How to Detect and Correct a Compaction - shallowed Inclination - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 47

tan I = f tan I

0 c

where f is the fl attening factor, I c is the compacted incli-
nation and I 0 is the initial, pre-compaction inclination,
Jackson et al . have essentially shown that f is related to
the bulk magnetic anisotropy and the individual parti-
cle anisotropy of the rock:

( ) −

Ka
Ka

+ 21

21

min

f

=

( ) −

+

max

where K min and K max are the minimum and maximum
axes of the ellipsoid used to represent the magnetic
anisotropy (Fig. 5.1) and a is a factor that represents
the anisotropy of the individual magnetic particles.
The factor a is simply the ratio of the magnetization
along the easy axis of an individual particle to the
magnetization along the perpendicular, or hard, axis.
One of the assumptions of Jackson et al . ' s approach is
that the anisotropy starts out very small or non-
existent when sediment is fi rst deposited and becomes
increasingly fl attened, i.e. oblate, with burial compac-
tion. The inclination is also assumed to start out paral-
lel to the ambient geomagnetic fi eld at deposition and
become shallower in a regular way as compaction pro-
ceeds. In Jackson et al .'s model there should be a direct
relationship between the development of the preferred
alignment of magnetic particle axes, the degree of
magnetic anisotropy and the amount of inclination
fl attening.
Sun & Kodama ' s (1992) laboratory compaction
experiments can be used to test these assumptions. One
of the sediments compacted in Sun and Kodama's
work was natural marine sediment containing natural
magnetite with a grain size of 2-3 μm. Although the
anisotropy of this sediment was not measured at
0 MPa of overburden, the pattern established between
anisotropy and compaction at low pressures indicates
that the anisotropy would extrapolate back near to 0
before compaction started (Fig. 5.2 ).
For this sediment the amount of shallowing at
0.15 MPa was 11.5° and the initial inclination 45°.
Subsequent work done on natural marine sediments
suggests that for natural magnetite the most typical a
value observed is a = 2. Using these parameters in
Jackson et al .'s correction equation gives an initial
inclination of 45°, exactly what was measured in
the laboratory at deposition. The most naturally realis-
tic sediment compacted in the laboratory shows
that the anisotropy-inclination shallowing correction

Fig. 5.1 Easy magnetic axes for hematite hexagonal plates
(top left) and for elongate magnetite particles (bottom left).
Oblate and prolate ellipsoids (right) graphically used to
represent the orientation distributions of the easy axes of
magnetic particles, shown by short lines.

lies in the basal crystallographic plane. The anisotropy
of an individual particle is a measure of how easy it is
to magnetize a particle along its 'easy' axis compared
to being magnetized perpendicular to it (or along a
particle ' s ' hard ' axis).
Paleomagnetists characterize different kinds of mag-
netic anisotropy, also called magnetic fabrics, as either
oblate or prolate because the second-rank tensor can
be represented graphically by an ellipsoid. Oblate mag-
netic fabrics are represented by 'fl attened ' ellipsoids
with magnetic particles lying closer to a plane (called
the foliation plane), while prolate fabrics are 'lineated'
with magnetic particles lying closer to a line (called the
lineation). Sometimes pancakes are used to visualize
oblate fabrics and cigars to represent prolate fabrics
(Fig. 5.1). For sedimentary rocks affected by compac-
tion, oblate fabrics lying parallel to the bedding plane
are typically observed.
Jackson et al . (1991) is an infl uential paper that pre-
sented the theoretical basis for the use of magnetic
anisotropy measurements to identify and correct shal-
lowed inclinations in sedimentary rocks. Jackson et al .
developed a theoretical relationship between the orien-
tation distribution of magnetic particle easy axes and
inclination shallowing. They assume that the orienta-
tion distribution can be measured by the magnetic
anisotropy of a sedimentary rock. In the King (1955)
relationship: