CRC LEME
Open File Report 114
ABSTRACT
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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