Post - Depositional Diagenesis and Chemical Remanent Magnetization - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 72

minerals was the cause of the red coloration of red
beds. Van Houten ' s (1973) review article and Turner ' s
(1980) topic, Continental Red Beds , are good propo-
nents of the view that red beds formed in arid environ-
ments and were the result of diagenesis - both early
diagenesis in the post-depositional history of rocks and
late diagenesis that could persist for tens of millions
of years after deposition. Obviously this view suggested
that the CRM of red beds would be secondary,
and probably not of much use paleomagnetically.
Walker (1974) studied recent deposits in Orinoco
which he suggested would turn into classic red beds
given enough time, and indicated that red beds could
form from clastics deposited in humid environments
and were not limited to an arid environment of
deposition.
A new idea about red beds was introduced by Dubiel
& Smoot (1994) soon after the publication of Butler's
(1992) topic. Red bed formation was favored by a mon-
soonal climate; alternating wet and dry seasons and
savannah-type vegetation would cause the diagenesis
needed to create red beds observed in the geologic
record. Seasonal aridity would cause the growth of
hematite geologically soon after deposition, probably
when the water table was deep in the dry season. Par-
rish's (1998) topic echoes the role of monsoonal
seasonality in red bed formation. In their study of the
Late Cretaceous Pozo Formation red beds of the
Salinian block in California, Whidden et al . (1998)
argue that the Pozo magnetization is due to early
diagensis, i.e. pedogenesis within 10 5 years of deposi-
tion, in a seasonally arid climate. The observation of a
magnetostratigraphy in the Pozo red beds supports an
early age of their magnetization. Whidden et al . argue
that the early formation of the hematite carrying
the Pozo magnetization occurred above the water
table in an overbank deposit. They provide good petro-
graphic and paleomagnetic evidence in support of
their interpretation.
The fi rst evidence that the remanence-carrying
hematite in red beds had a characteristic compaction
and/or depositional fabric came from the Mississippian
Mauch Chunk Formation and Cretaceous Kapusaliang
Formation red beds (Tan & Kodama 2002; Tan et al .
2003). Either stepwise dissolution of the magnetic
minerals in chemical demagnetization coupled with
AMS measurements or high fi eld laboratory - applied
magnetizations (IRMs) parallel and perpendicular to
the bedding plane showed that the specular hematite
grains that carried the high-temperature remanence

revealed a bedding-parallel oblate fabric. Earlier AMS
measurements of red beds (Bossart et al . 1990 ; Garces
et al . 1996) had shown similar fabrics, but the Mauch
Chunk and Kapusaliang studies showed that the hem-
atite carrying the ancient remanence had depositional/
compaction fabrics. This work had been performed as
part of an anisotropy correction for inclination shal-
lowing and did reveal shallowing for these red bed
units. Subsequently, as indicated in Chapter 5, the
anisotropy-based and EI techniques showed shallow
inclinations for a good number of red bed units from
all over the world. Both the anisotropy data and the
shallow inclinations support an early magnetization
age for red beds, if not a DRM then an early CRM, that
could be affected by compaction.
Sheldon (2005) has presented an interesting new
twist in the red bed formation story. Sheldon points out
that red beds had traditionally been interpreted paleo-
climatically to indicate desert conditions (Walker
1967), but more recently had been interpreted to indi-
cate monsoonal, seasonal aridity (Dubiel & Smoot
1994; Parrish 1998). Both of these interpretations
have informed paleomagnetists as they have attempted
to understand the timing and manner by which red
beds are magnetized. Sheldon (2005) studied the
Permian red beds of Cala Viola, Sardinia and showed
through elemental analysis of chemical weathering,
root traces and trace fossils, as well as pedogenic car-
bonate and salts, that the red beds formed pedogeni-
cally due to good drainage and not because of any
particular paleoclimate conditions. Their formation
could not be attributed to aridity or even seasonal
aridity during a strong monsoon. Sheldon concludes
that the red color of the Cala Viola pedosols is primarily
due to the hydrological conditions in which they
formed. To a paleomagnetist this would suggest that
the hematite carrying the Cala Viola red bed rema-
nence was formed by early diagenesis, not by extended
late diagenesis, and is consistent with the picture that
has developed from remanence and anisotropy
measurements.
To develop a model for diagenesis relevant to the
magnetization acquisition of red beds, it's best to sum-
marize all the observations we have about red beds.
First, red beds are typically continental sediments
deposited in fl uvial, lacustrine or eolian conditions.
Some red beds are formed from paralic or deltaic near-
shore marine sediments, but it is more likely that they
were deposited terrestrially. Metcalfe et al . (1994)
suggest that they are typically formed in rapidly subsid-