Jökull - 01.01.2014, Page 6

Leó Kristjánsson
netic polarity as determined by fluxgate magnetome-
ter measurements in the field. The succession of lava
flows dips towards the southeast, by an amount which
may vary with altitude. At the coast of Ísafjarðardjúp
it averages of the order of 2◦ from the western side
of Bolungarvík to Álftafjörður, where the stratigraphy
becomes complicated due to local buildup of acidic
rocks and other central-volcano products.
A few 40Ar/39Ar dates from Northwest-Iceland
were published by Hardarson et al. (1997). A sam-
ple from 290 m depth in a borehole situated between
the lower and upper parts of McDougall et al.’s (1984)
profile SU in Súgandafjörður yielded reasonably con-
sistent results of 15.7 Ma. However, the material in the
drill hole has suffered severe hydrothermal alteration,
possibly due to an intrusive body close to the hole. Its
significance is therefore doubtful like the one on SU
10 just mentioned. It should be noted that Hardarson
et al.’s profile SU is not identical to profile SU of Mc-
Dougall et al. (1984); it appears to be composed of
parts of McDougall et al.’s SU and SN along with re-
sults from additional field mapping. The correlations
between Skálavík, Súgandafjörður and Dýrafjörður in
Figure 2 of Hardarson et al. (1997) were made on the
basis of fluxgate measurements of lava polarities in
the field. Their normal-polarity zones in flows KE
22–24 in Skálavík, and TF 1 on the south side of
Dýrafjörður were later shown to be caused by inter-
fering factors like viscous remanence (Kristjánsson et
al., 2003). Hardarson et al.’s normal-polarity zone SU
3–5 has not been sampled for laboratory paleomag-
netic measurements. Their age determinations on two
fresh-looking tholeiites from just above and below a
lignite-sediment horizon in profile TF (Figure 1) gave
statistically indistinguishable ages of about 14.8 Ma.
Kristjánsson and Jóhannesson (1996) published a
stratigraphic/paleomagnetic study on several lava pro-
files (Figure 2) south of Ísafjarðardjúp, the western-
most (oldest) one being DO of Figure 3. Their com-
posite section (red line, Figure 1) crossed all the trib-
utary fjords to the southeast from Álftafjörður. No ra-
diometric dates are yet available from these profiles,
but the authors suggested how they might correlate
with the major part of the western composite section
of McDougall et al. (1984).
Research in 2000–2009
Kristjánsson et al. (2003) studied the paleomagnetic
directions in several profiles crossing the oldest lig-
nite sediments in Northwest Iceland (Figures 3 and 4).
Four of the profiles were in the Aðalvík and Fljóta-
vík inlets (Figure 1) north of Ísafjarðardjúp, and in
these a fairly consistent pattern of polarities was ob-
served: the lignite-bearing sediments were near the
base of a reverse-polarity zone, with an excursion to
low latitudes higher up in at least two profiles and an
R to N reversal still higher. See Figure 6 of Krist-
jánsson and Jónsson (2007). A thin normal-polarity
zone, which was originally noted to occur imme-
diately below and/or above the lignite sediments in
some profiles south of Ísafjarðardjúp (Kristjánsson,
1967, 1968) appears to thicken farther south, e.g. in
profile TF (Figure 1), see Figures 3 and 4 of Kristjáns-
son et al. (2003). If the lignite beds are everywhere of
the same age, it is possible that the absence of this
normal zone north of Ísafjarðardjúp is due to slower
buildup of the lava pile there. Alternatively, a vol-
canic hiatus in Aðalvík and Fljótavík could have hap-
pened in a time period different from that south of
Ísafjarðardjúp.
South of Ísafjarðardjúp the polarity pattern below
the lignite sediments is broadly similar in several pro-
files, with evidence of some geomagnetic reversals or
major apparent excursions being seen in each. Profile
KE on the north side of Skálavík (Kristjánsson et al.,
2003) for instance, has an N to R reversal between
flows KE 5 and 7, excursions in flows KE 8–10 and
21, and an R to N reversal between flows 26 and 28.
The lower reversal was also seen south of Skálavík,
between flows SK 10 and 12. It is not seen in profile
GD at Bolungarvik which begins an estimated 30 m
above sea level. A few excursions also occur here in
flows GD 5–6 and GE 3, 5 (the supplementary profile
GE filled inaccessible parts of GD), then a normal-
polarity flow occurs just below the sediments in GD
19. The same reverse zone is present in profile SW
which lies between GD and KE and was sampled up
to a scree-covered segment containing the sediments;
an excursion occurs in flow SW 11. In the two lowest
parts of profile SU, an N to R reversal similarly takes
place between flows 7 and 8. Excursions are found
6 JÖKULL No. 64, 2014