Interpreting the Rock Record: Sedimentary Rocks

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Interpreting the Rock RecordSedimentary Rocks
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Deformation

• Deformation involves
• Stress - force applied to a given area
• Types of stress
• Compressional stress shortens a rock body
• Tensional stress tends to elongate or pull
  apart a rock unit
• Shear stress produces a motion similar to
  slippage that occurs between individual playing
  cards when the top of the stack is moved relative
  to the bottom

• How rocks deform
• General characteristics of rock deformation
• Elastic deformation the rock returns to nearly
  its original size and shape when the stress is
  removed
• Once the elastic limit (strength) of a rock is
  surpassed, it either flows (ductile deformation)
  or fractures (brittle deformation)

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Most folds result from compressional stresses
which shorten and thicken the crust
Common types of folds Anticline upfolded or
arched rock layers Syncline downfolds or
troughs of rock layers
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Faults Brittle Deformation
Faults are fractures in rocks along which
appreciable displacement has taken place Sudden
movements along faults are the cause of most
earthquakes
Types of faults Dip-slip faults Reverse fault
hanging wall up (compression) Normal fault
hanging wall down (tension)
Strike-slip faults
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On a normal fault, the hanging wall moves up
relative to the footwalOn a reverse fault, the
hanging wall moves down
Concept of hanging wall and footwall along a fault
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Monoclines are often the result of movement along
buried faults
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The Grand Canyon Rock RecordPrecambrian Time
From Timmons et al., 2001
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The Grand Canyon Rock RecordProterozoic Time
From Timmons et al., 2001
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Reconstructing the Past

• The oldest sedimentary rocks in the Canyon
  (exposed in deep levels beneath the Great
  Unconformity) are the Grand Canyon
  Supergroup.Unkar Fm.Cardenas BasaltChuar
  Fm.60-mile Fm.
• These rocks contain evidence of tectonism and
  rifting, representing the breakup of giant
  supercontinent Rodinia.

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Reconstructing the Past 1.1 Gapart of
supercontinent Rodinia
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Interpreting the Rock RecordNeoproterozoic
Rifting and the Grand Canyon Supergroup