Glacial geology, deglaciation chronology and sea-level changes in the southern Telemark and Vestfold

NGU-BULL435,1999 -PA GE23

Glacial geology, deglaciation chronology and sea-level changes in the southern Telemark and Vestfold

counties, southeastern Norway

BJ0RN BERGSTR0M

Bergstrom, B.1999:Glacial geology,deglaciationchronologyandsea-levelchangesinthe southernTelemark and Vestfoldcounties,southeastern Norway.Norgesqeoloqiskeundersekelse Bulletin435,23-42.

A revised and more detailed deglaciationchronologyis presentedin the southern Telemark andVestfold, southeast- ern Norway, basedmain ly on informatio nandresul tsfrommappingprojects on the superficial deposits carried out by NGU during the last 20 years.The deglaciationof this area started about 13,000 years B.P.when thereced ing ice marginwas groundedalongthe coast.Distally to Jomfruland,subma rinemoraineridges depositedin frontof the ice may correlatewith the GoteborgMoraine on the westcoastofSwedendat edto12,900-12,600 years B.P.The icemar- gin retreate dsomedistanceproximallytotheJomf ruland islandbefore 12,240±80 years B.P. Aglacieradvance occurred at12,200-12,000 yearsB.P.and theicemargin reached Jomfruland,pro bably corr elating wit h the Tjeme- Hvaler ice-marginaldeposit s in theouter Oslofjordarea.Theposit ion of the Slagen-Onsoy ice marginabout11,400- 11,200 yearsB.P.istentat ivelyreconstr uctedwestwa rdfromOslofjord.DuringthelateAllerod/early Younge rDryas (after11,300yearsBP .),theglaciers readvancedandin theKrageroareatheicefront movedatleast10 km,and most likely 17-18 kmtofor mthe distinctRa (Younger Dryas)morain es atabo ut 10,800-10,600 yearsB.P.The submar ine Ra mo rain eshavebeen mappedby seismicprofi lin gandshow a conc avecalvin gice margi ninthe outer Langesundf- jordarea(t he Langesund Channel)during the deposition.TheEidangerice-margi nal deposits aredatedto 10,400- 10,300 years B.P.andcorrelated with theAs moraines.The Geiteryggen ice-marginaldeposit s are complexand indi- catea marked halt andreadvanceofthe ice margin 10,100-10,000 yearsB.P.correspon di ngtothe Ski morainesin the Oslofjord area.Afterthe Geiteryggeneven t, the ice marginrecededrapidlyand theAkkerhaugen(9,800 yearsB.P.) andNordag ut u(9,700 yearsB.P.)ice-marginal depo sitswere formed.An eq uidistan tshorelin ediagramhas been con- st ructedbased on the upper marine limitsin the st ud iedarea. Amodifiedshorelevel displacementcurve from the Krageroareaispresente d.

BjornBerqstte m,Norgesqeoloqiskeundersekelse,N-7497Trondheim,Norway.

Fig. 1.The exte nt oftheScand inavi an contin ent al ice sheet duringthe LateWeichselianmaximum and the YoungerDryas.Theinvest igated area(SW ofOslo) isshown by a small box.

- - - Ice marginduring theYoungerDryas ..a....a....&.. Icemarginduringthe

Weichselianmaximum (c.^23,000years^{BP )} Hardangervidda

~Theice divideinSouthNorway H

~ duringtheWeichselianmaximum

'--_---'I~a~~~~sduring the Weichselian

'--_---'I~e~~~a~duringtheWeichselian

Introduction

Quaternarygeologicalmapping carriedout by NGU(Geolog- ical Survey of Norway)during the last 20 yearsin Vestfol d and thesout hern partofTelemar k, southw estofOslo(Fig.1),has provided newinformation abou t the Late Weichselian and Early Holocenedeg laciation and shore level changes in the regio n west ofOslofjord(Berqst rom 1984,1985, 1988,1995c, 1997and in press,Olsen&Low e 1984,Barg el &Lien 1990, Serensen etal.1990,Klakeg g1991,Klakeg g&Se rensen 199 1, Berq strern et al. 1992,Dahl et al.1997).Refl ection seismic profiling in the Langesundsfjord/Langesund Chann el has provid ednew information about the submarine ice-contact depo sits whichcorrespo nd to theterrestrialRa moraines.

Based on thisinfo rmation and results from additional studies inthe coastalareas,it is nowpossibl e to present a more det ailedand somewhat revised deglaciat ion chrono- logy from this region.Correlations have tentatively been made with the classical Oslofjord regi onwhere a detailed chrono logyhasbeen presentedbyHolt edahl(1953) andsub- sequentlyrevisedbySerensen (1979, 1983,1990b, 1992) and Klakegg &Ser ensen(1991). Recent studies of the sho relevel displacement, based on radiocarbon dat ingsof isolation- contact s in basins and molluscs in marin edepo sit s(Berg- st re rn 1997, K.Hen ningsmoen,pers. comm. 1997),indicate that the previ ou s shorelevel curves constr ucted from the Kraqere area by 5tabell (1980)and Henningsmoen (1979) requiremodification.Anequidist ant shor eli nediagram has

NGU-BULL435,1999-PAGE 24 BJ0RNBERGSTR0M

been reco nst ruct ed on the basis of marine limit s in the mapped areasand previo usly publi sheddatafrom theOslo- fjord region(Se rensen 1979).

Bedrock and geomorphology

The bedrockintheregionwestof Oslofjord(Fig.2)consists offourmainunits(Dons& Jor de1978,Larsen et al. 1978).Pre- cam brian rocks, mainly gneiss,amphibolite and quartzit e, predominat e in the western part of the region . Vendian (EocambrianJ rocks, consisting of carbonatit es and feld- spathoid-rich rocks,occur in theFen area atNordsje,north- westof Skien.Cambro-Silurianrocks,mainlysandstone,shale and limestone,are present in a narrow belt in the Skien- Porsgrunn-Langesund area.Permian rocks,mainly plutonic (larviklte, alkali syenit e, ekerit e) and volcanic rocks (rhom b-porphyry, basalt), cover the eastern part of the region in theOslo district.

The topograph yis noticeabl y influ enced bythe bedrock, andin particul arbythemajorfaultsand fractures.Themaj or feat ures are the ice-eroded(U-shaped)valleysand fjordssur- roun ded by a hilly,undulating,highlandterraindissected by narrowfracture-con tr olled valleys.

The coastalarea is dominated by expo sed bedrockand characterisedbya coastlin e with numerousskerries, islands,

promontori es,soundsand bays,together with narrow inlets proje ct ing inlandinto the low-relief,undulat ingterrain.This coastaltype is classifiedasafj ard coast(Klemsdal 1982).The maturepaleic landform onthe mainlandisa peneplainthat slopes very gently tow ard s the sea.In thePrecam brian areas westof Lange sundsfjord,theland surfaceis controlledbythe 600 Ma old sub-Cambrianpeneplain,which is onlyslight ly modified by glacial erosion. Howe ver, glacial erosion has beenactiv e along zonesofweakness,resultingin a number of narrow fault- and joint -valleys in the bedrock.The two domina nttrendsarec.NE-SW(t he'Caledonian'trend)andc.

NW-SE, producing an uneven coastline and a dissected top ograph yinland(Fig.3).

In Skagerrak,parallel to the coastof Telemark,a deep nar- row trench, the Norwegi an Channel, curves around the sout hern coast.Itprobab lyoriginated asa combinedresultof tectonics and glacial erosion.From the mouth of Lange - sundsfj ord a submarinechan nel, the Langesund Channel, ext ends downto andjoi nsthe main chann el ata depth of nearly 600 m.The courseofthe Langesun d Channel is consid- ered to belargelystruct urally controlled ,situatedalongthe bounda rybetwee nthe Precam brian rocksto thewest and the Permian rocksintheOslo Region to theeast(Holtedahl 1986) (Fig. 2).Thechann el is strongly infl uenced byglacial erosion.

Fig.2. Simplified bedrockmap of theregi onwest of Oslofjord.Aft er Don s &Jo rde(1978)and Larsen etal.(1978).

o 25,-

l===t==*==f- t----.J Skag er r a k

Methods

Field mapping

Quat ernarygeolog ical mapping inscale1:50,000follo wing thestanda rd methodsused byNGU has been carriedoutin thestudied area. The mapsand the geolog ical information acqui red during the field work have been of fundamental importance forthe present study .The correlation ofthe ter- minal moraines and glacio flu vial ice-cont act deposit s and the reconstructi on of ice-frontoscil lations are main lybased on mor pholog y, lithostratigrap hy, ice-fl ow patterns and shoreline studies.

Radiocarbon dating

The radiocarbon dat ingshave beencarried outon molluscs from marinedepositsand onsilt ygyttja from the bottom of lacustrine basins.Dat ing by the conventional radiocarbon methodwascarried out atthe Radio logicalDating Labora- toryin Trondh eim(T).AMS(AcceleratorMass Spectromet ry) dateswere producedat the R.J. Van de Graaff Laboratorium , Utrecht (UtC) and theTandem AcceleratorLaboratory,Uni- versity of Upp sala (Tua).The dates of themarine molluscs havebeencorrect ed fora marine reservoirage of 440 years (Mangerud&Gulliksen 1975).All agesreferred toin thisarti- clearein radi ocarbonyears.

Geophysicalmeasurements

Reflectio n(CDP)and refractionseism icprofil ingandresistiv- itymeasuremen t s(pole-dipole configuration)havebeen car- ried out in ord er to obtain mor e info rma tion about the stratig raphyofthe Ra moraines.Subma rine reflectionseismic prof iling was carried outin the outerLangesundsfj or d and 59'

Precambrianrocks.

gneiss.grane.amphibolne Majorfautt

Fautt.prominentjoint Cauldron

o

Permianplutonicrocks.

larvi e.noremarete.granrte Permianvolcanicrocks.

rhomb-porphyry.basan Cambro-Silunansecments, limestone.shale CarnonalJtJcrocks (Fen area)

BJ0RNBERGSTR0M NGU-BULL435,1999-PAGE25

Fig.3.The Krage rocoastalareaisdominated by exposed bedr ock (Precam brian) and charact erisedbynumerousskerries,islands, sounds and narrow inletsintothelow-lyin gmainland.Thelandsurfaceis cont rolled bythesub-Cam brian peneplain.TheprominentJomfrulandislandin theforeground isan emergedpart of theRamor ainesand isclearly differentincharacterfrom theinterio rareas.Viewtoward sWSW.Phot o:Fjellanger WidemeAS.

theLangesund ChannelbytheNGUresearchvessel'Seisma' in1990.The acoust icsource'Elrna',whichis anelect romag- netic sound source operat ing at frequenciesbetween 240 and 1200 Hz, was used.Sedimentthicknessis presented in ms (millisec onds) tw o-w ay travel time (TWT)or in metres based on an estimated acoustic velocityof1600ms'.

Superficial deposits

Most of the areaabovethe marin e limit is dominated by exposedbedrockor a thin and discont inuo uscoverof till.A continuousand locally thickcoverof till is foundinafew val- leys and mainly on slopes facing the direction of the ice movement. Glacialtransport of tillmaterialcover adistance more than 10 km hasbeen recognised (Berqstrern 1984, 1988).

In the sout hern coastal areasbelow the marine limit, especially in the Precambrian area, the lackof superficial deposits isvery striking,except inareaswheremarked ice- marginaldepositswereformed duringthedeglaciation .The Ra morainesarethe most prom in ent of thesedepo sits.Some ofthe marginaldeposits inlocat ions exposed to thesea,such asJomfruland(Fig. 3) and M01en, havebeen heavily washed by wavesand currents duringthe postglacia lupliftand cov- ered by wellrounded boulders and cobbles.Fine-grained marine sediments,up to 60 mthick,dominat e inthe main valleys and the large basins.The largest and thickest gla- ciofluvial deposits were formed along the main drainage routes and built up as marine deltas,inpart icular along the ice-marginal zones, e.g., the Geiteryg gen ice-marginal deposit.

Theweat hering material is closely relatedtothetype of bedrock. Some rock-types are moresuscept ible to weather-

NGU-BULL 435,1999- PAG E26

ing processes than others.The products ofthe Cambro -Silu- rian shales and limestones are mainly fine-grained, while material weathere d from Permian basaltic rocks normally consistsof silt and fine sand(Berq st rom 1984,1995c).The weathering zones in the Permian larvikitearecharacte rised by coarse sand and gravel and pronounced 'core-stone' development(Serensen 1988). The carbo nat ites in the Fen area are covered by rust- coloured weathering material, mainly of sand and silt.

Most of the superficial deposits were formedduring the Lat e Weichselianand Holocene.Older sedimentshavebeen foun d in Hce rlandsdalen,a tributary valley to Lagendal en, where Roaldset(1980) has described overconsolidat ed sub- till clay sediments situated at about 250 m a.s.l.,whichis75m above the highest postglacial marin e levelin the area.The clay sedimentswereint erpreted to havebeendepo sitedina marine environment,based mainlyon their Ce-d eficient lan- thanid e abundance patterns,and they are consid ered to be of MiddleWeichselianage and correlatedwith theSandn es Interstad ial. During the geological mapping in the area,a radiocarbon(AMS)dating ofthe sub-till organic-bearingclay at Rundhaugen(Roald set 1980,Fig s.2and 3)gave an age of 32,000±30014C-years B.P.fortheNAOH-insoluble fractions (UtC-4728, Van de Graaff Laboratoriu m,Utre cht).This indi- cates a lateMiddle Weichselian ageandsup por t sRoaldsefs (1980)assumptionof a correlationwit h the SandnesIntersta- dial.Thismaj o rice recession about 30,000-40,000years ago has been recorded at many places in Norw ay and there appears to have been a nearlycompletedeglaciat ion of the country duringthisperiod (Olsen1997).

Ice-flow directions

The oldest ice movements detect ed in the regio n were directed tow ard s thesouth (Fig. 4).Themainice divide dur- ing the Late Weichselian maxim um, according to Vorren (1977), was situated at a considerable distance east and southeast of thewat ershedin Harda ng ervid da and the cen- tral part ofeaste rn Norw ay.Nesje et al.(1988), however,sug- gested a low -gradient,poly-cent red icesheet with the main ice divide located close to the wate rshed.

Inthe coastal areas,glacial striaeturning tow ards SSW indicate a convergenc eof the ice st ream s intothe deep Nor- wegian Channel where they joined the 'Skagerrak glacier', the majorice flowfromthe inner part ofSkagerrak(Long va &

Thorsnes 1997).

When the glacier intheSkagerr ak finallystarte dto break up,the ice flowin the area of the Telemar klVe stfol d coastline grad ually turned towards the south-south east and south- east. Anactive ice dome was regenerat edon the sout hwest- ern part of the Hardangervidda mount ain plateau, from which southeasterly directedice stream s flowedtow ard s the southern Telemark and Vestfold regi on s and gave the early Younger Dryasglacier advanceat 11,000-10,600years B.P. (Berg st ro m1995b). Theice flows were mostly not deflected by the local topography ,but there was a weakconvergence toward sthe main fjordsand valleys.

BJ0RNBERGSTR0 M

o 25·,.,

~-t-t=-+-t=i

- - - +

^{YOUllQeliceflows.}converglnfJ to't"o'a'dsSSWInthe coastala-eas

----+.

EarlyYounger DryasIceflows.c".000·10600yearsBP Youngestice-lIowdirections

Fig.4. Reconstruc tionofthe ice movem ent s during the LateWeichselian.

The final deglaciation was characterised by ice move- ments thatwere strong lydependent on the topographical conditions and the ice flo ws conv erged towards the larger valleys and fjords.

Late Weichseli an deglaciation

The Late Weichselian ice sheethad its maximum extens ion about 23,000yearsB.P.(Fig.1).Duringthe deglaciation,the ice sheet thinned and the glacier started to break up. In Skagerrak,the ice stream in the NorwegianChannelbroke up at approximately 15,000yearsB.P.and the ice margin was establishedalong the Norwegiancoast (Longva & Thorsnes 1997). Due to further thinning and calving, the margin ret reate d landwards and reached the outer coastal areas around 14,000-13,500 years B.P.Theicerecession occurred more slowly and partly stopped when the ice grounded along the coast.In the outer part of the Kragero area,parallel submarine moraine ridg esoccurring distallyto the Jomfru- land island (Figs.5, 7, 14) (Holtedahl & Bjerkli 1975) were probably formed in front of the ice when the margi n grounded in water depths of about 200 m.Radiocarbon dates from Jomfruland indicate that thisoccurred before 12,240±80 years B.P. (p. 36).These moraineridges may cor- relate with the Goteborg Moraineon thewest coast of Swe - den,which is datedto12,900- 12,600 yearsB.P.,and represent apronounced standstillin the icerecession (Berglund 1978, Passe 1986).

BJ0RN BERGSTR0M NGU-BULL435,1999-PAGE27

\ '

,

, \\ I1I

Skagerrak

Ice-marginal deposits

Ice-recession lines , tentative correlations The tentative Allemd line of ice recession

(the Slaqen-Dnsey ice margin)

o

I

."^"

..

o

(f)

~

o o ...

a...

o o

25 km I

The Ra moraines

The submarine Ra moraines The Geiteryggen ice margin

Fig.5.Ice-m arginal depositsandice-recession linesintheregion westofOslofjo rd.(D)Dalsvatn,(G)Gorningen, (H) Hze rlandsd alen,(Ho) Hogstad, (S) Sundsmoe n.

NGU-BULL435,1999 - PAG E 28 BJ0RNBERGSTR0M

TheTjeme-Hvalerice-marginaldeposits

Afterthe formationofthe submarinemoraine ridges,the ice margin retreated some distance proxima lly to the Jo mfru- landisland(Fig. 6).A radiocarbondate of a molluscfragment inthe boulder clayat Jomfruland(Fig.16)ind icates that the margin retreate d behind Jomfruland as early as during the lateB011ingChron,befor e 12,240±80yearsB.P.Thenthe gla- cieradvanced,buthowfar the icemarg in actually reach ed is uncertain.Most likely the margin reached theJomfruland island, due to the fact that the radiocarbon-dated mollusc, picked up by theice during the advance,wasinco rporate din the till (boul der clay)and deposited ontheisland.

Thisevent may correlate wit h the formationof the Tjerne- Hvaler ice-marg ina l deposits(Fig. 5) in the outerOslofj ord area(Se rensen 1983, 1992).At Tjerne,corresponding mar- ginalmorainesare overlainbyglaciomarin eclays wit hshell s of Portlandia aretica,radiocarbon datedto 11,975± 155years B.P., which indicates a minimum age for these margina l deposit s(Berqst rern et al. 1992).If this correlation betw een theOslofj ord and theKraqerearea is correct,the Tjeme-Hva- ler ice-marginaldepo sit s were formedbetween12,240±80 and11,975±155 yearsB.P.

Correspo nd ing ice-marg ina ldeposits and stratigraphical evidenceof glacieradvanceshavebeenfoundin many other areasalon gthe coast of Norway,suggestin g that a regi on al climati cdeterioration occurred when theTjo m e-Hvaler ice- marg inal depositswere formed duringthelat e BellinqChron (12,200-12,000years B.P.) (And ersen 1968, 1979,Mangerud 1970,1977,1980,Follestad 1989, Berqstrern et al. 1994,FolI- estad etal. 1994,Reite 1994,Berq stro rn 1995a,Sveian&Solli 1997,Olsen &Riib er in press).

In Bohuslan on thewest coast of Sweden,the Tjeme-Hva- lerice-marginal depositshave been correlated wit h the Troll- hat t anMoraine(Berg lund 1979,Serensen1979).

The

Allerod

ice-marginal deposits

Durin gthe early deglaciation of the coastalarea westof the Oslofj or d the ice margin was unstabledue to intensive calving,and the recessio nwas highly dependent onthe top- ographical conditions,part icularly on water depth. In the deep outer Oslofjord the calvin g continued and thefjo rd basin deglaciat ed rapi d ly. Westwards, the ice margin grounded in shallo w wateron the low,undulating coastal plainandthe ice recession took place more slowly.Towards

SE NW

Jomfruland

Submarine I

ice-marginal

t

Norweg ianChanne l

1

10

ridges _'§

'" '"

^c

oE a:co 20

Kragem Dobbe

t

30

Sannidal

40

Drangedal

1

~

^{50km YrsB.P}

9695±95 0

o

Lacustrine sediments,gyttja

18> Marine molluscsin submorainesediments

10000

13000 12000

I- I-

I- I- 11 000

i: ?

Covered by ice

&. Marinemolluscsnotoverrun by glaciers

EE Marinemolluscsin till 1O.280±90~.../~_~

~ ---

,, ,

10.690±150 ~

10.770±95 10,825±70

.

- - - -

11.295±60 ~

-- -- -

11.360±150

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11,460±90

- - - -- - - -

? C?1:?~

_{12.240 ±80EE __?}

- - - - -

-

- -

- -

-

10000

12000 11000

13000

14 000 10 20 30 40 50 km

Fig.6.Time-d istancediagram(SE-NW)from thesouthernpartofTelemark(the Jomfr uland- Kragero area)shows the glacieroscillat ionsduringthe deglaciat ion.

BJ0RN BERGSTR0M NGU-BULL435,19 99-PAGE 29

o 4km

N

A

M01en

G Ge iteroya

A = Ar0ya H = Helgero a M

. . , The Ra moraines

pi' The submarine Ra mo raines 200 Depth in metres

. ...

, , ,

Sup posed po sition of the ice margin

Position of the ice-mar ginal ridges outside Jomfruland De Geer mor aines

Fig.7.TheRamoraine sin theLang esund sfjord andadjacent marin e areas.Theint erp ret atio noft hesubmarinemor aine s is mainlybasedon the seismic reflectionprofiles.The positi on of theice-m arginal rid ge s outside Jomfrulandisbased onHoltedahl & Bjerkli(1975, Fig .1).Sitesfor seism icprofil es1-5 are shown(Fig s.8-12).

Langesundsfjord(Fig.5)theoldest ice-mar ginallines, recon- struct edfrom marginaldeposits,approacheach ot her.The marginaldeposits,however, are noteasilyseparated dueto the later wave and seaiceactivi ty and erosion during the crustaluplift.

Atabout 11,400-11,200yearsB.P.the glacierhadreceded to theSlaqen-Onseyice-margin aldepo sit s (Serensen 1992), which can be morphostr atig raphically correlat ed wit h the LeveneMorain e inBohuslan(Berglund1979,Serensen 1979).

Thecorresponding ice-marginal morainescan be followed westwards to St okke (Fig. 5) where they are cut by the

NGU-BULL435,1999-PAGE 30 BJ0RNBERGSTR0M

o

100 200 300 400 500 600 700 BOO 900 1000 m

Profile1

r~1

~ ! 1750nvs

6 1750ITVs

.

6

ma.s.l.

NW500 ITVs 400·450 ITVs 500 ITVs 600.700 ITVs 600-650 ITVs 60 Road • Road•

40 ";;1"800ITVs6

20 6Till ^{+ +} Bedrock

o

^4900ITVs

younger Ra moraines and no continuation can befound.The Slagen-Onsoyice margin in the western areas was mostlikely situa ted in a proximal position to the Ra moraine ridges (Sorensen 1992,Bergst ro m 1995b).In the Kragero area,the margin receded at least 10 km to Dobbe, and most likely more than 17-18 km,behind the zone of Ra moraines,during the Allerod Chron (Berqst rern 1993,1995b) (Figs. 5,6).

The Ra (Early Younger Dryas) ice-marginal deposits

Duringthe late Altered(after 11,300years B.P.)and the early Younge r Dryas Chrons the glaciers readvanced (Bergst ro m 1995b) (Fig.6)and the distinct Ra ice-marginaldeposits were formed(Fig.5).The Ra margincan be traced nearly continu- ously from Oslofjordand southwestwardsto Molenat Lange- sundsfjord as a lowand broad ridge complex(Hansen19 10, Sorensen 1983,1992,Andersen et al. 1995).From Molen,the moraine ridges cross the fjord and can partly be followed off the coastto Straholrnenand Jomfruland, wherethey emerge above the present sea level (Fig. 7). Farther towards the southwest the Ra moraine continues for almost 60km as a straight, submarine ridge parallel to the coast (Holt edahl 1989).

In general,the Younger Dryasmarginal morainesare very distinct in Norway and can be traced and followed almost continuously along the coast fromthe Swedish border in the southeastto the Russian border in the north(Andersen et al.

1995) (Fig. 1).

The M0/en area

The Ra moraineridges in Vestfoldwere mostlyformed belo w sea level (m arine limit 150-200m a.s.l.)and are thereforeusu- ally covered with beach sediments. The composition of the ridges is variable, but generally they consist of a clayey matrix-supported diam ict o n(bo ulder clay) (So rensen 1990a, 1992).In some areas the diamicton has a higher content of sand.Stratifieddiam ictonand glaciofluv ialdepositsmay also occur.

The Ra moraines in the area east of Langesunds fjordarea complexof ridges (Figs. 5,7).East of Helgeroa the Ra consist s of two parallelmoraine ridges (Berg st ro m in press).A seis- mic-refraction profile across these ridges (To nnesen 1991) indicatesa maximumthickness(dept h) to bedrock of more than 50 m (Fig. 8).The seism ic velociti es(1700-1800 m/s) indicate tilldeposits,except in the upper partwhere shore deposits(1-3 m thick)overli ethe ridges.Most of the ridges probably consistof a clayey diamicton(bo uld erclay).Partsof the Ra morain e ridges are rich in large boulders, erratics moved by the ice over considerabledistancesfrom different areas in southeastern Norway.

Towards Langesundsfjord the Ra moraines split into 4-5 ridges whi ch disappearbelowsea level(So rensen1992, Berg - strern1995b).Molenis thelarg est of theseridg es. During the crusta I upliftthe ridgeswere highly infl uenced by waves and currents and covered by coarse beach sediments.Along the seaward side of the Molen peninsu la there are numerous beach ridges consisting of well rounded stones and boulders.

Fig. 8. Seismicrefraction profile 1 acrossthe terrestrialRa moraines nort heast ofMelen,modifiedafterTonnesen(1991).Seismicvelocities aregiveninm/soFor location,seeFig.7.

Subma rine ice-marginal deposits can partly be traced from the Melenareaacro ssthe mou thoftheeast ern part of Langesundsfjord, based on data fro m bathymetric map s (Sorensen1985,1992).

Langesundsfj ord

To obtain more inform atio n about the submarineice-mar- ginal(Ra)morainesin the out er Langesu ndsfjor d,reflection seism icpro fil ing hasbeen carried outbyNGU(Fig.7).

Inthefj o rd northofMelen,a seism icprofile,profile2(Fig.

9), has revealed 5-6 subm arine ridgeswhich seem to corre- spond to the terrestr ialRa moraines(Fig.7).Themost distinct ridge(Fig . 9),outsid eAroya,is doublecrested and distally30- 40 m high,consistingof till underlainbydisturbed glacioma- rine/m arine sediments.

Thedisturb edsed imentswere mostlikely depositeddur- ing therecession in theAltered and thenoverriddenbythe glacierduringtheearly Yo ungerDryas readvance.The outer- most ridge,outsideGeite roya,situated at the edge of the channelprobab ly represent sthe maximum ice-fron t posi- tion during this readv ance. How ever,thisridgeislocat edin the most obviouspositionforthe grou nd ing line of thegla- ciers during the deglaciationin thepreYo ungerDryastime and, even though this is not clearly seen in the seism ic recor d s,the rid gemaybe co m posed ofsediments fromolder stagesas well. Most likelythe ridge maycontainsedim ent sof the Tjeme-H valer mor aine s fo rm edduringtheBoiling/Older DryasChro nsasdescr ibedabove.

A profilein thecentra lpart of theLangesun dChan nel, profile3, (Fig. 7)tshowsa doubleridgeat 75 mwater depth (Fig. 10).The ridg e liesjust out side thethreshold int o the main part of Lange sund sfjord and is the only margina l deposit in the trench between Geit eroya and Aroya. This infers that therewas astableice marginoverthe thresho ld while it retre ated towards the northeas t south of Aroya, whe re theRamorain essplit int oseveralridge s.Theproximal partconsist s of two rid geswhi chareabou t 25-30 m hig hon the ice-co nta ct side.They have steep proxima l and distal slopesof 10-12°and 13-15°,respectively.Onthe distalside the re are inclined reflections dipping 5- 10° sout hwa rds towards thedeeperpartofthechannel.Theyareint erpreted as foreset beds deposit ed in a subm arine fan and most of them are prob ably debris-flow and turbidite deposit s (Aarset het a1. 1997, Lysa & Vorren 1997). Till beds may be interb edd ed wit hthe foreset s layers. At the baseof thedistal

BJ0RNBERGSTR0M NGU-BULL43S,1999-PAGE 31

slope complex,hum m ockyand chao ti c reflections with tilted blocksindicate sliding and slum pi ng from the ridge.More distally,subparallel,low-angl e reflect io nsrepresentstratifi ed glaciomarinesedimentsdeposit ed during theearlyYounger Dryas.Anirreg ularsurface and partlydistu rbedsedim ent lay- ers indicate avalancheactivityduringthe dep osit io n.

In the southern part of profile 3(Fig. 7)there isa larg e bedrockbasinfilled up wit h sediment s (Fig. 10).The strati- graphycorrespondswiththegen eral st rat igrap hy fou nd in previous profilesfrom other partsof theLang esundChan nel (Holte dah l 1986, Olsen 1992). Four different aco usti c units are distinguished.Theupper 40 m (Unit 1) isacoustically transparentand interp ret ed as postglacial sed im ent s (pos t Ra deposits).Unit2hasverydistinctand mostly conti nuo us reflecto rs, which most likely repr esent glacio marine sedi- mentsdeposited mainl y during the earlyYo unger Dryas(Ra).

The lowerunits aredisturbed by sid eecho es,but themore transpa rent sediments below (Unit 3)indicat e a dist al posi- tion of the ice margin andwereprob ab lydepositedduring

themajorret reat in theAltered Chron.Thelowerunit(Unit 4) is int erp ret ed as prog lacialsedime nts depo sited during the deg laciatio nof this area(Holte dahl1986).

Westof theLangesund Channel, in the Steinr enn abasin, the seism icprofil e 4 (Fig.11, locat io n Fig.7)revealsathick beltof tilland disturbed glaciomarine/marinesediments(up to morethan 50 m thick),which can befollowed so ut hwest- wards along the coast in the directi o n of Jomfrul and.The small,bould er-cover ed island,Danm ark,representsa minor supramarine part ofthis morainebelt(Fig.7).

A profile5 (Fig. 12) alo ng the west ern marg in of the Langesund Channel shows a thick, broad, submarine moraineridgesitua te dat the cro ssing of profile4nearthe edge of theslop e.

Basedon the profiles 4 and5,the positi on ofthe subma- rineRa mo raine s southwestof theout er Lange sundsfj ordis tent at iv ely reconstructedas showninFig.7.The thickdepos- it s ofglacigenetic sedi men ts inthe Steinr ennabasin and the boulde risland Danm ark are interpretedasthemainco nti nu-

P rofile 2

"~.:'.J

: '::g~i O:: " !~J~j~~

• g I!,'~ ^{ii, f:Y :,} : ;J

+ + + + + + + + + + + + + + + +

NE

+ Bedrock

+ + + + +

+ + + + +

+ + + + + +

+ + + + + +

+ + + + +

B

⁺

+ + + + + +

+ + +

+ + + + +

+ +

+ +

+ + +

+Bedrock, TheRa moraines

+ +

+ +

+ +

+ + + + + 0 1 km' + + + +

1!::=====I===:::f1

+ + + + +

Ra maximumposition?

SW P3

~

mls

o

50

200

Lange- sund 100 Channel

150

Fig.9.A:Submarineseismic reflection profile2 in the Langesund sfjord northof Molen.Forlocation,seeFig.7.B:Interpretat ion oftheprofile 2.

NGU-BULL435,19 99-PAGE32 BJ0RNBERGSTR0M

ation of the Ramoraines from Jomfruland and Strahol men north w ardstotheLangesund Channel,where themoraines seem to turn north w ards along the edge of thechann el.

Morph olog ical evidenceofyoungerandsmallerice oscil- lations during the earlyYounger Dryas isfound proximally to themain Ra morainesin profile5 as two small,butdisti nct terminalmoraines. ln thesouthern partofthisprofile (Fig.12) many smallmoraineridgesoccur at depths betwe en 100 and 150 m.Theyare orientated transversely to theLangesund Channel and situated outside the Younger Dryas ice-limit, resembl ing very much the submarine De Geer moraines

describedfrom the More area,western Norway(Larsenetal.

1991,O.Longva,pers.comm.1999).Theseridgesare thoug ht to have beenformedat the grounding linebyglacierpush.If theinterpretation of the De Geer moraines in Langesunds- fjord is correct,the depositionof theseridgeshas occurredin front of the retreating glacierduring thedeglaciationof the LangesundChannel,most likely during the early ormiddle part of the Boiling Chron.However,some of the moraine ridges, particularly theinner ones,mighthavebeenformed during the lateBoiling glacier advance,correspon dingto the Tjerne-Hvalerice-marginal deposits.

s

2km

I

LangesundChannel

o

PostHa-deposits

1°

P2

!

mls

o

100

300

+

+ -Bedrock~ + + + +

400 + + + + + + + + +

+ ^~ + + + + + + +Bedrock. _{+ +}

+

B

^{+ + + + + + + + + + + + + + + +}

200

Fig.10.A:Submarineseism ic reflect ionprofil e3in the Lange sundsfj ordarea.For location,see Fig.7.B:Interpr et ation of the profile 3.

BJ0RN BERGSTR0M NGU-BULL 435, 199 9-PAGE33

I

Lange- sund Channel

+ +

+ +

-- -- -

NE

Lange- sund Channel

+ + + + + + + +

+ + +

+

P5

~

+ + + +

+ + + + +

+ + +

+ + + + + + +

+ + +

+ + +

+

+ + + + +

Bed rock + +

+

+ + + +

1 km

I

+ + + + + + + +

o

I + + + +

Steinrennabasin The Ra moraines

~~~~

+ + + + + + + +

sw

+ +

+ + + rnts

o

50

200 150 100

Fig.11. A:Su bma rineseismic refl ectionprofile4 in theLangesund sfjordarea.Forlocation, seeFig.7. B:Int er pret ation of theprofile4.

m/s P4

Moraine

~

0 Nridges?

S

The Ra moraines

0

2 km

I

100

De Geermoraines?

+

200 ⁺ Bedrock + / ....-_I'+.._~, ^-r

+ + ...~" ⁺

+ + + + + +

Multiple ^" ,

+ + + + + + + + + + + + ^{T T} + +' ..._-+--- + +

Fig.12.A: Su bma rineseismi c reflectionprofile 5intheLang esundsfj ord area.Forlocation,seeFig.7.B:Interpretati on of theprofile5.

NGU-BULL 435,1999 - PAG E34 BJ0RNBERGSTR0M

Seismic-reflectionprofile

D N N ,

A

^___Seismic-refractionprofile

A

^,

^,,

Resistivitymeasurements _,

,,

0

Log With ret.number

~

, ,

'---' '----' r,

,

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,

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/

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,

a

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d _t:b

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Fig.138 ad

CJ

^Themainlycovereoby^{Ra moraines}

marineshore 0

deposits

/ / ':>

c:=J

^{Exposecbedrock}

_0-

^//

A

^{0 lkm 0, Contour100, interval200,5 m 300, m}

13.A:Locatio n mapof Jomfruland. B:Location mapwit hsites ofseismicand electr ical profilesatJomfrula nd.

Fig.14.Profilefrom the outercoastal areaacross the Jomfrulandisland area(NW-SE).Sub marinepart modifiedafter Holt edahl &Bjer kli(1975).

Forlocat ion,seeFig. 13A.

r---

P rofile of the J omfruland isla nd ar ea

NWr--·----iTheE~~yYoUnger

Dfya$i--..-..---1

SE

m; Kragereflard L...!~~.!'2!~~~g!_J

!

m

4 0

^{~ coas} Jomfrulandisland Submarine areas ~

4 0

~ /'1..." ~ ..

: - - " A Presentsea level ;

0-;-"\ 1 '" ^{.J.J-- -} -;'_~ J_-

_\" - - - - - - - - - - - - 1-

0

-i- ....

deposits or gravelly/sandytill.This indicatesminor ice-mar- ginaloscillations,possibly duringthelateWeichseliandegla- ciation (late Bollinq/Older Dryas and lat e Allered/early Younger Dryas Chrons).This complex strat igraphy of the Jomfrulandmoraineridg e is quitesimilarto the stratigr aph y of the Ra moraine at Fokserud inVestfoldwhere glaciofluvial material(sandygravel)und erliesclayeydiam ict o n(Se rensen 1992).

The upper part (0-5m)ofthe loose deposit s has been st udiedinexposuresassociat ed with well digging. Mo stof the islandis coveredbybeachdeposits. In the most expo sed

~-80

H 20

~-160

- 4 0 -j: : : : : : : : : : : : :

-80

t

_-:-

~~i~---:..:---:---:.I:

_{~__1Bedrock}

1- • - - ... ^{: :} ~0

-120~- .,[,.JGlaciogenic 1- - - - j- 4 sediments(tills)

1- - - -

t

~

- -

1- - - - - - - - - - - - .. - -160--;- .,. Mannedeposrts 1-" - - - -

~_ -L L ..

Jomfrulandisland

Jomfruland isan emerged partof theRa moraineand the island is 7.5 km long and up to 1 km wide(Fig s.3,13). The highestpoint is about 20ma.s.l. Theisland has beenvisited and describ ed by many geologists (Keilhau 1842,Hansen 1910,Holtedahl1 953),but we still don't knowmuch about its internal compositionand structure.From old boreholesnear the lighthouse,2-5 m-thick beach depositsabove consoli- dated clay have been reported(Keilhau1842,Hansen 1910).

Lateron,a 40 m-deep drill ing downto bedroc k was carried out mainly through 'soft clay' near the new ligh thou se (Jansen 1982, 1987),but no further infor mati on about the strat ig raph yhas beenavailable. Parallel submarinemoraine ridges (Fig. 14)occur off the coast on the distal slope of Jomfruland(Holtedahl &Bjerkli1975).

Recently,geop hysical measurementswere carriedout at Lokstad and Hagane (Figs. 13,15) inorderto obtain more infor mati on about the sediments (Mauring & Tennesen 1992).At l.ekstad, refracti on seism icprofilesshowdepths to bedrock varying between 15 and 60 m (Fig.15).Refle cti on seismic measurements (CDP)show reflectors at a depth of 40-45m,whichindicate that marinesedim ents in the deep- est partof the basinformedin an early ice-fr ee period(older than theYounger Dryasadvance).Arefle ct orat a depth of 20-30mmostlikely represents anerosion bound ary formed bya glacialadvance.The sedimentsaboveareinterpretedas 'clayey till' (boulder clay).Resistivity measurements (pole- dipole configuration)have recorded threelayers with high resist ivit y which indicatecoarse materia lsuchas glaciofluvial

BJ0 RNBERGSTR0M NGU-BULL435,1999 -PAGE 35

tekstad , Jomfruland

PROFILE 1

- - - Reflector

- - Indicated rock surface

T

Intersecting profile

~22/. Layer with high resistivity

c=J

Till(boulder clay)

c:::=:J

Marinesediments 450nVs

I

Shoredeposits

Till 1700nVs

(boulderclay) .'

I Z

7.:7.7..

7.. 7.. 7.

-10 .::y':7- :Z.Z .., ., ., .

-20 450nVs

450nVs

:: Z 7.:/.:7.:7.::z:ir:»:zc:x:»:;:.

1750nVs 1750nVs

.... ·· / 7 7 ;'::/.:Z:<::,<:% ;,.

+ + ' .:... - - - - -

.~ :;.

-+-Be-+-droc-+-k

+ + + + + + + + + + + ' ';- '':r!7 /./../../

⁷

WZZZZZZZZZZZ .... ./

^-30

'-7"-:---;---:--'---=--=--=--::--;-~-r-~",f-^·40

+

+ +

+

+ + + + + + + + + + +

+

+ + + + +

450nVs

o

50 100 150 200m ^1700nVs Seismicvelocity

PROFILE 2

10

o

-10 -20 -30 -40 -50 rna.s.l.

SW

250m 200

Shoredeposits

150

+ + + + +

+

+

Bedrock

r , , + + + + +

+

+

100 Pl

o

50 -50 -40 -20

-30

: :~:-::~ ~~ Zf:~f! P~ _

+

+

+

+

+ + + + + + + + + + + + + + + + + + + + NE

ma.s.l.

10

l

!i' ....",_"'""""""""'"-= -.I

o

-10 i::7.:?7.7.7.Z7.:7.:Z:Z:? :z:LZ?ZZ7ZZ?':?,Z7.:Z]:Z7.7ZZZZ?:?2,Z7.:7.:Z:ZZZ 7.:7.:Z:<: Z Z 7.

Fig.15,Seismicand resistivityprofiles at Jomfruland,modifiedafter Mauring&Tonnesen(1992),

areasthere are a lot ofbeachridges containing well round ed cobbles and boulders.Theunderlyingtillconsistsof a consol- idat ed, clayey, mat rix-supported diamicton (bo ulder clay), partly withshe llfragmentsof arcti c mollu scs.Thecontent of clay and silt variesbetween 25 to 35 % and 30 to 35%, respectively.A borehol e (log 1)down to 10.5 m depth near the crestof thehighest ridgeat l.ekstad (Fig.16) showssoft er diamict materi al below the consolidated diamicton. The compositionof themateriallessthan16 mm isquite sim ilar, but the conte ntofcobb lesand bould ers isdecreasingdow n- ward. Thisindicatesthatthere is a grad ualtransitioninto the underlying glaciomarine sediments. Durin g the Yo unge r Dryas advance, much materi al from these sediments was picked up bythe glaci erandincor por atedintheRamorain es.

The highcontent ofsiltandclayin the bould erclay indicates a glaciomarin eorig in.Radiocarb o n dates of moll usc frag- ment s,mostlyMacomacalcaria and Portlandiaaraica,in the till giv e agesbet w een 10,690

±

150(UtC-2220) and10,825

±

70years B.P.(Tua-935).One frag me nt ofMacoma calcarea wasdated to12,240±80 years B.P.(Tua-629)andindicatesan earlierice-fr eeperiodduringthe late BoilingChron.

In thelow erpartof theproxim alslope ofJomfr uland, the co nsolidatedbou ld er clay is covered bymor ethan 3 m ofgla- ciomari ne/ma rine sed imentswhich arecoarseni ng upward fromclay tosandysilt.There is amajorhiat usto the upper

shore deposit s, which coarsen upwa rd from sand to big rounded cobblesand boulders,(log 2,Fig.16).Molluscshells from the lowerand midd lepart s of thesemarin e sedi men t s were dated, and gave agesbet w een 10,340±105(T-10777) and8585±105years B.P.(T-10776).

The st rati g raphy and theradiocarbondatesind icate that theicemarginretr eated some distanceproxim allyto Jomfru- land asearly asduring theBoiling Chron,before 12,240±80 years B.P.Then,theglacier advanced, due tothefactthatthe radiocarbo n-dated mollusc was picked up by the ice and incorporat ed in the till (boulderclay)on the island. Itis diffi- cult to interp ret how far the ice margin reach ed, but the strat ig raph yind icatesthat themarginextended at leastas far asJomfru land.Theglacier could haveoverridd en Jomfr uland and term inated on the distal side where the submarine moraine ridges were formed (Holt ed ahl&Bjerkli 1975) This impli esthat there must have been a major advanceand a co nsiderableincrease in the icethickness if theglacier shoul d have been ab leto form theseterminal ridges at adepthof more than150 m.How ever,thelack of stratigraphical reco rds intheadjacent areasof such a considera ble advanceindi- cate sonly a minorincrease in theglacialactivity.Most likely, Jom f rulandisland due toit s topographicallyfavo urable posi- tio n atthe edgeofthecoastal plain, representsthe maximum ice-f ront position during this advance.

NGU-BULL435,1999 -PAGE 36 SJ(oRN SERGSTR0M

Fig.16.Stratigraphicallogs from the upper part ofthe Jomfruland ice- marginaldeposit.14C-dates inyearsB.P.For location,see Fig.BB.The laborat ory numbersof thedat es(see methods,p.24)are listed fromthe toptothebottom ofthe logs. Log1:Tua-1211, UtC-220,Tua-935,Tua- 1210and Tua-629.Log2:T-'0776,T-'0775 and T-10777.

I I I I I I

island starte d later than 10,690± 150 years BP and before 10,340 ± 105 years B.P.,and most likely at around 10,600 years B.P.

After the deglaciat ion,Jomfruland was covered by the sea and fine-gra ined marine sediments were depositedat a water dept h of more than 100 m.During the emergence of the up perpart ofthe island,thefine-g rained marinesedi- ments were eroded andthe moraine ridgewas covered by boulder sand cobbles washed out from the tillbythewaves (log 1,Fig. 16).At the lower and lessexposedpart of theprox- imal slopeof theisland, some of the fine-gra inedmarinesed- iments still remain (log 2). Radiocarbon dates of molluscs ind icat ea min im um ageofthemoraine ridg eof 10,340±105 years B.P.Fragm ent s of molluscsfrom the coarse-gr ained shoredeposit s are washed out from the older sediments and the datesgiveonlya maximumage,8585±105yearsB.P.,for the time when the top of the ridge wasinfl uenced by the waves duringtheemergence of the island.

The Eidanger ice-marginaldeposits

After the Ra event, the ice margin began to recede tow ardsthe northwest(Fig. 5).The presence of dropstonesin the poorly sorted glaciomarine/marineclay and silt,overly- ingtheRa morainerid ge on the Jomfrulandisland,indicates that therewere drifti ngicebergsin this areaduringthe early part of the ice-free period, rad iocarbon-dat ed to ap proxi- mately 10,600-10,350years B.P.When theice marginreached the higher inland areas, the floating ice-margi nal parts became grounded and the calving processesdimin ished.

Somescatt ered glacio fl uvialfans orsmall deltasweredepos- ite dnearthis groundingline westofLang esund sfj ord(Fig.5).

They are correlated with the Eidangerice-margi nal deposit (Fig. 6),a glaciofluvia lfrontal ridge formed in a submarine posit ion(not built up to sea-level)duringashort haltor stag- nation in the ice recession in Langesundsfjord (Bergst rom 1995c). Northeast ofthe fjord,corresponding ice-marginal deposits have been mapped in the valleys Silj andalen and Lagendalen.

TheEidangerice-margi nal depositsaretentativelycorre- lated wit h the As Moraines (Berg strom 1995c, Sorensen 1983,1992).Aradiocarbondate ofgyttj a from a bog situated afewkmdistally to theEid anger icemargin indi cates thatthe area wasice-free from atleast 10,280±90 years B.P. (Hen- ningsmoen1979).Based on informati on from the Oslofjord area and Jomfruland,the most likely ag e of th e Eidanger eventis between 10,400 and 10,300years B.P.

c::o

~~

o

'"

^c::_o

'-' C>

.c;;c::

ee

e

'-'Q,)

Cl

1

Genesis

Shore deposits

Till (boulderclay)

Glacio- marine sediments

Shore deposits

Glaciomannernarlne

si~andclay Till (boulderclay) r8585::!:105

- 9925:!:70

L

10340::!:105

F6'-=-'----:-6-~-6-,-----j -6-6-~

o

I Log. 21

C l S i S G B D 4

3

m a .s.t. Lithology

^14C-

dates 10

9

10400 ::!: 100

8 c:

^f:::,

r-

10 690 ::!: 150

- 10 825 ::!: 70

7 c: --- L

10770::!: 95

12240 ::!: 80

6 c:

^f:::,

5 c:

4

^f:::,

c:

3

^f:::,

2 c:

f:::,

m a .s.l.

7

-rTl"TT/7V~"V"T-'

6 m~~

5

Subsequent ly, during the AlleredChron ,the icemargin recededat least17-18kminland fromthe Ra moraines(Berg- strern 1995b).The glacier readvance started at the end of the Allerod Chron,after 11,300years B.P.,andterminated off the coast at Jomfruland.The comp lex strat igraphy of the Ra morain es at Jomfrulandindicates smaller oscillations of the icemarg induringtheirform ation, which prob abl y starte dat about10,800 yearsB.P.The radi ocarbo n dates of the mollusc fragment s incorporated in the upperpart of the till sug gest that the lastreadvance tothe Ra morainesoccurred shortly after 10,690±150years B.P.The finalglacier retreat from the

The Geiteryggen ice-marginaldeposits

During the ice recession, the calving processes gradually decreased due to the genera lhighertopographical level of the inlandareaandmore of the ice margin was sit uated in a sup ramarine position.Only in the major valleyswas the ice still in contactwith thesea. Thisresult ed in concave calving bays and convergingice flows towards thesefj ord valleys.

In the valley between Skien and Porsgrunn, north of Eidanger,minor glaciofluvialdeposits were formed in the sea in front of the ice at Borqeasen and Nenset (Fig.5).Corre- spondingterm inalmoraine ridges and glaciofluvial ice-co n-

BJ0RNBERGSTR0M NGU-BULL 435,1999-PAGE37

Fig.17.DuringtheGeit eryggen event,theice frontwassituatedat thesout heast ern end oflake Nordsjo.The complexmarginalGeiteryggendeltawas built up tothemarinelimit(c.145ma.s.l.)in frontof the glacierin the Nordsjobasin.Viewtowardswestandthe formerglacier. TheNordsjolakein the background.

tact deposit s occurat lakeGorningen in Siljandalen and in Lagend alen, e.g. Grin i, Gaserud (Berqstrem 1988). To the southwest ofthe Langesund sfjord area, there are onlyminor scatte red marginal deposits (Berqst rom 1997).Theydo not representany readvancesor long sta ndsti lls.

From Nensettheicemarginretr eatedto the sout heastern end of Nordsjein theSkien area wherethelargeGeit eryg gen ice-margin aldepo sitwasform ed (Fig. 17) (Jansen 1980,1986, Bergstrom 1985).Thenort hern and midd lepartsconsist of ridges of coarse sand and gravel rich in cobbles.Georadar profiles(Storro et al. 1992)show distinct, but irregularundu- lating reflectors indicatinga complexformation of theridges close to the ice front.Dista lly,moreuniformforeset beds are sloping towards the east.Thesouthern part of Geiteryggenis a glaciofluvialdelta built up to the marine limi t,145-146 m a.s.1. Foreset beds ofsand and gravellysandare covered by 2 m-thick topset beds consisting mostlyofcoarse gravel and cobbles.Seismicrefraction profilesshow a maximumthick- ness ofthe Geiteryggen depositof morethan 100 m(Jansen 1980,Johansen 1980).

The com p lex ice- m argina l deposit at Ge it erygg en indi- cates minor oscillationsof the ice front during its formation. Cor responding terminalmoraines occur southwest ofGeite- ryg gen. Towards the nort heast, glaciofluvial ice-marginal deltas or fans atHogstadin Silj andalen arecorrelated with theGeit erygg en event. InHee rlandsdalen,a tribu taryvalley to Lagendalen,distinctlateralmoraines ind icateanactiveice

lobewith an ice-surface gradient of 5-10%near the front, whilethegradientinthe inner part is 1-2(1.5)%(Berqstre rn 1988).Based on thisreconstruction,an average gradient of c.3%hasbeen used bySerensen (1992)as a 'master profile' for the glacier lobesdurin gthe Geiteryggenevent.

TheGeit eryggenicemargin canbefoll owed,morpholog- ically, eastwa rdto Drammensfjo rd and Oslofjord (Fig.5).and corresponds to theSkiice-marginal ridges,dated to 10,200- 10,000 yearsB.P.(Serensen 1983, 1992).In thearea ofGeite- ryggen,a few radiocarbon datesof gyttja from bogs have been carried out in order to derive a more preciseage forthe formation,but contamination by roots has givendates that are mostly too young. Proximally to theGeiteryggen (5 km).

a radiocarbon dateof 9695±95 years B.P.(Tua-774A) pro- videsa minimum age for the deglaciation,but pollen analysis indicates a lossof the oldest organic materialduring the sam- pling.The lack ofpioneer flora in thebottomsediments indi- cates that the deglaciation occurred prio rto this date,but probably not more than 200-300years earlier. Theimmi gra- tion of tree birch was rapid during this period.Basedonthe datafromtheOslofjordareaand thelocal rad iocarbondate distally to the ice margin (Henni ngsmoen 1979), the ageof the Geiteryggen ice-margi nal delta is considered to be betw een10,100and10,000 yearsB.P.

NGU-BULL435,1999-PAGE38

The Akkerhaugen ice-marginaldeposits

Aft ertheGeit eryggen event the icemargin retreatedrapidly towards thenorth and northw est. In thedeep Nordsjo basin, a localcalving bay wasform ed.AtRomnes,wherethe basin is narrower,a short halt in the ice recession occurred and a marginal moraineridge wasformed(Fig.5) (Bergst rom1984).

Corresponding front deltas exist north of Valeb o (Fig. 5), datedto beabout 150to 200years younger thanthe endof the Geiteryggenevent(based on shoreline data).Theaver- age ice recessionin the Nordsjo basin is calculated to be c.125m/year.

At thenort hern endof the lake Nordsjo,a prominentice- front accumulation,not built up to sea-level,extends across the valley at Akkerhaug en (Fig. 18). It comprisestwo qla- cioflu vial ridges on eitherside of the riverand consistsmostly of sorted sand and gravel.Foresetbeds are dippingtowards the sou thwest. Thethickness of the deposit abovebedrock variesbetween 30 and70 m(Bergstrom 1984).Correspond- ing lateralmoraines aresitua tedsout heast of Akkerhaugen, and canbe followedtow ards thesoutheastinto the basin of Dalsvat n(Fig.5)where the ice lobe dammed alake.The main outlet was overthe col in thesout h at about 190m a.s.1.Gla- ciolacustrine fine sediments were deposited and glacioflu- vial terraces of sandwere builtupto thislevel.The existe nce of lateral,ice-dammed lakes at high er levels(up to 275 m a.s. l.)isindicatedby overf low channelsacross themountain ridge westofDalsvatn.Thelateralmoraines at Akkerhaugen

BJ0 RN BERGSTR0 M

indicateast eep front to the Akkerhauge n lobe inthe main valley with agradient up to 150m/km(15%)in the outer- most part(0-1 km).TheDalsvatnice lobe hasasurface gradi- entof 20-30m/km(2-3%)nearthefront(2-5.5kmfromthe front).

The Akkerhaug en ice-margi nal deposit has tentatively beencorrelatedwit h theEggar lat eralmoraine atPassebekk inLagendalenand with the Aker MorainesintheOslofjord area.BasedonaplotoftheML at Akkerhaug enin the eq ui- distant diagram (Fig. 20)and data from theOslofjord area, the Akkerhaugen event is datedto 9,800-9,750yearsB.P.

The Nordagutu ice-marginaldeposits

SubsequenttotheAkkerhaugenevent theicefrontretre ated abo ut5kmnort hwardtoNordagutu(Fig.5,18)wherea large ice-marginal delt awasform ed.Proximally,onthetop of the easte rn part of this delta (Sundsmoen), a frontal moraine ridge crossesthe terrace,indicating a short advan ceof the front atthe termina lstage of depositionof thedelta.Lateral glacioflu vial erosion and depositsinthe easternvalleyside, northeastofSundsmoen,were probablyformedduring the Nordagutu event. Pollen analysisand a radiocarbon date from a bog proximally to the Sundsmoen delta indicate a minimu m ageof 9,420±190 yearsB.P.(T-4266)(Bergstrom 1984).Thefirstpioneer floraisnotrepresentedin the pollen diagram.Thisindicatesthat theoldest organic material at the bottomofthe bog hasbeen lostduring the sampling,and

Fig.18.Akkerhaug en ice-marginal deposit(A)atthenorth ern endoflake Nordsjo, viewtoward s NNE.Duringthe deglaciation,large amounts of melt- water drainedlaterallyandsubgla ciallyalong themain valleys.Thelargest glaciofluvialdepositswere formedin front of the valleyglaciersduringstag- natio nsintheicerecession.Someofthedeposits were built uptosea-levelice-fro ntdeltas like Sundsmoen(5)inthe background,whileot herswere formedas submarine frontalridge s asatAkkerhaugen.The Akkerhaugenridgecrosses thevalleyandthe riverSaua at the bridge.