Regolith can be described as the entire unconsolidated or secondarily re-cemented cover that overlies more coherent bedrock. Regolith includes fractured and weathered bedrock, saprolites, soils, organic accumulations, volcanic material, glacial deposits, colluvium, alluvium, evaporitic sediments, aeolian deposits and ground water (adapted from Eggleton 2001, p. 101), or in a shorter form as ‘everything between fresh rock and fresh air’ (Eggleton 2001, p. 101). It has been formed by weathering and erosion of the older material, and by transport and/or deposition of younger overlying materials.

Regolith is continuously formed and modified, and what we observe in today’s landscape is a result of past, present and ongoing events. Regolith is an integrated expression of geology, climate, groundwater, topography, geomorphic processes and landscape evolution and has a very close empirical relationship with landforms, both present-day and past. Landforms are themselves a reflection of climate, geology and predominantly near-surface geomorphic processes (Craig 2005).

South Australia has a wide variety and diversity of regolith materials and landforms. Over 75% of the State is covered by transported regolith material (Krapf et al. 2012) and is therefore a major challenge that mineral exploration faces in many parts of South Australia in exploring efficiently and effectively through this extensive and thick cover. In addition, regolith itself hosts important resources.

Regolith profiles at Mount Gunson Mine. (Photo 045300)

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Age of major events

Preservation potential of regolith, already poor due to its formation close to the earth’s surface, decreases generally with time, thus only fortuitous and major culminations of regolith development will be retained in the older rock record.

The formation of regolith is closely related to weathering driven dominantly by changes in climate. This resulted in the widespread occurrence of extensively weathered rocks throughout South Australia, commonly reaching depths of 10 to 100 m.

• Extensive deep weathering occurred prior to the Middle Eocene and may be as old as early Mesozoic.
• Deep weathering after the Late Eocene – Early Oligocene appears to be dominantly related to the formation of duricrusts (Benbow et al. 1995) and paleodrainage and ancient coastal barrier systems (Hou et al. 2012).
• Duricrusts formed semi-continuously from Early Paleocene to Pleistocene and cap a variety of paleosurfaces of different ages and origins.
• Major silcrete and minor ferricrete formation occurred in the Paleocene to Early Eocene, Late Eocene and Late Miocene to Early Pliocene.
• Increased aridity and persistent strong winds during the Pleistocene caused the development of extensive dunefields like the Simpson and Great Victoria deserts. During this period a variable influx of aeolian carbonate dust, sourced from the exposed continental shelf during sea level low stands, lead to extensive calcrete and soil formation throughout South Australia.

Geological timescale (download PDF).

Prospective commodities

Regolith hosted REE deposits globally including, ion adsorption clay.

Deposits (eg Koppamurra), alluvial and placer deposits; Ni, Co, Au, PGE, Fe, U, heavy mineral sands (eg ilmenite, chromite, magnetite, zircon, rutile, leucoxene, garnet), precious stones (eg opal, chrysoprase, sapphire, diamond), extractive minerals – calcrete, gypsum, phosphate, salt, kaolin, halloysite, palygorskite, silica sand, peat, industrial clay, groundwater, and construction materials.

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Major exploration tools

• Regolith sampling: calcrete, soil, transported regolith (sediments), lag, siliceous material, ferruginous material, in-situ regolith (bedrock/saprock/saprolite)
• Remote sensing (DEM, multispectral and hyperspectral data)
• Geophysical data (radiometrics, gravity, AEM, TEM, shallow seismic, GPR)
• Biogeochemistry sampling
• Groundwater sampling
• Paleochannel and paleolandform mapping
  • Paleodrainage and Cenozoic coastal barriers map 2nd edition
    The updated version includes revised sediment boundaries based on newly acquired datasets; a modified approach to paleodrainage mapping following industry feedback (eg airborne electromagnetic (AEM) interpretation workshops); new data from mineral exploration activity (eg drilling data); and geological sections illustrating sediment and bedrock relationships.
  • The 2M Paleodrainage and Cenozoic coastal barriers GIS dataset is available from SARIG via the Geology - palaeodrainage systems layer group.
• Regolith mapping
  • The 1:2 000 000 regolith map of South Australia consists of three main data components — regolith materials and landforms, regolith material induration (ferruginous, calcareous, siliceous, gypsiferous, mixed and undifferentiated), and surface lag. The 2M regolith GIS dataset is available from SARIG via the Geology - regolith layer group.
  • Regolith material of the Greater Tarcoola Area of the Central Gawler Region (MALBOOMA, TARCOOLA and KINGOONYA 1:100 000 map sheets)
  • Regolith map of Coompana region
  • Regolith map of the Southern Gawler Range Volcanics
• Relevant regolith layers in SARIG
  • South Australia regolith 2M – state wide cover
  • South Australia regolith maps Coompana/S-GRV/Tarcoola
  • CRCLEME maps
  • South Australia paleodrainage and paleochannels
  • Remote sensing - regolith ratios (ASTER VNIR - SWIR)

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Summary geology

Regolith materials can be divided into five broad types — transported, in situ, indurated, lags, and soils. Observing and mapping these broad types of regolith materials, along with landscape setting, can assist in creating regolith-landform relationship models to aid in understanding and characterising regolith and cover.

Types of regolith

Transported sediments are the most generalised transported regolith material class, as it refers to regolith material that is either of mixed or unknown origin but has experienced transport, especially in areas of transitional or spatially complex regolith landform regimes. They are often overprinted by calcareous and/or siliceous induration or they represent the source sediments of an overlying lag.

In situ regolith accounts for 25% of South Australia’s regolith by area. There is hardly any outcropping bedrock in South Australia that hasn’t experienced some degree of weathering. Weathering intensity can vary dramatically over short distances within an outcrop area.

Examples of transported and in-situ regolith in South Australia:

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Induration

Induration of regolith material (both in-situ and/or transported) has and is still occurring as part of South Australia’s weathering history, mainly by the introduction and precipitation of silica, iron oxides, carbonates and sulphate (dominantly gypsum). The indurated material is generally present as distinctive cemented horizons (termed duricrust and hardpan/pan), which increase the substrate’s resistance to weathering and erosion. Hence the indurated regolith material is often preserved through topographic inversion as profiles capping topographic highs in the modern day landscape.

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Lag

Surface lags are common and widespread in many parts of South Australia - eg the Stony Desert and the Moon Plain northeast of Coober Pedy.

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Soil/Paleosol

Soils are common and widespread across South Australia but are most thickly developed in the south and southeastern part of the State or in areas of high rainfall and biological activity. Paleosols are relatively common across South Australia but are discontinuous, relict and tend to be incomplete.

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