Jökull - 01.01.2005, Side 81

A late Holocene jökulhlaup, Markarfljót, Iceland: nature and impacts
Table 3: Sedimentary descriptions (see Figure 1 for locations of sites). – Lýsingar á setlögum og setgerðum (sjá
staðsetningar á mynd 1).
SEDIMENT
SITES
(GPS location)
SITE
MORPHO-
LOGY
DESCRIPTION LITHOFACIES
CODES
STRATIGRAPHY
1: DRUMBA-
BÓT
63◦N 42’ 43”;
019◦W 06’ 57”
Braided sandur Bedded coarse black tephra includ-
ing lithics from fine sand - 0.8
cm clasts, olive brown silt at base,
lenses of fine sand, fine black tephra
at base. Deposit surrounds mature
well-preserved macrofossil trees.
Sh/Grh Below the ’flood’
unit: SILK YN1,
exact stratigraphic
relationship un-
clear. Trees have
radiocarbon date
1230±35BP
2: North Aurasel
63◦N 42’ 46”;
020◦W 04’ 41”
Braided sandur.
Riverbank
exposure
Horizontally bedded granules over-
laying structured fine sands (coarse
black tephra and coarse sand lithics)
with cross bedding (foresets), ripple
laminations and silt drapes. Deltaic
structures.
Upper: Grh
Lower: Ft (r)
Above the ’flood’
unit: V870 and 4+
black tephra layers.
Some may be re-
worked.
3: Einhyrnings-
flatir
63◦N 43’ 46”;
019◦W 28’ 27”
Rofabard in
soil terraces
above post-
glacial lavas
Approximately 2m. Bedded. Occa-
sional layers gravel-rich with clasts
up to 0.8–1 cm. Mainly fine to
coarse black tephra and lithic sand.
Some silt drapes.
Sh/Gh Above the ’flood’
unit: V870 and 2
black tephras
4: NORTH
ÞÓRSMÖRK
63◦N 43’ 44”;
019◦W 27’ 09”
Exposure on
the lava surface
just above
gorge top (305
m)
68 cm thick, 2 units: Upper 14
cm. Bedded coarse black tephra
with gravel up to 1 cm. Lower: 34
cm. Crudely bedded coarse black
tephra.
Upper: Gh
Lower: Sh
Above the ’flood’
unit: V870 Below
the ’flood’ unit: 5
black tephras and
Layer H.
file also represents an increase in flow strength with
time. These deposits are found beneath the Landnám
tephra (871±42AD, Grönvold et al. 1995) and at least
four black tephras.
Fluvial-type deposits (bedded sands, granules,
gravels and mud-flow type matrix supported diamicts)
have been identified in similar stratigraphic locations
in the middle of the valley at Einhyrningsflatir (Fig-
ure 3, Table 3) and at the top of the Markarfljót gorge
north of Þórsmörk. These deposits are all found be-
neath the Landnám tephra and/or above Layer H, a
prehistoric silicic tephra from Eyjafjallajökull dated
to 1540±50 14C yr BP (Smith 2004, GU-10383, cali-
brates to 1540-1330 cal BP), although the exact num-
ber of black tephras separating them from these key
horizons varies.
Upstream deposits at Einhyrningsflatir are ap-
proximately 2 metres deep, horizontally bedded black
tephra-rich sands with occasional gravel-rich beds
with clasts up to 1 cm. This unit is overlain by two
black tephra layers and the Landnám tephra layer. A
profile just above the top of the Markarfljót gorge,
north of Þórsmörk, shows a crudely bedded black
tephra-rich deposit with sand-sized lithic grains in the
upper layers, just beneath the Landnám tephra and
overlyingfive black airfall tephras and Layer H. There
is no evidence of a Katla flood at this time in soil sec-
tions in the upper slopes of hills north of Þórsmörk or
along Langanes.
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