1.1.3.2 Stratigraphy and rock type


1.1.3.2.1 The Gunnedah Basin

The Permian rocks in the Gunnedah Basin are a compositionally mixed assemblage of sandstone, siltstone, claystone, tuff and coal (Danis et al., 2010). Bedding is steeply dipping, and the correlation of coal seams across the basin is difficult (O’Neill and Danis, 2013). In contrast to the older Permian rocks, the Triassic strata in the Gunnedah Basin are dominantly sandstone. The stratigraphy of the Gunnedah Basin is shown in Figure 20. Detailed stratigraphic descriptions of all rock units in the Gunnedah and Surat basins are beyond the scope of this report. However, a brief summary of the main coal-bearing units of each basin is provided below.

Goonbri Formation

The Goonbri Formation is an Early Permian lake deposit characterised by a coarsening-upward sequence. The lowermost section is dark, massive siltstone, the central section is interbedded fine-grained sandstone and siltstone, and the uppermost section is fine- to medium-grained sandstone (Etheridge, 1986; Australian Stratigraphic Units Database, 2013). Thin coal seams are present in this unit (Australian Stratigraphic Units Database, 2013).

Maules Creek Formation

The Early Permian Maules Creek Formation contains mostly non-marine marsh plain deposits composed of sandstone, conglomerate and coal, with some clay pellet beds and carbonaceous claystone (Beckett et al., 1983). It occurs in the basal part of the Bellata Group and is up to 125 m thick (Beckett et al., 1983). The upper part of the unit shows evidence for marine transgression and change to a deltaic depositional environment (Beckett et al., 1983). Coal in this unit occurs in a series of up to 25 seams with a maximum thickness of nine metres (Beckett et al., 1983; Tadros, 1995; Ward and Kelly, 2013).

Pamboola Formation

The Pamboola Formation consists of sandstone, siltstone, claystone, conglomerate and coals (Australian Stratigraphic Units Database, 2013). A distinctive sub-unit, the Melvilles Coal Member, is a high quality and thick coal seam that covers a large portion of the basin (Hamilton, 1985). It is thickest in the east of the basin, up to 3.5 m, but thins towards the western boundary (Hamilton, 1985). In the south-east, this seam splits into two (Hamilton, 1985). This unit forms part of the Brothers Subgroup in the Black Jack Group (Australian Stratigraphic Units Database, 2013).

Hoskissons Coal

This coal unit varies from 2.4 to 18 m thick (Hamilton, 1985). It formed in a lagoonal and swamp environment, and is composed of coal and carbonaceous siltstone (Beckett et al., 1983; Hamilton, 1985). This formation is part of the Coogal Subgroup of the Black Jack Group (Australian Stratigraphic Units Database, 2013).

Benelabri Formation

The Benelabri Formation is composed of interbedded claystone, siltstone, sandstone and coal (Australian Stratigraphic Units Database, 2013). This formation is also part of the Coogal Subgroup of the Black Jack Group (Australian Stratigraphic Units Database, 2013).


Figure 20

Figure 20 Stratigraphic chart for the Central West subregion

This figure has been optimised for printing on A3 paper (297 mm x 420 mm).

The younger sediments overlying the Surat and Gunnedah basins are not shown.

Source data: derived from data presented in McKellar (1998); Totterdell et al. (2009); Cook and Draper (2013); Australian Stratigraphic Units Database (2013)


Clare Sandstone

The Clare Sandstone is a sandstone and conglomerate unit of the Black Jack Group’s Coogal Subgroup (Australian Stratigraphic Units Database, 2013). The Clare Sandstone contains the Breeza Coal Member, a coal-rich unit of variable quality coal and alternating claystone layers (Australian Stratigraphic Units Database, 2013).

Wallala Formation

The Wallala Formation is a unit composed of conglomerate, sandstone, siltstone, claystone, coal and minor tuff (Australian Stratigraphic Units Database, 2013). It forms part of the Black Jack Group within the Nea Subgroup (Australian Stratigraphic Units Database, 2013).

Trinkey Formation

The Trinkey Formation is in the Nea Subgroup, part of the Black Jack Group, and is composed of claystone, siltstone, sandstone, tuff and stony coal (Australian Stratigraphic Units Database, 2013). The Trinkey Formation is divided into six sub-units, four of which contain coal, namely the Clift Coal Member, Doona Coal Member, Springfield Coal Member and Whaka Coal Member. The Clift Coal Member is composed of five coal sections of variable quality, as well as inter-layered claystone that separates the coal seams (Australian Stratigraphic Units Database, 2013). The Doona Coal Member is a coal sub-unit with tuffaceous claystone laminae (Australian Stratigraphic Units Database, 2013). The Springfield Coal Member is predominantly coal with beds of claystone and tuffaceous claystone throughout (Australian Stratigraphic Units Database, 2013). The Whaka Coal Member is a coal unit with bands of claystone, siltstone and sandstone (Australian Stratigraphic Units Database, 2013).

1.1.3.2.2 The Surat Basin

The stratigraphy of the Surat Basin varies across its 300,000 km2 area. Distinct lithostratigraphic packages are defined for the main structural domains of the basin. The rock units defined for the southern NSW part of the Coonamble Embayment (Figure 20) differ from those in the better-known Queensland part of the Surat Basin. In particular, coal-bearing formations in the Coonamble Embayment are less common and markedly thinner than in Queensland. For example, the Walloon Coal Measures do not occur in this area, and the stratigraphic correlative, the Purlawaugh Formation, contains only thin coal seams that are uneconomic. Only minor coal-bearing units occur in the part of the Surat Basin that is in the Central West subregion.

Pilliga Sandstone

A Jurassic unit composed of largely quartzose sandstone and conglomerate but with a minor amount of interbedded mudstone, siltstone, shale, fine sandstone and coal, however carbonaceous fragments are common (Arditto, 1982; Herczeg, 2008; Australian Stratigraphic Units Database, 2013). The Pilliga Sandstone is an important aquifer unit within the Coonamble Embayment region (Herczeg, 2008).

Purlawaugh Formation

The Early to Middle Jurassic Purlawaugh Formation is composed of sandstone with beds of siltstone, mudstone and minor coal (Radke et al., 2012). Rare coal measures occur in this unit (Radke et al., 2012).

Drildool beds

The Cretaceous Drildool beds are composed of sandstone, siltstone, mudstone and coal, with rare breccia and pebble beds (Radke et al., 2012). This unit can be divided into three unnamed sub-units: the lowermost is a mixed assemblage of laminated mudstone, siltstone and muddy sandstone, the central section is composed of sandstone and siltstone that grade to carbonaceous mudstone and coal, and the upper sub-unit consists of laminated mudstone, siltstone and muddy sandstone, much like the lower sub-unit (Radke et al., 2012).

1.1.3.2.3 The Sydney Basin

The Permian to Triassic rocks in the Sydney Basin are a compositionally mixed assemblage of sandstone, siltstone, conglomerate, shale, claystone, tuff and coal (Mayne et al., 1974). The stratigraphy of the Sydney Basin is shown in Figure 21. Detailed stratigraphic description of all rock units within the Sydney Basin is beyond the scope of this report. However, a brief summary of the main coal-bearing units of the northern portion of the basin is provided below.

Greta Coal Measures

The Greta Coal Measures are a unit of varying facies such as siltstone, conglomerate, sandstone, carbonaceous shale with thinly bedded clay-ironstone and coal seams deposited in alluvial fan and fan delta environments (Mayne et al., 1974; Harrington et al., 1989; Australian Stratigraphic Units Database, 2013). The Greta Coal Measures are composed of several sub-units and coal members (Australian Stratigraphic Units Database, 2013).

Maitland Group

The Maitland Group is a unit with fewer subdivisions than the Greta Coal Measures, but one in particular is noted as containing up to eight coal seams: the Mulbring Siltstone (Harrington et al., 1989).

Singleton Supergroup

The Singleton Supergroup is a very largely diverse group composed of several facies units. Three of the four main subdivisions within the Singleton Supergroup are important coal-bearing sequences: the Newcastle Coal Measures, the Tomago Coal Measures and the Wittingham Coal Measures (Australian Stratigraphic Units Database, 2013). The Newcastle Coal Measures are composed of varying assemblages of sandstone, conglomerate, siltstone, shale, carbonaceous shale, coal and tuff subdivided into 32 units and coal seams (Mayne et al., 1974; Australian Stratigraphic Units Database, 2013). The Tomago Coal Measures are also subdivided into several units (29 sub-units) and are composed of siltstone, sandstone, coal, tuff, claystone and conglomerate deposited during a transition from prodelta to delta plain environments (Harrington et al., 1989; Australian Stratigraphic Units Database, 2013). The Wittingham Coal Measures also have a large number of subdivided units, totalling 30 internal divisions, composed of a variation of coal, claystone, siltstone, sandstone and conglomerate (Mayne et al., 1974; Harrington et al., 1989; Australian Stratigraphic Units Database, 2013).

Figure 21

Figure 21 Stratigraphic chart of the Sydney Basin

Source: FrogTech (2007)

1.1.3.2.4 Cenozoic stratigraphy

The youngest geological units in the Central West subregion are Cenozoic volcanic rocks, and alluvial to lacustrine sediments associated with modern rivers and paleovalleys. The dominantly mafic lavas and pyroclastic rocks of the Warrumbungle Volcanics have a restricted spatial distribution within about 40 km west and south-west of Coonabarabran. The basaltic Liverpool Range Volcanics occur only in the far east of the subregion. Both of these units were formed from localised volcanic eruptions during the Eocene (Liverpool Range Volcanics) and Miocene (Warrumbungle Volcanics), and locally intrude and overlie the Surat and Gunnedah basin strata.

The near-surface alluvial sequences of the main rivers and paleovalleys in the Central West subregion (which include the Macquarie, Castlereagh and Talbragar rivers) form important aquifer systems and are further discussed in Section 1.1.4 (hydrogeology). However, as the Cenozoic volcanic rocks and alluvial sediments do not contain any economic coal resources, they are not further discussed in this section.

Last updated:
5 January 2018