Inclination Shallowing in Sedimentary Rocks: Evidence, Mechanism and Cause - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 43

stones from the southern Alps to nannofossils and
found inclination shallowing via the EI technique, but
only a modest amount ( f = 0.89).
The last example of inclination shallowing in
magnetite-bearing rocks from the literature is Meijers
et al . (2010a) who studied Carboniferous marine lime-
stones and Permian red sandstones from the Donbas
Fold Belt in the Ukraine. They wanted to help resolve
the Pangea B problem, which holds that the overlap
between the northern continents and southern conti-
nents in the Late Paleozoic caused by the paleomag-
netic data is due to unrecognized inclination shallowing
forcing the sampling localities closer to the equator in
the paleogeographical reconstructions than they actu-
ally were (Rochette & Vandamme 2001). Meijers et al .
(2010a) identifi ed and corrected inclination shallow-
ing with the EI technique in Carboniferous magnetite-
bearing rocks. Inclinations steepened slightly but
signifi cantly with correction (average of f = 0.65). The
upper Carboniferous-Permian red beds which Meijers
et al . studied were not corrected because the results
agreed with similar age results from the Donbas Fold
Belt (Iosifi di et al . 2010) that were argued to be unaf-
fected by shallowing. Also, the Permian results of
Meijers et al . did not cause a Pangea B overlap, so were
not suspect.
Two studies that do not fall into our wet sediment
and dry rock categories should also be mentioned.
Iwaki & Hayashida (2003) studied Pleistocene water-
laid tephra deposits in central Japan. Anisotropy of
magnetic susceptibility (AMS) and anisotropy of
anhysteretic remanence (AAR), which will be dis-
cussed in the next chapter as a means of identifying
inclination shallowing, were used to identify modest
inclination shallowing in the tephra ( f = 0.8). Piper
et al . (2007) studied the paleomagnetism of travertine
deposits from the Sicak Cermik geothermal fi eld of
Turkey and found that bedded travertine recorded
paleosecular variation and the disturbance due to
earthquakes, and had anomalously shallow inclina-
tions ( f = 0.68).
Table 4.1 shows that when the fl attening factors for
these 13 studies of magnetite-bearing sediments and
sedimentary rocks are averaged, a fl attening factor of
0.69 ± 0.13 results. Bilardello & Kodama (2010b) pub-
lished an average fl attening factor of 0.65 + 0.14/ -
0.11 for nine magnetite-bearing rocks for which
anisotropy-corrected inclinations were reported (differ-
ent studies from those reported here). The agreement
between the two average f factors shows that magnetite-

bearing rocks and sediments can be assumed to have
inclination shallowing of typically f c. 0.65 - 0.69.

Hematite-bearing rocks

The reports of inclination shallowing in red sedimen-
tary rocks (red beds) occurred somewhat later than
with magnetite-bearing rocks. This delay was in no
small part because of the controversy over how red
beds are magnetized. Paleomagnetists have had long
discussions about whether red beds acquire their mag-
netization by a DRM process, or by secondary growth
of hematite grains sometime after deposition when
they acquire a chemical remanent magnetization
(CRM). This 'red bed controversy' consumed much of
some paleomagnetists' time, particularly in the 1970s.
The argument went that if the red bed carried a CRM
it would be immune from the effects of syn-depositional
inclination shallowing. This is a reasonable argument
but, with the acceptance that burial compaction could
shallow the inclination at burial depths of hundreds of
meters, an early CRM would not necessarily be immune
from compaction-caused inclination shallowing. The
issue is still not completely resolved but the evidence
from red bed anisotropy measurements (see Chapter 5)
suggest that most red beds, particularly those that are
demonstrated to provide reliable paleomagnetic meas-
urements, have either been magnetized by a DRM or a
very early CRM (early enough to be affected by burial
compaction).
The fi rst study that reports shallow inclinations in
red beds, not counting the early re-deposition experi-
ments with red beds covered in Chapter 2 (Lovlie &
Torsvik 1984; Tauxe & Kent 1984), is Garces et al .
(1996) who studied Miocene hematite-bearing sedi-
mentary rocks exposed in the Neogene Catalan basins
of Spain. They observed a correlation between AMS
intensity and inclination shallowing that argued to
them that the paleomagnetic inclinations were affected
by compaction or syn-depositional gravitational forces.
The greatest shallowing was observed in fi nely lami-
nated siltstones that were from distal alluvial fan facies
interbedded with transitional marine facies. Up to 40°
of shallowing was observed in these rocks ( f = 0.21).
Other lithologies investigated by Garces et al . (including
massive mudstones and breccias) had lower amounts
of shallowing, typically 20° ( f = 0.48).
The Mio-Pliocene Siwalik Group red beds of the
Himalayan foreland have long been a paleomagnetic