Sedimentary Geology, 52 (1987) 149-153 149 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

Short Note

T H E R O U N D N E S S OF AEOLIAN Q U A R T Z SAND GRAINS

D.S.G. THOMAS

Department of Geography, University of Sheffield, Sheffield SIO 2TN (U.K.)

(Accepted for publication October 8, 1986)

INTRODUCTION

The recent paper by Khalaf and Gharib (1985) on the roundness of aeolian quartz grains is to be welcomed as it deals with a relatively neglected aspect of the

sedimentology of unconsolidated sands. Their conclus ions-- tha t aeolian grains are mainly subrounded to subangular, that roundness increases as grain size increases and that roundness is inherited from source sed iments - -a re essentially in agreement with the findings of Folk (1978) and Goudie and Watson (1981). There are, however, a number of points raised by Khalaf and Gharib 's study which they do not discuss but which are worthy of some consideration.

THE RELATIONSHIP BETWEEN GRAIN SIZE AND MEAN ROUNDNESS

Although Khalaf and Gharib reaffirm that in aeolian sands mean roundness increases as grain size gets larger, their fig. 4 (p. 154) shows an interesting facet of

their work which is not considered. This is that the highest mean roundness values do not occur in the very coarsest size grade, but instead roundness is reduced in their - 1 . 0 to 0.0 ~ class from its peak at 0.0 to 1.0 q~. A similar trend (but not in

precisely the same size classes) is observed in the aeolian sands from the crests and straats of now stabilized late Quaternary linear dunes in northeastern Botswana and western Zimbabwe (Thomas, 1984, 1987; and see Fig. 1). Explanations may be suggested for this. Khalaf and Gharib cite studies (pp. 147-148) which suggested that grain rounding in aeolian sands results from abrasion during transportation.

The nature of aeolian transportation and grain mobilization may account for the reduced rounding of the very largest grains. Bagnold (1941) and others have noted that in aeolian sands coarser grains move as a creep load and finer grains are

saltated. In addition, greater wind velocities are necessary to mobilize the coarser grains. There are two implications here for particle roundness. Firstly, grains moved by creep are probably subjected to less grain-upon-grain impacts than those mobilized by saltation, creating fewer opportunities for edge abrasion. Secondly, the

potential for abrasion of the very largest grains may be further reduced by the likelihood that they are mobilized less often. These points clearly need to be

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Fig. 1. Relationship between mean roundness and grain size, stabilized dune crests (crosses, n = 18 samples) and interdune troughs (dots, n ~ f samples). Results expressed on the p scale (Folk, 19551, roudness measured by the method of Powers (1953).

considered along with those which reveal .that the rate of abrasion decreases as grain size falls (Khalaf and Gharib, 1985, p. 147): their interaction may explain why roundness increases and then decreases again with increasing particle size. There- fore, the particle size range in which maximum roundness in an aeolian sand occurs is at least partly a function of wind velocities and frequencies during sand mobiliza-- tion. As these vary considerably in desert sand seas (e.g. Fryberger, 1979), the roundness of quartz particles will vary accordingly. Indeed, Kuenen (1960) noted that abrasion is a function of both grain size and wind velocity.

R O U N D N E S S A N D A E O L I A N T R A N S P O R T A T I O N

Khalaf and Gharib (1985) found that the roundness of the aeolian sand particles in Kuwait was similar to that of their source material, the Dibdabba fluvial sands. Folk (1978) also found that Simpson Desert dune sands inherited roundness characteristics from parent material: in both cases it is implied that there has been insufficient abrasion for a high degree of particle rounding to be achieved. The Namib Desert in southern Africa has an extremely long, largely uninterrupted history of aridity (e.g. Ward et al., 1983) offering the potential for the sands of the Namib sand sea dunes to be reworked by the wind over a long time period, Yet Goudie and Watson (1981) found that these sands also displayed a relatively low degree of mean roundness.

Patro and Sahu (1977), however, were able to differentiate between dune, fluvial and beach sands on the bails of roundness and sphericity characteristics, implying

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that aeolian abrasion does modify roundness during transportation. Clearly, the relationship between the roundness of aeolian sands and of source materials is complex. In part this may relate to the distance over which sand has been transported from its source by the wind--al though Khalaf and Gharib (1985) found no spatial variations in roundness values in Kuwait. A further consideration is that particle roundness may vary according to the size and type of dune features. The large linear dunes of the Namib probably do not present the opportunity for a high frequency of aeolian reworking of the sands in which they are formed; whereas particles in smaller, or more mobile, dune forms may be subjected to a higher frequency of mobilization events, offering more opportunity for edge abrasion. If abrasion is a slow process in the wind, albeit 100 to 1000 times more efficient than in water (Kuenen, 1960) then the frequency of movement may be the key factor in determining particle roundness.

ROUNDNESS AND SPHERICITY

Finally, Khalaf and Gharib cite Richardson (1903) who suggested that aeolian shape sorting occurs because round and more spherical grains are more readily transported by the wind. It is important here to clarify the distinction between roundness and sphericity (or form) as these two attributes of particle shape are independent of each other (e.g. Barrett, 1980). Rounding results from abrasion during transport, whereas it is sphericity, the measure of how close a particle becomes to being uniaxial (Teller, 1976), which has an effect on selective aeolian

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Fig. 2. Relationship between mean sphericity and grain size. Sphericity measured by the method of Rfttenhouse (1943).

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transportation (e.g. Willetts et al,, 1982). ~ u s studies which wish to elucidate on the relationship between particle shape, aeolian transportation and source material need to investigate sphericity, which has largely been neglected, using, for example Rittenhouse's (1943) visual charts:

In the case of the quartz particles from the stabilized dunes in central southern Africa, mean sphericity values increase with grain size (Fig. 2), for it is more spheroid grains which are more readily mobilized by the wind, especially as grain size increases. It is interesting to note that, as with roundness, mean sphericity values fall in the largest size fraction, and is as yet unexplained. The lack of a statistically significant correlation between the roundness and sphericity data from individual samples supports the notion that they are indeed independent attributes.

CONCLUSIONS

The relationships between particle shape, sediment source and aeolian transpor- tation are extremely complex. Not only is it necessary to investigate particle sphericity as well as roundness, but it should be recognised that along with the particle shape characteristics of the parent material, dune type, frequency of aeolian mobilizati6n and wind velocity frequencies are all likely to influence the shape of quartz grains in sand bodies accumulated through the action of the wind.

REFERENCES

Bagnold, R.A., 1941. The Physics of Blown Sand and Desert Dunes. Methuen, London, 265 pp. Barrett, P.J., 1980. The shape of rock particles, a critical review. Sedimentology, 27: 291-303. Folk, R.L., 1955. Student operator error in determination of roundness, sphericity and grain size. J

Sediment. Petrol., 25: 297-301. Folk, R.L., 1978. Angularity and silica coatings of Simpson Desert sand grains, Northern Territo~

Australia. J. Sediment. Petrol., 48: 611-624. Fryberger, S.G., 1979. Dune form and wind regime. In: E.D. MeKee (Editor), A Study of Global Sand

Seas. U.S. Geol. Surv., Prof. Pap., 1052: 137-169. Goudie, A.S. and Watson, A., 1981. The shape of desert sand grains. J. Arid Environ., 4: 185-190. Khalaf, F.L and Gharib, I.M., 1985. Roundness parameters of quartz sand grains of recent aeolian sand

deposits in Kuwait. Sediment. Geol., 45: 137-158. Kuenen, Ph.H., 1960. Experimental abrasion. 4. Eolian action. J. Geol., 68: 427-449. Patro, B.C. and Sahu, B.K., 1977. Discriminant analysis of sphericity and roundness data of clastic

quartz grains in rivers, beaches and dunes. Sediment. Geol., 19: 301-311. Powers, M.C., 1953. A new roundness scale for sedimentary particles. J. Sediment. Petrol., 23: 117-119. Richardson, H., 1903. Sea sand. Annu. Rep. Yorks, Philos. Soc., 1902: 43-58. Rittenhouse, G., 1943. A visual method of estimating two-dimensional sphericity. J. Sediment. Petrol.,

13: 79-81. Teller, J.T., 1976. Equanticy versus sphericity. Sedimentology, 23: p. 47. Thomas, D.S.G., 1984. Ancient ergs of the former arid zones of Zimbabwe, Zambia and Angola. Trans.

Inst. Brit. Geogr., N.S., 9: 75-88.

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Thomas, D.S.G,, 1987. Discrimination of depositional environments using sedimentary characteristics, in the Mega Kalahari, Central Southern Africa. In: L.E. Frostick and I. Reid (Editors), Desert Sediments, Ancient and Modern. Geol. Soc. London, Spec. Publ.

Ward, T.C., Seeley, M.K. and Lancaster, N., 1983. On the antiquity of the Namib. S. Afr. J. Sci., 79: 175-183.

WiUets, B.B., Rice, M.A. and Swaine, S.E., 1982. Shape effects in aeolian grain transport. Sedimentology, 19: 409-417.