Jökull - 01.01.2009, Blaðsíða 33

Reviewed research article
A new study of paleomagnetic directions in the Miocene lava
pile between Arnarfjörður and Breiðafjörður in the Vestfirðir
peninsula, Northwest Iceland
Leó Kristjánsson
Institute of Earth Sciences, University of Iceland, Sturlugata 7, 101 Reykjavik, Iceland
leo@raunvis.hi.is
Abstract – A new paleomagnetic study has been carried out on basaltic lava flows in the promontories between
the fjords Arnarfjörður, Tálknafjörður, Patreksfjörður and Breiðafjörður, Vestfirðir peninsula of Northwest Ice-
land. This area is around 13 m.y. in age. The study involved mapping and sampling of 365 lava flows in 20
profile locations for laboratory measurement of magnetic remanence vectors; data from some 70 flows sampled
previously were also available. A tentative scheme of paleomagnetic polarities is proposed for a composite
section of 2.0 kilometers cumulative thickness, mostly overlapping with a stratigraphic column drawn up for
the major part of the area by J. Preston in the early nineteen-seventies. At least six polarity reversals are en-
countered in the lava pile, as well as some apparent major excursions of the geomagnetic pole to middle and
low latitudes. These are promising markers for stratigraphic correlation within and between the fjords, along
with various lithologically distinctive units such as thick clastic sediments and cumulate plagioclase lavas. The
rate of buildup of the lava pile is estimated to average 1.4 km/m.y. but it has been somewhat episodic, as near-
identical remanence directions are often recorded in two or more successive lavas. Additional work in the area
is required before detailed correlations with previously mapped and sampled composite stratigraphic sections
in the peninsula become feasible.
INTRODUCTION
Previous geological and geophysical work else-
where in the Northwest peninsula
The Northwest peninsula of Iceland (known as Vest-
firðir, Figure 1) is mostly composed of basalt lava se-
quences. Their tectonic dip is generally towards the
southeast in the south and west of the peninsula, turn-
ing towards the east when one moves north and east.
The age of these lavas reaches back to about 15 mil-
lion years (m.y.) on the northwest coast of the penin-
sula (McDougall et al., 1984), and no older rocks are
known to be exposed elsewhere in Iceland. How-
ever, this age estimate may be uncertain as much as
1 m.y., as few radiometric age determinations have so
far been carried out on the very oldest lava flows. The
time intervals spanned by various possible unconfor-
mities in the area, notably a thick lignite-bearing sedi-
ment horizon near the base of the lava pile (Harðarson
et al., 1997; Kristjánsson et al., 2003) are also uncer-
tain and may vary along strike. The youngest rocks
in the peninsula where it joins the mainland, are 8–9
m.y. old (Figure 7 of McDougall et al., 1984).
The Northwest peninsula is indented by scores of
fjords and inlets, the largest fjord being Ísafjarðardjúp
(Figure 1). Due to the excellent exposures of lava pro-
files in many parts of the peninsula, it provides oppor-
tunities for the stratigraphic mapping and sampling of
composite sections through the lava pile. Geological
JÖKULL No. 59 33