Jökull - 01.01.2005, Síða 44

Árni Hjartarson
tephra record based on data from ODP sites 907, 985,
919, 983, 984 and from the sites SU9029 and SU9032
(Figure 4). About 90% of all the identified tephra
layers were recognized in ODP sites 907 and 985, N
and NE off the Iceland coast. The lowest part of their
record is entirely based on these two sites. It is highly
likely that the Skati tephra will be found there. The
main attributes of this ash layer should be the follow-
ing:
• Discrete layer
• Age around 5.2 Ma
• Colourless glass shards
• Silica content around 75% SiO2 (on a water-
free basis)
• Reverse polarity (in the centre or lower part of
a polarity zone)
Four tephra layers from ODP-core 907A fulfil
these requirements (indicated as Z, AA, AB and AC
in Table 2 of Lacasse and Garbe-Schönberg 2001).
Chemical analysis indicates that one of them is alka-
line and originates in an off-rift volcanic zone in Ice-
land. It can therefore be eliminated. Two others are
near to the upper boundary of the Thverá subchron,
which makes them hard to correlate with the Skati
tephra. The only layer left is the oldest one in the
composite core, indicated as AC. If the Skati tephra is
to be found in the data collection at all, this is the best
candidate. Lacasse and Garbe-Schönberg (2001) give
it special attention, saying that its dispersal, thickness
and grain size clearly indicate that it was derived from
one of the largest explosive eruptions that ever oc-
curred in the Neogene rift zones of Iceland.
Comparing the chemistry (Table 3) it must be kept
in mind that the ODP-samples are made of a few glass
fragments from a tephra layer that were analysed us-
ing a microprobe, but the Skagafjörður samples are of
lava that was analysed by a conventional XRF tech-
nique. Nevertheless, comparison reveals similar com-
position in most of the major elements as well as in
the trace elements and does not indicate any differ-
ence that cannot be explained by the different analyt-
ical methods and sedimentary environment.
Lacasse et al. (1996) have described layer AC. It is
bimodal and the top differs from the bottom. The bot-
tom is crystal-poor tephra of 100% colourless glass
shards and can be considered as exclusively silicic.
The top contains 9% feldspar, 3% clinopyroxene and
up to 2% olivine. This sorting might have taken place
as the ash particles settled through the 1,800 m deep
water column.
Normal grading is also observed between the bot-
tom and top of the layer. It can also be interpreted
as the result of extensive size fractionation in the wa-
ter column. This difference has not been recognized
inside in the Skati tephra in the Skagafjörður Valleys.
ODP-site 907 is in the Arctic Ocean NNE of Ice-
land, 550 km away from the Tinná Volcano. This dis-
tance has not changed much since the eruption. The
holes (A, B and C) are 1,800m below sea level and the
tephra layer is at a depth of 85 m in the sediments. The
age, magnetostratigraphic alignment within subchron
C3r, its thickness (18 cm) and chemical composition,
all favour the distant correlation between the deep-sea
ash layer and the Skati tephra.
Volcanic history
The development of the Tinná Central Volcano
and its eruptive history can be divided into three
main phases, all experiencing rhyolitic volcanism at
or shortly after its beginning (Figure 7).
Phase 1: The initial stage took place ca. 6 Ma. An
acidic eruption started with the extrusion of the Ágúll
rhyolite dome. This was at least 250 m high and
0.6 km3 in volume. The stratigraphy of the Ábær
gorge indicates nearby eruptive vents. The volcan-
ism began with an explosive plinian phase and the ac-
cumulation of a thick and coarse tephra layer. Dur-
ing the plinian phase an acid lava was extruded. This
was very viscous and piled up around the conduit and
flowed slowly out over the light coloured tephra car-
pet, forming a layer of basal breccia and a massive
rhyolite dome (or a ridge). The acidic eruption was
followed by intense basaltic volcanism producing the
Ábær tholeiites that finally covered the dome. The
phase ended in a long period of quiescence during
which the Tinná lignite sediment was formed. The
lignite seams contain trunks and branches, indicating
44 JÖKULL No. 55