Transcript Biostratigraphic unit

Ch. 10—Key terms
• Principle of faunal succession
– Homotaxis
• Zones (or biozones)
– Taxon-range zone; interval zone; concurrent
range zone
• Biostratigraphic correlation
– Causes of diachroniety
• Index fossils (know examples)
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Chapter 10—Biostratigraphy
• Biostratigraphy = the branch of
stratigraphy that is concerned with the
spatial and temporal distribution of fossils
and fossil-bearing strata
• Biostratigraphic unit = body of rock
characterized by its (specified) fossil
content
– Present and recognizable only where its
characteristic taxa are present
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Region 3
Dev.
C
A B
Silurian
Pennsylvanian
Miss.
Devonian
Silurian
Region 4
E
H
G
F
D
Ordovician
C
Mississippian
B
Region 2
Devonian
A
C
Silurian
Devonian
Region 1
Unexplored
Region 5
(1) Determine the total
stratigraphic ranges of
species A–H on the basis
of known occurrences in
regions 1–4
Pennsylvanian
A B C D E F G H
(2) What is the age of a sample that
contains species H, C and E?
Mississippian
Devonian
(3) How would you interpret a sample that
contains species A, B, G and H?
Silurian
Ordovician
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Biostratigraphy
• William “Strata” Smith—father of
biostratigraphy (~1800–1815)
• Principle of faunal succession
– Homotaxis: Biotas follow one another in an
orderly succession through geologic time (as a
consequence of evolution)
• First documented in “Coal Measures” then
in Jurassic rocks in England
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Biostratigraphic units
• Principal biostratigraphic unit is the zone
(or biozone)
– Taxon-range zone = group of strata containing
the total stratigraphic range of a selected taxon
– Interval zone = group of strata between two
biohorizons
– Concurrent range zone = group of strata
containing the overlapping ranges of two or
more specified taxa
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Biostratigraphic units
• Regardless of the kind of zone employed, it
is imperative to define zones in such a way
as to make the boundaries unambiguous
and easily recognizable
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Taxon-range zone
C
E
Commonly used; lower and
upper limits of zone are
defined by lowest and
highest occurrence of
zonal name-bearer (taxon E)
A
B
D
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Interval zone
Most commonly used;
boundaries defined on first
or last occurrences of
specified taxa
C
E
A
B
D
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Concurrent range zone
Commonly used:
Zonal boundaries defined
by the overlapping ranges
of two or more specified
taxa (D & E)
C
E
A
B
D
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Practical recognition of zones
• Few, if any, biozones are recognizable
worldwide
– Most organisms live only in a particular
biogeographic province
– Within a given biogeographic province,
organisms only live where the environment is
favorable
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Shallow marine biogeographic
easward deflection of
provinces
Labrador Current
cold
easward deflection of Gulf Stream
warm
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Biostratigraphic correlation
• Biostratigraphic correlation = procedure by
which approximate age equivalency of strata in
separate areas is determined on the basis of fossils
• Starting point is always determining the local
stratigraphic ranges of taxa in a vertical section
of rock
• Then identify zonal boundaries and compare
with same at some distant section of interest
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Local stratigraphic
ranges and identification
of zonal boundaries
(Pennsylvanian
fusulinids)
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Accuracy of correlation
• Biostratigraphic correlations almost never
are exact time correlations
– Total stratigraphic range of a given taxon is
never preserved at any one locality
– Lowest and highest stratigraphic occurrences of
a given taxon are diachronous (timetransgressive) over large areas
• Evolutionary origin followed by dispersal
• “Last survivor” of a species’ extinction
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Accuracy of correlation (cont.)
• Despite shortcomings, zonal biostratigraphy is the
most accurate, most efficient, and most
practical method for correlating sedimentary
rocks
– Intercontinental resolution can approach ± 0.5 M.y. or
better
– Intrabasinal resolution even better
– Example: 26 conodont zones recognized worldwide in
Upper Devonian tropical carbonate biogeographic
province
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Accuracy of correlation (cont.)
• Best zonations utilize “guide” or “index”
fossils that exhibit:
–
–
–
–
Rapid rates of evolution
Widespread geographic distribution
Occurrence in variety of sedimentary facies
Abundant occurrences
• Typically marine, planktonic or nektonic
organisms
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Index fossils
•
•
•
•
•
Graptolites (most useful in Ordovician and Silurian)
Conodonts (useful throughout Paleozoic and Triassic)
Ammonoids (useful in Devonian through Cretaceous)
Planktonic forams (useful in Jurassic through Holocene)
Calcareous nannoplankton (useful in Jurassic through
Holocene)
• Acritarchs/dinoflagellates (useful throughout
Phanerozoic)
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Biostratigraphy & chronostratigraphy
• Chronostratigraphy = the branch of stratigraphy
concerned with organizing and classifying rocks
into named units that correspond with intervals of
geologic time
– Objective is to subdivide geologic time into units so
that there are no gaps or overlaps
• Chronostratigraphic unit = a body of rock that
formed during a specified interval of geologic
time
• Geochronologic unit = an interval of geologic
time (during which rocks of the corresponding
chronostratigraphic unit formed)
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Chronostratigraphic &
geochronologic units
Chronostratigraphic unit
Geochronologic unit
Eonothem
Erathem
System
Series
Stage
Eon
Era
Period
Epoch
Age
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Definition of chronostratigraphic
units
• Chronostratigraphic units (e.g., systems) are
defined by lower boundary stratotypes
– Physical stratigraphic horizon that defines
base of unit
– Boundary horizon must coincide with the
appearance of an agreed-upon guide fossil in a
depositionally continuous rock sequence
– Top of a chronostratigraphic unit is defined by
the base of the next higher unit
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Lower boundary stratotypes
• Base of Pennsylvanian Subsystem
– 82.9m above base of Bird Spring Formation at Arrow
Canyon, Nevada
– Evolutionary appearance of conodont
Declinognathodus noduliferus (Gnathodus girtyi  D.
noduliferus chronocline)
• Base of Permian System
– Base of limestone beds 19.5f at Aidaralash Creek
section, northern Kazakhstan
– Evolutionary appearance of conodont Streptognathodus
isolatus (S. wabaunsensis  S. isolatus chronocline)
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Triassic lower
boundary stratotype
Evolutionary appearance
of Hindeodus parvus in
Bed 27 at Meishan section
(South China)
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