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 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

Definitions

Fundamental Properties

Density (Rho (r))

Mass/unit volume

Water ~ 700x air

r = 0.998 g/ml @ 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

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)

Sediment load and viscosity co-vary

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 its 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 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)

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 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

Debris flow (laminated flow)

Traction deposits (turbulent flow)

What else impacts Fluid Flow?

Channels

Water depth

Smoothness of Channel Surfaces

Viscous Sub-layer

1. Channel

Greater slope = greater velocity

Higher velocity = greater lift force

More erosive

Higher velocity = greater inertial forces

Higher numerator = higher Re

More turbulent

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

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 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)

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 (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

Particle Settling Forces 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

Theory vs application

Increase velocity, increase turbulence and

entrainment

Material plays a role

Hjlstroms curve

Empirical measure of minimum Velocity required to

move particles of different sizes

Hjlstroms 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

Fm > 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)

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; 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

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 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

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 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)

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 (< ~1m) with increasing flow velocity

Dunes: (2d) sand waves with straight crests to (3d) dunes (>~1.5m)

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 upper-stage 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 0.8: 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 TurbiditesTurbidity 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