Jökull - 01.01.2014, Page 7

Paleomagnetic studies, Northwest Iceland
in SU 18 and 29, as well as in 33 (resampled in 2011,
Table 1). Furthermore, Kristjánsson et al. (2003) sam-
pled a profile GO on the north coast of Súgandafjörð-
ur. An N to R reversal occurs near the base of that pro-
file, at a gap in exposures between flows GO 2 and 3.
Excursions of the geomagnetic pole to mid-latitudes
were noted in flows GO 5 and 8; the authors assumed
in their Figure 3 that the level of the lignite sediments
lay between GO 22 and 23 but according to the au-
thor’s field notebooks it is more likely that the excur-
sion flow GO 23 is below the sediments (Figure 4).
Similarities between all the above profiles also include
the frequent occurrence of (usually single) feldspar-
porphyritic flows not far below and above the lignite
sediments, sometimes with cumulate textures.
Paleomagnetic measurements on the lignite-
bearing sediments themselves were made by
Kristjánsson et al. (2003) in three localities south
of Ísafjarðardjúp: in profile SN of McDougall et al.
(1984), in profile TF of Hardarson et al. (1997) and
of Kristjánsson et al. (2003), and in exposures at road
level east of Bolungarvík (cf. Roaldset, 1983). They
reveal that in each of these cases the polarity of the
sediments is uniform, indicating that their period of
emplacement was short.
Guðmundsson (2007) continued his mapping of
stratigraphy between Bolungarvík and Skutulsfjörður
in the field and in drill cores, again in preparation for
the construction of a road tunnel in the area. This ex-
tensive study has so far only been described in inter-
nal reports, and will not be dealt with in detail here.
A brief resumé of some of its findings was presented
by Kristjánsson and Guðmundsson (2013), and they
have aided in the construction of the lowest part of
the Ísafjarðardjúp polarity column of Figure 2.
NEW MAPPING AND
PALEOMAGNETIC WORK IN 2010–2012
Methods
New paleomagnetic sampling in Skálavík and Eyrar-
fjall was undertaken in 2010–2012. The sampling and
magnetic measurements was carried out in the same
fashion as in other recent surveys in the peninsula.
At least four 25-mm diameter core samples were col-
lected from each lava flow with a portable drill. These
were generally distributed over a few to several me-
ters laterally but within one meter vertically. Orien-
tation made use of sightings on the Sun or on objects
whose position was known from maps or GPS mea-
surements. The sightings as well as measurements of
core inclinations were made with a Brunton compass,
with an estimated accuracy of the order of 2◦. One
21–22 mm long specimen was cut from each sample.
Remanent magnetization vectors were measured
with an Institut Dr. Förster four-probe fluxgate mag-
netometer, before and after stepwise alternating-field
(AF) demagnetization at 10, 15, 20, 25 and 30 mT
peak fields in a Molspin two-axis tumbler device. Vis-
cous magnetization was generally minor, the direc-
tional change after the first AF step being 20◦ or less
in at least 80% of the samples. In successive steps,
directional changes were most often only a few de-
grees, showing that a stable primary remanence com-
ponent had been isolated. Typical curves of the de-
cay of intensity with increasing field amplitude were
similar to those in Figure 5 of Kristjánsson et al.
(2003) and Figure 5 of Kristjánsson (2015). Tradi-
tional statistical procedures (see Kristjánsson et al.,
2003) were applied in selecting the optimal estimate
of the mean primary remanence direction for each
flow. The between-sample agreement was quite sat-
isfactory, with 95% confidence limits on the mean
direction being 8◦ or less in most cases. The posi-
tive outcome of this crucial test of consistency makes
rock-magnetic studies unnecessary for the purpose of
the present project. In particular, the varied shapes of
thermomagnetic curves obtained from samples of rel-
atively unaltered Icelandic lavas do not appear to have
much relation to the stability or reliability of their pri-
mary remanence directions (Kristjánsson et al., 2003;
Kristjánsson, 2015).
The demagnetization results indicate that in the
area careful field measurements of magnetic polar-
ity in hand samples with a fluxgate magnetometer
(Kristjánsson, 1985, p. 129) can generally be trusted,
at least above the analcime zeolite zone which reaches
to about 100 m altitude according to Figure 3 of Mc-
Dougall et al. (1984). No tectonic tilt correction was
applied to the remanence-direction data from profile
JÖKULL No. 64, 2014 7