Transcript Lecture 10 Stratigraphy and Geologic Time

Lecture 10 Stratigraphy and Geologic
Time

Stratigraphy
 Basic principles of relative age dating
 Unconformities: Markers of missing time
 Correlation of rock units
 Absolute dating
 Geologic Time
How old is the Earth? When did various geologic events occur?
Interpreting Earth history is a prime goal of geology. Some knowledge
of Earth history and geologic time is also required for engineers in
order to understand relationships between geologic units and their
impact on engineering construction.

Stratigraphy:
 Stratigraphy is
the study of rock layers
(strata) and their relationship with each
other.
 Stratigraphy provides
simple principles
used to interpret geologic events.
Two rock units at a cliff in Missouri. (US Geological Survey)

Basic principles of relative age dating
Relative dating means that rocks are placed in their proper
sequence of formation. A formation is a basic unit of
rocks. Below are some basic principles for establishing
relative age between formations.

Principle of original horizontality

Principle of superposition

Principle of faunal succession

Principle of cross-cutting relationships

Principle of original horizontality:
Layers of sediment are generally deposited in a
horizontal position.
Thus if we observed rock layers that are folded or
inclined, they must, with exceptions, have been
moved into that position by crustal disturbances
sometime after their deposition.

Most layers of sediment are deposited in a nearly horizontal position. Thus,
when we see inclined rock layers as shown, we can assume that they must
have been moved into that position after deposition. Hartland Quay, Devon,
England by Tom Bean/DRK Photo.

Principle of superposition:
In an undeformed sequence of sedimentary
rocks, each bed is older than the one above and
younger than the one below.
The rule also applies to other surface-deposited
materials such as lava flows and volcanic ashes.
Principle of superposition. (W.W. Norton)

Applying the law of superposition to the layers at the upper portion of
the Grand Canyon, the Supai Group is the oldest and the Kaibab
Limestone is the youngest. (photo by Tarbuck).

Principle of cross-cutting relationships:
When a fault cuts through rocks, or when magma
intrudes and crystallizes, we can assume that the
fault or intrusion is younger than the rocks
affected.

Cross-cutting relationships: An intrusive rock body is
younger than the rocks it intrudes. A fault is younger
than the rock layers it cuts. (Tarbuck and Lutgens)

Unconformities: Markers of missing time
When layers of rock formed without interruption, we call them
conformable.
An unconformity represents a long period during which deposition
ceased and erosion removed previously formed rocks before
deposition resumed.

Angular unconformities

Disconformity

Nonconformity

Angular unconformities:
An angular unconformity consists of tilted or
folded sedimentary rocks that are overlain by
younger, more flat-lying strata.
It indicates a long period of rock deformation and
erosion.
Formation of an angular unconformity. An angular unconformity
represents an extended period during which deformation and erosion
occurred. (Tarbuck and Lutgents)
Angular unconformity at Siccar
Point, southern Scotland, that
was first described by James
Hutton more than 200 years ago.
(Hamblin and Christiansen and
W.W. Norton)

Disconformity:
A disconformity is a minor irregular surface
separating parallel strata on opposite sides of the
surface.
It indicates a history of uplifting above sea (water)
level, undergoing erosion, and lowering below the
sea level again.
Formation of disconformity. (W.W. Norton)

Disconformities do not show angular discordance, but an erosion
surface separates the two rock bodies. The channel in the central part of
this outcrop reveals that the lower shale units were deposited and then
eroded before the upper units were deposited. (Hamblin and Christiansen)
Nonconformity

A nonconformity is a break
surface that developed when igneous
or metamorphic rocks were exposed
to erosion, and younger sedimentary
rocks were subsequently deposited
above the erosion surface. (Tarbuck
and Lutgens)

A nonconformity at the Grand Canyon. The metamorphic rocks
and the igneous dikes of the inner gorge were formed at great depths
and subsequently uplifted and eroded. Younger sedimentary layers
were then deposited on the eroded surface of the igneous and
metamorphic terrain. (Hamblin and Christiansen)
Types of Unconformity

This animation shows the stages in the
development of three main types of unconformity
in cross-section, and explains how an incomplete
succession of strata provides a record of Earth
history. View 1 shows a disconformity, View 2
shows a nonconformity and View 3 shows an
angular unconformity. [by Stephen Marshak]
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Play Animation Windows version >>
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Play Animation Macintosh version >>

Distinguishing nonconformity and intrusive
contact

Nonconformity:
The sedimentary rock is younger. The erosion surface is generally smooth.
Dikes may cut through the igneous body but stop at the nonconformity.

Intrusive contact:
Intrusion is younger than the surrounding sedimentary rocks. The contact
surface may be quite irregular. A zone of contact metamorphism may
form surrounding the igneous body. Cross-cutting dikes may penetrate
both the igneous body and the sedimentary rocks.

Contrasting field conditions for (a) a nonconformity and
(b) an igneous intrusion. (West, Fig 9.4)

The three basic types of unconformities illustrated by this
cross-section of the Grand Canyon. (Tarbuck and Lutgents)
Geologic History

A cross-section through the earth reveals the
variety of geologic features. View 1 of this
animation identifies a variety of geologic features;
View 2 animates the sequence of events that
produced these features, and demonstrates how
geologists apply established principles to deduce
geologic history. [by Stephen Marshak]

Play Animation Windows version >>
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Play Animation Macintosh version >>

Principle of faunal succession:
Groups of fossil animals and plants occur
the geologic history in a definite and
determinable order and a period of geologic
time can be recognized by its characteristic
fossils.

Fossils are the remains of
ancient organisms. There are
many types of fossilization.
(Top) natural casts of shelled
invertebrates. (Middle) Fish
impressions. (Bottom)
Dinosaur footprint in finegrained limestone near Tuba,
Az.
The principle of fossil
succession. Note that
each species has only a
limited range in a
succession of strata.
(W.W. Norton)

Correlation of rock units
The method of relating rock units from one locality to
another is called correlation.

One way of correlation is to recognize the rock type or rock
sequence at two locations.

Another way of correlation is to use fossils. A basic
understanding of fossils is that fossil organisms succeeded
one another in a definite and determinable order, and
therefore a time period can be recognized by its fossil
content.
The principle of correlation of rock units. The rock columns can
be correlated by matching rock types. (W.W. Norton)

William Smith, a civil engineer and surveyor, could piece together the
sequence of layers of different ages containing different fossils by
correlating outcrops found in southern England about 200 years ago. In
this example, Formation II was exposed at both outcrops A and B, thus
Formation I and II were younger than Formation III. (Press and Siever).

Correlation of strata at three locations on the Colorado Plateau reveals
the total extent of sedimentary rocks in the region.
The geologic column was constructed by determining the
relative ages of rock units from around the world. (Next) By
correlation, these columns were stacked one on top of the
other to give relative ages of rock units (W.W. Norton)

Absolute dating
 The
geologic time based on stratigraphy and
fossils is a relative one: we can only say
whether one formation is older than the
other one.
 Absolute
dating was made possible only
after the discovery of radioactivity.

Radioactivity

At the turn of the 20th century, nuclear physicists
discovered that atoms of uranium, radium, and
several other elements are unstable. The nuclei of
these atoms spontaneously break apart into other
elements and emit radiation in the process known
as radioactivity.

We call the original atom the parent and its decay
product the daughter. For example, a radioactive
238 atom decays into a stable nonradioactive
U
92
206 atom.
Pb
82

example types of radioactive decay

Alpha decay: an a particle (composed of 2
protons and 2 neutrons) is emitted from a nucleus.
The atomic number of the nucleus decreases by 2
and the mass number decreases by 4.

Beta decay: a b particle (electron) is emitted from
a nucleus. The atomic number of the nucleus
increases by 1 but the mass number is unchanged.
Illustration of alpha and beta decays. (adapted from Tarbuck and Lutgens)

The decay of U238. After a series of radioactive decays, the
stable end product Pb206 is reached. (Tarbuck and Lutgents)

Decay constant

The rate of decay of an unstable parent nuclide is
proportional to the number of atoms (N) remaining at the
time t.
dN/dt=-l*N

The reason that radioactive decay offers a reliable means
of keeping time is that the decay constant l of a particular
element does not vary with temperature, pressure, or
chemistry of a geologic environment.

Half-life

The half-life of an radioactive element is the time required
for one-half of the original number of radioactive atoms to
decay:
T1/2=0.693/l.

The half-lives of geologically useful radioactive elements
range from thousands to billions of years. The age of the
Earth (4.6 billion years) was first obtained using U/Th/Pb
radiometric dating. The half-life of U238 is 4.5 billion years.

The radioactive decay is exponential. Half of the radioactive parent
remains after one half-life, and one-quarter of the parent remains after
the second half-life. (Tarbuck and Lutgens)
The concept of a half-life. The ratio of parent-to-daughter changes
with the passage of each successive half-life. (W.W. Norton)

Geologic Time
The geologic time scale subdivides the 4.6-billion-year history of the
Earth into many different units, which are linked with the events of the
geologic past.

The time scale is divided into eons: Precambrian and Phanerozoic and
eras: Precambrian, Paleozoic ("ancient life"), Mesozoic ("middle life"),
and Cenozoic ("recent life"). The eras are bounded by profound
worldwide changes in life-forms.

The eras are divided into periods.

The periods are divided into epochs.
The standard geologic
time scale was
developed using relative
dating techniques.
Radiometric dating later
provided absolute times
for the standard
geologic periods. (W.W.
Norton)

The awesome span of geologic time
The geologic time represents events of awesome
spans of time. If the 4.6-billion-year Earth history
is represented by a 24-hour day with the beginning
at 12 midnight, the first indication of life would
occur at 8:35am. Dinosaurs would appear at
10:48pm and become extinct at 11:40pm. The
recorded history of mankind would represent only
0.2 sec before midnight.

The KT extinction

At the boundary between Cretaceous (the last period of
Mesozoic) and Tertiary (the first period Of Cenozoic)
about 66 million years ago, known as KT boundary,
more than half of all plant and animal species died in a
mass extinction. The boundary marks the end of the era in
which dinosaurs and other reptiles dominated and the
beginning of the era when mammals became important.

The widely held view of the extinction is the impact
hypothesis. A large object collided with the Earth,
producing a dust cloud that blocked the sunlight from
much of the Earth’s surface. Without sunlight for
photosynthesis, the food chains collapsed, which affected
large animals most severely.