Transcript Sediment characterization - University of Washington

OCEAN/ESS 410
16. Sediment Transport
across continental shelves
William 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
Terminology
Shelf Break
Abyssal Plain
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
•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
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
1. Turbidity currents (and
hyperpycnal flow)
2. Fluidized sediment
flows
3. Debris Flows/Slides
4. 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)
Transition from Debrites to Turbidites
Turbidity Current Experiments
There is a good movie of a turbidity current available at
http://learningobjects.wesleyan.edu/turbiditycurrents/
Turbidity Currents – Erosion and Deposition
Classical Turbidite
Sediment Transport from Shelf to
Deep Waters
1. Turbidites
2. Fluidized sediment
flows
3. Grain supported
4. Debris Flows/Slides
Submarine Channels