Post - Depositional Diagenesis and Chemical Remanent Magnetization - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 78

Channell 1994) that were deformed during the Hercy-
nian orogeny (contemporaneous to the Alleghanian in
North America). All this evidence shows how wide-
spread the Kiaman age remagnetization was; despite
being concentrated in eastern and mid-continent
North America, its effects could be seen as far away as
the North American west and Hercynian England.
Fairly recent paleomagnetic studies are still affected
by this large-scale remagnetization. Cederquist et al .
(2006) and Hnat et al . (2009) showed that carbon-
ate and siliciclastic rocks from the Tennessee and
Pennsylvania salients are either totally or partially
remagnetized in the Late Paleozoic and the remagnet-
ized directions show no evidence of oroclinal bending,
so that any bending that occurred of a primarily
straight mountain belt must have occurred before the
Late Paleozoic.

highest-temperature magnetization component is
carried by magnetite and interpreted to be a CRM.
Gill et al . (2002) compared the magnetization of
equivalent-age rocks in the thrusted, disturbed belt of
Montana with relatively undeformed rocks in the
Sweetgrass Arch to the east. The rocks in the Sweet-
grass Arch have unaltered detrital smectite clay, while
the Montana deformed rocks have been illitized. The
deformed rocks have a stronger, more stable magneti-
zation carried by magnetite that is interpreted to be a
CRM.
In the Vocontian trough of southeastern France,
Jurassic and Cretaceous carbonates that have experi-
enced illitization have a secondary pre-folding normal
polarity magnetization carried by magnetite (Katz et al .
1998, 2000). The magnetization is interpreted to be a
CRM since the burial depths were not great enough to
cause a thermoviscous resetting of a primary magneti-
zation. Where the detrital magnetite is still present in
signifi cant amounts, the CRM magnetization is either
absent or very weakly developed. In the Vocontian
rocks, the intensity of the clay diagenesis CRM
increases as the degree of illitization increases. Sr, C
and O isotopic analyses show that an additional sec-
ondary reversed-polarity component of magnetization
in rocks closer to the Alps may be due to orogenic
fl uids, but that rocks further from the Alps have only
been in contact with ancient seawater. This suggests
that, for these rocks, illitization is the cause of the mag-
netite CRM.
In contrast to this is Zwing et al . ' s (2009) study of
rocks from the Rhenish Massif in Germany. In a
detailed study, Zwing et al . used K - Ar geochronologic
techniques to date the illitization and observed two illi-
tization events. Based on the age of the illitization
events and the age of the secondary CRMs, Zwing et al .
argue against illitization as the cause of the remagneti-
zation they observe in the rocks. Although they observe
that the age of remagnetization is equivalent in age to
the younger illitization event in Upper Devonian rocks,
it is not equivalent in age to Middle Devonian rocks.
Rare Earth element (REE) studies indicate that oro-
genic fl uids are not implicated.
Tohver et al . (2008) came to a different conclusion
when they dated the illitization of the clay residue
using 40 Ar/ 39 Ar geochronology in carbonate rocks
from the Cantabrian-Asturian arc and found it to be
the same age as the secondary magnetite carrying the
remagnetization of these rocks. They argued that the

Burial diagenesis and illitization

The primary mechanism proposed for remagnetization
due to the very general term 'burial diagenesis' is clay
diagenesis. More specifi cally, this entails the change of
detrital smectite clays to secondary illite clays during
burial, called illitization. Illitization provides the iron
necessary for the formation of secondary magnetite
that, in turn, would acquire a CRM as the newly formed
magnetite particles grew large enough to block-in a
stable magnetization. There are only a handful of studies
that support clay diagenesis as a cause of remagneti-
zation. Most are based on a 'presence-absence' argu-
ment in which the magnetizations of equivalent-age
rocks are compared at two localities where the rocks
have experienced illitization at one and detrital smec-
tite is still present in the rocks at the other. If the mag-
netizations are different at the two localities, the
argument is made that the magnetization of the illi-
tized rocks is secondary.
There are only three study areas that have been
used to support clay diagenesis remagnetization: Scot-
land, Montana and the Vocontian trough in France.
The study of Jurassic sedimentary rocks in Skye, Scot-
land is the best example of the presence-absence argu-
ment (Woods et al . 2002). The rocks with detrital
smectite and no evidence of illitization have a weak
magnetization, while equivalent-age rocks at a differ-
ent location that have clearly undergone illitization
have stronger multi-component magnetizations. The