NGU-BULL 435, 1999 - PAGE 23
Glacial geology, deglaciation chronology and sea-level changes in the southern Telemark and Vestfold counties, southeastern Norway
BJ0RN BERGSTR0M
Bergstrom, B. 1999: Glacial geo logy, deg laciation chronology and sea-level changes in the southern Telemark and Vestfold cou nties, southeastern Norway. NorgesqeoloqiskeundersekelseBulletin 435,23-42.
A revised and more detailed deglaciation chronology is presente d in the southern Telemark and Vestfold, southeast ern Norway, based mainly on informatio n and results fro m mapping projects on th e superficial deposits carried out by NGU during the last 20 years. The deglacia tion of this area started about 13,000 years B.P. when th e receding ice margin was grou nded along the coast. Distally to Jomfruland, submarine moraine ridgesdeposited in fron t of the ice may correlate wit h th e Goteborg Morai ne on th e west coast of Sweden dated to 12,900-12,600 years B.P.The ice mar gin retreated some distance prox imally to the Jomfruland island before 12,240 ± 80 years B.P. A glacier advance occurred at 12,200-12,000 years B.P. and the ice margin reached Jom fruland, probably correlating w it h the Tjeme Hvaler ice-marginal deposits in the outer Oslofjord area. The posit ion of th e Slagen-Onsoy ice margin about 11,400 11,200 years B.P. is tentatively reconstr ucted westwa rd from Oslofj ord . During t he late Allerod/early Younger Dryas (afte r 11,300 years BP.), the glaciers readvanced and in th e Kragero area the ice front moved at least 10 km, and most likely 17-18 km to form th e distinct Ra (Younger Dryas) moraines at abo ut 10,800-10,600 years B.P. The submar ine Ra moraines have been mapped by seismic profiling and show a conc ave calvin g ice margin in th e outer Langesundf jord area (the Langesund Channe l) du ring th e deposition. The Eidanger ice-marginal deposits are dated to 10,400 10,300 years B.P. and correlated with th e As mo raines. The Geiteryggen ice-marginal depositsare complex and ind i cate a marked halt and readvance ofthe ice margin 10,100-10,000 years B.P. correspon di ng to the Ski mo raines in the Oslofjord area. After the Geiteryggen event, the ice margin receded rapidly and the Akkerhaugen (9,800 years B.P.) and Nordag utu (9,700 years B.P.) ice-marginal depo sits were formed. An equidistan t shorel ine diagram has been con st ructed based on the upper marine limits in the studied area. A modified shorelevel displacement curve from the Kragero area is presented.
Bjorn Berqstte m, Norgesqeoloqiskeundersekelse, N-7497 Trondheim, Norway.
Introduction Quaternary geological mapping carried out by NGU (Geolog ical Survey of Norway) during th e last 20 yearsin Vestfol d and the sout hern part ofTelemar k, south west of Oslo (Fig. 1), has provided new informat ion abou t t he Late Weichselian and Early Holoce ne deglaciation and shore level chang es in t he region west of Oslofjo rd (Berqstrom 1984, 1985, 1988, 1995c, 1997 and in press, Olsen & Lowe 1984, Bargel & Lien 1990, Serensen et al. 1990, Klakegg 1991, Klakegg & Serensen 1991, Berqstrern et al. 1992, Dahl et al. 1997). Reflection seismic profiling in th e Langesundsfjord/Lan gesund Chann el has provided new information about th e submarine ice-contact depo sits which correspo nd to th e terrestrial Ra moraines. Based on t his info rmation and result s from addit ional studies in th e coastal areas, it is now possible to present a more detailed and somewhat revised deglaciation chrono logy from t his region. Correlat ions have te ntatively been made wi th the classical Oslofjord regi on w here a detailed chrono logy has been presented by Holtedahl (1953) and sub sequently revised by Serensen (1979, 1983, 1990b, 1992) and ~ The icedivide inSouthNorway H Hardangervidda ~ during theWeichselianmaximum Klakegg & Serensen (1991). Recent studies of th e sho relevel ..a....a....&.. Icemarginduringthe '--_-----'I ~e~~~a~ during the Weichselian Weichselian maximum displacement, based on radiocarbon datings of isolation (c. 23,000 yearsBP) contacts in basins and molluscs in marin e depo sits (Berg '--_-----'I ~a~~~~s during theWeichselian --- Ice marginduring st rern 1997, K. Henningsmoen, pers. comm. 1997), ind icate theYoung erDryas that th e previou s shorelevel curv es constr ucted fro m th e Fig. 1. The exte nt of the Scandinavian continent al ice sheet dur ing the Kraqere area by 5tabell (1980) and Henningsmoen (1979) Late Weichselian maximum and the Younger Dryas. The investigated requi re mo dification. An equidistant shoreline diagram has area (SW of Oslo) is show n by a small bo x. NGU-BULL 435 , 1999 - PAGE 24 BJ0RNBERGSTR0M
been reconstruct ed on the basis of mar ine limits in the promontories, sounds and bays, together wi th narrow inlets mapped areas and previously pu bli shed data from the Oslo projecting inland into the low-relief, undulating terrain. This fjord region (Serensen 1979). coastal type is classified as a fjard coast (Klemsdal 1982). The mature paleic landform on the main land is a peneplain that slopes very gen t ly tow ards t he sea.In the Precambrian areas Bedrock and geomorphology west of Langesundsfjord, the land surface iscontrolled by t he The bedrock in t he region west of Oslofjord (Fig. 2) consists 600 Ma old sub-Cambrian peneplain, wh ich is on ly slight ly of four main units (Dons&Jorde 1978, Larsen et al. 1978). Pre modified by glacial erosion. However, glacial erosion has cambrian rocks, mainly gne iss, amphibolite and quartzit e, been activ e along zones of weakness, resulti ng in a number predominat e in th e western part of the region . Vendian of narrow fault- and joint-valleys in the bedrock. The two (EocambrianJ rocks, consist ing of carbonatites and feld domina nt trends are c. NE-SW(the 'Caledonian' tre nd) and c. spathoid-rich rocks, occur in th e Fen area at Nordsje, nort h NW-SE, pro duc ing an uneven coastline and a dissected west of Skien. Cambro-Silu rian rocks, mainly sandstone, shale top ography inland (Fig. 3). and lim estone, are present in a narrow belt in the Skien In Skagerrak, parallel to the coast of Telemark , a deep nar Porsgrunn-Langesund area. Permian rocks, mainly pluton ic row trench, th e Norwegian Channel, curves around the (larviklte, alkali syenite, ekerite) and volcanic rocks sout hern coast. It probably originated asa combined result of (rhomb-porphyry, basalt), cover th e eastern part of the tectonics and glacial erosion. From the mouth of Lange region in th e Oslo distr ict. sundsfjord a subm arine chan nel, the Langesund Channel , The topography is noticeably influ enced by th e bedrock, extends do wn to and joins the main channel at a dep th of and in particul ar by the major faults and fractures. The major nearly 600 m.The course of the Langesun d Channel isconsid features are t he ice-eroded (U-shaped) valleys and fjordssur ered to be largely struct urally con trolled, situated along the rounded by a hilly, undulating , high land terrain dissected by bou nda ry bet wee n the Precambrian rocks to the west and narrow fracture-con trolled valleys. th e Permian rocks in the Oslo Region to the east (Holtedahl The coastal area is dominated by exposed bedrock and 1986)(Fig. 2). The channel is strongly influenced by glacial characterised by a coastline wi th numerous skerries, islands, erosion.
Methods Field mapping Quaternary geo log ical mapp ing in scale 1:50,000 fo llowing the standard methods used by NGU has been carried out in the stud ied area. The maps and the geo log ical information acquired dur ing th e fieldwork have been of fundamental importance for the present st udy. The correlation of the ter minal mo raines and glacioflu vial ice-contact deposits and the reconstruction of ice-front oscillations are main ly based on mor pholog y, lithostratig raphy, ice-flow patterns and shoreline stud ies.
Radiocarbon dating The radiocarbon datings have been carried out on mo lluscs from marine deposits and on silty gyttja from the bottom of lacustrine basins. Dating by th e conventional radiocarbon 59' method was carried ou t at the Radiolog ical Dating Labora tory in Tron dh eim (T). AMS (Accelerator Mass Spect romet ry) dates were produced at the R.J. Van de Graaff Laboratorium , Utrecht (UtC) and the Tandem Accelerator Laboratory, Uni
o 25,- versity of Uppsala (Tua). The dates of the marine molluscs l===t==*==f- t----.J have been correct ed for a marine reservoir age of 440 years Skag er r a k (Mangerud & Gulliksen 1975). All ages referred to in th is art i
Permianplutonic rocks. Precambrian rocks. cle are in radiocarbon years. larvi e. noremarete. granrte gneiss. grane. amphibolne Permianvolcanic rocks. Majorfautt rhomb-porphyry. basan Geophys ical measurements Fautt. prominentjoint Cambro-Silunansecments, limestone. shale o Cauldron Reflection (CDP) and refraction seismic pro fil ing and resistiv Carnon alJtJc rocks ity measuremen ts (pole-dipole configuration ) have been car (Fen area) ried out in order to obta in more information about the Fig. 2. Simplified bedrock map of the region west of Oslofjord. Aft er stratig raphy ofth e Ra mora ines. Submarine reflec tion seismic Dons & Jorde (1978) and Larsen et al. (1978). prof iling was carried out in th e outer Langesundsfjord and BJ0RNBERGSTR0M NGU-BULL 435, 1999 - PAGE 25
Fig. 3. The Kragero coastal area is dominated by exposed bedrock (Precambrian) and characterised by num erou s skerries, islands, sounds and narrow inlets int o th e low-lyin g mainland . The land surface is cont rolled by th e sub-Cambrian penep lain. The prominent Jomfruland island in the foreground is an eme rged part of th e Ra moraines and is clearly different in character from th e interio r areas. View toward s WSW. Phot o: Fjellanger Wid eme AS. th e Langesund Channel by th e NGU research vessel 'Seisma' In the southern coastal areas below th e marine limit, in 1990. The acoustic source 'Elrna', which is an elect romag especially in the Precambrian area, the lack of superficial netic sound source operating at frequencies between 240 deposits is very striki ng, except in areas where marked ice and 1200 Hz, was used. Sediment thickness is presented in marginal deposits were for med during th e deglaciation . The ms (millisec onds) two-way travel time (TWT) or in metres Ra mo raines are the most prominent of th ese deposits. Some based on an estimated acoustic velocity of 1600 ms'. of th e margi nal deposits in locationsexposed to the sea, such as Jomfruland (Fig. 3) and M01en, have been heavily washed by waves and currents during the postglacia l uplift and cov Superficial deposits ered by wel l rounded boulders and cobbles. Fine-grained Most of the area above the marine limit is dominated by marine sediments, up to 60 m thick, dominate in the main exposed bedrock or a thin and discont inuo us cover of till. A valleys and the large basins. The largest and thickest gla continuous and locally thi ck cover of till is fou nd in a few val ciofluvial deposits were formed along the main drai nage leys and mainly on slopes facing the direction of th e ice routes and built up as marine deltas, in particular along the movement. Glacial transport of till materia l cover a distance ice-marginal zones, e.g., the Geiteryggen ice-marginal more than 10 km has been recognised (Berqstrern 1984, deposit. 1988). The weathering materia l is closely related to the type of bedrock. Some rock-types are more susceptible to weat her- NGU -BULL 435 , 1999 - PAGE 26 BJ0RNBERGSTR0M
ing processes than others. The products of the Cambro -Silu rian shales and limestones are mai nly fine-grained, while mat erial weathere d from Permian basaltic rocks normally consists of silt and fine sand (Berqstrom 1984, 1995c). The weathering zones in the Permian larvikite are characte rised by coarse sand and gravel and pronounced 'core-stone' development (Serensen 1988). The carbo natites in the Fen area are covered by rust-coloured weathering material, mainly of sand and silt. Most of the superficial depos its wer e form ed during the Late Weichselian and Holocene. Older sediments have been 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., w hich is 75 m above the highest postglacial marine level in th e area. The clay sediments were interpreted to have been deposited in a marine environment, based mainly on their Ce-deficient lan thanide abundance patterns, and they are considered to be of Middle Weichselian age and correlated with th e Sandn es Interstadial. During the geological mapping in th e area, a radiocarbon (AMS)dating ofthe sub-till organic-bearing clay o 25·,., at Rundhaugen (Roaldset 1980, Figs. 2 and 3) gave an age of ~ - t -t=-+- t=i 32,000 ± 300 14C-years B.P. fo r th e NAOH-insoluble fractions (UtC-4728, Van de Graaff Laboratoriu m, Utre cht ). This indi cates a late Middle Weichselian age and support s Roaldsefs ----- + YOUllQel ice flows . converglnfJ to't"o'a'ds SSWIn the coastal a-eas (1980) assumption ofa correlation wi t h the Sandnes Int ersta ----+. Early Younger Dryas Ice flows . c " .000·10600yearsBP dial. This major ice recession about 30,000-40,000 years ago Youngest ice-lIowdirections has been recorded at many places in Norway and t here appears to have been a nearly complete deg laciation of the Fig. 4. Reconstruc tion of the ice movement sdu ring the Late Weichseli an. country during th is period (Olsen 1997). The fina l deg laciation was characterised by ice move ments that were stro ngly dependent on the topographical Ice-flow directions conditions and the ice flows converged towards the larger The oldest ice mo vements dete cted in th e region were valleys and fjo rds. directed to wards the south (Fig. 4). The main ice divide dur ing the Late Weichselian maxim um, according to Vorren (1977), was situated at a considerable distance east and Late Weichselian deglaciation southeast of the watershed in Hardangervidda and the cen The Late Weichselian ice sheet had its maximum extens ion t ral part of eastern Norway. Nesje et al. (1988), however, sug about 23,000 years B.P. (Fig. 1). During the deglaciation, the gested a low-gradient, po ly-cent red ice sheet with the main ice sheet th inned and the glacier started to break up. In ice divide located close to th e wate rshed. Skagerrak, the ice stream in the Norwegian Channe l broke up In th e coastal areas, glacial striae turning towards SSW at approximately 15,000 years B.P. and the ice margin was indicate a convergenc e of th e ice streams into the deep Nor established along the Norwegian coast (Longva & Thorsn es wegian Channel where th ey joined the 'Skagerrak glacier', 1997). Due to further thinning and calving, the margin t he major ice flo w from th e inner part of Skagerrak (Longva & ret reated landwards and reached the outer coastal areas Thorsnes 1997). around 14,000-13,500 years B.P. The ice recession occurred When the glacier in th e Skagerrak finall y started to break more slowly and partly stopped whe n the ice grounded up, the ice flo w in the area ofthe Telemar klVe stfol d coastline along the coast. In the outer part of the Kragero area, paralle l gradually turned towards the south -southeast and south submarine moraine ridg es occurring distally to the Jomfru east. An active ice dome was regen erated on th e sout hwest land island (Figs. 5, 7, 14)(Holtedahl & Bjerkli 1975) we re ern part of the Hardangervidda mount ain platea u, from probably formed in front of the ice w hen the margin which southeasterly directed ice streams flowed towards the grounded in water dep ths of about 200 m. Radiocarbon southern Telemark and Vestfo ld regi ons and gave the early dates from Jomfruland ind icate that this occurred before Younger Dryas glacier advanc e at 11,000-10,600 years B.P. 12,240 ± 80 years B.P.(p. 36). These mo raine ridges may cor (Bergstrom 1995b). The ice flo ws were mostly not deflected relate with the Goteborg Mora ine on the west coast of Swe by the local topography , but th ere was a weak convergence den, which is dat ed to 12,900-12,600 years B.P., and represen t to wards th e main fjo rds and valleys. a pronounced standstill in the ice recession (Berglund 1978, Passe 1986). BJ0RN BERGSTR0M NGU-BU LL 435, 1999 - PAGE 27
\', ,\ \ I1I
o (f) o ~ o...... a... o o
." .".
Skagerrak o 25 km I I
Ice-marginal deposits The Ra moraines Ice-recession lines , tentative correlations The submarine Ra moraines The tentative Allemd line of ice recession The Geiteryggen ice margin (the Slaqen-Dnsey ice margin)
Fig. 5. Ice-m arginal de po sits and ice-recession lines in th e region west of Oslofjo rd. (D) Dalsvatn, (G) Gorn ing en,(H) Hze rland sdalen, (Ho) Hogstad, (S) Sundsmoe n. NG U-BULL 435 ,1999 - PAGE 28 BJ0RNBERGSTR0M
The Tjeme-Hvaler ice-marginal deposits Corresponding ice-margina l depos its and strat igraphical After the format ion of t he subma rine moraine ridges, the ice evidence ofglacier advan ces have been found in many other margin retreated some distance pro xima lly to the Jomfru areas alon g the coast of Norway, sugge stin g t hat a region al land island (Fig. 6). A radiocarbon date of a mo llusc fragment clima ti c deterioration occurred when the Tjome-Hvaler ice in th e boulder clay at Jomfruland (Fig. 16) indicates that the marginal deposits were formed during the late Bellinq Chron margin retreated beh ind Jomfruland as early as du ring the (12,200-12,000 years B.P.)(Andersen 1968, 1979, Mangerud late B011in g Chron , before 12,240 ± 80 years B.P.Then th e gla 1970, 1977, 1980, Follestad 1989, Berqstrern et al. 1994, FolI cier adva nced, but how far th e ice marg in act ually reached is estad et al. 1994, Reite 1994, Berq strorn 1995a, Sveian & Solli uncertain. Most likely t he margin reached th e Jomfruland 1997, Olsen & Riiber in press). island, due to the fact that the radiocarbon-dat ed mo llusc, In Bohuslan on the west coast of Sweden , the Tjeme-Hva picked up by the ice during the advance, w as incorporated in ler ice-margi nal deposit s have been corre lated wi th the Troll the till (boul der clay) and deposited on t he island. hattan Moraine (Berglund 1979, Serensen 1979). This event may cor relate with the formation ofthe Tjerne Hvaler ice-marginal deposits (Fig. 5) in the outer Oslofj ord The Allerod ice-marginal deposits area (Serensen 1983, 1992). At Tjerne, correspond ing mar During the early degla ciation of the coastal area west of gina l mora ines are overla in by glaciomarine clays wi th shell s the Oslofj ord the ice margin was unsta ble due to intensive ofPortlandia aretica, radiocarbon dated to 11,975 ± 155 years calving, and the recession was highly dependent on the top B.P., which indica tes a minimum age for t hese marg inal ographical conditions, part icularly on wa ter depth.In the deposit s (Berqstrern et al. 1992). If this correlation bet w een deep outer Oslofjord th e calvin g continued and t he fjo rd t he Oslofj ord and the Kraqere area is correct, the Tjeme-Hva basin degla ciated rapidly. Westwards, th e ice mar gin ler ice-marginal depo sits were formed between 12,240 ± 80 grounded in shallo w water on the low, undulating coastal and 11 ,975 ± 155 years B.P. plain and th e ice recession took place more slowly. Towards
SE NW Norweg ian Channel Jomfruland Kragem Sannidal Drangedal Submarine I Dobbe ice-marginal t ridges '"c t 1 '§'" 1 o E co a: YrsB .P 10 20 30 40 ~ 50km
9695±95 0 10000 i:? 10000 ~ -----1O.280 ±90-~.../~_~ I- 10 .690 ±150 ~ 10.770 ±95 I- 10,825±70 11 000 . ---- I- 11 000 11.295±60 ~ --- I- 11.360±150 --:: ? 11,460 ±90 ------Covered by ice
12000 ? C?1:?~ 12000 12.240 ±80 EE __ ? o Lacustrine sediments, gyttja , , , &. Marine molluscs not overrun by glaciers 13000 ---- 13000 --- 18> Marine molluscs in submora ine sediments ---- EE Marine mo lluscs in till
10 20 30 40 50 km 14 000
Fig. 6. Time -distance diagram (SE-NW) from th e sout hern part ofTelemark (the Jomfruland - Kragero area) shows the glacier oscillat ions during the deg laciat ion . BJ0RN BERGSTR0 M NGU-BU LL 43 5, 1999 - PAGE 29
N A
M M01en G Geiteroya A = Ar0ya H = Helgero a
o 4km
.., The Ra moraines ...... Supposed position of the ice margin pi' The submarine Ra moraines Position of the ice-marginal ridges outside Jomfruland
200 Depth in metres , , , De Geer moraines
Fig. 7.The Ra moraine s in th e Langesund sfjord and adjacent marin e areas.The interpretati on ofthe sub marine moraine s is main ly based on the seismic refle ction profiles. The positi on of th e ice-marginal rid ges outside Jomfruland is based on Holtedahl & Bjerkli (1975, Fig. 1). Sites fo r seism ic profil es 1-5 are shown (Figs.8-12).
Langesundsfjord (Fig. 5) th e oldest ice-marginal lines, recon At about 11,400-11,200 years B.P. the glacier had receded structed from marginal depos its, approach each other. The to t he Slaqen-Onsey ice-marginal deposits (Serensen 1992), marginal deposits, however, are not easily separated due to wh ich can be morphostratig raphically correlated wit h the th e later wave and sea ice activi ty and erosion du ring th e Levene Moraine in Bohuslan (Berglund 1979, Serensen 1979). crustal uplift. The corresponding ice-marginal moraines can be followed westwards to Stokke (Fig. 5) where they are cut by the NGU-BULL 435, 1999 - PAGE 30 BJ0RNBERGSTR0M
younger Ra moraines and no continuation can be found. The Profi le 1 Slagen-Onsoy ice margin in the western areas was most likely ma.s.l. situa ted in a proximal position to the Ra moraine ridges NW500 ITVs 400·450 ITVs 500 ITVs 600.700 ITVs 600-650 ITVs 60 Road • Road • (Sorensen 1992, Bergst rom 1995b). In the Kragero area, the . ~ ! 40 ";;1"800 ITVs 6 margin recede d at least 10 km to Dobbe, and mos t likely 1750 ITVs 6 1750 nvs r~1 20 6 Till + + Bedroc k more than 17-18 km, behind the zone of Ra moraines, during o 4900ITVs 6 the Allerod Chron (Berqstrern 1993, 1995b)(Figs. 5, 6). o 100 200 300 400 500 600 700 BOO 900 1000 m
The Ra (Early Younger Dryas) ice-marginal Fig. 8. Seismic refraction profile 1 across the terrestr ial Ra morain es deposits nort heast of Melen, modified after Tonnesen (1991). Seismic velocities During t he late Al tered (after 11,300 years B.P.) and the early are given in m/soFor location, see Fig. 7. Younge r Dryas Chrons t he glaciers readvanced (Bergst rom 1995b )(Fig. 6) and the distinct Ra ice-marginal deposits were formed (Fig. 5). The Ra margin can be traced nearly continu Subma rine ice-marginal deposits can part ly be traced ously from Oslofjord and southwestwards to Mo len at Lange from the Me len area acro ss the mou th of the eastern part of sundsfjord as a low and broad ridge complex (Hansen 1910, Langesu ndsfjord, based on data fro m bathymetric maps Sorensen 1983, 1992, Andersen et al. 1995). From Molen, the (Sorensen 1985, 1992). moraine ridges cross the fjord and can partly be followed off the coast to Straholrnen and Jomfruland, w here they emerge Langesundsfjord above the present sea level (Fig. 7). Farther towards th e To obtain more information abo ut th e submarine ice-mar southwest the Ra moraine continues for almost 60 km as a ginal (Ra) moraines in the out er Langesundsfj ord, refle ction straight, submarine ridge parallel to the coast (Holtedahl seismic profil ing has been carried out by NGU (Fig. 7). 1989). In t he fj ord north of Melen, a seism ic profile, profile2 (Fig. In general, the Younger Dryas marginal moraines are very 9), has revealed 5-6 submarine ridges w hich seem to cor re distinct in Norway and can be traced and followed almost spond to the ter restr ial Ra moraines (Fig. 7 ). The mo st distinct continuously along the coast from t he Swedish border in the ridge (Fig. 9), outsid e Aro ya, is double crested and distally 30 southeast to the Russian border in the north (Andersen et al. 40 m high, consi sting of till underlain by distu rbed glacioma 1995)(Fig. 1). rine/ marine sediments. The disturb ed sed iments w ere mo st likely depos ited dur The M0/en area ing th e recession in th e Altered and then overridden by the The Ra moraine ridges in Vestfold were mostly fo rmed below glacier during the early Younger Dryas readvance. The outer sea level (marine limit 150-200 m a.s.l.) and are therefore usu most ridge, outside Geite roya, situated at th e edge of the ally covered with beach sediments. The composition of th e channel probably represents the maxim um ice-fron t posi ridges is variable, but generally they con sist of a clayey tion during th is readv ance. How ever, t his ridge is located in matrix-su pported diam icto n (boulder clay)(Sorensen 1990a, the most obvious position fo r the grou nd ing lin e of the gla 1992). In some areas the diamicton has a higher content of ciers during the deg laciat ion in the pre Younger Dryas time sand. Strat ified diam icton and glaciofluv ial deposits may also and, even t hough th is is not clearly seen in the seismic occur. recor ds, the ridge may be com posed of sediments fro m o lder The Ramoraines in the area east of Langesu ndsfjord are a stages aswell. Most likely th e ridge may co nta in sediments of complex of ridges (Figs. 5, 7). Eastof Helgeroa the Ra cons ists the Tjeme-Hvaler moraine s form ed during the Boiling/O lder of two parallel mora ine ridges (Bergstro m in pre ss). A seis Dryas Chro ns as described abo ve. m ic-refraction profile across these ridges (Tonnesen 1991) A profile in th e centra l part of the Langesund Channel, indicates a maximum thickness (depth) to bedrock of more profile 3,(Fig. 7) t shows a do uble ridge at 75 m water depth than 50 m (Fig. 8). The seism ic velociti es (1700-1800 m/s) (Fig. 10). The ridg e lies j ust outside the thresho ld into the indicate till deposits, except in the upper part w here shore ma in part of Lange sundsfjo rd and is the on ly mar gina l deposits (1-3 m th ick) overli e the ridges. Most of the ridges deposit in the tr ench betw een Geiteroya and Aro ya. This probably cons ist ofa clayey diamicton (boulder clay). Parts of infers that t here was a stab le ice margin over the thresho ld the Ra moraine ridges are rich in large boulders, erratics w hile it retre ated towa rds the no rtheas t south of Aroya, moved by the ice over cons iderable distances from different w here th e Ra mo raines split into several ridge s. The proxima l areas in southeastern Norway. part consists of two ridges w hich are about 25-30 m high on Towards Langesundsfjord the Ra moraines split into 4-5 the ice-contact side. They have steep proxima l and dista l ridg es w hich disappear below sea leve l (Sorensen 1992, Berg slopes of 10-12° and 13-15°, respectively. On the distal side strern 1995b). Molen is the largest of thes e ridg es. During the there are inclined refle ct ions dipp ing 5-10° sout hwa rds crusta I up lift th e ridges were highly influenced by waves and to wards th e deepe r part of the channel. They are interpreted currents and covered by coarse beach sediments. Along the as foreset beds de posited in a subm arine fan and most of seaward side of the Molen pen insu la there are numerous them are prob ably debris-f low and turbidit e deposits beach ridges consisting ofwell rounded stones and boulders. (Aarset h et a1. 1997, Lysa & Vorr en 1997). Till beds may be interb edd ed wi th t he foresets layers. At t he base of the distal BJ0RNBERGSTR0M NGU -BU LL 43S, 1999 - PAGE 31
slope complex, hu mmocky and chaoti c refl ections wit h tilted t he major ret reat in the Altered Chron. The lower unit (Unit 4) blocks ind icate slid ing and slumpi ng from the ridge. More is int erpret ed as prog lacial sedime nts depo sited during t he dista lly, subp arallel, low-angl e reflect io ns repr esent stratified deglaciation of th is area (Holte dahl 1986). glaciomarine sediments dep osited during th e early Young er West of th e Langesund Channel, in t he Steinrenn a basin, Dryas. An irreg ular surface and partly d istu rbed sediment lay t he seismic profile 4 (Fig. 11, location Fig. 7) reveals a th ick ers ind icate avalanche activity during t he deposit ion. belt of ti ll and disturbed glaciomarine/marine sediments (up In the southern part of profile 3 (Fig. 7) t here is a large to mo re than 50 m th ick), w hich can be followed southwest bedrock basin filled up with sedim ents (Fig. 10). The strati wards along the coast in the directi on of Jomfruland . The graphy corresponds wi t h t he general st rat igraphy fou nd in small, bo uld er-covered island, Danm ark, repre sents a minor previous pro files from other parts of th e Lang esund Channel supramarine part of t his mora ine belt (Fig. 7). (Holtedah l 1986, Olsen 1992). Four different acousti c units A profile 5 (Fig. 12) along the western margin of th e are di stinguished. The upper 40 m (Unit 1) is acoustically Langesund Channel shows a thi ck, broad, submarine tra nsparent and interpreted as po stg lacial sedim ents (post morain e ridge situa ted at th e cro ssing of pro fil e 4 near th e Ra deposits). Unit 2 has very di stinct and mostly conti nuous edg e of the slop e. reflectors, w hich mo st likely represent glacio marine sedi Based on the profiles 4 and 5, th e positi on of the subma men ts deposited mainl y during th e early Younger Dryas (Ra). rine Ra moraine s southwest of the outer Langesundsfj ord is The lower units are disturbed by sid e echoes, but t he more te ntatively reco nstructed as shown in Fig. 7. The thick de po s t ransparent sediments below (Unit 3) indicate a distal posi its of glacigenetic sedi men ts in t he Steinr enna basin and t he tion of th e ice margin and were prob ably deposited during bo ulde r island Danm ark are interpreted as th e main conti nu-
Profil e 2
"~.:'.J : '::g~i O:: " !~J~j~~ I ! ,' ~ • g ii,f:Y:,:;J
mls o SW Ra maxi mumposition? The Ra moraines NE P3
50 ~ Lange sund 100 Channel + Bedrock + + + + + + +
+ + + + + + + + + + + 150 + + + + + Bedrock, + + ++ + + + + + +
+ + + + + 0 1 km ' + + + + + + + + + + 200 1!::=====I===:::f1 + + + + + + + + + + + + + + + +
+ + + + + + + + + + + + + + + + + + + + + B +
Fig. 9. A: Submarine seismic reflection profile 2 in the Langesundsfjord north of Molen. For location, see Fig. 7. B:Interpretation of t he prof ile 2. NGU -BULL 435, 1999 - PAG E 32 BJ0RN BERGSTR0M
ation of th e Ra moraines from Jomfruland and Strahol men described from the More area, western Norway (Larsen et al. north wards to the Langesund Channel, where th e moraines 1991, O. Longva, pers.comm . 1999).These ridgesare thoug ht seem to turn north wards along the edge of th e channel. to have been for med at the grounding line by glacier push. If Morphological eviden ce of younger and smaller ice oscil the interpretation of the De Geer moraine s in Langesunds lations during th e early Younger Dryas isfound proximally to fjord iscorrect , the deposition of these ridges hasoccurred in the main Ra moraines in profile 5 as two small, but disti nct front of the retreating glacier during the deg laciation of the terminal moraines. ln th e southern part of th is profile (Fig. 12) Langesund Channel, most likely dur ing the early or middle many small moraine ridges occur at depths between 100 and part of the Boiling Chron. However, some of the moraine 150 m. They are orientated transversely to th e Langesund ridges, part icularly the inner ones, might have been formed Channel and situated outside the Young er Dryas ice-limi t, during the late Boiling glacier advance, corresponding to th e resembling very much the submarine De Geer mo raines Tjerne-Hvaler ice-marginal depos its.
mls P2 o ! s 1°
100
o 2 km I
200 Post Ha- deposits Langesund Channel
300 +
+ - Bedrock ~ + + + + 400 + + + + + + + + +
+ ~ + + + + + + + Bedrock. + + + B + + + + + + + + + + + + + + + +
Fig. 10. A: Submarine seismic reflection profil e 3 in the Langesundsfj ord area . For locat ion , see Fig. 7. B: Inte rpretation of the profile 3. BJ0RN BERGSTR0M NGU-BULL 435 , 199 9 - PAGE 33 I
Lang e sund Channel
P5 rnts o ~ sw Steinrennabasin NE The Ra moraines ~~~~ 50 Lange + + + sund 100 + + + Channel ----- + + + + + + + + + + + + + + + + + + + 150 + + + + + + + + + Bed rock + + + + + + + +
+ + + ++ + + + + + + + + + + + + + o 1 km 200 + + + + + + + I I + + + + + + + + + + + +
Fig. 11. A: Submarine seismic reflect ion profile 4 in t he Lange sund sfjord area. For location, see Fig. 7. B: Int erpretation of th e profil e 4.
P4 m/s Moraine 0 N ridges? ~ S The Ra moraines 0 2 km I
100 De Geer moraines? +
+ -_I' +.. _ ~ , 200 Bedrock + / .... -r ~ " + + + + + + + + ...... + Multiple " , + + + + + + + + + + + + T T + +' ... _-+-- - + +
Fig. 12. A:Submarine seismi c reflection profile 5 in t he Langesund sfjord area. For location, see Fig. 7. B: Interpretation of the profi le 5. NGU -BULL 435 , 1999 -PAGE 34 BJ0RNBERGSTR0M
Seismic-reflectionprofile N N , D ___ Se ismic-refractionprofile ,, ,, A Resist ivity measurements A ,, 0 Log Withret.number ~ , , '-----' '----' , , ,r ,-' ------/ ~
z:;$~ / / / I I '-.--/ , ~ 0° '-.--/ '----' I a -c:> '----" I I '---.-/ I ------I d I / 0 / t:b ---../ ,,/ ------'-.--/
Fig. 138 ad
The Ramoraines CJ mainlycovereoby marineshore 0 depo sits / / ':> / Exposecbedrock / c:=J 0- 0, 100,,200 300, m 0 l km A Contour interval 5 m
13. A: Location map of Jomfruland. B: Location map wit h sites of seismic and electr ical profiles at Jomfruland.
Jomfruland island deposits or gravelly/sand y till. This indi cates minor ice-mar Jomfruland is an eme rged part of th e Ra mo raine and the ginal oscillations, possibly during the late Weichselian deg la island is 7.5 km long and up to 1 km w ide (Figs. 3, 13).The ciation (late Bollinq/Older Dryas and late Allered/ early highest point is about 20 m a.s.l. The island has been visited Younger Dryas Chrons). This complex stratigraphy of the and described by many geo logists (Keilhau 1842, Hansen Jomfruland moraine ridg e is quite similar to the stratigr aph y 1910, Holtedahl1 953), but we still don't know much about its of th e Ra moraine at Fokserud in Vestfold where glaciofluvial internal com position and structure. From old boreholes near mat erial (sandy gravel) underlies clayey diamicton (Serensen t he lighthouse, 2-5 m-thick beach deposits abo ve consoli 1992). dated clay have been reported (Keilhau 1842, Hansen 1910). The upper part (0-5 m) of the loose deposits has been Later on, a 40 m-deep drill ing do wn to bedroc k was carried studied in exposures associated w ith well digg ing. Most of out mainly th rough 'soft clay' near the new ligh thouse the island is covered by beach depos its. In the most exposed (Jansen 1982, 1987), bu t no further infor mati on about the stratigraph y has been availab le. Parallel submarine moraine r------Profileof theJomfrulan disland area ridges (Fig. 14) occur off th e coast on the distal slop e of NW r--·---- iTheE~~yYo UngerDfya$i--..-..------1 SE Jomfruland (Holtedahl & Bjerkli 1975). m ; Kragereflard L...! ~~.!'2! ~~~g!_ J ! m Recently, geop hysical measurements were carried out at 40~ coas Jomfrulandisland Submarine areas ~ 40 ~ /'1..." ~ .. Lokstad and Hagane (Figs. 13, 15) in order to obtain more : - - " A Presentsea level ; 0-;-"\ 1 '" .J.J- - - -;'_~ J_- _\" ------1-0 information about t he sediments (Mauring & Tennesen -i- .... 1992). At l.ekstad , refracti on seismic profiles show depths to -40-j: ::::::::::: : bed rock varying between 15 and 60 m (Fig. 15). Reflecti on -80 t ~~i~ --- :. .: --- : ---:.I: :: ~0 ~ -80 seismic measurements (CDP) show reflectors at a depth of -:- ~ __1Bedrock 1- • -- ..... 40-45 m, wh ich indicate that marine sediments in the deep -120 ~- ., [,.J Glacioge nic 1------H 20 j- 4 sediments (tills) 1------est part of the basin formed in an early ice-free period (older t ~ - - 1------.. - -160 --;- ., . Mannedepo srts 1-" ------~ -160 than th e Younger Dryas advance). A reflector at a depth of ~_ -L L .. 20-30 m most likely represents an erosion bou ndary formed by a glacial advance . The sediments abo ve are interpreted as 'clayey ti ll' (boulder clay). Resistivity measurements (pole Fig. 14. Profile from the outer coasta l area across the Jomfruland island dipole configuration) have recorded t hree layers with high area (NW-SE). Submarine part mo dified after Holtedahl & Bjer kli (1975). resistivity which ind icate coarse materia l such as glaciofluvial For location, see Fig. 13A. BJ0 RN BERGSTR0M NGU-BU LL 435, 1999 - PAGE 35
tekstad , Jomfruland PRO FI LE 1 Shore deposits - -- Reflector 450nVs 450nVs I 450nVs 450nVs -- Indicated rock surface ::Z 7.:/.:7.:7.::z:ir:»:zc:x:»:;: . Till 1700nVs 1750nVs 1750 nVs (boulder clay) .' I Z 7.:7. 7.. 7.. 7.. 7. -10 T Intersecting profile .... ··/ 7 7 ;'::/.:Z:<::,<:% ;,. .::y':7-:Z. Z .., ., ., . + + ' .:..... ---- - . ~ :;. -20 ~22/. Layer with high resistivity
-+-Be-+-droc-+-k + + + + 7 -30 + + + ++ ' ';- +'':r!7/./../../WZZZZZZZZZZZ.... ./ c=J Till (boulder clay) '-7"-:---;---:--'---=--=--=--::--;-~-r-~", f- + + + + +++++++++++ + + + + + + ·40 c:::=:J Marine sediments
1700nVs Seismic vel ocity o 50 100 150 200m
PROFILE 2
SW NE Shoredeposits rn a.s.l. ma.s.l. Pl 10 !i' ....",_"'""""""""'"-= -.I l 10 o 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. -10 -20 -20 -30 -30 ::~:-::~ ~~Zf:~f!P~_ -40 -40 + + + + + + + + + + + + + + + + + + + Bedrock -50 ++++++++++++ r ,, + + + + + + + -50
o 50 100 150 200 250m
Fig. 15, Seismic and resistiv ity profiles at Jomfruland, mod ified after Mauring &Tonnesen (1992), areas th ere are a lot of beach ridges containing we ll rou nd ed shore deposits, which coarsen upward from sand to big cobbles and boulders.The underlying till consi sts of a consol rounded cobbles and bou lders, (log 2, Fig. 16). Mollusc shells idated, clayey, matrix-supported diamicton (boulder clay), from th e lower and midd le part s of th ese marine sedi men ts partly wit h shell fragm ents of arcti c molluscs. The content of were dated, and gave ages betw een 10,340 ± 105 (T-10777) clay and silt varies between 25 to 35 % and 30 to 35 %, and 8585 ± 105 years B.P. (T-10776). respectively. A borehol e (log 1) do wn to 10.5 m depth near The st rati graphy and th e radiocarbon dates indicate that the crest ofth e highest ridge at l.ekstad (Fig. 16) show s soft er th e ice margin retr eated some distance pro xim ally to Jomfru diamict material below the consolidated diamicton. The land as early as during th e Boiling Chron , before 12,240 ± 80 composition of th e material less th an 16 mm is quite similar, years B.P.Then, th e glacier advanced, due to th e fact th at th e but th e conte nt of cobb les and boulders is decreasing do w n radiocarbon-dated mollusc was pic ked up by th e ice and ward. This indicates th at there is a gradual tra nsition into th e incorporated in t he till (boulder clay) on th e island. It is diffi underlying glaci omarine sediments. During t he Younger cult to interpret how far th e ice margin reached, but t he Dryas advance, mu ch material from th ese sedim ents w as stratig raph y ind icates that th e marg in extended at least asfar picked up by the glacier and incor porated in t he Ra mo raines. asJomfruland.The glaci er cou ld have overridd en Jomfr uland The high content of silt and clay in the boulder clay ind icates and te rm inated on the distal side where the subma rine a glaciomarine origin. Radiocarbon date s of moll usc frag moraine ridges w ere fo rmed (Holtedahl & Bjerkli 1975) This ments, mostly Macoma calcaria and Portlandia araica, in the im pli es t hat t here must have been a major advance and a till give ages between 10,690 ± 150 (UtC-2220) and 10,825 ± considerable increase in the ice t hickness if th e glacier shoul d 70 years B.P. (Tua-935). One fragme nt of Macoma calcarea have been able to form th ese terminal ridges at a depth of was dated to 12,240 ± 80 years B.P. (Tua-629) and indicates an more than 150 m. How ever, th e lack ofstratigraphical records earlie r ice-free peri od du ring th e late Boiling Chron . in t he adjacent areas of such a considera ble advan ce indi In th e low er part ofthe pro xim al slope of Jom fruland, th e cates only a minor increase in t he glacial activity . Most likely, consol idated bou lder clay is covered by more than 3 m of gla Jom fruland island due to its topographically favo urable posi ciomarine/marine sediments w hich are coarseni ng upward tion at t he edg e ofthe coastal plain, represent s the maximum from clay to sandy silt. There is a maj or hiat us to the upper ice-front po sition during t his advance. NGU-BULL 435, 1999 - PAGE 36 SJ(oRN SERGSTR0M
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 deg laciation, Jomfruland was covered by the ma.s.t. Lithol ogy 14C - dates Genesis sea and fine -gra ined marine sediments were depos ited at a 10 water dep t h of more than 100 m. During the emergence of Shore th e upper part of th e island, the fine-grained marine sedi deposits 9 10400 ::!: 100 ments were eroded and the moraine rid ge was covered by f:::, r- 10 690 ::!: 150 boul der s and cobbles washed out from the ti ll by the waves 8 c: - 10 825 ::!: 70 (log 1, Fig. 16). At the lower and lessexposed part ofth e pro x 7 c: ----- L10770::!: 95 Till imal slope of the island, some of the fine -grained marine sed 12240 ::!: 80 (boulder clay) iments still remain (log 2). Radiocarbon dates of mo lluscs 6 f:::, c: 1c:: ind icate a minimum age ofth e mora ine ridge of 10,340 ± 105 o ~ years B.P. Fragm ent s of mo lluscs from th e coarse-grained 5 ~ c: o shore dep osits are washed out from the older sediments and c:: '"o the dat esgive on ly a maximum age, 8585 ± 105 years B.P., for 4 f:::, c: '-' C> .c;;c:: the time when the top of the ridge was influenced by the 3 f:::, eee waves during th e emergence of the island. '-' Q,) Cl 2 c: The Eidanger ice-marginal deposits After the Ra event, t he ice marg in began to recede f:::, Glacio towards the northwest (Fig. 5).The presence of dropstones in marine the poorly sorted glaciomarine/marine clay and silt, overly o sediments ing the Ra moraine ridge on the Jomfruland island, indica tes th at there were drifti ng icebergs in th is area during the early part of the ice-free period, radiocarbon-dat ed to approxi ILog. 21 mately 10,600-10,350 years B.P. When the ice marg in reached the higher inland areas, the floating ice-margi nal parts ma.s.l. became grounded and the calving processes dimin ished. 7 -rTl"TT/7V~"V"T-' Some scatt ered glaciofluvial fans or small deltas were depos ited near thisgrounding line we st of Langesundsfjord (Fig. 5). 6 m~~ Shore deposits They are correlated with the Eidanger ice-marginal depos it (Fig. 6), a glaciofluvial frontal ridge formed in a submarine 5 r8585 ::!: 105 posit ion (not built up to sea-level) dur ing a short halt or stag Glaciomanne rnarlne nation in the ice recession in Langesundsfjord (Bergstrom 4 - 9925 :!: 70 si~ and clay F6'-=-'----:-6-~-6-,-----jL 10340 ::!: 105 1995c). Northeast of the fjord, corresponding ice-margina l Till deposits have been mapped in the valley s Siljandalen and 3 -6-6-~ (boulderclay) Lagendalen.
II I I I I The Eidanger ice-marginal deposits are tentatively corre Cl Si SGBD lated with the As Moraines (Bergstrom 1995c, Sorensen 1983,1992). A radiocarbon date of gyttja from a bog situated Fig. 16. Stratigraphical logs from t he upper part of the Jomfruland ice a few km distally to th e Eidanger ice margin indi cates that the marginal deposit . 14C-dates in years B.P. For location, see Fig. B B. The area was ice-free from at least 10,280 ± 90 years B.P. (Hen laboratory num bers of the dates (see methods, p. 24) are listed from the ningsmoen 1979). Based on information from the Oslofjord top to the bottom of the logs. Log 1: Tua-1211, UtC-220, Tua-935, Tua 1210 and Tua-629. Log 2: T-' 0776, T-' 0775 and T -10777. area and Jomfruland, the most likely ag e of t h e Eidanger event is between 10,400 and 10,300 years B.P.
Subsequent ly, du ring the Allered Chron, th e ice margin The Geiteryggen ice-marginal dep osits receded at least 17-18 km inland from the Ra moraines (Berg During the ice recession, the calving processes gradually strern 1995b).The glacier readvance started at the end of the decreased due to the general higher topographical level of Allerod Chron , after 11,300 years B.P., and terminated off the t he inlan d area and more of t he ice marg in was sit uated in a coast at Jomfruland. The comp lex stratigraphy of the Ra supramarine position. Only in the major valleys was the ice moraines at Jomfruland indicates smaller oscillations of th e still in contact wi th the sea. This resulted in concave calving ice margin during th eir formation, w hich prob ably starte d at bays and converging ice flows towards these fjord valleys. abou t 10,800 years B.P. The radiocarbo n dates ofthe mollusc In the valley between Skien and Porsgrunn, north of fragments incorporated in the upper part of the till suggest Eidanger, minor glaciofluvial deposits were fo rmed in the sea that the last readvance to the Ra moraines occurred shortly in front of the ice at Borqeasen and Nenset (Fig. 5). Corre after 10,690 ± 150 years B.P. The final glacier retreat from the sponding terminal moraine ridges and glaciofluvial ice-con- BJ0RN BERGSTR0M NGU-BULL 435, 1999 - PAGE 37
Fig. 17. During t he Geiteryggen event, the ice fro nt was situated at the southeastern end of lake Nordsjo .The compl ex marginal Geiteryggen delta was built up to the marine limit (c. 145 m a.s.l.) in fro nt of the glacier in the Nordsjo basin. View towa rds west and the form er glacier. The Nordsjolake in the background. tact deposits occur at lake Gorningen in Siljandalen and in lobe with an ice-surface gradient of 5-10 % near the front, Lagendalen, e.g. Grini, Gaserud (Berqstrem 1988). To the while the gradie nt in the inner part is 1-2 (1.5) % (Berqstre rn southwest of the Langesundsfjord area, th ere are only minor 1988). Based on this reconstr uction, an average gradient of scattered marginal deposits (Berqstrom 1997). They do not c. 3 % has been used by Serensen (1992) as a 'master profile' represent any readvances or long standsti lls. for th e glacier lobes during th e Geiteryggen event. From Nenset th e ice margin retreated to th e southeastern The Geiteryggen ice margin can be followed, morpholog end of Nordsje in th e Skien area wh ere th e large Geiteryggen ically, eastwa rd to Drammensfjo rd and Oslofjord (Fig. 5). and ice-marginal deposit wasformed (Fig. 17) (Ja nsen 1980, 1986, corresponds to the Ski ice-marginal ridg es, dated to 10,200 Bergstrom 1985). The northern and middle parts consist of 10,000 years B.P. (Serensen 1983, 1992). In th e area of Geite ridges of coarse sand and gravel rich in cobbles . Georadar ryggen, a few radiocarbon dates of gyttja from bogs have profiles (Storro et al. 1992) show distinct, but irregular undu been carried out in order to derive a more precise age for th e lating reflectors indicating a complex formation ofthe ridges formation, but contamination by roots has given dates that close to the ice front. Distally, more uniform foreset beds are are mostly too young. Proximally to th e Geiteryggen (5 km). sloping towards the east.The southern part of Geiteryggen is a radiocarbon date of 9695 ± 95 years B.P. (Tua-774A) pro a glaciofluvial delta built up to the marine limit, 145-146 m videsa minimum age for the deglaciation, but pollen analysis a.s.1. Foreset beds ofsand and gravelly sand are covered by 2 indicates a lossof the oldest organic material during the sam m-thick topset beds consisting most ly of coarse gravel and pling. The lack of pioneer flora in th e bottom sediments indi cobbles . Seismic refraction profiles show a maximum th ick cates that the deglaciation occurred prior to this date, but ness of the Geiteryggen deposit of more than 100 m (Jansen probably not more than 200-300 years earlier. The immigra 1980, Johansen 1980). tion of tree birch was rapid during this period . Based on the T he com p le x ice-m arginal d epo sit at Geiteryggen indi d ata from the Oslofjord ar ea and the lo cal radiocarbon d ate cates minor oscillations of the ice front during its formation. distally to the ice margin (Henningsmoen 1979), the age of Corresponding terminal moraines occur southwest of Geite the Geiteryggen ice-marginal delta is considered to be ryggen. Towards the northeast, glaciofluvial ice-marginal betw een 10,1 00 and 10,000 years B.P. deltas or fans at Hogstad in Siljandalen are correlated with th e Geiterygg en event. In Hee rlandsdalen, a tributary valley to Lagendalen, distinct lateral moraines indicate an active ice NGU-BULL 43 5, 1999 - PAGE 38 BJ0 RN BERGSTR0 M
The Akkerhaugen ice-marginal deposits ind icate a steep front to the Akkerhaugen lobe in the main After th e Geiteryggen event the ice margin retreat ed rapidly valley with a gradient up to 150 m/km (15 %) in the outer towards th e north and northwest. In th e deep Nordsjo basin, most part (0-1 km). The Dalsvatn ice lobe has a surface gradi a local calving bay was form ed. At Romnes, wher e the basin ent of 20-30 m/km (2-3 %) near the front (2-5.5 km fro m the is narrower, a short halt in the ice recession occurred and a front). marginal moraine ridge wasformed (Fig.5) (Bergstrom 1984). The Akkerhaugen ice-marginal deposit has tentatively Corresponding front deltas exist north of Valebo (Fig. 5), been correlated with the Eggar lateral moraine at Pa ssebekk dated to be about 150 to 200 years younger than the end of in Lagendalen and with the Aker Moraines in the Oslofjord the Geiteryggen event (based on shoreline data). The aver area. Based on a plot of the ML at Akkerhaugen in the equi age ice recession in the Nordsjo basin is calculated to be distant diagram (Fig. 20) and data from the Oslofjord area, c. 125 m/year. the Akkerhaugen event is dated to 9,800-9,750 years B.P. At the northern end of the lake Nordsjo, a prominent ice front accumulation, not built up to sea-level, extends across The Nordagutu ice-marginal deposits the valley at Akkerhaug en (Fig. 18).It comprises two qla Subsequent to th e Akkerhaugen event the ice front retreated cioflu vial ridgeson either side of the river and consists mostly about 5 km northward to Nordagutu (Fig. 5, 18) whe re a large of sorted sand and gravel. Foreset beds are dipp ing to wards ice-marginal delta was form ed. Proximally, on the top of th e the southwest. The thickness of the deposit above bedrock eastern part of th is delta (Sundsmoen), a frontal moraine varies between 30 and 70 m (Bergstrom 1984). Correspond ridge crosses the terrace, indicating a short advance of th e ing lateral moraines are situated southeast of Akkerhaugen, front at the termina l stage of deposition of the delta. Lateral and can be followed towards the southeast into the basin of glacioflu vial erosion and depos its in th e eastern valley side, Dalsvatn (Fig. 5) where the ice lobe dammed a lake. The main northeast of Sundsmoen, were pro bably formed dur ing the outl et was over the col in the south at about 190 m a.s.1. Gla Nordagutu event. Pollen analysis and a radiocarbon date ciolacustrine fine sediments were depos ited and glacioflu from a bog proximally to the Sundsmoen delta indi cate a vial terraces of sand were built up to th is level. The existence minimu m age of 9,420 ± 190 years B. P. (T-4266) (Bergstrom of lateral, ice-dammed lakes at higher levels (up to 275 m 1984). The first pioneer flora is not represented in the pollen a.s. l.) is ind icated by overflow channels across th e mountain diagram.This ind icatesthat th e oldest organic material at the ridg e west of Dalsvatn. The lateral moraines at Akkerhaugen bottom of the bog has been lost dur ing the sampling, and
Fig. 18. Akkerhaugen ice-marginal depo sit (A) at the north ern end of lake Nordsjo, view towards NNE. During the degla ciation, large amounts of melt water drained laterally and subglacially along the main valleys.The largest glaciofluvial deposits were formed in front of the valley glaciers during stag nations in the ice recession. Some of the dep osits we re built up to sea-level ice-front deltas like Sundsmoen (5) in the background, while others were formed as submarine frontal ridge s asat Akkerhaugen. The Akkerhaugen ridge crossesthe valley and the river Sauaat the bridge. BJ0RN BERGSTR0M NGU-BULL 435, 1999 - PAGE 39 1
that the deglaciation of the area occurred at least 200 years previously. Based on information from shorelines and shore level changes in the area, the age of the Nordagutu ice-mar ginal delta is suggested to be about 9,700 years B.P., Le. 300 years younger than the Geiteryggen event.
Sea-level changes Shorelines During the deglaciation the sea followed the receding gla ciers and large areaswere submerged .The upper marine lim its have been determined in different parts of the studied area west of the Oslofjord (Fig. 19) and are the basis for the construction of an equidistant shoreline diagram (Fig. 20). The gradient of the tilt has been analysed by projecting syn chronous shorelines onto a projecting line, the direction of which gave the best fit to the marine limit observations in the area. This direction is estimated to be NlODE, and perpendic ular to the local isobase direction during the deglaciation. Data from previous work in the adjacent Oslofjord area (Serensen 1979) have provided a useful basisfor these calcu lations. In the U\gendalen area, Jerqensen & Sorensen (1979) have construc ted a shoreline diagram with a N-Strend forthe projection line.
Fig. 19. Sites of ML localities with projection line A-B (N1ODE) for the Fig. 20. Equidistant shoreline diagram. For location of the projection line shore line diagrams. and sites of ML localities, see Fig. 19.
m a.s.l. m a.s.1. 185 ,------::;,------,- 185
180 180 175 175 170 170
165 165 160 160
155 155 ».Ial\\oen 150 150
145 145 140 140
135 135 130 130
125 120
115
s I 6 6667 111 11 n 1314 15 76 ( ~ 4 ~~ 1" 55ss 57 60 63 "69 30 " .. LO CALITI ES "" LEGEND 1. Brynemoen 21. Solstad 41. Doksrod 61. Akkerhau gen Marinelimrt 2. Mo 22. Lensegrav 42. Rimstad 62. Folkestad (J) 3. Storfiskvatnet 23. Ronningen 43. Iorshon 63. Dterhon (J) Marine limit, uncertain determ ination 4. Fjolbuvatnet 24. Hoydal 44. Brenne 64. GAserud 5. KAsheia 25. Dampedal 45. vatesater 65. Sundsmoen (J) AfterJansen (1986) 6. NonsfJ ell 26. Hogebrun 46. Mo 66. HonsAs 7. Skogen 27. Klyve 47. Valebo 67. Ramberg (8) AfterBarg el & Lien(1990) 8. Sandvik 28. Geiteryggen 48. Sand 68. Brandsrud 9. Bostrak 29. Ronjum stua 49. Stulen 69. Kviberg 10. Drangedal 30. Teksle 50. Eikonrod 70. Nordby 11. Moen 31 . RamsAstjern 51. Gran dalen 71. Passebekk 12. Solberg 32. Svartangtjern 52. Rod 72. Goverud 13. Henneseid 33. Tjorna 53. Spiredalen 73. Alamoen(J) 14. Storefjell 34. Lia 54. Solverod 74. Nymoen (J) 15. Kallestad 35. Skifjell 55. Hogstad 75. Hove(J) 16. Torrdal 36. Mensvatn 56. (!sthaug 76. Skollenborg (B) 17. S The gradient of the early Younger Dryas (Ra) shoreline, shorelevel displacement curve based on pollen and diatom 1 m/km, is based mainly on data from the Lagendalen area, analyses and radiocarbon dates. due to the lack of suitab le localities in th e investigated area. During the mappin g of th e Kragero district, some radio The estimated age of the marine shorelines is based on the carbon dating of molluscs from marine terraces, shell banks height of the Geiteryggen shoreline, wh ich is dated to 10,000 and other shell-bearing sediments was carried out. These years S.P.The gradient is calculat ed as 0.95 m/km . The levels dates indicate some changes (Fig. 21) in th e early part of the of th e oldest marine shorelines, older than the Geiteryggen shorelevel fluctuation curve constru cted by Stabell (1980) shoreline, are very uncertain due to the lack of reliable ML from this area. A revision of the local shorelevel displace observations in the southern coastal areas. ment curve will be pub lished elsewhere . Information on the highest shorelines has been impor tant in the correlations of the ice-marg inal deposits and the reconstruction of the ice recession lines. Conclusions Anundsen (1985) hascalculated the approximate trend of The deglaciation of th e out er coastal areas in Vestfo ld and the 9,000 S.P.isobases to be about N800E - S800W based on Telemark started when the shelf ice in the Skagerrak broke up Stabell's (1 980) shoreleve l data from Porsgrunn and Kragero. and the glacier grou nded at the coast at aroun d 14.000 That means that the isobases may have shifted to a mo re 13,500 years S.P. The Jomfruland island was ice free before southwesterly direction dur ing the late Preboreal Chron. 12,200-12,300 years S.P. (12,240 ± 80 S.P.) Then, a glacier However, this calculation is partly based on inaccurate dates advance occurr ed and the ice margin reached at least as far of the isolation contacts in th e actual basins. Subseque nt as Jomfruland. A correlation with th e Tjerne-Hvaler ice-mar radiocarbon dates of molluscs and isolation contacts (Berq ginal deposits in the Oslofjord area, at 12,200-12,000 years strern 1997, K. Henningsmoen, pers. comm .1997) indicate S.P., seems most likely. that the basin from the Kragero area (basin no. 3 in Stabell During the Allerod Chron, the deep Oslofjord basin deg la 1980) used in the calculation was isolated earlier than 8,800 ciated rapidly due to calving processes, but towardsthe west years S.P. and probably at about 9,200 years S.P. This indi th e ice margin grounded in shallow water and retreated cates that there is no evidence of any shift of the isobase more slowly. The old est marg inal lines approach each other directions at 9,000 years S.P., as suggest ed by Anunds ens towards Langesundsfjord. The Slagen-Onsoy ice-marginal (1 985) calculat ions. deposits were form ed in Oslofjord at abou t 11 0400-1 1,200 years S.P. Westwards, the position of the Slagen-Onsoy ice Shorelevel displacement margin cont inu ed pro ximally to th e Ra moraines. In the Previously, three shorelevel displacement curves have been Kraqero area the distance between th e two marginal lines constructed from the studied area. In the Porsgrunn and was at least 10 km, but most likely more than 17-18 km. Kragero areas, Stabell (1980) has constructed two shorelevel During the late Allerod/ early Younger Dryas (aft er 11 ,300 displacement curves based on diatom analysis and radiocar years S.P.) the glaciers advanced and the marked Ra (early bon dates. The elevations of the studied basins have not Younger Dryas) ice-marginal deposits were formed at 10,800 been corrected for the uplift gradients. Anundsen (1985) 10,600 years S.P. The Ra morai ne ridges can be followed con attempted to make such corrections to the curves on th e tinuously from Oslofjord to Langesundsfjord, where there basis of uplift data from the Oslofjord area (Sorensen 1979). was a concave calving bay du ring the early Younger Dryas. In southern Vestfold, Henn ingsmoen (1979) published a From Langesundsfjord and sout hwestwards to Arendal, the Ra mo raine ridge sare tren ding parallel to the coast, mostly in a sub marine po sition. The Jomfruland island is an emerged part of the Ra moraines. ma.s.1. ma.s.1. After the Ra event, at about 10,600 years S.P ., th e ice mar 125 125 gin receded to wards the north west. In the Langesundsfjord area the Eidanger ice-marginal deposits were fo rmed at 100 100 • 100400-10,300 years S.P. Corresponding marginal deposits ·• occur in Siljandalen and Lagendalen. These depositsare ten 75 • Stabell (1985) uncorrected 75 •, tat ively correlated with the As moraines in the Oslofjord area. ·,, for uplift gradient During th e ice recession from Eidanger, minor ice-contact , 50 50 -, ' . depos its were formed without represent ing any readvance ...... or long standstill.The large and complex ice-marginal depos 25 25 itsat Geiteryggen indicate a readvance of the ice fro nt during their formation at 10,100-10,000 years S.P. Several term inal 10 9 8 7 6 S 4 3 o moraines and deltas correspond to th e Geiteryggen depo sit. 103 years a.p. Lateral moraines in Heerlandsdalen, a tri butary valley to Lagendalen, indicate an active ice lobe with a surface gradi Fig. 21. A mod ified shorelevel displacement curve for the Kragero area, based on 5tabell (1980). The curve is corrected for uplift gradient. The ent of 5-10 % near the front, and 1-2 % in th e inn er part. The older part of the curve (> 8000 years 8.P.) is based on new radiocarbon Geiteryggen ice-marginal depositsare correlated wi th the Ski dates (Bergstrom 1997, K. Henningsm oen, pers. comm. 1997). BJ0RN BERGSTR0M NGU-BU LL 435,1999 - PAGE 4 1 I ice-marginal moraines in the Oslofjord area wh ich are dated Bergst rom, B. 1995a: Late Weichselian deglaciation model of the coast al to 10,200-10,000 years B.P(Serensen 1992). areas of southern and cent ral Norway, wi t h special emphasis on t he From the Geiterygg en ice-marginal deposit, the ice mar period prior to t he Younger Dryas Chronozone. INQUABer lin 1995. Abstracts, Terra Nostra 2/95, 27. gin retreated rapidly with an average recession calculated to Bergstrom, B. 1995 b: St rat ig raphical evi dence of a considerable Younger be 125 m/year. The Akkerhaugen ice-marginal deposits indi Dryas glacie r ad vance in sout heaste rn Norway. Norsk Geolog isk cate a small readvance at c. 9,800 years B.P. Lateral morain es Tidsskrift 75, 127-136. indicate an active ice lobe with a surface gradient of 15 % in Bergstrom , B. 1995c: Por sgrunn.Quate rna ry m ap AM S M 711-1713 11 , the outermost part (0-1 km from the front), while the gradi scale 1:50,000 w it h description (in Nor weg ian). (Eng lish sum ma ry). Norges geologiske undersekelse Skrift er lIS, 1-40. ent of the tributary lobe at Dalsvatn is 2-3 % in the inner part Bergstr om , B. (in press): Lang esund and Stave rn. Quate rna ry geol ogic al (2-4 km from the front).These deposits are correlated with maps 1712 I and 181 2 IV, scale 1:50,000, wi th descript ion on th e the distinct Eggar lateral moraine in U\gendalen and wit h the maps (in Norwegian). Norges geolog iske undersekelse. Aker moraines in the Oslofjo rd area. Bergstrom, B. 1997: Kragero and Riser, Qua ternary geological m aps 1712 The large Nordagutu ice-marginal deltas, deposited IV and 1712 Ill, scale 1:50,000, wi th description on the maps (in Nor we gi an). Norges geologiske undersekelse. about 5 km proximally to the Akkerhaugen margin al depo s Bergstrom, B., Olsen, K.5. & Serensen , R.1992: Tj em e. Qua ternary geolog its, are calculated to be 50-100 years younger. Frontal ical map 181311, scale 1:50,000, wi t h description on the map (in Nor moraine ridges on top of the delta terrace indicate small wegian). Norge s geologiske underse kelse. oscillations ofth e ice front during th eir formation. Berg strom, B.,Olsen, L.& Sveian, H. 1994: Sist e g lasiasjon og de glasiasjon An equidistant shoreline diagram has been constructed av Namdalskyst en, Nord-Tro nde lag. 21. Nord iska Geol ogiska Vint er rnet et , l.ulea, Abstract, 23. on the basisof marine limits in the investigated area and data Dahl, A., Olsen, K.5. & Serensen, R. 1997: Holmestrand. Quaternary geo from the adjacent Oslofjord area. logical ma p 1813 IV, scale 1:50,000, with descripti on on t he ma p (in Nor weg ian). Norges geologiske undersekelse. Acknowledgements Dons, J.A. & Jo rde, K. 1978: Geo logi sk kart over No rge, berggru nnskart Skien 1:250,000. Norges geologiske underse kelse. Th is art icle summarises the resul ts from the Quaternary mapping and Follest ad, BA 1989: Tingvoll. Qu aternary geological m ap 1320 I, scale studies in the southern Telemark and Vestfold counties carried ou t by 1:50,000, with description on the map (in Norwegian). Norges geol th e Geological Survey of No rway during th e last tw o decades. S. F. Selvik ogiske undersekelse. has carried out some prel iminary analyses of macroscopic plants and K. Follestad, BA, Larsen, E.,Blikra, H., Longva, 0., Anda, E., Sonstegaard, E., Henningsmoen som e diatom analyses which will be pu blished else Reite, A. & Aa, A.R. 1994: Quaternary geological map of More og where. Arne Reite and Oddvar Longva are thanked for their critical read Romsda l Cou nty, Norway. Scale 1:250,000 wi th description (in No r ing of an early ve rsion of the man uscri pt, and D. Robe rts has co rrect ed th e English language. Inge Aarset h and Rolf Sorensen are t hanke d for weg ian). Norges geo logiske undersekelse Skrift er 112, 1-52. Hansen, A.M. 1910: Fra ist ide rne - Vest-Raet . Norges geologiske under t heir co nstructive and hel pful reviews. The illustra t io ns were d raw n by I. Lundquist. To all th ese person s I expr ess my sincere th ank s. sekelse54, 265 pp . Henningsmoen , K. 1979: En karbo nda ter t strandfo rskyvni ng skurve fra sendre Vestf old . Fortiden i sekelyset. Laboratoriet for Radiologisk References Datering, 239-247. Aarset h.I ., Austb e, P.K.& Risnes, H. 1997: Seismic st rat igraphy of Younger Ho lte dahl , 0 .1953: Norges geo logi, Bd. 11. Norges geolog iske unde rsekeise Dryas ice-m arginal deposit s in western Norwegian fjords. Norsk 164,587-1118. Geologisk Tidsskrift 77, 65-85. 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Robinson , J.E. (ed s.), Studies in the Lateglacial ofNorth -West Europe. Sorensen, R. 1990a: Raets oppbygging og dannelse i Vestfold , Sores t Pergamon Press, Oxford, 23-30. Norge. Geonytt 1, 113 (ab st ract) . Mangerud , J. & Gulliksen, 5.1 975: Apparent radiocarbon ag e of recent Sorensen, R. 1990b: The Alle ro d-Younger Dryas bio logical and glacial m arin e shells fro m Norway, Svalbard and Ellesmere Island. Quater en vi ronment in the Oslofjord area, so uth eastern Norway. (Abstract). nary Res earch 5,263-273. In J. Lundqvist & M. Saarn isto (ed s.): Termination of the Pleistocene. Mau ring , E. & To nnesen, J.F. 1992: Seism iske og elektriske m alinger pa IGCP-253 Field conference - Excursio n and abstracts, 26-28. Jomfru land, Kraq ero kommmune, Telemark. Norges geologiske Soren sen, R. 1992: Th e physica l en v ironment of Late Weichselia n degla undersekelse Rapport 92.216 , 11 pp. ciation of th e Oslofjord regi o n, sout heastern Norwa y. Sverigesqeol Nesje, A., Dah l, 5.0 ., And a, E. & Rye, N. 1988: Block fie lds in southern Nor oqiska undersokninq Ser. Ca 81, 339-346. way: Sig nificance fo r the Late Weichselian ice sheet. Norsk Geologisk Soren sen, R., Li, K.T. & Ny bakken , S.E. 1990 : Drobak, Qua ternary geologi Tidsskrift 6B, 149-169. cal map 181411, scale 1:50 000, wi t h des cription on the map (in Nor Olsen; HA 1992: Kvart Manuscript received June 7998; revised manuscript accepted May 7999.