Summary:
Rolleston Coal Mine lies on the western edge of the Bowen Basin in a tectonic region known as the Denison Trough, in which thick sequences of Permian and Triassic sediments are located. Deformation of the sediments is regarded as being contemporaneous, but accentuated by further movement during a mild orogeny in the late Triassic. Sedimentation continued throughout the Mesozoic, while uplift in the early Tertiary was followed by erosion and the extrusion of basalt flood volcanics. Deposition of alluvium occurred during recent geological times.
The regional stratigraphy of the area consists of the dominantly marine Lower to Upper Permian Back Creek Group (comprising in ascending order; the Cattle Creek Formation, Aldebaran Sandstone, Freitag Formation, Ingelara Formation and Catherine Sandstone, Peawaddy Formation and Black Alley Shale). This is overlain conformably by the dominantly terrestrial Upper Permian Blackwater Group (comprising in ascending order; the Burngrove Formation and the coal bearing Rangal Coal Measures which is equivalent to the Bandanna Formation in this part of the Denison trough). The Permian strata were conformably overlain by Triassic strata, however in much of the Rolleston area the Triassic sediments have been removed and a large unconformity exists between the Permian and Tertiary strata. The Tertiary consists primarily of sequences of basalt flows with some sedimentary strata developed between the basalts.
Tectonic deformation along the western edge of the Denison Trough has been minimal and the geological structure is relatively simple. The Rolleston lies between two of major structural features which generally strike north-south. The eastern structure, the Albinia Fault is a fault downthrown 500 m to the east. The western structure is a large fault which defines the edge of the Springsure Anticline and lies to the west of the Project Site. Between these two structures gentle folding of the strata into a series of broadly north-south trending synclines and anticlines has occurred. The axial trends of the folds vary from north- northeast to north-northwest.
The main structural features controlling the occurrence of the coal measures in the Rolleston are two double plunging anticlines – the Consuelo and Inderi anticlines; and two complementary synclines – the Meteor Park and Meteor Downs synclines. A third syncline – the Rewan Syncline – has been recognised from exploration to be the main structure in the south-western part of MLA70415 (mining lease applications).
Economically important coal seams occur along the western limb of the Inderi Anticline in the north of MLA70415 where dips are of the order of 2º to the west, though coal is interpreted to exist on the steeper eastern limb. The Consuelo Anticline in the south of MLA70415 is a steeper and tighter structure, which limits the south western extent of coal seams. Dips on the eastern limb of the Consuelo Anticline are of the order of 3.5º, but steepen to 7.5º towards the south. Current Rolleston Coal Mine operations have identified smaller scale fault structures. Faults appear to be thrust structures as indicated by the amount of seam thickening observed in drill core, occasionally full seam repetition has been encountered.
Eight coal seams were originally identified across the Rolleston and named, in top-down stratigraphic order – X1, X2, U, A1, A2, B, C, D. Subsequent re-evaluation has revised the coal stratigraphy into six seams: A (A1 and A2), B, C and D. These seams occur in the upper part of the Blackwater Group sequence.
A Seam
The A Seam occurs only in its un-split form within ML70307. The main area of the A Seam is centred north of the Springwood Road region and extends into the central portion of ML70307 where it maintains a thickness of approximately 3.5 m. The A Seam is present in MLA70416 but degrades to a carbonaceous unit in MLA70415.
B Seam
The B Seam is present over MLA70415 and MLA70416 and reaches a maximum thickness of around 3.3 m and an average thickness of 2.5 m. The quality of the B Seam is poorer with a higher ash content and lower calorific value.
C Seam
The C Seam is present in MLA70416, but rarely exceeds more than 0.5 m thick. The C Seam occurs approximately 3 m below the B Seam.
D Seam
The D Seam is the most important individual seam in the deposit. Seam thickness is consistent across the deposit with an average thickness of approximately 4.4 m ranging from about 0.7 to 6.1 m. The lithotype profile comprises a generally dull top section (some 0.5 – 1.0 m); a dull to bright middle section (around 2.5 – 3.0 m); and a generally bright lower section (generally 1.5 – 2.0 m). A thin coaly, carbonaceous, stony ply is usually present at the immediate base of the seam. This ply is not consistently present in all drill holes and in some cases may have been recorded as floor material rather than included in the seam. It can be up to 0.2 m thick, but is generally less than 0.1 m thick. Inorganic bands are uncommon resulting in a remarkably ‘clean’ coal.