Palustrine wetland formation during the MIS 3 interstadial: implications for preserved alluvial records in the South African Karoo

Dryland alluvial successions across interior South Africa are a focus of pedosedimentary research because they provide insights into past hydrogeomorphic changes and improve our understanding of allogenic and autogenic controls on fluvial systems. This paper focuses on an alluvial succession preserved within the Wilgerbosch River, on the Great Escarpment, South Africa. The pedosedimentary architecture of a calcretized palaeosol formed within an abandoned MIS 3 (51–39 ka) floodplain was investigated using grain size, micromorphological, XRD and mineral magnetic analyses. The primary controls on pedogenesis are inferred and the impacts of their products on subsequent landscape development outlined. The biologically-dominated calcrete micromorphology is consistent with the rhizogenic calcrete variety, which together with XRD, grain size and soil magnetic data provides evidence for an MIS 3 palustrine palaeowetland system. Palustrine wetlands have been documented across interior South Africa, but have not been previously associated with rhizogenic calcrete formation. We interpret these wetlands to be representative of basin-wide hydrogeomorphic adjustments rather than localised discontinuous channel and floodout processes. Magnetic enhancement (pedogenic maghemite) of the soil column overlying the calcrete compared to the overlying floodplain deposits is indicative of moister climate conditions ~51 ka relative to ~39 ka. We hypothesise that the longevity of the geomorphic quiescence, coupled with relatively moist climate conditions in early MIS 3 enhanced the weathering of calcic plagioclases, resulting in soil CaCO3 accumulation. The resultant cementation of this abandoned floodplain deposit accounts for an alluvial record that is dominated by older rather than younger (e.g. Holocene) deposits, despite the narrow valley context (<150 m). These findings illustrate the interplay of factors that may generate sedimentary records in this and other confined catchment systems. Such insights may aid interpretation of tectonically-stable dryland inland valleys preserved within the geological record.