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GE0-3112 Sedimentary processes and products

Lecture 3. Sedimentary structures I – fluid flows. GE0-3112 Sedimentary processes and products. Geoff Corner Department of Geology University of Tromsø 2006. Literature: - Leeder 1999. Ch. 7, 8 & 9. Sediment transport and structures. Contents. 3.1 Introduction

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GE0-3112 Sedimentary processes and products

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

  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. 35o) 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 and re-attachment Flow re-attachment Flow separation

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

  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 Straight Sinuous 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): • draas 20-450 m high • dunes 0.1-100 m " • ripples 0.005-0.1 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.20o) 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 Longitudinal Transverse 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.

  37. 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).

  38. Seif dunes

  39. Complex dunes • Star dunes. • Height 50 – 150 m, wavelength 500 – 1000 m. • Multidirectional winds

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

  41. 3.4 Combined flows and tides • Waves • Tides

  42. Wave motion

  43. 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.

  44. 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

  45. Wave and wave-current ripples

  46. Wave-ripple structure

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