GE0-3112GE0-3112 Sedimentary processes and products Sedimentary processes and products

Lecture 11. ShelvesLecture 11. Shelves

Geoff CornerGeoff CornerDepartment of GeologyDepartment of GeologyUniversity of TromsøUniversity of Tromsø20062006

Literature:Literature:- Leeder 1999. Ch.25. Shelves.Leeder 1999. Ch.25. Shelves.- Dalrymple 1992, In Walker & James (eds)Dalrymple 1992, In Walker & James (eds)

ContentsContents

►Continental shelvesContinental shelves►Shelf processesShelf processes

TidesTides Storm waves and currentsStorm waves and currents

►Tide-dominated shelvesTide-dominated shelves►Weather- (storm-) dominated shelvesWeather- (storm-) dominated shelves

Continental shelvesContinental shelves

► Transitional areasTransitional areas for sediment transport for sediment transport from continents to deep sea.from continents to deep sea.

► Permanent ’sinksPermanent ’sinks’ for sediment because of ’ for sediment because of subsidence.subsidence.

► In situIn situ (calcareous) sediment (calcareous) sediment production production important (<40%) in some areas. important (<40%) in some areas.

► Complex fluid dynamicsComplex fluid dynamics: tides, waves, : tides, waves, oceanic and density currents.oceanic and density currents.

► >100m sea-level fluctuation during the >100m sea-level fluctuation during the Quaternary.Quaternary.

Shelf morphologyShelf morphology

► Depth:Depth: Shelf (shoreface to shelf edge): Shelf (shoreface to shelf edge): ~~5 - 550 m.5 - 550 m. Shelf-edge break: 20 – 550m.Shelf-edge break: 20 – 550m.

► Width:Width: 2 – 1500 km. 2 – 1500 km.

Active and passive margin Active and passive margin shelvesshelves

► Passive margin shelves: widerPassive margin shelves: wider► Active magin shelves: narrower Active magin shelves: narrower

Glaciated shelvesGlaciated shelves► Last glacial maximum (LGM) extent of Scandivavian-Barents Sea Last glacial maximum (LGM) extent of Scandivavian-Barents Sea

ice sheet.ice sheet.

Svendsen 2004

Dag Ottesen 2006

Peri- and epicontinental Peri- and epicontinental shelvesshelves

► Pericontinental shelves:Pericontinental shelves: e.g. Mid-Norway, e.g. Mid-Norway, e.g. Gulf of Cadiz, SW Spaine.g. Gulf of Cadiz, SW Spain

► Epicontinental (epeiric) shelves:Epicontinental (epeiric) shelves: e.g. North Seae.g. North Sea e.g. Yellow Seae.g. Yellow Sea e.g. Timor-Arafura Sease.g. Timor-Arafura Seas

Shelf structureShelf structure

► Pericontinental shelfPericontinental shelf

Simple, pericontinental shelf prism, Gulf of Cadiz, SW Spain

Shelf structure (and facies)Shelf structure (and facies)► Epicontinental shelvesEpicontinental shelves

e.g. North Seae.g. North Sea e.g. Tethyse.g. Tethys e.g. Mesozoic Western Interior e.g. Mesozoic Western Interior

Seaway, North America Seaway, North America

Western Interior Cretaceous shoreface sediments, Wyoming

Western Interior Cardium Fm oil and gas fields, Alberta

Shelf processes and typesShelf processes and types► Shelves have been classified as:Shelves have been classified as:

Tide-dominatedTide-dominated Wave- (weather-) dominated (tidal range <1m)Wave- (weather-) dominated (tidal range <1m)

► Complexity: tidal range may vary across a shelf.Complexity: tidal range may vary across a shelf.► Generalized model for shelf physiography and Generalized model for shelf physiography and

water characteristics:water characteristics: Inner shelf mixed layer (waves and tides)Inner shelf mixed layer (waves and tides) Mid- to outer shelf: surface, core and bottom layersMid- to outer shelf: surface, core and bottom layers

Shelf water dynamicsShelf water dynamics

►TidesTides►WavesWaves►WindWind►Oceanic currentsOceanic currents►Density currentsDensity currents

Open ocean tidesOpen ocean tides

► In the open ocean, the tidal wave In the open ocean, the tidal wave shows:shows: long wave-length (c. 10,000 km)long wave-length (c. 10,000 km) low amplitude (c. 0.5 m) low amplitude (c. 0.5 m) high wave (propagation) velocities (c. high wave (propagation) velocities (c. low tidal current velocities (few cm/s) low tidal current velocities (few cm/s)

Shelf tidesShelf tides► On shelves:On shelves:

tidal wave velocity decreasestidal wave velocity decreases tidal amplitude increasestidal amplitude increases tidal current strength increasestidal current strength increases

M2 high-water tidal ranges

► Resonant tidal wave effects Resonant tidal wave effects cause:cause: standing waves with nodes and standing waves with nodes and

antinodes.antinodes. rotating tidal waves (Kelvin rotating tidal waves (Kelvin

waves).waves). tidal amplification (increases tidal amplification (increases

height and current strength).height and current strength).

Wind drift currentsWind drift currents

► Winter winds cause net residual currents arising Winter winds cause net residual currents arising from:from: wind drift (wind shear stress wind drift (wind shear stress drift currents) drift currents) wind set-up (wind shear and horizontal pressure wind set-up (wind shear and horizontal pressure

gradients gradients surface gradients surface gradients set-up currents) set-up currents) storm surge (shear and pressure set-up storm surge (shear and pressure set-up

geostrophic currents)geostrophic currents)► Water moves at an angle Water moves at an angle

to the dominant wind direction to the dominant wind direction due to Ekman effect/Coriolis force. due to Ekman effect/Coriolis force.

Ekman spiral; water c. 100 m deep

Storm surges (set-up)Storm surges (set-up)► Storms cause major shelf erosion and deposition.Storms cause major shelf erosion and deposition.► E.g. Hurricane storm surge in Gulf of Mexico: E.g. Hurricane storm surge in Gulf of Mexico:

up to 4 m above mean high-water. up to 4 m above mean high-water.► E.g. southern North Sea, E.g. southern North Sea,

1953: up to 3 m.1953: up to 3 m.

Numerically modelled storm surge for the 31.1-2.2.1953 flood, North Sea

Wind-forced (geostrophic) Wind-forced (geostrophic) currents currents

► Gradient currents from wind set-Gradient currents from wind set-up.up.

► Especially common during storms.Especially common during storms.► Coastal set-up causes Coastal set-up causes

compensatory bottom flow.compensatory bottom flow.► Velocities > 1m/s. Velocities > 1m/s. ► Deflection due to Coriolis force.Deflection due to Coriolis force.► Major cause of coast to shelf Major cause of coast to shelf

sediment transport.sediment transport.

Walker & James 1992

Shelf density currentsShelf density currents► Buoyant plumes (hypopycnal flow) of Buoyant plumes (hypopycnal flow) of

suspended sediment.suspended sediment.► May reach mid-shelf or shelf edge.May reach mid-shelf or shelf edge.► Sensitive to coastal upwelling and Sensitive to coastal upwelling and

downwelling currents caused by winds.downwelling currents caused by winds.► Generated by river outflow or storms.Generated by river outflow or storms.

Recent shelf faciesRecent shelf facies

►Modern shelves are ’highstand’ Modern shelves are ’highstand’ shelves.shelves.

►Great variability in facies distribution:Great variability in facies distribution: increasing muds offshore where current increasing muds offshore where current

strength low.strength low. sands and lag gravels where current sands and lag gravels where current

strengths high.strengths high. relict topography (incised valleys, relict topography (incised valleys,

moraines, lowstand barriers, etc.) moraines, lowstand barriers, etc.) influences sediment distribution. influences sediment distribution.

sediment source and regime influence sediment source and regime influence sediment distribution. sediment distribution.

Tide-dominated shelvesTide-dominated shelves►Tidal currents:Tidal currents:

uni- to multidirectionaluni- to multidirectional tidal current strength tidal current strength

variesvaries bedforms and facies vary bedforms and facies vary

downcurrent.downcurrent.

Dalrymple 1992

Tidal current transport pathsTidal current transport paths

► ’’Bedload partings’ (separating Bedload partings’ (separating transport directions) located over transport directions) located over amphidromic points or coastline amphidromic points or coastline constrictions.constrictions.

► Decreasing grain size along Decreasing grain size along tidal tidal current paths.current paths.

► Ebb and flood tides may Ebb and flood tides may follow different paths. follow different paths.

Tidal bedformsTidal bedforms► Downcurrent bedform successionDowncurrent bedform succession

Furrows and gravel wavesFurrows and gravel waves Sand ribbonsSand ribbons Sandwaves (dunes).Sandwaves (dunes). Rippled sand sheetsRippled sand sheets Sand patches and mudSand patches and mud

► Large composite bedformsLarge composite bedforms Tidal sand ridges (banks) Tidal sand ridges (banks)

► Sand ribbonsSand ribbons Velocity > 1 m/sVelocity > 1 m/s Depth 20-100 m Depth 20-100 m Length <20 kmLength <20 km Width <200mWidth <200m Height <0.1 mHeight <0.1 m

► Sandwaves (dunes)Sandwaves (dunes) Velocity 0.5-0.8 m/sVelocity 0.5-0.8 m/s Abundant sand (sheets)Abundant sand (sheets) Large areas (>100 kmLarge areas (>100 km22)) Wavelengths < 600 mWavelengths < 600 m Height 3 – 15 mHeight 3 – 15 m Asymmetrical where tidal Asymmetrical where tidal

ellipse asymmetricalellipse asymmetrical Unimodal cross-Unimodal cross-

stratification?stratification?

Dalrymple 1992

► Tidal sand ridges (banks)Tidal sand ridges (banks) cf. to linear seif dunes and draas for size and cf. to linear seif dunes and draas for size and

orientation. orientation. Length 60 kmLength 60 km Width 2 kmWidth 2 km Height 40 mHeight 40 m Spacing 3 – 12 kmSpacing 3 – 12 km Asymmetrical; lee face <6Asymmetrical; lee face <6˚̊ Superimposed dunesSuperimposed dunes

Dalr

ym

ple

199

2

Internal structure of tidal current sand ridges in the north Sea

Zeeland

► Distal storm sand and mudDistal storm sand and mud Bioturbated mudBioturbated mud Graded and and shell storm Graded and and shell storm

layers (’tempestites’)layers (’tempestites’)

Wave- (weather-) dominated Wave- (weather-) dominated shelvesshelves

►Offshore decrease in grain sizeOffshore decrease in grain size►Attenuating wave power with depth Attenuating wave power with depth

(NB. ripples down to 200 m on Oregon (NB. ripples down to 200 m on Oregon shelf).shelf).

►Fair-weather bioturbation may destroy Fair-weather bioturbation may destroy storm laminae.storm laminae.

► Middle Atlantic Bight – a weather dominated shelfMiddle Atlantic Bight – a weather dominated shelf 75-180 km wide75-180 km wide c. 20 – 50-150 m deepc. 20 – 50-150 m deep incised valleys (lowstand channels)incised valleys (lowstand channels) sand sheets with oblique linear ridgessand sheets with oblique linear ridges shoreface shoalsshoreface shoals Present sediment distribution related to Holocene Present sediment distribution related to Holocene

transgression combined with present storm-generated transgression combined with present storm-generated currents. currents.

Bedforms on the Middle Atlantic Bight

Ancient clastic shelf faciesAncient clastic shelf facies

► Cretaceous western Cretaceous western North American North American seaway (Colorado – seaway (Colorado – Alberta): wave – and Alberta): wave – and tidal processes.tidal processes.

► Precambrian of Precambrian of Varanger, Finnmark – Varanger, Finnmark – storm-dominated storm-dominated shallow marine.shallow marine.

Further readingFurther reading► Dalrymple 1992. Tidal depositional systems. In Walker & Dalrymple 1992. Tidal depositional systems. In Walker &

James (eds). Facies Models: Response to Sea Level Change. James (eds). Facies Models: Response to Sea Level Change. ► Walker & Plint 1992. Wave- and storm-dominated shallow Walker & Plint 1992. Wave- and storm-dominated shallow

marine systems. In Walker & James (eds). Facies Models: marine systems. In Walker & James (eds). Facies Models: Response to Sea Level Change. Response to Sea Level Change.

► Johnson & Baldwin 1996. Shallow clastic seas, In Reading (ed.) Johnson & Baldwin 1996. Shallow clastic seas, In Reading (ed.) Sedimentary Environments. Sedimentary Environments.