Jökull - 01.12.1999, Page 82

Figure 6. Shapes of the kettles according to their
axes ratios (W/L : DAV) on the Höfðabrekkujökull
(circles, N=16) and Skeiðarárjökull forefields (trian-
gles, N=18). — Samanburður á stœrðarhlutföllum
jökulkerja á Mýrdalssandi (hringir) og Skeiðarár-
sandi (þríhyrningar).
with a shallow, periodically changing and disappear-
ing water table, surfacial sands (H) and denudative
sediments (I) from the young walls of the cleft of the
kettle.
Deposits and evolution of a rimmed-normal kettle
The sediments of the substratum of this kettle are
represented by two lithofacies sets (Figures 9-13).
Moraine diamicton (A on Figure 11A), gravelly, and
massive, with lenticles of fine sand. Its roof is ero-
sionally lowered beneath the out-melting-denudative
series but lies about 2 m higher under the fluvioglacial
series. The other set consists of massive fine-grained
sand (B) with lenticles and strips of silt and coarse-
grained sand. The original lamination of silts is dis-
sected by a few, small normal and reverse faults. The
textural characteristics of the sediment and its slightly
disturbed structure indicate an environment of stand-
ing waters with episodic flux. However, the deforma-
tional structures of the deposits of both lithofacies sets
and their erosional roofs may be genetically linked to
the deposition of a block of ice above them.
Figure 7. Morphology and geological structure of the
bottom of a normal melt-out kettle 10. — Þversnið af
jökulkeri 10.
Fluvioglacial sediments (C) with a thickness of
1.4-2.2 m, make up the primary surroundings of the
kettle (Figures 10, 11A and 13). They are composed
of horizontal layering of gravel and sand with boul-
ders, slightly inclined towards the rounded bottom
of the kettle. The development of rims around the
kettle, is linked with the deposition of out-melting-
denudative deposits (sets F, G, and H, on Figure 13:
1-3). The following deposits form the rim:
(a) sediments of melt-out diamicton (F), with a pre-
dominance of coarse gravel and boulders, tightly
packed and reaching up to 20 cm. They might have
been melted out of moraine material from the bottom
of the dead ice (Figure 13: 3-5).
(b) fine and medium sands with diamicton (G) and
boulders in the central part of the lithofacies set, along
with small laminated layers of fine sand. This sedi-
ment represents the lithofacies of a small depression,
permanently filled with water. It was formed under
and between a melting wall of ice with diamicton,
which was flowing and falling from the highest parts
of the ice mass and from the edge of the sandur (Fig-
ures 10, 11A and 13: 2-3). The complex, which orig-
inally had slight inclining layers of fine sand, was
greatly deformed by the gravitational push of the over-
laying sediments (H) and the melting of smaller pieces
of the block of ice lying below, formed by the break-
up of the original block (Figure 13: 3-5). In connec-
tion with this, the depositional structure of the satu-
rated sediment underwent folding and severe shatter-
80 JÖKULL, No. 47