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 Chronostratigraphy
is the element of stratigraphy that
deals with the relative time relations and ages of rock bodies.
A
chronostratigraphic classification is the organization of
rocks into units on the basis of their age or time of origin.
 The
purpose of chronostratigraphic classification is to
organize systematically the rocks forming the Earth's crust
into units corresponding to intervals of geologic time to
serve as a basis for time-correlation and recording events of
geologic history.
 Traditionally,
biostratigraphy has formed the
important basis for chronostratigraphic classification.
classification.
most
The Geologic Time Scale: an arrangement of the earth’s
geologic record into a standard sequence of units.
• relative age order
- Principle of Original Horizontality
- Principle of Original Lateral Continuity
- Principle of Superposition
- Principle of Inclusions
- Principle of Cross-Cutting
Cross Cutting
- Principle of Unconformities
• boundaries based on fossils, lithology, polarity
• absolute ages by radiometric dating
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absolute dating
Calibrate
the Time
Scale
relative age biozone
absolute dating
relative age biozone
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Stratigraphic Units
Lithostratigraphic
Units (Rock Units)
Biostratigraphic
Units
Time Units
Time-Rock Units
Lithostratigraphic Units
(Rock Units)
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- Geochronologic Units
- Geochronometric Units
- Chronostratigraphic
Units
Biostratigraphic
Units
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Time unit
Time-rock unit
Example: Devonian Period
A division of time
distinguished on the basis of
the rock record preserved in a
chronostratigraphic unit
Example: Devonian System
A body of rock established to
serve as the material reference
for all rocks formed during the
same span of time
Late
Cretaceous
Time Cretaceous Epoch
Early
Unite Period
Cretaceous
Epoch
Upper
Cretaceous
Time- Cretaceous
Series
Rock System
Lower
Unite
Cretaceous
Series
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Late
Cretaceous
Early
Cretaceous
Upper
Cretaceous
Lower
Cretaceous
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Geochronometric units
Geochronologic units
They are time units, not material ones
Units established through the direct division of
geologic time, expressed in years.
Not based on the time span of designated
A division of time distinguished on the
chronostratigraphic units, but are simply time
b i off the
basis
th rockk record
d preserved
d in
i a
divisions of convenient magnitude for the
chronostratigraphic unit.
purpose for which they are established
Their boundaries are arbitrarily chosen or
agreed upon ages in years
Geochronometric units
Geochronologic units
They are time units, not material ones
Units established through the direct division of
geologic time, expressed in years.
Not based on the time span of designated
A division of time distinguished on the
chronostratigraphic units, but are simply time
basis of the rock record preserved in a
divisions of convenient magnitude for the
chronostratigraphic unit.
purpose for which they are established
Their boundaries are arbitrarily chosen or
agreed upon ages in years
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How do we recognize the same time
period in different vertical sequences
p
q
of
rock that are sometimes separated by
large distances?
 Demonstrating equivalency between rock units.
 Determination of the equivalence of bodies of rock at
different locations.
locations
Matching of rocks of similar ages in different regions.
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Matching
Matching of rocks of
similar age
(Correlation)
Basis of Correlation:
1 Trace of lateral continuity
1.
2. Lithology*
3. Rock properties (surface samples - cuttings - cores)
4. Stratigraphic successions
5 Fossil
5.
F
il content
t t
6. Chemical, magnetic, or geophysical properties
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Main types of correlation:
- Local
- Regional
- Inter-regional
Lithostratigraphic Correlation:
- correlating
g similarities in the sequence
q
of rock types
yp
- requires detailed understanding of rocks and their
structures
Biostratigraphic Correlation:
- correlating similarities in the fossil assemblages
present
- requires
req ires complete understanding
nderstanding of fossils and their
diversity
Chronostratigraphic Correlation:
- correlating time-contemporary rock units
- requires some absolute age constraints
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Main types of correlation:
- Over short distances lithostratigraphic
correlation is the same as chronostratigraphic
correlation.
- Over medium distances they are not the
same.
- Over long distances only chronostratigraphic
correlation can be used.
To correlate lithostratigraphic units, geologists define facies, and
look for key beds or key sequences that can be used as a datum.
 Direct correlation: based on lithology, color, structure, thickness…
 Indirect correlation: electric log correlation (gamma-ray, density,
resistivity…)
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How to categorize rocks




Supergroup
Group
Formation
Member
Correlation is based on
Nukhul Fm.
Abu Zenima Fm.
Tayiba Fm.
1. Composition (lithology
lithology))
2. Position in a sequence
Tanka Fm.
Fm
3. The presence of distinctive
key beds or marker beds
Superposition cannot be used to assign relative
ages in areas of complex structure
Original lateral continuity permits
lithostratigraphic correlation
Correlation based on similarity
of rock type and position.
A formation may has changed somewhat in appearance
between 2 localities, but if it always lies above or below a
distinctive stratum of consistent appearance, then the
correlation of the formation is confirmed
confirmed..
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- Examples:
Marker Bed – Ash Fall
- Examples:
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- Examples:
Unconformity is complicating the lithostratigraphic correlation
- Examples:
Use of
i
igneous
rocks (marker
bed) for
correlation
?
Cyclical
sequence
In case of cyclical
d
deposition,
iti
correlation by
position in
sequence is
rendered more
difficult
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 Despite
advances in geochronology and geophysics,
palaeontology remains a popular tool in stratigraphy.
 Fossil groups are therefore characteristically associated
with geological time units. Some groups may have existed
during longer time periods than others. Some fossil groups
have externally restricted time period of occurrence.
 Such fossils are known as index fossils as they indicate the
age of rock formations much more accurately than others.
1. short stratigraphic range
2. broad geographic distribution
3. broad ecological tolerance
Examples: (often planktic or
swimming) Ammonites, Conodonts,
Trilobites, Graptolites, Planktic
foraminifera, etc.
Using of fossils to
compare rocks from
different
ff
localities
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Biozone 4
Biozone 3
Disconformity
Biozone 2
Biozone 1
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(Assemblage, Abundance & Range Biozones)
 They tend to be of greatest value in local correlation.
 Have particular significance as an indicator of
environment, which may vary regionally.
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 They are unreliable and satisfactory for time-stratigraphic
correlation.
 Peak abundances may be related to favorable local
ecological conditions that can occur at different times in
different areas and that may persist in one area much longer
than in another.
Area 1
Area 2
Area 3
Relative
abundance
Relative
abundance
Time
Time line
Correlation line
Relative
abundance
- Biostratigraphic correlation of range zones
demonstrates that rocks in different areas are of the
same relative
l ti age, even with
ith different
diff
t compositions.
iti
Range biozones yield correlation lines
that coincide in general with time lines
lines..
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- None of these methods
(i.e., lithoand biocorrelation)
functions
alone.
- Information from all give
the
most
complete
description of the rock
record.
Using
biostratigraphy,
disconformities can be quite
easy to find. If you know the
evolutionary progression of
fossils in a time period, you
can find a disconformity by
the gap in the fossil record
 Note that, we cannot rely on lithostratigraphic
correlation to demonstrate time equivalence.
 The most effective way to demonstrate time
equivalence is time-stratigraphic correlation
(chronostratigraphic
chronostratigraphic
biozones.
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correlation
correlation)
using