THE GEOMORPHIC EVOLUTION AND HYDROGEOLOGICAL ARCHITECTURE OF THE LOWER BALONNE FLOODPLAIN, QUEENSLAND, AUSTRALIA

The floodplain of the lower Balonne river is part of the upper reaches of the Murray Darling Basin, an area of considerable agricultural value for irrigated cotton farming and cattle grazing. The sedimentary architecture beneath this floodplain is of crucial importance for managing groundwater levels and salinity loads in an area increasingly relying on groundwater resources.

The present landscape consists of a series of juxtaposed depositional surfaces, surface expressions of an incised and infilled valley succession formed during the Pliocene onwards by the paleo-Balonne, Moonie, Maranoa and Condamine Rivers. Surface regolith landform and geomorphic map units were delineated from interpretations of remotely sensed data and field validation. Landsat TM, airborne gamma-ray radiometrics and aerial photography were heavily utilised. The surface distribution of regolith materials in the lower Balonne gives a fair indication of the complexity of regolith materials at depth, and their character and distribution in the major alluvial geomorphic units present can be described, if their actual locations at depth can not be predicted.

An extensive program of high resolution Airborne-Electro Magnetics (AEM) was also undertaken to improve the understanding of architecture at depth. Data from the survey was calibrated with down-hole geophysics, and considered with geological and hydrological information from boreholes.

The AEM proved to be an effective technique in the lower Balonne area for mapping buried bedrock topography, conductive weathered bedrock and areas of high conductivity associated with near-surface salinity anomalies. It was also effective in showing the variability within these geologic and hydrologic units which previously had been interpreted as lateral ‘layers'.

AEM data has been an extremely useful tool for interpolating between drillhole information to produce 3D sedimentary architecture in the Lower Balonne catchment. Coupled with surface data and mapping, this integrated geoscience approach has revealed a complex geomorphic evolution and sedimentary architecture concealed beneath a low relief landscape. This understanding of the complexity of the area will help future management plans for groundwater resources in the area.