New regolith mapping approaches for old Australian landscapes.

Abstract

The regolith, or ‘critical zone’, forms a discontinuous layer that covers large areas of Earth’s terrestrial surface. It is a dynamic zone that forms and changes through time in response to interactions between air, rocks (minerals), water and biota. Knowledge of regolith is critical because of its key role in supporting terrestrial life, through physical, chemical and biological processes that operate at mineral-water interaction scales up to the regional scale through geological and tectonic activity. There are many disciplines or areas of applied integrated research that rely on an improved understanding of regolith formation and information on surface and sub-surface regolith properties at appropriate spatial scales. These areas of study include; agriculture, land use sustainability, hydrology, salinity management, ecology, mineral exploration, natural hazard risk assessment and civil engineering. Furthermore, mapping regolith is critical in understanding the origin and evolution of regolith through space and time. Mapping the regolith and formulation of associated robust process models are in their infancy compared with geological and soil mapping, which have had a long history of development and refinement. Regolith mapping can be seen as a hybrid approach combining elements from the existing mapping disciplines of geology, soil and geomorphology. The regolith-landform approach, used extensively in Australia, is broadly similar to soil-landscape mapping where landforms are used as the principal surrogate to map regolith. Regolith-landform and soil-landscape mapping are inherently empirical and qualitative. However, in the last ten years there has been a move from the qualitative land resource survey (i.e. soil-landscape mapping) approaches to quantitative, digital survey underpinned by statistical methods. These new quantitative approaches are enabling the prediction of specific soil properties with associated estimates of model confidence or uncertainty not possible using traditional approaches. The aim of the thesis is to demonstrate and assess the application of quantitative soil mapping approaches in predicting regolith properties. Four case studies are presented that illustrate the application of quantitative mapping approaches in predicting regolith across a range of spatial scales and within different landscape settings. These four investigations include: 1. A continent-wide prediction of weathering intensity using a step-wise multiple regression-based model using airborne gamma-ray imagery and terrain relief; 2. A continent-wide prediction of near-surface secondary carbonate using environmental correlation and regolith geochemistry; 3. A regional-scale prediction of soil-regolith thickness over the Mt Lofty Ranges in southern South Australia using environmental correlation, drilling and legacy data, and 4. A regional-scale 3D regolith-landscape evolution model of valley-fill deposits from the Jamestown area in South Australia based on dataset integration, regression analysis and optically stimulated luminescence dating. The investigations are interpreted within a landscape evolutionary framework and future research directions are discussed. Digital regolith mapping shows considerable potential in predicting regolith properties over different landscape scales. This mapping is also important for understanding the complex interaction of environmental factors that control regolith formation, removal and preservation. Addressing gaps in predictive datasets that describe or reflect properties within the sub-surface (i.e. 5–100 m depth interval) and systematic collection of quantitative regolith attributes such as weathering depth and geochemistry will greatly enhance the future applications of digital regolith mapping in Australia.