Sedimentary Process

1. Sedimentary Process
Lecture 2#

2. Sediment Transport and Deposition
Media:
1. Water
 Overland flow, channel flow
 Waves, tides, ocean currents
2. Air (small particle)
3. Ice
4. Gravity
 Rock falls (no transport medium
involved)
 Debris flows, turbidity currents
(water involved)

3. Different kinds of mass movements,
variable velocity (and other factors)

4. From weathering to deposition:
sorting and modifification of clastic particles

5. Effects of transport on rounding, sorting

6. Three types of flow:
i. Laminar
ii. Turbulent
iii. Transitional

7. Laminar flow
Turbulent flow

9. Flow regimes in a stream

10. Laminar flow vs Turbulent Flow
 Two modes of flow dependent upon:
 1. Velocity
2. Fluid viscosity
3. Bed roughness
• Laminar Flow: streamlined, uniform current.
Requires: Low fluid velocity or High viscosity or Smooth
beds
• Turbulent Flow: discontinuous, distorted, flow
w/considerable motion perpendicular to primary flow direction.
e.g: Eddies - highly turbulent flow (water and air)
Eddy Viscosity - internal friction at a larger scale

11. Laminar flow vs Turbulent Flow
 Laminar flow occurs at low flow velocities
 Turbulent flow occurs at higher velocities
 Reynolds number (Re) is used to determine whether
flow is laminar or turbulent
Re =ULρ
μ
When μ dominates, Re is small (<500), flow is laminar
e.g., low velocity, shallow, or ice flows
At high U and/or L, flow becomes turbulent (>2000)
U = mean velocity, L = water depth, ρ=density, μ = fluid viscosity

12. Surface Waves & Froude Number
Fluid Flow variables:
– Viscosity (ignore for water)
– Inertial forces
– Gravity & depth (surface wave velocity)
Froude # (Fr) - ratio of inertial forces to gravity,
Fr= U , U= Mean velocity, g = gravity, L= water depth
√gL
Fr= 100 cm/s
√9.8m / s2 * 50cm
Froude #
< 1, max wave velocity exceeds current, tranquil (sub-critical)
>1, current velocity exceeds max wave velocity (supercritical)

13. Deeper
Subcritical
(Fr<1)
Shallow
Supercritical
(Fr>1)

14.  Critical velocities are different for sediment
entrainment and deposition, especially in the finer
fractions
 Fluid density and viscosity play a key role in
determining which particle sizes can be transported
 The amount of sediment transport is not only related to
flow velocity (or bed shear stress) and grain size, but
also to:
i. Grain density
ii. Grain shape

15. Sediment transport and deposition
Stokes’ Law (settling velocity in a static fluid)
( ) 2
 
18
g f
vg= settling velocity; D=grain diameter; ρg = grain
density; ρf= fluid density; μ=dynamic viscosity
Stokes’ Law only applies to fine (<100 μm), quartz-density
grains in water

g
gD
V



16. How are particles transported by fluids?

17. Transport modes in a turbulent fluid
 Traction (rolling over the bed surface)
 Saltation (jumping over the bed surface)
 Suspension (permanent transport within the fluid)
 Solution (Chemical transport)

19. A. Suspension; B. Bouncing ; C. Rolling

20. Sedimentology Concepts
Fluid flow and bedforms
 Unidirectional flow leads predominantly to
asymmetric bed forms (two-or-three dimensional) or
plane beds
 Current ripples
 Dunes
 Plane beds
 Anti-dunes
 Oscillatory flow due to waves causes predominantly
symmetric bedforms (wave ripples)
 Combined flow involves both modes of sediment
transport and causes low relief mounds and swales.

23. Bed forms and sedimentary structures for different flow regimes.
(after Harms and Fahnestock, 1965 and Simons et al., 1965)

24. FLOW REGIMES
Flow velocity

26. Current Ripples
 Once movement of sand grains (<0.7 mm) occurs, current
ripples are formed as a result of boundary layer
separation, commonly accompanied by a separation
vortex
 Current ripples have a stoss side (erosion and transport)
and lee side (deposition), the latter with a slope of ~ 30°
(angle of repose)
 Current ripples only form under moderate flow velocities,
with a grain size < 0.7 mm
 Height: 0.5-3 cm; wavelength: 5-40 cm

27. Dunes
 Dunes are distinctly larger than current ripples
 There is a relationship between boundary layer
thickness (≈ flow depth in rivers) and the dimension of
dunes
 Dunes only form in grain sizes > 0.2 mm
 Low flow velocities (bed shear stresses) yield straight-crested
bedforms (valid for both dunes and current
ripples); higher shear stresses result in sinuous to
linguoid crest lines.
 Sand waves constitute the largest category of
subaqueous dunes

28. Plane beds and anti-dunes
 In coarse sands (> 0.7 mm) lower-stage plane beds develop
instead of current ripples
 At high (but still subcritical) flow velocities upper-stage
place beds are formed in all sand grain sizes
 Supercritical flow conditions (Fr≈1 or higher) enable the
formation of anti-dunes, characterized by bedform
accretion in an upstream direction

29. Waves
 Waves are wind generated oscillatory motions of water
 Wave height is dependent on wind strength and fetch
 The depth to which the oscillatory motion due to wave
action extends is known as the wave base; shallow water
leads to breaking waves
 Wave ripples are distinct from current ripples due to their
symmetry, and include low energy rolling grain ripples
and high energy ‘vortex ripples’

30. Tides
Tides are formed by the gravitational attraction of
the moon and sun on the earth, combined with the
centrifugal force caused by the movement of the
earth around the center of mass of the earth moon
system
 Semi-diurnal or diurnal tidal cycles
 Neap-spring tidal cycles
 Annual tidal cycles

31. Ocean currents
 The circulation of sea water in the world’s oceans is
driven by wind and contrasts in density due to
variable temperature and salinity (thermo-haline
circulation), combined with the Coriolis effect
 Ocean currents transport clay and silt in
suspension, and sand as bed load, and their effects
are especially important in deep waters, where
storms and tides are less important.

32. Gravity flows
 Debris flows have a high (> 50%) proportion of sediment to water can
be both subaerial and subaqueous  low Reynolds numbers
 Turbidity currents have a higher proportion of water, are always
subaqueous and move due to density contrast  higher Reynolds
numbers
Re = 휌vL = vL
μ υ
where:
v is the mean velocity of the object relative to the fluid (SI units: m/s)
L is a characteristic linear dimension, (travelled length of the fluid;
hydraulic diameter when dealing with river systems) (m)
μ is the dynamic viscosity of the fluid (Pa·s or N·s/m² or kg/(m·s))
υ is the kinematic viscosity (υ = μ / 휌 ) (m²/s)
휌 is the density of the fluid (kg/m³).

34. Ripple marks
 Fine silt to fine sand
 Slow to fast flow velocity

35. Dunes
 Fine to coarse sand
 Medium to high flow velocities
 Sand loves to make dunes
Modern Ancient

36. Lower plane beds
 Fine to coarse sand, low flow velocity
 Sand grains like to make ripples
 Really slow velocity

37. Upper planar beds (laminae)
 Fine silt
 High flow velocity
 Hard to tell apart from lower planar beds
(Use grain size)

38. Anti-dunes
 Fine silt to coarse sand
 High flow velocity
 Rarely seen in rocks

39. Flow direction
 From looking at the rock structures, flow direction can
be determined even if the rock is millions of years old
When the flow has changed direction,
the structure known as crossbeds

41. Conclusion
 Planar stratification is primarily the product of aggrading plane
beds
 Cross stratification is formed by aggrading bedforms
 Planar and trough cross stratification are the result of straight
crested (2D) and linguoid (3D) bedforms, respectively
- Small scale cross stratification (current ripples)
- Large scale cross stratification (dunes)
-Wave cross stratification (wave ripples)
- Hummocky cross stratification [combined flow] (mounds and
swales)
 A single unit of cross stratified material is known as a set;
multiple stacked sets of similar nature form co-sets.