Although simple in principle, matching of a particular
sequence to a segment of the magnetostratigraphic scale is far
from easy in practice. The ideal condition for a record of
magnetic polarities to be perfectly continuous would be for
the “rain” of sedimentary particles to accumulate continuously over time. But this is rarely the case. In general, the rate
of accumulation of a sedimentary sequence is variable, with
potential differences of an order of magnitude, depending, for
instance, on the changing climatic/environmental conditions
over time. Hiatuses can also occur. All these factors change
the appearance of the sequence of polarities, making its
identification in GPTS complicated. However, in marine
sequences, close to ideal conditions can be found for periods
of about a few million years.
It is certainly a major advantage for magnetostratigraphic
studies to have thick sections (10
2
–10
3 m). Indeed, the
thicker a section is, the more likely it is that it will contain
several polarity zones, and therefore can be more easily
correlated with a defined segment of the reference magnetostratigraphic scale. Our colleague, Robert Butler, made an
amusing analogy between identification of a particular
sequence in the magnetic GPTS and fingerprinting in a
police investigation. Usually, a full fingerprint can identify a
person, half a fingerprint leaves identification open to discussion but a quarter fingerprint is unlikely to be accepted as
irrefutable proof in a court of law.
In practice, even in favorable cases, the researcher is
faced with a series of zones of normal and reverse polarity,
which is often difficult to unambiguously correlate with the
GPTS scale. The assumption made at the outset is that the
rate of sedimentation of the sampled section is more or less
constant. So, different ways of correlating the sequence to
GPTS are tested. In most cases, however, it is necessary to
have an independent marker in time (radiometric dating or
biostratigraphic data already independently correlated to the
GPTS) so that magnetostratigraphy can correlate the full
section being studied to the GPTS, and thus, to specify its
age, temporal thickness and rate of accumulation.
Cross-correlation has been used by some authors to assess
the agreement between the sequence being studied and a
particular segment of the GPTS, by calculating the correlation coefficient corresponding to the various, visually evaluated, solutions. A coefficient of maximum value indicates
the most likely correlation, while a value close to zero allows
rejection of that hypothesis. In practice, this method is only
usable when the studied section contains a large number of
polarity intervals.
Regardless of the method used to associate the studied
sequence with a particular segment of the GPTS, the quality
of a magnetostratigraphic study is primarily based on
unambiguous identification of the magnetic polarity at each
level of the studied sequence. This can sometimes be
achieved if there is agreement between determination of
polarity in two parallel and close sections. In general,
however, this requires a complete paleomagnetic study
(determination of the magnetic mineralogy, field test, progressive demagnetizations either by heating or by applying
alternating fields) to establish the stability of the magnetization and its acquisition at the time of deposition. For this
purpose, Opdyke and Channell (1996) established ten criteria to assess the quality of a study. The authors themselves
recognize that it is very difficult to simultaneously comply
with all ten for one section, but at least five should be present
in a modern magnetostratigraphic study.
A High-Quality Magnetostratigraphic Study: The
Siwalik Sequences in Pakistan
The deposits in the foothills of the Siwalik Basin in northern
India and Pakistan are among the most studied fluvial sediments. Indeed, in them, many fossils of mammals, including
primates, have been discovered. Their precise chronological
study and the correlation of different outcropping sections is
of crucial importance for understanding the evolution and
migration of these hominids and their land use.
From a paleomagnetic point of view, the main problem
with this study is that the deposits of Siwalik are “red beds”
and magnetization is carried by hematite. The magnetization
of this type of sediment is very complex, and it has been
shown in some cases that magnetization was acquired as a
result of chemical reactions in the sediment, considerably
after deposition. It is therefore essential for a magnetostratigraphic study of Siwalik to establish that magnetization was acquired during deposition or immediately
afterwards so as to prevent invalidating correlation with the
magnetic polarity scale (GPTS).
The study of the magnetic mineralogy showed that the
components of magnetization of these red beds fall under two
different categories of hematite: a red pigmentation phase and
a specular hematite phase. The red pigmentation phase
acquired its magnetization well after deposition (at least one
polarity interval later). However, the magnetization carried
by specularite was acquired during deposition or immediately
afterwards. These two components can be separated by
thermal demagnetization, the magnetization of the specularite
being isolated between 525 and 600 °C. This component can
then be used to establish the magnetic polarity of the sampled
sections (Fig. 7.6) which is then correlated with the scale of
Mankinen and Dalrymple (1979) (Fig. 7.7).
7 Magnetostratigraphy: From a Million to a Thousand Years
107
sequence to a segment of the magnetostratigraphic scale is far
from easy in practice. The ideal condition for a record of
magnetic polarities to be perfectly continuous would be for
the “rain” of sedimentary particles to accumulate continuously over time. But this is rarely the case. In general, the rate
of accumulation of a sedimentary sequence is variable, with
potential differences of an order of magnitude, depending, for
instance, on the changing climatic/environmental conditions
over time. Hiatuses can also occur. All these factors change
the appearance of the sequence of polarities, making its
identification in GPTS complicated. However, in marine
sequences, close to ideal conditions can be found for periods
of about a few million years.
It is certainly a major advantage for magnetostratigraphic
studies to have thick sections (10
2
–10
3 m). Indeed, the
thicker a section is, the more likely it is that it will contain
several polarity zones, and therefore can be more easily
correlated with a defined segment of the reference magnetostratigraphic scale. Our colleague, Robert Butler, made an
amusing analogy between identification of a particular
sequence in the magnetic GPTS and fingerprinting in a
police investigation. Usually, a full fingerprint can identify a
person, half a fingerprint leaves identification open to discussion but a quarter fingerprint is unlikely to be accepted as
irrefutable proof in a court of law.
In practice, even in favorable cases, the researcher is
faced with a series of zones of normal and reverse polarity,
which is often difficult to unambiguously correlate with the
GPTS scale. The assumption made at the outset is that the
rate of sedimentation of the sampled section is more or less
constant. So, different ways of correlating the sequence to
GPTS are tested. In most cases, however, it is necessary to
have an independent marker in time (radiometric dating or
biostratigraphic data already independently correlated to the
GPTS) so that magnetostratigraphy can correlate the full
section being studied to the GPTS, and thus, to specify its
age, temporal thickness and rate of accumulation.
Cross-correlation has been used by some authors to assess
the agreement between the sequence being studied and a
particular segment of the GPTS, by calculating the correlation coefficient corresponding to the various, visually evaluated, solutions. A coefficient of maximum value indicates
the most likely correlation, while a value close to zero allows
rejection of that hypothesis. In practice, this method is only
usable when the studied section contains a large number of
polarity intervals.
Regardless of the method used to associate the studied
sequence with a particular segment of the GPTS, the quality
of a magnetostratigraphic study is primarily based on
unambiguous identification of the magnetic polarity at each
level of the studied sequence. This can sometimes be
achieved if there is agreement between determination of
polarity in two parallel and close sections. In general,
however, this requires a complete paleomagnetic study
(determination of the magnetic mineralogy, field test, progressive demagnetizations either by heating or by applying
alternating fields) to establish the stability of the magnetization and its acquisition at the time of deposition. For this
purpose, Opdyke and Channell (1996) established ten criteria to assess the quality of a study. The authors themselves
recognize that it is very difficult to simultaneously comply
with all ten for one section, but at least five should be present
in a modern magnetostratigraphic study.
A High-Quality Magnetostratigraphic Study: The
Siwalik Sequences in Pakistan
The deposits in the foothills of the Siwalik Basin in northern
India and Pakistan are among the most studied fluvial sediments. Indeed, in them, many fossils of mammals, including
primates, have been discovered. Their precise chronological
study and the correlation of different outcropping sections is
of crucial importance for understanding the evolution and
migration of these hominids and their land use.
From a paleomagnetic point of view, the main problem
with this study is that the deposits of Siwalik are “red beds”
and magnetization is carried by hematite. The magnetization
of this type of sediment is very complex, and it has been
shown in some cases that magnetization was acquired as a
result of chemical reactions in the sediment, considerably
after deposition. It is therefore essential for a magnetostratigraphic study of Siwalik to establish that magnetization was acquired during deposition or immediately
afterwards so as to prevent invalidating correlation with the
magnetic polarity scale (GPTS).
The study of the magnetic mineralogy showed that the
components of magnetization of these red beds fall under two
different categories of hematite: a red pigmentation phase and
a specular hematite phase. The red pigmentation phase
acquired its magnetization well after deposition (at least one
polarity interval later). However, the magnetization carried
by specularite was acquired during deposition or immediately
afterwards. These two components can be separated by
thermal demagnetization, the magnetization of the specularite
being isolated between 525 and 600 °C. This component can
then be used to establish the magnetic polarity of the sampled
sections (Fig. 7.6) which is then correlated with the scale of
Mankinen and Dalrymple (1979) (Fig. 7.7).
7 Magnetostratigraphy: From a Million to a Thousand Years
107