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Interpreting the Rock Record: Sedimentary Rocks

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Compressional stress shortens ... Shear stress produces a motion similar to slippage that occurs between ... Syncline downfolds or troughs of rock layers ... – PowerPoint PPT presentation

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Title: Interpreting the Rock Record: Sedimentary Rocks


1
Interpreting the Rock RecordSedimentary Rocks
2
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)

3
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
4
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
5
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
6
Monoclines are often the result of movement along
buried faults
7
The Grand Canyon Rock RecordPrecambrian Time
From Timmons et al., 2001
8
The Grand Canyon Rock RecordProterozoic Time
From Timmons et al., 2001
9
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.

10
Reconstructing the Past 1.1 Gapart of
supercontinent Rodinia
11
Interpreting the Rock RecordNeoproterozoic
Rifting and the Grand Canyon Supergroup
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