Jökull - 01.01.2009, Blaðsíða 35

A new study of paleomagnetic directions in the Miocene lava pile, NW Iceland
tensive contemporaneous horizons (Jóhannesson and
Sæmundsson, 1998). Eroded remains of at least seven
central volcano complexes are known in the peninsula
(Figure 1). Results from mapping of one of these, at
Króksfjörður on the south coast, have been published
(Hald et al., 1971).
One of the techniques employed for correlation
between mountainside profiles (up to 10 km or more
apart) is the measurement of remanent magnetization
vectors in the lava flows. Their directions faithfully
reproduce those of the geomagnetic field at the time of
emplacement. From the remanence direction recorded
in each flow, it may be estimated (with simplifying as-
sumptions) where the geomagnetic pole was located
on the globe at that time. Such an estimated location is
called a “virtual geomagnetic pole” (VGP). The VGP
is most of the time wobbling slowly around either of
the Earth’s geographic poles, but it moves from one
pole to the other at irregular intervals. On average,
such polarity reversals of the geomagnetic field occur
at least eight times per m.y. The VGP is also known
to have undertaken occasional large excursions to low
latitudes. Both the reversals and excursions have in
past decades supplemented the geological methods
available for the stratigraphic correlation studies in
Iceland. The mean thickness of a single-polarity zone
in the lava pile between ages of 2 and 15 m.y. is of the
order of 15–20 lava flows (Kristjánsson and Jónsson,
2007).
The major field effort in stratigraphic/paleomagn-
etic mapping in the Northwest peninsula took place
in 1975–1978 (McDougall et al., 1984) on composite
sections through both the western and eastern parts of
the peninsula (1 and 2 in inset of Figure 1), as well
as in two profiles on its south coast (not shown). De-
tailed laboratory measurements of paleomagnetic di-
rections and intensities were carried out on 1261 lava
flows, and radiometric age determinations were made
on over 70 flows.
Similar stratigraphic mapping and paleomagnetic
work was undertaken in 1982–1985 on a shorter com-
posite section in the Northwest peninsula (Kristjáns-
son and Jóhannesson, 1996, 3 in the inset of Figure
1). 307 lavas were sampled in the fjords south of
Ísafjarðardjúp, but an opportunity for radiometric dat-
ing was not available. Kristjánsson and Jóhannesson
(1996, Figure 8; Kristjánsson and Jónsson 2007, Fig-
ure 5) suggested a tentative correlation between their
composite section and part of the western one of Mc-
Dougall et al. (1984), based on paleomagnetic polari-
ties and other evidence.
One more stratigraphic-paleomagnetic survey in
the northwestern part of the peninsula was carried
out in 1998–2001 (Kristjánsson et al., 2003), greatly
extending an early pilot study (Kristjánsson, 1967).
Many profiles were sampled at intervals of several
kilometers to tens of kilometers, mostly along strike
(4 in inset of Figure 1) around the level of the oldest
thick lignite-bearing sediment layer in the peninsula.
One purpose of that survey was to find out whether
the pattern of paleomagnetic polarity zones was the
same at these profiles. The sediment layer is often
missing in places where it might be expected, and
consistent patterns in lava lithology and magnetic di-
rections would help in locating its stratigraphic posi-
tion in those cases. Kristjánsson et al. (2003) found
a fairly good consistency in these properties in four
profiles north of Ísafjarðardjúp (see also Figure 6 of
Kristjánsson and Jónsson, 2007), whereas to the south
the correspondence between profiles was less clear.
One possible reason for this is that the lava pile was
built up by episodic and alternating extrusive activity
at a number of sources (dike swarms) spaced in such
a way that overlap between their products is limited.
It should be noted that these mapping and sam-
pling projects only furnish geological information on
narrow sections through the lava pile. It would be de-
sirable to carry out in Northwest Iceland much more
detailed mapping like that of G.P.L. Walker and his as-
sociates in parts of East Iceland in 1955–1965. Their
work covered areal exposures (e.g. Walker, 1959)
along with studies of hydrothermal alteration, intru-
sions, and tectonics. Furthermore, extensive Ar-Ar
radiometric dating combined with geochemical inves-
tigations will be needed for the resolution of strati-
graphical questions. A desirable goal of such strati-
graphic mapping, which however will hardly be re-
alized in the near future, includes obtaining a three-
dimensional picture of all the above aspects of the
basalt formations at least down to sea level.
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