How to Detect and Correct a Compaction - shallowed Inclination - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 56

extend back in time 200 million years. Before that
time, paleogeographic reconstructions of the conti-
nents rely heavily on averaging of individual paleo-
magnetic poles in different time periods and, as we
have shown, many of those paleopoles are derived from
red beds. In Besse & Courtillot's (2002) construction of
synthetic APWPs for the continents, errors that may
occur in one type of lithology (i.e. sedimentary inclina-
tion shallowing) are probably minimized by averaging
sedimentary and igneous paleopoles. However, any
errors are still buried in the data and probably cause a
small bias in the synthetic paths, particularly when
they're dominated by sedimentary rock results.
The validity of an inclination-shallowing correction
for red beds depends on whether the red beds acquired
their paleomagnetism at or soon after deposition. Some
paleomagnetic studies have suggested that hematite-
bearing red sedimentary rocks carry a depositional
remanence. If this is the case, red bed remanence prob-
ably acquires a magnetization signifi cantly shallower
than the geomagnetic fi eld. The natural and laboratory
re-deposition of the Siwalik Group hematite-bearing
sediments studied by Tauxe & Kent (1984), already
discussed in Chapter 2, showed approximately 25° of
syn-depositional inclination shallowing. A red bed
depositional remanence would also be susceptible to
burial compaction. In fact, the inclination of hematite-
bearing red beds may be affected more than magnetite-
bearing sediments because a red bed depositional
remanence could be fl attened by both depositional and
compaction processes. The reason for the possibility of
greater shallowing in red beds is that, in contrast to
magnetite particles, relatively large hematite grains
(5 - 20 μm) are stable uniformly magnetized grains
(called single domain by paleomagnetists) that carry
the red bed's demagnetized remanence. These large
grains will be the most susceptible to gravitational
forces during deposition and, because hematite has an
intrinsic magnetization 200 times smaller than mag-
netite, will be less likely to be realigned parallel to the
geomagnetic fi eld by post-depositional processes.
A chemical origin for red bed remanence, acquired
by the growth of secondary hematite grains, has been
the more generally accepted magnetization mecha-
nism for red beds. In this case, red bed paleomagnetism
should be free of the errors inherent to a depositional
magnetization but then the timing of the remanence
acquisition becomes an important consideration. Some
early workers had suggested that red bed remanence is
a chemical remanence acquired millions of years after
deposition (Walker et al . 1981 ; Larson et al . 1982 ).

Thus, red bed remanence would not be useful as a
record of the geomagnetic fi eld at or near the time of
the rock's deposition. However, subsequent extraction
of detailed magnetostratigraphies from red bed units
shows that even if red bed remanence is a secondary
CRM, it must be acquired soon after deposition (thou-
sands or maybe tens of thousands of years). Evidence
from one study shows that a red bed chemical rema-
nence was acquired before burial by only 1 m of over-
burden (Liebes & Shive 1982). These observations
indicate that red bed remanence (even if a secondary
chemical magnetization) can be acquired soon after
deposition and could still suffer from inclination shal-
lowing caused by burial compaction. Even a late-
acquired post - compaction chemical magnetization
could suffer from inclination shallowing if hematite is
formed from precursor minerals with depositional or
compaction fabrics.
A compilation of magnetic fabrics measured in red
beds is consistent with red sedimentary rocks being
dominated by a primary depositional remanence or
an early compaction-affected chemical remanence
(Fig. 5.9 ).
These data are particularly compelling because they
come from Paleozoic and Mesozoic formations that
are considered 'classic' red beds. They all show the
typical magnetic fabric, both for anisotropy of mag-
netic susceptibility (AMS) and anisotropy of rema-
nence (anisotropy of isothermal remanence or AIR)
that occurs during deposition and compaction. The
oblate fabrics have minimum principal axes oriented
perpendicular to the bedding plane and the maximum
and intermediate principal axes are distributed in the
bedding plane.

THE RED BED MAGNETIC
ANISOTROPY-INCLINATION
CORRECTION

Because of the high coercivity of hematite, paleomag-
netists have tried different methods for measuring the
remanence anisotropy of the hematite grains. Tan
et al .'s (2003) study of the shallow inclinations in
central Asian Cretaceous red beds illustrates an
approach that uses both chemical demagnetization
and the measurement of susceptibility anisotropy
(AMS). In this technique, chemical demagnetization is
used to isolate the ancient magnetization in the rocks.
In chemical demagnetization the rock samples are
soaked for progressively longer time periods in concen-