Jökull - 01.01.2005, Blaðsíða 79

A late Holocene jökulhlaup, Markarfljót, Iceland: nature and impacts
Table 2: Electron microprobe analysis of tephra layer 76–2 (see Figure 3 for location of layer). – Greining á
gjóskulagi 76-2 (sjá staðsetningu lags á mynd 3).
76-2 SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O Total
(a) 70.33 0.77 13.67 3.76 0.06 0.52 1.47 4.5 3.34 98.42
(b) 66.51 1.16 14.17 5.64 0.20 1.15 2.95 4.60 2.94 99.31
66.39 1.25 13.98 5.45 0.09 0.97 2.89 4.44 2.67 98.13
66.18 1.24 14.05 5.56 0.19 1.11 2.72 4.06 2.72 97.84
66.12 1.26 14.19 5.89 0.15 1.10 2.76 4.55 3.00 99.02
66.04 1.31 13.79 5.53 0.15 1.06 2.80 4.48 2.87 98.05
66.00 1.20 13.69 5.49 0.19 1.04 2.94 4.61 2.78 97.93
66.00 1.18 14.07 5.75 0.11 1.03 2.76 4.73 2.76 98.39
65.83 1.28 13.86 5.61 0.16 1.09 2.93 4.52 2.71 97.99
65.82 1.29 14.05 5.73 0.18 1.10 2.95 4.33 2.57 98.03
65.65 1.20 13.83 5.43 0.16 1.12 2.83 4.39 2.72 97.33
65.59 1.21 13.98 5.41 0.10 1.13 2.91 4.39 2.78 97.50
65.58 1.24 14.05 5.66 0.16 1.06 2.90 4.61 2.81 98.07
mean 65.98 1.23 13.98 5.60 0.15 1.08 2.86 4.47 2.78 98.13
s.d. 0.30 0.05 0.15 0.15 0.04 0.05 0.08 0.17 0.12 0.56
(c) 47.45 4.50 12.75 15.02 0.18 5.17 10.00 2.24 0.80 98.11
46.49 4.26 12.70 14.47 0.15 5.05 9.57 3.08 0.79 96.56
46.39 4.29 12.45 14.20 0.23 5.02 9.74 3.04 0.74 96.08
46.33 4.36 12.61 14.43 0.23 4.98 9.69 3.08 0.78 96.50
mean 46.66 4.35 12.63 14.53 0.20 5.06 9.75 2.86 0.78 96.82
s.d. 0.53 0.11 0.13 0.35 0.04 0.08 0.18 0.41 0.02 0.89
Soil and tephra layers stratigraphically below
the Drumbabót sand and gravel deposit are exposed
within a small hillock at Drumbabót. A pale tephra
layer (76-2, Figure 3) within this sequence was anal-
ysed using a Cameca Camebax Microbeam elec-
tron microprobe using procedures recommended by
Larsen et al. (1999) and has a geochemical compo-
sition very similar to the silicic Katla tephra SILK
YN with 67% SiO2 (Table 2). This tephra layer
dips below the Drumbabót bedded deposit indicating
that the sand and gravel deposit overlies SILK YN
(weighted mean of 19 dates from GU-7070 to GU-
7096; 1660±12 14C yr BP, 1575–1525 cal BP, Dug-
more et al. 2000).
The deposit which surrounds the Drumbabót trees
is a bedded vesicular, black, tephra-rich sand to gravel
deposit (Figure 3). The dominant black tephra is
basaltic tephra from Katla. Beds rich in gravel (clasts
between 0.8 cm and 5 cm) and lithic sands are in-
terbedded with tephra-rich fine to coarse sands. Bed-
ding is predominantly horizontal with some cross-
bedding in upper layers. Lenses of fine sand and fine
black tephra are found in the base of the deposit. The
thickness of this bedded deposit varies but was once
somewhat deeper, this overlying material having been
eroded in strong winds and by river action (Markús
Runólfsson, pers. comm.). This means that as well as
once being a deeper deposit, the structure and nature
JÖKULL No. 55 79