Eolian Desert Systems
Eolian (wind-driven)
• Distribution
– Desert belts
• 10-30° N and S of equator (Hadley circulation cells)
– trades; downwelling cool, dry air
– more land, more absoprtion (less if reflective) and shift in
circulation
– Rainshadows
• downwind side of mountain
– in US, winds blow W to E
– lose moisture over mountains
– Far inland
• center of continent away from moisture source
Deserts
• 20-25% of E’s surface
• low rainfall (<25 cm/ yr on avg.)
– NOT ALL HOT! (freeze at night!)
– NOT ALL DRY (ephemeral)
• usually sparsely vegetated
– any rain usually generates flash floods
• Drains to center of basin to form pans and lakes
Transport
• Types dependent on wind velocity
– Traction
• coarse grains
– Saltation
• coarse grains
• grain collisions aid transport through dispersion
– Suspension
• fines (<0.05 mm) are winnowed out and put into suspension
• at very high winds, coarser grains are blown
Deposits
• Dust deposits (silt)
– loess
– travels great distances
• Glacial eolian
– Indiana; Ohio, China, Mississippi
– Dust storms
• Sand deposits
• Lag deposits
Deposits
• Sand deposits
– well sorted since fines were winnowed away
• Lag deposits
– leftovers
• gravel sized
– deflation pavement
Subenvironments
– alluvial fans
– ephemeral lakes (playa)
• water may collect in center of basin
– dune fields/ sand seas
• sand- poor areas exist between dunes
Dunes
• Ripples, dunes
– Cross-beds
• Draas
– HUGE!
–  up to 5.5 km;
– h >0.4 km
• Controls
– as wind V increases,  lengthens (smear out)
– as grain size increases, height goes up (piles up)
Ripples
• Ripples
– amplitude = .01 – 100 cm, up to 20 cm apart
– size fnc. (wind V, particle size, ripple type)
** most formed by bombardment of saltating
grains and associated creep
– impact ripples
Dune Morphology
• Well-sorted, Q-rich
• Heavy minerals
– smaller grain size
• Shell fragments possible
• Gross morphology similar to water ripples
– Generally only preserve lower portion of foreset
– Contains smaller scale bedforms (ripples)
– Hard to extract wind directions since winds often
change direction
Dunes
• Dune height and spacing are regular
• Described by power function
Dh=cDsn
• Dh
– dune H
• Ds
– spacing
• N
– function of sand supply, wind speed
• C
– constant
Dune types
• Classifed by patterns formed by slip faces
(lee)
– Different patterns due to different # wind
directions
No slip faces
• Eolian sand sheet
– Ergs
• smaller
– Sand seas (Namibia)
• larger
• flat undulating sand bodies
– low < x strata (0-20°)
– Ephemeral stream deposits possible
Form by unidirectional winds
• barchan/barchanoid ridge/transverse continuum
•
•
•
•
function of sed supply
horns point downwind
ridge – commected crescents
(
• parabolic/ blow-out dune
•
•
•
•
function of vegetation
horns point upwind
lateral edges anchored by vegetation so, middle migrates
)
– Differentiated by regional patterns
2 wind directions
– Linear~ symmetrical ridge
• uniform sand accumulation but wider and steeper upwind,
tapers downwind
– Navajo (NW Arizona)
– linear dunes with 100 m sand free between
– often vegetated
• elongate, sharp-edged ridge of opposing crescents
– depositional and erosional
– Sinai Desert up to 300 km!
• Synonyms
– Sief
– Longitudinal
– Reversing ~ assymetrical ridges
• slip faces form @ time 1; altered @ time 2
•
This is a C-band, VV polarization radar image of the
Namib desert in southern Namibia, near the coast of
South West Africa. The image is centered at about 25
degrees South latitude, 15.5 degrees East longitude.
This image was one of the first acquired by the
Spaceborne Imaging Radar-C/X- Band Synthetic
Aperture Radar (SIR-C/X-SAR) when it was taken on
orbit 4 from the shuttle Endeavour on April 9, 1994.
The area shown is approximately 78 kilometers by 20
kilometers. The dominant features in the
image are complex sand dune patterns
formed by the prevailing winds in this
part of the Namib desert.
•
The Namib desert is an extremely dry area formed
largely because of the influence of the cold Benguela
ocean current that flows northward along the coast of
Namibia. The bright areas at the bottom of the image
are exposed outcrops of Precambrian rocks. This
extremely barren area is a region rich in diamonds that
through the centuries have washed down from the
mountains.
3 wind directions
– Star
• HUGE! > 400 m high
• arms radiate from center
• need high speed, variable winds AND lots of
available sand
Interdunes
• Deflation
– low sand accumulation
– Lags
• Deposition
– Dry or Wet
Interdune Areas
• Dry
– Ripples formed from sands avalanching off dunes
– Poor sorting
• gently dipping layers with poor structure
– Bioturbated
• WET
• water accumulates and traps silts/ clays
• look for evidence of terrestrial life
– Tracks
– Diatoms
– gastropods, etc.
– Evaporites
• when lake dries up, can get evaporates
– Gypsum
– mudcracks, other evidence of dessication