Transcript Stratigraphy & Relative Dating

Stratigraphy
Stratigraphy is the branch of
geology that deals with the
arrangement of rocks in layers.
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Relative Dating
The procedure by which scientists
determine the geologic events which
formed these rocks and the order in
which these events occurred.
Law of Uniformitarianism
The geologic processes which
shape the earth today are the same
as they were in the past.
“The present is the key to the past”
Law of Superposition
In an undisturbed sequence, each
rock later is older than the layers
above it.
“Oldest on the bottom, youngest on
the top.”
Law of Original Horizontality
Sediments are always deposited in
horizontal layers. If sedimentary rock
layers are not horizontal, then an
event like folding, faulting,intrusions,
etc. took place to change the layers
after they were deposited.
Law of Cross-Cut Relationships
If a fault or an igneous intrusion
cuts across or through rock layers,
then the fault or intrusion is
younger than the rock layers.
the Law of Crosscutting Relationships stated that if a fault or other
body of rock cuts through another body of rock then it must be
younger in age than the rock through which it cuts and displaces.
Principle of Unconformity
Often the deposition of rock layers is
stopped for one reason or another
and the layers undergo erosion. This
causes a break in the geologic record.
This break or missing time sequence
is called an unconformity. There are
3 types of unconformities.
A. Disconformity
The sedimentary rock area is
eroded, then undergoes deposition
without any change in the angle of
the rock layers. The old erosional
surface is parallel to the beds
above and below it.
DISCONFORMITIES
Disconformities are characterized by an irregular erosional surface which
truncates flat-lying sedimentary rocks. The layers of sedimentary rocks
above and below the unconformity are parallel with one another.
B. Angular Unconformity
When sedimentary rock layers are
tilted, exposed to the new surface
and then eroded, and covered by
new deposits, an angular
unconformity results. The new
layers ARE NOT parallel to the
older layers.
ANGULAR UNCONFORMITIES
Angular unconformities are characterized by an erosional surface which
truncates folded or dipping (tilted) strata. Overlying strata are deposited
basically parallel with the erosion surface. The rocks above and below the
unconformity are at an angle to one another.
C. Nonconformity
This occurs when igneous or
metamorphic rocks are eroded and
younger sedimentary rocks are
deposited on the erosional surface.
NONCONFORMITIES
Nonconformities are characterized by an erosional surface which
truncates igneous or metamorphic rocks. At a nonconformity, sedimentary
rocks unconformably overlie igneous or metamorphic rocks.
Disconformity
Angular
Unconformity
Nonconformity
Cambro-Ordovician sea floor life.
Relative time scale
Faunal succession. Selected members of the long-extinct group, trilobites,
indicate changing styles with time
Evolution of life
Correlating rocks, etc.
Correlation by fossils. Certain index fossils are keys
to matching sedimentary strata in widely separated
outcrops
Fly reserved in amber for over 3 million years.
Carbonization
conversion to coal
carbon film impressions
PRACTICE
1. Shale
2. Sandstone
3. Shale
4. Conglomerate
5. Shale
6. Limestone
7. Tilting
8. Angular unconformity
9. Igneous (granite)
10.Shale
11.Sandstone
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