1.2.1.1.5 Winton Formation

Within the Galilee subregion, large tonnage, low rank coal seams at relatively shallow depth (below surface) occur in the Winton Formation of the Eromanga Basin. These were deposited in the Late Cretaceous (Cenomanian stage), approximately 100 million years ago (Figure 4), and are thus considerably younger than the more extensive Permian coals. The Winton Formation rests unconformably on the older Mackunda Formation of the Eromanga Basin, with the unit’s upper erosional surface overlain by Cenozoic sediments of the geological Lake Eyre Basin (Radke et al., 2012). These relationships are further described in product 1.1 for the Galilee subregion (Evans et al., 2014).

The Winton Formation has a maximum thickness of 1100 m (northern Patchawarra Trough in the south-west of the basin) and forms the uppermost part of the Rolling Downs Group (Figure 4). Throughout most of the Central Eromanga Depression formation thickness varies from 400 to 1000 m (Radke et al., 2012). The Winton Formation is a heterogeneous package of sedimentary rocks that include lithic and feldspathic sandstone, mudstone, siltstone, coal, minor conglomerate and layers of volcanic-derived detrital material (GA and ASC, 2014).

At a local-scale, significant resources of low rank coal (lignite to sub-bituminous coal) are known to occur. The coal varies from brown to black and is commonly interbedded with carbonaceous mudstone. There are two such large-scale resources within the Galilee subregion (the Inverness and Blackall deposits) as well as one to the south of the subregion boundary (South Blackall – not further considered in this report as it is not within the Galilee subregion). Considerable exploration effort is focused on similar deposits (in other nearby parts of the subregion) to assess their potential for future large-scale open-cut mining. These are further reported in Section 1.2.3.

The coals hosted in the Winton Formation commonly occur at less than 50 to 100 m below surface and form laterally extensive seams with low angle dips and minimal folding or faulting. Consequently, they may form large-tonnage deposits with low in situ strip ratios, with cumulative coal thicknesses of up to 10 to 12 m, in places (EER, 2014). These coals typically have raw ash content of 12 to 20% and generally low sulfur (Table 3). However, moisture content is typically much higher than in the older Permian coals of the Galilee Basin, and commonly exceeds 20%. These elevated moisture levels make the Cretaceous coals of the Galilee subregion less marketable as an export product.

Minor coal seams also occur in several other formations within the Eromanga Basin. These include the Mackunda Formation and the Birkhead Formation (Figure 4). Within the Galilee subregion, coals in these formations are not known to constitute economically viable resources.

Table 3 Typical composition of sub-bituminous coal in the Winton Formation in the Galilee subregion

Source data: Resolve Coal (2012)

MJ/kg – megajoules/kilogram