Jökull - 01.01.2009, Síða 79

Deglacial and Holocene sediment distribution in Hestvatn, South Iceland
Figure 7. Paleobathymetry of Hestvatn. Paleobathymetry I depicts the configuration of the basin prior to any
sediment infill. Paleobathymetry II shows the basin after seismic unit II is deposited. Both are superimposed on
a subareal map, published with permission from the National Land Survey of Iceland. –Dýptarkort af Hestvatni
á mismunandi tímum. Mynd I sýnir botn vatnsins áður en nokkurt set hefur sest til. Mynd II sýnir botninn eftir
að eining II hefur sest til. Landakort er birt með leyfi Landmælinga Íslands.
of the south basin, protruding into the basin from the
lake shore. Evidently sediment accumulation occurs
in this cove of the lake, with transport from the west
to the deepest part of the south basin. An apparent
delta is observed at the mouth of Krákulækur (Fig-
ure 2b), where there is only minimal inflow today. A
vague deltaic form (or sediment pile) is also visible in
front of the middle cove at the north side of the lake.
Small fan-like structures are observed along the steep
bedrock walls on the eastern side of the lake. Multi-
ple ridges are found in the area dividing the north and
south basins of the lake, protruding up from the oth-
erwise relatively flat lake bottom, thus narrowing the
channel between the basins (Figure 2b).
INTERPRETATION
All but one seismostratigraphic unit (I) can be mapped
across the Hestvatn basin and isopach maps of sedi-
ment thickness for each unit, derived using GIS soft-
ware, show that sediment is not uniformly distributed
across the lake basin. Seismic unit I is only found in a
few of the lines from the north basin. From the acous-
tic character of seismic unit I and its small core sample
we interpret this unit to be till. Since it is not possi-
ble to create a separate isopach map for seismic unit
I it is combined with seismic unit II for the pertinent
isopach map. These maps imply substantial changes
in the dominant sediment sources from the deglacia-
tion (seismic units I and II) and through the Holocene
(seismic unit III).
Comparison of lithofacies and acoustic character-
istics for seismic unit II in the south basin show the
glaciomarine origin of the unit. The sediments were
deposited in the submerged south basin before and
during the deposition of the Vedde Ash, which is only
found in the south basin. The till unit (seismic unit I)
found in the north basin suggests occupation of the
north basin by an outlet glacier at the time of the
Vedde Ash formation. At this time most of seismic
sub-unit IIa, was deposited in the south basin, ex-
plaining the thicker accumulation in that basin com-
pared to the north basin. The fact that this seismic
sub-unit IIa is only found in parts of the easternmost
lying seismic line of the north basin (lies along the
drumlin-like ridges), suggests that the accumulation
took place during the retreating phase of the glacier
that formed seismic unit I. Sediment accumulation
rates for the seismic units show rapid sediment de-
livery during the marine depositional phase (Table 3).
The transitional change from sub-unit IIa to sub-unit
JÖKULL No. 59 79