Geology Reference
In-Depth Information
In a conglomerate test (Butler 1992; Tauxe 2010), the magnetizations of
individual conglomerate clasts are compared and should be randomly scattered
if the magnetization is primary. A blanket remagnetization of the conglomerate
unit would produce a magnetization that is uniform. A statistical test of the
clast magnetizations checks if the scatter can be considered to be truly random
at some level of confidence (e.g., Watson 1956). The conglomerate test is not
commonly performed since many sedimentary rocks don't have a conglom-
erate bed available for the test.
An important test to constrain the age of magnetization for a magneto-
stratigraphic study is a reversals test. In a simple reversals test, the reversed
polarity and normal polarity mean directions are compared to determine if
they are antipodal (180° apart) within their 95% confidence limits. When
one of the polarity means is rotated 180°, both polarity mean directions
should lie within the other polarity mean's 95% confidence limits. The
confidence limits are usually calculated with Fisher statistics (Fisher (1953),
see Butler (1992)). Non-antipodality of normal and reversed polarity mean
directions is interpreted to mean that an incompletely removed overprint,
either normal or reversed polarity, has contaminated the results. A more
sophisticated application of the reversals test has been developed by
McFadden and McElhinny (1990) based on the advances in statistical
theory, but basically all versions of the reversals test check whether an over-
print persists in the data despite the demagnetization conducted in a stan-
dard paleomagnetic study. Pares and Van der Voo (2013) studied the
non-antipodality of Tertiary age rocks worldwide and suggested that the
shallower inclinations in reversed polarity rocks may be due to the structure
of the geomagnetic field rather than a persistent, partially unremoved
normal polarity overprint.
In any event, overprinting by secondary magnetizations is a reality for
paleomagnetic data and the reversals test is a good way to check for unre-
moved or incompletely removed overprints.
3.2.5 Plotting of Magnetostratigraphic Data
and Determining a Reversal Stratigraphy
To determine a magnetostratigraphy for a sedimentary sequence, the data
can be plotted in a variety of ways. In most cases, however, each sample's
demagnetized magnetization is converted to a
virtual
geomagnetic pole
(VGP)
. A VGP is the position of the north geomagnetic pole consistent
with the sample's magnetization assuming that the Earth's field is
dipolar in geometry. Reversed polarity directions will have their VGPs
distributed around the south geographic pole for younger data in
which the continents have not strayed far from their present positions.
In calculating a VGP position, a magnetization's declination indicates the
direction from the site location to the VGP and the magnetization's
inclination indicates how far away the VGP is from the site on the surface
