Jökull - 01.01.2016, Blaðsíða 59

The marginal zone of Tungnaárjökull, Iceland
ers, ice-marginal lakes. The deltas in the forefield of
Tungnaárjökull clearly record the development of an
ice-marginal glacilacustrine environment with a large
supply of material in conditions of hyperpycnal flow
(Boggs, 1995), corresponding to a situation in which
the density of the water in the lake is lower than that of
glacial meltwaters. Such conditions are conducive to
the formation of fan deltas (Hjulström, 1952; Holmes,
1965), which develop where an alluvial fan enters a
water body (Nemec and Steel, 1988). The fossil sedi-
ment record of such a fan is probably represented by
the gravel and sand facies of horizontal stratification
observed in the profile (Figure 5A).
THE SOUTHERN STUDY AREA
The southern study area, located approximately four
kilometres southwest of the northern area (Figures 1B
and 6) and c. 900 m SE of the present glacier margin,
is characterized by a complex of diverse landforms
and ice-contact sediments, mainly in the form of ice-
cored moraines (Figure 7A, B, C).
The focus of study in this area was an irregular
fragment (200 m long, 130 m wide) of a kame terrace,
whose surface lies at an elevation of approximately
775 m a.s.l., adjacent to the Jökulgrindur bedrock
ridge, which reaches approximately 800 m a.s.l. (Fig-
ure 6). The terrace’s relative height is approximately
12 m. A pronounced shoreline of an ice-dammed lake
lies approximately 15 m above the kame terrace, at an
altitude of 790 m a.s.l. along the glacier-facing slope
of Jökulgrindur (Figure 7D). A small hill of 50 by
100 m, about 100 m south of the terrace, is interpreted
as being cut off from the kame terrace, by glacifluvial
erosion (Figure 7C). The slopes of the kame terraces
are undergoing intense denudation. Lithofacial analy-
sis were conducted on the northern slope of both ter-
races (profiles A and B, Figure 6).
Kame terrace deposits
Sediments exposed within the two kame terrace pro-
files (A and B on Figures 6 and 7) were divided into 6
discrete lithofacies, namely A1, A2 and A3 in profile
A (Figure 8A) (Molewski et al., 2005) and B1, B2, B3
in profile B (Figure 8B).
The silty sands of unit A1 (Figure 8A) are 5.6 m
thick and lie directly on buried "dead" glacial ice (Fig-
ure 9A). This silty lithofacies is characterised by a
horizontal or wavy lamination developed on a range
of scales and differing extent. They include secondary
Figure 6. Geomorphological sketch of the southern study area (Molewski et al., 2005, modified). – Landmót-
unarkort af syðra rannsóknarsvæðinu. Snið í sethjalla eru merkt A og B.
JÖKULL No. 66, 2016 59