Tilting of Lake Shorelines in Jølstravatnet, Western Norway, Caused by Glacioisostatic Rebound
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Tilting of lake shorelines in Jølstravatnet, western Norway, caused by glacioisostatic rebound OVE NORALF RYE KLAKEGG & Klakegg, O. Rye, N.: Tilting of lake shorelines in Jølstravatnet, western Norway, caused by glacio & isostatic rebound. Norsk Geologisk Tidsskrift, Vol. 70, pp. 47-59. Oslo 1990. ISSN 0029-196X. Based on studies of shore levels and lake sediments, a change in outlet from the eastern to the western end of lake Jølstravatnet is demonstrated. The change was caused by the faster uplift in the eastern end. The lake history is subdivided into five phases. During phase I ( -9500 BP) lake Jølstravatnet was an ice-dammed lake. During phases Il and Ill (9500-7500BP) there was an eastern outlet and glacioisostatic tilting caused a transgression in the western part of the lake until the lake leve) had risen to the threshold in the western end. Phase IV (7500--6000 BP) is characterized by outlets at both ends of the lake. The emergence of the eastern outlet (ca. 6000 BP) marks the transition to phase V, i.e. the present situation, with an outlet over the western threshold. By making a tentative comparison between the Jølstravatnet time-gradient curve and two marine based curves from adjacent areas some trends in the glacioisostatic rebound of western Norway are deduced. Klakegg, Geological Survey of Norway, P. Box 3006, Lade, N-7002 Trondheim, Norway. Present O. O. address: Norwegian Institute of Land Inventory, P. Box 115, N-1430 As, Norway; Rye, University O. N. of Bergen, Department of Geology, Sec. B, A/legaten 41, N-5007 Bergen, Norway. Tilted marine shorelines is a classical field of Kræmer 1977; Holm-Larsen 1982). The divide study both in western Norway (e.g. Rekstad 1907; between the two lobes was situated at the water Kaldhol 1913; Reite 1968; Aarseth Mangerud shed at Skei at the eastern end of the lake (Klak 1974; Kaland 1984; Anundsen 1985; &Fareth 1987; egg 1981, 1987). Svendsen Mangerud 1987) and in other formerly glaciated& regions. The effect of glacio isostatic rebound on lake shorelines and location of outlet has, however, not been studied in Methods western Norway before. The lake Jølstravatnet, The development of the lake is reconstructed by situated at the watershed between Nordfjorden a synthesis of studies of the deglaciation history, and Førdefjorden (Fig. 1) , is well-suited for such former lake levels and the stratigraphy in lake a purpose because of the only 3 m difference Fuglevatnet at the eastern (Skei) threshold (Figs. in elevation of the thresholds at both ends and 2, 3). The terrace levels are plotted in an equi because the isobases cross the lake at an oblique distant shoreline diagram perpendicular to the angle (Fig. 3). During the Holocene uplift the Nor marine shore level isobases from Fareth outlet switched from the eastern to the western (1970, 1987). A Meridian clinometer, a Paulin end of the lake, causing several interesting fea aneroid barometer and maps at a scale of l : 1000 tures in the paleolake. (contour interval l m) are used to determine the The aim of this paper is thus to describe the elevation of the terraces. Holocene changes of lake Jølstravatnet and to put The sediments of lake Fuglevatnet were studied this development into the context of glacioiso to identify and date the start and the end of the static rebound. drainage across the Skei threshold. The corings The lake area was deglaciated ca. 9500 years were carried out using a 'Russian' peat corer BP, during the Preboreal time (Klakegg 1981). (Tolonen 1967) and a modified 110 mm diameter Afterthe formation of the Nor moraines (Fig. l) piston corer. The X-ray photographing of the in Younger Dryas (Mangerud et al. 1979; Fareth cores follows the methods described by Bouma 1987) the glaciers in Førdefjorden and Nordfjor (1969). Grain-size analyses were performed on den (Fig. l) retreated to Jølstravatnet (Rye 1963; sediments with loss on ignition below 10%. 48 O. Klakegg N. Rye NORSK GEOLOGISK TIDSSKRifT 70 (1990) & N 4° 6° SOUTH NORWAY o 10 20 C====l50 km Fig. A map of the area showing lake Jølstravatnet situated just inside the Nor moraines (Younger Dryas). loset: key map of southernl. Norway. Organic matter was removed with 10% H202 curve) can be constructed (Fig. 10), giving age solution. The coarse fraction (>63 ,um) was wet estimates of the different lake phases. sieved and the fine fraction analysed by the hydrometer method. The minerogenic sediments are classified according to Wentworth (1922). Where loss on ignition exceeds 10%, the sediment The present lake is named gyttja. Loss on ignition was performed Lake Jølstravatnet (207 m a.s.l., Fig. 2) is a typical at a temperature of 550°C (Sønstegaard fjord lake (Strøm 1935) situated well above the & Mangerud 1977). The results are shown as percent marine limit. The present outlet Iies at Vassenden ash content. Pollen preparation and identification at the western end of the lake (Fig. 3). The 80 m are mainly according to Fægri Iversen (1975). broad outlet river (the Jølstra) passes across a & By combining the radiocarbon datings from bedrock threshold which has a veneer of boulders. lake Fuglevatnet with synchronous lake leve! The level of the threshold is 205.9 m a.s.l., being gradients of lake Jølstravatnet, an approximate the lowest recorded water leve! in the lake. time-shore leve! gradient curve (time-gradient Lake leve! ftuctuations have been recorded NORSK GEOLOGISK TIDSSKRIFT 70 (1990) Ti/ting of lake Jølstravatnet, W. Norway 49 E w Fig. 2. Lake Jølstravatnet seen towards south, with present (Vassenden) and former (Skei) outlets. (Photo: Normann.) daily since 1903 by the Norwegian Water A threshold at the northeastern end of the lake Resources and Energy Administration (NVE). (at Skei) is only ca. 3m higher than the present Before regulation of the water level in 1951 the outlet threshold. Due to road constructions it is highest flood peaks reached 2.1-2.5 m above the impossible to determine the exact threshold level, threshold level. The mean water level lies 1.1 m but allowing for minor errors the level of the Skei above this level. threshold can be estimated to Iie at 209 m a.s.l. l l l l l l l l l l l l l l 217 l IH.HYKL.EBUST l "' l •226 / Yl M'l'lUBUST •zzl'8 / N.ÅS�2� l/ l l l l l l 'lOm 100m l l Fig. 3. Terrace levels (m.a.s.l) along Jølstravatnet. For Vassenden and Skei the elevations of the thresholds are given. Isobases for the Nor leve! (Y ounger Dryas marine shore leve!) are from Fareth (1987). 50 O. Klakegg & N. Rye NORSK GEOLOGISK TIDSSKRIFT70 (1990) Table l. Raised terraces at Jølstravatnet. Locality Description ma.s.l. Interpretation Hammar Small delta plane dipping 5°-7" towards the lake 230 Lateral Strand Remnant of a small fan delta 217 Lateral Tåhaug Shoreline, ca. 50 m long 214 Lake phase I Sanddal (l) Fan delta with surface dipping � towards the edge 218 Lake phase l, early? (2) Shoreline and small delta 214 Lake phase l Nedre Åsen (l) Fan delta 228 Lateral (2) Small delta 221 Lake phase I Ytre Myklebust (l) Small delta and shoreline 219 Lake phase I (2) Fan delta eroded in front 222-226 Lake phase l, early? Indre Myklebust (l) Shoreline 219 Lake phase I (2) Small fan delta and discontinuous shoreline 217 Lake phase I Dvergsdal Fan delta, eroded 235 Lateral Sandvikja Small delta 208.5 Lake phase Il Breelvskaret Fan with frontal delta sedimentation 213 Lake phase Il Sygnesand Eastem part of raised delta 215.5 Lake phase Il Vik ja Small delta 209 Lake phase Il Årdal Raised delta, foreset dipping towards the lake 218 Lake phase I Ålhus Raised delta plain with low dip towards the lake 212 Lake phase I We assume that the two thresholds were sta mark older Jateral lakes to the glacier and are not bilized by ftoodsduring the deglaciation and that used for the history of Jølstravatnet. The levet of no postglacial erosion has Jowered them sig the ice-dammed lake is constructed on the basis nificantly. of terraces at the mouth tributary valleys which we assume were deglaciated nearly at the same time as the lake itself. Such a deglaciation pattem seems to be the case in Ålhus and Årdal, and the The Jølstravatnet glaciallakes (lake terraces at these sites are interpreted to represent phases I and Il) Jevels during lake phase I. Corresponding terraces The Jølstravatnet glacial lake formed when gla in Sanddal, Tåhaug and Myklebust (Table l) sup eiers in Førdefjorden (Fig. l) retreated to Jøl port this interpretation. From these terraces the stravatnet white a glacier from the Kjøsnesfjorden shoreline gradient of the ice-dammed lake can be down towards the lake Breimsvatnet still existed estimated to be 1.2-l.Om/km (Fig. 4). (lake phase l, Fig. 4, Klakegg 1981). Terraces at Some terraces (e.g. at Sanddal, N. Åsen and the mouth of tributary valleys are evidence of this Myklebust) which are situated a few metres above lake phase (Table l and Fig. 3). An equidistant this shore levet are also assumed to represent this shoreline diagram perpendicular to the Yo unger lake phase, as lateral ice-damrning at such Jow Dryas marine shore levet isobases (Fareth 1987) levels can be excluded. Because of the short time has been constructed on the basis of these terraces available it seems unlikely that the gradient of the (Fig. 4). Two glacial lake phases, an ice-dammed ice-dammed lake has changed beyond the inter phase (lake phase I) and a frontal glacial lake val estimated above.