POLYPHASE DUNE AND SOIL FORMATION ON THE MAPUTALAND COASTAL PLAIN

Maputaland forms the southern end of the East African coastal plain. Dunes have accumulated since the Neogene and the general coast-parallel arrangement of dune systems stabilised by grassland and forest has been assumed by previous researchers to young coastward. Detailed mapping of the dune morphology revealed that composite parabolic and hummocky dune systems represent polyphase aeolian reworking. The project assessed relative- and numeric dating techniques that can be used to differentiate dune systems. Soil profile characteristics were used to calculate soil development indices (SDI) for sampled horizons from holes hand augered to 6m depth. Weathering indices were calculated from geochemical data, magnetic susceptibility was measured and infrared stimulated luminescence dating of representative dune sands defined the period of pedogenesis since deposition.

The quartzose dune sands have weathered to form deep reddened soil profiles with higher SDI horizon values reflecting pH decrease, advanced rubification, clay increase and harder consistency down profile. The Holocene profiles exhibit decreasing horizon index values below the A-horizon whereas late Pleistocene profiles display downward increasing horizon index values within the upper 3m of the profile. Greater complexity and horizonation within some mid- to late Pleistocene aeolian sands manifests as distinctly mottled, clay-enriched horizons at depths of 2-5m below the surface. The SDI profiles from specific dune systems or stratigraphic units cluster well and effectively contrast the characteristic soil development on dunes of different ages. Plots of depositional age vs. sampled horizon index values enable the derivation of generalised rates of soil profile development and comparison of pedogenic processes on the high rainfall coastal barrier dunes relative to the drier interior. SDI horizon values distinguish composite profiles and highlight possible localised surficial reworking or profile burial. The technique is less effective in differentiating profiles where longer-term accretion occurred on a dune. When plotted against geochemical weathering index values or magnetic susceptibility, the SDI methodology proved effective in differentiating dune sands or provenances.