Environmental Engineering Reference
In-Depth Information
Talnakh section contains the two lowermost units (Ivakinsky and Syverminsky
formations).
5.3 Paleomagnetism
All samples were thermally demagnetized up to 580
-
680
C with an average
of 12 steps to isolate the components of the natural remanent magnetization.
The measurements were performed in the paleomagnetic laboratories of the Institut
de Physique du Globe de Paris (France), the Institute of Physics of the Earth and
Lomonosov Moscow State University (Moscow, Russia).
The characteristic remanent magnetizations were determined after inspection
of orthogonal projections (Zijderveld,
1967
) using principal component analysis
(Kirschvink,
1980
). Site mean directions based on four to ten sample results were
calculated using Fisher statistics (Fisher,
1953
). The paleomagnetic analysis was
performed using the PaleoMac software (Cogné,
2003
) or R. J. Enkin
'
s (Enkin,
1994
) paleomagnetic software packages.
The quality of the paleomagnetic record differs from
ow. In most
samples, from all studied sections two components of magnetization can be
isolated. A low-temperature component, destroyed by heating at 200
-
250
C,
has a direction close to the modern geomagnetic
ow to
field. We therefore consider this
component as a recent viscous overprint and do not discuss it further. The second,
high-temperature component decays to the origin of an orthogonal vector diagram
between 200
-
600
C(
Figure 5.3a
,
d
,
g
,
j
).
We sampled 20 lithic clasts from a tuff layer from the lower part of the
Arydzhangsky Formation. In 19 of them we isolated the high-temperature com-
ponent of magnetization, which decays to the origin of the Zijderveld diagram
between 350 and 560 ºC. The distribution of vectors of this component is shown in
Figure 5.3c
. The length of the resultant vector,
R
¼
2.33, is much less than the
critical (at the 95% con
19.
Therefore, the result of the conglomerate test is positive, and indicates a primary
origin of high-temperature component isolated in the Arydzhangsky Formation.
The lowermost Arydzhangsky Formation (sometimes referred to as the Khar-
dakhsky Formation (Egorov,
1995
)) in the lower part of the composite Kotuy
section has a reversed polarity (
Table 5.1
and
Figure 5.3f
). Above this, the larger
part of the Arydzhangsky and the lowermost
dence level) value
R
o
¼
6.98 (Watson, 1956) for
N
¼
flows of the Onkuchaksky forma-
tions yield a normal polarity, while the overlying middle and upper parts of the
Onkuchaksky Formation are again reversely magnetized.
A reversal test (McFadden and McElhinny,
1990
) applied to the whole section
indicates some minor non-antipodality (see
Figure 5.3b
) that could result either
