Jökull - 31.12.2001, Page 60

Guðmundsson et al.
Skeiðarárjökull
■0/°/10° 135/ÆSS
25/72/13
10/160/-75
0/75/10
'148/210/-140 178/1
83/83/-33
90/220/-193
Skeiðarársandur
155/250/-245
0 5 10km
17° 00’ W
Atlantic Ocean
Figure 5. Map of Skeiðarársandur showing the sediment thickness; 90/220/-193 denotes the thickness of un-
consolidated glaciofluvial sediments/total sediment thickness/height of bedrock above sea level. The thickness
of the unconsolidated sediments (V<1.9 km s_1) is small in the areas that have been under glacier cover during
the Little Ice Age or in earlier advances during the Holocene. - Setlagaþykkt á Skeiðarársandi; 90/220/-193,
þykkt lausra setlaga/heildarþykkt setlaga/hœð berggrunns yfir sjó. Þykktin er lítil á þeim stöðum sem lágu undir
jökli á litlu ísöldinni eða á fyrri skeiðum jökulframskriðs frá því síðasta jökulskeiði lauk. Þykkt lausa setsins
(V<1.9 km s_1) ermest neðan til á sandinum og austan til, nœrri Skaftafelli.
possibility of the deepest observed reflection being
from the top of a layer of sedimentary rock cannot
be ruled out. However, this is considered unlikely.
Firstly, there are no indications of reflections deeper
than the assumed bedrock. Secondly, the bedrock out-
crop found about 2 km east of SKS4 (Óskar Knudsen,
pers. comm. 2002) may be taken as evidence against
considerably greater bedrock depths than found in this
study. Thirdly, on Breiðamerkursandur at the other
side of Öræfajökull, good agreement was obtained be-
tween bedrock depths from reflection and refraction
(Bogadóttir et al., 1987). The reflections from bound-
aries above the deepest reflection must arise from lay-
ering of the sediments. The deeper layers have higher
velocities indicating progressively greater consolida-
tion with increasing depth.
DEPTH TO BEDROCK
Although bedrock depth has been determined at only
10 points an interesting picture emerges from the
soundings (Figures 4 and 5). Sediment thickness is
smallest, 70-80 m, near Skeiðarárjökull, where the
bedrock lies slightly above the present sea level. The
same applies to profile S V near the terminus of Svína-
58 JÖKULL No. 51