Jökull - 01.01.2014, Blaðsíða 12

Leó Kristjánsson
both composite sections. Use is made of the paleo-
magnetic study of Kristjánsson et al. (2003) around
the oldest lignite-bearing sediments of the peninsula,
augmented by new paleomagnetic work on some 60
lava flows (Figure 5, Table 1). Results from Guð-
mundsson’s (1989, 2007) mapping of lava sequences
in the Álftafjörður-Önundarfjörður-Bolungarvík area
have also been a helpful reference on some aspects
of the local stratigraphy. Further geological and pa-
leomagnetic studies would have been desirable in the
project reported here, but local conditions for this are
rather difficult especially in steep outcrops of thin
central-volcano tholeiite lava series such as occur in
Skutulsfjörður, Álftafjörður, and Önundarfjörður (cf.
profile AJ of Figure 2 in McDougall et al. (1984)).
Several geomagnetic excursions have been
recorded in the present study, and are useful in short-
distance correlations as in previous paleomagnetic
surveys in the older parts of the peninsula. Their pres-
ence is related to the fact that the general scatter of
paleomagnetic poles around the geographic poles was
greater at >12 Ma than in younger rocks, cf. Figure
9 of Kristjánsson (2013). An extended episode of
geomagnetic instability was recorded in over 15 suc-
cessive lavas of about 13 Ma age at the eastern end
of Ísafjarðardjúp by Kristjánsson and Jóhannesson
(1996). The present author (Kristjánsson, 2015, and
references therein) who reports new observations on
this feature, has not found in rocks of the Northwest
peninsula any evidence for other episodes compara-
ble in duration to that one or to the complex 16.7 Ma
transition at Steens Mountain in Oregon.
Statistical considerations predict that a very slow
rate of eruptions relative to the rate of geomagnetic
reversals would result in an average of 2 lavas per po-
larity zone in the pile. The average number of lavas
per polarity zone in the Pliocene and Miocene regions
of Iceland is 15–20 (Kristjánsson and Jónsson, 2007),
representing a time interval of order 0.2 M.y. and a
buildup rate of order 1 km/M.y. Only 0 to 3 rever-
sals are observed to occur in the 10–30 flows between
sea level and the lignite sediments, in the profiles of
Figure 4 as well as to the north and south, cf. Fig-
ures 2 and 4 of Kristjánsson et al. (2003). The very
slow (<0.3 km/M.y.) rate of emplacement of lavas be-
low these sediments suggested by Pringle et al. (1999)
therefore does not seem realistic. Hopefully, this
question will be resolved by new 40Ar/39Ar age de-
terminations from the Northwest peninsula (Riishuus
et al., 2013).
The lithological characteristics of the lava flows
below and above the lignite sediments in the area of
Figure 3 are fairly similar, as regards for instance the
occurrence of flows with large feldspar phenocrysts.
This would tend to agree with a small (∼0.2 M.y.)
rather than a large (>2 M.y.) age difference between
them, especially if lateral movements or other major
changes in the mode of volcanism are supposed to
have taken place within the latter time gap. It has not
yet been established whether for instance removal of
a significant thickness of lavas by erosion occurred
before the deposition of the sediments. Hydrother-
mal effects around their level in Skálavík are quite
minor, but microscope inspection (M.S. Riishuus,
pers. comm. 2013) of thin sections indicated some-
what greater alteration of the glassy matrix to clay in
two lavas below it (SY 8, KE 23) than in two lavas
above it (SY 18, KE 31). Judging from a preliminary
analysis of the magnetic behavior of samples in SY
and in other profiles around Ísafjarðardjúp (Kristjáns-
son et al., 2003, section 3.7; Kristjánsson, 2013, p.
547), no major systematic reduction in the overall in-
tensity and stability of remanence occurs below the
sediments; these properties are adversely affected by
even a moderate rise in the maximum temperature en-
countered by lavas during burial (Watkins and Walker,
1977, Figure 13; Kristjánsson, 2015). The amount of
angular unconformity between the sequences above
and below the lignite-bearing sediments is uncertain;
published estimates of tectonic tilts in that area are
somewhat incongruous and await improvement with
the aid of modern techniques. As an example, it has
been stated by Hardarson et al. (1997) and some sub-
sequent writers that the tectonic tilt below the oldest
sediments in the peninsula is about 5◦ towards north-
west or west. This is evidently not the case in our
Figure 6; available photographs from the coastline
mountainsides in fact reveal no westerly dips between
the south side of Dýrafjörður and the north side of
Fljótavík.
12 JÖKULL No. 64, 2014