The Paleomagnetism of Sediments and Sedimentary Rocks: Importance and Reliability - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 14

grams of C/m 2 yr to depths of only 1 cm for tens of
grams of C/m 2 yr (Schulz & Zabel 2006 ). The second-
ary ferromagnetic iron sulfi de minerals that are formed
essentially give a delayed NRM acquisition for the sedi-
ments affected, and the sediments are no longer mag-
netized by a DRM but by a CRM. Since this CRM is
typically formed very early in the post-depositional
history of the sediments, it could be affected by subse-
quent burial compaction inclination shallowing even
though it will probably have an accurate directional
recording of the geomagnetic fi eld when the secondary
minerals form. Efforts to use an early CRM in marine
sediments for relative paleointensity measurements
will be complicated by changes in paleoenvironmental
conditions, causing more or less organic carbon to be
deposited in the sediments (Schulz & Zabel 2006 ).

growth of secondary fi ne - grained, perhaps pigmentary,
hematite in clastic sedimentary red rocks.
If late stage secondary CRMs occur in sedimentary
rocks, the direction of the remagnetization will most
likely be an accurate record of the geomagnetic fi eld
direction. It is unlikely that a late stage CRM will be
affected by burial compaction; however, the remagneti-
zation age will be diffi cult to determine. Field tests such
as the fold test may help constrain the age of remag-
netization, but it will not be accurately known. Because
the paleohorizontal cannot be determined at the time
the CRM formed, particularly if the rock has been in a
tectonically active area, paleopoles or paleolatitudes
determined from the CRM will be dubious.

Tectonic strain

Later secondary CRMs Tectonically deformed regions are important targets

for paleomagnetists, primarily because folded rocks
allow them to constrain the age of magnetizations in a
rock using the fold test (Graham 1949). Paleomagnetic
results can also provide important insights about the
tectonic events that occurred in a region. Most paleo-
magnetists only remove the tilt of folded strata in
Graham's fold test and not the effects of grain-scale
strain that may have occurred during the folding. It is
interesting that in his landmark paper suggesting the
fold test, Graham (1949) considered that both rigid
and grain-scale rotations could affect the paleomag-
netic remanence. Rock deformation can indeed affect
the accuracy of a sedimentary rock's paleomagnetism.
Rotation of a rock's remanence can be caused by
bedding-parallel simple shear strain during fl exural
fl ow/slip folding (Kodama 1988). The geometry of the
simple shear not only causes an inaccuracy in the
direction of remanence, but an inaccurate estimate of
the age of magnetization in that the simple shear rota-
tion can make a pre-folding magnetization appear to be
syn-folding in age. This has been demonstrated in
folded red beds (Stamatakos & Kodama 1991a) and
shows that magnetic grains can rotate as active parti-
cles. If the remanence behaves as a passive line, then
pure shear during buckle folding will rotate the rema-
nence. Sense and magnitude of rotation will however
be case specifi c, depending on the relative angles
between the strain ellipsoid and the remanence.
Grain-scale strain in a deformed rock can cause sec-
ondary remagnetizations since secondary magnetic

Of course, the formation of secondary magnetic min-
erals can occur at any stage in the post-depositional
history of a rock. A rock can then have both a primary
magnetization, typically carried by depositional mag-
netite, and a secondary magnetization, usually carried
by iron sulfi des or hematite. If the secondary magnetic
minerals have the appropriate grain size distribution
they may have higher coercivities or unblocking tem-
peratures than the primary magnetic minerals and
their remanence will be isolated by demagnetization as
the characteristic remanence of the sedimentary rock.
In some cases, the primary magnetization of the rock
may be destroyed during the growth of the late stage
magnetic minerals or the primary magnetization may
be so weak initially that it is swamped by the secondary
CRM. The best example of this is the nearly ubiquitous
Late Paleozoic remagnetization of rocks throughout
eastern North America that occurred during the
Alleghanian orogeny (McCabe & Elmore 1989). This
remagnetization has totally reset the remanence of
Paleozoic carbonates throughout eastern North
America, but it is also present as one of several compo-
nents of magnetization in red clastic sedimentary
rocks from the Appalachians. One possible explanation
for the formation of the secondary magnetization is
that fl uids squeezed through the rocks of North
America during the Alleghanian orogeny, causing the
growth of secondary magnetite in the carbonate rocks
(Oliver 1986 ). These fl uids also apparently caused the