This paper identifies and quantifies the clay assemblage within coastal dunes of northern KwaZulu-Natal, South Africa, and proposes a model for clay distribution and abundance. Distinct dune units identified by previous authors on the basis of the sand fraction are redefined on the basis of the composition of the clay fraction. The most mature dune unit is classified as Unit 0 (250–350? ka) and is characterised by a clay assemblage rich in kaolinite and goethite. Units 1 and 2 (19–250 ka) are of intermediate maturity and comprise kaolinite, interstratified clays, quartz, and goethite. The relatively immature Units 3 and 4 (Recent to 19 ka) are characterised by calcite, quartz, vermiculite, and montmorillonite.
The relative abundance of clay minerals in dune units allows weathering and postdeposition alteration processes within the dunes to be investigated. From these, a simplified weathering sequence with increasing maturity can be summarised as follows: quartz feldspars calcite heavy minerals → illite vermiculite → illite interstratified clays → illite montmorillonite → halloysite interstratified clays kaolinite → kaolinite. Past work and field studies indicate that the dune cordon acts as an open system of high permeability, resulting in carbonates and mobile ions being readily leached and lost from the system. However, when prolonged weathering has caused an accumulation of fine-grained material, permeability decreases substantially, whereby mobile cations and silica cannot be readily evacuated. This can result in the formation of montmorillonite rather than kaolinite, even when carbonates have been leached. The dominant controlling factors in determining clay assemblages are pH and leaching conditions. This observed weathering profile is complicated by reworking, the type and distribution of vegetation, and water table changes because of sea level changes.