Relative Age Dating
Students will:
1) understand how the age sequence of rocks can
be determined from strata
2) conduct stratigraphic investigations (interpret
strata)
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Students will:
1) understand the scientists who developed the
principles of Stratigraphy.
2) understand Steno’s 4 principles of Stratigraphy
3) use Steno’s principles to interpret rock
sequences.
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The Father’s of Modern Geology
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William “Strata” Smith
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This principle, developed by the British
Geologist William Smith, suggests that
the landscape developed over longer
periods of time through very slow
geological processes instead of one big
catastrophic event.
This principle gave Darwin the basis
for his theory of evolution. Darwin
suggested that evolutionary change
occurs very slowly, requiring
generations.
Smith’s theory of slow deposition of
rock layers (strata) conformed well to
this scientific idea.
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An example of the
slow processes of
Geology can be seen
at Niagara Falls. The
erosion of the edge of
the waterfall can be
traced since its
discovery by
Europeans in 1678.
Early Geologists
referred to this slow
rate of change as
Uniformitarianism.
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A classic unconformity – two
layers of rock meeting at an angle.
The boundary between the
Permian Yellow Sands
Formation (above), and the
Carboniferous Pennine Upper
Coal Measures (below)
A Scottish farmer known as the
“Father of Modern Geology”
Lived during a time when
natural phenomena were being
explained by natural causes.
Hutton observed unconformities,
and believed in cycles in science.
Hutton proposed the Rock Cycle
and explained earth's natural
phenomena by natural causes
which he could see operating
today such as uplift of rock
layers, erosion, transportation,
deposition, lithification and
volcanism.
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This led to what we call
THE PRINCIPLE OF
UNIFORMITARIANISM
THE CONCEPT REVELED
IN THE CONCEPT OF
DEEP TIME
He viewed the Earth and
Earth history as having, "no
vestige of a beginning, no
prospect of an end."
Geologic Process:
In One Year:
deposition of deep-sea sediment
0.0002 cm
deposition of continental-shelf
sediments
0.004 cm
erosional lowering of continents
0.004 cm
uplift of mountains
0.3 cm
plate movement
4 cm
slip on San Andreas fault
4.5 cm
For Comparision:
growth of fingernails
10 cm
growth of a sapling
18 cm
From Bucke, David, Physical Geology
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Age of canal building - Smith
was a canal engineer.
Smith discovered that different
rock formations contain distinct
fossil assemblages and that even
isolated areas of rocks could be
identified in their sequence by
the fossils they contained.
Made a map of England
published in 1814-1815
Strata identified by organized
fossils 1816-1819
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Smith developed the Principle
of Biotic Succession: The
sequence of life forms in the
stratigraphic record follows a
specific order.
This principle allowed rocks in
distant areas to be ordered.
20 years of intense work by
geologists in Europe and North
America ordered rock
sequences and resulted in the
naming of most "time periods".
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Lyell’s stratigraphic principles,
geological evidence, and results
were published brilliantly in
"Principles of Geology" 1830;
went through 11 editions.
Very strong Uniformitarian.
Felt that speed of Geological
Processes had not changed.
Earth must be very old.
All the big changes in Earth
History were all due to the sum
of small effects.
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Applied Lyellian Uniformitarianism to
organisms.
Theorized that evolution occurred by
natural selection and that vast amounts of
time were needed for evolutionary
changes to occur.
Wrote the groundbreaking, influential On
the Origin of Species in 1859.
His theory provided a mechanism to
explain William Smith’s PRINCIPLE OF
BIOTIC SUCCESSION.
Today Geologists USE SIMILARITY IN
THE FOSSIL ASSEMBLAGES TO
INFER PROXIMITY IN TIME.
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Stratigraphy is the study of the
rock layers and the order of
events that happened to them.
Not only does this include
deposition of the layers and the
order that they were deposited
BUT ALSO anything that
happened to the rocks, like
faults, folding, tilting,
metamorphism, and eroding.
This is one of the
simplest things to
do in basic geology
and usually one of
the more enjoyable
things since it is
like a big puzzle
that needs to be put
together.
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Steno was a Danish scientist of anatomy & geology.
He is thought to be the originator of geology and
stratigraphy and was beatified by Pope John Paul II in 1987.
Steno was also something of a theologist, having being
brought up a Lutheran he later converted to Catholicism as
he considered this branch of Christianity to better facilitate
his curiosity in the natural world.
He was eventually ordained as a priest in Florence.
He was memorialized by Google on January 11, 2012.
In 1669 Niclaus Steno postulated 4 principles of stratigraphy:
1)
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2)
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3)
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4)
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Principle of superposition of strata
In a sequence of strata, any stratum is younger than the sequence of
strata on which it rests, and is older than the strata that rest upon it.
Principle of initial horizontality of strata
Strata are deposited horizontally and then deformed to various
attitudes later.
Principle of stratal continuity
Strata can be assumed to have continued laterally far from where they
presently end.
Principle of cross cutting relationships
Things that cross-cut layers probably postdate them. (i.e. they are
younger than them)
since then we have added the Principle of Biotic Succession and the
Principal of Inclusions.
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The law states that strata
that are younger will be
deposited on top of
strata that are older,
given normal conditions
of deposition.
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In a sequence of strata,
any stratum is younger
than the sequence of
strata on which it rests,
and is older than the
strata that rest upon it.
•
Strata are deposited
horizontally and then
deformed to various
attitudes later.
•
The Law of Original
Horizontality states that
when strata such as
sediments (sand, silt, or
clay) are formed, they are
laid down in horizontal,
flat-lying layers. If you pour
sand into a tub of water, the
layers will build up more or
less horizontally and will be
basically flat
This figure shows both the
Principle of Superposition
(younger layers form on top of
older layers) and the Principle
of Original Horizontality (layers
are deposited horizontally
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Strata can be assumed to
have continued laterally far
from where they presently
end.
This principle allows
geologist to follow layers of
rock that are kilometers
apart often using drill core
samples.
Drill core samples allow geologist to
trace the layers of rock across vast
distances.
3) Principle of Stratal Continuity
Stratal Continuity of rock layers in 3
national parks found near each other in
the Southwest USA.
3) Principle of Stratal Continuity
Stratal Continuity of rock layers in 3 national parks
found near each other in the Southwest USA.
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Things that cross-cut
layers probably postdate
them. (i.e.they are
younger than them)
Often sedimentary rocks
have intrusions of
younger rocks that cut
through them
Often faults displace
rocks from the original
sequence of events
Determine the cross-cutting
layers in the sequences above
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Since Steno’s time we
have added the principle
of biotic succession
A layer of rock will
transition from limestone
to shale to sandstone as
you move down a
continental shelf
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Since Steno’s time we have
added the Principle of
Inclusions
Chunks of rocks can fall
into igneous lavas or glacial
eratics can be included in
older sediments
Inclusions are older than the
layers
Stratigraphy of the Grand Canyon
The Diagram at left
shows how geophysical
testing techniques such
as Seismology can be
used to determine
strata found below the
surface.
This technique is used
to determine layers
associated with oil and
gas deposits.
Geophysics is essential
tool in Oil and Gas
Exploration.
What happened here?
Interpret the geological processes that would
produce this strata?
1)
a)
b)
c)
d)
e)
2)
Stratigraphy does not only include deposition of the
layers and the order that they were deposited but
also anything that happened to the rocks, for
example:
faulting
folding
tilting
metamorphism
eroding.
This is one of the simplest things to do in basic
geology and usually one of the more enjoyable thing
since it is like a big puzzle that needs to be put
together.
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An unconformity is a buried
erosional surface.
What this means is that an
old surface that was exposed
to the surface for an extended
period of time was eroded
then buried.
It usually means the
structures below it are
truncated in some variety.
There are 3 types of
unconformities:
1) Angular Disconformities
2) Disconformity
3) Nonconformity
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Angular Disconformity - This is an unconformity
between strata that are not parallel with one another.
Usually when one set is folded or tilted and the above
layers are not.
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Angular Disconformity - This is an unconformity
between strata that are not parallel with one another.
Usually when one set is folded or tilted and the above
layers are not.
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Disconformity - This is an unconformity between
parallel layers. It indicates a gap in time and is often
hard to identify. Some identifying features are an
irregular surface (wavy instead of flat) and inclusions
(portions of the underlying rock within the above rock).
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Disconformity - This is an unconformity between
parallel layers. It indicates a gap in time and is often
hard to identify. Some identifying features are an
irregular surface (wavy instead of flat) and inclusions
(portions of the underlying rock within the above rock).
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Nonconformity - This is an unconformity between
igneous/metamorphic rocks and sedimentary rocks.
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Nonconformity - This is an unconformity between
igneous/metamorphic rocks and sedimentary rocks.
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Erosion - The current surface that is actively being
eroded. This is always the last thing to be added
on a stratigraphy section.
Tilting - Rock strata always form horizontally, so
anything not horizontal has been acted upon. If
they are straight but at an angle they have been
tilted.
Folding - If the rocks are bent, they have been
folded.
Faulting - If there is a break and movement within
the rocks this is a fault.
suggest the sequence of
events that led to this
cross-section:
 What knowledge and
terms do we need to
understand?
1) Steno’s principles of stratigraphy (+ 2 more)
2) terminology – unconformities, faulting, folding
intrusions, etc.
So let’s fill in the gaps! Look at the Stratigraphy
exercise that was distributed earlier!
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Each layer is labeled (A
through K)
Determine which layer is
youngest
1 (youngest):
2:
3:
4:
5:
6:
7:
8:
9:
10:
11 (oldest):
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Principles of Stratigraphy and Cross-Cutting Relationships:
http://www.youtube.com/watch?v=rc3da3znK4&feature=related
Law of Superposition:
http://www.youtube.com/watch?v=EadTLGMu3LI&feature
=related
3D Seismic:
http://www.youtube.com/watch?v=hxJa7EvYoFI&feature=r
elated
Folds, Dips and Strike:
http://www.youtube.com/watch?v=UzZFMWHlSQ&feature=rellist&playnext=1&list=PL2B2C1DA2EECFE20
F