The Late Weichselian - Early Holocene Lindhov clay sequence in the Varberg area, southwestern Sweden

The Late Weichselian - Early Holocene Lindhov clay sequence in the Varberg area, southwestern Sweden

FREDRIK KLlNGBERG

Klingberg,F. 1996:TheLate Weichselian-EarlyHoloceneUndho v clayseq uencein theVarbe rg area,sout hwestern Sweden.Nor.geol.unders.Bull.430,17-24.

Aclaysequencewit hspecific depositional characte risticshas beenfound,bothon landandinthe sea,in the area sur rou ndi ng the townof Varbergand ext endi ngnorth alongthe coast.Theclaysequence,here provisionally refer- red toasthe'Undhovclay',has been identified andinvestigatedby refl ect ionseismics andsub-bottom profiling at seaandbymapping onland.On land,theUndhovclay isfound atalt it udesbelow10 mabove thepresentsealevel, andinthesea inac.10kmwidezoneoutsidethepresentcoastline.TheUndhov clay isIit hostratigraphicallysit ua- tedin thetransition zonebetwee nglacialand postglacial depo sits.For aminiferalanalysesof the clayseq uence showthat thedepositionoccurredin low-salinitywat ersduringarctic -borealtoborealconditi ons.The5-20 m thick clay sequence mayhavebeen formedbyredeposition duringtheregr essionwhen largelandareaswere exposed and subjectedto erosion.Thesubsequentredeposition of the materialresulted inthe formation of theclay.

FredrikKlingberg,Geological SurveyofSweden,DivisionofMarine Geology, Box670,S-75 128Uppsala,Sweden.

Introduction

The Quaternary deposits in the Swedish part of the Kattegat have recently been investigated by the GeologicalSurvey of Sweden(SGU)as a part of aregular long-term survey of the Swedish continental shelf area.

The results of thissurveyarepresented as amap series:

The marine Geological Map, SGU SerieAm,inthe scale 1:100,000(SGU, 1989,1994, in prep).The marine survey was carriedout fromthevesselsRN Caroli na(1985-1988) and from SN Ocean Surveyor (1989-1992). During this work a characteristic clay sequence, here provisiona lly referred to as the'Lind hov clay',was first observedin the sea northwest of the town of Varberg (SGU 1994) (Fig. 1).

Stratigraphically, the Lindhov clay sequence is embed- ded between glacial and postglacial deposits. In this paper,the genesisand ageof the clay sequencewill be discussed.

The geol ogicalsetting of the Kattegat region hasbeen treated by many authors.A review ofimportant investi- gationsdocument ing the distrib ution andst ratigraphy of bot h pre-Quaternary andQuate rnarysedimen ts hasbeen made by Lykke-Andersenet al.(1993).Late Weich selian deposits have been st udied by Michelsen(1967).M6rner (1969), Fait (1982), Bahnson et al. (1986), Knudsen &

Nordberg (1987), Bergsten & Nordberg (1992), Seidenkrantz&Knu dsen(1993)andBergsten (1994).The st ratigraphy of Holocene sediments in the sou th ern Katte gat has been stu died by Nordberg (1989) and Conradsenet al.(1994). Gyldenho lm et al.(1993)sugge- sted a divisionof the Quaternarysedi ments intofourseis- mic units(unit s 0-3).Unit 3 containsdepositsof supposed Late Saalian to Middle Weichselian age, unit 2 represents the Late Weichselian till, unit 1 represents the Late Weichselian wat erlain deposits and isup to 100 m thick, and unit0 represents the Holocenesediments.

The uppermost part of the clay sequence at Klosterfjorden was cored and investigated by M6rner (1969) (Fig. 1).This clay was classified by him as being postglacial and dating to the Boreall pollen zone.

Bergsten & Nordberg(1992) studied38 cores from the southern Kattegat and subdivided the Late Weichselian and the Holoceneint o four facies. The uppermost,facies I, contains faunas characterised by species typical of Holocene temperate conditions in the Kattegat and Skagerrak regi ons. The deposition correspond ingto faci- es I was estimated to have occurred between 10,300 to 9,500 years B.P.Facies I was subdivided into two parts representing different water depths. The sublittoral facies Ib was characterised by foraminifer assemblages with species such asElphidium albiumbilicatum,Bulimina mar- ginata and Elphidium excavatum representing relatively shallow conditions. Facies la is characte rised by shore/near shore foraminiferal assem blageswit h species such as Elphidium williamsoni, Haynesina germanica, Elphidium guntheri and Amm onia beccari.The sediment correspo ndingto Facies 11 wasdeposited between 10,300 and 12,700yearsB.P.The fauna contain arctic,temperate, normalmarine and low-salinity elements with Elphidium albiumbilicatum, Elphidium excavatum, Cassidulina reni- forme and Bulimina marginata. In more shallow parts, Elphidiumalbiumbilicatum and Elphidium excavatum are themost abundantspecies(Bergst en&Nordberg1992).

Geology and palaeoceanoqraph,

The deglaciation of the eastern Kattegat started at about 14,000 years B.P.(Lagerlund &Houmark-Nielsen 1993) and the Varberg area was deglaciated at about 13,400- 13,500 years B.P. (Passe 1990).During the deglaciation the sea entered the Kattegat basin.Thisearly Kattegat

18 FredrikKlingberg GU-BULL 430.1996

'.

20'.m

5km 40m

oI

Fig. 1. Location map of southwestern Scandinavia showing the investigated area and the cores mentioned in thetex t. The seis- mic sectionsAI-A2,81-82,CI-0,01-02 and El-E2areplacedattheircorrect positions. The broken lines indicate present bathymetry.

Point 1is core 8502 and point2iscore 8523by Berqsten&Nordberg(I992}.140,141and142 refertocores byMorner(l969).Extension of the Lindhovclaysequenceaccording to the

"} G,o'ogi<aJS''''10/

s .'," "

geologicalsectionsat sea(widehatched)and

' - - ---'- --'- • Jl_an_'"'_G_<oI--=ov:....--' from the survey onland(close hatched).

had a bay-likesetting with the openingto the north and with contact with At lant ic water through the Skagerrak and the North Sea.In the south the BalticIce Lake was drained into the Kattegatthroughthe Oresund strait bet- ween 12,700 to 10,300 yearsB.P. (Bergsten & Nordberg 1992).The influxof highly saline waterbelow the meltwa- ter in the early Kattegat createdestuarine conditions with a stratified watercolumn.During this phase,the Varberg areawasstil l depressedbelowthe sealevel(Passe 1990).

Meltwater carrying large amounts of suspended matter

m above

.... ..lev elI

submerged the area and sedi ments, mainly clay, were deposited during these glaciomarineconditions.Close to thehighest coastline,at about 75 m above sea level,gla- cifluvial coarse sediments were deposited. The shore level displacement was an import ant factor infl uencing the development of different sediment type s. After deglaciation,relatively fast regressio n tookplace in the area(Passe 1990) (Fig. 2).During the regression ,redeposi- tionofboth coarseand fine glacialsediment s occurred.

In the invest igatedarea,moraine ridge srunparallelto the ice margin.These ridges were orig inally interpreted to be end-moraines (De Geer 1893),buthave been rein- terpreted to be glaciotectonic ridge s (Fernlund

1988,1993 ) formed during an ice ad v a n ce called the

'Halland advance' (Fern lund 1993). Parts of the ridges were thrust over the earlierdepositedglaciomarineclays.

Shells from the clay have been "C-dated to c. 12,400 yearsB.P.which im pli es that the Halland advance post- datesthe depositionof thisclay(Fern lund1993).

20

10

Methods

12000 10000 8000 6000 4000 2000 0

YearS.P. Acousticprofiling andsampling

Fig.2.Shore leveldisplacem ent in the Varb ergarea.FromPasse(1990). Acousticprofiling andsam plingwere carried out by the

NGU-BULL430,1996 FredrikKlingberg 19

A1 ^{Sea level} A2

12685

B2

Sea level

. .

gf~

Om~_B_1 ~et::J'O - t

cP

-50 m ~----=

.::.:-::::-,.:,:.:,::. :.:'~

1270

C1 ^{Sea level} C2

Om-I-- - - - -- - -- -- - - -- -- - - ---:t

12724

Om-j-- --t-- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ----II-t

01 ^Sealevel 02

Legend:

E1 ^Sealevel

1274

E2

r.,·::':::1 Postglacial clay

1 ---1

Lindhov clay sequence _ Late glacial clay

~Interglacial I

~•.: glacial deposits

~Sedimentary bedrock

o

Crystalline bedrock

SGU

127112 ^GeologicalMilr/ne Geology^{Surveyof Sweden}

Fig.3.Fivegeological crosssectionsinterpretedfromreflection shallowseismics. End-pointsaccordingtoTable1and locationaccordingtoFigure1.The positi- onofcore 05B-030isshown.

GeologicalSurvey of Sweden in order to establi shast rati - graphy of the Quaternarysedi ments in the investi gat ed area. A3.5/7 kHz EDO Western sub-bottom profiler was used to obtain high resolution information.To acquire info rmati on on the sed iment s out-of-range of the sub- bottom profiler,single-channel shallow seismicreflection data were collected. The seismic sound sources used were an EG&GSparker and an EG&GUniboom.The recei- ved signalswerefilteredbetween 0.2and 1 kHz.

Sampling

Surface sam ples and vibrocoreswere taken to verify the interpretati on ofthe acoustic profiles.The coring was car- ried out with a 6 m vibrohammercore and the surface was sampled with an'Orange Peel Bucket'grab sampler.

The results from the surface samples are not presentedin this paper.The grain- sizedistribution and biostratigrap- hy,especially concerning foraminifers,were analysed in subsamples of the sediments obtained. The percentage

20 FredrikKlingberg

Table 1. Lengt h and posit ions ofthe end-point s of the cross sections.

The locationsof the cross sectionsareshowninFig.1.

Section name Point East Nort h Lengthin m

12685 A1 12"02'01 57"09'37 7831

A2 12"07'53 57°12'22

1270 B1 12"01'15 57"08'06 10000

B2 12"08'24 57°11'50

12724 Cl 12"03'06 57"05'39 6997

C2 12"08'16 57"08'10

1274 D1 12"05'30 57"05'00 6000

D2 12"09'55 57"07'10

12752 El 12"09'11 57"03'25 6710

E2 12°13'47 57"06'0 1

Table 2.Lithology ofcore 05B-030.

Depth below surface

incm Lithost rati g raphy

0-20 Siltyfine sand.Shell-bearing.ShellsfromAretica istandica,Macoma balticaand Ba/anus sp.A sto ne at10-12.Olive grey,5Y4/2

20-565 Fine clay, in the uppermost part gyttja clay.

Scatte red shell fragments. Homogeneous. 20- 120 worm burrows filled with fine sand. 290- 295 clayey, coarse silt with shell fragments.

Sulphide spotsat500-510and 555-565.Colour 20-350 grey 5Y 6/1,350-560,grey,5Y5/1 .

of sedim ent particleslarger than 0.125mm of the total dry weight was calculated (Figs. 4 & 5).The sedim ent colours were estimatedaccord ingto a Munsell soilcolour chart(Tables 2&4).

Foraminifers

The laboratory treat ment of the foramin iferal samp les was carried out accord ing to the met hods described by Meldgaard &Knudsen (1979).For quantitative analysis, when possible at least 300 specimens of foram inifers were counted from eachsam ple.The num ber ofspeci- mens in each sample and the relative frequencies of selected taxa in thetwo profilesare illustrated inFigs. 4 and 5.When a samplecontainedlessthan 100specimens the percentagewas notcalculat ed;instead,theoccurren- ce of specieswas ente redinthe range chart bytheir actu- al numbers.Faunal diversity and thenumberof species are given in therangechart. Faunal diversityiscalculated as the numberof rankedspeciesthat accountfor 95%of the counted fauna. LFAZ are defined as Local Foraminiferal AssemblagesZones,according to Hedberg (1976).

Radiocarbondating

Two radiocarbon dating s from marin e mollus cs were obtained byusing AMS,carried out by theTandem labo- ratory at Uppsala,Swede n.The radiocarbon ageswere

GU-BULL430.1996

reducedby 400 years to account forthe reservoireffe ctof seawater(Mangerud&Gulliksen 1975).

Resu lts

Of the five NE-SW trending cross sections presented in thispaper,two(A1-A2and 01-02)have previouslybeen published as part of the Marine Geolog ical map (SGU 1994).The sections,which are between 6 and 10kmlong, run parallel to the main ice movementdirect ion (Table 1, Figs. 1 & 3). This means that the cross-sections cut through sediments deposited parallel to the ice margin, e.g. glacio-te ctoni sed ridge s.

Six diff erent st ratigr aphi c units (SGU 1989,1994, in prep.)were mappe dduring the geological interpretation of thecross secti ons. ThelowestisthePrecam brian base- ment which cropsout bythe Swedish coastline. Tothe west, this is overlain by sedi mentary bedr ock. The bedrock is covered by sedimentsinterpreted to beinter- glacial/stadial/interstadial sediments deposited during the Saale,Eemian,Early and MiddleWeichselianperiods.

These sedimentswere,to a certainextent,glacio-tecton i- sed duringthe Late Weichselianand ridgestrending NW- SE were formed.The Late Weichselian deposits, mainly tills,cover the interglacial/stadi al/interstadi al sediments.

It isdifficultto distingu ishthe intergl acial/ stadial/ inter- stadial deformed sediments from the Late Weich selian sediments in the glacio-tectonisedridges .During degla- elat ion . glaciomarine clay was deposited conformably upon the glacialsubstratum.

The seismicprofiles(Fig.3)indicate that alo ngthe pre- sent coastline,a clay sequence wit h specific deposit iona l characteristics,provisionally referred to as the 'Lindhov clay', was deposited above the glaciomarine clay.The base of the Lindhov claywas deposited conformably on top of the glaciomarin eclay. The Lindhov clayis designa- ted glacial clay on the marine geological maps (SGU, 1989, 1994,in prep).The top surface of the sequence is quiteflat and the shift towardsthe Holocene clayis mar- ked by a sharp acoustical boundary. The Lindhov clay sequences maythen be cut through by Holocene sedi- ment-filled channels.The layering of the Lindhov clay sequence dips westwardsand the clay extends at most to 10 km from the coastline.The thickness is generallybet- ween 5 to 10 m and reachesa maximum of about 20 m.

The Lindhov clay is silty, partly banded and hasalow organiccontent. Abovethe Lindh ovclay sequencethere is a younge rclay that has beendepositedthroughoutthe Holocene unt il recent time . The Holocene clay has a slightly higher organic content compared with the Lind hov clay.

Co re 05B-030

The 5.65 m long core058-030,posit ioned at N 57°10'08E 12"05'07,wastaken withthe purpose ofinvesti gati ng the uppermost part of the Lindhov clay sequence (Fig. 1,

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22 ^{FredrikKlingberg}

Table3.Local foraminiferalassemblageszones(LFAZ) in058-030

Zone Dep thbelow LFAZ surfaceincm

NGU-BULL 430,1996

mainly boreal species.The clayey, coarse silty layer at 290-295cmprobably represents a hiatus.

82 0-80 Elphidium excavarum-Elphidium albiumbilicaru m -

8uli mina marginara- Ammonia beccari

Lindhov

Table4.Lit hologyatthe Lind hov road-cut . 81,b 80-275

81,a 275-565

Depth below surfacein cm 0-70 70-270 270-300

Elphidium excavarum- Elp hidium albiumbilicarum - Amm oniabeccari

Elphidium excavarum -Elphidium albiumbilicatum

St ratigraphy Dry crust

Fine clay,grey,5Y 5/1 Fineclay,grey,5Y5/1, fragments ofshells,diffu se sulp hide banded,5Y3/1

The samples were collected in a 3 m deep cut during a road construction at Lindhov,3 km north of Varberg at the position N57"08'18 E12°15'52 (Fig. 1, Table 4).

Lindh ovissituatedonaveryflat coastalplainonlyafew metr es above sealevel. The distributionof thesedim ent can beseen onthemap-sheetVarberg NO(Passe1990).

The highestcoastlineis about 70 m above sea levelin the Varbergarea (Fig. 2).

Radiocarbon dating was performed on foramin ifers in sample275 in AMS.Theuncorre cted ageis 11,070±215 yearsB.P.,and thecorrected age 10,770±215yearsB.P.

Foraminifers

Table 2).Holocene sediment is presentin the uppermost part of the core.The core was taken at a water depth of 22m.

Radiocarbon dating at core-leve l 160cm on unspecifi- ed shellfragmentsgave an uncorrected age of 8,975±95 yearsB.P.on the insolublefraction (Ua-4801).Thecorrec- ted age is 8,575±95year B.P.

Foramin ifers

Twel ve sam pl eswereinvestigated fortheconte nt of fora- minifer s(Fig. 4).The sequence canbe divided into two LFAZ,B1 and B2(Table3).The number of specimensper 100 g sediment in the lowermost zone B1 isrelativel y low andvaries between 70 and 550. In the uppermost zone B2 theconcentrationsofspecime ns range betwe en 1200 and 3000.The num ber ofspecies persampl evariesbet- ween3 and23inLFAZB1and29 and39in LFAZ B2. The most im port ant species are Elphidium excava tum and Elphidium albiumbilicatum.In LFAZ B1, the arctic species Astrononion gal/owayi and Cassidulin a reniform e are import ant accessory species. Elphidium magel/anicum and Cibicides lobatulus are also common. In LFAZ 82, import ant accessory species are the boreal species Amm onia beccari,Buliminamarqinataand Nonionel/a tut- gida. These species usually occur in recent sediment in theKattegatreg ion(e.g.Conradsenet al.1994).

Interpretation

Nine samples were investi gated for their foraminiferal cont ent (Fig.5),and thesequ encewas divided intoLFAZ Li1 andLi2 (Table5).Thenumber ofspecimensper100 g sediment isbetw een 1,000 and5,700and the num ber of speciesranges between 14 and 26 persample. Thepre- dominat ing species are Elphidium excavatum and Elph idium albiumb ilicatum which accou nt for > 95% of the fauna in all samples.The arct ic species Cassidulina reni formeis themostimportantaccessoryspecies in LFAZ L1,butitislacking in LFAZL2.

Interpretation

Theforam inife rs indicateanarct ic-borealbotto m water environ me nt.The extre mely low diversity refl ects ext re- meecologicalconditions,caused by a lowered salinity in thebottomwater duringdeposition.

Discussion

TheLindhovclay found off the coast of Varberg has also beenfound on land.Onland,theclaysequence occursat levels below 10mabovethepresent sealevelin the area northof Varberg. Thetopsurfaceoftheclay sequ enceis quiteflat onland,whichisa usual feature inthe region, andexplains whythis sequence was termed postg lacial clayon the landmaps(Passe1986, 1990)andprobably in

Table5.Localforamin iferalassemb lages zo nes(LFAZ)atLind hov. Dept hbelow LFAZ

surface incm

90-70 Elphidiumexcavatum - Elph idiumalb iumb ilicat um

The foraminifersdisplay boreal-arctic to boreal bottom water conditions. The foraminiferal concentration in LFAZ B1suggestsa high accum ulat ionrate,whichis not seen inLFAZ B2. Both forami nifer assemblagesarecha- racterised by a fauna showing low salinity.In LFAZ B1 thereareafew arct ic specieswhereasLFAZB2 con tains

Zone Li2

Li1 300-90 Elph id iumexcavatum - Elph id ium albiumbilicatu m - Cassidulinarenifo rme

NGU-BULL430,1996 FredrikKlingberg 23

other areas (Freden 1983, Passe 1988, Adrielsson &

Klingberg, 1989) as well. The sequence at Lindhov con- firms the interpretation that the clay sequence onland is the same sequence that was formed off the coast.The 'Lindhov clay' is here provisionally named after the locali- ty where the clay was first observed on the Lindhov farm, situated 3 km north of the town of Varberg. The distribu- tion of the Lindhov clay sequence has been tentatively estimated (Fig.1).

A core (8502) taken 40 km south of the investigated area penetrated a clay sequence which was dated to 10,000 and 10,400 years B.P. (Bergsten&Nordberg 1992) (Fig 1).These authors suggested that the clay sequence was deposited during the Younger Dryas. Comparison of the foraminiferal assemblages shows a similarity to those at Lindhov and core 05B-030.They are characterised by the same dominantspecies and also by the same acces- sory species.Therefore, it is believed that the Lindhov clay sequence found in the Varberg area is the same as that found south of the investigated area.

Core 8523 was taken about 70 km south of the investi- gated area (Bergsten &Nordberg 1992)(Fig 1).The fora- miniferal assemblages have very high percentages of Elphidium albiumbilicatum,which suggests lower salinity conditions than in the investigated area.The sediment sequence in this core can also be related to the Lindhov clay sequence. The sediment in core 8523 was estimated to have been formed between 11,700 and 10,300years B.P.

However, it is not known whether the Lindhov clay extends further to the south and to the north.

The foraminiferal analyses show that deposition of the Lindhov clay took place in boreal to arctic - boreal bot- tom water conditions.A strong freshwater infl uence can be determined from the large number of specimens which are tolerant to brackish water,mainlythe species Elphidium albiumbilicatum and Elphidium magellanicum.

Arctic species occur with low frequency in LFAZ Li1 and Li2.The assemblages indicate that the temperature of the bottom waterhad begun to increase but that it was still cold enough for these arctic species to survive in small numbers.The arctic species might, however,also have been redeposited.The sedimentation at Lindhov appears to have ended earlier than at 05B-030 and it thus relates to an earlier phase. The accumulation rate was slower at Lindhov compared to 05B-030.

The dating of the Lindhov clay sequence is uncertain.

One"C-date from Lindhov gave the age as 10,700 B.P..

The age of 8,575 B.P. at 05B-030is more recent than the presumed hiatus and therefore represents Holocene sediment.The lowermost part of core 05B-030,which dis- plays the Lindhov clay,is not dated.

Even if the dating of the clay sequence is unclear, it is suggested that deposition of the Lindhov clay began sh o rt ly aft e r de gl acia ti on of the area.During the subs e - quent regression of the sea, the areasaround Varberg were eroded. Redeposition of this material together with

suspended load

from the rivers and streamrunoff, resul-

ted in the formation of the Lindhov clay sequence.The sediments were deposited upon the late glacial clay sequences, showing a transition from conformable depo- sition in the lower part to typical basin fill in the upper- most part. The accumulation of the Lindhov clay may have ended at the same time as the peak of thereg ressi- on at 9,300 years B.P.

Acknowledgements

This is aDivision of MarineGeology,Geologi calSurveyofSweden,rese- arch document ationpaper.

Iam greatly indebt ed to my colleaguesat theDivision of Marine Geology, whowereall invol ved inthis st udy,andinpartic ulartoAnders Elhammerfor the seismicandsub-bottomprofi ler int erpretation and for improve ments to the manuscript.Iam alsograteful toTorePasse and Ingemar Catoforconstr ucti ve criticism anddiscussions,and tothe refereesKaren Luise KnudsenandMarit-Solveig Seide nkrantzforcon- str uctive comments on the manuscript . G6ran Possnert and Maud S6derman at Tandem laborat ory, UppsalaUniversity, perfo rmed the radiocarbo n datings;Kjell Helge at Minoprep did thefor amin iferalpre- parat ion;andKerstinBerntssonandKristi naKling helped wit hdrawings.

To allofthemIwouldlike to expressmy gratitude.

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