The Magnetization of Sedimentary Rocks: Processes and their Interpretation - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 133

extracts can also be examined by energy-dispersive
spectrometry (EDS) or microprobe to more directly
determine their chemical make-up, for example
whether they contain sulfur, to aid in their identifi ca-
tion. X-ray diffraction of the extract, if there is enough
of it, can also be important to magnetic mineral
identifi cation.
To check the effi ciency of the magnetic extraction, it
is useful to compare a rock magnetic measurement
(e.g. an IRM acquisition curve) for the whole rock and
for a sample made up from the magnetic extract.

maximum inaccuracies in paleolatitude by about the
same order of magnitude.
A sedimentary rock's remanence can also be
defl ected by grain-scale tectonic strain. Theoretically
the size of the defl ection can be quite large, particularly
if the magnetization behaves in simple shear strain as
if it were carried by actively rotating rigid particles. If
the magnetization behaves more as a passive line, then
it can only rotate as far as the shear plane. For a fl exu-
ral slip fold, the shear plane is the bedding plane so
inclinations would not change their sign for passive
line strain behavior. Most paleomagnetists would avoid
sampling severely strained sedimentary rocks, so the
size of tectonic defl ection is more likely to be of the
order c. 10° if it has occurred at all in rocks typically
sampled for paleomagnetism.
One way to check for strain defl ection of remanence
is to compare the paleomagnetic directions in a rock
with rock strain measured by standard structural
geology techniques, such as R f - ϕ or center - to - center,
across a region or locale of varying strain. Another
possible indicator of strain-induced remanence defl ec-
tion is a syn-folding magnetization, although the sim-
plest interpretation of a syn-folding magnetization is a
secondary remagnetization that occurred during
folding. The observation of a syn-folding magnetiza-
tion should be followed up with rock strain measure-
ments to check for the possibility of remanence
defl ection by strain.
As detailed in Chapter 6, chemical remanence that
has resulted from remagnetization or partial remag-
netization of a sedimentary rock is of course an impor-
tant source of inaccuracy of a paleomagnetic result.
The CRM resulting from a remagnetization is not inac-
curate per se - it is probably a very faithful record of the
geomagnetic fi eld at the time of remagnetization - but
the age of magnetization is quite different from the
depositional age of the rock. Fold tests and other fi eld
tests can be used to check for secondary magnetiza-
tions. Examination of a rock's magnetic minerals,
either in situ or in a magnetic extract, can determine
whether they are secondary in nature from their com-
position or from their morphology.
Viscous remanent magnetization (VRM) is another
source of inaccuracy in a paleomagnetic direction, if it
contributes to the fi nal demagnetized paleomagnetic
direction. Viscous remanence is a secondary magneti-
zation acquired by rock over time when it is exposed to
a post-depositional magnetic fi eld. The mechanism of
VRM acquisition is discussed in detail by both Butler

ACCURACY OF THE PALEOMAGNETIC
DIRECTION: DEFLECTION DUE TO
PHYSICAL PROCESSES

Chapter 4 shows that burial compaction and some-
times syn-depositional processes can cause the defl ec-
tion of a sediment's or sedimentary rock's DRM or
pDRM inclination toward the horizontal, i.e. inclina-
tion shallowing. Since magnetite is unquestionably a
depositional magnetic mineral, magnetite-bearing lake
and marine sediments and sedimentary rocks should
be checked for the effects of inclination shallowing
once the characteristic magnetization has been iso-
lated by demagnetization and the characteristic rema-
nence has been shown likely to be primary by fi eld
tests. The anisotropy of remanence applied to the
characteristic remanence - carrying magnetic particles,
usually with an ARM, is a relatively straightforward
way to check for inclination shallowing and to correct
it. If there are enough sites (>100) then the EI tech-
nique (Tauxe & Kent 2004) is another very good way
to check for and correct any inclination shallowing in
a magnetite - bearing rock.
Hematite-bearing red beds have also been shown to
suffer from inclination shallowing; however, it is not
always clear that the magnetization of red beds is a
primary depositional remanence. Red bed remanence
could have been acquired early enough after deposi-
tion, by an early CRM for example, that it was subse-
quently affected by burial compaction. The anisotropy
of remanence technique, this time using an IRM
applied to the high-coercivity hematite particles, or the
EI technique can be used to correct any inclination
shallowing found in the red beds.
For magnetite and hematite-bearing sedimentary
rocks the magnitude of inclination shallowing effect is
about the same, ranging c. 10 - 20 ° , enough to give