1 / 56

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

Download Presentation

GE0-3112 Sedimentary processes and products

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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

More Related