The Magnetization of Sedimentary Rocks: Processes and their Interpretation - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 130

It is of course important to quantitatively assess at
what level of confi dence the magnetization has passed
or failed the fold test. For this assessment, statistics are
needed to determine whether the clustering in a given
orientation is better than in another confi guration.
Many different statistical tests have been proposed and
used (e.g. McElhinny 1964; McFadden & Jones 1981;
McFadden 1990 ; Tauxe & Watson 1994 ; Enkin 2003 ).
Both Butler (1992) and Tauxe (2010) provide good
descriptions of these tests and how to use them, but
they all have one result in common: they indicate that
at some level of confi dence, usually 95% in most paleo-
magnetic studies, whether the best clustered magneti-
zation is signifi cantly different in its clustering than in
its worst clustered confi guration. For the fold test to be
meaningful, it is helpful if the folding occurred geologi-
cally soon after the sedimentary rocks were deposited.
A conglomerate test constrains the age of magneti-
zation by comparing the magnetization in conglomer-
ate clasts with the magnetization in the matrix. The
magnetic directions in the clasts should be randomly
oriented if the rock has not been affected by a pervasive
remagnetization. Statistical tests for random directions
are used to quantify the conglomerate test (Watson
1956). The conglomerate test is not used as frequently
as a fold test, probably because conglomerate layers
are not particularly good paleomagnetic targets
because they indicate a high-energy environment of
deposition.
A baked contact test checks if the rocks heated by an
igneous intrusion have been thermally remagnetized
in a direction similar to that of the igneous rocks and
different from the surrounding unheated rocks. This
confi guration of magnetizations would suggest that
the magnetization in the surrounding unheated rocks
(and of course, the baked rocks and the igneous rocks)
is not the result of a large-scale remagnetization and
is, therefore, assumed to be primary. Although it is an
important test, it does not constrain the age of mag-
netization as defi nitively as a fold test. The test can be
quantifi ed by using statistics to check whether the
mean directions of the different rock units are the same
or different at a given level of confi dence, typically
95%. McFadden & Lowes (1981) have provided a dis-
crimination of means statistical test that is typically
used.
Workers often support an ancient age of magnetiza-
tion by comparing the characteristic magnetization
isolated by demagnetization to the present-day fi eld
direction, the axial dipole fi eld direction during the

most recent normal polarity chron (Brunhes chron) or
to an ancient, but younger, paleomagnetic fi eld direc-
tion from the tectonic unit being studied (e.g. conti-
nent, plate). Although these comparisons are not as
powerful as the three fi eld tests described above, they
provide additional evidence for or against a remagneti-
zation in either the present-day fi eld, the most recent
normal polarity chron or at some time after deposition
of the sedimentary rocks.
Finally, the magnetic fabric of a sedimentary rock can
be used to argue whether or not the rock is carrying a
primary depositional remanence. This test is not gener-
ally used, but could become important if magnetic
fabric were more routinely measured, for example, to
check for inclination shallowing. It is not as powerful as
the fold test nor can it be quantifi ed by statistics, but
the observation of depositional/compactional magnetic
fabric - particularly a remanence fabric carried by the
same magnetic grains that carry the ChRM, with
minimum principal axes perpendicular to bedding and
maximum and intermediate axes scattered in the
bedding plane - is a strong argument that the magneti-
zation is either depositional or acquired very soon after
deposition before much burial compaction has occurred.

SOURCE OF A ROCK'S
MAGNETIZATION: MAGNETIC
MINERALOGY

It is incredibly important to identify the magnetic min-
erals carrying the magnetization of a sedimentary rock
in order to gain insight into the source of the magneti-
zation. Different magnetic minerals are likely to be a
primary depositional magnetic mineral or a secondary
magnetic mineral. Magnetite is the expected primary
depositional mineral for marine, fl uvial and lake sedi-
ments. It is transported to the depositional basin from
either igneous, metamorphic or sedimentary rock
source areas. The magnetite has probably been initially
magnetized by thermal processes. In some cases, mag-
netic minerals that are usually thought to be second-
ary, e.g. maghemite or pyrrhotite, have been shown to
carry a depositional remanence (Kodama 1982; Horng
& Roberts 2006), but magnetite is the premier primary
magnetic mineral. Hematite, particularly specular
hematite, is the expected primary magnetic mineral in
red sedimentary rocks, i.e. red beds. As described in
Chapter 6, it is 'up in the air' whether the hematite
in red beds is primary depositional or secondary; if it is