How to Detect and Correct a Compaction - shallowed Inclination - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 64

there will be some averaging of secular variation
within a site. The net result will be to smooth some of
the features of secular variation and hence reduce the
elongation of the site mean distribution recorded by
the sedimentary rocks. This could lead to an underes-
timate of the initial corrected inclination. For example,
typical deep-sea sediment accumulation rates are less
than 1 cm/kyr; in Chapter 3 deep - sea sediments studied
to understand pDRM acquisition varied over the range
1 - 8 cm/kyr. If a typical deep - sea paleomagnetic sample
is a cube of side length 2 cm, it could integrate the fi eld
over 250 years to more than 2000 years. Secular vari-
ation is averaged out over thousands of years, so for
very slow sediment accumulation rates (< 1 cm/kyr)
the scatter due to geomagnetic secular variation, and
needed for an accurate EI correction, would not be
observable in a collection of samples. The situation
becomes worse if samples at a site are collected over a
fi nite thickness of section. If for instance samples at a
site are collected over c. 0.3 m of stratigraphic thick-
ness (not unlikely in paleomagnetic studies), the site
could average secular variation over 3750 to > 30,000
years, more than enough to completely remove the
scatter due to secular variation. The EI technique has
been applied to many red bed results since its introduc-
tion, and for red beds the situation is similar. Sadler
(1981) suggests that the sediment accumulation rates
for fl uvial, continental sediments ranges 1-10 cm/kyr.
A major difference between the EI and anisotropy
techniques is that the EI technique does not collect
or use any information about the rock magnetic
behavior of the sedimentary rocks being studied. The
anisotropy-based correction requires that the user
measure the remanence anisotropy of at least repre-
sentative samples, if not all of the samples being
studied. The anisotropy technique also requires that
the individual particle anisotropy be determined. These
data in turn tell the user about the nature of the mag-
netic fabric, i.e. whether it is primary or secondary and
whether a primary or secondary magnetic mineral is
carrying the paleomagnetism of the rock. The anisot-
ropy technique requires the user to learn enough about
the rock magnetics to help judge the quality of the
result. For the EI technique, this information can be
assessed with additional rock magnetic measurements
not required by the technique.
Tauxe et al . (2008) present a comparison of the EI
and anisotropy-based correction techniques for two
case studies: (1) the red beds of the Triassic Passaic
Formation corrected by Tan et al . (2007) using the ani-

sotropy technique and (2) the Nacimiento Formation
continental sediments corrected by Kodama (1997)
using ARM anisotropy (Table 5.2). In each case the
anisotropy and EI corrections are statistically indistin-
guishable from each other. For the Passaic Formation
rocks, the anisotropy technique yielded a 10-11° cor-
rection while the EI technique gave a 16° correction.
The corrected inclinations for the Passaic Formation
rocks are 29° for the anisotropy technique and 34° for
the EI correction. Tauxe et al . (2008) argue that the
200 Ma paleomagnetic pole for North America indi-
cates that the EI correction is closer to reality; however,
Tan et al . point out that nearby Newark basin volcanics
of the same age show a 27° inclination, in better agree-
ment with the anisotropy correction. For the Paleocene
Nacimiento Formation of New Mexico, the anisotropy
and EI techniques gave remarkably close corrected
inclinations (anisotropy: 57°; EI: 56°) that are in
agreement with the Paleocene pole for North America
that is based on volcanic results (Diehl et al . 1983 ).
The agreement the EI and anisotropy corrections for
the red beds of the Passaic Formation and siltstones
and claystones of the Nacimiento Formation, as well as
with coeval volcanic rock results, bodes well for the
robustness and accuracy of each technique. The sam-
pling for the EI corrections was particularly well suited
for the technique. The samples from the Passaic Forma-
tion were collected from deep cores drilled into the
Newark Basin; each data point only sampled the geo-
magnetic fi eld over the time it took the 25 mm sample
to be deposited. The Nacimiento Formation data were
from a magnetostratigraphic study with three stand-
ard paleomagnetic samples scrupulously collected
from a single sedimentary horizon. The average sedi-
ment accumulation rate for the Passaic was about
160 m/myr (Kent & Olsen 1997 ), so a 25 mm sample
would average the geomagnetic fi eld over only about
150 years. The average sediment accumulation rate
of the Nacimiento Formation was about 60 m/myr
(based on the magnetostratigraphy), indicating that a
20 mm sample cube would average the fi eld over about
330 years. Each interval is short enough not to average
out secular variation, perfect for the EI correction
technique.
One of the pitfalls of using the EI technique is unrec-
ognized vertical axis rotations between sites in a paleo-
magnetic study. If these rotations are subtle, they could
cause an east-west elongation in the site mean distri-
bution that would be misinterpreted to be caused by
inclination fl attening and yield an overcorrection of