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Negatively skewed aeolian sands from a humid tropical coastal Dunefield, Northern Australia
Sedimentary Geology, 31 (1982) 249--266
249
Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
NEGATIVELY SKEWED AEOLIAN SANDS FROM A HUMID TROPICAL COASTAL DUNEFIELD, NORTHERN AUSTRALIA
KENNETH PYE
Department of Earth Sciences, Cambridge (Great Britain) (Received March 17, 1981 ;revised and accepted October 15, 1981)
ABSTRACT Pye, K., 1982. Negatively skewed aeolian sands from a humid tropical coastal dunefield, northern Australia. Sediment. Geol., 31: 249--266. This paper describes the nature and environmental significance of variations in textural parameters within a large coastal dunefield in tropical northern Australia. Grain-size parameters of modern beaches and foredunes are very similar b u t are distinct from those of parabolic dune and sandplain environments. On average, beach and foredune sands have a normal grain-size distribution, but parabolic dune sands are predominantly negatively skewed. Negative skewness is interpreted as primarily reflecting sorting of dune sediments by winnowing at dune crests and by avalanching on slip faces. A general relationship between mean size and skewness is supported, with finer sediments showing increased negative skewness. Positive skewness can therefore not be used as a diagnostic property of aeolian sediments.
INTRODUCTION
The nature and significance of textural parameters in aeolian sediments have been discussed in several previous papers (Mason and Folk, 1958; Friedman, 1961; Folk, 1971; Ahlbrandt, 1979; Chaudhri and Khan, 1981). Much of the work on coastal dune sediments has been undertaken in the United States (e.g. Shepard and Young, 1961; Schlee et al., 1964), Western Europe (Greenwood, 1969, 1970, 1972), and other temperate parts of the world (e.g. Sevon, 1966). Fewer studies have been conducted in subtropical areas, though important exceptions include the work of Martins (1965) and Bigarella et al. (1969, 1971), while very little is known of dune sediments on humid tropical coasts. Furthermore, many previous studies have been concerned only with foredunes or minor transgressive dunes bordering beaches; very few major transgressive dune complexes have been investigated in detail. The purpose of this paper is to describe and discuss the possible significance of textural parameters from a major parabolic coastal dunefield on the seasonally-humid tropical part of the Australian coast. The results presented 0037-0738/82/0000--0000/$02.75 © 1982 Elsevier Scientific Publishing Company
250
here were obtained as part of a wider investigation into the formation and history of coastal dunes on the North Queensland coast (Pye, 1980). During this study textural parameters were examined in order to throw light on the provenance and post
The grain-size data discussed in this paper were obtained from the Cape Bedford--Cape Flattery dunefield, located at approx. 14050 ' south on the east coast of Cape York Peninsula, North Queensland (Fig. 1). The dunefield covers an area of approx. 600 km 2 and lies to the east of an upland area, the Deighton Tableland, which consists of gently westward
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252 METHODS The grain~size characteristics of four main sedimentary environments have been investigated; namely, ocean beaches, foredunes, parabolic and elongate parabolic dunes and interdune sandplains. Beach samples were collected in all cases from the mid-foreshore. Both of the main ocean beaches, Cape Bedford Beach and Fourteen Mile Beach, have broad, gently-sloping profiles, ("dissipative" beaches in the terminology of Wright et al., 1979), with little apparent cross-beach variation in grain-size. No a t t e m p t was made to sample grain-size distributions of individual beach sand laminae (cf. Emery, 1978; Grace et al., 1978). Both beach and other surface samples were collected from the uppermost 5 cm. Subsurface dune and sandplain sediments were sampled at 1 m depth intervals using a combination of hand-auger and petrol-engined "Sandrill". The location of auger and drill holes was generally restricted to a number of specific transects and determined by "purposeful" sampling. Beach sands were sampled systematically at 500 m intervals along the entire shoreline. The sand samples were pre-treated to remove salts and other coatings where necessary and sieved at quarter phi intervals on a Ro-tap mechanical shaker as described by Ingram (1971). The weight percentage of sand retained in each size category was then plotted as a cumulative frequency curve and the graphical statistical parameters of Folk and Ward (1957) calculated. Despite criticisms of this technique (Chappell, 1967; Swan et al., 1978) it was preferred to other methods because the convenience of calculation outweighs the significance of the errors involved (Davis and Ehrlich, 1970; Warren, 1974), and because it is easier to use with samples which contain up to 5% fines. INTERPRETATION OF RESULTS A summary of the grain-size parameters in beach, foredune, parabolic dune and sandplain environments at Cape Bedford and Cape Flattery is presented in Table I. Beach sands
The m o d e m beaches consist of medium and fine wands which are well to very well sorted according to Folk and Ward's (1957) classification. On average the Fourteen Mile Beach sands are slightly finer and better sorted than the Cape Bedford Beach sands (both significant at 0.05% level; Mann-Whitney U-test). This probably reflects preferential northerly movement of fines under the influence of southeasterly wave action. The beach sands are typically unimodal and leptokurtic (Fig. 2A), with an absence of particles coarser than 0.5 ~ or finer than 3.0 ~. Skewness is variable, ranging from --0.47 to +0.25, b u t on average the sands approximate a normal distribution.
253 TABLE I Summary of grain-size parameters in beach, foredune, parabolic dune and sandplain environments at Cape Bed ford and Cape Flattery Mean
Sorting
Inclusive graphic skewness
Graphic kurtosis
Cape Bedford Beh. (n = 13) Mean 1.51 Range 1.12 to 2.63
0.45 0.24 to 0.71
--0.01 --0.47 to +0.25
1.24 0.78 to 2.38
Fourteen Mile Bch. (n = 7) Mean 1.86 Range 1.59 to 2.26
0.34 0.25 to 0.43
+0.08 +0.03 to +0.23
1.00 0.90 to 1.15
Cape Bedford Bch. (n = 15) Mean 2.09 Range 1.72 to 2.72
0.36 0.22 to 0.51
+0.01 - 0 . 1 6 to +0.16
1.11 0.92 to 1.27
Fourteen Mile Bch. Mean 1.97 Range 1.80 to 2.14
0.32 0.26 to 0.40
+0.04 - 0 . 0 7 to +0.16
1.12 0.94 to 1.22
Beaches
Foredunes
Parabolic dunes Cape Bedford (n = 25) Mean 2.32 Range 1.75 to 2.66 Cape Flattery (n = 15) Mean 2.27 Range 2.19 to 2.56
0.51 0.35 to 0.70
-0.12 - 0 . 2 7 to 0
1.04 0.63 to 1.58
0.45 0.38 to 0.51
-0.18 ---0.43 to +0.05
0.99 0.87 to 1.11
Sandplains (n = 13) Mean 2.27 Range 1.90 to 2.52
0.57 0.41 to 0.66
-0.02 --0.23 to +0.47
1.01 0.87 to 1.17
Measures of Folk and Ward (1957).
F o r e d u n e sands T h e f o r e d u n e sands are m e d i u m t o fine and well t o very well s o r t e d , unim o d a l and l e p t o k u r t i c (Fig. 3A). As in t h e case o f t h e a d j a c e n t b e a c h sands, skewness values are variable, ranging f r o m - - 0 . 1 6 t o + 0 . 1 6 , b u t o n average t h e sands a p p r o x i m a t e a n o r m a l d i s t r i b u t i o n . Particles coarser t h a n 1.0 ~ o r finer t h a n 3.0 ~ are rare. A graphic p l o t o f m e a n against sorting s h o w s a considerable overlap b e t w e e n the b e a c h and f o r e d u n e sands (Fig. 4), b u t o n average t h e f o r e d u n e sands are slightly finer t h a n t h e b e a c h sands (significant at t h e 0.05% level). This is in a g r e e m e n t w i t h previous findings in Texas (Mason and F o l k , 1 9 5 8 ) ,
254
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New South Wales (Gibbons, 1967), and elsewhere, and may be attributed to selective wind transport which leaves coarser grains on the beach (cf. Visher, 1969).
Parabolic and elongate parabolic dune sands Samples collected from the upper 10 m of active and partially active parabolic dunes consist of fine, well-sorted sand which is predominantly nega-
256
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tively skewed. Individual samples show considerable variability in their grainsize distributions. Some are unimodal and mesokurtic, with a marked absence o f particles finer than 3.0 ~ (e.g. Fig. 3B), while others are bimodal or multimodal b u t still negatively skewed (compare samples C F 1 6 A and CF241B in Fig. 5). A graphic plot of mean size against skewness shows that the parabolic dune sands can be readily differentiated from t h e foredune sands (Fig. 6). Statistical tests have also shown that differences in the mean size, sorting, and skewness values of the foredune and parabolic dunes are significant at the 0.05% level, the parabolic dune sands being finer, less well sorted, and more negatively skewed. The poor sorting values of the parabolic dune sands reflect the presence of significant tails of both coarse and fine particles. The most strongly negatively skewed samples have particularly large tails of coarse grains. The possible significance of this tendency for negative skewness is discussed below.
Sandplain sediments The surface inter
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(Fig. 2D). Most contain both a tail o f coarse grains and 1--5% fines. Skewness values are highly variable, ranging from --0.23 to greater than +0.47, depending largely on the percentage of fines present. Similar sediments have been reported from interdune or serif areas in many desert areas of the world (Cooke and Warren, 1973; Ahlbrandt, 1979). Although the sandplain sediments at Cape Flattery are less well sorted than the parabolic dune sands, an aeolian origin is indicated by their similarity to the sands found within intradune deflation corridors of active dunes which are currently being reworked (compare curves 4 and 6 in Fig. 7). In many places the sandplain sediments are ferruginous and contain ferric iron nodules o f pedogenic origin. These sands appear to represent the e x h u m e d B horizons o f former podzol soil profiles, the bleached A horizons of which have been stripped by deflation. Clayey B horizon material of this
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Fig. 6. Scattergram plot of mean versus inclusive graphic skewness values of foredune and parabolic dune samples. type would have acted as a local "deflation base" in the sense of Cooper (1958). The gently undulating to flat nature of the sandplain surface reflects the fact that the thickness and position of podzol B horizons are controlled to a large extent b y the position of the water table. In places the ferruginous sandplain sediments have been disrupted by blowouts to form low orange-coloured parabolic dunes. The latter consist of medium to fine-grained sands from which most of the fines present in the parent sandplain sediments have been stripped b y winnowing (Fig. 2C; curve 5 in Fig. 7). Elsewhere the bleached A horizon sands of the former podzols have been incompletely stripped, or have themselves been remobilized into small parabolic dunes. Particularly along the western limit of the dunefield, b u t also downwind of currently active dunes, the sandplaln surfaces have been covered by a thin (1--10 cm) veneer of wind-blown quartz silt which is probably derived f r o m abrasion during aeolian sand transport. SIGNIFICANCE OF NEGATIVE SKEWNESS IN THE PARABOLIC DUNE SANDS The diagnostic significance of skewness in aeolian sediments remains an area of uncertainty. Many previous investigators have concluded that dune sands are predominantly positively skewed (Mason and Folk, 1958; Friedman, 1961; Hails, 1967; Ly, 1978; Chaudhri and Khan, 1981}, whereas
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beach sands are predominantly negatively skewed. This difference has generally been attributed to selective removal of finer particles from the beach by wind. According to Visher (1969, p. 1083), for example, "the presence of a suspension population and the truncation of the coarse population account for the positive skewness characteristic o f dune deposists". Certain other workers, on the other hand, have been less convinced of the environmental significance of textural parameters. Shepard and Young (1961), Schlee et al. (1964), Gees (1965), Sevon (1966) and others have presented data which suggest that textural parameters including skewness are not environmentally sensitive and cannot be used to differentiate between beach and dune sands. Highly variable skewness values have been reported
260 b y Moiola and Wiesner (1968), while predominantly negatively skewed dune sands occur in eastern England (Roy, 1967; Psilovikos, 1979) and southeastern Brazil (BigareUa et al., 1969). The results of the latter workers suggest that skewness is partly dependent on grain-size, increasing negative skewness being associated with progressively finer sizes. A similar relationship was established in the Killpecker Desert dunefield of Wyoming by Ahlbrandt (1974, 1975). In this instance, sands coarser than 1.70 ~ were found to be positively skewed, while sands finer than 1.70 showed negative skewness. Results from Cape Flattery also show that finer samples are characterized b y greater negative skewness. Considering b o t h foredune and parabolic dune sands together, a positive correlation is significant at the 0.05% level. However, the reasons w h y finer sand should be negatively skewed are not altogether clear. Folk and Ward (1957), in their study of Brazos River bar sediments, concluded that skewness is indicative of the bimodality of a sediment, even when the modes are n o t immediately apparent. According to this argument, the further apart the modes, and therefore the poorer the sorting, the more pronounced the skewness, although in the extreme case of two equal sub-populations the combined distribution would have the worst possible sorting and would also be symmetrical. Such a pattern is evident at Cape Flattery, where the negatively skewed parabolic dune sands are significantly less well sorted (average phi sorting 0.48) than the more normally distributed foredune sands (average phi sorting 0.34). This stands in contrast to Ahlbrandt's results from Killpecker dunefield, which indicated that finer sizes and increased negative skewness are associated with better sorting. The characteristic poor sorting of the Cape Flattery parabolic dunes in comparison with the foredunes may be explained by the fact that the t w o are sedimentologically distinct units which are not genetically related. The foredunes are composed of sand which has been deflated from the compositionally similar adjacent beaches, b u t little or no sand is being supplied from the beach to the parabolic dunes at the present day. The latter have in many cases been initiated as spot blowouts well away from the shoreline in much older and deeply weathered sand deposits. These older deposits consist chiefly of degraded dunes of Pleistocene age, b u t also appear to include some residual sand, alluvial deposits, and beach ridges. The coarse grain population, which is significant in the modern parabolic dunes but much less so in the foredunes, appears to be derived b y reworking of the older deposits. The relative absence of particles finer than 3.0 ~ in the parabolic dune crest samples may be explained by a process of winnowing as the dunes migrate downwind. Sand removed from intra
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avalanching occurs (cf. Allen, 1970). Further sorting takes place during movement of sand across the slip face. This results from internal dispersive pressure within flowing sand which varies with particle diameter for any given shear stress. Larger grains tend towards the surface of the flow, where shear stress is at a minimum (Bagnold, 1954; Ahlbrandt, 1975). Smaller grains, on the other hand, move towards the base of the avalanche where shear stress is greatest. Thus coarser grains move further and faster than finer grains, and tend to concentrate in the basal layers of the dune (cf. Anan, 1969). Mean grain-size therefore becomes coarser with depth within a dune, and, since it has previously been shown that coarser grain-sizes are associated with increasing positive skewness, the sign of skewness also changes from negative to positive with depth. These trends can clearly be seen in Fig. 8, in which grain-size parameters have been plotted against depth for a single dune
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263
at Cape Flattery. Drilling and sedimentological analyses have shown this pattern to be extremely common in this area. P O S T - D E P O S I T I O N A L M O D I F I C A T I O N O F GRAIN-SIZE C H A R A C T E R I S T I C S
The grain-size distributions of the older stabilized dunes at Cape Bedford and Cape Flattery have been modified to a varying extent by weathering, input of aeolian dust, and by pedotranslocation. Podzol soil profiles are welldeveloped on the older dunes, the B horizons of which are typically strongly positively skewed due to the presence of up to 10% silt and clay. These fines represent both airfall deposits which have been translocated dow~ the profile by percolating rainwater (Wright and Foss, 1968; Crone, 1975) and authigenic weathering products. In some profiles the B horizons have been partially cemented by iron and aluminium hydroxides or organic matter (humate). Figure 9 shows the cumulative frequency curves of a sample of iron-stained sand before and after removal of the iron pigment. Further details of the nature of post
264
and Cambridge University. Invaluable logistic assistance was provided by Cape Flattery Silica Mines Pty. Ltd. Permission to work on tribal lands was granted by the Hopevale Aboriginal Community.
REFERENCES Ahlbrandt, T.S., 1974. The source of sand for the Killpecker dunefield, southwestern Wyoming. Sediment. Geol., 11: 39--57. Ahlbrandt, T.S., 1975. Comparison of textures and structures to distinguish eolian environments, Killpecker dunefield, Wyoming. Mt. Geol., 12: 61--73. Ahlbrandt, T.S., 1979. Textural parameters of eolian deposits. In: E.D. M c K e e (Editor), A Study of Global Sand Seas. U.S. Geol. Surv., Prof. Pap., 1052: 23--58. Allen, J.R.L., 1970. The avalanching of granular solids on dune and similar slopes. J. Geol., 78: 326--351. Anan, P.S., 1969. Grain-size parameters of the beach and dune sands, northeast Massachusetts and N e w Hampshire coasts. In: Coastal Environments: Northeast Massachusetts and N e w Hampshire. Coastal Res. Group, Dept. Geol., Univ. Massachusetts, Mass., pp. 266--280. Bagnold, R.A., 1954. The Physics of Blown Sand and Desert Dunes. Methuen, London, 265 pp. Bigarella, J.J., 1969. Dune sediment characteristics, recognition and importance. Bull. Am. Assoc. Pet. Geol., 53, p. 707. Bigarella, J.J., Alessi,A.H., Becker, R.D. and Duarte, G.M., 1969. Textural characteristics of the coastal dune, sand ridge, and beach sediments. Bol. Paran. Geociencias, 27: 15-80. Bigarella, J.J., Duarte, G.M. and Becker, R.D., 1970--71. Structural characteristicsof the dune, foredune, interdune, beach, beach-dune ridge, and sand ridge deposits. Bol. Paran. Geociencias, 28/29: 9--72. Bird, E.C.F., 1965. The formation of coastal dunes in the humid tropics: some evidence from North Queensland. Aust. J. Sci., 27: 258--259. Chapp.ell, J., 1967. Recognizing fossil strandlines from grain size analysis. J. Sediment. Petrol., 37: 157--165. Chaudhri, R.S. and Khan, H.M.M., 1981. Textural parameters of desert sediments --That Desert (India). Sediment. Geol., 28: 43--62. Cooke, R.U. and Warren, A., 1973. Geomorphology in Deserts. Batsford, London, 374 pp. Cooper, W.S., 1958. Coastal sand dunes of Oregon and Washington. Mem. Geol. Soc. Am., 72: 1--169. Crone, A.J., 1975. Laboratory and Field Studies of Mechanically InfiltratedMatrix Clay in Arid Fluvial Environments. PhD. Thesis, Univ. Colorado, Boulder, Colo., 162 pp. Davis, J.H. and Ehrlich, R., 1970. Relationships between measures of sediment size-frequency distributions and the nature of sediments. Geol. Soc. Am. Bull., 81: 3537-3548. Duane, D.B., 1964. Significance of skewness in Recent sediments, Pamlico Sound, North Carolina. J. Sediment. Petrol., 34: 864--874. Emergy, K.O., 1978. Grain size in laminae of beach sand. J. Sediment. Petrol., 48: 1203-1212. Folk, R.L., 1971. Longitudinal dunes of the northwestern edge of the Simpson Desert. Northern Territory, Australia: Geomorphology and grain-size relationships. Sedimentology, 16: 5--54. Folk, R.L. and Wardl W.C., 1957. Brazos River bar: a study in the significance of grain size parameters. J. Sediment. Petrol., 27: 3--26.
265 Friedman, G.M., 1961. Distinction between dune, beach, a n d river sands from their textural characteristics. J. Sediment. Petrol., 31 : 514--529. Gees, R.A., 1965. Moment measures in relation to the depositional environment of sands. Eclogae Geol. Helv., 58: 209--213. Gentilli, J., 1971. The main climatological elements. In: J. Gentilli (Editor), World Survey of Climatology, 13. Elsevier, Amsterdam, pp. 119--188. Gibbons, G.S., 1967. Shell content in quartzose beach and dune sands, Dee Why, New South Wales. J. Sediment. Petrol., 37: 869--878. Grace, J.T., Grothaus, B.T. and Ehrlich, R., 1978. Size frequency distributions taken from within sand laminae. J. Sediment. Petrol., 48: 1193--1202. Greenwood, B., 1969. Sedimentary parameters and environmental discrimination: an application o f multivariate statistics. Can. J. Earth Sci., 6: 1347--1358. Greenwood, B., 1970. Sediment Studies and the Discrimination of Certain Environments of Deposition. PhD. Thesis, Univ. Bristol. Greenwood, B., 1972. Modern analogues and the evaluation o f a Pleistocene sedimentary sequence. Trans. Inst. Brit. Geogr., 56: 145--169. Hails, J.R., 1967. Significance o f textural parameters for distinguishing sedimentary environments in New South Wales. J. Sediment. Petrol., 37: 1059--1069. Ingram, R.L., 1971. Sieve analysis. In: R.E. Carver (Editor), Procedures in Sedimentary Petrology. Wiley Interscience, New York, N.Y., pp. 49--67. Lourensz, R.S., 1977. Tropical Cyclones in the Australian region, J u l y 1909 to June 1975. Meteorol. Summary for Bur. Meteorol., Dept. of Science. Aust. Govt. Publ. Service, Canberra, 111 pp. Lucas, K.G. and De Keyser, F., 1965a. Cocktown, Queensland. 1 : 250,000 Geol. Set. Explan. Notes Sheet SD/55-13. Aust. Bur. Min. Res. Geol. Geophys., Canberra. Lucas, K.G. and De Keyser, F., 1965b. Cape Melville, Queensland. 1 : 250,000 Geol. Set. Explan. Notes Sheet SD/55-9. Anst. Bur. Min. Res. Geol. Geophys., Canberra. Ly, C.K., 1978. Late Quaternary deposits o f the Newcastle--Port Stephens area revealed by grain-size analysis and scanning electron microscopy. J. Proc. R. Soc. N.S.W., 111: 77 --88. Martins, L.R., 1965. Significance of skewness and kurtosis in environmental interpretation. J. Sediment. Petrol., 35: 768--770. Mason, C.C. and Folk, R.L., 1958. Differentiation o f beach, dune, and aeolian fiat environment by size analysis, Mustang Island, Texas. J. Sediment. Petrol., 28: 211--226. Maxwell, W.G.H., 1968. Atlas o f the Great Barrier Reef. Elsevier, Amsterdam, 258 pp. Moiola, R.J. and Wiesner, D., 1968. Textural p a r a m e t e r s - - a n evaluation. J. Sediment. Petrol., 38: 45--53. Psilovikos, A.A., 1979. Sediment analysis at Gibraltar Point. East Midland Geogr., 7: 128--133. Pye, K., 1980. Geomorphic Evolution o f Coastal Sand Dunes in a Humid Tropical Environment: North Queensland. PhD. Thesis, Univ. Cambridge, 413 pp. l~ye, K., 1981. Rate of dune reddening in a humid tropical climate. Nature, 290: 582-584. Pye, K., in press a. Morphological development of coastal dunes in a humid tropical environment, Cape Bedford and Cape Flattery, North Queensland. Geogr. Annal. Set. A. Pye, K., in press b. Post-depositional reddening in late Quaternary coastal dune sands, Northeast Australia. J. Geol. Soc. Pye, K. and Jackes, B., 1981. Vegetation of the coastal dunes at Cape Bedford and Cape Flattery, North Queensland. Proc. R. Soc. Qld., 92: 37--42. Pye, K. and Switsur, V.R., 1981. Radiocarbon dates from the Cape Bedford - - Cape Flattery dunefields, North Queensland. Search, 12: 225--226. Roy, P.S., 1967. The Recent Sedimentology of Scolt Head Island. PhD. Thesis, Univ. London, 2 Vols. Schlee, J., Uchupi, E. and Trumbull, J.V.A., 1964. Statistical parameters of Cape Cod beach and eolian sands. U.S. Geol. Surv., Prof. Pap., 501D: D118--D122.
266 Sevon, W.D., 1966. Distinction o f New Zealand beach, dune, and river sands by their grain~size distribution curves. N . Z . J . Geol. Geophys., 9: 212--223. Shepard, F.P. and Young, R., 1961. Distinguishing between beach and dune sands. J. Sediment. Petrol., 31: 196--214. Swan, D., Claque, J.J. and Luternauer, J.L., 1978. Grain-size statistics. 1. Evaluation of the Folk and Ward graphic measures. J. Sediment. Petrol., 48: 863--868. Visher, G.S., 1969. Grain-size distributions and depositional processes. J. Sediment. Petrol., 39: 1074--1106. Warren, G., 1974. Simplified form of the Folk-Ward skewness parameter. J. Sediment. Petrol., 44, p. 259. Wittwer, E.L., 1973. Seasonal changes in wind regimes of the North Australian and adjoining intertropical area. Aust. Geogr., 12: 340--362. Wright, L.D., Chappell, J., Thorn, B.G., Bradshaw, M.P. and Cowell, P., 1979. Morphodynamics of reflective and dissipative beach and inshore systems: southeast Australia. Mar. Geol., 32: 105--140. Wright, W.R. and Foss, J.E., 1968. Movement of silt-sized particles in sand columns. Proc. Soil Sci. Soc. Am., 32: 446--448.
249
Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
NEGATIVELY SKEWED AEOLIAN SANDS FROM A HUMID TROPICAL COASTAL DUNEFIELD, NORTHERN AUSTRALIA
KENNETH PYE
Department of Earth Sciences, Cambridge (Great Britain) (Received March 17, 1981 ;revised and accepted October 15, 1981)
ABSTRACT Pye, K., 1982. Negatively skewed aeolian sands from a humid tropical coastal dunefield, northern Australia. Sediment. Geol., 31: 249--266. This paper describes the nature and environmental significance of variations in textural parameters within a large coastal dunefield in tropical northern Australia. Grain-size parameters of modern beaches and foredunes are very similar b u t are distinct from those of parabolic dune and sandplain environments. On average, beach and foredune sands have a normal grain-size distribution, but parabolic dune sands are predominantly negatively skewed. Negative skewness is interpreted as primarily reflecting sorting of dune sediments by winnowing at dune crests and by avalanching on slip faces. A general relationship between mean size and skewness is supported, with finer sediments showing increased negative skewness. Positive skewness can therefore not be used as a diagnostic property of aeolian sediments.
INTRODUCTION
The nature and significance of textural parameters in aeolian sediments have been discussed in several previous papers (Mason and Folk, 1958; Friedman, 1961; Folk, 1971; Ahlbrandt, 1979; Chaudhri and Khan, 1981). Much of the work on coastal dune sediments has been undertaken in the United States (e.g. Shepard and Young, 1961; Schlee et al., 1964), Western Europe (Greenwood, 1969, 1970, 1972), and other temperate parts of the world (e.g. Sevon, 1966). Fewer studies have been conducted in subtropical areas, though important exceptions include the work of Martins (1965) and Bigarella et al. (1969, 1971), while very little is known of dune sediments on humid tropical coasts. Furthermore, many previous studies have been concerned only with foredunes or minor transgressive dunes bordering beaches; very few major transgressive dune complexes have been investigated in detail. The purpose of this paper is to describe and discuss the possible significance of textural parameters from a major parabolic coastal dunefield on the seasonally-humid tropical part of the Australian coast. The results presented 0037-0738/82/0000--0000/$02.75 © 1982 Elsevier Scientific Publishing Company
250
here were obtained as part of a wider investigation into the formation and history of coastal dunes on the North Queensland coast (Pye, 1980). During this study textural parameters were examined in order to throw light on the provenance and post
The grain-size data discussed in this paper were obtained from the Cape Bedford--Cape Flattery dunefield, located at approx. 14050 ' south on the east coast of Cape York Peninsula, North Queensland (Fig. 1). The dunefield covers an area of approx. 600 km 2 and lies to the east of an upland area, the Deighton Tableland, which consists of gently westward
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252 METHODS The grain~size characteristics of four main sedimentary environments have been investigated; namely, ocean beaches, foredunes, parabolic and elongate parabolic dunes and interdune sandplains. Beach samples were collected in all cases from the mid-foreshore. Both of the main ocean beaches, Cape Bedford Beach and Fourteen Mile Beach, have broad, gently-sloping profiles, ("dissipative" beaches in the terminology of Wright et al., 1979), with little apparent cross-beach variation in grain-size. No a t t e m p t was made to sample grain-size distributions of individual beach sand laminae (cf. Emery, 1978; Grace et al., 1978). Both beach and other surface samples were collected from the uppermost 5 cm. Subsurface dune and sandplain sediments were sampled at 1 m depth intervals using a combination of hand-auger and petrol-engined "Sandrill". The location of auger and drill holes was generally restricted to a number of specific transects and determined by "purposeful" sampling. Beach sands were sampled systematically at 500 m intervals along the entire shoreline. The sand samples were pre-treated to remove salts and other coatings where necessary and sieved at quarter phi intervals on a Ro-tap mechanical shaker as described by Ingram (1971). The weight percentage of sand retained in each size category was then plotted as a cumulative frequency curve and the graphical statistical parameters of Folk and Ward (1957) calculated. Despite criticisms of this technique (Chappell, 1967; Swan et al., 1978) it was preferred to other methods because the convenience of calculation outweighs the significance of the errors involved (Davis and Ehrlich, 1970; Warren, 1974), and because it is easier to use with samples which contain up to 5% fines. INTERPRETATION OF RESULTS A summary of the grain-size parameters in beach, foredune, parabolic dune and sandplain environments at Cape Bedford and Cape Flattery is presented in Table I. Beach sands
The m o d e m beaches consist of medium and fine wands which are well to very well sorted according to Folk and Ward's (1957) classification. On average the Fourteen Mile Beach sands are slightly finer and better sorted than the Cape Bedford Beach sands (both significant at 0.05% level; Mann-Whitney U-test). This probably reflects preferential northerly movement of fines under the influence of southeasterly wave action. The beach sands are typically unimodal and leptokurtic (Fig. 2A), with an absence of particles coarser than 0.5 ~ or finer than 3.0 ~. Skewness is variable, ranging from --0.47 to +0.25, b u t on average the sands approximate a normal distribution.
253 TABLE I Summary of grain-size parameters in beach, foredune, parabolic dune and sandplain environments at Cape Bed ford and Cape Flattery Mean
Sorting
Inclusive graphic skewness
Graphic kurtosis
Cape Bedford Beh. (n = 13) Mean 1.51 Range 1.12 to 2.63
0.45 0.24 to 0.71
--0.01 --0.47 to +0.25
1.24 0.78 to 2.38
Fourteen Mile Bch. (n = 7) Mean 1.86 Range 1.59 to 2.26
0.34 0.25 to 0.43
+0.08 +0.03 to +0.23
1.00 0.90 to 1.15
Cape Bedford Bch. (n = 15) Mean 2.09 Range 1.72 to 2.72
0.36 0.22 to 0.51
+0.01 - 0 . 1 6 to +0.16
1.11 0.92 to 1.27
Fourteen Mile Bch. Mean 1.97 Range 1.80 to 2.14
0.32 0.26 to 0.40
+0.04 - 0 . 0 7 to +0.16
1.12 0.94 to 1.22
Beaches
Foredunes
Parabolic dunes Cape Bedford (n = 25) Mean 2.32 Range 1.75 to 2.66 Cape Flattery (n = 15) Mean 2.27 Range 2.19 to 2.56
0.51 0.35 to 0.70
-0.12 - 0 . 2 7 to 0
1.04 0.63 to 1.58
0.45 0.38 to 0.51
-0.18 ---0.43 to +0.05
0.99 0.87 to 1.11
Sandplains (n = 13) Mean 2.27 Range 1.90 to 2.52
0.57 0.41 to 0.66
-0.02 --0.23 to +0.47
1.01 0.87 to 1.17
Measures of Folk and Ward (1957).
F o r e d u n e sands T h e f o r e d u n e sands are m e d i u m t o fine and well t o very well s o r t e d , unim o d a l and l e p t o k u r t i c (Fig. 3A). As in t h e case o f t h e a d j a c e n t b e a c h sands, skewness values are variable, ranging f r o m - - 0 . 1 6 t o + 0 . 1 6 , b u t o n average t h e sands a p p r o x i m a t e a n o r m a l d i s t r i b u t i o n . Particles coarser t h a n 1.0 ~ o r finer t h a n 3.0 ~ are rare. A graphic p l o t o f m e a n against sorting s h o w s a considerable overlap b e t w e e n the b e a c h and f o r e d u n e sands (Fig. 4), b u t o n average t h e f o r e d u n e sands are slightly finer t h a n t h e b e a c h sands (significant at t h e 0.05% level). This is in a g r e e m e n t w i t h previous findings in Texas (Mason and F o l k , 1 9 5 8 ) ,
254
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New South Wales (Gibbons, 1967), and elsewhere, and may be attributed to selective wind transport which leaves coarser grains on the beach (cf. Visher, 1969).
Parabolic and elongate parabolic dune sands Samples collected from the upper 10 m of active and partially active parabolic dunes consist of fine, well-sorted sand which is predominantly nega-
256
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tively skewed. Individual samples show considerable variability in their grainsize distributions. Some are unimodal and mesokurtic, with a marked absence o f particles finer than 3.0 ~ (e.g. Fig. 3B), while others are bimodal or multimodal b u t still negatively skewed (compare samples C F 1 6 A and CF241B in Fig. 5). A graphic plot of mean size against skewness shows that the parabolic dune sands can be readily differentiated from t h e foredune sands (Fig. 6). Statistical tests have also shown that differences in the mean size, sorting, and skewness values of the foredune and parabolic dunes are significant at the 0.05% level, the parabolic dune sands being finer, less well sorted, and more negatively skewed. The poor sorting values of the parabolic dune sands reflect the presence of significant tails of both coarse and fine particles. The most strongly negatively skewed samples have particularly large tails of coarse grains. The possible significance of this tendency for negative skewness is discussed below.
Sandplain sediments The surface inter
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(Fig. 2D). Most contain both a tail o f coarse grains and 1--5% fines. Skewness values are highly variable, ranging from --0.23 to greater than +0.47, depending largely on the percentage of fines present. Similar sediments have been reported from interdune or serif areas in many desert areas of the world (Cooke and Warren, 1973; Ahlbrandt, 1979). Although the sandplain sediments at Cape Flattery are less well sorted than the parabolic dune sands, an aeolian origin is indicated by their similarity to the sands found within intradune deflation corridors of active dunes which are currently being reworked (compare curves 4 and 6 in Fig. 7). In many places the sandplain sediments are ferruginous and contain ferric iron nodules o f pedogenic origin. These sands appear to represent the e x h u m e d B horizons o f former podzol soil profiles, the bleached A horizons of which have been stripped by deflation. Clayey B horizon material of this
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Fig. 6. Scattergram plot of mean versus inclusive graphic skewness values of foredune and parabolic dune samples. type would have acted as a local "deflation base" in the sense of Cooper (1958). The gently undulating to flat nature of the sandplain surface reflects the fact that the thickness and position of podzol B horizons are controlled to a large extent b y the position of the water table. In places the ferruginous sandplain sediments have been disrupted by blowouts to form low orange-coloured parabolic dunes. The latter consist of medium to fine-grained sands from which most of the fines present in the parent sandplain sediments have been stripped b y winnowing (Fig. 2C; curve 5 in Fig. 7). Elsewhere the bleached A horizon sands of the former podzols have been incompletely stripped, or have themselves been remobilized into small parabolic dunes. Particularly along the western limit of the dunefield, b u t also downwind of currently active dunes, the sandplaln surfaces have been covered by a thin (1--10 cm) veneer of wind-blown quartz silt which is probably derived f r o m abrasion during aeolian sand transport. SIGNIFICANCE OF NEGATIVE SKEWNESS IN THE PARABOLIC DUNE SANDS The diagnostic significance of skewness in aeolian sediments remains an area of uncertainty. Many previous investigators have concluded that dune sands are predominantly positively skewed (Mason and Folk, 1958; Friedman, 1961; Hails, 1967; Ly, 1978; Chaudhri and Khan, 1981}, whereas
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beach sands are predominantly negatively skewed. This difference has generally been attributed to selective removal of finer particles from the beach by wind. According to Visher (1969, p. 1083), for example, "the presence of a suspension population and the truncation of the coarse population account for the positive skewness characteristic o f dune deposists". Certain other workers, on the other hand, have been less convinced of the environmental significance of textural parameters. Shepard and Young (1961), Schlee et al. (1964), Gees (1965), Sevon (1966) and others have presented data which suggest that textural parameters including skewness are not environmentally sensitive and cannot be used to differentiate between beach and dune sands. Highly variable skewness values have been reported
260 b y Moiola and Wiesner (1968), while predominantly negatively skewed dune sands occur in eastern England (Roy, 1967; Psilovikos, 1979) and southeastern Brazil (BigareUa et al., 1969). The results of the latter workers suggest that skewness is partly dependent on grain-size, increasing negative skewness being associated with progressively finer sizes. A similar relationship was established in the Killpecker Desert dunefield of Wyoming by Ahlbrandt (1974, 1975). In this instance, sands coarser than 1.70 ~ were found to be positively skewed, while sands finer than 1.70 showed negative skewness. Results from Cape Flattery also show that finer samples are characterized b y greater negative skewness. Considering b o t h foredune and parabolic dune sands together, a positive correlation is significant at the 0.05% level. However, the reasons w h y finer sand should be negatively skewed are not altogether clear. Folk and Ward (1957), in their study of Brazos River bar sediments, concluded that skewness is indicative of the bimodality of a sediment, even when the modes are n o t immediately apparent. According to this argument, the further apart the modes, and therefore the poorer the sorting, the more pronounced the skewness, although in the extreme case of two equal sub-populations the combined distribution would have the worst possible sorting and would also be symmetrical. Such a pattern is evident at Cape Flattery, where the negatively skewed parabolic dune sands are significantly less well sorted (average phi sorting 0.48) than the more normally distributed foredune sands (average phi sorting 0.34). This stands in contrast to Ahlbrandt's results from Killpecker dunefield, which indicated that finer sizes and increased negative skewness are associated with better sorting. The characteristic poor sorting of the Cape Flattery parabolic dunes in comparison with the foredunes may be explained by the fact that the t w o are sedimentologically distinct units which are not genetically related. The foredunes are composed of sand which has been deflated from the compositionally similar adjacent beaches, b u t little or no sand is being supplied from the beach to the parabolic dunes at the present day. The latter have in many cases been initiated as spot blowouts well away from the shoreline in much older and deeply weathered sand deposits. These older deposits consist chiefly of degraded dunes of Pleistocene age, b u t also appear to include some residual sand, alluvial deposits, and beach ridges. The coarse grain population, which is significant in the modern parabolic dunes but much less so in the foredunes, appears to be derived b y reworking of the older deposits. The relative absence of particles finer than 3.0 ~ in the parabolic dune crest samples may be explained by a process of winnowing as the dunes migrate downwind. Sand removed from intra
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avalanching occurs (cf. Allen, 1970). Further sorting takes place during movement of sand across the slip face. This results from internal dispersive pressure within flowing sand which varies with particle diameter for any given shear stress. Larger grains tend towards the surface of the flow, where shear stress is at a minimum (Bagnold, 1954; Ahlbrandt, 1975). Smaller grains, on the other hand, move towards the base of the avalanche where shear stress is greatest. Thus coarser grains move further and faster than finer grains, and tend to concentrate in the basal layers of the dune (cf. Anan, 1969). Mean grain-size therefore becomes coarser with depth within a dune, and, since it has previously been shown that coarser grain-sizes are associated with increasing positive skewness, the sign of skewness also changes from negative to positive with depth. These trends can clearly be seen in Fig. 8, in which grain-size parameters have been plotted against depth for a single dune
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Fig. 9. Cumulative f r e q u e n c y curves o f a sample of ferruginous sand after (a) and before (b) removal of the iron oxides and silt and clay fraction. The f r e q u e n c y histogram illustrates the grain~ize distribution after treatment.
263
at Cape Flattery. Drilling and sedimentological analyses have shown this pattern to be extremely common in this area. P O S T - D E P O S I T I O N A L M O D I F I C A T I O N O F GRAIN-SIZE C H A R A C T E R I S T I C S
The grain-size distributions of the older stabilized dunes at Cape Bedford and Cape Flattery have been modified to a varying extent by weathering, input of aeolian dust, and by pedotranslocation. Podzol soil profiles are welldeveloped on the older dunes, the B horizons of which are typically strongly positively skewed due to the presence of up to 10% silt and clay. These fines represent both airfall deposits which have been translocated dow~ the profile by percolating rainwater (Wright and Foss, 1968; Crone, 1975) and authigenic weathering products. In some profiles the B horizons have been partially cemented by iron and aluminium hydroxides or organic matter (humate). Figure 9 shows the cumulative frequency curves of a sample of iron-stained sand before and after removal of the iron pigment. Further details of the nature of post
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and Cambridge University. Invaluable logistic assistance was provided by Cape Flattery Silica Mines Pty. Ltd. Permission to work on tribal lands was granted by the Hopevale Aboriginal Community.
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