Transcript Slide 1
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)
Contents
► Continental
shelves
► Shelf processes
Tides
Storm waves and currents
► Tide-dominated
shelves
► Weather- (storm-) dominated shelves
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.
Shelf morphology
► Depth:
Shelf (shoreface to shelf edge): ~5 - 550 m.
Shelf-edge break: 20 – 550m.
► Width:
2 – 1500 km.
Active and passive margin shelves
► Passive margin shelves: wider
► Active magin shelves: narrower
Glaciated shelves
►
Last glacial maximum (LGM) extent of Scandivavian-Barents Sea ice sheet.
Svendsen 2004
Dag Ottesen 2006
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
Shelf structure
► Pericontinental
shelf
Simple, pericontinental shelf prism, Gulf of Cadiz, SW Spain
Shelf structure (and facies)
► Epicontinental
shelves
e.g. North Sea
e.g. Tethys
e.g. Mesozoic Western Interior
Seaway, North America
Western Interior Cardium Fm oil and
gas fields, Alberta
Western Interior Cretaceous shoreface
sediments, Wyoming
Shelf processes and types
► Shelves
have been classified as:
Tide-dominated
Wave- (weather-) dominated (tidal range <1m)
► Complexity:
► Generalized
tidal range may vary across a shelf.
model for shelf physiography and water
characteristics:
Inner shelf mixed layer (waves and tides)
Mid- to outer shelf: surface, core and bottom layers
Shelf water dynamics
► Tides
► Waves
► Wind
► Oceanic
currents
► Density currents
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)
Shelf tides
► On
shelves:
tidal wave velocity decreases
tidal amplitude increases
tidal current strength increases
M2 high-water tidal ranges
►
Resonant tidal wave effects cause:
standing waves with nodes and
antinodes.
rotating tidal waves (Kelvin waves).
tidal amplification (increases height
and current strength).
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
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
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
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.
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.
Tide-dominated shelves
► Tidal
currents:
uni- to multidirectional
tidal current strength varies
bedforms and facies vary
downcurrent.
Dalrymple 1992
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.
►
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)
► Sand
ribbons
Velocity > 1 m/s
Depth 20-100 m
Length <20 km
Width <200m
Height <0.1 m
► 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
► Tidal
Dalrymple 1992
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
Internal structure of tidal current sand ridges in the north Sea
Zeeland
►
Distal storm sand and mud
Bioturbated mud
Graded and and shell storm layers
(’tempestites’)
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.
► 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.
Bedforms on the Middle Atlantic Bight
Ancient clastic shelf facies
► Cretaceous
western North
American seaway
(Colorado – Alberta):
wave – and tidal
processes.
► Precambrian of Varanger,
Finnmark – stormdominated shallow
marine.
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.