Jökull - 31.12.2001, Qupperneq 55

Reviewed research article
Seismic soundings of sediment thickness on Skeiðarársandur,
SE-Iceland
Magnús T. Guðmundsson1, Aurélie Bonnel2 and Karl Gunnarsson3
lScience Institute, University oflceland, Haga Hofsvallagötu 53, IS-I07 Reykjavík, Iceland; mtg@raunvis.hi.is
2 Université Louis Pasteur, Strasbourg, France; aureliebonnel@yahoo.fr
3National Energy Authority, Grensásvegur 9, IS-108 Reykjavík, Iceland; kg@os.is
Abstract - Seismic soundings on Skeiðarársandur show clear reflections from bedrock and inter-sedimentary
layers. Ten seismic profiles were collected at scattered locations in 1997 and 1999. They indicate a sediment
thickness of 80-100 m near the terminus of Skeiðarárjökull, increasing to about 250 m at the coast. The
soundings suggest that a 100 m deep valley is present in the bedrock south of Skaftafell, probably eroded by
a Pleistocene ice stream. Three seismic units are detected in the sediments. The uppermost unit and the most
voluminous consists of unconsolidated glaciofluvial sediments with a seismic velocity of 1.4—1.8 km s-1. A
second unit with a slightly higher seismic velocity (1.9-2.2 km s-1) is found inside the outermost moraines
of Skeiðarárjökull and Svmafellsjökull. A comparison with studies on Breiðamerkursandur suggests that this
unit may be glaciofluvial Holocene sediments compacted by loading ofice during the Little Ice Age and earlier
Holocene advances. Alternatively, the higher velocities may be due to larger proportion of coarse-grained
sediments in the vicinity ofthe glacier. A third unit, with seismic velocity of 2.5-2.7 km s-1, is found in the
southern and central parts ofthe sandur, buried under 100-150 m of sediments. The velocity is consistent with
consolidated sedimentary rock of Pleistocene age. The total volume of sediments on Skeiðarársandur is 100-
200 km3. The majority ofthis material has not been subjected to compaction under glaciers and must therefore
date from the Holocene. There may have been large variations in sedimentation rates over the Holocene, but
the average growth ofthe sandur body over the last 10,000 years has been about 1 kmfl/century.
INTRODUCTION
The lowland areas in south and southeast Iceland
are predominantly outwash plains or sandur, created
by deposition of glaciofluvial jökulhlaup sediments
(e.g. Hjulström et al., 1954; Þórarinsson, 1974; Har-
aldsson, 1981; Maizels, 1991). The sandur contains
large volumes of sediment; seismic depth soundings
have revealed thicknesses of 100-200 m on Breiða-
merkursandur (Bogadóttir et al., 1987), the Markar-
fljót sandur (Haraldsson and Palm, 1980) and Mýr-
dalssandur (Björnsson, 1964; Þórarinsson and Guð-
mundsson, 1979). Since the sandur are major traps
for glaciofluvial sediment, both from regular runoff
and high magnitude jökulhlaups, their stratigraphy
and sediment volume provide important information
on erosion and sedimentation in Iceland. Studies of
sandur thickness and stratigraphy are therefore impor-
tant in quantifying rates of these processes. Until re-
cently no data existed on the thickness and stratigra-
phy of Skeiðarársandur (~ 1000 km2), the largest of
the active sandur in Iceland, located between the At-
lantic Ocean and Skeiðarárjökull, an outlet glacier of
Vatnajökull (Figures 1 and 2). In this paper the results
of a small-scale reconnaissance seismic reflection sur-
vey are presented. The data consist of 10 soundings
made at scattered locations on the sandur during field
courses in exploration geophysics at the University of
Iceland in May 1997 and 1999.
JÖKULLNo. 51 53