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For the proper comparison, Tan & Kodama (1998)
also conducted re-deposition experiments with
magnetite-bearing sedimentary rocks of the Creta-
ceous Holz Shale and the Point Loma Formation, again
without applying an IRM to the sediments before re-
deposition. These re-deposited sediments had DRM:
SIRM ratios of 2.3% and 1.1%, respectively. The differ-
ence in alignment between re-deposited hematite-
bearing sediments and magnetite-bearing sediments
cannot be explained by the 200-fold difference in spon-
taneous magnetization of the different magnetic min-
erals, so a misaligning mechanism must also be
affecting the DRM acquisition of hematite. Brownian
motion is unlikely to affect detrital specular hematite
particles that are typically several microns in size, so
fl occulation is the preferred mechanism. Since organic
material is unlikely in the highly oxidized red sedimen-
tary rocks, it is more likely that clay domains or van
der Waals attraction of hematite to clay particles, as in
magnetite-bearing rocks, reduces the alignment of the
hematite grains.
the laboratory or directly in the cryogenic magnetom-
eter showed no, or at least a very small (< 3 ° ), syn -
depositional inclination error. It was then thought that
the syn-depositional inclination error originally
observed by King and Griffi ths
et al
. was an artifact of
their experimental procedure (Verosub 1977; Butler
1992). Measurements of natural sediments, particu-
larly Opdyke & Henry's (1969) study of recent marine
sediments from the world oceans, showed no inclina-
tion error. We have also shown (Chapter 1) that many
recent lake and marine sediments appear to give accu-
rate records of the geomagnetic fi eld.
How can we explain the King results, the slower
second - generation re - deposition experiments and the
observations of an apparently accurate DRM in recent
natural sediments? The prevailing idea at the time
(Verosub 1977) was that, even if the magnetic parti-
cles initially had an inclination error at touchdown,
they would quickly reorient themselves with the
Earth ' s magnetic fi eld in an early post-depositional
remanent magnetization (pDRM) that accurately
recorded the direction of the geomagnetic fi eld. pDRM
will be discussed in more detail in the next chapter. It
is interesting to note that, in the recent re-deposition
experiments and fl occulation models of DRM acquisi-
tion, post-depositional remanence is not considered
important because the magnetic particles are trapped
in fl ocs and cannot easily reorient after deposition.
Indirect evidence from Sun and Kodama's laboratory
compaction experiments also indicates that the mag-
netic particles are locked onto clay particles at deposi-
tion by electrostatic or van der Waals forces, and are
essentially immobilized.
Can the results of the fi rst - and second - generation
re-deposition experiments be explained by the fl occula-
tion model of DRM acquisition? In all the re-deposition
studies, the salinity of the water is not explicitly men-
tioned but they all use freshwater sediments: King
(1955) and Griffi ths
et al
. (1960) used re - deposited
glacial varves; Barton & McElhinny (1979) and Barton
et al
. (1980) used fi ne - grained organic - rich lake sedi-
ments; and Levi & Banerjee (1990) used lake sediments
from Lake St Croix in Minnesota.
In the Griffi ths
et al
. (1960) study, re - deposition
experiments were conducted with different sediment
grain sizes of
c.
2 - 30 μm. All the experiments showed
roughly the same magnitude inclination error (10-
20°). The grain size and the sediment type (glacial
varves) would suggest that the sediment was mainly
quartz grains, so fl occulation is unlikely for these
THE ACCURACY OF A SYN-
DEPOSITIONAL REMANENCE
The idea of a syn-depositional inclination error came
from the fi rst round of re-deposition experiments con-
ducted by the fi rst generation of paleomagnetists. In
these experiments, typically varved glacial lake sedi-
ments were re-deposited in the laboratory in fresh
water (King 1955; Griffi ths
et al
. 1960 ). The original
defi nition of the tangent-tangent relationship used to
describe inclination shallowing resulted from this
work:
tan
I
=
f
tan
I
f
m
where
I
m
is the measured inclination of the re-deposited
sediment,
I
f
is the laboratory magnetic fi eld inclination
and
f
is the fl attening factor which lies within the range
0 <
f
≤ 1. For the early re-deposition experiments, fl at-
tening factors of 0.4 were observed. The work of King
and Griffi ths
et al
. was also important because it showed
that bedding slope and deposition from a current could
affect the accuracy of the remanent magnetization
acquired by the re-deposited sediments.
Subsequently, second - generation re - deposition experi-
ments by Barton & McElhinny (1979) and Levi &
Banerjee (1990) showed that slow re-deposition in
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