datings approaching an absolute time scale, but radiometric datings are not absolute with respect to time.
We thus distinguish between two types of time
stratigraphy: (1) Geochronology, which is the subdivision of the Earth’s history into finite time units. A
geochronological unit is a specific interval of geological time. (2) Chronostratigraphy or time-rock stratigraphy is the subdivision of sedimentary successions,
and their correlation, on the basis of time. Chronostratigraphic units are by definition synchronous.
A geochronological unit represents a specific interval in the geological time scale. For example, the
Jurassic period is a geochronological unit. A
geochronological unit may define the time span
between two specific geological events. We can say,
for example, that some of the rocks in the North Sea
were deposited during the Jurassic period. The
corresponding chronostratigraphic unit (system)
signifies the rocks which were formed during the
same period. We therefore say that some of the rocks
in the North Sea belong to the Jurassic system.
The basic chronostratigraphic unit is a chronozone.
A chronozone includes all the deposits formed during
a particular, relatively short, geologic time interval
and which are defined by a geological phenomenon
or by a particular interval of a rock succession. In most
cases a chronozone is a taxon chronozone, which is
defined as the period between the first appearance and
last occurrence of a particular fossil taxon (Fig. 7.8).
Whereas a biozone can only be defined where the
fossil is present, a chronozone represents all the
rocks that were formed during this period, regardless
of whether they contain fossils. A chron is the interval
of time during which the rock in a chronozone was
formed. It is thus the geochronological equivalent of a
chronozone.
A chronozone may be named after a biostratigraphic unit, e.g. a Didymograptus extensus
chronozone, or after a lithostratigraphic unit, which
can be recognised over large areas. Such characteristic
strata are often called marker beds, key beds, or datum
beds. Chronostratigraphic horizons are important for
correlation within sedimentary basins, and form the
framework for all facies reconstructions. If we have
two chronohorizons or datum beds, we can measure
the variation in thickness and composition of the
sediments which were deposited during a particular
time period. This may make it possible to map
variations in sedimentation rate and the ratio of sandstone to shale. Maps showing the sediment thickness
between two marker beds are called isopach maps.
This is very useful for reconstructing sedimentary
facies on the basis of borehole data, a standard method
in connection with oil prospecting.
The best chronostratigraphic horizons (marker
beds) are bentonite (ash) layers; on a smaller regional
scale also coal beds, phosphate beds, thin limestone or
sandstone beds, or particular fossil horizons. When
analysing stratigraphic records from wells (logs) one
tends to use beds which produce a distinctive log
pattern. Seismic reflectors can also in certain instances
be used as datum beds (horizons).
A stage is a chronostratigraphic unit which includes
one or more chronozones but which nevertheless
covers a limited period of time, usually 3–10 million
years (Fig. 7.9). This is the smallest unit in the
chronostratigraphic hierarchy which is used for correlation all over the world. A stage is defined in a type
section and usually designated by a geographical name
near the type profile. For example, the Kimmeridgian
stage is well exposed on the Dorset coast at
Kimmeridge. The correlation of a stage is usually
based on biostratigraphy. An age is the period of
time (geochronological unit) which corresponds to a
stage.
A series is a chronostratigraphic unit larger than a
stage. For example, the Late Jurassic is a series
constituting part of the Jurassic system. The
geochronological unit which corresponds to a series
is an epoch. We can say that a certain limestone was
deposited during the Late Jurassic epoch.
A geochronological period varies in duration from
about 20–30 million years (Silurian) to about 60–70
million years (Cretaceous). The Quaternary period,
however, is much shorter, only about 2.5 million
years. The rocks formed during a period constitute a
system. An era is comprised of two or more periods.
The Palaeozoic era had a duration of about 300 million
years, but the Cenozoic era did not last longer than the
longest Palaeozoic periods (65 million years).
The largest units in the chronostratigraphic scale,
erathem and enothem, are not used much, since it is
seldom relevant to group rocks which were deposited
over such long periods of time. However, when we
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J. Nagy and K. Bjørlykke
We thus distinguish between two types of time
stratigraphy: (1) Geochronology, which is the subdivision of the Earth’s history into finite time units. A
geochronological unit is a specific interval of geological time. (2) Chronostratigraphy or time-rock stratigraphy is the subdivision of sedimentary successions,
and their correlation, on the basis of time. Chronostratigraphic units are by definition synchronous.
A geochronological unit represents a specific interval in the geological time scale. For example, the
Jurassic period is a geochronological unit. A
geochronological unit may define the time span
between two specific geological events. We can say,
for example, that some of the rocks in the North Sea
were deposited during the Jurassic period. The
corresponding chronostratigraphic unit (system)
signifies the rocks which were formed during the
same period. We therefore say that some of the rocks
in the North Sea belong to the Jurassic system.
The basic chronostratigraphic unit is a chronozone.
A chronozone includes all the deposits formed during
a particular, relatively short, geologic time interval
and which are defined by a geological phenomenon
or by a particular interval of a rock succession. In most
cases a chronozone is a taxon chronozone, which is
defined as the period between the first appearance and
last occurrence of a particular fossil taxon (Fig. 7.8).
Whereas a biozone can only be defined where the
fossil is present, a chronozone represents all the
rocks that were formed during this period, regardless
of whether they contain fossils. A chron is the interval
of time during which the rock in a chronozone was
formed. It is thus the geochronological equivalent of a
chronozone.
A chronozone may be named after a biostratigraphic unit, e.g. a Didymograptus extensus
chronozone, or after a lithostratigraphic unit, which
can be recognised over large areas. Such characteristic
strata are often called marker beds, key beds, or datum
beds. Chronostratigraphic horizons are important for
correlation within sedimentary basins, and form the
framework for all facies reconstructions. If we have
two chronohorizons or datum beds, we can measure
the variation in thickness and composition of the
sediments which were deposited during a particular
time period. This may make it possible to map
variations in sedimentation rate and the ratio of sandstone to shale. Maps showing the sediment thickness
between two marker beds are called isopach maps.
This is very useful for reconstructing sedimentary
facies on the basis of borehole data, a standard method
in connection with oil prospecting.
The best chronostratigraphic horizons (marker
beds) are bentonite (ash) layers; on a smaller regional
scale also coal beds, phosphate beds, thin limestone or
sandstone beds, or particular fossil horizons. When
analysing stratigraphic records from wells (logs) one
tends to use beds which produce a distinctive log
pattern. Seismic reflectors can also in certain instances
be used as datum beds (horizons).
A stage is a chronostratigraphic unit which includes
one or more chronozones but which nevertheless
covers a limited period of time, usually 3–10 million
years (Fig. 7.9). This is the smallest unit in the
chronostratigraphic hierarchy which is used for correlation all over the world. A stage is defined in a type
section and usually designated by a geographical name
near the type profile. For example, the Kimmeridgian
stage is well exposed on the Dorset coast at
Kimmeridge. The correlation of a stage is usually
based on biostratigraphy. An age is the period of
time (geochronological unit) which corresponds to a
stage.
A series is a chronostratigraphic unit larger than a
stage. For example, the Late Jurassic is a series
constituting part of the Jurassic system. The
geochronological unit which corresponds to a series
is an epoch. We can say that a certain limestone was
deposited during the Late Jurassic epoch.
A geochronological period varies in duration from
about 20–30 million years (Silurian) to about 60–70
million years (Cretaceous). The Quaternary period,
however, is much shorter, only about 2.5 million
years. The rocks formed during a period constitute a
system. An era is comprised of two or more periods.
The Palaeozoic era had a duration of about 300 million
years, but the Cenozoic era did not last longer than the
longest Palaeozoic periods (65 million years).
The largest units in the chronostratigraphic scale,
erathem and enothem, are not used much, since it is
seldom relevant to group rocks which were deposited
over such long periods of time. However, when we
242
J. Nagy and K. Bjørlykke