How to Detect and Correct a Compaction - shallowed Inclination - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 53

particles, but becomes more diffi cult when an anisot-
ropy correction is applied to high-coercivity hematite-
bearing red bed sedimentary rocks.
It is important to stress that remanence anisotropy
is far superior to susceptibility anisotropy (AMS) for
an inclination - shallowing correction. The magnetic
response of a rock to an applied magnetic fi eld (suscep-
tibility) is the result of all the magnetic minerals in a
rock: clays, ferromagnesian silicates and ferromag-
netic minerals such as magnetite and hematite. Even
calcite in a carbonate rock can contribute to an AMS.
The accuracy of an inclination correction relies on
measuring the anisotropy of only the ferromagnetic
minerals in a rock, particularly the same subpopula-
tion of ferromagnetic grains that carry the demagnet-
ized remanence.

forced Baja BC to reach no further south than northern
California (40°N) based on the inclination corrected
locations of rudist fossils. Kim & Kodama (2004) then
made a direct inclination-shallowing correction to
Baja BC rocks exposed near Vancouver Island. The
Nanaimo Group rocks yielded a corrected inclination
that beautifully placed the southern end of Baja BC at
about 41°N, in agreement with the rudist distribution
from corrected paleolatitudes. Krijgsman & Tauxe
(2006) also worked on the inclination shallowing of
Baja BC rocks, but approached it using the EI tech-
nique. Interestingly, Krijgsman and Tauxe came up
with the same size correction as Kim and Kodama (Kim
and Kodama: f = 0.7; Krijgsman and Tauxe: f = 0.68),
but arrived at different tectonic conclusions based on
their correction. Part of the reason for this is that they
also included data from the original study (Ward et al .
1997) that had initial inclinations much lower than
those obtained by Kim & Kodama (2004) and Enkin
et al . (2001) in subsequent studies of the rocks.
The remaining two ARM anisotropy studies in Table
5.1 pushed back the inclination-shallowing correction
into Carboniferous age rocks in order to help resolve
disagreements about the paleogeographic reconstruc-
tions of the continents in this time period. This work,
and Tamaki et al . ' s (2008) study that used an IRM
applied at 45° to bedding (Hodych & Buchan 1994)
will be discussed when we consider the inclination-
shallowing correction for hematite-bearing rocks.

ANISOTROPY-INCLINATION
CORRECTIONS: THE NEXT
GENERATION

A survey of the literature shows that by early 2011 ten
studies had been reported that used ARM to measure
the magnetic anisotropy of a rock in order to make
an inclination-shallowing correction (Table 5.1). The
magnetic mineralogy of these rocks was typically mag-
netite; in one case (the Nacimiento Formation) the
magnetic mineralogy was a titano-hematite that had a
low coercivity like magnetite and therefore ARM could
be effi ciently applied. Six of these studies (Collombat
et al . 1993 ; Hodych & Bijaksana 1993 ; Kodama & Davi
1995 ; Kodama 1997 ; Tan & Kodama 1998 ; Vaughn
et al . 2005) have already been discussed.
The Kodama & Ward (2001) study listed in Table 5.1
did not make any actual inclination corrections, but
used previously corrected rock units to establish the
northern limits of rudists in the Cretaceous along
western North America in order to constrain the
southern limit of British Columbian tectonostrati-
graphic terranes. The Baja BC controversy (Cowan
et al . 1997) about the shallow paleomagnetic inclina-
tions in rocks from coastal British Columbia that put
that part of the world near to Baja California in the
Cretaceous needed to be addressed by inclination-
shallowing corrections. The Kodama & Ward (2001)
study attempted to do that by recognizing that because
Baja BC rocks did not contain rudist fossils, Baja BC
could be no further south than the northern recog-
nized limit of rudist corals in the Cretaceous. This

THEORETICAL BASIS OF THE
ANISOTROPY-INCLINATION
CORRECTION FOR HEMATITE-
BEARING ROCKS

The theoretical basis of the anisotropy-inclination
correction provided by Jackson et al . (1991) assumes
that the magnetization of the magnetic particles in
a rock lies parallel to the long axis of the particle
(the 'easy' axis). This is certainly true for a magnetic
mineral such as magnetite whose magnetization is
governed by the shape of the magnetic particles. Hem-
atite, the magnetic mineral typically carrying the
paleomagnetism of red sedimentary rocks (i.e. red
beds), has a magnetization controlled not by the shape
of the grains but by the internal crystallographic struc-
ture of the hexagonal hematite crystal. In the case of
hematite, the magnetization of the mineral is con-
strained to lie in the basal crystallographic plane. The