Geology Reference
In-Depth Information
polarity interval in continental sedimentary rocks with
an average sedimentation rate of 100 m/million years
would require a stratigraphic sampling interval of
about 5-10 m. However, for much more slowly depos-
ited marine sediments with a sediment accumulation
rate of 10 m per million years or less, the sites must be
spaced at least every 75 cm. At the other end of the
timescale for a magnetostratigraphic study, at least
10-20 polarity intervals need to be observed for a good
fi t to the geomagnetic polarity time scale (Butler 1992),
so at least 2 million years of rocks should be sampled.
Because many more sites are collected in a magneto-
stratigraphic study compared to a paleomagnetic pole
study, only 3 or 4 samples are collected per site.
For secular variation studies of the geomagnetic
fi eld, lake or marine sediments are sampled nearly con-
tinuously in a core. The fi ne - grained sediments meas-
ured usually ensure a fairly continuous sedimentation
rate, important for observing in detail changes in the
geomagnetic fi eld direction and intensity.
Field tests for the stability and age of the remanence
are always critical to a good paleomagnetic study so
sampling the limbs of folds, or at least strata with dif-
ferent tectonic tilts, is made whenever possible. Sam-
pling the baked zone around an igneous intrusion and
the unbaked rocks further away from the intrusion for
a baked contact test, or sampling the clasts and matrix
of a conglomerate layer for a conglomerate test, are
also important if the opportunity arises.
these topics for the theoretical and practical details of
each type of demagnetization. The underlying assump-
tion of demagnetization is that secondary magnetiza-
tions acquired by sedimentary rocks since the time of
their deposition have either low coercivities or low
unblocking temperatures, so progressive demagnetiza-
tion is designed to isolate the magnetization direction
of the oldest, primary magnetization of a rock. This
assumption is not always borne out and needs to be
rigorously tested with fi eld tests that constrain the age
of a rock's magnetization, i.e. fold tests, baked contact
tests or conglomerate tests.
When detailed progressive demagnetization is
applied to a rock, typically for ancient sedimentary
rocks, multiple components of magnetization are
removed at different ranges of unblocking tempera-
tures or alternating fi elds (Fig. 9.1). The smaller the
incremental steps of demagnetization, the easier it is to
resolve the different components of magnetization in a
rock. It is often advisable to be conservative in designing
a demagnetization strategy and space demagnetization
steps relatively closely together because, once a rock
has been demagnetized at a particular fi eld or tempera-
ture, that magnetization has been removed and the
rock can never be measured again at lower steps. This
last statement is perhaps obvious but it cannot be over-
stated: once a magnetization has been demagnetized,
it's gone. When demagnetizing a rock it is particu-
larly important to isolate the highest coercivity or
unblocking-temperature magnetization component in
a rock, i.e. the component that is removed just before
the rock is completely demagnetized or the Curie
temperature is reached. This is the most stable magneti-
zation in a rock and is most likely to be the most ancient,
perhaps the primary magnetization. Because it is not
necessarily the primary magnetization, however, the
last component of magnetization removed is denoted
the characteristic remanent magnetization (ChRM).
A robust paleomagnetic result depends on the meas-
urement of many samples from many sites from a
sedimentary rock unit. Typically 8-10 samples are col-
lected at each paleomagnetic site (less for a magneto-
stratigraphic study), and at least 15-20 sites are
collected from a sedimentary formation for a reasona-
bly well-constrained paleomagnetic pole. Many more
sites are collected for a magnetostratigraphic study.
There is always a tradeoff between minimizing the
confi dence limits around the mean paleomagnetic
direction obtained by measurement and demagnetiza-
tion of the multiple samples from a site and the multi-
STANDARD LABORATORY
MEASUREMENTS AND ANALYSIS
Standard demagnetization techniques are used to
remove secondary magnetizations acquired by rocks.
In either of the two widely used techniques - alternat-
ing fi eld demagnetization and thermal demagnet-
ization - the magnetization of a rock is removed
incrementally either by increasingly higher alternat-
ing magnetic fi elds or increasingly higher tempera-
tures. The magnetizations for subpopulations of
magnetic grains in the rocks with either coercivities or
unblocking temperatures affected by the demagnetiza-
tion fi elds or temperatures are essentially randomized
by the procedure and no longer contribute to the mag-
netization of the rock when it is measured at each of
the demagnetization steps. Both Butler (1992) and
Tauxe (2010) provide excellent detailed treatments of
progressive demagnetization; the reader is referred to
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