1.1.3.2 Stratigraphy and rock type


The stratigraphy and rock types in the Cooper subregion are summarised below and in Figure 15. The stratigraphy includes crystalline basement rocks of the Thomson and Delamerian orogens, sedimentary rocks of Warburton Basin and Adavale Basin system, the thick sequences of the Cooper Basin, rocks of the Eromanga Basin and sediments of the geological Lake Eyre Basin.



1.1.3.2.1 Pre-Permian Basement

The rocks of the Thomson Orogen consist of low-grade metamorphosed sedimentary rocks of Cambrian to Ordovician age. Sandstone and mudstone are the dominant metasedimentary rock types encountered in the Thomson Orogen underlying the Cooper Basin. Granites have been intersected by drilling in the Thomson Orogen with one sample from north of the Cooper Basin dated at 428 ± 5 Million years ago (Ma) (Fergusson and Henderson, 2013).

The Cooper Basin unconformably overlies the sedimentary and volcanic rocks of the Cambrian – Ordovician Warburton Basin. The Warburton Basin consists of a sequence of marine sedimentary rocks deposited in a range of environments from the continental shelf to the deeper ocean. These include carbonate rocks and shale, siltstone and chert (Fergusson and Henderson, 2013).

The Adavale Basin System as defined by Draper et al. (2004) refers to the Adavale Basin and remnants of Devonian sedimentary rocks in the region. Devonian rocks are intersected in the Warrabin and Barrolka troughs, beneath the north-east Cooper Basin and can be identified from seismic data (Draper et al., 2004; Fergusson and Henderson, 2013; Murray, 1994).

Numerous granite bodies intrude the Warburton Basin and underlie the Cooper Basin. Early Devonian granites occur beneath the southern part of the Cooper Basin in Queensland and SA. middle Carboniferous and early Permian granites are present beneath the Nappamerri Trough, and include the Big Lake Suite granodiorite (Gatehouse et al., 1995; Meixner et al., 2012).

1.1.3.2.2 Cooper Basin

The Cooper Basin is Australia’s largest producing onshore hydrocarbon basin. As such, the geology of the Cooper Basin has been studied in detail over a long period of time. The following summary is based on regional work carried out in SA (e.g. Alexander et al., 1998; Boucher, 2001; Gravestock and Jensen-Schmidt, 1998; Hill and Gravestock, 1995) and Queensland (e.g. Draper, 2002b; Fergusson and Henderson, 2013), as well as basin-wide studies (e.g. Meixner et al., 2012; Radke, 2009).

Gidgealpa Group

The Gidgealpa Group contains rocks which are entirely of non-marine origin. Initially formed in glacial environments, it is characterised by coal measures that are separated by lacustrine shales. The Gidgealpa Group is thickest in the south-western part of the Cooper Basin, where it reaches a maximum thickness of 1200 m (Geoscience Australia and Australian Stratigraphy Commission, 2012). On the Jackson-Naccowlah-Pepita (JNP) Trend, the Group is 200 to 400 m thick, whereas in areas immediately adjacent it is less than 100 m thick. In the northern and north-eastern Cooper Basin, intersected thickness is from 50 to 150 m (Gray and McKellar, 2002). The Gidgealpa Group incorporates rocks ranging in age from Late Carboniferous to the end of the early Permian. The group comprises rocks of the Merrimelia Formation, Tirrawarra Sandstone, Patchawarra Formation, Murteree Shale, Epsilon Formation, Roseneath Shale, the Daralingie Formation and the Toolachee Formation (Geoscience Australia and Australian Stratigraphy Commission, 2012).

The Merrimelia Formation includes sediments derived from the terminal stages of glaciation. This unit interfingers with the overlying Tirrawarra Sandstone, which includes conglomerate, sandstone, mudstone, siltstone and shale. The Tirrawarra Sandstone is a fine- to coarse-grained and pebbly sandstone, with minor carbon-rich silt beds, shale and coal (Alexander et al., 1998; Draper, 2002b; Radke, 2009).

The Patchawarra Formation is a widespread, upward-fining, interbedded sandstone, siltstone, mudstone and coal-bearing succession. At its thickest, three assemblages are distinguishable:

  • a lower carbonaceous siltstone with minor sandstone and thin coal seams
  • a middle assemblage dominated by sandstone, with grey-black shale interbeds and thick coal seams. The thickest seam, up to 30 m thick and most laterally extensive, corresponds to a distinct seismic horizon (known as the Vc horizon)
  • an upper assemblage of siltstone and shale with minor sandstone interbeds.

The Patchawarra Formation overlies and interfingers with the Merrimelia Formation and Tirrawarra Sandstone, and is overlain by the Murteree Shale. Adjacent to the basin margin in SA, the Toolachee Formation and Eromanga Basin rocks overlie the Patchawarra Formation. The Patchawarra Formation is the thickest unit in the Gidgealpa Group, up to 680 m in the Nappamerri Trough (Draper, 2002b; Gray and McKellar, 2002; Radke, 2009).

The unit was deposited by fluvial systems on floodplains with peat swamps, lakes and coal swamps. It is thought that coal in the Patchawarra Formation was deposited in raised swamp environments (Alexander et al., 1998; Radke, 2009). Coal seams within the Patchawarra Formation can be up to 30 m thick, with an average of 2.1 m (Alexander et al., 1998). The cumulative thickness of coal in the Patchawarra Formation is spatially variable, and concentrated in various troughs, ranging from greater than 60 m in the Weena Trough, to 40 m in the Patchawarra Trough, less than 10 m in the Nappamerri Trough and between 10 and 15 m thick north of the JNP Trend (Draper, 2002b). The distribution and thickness of the Patchawarra Formation is shown in Figure 16. The Patchawarra Formation is a major coal-bearing formation in the subregion.

Figure 16

Figure 16 Distribution and thickness of the Patchawarra Formation in the Cooper subregion

Data: Draper (2002b); DMITRE (Dataset 10)

Discontinuities at state boundaries are a result of using different datasets compiled at different times by state agencies.

The Murteree Shale is a clay-rich siltstone with minor fine-grained sandstone, and some carbonaceous, muscovite and pyrite-rich sections. The unit becomes sandier towards the south of the Cooper Basin. The Murteree Shale averages 50 m thick, and is up to 80 m thick in the Nappamerri, Wooloo, Allunga and Tenappera troughs. The depositional environment is interpreted as deep lakes with minimal disturbance (Alexander et al., 1998). The Murteree Shale conformably overlies and interfingers with the Patchawarra Formation, and is in turn conformably overlain by the Epsilon Formation, although in places it is unconformably overlain by the Toolachee Formation (Alexander et al., 1998; Draper, 2002b).

The Epsilon Formation is a series of thinly bedded fine- to medium-grained sandstone interbedded with carbonaceous siltstone and shale and coal seams overlying the Murteree Shale and underlying the Roseneath Shale. The Epsilon Formation unconformably underlies the Toolachee Formation where the intervening units are absent. The Epsilon Formation is widespread across the southern Cooper Basin, but restricted to the Arrabury Trough across the JNP Trend. The maximum thickness of the Epsilon Formation is 156 m in the Nappamerri Trough, with the average thickness being 30 to 40 m. Three depositional stages are recognised in the Epsilon Formation, all representing fluvial, delta and lake environments. The lowermost stage is a coarsening upward sandy interval capped with coal and shale. This is followed by a coal-dominated stage, which is overlain by upward-coarsening sandstone. These represent deltaic and shoreline deposits, distributary channel, backswamp and peat swamp environments and lacustrine environments respectively (Alexander et al., 1998; Draper, 2002b).

The Roseneath Shale is restricted to the southern part of the Cooper Basin, and is found across the Nappamerri, Wooloo, Allunga and Tenappera troughs. The Roseneath Shale conformably overlies and interfingers with the Epsilon Formation, and is conformably overlain by and interfingers with the Daralingie Formation. The Roseneath Shale is a siltstone and mudstone, with minor fine-grained sandstone interbeds. This unit was deposited in a lacustrine setting. The unit is generally 50 to 80 m thick, and has a maximum thickness of 110 m in the Nappamerri and Tenappera troughs (Alexander et al., 1998; Draper, 2002b; Hill and Gravestock, 1995).

The Daralingie Formation consists of sandstone, shale and coal conformably overlying the Roseneath Shale. It is disconformably overlain by the Toolachee Formation. The Daralingie Formation is dominated by carbonaceous and micaceous siltstone and mudstone with interbedded, fine- to very fine-grained sandstone. These rocks form upward-coarsening cycles. The unit is restricted to the southern part of the Cooper Basin, averaging 15 to 30 m thick, with a maximum thickness of 120 m in the Nappamerri and Allunga troughs. The Daralingie Formation was deposited in prograding deltas and beaches developed as the lake in which the Roseneath Shale was deposited receded (Alexander et al., 1998; Draper, 2002b).

The uppermost and most extensive unit of the Gidgealpa Group is the Toolachee Formation. The Toolachee Formation comprises interbedded sandstone, siltstone, mudstone and shale with thin coal seams and conglomerates. Rocks are upward-fining in the lower part, and upward-coarsening in the upper part. Coal occurs as laterally continuous, thin seams (averaging 4.3 m, and up to 22 m thick, but generally less than 2 m thick; Alexander et al. (1998)). Basal conglomerate occurs adjacent to structural ridges. The lower part of the Toolachee Formation was deposited by meandering streams and in back swamps on floodplains. The upper part was deposited in flood basin lakes and during overbank flooding. Coarse-grained, upward-fining packages may represent higher energy alluvial channels (Alexander et al., 1998; Gray and McKellar, 2002). The Toolachee Formation disconformably overlies the Daralingie Formation, and unconformably overlies older units where the Daralingie Formation is absent. The Arrabury Formation conformably overlies the Toolachee Formation, or where the Arrabury is absent, Eromanga Basin rocks unconformably overlie the Toolachee Formation (Alexander et al., 1998).

In SA, the Toolachee Formation is thickest in the Nappamerri and Patchawarra troughs, with a maximum thickness of at least 200 m (Gray and McKellar, 2002; Meixner et al., 2012). In Queensland it reaches up to 120 m in the Windorah Trough (based on well intersections; Real Energy Corporation Limited (2014)). The Toolachee Formation is generally 25 to 50 m thick, and thickens from north to south and south-west, approaching 130 m thick immediately north of the JNP Trend, and 150 m thick to the south (Alexander et al., 1998; Gray and McKellar, 2002). Coal in the Toolachee Formation is thickest in the Patchawarra and Arrabury troughs, where it reaches a cumulative thickness of up to 40 m. It also occurs in the Nappamerri and Tenappera troughs. Coal reaches cumulative thicknesses of up to 13 m in the Ullenbury Depression, and is also in the Windorah Trough and the Yamma Yamma Depression. It is thought that coals were deposited in raised swamps, with restricted overbank flooding (Gray and McKellar, 2002). The thickness and distribution of the Toolachee Formation in the Cooper subregion is shown in Figure 17.

Nappamerri Group

The late Permian to Middle Triassic Nappamerri Group occurs across the entire Cooper Basin and thickens substantially in the Nappamerri, Patchawarra, Arrabury and Windorah troughs (where it may be up to 550 m thick). The Nappamerri Group thins over structural ridges except the Dunoon and Murteree ridges, and has been eroded around the margins of the Cooper Basin in SA. In Queensland, the Nappamerri Group has a much broader extent than the underlying Gidgealpa Group (Radke, 2009). The Nappamerri Group consists of the Arrabury and Tinchoo formations (Geoscience Australia and Australian Stratigraphy Commission, 2012). The distribution and thickness of the Nappamerri Group is shown in Figure 18 .

The basal unit of the Nappamerri Group is the Arrabury Formation. This unit is characterised by thin fine- to medium-grained sandstone interbeds overlain by sandstone with minor siltstone interbeds. The Arrabury Formation is widely distributed throughout the Cooper Basin. The Arrabury Formation unconformably overlies the Toolachee Formation and rocks of the Warburton Basin. It is unconformably overlain by Eromanga Basin rocks in the southern part of the Cooper Basin and conformably overlain by the Tinchoo Formation north of the JNP Trend. Where it overlies the Toolachee Formation, the base of the Arrabury Formation is distinguished by the lack of organic material in comparison to the underlying siltstone and coal.

Three members have been described from within the Arrabury Formation, the Callamurra, Paning and Wimma Sandstone members. The Callamurra and Paning members are mudstone and siltstone with interbedded fine- to medium-grained sandstone, whereas the Wimma Sandstone Member consists of sandstone with minor siltstone interbeds. These units may be indistinguishable in places, although a depositional hiatus of up to 10 million years has been postulated between the Callamurra and Paning members (Alexander et al., 1998). The Arrabury Formation has a maximum thickness of 412 m in the Arrabury Trough, and is also thickest in the Nappamerri and Windorah troughs. The remainder of the Arrabury Formation is between 50 and 100 m thick (Gray and McKellar, 2002).

Deposition of the Arrabury Formation is interpreted as being in vegetated floodplains with ephemeral lakes in lowlands, with pedogenesis in exposed areas. The floodplain was cut by low sinuosity rivers confined to north-east-oriented channel belts in the Patchawarra and Nappamerri troughs (Alexander et al., 1998).

Figure 17

Figure 17 Distribution and thickness of the Toolachee Formation in the Cooper subregion

Data: Draper (2002b); DMITRE (Dataset 7, Dataset 10)

Discontinuities at state boundaries are a result of using different datasets compiled at different times by state agencies.

Figure 18

Figure 18 Combined distribution and thickness of the Nappamerri Group in the Cooper subregion

Data: Draper (2002b); DMITRE (Dataset 10)

Discontinuities at state boundaries are a result of using different datasets compiled at different times by state agencies.

The youngest unit in the Cooper Basin (and the top of the Nappamerri Group) is the Tinchoo Formation. The Tinchoo Formation is a fining-upwards fine- to medium-grained sandstone with thin siltstone interbeds which conformably overlies the Arrabury Formation. In Queensland, it is subdivided into the Gilpepee and Doonmulla members, but these members are not readily identifiable to the south-west in SA where the formation has been eroded and is relatively thin (Alexander et al., 1998; Gray and McKellar, 2002). The Tinchoo Formation unconformably overlies the Warburton Basin to the north-west of the Cooper Basin towards the Birdsville Track Ridge. The Tinchoo Formation is unconformably overlain by the Cuddapan Formation or units of the Eromanga Basin. A maximum thickness of 109 m is preserved in SA where the formation has been eroded from crests of major ridges and towards the edge of the Cooper Basin. In Queensland the formation is mainly 125 to 200 m thick and thickest (to 263 m) adjacent to the Windorah Trough (Alexander et al., 1998; Golder Associates, 2011; Gray and McKellar, 2002).

The Tinchoo Formation is interpreted to have been deposited in a fluvial environment, which graded into a fluvial and lacustrine setting towards the end of deposition (Alexander et al., 1998; Gray and McKellar, 2002).

Cuddapan Formation

The Cuddapan Formation is not assigned to either the Cooper Basin or Eromanga Basin, and may be an outlier of the Simpson Basin to the west (Alexander et al., 1998). The Cuddapan Formation is lithologically similar to the overlying Poolowanna Formation in the Eromanga Basin and their distinction relies on dating. The Cuddapan Formation comprises a basal sandstone package with increasing siltstone and coal interbeds upwards. The Cuddapan Formation disconformably overlies the Tinchoo Formation, and is disconformably overlain by the Poolowanna Formation. This unit reaches a maximum of 67 m (Alexander et al., 1998) and occurs only as eroded remnants in SA, it is not present in Queensland. A fluvial system with some flood basin coal swamps is interpreted as the environment of deposition where the Cuddapan Formation overlies the Cooper Basin (Alexander et al., 1998).

1.1.3.2.3 Eromanga Basin

The Mesozoic Eromanga Basin overlies and extends well beyond the boundary of the Cooper subregion. The Eromanga Basin is thickest where it overlies the Cooper Basin, in the Central Eromanga Depocentre. Structural features of the Eromanga Basin in the Cooper subregion are shown in Figure 19. More detail on the rocks of the Eromanga Basin is provided in Alexander et al. (2006); Cook et al. (2013); Gray et al. (2002); Radke (2009); and Ransley et al. (2012a), which are summarised here.

Early Jurassic to Late Cretaceous deposition within the Eromanga Basin was relatively continuous and widespread. Deposition was controlled by subsidence rates and plate tectonic events on the margins of the Australian Plate to the east. Volcanic activity on the evolving continental boundary to the east influenced sediment provenance and depositional environment. On the southern margin, separation of Australia and Antarctica during the Late Cretaceous also influenced deposition within the Eromanga Basin (Alexander et al., 2006). The stratigraphy of the Eromanga Basin within the Cooper subregion is shown in Figure 15.

The Eromanga Basin rocks were deposited in terrestrial and marine sedimentary environments. There is a basal succession of terrestrial sedimentary rocks, followed by a middle marine succession, and an upper terrestrial succession. In the Early Jurassic to Early Cretaceous lower non-marine succession, large sand-dominated, braided fluvial systems drained into lowland lakes and swamps. The Early Cretaceous marine succession is dominated by thick transgressive shales, with thin sandstone units reflecting regressive cycles. In the Late Cretaceous (upper) non-marine succession, meandering fluvial systems were dominated by coal swamps and lakes (Alexander et al., 2006). The stratigraphy of the Eromanga Basin is summarised in Table 5.

Figure 19

Figure 19 Structural features of the Eromanga Basin in the Cooper subregion

Data: Ransley et al. (2015)

Table 5 Stratigraphy of the Eromanga Basin in the Cooper subregion


Unit name

Age

Lithological description

Thickness

Depositional environments

Stratigraphic relationships

Poolowanna Formation

Early Jurassic

Interbedded carbonaceous siltstone, fine- to medium-grained pebbly sandstone and rare coal with carbonate and clay mineral cements. In Queensland, the lower part is coarser-grained sandstone

up to 205 m

Alternating fluvial floodplain with minor coal swamps and lacustrine

Unconformable on Cooper and Warburton basins and Cuddapan Formation. Interfingers with Algebuckina Sandstone in the south and west and Hutton Sandstone in the north and east

Algebuckina Sandstone

Middle Jurassic to Early Cretaceous

Fine- to coarse-grained sandstone with coarser layers and shale intraclasts. Minor shale and siltstone lenses

up to 800 m

Braided fluvial

Lateral equivalent to Hutton Sandstone, Birkhead Formation, Adori Sandstone, Westbourne Formation, Namur Sandstone and Murta Formation

Hutton Sandstone

Middle Jurassic

Fine- to coarse-grained quartzose sandstone with minor siltstone interbeds. Upper part is generally sandier than the lower part

40 to 360 m

Braided fluvial

Unconformable on Cooper and Warburton basins and Warrabin Trough. Interfingers with Birkhead Formation. Laterally equivalent to Algebuckina Sandstone

Birkhead Formation

Middle Jurassic

Interbedded siltstone, mudstone and fin-to medium-grained sandstone with thin coal seams

40 to 100 m, maximum 150 m

Lacustrine and coal swamp with some meandering channels and deltas

Conformable on and interfingers with Hutton Sandstone, unconformably overlain by Namur and Adori sandstones

Adori Sandstone

Late Jurassic

Upward-fining, very fine- to coarse-grained sandstone with minor siltstone and conglomerate

20 to 130 m

Braided fluvial

Conformable on Birkhead formation and unconformably underlies Westbourne Formation in SA. Unconformable on Birkhead and Conformably underlies Westbourne in Queensland

Westbourne Formation

Late Jurassic

Shale and siltstone interbedded with minor fine- to very fine-, and some medium- to coarse-grained sandstone

30 to 140 m, maximum 166 m

Transition from fluvial to lacustrine and lake-shore

Conformably overlies Adori Sandstone. Conformably underlies and interfingers with Hooray, Namur and Algebuckina sandstones

Namur Sandstone

Late Jurassic

Fine- to coarse-grained sandstone with minor siltstone and mudstone interbeds and rare conglomerate interbeds with carbonaceous mudclasts

40 to 240 m thick

Fluvial

Conformably overlies Birkhead Formation or interfingers with Westbourne Formation

Murta Formation (includes McKinlay Member)

Late Jurassic

Thin interbeds of shale, very fine- to fine-grained sandstone with minor medium- and coarse-grained sandstone. Base marked by siltstone

30 to 60m, max 90 m

Lacustrine, possible marine transgression at the top

Lateral equivalent to upper part of the Hooray Sandstone. Interfingers with and conformably overlies the Namur Sandstone, gradation with Cadna-owie Formation

Cadna-owie Formation

Early Cretaceous

Silty mudstone, siltstone and very fine- to fine-grained sandstone. Sand increases upwards. Rare coal and carbonaceous fragments, and shale clasts

60 to 115 m

Fluvial, lagoonal, shoreface, beach, offshore marine and lacustrine. Transition between terrestrial and marine environments

Gradational, conformably overlies Murta Formation, or conformably overlies the Algebuckina Sandstone. Unconformably underlies the Bulldog Shale or Wallumbilla Formation

Rolling Downs Group: Bulldog Shale

Early Cretaceous

Fossiliferous mudstone, with minor siltstone and very fine-grained sandstone interbeds. Basal portion is carbonaceous

generally 200 m, maximum greater than 340 m

High latitude marine shelf

Conformably overlies the Cadna-owie Formation, lateral equivalent of the Wallumbilla Formation. Conformably underlies the Coorikiana Sandstone

Rolling Downs Group: Wallumbilla Formation

Early Cretaceous

Fossiliferous, interbedded mudstone, siltstone, sandy mudstone, sandstone and minor limestone. Coarsening up to sandstone-dominated top

200 to 375 m, maximum 596 m

High latitude marine shelf

Conformably overlies the Cadna-owie Formation, lateral equivalent of Bulldog Shale. Conformably underlies the Toolebuc Formation

Rolling Downs Group: Coorikiana Sandstone

Early Cretaceous

Fine-grained, silty sandstone with minor conglomerate. Siltstone and mudstone interbeds at the base

20 m

Near-shore

Conformably overlies the Bulldog Shale, conformably underlies Oodnadatta Formation

Rolling Downs Group: Oodnadatta Formation

Early Cretaceous

Laminate claystone and siltstone with fine-grained sandstone interbeds. Lower part contains calcareous siltstone and fossil-rich limestone

up to 300 m

Low-energy, shallow marine

Conformably overlies the Coorikiana Sandstone. Interfingers with and conformably underlies the Mackunda Formation. Transitions laterally into the Wallumbilla and Toolebuc formations and Allaru Mudstone

Rolling Downs Group: Toolebuc Formation

Early Cretaceous

Oil shale, kerigenous shale, coquinitic limestone, nodular limestone with minor sandstone

20 to 45 m

Restricted marine at maximum high stand

Conformably overlies the Wallumbilla Formation and conformably underlies the Allaru Mudstone

Rolling Downs Group: Allaru Mudstone

Early Cretaceous

Mudstone, siltstone, calcareous mudstone with minor limestone and very fine-grained sandstone interbeds towards the top

100 to 240 m, maximum greater than 600 m

Quiet shallow marine

Conformably overlies the Toolebuc Formation and conformably underlies the Mackunda Formation

Rolling Downs Group: Mackunda Formation

Early Cretaceous

Interbedded, calcareous, very fine-grained sandstone, siltstone and shale

60 to 120 m

Alternating deep-marine and shoreface

Conformably overlies the Allaru Mudstone, conformably overlies and interfingers with the Oodnadatta Formation. Conformably underlies the Winton Formation

Rolling Downs Group: Winton Formation

Early to Late Cretaceous

Interbedded fine- to coarse-grained sandstone, carbonaceous shale, siltstone and coal seams with intraclast conglomerates

more than 400 m thick, maximum 1100 m

Fluvial and lacustrine in a coastal plain setting

Conformably overlies the Mackunda Formation. Unconformably overlain by Lake Eyre Basin rocks. Crops out in parts of the subregion

Source: Alexander et al. (2006); Cook et al. (2013); Geoscience Australia and Australian Stratigraphy Commission (2012); Golder Associates (2011); Gray et al. (2002); Radke et al. (2012); Santos (2003)

1.1.3.2.4 Lake Eyre Basin

The geological Lake Eyre Basin is a thick sedimentary succession overlying the Eromanga Basin, covering parts of northern and eastern SA, south-eastern NT, western Queensland and north-western NSW. The basin is up to 400 m thick and contains sediments deposited from the Paleocene (66 Ma) through to the Quaternary (Stewart et al., 2013). The Lake Eyre Basin is further subdivided into three sub-basins, with the Callabonna sub-basin encompassing part of the Cooper subregion (Callen et al., 1995). Three phases of deposition are recognised within the basin, the first from the late Paleocene to the middle Eocene, represented by the Eyre, Glendower and Marion formations; the second from the end of the Oligocene to the Miocene, represented by the Namba, Whitula and Doonbara formations and the Cadalga Limestone; the third phase occurred during the latest Pliocene to the Quaternary (Alley, 1998; Callen et al., 1995).

Eyre, Glendower and Marion formations

The basal unit in the Cenozoic Lake Eyre Basin in SA is the Eyre Formation. This unit consists of carbonaceous sand, silt and gravel, with some lignite and clay beds. The base of the Eyre Formation is marked by polished gravel. Silcrete and other weathering products are common within the Eyre Formation. The Eyre Formation is a lateral equivalent of the Glendower and Marion formations in Queensland. The Eyre Formation unconformably overlies weathered Winton Formation across the subregion, and unconformably underlies the Namba Formation in SA. The Eyre Formation is up to 140 m thick, but is generally 10 to 20 m thick. The Eyre Formation is interpreted as representing deposition in a braided stream fluvial setting (Alexander et al., 2006; Alley, 1998).

The Glendower Formation is a lateral equivalent of the Eyre Formation, which occurs over much of the central Eromanga Basin in Queensland. The unit comprises sandstone, conglomerate and minor siltstone. The base is marked by reworked Winton Formation clasts. The Glendower Formation also contains extensive silcrete and other weathering products. It unconformably overlies the Winton Formation, and is overlain by the Whitula Formation and undifferentiated Quaternary sediments. The Glendower Formation, with a recorded thickness of about 70 m, was deposited in braided stream fluvial settings (Draper, 2002a).

The Marion Formation consists of sandstone and conglomerate, and has been extensively silicified such that it is capped by silcrete. It unconformably overlies the Winton Formation, and is overlain by Austral Downs Limestone and undifferentiated Quaternary sediments. The Marion Formation is up to 8 m thick in the Cooper subregion, and is correlated with the Glendower and Eyre formations. It is recorded as scattered occurrences in the northern and north-western part of the subregion in Queensland (Draper, 2002a).

Namba and Whitula formations

The Namba Formation comprises alternating fine- to medium-grained sand, silt and clay with thin dolomite interbeds (Callen et al., 1995). The Namba Formation reaches a maximum thickness of 210 m in the Cooper subregion (Alexander et al., 2006). It unconformably overlies the Eyre Formation, and is overlain by undifferentiated Quaternary sediments. The Namba Formation is restricted to the Callabonna sub-basin in SA (Callen et al., 1995).

The Whitula Formation is confined to the Windorah Trough, Yamma Yamma Depression, Farrars Syncline and Thomson Depression in the Queensland portion of the Cooper subregion (see Figure 14 for locations). It is an interbedded sandstone, siltstone, mudstone and claystone with minor conglomerate, lignite and gypsum. The unit contains some silica or iron oxide indurated portions. It unconformably overlies the Glendower Formation or weathered Winton Formation and is conformably overlain by undifferentiated Quaternary alluvium. The maximum recorded thickness is 160 m. The Whitula Formation was deposited in a fluvial and lacustrine environment (Draper, 2002a).

Doonbara Formation and Cadelga Limestone

The Doonbara Formation is an iron-rich fine- to medium-grained sand overlying the Eyre Formation. It is generally 7 to 10 m thick, with a maximum thickness of 40 m recorded in places. The Doonbara Formation may represent a lateral fluvial equivalent to the top of the Namba Formation in the north and east of the SA part of the Lake Eyre Basin in the Cooper subregion, and correlate with the Whitula Formation. The Cadelga Limestone is a cherty dolomitic limestone which interfingers with the Doonbara Formation. This unit is up to 5 m thick, and formed in a lacustrine depositional setting (Alley, 1998).

Last updated:
5 January 2018
Thumbnail of the Cooper subregion

Product Finalisation date

2015

ASSESSMENT