CATENA Vol. 8,369-382 Braunschweig 1981

FORMATION OF THE HOLOCENE LAKE CHILWA SAND BAR

Southern Malawi

N. Lancaster, Walvis Bay

SUMMARY

A sand ridge up to 25 m high separates Lake Chilwa, Malawi, from its former Indian Ocean outlet. Consisting of beach sands capped by dunes, the sand bar was formed by the growth of spits across the northern end of the lake. Mineralogical studies of the sands indi- cate that they were derived from northern and western areas of the Chilwa basin and were brought to the lake during a period of increased runoffand rainfall intensities. The formation of the sand bar in the early Holocene represented a radical change in the hydrology and biolo- gy of Lake Chilwa, from an open fresh water lake to a closed saline lake with a marked ten- dency to fluctuate in level.

I. INTRODUCTION

East of the main Rift Valley in southern Malawi lies a shallow NE - SWtrending down- warp. In the southern part of this, at an altitude of 622 m, lies Lake Chilwa, a shallow saline lake. Today Lake Chilwa has an open water area of some 700 krn 2 which is surrounded by ex- tensive reed swamps and seasonally inundated grasslands. The northern part of the depres- sion is occupied by Lake Chiuta, a small shallow freshwater lake. Lake Chiuta, roughly trian- gular in shape, is linked by a shallow channel to Lake Amaramba in Mozambique. In turn, Lake Amaramba is the source of the Lugenda river, which leaves it by a narrow incised chan- nel and flows northeastwards to join the Rovuma river, which enters the Indian Ocean near Mtwara (Fig. 1).

Separating Lake Chilwa from the northern part of the depression and its Indian Ocean drainage is a prominent ridge of sand, generally known as the Chilwa sand bar, which rises up to 25 m above the seasonally inundated grasslands north of the lake and the swamps border- ing Lake Chiuta.

Evidence from two main sources indicates that, until the late Pleistocene, Lake Chilwa was an open lake, draining to the Indian Ocean via the Lugenda river. KIRK (1967) first described the close affinities of the fish fauna of Lake Chilwa with that of rivers in Mozam- bique draining to the Indian Ocean. At the same time, he contrasted the Chilwa fauna with that of the Lake Malawi-Shire system. TWEDDLE (1979), in a more detailed study, noted that all 36 species offish found in lakes Chilwa and Chiuta were also present in the Rovuma or neighbouring rivers in Mozambique.

The biological evidence is supported by that from high level shorelines in the Chilwa basin. Prominent shorelines at 637, 631 and 627 m or 15, 9 and 5 m above mean lake level have been mapped (Fig. 2) and described by the writer (LANCASTER 1979). At the 637 m stage Lakes Chilwa and Chiuta formed a single open lake draining to the Indian Ocean. However, by the 631 m stage Lake Chilwa had become a closed lake, separated from its out-

370 LANCASTER

Indlon

Ocean

io °

260 3o0 km

15 °

Fig. 1: Location map: Lake Chilwa in relation to the inland waters ofeastern Central Africa.

let by the formation of a sand bar across its northern end. The origins of the sand bar are thus of considerable significance in the geomorphological evolution of the Chilwa basin. They al- so mark a radical change in the hydrology and biology of Lake Chilwa, from an open fresh- water lake to an enclosed saline lake, with a marked tendency to fluctuate in level. This paper describes the geomorphic and sedimentary character of the Chilwa sand bar, as a basis for an understanding of the events leading to its formation and the enclosure of Lake Chilwa.

2. TOPOGRAPHY OF THE SAND BAR

The Chilwa sand bar (Fig. 3) extends for 30 km from the Nkande river to Nayuchi on the Malawi-Mozambique border. Related beaches and low sand dunes extend for a further 10 km southwestwards to the Sumulu river. It consists of a hummocky sand ridge varying in width from 100 m to 1 km and rising to 25 m above the north Chilwa plains. The vegetation of

HOLOCENE LAKE, SOUTHERN MALAWI 371

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o

oj to

637rn~

~,o~'~ 627 m

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627m

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Fig. 2: High level shorelines and associated features ofthe Chilwa basin.

372 LANCASTER

N / N.

J ~ ~ w a s h o v e r .

'1~, ~ f ~A D ~ .

f deltas grass land 2 /

/

~ eita

\ 631 m Shoreh'ne

/ g

I

/ [ Reed ~

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Fig. 3: The sand bar: topography and relationship to drainage system ofthe northern Chilwa basin. A- D location oflevelled profiles of Fig. 4

the sand bar consists of grassland with Terminalia sericea scrub and low trees, although much of this has been cleared for agriculture. On the southern side of the sand ridge a gently sloping bench or terrace up to 100 m wide at an altitude of 631 - 633 m, extends most of the way along the bar. From its altitude, sediments and general form this can be correlated with the 631 m shoreline found elsewhere in the basin. Only one river, the Mikoko, breaches the sand bar, cutting through the dunes and the laterites underlying them in a narrow valley. All other streams, some of which, like the Mpili, carry a considerable wet season flow, have been forced to back up behind the sand bar and have formed extensive areas of swamp.

The sand bar can be divided into three sections. In the west a series of undulating tongues of sand, 500 - 800 m wide and up to 2 km long, extend north into the marsh and swamp zone between the sand bar and Lake Chiuta. The crests ofthese sand accumulations

HOLOCENE LAKE, SOUTHERN MALAWI 373

rn 15

0

Nocth South

~ ~ A

631 m

627m

15

1

15

1

~627m

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rn 10

0 _ _ ~ B27 m

0 100 200m

Fig. 4: Levelled profiles across the sand bar showing 631 m and 627 m shorelines. For locations see Fig. 3. Vertical exaggeration 5x.

rise to 645 m, or 23 m above the mean level of Chilwa. Between these sand accumulations the ridge narrows to a width of 100 - 200 m and is much lower at 638 - 640 m (Fig. 4a). The 631 m beach reaches its maximum width of 300 m in this section of the bar.

The central section ofthe bar extends for 12 km from east of Namanja to the vicinity of Likhonyowa. In this section the sand bar becomes generally wider and more continuous. Two, and locally three, sand ridges can be identified. The southermost is the highest (640 - 642 m) and the most continuous. As the profiles show (Fig. 4b) it is asymmetric, with steeper slopes on its southern side. The other sand ridges are much lower and very hummocky in form. Locally they degenerate into a series of blowouts and subparabolic dunes. Towards the east the main sand ridge becomes much wider and merges with the subsidiary ridges to form one broad sand ridge (Fig. 4c). The 631 m beach in this section is well developed, with a width of 100 - 200 m, as Fig. 4b shows. Its southern margins are marked by a break of slope at about 627 m below which it slopes gently into the clayey plains, with low sandy ridges, which lie on the northern side of Lake Chilwa (Photo 1). Approximately half way along its length, the character of the sand bar begins to change. The hummocky sand accumulations to the north of the main sand ridge become much lower and narrower and are replaced by a zone up to 2 km wide of sandy clays and clayey sands, which grade into the Chiuta marshes.

East of Likhonyowa the eastern section of the bar begins. The main sand ridge declines in height to 635 - 636 m and narrows to a width of 100 - 200 m. The distinct beach on its southern side disappears and is replaced by a slight break ofslope at about 631 m, as shown by Fig. 4d. The break of slope at 627 m becomes much more prominent and to the east be-

374 LANCASTER

Photo 1: View east along sand bar, dense vegetation marks position of 631 m beach.

comes a gently sloping terrace of clayey sand at 627 - 629 m. North of the bar in this section is an extensive area of sandy clays and clayey sands. By the Mozambique border the sand bar is further reduced in height to 633 m, barely 7 m above the Chilwa plain on its southern side.

3. COMPOSITION OF THE SAND BAR

The composition of the sand bar was studied by collecting pairs of surface samples from dune and beach at 1.5 - 2 km intervals along the bar. In addition auger holes and pits were put down along the levelled profiles to study the relationship between beach and dune sands. Sorting and size parameters were calculated using the graphical methods of FOLK & WARD (1957). Some additional data on the mineralogy of the sands was obtained from investi- gations made by the Malawi Geological Survey Department (DAWSON 1970).

The sedimentary evidence supports the morphological evidence and confirms that the sand bar is a composite feature made up ofwo groups of related but quite distinct sands, as Table 1 and Figs. 5 and 6 indicate.

In the central and western sections of the bar the crest and areas to the north ofthe main ridge are composed of subangular to subrounded medium sands, with a mean grain size of 1.70 to 1.90 phi, with local areas of finer sands with a mean grain size of 2.10 to 2.20 phi. The sands are moderately sorted, with a standard deviation of 0.50 to 0.70. The majority of sam-

HOLOCENE LAKE, SOUTHERN MALAWI 375

Tab. 1: COMPARISON OF SORTING AND SIZE MEASURES FOR BEACH AND DUNE SANDS

West East

Sample Number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

Beach

Mean 0.64 0.88 1 .22 1 .55 0 .93 0.90 1 .27 0.77 1 .13 1 .80 1 .45 1.53 1.25 1 .58 1 .50 1.63

Standard

Deviation 1.18 0.94 1 .00 0.90 0.72 0.84 1 .47 0.75 0 .54 0.70 0.55 0.56

Skewness-0.01 -0.32 -0.02 -0.10 -0.09 0.20 -0.12 0.06 -0.13 -0.06 0 .11 0.20

Kurtosis 0.92 0 .93 0.99 1.00 1 .16 1 .50 1 .64 0.96 0.98 1.21 1 .64 1.43

Dune

Mean 1.73 1 .73 1 .77 1 .58 1 .58 1.32 1.71 1 .82 1 .82 1 .65 1 .85 1.77

Standard

Deviation 0.67 0.64 0 .61 0.59 0.44 0.72 0.53 0.58 0.60 0 .51 0.63 0.59

Skewness 0.15 0.43 0.34 -0.14 0.22 0.07 0.48 0.38 0.19 0.36 0.17 0.29

Kurtosis 1 .13 0.68 0.84 1 .23 1 .19 1 .10 1 .27 0.89 0.78 1 .09 0.86 0.89

0.92 0.91 0 .41 1.15

0.16 -0.34 -0.18 -0.27

1.10 0.88 1 .27 1.13

1.77 1 .70 1 .90 1.32

0.71 0.68 0.64 1.08

0.14 0.10 0.07 -0.02

1.04 1 .28 1 .04 1.10

pies were quite strongly positively or fine skewed, with phi skewness values of 0.02 to 0.50: a common characteristic of dune sands. Little significant variation in grain size or sorttng para- meters was detectable along the length of the main section of the bar, although in the eastern section mean grain size tends to decrease somewhat and sorting becomes poorer, as a result of a significant admixture of fines. There is little variation of the dune sands with depth: in the centre of the bar three auger holes showed no significant variation in grain size or sorting parameters to depths of 5 m.

All samples contained quite a significant proportion of heavy minerals, generally 2 - 5%, with the percentage tending to decrease eastwards, but in an irregular fashion. Some 70-80% of the heavy mineral content consisted of magnetite, which was particularly abundant in the 3 - 4 phi size fraction. Zircon was quite common in many samples, with rare rutile and occasional garnet.

The area to the south of the main sand ridge forming the 631 m beach is composed generally of coarse angular to subangular sand with a mean grain size ranging from 0.64 in the west to 1.65 phi in the east. There is a much wider range of sorting values, from 0.40 or well sorted, to 1.20 or poorly sorted. Sorting tends to improve in an easterly direction, with samples in the central section of the bar tending to be quite well sorted, and samples in the western section tending to be poorly sorted to moderately sorted. In the eastern section of the bar the 631 m beach becomes much less well marked, and at the same time the sorting of sands associated with it deteriorates sharply. Most samples are near symmetrical or negatively (coarse) skewed, with skewness values of 0.06 to -0.35. Frequently the beach sands have much higher kurtosis values than adjacent dune sands.

Auger holes and pits dug into the 631 m beach show that the beach sands become coar- ser with depth. As mean grain size increases, so sorting becomes poorer, mainly as a result of the addition of very coarse ( > -1 phi) particles. Some of the coarsest sands are in fact bimodal, and contain 10 - 15% of small pebbles, often very well rounded, suggesting de- position under conditions of quite high wave energy. However, in general it seems that dur- ing the formation of the 631 m beach the supply of very coarse sediment declined rapidly, as very coarse particles are absent in all but basal samples.

At the western end of the sand bar the beach sands contain up to 20% feldspar, with the percentage declining rapidly eastward to 2 - 3%. As with the dune sands, the dominant heavy

376 LANCASTER

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100 90 80 70 60 50

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0

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100' l 90' 80 70 60' 50' 40 . ~ 30' 20' 10

' 0

0 1 2 3 4 phi -1 0 1

f I 3 4 phi

Fig. 5: Grain size distributions of related beach and dune sands. Located on profiles of Fig. 4.

HOLOCENE LAKE, SOUTHERN MALAWI 377

1-0

0"5

0.0 0"5

S,tandard dev,ahon

X

• O 0

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• beach

X dune

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× x x,-,x

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Skewness

Mean

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X • X x

X X X

X × X

• X

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0"5 1"0 1"5 2"0 Mean

Fig. 6: Scatter plots of mean grain size against standard deviation and skewness to show relationship between beach and dune sand.

378 LANCASTER

Tab. 2: HEAVY MINERAL ANALYSES OF SANDS (after DAWSON 1970)

Sample Nos. DAWSON N.L. equivalent (Table 1)

Magnetite Zircon Rutile Garnet Amphibole and Pyroxene Total heavy minerals

AD13 AD12 ADll 3 5 8 % % %

1.37 0.67 1.23 0.04 0.04 0.32 0.04 nil 0.01 1.07 0,05 0.01 0.08 0.02 0.01 2.60 0.78 1.58

mineral is magnetite, concentrated in the very fine sand fraction. Again, the heavy mineral content of the beach sand declines eastwards, from 3 - 4% to 1 - 2%. Generally the beach sands become more rounded towards the east. In the western parts of the sand bar the beach sands are commonly angular to subangular, but in the central section they are subangular to subrounded.

Auger holes indicate that the coarser members of the beach sands pass below the dune sands on the southern side of the main sand ridge, but no beach sands were intersected below the crest of the ridge, and they appear to reach a maximum height of 633 m. The upper parts of the 631 m beach are covered by a thin layer of dune sand, probably washed down from the main ridge. The bulk of the central and western sections of the sand bar are thus composed of dune sands, which pass below the wamp deposits to the north. However, in the eastern sec- tion the capping of the dune sand is much thinner, and in many places the area north of the main sand ridge is composed of sandy clays and clayey sands, deposited in a series of fans ex- tending north into the marshes. The general appearance of these features suggests that they are washover fans, formed by waves breaking over the crest of the beach and depositing material in the slack waters beyond.

4. SOURCE OF THE SANDS MAKING UP THE SAND BAR

The considerable amount of quartz sand making up the sand bar implies a high movement of coarse quartz rich sediments to the lake during the time of its formation. However, modern river sediment loads appear to be dominated by silt and clay size particles, derived from the deeply weathered regolith of the Chilwa basin.

Most of the Chilwa basin is underlain by metamorphic rocks of the Malawi Basement Complex, with intrusive igneous rocks of the Chilwa Alkaline Province forming the prominent mountain masses ofZomba-Malosa, Mulanje and the Chikala Hills (BLOOM- FIELD 1965). The majority of the Basement Complex rocks are charnokitic gneisses and granulites containing little or no quartz. The pulaskites and nepheline syenites making up the Chikala Hills similarly do not contain quartz. The only quartz rich rocks in the northern part of the Chilwa basin are the quartz syenites of the eastern part of the Malosa mountain and the quartz plagioclase granulites and gneisses of the Makongwa escarpment area. According to BLOOMFIELD (1965) the Malosa quartz-syenites contain up to 10% quartz. However, sediments from this source would have had to travel a considerable distance to become incorporated in the sand bar, and it is considered that most of them would have been

HOLOCENE LAKE, SOUTHERN MALAWI 379

incorporated in the large sand bar which occurs between the Domasi river and the Chikala Hills.

In view of this, the quartz plagioclase gneisses and granulites of the Makongwa escarp- ment zone probably represent the major source of the sands of the safid bar. BLOOMFIELD (1965) describes these rocks as containing 25 - 400/0 quartz. They weather to a loose very san- dy regolith, which is easily eroded and transported by the many streams which drain from this area to the northwest shores of Lake Chilwa. The larger streams, particularly the Mikoko and Sumulu, have built large, probably paleoform, deltas into the grasslands surrounding Chilwa, indicating a former high rate of sediment transport.

Examination of heavy mineral data for these rocks in BLOOMFIELD (1965) (see table 2) tends to support these hypotheses, Magnetite is the principal heavy mineral of the suite found in the sand bar. This is not surprising, as it is a common dark mineral in all the rocks of the basin. However, the quartz plagioclase gneisses contain particularly abundant magnetite (in some cases 2 - 3% of the rock mass). Pink zircon is also a common heavy mineral, and is the second most abundant in the sand bar suite. Ruffle is a much rarer component of both suites.

It can thus be concluded that the bulk of sands making up the sand bar were derived, via the Sumulu and Mikoko rivers, from the Makongwa escarpment area northwest of Lake Chilwa.

5. FORMATION OF THE SAND BAR

The morphology and composition of th~ Chilwa-Chiuta sand bar indicates that it is comparable to the barrier beaches or islands found in many gently sloping coastal plains throughout the world. Various hypotheses, summarised by HOYT (1967), have been proposed to explain formation of such features. Generally they form by the building of a ridge of sand at the high water mark by waves and wind action on coasts of moderate to low swell intensity. Submergence, due to rising water levels creates a lagoon behind the sand ridge which then forms a barrier island offthe actual coastline. Conversely, emergence of the shoreline or lowering of water levels results in the abandonment and "fossilisation" of the barrier system. It is the latter which has taken place in the Chilwa basin.

Although barrier beaches are common features of lowland coasts today, comparable examples from lacustrine environments are relatively rare. However, they do occur in some of the larger paleolake basins of Africa, where they often define the position of former shorelines. One of the most impressive is the Bama Ridge, described by PIAS & GUICHARD (1960) and GROVE & PULLAN (1964) which runs for 400 km along the southwestern margins of a much enlarged Lake Tchad. In Botswana, the Gidikwe Ridge stretches for 180 km along the western side of the Makgadikgadi depression; and similar features occur southwest of Lake Ngami and in the Magwikwe depression (COOKE 1980). They mark the shoreline of a massive 60,000 km 2 paleolake in the area.

Extensive systems of beach ridges have been described by BUTZER (1980) from the Kibish lake plain north of Lake Turkana. They have a relief of 5 - 10 m and are commonly 10 - 20 km long. In terms of their composition and morphology these beach ridges are very similar to the Chilwa sand bar with beach sands in lower areas and aeolian reworking of their crests.

Most barrier beaches form by piling up of sediments derived from the offshore zone by wave action, although they may form from spits and bars generated by longshore sediment

380 LANCASTER

transport, as originally proposed by GILBERT (1885). It is this latter mode of formation that appears to be most applicable to the Chilwa-Chiuta sand bar. This is confirmed by the grain size and sorting measures for the 631 m beach which show a movement ofsand from west to east. Heavy mineral and feldspar contents of the beach sand similarly decrease from west to east, indicating transport in an easterly direction.

The Chilwa-Chiuta sand bar can be seen as having been formed by the eastward movement of material along the northwest side of the lake and across its northern end by longshore movements generated principally by southeasterly winds. The souce of sands was primarily the Sumulu and Mikoko rivers, draining the northern Chikala Hills and the Makongwa escarpment areas.

A series of spits extended across the former open lake from the vicinity of the Mikoko river. The alternation of narrow ridges with wide sand accumulations probably marks a series of points where these spits were recurve& As the spits extended eastwards so they recurved more frequently forming a more continuous sand bar. Wind action on the beaches, exposed perhaps by seasonal lowering of water levels, winnowed out fine sands to accumulate on their upper sides as low foredunes. Towards the eastern side of the lake, wave energy tended to diminish, in the lee of the eastern shoreline of the lake, as did sediment supply, and so the bar becomes lower. With the contraction of the outlet of Lake Chilwa washover features for- med as the growing bar was locally overtopped by waves. Behind the spits, rivers such as the Mpili deposited clays and clayey sands in lagoons behind the sand bar.

6. DISCUSSION AND CONCLUSIONS

The events leading to the formation of the sand bar and the closing of Lake Chilwa's out- let to the Indian Ocean represent a significant change in the sedimentary environment of the basin. At the 637 m stage of the lake the shorelines are mainly wave cut notches and cliffs in laterites and colluvial material. Locally, as in the Chikala Hills, the shoreline is cut into bed- rock. Sand formations and sandy features are rare or absent. In most localities the best deve- loped shorelines face northeast, suggesting formation by winds from that direction.

However, at the 631 m stage the character of the shoreline is substantially different. In every part of the basin the 631 m shoreline is marked by sandy beaches, spits and bars (Fig. 2). These are best developed on the western side of the lake, from the Likangala river northwards. Southeast of Mpyupyu Hill lie a series ofrecurved spits composed of coarse feld- spathic sands, fining upwards into rather muddy sands. The 631 m shoreline is continued by a series of discontinuous barrier beaches as far as the Domasi river. Between here and the Chikala Hills is a near continuous barrier beach of coarse sands with a high content of feld- spars, which merges with the promontary at the end of the Chikala Hills. North of this is a wide sandy area of coalesced beach ridges which continues to the Sumulu river, where the sand formations associated with the sand bar commence. These sand formations clearly im- ply a much increased supply of sandy sediments to the lake, which is confirmed by the dominantly sandy composition of the river terraces in the Domasi river rally which correlate with the 631 m stage of the lake. 14C dates (Pta 2749 : 9140 + 80 BP and Pta 2750 : 5660 + 1100 BP) from calcareous lacustrine clays at Nchisi Island indicate that Lake Chilwa was at this level from just prior to 9,000 BP to sometime after 6,000 BP. It would appear therefore that the 631 m stage of Lake Chilwa coincides with the widespread early to mid Holocene period of high lake levels in intertropical Africa recognised by STREET & GROVE (1976, 1979). During this time Lake Rukwa, 800 km north of Chilwa reached a level 50 - 60 m above

HOLOCENE LAKE, SOUTHERN MALAWI 381

its present one (BUTZER et al. 1972, GROVE, personal communication). Unfortunately, nothing is known about the state of Lake Malawi during this period. Evidence from further south is patchy, but HEINE (1980) suggests that the northern Kalahari was sub humid dur- ing the period 9,000 - 8,000 BP, with intermediate lake levels in the Makgadikgadi depres- sion. This suggests that Lake Chilwa was close to the southern margins of the zone of high lake levels and much increased precipitation.

Paleohydrological estimates (LANCASTER 1979) suggest that a rainfall 135 - 1400/0 present amounts would have been necessary to support Lake Chilwa at the 631 m level, assuming present day evaporation rates. This implies a 2.6 fold increase in runoff and inflow to the lake. It was this much increased runoff, promoted probably by greater rainfall intensi- ty, which was responsible for the increases in production of sandy sediments in the basin.

From personal experience, years of above average rainfall in Malawi are accompanied by much higher precipitation intensities. There is evidence to suggest, that in high relief areas, this leads to widespread slope instability, landslide formation and increased sediment production (TALBOT-EDWARDS 1948, SHRODER 1976), although the effects of vegetation disturbance by man must not be discounted.

The sandy sediments were then concentrated by wave action generated by southeasterly winds towards the northwest comer of the lake, where they were built into a series of sand spits and bars. Wind action during seasonal falls in lake level winnowed out the finersands and deposited them on top of the spits. During the duration of the 631 m stage of Lake Chilwa the spits built out across its northern end, finally isolating it from its outlet to the Indian Ocean. The subsequent fall in lake level after about 5,000 BP ensured isolation of the lake, and promoted development ofa specialised biota, adapted to periods of seasonally low lake levels and periodic partial or complete dessication of the lake.

ACKNOWLEDGEMENTS

The research on which this paper is based was supported by grants from the University of Malawi, Research and Publications Committee. The 14C determinations were carried out by Dr. J.C. Vogel, National Institute for Physical Research, C.S.I.R., Pretoria.

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382 LANCASTER

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Anschrift des Autors: N. Lancaster, Desert'Ecological Research Unit, P.O. Box 953, Walvis Bay, 9190, South West Africa/Namibia.