Last Time
UNIVERSITY OF SOUTH ALABAMA
Shelves
MAS 603: Geological Oceanography
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Definition and a bit of history
Factors controlling shelf sedimentation
The Bahamas
Lecture 14: Submarine Fans
Continental Shelves
Factors influencing shelf sedimentation
Six factors are recognized as influencing shelf sedimentation. The last three
are designated as “biggie” factors.
By definition: shelves are: “oceanic environments characterized by moderate
water depths (10m to 150/200m) in which a variety of shallow marine and
moderate depth facies occur”
1) Sea level fluctuations (controls distribution of relict sediments,
barrier reefs etc.)
2) Animal-sediment interactions (modifies the substrate)
3) Chemistry (produces authigenic minerals)
4) Type and rate of sedimentation
5) Climate
Biggies
6) Type and intensity of shelf hydraulic regime (“energy”)
http://faculty.gg.uwyo.edu/heller/Sed%20Strat%20Class/SedStrat%207/SedStrat7.html
Continental Shelves
Factors influencing shelf sedimentation
Type and rate of sedimentation
Climate
very-high sedimentation
250
0
River
Temperature (°C)
Sediment
Wedge
Modified from McCave (1972)
Relict?
Relict
0
15% + gravel
content
45% + sand content
Shelf break
Controls shelf sedimentation by its effects on the
surrounding land mass (siliciclastic source area).
• Wet and hot (tropical) = intense chemical
weathering and clay production
• Cold and dry (temperate) = physical weathering
and sand/gravel production
40% + mud
content
40
Relict
Shelf break
Rainfall (cm)
ex
am
pl
es
High-energy shoreline
Fe
w
Normal
Modified from Hayes (1967)
Shelf break
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Continental Shelves
Today’s Agenda
Hydraulic regime
Three dominant processes are responsible for transporting, reworking and
sorting sediment on shelves (including relict areas).
1) Tide-dominated shelves (17% of pericontinental shelf areas)
2) Ocean current-dominated shelves (3% of pericontinental shelves)
3) Storm-dominated shelves (80% of pericontinental shelf areas)
Submarine Canyons and Fans
Submarine Fans
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Definition and morphology
Transport mechanisms (density currents)
Submarine fan facies/sedimentary sequences
Submarine Canyons (Basic Info)
1. Submarine canyons cut all
continental shelves and slopes and
carry sediment to the deep ocean
basins.
2. There are many small submarine
canyons and few large ones.
3. Large canyons are always associated
with large coastal rivers.
Submarine canyons: major conduits
of siliciclastic sediment from the
continental shelf to the abyssal plain
Submarine fans: accumulations of
siliciclastic sediment from the shelf
break onto the base of the
continental slope
http://www.dkimages.com/discover/previews/851/20117924.JPG
http://www.geol.umd.edu/~jmerck/
Text from http://www.ocean.uni-bremen.de/EInfo/materialien/elemOc/top12-17/topic13.html
Submarine Canyons (Basic Info)
Submarine Canyons (Basic Info)
4. Submarine canyons may be formed, in part, by downcutting
of coastal rivers during intervals of low sea level. Later
continued flow of water and suspended sediment through the
submerged channels might lead to their further development
and extension into the deep sea.
5. Submarine canyons may also be periodically “flushed” by
submarine slides. Continued flow through the depressions
might lead to their further development and extension into the
deep sea.
Before
Sediment-filled
After
Sediment-flushed
Channel lag
Text from http://www.ocean.uni-bremen.de/EInfo/materialien/elemOc/top12-17/topic13.html
Text from http://www.ocean.uni-bremen.de/EInfo/materialien/elemOc/top12-17/topic13.html
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Submarine Fans
Submarine Fans (Basic Info)
Submarine fans are found at the base of
canyons around the world.
Submarine fans can be
divided into 3 zones
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1: Proximal (upper)
2: Middle
3: Distal (lower)
2
3
http://www.geol.umd.edu/~jmerck/geol100/lectures/31.html
http://faculty.gg.uwyo.edu/heller/Sed%
http://www.indiana.edu/~g105lab/images/gaia_chapter_13/sfs026.gif
Submarine Fans (Basic Info)
Like their alluvial fan counterparts, submarine fans are fanshaped and “fine” in a proximal to distal direction
A
A
B
Submarine Fans: Transport Mechanisms
Sediment transport, in general can occur in one of 5 modes:
1. Rolling
Traction 2. Sliding
3. Bouncing (saltation)
4. Suspension
5. Mass movement
B
From Walker, R.G. (1980). Facies Models. Geological Association of Canada.
Submarine Fans: Transport Mechanisms
Submarine Fans: Transport Mechanisms
Mass movement is particularly important as a sediment
transport mechanism on submarine fans.
Mass movement is particularly important as a sediment
transport mechanism on submarine fans.
Increase in water:sediment ratio
From Walker, R.G. (1980). Facies Models. Geological Association of Canada.
From Walker, R.G. (1980). Facies Models. Geological Association of Canada.
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Submarine Fans: Transport Mechanisms
Submarine Fans: Transport Mechanisms
The whole process is initiated as an underwater landslide.
On the proximal fan, the slide has a low water:sediment ratio.
http://instruct.uwo.ca/earth-sci/200a-001/07turbidity.jpg
Submarine Fans: Transport Mechanisms
Debris flow facies
http://instruct.uwo.ca/earth-sci/200a-001/07turbidity.jpg
Submarine Fans: Transport Mechanisms
On the distal fan, the flow has a high water:sediment ratio
and moves as a density current.
Density currents have
long been studied by
geologists and
engineers. Ultimately
they produce one of the
most diagnostic rock
types in geology…
…the turbidite.
http://serc.carleton.edu/
http://instruct.uwo.ca/earth-sci/200a-001/07turbidity.jpg
Density Currents
Our first quantitative experience with density currents was in
1929 following the M 7.2 Grand Banks Earthquake.
http://twister.ou.edu/density-current-2.images/LS1(21B).002400.gif
Density Currents
The earthquake triggered an underwater landslide/ density
current as evidenced by a series of underwater telephone cable
breaks.
Cables near the epicenter of the earthquake
broke at the same time as the quake itself
(yellow dots). The last cable, 600 km
south, broke 13 hours after the earthquake.
http://earthquakescanada.nrcan.gc.ca/historic_eq/20th/1929/1929_e.php
Estimated tsunami travel times (in
hours) for waves generated by the 1929
Grand Banks earthquake.
http://earthnet-geonet.ca/images/communities/earthquakemap.gif
Twenty-five years later, south of the
earthquake site, a layer of sand was
discovered on the sea floor, covering an
area the size of the province of Quebec
(shown above in light blue). And evidence
of vast underwater landslides was found
near the epicentre
http://www.benfieldhrc.org/activities/hrsr/h&rsr_2005
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Density Currents
Turbidites
The velocity of the density currents were incredible
Cable
Time to break
(mins)
Turbidites are formed
as a result of waning
flow.
Velocity
m/s
1
183
20.3
2
541
`14.4
3
618
12.8
4
797
11.4
km/hr
73
41
A classic Bouma sequence
1
http://faculty.gg.uwyo.edu/heller/Sed%20Strat%20Class/SedStrat%208/bouma.jpg
White board
http://www.geol.lsu.edu/jlorenzo/PhysicalGeology_F2004/images/Turbidity.gif
http://www.ocean.uni-bremen.de/
Turbidites
Turbidite sequences can consist of 1000’s of individual beds
Turbidites
Turbidites are primarily
distinguished on the basis of
their graded nature.
Upper Carboniferous (Namurian), Ireland. Note bed tabularity.
http://regentsprep.org/Regents/earthsci/units/weathering/gradedbedding.gif
http://www.geoclastica.com/images/MisoaFm.jpg
Turbidites
The character of turbidites (e.g., the thickness and number of
Bouma units varies with distance down the fan).
Submarine Fans
Unlike most depositional
environments which have
easily accessible modern
analogs, almost all of our
knowledge of submarine
fans has come from
ancient environments and
geophysics.
http://strata.geol.sc.edu/images/PosmentierKolla-elementsFan.gif
http://csmres.jmu.edu/geollab/fichter/SedRx/subfan.html
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Submarine Fans
It is interesting to note
that even though these
environments are 1000’s
of metres below the
surface of the ocean, they
apparently have features
that look very much like
those produced by
surface processes.
The Submarine Fan Model
This is the best (and
simplest) model we have
of a submarine fan
depositional facies.
From Walker, R.G. (1980). Facies Models. Geological Association of Canada.
http://strata.geol.sc.edu/images/PosmentierKolla-elementsFan.gif
The Submarine
Fan Model
From Walker, R.G. (1980). Facies Models. Geological Association of Canada.
And this is the best vertical
section.
Next Week
1. Monday: Discussion: Sea Water evolution (Lauren)
2. Icehouse vs Greenhouse Earth’s
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