1.1.3 Geology


The Sydney Basin bioregion is within the geological Sydney Basin (simply referred to henceforth throughout this section as the Sydney Basin), part of the greater Sydney-Gunnedah-Bowen Basin complex (SGBB). The Sydney Basin extends offshore to the continental shelf, which marks its easternmost boundary. It is further bounded by older rocks of the Lachlan Fold Belt to the south and west, the Mount Coricudgy Anticline to the north-west – across which is the Gunnedah Basin – and the New England Fold Belt to the north and west. The Sydney Basin bioregion abuts the Hunter subregion of the Northern Sydney Basin bioregion to the north. The rocks of the SGBB complex are of Carboniferous to Triassic age, and about 325 to 230 million years old.

The evolution of the Sydney Basin – and the wider SGBB – began during the orogenic phase that built the Tasman Fold Belt, initially during the Devonian and continued intermittently until the Cretaceous, when major tectonic activity ceased. The stratigraphic record preserved within the basin reveals an active depositional history from the Carboniferous to the Triassic, although Jurassic and Cretaceous rocks were likely to have been present at some stage (and have since been eroded). Sedimentation took place across the SGBB as a zone of crustal depression was gradually infilled by sediment supplied from the nearby New England and Lachlan fold belts. The dominant control on this deposition was the cyclic nature of marine transgression and regression. The major coal-bearing units were deposited from the latest early Permian to the earliest middle Permian, and again from the latest middle Permian to the end of the Permian. These coal-rich stratigraphic units are known as the Greta, Clyde, Illawarra, Wittingham, Tomago and Newcastle coal measures. The Sydney Basin is divided into five coalfields, namely the Hunter, Newcastle, Western, Central and Southern coalfields, based largely upon variations in stratigraphy, including the specific coal measures.

In the Sydney Basin bioregion, the main coalfields are the Western, Central and Southern coalfields. The most economically important coal-bearing unit in each of these areas is the Illawarra Coal Measures, although lesser coal units also occur.

Last updated:
21 January 2019
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