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
Time-stratigraphic context
The alignment of sheetfl ow tracks, fanning out to
the southeast and south indicates drainage of large
subglacial reservoir beneath the FIS after YD (Fig.
1). The former Lokka ice-lake most likely was the
source for the sheetfl ow system. The sheetfl ow pattern
crosses the N-S oriented esker system (Fig. 1),
previously interpreted as Mid-Weichselian by Sutinen
(1992). Also, maximum σ a-anisotropy of the
tills exhibited a rather concurrent pattern with the
N-S eskers (Fig. 2) suggesting that the N-S oriented
eskers were generated, probably through time-transgressive
evolution (Banerjee & McDonald 1975),
prior to the sheetfl ow drainage of Lake Lokka. The
preservation of Mid-Weichselian eskers and tillsequence
indicates that the erosion rate of the Late
Weichselian ice-fl ow had been low in the Lokka region.
Morphologically this is supported by the lack
of erosional streamlined features commonly found
inside the YD End Moraines (YDEMs; Nordkalott
Project 1987). We also argue against high velocity
fl ood events during the sheetfl ow drainage because
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RESULTS AND DISCUSSION
large regions had not eroded out of unconsolidated
sediments (cf. Kor & Cowell 1998).
Sheetfl ows vs. arborescent eskers
The anastomosing pattern of sheetfl ow tracks in
the Lokka region (Figs. 1 and 3) is different from
those in lower parts of northern Fennoscandia and
the Canadian Shield displaying river-like drainage
pattern of widely spaced tributary branches. The
arborescent esker chains can often be followed tens
or even hundreds of kilometres (Nordkalott Project
1987; Clark & Walder 1994). According to Clark
and Walder (1994), anastomosing or braided channel
systems are attributed to sedimentary terrains with
fi ne-grained and low-permeability tills, not high-permeable
tills underlain by crystalline bedrock as is the
case in Lokka region. We found network patterns of
some eskers (Fig. 4) rather similar to those described
earlier by Prest (1968) and, Shaw and Kvill (1984)
in Saskatschewan, Canada. However, discontinuities
or even offsets in the esker tracks are diffi cult to explain
without subglacial fault instability.
Fig. 2. Anastomosing network of sheetfl ow tracks crosses the N-S-oriented esker ridges in the Lokka area. Note the majority of
maximum σ a-anisotropy of tills oriented transversally to the sheetfl ow system, but rather than concurrently with the N-S eskers.