Sedimentary Structures

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Sedimentary Structures

• and what they can tell us

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Two Types

• Primary
• Produced by the currents that deposit the
  sediment
• Secondary
• Produced after the sediment has been deposited

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Graded Bedding
(fining upward sequence)
UP
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Graded bedding is typical of turbidites
Turbulent mixtures of sand, gravel, mud and
water that are produced by submarine landslides,
deposits fine upward and basinward, submarine
mudflows/debris flows
Bouma Sequence
Distalfar from source
Proximalclose to source
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Distal Turbidites (Washington coast)
Scour surface
White sand, coarser
Gray shale, finer
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Mud cracks are concave UP
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Scour and Fill

• Process by which sediments (channel usually) are
  excavated by the action of current or high flow
  subsequent flow results in deposition and filling
  of the scour.

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Current direction indicators
DOWNSTREAM
Asymmetric Ripple-marks
Steeper slope is downstream
DOWNSTREAM
Imbricated clasts
Elongate clasts dip upstream (pushed like falling
dominoes)
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Ripples
Assymetrical Ripples
Symmetrical Ripples
Diagrams by Stephen A. Nelson Tulane University
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Cross-beds tell current direction and sometimes
UP direction
Caused by avalanching sand down lee slope of
wave-like structures
DOWNSTREAM
Note that cross-beds are cut off on top,
asymptotic at bottom
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Sand Waves

• Large Scale Ripples
• Straight Crests
• Planar Crossbeds

Diagrams by Stephen A. Nelson Tulane University
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Dunes

• Large Scale ripples
• Sinuous crests
• Form trough crossbeds

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Cross-beds in wind-blown dunes, Supai Sandstone,
Sedona AZ
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Cross-beds in unlithified sands
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Cross-beds and laminar beds in beach rock
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Flute Casts Spoon-shaped sole markings on
bottom of beds, commonly caused by turbidity
currents
Top view
Side view
deepest
DOWNSTREAM
DOWNSTREAM
Flute casts are preserved on the bottom of
overlying sand beds
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Examples of flute casts note flute shape(?)
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Trace fossilstracks of organisms
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Dinosaur Tracks, Tazones, Spain
dinoturbation
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Soft-sediment deformation
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Load casts or ball-and-pillow structures tell WAY
UP
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This is just the beginning of what one can learn
from sedimentary structures (more in Strat/Sed)