Náttúrufræðingurinn - 1993, Blaðsíða 27

constitute less than one percent by volume.
The tephra is ntoderately sorted and
consists of fine and mediurn ash. On the
basis of texture analyses, it can be con-
cluded that these tephra layers formed in
hydromagmatic eruptions, when hol
magma comes into contact with suriace
water or ice. This conclusion is in
agreement with the locations oi the most
potential sources, i.e. Grfmsvötn or
Kverkfjöll volcanic systems, which are
mostly covered by ice of the Vatnajökull
ice cap.
No direct dating method has been
applied on the tephra layers or the peat
deposits in Sogamýri, but several criteria
can be used to evaluate the age of the
tephra layers. Firstly, the close strati-
graphic proximity of the tephra layers to
the underlying clay indicates that they
must be from early postglacial time. The
ice of the last glaciation (Weichselian)
retreated from the Reykjavík area, approxi-
mately 10000 years ago. Secondly, with
the aid of pollen analyses and radiocarbon
dating of the peats in the Reykjavtk area it
can be concluded that these two tephra
layers are about 9000 years old. Thirdly,
the upper tephra layer in Sogamýri can
possibly be correlated with a tephra layer
found in a Late Weichselian-Early Holo-
cene lake sediments in North Iceland. The
age of this tephra layer is about 8900 years
B.P., according to radiocarbon dating. The
geochemical composition of this layer and
the upper Sogamýri tephra are sufficiently
congruous to support this correlation.
Recently, a basaltic tephra layer has
been identified in the Saksunarvatn lake
sediments in the Faroe Islands (Mangerud
et al. 1986). Radiocarbon dating indicates
an age of 9000-9100 years BP. A com-
parison of textural and geochemical data
reveals that the upper tephra layer in
Sogamýri, rather than the lower one,
closely resembles the Saksunarvatn lephra
layer (Table 1). On the basis of this
comparison, it is reasonable to conclude
that these two tephra layers were probably
formed by the same eruption event in
either the Grímsvötn or Kverkfjöll volcanic
systems about 9000 years ago.
The thickness and areal distribution of
the tephra layers found in Sogamýri is not
known yet, but present data indicates that
il must be substantial if thcir sources are
subglacial in the Vatnajökull ice cap, as
postulated here. Plausible equivalents to
these tephra layers have recently been
identified in several scattered locations in
Iceland (Fig. 3).
137