Jökull - 01.12.1994, Blaðsíða 15

NW-Iceland that the lower boundary of the long
"Anomaly 5" chron was at least 11.1 Ma old. This is
about 0.7 Ma greater than the age found previously in
two other composite sections in Iceland, a difference
which is well beyond analytical uncertainties in the
dating. Tauxe et al. (1985) suggested that some of the
ages from the McDougall et al.'s (1984) eastern NW-
peninsula section are too high, preferring a value of
10.0 Ma for the lower boundary of Anomaly 5. In the
new ocean-floor geomagnetic polarity time scale of
Cande and Kent (1995) the age of Anomaly 5 is how-
ever estimated to be from 10.95 to 9.75 Ma ago, a
swing back towards the results of McDougall et al.
(1984). It is necessary to date outcrops of Anomaly 5
in other locations in the country to help settling the
question of its age.
Trends in magnetic anomalies and dykes
Kristjánsson et al. (1983) observed that the strike
of aeromagnetic anomalies measured in 1972-74 by
Sigurgeirsson (1984) over the northwestem peninsula
appears to bend from a northeasterly direction to a
northerly direction (characteristic of rifting on the
Kolbeinsey Ridge), as one moves from the south
coast of the peninsula to its northeast coast. This
change of strike can be seen in the colour maps of
Jónsson et al. (1991) and Kristjánsson and Jónsson
(1996). Over the area of our profiles DK to DM the
anomalies have a NNE-SSW direction, in agreement
with the observed strike of the lava pile.
The number of magnetic anomaly lineations over
the NW-peninsula (see Jónsson et al. 1991) is much
less than the number of polarity zones in the lava pile.
The problem of correlating these has been discussed
by Kristjánsson and Helgason (1988). The local to-
pography does not affect the aeromagnetics much, ex-
cept that a small positive anomaly seems to follow
Skötufjörður, and a small negative anomaly runs
along ísafjörður.
A gradual change in dominant dyke trends, from
ENE-WSW in the southwestem part of the peninsula
to NNE-SSW farther north and east, is seen in Fig. 1
of Guðmundsson (1984) and Fig. 1 of McDougall et
al. (1984).
New interpretation of stratigraphy
The K/Ar dates from two sides of the NW-penin-
sula as processed by McDougall et al. (1984) indicat-
ed that the eastem composite section overlapped with
the uppermost part of the western one by about 0.8
Ma, i.e. down to 0.5 km or more below the Brjáns-
lækur sediments. The inferred presence of the over-
lap agreed with the estimate by these authors of a gen-
eral northeasterly strike between Brjánslækur and
Reykjarfjörður. However, from our observations of a
NNE strike in Isafjörður, we infer that the strike di-
rection rotates towards the north across the peninsula.
This is supported by the regional anomaly and dyke
trends mentioned above, as the strike of anomalies
and dyke swarms is known to generally agree with
that of the lava pile elsewhere in Iceland. A tectonic
tilt of 4-6° in Isafjörður indicates (unless much repeti-
tion by faulting occurs) that there is a thickness of 1
km or more of lavas between the top of profile DM
and the base (in Reykjarfjörður) of the eastern com-
posite section of McDougall et al. (1984). The un-
mapped segment of the lava pile may be accessible in
the coastal region north of the Arnes central volcano.
Long-distance paleomagnetic correlations
It is important for various studies in the historical
geology and tectonics of Iceland to establish definite
correlations between stratigraphic horizons in differ-
ent parts of the country. The only magnetic polarity
chron in the age range 5-15 Ma which has so far been
of practical use for this purpose is "Anomaly 5". Other
anomalies are generally too short, in relation to local
variations in build-up rates, to be reliably identified.
Kristjánsson and Jóhannesson (1989) point out that
the long episode of geomagnetic field instability in
profiles DD to DF is probably also seen within profile
JD of McDougall et al. (1984) as may be expected
from the above stratigraphic ties. It should be noted
that the within-lava directional uncertainties in some
of the lavas in profile JD are rather large, and detailed
correlations cannot be attempted. The above authors
suggested that this period of geomagnetic instability is
possibly recorded in the lava pile in Watkins and
Walker's (1977) profiles G and H at Gerpir, the east-
ernmost promontory of Iceland. Another possibility
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