Jökull - 01.01.2016, Page 56

Paweł Molewski and Leon Andrzejewski
Figure 3. Structure of the ice-dammed lake’s bottom. A: Morphology of the central part of the northern study
area; B: Lithofacies profile. – Yfirlitsmynd og setlagasnið af fyrrum jökulstífluðu lónstæði á nyrðra rannsókn-
arsvæðinu, við Jökulgrindur.
The sand-dominated lower part of the sequence
exposed in the excavation is interpreted as being wa-
terlain and deposited in a high-energy (upper flow
regime) environment. The overlying finer grained silts
and silty sands are consistent with deposition having
occurred in a low-energy environment possibly within
standing or very slow moving water within the lake
(glacilacustrine). In contrast, the overlying c. 0.5 m
thick sequence of gravel, sand and silt represents the
development of a relatively higher energy environ-
ment within the lake. The inclined surfaces (stratifi-
cation) which characterise this part of the sequence
are interpreted as foresets which record the north-
westerly progradation of a small fan or apron into the
lake. The top of this fan/apron was subsequently trun-
cated by the erosive base of the overlying gravel. This
coarse-grained lithofacies may record the establishing
of high-energy, shallow water conditions within the
lake. In contrast, the fine-grained sediments at the top
of the sequence mark a return to a much lower energy
environment, with deposition of these cross-stratified
sands and silts possibly having occurred at the distal
edge of a subaqueous fan/apron. Changes in grain size
within the deposits of the fan/apron probably result
from a deepening of the lake and a shift in the pat-
tern of deposition. The cross-stratification records a
palaeoflow direction towards the west-northwest (Fig-
ure 3B).
Erosion and truncation of the top of this sequence
probably occurred during the development of the
palaeoshoreline. Apart from the lower part of the se-
quence, these sediments probably record deposition
close to the shore of a shallow proglacial lake.
56 JÖKULL No. 66, 2016