Residual soil mineralogy and dune subdivision, Swan Coastal Plain, Western Australia

Yellow sands in the near-coastal parts of the Swan Coastal Plain of the Perth region are of residual origin, developed from the weathering of Tamala Limestone. An alternative hypothesis that they were emplaced from a vast extension of sand dunes from the Central Australian Desert, is not supported. Samples collected mainly from the upper B-horizon of the yellow sands, along a series of east-west transects, have been studied mineralogically. A 'maturity index' was devised to compare the combined percentages in the heavy-mineral fraction of ilmenite plus leucoxene against the combined percentages of three minerals which weather away rapidly from the soils, namely garnet, amphibole and epidote. The results reveal a pronounced east-west zonation, ranging from a distinctive 'young' assemblage on dune ridges near the coast to an 'old' assemblage depleted in the three silicate minerals in areas further inland. Used in conjunction with geomorphic features this zonation has facilitated a subdivision of the Spearwood Dune System into several dune subsystems, named herein as the Trigg, Karrinyup, Gwelup, Balcatta and Yokine Dunes, plus a dune subsystem along the western edge of the Bassendean Dune System defined herein as the Gnangara Dunes.