39

The Tjøtta glacial event in southem , North

BJØRN G. ANDERSEN, FREDE BØEN, ARNE RASMUSSEN, KÅRE ROKOENGEN & PÅL N. VALLEVIK

Andersen, B. G., Bøen, F. , Rasmussen, A., Rokoengen, K. & Vallevik, P. N.: The Tjøtta glacial event in southem Nordland, North Norway. Norsk Geologisk Tidsskrift, Vol. 62, pp. 39-49. Oslo 1982. ISSN 0029-196X.

Large and distinctive marginal moraines representing the Tjøtta glacial event have been mapped on land and across fjords. They were radiocarbon dated at about 10,500 years BP - 11,000 years BP, wich is an early Younger Dryas age. A Portlandia arctica molluscan fauna and a distinctive Main shore line correlate with the moraines, and the glaciation limit was 500 m to 550 m below the modem glaciation limit during the Tjøtta event. The evidence suggests a cooling, incursion of arctic water with a Portlandia arctica fauna and a glacier advance to the Tjøtta event moraines about 11,000 years BP. Climatic amelioration, with a corresponding retreat of the glaciers from the Tjøtta moraines, and the disappearance of the Portlandia arctica fauna took place shortly after 10,500 years BP. About 10,100 years BP the glaciers advanced and deposited the younger marginal moraines of the Nordli event.

Bjørn G. Andersen and Ame Rasmussen, Department of Geology, University of Bergen, A/legt. 41, N- 5014 Bergen-univ, Norway. Frede Bøen, Oljedirektoratet, Box 6000, N-4001 Stavanger, Norway. Kdre Rokoengen, Continental Shelf Institute (JKU) Box 1883, N-7001 Trondheim, Norway. Pdl N. Vallevik, Krokeidev. 21, N-5047 Stend, Norway.

Moraines representing several glacial events of southern, western and centnil Norway to the have been mapped and studied on the coast of county boundary between North Trøndelag and the southern part of the county of Nordland, Nordland. In the same mann er, moraines, in­ North Norway (Fig.1). The field work on land cluding the Tromsø-Lyngen moraines, have been was carried out as a part of the IGCP-project traced and mapped along the coast of northern­ 'Quaternary Glaciations in the Northern Hemi­ most Norway southward to central Nordland at sphere'. Saltfjord (Andersen 1979). Radiocarbon dates of As a special part of a regional off-shore map­ the moraines suggested that they are of similar ping program, the Continental Shelf Institute Younger Dryas age. (IKU) in cooperation with the Hydrographic Of­ However, the approximately 300 km long ficeof Norway carried out seismic profiling along coast of southern Nordland represented an area the coast of the county of Nordland. About 300 where the Y ounger Dryas moraines had not been km of seismic profiling was run in the fjords identified. In fact, very few moraines were during September 1979. Several large and dis­ known from this coast, which is dissected by tinctive submarine end-moraine ridges were ob­ many deep fjords. Several scientists have studied served and tentatively correlated by Rokoengen glacial striations and the shore lines which are (1979). well developed along parts of the coast (Rekstad The work by the IGCP-group and the profiling et al. 1900, Rekstad 1922, Grønlie 1940, Marthi­ by IKU supplement each other, and it was de­ nussen 1962). A few moraines were mentioned cided to write a joint presentation. Only the by Rekstad (1917, 1925), Grønlie (1940) and results related to the Tjøtta glacial event in the Svensson (1957, 1959a). Within the western Vefsnfjord-Ranafjord area (Fig. 2) will be dis­ Vefsnfjord-Ranafjord area, only the Tjøtta mo­ cussed in this paper. raine (Rekstad 1925), and the lateral moraine and the cirque moraine near Breimo (Svensson 1959b), have been described before. None of the Review of previous studies moraines were dated, but Svensson (1959a) and Moraines, including the Ra moraines in the Os­ Marthinussen (1962) suggested that they could lofjord area, have been mapped along the coast be of Younger Dryas age. 40 B. G. Andersen et al. NORSK GEOLOGISK TIDSSKRIFT l (1982) Rasmussen (1981) mapped moraines of a Vassdal �"" event which is tentatively correlated with the Vega event. Several radiocarbon dates show that the Vassdal event is older than 11,700 years BP, USSR ···�. and probably of late Bølling or Older Dryas age. Distinctive moraines which Iie about 20 km to

OESCRIBEO AREA ---+ the east of Tjøtta moraines represent the Nordli event, which was dated at 10,100± 200 years BP. Younger moraines of the Mosjøen-Hemnes events are situated about 30 km to the east of the

SlleUond Nordl(moraines. They are of Preboreal age. The terms Bølling, Older Dryas, Allerød, 1 HERDLA MORAl NES Younger Dryas and Preboreal are used in this paper for the chronozones 13,000-12,000, 12,000-11,800, 11,800-11,000, 11,000-10,000 and 10,000-9,000 years BP, respectively (see Mangerud et al. 1974).

Descriptionof the marginal moraines The Tjøtta moraine, covering the western part of Tjøtta island, was described by Rekstad (1925). Most of the small Tjøtta community, including several farms, Iies on the moraine itself, which

Fig. l. Location of the investigated area and of the Younger forms a low broad ridge. The moraine is situated Dryas moraines in Scandinavia. near the present-day sea leve!, and has been strongly attacked by marine wave abrasion. It is mostly covered with shore deposits. The Tjøtta moraine was deposited at the front of a wide outlet glacier which occupied the Vefsnfjord and Field studies moved in southwesterly direction. Field studies on land were carried out along the An interpretation of a sparker profile across entire coast of southernNordland. Correlation of the submarine part of the Tjøtta moraine on the the known Younger Dryas moraines in southern south-eastern side of the island, is shown in Fig. Norway with the moraines in 3. The ridge is 1.5 km broad and consists of an up was found to be possible. The studies were car­ to 100 m thick unit of morainic deposits resting ried out along the following lines: on a bedrock threshold. The crest of the ridge Iies at about 100 m below sea leve!. l) Marginal moraines and other ice-marginal de- posits were identified and mapped; The continuation to the north of Tjøtta is a 2) Moraines were correlated; gravelly spur which grades into a broad 4-6 m 3) Dating of the moraines was carried out. high ridge at Alstadhaug, representing a north­ em segment of the front moraine. Marine clays Profilingin the fjords (Fig. 2) was carried out on cover the lowland to the north of the Alstadhaug board the coastguard ship KN 'Hydrograf. An moraine, and a Portlandia arctica fauna was ob­ EG & G sparker was used with 1000 joules ener­ served in a road cut about 500 m from the mo­ gy and 50-500 Hz band pass filter. raine, 10-20 m above sea leve!. Moraines representing several glacial events A small glacier branch moved in a northerly were mapped, studied and dated. The moraines direction from the main trunk glacier in ­ of the Vega event Iie on the Vega Island 30 km- fjord into the small Søvikskar valley where it 40 km to the west of the Tjøtta moraines (see deposited an about 150 m broad and 10-16 m Fig. 2). A radiocarbon date of shells from till in high end moraine (Fig. 4). The ridge was depos- these moraines suggest that they are younger than about 13,400 years BP. In the Glomfjord area, about 80 km to the northwest of Ranafjord, Fig. 2. Glacial map of parts of southem Nordland. NORSK GEOLOGISK TIDSSKRIFT l (1982) The Tjøtta glacial event 41

lE GEN O

Marginal moroine and inferred locotion of ice �in correloted with the .. /·· Tj�ta Event Conlirmed and inferred submarine €;![�( nes cotreloted with :1J;'�7v. �1 Younsf�:rMoldermorvi"ol morait�e ond /'' infered loc:otion oflc:e rnorgin

c Cir� moroine

f'10 Rodioc:otbon date of deposit older thon 9<0l the momine (<10•601 Rodioc:Otbon date cf deposit younger thon the moroine

Rodioccwbon date of deposit correlated 1-•oeool with the moroir:e

_...... The Moin shore line -� lsobose for the Moin �hore line .- Glodol striotlon obMrved by the ..-

- �ocier or pMeMlol snow field

� Sec:tion shown in the text

Altitvdes in rnetres �,/ tlf l l l l l l l l l 70 l l fo' l l l N 42 B. G. Andersen et al. NORSK GEOLOGISK TIDSSKRIFT l (1982)

sw NE s N 2km Large boulders Proximal-� . / +--60m a.s.l.

A �B •:.�-=---=----=---=-= c -x.... � 45m a.s.l.

SCALE

O 20 40 60 80 100m

Fig. 4. The Søvikskar end moraine. A: South dipping sand and gravel beds. B: North dipping sand and gravel beds. C: Marine clay. X: Marine blue clay radiocarbon dated at 10,940 ± 120 years BP (T-3271).

Fig. 3. Interpretation of sparker profile showing the submarine part of the Tjøtta moraine on the southeast side of the Tjøtta island. kar moraine and the large Breimo moraine (Fig. 2). Clays exposed in a 3.5 m deep well which was dug 5-10 m above sea leve! at Søvik, 1.5 km from ited in the sea, and beds exposed in a grave! pit the Søvikskar moraine and 4 km from the consist mainly of sand and grave! with many Breimo moraine, showed the following strati­ large erratics. The beds dip about 15°-20° in a graphy: northerly direction, some beds on the proximal 0.1-0.2 m beach gravels with well-rounded side dipping in a southerly direction. Large erra­ cobbles tics Iie on the surface of the ridge. The sand and 0.25 m clayey silt without stones grave! unit overlies a unit of blue clay of which 0.15 m clayey silt with some stones about 2 m was exposed. The clay contains a few 1.7 m blue clay with frequent dropstones scattered dropstones and many shells of large and shells, many Hiatella arctica Mya truncata and Hiatella arctica. Several shells and a few Portlandia arctica and Iie in their position of growth. They were radio­ Macoma calcarea in the upper 0,5 carbon dated at 10,940 ± 120 years BP (T-3271), m, and many Portlandia arctica and the moraine being younger. a few Hiatella arctica in the Jower Glaciomarine clays more or less completely 1,2 m cover the Jow-lying districts between the Søviks- 1.3 m blue clay with scarce scattered stones and a few shells of Portlandia arctica. sw NE "' ... .. The 1,7 m thick bed of blue clay with dropstones was correlated with the described moraines, and Portlandia arctica from this clay was radiocarbon dated at 10,800 ± 180 years BP. (T-3081). A large, about 400 m broad end-moraine de­ posited by a southwesterly moving branch of the glacier crosses the valley floor at Breimo, southwest of Sandnessjøen. The end moraine was deposited in the sea, and it has been

Fig. 5. The Breimo end moraine. A: 0. 8 m stratified sand exposed to strong marine wave abrasion. Its sur­ (shore deposit). B: 0. 5 gravel with cobbles and large boulders. face is covered with beach gravels and large erra­ Many stones are well rounded (shore deposit of reworked till). tics. About 6-8 m deep grave! pits in the ridge C: 1-2 m bouldery till with large erratics. D: 0. 5 m gravelly silt with dropstones and shell fragments, strongly tectonized (gla­ exposed grave!, sand and till beds. Of particular ciomarine?). E: 0. 5 m gravel with cobbles and shell fragments, interest is a section with very steep to almost strongly tectonized (glaciofluvial?). F: 6 m fine sand and silt vertical beds which must have been deformed by beds, strongly tectonized and folded, with no fossils (marine or the overriding glacier (Fig. 5). A bed with very lacustrine?) Mya truncata and Hiatella arctica were identified in beds D and E. They were radiocarbon dated at 11,400± 100 bouldery till (C) 'overlies' beds of gravelly silt years BP (T-3082). (D) and grave! (E) with shell fragments. Mya NORSK GE OLOGISK TIDSSKRIFT 1 (1982) The Tjøtta g/acial event 43

NW Crest of moroine ridge SE

Distol l Proximol

100m

Fig. 6. The Angersnes end moraine. A: 2 m grave) with many large boulders (marine-wave washed). B: 0.1-1 m m clayey, bouldery till. C: 3-4 m grave) and sand, foreset beds with shell fragments, radiocarbon dated at 11,600± 160 years BP (T- 3944). D: 0.3-0.7 m bouldery gravelly ela y with dropstones and shells, radiocarbon dated at 10,460 ± 190 years BP (T-3083). E: Fig. 7. Portlandia arctica locality (A) at Fagervik. B is a Iarger 0.5 m very gravelly and bouldery elay ( till?) with shells, radio­ terrase remnant with glaciofluvialgravels on top of ela y. Heavy carbon dated at 10,540 ± 90 years BP (T-3272) BedE probaLiy lines: Marginal moraines. Hatched line: The highest-lying corresponds with bed B. shore leve) (marine limit). Long arrow: Canyon.

truncata and Hiatella arctica were identified and An interpretation of the sparker profiles across radiocarbon dated at 11,400 ± 100 years BP (T- the submarine ridge in this fjord is difficult due 3082). This is a maximum age for the glacier to side reflections from the steep sides of the advance which caused tilting of the beds and fjord, but the ridge may be a deposit similar to deposition of the end moraine. that to the south of Tjøtta. The largest gravel pit, 10 m-20 m deep, Iies on A northern segment of the end moraine depos­ the proximal side of the moraine on a northeast­ ited by this glacier branch is situated at An­ southwest-oriented ridge which grades into the gersnes. Here, a gravelly and bouldery morainal end moraine. Beds of well sorted stratifiedgravel spur grades into an about 100 m broad, 4-8 m and sand are exposed, and the ridge is probably high and 500 m long end-moraine, which ends at an esker which has been levelled by marine the steep mountain side. A gravel pit in the ridge abrasion. exposed the section shown in Fig. 6. Corresponding to the Breimo end moraine is a A unit with foreset gravel beds (C, Fig. 6), of very sharp 3-8 m high and about 2.5 km long which 2 m to 4 m were exposed, represents the lateral moraine on the southeastern valley side. core and main unit of the moraine. A till (B), a The ridge slopes about 60-80 m/km in a south­ till-like bed (E) and a bouldery and gravelly westerly direction and ends about 100 m above glaciomarine clay (D) which overlies the foreset sea level. This shows that the glacier which de­ unit, must represent a later advance to the mo­ posited the Breimo moraine was very steep. A raine. Shells of Hiatella arctica from the till-like series of well-developed cirque moraines Iie im­ bed (E) were radiocarbon dated at 10,540 ± 90 mediately above the lateral moraine, and are years BP (T-3272). The glaciomarine clay (D) correlated with the moraine. However, one of which overlies the till-like bed is correlated with the cirque moraines crosses the lateral moraine a later phase of the glacial event or the start of and shows that cirque glaciers were active also the glacier retreat from the moraine. Shells from after the retreat of the trunk glacier. this ela y, Hiatella arctica and Yoldiella lenticu/a, A 150-190 m high bedrock ridge Iies along the were radiocarbon dated at 10,460 ± 190 years BP northwestern side of the valley, between Breimo (T-3083). The dates suggest that the glacier was and Sandnessjøen. This ridge is more or less located at the Angersnes moraine at about covered with drift which must represent ice-mar­ 10,500 years BP. Small shell fragments, mainly ginaldeposits associated with the Breimo moraine. of Mya truncata collected from the foreset beds, A grave! spur with boulders which projects into were radiocarbon dated at 11,660 ± 160 years BP the fjord at the southwestern end of Sandnes­ (T-3944). They are correlated with an interval sjøen is another segment of the marginal mo­ before the final glacier advance to the moraine. raine. This part of the moraine was deposited by The northern side of the Leirfjord glacier must a broad branch of the Leirfjord glacier which have been located along the steep southern moved in a westerly direction. slopes of the Lifjellene mountains, where small 44 B. G. Andersen et al. NORSK GEOLOGISK TIDSSKRIFT l (1982) sw NE 50 Fig. 7). This moraine possibly represents an older phase of the described morainal complex, 100 and the glaciofluvial grave) on top of the marine

150 clay and silt is correlated with this phase or with an early phase of the Brunes moraine. Conse­ 200 quently, the elay with Portlandia arctica is corre­ lated with an early phase of the Tjøtta glacial 250 event, or with a phase immediately before this 300 event. The Jack of dropstones in the clay sup­ ports the latter interpretation. Shells of Portlan­ dia arctica were radiocarbon dated at 11,210 ± 200 years BP (T-3084). The Ranafjord end moraine is a distinctive, � Cloy i:=.:]Sih [JSond large ridge which crosses the mouth of Ranafjord between Brunes and Huglnes. A sparker profile 8. Fig. Interpretation of a sparker profile across the submarine across the moraine shows a 300 m thick strati­ Ranafjord end moraine. Heavy lines: reflectors observed on fied deposit (Fig. 8). The ice-contact slope of the sparker profile. The suggested sediments and units (A-D) are based on correlation with well studied raised marine end mor­ moraine is about 325 m and the distal slope aines (Andersen 1979). A: lce-contact deposits, mainly till. B: about 200 m high. A parallel profile further "Foreset" beds, mainly of glaciofluvial grave! and sand with north shows that the thickness of the stratified some flow til!. C: Marine or glaciomarine sand, silt and ela y, in part older than the moraine. D: Glaciomarine sediments, unit exceeds 500 m. The bedding is typical for mainly silt and clay with numerous dropstones. end moraines deposited in a marine environment (Andersen 1979). Unit A on the ice-contact slope is an approximately 30 m thick weakly stratified unit with beds dipping less than 10°, scattered remnants of a lateral moraine were parallel with the surface. The unit could have found. However, distinctive marginal-moraine been formed by sliding of sediments from unit B ridges cross the north-eastem end of Lifjeldene, when the ice retreated from the moraine. How­ 400-550 m above sea level. They continue in a ever, the low angle of the beds together with Jack well-developed lateral-moraine ridge on the of unit A beds at the deepest levels suggest that northem slope of the mountains. The moraine this interpretation is less likely. Similar units drops steeply in a westerly direction towards have been found in almost all well-studied Brunes on the southem side of Ranafjord, and emerged marine end moraines in Norway. They Brunes is a long morainal grave) spur which pro­ consist of ice-contact deposits, tills and poorly jects into the fjord (Fig. 7). sorted glaciofluvial sediments, and this is prob­ A marine clay with Portlandia arctica was ob­ ably also the origin of unit A. served by Rekstad (1904) at Fagervik, in the Unit B with beds dipping about 100 in a distal valley to the southeast of Brunes (Fig. 7). An (westerly) direction is interpreted as a foreset exposure 60-70 m above sea Jevel in a small hill unit of glaciofluvial sand and gravet. They grade about l km from the Brunes moraine showed: 2 into a bottomset unit D of pro bable glaciomarine m of glacio-fluvial grave! on top of 4 m of clayey clay and silt with dropstones. The thickness of silt with no fossils which rests on 4 m of silty clay this unit decreases in a westerly direction and it with many Portlandia arctica and a few Hiatella more or less ends at a bedrock threshold about 4 arctica (A, Fig. 7). The clayey silt and silty clay km from the moraine ridge. No unit of clearly units are in part Jaminated, but no dropstones post-glacial sediments was observed on top of the were seen. Terraces and a distinct shore line glaciomarine unit, and the post-glacial sedimen­ about 90 m above sea leve! were found in adja­ tation must have been very limited. cent parts of the valley, and a larger terrace Unit C consists of flat-lying beds. They could remnant with glaciofluvial grave! resting on ma­ be bottomset beds with fine-grained sediments or rine clay Iies 70 to 80 m above sea leve! next to beds with coarser glaciofluvial sediments. the fossil locality (B, Fig. 7). Further upstream a The evidence presented shows that the Rana­ small lake is dammed by an end moraine which fjord end moraine is a typical marine end mo­ must be slightly older than the Brunes moraine raine, and it is one of the !argest moraines known and the lateral moraine mentioned above (see in Norway. NORSK GEOLOGISK TIDSSKRIFT l (1982) The Tjøtta glacial event 45

To the north of Ranafjord, a low moraine w ridge on Huglnes, a sharp 2 km long moraine 2km ridge 200 m to 250 m above sea level on the northeastem part of island, and a small morainal spur on the north side of the island, represent the associated marginal moraines. Between Hugla and Handnesøy there is also a very marked submarine end-moraine ridge (Fig. 9) which resembles the Ranafjord end moraine (Fig. 8). Unit B grades into unit D which has a rather irregular surface. A 300 m broad and 50 m deep scar is probably caused by slumping of the

fine-grained sediments. Unit A is very thick and Fig. 9. Interpretation of a sparker profile across the submarine two distinctive ridges can be distinguished. The end moraine between Hugla and Handnesøy. See Fig. 8 for proximal (eastem) ridge must have been formed explanation. during a re-advance of the ice front. Several of the moraine ridges described on land also branch into two or more ridges within limited areas, mlkm were deposited by the steep outlet gla­ suggesting that the ice-front fluctuated during the eiers. deposition of the moraine. Unit E (Fig. 9) is probably of glaciomarine origin, but is younger The radiocarbon dates, a brief review than the end moraine. Segments of a correspond­ ing marginal moraine were observed on Hand­ Altogether 15 of the shell samples collected in nesøy. A morainal ridge about 800 m long covers the area described were radiocarbon dated. All a bedrock point at the southwestern end of the of the samples were collected to date the mo­ island. Small moraine ridges Iie on the eastern raines. On the basis of stratigraphical evidence and northem slopes, and a small morainal spur they are correlated with different phases of mo­ projects into the Sjonafjord on the north side of raine formation or phases immediately before or the island. An about 4 km long and in part very after the formation of the moraines. A list of the sharp lateral moraine Iies along the northem side dates together with their correlation with the of the Sjonafjord. The moraine drops steeply in a moraines is presented in Fig. 11. They show that westerly direction fromabout 450 m to about 200 the Tjøtta event is 11,000 ± 100 to 10,500 ± 100 m above sea level. The equivalent end moraine radiocarbon years old. The two parallel ridges which crosses Sjonafjord is shown on Fig. 10. observed in some areas could represent glacier­ The water depth on the proximal side of the fluctuationsof very early Younger Dryas age and ridge is more than 500 m. The thickness of sedi­ middle Younger Dryas age. ments within the ridge exceeds 100 m, and the crest Iies at about 170 m below sea level. The nearest rock basin on the distal side of the mo­ w raine is occupied by glaciomarine sediments. The 2km next basin is half filled, and there are almost no sediments in the third basin about 4 km from the moraine. A similar rapid decrease in the thick­ ness of the glaciomarine sediments in a distal direction has been observed at several marine end moraines. Small segments of lateral moraines on the steep valley slopes further to the north of ­ fjord together with distinctive marginal moraines deposited by small outlet glaciers in the west­ and north-facing valleys show rather accurately �Cioy IZJSi• [JSond �G ..... 63TIII the location of the former ice margin. Most of

the west- and north-facing valleys are very steep, Fig. 10. Interpretation of a sparker profile across the subma­ and lateral moraines with gradients up to 200 rine end moraine in Sjonafjord. See Fig. 8 for explanation. 46 B. G. Andersen et al. NORSK GEOLOGISK TIDSSKRIFr l (1982)

OLDER THAN BET WEEN BETWEEN GLACIAL EVEN TS T VEGA TJØT TA YOUNGER THAN VEGA VEGAond TJØT TAond NORDL I LAB . NO. EVE NT EVE NT THE NORDL IEVENT sTY EVENT TJØT TA E. NORDL I E.

T- 3941 VEGA .. J3.420i170o.. SØVIKSKAR ....10.940± ]2Q... . T- 3271

SØVIK ··10.800t180·.. T- 3081

BREI MO .. ll • .WO±lQ()o.. T- 3082 T- 3275 GRØN LIAKSLEN ...9 .620!1lo-... ANGER SNES l .. ]0.540±80... . T- 3272

ANGER SNES11 ""10.460±190 .... T- 3083

ANGER SNES111 •• 11.660"!160 ... T- 3944 T- 3273 FORSLANO ...J O.lOOi 100·......

• T- NORDLI "10.140t 180 ... 3085

FAGER VI K ....11.210! 200.... T- 3084 T- 3521 SK RAV LÅEN ... 9.780±130·"

HEM NES ... 9.460±270 ... T- 3943 T- 3276 LAU K BA K KEN • · 10.510t 190 • • T- 3086 JE KT VIK .. 11.7oot8o....

Fig. 11. Radiocarbon dates of shells related to the glacial ever.ts. Exarnple: When a date with the dotted lines on both sides Iie entirely within the Tjøtta event column it shows that the dated sarnple was correlated with the Tjøtta event. If one of the dotted lines cross into one of the adjacent columns it shows that a correlation with a slightly older/younger phase can not be excluded. If the date is placed across the line between the Tjøtta event column and an adjacent column it shows that the dated sample could as well be slightly older/younger than the Tjøtta event.

tion of the glaciomarine beds with the moraine The local cirque moraines seems likely, although it is not possible to prove An abundance of cirque moraines Iies in the this correlation definitely. On the basis of the mountain districts immediately on the distal side evidence presented, we conclude that most of the of the marginal moraines described. On the distinctive cirque moraines on the distal side of proximal side, there are only a few cirque mo­ the Tjøtta event ice margin are most probably of raines, and they Iie within the very highest moun­ Younger Dryas age and correspond with the tain areas with many glaciers today. Andersen Tjøtta event. (1968, 1975) described the same conditions in connection with the Younger Dryas (Tromsø­ Lyngen) moraines in Troms and northem Nord­ The glaciation limit land. Apparently the Younger Dryas cirque gla­ eiers were very active, and the activity decreased The local moraines were used to calculate the drastically after that time due to climatic amelio­ altitudes of the glaciation limit in two areas, to ration. the north of Melfjorden and between the islands Shells from a glaciomarine deposit at Lauk­ of and Lurøy. The calculations were bakken to the north of were radiocarbon based on the 'top method' as described by An­ dated at 10,510 ± 190 (T-3276) years B.P. The dersen (1975), and aerial photographs from 1978 shells, Macoma calcarea, Mya truncata, Astarte were used to distinguish the glaciers for the cal­ elliptica and Astarte borealis lay in a silty sand culation of the modem glaciation limit. The re­ with many stones, some of which were striated. sults showed that the Younger Dryas glaciation Most shells were whole, and some lay in their limits were 610 m and 590 m above the Younger original positions. Only l m of the sand was Dryas sea level in the two areas. Near the exposed, and it was overlain by a 1.5 m thick bed Høgtuva glacier the modem glaciation limit is of glaciomarine, gravelly clay with dropstones about 1175 m above present-day sea level. The and shells of Macoma calcarea and Mya truncata. altitude of the modem glaciation limit descends About 0.5 m of mixed beach grave! and slumped in westerly direction towards the sea, and it Iies material covers the beds. A cirque moraine Iies about 1100 m above the sea leve! at Tomma­ only 100 m-200 m fromthe locality and a correla- Lurøy. Therefore the glaciation limit during the NORSK GEOLOGISK TIDSSKRIFf l (1982) The Tjøtta glacial event 47

Tjøtta event was approximately 500 m-550 m o 0·1""' lower than the modem glaciation limit. I:·.S% ' z A depression of the early Younger Dryas gla­ ® .s'-10"11. � l ! • 10- 20"11. l .. f g ciation limit in the region of 500 m corresponds .l! i J � � z 20-40% � 1. l � ..J ' ... well with observations in the Glomfjord-Skjer­ 40-60"11. ' � j l j :; l � ' � l 60- 100"' i � � � i � � stadfjord areas, immediately to the north of the � ! .. � e z i. i. i E l j � Tjøtta-Nesna area (Andersen 1975, Rasmussen lOCALITY ;;; ;;; V :1 ;;: ;;: O � � � � 1981, Vallevik 1981), and with observations in SØVIK l l • • ® o • ® • 31 o . o o o o other parts of Norway (see review in Andersen SØVIK 2 • • • 71 SØVIK 3 • . • ® . 5 70 1975). SØVIK 4 • o l o 2 73 SØVIKSKAR • • . o 5 n BREIMO • o • ® o • o • .. The Main shore line ANGEASNES 1 • o ® ® . 82 ANGEASNES 2 • o ® • • 73 A distinctive Main shore line is well developed in F.4.GERVIK l l ® 7 ,. several areas on the distal side of the Tjøtta Fig. 12. Distribution of lO foraminifera species collected from event moraines. This shore line represents the glaciomarine or marine sediments connected to the Tjøtta marine limit dose to the moraines and it must moraines. have formed during the Tjøtta event. The re­ corded Main shore lines are plotted on Fig. 2 together with the corresponding isobase. The iso­ line had been at least 4 cm in some areas near base shows that the Main shore line has a gradi­ Glomfjorden. ent of about l m/km. This agrees well with gradi­ ents further to the north in Nordland (about l The fauna mlkm, Rasmussen 1981) and in Troms and Finn­ mark (about 0.9 mlkm, Marthinussen 1960, An­ Thick units of glaciomarine clays with a Port/an­ dersen 1968 and Sollid et al. 1973). In all these dia arctica molluscan fauna and an Elphidium areas the Main shore line was correlated with the excavatum - Cassidulina reniforme foraminiferal Younger Dryas end moraines. Both Andersen fauna (Fig. 12) were found on the distal side of (1968) and Rasmussen (1981) showed that the the Tjøtta moraines. This fauna lived in the sea shore line is frequently best developed in shel­ at ice fronts during the Tjøtta event. As de­ tered areas near the early Younger Dryas end scribed by Hessland (1943), Portlandia arctica is moraines. In addition they observed that the a high-arctic species which thrives in shallow wa­ rock surfaces are generally shattered and not ter ((}...100 m) with temperatures between - 2°C well rounded at the shore lines which have been and + 2,5°C. In addition Portlandia arctica formed in bedrock. They concluded, therefore, thrives in the muddy, slightly brackish water at that frost shattering and the periodical break-up ice fronts (Thorson 1934). Many younger glacio­ of the sea-ice were the most important factors in marine clays which have been studied in Nord­ the formation of the lines. The cold, slightly land contain no Portlandia arctica fauna. Other brackish water in front of the glaciers probably arctic species like Bathyarca glacialis and Yol­ promoted the formation of sea ice, and the tide diella lenticula dominate in several of these clays. which has a range of about two metres in this This indicates that Portlandia arctica disappeared area today probably caused the sea ice to move from the coast or became very rare after the up and down every day and created tension at Tjøtta event. Studies of the benthonic and planc­ the ice/rock contact. In addition, water derived tonic foraminiferal faunas in cores from the shelf from melted snow at this contact could freeze off the coast of Trøndelag ( 65°N) show a marked and shatter the rock surface. During the seasonal change from cold water to warmer water faunas break-up of the sea ice and the ice foot, rock in late Younger Dryas time (Løfaldli & Rokoen­ fragments could be plucked out and transported gen 1980). Based on extrapolation between two away from the shore. This mechanism was de­ core levels that were radiocarbon dated at scribed by Nansen (1922), who considered it to 11,230 ± 100 years BP (T-2929) and 9,920 ± 110 have been an important factor in the formation BP (T-3236), an age of about 10,400 ± 200 years of the 'strandflat'. Rasmussen (1981) found that BP was suggested for the change. A similar tran­ the average annual cliff retreat at the Main shore sition in foraminiferal fauna near the end of 48 B. G. Andersen et al. NORSK GEOLOGISK TIDSSKRIFT l (1982)

Younger Dryas time has been recorded by sever­ Acknowledgements. - We are very grateful to the following al scientists in different areas along the coast, but institutions and persons: The Norwegian Research Council for Science and Humanities (NAVF) and The Royal Norwegian not dated exactly (Vorren et al. 1978, Sejrup et Council for Scientific and lndustrial Research (NTNF) for . al. 1980, Rasmussen 1981). financial support, The Hydrographic Office of Norway for Considering the available evidence, a rise in assistance and cooperation, The Radiological Da ting Laborato­ temperature caused by the incursion of warmer ry of Norway for radiocarbon dating, cand. real. Dagfinn Bøe for the foraminiferal analysis, Department of Geology, Univer­ Atlantic water seems to be the most likely reason sity of Bergen and IKU for help with drawing and typing and for the disappearance or decrease of Portlandia Dr. Brian Robins for correcting the English language. arctica, and the retreat of the glaciers from the Manuscript received November 1981. Tjøtta moraines. References The early Younger Dryas climate Andersen, B. G. 1968: Glacial Geology of Western Troms, The distinctive and in places large, marginal mo­ North Norway. Nor. geo/. unders. 256, 1-160 raine-ridges of the Tjøtta event which have been Andersen, B. G. 1975: Glacial Geology of northern Nordland, found along the entire coast of Nordland must North Norway. Nor. geo/. unders. 320, 1-74 Andersen, B. G. 1979: The deglaciation of Norway 15,000- have been deposited during a glacial advance. 10,000 B.P. Boreas 8, 79--87 The folded marine sediments and marine shells Andersen, B. G., Bøen, F., Rasmussen, A. & Vallevik, P. N. within the till beds in several end moraines also 1979: The deglaciation between Skjerstadfjord and Svart­ show that the glacier advanced to the moraines isen, North Norway. Boreas 8, 199--201 Grønlie, O. T. 1940: On the Traces of the lee Ages in Nord­ across marine deposits. This advance, observed land, Troms and the South-Western part of Finnmark in in a wide area, was definitelycaused by a climatic northern Norway. Nor. Geo/. Tidsskr. 20, 1-70 deterioration, a lowering of the summer tem­ Hessland, l. 1946: Marine Schalenablagerungen ;-!ord-Bo­ peratures and/or an increase in winter precipita­ huslans. Bull. Geol. Inst. Uppsala, 31, no. 149 Jansen, E. , Sejrup, H. P., Fjæran, T., Hald, M., Holtedahl, H. tion. & Skarbø, O. in prep.: Late Weichselian Paleoceanography The calculated depression of the glaciation in the south-eastern Norwegian Sea. limit during the Tjøtta event was in the order of Løfaldli, M. & Rokoengen, K. 1980: Late- and post-glacial 500 m, and this information allows some conclu­ foraminifera and sediments on Viknaryggen and Haltenban­ ken off Mid-Norway. Continental Shelf Institute, no. 103, 1- sions to be reached about the early Younger 62 Dryas climate. Assuming that the precipitation Mangerud, J. , Andersen, S. Th. , Berglund, B. E. & Donner, was about as high as at present, the depression J. 1974: Quaternary stratigraphy of Norden, a proposal for corresponds with an early Younger Dryas sum­ terminology and classification. Boreas 3, 109--128. Marthinussen, M., 1960: Coast and fjord area of Finnmark. mer temperature about 3.0-3.5°C lower than the With remarks on some other districts. Nor. geo/. unders. present (Andersen 1968). On the basis of the 208, 416-429 pollen flora in a core from lake Endletvann on Marthinussen, M. 1962: C-14 datings referring to shorelines, the island of Andøya in northern Nordland, K. transgressions and glacial substages in northern Norway. Nor. geo/. unders. 215, 37-67 D. Vorren (1978) also arrived at the conclusion Nansen, F. 1922: The strandflat and isostasy. Skr. norske that the Younger Dryas summer temperature Vidensk.-Akad. Mat.-naturv. Kl. 313 pp. was about 3.0-3.5°C lower than today. Rasmussen, A. 1981: The deglaciation of the coastal area NW A verification of the presumed high precipita­ of , northern Norway. Nor. geo/. unders. 369, 1-31. Rekstad, J. 1904: Beskrivelse til kartblad Dønna. Nor. geo/. tion is more difficult to find. However, the sea unders. 37, 32 pp. must have been open offthe coast of Nordland in Rekstad, J. 1917: Vega. Beskrivelse til det geologiske general­ order to obtain a high precipitation. On the basis kart. Nor. geo/. unders. 80, 85 pp. of foraminiferal analysis of cores from the ocean Rekstad, J. 1922: Norges hevning efter istiden. Nor. geo/. unders. 96, 27 pp. floor off the coast, both Løfaldli & Rokoengen Rekstad, J. 1925: Træna. Beskrivelser til det geologiske gener­ (1980) and Jansen et al. (in prep.) arrived at the alkart. Nor. geo/. unders. 125, 36 pp. conclusion that the sea off central Norway was Rekstad, J. & Vogt, J. H. L. 1900: Søndre . Nor. open during Younger Dryas time. Therefore, geo/. unders. 29, 62-112. Rokoengen, K. 1979: Foreløpig tolkning av grunnseismikk. conditions were probably favourable for a high Tokt 7911, Træna banken og fjordene innenfor. Continental precipitation. Consequently, the early Younger Shelf Institute, P-155119179, 28 pp. Dryas climate along the coast of Nordland was Sejrup, H-P., Holtedahl, H., Norvik, O. & Miljeteig, 1., 1980: probably as humid as at present, with summer Benthonic foraminifera as indicators of the paleoposition of the Subartic Convergence in the Norwegian - Greenland temperatures in the order of 3.0-3.5°C lower Sea. Boreas 9, 20�207. than at tJresent. Sollid, J. L., Andersen, S. , Hamre, N., Kjeldsen, 0., Salvig- NORSK GEOLOGISK TIDSSKRIFT l (1982) The Tjøtta glacial event 49

sen, 0., Sturød, S., Tveitå, T. & Wilhelmsen, A., 1973: Thorson, G., 1934: Contributions to the Animal Ecology of the Deglaciation of Finnmark, North Norway. Norsk Geogr. Scoresby Sound Fjord Complex (East Greenland). Medd. Tidsskr. 27, 23:>-325. Grønland 100 (3), 67 pp. Svensson, H. 1957: Ra-moraner vid Helgelandskysten? Nor. Vallevik, P. N. 1981: Kvartærgeologiske undersøkelser og jord­ Geo/. Tidsskr. 37, 275-278. artskart/egging i Beiarn, Nordland. Unpubl. cand. real. the­ Svensson, H. 1959a: Glaciation och morfologi. En glacialgeo­ sis, Univ. of Bergen. grafisk studie i ett tvarsnitt genom Skanderna mellan S. Vorren, T. 0., Strass, 1., Lind-Hansen, O. W. 1978: Late Helgelandskusten och Kultsjooalen. Meddn. Lunds Univ. Quaternary sediments and stratigraphy on the Continental Geogr. Inst. Avh. 36, 283 pp. Shelf off Troms and west Finnmark, northern Norway. Quat. Svensson, H. 1959b: Syv søstre. En glacialmorfologisk studie Res. JO, 340-365. av ett fjållområde ved Nordlandskusten. Norsk Geogr. Vorren, K. D. 1978: Late and Middle Weichselian stratigraphy Tidsskr. 16, 249-270. of Andøya, North Norway. Boreas 7, 19-38.

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