The Monitoring Well, Reflections on Geology & Society

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August 2, 2021

Notes, News and Comments

Here are some news items I thought were interesting:

 August 2, 2021

Depositional Environments for Sedimentary Rocks, Part 4, Sand Dunes

Figure 1 - Fishing at Athabasca Sand Dunes Provincial Park
Credit: Tourism Saskatchewan, J.F.Bergeron, Enviro Foto

I am going to continue the terrestrial depositional environments with a discussion of aeolian depositional environments, Table 1.

Credit: Steven Earle, Creative Commons Attribution-NonCommercial-Share Alike 4.0 International License

Figure 2 - Mesquite Flats Sand Dunes in Death Valley
Credit:Tuxyso, Creative CommonsAttribution-Share Alike 3.0 Unported license

When we think of sand dunes, we often think of desert scenes such as in the Death Valley scene in Figure 2 above, or in movies such as March or Die (one of my favourites).  However, windblown sand deposits are found in many terrestrial environments, often in places where sand was deposited as the result of glacial process, as in the outwash sands reworked by the wind at Lake Athabasca, see in Figure 1, or at the so-called Carberry Desert in Manitoba where sand deposits from Glacial Lake Agassiz were re-worked into sand dunes.

Wind Depositional Processes

Wind will blow sand along the surface of the soil, bouncing as the grains go along, a process called saltation.  The wind drives the grains upslope until they reach the top of the dune, where they fall down the slip face of dune.  The angle of the slip face will be approximately the angle of repose for dry sand, usually around 30 - 40 degrees.

The bouncing and abrasion result in well rounded grains of relatively uniform size; the grains are often frosted in appearance.  Figure 3 shows the processes graphically

Figure 3 - Wind Blown Sand Processes 
Credit: Po ke jung, Geography at College of Marin, Section 4, Aeolian Processes

 Types of Sand Dune

The basic types of dunes are: crescent, linear, transverse, star, dome, and parabolic.  The types of dunes that form will depend upon the amount of sand available, the strength and direction of the winds, and the presence of vegetation, as in Figure 4.

Figure 4 - Dune Forms
Credit: Harish Kumar

Crescent Shaped Dunes 

Figure 5 - Crescent or Barchan Dune
Credit: Po ke jung, Creative Commons Attribution-Share Alike 3.0 Unported license

 Crescent dunes form under winds that blow from one direction.  They also are known as barchans, or transverse dunes. These dues can migrate fast: in China's Ningxia Province a group of dunes moved more than 100 meters per year between 1954 and 1959; similar rates have been recorded in the Western Desert of Egypt.  China's Taklimakan Desert has the largest crescentic dunes on Earth, with mean crest-to-crest widths of more than 3 kilometres.

Linear and Transverse Dunes

Figure 6 - Linear Dunes, Namibian Desert
Credit: NASA International Space Station Program, public domain 

Linear and transverse dunes are straight or slightly sinuous sand ridges that are typically much longer than they are wide; some may be more than 160 kilometres long.  Linear dunes generally form sets of parallel ridges separated by miles of sand, gravel, or rocky corridors between the dunes. They may occur as isolated ridges. In some  cases, linear dunes merge to form Y-shaped compound dunes.  Many linear dunes form in where the wind comes from two different directions over the year with the long axes of these dunes extending along the resultant direction of sand movement.

Star Dunes 

Figure 7 - Star Dune, Badain Jaran Desert
Credit: Śľ†ť™ź, Creative Commons¬†Attribution-Share Alike 3.0 Unported license

 Star Dunes are pyramidal shaped sand mounds made up of three or more arms that radiate from the high center of the mound. They a generally found in areas where the wind blows in many different directions throughout the year. With the winds pushing from many sides, Star dunes grow upward rather than laterally. They are the most common type of dune in the Grand Erg Oriental of the Sahara. Elsewhere, they occur around the margins of the so-called sand seas, areas of extensive sand, particularly near topographic barriers.  The star dunes are up to 500 meters tall in the southeast Badain Jaran Desert of China and may be the tallest dunes on Earth.

Dome Shaped Dunes 

Figure 8 - Dome Dune
Credit: Harish Kumar

Dome Shaped Dunes are rare and similar to star dunes; they are low circular-shaped dunes that lack a slip face.  The dome shape is due to unstable wind patterns.

Parabolic Dunes

Figure 9 - Parabolic Dunes
Credit: Po ke jung, Creative Commons Attribution-Share Alike 3.0 Unported license

Parabolic Dunes are U-shaped mounds of sand with convex noses trailed by elongated arms.  Also called U-shaped, blowout, or hairpin dunes, they are common in coastal deserts.  With parabolic dunes, the crests point upwind. The elongated arms of parabolic dunes are fixed by vegetation, thus they follow rather than lead as the due migrates.  The longest known parabolic dune has a trailing arm 12 kilometres long.

Standard Caveat

The purpose of my weblog postings is to spark people's curiosity in geology.  Don't entirely believe me until you've done your own research and checked the evidence.  If I have sparked your curiosity in the subject of this posting, follow up with some of the links provided here.  If you want to, go out into the field and examine some rocks on your own with the help of a good field guide.  Follow the evidence and make up your own mind.

In science, the only authority is the evidence.