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doi:10.1016/j.qua

Quaternary Science Reviews ] (]]]]) ]]]–]]]

Late Quaternary seismic stratigraphy and geological development ofthe south Vøring margin, Norwegian Sea

Berit Oline Hjelstuena,*, Hans Petter Sejrupa, Haflidi Haflidasona, Atle Nyg(arda,Ida M. Berstada, Gregor Knorrb

aDepartment of Earth Science, University of Bergen, All!egaten 41, N-5007 Bergen, Norwayb Institute for Meteorology, University Hamburg, Bundesstr. 55, 20146 Hamburg, Germany

Received 22 August 2002; accepted 18 March 2004

Abstract

High-resolution seismic data and sediment cores show that an up to 280m thick sedimentary sequence has been deposited on the

south Vøring margin, off mid-Norway, the last ca 250 ka. The sedimentary succession has been divided into six seismic units,

dominated by hemipelagic sediments. Five wedge-shaped massive sequences, of marine isotope stages 8, 6 and 2, interfinger the

hemipelagic deposits on the upper slope. The wedge-shaped sequences represent glacigenic debris flows that have been fed by till

transported to the shelf edge by grounded ice sheets during maximum glaciations. The hemipelagic units show well-defined

depocentres, of various thicknesses, on the upper continental slope. Seismic facies interpretation indicates that the sediment

distribution locally has been controlled by currents. Commonly, the hemipelagic units are characterised by parallel and continuous

reflectors. However, the second youngest unit identified, deposited between 15.7 and 15.0 14C kaBP, is acoustic transparent.

We suggest that this unit has been sourced by along-slope transported meltwater plume deposits, released during the initial stage of

the last deglaciation of the Norwegian Channel. The hemipelagic sedimentation rates have varied considerably throughout the

studied time period. Until ca 21 14C kaBP the rates did not exceed 1.4m/kyr, whereas during the Last Glacial Maximum the rates

increased and reached values of about 36m/kyr before decreasing again at ca 15 14C kaBP. Observation of iceberg scourings, of MIS

8 age, about 800m below the present day sea level, suggest that the south Vøring margin has subsided by a rate of 1.2m/kyr in

the Late Quaternary.

r 2004 Elsevier Ltd. All rights reserved.

1. Introduction

The Vøring margin, off mid-Norway (Fig. 1), has forthe last 20 years been the location for both commercialinterests and geological/geophysical scientific studies.The investigations have to a large degree dealt with thetectonic evolution, crustal structure, rifting and break-up of the margin segment (e.g. Eldholm et al., 1989;Skogseid et al., 1992, 2000; Dor"e et al., 1999; Mjeldeet al., 2003). However, studies have also focused onvertical movements and Cenozoic sediment stratigraphy(e.g. Skogseid and Eldholm, 1989; Stuevold andEldholm, 1996; Hjelstuen et al., 1999), whereas DeepSea Drilling Project (DSDP) and Ocean Drilling

g author. Tel.: +47-55-58-35-07; fax: + 47-55-58-36-

s: [email protected] (B.O. Hjelstuen).

front matter r 2004 Elsevier Ltd. All rights reserved.

scirev.2004.03.005

Program (ODP) legs have increased our knowledge onsedimentary environments and paleoclimatic changeswithin the region (Henrich et al., 1989; Jansen et al.,1989). In spite of the broad investigations, rather fewstudies (e.g. Dahlgren et al., 2002; Dahlgren and Vorren,2003) have in detail focused on the Late Quaternarymargin evolution, when Fennoscandia and the adjacentcontinental shelf periodically were covered beneathextensive ice sheets.The Quaternary sediments on the mid-Norwegian

continental margin strongly reflect changes related toclimatic oscillations (King et al., 1987; Jansen et al.,1989, 1990). Thus, this area is important when describ-ing geological evolution and sedimentary processesacting on glacial continental margins. Increased hydro-carbon exploration on continental slopes has alsocaused the need for studies of geological processeswithin these environments. Furthermore, knowledge on

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the sedimentary regime off mid-Norway is also ofimportance for the geological understanding of thecontinental slope to the south. This, as it is believed thatsimilar deposits as the Quaternary sediments on thesouth Vøring margin were deposited in the StoreggaSlide region (Fig. 1) before the Storegga Slide eventremoved these sediments for about 7200 14C years ago(Haflidason et al., 2003).In this study we present interpretation of recently

collected high-resolution seismic data, that in combina-tion with shallow cores make it possible to address, indetail, the Late Quaternary geological evolution of thesouth Vøring margin. We aim to: (1) establish a LateQuaternary seismic stratigraphy, (2) map any changes insediment distribution and sedimentation rates through

time, (3) identify source areas, and (4) study sedimentaryprocesses.

2. Geological background

2.1. Structural setting and morphology

The structural configuration of the Vøring marginincludes the Trøndelag Platform, the Vøring Basin andthe Vøring Marginal High (Fig. 1), and is a resultof post-Caledonian lithosphere extension (e.g. Brekke,2000), which culminated with break-up of theNorwegian–Greenland Sea at the Paleocene–Eocenetransition for about 55 million years ago (Skogseid

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et al., 1992, 2000). The Cretaceous Vøring Basin wasstructured into several ridges and heights in the LatePaleocene–Early Eocene, and throughout Cenozoic timedomes and arches evolved within the basin (Fig. 1)(Dor!e and Lundin, 1996; Hjelstuen et al., 1997; Lundinand Dor!e, 2002). These structural elements have playeda major role for the sediment distribution on thecontinental margin throughout the Tertiary and Qua-ternary.The Vøring margin is furthermore characterised by a

broad continental shelf with bank areas and cross-shelftroughs (Fig. 1). Along the western part of the shelf,close to the shelf edge, a 100 km long, 200m high and10 km wide moraine ridge complex, the Skjoldryggen,has evolved. The continental slope is broken by theVøring Plateau (Fig. 1), a broad marginal plateau at awater depth of about 1100 and 1400m. Mud diapirs arelocally rising tens of metres above the seafloor in thisregion and disturb the smooth plateau seabed (Hjelstuenet al., 1997; Hovland et al., 1998). The slope bathymetryalso reflects the major slide events, i.e. the Storegga andTrænadjupet slides, which affected the Norwegiancontinental margin in the Holocene (Laberg and Vorren,2000; Laberg et al., 2002; Bryn et al., 2003) (Fig. 1).

2.2. Glacial history and ocean circulation pattern

The climate on the Northern Hemisphere started todecline during the Miocene, and at about 12Ma the firstoccurrence of ice rafted material (IRD) is observed onthe Vøring margin (Fronval and Jansen, 1996); probablyindicating that ice caps large enough to reach theNorwegian coast existed. Throughout the Late Plio-Pleistocene time period stepwise increase in the amountof IRD supply to the continental slope reflects theintensification in the Northern Hemisphere glaciations(Henrich et al., 1989; Jansen and Sjøholm, 1991).The first extent of an ice sheet to the shelf edge within

the studied region is suggested at about 1.1Ma(Haflidason et al., 1991). However, repeated ice sheetadvances across the mid-Norwegian continental shelfhave only been documented since ca 0.5Ma (Dahlgrenet al., 2002), culminating with the Last Glacial Max-imum (LGM) in the Late Weichselian. Around 1514C kaBP LGM terminated and the final deglaciation ofthe Norwegian margin occurred (Vorren and Plassen,2002; Dahlgren and Vorren, 2003; Sejrup et al., 2003).We note that Nyg(ard et al. (2004) identified an icereadvance, the Bremanger Event, on the Norwegianshelf in the time period between ca 15 and 13.314C kaBP, i.e. when the main Fennoscandian deglacia-tion was well underway. The repetitive glaciationscaused erosion of the continental shelf and formed thepresent day morphology of the Trøndelag Platform(Fig. 1), where the troughs acted as pathways for icestreams (Ottesen et al., 2001, 2002).

The present hydrographic regime within the studiedregion is dominated by three water masses: NorwegianCoastal Current water, Norwegian Current Atlanticwater, and the deep-water in the Norway Basin. TheAtlantic water, which has a salinity of about 35% andtemperatures of 6–9�C (Henrich and Baumann, 1994),covers the outer continental shelf and the uppermostcontinental slope down to a water depth of about600–700m, and is sharply bounded both towardsthe Norwegian Coastal Current, which has a salinityof o33%, and the Norway Basin deep-water havingsalinities >34.9% and temperatures o0�–1� (Thiedeet al., 1989).It appears that the seabed sediment distribution is

strongly affected by the characteristic of the overlyingwater masses. The sea floor beneath the Atlantic waterconsists of sorted sand or gravelly lags. Terrigeneousmud of variable thickness covers the sea floor between700 and 1200m, whereas in water depths >1200m, aless than 20 cm thick cover of foraminiferal ooze with ahigh carbonate content has been deposited (Holtedahl,1981; Sejrup et al., 1981). Throughout the LatePlio-Pleistocene time period the magnitude of theNorwegian Current has varied. Between ca 2.6–1.0MaAtlantic water intruded the Norwegian–GreenlandSea only episodically, whereas from about 1Ma theNorwegian Current was strengthened (Henrich andBaumann, 1994).

3. Data and methods

Two IMAGES cores, two boreholes and about3000 km with deep-tow boomer profiles define the maindatabase in this study (Fig. 1). Mini-sleeve gun recordsand a 3D survey have also been investigated.

3.1. Images cores and boreholes

IMAGES cores MD99-2289 and MD99-2291 aregiant piston cores sampled during the 1999 IMAGES-

V cruise. MD99-2289 is 23.69m long and was raisedfrom a water depth of 1262m, near the northernStoregga Slide scar (Figs. 1–3). From this core, sedimentsamples have been taken every 2–10 cm for stableisotope and magnetic susceptibility measurements andfor analyses of bulk intensities of selected chemicalelements (Berstad, 2003). The core has been dated byusing Accelerator Mass Spectrometry (AMS) on 12monospecific samples of the planktonic foraminiferaspecies Neogloboquadrina pachyderma (sin) (Fig. 3). The25.75m long IMAGES core MD99-2291 is taken on theupper continental slope, in a water depth of 577m(Fig. 1). Ninety sub-samples have been taken from thiscore for analyses of bulk density, porosity, watercontent, and grain-size distribution (Knorr, 2000). The

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Fm. Profile and core (MD99-2289 and 6404/5) locations in Fig. 1. T1 and T3: Massive seismic sequences comprising glacigenic sediments. MIS:

Marine isotope stage.


core chronology is based on six radiocarbon (AMS)dates on samples of N. pachyderma (sin) (Fig. 3).Borehole 6404/5, which is 310m long, was done in a

water depth of 966m whereas the 123.6m long borehole6405/2 is localized only 5 km east of MD99-2291 (Fig. 1).The recovery for both boreholes is low, only between 8.5and 11.6%. Radiocarbon analyses, based on N.

pachyderma (sin), paleomagnetic analyses, strontiumisotope analyses and amino acid diagenesis wereobtained to establish an age model for the boreholes(Haflidason et al., 1998, 2001). The preparation andcounting procedures for the foraminiferal analyses werein accordance with methods described by Feyling-Hansen (1958) and Meldgaard and Knudsen (1979).The lithological descriptions are based on visualinspection, X-radiographs, sediment petrography andgeochemical measurements of carbon content and totalorganic carbon.

3.2. Seismic data

A regional grid of deep-tow boomer (DTB) profiles(Fig. 1), collected by the British Geological Survey (BGS)and Norsk Hydro in 1993 and 1996–97, respectively, havebeen interpreted. The BGS data, which were collectedalong the northern flank of the Storegga Slide (Fig. 1), areof high quality and have a maximum penetration of about300 milliseconds (ms) two-way traveltime (twt) (Evanset al., 1996). The Norsk Hydro data are collected by aHuntec DTB system operating at a frequency of 8kHz.These data have a penetration of 100–200ms (twt) andshow variable quality, thus making the stratigraphiccontrol locally uncertain.Along the surveyed Norsk Hydro DTB profiles, mini-

sleeve gun data were also collected. These profiles havebeen used to establish seismic stratigraphic control

beyond the range of the DTB. Furthermore, a 3D surveyfrom the Helland-Hansen Arch has been studied (Fig. 1).

4. Seismic stratigraphy and chronology

The Cenozoic succession on the Vøring margin hasbeen divided into three formal units (Dalland et al.,1988): the Eocene to Oligocene Brygge Formation (Fm),the Miocene to Early Pliocene Kai Fm, and the LatePliocene to Recent Naust Formation. Rokoengen et al.(1995) and McNeill et al. (1998) subdivided the NaustFm into several units, and at present five units, W(oldest), U, S, R and O (Fig. 2), have been identified(Norsk Hydro, 2003). Naust units W, U and S arecharacterised by westward dipping high-amplitudereflectors that separate sediment packages showingweak layering or a chaotic seismic pattern (Fig. 2).Naust R is dominated by a seismically massive sedimentunit, whereas Naust O, which is the focus of this paper,is composed of well-stratified sediments and seismicmassive wedge-shaped units (Fig. 2). As will bediscussed later, Naust O has been deposited throughoutthe last B250 ka.

4.1. Naust O

Seven sequence boundaries, Rf1–Rf7, have beenidentified within Naust O (Figs. 3–5). To be able to tiethe core data with the seismic profiles (Fig. 3), aninterval velocity of 1500m/s is suggested for thesediment sequences above Rf5, whereas an intervalvelocity of 1600m/s has been used for the underlyingdeposits.The oldest reflector mapped, Rf1, defines the upper

boundary of the Naust R unit (Figs. 4 and 5). Rf1 is

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Berstad, 2003). The radiocarbon dates have been corrected for the marine reservoir effect by 400 years (Stuiver et al., 1998). Estimated sedimentation rates are based on maximum sequence

thicknesses as observed in the seismic data. Core locations in Fig. 1. G: Glacigenic; GM: Glacimarine; M: Marine; MIS: Marine isotope stage; MS: Magnetic susceptibility; WD: Water depth.

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showing a highly irregular appearance in areas whereNaust R is represented by a seismic massive unit (Fig. 2),elsewhere the reflector is smooth and featureless. Rf2 iscommonly observed as a high amplitude reflector withinthe seismic data. The reflector, which locally issegmented, is traced to IMAGES core MD99-2289where it is identified at a core depth of about 20m. Theplanktonic foraminiferal assemblage and stable isotoperecords indicate that the last interglacial period (MarineIsotope Stage (MIS) 5e) is located at a core depth ofabout 18.6m (Berstad, 2003). Thus, as Rf2 is correlatedto a level just below MIS 5e, we envisage MIS 6 as aminimum age for the reflector.Sequence boundary Rf3 (Fig. 4), which also has been

termed INO3 (Norsk Hydro, 2003), is commonlyobserved as a smooth and featureless reflector within thedata set. We note that in an area across the southern partof the Helland-Hansen Arch the reflector is showing anirregular appearance (Fig. 4). In MD99-2289 Rf3 appearsto coincide with the identified MIS 5e level (Berstad,2003), thus revealing a reflector age of about 120ka.The medium to high amplitude, locally discontinuous,

reflector Rf4 is correlated to a core level of about 32m inborehole 6405/2. Thus, suggesting that Rf4 has aminimum age of 19.1 14C kaBP, which is the age foundat about 30m core depth. Seismic correlation to MD99-2289 indicates that Rf4 must be younger than ca 2114C kaBP (Fig. 3). This age therefore represent amaximum age for Rf4. It should be noted that thecorrelation to MD99-2289 is complicated by the factthat the core is located about 600m from the closestseismic line. The sedimentary sequences also thinsignificantly towards the IMAGES core, reducing theaccuracy of the seismic tie.Reflectors Rf5, Rf6 and Rf7 are observed as high

amplitude continuous reflectors in the seismic profiles,and are easily traceable throughout the data set. ForIMAGES core MD99-2291, a good geochronologicalframework has been established (Knorr, 2000) (Fig. 3).Rf5 coincides with a level dated to about 16.2 14C kaBPwithin this core, whereas Rf6 is correlated to a coredepth of about 15m, suggesting a maximum reflectorage of 15.7 14C kaBP. At a level of 2.8m in MD99-2291an age of 15.0 14C kaBP has been found. Rf7 is tied to alevel just below this age; therefore we suggest 15.014C kaBP as a minimum age for the uppermost reflectoridentified in Naust O. Seismic correlation to the othercores also confirm the age constraints of the upper threeidentified sequence boundaries.

5. Seismic sequences

Reflector Rf3 divides Naust O into two main units:Lower and Upper Naust O, where Upper Naust O hasbeen divided into five sub-units, Se1 (oldest) to Se5

(Fig. 3). The seismic data reveal that the sub-unitseither are acoustic transparent or are composed ofcontinuous parallel medium to high amplitude reflec-tors. On the uppermost slope, Rf2, Rf4, Rf5 andRf6 coincide with wedge-shaped seismic massive units(T1–T4 in Figs. 3–5).

5.1. Acoustic well-stratified units

Lower Naust O, which is limited by Rf1 and Rf3, isdominated by parallel to sub-parallel reflectors. In alocal area over the southern Helland-Hansen Arch non-continuous reflectors and a slightly chaotic seismicpattern characterise the unit (Fig. 4). Borehole 6405/2(Fig. 3) shows that the upper part of the unit consists ofsandy silt with sporadic occurrence of gravel, changinginto gravelly, sandy mud near the lower sequenceboundary (Haflidason et al., 1998, 2001). The sedimentdistribution of the Lower Naust O unit envisages a maindepocentre in a water depth of 800–1000m, close to thenorthern Storegga Slide scar (Fig. 6). Within thedepocentre the sediments are reaching a thickness>200ms (twt). A local depocentre has also evolvedwithin the Sklinnadjupet Slide scar, where the seismicprofiles show that Lower Naust O has a moundeddepositional pattern (Figs. 6 and 7).The five sub-units of Upper Naust O are all, except

for Se4, dominated by parallel, medium to highamplitude reflectors (Fig. 4). The geotechnical boringsshow that units Se1 and Se2 consist of sandy silty mud,which changes into silt in the uppermost part of Se2.IMAGES core MD99-2291 indicates that the well-layered Se3 unit consists of clay with various amountsof sand and silt, whereas the youngest sub-unitidentified, Se5, is composed of silty sand on the upperslope.During deposition of Upper Naust O, the depocentre

on the southern Vøring margin persisted (Fig. 6),reaching a maximum thickness of 115ms (twt) nearthe northern scar of the Storegga Slide. Duringdeposition of sub-units Se1, Se2, and Se3 (Fig. 3) themain part of the sediments were deposited in waterdepths between 500 and 800m, i.e. along the uppercontinental slope (Fig. 7). For unit Se5 the isopach mapshows a well-defined depocentre, reaching a maximumthickness of about 15m, within a water depth of about1000m (Fig. 7).We note that the acoustic well-stratified units locally

are disturbed by pockmarks, vent-like features, muddiapirs (Fig. 7) and small-offset faults (Evans et al.,1996; Hjelstuen et al., 1997; Bryn et al., 1998; Fulop,1998; Gravdal, 1999; Bouriak et al., 2000; Gravdal et al.,2003). The fluid expulsion features, in addition toobservation of bottom simulating reflectors (BSRs)(Mienert et al., 1998), imply that shallow gas is, or hasbeen, present in the sediments.

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surface. Based on interpretation of Helland-Hansen 3D survey, and modified from Nyg(ard et al. (2000). Curvi-linear features represent iceberg ploughmarks. (c) DTB example showing seismic facies

characteristics of the Naust O unit.

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seismic units (T1–T4). Profile location in Fig. 1.

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Slide (SkS) from Dahlgren et al. (2002). (b) Isopach map of Upper Naust O hemipelagic sediments. Contour interval 25ms (twt). Stippled line

represents western limit of identified glacigenic units. Bathymetry in metres.

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~2.5 km

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Fig. 7. (a) Location of main depocentres of Upper Naust O sub-units Se1, Se2, Se3 and Se5. Thickness, in ms (twt), of each unit is indicated within

parentheses. Outline of Sklinnadjupet Slide from Dahlgren et al. (2002). (b) Mini-sleeve gun profile (NH9651-301) across the southern Sklinnadjupet

Slide scar. Profile location in Fig. 7a.

B.O. Hjelstuen et al. / Quaternary Science Reviews ] (]]]]) ]]]–]]] 9

5.2. Transparent units

Unit Se4 is limited by reflectors Rf6 and Rf7, and ischaracterised by an acoustic transparent seismic pattern(Figs. 4 and 5). Weak layering is locally observed. Thus,Se4 differs significantly in its seismic facies appearancefrom the other Upper Naust O sub-units. This difference

is also documented in MD99-2291, where Se4 corre-sponds to lithological unit L3 that is composed of siltyclay and minor amounts of material >150 mm (Fig. 3).The isopach map reveals that an elongated depocentre,reaching a maximum thickness of about 20m, developedalong the upper continental slope during Se4 deposition(Fig. 8).

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Fig. 8. (a) Isopach map of acoustic transparent unit, Se4, deposited between 15.7 and 15.0 14CkaBP. Contour interval is 2.5/5m (assumed interval

velocity is 1500m/s). LGM: Last Glacial Maximum. (b) Sedimentation rates the last 30,000 years in selected cores along the margin (based on

Dokken and Jansen, 1999; Knorr, 2000; Lekens, 2001; Berstad, 2003). Core locations in Fig. 8a.


5.3. Seismic massive units

Unit T1 represents the oldest massive unit mappedwithin Naust O. The unit reaches a maximum thicknessof 250ms (twt), and is restricted to the continental slopejust west of Skjoldryggen (Fig. 9). It extends down to awater depth of about 1200m, and is dominated by anacoustically non-structural signature. A few internalcontinuous reflectors are locally observed, and the unitsplit up into several smaller-sized acoustic transparentlobes at its westernmost tip. On the uppermostcontinental slope the T1 surface shows an irregularappearance; elsewhere the surface of the unit is verysmooth (Figs. 4 and 5).Units T2 and T3 show large variations in extent and

thickness within the study area (Fig. 9), and areseparated from each other by a less than 10ms (twt)thick well-laminated sequence (Figs. 4 and 5). WhereasT2 is limited to the southernmost study area, the above-lying T3 unit can be identified along the entirecontinental shelf and upper slope. The limited extentof T2 may be due to the erosive nature of the T3 unit.Both units, which extend down to a water depth of600–800m in the southern part of the study area, arecommonly characterised by an acoustic homogeneousand non-structural acoustic seismic facies (Fig. 2).

Locally, internal medium amplitude reflectors dividethe units into lobe-shaped lenses. The uppermostidentified seismic massive unit within Naust O, T4, isrestricted to the continental shelf, where it reaches amaximum thickness of about 100ms (twt) (Fig. 9). T4,which has an erosive base and an irregular uppersurface, has a similar seismic facies as units T1–T3.The seismic massive unit within Naust R reaches a

maximum thickness of about 350ms (twt) (Fig. 9). Theunit pinches out in water depths between 1200 and1400m. In depths o800m the Naust R surface ischaracterised by curvi-linear scours (Fig. 4), whereasdown-lapping reflectors are observed near the base ofthe unit (Fig. 2). In water depths >1000m the Naust Rmassive unit is composed of well-defined lobes (Fig. 10).Dahlgren and Vorren (2003) found from analyses of

two gravity cores from the northern Vøring margin thattheir mapped Naust A2 unit, which correlates to T3 inthis study, has high density, relative high shear strength(about 30 kPa) and is composed of sand, silt and claywhere the sand fraction is dominated by quartz grains.The deposits are furthermore unsorted and containclasts of striated sedimentary and angular crystallinebedrock fragments. Borehole 6405/2 penetrates theoutermost parts of T3 and T2 in our study area;showing that T3 consists of silty sand with gravel and

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Fig. 9. Isopach maps of: (a) Naust R glacigenic sediments. Modified from Dahlgren et al. (2002), (b) Unit T1 (MIS 6), (c) Unit T2 (MIS 2), (d) Unit

T3 (MIS 2) and (e) Unit T4 (MIS 2). Countour interval 50ms (twt). Bathymetry in metres.


pebbles, whereas T2 is characterised by silty sedimentsand laminated sand-rich zones that sporadic containgravel. Borehole 6405/2 also penetrates the uppermostpart of the seismic massive sequence within Naust R(Haflidason et al., 1998, 2001), revealing that it iscomposed of gravelly sandy silt and/or silty sand withclasts.

6. Margin subsidence

The amount of subsidence and local variations insubsidence rates strongly influence the distribution andgeometries of sedimentary units. Differential subsidencemay also influence the evolution of structural elements,as has been suggested for the Helland-Hansen Arch onthe Vøring margin (Fig. 1) (Stuevold et al., 1992). Thestudy area subsided rapidly due to thermal cooling justafter margin break-up at ca 55Ma, and throughout theEocene time period the Vøring margin subsided byabout 1000m (Eldholm et al., 1989). The subsidence was

most rapid in the west, diminishing eastwards. Sub-sidence analyses in wells along the Atlantic margin havefurther revealed low subsidence rates, commonlyo2 cm/kyr, during the Oligocene and Miocene, whereasincreased rates, up to ca 10 cm/kyr, are observed duringthe Pliocene time period (Cloetingh et al., 1990).The characteristics of the linear criss-crossing features

observed at the Naust R surface (Fig. 4), about 800mbelow the present day sea level, make us infer that theyrepresent iceberg ploughmarks. Our seismic recordsshow that the present day seabed, in water depths lessthan ca 500m, is heavily scoured by icebergs that calvedoff a waning ice margin during the last deglaciation ofthe Norwegian continental shelf. This observation isalso supported by side-scan sonar data from thenorthern flank of the Storegga Slide (Gravdal, 1999),the North Sea Fan region (Nyg(ard et al., 2002) and theeastern part of the Faroe-Shetland Channel (Masson,2001). Furthermore, and as will be discussed later, theupper part of Naust R has a minimum age of MIS 8.Thus, if we assume that icebergs only reached a

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Fig. 10. (a) Mini-sleeve gun profile (NH9651-110), showing configuration of Late Pliocene–Recent Naust sediments on the south Vøring margin.

Profile location in Fig. 1. HHA: Helland-Hansen Arch, (b) seismic facies characteristics of Naust R in water depths >800m. GDFs: Glacigenic

debris flows, (c) seismic facies characteristics of Naust R in water depthso800m, (d) 3D time slice showing internal acoustic facies of the Naust R

unit.


maximum water depth of about 500m during previousglaciations, the Vøring margin has subsided by B300msince MIS 8; giving a maximum margin subsidence rateof B1.2m/kyr during the last ca 250 ka.Based on identification of paleo-ice sheet grounding

lines, Dahlgren et al. (2002) found similar Quaternarysubsidence rates for the northern Vøring margin. HighQuaternary subsidence rates have also been reported

elsewhere along the Norwegian continental margin.King et al. (1996) estimated a minimum subsidence rateof 0.8m/kyr for the northern North Sea margin,whereas along the Svalbard margin subsidence ratesof up to 0.48m/kyr have been found (Solheim et al.,1996). Cloetingh et al. (1990) related the increased ratesto major changes in the spreading direction andspreading rate along the Mid-Atlantic spreading ridge.


Nonetheless, the high sediment input to the mid-Norwegian margin during the Pleistocene time periodmust also have influenced on the observed rates. It hasbeen inferred from modelling that sediment loading canamplify the subsidence by an order of 2–4 (Gvirtzman,2001). Most likely the high rates are a result of acomplex interaction between sediment loading, eustacyand tectonic events, as also have been suggested by Jordtet al. (2000).

7. Margin evolution

The detailed chronology and seismic facies charac-teristics of the south Vøring margin sedimentarysuccession (Fig. 3) make it possible to identify, andto study, sedimentary processes that have been acting ona continental margin through a glacial–interglacialcycle.

7.1. Glacial environments

7.1.1. Glacigenic sediments

Seismic correlations show that our identified massiveunits coincide with sequences on the continental shelf,which King et al. (1987, 1991) ascribed to be of aglacigenic origin. The seismic ties envisage that the‘‘Upper Till’’ unit identified by King et al. (1987)correlates to our unit T4, whereas the ‘‘Middle Till’’ unit(King et al., 1987) corresponds to T3, and possibly T2,on the slope. We also note that the massive units show asimilar seismic facies as sediments which on theCanadian and West Shetland continental margins havebeen interpreted to represent glacigenic material(Stoker, 1997; Stravers and Powell, 1997; Josenhansand Lehman, 1999; Hewitt and Mosher, 2001), eventhough seismic facies cannot be used conclusively toidentify, or distinguish between, different lithologies andgenesis. The uppermost identified shelf sequence of Kinget al. (1987, 1991) is cored SE of Skjoldryggen (Sættemet al., 1996), showing that the anticipated glacigenic unitconsists of silty, sandy clay with some gravel. High shearstrength and overconsolidation characterise these de-posits (Rokoengen and Frengstad, 1999). Shallowcoring from the Haltenbanken area (Fig. 1) also revealthe glacigenic nature of the mid-Norwegian shelfsediments (Haflidason et al., 1991), even though a directcorrelation to the continental shelf units of King et al.(1987, 1991) has not been established.Our chronostratigraphy (Fig. 3) show that T1–T4

have been deposited in time periods when ice capscovered the Norwegian mainland and the adjacentcontinental shelf (Fig. 3). Unit T1 coincides withreflector Rf2 (Fig. 5), showing that the lowermostmapped massive Naust O unit is related to MIS 6. UnitsT2–T4 coincide with sequence boundaries Rf4, Rf5 and

Rf6; thus suggesting that these units were deposited atabout 21, 16.2 and 15.7 14C kaBP, i.e. during LGM(MIS 2).Studies of a large number of cores from the northern

North Sea have shown that LGM is divided into twophases (Sejrup et al., 1994, 2000): A first phase betweenca 29–22 14CkaBP and a final phase between about 18.5and 16 14C kaBP, with open marine conditions in thenorthern North Sea between ca 22 and 18.5 14C kaBP.Valen et al. (1996) confirmed this observation, findingthat the Scandinavian ice sheet had retreated to theNorwegian coast by ca 20 14C kaBP. Sejrup et al. (1994)concluded that the Scandinavian ice sheet coalesced withthe British ice sheet as early as ca 29 14C kaBP. Boultonet al. (2002) have however questioned ice sheetconfluence as early as this, as radiocarbon dates suggestice free regions on the Scotland mainland until ca 2614C kaBP.From the Andøya region, northern Norway, it has

also been reported that the Fennoscandian ice sheetadvanced twice during LGM; i.e. between about 23 and22 14C kaBP and from 18 to 14.5 14C kaBP (Vorren andPlassen, 2002). We further note that Dahlgren andVorren (2003) from IRD maxima, deposition ofstratified diamicton and glacigenic debris flows sug-gested five ice sheet fluctuations during LGM on thenorthern Vøring margin. The chronology and stratigra-phical positions of units T2–T4, divided as they are by athin layer of well-stratified sediments, thus suggest atleast three LGM ice sheet fluctuations within the studyarea.The chronology of the Naust O massive units and

their relationship to glacial periods indicate that themassive Naust R unit must be older than MIS 6. On theother hand, amino acid stratigraphy (Haflidason et al.,1998, 2001) suggests that the uppermost part of this unitis younger than MIS 9. Hence, we suggest MIS 8 as aminimum age for Naust R.

7.1.2. Deposition mode

Different depositional models have been proposed fortransportation and distribution of glacigenic material oncontinental margins. Fifteen years ago King et al. (1987)established the till-tongue model, which described thewedge-shaped massive slope units (Fig. 5) on the Vøringmargin as purely deposits from grounded ice sheets.Since then, this model has been modified (King, 1993)and distribution of glacigenic material on continentalslopes is today commonly considered to be related todifferent mass movements (e.g. Stoker, 1997; Straversand Powell, 1997). The glacigenic unit within Naust Rhas the best data coverage, including both 2D and 3Dseismic, and is thereby suitable for a detailed study ofprocesses acting during deposition. It is howeverreasonable to assume that units T1–T4 have beendeposited in a similar way.


The geometry of Naust R, with its flat surfacebounded westward by a sharp edge in a present daywater depth of 700–800m, suggests that a paleo-continental shelf and a paleo-shelf edge have existedfurther to the west at MIS 8 time then at present(Fig. 10a). The observed ploughmarks at the Naust Rsurface (Fig. 4), in addition to the sediment character-istics of the unit, imply that an ice sheet could haveextended all the way out to the observed paleo-shelfedge. If so, the glacigenic material of Naust R has beentransported, and deposited, by a grounded ice sheet. Theinternal down-lapping reflectors within this unit (Fig. 2)might thus in fact represent a westward prograding tilldelta (Alley et al., 1989), developed beneath a westwardgrowing ice sheet. The 3D data show that the down-lapping reflectors bound lobe-like structures, having aninternal facies compatible with mass movements duringdeposition (Fig. 10d). Observation of internal flat-lyingsurfaces within Naust R might represent erosionalsurfaces caused by previous shelf edge glaciations.In water depths >800m the Naust R massive unit is

thinning (Figs. 2, 9 and 10a), and seismic cross sections(Fig. 10b) show that it display a similar seismic patternas glacigenic debris flows (GDFs) which build the hugetrough mouth fans on the Norwegian-Barents Seamargin (Laberg and Vorren, 1995; Sejrup et al., 1996;Elverhøi et al., 1997; King et al., 1998; Nyg(ard et al.,2002). Thus, to summarize, we suggest that during MIS8 time, and also during deposition of units T1–T4,tills were transported beneath a grounded ice sheet tothe shelf edge where the glacigenic material wasredistributed downslope by gravity flows (Fig. 11a).This model is also supported by results from the NorthSea Fan to the south (Nyg(ard et al., 2004) and studiesfrom the northern part of the Vøring margin (Dahlgrenet al., 2002).The T3 unit shows that the gravity flows on the

continental slope have eroded into the underlyingsedimentary units during deposition. However, in waterdepths >600m T3 lose its erosional effect, and theglacigenic material has been deposited conformableabove the underlying stratified sediments. Comparedto the GDFs observed on the North Sea Fan, whichhave been found as far out as 500 km from the shelf edge(Nyg(ard et al., 2004), the gravity flow lobes on theVøring margin have a much shorter run-out distance.This may be due to differences in the physical propertiesof the source material, amount of sediment delivered tothe shelf edge and differences in flow mechanisms (Marret al., 2002).

7.1.3. Source areas and sediment pathways

The isopach maps of the Naust O glacigenic unitsreach their maximum thickness at the mouth of shallowtroughs that intersect the continental shelf (Fig. 9). Ithas been inferred that these troughs acted as ice stream

pathways during maximum glaciations (Ottesen et al.,2001). Thus, it is reasonable to anticipate that the mainpart of the sediments have been transported by thecanalised ice streams to the shelf edge, where thematerial was deposited in depocentres at the ice streamfront.

7.2. Glacimarine environments

7.2.1. Sedimentation rates

Throughout the last ca 250 ka the maximum averagesedimentation rates on the south Vøring margin havebeen about 1.0m/kyr. However, as is well documentedfor the Upper Naust O unit (Fig. 3) the rates have variedsignificantly from this value through time. Between MIS8 and MIS 5e, during deposition of Lower Naust O, themaximum average sedimentation rates did not exceed1.4m/kyr. Low rates, o0.5m/kyr, persisted on themargin throughout MIS 5, 4 and 3, before increasing atabout 21 14C kaBP; most likely as a consequence of theLGM ice advance. Dahlgren and Vorren (2003) havereported a similar change in the accumulation rates atthe northern Vøring margin. Within the study area thesedimentation rates reached a maximum (36m/kyr)between 16.2 and 15.7 14C kaBP, when the well-laminated Se3 unit was deposited (Fig. 3). Highsedimentation rates, up to 27m/kyr, persisted between15.7 and 15.0 14C kaBP, during the initial phase of thefinal deglaciation of the Norwegian continental shelf.A significant drop in the sedimentation rates tookthereafter place at ca 15 14C kaBP (Fig. 3), and coreMD99-2291 reveals that the Holocene accumulationrates were as low as 0.1m/kyr on the upper continentalslope. Similar drops, when going from glacial to fullyinterglacial conditions, have also been reported from thewestern Svalbard margin (e.g. Andersen et al., 1996) andfrom the Laurentide shelf (Andrews, 2000). We notehowever that high Holocene rates are observed in theStoregga Slide scar (Fig. 1), where an up to 25–30mthick sediment package has been deposited since ca 7.214C kaBP (Sejrup et al., 2004).

7.2.2. Depositional pattern and source areas

The isopach maps of the Lower and Upper Naust Ounits show that the Pleistocene hemipelagic sedimentshave been deposited in major depocentres on the uppercontinental slope, in front of the shallow SuladjupetTrough (Figs. 6–8). As the Holocene Storegga Slideevent (Fig. 1) has removed large quantities of sedimentsfrom the Møre margin, it is not possible to infer if theidentified depocentres have extended further to thesouth or not.When discussing possible source areas and deposi-

tional environments for the hemipelagic Naust Osequence, the following two points must be taken intoconsideration: (1) The sediments have been transported,

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Fig. 11. Schematic models of the south Vøring margin during: (a) glacial maximum, (b) late glacial/early deglaciation and (c) interglacial. Present

day seabed sediment distribution and ocean currents are shown.



deposited and/or reworked under the influence of along-slope currents, and (2) The sediment distribution reflecta local point source on the Vøring margin. Currentinfluenced sediments are commonly observed in theseismic sections as various types of mounded features(Faug"eres et al., 1999), and Laberg et al. (1999, 2001)have identified several typical current-related depositson the Norwegian continental margin. On the Vøringmargin the isopach map of the Lower Naust O unitshows a well-defined depocentre within the Sklinna-djupet Slide scar (Fig. 6). The depocentre is identified asa mounded structure within the seismic sections (Fig. 7).Similar features are also locally observed along thewesternmost edge of the glacigenic T1 unit, thusshowing that bottom currents have influenced thesediment distribution within the study area in pre-MIS5e time. Observation of truncated reflectors within waterdepths of about 750–900m, along the eastern edge of theSe5 depocentre (Fig. 7), also indicate that along-slopecurrents have affected the sediment distribution thelast 15 ka.Even though the seismic records show that oceanic

currents affected the Late Quaternary depositionalpattern, we infer that the currents mainly have had alocal effect, and influenced the sediment pattern only inplaces along the margin were it is likely that strongcurrents could be initiated, as for instance within slidescars. As shown, the accumulation of sedimentsincreased in periods when the Fennoscandian ice sheetextended onto the continental shelf (Fig. 3). Thus, amajor part of the sediments in the well-stratified NaustO sub-units must have been delivered by icebergs andsuspension fall-out in front of a grounded ice margin(Fig. 11). It is also likely that the margin configuration(Fig. 2) prior to the deposition of Naust O has affectedthe sedimentary distribution pattern the last ca 250 ka.

7.2.3. Meltwater plumes

In the region from the North Sea Fan to the centralpart of the Vøring Plateau well-dated sediment coresshow that the sedimentation rates on the Norwegianmargin have varied considerably throughout the last30 ka (Fig. 8). Both on the North Sea Fan and on theVøring margin the accumulation rates increased atabout 25 14C kaBP. On the southwestern part of theNorth Sea Fan the rates declined at about 20 14C kaBP,whereas high rates persisted on the Vøring margin untilca 15.0 14C kaBP. In the time period preceding 15.714CBP well-laminated sediments were deposited on themargin. However, as is revealed both from seismic faciespattern and lithology a major change in the depositionalenvironment occurred at 15.7 14CBP (Figs. 3 and 4).This change in depositional environment occurred atabout the same time as the start of the deglaciation ofthe Norwegian Channel (Fig. 8), which had beenoccupied by an ice stream during LGM (e.g. Sejrup

et al., 2000). We infer that large amounts of meltwaterwere released from this waning ice stream, and thatalong-slope currents transported the sediment-ladenmeltwater, as a plume, towards the Vøring margin.Gradually, the fine-grained sediments settled out inthe up to 20m thick Se4 unit on the Vøring Plateau(Figs. 8 and 11b).

8. Conclusions

The Late Quaternary evolution of the south Vøringmargin has been studied by the use of high-resolutionseismic data and sediment cores. The sedimentarysuccession (Naust O) has been divided into six hemi-pelagic units, separated by the medium to highamplitude reflectors Rf1 (oldest) to Rf7. The seismicprofiles reveal that the units either are acoustictransparent or are composed of parallel reflectors. Onthe upper continental slope four massive seismic units,T1–T4, interfinger the hemipelagic sediments.The seismic massive units continue onto the con-

tinental shelf and are composed of glacigenic material.During maximum glaciations, when the ice sheetsreached the shelf edge and ice streams occupied theshallow troughs on the continental shelf, the glacigenicmaterial was transported to the shelf break by groundedice sheets where gravity flows distributed the glacigenicsediments down slope (Fig. 11a). The isopach mapsshow that the depocentres of these sediments are at themouth of shallow troughs (Fig. 9).Throughout the entire Late Quaternary time period

the south Vøring margin has been a major depocentrefor hemipelagic sediments (Figs. 6–8). The sedimentsuccession reaches a maximum thickness of about350ms (twt), i.e. about 280m (assuming a velocity of1600m/s), within the study area. Locally, these sedi-ments have been influenced by current activity, as isexemplified by the mounded deposits within theSklinnadjupet Slide scar (Fig. 7).The sedimentation rates on the margin have varied

considerably through time. Low average rates existed onthe south Vøring margin in pre-LGM times. DuringLGM the sedimentation rates increased dramatically,reaching a peak of 36m/kyr between 16.2 and 15.714C kaBP. High rates persisted on the margin between15.7 and 15.0 ka 14CBP, when Naust O sub-unit Se4 wasdeposited. We relate the deposition of this unit to thelast deglaciation of the Norwegian continental shelf,assuming that Se4 has been fed by sediments from ameltwater plume released from the disintegratingNorwegian Channel Ice Stream and transported to thesouth Vøring margin by along-slope currents (Fig. 11b).Throughout the Holocene the sedimentation rates onthe margin have been low, and erosion by oceaniccurrents occurred on the upper slope (Fig. 11c).


The high sedimentation rates which periodically havebeen observed on the Vøring margin during thePleistocene time period have presumably affected themargin subsidence. Observation of iceberg ploughmarksabout 800m below the present day sea level suggest thatthe margin has subsided by about 300m through theLate Quaternary, indicating maximum subsidence ratesof 1.2m/kyr since MIS 8.

Acknowledgements

We acknowledge the SEABED and Ormen Langeindustry consortiums, and the British Geological Surveyfor making seismic data available. This work has beenfunded by Norsk Hydro ASA, Enterprise Oil NorwayLimited and the EC-supported STRATAGEM (Strati-graphical Development of the Glaciated EuropeanMargin) project (Contract Number EVK3-CT-1999-00011). We are grateful to Professor J .orn Thiede andEditor in Chief, Professor Jim Rose for their construc-tive reviews of the paper.

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