7.3.2.3 Redeposition
Erosion of fossil-bearing sediments with subsequent
transport and redeposition of microfossils can lead to
serious problems for dating and correlation.
Palynomorphs are particularly liable to redeposition
owing to their small size and weight. Conodonts also
have a high redeposition potential because their calcium phosphate skeletons are strongly resistant to
corrosion by weathering and transport. Redeposition
is suspected if microfossils appear at higher stratigraphic levels than expected. Reworked microfossils
can be distinguished by changes in colour and traces of
wear.
7.3.2.4 Facies Barriers and Biostratigraphy
Sediment cores from deep oceanic settings usually
provide undisturbed sections containing a continuous
fossil record, commonly of high stratigraphic resolution. Based on this type of material, biostratigraphy
combined with magnetostratigraphy and chemostratigraphy contributed heavily to development of a
global biochronozonal framework. Although with
marked reductions, this framework has been extended
to formations deposited in marine shelf, deltaic and
coastal areas, where it forms an important basis for
dating and correlation.
The basic unit of biostratigraphic correlation is the
taxon chronozone, also called biochronozone. As defined
below, a taxon chronozone represents all sediments
deposited between the evolutionary inception and
extinction (total range) of its marker species (Fig. 7.8).
In contrast, the biozone represents only those sediments
at a given place that contain one or several marker
species. In the case of a single marker, the biozone can
be defined by the local appearance and disappearance
(local range) of the species. The time interval represented
by the biozone may differ from place to place.
In deep oceanic regimes, the latitudinal distribution
of each species is generally defined by the water mass
temperature and chemistry. Therefore, the regional
(lateral) extent of biozones and taxon chronozones is
of restricted nature. Outside its optimal stratigraphic
distribution area, the range of the marker species can
vary considerably owing to environmental factors. At
such sites the chronozone cannot be defined and the
term biozone is used. Correlation between tropical and
temperate oceanic regions is difficult, because of the
low number of biota that include reliable zonal indicator species common to both areas. Interfingering of
biozones (local ranges) and integration with
magnetostratigraphy and chemostratigraphy is a useful approach to this type of correlation.
7.3.3 Biozones
The basic unit of biostratigraphy is the biozone which
is a body of strata defined by its fossil content. During
the history of biostratigraphy, a variety of names and
definitions of units have been proposed, but there are
only three basic types of zones: interval zones, assemblage zones and abundance zones. Zones defined in
sedimentary successions are usually arranged into a
zonal scheme, which can be developed for an extensive region and even for global application.
7.3.3.1 Interval Zones
These are the most commonly used type of zones, with
boundaries defined by particular fossil events, specifically the first and last occurrence of taxa (usually
species). Thus, the interval zone comprises the strata
deposited between these two events. There are five
types of interval zones (Fig. 7.6).
1. Taxon range zone is defined by the first and last
occurrence of a particular taxon.
2. Concurrent range zone is the interval between the
first occurrence of a taxon and the last occurrence
of another taxon.
3. Partial range zone is the interval between the last
occurrence of a taxon and the first occurrence of
another taxon partitioning the range of a third
taxon.
4. Successive first appearance zone represents the
interval between the successive first appearance of
two taxa.
5. Successive last appearance zone is the interval
between the successive last appearance of two taxa.
The successive last appearance zone is the type
most commonly used in industrial biostratigraphy,
because the samples are mainly derived from well
drilling cuttings. In such sample sets, the actual first
appearances are unreliable owing to downhole
(caving) contamination. Precise positions of first
appearances are required in order to recognise the
first four interval zone types. Taxon range zone and
successive first appearance zone are the most applied
in zonation of onshore sections.
238
J. Nagy and K. Bjørlykke
Erosion of fossil-bearing sediments with subsequent
transport and redeposition of microfossils can lead to
serious problems for dating and correlation.
Palynomorphs are particularly liable to redeposition
owing to their small size and weight. Conodonts also
have a high redeposition potential because their calcium phosphate skeletons are strongly resistant to
corrosion by weathering and transport. Redeposition
is suspected if microfossils appear at higher stratigraphic levels than expected. Reworked microfossils
can be distinguished by changes in colour and traces of
wear.
7.3.2.4 Facies Barriers and Biostratigraphy
Sediment cores from deep oceanic settings usually
provide undisturbed sections containing a continuous
fossil record, commonly of high stratigraphic resolution. Based on this type of material, biostratigraphy
combined with magnetostratigraphy and chemostratigraphy contributed heavily to development of a
global biochronozonal framework. Although with
marked reductions, this framework has been extended
to formations deposited in marine shelf, deltaic and
coastal areas, where it forms an important basis for
dating and correlation.
The basic unit of biostratigraphic correlation is the
taxon chronozone, also called biochronozone. As defined
below, a taxon chronozone represents all sediments
deposited between the evolutionary inception and
extinction (total range) of its marker species (Fig. 7.8).
In contrast, the biozone represents only those sediments
at a given place that contain one or several marker
species. In the case of a single marker, the biozone can
be defined by the local appearance and disappearance
(local range) of the species. The time interval represented
by the biozone may differ from place to place.
In deep oceanic regimes, the latitudinal distribution
of each species is generally defined by the water mass
temperature and chemistry. Therefore, the regional
(lateral) extent of biozones and taxon chronozones is
of restricted nature. Outside its optimal stratigraphic
distribution area, the range of the marker species can
vary considerably owing to environmental factors. At
such sites the chronozone cannot be defined and the
term biozone is used. Correlation between tropical and
temperate oceanic regions is difficult, because of the
low number of biota that include reliable zonal indicator species common to both areas. Interfingering of
biozones (local ranges) and integration with
magnetostratigraphy and chemostratigraphy is a useful approach to this type of correlation.
7.3.3 Biozones
The basic unit of biostratigraphy is the biozone which
is a body of strata defined by its fossil content. During
the history of biostratigraphy, a variety of names and
definitions of units have been proposed, but there are
only three basic types of zones: interval zones, assemblage zones and abundance zones. Zones defined in
sedimentary successions are usually arranged into a
zonal scheme, which can be developed for an extensive region and even for global application.
7.3.3.1 Interval Zones
These are the most commonly used type of zones, with
boundaries defined by particular fossil events, specifically the first and last occurrence of taxa (usually
species). Thus, the interval zone comprises the strata
deposited between these two events. There are five
types of interval zones (Fig. 7.6).
1. Taxon range zone is defined by the first and last
occurrence of a particular taxon.
2. Concurrent range zone is the interval between the
first occurrence of a taxon and the last occurrence
of another taxon.
3. Partial range zone is the interval between the last
occurrence of a taxon and the first occurrence of
another taxon partitioning the range of a third
taxon.
4. Successive first appearance zone represents the
interval between the successive first appearance of
two taxa.
5. Successive last appearance zone is the interval
between the successive last appearance of two taxa.
The successive last appearance zone is the type
most commonly used in industrial biostratigraphy,
because the samples are mainly derived from well
drilling cuttings. In such sample sets, the actual first
appearances are unreliable owing to downhole
(caving) contamination. Precise positions of first
appearances are required in order to recognise the
first four interval zone types. Taxon range zone and
successive first appearance zone are the most applied
in zonation of onshore sections.
238
J. Nagy and K. Bjørlykke