Lecture 10 Stratigraphy and Geologic Time

1
(No Transcript)
2
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.

3

• Stratigraphy
• Stratigraphy is the study of rock layers (strata)
  and their relationship with each other.
• Stratigraphy provides simple principles used to
  interpret geologic events.

4
Two rock units at a cliff in Missouri. (US
Geological Survey)
5

• 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

6

• 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.

7

• 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.

8

• 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.

9
Principle of superposition. (W.W. Norton)
10

• 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).

11

• 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.

12

• 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)

13

• 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

14

• 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.

15
Formation of an angular unconformity. An angular
unconformity represents an extended period during
which deformation and erosion occurred. (Tarbuck
and Lutgents)
16
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)
17

• 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.

18
Formation of disconformity. (W.W. Norton)
19

• 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)

20
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)

21

• 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)

22
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
• Play Animation Windows version gtgt
• Play Animation Macintosh version gtgt

23

• 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.

24

• Contrasting field conditions for (a) a
  nonconformity and (b) an igneous intrusion.
  (West, Fig 9.4)

25

• The three basic types of unconformities
  illustrated by this cross-section of the Grand
  Canyon. (Tarbuck and Lutgents)

26
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 gtgt
• Play Animation Macintosh version gtgt

27
(No Transcript)
28

• 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.

29

• 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
  fine-grained limestone near Tuba, Az.

30
(No Transcript)
31
The principle of fossil succession. Note that
each species has only a limited range in a
succession of strata. (W.W. Norton)
32

• 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.

33
The principle of correlation of rock units. The
rock columns can be correlated by matching rock
types. (W.W. Norton)
34
(No Transcript)
35

• 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).

36

• Correlation of strata at three locations on the
  Colorado Plateau reveals the total extent of
  sedimentary rocks in the region.

37
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)
38
(No Transcript)
39

• 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.

40

• 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 92U238 atom decays into a stable
  nonradioactive 82Pb206 atom.

41

• 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.

42

• Illustration of alpha and beta decays. (adapted
  from Tarbuck and Lutgens)

43

• The decay of U238. After a series of radioactive
  decays, the stable end product Pb206 is reached.
  (Tarbuck and Lutgents)

44

• 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-lN
• 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.

45

• 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
• T1/20.693/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.

46

• 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)

47
The concept of a half-life. The ratio of
parent-to-daughter changes with the passage of
each successive half-life. (W.W. Norton)
48

• 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.

49

• 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)

50

• 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 835am. Dinosaurs would
  appear at 1048pm and become extinct at 1140pm.
  The recorded history of mankind would represent
  only 0.2 sec before midnight.

51
(No Transcript)
52

• 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 Earths surface.
  Without sunlight for photosynthesis, the food
  chains collapsed, which affected large animals
  most severely.