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Drumlin Formation Time: Evidence from Northern and Central Sweden Drumlin Formation Time: Evidence from Northern and Central Sweden

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Drumlin Formation Time: Evidence from Northern and Central Sweden Drumlin Formation Time: Evidence from Northern and Central Sweden DRUMLIN FORMATION TIME: EVIDENCE FROM NORTHERN AND CENTRAL SWEDEN DRUMLIN FORMATION TIME: EVIDENCE FROM NORTHERN AND CENTRAL SWEDEN BY CLAS HÄTTESTRAND1, SVEA GÖTZ1, JENS-OVE NÄSLUND1, DEREK FABEL2, AND ARJEN P. STROEVEN1 1Department of Physical Geography and Quaternary Geology, Stockholm University, Sweden 2Research School of Earth Sciences, Australian National University, Canberra, Australia Hättestrand, C., Götz, S., Näslund, J.-O., Fabel, D. and Stroeven, analysis of the glacial geomorphological record, A.P., 2004: Drumlin formation time: evidence from northern and and the timing of different glacial events is con- central Sweden. Geogr. Ann., 86 A (2): 155–167. strained by geochronology and stratigraphical ABSTRACT. Large-scale drumlins occur abundantly records (Boulton and Clark 1990; Kleman and throughout central and northern Sweden. Whereas Borgström 1994). Of the glacial landforms that are many drumlins in the north are an integral part of a used to infer ice flow patterns, flow-parallel glacial relict glacial landscape >100,000 years old, those to lineations are by far the most commonly used (e.g. the south are generally interpreted as of last degla- ciation age. Typically, the latter ones have not been Boulton et al. 1985, 2001; Kleman 1990; Clark overprinted by younger glacial landforms. Despite 1993). Flow-parallel glacial lineations include this apparent difference in formation history, drum- drumlins, flutes, and glacial striae. They are partic- lins in both regions have similar directional and ularly well suited for this type of analysis because morphological characteristics. A systematic analy- they all show fairly accurately the direction of sis of >3000 drumlins in (i) areas within relict land- scapes, (ii) areas with an ambiguous deglaciation warm-based ice flow. Unfortunately, these land- age assignment, and (iii) areas within deglacial forms are often treated together, despite large var- landscapes, indicates that these latter deglaciation iations in size or shape. If we regard glacial geo- drumlins differ clearly in both shape and size from morphological maps, for example, a drumlin sym- drumlins in the other two types of landscapes. In ad- bol (e.g. Fig. 1) can either represent individual dition, numerical modelling indicates that basal forms, from a 10 m long flute up to a 70 km long melting conditions, a prerequisite for drumlin for- mation, prevailed only for a very limited time over mega-drumlin (cf. Clark 1993), or be used as a gen- much of northern Sweden during the last deglacia- eral map symbol to indicate the general direction of tion, but lasted for longer periods of time during multiple features. Further, sometimes drumlins are earlier stages of the Weichselian. A reconnaissance used together with information on the direction of radionuclide bedrock exposure date from the crag glacial striations, which typically are 10–100 cm of a large drumlin in the relict landscape indicates that glacial erosion, and presumably drumlin for- long. These mapped features may differ in length mation, at this location predated Marine Isotope by seven to eight orders of magnitude. Moreover, if Stage 7. We conclude, therefore, that the large-scale we compare the bulk of material required to exca- drumlins of central and northern Sweden did not vate or accumulate these landforms, there is a vol- form during the last deglaciation, or during any oth- ume variation of 16 orders of magnitude, from c. er specific ice flow event. Instead, we suggest that –7 3 10 m (glacial striae; 10 cm long, 1 mm wide, and they were formed by successive phases of erosion 9 3 and deposition by ice sheets of similar magnitude 1 mm deep) to c. 10 m (mega-lineation; 70 km and configuration. long, 500 m wide, and 50 m high). Hence, the time it takes to form large drumlins, for example, is like- Key words: drumlins, ice sheet, Sweden ly vastly different from the formation of striae and small drumlins alike. If all these landforms were to Introduction be treated indiscriminately within an ice sheet re- Reconstructions of former ice sheets utilize a wide construction, biased or erroneous results may arise. range of glacial geology data. The spatial evolution If drumlins are crosscut by younger glacial land- of the ice sheets is typically reconstructed from forms, the inference is that they have formed during Geografiska Annaler · 86 A (2004) · 2 155 C. HÄTTESTRAND, S. GÖTZ, J.-O. NÄSLUND, D. FABEL AND A.P. STROEVEN Fig. 1. Location map. General pattern of drumlin distribution and direction in central and northern Sweden, after Hät- testrand (1998). The outline of the relict area of northeastern Sweden is a compilation from data in Kleman et al. (1997, 1999), Hättestrand (1998), and Kleman and Hättestrand (1999). Ice divide positions are derived from Kleman et al. (1997). an earlier ice flow event. If, on the other hand, they places crosscut by younger small-scale flutes and lack the signs of younger crosscutting, they are lateral meltwater channels, formed during the last considered to have formed during the last deglaci- deglaciation (Fig. 2). The relict landscape is sharp- ation (Clark 1993; Kleman and Borgström 1996; ly truncated to the north by a well-defined deglacial Hättestrand et al. 1999). This strategy in recon- landform system dominated by drumlins and esk- structing the conditions during past ice flow events ers, the ‘Kiruna fan’, extending through Finland to- appears straightforward and sound. However, the wards the Barents Sea coast (Kleman et al. 1997; implicit assumption is that the ice flow direction Hättestrand et al. 1999). To the south, the boundary during the last deglaciation differed in direction of the relict glacial landscape is not well expressed. from previous glacial events. This is because the ice flow direction during the last A well-developed relict glacial landscape con- deglaciation closely matched the ice flow direction sisting of large drumlins, eskers, and hummocky indicated by the relict landscape in this region, i.e. moraine, of an inferred Early Weichselian age, ex- ice flow from the northwest. Hättestrand (1998) ists in northern and central Sweden (Lagerbäck noted that the drumlins of the relict landscape ap- 1988; Lagerbäck and Robertsson 1988; Kleman et pear to have a morphology and direction very sim- al. 1997; Hättestrand 1998). These landforms are in ilar to the drumlins in the last deglaciation land- 156 Geografiska Annaler · 86 A (2004) · 2 DRUMLIN FORMATION TIME: EVIDENCE FROM NORTHERN AND CENTRAL SWEDEN Fig. 2. Aerial photograph and contour map (interval 5 m) of drumlins in area O1. Most drumlins in this area have a crag-and-tail mor- phology. Note the lateral meltwater channels indicated by arrows, crossing the tail of one of the drumlins in the centre. These channels indicate a deglaciation ice surface slope, and, hence, ice flow direction, at right angle to the drumlins. The drumlins themselves are in- terpreted to be older than the early Weichselian. The location of the area covered by the aerial photograph and map is show in Fig. 4. scape to the south. He therefore suggested that the Subsequently, we compare the morphometric drumlins of both landscapes may have experienced analysis with numerically modelled Fennoscandi- a similar formation history, and, therefore, that they an ice sheet dynamics for the last glacial cycle were formed by consecutive ice sheets with rough- (Näslund et al. 2003). One output parameter of the ly the same ice flow direction. model is the basal sliding distance, i.e. the accumu- In order to test whether the drumlins in northern lated length of wet-based ice flow over a particular and central Sweden belong to two different popula- area. This yields a measure of the amount of ice that tions, or if they all have a similar formation history, was available to form drumlins of a particular size we mapped and measured more than 3000 drumlins and direction (in this case ice flow from the NW distributed over nine areas in northern and central sector is of interest), for a particular time period. Sweden (Fig. 1). Three of the areas are located in Finally, we present cosmogenic nuclide data the relict glacial landscape (O1–O3), three are lo- from one of the larger drumlins in the relict land- cated in landscapes of last deglaciation age (such as scape of northeastern Sweden, to gain insight into the Kiruna fan; Y1–Y3); and three are located in ar- the absolute age of the drumlins in this area. eas with an ambiguous deglacial age assignment in the inner part of central and northern Sweden to the south of the relict landscape (U1–U3). Area description We follow the premise that careful mapping and Y1–Y3 analysis of the drumlins will reveal systematic sim- Drumlins occur in distinct deglaciation landform ilarities or differences in morphology between the assemblages within the Kiruna fan (Y1 and Y2) and three regions, thus improving our ability to corre- on the coast of northern Sweden (Y3). At these lo- late areas where the age of the drumlins is unknown cations, they are situated in the proximal part of to either the old relict landscape or to the young de- well-defined landform systems that extend from glaciation landscape. the Younger Dryas moraines in northern Norway Geografiska Annaler · 86 A (2004) · 2 157 C. HÄTTESTRAND, S. GÖTZ, J.-O. NÄSLUND, D. FABEL AND A.P. STROEVEN and eastern Finland, respectively, to the eastern gued that the time available to construct such large flank of the northern Swedish mountains (Kleman drumlins by ice flow from the northwest was too et al. 1997). Prior to deglaciation, area Y3 was sit- short during the deglaciation. In this study, we test uated underneath the proximal part of an ice stream the hypothesis that these drumlins are of deglacia- in the Bothnian Bay, which terminated in the west- tion age (presently uncertain age; U1–U3), by com- ward continuation of the Salpausselkä end moraine paring their morphology to the morphology of rel- belt of Younger Dryas age (Kleman et al.
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