Jökull - 01.01.2016, Blaðsíða 57

The marginal zone of Tungnaárjökull, Iceland
Glacilacustrine deltas
A number of deltas have been identified within the
lower morphologic level in the northern part of the
forefield of Tungnaárjökull (cf. Andrzejewski et al.,
2005). They occur adjacent to the sloping edge of
this higher morphological level on the north-western
edge of a complex area of stagnating and dead ice
landforms (kettle holes) (see Figures 2 and 3A). The
delta examined during this present study is an approx-
imately 80 m long, fan-shaped landform which at its
culmination rises 12 m above the surface of the sur-
rounding outwash plain (Figure 4). The delta slopes
downwards in a north-westerly direction at an angle
of about 15◦. On its glacier side, the delta currently
ends with a 30◦, 17 m high slope. The steep slope of
a fan facing the glacier is interpreted as being an ice-
contact slope. This interpretation is supported by the
presence of dead-ice landforms on the up-ice side of
this proposed former ice marginal position. The gentle
"down-ice" slope of the delta has been locally modi-
fied by the development of ten distinct palaeoshore-
lines which record the subsequent phased lowering
and eventual disappearance of the lake. The upper
3.1 m of the sedimentary sequence is exposed in the
central part of the delta (Figures 4 and 5).
The sedimentary sequence is dominated by sands,
sands and gravels, and gravels (Si, SGi, GSi, Gi)
with a well-developed inclined (c. 25◦) stratification
recording a palaeoflow direction toward 295◦ to 345◦
(Figure 5A). The lower part of the exposed sequence
can be divided into two units composed of rhythmi-
cally bedded (average bed thickness c. 20 cm) sand
and gravel. A prominent erosion surface located at
a depth of c. 1 m separates the upper of these units
from an overlying 10 cm thick layer of horizon-
tally bedded/laminated sand and gravel (GSh) (Fig-
ure 5B). The gravel is overlain by an overall upward-
coarsening sequence of sand and gravel (Figure 5A)
characterised by a well-developed inclined stratifi-
cation recording a palaeoflow direction towards the
northwest.
The exposed sedimentary sequence is interpreted
as recoding deposition on the steep underwater slope
of a small delta as a result of the gravitational fall
of sediment grains (foreset). The increase in gravel
upward through the sequence, including the occur-
rence of rhythmically stratified sandy gravels (Si/Gi),
is interpreted as recording the variability of the hy-
drodynamic conditions related to the ice ablation rate.
Lithofacies comprising horizontally stratified grav-
els and sands (GSh) in the upper part of the se-
quence topsets (Zieliński, 2014) were deposited prob-
ably on the subaerially exposed upper surface of the
delta/alluvial fan (Figure 5C). The overlying gravel–
sand sediments which are characterised by a well-
developed inclined stratification are interpreted as
having been deposited on the delta slope, and as
Figure 4. Longitudinal cross-section of the studied lake delta. – Snið yfir aurkeilu við lónstæðið á 3. og 5. mynd.
JÖKULL No. 66, 2016 57