Jökull - 31.12.2001, Side 61

Seismic soundings on Skeiðarársandur
fellsjökull. Sediment thickness increases towards the
sea and bedrock is 200-250 below sea level on the
lower part of the sandur (Figure 5).
Compared to the upper central part, depth to
bedrock is much greater on the eastern part of the
sandur, near the bridge over Skeiðará (Figure 4). The
two profiles located in its vicinity (SKS2 and SKS3)
reveal a depth of 180-210 m. Thus an erosional
channel may exist in the eastern part of the sandur.
Depth to bedrock decreases again near Svínafellsjök-
ull where it is 75 m in front of the glacier terminus.
LAYERING OF SEDIMENTS
While the interval velocities obtained through the Dix
equation are very approximate and subject to consid-
erable uncertainty, the determination of distinct layer-
ing above bedrock is robust. Moreover, there is a clear
pattern of higher velocities in the deeper sedimentary
layers. The uppermost layer (V = 1.4-1.8 km s_1)
has velocity characteristic of unconsolidated water-
saturated fluvial sediments. It is therefore consid-
ered to consist of glaciofluvial sediments that have
not been subjected to any appreciable compaction.
This layer is thin or absent inside the outermost termi-
nal moraines of Skeiðarárjökull and Svínafellsjökull.
Its thickness increases rapidly outside the moraines
reaching over 150 m in vicinity of the Skeiðará bridge
and near the coast (Figures 4 and 5).
On the basis of sediment grain size, Boothroyd
and Nummedal (1978) divided Skeiðarársandur into
four areas, with coarseness of sediments gradually de-
creasing with increasing distance from Skeiðarárjök-
ull. Proximal to the glacier are tills, grading to coarse
gravel through to fine gravel, with the lower half of
the sandur largely composed of sand-sized material.
The velocities recorded in our profiles show little cor-
relation with this classification apart from the signifi-
cantly higher velocities in the till facies area.
The layers underneath the unconsolidated glacio-
fluvial sediments have velocities in the range 1.9-
2.7 km s_1 (Figures 4 and 6). The lower values
(1.9-2.2 km s_1) may be due to coarse-grained sedi-
ments; Flaraldsson and Palm (1980) obtained a good
correlation between coarseness and seismic velocity
in the Markarfljót sandur. An alternative explana-
tion would be that these velocities represent somewhat
compacted sediments of the same type as the top layer.
On Breiðamerkursandur, Bogadóttir et al. (1987)
recorded velocites of 1.5 km s_1 outside the Little Ice
Age moraines but 1.9-2.0 km s_1 in the same type of
sediments within the moraines. They suggested that
the higher velocities arise because of compaction by
ice loading. Boulton and Dobbie (1993) presented a
model explaining how the flux of groundwater in sed-
iments under a glacier may lead to such consolidation.
On Skeiðarársandur, the layer with velocity 1.9-
2.2 km s-1 reaches almost to the surface within the
Skeiðarárjökull Little Ice Age moraines while the
thickness of the overlying unconsolidated sediments
increases rapidly outside the moraines. Although in-
creased coarseness undoubtedly plays a role in in-
creasing seismic velocity in the proximal zone of
Skeiðarárjökull, the close correlation between maxi-
mum extent of the Little Ice Age moraines and sed-
iment velocity suggests that compaction by glacier
load is an equally plausible mechanism. At Svína-
fellsjökull (Figures 7 and 8) the relationship be-
tween seismic velocity and Holocene glacial extent
is particularly instructive. The profile HS coincides
roughly with the maximum extent of the glacier in
the Holocene, the Stóralda stage considered to date
back to the onset of climatic deterioration 2500 years
BP (Þórarinsson, 1956). This profile has a velocity of
2.2 km s-1 at about 10 m depth while no such layer is
found in profile FR, only 0.8 km to the west.
The highest velocities found in the Skeiðarársand-
ur sediments (2.5-2.7 km s-1) indicate consolidated
sedimentary rocks. The fact that the upper surface of
this layer shows up as a reflection indicates that it is
an unconformity; this lowermost layer may be sedi-
mentary rock of Pleistocene age.
In profile SKS3 a layer with a velocity of 3.7 km
s-1 showed up as a refraction (Figure 4) at about 40
m depth. The origin of this layer is unknown. The ve-
locity is unusually high for a sedimentary layer and it
seems to be underlain by more than 100 m of uncon-
solidated sediments. The likelihood of a buried lava
flow at this location is small. This may be a thin fully
consolidated layer within the sediments, perhaps due
to palagonitization or some other alteration process.
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