How to Detect and Correct a Compaction - shallowed Inclination - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 60

rection (Bilardello & Kodama 2010b) that there was a
15% uncertainty in determining f . This scale uncer-
tainty led only to a 0.31° increase in the 95% cone of
confi dence for the mean corrected direction and a
steepening of the corrected mean inclination by 0.32°.
Therefore, although propagating uncertainties in f
should be calculated for a complete inclination correc-
tion, it appears that error propagation has a very small
effect on the mean corrected direction inclination and
cone of confi dence.

coercivity red beds. Tan et al . (2002) re - deposited red
bed sediment, which spontaneously separated into
fi ne-grained and coarse-grained fractions (see Chapter
4), and observed signifi cant compaction shallowing for
the fi ne-grained fraction and little compaction shal-
lowing for the coarse-grained fraction. The application
of the IRM at 45° (Hodych and Buchan's prescription)
and 60° (the initial inclination during Tan et al . ' s re -
deposition) accurately indicated signifi cant anisotropy
and defl ection for the IRM applied to the fi ne - grained
sediments. The amount of defl ection for the 60° IRM
was similar to the amount of compaction shallowing.
Similarly, the coarse-grained fraction showed little
defl ection of the IRMs.
The results of this experiment are good news for the
Hodych and Buchan approach. Unfortunately however,
for a separate set of re-deposition experiments in which
signifi cant syn-depositional inclination shallowing of
the coarse-grained fraction occurred, the 45° IRM was
unable to detect any anisotropy and shallowing even
though it existed. Tan et al . (2002) further tested the
45° IRM approach on two units corrected by complete
measurement of the anisotropy tensor, the Mauch
Chunk Formation and the Passaic Formation. The 45°
IRM technique worked well for the Mauch Chunk, but
did not detect shallowing for the Passaic.
We will show in the following section that the
Passaic result agrees nicely with an EI correction for
the same unit, so it is particularly troubling that the
45° IRM method doesn't detect any shallowing for
the Passaic Formation. The 45° IRM approach appears
to be particularly sensitive to magnetic particle grain
size and can give false negative results (i.e. no shallow-
ing when there is some). If the technique detects
shallowing it is probably correct, but if it indicates
no shallowing this negative result should be checked
by the measurement of the complete anisotropy
tensor.
Schmidt et al . ' s (2009) inclination shallowing study
of the Snowball Earth deposits in Australia (Table 5.1)
and the Eltanin and Nucaleena formations is similar in
that application of high fi eld IRMs (12 T) parallel and
perpendicular to bedding did not show signifi cant ani-
sotropy or shallowing. Although this study was more
complete than a simple 45° IRM application, it perhaps
should be further tested with the measurement of a
complete anisotropy tensor. Schmidt et al . (2009) did
check their results with application of the EI technique
and it yielded slightly steeper corrected inclinations (by
5°) than the three-axis application of an IRM, suggest-

THE 45° IRM CORRECTION

Tamaki et al . (2008) and Wang & Yang (2007) are two
of the three remaining inclination-shallowing correc-
tion studies (Table 5.1) that have not yet been dis-
cussed. Both Tamaki et al . and Wang and Yang used
Hodych & Buchan's (1994) approach of applying an
IRM at 45° to bedding to see if the rock had a strong
enough remanence anisotropy to defl ect its remanence
from the geomagnetic fi eld inclination during the
rock's acquisition of its paleomagnetism. Tamaki et al .
studied magnetite-bearing rocks and Wang and Yang
studied hematite-bearing rocks. Hodych and Buchan
used the technique to study the Silurian Springdale
Group red beds of Newfoundland. They saw little
defl ection of the IRM they applied and concluded that
inclination shallowing had not occurred. Stamatakos
et al . (1994) commented on this study, indicating that
the uncorrected remanence of the red beds was much
shallower than the magnetization of nearby volcanics
of the same age and did not give a reasonable paleogeo-
graphic interpretation for the Silurian. Smethurst &
McEnroe (2003) restudied the paleomagnetism of the
Springdale volcanics and red beds showing that, with
more rigorous demagnetization, the difference in incli-
nation between the volcanics and red beds disappeared,
thus vindicating the conclusion of Hodych & Buchan
(1994) that inclination shallowing had not affected
these red beds.
Tan et al . ' s (2002) re - deposition work with red bed
material from China was designed to test Hodych and
Buchan's 45° IRM approach for detecting inclination
shallowing. The test is much easier to conduct than the
time- and labor-intensive measurement of a rock's
complete remanence anisotropy with many applica-
tions of laboratory remanences in different orienta-
tions. It would also allow a typical paleomagnetism
laboratory to easily check the anisotropy of high-