Summary:
The Cameby Downs is located on the eastern flank of the northern Surat Basin, which stratigraphically overlies the Bowen Basin. The Surat Basin comprises Jurassic to Cretaceous aged sandstones, siltstones, and mudstones deposited in a predominantly fluvio-lacustrine depositional environment.
The oldest sediments throughout the Surat Basin are the early to middle Jurassic aged Precipice Sandstone, Evergreen Formation, and Hutton Sandstone. These units subcrop approximately 25 km northeast of the Project area and dip to the southwest. Within the Cameby Downs area, these units are between 600 m to 1,100 m below ground level (mbGL). The Hutton Sandstone is unconformably overlain by the Eurombah Formation, which in turn is overlain by the Walloon Coal Measures. The Eurombah Formation is approximately 95 m thick in the Project area and comprises lithic conglomeratic sandstone with mudstone and siltstone beds.
The Cameby Downs Mine intersects coal seams associated with the Walloon Coal Measures of the Injune Creek Group. The Walloon Coal Measures subcrop in a broad, northwest trending arc, up to 70 km wide along the northeast and northern margins of the Surat Basin. The Walloon Coal Measures are further split into three distinct units, the Juandah Coal Measures, Tangalooma Sandstone, and the Taroom Coal Measures. The existing Cameby Downs Mine and the Project target the economic coal seams (i.e. Kogan, Macalister and Wambo seams) of the Juandah Coal Measures.
The Walloon Coal Measures are unconformably overlain by the late Jurassic Kumbarilla Beds, which are stratigraphic equivalent of the Springbok Sandstone, Westbourne Formation and the Gubberamunda Sandstone. Only the Springbok Sandstone is present in the Cameby Downs Operations area, and outcrops in the northern part of the area. The Springbok Sandstone comprises labile sandstones, siltstone, mudstone and some conglomerate. The Springbok Sandstone thickens to the southwest with the general dip of stratigraphy. The Westbourne Formation and the Gubberamunda Sandstone occur as outcrop 7 km southwest (down-dip) of the Project, but do not occur in the Project area.
The following main stratigraphic units occur within the Cameby Downs Operations area and surrounds (from oldest to youngest):
- Walloon Coal Measures;
- Springbok Sandstone;
- Tertiary sediments; and
- Quaternary sediments.
Walloon Coal Measures.
The Walloon Coal Measures, part of the Injune Creek Group, unconformably overly the Eurombah Sandstone. The Walloon Coal Measures subcrop in a broad, northwest trending arc, up to 70 km wide along the northeast and northern margins of the Surat Basin. The sub-group is separated in to three stratigraphic units. The lowermost unit in the Project area is the Taroom Coal Measures. The unit is up to 145 m thick in the Project area and comprises sub-labile, medium grained sandstone grading upwards to interbedded sandstone, siltstone, mudstone and coal. The Tangalooma Sandstone is approximately 50 m thick and conformably overlies the Taroom Coal Measures. The unit comprises lithic, labile, medium grained sandstone with an argillaceous matrix and numerous intra-formational conglomerate beds.
The uppermost unit of the Walloon Coal Measures in the Cameby Downs Operations area is the Juandah Coal Measures which comprises lithic, labile sandstone interbedded with siltstone, mudstone and coal. Coal deposition is more frequent towards top of the unit. The Juandah Coal Measures are up to 220 m thick.
The Walloon Coal Measures subcrop in the north east of the area and dip between one and three degrees to the southwest. The subcrop forms the northeast boundary of the proposed open cut mine. The main economic coal seams in the region are within the Walloon Coal Measures and include the target coal seams for the Cameby Downs as well as for CSG production within the region (OGIA, 2016).
The Juandah Coal Measures occur at depths of between 30 mbGL to 125 mbGL in the Cameby Downs area and are the coal seams being mined at the Cameby Downs. Exploration drilling has identified three coal horizons and 12 individual coal seams, including:
- Kogan (KG1, KG2, KG3, and KG4);
- Macalister (MA1, MA2T, MA2B, MA3, and MA4); and
- Wambo (WM1, WM2, and WM3).
The Surat Basin is characterised by several anticlinal and synclinal structures that are largely controlled by reactivation of Triassic basement fault systems (QGC, 2012). One of these structures is the north-south trending Burunga-Leichhardt thrust fault, located approximately 12 km west of the Cameby Downs area.