Using airborne geophysics to define the 3D distribution and landscape evolution of Quaternary valley-fill deposits around the Jamestown area, South Australia

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.