Jökull - 01.01.2012, Page 176

Í. Ö. Benediktsson
Figure 4. Section 1. A) Detailed drawing of the section with structural data. B) Photograph of the section. C)
Tectonically foliated sheath folds in the section core. D) Recumbent folds in the proximal part of the core. Ice
flow from right to left. – A) Teikning af sniði 1. Gröfin sýna þrívíða legu setlaga, misgengja og fellingaása
í sniðinu, sjá skýringar við 3. mynd. B) Ljósmynd af sniði 1. C) Hringlaga slíðursfellingar og D) liggjandi
fellingar í kjarna garðsins. Þrýstingur jökuls frá hægri til vinstri.
of the advancing ice. As the pressure from the glacier
increased, the uppermost strata sheared over the strata
below to form the asymmetric box and sheath folds
and the anticline-syncline pair, which constitutes the
ridge form of the end moraine. Simultaneously, the
lower part below the shear zone deformed further
and became largely homogenised, and a new anti-
cline formed in front of the syncline. Subsequently,
this new anticline overturned and thrust over slightly
folded strata in the distal part before the deformation
ceased.
The upwards decrease in strain in the central part
of the section is consistent with variable amounts of
horizontal shortening. While the layers of the asym-
metric box folds in the shear zone below the anticline
have been shortened by 46%, the upper strata form-
ing the anticlines and synclines have been shortened
by 23%. If the depth to the basal décollement is cal-
culated on the basis of these numbers, the depth to
the basal décollement would be 5–9 m. These num-
bers do not correspond to the surface of the gravel
body at the base of the section and indicate a some-
what deeper deformation at this site than previously
suggested (Benediktsson et al., 2008) and that the
gravel body was indeed incorporated in the deforma-
tion. The large differences in horizontal shortening
within the section reflect contrasting strain accom-
modation within the sediment pile, likely indicating
polyphase deformation of the foreland. The structural
data within section 1 show that fold axes are dispersed
and barely reliable indicators of the direction of stress,
while LPT beds and faults strongly suggest stress ap-
plication from the south, as expected (Figure 4).
Sections 2a and 2b
Sections 2a and 2b are located in the eastern part of
the central forefield in Kringilsárrani (Figure 1) in a
wind eroded trench that exposes the central part of the
end moraine. Three sediment facies were recognized
in the sections; interbedded sand and silt (F3), LPT
(F4), and tephra (F5) (Table 1). Section 2a has not
been described before and gives an insight into the ar-
chitecture of the backslope of the end moraine and the
glaciotectonic processes operating beneath or just in
front of the ice margin. Section 2b, which covers the
proximal to distal core of the moraine, was previously
described by Benediktsson et al. (2008). Their de-
scriptions are slightly revised here. The end moraine
is up to 10 m high and 50 m wide at this site.
174 JÖKULL No. 62, 2012