Jökull - 01.01.2013, Síða 43

Stratigraphy, 40Ar–39Ar dating and erosional history of Svínafell, SE–Iceland
PALEOMAGNETISM
The Skjólgil lava group (S2) forms a 120 m high hill
between two basins; a small basin in Bæjargil to the
northwest and a greater basin into which the Svínafell
sediments were deposited to the southeast. Sedimen-
tary rocks bank up against the hill on both sides. This
hill is the only preserved fragment in Svínafell that
can be correlated with the upper Matuyama chron.
The Skjólgil lavas are stratigraphically older than the
Svínafell sediments (group S3) on the east/south side.
Although the Skjólgil lavas terminate abruptly toward
the east, and do not continue below the Svínafell
sediments, their magnetic polarity is highly signifi-
cant for constraining the age of the sediments. The
same comment applies to the volcanic units further
up-section; i.e., group S4. Cooling units within S4
are finer grained and fresher in hand sample com-
pared with lavas of group S2 and appear considerably
younger. Generally, basaltic rocks of Brunhes age are
fresh, have strong remanent magnetic signature, and
thus a hand-held magnetometer suffices to determine
their magnetic polarity in the field and to map their
areal extent.
Sampling for paleomagnetic measurements
We drilled 12 lavas of stratigraphic profile L in Skjól-
gil gully (Figure 7) for paleomagnetic purposes, as
these are the cooling units below the Svínafell sedi-
ments. Sampling was conducted using a two-stroke
Pomeroy drill. Core tube diameter is 2.5 cm and core
length about 5 cm. Normally four cores were sampled
per cooling unit. The samples were demagnetized in
Dr. Leó Kristjánsson’s laboratory in the Geophysics
Division, Science Institute, University of Iceland, us-
ing Molspin AF demagnetizing equipment. The sam-
ples were measured after treatment at 0, 10, 15 and 20
mT peak field strength, respectively, and the best re-
sults chosen. A correction for tilt, 6◦/150◦, was made
for profile L.
Paleomagnetic results are presented in Table 1.
The magnetic polarity in Skjólgil changes from R to
N to R. The N-lavas are correlated with the Olduvai
magnetic chron based on their lithological similarity
with a dated R/N/R-sequence in Hafrafell (Helgason
and Duncan, work in progress). The Hafrafell se-
quence has been defined as "valley filling" and one of
the R-units, 40 m above the N-lavas, was dated at 1.69
Table 1. Paleomagnetic results for Svínafell: section L in Skjólgil. – Niðurstöður segulmælinga fyrir Svínafell:
snið L í Skjólgili.
Form. No N D I PLong PLat α95 J10 Pol
SV9 LA 2(2) 168.8 –50.3 1.0 –56.3 10.6 2.2 R
SV9 LB 3(1) 200.7 –71.6 280.8 –77.2 5.6 2.3 R
SV9 LC 4 187.4 –65.9 325.7 –73.7 4.1 2.7 R
SV9 LD 4 198.5 –70.0 291.8 –76.2 3.0 4.3 R
SV8 LE 4 255.7 –62.9 241.5 –44.8 12.6 1.3 R
SV8 LF 3(1) 231.9 –61.7 264.2 –54.1 2.0 5.0 R
SV8 LG 4 183.7 –73.9 317.9 –85.7 4.5 4.8 R
SV8 LH 3(1) 188.4 –75.3 272.6 –85.9 9.1 2.0 R
SV7 LI 4 12.5 70.3 124.0 78.5 2.0 6.0 N
SV7 LK 3 16.4 73.0 101.0 80.4 5.4 2.3 N
SV6 LL 3 218.6 –61.4 279.3 –59.3 10.3 1.0 R
SV6 LM 4 211.3 –58.6 291.7 –59.3 2.6 2.2 R
Explanations: Form: stratigraphic formation number. No: Unit number. N: Number of measured samples with number of
rejected samples in brackets. D, I: Declination and inclination after tilt correction. PLong: Longitude of virtual geomagnetic
pole (VGP). PLat: Latitude of virtual geomagnetic pole (VGP). α95: Mean field 95% confidence radius. J10: Remanence
intensity (Amperes/m) after demagnetization in 10 mT AF. N: Normal polarity. R: Reverse polarity.
JÖKULL No. 63, 2013 43