PERMIAN

The Gunnedah Basin is the central part of the Sydney-Bowen Basin, a zone that stretches from south of Wollongong to central Queensland. This geological feature hosts the extensive Permian age coal deposits that have been exploited for over 100 years in NSW and Queensland to provide fuel for electricity generation, coal for steel production, and today is the basis of a huge export industry. The accompanying map summarises the geology of the ACM assets.

The geological development of the Gunnedah Basin extends back to the very start of the Permian Period - about 300 million years ago - when plate movements caused tensions along the axis of what became the Sydney-Bowen Basin. Such tensions resulted in pulling apart and thinning of the Earth’s crust, allowing injection of magma which reached the surface as lavas and extensive volcanic ash deposits . The initial volcanic phase was followed by sagging to form a basin in which sedimentary rocks were deposited from material washed in from land to both the west and east of the basin.

Early Permian deposition - the lower coals

Initial deposition during the Early Permian Epoch was in a complex of valleys, lakes and marshes formed on a deeply weathered and irregular land surface. Braided rivers deposited sand and gravels eroded from high ridges and in the swampy areas between river systems, coal accumulated from vegetation. These sediments which are up to 800 metres thick, constitute the Maules Creek Formation and its coal seams are important targets for seam gas exploration in the Gunnedah Basin .

Middle Permian – the sea invades and retreats

Eventually, continued basin subsidence allowed the sea to inundate the river systems . Deposition of sediments continued with a poorly sorted mix of sand, coarse gravel and silt being deposited, the Porcupine Formation. With increased deepening of the sea, sediments became finer - sandy muds and finally silts and clays of the Watermark Formation.

During this time a change in tectonic stresses from tensional to compressional occurred, and the New England region to the east of the Gunnedah Basin began to rise. Rivers began to flow from the New England region into the Gunnedah Basin and provided a fresh source of sediment. These river systems formed deltas extending out into the marine environment and there was a return to non-marine conditions. Continued compression began to force the western edge of the New England block to ride over the eastern side of the Gunnedah Basin, resulting in the Hunter-Mooki Thrust system. This structure was active at various periods over the remainder of the geological history of the Gunnedah Basin and today forms the eastern boundary of the basin.

Late Permian – and more coal

Deposition in the Late Permian Black Jack Group is controlled by the major river systems, initially in a brackish lower delta plain environment and then beyond tidal reach in an upper delta plain environment. Sediment was supplied largely by rivers draining from the rising country of the New England block, but quartz-rich sands were also contributed by rivers from the Lachlan Fold Belt to the west. A short-lived return to marine conditions (the Arkarula Formation) is evident over much but not the entire basin. Extensive coal deposits occur throughout the Black Jack Group with the principal seam, both in thickness and extent being the Hoskissons Coal.

Fossil evidence indicates that the Hoskissons Coal and its equivalents in the Sydney Basin, formed at much the same time across a vast area of the Sydney-Gunnedah Basin. This suggests that a widespread drop in sea level exposed a vast flat area of bay and delta sediments and allowed the development of the swampy and shallow lagoonal conditions suitable for coal deposition. Prolonged stable conditions permitted the accumulation of coal thicknesses of up to almost 20 metres in this single seam. Because of its extensive and thick development, the Hoskissons Coal is a primary target for seam gas exploration. Numerous other coals are developed both above and below the Hoskissons, and locally these too may be important seam gas targets.