan. EOS, 74: 449, 452-453.

Vorren, T.O., Hald, M. and Thomsen, E., 1984. Quaternary sediments and environments on the continental shelf off north- em Norway. Mar. Geol., 57: 229-257.

Vorren, T.O., Lebesbye, E., Andreassen, K. and Larsen, K.-B., 1989. Glacigenic sediments on a passive continental margin as exemplified by the Barents Sea. Mar. Geol., 85: 251-272.

Vorren, T.O., Lebesbye, E. and Larsen, K.B., 1990. Geometry and genesis of the glacigenic sediments in the southern Barents Sea. In: J.A. Dowdeswell and J.D. Scourse (Editors), Glaciomarine Environments: Processes and Sediments. Geol. Soc. Spec. Publ., 53: 269-288.

Vorren, T.O., Richardsen, G., Knutsen, S.-M. and Henriksen, E., 1991. Cenozoic erosion and sedimentation in the western Barents Sea. Mar. Pet. Geol., 8: 317-340.

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