Star dune

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Star dunes (also known as cross dunes or pyramid dunes ) are particularly high, complex dunes that are counted among the transport bodies of the Draa . They are caused by strongly changing, seasonally overlapping wind directions and when large amounts of sand are present . Star dunes do not move, rather the existing sand is constantly being rearranged.

etymology

The name star dune is derived from its uniform, star-shaped floor plan, cross dune from the cross-shaped side arms. The term pyramid dune refers to its pyramidal shape . The Arabic Ghourd or Rhourd means sand dune .

description

In many sand deserts ( English sand seas ), the star dune is the largest dune shape, which usually rises above the surrounding deflation levels by 150 to 250 meters and in rare cases can reach a height of 400 meters. The volume of sand is the most important of all types of dunes. Star dunes consequently occur in the deposition centers of the respective sandy deserts. Their spatial distribution is generally complex to random, but sometimes their tips protrude from longitudinally aligned ridges, for example in the Namib .

The distances between star dunes vary between 150 and 3000 meters. In rare cases up to 4000 meters can be reached, whereby individual dune fields show clear differences for this parameter. In Gran Desierto Mexico, for example, the distances are 150 to 500 meters, whereas in the Namib, at 1200 to 3000 meters, much higher values ​​are achieved. Their height distribution can vary between 20 and 300 meters (Gran Desierto 20 to 150 meters, Namib 70 to 300 meters). Their ratio of height to distance (h / L) is between 0.12 and 0.43 and thus reaches significantly higher values ​​than z. B. with sickle or soap dunes.

Star dunes have a characteristic pyramidal structure with several avalanche faces or slip faces , with three, four or more arms usually radiating from a central peak. Usually each arm has its own well-formed sliding slope that comes into action at different times. Occasionally the arms start from several peaks, which are connected to each other by high ridges. Each arm consists of a sharp, curved ridge, the sliding slope of which can change sides depending on the prevailing wind direction. The individual arms are not designed uniformly, rather star dunes show longer primary arms in very specific preferred directions. The upper section of many star dunes is relatively steep with angles of incidence of 15 to 30 degrees. Its broad, apron-like substructure is flatly inclined (with angles of 5 to 10 °) compared to the blow-out planes, in which what is in store can also emerge. The substructure can be overlaid by barchanoid or reverting secondary forms.

Emergence

The following explanations are being considered for the formation of star dunes:

  • As the center of convection cells, whose wavelengths roughly correspond to the thickness of the atmospheric boundary layer. One difficulty with this explanation, however, lies in the hypothetical location of the convection cells.
  • At nodes of standing waves in oscillating currents.
  • At junctions of complex dune alignment patterns, which are formed by intersecting or converging sand transport paths.

Comparisons in the spatial distribution of star dunes with prevailing wind patterns suggest that they form under complex and multidirectional flow patterns. This fact is also expressed in the ratio of the resulting (RDP) to the total (DP) sand flow rate (ratio RDP / DP). Star dunes with a complex flow distribution have a significantly lower RDP / DP ratio than linear or transverse dunes, averaging 0.19 (for comparison: longitudinal dunes with bimodal currents have an average value of 0.45, and unimodal transverse dunes even at 0.68). The occurrence of star dunes has also been linked to topographical obstacles. Topographical conditions have a very strong influence on wind patterns. In Erg Fachi Bilma (Sahara) and in the Namib, for example, they significantly change the directional dependence of the currents. At the Kelso Dunes and the Great Sand Dunes , topographical obstacles act as sand traps.

In the development model by Lancaster (1989), star dunes emerge from transverse dunes that immigrate to an area with changing wind regimes. Due to the wind reversal, the original transverse dunes reverse. After another wind reversal in the initial direction, a first arm forms on the leeward side due to cross currents. When the wind reverses again and then turns slightly in a second wind direction, a second arm grows on the opposite side. Another strong turn in a third wind direction finally accentuates the arms into slightly curved shapes, the sliding slopes of which now point in the opposite direction.

Internal structure

Star dunes have a very complicated internal structure, which is partly due to their prehistory and to their superimposition by secondary forms. The steep foreset layers of their sloping bodies, which dip at 31 to 33 °, can have three or more incidence maxima due to the changing wind directions.

Occurrence

Tsauchab , Namibia
Erg Issaouane , Algeria; Star dune right (marked)
Rub al-Kali , Saudi Arabia; Star dunes in the right part of the picture

Star dunes and associated reversing dunes are very common in the Sahara and represent 40% of all dune shapes in the Grand Erg Oriental , for example . In the Namib, in the Gran Desierto of Mexico and in the sandy deserts of Central Asia , they represent 9 to 12%. Strangely enough, star dunes are absent in the deserts of Australia , the Kalahari, and the deserts of India and Pakistan ( Thar ).

Occurrence in detail:

Examples of star dunes from the geological past are also known, for example from the Permo-Triassic Hopeman Sandstone on the Moray Firth in Scotland .

Web links

Commons : Star Dunes  - Collection of images, videos, and audio files

Individual evidence

  1. ↑ A. Clos-Arceduc: Essai d'explication des formes dunaires Sahariennes . In: Étud. Photo interpretation . No. 4 . Inst. Geogr. Nat., Paris 1969.
  2. ^ MR Leeder: Sedimentology, Process and Product . G. Allen & Unwin, London / Boston / Sydney 1982, p. 344 .
  3. ^ RJ Wasson, R. Hyde: Factors determining desert dune type . In: Nature . tape 304 , 1983, pp. 337-339 .
  4. ^ N. Lancaster: Controls of dune morphology in the Namib sand sea . In: TS Ahlbrandt, ME Brookfield (Ed.): Aeolian sediments and processes . Elsevier, Amsterdam 1983, pp. 261-289 .
  5. ^ N. Lancaster: Grain size characteristics of Namib Desert linear dunes . In: Sedimentology . tape 28 , 1981, pp. 115-122 .
  6. ^ RJ Wasson, R. Hyde: A test of granulometric control of desert dune geometry . In: Earth Surface Processes and Landforms . tape 8 , 1983, p. 301-312 .
  7. ↑ H.-E. Reineck, IB Singh: Depositional Sedimentary Environments . Springer-Verlag, Berlin / Heidelberg / New York 1980, ISBN 0-387-10189-6 .
  8. ^ N. Lancaster: Dune Morphology and Dynamics . Ed .: AD Abrahams, AJ Parsons. Chapman & Hall, London 1994, ISBN 0-412-44480-1 .
  9. ↑ IG Wilson: Aeolian bedforms - their development and origins . In: Sedimentology . tape 19 , 1972, p. 173-210 .
  10. ^ A b N. Lancaster: The dynamics of star dunes: an example from the Gran Desierto, Mexico . In: Sedimentology . tape 36 , 1989, pp. 273-289 .
  11. ↑ a b S. G. Fryberger: Dune forms and wind regimes . In: ED McKee (Ed.): A study of global sand seas (=  United States Geological Survey Professional Paper . Volume 1052 ). 1979.
  12. ↑ CS Breed, T. Grow: Morphology and distribution of dunes in sand seas observed by remote sensing . In: ED McKee (Ed.): A study of global sand seas (=  United States Geological Survey Professional Paper . No. 1052 ). 1979.
  13. ↑ M. Mainguet, YI Callot: L'erg de Fachi-Bilma (Tchad-Niger) . In: Mémoires et Documents CNRS . tape 18 , 1978, p. 178 .
  14. ↑ E.McKee: Sedimentary structures in dunes of the Namib desert, South West Africa . In: Geological Society of America Special Paper . tape 188 , 1982.
  15. ^ RP Sharp: Kelso Dunes, Mohave Desert, California . In: Bulletin of the Geological Society of America . tape 77 , 1966, pp. 1045-1074 .
  16. ^ S. Andrews: Sedimentology of Great Sand Dunes, Colorado . In: Society of Economic Paleontologists and Mineralogists Special Publication . tape 31 , 1981, pp. 279-291 .
  17. ^ ED McKee: Structures of dunes at White Sands National Monument, New Mexico (and a comparison with structures of dunes from other selected areas) . In: Sedimentology . tape 7 , 1966, pp. 1-69 .
  18. ^ MP Petrov: Deserts of the World . Wiley, New York NY 1976.
  19. ↑ RU Cooke, A.Warren: Geomorphology in Deserts . Batsford, London 1973.
  20. ^ KW Glennie: Desert Sedimentary Environments . Elsevier, Amsterdam 1970, p. 222 .
  21. ↑ LB Clemmenson: Complex star dunes and associated Aeolian bedforms, Hopeman Sandstone (Permo-Triassic), Moray Firth Basin, Scotland . In: L. Frostick, I. Reid (Ed.): Desert Sediments: Ancient and Modern (=  Geological Society of London, Special Publication . No. 35 ). 1987, p. 213-231 .