Jökull - 01.01.2009, Blaðsíða 78

Hannesdóttir et al.
Figure 6. Mean thickness and volume of seismic units within specified depocenters (where total sediment
thickness >20 m). North Basin sediments indicated in dark grey, South Basin sediments in light grey. a. Mean
thickness of seismic units. b. Volume of the same seismic units. – Meðalþykkt og rúmmál seteininga innan
suður- og norðurdældar (skilgreint svæði þar sem setþykkt >20 m). Einingar í norðurdæld dökkgráar og í
suðurdæld ljósgráar. a. Meðalþykkt seteininga. b. Reiknað rúmmál seteininga.
thick in the north basin, with a maximum thickness of
13 m, but on average 8 m thick in the south basin and
maximum of 11.5 m (Table 3).
Isopach maps of seismic units in Hestvatn and
multibeam bathymetric map
Sedimentation accumulation rates (SAR) are given in
Table 2 for the seismic units, except for the lowermost
sediments (Figures 3 and 4). The highest SAR is ob-
served in themarine sediments of seismic unit II. Seis-
mic sub-unit IIIa corresponding to multiple graded se-
quences has twice the SAR in the north basin com-
pared to the south. A more even SAR between the
two sub-basins is recorded in seismic sub-unit IIIb,
which also possesses the lowest SAR of the sedimen-
tary record (Table 3).
The isopach maps, based on the identification of
seismic units from Hestvatn sediments, suggest that
the different sediment deposition between the two
sub-basins is mostly controlled by past and present
hydrology of the watershed and the landscape of the
lake basin, including the narrow channel between the
south and north basin (Figure 2). Isopach maps of
total sediment for the whole lake show thickest sed-
iment in the south basin (44 m) with a little over 30
m in the north basin (Figure 5). Volume and thick-
ness calculations for the seismic units were confined
to specified areas within the two basins, where total
thickness is >20 m (Table 3). Seismic unit II makes up
half of this thickness, or 19 m on average with maxi-
mum of 35 m. In the north basin it is 16 m on average
with maximum thickness of 25 m (Figure 6a). How-
ever, due to the south basin’s smaller size compared to
the north, the volume is substantially less (Figure 6b).
Seismic sub-unit IIIa is thicker in the north basin than
the south basin. The unit shows a thickening towards
the southern parts of the seismic lines in the north
basin but it is thickest in the middle of the seismic
lines in the south basin (Figures 3 and 4). If the axis of
maximum thickness is followed from the north basin
to the south basin it points to a source around the inlet
of Krákulækur (Figure 1b). Seismic sub-unit IIIb is
also considerably thicker in the north basin compared
to the south basin (Figure 6a). This sedimentation pat-
tern is probably related to the sediment source becom-
ing restricted to a northern inlet- although it is also
possible that the narrow channel connecting the two
basins limited sediment delivery to the southern basin
to suspended load sediment only. Paleobathymetric
maps of the lake illustrating the configuration of the
basin prior to any sediment deposition, and prior to
deposition of seismic sub-unit IIIb (Figure 7) show
that the bathymetry, after deposition of unit II, resem-
bled modern-day bathymetry; the lake floor had been
leveled out and the basins widened.
Various features of the bottom sediments are visi-
ble from the multibeammap revealing sediment forms
and transport pathways hitherto unknown (Figure 2b).
A distinctive platform is seen in the southwestern part
78 JÖKULL No. 59