Jökull - 01.01.2013, Page 4

L. Kristjánsson
opposite to it. The average VRM intensity in lavas
may be of the order of 0.3 Am−1, but in some rock
samples it exceeds their TRM intensity. Remanence
intensities of 40 Am−1 or more are only found in very
rare and isolated cases (apart from lightning strikes).
Fresh Quaternary basalts may have a stronger average
remanence than the Tertiary lavas, possibly 6 Am−1
(Kristjánsson, 1970, and other data) and distinctly
lower susceptibility. Regional hydrothermal alteration
tends to cause a progressive decrease in the intensity
of primary remanence of lavas (Fig. 13 of Watkins and
Walker, 1977). This decrease, along with enhanced
tendency for VRM buildup, may reach significant pro-
portions already at the upper boundary of the analcime
zeolite zone, but the matter requires further study.
Geological research
Detailed geological studies in the Stardalur anomaly
area (Friðleifsson and Tómasson, 1972; Friðleifsson
and Kristjánsson, 1972; Friðleifsson, 1973, 1985) re-
vealed the presence of an extinct and eroded volcanic
complex. This complex is manifested by a caldera of
about 6.5 km in diameter as well as cone sheets, rhyo-
lite and dolerite intrusions, and small plugs around the
caldera rim. See Fig. 1 of Friðleifsson and Kristjáns-
son (1972), which is also reproduced as Fig. 1b of
Vahle et al. (2007). The southern half of the rim
is covered by the so-called „Reykjavik gray basalts“,
a lava sequence of Late Quaternary interglacial age
which is widespread in this region. The main peak
of the ground magnetic anomaly lies about 0.5 km in-
side the easternmost part of the caldera fault (Figure
1, inset).
Pálmason (1971) found by refraction seismic
measurements that „Layer 3“ (with a p-wave veloc-
ity of 6.5 kms−1) of the crust reaches up to 0.5 km
depth under the Stardalur area. According to his re-
sults, the upper surface of this layer (presumed to be
an intrusive complex of gabbroic cumulates) usually
lay at 2.5–4 km depth outside the volcanic zones of
Iceland.
An extensive project of sampling lava flows
for stratigraphic mapping and paleomagnetic studies
in mountains northwest and north of the Stardalur
caldera was initiated in 1973 (Kristjánsson et al.,
1980). Most of these lava flows had reverse mag-
netic polarity. A K-Ar age determination on a nor-
mally magnetized rhyolitic hyaloclastite belonging to
the latest phase of the volcanism yielded an age of
about 1.9 m.y. (when recomputed with current de-
cay constants). It may therefore be expected that the
thick mostly reversely magnetic lava sequence found
in the above sampling project belongs to the lower
part of the Matuyama geomagnetic chron. Also, rocks
of Matuyama age probably cause the wide negative
anomaly lineation through Stardalur. If the localized
magnetic anomaly there is due to rocks of compara-
ble age as the acidic hyaloclastite, these rocks might
accordingly date from the Reunion or Olduvai sub-
chrons at 2.1 and 1.9–1.8 m.y. respectively. They may
also be of the same age as the thin N3 polarity zone
of lava flows originally mapped by T. Einarsson and
Þ. Sigurgeirsson in the 1950s (see Goguitchaichvili et
al., 1999).
DRILLING AT STARDALUR, AND
STUDIES ON THE DRILL CORE IN
1969–1973
Drill holes and sampling of nearby outcrops
The enigmatic nature of the Stardalur magnetic
anomaly contributed to a decision to drill three holes
within the Stardalur caldera in 1969–1970, by the Na-
tional Energy Authority of Iceland. Hole 1 was lo-
cated at the main peak of the magnetic anomaly. Be-
low fresh olivine tholeiite lavas (of the Reykjavik gray
lava sequence) and tuff-breccia, the drill entered al-
tered but strongly magnetic olivine tholeiite lava flows
at 41 m depth (Friðleifsson and Tómasson, 1972). A
continuous 6 cm diameter core was recovered down to
143 m, and cuttings to 200 m. According to drill logs
and inspection of the core, there were some 20 lavas
present in the 41–200 m interval, without significant
interbeds. Drill hole 2 inside the caldera about 2 km
to the west of the first hole encountered mostly tuffs
and minor intrusives to 240 m depth. Hole 3 another
2 km to the west passed through 90 m of lavas and in-
trusives which to our knowledge did not have unusual
magnetic properties.
Various surface outcrops (dikes, cone sheets, pil-
lows, gabbros etc.) were sampled in the early 1970s
4 JÖKULL No. 63, 2013