NGU-BULL435,19 99 -PAGE43
Middle and Late Weichselian high relative sea levels in Norway: implications for glacial isostasy and
ice-retreat rates
LARS OLSEN
&KARI GR0SFJELD
Olsen,L.& Gresfje ld,K.1999:Middle and Lat e Weichselia n highrelat ive sealevelsin No rw ay:implicati on sfor glacia l iso stasyandice-ret reat rat es.Norges geolog iske underse kelseBulletin435,43-51.
HighlyupliftedMiddle Weic hselian marin e sed ime nts at Heqj eeren , SWNor w ay,hav e prev io usly beenexpl ain edby tect on icmovem ents,glacia l isostasy ora com binatio nof thesefact or s,whe reas mo strecent st ud iesseem tofavo ur amod elbased on glacialisostasy.New evidenceofup lifte d Middleand Lat eWeich selianmarin e sedi m ents incoasta l areas ofcent raland no rth ernNor w ay, as wellasfrominlan d areasofcentra landsout heaste rn Nor w ay, stro ng lysug- gestsa glacio-isosta t iccause forthehigh -altitud elocati on of most of these occu rrences. Thepositi on s of these sed- ime ntsalso ind icat e afrequentlyfluctu at in g ice shee t duringthe inte rval 18- 50kaBP. Repeate drapid iceret reat foll owingheavyice loadingseemstobethemost likelywaytogetmar in e sedi me ntsof boththe sameand different age inte rvalsinsuc hup lifte dpositio nsover sucha wid egeograp hicalarea.Sig nif ican t devi ati on sin the gene ralgla- cial reb oun dtrend s of postg lac ialsho re li ne displ acementinanyof theareasdiscussedherehav enot been reported earlie r. Hen ce,a tecto niccause forthelandupliftofanyof theselocati on sis conside red tobequiteunl ikely.
Lar s Olsen& Kari Grosfje ld,Norges geolog iske undersek eise,749 7Trondheim,Norw ay.
Weichselian age, based on geomorphological and sedimen - tological evidence , have been suggested for th e Gudbrands- dalen - Mjosa regi on in southea stern Norway (Bergersen 1964, Olsen 1979, 1985a , Bergersen & Garnes 1981, Rokoen-
Fig.1.Map wit hallsitesand namesofthemo stcentralplacesandareas discu ssedin thispap er.1- Sto re lva, Gryt oya; 2-Mag elva11,Hinnoya;3-Sit- ter;4-Nam sen ; 5-Ro kob erget;and6- Rundhau gen ,Herl and sdalen .N- Nam sfjor den ,T - Tron dh eim sfjorden,L- Lagjaeren, H-He qj eeren .M - Mjosa, A-Alesundarea.
Introduction
The coast of Norway and the Norwegian fjord valleys reach- ing far inland have been exposed to major climatic, sea-level and glacial changes during the Quaternary. Evidence of high relative sea levels (marine sediment s, marine erosion of coastal caves, etc.) from ice-free intervals during the last gla- ciation have been known for a long time from coastal sites in western Norway (Bjerlykke 1908, Grimnes 1910, Holtedahl 1955, Andersen 1964, Feyling-Hans sen 1966, Andersen et al.
1987, 1991, Janocko et al. 1998). At Hoqj eeren, SW Norway (Fig. 1), uplifted (above the late-/postglacial marine limit) Middle Weichselian in situ marin e sediments > 200 m a.s.l, have been explained by tectonic movements (e.g. Feyling- Hanssen 1966, Fugelli & Riis 1992), glacial isostasy (e.g.
Andersen et al. 1987, Sejrup et al. 1998) or a combination of tectonic and glacio-isostatic upl ift (Andersen et al. 1991).The main problem for the glacial rebound theory has been to expla in why such a model , while successf ully explaining the uplifted position of the Middle Weichselian clays, has failed to explain th e altitude of the late-/postglacial marine limit at Jeeren, which is generall y low
«25 m a.s.l.), In a comprehen- sive study of sea levels, sediment at ion and erosion at Jeeren, L arsen et al. (in press) review this debate and provide new evidence wh ich seems to expl ain both high Middle Weichse- Iian clays and the low lategl acial marine limit w it h glacial rebound and differences in ice-retreat history .To reach a bet- ter understanding of this problem, more regional data should also be con sidered.
In spite of comprehensive st udies, proper evidence of marine influence at pre-Iategl acial sites located far inland have been difficult to find. E xcept for o ne find of marine shells at Lillehammer (Fig. 2), no marine fossils of pre-Late Weichselian age in such locations have previou sly been
reported. However, highrelative sea levels of Middle to Early
SWEDEN
--- -\--:jJ
\
--
--
NGU-BULL435,1999-PAGE44 LARS OLSEN
s
KARtGR0SFJELDFig.2. Locatio nmap,sout heastern Norway, modified from Rokoengen etal.(1993a). During atleastoneMidd leWeichseli aninterval, witha sea levelreaching
c.
240 m a.s.l,(atleast233 m a.s.l,at Ro kobergetl.the sea coveredawidearea ofSENorway,wit h the southe rn parts ofGudbrands- dalenand0ste rdalen proba blyoccurr ingasnarrowfjords,inwhich pos- siblebeachterraces weredeveloped intills.gen et al. 1993a). The uplift ed posit ion of th e locations of th ese relati ve sea levels inferred from th e inland area has sim- ply been explained by glacial loading and unload ing.
The aim of this paper is to present pr eliminary data from localities with Middle to early Late Weichselian marine fossils (shells, dinoflagellates ) in up lifted positions in Norway. The record includesthree coastal sites and two inland site s , w hich have been used, together wi th other relevant data, to discuss the most probab le cause of land upli ft and the consequences for th e glacial conditio ns du ring th e interval 18-50 ka BP.
transp ort at ion (e.g. Vor ren et al. 1981). Such data cannot eas- ily be used to reconstr uct previous sea levels, whereas other data, fo r examp le the uplifted in situ marine sediments at Heqjeeren (Fig. 1), may have a pot enti al as a sea-level indica- tor, if the absolute and relative magnitude of the tect onic movem ent s and the glacial isostasy can be sorted out.
The hy pothesi s of a possible marine influ ence in the inland locatio ns has not previously been confirmed by find s of marine fossils, except for one site at Lillehammer in the nort hern Mjo sa district, SE Norway, but is based on various secondary evidence. These includ e lanth anide d istribution wi th typical marine Ce deficiency (e.q. R oaldset 1980, Olsen et al., in prep. b), pollen cont ent indic ating vegetat ion w ith a sea-shore affili ation (e.g. Rokoengen et al. 1993a), sea-level reconst ruction s from accumu lation/erosion level of delta ic and other glaciofl uvial sediments in subt ill posit ions (e.g.
Be rgersen & Garnes 1981), and sub -surface terraces resem- bling beach terraces develop ed by erosion in tills of Mid - Weichselian age (e. g. Bergersen 1964, Olsen 19 79, 1985a).
A find of paired shells of the low bo real marine species Glycimeris redepo sited from pre-Ho locene sediments in the de lta at the outlet of the Gausdal and Gudbrandsdalen val- leys in lake Mjesa at Lillehammer (Figs. 2 and SA), indicat es the occurrence of a pre-Late Weichsel maximum interval wit h a relative sea level of more t han 122 m a.s .l, (Olsen, unpub- lished material). Pollen of spruce (Picea) fou nd in a calcareous concret ion fi lling th e paired shell (S . Funder, pers. comm.
1990) may indic ate a pre-Mid d le Weichselian agefor the shell because spruce is not supposed to have grown north of Ger- many in any Weichselian interstadial younger than the Ea rly Weichselian (Mangerud 1991). The precise age of the mo llusc shell, however, is no t known , but the shell has to derive from a warm interval, i.e.a w arm int erstadial or perhaps most likely an int erglacial, e.g. the E emian. If th e upli ft ed position of the shell site is caused mai nly by gl acio-isostasy, this impl ies tha t the mollusc lived in an early part of an ice-free interval. Theo- ret ically, the uplifted posit ion cou ld be caused by tectonic moveme nts, but there are no other data wh ich might sug- gest a significant tectonic component dur ing the Late P leis- tocene or th e Holocene in this area. This is d ifferent from the situat ion in north ern cent ral F ennoscandia where postg lacial faults wi t h vertical displacem ents of 7.5-30 m have been recorded (e.g. Lundqu ist & l.aqerback 1976, Olesen 1988, Olsen et al. 1999), and in S W Norway where possib le active fault zones in northern Jeeren may have caused sign ificant uplift of the eastern landblock (e.g. Feyling -Hanssen 1966, F ugelli & Riis 1992, Jorde et al. 1995). Moreover, lith o-iso- stati c subsiden ce still seems to be occurring in the southe rn Jzeren - E gersund area (Bakkelid & S kjet hauq 1984, Anund-
sen & Gabrielsen 1990).
Stratigraphy, marine fossils and chronology
Coastal sites
Fragment s of marin e mollu sc shells have been found in the lower part of a 1.5 m-th ick sandy g ravel whi ch is overly ing
,, , ,, ,
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pairedshell of Glycimeris.
122m as.1.
62"
58"
S Sorperoa . , . Northernlim it of , / 240m sea-level G Gudbrandsdalen
Setting and background
The Quaternary geology of No rway has been investigated by the Geolog ical Survey of Norway (NGU) and t he Norwegi an universit ies in several mappi ng prog rammes comprising regional surficial mapping (e.g. Olsen et al. 1996b, B erqstre rn et al. 1997, Sveian et al. 1997) and mapping of th e Quaternary strat igraphy at selected localities (e.g. Roaldset 1980, Berger-
sen & Garnes 1981, Vorr en et al. 1981, Andreassen et al. 1985,
Olsen 1985b, 1993, 1995, 1997 , in prep, Andersen et al. 1987,
1991,Landvik & Hamborg 1987, Larsen & Ward 1992, Rokoen-
gen et al. 1993a, Janocko et al. 1998,Olsen et al., in prep. a, b,
c). ln all these sou rces , wh ich together cover bo t h coastal and
inland areas, up lifted Weichselian int erstadial sedimen ts of
possible marine character occur. The upl ifted po sition of
some of the mari ne sediments, or sediments wit h marin e
mo llusc shells at the coast, have been explained by glacial
LARS OLSEN&KARIGR0SFJEW NGU-BULL435,1999-PAGE 45
I
Jreren uplands (H0gjreren)
2 3 7
Mage lva IT Sitter Elgane - 1,2
Si SGD SiSGD SiSG D
' m Q
c.162ma.s.l. Om c.50 ma.s .l, Om c.196 m a.5.1.\
1 6 6 10
46,000 M
34,82 0} 12,490 M
33480 Sa nd ne s
20
M
lnterstad.21,150 M
3 30
30,200 T
----
Bedr oc k
Coast
1 Sto relva
SiSGD
Om c.140 ma.s .1.
5
"-
6
10 6
6
15
i
41,000 M alleIlle=0.08 20
River
\ Inland
4 Namsen
Si SG D
Om c.194ma.s .1.
6
10 6
6 6
-
16,11018,580 M20 6 66
-
30 18,020 M?
6
-
~
c.100 m~
Bed rock
6 Rundhaugen
SiSGD
om ~
_ 6_ __ 6_
...lJ. -1:.-
2
4 •
c. 245ma.s.l.
32,000 M?
5 Rokoberget
SiSGD
o
mp 6 -6" C.235ma.s.l.:. A
2
i : >
33,800 M4 ; ... . h iatU:: ,ooo M
Legend
~ Till
c:::::::::J
Gravelr=:::=J
SandI··~<:I Pelite
D Diamicton G Gravel S Sand Si Silt
Deform ation Environment M Marine influe nce T Terrestrial
Deformationor ice-f low direction 200m
- 7
Deform ationor fabric-100+ - - ----.-- - .---- ---,-- ---,---.---,---,-- - .,.-- --,,--- --,-- --,
s
Striation
100 km 80
Inland
40 60 20
- 3 o Coast (sea, fjord)
- 2
- 1 - 6 -5
- 4
o - - postglacial mar ine limit
100
Fig.3.Simplifiedstratigraphiesfromthecoastal andinland areas.The Sto relvast ratigraphyis after A.Lysa (unpublished material1997),and thesubtill sediment satRundhaugen andRokobergetare previouslypublishedbyRoaldset(1980)and Rokoeng enet al.(1993a),respectively.Coringdatafrom ElganeatJeeren(Hogjceren)areincluded for comparison (afte r Janocko etal. 1998,Sej rupetal.1998).Comp arisonbetweenminimumsea levelsrepre- sented bythe marine sed imentsfromthe differentsites and the postglacialmarine lim it (definedbythe O-Ievel)is also indicated.
Sm-thick lam inat ed sediments at Stor elva c. 12S m a.s.l, on th e island Gryteya, nort hern Norway (Figs. 1, 3 & 4). The waterlain sedim ents, whic h are int ercalated b etw een tills, have a general coarsening up w ards tre nd and are inferred to represent an i n situ marin e succession of pro grading or
regressive character (A. Lysa,pers.comm . 1997, Olsen et al. in
prep.c).One fragmen t of Arcticaislandica fro m the gravel has
been
14C-AMS dated to c. 41,000 yr BP(Olsen et aI., in prep. a,
cl. w hereas another fragment from the same species gave
alle/ll e ratios of 0.081 (HYD) and 0.289 (FREE)from amino acid
NGU-BULL435,1999-PAGE46 LARS OLSEN&KARtGR0SFJELD
e-
\ ® a~ ' \ ®
~ '00 ~
, 0 ,
I I I I Ie- Altitude of mar ine-influenced sediments indi cated in Fig . 3
o
CID
10 6
kaBP
m
a.s.l.
~
l 00~_
o ~
I i i I I ,10 6 0
kaBP
o
o
®
6
ka BP6
ka BP10
10
m CD m
a.s.1.]
a.s.l.~
B- lOO ~
lOO I
0 0
la 6 0
kaBP
m @ m e-
a.s.l. a.s.l.
lOO 0 10 ~ 6 0 l OO 0
kaBP
m 0
a.s.l.
100
0
~
10 6 0
kaBP
Fig.4.Simplifiedlate -/postglacialshoreline displac eme ntcurves fro meach areadiscussed inthe text.
The altitudeof each sitewith subtillsedimentsof pre-Iateglac ial ageand possibl emarine origin,as shown inFig.3,is indicatedin each diagram.The shorelinedata aremod ifi edfromAnd ersen (1968) (1&
2),Ramfjo rd(1982) andKjemperud(1986)(3),5veian&Olsen(1984) (4), Hafsten(1956)(5),Berg st rom (1988)(6), and Hafsten(1983)(7).All the curvesare stronglygeneralised.
analysis, w hi ch also i nd icat es a Mid dl e Weichseli an age (H.P.
Se jrup, pe rs. com m . 1998).
One fragment of Mya truncata from a t ill at Ma gelva c. 160 m a.s,l. on the island Hinn eya (Fig s. 1, 3 & 4) ha s be en 1 4 C-AM S d at ed to c. 46 ,000 yr BP (Olsen et al., in pre p. a, c). The lategla- cia l marin e limit (M L) is c. 70 m a.s .l, in the Gryteya-Hlnneya area, w hich indica tes a co nside rab ly low er uplift t han in dica - ted by t he highly raised posit io ns of t he Middle Weichselian she ll finds. However, the shell in this till may hav e bee n gla - cially tr ansported from lo wer ground 6-7 km fro m t he Mag-
elva sit e in the 'up st ream' direction of the reg io na l ice move- ment.
At a third coasta l site, at Sitter in centra l No rway, dinoflag- ella te cysts hav e been recorded in reworked sediments in a t ill c. 50 m a.s.l., wh ich is 60 -70 m below t he lateglac ial ML in t his area (Figs.1, 3,4 & 5B-E). According to 1 4 C-AM S dating of the insolubl e (INS) fraction of on e b u lk samp le from these sedim ents, t he ag e of t he interval t hat mo st likely include a p hase wi th a high relat ive sea leve l (> 50 m a.s.l.) is c. 21,000 yr BP (Table 1 ). Underlying t he dinocyst-bearing t ill there is
Table1.Record of high lyupliftedNorweg iansiteswith Midd leandearly LateWeichselian sediments, includi ng marinefossilsand 14C·AM5 dates from the samest ratig raph ical levels.
Note that the sedimentdates are from sediments wit h a generally lowconten toforganicmat erial(total organic carbon1- 2%or less).Possib le contamination of such sedimentsbyyoung or old carbonisdiscussed byOlsen et al.(in prep.a).
Marine fossils 1. Gryt"ya* 2. Hinn"ya** 3. Sitter 4.Namsen 5.Rokoberget 5.Rokoberget 6.Rundhauge n 125 m a.s.1. 160 m a.s.l, 50 m a.s.l, 179 m a.s.l. 233 m a.s.l, 231 m a.s.1. 243ma.s.l.
Molluscshells
XX X
Dinofiagellates
XX X X X
Possible diatoms(marine)
X
14C-AM5age, yr BP; 41,660+/- 1500 45,560+/- 2400 21,150 +/-130 18,580+/-1 40...33,800+800/-70047,000
-t-
4000 32,000+/- 300 dated materialArt ica isl andica M yatruncata
INSfraction,sed. INS fraction,sed. INSfraction,sed.
INSfraction,sed. INSfraction,sed.*}Loe. 5torelva (A. Lysa,unpublished); **}Loe. Magelva 11(Olsen etaI.,inprep.c); *** )Dating ofthe hexane extra ctedfraction :16,110 +/- 120yr BPfrom the same unit.and of theINS fraction: 18,020 +/-170 yr BPfrom the underlyingunit. x=some; xx=abundant.
LARSOLSEN&KARI GR0 SFJELD NGU-BULL 435,1999-PAGE47
A B c
D F
G H
Fig.5.Photogr aphsofsom eexam plesofmarin efo ssilsrecord edinthe sedime nts.A)Glycimeris, paired shell;lowbor ealmollusc species ,Lilleham mer.
Age:>74kaBP(?).Co llected by Thor Nord ah l,1989. B- H)Dinoflag ellat e cysts,bright field.All givensizesof thecystsrefer tothelon gaxisofthecyst body.Prep arati on follow ed stan da rd palyn olog ical methods, usingcold HCI and HFtorem ove carbo nateandsilicate particles (Rochon&deVern al 1994).Thedin ocystno men clat urefollow s that of Rocho net al.(1999)andLentin& Williams(1993).Fig s.B, C &H.Bitectatodiniumtepikiense.B,C:Sitter, N.-Flat anger.Sampleid.950557.Age:c. 21kaBP.B:Opticalsectio nshowingwallst ructure,cystbody diameter:47.5micron s.C:Dorsalview,hig h focus, showingthearcheopyle (theopeningin the cystwallthrou gh which the proto plastemerges dur ingexcyst m ent) form edby1055of tw oprecing ular paraplat es, cyst bod ydiamet er:49micron s.H: Rokoberget,sampleid.4-11/3-90,4mdept h.Age:c. 47kaBP. Cystbody diam eter:37.5microns, optical section.Figs.D-G:Operculodiniumcentroca rp um.D,E:Sitter,N.-Flata nge r.Sample id.950557.D:Cystbod ydiameter:42.5micron s.E:Specime nwit h sho rtprocesses.F: Roko berg et,samp leid.2-27/9-91,2.5 mdept h.Age:c. 34 ka BP.Speci me nwithshortprocesses;cystbodydiamete r:35 microns.G:
Nam sen, sampleid.2-10/10-95,18m depth.Age:c. 18 ka BP.A st ro nglyoxid isedspecime n classified asOperculodiniumcentrocarpum.
anot her till, w hich overlies a th in (0.1- 0.3 m) diamictic silt.
This unit is inferred to be of glaciolacustrine or glacioma rine origi n. It cont ains no marine fossils, but terrestri al organic plant resid ue suggests deposition above sea level and impliesa corre spondin g sealevel below 50 m a.s. l, A 1 4 C-AMS dati ng of t he INS -fract io n of a bulk sample from th e s ilt indi- ca tes t hat this may have ha p p ene d c. 30 ,200 yr BP (Fig. 3;
Olsen et aI., in prep. a, c). The sediments w ith the dinocys ts at S itter may be slightly dislocated, but in that case they were prob ably tran sported from high er grou nd in accordance
with t he ice-movement di rection from eas t to west (Fig . 3;
Olsen, unpublished material).
Inland sites
Dinocystsare also recorded in subtil l lami nated se di ment s at two inland sites, at Namsen near the vil lage of Trones in cen- t ral No rway and at Rok o b er g et in so ut h easte rn Nor w ay (Fig s.
1, 2, 3,4 & Table 1). The sediments may th erefore be of shal-
low-marine origi n. The Namsen sediments are inte rcalate d
betw een tillsand located c. 179 m a.s.l., w hich is s ome 37 m
I
NGU-BULL435,1999 -PAGE48above the postg lacial ML in th is area. The sediments at Rokoberget are situated c. 233 m a.s.l . , wh ich is mo re than 40 m above th e early Holocene M L (190 m a.s.l.) in the Mj0sa area (Fig. 2). All the dinocysts from the Namsen and Rokober- get sites are badly preserved and difficu lt to ident ify. Hence, th e d inocysts recorded from these sites refer to unidentifi ed species, except for thos e which are illu str ated in F igs. 5F-H, w here namesof inferred species types are incl uded.
Three
14 C-AMS dates of extracted fractions from bulk samples of the Namsen sediments and reworked clayey silt in the underlying till give ages of c. 16,000-18,500 yr BP (Table 1), which together with the sediments at Sitter ind icate a major ice retreat and ice-margi n oscillat ions in the Namsfjo r- den-Namda len area, and thus the ent ire Trondheimsfjord en area, du ring the interval 16,000-21,000 yr BP .
Dinocysts are record ed at two stratigraph ical levels at Rokoberget (Figs.3 & SF, H;Table 1). 1 4 C-AMSdat esof the INS fraction of bulk samples of the sediments from these two stratigra phical levels yield ages of c. 34,000 and 47,000 yr BP, respectively (Rokoengen et al. 1993a ).Vegetat ion wit h a sea- shore affiliat ion, reflect ed in the po llen content, is thought to be represent ed at both stratigrap hical levels (Rokoengen et al. 1993a). The boundary zone in the sedim ents between these stratigraph icallevels is not observed in detail. A hiatus or unconformity, corresponding to the regiona l ice advance which is known to have occurred in other part s of Norway c.
40,000 yr BP(Larsen et al. 1987,Olsen et al. 1996a), and in the Norwegian Channel (Longva & Thorsnes 1997, Se jrup et al.
1998), may well be represented.
At Rundhaug en in Herlandsda len, SE Norway (Figs. 1,2,3
& 4), glaciotecton ised sub-till clay located c. 243 m a.s.l, has been studied by Roaldset (1980). Based on a Ce-deficiency in the sediments typical of marine cond it ions, she conclud ed that the clay was most likely of marine origin. She also sug- gested a correlation with the Sandnes interstadial clays (30- 40 ka BP) based on general considerations and particularly on the ir sim ilarly high ly uplifted posit ions.
The Geolog ical Survey of Norway has recent ly reinvest i- gated the sub-t ill sediments in th e Herlandsdale n valley, and a Middl e Weichselian age has been confirmed by tw o 1 4 C_
AMS dates of c. 28,000 and 32,000 yr BP (Berqst rern 1999, Olsen et aI. , in prep . a, c). The sub-till sediment s at Rundhau - gen conta in some microfossils (possible diatoms) w hich have not yet been identi fied.
Contrary to th e coastal area of SW Norw ay, th ere are no published records of anomalous postglac ial shore displ ace- ments or oth er data w hich suggest that active fault zones, w ith significant vert ical displacement of landblocks, exist in any of the coastal or inland regions discussed here.
Discussion and conclusions
The results presented here demonstrate the existence of ice- free condition s wit h a marine infl uence both at coastal and in land sites in Norway at around 4 1,000-47,000 yr BP, 32,000- 34,000 yr BP, and 16,000-21,000 yr BP. The uplifted posit ions of the sediments imply most likely a g lacial isostat ic cause, which unfortunately and erroneously was printed as a 'g laci-
LARS OLSEN&KARtGR0SFJELD
ot ectonic' cause by Olsen (1 997, p.55), wh ereas the context, wi t h many different uplifted sites distrib ut ed over a wide region , indicates clearly that it was th e term ' glacioisost at ic' w hich was meant to be used. Theoretic ally, tectonic mov e- ments could have expla ined the uplift ed position of the se sit es, bu t we fi nd it most un likely that repeated and alternat- ing down ward and up ward tectonic movements could bring these sediments to comparable high alt itud es at such diff er- ent locat ions du ring the same time interval (e.g. S itter vs.
Namsen, Gryt eya and Hinneya vs. Rokob erget ), or to t he same alt itud e and location du ring d iff erent time int ervals (e.g. Rokoberget ).
A g lacio-isostatic explanat ion for the inferred upl ift of t he present ed locat ions seems evident based on general consid- erations.The consequence for t he glacial developme nt must therefore include a dynam ically unstable ice-sheet wi t h repeate d interva lsof rapid ice advance alt ernat ing wi th rapid ice retr eat. If the tect onic factor is neglig ib le, asw e think it is for the sites discussed here, then such high relative sea level s can on ly occur in connection with rapid ice retreat. The g la- cio-isostatic compo nent seems to have been even more impo rt ant if we incl ude the possib le sho reline w hich was developed over a wi de area as a rough ly formed, elongat ed terrace in t ills c. 240-260 m a.s,l. in th e sout hern part of the Gudbrand sdalen valley, and along th e bord ers of t he Mjesa drainag e basin (F ig. 2). The terrace has prev iously been expla ined pr imarily asa result of glacial and glaciofluvial ero- sion (Bergersen 1964, Olsen 1979, 1985a), bu t it s extensive and partia lly almo st horizonta l character suggests that it should rather b e considered to reflect a previous shoreline.
However, a better description of t his landform would prob a- bly be a previous tem porary fluvial and colluvial erosion - accumu lat ion basis. This possible 'shoreline' seems to be of comple x origi n as it was proba bly develop ed both prior to and after deposition of t he fine-grained, blu ish-grey Jerstad till, wh ich is young er than 31,000-36,000 yr BP according to three 1 4 C-AMS dates from laminat ed sediments underlyin g this till at Lillehamm er (Olsen 1985b, 1995). The older part of the 'shoreline' corresponds ro ughly wi th the lat eral d ist ribu- tion of the Jerstad till in the main valleys, and wi th t he accu- mulation level of the younger part of th e subti ll sedi ments in the main Gudbrandsdalen valley (Bergersen & Garnes 198 1 ).
This part of the ' sho reline' may well roug hly correlate wi th th e S orperoa intersta dial, wh ich is represented by win d- blow n sediments , TL-d ated to c. 35,000-40,000 yr BP, and described from t he mid-Gudb randsda len v alley (Bergersen et al. 1991). The younger part of the 'shoreline' is less d istinct and less know n. I t is not yet proven that th is part reflects a real shor eline, because the zone is represent ed mainly as an erosion level on to p of t he Jerstad till along the valley sides around Lillehammer. S mall envelopes of sorted sed iment s occur in places along this zone betwe en the Jerstad till and the overlying tills, but th ese have not been stu died in detail.
Moreover, it has not been demonst rat ed that th e area around
Lillehamme r was com plet ely ice-f ree and expo sed to direct
infl uence by the sea in any interval betw een the deposit ion
of the Jerstad till and the last deglaciation period .
LARS OLSEN&KARIGR0SFJELD
The marine in f lue nce, reaching f ar in land and up t o at least 233 m a.s.l, aro u nd 34,000 yr BP, is proven from th e site Rokoberget in th e so ut heastern pa rt of th e Mj 0 sa regio n.
Given th at there we re ice-f ree cond itions, t he sea must th ere- fo re hav e reach ed mu ch f arth er in centra l sout he aste rn No r- way, including the southern part of th e Gud br an d sdalen and 0sterd alen v all ey s (Figs. 2, 3 & Tabl e 1). A marin e infl uen ce in the int erva l so mewhe re b et w een t he last degl aciat ion and 31,000 yr BPis not yet proven f or th e Mj0 sa reg ion, but seems qu it e li kely. Thi s sup pos it io n is based o n th e we ll-known gla- cio-isostatic d ep ressio n during th e Lat e W eich seli an maxi- mum (LWM) at around 22,000-24,000 yr BP, and th e su bseq uent (e. 18,000 -21,00 0 yr BP ), extre me ly rapid, ice retreat even far inland in sout hea stern No rwa y (Olsen 1993, 1997, 1998, Olsen et al., in prep . a, b, c). A possib le marin e influence aro u nd Mj 0sa du rin g ice ret reat sub sequ ent to t he LWM is supported by th e sugges te d m arine influenc e at Namsen, central Norway, around 16,000- 18,500 yr BP (Figs. 1, 3 & Tabl e 1).
Olsen (1997) and Olsen et al. (in p rep. a, b, c) have f ound a semi -cycl icit y of 5-7 ka in glacie r variat io n du rin g th e int erva l 15-45 ka BP. If glac ial isostasy is th e on ly sig ni ficant cause of uplift fo rthe highly up lifted 'mari ne' lo calities di scu ssed here, the requ irement of ice lo ading, and th erefor e duration of th e ice-g rowth interval, shou ld match w it h this semi-cyclic ity . We will try to show th at th is is possibl e, using the followin g gen - eral approach. Th e highest levels of uplifted m arine -influ- enced sediments are e. 260 m a.s.l. This requ ires a m in imum load of ice which equals an averag e ice t hickness of 260 m x 3= 780 m, where the factor 3 is the averag e rat io bet w een specific w eights of roc k and ice (Pate rson 1994). With a net accu m ulati on rat e of 0.1 m/yr. w hi ch is sim il ar t o th e averag e Holocene net accumu lati on rat e for th e Greenland ice sheet, and an ice-growth interv al of 7800 yr, an ice shee t w it h a thickness of 780 m co uld be deve lo ped. How ever, this approach sta rt s from equilibriu m, whic h m ean s ice growth in a context w here the rebound each t ime has bee n complet ed.
A more realistic co nt ext w ould be a sit uat io n w here th e land - blocks were, to some exte nt, gl acial-i sostati cally dep ressed co nst ant ly during th e interv al 15-18 to 45-50 ka BP. In th e area s we discuss here , an ave rage dep ression of m inimum 50 m seem s reasonabl e. With th is adj ust me nt as a st art ing lev el, the sam e ap p roach as outlined above gives an ice-growt h interval of 6300 yr, w hich seems to mat ch th e semi -cy clicity of 5000 -7000 yr of gla cier variation very we ll.
The gl acio -iso stat ic cause w hi ch seems t o exp lai n th e uplifted posit ion of all th e marin e inf luenced sed iments dis- cussed here, suggests a sim ilar cause for th e highl y up lifte d Middle We ichsel ian marine sed imen ts at Heqj eeren, 5W Nor- way (Fig.1). This acco rds we ll wi t h m o st of the recent reports which di scuss th e uplift histor y of th ese sedim ent s. For exam p le, Sejrup et al.
(19 98)exp lain th e up lifted position of the Middle Weich sel ian clays at Heqj eeren by rebo u nd afte r unloading of th e bi g ice-l o ad forme d by a major ice st ream e.
40 ka BP in th e Norw eg ian Chann el adja cent to th e coas t of SW Norw ay. It is w ell est ab lished th at a maj o r ice st ream dev elop ed in th e No rw eg ian Chann el also during t he LWM (24-22 ka BP) and during th e Tampen readva nce (18- 15 ka BP)
NGU-BULL 435,1999-PAGE49
as we ll (Sej ru p et al. 1996, King et al. 1998, S ej rup et al. 1998), w hich th erefore sho u ld give gen erally hig h lateglac ial ma rine lim it s in th e Jee ren region. Thi s is no t th e case, as all rep orts of lategla cial MLs in t hi s area ind icate low ML alt it udes. The high est of th ese is that of 20-25 m a.s.l, in th e north ern part of Jeeren (Thomsen 198 1, And ersen et al. 1987). Hence, to exp lain th ese differen ces in sea levels and glacia l co nd iti o ns Sejrup et al. (1998) sugges t ed th e pr esence of different degl aciati on m ech anism s and eust atic co ndi ti o ns bet w een th e pre-LWM interstadi als an d th e last ice-ret reat inte rval.
Larsen et al. (in press) ref in e this mod el and conclude that th e lo w ML at Jeeren can be exp lai ned by gl acial rebo u nd, but in th at case th e ice sheet cover ing Jeeren du ring th e Lat e Weich - selian must hav e been thin, in accorda nce w it h the model pre sented by Nesje et al. (1988) and Nesj e & Sejrup (1988).
The infer red 'low '
«50 m a.s.l.) sea leve l at Sitt er e. 30,200 yr BP (Figs. 1 & 3) ma y corres po nd wit h th e low (lower th an th e present) sea lev el at e. 30,000 yr BP report ed from th e North Sea Plat eau so ut hwest to wes t of Alesund (e.g. Roko - enge n et al. 1993b ). It w ou ld also seem t o be in acco rd ance wi t h the sea lev el at e. 29,000 - 34,000 yr BP (t he Alesund interstad ial) in th e Alesund area, w h ich never exceede d th e po stgl aci al marine limi t in that area (Larsen et aI., in press).
The alt it udes well ab ove th e postgl acial m arine limit of th e sea level s du ring th e supposed co rrelat ive inte rvals at Rokob erg et (e. 33,800 yr BP) and Rundhaugen (e. 32,000 yr BP) ma y, however, ind icate a different tre nd (Fig . 3). Altern a- tively, thi s ap pare nt discrep ancy may sim pl y be caused by an earlie r sta g e of gla cial recove ry at the se sit es.
Acknowledgements
Thisis cont rib utio n no.1toproje ct5.3.3in the NORPAST(PastClimates of theNorw egi anRegi on )project, a COllabo rationbet weenNorwe gi an geol ogical instit ut ions.Thepaperismain lydealingwit hresults achieve d in a pilot-projectcarried outat NGU.The field w or khasbeenfinancedby NGU.Astridtysa,KariSlette nand OddvarLongvahave carriedoutthe stratig rap hi calworkatSto relva, Grytoya.Themanuscriptrefere es,Atle Nesje and ArneReit e,providedconstructivereview s.Davi d Robertscor- rectedthe Eng lishtext inthefinalversion andIreneLu nd qvisthas pre- pared the illustrati ons. We are grat efu l to all these person s and institution s.Thanks alsotoThor No rda hl,who found theGlycim erisshell at Lilleh amm er, and to SvendFun der whosent the sam ple and the back- grounddata to usfor futurepublica t io n.
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