1. Eolian Desert Systems

2. Eolian (wind-driven) Distribution Desert belts Rainshadows Far inland
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

6. 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

7. Transport Types dependent on wind velocity Traction Saltation
coarse grains
Saltation
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

8. Deposits Dust deposits (silt) Sand deposits Lag deposits loess
travels great distances
Glacial eolian
Indiana; Ohio, China, Mississippi
Dust storms
Sand deposits
Lag deposits

10. Deposits
Sand deposits
well sorted since fines were winnowed away

11. Lag deposits
leftovers
gravel sized
deflation pavement

12. Subenvironments alluvial fans ephemeral lakes (playa)
water may collect in center of basin
dune fields/ sand seas
sand- poor areas exist between dunes

13. Dunes Ripples, dunes Draas Controls Cross-beds 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)

14. 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

16. 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

17. 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

18. Dune types Classifed by patterns formed by slip faces (lee)
Different patterns due to different # wind directions

21. No slip faces Eolian sand sheet Ergs Sand seas (Namibia)
smaller
Sand seas (Namibia)
larger
flat undulating sand bodies
low < x strata (0-20°)
Ephemeral stream deposits possible

22. 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

25. 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 time 1; time 2

28. 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, 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.

29. 3 wind directions Star HUGE! > 400 m high arms radiate from center
need high speed, variable winds AND lots of available sand

33. Interdunes
Deflation
low sand accumulation
Lags
Deposition
Dry or Wet

35. Interdune Areas Dry WET Evaporites
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