Surface texture analysis of southern Tuli Basin sediments: Implications for Limpopo Valley geoarchaeological contexts

The Hackthorne 1 site (southern Tuli Basin, South Africa) is situated on a sand-covered plateau adjacent to the Limpopo River Valley. Although the site is well known for its Stone Age archaeology, the past environmental contexts (particularly sedimentological/geomorphological) are not well known. We examine the Hackthorne sand grain surface textures, so as to provide some insight on the site specific and regional depositional history. Quartz sands at Hackthorne were collected from surface sands and from underlying weathered calcrete. SEM analysis was performed on sand grains, through which several mechanical and chemical microtextures were identified. Microtextures typical of fluvial environments were found only on grains derived from the plateau calcrete host sediment, whilst the surface sands exhibited only textures associated with aeolian environments. The results indicate that the calcrete host sediment is composed of alluvium, and that the surface sands mantling the Hackthorne Plateau are not deflated from the alluvial deposits in the Limpopo Valley, but may rather be derived from distant aeolian sources. The deposition of aeolian sands is consistent with OSL dates which place sand deposition, or remobilization, at 23 and 15 kya, periods in southern Africa associated with increased aridity.