Geological structural framework Introduction

The Galilee subregion encompasses the entire geological Galilee Basin, which is the principal geological feature of interest in the Galilee subregion bioregional assessment. The Galilee Basin encompasses about 248,000 km2 of Central Queensland and is a large, intracratonic, coal-bearing basin. It was an active depositional centre from the Late Carboniferous through to the Middle Triassic (Scott et al., 1995), around 323 to 238 million years ago. Galilee Basin rocks outcrop as an elongated corridor some 600 km long and up to 80 km wide along the eastern margin of the Galilee subregion (Figure 14).

There are three major geological sequences of interest to this bioregional assessment in the Galilee subregion: the Galilee Basin, the Eromanga Basin, and Cenozoic sediments and volcanics. The cross-sections in Figure 15 and Figure 16 demonstrate that rocks in the Galilee Basin are for the most part buried by younger rocks belonging to the Eromanga Basin. Cenozoic sediments and volcanics overlie Eromanga and Galilee basin rocks, but are too thin to be shown on the cross-sections. On the section in Figure 15, the faults that occur near the eastern margin of the Galilee Basin are likely to have a flatter east dip than what is shown.

Crystalline basement rocks underlie the Galilee subregion (Figure 17). The oldest rocks belong to the Proterozoic Mount Isa Inlier, which formed around 1870 to 1500 million years ago (GSQ, 2011) and underlies the north-west Galilee subregion. The rest of the Galilee subregion is underlain by rocks belonging to the Late Proterozoic to Early Paleozoic, Thomson Orogen. It consists of polydeformed, Neoproterozoic to Devonian, volcanic and sedimentary rocks that are intruded by mafic to acidic sills, dykes and plutons (Purdy et al., 2013).

In some parts of the Galilee subregion, the Adavale, Belyando, and Drummond basins overlie the Thomson Orogen basement rocks (Figure 17). The Early to Middle Devonian Adavale Basin sequence includes volcanic rocks, fluvial, marine and evaporative units. The basal unit is the Gumbardo Formation which mainly comprises felsic volcanics and interbedded volcaniclastic sediments. Subsequent tectonic events led to the formation of half-grabens and the deposition of fluvial sediments of the Eastwood beds and Log Creek Formation. Ensuing marine incursion led to the deposition of carbonate and siliciclastic strata of the Lissoy Sandstone and Bury Limestone. A change to arid climate involved the deposition and transformation of carbonate rocks to dolomite, and widespread formation of halite-rich sediments (i.e. Cooladdi Dolomite and Boree Salt formations). Following a hiatus, the Etonvale and Buckabie formations were deposited in fluvial to marginal marine environments (McKillop, 2013).

The Late Devonian to Early Carboniferous Drummond Basin units outcrop to the east of the Galilee subregion, but also underlies the eastern margin of the Galilee Basin. These units consist of sedimentary and volcanic rocks that were deposited in three major depositional cycles (Henderson and Blake, 2013). The stratigraphy of the final cycle, which is in contact with the Galilee Basin, includes the Ducabrook, Natal, Star of Hope and Bulliwallah formations. These units can be up to 1800 m thick and consist primarily of volcaniclastic sandstone, siltstone, mudstone and tuff. Little is known about the Belyando Basin as it has not been drilled, but it predates the Drummond and Galilee basins and is younger than the Thomson Orogen (Draper, 2013).

Figure 14

Figure 14 Surface geology of the Galilee subregion and surrounds

Figure 15

Figure 15 Cross-section through Koburra Trough and eastern margin of the Galilee Basin

Source: Figure 3.111 McKellar and Henderson (2013) TWT equates to Two Way Time. This interpreted geological section is derived from a seismic survey section. Two way time is a measure of the amount of time it takes for sound wave generated by a seismic survey to be reflected off a feature in the earth then return to surface.

Figure 16

Figure 16 Cross-section through southern Galilee subregion

Source: Figure 3.113, McKellar and Henderson (2013) TWT equates to Two Way Time. This interpreted geological section is derived from a seismic survey section. Two way time is a measure of the amount of time it takes for sound wave generated by a seismic survey to be reflected off a feature in the earth then return to surface.

Figure 17

Figure 17 Geological features underlying the Galilee Basin Subregion structure

There are many significant geological structures in the Galilee subregion (Figure 15, Figure 16, and Figure 18). They have played an important role in determining the present day spatial and stratigraphic distribution of rock types (including coal) in the Galilee Basin. Structural fabrics in the Galilee Basin have also influenced the structural architecture in the overlying Eromanga Basin (Cook et al., 2013). The following describes the key architectural elements within the Galilee subregion.

Figure 18

Figure 18 Structures and depositional centres in the Galilee Basin

Source data: Top of Permian surface mapping derived from Bradshaw et al. (2009). While Permian exists throughout the basin, not all of it was shown in the mapping presented in Bradshaw et al.(2009). mBGL – Metres below ground level

Maneroo Platform: Broad structural high composed of Paleozoic basement situated between the Powell and Lovelle depressions on the western side of the Galilee Basin. The Maneroo Platform existed as a structural high during the Permian and Triassic. It exerted significant influence on the paleogeography of the Galilee Basin and has also affected structuring in the overlying Eromanga Basin. Monoclinal folds found in the vicinity of the Maneroo Platform margin deform both the Galilee and Eromanga basins.

Canaway Fault/Canaway Ridge: North-trending structures that separate the Galilee from the Cooper Basin. The Canaway Fault is a normal fault downthrown to the east (Galilee side), which was active during the Early Permian. Late Permian to Triassic sedimentary rocks can be correlated between the two sedimentary basins, across this major fault. Reactivation during the Paleogene increased displacement across the fault by about 400 m (Ransley et al., 2012).

Nebine Ridge: North-trending basement high that forms a boundary between sedimentary sequences in the Bowen Basin – Surat Basin to the east and the Galilee Basin – Central Eromanga Basin to the west.

Barcaldine Ridge: A major easterly-trending ridge composed of crystalline basement rocks, situated around 24° South latitude near the town of Barcaldine. It segregates the northern Galilee Basin from the southern Galilee Basin. The Barcaldine Ridge exerted significant influence on the deposition of stratigraphic sequences during the Permian, acting as a partial barrier between the north and south Galilee Basin.

The terms north Galilee Basin and south Galilee Basin are not formalised geological subdivisions. However, they are commonly used terms in the literature and will be adopted for this report. Some major structural features in the northern and southern Galilee Basin are:

North Galilee Basin

Lovelle Depression: North-east trending depression infilled with up to 700 m of Galilee Basin rocks (Hawkins and Green, 1993). It was also an active depositional centre for the overlying Eromanga Basin.

Cork Fault: A basement-intersecting normal fault, downthrown to the west. It occurs along the south-eastern margin of the Lovelle Depression. This fault was active during the development of the Lovelle Depression in the Early Permian (Van Heeswijck, 2010). It was later reactivated in the Paleogene, with a further 420 m of displacement (Ransley et al., 2012).

Beryl Ridge: North-trending ridge of crystalline basement that forms an extension from the Maneroo Platform.

Aramac Trough: North-easterly trending depression running from the edge of the Maneroo High towards the Koburra Trough.

Koburra Trough: Significant north-trending trough near the eastern margin of the Galilee Basin, mainly north of the Barcaldine Ridge. The Koburra Trough is interpreted as one of the earliest features to form in the Galilee Basin and contains up to 2800 m of sedimentary rocks. Part of the eastern margin of the trough is structurally defined by a series of folds and east dipping thrust faults Van Heeswijck, 2010). The Koburra Trough was not an active depositional centre for the Eromanga Basin.

South Galilee Basin

Powell Depression: North-easterly trending depression adjacent to the Maneroo Platform and west of the Pleasant Creek Arch. This is the main depositional centre for the south Galilee Basin. It was also active as a depositional centre for the Jurassic Eromanga Basin.

Pleasant Creek Arch: North-east trending structure between the Springsure Shelf and south-eastern margin of the Powell Depression. It is associated with the Warrego Fault.

Springsure Shelf: The area located between the Nebine Ridge and Pleasant Creek Arch near the south-eastern margin of the Galilee Basin. Significant north-east trending fold structures that occur in this region are not evident in the north Galilee Basin (Van Heeswijck, 2006). The folds developed during the Paleogene (around 50 Ma) and have deformed sedimentary sequences in the Galilee Basin and overlying Eromanga Basin.

Last updated:
3 January 2019