Deciphering a colluvial mantle

Recent research in the Sydney water catchment area has revealed a colluvial mantle that does not conform to expectations based on soil landscape and vegetation mapping. The site is a section of valley at Blue Gum Creek, located in the Nattai catchment, south-west of Sydney. The geology of the catchment includes Hawkesbury Sandstone that forms a dissected plateau and cliffs, overlying units of shale, lithic sandstone and quartz sandstones of the Narrabeen Group. The dip is easterly, resulting in a steep, asymmetrical valley and the development of an extensive, continuous soil mantle on the north-east facing slope beginning below the cliff line and extending to the valley base. The main vegetation in the valley includes Eucalyptus crebra, E. piperita, E. punctata and Allocasuarina tortulosa with E. deanei dominating the valley floor. Soil landscape mapping classifies the site into two soil landscapes: Hassans Walls and Hassans Walls variant-a, however, a different arrangement of soil types was found, with Clastic Rudosols dominating the upper – mid slopes, Dermosols and Kandosols forming the middle slopes and Tenosols forming on the lower slopes, footslopes and valley floor. A comparison with other soil landscapes described for the Sydney Region with similar lithology or vegetation identified the Watagan and Blue Gum soil landscapes as the most likely alternatives, however, neither matched the arrangement of soil types found at the study site. It is clear that the colluvial mantle at Blue Gum Creek forms a different soil landscape, not described elsewhere. The vegetation is also somewhat different from what would be expected. Regional vegetation mapping describes it as Sydney Sandstone Gully Forest but the dominant species present, E. deanei, is not a recognised element.