14. Passive Margins and Sediment Transport William Wilcock (w/ some slides from Dan Nowacki)...
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Transcript of 14. Passive Margins and Sediment Transport William Wilcock (w/ some slides from Dan Nowacki)...
14. Passive Margins and Sediment Transport
William Wilcock (w/ some slides from Dan Nowacki)
OCEAN/ESS 410
1
Lecture/Lab Learning Goals• Know the terminology of and be able to sketch
passive continental margins• Understand how passive margins are formed• Understand differences in sedimentary
processes 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.
2
Passive Margins
Transition from continental to oceanic crust with no plate boundary.
Formerly sites of continental rifting3
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 > 4000 m) - Average slope <0.1°
Shelf Break Abyssal Plain
4
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.
13
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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10 largest rivers in world supply 40% of freshwater and sediment to ocean
90% of carbon accumulating in ocean does soon continental shelves
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 18
Sediment Mobilization - 2. Bottom Currents
•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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Holocene deposits (<20,000 y)on passive continental shelves
Boundary between modern inner-shelf sand and modern mid-shelf mud depends on waves
70% of shelf surfaceshave exposed relict deposits
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ShelfSedimentation
• Coarse grained sands - require strong currents/waves to mobilize
• 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
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Debrites and Turbidites
• Debrites– Weakly 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– Weakly 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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Turbidity Current Experiments
There is a good movie of a turbidity current available athttp://learningobjects.wesleyan.edu/turbiditycurrents/ 27