1. GE0-3112 Sedimentary processes and products Lecture 3. Sedimentary structures I – fluid flows Geoff Corner Department of Geology University of Tromsø 2006 Literature: - Leeder 1999. Ch. 7, 8 & 9. Sediment transport and structures. Sediment transport and structures.

2. Contents ► 3.1 Introduction ► 3.2 Unidirectional water flows ► 3.3 Atmospheric flows ► 3.4 Combined flows and tides ► Further reading

3. 3.1 Introduction ► Bedforms and structures (definition) ► Plane bed, ripples and dunes ► Bed shape changes with flow strength ► Feedback: bedforms modify flow

4. Bedforms and structures

5. Classification of primary sedimentary structures

6. Plane bed, ripples, dunes Ripples Dunes Plane bed

7. 3.2 Unidirectional water flows ► Current ripples ► Lower-stage plane bed ► Dunes ► Upper stage plane beds ► Antidunes ► Bedform relationships

8. Current ripples ► Are stable bedforms at low flow strength in fine sand. ► Do not form in sand coarser than 0.7 mm (c.s.). ► Asymmetric profile parallel to flow: gentle stoss, steep (c. 35 o ) lee. ► Height (h): <4 cm; wavelength (λ): <0,5 m. ► Ripple index (λ/h): 10-40. ► Ripple size varies clearly with grain size (λ ≈ 1000d) but not with flow strength or water depth.

9. Ripple shapes ► Ripple crests are straight, sinuous or linguoid (tongue-shaped). ► Straight- and sinuous ripples are metastable and change to linuoid with time.

10. Flow over a rippled bed Flow separation Flow separation and re-attachment Flow re-attachment

11. Ripple cross-bedding Planar cross-sets Trough cross-sets Climbing-ripple cross-lamination

12. Dunes ► Similar to ripples in general shape but distinctly different because:  ripple and dune form indices do not overlap.  ripples occur on the backs of dunes in apparent equilibrium. ► Height: 5 cm - 10 m; wavelength: 0,6 – 100’s m. ► Modification during stage variation may produce ’reactivation’ surfaces.

13. Dunes Sinuous Straight Rhomboid

14. Dune formation

15. Upper-stage plane beds ► Bed and water surface in phase; rapid flow. ► Plane bed actually comprises very low amplitude (c. 1 – 10 med mer) bedwaves that move downstream. ► Each bedwaves may deposit a thin lamina some few grains thick. ► The bed surface shows primary current lineation (parallel heavy-mineral streaks, etc.)

16. Upper-stage plane lamination Parting lineation

17. Antidunes ► Bedforms are stationary or migrate slowly upstream.

18. Bedform phase diagrams

19. Froude number and flow regime ► Froude number: ratio of inertial to gravity forces in water flow having free surface ► Fr < 1: Tranquil flow  Lower flow regime; water surface and bed out of phase. ► Fr > 1: Rapid slow  Upper flow regime: water surface and bed in phase. (NB. Upper and lower flow-regime concept not as clear cut as previously thought.)

20. 3.3 Atmospheric flows ► Differences between air and water flows ► Ripples ► Dunes

21. Comparison of air and water ► Low shear stresses in air limits maximum bedload grain size to v.coarse sand/v.f.pebble. ► Collision effects and saltation more important in air. ► Energetic kollisions promote abrasion of grains and substrate. (NB. Snow particle abrasion is effective in periglacial regions). ► Suspension transport of sand is more difficult in air than in water because of lower buoyancy. ►

22. Aeolian sediment transport

23. Aeolian sediments ► Gravel  transport by rolling and saltation (< 4 mm)  gravel normally forms protective lag ► Sand  median typically (fine sand)  aeolian sand ideally better sorted than beach sand  sorting varies  bedforms: ripples and dunes ► Silt  typically coarse silt (loess)

24. Aeolian bedforms ► Two major groups: ripples and dunes. ► Draas are large composite bedforms made up of smaller dunes. Previous classification acc. to size (Wilson 1972): ► draas20-450 m high ► dunes0.1-100 m " ► ripples0.005-0.1 m high Ripples Dunes

25. Ripple types ► Ballistic ripples ► Adhesion ripples

26. Ripples (ballistic ripples) ► Asymmetic profile parallel to flow: gentle, slightly convex stoss, steep (c.20 o ) lee. ► Height (h): few mm-10 cm; wavelength (λ): 2- 200 cm. ► Ripple index (λ/h): 8 – 50. ► Wavelength increases with grain size and wind strength.

27. Ripple shapes ► Persistent sinuous crests common. ► Barchanoid shapes form where sediment is sparse.

28. Ripple variability ► Wavelength increases with increasing grain size and wind strength.

29. Formation of wind ripples ► Ballistic collisions due to saltation cause up to 25% transport as ’creepload’. ► Lee slopes migrate more from effects of saltation bombardment than avalanching (hence lower angle than in water ripples) ► Crests contain coarser grains more resistent to bombardment (gives inverse grading in structures)

30. Internal structure of wind ripples ► No clear internal structure. ► Parallel bedding shows inverse frading

31. Internal structure of wind ripples ► Climbing ripples form where net accumulation of sand

32. Aeolian dunes ► Simple division into:  Transverse  Longitudinal  Complex forms Transverse Longitudinal Complex

33. Flow-transverse dunes ► Occur where predominant seasonal winds are unidirectional. ► Sand supply influences dune shape:  barchans: low sand supply.  sinuous-crested (aklé) dunes: plentiful supply.

34. Transverse-dune morphology

35. Formation of flow-transverse dunes

36. Internal stucture of transverse dunes ► Large-scale cross sets (cosets) ► First-, second- and third-order bounding surfaces record bedform migration.

38. Flow-parallel dunes ► Longitudinal (linear) dunes (’seif’ dunes). ► Height up to 50 m, separation several 100 m’s. ► Two wind directions may be important (transition from barchanoid to linear).

39. Seif dunes

41. Complex dunes ► Star dunes. ► Height 50 – 150 m, wavelength 500 – 1000 m. ► Multidirectional winds

42. Parabolic dunes ► Sand source in ’blowout’ (deflation hollow) in vegetated area. ► Tails upwind (opposite of barchan). ► Common on coasts.

43. 3.4 Combined flows and tides ► Waves ► Tides

44. Wave motion

46. Wave ripple formation ► Shallow-water waves (d=λ/20) cause horisontal bottom motion. ► Above threshold of motion movement occurs rolling and saltation. ► Initial ripple crests are low (< c. 20 grain diameters high) with broad troughs. ► Increased shear stress gives flow separation vortices on either side of symmetrical ripples.

47. Wave ripples ► Wavelength: c. 0.9 cm – 2 m. ► Height: c. 0.3 – 25 cm. ► RI (L/H): c. 4 – 13. ► Wavelength increases with increasing wave period. ► Bifurcation common

49. Wave and wave-current ripples

50. Wave-ripple structure

51. Combined flows ► Combined flow: current + wave motion. ► Bottom shear stress greater than for waves alone. ► Wave-current ripples RI<15; wave ripple RI<40.

52. Hummocky cross-stratification ► Formed by storm waves of long period (below fair-weather wave base). ► 3-D convex-up domes and convex-down troughs.

54. Tides

55. Tidal ellipses

56. Further reading ► Allen, J.R.L. 1970. Physical processes of sedimentation.  Chapter 1 covers the same ground as Leeder and explains clearly the principles involved; good supplementary reading for aquiring a sound grasp of the physics of fluid dynamics and sedimentation. Alternatively consult the more encyclopedic: ► Allen, J.R.L 1984. Sedimentary structures: their character and physical basis.  A more encyclopedic alternative to the above if it is unavailable.