GE0-3112 Sedimentary processes and products

1. Lecture 11. Shelves GE0-3112 Sedimentary processes and products Geoff Corner Department of Geology University of Tromsø 2006 Literature: • Leeder 1999. Ch.25. Shelves. • Dalrymple 1992, In Walker & James (eds)

2. Contents • Continental shelves • Shelf processes • Tides • Storm waves and currents • Tide-dominated shelves • Weather- (storm-) dominated shelves

3. 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.

4. Shelf morphology • Depth: • Shelf (shoreface to shelf edge): ~5 - 550 m. • Shelf-edge break: 20 – 550m. • Width: • 2 – 1500 km.

5. Active and passive margin shelves • Passive margin shelves: wider • Active magin shelves: narrower

6. Glaciated shelves • Last glacial maximum (LGM) extent of Scandivavian-Barents Sea ice sheet. Svendsen 2004

7. Dag Ottesen 2006

8. 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

9. Shelf structure • Pericontinental shelf Simple, pericontinental shelf prism, Gulf of Cadiz, SW Spain

10. 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

11. 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

12. Shelf water dynamics • Tides • Waves • Wind • Oceanic currents • Density currents

13. 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)

14. Shelf tides • On shelves: • tidal wave velocity decreases • tidal amplitude increases • tidal current strength increases M2 high-water tidal ranges

15. Resonant tidal wave effects cause: • standing waves with nodes and antinodes. • rotating tidal waves (Kelvin waves). • tidal amplification (increases height and current strength).

16. 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

17. 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

18. 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

20. 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.

21. 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.

22. Tide-dominated shelves • Tidal currents: • uni- to multidirectional • tidal current strength varies • bedforms and facies vary downcurrent. Dalrymple 1992

23. 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.

24. 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)

25. Sand ribbons • Velocity > 1 m/s • Depth 20-100 m • Length <20 km • Width <200m • Height <0.1 m

26. 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

27. 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

28. Internal structure of tidal current sand ridges in the north Sea Zeeland

29. Distal storm sand and mud • Bioturbated mud • Graded and and shell storm layers (’tempestites’)

30. 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.

32. 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.

33. Bedforms on the Middle Atlantic Bight

34. Ancient clastic shelf facies • Cretaceous western North American seaway (Colorado – Alberta): wave – and tidal processes. • Precambrian of Varanger, Finnmark – storm-dominated shallow marine.

35. 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.