Post-Gondwana pedogenic ferromanganese deposits, ancient soil profiles, African land surfaces and palaeoclimatic change on the Highveld of South Africa

Several ferromanganese wad deposits are developed on the Archæan Malmani dolomite succession of the Transvaal Supergroup along the plateau forming the watershed between rivers draining to the Indian and Atlantic Oceans from the Highveld area between Johannesburg and Lichtenburg. The deposits were studied at the Wes Wits Gold Mine and the Ryedale, Houtkoppies and Klipkuil ferromanganese mines. The ferromanganese wad deposits are located in the Waterval saprolite, which formed by deep chemical weathering along the post-Gondwana African surface. The parent rocks for the wad are the Fe- and Mn-rich dolomites of the Malmani Subgroup of the Transvaal Supergroup at Wes Wits Mine and at Klipkuil, and Fe- and Mn-rich blackband Fe ores of the Ecca Group of the Karoo Supergroup at Ryedale and Houtkoppies. This saprolite is unconformably overlain by a ferruginous alluvial succession, informally defined as the Weswits formation. Diamondiferous gravel bars occur in the lower part of the fluvial succession. The unconformity at the base of the succession with incised valleys is thought to correspond to the post-African I event of uplift and erosion. Manganiferous soil nodules, derived from a lateritic weathering profile that originally covered the Waterval saprolite, are concentrated in the lower part of the Weswits formation. Climatic conditions became drier during the deposition of the Weswits formation, and plants with deep taproots vegetated the surface of the alluvium, giving rise to the formation of a ferric poclzol. Further aridification took place and eventually the plants with deep taproots died and a pediment developed along which a stone lag was concentrated. This pediment is thought to represent the post-African II surface of erosion. The pediment is overlain by the Hutton soil, representing Kalahari sand and dust that have been reworked by fluvial and peclogenic processes. Massive ferruginous soil nodules grew in situ in the Hutton soil, indicating more pluvial climatic conditions at times, most probably corresponding to the Quaternary ice age events in the Northern Hemisphere. A thin, modern orthic soil developed on top of the Hutton soil in the present day mild, subhumid climatic conditions.