Jökull - 01.12.1994, Page 6

free of disturbance from tectonics and from hydro-
thermal effects of volcanic centers. Good access to
the lava pile is provided by the numerous fjords and
valleys of the peninsula.
The present project of stratigraphic mapping was in
part initiated to test the use of paleomagnetic polarity
zones for correlation over distances of up to 50 km or
more. This can be done by comparing the pattem of po-
larity reversals in a composite WNW-ESE section
through the tributary fjords south of Isafjarðardjúp
(Fig. 1) with a pattem previously obtained from pro-
files along the westem side of the NW-peninsula (Mc-
Dougall et al. 1984). The paleomagnetic directions
from the lavas in the section can also be used for statis-
tical studies of the geomagnetic secular variation, and
some such studies using the directions from the present
collection have already been published (Kristjánsson
Fig. 1. Schematic geological map of the Vestfirðir peninsula,
NW-Iceland. 1 Exposed central volcano, 2 Down-dip central
volcano, 3 Series of compound lava flows in Isafjarðardjúp,
4 Flexure zone, 5 Unconformity, 6 Dip and slope. H Hrafn-
fjörður central volcano, Á1 Álftafjörður c.v., T Tjaldanes c.v.,
G Gláma c.v., F Flatey c.v., R Reiphólsfjöll c.v., Ár Árnes
c.v., K Króksfjörður c.v., M Mókollsdalur c.v., S Sælings-
dalur c.v. and P Prestbakki c.v.
1. mynd. Einfaldað jarðfræðikort afVestfjörðum. Megineld-
stöðvar eru merktar með bókstöfum. Syrpa dyngjuhrauna við
innanvert Isafjarðardjúp er sýnd sérstaklega, svo og halla-
belti og mislœgi. Strik og halli jarðlaga eru gefin á all-
mörgum stöðum.
and Jóhannesson 1989; Kristjánsson 1995).
General geological setting of the Vestfirðir peninsula
Iceland is sitting on a divergent plate boundary in
the North Atlantic Ocean. The rift axis branches
through Iceland from southwest to northeast, joining
the ridge segments southwest and north of Iceland.
The basalt plateau of Iceland consists predominantly
of tholeiitic basalt lava flows which have been erupt-
ed within a rift zone and subsequently drifted away to
the east and west. The currently active rift zone struc-
ture is a relatively recent phenomenon: the main part
of the lava pile in NW-Iceland was produced by an
older rift zone which was located west of the present
one (Sæmundsson 1979, Jóhannesson 1980) and be-
came extinct about 6-7 million years ago. The extinct
zone (named the Snæfellsnes rift zone) can be traced
as a syncline structure running obliquely across the
Snæfellsnes peninsula from SW to NE and continuing
east along the Hvammsfjörður Bay. Farther to the east
the structure takes a northerly trend to join the Kol-
beinsey ridge north of Iceland.
The geology of the Vestfirðir region is relatively
poorly known. The lava succession is 4-5 km thick
and consists mainly of tholeiitic lava flows intercalat-
ed with thin (1 m or less) sedimentary beds, usually
red lateritic paleosoil. A few thicker sedimentary hori-
zons do occur within the succession representing times
of very slow or no volcanic activity (Sæmundsson
1979). These horizons are usually accompanied by
minor unconformities. The sedimentary horizons are a
few tens of m in thickness and some are rich in plant
impressions (Friedrich 1966) which give indications
about the climate of the time when they were formed.
The lava flows in the westem coastal region of the
Vestfirðir peninsula have a gentle (often 2-3°) south-
easterly dip. The dip increases gradually towards the
southeastern part of the peninsula until it reaches
about 15° in a monoclinic flexure zone which stretch-
es from Þorskafjörður to Steingrímsfjörður. Central
volcanoes are buried within the succession and 7-8
have been identified (Fig. 1) but there is evidence for
a few more down-dip. Results of mapping have been
published for only one volcanic center, the Króks-
fjörður volcano in the southeastem part of the area
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JÖKULL, No. 44