The Magnetization Mechanism of Sediments and Sedimentary Rocks: Depositional Remanent Magnetization - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 16

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The Magnetization Mechanism of
Sediments and Sedimentary Rocks:
Depositional Remanent Magnetization

TRADITIONAL DRM THEORY:
MAGNETITE-BEARING ROCKS

omagnetism. The reader is referred to these topics for
DRM processes presented in the context of other mag-
netization processes, for instance thermal remanent
magnetization (TRM) for igneous rocks and chemical
remanent magnetization (CRM) for secondary mag-
netizations in sedimentary rocks.
The traditional conception of how magnetite-bearing
sedimentary rocks acquire their magnetization can be
traced back to Nagata's important textbook on rock
magnetism (Nagata 1961). Collinson (1965) further
developed Nagata's basic idea of DRM acquisition. The
main point made by these workers (the traditional view
of DRM acquisition) is that the geomagnetic fi eld
causes a torque on the magnetic moment of a mag-
netic nano-particle, aligning it parallel to the geomag-
netic fi eld as it falls through the water column and is
deposited at the sediment-water interface (see Fig. 2.1
and treatment in Butler 1992).
The classic DRM equation that mathematically rep-
resents the torque of the Earth's magnetic fi eld on the
magnetic moment of a ferromagnetic mineral grain is
(Nagata 1961 ):

Even though paleomagnetists have measured and
interpreted the magnetization of sedimentary rocks
from the very beginning of paleomagnetism, they
are still investigating and debating how sediments
and sedimentary rocks acquire their paleomagnetism.
This chapter will review the essentials of how sedi-
ment acquires a depositional (or, some say, detrital)
remanent magnetization (DRM) and also show some
of the insights that experimental compaction studies
can provide for understanding the DRM of rocks.
For the purposes of this discussion we will treat
the magnetization of hematite-bearing sediments and
magnetite-bearing sediments separately, although
similar processes probably operate for both types of
magnetic minerals.
There are two paleomagnetism text books that
provide excellent summaries of the DRM process:
Butler ' s (1992) Paleomagnetism: Magnetic Domains to
Geologic Terranes and Tauxe ' s (2010) Essentials of Pale-