Jökull - 01.01.2012, Page 175

Structural evolution of the 1890 Brúarjökull end moraine
pair, which has a general plunge to the east (Figure 3).
Patches of the Öræfajökull AD 1362 tephra near the
surface can possibly be connected to the same tephra
layer approximately 0.5 m below via another proximal
recumbent syncline. Balancing of the section, with
fold hinges inferred, shows that the minimum hori-
zontal shortening is 6 m or 67%, and that the décolle-
ment surface lies at 1 m depth, which coincides with
the bedrock surface at the base of the section. The
strike and dip of the principal plane (89◦/14◦N) con-
firm that stresses were induced from the south (Figure
3). This particular segment of the 1890 end moraine
indicates that the glacier coupled to the foreland at
least a few metres upglacier from its terminal position
and deformed the foreland strata into an open fold that
subsequently overturned.
Section 1
Section 1 is located along one of the major meltwa-
ter outlets of the 1890 surge (Figure 1), now occupied
by the Kringilsá River. The moraine ridge is 20–25 m
wide here and 1.5–2.5 m high and symmetric in shape
(Figure 4). Section 1 is orientated approximately per-
pendicular to the moraine strike. Four sediment facies
were identified: diamict (F1), gravel (F2), LPT (F4),
tephra (F5) (Table 1). A sub-horizontal gravel bed
(F2) underlies the entire section and forms a décolle-
ment. Above this bed, the sediments are deformed.
Based on the style and magnitude of glaciotectonic
deformation, the facies architecture can be divided in
three parts: (i) the proximal part (0–3 m), (ii) the cen-
tral part (3–15 m), and (iii) the distal part (15–22 m).
Proximal part, 0–3 m.
The proximal part is characterized by deformed
pre-surge diamict which interfingers with LPT, in par-
ticular. The LPT includes boudinaged and small-scale
folded sand, indicative of both tensile and compres-
sive stresses, but is otherwise crudely bedded and
contains few sedimentary and deformation structures,
mainly due to destruction by roots in the upper levels
(Figure 4).
Central part, 3–15 m.
This part of the section reveals the heaviest glacio-
tectonic deformation in the end moraine. The moraine
is characterized by shear zones, frequent small scale
folds, and foliation (Figure 4). A number of recum-
bent and inclined folds exist in the lower core. For
example, just above the gravel bed at 3–5 m there are
small overturned and recumbent folds with axial dips
to north and south and vergence to west and south-
west, respectively, indicating principal stress applica-
tion from easterly directions (Figure 4D). In many
places of the lower central part, but particularly at
10–13 m, the different sediment facies have been ho-
mogenised by continuous deformation (van der Wa-
teren 1995) and therefore contain no sedimentary or
deformation structures. A shear zone occurs above
the lower central part and is typified by non-periodic
asymmetric box folds verging north, and attenuated
sheath folds verging west (Figure 4C). This shear zone
marks the lower boundary of an anticline-syncline
pair that overrode the lower part. The anticline-
syncline pair is asymmetric in the sense that the an-
ticline is considerably wider than the syncline. Mea-
surements on the syncline show that the proximal and
distal limbs dip steeply towards north-west and south-
west indicating that the syncline closes towards west.
The dip of the sedimentary beds of the anticline could
not be measured due to poor accessibility.
Distal part, 15–22 m.
This part of the section is dominated by deformed
pre-surge diamict (F1) in the lower part but sand and
silt (F3), LPT (F4), and tephra (F5) in the upper part.
The diamict interfingers with the other sediment fa-
cies but attenuates in the distal extremity of the sec-
tion. Above the diamict, there are minor overturned
folds at 14–16 m and thrusts at 17 m which indi-
cate that there is an overturned anticline that has been
thrust up in front of the syncline (Figure 4). Appar-
ently, the hinge of this anticline has been either trun-
cated by syn-tectonic slope failure or eroded by syn-
or post-tectonic meltwater. Soil and roots at the sur-
face prevented this to be further determined. Distal
to the minor thrusts there is open folding of LPT and
gravel that fades out and merges imperceptibly with
the undeformed sandur in the foreland (Figure 4).
The deformation in section 1 can be divided in
three phases. It commenced with folding and, to a
lesser extent, thrusting, of the foreland strata in front
JÖKULL No. 62, 2012 173