Cisuralian coals of the Mullaley Sub-basin, northern New South Wales

Abstract

ABSTRACTThe Mullaley Sub-basin of the Gunnedah Basin extends from Quirindi in the southeast, to north of Narrabri, to west of Dunedoo in northern New South Wales. There have been more than 100 boreholes sunk to basement investigating the (lower Permian) Cisuralian coal and coal seam gas resources of the Mullaley Sub-basin since the early 1990s. A desktop review of this open file information has allowed the formal correlation and naming of six Cisuralian coal members attaining a maximum 35 m of cumulative thickness within an upward coarsening sedimentary package totalling no more than 150 m. In ascending order, the coal members are: Bibblewindi (0–10 m), Bohena (3–18 m), Collygra (0.5–3 m), Coxs (1.5–4 m), Tullamullen (0.5–4 m) and Mooki (0.5–3 m).Cisuralian coal seams in the Maules Creek Formation of the southern Mullaley Sub-basin are here correlated with those of the Greta Coal Measures at Werris Creek and Muswellbrook. It is apparent that basement paleotopography played a significant role in the Cisuralian coal development as coals are best developed where the sedimentary sequence is greater than 60 m thick, as there the thick seams (Bohena and Bibblewindi coal members) occur towards the base of the sequence. The maximum western limit of the Cisuralian coals (Rocky Glen Ridge) is further east than previously inferred with new drilling information showing the Porcupine Formation directly overlying the barren pelletoidal claystones of the Leard Formation or the underlying volcanics (Boggabri Volcanics/Werrie Basalt). Early marine transgressions at the top of the Maules Creek Formation have stopped development of the Mooki, Tullamullen and Coxs coal members in the northern and eastern Mullaley Sub-basin and allowed the development of localised paraconglomerate (diamictite) intervals up to 10 m thick. Thick (>20 m cumulative) coal occurrences are localised to the Jacks Creek and Pilliga East State Forest areas southwest of Narrabri. The coal resource potential of the Mullaley Sub-basin is estimated at 13–28 billion tonnes.