Browsing by Author "Majodina, Thando Olwethu"
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- ItemGeochemical and mineralogical characterisation of Vaalputs palaeosols : inference of paleoclimates(Stellenbosch : Stellenbosch University, 2013-03) Majodina, Thando Olwethu; Clarke, Catherine E.; Mikes, Daniel; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.ENGLISH ABSTRACT: Vaalputs radioactive waste disposal facility is situated in an arid region of Bushmanland currently with evapotranspiration potential that far exceeds precipitation. Dominant soil features in Vaalputs are palimpsests of climates under which they formed. Particle sizes vary drastically between horizons which suggest different modes of sediment transport. Petrographic analyses revealed euhedral habits of primary mineral feldspar within the soils of Vaalputs. This suggests a proximal source of sediments and minimal primary mineral weathering under an arid climate where euhedral grains of feldspar are maintained. The surface horizon of the soils is covered by an equigranular coarse sand of residual aeolian origin. The transition from the surface horizon to the subsurface horizons is widely marked by a pebble sized stone-line. The pebble sized material of the stone-line suggests residual accumulation during the weathering of a previously surface exposed horizon. Since deposition of subsurface sediments (15 Ma) pedogenic alteration has been active in Vaalputs. This has resulted to a complex soil system which displays varied forms of thick dorbank horizons including massive polygonal peds and platy horizons. The polygonal peds are defined by desiccation cracks where vertical and horizontal laminations are hosted. Slaking tests as well as bulk chemistry confirmed that the laminations are composed largely of secondary calcite, however elemental mapping revealed numerous illite bands alternating with calcite layers. It is proposed that calcite layers represent solution features rather than cutanic features. Signs of hydromorphy are commonly observed within the dorbank horizons, since an arid climate currently prevails in Vaalputs such hydromorphic features may indicate formation under past climates. The occurrences of palygorskite, sepiolite and dorbank horizons in Vaalputs require high soil pH (generated by high concentrations of Na) for their formation. Vaalputs soils, however, measured circumneutral pH and relatively low Na concentrations which suggests that palygorskite, sepiolite and dorbank horizons are relic features. Salt casts of lenticular texture occur between polygonal peds of massive dorbank horizons and their enveloping vertical and horizontal laminations. Scanning Electron Microscope analyses indicate high concentrations of Mg, Al, Si and O which suggests sepiolite and palygorskite accumulation through a replacement of gypsum. Elemental maps in conjunction with x-ray tomography and bulk chemical analyses revealed that high concentrations of secondary barite occur along the contact surfaces between dorbank horizons and the laminations. The solution chemistry of all horizons show supersaturation with respect to barite suggesting that the Ba accumulation adjacent to the laminations is likely to have taken place at lower sulphate conditions than those present in the soils today. Evidence shows that Vaalputs soils have experienced at least one climate shift. The preserved soil mottles are indicative of soil environments that remain wet for an extended period. A fine textured platy dorbank horizon is an extensive feature in Vaalputs. The presence of this horizon indicates that the sediments were deposited from a low energy fluvial system. The large polygonal ped units in the lower dorbank units as well as the barite enrichments in pore spaces suggests a climate shift from wet to dry began after the sediments were deposited.