How to Detect and Correct a Compaction - shallowed Inclination - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 46

5

How to Detect and Correct a
Compaction - shallowed Inclination

As the evidence from laboratory experiments in the
late 1980s and early 1990s began to mount that com-
paction could shallow the inclination of sediments at
burial depths of hundreds of meters, the next logical
step for paleomagnetists was to develop techniques
that could discern if shallowing had occurred in par-
ticular sedimentary rocks and, if it had, to accurately
correct for it. The fi rst papers attempting a correction
used an approach based on the magnetic anisotropy of
sedimentary rocks. They are, in order of appearance,
Hodych & Bijaksana (1993), Collombat et al . (1993)
and Kodama & Davi (1995) . Tauxe & Kent ' s (2004)
elongation-inclination (EI) technique came along
about a decade later and uses a totally different and
independent approach for identifying and correcting
inclination shallowing in sedimentary rocks. We will
consider the EI technique later in this chapter after we
have discussed the anisotropy technique; we will then
compare the two techniques.

bulk magnetic anisotropy of a rock sample is deter-
mined from measurements, made in different orienta-
tions, of the ease with which a rock or sediment sample
is magnetized by an applied magnetic fi eld. The
measurements made in different orientations are fi t
to a second-rank tensor that describes the three-
dimensional relationship between the fi eld applied to
the sample and the resulting magnetization (see Menke
& Abbott 1990 for an excellent physically intuitive
explanation of second-rank tensors). The tensor repre-
sents the magnetic anisotropy of the sample. Ulti-
mately, the bulk anisotropy depends on two factors: the
preferred alignment of the magnetic particles in the
rock and the magnetic anisotropy of the individual
magnetic particles in a rock. Without delving too
deeply into the physics of the small (micron and sub-
micron) magnetic particles that carry the paleomag-
netism of sediments and sedimentary rocks, it is
important to realize that a rock's magnetic particles
have 'easy' directions along which they are typically
magnetized, with a magnetic north pole at one end and
a south pole at the other. For some magnetic minerals
such as magnetite, the physical shape of the particle
dictates the alignment of the magnetic 'easy' axis: it is
parallel to the long axis of the particle. For other mag-
netic minerals such as hematite, the 'easy' magnetiza-
tion axis is controlled by crystallography and typically

THEORETICAL BASIS OF THE
MAGNETIC ANISOTROPY-INCLINATION
CORRECTION

Magnetic anisotropy is simply a measure of the pre-
ferred alignment of magnetic particles in a rock. The