Jökull - 01.01.2001, Blaðsíða 36

Leó Kristjánsson and Ágúst Guðmundsson
clearly required.
Kristjánsson and Sigurgeirsson (1993, profile SH)
sampled 21 flows at the R5–N5 polarity zone bound-
ary (profile SH, Figure 2) in the Villingadalur corrie
in the north–eastern part of Skarðsheiði, close to a
site where Wilson et al. (1972, profile C) had found
several intermediate poles (Dagley and Lawley, 1974,
Figure 8). Kristjánsson and Sigurgeirsson (1993, Ta-
ble 2 and Figure 7) confirmed these early results, find-
ing low–latitude poles (scattered widely around Aus-
tralia) in flows SH 9–12 and 15–18, separated by two
flows with mid–latitude reverse directions.
NEW STRATIGRAPHIC AND
PALEOMAGNETIC MAPPING IN
SKARÐSHEIÐI
Given the confirmed presence of at least 8 interme-
diate virtual poles at the R5–N5 boundary in north-
eastern Skarðsheiði, it was of interest to determine if
there are also several intermediate poles occurring at
other locations of this boundary in Skarðsheiði and/or
nearby mountains.
Stratigraphic mapping in new profiles SI and SJ
Profile SI
We wished to map a composite profile up through the
various volcanic formations of Skarðsheiði, for pale-
omagnetic sampling. Suitable locations are however
limited, in some parts of the mountain due to scree
cover, in others due to steepness or the presence of in-
trusions, thick rhyolite lavas etc. The location selected
is in the south–western part of Skarðsheiði (Figures 2
and 3). At the base of the composite profile, units
1–23 in profile SI are tholeiite lavas exhibiting con-
siderable alteration with infillings of chalcedony (up
to fist–size) and calcite. The thick andesite flow SI 28
forms a conspicuous scarp above this part of the pro-
file. Some stratigraphic complications occur beyond
this flow, possibly due to a small unconformity or to
landscape effects during emplacement. We sampled
a tholeiite outcrop numbered 28O above the andesite
(contact not seen) and then the flows 28A–F to the
northwest. The profile is continued still farther to the
northwest, beyond a fault zone, but probably only 10–
20 m are missing between the SI 28 series and SI 29.
The flows SI 29–47, exposed on the west side of the
peak Litlahorn, are largely of feldspar– and olivine–
porphyritic composition.
Profile SJ
This profile (Figure 3) begins with a few porphyritic
flows at a pass about 1 km to the northwest of Litla-
horn, between the Skarðshyrna peak and a small peak
of acidic composition. Below them, some thin flows
may be seen, getting more numerous (in very steep
outcrops) to the east. The thick and massive flow SJ 4
is petrographically somewhat similar to the flow SI
44 but their paleomagnetic directions are not iden-
tical, so there may be a stratigraphic gap of a few
tens of m between SI 47 and SJ 2. According to
Franzson (1978) this porphyritic sequence which is
reversely magnetized, is also found in the lower part
of Akrafjall. However, the top part of this reverse
zone in Skarðsheiði (SJ 5–11) includes some tholeiitic
basalts (and one fairly thick clastic horizon), whereas
corresponding reverse tholeiitic lavas are not seen in
Akrafjall. Further mapping and radiometric dating of
the exposed lava sequences in the Hvalfjörður area
is needed to determine their relation to the extrusive
activity of various volcanic centers in South–Western
Iceland (see Figure 1). The hydrothermal alteration at
this part of the composite section is minor, the zeolite
chabazite being noted in several flows.
The profile SJ continues up the western shoul-
der of the Skarðshyrna peak to flow SJ 24, and in-
cludes additional sediment horizons above flows SJ
12 and 14. The sediments are conspicuous (visible
from Reykjavík, 35 km away, as a light–brown band
in the cliff face) but they seem to be of variable thick-
ness or even absent in some parts of Skarðsheiði.
The top part of the profile runs north along the ridge
from Skarðshyrna to the Heiðarhorn peak and consists
mostly of tholeiites (very fresh at the top) with a thick
horizon of conglomerate and other sediment coinci-
dent with a polarity change at SJ 33/34. Only a much
thinner sediment occurs immediately above the four
transitional flows FA 45–48 of the normal to reverse
transition in Akrafjall, so that Franzson (1978) cor-
related the SJ 33/34 sediment with a 4 m thick con-
36 JÖKULL No. 50