Jökull - 01.01.2001, Blaðsíða 40

Leó Kristjánsson and Ágúst Guðmundsson
ter 10 mT treatment) is less than 2 A/m in all except
one of the flows SI 1–26, as compared to an overall av-
erage in lavas of similar or greater age which is about
3.5 A/m. However, these alteration processes do not
seem to disturb the remanence directions: the nor-
mally magnetized flows SI 1– 14 which exhibit typ-
ical secular variation behavior, are followed by two
transitional flows and then a thick reverse series. We
suggest that SI 1–14 are from the Cochiti subchron,
which is not seen in Wilson et al.’s (1972) profile C
in North–Eastern Skarðsheiði. Much additional work
is needed, however, before this polarity zone can pos-
sibly be correlated e.g. with the thin zone FA 4–7 in
Akrafjall, or with Einarsson’s N6 in hills northeast of
Skarðsheiði (see Wilson et al. 1972, Figure 1).
The remainder of SI as well as the porphyritic bot-
tom part of profile SJ up to SJ 10 forms a single thick
reverse–polarity zone. This zone includes three reli-
ably determined excursions of the virtual pole to lati-
tudes of 40 or less, namely at SI 28B, 32 and 47. We
then find the transitional flow SJ 11, sampled in two
outcrops several tens of m apart which yielded iden-
tical directions. Their mean is very similar to the di-
rection in SH 10 but otherwise it was somewhat dis-
appointing to find only a single intermediate direction
at this point of the section.
Profile SJ continues in normally magnetized lavas
to SJ 33, with only one VGP excursion to mid–
latitudes in SJ 24 and 24A. Above the conglomerate
bed overlying SJ 33, the profile contains relatively
fresh reverse flows (presumably R4, from the Mam-
moth subchron) up to the top of Heiðarhorn.
The mean field direction in 92 flows from profiles
SI and SJ has a declination of 4 and an inclination
of +74.4 . It is thus quite close to the central axial
dipole direction D = 0 , I = +76.5 . The vector sum R
is 87.1, giving an angular standard deviation of 19 .
If VGPs are averaged, their mean position is at lati-
tude 87 N, longitude 109 E. The vector sum in this
case is 81.7, yielding an angular standard deviation of
27.5 , which is comparable to values found in rocks of
similar age elsewhere in Iceland (Kristjánsson, 1995,
Figure 4).
Further mapping and paleomagnetic results at the
presumed R5–N5 boundary
New profile, SW, in Skarðsheiði
It was considered possible – although unlikely – that
the unusual occurrence of many transitional directions
in profile C of Wilson et al. (1972) and profile SH of
Kristjánsson and Sigurgeirsson (1993) was caused by
some disturbance such as localized secondary alter-
ation of the lavas. We therefore sampled lavas west of
the Villingadalur corrie, at about 650–740 m altitude
(Figure 2) some 1.5–1.8 km northwest of SH. Expo-
sures along the new profile SW are not as good as
in SH, and a stratigraphic map of SW has not been
prepared. We cored flows numbered SW 10 through
23, except two thin flows SW 12–13. The lavas in
profile SW were good material for paleomagnetic di-
rection measurements, only four samples having to be
discarded due to lightning effects.
Results are listed in Table 1. A comparison with
profile SH shows excellent agreement in paleomag-
netic directions between these profiles, for example:
SW 10 = SH 4 or 5; SW 11 = SH 7; SW 15 = SH 8
(both sites are also porphyritic); SW 16 = SH 9 (ap-
prox.); SW 17 = SH 10; SW 18 and 19 (which may
be the same flow) = SH 12; SW 20–23 = SH 13–16.
These flows cover most of the R to N transition, but
profile SW was not continued farther upwards due to
increasing lateral distance between outcrops and the
presence of some intrusions.
New profile, SS, in South–Eastern Skarðsheiði
In an attempt to find a R to N transition zone thicker
than one flow at the presumed Gilbert–Gauss bound-
ary on the south side of Skarðsheiði, we mapped a
new supplementary profile SS (Figures 2 and 5) con-
sisting of 23 flows in the south–eastern part of the
mountain, on the eastern slope of the Súlárdalur cor-
rie. The lower part of the exposed lava sequence is
largely porphyritic, but tholeiitic flows are more com-
mon in the upper part. Profile SS contains at least
three sediment beds but further studies are needed be-
fore they may be correlated individually with those in
SJ and SH. Only flows SS 7–23 were cored for paleo-
magnetic measurements.
40 JÖKULL No. 50