1.1.3 Geology

Summary

The Clarence-Moreton Basin covers approximately 38,000 km2 on-shore in south-eastern Queensland and north-eastern New South Wales, and extends off-shore over an area of at least 1,000 km2 in New South Wales. The basin is bounded on three sides by Paleozoic basement blocks, which are part of the New England Orogen.

Following the initiation of the Clarence-Moreton Basin and associated precursor basins by tectonic extension during the Middle Triassic, a period of thermal cooling continued from the Late Triassic probably into the Cretaceous. Thermal relaxation was also the dominant driving force for subsidence in the Clarence-Moreton Basin. The Clarence-Moreton Basin has three sub-basins separated by basement highs. These discrete depositional centres, which are bounded by transtension-related strike-slip faults, are from north-west to south-east: the Cecil Plains sub-basin, the Laidley sub-basin and the Logan sub-basin. Both the Laidley and Logan sub-basins form part of the Clarence-Moreton bioregion, whereas only the eastern Cecil Plains sub-basin is included (the western Cecil Plains sub-basin is part of the Northern Inland Catchments bioregion). In Queensland, the north-western Clarence-Moreton Basin is connected to the Surat Basin across the Kumbarilla Ridge, a shallow basement high. However, only four stratigraphic units are common to both basins.

The Clarence-Moreton Basin contains sedimentary sequences of Late Triassic to Lower Cretaceous age, with a combined estimated thickness of approximately 3 to 4 km. The sediments were originally deposited in non-marine depositional environments by northwards flowing rivers in a humid climate. The mixed nature of the sedimentary facies indicates that there were frequent environmental changes, mostly associated with tectonic processes. This resulted in the deposition of interbedded sequences of fluvial, paludal (swamp) and lacustrine deposits.

The oldest coal-bearing units of the Clarence-Moreton Basin sedimentary succession are the Middle Triassic Nymboida Coal Measures and the Late Triassic Ipswich Coal Measures, which are separated from each other by an angular unconformity. Deposition of the overlying Bundamba Group (including the Woogaroo Subgroup, the Gatton Sandstone and the Koukandowie Formation) occurred during the Late Triassic to Early Jurassic. These lithostratigraphic units comprise mixed facies dominated by sandstone, siltstone, mudstone and conglomerate, with minor coal also reported, particularly from the Koukandowie Formation. Deposition of the Walloon Coal Measures followed the Bundamba Group during the Middle Jurassic. This formed the major coal-bearing stratigraphic unit in the Clarence-Moreton Basin and the primary target for coal and coal seam gas exploration. Its deposition marked a change towards lower energy depositional environments characterised by sinuous rivers meandering across a wide floodplain. During the Late Jurassic to Cretaceous, the Orara Formation and the Grafton Formation were deposited. As a result of widespread erosion, these youngest stratigraphic units are restricted to the New South Wales part of the Clarence-Moreton Basin.

Throughout the Clarence-Moreton bioregion, there are only two currently operating coal mines (both in south-east Queensland), but the basin has a long history of coal mining dating back to the 1870s. The Clarence-Moreton Basin has also been explored for conventional hydrocarbon (oil and gas) in the past and more recently for unconventional gas resources (coal seam gas), primarily targeting the Walloon Coal Measures.

Last updated:
23 March 2016
Thumbnail images of the Clarence-Moreton bioregion

Product Finalisation date

28 May 2014