Geologic Time Ch'8

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Geologic Time(Ch.8)
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Geologic Time

• The Geologic Time Scale

- a dual scale based on relative ages of rock
sequences with absolute ages (radiometric ages)
added to it
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The Geologic Time Scale
The time scale is divided into two large periods
of timePrecambrian Phanerozoic
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Comprehending Geologic Time

• Precambrian first 87 yards

• Phanerozoic last 13 yards

Paleozoic - 13 to 5 yard line
Mesozoic - 5 to 1.5 yard line
Cenozoic - 1.5 yard line to goal line
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Geologic Time

• Precambrian (4500-545 Ma)

• birth of Earth to before complex life forms
  developed

• algae, bacteria, some fossils w/o shells, etc.

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Geologic Time

• Paleozoic (545-245 Ma) - Early Life

• Abundant marine life (trilobites, fish, clams,
  coral) amphibians, ferns, primitive trees, etc.

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Geologic Time

• Mesozoic (245-66 Ma) - Middle Life

• Reptiles (dinosaurs others), scrawny mammals,
  flowers, etc.

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Geologic Time

• Cenozoic (66-0 Ma) - Recent Life

• Mammals (us), grass, etc.

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Establishing the Geologic Time Scale Relative
Dating

• How do geologists figure out the relative ages of
  rocks?

• -Using some basic relationships
• such as?

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Relative (Geologic) Dating

• Place events in a proper sequence, but does not
  tell us the exact age of the rock

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Relative (Geologic) Dating Principles
1) Principle of Original Horizontality

• Many rock layers are originally deposited
  FLAT(water-laid sedimentary rocks, lava flows,
  tuff)

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Relative Dating Principles
1) Principle of Original Horizontality

• If layers are TILTED, then something happened
  since their deposition

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Relative Dating Principles
2) Principle of Lateral Continuity

• Unless confined, sediment/tuff tends to extend
  out in all directions

• Good for correlating rocks in different areas

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Relative Dating Principles
3) Law of Superposition

• In an upright sequence of sedimentary or volcanic
  rocks, the layers get YOUNGER from bottom to top

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Relative Dating Principles
4) Principle of Cross-Cutting Relationships

• Faults intrusions are YOUNGER than the layers
  they cross, cut or bake

Alice I got to here
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Relative Dating Principles
5) Principle of Inclusions

• A younger rock (host rock) may contain pieces of
  older rocks

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Relative Dating Principles
5) Principle of Inclusions
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Somethings Missing Unconformities

• An unconformity is a contact that represents a
  significant time difference between adjacent rock
  units

much younger
much older
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Unconformities
Angular Unconformity

• Contact separating younger older beds that are
  at an angle to each other

• Older beds typically dip more steeply

• Relatively easy to see

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Unconformities
Angular Unconformity
Making an angular unconformity
Deposition Folding/Uplift Erosion Subsidence/more
erosion
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Unconformities
Nonconformity

• Where sedimentary rocks overlie older plutonic
  igneous or metamorphic rocks

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Unconformities
Nonconformity
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Unconformities
Disconformity

• A surface between parallel younger older
  sedimentary layers where there is a gap in time

• Relatively hard to see need to look for fossils
  inclusions

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Faunal (fossil) Succession

• Knowing the changes in the fossil record helps
  put stratigraphy into a general time frame

• Certain index fossils can be found worldwide

• What makes a good index fossil?
• Widespread
• Short-lived

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Combining Relative Absolute Dating

• Absolute ages can tell us (indirectly) how old
  adjacent rocks are

• Age of Layer ? 200,000-400,000 a