Jökull - 01.12.1987, Page 78

water in reducing shear strength of pro- and subglacial
sediments (e.g. Banham 1975, Sharp 1985). But se-
quences also exist where a combination of high pore-
water pressure and differential permafrost in the strata
subject to glacial pressure are invoked to explain large-
scale deformations (e.g. Thomas 1984b). Recently Aber
(1985, p. 389) stated that “glaciotectonic features may
. . . affect materials that were either frozen or thawed at
the time of deformation”.
There are no structural methods available yet with
which to get objective information from deformed sedi-
ments on the basal conditions of a glacier at the time of
glaciotectonic deformation. The information is sparce
from recent glaciers on the effects of different combina-
tions of basal temperature, different substratum, hydro-
dynamic situation, differential loading, compressive ice
flow etc. on the development of glaciotectonic deforma-
tion. Thus at this stage, the models are somewhat cir-
cumstantial and should be used carefully. In the case of
the Melabakkar-Asbakkar glaciotectonics, the com-
bined effect of frontal push, differential ice loading and
hydrodynamic mechanisms is to me the most attractive
explaination. The two ice advances occured into a sub-
merged fjord basin, where the presence of any perma-
frost is very unlikely. There is also evidence of abundant
meltwater in connection with the glacial events.
SUMMARY AND DISCUSSION
It is my conclusion, that the Melabakkar-Ásbakkar se-
quence, bounded by a lower surface of glacial erosion
and by an upper surface of wave erosion, contains a
fairly continuous record of glacial episodes in the lower
Borgarfjördur region for the later part of the Late
Weichselian, after ca. 12.500 BP. The sequence was
deposited in a glacio-isostatically depressed fjord basin,
where the major controls of lithofacies distribution and
stratigraphical associations were waterdepth and the
proximity to an ice margin and a source of meltwater
input. The total thickness of the exposed strata is about
145 m, of which the glaciomarine sequences constitute
about 85 m, ice-proximal/ice contact outwash sedi-
ments, debris flows and tills 40-45 m, and an emergence
facies of sand and gravel about 15 m. A composite verti-
cal section is shown in Fig. 3.
The striated bedrock, which constitutes the lower
boundary of the sequence, bears witness to a glaciation
event when the ice reached beyond the present coast —
some time prior to about 12.500 BP. The development of
the Melabakkar-Ásbakkar sequence can be divided into
nine stages (Fig. 19): During the first stage (stage A),
mollusc-bearing glaciomarine sediments (the Ásbakkar
diamicton) accumulated from suspension, random un-
derflows and ice-rafted debris. Around 12.300 BP, the
relative sea level was at least 70 m above the present sea
level, possibly reaching the marine maximum level at
80-90 m a.s.l. Around 12.000 BP a glacial advance down
the Borgarfjördur valley/fjord (stage B) caused the mol-
lusc populations to disappear, and subaqueous ice-mar-
ginal/ice-proximal stratified sediments (the Ás beds)
were deposited from subglacial meltwater streams and
- .V, ' 'T-
' . s '
‘ Y\‘ -

Fig. 17. A composite oblique photograph of alarge fold- synclinal (trough) hinge zone. The deforming force has
ed structure developed in the Ásbakkar diamicton at acted sub-parallel to the outcrop, from left to right. The
around 160 m. The structure is truncated by a gravel lag section is about 50 m long.
horizon and overlain by littoral gravels. The arrow 17. mynd. Samsett Ijósmynd afhögguðum jarðlögum við
points at axial plane foliations developed close to the 160 m.
76