Progress in Physical Geography

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Star dunes N. Lancaster Progress in Physical Geography 1989 13: 67 DOI: 10.1177/030913338901300105 The online version of this article can be found at: http://ppg.sagepub.com/content/13/1/67

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Star dunes by

N. Lancaster

Star dunes, characterized by their large size, pyramidal morphology and radiating sinuous arms are a distinctive dune type. They have been described from many sand seas, under a variety of names. Star dunes are known as demkhas, ghourds, rhourds or oghrouds in Saharan sand seas (Aufrere, 1935; Capot-Rey, 1945; 1953; Alimen et al., 1958; Mainguet and Callot, 1978; Wilson, 1972). Other terms which have been applied to star-like dunes include sand mountains (Cooke and Warren, 1973), sand massifs (Bagnold, 1951), pyramidal dunes (Holm, 1960) and stellate dunes (Glennie, 1970). Star dunes are the largest aeolian bedforms in many sand seas and may reach heights of more than 300 m. They contain a greater volume of sand than any other dune type (Wasson and Hyde, 1983) and appear to occur in areas which represent depositional centres in many sand seas (Mainguet and

Callot, 1978; Lancaster, 1983; Brookfield, 1984).

Although a large amount of information on star and star-like dunes is scattered throughout the literature on deserts and dunes, little is known about this dune type other than its general characteristics. Relatively little has been published to date about star dune dynamics and morphology, or the environments in which these dunes form. The purpose of this review is to provide an overview of the characteristics and environments of for the formation of this dune type. 1

Occurrence

star

dunes, and

to

put forward

new

models

of star dunes

and Goudie (1981) have estimated that approximately 8.5 per cent of of star type, representing some five per cent of aeolian depositional surfaces. Star dunes occur in many sand seas (Figure 1), including the Grand Erg Oriental in Algeria; the Ala Shan Desert in China; the southeastern Rub-al-Khali in Arabia; the Gran Desierto in Mexico; the Erg Fachi-Bilma in Niger and the Namib Sand Sea in southern Africa (Breed and Grow, 1979; Breed et al. , 1979; Capot-Rey, 1945; Holm, 1960; McKee, 1966; 1982; Mainguet and Callot, 1978; Lancaster, 1983; Lancaster et al., 1987). In the Basin and Range province of the north American desert there are many occurrences of star-like dunes (Sharp, 1966; Andrews, 1981; Smith, 1982; Breed et al., 1984; Nielson and Kocurek,

Fryberger all dunes

are

1987).


68

.

,

(1) Namib Sand Sea; (2) Erg Fach-Bilma; (3) Figure Major Grand Erg Oriental; (4) Rub-al-Khali; (5) Badain Jaran Sand Sea; (6) Gran Desierto; (7) Mojave Desert. 1

areas

star

dunes:

dunes varies considerably from one sand sea to than 10 per cent. Star dunes are absent in Australian, Kalahari and Indian sand seas, as well as those in the eastern, western and southern Sahara. They cover 9-12 per cent of the dune area of sand seas in the northern Sahara, central Asia, the Namib and the Gran Desierto (Fryberger and Goudie, 1981; Lancaster, 1983; Lancaster et al., 1987). The only sand sea in which star dunes cover a large proportion of its area is the Grand Erg Oriental in Algeria, where 40 per cent of dunes are of this type (Breed et al., 1979). In many sand seas, complex dunes (see McKee, 1979) are transitional to star dunes. Star dunes may develop from complex linear dunes in the Namib Sand Sea (Lancaster, 1983) and complex crescentic dunes in the Gran Desierto, northern Saharan, Arabian and central Asian sand seas (Breed et al., 1979; Lancaster et al., 1987; Walker et al., 1987)

The

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I

Characteristics of star dunes

1

Morphology

and

morphometry

Star dunes are characterized by a pyramidal shape, with three or four arms radiating from a central peak and multiple avalanche faces. Each arm has a sharp sinuous crest, with avalanche faces which alternate in aspect as wind directions change. The arms may not all be equally developed and many star dunes have dominant or primary arms on a preferred orientation. Deep hollows often occur between the arms. The upper parts of many star dunes are very steep with slopes at angles of 15-30°. The basal parts of star dunes consist of a broad, gently sloping


69

(5-10°) plinth or apron. Small crescentic or reversing dunes are often superimposed on the lower flank and upper plinth areas of star dunes. Breed and Grow (1979) distinguished between simple star dunes with three to four equally developed arms and slipfaces; compound star dunes, with large primary arms and smaller subsidiary arms and complex star dunes which are combinations of star and other dune types with the star dune morphology being dominant. They recognized three varieties of star dunes on Landsat images. Star dunes with a sharply pointed crest, pyramidal shape and radiating arms are common to many sand seas. In the southern parts of the Grand Erg Oriental, many star dunes are rounded, with arms which do not project far from the main dune. Sharp crested dunes with arms which extend in a preferred direction occur in the Namib, Gran Desierto and Rub-al-Khali sand seas. Mainguet and Callot (1978) describe isolated star dunes (ghourds en étoiles isolées) from Algerian sand seas and ghourds en chdines or chains of star dunes from the Fachi Bilma erg. Mainguet (1976) recognized five ways in which star dunes could be grouped together: isolated, unorganized, coalescing, triangular or rectangular groupings, and parallel or lozenge-like chains. Examination of Landsat images of star dunes suggests, however, that few star dunes are randomly distributed; most star dunes occur in clusters and chains, or on distinct lineaments (Figure 2). 2

Morphology of star

dunes in the Namib Sand Sea

Star dunes cover about 10 per cent of the area of the Namib Sand Sea in five main localities along the eastern margin of the sand sea (Lancaster, 1983). Few of the dunes identified as star types in the Namib Sand Sea have a truly stellate form (Lancaster 1983). Many consist of a single narrow, relatively short (0.5-2 km), steep-sided ridge with a straight or sinuous crestline, which is near symmetrical in profile and has a southeast-northwest or south southeast-north northwest alignment. From the crestal ridge, curving arms descend on alignments which are roughly perpendicular to the crest. Slip faces develop on both sides of the crest and the subsidiary arms, their orientation depending on the winds of the time. They face northeast to east in summer and southwest to west in winter. In some southern parts of the sand sea, slip faces with a southeast or south southeast orientation may also develop in winter. The lower slopes of the star dunes form a wide, undulating plinth with slope angles of 2-5°. In many areas, there are small crescentic and reversing dunes on the plinths. These dunes merge with the arms descending from crestal ridges. Plinths of star dunes are often quite well vegetated, with large clumps of Stipagrostis sabilicola and Eragrostis spinosa. There are deep hollows and blowouts between and adjacent to the arms descending from the crestal areas. Interdunes between star dunes are often irregular, with areas of small crescentic and reversing dunes at intervals. Star dunes are the largest dunes in the Namib Sand Sea, averaging a height of 200-350 m around Sossus Vlei. In this area, and also north of Tsondab Vlei, many star dunes coalesce to form chains of star dunes, similar to the chdines


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