Mineralogy of Soil‐Free Bare Crusts on Native Dryland Plant Habitats in Mackenzie Basin, South Island, New Zealand

Late Cenozoic silty and sandy sediments in the semi‐arid Mackenzie basin have been eroded and redeposited almost entirely from nearby Mesozoic greywacke basement. Fine (commonly micron scale) phyllosilicate flakes, including clays, are randomly distributed and randomly oriented, and the sediments are porous. Dry surface crusts (mm‐cm scale) on bare soil‐free areas are locally cemented by clay minerals and biofilm but remain friable and porous. Surface waters drain through the sediments faster than evaporation occurs, even from tarns in depressions. Hence, evaporative salts are absent apart from traces of calcium sulphate and sodium chloride in the immediate subsurface of clay‐rich gravel. Mackenzie basin bare ground chemistry (pH 5–8; electrical conductivity, EC < 1 mS/cm) is different from saline bare ground derived from Otago Schist (pH 7–11; EC 1–60 mS/cm) where stacked coarse mica flakes accentuate impermeability and evaporative salt accumulation. Greywacke‐sourced Mackenzie basin bare ground hosts dryland vegetation that is mostly not salt‐tolerant. This contrasts with the salt‐tolerant plant species that grow in association with evaporative ephemeral surface water on schist‐derived saline sites in inland Otago. Absence of high salinity allows exotic plant encroachment on the Mackenzie basin bare ground.