CATENA Vol. 10, 281-283 Braunschweig 1983

N. LANCASTER: FORMATION OF THE HOLOCENE

LAKE CHILWA SAND BAR

A DISCUSSION

P. Shaw, Gaborone 1L Crossley, Zomba

We refer to the paper by Dr. N. Lancaster entitled "Formation of the Holocene Lake Chilwa sand bar", published in CATENA, Volume 8, pp 369-382. Although we are in agreement with Dr. Lancaster on the morphology and formation of the Chilwa sand bar, the dating of events in the Chilwa basin suggested in the conclusion (pp 380-1) required c(itical reappraisal.

There is no doubt that the Chilwa sand bar was formed during a period of prolonged, high energy wave action at or about the 631 m.a.s.l, lake level. To conclude that such a level was maintained during the early to middle Holocene by comparing Lake Chilwa to Lake Rukwa (BUTZER et al. 1972) and other lakes in intertropical Africa (STREET & GROVE 1976, 1979) is risky, for, as LANCASTER(1979, 49) himself points out, Lake Rukwa is out of phase with Lake Chilwa at the present time. Correlation with the northern Kalahari is even more tenuous, for the intermediate lake levels in the Makgadikgadi basin are still open to de- bate (see HEINE 1978, 1979, COOKE 1979). It would be convenient for Lake Chilwa to fit the emerging patterns of high lake levels in Africa, but the lake is geographically remote from comparable water bodies, and more substantial evidence is required.

In this context the C 14 dates quoted for Nchisi Island (Pta 2749:9140 + 80 B.P. & Pta 2750:5660 + 1100 B.P.) are misleading in two respects. First, Nchisi Island is composed of Chilwa Series carbonatite complex rocks (GARSON 1960), providing a rich supply of cal- cium carbonate to the surrounding marshes. The chances of obtaining a reliable date for cal- crete-type deposits in such an environment are remote indeed. Furthermore, the three thou- sand year interval between the two dates must be arbitrary.

Second, the C 14 saples in question were collected from the lacustrine fiats to the west of Mkumbira Village (LANCASTER, personnal communication to Crossley), itself sited on the 631 metre raised beach. Not only do the samples represent shallow, still water deposition, a different sedimentary facies from the beach, but they also occur at a lower altitude. Without stratigraphic correlation it is difficult to see how the samples can date the formation of the beach.

The model of a high lake level in the early Holocene, succeeded by fluctuating lower lake levels upto the present day, is oversimplified in the case of Lake Chilwa. Further research (SHAW, CROSSLEY& DAVISON-HIRSCHMAN, in preparation) indicates that the 631 metre lvel has been obtained, with associated high energy wave action, as late as the late Iron Age. Further research into the evolution of the Lake Chilwa system is neces- sary.

282 SHAW & CROSSLEY: A DISCUSSION

REFERENCES

BUTZER, K.W., ISSAC, G.L., RICHARDSON, J.L. & WASHBORN-KAMAU, C.K. (1972): Radio- carbon dating of East African lake levels. Science, 175, 1069-1076.

COOKE, H.J. (1979): K. Heine: Radiocarbon chronology of the late Quaternary lakes in the Kalahari, Southern Africa: A discussion. CATENA, 6, 107.

GARSON, M.S. (1960): The geology of the Lake Chilwa area. Geol. Surv. Nyasaland Bull. 21 Govern- ment Printer, Zomba. 157 pp.

HEINE, K. (1978): Radiocarbon chronology of late Quaternary lakes in the Kalahari, Southern Africa. CATENA, 5, 145-149.

HEINE, K. (1979): Reply to Cooke's discussion of: K. Heine: Radiocarbon chronology of late Quater- nary lakes in the Kalahari, Southern Africa. CATENA, 6, 259-266.

LANCASTER, N. (1979): The changes in the lake level. In: Kalk, M., McLachan, A.J. & Howard- Williams, C. (eds.), Lake Chilwa: Studies of change in a tropical ecosystem. Monographiae Biologicae, 35. Junk, The Hague, 41-58.

STREET, F.A. & GROVE, A.T. (1976): Global maps of lake level fluctuations in Africa. Nature, 261, 385-390.

STREET, F.A. & GROVE, A.T. (1979): Global maps of lake level fluctuations since 30,000 yr BP. Quaternary Research, 12, 83-119.

Addresses of authors: P. Shaw, Department of Environmental Science, University College, P/Bag 0022, Gaborone, Botswana R. Crossley, Department of Geography, Chancellor College, P.O. Box 280, Zomba, Malawi.