Stratigraphy and rock type Gunnedah Basin

Following a review of work by researchers between the 1940s and 1990s, Tadros (1993) provided a detailed description of the stratigraphy of the Gunnedah Basin and also proposed a new stratigraphy. Subsequently, the stratigraphic nomenclature has undergone minor revisions (Figure 21[3]). In recent sequence stratigraphic studies that correlated the lithostratigraphy, sedimentary sequences, main tectonic events and basin phases of the Surat and Bowen–Gunnedah basins, Totterdell et al. (2009) have shown that the same series of basin-forming processes recognised in the Queensland portion of the Surat and Bowen basins also occurred in the Surat and Bowen–Gunnedah basins in NSW. However, there are minor differences in the timing and duration of the basin phases (Totterdell et al., 2009).

Figure 21

Figure 21 Stratigraphic column for the Gunnedah Basin and the overlying Surat Basin sediments

The column ‘Reservoir Potential’ indicates the potential of the rocks of the formation to store hydrocarbons. The column ‘Source Potential’ indicates the potential of the rocks of the formation to produce hydrocarbons.

Source: NSW Department of Trade and Investment (2013). This figure is not covered by a Creative Commons Attribution licence. It has been reproduced with permission from NSW Department of Trade and Investment.

Bellata Group

  • Leard and Goonbri formations: the Lower Permian Leard Formation is the basal unit in the Gunnedah Basin sequence and rests unconformably on the weathered volcanic basement (Hamilton et al., 1989). These sedimentary rocks were deposited in dominantly lacustrine environments which were a relatively common feature of the early stages of continental rifting for the upper part of Supersequence A (Totterdell et al., 2009). The formation is commonly 12 to 18 m thick (maximum thickness of 32 m) and consists of buff coloured kaolinised pelletoidal claystone, conglomerate, sandstone and siltstone, commonly interbedded with coal (Tadros, 1993). Clasts contained within the strata are kaolinite clay pellets generally <25 mm in diameter. Many of the colluvial deposits of the Leard Formation are derived from the weathering of the basal volcanic rocks (Tadros, 1995). The Goonbri Formation is a lacustrine lithic sandstone and siltstone. In the Maules Creek Sub-basin it is >125 m thick, and in the Bellata and Bohena troughs of the Mullaley Sub-basin it may be up to 106 m thick. It consists mainly of dark, organic‑rich siltstone, thin layers of coal and siltstone-claystone laminite, and fine to medium-grained sandstone (Tadros, 1993).

Boggabri Volcanics and Werrie Basalt

The floor of the Gunnedah Basin sequence consists of Lower Permian silicic volcanic rocks in the west, and intermediate to mafic and silicic volcanic rocks in the east (Tadros, 1993). The silicic volcanic rocks in the east are exposed along the Boggabri Ridge and are known as the Boggabri Volcanics, whereas the intermediate to mafic rocks which crop out in the east are correlatives of the Lower Permian Werrie Basalt (Hamilton et al., 1989). Towards the west of the basin, underneath the Rocky Glen Ridge and the Gilgandra Sub-basin, there are metavolcanic and metasedimentary sequences of the Lachlan Fold Belt (Tadros, 1995). Together with the lacustrine deposits of the Goonbri and Leard formations, these mafic and felsic volcanic rocks comprise Supersequence A as defined by Totterdell et al. (2009).

  • Maules Creek Formation: this formation is a thick unit in the Maules Creek Sub-basin. It is well developed east of the Boggabri Ridge and reaches a thickness of >800 m adjacent to the Hunter-Mooki Thrust System. It is equivalent to Supersequence B as recognised by Totterdell et al. (2009), and consists of a thick succession of coarse-grained clastic sedimentary rocks, mudstone and coal. West of the Boggabri Ridge in the other sub-basins it is normally <100 m thick. In the Maules Creek Sub-basin it consists of lithic conglomerate, sandstone, siltstone, claystone and coal seams of up to 8 m thick (Tadros, 1993). Sedimentary sequences occur as both large- and small-scale fining-upward cycles of conglomerate and coarse-grained sandstone to mudstone and coal. Deposition occurred mainly in braided streams (Totterdell et al., 2009). To the west of the ridge the Maules Creek Formation is divided into three lithologically distinct zones: a quartz-rich sandstone zone in the north, a central volcanogenic zone in the central region and a south-eastern zone with fine-grained sedimentary rocks rich in coal (Tadros, 1995).

Millie Group

  • Porcupine Formation: a marine transgression followed the end of sedimentation of the Maules Creek Formation and led to the deposition of the Porcupine Formation. This unit is up to 10 m thick along the western margin of the Mullaley Sub-basin, 20 to 60 m thick in the north, and 30 to 186 m thick in the south and south-east of the sub-basin (Tadros, 1993). The Porcupine Formation forms part of Supersequence C, which includes shallow marine to deltaic sedimentary rocks of the entire Millie Group up to the lower Black Jack Group (Pamboola and Arkarula formations) that unconformably overlie the dominantly fluvial Supersequence B (Totterdell et al., 2009). The Porcupine Formation consists of an upwards-fining sequence with pebble conglomerate at the base through sandstone to shale and siltstone near the top. Deposition began when fan deltas deposited conglomeratic sediments onto the marine shelf (Totterdell et al., 2009). Bioturbation occurs throughout the unit.
  • Watermark Formation: the shallow marine Watermark Formation occurs over much of the Mullaley Sub-basin and has a maximum recorded thickness of 230 m in the Breeze-Quirindi area (Tadros, 1995). The lower part of the formation forms an upward-fining, sandy siltstone, dark grey siltstone and claystone sequence containing common fossil zones and intense bioturbation and burrowing. The upper part is divided into two units, which together form a major upward-coarsening sequence (Hamilton et al., 1989).

Black Jack Group

  • Brothers Subgroup
    • Pamboola Formation: the Upper Permian Pamboola Formation comprises lithic sandstone, siltstone, claystone, conglomerate and intercalated coal, and varies in thickness from 89 m in the north to >206 m in the south-east (Tadros, 1993). The sequence is generally upward-coarsening and contains fine- to coarse-grained sandstone with finely macerated organic matter and coaly fragments (Tadros, 1993). The most significant coal seams of the formation formed within a lower delta plain environment and are contained in the Melvilles Coal Member (Hamilton et al., 1989). The unit is generally 2.5 to 3.5 m thick in the eastern part of the Mullaley Sub-basin but may be up to 5 m thick in the area of Boggabri. These are moderate to high-vitrinite coals with layers of fine-grained sandstone, carbonaceous siltstone, and claystone (Tadros, 1995).
    • Arkarula/Brigalow Formation: this formation occurs over much of the Mullaley Sub-basin south of Narrabri and ranges in thickness from 22 m in the Gunnedah Colliery area to 51 m in the north of the sub-basin. It is a shallow marine, upward-fining sequence, characterised by fine- to medium-grained sandstone at the base and finely interbedded sandstone and siltstone, laminated organic-rich siltstone, oscillation ripples, load casts and mud drapes at the top. To the north of Narrabri well-sorted, medium-grained sandstone with a pebbly base is common (Tadros, 1995). In the western and northern areas of the Mullaley Sub-basin the Arkarula Formation laterally grades into and is locally overlain by the Brigalow Formation, which consists of medium and coarse-grained pebbly quartzose and sandstone with subordinate fine-grained sandstone and thinly bedded siltstone and carbonaceous siltstone (Tadros, 1995).
  • Coogal Subgroup: the sedimentary sequence from the onset of the Coogal Subgroup deposition to the top of the Trinkey Formation is equivalent to Supersequence D as recognised by Totterdell et al. (2009). It marks a return to a non-marine depositional environment with fluvial channel to floodplain and peat swamp deposits, resulting in regionally extensive coal formation.
    • Hoskissons Coal Member: consists of inertinite-rich coal with subordinate layers of fine-grained sandstone, carbonaceous siltstone, claystone and tuff, which extends over much of the Mullaley Sub-basin. It ranges in thickness between >1 m and 18 m (Tadros, 1993). The coal is upward-dulling with the lower section containing lower mineral content compared to the upper section. The lower section is divided into two plies and the upper section is divided into three plies. Along the western margin of the sub-basin the coal is split by the Clare Sandstone (Tadros, 1995). The fluvially-influenced coal was formed during peat accumulation across an extensive floodplain.
    • Benelabri Formation: this formation is an organic-rich, mudstone-dominated unit above the Hoskissons Coal Member, present mainly in the eastern half of the Mullaley Sub-basin north of the Liverpool Range (Tadros, 1993). Totterdell et al. (2009) interpreted the formation as being deposited as the base-level continued to rise and accommodation reached its maximum. Contained within the Benelabri Formation are the Caroona and Howes Hill coal members. The formation thickness varies between 20 and 30 m but is up to 35 m in the north and ~69 m in the southeast (Tadros, 1995). The formation consists of upward-coarsening sequences, each consisting of organic-rich mudstone at the base, grading into mudstone/siltstone in the middle and mainly sandstone at the top. The Caroona Coal Member is confined to the southeastern corner of the Mullaley Sub-basin and is 2.3 to 3.4 m thick. It consists of a bright basal section, a dull and minor bright layered middle section and a bright upper section. The Howes Hill Coal Member is more extensive than the Caroona Coal Member and varies in thickness between 1 and ~5 m, with the coal having an upward-dulling profile (Tadros, 1995).
    • Clare Sandstone: in the Mullaley Sub-Basin the Clare Sandstone is a few metres thick in the west and up to 95 m in the southeast. The rocks of this formation are mainly medium- and coarse-grained sandstone with subordinate quartz conglomerate (Tadros, 1995). The top of the unit contains an upward-fining sequence of interlaminated siltstone and claystone topped with the Breeza Coal Member. This coal unit extends over much of the southern Mullaley Sub-basin and is between 5 and 7 m thick.
  • Nea Subgroup
    • Wallala Formation: this formation consists of lithic conglomerate, sandstone, siltstone, claystone and coal, with minor tuffaceous units and is up to 55 m thick (Tadros, 1995). These sediments are thought to be derived from the New England Orogen to the east (Totterdell et al., 2009). A conglomerate, which is up to 5 m thick and contains variable amounts of silicic and mafic volcanic rock fragments, dominates the sequence in the east and south-east. The top of the formation is commonly upward-fining.
    • Trinkey Formation: this formation occurs from Narrabri to Quirindi where it is up to 258 m thick. The formation consists mainly of finely bedded claystone, siltstone and fine-grained sandstone, and has some thinly bedded to massive tuffaceous sedimentary layers and coaly matter or stony coal seams. It also contains conglomerate beds up to 40 m thick in the areas between Gunnedah, Mullaley and Quirindi (Tadros, 1995). The formation contains several coal seams, including the extensively developed Clift and Springfield coal members. The former coal member is an upward-dulling coal which can exceed 10 m thickness, whereas the latter consists of several upward-dulling cycles up to ~5 m thick (Tadros, 1995).

Digby Formation

Deformation due to a contractional event in the Upper Permian resulted in erosion and led to the development of an essentially flat peneplain. Consequently, Supersequences E and F, which are recognised in the Bowen Basin, are not present in the Gunnedah Basin. The Lower Triassic, coarse-grained clastic sedimentary rocks of Supersequence G (i.e. the Digby Formation) were thus deposited directly on the Trinkey Formation (top of Supersequence D) in the Gunnedah Basin (Totterdell et al., 2009). The three lithological units of the Digby Formation are variably distributed across the basin, varying in thickness from 20 m in the north and northeast to >220 m in the south-east (Tadros, 1993). According to Hamilton et al. (1989) the formation appears to have formed from a series of major coalesced conglomerate fans which inundated and buried the older coal swamp delta plain of the Black Jack Group in the Upper Permian. The formation unconformably truncates the rocks of the Black Jack Group, the Watermark Formation and the Porcupine Formation in the northern part of the Gunnedah Basin. South of Narrabri it rests on various horizons of the Black Jack Group (Tadros, 1993). The Digby Formation consists mainly of lithic and quartz conglomerate, sandstone, and minor fine-grained sedimentary rocks (McDonald and Skilbeck, 1996).

Napperby Formation

The lacustrine to regressive deltaic sequence of the Triassic Napperby Formation (the Lower Triassic Supersequence H as recognised by Totterdell et al. (2009)) has three units. The lowermost unit is a finely laminated, dark grey siltstone 18 to 45 m thick which contains siderite and Fe-sulfides, indicative of deposition in a restricted near-shore environment. The middle unit is a sandstone/siltstone laminite that may have been deposited in a tidal flat setting. The uppermost unit consists of siltstone with minor interbedded claystone and fine-grained sandstone which developed in a regressive deltaic system (Hamilton et al., 1989; Othman et al., 2001). According to Totterdell et al. (2009) it is an aggradational to progradational coarsening-upward succession of lacustrine delta claystone, siltstone and sandstone in most parts of the basin. The units of the Napperby Formation are widely distributed across the basin, except for areas of the Maules Creek Sub-basin (Tadros, 1993).

Deriah Formation

The Deriah Formation is divided into a lower and an upper part, the former interpreted as deposits of a sandy alluvial fan and the latter as deposits of mixed load streams in point bars, levees, crevasse splays and poorly to well-drained swamps, as well as freshwater lacustrine environments (Tadros, 1993). Coarse to very coarse-grained granule-bearing sandstone occurs at base, and fine- to medium-grained green lithic sandstone and rare siltstone and claystone beds occur at the top of the lower unit. The upper unit comprises lithic sandstone and dark grey mudstone with minor plant roots and coal layers (Tadros, 1993). This formation is equivalent to Supersequence I which was deposited in fluvial, floodplain and lacustrine environments by a westerly prograding fluvial system (Totterdell et al., 2009). Surat Basin

In the Namoi river basin the Jurassic and Lower Cretaceous sedimentary rocks of the Surat Basin unconformably overlie much of the Permian-Triassic sequence in the western half of the Gunnedah Basin (Gurba et al., 2009). The Garrawilla Volcanics generally form the base of the Surat Basin (Figure 21). Totterdell et al. (2009) noted that the Surat Basin megasequence consists of the fluvial to marginal marine sedimentary rocks of the Lower Jurassic to Lower Cretaceous, and includes four major depositional cycles, Supersequences J to M, of which Supersequences K to M are the equivalents of the Purlawaugh, Pilliga, Orallo and Mooga formations (Stewart and Alder, 1995; Totterdell et al., 2009).

Garrawilla Volcanics

According to Totterdell et al. (2009) the Late Triassic–Early Jurassic igneous rocks of the Garrawilla Volcanics, which consist of tuffs and flows of mafic volcanic rocks, have formed topographic highs onto which the Surat Basin sequences were deposited, indicating that this magmatism pre-dated deposition of the Surat Basin sediments. Ages reported for these igneous rocks range from 218+8 to 142+6 Ma (Martin, 1993). Martin (1993) has attributed an upper mantle origin and the geochemical signature of intraplate basalts to these extrusive and intrusive rocks which occur mainly as bodies or sills.

Purlawaugh Formation

The Purlawaugh Formation, which is part of the Surat Basin sequence (Supersequence K as recognised by Totterdell et al. (2009)) is of Lower to Middle Jurassic age. The meandering fluvial and lacustrine sedimentary sequences, which extend northward into Queensland, have an average thickness of 30 m around Narrabri, but can be up to 76 m thick in other areas. Overlying the Gunnedah Basin, the formation commonly consists of a basal unit of mainly sandstone formed by meandering fluvial channel/point bar deposits, and is medium- to coarse-grained and partly conglomerate-rich. The upper unit consists of interbedded siltstone, shale, thin coal and sandstone interpreted as floodplain, channel margin and meandering stream deposits (Stewart and Alder, 1995).

Pilliga Sandstone

The Pilliga Sandstone was deposited by braided fluvial streams (Stewart and Alder, 1995). It also contains minor interbeds of siltstone and mudstone (Goscombe and Coxhead, 1995). The Pilliga Sandstone and the Orallo Formation are the equivalents of Supersequence L, the beginning of which marks a period of basin-wide fluvial incision prior to the deposition of a thick succession of braided-stream sandstone sequences (Totterdell et al., 2009).

Orallo Formation and Mooga Sandstone

In the far west of the Namoi subregion the Pilliga Sandstone is conformably overlain by non-marine to marine sandstone, siltstone, shale and mudstone of the Orallo and Mooga formations (Stewart and Alder, 1995). Cenozoic fluvial stratigraphy

The Cenozoic alluvium includes the Upper and Lower Namoi formations, and the Gunnedah and Narrabri formations. These are described in the Hydrogeology Chapter.

Last updated:
5 January 2018