CRC LEME
Open File Report 172
ABSTRACT
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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