SOURCES OF SILICATE DUST IN TERRA ROSSA SOILS AND SURFICIAL DEPOSITS, SOUTHEAST SOUTH AUSTRALIA

Strontium isotopes (Sr87/86) combined with mineralogy and mass balance geochemistry are used to trace silicate material from possible regional sources through local playa-lunette systems and ultimately into Terra Rossa soils in the Southeast of South Australia. Results show good correspondence between the clayey Terra Rossa soil horizons and nearby lunette deposits, indicating that the soil is largely derived from silt and clay blown-off this extensive playa system. The silicate material in the underlying limestone is easily discounted as a source for the Terra Rossa soil through a combination of mass balance geochemistry and comparisons of strontium isotopes. A variety of potential local to regional sources for the fine-grained material in the lunette-playa system are possible. These include Quaternary basaltic volcanics, early Paleozoic granitic rocks, early Paleozoic syn-tectonic shale-sandstone sequences, late Precambrian shale-sandstone sequences, Murray River mud, and far-travelled dust. The strontium isotopes and XRD mineralogy strongly indicate the fine-grained material that is widespread in the lagoons and soils of this district have a source dominated by the early Palaeozoic shale-sandstone units. Thus, the deposition in this district over the last million years, just south from the Murray River mouth, shows local provenance dominating over inputs from the Murray River. Reasons for this include the abundance and durability of mica and illite from these syn-tectonic shale-sandstone units as well as their extensive exposure and ease of weathering. The local volcanic rocks are of limited aerial extent. The granitic rocks are proximal to the soils in question but have restricted outcrop distribution. An unexpected finding is that Murray River mud is not a significant contributor to the fine-grained material. It is possible that Murray River mud is largely trapped in up-stream floodplains, playas, and yazoo-like streams where a significant portion is deflated and blown to the east.