Parabolic dune

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Parabolic dunes are bound, simple dune forms that occur mainly on coasts when the wind direction is constant . They are partly stabilized by wetlands and vegetation.

Parabolic dune on the French North Sea in the Dune du Perroquet area

etymology

The terms parabolic dune , parabolic dune or arched dune are derived from the convex, parabolic floor plan of these dunes, which is curved in the direction of the wind .

description

Parabolic Dune in Hoffmaster State Park in Michigan

Parabolic dunes, in English as parabolic dunes , blow-out dunes or upsiloidal dunes designated, are sickle dunes similar. However, they are narrower and have a U-shaped curvature opposite to the sickle dunes , ie their convex sides point leeward , but their legs (or arms ) point in the direction of the wind. They are partially overgrown with vegetation. The middle part of the dunes, almost free of vegetation, moves faster than the legs, which are up to 2 kilometers long in extreme cases and cannot follow the main ridge because they are "anchored" by their dense vegetation (heather, crowberry, etc.). The more or less vegetation-free middle section (or nose ) moves forward through avalanche-like sand slides over its leeward slope.

The size of parabolic dunes varies from tens to hundreds of meters, and their length can be many times (usually> 3) their width. Due to the sand blown out ( deflation ) between the legs, an erosion- related, occasionally water-filled depression is created , which is surrounded in a semicircle downstream by a rounded to sharp dune ridge, from which the leeward slide slopes emanate. The dune ridge usually reaches heights of 10 meters above the deflation basin, but in extreme cases it can grow up to 70 meters.

Parabolic dunes usually occur more frequently and in an irregular spatial distribution. But they can also be arranged in nests or offset from one another.

Typology

Pye divides parabolic dunes into simple, complex, and compound shapes. With the simple forms he differentiates with increasing curvature crescent-shaped (lunate), semicircular (hemicyclic), bulbous (lobate) forms (with a length / width ratio of <3) and elongated (length / width ratio> 3) ).

Complex shapes can be overlaid by individual sickle dunes, barchanoid ridges or aklé . Sometimes they also carry smaller parabolic dunes with deflation troughs, as well as remnants of elongated and fingered parabolic dunes. The simplest composite forms arise from the mutual superposition of two parabolic dunes. Compound parabolic dunes can also, as for example in the Thar desert , grow together laterally to form rake-like structures, with the arms being divided by two adjacent dunes. This case always occurs when individual parabolic dunes in a dune field have different rates of movement. Larger parabolic dunes sometimes also house smaller parabolic dunes nest-like in their interior. Lateral adhesions to form fingered structures in the form of a hand are also known. Downwind, parabolic dunes can change into sickle dunes.

When the sand supply is running low, it happens that the central part of the parabolic dune is blown through and a tongue of sand is deposited. The only remaining relief forms are the elongated side arms, which on the surface look like longitudinal dunes .

Internal structure

The main characteristic of the internal structure of parabolic dunes are their slightly convex downwardly curved foreset layers, which, due to the partial vegetation, adopt this geometric arrangement. Due to the semi-circular sliding slope, the stroke directions of the foreset angles of incidence can vary by up to 200 °.

The internal sloping bodies , separated from one another by discordances ( bounding surfaces ), can have a rather complicated structure. Ahlbrandt (1975) found convex upward sloping stratifications in the Wyoming dunes .

On the windward side , the angle of incidence of the layers is usually less than 10 °. The positions on the leeward side are much flatter compared to other dunes and generally fall downwind at around 20 °, steeper angles are rare.

The plant growth follows the individual layers that are covered with organic material. However, rhizomes penetrate the layers perpendicularly.

Emergence

Parabolic dunes arise in regions with significant precipitation ( semi-arid to humid ), which enables the development of a loose vegetation cover - often behind smaller lakes or wetlands. Cooper (1958) names the following conditions for their creation:

  • A generally stable base that enables the wind to attack various weak points in the sand cover.
  • Sufficient sand thickness so that the forward migration of the dune can be restricted to relatively narrow frontal areas.
  • A unidirectional wind direction.

After a sufficiently large deflation basin ( blow-out ) has formed, a dune ridge ( blow-up ) with a semicircular sliding slope can establish itself, which gradually moves downwind. The legs, which remain passive, are thereby elongated.

Occurrence

Parabolic dunes are largely tied to dune fields in the coastal area. They occur predominantly in temperate climates, but can also be observed in warmer and occasionally in periglacial climates. They are rare in the hot, sandy deserts of the interior, with the exception of the Thar deserts of India and Pakistan . Smaller occurrences can also be found in the southwest of the Kalahari , in Saudi Arabia , in northeast Arizona and in the White Sands National Monument of New Mexico .

Occurrence in detail:

Individual evidence

  1. ^ A b J. RL Allen: Sedimentary Structures - their Character and Physical Basis . Elsevier Science Publishers, Amsterdam 1984, ISBN 0-444-42232-3 .
  2. ^ K. Pye: Late Quaternary development of coastal parabolic megadune complexes in northeastern Australia . In: K. Pye, N. Lancaster (Eds.): Aeolian sediments: ancient and modern (=  Special Publication ). No. 16 . Blackwell Scientific Publications, Oxford 1993, pp. 23-44 .
  3. ^ JE Ash, RJ Wasson: Vegetation and sand mobility in the Australian desert dunefield . In: Journal of Geomorphology . tape 45 , 1983, pp. 7-25 .
  4. ^ K. Pye: Morphological development of coastal dunes in a humid tropical environment, Cape Bedford and Cape Flattery, North Queensland . In: Geografiska Annaler . Volume A64, 1982, pp. 213-227 .
  5. ^ A b D. Anton, P. Vincent: Parabolic dunes of the Jafurah Desert, Eastern Province, Saudi Arabia . In: Journal of Arid Environments . tape 11 , 1986, pp. 187-198 .
  6. ↑ M. Leeder: Sedimentology and Sedimentary basins - From Turbulence to Tectonics . Blackwell Science, 1999, ISBN 0-632-04976-6 .
  7. ↑ JJ Bigarella: Dune sediments: characteristics, recognition and importance . In: Bull. Am. Assoc. Petroleum Geologists . tape 53 , 1969, p. 707 (abstract) .
  8. ^ A b E. D. McKee: Structures of dunes at White Sands National Monument and a comparison with structures of dunes from other selected areas . In: Sedimentology . tape 7 , 1966, pp. 1-69 .
  9. ↑ TS Ahlbrandt: Comparison of textures and structures to distinguish Aeolian environments, Killpecker dune field, Wyoming . In: The Mountain Geologist . tape 12 , 1975, p. 61-73 .
  10. ↑ H.-E. Reineck, IB Singh: Depositional Sedimentary Environments . Springer-Verlag, Berlin / Heidelberg / New York 1980, ISBN 3-540-10189-6 .
  11. ↑ WS Cooper: Mem. Geol. Soc. At the. No. 72 , 1958.
  12. ^ N. Lancaster: Dune Morphology and Dynamics . In: A. Abrahams, AJ Parsons (eds.): Geomorphology of Desert Environments . Chapmann & Hall, London 1994, ISBN 0-412-44480-1 , pp. 474-505 .
  13. ↑ K. Gripp: On the recent geological history of Hörnum / Sylt and Amrum with an overview of the formation of the dunes in North Friesland . In: The coast . tape 16 , 1968, pp. 76-117 .
  14. ↑ NP Psuty: The geomorphology of beach ridges in Tabasco, Mexico . In: Technical Report . No. 30 . Louisiana State University, Coastal Studies Institute, 1966.
  15. ^ RJ Wasson et al .: Geomorphology, late Quaternary stratigraphy and paleoclimatology of the Thar dunefield . In: Journal of Geomorphology . tape 45 , 1983, pp. 117-151 .
  16. ↑ JT Hack: Dunes of the western Navajo Country . In: Geographical Review . tape 31 , 1941, pp. 240-263 .