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Abstract
The combined analysis of airborne electromagnetics, airborne gamma-ray spectrometry, magnetics and a digital elevation model with ground-based calibration has enabled construction of a 3D architectural and landscape evolution model of valley-fill deposits around the township of Jamestown in South Australia. The valley-fill sediments consist of traction, suspension and debris-flow deposits that range in age (optically stimulated luminescence dating) from 102 ± 12 ka to the present day. A sediment isopach map generated from the airborne electromagnetics dataset reveals the 3D structure of the valley-fill deposits. The sediments are up to 40 m thick in asymmetrical valleys and are the result of colluvial fan, floodplain and sheet-wash processes. The sediments fine upwards with a higher proportion of coarser bedload deposits toward the base and fine sand, silt and clay towards the top of the sequence. A strong linear correlation between airborne K response and soil texture allowed the percentage of surface silt to be modelled over the depositional landforms. The sediments are thought to have been derived by a combination of eolian dust accessions, and weathering and erosion of bedrock materials within the catchment. Older drainage lines reflected in the distribution of relatively closely spaced and well-connected ‘magnetic channels’ differ markedly from present-day streams that are largely ephemeral and interrupted. This is thought to reflect a change in local hydrology and associated geomorphic processes from relatively high to lower energy conditions as the valley alluviated. These hydrological changes are likely to be associated with a drying climate, lower recharge and runoff.
Acknowledgements
I wish to thank Richard Cresswell, Craig Liddicoat, Mark Thomas and Chris Henschke for support in the field and in providing datasets as well as an opportunity to exchange views and debate science, and gratefully acknowledge the assistance of Mark Raven for XRD analysis, Malcolm Nicoll for the GOCAD model and Tim Pietch who provided the OLS dates. Thanks to Mark Thomas, Graham Heinson, David Gibson and Wilf Wilford for comments on the paper, and to Colin Pain and Ian Roach for reviewing the final draft. I also wish to thank Colin Pain and Graham Heinson for reviewing the paper. Angelo Vartesi and Travis Naughton, Visual Resources Unit, CSIRO Exploration and Mining, Perth, prepared and . Published with permission of the CEO of Geoscience Australia and CRC LEME.