1.1.3.2.1 The Bowen Basin
There are almost 50 stratigraphic units recognised throughout the Bowen Basin with varying distribution, both spatially and stratigraphically (Australian Stratigraphic Units Database, 2013). However, the only section of the Bowen Basin to fall within the Gwydir subregion is the southern Taroom Trough, which has a distinct stratigraphic assemblage. The most important coal-bearing rock units in the Bowen Basin likely to occur in the Gwydir subregion are summarised below, and are displayed in the Gwydir stratigraphic chart (Figure 19). However, there are currently no coal mining operations or coal seam gas developments in the Gwydir subregion and, given the thickness of the overlying Surat Basin strata in this subregion (up to 1200 m, Hawke and Bourke, 1984), future resource development of this part of the southern Bowen Basin is considered unlikely.
Flat Top Formation
The Flat Top Formation formed in a delta fan to shallow marine setting and is composed of siltstone, sandstone, mudstone, conglomerate, coal and tuff (Draper, 2013).
Banana Formation
The Banana Formation is an assemblage of mudstone, shale, siltstone, sandstone and with minor coal formed during the Late Permian in brackish to marine settings (Green et al., 1997).
Burunga Formation
The Burunga Formation is a Late Permian unit of sandstone, shale, siltstone, tuff and clay with significant lithological variations from region to region (Green et al., 1997). In the southern Bowen Basin this unit appears in three distinct layers, with the upper and lower layers coal measures and the middle layer a marine mudstone (Green et al., 1997). A sub-unit of the Burunga Formation is the Scotia Coal Member, which contains sandstone, shale, siltstone, coal and tuff (Green et al., 1997).
Baralaba Coal Measures
A marine and fluvial unit deposited during the latest Permian, the Baralaba Coal Measures contain a mixed package of sandstone, siltstone, mudstone, coal and tuff (Green et al., 1997). The basal sub-unit of the Baralaba Coal Measures, the Kaloola Member, consists of siltstone, sandstone, tuff and conglomerate (Green et al., 1997).
1.1.3.2.2 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 19. Detailed stratigraphic descriptions of all rock units in the Gunnedah Basin are beyond the scope of this report. However, a brief summary of the main coal-bearing units of the basin is provided below, although there are currently no active Gunnedah Basin coal mines in the Gwydir subregion.
Figure 19 Stratigraphic chart of the Gwydir subregion
Source data: McKellar (1998); Totterdell et al., (2009); Cook and Draper (2013); Australian Stratigraphic Units Database (2013)
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.
The Maules Creek Formation
The Maules Creek Formation is an Early Permian non-marine formation of marsh plain deposits composed of sandstone, conglomerate, coal, clay pellet beds and carbonaceous claystone (Beckett et al., 1983). This unit can reach a thickness of up to 125 m (Beckett et al., 1983). The coal in this unit is present in a series of up to 25 seams that can reach up to 9 m in thickness (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). The Melvilles Coal Member is an important component of this unit and is a high quality and thick coal seam, covering a large area (Hamilton, 1985). It is thickest in the east of the Gunnedah 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
The Hoskissons 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 part of the Coogal Subgroup of the Black Jack Group (Australian Stratigraphic Units Database, 2013).
Clare Sandstone
Clare Sandstone is a sandstone and conglomerate unit of the Coogal Subgroup (Australian Stratigraphic Units Database, 2013). The Clare Sandstone also contains the Breeza Coal Member, a coal unit of variable quality coal and 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 a formation within the Nea Subgroup, part of the Black Jack Group, composed of claystone, siltstone, sandstone, tuff and stony coal (Australian Stratigraphic Units Database, 2013). The Trinkey Formation is divided into six sub-units, of which four contain coal, namely the Clift Coal Member, the Doona Coal Member, the Springfield Coal Member and the Whaka Coal Member. The Clift Coal Member is composed of five coal sections of variable quality, as well as inter-layered claystone (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 mostly coal with minor 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).
1.1.3.2.3 The Surat Basin
The stratigraphy of the Surat Basin varies across its 300,000 km2 area, and distinct lithostratigraphic packages are defined for the main structural domains of the basin. The rock units defined for the NSW part of the Surat Basin (which includes the Coonamble Embayment and the Boomi Trough) differ from those in the better-known Queensland part of the basin. In particular, coal-bearing formations in the Coonamble Embayment region 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. Below are brief summaries of the coal-bearing stratigraphic units within the Gwydir subregion of the Surat Basin, and the hydrogeologically important Pilliga Sandstone.
Evergreen Formation
The Evergreen Formation is more commonly described from Queensland, although it may occur in far northern NSW. It consists of siltstone, shale, sandstone and coal which formed in a fluvial depositional setting on a coastal plain, with possible deltaic influence (Exon, 1976; Cook and Draper, 2013). The formation contains two sub-units, the Boxvale Sandstone Member and the Westgrove Ironstone Member (Exon, 1976; Cook and Draper, 2013). The Boxvale Sandstone Member, consisting of sandstone, siltstone and coal, was deposited during a brief marine transgression (Exon, 1976).
Walloon Coal Measures
The Walloon Coal Measures are the most significant coal resource in the Surat Basin and the unit targeted for coal mining and CSG within the basin. It is a compositionally varied formation that consists of thinly bedded claystone, shale, siltstone, sandstone (lithic and sublithic to feldspathic arenite), coal seams and minor limestone (Exon, 1976). A muddy pebble conglomerate also occurs in several locations (Exon, 1976). The depositional environment is interpreted as fluvial and swampy plains (Exon, 1976). In Queensland, the Walloon Coal Measures can be sub-divided into four units: the Durabilla Formation, Taroom Coal Measures, Tangalooma Sandstone and the Juandah Coal Measures (Exon, 1976; Swarbrick, 1973). The Taroom Coal Measures are a mixed sequence of sandstone, siltstone and mudstone, with abundant coal and carbonaceous mudstone interbeds (Scott et al., 2004). A mixed assemblage of sandstone, siltstone, mudstone, claystone, tuff and thin coal to carbonaceous mud seams form the Tangalooma Sandstone (Rohead-O’Brien, 2011). The sandstone is commonly heavily cemented or infilled with clay (Rohead-O’Brien, 2011). The Juandah Coal Measures are similar in composition and coal volume to the Taroom Coal Measures (Scott et al., 2004).
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, with common carbonaceous fragments (Arditto, 1982; Herczeg, 2008; Australian Stratigraphic Units Database, 2013). The Pilliga Sandstone is an important aquifer within the Coonamble Embayment region (Herczeg, 2008).
Purlawaugh Formation
The Purlawaugh Formation is an Early to Middle Jurassic unit within the Coonamble Embayment composed of sandstone with beds of siltstone, mudstone and rare coal (Radke et al., 2012). The Purlawaugh Formation is the stratigraphic equivalent of the Walloon Coal Measures in the Mimosa Syncline further to the north-north-east of the Gwydir subregion (Radke et al., 2012).
Westbourne Formation
The Westbourne Formation is a Late Jurassic formation of interbedded mudstone, siltstone, sandstone and some coal (Exon, 1976).
Orallo Formation
The Orallo Formation is composed of thin beds of siltstone and mudstone with some sandstone and minor conglomerate and coal deposited by fluvial channels in the Early Cretaceous to Late Jurassic (Exon, 1976).
Drildool beds
The Drildool beds are a Cretaceous unit composed of sandstone, siltstone, mudstone and coal, with some rare breccia and pebble beds (Radke et al., 2012). This unit can be divided into three unnamed sub-units: the lowermost unit is a mix of laminated mudstone, siltstone and muddy sandstone, the middle subdivision is composed of sandstone and siltstone that grade to carbonaceous mudstone and coal, and the upper unit is a mix of laminated mudstone, siltstone and muddy sandstone, much like the lower sub-unit (Radke et al., 2012). The Drildool beds occur in the Coonamble Embayment but are the lateral equivalent of the Bungil Formation in the northern Surat Basin (Radke et al., 2012).
Griman Creek Formation
Griman Creek Formation is a sandstone-dominated unit grading in places to siltstone and mudstone. Minor conglomerate and coal bands occur throughout and are more common in the upper section (Exon, 1976). The lower part of the Griman Creek Formation (Wallangulla Sandstone Member) is interpreted to have formed in a marine environment whereas the upper section (Coocoran Claystone Member) is interpreted as non-marine (Exon, 1976).
1.1.3.2.4 Cenozoic stratigraphy
The youngest geological units in the Gwydir subregion are Cenozoic volcanic deposits and alluvial to lacustrine sediments associated with modern rivers and paleovalleys. The basaltic lavas of the New England Tablelands (Central Province Volcanics) occur in the east of the subregion, and the Nandewar Volcanic Field (hawaiite, trachyandesite, tristanite, trachyte and tuff) (Stroud and Brown, 1998) in the south-east. These were formed from localised volcanic eruptions during the Paleogene and Neogene, and locally intrude and overlie the Surat and Gunnedah-Bowen Basin strata.
The near-surface alluvial sequences of the main rivers in the Gwydir subregion 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.
