Jökull - 01.01.2001, Blaðsíða 42

Leó Kristjánsson and Ágúst Guðmundsson
Figure 6. Virtual poles from the R5–N5 transition in profile SH (Kristjánsson and Sigurgeirsson, 1993; open cir-
cles) with the poles from flows SJ 10–12 (squares) and SS 16–20 (triangles) added. All transitional poles found
in profile SW are very similar to poles from SH. – Staðsetning sýndarsegulskauta við mót hraunasyrpanna R5
og N5 úr sniðinu SH í Villingadal (Kristjánsson og Sigurgeirsson, 1993) ásamt segulskautum úr hraunlögum í
sniðum SJ og SS.
Flows SS 17,18 and 19 were found to have transi-
tional directions (Table 1; Figure 6). The virtual pole
in SS 17 (NE–Australia) is similar to that in SJ 11 and
SH 10 while those in SS 18–19 (SE–Asia) are not far
from those in SH 15–16. The fact that several inter-
mediate directions are seen in SH and SW, indicates
that eruptive activity at the time of the transition was
greater in the north–eastern part of the Skarðsheiði
mountain than to the south and west. Judging from
the results of Wilson et al. (1972), the N5 polarity
zone is also thicker in their profile C in North–Eastern
Skarðsheiði (around 50 flows, some thin) than in our
SJ in South–Western Skarðsheiði (22 flows).
Search for a possible R5–N5 transitional direction in
profile FA in Akrafjall
Kristjánsson et al. (1980) showed that the presumed
N5 lavas above a clastic bed between flows 28 and
29 in their profile FA in Akrafjall (Figure 1) have
regular normal–polarity directions. Those below that
bed have regular reverse–polarity directions, with the
possible exception of FA 28 which was quite unsta-
ble. We have carried out additional sampling of a few
lava flows at this transition, at a locality some 300 m
west of the original FA profile. Some of the results
are shown at the bottom of Table 1. It appears from
these results that no intermediate VGPs occur at this
locality. We have used the opportunity to resample
the compound flow FA 17, confirming the presence of
a low–latitude pole position. This excursion may be
of similar age as that in the Skarðsheiði lava flows SI
28B or SI 32.
Thermomagnetic measurements
In order to ascertain the nature of the carrier of mag-
netization in the Skarðsheiði lavas, we measured the
temperature dependence of the magnetic susceptibil-
ity of paleomagnetic cores, at several levels in the lava
pile. Measurements were made on 10–gram samples
of crushed rock in air, using a Bartington MS2 W/F
furnace. Heating to 600 C took about one hour.
Results from four typical samples are shown in
Figure 7. The only one whose remanence was un-
stable during AF demagnetization is SI 29–1; the oth-
ers exhibited MDF values exceeding 20 mT and min-
imal direction change during AF treatment. The main
42 JÖKULL No. 50