The Magnetization of Sedimentary Rocks: Processes and their Interpretation - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 129

struck by lightning. Since magnetic fi elds fall off in
intensity by the inverse cube of distance, lightning
effects tend to be very localized.
There are other demagnetization techniques that
are much less commonly used, but are sometimes good
for attacking a particular problem. Chemical demag-
netization entails soaking paleomagnetic samples in a
concentrated acid (usually HCl) for increasingly longer
periods of time (up to several months) and measuring
them at increments of increasingly long duration. The
underlying assumption is that the fi nest magnetic
grains are dissolved fi rst by the acid soaking into the
rock, leaving the largest magnetic grains at the longest
periods of chemical demagnetization. This assumption
is reasonable for hematite-bearing red beds because
hematite, the magnetic mineral of red beds, has a rela-
tively large SD-MD critical diameter close to c. 10 -
20 μ m (Dunlop & Ozdemir 1997 ). The submicron
pigmentary hematite grains that give red beds their
color are assumed to be secondary and should be
removed at earlier steps of chemical demagnetization
while the larger micron-sized supposedly detrital hem-
atite grains are removed last. This assumption may not
always be borne out and should be checked by thermal
demagnetization that works quite well for hematite-
bearing red beds. Because chemical demagnetization is
messy and takes a very long time to complete (months)
it is no longer widely used. It is also not always as suc-
cessful as thermal demagnetization in isolating a rock's
primary magnetization. Tan & Kodama (2002) and
Tan et al . (2003) used chemical demagnetization to
isolate the AMS of the hematite grains carrying the
characteristic magnetization (ChRM) of red beds for
inclination shallowing studies.
Microwave demagnetization can be used to preferen-
tially heat the magnetic minerals in a rock without
appreciably heating the whole specimen and causing
unwanted mineralogic changes. Samples are exposed
to microwave radiation in a resonant cavity for several
seconds in microwave demagnetization. The micro-
wave radiation affects the spin moment alignment of
the Fe atom electrons that generate the spontaneous
magnetization of a magnetic mineral. The technique is
more typically used to apply remanences to rocks for
absolute paleointensity measurements. In order to
demagnetize a sample with microwaves, some energy
is passed to the surrounding non-magnetic minerals
and heats the specimen to temperatures of 150°C or
less (Shaw 2007).

In laser selective demagnetization (LSD; Renne &
Onstott 1988) magnetic minerals are selectively
removed in thin section by laser pulses, thus allowing
the removal of specifi c magnetic phases and the
determination of what subpopulation of magnetic
grains carries what component of remanence. This
can be useful for unraveling the relative ages of multi-
component magnetizations in a rock, since cross-
cutting relationships between different magnetic
phases can be observed by microscopic examination of
thin sections.
Once progressive demagnetization has been com-
pleted for a collection of samples from a sedimentary
rock and the characteristic magnetization of the rock
has been isolated, several laboratory measurements
need to be made to check the age, the source and the
accuracy of the characteristic remanence. These tests
are the focus of this chapter.

CONSTRAINING THE AGE OF
MAGNETIZATION

Graham's fold test is an important fi eld test that paleo-
magnetists use to constrain the age of magnetization
in a rock. After demagnetization is completed and the
characteristic remanent magnetization (ChRM) has
been isolated, it is crucial to test whether the magneti-
zation is in fact primary or at least nearly the same age
as the age of the sedimentary rock. The fold test has
been discussed already in Chapter 7 as grain-scale rock
strain can affect the interpretation of the fold test, par-
ticularly the syn-folding magnetization that occurs
when the paleomagnetic directions on either limb of a
fold cluster best when the bedding tilts are only par-
tially removed during the test. Basically, the fold test is
conducted by sampling both limbs of a fold or region-
ally sampling beds with different bedding orientations.
If the magnetization clusters better in 'untilted' coor-
dinates when the limbs have been restored to their
horizontal pre-folding orientation, the magnetization
was acquired before folding occurred. If the magnetiza-
tion clusters better before the beds are 'untilted', the
magnetization was acquired some time after tilting of
the beds. Chapter 7 considers in detail the case when
the magnetization clusters best about half-way in
between. The most straightforward interpretation is of
course that the magnetization was acquired during
folding, but strain can have an effect.