Jökull - 31.12.2001, Page 63

Seismic soundings on Skeiðarársandur
approx
Stóraída maximum
km km
Figure 8. Seismic stratigraphy of the SV-HS-FR profile in front of Svínafellsjökull. The unconsolidated sedi-
ments are thin in the area that has been covered by ice during the Holocene but the compacted layer is absent
outside that area. - Lagskipting setsins framan við Svínafellsjökul. Bylgjuhraði íþví seti sem legið hefur undir
jökli er hœrri en þar sem jökull hefur ekki gengið yfir á nútíma.
SEDIMENT VOLUMES
The seismic soundings show sediment thicknesses in
excess of 200 m for the lower reaches of the sand-
ur and 70-180 m for the upper parts. Although the
soundings are widely spaced and therefore cannot be
used for detailed volume estimates, the results indi-
cate a total volume of sediments above bedrock of
100-200 km3 over the 1000 km2 area of Skeiðarár-
sandur. Unconsolidated and uncompacted glacioflu-
vial sediments (1.4-1.8 km s_1) seem to make up
one half to two-thirds of this volume. Material of
intermediate seismic velocity (1.9-2.2 km s_1) oc-
curs mainly inside the moraines of Skeiðarárjökull
and Svínafellsjökull. The volume of this formation
inside the moraines is only a few km3, but if it also
occurs in appreciable thicknesses elsewhere, as indi-
cated by SKS2, its volume may be greater. The third
sedimentary unit, the consolidated sedimentary rocks
(2.5-2.7 km s_1), is about 100 m thick in the two pro-
files on the central and southem part of the sandur. If
these profiles are characteristic for the lower reaches
of the sandur this unit may well have a total volume
of the order of 50 km3. More detailed seismic work is
required to test these estimates.
DISCUSSION
The bedrock depth observed on Skeiðarársandur is
similar to that found on Breiðamerkursandur (Boga-
dóttir et al., 1987) and the Markarfljót sandur (Har-
aldsson and Palm, 1980). In both Breiðamerkursand-
ur and in the Markarfljót area buried bedrock troughs
were found, considered eroded by Pleistocene ice
streams (Haraldsson, 1981; Bogadóttir et al., 1987;
Bjömsson, 1996). The valley southwest of Skaftafell
may be of similar origin. It lines up with the Skeiðar-
árdjúp, a submarine gorge in the insular shelf off the
coast south of Skeiðarársandur. The erosional valley
may also connect to the overdeepening found in radio-
echo soundings under the eastern part of Skeiðarár-
jökull (Björnsson, 1998; Björnsson et al., 1999). Dur-
ing the height of recent Pleistocene glaciations most
of Iceland was ice covered (e.g. Norðdahl, 1990) and
glaciers probably calved into the sea off the south-
east coast of Iceland. South of Skaftafell large ice
streams/valley glaciers from Morsárdalur and Skafta-
fellsjökull must have merged with the main ice stream
of Skeiðarárjökull in the area of this bedrock valley.
Merging of ice streams may have caused enhanced
erosion forming the valley.
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