Do sand dunes of the lower Lachlan floodplain contain the same dust that produced parna?

Abstract

Æolian dust deposits are known to be widespread in south-eastern Australia, with the dominant model being that of ‘parna’, an aggregated material comprising clay, calcium carbonate, and silt-sized quartz. Despite a general acceptance of the parna model, there is surprisingly little evidence of silt-sized clay aggregates remaining in parna profiles. To investigate a relatively pure æolian dust deposit within the proposed zone of parna distribution, we examined the various phases of 3 source-bordering sand dunes on the lower Lachlan River floodplain of south-western NSW. In each dune exists an upper-slope phase of coarse brown sand, a mid-slope layer of reddish, clay-enriched sand, and a lower-slope phase of coarse sand dominated by an accumulation of carbonate glaebules. Granulometric analyses of the clay-enriched phase(s) of each dune identified a conspicuous particle population in the 20–60 μm range, and another in the fine-silt/clay range (<10 μm). Mineralogical characterisation revealed an abundance of illite and kaolinite in the upper 2 dune phases, coupled with a minor amount of smectite, further suggesting an allochthonous æolian origin, as the surrounding floodplain is smectite-rich and relatively poor in illite. Micromorphological features within the clay-enriched phase, including abundant argillans and laminar bands of well-sorted fine quartz grains, indicate that this clay is a depositional feature, illuviated from surface horizons and re-deposited at depth. A similar illuvial origin is suggested by the fine crystalline nature of the calcium carbonate accumulation, ubiquitously coating the matrix mineral grains of the lower dune phase. No discrete clay aggregates were identified; however, all the assumed components of parna (silt-sized quartz grains, clay, and calcium carbonate) were identified, spatially separated within each dune. The consistency of these features among the 3 dunes indicates an analogous æolian dust accession, but it is not clear whether the clay component of this dust was transported as coatings on quartz grains, or as silt-sized clay aggregates accompanied by silt-sized quartz grains.