The Magnetization of Sedimentary Rocks: Processes and their Interpretation - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 124

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The Magnetization of Sedimentary Rocks:
Processes and t heir Interpretation

This last chapter is designed to bring together most of
the major points from the topic about processes that
can affect the magnetization of a sedimentary rock,
and to give an outline of the procedures that will, hope-
fully, ensure the best paleomagnetic results from a sedi-
mentary rock. There is no guarantee that these
techniques will always yield great results. As can be
seen from Chapter 6 on reduction diagenesis and
remagnetization, some sedimentary rocks have com-
pletely lost their primary magnetization. The experi-
mental techniques outlined in this chapter will help
the worker determine when this may have happened
and incorporate that reality into their interpretation of
the results. Of course, as in any endeavor, a prescribed
set of procedures will be rigid and give a false sense of
security of incontrovertibly robust results. We need to
be creative in the design of a measurement procedure
and a sampling scheme to meet the objective of the
investigation. Knowing something about the different
processes that could affect the accuracy of a paleomag-
netic result will help in the design of the fi eld and labo-
ratory work so that many of the pitfalls in interpreting
paleomagnetic data can be minimized and the best
results obtained.

FIELD WORK: ROCK TYPE AND
SAMPLING SCHEME

Practiced paleomagnetists can look at an outcrop of a
sedimentary rock and immediately have a sense of
whether it is likely to yield good results. This can only
come from years of experience trying many different
rock types; even with that experience, however, a good
paleomagnetist can be fooled. Sometimes rocks that
are not expected to give good results do; other times
rocks that typically yield good results do not. Know-
ing something about how a sedimentary rock is mag-
netized and how a primary magnetization could be
affected after it has been acquired can be a guide in the
fi eld, so that time is spent sampling the rocks that will
most probably yield good results.
The best sedimentary rocks for paleomagnetism are
typically fi ne grained. Mudrocks, siltstones or very fi ne-
grained sandstones are the best for good paleomag-
netic results because these grain sizes indicate a quiet
depositional environment relatively free from the mis-
aligning infl uences of currents and waves in a high-
energy environment. Lake, hemipelagic marine, deep
marine or fl uvial overbank environments ensure rela-