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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
Lecture 3. Sedimentary structures I – fluid flowsGE0-3112 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
Contents
  • 3.1 Introduction
  • 3.2 Unidirectional water flows
  • 3.3 Atmospheric flows
  • 3.4 Combined flows and tides
  • Further reading
3 1 introduction
3.1 Introduction
  • Bedforms and structures (definition)
  • Plane bed, ripples and dunes
  • Bed shape changes with flow strength
  • Feedback: bedforms modify flow
plane bed ripples dunes
Plane bed, ripples, dunes

Dunes

Plane bed

Ripples

3 2 unidirectional water flows
3.2 Unidirectional water flows
  • Current ripples
  • Lower-stage plane bed
  • Dunes
  • Upper stage plane beds
  • Antidunes
  • Bedform relationships
current ripples
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.
ripple shapes
Ripple shapes
  • Ripple crests are straight, sinuous or linguoid (tongue-shaped).
  • Straight- and sinuous ripples are metastable and change to linuoid with time.
flow over a rippled bed
Flow over a rippled bed

Flow separation and re-attachment

Flow re-attachment

Flow separation

ripple cross bedding
Ripple cross-bedding

Climbing-ripple cross-lamination

Planar cross-sets

Trough cross-sets

dunes
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.
dunes1
Dunes

Straight

Sinuous

Rhomboid

upper stage plane beds
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.)
antidunes
Antidunes
  • Bedforms are stationary or migrate slowly upstream.
froude number and flow regime
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.)
3 3 atmospheric flows
3.3 Atmospheric flows
  • Differences between air and water flows
  • Ripples
  • Dunes
comparison of air and water
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.
aeolian sediments
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)
aeolian bedforms
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

ripple types
Ripple types
  • Ballistic ripples
  • Adhesion ripples
ripples ballistic ripples
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.
ripple shapes1
Ripple shapes
  • Persistent sinuous crests common.
  • Barchanoid shapes form where sediment is sparse.
ripple variability
Ripple variability
  • Wavelength increases with increasing grain size and wind strength.
formation of wind ripples
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)
internal structure of wind ripples
Internal structure of wind ripples
  • No clear internal structure.
  • Parallel bedding shows inverse frading
internal structure of wind ripples1
Internal structure of wind ripples
  • Climbing ripples form where net accumulation of sand
aeolian dunes
Aeolian dunes
  • Simple division into:
    • Transverse
    • Longitudinal
    • Complex forms

Longitudinal

Transverse

Complex

flow transverse dunes
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.
internal stucture of transverse dunes
Internal stucture of transverse dunes
  • Large-scale cross sets (cosets)
  • First-, second- and third-order bounding surfaces record bedform migration.
flow parallel dunes
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).
complex dunes
Complex dunes
  • Star dunes.
  • Height 50 – 150 m, wavelength 500 – 1000 m.
  • Multidirectional winds
parabolic dunes
Parabolic dunes
  • Sand source in ’blowout’ (deflation hollow) in vegetated area.
  • Tails upwind (opposite of barchan).
  • Common on coasts.
wave ripple formation
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.
wave ripples
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
combined flows
Combined flows
  • Combined flow: current + wave motion.
  • Bottom shear stress greater than for waves alone.
  • Wave-current ripples RI<15; wave ripple RI<40.
hummocky cross stratification
Hummocky cross-stratification
  • Formed by storm waves of long period (below fair-weather wave base).
  • 3-D convex-up domes and convex-down troughs.
further reading
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.