Jökull - 01.01.2013, Blaðsíða 44

J. Helgason and R. Duncan
Figure 12. Stratigraphic position of Skjólgil showing conditions where dyke D2 cuts lavas LA and LB (inlet).
– Skipan jarðlaga í Skjólgili. Á minni myndinni er sýnt hvar gangur (D2) sker hraunlög LA og LB.
(±0.29) Ma, thus providing a correlation with the ge-
omagnetic polarity time scale to the normal Olduvai
magnetic chron (C2n: 1.78–1.95 Ma). The N-lavas in
Skjólgil have a high VGP-latitude.
AGE DETERMINATIONS
Volcanic units above the Svínafell sediments (group
S3) proved normally magnetized when measured with
a handheld magnetometer in the field and were there-
fore inferred to be of Brunhes age or younger than
about 0.781 Ma. As the uppermost volcanic units be-
low the Svínafell sediments are reversely magnetized,
i.e., lavas in Skjólgil, it is clear that these could be
from the Matuyama reversed chron or older. There-
fore, we emphasized sampling units for dating from
below as well as above the Svínafell sediments to
constrain their age. Geological mapping showed the
Skjólgil lavas to be stratigraphically older than the
Svínafell sediments (Figure 12).
We collected the freshest samples, based on mini-
mal vein and vug-filling secondary minerals, from the
Skjólgil paleomagnetic L-section as well as from vol-
canic units above the Svínafell sediments. Initially,
several attempts using 40Ar–39Ar incremental heat-
ing experiments revealed that no lava samples from
the Skjólgil L-section provide reliable crystallization
ages due to alteration effects and variable amounts of
excess 40Ar. This indicated that the Skjólgil lavas are
considerably older and more altered than the Brun-
hes age rocks above the Svínafell sediments, which
provided reliable age determinations. When dating
the Skjólgil lavas proved unsuccessful we examined
fresher dykes that could provide a minimum age for
the strata below the Svínafell sediments (Figure 12,
enlarged insert). Two dykes cut lavas LA and LB, i.e.,
the two top lavas in formation SV9. The dykes are
clearly part of the lower strata and do not extend above
surface SR3. Therefore, their age must be somewhat
44 JÖKULL No. 63, 2013