1.1.3 Geology

Summary

The Hunter subregion is within the geological Sydney Basin and the minor Werrie Basin, which in turn form part of the Permian-Triassic Sydney-Gunnedah-Bowen Basin (SGBB) that developed as a contiguous geological complex. The Sydney Basin is structurally constrained to the west by the Lachlan Fold Belt, to the north-east by the New England Fold Belt, and to the east and south where the basin extends to the edge of the continental shelf. A basement high, the Mount Coricudgy Anticline, forms the basis of a structural boundary between the Sydney and Gunnedah basins. However, sedimentary rocks typical of the northern Sydney Basin occur north of the Mount Coricudgy Anticline, implying connectivity between the basins. The Werrie Basin stratigraphy is closely associated with the Sydney and Gunnedah basins.

Coal measure sedimentation in the Sydney Basin began in the early Permian and was terminated towards the end of the Permian by major uplift and basin tilting. Permian fluvial, coastal plain and marine sediments were deposited on Paleozoic basement, after which rapid subsidence led to deposition of coal-bearing sequences in the late Permian. The Hunter subregion straddles three of the five coalfields that make up the Sydney Basin: mainly the Hunter and Newcastle coalfields and part of the Western Coalfield. The Hunter and Newcastle coalfields each host three coal measure sequences: the Greta Coal Measures, the Wittingham Coal Measures (in the Hunter with its equivalent in the Newcastle Coalfield, the Tomago Coal Measures), and the Newcastle Coal Measures. The main coal of economic interest in the Western Coalfield is the Illawarra Coal Measures.

The Greta Coal Measures in the Sydney Basin form a wedge-like sequence ranging from 60 to 90 m thick. The Homeville Coal Member of the Greta Coal Measures is up to 8 m thick and outcrops at Kurri Kurri. The coals were formed as a result of peat accumulation behind advancing barrier islands. The informally named Greta seam of the Greta Coal Measures, however, is sulfur-rich, indicating that it was deposited in a marine environment. The Foybrook Formation, which is part of the Wittingham Coal Measures, contains coal seams interbedded with sandstone, siltstone, claystone and tuff, formed by a river-dominated delta system. The coals are particularly well developed in the Muswellbrook area but are characterised by erratic splitting, which is also true of other coals in the Wittingham Coal Measures. The Jerrys Plains Subgroup is the upper part of the Wittingham Coal Measures that developed as part of a river-dominated sedimentary sequence and consists of multiple coal seams laid down in back-barrier coal swamp and delta plain environments. Coals formed in the upper delta plain are thicker and laterally continuous. In the Newcastle Coalfield the coals (such as those of the Tomago Coal Measures) formed in terrestrial, lower delta plain and brackish marine environments. The Newcastle Coal Measures were deposited under fluvial (river) conditions and are subject to splitting and erosion caused by rapid channel migration of the river system.

The Illawarra Coal Measures are divided into four subgroups in the Western Coalfield which is within the Hunter subregion: the Nile Subgroup that consists of prodelta to lower delta-plain sediments; the Cullen Bullen Subgroup that hosts the Marrangaroo Formation, the Lithgow Coal, the Blackmans Flat Formation and the Lidsdale Coal, that were deposited in fluvial and deltaic environments; the Charbon Subgroup that consists of several formations, coals and oil shales; and, the Wallerawang Subgroup that typically consists of sediments likely to have been deposited in alluvial, point-bar, levee and floodplain environments. Deposition of the Charbon Subgroup occurred in a delta system (such as Long Swamp Formation) and overbank swamps (such as Irondale Coal), in addition to distributary mouth bar-crevasse splays (Angus Place Sandstone) and lower delta plains (State Creek Mine Formation). Some of the coals are economic whereas others are thin, discontinuous and uneconomic. A marine interval is represented by the Baal Bone Formation, which is 10 to 50 m thick and represents possibly a lower delta-front environment and a back-barrier swamp environment, such as the Moolarben Coal Member. The Werrie Basin contains coal that correlates with the Greta Coal Measures in the Hunter Coalfield.