Aeolian environments Lecture 9

Introduction Depositional environments. 1. Where? a. Wherever there is available sand and silt b. Availability requires minimal vegetative cover c. Hence, beaches, glacial outwash plains, deserts, dry lakes, loess plains

2. Our focus today: deserts a. 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 70’s)

The Desert system 1. Locations and names

2. Relationship to mid-latitude highs, cool oceanic currents, and rain shadows

3. Variations as a function of time Modern aeolian regions Glacial-maximum (18ka) aeolian regions Glacial-maximum loess and aerosol dust tracks

Why more extensive in time of glacial maximum? 4. Principal factors controlling deposition: source, wind energy and aridity were different

Sand movement in the Sahara desert: an example of the influence of air movement

Ripples and Dunes Some names of landforms and features Ergs (sand seas) Yardangs (erosional ridges) Hoodoos (erosional pillars) Desert pavements (deflation lag) Ventifacts

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

Airflow over dunes

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.

Grain fall and grain flow processes Grain flow produces inverse sorting and shear sorting. Beds thicken downslope. Grain fall produces indistinct lamination. Beds thin downslope but may continue to base of dune if dune is small enough.

Grain fall and grain flow processes Ripples can occur as ornamentation on lee face. Slumps occur only on wet dunes, because cohesion is needed.

An example of seasonal effects on stratigraphy (wet vs. dry systems) Wintertime accumulations Summertime accumulations

Classification of dunes

Accumulation space and preservation space

The generation of cross-sets and sequences For accumulation to occur, flux in must exceed flux out, q i > q o

The generation of cross-sets  y = rate of accumulation  x = rate of migration  y/  x = tan 

The generation of cross-sets The nature of the stratification is critically dependent on whether  is greater than or less than the slope of the stoss face.

Generation of a sequence of beds

Accumulation space and preservation space Surfaces and super- surfaces

Wet aeolian system. Accumulation with a rising water table with time.