Jökull - 01.01.2009, Blaðsíða 81

Deglacial and Holocene sediment distribution in Hestvatn, South Iceland
ronment and delivery paths to the lake. The multiple
ridges between the north and south basins form possi-
ble pinning points for the calving glacier terminating
in the paleobay west of Hestfjall. This position of the
glacier terminus explains the thick glacio-marine sed-
iments only found in the south basin of the lake. In
the southwestern part of the south basin a platform
extends into the lake from the shore, possibly related
to the delta on the south shore of the lake (Figure
1b). The delta probably formed in front of a glacier
tongue that curved around Hestfjall into the paleoma-
rine environment during the accumulation of seismic
sub-unit IIa. The fan-like structure at the mouth of
Krákulækur, is the only profound deltaic feature in the
north basin. This correlates with the thickness distri-
bution of seismic sub-unit IIIa, which axis of maxi-
mum thickness points to a source around the inlet of
Krákulækur. This may possibly have been the main
inflow for the sediment-laden water forming the tur-
bidites of seismic sub-unit IIIa.
DISCUSSION
A dynamic environment from a marine to a lacus-
trine sedimentation
The sediment record preserved in Hestvatn reflects
a dynamic environment, which suggests a balance
between glacial, marine and fluvial processes. The
isopach maps of the seismic units demonstrate a shift
of the main sediment pathways to the lake basin. Dif-
ferences in the seismic units’ spatial distribution re-
flect changes in the sediment source, mainly affected
by deglaciation and the subsequent isostatic rebound,
isolating the lake basin from the sea. The new sedi-
ment cores reveal a succession of turbidites (seismic
unit IIIa, hitherto unrecovered from Hestvatn), which
allow re-interpretation of the seismic data. The asso-
ciated seismic unit has a clear lower boundary - an
erosional surface, and was deposited during an inter-
val of only 600 years (Hannesdóttir, 2006).
The seismic survey and the isopach maps of seis-
mic units I and II demonstrate an environment domi-
nated by a tidewater glacier that was pinned between
the two sub-basins. Although the tidewater glacier in
the north basin delivered substantial sediment to the
southern basin, a sandy-gravelly ice-contact marine
delta at the south end of the lake graded to 50 m a.
s. l. suggests that some of the gravelly layers of seis-
mic unit II in the south basin may have been sourced
from another glacier tongue that curved around the
east side of Hestfjall. This interpretation is supported
by the platform in the southwest part of the south
basin on the multibeam map. Longshore drift may
have also contributed a substantial volume of sedi-
ment to the south basin prior to isolation from the
sea. Changes within seismic unit II indicate retreat
of the glacier and subsequent isolation of Hestvatn
as it changed into a lacustrine environment. A shift
in sediment source is evident from the isopach maps,
with the primary locus of sedimentation shifting from
the south basin to the north basin as deglaciation pro-
gressed (Figures 5 and 6). A succession of thick,
distinctly graded sedimentary units interbedded with
finely laminated sediment reflects the episodic input
of turbidites associated with failures of ice dams up
stream of Hestvatn. The timing of these turbidites co-
incides with the retreat of the main Iceland Ice Sheet
from the Kjölur highland area !80 km north of the
Hestvatn basin (Figure 1a) and suggests their origin
to be the results of repeated release of ice-dammed
lakes (Kjartansson, 1964; Tómasson, 1993; Kaldal
and Víkingsson, 1990). The turbidites were deposited
as jökulhlaups in the newly established lake, and are
not observed in the marine section of the sedimentary
record. However, the preservation potential of flood
deposits entering the sea is not high for modern day
jökulhlaups (e.g. Maria et al., 2000), and jökulhlaups
prior to isolation of Hestvatn may not be apparent.
Isolation of the lake basin provides information on
glacial rebound during last deglaciation. The rapid
change in diatom flora from marine to freshwater as-
semblages, recorded in the sediment cores (Hannes-
dóttir, 2006), indicate rapid uplift, in accordance with
the sensitivity of the Icelandic crust to glacial load-
ing and unloading (Sigmundsson, 2006). According
to lithological and diatom analysis the Hestvatn basin
became isolated around 10.6 ka, hence sea level was
at that time 50 m a. s. l. This data point adds to a set
of a few dated sea level stands in south Iceland (Hjart-
arson, 1988; Hjartarson and Ingólfsson, 1988; Geirs-
JÖKULL No. 59 81