Jökull - 01.07.2003, Blaðsíða 27

Paleomagnetic observations on Late Quaternary basalts, SW-Iceland
N
Figure 2a. Stereographic projection of paleomagnetic
directions (Table 1) from the Reykjavík gray lavas.
Note that many of the points correspond to two or
three flow units each. The direction of the geocen-
tric axial dipole field in Reykjavík is shown. – Wulff-
vörpun segulstefna úr Reykjavíkurgrágrýtinu (Tafla
1) ásamt áætlaðri meðalstefnu jarðsegulsviðsins á
löngum tíma.
General results on magnetic properties
The Reykjavík gray lavas are excellent material for
paleomagnetic studies. Their remanence intensity (J
in Table 1) after alternating field treatment at 10 mT
peak field varies from less than 1 A/m to over 15 A/m,
but the stability is generally high; in the lavas of Table
1a for instance, the value of J is on average over 80%
of the natural remanence intensity. The within-unit
95% confidence angle for mean directions in those
lava units where four samples were collected, is usu-
ally in the range 2–7Æ, with an average of about 4Æ.
The author has obtained a strong-field thermo-
magnetic curve (in air) on one sample from site KA
and a weak-field curve on one sample from KH. The
former indicates a single Curie point (T ) of about
540ÆC, with 10% reduction in room- temperature sat-
uration remanence after heating to 580ÆC. The latter
sample has a reversible thermomagnetic curve with
T = 570ÆC. Samples from sites RE 7, RE 26 and
RE 30 exhibit fairly reversible curves in vacuum with
Curie points similar to these (E. Schnepp, pers. comm.
1996). Two samples from unit MI 2, however, had
strongly irreversible curves with T of about 200ÆC
on heating. Susceptibilities are of the order of 1.10 cgs volume units (0.013 SI), which is about half the
average value for Tertiary basalts. These results along
with an observed low tendency of the gray basalts
to acquire viscous remanence, indicate that many of
them are a more promising material for paleointensity
determination than Icelandic lava flows in general.
Mikligarður geomagnetic excursion site 1988, and
subsequent sampling nearby
In 1988, large excavations for building and harbour
development had been undertaken through the gray
lavas about 700 m SSE of the Mikligarður (later
IKEA/Samskip) shopping mall at the Sundahöfn har-
bour in Reykjavík (site 10 of Figure 1). These re-
vealed a sediment horizon with lignite between two
lava flows (Figure 3a). At the request of J. Eiríks-
son of the University of Iceland, three lava units were
cored for paleomagnetic measurements, as well as the
sediment. Attempts have been made to date the lignite
by means of radiocarbon measurements by accelera-
tor mass spectrometry (Á. E. Sveinbjörnsdóttir, pers.
comm. 1996) but it shows no significant
C content
and must therefore be more than 50 kyr old. The sites
are no longer accessible.
Paleomagnetic measurements revealed that the
bottom outcrop, MI 0 of Table 1c (see Figure 3b), car-
ried a transitional remanence direction (seen on the
right-hand side of Figure 2a). The corresponding vir-
tual geomagnetic pole (V.G.P.) is at lat. 9ÆN, long.
52ÆE, see Figure 2b. Only the very top of the lava
could be sampled and demagnetization treatment to
30 mT was needed, possibly due to baking by the
overlying flow MI 1. The lava has a typical gray-
basalt appearance and it is not felspar-porphyritic.
Hjartarson and Guðjónsson (1984) have previ-
ously reported that a 1963 drillhole south of Mikli-
garður penetrated crystalline rock within the Elliða-
vogur sediments, although this was possibly a broken-
up lava margin rather than solid lava. Richter (1995)
made a detailed study of six long drill cores through
the Reykjavík gray basalt sequence. One was located
JÖKULL No. 52, 2003 25