Sediment Erosion,Transport, Deposition, and Sedimentary - PowerPoint PPT Presentation

Loading...

PPT – Sediment Erosion,Transport, Deposition, and Sedimentary PowerPoint presentation | free to view - id: 3bc3af-OGI5O



Loading


The Adobe Flash plugin is needed to view this content

Get the plugin now

View by Category
About This Presentation
Title:

Sediment Erosion,Transport, Deposition, and Sedimentary

Description:

Sediment Erosion,Transport, Deposition, and Sedimentary Structures An Introduction To Physical Processes of Sedimentation Sediment transport Fluid Dynamics ... – PowerPoint PPT presentation

Number of Views:1056
Avg rating:5.0/5.0
Slides: 103
Provided by: njcuInfod5
Category:

less

Write a Comment
User Comments (0)
Transcript and Presenter's Notes

Title: Sediment Erosion,Transport, Deposition, and Sedimentary


1
Sediment Erosion,Transport, Deposition, and
Sedimentary Structures
  • An Introduction To
  • Physical Processes of Sedimentation

2
Sediment transport
  • Fluid Dynamics
  • COMPLICATED
  • Focus on basics
  • Foundation
  • NOT comprehensive

3
Sedimentary Cycle
  • Weathering
  • Make particle
  • Erosion
  • Put particle in motion
  • Transport
  • Move particle
  • Deposition
  • Stop particle motion
  • Not necessarily continuous (rest stops)

4
Definitions
  • Fluid flow (Hydraulics)
  • Fluid
  • Substance that changes shape easily and
    continuously
  • Negligible resistance to shear
  • Deforms readily by flow
  • Apply minimal stress
  • Moves particles
  • Agents
  • Water
  • Water containing various amounts of sediment
  • Air
  • Volcanic gasses/ particles

5
Definitions
  • Fundamental Properties
  • Density (Rho (r))
  • Mass/unit volume
  • Water 700x air
  • 0.998 g/ml _at_ 20C
  • 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 down slope
    movement of particles
  • ?H20 gt ? air

6
Definitions
  • Fundamental Properties
  • Viscosity
  • Mu (m)
  • Water 50 x air
  • ? 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)
  • Sediment load and viscosity co-vary
  • Not always uniform throughout body
  • Changes with depth

7
Types of FluidsStrain (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

8
Types of FluidsStrain (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??

9
Why do particles move?
  • Entrainment
  • Transport/ Flow

10
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

11
Bernouli Force
  • (rgh) (1/2 rm2)PEloss 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 its not so simple grain size, friction,
    sorting, bed roughness, electrostatic attraction/
    cohesion

12
(No Transcript)
13
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

14
Geologically SignificantFluid Flow Types
(Processes)
  • Laminar Flows
  • straight or boundary parallel flow lines
  • Turbulent flows
  • constantly changing flow lines. Net mass
    transport in the flow direction

15
Flow fight between inertial and viscous forces
  • Inertial 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
    its 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)

16
Reynolds
  • 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 lt500 (glaciers some mud flows)
  • High Re turbulent gt 2000 (nearly all flow)

17
Geologically SignificantFluid Flow Types
(Processes)
  • Laminar Flows
  • straight or boundary parallel flow lines
  • Turbulent flows
  • constantly changing flow lines. Net mass
    transport in the flow direction

18
Geologically Significant Fluids and Flow Processes
Debris flow (laminated flow)
  • 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)
19
What else impacts Fluid Flow?
  • Channels
  • Water depth
  • Smoothness of Channel Surfaces
  • Viscous Sub-layer

20
1. Channel
  • Greater slope greater velocity
  • Higher velocity greater lift force
  • More erosive
  • Higher velocity greater inertial forces
  • Higher numerator higher Re
  • More turbulent

21
2. Water depth
  • Water flowing over the bottom creates shear
    stress (retards flow exerted parallel 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 boundary layer is
    retarded
  • Consider velocity in broad shallow stream vs deep
    river

22
2. Water Depth
  • Boundary Shear stress (?o)-stress that opposes
    the motion of a fluid at the bed surface
  • (?o) gRhS
  • ? 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)

23
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)

24
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

25
4. Viscous Sub-layer
  • 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 sub-layer
  • 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)

26
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

27
Deposition
  • Occurs when system can no longer support grain
  • Particle Settling
  • Particles settle due to interaction of upwardly
    directed forces (buoyancy 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

28
Particle SettlingForces opposing entrainment and
transport
  • VS    (?g - ?f)gd2/18 m
  • VS settling velocity
  • ?g grain density
  • ?f fluid density
  • m fluid viscosity
  • d grain diameter
  • Stokes law of settling
  • Applies to grains lt0.1mm in water
  • lt0.06mm in air

29
Theory vs application
  • Increase velocity, increase turbulence and
    entrainment
  • Material plays a role
  • Hjülstroms curve
  • Empirical measure of minimum Velocity required to
    move particles of different sizes

30
Hjülstroms 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

31
Critical Threshold for Particle Entrainment
  • Fm     gt    Fi
  • Hjulstrom Diagram
  • 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
  • For clay size particles electrostatics requires
    increased flow velocity for entrainment
  • (gray area is experimental variation)

32
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 gt 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

33
Transport Modes and Particle Entrainment
  • With a grain at rest, as flow velocity increases
  • Fm     gt    Fi initiates particle motion
  • Grain Suspension (for small particle sizes, fine
    silt lt0.01mm)
  • When Fm  gt  Fi
  • U (flow velocity) gtgtgt VS (settling velocity)
  • Constant grain Suspension at relatively low U
    (flow velocity)
  • Wash load Transport Mode

34
Transport Modes and Particle Entrainment
  • With a grain at rest, as flow velocity increases
  • Fm     gt    Fi initiates particle motion
  • Grain Saltation for larger grains (sand size
    and larger)
  • When Fm  gt  Fi
  •  U   gt VS  but through time/space U lt VS
  • Intermittent Suspension
  • Bedload Transport Mode

35
Transport Modes and Particle Entrainment
  • With a grain at rest, as flow velocity increases
  • Fm     lt    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

36
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

37
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

38
Grain size and Water Depth-Bedform
  • Grain size impacts bedform formation
  • coarse grains, no ripples are formed
  • fines (clays), no dunes form
  • Water depth affects bedform
  • Increase with depth, increase velocity at which
    change from low to upper flow regime occurs

39
Flow Regime and Sedimentary Structures
  • An Introduction To
  • Physical Processes of Sedimentation

40
(No Transcript)
41
Sedimentary structures
  • Sedimentary structures occur at very different
    scales, from less than a mm (thin section) to
    100s1000s 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)

42
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

43
(No Transcript)
44
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 gt
    sediment output)
  • Straight-crested bedforms lead to planar cross
    stratification sinuous or linguoid bedforms
    produce trough cross stratification

45
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
  • 2D3D ripples and dunes
  • Bi-directional (oscillatory)
  • Straight crested symmetric ripples
  • Combined Flow
  • Hummocks and swales

46
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.

47
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 (lt 1m?) with
    increasing flow velocity
  • Dunes (2d) sand waves with straight crests to
    (3d) dunes (gt1.5m?) with sinuous crests and
    troughs

48
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 (lt 1m) with
    increasing flow velocity
  • Dunes (2d) sand waves with straight crests to
    (3d) dunes (gt1.5m) with sinuous crests and
    troughs

49
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)

50
(No Transcript)
51
(No Transcript)
52
(No Transcript)
53
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

54
(No Transcript)
55
(No Transcript)
56
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)

57
Sedimentary structures
  • Planar stratification
  • Planar lamination (or planar bedding) is formed
    under both lower-stage and upper-stage flow
    conditions
  • Planar stratification can easily be confused with
    planar cross stratification, depending on the
    orientation of a section (strike sections!)

58
Bed Response to Steady-state, Unidirectional,
Water Flow
  • Consider Variation in Grain Size Flow Velocity
  • for sand lt0.2mm No Dunes
  • for sand 0.2 to 0.8mm Idealized Flow Regime
    Sequence of Bed forms
  • for sand gt 0.8 No ripples nor lower plane bed

59
(No Transcript)
60
Flow regime Concept (summary)

61
(No Transcript)
62
(No Transcript)
63
(No Transcript)
64
(No Transcript)
65
(No Transcript)
66
Application of Flow Regime Concept to Other Flow
Types

67
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

68
(No Transcript)
69
(No Transcript)
70
(No Transcript)
71
(No Transcript)
72
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

73
(No Transcript)
74
Debrites
  • Debris flow deposits
  • See Turbidites?Turbidity current deposits

75
(No Transcript)
76
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

77
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

78
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.
79
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

80
Fluidal Flows
  • Turbidity Currents
  • deposition from bedload suspended load
  • initial deposits are coarsest transported
    particles deposited (ideally) under upper (plane
    bed) flow regime

81
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

82
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

83
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)

84
(No Transcript)
85
(No Transcript)
86
(No Transcript)
87
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)

88
(No Transcript)
89
(No Transcript)
90
(No Transcript)
91
(No Transcript)
92
(No Transcript)
93
Dewatering Structures
94
(No Transcript)
95
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

96
(No Transcript)
97
(No Transcript)
98
(No Transcript)
99
(No Transcript)
100
(No Transcript)
101
(No Transcript)
102
(No Transcript)
About PowerShow.com