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Open File Report 114

Wonnaminta regolith-landforms.

Gibson, D.L.

The Wonnaminta regolith-landform map is a product of the first attempt at 1:100 000 scale regolith mapping within the Koonenberry Belt, an inlier of dipping late Precambrian to Devonian basement rocks northeast of Broken Hill in far western NSW. The map and this report have been produced to show the range and distribution of regolith materials present in an area where basement rocks are overlain by the remnants of a veneer of Mesozoic Eromanga Basin sediments, and to document known aspects of regolith and landform history of the area. Relief is generally low, apart from ranges and plateaux of Devonian sandstone and several steep low hills of Eromanga Basin sediments. Most pre-Devonian basement rocks and the Mesozoic sediments have been deeply weathered. Outcrop of pre-Devonian rocks over much of the area is limited to saprolite exposed in watercourses incised into erosional plains and rises, which are dominated by a stony mantle over desert loam soils. This mantle is considered to be colluvial rather than residual lag, as it includes fragments that have not originated from the underlying rocks (eg cemented fragments of now-removed sediment), and in some areas has clearly been transported. Some of the basin sediment has been silcreted, resulting in local cappings of in situ silcrete and extensive areas of a mantle of silcrete fragments over both sediment and basement saprolite. Cementation of sediments is by iron compounds is also locally important. Aeolian deposits dominate the southern part of the area.

Relatively resistant Devonian sandstones make up two areas of high relief within the area, the Turkaro Range and Koonenberry Mountain. Analysis of these landforms and their relationships with the adjacent sediments suggests that at least the former, and probably both features were present in the Mesozoic, buried beneath Jurassic to Cretaceous sediments, and exhumed by differential erosion after lowering of relative base levels in the area by regional post-sedimentation warping. It is also interpreted that bedrock has been eroded to a maximum of a few tens of metres below the Mesozoic landsurface over most of the area.

Few metallic mineral occurrences have been recognised in the area. Possible geochemical sampling media for future exploration include surface stony materials, soil, stream sediments, regolith carbonate, ferruginous saprolite, rock chips, and silcrete. Ferruginous surface fragments are locally common, comprising one or more of the following: ferruginised basement saprolith, fragments of quartz-iron oxide veins from basement rocks, ferruginous silcrete, iron-cemented sediment, and magnetic "buckshot" gravels. The last are interpreted to probably be derived from the weathering of both Mesozoic and basement materials. The variety of surface ferruginous material present implies that sampling of undifferentiated surface ferruginous material may lead to erroneous geochemical results. Soil and stream sediments are likely to have an aeolian component that will dilute any bedrock geochemical signal. Silcrete has most probably formed exclusively in cover sediment, and is thus not presently considered to be a useful sampling medium for basement mineralisation. Regolith carbonate is present in many soils in the area. There has been no investigation of this as a potential gold exploration sampling medium. It is possible that the basal Mesozoic sediments could be used as palaeo-stream sediment samples. However, there are few exposures of cross-bedded sandstone that could be used to determine Mesozoic palaeocurrent directions, so this technique might only give a general indication of mineralisation.

Last updated: Friday, January 25, 2002 3:56 PM


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