1.2.1.1.1 Sydney Basin
Only coals of the northernmost extent of the Western Coalfield of the Sydney Basin are currently being mined in the Central West subregion, as discussed in Sections 1.2.2 to 1.2.4 . The main mined coal horizons in the Western Coalfield are within the Cullen Bullen and the Charbon subgroups of the Illawarra Coal Measures (Bayly, 2012, p. 3), which are of Permian age (see Figure 21 in companion product 1.1 for the Central West subregion (Welsh et al., 2014)). These are overlain by sedimentary formations of varying thickness, age and type including sandstone, shale and alluvium (Ulan Coal, 2014a). Other important coal seams, such as the Moolarben Coal Member, and Middle River Coal Member and Katoomba Coal Member, are within overlying units such as the Farmers Creek Formation.
In the Bylong (Figure 3) area the Lithgow Coal comprises mainly dull coal with a raw ash yield of 20%. The seam rapidly thins west of Bylong (Yoo et al., 1995, p. 241). Similar to the rest of the coalfield the Lidsdale Coal persists in the Ulan (Figure 3) area, where it makes up the lower section of the Ulan Coal. It is around 7 m thick, and has a low ash yield (11 to 13%, air-dried) (Yoo et al., 2001, p. 89). In the north of the Western Coalfield the upper half of the Cullen Bullen Subgroup and the lower half of the Charbon Subgroup are referred to as the Ulan Coal (Yoo et al., 2001, p. 89). The upper section of the Ulan Coal in the same area has raw ash yields ranging from 11 to 45% (Yoo et al., 1995, p. 243) and is typically layered (Yoo et al., 2001, p. 89). The total thickness of the Ulan Coal is around 14 m in the Ulan area (Yoo et al., 2001, p. 89). Multiple coal seams occur within the Illawarra Coal Measures and ten are referred to by Ulan Coal Mines Limited at the Ulan coal site (Ulan Coal, 2014a). Of these, nine are situated in the west, the bottom-most of which is the seam commonly referred to by Ulan Coal Mines Limited as the ‘Ulan Seam’ (Ulan Coal, 2014a). The seams range in thickness from 0.4 to 10 m, with the Ulan Seam being the thickest (Ulan Coal, 2012, p. 23). The Ulan Seam thickness averages 5 to 7 m and dips towards the north-east at 1 to 2° (Ulan Coal, 2014b). The dip of the coal and changes in topography generally increase the depth of cover towards the north-east (Ulan Coal, 2014b), from 40 to 50 m at the Open-Cut extension area to 320 m at Ulan No. 3 underground mine (Ulan Coal, 2012, p. 24). Apart from the Ulan Seam, all seams within the Ulan mining leases are discussed in Sections 1.2.2 and 1.2.3 (Ulan Coal, 2014a). At the northernmost extent of the coalfield, in the Coolah‑Turill area, the Lidsdale Coal, which has raw ash yield from 20 to 34%, is up to approximately 4 m thick and increases in depth and thickness towards the Liverpool Range (Figure 3) (Yoo et al., 1995, p. 243).
The Irondale Coal has economic potential in the Wolgan (Figure 3) area, where it is up to 2.4 m thick. It is a high-volatile, vitrain-rich, coking coal, although most of the resources are within the Wollemi National Park (Figure 3) (Yoo et al., 1995, p. 243). According to Yoo et al. (1995, p. 243) the Turill Coal Member is potentially mineable in the Turill-Cassilis area (Figure 3) (northern part of the coalfield) where it is up to 4 m thick at Cassilis. It commonly consists of bright bands and some stone bands and has raw ash of 23 to 45%. The Middle River and Katoomba coal members are economic towards the south of the coalfield, although outside of the Central West subregion (Yoo et al., 1995, p. 243).
The main coal horizons include the Lithgow Coal and the Lidsdale Coal within the Cullen Bullen Subgroup (southern area), the Ulan Coal (northern area), and the Irondale Coal stated by Yoo et al. (1995, p. 235) as being within the Charbon Subgroup, although the Australian Stratigraphic Units Database (ASUD, n.d.) does not formally deem this to be part of the Charbon Subgroup.
Figure 3 Geological features and geographic locations of and around the Central West subregion
1.2.1.1.2 Gunnedah Basin
The southernmost areas of the Gunnedah Basin (Figure 3), which fall within the Central West subregion, include the Oxley sub-basin (Figure 3), (O’Neill and Danis, 2013, p. 32), which is possibly synonymous with the Gilgandra sub-basin referred to by Yoo et al. (1995, p. 251), the Gilgandra Trough, referred to in EMM (2012, p. 4) and the western edge of the Mullaley sub-basin (see Figure 17 in companion product 1.1 for the Central West subregion (Welsh et al., 2014)). The Gilgandra Trough is separated from the Western Coalfield by Paleozoic rocks of the Rocky Glen Ridge (Figure 3), and the Dunedoo High to the north (EMM, 2012, p. 4). According to Yoo et al. (1995, p. 251) areas west of the Rocky Glen Ridge are largely unexplored for coal, although Permian rocks have been intersected in exploration drilling.
Apart from the Ulan Seam, the remaining strata of the Cobbora coal deposit has proven challenging to correlate between the Western Coalfield of the Sydney Basin and the Gunnedah Basin (Figure 3) (EMM, 2012, p. 22). The region contains pre-Permian metamorphic basement rocks overlain by coal-bearing strata of Permian, Triassic and Jurassic age (AMM, 2012, p. 22). Over 500 holes have been drilled in the area since the late 1970s and recent exploration has identified that the Cobbora coal deposit is primarily in a sinuous syncline where the regional dip increases on its flanks and where sedimentary rocks lap unconformably onto underlying basement to the west (EMM, 2012, p. 23). The Upper Permian strata contain five potentially mineable coal seams, which in stratigraphic descending order are the Trinkey Formation (2 to 5 m thick), Whaka Coal Member (2 to 14 m thick), Flyblowers Creek Coal (3 to 5 m thick) and two split seams of the Ulan Coal (each 2 to 5 m thick), of which the main mining targets are the two Ulan seams and the Flyblowers Creek Coal, which have consistent distribution with shallow overburden (EMM, 2012, p. 24–25). The seams in the Trinkey Formation and Whaka Coal Member have higher ash content than the other seams (EMM, 2012, p. 23). Coal seams subcrop continuously in a north-west line to the west of Laheys Creek and crop out at several locations (EMM, 2012, p. 25).
1.2.1.1.3 Surat Basin
Overlying the western Gunnedah Basin in the Central West subregion are sedimentary deposits of the Coonamble Embayment (Figure 3), part of the southern Surat Basin. Little is known about the coal-bearing formations in the Coonamble Embayment, but Radke et al. (2012, p. 66, 122) mentioned that coal seams are rare, with only thin, uneconomic coals present in the Purlawaugh Formation (see Figure 20 in companion product 1.1 for the Central West subregion (Welsh et al., 2014)) and the Drildool beds.
