Jökull - 01.01.2009, Page 80

Hannesdóttir et al.
IIb, reflects changes in sedimentary environment. Re-
treat of the ice lobe from the north basin was first fol-
lowed by a marine transgression of that basin and de-
position of glacial marine sediment of similar affinity
as seismic sub-unit IIb in the south basin (Figure 8).
Continuing isostatic rebound resulted in isolation
of the lake basin from the sea, dated to 10.6 ka BP
(Hannesdóttir, 2006). Subsequently sediments accu-
mulated in a freshwater system. Diatom analyses as
well as changes in sediment lithology support this in-
terpretation.
The strong irregular seismic reflector that sepa-
rates seismic sub-unit IIb and the overlying acousti-
cally well-stratified seismic sub-unit IIIa indicates an
erosional event. When compared with core lithofa-
cies of both cores (Hannesdóttir, 2006), seismic sub-
unit IIIa marks a series of rhythmic units, each start-
ing with coarse silt which grades into a clay cap. The
rhythmites are separated by finely laminated grayish
sediments. Seismic sub-unit IIIa is twice as thick in
the north basin as in the south basin, and sediment ac-
cumulation rates are close to what is observed in the
marine section (Table 3). We interpret these rhythmic
sediments as turbidites, reflecting episodic sedimenta-
tion. In some areas, the turbidites eroded the underly-
ingmarine seismic unit II, resulting in the channelized
boundary between the two units. The axes of chan-
nels and the maximum thickness of this unit points
to a source through the Krákulækur inlet, which also
could explain the relict form of a delta mouthing that
inlet (Figure 2b).
The transition from seismic sub-unit IIIa to the
overlying seismic sub-unit IIIb is indistinct in the seis-
mic sections but very distinct in the sediment cores
from both basins, which probably reflects small den-
sity differences in the fine grained graded sediment.
Deposition of the Saksunarvatn Ash occurred just
prior to the transition from seismic sub-units IIIa to
IIIb (Jóhannsdóttir, 2007) (Figures 3 and 4). The well
stratified appearance of sub-unit IIIb and the organic
rich sediments revealed by the sediment cores from
both basins indicate that Hestvatn reached a new equi-
librium as a freshwater system. Sediment accumula-
tion rates are the lowest for this seismic unit, showing
the decreasing glacial influence on the sediment sup-
ply to the lake.
The multibeam map reveals important sedimen-
tary features, which reflect changes in sediment envi-
Figure 8. Seismic profiles containing the core sites from the north and south basins. The different pattern of the
earliest sedimentation of the two basins is evident, with seismic unit I only present in the north basin, and thick
seismic unit IIa characteristic of the south basin sediments. The north basin holds a thicker lacustrine sequence,
because sediment delivery was restricted then to the northern basin. – Endurvarpslínur sýna kjarnatökustaði í
norður- og suðurdæld. Mismunandi setmyndun í neðstu setlögum Hestvatns er greinileg, þar sem eining I finnst
einungis í norðurhluta vatnsins og þykk eining IIa er einkennandi í suðurhluta vatnsins. Stöðuvatnaset er mun
þykkara í norðurdældinni, þar eð setflutningar voru aðallega frá lækjum á norðurströnd vatnsins.
80 JÖKULL No. 59