Pleistocene development of the SE Nordic Seas margin

Marine Geology 213 (2004) 169 – 200 www.elsevier.com/locate/margeo

Pleistocene development of the SE Nordic Seas margin Hans Petter Sejrupa,*, Haflidi Haflidasona, Berit Oline Hjelstuena, Atle Nyg3rda, Petter Brynb, Reidar Lienb a

Department of Earth Science, University of Bergen, Alle`gaten 41, N-5007 Bergen, Norway b Norsk Hydro ASA, 0256 Oslo, Norway Accepted 30 September 2004

Abstract Throughout the Pleistocene the sedimentary environment on the SE Nordic Seas continental slope/outer shelf, off western Norway, has been strongly controlled by variability in the Norwegian Atlantic Current (NwAC), glaciations of the shelf areas and sea level changes. Acoustic and core data from the southern Vbring Plateau show a Pleistocene sequence characterised by hemipelagic sediments interfingered by diamictons on the upper slope. The area of the 7.25 14C ka BP Storegga Slide shows evidence of a long history of Pleistocene mega-slides. During the last interglacial, and most likely also during previous interglacials, the slide region has been the locus of rapid deposition between water depths of 800 and 1200 m, as a result of NwAC winnowing along the upper slope. The North Sea Fan region is strongly influenced by glacigenic debris flows (GDFs) deposited during glacial advances reaching the shelf edge, when the Norwegian Channel was occupied by the Norwegian Channel Ice Stream. It appears that GDF activity was initiated at ca. Marine Isotope Stage 12. Interbedded between the debris flow sequences, mega-slide events such as the Mbre and Tampen slides have been identified. During glaciations, when the entire SE Nordic Seas continental shelf was covered by extensive grounded ice sheets, basal till were transported to the shelf edge from where subsequent mass movement occurred. During late glaciation/early deglaciation meltwater plumes were released at the time of disintegration of ice streams in the Norwegian Channel, as is evidenced from the last deglaciation of the margin at ca. 15 14C ka BP. The plume material was transported northwards by currents, before rapidly deposited as a thick package within the Storegga Slide area and on the south Vbring Plateau. Based on identification and dating of iceberg scourings, glacial erosion surfaces and delta deposits on the shelf, subsidence rates between 0.7 and 1.2 m/ky have been calculated for the last ca. 250 ka. D 2004 Elsevier B.V. All rights reserved. Keywords: Pleistocene; Nordic Seas; continental margin; sedimentary processes; slope instability; glaciation; interglacial

1. Introduction and geological setting * Corresponding author. Tel.: +47 55 58 35 05; fax: +47 55 58 36 60. E-mail address: [email protected] (H.P. Sejrup). 0025-3227/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.margeo.2004.10.006

Since the early 1960s there has been an increasing geo-scientific research effort on the SE Nordic Seas continental margin (Figs. 1 and 2). Commonly, the

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Fig. 1. Bathymetry and major surface current pattern in the North Atlantic. The Nordic Seas are defined to the region delineated by the Iceland, Norwegian and Greenland seas. Study area within box. Bathymetry in metres. NC: Norwegian Channel; NCC: Norwegian Coastal Current; NwAC: Norwegian Atlantic Current. NwAC flow path from Orvik and Niiler (2002).

investigations have been related to hydrocarbon exploration; however, the economic interests have also strengthened the research on Late Cenozoic sedimentary processes and margin evolution. The margin offers high-resolution sedimentary sequences, which have large potential for paleoclimatic studies.

The possibility to correlate these sequences to the Fennoscandian Pleistocene glaciations has also enhanced the interest from palaeoceanographic research teams for continental margin research. The morphology and distribution of Pleistocene sediments on the SE Nordic Seas margin, defined by

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the south Vbring, the Mbre and the northern North Sea margin segments (Figs. 1 and 2), strongly reflect impact of glaciations, variability in ocean circulation and sea level changes (Holtedahl and Bjerkli, 1975, 1982; Jansen et al., 1989; Henrich and Baumann, 1994; King et al., 1996; Sejrup et al., 2003). Thus, to understand sedimentary depositional environments and sedimentary processes that have been active during the Pleistocene, it is of prime importance to know how the above control factors have changed through time and the effect they have had on the sedimentary regime. In this paper new and previously published results from sediment cores and acoustic data sets from the SE Nordic Seas margin are compiled, and geological models for the Pleistocene development of an ice stream influenced (northern North Sea), a mega-slide dominated (Mbre) and hemipelagic dominated (southern Vbring) margin are presented and discussed.

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Structurally, the studied margin is characterised by deep sedimentary basins, the Vbring and Mbre basins, which are limited by the ca. 55 Ma break-up related volcanic Vbring and Mbre marginal highs to the west (Fig. 2a) (Eldholm et al., 1989a; Skogseid et al., 1992). The boundary between the basins and the marginal highs are defined by the Vbring and Faroe– Shetland escarpments (Fig. 2a). The Jan Mayen Fracture Zone and its landward continuation, the Jan Mayen Lineament, separate the Mbre and Vbring margins (Blystad et al., 1995) and have complexly divided the SE Nordic Seas margin during post-breakup geological events. In the NW part of the studied area, the former %gir Ridge spreading axis (Fig. 2a), which terminated during the Late Oligocene–Early Miocene (Brekke, 2000), defines the outermost boundary of the North Sea Fan and the Storegga Slide. 1.2. Oceanography

1.1. Topography and structural framework To the north, the SE Nordic Seas margin is characterised by a broad continental shelf, extending about 200 km from the Norwegian coastline (Fig. 2a). The shelf narrows southwards, and is only 50 km broad at the Mbre margin. Shallow troughs intersect the entire studied continental shelf. The Norwegian Channel, which runs parallel to the southern and southwestern coast off Norway (Figs. 1 and 2) is, by far, the largest and deepest of these troughs, which acted as pathways for fast moving ice streams during glaciations (Sejrup et al., 1998, 2000; Ottesen et al., 2001). At the Vbring margin the broad, flat-lying Vbring Plateau intersects the continental slope between 1100 and 1400 m water depth (Fig. 2). Further south, the present-day seafloor reveal both the impact of the huge submarine Storegga Slide at the Mbre margin and the seaward convex bulge of the North Sea Fan in front of the Norwegian Channel.

Three water masses, the Atlantic Water in the Norwegian Atlantic Current (NwAC), the Coastal Current Water in the Norwegian Coastal Current (NCC) and the homohaline deep-water in the Norway Basin, dominate the present-day hydrographic regime along the SE Nordic Seas Margin (Fig. 1) (Mosby, 1972; Swift and Aagaard, 1981; Hansen and Østerhus, 2000). The Atlantic Water enters the study area over the Iceland–Faroe Ridge and through the Faroe– Shetland Channel, as two separate branches (Fig. 1) (Orvik and Niiler, 2002). The western NwAC branch follows the slope of the Vbring Plateau towards Jan Mayen before turning northeastward along the slope of the Mohns Ridge, whereas the eastern branch follows the outer part of the Norwegian continental shelf. The Atlantic Water has a temperature of 6–8 8C and salinity values between 35.1x and 35.3x, and is sharply bounded towards the Coastal Current Water which has salinities of b33.1x and is characterised by

Fig. 2. (a) Bathymetry and main structural elements of the SE Nordic Seas margin. Outline of Storegga Slide (from Haflidason et al., 2004), Tampen Slide (from Nyga˚rd et al., in press), Mbre Slide (from Nyga˚rd et al., in press), Sklinnadjupet Slide (from Solheim et al., submitted) and North Sea Fan (from Nyga˚rd et al., in press) are shown. Pleistocene subsidence rates along the margin are indicated within grey boxes. Bathymetry in metres. Structural elements from Blystad et al. (1995). FSE: Faroe–Shetland Escarpment; HHA: Helland Hansen Arch; JMFZ: Jan Mayen Fracture Zone; MA: Modgunn Arch; MMH: Mbre Marginal High; ND: Naglfar Dome; OL: Ormen Lange; SkS: Sklinnadjupet Slide; VD: Vema Dome; VE: Vbring Escarpment; VMH: Vbring Marginal High. (b) Location of cores and profiles discussed in the text. The main morpho-sedimentary regions (see figure legend) that have been identified within the Storegga Slide are outlined. Batymetry in metres. Abbreviations as in (a).

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large seasonal variations. The Atlantic Water is also sharply bounded towards the deep-water in the Norway Basin, which is characterised by a uniform salinity of 34.95x and temperatures close to 1 8C. The deep-water is formed by cooling and evaporation of Atlantic Water that sinks in the northern central areas of the Nordic Seas (Mosby, 1972; Swift and Aagaard, 1981). Deep-water is also renewed by brineformation on the adjacent shelf areas (Aagaard et al., 1985; Dokken and Jansen, 1999). The dense, saline and oxygen-rich brines originate during winter-ice formation and are transported across the shelf edge into the deep-sea basins. Several studies in the Nordic Seas region have demonstrated that this present-day interglacial circulation system cannot be extrapolated to the entire Quaternary period. It most likely developed during some interglacial periods (Kellogg, 1980; Sejrup et al., 1981, 1995, 1999; Sejrup and Knudsen, 1993; Henrich and Baumann, 1994; Fronval and Jansen, 1997).

2. Margin configuration The Pleistocene sedimentary sequences on the SE Nordic Seas continental margin have been deposited by: (1) marine and glacimarine processes, (2) largescale sliding, (3) glacigenic debris flows related to fast moving ice streams and (4) grounded ice sheets (e.g. Bugge, 1983; King, 1993; King et al., 1996; Sejrup et al., 1996; Taylor et al., 2002; Nyga˚rd et al., 2002; Hjelstuen et al., in press). These depositional processes display large temporal and spatial variations along the studied margin which can be divided into three distinct regions dominated either by ice stream-related processes (North Sea Fan, northern North Sea margin), submarine mega-slides (Storegga Slide area, Mbre margin) or hemipelagic/glacimarine sedimentation (south Vbring Margin). 2.1. North Sea Fan—ice stream margin During the last decade a number of papers have described the huge submarine fans bordering the Nordic Seas (Eidvin et al., 1993; Vogt et al., 1993; Sættem et al., 1994; King et al., 1996; Laberg and Vorren, 1996; Sejrup et al., 1996; King et al., 1998; Solheim et al., 1998; Vorren et al., 1998). These fans,

which have been termed Trough Mouth Fans (Vorren and Laberg, 1997), as they are located in front of depressions on the shelf, represent localised regions of rapid Late Pliocene to Pleistocene margin build-out (e.g. Faleide et al., 1996). The North Sea Fan, found at the mouth of the Norwegian Channel (Figs. 1 and 2), consists of an up to 1800 m thick Late Plio–Pleistocene sedimentary wedge (Nilson, 1996). Thus, the North Sea Fan complex is the third thickest fan on the NE Atlantic margin, ranging after the 3500 m thick Bear Island Fan and the 4500 m thick Storfjorden Fan on the western Barents Sea margin (Fiedler and Faleide, 1996; Hjelstuen et al., 1996). The North Sea Fan has been divided into a proximal and a distal depositional province, separated at ca. 2000 m water depth by the Mbre Marginal High (Figs. 2a and 3). The proximal depositional province has been divided into several seismostratigraphical units (e.g. King et al., 1996; Sejrup et al., 1996; Nyga˚rd et al., in press), and two main depositional facies have been identified; stacks of glacigenic debris flows (GDFs) and major slide debrites. Hemipelagic sediments have contributed to the total sediment volume but to a much smaller extent than the debris flows and the slides (Nyga˚rd et al., in press). The distal depositional province is dominated by GDFs and slide debrites, deposited in an up to 500 m thick glacigenic wedge pinching out towards the %gir Ridge (Fig. 3). This wedge, with a surface slope of ca. 0.28, accumulated on top of a seismically transparent package assumed to correspond to the hundreds of metres thick biosiliceous muds and oozes of Oligocene–Miocene age found in ODP sites from the outer Vbring Plateau (Eldholm et al., 1989b). The ooze locally penetrates the overlying glacigenic wedge due to diapirism (Fig. 3). The GDFs have been extensively studied on the proximal North Sea Fan where the debris flows are organized in sequences. Each sequence is represented by a broad basinward thinning apron of stacked lobes built out from the shelf edge. The GDFs develop their characteristic morphology from approximately 700–800 m water depth (Fig. 4), and occur from this depth on as continuous elongated lobes, lensoid in cross-section, 2–40 km wide and with thicknesses up to 60 m (King et al., 1996; Nyga˚rd et al., 2002). In the distal wedge, west of the Mbre

H.P. Sejrup et al. / Marine Geology 213 (2004) 169–200 Fig. 3. Geoseismic section across the North Sea Fan. P1–P10: identified Late Plio–Pleistocene seismic sequences on the proximal North Sea Fan. GDFs: Glacigenic debris flows. Figure is modified from Nyga˚rd et al. (in press). Profile location in Fig. 2b.

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Fig. 4. (a) TOBI side scan sonar data from the outer Norwegian Channel and uppermost North Sea Fan. Modified from Nyga˚rd et al. (2002). Bright and dark tones reflect strong and weak backscatter, respectively. Deeper than about 600 m water depth the seabed is covered by GDFs. Figure location in Fig. 2b. (b) Boomer profile across GDFs, revealing their seismic expression. Figure modified from Nyga˚rd et al. (2002).

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Marginal High, the lowermost glacigenic unit identified consists of well-developed lensoid GDF bodies (Fig. 3). Subsequent GDF deposits in this region are locally observed as far west as the %gir Ridge (Vogt, 1997), almost 500 km from the shelf edge, and are volumetrically subordinate compared to the observed slide debrites. Due to repeated sliding events, only a thin sequence of GDFs (b75 m), presumably of Weichselian age, has been preserved on top of the Mbre Marginal High. Lithologically, the GDFs are made of finegrained, homogenous, structureless grey diamictons, with remarkably uniform properties across the whole North Sea Fan. Both seismic and lithological evidences show that the slope GDFs and shelf tills are closely associated. Therefore, it is assumed that GDFs have been directly sourced from basal till transported subglacially to the shelf edge during glaciations maxima (King et al., 1998). Dating of glacimarine sediments overlying the youngest GDFs indicate that they ceased to form close to 15 14C ka BP (King et al., 1998) (Fig. 5), when the Norwegian Channel was deglaciated. Seismic correlations of the lowermost GDF sequence identified on the North Sea Fan to the Troll 8903 borehole in the Norwegian Channel (Sejrup et al., 1995, 1996; King et al., 1996) (Fig. 2b) and to borings on the upper Mbre continental slope (STRATAGEM Partners, 2002) indicate that processes leading to the formation of GDFs initiated at ca. Marine Isotope Stage (MIS) 12. The large Tampen and Mbre slide events have severely dissected the proximal North Sea Fan (Figs. 2a and 3). Observations indicate that the smooth surfaces of older GDF sequences, possibly draped with interglacial/interstadial sediments, have acted as glide planes for the Mbre Slide and in particular the Tampen Slide. Note that deposition of GDFs during subsequent glaciations effectively filled the slide scars left by older events. Basinward of the Mbre Marginal High, deposition of large slide debrites alternated with the deposited gravity flows/GDFs. The slide debrites show a chaotic internal signature, and are pincing out towards the %gir Ridge. The top surfaces of the debrites also get smoother westward, and nearby the %gir Ridge debris flow extension of the main slide debrites are presumably found. At least three slide debrites, with varying run-out, are identified on the distal North Sea

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Fan (Fig. 3). The youngest two slide events are of significant size. Seismic correlation between the distal and the proximal fan complex is not obvious due to heavy erosion and thinning across the Mbre Marginal High. However, the two youngest slide debrites west of the Mbre Marginal High are tentatively correlated to the Tampen and Mbre slides, respectively (Nyga˚rd et al., in press). 2.2. Storegga Slide—mega-slide margin The acoustic data from the Mbre continental margin demonstrate that this region has been affected by repeated slide events (Evans et al., 1996; Bryn et al., 1998, 2003; Evans et al., 2002), of which the present-day exposed Storegga Slide is the last mega-slide affecting this part of the SE Nordic Seas margin (Fig. 2, Figs. 6–8). Since most of the large and medium scale slides identified on the SE Nordic Seas margin are buried, it is of prime importance to study in detail the exposed Storegga Slide as an analogue for the development of older slides in the region. The Storegga Slide scar was first described by Bugge (1983). He suggested that the slide scar had been created through three major slide events, where the oldest event was N30–50 ka old and the youngest event was generated at about 6–8 14C ka BP (Bugge et al., 1987; Jansen et al., 1987). Observations of distorted sediment sequences in shallow marine/lacustrine basins in western Norway and in Scotland furthermore led to the conclusion that a tsunami flooded the coasts of Norway and Scotland at about 7.2 14C ka BP. This tsunami was therefore related to the youngest Storegga Slide event as defined by Bugge (1983) (Bondevik et al., 1997a,b; Dawson et al., 1988; Dawson, 1994). Studies by Haflidason et al. (2004; in press) suggest that the Storegga Slide scar was created during one single retrogressive event that took place at about 7250F250 14C years BP (8100F250 cal years BP). The Storegga Slide event is found to have affected an area of about 95,000 km2 and involved a total sediment volume of ca. 2500–3200 km3 (Haflidason et al., 2004). Even though this volume is found to be significantly less than the sediment volume first estimated by Bugge (1983), the Storegga Slide is still one of the largest exposed slides in the world and it is

178 H.P. Sejrup et al. / Marine Geology 213 (2004) 169–200 Fig. 5. (a) Magnetic susceptibility (MST) curves from 3 m long 14C AMS dated gravity cores from the North Sea Fan. The datings are reservoir corrected by 440 years. Core location in Fig. 2b. Figure modified from King et al. (1998). GDFs: Glacigenic debris flows. (b) Schematic profile showing stratigraphical location of three of the cores in (a). GDFs: Glacigenic debris flows; MMH: Mbre Marginal High; NSF: North Sea Fan. Figure modified from King et al. (1998).

H.P. Sejrup et al. / Marine Geology 213 (2004) 169–200 Fig. 6. (a) TOBI imagery from the northern Storegga Slide escarpment. Imagery location in Fig. 2b. (b) Close-up view showing well-defined, sub-parallel, concentric ridges and cracks at the northern Storegga Slide flank. Imagery location in (b). 179

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Fig. 7. Multichannel seismic profile (MB-0400-91), showing seismic facies characteristics of the Late Plio–Pleistocene deposits within the Storegga Slide region. The position of INO3 (MIS 5e) in the well-stratified hemipelagic/glacimarine Naust O unit on south Vbring Plateau is indicated. Profile location in Fig. 2b.

certainly the largest known slide from the continental margins of Europe (Canals et al., 2004). The Storegga Slide scar defines a depression bounded by a ~300 km long headwall along the present-day shelf edge (Fig. 2, Figs. 6–8). The scar is, however, reduced to a chute of only ~50 km in width at a water depth of about 2000–2200 m where the slide is crossing the Faroe–Shetland Escarpment (Fig. 2b). The exposed Storegga Slide surface can be divided into four morpho-sedimentary areas, which from shallow to deep are (Fig. 2b): (a) erosional, (b) erosional/depositional, (c) compressional and (d) accumulational. The areas of erosion are associated with the upper slide region where the sediments have mostly failed along planes of weakness. The planes are commonly parallel to the stratigraphic layering and can be followed for long distances. One of the most pronounced failure planes identified in the upper Storegga Slide area corresponds to the INO3

reflector (Figs. 7–9). The INO3 reflector is dated to be of last interglacial age, i.e. the Eemian Interglacial (MIS 5e) (Haflidason et al., 2003; Hjelstuen et al., in press). The erosional/depositional area are associated with the region where the width of the slide scar is considerably reduced and where the slope gradient is slightly less than on the upper slope. This area is also characterised by deep erosion channels. Both inside and along the rims of these channels, partly disintegrated slide deposits have been observed. These slide deposits have been transported downslope as blocks/ridges, and are frequently found as well-defined mounds or lobes. The two most extensive compressional areas observed within the slide scar are identified below the erosional area and in a region just west of the Faroe–Shetland Escarpment (Figs. 2b and 7), whereas the accumulation area is associated with the distal parts of the

H.P. Sejrup et al. / Marine Geology 213 (2004) 169–200 Fig. 8. Seismic profile NH9651-107 showing seismic facies characteristics of the deposits within the Storegga Slide region. The Storegga Slide glide plane (INO3) and sequence boundaries Top Naust R (TNR) and Top Naust S (TNS) are indicated. Profile location in Fig. 2b.

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Fig. 9. Mini-sleeve gun profile (NH9753-205) showing seismic facies characteristic of the two main depositional types, Lithofacies I and Lithofacies II, on the south Vbring margin. Identified Naust units (Naust W–Naust O) and their ages are indicated. Figure is modified from Hjelstuen et al. (in press). Profile location in Fig. 2b.

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Storegga Slide, and consists of both debris flow and turbidite sediments that have been eroded and transported from the upper and middle slope. The Storegga Slide region is furthermore characterised by sedimentary units composed of diamicton and/or ice-proximal upper slope deposits and hemipelagic, contouritic and/or glacimarine, seismically well-layered deposits on the middle and lower slope (Figs. 7 and 8). The boundaries of these units, which vary considerably in thickness, are usually regional. The sediments that failed during the Storegga Slide events are composed of glacial and interglacial sediments. To reconstruct these units, major emphasis has been put on analysing the geometry and stratigraphic context of the undisturbed sediment units found adjacent to the slide (Figs. 3 and 9). Analyses carried out on sediments from the last glacial–interglacial cycle have revealed that these cycles commonly result in stacks of rather well-defined bedforms/units related to sedimentation processes that have been strongly influenced by climatic and oceanographic oscillations. Recent studies have also shown that the delivery of sediments to the Storegga Slide area varied dramatically during the last glacial–interglacial period (Hjelstuen et al., in press), depending on the position of the ice sheet on the shelf and on the stability of the Norwegian Channel Ice Stream (NCIS) (Sejrup et al., 2003). The sediment flux as well as the geometry and the genesis of the Pleistocene units are considered to be important for the understanding of the stability of the Storegga Slide region. 2.3. South Vøring Plateau—hemipelagic dominated margin During the Late Pliocene and Pleistocene an up to 1500 m thick prograding wedge was deposited on the Vbring margin (Fig. 9) (Henriksen and Vorren, 1996; Hjelstuen et al., 1999). This wedge reflects the increased delivery of sediments to the margin due to intensified erosion caused by the Northern Hemisphere glaciations and the Neogene uplift of Fennoscandia. Detailed investigation of the 300 m thick uppermost part of this wedge, covering the Middle and Late Pleistocene time period, shows that it consists of two main lithofacies (Hjelstuen et al., in press); here termed Lithofacies I and Lithofacies II.

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Lithofacies I consists of hemipelagic sediments with a variable admixture of coarse material. In seismic profiles, Lithofacies I deposits display parallel medium to high amplitude reflectors or an acoustically transparent seismic pattern (Fig. 9). Locally, the Lithofacies I sediments are intersected by pockmarks and other fluid escape structures (Gravdal et al., 2003). Small-offset faults and diapirs are also preserved in these deposits. Observation of bottom simulating reflectors (BSRs) (e.g. Mienert et al., 1998), furthermore indicate that gas hydrates are, or have been, present. Isopach maps (Hjelstuen et al., in press) reveal that the southernmost part of the Vbring Plateau has been a main depocentre for Lithofacies I sediments throughout the entire Pleistocene. The core data (Figs. 10 and 11) furthermore show that such sediments were deposited rapidly during glacial stages. For MIS 2 it has been documented that between 16.2 and 15.7 14C ka BP sediment rates as high as 36 m/ky existed on the upper Vbring continental slope (Hjelstuen et al., in press). Lithofacies II consists of glacigenic material (Haflidason et al., 1998; Dahlgren and Vorren, 2003) that interfinger Lithofacies I sediments as wedge shaped units on the upper slope (Fig. 9). The Lithofacies II units show a large variation in thickness and extend along the continental slope. The oldest glacigenic unit identified is of MIS 8–10 age (Fig. 9) and reaches a maximum thickness of about 250 m (Dahlgren et al., 2002; Hjelstuen et al., in press). This unit wedge out in a water depth of about 1400 m, and shows the most extensive distribution on the Vbring margin of the mapped glacigenic units. Lithofacies II units deposited during MIS 6 and MIS 2 reach their maximum thickness at the mouth of shallow troughs, and wedge out in water depths of about 1000 and 700 m, respectively. The seismic profiles reveal that the outermost part of the identified MIS 8–10 unit is composed of lensoid features, whereas downlapping reflectors are locally observed within the unit further upslope. Similarly, the younger glacigenic units also display a lensoid pattern at their tip, however these are less well developed than those observed in the MIS 8–10 unit. The high-resolution seismic records locally reveal erosion at the base of these units.

184 H.P. Sejrup et al. / Marine Geology 213 (2004) 169–200 Fig. 10. Compilation of geotechnical borings and IMAGES cores from the south Vbring margin. Core information from Haflidason et al. (1998), Knorr (2000), Haflidason et al. (2001) and Berstad et al. (in press). Location of borings and cores in Fig. 2b.

H.P. Sejrup et al. / Marine Geology 213 (2004) 169–200 Fig. 11. (a) Isopach map of lithological unit L3 (see IMAGES core MD99-2291 in Fig. 10), deposited between 15.7 and 15.0 14C ka BP. (b) Sedimentation rates the last 30,000 years in selected cores along the studied margin. Core information from Dokken and Jansen (1999), Knorr (2000), Lekens (2001) and Berstad et al. (in press). Core locations in (a).

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2.4. Alongslope seismic correlation The above-described Pleistocene sediments belong to the Naust Formation (Dalland et al., 1988), and have been divided into several sub-units based on seismic expression and sedimentary composition (e.g. McNeill et al., 1998; Dahlgren et al., 2002; STRATAGEM Partners, 2002; Hjelstuen et al., in press; Nyga˚rd et al., in press). Nyga˚rd et al. (in press) have divided the Pleistocene succession on the proximal part of the North Sea Fan into several seismostratigraphic units termed P1 (youngest) to P10 (Figs. 3 and 12). As stated these units are commonly composed of GDFs/gravity flows. However, mega-slides, as the Tampen and Mbre (Figs. 3 and 12), have caused deep erosion both on the proximal and distal parts of North Sea Fan. On the south Vbring margin and in the Storegga Slide region the Naust Formation has been subdivided into five hemipelagic/glacigenic units, termed Naust O (youngest), R, S, U and W (Norsk Hydro, 2003). Within the Storegga Slide area Naust units S, R and O have been severely dissected (Fig. 12). The Naust units have been correlated to dated cores. The cores have been dated by 14C AMS datings, amino-acid analyses, biostratigraphical analyses, stable isotopes and paleomagnetism (Haflidason et al., 1998, 2001, in press; Knorr, 2000; Berstad et al., in press). The shallow IMAGES cores give a wellconstrained chronology of the upper Naust O unit on the south Vbring margin (Hjelstuen et al., in press) (Fig. 10). Mainly based on amino-acid analyses the upper part of the Naust R unit is considered to be of MIS 8 age (Figs. 9 and 10). Naust R probably also embraces MIS 10, whereas Naust S most likely is of MIS 12–14 age (Fig. 12). In the North Sea Fan region it has been inferred that GDFs units P1, P4, P5 and P8 have been deposited during MIS 2, 6, 8 and 10 (Nyga˚rd et al., in press). Units P2 and P6 have most likely been deposited during late stages of MIS 6 and MIS 8, respectively (Fig. 12); whereas the lowermost unit identified, P10, is possibly related to MIS 12. Correlations of the identified Naust units from the Vbring margin to the Storegga Slide/North Sea Fan region indicates that the reflector defining the upper boundary of the P5 unit to the south may correspond to the upper boundary of Naust R unit on the Vbring Margin, whereas the upper boundary of P9–P10 appears to tie to the upper boundary of the Naust S

unit to the north (Fig. 12). However, it should be stated that the complicated stratigraphical setting of the SE Nordic Seas margin with numerous slide events and deep erosion, introduce uncertainties in the correlation.

3. Margin evolution 3.1. Subsidence To understand the geometry of the sedimentary sequences forming the SE Nordic Seas margin it is essential to take into account subsidence rates through time. In the time period just after the ca. 55 Ma breakup of the Norwegian–Greenland Sea, the region subsided rapidly primarily due to thermal cooling and contraction. Based on a limited number of data points, Eldholm et al. (1989a) performed backstripping analyses on ODP Leg 104 sites and showed that the outer Vbring Plateau subsided by up to 3 km during post-break-up times. Furthermore, Pedersen and Skogseid (1989) demonstrated that the region just east of the Vbring Escarpment subsided by 1.8–2 km at a constant rate during the same time period. This is in contrast to the established subsidence curve close to the shelf edge, where it appears that the subsidence rate increased for about 5 million years ago (Pedersen and Skogseid, 1989). This Late Neogene increase in subsidence rates was also observed by Cloetingh et al. (1990) from analyses of 85 wells within the North Atlantic. Seismic data from the south Vbring margin reveal that iceberg scourings appear at depths of 1.1 s(twt) (about 800 m) below the present-day sea level (Hjelstuen et al., in press). These features are observed on the upper bounding surface of the glacigenic unit of MIS 8–10 age (Fig. 9). If we anticipate that iceberg keels did not reach water depths in excess of about 500 m, as is supported by the distribution of iceberg scourings on the modern seafloor in the region (Gravdal, 1999; Masson, 2001; Nyga˚rd et al., 2002), the south Vbring Plateau has subsided by about 300 m since MIS 8. This results in a maximum subsidence rate of about 1.2 m/ky for the last ca. 250 ka. Dahlgren et al. (2002), who based their study on observed paleo-ice sheet grounding lines, found similar subsidence rates at the northern Vbring margin for the last 350 ka.

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Fig. 12. Geoseismic profile crossing the North Sea Fan, Storegga Slide and the southern Vbring margin showing distribution and correlation of identified Pleistocene units along the SE Nordic Seas margin. Stratigrapic nomenclature on the North Sea Fan from Nyga˚rd et al. (in press), whereas the nomenclature in the Storegga Slide region and the Vbring Plateau are from Norsk Hydro (2003). Insert tables show the proposed ages for the identified seismic units. Profile location in Fig. 2b.

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3D seismic data from southern Mbre continental shelf reveal a delta complex below tills of Weichselian and MIS 6 age (Fig. 13). The subaerially developed delta plain is interpreted to be represented by a subhorizontal unconformity observed at ca. 280 m below the present-day sea level. The unconformity slopes, ca. 0.1–0.28, towards northwest. Amplitude analyses of the 3D data revealed elongated features associated with this unconformity, interpreted to represent fluvial channels. Age brackets for the delta is based on depth below sea level, seismic correlation to the Troll 8903 core (Fig. 2b) (Sejrup et al., 1995), which indicate an age NMIS 5e, and correlation to borings on the Mbre slope, which give an age VMIS 8. Therefore, assuming that the delta was constructed when sea level was lowest and that there was no glaci-isostatic depression, a minimum subsidence rate of 0.7 m/ky is obtained for the last ca. 250 ka for the southern Mbre continental shelf. Based on the depth to the paleoshelf edge of the oldest glacial unconformity identified, King et al. (1996) estimated a minimum subsidence rate of 0.8 m/ky within the same region. We note that these high subsidence rates occur contemporaneously with increased Late Plio–Pleistocene sediment input to the margin. Thus, it appears that the enhanced subsidence could be an effect of the increased sediment loading, as also is noted by Skogseid and Eldholm (1989). However, tectonic events appear to have influenced on the subsidence pattern. Jordt et al. (2000) suggested that lithospheric stress, induced by ridge-push from the Atlantic rift zone and the Alpine collision, caused enhanced Plio– Pleistocene subsidence. Cloetingh et al. (1990) also stated that the increased subsidence rates were related to changes in spreading direction along the Atlantic ridge system. It seems, therefore, that the enhanced subsidence pattern was caused by a complex interaction between sediment loading, thermal subsidence and regional tectonic events. 3.2. Deposition of glacigenic sediments and GDFs Large-scale ice rafting in the Norwegian Sea started at ca. 2.74 Ma, reflecting the initiation of the Late Plio–Pleistocene Northern Hemisphere glaciations (Jansen et al., 2000). However, until 1.1 Ma the ice caps most likely were of moderate size, only reaching the coastal area of Norway (Jansen and

Sjøholm, 1991). From about 1.1 Ma larger ice caps developed, and the continental shelf was periodically covered beneath extensive ice sheets that have reached the shelf edge several times since ca. 0.5 Ma (Haflidason et al., 1991; Sejrup et al., 2000; Dahlgren et al., 2002; Hjelstuen et al., in press; Nyga˚rd et al., in press). It has been inferred that NCIS was activated and grounded at the shelf edge for the first time at 1.1 Ma, during the Fedje glaciation (Sejrup et al., 1995). The last deglaciation of the Norwegian Channel, at ca. 15 14C ka BP, is well documented by a number of 14C datings from the North Sea Fan and the Northern North Sea (Lehman et al., 1991; Sejrup et al., 1995; Haflidason et al., 1995; King et al., 1998). Recent studies have, however, indicated that a regional glacial readvance occurred on the Mbre–Vbring margin between ca. 15 and 13.3 14C ka BP (Rokoengen and Frengstad, 1999; Nyga˚rd et al., 2004). Ice sheets are capable of transporting huge quantities of material, and studies from glacial continental margins have shown that grounded ice sheets may leave basal till sheets, often streamlined, on continental shelves (Vorren et al., 1989; Canals et al., 2000, 2002). Within the Norwegian Channel (Fig. 1) Sejrup et al. (1996) demonstrated that a typical channel unit comprises a 30–40 m thick till sequence, which is capped by marine/glacimarine sediments. Successive till units are separated from each other by extensive glacial erosion surfaces. Based on lithological, textural and seismic evidence it has been demonstrated that the GDFs of the North Sea Fan represent the downslope continuation of the till units on the shelf (King et al., 1998). Thus, during glacial maxima, when ice sheets were grounded at the shelf edge, NCIS transported basal till to the shelf edge where the sediments were distributed further down slope as GDFs (Figs. 14 and 15). Nyga˚rd et al. (2002) attributed the release and mobility of GDFs on the North Sea Fan to elevated pore pressures, while other authors have advocated the concept of hydroplaning to explain the extreme run-out of comparable deposits (Mohrig et al., 1999; Laberg and Vorren, 2000). At the Vbring margin, sediment cores and seismic reflection profiles reveal that basal till deposits on the continental shelf were deposited as acoustic nonstructural units characterised by high shear strength and overconsolidation (King et al., 1987; Rokoengen

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Fig. 13. (a) Seismic profile from the southern Mbre continental shelf (Fig. 2b), showing the constructional elements of a deltaic sediment body. Profile location in (b). (b) Isochron map, with a contour interval of 20 ms (twt), of the unconformity defining the delta surface. The delta front, as indicated in (a), is shown by a stippled line.

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Fig. 14. Conceptual models of the geological development, during one single glacial–interglacial cycle, of the: (a) North Sea Fan. m.b.s.l.: metres below sea level; MMH: Mbre Marginal High; NCIS: Norwegian Channel Ice Stream. (b) Storegga Slide area. m.b.s.l.: metres below sea level. (c) South Vbring margin. m.b.s.l.: metres below sea level.

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Fig. 14 (continued).

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Fig. 14 (continued).

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Fig. 15. Setting of the SE Nordic Seas margin during interglacial (a) and glacial (b) conditions. GDF: Glacigenic debris flow.

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and Fregnstad, 1999). These units can be followed beyond the present-day shelf edge, where they correlate to Lithofacies II units (Hjelstuen et al., in press) (Fig. 9). The seismic facies pattern suggest that the slope part of these glacigenic units have been deposited in a similar way as the GDFs on the North Sea Fan (Fig. 14). If each wedge shaped glacigenic unit on the Vbring margin represents one shelf edge glaciation, grounded ice reached the shelf edge in this region at MIS 10, 8, 6 and 2 (Hjelstuen et al., in press). The run-out distances of the glacigenic material differ along the margin, with the largest run-outs at the North Sea Fan. These differences can be ascribed to the varying amounts of sediments transported to the shelf edge, to the physical properties of the sediments and to differences in flow mechanisms (Marr et al., 2002). 3.3. Deposition of marine and glacimarine sediments Core data show that Lithofacies I units on the south Vbring margin mainly represent glacimarine sediments deposited from suspension, and as icerafted debris (IRD). Mounded sediment packages within slide scars (Laberg et al., 2001) and seismic truncation patterns within a water depth of 750–900 m reveal that currents locally have influenced the sediment distribution during the Pleistocene. It also appears that the margin configuration established after deposition of the massive MIS 8–10 glacigenic unit (Fig. 9) strongly affected the glacimarine depositional pattern. Between 15.7 and 15.0 14C ka BP, when the lithological unit L3 in core MD99-2291 (Fig. 10) was deposited on the south Vbring margin, both the composition of the sediments and the seismic pattern changed significantly. The isopach map shows that unit L3 reaches a maximum thickness of about 20 m on the upper slope, thinning northwards and westwards (Fig. 11). Due to the 7.25 14C ka BP Storegga Slide event (Haflidason et al., in press) unit L3 cannot be mapped south of the Vbring Plateau. Several cores from the Vbring margin show high sedimentation rates at the time of deposition of L3, whereas within the same time interval cores on the southwestern flank of the North Sea Fan show low sedimentation rates (Fig. 11). The established chronology also reveals that deposition of unit L3

roughly coincides with the deglaciation of the Norwegian Channel. It is therefore suggested that this fine-grained, acoustically transparent unit represent meltwater plume deposits generated during the disintegration of the NCIS. The meltwater plumes were deflected northwards by currents allowing for deposition in the Storegga Slide region and on the south Vbring margin (Fig. 14). 3.4. The interglacial system The present-day circulation regime and the observed distribution of Holocene sediments are good analogues for the sedimentation patterns on the SE Nordic Seas margin during past interglacials. NwAC enters the Norwegian Sea through the Faroe– Shetland Channel and follows the Norwegian continental margin northwards (Hansen and Østerhus, 2000; Hansen et al., 2001; Orvik and Niiler, 2002) (Fig. 1). The boundary between the warm, saline Atlantic Water and the slower water masses with temperatures close to 1 8C and salinity close to 34.9x occurs at ca. 700 m deep on the upper continental slope. We note that current velocities of up to 50 cm/s have been recorded on the shelf and upper slope. The surface sediment distribution along the margin appears to be strongly related to this water mass structure (Figs. 14 and 15). Within water depths less than ca. 700 m the surficial sediments consist of a b0.5-m-thick layer of sorted sand/ gravely lag (Holtedahl, 1981). The lower boundary of these sediments coincides with the transition zone between the Atlantic Water and the underlying homohaline deep-water (Sejrup et al., 1981; Holtedahl and Bjerkli, 1982). Between about 700 and 1200 m water depth, up to 25 m thick terrigeneous muds are found, whereas at depths exceeding 1200 m the seabed sediments are b50 cm thick and consist of foraminiferal oozes with a carbonate content of N30% (Sejrup et al., 1981). In the Storegga Slide area, up to 25 m of finegrained terrigeneous sediments have been deposited after the Storegga slide event (Berstad et al., 2003; Haflidason et al., in press). The term bcontouriteQ have been used for these deposits (Bryn et al., 2002, 2003). There is no indication for similar Holocene high sedimentation rate localities along

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the SE Nordic Seas margin. It has been inferred that NwAC could have acted in a similar manner during earlier interglacial periods. This is especially plausible for the last interglacial (MIS 5e), between 128 and 115 ka ago. Taken into account the subsidence rates for this part of the margin (Fig. 2b), MIS 5e countouritic sediments should be found in water depths exceeding 800 m. There is evidence for older contourites within the Storegga Slide complex (Fig. 8). This could indicate that the Holocene type of sedimentation regime has been active also during earlier interglacials.

4. Pleistocene development and mega-slides Several minor, major, and mega-slides intersect the sedimentary succession of the SE Nordic Seas continental margin (Fig. 2a). The oldest slide event is recorded within the lower part of the Naust Formation, and is interpreted to be as old as late Pliocene or early Pleistocene (Evans et al., 2002). In addition to the exposed Storegga Slide, the Tampen and Mbre slides on the North Sea Fan and the Sklinnadjupet Slide on the central Vbring Plateau are three of the largest slides on the investigated margin segment (Fig. 2a). The Tampen Slide has affected an area of ~21,000 km2 (Nyga˚rd et al., in press). This should however be regarded as a minimum estimate, since the eastern flank of the slide might have extended further into the region subsequently eroded by the Storegga Slide. Observation of possible Tampen Slide remnants within the exposed Storegga Slide scar supports this interpretation (Fig. 7). The Mbre Slide is confined to the North Sea Fan (Figs. 2a and 3) and covers an area of ~14,000 km2. This slide event has been correlated to Palaeoslide 2 identified within the Storegga Slide region (Evans et al., 2002). Bryn et al. (2003) suggested a link with climate cyclisity and mega-slides in the region. Based on the stratigraphical position of these slides within the Late Plio–Pleistocene sedimentary succession (Fig. 3), Nyga˚rd et al. (in press) suggested a late MIS 6 age as a maximum age for the Tampen Slide, whereas the Mbre Slide has an age between MIS 10 and MIS 8. The Sklinnadjupet Slide on the Vbring Plateau mainly affects the glacigenic unit of MIS 8– 10 age (Figs. 2a and 9), thus revealing that the

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Sklinnadjupet event might be coeval with the Tampen Slide. The slide frequency of the minor, major and megaslides on the investigated margin segment apparently increased considerably after 0.5 Ma. Throughout the Middle and Late Pleistocene the slide events occurred with a periodicity close to 0.1 Ma (Britsurvey, 2002). The increase in slide activity seems to have been contemporaneous with the onset of larger-scale Fennoscandian glaciations (Haflidason et al., 1991; Sejrup et al., 2000; Nyga˚rd et al., in press). The ages of the Tampen and Sklinnadjupet slides also reveal that these events are closely related to glaciations, when the ice sheets reached the continental shelf edge. Furthermore, it appears that the structural setting along the margin controls to some extent the location and generation of slides.

5. Summary and conclusions The Pleistocene sedimentary sequences of the SE Nordic Seas continental margin have been deposited by: (1) marine and glacimarine processes, (2) slide events, (3) GDFs related to fast moving ice streams, and (4) grounded ice sheets. The effect of these depositional processes shows large temporal and spatial variation along the studied margin segment. The Vbring continental margin is dominated by deposition of hemipelagic/glacimarine sediments, which on the upper slope are interfingered with glacigenic sedimentary wedges deposited during glacial advances. Fine-grained meltwater plume deposits were rapidly deposited in this region between 15.7 and 15.0 14C ka BP. The North Sea Fan, on the northern North Sea margin, is divided into two sedimentary provinces by the Mbre Marginal High (Fig. 3). The fan complex is dominantly composed of GDFs and slide debrites. The deposition of GDFs on the North Sea Fan was most likely initiated at MIS 12. The sedimentary succession on the Storegga megaslide margin is characterised by units composed of diamicton and/or ice-proximal upper slope deposits and hemipelagic, contouritic and/or glacimarine seismically well-layered deposits on the middle and lower slope. The SE Nordic Seas continental margin is thus defined by an ice stream influenced margin segment

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(northern North Sea), a mega-slide dominated margin segment (Mbre) and a hemipelagic dominated margin segment (southern Vbring). These three segments show several similarities in their Pleistocene geological development (Figs. 14 and 15). During interglacial conditions low sedimentation rates and winnowing along the continental shelf edge prevailed along the SE Nordic Seas margin. Local exceptions only existed in the Storegga Slide segment, as exemplified by up to 25 m of soft sediments accumulated during the present-day interglacial (Fig. 14). During glaciations ice sheets were grounded on the shelf edge and ice streams were transporting basal till to the shelf break from where till was transported down slope by mass movements as GDFs (Figs. 14 and 15). In addition, thick packages of glacimarine/hemipelagic sediments were deposited on the Vbring and Mbre margins during glacials. Meltwater plumes appear to have been generated especially during disintegration of the NCIS. Northwards flowing currents transported plume suspensates to the Storegga Slide region and to the south Vbring Plateau where particle settling resulted in thick units (Fig. 14). Apparently, the large slides on the Northern North Sea and the Mbre margins were initiated during late glaciation/early deglaciation phases. The SE Nordic Seas margin was also characterised by high subsidence rates during the Pleistocene (Fig. 2a). On the southern Vbring margin, a subsidence rate of 1.2 m/ky has been estimated for the middle and late Pleistocene. The Northern North Sea margin subsided at a rate of 0.8 m/ky throughout the same time period, whereas a maximum subsidence rate of 0.7 m/ky has been estimated on the Mbre continental shelf for the last ca. 250 ka. The high sedimentation rates found along the SE Nordic Seas margin during this time span cannot, however, account for all the observed Pleistocene subsidence, and additional mechanisms, such as tectonism, must be taken into consideration.

Acknowledgements This study has been supported by the EC commission, through the COSTA (EVK3-1999-00028) and STRATAGEM (EVK3-CT-1999-00011) projects, by

the Norwegian Research Council, Enterprise Oil Norge Ltd and Norsk Hydro ASA. We are grateful to the SEABED and Ormen Lange joint industry consortiums for providing seismic and core data. Miquel Canals, Jan Sverre Laberg and David Long reviewed the paper, and gave valuable suggestions for improvements.

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