Jökull - 01.12.1994, Page 5

Stratigraphy and Paleomagnetism of the Lava Pile South of
s
Isafjarðardjúp, NW- Iceland
Leó Kristjánsson
Science Institute, University of Iceland, Dunhaga 3, 107 Reykjavík
Haukur Jóhannesson
Icelandic Institute of Natural History, Hlemmi 3, 105 Reykjavík
Abstract - A composite 2.6 km stratigraphic section has been constructed from 12 well-exposed
lava profiles in thefjords on the south side of Isafjarðardjúp, in the Vestfirðir peninsula ofNW-
Iceland. Its age is around 13 Ma. Oriented cores for paleomagnetic measurements were collect-
edfrom 307 lava sites, 225 ofthem in the composite section. The lavas carry a stable primary re-
manence, and ten or more reversals of geomagnetic polarity are recorded in the composite sec-
tion. There is also evidence for extended excursions (periods of instability) of the geomagnetic
field, with some 15 lavas in the section being erupted during one ofthese.
The paleomagnetic polarites are used along with geological information for correlation with the
composite section of McDougall et al. (1984) along the western side ofthe Vestfirðir peninsula.
Sediments correlated with the Brjánslœkur sedimentary horizon occur near the top of the Isa-
fjarðardjúp section. We suggest that the two sections mapped by McDougall et al. on the western
and eastern sides ofthe peninsula fail to overlap, possibly by over 1 km oflava thickness.
INTRODU CTION
Scope of the present study
In Iceland, the chief difficulties in regional strati-
graphic mapping include the episodic character of the
volcanism, and tectonic complications connected with
shifts in the position and direction of the volcanic
zones. The coverage of extended stratigraphic sections
is sparse, and few radiometric age determinations are
available. These circumstances make long-distance
correlations by conventional mapping methods quite
uncertain, so it is becoming necessary to develop new
and improved methods.
Under favorable circumstances, lava sequences
can fumish important data on the age of geomagnetic
chron boundaries, to supplement and confirm similar
data available from sediments and ocean-ridge anoma-
ly inversion. The chief requirements are a constant rate
of build-up of a lava pile, preferably without any
breaks exceeding 10,000 years, and accurate radio-
metric dating. In the nineteen-sixties good progress
was made world-wide in establishing the age of some
Quatemary reversals, and it seemed reasonable to ex-
pect that a complete geomagnetic polarity time scale
would be established before long. Unfortunately, com-
plications including the presence of numerous short
chrons and variable fidelity of the available "tape
recorders" have tumed up. In spite of individual suc-
cesses such as the dating of Gilbert subchrons in Ice-
land by McDougall et al. (1977), the various ap-
proaches to a polarity time scale have still not con-
verged sufficiently.
The Vestfirðir (Western fjords) peninsula of NW-
Iceland is a region very suitable for detailed strati-
graphic studies and for age dating of chron bound-
aries. It is a basalt lava pile of 15 - 8 Ma age, largely
JÖKULL, No. 44
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