Geology 3120 Sedimentary Structures

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Geology 3120 - Sedimentary Structures
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Outline

• Review the geologic history exercise from last
  time
• Contacts, primary structures, and secondary
  structures
• How to determine which way is up
• Cross-bedding, graded-bedding, reverse
  graded-bedding
• Determining way up using top surface features -
  ripples, mudcracks, raindrops, footprints
• Determining way up using bottom surface features
  - load casts
• Determining way up using features within rocks -
  geopetal, bioturbation, stromatolites, flame
  structures, pillow lavas

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Block model for exercise
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Geologic History

• 12 Ma dike
• 15 Ma dike
• Normal fault
• 20 Ma sed
• 22 Ma sed
• Erosion
• Thrust fault
• Folding
• Layer B
• 60 Ma sed
• Layer G
• 70 Ma sed
• 80 Ma sed

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Contacts, Primary Structures Secondary
Structures

• Contact - a boundary between rock units
• Primary structure - structures that form during
  lithification
• Secondary structure - structures that form after
  lithificaiton

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Contacts, Primary Structures Secondary
Structures

• Contact - a boundary between rock units
• Primary structure - structures that form during
  lithification
• Secondary structure - structures that form after
  lithificaiton

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Which way is up?
Today
Option 1 Overturned syncline
Option 2 up side down overturned anticline
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Cross-bedding
Checkerboard Mesa, Zion National Park
Younger
Flow direction
Y
Concave up
Older
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Graded bedding
Younger
Y
Older

• Decrease in depositional energy with
  sedimentation
• Example flood deposits, turbidity currents

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Graded bedding
Younger
Y
Older

• Decrease in depositional energy with
  sedimentation
• Example flood deposits, turbidity currents

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Reverse (Inverse) Graded Bedding
Younger
Y
Older

• Increase in depositional energy with
  sedimentation
• Example debris flows (a lot less common than
  normal graded beds)

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Ripple marks

• Symmetric ripples indicate bi-modal current
• Concave up

• Asymmetric ripples indicate unidirectional
  current

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Mud cracks

• Desiccation of
• muddy sediments

• Mud cracks

5 cm
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Raindrops

• Limited to terrestrial sediments

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Footprints

• Limited to terrestrial sediments

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Load casts

• Protrusion of material into a layer below
• Load casts indicate the base of a layer, not the
  top of a layer
• Determining the current direction may be possible

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Geopetal Structures

• A natural carpenters level

Matrix
Shell or cavity in the rock
Infill material (I.e., calcite)
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Bioturbation

• Habitation burrows
• Feeding burrows
• Movement

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Stromatolites

• Sharks Bay, Australia
• Cyanobacteria grow upward toward the surface

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Flame structures

• Less dense material intrudes into material above

• Caused by rapid loading of turbidite sands

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Pillow lava
Upper curved surface
V notch
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References
Slide 8 http//www.utahpictures.com/Checkerboard.h
tml Slides 9-15, 17 Busch, R. M. and D. Tasa,
Laboratory Manual in Physical Geology, 3rd. Ed.,
American Geological Institute and National
Association of Geology Teachers, 260 p.,
1990. Slide 18 http//www.discoverwest.com.au/ha
blin.html Slide 20 http//volcanoes.usgs.gov/Prod
ucts/Pglossary/PillowLava.html http//volcanoes.us
gs.gov/Products/Pglossary/ancientseq.html