Post- Depositional Remanent Magnetization - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 26

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Post - Depositional Remanent Magnetization

One of the conundrums brought on by the early DRM
re-deposition experiments was how to explain the large
inclination error found in laboratory re-deposited sedi-
ments when paleomagnetists sensed that at least
recent sediments were accurate recorders of the geo-
magnetic fi eld. This hunch was supported by a classic
1969 paper by Opdyke & Henry (1969) that looked at
the paleomagnetism of young marine sediments over
a range of latitudes. The inclination of these sediments
fi t the predicted inclination for their latitude, not only
supporting a geocentric axial dipole but accurate sedi-
mentary paleomagnetism and no, or little, inclination
shallowing.
Ted Irving, one of the important early contributors
to paleomagnetism, had already proposed in 1957
(Irving 1957) a way in which sediments could accu-
rately record the geomagnetic fi eld despite the results
of the re-deposition experiments, i.e. the small mag-
netic mineral grains in a rock were mobile when
initially deposited and could reorient parallel to the
geomagnetic fi eld after deposition. He based his idea on
the observation of a uniform magnetization in soft-
sediment slumps in the Torridonian sandstone, indicat-
ing that the sediments had to have acquired their
magnetization after slumping. He followed this pro-
posal with an experimental result (Irving & Major
1964) in which a dry silt-sandstone/magnetite

mixture, fl ooded with water in the presence of a range
of magnetic fi eld inclinations in the laboratory, accu-
rately recorded the laboratory magnetic fi eld.
Thus was born the concept of a post-depositional
remanent magnetization (pDRM) in its simplest form.
A pDRM not only explains accurate sedimentary
records of the geomagnetic fi eld, but has other implica-
tions for the interpretation of the paleomagnetism of
sedimentary rocks. The main implication is that, if
pDRM is the way sediments are typically magnetized,
a sediment's paleomagnetism is younger than the dep-
ositional age of the sediment. To complicate matters,
further down in the sediment column the sediment's
magnetization will appear older than its depositional
age. A second implication, that comes from the theo-
retical modeling of the acquisition of a pDRM, is that
the geomagnetic fi eld variations recorded by a sedi-
ment will be smoothed if different sub-populations of
the magnetic mineral grains in a rock acquire their
magnetization at different times after deposition.
Irving's original proposal of a pDRM was particu-
larly far-sighted, given that very early paleomagnetic
results from natural varved sediments from Scandina-
via showed magnetizations pre-dating soft sedi-
ment deformation slumps and also had anomalously
shallow inclinations (e.g. Johnson et al . 1948 ; Granar
1958 ).