Aeolian systems Flashcards
Define aeolian
Processes of erosion, entrainment, transportation, and deposition by the wind
Why is it harder to move particles on land than it water?
The difference in density is greater
How are grains on land held together?
Gravity, friction, vegetations, cements, moisture
What forces promote entrainment of grains on land?
Ballistic impacts and aerodynamic lift
What forces prevent entrainment of grains on land?
Weight, friction, drag, cohesive inter-particle forces
How are sand grains primarily transported? What does their transport and deposition allow?
As a bedload
Allows the construction of dunes
How are silt and clays primarily transported?
In suspension
What is a deflation lag deposit?
Formed by the grains that are too heavy to move e.g. gravel
Give the three ways (or combination of these ways) that sand can be carried as a bedload
Rolling/creep due to drag on the bed
Saltation
Reptation
Describe saltation in the bedload transport of sand grains
Particles are temporarily lifted into the airflow, collide with other particles, introducing a chain reaction of interparticle collisions
This process is the most important
Describe reptation in the bedload transport of sand grains
Saltating particles return to the bed and impacting other particles, causing them to hop short distances downwind
Where are aeolian processes most active and widespread?
Arid, hot/cold deserts
These cover ~30% of land surface
Why are grains in deserts so suseptible to aeolian processes?
They lack vegetation (to provide strength and cover) and moisture
Describe the back-erg of a dunefield
Represents the trailing, upwind margin of the erg
Describe the fore-wind of a dunefield
Represents the leading, downwind margin of the dunefield
Define an erg
Wind-formed desert landscapes characterised by extensive sand dunes that cover the terrain
Give three examples of geomorphic elements that can be associated with dunes
Zibars, sandsheets, interdunes
What are the six types of sand dune?
Barchan, transverse, linear, star, dome, parabolic
What is a stabilising aeolian system?
Develop where physical, chemical, or biogenic agents promote aeolian system construction and accumulation
What are the three classifications of aeolian systems?
Dry, wet, stabilising
Describe a dry aeolian system
Accumulation is controlled by aerodynamic configuration
Airflow deceleration causes sediment-laden winds to deposit some of their load
Describe a wet aeolian system
Accumulation is controlled by progressive water-table rise concomitant with ongoing aeolian activity
Where do siliciclastic coastal dunefield typically occur?
Along humid, mid-latitude coasts
Where do carbonate dunefields typically form?
Preferentially along mid-low latitude coasts, esp those bordering productive shallow marine platforms (supply of carbonate sediment)
How are aeolian processes inhibited in humid (inc. glacial regions) settings?
Vegetation and moisture
What is aeolian loess?
A layer of fine, wind-blown silt and clay particles
Where are loess deposits most typically found?
Areas that have experienced extensive glacial activity during past ice age
Areas where strong winds carry fine grains from exposed surfaces (e.g. glacial outwash plains, river valleys, deserts)
What impacts do dust/sand storms have?
Reduced visibilty, productivty, resp diseases
Fertisation of oceans (stimulates growth of phytoplankton)
What are the three scales of aeolian bedform?
Ripples, dunes, megadunes
What are the three types of small-scale ripple structures that occur on wind-blown surfaces?
Impact ripples, megaripples, adhesion ripples
What is the most widespread ripple type?
Wind ripples
Describe wind ripples
Low relief and form preferentially form the coarser-grained fraction of sand in transport
What are the three stages in the formation of wind ripples?
Initiation of development from perturbation on a bed
Impact ripple spacing determined by saltation path length
Upwind-facing stoss slope (subject to impact of infoming grains)
What generates a grain flow sand avalanche?
Gravitational collapse of dry, loose sand on aeolian dune/megadune lee slopes inclined at the angle of repose (~32-36deg)
What is an aeolian grain flow strata?
Thick package of multiple stacked aeolian grain flow avalanche deposits
When does gravity-driven grain fall occur?
Where wind carrying clouds of saltating grains over a dune brink decelerates as it passes into the lee-side depression, resulting in a reduction in sediment carrying capacity
Give another example of when gravity-driven grain fall can occur
On the apron of a dune where secondary cross-wind airflow is insufficient to form ripples
Describe what happens when net aeolian sediment budget is positive
Accumulation occurs where bedforms may climb over one another at varying angles
Sets and cosets of cross strata are produced
Describe the sets of cross strata formed when bedforms climb over one another
Contain smaller-scale facies units of wind-ripple, grainflow, grainfall, and plane-bedded strata
Therefore, dune elements are larger scale strata units
Why is one bedform truncated by another?
They climb at low angles, the lower bedform is accumulated as the second overrides it
Define aeolian interdunes
Elongated, relatively flat and sandy landforms that exist between larger sand dunes in desert and arid environments
Describe a dry interdune
Water table is not in contact with depositional surface
Describe a damp interdune
Capillary fringes of the water table are in contact with the depositional surface
Describe a wet interdune
Water table is in contact with the depositional surface
How do adhesion strata arise?
Arise from the adhesion of moving grains to a damp surface e.g. a damp interdune
Describe adhesion strata
Low relief
Sub-horizontal structures with irregular surfaces
Describe megadunes (draas)
Support development of superimposed dune-scale bedforms
Describe a compound draas
Where a smaller dune is superimposed on a larger dune of the same morphological type
Describe a complex draas
Where a smaller dune is superimposed on a larger dune of a different morphological type
What is an aeolian sand sheet?
Areas of sand accumulation characterised by flat areas of impact wind ripples
Limited by sediment supply
What is a zibar?
Low relief ridges and domes with gently inclined sloping surfaces
What is required for the long-term preservation of aeolian sequences?
The strata is placed below a regional baseline, below which erosion does not occur
What does subsidence allow?
It is the principal agent that generates accommodation and promotes preservation
Give two examples of how aeolian strata can achieve long-term preservation
Accumulation occurs in a subsiding basin
Accumulation occurs below the water table and is protected from deflation
