Jökull - 01.01.2009, Side 68

Hannesdóttir et al.
coastal sites contain evidence for fluctuating tidewater
glacier termini occupying paleo fjords and bays (e.g.,
Geirsdóttir et al., 1997; Geirsdóttir et al., 2000; Geirs-
dóttir et al., 2007; Norðdahl and Pétursson, 2005;
Hannesdóttir, 2006). The deglaciation in Iceland also
coincided with an increase in volcanic activity (Jak-
obsson et al., 1978; Sigvaldason et al., 1992; Jull
and McKenzie, 1996; Sinton et al., 2005). Subglacial
lakes and subaerial ice-dammed lakes drained in re-
peated jökulhlaups (e.g. Kjartansson, 1964; Tómas-
son, 1993) that delivered large pulses of sediment to
lake basins and the marine environment (Geirsdóttir
et al., 1997; Lacasse et al., 1998; Geirsdóttir et al.,
2000; Hannesdóttir, 2006).
The history of glacier growth and retreat in Ice-
land during the last deglaciation and the Holocene
is mostly based on morphological studies of glacial
features and palynological studies of peat sections
and raised shorelines/terraces showing the step-wise
retreat/advance of the main ice sheet through the
Allerød/Bølling, Younger Dryas and the PreBoreal
times (Rundgren et al., 1997; Ingólfsson and Norð-
dahl, 1994; Norðdahl and Pétursson, 2005). The
Búði moraine complex in South Iceland is probably
the strongest evidence for this step-like retreat (or ad-
vance) of the main ice sheet and the associated se-
ries of relative sea-level changes. Although most
14C dates indicate a Preboreal age for their formation
(Hjartarson and Ingólfsson, 1988), a recent study of
the moraine complex, supported by studies on sed-
iment cores obtained from Hestvatn, a lake located
25 km in front of the moraines, suggests parts of the
moraines were formed during Younger Dryas time
when the southern lowlands were a marine embay-
ment (Geirsdóttir et al., 1997, Geirsdóttir et al., 2000;
Harðardóttir et al., 2001a). A tephra layer geochemi-
cally identified as the Vedde Ash (11.8 ka (calibrated
years before present), Grönvold et al., 1995) found
in Hestvatnt’s marine deposits underlying lacustrine
sediment confirms local deglaciation prior to Younger
Dryas time (Geirsdóttir et al., 1997, 2000; Harðar-
dóttir et al., 2001a; Hannesdóttir, 2006).
In this paper we examine the sedimentation pat-
tern in Hestvatn from before the deposition of the
Vedde Ash to present day based on over 100 km
of seismic reflection profiles of the sediment fill, a
new high-resolution multibeam survey, and sediment
cores. Isopach maps built on the re-evaluation of
the seismic survey indicate striking shifts in sediment
source and depositional environments from the ear-
liest phase of sediment infill and after isolation of
the lake basin. Hestvatn is ideally positioned to pro-
vide information on the shift from glacial to inter-
glacial regime, isostatic rebound, andmarine to terres-
trial environment. Efficient glacial erosion of the soft
bedrock of Iceland has resulted in high sedimentation
rates, approaching 1 m ka!1 for most of the Holocene.
Seismic reflection surveys on the lake, studies of new
sediment cores and underlying marine sediment pro-
vide essential information on sediment distribution,
thickness and depositional processes where acoustic
properties allow tracing of seismic units within the
lake. A change in depositional environment is seen
from glacial to glacial marine and lacustrine sedimen-
tation with episodic turbidite formation concurrent to
isostatic uplift and catastrophic release of meltwater
from a retreating ice margin.
PHYSICAL SETTING
Hestvatn (6.8 km2) is situated 49.5 m a. s. l. in
the lowlands of South Iceland, seaward of the Búði
moraines (Figure 1). Basalt and hyaloclastite consti-
tute themain rock types of the Hestvatn basin (Tómas-
son, 1961). Mt. Hestfjall (319m a. s. l.), partly formed
subglacially, shows classic table mountain character-
istics, with glacial striations on top, dominantly a
southwesterly direction, but westerly and southerly
striations exist as well (Tómasson, 1961). Marine ter-
races are preserved on the western side of the moun-
tain, at 94 m, 75 m and 60 m a. s. l. (Kjartansson,
1939; Tómasson, 1961) and an excavation on the
south shore of the lake reveals a sandy gravelly ma-
rine delta (Figure 1b).
The lake basin has been subject to both tectonic
activity as well as glacial erosion, resulting in a N-S
orientation, with two >60 m deep basins, separated by
a deep but narrow (200 m wide) channel (Figure 2).
In the present regime inflow is limited to a few small
creeks on the northern side of the lake. Slauka, for-
merly known as Hestlækur is the only outflow (Fig-
68 JÖKULL No. 59