Geological Survey of Finland, Special Paper 46 - arkisto.gsf.fi

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Geological Survey of Finland, Special Paper 46 Raimo Sutinen, Mari Jakonen, Pauliina Liwata and Eija Hyvönen Eskers and tunnel valleys have been understood to be meltwater forms, but the organization of these landforms, however, exhibits evidence of different meltdown drainage patterns of Pleistocene ice sheets. The conceptual model regards eskers as products of time-transgressive glaciofl uvial deposition within close proximity of the retreating ice margin (Banerjee & McDonald 1975). On a subcontinental scale, arboreal esker systems also show time-transgressive features (Clark & Walder 1994). Eskers are commonly associated with anastomosing tunnel valleys that are found to be common in regions typifi ed by fi ne-grained tills derived from sedimentary bedrock (Wright 1973, Clark & Walder 1994). It is assumed that high-velocity fl ood event(s) preceded the evolution of eskers found in the tunnel valleys (Wright 1973, Kozlowski et al. 2005). Morphological evidence has also shown that catastrophic sheetfl ood events are capable of eroding large regions out of pervasive cover of unconsolidated sediments (Kor & Cowell 1998). The anastomosing network of tunnel valleys, esker tracks or sheetfl oods are evidently attributed to subglacial lakes (Wright 1973, Shaw We compiled DEM and low altitude AGR (γ K ) to outline the anastomosing network of sheetfl ow tracks within an area of 100 by 100 km in east-central Finnish Lapland (Fig. 1). The time-stability of spatial structures in the soil γ K (Hyvönen & Sutinen 1998) implies that the terrain anisotropy, whether due to glacial streamlining or sheetfl ow outbursts, is revealed by the AGR-γ K . The regimes of low soil volumetric water content (0v), such as sheetfl ow esker ridges, are indicated by high γ K -signatures, while low γ K -signatures (high 0v) typify topographic lows, often with organic or fi ne-grained materials (Hyvönen et al. 2003, Sutinen et al. 2005). The DEM-AGR enhancement of the sheetfl ow associations was viewed closer in the subset areas of Tanhua and Suoltijoki (Fig. 1.). Compilation of AM and DEM was applied to search candidate lines to represent neotectonic faulting. The coincidental appearance of topographic escarpments and AM-anomalies, showing, for example, pre-existing hydrothermal alteration zones, 56 INTRODUCTION MATERIALS AND METHODS & Kvill 1984, Sutinen 1992, Kor & Cowell 1998, Sjogren et al. 2002, Kozlowski et al. 2005), but the outburst release mechanisms of former subglacial meltwater bodies have remained unknown. Radar evidence has indicated large subglacial lakes (e.g. Lake Vostok, 500 by 300 km in size) beneath the Antarctic Ice Sheet (AIS) (Robin et al. 1970, Kapitsa et al. 1996, Siegert 2000), hence potential water volumes for tunnel valley formation evidently existed beneath the former Pleistocene ice sheets. Following the concept of glacio-seismotectonics at the margin of FIS (Stewart et al. 2000), we suspect that large-magnitude seismic impacts (Arvidsson 1996, Wu et al. 1999) have been part of an abrupt release of subglacial water bodies as outburst sheetfl ows beneath the FIS. In this study, we present morphological/geophysical evidence that neotectonic fault instability likely contributed not only to paleolandslides (Sutinen 1992, 2005, Sutinen et al. this issue), but also to glaciofl uvial landform genesis shortly after the Younger Dryas (YD) episode in Finnish Lapland. marks potential zones where faults were likely reactivated within the glacio-seismotectonic subsidence-rebound processes (Arvidsson 1996, Fjeldskaar et al. 2000, Stewart et al. 2002). However, not all escarpments are neotectonic and not every AManomaly is present at sites with neotectonic faults (Ojala et al. 2004). Thus, glacio-morphological investigations are often needed to verify neotectonic origin (Lagerbäck 1990, Dehls et al. 2000). For the time-stratigraphic purposes, additional data on the preferred orientation of till(s) were acquired by applying non-destructive azimuthal measurements of apparent electrical conductivity (σa) (Taylor & Fleming 1988, Penttinen et al. 1999, Sutinen et al. this issue). We determined the maximum σaanisotropy (orientation) of the tills at 12 sites with a Geonics EM38 (Geonics Ltd. Mississauga, Ont. Canada) device designed to a depth range of 1.5-2 m (depending on material) in vertical coil position EM38(V). The fi eld campaign was carried out in August 2005.
Geological Survey of Finland, Special Paper 46 Late Weichselian sheetfl ow drainage of subglacial Lokka ice lake Fig. 1. Anastomosing network of sheetfl ow tracks in the Lokka area, east-central Finnish Lapland. The Tanhua and Suoltijoki subsets are viewed in more detail in the text. The Tanhua fault line (tentatively Late Glacial Fault; LGF) shown with barbs turned towards the lower block. Descriptions of pre-Late Weichselian NE-W (top of the fi gure) and N-S (in the mid of the fi gure) oriented esker tracts were given in Sutinen (1992). 57
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GEOLOGICAL SURVEY OF FINLAND Specia
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Johansson, P. & Sarala, P. (eds.) 2
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4 CONTENTS Preface, Peter Johansson
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