Jökull - 01.12.1999, Blaðsíða 84

ing with cutting faults (including inverted faults in a
rotational position). At the same time, deposition of
a package of deposits of reservoir sedimentation oc-
curred (Figure 13: 3, Ji) on the southern side of the
kettle in a crack between the iceberg and the basic di-
amicton.
(c) basic diamicton (H), with many boulders and
coarse gravel is the main lithofacies set filling up the
kettle. It comes from the ablation of the moraine
load contained in the ice and occurs across the whole
width of the kettle, although it is thickest (2.1 m) at
its sides. Layer H forms a circular rim (ring) of di-
amicton around the kettle at a depth of 1 m and lies on
top of a fluvioglacial shelf (C) in its northem part. We
are not aware of other researchers having recognized
such rounded forms and therefore claim that we have
discovered a new type of rimmed kettle.
In cross-section, both the internal and extemal
slopes of the rim mound are steep. The sediments of
the external slope are in contact with the sandur de-
posits, while the internal slope was formed by grav-
itational processes, i.e. downwards displacement of
younger sediments. Deformational structures occur
in the root of the rim, e.g. small folds of sand in the
diamicton, and rotational microfolds. Thus the inter-
nal structure of the rim ridges indicates that they are
not exclusively formed by gravitational scattering of
material but also by consolidation and deformation in
their root zone.
Further deposition of younger sandur deposits (D)
marks the beginning of the second evolutionary phase
of the kettle, changing it from a rimmed kettle to a
more normal looking one. Further deposits of vari-
grained sands with diamicton (I) gradually fill the
kettle (Figure 13: 4-6). Three separate phases could
be distinguish on the northern side of the kettle. A
lower section (L) composed of sandy diamicton with
a continuous layer of undulationally disturbed fine
sand at its bottom, a middle section (I2), composed of
sandy diamicton with numerous lenticles and irregular
lumps of fine- and medium-grained sands exhibiting a
torrent and involutional structure (Grzybowski, 1970),
and a upper section (I3), composed of unlayered, vari-
grained sands, gravel and small boulders. The thick-
ness of these sections increases towards the centre of
Figure 9. Geological structure of the bottom of kettle
16 and its immediate surroundings. — Þversnið af
jökulkeri.
the kettle opposite to the lower lithofacies complex.
Fragments of the (I2) and (I3) sections which rep-
resent a clear, integral extension of the fluvioglacial
deposits which make up the surrounding sandur (D)
are also deposited above the basic rim mound. The
evolution of the normal type kettle is thus linked with
sediment sinking and gentle gravitational flow on its
northem side, as well as block displacement or sub-
sidence on its southern side (Figures 10, 11A and 13:
4-5). The depression formed on the surface after the
ice melted collected water temporarily, in which ac-
cumulation of fine-grained deposits, silts and fine- and
very fine-grained sands with the character of sediment
from ice-dammed lakes took place (J2, J3).
Section J formed in the kettle during the final sed-
imentation period when the kettle was being morpho-
logically shallowed by the deposition of fluvioglacial
sediments (E). These sediments, deposited before the
ultimate disappearance of the embedded ice remains
do not extend to the southern edge of the kettle, i.e.
level VI (Figures 10, 11A and 13: 6).
DISCUSSION
Maizels (1991; 1992) described rim-shaped, sur-
face diamicton-boulder microforms created during the
1918 jökulhlaup on Mýrdalssandur. Fuller (1914) de-
scribing similar forms in the state of New York, distin-
guished three lithological types of rims; i.e. outwash
82 JÖKULL, No. 47