Glacial geology and deglaciation chronology of the area between inner Nordfjord and Jostedalsbreen Strynefjellet, western Norway NORALF RYE, ATLE NESJE, RUNE LIEN, LARS HARALD BLIKRA, OLIANNE EIKENÆS, PER AUDUN HOLE & INGRID TORSNES

Rye, N., Nesje, A., Lien, R., Blikra, L. H., Eikenæs, 0., Hole, P. A. & Torsnes, 1.: Glacial geology and deglaciation chronology of the area between inner Nordfjord and Jostedalsbreen - Strynefjellet, western Norway. Norsk Geologisk Tidsskrift, Vol. 77, pp. 51-63. Oslo 1997. ISSN 0029-196X. A lower limit of blockfields is inferred to indicate the maximum heights and thus thickness of the Late Weichselian iee sheet in the inner Nordfjord region. lee movements in this area have been topographically controlled during the entire Weichselian glaciation. Prominent lateral moraines de1imit the Younger Dryas valley glaciers in inner Nordfjord. Subsequent to the Younger Dryas Chronozone, the glaciers retreated rapidly due to calving in the fjord and climatic amelioration. In a later phase of deglaciation, in all probability around the early and middle part of the Preboreal Chronozone, an iee eentre east of Strynefjellet dominated, while the Jostedalsbreen area is thought to have played a minor role as a eentre of iee dispersal. The final deg1aciation was dominated by vertically down-wasting iee remnants in the lake basins and tributary valleys. Terminal moraines in front of several outlet glaciers of Jostedalsbreen beyond the 'Little lee Age' moraines indicate a climatic deterioration at the end of the Preboreal Chronozone. N. Rye, Department of Geology, University of Bergen, A/legt. 41, N-5007 Bergen; A. Nesje, Department of Geography, University of Bergen, Breiviken 2, N-5035 Bergen-Sandviken; R. Lien, Statens Vegvesen, P.O. Box 608, 9800 Vadsø; L. H. Blikra, Geological Survey of Norway, P.O. Box 3006, N-7002 Trondheim; O. N-0301 Oslo; P. A. Hole, Statoil, N-5020 Bergen;

l.

Eikenæs,

Norges vassdrags- og energiverk, P.O. Box 5091 Majorstua,

Torsnes, Mø//esvingen 2, 0854 Oslo, Norway.

Introduction

Recent investigations in the area between ioner Nord­ fjord and Jostedalsbreen (breen = glacier) - Strynefjellet (fjellet= mountain) (Rye et al. 1984, 1987; Nesje 1984; Lien 1985; Hole 1985; Blikra 1986; Nesje et al. 1987, 1991, Nesje & Kvamme 1991; Nesje 1992; Dahl & Nesje 1992; Nesje & Dahl 1992; McCarroll & Nesje 1993; Torsnes et al. 1993) make it possible to present a deglaci­ ation history from the Late Weichselian maximum up to the present in this part of western Norway. Marine terraces have previously been mapped by Kal­ dhol (1912). Rye (1963, 1978) and Fareth (1970, 1987) described the deglaciation in the middle and inner parts of Nordfjord, while the areas between Jostedalsbreen and Strynefjellet have been less well known. Stokke (1982) mapped the Quatemary deposits in the valley bottom in Stryn and Hjelledalen.

Bedrock

The bedrock in Nordfjord consists of Precambrian gneisses and granites which are l 000-1800 million years old. The rocks are divided into two main units; the Fjordane Complex (at surface) and the Jostedal Complex (at depth). Ioner Nordfjord belongs to the Jostedal Com­ plex. The dorninating rocks are banded gneiss and granitic gneiss. The tectonic history of the bedrock in

ioner Nordfjord is complicated and the rocks have been subject to several deformation phases during the Precam­ brian and Caledonian orogenies.

Landforms

The landscape of ioner Nordfjord (Figs. l, 2) has evolved from a plateau landscape which was developed close to sea level during the Mesozoic and during a subsequent uplift in the Tertiary. At present, remnants of this plateau landscape can be seen as rather smooth, undulat­ ing sumrnit areas along Nordfjord, gently sloping from 1800-2000 m at Jostedalsbreen to 400-600 m at the coast (Fig. 3). During the Quatemary glaciations, the plateau landscape was exposed to glacial erosion. During the ice-free interglacial periods, fluvial and avalanche activity modified the landscape. The valleys in ioner Nordfjord have topographic features characteristic of glaciated areas: steep valley sides with U-shaped trans­ verse profiles, bedrock basins occupied by lakes or filled in with sediments, rock thresholds, cirques, and hanging valleys. The fjord and the main valleys in ioner Nord­ fjord are good examples of topographic features, whose direction and glaciated form were controlled by fracture zones more so than in the homogeneous and resistant areas. In the main valleys there are both wide, deep basins and short, narrow gorges where rivers form water­ falls.

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SUNNMØRE

Skjlk

o

Fig.

10

20

30km

l. Location map of Nordfjord.

One branch of the Nordfjord, Oldedalen (dalen = valley), stretches about 20 km southward to Brigsdalen where two outlet glaciers from Jostedalsbreen, Brigsdals­ breen and Melkevollsbreen are situated. Another branch, Lodalen, reaches 15 km southeast where the three valleys Bødalen, Nesdalen and Kjenndalen coalesce. In these valleys the glacier outlets from Jostedalsbreen; Bø­ dalsbreen, Ruteflotbreen and Kjenndalsbreen are lo­ cated. Strynedalen branches north from Nordfjord, swings east, continuing about 20 km to the east, where the valleys Hjelledalen/Videdalen and Erdalen coalesce. Glomsdalen is a north-south oriented, hanging valley to Strynedalen at the eastern end of Strynevatnet ( vat­ net = lake). The valleys Skjerdingsdalen, Grasdalen and Sunndalen are tributary valleys to Hjelledalen.

Late Weichselian glacier extent, glacial maximum and ice movements

Mapping of blockfields in the mountain areas has demonstrated that the weathering limit is found to be about 1750 m a.s.l. at Strynefjellet, descending to about 1500 m a.s.l. between inner Nordfjord and Sunnmøre (Nesje et al. 1987; Rye et al. 1987, Fig. 4). The lower limit of the blockfields is interpreted by Nesje et al. (1987) to represent the upper limit of the Late Weichse­ lian maximum ice sheet. At that time, the glacier front reached the edge of the continental shelf off the Møre coast (Andersen 1979, 1981; Rokoengen 1979; Bugge 1980). Resistance to the concept of Late Weichselian nuna­ taks has none the less persisted, most recently by Fol­ lestad (1990), who stresses the consistent pattern of striations and till fabrics in the Nordmøre region, sug­ gesting ice movement largely independent of the local

terrain and an ice surface above the level of mountain summits. He notes the presence of erratics and tills in same blockfields reaching a level of at least 600-700 m. However, Pollestad seems to have included al­ lochthonous blockfields and boulder-rich till in his blockfield definition, and consequently his blockfield limit is 200-300 m lower than the autochthonous blockfield boundary described by Nesje et al. ( 1987) in the same region. In a recent paper, Larsen et al. (1995) conclude that the lower limit of the blockfield cannot be taken as the upper glacial surface during the Weichselian max1mum. Glacial striations show that ice movements in this area have been topographically controlled throughout the Late Weichselian glaciation, especially during the late phases (Fig. 5). The oldest striations in the area are found on Langvasseggi (1600 m a.s.l.). Striations north­ west of Videdalen and Djupvassegga (1500 m a.s.l.) show ice movements toward the northwest crossing Videdalen and Grasdalen towards Geiranger during the most exten­ sive phase of the last glaciation (Blikra 1986). The tribu­ tary valley Glomsdalen has two northern pass-points at 1360 and 1400 m a.s.l. toward Holedalen and Hellesylt, while Skjerdingsdalen has a pass-point of 1200 m a.s.l. toward Flydalen and Geiranger. Hole (1985) and Blikra (1986) mapped glacial striations in these pass-points (Fig. 5), showing ice movements from south to north. Lateral moraines deposited during the Younger Dryas Chronozone, indicating an ice surface around 800 m a.s.l. in Holedalen and around 900 m a.s.l. in Flydalen (Kalstad 1993). This indicates that the striations mapped in the pass-points must be older than the Younger Dryas Chronozone. These northerly striations might possibly represent the Late Weichselian maximum ice movement. In two pass-points between Glomsdalen and Skjerdings­ dalen (1340 and 1260 m a.s.l.) striations show glacier

NORSK GEOLOGISK TIDSSKRIFT

Fig. 2.

77 (1997)

Location map of inner Nordfjord and the Geirangerfjord area.

Glacial geology. Nordfjord-Jostedalsbreen

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77 (1997)

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Longitudinal profile of Nordfjord showing altitude of mountains on the northem and southem sides of the fjord together with the fjord bottom. The submarine part is adapted from Giskeødegaard (1983).

Fig. 3.

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50

movements towards the west and southwest (Fig. 5). The northern pass-point is probably so high that no active ice movement from Skjerdingsdalen to Glomsdalen took place during the Younger Dryas Chronozone. The other pass-points between these two valleys are lower and situated further south, which favour an active ice movement from east to west. In addition, Glomsdalen has a pass-point toward a valley between Flo and Holedalen (1160 m a.s.l.). Glacier movements towards the southwest out of Glomsdalen, as demonstrated by glacial scouring, probably took place both during the Late Weichselian glacial maximum and the Younger Dryas Chronozone (Fig. 5) (Hole 1985).

Younger Dryas glaciation

The investigated area is situated at the borderline of the inland ice during the Younger Dryas Chronozone. Be-

2400

Distance

(km)

yond the inland ice, local glaciation, especially lateral­ frontal moraine, was common (e.g., Reite 1967; Fareth 1970, 1987; Mangerud et al. 1979; Larsen et al. 1984). Evidence of Younger Dryas local glaciation, especially lateral-frontal moraine, is found in the cirque valleys south of Holedalen (east of Hellesylt). Fareth (1970, 1987) mapped the extent of the Nord­ fjord glaciers during the Younger Dryas readvance on the basis of lateral moraines. In Stryn, lateral moraines from this advance are deposited in Vikadalen (760-800 m a.s.l.) and Staurnibba (1084 m a.s.l.) (Fig. 5). On the basis of these moraines, Fareth (1970, 1987), in his reconstruction, indicates an ice surface at 1000-1100 m a.s.l. in Stryn. In Olden and Loen he placed the ice surface at 1100-1200 m a.s.l. In Olden two sets of lateral moraines 1170 and 1120-1130 m a.s.l. on the plateau north of Sisiliekruna may also be correlated to the Younger Dryas lateral moraines at Skarsteinfjellet fur­ ther to the north. On the eastern side of Oldedalen, lateral moraines at altitudes from 1080 to 1200 m a.s.l. E

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