Post - Depositional Diagenesis and Chemical Remanent Magnetization - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 71

depositional magnetite and secondary iron sulfi des.
Larrasoana et al .'s (2003) study of the same rocks
demonstrated similar polarities for the Fe sulfi des and
magnetite in a magnetostratigraphic study. Huesing et
al . ' s (2009) high - resolution cyclostratigraphic study of
marine sediments from northern Italy showed good
correlation between a magnetostratigraphy recorded
by greigite with the biostratigraphy and argues strongly
for an early 'syn-depositional' age of the greigite.
Spahn et al . ' s (2011) magnetostratigraphy from the
Triassic limestones of the Dolomites in northern Italy
showed similar directions carried by greigite and mag-
netite, suggesting an early nearly depositional age for
the secondary greigite.
The presence of secondary iron sulfi de magnetic
minerals in a sedimentary rock does not necessarily
spell doom for nearly syn-depositional age of the rema-
nence, although this possibility should be rigorously
checked with whatever means are possible such as
reversal and fold tests.

netization of red beds however suggests that a
larger-grained hematite carries the highest tempera-
ture magnetization in the rocks, and it may be different
in direction from the magnetization carried by the pig-
mentary hematite. This magnetization is typically
attributed to specular hematite (or specularite). The
age of this hematite is what is at issue in the 'red bed
controversy', i.e. whether it is depositional or second-
ary in age.
Butler (1992) points out that there is evidence that
in some red beds the specular hematite carries a DRM.
These rocks are likely to be more 'mature' sediments
and relatively coarse-grained clastic rocks. Butler char-
acterizes this interpretation as the 'minority view'
among paleomagnetists. The ' majority view ' , accord-
ing to Butler's reading of the literature in the early
1990s, was that red beds carry a CRM that was created
by early diagenesis of the sedimentary rocks. The
early diagenetic CRM of red beds, carried by specular
hematite, can still be a paleomagnetically useful signal
if it is formed within 10 3 - 10 4 years of deposition,
according to this view. For a Mesozoic or Paleozoic rock
that is tens or hundreds of millions of years old, this is
an inconsequential difference from the true deposi-
tional age.
One of the new pieces of paleomagnetic information
about red bed magnetization that has come to light
since Butler's (1992) topic is that the magnetic fabric,
particularly the anisotropy of the hematite carrying
the high-temperature characteristic remanence, has
maximum and intermediate principal axes lying in the
bedding plane and the minimum principal axis perpen-
dicular to the bedding plane. This is a fabric character-
istic of depositional or compaction processes, or the
result of both, acting on the magnetic minerals in a
rock. The other piece of new information is that both
the anisotropy - based inclination - correction and EI
correction techniques have identifi ed shallow inclina-
tions in red bed units. This new information is relevant
to our understanding of the acquisition of red bed
remanence.
Before we integrate our new information about red
bed remanence, it is useful to briefl y summarize the
understanding of red beds that has evolved in the lit-
erature. The early prevailing view in the 1960s and
1970s was that red beds formed in desert conditions.
Meticulous work by Walker (1967) on recent alluvial
deposits in Baja California and Walker et al . (1978,
1981) and Larson et al . (1982) on more ancient red
beds suggested that inter-stratal alteration of Fe silicate

EARLY DIAGENESIS IN TERRESTRIAL
RED BED SEDIMENTARY ROCKS

We have already discussed the magnetization (Chapter
2) and magnetic anisotropy of red sedimentary rocks
(red beds; Chapters 4 and 5). We have seen that red
beds are very important sources of paleomagnetic
data, particularly for the Mesozoic, Paleozoic and
earlier in Earth history. We also mentioned that in the
1970s and 1980s there was a good deal of controversy
in paleomagnetic circles about whether the character-
istic magnetization of sedimentary red beds was due to
a primary DRM or a secondary CRM. Butler (1992)
does an excellent job of summarizing the 'red bed con-
troversy'; many of the main points that he made in the
early 1990s are still relevant today.
Before repeating those points here to aid our discus-
sion, it is important to realize that the magnetization
of red beds is due to hematite (Fe 2 O 3 ) and that typically
for red beds there are two forms of hematite (and hence
two magnetizations) in red beds. One is the obvious
pigmentary hematite that gives the red bed its charac-
teristic color. Many studies, particularly by T.R. Walker
and his colleagues (e.g. Walker 1967, 1974; Walker et
al . 1978, 1981; Larson et al . 1982), indicate that the
pigmentary hematite is submicron in size and most
likely due to the post-depositional inter-stratal altera-
tion of primary Fe silicate minerals. Thermal demag-