Transport processes and particle properties Flashcards
(19 cards)
how is density and viscosity related to transport processes
fluid density affects settling velocity which impacts deposition and influences movement of fluid
density of water is >700 times greater than that of air
◼ Water transport»_space; larger particles than wind
Fluid dynamic viscosity is the force required to move the fluid at a certain velocity
◼ ice»_space; water»_space; air
◼ ↓ temperature =↑ viscosity
◼ ↓ turbulence = ↑ viscosity
bed load vs suspended load
bedload
-traction transport (in constant contact with the bottom, rolling, sliding, creep)
-uninterrupted saltation (sort of hops across the bottom, gets some lift)
-interrupted saltation (interrupted by turbulence or collision with other grains)
suspended load
◼ Continuous suspension of very fine-grained sediments
◼ May be intermittent due to erratic lift forces
◼ Move with the fluid (wash load)
list the main evidence for wind vs fluvial vs marine sediments
wind:
- fine grained
- frosted
-graded arcs
-crescentic percussion marks
-abrasion fatigue
-rounded outline
-upturned plates
-low relief
-small conchoidal fractures
-meandering ridges
-elongated depressions
high energy fluvial:
-rounded/subangular
-small conchoidal fractures
-v shaped percussion cracks
-medium relief
low energy fluvial
-medium relief
-silica pellicle/globules/flowers
marine
-subangular/rounded
-small conchoidal fractures
-straight/curved grooves/scratches
-v shaped percussion cracks
-oriented etch pits
-silica globules/flowers
-medium relief
glacial
-angular
-small/medium/large conchoidal fracture
-arcuate/straight steps
-parallel striations
-high relief
-chatter marks
techniques to measure particle size distributions
Large particles
Caliper or tape
Granule- to silt-size particles
Sieving techniques
Image analysis
–> AI -> particle shape analysis
Silt and clay-size particles
Laser-diffraction size analyser
Hydrometer and pipette analysis
Etc.
laser defraction:
laser through transparent flow cell
Particles of a given size scatter/diffract light through a
given angle, the angle is inversely proportional to particle size
Dynamic Image Analysis (DIA)
-measure particle size and shape by capturing and analyzing high-speed images of particles as they move (in a flow of air/liquid).
-assess the size distribution and morphology of particles ranging from sand to silt-sized grains
statistical analysis of grain size data
Mode, median, mean, standard deviation
Grain size data typically
-not normally distributed
-reported as wt. percent
-variables not independent
–>As the proportion of one constituent (e.g. sand %) increases, the proportions of the other constituents must decrease
-‘closed’ and non-negative
Log-ratio transformations
- used to address the ‘closure’ problem
- Produce independent values that are free to vary from -∞ to +∞
-necessary step in the preparation of grain size data prior to statistical analysis
lower vs upper flow regime
(in order of increasing flow strength)
lower:
ripples
dunes
upper:
antidunes
chutes and pools
asymmetric vs climbing vs symmetric ripples vs antidunes
Asymmetric ripples
-lee side shorter than stoss
-eddy in trough
-lee side downcurrent
* Sand particles move up the stoss-side
* Some particles avalanche down the lee-side (grain flow)
* Others will go in brief suspension and fall (near bottom of lee-side); (grain fall)
* Fine sand in suspension may settle-down as well (particle settling)
* The balance between deposition from bedload versus suspended load will affect the migration rate and final shape of ripples
Planar cross laminations created by straight crested ripples
Trough cross laminations created by sinuous crested ripples
climbing ripples
subcritically climbing ripples
angle of climb < stoss side angle
only toe of foreset preserved
critically climbing ripples
angle of climb = stoss side angle
entire lee side is preserved and stoss side is barely preserved
supercritically climbing ripples
angle of climb > stoss side angle
entire lee side is preserved and entire stoss side is preserved
symmetric
-Wave-oscillation ripples
-waves going up and down a beach
Antidunes
- produced by in-phase, shallow flow
- migrate upstream
- low angle cross laminations that are dipping upstream.
- Migration is due to grain accretion
temporal changes
What happens when processes change with time?
How is it recorded in sedimentary deposits?
fining/coarsening up
eg * Brown/buff sand-rich layers transition to Grey silt/clay rich layers
turbidity currents
surge type vs steady uniform type
low vs high density flows
-turbulent flow
-non cohesive
dense turbid cloud of sediment flows down slope along the bottom of an ocean or lake due to density contrasts between the current and the surrounding (ambient) fluid
Frequently initiated by:
-Sediment failure
-Storm- or earthquake-triggered flow of sediment
-Bedload inflow from rivers
surge-type
head
Twice as thick as the rest
Intense turbulence
Divided transversely into lobes and clefts
Coarser material, may be a region of erosion
body
Flow is nearly steady and uniform in thickness
The body flows at a faster velocity than the head
–> Process of mixing
Region of deposition
tail
Thins abruptly away from the body and becomes more dilute
Turbidity currents (steady, uniform type)
Uniform flows lack a turbulent head
Similar velocities as surge-type flows
Flow can occur on slope < 1 degree
Sediment-laden rivers run into lakes (muddy rivers discharge)
Less likely in marine settings due to lower density contrasts
Low-density flows
< 20%-30% grains (may vary depending on classification used)
Clay, silt and fine- to medium sand
Suspended entirely by turbulence
High-density flows
>30 % grains (may vary depending on classification used)
May also include coarse-grained sands and pebble- to cobble-size clasts
Turbulence and hindered settling from high sediment concentrations
The two types may occur along a single current
turbidites
Turbidity current deposits
Deposited in marine and lacustrine environments.
Can be classified by their flow type (high vs low)
high-density flows
Thick-bedded turbidite successions (e.g. coarse-grained sandstones)
Relatively poor sorting
Few internal laminations
Basal scour marks may be rare and poorly developed
May have normal grading
Traction structures (e.g. laminations)
irregular/wavy lower contact
low-density flows
Thin-bedded turbidite successions
Fine-grained at the base
Well developed sedimentary structures
Basal scour marks are typically abundant
The Bouma sequence
-applies to turbidity currents
-Record the decay of flow strength with time
-Progressive development of sedimentary structures as flow velocity wanes
-rippled or flat top
-ripple drift micro cross laminations
-laminated
-good grading (normal)
-flutes/tool marks on base
bottom to top:
A - Massive sand unit, flutes/tool marks on base, erosional contact with underlying material, normal grading, few to
no sedimentary structures
*Results from rapid deposition at the base of the flow
B - Parallel laminated coarse–medium sand (upper plane beds).
*Results from deposition of bedload transport sediment (upper
flow regime).
C - Cross laminated medium-fine sand (dunes, ripples), Lower flow regime deposition, can include shearing and dewatering structures
D - Laminated fines, Deposited within the tail of the turbidity current
E - *Hemipelagic sedimentation of fines.
liquefied flows
-mix of laminar and turbulent flow
-mix of cohesive and non cohesive
Liquefaction is a process by which water-saturated sediment temporarily loses strength and acts like a fluid
Resedimented sand has tighter packing
liquefied (/fluidized) flow deposit:
grooves, striations on base, flame&load structures
poor grading (coarse tail)
convolute lamination
Thick deposits (10s of cm+)
Sand units (sometimes poorly-sorted)
Characterized by fluid escape structures.
- Internal dish structures
- Pipes of liquefied sediment leading to sand “volcanoes
Some sedimentological characteristics
-wavy contact
-gravel injections
-hydrofractures, sand dikes
Grain flows
Cohesionless sediment moves, as driven by gravity.
-Occur on slopes that approaches or exceeds the angle of
repose
Angle of repose
The gravitational force acting on a slope can be divided into two components:
Shear Stress pulling the grain down the slope
Shear Strength of the material holding the grain in place (normal stress, cohesion to other grains)
The angle of repose is the steepest slope where Shear Strength > Shear Stress
Dispersive pressures push the grains apart
In air: provided by grain-to-grain collision
In water: grain collisions and close encounters
Grain flow deposits
common on the lee slope of dunes
Single grain flow are usually just a few cm thick for sand-sized grains.
Deposition occurs when kinetic energy of the particles falls below threshold.
Tend to form reverse graded beds (slight, subtle)
–> Smaller particles filtering down through the larger particles during dispersed state
–> Kinetic sieving
top to bottom:
-flat top
-no grading
-massive, grain orienation parallel to flow
-reverse grading near base
-scours, injection structures
Debris flows and mud flows
- laminar flow
- cohesive
Slurry-like flows composed of a mass of liquefied poorly-sorted mixture that move downhill under the force of gravity
Clasts are supported by strength and buoyancy of the matrix.
Triggered by the near saturation of loose sediment on steep slopes (>10°)
can continue to flow on gentle slopes
common
in mountainous regions
in arid and semiarid regions.
near volcanoes with saturation of loose pyroclastic material.
- Deposition via mass emplacement
- Multiple grain sizes
- Complex orientation of clasts
- Can be reverse graded beds.
- Massive to crudely stratified
- Often matrix-supported
-clast to matrix supported
-sharp base, erosive to non erosive
-crude bedding
top to bottom:
Deltas
– “Discrete shoreline protuberances formed where rivers enter
oceans, semi-enclosed seas, lakes or lagoons and supply
sediment more rapidly than it can be redistributed by basinal
processes”
Grain size and shape of sediment bodies
The size and gradients of terrigenous depositional
systems are related to the grain size or calibre of the
sediment
– Coarse-grained alluvial, deltaic and deep-sea systems are
relatively small and steep
– Sand-rich systems tend to be intermediate in size, with
moderate gradients
– Mud-rich systems are generally large with low gradient
Fluvial-dominated delta system
facies associations and successions
gilbert type
- Density contrast between river and basin waters– Homopycnal jet flow
- Little or no density contrast (river outflow in a freshwater or low salinity basin)
- Mixing and rapid deposition
- Gilbert-type deltas
– Hyperpycnal plane-jet flow - Density current (common during floods)
– Hypopycnal plane-jet flow - Buoyant dominated river mouth
– Rivers flowing into denser seawater or a saline lake - Tends to generate a large, active and low dipping delta-front area
Facies associations and successions
* Seaward progradation
– Coarsening-upward sequence
* May be affected by
– Tectonism
– Climate change
– Diversions of rivers
– Sea level change
– Switching of delta lobes, distributaries,
or tidal channels
Coarse-grained river-dominated delta (Gilbert-type)
-topsets proximal fan delta to transition zone
-foresets delta front
-bottomsets prodelta
-basin
Wave vs tide dominated deltas
- Strong waves cause
– Rapid diffusion and deceleration of river outflow - Constricted or deflected river mouths
– Distributary-mouth sediments are - Reworked by waves
- Redistributed along the delta front by longshore currents
– Wave-built shoreline features (e.g. beaches)
Tide-dominated deltas
* Tidal currents are stronger than river outflow
* Bidirectional currents redistribute river-mouth sediments
– Sand-filled, funnel-shaped distributaries
– Mouth bar may be reworked into a series of linear tidal ridges
* Extend out onto the subaqueous delta-front platform
Tidal-dominated example
- Coarsening-upward
- Bioturbated sand bodies
Estuarine systems
* Seaward portion of a drowned valley system
– Influenced by fluvial, tidal and wave processes
– Form under trangressive (SLR) conditions
– May evolve into deltas
-tide or wave dominated
Facies successions
* Cross-bedded, bioturbated sand near the mouths and in fluvial-tidal channels
* Laminated, well-bioturbated muds occupy the nonchannel middle and upper parts of the estuary
* Many are subjected to transgression
– Facies Associations
* Fluvial or fluvial-deltaic facies overlain by facies showing more tidal influence
* Regression causes filling and destruction of the estuary and seaward progradation
– Estuary may evolve into a delta
coastal systems
ichnofacies
barrier island systems
lagoonal systems
types of coasts:
-wave dominated
-tide dominated
-mix
What are ichnofacies?
* Ichnofacies are spatially and temporally recurring groupings of organism behaviours, as recorded in trace fossil suites
Barrier-island systems
Beaches separated from land by shallow lagoon, estuary, or marsh, Commonly dissected by tidal channels or inlets
origin:
* Upward building and eventual emergence of the longshore bar
* Spit segmentation
* Mainland ridge engulfment
* Lateral shifting of coastal sands during transgression
Barrier islands
Facies associations:
* Other shoreface and offshore facies
* Tidal channels and mudflats
* Tidal deltas
* Lagoon facies
Sea-level change
-slow slr: transgression by shoreface retreat
-rapid slr: trangression by in place drowning
-regression
lagoonal systems
Dominance of low-energy conditions; little or no freshwater input
types:
*choked
-water movement by wind forcing
-Long residence time of water
*restricted
-Well-defined tidal circulation
-strongly influenced by winds
-Silty or muddy seds. Carbonates, evaporites
*leaky
-Important tidal currents
-Efficient water exchange with the ocean
Ancient lagoonal deposits
* Evidence for restricted circulation
– Evaporites
– Anoxic facies (black shales)
– Lack of strong tidal influence
– Low faunal diversity
* But there can be extensive bioturbation
- Beach/barrier systems
– lithology – sand and conglomerate
– mineralogy – mature quartz sands and shelly sands
– texture – well sorted, well rounded clasts
– bed geometry – elongate lenses
– sedimentary structures – low-angle stratification and wave reworking
– palaeocurrents – mainly wave-formed structures
– fossils – robust shelly debris
– colour – not diagnostic
– facies associations – may be associated with coastal plain, lagoonal or shallow-marine facies - Lagoons
– lithology – mainly mud with some sand
– mineralogy – variable
– texture – fine-grained, moderately to poorly sorted
– bed geometry – thinly bedded mud with thin sheets and lenses of sand
– sedimentary structures – may be laminated and wave rippled
– palaeocurrents – rare, not diagnostic
– fossils – often monospecific assemblages of hypersaline or brackish tolerant organisms
– colour – may be dark due to anaerobic conditions (e.g. black shales)
– facies associations – may be associated with coastal plain or beach barrier deposits - Tidal channel systems
– lithology – mud, sand and less commonly conglomerate
– mineralogy – variable
– texture – may be well sorted in high energy settings
– bed geometry – lenses with erosional bases
– sedimentary structures – cross-bedding and cross-lamination and inclined heterolithic stratification
– palaeocurrents – bimodal in tidal estuaries
– fossils – shallow marine
– colour – not diagnostic
– facies associations – may be overlain by fluvial, shallow marine, continental or delta facies - Tidal mudflats
– lithology – mud and sand
– mineralogy – clay and shelly sand
– texture – fine-grained, not diagnostic
– bed geometry – tabular muds with thin sheets and lenses of sand
– sedimentary structures – ripple cross-lamination and flaser/lenticular bedding
– palaeocurrents – bimodal in tidal estuaries
– fossils – shallow marine fauna and salt marsh vegetation
– colour – often dark due to anaerobic conditions
– facies associations – may be overlain by shallow marine or continental facies
