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

Geological Survey of Finland, Special Paper 46
Maris Rattas
Eskers in Estonia are located in the northern part
of the territory and are associated with Paleozoic
carbonaceous bedrock. The morphology and sedimentology
of eskers in Estonia were investigated in
more detail in the 1960s (Rähni 1957, 1960, Raukas
et al. 1971, cf. Karukäpp 2005). Entire esker ridges
can vary from tens of metres to several kilometres
in length. They are generally less than 500 m in
width, but may exceed one kilometre. The relative
height reaches up to 35 m, but is usually 10-20 m,
and the gradient of the slopes is up to 40° (Raukas
et al. 1971). The largest esker system crosses almost
north to south and the central part of the Pandivere
Upland has a length of about 60 km (Fig. 2). Eskers
show a large variation in facies characteristics
A preliminary model of meltwater drainage pathways
has been presented according to the spatial
distribution of eskers and bedrock valleys (Rattas
2005, 2006). Several studies in the areas of the
former ice sheets demonstrate that eskers and incised
bedrock valleys are often interconnected (e.g.
Attig et al. 1989, Fisher et al. 2005, Kozlowski et al.
2005, Jørgensen & Sandersen 2006). In northern Estonia,
both bedrock valleys and eskers are arranged
in a sub-parallel pattern, mostly from northwest to
southeast, or sub-parallel to palaeo-ice streams refl
ecting former meltwater fl ow towards the ice margin
(Fig. 2). Most of the esker systems are confi ned
to the valley limits following either along the valley
fl oors or lying upon valley shoulders or sometimes
replacing a valley if it disappears over a short distance.
Occasionally, some eskers cross valleys at
rather sharp angles. This has been associated with
late-glacial tectonic movements along tectonic fault
zones (Rähni 1973) leading to the ice splitting and
formation of open crevasses above the valleys. Considering
that the main precondition for esker formation
is low-permeability, and especially rigid substratum,
then esker formation within valleys fi lled
66
ESKERS
DISCUSSION
indicating non-uniform sedimentation due to fast
changes in meltwater volume, energy and velocity.
Two general types of esker systems have been
observed – those that have formed subglacially in
tunnels and those that appear on the ice-marginals
in deltaic or fan environments. Earlier investigations
suggest that the majority of the radial eskers
were formed in the open crevasses of the dead or
passive ice (Raukas et al. 1971). There has been
signifi cant development in glacier hydrology and
glacial sedimentology, and revision of the defi nition
and classifi cation of Estonian eskers is needed
regarding the morpho-sedimentary relations of eskers
and the hydraulic processes responsible for their
formation.
with permeable soft sediments, or above permeable
tectonic fault zones could presumably be responsive
to changes in subglacial hydraulic conditions
leading to tunnel creation. When the tunnel forms,
the water pressure rapidly decreases causing strong
groundwater fl ow towards the tunnel. The large
upward fl ow in tunnels and rapid pressure release
seems to be responsible for carbonate precipitation.
Such carbonate-cemented sand and gravel deposits
are common in eskers and valley fi lls indicating
their subglacial origin (Figs 3 and 4).
The sedimentary record suggests that some valley
infi ll and esker sediments may have developed as
a successive formation during periodical sedimentation
by a number of meltwater fl ow episodes. Alternatively,
some of the buried valleys or segments of
valleys may be the result of tunnel valley formation
with clear morphological characteristics of a tunnel
valley, like abrupt termination and undulating
or convex-up longitudinal profi les. This suggests
that the initial focusing of meltwater may have been
controlled by the location of bedrock valleys, and
the steady-state discharges most likely preferred to
follow pre-defi ned meltwater pathways.