16. Sediment Transport across continental shelves William Wilcock

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16. Sediment Transport across continental
shelvesWilliam Wilcock
OCEAN/ESS 410
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Lecture/Lab Learning Goals

• Be able to sketch passive continental margins and
  label features
• Understand how sedimentary processes differ
  between active and passive margins
• Know how sediments are mobilized on the
  continental shelf
• Understand how sediments are transported into
  deep water and be able to explain the difference
  between turbidites and debrites.
• Interpret bathymetric maps of submarine canyons
  and other coastal features - LAB

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Passive Margins

• Transition from continental to oceanic crust with
  no plate boundary.
• Formerly sites of continental rifting

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Terminology
Continental Shelf - Average gradient 0.1 Shelf
break at outer edge of shelf at 130-200 m depth
(130 m depth sea level at last glacial
maximum) Continental slope - Average gradient
3-6 Continental rise (typically 1500-4000 m) -
Average gradient 0.1-1 Abyssal Plain (typically
gt 4000 m) - Average slope lt0.1
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Active Margins

• Plate boundary (usually convergent)
• Narrower continental shelf
• Plate boundary can move on geological time scales
  - accretion of terrains, accretionary prisms

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Sediment transport differences
Active margins - narrower shelf, typically have a
higher sediment supply, earthquakes destabilize
steep slopes
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Sediment Supply to Continental Shelf

• Rivers
• Glaciers
• Coastal Erosion

Sediment Transport across the Shelf

• Once sediments settle on the seafloor, bottom
  currents are required to mobilize them.
• Wave motions
• Ocean currents

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Sediment Mobilization - 1. Waves
The wave base or maximum depth of wave motions is
about one half the wave length
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Shallow water waves

• Wave particle orbits flatten out in shallow water
• Wave generated bottom motions
• strongest during major storms (big waves)
• extend deepest when the coast experiences long
  wavelength swell from local or distant storms

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Sediment Mobilization 2. Bottom Currents

• The wind driven ocean circulation often leads to
  strong ocean currents parallel to the coast.
• These interact with the seafloor along the
  continental shelf and upper slope.
• The currents on the continental shelf are often
  strongest near outer margins.

Aguihas current off east coast of southern
Africa. The current flows south and the contours
are in units of cm/s
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Sediment Distribution on the Continental shelf

• Coarse grained sands - require strong currents to
  mobilize, often confined to shallow water where
  wave bottom interactions are strongest (beaches)
• Fine grained muds - require weaker currents to
  mobilize, transported to deeper water.

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Sediment Transport from Shelf to Deep Waters

1. Turbidity currents (and hyperpycnal flow)
2. Fluidized sediment flows
3. Debris Flows/Slides
4. Grain supported

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Debrites and Turbidites

• Debrites
• Weakley Inversely graded (upward coarsening)
• Thick, but pinch out quickly
• Convoluted bedding
• Turbidites
• Normally graded
• (upward fining)
• Laterally extensive
• Thin
• Horizontal bedding

Lahars and pyroclastic flow deposits, Mt. St.
Helens, WA.
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Debrites and Turbidites

• Debrites
• Weakley Inversely graded (upward coarsening)
• Thick, but pinch out quickly
• Convoluted bedding
• Turbidites
• Normally graded
• (upward fining)
• Laterally extensive
• Thin
• Horizontal bedding

Turbidite in sandstone, unknown location (from
http//uibk.ac.at)
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Transition from Debrites to Turbidites
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Turbidity Current Experiments
There is a good movie of a turbidity current
available at http//learningobjects.wesleyan.edu/
turbiditycurrents/
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Turbidity Currents Erosion and Deposition
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Classical Turbidite
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Sediment Transport from Shelf to Deep Waters

1. Turbidites
2. Fluidized sediment flows
3. Grain supported
4. Debris Flows/Slides

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Submarine Channels
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