PIRSA Minerals

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Mesozoic Basins

Age

Early Triassic to Late Cretaceous.

 

Prospective commodities

Coal, hydrocarbons (including oil shale), precious opal, celestite, clay minerals, sedimentary uranium, placer gold, diamonds and dimension stone.

 

In the northeast, epicratonic sediments were laid down over much of inland South Australia forming the Eromanga Basin. Further south, crustal extension and rifting between the Australian and Antarctic plates led to the formation of elongate, narrow and deep rift basins such as the Otway, Duntroon, Polda and offshore Bight Basins.

 

 

Coal

 

The only currently producing coalfield in South Australia is near Leigh Creek, where coal occurs in the Late Triassic part of a Late Triassic – Early Jurassic sequence of freshwater sediments. The low-grade sub-bituminous coal occurs in five discrete basins over a distance of 20 km. Lobe B (Telford Basin) is the largest and hosts the current mining operation. The coal seams are interbedded with mudstone, siltstone and sandstone, and range in thickness from a few metres up to 18 m for the main seam in Lobe B. Basin sediments dip radially towards the centre to form regular saucer-shaped depressions. The best coal in these deposits was worked out by 1976, after which operations returned to the larger but deeper deposit in the Telford Basin.

 

The present mine is owned and operated by NRG Flinders, which currently extracts ~3Mt per year of coal by open-cut methods. The mining operation involves drilling, blasting and removal of overburden and coal by shovels and trucks. After mining, the crushed coal is railed to the Port Augusta power stations where it is used as fuel to produce ~30% of Adelaide’s electricity.

 

Eromanga Basin

 

The Eromanga Basin covers approximately 1 000 000km2 of the interior of eastern Australia, ~360 000km2 of which lie in South Australia and, along with the Carpentaria and Surat Basins, forms part of the Great Australian Basin.

 

The Eromanga Basin overlies Neoproterozoic to late Palaeozoic sedimentary basins, and Precambrian basement. It is overlain by Cainozoic Lake Eyre Basin sediments.

 

The South Australian portion of the basin can be divided into a thin (generally <200m), marginal shelf or platform area in the southwest, and a deeper basinal area to the northeast. A hinge zone, termed the Denison–Willouran divide, separates these two areas.

 

The deeper northeastern area contains two major depocentres with up to 3000m of sediment (Poolowanna Trough and Cooper region) that are separated by the Birdsville Track Ridge.

 

Sediments within the Eromanga Basin can be divided into three packages

  • Lower non-marine
  • Marine
  • Upper non-marine

In the Early Jurassic to Early Cretaceous lower non-marine succession, large sand-dominated, braided fluvial systems drained centripetally into lowland lakes and swamps. The Early Cretaceous marine succession is dominated by thick highstand mudstones, with thin transgressive shoreface sandstone units. In the Late Cretaceous upper non-marine succession, meandering fluvial systems traversed a flood basin dominated by coal swamps and lakes.

 

The Eromanga Basin contains large resources of groundwater and hydrocarbons. The groundwater is contained in a basal sandstone aquifer (Algebuckina Sandstone – Cadna-owie Formation and equivalents). The groundwater system is generally referred to as the Great Artesian Basin, and is of great importance to the pastoral and mining industry.

 

The sedimentation rate during deposition of the upper non-marine succession was 5–10 times greater than during the older successions with almost half of the total sediment thickness being deposited. Following this phase of rapid deposition, erosion and deep weathering occurred during the Late Cretaceous to Paleocene.

 

 

Precious opal is mined at Coober Pedy, Andamooka and Lambina from Early Cretaceous Bulldog Shale that has been affected by Late Cretaceous – early Tertiary deep weathering. Numerous opal occurrences have been recorded between Lambina in the north and Andamooka in the south so the potential for additional discoveries of opal is considered high.