Weichselian till stratigraphy and ice movements in Ottadalen, central south Norway JARLE HOLE & OLE FREDRIK BERGERSEN Hole, J. & Bergersen, O. F.: Weichselian til! stratigraphy and ice movements in Ottadalen, central south Norway. Norsk Geologisk Tidsskrift, Vol. 61, pp. 25-33. Oslo 1981. ISSN 0029-196X. Til! stratigraphy and ice movement have been studied in the Skjåk area in the valley Ottadalen. The stratigraphyof one locality, Gubbhågå, is described in detail. Both at Gubbhågå and at other localities in Skjåk waterlaid sediments have been found which are presumed to be the sediments from the end of an interstadial of Mid/Early Weichselian age, most probably Gudbrandsdalen Interstadial. On the basis of fabric analyses the overlying tills are correlated with different phases of ice movements and a four-phase model of these movements during the last lee Age has been established. These results are consistent with others from other parts of Ottadalen and Gudbrandsdalen. Investigations show that the effects of the ice erosion, even in large glacial troughs dose to the main water divide, have been surprisingly little during the last lee Age. Jarle Hole, Jordregisteringsinstituttet, Postboks 115, N-1430 Ås, Norway. Ole Fredrik Bergersen, Geologisk institutt, Avd. B, Universitetet i Bergen, A/Legt. 41, N-5014 Bergen, Norway. Localities where tbere are waterlaid sub-till sedi­ of basal gneisses of Precambrian age. About 8 ments from a Weichselian Interstadial in the km south of the valley, there is a 13 km wide Gudbrandsdal area have been recently described zone stretching east-west with Eocambrian and (e.g. Bergersen & Garnes 1971) (Fig. 1). Such Cambro-silurian sediments. Further south, large sediments are normally covered by different tills areas with the so-called Jotun rocks, mainly dark which are correlated with reconstructed ice basic crystalline rocks, occur. movement phases from the last ice age ((Garnes & Bergersen 1977, Garnes 1979) cf. Fig. 6). Several till sections where the stratigraphy is Stratigraphy at Gubbhågå thought to show the development of the last glaciation have also been found in Ottadalen (Lie There are a number of localities in Skjåk, mainly 1974, Skjerven 1978, Hole 1979). dose to the valley bottom or along the lower The 1ocalities which have been studied in the parts of the valley sides, where sorted sediments Skjåk area Iie in the upper part of the valley occur below tills that were deposited by different Ottadalen (Fig. 2) and one of these, at ice movements. In most places the general Gubbhågå, situated at about 400 m a.s.l. near the stratigraphy is as sketched in Fig. 2, though this valley bottom, is described here. is usually incomplete because the bedrock is not Ottadalen, a typical glaciated valley with large exposed. The lowermost unit observed consists overdeepened troughs, is the largest tributary of sorted sediment and is overlain by till which valley to Gudbrandsdalen, and extends from appears to have incorporated older (glacio-)flu­ Otta westwards to the water divide towards vial sediments, as suggested by the abundance of Sunnmøre in west Norway. Most of the popu­ rounded stones. One or more additional tills lated areas of Skjåk are concentrated in a large occur at the top (Fig. 3). through with very steep U-shaped valley sides. The locality Gubbhågå Iies at a former gully The bottom of the valley Iies only ca. 400 m section, oriented north-south, where one of the a.s.l., though the rivers have a ca. 480 km long sides has been levelled off due to land reclama­ path to the sea in Oslofjorden area. Surrounding tion (Fig. 7). In this section three different tills mountain plateaus are found up to 2000 m a.s.l. have been distinguished on the basis of textural and from the plateau to the south there are three and structural analyses. Beneath the tills there large tributaries which descend to the valley. are more than 7 m of sorted sand (Fig. 4). The The bedrock in the area consists predominantly sand was homogenous with several cm thick silt 26 J. Hole & O. F. Bergersen NORSK GEOLOGISK TIDSSKRIFT l (1981) '--- ..+ --.,"----.... Va l dr es HARDANGER­ VIDDA Waterlaid sediments presumed older than Late-Weichsel Fig. l. Localities with sediments and fossils presumed older than /v\ammoth finds Late-Weichsel in central south Norway. After Garnes & Other fossils older than Late-Weichsel Bergersen (1977). layers, and showed few clear structures, till unit. The overlying till (C), some 3 m thick although the layers had obviously been dis­ and also without clear structures, was very sand­ turbed, probably due to the overriding ice. rich but poor in coarser fragments. Roundness Neither macrofossils nor pollen have been analyses showed that the stones were more found in the section. There was a 0.5 m gradual angular than those in the till below, while fabric transition up to the overlying 7 m thick, fine­ analyses indicated a direction of ice movements grained til!(D ). The til! contained few pebbles or to the northwest (cf. Fig. · 4) . A diffuse layer blocks and was homogenous and compact with­ marked the transition between tills D andC. out visible structures. Fabric analyses showed a The uppermost till (B) followed without a maximum direction east- west, with a slight dip distinct border to tillC. This till, some2m thick, to the west, indicating that the till had been was loose, gravelly and lacked clay. Fabric deposited by glacier streams eastwards, i. e. measurements showed an east-northeast direc­ along the valley. In this til! we found two very tion, with a slight dip to the west-soutwest. On distinct, dark clay layers, respectively 20 cm (a) the top of the section was a0.5-l m thick layer of and lO cm (b) thick (cf. Fig. 4). The layers, silt and sand. These sediments, which are wide­ dipping slightly to the south, contained42% elay spread in the lower part of the valley sides, are and were dried up. They could be traced through correlated to the deglaciation period when a the whole section, and in another section 250 m series of glacier dammed lateral !akes existed away a similar layer was observed in the same (Hole 1979). NORSK GEOLOGISK TIDSSKRifT l (1981) W eichselian til/ in Ottadalen 27 [0Mammothf ind ed 0 ����'\�i�n•t�\�:��c����um r;;;, Till with older sediments l� incorpora1ed Fig. 2. Localities with mammoth-finds, waterlaid sediments presumed older than Late-Weichsel, and till with older sediments incorporated in the Ottadalen area. For location see Fig. l. however, block layers can be useful in till Interpretation of the stratigraphy at stratigraphic studies. Gubbhågå The origin of the two clay layers in till D is Few conclusive results have been drawn from uncertain, though it seems probable that they are the investigations. Mineralogical/petrographical syngenetic and therefore suggest the existence of studies also failed to allow distinctions between a wet base glacier during the till formation. the various tills. The characteristic Jotun rocks, As found in other studies of till stratigraphy which elsewhere in the Gudbrandsdalen region (e.g. Garnes 1979, Garnes & Bergersen 1977), have proved to be useful indicators of transport fabric analyses have been the most important direction, are few in all samples. method in distinguishing tills. In the Gubbhågå Roundness analyses showed that the low­ section five analyses were carried out. The ermost till, D, contained more rounded material orientation and dip of the longest axes of stones than the other tills. were measured and plotted on a Schmidt net. In the transition zone from one till layer to The fabric analyses indicate that the tills cor­ another, for example between the tills D and C, respond to three different ice movement phases; there often occur boulder layers. Evidently namely, along the valley (oldest), across the block layers may have different genesis, some­ valley towards the northwest (medium), and times also occurring interlayered in tills (e.g. across the valley to the northeast (youngest) Dreimanis 1976). U sed with combined analyses, (Fig. 5). These directions are in full agreement 450 SOUTH 400 Fig. 3. The Weichselian till Til\ with older sediments incorporated (0) 350 stratigraphy in Skjåk, schemati­ cally drawn. The letters A-D correspond to the layers at Gubbhågå, Fig. 4. 28 J. Hole & O. F. Bergersen NORSK GEOLOGISK TIDSSKRIFT l (1981) 1.20 ma sl ROUNDNESS GRAIN SIZE B + :!!!!!!!!!!!W!Wiii!!!t�::;·;·�6 0-76 c ki==============�:!:!:::::::::! E:t=================�i:H::::::::::::::::::::::::::::::J::;·;J 2 -77 + E1========�======�::i:m::m::m::m:mm::::I>;=:=>;·=:=J 17 -76 E=i===�======�==�m ::i:i:li:li!!ii::::�:::::::::t-:::::-;<=1 61-76 o r.=:::�::: �:::: ::: g=============�:::m:::::::::�-;:_;_j ,a-76 : E = �=======:s=====1::::mm::::::m::::::::::::t:::·;·::::;·;·!19 -76 l: =�======�======l:::::::ii:::::::::l::H:::::i::![:::-;-:::>:·:J6 2-76 Fig. 4. Sketch of the Gubbhågå + section showing the stratigraphy and the localities of the samples. E The diagrams illustrate the 50 roundness analyses. grain-size % % distribution, and fabric analyses 40 in the section. Roundness cias­ CLAY SILT SAND GRAVEL ses are angular (k), abraded ang­ 20 ="' = les (kr), rounded (r), and well L E i ===== === rounded (gr). Definitions, see o = = =3::m:::::::::::::t·::=::·;·::::;·;·::=::·l k kr gr <0.002 0002-0.6 0.6-2 2-16mm Bergersen (1973). NORSK GEOLOGISK TIDSSKRIFT l (1981) Weichselian till in Ottadalen 29 Orumlin field or distinct ftuted surface Oldest � Glacial striae Youngest o Fig. 5. Striae and drumlinoid forms in Ottadalen and in the upper Gudbrandsdalen area. Only a limited number of the total striae measured are presented. Some of the striae in the lower part of Ottadalen are taken from Lie (1974), Garnes (1978), and Skjerven (1978). For location, see Fig. l.