How to Detect and Correct a Compaction - shallowed Inclination - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 58

Central Asia had presented a geological problem for
paleomagnetic data because there was no evidence of
over 1000 km of intra-continental shortening to the
north.
Another approach that was used to measure the
fabric of red bed hematite grains employed an isother-
mal remanent magnetization (IRM) and many high-
temperature heatings. The IRM was applied in high
fi elds ( c. 1 T) in the same orientations used in the ARM
anisotropy experiments. The IRM application was fol-
lowed by a thermal demagnetization at about 630°C
for each orientation to measure the IRM carried by the
same hematite grains that carried the ancient mag-
netization isolated by standard thermal demagnetiza-
tion of the samples. To apply an IRM, the rock sample
is simply exposed to a DC magnetic fi eld. It is a different
way of applying a laboratory remanence and can
reach higher fi elds than an ARM application, thus
allowing activation of very high-coercivity hematite
grains. After measurement of the thermally demagnet-
ized IRM, the samples were further thermally demag-
netized at 690-700° to totally remove the IRM applied
at each orientation before moving on to the next orien-
tation. This approach was used in the Tan et al . (2007)
study of the Triassic Passaic Formation red beds of
New Jersey and requires many high-temperature heat-
ings that can cause chemical and mineralogical altera-
tion of the hematite grains. Despite this, the study of
the Passaic Formation red beds gave very reasonable
results and provided an important comparison to the
EI correction technique (see next section).
Bilardello & Kodama (2009a) developed one more
way to measure the remanence anisotropy of the high-
coercivity hematite grains in red beds. Their approach
was initially proposed by Kodama & Dekkers (2004),
but Bilardello & Kodama (2009a) provided a full devel-
opment and test of the procedure. Kodama and Dekkers
applied IRMs in very high fi elds (13 T) at a special
strong magnet facility in Nijmegen, Holland so that no
demagnetization was necessary between orientations;
the magnetization of the grains was totally reset at
each orientation. Obviously, this approach was not
practical for most paleomagnetic studies since only
four of these strong magnet facilities exist in the world.
Bilardello & Kodama (2009a) used very small samples,
so that a standard impulse magnetizer with a high fi eld
coil could be used to apply 5 T IRMs to samples in the
different orientations used to determine the anisotropy
of isothermal remanence (AIR). Using an alternating
fi eld demagnetization, Bilardello and Kodama then

demagnetized the samples at 100 mT to remove the
contribution of any magnetite that might be in the
samples. Rock magnetic measurements had shown the
presence of magnetite in addition to the hematite typi-
cally found in red beds (Kodama & Dekkers 2004;
Bilardello & Kodama 2009a ; Kodama 2009 ). Finally,
Bilardello and Kodama used low-temperature thermal
demagnetization at 120°C to remove the contributions
due to goethite, a secondary magnetic mineral that can
be common in red beds. The high fi elds used in these
experiments to totally reset the IRM at each orientation
would also activate any goethite present.
The technique presented in Bilardello & Kodama
(2009a) was successfully used to isolate the rema-
nence anisotropy for inclination corrections of the
Carboniferous red beds of the Maritime Provinces of
Canada (Bilardello & Kodama 2009b ). In this study,
the inclinations of the Shepody Formation and
Maringouin Formation were corrected (Shepody:
f = 0.64; Maringouin: f = 0.83) to provide an improved
Carboniferous paleomagnetic pole for North America.
Interestingly, the corrected paleopoles for the nearly
coeval Shepody and Maringouin Formations fall almost
exactly on top of each other while the uncorrected
poles were several degrees apart. The Shepody and
Maringouin results could be compared to a magnetite
correction of the Glenshaw Formation of western
Pennsylvania and a hematite correction of the Mauch
Chunk Formation red beds of eastern Pennsylvania
(Kodama 2009 ; Bilardello & Kodama 2010a ). The cor-
rected and coeval paleopoles were found to be in good
agreement. The Mauch Chunk correction reported by
Bilardello & Kodama (2010a) used the high fi eld AIR
technique of Bilardello & Kodama (2009a) and had
less of a correction ( f = 0.49) than the chemical
demagnetization correction of the Mauch Chunk
( f = 0.22) by Tan & Kodama (2002) . The high fi eld AIR
correction seems to be the best way to make an anisot-
ropy correction to hematite-bearing red beds because
it involves less potential mineralogical alteration of the
magnetic grains by either acid leaching or repeated
high temperature heating.

A SIMPLIFIED ANISOTROPY
CORRECTION

One of the most diffi cult aspects of the anisotropy-
inclination shallowing correction for red beds is the
determination of the individual particle anisotropy,