Inclination Shallowing in Sedimentary Rocks: Evidence, Mechanism and Cause - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 35

could not determine whether a sediment's inclination
changed during volume loss. The Hamano device
worked by dripping water into a Plexiglas tank that
vertically loaded a sediment slurry sample. The highest
pressure that the device could reach was c. 0.15 MPa;
higher pressures (up to 2.53 MPa) were achieved with
a soil test consolidometer. Anson and Kodama used
distilled pore water and kaolinite clay to mimic a
natural sediment. They added both acicular and equi-
dimensional magnetite to the clay to see the effects of
magnetic particle shape on any inclination shallowing
that they observed. The clay/magnetite slurries were
compacted slowly in the laboratory so that the pore
fl uid could escape fast enough to avoid over-pressuring
the samples, thus ensuring that the volume loss would
mimic natural dewatering as closely as possible.
To magnetize the samples, the clay-magnetite-dis-
tilled water slurry was stirred in magnetic fi elds with
inclinations 20-80° and intensities comparable to the
Earth ' s magnetic fi eld (nominally 50 μT). Stirring was
used to mimic the acquisition of a pDRM, assumed at
that time to be the mechanism of marine sediment
magnetization (Kent 1973). Anson and Kodama also
stirred a sample in a nearly zero intensity fi eld (several
nT) and found that the sediment acquired a magnetiza-
tion at least two orders of magnitude smaller.
The main results of these experiments were that the
samples lost 32-64% of their volume and inclination
shallowed by c. 10 - 12 ° for initial inclinations close
to 45°. Anson and Kodama observed a tangent-
tangent relationship between initial inclination and
compaction-shallowed inclination following Blow &
Hamilton (1978) who followed King (1955):

tions from these experiments was Anson and Kodama's
suggestion of a mechanism for compaction-caused
inclination shallowing. They proposed that magnetite
particles were sticking to clay particles by electrostatic
attraction and were constrained to move with the clay
particles as they rotated into the horizontal due to the
vertical loading of the sediment slurry. This model was
at odds with the ideas of the time which argued that,
as sediment volume and its pore spaces decreased
during compaction, the largest magnetic particles
would be affected earlier than the smallest magnetic
particles and thus should have greater amounts of
inclination shallowing. Anson and Kodama's electro-
static sticking model predicted exactly the opposite
behavior: the smallest magnetic particles would be
the easiest to stick to clay particles and thus should
experience the most shallowing. Alternating fi eld
demagnetization could separate the effect of compac-
tion shallowing for large (low coercivity) and small
(high coercivity) magnetite particles. In fact, alternat-
ing fi eld demagnetization of the samples in Anson and
Kodama's experiments showed that in two-thirds of
the runs the smallest (highest coercivity) magnetite
particles were fl attened the most during compaction.
Deamer & Kodama ' s (1990) laboratory compaction
experiments followed those of Anson and Kodama's.
Deamer and Kodama's work was designed to test the
' electrostatic - sticking model ' for inclination shallow-
ing. In these experiments, four different single-clay
synthetic sediments were compacted in the Hamano
water tank consolidometer, as well as two natural
marine sediments from off the coast of Oregon. Acicu-
lar and equi-dimensional magnetite of grain size
0.5 μm was also used for the single-clay synthetic sedi-
ments, as in the Anson and Kodama experiments. The
main difference in the experiments was the use of
saline pore waters to mimic natural marine sediments.
In fact, Deamer and Kodama used Instant Ocean™ as
the pore fl uid. It is used for saltwater aquariums and
has the same salts in the same proportion as ocean
water.
Deamer and Kodama also varied the pH in their
experiments to manipulate the electrostatic sticking
effect. The pH of natural marine waters causes clay
particles to have negative electric charges on their
surface and for magnetite particles to have positive
electric charges. The 'zero point of charge' for magnet-
ite occurs at a pH of 6.5. For pH values < 6.5 magnetite
has positive surfaces charges; for pH values > 6.5 it has
negative charges. Altering the pH of the sediment

tan I =− ( 1 a Δ V )tan I

c 0

where I c is the compacted inclination, Δ V is the volume
lost, I 0 is the initial inclination and a is an empirical
factor related to acicular or equi-dimensional magnet-
ite, of value c. 0.55 for their experiments. For a more
useful comparison to the other inclination-shallowing
work described in this chapter, Anson and Kodama's
experiments found f = 0.61 for acicular magnetite and
f = 0.70 for equi - dimensional magnetite where
tan I f = f tan I 0 and f is the fl attening factor of King
(1955) , I f is fi nal inclination and I 0 is the initial
inclination.
Aside from the main result that volume loss like that
experienced by natural sediments could cause signifi -
cant inclination fl attening, one of the main contribu-