Potential for using soil particle-size data to infer geological parent material in the Sydney Region

Ecological communities are more than assemblages of species. In assessing the presence of many ecological communities, interpretation of soil properties and associated parent material has become a definitive component under environmental legislation worldwide, and particularly in Australia. The hypothesis tested here is that the geological parent material of a soil sample can be determined from particle size fraction data of the Marshall soil texture diagram. Supervised statistical classifiers were built from data for four particle-size fractions from four soil landscape publications. These methods were modified by taking into account possible autocorrelation between samples from the same site. The soil samples could not be classified with certainty as being derived from Wianamatta Group Shale or Hawkesbury Sandstone parent material. The classification of alluvial/fluvial-derived soils was no better than chance alone. A good classifier using four-fraction compositional data could not be built to determine geological parent material. Hence, the three size fractions of the Marshall soil texture diagram are insufficient to determine the geological parent material of a soil sample.