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Explore the differences in sedimentary processes between active and passive margins, learn about sediment mobilization and transport on continental shelves, and analyze bathymetric maps. Discover the terminology of passive and active margins, sediment mobilization, and distribution, as well as the mechanisms of turbidites and debrites formation. Delve into sediment transport from shelves to deep waters and study submarine canyons.
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OCEAN/ESS 410 16. Sediment Transport across continental shelvesWilliam Wilcock
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
Passive Margins Transition from continental to oceanic crust with no plate boundary. Formerly sites of continental rifting
Shelf Break Abyssal Plain 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°
Active Margins Plate boundary (usually convergent) Narrower continental shelf Plate boundary can move on geological time scales - accretion of terrains, accretionary prisms
Sediment transport differences Active margins - narrower shelf, typically have a higher sediment supply, earthquakes destabilize steep slopes
Sediment Supply to Continental Shelf Sediment Transport across the Shelf Rivers Glaciers Coastal Erosion • Once sediments settle on the seafloor, bottom currents are required to mobilize them. • Wave motions • Ocean currents
Sediment Mobilization - 1. Waves The wave base or maximum depth of wave motions is about one half the wave length
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
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
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.
Sediment Transport from Shelf to Deep Waters Turbidity currents (and hyperpycnal flow) Fluidized sediment flows Debris Flows/Slides Grain supported
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.
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)
Turbidity Current Experiments There is a good movie of a turbidity current available at http://learningobjects.wesleyan.edu/turbiditycurrents/
Sediment Transport from Shelf to Deep Waters Turbidites Fluidized sediment flows Grain supported Debris Flows/Slides