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.