sediment erosion,transport, deposition, and sedimentary structures an introduction to physical...

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

• Water• Water containing various amounts of sediment• Air• Volcanic gasses/ particles

Definitions

• Fundamental Properties– Density (Rho ())

• Mass/unit volume– Water ~ 700x air

= 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 H20 > air

Definitions

• Fundamental Properties– Viscosity

• Mu ()– Water ~ 50 x air

= measure of ability of fluids to flowresistance 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 gh) + (1/2 2)+P+Eloss = constant

Static P + dynamic P • Potential energy= gh• Kinetic energy= 1/2 2

• 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 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

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/(/ dimensionless #

– V= current velocity– l= depth of flow-diameter of pipe– = density– = viscosity

/kinematic viscosity• Fluids with low (air) are turbulent• Change to turbulent determined experimentally

• Low Re = laminar <500 (glaciers; some mud flows)• High Re = turbulent > 2000 (nearly all flow)

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

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 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) = RhS

• = 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 SettlingForces opposing entrainment and transport

• VS = [(ρg - ρf)g/18 ]d2

– VS : settling velocity

– ρg = grain density

– ρf = fluid density

= 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

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 (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


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


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 Depth-Bedform

• 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)


• 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


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.


• 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


• 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)


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


• 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)


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!)


• Consider Variation in Grain Size & Flow Velocity– for sand <~0.2mm: No Dunes

– for sand ~0.2 to 0.8mm Idealized Flow Regime Sequence of Bed forms

– for sand > 0.8: No ripples nor lower plane bed

Flow regime Concept (summary)

Application of Flow Regime Concept to Other Flow Types


• 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


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.

http://volcanoes.usgs.gov/Hazards/What/Lahars/RainLahar.html

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


• 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)


• 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

sediment erosion,transport, deposition, and sedimentary structures an introduction to physical...

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