A geobotanical investigation of the Koedoesfontein Complex, Vredefort Dome, South Africa

Abstract

The Vredefort Dome (impact structure) in South-Africa hosts several unique peralkaline to ultramafic intrusive bodies that were emplaced into its core and collar. Because of the broad difference in mineral composition, between the intrusions as well as the abundant siliciclastic country rocks, abrupt transitions in soil chemical characteristics are produced that can ultimately affect the floristic and physiognomic characteristics of the associated vegetation in the dome. Despite various efforts to study the ultramafic-adapted vegetation of southern Africa, little is known about the effect these particular intrusions have on the soil and floristical components of the dome. The aim of the present study was to investigate the geobotany of one of the ultramafic-peralkaline complexes in the dome, namely the Koedoesfontein Complex. The complex hosts several different intrusions that collectively form a mineralogical gradient ranging from mafic- to felsic-dominant silicates. Three primary components (geology, soil and woody vegetation) were sampled along this gradient and analysed to determine the effect of these intrusions on the residual soil chemistry, and the floristics and physiognomy of the supported vegetation type. Weathering of the peralkaline to ultramafic lithologies greatly affected the soil’s chemical properties and related species composition and physiognomy. Major chemical variation in the soil, including the total chromium and nickel content, is correlated with the difference in abundance of sodium-, calcium-, iron- and magnesium-rich silicate minerals. Significant variations in woody species composition (particularly across Senegalia and Vachellia genera) have been established between soils originating from ultramafic–mafic, intermediate and felsic rock types and can be utilised as indicators for different geological substrates within the dome.