1.1.3 Geology


There are three large coal-bearing sedimentary basins within the Central West subregion, namely the Surat Basin, the Gunnedah Basin and the Sydney Basin (Figure 16). The geology of the underlying basement rocks and the overlying Cenozoic alluvial and volcanic deposits is not discussed in detail here (refer to Section 1.1.4 for information on the hydrogeology of the Cenozoic sequences). The part of the Surat Basin within the Central West subregion is the Coonamble Embayment. For the Gunnedah Basin, which structurally and stratigraphically underlies the Surat Basin, this section focuses on the Oxley sub-basin and the western part of the Mullaley sub-basin (Figure 17). Only the northern-most section of the Sydney Basin falls within the Central West subregion.

The Gunnedah Basin started forming in the Late Carboniferous, during a period of volcanic activity and mechanical plate extension that resulted in rapid subsidence. Passive thermal subsidence followed this phase, marked by a period of uniform sedimentation across the basin. Deposition centred on a series of half-grabens that formed deeper troughs across the basin, particularly along the western margin. The Gunnedah Basin is bounded largely by older geological fold belts and structural highs, such as the New England and Lachlan orogens. The eastern margin coincides with a major thrust fault system known as the Hunter-Mooki Thrust. The rocks of the Gunnedah Basin are sandstone-dominated, with siltstone, claystone, tuff and coal less abundant, although these are more common in the upper half of the basin infill sequence.

After the initial phase of extension and subsidence, the Gunnedah Basin was affected by the Late Permian to Early Triassic Hunter-Bowen Orogeny that caused widespread contractional deformation. This tectonic phase was later overwhelmed by rapid tectonic subsidence that brought an end to the evolution of the basin in the Middle Triassic.

The Sydney Basin was deposited continuously with its northern counterpart, the Gunnedah Basin. Several phases of volcanic activity have been noted throughout the basin. Sedimentation was largely controlled by changed in the sea level during formation. Like the Gunnedah Basin, the Sydney Basin is also bounded by the New England Fold Belt and the Hunter-Mooki Thrust. However the Sydney Basin’s easternmost edge is bounded by the offshore continental margin.

Deposition in the younger Surat Basin commenced in the Early Jurassic. The sedimentary rocks of the Surat Basin formed above volcanic rocks (the Garrawilla Volcanics) during renewed subsidence. This was followed by a compressional tectonic regime, with active volcanic intervals and fault reactivation phases. During the latest Early Cretaceous, uplift brought an end to the deposition of the Surat Basin infill sequence. The Surat Basin, which contains up to 2500 metres of sedimentary infill, is largely composed of sandstone, siltstone, mudstone, shale, coal, conglomerate, volcanic and tuff deposits.

Last updated:
5 January 2018