Jökull - 01.12.2006, Side 42

Kristjánsson et al.
0.5 Ma, which has limited their potential in detailed
work on stratigraphic correlations and on the exact
timing of major events such as lateral movements, or
jumps, of the volcanic zones.
The other technique employed in the stratigraphy
involves measurement of magnetic remanence direc-
tions. It was pioneered in Iceland by J. Hospers, T.
Einarsson and T. Sigurgeirsson in the early 1950’s,
chiefly in Pliocene and Pleistocene formations. It
makes use of temporal variations in the geomagnetic
field, in particular reversals of its dipole moment,
which are permanently recorded in the basalts when
they cool after emplacement. This technique has been
of considerable use in the Tertiary areas, where rever-
sals of polarity occur on average once in every 15-20
lava flows (Kristjansson and Jónsson 2006). Mapping
of the polarity zones may be conveniently carried out
with portable magnetometers in the field, which in the
present project gave reliable and useful results in al-
most all cases. However, it is in general advisable
to follow up such mapping by laboratory studies (es-
pecially in hydrothermally altered sequences), due to
occasional polarity ambiguities arising from the pres-
ence of viscous remanence of secondary origin (see
Kristjansson 1985, Kristjansson and Jonsson 2006).
The frequent occurrence of short-lived reversal
events (subchrons), combined with the apparently
episodic nature of the volcanism, may also cause dif-
ficulties in the use of magnetic polarities in corre-
lation between profiles more than a few kilometers
apart. It is desirable whenever possible to corre-
late directly dated magnetic polarity transitions with
published geomagnetic polarity time scales; however,
several assumptions are involved in the derivation of
these scales and they have been subject to revisions in
the past.
GEOLOGICAL WORK IN
SKAGAFJÖRÐUR-EYJAFJÖRÐUR
The first stratigraphic study in the mountainous
Tröllaskagi peninsula between the fjords Eyjafjörður
and Skagafjörður (Figure 1a), largely carried out
in 1974-1976, was reported by Saemundsson et al.
(1980). Much of the mapping work in that exten-
sive study which also included detailed paleomagnetic
studies and K-Ar dating, was performed by Ágúst
Guðmundsson and Árni Hjartarson in collaboration
with Jóhann Helgason. In the mountains east of
the main valley of Skagafjörður, Saemundsson et al.
(1980) sampled three profiles: PK (Bólugil gully, 20
flows of 300 m thickness in total, including thick sed-
imentary rocks in its lower part), PF (Mt. Sólheima-
fjall, about 50 flows, 800 m), and PG (Bakkadalur
tributary valley of Austurdalur, 68 flows, 650 m).
Along with these profiles, they also published re-
sults of geological mapping and field measurements
of magnetic polarity in five other profiles including
TB (86 flows, almost 800 m) at the Geldingsgil gully
in Norðurárdalur.
One unexpected result obtained by Saemundsson
et al. (1980) concerned the rate of build-up of the lava
pile in Skagafjörður. Although dating results in their
Figure 4 are somewhat scattered and did not include
any samples from the profiles TB and PG, they con-
cluded that some 1.9 km thickness near the top of
their composite section might have been emplaced in
the time interval 9.5–9.0 Ma ago. This rate of build-
up, i.e. 3.8 km/Ma, is several times higher than has
been found in other comparable surveys in the Neo-
gene of Iceland (see Table 2 of Kristjánsson and Jóns-
son 2006). The rapid build-up was considered (by the
field mappers) to be related to the proximity of a cen-
tral volcano located in the valleys at the south end of
the present study area. No further radiometric dating
results have been published from the region mapped
by Saemundsson et al. (1980).
In the mid-1970s, Björnsson (1975) mapped the
bedrock geology and tectonics along the valley bot-
toms of the rivers Héraðsvötn and Austari Jökulsá
in Skagafjörður, and Kaldal and Víkingsson (1978)
made a brief survey of the bedrock in the Skagafjörður
valleys and adjacent highland. In the years 1983–
1989 Á. Guðmundsson mapped the bedrock of the in-
ner Skagafjörður valleys. This work has been partly
published in Harðarson and Guðmundsson (1986).
Jóhannesson (1991) described various aspects of
the geological structure of Tröllaskagi, including the
southwards continuation of its mountain range be-
tween Eyjafjörður and Skagafjörður. He suggested
40 JÖKULL No. 56