Sediment Erosion,Transport, Deposition, and
Sedimentary Structures
An Introduction To
Physical Processes of
Sedimentation
Sediment transport
• Fluid Dynamics
• COMPLICATED
– Focus on basics
• Foundation
• NOT comprehensive
Sedimentary Cycle
• Weathering
– Make particle
• Erosion
– Put particle in motion
• Transport
– Move particle
• Deposition
– Stop particle motion
• Not necessarily continuous (rest stops)
Definitions
• Fluid flow (Hydraulics)
– Fluid
• Substance that changes shape easily and continuosly
• Negligeable resistance to shear
• Deforms readily by flow
– Apply minimal stress
– Moves particles
– Agents
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•
•
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Water
Water containing various amounts of sediment
Air
Volcanic gasses/ particles
Definitions
• Fundamental Properties
– Density (Rho (r))
• Mass/unit volume
– Water ~ 700x air
r = 0.998 g/ml @ 20°C
• Density decreases with increased temperature
– Impact on fluid dynamics
• Ability of force to impact particle within fluid and on bed
• Rate of settling of particles
• Rate of occurrence of gravity -driven downslope movement of
particles
– rH20 > r air
Definitions
• Fundamental Properties
– Viscosity
• Mu (m)
– Water ~ 50 x air
• m = measure of ability of fluids to flow (resistance of substance
to change shape)
– High viscosity = sluggish (molasses, ice)
– Low viscosity = flows readily (air, water)
• Changes with temperature (Viscosity decreases with
temperature)
– Sed load and viscosity covary
• Not always uniform throughout body
– Changes with depth
Types of Fluids:
Strain (deformational) Response to Stress
(external forces)
• Newtonian fluids
– normal fluids; no yield stress
• strain (deformation);
proportional to stress, (water)
• Non-Newtonian
– no yield stress;
• variable strain response to stress
(high stress generally induces
greater strain rates {flow})
– examples: mayonnaise, water
saturated mud
Types of Fluids:
Strain (deformational) Response to Stress
(external forces)
• Bingham Plastics:
– have a yield stress (don't flow
at infinitesimal stress)
• example: pre-set concrete; water
saturated, clay-rich surficial
material such as mud/debris flows
• Thixotropic fluids:
– plastics with variable
stress/strain relationships
• quicksand??
Why do particles move?
• Entrainment
• Transport/ Flow
Entrainment
• Basic forces acting on particle
– Gravity, drag force, lift force
• Gravity:
• Drag force: measure of friction between water and
bottom of water (channel)/ particles
• Lift force: caused by Bernouli effect
Bernouli Force
 (rgh) + (1/2 rm2)+P+Eloss = constant
•
•
•
•
Static P + dynamic P
Potential energy= rgh
Kinetic energy= 1/2 rm2
Pressure energy= P
Thus pressure on grain decreases, creates lift force
Faster current increases likelihood that gravity, lift and drag
will be positive, and grain will be picked up, ready to be
carried away
Why it’s not so simple: grain size, friction, sorting, bed
roughness, electrostatic attraction/ cohesion
Flow
• Types of flow
– Laminar
• Orderly, ~ parallel flow lines
– Turbulent
• Particles everywhere! Flow lines change constantly
– Eddies
– Swirls
– Why are they different?
• Flow velocity
• Bed roughness
• Type of fluid
Geologically Significant
Fluid Flow Types (Processes)
• Laminar Flows:
– straight or boundary parallel flow lines
• Turbulent flows:
– constantly changing flow lines. Net mass transport in the flow
direction
Flow: fight between inertial and
viscous forces
• Intertial F
– Object in motion tends to remain in motion
• Slight perturbations in path can have huge effect
• Perfectly straight flow lines are rare
• Viscous F
– Object flows in a laminar fashion
– Viscosity: resistance to flow (high = molasses)
• High viscosity fluid: uses so much energy to move it’s more
efficient to resist, so flow is generally straight
• Low viscosity (air): very easy to flow, harder to resist, so flow
is turbulent
• Reynolds # (ratio inertial to viscous forces)
Reynold’s #
Re = Vl/(r/m)
dimensionless #
– V= current velocity
– l= depth of flow-diameter of pipe
– r= density
– m= viscosity
u=(r/m)- kinematic viscosity
• Fluids with low u (air) are turbulent
• Change to turbulent determined experimentally
• Low Re = laminar <500 (glaciers; some mud flows)
• High Re = turbulent > 2000 (nearly all flow)
Geologically Significant
Fluid Flow Types (Processes)
• Laminar Flows:
– straight or boundary parallel flow lines
• Turbulent flows:
– constantly changing flow lines. Net mass transport in the flow
direction
Geologically Significant Fluids and
Flow Processes
• These distinct flow mechanisms
generate sedimentary deposits with
distinct textures and structures
• The textures and structures can be
interpreted in terms of hydrodynamic
conditions during deposition
• Most Geologically significant flow
processes are Turbulent
Traction deposits
(turbulent flow)
Debris flow (laminated flow)
What else impacts Fluid Flow?
•
•
•
•
Channels
Water depth
Smoothness of Channel Surfaces
Viscous Sublayer
1. Channel
– Greater slope = greater velocity
– Higher velocity = greater lift force
• More erosive
– Higher velocity = greater intertial forces
• Higher numerator = higher Re
• More turbulent
2. Water depth
• Water flowing over the bottom creates shear stress (retards
flow; exerted // to surface)
– Shear stress: highest AT surface, decreases up
– Velocity: lowest AT surface, increases up
– Boundary Layer: depth over which friction creates a
velocity gradient
• Shallow water: Entire flow can fall within this interval
• Deep water: Only flow within B.L. is retarded
– Consider velocity in broad shallow stream vs deep river
2. Water Depth
• Boundary Shear stress (o)-stress that opposes the
motion of a fluid at the bed surface
(o) = gRhS
• g= density of fluid (specific gravity)
• Rh = hydraulic radius
– (X-sectional area divided by wetted perimeter)
• S = slope (gradient)
– the resistance to fluid flow across bed (ability of fluid
to erode/ transport sediment)
– Boundary shear stress increases directly with increase
in specific gravity of fluid, increasing diameter and
depth of channel and slope of bed (e.g. greater ability to
erode & transport in larger channels)
2. Water depth
• Turbulence
– Moves higher velocity particles closer to stream
bed/ channel sides
• Increases drag and list, thus erosion
– Flow applies to stream channel walls (not just
bed)
3. Smoothness
• Add obstructions
– decrease velocity around object (friction)
– increase turbulence
• May focus higher velocity flow on channel sides or
bottom
• May get increased local erosion, with decreased
overall velocity
4. Viscous Sublayer
• At the surface, there is a molecular attraction that
causes flow to slow down
– Thin layer of high effective viscosity
• Reduce flow velocity
• May even see laminar flow in the sublayer
• Result? Protective “coating” for fine grains on
bottom
– Smallest grains are within the layer
– (larger grains can poke up through it, causing
turbulence and scour of larger particles)
Flow/Grain Interaction:
Particle Entrainment and Transport
• Forces acting on particles during fluid flow
– Inertial forces, FI, inducing grain
immobility
FI = gravity + friction + electrostatics
– Forces, Fm, inducing grain
mobility
Fm= fluid drag force + Bernoulli force
+ buoyancy
Deposition
• Occurs when system can no longer support
grain
• Particle Settling
– Particles settle due to interaction of upwardly directed
forces (bouyancy of fluid and drag) and downwardly
directed forces (gravity).
• Generally, coarsest grains settle out first
– Stokes Law quantifies settling velocity
– Turbulence plays a large role in keeping grains aloft
Particle Settling
Forces opposing entrainment and transport
• VS = [(ρg - ρf)g/18 m]d2
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–
–

–
VS : settling velocity
ρg = grain density
ρf = fluid density
m = fluid viscosity
d = grain diameter
Stoke’s “law” of settling
Theory vs application
• Increase velocity, increase turbulence and
entrainment
– Material plays a role
– Hjülstrom’s curve
• Empirical measure of minimum Velocity required to
move particles of different sizes
Hjülstrom’s curve
• EMPIRICAL
– Series of grain sizes in straight sided channel
– Increased velocity until grains moved
• Threshold velocity (min. V) to entrain particles
– Transition zone (specifics like packing
– Intuitive except for clays
• Cohesion (consolidated fines)
• Electrostatic attraction (unconsolidated fines)
• Viscous sublayer
Critical Threshold for Particle Entrainment
• Hjulstrom Diagram
Fm
> Fi
– Empirical relationship between grain size (quartz grains) and current velocity
(standard temperature, clear water)
– Defines critical flow velocity threshold for entrainment
– As grain size increases
entrainment velocity increases
(sand size and > particles)
– For clay size particles
electrostatics requires
increased flow velocity for
entrainment
– (gray area is experimental
variation)
Grains in Motion (Transport)
• Once the object is set in motion, it will stay in motion
• Transport paths
– Traction (grains rolling or sliding across bottom)
– Saltation (grains hop/ bounce along bottom)
– Bedload (combined traction and saltation)
– Suspended load (grains carried without settling)
• upward forces > downward, particles uplifted stay aloft
through turbulent eddies
• Clays and silts usually; can be larger, e.g., sands in floods
– Washload: fine grains (clays) in continuous suspension derived
from river bank or upstream
• Grains can shift pathway depending on conditions
Transport Modes and Particle Entrainment
•
With a grain at rest, as flow velocity increases
Fm
> Fi ; initiates particle motion
• Grain Suspension (for small particle sizes, fine silt; <0.01mm)
– When Fm > Fi
• U (flow velocity) >>> VS (settling velocity)
– Constant grain Suspension at relatively low U (flow velocity)
– Wash load Transport Mode
Transport Modes and Particle Entrainment
•
With a grain at rest, as flow velocity increases
Fm
> Fi ; initiates particle motion
• Grain Saltation : for larger grains (sand size and larger)
– When Fm > Fi
•
U > VS but through time/space U < VS
– Intermittent Suspension
– Bedload Transport Mode
Transport Modes and Particle Entrainment
•
With a grain at rest, as flow velocity increases
Fm
<
Fi , but fluid drag causes grain rolling
• Grain Traction : for large grains (typically pebble size and larger)
– Normal surface (water) currents have too low a U for grain entrainment
– Bedload Transport Mode
Velocity/Particle Size Fields and Entrainment,
Transport Mode, and Deposition Model
• Entrainment/Transportation
– Suspension
– Saltation
– Traction
• Settling/Deposition
Depositional structures indicate
flow regime of formation
• Traction Currents
– Air and Water
• Bed is never perfectly flat
– Slight irregularies cause flow to lift off bottom slightly
– Leads to pocket of lower velocity where sediments
pushed along bottom can accumulate
– Bump creates turbulence, advances process
– Bedform height and wavelength controlled by:
• Current velocity
• Grain Size
• Water depth
Theoretical Basis for Hydrodynamic
Interpretation of Sedimentary Facies
• Beds defined by
– Surfaces (scour, non-deposition) and/or
– Variation in Texture, Grain Size, and/or Composition
For example:
• Vertical accretion bedding (suspension settling)
– Occurs where long lived quiet water exists
• Internal bedding structures (cross bedding)
– defined by alternating erosion and deposition due to spatial/temporal
variation in flow conditions
• Graded bedding
– in which gradual decrease in fluid flow velocity results in sequential
accumulation of finer-grained sedimentary particles through time
Grain size and Water DepthBedform
• Grain size impacts bedform formation
– coarse grains, no ripples are formed
– fines (clays), no dunes form
• Water depth affects bedform
– Increase w.d., increase velocity at which change
from low to upper flow regime occurs
Flow Regime and
Sedimentary Structures
An Introduction To
Physical Processes of Sedimentation
Sedimentary structures
• Sedimentary structures occur at very different
scales, from less than a mm (thin section) to
100s–1000s of meters (large outcrops); most
attention is traditionally focused on the
bedform-scale
• Microforms (e.g., ripples)
• Mesoforms (e.g., dunes)
• Macroforms (e.g., bars)
Sedimentary structures
• Laminae and beds are the basic
sedimentary units that produce stratification;
the transition between the two is arbitrarily
set at 10 mm
• Normal grading is an upward decreasing
grain size within a single lamina or bed
(associated with a decrease in flow velocity),
as opposed to reverse grading
• Fining-upward successions and
coarsening-upward successions are the
products of vertically stacked individual beds
Sedimentary structures
Cross stratification
• Cross lamination (small-scale cross stratification)
is produced by ripples
• Cross bedding (large-scale cross stratification) is
produced by dunes
• Cross-stratified deposits can only be preserved when
a bedform is not entirely eroded by the subsequent
bedform (i.e., sediment input > sediment output)
• Straight-crested bedforms lead to planar cross
stratification; sinuous or linguoid bedforms produce
trough cross stratification
Bed Response to Water (fluid) Flow
• Common bed forms (shape of the unconsolidated bed) due to
fluid flow in
– Unidirectional (one direction) flow
• Flow transverse, asymmetric bed forms
– 2D&3D ripples and dunes
– Bi-directional (oscillatory)
• Straight crested symmetric ripples
– Combined Flow
• Hummocks and swales
Bed Response to Steady-state,
Unidirectional, Water Flow
• FLOW REGIME CONCEPT
– Consider variation in: Flow Velocity only
• Flume Experiments (med sand & 20 cm flow depth)
– A particular flow velocity (after critical velocity of
entrainment) produces
– a particular bed configuration (Bed form) which in
turn
– produces a particular internal sedimentary
structure.
Bed Response to Steady-state,
Unidirectional, Water Flow
• Lower Flow Regime
– No Movement: flow velocity below critical entrainment velocity
– Ripples: straight crested (2d) to sinuous and linguoid crested (3d)
ripples (< ~1mλ) with increasing flow velocity
– Dunes: (2d) sand waves with straight crests to (3d) dunes (>~1.5mλ)
with sinuous crests and troughs
Bed Response to Steady-state,
Unidirectional, Water Flow
• Lower Flow Regime
– No Movement: flow velocity below
critical entrainment velocity
– Ripples: straight crested (2d) to
sinuous and linguoid crested (3d)
ripples (< ~1m) with increasing flow
velocity
– Dunes: (2d) sand waves with straight
crests to (3d) dunes (>~1.5m) with
sinuous crests and troughs
Dynamics of Flow Transverse
Sedimentary Structures
• Flow separation and planar vs. tangential fore sets
– Aggradation (lateral and vertical) and Erosion in space and
time
• Due to flow velocity variation
• Capacity (how much sediment in transport) variation
• Competence (largest size particle in transport) variation
– Angle of climb and the extent of bed form preservation
(erosion vs. aggradation-dominated bedding surface)
Sedimentary structures
Cross stratification
• The angle of climb of cross-stratified deposits
increases with deposition rate, resulting in ‘climbing
ripple cross lamination’
• Antidunes form cross strata that dip upstream, but
these are not commonly preserved
• A single unit of cross-stratified material is known as a
set; a succession of sets forms a co-set
Bed Response to Steady-state,
Unidirectional, Water Flow
• Upper Flow Regime
– Flat Beds: particles move continuously with no relief on the bed surface
– Antidunes: low relief bed forms with constant grain motion; bed form
moves up- or down-current (laminations dip upstream)
Sedimentary structures
Planar stratification
• Planar lamination (or planar bedding) is
formed under both lower-stage and upperstage flow conditions
• Planar stratification can easily be confused
with planar cross stratification, depending on
the orientation of a section (strike sections!)
Bed Response to Steady-state,
Unidirectional, Water Flow
• Consider Variation in Grain Size & Flow Velocity
– for sand <~0.2mm:
– for sand ~0.2 to 0.8mm
– for sand > 0.8:
No Dunes
Idealized Flow Regime Sequence of Bed forms
No ripples nor lower plane bed
Flow regime Concept (summary)
Application of Flow Regime Concept to
Other Flow Types
Sedimentary structures
• Cross stratification produced by wave ripples can be
distinguished from current ripples by their symmetry and by
laminae dipping in two directions
• Hummocky cross stratification (HCS) forms during storm
events with combined wave and current activity in shallow seas
(below the fair-weather wave base), and is the result of
aggradation of mounds and swales
• Heterolithic stratification is characterized by alternating
sand and mud laminae or beds
• Flaser bedding is dominated by sand with isolated, thin mud drapes
• Lenticular bedding is mud-dominated with isolated ripples
Sedimentary structures
Gravity-flow deposits
• Debris-flow deposits are typically poorly sorted,
matrix-supported sediments with random clast
orientation and no sedimentary structures; thickness
and grain size commonly remain unchanged in a
proximal to distal direction
• Turbidites, the deposits formed by turbidity
currents, are typically normally graded, ideally
composed of five units (Bouma-sequence with
divisions ‘a’-‘e’), reflecting decreasing flow velocities
and associated bedforms
Debrites
• Debris flow deposits
– See TurbiditesTurbidity current
deposits
Application of Flow Regime Concept to
Other Flow Types
• Deposits formed by
turbulent sediment gravity
flow mechanism
– “turbidites”
– Decreasing flow regime
in concert with grain
size decrease
• Indicates decreasing flow
velocity through time
during deposition
Sediment Gravity Flow Mechanisms
• Sediment Gravity Flows:
– 20%-70% suspended sediment
– High density/viscosity fluids
• suspended sediment charged fluid within a lower density, ambient fluid
• mass of suspended particles results in the potential energy for initiation of
flow in a the lower density fluid (clear water or air)
• mgh = PE
–
–
–
–
M = mass
G = force of gravity
H = height
PE= Potential energy
Sediment Gravity Flows
• Not distinct in nature
• Different properties within different portions of a
flow
Leading edge of a debris flow triggered by
heavy rain crashes down the Jiangjia Gully
in China. The flow front is about 5 m tall.
Such debris flows are common here
because there is plenty of easily erodible
rock and sediment upstream and intense
rainstorms are common during the summer
monsoon season.
Fluidal Flows
• Turbidity Currents
– Re (Reynolds #) is large due to (relatively) low
viscosity
– turbulence is the grain support mechanism
– initial scour due to turbulent entrainment of
unconsolidated substrate at high current velocity
• Scour base is common
Fluidal Flows
• Turbidity Currents
– deposition from bedload & suspended load
– initial deposits are coarsest transported particles
deposited (ideally) under upper (plane bed) flow
regime
Fluidal Flows
• Turbidity Currents
– as flow velocity decreases (due to loss of minimum mgh)
finer particles are deposited under lower flow regime
conditions
• high sediment concentration commonly results in climbing ripples
– final deposition occurs under suspension settling mode with
hemipelagic layers
Fluidal Flows
• The final (idealized) deposit: Turbidite
– graded in particle size
– with regular vertical transition in sedimentary structures
• Bouma Sequence and
“facies” tract in a
submarine fan
depositional
environment
Sedimentary structures
• Imbrication commonly occurs in water-lain gravels and
conglomerates, and is characterized by discoid (flat) clasts
consistently dipping upstream
• Sole marks are erosional sedimentary structures on a bed
surface that have been preserved by subsequent burial
• Scour marks (caused by erosive turbulence)
• Tool marks (caused by imprints of objects)
• Paleocurrent measurements can be based on any sedimentary
structure indicating a current direction (e.g., cross stratification,
imbrication, flute casts)
Sedimentary structures
• Soft-sediment deformation structures are
sometimes considered to be part of the initial
diagenetic changes of a sediment, and
include:
• Slump structures (on slopes)
• Dewatering structures (upward escape of water,
commonly due to loading)
• Load structures (density contrasts between sand
and underlying wet mud; can in extreme cases
cause mud diapirs)
Dewatering Structures
Biogenic Sedimentary Structures
• Produced by the activity of organisms with
the sediment
– Burrowing, boring, feeding, and locomotion
activities
– Produce trails, depressions, open burrows,
borings
• Dwelling structures, resting structures,
crawling and feeding structures, farming
structures