Jökull - 01.01.2016, Blaðsíða 86

Leó Kristjánsson
demagnetization of 5 or 6 samples from each flow, up
to 30 mT peak field. The 95% confidence angles α95
for mean directions are less than 10◦, except for those
in the three youngest units which are 12–15◦ due to
poor magnetic stability in some samples. A 9-m layer
of glacially deposited clastic sediments occurs imme-
diately above the sequence of Figure 1, but otherwise
there is little or no sediment between the flows.
Several cases of the occurrence of widely scat-
tered VGPs during apparent transitions or excursions
have been noted in lava collections from Iceland.
They generally include less than 10 successive flows.
Some are listed in the paper by Kristjánsson (2015, p.
311–312), who agrees with the conclusion of Harri-
son (1980) that randomized behavior of the field di-
rection occurs when the main dipole source is of di-
minished strength. At these times, the field config-
uration is mainly due to multipole sources. Due to
their complex geometry, observed paleomagnetic di-
rections will vary more rapidly and irregularly than
they do when a near-axial dipole dominates. Fields
during transitions and major excursions may also be
expected to be relatively weak, as is well established
in Iceland (Kristjánsson, 2013, p. 553-555).
THE KLEIFAKOT EXCURSION
Sampling in Ísafjarðardjúp and in profile JD, to
2012
Two unusually thick zones involving more than 10
flows with irregularly varying VGPs were described
respectively by McDougall et al. (1984) and by Krist-
jánsson and Jóhannesson (1996). Both of these zones
are in the Northwest peninsula, and are flanked by
normal-polarity sequences above and below. The for-
mer zone occurs in flows number 16–26 spanning
about 120 m thickness in profile JD, one of some
forty profiles sampled in 1975–1978 (dots in Figure
2a) to make composite sections along A-B and D-
E. The latter zone some 30 kilometers to the north-
east occupies segments of three profiles in the two in-
nermost tributary fjords of Ísafjarðardjúp. They are
part of a composite section of twelve profiles sampled
in 1982-1985 (circles in Figure 2a). These segments
were flows DD 21-23 and DE 2-16 on the southeast
side of a valley at the head of Mjóifjörður, and DF 1-
12 similarly situated in Ísafjörður. Their approximate
locations are shown by small capitals in Figure 2b.
According to Kristjánsson and Jóhannesson
(1989, 1996), flows DD 21–23 are the same as the
flows DE 2,4 and 5 respectively (c. 500 m away),
based on very similar remanence directions and lithol-
ogy. The flows DE 9 and 14 also have the same direc-
tions as DF 2 and 8 sampled more than 4 kilometers
downdip, in agreement with prior conclusions from
geological observations. Allowing for the overlap, the
excursion event covers at least 15 successive sampled
flows (i.e. DD 21–23, DE 7–13, DF 8–12), plus pos-
sibly some unexposed units. Their total thickness is
150 m including minor sediments.
It seems possible that the two excursion events
recorded in the projects of McDougall et al. (1984)
and Kristjánsson and Jóhannesson (1996) are of sim-
ilar or even identical age, estimated by Kristjánsson
(2015) to be about 13 Ma. The estimate was based on
K-Ar determinations by McDougall et al. (1984) in
JD and nearby profiles. Kristjánsson (2015) wished
to investigate whether the excursion in Ísafjarðardjúp
which he called the Kleifakot event, could be stud-
ied in additional localities. It should be noted that
while the actual tectonic dip is locally about 4◦ in a
direction 105◦ east, apparent dips to the northeast are
only of the order of 1◦ as the fjords run close to the
strike (Figures 2b, 3). It should also be noted that the
landscape relief between Mjóifjörður and Ísafjörður
and east of Ísafjörður drops to below 300 m and be-
comes more rounded than in most profiles previously
sampled for paleomagnetic studies in the peninsula.
Exposures in lava flows here are therefore somewhat
intermittent, often laterally discontinuous and only 1-
6 m in height at altitude intervals of 10-20 m on the
soil-covered hillsides.
Kristjánsson (2015) selected four locations for
new profiles, where 51 sites were sampled in 2012–
2014. Profile DX is situated in the valley of Mjói-
fjörður near the lower part of DE (which was under
snow cover at the time), and DV mostly follows the
northeast edge of a gully at Hestakleif on the north-
west side of innermost Ísafjörður between DE and
DF. Two profiles DT and DU are close together about
86 JÖKULL No. 66, 2016