Late Quaternary glacial development of the mid-Norwegian margin—65 to 68°N

Marine and Petroleum Geology 19 (2002) 1089–1113 www.elsevier.com/locate/marpetgeo

Late Quaternary glacial development of the mid-Norwegian margin—65 to 688N K.I. Torbjørn Dahlgren*, Tore O. Vorren, Jan Sverre Laberg Department of Geology, University of Tromsø, N-9037 Tromsø, Norway Received 31 May 2002; received in revised form 19 December 2002; accepted 11 January 2003

Abstract We have studied the younger part of the large Plio-Pleistocene prograding wedge on the northern part of the mid-Norwegian margin, using high-resolution seismic data. The wedge is mainly composed of thick glacigenic debris flow packages interbedded with hemipelagic and contouritic sediments. Age constraints of the seismic stratigraphy obtained through correlation to ODP 644A show that the Fennoscandian Ice Sheet advanced to the shelf break during Marine Isotope Stages (MIS) 2 (ca. 15 – 22 ka BP), 6 (128 – 186 ka BP), 10 (339 – 362 ka BP), 12 (423 – 478 ka BP) and 14 (524– 565 ka BP), and to the inner shelf during MIS 8 (245– 303 ka BP). The stratal geometry and sedimentary pattern locally indicate rapid subsidence (up to ca. 1.2 m/ka) of the outer shelf and slope area since 350 ka BP. The subsidence is most likely a response to the rapid deposition of glacigenic sediments during the Plio-Pleistocene. It is inferred that sediment loading was an important factor behind the diapirism in the Vema Dome area. q 2003 Elsevier Science Ltd. All rights reserved. Keywords: Continental margin; Late Quaternary; Debris flow

1. Introduction The mid-Norwegian shelf (Figs. 1 and 2A) has repeatedly been inundated by glaciers from mainland Norway throughout the Plio-Pleistocene, as is evident both from seismic records (Henriksen & Vorren, 1996; Rokoengen et al., 1995) and proxy data from deep-sea sediment cores (Baumann et al., 1995; Henrich & Baumann, 1994; Jansen, Bleil, Henrich, Kringstad, & Slettemark, 1988; Jansen, Fronval, Rack, & Channell, 2000). Accordingly, glacial processes of erosion, transport and deposition have played a major role in controlling the sediment composition, distribution, architecture and the surficial morphology of the shelf deposits (Bugge, 1980; Henriksen & Vorren, 1996; King, Rokoengen, & Gunleiksrud, 1987; Sættem, 1990). Age-determination of the glacigenic shelf sediments offshore mid-Norway has, however, proven problematic (cf. Eidvin, Brekke, Riis, & Renshaw, 1998; Eidvin, Jansen, Rundberg, Brekke, & Grogan, 2000; Haflidason et al., 1991; Poole & Vorren, 1993; Rokoengen et al., 1995; Sættem, Rise, Rokoengen, & By, 1996). The continental slope has, in * Corresponding author. E-mail address: [email protected] (K.I. Torbjørn Dahlgren).

contrast to the shelf, drawn little interest in the past, although it should hold a key for linking the shelf stratigraphy to the better-dated deep-sea proxy-records of glaciation. Like the records archived in Trough Mouth Fans (TMFs) (Vorren & Laberg, 1997), slope sediments offshore midNorway have the potential to record the timing, extent and activity of past glaciations. Such information can in turn make it possible to reveal the effect of the interplay between climate, glaciers, tectonics and sea level changes. Possible information regarding late Cenozoic uplift/subsidence is of importance for elucidating both the burial history and the migration paths for hydrocarbons. Exploration for hydrocarbons in deep-water areas off mid-Norway also meets several challenges regarding geo-hazards such as slides and mud diapirism. In this respect a detailed stratigraphic framework and a sedimentological model with good chronological resolution is essential for establishing a reliable risk-assessment. We approach these problems through the interpretation of high-resolution seismic data from the continental slope and the Vøring Plateau. Chronological/lithological constraint is achieved through correlation of the seismic data to scientific boreholes and short gravity cores.

0264-8172/02/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0264-8172(03)00004-7

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Fig. 1. Overview of the Norwegian Sea and the large scale topography of mainland Norway and Sweden. TMFs and slides are adapted from Vorren et al. (1998). The shaded box marks the study area which is shown in detail in Fig. 2A. The bathymetric contours are shown in 100 m increments down to 500 m depth, 500 m interval between 500 and 2000 m depth, and below 2000 m the contour interval is 1000 m. The contours on land are enveloping topographic curves in km.

2. Physical setting The mid-Norwegian margin is characterised by an up to 200-km-wide continental shelf and a gentle slope down to the inner Vøring Plateau (Fig. 2A). The slope from the outer Vøring Plateau stretches gently into the abyssal plain of the Lofoten Basin northwards, and south-westwards into the Norwegian Basin (Fig. 1). On the shelf, depths range from less than 150 m on shallow bank areas to more than 450 m in the deep transverse glacial troughs (Fig. 2A). The shelf break is at depths between 280 and 410 m (Fig. 2A). The minimum depths are found on Skjoldryggen, and the maximum depths occur where open glacial troughs reach the shelf break (e.g. Trænadjupet). The continental slope along the central part of the study area has a gradient of 0.58, the gradient increasing to 1.28 off Gamlembanken (Fig. 2A). To the north of Trænadjupet, off Lofoten, the margin is steeper, up to 58. In the southernmost part of the study area the upper slope

has a very low gradient, approaching 0.38. The seafloor on the Vøring Plateau is relatively smooth, with depths between 1200 and 1400 m. More variable relief is found within the diapir fields (Fig. 3) and along the Vøring marginal escarpment (Fig. 2B) on the outermost part of the plateau. 2.1. Geology The deeper structures of the Vøring marginal plateau are divided into the Vøring Marginal High and the Vøring Basin, with the Trøndelag Platform situated to the east (Fig. 2B). Rifting and associated subsidence during the late Mid Jurassic to Early Cretaceous created the deep Vøring Basin to the west of the Trøndelag Platform (Skogseid, 1994). The Vøring Marginal High, which defines the western extent of the plateau, developed during the latest rifting episode (Late Cretaceous to Early Eocene) and the opening of the Norwegian Sea (Eldholm,

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Fig. 2. (A) Bathymetric map of the study area. The heavy dashed line marks the shelf break. This also marks the maximum extent of the Fennoscandian Ice Sheet during the last glacial maximum. Note the deep (.400 m) transverse glacial troughs, and the shallow (,300 m) intervening bank areas. (B) Map showing the major structural highs, arches and domes (in pale grey shading) on the mid-Norwegian margin. The area shaded in dark grey along the coast marks the border between out- or subcropping crystalline basement and the sedimentary rocks to the west. (C) Thickness of Upper Pliocoene/Pleistocene sediments off mid-Norway, adapted from Hjelstuen, Eldholm, and Skogseid (1999). Only areas where the sediment thickness exceeds 0.5 s TWT are shaded (1 s TWT is roughly equivalent to 1 km of strata). Note that the depocentres are situated along the shelf break (heavy stippled line), at the mouth of transverse glacial troughs on the shelf. (D) Map showing the seismic database and locations of cores referred to in the text.

Thiede, & Taylor, 1989). During the final stage of rifting and early sea-floor spreading in the Norwegian Sea (Late Paleocene to Eocene) the area was subjected to widespread subaerial flood-basalt volcanism (Eldholm et al., 1989). Seaward of the crystalline basement, on the innermost shelf, Mesozoic rocks sub-/outcrop (Bugge, Knarud, & Mørk, 1984; Fig. 2B). The Miocene to early Pliocene Kai Formation underlies the large Plio-Pleistocene prograding wedge (The Naust Formation). In the Vøring basin the Kai Formation is thought to mainly consist of siliceous

ooze (cf. McNeill, Sailsbury, Østmo, Lien, & Evans, 1998; Talwani et al., 1976), and on the shelf it mainly comprises clay-, silt- and sandstone with limestone stringers (Dalland, Worsley, & Ofstad, 1988). The PlioPleistocene prograding wedge and its distal sediments comprise the Naust Formation. The prograding wedge is thought to be largely composed of glacigenic debris flows interbedded with hemipelagic sediments (cf. Henriksen & Vorren, 1996; McNeill et al., 1998). During its deposition the shelf break migrated some 100 km seaward (Henriksen & Vorren, 1996).

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Fig. 3. Late Quaternary morphological and sedimentary features within the study area. The Nyk drift and the Sklinnadjupet Slide are adapted from Laberg et al. (2002). The Vøring Plateau Slide (paleoslide 2 in McNeill et al., 1998) is a possible slide feature. Due to scarce data cover in that area it is difficult to determine whether it is related to the Sklinnadjupet Slide event or if it is a separate event. The diapirs marked by dark grey shading were identified from high-resolution seismics in this study; the outlines of the diapir fields marked by pale grey are adapted from Hjelstuen, Eldholm, and Skogseid (1997).

2.2. Glacigenic sediment distribution The main Plio-Pleistocene depocentre is situated along the shelf break (Fig. 2C). Two areas where the Pliocene – Pleistocene deposits reach a thickness in excess of 1.5 s Two Way Time (TWT) are observed, at the mouth of the Trænadjupet Trough and along the shelf break off the Sklinnadjupet Trough. This is equal to about 1.5 km of sediments based on an acoustic velocity for the sediments of about 2 km/s. To the south of the study area, Pliocene – Pleistocene depocentres occur landward of the Storegga Slide area (Stuevold & Eldholm, 1996) and at the mouth of the Norwegian Channel where the North Sea TMF is situated (King, Haflidason, Sejrup, & Løvlie, 1998; King, Sejrup, Haflidason, Elverhøi, & Aarseth, 1996). All these depocentres are situated seaward of glacial troughs or main drainage paths for past glaciers. 2.3. Slides and Paleoslides Both the Trænadjupet and the Storegga Slide complexes (Fig. 1) have repeatedly been affected by large-scale slope failure (during the Holocene and the recent geological past)

(e.g. Bryn, Østmo, Berg, & Tjelta, 1998; Bugge, 1983; Evans, King, Kenyon, Brett, & Wallis, 1996; Laberg & Vorren, 2000; Vorren et al., 1998). Consequently, large portions of the sediments deposited in the past have been transferred to the basin areas. The extent of sliding in the deeper part of the stratigraphy is less well known. One large buried slide has so far been identified in the area between the Storegga and Trænadjupet Slide complexes, namely the Sklinnadjupet Slide (Laberg, Dahlgren, Vorren, Haflidason, & Bryn, 2002). The Sklinnadjupet Slide (Fig. 3) was previously referred to as the Trænabanken Slide (Evans, McGiverson, Harrison, & Bryn, 2003).

3. Material and methods Mini sleeve airgun and deep-tow boomer data (Fig. 2D) was provided to us through the Seabed project. The data are generally of very good quality. The mini sleeve airgun data was, however, cut at a depth of 1 s below the seabed. The sparker/3.5 kHz and the mini sleeve airgun/3.5 kHz data (Fig. 2D) was collected during cruises in August 1998, July 1999, 2000 and June 2001, using the University of Tromsø’s

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Table 1 Core locations, water depth, core/hole length and references for the cores referred to in the text Core

Location

Water depth (m)

Core/hole length (m)

Core description/age model

JM98-624/1 JM98-628/1 ODP 644A MD95-2010

N668430 N668490 N668410 N668680

E078420 E068180 E048350 E048570

487 1068 1226 1226

2.6 3.9 254 32.5

N678130 E068190 N678120 E068180 N678200 E068070

1262 1206 1439

107.5 104.0 398.8

Dahlgren and Vorren, 2003 Dahlgren and Vorren, 2003 Henrich and Baumann, 1994 IMAGES database (Dokken & Jansen, 1999) Talwani et al., 1976 Talwani et al., 1976 Talwani et al., 1976

DSDP 339 DSDP 340 DSDP 341

research vessel F/F Jan Mayen. The data were printed online and digitally recorded on a Delph system, allowing for later playbacks. Seismostratigraphic units were defined based on the amplitude and continuity of bounding reflections, the internal acoustic signature as well as the outer geometry and unit architecture. Features such as the Vema diapir field, and the Trænadjupet, Nyk and Sklinnadjupet Slides (Fig. 3) complicate the seismic stratigraphy as they disrupt seismic reflections and units. Further, the occurrence of strong seabed multiples on the upper slope and shelf areas limits the mapping in the landward direction. Cores and boreholes referred to in this study are listed in Table 1. Age control for the stratigraphy was obtained through correlation of the deep tow boomer data to ODP site 644A (Fig. 2D). The applied age-model for ODP hole 644A is from Henrich and Baumann (1994). For correlation of the seismic record to ODP site 644A a p-wave velocity in sediment of 1500 m/s was applied, as indicated by p-wave logging data of core MD95-2010, which was retrieved from the same locality (IMAGES database).

4. Results In the following sections, we first present a general description of the two main seismic facies found in the study: laminated acoustic facies and homogenous (transparent) acoustic facies. We then outline a detailed seismostratigraphy for the late Cenozoic sediments on the mid-Norwegian continental slope and the inner Vøring Plateau between 65 and 688N. This stratigraphy represents a refinement of the regional seismostratigraphic framework put forward by Evans et al. (2003) and McNeill et al. (1998). We also significantly improve the chronological constraints of the seismic units and correlate the Vøring Plateau and continental slope stratigraphy to the stratigraphy on the continental shelf (cf. King et al., 1987). 4.1. Laminated acoustic facies The acoustically laminated facies is found on the continental slope and on the Vøring Plateau. On the

continental slope it interdigitates with and onlaps the acoustically homogenous Naust units and sub-units A2 –D (Figs. 4 – 9). The acoustically laminated sediment packages that separate the Naust sub-units (e.g. A1 and A2, and B1, B2, B3 and B4, and C1 and C2) are thinner than those separating the Naust units A, B, C, D and E (Figs. 4 and 7). The thickness of the acoustically laminated sediments that are interlayered with the sub-units of Naust B is less than 10 ms TWT (Fig. 7). However, the exact thickness is often difficult to determine since in most cases only a single reflection separates the acoustically homogenous sub-units (Fig. 8). The thickness of the acoustically laminated sediments that separates Naust C1 and C2 is ca. 10 ms TWT to the north (Fig. 7). To the south, however, these sediments are thicker (ca. 20 ms TWT). In contrast, the acoustically laminated sediment packages that are found inbetween the Naust units A and E generally exceed 20 ms TWT in thickness (Fig. 8). On the Vøring Plateau, a relatively thick (70 m at ODP 644A) package of acoustically laminated sediments has been deposited during the last ca. 600 ka (Fig. 5), which is the time interval considered in this study. However, the distal facies of both the Plio-Pleistocene Naust Formation and the Mio-Pliocene Kai Formation consists of acoustically laminated sediments (McNeill et al., 1998). Thus, the total thickness of acoustically laminated sediments is much greater. The laminated acoustic facies also occurs as infill of slide scars, e.g. the Sklinnadjupet Slide (Figs. 5, 7 and 8; Dahlgren & Vorren, 2003; Laberg et al., 2002). The laminated acoustic facies shows marked stratigraphical variations in reflection strength between sets of individual reflections as recorded on both deep-tow boomer and mini sleeve airgun. Intervals showing strong coherent reflections and others of a more transparent and chaotic character occur throughout the stratigraphy (Figs. 4– 9). The laminated facies is in areas affected by syn-sedimentary faulting (Figs. 5, 7 and 9). Most faults root and terminate in the Kai Formation, some, however, penetrate younger sediments and reach the seabed (or close to). Generally, the acoustically laminated sediment packages, which overlie Naust B and interfinger Naust D – B, are thickest on the lower part of the continental slope (Figs. 4 and 5). However, a local thickness maximum of the

1094 K.I. Torbjørn Dahlgren et al. / Marine and Petroleum Geology 19 (2002) 1089–1113 Fig. 4. Interpreted mini sleeve airgun line NH96-407. Also shown is a simplified log displaying the lithology of the DSDP hole 341 with a tentative correlation to the seismic record (the log and the seismic are scaled assuming an acoustic velocity of 1500 m/s in the sediments). Note the prograding/aggrading clinoforms composed of acoustically homogenous facies on the continental slope interbedded with thinner packages of acoustically laminated facies. A thicker partly buried (by Naust A2) package of acoustically laminated sediments is also present on the lower part of the slope. Extensive diapirism has disrupted and/or deformed parts of the Naust units E –B4.

K.I. Torbjørn Dahlgren et al. / Marine and Petroleum Geology 19 (2002) 1089–1113 Fig. 5. Interpreted mini sleeve airgun line NH96-403 and a section of deep-tow boomer with a correlation to ODP hole 644A (the correlation is based on the assumption of a sediment velocity of 1500 m/s). The depth/age model is adopted from Henrich and Baumann (1994), with even numbered MIS shown in black. Also shown is the Bruhnes/Matuyama boundary. The marked reflections on the boomer profile are reflections on which the individual Naust units pinch out. The reflections marked on the left-hand side were traced from the north and west, and the reflections marked on the right-hand side were traced from the south and east. The Sklinnadjupet Slide cuts into Naust B. The slide scar is filled in with contouritic sediments (acoustically laminated) and glacigenic debris flows (acoustically homogenous) of Naust A1. Note that the Helland Hansen Arch has acted as a barrier to Naust D and was first overstepped by Naust C1. 1095

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Fig. 6. Interpreted seismic profile comprising a mini sleeve airgun profile from the upper slope, NH96-405, and an interpreted sparker profile on the shelf (King et al., 1987; Profile B81-114). The sparker record (V98-84) illustrates the correlation between the seismostratigraphic units on the continental slope (McNeill et al., 1998) and the corresponding till units of King et al. (1987). Note the overall seaward dip of the URU surface and the middle till, which is inferred to be due to outer shelf subsidence.

laminated package that overlies Naust B is related to the infill of the buried Sklinnadjupet Slide (Figs. 3, 5 and 7). The acoustically laminated packages thin basinward on the Vøring Plateau (Figs. 4 and 5). They also thin up-slope although on the upper slope this thinning is concurrent with onlap in an apparently up-slope prograding fashion (Fig. 4). The acoustically laminated sediments that overlie Naust B4 on the upper slope show thickness variations related to infill of the subsurface relief (Fig. 8). Here, the laminated sediments display an onlapping pattern of infill. Also within these infilling packages, faintly developed unconformities can be observed (Fig. 8; cf. King et al., 1987). 4.2. Homogenous acoustic facies Acoustically homogenous and transparent sediments (Figs. 4– 9) are found on the continental slope. This facies comprises seismic units Naust A – H in the study area (Evans et al., 2003). These units constitute a substantial part of the Late Cenozoic progradation of the shelf. Some of these units extend down onto the inner Vøring Plateau (Figs. 4, 5 and 9).

On the lower slope, the acoustically homogenous facies is found in stacked lensoid bodies with a maximum width and thickness of ca. 10 km and ca. 50 ms TWT, respectively, (Fig. 7). On the upper slope, on the other hand, the geometry/architecture is more chaotic with few easily identifiable single lensoid bodies (Fig. 8). In addition, diapiric bodies and associated small-scale slump deposits also exhibit a homogenous acoustic character (Figs. 4 and 9). Diapirs that pierce, or reach close to, the seabed occur in two main areas, the Vema Dome and the Vigrid diapir field (Fig. 3). Diapirs are also occasionally found deeper within the stratigraphy (Fig. 5). Mud mounds coupled to fluid or gas escape also occur sporadically within the area of investigation. 4.3. Seismic stratigraphy We follow the stratigraphic grouping and nomenclature of the seismic units Naust A – D of Evans et al. (2003) and McNeill et al. (1998). However, we will demonstrate that a further subdivision of Naust units B and C is possible. The

K.I. Torbjørn Dahlgren et al. / Marine and Petroleum Geology 19 (2002) 1089–1113 Fig. 7. Interpreted mini sleeve airgun profiles NH96-203 and 301. The profiles are located along the lower continental slope. The fact that Naust D and E are not seen on the south-western part of this profile is due to the Helland Hansen Arch acting as a barrier (cf. Fig. 5). Note the progressively more frequent occurrence of internal reflections from Naust E to B. There is also a gradual change in character of the external geometry with Naust C, D and E displaying a much smoother large-scale lensoid geometry compared to Naust B, which shows a much more complex external geometry. The Sklinnadjupet Slide side wall is seen to the south, and debris flow input from the Trænadjupet Trough to the north. A possible drift accumulation is seen underlying Naust C1 in the south.

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1098 K.I. Torbjørn Dahlgren et al. / Marine and Petroleum Geology 19 (2002) 1089–1113 Fig. 8. Interpreted mini sleeve airgun profile 99-091. The north-eastern part of the profile shows the internal reflection pattern and external geometry of the Naust units along the upper part of the continental slope. The south-eastern part runs obliquely down slope. Note that the internal reflection pattern which allows a sub-division of Naust B, is less defined on the upper continental slope. There are also strong signs of basal erosion during deposition of Naust B4 on the upper slope. In the vicinity of the Sklinnadjupet Slide side wall, more indications of past slope instability can be seen, possibly suggesting that there have been an additional number of slide events.

K.I. Torbjørn Dahlgren et al. / Marine and Petroleum Geology 19 (2002) 1089–1113 Fig. 9. Interpreted mini sleeve airgun profile composed of profiles 99-092 (to the south) and 00-003B (to the north), running through ODP site 644A on the Vøring Plateau. For profile 99-092 a single 0.6 l sleeve gun was used and for 00-003B a set of two tuned 0.6 l guns were used. The distal Sklinnadjupet Slide deposits are to be found on the southern profile; underlying these is Naust C2. Note the diapirs to the south, which appear to originate from the slide masses, although the overburden is thin. Note also the heavily faulted acoustically laminated sediments.

1099

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sub-division is based on the fact that the Naust B and C units are composed of several discrete bodies separated by distinct, medium- to high-amplitude reflections (Figs. 4, 5 and 7). These reflections are inferred to represent acoustically laminated sediments interbedded with the acoustically homogenous sub-units. This is exemplified by a ca. 20 ms TWT thick acoustically laminated package separating Naust C1 and C2. This package gradually thins to the north, and is here represented in the seismic data by a single reflection (Fig. 7). Isopach maps of each unit are presented together with the corresponding upper surface unconformity on the continental shelf (the upper surface of the unit on the shelf) based on maps from King et al. (1987). In effect these maps are isotime maps since the acoustic velocity in both the water and sediment column was assumed to be 1500 m/s (King et al., 1987). The correlation of our seismic stratigraphy to that of King et al. (1987) is addressed in ‘correlation and chronology’. The surface topography is available for Naust A2 (mirrored by the seabed topography) and Naust A1. Naust A1 corresponds to the Middle till of King et al. (1987) (Fig. 6). Since, only a thin layer of acoustically laminated sediments separates the base Naust A2 (Upper till) as mapped by King et al. (1987) and the upper surface of Naust A1 (Middle till), we infer this surface to closely mimic the top of Naust A1. Naust units B, C and D are shown together with the topography of the Upper Regional Unconformity (URU), an erosion surface last reshaped in the central part of our study area during deposition of Naust B4 (Fig. 6). In the Trænadjupet Trough

the URU was most likely reshaped during the deposition of the Naust A2/Upper till, since the latter lies directly on top of the URU in the trough (King et al., 1987). The thickness of each unit is given in milliseconds two-way travel time (TWT). In the following, we describe Naust units A –D in descending stratigraphic order. We follow this approach because the stratigraphically uppermost units (Naust A and B) are well depicted in both deep-tow boomer and mini airgun seismics. This allows a more detailed interpretation to be made, and also serves as a ‘calibration’ of the mini airgun data. For the stratigraphically deeper units (Naust C and D) we mainly rely on mini airgun seismics. However, in the areas where these units (Naust C and D) extend beyond the limits of the overlying Naust A and B units, deep-tow boomer seismic data also depict these units. 4.3.1. Naust A Naust A is composed of two sub-units: A1 and A2 (Fig. 10A and B). Both sub-units are composed of prograding and abruptly terminating sediment wedges on the shelf; ‘till tongues’ in the sense of King et al. (1987). Where these spill over the shelf break, they stretch down the slope as ‘flow-lobes’ (cf. Figs. 7, 8, 10A and B). Naust A2 is present along the margin from the southern boundary of the study area to the Trænadjupet Slide (Fig. 10A). Naust A2 has been sampled at several locations (Table 2), and is composed of a structureless grey diamicton. Naust A2 is thickest, more than 200 ms, on the uppermost part of the slope. It makes up the large morainal

Fig. 10. Thickness maps (in milliseconds TWT) of (A) Naust A2, and (B) Naust A1. Also shown is the surface topography of the corresponding shelf unit, i.e. the Upper and Middle tills of King et al. (1987). The Nyk and Trænadjupet Slide events have removed parts of the originally deposited sediments. Abbreviations: S, Sklinnadjupet Trough; Td, Trænadjupet Trough; and T, Trænabanken.

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Table 2 Lithology of the sediments corresponding to the seismic units at each core location. The glaciomarine/marine muds with dropstones represents hemipelagic sedimentation distal to the glacigenic debris flow units (Naust A –D). Note that DSDP 339/340 are located within the Vema diapir field (cf. Figs. 2D and 3); sampling diapiric core material (Talwani et al., 1976) Hole/core with the sampled sediment-type indicated Seismic unit

DSPD 341

DSDP 339/340

JM98-624

JM98-628

ODP 644A MD95-2010

Naust A2

Pleistocene glaciomarine /marine muds with dropstones

Pleistocene glaciomarine /marine muds with dropstones

Glacigenic debris flow diamicton

Glacigenic debris flow diamicton

Pleistocene glaciomarine /marine muds with dropstones

Naust A1 Naust B Naust C Naust D

Mass flow diamict with shallow water bentonic fauna (Glacigenic debris flow diamicton) Naust E and older units glaciomarine /marine muds with dropstones

Eocene diapiric core material

bank-complex Skjoldryggen (Figs. 2A, 5 and 10A). Farther north the sub-unit constitutes a fan-shaped accumulation with several lobes reaching down to the base of the slope (Fig. 10A). In the north the distribution of Naust A2 shows sediment input down to the base of the slope, sourced from the Trænadjupet Trough. However, only part of Naust A2 is preserved here, as the Trænadjupet and Nyk Slides (Laberg & Vorren, 2000; Lindberg, 2000) disrupted these strata. The surface of sub-unit A2, which is subparallel to the present sea-floor, displays a hummocky topography east of the Skjoldryggen moraine complex, a relatively smooth surface in and to the north of the Sklinnadjupet trough (Fig. 10A). Iceberg ploughing is registered on seismic profiles as a hummocky surface down to ca. 500 m water depth, cf. Dahlgren and Vorren (2003) and Lien (1983). Naust A1 is present in the central part of the study area, at the mouth of the large paleo-Sklinnadjupet Trough and in a small lobe (, 50 ms thick) off the Trænadjupet Trough (Fig. 10B). Later sliding has probably removed most of the sediments originally present here (cf. Laberg & Vorren, 2000; Lindberg, 2000). The unit exceeds 250 ms TWT in thickness at the depocentre off Sklinnadjupet, just proximal to the paleo-shelf break (Fig. 10B). The Naust A1 sediments extend furthest down-slope in the slide scar of the buried Sklinnadjupet Slide (cf. Figs. 3 and 10A). The break in slope representing the paleo-shelf break (and accordingly the paleo-grounding line), is to be found at a depth of ca. 5 – 600 m below present sea level. Irregular surface topography of Naust A1 deeper than the inferred paleo-grounding line is interpreted to represent iceberg ploughing. These features occur down to a depth of an additional 100 m from the paleo-grounding line. 4.3.2. Naust B Naust B is grouped into four sub-units; B1 – B4 to the north of the Sklinnadjupet Slide (Fig. 11A –E). These subunits cannot, however, be followed to the south of the slide.

Consequently, Naust B was not sub-divided south of the Sklinnadjupet Slide. On the lower slope, the upper and lower bounding reflections of the sub-units are well defined and of medium to high amplitude (Figs. 4 and 7). On the upper slope, however, the bounding reflections are intermittently disturbed (Fig. 8). The top of Naust B4, landward of the paleo-shelf break, marks the latest major reshaping of the URU on the shelf (Fig. 6). The morphology of this horizon has large shelfcrossing analogues to the present-day Sklinnadjupet and Trænadjupet troughs. In addition, there is a large trough extending in the continuation of Vestfjorden in a NE – SW direction (Fig. 11). Naust B4 comprises several acoustically homogenous and transparent bodies on the middle and lower slope, which are lensoid in cross-section and lobe-shaped in plan view (Figs. 7 and 11B). On the upper slope Naust B4 displays a continuous acoustically transparent cover (Figs. 8 and 11B). The lobes are thicker (50 – 100 ms) on the middle to upper slope, and thin in a downslope direction, where they terminate abruptly (Figs. 4, 5 and 11B). A substantial part of Naust A4 was removed by the Sklinnadjupet Slide (Figs. 7, 8 and 11B). The geometry of the basal reflection indicates some erosion of the underlying sediments below individual lobes, especially on the uppermost continental slope (Fig. 8). In general, however, the lower bounding surface is seemingly non-erosive, as observed within the resolution of the data. The break-of-slope indicating the position of the paleoshelf break, and likewise, the inferred paleo-grounding line (Figs. 4– 6 and 11B) is found at a present water depth of 650 – 800 m. On the upper part of the paleo-slope, the sediments of the upper part of Naust B4 are disturbed (Fig. 8), and in-between the lobes where Naust B4 is absent (Fig. 11B), the upper part of Naust B3 is disturbed. The disturbance is of a more intense character than the inferred

1102 K.I. Torbjørn Dahlgren et al. / Marine and Petroleum Geology 19 (2002) 1089–1113 Fig. 11. Thickness maps (in milliseconds TWT) of (A) Naust B (total) and sub-units, (B) Naust B4, (C) Naust B3, (D) Naust B2, and (E) Naust B1. The Sklinnadjupet Slide event has eroded a substantial part of the originally deposited sediments, and it is not possible to follow the sub-division of Naust B to the south of this slide. Also shown is the topography of the URU as mapped on the shelf (modified from King et al., 1987). The URU was last reshaped during the deposition of Naust B4. On the upper Naust B4 paleo-slope the surface is heavily disturbed (this is also seen in Fig. 9). Note also a small slide scar in the northern part of Naust B1. Abbreviations: S, (paleo) Sklinnadjupet Trough; Td, Trænadjupet Trough; and T, Trænabanken.

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iceberg scouring on the overlying units (Naust A2 and A1), and also affects the sediments to a deeper level. Naust B3 displays a more continuous cover on the slope than Naust B4 (Figs. 7 and 11C), but is limited in its northward extent. The lobes are larger and thicker compared to those of Naust B4, and are composed of sets of stacked lensoid bodies (Fig. 7). The overall sediment distribution indicates a depocentre off the paleo-Sklinnadjupet Trough (Fig. 11C). Part of this unit was removed by the Sklinnadjupet Slide. Naust B3 reaches a thickness in excess of 150 ms on the upper slope, and proximal to the paleoshelf break. The architectural pattern of stacked lensoid bodies is seen in the topography of the upper bounding surface (Figs. 7 and 8) as well as in the isopach map (Fig. 11C). Naust B2 is very comparable to the overlying Naust B3 with regard to its acoustic and architectural characteristics (Figs. 7 and 8), and similarly shows a limited extent towards the north (Fig. 11D). A depocentre is located at the mouth of the paleo-Sklinnadjupet Trough (Fig. 11D), where the unit reaches a thickness in excess of 100 ms TWT. The Sklinnadjupet Slide removed sediments also at the stratigraphic level of Naust B2 (Figs. 5, 7 and 8). Accordingly, the mappable southward extension of Naust B2 is limited by the northern fringe of the slide (Fig. 11D). Naust B1 is relatively thick, exceeding 200 ms TWT on the upper slope (Fig. 11E). The unit is very similar to the overlying Naust B2 and B3 with regard to its internal reflection pattern and architecture (Figs. 7 and 8), and is also limited northward (Fig. 11E), indicating sediment sourcing from the paleo-Sklinnadjupet Trough. The depocentre of Naust B1 is, however, somewhat shifted to the north as compared to the overlying sub-units. A minor surficial slide scar is imaged on the thickness map (Fig. 11E). The headwall of the slide is situated close to the inferred position of the paleo-shelf break. 4.3.3. Naust C Naust C is divided in two sub-units: C1 and C2. These sub-units differ somewhat in internal acoustic character and architecture as compared to the sub-units belonging to the overlying Naust B. Naust C1 and C2 display a weaker pattern of internal reflections (Figs. 7– 9), and consequently have a more homogenous appearance. The upper and lower bounding reflections of Naust C1 and C2 are of medium to high amplitude (Fig. 7). To the south, Naust C1 and C2 are interlayered by a ca. 20 ms TWT thick acoustically laminated sediment package. Naust C2 is present along most of the margin (Fig. 12A). It reaches a thickness in excess of 200 ms TWT on the upper slope. Two lobe-shaped depocentres extend onto the Vøring Plateau. The relatively uniform thickness on the slope (ca. 100 ms TWT), and the larger extent of the sub-unit in the downslope direction, both contrast with the depositional pattern of Naust B1 –4 (cf. Figs. 11 and 12A). Part of the sediments of the south-western lobe (Fig. 12A) have been

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removed by the Sklinnadjupet Slide or possibly an older slide event in the same area. Naust C1 has a more limited extent and is thinner, only exceeding 100 ms TWT in thickness within its mappable confines (Fig. 12C). The unit is composed of several lobes (Fig. 12C) which seem to have been deposited at the flanks of the lobes belonging to the underlying Naust D (cf. Fig. 12C and D). Sub-unit C1 is similar to the overlying Naust C2 with regard to its relatively uniform thickness on the slope and widespread downslope extent (Fig. 12B and C). 4.3.4. Naust D Naust unit D is present all along the investigated part of the margin and comprises two depocentres, each exceeding 200 ms TWT in thickness (Fig. 12D). Naust unit D is acoustically homogenous and shows very few signs of any structured internal reflections. Sediments of the southern depocentre are ponded behind the northernmost extension of the Helland Hansen Arch (Fig. 5). The northern depocentre extends far out onto the northern Vøring Plateau (Fig. 12D). The sediments have a relatively uniform thickness across the northern slope area, the sediments being ca. 150 ms TWT thick (Fig. 12D). However, the sediments increase in thickness in a north-westward direction at the base of the northern part of the slope (Fig. 12D). Lithological information on Naust D is obtained from DSDP site 341 (Fig. 4; cf. Talwani et al., 1976). The lithology of unit 1C (the equivalent of Naust D in the borehole) was originally described as calcareous sandy mud and mud with pebbles, and siliceous and calcareous fossils (White, 1978). Colour photographs supplied to us by the ODP core curatory revealed a striking similarity between the sediments of Naust A2 and that found for unit 1C (equivalent to Naust D). Both units being composed of a grey structureless diamicton. Thus, we infer that Naust A2 and D are very similar in their lithological composition.

5. Correlation and chronology The correlation of the seismic stratigraphy to the ODP 644A borehole is shown in Fig. 5. We used the deep-tow boomer data which has a resolution and penetration to allow a correlation between the Naust D –A units and the ODP 644A hole to be made. We traced the reflections, on which individual acoustically homogenous units pinch out, to the borehole, as marked on the boomer profile in Fig. 5. Naust B3 and B4 pinch out within the acoustically laminated sediment package on the profile in Fig. 5. The other marked reflections were traced to the borehole using the extensive grid of tie-lines (Fig. 2D). The reflections were traced from areas where the respective units pinch out within the acoustically laminated sediment package, and where there are no overlying acoustically homogenous units that mask

1104 K.I. Torbjørn Dahlgren et al. / Marine and Petroleum Geology 19 (2002) 1089–1113 Fig. 12. Thickness maps (in milliseconds TWT) of (A) Naust C (total) and sub-units, (B) Naust C1, (C) Naust C2, and (D) Naust D. The Sklinnadjupet Slide event has only affected a minor part of the originally deposited sediments in Naust C2. Diapirs pierce Naust C2 and D, especially in the Vema diapir field to the north. Also shown is the topography of the URU as mapped on the shelf (modified from King et al., 1987). The URU was last reshaped during the deposition of Naust B4. Abbreviations: S, (paleo) Sklinnadjupet Trough; Td, Trænadjupet Trough; and T, Trænabanken.

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the deep-tow boomer records. We used the depth-age model of Henrich and Baumann (1994) for the ODP 644A hole. Based on this model, the correlation (Fig. 5) suggests that Naust A2 was deposited during Marine Isotope Stage (MIS) 2 (ca. 15– 22 ka BP), Naust A1 during MIS 6 (128 – 186 ka BP), Naust B during MIS 10 (339 – 362 ka BP), Naust C during MIS 12 (423 –478 ka BP) and Naust D during MIS 14 (524 – 565 ka BP). Additional age and lithological constraint on Naust A2 also comes from core JM98-628/1 (Fig. 2D, Table 2). In this core, a radiocarbon date on foraminifera in glaciomarine mud directly above a structureless grey diamicton representing Naust A2 yielded an age of ca. 17 14C ka BP (Dahlgren & Vorren, 2003), confirming that Naust A2 is of MIS 2 age. The correlation of our seismic stratigraphy on the continental slope to the ‘till tongue’ stratigraphy on the shelf (King et al., 1987) is displayed in Fig. 6, and is summarised in Table 3. The lower till on the shelf is sandwiched between the URU/Naust B4 and the middle till/ Naust A1. Based on its stratigraphic position we tentatively infer that the lower till was deposited during MIS 8. The deposition of glaciomarine diamiction recorded in ODP 644A during MIS 8 (Ho¨lemann & Henrich, 1994) supports this conclusion. Similarly, glaciomarine diamict sedimentation was found in the three ODP leg 104 sites during MIS 14, 12, 10 and 6 (Ho¨lemann & Henrich, 1994). Moreover, the pattern of reworked nannofossils (Henrich & Baumann, 1994) in site 644A indicates (glacial) erosion of Mesozoic sediments on the inner shelf, especially during MIS 6 and 10. Our results are partly in contrast to earlier correlations (Table 3), and the differences are mainly related to the

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chronology. For the southern part of the mid-Norwegian shelf between 64 and 658N a regional upper Cenozoic stratigraphy was established from a multichannel seismic grid and IKU sparker profiles by Rokoengen et al. (1995). They recognised 11 (L – A and U) informal units comprising the Plio-Pleistocene prograding wedge. Rokoengen et al. (1995) correlated their unit B to the Lower till, unit A to the Middle till and unit U to the Upper till of King et al. (1987). They further assigned a Saalian age of the URU (base of their unit D) and a Weichselian age of the three units B, A and U. In contrast, Sættem et al. (1996) correlated units A and U with the Upper till and unit B with glaciomarine sediments associated with, and present below, the Lower till of King et al. (1987). Sættem et al. (1996) further assigned an Eemian age of the reflection separating units B and D, based on amino-acid ratios of foraminifer shells. Our age constraints of the Naust units and our correlation to the ‘till tongue’ stratigraphy of King et al. (1987), together with the overall geometry of the units of Rokoengen et al. (1995) and Sættem et al. (1996), implies that further revision is necessary. We infer that unit U of Rokoengen et al. (1995) and Sættem et al. (1996) corresponds to Naust A2/Upper till, and unit A to Naust A1/Middle till, and that unit B and D correspond to the Lower till and associated glaciomarine sediments. We further suggest that the regional unconformity below unit D corresponds to our URU, which was last reshaped during the deposition of Naust B4. It must be stressed, however, that the seismic stratigraphy on the continental shelf is complicated in areas where different depositional systems (glacier tongues) have interacted. In the outer Sklinnadjupet area, where we managed to correlate to the ‘till tongue’ stratigraphy, the stratigraphic relationships are relatively

Table 3 Compilation of seismostratigraphic work on the mid-Norwegian continental slope and shelf. Shown is also a tentative age-correlation to the seismostratigraphy on the southern part of the mid-Norwegian shelf Age

MIS 2 ca. 15 –22 Ka MIS 6 128–186 ka MIS 8 245–303 ka MIS 10 339–362 ka MIS 12 423–478 ka MIS 14 524–565 ka

Area

Tentative age-correlation to units in the southern area

Inner Vøring slope This work

Middle shelf (658 –678300 N) King et al. (1987); King, Rokoengen, Fader, and Gunleiksrud (1991)

Northern shelf (668 –688N) Henriksen and Vorren (1996) (tentative ages)

Southern shelf (648–658N) Rokoengen et al. (1995)

Naust A2 /Upper till Naust A1 /Middle till Lower till, URU URU, Naust B (1–4) Naust C (1–2) Naust D

Upper, Middle and Lower tills URU

D , 0.8 ^ 0.2 Ma

UAB

U þ A, B (MIS 5)

U

D, URU

B (MIS 5), D, URU

A

URU, E–L (Late Quaternary)

URU, E–L

D, URU

Sættem et al. (1996)

URU, E Fþ? URU 0.8 ^ 0.2 Ma C . 0.8 ^ 0.2 Ma

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uncomplicated because the glaciers seem to have followed the Sklinnadjupet Trough or its ‘paleo-equivalents’ during each advance. At the fringes of the Sklinnadjupet depositional system interactions with other glacier tongues may have complicated the situation, e.g. to the south where Rokoengen et al. (1995) established their stratigraphy.

6. Discussion The origin of the acoustically homogenous and laminated facies will be discussed first. We will then reconstruct and discuss the history of fluctuations of the Fennoscandian Ice Sheet on the mid-Norwegian continental shelf. Based on the presented results and the discussions above, we synthesise the stratal geometric patterns and the development of the margin, including evidence for subsidence. 6.1. Debris flows Several results indicate that the acoustically homogenous portions of the Naust A – D units are largely composed of glacigenic diamicton (Dahlgren & Vorren, 2003; Dahlgren, Vorren, & Laberg, 2002; Evans et al., 2000; Henriksen & Vorren, 1996; King, 1993; McNeill et al., 1998; Sættem et al., 1996). The glacigenic diamictons were deposited at the grounding-line of the Fennoscandian Ice Sheet. Subsequently, they were transported by gravity flows to be redeposited on the continental slope. Also by analogy to other parts of the Norwegian continental margin a similar conclusion can be reached (cf. Elverhøi et al., 1997; King et al., 1996, 1998; Laberg & Vorren, 1995, 1996; Vorren & Laberg, 1997; Vorren, Lebesbye, Andreassen, & Larsen, 1989). Thus, we do not wish to repeat the general discussion on the glacigenic nature of these and similar deposits on the Norwegian margin. Instead, we focus on the differences observed between and within, Naust units A to D, and what might account for the observed differences. It is puzzling that Naust B shows a much more well developed lensoid internal facies than the other units (Fig. 7), despite the likelihood that they are all composed of similar material and were deposited by similar processes. Similar differences have been observed elsewhere, e.g. on the North Sea Fan (King et al., 1996). Differences in seismic expression due to higher resolution in the uppermost layers may be an explanation. However, this cannot fully explain these differences since Naust A to some extent shares the more internal homogenous character with Naust C and D (Fig. 7). The differences in internal reflection pattern could arise from varying water content and/or compaction of the units. This may be related to the water content of the parental material of the debris flows (glacigenic diamicton) and/or the magnitude/duration of the sediment supply. Variations in water content of the parental material could possibly be related to varying subglacial conditions during the different

ice-sheet advances, e.g. deforming versus non-deforming bed, basal melting rates, ice-flow velocities, etc. A steady sediment supply over a relatively longer period may be reflected by the internally more homogenous units (Naust C and D). An indication of this could be that the internally homogenous units are of surprisingly uniform thickness on the slope, with a thickness maximum at the base of the slope (cf. Fig. 12). Such a sedimentary pattern in response to the sediment supply is suggested in a modelling study of sedimentation and debris-flow triggering on TMFs by Dimakis et al. (2000). If this model is representative of the prevailing conditions, a relatively uniform sediment thickness on the slope and longer run-out distance (possibly causing thicker accumulation at the base of slope) can be explained through a relatively high and steady sediment supply to the shelf break, in combination with relatively lower slope gradients. In contrast, a reduced sediment supply would result in more sporadic and deeper-seated slope failures, and in combination with a higher slope gradient relatively shorter run-out distances can be expected (e.g. Naust B4, Figs. 4 and 11B). Supporting these results is the observed relationship between run-out distance and slope gradient on the TMFs along the Norwegian margin (Vorren & Laberg, 1997; Vorren et al., 1998). Thus, the progressive change from sheet like internally homogenous units (Naust D) to singular lensoid bodies in Naust B4 could be related to varying sediment supply in combination with a progressively steeper slope. The evolution of the margin topography may also have played a role in this change in internal acoustic character. The progressively stronger lensoid pattern within Naust B (and from Naust D to B) is contemporaneous with a shift from more focused deposition on a concave slope profile along the base of the slope, to more dispersed deposition on a convex slope. A characteristic feature for all the acoustically homogenous units is the changing spatial pattern of sedimentation (i.e. the depocentre of a following unit is shifted to where the previously deposited unit is thin or absent). The underlying topography caused by structural features has also had an important role in ‘ponding’ the sediments of units or parts of units, or just focusing the sedimentation. The thickness maxima of Naust D and C at the base of the slope can be favourably compared with the longitudinal geometry of debris flows on the Bear Island Fan (cf. Laberg & Vorren, 1995). Many debris flows on the Bear Island Fan are thickest near their snout; a similar sediment distribution within the debris flows building units Naust C and D could explain the spatial sedimentary pattern. Other possible explanations of the change from homogenous internal facies to a lensoid pattern encompasses differences in rheology, e.g. grainsize and clay mineralogy. Differences in clay content/mineralogy could arise from different lithology of the eroded parental material of the till delivered at the shelf break. Differences in source material over time were speculated to be responsible for similar

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differences in the seismic expression of debris flows in Baffin Bay (Hiscott & Aksu, 1994). 6.2. Hemipelagic sedimentation and along-slope current influence Dahlgren and Vorren (2003) studied the acoustically laminated facies overlying Naust A1, using short sediment cores and high-resolution seismics. In the following we partly build upon their results to shed light upon the sedimentary environment during the last ca. 600 ka, using seismic data of somewhat lower resolution (airgun instead of deep-tow boomer and 3.5 kHz). During the last ca. 40 14C ka, the accumulation of sediments corresponding to the acoustically laminated facies increased by several orders of magnitude during periods when the Fennoscandian Ice Sheet was calving in the sea. A large portion of these sediments was thus supplied through iceberg rafting and suspension fallout from meltwater plumes originating from the glacier margin (Dahlgren & Vorren, 2003). A decrease in sedimentation with distance from the source can then explain the basinward thinning of these sediments. Deposition from turbidity currents seems to have been of little importance, since no turbidites have been reported from the large number of short cores retrieved from the Vøring Plateau for paleoceanographic studies. The lack of associated morphological features on the sea-floor and deeper in the laminated sediments package, such as channels and levees, further suggests that turbidity currents were of minor importance in transporting sediments downslope. The relatively thin package of acoustically laminated sediments on the upper slope can be attributed to strong bottom currents, causing extensive winnowing during interglacials and reduced winnowing/sediment by-pass during glacials (Dahlgren & Vorren, 2003). The fact that the acoustically laminated sediment package that interfingers with Naust C has a similar geometry (Fig. 4) suggests to us that similar processes were responsible for its deposition. The thick onlapping package of acoustically laminated sediments overlying Naust B4 on the lower slope (Fig. 4), is related to the Nyk sediment drift (cf. Laberg et al., 2002). The onlapping pattern of the contouritic sediments is better developed further north where the drift is situated in a bathymetric embayment (Fig. 3; Laberg et al., 2002). That this thickness maximum on the lower continental slope gradually develops into a well-developed sediment drift indicates that bottom currents and along-slope sediment transport have had a strong influence upon sedimentation along the whole mid-Norwegian margin. As the distribution of acoustically laminated sediments on the slope, including the Nyk drift, is related to the surface and intermediate ocean circulation (cf. Dahlgren & Vorren, 2003; Laberg et al., 2002), this implies that sediments are carried into the area from the south by ocean currents. Subsequently, sediments are exported from the area towards the north.

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The subtle unconformities found on the upper slope within the acoustically laminated package overlying Naust B4 (Fig. 8) were possibly developed during interglacials when sedimentation was minimal and bottom currents were strong and caused winnowing. Alternatively, these unconformities could also have formed in response to low (glacioeustatic) sea level during glacial periods, prior to advances of the Fennoscandian Ice Sheet on the shelf. Another possible origin is that the unconformities were formed by erosion due to the down-slope flow of cold saline water masses (brines) formed on the shelf during sea iceformation in glacial periods (Dokken & Jansen, 1999). However, as observed in sediment cores from the upper slope, glacial periods are characterised by high sedimentation rates and interglacial periods by sediment starvation (Dahlgren & Vorren, 2003). This suggests to us that these unconformities were most likely formed during interglacials. Evidence of bottom-current influence in older sedimentary units is observed in the acoustically laminated sediments underlying Naust C in the Sklinnadjupet Slide (Fig. 7). At the southern end of profile NH96-301 a ca. 250 ms TWT thick sequence of acoustically laminated sediments displays a progressive development from onlapping infill of a local bathymetric low into a mounded aggradational sediment body (Fig. 7). 6.3. Glaciation history As discussed above, the existence of several sets of glacigenic debris flows on the continental slope reflects shelf-wide advances of the Fennoscandian Ice Sheet during MIS 2 (ca. 15 – 22 ka BP), 6 (128 – 186 ka BP), 10 (339 – 362 ka BP), 12 (423 –478 ka BP) and 14 (524 –565 ka BP), (Fig. 13). The Lower till on the inner part of the shelf is chronologically bracketed between MIS 6 and 10 and is therefore, inferred to reflect an advance of the Fennoscandian Ice Sheet during MIS 8. Vorren and Laberg (1997) correlated the glacigenic debris flow units in the TMFs along the western Barents Sea, Norwegian and British continental margins to the deep-sea oxygen isotope stratigraphy. They, however, used a count from the top approach. They found that the ice sheets generally did not extend to the shelf break for any appreciable time before the early mid-Pleistocene (ca. 0.6 Ma). They also found that the TMFs on the western Barents Sea margin contained a longer record of shelf-wide glaciations than the fans further south. The TMFs off western Spitsbergen reflect shelf-wide glaciations even earlier, at ca. 1.5 Ma (Butt, Elverhøi, Solheim, & Forsberg, 2000). The period covered by Naust A – D thus roughly corresponds to the interval when the western Barents Sea TMFs record shelf-wide glaciations. In this study we have not considered Naust E – H (cf. Evans et al., 2003) and sedimentary units older than Naust H in the Plio-Pleistocene prograding wedge. They also probably have a glacigenic origin (Eidvin et al., 1998; Henriksen & Vorren, 1996;

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Fig. 13. A conceptual model of margin evolution displaying the interplay between subsidence, glaciation and sedimentation. (A) Shows the geometry of the deposits related to glaciations and ice margin positions during the past ca. 350 ka. (B) Shows a glaciation curve for the mid-Norwegian continental shelf during the last 600 ka. Also shown is a global sea level curve adapted from Skene, Piper, Aksu, and Syvitski (1998). The paleo-grounding line positions and depths are plotted versus age. The increasingly greater depths at which the paleo-grounding line is to be found, indicates a rapid subsidence of the margin, if one assumes that the glaciers were of equal thickness.

Poole & Vorren, 1993). However, we do not know the marginal position of the continental ice sheet. It may have been on the coast, and the shelf might have been subaerial, proglacial, sandur plains at this time. On the other hand, the first occurrence of ice-rafted thermally mature organic matter in the Vøring Plateau ODP sites occurs at ca. 2.4 Ma (Ho¨lemann & Henrich, 1994). This reflects glacial(?) erosion of Mesozoic organic-rich sediments on the shelf, and might represent evidence of the first advance of continental ice sheets onto the shelf (Ho¨lemann & Henrich, 1994). In southern Norway the first glacial advance in the Norwegian Trench and the northern North Sea is recorded at ca. 1.1 Ma (Sejrup et al., 1995).

Glaciers reaching the shelf break off mid-Norway reflect full-scale Fennoscandian glaciations. The Fennoscandian mountains are at their lowest inland from the southern part of the mid-Norwegian shelf (Fig. 1) and ice-flow from east of the water divide during ice sheet maximums is likely to have had a much larger influence here than elsewhere along the coast of Norway. This does not necessarily imply that much sediment was transported sub-glacially by the ice sheets from east of the water divide, as the Fennoscandian Ice Sheet was largely cold-based along the axis of the mountains during glacial maximums (cf. Kleman & Ha¨ttestrand, 1999). This is supported by 40Ar/39Ar dates on hornblende, biotite and muscovite grains from the

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structureless grey diamicton in core JM98-624/1 (Tables 1 and 2) which correspond to Naust A2. These dates yielded mainly Paleozoic ages, suggesting that the sediments in Naust A2 were derived from the Caledonides or sedimentary rocks containing erosional products from the Caledonides (Hemming, Vorren, & Kleman, 2002). The record of glaciation on the mid-Norwegian continental shelf can possibly be correlated to the record of large Fennoscandian glaciations reaching continental Europe. The mid-Pleistocene stratigraphy of the Netherlands records two large Fennoscandian glaciations (till deposits), the Saalian and the Elsterian (Mangerud, Jansen, & Landvik, 1996 and references therein). In addition, there is evidence for Fennoscandian glaciations (glaciofluvial sediments and/or boulders of Fennoscandian provenance) during the Menapian (about 1 Ma) and Cromerian (about 0.4 –0.7 Ma), Zagwijn (1992). The Saalian complex is correlated to MIS 6 – 10 (128 –362 ka BP), whilst the Elsterian is largely dependent on the age of the following interglacial, the Holsteinian. Although the age of the Holsteinian has been highly debated, more and more evidence suggests that the Holsteinian is to be correlated to MIS 11 (362 – 423 ka BP) (de Beauliue et al., 2001; Reille, de Beauliue, Svobodova, Andrieu-Ponel, & Goerury, 2000; Vandenberghe, 2000), implying that the Elsterian is of MIS 12 age (423 – 478 ka BP). This would then suggest that Naust C corresponds to the Elsterian of central Europe. The older Naust D can be correlated to a glacial phase in the Cromerian, if the correlation of the youngest Cromerian interglacial (IV) to MIS 13 (478 – 524 ka BP) as suggested by Vandenberghe (2000) is correct. The varying geometry and internal character of the Naust A – D units may reflect differences between the mid and late Pleistocene glaciations. Naust A2, A1 and the Lower till each comprises several ‘till tongues’ on the shelf (King et al., 1987). The progradational pattern of the ‘till tongues’ comprising Naust A2 on the shelf (Fig. 6) indicates glacier fluctuations, with retreats of 30 – 40 km in-between advances. Several fluctuations extending to the shelf break during MIS 2 are also indicated by a highly variable pattern of Ice Rafted Detritus accumulation rates on the Vøring Plateau (Dahlgren & Vorren, 2003). A similar progradational pattern, indicating fluctuations in ice sheet extent, is also found within Naust A1 (MIS 6) and the Lower till (MIS 8), cf. King et al. (1987). Here, retreats of 30 –40 km inbetween advances can be inferred from the internal progradational pattern of the units. Thus, the glaciations during MIS 2, 6 and 8 were possibly characterised by frequent fluctuations in ice sheet extent on the midNorwegian continental shelf. The sediment distribution of the Naust B (MIS 10) subunits indicates that the ice-flow pattern was slightly different during the deposition of each sub-unit (Fig. 11). The reflections separating each of the sub-units probably represent hemipelagic sediments deposited between the debris flow packages of the Naust B1 – B4 sub-units.

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Together, this suggests that the Naust B sub-units were deposited through a sequence of four fluctuations of the icesheet margin. Similarly, it is inferred that the two sub-units of Naust C (MIS 12) reflect deposition during two fluctuations in the ice-sheet margin. In contrast, the geometry of Naust D (MIS 14) indicates deposition during a single ice-sheet maximum event. These units are limited to the continental slope, and there is no preserved sedimentary record on the shelf which allows an estimate of how far the ice sheet retreated between advances. Thus, the MIS 10 (Naust B) glaciation was probably characterised by four fluctuations in ice-sheet extent to the shelf break, and the MIS 12 (Naust C) glaciation by two fluctuations. The MIS 14 (Naust D) glaciation was likely characterised by a single maximum event. 6.4. Stratal geometry and tectonics The seaward tilt of the URU (Figs. 6 and 11) in combination with the successively deeper buried paleo-shelf breaks (paleo-grounding lines) suggests to us that they possibly are related to subsidence of the outer continental margin. To our knowledge there are no glaciated margins that show such a seaward deepening of the present day shelf as observed for the URU on the mid-Norwegian margin. In fact, on the contrary, modern glaciated shelves, and in particular transverse troughs, are commonly overdeepened, (cf. Antarctica; Anderson, 1997). This might indicate that the unconformities have been affected by later tectonic tilting. We use a simple and justifiable model to quantify this subsidence (Fig. 13A). We assume that the glaciers depositing the Naust B (MIS 10), Lower till (MIS 8), Naust A1 (MIS 6) and A2 (MIS 2) units were of similar thicknesses at the margin. Furthermore, we find it unlikely that the glacier depositing Naust B (MIS 10) advanced across a more than 200 km wide seaward sloping shelf while terminating in more than 600 m deep water. Following our assumption, the depth at which the paleo-grounding line of each unit is found today should record a history of subsidence. If the depths of the paleo-grounding lines are plotted versus age (Fig. 13B) and further are corrected for differences in global eustatic sea level, the best fit line is indicating a subsidence rate of ca. 1.2 m/ka (Fig. 13B). In Antarctica, many buried glacigenic shelf unconformities similarly display a seaward dip (cf. Cooper et al., 1991; ten Brink, Schneider, & Johnson, 1995). This is generally attributed to erosion of the inner shelf and subsequent uplift, and subsidence of the outer shelf and continental slope due to deposition (Cooper et al., 1991; ten Brink et al., 1995). A similar pattern of gradually deeper buried paleo-shelf breaks and increasingly dipping glacial unconformities is found on the North Sea Fan (Fig. 1; King et al., 1996). King et al. (1996) estimated a minimum subsidence rate of 0.8 m/ka from the depth at which the paleo-shelf break of the oldest glacial unconformity was

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buried. Similarly, Solheim, Andersen, Elverhøi, and Fiedler (1996) inferred subsidence rates of 0.21 – 0.45 m/ka of the western Spitsbergen continental shelf from tilted glacial unconformities. Finally a study of inferred wave-cut submerged terraces in the Bear Island Fan area (Fig. 1) yielded estimates of recent subsidence rates (0.2 –0.8 m/ka; Lebesbye & Vorren, 1996) which compares favourably with our estimates from the mid-Norwegian margin. The pattern of subsidence is suggested to vary spatially, with a larger subsidence at the mouth of the Sklinnadjupet Trough (668N and 78E) as indicated by the overall geometry of the URU (Figs. 6 and 11). This is also where the Naust units A1 and 2 are found to be thickest and the aggradational pattern is best developed (Figs. 6 and 10). In contrast, the area around the Trænadjupet Trough seems to have experienced little or no subsidence during the last 350 ka, as indicated by the fact that this trough is scoured down to bedrock during all glaciations following MIS 10 (Naust B). This could possibly be due to the further transfer of sediments to the Lofoten Basin through repeated remobilization by sliding within the Trænadjupet Slide complex (cf. Laberg & Vorren, 2000). This would limit the subsidence due to sediment loading in this particular area. A combination of mechanisms can probably be related to subsidence. Sediment loading through the rapid emplacement of more than 1.5 km thick Naust Formation (the PlioPleistocene prograding wedge, Fig. 2C) is potentially one main factor. This is supported by the correspondence of the depocentre during the last ca. 600 ka (cf. Figs. 10 –12) and the area of highest subsidence as indicated by the geometry of the URU (Fig. 11), both being located on the outermost shelf and the continental slope off the Sklinnadjupet Trough and its paleo-equivalents (Figs. 10 – 12). The overall change in stratal geometry, which is observed across the URU (from progradation to aggradation), can be attributed to several causes. The relatively long interval between the shelf-wide glaciations of MIS 10 and MIS 6 age would have allowed for a long period of subsidence that was not compensated for by deposition of new strata on the outer shelf during the intervening glaciation (MIS 8), which only reached the inner shelf. Thus, the MIS 6 glacier had to fill the accommodation space created by subsidence during two glacial cycles, before being able to shed sediments on the continental slope. Such a response in the stratal geometry (i.e. progradation to aggradation) of the deposits related to the sequential positions of the grounding-lines (i.e. the glacier does not reach the shelf break for an extended period of time) is also supported by modelling studies of the Antarctic margin (ten Brink et al., 1995). However, the same modelling studies have also shown that a shift from progradation to aggradation can simply arise from margin maturation, i.e. the progressive widening of the shelf with time, or a decrease in sediment flux with time (ten Brink et al., 1995). Another consequence of the margin subsidence is that the long-term, upslope, onlap of the acoustically laminated

sediments on the continental slope (including the Nyk drift) might partly be related to margin subsidence. Hence, subsidence would over time allow sediments to be deposited in areas where the bottom currents previously were too vigorous. It must be stressed, however, that no simple relation between subsidence and upslope onlap pattern can be expected, since, the bottom currents are expected to vary both with depth and in strength during glacial/interglacial cycles. Consequently, we infer that the observed stratal geometry is a result of the interplay between glaciation magnitude (both in terms of extent and sediment delivery), glacioeustatic sea level changes, and subsidence of the margin. 6.5. Sklinnadjupet Slide The buried Sklinnadjupet Slide (Fig. 3) mainly affected Naust B (Fig. 11), but also a minor part of Naust C2 (Fig. 12B). This slide may be a complex of several events as is indicated by several possible buried side walls within Naust B4/B3 (Fig. 8). However, the main slide event was triggered after the deposition of Naust B4 (Figs. 5, 7 and 8). The minimum age estimate is based on the oldest reflection within the acoustically laminated package that it is possible to trace out of the slide scar. Thus, the age of the slide is bracketed between MIS 8 and 10 (245 –339 ka BP) (Fig. 5). The fact that acoustically laminated sediments infill the base of the slide scar (Fig. 5) suggests that the slide was probably triggered at the very end of a glacial or during an interglacial. Over large areas, the glide plane of the Sklinnadjupet Slide is located within the acoustically laminated sediment package which separates Naust B and C (Figs. 5, 7 and 8). Similarly, the Trænadjupet Slide (Fig. 3) glide plane(s) are located along acoustically laminated sediment packages overlain by acoustically homogenous debris flow deposits (Laberg & Vorren, 2000). This highlights the role of the acoustically laminated sediments as potentially weak layers, susceptible to slope failure. 6.6. Diapirism The large lobe of Naust unit D extending out on the northern Vøring Plateau (Fig. 12D) is penetrated by several diapirs (Figs. 4 and 12D). This raises the question; were these diapirs present at the time of deposition of Naust D, or are they a later phenomenon? Since Naust D extends beyond the diapirs, we consider it unlikely that the diapirs were well developed at the seabed at the time of deposition of Naust D. If this were not the case, Naust D would have been dammed behind the obstacle that diapirs at seabed would represent. Information from DSDP 341 also suggests that subsequent to the deposition of Naust D, diapirism was very active, as a mixture of Eocene and younger sediments with ambient deformation and flow structures superimpose the undeformed Naust D (and underlying sediments) at the rim

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of the diapirs (Fig. 4). The location of the diapir field corresponds to the major depocentre of Naust D (Fig. 12D), indicating that the loading induced by Naust D on the underlying sediments acted to trigger diapirism. However, several other factors were probably important in addition to the loading induced by the Naust units, e.g. hydrocarbon migration guided by volcanic intrusions and vents, below and through easily mobilized low density oozes (Hovland, Nygaard, & Thorbjørnsen, 1998).

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consequence of sediment loading due to the Late Quaternary sedimentary pattern. † The deposition of Naust D in the Vema Dome was followed by a period of extensive diapirism. This suggests that diapirism occurred in response to sediment loading in that area. † A large buried slide, the Sklinnadjupet Slide, occurred between MIS 8 and 10 (245 –339 ka BP). Since the slide scar is infilled by contouritic sediments, it is suggested that the slide was likely triggered at the very end of glacial or in interglacial times.

7. Conclusions Based on detailed studies of high-resolution seismic records and correlation to boreholes and previously established seismic stratigraphies we observe and conclude that † The Fennoscandian Ice Sheet probably reached the shelf break during MIS 2 (ca. 15– 22 ka BP), 6 (128 – 186 ka BP), 10 (339 –362 ka BP), 12 (423 – 478 ka BP) and 14 (524 – 565 ka BP), and to the inner shelf during MIS 8 (245 –303 ka BP). The advances are reflected in the stratigraphical record as thick glacigenic debris flow units which exhibits a homogenous acoustic signature. † The observation that many of the glacigenic debris flow diamicton units comprising one glacial can be subdivided, and that sub-units reflect shifts in the depositional pattern, suggests that the sub-units may reflect ice-sheet fluctuations within one glacial. † The glacigenic debris flows are found both as readily identifiable singular lobes and as larger internally homogenous bodies composed of many stacked glacigenic debris flows. The varying depositional mode of the debris flows (singular lobes/larger homogenous bodies) can possibly be related to a variety of causes: (1) a change from homogenous internal facies related to focused deposition on a concave slope profile along the base of slope to a lensoid pattern related to dispersed deposition on a convex profile; (2) steepening of the slope, with deposition on a low gradient slope resulting in homogenous units and deposition on a high gradient slope in lensoid units; (3) differences in rheology related to water content and/or differing mineralogy, in turn related to differing glacial regimes and/or erosive pattern; and (4) variations in the rate of sediment supply, with high rates corresponding to homogenous units and low rates to lensoid units. † Bottom-current control upon hemipelagic/contouritic sedimentation is evident during both interglacials and glacials. † The outer shelf subsided at a rate of up to ca. 1.2 m/ka since ca. 350 ka BP. The area reflecting the largest subsidence corresponds to the depocentre during the same period. Hence, the subsidence is most likely a

Acknowledgements STATOIL most kindly provided a research fellowship to the first author. The contribution of JSL was funded through the EU-project STRATAGEM (Contract no. EVK-CT-9900011). The Norwegian Deepwater Programme’s Seabed Project (JIP sponsored by Norsk Hydro; BP; Statoil; Conoco; Shell; TFE; Esso; Phillips Petroleum; RWEDEA; Petoro) supplied a substantial part of the seismic database. The captain and crew of the University of Tromsø’s F/F Jan Mayen and engineer Steinar Iversen contributed to successful cruises. Paula Clark, data librarian at Ocean Drilling Program most kindly supplied colour slides of the DSDP hole 341 cores. Part of this work was undertaken during the first author’s visit to the Marine and Petroleum Studies Group at the British Geological Survey, Edinburgh. Reviewers Tom Bugge and Sverre Henriksen provided constructive comments for improving the paper. Martyn Stoker kindly made suggestions for improving the language and the clarity of the paper.

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