Geology Reference
In-Depth Information
ments having the vector rotate by up to twice the
amount of dip. In this case, if the initial dip of the bed
was unknown to the paleomagnetist, the rotation of
the paleomagnetic data into stratigraphic coordinates
would essentially 'undo' the bedding error for this
special case of beds dipping exactly in the direction of
the magnetic meridian. However, King's (1955) re-
deposition experiment showed that, for beds dipping
opposite to the fi eld direction, the inclination would
shallow by about the dip of the bed. Griffi ths
et al
.
(1960) also looked at dipping natural sediments
(glacial varves) and saw that the rotation of the vector
due to initial dip was always around a horizontal axis
parallel to the strike of the bed. In this more general
case, the declination as well as the inclination of the
remanence would be affected. In all cases the rotation
could be explained as though the magnetic grains were
rolling down the slope; the magnitude of the effect was
restricted to the small slopes investigated (≤ 10 ° ).
Deposition from moving water causes about the
same magnitude directional error in the syn-
depositional DRM as deposition on a sloping surface.
The sense of the rotation of the DRM vector is around
a horizontal axis perpendicular to the current direction
and the vector rotates into the current. The early
current deposition results by Griffi ths
et al
. (1960) and
Rees (1961) are complicated by the inclination error
that was ubiquitous for the deposition of the Swedish
varved material used almost exclusively in these exper-
iments. For instance, in the fl ume studies of Rees
(1961), the magnitude of the 'current error' depends
on an assumption of an inclination error that varied
over 10-23° for an initial fi eld inclination of 67°
(0.41 ≤
f
≤ 0.65). Based on this assumption, the
' current error ' ranged over 6 - 15 ° . All of the sloping
bed and current work was done with glacial varves or
re-deposition of glacially varved material in fresh
water, so the effects of fl occulation are unknown.
Before any general conclusions can be drawn about the
size of current and bedding slope errors, work should
be done with sedimentary material other than glacial
varved sediments.
SUMMING UP: SYN-DEPOSITIONAL
DRM
It is clear that although paleomagnetists have made
good progress in understanding the primary magneti-
zation process for sediments and sedimentary rocks,
the mechanism of syn-depositional DRM is still being
worked out. At the most fundamental level, it involves
rotation of magnetic nano-particles into alignment
with the ambient magnetic fi eld. The typical intensities
of DRM in natural rocks however shows that the mag-
netic nano-particles are not perfectly aligned with the
magnetic fi eld, although simple theory predicts it.
Some kind of misaligning force needs to be postulated.
As recently suggested by a series of workers, fl occula-
tion is a strong candidate.
Syn-depositional DRM can be quite accurate based
on evidence from recent natural sediments, assuming
that burial compaction has not yet occurred. In con-
trast to this, early re-deposition experiments do show a
large inclination error, typically with
f
= 0.4 where
tan
I
=
f
tan
I
,
m
f
leading to shallowing as great as 20° in the worst case.
Very slow re-deposition experiments taking several
weeks up to several months would suggest that the
syn-depositional inclination error could be mitigated
by post-depositional reorientation of the magnetic
nano-particles. Deposition on a sloping surface or
from moving water has been shown to defl ect the
remanence from the ambient fi eld by up to 10°;
however, these results are based on very specialized
conditions (Swedish varved material) and it is therefore
diffi cult to know if the results can be applied more
generally.
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