Betts Creek beds

The thickest and most areally extensive coal-bearing units in the Galilee Basin are of Late Permian age and are equivalent to the Group IV coals of the nearby Bowen Basin (Smith, 2013). These units occur across most of the basin (Figure 3), commonly at depths of between 500 and 1000 m, with maximum depths exceeding 1500 m in the Lovelle Depression and the Powell Depression. However, close to the eastern margin of the basin, the coal-bearing Permian strata occur at relatively shallow depths beneath younger (mainly Cenozoic) sediments, and outcrop sporadically in a zone parallel to the basin margin. Along the eastern margin of the basin and into the Koburra Trough, the Late Permian coal seams are well developed and continuous (Scott and Hawkins, 1992). However, south of the Barcaldine Ridge, Late Permian coals are poorly developed, and typically occur as thin and discontinuous seams (evident in Figures 15 and 16 of product 1.1 for the Galilee subregion (Evans et al., 2014)). Consequently, the southern Galilee Basin is unlikely to be an area of future mining for Permian coal deposits.

In the northern, western and north-eastern parts of the basin (including the Lovelle Depression and the Koburra Trough, Figure 5) the main Late Permian coal-bearing unit is known as the Betts Creek beds. However, in the central-east and south of the basin, especially in areas south of the Barcaldine Ridge such as the Powell Depression, the stratigraphic equivalents of the Betts Creek beds are the coal-poor Peawaddy Formation and the Black Alley Shale, and the coal-bearing Colinlea Sandstone and Bandanna Formation (Figure 4, and discussed further below). These equivalents are also known from the south-western Bowen Basin, which is interpreted to link across the Springsure Shelf to the Galilee Basin (Smith, 2013). According to McKellar and Henderson (2013), the Betts Creek beds do not occur in areas of the Galilee Basin that overlie the Drummond Basin. In these areas, Bowen Basin equivalents are present.

The Betts Creek beds are a mixed sedimentary sequence comprising lithic sandstone, conglomerate, mudstone, carbonaceous shale, coal, pebbly mudstone, and minor tuff (GA and ASC, 2014). In some parts of the basin the Betts Creek beds form an almost continuous sequence of relatively uniform thickness and contain coal seams that dip regionally at low angles (generally less than 5°) towards the west and south-west (Smith, 2013). However, in other areas the thickness of the Betts Creek beds varies considerably (from 50 to 390 m) due to irregular basement topography at the time of deposition (Wells, 1989).

The Late Permian coals in the Galilee Basin are mostly low rank, sub-bituminous A type coals (based on the American Society for Testing and Materials (ASTM) classification, see Appendix A.1) although, in places, there are also some high volatile bituminous C type coals. Vitrinite reflectance of the Betts Creek beds coals usually ranges from 0.4 to 0.6% (Rv, max), and most are generally dull and relatively high in mineral matter (up to 25%). They are typically low sulfur and high ash content, and are regarded as non-coking coals, suitable for thermal uses such as generating steam for electrical power plants (Wells, 1989; Huleatt, 1991). The maceral compositions of individual coal seams are variable, for example, some plies are relatively high in inertinite, whereas others have elevated liptinite (I’Anson, 2013). In general, Galilee Basin coals are lower in vitrinite and duller than their Bowen Basin equivalents (APLNG, 2011).

Scott et al. (1995) suggested that a typical proximate coal analysis (air-dried) for the high volatile bituminous C type coals of Betts Creek beds that occur in the eastern Koburra Trough is about 46% fixed carbon, 30% volatile matter, 9% moisture and 15% ash.

Last updated:
8 October 2018
Thumbnail of the Galilee subregion

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