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The Norwegian Strandflat: a Reconsideration of Its Age and Ongtn

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The Norwegian Strandflat: a Reconsideration of Its Age and Ongtn The Norwegian strandflat: A reconsideration of its age and ongtn EILIV LARSEN & HANS HOLTEDAHL Larsen, E. & Holtedahl, H.: The Norwegian strandflat: A reconsideration of its age and origin. Norsk Geologisk Tidsskrift, Vol. 65, pp. 247-254. Oslo 1985. ISSN 0029-196X. Based mainly on stratigraphical data and information on sea-leve! and cirque glacier erosion rates, the age and processes for the formation of the Norwegian strandflat are discussed. It is concluded that the main processes are frost-shattering in combination with sea-ice transportation and planation during glacial stages in the last 2.5 Ma. E. Larsen, Geological Survey of Norway, P. O. Box 3006, N-7001 Trondheim, Norway. H. Holtedahl, Department of Geology, Section B, University of Bergen, A/legt. 41, N-5000 Bergen, Norway. Since Reusch (1894) introduced the term 'strand­ sion (0. Holtedahl 1929), frost-shattering and flat' and discussed its origin, a number of scien­ sea-ice erosion (Nansen 1904, 1922), or some tists have proposed different geneses and ages kind of combination of different processes (Hol­ for its formation. With quantitative data at hand, tedahl 1959, Klemsdal 1982). Biidel (1978) ar­ it is now possible to exclude some of the earlier gued that the strandflat was formed under a hypotheses. tropical climate as an etch-plain with inselbergs. It is difficult to define precisely the strandflat, Clearly, the different processes imply very dif­ but it is an uneven and partly submerged rock ferent ages for the formation of the strandflat, platform extending seawards from the coastal and suggestions have therefore varied from pre­ mountains. Where well developed, the strandflat Quaternary to glacial or interglacial stages in the consists of numerous low islands, skerries, shal­ Quaternary. Holtedahl (1959) reviewed the dif­ low sea areas and a low rock platform that abuts ferent theories with respect to formation and onto a steep slope. In many cases the supramar­ age. ine strandflat can be seen as a brim of low lying land around islands (Fig. 2). Along the Norwegian coast, the strandflat is Age of the strandflat developed from Stavanger to Magerøya (Fig. 1). In this region, its width varies from 50-60 km in In the Møre area a number of marine caves are Helgeland and the northern part of Møre to a situated above the Late Weichselian marine limit more or less total absence in the Stad area (Fig. (Holtedahl 1984). The caves are developed in l) (Holtedahl 1959). The strandflat is not con­ zones of structural weakness on the steep slopes fined to the Norwegian coast, but is developed that form the inner limitation of the strandflat. along Arctic and Antarctic coasts in many parts The age of the caves is thus a minimum age for of the world. Nansen (1904) arrived at the con­ the formation of the strandflat. Recent stratigra­ clusion that typical strandflat areas are only phical investigations in the cave Skjonghelleren found in formerly glaciated regions. This howev­ (Fig. l) have shown that it contains same 15-20 er, has been disputed by Fairbridge (1977), who metres of unconsolidated sediments (Larsen et claimed that shore platforms in formerly ungla­ al. 1984). An excavation in the inner part of the ciated areas of western France and along the cave down to about 6 metres below the sediment west coast of U.S.A. should be considered surface shows bouldery beds in alternation with strandflats. fine-grained laminated sediments (Fig. 3). The A wide range of processes has been suggested laminated beds were deposited subglacially in a for the formation of the strandflat. These include waterfilled cave while the bouldery beds were marine abrasion (Reusch 1894), subaerial denu­ formed during ice-free periods (Larsen et al. dation (Ahlmann 1919, Evers 1962), glacial ero- 1984). Two 14C dates on fossil bones and three 17 - Geologisk Tidsskr. 4/85 248 E. Larsen & H. Holtedahl NORSK GEOLOGISK TIDSSKR!Ff 4 (1985) �.�....:.-.--- ......,' : ' l ' � l l ' ' { N ��- 1 l l l .... : � , «- ,' C) 'o � ,, ....... .;,' / l l -t-f>�'t. l SKJONGHELLEREN ) STAD KRÅKENES � 11 NORTH : l SEA ' 'l l BERGEN , l' l l l )l r l l 400 Km Fig. l. Map of Norway with geographical names mentioned in the text. The strandflat is developed from the Stavanger area to Magerøya. Uffh dates on speleothems from the upper inter­ the cave was formed during the initial phases of stadial bed all gave ages around 30,000 yrs. B.P. the last glacial stage. If correct, this implies that (Larsen et al. 1984, in press) (Fig. 3). This age is considerable strandflat development might have thus a minimum age for the formation of the taken place in this area in Early Weichselian time cave. Considering the shallow depth of the dated and that practically no strandflat development bed, the nature of the excavated sediments be­ has taken place since then. This seems to be true low that bed (Fig. 3), and the total sediment in spite of the assumption that western Norway thickness, it is evident that the cave is far older was ice-covered only for some 30% of the last than the age obtained. It has been suggested by glacial cycle (Miller et al. 1983). Dated sedi­ Larsen et al. (in press) that the base of the ments located on the strandflat, other than cave sediment sequence is Early Weichselian, and that sediments, also provide minimum ages for the NORSK GEOLOGISK TIDSSKRIFT 4 (1985) The Norwegian strandflat 249 Fig. 2. The island of Valderøya with the low brim belonging to the strandflat. The cave Skjonghelleren is situated on the steep mountain slope that abuts on the strandflat. Photo looking south. formation of the strandflat. No pre-Eemian sedi­ by Late Pliocene time. It is, however, reasonable ments are known to exist at the Norwegian to assume that a subsidence of the North Sea strandflat, but at Brøggerhalvøya, Spitsbergen, a occurred in connection with the Tertiary uplift thick stratigraphical sequence is situated on the (Rokoengen & Rønningsland 1983). This indi­ strandflat. The oldest sediments have an absolute cates that at !east most of the Tertiary uplift had age between 0.3 and l Ma inferred from amino come to an end by Late Pliocene time. From this acid ratios in molluscs (Miller 1982). This implies it follows that the strandflat is younger than the that the strandflat in that area is older than the Middle Pliocene. One piece of seemingly con­ cited time span. flicting evidence is found in the results from the The Norwegian strandflat must be younger Fjøsanger section near Bergen (Mangerud et al. than the cessation of the Tertiary uplift of Scan­ 1981) (Fig. 1). Based on stratigraphical reason­ dinavia (e.g. Holtedahl 1959), which provides a ing, these authors have determined a net uplift maximum age for its formation. Faults in the between 10 and 40 metres since the Eemian. border zone between the North Sea sedimentary Mangerud et al. (1981) concluded that this uplift rocks and the crystalline rocks to the east may is mainly a result of longterm tectonic move­ have been active in Pliocene time or even later ments, possibly with some additional effect of (Egeberg 1977). No faults have been detected on isostatic compensation for glacial erosion. Holte­ sparker records of the Late Pliocene sediments dahl (1984) is also of the opinion that neotectonic from the northern North Sea (Rokoengen & movements have been active and responsible for Rønningsland 1983). This implies that Late Plio­ the high positions of sea-formed caves along the cene tectonic movements either took place solely coast of Møre. If back-cutting of cliff walls has within the basement area or that they had ceased kept up with the rate of uplift, there might be no 250 E. Larsen & H. Holtedahl NORSK GEOLOGISK TIDSSKRIFT 4 (1985) ma.s.l. conflict. Another point worth making is that the conclusions regarding tectonic movements (Man­ w E Beds gerud et al. 1981, Holtedahl 1984) are not quite so straightforward. The uplift deduced from Fjøsanger, for instance, might well have been episodic. There is also the possibility that the 62 Fjøsangerian Interglacial is older than the Ee­ mian (J. Mangerud, pers. comm. 1984), in which _clay D case the uplift rate will be greatly reduced. Hol­ Clay with tedahl's (1984) conclusion on tectonic move­ intraclasts E ments is based on the assumption that all the high-lying caves were formed at the same time. This assumption cannot be tested at present. Laminated ela y F Processes The different theories for the formation of the strandflat have been listed above. Based on ob­ servations in Antarctica, O. Holtedahl (1929) strongly favoured the idea of cirque glaciers cut­ ting backwards into mountain slopes. This was Silt H later supported by Dahl (1947). From measure­ ments of the volume of sediments deposited from a Younger Dryas cirque glacier at Kråkenes (Fig. 1), Larsen & Mangerud (1981) found that the erosion rate of that glacier (0.5-0.6 mm·a-1) was in accordance with the estimated rates from modem and other former glaciers. Using this erosion rate, an estimate of 65 to 80 Ma is ob­ 60. Laminated ·clay l tained for a cirque glacier to erode 40 km back­ wards. The erosion rate of Larsen & Mangerud (1981) is a mean for the whole glacier bed, and the backward erosion is thought to have been somewhat greater. Even if the coast were to a large extent dissected into fjords before the Quaternary (e.g. Rokoengen & Rønningsland 1984), so that cirque erosion could have started from many sides, the erosion rate is so low that cirque glacier erosion can be excluded as the main process for strandflat formation.
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