Sub-till sediments at Ro koberget, southeastern Norvvay
KARE ROKOENGEN,LARS OLSEN & SYNr;WEF,SELVIK
Rokoengen, K., Olsen, L. & Selvik,S.F. 1993:Sub-till sediments at Rokoberget.southeastern Norway.Nor. geol. unoers.Bull.424,1-12.
A dark.blue-g rey.siltyclay below till isdescribed fromalocality230 m a.s.l. intheRokos joen depressioneast of Mjos a.Thesedimentis glaciallydeformedbutinterpretedto represen t a depo- sitfromaperiod withalocal lake-level at least 240-250 mabovepresent sea-level.Thedeposi- tional environments are complex and believed partly tohavebeen a shallow fjord embayment, but glaciolacustrine conditions mayalso have been lnvolved.The pollen tlor a ofthesedimentis reflectin g a grass-dominated,treeless.lnterstadtalvegetation.Two"C-AMSdates atc.34andc.47 ka suggestaMiddleWeichselian ageforthe sedimentsin thedescribedRokoberqet inters tadial.
Similar glaciolacustr ine/lacustrine/marinedeposits mayhave been widespreadin severalperiods, playingan importantroleas parentmaterialfor parts ofthe'old blue tills'andoverconsolidated sub-tillsediments intheMjosaregion.Possible correlations with other known interstadial deposits insoutheastern Norwayare discussed.but only fragments of the Weichselian development are known at thepresent time.
«sre Rokoengen. Institutt for Geol ogi og Bergteknikk. NTH, Universitetet i Trondtie im. N-7034 Tro ndheim-NTH,Norwa y.
Lars Otsen & Syn ove F. Selvik, Norges geolo giske unaers eke tse, P.O.Box 3006-Lade. N-7002 Trondheim,Nor way.
some of the 'blue tills' inthe region.Possible corr elationswith similarsequencesinsurrou n- ding areas will also be discussed.
G- Gu dbrandsdalen
0- Oslerdal en S - Sorper oa F - Ftskvik Li - Lillehammer A - Asldale n B - Brumunddal lo- Lote n E - Elveru m T - Tange n R - Rokob erg el N - Numedal
Fig. 1. Location map. southeastern Nor way. Quaternary geological maps in scale 1:50,000 forthe map-sheets t.e- ten. Elverum and Tangen (Fotlestac 1973. 1974. Bargel 1983)are shown.The shadedarea on the insetmap indica - tes areas with fine-gra ined , blue-qrey.lodgement till (after Olsen 1985a).
12' 8'
Introduction
Since the description of the complex Gud- brandsda len interstadial(Bergersen& Garnes 1971), some ten occurrences of interstadial sediments have been reported from the sout- hern part of Central Norway.Three of these are situated in the Miesa region (Fig.1):near Brumunddal (Helle et al. 1981), in Astdalen (Haldorsen&RappoI 1990),andintheLilleham- mer area (Olsen, unpublished).
During the summer 1990, a new locality with sub-till sediments was disco vered and sampledin an extended slope excavationalong a for est road at Rokobe rget east of Miesa (Figs. 1 & 2). Preliminary investigations sho- wed the presence of both pollen andmacrosco- pic plant remains (J.O. Vigran pers. comm.
1990).In 1991the section was thereforeexami- ned more close ly,andboth a partly diamictic sand anda lodgement till were recognised in the upper parts of the section. The sediment below the diamictons is a dark,blue-grey,sil- ty clay affected by glaciotectonic deformation (Fig.3).
The purpose of the pres ent paper is to describe the sub-tillsediments from Rokob er- get, and thereby to contribute with another puzzle to the complex picture of Weichselian interstadialsin Norwa y.Itisour hopetothrow some new light on the origin of the matrix of
2 KereRokoe ngen.LarsOlsen& SynoveF.Selvik GU-BULl.424,1993
.V
-_ /
N
?
\
\ ,\
\ ( \
...-<.-1 \_ -
\ I (- 1km!
o
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!
g
Rokosj"en
215
- - Main ro ad - - - Small road
'-RS - Rokobe rge t section P - Presllokken R - Rokoengen V - Veideborg
... -.- Lo w er limi t of continuous sub-t ill se diments
o!
Fig.2.Topography of the Rokobe rge tarea and location.south of the forest road.of the described section(RS)at the northern end of a sma ll ridge thought to bea drumlin.The dashedlineindicates the extentof thesub-tillseciments towards the northandeast.
Physiography and geology Topography and bedrock
The lake Rokosj0en (Figs. 1 & 2) forms the centralpartof a c. 30 km transverse depressi- on between the two main N -S trending val- ley systems in southeastern Norway;the lake Mj0sa and Gudbra nds da len in the west and 0sterdalen with theriverGlommain theeast.
The present water-level in Rokosjee n is c.
215m,in Mj0sa c.125 m and in the Glomma east of Rokosj0en c. 160 m (all values above present sea-level;a.s.I.).South of Rokosj0en, the hill Rokoberget has its highest point at 355 m a.s.1. (Fig.2).
The local bedrock around Rokosj0en is dominatedbyPreca mbriangranites and gneis- ses. Rokobe rget is underlain by one of seve- ral gabbro bodies forming low smooth hills.
All these rock types are about 1,600 Ma.To the north and west the basement platform is overlain by younger sands tones and shales of Vendian and Cambro-Silurian age (Gvein et al. 1973).
Quaternary geology
The superficial Quaternary deposits east of Miesa in the vicinity of aokosjcen (Fig. 1) have been mapped by NGU (Follestad 1973, 1974, Bargel 1980, 1983), and are of Late Weichselian and Holocene age.
In the valleys of the Glomma and other ri- vers there are glaciofluvial, fluvial and some aeolian deposits, while till is the dominant superficial sediment outside the river valleys.
The gravel fraction (4- 8 mm)in the till south of Hokosieen is totallydominated by Precam- brianrocks and quartzite/sandstone (Foll estad 1973, Bargel 1983).
The investigated area south of Hokosjee n
(Fig. 2) has a continuous till cover, partly of great thickness. In a road-cut, a till with a thickn ess of mor e than 2 m was described by Bargel (1980), before a new and deeper section was excavate d in 1990.
The oldest recordedregional ice movement in the area (o f suppose d Weichselian age) is towards the south. The younger ice move-
NGU-BUL L.424.1993 Sub-tillsedimentsat Rokoberget 3
ments recorded during NGU's mapping have a slightly more southeasterly component (Folle- stad 1973,Bargel 1983).
Locality desc ription
Theinvestigated Rokobergetsection lies south of Rokosj0en (Figs. 1& 2)about 230 m a.s.1.
It encompasses a cross-section through a small ridge which is elongated N-S or slightly towardsNNW-SSE (Fig.2).The ridge isinterp- reted as a possib ledrumlin made by erosion of the substrate during an ice movement to- wards the south.
Examination of exposures of granite in the area not far from the Rokobe rget locality in 1991 revealed that the oldest ice movement recorded in thearea is represented by crude grooves with a direct ion towards southeast or east-southeast. The subsequentice move- ment left coarse striations directed towards the south.The younger,less pronounced, ice movement structures observed near thelocali- ty have a mainly southeaste rly component , while the lastice movement in the Hokosjee n area was directed tow ards east-southeast.
Striations from this phase are found asweak and thin striae on the topmost parts of bed- rock exposures.
The ice movement that form ed the sugge- sted drum linmay therefore be that whichpro- duced the old coarse striation towards the south; alternatively it may correlate with a younger regional southerly ice movement that is not so well documented in this particular area.
The Rokobe rget section is about 75 mlong, up to 4 - 5 m high and can be divided into three main lithostratigraphical units (Fig. 3):
a) Glaciotectonised sub-t ill sediments.
- Blue-grey silty clay.
- Silt and sand.
b) Sand and gravelly sand.
c) Till.
The blue-grey silty clay is exposed at the base of the excavation.It is compact and the exposed part of thesediment hasbeen strong- lydisturbed byglaciotecton ics with structures (mainlyfolding) showing an ice movementfrom north-northwest to south-so utheast. The clay is overlainby siltandsandthatarealsodistur- bed by glaciotectonic structures (Fig.3).
In the western part of the section the sub- till sediments are overlainby a brownish dia-
ROKOB ERGE T
(E) m a.s.1.
233 232 23 4 23 5
23 1
o
1 2mGlac iot e c t o nis e d sedimen t s
[
rt~;:.§l Blu e-g r ey silty cl ay a)
1 =-= 1
Silt and sandr=::::=l Sand and b)
c:::=:::.J
gravelly sand c)~:~f~1 TillClast fabric position
o
Max. -> "SS E"( 1659)9 Position and number
• of the samples
fig.J. I neaokoberqetsection . looking towardsthesouth.The positions of the samplesites areindicated.The "C-AMS dates are from samples nos.9 and 12.
4 KareRokoengen.Lars Otsen& Syno veF.Se/vi
micton interpreted to represent a lodgement till (Fig.3).The tillcontains local granite boul- ders up to more than 1 m' in size. A ctast fabricanalysis in thelower part ofthetillrevea- led a preferred orientation indicating an ice movement towards the south-so utheast. This is in agree ment with measur ements of the axes of ten glaciotectonicfoldsinthe underly- ing deformed sediments .
The eastern part of the section has a thin till cover (ablation till ?) but is dominated by sand and gravelly sand (Fig.3).The sandhas a somewhatdeformed appearance,which may have been caused by either glaciotec tonic deformation or deep frost action (perm afrost), or by a combinat ion of both processes.
Theboundary between the unitsvariesfrom dist inctand sharp toindistinct and mixed.The chaotic defor mation structures in the upper- middlepart of the section(Fig.3)makeit diffi- cultto decide on therelative agebetw eenthe sandin the eastern,andthetillin thewestern part of the section.The sand maygradelate- rally into a grey to brown-g rey till with sand, gravel and some large boulders outside the section. Our present favoured interpretation, however, is that the sand is older than the till and thus constitutes the younges t part of thesub-tillsediments.To test this hypothesis, more extensive field-w ork, including digg ing , is required.
In this work we have not given the units any formal or informal names,except for the lower unitwhich we have called the Rokober-
get interstadial sediments.
Laboratory investigations
As the till in the section resembles the com- mon,well-known, Quaternary deposits in the region (Follestad 1973,Bargel 1983),only the sub-tillsediments have beensubject to close;' labor atory investigation in the prese nt work and are described in more detail below. A search was also made for for aminifera (J.G.
Verde nius pers.comm. 1990)and diatom s(B.
Stabellpers.comm.1992)in the claybut none was found.
Grain-size distribut ion
The grain-size distribut ion has been determi- ned on selected samplesfrom the Rokoberget
GU·BULL.:2:.1993
section (Fig. 4). As a compa rison, the grain- size distribution of the matrix-richJerstac Till from the Lillehammer area is also shown in this figur e.
In Fig.5,grain-size parametersarecompa- red with Norwegian Quaternary sediments of know n origin (after Selmer-Olsen 1954). The Jerstac Tillsamples group within the till field, whereas the Roko berget samples indicate a complex depositional history. The silty clay from the Rokoberget interstadial sediments (sample 12) fallswithin the field for lacustrine sediments while the sand above (sample 13) mayhave severalpossible origins.Thegravel- lysand(sample 14)isclearly bimodaland the most proba ble explanation for thegravel.con- tent would seem to be ice rafting.
Minera logy
Bulk samples of the silty clay from Ro ober- get have been analysed by X-ray diffraction anddifferentialther malmethods.Semi-quanti- tativeevaluationindicatesthefollowing minera-
logical composition: 40-50% quart z. 20-30%
feldspar, 15·20% mica/iIIite, 10-15% chlor ite and abo ut 5% pyrite. This is a composition which resembles that of the Late Weichselian claysfrom the 0stlande area(L;!jg 1948,Sel- mer-Olsen 1977).
Geotechnical prope rties
In the section, the clay was very hard and probably partly dehydrated, a feature which addsto the uncert aintyof someoftheanalyses. The plastic properties gave the following values:Liquid limit, WL= 29%;plasticlimit, Wp
=
23% ,and plasticityindex,Ip=
WL- Wp=
6% . The sediment was very sensitive to small changes in water content. The natural water content was measur ed to 13-14% . Although thismay be too low due to drying,it isbelie- ved to be below the plastic limit.Thegeotechnicalpropertiesof theclayindi- cate a sediment with a clay fraction of low activity (as confirmed by the mineralogical analysis)and with some organic material.Tre- atment with H,O, gave no weight loss, while the loss-on-ignition at 1000°C was abo ut 5%
and at550°C 2-3% insamplesfrom thelower- most part of the sediments(sample no. 12 in Fig.3).
NGU·BULL.424.1993 Sub-tillsedimentsatRokoberget5
CLAY SILT SAND GRAVEL
100
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Grain size d(mm)
R12-Sill y clay R13-Sa nd
R
14- Grave lly sand J-Jo r sl adlillFig.4.Grai n-size distribut ion fo rselected samples fro mthe Rok ob er getsection.Forlocation ofsamples.see Fig.3.The othercurves showgrain -size distribu tion for 33 samples of the fine·grained basalJer stacTill fromthe Lillehammerarea. About 50%ofthe curves are situated in theshaded area (afterOlsen 1985a).
3.0
0.5 Cl)
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- -...:" 1dune s ~sedimen ts ......0.00 2 0.0 0 6 0.02 0.06 0.2 0.6 2 6 20 60
Median grain size,Md
=
d5 0 (m m)Fig.5.Grain-siz e datafro m samples in the Ro koberg et secti o n and the fine-grained basal Jersta o Till in the l.illeharnmer area.compared with sediments of known origin(after Selmer-O lsen 1954).
6 «ereRok oenge n.Lars Otsen&Synove F.Selvik
Pollen
Pollen analysis was carried out on five diffe - rent samples (Table 1), four(9 - 12)from vari- ous depths in the sub-till sediments and one (8) from the bottom part of the lodgement till overlyingthe waterl ainsediments(Fig. 3).The samples were treated with HF and acetolysis as describedin Fcegri&Iversen(1989).Lycopo - dium tablets were added to allow absolute pollen analysis (Stockmarr 1972).
The pollen concentration was high (9x10' - 2x 10' pollen grains/cm') in the waterlain sedi- ments, but clearly lower (5x10' pollen grains/
cm') in theoverlying till.Due to relatively poor preservation, the frequency of unidentified pollen grains ishigh in all samples. Althoug h only a small number of pollen grains were analysed (LP=106 at most), the result s are interesting enough to warrant some com- ments. A difference in composition of the pollen flora canbe observed from the tillsamp- le (8) to the waterlain sediments (9 - 12), but thesparse nature of the pollen inthe tillsamp- le does not allow an ecolog ical interpr etation. The composition of the pollen flor a is very similar in the samples from thewaterlainsedi- ments (9 - 12). Herb pollen predominates, withgrasses(Poaceae)and sedges(Cyperace- ae)being themost frequentlyoccurring pollen
Table 1.Pollendata.
NGU-BUL L.424.1993
types, and wormwood (Artemisia) present in the four samples (Table 1).
The predominance of herb pollen types is so clear that the pollenflorais interpreted as reflecting a grass-dominated,treeless vegeta- tion.The compositionof the herb pollenflora may indicate an arctic-alpine,tundra-like veg- etation ,but the possible existence of seasho- re conditions must also be considered.
Mac rosc opic plant remains
The contents of macro scopic plant remains were examined in samples taken from th'ree differentdepths withinthesiltyclay,and from the bottompart of the overlyingtill.Thesamp- les,each c.50 cm' in volume,corresp ond to four of the pollen samples indicated in Fig.3 (8-11). The material was washed with water and sieved through metal sieves with mesh width s from 2 mm to 0.125 mm, and exami- ned in a low-power microsco pe.
Smallamounts ofunidentified plant remains up to 4 mm in length were present, and in one samplefrom the silty clay there were frag- ments of at least five,different,but notidenti- fied,moss species .
Rokobergetinterstadial sediments
AP N AP TOTAL
SPORESC)
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'~E ro
'"
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(L 0 CO ::J Cl) CO 0. o DJ
Qi E CO 0 ::J c .I? Cl) Q.. <t 'c
2 Q.. e CJl "5 ~ CJl 0. U Qi e ::J :g CO CO
Cl> ::J ::J 'c .~ CO 0. Cl) C- Ol c ro :g <t Z :::J Qi s: '6
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c Q5 0 c ::J ro 0 '>, ~en
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2
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8 14 5·10 6 1 1 1 5 57 7 36 2 X
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9 87 9'10 1 3 1 9 2 2 69 6 15 79 1
10 1062'10' 1 4 3 1 43 18 1 1 33 85 60 31.5 5 1
.
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12
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NGU·BULL.424.1993 Sub-tillsedimentsatRok oberget 7
Discussion
Dating with
14C-AMSTwo organic-bearing silty clay samples from theinterstadialsediments atRokoberget have beenradiocarbon- AMSdated in Utrecht,the Netherlands , withthe following results:
The first sample (UtC 1963) isfrom theupper- most part of the sub-till sediments (sample 9 in Fig.3),andthe second from thelowermost part ofthe sediments more than 1 metre be- low the first (sample 12).The oBC-values, at c. -30 °/00, are suggestive of a terrestrial ori- gin,andindicate that any possible contaminati- on of older or younger carbon with biased isotopicfractionation shouldbefairlylow.From the fieldwork carried out on the existing sec- tion it is not possibleto say for sure if there areany unconf ormitiesin the sediments betw e- en the sites of the dated samples.
Extent and genes is of the sub-till sediments
Blue-gre y sub-till sediments or blue-grey till are presentbelow the sandy cover -till (general- ly ablationtill) in several localities at Rokober-
get. 'Blue clay'was observedbythe first aut-
hor during well excavation in 1950. Earlier, such clay had also been encountered during excavation for a farm building,where digging proved to be very difficult.The highest of the- selocalities at Rokoengen (Fig.2) lies at about 275 m a.s.I.
Other similar localities have also been recor- ded (Fig. 2).Blue-grey sub-till sediments with the local name 'blakvabb' is reported from a smallriver near Veidebor g. According tolocal tradition, 'blakvabb' was also excavated at Prestlekkenand used as construction material for the old church on the top of Rokoberget.
The relative ages of the sediments from the different sitesare unknown at the presenttime.
The blue-grey clay, or a till with a similar matrix,may thus be prese nt belowthe youn- ger tilloveranarea ofatleast 1 km' atRoko-
berget.The lower boundary,towards the north and northeast, is partly marked as a fairly steep slope (Fig.2).The change in sediment thicknessis striking,froma thick cover inthe higher areas to sparse and very coarse-grai- ned sediments in the lower areas around the lake Hokosjeen. This is also reflected in the vegetationwhich shows a change from amix- ed spruce and deciduous forest inthe higher areas to pine forest in the lower areas.
The sediment distribut ion maybe explained by a stoss-side accumulation or stac king ef- fect due to the south-southeastward moving ice which subsequently rework ed or defor- med, com pacted and thereby prese rved the older sediments on the upper southern side of the Rokosj0en depression.Later ice move- ments towards the east-southeast and meltwa- ter erosion and transport may have removed the old fine-grained sediments in the lower partofthe E-Wtrending Hokos jeen depressi- on.
The distribution of the blue-grey deposits towards the south and west is more diff icult to evaluate, but they may be present below ablation till in areas witha thicksedimentcover.
Thesituationat Rokoberget shows a strong resemblance to the Lillehamme r area which has been investigated in more detail (Olsen 1985a, b), and where clay-rich blue-grey till (the Jerstad Till) is found to dominate below a depth of about 2 m over most of the area.
It seems evident that the matrix in this blue- greytillmusthaveits originindepositsresem- bling the Rokoberget sub-till sediments.
Ourinterpretation is that the silty clay in the Rokobe rget section was deposited in a Midd- le Weichselian interstadial period where the local water-level in the Rokosjee n basin was raised by at least 25 m compared with the presentlake-level (Fig.2). One possibledeposi- tional environment could therefore be that of a lake dammed between ice-lobes in the Mj0- sa basin in the west and the Glomma valley in the east (Fig. 1).
If the rise in water-level occurred without
ice-damming and with no connection to the
sea,thenthe thresholds bothin thewest and
in the east would have had to have been
much higher than today.The lowland both to the west and to the east of Roko sjeen impli- es,in thiscase,that a post-Ro kobe rget inter- stadial erosionto a depthof atleast 15-20 m below the ground surface must have occured over a wide area.
AgeinI4C-years B.P.
33,800+800/-700 47,000+4000/-3000 -29.6
-30.4 UtC 1963 UtC1962 No.
8 KareRokoenqe n.Lars Olsen&Syno veF.Selvik
Although some east- or west-directed melt- water erosion is a probable explanation for the rather abrupt lower limit of thethick sedi- ment cover along the southern side of the Rokosj0enbasinarea (Fig.2),wefind it more likely that thethres holds were notmuch high- er during the Rokoberget interstadial period than today. Therefore, we suggest that the silty clay sediments in the lower part of the Rokoberget section were deposited in a shal-
low fjord embayment, even though a marine
origin is still to be proven.
Thesilt,sandand gravelly sandin the Roko- berget section (Fig. 3)could be ofglaciofluvial origin, deposited in a fjord or lake environ- ment. Aeolian transport should,however,also be considered for the sand and sili fraction, while the gravel component could have been ice rafted.
Regional correlations and age
Follow ingthe 1980 conference on the Weichse- lianinthe Nordiccountriesbefore 15,000years B.P., Mangerud (1981a) stated that the two most striking points were, firstly, the large number of Earlyand Middle Weichselian sites thathad beendiscoveredandstudied in deta- il during the last decades;and,secondly,the problems associated with dating and correla- ting these sequences with each other or with sequencesoutside Norden.Duringthedecade after the conference,severalmore sites were discovered (Mangerud 1991), especially in North Sweden and North Norway (Laqerback
& Robertsson 1988,Olsen 1988,1989a,b)and several attempts at dating have been carr ied out. Theproblems withdatingand correlations are,however, still present.
The dates obtained from the Rokoberget interstadialsediments clearlyexclude the pos- sibility of a Late Weichselian age. If the sedi- ments described in this paper are from an uninterrupted ice-free period,then theywould, within plus/minus one standard deviation of the datingresults,represent a MiddleWeichse -
lian inter s tadi al la s tin g for at le a st 10-15,000
years in the Rokoberget area.
The correlations between the sub-till sedi- ments of the Rokoberget sectionandthesedi- ments in the coastal areas (Andersen et al.
1981, Mangerud 1981b, Sejrup 1987) and in areas outside Norwa y(Lundquist1981,Lager- back & Robertsson 1988, Hirvas 1991) are veryuncertainat presentand willnotbediscu s-
GU -BULL. 424.1993
sed here.Wewillalsoexclude from thediscus- sion the stratigraphic information from e.g.
Hardangervidda (Vorren & Roaldse t 1977), Setesdalen (Blystad 1981), Trencetaq (Berg- stre rnunpubl.,Olsen unpubl.),Nor dland (Lau- ritzen 1991,Olsen unpubl.)and Finnmark(01- sen 1988, 1989a,b), and concentr ate on the more nearby localities east and south of the present watershed (Fig. 1).
Thestrongestlithologicalresemblanceto the clay at Rokoberget is perhaps shown by the overcon solidated clays at about 260 m a.s.1.
in Numedal described by Roaldse t (1973, 1980). There too,clay sediment s were fou nd belowa younge rtill.Despite a close examinati- on of the sediment s no fossils were found. Roalds et (1980), however, favoured amarine origin for the claysbasedon physicalparame- ters.
East of thelake Mj0 sa,some 30 km north- west of Rokosj0en and at ca. 395 m a.s.l., Helle et al. (1981) reported a sub-tillinterstaci - aldepo sit(theBrumunddaleninterstadialbeds) with a peat horizon that was submerged dur- ing deposition of an overlying, more than 2m-thick,silt sequence.The sub-tillsediments have afull interstadial,cold-warm-cold,pollen signature. It is possible that the upper silt sequence in theinterstadialsedimentsat Bru- munddalen was depo sited in a small ice- dammed lake. If the Roko berge t interstadial sediments observed so far represent onlythe upper and last part of an interstadial period, then they may correlatewith the Brumunddal interstadial. Otherwise, such a correlation is not likely because the optimal conditions du- ring the Brumunddal interstadial, as indicated by the pollen content, imply a warm Early Weichselian interstadial climate (Helle et al.
1981),much warmerthanthatindicatedby the preliminary results from Rokoberget.
In Astdalen, some 45 - 50 km northwest of Rokos j0en (Fig. 1), Haldor sen & Rappol (1990)reportedastrat igraphycomprisinginter- stadial silty clay overlain bytills and waterlain sediments.The area is situated at more than 700 m a.s.1. The ice movement phases repre- sented above the interstadial sediments in Astdalen show a one-to-one correlation with the three youngesticemovementphasesfo und intheLillehamm er area(Olsen 1985b).Allthe- se phases are younger than the regionally distributed, fine-grained,blue-gr ey till (the J0r- stad Till).This impliesthat also the interstadi- alsedimentsinAstdalenmay beyoungerthan
NGU-BULL.424.1993
this till. If not, then the situat ion may have been that the glacier that deposited the blue- grey Jerstad Tilldid not completelycover the Astdalen area or, alternat ively, that the area may have been situated relatively unaffected between ice-lobes or perhaps between ice- streams within this glacier.Haldorsen & Rap- pol(1990)suggested that theinterstadial sedi- ments in Astdalen may have been deposited
in an ice-dammed lake, probably during a
periodof deglaciation.Thisphase may correla- te with the newly recorded ice-free period between the Jerstad Tillevent and the previ- ous stadiai represented by the underlying, coarse-gra ined, blue-grey Mesna till (Olsen 1985b and unpubl.).The sub-till sediments in the Lillehammer area are "C-AMS dated to 31.5,32 and 36 ka,whichgivethe Jerstac Till a late Middle to early Late Weichse lian age, as suggested by Olsen(1985a,b).The radiocar - bon dates from the interstadial sediments in the Lillehammer area suggest a possiblecorre- lation with the younger part of the Rokober- get interstadial sediments.
Radiocarbo n dates from Brumunddal (Helle et at.1981) and Astdalen(S.Haldorsen,pers.
comm. 1991) have given infinite ages (>50,
>48 and >47 ka), and a preliminary UlTh dating of peat from Brumunddalen yielded c.
60 ka (S.E. Lauritzen & N. Rye, pers. comm.
1991).The UlTh date of thepeat may repre- sent the approximate age of the time when the peat becamea closed system with regard to exchange ofU withthe surroundingground- water. This situation does not necessarily correspond with the age of the peat; it may well be that the closing time correspo nds better with the compact ion phase during the subsequent glaciation. Thus, the UlT h date could well be indicating an approx imate age for this ice advance and, in addition,a mini- mum age of the peat.Helle et at. (1981) sugge- sted for the Brumunddalen interstadial a corre- lation with the regional Brerup interstadial(c.
100 ka).This,and even a correlationwith the regional Odderade interstadial (c. 80 ka), will therefore possibly be in approximate concor- dance with the UlTh date. If this is correct, the"C-AMS datesof the Rokoberget interstadi- al would indicate that it is younger than the Brumu nddalen interstadiat.
A correlationbetweentheRokobe rgetinter-
staola l
sediments and thelower sub-till sedi- ments in Astdalen is also difficult. Both the interstadial sediments and the younger ice-Sub-tillsedimentsatRokoberget 9
phasehisto ryare close ly comp arablein these areas.The pollensignature reported from the interstadial sediments in Astd alen (Haldorsen
& Rappol 1990) also resembles that in the
Rokoberget interstadial sediments. The Ast- dalen locality, however, is locat ed some 470 m higher than the Rokoberget section which makes the correlation more uncertain. The radiocarbon dates from Astdalenalsoindicate that this interstadialis older than theRokobe r- get interstadiat.
In Gudbrandsdalen,extensive studies have been carriedout on the waterlainsub-tillsedi- ments (Bergersen& Garnes 1971,1981,Ber- gersen &Thoresen pers.comm.1991). Based on some 20 TL - dates, most of them with ages of about 50to 70 ka but ranging from35 - 40 ka to 80 - 100 ka and even more, it se- emslikelythat the sediments from the defined Gudbrandsdalen interstadial (Bergersen &
Garnes 1971,1981) belong to more than one ice-free period. Bones of mammoth found in the sub-till sediments have been radiocarbon dated to infinite ages (>40 ka, Bergersen &
Garnes 1981); and some prelimin ary UlTh datings of mammoth bones from Gudbra nds- dalen indicate aqas of about 60-80 ka (S.E.
Lauritzen pers. comm. 1991). This seems to fit well with several, but definitely not all, of the TL-ages from the sediments.
Some of the reported sub-tillsediments from the Gudbrandsdalen interstadialmaycorr elate withthe Rokoberget sediments.Likely candida- tes are, for instance, the Favang and the Haugalia sub-till sandur sediments (Bergersen
& Garnes 1971, 1981). Both these localities
require a water-level in Mj0sa at about 220 - 240 m a.s.l., which probably would be the same for the corresponding sea-level at that time.The high sea-level may be explained by glacial isostatic depression,which in that case would have been of a magnitude comparable to but even greater than that during the last glaciation in thisarea,expressed by the post- glacial marine limit at c. 190 m a.s.1. in the Mj0sa basin. However,the tectonicbehaviour of the Earth's crust in this area may have been different during the Gudbrandsdalen in- terstadial than during the last deglaciation period. It is therefore not particularlyeasy to estimate the size of the ice-sheet which cau- sed such a huge depression of the crust. A forebulge effec t should also be considered if theinterstadial sandursediments weredeposi- ted as proglacialsediments during an advan-
10 KareRokoenge n.LarsOlsen& SyntiweF.Selvik
cmq ice-lobe event, as suggested by Berger- sen & Games (1981). It seems more likely, however,that these sedimentswere deposited during ice retreat phases,perhaps with some more orlessfloatingice-bodiesleft as melting ice-remnantsin the biggest basinsin the sout- hern Gudbrandsdalen area.
A sea-level about 220 - 240 m higher than today would have resulted in a direct marine incursion in the Rokosj0en basin area, inclu- ding the Rokoberget locality (Figs. 1 & 2). If there was a marine influence connected with the Rokoberget interstadial sediments, then the required base-level for deposition of the Gudbrandsda leninterstadialsedimentsexisted duringthe Rokobergetinterstadial(Bergersen
&Games 1981).Thiswould support a correla-
tion betwee n these interstadials.
The younger,possibly ice-dammed,intersta- dial sediments at Rokoberget are most likely younger than the sandur deposits at Haugalia and Favanq,and they may well correlate with the tillsdepositedduring the initial,valley glaci- er,ice-phase A,and even the first part of ice- phase B, in the established ice-phase model for 'the last ice age' in the Gudbrandsdalen region (e.g. Games 1978, Olsen 1985b). The newly recorded interstadial sediments in the Lillehammer area,situated betweentills repre- sentingice-phases A and B,respectively,sup- port this correlation. The ice movement that formedthe assumed drumlinwhere the Roko- berget section is located, may therefore be- long to the youngest part of ice-phase B,or to the ice-phase C that includes the last gla- cial maximum event and most of the Late Weichsel ianperiod.Both theseice-phases had south-directed ice movements in the Mj0sa region (Olsen 1985b).
The Sorperoa interstadialin Gudbrandsda len (Fig. 1) is defined and represented by wind- blown sand that has been TL-dated to about 40 ka (Bergersen et al. 1991).This interstadi- al may correlate with the younger part of the complex Rokoberget interstadial.
Based on "C-dates of gyttja remains below the sub-till clay at Grarnobekken in Folldal, these sediments may be younger than 40 ka (Thoresen & Bergersen 1983),and their pollen signature (Selvik unpubl.) resembles that of the Rokoberget sub-till sediments. Another correlationcandidate in thisnortheastern regi- on is that of the sub-till sediments at Fiskv ik in Rendalen (Thoresen & Selvik unpubl.).The Fiskv iklocality(Fig.1) lies about 85km north-
NGU-BULL.424,1993
northw est from the Roko berget section and at c.325 ma.s.1. However,thepollensignatu- re for parts of these sediments indicates a warm Early Weichselian interstadial climate similarto that for theBrumundda leninterstadi- al, and these sediments may therefo re also be olderthanthe Rokober get interstadial. One
"C-dating gave an age of about 46 ka (or more)and two TL-dates gaveages of 50 and 61 ka from these sediments (M. Thoresen, pers.comm. 1991).
Conclusions
Thesub-tillsediments in thedescribed Roko- berget sectionare disturbedandpartlyrewor- ked by ice, but still believed to represent an interstadial glaciolacustrine/lacustrine/marine sediment of regional import ance. The local water-level during deposition is assumed to have been at least 240-250 m above present sea-level. This should imply a connection to the sea,but in the Rokosjeen area themarine influence would probably have been ratherlimi- ted.
Two "C-AMS dates at c. 34 and c. 47 ka suggest a Middle Weichselian age for the Rokobergetsub-tillsediments,and theprelimi- nary pollen data for these sediments resem- ble thosefrom sites further north insoutheas- tern Norway,which are also considered to be of Middle Weichse lian interstadial origin. The most probable correlatives of the complex Rokobergetsub-tillsediments are the waterla- in sub-till sediments at Favang and Haugalia in Gudbrandsdalen(Bergersen & Games 1971, 1981),at Mesna and Starnpes letta in the Lille- hammer area (Olsen,unpubl.),and at Grarno - bekken in Folldal (Thoresen & Bergerse n 1983). Another correlation candidate is the interstadial aeolian sandat Sorperoa in Gud- brandsdalen, although this sand has been convincingly argued to be younger than the Gudbrands dalen interstadial sediments (Ber- gersen et al. 1991).
Correlationbetween alltheselocalinterstadi- alsseems to be quitepossibleifthe Rokober- get interstadial represents a long, 'multi-sta- ge',MiddleWeichselian,complex ice-fr ee peri- od,as suggested bythe dates and the appa- rently complex depositional history.
Reconstruction of the regional distribution of such glaciolacustrinellacustrine or marine sedimentsisimportant asthe subseq uent91a- ciation shave rework ed and/or overco nsolida-
NGU·BULL.424.1993
ted the sediments and formed new deposits with special geotechnical and geological pro- perties. This includes very compact silt and clay deposits andclay-r ichtills withproperties different from those of the younger tills for- med duringLate Weichse liantimein theinland areas far from the coast.
Many questions still remain unanswered regarding the number,time and extent of the differentWeichselian interstadials.New dating methods and careful examination of old and new sections willhopefully reveal more about theWeichse lian historyin the comingdecades.
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Manuscrip treceived August 1992; revised typescript Desember 1992; accepte d January 1993.
