Jökull - 01.12.1999, Blaðsíða 81

Table 3. Morphological groups of the kettles in the light of the relation of their morphographic features. Average values of the
morphometric indexes (D/L; L+W/2D; L/W-l) are given for individual groups. — Mismunandi stœrðarhlutföll
og lögun jökulkerjanna.
morphographic elements and their features
outline transversal outline of edge transversal
no. of edge cross-section of kettles cross-section
(a) (b) (a) (b) (c) irregular (a) oval (b)
1 + +
2 + + (A) (B)
3 + + no. 5, 14 No. 3,7, 11, 17, 18 bowl-shaped (a)
4 + + (0.245/5.965/0.96) 0.196/4.978/0.61)
5 + +
7 + +
8 + +
9 + + (C) (D)
10 + + no. 2, 8, 10 No. 1,4, 12, 15 one bottom (b)
11 + + (0.163/7.113/0.42) (0.195/4.63/0.431)
12 + +
14 + +
15 + +
16 + + (E) (F)
17 + + no. 9, 16 - two bottoms (c)
18 + + (0.165/5.135/0.99)
GEOLOGICAL STRUCTURE AND
EVOLUTION OF SELECTED KETTLES
A normal kettle
The sedimentary surroundings of kettle 10 (Figures
5, 7 and 8) are composed of complex lithofacies de-
posits (Ai ,Figure 8) made up of layered and varifrac-
tional sand and gravel with boulders, inclined towards
the kettle. These deposits together with the sediment
lithofacies (A2) occurring below the kettle, indicate
that a small block of ice situated there earlier was
completely covered up by fluvioglacial sediments.
A total of six sedimentation sets, A2 to F, make
up the kettle (Figure 8). These include a basic di-
amicton (B) a gravelly-sandy-clayey sediment with a
massive structure, melted from the moraine load of
the embedded glacier ice. The next lithofacies set
(A2) belonged originally to the fluvioglacial deposits
(Ai). When the ice melted massive gravelly diamic-
ton appeared, greatly disturbing the original lamina-
tion. The roof of the gradually sinking lithofacies
set (A2) thus became the bottom of the thermokarsti-
cally deepening hole, with gradually increasing sur-
face area. The tripartite lithofacies set (C) appeared
initially on the melting ice. It is composed of fine-
grained black sands (Ci), medium- and fine-grained
sands (C2) and fine-grained sands (C3) with defor-
mational structures, such as weighted-down structures
and small landslip folds creating a flat-bottomed ket-
tle temporarily. The next underwater lithofacies set
(D) is made up of yellow-olive, gravelly-clayey di-
amicton, formed in pockets of gravity-slide (turbid-
ity) diamicton interfering with the sediments below.
The creation and development of such structures can
be observed along the shores of water reservoirs in ac-
tive kettles formed by recent jökulhlaups, such as the
November 5, 1996 jökulhlaup on the Skeiðarárjökull
forefield. Yellow-olive silt and clay (E), deposited in
calmer conditions within a closed water reservoir, lie
above the diamicton (D) and beneath laminated, fine-
grained sands with silty interbedding (F). As the depth
and surface area of the water in the kettle diminished
a kettle hole started to form (Olszewski and Weckw-
erth, 1998b). The threefold nivation-denudative com-
plex composed of three lithofacies sets G, H, I formed
last. They consist of black silt and fine sand (G) linked
JÖKULL, No. 47 79