Jökull - 01.01.2016, Síða 54

Paweł Molewski and Leon Andrzejewski
terraces were selected for more detailed laboratory-
based grain-size analysis. Sandy sediments and sand
fractions in silty-clay samples were analysed using the
sieve method. Fractions finer than 0.1 mm were mea-
sured using a laser grain-size analyser. Based on these
methods the deposits were classified by grain-size us-
ing nomenclature and fractional categories from the
Polish Geological Institute (Wydawnictwo Geolog-
iczne, 1977) which subdivides the sediments into five
main categories: clay <0.01 mm; silt 0.01–0.1 mm;
sand 0.1–2.0 mm; gravel 2.0–100.0 mm; and boulders
100.0–1000.0 mm.
For selected deposits, average deposition velocity
was estimated in cm/s according to Koster’s (1978)
formula V=49D0.381, where the variable is the me-
dian grain size (D50) in cm. Maximum Particle Sizes
(MPS) were also determined as an indicator of flow
rate competence for large-grain deposits. MPS is de-
termined in the field based on measurement of the in-
termediate axis (b) of the ten largest grains originating
from a monogenetic lithosome or lithofacies, or based
on the cumulative grain-size curve with MPS fraction
size being determined as that of the 95th percentile
(D95). The estimated palaeocurrent competence is cal-
culated using Koster’s (1978) formula.
THE NORTHERN STUDY AREA
The northern study area is located in the northernmost
part of Tungnaárjökull’s marginal zone in the vicinity
of the volcanic cone of Kerlingar (1,286 m a.s.l.) (Fig-
ure 1B), which separates Tungnaárjökull from Sylgju-
jökull. The area is bordered to the southeast by the
glacier snout and to the northwest by the volcanic
ridge of Jökulgrindur (Figure 2). The present glacier
margin lies parallel to Jökulgrindur at a distance of 1.2
to 1.5 km to the southeast of this bedrock ridge. The
area extends from northeast to southwest over about
3.5 km and comprises two distinct structural morpho-
logical levels. The higher level ranges between 300 to
950 m across at an altitude of 860 to 900 m a.s.l. The
lower morphological level is between 250 and 800 m
wide, at an altitude of 800 to 840 m a.s.l. and extends
to the foot of Jökulgrindur.
The higher morphological level consists mainly of
bedrock covered with subglacial traction till and mass
flow deposits of variable thickness (0.5–1.5 m) in the
form of a very poorly sorted sand–gravel–boulder di-
amicton. Its morphology is currently characterised by
dry (abandoned), proglacial meltwater channels and
flutings. In the northern part of this higher morpho-
logical level there is a complex of ice-marginal and
proglacial lakes which are fed by glacial meltwater
and drained by a river flowing towards the southwest
along the glacier’s snout (Figure 2A). Water levels in
these lakes are at different altitudes ranging from ap-
proximately 850 to 895 m a.s.l.
The lower morphological level is dominated by a
series of variably developed outwash levels (Figure
2B). The surfaces of the outwash levels descend to
an altitude of about 845 m a.s.l. in the north, down
to about 820 m a.s.l. in the southern part. The lowest
parts of the lower morphological level are represented
by the bottom of the former ice-dammed lake. A rem-
nant of this lake is preserved by a small, isolated (no
outflow) groundwater-supplied lake which possesses
a small coastal bench cut into its southern side. The
average water level of the lake is approximately 840 m
a.s.l. (Figure 2A).
A diverse set of dead and stagnating ice forms are
present in the central part of the northern study area, at
the contact with the higher morphological level, along
with several preserved delta fans (Figure 2), whereas
end moraines characterise its southern part, on the
glacier-facing slope of the Jökulgrindur. To the north-
west, at the lower morphological level, an elongated
end-moraine rampart is undercut by erosion.
Ice-dammed lake shorelines and glacilacustrine
deposits
One characteristic feature of the slopes of the upper
morphological level, deltas and glacier-facing slope
of Jökulgrindur is the traces of ancient shorelines of
the ice-dammed lake (Figure 3A). The former shore-
lines are marked by small wave-cut platforms of up
to 2 m wide which gently slope in the direction of
the lake. The highest shoreline is situated 37 m above
the bottom of the lower morphological level, i.e. up
to 877 m a.s.l. Differences in the spacing between
the palaeoshorelines (0.5 to 1.5 m) are interpreted as
recording the variation in the rate of lowering of the
former ice-dammed lake.
54 JÖKULL No. 66, 2016