Deserts Flashcards
rain or groundwater fed are…
interdunes: have evaporites, microbialite crusts preserved. thin mudstone, deflation pans (saline muds)
how are interdunes made?
theres a depression because of the deflation. the sand will be removed from depression because of wind and will be deposited as dunes. water sweeps out of the aquifer and forms swamps/oasis. salt weathering will occur after deflation which will create accommodation space and provides sediment
desert pavements
- regs, serirs, gibber
- may become basal conglomerates
ventifacts
-pepples have 2 or 3 faces. paleocurrent indicator
yardings
ridges of bedrock or indurated (lake) sediment made by sand abrasion. teardrop shaped with steep uwind slope. paleo wind indicator
textures of eolian particles
well sorted: fine coastal dunes
mod sorted: fine to med inland dunes
poor sort: interdune and reg (stony desert) sands
what are some eolian surface structures?
- adhesion warts: wart/blister. made by wind blowing dry sand over damp surface
- adhesion ripples: subparallel ridges formed -perpendicular to wind blowing dry sand over a moist surface
- invertebrate trail
climbing ripple lamination
cant recognize easily bc of low relief, have inverse grading
grainfall lamination
- deposited from suspension on lee side of dunes
- poor grain size segregation and weak lamination
- grading from falling wind velocity
- they thin downwards
- variable in thicknesses
grainflow lamination
- avalanche cx beds
- slumping and grainflow down slipface of dunes
- interfingers with grainfall laminae
- may see deformed laminae
eolian bedforms
- ripples
- dunes
- draas
eolian ripples
- grains move by saltation (carpet of sand moving)
- surface irregularities and turbulence create patches where grains are slightly elevated
- when theres constant velocity: medium grains move same distance during saltation, which leads to equidistant piles of grains aligned 90 degrees to wind flow
- finer grains will be sheltered in ripple troughs
- coarser grains will be on crests because of winnowing
- inverse grading caused by accommodation on ripples
- oversteepening and avalanching creates cx lamination
eolian dunes
- at least 3 m-600m in wavelength
- saltation on stoss side (rippled there),
- grainflow/fall on lee side, which produce cx beds
seif dunes
- longitudinal: two wind directions at 90 degrees
- avalanche cx beds are orientated in diff direction
draas
-made of dunes on stoss and lee side which have star, linear, and transverse forms
transverse bedform model
- dune types: barchan, transverse, interdune, eolian lamination, 1st, 2nd order surfaces.
- at least 25 m
- sediments may interfinger with fluvial and lake deposits
- SEQUENCE: left to right above
- draa during max aridity
- alluvial fans supply the sand
- wind direction is unidirectional
longitudinal bedform model
- at lease 100m
- draa, interdraa, draa sequence
- has similar development to transverse but early sand patches and barchans are followed by linear dunes/draa. or star draa when its complex
differences between arid and hyper arid conditions for eolian systems
HYPER ARID
- alluvial fans are abandoned and cut off. the fans will cause deflation which will be the sand supply for the system
- abandoned braided system
- badland topography
- basin bounded fault
- evaporitic playa lake
- climbing dunes with dry interdune depressions
ARID
- alluvial fans will be reactivated because there is precipitation
- will have ephermal streams between interdunes
- not a lot of vegetation
- the ephermal system will replenish playa lake
- enhanced badland development
- playa lake expansion, permanently there
- salt precipitates will dissolve again
- wedge bodies of alluvial fan strata
semi arid climate regimes
- alluvial fans will overlap
- periodic lobe switching
- deltas form
- damp water table is controlled by sand sheet
- alluvial fans pass into subaqueous deltas at lake margins
- partial stability bc of vegetation
- dunes do not climb
- not a lot of sediment available
