Stratigraphy and rock type

The Hunter subregion includes parts of the Hunter, Newcastle and Western coalfields, and the stratigraphy and rock types of these areas are discussed in this section. More detailed information on the Southern Coalfield of the Sydney Basin can be found in the Sydney bioregion context statement report. The Central Coalfield is not discussed further as it is not a prospective area and it is currently unlikely that it would be developed in future. This section is not focused on discussing in detail all of the stratigraphic units known from the three coalfield areas. The focus here is on providing a suitable overview of the main coal-bearing stratigraphic units and some of the key non-coal bearing units. More information about the coal and coal seam gas resources, mines and proposed developments can be found in the Hunter subregion product 1.2. Hunter Coalfield

In the north-east of the Sydney Basin the Hunter Coalfield hosts the Newcastle, Wittingham and Greta coal measures. The main stratigraphic units for the coalfield are shown in Figure 33. According to Glen and Beckett (1989, p. 592), most of the sediments were sourced nearby from the uplifted New England Fold Belt. Sedimentation in the coal basins is characterised by four major episodes of deltaic to fluvial deposition, separated by three marine transgressive events.

Greta Coal Measures

The middle Permian Greta Coal Measures (Figure 33) were deposited by fluvial and deltaic sediment systems that prograded into the basin, and are exposed in the northern part of the coalfield near Muswellbrook, and in the south along the western limb of the Lochinvar Anticline. In the Muswellbrook area the Greta Coal Measures (comprising sandstone, siltstone, claystone, chert and coal) are characterised by crevasse-splay, marsh or lacustrine, and coal swamp deposits (Sniffin and Beckett, 1995, p. 177). The Greta Coal Measures are stratigraphically subdivided into several constituent formations. The Rowan Formation, which contains bright coal seams, igneous intrusions and fine-grained sedimentary rocks with some clastic sand bodies (Boyd and Leckie, 2000, p. 261), overlies the basal Skeletar Formation, which consists of colluvial/alluvial mudstone, sandstone and conglomerate. Around the Lochinvar Anticline the coal measures overlie the marine sedimentary rocks of the Rutherford and Farley formations, and have characteristic fine-grained conglomerate with associated sandstone, siltstone, mudstone and shale. The massive, fine-grained Neath Sandstone rests on the formations mentioned above and is overlain by the Kurri Kurri Conglomerate, which hosts the Lower and Upper Homeville Coal Members. Overlying that are the Kitchener and Paxton formations, which are also coal-bearing (Van Heeswijck, 2001, p. 420). Underground it is separated from the overlying Greta seam (Greta seam is an informal name according to the Australian Stratigraphic Units Database) mainly by conglomerate up to 30 m thick (Hutton, 2009, p. 48). The Greta seam in the Cessnock area is up to approximately 11 m thick. The coals are thicker in the Muswellbrook area, where the seams are associated with the Muswellbrook Anticline. In the south near the Lochinvar Anticline, the deepest coals, the Lower and Upper Homeville seams are low ash yielding coals with high volatile matter. Seam continuity is commonly disrupted by faulting and some seams are affected by igneous intrusions (Basden, 1969, p. 325).

Maitland Group

The Branxton Formation of the Maitland Group (Figure 33) was formed under transgressive marine conditions, followed by a significant marine regression depositing conglomeratic fan-delta and interbedded sandstone and siltstone forming the Muree Sandstone. Large amounts of silt and clay were then deposited in a deep marine shelf environment to form the Mulbring Siltstone (Sniffin and Beckett, 1995, p. 185).

Wittingham Coal Measures

The sandstone-dominated Saltwater Creek Formation represents a transition from a marine delta front to river-dominated lower delta plain deposits on which the overlying Vane Subgroup of the Wittingham Coal Measures rests. The Foybrook Formation of the Vane Subgroup contains coal seams interbedded with sandstone, siltstone, claystone and tuff layers, and was formed by a river-dominated delta system with sediments derived from two main source areas (Sniffin and Beckett, 1995, p. 180). A marine incursion followed deposition of the Foybrook Formation resulting in deposition of the sandstone-rich Bulga Formation and the Archerfield Sandstone; the latter comprises beach/barrier beach-lagoon deposits. Near Muswellbrook the Bulga Formation-Archerfield Sandstone sequence seems to form a single unit, whereas the Bulga Formation is absent towards the south (Sniffin and Beckett, 1995, p. 186). The coals of the Foybrook Formation are well developed in the Muswellbrook area but are characterised by erratic splitting. Many of the coal seams of the Wittingham Coal Measures are characterised by multiple splitting. Individual coal seams tend to be thin and of inferior quality, and have fewer igneous intrusions than the seams in the Greta Coal Measures (Sniffin and Beckett, 1995, p. 181).

The Jerrys Plains Subgroup developed as a river-dominated sequence from major source areas outside of the coalfield. The Bayswater Coal Member is the lowest coal seam in this sequence, and was formed by progradation of a back-barrier coal swamp. Deposition of alternating interdistributary bay laminites and upwards coarsening crevasse-splay sandstones occurred in a lower delta plain environment, with the thin and banded Broonie Coal Member and Vaux Coal Member forming part of this sequence. Upper delta plain conditions then resulted in thicker and laterally continuous seams such as the Piercefield Coal Member and Mount Arthur Coal Member, after which lower delta plain conditions were re-established with the deposition of the Glen Munro through to the Whybrow Coal Member. Deposition of the Jerrys Plains Subgroup ended with a marine transgression, forming the base of the Denman Formation (Sniffin and Beckett, 1995, p. 181–182). Most of the coal mined in the Hunter Coalfield is sourced from the Jerrys Plains Subgroup.

Newcastle Coal Measures

The siltstone-sandstone laminites of the Denman Formation and the coarse arenites of the Watts Sandstone represent a depositional transgressive-regressive event between the Wittingham Coal Measures and the Newcastle Coal Measures. Rapid change to lower delta plain conditions following the progradation of the Watts Sandstone is the unit in direct underlying contact with the Newcastle Coal Measures. The Newcastle Coal Measures were deposited under fluvial conditions with rapid channel migration. Seam splitting and erosion are common (Sniffin and Beckett, 1995, p. 189).

Structural elements of the Hunter Coalfield

The major structures in the Hunter Coalfield include the Lochinvar, Muswellbrook, Camberwell and Sedgefield Anticlines, the Belford and Loder Domes, and the Bayswater and Rixs Creek Synclines. These are the major synsedimentary structures which formed during the Permian and affected the distribution of most sedimentary sequences in the coalfield, as well as thickness and architecture of the sequences (Sniffin and Beckett, 1995, p. 178). The major fault systems, namely the Hunter-Mooki Thrust Fault in the east, and the Mount Ogilvie Fault in the west, can be seen in Figure 34. The Hunter River Cross Fault in the south is not shown in Figure 34 as insufficient information on the trace of the fault was available during map preparation.

Figure 34

Figure 34 Structural elements of the Hunter Coalfield (Hunter Thrust shown here is the same as Hunter-Mooki Thrust Fault)

Source: Sniffin and Beckett (1995) Newcastle Coalfield

Dalwood Group

The Dalwood Group comprises a mixed assemblage of marine sedimentary rocks and volcanic rocks, with the basal section consisting of shale, siltstone, and lithic sandstone alternating with basalt, volcanic breccia and tuff. The rest of the sequence consists of conglomerate, lithic and feldspathic sandstone, siltstone and shale, with minor limestone, marl and coal (Agnew et al., 1995, p. 197).

Greta Coal Measures

In the Newcastle Coalfield the Greta Coal Measures occurs as a stratigraphic wedge commonly 60 to 75 m thick (Agnew et al., 1995, p. 197), with a maximum known thickness of 90 m (McClung et al., 1980, p. 61). It comprises mainly sandstone and conglomerate with minor siltstone, shale and coal (Agnew et al., 1995, p. 197). The unit thins towards the south (McClung et al., 1980, p. 61). The Greta Coal Measures are confined in outcrop to the Lochinvar Anticline (McClung et al., 1980, p. 61). The massive and even-grained Neath Sandstone is the lowest unit in the coal measure sequence, on which the Lower and Upper Homeville Coal Members were formed as a result of peat accumulation behind advancing barrier islands. These seams coalesce and thin south of Kurri Kurri and are contained within the Kurri Kurri Conglomerate (Agnew et al., 1995, p. 203). The sulfur-rich Greta seam within the Kitchener Formation was deposited in front of a marine transgression and is split towards the east by the Kearsley Lens. The marine transgression was interrupted by coarse clastic sedimentation and deposition of the channel sands of the Paxton Formation, which hosts the Pelton Coal Member (Agnew et al., 1995, p. 204). Tuffaceous layers are fairly rare in the Greta Coal Measures (Agnew et al., 1995, p. 203). Unlike the Greta Coal Measures in the Hunter Coalfield, igneous dykes and sills are common in the Greta Coal Measures of the Newcastle Coalfield (Agnew et al., 1995, p. 204).

Maitland Group

The 1200 m thick sequence of marine sedimentary rocks of the Maitland Group is divided into three main formations: the Branxton Formation, the Muree Sandstone and the Mulbring Siltstone (Agnew et al., 1995, p. 197). The basal units consist of sandstone and sandy siltstone (Agnew et al., 1995, p. 199). The Branxton Formation, deposited during a steady marine transgression, contains mainly sandstone and conglomerate at the base and silty sandstone and siltstone are more common at the top (McClung, et al., 1980, p. 61). The Muree Sandstone consists of thick conglomerate adjacent to the Hunter-Mooki Thrust Fault, and sandstone and interbedded sandstone-siltstone facies further south (McClung, et al., 1980, p. 63). The Mulbring Siltstone approximately 330 m thick comprises mainly dark siltstone with minor claystone and sandstone (Agnew et al., 1995, p. 199) deposited under shallow marine conditions.

Tomago Coal Measures

The equivalent of the Wittingham Coal Measures in the Hunter Coalfield, the Tomago Coal Measures, thickens to the east, ranging from approximately 600 m near Maitland to greater than 1200 m in the Williamtown area (Agnew et al., 1995, p. 199). During deposition of the coal measures, marine to brackish environments prevailed (Diessel, 1980, p. 104). The coal measures contain one formation and two subgroups: the Wallis Creek Formation, Four Mile Creek and Hexham subgroups. The approximately 300 m thick Wallis Creek Formation is a cyclical sequence of terrestrial coal formation and brackish marine phases of bioturbated siltstone, fine-grained sandstone, mudstone and laminated shale. The main coal seams include the Morpeth and Rathluba Formations (Agnew et al., 1995, p. 204). The Four Mile Creek Subgroup has a higher coal to interseam sediment ratio compared to the other subgroups and shows no evidence of marine influence. It is approximately 160 m thick and the major economic horizons include the Donaldson seam and Big Ben Coal Member (Agnew et al., 1995, p. 204). The 140 m thick Hexham Subgroup is dominated by brackish to marine mudstone and laminite of the Dempsey Formation and the marine influence is reflected in sulfur values of the lower coals, such as the Buttai Coal. The uppermost section of the Hexham Subgroup is indicative of a lower delta plain setting and a return to terrestrial conditions of the Newcastle Coal Measures (Agnew et al., 1995, p. 204). The Tomago Coal Measures contain abundant thin but persistent tuffaceous claystone horizons (Agnew et al., 1995, p. 203).

Newcastle Coal Measures

The dominantly fluvial sequence of the Newcastle Coal Measures has a maximum known thickness of approximately 450 m, and consists of conglomerate, sandstone, siltstone, tuff and numerous coal seams (Agnew et al., 1995, p. 199). The coal measures were deposited in a high-energy terrestrial setting, resulting in significant amounts of coarse-grained sediments (Diessel, 1980, p. 104). Directly underlying the base of the Newcastle Coal Measures is the medium-grained, well sorted Waratah Sandstone which contains low- to high-angle cross bedding. On these sands the Borehole coal seam (informal name) developed in swamps covering back-barrier lakes and lagoons. Organic-rich mud, laminated shale and siltstone and sandstone form the base of the overlying interval of the Yard and Victoria Tunnel seams (informal names), an interval dominated by swamps and river deposits. Meandering river depositional conditions then changed to become braided channels. Following deposition of the Victoria Tunnel and Australasian seams (informal names), depositional conditions evolved from a braided channel to a higher energy piedmont-alluvial fan environment, culminating in the deposition of a conglomerate within the Adamstown Formation.

The coals within the interval between the Australasian and Fassifern seams (informal names) split and coalesce over short distances, and the interval is characterised by large volumes of tuffaceous material (Agnew et al., 1995, p. 205). Among the sedimentary formations in the Newcastle Coalfield, the Newcastle Coal Measures contain the greatest volume of pyroclastic rocks, with tuffaceous claystones of variable thickness and lateral extent occurring throughout the sequence (Agnew et al., 1995, p. 203). The clastic sediments of the upper Newcastle Coal Measures were deposited in well-defined channels by high-energy braided rivers. The Fassifern seam is the thickest and most widespread coal seam in the upper Newcastle Coal Measures. The lower part of the Fassifern seam is up to 8 m thick, and contains several mudstone and tuffaceous claystone bands in its basal section. The upper Fassifern seam is separated from the lower Fassifern seam by the lower phase of a conglomerate in the Boolaroo Formation, and the upper Fassifern seam is also separated from the Awaba Tuff by a conglomerate in the Boolaroo Formation.

The Awaba Tuff, which ranges from 1 to 27 m, is the most widespread unit in the Newcastle Coal Measures (Agnew et al., 1995, p. 206). Above the Awaba Tuff the Great Northern seam is associated with several phases of a major alluvial channel system deposited contemporaneously with the seam. At the top of the Newcastle Coal Measures the Wallarah seam is present over all but the north-western portion of the coalfield, where it is known as the Vales Point seam (Agnew et al., 1995, p. 208).

Structural elements of the Newcastle Coalfield

The Hunter-Mooki Thrust Fault forms the north-western boundary of the coalfield (Agnew et al., 1995, p. 208). Various fold structures influence the outcrop pattern, thickness variation and distributions of the Tomago and Newcastle Coal Measures in the Newcastle Coalfield (Diessel, 1980, p. 111). Major fold structures include the south-plunging Lake Macquarie Syncline in the centre of the coalfield. The Lochinvar Anticline is present in the west, as shown in Figure 35. Minor fold structures include the Delta Syncline at the Hunter River mouth and the Shepherds Hill Anticline (Diessel, 1980, p. 113). North-trending faults disrupt the Permian and Carboniferous units in the coalfield, some with displacements of greater than 60 m. Faults with less significant displacement occur throughout the coalfield, including normal, reverse, strike-slip and bedding plane faults. Igneous dykes are common (Agnew et al., 1995, p. 209).

Figure 35

Figure 35 Structural elements of the Newcastle Coalfield

Source: Agnew et al. (1995) Western Coalfield

Talaterang Group

In the Western Coalfield the Talaterang Group forms the basal part of the Sydney Basin succession and the rocks are relatively undeformed (Tye et al., 1996, p. 58). It is overlain by the Shoalhaven Group and includes the Clyde Coal Measures and the Wasp Head Formation (Tye et al., 1996, p. 58). Hutton (2009, p. 43) stated that there was little possibility of economic seams being found in the Clyde Coal Measures of the Western Coalfield as the seams are discontinuous and thin.

Shoalhaven Group

The relatively undeformed rocks of the Shoalhaven Group disconformably overlie the Talaterang Group, and are largely interpreted as being of marine shelf to coastal plain origin (Tye et al., 1996 p. 58, 63). The Shoalhaven Group is of early Permian age and includes the Yarrunga Coal Measures. Hutton (2009, p. 43) stated that there was little possibility of economic seams in the Yarrunga Coal Measures, since, similar to the Clyde Coal Measures, the seams of the Yarrunga Coal Measures are discontinuous and thin. In the Western Coalfield the Shoalhaven Group unconformably overlies basement metamorphic rocks of Silurian and Devonian age and Carboniferous granite (Yoo et al., 2001, p. 9). It consists of the Snapper Point Formation, an approximately 90 m thick medium-grained sandstone, and the Berry Siltstone, a predominantly grey micaceous sandy siltstone which can be up to 210 m thick (Yoo et al., 2001, p. 10).

Illawarra Coal Measures

The Illawarra Coal Measures dip gently to the east in the south and to the north-east in the north of the coalfield (Yoo et al., 1995, p. 231). In the Western Coalfield the sequence is divided into four subgroups: the Nile, Cullen Bullen, Charbon and Wallerawang subgroups.

The Mount Marsden Claystone is the lowermost formation of the Nile Subgroup, and consists mainly of sandstone and claystone (Yoo et al., 1995, p. 234). It is overlain by the quartz-lithic Coorongooba Creek Sandstone, which grades into the Gundangaroo Formation at the top. The latter is an interbedded quartz-lithic sandstone and carbonaceous siltstone with some coal beds (Yoo et al., 1995, p. 234, 239). The three formations of the subgroup, which were deposited under prodelta to lower delta plain conditions, are not consistently recognisable throughout the coalfield (Yoo et al., 2001, p. 15).

The main lithological units of the Cullen Bullen Subgroup are the Marrangaroo Conglomerate, the Lithgow Coal, the Blackmans Flat Conglomerate and the Lidsdale Coal (Yoo et al., 2001, p. 15). The Marrangaroo Conglomerate is an upwards fining pebbly sandstone or conglomerate which grades into a medium-to-fine-grained sandstone, between 2 and 24 m thick. Sediments of the Marrangaroo Conglomerate were incorporated into a fluvial environment which prograded into fan deltas. Extensive swamps in the interlobe areas of these deltas provided a suitable environment for the deposition of the dull coal and interbedded carbonaceous claystones of the Lithgow Coal (Yoo et al., 1995, p. 239). The Lithgow Coal overlies the Marrangaroo Conglomerate, and is the major economic coal unit in the Lithgow, Rylstone and Bylong areas, ranging in thickness from less than 1 to 9 m (Yoo et al., 1995, p. 236; Yoo et al., 2001, p. 21). It is overlain by the coarse-grained, commonly pebble-bearing, quartzose sandstone of the Blackmans Flat Conglomerate, which was deposited in an alluvial plain. The latter is up to 20 m thick in the south of the coalfield, and ranges from approximately 3 to 11 m at Bylong, and from 2 to 5 m in the Ulan area (Yoo et al., 2001, p. 24). The Lidsdale Coal consists of predominantly dull coal with minor bright coal layers, thin claystone, carbonaceous and tuffaceous claystone and siltstone, and ranges in thickness from less than 1 to 5 m. In the north of the coalfield the Lidsdale Coal is persistent in the Ulan area, where it makes up the lower section of the Ulan Coal. The Ulan Coal has a total thickness of approximately 14 m in the Ulan area (Yoo et al., 1995, p. 238–240; Yoo et al., 2001, p. 26).

In ascending stratigraphic order the Charbon Subgroup includes the Long Swamp Formation, Irondale Coal, Newnes Formation, Glen Davis Formation, Baal Bone Formation (Denman Formation equivalent), Angus Place Sandstone (Watts Sandstone equivalent) and State Mine Creek Formation. The Long Swamp Formation consists of claystone and siltstone, which are commonly bioturbated, tuff, sandstone, and thin discontinuous coal layers, formed within delta systems prograding from the north-east. The mean thickness of the formation in the west of the coalfield is 30 m and increases to 60 m in the area east of Rylstone and 100 m south-west of Mount Coricudgy (Yoo et al., 1995, p. 238; Yoo et al., 2001, p. 28).

The dull and bright coal with stone bands of the Irondale Coal is relatively thin (1.3 to 1.5 m) but persistent and formed in an overbank swamp environment. In the Ulan-Bylong area the uppermost ply of the Ulan Seam is correlated with the Irondale Coal (Yoo et al., 1995, p. 238–240; Yoo et al., 2001, p. 29).

The Newnes and Glen Davis formations were deposited as overbank and swamp deposits (Yoo et al., 1995, p. 240). The Newnes Formation is between 8 and 14.5 m thick between Lithgow and Ulan, and approximately 4 m near Bylong. It generally consists of a fine- to medium-grained, lithic sandstone, and interbedded siltstone and claystone west of the Wollar Hingeline, and an upward-fining lithic sandstone east of the Wollar Hingeline (Yoo et al., 1995, p. 238; Yoo et al., 2001, p. 29).

The coal, carbonaceous claystone, claystone, siltstone and sandstone of the Glen Davis Formation have a total mean thickness of approximately 8.1 m and generally range between 17 and 26 m thick in the north of the coalfield. The Glen Davis Formation contains two thin, uneconomic coal seams, the upper of which is the Bungaba Coal Member, which consists of dull coal (with minor bright coal) and numerous carbonaceous and tuffaceous claystone layers. At Newnes and Glen Davis the formation hosts oil shales. The thick quartzose sandstone of the Cockabutta Creek Sandstone Member is also hosted within the Glen Davis Formation and ranges in thickness from 2.5 to 9.8 m.

The Glen Davis Formation is overlain by the Baal Bone Formation east of the Ulan Hingeline and by the Moolarben Coal Member of the State Mine Creek Formation west of the Ulan Hingeline (Yoo et al., 1995, p. 238; Yoo et al., 2001, p. 35–37). The marine interval represented by the Baal Bone Formation is 24 m thick in the Lithgow area and as far north as Rylstone and Bylong. It is 10 m thick near Ulan and thickens in an easterly direction from 20 m at Bylong to 50 m at Denman. The rocks consist of dark grey claystone, laminated claystone and fine-grained sandstone with common bioturbation, into fine-grained, which grades into lithic sandstone of possible lower delta-front environment (Yoo et al., 1995, p. 238; Yoo et al., 2001, p. 40).

The coarsening-up lithic sandstone of the Angus Place Sandstone, a distributary mouthbar-crevasse splay facies, is white, coarser grained, cross-bedded and with calcareous cement and ranges in thickness from 5 m to a maximum of 15.5 m (Yoo et al., 1995, p. 239; Yoo et al., 2001, p. 41).

The lower delta plain facies of the State Mine Creek Formation consist of claystone, mudstone, siltstone, and minor sandstone, and three coal seams are generally present: the Moolarben and Turill Coal Members, and the ‘Lennox seam’ or ‘Goulburn seam’ (informal names), which is only locally developed in the Ulan area. The formation ranges in thickness from 5 to 10 m along the western margin and thickens towards the east. The Moolarben Coal Member, deposited in a back-barrier swamp, consists predominantly of dull coal with some bright coal layers at its base. Similarly the Turill Coal Member consists of dull coal with bright layers and numerous thin carbonaceous claystone layers in the lower half (Yoo et al., 1995, p. 239; Yoo et al., 2001, p . 41 – 43).

The Wallerawang Subgroup consists of the Gap Sandstone and the overlying Farmers Creek Formation; generally reaching a total thickness of approximately 27 m with a maximum recorded thickness 59 m (Yoo et al., 2001, p. 43). The fluvial channel Gap Sandstone is an off-white upward-fining medium-to-coarse-grained quartz-lithic to lithic sandstone, with a consistent thickness ranging from 3 to 5 m across most of the coalfield and locally thicker (11 m) in the Wilpinjong area (Yoo et al., 2001, p. 43). The Farmers Creek Formation hosts the Middle River Coal Member at the base, the Woodford Coal Member in the middle and the Katoomba Coal Member at the top (Yoo et al., 1995, p. 239). It consists of claystone, carbonaceous claystone, siliceous claystone, siltstone, sandstone and coal and oil shale (Yoo et al., 2001, p. 43). The formation consists of alluvial, point-bar, levee and floodplain sequences with floodplain swamps (Yoo et al., 1995, p. 240).

Narrabeen Group and Digby Formation

The quartz-lithic sandstones of the Narrabeen Group form near-continuous and mesa-like plateaux, which are characteristic morphological features in the Western Coalfield. The sequence can be up to 656 m and moderately thins westwards (Yoo et al., 2001, p. 52). In the north of the coalfield equivalents of the Narrabeen Group were referred to as the Wollar Sandstone, although this formation has lithological composition and lithofacies characteristics similar to the Digby Formation of the Gunnedah Basin. The four different rock types recognised in the northern part of the coalfield include conglomerate, quartz-lithic sandstone, quartzose sandstone siltstone and sandstone (Yoo et al., 2001, p. 52).

Hawkesbury Sandstone

The Hawkesbury Sandstone has a thickness of approximately 244 m in the Kurrajong Heights area and thins westwards to 52 m at Mount Tomah and 55 m at Mount Banks. It is massive, quartzose sandstone with numerous quartz conglomerate layers and sporadic shale lenses (Yoo et al., 2001, p. 53).

Napperby Formation and Wianamatta Group

The approximately 35 m thick Napperby Formation is recognised in the northern part of the Western Coalfield, and consists of an upwards coarsening lacustrine sequence, with finely laminated dark grey claystone at the base, finely layered siltstone and sandstone laminite in the middle, and lithic sandstone at the top (Yoo et al., 2001, p. 53). The Wianamatta Group is represented in the Western Coalfield by the Ashfield Shale. Rocks of the Napperby Formation in the northern part of the coalfield have similar characteristics to those of the Wianamatta Group (Yoo et al., 2001, p. 53).The Ashfield Shale consists of a lower sequence of dark grey to black, sideritic claystone-siltstone and grades upwards into a fine sandstone-siltstone laminite (Herbert, 1980, p. 262), which is preserved under basalt on the eastern side of Mount Tomah and at Mount Irvine (Yoo et al., 2001, p. 54).

Jurassic and Cretaceous units

In the northern part of the Western Coalfield, Lower to Middle Jurassic fluvial and lacustrine sedimentary rocks fluvial and lacustrine sedimentary rocks of the Purlawaugh Formation unconformably overlie the Triassic rocks of the Napperby Formation. The Purlawaugh Formation consists mainly of quartz-lithic sandstone, siltstone and sideritic ironstone lenses with minor layers of kaolinitic claystone, and is disconformably overlain by the medium-to-coarse-grained Pilliga Sandstone (Yoo et al., 2001, p. 54). North-east of Coolah approximately 90 m of fine-to medium-grained lithic sandstone, with mudstone, claystone, thin limestone and thin coal beds of the Bungil Formation overlie the Pilliga Sandstone (Yoo et al., 2001, p. 55). The thin coal beds of the Bungil Formation are not of economic interest.

Structural elements of the Western Coalfield

The northern part of the Western Coalfield is on the Gunnedah Basin’s Wollar Shelf in the west and the Murrurundi Trough in the east. The southern part of the Western Coalfield occupies the Sydney Basin’s Blue Mountains Shelf in the west and the Macdonald Trough in the east. Areas of the coalfield on the Wollar and Blue Mountains shelves are separated from the Murrurundi Trough and the Macdonald Trough by the Mount Tomah Monocline (Yoo et al., 2001, p. 57). Along with the Lapstone Structural Complex, a north-trending monocline and fault system within the Macdonald Trough, the Mount Tomah Monocline is the most significant structural feature of the Western Coalfield, as shown in Figure 36. The Mount Coricudgy Anticline is a major basement growth-feature with present elevation of approximately 2000 m above the floor of the Macdonald Trough (Yoo et al., 2001, p. 57). Structural hingelines in the coalfield include the Ulan, Wollar and Bylong hingelines (Yoo et al., 2001, p. 59 (Figure 9). One of the few major faults in the north-western part of the coalfield is the Kurrajong Fault, an east-dipping, high-angle reverse fault system. Several other faults are recorded in the western margin of the coalfield, particularly where intersected by mine workings (Yoo et al., 2001, p. 60).

Figure 36

Figure 36 Structural elements of the Western Coalfield

Source: Yoo et al. (1995)

Last updated:
18 January 2019
Thumbnail of the Hunter subregion

Product Finalisation date