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Open File Report 177

Regolith-landforms and salt stores in the Angas-Bremer Hills

Wilford J

The combined interpretation of airborne gamma-ray spectrometry and terrain indices derived from a digital elevation model along with ground data has provided new insights into the distribution of regolith materials and related salinity in the Angas Bremer Hills of South Australia.

Modeling of the gamma-ray data has been able to separate highly weathered landforms from areas characterised by thin soil and slightly weathered bedrock.Highly weathered materials include leached ferruginous soils,mottled colluvial and alluvial sediments and highly weathered kaolinised bedrock. Weathered bedrock or saprolite is typically mottled with iron fragments and nodules common in the upper part of the profile.Iron induration is mostly associated with segregations of iron in the mottled zone of the weathering profile.In places these weathering profiles are covered by alluvial and colluvial sediments that in places are also highly weathered (mottled and ferruginised).

The gamma-ray data has the potential to improve existing soil-landscape and geological mapping. Variations in the concentration of the radioelements can be used to modify soil-landscape and geological unit boundaries and assist in describing variations of specific attributes within individual units.As similar gamma-ray responses can relate to different materials at the surface it is recommended that the imagery be interpreted within different geological formations and landform units (e.g.erosional vs depositional landscapes).

In the Angas Bremer Hills,a long weathering history,dating back to the Middle Mesozoic,combined with more recent tectonic activity resulting in faulting,uplift and associated erosion has led to complex landscapes where highly weathered landforms are juxtaposed with youthful landforms with little regolith development.Analysis of the gamma-ray imagery and digital elevation model has provided improved clarity on the weathering and geomorphological evolution of the region.

Regolith thickness and composition governs the capacity of the landscape to store cyclic salts (e.g. those salts derived from rainfall).Weathering is important when considering salts derived from bedrock minerals.In both cases (e.g.salt derived from rainfall and bedrock)delineating areas of deep weathering is important in predicting likely salt stores.For a local region the highest salt stores are associated with catchments that have thick soils and deeply weathered bedrock.The stores are associated with valley alluvium,colluvial fans and highly weathered bedrock.Stream ECs tend to be higher from these catchments compared to those catchments with relatively shallow regolith.

Across the whole region the main driver for controlling the amount of salt in the landscape is rainfall. The thickness and composition of the regolith might determine the capacity of the landscape to store salts,but rainfall largely determines the relative abundance of salts in the profile.When comparing similar regolith profiles across the study area,those profiles in the high rainfall zone (western side of the Hills)store considerably less salt than those profiles on the drier eastern side.

The highest salt fluxes are associated with Western Flat Creek in the Mt Barker catchment.The average annual rainfall is the highest in the study area.Modelling of the gamma-ray imagery indicates that a high proportion of this catchment is deeply weathered,suggesting that the highest salt exports are associated with deeply weathered landscapes in high rainfall areas.Conversely,fresh water runoff is commonly associated with less weathered catchments in areas of high rainfall.

These relationships are summarised in a series of 3Dconceptual models which detail relationships between the distribution and thickness of regolith materials,salinity and hydrogeomorphic processes. These models provide a basis for understanding processes controlling the distribution of soil and river salinity across the Hills region.


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