Jökull - 01.12.1994, Síða 16

for correlation with Eastem Iceland is that this period
of unstable directions is recorded in lavas at the top of
the Dalatangi profile DB of Kristjánsson et al. (1995).
SUMMARY AND CONCLUSIONS
Paleomagnetic directions in twelve lava sections
totalling 3.9 km in thickness were measured along a
35 km stretch (Fig. 2) on the southem coast of Isa-
fjarðardjúp in the Vestfirðir peninsula. A composite
section of 2.6 km thickness was established for the
area. The lava flows which make up 96% of the suc-
cession are entirely basaltic in composition. The pri-
mary paleomagnetic signal in most of the lavas is
very stable and the within-lava agreement of direc-
tions is excellent. This is probably related to the fact
that the profiles are situated away from major vol-
canic centers and geothermal alteration has at most
reached only the chabazite zeolite stage.
The primary geomagnetic polarity which reversed
at least ten times during the build-up of this part of the
lava pile has been used for correlations between pro-
files. The polarity zones and other stratigraphical evi-
dence also allow correlation with profiles SB to JF in
the composite section of McDougall et al. (1984)
where detailed K-Ar dating is available. The age
range of the Isafjarðardjúp composite section can thus
be estimated as being 13.7 to 12.0 Ma. We conclude
that sediments in our section DL correspond to the
Brjánslækur sediments. Instead of overlap of western
and eastem composite sections of McDougall et al.
(1984, see their Fig. 8) on the peninsula, there is a
stratigraphic gap of over 1 km. This is a consequence
of strike directions around the eastern end of Isa-
fjarðardjúp being more northerly than assumed by
McDougall et al. (1984).
The mean geomagnetic field direction during the
emplacement of the Isafjarðardjúp section was very
close to that of a central axial dipole field. The secular
variation of paleomagnetic virtual poles around their
mean is unusually large for Icelandic lava sections,
due in part to a zone of some 15 lavas with erratically
varying directions in profiles DD, DE and DF
(Kristjánsson and Jóhannesson 1989).
ACKNOWLEDGEMENTS
This work was supported in part by the Icelandic
Science Fund (Vísindasjóður) and the Fjórðungssam-
band Vestfirðinga. Gylfi Sigurðsson, Jónas Valdi-
marsson, Kjartan Ólafsson and Kristján Leósson as-
sisted in paleomagnetic sampling and measurements.
REFERENCES
Cande S.C. and D.V. Kent 1995. Revised calibration of
the geomagnetic polarity time scale for the Late
Cretaceous and Cenozoic. Journal of Geophysical
Research 100, 6093-6095.
Coe R.S., M. Prévot and P. Camps 1995. New evidence
for extraordinarily rapid change of the geomagnet-
ic field during reversals. Nature 374, 687-692.
Friedrich W. 1966. Zur Geologie von Brjánslaekur
(NW-Island) unter besonderer Berucksichtigung
der fossilen Flora. Geologisches Institut, Univer-
sitát Köln. Sonderveröffentlichungen 10, 108 pp.
Guðmundsson, Á. 1984. Tectonic aspects of dykes in
Northwest Iceland. Jökull 34, 81-96.
Hald N., A. Noe-Nygaard and A.K. Pedersen 1971.
The Króksfjörður central volcano in north-west
Iceland. Acta Naturalia Islandica 10, 29 pp.
Jóhannesson H. 1980. Jarðlagaskipan og þróun rek-
belta á Vesturlandi (in Icelandic). Náttúrufræðing-
urinn 50, 13-21.
Jónsson G., L. Kristjánsson and M. Sverrisson 1991.
Magnetic surveys of Iceland. Tectonophysics 189,
229-247.
Kristjánsson L. 1968. The paleomagnetism and geolo-
gy of North-western Iceland. Earth and Planetary
Science Letters 4, 448-450.
Kristjánsson L. 1995. New palaeomagnetic results
from Icelandic Neogene lavas. Geophysical Jour-
nal International 121, 435-443.
Kristjánsson L. and J. Helgason 1988. Some properties
of basalt lava sequences and volcanic centres in a
plate-boundary environment. In A.C. Morton and
L.M. Parson eds. Early Tertiary Volcanism and the
Opening of the NE Atlantic. Geological Society
Special Publication 39, 147-155.
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