Regolith Material of South Australia mapAge of major events

Regolith is continuously formed and modified, and what we observe in today’s landscape is a result of past, present and ongoing 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. Dating of regolith-forming events is frequently difficult and often only bracketed by stratigraphic units.

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 SA, commonly reaching to depth 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 Tertiary 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's during sea level low stands, lead to extensive calcrete and soil formation throughout SA.

Prospective commodities

Ni, Co, Au, PGE, Fe, U, heavy mineral sands (e.g. ilmenite, chromite, magnetite, zircon, rutile, leucoxene, garnet), precious stone (e.g. opal, chrysoprase, sapphire, diamond), calcrete, gypsum, phosphate, salt, kaolin, halloysite, palygorskite, silica sand, peat, industrial clay, groundwater, construction materials.

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

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Summary geology

Regolith can be described as the entire unconsolidated or secondarily re-cemented cover that overlies more coherent bedrock. It has been formed by weathering and erosion of the older material, and by transport and/or deposition of younger overlying materials. 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).

Regolith is continuously formed and modified, and what we observe in today’s landscape is a product/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 materials can be divided into five broad types: transported, in situ, indurated, lags, and soils.

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Transported regolith

Transported regolith refers to material of exotic or redistributed origin such as alluvium, colluvium, lacustrine, aeolian and marine sediment that blanket fresh or weathered bedrock. Transported sediment may be friable, or partially or wholly consolidated, or cemented (Eggleton 2001).

Aeolian sediments are the dominant regolith material in South Australia, covering more than 43% of the State. Aeolian landforms, such as dunefields and sandplains, are widely preserved in today’s landscape and form extensive parts of the Great Victoria, Tirari, and Pedirka-Simpson-Strzelecki deserts. Coastal dunes and foredunes are also included in this regolith unit.

Alluvial sediments include fluvial channel and overbank/floodplain as well as alluvial plain deposits. Alluvial deposits occur mainly along rivers and creeks, with the Murray, Cooper, Warburton, Macumba and Neales rivers accounting for the largest river systems in the State. Most of the river systems in South Australia are ephemeral and experience discharge/flooding only on an irregular basis.

Colluvial sediments are widespread along range fronts, slopes and rises, and on depositional and erosional plains. They are variably thick and bedrock-masking heterogeneous deposits of variable grain size. They are typically coarse and angular on upper slopes, where they dominantly represent debris flow deposits (Eggleton 2001). Downslope they decrease in grain size and transition into sheet flow deposits.

Sheet flow deposits are a subclass of colluvial sediments. They often have a finer grain size spectrum than higher angle colluvial slope deposits and therefore represent a more homogeneous sampling medium within the colluvial class. Sheet flow deposits are associated with lower slopes and generally display a distinct contour banding/tiger bush/striping surface pattern. This surface expression is due to banded mosaic vegetation patterns separated by bare ground, that run roughly parallel to contour lines of equal elevation, formed on gently sloping plains (Wakelin-King 1999).

Lacustrine sediments comprise fresh water lake deposits as well as playa and salt lake deposits, characterised by stratified clay, silt or sand. Soluble salts form often as a thin salt crust on the surface.

Lacustrine and playa beach sediments are situated along the shores of the large inland lakes and playas (e.g. lakes Blanche, Callabonna, Eyre, Frome, Gairdner, Harris and Torrens) and also include prominent remnants of Pleistocene beach ridges near Lake Eyre South.

Coastal sediments occur along the marine coastal zone of South Australia and include coastal barrier, back barrier lagoon, shoreface, tidal and delta deposits.

Paludal sediments occur only along and inland of the State’s South-East coast as carbonate-precipitating lakes and swamps. They represent almost level, closed or almost closed depressions with a seasonal or permanent water table at or above the surface, commonly aggraded by overbank stream flow and sometimes with biological (peat) accumulation (Eggleton 2001). Today most of these areas are drained and dry.

Spring deposits are associated with the mound springs of the southern margin of the Great Artesian Basin. These springs can arise from a mound, rock fracture or a seep, and form low hills of limestone and gypsiferous, calcareous and carbonaceous/organic silt surrounded by wetlands around their base (Habermehl 1982; Sheard and Smith 1993).

Relaitvely young volcanic material is restricted to the Quaternary basaltic lava and ash deposits of the Mount Gambier and Mount Burr groups in the Lower South-East of the State. Several cones, domes and maars that punctuate today’s landscape characterise the volcanism in this area (Sheard 1993).

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.

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In situ regolith

In situ regolith refers to weathered rock that has undergone either no or minimal physical transport (Eggleton 2001) and 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.

Variably weathered bedrock can be found in many areas of the State and occurs in all geological provinces, with extensive exposures of bedrock mainly in the Flinders, Willouran and Mount Lofty Ranges, Gawler Ranges, Peake and Denison Ranges, Musgrave Ranges and the tablelands of the Eromanga Basin. Landforms associated with in-situ regolith are exclusively erosional and include eroding uplands and mountains, ranges, hills, tablelands, inselbergs and whalebacks, ridges, and erosional windows in areas of transported regolith.

In-situ regolith also includes residual material which ‘results from the weathering of rock without significant lateral movement of the solid weathered products’ (Eggleton 2001, p. 103) and is characterised by loss of volume mainly through solution.

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Induration is defined as ‘the hardening of a rock, rock material or regolith by the action of heat, pressure, or the introduction of some cementing material not commonly contained in the original mass: especially the process by which relatively consolidated rock is made harder or more compact’ (Eggleton 2001, p. 51). 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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Surface lags are common and widespread in many parts of South Australia, e.g. the Stony Desert and the Moon Plain northeast of Coober Pedy. Lag is a general term for a surface accumulation of usually loose clasts of diverse origin, such as various regolith materials, rocks, and mineral particles. Most lags range from granules to cobbles (2 to 256 mm) and are dominated by chemically and physically resistant minerals and rocks. They result from the removal of finer material by aeolian and/or sheet flow alluvial processes, or by matrix removal as a result of differential weathering (Eggleton 2001, p. 57). In Australia the term ‘gibber’ is often used as a synonym for particularly pebble- to cobble-sized lags. The type of lag present in an area is partly a function of the local regolith, landform and bedrock lithology. It can reflect the immediately underlying bedrock or regolith material, a local source, a distant source or a mixture of all three.

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Soil is the unconsolidated mineral matter on or near the surface that has been subjected to and influenced by pedogenic, genetic and environmental factors such as climate (including moisture and temperature effects), macro- and micro-organisms and topography. Soil differs from the material from which it is derived in many physical, chemical, biological, morphological properties and characteristics (Eggleton 2001). 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 fossil soils formed under previous environmental conditions that were different from those of the present. Paleosols are relatively common across South Australia but are discontinuous, relict and tend to be incomplete.

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Benbow MC, Callen RA, Bourman RP and Alley NF 1995. Deep weathering, ferricrete and silcrete. In Drexel JF and Preiss WV eds, The geology of South Australia. Volume 2, The Phanerozoic. Department of Primary Industries Resources South Australia, Adelaide. Bulletin 54:201-207

Craig MA 2005. Regolith-landform mapping, the path to best practice. In Anand RR and de Broeckert R, Regolith landscape evolution across Australia: A compilation of regolith landscape case studies with regolith landscape evolution models. CRC LEME, Perth, 354.

Eggleton RA (editor) 2001. The regolith glossary: surficial geology, soils and landscapes. CRCLEME, Canberra, ACT: 144.

Hou B, Fabris AJ, Michaelsen BH, Katona LF, Keeling JL, Stoian L, Wilson TC, Fairclough MC, Cowley WM and Zang W (compilers) 2012. Palaeodrainage and Cenozoic coastal barriers of South Australia. Digital Geological Map of South Australia, 1:2 000 000 Series (2nd Edition). Department for Manufacturing, Innovation, Trade, Resources and Energy, South Australia, Adelaide.

Krapf CBE, Irvine JA and Cowley WM 2012. Compilation of the 1:2 000 000 State Regolith Map of South Australia – a summary, Report Book 2012/00016. Department for Manufacturing, Innovation, Trade, Resources and Energy, South Australia, Adelaide.

Sheard MJ 1993. Quaternary volcanic activity and volcanic hazards. In Drexel JF and Preiss WV eds, The geology of South Australia. Volume 2, The Phanerozoic. Department of Primary Industries Resources South Australia, Adelaide. Bulletin 54:264-268

Sheard MJ and Smith PC 1993. Karst and mound spring deposits. In Drexel JF and Preiss WV eds, The geology of South Australia. Volume 2, The Phanerozoic. Department of Primary Industries Resources South Australia, Adelaide. Bulletin 54:257-261

Wakelin-King GA 1999. Banded mosaic (‘tiger bush’) and sheetflow plains: a regional mapping approach. Australian Journal of Earth Sciences 46:53-60.

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