GE03112 11 Shelves.06a

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GE0-3112
 Sedimentary processes and products
Lecture 11. Shelves
Geoff Corner
Department of Geology
University of Tromsø
2006
Literature:
Leeder 1999. Ch.25. Shelves.
Dalrymple 1992, In Walker & James (eds)
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Contents
Continental shelves
Shelf processes
Tides
Storm waves and currents
Tide-dominated shelves
Weather- (storm-) dominated shelves
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Continental shelves
Transitional areas for sediment transport from continents to deep sea.
Permanent ’sinks’ for sediment because of subsidence.
In situ (calcareous) sediment production important (<40%) in some areas. 
Complex fluid dynamics: tides, waves, oceanic and density currents.
>100m sea-level fluctuation during the Quaternary.
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Shelf morphology
Depth:
Shelf (shoreface to shelf edge): ~5 - 550 m.
Shelf-edge break: 20 – 550m.
Width:
2 – 1500 km. 
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Active and passive margin shelves
Passive margin shelves: wider
Active magin shelves: narrower 
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Glaciated shelves
Last glacial maximum (LGM) extent of Scandivavian-Barents Sea ice sheet.
Svendsen 2004
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Dag Ottesen 2006
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Peri- and epicontinental shelves
Pericontinental shelves:
e.g. Mid-Norway, 
e.g. Gulf of Cadiz, SW Spain
Epicontinental (epeiric) shelves:
e.g. North Sea
e.g. Yellow Sea
e.g. Timor-Arafura Seas
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Shelf structure
Pericontinental shelf
Simple, pericontinental shelf prism, Gulf of Cadiz, SW Spain
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Shelf structure (and facies)
Epicontinental shelves
e.g. North Sea
e.g. Tethys
e.g. Mesozoic Western Interior Seaway, North America 
Western Interior Cretaceous shoreface sediments, Wyoming
Western Interior Cardium Fm oil and gas fields, Alberta
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Shelf processes and types
Shelves have been classified as:
Tide-dominated
Wave- (weather-) dominated (tidal range <1m)
Complexity: tidal range may vary across a shelf.
Generalized model for shelf physiography and water characteristics:
Inner shelf mixed layer (waves and tides)
Mid- to outer shelf: surface, core and bottom layers
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Shelf water dynamics
Tides
Waves
Wind
Oceanic currents
Density currents
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Open ocean tides
In the open ocean, the tidal wave shows:
long wave-length (c. 10,000 km)
low amplitude (c. 0.5 m) 
high wave (propagation) velocities (c. 
low tidal current velocities (few cm/s) 
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Shelf tides
On shelves:
tidal wave velocity decreases
tidal amplitude increases
tidal current strength increases
M2 high-water tidal ranges
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Resonant tidal wave effects cause:
standing waves with nodes and antinodes.
rotating tidal waves (Kelvin waves).
tidal amplification (increases height and current strength).
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Wind drift currents
Winter winds cause net residual currents arising from:
wind drift (wind shear stress  drift currents) 
wind set-up (wind shear and horizontal pressure gradients  surface gradients  set-up currents)
storm surge (shear and pressure set-up  geostrophic currents)
Water moves at an angle to the dominant wind direction due to Ekman effect/Coriolis force. 
Ekman spiral; water c. 100 m deep
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Storm surges (set-up)
Storms cause major shelf erosion and deposition.
E.g. Hurricane storm surge in Gulf of Mexico: up to 4 m above mean high-water.
E.g. southern North Sea, 1953: up to 3 m.
Numerically modelled storm surge for the 31.1-2.2.1953 flood, North Sea 
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Wind-forced (geostrophic) currents 
Gradient currents from wind set-up.
Especially common during storms.
Coastal set-up causes compensatory bottom flow.
Velocities > 1m/s. 
Deflection due to Coriolis force.
Major cause of coast to shelf sediment transport.
Walker & James 1992
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Shelf density currents
Buoyant plumes (hypopycnal flow) of suspended sediment.
May reach mid-shelf or shelf edge.
Sensitive to coastal upwelling and downwelling currents caused by winds.
Generated by river outflow or storms.
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Recent shelf facies
Modern shelves are ’highstand’ shelves.
Great variability in facies distribution:
increasing muds offshore where current strength low.
sands and lag gravels where current strengths high.
relict topography (incised valleys, moraines, lowstand barriers, etc.) influences sediment distribution. 
sediment source and regime influence sediment distribution. 
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Tide-dominated shelves
Tidal currents:
uni- to multidirectional
tidal current strength varies
bedforms and facies vary downcurrent.
Dalrymple 1992
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Tidal current transport paths
’Bedload partings’ (separating transport directions) located over amphidromic points or coastline constrictions.
Decreasing grain size along tidal current paths.
Ebb and flood tides may follow different paths. 
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Tidal bedforms
Downcurrent bedform succession
Furrows and gravel waves
Sand ribbons
Sandwaves (dunes).
Rippled sand sheets
Sand patches and mud
Large composite bedforms
Tidal sand ridges (banks) 
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Sand ribbons
Velocity > 1 m/s
Depth 20-100 m 
Length <20 km
Width <200m
Height <0.1 m
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Sandwaves (dunes)
Velocity 0.5-0.8 m/s
Abundant sand (sheets)
Large areas (>100 km2)
Wavelengths < 600 m
Height 3 – 15 m
Asymmetrical where tidal ellipse asymmetrical
Unimodal cross-stratification?
Dalrymple 1992
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Tidal sand ridges (banks)
cf. to linear seif dunes and draas for size and orientation. 
Length 60 km
Width 2 km
Height 40 m
Spacing 3 – 12 km
Asymmetrical; lee face <6˚
Superimposed dunes
Dalrymple 1992
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Internal structure of tidal current sand ridges in the north Sea
Zeeland
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Distal storm sand and mud
Bioturbated mud
Graded and and shell storm layers (’tempestites’)
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Wave- (weather-) dominated shelves
Offshore decrease in grain size
Attenuating wave power with depth (NB. ripples down to 200 m on Oregon shelf).
Fair-weather bioturbation may destroy storm laminae.
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Middle Atlantic Bight – a weather dominated shelf
75-180 km wide
c. 20 – 50-150 m deep
incised valleys (lowstand channels)
sand sheets with oblique linear ridges
shoreface shoals
Present sediment distribution related to Holocene transgression combined with present storm-generated currents. 
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Bedforms on the Middle Atlantic Bight
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Ancient clastic shelf facies
Cretaceous western North American seaway (Colorado – Alberta): wave – and tidal processes.
Precambrian of Varanger, Finnmark – storm-dominated shallow marine.
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Further reading
Dalrymple 1992. Tidal depositional systems. In Walker & James (eds). Facies Models: Response to Sea Level Change. 
Walker & Plint 1992. Wave- and storm-dominated shallow marine systems. In Walker & James (eds). Facies Models: Response to Sea Level Change. 
Johnson & Baldwin 1996. Shallow clastic seas, In Reading (ed.) Sedimentary Environments.