The Bethungra area is located between Junee and Cootamundra in NSW. Gamma-ray and DEM analysis have been used to map regolith materials, salt stores and saline discharge sites. Bedrock in the areas consists of acid volcanic and igneous bedrock (e.g., rhyolite and granite). The following desciptions refer to images in the "Surfaces" drop-down box in the bottom left of the window above. You can change the images as you work through the text below.
Gamma-ray image responses relate to both bedrock and regolith materials. A residual analysis technique is used to separate bedrock responses (shallow regolith) from regolith materials consisting of saprolite and clay. Thick regolith typically has a much higher salt store than thin soils over bedrock. The residual analysis approach uses the loss of K for each individual geological unit as a surrogate for highlighting areas in the landscape where relatively thick regolith and high potential salt stores occur.
The residual image shows shallow bedrock-dominated landscape with thin regolith in reddish hues and relatively thick regolith in blue hues.
A topographic wetness index is then used to highlight likely groundwater discharge sites (high wetness values shown in red hues). Saline discharge sites are likely to occur where the modelled regolith thickness map and wetness index intersect. Fresher groundwater flow occurs in discharge sites with shallow regolith cover. Photo-interpreted salt scalds are superimposed in yellow. See the salinity discharge layer – thick regolith is shown in blue hues and potential discharge areas in red.
Figure captions (right) :
Figure A. Bedrock geology
Figure B. Shallow soils on saprock (slightly weathered bedrock)
Figure C. Thick clay-rich soil over highly weathered bedrock (clay likely to have an aeolian dust component)
Figure D. Colluvial foot-slopes consisting largely of clay from local salt stores