Fabric of soil derived from parna and the riddle of transported pellets

Abstract

It is generally accepted that large areas of southern New South Wales (NSW) and northern Victoria are mantled by soils derived from clayey loess deposits called parna. These parna deposits are believed to have formed during arid phases of the Pleistocene and the mineral constituents are assumed to have been transported as silt- and fine sand-sized pellets of calcareous clay, with some companion quartz grains of a similar size. A common property of parna-derived soils is subplasticity, where the apparent field texture grade becomes more clayey with increasing mechanical working of the bolus. This propensity for subplastic behaviour suggests that parna-derived soils contain stable silt- and fine sand-sized pellets of clay, yet there has been little direct micromorphological evidence of these pellets ever published. In the present study, thin section samples from several parna type-sites in southern NSW were examined micromorphologically to reveal the presence of very well size-sorted quartz grain populations (companion grains) and identifiable prolate clay aggregations of a similar silt to fine sand size. These prolate pellets, comprised of quartz, illite, kaolinite and chlorite, have a very distinctive mosaic-speckled b-fabric and are more widely distributed throughout the soil than the similarly sized and shaped faunal faecal pellets. Where these pelletal aggregations are not evident, such as in the deeper parna deposits, abundant illuviation features suggest that clay particles deposited within the parna, whether as pellets or coatings on grains, have subsequently undergone considerable weathering and a range of pedogenic processes.