Aklé

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Aklé are barchanoid transversal dunes with a winding back, which emerge from the lateral merging of sickle dunes when the supply of sand increases .

description

Aklé are the most common form of transverse dunes and are characterized by great stability. The dune ridges are almost straight to wavy and curved transversely to the direction of the wind. It is therefore a three-dimensional shape, which suggests quite frequent changes in the wind direction. In reversal of the wind direction arise from Akle cross dunes ( engl. Reversing dunes ), are reversed in their comb section windward and leeward slope. The fairly constant wavelength of the Aklé forms is usually between 10 and 100 meters and their height can vary between 1 and 10 meters. The ratio of wavelength to height is around 10: 1. Within a dune field, the ratio of height to wavelength usually remains fairly constant. Aklé with very long wavelengths between 1000 and 3000 meters are called Transversal Draa .

The grain size of the available sand has a decisive influence on the wavelength that occurs, which increases significantly with coarser grains.

The windward side of the Aklé is only slightly inclined towards the wind direction and curved convexly upwards, but the leeward side drops sharply with 30 to 35 ° in the wind direction. The slightly curved and sometimes irregularly running ridge shoulder shows a gentle flattening in the low-lying saddle sections, and bulges can arise at their higher positions. The ridge bulges can expand into ridges, and less often ridges also pull away from the saddle areas. In some dune fields, these ridges have an asymmetrical outline and are usually not exactly at right angles to the dune line. The observed asymmetry can be explained by differential erosion or sediment accumulation on the sides of the ridges. Under the influence of seasonal cross winds, the ridges can also develop into independent dune shapes; in extreme cases, networked dunes are created whose crests form an L-shaped pattern in the ground plan.

Internal structure

Aklé forms have a very clear and regular oblique stratification of the planar-tabular type. The long-lasting, evenly formed slippery slope layers (English foreset ) dip at a very high angle of 30 to 34 ° in the direction of the wind. At right angles to the wind, their dip is only 1 to 6 °. By blowing out more forcefully, an asymmetrical trough layering can also appear discordantly in saddle areas across the wind direction . Very high aklé can develop a horizontal stratification on the windward side, which slopes gently (2 to 5 °) against the wind direction. The inclined stratification bodies can form separated, large internet sets due to discordances (reactivation surfaces), which can be traced back to changing wind directions. Among all types dunes Akle show the lowest spatial dispersion of their sliding slope angle of incidence and thus confirm the minimal disturbance of the flow pattern by extending downwind secondary vortex .

Emergence

The origin of the Aklé forms has not yet been fully clarified. It is generally assumed that Aklé forms emerge from the lateral merging of sickle dunes (barchans) with increasing sand supply.

Glennie noticed her performing in Inland Sabchas in 1970 . He therefore suspected that the moist surface of the Sebkha had prevented the growth of sickle dune horns and that only Aklé transversal forms could therefore arise. When the Sekkha areas between the dune ridges dried out later, the characteristic sand fields remained.

An analogy may be offered by underwater dunes whose wavelengths are directly dependent on the thickness of the boundary layer of the surrounding medium. The latter is responsible for the spatial distancing of the vortices that form in it, which set up downstream from the current separation. In the case of the Aklé, this analogy at wavelengths in the hundred meter range will indicate an atmospheric boundary layer that is also several hundred meters thick.

Occurrence

Aklé are found widely in hot desert landscapes. Good examples can be found in the Sahara , in the deserts of Arabia , South Africa , Asia and in North and South America .

But they can also be found on dry and windy coastlines, with their wavelength increasing towards the hinterland. Coastal aklé can unite with sickle dunes and other transverse shapes to form triangular, in the wind, standard structures (English banners ).

Aklé transversal structures also arise in the river environment . In the Antarctic , transverse dunes are created by katabatic winds , which consist of a granular mixture of snow and sand or just granular snow.

Since aklé can be anchored by plants, fossil examples are known in both periglacial and hot climates. The arrangement of the dunes then suggests the paleowind direction.

Aklé has also been discovered on other planets, such as Mars .

Examples

Individual evidence

  1. ^ A b W. S. Cooper: Coastal Dunes of Oregon and Washington . In: Geol. Soc. At the. Mem. Band 104 , 1958, pp. 1-169 .
  2. ↑ H.-E. Reineck, IB Singh: Depositional Sedimentary Environments . Springer-Verlag, 1980, ISBN 0-387-10189-6 .
  3. ↑ IG Wilson: Aeolian bedforms-their development and origins . In: Sedimentology . tape 19 , 1972, p. 173-210 .
  4. ^ V. Cornish: Waves of Sand and Snow . Fisher Unwin, London 1914.
  5. ↑ Th. Monod: Majabat Al-Koubra. Contributions a l'etude de T'Empty Quarter 'Quest Saharians. Mem . In: Mem. Inst. Fr. Afr. Noire . tape 52 , 1958.
  6. ↑ G. Solle: Notblatt Hess. Country. Soil research. tape 95 . Wiesbaden 1966, p. 54-121 .
  7. ↑ a b E. D. MCKE: Structures of Dunes at White Sands National Monument, New Mexico (and a comparison of dunes from other selected areas) . In: Sedimentology . tape 7 , 1966, pp. 1-69 .
  8. ^ M. Leeder: Sedimentology and Sedimentary Basins . Blackwell Science, 1999, ISBN 0-632-04976-6 .
  9. ^ KW Glennie: Desert sedimentary environments . In: Developments in Sedimentology . tape 14 . Elsevier, Amsterdam 1970, p. 222 .
  10. ↑ HTU Smith: Eolian geomorphology, wind direction, and climatic change in North Africa . In: US Air Force, Cambridge Res. Lab. Rep. 1963, p. 63-443 .
  11. ^ HJ Walker: Arct. Inst. North America, Tech. Pap. Ed .: ME Britton, Alaskan Arctic Tundra. tape 25 , 1973, pp. 49-92 .
  12. ↑ JA Cutts, RSUSmith: Eolian deposits and dunes on Mars . In: Journal of Geophysical Research . tape 78 , 1973, p. 4139-4154 .
  13. ^ PA Hesp: Coastal Dunes in the Tropics and Temperate Regions: Location, Formation, Morphology and Vegetation Processes . In: ML Martinez, NP Psuty (Ed.): Ecological Studies . tape 171 . Springer Verlag, Berlin / Heidelberg 2004.
  14. ^ RU Cooke, A. Warren: Geomorphology in Deserts . Batsford, London 1973.