The precise geometric definition of palaeochannels on the north-western Gawler Craton, South Australia, is important in the exploration for placers (e.g. gold and heavy minerals), secondary geochemical deposits (e.g. uranium) and even for lignite...
The precise geometric definition of palaeochannels on the north-western Gawler Craton, South Australia, is important in the exploration for placers (e.g. gold and heavy minerals), secondary geochemical deposits (e.g. uranium) and even for lignite and groundwater resources which probably occur in the channel sediments. Knowledge of palaeochannel architecture and any concentration of minerals in the channels is also useful to lend guidance to the location of both palaeochannel and bedrock lode deposits in the craton. Geoscientific data sets have lately been integrated in an investigation of Tertiary palaeochannels that once drained the north-western Gawler Craton and that have significance for mineral exploration. The objective of the study is the development of a comprehensive model to assist exploration in palaeodrainage terrains. This has been achieved through the combination of results from several geological and geophysical methods. These include interpretations from field exposures, a compendium of geological and drilling data, computer modelling of ancient landscapes, topographic and evaluated digital elevation models, remote sensing imagery, magnetics, seismic, gravity, airborne and transient electromagnetics, and radiometrics, all of which have contributed to a systematic investigation of both shape and depth of the channels. Physical property contrasts that exist between the channel sediments and underlying bedrock, for instance, can be differentiated by geophysical methods to locate the channel thalweg. Evidence from sedimentology has been combined with that of other geological and geophysical characteristics to arrive at a general reconstruction of palaeochannel architectures and depositional environments. The palaeochannels were originally incised into the pre-Middle Eocene landscape, mostly weathered basement, and became the sites where Tertiary fluvial, lacustrine and even estuarine sediments accumulated during the Middle-Late Eocene and Middle-Late Miocene. The application of detailed sequence stratigraphy and facies analysis over the palaeodrainage network has established what depositional changes were taking place in the palaeochannels as critical conditions, notably sea level and sediment supply, fluctuated. Present-day major refinements in remote sensing and geophysical techniques, data processing, sedimentology and computer-aided interpretations are now providing an effective, economic and efficient model for exploring these highly prospective terrains. The most successful procedure for defining the palaeochannels is to combine imagery and geological and geophysical methods to yield architectural and evolutional models of channel development. Though no palaeochannel deposits in the region have yet been mined, on-going research seeks to predict where placer and other deposits, as well as any related bedrock mineralisation, might be located.
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