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Open File Report 172

An enhanced framework for natural resource studies in the Angus-Bremer Plains area, South Australia

Gibson D

This report presents an interpretation of the geology, regolith and landscape of the Angas-Bremer alluvial plain and surrounding areas. The study area is centred on the town of Strathalbyn and villages of Langhorne Creek and Milang in South Australia, and is based on interpretation of new airborne geophysical data. It links their interpretation to field investigations and targeted project drilling, along with a distillation of existing regolith, landscape, and geological data. The area was selected because of concerns regarding quantity and quality of surface and groundwater used for irrigation in the rapidly expanding prime grape-growing region around Langhorne Creek.

The new airborne geophysical data give new insight into the distribution of materials and groundwater quality of the area. The digital elevation models (DEMs) provide additional details on topography and surface processes. The airborne electromagnetics (AEM) conductivity data are interpreted to determine the distribution of salinity and sediments, and geological structure. Radiometric coverages give great detail on variations in surface materials, which in turn give clues to the underlying regolith/rock type. Field examination and project drilling helped provide additional information on the 3D distribution of materials.

The geological and geomorphological framework of the area is related to the deposition of Cenozoic sediments at the western margin of the Murray Basin, erosion of basement areas in the eastern Mt Lofty Ranges, and tectonic deformation, including tilting and faulting. Some of the fault movements may be quite recent in geological terms. The geometry of aquifers in sedimentary rocks has been affected by the faulting and tilting. In particular, faulting along the previously unrecognised Sandergrove Fault has resulted in a small groundwater basin southwest of Strathalbyn, while the main Tertiary aquifer further to the east has been offset by up to 80 m along the Bremer Fault. Downwarp associated with faulting has also resulted in areas of internal drainage which may be important for local aquifer recharge in tomes of higher rainfall.

Structure contours on the base of the Cenozoic sequence have been estimated from drill hole intersections and AEM data, and the Quaternary/Tertiary contact has been identified in many hundreds of drill holes. These surfaces can be used in new groundwater modelling of the aquifers. A fault previously thought to control the northwestern margin of the Milang groundwater basin, and to influence recharge to the basin is not evident in the new geophysical data. It is proposed that rather than having a faulted margin, the aquifer thins beneath Quaternary cover at this point. Conductivity data from the AEM suggest that groundwater recharge occurs along the Angas and Bremer Rivers across the entire alluvial plain.

The survey area is divided into ten broad areas, each with a relatively uniform distribution of soils, sediments, landscape and hydrogeology. It is postulated that each requires application of different natural resource management techniques.


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