Jökull - 01.01.2010, Síða 152

L. Kristjánsson
Their results along with magnetic polarity measure-
ments made in the field have aided in the compilation
of regional geological maps of surface outcrops, such
as the recent map by Sæmundsson et al. (2010).
THE PRESENT PROJECT
Although some of the publications listed in the
last paragraph (as well as various student theses
and institute reports of limited circulation) have fol-
lowed up on details of T. Einarsson’s paleomagnetic-
stratigraphic mapping efforts, much remains to be
done in that field. The work described in this pa-
per continues some of Einarsson’s (1962) studies and
will hopefully stimulate others to extend them further.
Three isolated areas were chosen, all presumably be-
longing to the Matuyama geomagnetic chron. These
were in the Brynjudalur valley south of Hvalfjörður
(Figures 1 and 2), on both sides of the Kjósarskarð
pass (Figures 1 and 4), and near the Fossnes farm in
the Þjórsárdalur valley (Figures 1 and 7). As in previ-
ous studies by the author and his collaborators in other
profiles of Brunhes-Matuyama age (e.g. Kristjánsson
et al., 1988; Kristjánsson and Sigurgeirsson, 1993),
the chief aim of the project was to locate geomagnetic
excursions and boundaries of polarity zones. These
might be promising for radiometric dating and for use
as stratigraphic markers across larger areas.
Another motive for sampling these profiles was to
obtain new data on the long-term secular variation of
the geomagnetic field in the Pleistocene. The angu-
lar standard deviation (a.s.d.) of VGPs from Icelandic
lavas seems to have had a significantly smaller am-
plitude in the last few million years than for instance
during the period 10–15 Ma ago (Kristjánsson, 2008).
Few if any other parts of the world seem capable of
providing coverage of comparable intervals with mag-
netic direction data of comparable quality and quan-
tity. Therefore, it is still not known whether this re-
duction in the a.s.d. is related to changes in other prop-
erties of the geomagnetic field such as its reversal fre-
quency or its long-term mean intensity.
METHODS
Sampling of 25-mm diameter cores was carried out
with a two-stroke gasoline-powered drill. The cores
were oriented in situ with a Brunton compass, using
sightings on the Sun or distant geographic objects for
azimuth reference. At least four core samples were
collected at each site. All remanence measurements
on 21 mm long specimens from these cores were
made with an Institut Dr. Förster four-probe fluxgate
magnetometer. Many samples contained significant
viscous magnetization (VRM) whose intensity could
even exceed that of the primary remanence. How-
ever, this VRM was easily removed by stepwise treat-
ment in a Molspin alternating-field demagnetizer. The
peak fields applied were 10, 15, 20 and 25 milli-Tesla.
A 30 mT step was also added in the Þórisgil and
Kjósarskarð profiles, but it turned out to be unnec-
essary. An average direction was computed for each
lava after each step, and the average with the smallest
95% confidence angle (α95) was selected for inclu-
sion in Appendix 1. The lava flows in all the sampled
profiles generally carry a very stable primary rema-
nence of moderate intensity, giving excellent direc-
tional agreement between the core samples. Only a
few samples were rejected in each of the three areas,
due to instability, suspected orientation errors or out-
crop movement.
In the diagrams of the profiles, the sampled
lavas have been classified roughly into lithologi-
cal categories based on field criteria and inspection
of the cores: tholeiites, olivine tholeiites, feldspar-
porphyritic flows, and andesites. The clastic units in
the profiles are also described in broad terms only.
PROFILE WB IN ÞÓRISGIL,
BRYNJUDALUR, SW-ICELAND
A stratigraphic profile of the Þórisgil gully (Figure 2)
in the Brynjudalur valley south of Hvalfjörður was
published in Figure 31 of Einarsson (1962). It indi-
cates that the lower part of the profile is occupied by
the R2 series. This is followed by a 10 m layer of sec-
ondary breccia and fine conglomerate at an unspeci-
fied altitude and then 15 lava flows belonging to the
N2 zone on the slopes of the Botnssúlur mountain.
152 JÖKULL No. 60