Sediment transport and sedimentary structures - lecture 3

transport media gravity

rock falls, usually gravel

transport media water

most important, currents driven by gravity, tides, wind

transport media air

limited transport capacity due to low density

transport media ice

large transport capacity

transport media dense sediment and water mixtures

gravity-driven as debris flows and turbidity currents

Laminar flow

no mixing, high viscosity: ice, debris flows, lava

Turbulent flow

mixing, low viscosity: water, air

Transport particles in a fluid: bedload

rolling, low viscosity

Transport particles in a fluid: lighter bedload

saltation, middle viscosity

transport particles in a fluid: suspended load

suspension: high viscosity

Bernoulli’s equation

Total energy = kinetic energy + potential energy + pressure.
velocity (kinetic energy) increase over an object results in a pressure drop (lift force), because of conservation of total energy with potential energy remaining constant.

Stokes law

larger particles reach higher settling velocities

Normal grading

settling of sediement out of suspension in standing water, settling of sediment from decelerating flow

reverse grading (less common)

results from deposition with increasing flow velocity (or from kintetic sieving in grain flows0

Fining upward and coarsening upward

refer to grain size trends in a series of beds, caused by a series of depositional events.

Smooth boundary layer, bedform formation

thick viscous sublayer (low velocities) and/or small grain sized. All sediments are laying down fully within the viscous sublayer (current ripples)

rough boundary layer: bedform formation

thin viscous sublayer (high velocities) and/or large grain diameters the sediments cros the viscou sublayer (dunes)

The boundary layer within water

within the boundary layer the bottom flow velocities are lower and on top are higher. Above the boundary layer the flow is constant.

Turbulent sweeps

krachtige, neerwaartse bewegingen van wind of vloeistof in een turbulente grenslaag, these lead to local flow seperation

Avalanching

describes a sudden, massive downarde slide of material, it builds a slope of 30 degree, max angle possible.

Cross-lamination

a small-scale sedimentary structure featuring tiny inclined layers (foresets) that lie at an angle to the main horizontal bedding plane

Curren ripples and cross-lamination

the higher the flow velocity and duration the planar cross-laminnation will progress to a trough cross-lamination. So from straight crested beforms to sinuous crested bedforms

constraints on current ripple formation

Only in sand finer than 0.6 mm

on hydrodynamically smooth beds

formation independent of water depth

maximum height 40 mm, maxiumum wavelenght 500 mm

dunes

not just large ripples, contorlled by large-scale turbulenc in the whole flow, not just the boundary layer, controlled by water depth, can never be higher then the water depth. Current ripples ofter superimposed on dunes

Dunes and cross-bedding

formation by avalanching on lee slope. structure is cross-bdding. Depending on strenght of rollervortex: planar or trough cross-bedding. If very strong roller vortex: counter flow ripples.

constraints on dune formation

not in silt and very fine sand (<0.125 mm)

on hydrodynamically rough beds

formation dependent on water depth

height ranges from a few cm to over 10 m

wavelengths ranges from 60 cm to a few hundreds of metres

requires long flow events to build up.

Bars

Bars are larger than dunes. made up of large range of grain sizes. cross-lamination, cross-bedding and (larger scale) cross-stratification

plane bed

in coarser sands at low flow velocities, close to threshold of motion, insufficient for dunes to form (ripples do not form because of rough bed conditions). In very fine to coarse-grained sands at high flow velocities (ripples and dunes are washed out). Primary current lineation forms on (upper) plane bed at high flow velocities) sedimentary structure: planar lamination

Bedform stability…

lower flow regime: ripples, dunes, lower flat (plane bed

upper flow regime: upper flat(plane) bed, antidunes

antidunes are upstream migrating dunes that occur at high-energy flow conditions (are rarely preserved)

Waves

oscillatory motion in deep water, without net horizontal water movement

Breaking in shallow water creating horizontal movement of water

generated by wind or single events (tsunamis)

Wave ripples and wave ripple cross-lamination

formed in coarse silt an sand, symmetrical, few mm to few cm, overlapping laminae dipping in both directions

wave ripples

long, sinuous to straight crests, which may bifurcate

symmetrical in cross-section

laminae dip in both directions

Current ripples

short crests that ar very sinuous/curved

assymmetrical in cross-section

laminae dip consistently in one direction

sediment gravity flows

mixtures of sediment (detritus) and fluid (or air) moving under gravity

debris flows

dense, viscous mixtures of sediment and water, more sediment than water

turbidity currents

turbid mixtures of sediment temporarily suspended in water, more water than sediment

grain flow

avalanches of particales (usally well sorted) down a steep slope

debris flows

laminar flow, no or little sorting, varriable grain sizes

significant slope is needed

in arid areas (limited water) and on submarine slopes

deposits are typically matrix-supported gravels/conglomerates

chaotic fabric, hardly any structure; modderstroom!

trubidity currents

turbulent flow

always on submerged (submarine) slopes

driven by density differences

deposits a normally graded bed

turbidites

are graded beds deposits by turbidity currents, differt types based on grain size

mudcracks

desiccation cracks form under subaerial conditions due to drying of cohesive material (clay)

spacing dependent on thickness of clay

taper downward in cross-section

detached edges may form mud-chips or mud-flakes in overlying deposits

erosional structures

large scale: channels

small scale: sole marks

scour marks: flue marks (turbulent eddies), obstacle scours, ridges and furrows (mm scale), gutter marks (cm scale)

tool marks: grooves and prod, skip, bounce marks

bed

unit of sediment which is generally uniform in character and contains no distinctive breaks

interbedded

alternations of thin layers of different lithologies

bed thickness

thin, medium, thick, very thick

cross-stratification

any layering oriented at an angle to the (depositoinal) horizontal (general term, independent of origin or scale)

cross-lamination

inclined strata formed by ripples, smaller then 30 mm height

cross-bedding

inclined stata formed by dunes, tens of cm to tens of m height

bed-set

single unit of cross-laminated, cross-bedded or cross-stratified sediment

coset

stack of multiple sets with the same type of structure

mixtures of sand and mud: flaser lamination

mostly sand

Mixtures of sand and mud: wavy lamination

half sand half mud

mixtures of sand and mud: lenticular lamination

mostly mud