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ing fault-bounded basins. The molasse sediment of the
classic Paleozoic red beds of the Appalachians would
fi t this description, as would the Mesozoic basins of the
North American east coast (e.g. Newark and Hartford
basins).
Petrographic examination of classic, paleomagneti-
cally important red beds indicate that inter-stratal
alteration of ferromagnesian silicates and oxides
(pyroxenes, biotites, amphiboles, olivine, magnetite,
etc.) was the primary source of the pigmentary hema-
tite in the rocks. Iron oxyhydroxides, which form
during diagenesis, are the source of the hematite
through dehydration reactions. In an early study of red
bed genesis, Walker (1967) reported that the distinc-
tive red bed coloration and inter-stratal alteration
of alluvium in northeastern Baja California was not
evident until the sediments were Pliocene in age
(2-5 Ma). Pleistocene sediments at the ground surface
were red due to soil formation but were more typically
yellow/red colored. This observation is consistent with
a survey of the 2005 version of IAGA Global Paleo-
magnetic Database (McElhinny & Lock 1990) that
shows the youngest reported red sediments in the data-
base are 2-3 million years old (the red clays of the
Dnieper River sediments, Ukraine and the Kuban River
sediments, Russia). The youngest rocks designated as
red beds are the 3-5 Ma sediments of the Karanak
Group from Tajikistan.
Thermal demagnetization of red bed remanence
typically shows a distributed unblocking-temperature
spectrum at intermediate temperatures (300-650°C),
usually attributed to the fi ne - grained pigmentary hem-
atite, and a narrow unblocking-temperature spectrum
at the highest temperatures (660-680°C) near to the
highest unblocking temperatures for hematite (Neel
temperature c. 685°C). The narrow highest unblock-
ing temperatures are usually attributed to the specular
hematite that carries the characteristic remanence of
a red bed (Fig. 6.2). Red beds can carry a well-defi ned
magnetostratigraphy with polarity zones confi ned to
strata. Most recently, the characteristic remanence-
carrying hematite grains in red beds are observed to
have a compaction and/or depositional fabric (Fig. 5.9)
and to have suffered from inclination shallowing,
similar in magnitude to the shallowing observed in red
bed laboratory re-deposition experiments (Tauxe &
Kent 1984 ; Tan et al . 2002 ).
These observations suggest that, for paleomagneti-
cally important red beds, the remanence is either depo-
sitional or very early diagenetic (within 10 5 years if not
Fig. 6.2 Intensity decrease during thermal
demagnetization of NRM for representative red bed
magnetizations. Data from Cretaceous Kapusaliang, the
Mississippian Mauch Chunk and the Paleocene Suweiyi
formations.
10 3 - 10 4 years of deposition). If it is diagenetic, the
hematite carrying the remanence grew early enough
after deposition to acquire a burial compaction fabric.
Another possibility is that the hematite formed diage-
netically inherited the compaction/depositional fabric
of the hematite's precursor minerals, although this
scenario would not be suffi cient to explain the bedding-
confi ned polarity zones that allow good magnetostrati-
graphies in red beds.
The question raised in Butler's topic is still unre-
solved at the end of the analysis in this section: what
is the source of the specular hematite in red beds that
carries their characteristic high - unblocking - temperature
remanence? Are the specular hematite grains deposi-
tional or secondary? I would argue that the evidence
should swing paleomagnetists more toward the 'minor-
ity view' in Butler's topic, that is, that the specular
hematite is likely to be depositional in many cases, but
that there can certainly be cases where specularite is
formed from diagenesis (even diagenesis that has
occurred over hundreds of thousands or even a million
years since deposition). The age of red bed remanence
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