Passive Margins and Sediment Transport

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OCEAN/ESS 410
14. Passive Margins and
Sediment Transport
William Wilcock (w/ some slides from
Dan Nowacki)
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 rifting
3
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°
4
Volcanic Rifted Margins
1. Mantle Plumes
5
Volcanic Rifted Margins
2. Slab Pull Driven Extension
6
Mantle warm enough to convect
and melt
7
Sequences of up to 20 km of
basalt
8
Non-Volcanic Rifted Margin – Mantle
too cold to melt
9
10
11
Active Margins
Plate boundary (usually convergent)
Narrower continental shelf
Plate boundary can move on geological time
scales - accretion of terrains, accretionary prisms
12
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
14
10 largest rivers in world supply 40% of freshwater and
sediment to ocean
90% of carbon accumulating in ocean does so
on continental shelves
15
16
Sediment Mobilization - 1. Waves
The wave base or maximum depth of wave motions is about
one half the wave length
17
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
19
Aguihas current off east coast of southern Africa. The
current flows south and the contours are in units of cm/s
Holocene deposits (<20,000 y)
on passive continental shelves
70% of shelf surfaces
have exposed relict deposits
Boundary between modern inner-shelf sand and
modern mid-shelf mud depends on waves
20
Sedimentation on active margins
Washington continental shelf
21
Shelf
Sedimentation
• Coarse grained
sands - require
strong
currents/waves to
mobilize
• Fine grained muds require weaker
currents to mobilize,
transported to
22
Sediment Transport from Shelf to
Deep Waters
1. Turbidity currents (and
hyperpycnal flow)
2. Fluidized sediment
flows
3. Debris Flows/Slides
23
Debris Flows and Turbidity Currents
24
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.
25
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)
26
Turbidity Current Experiments
There is a good movie of a turbidity current available at
http://learningobjects.wesleyan.edu/turbiditycurrents/
27
Turbidity Currents – Erosion and Deposition
28
Classical Turbidite
29
Submarine Channels
30
31
32
33
Missoula Floods
Flow rates of
up to 50
km3/hr
Modern day
Columbia
River is
~0.02 km3/hr
34
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