Thickness, distribution and depositional environment of Holocene sediments in the Norwegian part of the Skagerrak

Thickness, distribution and depositional environment of Holocene sediments in the Norwegian part of the Skagerrak

LEIFRISE,HEIDI ANITAOLSEN,REIDULV B0E&DAG OTIESEN

Rise,L.,Olsen,HA,Bee,R. &Ottesen,D1996.:Thickness,distr ibutionand depositional env ironmentof Holocene sediments in the Norwegianpart of the Skagerrak.Nor.geol.unders.Bull.430,S-16.

A regionalseismi c reflector separating an uppermost low-reflectivityHolocene unit from anacoust ically layered, Late Weichselian,glaciomarineunit belowhasbeen mappedinthe NorwegianSkagerrak between Langesu ndand Egersund.The thickestHolocene depositsoccur in the easternpart of the Skagerrak,with a generallydecreasing thickness westward s. In the central partofthe NorwegianTrench east of Lista,the Holocene soft clay is 7.5-15 m thick,while further west it isgenerallylessthan 7.5 m thick.On the northern slope of the trench,east of Mandal,the thicknessvariesbetween 15 m and35 m. WestofMandal,on the northern slope,Holocenesed imentsoccuronly wit hi n local basins.

Onthesteepest part ofthesouth ernandsout heaste rnslopeof theNor wegianTrench,and along thenorthernpart of the plateausout hof thetrench,theHolo ceneunitisabsent.Thisareaoferosion/no n-deposit ionexte ndseast- northeastfor abo ut100 km,and termin atesinanarrowtrench in the east ernmost part, indicatin g that conto ur- parallel curre ntshavebeenrespon sibleforthe erosion.A substant ial portion of the erodedsediments hasbeen deposited tothe east and northeastofthe areaof erosion,where upto100mof Holocene sedim entsoccur. Onthe plateausout h of theNorwegian Trench the Holocenethickness increasestowards the east-southe ast,to a maxi- mum of40-50m near theDanishborder.Largesandwavesmigrating towardsthe southeastoccurin this area,in water dept hs shallower than200m.The migrati ondirection indicates that they are relict andthattherewas an important oceanographicchangeinthe Late Weichselian/EarlyHolocene.Acoust ic blanking ofhigh-freq uencyseis- micrecord s indicates that shallow gas occurs in theHolocene sedimentsinthe relictsandwaveareaandinthe upperslop etothenorthof this.

LeifRise,HeidiAnitaOisen,Reidu lvB0e&Dag Ottesen,Norgesgeologiske undersekelse,Postboks3006-Lad e,7002 Trondbeim,Norwa y.

Introduction

Holocene sedimentation in the Skagerrak was recently addressedin a special issue of MarineGeology(Liebezeit et al.1993).Althougha number of authors describedvari- ous aspects of the sediments and the sedimentary pro- cessesin the Skagerrak,it was concluded that several sig- nificant gaps exist in our present knowledge (Van Weering et al. 1993a).The main objectiveof the present paper is to try to close one of these gaps by presenting a map of the thickness and distribution of sediments depo- sited over the last 10,000-11,000years.

Severalinvesti gat ions have shown that a substantial part of the sediment supply to the North Sea,and sedi- ments reworked duringstorms or extremecurrent episo- des on the shelfand alongthe coasts,have ended orwill end theirtransport path in the Skagerrak.Compared wit h the shallower parts of the North Sea and the Norwegian Trenchalong the Norwegianwest coast(Van Weering et al. 1973,Rise&Rokoengen 1984, Stoker et al.1985,Long et al. 1986,Sejrup et al.1989,Andersenet al. 1995),very high sedimentation rates have been measured in the Skagerrak (e.g. Van Weering et al. 1987, 1993b).

Correlationof seismicdata with agesobtainedfrom stra- tigraphic coresindi cates that the uppermost acoustically transparent unit in the central part of the Norwegian Trench represents sediments deposited during the last

10,000-11,000years.The uppermost unit changes seismic character within the area, and although the character may vary due to a variable quality of the seismic records, it has been possible to provide a consistentint erpret ati o n for mostof theinvest igated area. There arefew cores that penetrate the reflector at the base of the uppermost transparent unit,but recent sedimentaccumulation rates indirect ly support our interpretation of the Holocene sediment thickness and distribution(B0e et al.,this volu- me).An isopach map of the uppermost transparent seis- mic unit in the Norwegian Trench has previously been presented by Van Weering et al. (1973). This map was based on a very open grid of penetration echosounder data,and comparison of the interpretations is possible only withinparts of the area.

Methods

During the period 1991-1994 the Geological Survey of Norway,together with several other institutions,collec- ted more than 16,000 km of shallow seismic data (B0e et al. 1991,1993,Thorsnes et al.1992, 1993,Ottesen et al.

1994) (Fig.1),and short cores from 133 stations in the Norwegianpart of the Skagerrak. Bolt single airgun and Sleevegun (20 and 40 cubic inches) were used as seismic sources,together with the high-frequencyseismicsource

6

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LeifRise,HeidiAniraO/sen,ReidulvBee&Dag Ortesen GU-BULL430.1996

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Fig.I.NGU's shallowseismiclinesin the Norwegianpart of the Skagerrak usedin the presentstudy.

Geopul se.Due to bad weat herthelatter was not deploy- edonall lines.The GlobalPosit ion ing System (GPS)was appli edwith diff erenti al correct ions fromfixedlandstati- ons. Insomeareaswit h a thinHoloceneunit theinterpre- tation was difficult,mainly whereGeo pulse data werelac- king.The accurate positioning of the zero milliseconds tw o-way travel time (msTWT) line was also difficult in someareaswhereGeop ulse datawere available,because the sea-bed pulse often hides weak reflectors in the uppermost metres.However, accessto high-resolu ti on, deep-to wed boomer dat a enabled control and adjust- mentoftheinterpretati onwit hinsomeareas.

Shallow cores(59 mm core diameter and core length generally lessthan 50cm)were taken at 133 locationsin the midd le/eastern part sof the study area.A descript io n of these coresas well as grain-sizeanalysis are given in repo rtsbyBee(1993, 1994,1995).A paper summarising the result sof the sedi mentologic al analysis as wellaspre- sentinga compilationofallavailablesedimentationrates, isgivenby Bee et al.(t hisvol um e).

Bathymetry and oceanography

The Norw egian Trench starts southeast of Langesund, and contin uesalong thesout h and westcoastofNorway

forapproximately900km(Fig. 2).Thedeepest part of the Skagerrak is wit hi nthe eastern mo st part ofthis trench, with maximumwater depthsof ca.700 m at a distance of 50 kmoffthe coast betweenArendal and Langesund .The trenchbecomes graduallyshallowe rtoward sa sill at 290 m water depth west of Egersund.East of the deepest trough thereis a shallow ing towards the outlet of the Katt egat.Towards main landNorwaythe slopeis general- lysteeperand moreirregular than onthe south ernside, result ing in an asymmetricalshape of the trench. Thesea floor is particularly irregularalong thecoast,where crys- tallinerockscrop out,andinthe northeastern part of the stu dy area.

The boundarybetween the Norweg ian Trench and the gentl ynorthwesterlydippingplateau to the sout his mar- ked by a change in slope angle, at 250-300 m water depth.In the southwestern part ofthest udy areathere is a moregradualtransitionbetweentheslope and the pla- teau.The slope angle decreases southwards,resulti ngin a large-scaleconvextopographyalong the sout hernslo- pe ofthe trench. Thelow ermost partofthe south ernslo- pe south of Lista is much steeper (about 10") than the averageslope angleto the eastandwest.

Betw een Langesund and the deepest parts of the Norweg ian Trench the topography is characterised by severalsouth to southwesttrending trenches(Holtedahl

NGU-BULL430,1996 LeifRise, HeidiAnitaOlsen, Reidulv B0e&DogOttesen 7

NORWAY

-, -,

: :: ::~.:::

KATTEGAT

Fig. 2.General pattern of oceancurrents in the Skagerrak and adjacent areas. Openand filled arrowsindicate subsurface and surface water, respectively(modified from Danielsen et al.

(1991)). AW: Atlantic water; CNSW: Central North Sea water;JeW:Jutland coastal water;

BW: Baltic water; NCW: Norwegian coastal water;NT: NorwegianTrench.

1986).Southeast of Arenda l(between8°30'E and 9°30' E) there is an extens ive subhorizonta l area wit h wat er depths between 400 m and 450 m (here informally named the 'Arendal terrace').On a small scale the topo- graphy of this area is irregular,with depressionsand rid- ges a few hundred metres apart, and in the south the ter- race-like feature is bounded by a steep slope down to the deepest parts of the Skagerrak.Along the northernmost part of the terrace a 15-30 m deep and up to 2 km wide, coast-parallel trench occurs.To the north of the terrace the topography of the slope isirregular.

The general and well-known cyclonic circulation of water masses in the Skagerrak is mainly regulated by in- and outflowingwater from the North Sea and the locally steep topography of the Norwegian Trench (Fig. 2).

Atlantic water enters the North Sea between Norwayand Shetland,and follows the western sideof the Norweg ian Trench downto the Skagerrak.Thismajor inflow (1 kmvs) ofhigh- salin ewater(salinity>35%0)consistsof two part- ly separablecurrents.Oneisthe main and direct infl ow along the westernandsou th ern slopeofthe Norwegi an Trenchwith adeep plume locatedabovethe150 mbot- tom contour,and theother isa branchofAt lant ic water which hastaken amore southerly route and is steered int o the Skagerrakapproximatelyalongthe 70 mbottom contour (Aure et al. 1990). The depths of the major inflows are thought to increase during extrememeteoro- logical/oceanograph icalconditions.

The different water massesfrom the North Seaenter the Skagerrak in an area of about 50 km width north of

Hanstholm,and a mixing between allof them canoccur (Fig.2).The infl ow to the Skagerra k iscomplex,andvaria- ble weather condit ions frequently change the direct ion and the magnitudeof several of the major ocean currents (e.g.Larson&Rodhe 1979).A general pict ureis thatthe currents from the central North Sea join the southern branch of the Atlantic Current to continue north and nor- theastwards.Jutland coastal water,which isa mixtureof continental river water and water from the English Channel/southern North Sea, flows northwards closeto the western Danish coast, parallel to the currents from the so u t h e rn North Seain deeper water furthe rwest.

Aure et al.(1990) pointedout the pulsating nature of the Jutland Current into the Skagerrak, and that large amountsof water can flow into the Skagerrak and the Kattegat over relatively short time period s.The speed and partly also the direction are highly dependent on the strengthand direction of the prevailing winds. Also,the infl ow of Atlantic water is known to vary from year to year, probably as a result of large-scale hydr ogr aphi c changesin the NorthAtlantic(Sancetta et al. 1973). Water massesofAtlantic orig inenteringthe Skagerrakoutside Hanstholmseemto have a greatinflu ence ontheJut land Current.In periods wit h st rong infl ow of Atlantic water, theJutland Currentand water from the southern North Sea maybe stopped,andin period s wit hreducedinflow of Atlantic water accumulated water in the southern North Sea can flush into the Skagerrak for 2-3 weeks (Danielsen et al. 1991). Jutland Current water is mixed with Baltic water at the outlet of the Kattegat,and contin-

8 LeifRise,HeidiAnitaOlsen,ReidulvBee&Daq Ottesen GU-BULL430,1996

Seismic stratigraphy

ues alo ng the Swed ish west coast to Norway where it turnswestwards as theNorweg ian CoastalCurrent.

Fig.3.Geopulserecordshowing the generalsrrarigraphyof the cenrralpartof the NorwegianTrench. SouthwarddippingMesozoicbedrock(B)isovettainby till

m

internalreflecrors.The reflectorsnear the sea-bed are artificial,and probably

caused by a delayed sea-bed rerumvia the sea surface.Seismic sectionlocated at N57°46.1 :£7°38.8:

Theuppermostseismi c unitis fairlyeasily distinguisha- ble from the layered succession below. It is acoustically light or transparent,with onlya veryweak internal laye- ring.Locally,the unitshow sa more pronouncedlayering, part icularly along the slopesof the Norwegian Trench, but a strong and continuous reflector at the baseof the unithas made a confidentinterpretati on of thebounda ry possib le. On the nort herly dipping plateau near the Danish border the seismic records become dark due to shallow gas masking geological informat ion below the sea-bed,in particularon the Geopulse records(acoust ic blanking). Large sediment waves in water depths less than 200 mindi cate that fine andvery fine sandconstitu - tesasubstantial part of thesedi mentsin thisarea.

Thickness of Holocene sediments

Shallow seismiclines were run through the location of a multi-disiplinarily investi gate d core takento the sout h of Mandal (t he OSKAP core,Stab ell et al. 1985),where the baseof the uppermo stseismicunit occursat 11 ms(8.5 m).Inthecore,the base of the Holocenewas found at 6.8 m, and Younger Dryas sediments were present in the lowe r part of the co re at approxim ate ly10.5m.Ona com- posite piston-coretakenbythe University of Bergen and NGU in June 1991(PC61/62,

5 r

water depth), detailed sedimento log ical and biostrati- graphical analyses were carriedout,and several radi ocar- bon ages were obtained (Hellesund 1994). The upper- most acoustically transparent unit wasmeasured, on a Geopulse record,at 10-1 1ms(approximately8 m).The coredataindicated ,however,thatthebase Holocene(- 10,000 years BP) is at 5 m depth, and that the base Younger Dryas(-11,000yearsBP)levelis at 7m.

Due to friction and a tendency of plugg ing during the penetra t ion ofthe corebarrel,cores of soft sedimentsare often shorter than the corresponding in situ section.This may exp lain why the base of thetransparent unitusually is interpreted as being at greater depth on the seismic recordsthan in the corresponding cores. In previousstu- dies the uppe rmost acoustically transparent layer has been regarded to be of Holocene age(Van Weeringetal.

1973,Van Weering 1975,1982a,Rise et al. 1984,Stabellet al.1985,Andersenet al. 1995).Our data indi cate that the transparent layer may also include the Younger Dryas strata, or a partofthe YoungerDryas strata.A map sho- wing the thicknessof Holocene sedime nts isshown in Fig.4.

Inazonealong the coastofNorway,areas of Holocene sed imentsalternate with areas of bedrock,glacial/glacio- marine sediments and reworked sediments.Sout h of this zone(about 10 km off the coast east of Lista,andup to 40 km off thecoast between Lista and Egersund)thereis a continuous Holocenesediment cover(Fig. 4)thatincrea- ses inthicknessfrom thesouthwest towardsthe northe- ast.On the northern slope of the NorwegianTrench the thickestdepositsoccurin NE-SW trending basins,withup

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The seismicsecti on shown in Fig . 3 givesa general view of the seismi c st rati g raphy in the central part of the NorwegianTrench. The bedr ock,consisting of Mesozoic sedimentaryrocks,is truncatedby glacialerosion,and is overlain byoneor two thin, acousticallychaot ic till units (J.Seettern,pers.comm. 1996).In the sou t hern andsou t h- western parts of the investi gated area severaltill units may occur (L.Rise,unpubl. data),and make up alarger part of the Quat ernary succession. In most of the NorwegianTrench (also alongthewest coast of southern Norway)thetill isoverlainby acou st icallyparallel-layered deposits, mainl y glaciomarin e sedi ments (Rise &

Rokoengen 1984,Andersen et al. 1995,Sej rup et al.1995).

Fig. 4.Isopach mapofHolocenesediments based onseismic interpr etationof the gridshown inFig.1.Thethickness is show nin millisecond s (ms) two way trave l time (TWT). P·wave velocity insoft marine claysin the Norw egian Trenchis1500·1550rn/s,and milliseconds should bemultipliedby0.75inorde r toobtainthe thicknessin metres.Themore sandy depositsin the southern part ofthe investigatedarea haveprobably a slightly higher sound vel- ocity(1600- 1700 m/s)duetohigherbulkdensities (Rise&Boe1995), and a multiplicationfactorof0.80-0.85 should beusedfor transformation frommillisecondstometres.

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10 LeifRise,HeidiAniraOisen,Reidulv8IJe&Dog Orresen GU-BUll4 30,1996

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Fig, S.Isopachmap of Holocene sedim entsshowing theeastern partoftheareaof erosion/no n -depositionontheso uthern slopeof theNorwegian Trench andthenorthernparrof theplat eaualongtheDanishborderlin e.The locationsof Figs.6, 7,8and9and the northern boundary of therelict sandwavearea, are shown.SeeFig.4forloca tionofmaparea.

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Fig.6.Sleeveg unrecordfrom tbe southeasternslopeillustr atingincreasing Holocenethicknessup theslope,Note theabrupt shin inacoustic reflectivity in the sedi m ents due toshallow gas.8:Mesozoic bedrock;T:till;G:glacioma rine clay;H:Holoceneclay. Forloca tion,see Fig.5.

to 50-60 msthickness,onthe innerpart of the Arendal terrace(seealso Hol t edahl 1964,1989,1993,Haugwitz&

Wong1993)(Fig. 4).On the steep slopesouthof theterra-

ce,the Holocen eunit isverythin and locally absent. To the northeastoftheArendal terrace,thereis a large varia- tio n in Holocene sedi ment thickness,pro bably due to a

NGU-BULL430,1996 Leif Rise, Heidi Anita O/sen, Reidu/v80e&Dag Orresen 11

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Fig.7.(a)InterpretedGeopulseprofile showing pinch out of the Holoceneunitandtransitiontonon-depositiononthe southernslopeoftheNorwegian Trench,anderosionalcuttingofthebaseHolocenereflector(and older reflectors)at thegen tlynort hwestwardslop ingpla teau.The arealextentoftheareaof erosion/non-deposition isshown inFig.4.(b)Profilefromtheplateau.SeeFig.S for locationandFig.6forexplanationof abbreviation s.

complextopography (Fig. 4).In Langesundsrenna sout h- west of Langesund, several trenchesare partlyfilled with sediments,separat ed by bedrock ridges with sparse over- burden (see also Holtedahl 1986, 1989).The thicknessof Holocene sediments may be up to 100 ms on the western slopes of the trenches, whereas no sediments have been deposited on the eastern slopes (Fig. 4).

The thickness of the Holocene succession in the Norwegian Trench east of Listais 10-20 ms (Fig. 4).In the sout heastern part of the area with asub-horizontal sea bottom, near the transition to the slope, there is an accu- mulation zone with more than 30 mssedi ment thickness.

In the southern part of the trench south of Lista the lar- gest thickness of the Holocene succession is approxima- tely 30 ms. West of Lista the thicknessof Holocene sedi-

ments is generally less than 10 ms, and the seismic unit pinches out in the northern part of the trench.

On the southern slope southeast of Kristian sandthere is an areaof erosion /non-deposition,whichterminatesin the eastin a 500-1000m wide and 20-30m deep trench.

Thegreatestthickness of the Holocene succession (120- 130 ms,ca.100 m) is found to the east and north of this trench (Figs. 4 and 5). Southeast of the area of erosion /non-deposition,on the northwestsloping plate- au, the thickness of the uppermost unit reachesa maxi- mum of 60-70 ms near the Danishborder (Fig. 5).In this areathe resolutionof the map isslig ht ly reduced due to acoustical blanking caused by shallow gas in the sedi- ments (Fig. 6).Along the northern margin of the area of erosion /non-deposition, the Holocene unit onlapsand

12 LeifRise,HeidiAnirootser;Reidulv Bee&DagOttesen GU-BULL430,1996

pinches out,whileat the northwesterly slop ing plateau justsou th of the steepest slope(Fig. 7)thebase Hol ocene reflectorandsomeolde rreflectors are truncated .

On the southern slope southwest of l.ista, south of where the Holocene clay pinches out, side scan sonar records indi cate that hardclayscrop outmain lyat ridges and thatlow er areasinbetw een are infi lledwit h1-2 m of sandy and silty fine-gra ined sediments, probably of Holocene age(Lien 1995).In shallower water furtherto the sout h,exte nsive erosionalareashave beenidenti fied, as well asactivesandwaves(Lien 1995).

Di scussion

Several st udi eshaveshown that a substantial portion of the fine-grained material supplied to the North Sea is deposited in theNorweg ianTrench(Eisma 1981,Eisma &

Kalf1987,Van Weeri ngetal.1987,1993a,b).Com parison wit h the thickness of Holocene sediments in the Norw eg ianTrenchalong western Norway (VanWeering et al.1973,Rise&Rokoengen 1984,Andersen etal. 1995) shows that theSkagerrak hasbeen the main catchment area forfine-grainedsedimentsin the North Seathroug- hout the Holocene.Our seismicinterpretationshowsthat the major depocent ersare onthesoutheastern slope,in the eastern part of the Skagerrak and partly in local depression s on the nort heastern slope. Althoug h the palaeoceanographyand the relat ive im po rtance of sedi- ment sources in theNorthSea havechanged during the Holocene (Nord berg &Bergsten 1988),recent sedime nt accum ulatio nrates(Van Weering et al. 1987, 1993b,Boe et al.,this volu me)seem to broadlycorrespond with the Holo cene thickness.

In the centra l, deep parts of the Norweg ian Trench sedimen t accumulat ion rates of 10-20cm/100 years cor- respondwit h our int erpretati on of 10-20m of Holocene sediments,indi cating that the sedimentation rate in that area has been more or less constant throug hout the Holocene.On the plateau along theDanish borderline, present sed imentation rates of around 20cm/100 years indicate thatmuchof the up to 50 mthick successionwas depositedin the LateWeichselian/Ea rly Holocene.Locally on the south eastern slope and in the eastern most part s of the trench, sedi mentati on rates of 50-100 cm/1 00 years (Boe et aI., this volume) correspond well with Holoce n e sedi m e nt thickn essesof50-100m.

The displacement of shorelines in southe astern Norway and western Sweden, causedby rapid isostati c upliftintheEarlyHolocene,probablyexpo sed a substan- tialamoun tofmaterialto erosio nand transport intothe Skagerrak. Most of the thick,fine-g rained succession in theLangesundsrenn ais thoug ht to have beendeposited during the Early Holocene (see also Holtedahl 1986). A hig h deposition rat e at that time may alsoexp lain the thickHolocene depositsfound in basin salong the sout - heastern coast of Norway(Fig.4).

During the degl aciation (ca.15,000-10,000 yearsBP.)

Fig.B.Narrow trench in the eastern terminationof rhe area of erosion/non- deposition on thesouthem slope of the NorwegianTrench. Nore erosionof the base Holocene retlector,and ouruoppingCreraceousbedrock in tbe southern slopeof tbe trench.The Holocene successionisupro50mtbic northeast of the channel.SeeFig.5forlocation.

the Skage rrak was a deep fjord-lik e basin (Stabell &

Thiede 1985,1986,Thiede 1987),with land areasto the southandacalving icefront alo ngmuch of thenorthern and easternflanks.At that time thelargerivers incent ral Europ e and Britain carried suspended material north- wards tothe NorwegianTrench area.The establishment of thetempe rate Atl anti c Current along the rim of the western and southern slope s of the Skagerrak had an important impact on the sediment ological conditi onsin the Skagerrak from about10,000 yearsB.P.Extensive win- nowing of the sedi ments in the very shallow north ern Nort h Seaat that time (Rokoengen et al. 1982, Rise et

al.1984,Rise&Rokoengen 1984,Carlsen etal. 1986)may

also have made fine-grained sedimen ts available for transport to the Skagerrak inthe latest Weichselian/earli- est Holocene. The Balt icIceLake,whichwas emptiedinto the Skag e rr a k from around 10,200 years S.P., probably also influencedthepalaeocurrents and the deposit ion of fine-grained sediments.

The land area south of the Skagerrakwastransgressed in the period 10,000- 8,000 years B.P., and the English Channelopened atabout 7,800 years B.P.,probably initia- ting a circulation patt ernsimilarto thepresent (Stabell&

Thiede 1986).The flooding of the southern North Sea with coastal erosion and reworking must have had an important impact on the transport of sand and fines towards the Skagerrak. Largesandwaves at present-d ay water depth s shallower than200 m are seeninthe sout-

NGU-BULL430,1996 LeifRise,HeidiAniraO/sen, Reidulv80e& DogOttesen 13

Fig.9.Relictsand waves ontheplateau alongtheDanish borderlin e.Internal reflectorsindicate sediment transport towardsthe so utheast,whichisdifferent from thepresentl ydominating sedi me nt tran sportto wardsthenortheast.Notethe sligh tly rounded sandw avecrests,indicatingmodificationofthesea-b ed after the sandw aveswereactive.Fig.5showsthe nor th ernboun da ry of the relict sandwave area.SeeFig.5forlocation.

heastern part of the investigated area, near the Danish border line(Figs. 5 and 9).These have migratedtowards the southeast,and cannot be explained by the present hydrographic regime with dominating sediment trans- port tow ards the east/northeast. Most of the 20-50 m thicksand successionwas probablydeposited in theEarly Holoc enein shallow erwater than occurs at the present.

The seismic dat a indicate that modifications of the sea floorhave occurred since the sandwaves were active,e.g.

the sandwa vecrestsarecommo nly sym metri cally roun- ded, giving them a sinuousappearance.5alge &Wong (1987, 1988), however, sug gested that Holocene sedi- ments are absent and thatPleistocenedepo sitscon stitu - tethe seafloo r in a zone from about 70m wate rdepth north ofDenmark to the shelf edge (about 200m).They interpreted the depositsin this area to represent a pro- deltaslump facies.Ourinterpret at ion of a thick Holocene succession in this area is,however,supported by the seis- micinterpretation of Haugwitz&Wong (1993).

50 far we have only a limited understanding of the observed sand waves,though a northerly sediment sour- ce area in deeperwateris unlikely.An explanation could be that sediments transported from the westwere diver- ted towardsthe southeast due to strong currents formed by a cyclonic gyre in the sout heast ern 5kagerrak.The drainage of the huge BalticIce Lakeand/orthe Ancylus Lake(8,500years B.P.)could theoretically be such events, but it seem sunlikely that the duration of these floods were longenoughto form such a thicksand unit.

The most conspicuous features of the Holocene thick- nessmap arethe extensivearea of erosion/non-depositi- onon thesout hern slopeandthetrench ('moat') in the eastern termina tio nof thisarea(Figs.5 and 8).It isproba- blethat thesefeatures have originated due to conto ur-

pa rallel c urrents directed

thesout hern slopeof the Norwegian Trench,and that the thick Holocene depositsto the north and east of thenar- rowtrench can be interpreted ascontourites.A contouri- te-like sedimentation pattern in this area has also been indicated by Jerqensen et al.(1981),Hass(1993) and B0e et al.(this volume)on the basis of sedimentary structures. Erosion of the baseHolocene reflector,to the south of the area of erosion/non-deposition,showsthat at some time duringtheHolocene contour-parallelcurrents incre- asedinimporta nceand caused reworking and transpo rt of sea-bed sedimen ts, includi ng Early Holocene sedi- ments. Also the present sea-floor morph ology of the sou theastern slo pe of thetrench ind icatesthat the sedi- mentologica l conditi ons locally became more variable during deposit io n oftheupper part of the Holocenesuc- cession,e.g.elongated erosional depressions and sedi- mentgravit y slides(B0e et aI.,in prep.,Riseet aI.,in prep.).

The underlying glaciomarine, str at ifi ed sediments and also the Early Holocenesediments show a higherdegree of lateral continuity,or conform sedimentation thro ug h time(sheet drape sedimentation).

Erosion/non-deposition along the upper part of the southern slope has also been suggested in previous st u- dies, e.g. Van Weering (1982b, 1987), Quale & Van Weering (1985) and Delhez &Martin (1992). Also Lien (1995)hasshownthat inflow of Atlanticwater may be of suff icient strengthto resuspend fines and transportsand- sized particleseastwardson thesout hw est ern slope(Fig.

4).Thisprocess was corroborated by Eisma&Kalf(1987), who noticed high contentsof suspended matternear the bottom along the southern slope, probablycaused by resuspension of bottomsedi ments.

Erosion of the Holocene succession has also been observed on the westernslopeofthe Norw egi an Trench west of Bergen. High-resolut ion deep-towed boomer

14 Lel!Rise,HeidiAniraOtsen,ReidulvBee & DogOrresen

data show that the acoustically transparent Holocene unit, andalso glacio marine sediments,havebeen subject to exte nsive erosion,part icularly inconto ur-p arallel elon- gated depressions(Hovland 1983,1984),Thisauthoresti- mated that a 5-10 m thick layerofsedim ents had been eroded in some areas during the Late Holocene,andsug- gested that acombined effect of bottom current s and gas escapehad caused the erosion,

Inthe southern Kattegat,Nordberg &Bergsten(1988) have presented evidencefor a pronounced hydrograp hic change at about 4000 years B.P. The hydrographic shift wasat t ributed to changes in the inflow-outflow regime between the North Sea and the Skagerrak-Kattegat- Baltic,and tentativelyto changesin the large-scalecircu- lationpatterns of the North Atlantic.

The present configuration of the Jutland Current and the Southern NorthSea Current was probably established about 4000 years ago (Van Weering et aI.1993a,b).

Alth ough these currents are smallinvolume comparedto the Atlantic Current,it is generally considered that the bulk of the Holocene sedimentsin the Skager rak arederi- vedfrom thesou t hw est(Svansson 1975,Eisma 1981,Van Weering 1981,1982a,Kuij pers et al.1993a,b,Zollrner&

Irion 1993,Van Weering et al.1993b).Van Weering et al.

(1987) suggested thatmost ofthe inferredsea-floorerosi- on takes place in the sout hern half of the North Sea during extens ive storm periods in thewinter. Ourdata show increased sea-floor erosion duringthe later partof the Holocene,and suggestthat erosion still occursonthe southern slope of the NorwegianTrench and on the pla- teau further to the south .

The depocenter of Holocene fine-grained sediments eastof the areaof erosion/non-deposition(Figs. 4 and 5) requiresthat alarge portionof these sedime nt s aretrans- ported from the west;however,mostof the fines have probably been carried by the Jutland and the Southern NorthSea Currents duringext reme hydrographicevents.

There are probably both westerly and sout hwe sterly sources for the thick succession of fine-grained sedi- ment sof Holoceneageonthe sou t heast ern slope. This was also indicated by lbllmer&Irion (1993)who interpre- ted smectite-rich sedimentsfromthe sout hernandsout- heastern North Sea to be carried by theJutlandCurrent into the Skagerrak,where anorthward decreasing pro- port io n of smectite suggest san increasing infl uence of the Centraland Northern North Sea Current s(more illite andchlorite-richsediments).

The Holocene depocenter in the easternmo st Skagerrak isin the areawhere Atlanticwater and water from the North Seajoi n the Balt icCurrent and thenturn to thenort h to form the Norweg ian CoastalCurrent. In thisarea a large cyclonic circulat ion pattern with slow currents ispresent,which may explain the high rates of deposition.

Suspended matter from the Norweg ianrivers is mainly trapped in the fjo rds (Pederstad et al. 1993),and only minor amount s of suspended matt er are thoug ht to reach the Skagerrakvia the BalticCurrent. Measurements

GU-BULL 430.1996

of the bottom nepheloidlayer(Van Weering et al. 1993b), suggested local current induced erosion of the eastern Skagerrakmarg in,but the presenteddata giveno idea of the import ance of such a process.Sediment spresently depositedalongthe northernslo pearetherefore to alar- ge extentthoughtto be derived from the southwest,and to have beendeposited after a long transport insuspen- sion.

Summary and co ncl ud ing remarks

The thicknessand distributionof Holocene sedi mentsin the Norwegi an part of the Skagerrakhas been mapp ed basedon adense,reg ular grid ofshallow seismiclines. A well expr essed basal Holocene reflector separates the uppermostlow-reflectivityHoloceneunitfrom theacous- ticallylayered unit below,con sisting of Late Weichselian glaciomarinesed iments. Theisopach map(Figs.4and 5) shows that thethickest Holocene deposits occur on the sout h-eastern slo peof the NorwegianTrench,in an area where alsothepresent sedimentation rates are high,and on the eastern slop es of the Norwegian Trench.In the deepest trench there is a general decrease in thickness towards the west. TheHolocene succession is 7.5-15 m thick in thecent ralNorwegianTrench east ofLista,and generallylessthan 7.5m thick further west. Recent sedi- ment ation ratesin the deepest part of the Norwegian Trench are generally 10-20 cm/lOOyears(B0e et al.this vol ume),indicating that sedimentation has been stable and continuousthroughout the Holocene.On the nor- thern slope,east of Mandal,the thicknessof Holocene sedimen t s variesbetw een 15 m and 35 m, while to the west of Mandal Holocene sediments occur onlyin local basins.

Erosion of the Holocene successionis evident on the stee pest part of the sout hern and south-easternslopeof the Norwe gianTrench,and alongthe nort hern part of the northwestslop ing plateau (Figs.4,7aandb).The area of erosion/non-deposition ,which can be traced alonga NE- SWtrendfor about 100km,indicates strongcurrentswit- hin this part of the Norw egianTrench.Sediment s eroded fromthisarea,and suspended sediments brought in from the west and thesouthwest,haveto alargeext ent been deposited by contour-paralle l currentsto thenortheast of a narrow trench in theeasternmostpart ofthe eroded area(Figs.5and8).Thisisthe most extensive Holocene depocenterinthe Norwegian part of the Skagerrak,with up to 100 m of Holocene sediments.

On the plateauinthesouth thethicknessof Holocene sedi mentsincreases to a maximum of 40-50 m near the Danish border.Large relict sandwaves (Fig.9) occur in wat er depths shallow er than 200m.The sandwaveswere probably formed in shallow er water than the present during the latest Weichselian/Early Holocene.Acoustic blankingof high -frequencyseismicrecordsindicates that shallowgas occursin the Holocenesedi mentsin the relict sandw ave area andinthe upper slope tothe north of this.