Pans of the southern Kalahari, Botswana

Abstract

The main geomorphic features of the southern Kalahari in Botswana are the 1000 pans or small dry lakes which lie along the broad watershed between the Nossop - Molopo and Makgadikgadi drainages. No previous detailed studies exist of the pans, which appear to have developed under climatic conditions unlike those of today, and appear to represent further evidence for Quaternary climatic change in the region. The pans, whioh may have a grassed, partly grassed or bare clay surface, are contained in shallow sub circular to sub elliptical isolated depressions, on the southern side of which is an area of fringing dunes, indicative of a deflation origin for the pan depressions. Analysis of the distribution of the pans shows that they are strongly clustered but do not form aligned groupings. Two dune ridges, both formed by northerly winds, are identified. The composition of the dunes shows that the outer dunes were formed by deflation from the site of the pan depressions and the inner dunes by deflation of sandy pan deposits. The nature of the deposits which underlie the pans and the flanks of the inner dunes is described, and upper sandy and lower clayey phases identified. Their composition indicates that the pan depressions once held extensive permanent lakes, which gradually contracted as the climate became drier. The nature of the pan surfaces today is shown to represent a sequence as the pans are excavated by deflation. The paleoclimatic significance of the three main periods in the origin and development of the southern Kalahari pans is discussed. The pan deposits provide further evidence for a major wet period in the Kalahari some 12000 to 20000 years ago. Deflation to form the outer dunes took place in the arid period that occured 20000 to 25000 years ago, and the inner dunes were formed during dessication of the climate some 10000 to 12000 years ago.