Jökull - 01.01.2016, Blaðsíða 63

The marginal zone of Tungnaárjökull, Iceland
At a depth of approximately 3.2 m from the top of the
profile, lithofacies A2 contains a layer of ripple cross-
laminated medium- and fine-grained sand. Immedi-
ately overlying the sand is a several-cm-thick layer
of horizontally laminated silt which contains a large
(7-cm diameter) cobble which distorts the lamination
(Figure 9B). Overlying the silts is a 2 cm thick layer
of massive sand and fine gravel (SGm).
Sediments of unit A2 are interpreted as having
been deposited in standing or low-flow water condi-
tions. Their character, however, indicates a higher en-
ergy sedimentary environment than the underlying A1
lithofacies. The estimated deposition velocities of the
currents responsible for the deposition of lithofacies
A2 sediments range from 7.6 to 12.8 cm/s (Koster,
1978). An increase in the grain diameter of the sand
fraction in the top of the unit indicates an increase in
flow energy. The domination of sand, as well as the
presence of ripple cross-lamination, indicates trans-
portation of the material in bottom and fractional sus-
pension. The layer of ripple cross-laminated medium-
and fine-grained sand (Sr) is interpreted as episodes of
lower flow regime conditions. The layer of silt (Fhd),
which contains a cobble of approximately 7 cm di-
ameter, interpreted as a dropstone (Figure 9B), was
deposited in an environment of standing water. A thin
layer of SGm lithofacies proves the episodic growth
of current energy with the participation of traction.
Unit A3 is 2.05 m thick (Figure 8A) and comprises
horizontally stratified gravel, sandy-gravel, and sand
(Gh, SGh, Sh). The base of the unit, between 2.0 to
1.5 m depth, is marked by 0.5 m thick gravel, which is
in turn overlain by a sand dominated sequence (fine to
coarse sand) containing occasional sandy-gravel beds.
The MPS in the gravel and sandy-gravel layers ranges
from 80 to 150 mm.
Unit A3 was accumulated under the conditions of
an upper flow regime and high-energy current. Under
these conditions, sand and gravel grains are thought
to have been transported in traction and bottom sus-
pension (saltation). The average water flow (deposi-
tion velocities) rate, estimated on the basis of average
grain diameter, is from 30 to 40 cm/s (Koster, 1978).
To determine the energy of the depositional environ-
ment, the flow competence indicator (MPS) was also
used. This was calculated on the basis of the ninety-
fifth percentile (D95) and field measurements of the
average axis (b) of the ten largest grains from the
monogenetic facies Gh, GSh and SGh. The current
velocity values calculated from D95 range from 0.7
to 0.8 m/s, while those based on field measurements
range from 1.1 to 1.4 m/s.
The lowest unit of profile B, (B1) has a thickness
of 1.2 m on top of dead glacial ice (Figure 8B). Unit
B1 consists of sandy-silty lithofacies of massive struc-
ture and wavy laminating or climbing ripples (Figure
9C). The sediments of unit B1 were deposited in an
environment of standing water (Fm, SFm, FSw, SFw,
Sw) or under low-energy flow conditions (Src, SFrc).
Unit B2 has a thickness of 1.75 m and con-
sists mainly of fine-medium-coarse grained sand. The
lower 1.4 m of the unit has a well-developed low-
angle cross stratification (layers dip 10◦, with dip
direction of approximately 100◦) which records a
palaeoflow direction towards the east (Figure 9D).
The upper part of lithofacies B2 comprises a 25 cm
thick layer of horizontally bedded sand (Sh) capped
by a 10 cm thick bed of massive silt (Fm). The upper
part of unit B2 defines a coarsening upward cycle.
The thick series of sediment with low-angle cross
stratification (Sl) and horizontal stratification (Sh) at
the top, is interpreted as being deposits of an alluvial
fan. The massive silt which caps Unit B2 can be inter-
preted as having been deposited in an environment of
standing water in the channel.
Unit B3 (3.35 m thick), consists mainly of grav-
elly sand inclined layers of deposits with boulders
of massive structure and clast-supported (Gcm, GSm
SGm), secondary horizontal or low-angle cross strat-
ification (GSh, Sl, SFl). At the top of this unit’s sedi-
ments there is a fining upward cycle. The maximum
diameter of the grains in the gravel–boulder layers
range from 100 to 140 mm.
The gravel-rich GSh lithofacies was accumulated
under upper flow regime and high-energy current con-
ditions. The mainly inclined layers of the sediment of
unit B3 (Figure 8B) were probably the result of a de-
bris flow (e.g. Zieliński, 2014), attested by the mas-
sive structure of the deposits, formed as several thick
series of boulder–gravel–sand diamicton, and the low
JÖKULL No. 66, 2016 63