The Paleomagnetism of Sediments and Sedimentary Rocks: Importance and Reliability - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 1

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The Paleomagnetism of Sediments
and Sedimentary Rocks:
Importance and Reliability

THE IMPORTANCE OF SEDIMENTARY
PALEOMAGNETISM

magnetic axial dipole (GAD) hypothesis. The GAD
hypothesis, that the Earth's magnetic fi eld has been a
dipole at the center of the Earth oriented parallel to the
Earth's rotation axis, is central to the widespread and
successful use of paleomagnetism in the Earth sci-
ences. Without it, paleomagnetism would probably not
be a subdiscipline of geology. Using the GAD hypothe-
sis, paleomagnetists can calculate the paleolatitudes of
rocks and the amount of vertical axis rotation that
may have occurred in an area. The amount of time
needed to adequately average the effects of secular
variation is not easily determined. In fact, some paleo-
magnetists would argue that the departure of the
time - averaged fi eld from the GAD fi eld is caused by a
bias in secular variation that persists for millions of
years, particularly back in the Paleozoic or Precam-
brian. Based on the behavior of the Earth's fi eld over
the past 5 million years, when plate motions would not
be large enough to affect the observation of geomag-
netic secular variation, averaging over several thou-
sand years is generally considered a long enough time
to ensure that a GAD fi eld is observed.

Sediments and sedimentary rocks, both lacustrine and
marine, are important targets for paleomagnetists who
want to answer questions about global and regional
tectonics, about paleoclimate, and about the behavior
and history of the Earth's magnetic fi eld. Of the 9259
results reported in the Global Paleomagnetic Database
in 2009, 4971 (54% of them) were attributed to sedi-
mentary rocks. Although estimates indicate that
sedimentary rocks make up only about 8% of the total
volume of the Earth's crust (Buchner & Grapes 2011),
they are ubiquitous in the thin veneer of crustal rock
available to geologists.
There are two signifi cant reasons why sedimentary
rock is an important target for paleomagnetic studies.
The fi rst is that sedimentary rocks give a nearly conti-
nuous record of the geomagnetic fi eld. This is critically
important to paleomagnetic studies because it allows
undisputed time averaging of the secular variation of
the geomagnetic fi eld and the application of the geo-