greatly over geologic time (Fig. 3.8). By analysing this
ratio in minerals precipitated in marine environments
the ages can be constrained. During Tertiary time, the
87 Sr=
86 Sr ratio increased and analyses of calcareous
organisms will give a unique age, in some cases with a
resolution of 1–2 million years. This method can be
applied also to calcareous microfossils such as foraminifera and coccoliths. In carbonate the Sr/Ca ratio may
be useful also in detecting depositional facies, because
the data will reflect the primary aragonite/calcite ratio.
7.7.2 Bulk Chemical Composition Analyses
Chemical analyses of bulk samples of cuttings and
cores may be useful in correlation between wells,
because the chemical composition reflects changes in
the composition of sediments supplied to the basin.
Particularly good results are expected in correlation of
reservoirs. For this purpose major elements, trace
elements and isotopic composition can be used. The
samples can be analysed by XRF, by energy dispersive
systems in a SEM, or in mass-spectrographs. Analyses
can be performed for bulk chemical composition.
The bulk chemical composition varies as a function
of grain size but the ratio between major elements such
as Na/Al and K/Al may be characteristic for the provenance rocks of sediments. Both sandstones and shales
may have very different ratios between Na-feldspar
and K-feldspar. Trace elements may be useful also in
provenance studies.
7.8
Magnetostratigraphy
7.8.1 Palaeomagnetism
When clastic sediments are deposited or when volcanic rocks solidify, minerals orientate themselves in the
prevailing magnetic field. Magnetic minerals in clastic
sediments can orient themselves according to the magnetic field during deposition, while diagenetic
minerals will become oriented in the magnetic field
during diagenesis. The magnetic minerals in rocks
thus define a magnetic vector, which indicates the
direction and strength of the magnetic field during
formation. By compensating for later magnetic effects,
the orientation of this remanent magnetism can be
measured. The data will reveal the position of the
geographic pole, and indicate the palaeogeographical
latitude and longitude.
Palaeomagnetic measurements are of great help in
reconstructing the positions of the continents during
the geological past, and are important for plate tectonic and palaeoclimatic reconstructions. From the
Palaeozoic onwards there is quite good agreement
between palaeomagnetic determinations of latitude
and palaeoclimatic indications like biogeography, glacial deposits and evaporites, but this is not the case
with Precambrian deposits, which are more difficult.
7.8.2 Magnetic Field Polarity Changes
It has become evident that the polarity of the Earth’s
magnetic field has been reversed for numerous time
periods of varying duration. A number of
measurements of magnetism in rocks of known age
have provided us with a time scale based on periods of
normal and reversed magnetic field. We thus find that
we can divide geological time into periods with dominantly normal or reversed polarity (Fig. 7.11). Within
these we also find several shorter intervals when the
magnetic field switched between the two polarities.
Since we must assume that the switching between
normal and reversed magnetism has taken place simultaneously and suddenly all over the world, such physical changes offer an ideal basis for correlation.
There are often major practical problems, however,
since many periods of the Earth’s history are
characterised by a predominance of successions with
rapidly changing reversals. Where we have many
measurements and continuous profiles, e.g. in deepsea cores, we will be able to correlate with relative
certainty on the basis of the longer periods of magnetic
field stability. Volcanic rocks and sediments deposited
in fluvial or shallow-water environments, however,
will have numerous hiatuses between beds, hampering
registration of continuous variations in the residual
magnetism. During the last 700,000 years we have
had apparently normal polarity, possibly with the
exception of a short period about 200,000–300,000
years ago. If we find sediments or volcanic rocks
246
J. Nagy and K. Bjørlykke
ratio in minerals precipitated in marine environments
the ages can be constrained. During Tertiary time, the
87 Sr=
86 Sr ratio increased and analyses of calcareous
organisms will give a unique age, in some cases with a
resolution of 1–2 million years. This method can be
applied also to calcareous microfossils such as foraminifera and coccoliths. In carbonate the Sr/Ca ratio may
be useful also in detecting depositional facies, because
the data will reflect the primary aragonite/calcite ratio.
7.7.2 Bulk Chemical Composition Analyses
Chemical analyses of bulk samples of cuttings and
cores may be useful in correlation between wells,
because the chemical composition reflects changes in
the composition of sediments supplied to the basin.
Particularly good results are expected in correlation of
reservoirs. For this purpose major elements, trace
elements and isotopic composition can be used. The
samples can be analysed by XRF, by energy dispersive
systems in a SEM, or in mass-spectrographs. Analyses
can be performed for bulk chemical composition.
The bulk chemical composition varies as a function
of grain size but the ratio between major elements such
as Na/Al and K/Al may be characteristic for the provenance rocks of sediments. Both sandstones and shales
may have very different ratios between Na-feldspar
and K-feldspar. Trace elements may be useful also in
provenance studies.
7.8
Magnetostratigraphy
7.8.1 Palaeomagnetism
When clastic sediments are deposited or when volcanic rocks solidify, minerals orientate themselves in the
prevailing magnetic field. Magnetic minerals in clastic
sediments can orient themselves according to the magnetic field during deposition, while diagenetic
minerals will become oriented in the magnetic field
during diagenesis. The magnetic minerals in rocks
thus define a magnetic vector, which indicates the
direction and strength of the magnetic field during
formation. By compensating for later magnetic effects,
the orientation of this remanent magnetism can be
measured. The data will reveal the position of the
geographic pole, and indicate the palaeogeographical
latitude and longitude.
Palaeomagnetic measurements are of great help in
reconstructing the positions of the continents during
the geological past, and are important for plate tectonic and palaeoclimatic reconstructions. From the
Palaeozoic onwards there is quite good agreement
between palaeomagnetic determinations of latitude
and palaeoclimatic indications like biogeography, glacial deposits and evaporites, but this is not the case
with Precambrian deposits, which are more difficult.
7.8.2 Magnetic Field Polarity Changes
It has become evident that the polarity of the Earth’s
magnetic field has been reversed for numerous time
periods of varying duration. A number of
measurements of magnetism in rocks of known age
have provided us with a time scale based on periods of
normal and reversed magnetic field. We thus find that
we can divide geological time into periods with dominantly normal or reversed polarity (Fig. 7.11). Within
these we also find several shorter intervals when the
magnetic field switched between the two polarities.
Since we must assume that the switching between
normal and reversed magnetism has taken place simultaneously and suddenly all over the world, such physical changes offer an ideal basis for correlation.
There are often major practical problems, however,
since many periods of the Earth’s history are
characterised by a predominance of successions with
rapidly changing reversals. Where we have many
measurements and continuous profiles, e.g. in deepsea cores, we will be able to correlate with relative
certainty on the basis of the longer periods of magnetic
field stability. Volcanic rocks and sediments deposited
in fluvial or shallow-water environments, however,
will have numerous hiatuses between beds, hampering
registration of continuous variations in the residual
magnetism. During the last 700,000 years we have
had apparently normal polarity, possibly with the
exception of a short period about 200,000–300,000
years ago. If we find sediments or volcanic rocks
246
J. Nagy and K. Bjørlykke