14. Passive Margins and Sediment Transport

William Wilcock (w/ some slides from Dan Nowacki)

OCEAN/ESS 410

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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.

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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

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Volcanic Rifted Margins 1. Mantle Plumes

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Volcanic Rifted Margins 2. Slab Pull Driven Extension

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Mantle warm enough to convect and melt

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Sequences of up to 20 km of basalt

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Non-Volcanic Rifted Margin – Mantle too cold to melt

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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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10 largest rivers in world supply 40% of freshwater and sediment to ocean

90% of carbon accumulating in ocean does soon continental shelves

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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 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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Washington continental shelf

Sedimentation on active margins

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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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Debris Flows and Turbidity Currents

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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

Turbidity Currents – Erosion and Deposition

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Classical Turbidite

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Submarine Channels

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Missoula Floods

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Flow rates of up to 50 km3/hr

Modern day Columbia River is ~0.02 km3/hr