1.2.1.1.2 Clarence-Moreton Basin

The Clarence-Moreton Basin merges with the Surat Basin across the Kumbarilla Ridge, a basement high consisting of Carboniferous Texas beds that separate the Cecil Plains sub-basin within the Clarence-Moreton Basin (New Hope Group, 2014, p. 4-19). The economic coal-bearing strata of the Surat and Clarence-Moreton basins are in the Walloon Coal Measures, which are Middle to Upper Jurassic in age, and part of the Injune Creek Group (see Figure 9 (stratigraphic column) in companion product 1.1 for the Maranoa-Balonne-Condamine subregion (Welsh et al., 2014)). The Walloon Coal Measures occur on both sides of the Kumbarilla Ridge, and are laterally continuous (New Hope Group, 2014, p. 4-19). In the Clarence-Moreton Basin, the Injune Creek Group is divided into a productive, coal-bearing lower unit, the Walloon Coal Measures, and a coal resource-barren upper unit, the Kumbarilla beds (New Hope Group 2014, p. 4-20).

The coal seams of the Walloon Coal Measures in the Clarence-Moreton Basin are generally interbanded with mudstone and siltstone, and minor thin shale bands. Individual coal plies are mostly less than 1 m thick (Goscombe and Coxhead, 1995, p. 501). The coals are generally vitrinite‑rich and contain up to 20% liptinite in some cases (Goscombe and Coxhead, 1995, p. 502).

Major coal deposits of the Clarence-Moreton Basin occur in the Rosewood, Oakey and Millmerran areas. Washed ash yields are around 16 and 17% and volatile matter contents vary between 38 and 42% (Goscombe and Coxhead, 1995, p. 504). Working sections of coal are reported to be 10 to 20 m in the Oakey area, whereas banded seams range from 2 to 15 m in thickness in the Millmerran area (Goscombe and Coxhead, 1995, p. 505).

New Hope Coal (2014, p. 4-21) state that at their project site, New Acland Coal Mine Stage 3, the Walloon Coal Measures are about 120 to 130 m thick, with economically recoverable coal reserves less than 75 m below ground level at their deepest point (New Hope Coal, 2014, p. 4-21). Within the lower Walloon Coal Measures both at the New Acland Coal Mine Stage 3 site and beyond, there are three major coal intervals: the Waipanna, Acland-Sabine and Balgowan.

The Acland-Sabine Sequence (New Hope Coal, 2014, p. 4-20) is a major coal-bearing unit of the Walloon Coal Measures and occurs in the lower coal-bearing unit, equivalent to the Taroom Coal Measures. The Walloon Coal Measures are unconformably overlain by Cenozoic basalts in some areas, whereas other areas are covered with Quaternary alluvium deposited by creeks and rivers (New Hope Coal, 2014, p. 4-21). The coal measure sequences dip 1 to 3 degrees towards the south-south-west, although local faulting and folding causes local variation (New Hope Coal, 2014, p. 4-21). Faulting typically occurs along two main trends striking north-east and north-west.

The Acland-Sabine interval contains six groups of seams, each containing up to 10 plies, with a total of 47 plies and a mean ply thickness of 0.23 m. These groups of seams are unlikely to extend laterally, but instead form isolated pods of coal (New Hope Coal, 2014, p. 4-21). The Waipanna interval overlies the Acland-Sabine interval and contains six seams that contain a total of 53 plies. The Balgowan interval, which underlies the Acland-Sabine interval, contains seven seam groups with a total of 21 plies (New Hope Coal, 2014, p. 4-21). Coal seams in the Waipanna interval, within the lower part of a 75 m unit of thinly bedded fine-grained sandstone, exhibit rapid, lateral facies changes, and one seam reaches 6 m in thickness but is of poor quality and banded (New Hope Coal, 2014, p. 4-52). Within the Acland-Sabine interval a banded coal 18 m thick has been identified in a 30 to 60 m unit of predominantly thinly bedded fine-grained sandstone and mudstone (New Hope Coal, 2014, p. 4-52). In the Balgowan interval numerous thin coal seams extend though a 30 m unit of medium- to fine-grained sandstone (New Hope Coal, 2014, p. 4-52).

At the Commodore Mine the Walloon Coal Measures are gently folded with coal dipping at less than 3 degrees to the west. The seams mined are the Kooroongarra seam, which is up to 3 m thick, the Commodore seam, which averages 5.2 m in thickness and the Bottom Rider seam, which is 1.5 to 0.9 m thick. The Top Rider and Bottom Rider seams are considered to be minor seams and the typical in situ ash of coals at the Commodore deposit is about 36% (Goscombe and Coxhead, 1995, p. 505).