How to Detect and Correct a Compaction - shallowed Inclination - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 48

Kodama 1997, described below) and similar results
from the EI correction technique of Tauxe & Kent
(2004) and the anisotropy technique for the same rock
units suggests that the commonly observed horizon-
tally foliated fabric in natural sediments is probably
always due to some amount of inclination shallowing,
whether it is caused by compaction or mis-orientation
of the magnetic grains at deposition.

EARLY INCLINATION-SHALLOWING
CORRECTIONS

In the fi rst inclination-shallowing study in the litera-
ture, Hodych & Bijaksana (1993) studied Cretaceous
limestones collected from fi ve equatorial Pacifi c DSDP
cores. Hodych and Bijaksana closely followed the
theoretical reasoning of Jackson et al . (1991) for an
inclination-shallowing correction using magnetic
anisotropy. The inclination of their samples was on
average 17° shallower than the expected paleofi eld
direction of 44 ° ( f = 0.53). Magnetic anisotropy was
measured using anhysteretic remanent magnetiza-
tions (ARM) applied to their samples in nine different
orientations.
ARM is a laboratory-induced magnetization that
can activate magnetic minerals such as magnetite with
relatively low magnetic 'hardness' or coercivity. Mag-
netic hardness is a measure of how easy or how diffi -
cult it is to change the magnetization of a sample by
the application of a magnetic fi eld. Samples with high
coercivities or high magnetic hardness need compara-
tively strong magnetic fi elds to alter their magnetiza-
tion. ARM is a special manner of applying a magnetic
fi eld using a combination of alternating and direct
magnetic fi elds. ARM is used in many experiments to
measure the magnetic anisotropy of magnetite-bearing
rocks.
Hodych & Bijaksana (1993) were able to correct the
shallow magnetization of the limestone samples to the
expected paleofi eld direction using the ARM magnetic
anisotropy of their samples. Following Jackson et al . ' s
theoretical approach, they measured the individual
particle anisotropy of their samples as well as the bulk
anisotropy. Hodych and Bijaksana also looked at the
degree of compaction of their samples based on the
samples' porosity, and determined that compaction
was the cause of the inclination fl attening. Despite this
pioneering effort of using Jackson et al . ' s approach to
correct inclination shallowing, Hodych did not pursue

Fig. 5.2 Increase in magnetic anisotropy of natural
marine sediment compacted in the laboratory by Sun &
Kodama (1992). Although the magnetic anisotropy was not
measured at 0 pressure, the extrapolation of the data back
to 0 pressure indicates that the anisotropy was near to 0%.
W-W Sun and KP Kodama, Magnetic anisotropy, scanning
electron microscopy, and x ray pole fi gure goniometry study
of inclination shallowing in a compacting clay-rich
sediment, Journal of Geophysics Research , 97, B13, 19599-
19615, 1992. Figure 7, page 19608. Copyright 1992
American Geophysical Union. Reproduced by kind
permission of American Geophysical Union.

technique can give accurate results. The sediments
in Sun and Kodama's experiments were all stirred to
obtain an analogue of a pDRM acquired during
bioturbation in marine sediments. Slow settling in a
geomagnetic fi eld may give an entirely different initial
pre-compaction magnetic fabric. Recent work by
Schwehr et al . (2006) supports low magnetic anisot-
ropy at the deposition of natural sediment. They meas-
ured the anisotropy of magnetic susceptibility (AMS)
of sediment downcore in the Santa Barbara basin and
observed very low-intensity AMS fabrics for the surface
sediments.
The assumptions in the Jackson et al . (1991) theo-
retical model for an anisotropy-based inclination cor-
rection could lead to overcorrection for an initial
horizontally foliated fabric associated with an accurate
syn-depositional inclination. However, the success
of the anisotropy-based inclination correction (see