Geology

The search for underground water and mineral wealth was the principal stimulus to the growth of geological knowledge about South Australia.

For the first fifty years, exploration for minerals was conducted by private companies and individuals; it was not until 1882 that the Geological Survey was established to carry out systematic mapping and investigation of mineral deposits and underground water resources. H.Y.L. Brown was appointed the first Government Geologist. Travelling by camel, often under considerable hardship, he undertook extensive fieldwork which resulted in the first geological maps of South Australia. Early editions of the map in 1883 and 1886 were superseded in 1899 by a State geological map at a scale of sixteen miles to the inch. This map, still broadly accurate, was the foundation of future geological work.

Brown's pioneering work was supplemented later by contributions from R. Tate, W. Howchin, D. Mawson and R.L. Jack. Since then many other geologists in government organisations, mineral exploration companies and universities have significantly advanced our understanding of the geological evolution of South Australia.

The surface geology map shows the broad surface distribution of rocks in South Australia. The subdivisions used are based not only on rock or sediment type but also on time. Appreciating the importance of time in geological processes is fundamental to understanding the geological record. Natural radioactive isotopes provide a means for reducing geological time; they indicate that the earth formed at least 4500 million years ago (Ma). Major geological time subdivisions are Archaean, Proterozoic, Palaeozoic, Mesozoic and Cainozoic. Each of these is further subdivided, as shown on which depicts characteristic South Australian fossils for each time period.

The main theme for the surface geology map is the succession of sedimentary and metamorphosed sedimentary strata and igneous events. Sedimentary sequences are grouped so as to define periods which deposition in the basin or on a shelf was controlled by uniform or interrelated processes. Similarly, the igneous events defined distinct pulses of magnetism. On the surface geology map, each igneous event and sequence of sediments is assigned a colour or symbol.

Archaean (Pre-2500 Ma)

The oldest rocks in South Australia are restricted to southern and central Eyre Peninsula and the Tarcoola areas. The original sediments of quartzite, shale, limestone and ironstone were deposited about 2700 Ma and were highly folded, deformed and intruded by granites, granodiorites and amphibolites about 2500 Ma. Their antiquity was determined only in the 1970s.

Early-Middle Proterozoic (2500-1100 Ma)

After deformation and metamorphism of Archaean rocks, there was a 300 to 500 million year period of tectonic quiescence and erosion. On the eastern and possibly also the northern margins of this ancient continent were deposited sediments of the Hutchison Group and its equivalents. These classic and chemical sediments host iron ores in the Middleback Range, jade near Cowell and barite near Olary.

The interval 1800-1580 Ma saw a major period of folding, metamorphism, intrusion of granite and volcanic activity known as the Kimban Orogeny. This orogeny influenced all rocks older that 1580 Ma, particularly those of eastern Eyre Peninsula and the Olary-Broken Hill area (Willyama Inlier).

Immediately following the orogeny, there was a period of extensive volcanism (Gawler Range Volcanics), associated sediments and volcaniclastics (Corunna Conglomerate) and further granitic intrusions. Probably related to this phase is the Olympic Dam copper-uranium-gold depots.

Other significant events during this period include deposition of very thick fluvial sediments in the Mount Gunson area (Pandurra Formation), and high grade metamorphism and deformation accompanied by the intrusion of granite and the basic-ultrabasic rocks of the Giles Complex about 1200-1100 Ma.

Adelaidean-Cambrian (1100-500 Ma)

Sedimentary rocks of these two periods accumulated on marine shelves and within basins or rifts. In the Adelaide Geosyncline, which now forms the Mount Lofty and Flinders ranges, rifting and subsidence resulted in extensive basaltic volcanism (Wooltana Volcanics) and the deposition of marine and evaporitic sediments (Burra and Callanna Groups). Subsidence continued, resulted in the formation of a sedimentary succession over 24 kilometres thick containing the Umberatana Group, the Wilpena and Kanmantoo groups, and the Truro Volcanics. Folding and metamorphism of the sediments and granitic intrusion along the eastern side of the geosyncline occurred in the late Cambrian Delamerian Orogeny. Copper and gold deposits of the Mount Lofty-Flinders Ranges are related to these vents.

Ordovician-Permian (500-233 Ma)

Ordovician and Devonian sediments crop out in the north-west of the State (Officer Basin) and may also be present at depth in the north-east (Warburton Basin).

The Carboniferous-Permian rocks record a very widespread glaciation. Glacial till, sand and clay may be seen south of Adelaide at Hallett Cove, in Inman Valley and at Cape Jervis; till and coal also occur extensively subsurface in the north and north-east of the State (Arckaringa and Cooper basins). Oil and gas are currently recovered from sediments of this age in the Cooper Basin.

Triassic-Cretaceous (233-65 Ma)

During the Triassic-Jurassic period, continental fluvial and swamp sediments were deposited in the north-east of the State. These sediments are of great economic importance as they contain coal deposits at Leigh Creek and oil near Moomba, as well as the main aquifers in the Great Artesian Basin (Eromanga Basin).

Basalt on Kangaroo Island probably erupted from crustal fractures just prior to the separation of Australia and Antarctica which began during this period.

Early Cretaceous shale and sandstone are widespread in the north-east of the State and represent a major marine transgression. Opal occurs within these rocks at Coober Pedy. Rocks of similar age occur in the Otway Basin in the South East.

Tertiary (65-1.8 Ma)

As Australia and Antarctica drifted apart during this period, shallow water marine sediments marginal to the continent were deposited in the Eucla Basin and other basins resulting from crustal warping and fracturing. Marine fossils are common in limestone cliffs along the River Murray and near Port Willunga.

Fluvial sands and lacustrine deposits accumulated within interior basins, and contain fossil remains of Diprotodon and other precursors of the present day marsupial fauna. Tertiary fluvial sands near Adelaide are an important source of construction material, while Tertiary lignites (brown coal) are a potential source of energy.

Quaternary (1.8 Ma-Present)

In this period Pleistocene glaciation resulted in considerable fluctuations in sea-level and periods of aridity more severe than the present climate. Dune ridges parallel to the Coorong coast and extending well inland were built up during times of high sea-level. The Bridgewater Formation, comprising coastal aeolian and sand-dunes, produces spectacular cliffs along some present-day southern and western coasts. Equivalent sediments inland are the long, parallel sand-dunes of the The Great Victoria and Simpson deserts. Salt lakes (playas), such as Lake Eyre and Lake Frome, also formed at this time.

The basaltic flows and tuffs of Mount Gambier are dated at about 4800 years ago.

Groundwater

Underground water occurs in three types of aquifer: unconsolidated deposits (alluvial, aeolian or dune aquifers), sedimentary basins and fractured rocks. All three types are represented in South Australia; the latter two occupy the greater area. Important local groundwater resources, however, are also obtained from relatively thin deposits of wind blown sandstone (aeolianite), particularly on Eyre Peninsula.

Large-scale supplies of water are obtainable from the larger sedimentary basins (Murray, Otway, Great Artesian and St Vincent). Fractured rock groundwater environments are fund in the Adelaide Hills, Flinders Ranges, Musgrave Block and parts of the Stuart Shelf. Groundwater supplies in these rocks are more restricted than in the sedimentary basins, but they have provided town water supplies at Leigh Creek South and in the Mid North and enabled the irrigation of market gardens in the Adelaide Hills.

The groundwater map broadly summarises the present knowledge of the quantity and quality of the State's underground water resources. The colour shading indicated the range of salinity levels in milligrams of dissolved salts per litre; the 'best' quality water with the largest range of uses is shown in blue. Four about half the area of South Australia the underground water is too saline for most livestock. Investigation of the groundwater resource is most advanced in the smaller basins and in a few localities where the intensity of use has reached a critical stage.

The largest sedimentary basin is the Great Artesian Basin; the South Australian portion covers 310 000 square kilometres, almost one-third of the State. Its exploration commenced in 1881 with the drilling of a successful flowing well at Anna Creek. Deep bores are necessary to tap these waters and very large flows have been obtained from stock-watering bores. It is proposed that the industrial and domestic needs of the Olympic Dam mining venture will be met by drawing from the resources of the Great Artesian Basin.

The Murray Basin contains groundwater of relatively low salinity in its southern portion. Development of this groundwater resource began with a well drilled at Coonalpyn in 1886. However, salinity increases northwards and in the vicinity of the River Murray the groundwater becomes unusable. The South Australian portion of the Otway Basin contains large supplies of low-salinity groundwater at shallow depth in the Gambier Limestone aquifer. Good quality groundwater has been obtained from the Adelaide Plains portion of the St Vincent Basin. However, withdrawal for irrigating market gardens has greatly exceeded the groundwater replenishment rate in recent years.

The number of pumping or flowing wells in South Australia is estimated to be approximately 150 000. Before 1976, there were very limited governmental powers to manage groundwater resources. The Water Resources Act of 1976 requires permits prior to drilling all wells to depths greater than 2.5 metres and provides controls on activities which may lead to groundwater contamination and over-exploitation; it also controls well construction methods in environmentally sensitive areas.

Tectonic Geology

Tectonic geology is the study of the history and evolution of the earth's crust. The two main tectonic elements are cratons and sedimentary basis. Cratons are stable crustal areas consisting of igneous and metamorphic rocks, while sedimentary basis are crustal troughs and basins filled with sediment from the erosion of cratons.

Cratons are usually highly deformed and show complex geological structures; however, sedimentary basis may remain relatively undisturbed or may be folded and metamorphosed to form fold belts welded on to existing cratons. The Adelaide geosyncline is an ancient sedimentary trough which has been folded and welded on to the eastern edge of the Gawler Craton.

The folding of a sedimentary basis to form a mountain chain is know as an orogeny. The earth's curst contains the record of many orogenies whereby successive generations of sedimentary basins have been folded and welded on to pre-existing cratons. This process is caused by the movement and collision of crustal plates.

The tectonic geology map shows that South Australia consists of a mosaic of cratons and sedimentary basins criss-crossed by linear fault zones. This arrangement is the result of the following events:

Sleafordian Orogeny (2500 - 2300 Ma)

This is the earliest known tectonic event in South Australia and is recorded in rocks found in the western part of the Gawler Craton. Rocks from this orogenic period are the oldest known in South Australia and probably formed part of the earth's first continental crust.

Kimban Orogeny (1820 - 1580 Ma)

Early Proterozoic sediments were intensely folded, fused on the Sleafordian rocks and intruded by granites to form the Gawler Craton. At the end of this orogenic period the northern part of the craton was covered by lavas which now form the Gawler Ranges.

Wartakan Event (1500-1400 Ma)

This was a period of minor folding, faulting and shearing affecting part of the Gawler Craton; the Cowell jade deposits were formed during this orogenic event.

Musgravian Orogeny (1200-1050 Ma)

Extending into Western Australia and Northern Territory, the Musgrave Block consists of a large craton of crystalline rocks forming a prominent series of ranges.

Petermann Ranges Orogeny (600-550 Ma)

This orogeny was concentrated in North Territory but had minor effects on rocks in the Musgrave Block.

Adelaide Geosyncline (900-500 Ma)

During this period the rocks which now form the Mount Lofty and Flinders ranges were deposited in a long shallow crustal trough know as the Adelaide Geosyncline. These sediments reached a maximum thickness of 24 kilometres and the development of this sedimentary basin was a very significant event in South Australia's geological history. The development of the Kanmantoo Trough on the southeastern flank of the geosyncline in Early Cambrian times led to the deposition of a further 8000 metres of sediment.

Delamerian Orogeny (500-480 Ma)

Sediments of the Adelaide Geosyncline were folded and faulted from Kangaroo Island to the far northern region of the State. Crustal movements related to this orogeny, which were accompanied by intrusion of granites such as those at Palmer and Victor Harbor, did not affect sediments of the Stuart Shelf on the northeastern part of the stable Gawler Craton. This area now forms an important mineral province which includes the Olympic Dam deposit.

Post Delamerian Basins

Since the Delamerian Orogeny there have been no major periods of crustal deformation in South Australia. However, a number of sedimentary basins, many of them controlled faults, developed in low-lying areas between the cratons. These basins contain South Australia's coal, oil and gas deposits.

Separation from Antarctica

Until the Late Jurassic Period, Australia formed part of the great Gondwanaland southern continent. About 150 Ma, a rift began to develop along the southern margin of the continent and separation of the Australian and Antarctic crustal plates began. The northwards movement of Australia was accompanied by a widening of the gap between the continents to form the Polda, Great Australian Bight, Duntroon and Otway sedimentary basins. The sea advanced from the north to form the Eromanga (Great Artesian) Basin. Subsequently during the Tertiary Period, the Murray and St Vincent basins were formed and further sedimentation took place in existing basins.

Faulting

The fault pattern shown on the tectonic geology map has resulted from the combined effects of all periods of tectonism. Many of the faults were formed during the Delamerian Orogeny and represent zones of major crustal weakness which was reactivated during subsequent orogenic movements. New faults were also formed and older ones reactivated during the separation of Australia from Antarctica. Small fault displacements are still taking place and cause minor earthquake activity.