Aeolian environments

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Aeolian environments
Lecture 9
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Introduction

• Depositional environments.
• Where?

• Wherever there is available sand and silt

• b. Availability requires minimal vegetative
  cover

• c. Hence, beaches, glacial outwash plains,
  deserts, dry lakes, loess plains

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• 2. Our focus today deserts

• The predominant environment of aeolian processes

• b. Importance (paleoclimatic studies,
  desertification and soil erosion)

• c. Perspective Environment going to Hell may
  just be climatic change (for example, the Sahel
  in the early 70s)

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The Desert system
1. Locations and names
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2. Relationship to mid-latitude highs, cool
oceanic currents, and rain shadows
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3. Variations as a function of time Modern
aeolian regions Glacial-maximum (18ka) aeolian
regions Glacial-maximum loess and aerosol dust
tracks
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Why more extensive in time of glacial maximum?
4. Principal factors controlling deposition
source, wind energy and aridity were different
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Sand movement in the Sahara desert an example
of the influence of air movement
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Some names of landforms and features

• Ripples and Dunes

• Ergs (sand seas)

• Yardangs (erosional ridges)

• Hoodoos (erosional pillars)

• Desert pavements (deflation lag)

• Ventifacts

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• Bedforms

• Ballistic ripples and ridges
• (H 0.1 m ? 0.02 to 2.0 m) Caused by
  separation of flow
• Lee and stoss (leeward and windward)

• Dunes
• Lee-face processes and stratification

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Airflow over dunes
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Grain fall and grain flow processes
Grain fall occurs as saltating grains on the
stoss slope fly over the brink and come to rest
on the lee face.
Grain flow occurs as the upper lee face steepens
from grainfall accumulation and then fails to
static angle of repose.
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Grain fall and grain flow processes
Grain fall produces indistinct lamination. Beds
thin downslope but may continue to base of dune
if dune is small enough.
Grain flow produces inverse sorting and shear
sorting. Beds thicken downslope.
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Grain fall and grain flow processes
Ripples can occur as ornamentation on lee face.
Slumps occur only on wet dunes, because cohesion
is needed.
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An example of seasonal effects on stratigraphy
(wet vs. dry systems)
Wintertime accumulations
Summertime accumulations
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Classification of dunes
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Accumulation space and preservation space
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The generation of cross-sets and sequences
For accumulation to occur, flux in must exceed
flux out, qi gt qo
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The generation of cross-sets
Dy rate of accumulation Dx rate of
migration Dy/Dx tan q
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The generation of cross-sets
The nature of the stratification is critically
dependent on whether q?is greater than or less
than the slope of the stoss face.
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Generation of a sequence of beds
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Accumulation space and preservation space
Surfaces and super-surfaces
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Wet aeolian system. Accumulation with a rising
water table with time.
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