Sedimentary Structures

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Sedimentary Structures

• Clues to depositional environments

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Bedding - layering!

• Layering of sediments is common due to episodic
  deposition
• In most cases the layers are close to horizontal
  at the time of deposition
• Layering may survive lithification

Loose sediment - Merrimack Valley
East Berlin formation - Holyoke
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What does a geologist see?
Colorado River - Grand Canyon National Park, AZ
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Ripple marks - indicate currents
Pyramid Lake, NV
Grand Canyon Natl. Park, AZ

• Ripples form on the surface of sediment (mud or
  sand) over which a current of wind or water is
  flowing
• Current direction may be indicated by the
  asymmetry of the ripples (steep side upstream)

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Cross-bedding (inside ripples or dunes)

• Sediment layers on the front slope of a ripple or
  dune will be inclined at the time of deposition
• Current direction is indicated by the direction
  of tilt of those cross-beds

Interior of ripples in outwash - NH
De Chelley sandstone - AZ
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• Note large scale cross-bedding in these
  sandstones (interpreted as old desert dunes,
  lithified to stone!)

Canyon de Chelly, AZ
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Mudcracks (drying of fine sediment)
In sediment, near Albany, NY
In mudstone, Glacier Natl. Park, MT

• Mudcracks form when wet, muddy sediment dries out
  and shrinks
• Other sediment may fill in the cracks, preserving
  these features after lithification

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Implications for environments?

• Bedding - episodic deposition, undisturbed
  sediment (no burrowers)
• Ripples marks - margins of streams, windy
  environments (current of wind or water)
• Cross-bedding - margins of streams, desert dunes
  (similar to ripples - the cross-beds are inside!)
• Mudcracks - seasonal lakes, margins of rivers or
  lakes (occasional drying out of environment)