Due to Time Constraints

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Due to Time Constraints

• We wont cover
• Contact, Burial, Hydrothermal, or Regional
  Metamorphism in class
• See lecture notes and the book!
• My notes cover this in GREAT detail as does the
  book

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Where Does It all End?
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Timing the Geologic Record

• Professor Burkett
• Wednesay, 2/4/08

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We have actual records

• We can directly measure short
  term processes
• Beach erosion
• Movement of glaciers (meters per year)
• Plate movements (centimeters per year)
• Historical records that can date back thousands
  of years
• Vesuvius in AD 79

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How far back can we go?

• Have we ever witnessed a
• Chicxulub meteorite impact?
• Yes, in the movies

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Human observation is too short

• We must rely on information preserved in rocks
  that have survived destruction and erosion
• Oceanic crust only up to 200 million yrs old
• So, must rely on the older continental crust
• But how do we put ages on prehistoric geologic
  phenomena?

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The Early Days Relative Dating

• We originally could only relate how old one event
  was relative to another
• For example
• These fish bones were deposited in marine
  sediments before mammal bones appeared
  in land sediments
• Fossils preserved remnants of ancient life

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Steno

• 1667 discovered that tongue stones found in
  Mediterranean sediments
• Looked like teeth of modern sharks
• Concluded they were teeth of ancient sharks
• Stratigraphy study or rock layers

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Principle of Original Horizontality

• Sediments are originally deposited by gravity as
  horizontal beds
• If we find folded or faulted strata, we know the
  layers were deformed after deposition

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Principle of Superposition

• Each sedimentary layer of an undisturbed sequence
  is younger than the one beneath it and older than
  the one above it

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The usefulness of all this

• Formations Rock layers have distinct properties
  that can be grouped together
• If we can understand the relative age
  relationships at one outcrop, we can relate these
  to other outcrops at a distance

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Problems

• There are almost always gaps in successions of
  layers
• Say, uplift due to mountain-building causes large
  amounts of sediment to be removed by erosion
• Also, rock layers generally cannot correlate over
  continental scales
• i.e. similar mud layers in England and China?

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Fossils provide the answer

• Smith, 1793
• Fossils are present in certain layers of rock
• Different layers contain different sets of
  fossils
• Able to distinguish one layer from another

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Principle of Faunal Succession

• Layers of sedimentary rock in an outcrop contain
  fossils in a definite sequence
• The same sequence can be found in rocks at other
  locations and so strata from one location can be
  matched to strata from another location
• Often faunal successions on different continents
  showed the same changes in fossil type
• Could determine relative ages of rock on a global
  scale

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Unconformities Gaps in the Record

• Sometimes formations are missing
• Either it was never deposited (a rise) or it was
  eroded before the next layer was deposited
• The boundary between two such formations is
  called an unconformity

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Disconformity

• Upper formation overlies an erosional surface on
  undisturbed lower beds
• Ex Sea level drops caused by glaciations
• Characterized by irregular
• or wavy surface

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Nonconformity

• Sedimentary beds overlie igneous or metamorphic
  rocks

Stream Cut
Smaller volcanic (ash) eruption
Giant volcanic eruption
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Angular Unconformity

• Upper beds overlie lower beds that have been
  folded by tectonic processes and then eroded to
  more or less an even plane

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Cross-Cutting Relationships

• Other disturbances provide clues for determining
  relative age
• Dikes Faults

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5.5
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Homework

• Place each labeled unit into the proper order
  from oldest to youngest list processes

Read Ch. 4, 73-84