Jökull - 01.01.2012, Blaðsíða 123

Glacier changes in the marginal zone of Sólheimajökull
Table 2. Description and interpretation of sedimentary units in section 1. See also Figure 6. – Tafla 2. Lýsing
og túlkun á seteiningum í sniði 1. Sjá einnig 6. mynd.
Unit Description Interpretation References
1 0–0.1 m: Massive gravel with <50 cm, sub-rounded to
angular clasts. Upper boundary is sharp conformable
but undulating, probably indicating varying thickness.
Lower boundary is not exposed.
Glaciofluvial sediments indicating high discharge, pos-
sibly a jökulhlaup.
Maizels, 1997; Marren,
2005; Benn and Evans,
2010.
2 0.1–3 m: Massive gravel consisting mainly (c. 90%)
of <1 cm, very angular, black basaltic pumice grains,
slightly mixed with other light- coloured lithologies (c.
10%). Vague cross-bedding observed with beds dipping
towards south-west. 0.7–0.8 m: Horizontal silt and fine
sand with lenses of pumice.
Unit 2 represents either primary deposition of pumice
into running water, or fluvial transport immediately af-
ter deposition. The angular nature of the grains suggests
very short transport. The vague cross-bedding indicates
deposition in running water.
Francis, 1993. Marren,
2005.
3 2.4–2.6 m: Massive gravel with up to 100 cm sub-
rounded to angular clasts. Thickness varies from 0.2–1.2
m. Lower boundary is an unconformity. Upper bound-
ary is sharp conformable but greatly undulating due to
variable thickness.
Glaciofluvial deposits indicating a high discharge event,
probably a jökulhlaup.
Maizels, 1997; Marren,
2005; Benn and Evans,
2010.
4 2.6–3.1 m: Massive gravel consisting mainly (c. 90%)
of <1 cm, very angular, black basaltic pumice grains,
slightly mixed with other light- coloured lithologies (c.
10%).
Unit 2 represents either primary deposition of pumice
into running water, or fluvial transport immediately af-
ter deposition. The angular nature of the grains suggests
very short transport.
Francis, 1993. Marren,
2005.
5 3.1–3.4 m: Brownish, heterogeneous, matrix-supported
and clast-rich diamict. Matrix is clayey-silty. The di-
amict is massive in some places but banded in others.
Clasts are <20 cm and very angular to sub-rounded.
Subglacial traction till. Evans et al. 2006
6 3.4–4 m: Horizontally laminated, normal graded rhyth-
mites. The laminae gradually coarsen upwards from
light-brown, clayey-silty laminae to dark gray, sandy
laminae. Three layers of medium grained sand interrupt
the laminated sequence. The base of the sand layers is
an unconformity. A few outsized <2 cm pumice clasts
are scattered around the unit.
Unit 6 indicates deposition in a proglacial lake and is
interpreted as bottomsets of a delta. The rhythmites in-
dicate fluctuating discharge into the water body, possibly
on a diurnal basis. The overall coarsening upwards indi-
cates either gradually increasing discharge or increasing
proximity to meltwater source. The sandy layers sug-
gest high-discharge pulses into the lake. The outsized
pumice clasts are dropstones that initially floated on the
water until they became water-soaked.
Benn and Evans, 2010.
7 4–4.6 m: Planar cross-bedded, medium to coarse sand.
Thickness of individual layers is 1–3 cm. Layers dip
220◦ towards SW.
Unit 7 indicates deposition on the foreslope of a delta.
Layers are interpreted as delta foresets.
Benn and Evans, 2010.
8 4.6–7.8 m: Horizontally bedded medium to coarse sand.
Occasional clasts of 2-5 cm in diameter occur in the
coarser layers. Bed thickness is 1–2 cm.
Unit 8 is interpreted as delta topsets, thus representing
glaciofluvial deposition on a sandur plain.
Benn and Evans, 2010.
9 7.8–9.1 m: Gray, massive, matrix-supported diamict,
with moderate clast content. Clasts are elongated to
blocky and sub-rounded to angular. Matrix is silty-
sandy. Fissility is very distinct in the lower part but less
in the uppermost 60 cm. Diamict is firm.
Subglacial traction till. Evans et al., 2006.
10 9.1–9.3 m: Brown, massive, matrix-supported diamict,
with moderate content of <50 cm large clasts that are
elongated to blocky and sub-rounded to angular. Matrix
is silty-sandy.
Subglacial traction till. Evans et al., 2006.
11 9.3–9.5 m: Gray, massive, matrix-supported diamict,
with high content of <50 cm large clasts that are elon-
gated to blocky and sub-rounded to angular. Striations
on clasts are moderately common but with non-preferred
orientation. Matrix is silty-sandy. Lower boundary is
defined by a lens of coarse sand.
Subglacial traction till. Evans et al., 2006.
12 9.5–9.8 m: Massive, medium sand abundant with out-
sized clasts up to 40 cm in diameter.
Glaciofluvial sediments. Marren, 2005; Benn
and Evans, 2010.
JÖKULL No. 62, 2012 121