Jökull - 01.01.2009, Page 49

A new study of paleomagnetic directions in the Miocene lava pile, NW Iceland
reksfjörður and FH, HM on its south coast. A more
tentative correlation is suggested between profiles
TW north of Tálknafjörður and HP south of Patreks-
fjörður. As the distances between TR/TS and FH are
around 12 kilometers, and so the distance from TW
to HP, it is desirable to attempt confirming this by
sampling at intervening locations. Similar uncommon
pole positions are also seen in overlapping parts of
profiles VR and VS which are over 1 kilometer apart.
At the moment, it seems premature to suggest def-
inite correlations between the polarity patterns in the
lava pile of the Arnarfjörður-Breiðafjörður area and
any part of the 4-km thick western composite section
of McDougall et al. (1984) sampled along line 1 in
Figure 1. As emphasized in the Introduction above
and by Kristjánsson et al. (1975, p. 214), the lava pile
may be composed of overlapping lens-shaped units,
which could cause thicknesses of polarity zones to
vary laterally. As the distances between sampled pro-
files in the work of McDougall et al. (1984) mostly
were in the range 5–20 kilometers, correlations be-
tween these will also need checking by mapping and
sampling of additional profiles. The tectonic tilt on
the way north through the central part of the peninsula
is probably increasing and its direction changing to-
wards east, but these changes have not been charted in
detail. Tilt changes at possible unconformities at sed-
imentary formations such as at the top of the profile
AB (Figures 2 and 3) are not known, nor are the time
intervals represented by these. In order to establish
some correlations with the section of McDougall et
al. (1984), it is advisable to carry out another project
comparable to the present one (with more detailed
mapping, and preferably a number of Ar-Ar age deter-
minations) in the eastern part of the area of Figure 1,
also including some profiles north of Arnarfjörður.
As a rough estimate, the present composite sec-
tion of 2.0 kilometers (250–2250 m in Figure 3) may
correspond to the 2.6-km interval from 900 to 3500
m cumulative thickness in Figure 4 of McDougall et
al. (1984). At the lower end, this estimate is mostly
based on the strike direction, while at the upper end
the polarity observations mentioned in the stratigraph-
ical notes above are also taken into account. Omitting
very short reversal events and apparent excursions, re-
spectively 6 and 8 polarity reversals are encountered
in these stratigraphic columns. It would also follow
that the pile of Figure 3 spans a Middle Miocene age
interval from about 13.6 to 12.2 m.y. according to
the averaged K-Ar dates of McDougall et al. (Lower
ages for the present Arnarfjörður-Breiðafjörður col-
umn were indicated by four previous K-Ar determi-
nations quoted by Pálmason and Sæmundsson (1974)
and by Kristjánsson et al. (1975), but not published
in detail). This would in turn imply an average rate
of buildup of 1.4 kilometers per m.y., somewhat less
than in composite profiles 1 and 3 of Figure 1 (Table
2 of Kristjánsson and Jónsson, 2007).
Acknowledgements
This study was supported by grants from the Univer-
sity of Iceland Research Fund. Especially valuable as-
sistance and companionship in the mapping and sam-
pling work in 2004–2006 was provided by Eyjólfur
Magnússon. Other field assistants were Kelly-Marie
Hayes in 2006, Gísli Örn Bragason in 2007, and Ásdís
Benediktsdóttir in 2008. Geirfinnur Jónsson prepared
Figures 2 and 4 and Table 2, Rósa Ólafsdóttir drafted
Figures 1 and 3. John Preston of the Queen’s Univer-
sity of Belfast made various results from his detailed
mapping work in 1970–1973 available to the author.
Ásta Rut Hjartardóttir, Gísli Örn Bragason and oth-
ers supplied information acquired in student mapping
projects. J. A. Karson and M. S. Riishuus provided
very useful reviews of the original manuscript.
Nýjar mælingar á segulstefnum í hraunlagastafl-
anum milli Arnarfjarðar og Breiðafjarðar
Jarðlagaskipan Vestfjarða og þar með að líkindum
elsta gosbergs ofansjávar á Íslandi, er enn ekki vel
þekkt fremur en flestra annarra landshluta frá Míó-
sen og Plíósen tíma. Þó hafa nokkur allstór verk-
efni við kortlagningu á legu jarðlaganna verið unnin
þar. Í því viðamesta voru könnuð tvö löng þverskurð-
arsnið gegnum staflann, ásamt bergsegulmælingum
og kalíum-argon aldursgreiningum (McDougall og
fl., 1984). Fjallað er hér um niðurstöður bergsegul-
mælinga á borkjarnasýnum úr 365 hraunlögum sem
safnað var í margþættum tilgangi úr um 20 sniðum
JÖKULL No. 59 49