Hunting for parna pellets in Australia’s clayey loess

Abstract

The best-known loessic material in eastern Australia is the so-called parna of southern NSW and northern Victoria. Parna deposits are believed to have formed during glacial phases of the Pleistocene, when huge amounts of dust were transported from arid and semi-arid regions of southern Australia, across the more humid eastern parts of Australia, towards the Tasman Sea. The mineral constituents of this dust are assumed to have been calcareous clays, transported as silt- and fine sand-sized pellets, with some companion quartz grains of a similar size. Clay-rich Alfisols are the most commonly found soils to weather from these parna deposits. 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. One train of thought is that post-depositional pedogenesis has destroyed these pellets. Here, thin section samples from a number of parna type-sites in southern NSW have been examined micromorphologically, to reveal the presence of very well size-sorted quartz grain populations (companion grains), and, in the drier locations, identifiable prolate clay aggregations of a similar silt to fine sand size. Where these pelletal aggregations are not evident, such as in the older parna deposits and in the wetter locations, 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. A complicating factor in the positive identification of parna pellets is that faecal pellets of soil mesofauna are often of a similar size and colouration, and similar morphologically. Nevertheless, the apparent ubiquity of the silt-sized pellets in parna soils, and the presence of these outside obvious faunal chambers and pores, suggests that the majority of these features are not of biologic origin.