Terrestrial sedimentary environments IV - Deserts and Eolian

But first, Outcrop du jour:

Eolian environments

Eolian environments are those where the wind is the primary mode of sediment transport. This fact limits their geographic distribution to regions where water driven transport is rare because:

Water is 700 times more viscous than air. Thus, Stokes' Law indicates that particles should sink much more rapidly in it. This is, indeed, the case. The wind can usually transport nothing coarser than sand as bed load. Any wind capable of moving sand, however, is able to loft finer particles long distances. Thus: eolian environments tend to be sand dominated.

Deserts: Continental environments characterized by low precipitation. Generally:

Because stream transport of sediments is sporadic, most eolian depositional environments occur in deserts, even though the bulk of desert sediment is still transported by streams - i.e. Not eolian!. Often, deserts are seasonally hot, but not always. Polar deserts (E.G. Dry Valleys of Antarctica, right) are deserts but glaciers play the role of streams.

Sediment transport:

Eolian flow dynamics:

Bed load transport:

Kansas dust storm from Symon sez
Suspended load transport:

Suspended load: Silt & clay sized particles resist being taken into suspension, but once they are up, they tend to remain suspended for long periods, often removing them from the environment of origin. Air, like water, experiences friction with the ground. Therefore the layer of air in the few centimeters next to the ground is moving slowly. Saltating sand grains smashing into clay and silt particles kick them up into regions of faster moving air. From here on, they can be kept in suspension indefinitely.

  • Dust storms: Strong winds can mobilize large amounts of clay and silt: Patterns of eolian erosion.

    Frosted grains from Through the Sandglass
    Abrasion (sandblasting): impact of saltating grains causes objects in eolian envoronments to have a frosted patina, like the frosting of glass infancy restaurant windows. Indeed, grains of eolian sand deposits are called frosted grains. The mechanical weathering that this imposes on the grains tends quickly to winnow out non-quartz grains. Thus, most eolian sands are pure quartzarenites.

    Ventifacts: The products of the abrasion of larger objects by sand. These include pebbles, cobbles, boulders faceted by the wind.
    Yardangs [Turkic - "steep slope"]: Hunks of bedrock sticking up above the average surface level tend to be eroded into characteristic shapes called yardangs. A yardang is generally sloping on its stoss side and steep on its lee side. Often, the dead giveaway is that yardangs occur in groups that are oriented toward the direction of the prevailing wind. (Earth, Mars.)
    Surface textures caused by eolian weathering and erosion:

    Desert varnish: Sediments are often oxidized or coated with iron oxides, largely because the products of weathering reactions aren't removed from the environment of weathering by water. One interesting consequence is desert varnish, the dark shiny patina of clay minerals and manganese and iron oxides that forms as a result of a combination of action of windblown dust, chemical weathering, and water in dew. Desert varnish forms very slowly. A thin coat may be 2000 years old.

    Deflation: The removal of smaller clasts from unconsolidated regolith. Assume we have framework clasts in a silty-muddy matrix.

    Desert pavement: The common name for deflation surfaces in eolian settings. Link to a more unusual form of deflation surface.

    Eolian sedimentation:

    Eolian dunes are like subaqueous dunes, only different. Any topographic irregularity can create a wind shadow in which sand will be deposited, adding to the irregularity and provoking more deposition.

    Dune morphology: A typical eolian dune forms from the deposition of saltating sand grains. Typically a dune is an ephemeral structure that migrates, divides or fuses with other dunes. It displays:

    Conditions for dune formation: Most deserts are rocky, so dunes are actually rare. 10% of Sahara, tiny % of SW US. Still, in some places large amounts of sand accumulate in sand seas called ergs. Examples occur in Sahara and Empty quarter of the Arabian Penninsula. What governs the location, size, and shape of dune formation?

    Dunes tend to form in topographic pockets where the sand is confined. The White Sands dunefield (right) in New Mexico, for instance, is trapped against a range of high mountains by prevailing winds.

    Dune morphology: Depend mainly on 3 factors

    Depending on how these factors covary we can get four basic dune types.

    Associated environments:

    Eolian dune deposits tend to interfinger with other common desert depositional environments:

    Distinguishing form other supermature sands:

    Eolian sandstones resemble beach deposits in being:

    Where they differ: