Jökull - 01.01.2009, Page 82

Hannesdóttir et al.
dóttir et al., 1997). The Hestvatn sedimentary record
is the only archive where the isolation contact (transi-
tion frommarine to a terrestrial environment) in South
Iceland has been dated.
The uppermost seismic sub-unit IIIb with the very
fine acoustic reflectors matching tephra layers in the
sediment cores reflects sedimentation in a freshwater
system. Most of the sediment of this unit was formed
from a suspended load, which also results in a much
better preservation of tephra layers than in the under-
lying seismic units. The multibeam bathymetric map
reflects the most recent conditions in Hestvatn. Relict
geomorphic structures are thought to reflect sedimen-
tation during earlier times.
Implication for the deglacial history of the south-
ern lowlands of Iceland and correlation with the
North Atlantic region
Different views on the size of the Younger Dryas
glacier in South Iceland have been suggested. Kjart-
ansson (1939) described a limited Late Weichselian
ice sheet, with thin lobate glaciers from the high-
lands, following themodern river courses of Hvítá and
Þjórsá. A heavy Younger Dryas glaciation of South
Iceland was inferred by Hjartarson and Ingólfsson
(1988) and Hjartarson (1991) based on deficit of shells
of Allerød age in the southern lowlands. Geirsdóttir
et al. (2000) argued a more limited Younger Dryas
glaciation based on research of the Búði moraines and
lacustrine studies from South Iceland (also Harðar-
dóttir et al., 2001a). Re-interpreted seismic data and
multibeam images from Hestvatn have revealed a dy-
namic depositional environment during the deglacia-
tion of the southern lowlands.
The occurrence of glacial deposits in the north
sub-basin of Hestvatn and the restriction of the Vedde
Ash to the marine sediments in the south-basin, in-
dicates that during Younger Dryas time (12.7–11.5
ka) outlet glaciers in southern Iceland terminated in
a marine embayment inland from the current coast-
line, at or just beyond the Búði moraines in southern
Figure 9. Possible position of ice margin at various times during Younger Dryas, deglaciation and Early
Holocene derived from available data. Images based on Digital Elevation Maps (http://edcdaac.usgs.gov), with
a resolution of about 1 km per grid cell. Glacial striations are marked with black lines and fluted moraines with
blue lines (Kaldal and Víkingsson, 1990). A. Younger Dryas glacier terminating in Hestvatn (!12.0 cal. kyr
BP), various evidence of the glacier terminating in the north basin of Hestvatn (see text). B. Búði morainal
complex (!11.0 cal. kyr BP), the configuration of the moraines (orange) is from Kjartansson (1939), Hjart-
arson and Ingólfsson (1988) Geirsdóttir et al. (1997), and evidence of ice-marginal lakes is from Áskelsson
(1942), Kjartansson (1964), Tómasson (1993), and Geirsdóttir et al. (2000). Red dots are sections with jökul-
hlaup deposits mapped by Axelsdóttir (2005). C. Isolation of Hestvatn and jökulhlaups (!10.6 cal. kyr BP).
Extent of ice-dammed lakes based on measurements by Tómasson (1993), end-moraines mapped by Kaldal
and Víkingsson (1990). D. Early Holocene (!10.0 cal. kyr BP), glacier retreating towards the mountainous
area in SE Iceland, most of the highlands ice free. – Möguleg staða ísaldarjökulsins á mismunandi tímum frá
Yngra Dryas til upphafs Nútíma byggt á margvíslegum gögnum. Grunnmyndir frá http://edcdaac.usgs.govmeð
1 km upplausn. Jökulrákir merktar með svörtum línum og jökulgarðar með bláum línum (Ingibjörg Kaldal and
Skúli Víkingsson, 1990). A. Yngri Dryas jökullinn kelfir í Hestvatni (sjá texta). B. Búðagarðarnir (um 11.000
ára gamlir) sýndir á einfaldan hátt með appelsínugulum línum samkvæmt Guðmundi Kjartanssyni (1939) og
Árna Hjartarsyni og Ólafi Ingólfssyni (1988), Áslaugu Geirsdóttur o. fl. (1997). Gögn um jökulstífluð vötn
eru fengin frá Jóhannesi Áskelssyni (1942), Guðmundi Kjartanssyni (1964), Hauki Tómassyni (1993), og Ás-
laugu Geirsdóttur o. fl. (2000). Rauðir punktar tákna opnur þar sem jökulhlaupaset hefur verið kortlagt (Hulda
Axelsdóttir, 2005). C. Einangrun Hestvatns og fyrstu jökulhlaup (um 10.600 ár). Jökulstífluð vötn á Kili eru
teiknuð samkvæmt mælingum frá Hauki Tómassyni (1993), jökulgarðar eftir Skúla Víkingssyni og Ingibjörgu
Kaldal (1990). D. Upphaf Nútíma (10.000 ár) og jökullinn hörfar inn til landsins. Á þessum tíma er mestur
hluti hálendisins orðinn jökullaus.
82 JÖKULL No. 59