Sedimentation and Stratigraphy Geology 5142 Dr' Thieme

1
Sedimentation and StratigraphyGeology 5142Dr.
Thieme

• Lecture 19 Aeolian Deposits, Alluvial Fans,
  Erosion Surfaces

2
Desert Environments

• Arid conditions where annual rainfall is less
  than 250 mm
• Semiarid were annual precipitation ranges from
  250-500 mm
• Most modern deserts have a winter season where
  temperatures reach below 0ºC
• Sparse vegetation due to low rainfall and cold
  temps -gt wind blows unimpeded

3
Desert Sedimentary Facies

• Aeolian dune deposits of texturally and
  compositionally mature sand
• Wadi or alluvial fan deposits of poorly sorted
  conglomerate or breccia
• Playa lake sediments - evaporite minerals such as
  gypsum and anhydrite, smectite clays, some
  zeolites and other rare minerals

4
(No Transcript)
5
Aeolian Transport

• air has lower viscosity than water
• therefore higher velocities are required to
  achieve enough lift force to raise particles up
  off the bed into the free stream
• while gusts may blow at up to 55 m s-1 (200 km
  h-1) sustained wind velocities rarely exceed 30 m
  s-1
• particles larger than 0.5 mm in diameter cannot
  be entrained

6
(No Transcript)
7
(No Transcript)
8
Coarse particles, if entrained, also settle much
more rapidly out of air than water.

• Threshold below which falling velocity is
  proportional to the square of the diameter
• 0.18 mm for water
• 0.05 mm for air

9
Aeolian Transport

• saltation is much more effective in air because
  the falling particles do not encounter as much
  friction
• for a given diameter, a grain transported in air
  will
• rise higher
• settle faster
• impact grains on the bed with more force

10
(No Transcript)
11
Wind Ripples

• piles of saltating grains align perpendicular to
  the wind direction
• separation and attachment point are closer
  together than for sand of the same diameter
  transported by water
• faster falling velocity results in a "shadow
  zone" on the lee side
• coarsest grains concentrate at each crest

12
Aeolian Dune

• range from 10 cm - 100 m in height
• more than just a very large wind ripple because
  one saltation "hop" will not carry grains between
  dune crests

13

• dune and "draa" wavelengths are much larger than
  the distance that sand grains travel in saltation
  "hops"
• very large sand grains occur on some wind ripple
  crests
• only dunes or "draas" have laminated "avalanche"
  deposits on the lee side

14
(No Transcript)
15
Dunes and Bedforms

• Transverse dunes
• form under consistent, unidirectional winds
• with abundant supply of sand
• create planar cross beds as straight-crested
  ripples migrate downwind
• Barchan dunes
• form when sand supply is limited and winds are
  more variable
• form trough cross beds with steep dips

16
Dunes and Bedforms

• Linear or "seif" dunes
• form when there are two prominent wind directions
  at approximately 90
• create "herringbone" cross beds, where steeply
  dipping beds are oriented in different directions
• Star dunes
• form in areas of multiple wind directions
• cross beds have greatest variety of directions

17
Steeply dipping crossbeds on an active linear dune
18
Cross-bedded Aeolian Sands
Planar cross-bedding in Wingate sandstone at
Arches National Park
Trough cross-bedding in Navajo sandstone, Zion
National Park
19
Facies found in an "interdune" area where aeolian
sands advance over inter-mittent marsh
20
Desert pavement or "serir" deposits form as fine
material filters downward and coarser clasts are
"borne at the surface"
"STONE LINE"
21

• Depositional subunits of an alluvial fan
• feeder canyon
• fan apex
• fan-head trench
• fan lobe
• fan toe
• Fan deposits resemble other alluvium but will be
  reverse graded near the fan apex

22

• concentrated at the fan apex

• extend out and build a depositional lobe

23
Wadi gravel deposits often interfinger with
interdune muds and aeolian dune deposits
24
(No Transcript)