Jökull - 01.01.2016, Page 92

Leó Kristjánsson
sis on cases with clustering of low- to mid-latitude
VGPs in successive flows (e.g. Figure 1) and on cases
where the VGP moves about the globe in an irreg-
ular fashion. An unusual example of the latter kind
was originally described by Kristjánsson and Jóhann-
esson (1989, 1996) from lava profiles sampled in Ísa-
fjarðardjúp, Northwest Iceland. Kristjánsson (2015)
confirmed the persistence of this geomagnetic excur-
sion in the lava pile by detailed sampling at 51 sites
in four new profiles, and thorough demagnetization
of secondary remanence components. He estimated
that this Kleifakot instability event is around 13 Ma
age, if contemporaneous with the K-Ar dated excur-
sion in profile JD of Figure 2a. Covering the emplace-
ment of some 25 successive lava flows of 220 m total
thickness, the event may have lasted for perhaps 100
thousand years judging from overall rates of buildup
of the lava pile in the peninsula (McDougall et al.,
1984; Kristjánsson, 2015, p. 322–323). This paper
adds 20 lava sites from five locations to the excur-
sion sites already described, extending significantly
the area in Ísafjörður where the event is recorded. In
the present study some samples had to be rejected due
to discordant directional results, but excellent within-
site agreement was eventually obtained.
Some previous instances of the use of intermedi-
ate remanence directions to support stratigraphic cor-
relations in Icelandic lavas were quoted above. The
results in Table 2 used in Figure 4 further demonstrate
the value of this method and confirm the good qual-
ity of direction results from our lavas. The angular
differences between the 39 correlated site pairs rarely
exceed 8◦, averaging less than 5◦. Comparable agree-
ment occurred in the paired sites discussed by Krist-
jánsson and Guðmundsson (2001, p. 40) as well as
in profile JD (Table 2 of Kristjánsson, 2015), where
fewer samples were generally collected. The angu-
lar differences include effects of all random and sys-
tematic errors in orientation and measurement of the
samples. The measured remanence directions can also
have been influenced by unavoidable noise sources
such as local magnetic anomalies at the time of ini-
tial cooling, slight movement of outcrops by recent
erosional processes, and undetected lateral variations
in the tectonic tilt vector. In some cases, one may
be comparing two units emplaced at slightly different
times.
Figure 6. A typical appearance of the contact be-
tween a lava flow and an undisturbed clastic sediment,
in the fjords of Ísafjarðardjúp. Total height about
1.2 m. – Dæmi um hvernig hraunlag leggst á set í
staflanum sunnan Djúps. Ekkert gjall er þar á milli.
Ljósm./Photo. L.Kr. 2015.
The rate of buildup of the lava pile seems to vary
somewhat within the area of the sampled profiles. For
this reason it is not often possible to follow individ-
ual lava flows for more than a couple of kilometers
laterally, in particular where exposures are incom-
plete. Thus, units DF 2 and 3 are the only ones of
the excursion flows DF 1–12 whose remanence direc-
tions find definite counterparts in profiles DT, DU and
DW along the fjord (Table 2). Even so, the Kleifakot
event as a whole promises to be a useful stratigraphic
marker in the coastal areas of inner Ísafjarðardjúp
where little geological work has yet been carried out,
and also farther away. A transitional series of a few
flows was correlated similarly by Kristjánsson et al.
(2004, p. 590) across some 5 kilometers between their
profiles GL and AF in central North Iceland, with only
a partial match in individual directions.
The presence of paleomagnetic directions which
can be correlated with ease and some certainty over
distances of kilometers, offers further opportunities
for research. For instance, spatial variations in the
chemical composition of extensive lava flows can be
studied, and also their provenance. The mode of em-
placement of the flows in Northwest Iceland, which
has hardly been studied at all, may be quite different
92 JÖKULL No. 66, 2016