Jökull - 01.01.2012, Page 172

Í. Ö. Benediktsson
sediment wedge, in which the sediment thickness in-
creases from 1–3 m at the upglacier end to >7 m at
the end moraine crest, was formed (Benediktsson et
al., 2008; Figure 2). By comparing the length of the
wedge and observed ice-flow velocities (∼120 m-day)
from the 1963-64 surge of Brúarjökull, Benediktsson
et al. (2008) deduced that the wedge was formed in
about five days. Moreover, they claimed that the 1890
end moraine formed on the distal top of the wedge
during the last day of the surge, approximately, in re-
sponse to a drop in submarginal porewater pressures
following blow-out of overpressurized water in front
of the glacier. Thus, the sediment wedge and the end
moraine form a dual, inseparable marginal end prod-
uct of the 1890 surge (Figure 2). A detailed descrip-
tion of the structural evolution of the end-moraine
ridge has not been given before and follows below.
GEOMORPHOLOGY, SEDIMENTO-
LOGY AND INTERNAL ARCHITEC-
TURE OF THE 1890 END MORAINE
Apart from low and narrow dump moraines in areas
of elevated bedrock and thin sediment cover, the 1890
end moraine in Kringilsárrani is 5-20 m high and 40-
80 m wide and most prominent in topographic de-
pressions where the sediment cover is thick (Figure
1). Usually, the foreslope is steep and the backs-
lope gentle with ice-free hummocky moraine. Active
downwasting of buried ice is rare where the moraine
consists of fine-grained sediments (Schomacker and
Kjær, 2007; Benediktsson et al., 2008). The internal
sedimentary composition and architecture of the 1890
end moraine was studied in four cross-sections. Five
sediment facies were identified; F1 diamict, F2 gravel,
F3 interbedded sand and silt, F4 deformed LPT, and
F5 tephra. F4 and F5 are most dominant and occur in
all sections while F1, F2, and F3 were found in two
sections. Descriptions and interpretations of the sed-
iment facies follow in Table 1. Three of the sections
(1–3) were described by Benediktsson et al., 2008 but
are now revised. Two new sections are added and de-
scribed for the first time; section 0 and sub-section 2a.
Section 0
Section 0 is located in the central part of the 1890
end moraine in Kringilsárrani (Figure 1) along a small
stream running on bedrock. This stream occupies a
Figure 2. Conceptual model illustrating the formation of the marginal sediment wedge with the actual end-
moraine ridge on the distal top. See text for details. Modified from Benediktsson et al., 2008. – Líkan sem
sýnir myndun setfleygs undir sporði Brúarjökuls á síðustu dögum framhlaupsins 1890. Sjálfur jökulgarðurinn
myndaðist á toppi fleygsins á síðasta degi framhlaupsins.
170 JÖKULL No. 62, 2012