Environmental Engineering Reference
In-Depth Information
field recorded in the sections allow clear
correlation among these sections (
Figure 5.5
). The correlation indicates that at least
one-quarter of the Norilsk region volcanic sequence was likely formed during a
relatively short time interval at the time of a reversal of the geomagnetic field.
Studies converge on estimates of the duration of a geomagnetic field reversal of
between 5000 and 10 000 years, though durations as long as 20
-
30 thousand years
have been suggested (Merrill
et al
.,
1996
). Recently, Valet
et al
.(
2012
) analyzed
similarities of several well-recorded reversals of different ages. Three phases can
be observed during a reversal: a precursor, the transit, and a rebound. In our case,
only the transit and rebound are present, as in the Karoo Traps (Moulin
et al
.,
2011
). Valet
et al
.(
2012
) have estimated that the transit and the rebound may have
not exceeded 1 kyr and 2.5 kyr, respectively. Thus, our data indicate that a
signi
Thus, peculiarities of the geomagnetic
cant part (up to 1200 m thick) of the volcanic sequence of the Norilsk region
may have been formed over periods as short as several thousand years and even
shorter.
It must be noted, however, that at this stage of the study, we cannot completely
exclude the possibility that the observed transitional and excursional intervals are
linked with independent geomagnetic events that do not constitute parts of the same
reversal. If so, the time estimation above should be considered very conservative.
It is interesting to note that the transitional interval (referred to as Group 2 by
Gurevitch
et al
.(
2004
)), corresponding to the Gudchikhinsky and Syverminsky
formations, is characterized by directions with, on average, smaller inclinations
than those of the overlying intervals. We can therefore try to make correlations
between our measurements in this interval and those from magnetostratigraphic
borehole studies (Gurevitch
et al
.,
2004
; Mikhal
tsov
et al
.,
2012
). Indeed, in two
of three paleomagnetically studied boreholes crossing the Gudchikhinsky and
Syverminsky formations (CD28 and HS59, see
Figure 5.1
), mean inclinations
calculated for these formations are close to those obtained for the transitional
interval (Heunemann
et al
.,
2004
; Gurevitch
et al
.,
2004
) and, at a 95% level of
con
'
dence, are smaller than the mean inclinations calculated for overlying forma-
tions. Therefore, the data suggest that a magmatic event corresponding to the
transitional and, most probably, excursional intervals occurred over a large region,
including at least the Kharaelakh, Norilsk and Imangda troughs. Using the total
area of these troughs (more than 18 000 km
2
) and the thickness (
1100 m) of the
transitional and excursional intervals in the studied sections, we obtain a minimum
volume estimate for the lava of 20 000 km
3
(the greater thickness in the centers
of the troughs is not considered, and thus this is a minimum estimate). If the
subdivision of our studied sections is correct, all this volume was erupted during
only a few volcanic pulses and individual eruptions.
~
