Post - Depositional Diagenesis and Chemical Remanent Magnetization - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 74

needs to be addressed carefully on a case-by-case basis
and informed by anisotropy measurements and tests
for inclination shallowing.

ite. In the Appalachians the magnetization is post- or
syn-folding in age in the carbonates (Fig. 6.3). The
magnetite carrying the remagnetization appears to be
framboidal in morphology, suggesting that it is diage-
netically grown (Fig. 6.4 ).
2. Red siltstones and sandstones carry a partial remag-
netization. The lower - unblocking - temperature mag-
netization, perhaps associated with the pigmentary,
submicron hematite in the rocks, carries a syn- or post-
folding Kiaman - age overprint on a high - unblocking -
temperature primary characteristic magnetization
carried by specular hematite.
3. Despite some controversy in the 1980s about the
possibility that the remagnetization was due to a ther-
moviscous effect, i.e. elevated temperatures over long
periods of time, the evidence of diagenetic morpholo-
gies for the magnetite particles in the carbonate rocks,
along with their syn- or post-folding magnetization
ages in the Appalachians, argued for basinal fl uid fl ow
as the cause of a CRM magnetization. From some fl uid
inclusion studies, the fl uid was observed to be warm
brine which was rich in potassium. It would have
caused clay diagenesis that in turn provided the Fe for
the secondary magnetite. Interestingly enough, the
same fl uids would have had to produce secondary pig-
mentary hematite in the siltstones and sandstones at
the same time, so the chemical changes were lithologi-
cally dependent.
4. There was an initially puzzling observation of mag-
netic fabric data from remagnetized carbonates that
appeared to be primary. The magnetic fabric for the
remagnetized carbonates, the prime example being the
Onondaga Limestone, was measured by magnetic sus-
ceptibility (AMS). The evidence for a primary magnetic
fabric for obviously remagnetized rocks is important to
this topic, since in the previous section we argued that
primary magnetic fabrics suggested DRMs or very early
CRMs for red beds. We will return to the primary-
seeming AMS fabrics in the Onondaga Limestone later
in this section.
Since the publication of the McCabe & Elmore (1989)
review article, there has been additional work done on
the Late Paleozoic remagnetization event. Jackson
(1990) and Jackson et al . (1992) conducted detailed
rock magnetic studies of remagnetized carbonates and
made some important observations. The most impor-
tant was that the remagnetized carbonates had a char-
acteristic signature in their hysteresis plots where the
magnetization of a rock sample is cycled slowly back
and forth in strong magnetic fi elds that saturate the

LATE DIAGENESIS AND
REMAGNETIZATION

There are several processes that cause a late-stage dia-
genesis, tens to hundreds of millions of years after
deposition of a sedimentary rock, that through the
growth of secondary magnetic minerals create a
'classic' remagnetization in the rocks. The remagneti-
zation may be partial or complete, either totally reset-
ting the rock's magnetization or only adding another
component of magnetization that can hide the primary
magnetization of the rock. The three major processes
outlined in the literature over the last several decades
are: basinal fl uid fl ow associated with orogenesis, clay
diagenesis and hydrocarbon maturation. The process
that by far has received the most attention is basinal
fl uid fl ow associated with diagenesis, so it will be dis-
cussed fi rst. It is anticipated that readers can also refer
to Van Der Voo & Torsvik's review on the subject which,
at the time of writing (March 2012), is due to be
published.

Basinal fl uid fl ow during orogenesis

The Late Paleozoic remagnetization event, primarily in
North America, is one of the most extensively studied
and documented remagnetizations in the literature. It
is the paleomagnetic consequence of the 'squeegee tec-
tonics' proposed by Oliver (1986) in which fl uids were
squeezed through rocks during orogenesis. The remag-
netization occurred during the Kiaman Permian-age
reversed-polarity superchron so remagnetized rocks
typically have a very shallow, nearly due south, paleo-
magnetic direction in North America. The geographic
extent of the remagnetization is typically in Appala-
chian Paleozoic rocks from eastern North America and
in the fl at-lying Paleozoic rocks of the mid-continent.
McCabe & Elmore (1989) provide an excellent review
paper summarizing results up to the late 1980s. The
major points that McCabe and Elmore provide about
the Kiaman age remagnetization in North America are
as follows.
1. Typically carbonate rocks are totally remagnetized
and the magnetization is carried by diagenetic magnet-