Post- Depositional Remanent Magnetization - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 33

both for and against a post-depositional remanence
with a 15 cm or so lock-in depth in marine sediments.
The best way to proceed when considering the accu-
racy of marine sedimentary rocks is probably to assume
that a pDRM could be present and that the age of mag-
netization could be offset from the depositional age of
the sediments by some 2-15 kyr, given sediment accu-
mulation rates of 1-8 cm/kyr. For most tectonic uses
of paleomagnetism, this amount of offset is probably
inconsequential. For faster sediment accumulation
rates, for instance 10 - 100 cm/kyr for near - shore
marine sediments, the pDRM could be locked in rela-
tively soon (< 1 kyr) after deposition and only be
smeared over hundreds of years (an inconsequential
amount). For slower sediment accumulation rates (<
1 cm/kyr for deep-sea marine sediments), the pDRM
could be locked in tens of thousands of years after
deposition and the geomagnetic fi eld record could be
smeared over thousands of years, compromising the
record of paleosecular variation.

If the paleomagnetism of a sedimentary rock is
being used for magnetostratigraphy, the user should be
aware that transition boundaries may be shifted
slightly in time. This caveat is probably more important
for marine sediments however, and not as much for the
terrestrial sedimentary rocks often used for magneto-
stratigraphic studies. There is very little observational
evidence regarding pDRMs in terrestrial sediments but,
even if it exists with the same magnitude of offset (10-
15 cm), the higher sediment accumulation rates of ter-
restrial sediments would make the actual time offset
between depositional and magnetic age smaller than in
marine sediments. The smoothing that accompanies a
lock-in depth of magnetization in slowly deposited
marine sediments may attenuate the records of some
high-frequency variations of the geomagnetic fi eld, but
could actually help in tectonic and magnetostrati-
graphic studies by partially averaging secular variation
for a better resolution of the geocentric axial dipole
fi eld.