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A Kernich, C Pain, P Kilgour and B Maly

October 2004


This report presents results from a study of landforms and surface regolith in the Lower Balonne Airborne Geophysics Project (LBAGP) area. Data sources included Landsat Thematic Mapper (Landsat TM), and Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) satellite images, radiometrics and digital elevation models (DEM) from airborne surveys and aerial photographs. These remotely sensed data were supplemented by data derived from field and laboratory work.

The aims of the LBAGP were to provide a better understanding of the nature of regolith materials and groundwater in the project area; a three-dimensional framework of salt stores and regolith characteristics influencing water and salt movement; an overview assessment of areas (land and water) at risk from salinity; and an evaluation of the application of airborne geophysics for salinity investigations. This report contributes to the first part of these aims, and significantly expands knowledge of the surface regolith and soil materials, and the arrangement of the weathered rock, unconsolidated clay and sand which form the subsurface and within which water and salt can move.

The area was subdivided into 8 geomorphic units and 22 regolith landform units on the basis of landforms, surface regolith materials, and geomorphic evolution. All but one of the geomorphic units are sedimentary in nature, and include a major Plio-Pleistocene surface (the Maranoa surface) in the west, Pleistocene anastomosing alluvial surfaces in the centre and east, and the modern Balonne and Moonie River floodplains. One geomorphic unit consists of residual regolith on an erosional landscape formed on the Cretaceous Griman Creek Formation.

It is convenient to begin the geomorphic history of the area with the erosional landscape formed on the Griman Creek Formation during the Tertiary. Towards the end of the Tertiary the Dirranbandi palaeovalley, perhaps fault-bounded, began to fill with sediments. By the beginning of the Pleistocene the palaeovalley was full, and sediments began to spread out across the erosional landscape, to form the Maranoa surface. Deposition of sediments on the Maranoa surface forced rivers to shift to the east, and the Balonne River began depositing sediments in a fan extending from St George to the south, eventually surrounding the Noondoo rises. The Balonne River appears to have shifted gradually westwards during the Quaternary, and now occupies a floodplain adjacent to the Maranoa surface.

During the same period the Moonie River filled its valley, and now occupies a floodplain east of the Balonne surfaces. Landsat TM and radiometric data were the most useful for the work reported here. Airborne electromagnetic (AEM) imagery was compared with the maps produced for this report to see if there was any relationship between conductivity and surface materials. As expected there is some correspondence between the shallow AEM slices and the regolith landform features of the area.

Geology, soil and land system maps of the area were available before the airborne geophysical data were acquired. Of these, the land system map most closely provided the kind of information sought from the geophysics and the regolith landform map. However, this study has been able to add significantly to the information contained in the land system map. The radiometrics provides spatially explicit information on the location of different surface materials in the area, while the regolith landform map and this report provide the interpretation required to provide a basis for interpretation of the 3D information obtained from drill hole data, and from the AEM.



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