EARTH SURFACE PROCESSES AND LANDFORMS, VOL. 7, 575-587 (1982)

DUNES ON THE SKELETON COAST, NAMIBIA (SOUTH WEST AFRICA):

GEOMORPHOLOGY AND GRAIN SIZE RELATIONSHIPS

N. LANCASTER Desert Ecological Research Unit, P.O. Box 953, Waluis Bay 9100, South West Africa

Receiued 2 April 1981 Revised 10 September 1981

ABSTRACT

Simple, and locally compound, transverse and barchanoid dunes dominate the 2000 km2 Skeleton Coast dunefield in northwestern Namibia/South West Africa. Dune height and spacing are closely correlated ( r = 0.89) and decrease across the dunefield from southwest to northeast, with an accompanying change from transverse to barchanoid ridges and ultimately barchans.

The dunes are aligned transverse to the dominant strong south and south southwest onshore winds. Alignment patterns indicate that surface roughness changes between coastal plain and dunes cause dune-forming winds to swing to the right over the dunes, but resume their original direction beyond.

Grain size and sorting vary at three scales: the dune, the dune landscape and through the dunefield. Overall the sands, derived from three localities by deflation from beaches supplied by vigorous longshore drift, become progressively finer and better sorted across the dunefield paralleling changes in dune height and spacing.

A statistically significant relationship ( r = -0.65) was established between dune spacing and the phi grain size of the coarser fraction of the dune sands, demonstrating the importance of the protective effects of coarse grains, and suggesting that the morphometry of simple transverse dunes may be controlled by the scale of turbulence associated with the threshold wind speed required to move the coarsest fraction of the dune sand.

KEY WORDS Transverse and barchanoid dunes Granulometry Aeolian processes

INTRODUCTION

The Skeleton Coast, that part of the Namib coast between the Ugab and Cunene rivers (Figure 1) was until recently inaccessible and is still almost unknown scientifically save for descriptions of the diamond- bearing raised beach deposits (Hallam, 1964). Little is known about the geomorphology of the area north of the Ugab, as neither Gevers (1936) nor Mabbutt (1952) carried their investigations into this area.

Between Torra Bay (20'20's) and the Hoarusib River (19"s) there is a belt of transverse and barchanoid dunes up to 15 km wide paralleling the coast and 2-5 km inland. Together with other areas of similar dunes to the north, these constitute the northern Namib Sand Sea of Seely (1978).

This paper reports the results of an investigation of the dunes between Torra Bay and Mowe Bay in November 1980, in which studies of the morphology and sedimentary characteristics of the dunes were carried out at a total of 16 sites throughout the dunefield, in order to examine factors influencing aspects of dune morphology.

Regional setting Behind the long straight beaches of the Skeleton Coast a coastal platform 20-40 km wide rises gently

to the broken hills of Damaraland and the Kaokoveld. Over most of the area the platform is cut into schists and occasional granite intrusions of the Damara System, but locally areas of Karoo age basalts

0197-9337/82/060575-13$01.30 @ 1982 by John Wiley & Sons, Ltd.

576 N. LANCASTER

Figure 1. The northern Namib Desert: location map

and related volcanic rocks outcrop, especially along the coast. A number of seasonal and ephemeral streams, notably the Uniab, Hoanib and Hoarusib, cross the platform in shallow valleys. Along the coast are a series of salt pans (sebkhas) and inland raised beaches at altitudes of 4.6, 9.1, 15.2 and 22.9m are found (Hallam, 1964).

From Torra Bay northwards the coastal platform is covered by a discrete belt, up to 15 km wide, of barchanoid and transverse dunes running parallel to the coast and 2-5 km inland. These terminate at the deep valley of the Hoarusib river which floods strongly in most years. Only at its western end do dunes cross the valley as a 1-2 km wide train of individual and linked barchans, which continue a further 25 km north to the Khumib river where they are effectively terminated. North of the Khumib river scattered barchans continue to 18"30'S, dispersing northward and eastward. North of Cape Frio, at 18"s barchan trains and linear sand streaks restart near the coast and develop into a continuous dunefield at 17'40's. From there to the Cunene at 17'15's there is a continuous area of barchanoid and low transverse dunes up to 40 km wide which runs into the Cunene. In Angola further dunes of a similar type continue to the Curoca river at 15'40's.

Climatically the area continues the hyperarid but cool and foggy coastal Namib desert climate, although rainfall is rather higher than in areas to the south. During the period 1974-1977 rainfall at Mowe Bay averaged 22-25 mm yr-', but with an annual variability of 72 per cent. Mean daily maximum temperatures range from 13-24°C and minima from 9-15°C. Fog occurs on 11-25 days per month with a maximum frequency during the period July to October. All year strong onshore south or south southwesterly winds blow. An autographic wind recorder was maintained at Mowe Bay during the period 1973-1977. During this time 98 per cent of the annual sand flow, calculated using the Bagnold formula (Bagnold, 1941), was from directions between south southeast and south southwest. Winds were able to move sand (i.e. blowing at >4 m s-') for 47.16 per cent of the time. Resultant sand flow was from 188" and totalled 369.3 tonnes m-' yr-*. Using Fryberger and Dean's (1979) weighting equation annual total drift potential

DUNES ON THE SKELETON COAST, NAMIBIA 577

is 397 units. Such a wind regime rates as an intermediate to high energy narrow unimodal type. A seasonal pattern in sand flow is apparent, with maximum sand flow in August to October.

GEOMORPHOLOGY OF THE DUNES

Form of the dunefield The general form of the Skeleton Coast dunefield (Figure 2) as visible on Landsat imagery and aerial

photographs is a 165 km long belt of dunes running parallel to the coast and 2-5 km inland, with an area of approximately 2000 km2. Two subsidiary areas of dunes join the main belt northeast of Terrace Bay and Mowe Bay. Immediately north of these points the dunefield widens out from its usual 6-8 km width to 15-20 km. Two ephemeral rivers, the Uniab and Hoanib, cut through the dunefield. Dunes cross the former at several places, but the bed of the Hoanib river is almost completely blocked. During periods when these rivers flood, they are apparently able to remove much of the sand which has blocked their courses. Patches of playa silts west of the present end points of the Hunkab and Samonab rivers testify to periods when they were able to penetrate further westwards.

, I 1;. 13O30’

1 go

Atlantic Ocean

\ \152Iu

19’30‘

2OC

u barchans

prominent p i 9 $

23O30’

t sand streams

Point

.1_

0 10 20 km

Figure 2. The Skeleton Coast dunefield with location of sampling sites. Dunes shown schematically. Position of sand streams were determined from aerial photographs by sand streaks and concentrations of shrub coppice dunes. Map compiled based

on Landsat imagery

578 N. LANCASTER

The dunefield starts quite abruptly as a series of barchans and large compound barchan dunes 2.5-3 km north of a granite outcrop 40-50 m high and 4.5 km across. It seems that these low hills create a disturbance in the south and south southwest winds which move sand inland from the coast, resulting in deposition downwind. Thus a topographic obstacle fixes the southern limits of the dunefield. The northern limit is the deep valley of the Hoarusib river. Like the Kuiseb river which forms the northern boundary of the main Namib sand sea, the upstream parts of the valley are too deep and floods too large and frequent to permit sand to invade and cross them. In both cases the valleys are less incised westwards and floods not as frequent, allowing sand to cross in small amounts.

The western margin of the dunefield is distinctive and is marked by a prominent ‘dune wall’ 20-80 m high with a large slip face to the east. The ‘wall’ is apparently formed by the coalescence of the western arms of crescentic or barchanoid dune ridges. Rather similar features occur on the western side of the Algodones dunes, California and in Libyan and Tunisian sand seas (Breed, 1977) and also along the coastal dunes of the Namib sand sea. Subsequently the ‘wall’ has tended to grow larger and more continuous as it traps sand coming into the dunefield from the coastal plain. Coarse sand is also diverted along the base of the ‘dune wall’ and locally forms a zone of mega ripples up to 30 m wide.

Simple barchanoid and transverse ridges dominate the dunefield with areas of barchans along the eastern and southern margins. Locally, compound varieties of all dunes occur. Sand cover is generally thin except where sand streams join, and the subdune surface of angular wind eroded basaltic gravels is commonly exposed between dune ridges. Dune height gradually diminishes northeastward from the ‘dune wall’ and transverse ridges grade into barchanoid ridges and finally individual barchans. The eastern margin of the dunefield is quite distinct. Dunes become rapidly smaller and ultimately disappear entirely within 500 m to 1 km. Some sand leaves the dunefield as a series of indistinct sand streams, but the eastern margin appears to result from a failure of sand supply for dune building as most of the available sand has been trapped in the dunes upwind.

Varieties of dune identified Comparisons between dunes in different localities are often hindered by the lack of an accepted

classification of dune types. The morphological classification adopted by McKee (1979) provides a good general scheme, and as such is followed by this paper.

The Skeleton Coast dunefield is dominated by varieties of crescentic dunes. Simple crescentic or barchan dunes are common along the eastern margin of the dunefield and locally on its western and southern edges. A few compound crescentic (mega barchan) dunes exist in these areas.

Simple crescentic or transverse ridges are the most widespread dune variety identified. Two subtypes exist: the barchanoid ridge with prominently crescentic slip faces, and transverse ridge with a relatively straight slip face. Northeast of Terrace Bay and Mowe Bay large compound transverse ridges occur, often with a subsidiary linear element perpendicular to the main ridge trend, and frequently with small barchanoid or transverse ridges developed on both windward and leeward slopes.

Low rolling dunes without slip faces, comparable to the ‘zibar’ of Holm (1960) and Warren (1972) are developed locally on the lower slopes of the western ‘dune wall’ and are widespread in the angle between the main and subsidiary sand streams east of Terrace Bay. Mega ripples, equivalent to the ‘granule ripples’ of Sharp (1963), are often associated with these dunes.

In the area between the main dunefield and the coast shrub coppice dunes or nebkha have developed as sand is trapped by bushes of Salsola spp. Linear concentrations of these dunes mark the position of major sand streams supplying sand from beaches to the dunefield.

Characteristics of dune wrieties Barchans. Dunes of barchan type are found mainly along the eastern margins of the dunefield where

they represent the final stages of breakdown of crescentic ridges as sand supply decreases. These barchans, most of which are only 2-5 m high, do not leave the main dune body but frequently travel subparallel to it on an alignment to the left (west) of the barchanoid and transverse ridges.

DUNES ON THE SKELETON COAST, NAMIBIA 579

At the southern end of the dunefield barchans are quite frequent. They can be seen to migrate into and add to the main dunefield. Others miss the main mass of the dunes and continue northwards to the Uniab valley. In this area there are a number of very large (25-30 m high) mega barchans or compound crescentic dunes with multiple small barchanoid slip faces on their upper windward and leeward slopes, similar in many ways to the Pur Pur dune in Peru (Simons, 1956).

A characteristic of many of the barchans is the pronounced elongation of their western horns, which becomes greater towards the main dunefield. In this area at least, it probably results from an asymmetry in sand supply with more sand being available towards the dunefield. Once the horn has become elongated it reinforces a prominent eddy which tends to move sand along the horn even with a wind blowing parallel to the axis of the main mass of the dune. This phenomena is akin to the illustration in Warren (1979) and was observed during a period of strong southerly winds.

Barchanoid and transverse dune ridges. Barchanoid and transverse dune ridges with slip faces on their northeastern sides are the most widespread dune type in the Skeleton Coast dunefield. Barchanoid ridges (Figure 3) with connected crescentic slip faces tend to be more common towards the east of the dunefield where sand supply is less and the subdune surface more irregular, whilst transverse dunes with straight or very gently curved slip faces are more common to the west where sand supply is greater (Figure 4).

All of these dunes have a characteristic profile with a windward slope of 10-12" in its middle part decreasing to 2-3" at the crest, which is often almost flat or gently convex. On the lee side a slip or avalanche face at an angle of 31-34" reaches down to the interdune flats, in places with a small 'toe' at the base. Many dunes, especially those with straight slip faces, have sharp crests at the top of the slip face, but with others the convex crest line lies behind the brink of the slip face. Height of the simple transverse and barchanoid dunes varies from 10-20 m in the western parts of the dunefield to 3-10 m in the east.

Along the western side of the dunefield north and east of Terrace Bay and east and north of Mowe Bay large compound crescentic dunes occur. Individual dunes here are 30-50 m high and are distinguished from larger versions of ordinary transverse ridges by the presence of two orders of dune wavelengths with 2-3 m high barchanoid dunes on their upper windward slope and multiple small slip faces on their leeward sides, In some areas there is a prominent linear element parallel to the dominant sand moving

Figure 3. Small barchanoid dunes on eastern margin of dunefield

580 N . LANCASTER

Figure 4. Larger transverse ridges on western side of dunefield

wind which crosses interdune areas, often enclosing deep hollows and giving an almost rectilinear or akl6 pattern.

Crest to crest spacing of the crescentic dunes as measured from aerial photographs varies with height through the dunefield. The largest dunes, often of compound form, are found along the western side of the dunefield. Here dune spacings are greater than 350m, with compound forms being spaced at 550-700 m apart. Towards the east of the dunefield spacing decreases to less than 200 m and averages 150 m along the eastern margins.

There is a very good correlation ( r = 0.89, significant at the 0.05 and 0.01 levels) between dune height and spacing as Figure 5 illustrates. Similar relationships have been established by Wilson (1973) for Saharan dunes and by Lancaster (in press) for linear dunes in the main Namib sand sea ( r=0*59 , significant at 0.05 level), implying some overall control, probably aerodynamic, of dune morphology.

H: -4 67 + 0.10 r = O W r2= 0.80

0 100 200 300 LOO 500 600 700 Dune spacing Im)

Figure 5. Plot of mean dune height against mean spacing at each site

DUNES ON THE SKELETON COAST, NAMIBIA 581

In terms of their size and spacing most of the dunes of the Skeleton Coast dunefield are much smaller than those in the coastal parts of the Namib sand sea south of the Kuiseb, but apparently similar to barchanoid dunes north of the Kuiseb near Walvis Bay. However, the compound crescentic dunes north of Terrace Bay and Mowe Bay are similar in size and spacing to compound crescentic dunes in the coastal parts of the Namib sand sea. Comparisons with data on dunes of a similar type elsewhere (Breed, 1977), indicates that many of the dunes on the Skeleton Coast are similar to barchanoid ridges at White Sands, New Mexico, whilst smaller barchanoid ridges on the eastern margins of the dunefield compare in terms of overall morphology, size and spacing with dunes at Guerrero Negro, Baja California (Inman et al., 1966). The spacing of the compound crescentic dunes compares well with those in Qatar, the United Arab Emirates and A1 Jiwa area of Saudi Arabia, and the Thar desert near Umarkot, Pakistan (Breed and Grow, 1979).

Alignments of the crestlines of the crescentic dunes (Figure 6) are 280-308" or transverse to a wind blowing from 190-218" (south to south southwest). There are some small variations with crest aligned

1 4TB 0 10 20 km

Figure 6. Variation of dune alignments through area investigated. Compiled from aerial photographs. Arrows indicate winds at 90" to dune crests but parallel to sand streams

582 N. LANCASTER

at 90" to southwest winds in the south and also towards the east of the dunefield. In the northern areas dune alignments tend to be aligned to more southerly winds. These changes may represent slight regional variations in wind regimes with winds being more southwesterly in southern and eastern areas. However there is an alternative explanation in terms of interaction between the dunes and the wind regime in the manner suggested by Warren (1976). Measurements of the alignments of linear sand streaks and shrub coppice dune groups on the coastal plains show that their alignment is consistently parallel to a wind from 180-190". Similarly, barchan dune and sand streak alignments to the east of the dunefield fall between 182-195". Thus dunes and aeolian features outside the main dunefield are aligned consistently to the left of dunes within the dunefield. Comparison of dune alignments with the resultant sand flow direction at Mowe Bay indicates that the sand streams and barchan dunes are aligned with the resultant wind, but the transverse and barchanoid ridges of the dunefield are aligned with a more westerly wind. Rather similar observations were made by Norris and Norris (1961) who found that mega barchans were oblique to the wind, yet small barchans transverse to it. Personal observations of wind directions during the study period also suggest that winds are more southerly on the coast than in the dunefield. These observations seem to confirm Warren's proposition that winds are deflected to the right (in the southern hemisphere) by increasing friction as they pass from the relatively smooth ocean and coastal plain to the rougher surface of the dunes. On the Skeleton Coast they then resume their original direction on leaving the dunes for the smoother alluvial surfaces beyond. Low rolling dunes. Low rolling dunes without slip faces are particularly common in the angle between

the main dunefield and Terrace Bay sand stream. Typically they have a spacing of 100 m and are 1-2 m high. To the west and north dune spacing and height increases until 1-2m high slip faces develop. Particularly in the lower areas these dunes are covered by mega ripples up to 20 cm high and 50-100 cm apart developed in coarse basaltic sand. Elsewhere low rolling dunes are found along the lower slopes of the 'dune wall' and locally in the southern parts of the dunefield.

Shrub coppice dunes. Shrub coppice dunes or nebkha fixed by bushes of Salsola spp are common in the area between the coast and the dunefield. Frequently they are grouped into linear zones up to 3 km wide which mark the position of major sand streams from beaches to dunes. Extensive areas of these dunes are located north of the Koigab river south of the main dunefield; in the vicinity of the Uniab river and between the coast at the mouth of the Hoanib river and the dunes east of Mowe Bay. Maximum height of the shrub coppice dunes appears to be about 3 m, but most are 1 m or less in height. There appears to be no consistent distribution of dunes within shrub coppice dunefields, apart from an overall decrease in height downwind accompanied by an increase in spacing between the dunes as sand supply decreases.

GRAIN SIZE CHARACTERISTICS OF THE DUNES

Sedimentary characteristics of the dunes were studied at a total of 16 sites throughout the dunefield (for locations see Figure 2). At each site surface (0-5 cm) samples were taken from the crest of 3 to 8 dunes and one dune was sampled in detail at the following points, under active sand transport conditions: the crest; middle and base of slip face; and middle and base of the windward slope. Following sieving through a nest of sieves at 0.5 phi intervals, grain size and sorting parameters were calculated from graphical data and are summarized in Table I.

Generally the sands of the Skeleton Coast dunefield are relatively coarse with 67 per cent of all samples having a mean grain size between 1.70 and 2.20 phi (0.31-0-22 mm) with a modal group of 2.00-2-09 phi (0.23-0.25 mm). Considering crest sands from crescentic dunes only, 69 per cent of samples had a mean grain size between 1.80 and 2.30 phi (0.20-0.29 mm). Thus the sands fall towards the coarse side of Ahlbrandt's (1979) sample of aeolian sands. Significantly, they are coarser than most dune crest sands from the main Namib sand sea, which have mean grain sizes finer than 2.30 phi (Lancaster, 1981).

Sorting values, as measured by phi standard deviation, have two modes: the first (57 per cent of all samples) between 0.20 and 0.49phi or very well to well sorted; and the second (23 per cent of all samples) between 0.60 and 0.79 phi, or moderately sorted. Again considering crest sands only, 59 per cent of samples fall in the first modal group and 30 per cent in the second.

DUNES ON THE SKELETON COAST, NAMIBIA 583

Table I. Means of grain size and sorting parameters of dune crest sands at each site (phi units)

Site Mean Standard deviation Skewness Kurtosis

1 2 3 4 5 6 7 8 9

10 11 12 13 14 15 16

2.07 2.27 1-82 2.10 1.67 2.17 2.16 2.07 1.86 2.16 2.07 2.02 2-02 2.07 1.87 1.88

0.46 0.36 0.45 0.26 0.47 0.58 0.52 0.27 0.52 0.46 0.61 0.68 0.29 0.69 0.80 0.66

0.18 0.10 0.28 0.17 0.44 0.24 0.27 0.16 0.30 0.23 0.41 0.40 0.14 0.40 0.35 0.44

0.50 0.49 0-53 0.52 0.57 0.52 0.50 0.51 0.5 1 0.48 0.56 0.49 0.50 0.43 0.49 0.47

For location of sites see Figure 2.

Grain size and sorting patterns in the dunefield Variability in grain size and sorting occurs at three scales in the Skeleton Coast dunefield: on individual

dunes, between dunes of different types in the same area, and from area to area in the dunefield. Across dune variability. The crests of barchanoid and transverse ridges are generally somewhat finer,

and sometimes better sorted than windward slopes. Slip face sands are frequently the best sorted of all those on the dunes, and are often negatively skewed. There is little change in kurtosis values over the dunes. On barchan dunes the crest is again finer than upwind slopes. The outward slopes of both horns and the base of the windward slope are significantly coarser than the rest of the dune.

These grain size patterns can be explained in terms of the movement of sand over the dunes. As sand is moved up windward slopes the coarser sands moved as creep load are steadily left behind, resulting in a fining and improvement in sorting of sands towards the crest. Similar processes also take place on dunes in the main Namib sand sea as described by Lancaster (1981). On barchan dunes, coarser grains are also diverted around the dune base as they take the path of easiest creep movement, and add coarse grains to the outer slopes of the horns. This effect has been noted elsewhere by many workers e.g. Finkel (1959), Hastenrath (1967) and Tricart and Mainguet (1965).

Essentially similar processes appear to take place over low rolling dunes without slip faces formed in poorly sorted sands with a prominent mode at 1-1.5 phi. Windward slopes and hollows between dunes are dominated by very coarse sands and often have mega ripples developed on them. Crests are somewhat finer and lee slopes finer still.

Variability within the dune landscape. In most areas only one dune variety occurs, but there is some evidence to indicate that at site 2 barchan dunes are finer and better sorted (average mean grain size = 2.45 phi, Crl = 0.26) than adjacent transverse and barchanoid ridges (mgs = 2.27 phi, Crl = 0.36).

At sites 6, 7, 9 and 10 much more variability exists and dunes of different varieties exist in close proximity. Here flat or gently undulating sand sheets, often with mega ripples, grade into low rolling dunes without slip faces. Locally these develop into transverse ridges, whilst elsewhere barchanoid dunes can be seen to be moving over zibar sands. These differences in dune types are paralleled by changes in grain size and sorting of the sands from which they are composed, as illustrated in Figure 7.

Sand sheets and zibar are composed of poorly sorted sands with phi standard deviation of >1.00, they are also relatively coarse with mean grain sizes between 1.5 and 2.0 phi. In contrast, crests of transverse and barchanoid dunes are well to moderately well sorted with a phi standard deviation of 0-35-0.56,

5 84 N. LANCASTER

R = rolling dunes S = sand sheet C = crest of

barchanoid or transverse dune

C C

C

0 1.5 2.0 2.5 Mean grain size (phi units)

Figure 7. Plot of mean grain size and standard deviation for dunes of different varieties at sites 6, 7, 9 and 10

and much finer (mgs 2.00-2-30phi). Similar relationships have been noted from areas of adjacent transverse and rolling dunes north of the Tsondab Flats in the Namib sand sea by the author (unpublished data) and the TCnCrC desert by Warren (1972).

The occurrence of dunes of these types in this area is probably a large scale version of the sorting process on barchans described above. Coarse sands are diverted around the margins of the dunefield and concentrated in lower parts of the dune landscape. This process also occurs in the southern parts of the dunefield where the base of the ‘dune wall’ is composed of very coarse sand. Coarse sands are also widespread on the flanks of the large compound crescentic dunes which mark the start of the dunefield.

Variarion ouer the dunefield. Mean grain size of the crest sands of transverse and barchanoid ridges varies through the dunefield (Table I). There is no consistent fining trend from south to north. Rather sands become somewhat finer from southwest to northeast across the dunefield. Similarly, sorting tends to improve in a similar direction. In addition, dunes are generally better sorted at the southern end of the dunefield than at the northern.

This pattern can be explained in the following way. Although the dunefield runs parallel to the coast, it extends obliquely to the direction of sand supply to it. Sand is supplied to the dunefield from the coast throughout its length, but principally at three places: east of Torra Bay where the dunefield starts; east of Terrance Bay from two sources, the beaches southeast of Terrace Bay and from those south of the Uniab river mouth; and the beaches south of Mowe Bay at the mouth of the Hoanib river. At each place the dunefield receives a new input of relatively coarse moderately sorted sand. Dune sands downwind of these are correspondingly coarse and moderately sorted. Most of the coarse sands are trapped in the dunes upwind and to the west, and finer sands move downwind to the eastern side of the dunefield.

Relations between sand gram size and dune morphology As outlined above, dune height and spacing vary together in a systematic way through the Skeleton

Coast dunefield. It often seems that there is a correlation between areas of relatively coarse and often less well sorted sands and the larger and more widely spaced barchanoid and transverse dunes along the western side of the dunefield, and also between fine sands and small closely spaced barchanoid dunes in the east.

DUNES ON THE SKELETON COAST, NAMIBIA 585

600

_ _ _

A number of other workers have made similar associations. Bagnold (1941, p. 220) observed ‘that in general an absence of coarse grains on the floor is associated with a close packing of the barchans, and a plentiful supply with a large spacing’. Finkel (1959) noted that ‘particle size analysis indicates a somewhat finer texture in smaller dunes’. In the southern Namib, Rogers (1977) found that small barchans were finer (average mean grain size = 3.00 phi) than larger ones (average mean grain size = 2.55 phi). Wilson (1972) refined these observations to show that dune wavelengths were correlated with the grain diameter of the coarse 20th percentile; and that spacings were a function of the wind speeds necessary to move the coarse sand on a dune, which had the effect of ‘protecting’ finer sands from movement.

On the Skeleton Coast, the average size of the coarsest fraction of the sands is approximated by the grain size of the coarse 5th percentile. To test Wilson’s hypothesis and also to seek an explanation for dune size and spacing variations in the dunefield, the spacing of dunes at each size was plotted against the mean values of the median grain size and of the 5th percentile. There was no statistically significant correlation between median grain size and dune spacing, but a highly significant correlation (at the 0.05 and 0.01 levels) between the phi grain size of the 5th percentile and dune spacing ( r = -0.65) as shown in Figure 8.

This provides the first independent confirmation of Wilson’s hypothesis on the granulometric control of dune spacings and strongly suggests that the protective effects of coarse grains play a major role in determining dune morphometry particularly in simple situations, by controlling the effective threshold wind velocity for general sand movement.

Origin of the dune sands Composition of the dune sands. Preliminary studies of the composition of the dune sands indicates that

they are composed of four types of grains: subangular to angular clear quartz; subrounded frosted quartz, often with a significant haematite patina; subrounded often slightly platy garnets, and subangular to subrounded rock fragments, mostly basaltic, but also including schists, quartzites and granites.

Proportions of angular clear quartz decline rapidly from southwest to northeast and most sands are dominated by subrounded to subangular quartz, becoming more frequently frosted and reddened east- wards. Beach sands and those in shrub coppice dunes on the coastal plain are very pale grey (10YR

X

s = l O L l - 559 6 gs r I -0.65 r2= 0 4 3 \ n = 1L

7001

I 5 7

a

D 3001

2ool 100.

\ ”

\ X X

\ X

x \

X

x \

”. 05 1 0 1.5 2 0

Groin size of 5th percentile (phi units)

Figure 8. Plot of phi grain size of 5th percentile of crests of transverse and barchanoid dunes against dune spacing. Note: slope regression line is negative as phi units used for grain size

5 86 N. LANCASTER

7/2), whilse most sands in the western and central parts of the dunefield are very pale brown (10 YR 7/3, 7/4) and those along the eastern edge are light yellowish brown (10YR 6/4). Garnet-rich sands are common in eastern areas, giving strikingly pink dunes. All dune sands contain varying proportions of rock fragments, mostly coarser than 1.5 phi. These are mostly derived from deflation of adjacent areas of the coastal plain and gravel interdunes. This is confirmed by the close links between the lithology of the rock fragments on the dunes and adjacent interdunes. For example, many dunes contain fragments of granite in the area northeast of Torra Bay, and the adjacent coastal plains here have widespread granite outcrops. Only in the areas northeast of Terrace Bay does it seem that the rock fragments in the dune sands are derived from beach sands. South of Terrace Bay all the beach sands contain up to 10 per cent fragments of basalt, presumably derived from the Uniab river, and these are ultimately responsible for the widespread low rolling dunes and mega ripples east of Terrace Bay.

Relationships between dune and beach sands. Beach sands were sampled from areas upwind of the main sand streams leading to the dunes. Commonly they are medium sands, moderately to moderately well sorted and frequently negatively skewed. The sands are composed of subangular to angular clear quartz, often showing prominent crescentic fracturing, with a significant (up to 10 per cent) content of garnets, especially in northern areas. Sands southeast of Terrace Bay also contain up to 10 per cent basaltic rock fragments.

By comparison, dune sands are somewhat finer than the beach sands, rather better sorted and positively skewed. Sand sampled from the shrub coppice dunes near the beach sand is intermediate in composition, but downwind closely resembles a dune sand.

The beach sands themselves are probably derived from the deposits of the ephemeral streams which reach the coast, and have often been moved some distance by longshore drift resulting from the strong onshore south and south southwest winds. Most of the sand probably comes from the larger rivers, notably the Huab and Ugab, with smaller contributions from the Uniab and even from as far south as the Omaruru river. Significantly, the Huab river also drains an area of the aeolian Etjo sandstone of Karoo age.

Thus the accumulation in its present position of the Skeleton Coast dunefield is perhaps best explained by a combination of suitable coastal configurations relative to onshore sand moving winds, together with an abundant supply of sand from these ephemeral rivers. Further south, supply of sands to the beaches is probably insufficient to permit sustained deflation and the transport inland of enough sand for dune formation. In addition much of the coast in this area is backed by a cliff 3-5 m high, so inhibiting sand movement away from the beach.

This finaly balanced combination of coastal configuration and sediment supply is probably a major reason for the absence of dune development on the coast between Swakopmund and Torra Bay, and explains the gap of 250 km separating the southern and northern Namib sand seas.

CONCLUSIONS

The Skeleton Coast dunefield is made up of transverse and barchanoid ridges and barchans of simple and compound forms. Dune spacing and height vary together over the dunefield and are in turn correlated with the grain size of the coarse 5th percentile of the sands composing them. This confirms Wilson’s (1972, 1973) hypothesis of the role of grain size in determining the size of aeolian bedforms and implies an overall aerodynamic control of dune morphometry which is particularly strong in the case of simple transverse forms. Further work is now needed to observe the structure of winds over dunefields to determine the nature of interactions between winds, sands and dunes.

ACKNOWLEDGEMENTS

I thank the Division of Nature Conservation, South West Africa Administration for permission to work in the Skeleton Coast Park and for facilities at Gobabeb. The C.S.I.R. and Transvaal Museum are further thanked for financial support.

DUNES ON THE SKELETON COAST, NAMIBIA 5 87

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