220.127.116.11.1 Tectonic setting
The Bowen Basin began forming during the Late Carboniferous and Early Permian during a phase of tectonic extension (Cadman et al., 1998; Elliott, 1989; Draper, 2013). Extensional subsidence during the Early Permian led to the deposition of the earliest known sediments of the basin (Cadman et al., 1998). During this time volcanic rocks were also deposited to the east of the Roma Shelf and into the basin, and andesitic extrusions occurred near the Auburn Arch (Cadman et al., 1998). Deposition ceased in the Bowen Basin during the Late Triassic (Cadman et al., 1998).
The evolution of the Gunnedah Basin is broadly similar to the Bowen Basin, as they both form part of the larger Bowen-Gunnedah-Sydney Basin depositional complex. Mechanical extension during the Early Permian led to rapid subsidence, resulting in development of the Gunnedah Basin (Korsch and Totterdell, 2009; Stuart-Smith et al., 2010; O’Neill and Danis, 2013). The next major phase of basin subsidence was caused by plate flexure due to foreland loading. This was the dominant process driving basin evolution from the latest part of the Early Permian until the Middle Triassic, and led to the deposition of all stratigraphic units in the basin that post-date the Maules Creek Formation. However, subsidence associated with foreland loading was interrupted by a Late Permian to Early Triassic deformational event which halted deposition and caused localised uplift and erosion (Korsch and Totterdell, 2009).
Subsidence continued steadily into the Early Triassic driving deposition (Draper, 2013). During the Late Triassic, deposition ceased, with approximately a 30 million year period of erosion marking the divide between the Permian-Triassic Bowen and Gunnedah basins and the Jurassic-Cretaceous Surat Basin (Cadman et al., 1998). The Surat Basin began forming due to a new phase of thermal subsidence following the Hunter-Bowen Orogeny, after deposition of sedimentary rocks in the Bowen and Gunnedah basins (McKellar, 1998; Fielding et al., 1993). Deposition of most sedimentary sequences is attributed to large inland fluvial systems developed across an alluvial plain, interspersed with swamps, lakes and deltas (Exon, 1976; Rohead-O’Brien, 2011). Sediment input was largely controlled by the steady rate of subsidence (Fielding et al., 1993). Early sedimentation patterns provide evidence for periods of erosion and tectonic reactivation from underlying faults in the Bowen Basin (McKellar, 1998; Fielding et al., 1993). Basin formation ceased once widespread uplift began during the Middle Cretaceous (Yago, 1996). Subsequent volcanic activity in the Cenozoic resulted in localised compression and some folding (Yago, 1996; Fielding et al., 1993; Finlayson et al., 1990).
18.104.22.168.2 Paleozoic volcanism and intrusives
Early Permian volcanic activity in the Bowen Basin formed the Combarngo Volcanics and the Camboon Volcanics, which are the basal formations of the Bowen stratigraphic sequence (Cadman et al., 1998). Further south, tectonic extension during the Late Carboniferous and Early Permian initiated volcanism that gave rise to the Boggabri Volcanics and Warrie Basalt (Danis et al., 2010; O’Neill and Danis, 2013). Thin layers of tuff also occur sporadically throughout some parts of the Gunnedah Basin sequence, for example in the Melvilles and Hoskissons coals (Hamilton, 1985). These provide evidence for minor pyroclastic volcanism in the Late Permian sequences of the Gunnedah Basin.
Reactivation of volcanic activity in the vicinity of the Gunnedah Basin during the Early Jurassic, caused by the separation of Pangaea and eastern Gondwana, led to deposition of up to 180 m of volcanic rocks in the north and west (Danis et al., 2010, O’Neill and Danis, 2013). This Jurassic volcanism, which represents the base of the southern Surat Basin sequence and is known as the Garrawilla Volcanics, also contributed pyroclastic material (e.g. tuff layers) to several younger stratigraphic units, such as the Purlawaugh Formation (Exon, 1976).
Igneous intrusions, such as the Black Jack Sill and Ivanhoe Sill, are common throughout the Gunnedah Basin sequence, and are mostly of Jurassic and Early Cretaceous age (Ward and Kelly, 2013; Pratt, 1998). The thickness of sills varies greatly, from a few centimetres to over 120 m thick (Ward and Kelly, 2013). In places where intrusions are adjacent to coals, the coal seam gas contents are commonly elevated due to micro fracturing of the seam (Ward and Kelly, 2013). Sills have also increased the thermal maturity of coals and led to enhanced hydrocarbon production (Ward and Kelly, 2013).
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- 1.1.1 Bioregion
- 1.1.2 Geography
- 1.1.3 Geology
- 1.1.4 Hydrogeology and groundwater quality
- 1.1.5 Surface water hydrology and water quality
- 1.1.6 Surface water – groundwater interactions
- 1.1.7 Ecology
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