Pan floor ‘barchan’ mounds, Ntwetwe Pan, Makgadikgadi, Botswana: Their origin and palaeoclimatic implications

Abstract

The variety of forms, assumed to be dunes, and palaeolakes of northern Botswana have long been perceived as geoproxies for the elucidation of Quaternary palaeoclimates. Mounds on the floor of Ntwetwe pan have been regarded as modified barchans, pertaining to a time in the past when conditions were drier than at present. The morphology, hydrogeology, and biology of these, placed in the wider context of the palaeolacustrine hinterland, indicate that they are not dunes but subaqueous features, spring mounds. The morphology is inconsistent with barchans. Conspicuous features are the elevated ‘eyes’ within the enclosing arms of the crescentic forms that occur furthest from the pan edge. Seepages discharge from their flanks, commonly about 7 m higher than the dry, saline pan floor. On the top of forms nearer the pan margins, permanent pools of potable water occur, similarly elevated above the floor. Hydraulic pressure to sustain the upward discharge of groundwater is provided by the excellent recharge through silicate karst depressions in the hinterland, particularly those of the 936 m palaeolake floor. Groundwater discharge, still operating today, was formerly much stronger and associated with lacustrine conditions. A chronological framework is provided by archaeological remains on the palaeolake floors of the hinterland. These indicate that the oldest palaeolake floor pertaining to the Quaternary is the 920 m level. Periods of spring mound formation are associated with both this and the 912 m palaeolake. The implications for future luminescence dating targeting palaeoenvironmental reconstruction are discussed in the light of this new model of mound formation.