Assessing the evidence for aeolian origins of mima-like mounds in South Africa

Abstract

The genesis of regularly spaced mounded landscapes worldwide remain mysterious. Mima-like heuweltjies (Afrikaans for small mound, ca 30 m diameter, 1 m high) of South Africa have been variously suggested to arise from (1) faunal accumulation of sediment, (2) aeolian deposition around termite mounds and/or vegetation patches, or (3) differential erosion of sediment from between vegetation patches. We adopted nabkhas as a model system for aeolian deposition and hypothesised that heuweltjies retain at least some of the morphological, spacing or sediment characteristics of nabkhas. We conducted aerial surveys of three geographically separate sites and sedimentological analysis (e.g. texture, soil carbon, soil compaction) at one site. Aerial surveys allowed analysis of spatial distributions and morphological characteristics. We found no evidence for the origins of heuweltjies as nabkhas from mound morphology, spatial distribution, or variation in sediment texture around the mounds. For example, unlike many nabkhas, the plan shape of the heuweltjies was not elongated in the prevailing wind direction and did not have steeper windward slopes. Spacing of heuweltjies was also not consistent with wind flow characteristics across nabkhas. There was no evidence of anisotropic distribution of heuweltjies, which sometimes manifests with nabkhas. Unlike many nabkhas, there was also no differentiation of sediment characteristics across various aspects. The fact that heuweltjies are elongated downslope and have steeper downslope flanks suggests that the mounds have been subjected soil creep and possibly water erosion. We suggest that heuweltjie spatial pattern and morphology is initiated by hydrological processes, with redistribution of aeolian sediments contributing to subsequent mound development. The association of heuweltjies with relatively arid winter-rainfall climates results in sparse low-stature and low-cover vegetation as a result of summer drought. Sparse vegetation, erosive winter rains and summer winds may contribute to the mound morphology.