Lecture Twenty Seven - Surface processes and sedimentary transport Flashcards
In what ways can sediments be transported?
Involvement and nature of interstitiual medium between deiment fragments:
Water, air, ice or none (or combination).
Whether sediments move:
1) Independantly
- Aka particulate transport.
2) As a large volume of sediment en masse and rapidly deposited.
- Aka mass movement.
–> Driven by oarticle interation (and gravity).
- E.g. Debris flows, flumps, turbidite structures.
These are relatively rare events.
They are over represented in the geological record as they involve a big dump of sediments, which are then preserved on the surface usually.
Big time disconuities between sediment dumps and the layers of rock the sediments were dumped on.
What are the roles of water in sedimentary transportation?
Water buoys/suspends light larticles (Stokes’ Law).
Exerts an (extra) force on sediment particles -> drives sediment movement.
Water acts as a lubricant in some mass flows, reducing the coefficient of the internal friction -> Allows mass flows to move more easily over the substrate.
Water flows downslope due to its potential energy with only minor disturbance.
Provide an overview of sedimentary transport processes.
Describe mass movement of sediments.
River (fluvial) torrent flow or hyperconcentrated flood flow.
- ‘Upper flow regime’ flood flow (FR>1).
- > High suspended granular sediment load.
- > Highly erosive.
- When flow velocity wanes.
- > Massive sandstone and conglomerate beds meters thick.
Describe turbidity currents.
Gravity driven mass flows - AKA ‘Density currents.’
Consist of deiment roch fluid.
Deposits called turbidites.
Co-efficient of friction extremely low - flow on minimum slope angle of ~0.25 degrees.
Flow up to 000’s of kms.
Most commonly in deep oceans and lake slopes/floors.
Turbidites form:
Low sediment concentration/density turbidity currents.
- ~20-30% grains, typically fine particles.
- Highly turbulent (low viscosity).
- Normal grain size graded beds.
- ‘Bouma sequence.’
High sedimentary concentration/density turbidity currents:
- >30% grains.
- Poorly expanded, lower turbitites (higher viscosity).
- Massive beds, poor grading.
Bouma sequence (low concentration):
Overall normal grading:
- High density sediments deposited first.
- Fine sediments initially in suspension, eventually fall out.
Bouma sequence can be subdivided:
- Preserve structures and grain sizes differ.
- Upwards trend represents wanning of current flow rate.
What are flame casts and flute casts?
Flame casts:
Formed when there is a dump of heavy sediments on top of finer grained sediments. The weight of the courser heavier sediments cause the fine sediments to flow into the gaps in the overlying sedimentary layer, casing flame shaped intrustions.
Flute casts:
Formed when there is a flow of sediments and a ditch is dug out of the underlying sediments due to turbulent flows.
Describe mud flows and debris flows.
Viscous, cohesive flows of mud and suspended course grained debris.
- As opposed to hyperconcentrated flood flows (HFF have more water and less sediments).
Water acts as a lubricant in mud flow.
- Massive, unsorted deposit with variable degree of course debris supported by mud matrix.
Deposits 10’scm to m’s thick.
Move over slopes as low as 1-2 degrees.
Subaerial or subaqueous.
Describe slumps.
Watery, coherent slides.
Coherent soft sediment, deformed deposits.
triggered by tectonic/volcanic/climatic events.
In subsequent environments slumps may evolve into -> debris flow -> turbidity currents.
Post-depositional slup structure - commonly form within sediments deposited on slope.
Slumps can be differentiated from deformation structures caused by tectonic movement because slumps have linear beds above and below them.
Describe soil creep.
Uppermost soil layers creep downslope.
Enhanced by water saturated clay rich soil on slope.
More prevelent nowadays due to deforrestation - trees can be used to hold soils in place.
What mechanisms promote grain transport in mass flow processes?
1) Turbulent eddies suspend particles e.g. low concentration/density turbidity currents.
2) Upward drag force (velocity) of escaping water during deposition retards deposition e.g. turbidity currents.
3) Grains to grain collisions transfer momentum e.g. grain flows, high concentration/density turbidity currents, rock avalanches.
4) Buoyancy where the fluid is high density cohesive mud e.g. mud and debris flows.
5) Body strnegth of sliding coherent mass e.g. landslides.
From 1 –> 5, viscosity is increasing.
Describe air as a interstitial medium.
**Particulate (in this case wind blown) movement: **
_Traction (particles transported along substrate). _
Remember Stokes’ Law = Larger density difference between medium and grains.
- Particles are less buoyant in air than in water.
- Resist transport (high angle of repose).
- Can form large sedimentary structures (ripples to large dunes with huge X bends to draa (>50 meters)).
Depositional environments - coastal dunes and deserts.
Suspension (particles held above substrate):
Aeolian turbidity currents (dust/sand stormes).
Mass movement:
Air lubricated rock avalanches.
