Jökull


Jökull - 31.12.2001, Side 60

Jökull - 31.12.2001, Side 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
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