Transcript Slide 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.
Contents
► 3.1
Introduction
► 3.2 Unidirectional water flows
► 3.3 Atmospheric flows
► 3.4 Combined flows and tides
► Further reading
3.1 Introduction
► Bedforms
and structures (definition)
► Plane bed, ripples and dunes
► Bed shape changes with flow strength
► Feedback: bedforms modify flow
Bedforms and structures
Classification of primary sedimentary
structures
Plane bed, ripples, dunes
Dunes
Plane bed
Ripples
3.2 Unidirectional water flows
► Current
ripples
► Lower-stage plane bed
► Dunes
► Upper stage plane beds
► Antidunes
► Bedform relationships
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
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 separation and re-attachment
Flow re-attachment
Flow separation
Ripple cross-bedding
Climbing-ripple cross-lamination
Planar cross-sets
Trough cross-sets
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.
Dunes
Straight
Sinuous
Rhomboid
Dune formation
Upper-stage plane beds
► Bed and water surface in phase; rapid
► Plane bed actually comprises very low
flow.
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.)
Upper-stage plane lamination
Parting lineation
Antidunes
► Bedforms
are stationary or
migrate slowly upstream.
Bedform phase diagrams
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
► Differences
► Ripples
► Dunes
between air and water flows
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 sediment transport
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
► 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
► Ballistic
ripples
► Adhesion 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): 2200 cm.
► Ripple index (λ/h): 8 – 50.
► Wavelength increases with grain size and wind
strength.
Ripple shapes
► Persistent
sinuous crests common.
► Barchanoid shapes form where sediment is sparse.
Ripple variability
► Wavelength
increases
with increasing grain
size and wind strength.
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
► No
clear internal structure.
► Parallel bedding shows inverse frading
Internal structure of wind ripples
► Climbing
ripples form
where net
accumulation of sand
Aeolian dunes
► Simple
division into:
 Transverse
 Longitudinal
 Complex forms
Longitudinal
Transverse
Complex
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.
Transverse-dune morphology
Formation of flow-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
► 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).
Seif dunes
Complex dunes
► Star
dunes.
► Height 50 – 150 m, wavelength 500 – 1000 m.
► Multidirectional winds
Parabolic dunes
► Sand
source in ’blowout’ (deflation hollow)
in vegetated area.
► Tails upwind (opposite of barchan).
► Common on coasts.
3.4 Combined flows and tides
► Waves
► Tides
Wave motion
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
► Wavelength: c. 0.9 cm –
► Height: c. 0.3 – 25 cm.
► RI (L/H): c. 4 – 13.
► Wavelength increases
with increasing wave
period.
► Bifurcation common
2 m.
Wave and wave-current ripples
Wave-ripple structure
Combined flows
► Combined
flow: current + wave motion.
► Bottom shear stress greater than for waves
alone.
► Wave-current
RI<40.
ripples RI<15; wave ripple
Hummocky cross-stratification
► Formed
by storm waves of long period (below
fair-weather wave base).
► 3-D convex-up domes and convex-down
troughs.
Tides
Tidal ellipses
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.