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Open File Report 178

3D regolith architecture of the Jamestown area - implications for salinity

Wilford J

The combined analysis of a digital elevation model, gamma-ray spectrometry, magnetics, airborne electromagnetics (AEM) and drill holes has provided new insights into the regolith-landform evolution of the Jamestown region, located in the Mid North of South Australia. In particular, the 3D architecture (geometry and composition) of the valley fill deposits is better understood. Three dimensional models generated from the integration of these datasets show the distribution of regolith materials, salt stores and possible preferential flow paths for groundwater in the region.

The Jamestown region is characterised by a range of landforms from low relief colluvial and alluvial fans, floodplains and pediments through to rises, low hills and hills. Depositional materials occur in three main valleys: the Belalie, Bundaleer and Caltowie. Coalescing colluvial and alluvial fans of Quaternary age have filled these valleys to depths of up to 40 metres. The thickest sediments consisting of silt, clay, fine sand and minor gravels occur in the Belalie and Bundaleer valleys. The Caltowie valley has a thinner sediment cover which appears to have a lower electrical conductivity when compared with the other two catchments. Low angle pediments characterise the upper parts of the Caltowie catchment.

Airborne gamma-ray spectrometry was effective in separating a variety of bedrock types and regolith materials based on their radioelement characteristics. A relationship between the surface texture of soils on depositional plains and airborne K concentrations was used to predict the surface silt content in each of the main valleys. The Caltowie valley has less silt and a higher percentage of medium to fine sand than the other two catchments. Whether these sandier soils are representative of materials at depth is not known. Most of the sediments in these valley systems are dominated by poorly sorted debris or mudflow deposits that consist mostly of silt and clay with minor gravels. Available drilling suggests that scattered throughout the valley fill, but more common in the basal part of the alluvial sequence, are traction or bed load deposits associated with river channels. Many of these buried channel sediments contain maghemite that are clearly delineated in high frequency component of the airborne magnetic imagery. Magnetic channels show rill, dendritic and braided patterns. In areas of thicker sediment cover, inter-woven and possibly stacked, buried palaeo-channel networks are observed in the magnetic imagery. These buried palaeo-channels define a palaeo-darainge system that is not reflected in the contemporary drainage, which is widely spaced and discontinuous. As a result of the Belalie valley being partly filled with sediment there is evidence of superimposed drainage diverting palaeo-drainage lines east of Jamestown. With the exception of some local discrete conductive basement rocks, the valley fill sediments and some areas of highly weathered saprolite appear, in the airborne EM, as the most conductive materials in the landscape. An isosurface (3D surface of equal value) of the high, near-surface conductivity in most cases delineates the geometry of the valley prior to sedimentation and infilling. Both the fine grained sediments of the valley fill and highly weathered bedrock are likely to contain the highest salt stores as implied by an interpretation of the AEM data. Overlaying the salt stores (defined using the AEM) on the magnetic channels highlights possible groundwater flow directions. The surface expression of dryland salinity within the valleys occurs where salt stores and associated regolith materials thin and where basement barriers or valley constrictions impede groundwater flow. Valley constrictions and associated dry land salinity occur in the Bundaleer and Caltowie valleys but not the Belalie valley, despite the fact that the Belalie valley has the highest salt store. This might be explained by the fact that the Belalie system has a relatively thick transported cover with numerous buried, and more transmissive and inter-connected channel networks. It is proposed that this more effective sub-surface drainage may prevent the build up of shallow water tables and salts in the upper soil layer.


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