Inclination Shallowing in Sedimentary Rocks: Evidence, Mechanism and Cause - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 38

mechanism, it did show that signifi cant shallowing
occurred at low pressures and high porosities when the
pore spaces would be larger than the 0.5 μm sized mag-
netite particles so some kind of clay-magnetite sticking
mechanism was still probably occurring. They postu-
lated that van der Waals forces could be responsible.
Sun & Kodama (1992) followed Deamer and Koda-
ma's laboratory compaction experiments with more
comprehensive experiments to test the mechanism for
inclination shallowing, specifi cally the clay-magnetite
sticking model. These experiments again used the
Hamano water tank consolidometer to compact syn-
thetic, single-clay sediments and natural marine sedi-
ments. The single-clay sediments used kaolinite and
illite, 0.5 μm acicular magnetite and distilled water.
The natural marine sediments contained about 50%
clay content and had 2-3 μm natural magnetite. In the
compaction experiments, saline pore water was used
for the natural sediments. In all experiments at least
50% volume decrease occurred with up to 0.16 MPa of
pressure. Inclination fl attening and intensity decreases
similar in magnitude to the previous experiments
(Anson & Kodama 1987 ; Deamer & Kodama 1990 )
were observed, but some additional important observa-
tions were made. One of the most important came
when the compacted and uncompacted sediments
were critically point dried (to avoid surface tension
effects during evaporative drying) and examined with
a scanning electron microscope (SEM). These pictures
clearly showed acicular magnetite grains sticking to
clay particles in high-porosity sediments in which the
pore spaces were much larger than the magnetite par-
ticles (Fig. 4.4 ).
In addition to this important observation were
observations that provided a stronger hint about the
microscopic mechanism of inclination shallowing. The
inclination shallowing was not a simple linear function
of void ratio (or volume) decrease. Most of the shallow-
ing occurred at the lowest pressures (0.02-0.03 MPa),
where the greatest volume losses occurred. During this
period, volume loss occurred as pore spaces decreased
in size with little reorientation of the clay grains
making up the sediment. At higher pressures, inclina-
tion shallowing and volume loss occurred at a slower
rate with increasing pressure. Here the mechanism of
shallowing was the reorientation of clay grains into
the horizontal with magnetite particles attached to
clay grains and incorporated into clay domains. Clay
domains develop in natural clay-rich sediments and
are part of the clay fabric (Bennett et al . 1981 ). Clay

Fig. 4.3 Results of cross-correlation between Site 522
paleointensity results (Tauxe & Hartl 1997) and inclination
record. Negative cross-correlations at depths between
7000 cm and 10 000 cm could be due to signifi cant porosity
loss at these depths and high enough clay content to make
compaction effects on intensity apparent.

quite high and at 100 m depth increases to greater
than 93%. We will show later in this section that clay
content is an important factor controlling compaction-
caused inclination shallowing. Clay content as low as
15% has been shown to cause inclination shallowing
in Cretaceous marine sedimentary rocks from south-
ern California (Tan & Kodama 1998). The clay content
is c. 15% between depths of 60 and 100 m in the Site
522 core. The depth range of 70-100 m is perfect for
observing if inclination is anti-correlated with paleoin-
tensity, and the results from this one core suggest that
variable amounts of compaction-caused intensity
decrease can affect the paleointensity record. In most
cases however the effect is probably small or insignifi -
cant because of small variations in inclination and
large genuine changes in paleointensity, which tend to
minimize or swamp the compaction effect. The paleoin-
tensity record, stacked over the past 800 kyr (Guyodo
& Valet 1999), shows good agreement over the world
ocean suggesting that compaction has not appreciably
affected these records.
Deamer and Kodama also pointed out that although
their results did not support the electrostatic sticking