Jökull - 01.01.2005, Page 58

Bergrún Arna Óladóttir et al.
ditional tephra layers have been assigned an approx-
imate radiocarbon age (Table 1a, 1b; Larsen et al.,
2001). A total of four silicic Hekla layers have been
identified in the prehistoric part of the studied sec-
tion: HM, HN, HS and H4. They are all characterised
by light-coloured lower parts and bluish-black upper
parts (Figure 2). The last two tephra layers have been
dated, having ages of ∼3860 and ∼4260 years, re-
spectively (14C-age of 3515±55 and 3826±12 years
BP, Table 1a; Larsen et al., 2001; Dugmore et al.,
1995b). The HM and HN tephra layers have not been
dated but their stratigraphic position indicates an age
of∼3000 years. The soil thickness between them and
the SAR derived in this study indicates that these two
layers were formed by eruptions separated by ∼50
years.
Figure 2. An example of a marker tephra layer: H4
(3826±12 years BP; Dugmore et al., 1995b) from
the Hekla volcano, S-Iceland. – Dæmi um leiðar-
lag, Heklugjóskulagið H4 (3826±12 geislakolsára,
Andrew Dugmore og fl. 1995b) í jarðvegssniði í Atley.
Two of the felsic marker layers, labelled A6 and
A13 by Larsen et al., (2001), are distinguished by
white pumice grains in a matrix of olive-grey ash. The
latter has an age of∼8380 years (14C age of 7505±42
years BP, Larsen et al., 2001), whereas the former has
an estimated eruption age of ∼7100 years (calculated
from SAR).
Other layers that are readily recognised include
the dacitic SILK layers from the Katla volcano. These
layers are characterised by their olive-green colour
and needle-like pumice grains that resemble chips
of wood, making these layers unique among the
Holocene felsic tephra layers in Iceland. Ten SILK
layers have been identified in the study area and three
of those (SILK-UN, -MN, -LN) have been dated, hav-
ing ages of ∼2855, ∼3235 and ∼3445 years, respec-
tively, (14C age of 2660±50, 2975±12 and 3139±40
BP, Table 1a; Larsen et al., 2001).
Basaltic tephra layers from the Katla volcano are
readily distinguished from other basaltic tephra layers
by their brownish-black to coal-black colour and com-
ponents. They consist of poorly- to highly-vesicular
ash- to lapilli-size glass grains (Table 2a) charac-
terised by curvi-planar fracture surfaces. Clasts with
fluidal surfaces (i.e. achneliths as defined by Walker
and Croasdale, 1972) are present in most tephra lay-
ers, but in minor amounts. The glass grains contain
1-5 modal% phenocrysts and groundmass crystals
of plagioclase, olivine, clinopyroxene and rare mag-
netite. Crystalline mafic and felsic wall-rock lithics,
as well as hyaloclastite, and crystal fragments of pla-
gioclase and olivine, are present in very trace amounts
(< 1%).
Construction of the composite soil section
The composite tephra profile measured and sampled
in this study contains the prehistoric part of the tephra
succession as preserved in soil profiles east of the
Katla volcano (Figures 1 and 3). The historical part
of the succession, with at least 18 tephra layers from
various sources, was previously sampled and analysed
by Larsen (1981, 2000). Twenty-one historical tephra
layers produced by the Katla volcanic system are
known from soil sections around the volcano (∼870
AD to 1918 AD). Ten historical Katla layers and the
Eldgjá tephra layer have been identified east of the
Katla volcano (Larsen, 1981, 2000). For this study,
nine historical Katla layers were analysed from three
separate soil sections: at Hrífunes, about 8 km south-
east of Atley; at Tjaldgilsháls, about 20 km northwest
of Atley; and at Ytri-Dalbær, about 30 km east-north-
east of Atley (Figure 1).
The prehistoric tephra section is a composite one,
stitched together from three separate sections mea-
sured at closely-spaced locations. The three locations
are Atley (AT and HA) and Rjúpnafell (RF) (Fig-
ures 3a-3d). This composite section has an accumu-
58 JÖKULL No. 55