Sediment Erosion,Transport, Deposition, and Sedimentary

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Sediment Erosion,Transport, Deposition, and
Sedimentary Structures

• An Introduction To
• Physical Processes of Sedimentation

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Sediment transport

• Fluid Dynamics
• COMPLICATED
• Focus on basics
• Foundation
• NOT comprehensive

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Sedimentary Cycle

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

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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

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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

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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

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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

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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??

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Why do particles move?

• Entrainment
• Transport/ Flow

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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

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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

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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

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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

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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)

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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)

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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

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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)
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What else impacts Fluid Flow?

• Channels
• Water depth
• Smoothness of Channel Surfaces
• Viscous Sub-layer

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1. Channel

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

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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

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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)

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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)

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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

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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)

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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

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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

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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

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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

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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

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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)

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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

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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

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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

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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

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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

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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

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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

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Flow Regime and Sedimentary Structures

• An Introduction To
• Physical Processes of Sedimentation

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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)

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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

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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

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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

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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.

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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

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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

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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)

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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

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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)

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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!)

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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

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Flow regime Concept (summary)

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Application of Flow Regime Concept to Other Flow
Types

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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

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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

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Debrites

• Debris flow deposits
• See Turbidites?Turbidity current deposits

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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

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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

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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.
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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

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Fluidal Flows

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

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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

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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

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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)

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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)

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Dewatering Structures
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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

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