Loss of K and/or preferential enrichment in Th and U can, in places, be used to recognise highly weathered and leached soils. Highly weathered landscapes over parts of Cape York Peninsula (Wilford 1992) have either low abundance for K, Th and U (e.g., quartz sands) or low K and high Th and U associated with the accumulation of Fe- and Al-oxyhydroxides and resistate minerals in the upper part of the regolith (see Ebagoola virtual field trip and Selwyn 3D interactive model). For example, highly leached aluminous and ferruginous bauxitic soils around the Weipa region are identified on the gamma-ray imagery by their low K and elevated Th and U values. These radioelement responses are reflecting soils with very low exchangeable cations and relatively high abundances of Fe- and Al-oxyhydroxides and resistate minerals (e.g., zircons) (Figure 23). A similar situation is seen in the Mt Lofty Ranges in South Australua (Figure 24).
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Figure 23 (above). Left - Ternary image with K in red, Th in green and U in blue. High Th concentrations define the Weipa bauxites. Extensive alluvial fans (some of the largest in the world) are clearly visible on the western side of Cape York Peninsula (Bain and Draper 1997). Right - Bauxite pisoliths at Weipa. |
Figure 24. Ternary image with K in red, Th in green and U in blue. A - High Th and low K saturation correlates to a partly preserved highly weathered palaeosurface over the Mt Lofty Ranges east of Adelaide. B - Lithosols and exposed bedrock correlate to high gamma-ray emissions, particularly K.
Depleted K has also been shown to be associated with highly leached podzolic soils in the Wagga Wagga region of New South Wales (NSW) (Bierwirth 1996). Ratios of airborne gamma-ray K and Th have been use to separate different regolith materials from bedrock responses in the Yilgarn Craton of Western Australia (Dauth 1997). Dauth (1997) used K/Th ratio bands to define landscapes with highly weathered and ferruginous saprolite. A combination of Landsat TM and gamma-ray ratios were used to identify areas of intense bedrock leaching in the Selwyn region, southeast from Mt Isa in Queensland (Qld) (see Figure 13 in the Data Analysis - Imaging Gamma Rays section).