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

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.

Automated summary

The origins of regularly spaced mounded landscapes worldwide remains a mystery, including the Mima-like heuweltjies in South Africa. This study explored the hypothesis that these mounds retain some characteristics of aeolian deposition known as nabkhas. Aerial surveys and sedimentological analysis were conducted, revealing no evidence of heuweltjie origins as nabkhas based on mound morphology, spatial distribution, or sediment characteristics. Instead, the elongated downslope shape and steeper downslope flanks suggest soil creep and water erosion as the primary factors. This study suggests that hydrological processes and the redistribution of aeolian sediments contribute to the spatial pattern and morphology of heuweltjies.