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magnetization of red beds is therefore carried by the
largest single-domain hematite grains. The steep
decrease in intensity during thermal demagnetization
of some red beds then can be interpreted as evidence
for specular hematite carrying the paleomagnetism of
the red bed.
Other evidence used to argue for a DRM in red beds
came during the heyday of the red bed controversy
from the measurement of red bed paleomagnetism on
the topset, bottomset and foreset beds of crossbedding
(Steiner 1983). A difference in the inclination carried
by the fl at-lying bottomset or topset beds and the ini-
tially dipping foreset beds would suggest a DRM affected
by an initial bedding slope by magnetic grains rolling
down the slope. Another powerful argument that the
magnetization of red beds is primary (and therefore a
DRM) comes from the observation that many red beds
record a magnetostratigraphy, in which geomagnetic
fi eld polarities are constrained to stratigraphic hori-
zons. More recent evidence that red beds are magnet-
ized by a DRM come from what is interpreted to be
records of paleosecular variation in red sedimentary
rocks. Kruiver et al . (2000) report records of paleose-
cular variation (PSV) in Permian-age red beds from
Dome de Barrot in southeastern France. They even go
as far as to analyze the characteristics of secular vari-
ation in the Permian and observe looping of the geo-
magnetic fi eld vector, behavior suggesting paleosecular
variation. Much earlier, Evans & Maillol (1986) meas-
ured red bed samples taken from unoriented cores and
interpreted the directional variations as due to PSV.
They modeled the PSV with dipole wobble plus shorter
non-dipole variations, an approach typically used with
the secular variation recorded by recent marine and
lake sediments. Finally, depositional magnetic fabrics
carried by the anisotropy of magnetic susceptibility
(AMS) and magnetic remanence (AMR) of red beds
suggest that their paleomagnetism is carried by
depositional magnetic minerals (Kodama & Dekkers
2004). This evidence will be covered in more detail in
Chapter 5 .
An interesting conundrum about the accuracy of
red bed remanence arises from the different possibili-
ties for the manner in which a red continental sedi-
mentary rock becomes magnetized. If the red bed is
magnetized primarily by a post-depositional chemical
remanent magnetization (CRM), then the direction
carried by the red bed may be an accurate record of the
geomagnetic fi eld direction when the secondary hema-
tite grains grew. The age of the magnetization will not
however be the age of the rock, and it will be diffi cult
to determine accurately. If on the other hand the red
bed is magnetized by a DRM, then the timing of mag-
netization is exactly the age of the rock and will be
accurately known. The direction of the magnetization
will most likely not accurately record the direction of
the geomagnetic fi eld at deposition, however. The low
spontaneous magnetization of hematite will cause
detrital hematite grains in red beds to be more easily
affected by gravity and other misorienting forces at
deposition (water currents, initial slope). Tauxe & Kent
(1984) showed this beautifully with their pioneering
work on the Siwalik River sediments of Pakistan.
Redeposition of these hematite-bearing sediments in
the laboratory revealed a large inclination error as did
observation of naturally redeposited sediments on the
river fl oodplain. Tan et al . (2002) also redeposited red
sedimentary rocks in the laboratory, in this case rocks
from western China. They found a large compaction-
caused inclination error for the fi nest - grained sedi-
ments and a large syn-depositional inclination error
for the coarser-grained sediments. Therefore, accurate
red bed directions will probably not have their age well
known, and accurately dated red bed magnetization
ages will probably not be an accurate record of the
Earth ' s fi eld direction at the time of deposition.
POST-DEPOSITIONAL PROCESSES
THAT AFFECT THE MAGNETIZATION
OF SEDIMENTS AND SEDIMENTARY
ROCKS
Even though it's quite clear that the DRM acquired by
recent sediments is reproducible when multiple coeval
records are compared, this still begs the question of
how accurately the DRM records the Earth's magnetic
fi eld. Chapter 2 will examine the accuracy of DRM in
detail. However, even if it turns out that the DRM of
recent sediments is not only reproducible but accurate,
there are post-depositional processes that occur in sedi-
ments and sedimentary rocks that can either totally
obliterate, distort, or bias the direction and intensity of
the initial DRM acquired by the rock. These processes
are: post-depositional remanence acquisition, burial
compaction, reduction diagenesis, secondary growth
of magnetic minerals, and tectonic strain. Not all of
these processes, and in some cases none, occur in any
given sediment or sedimentary rock, but we need to be
cognizant of them all to be aware of their potential
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