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

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.