Stratigraphic architecture of Lopingian (upper Permian) strata in the Galilee Basin, NE Australia

Abstract

This study reviewed the stratigraphic framework of the Koburra Trough, Galilee Basin and proposed a modified stratigraphy of the Betts Creek beds based on regional correlation of coal and interburden facies character observed in wireline log signatures, core and chip descriptions. The new stratigraphy was tested through radiogenic isotope dating of tuffs and palynology. This new framework provides the foundation for a regional palaeo-environmental model, coupled with an investigation of sediment provenance through U-Pb analysis of detrital zircons and their implications for the tectonic setting of the Galilee Basin. Through the regional correlations of 76 public domain and 1205 proprietary geophysical wireline logs complemented by core descriptions, a series of regional coal seam correlations were developed. Using the C seam, with a distinctive tuffaceous character, as a marker horizon, the Lopingian coal seam architecture was established. The C seam is overlain by the B and A seams that merge to the north and diverge to the south above a coarsening upward sequence correlative to the Black Alley Shale. This architecture is similar to the splitting of the Fort Cooper Coal Measures into the Burngrove and Fair Hill formations in the neighbouring Bowen Basin. The C seam is underlain by three seams, D, E and F, that exhibit complex splitting patterns reflecting compensational stacking of interburden strata and coals. These form the 'Colinlea Sandstone equivalent' that, to the south, transition to the Peawaddy Formation and Catherine Sandstone. The combination of these formations elevates the Betts Creek beds to group status. From east to west the Betts Creek Group incorporates units of coal seams known as the Crossmore and Glenaras sequences. However, these seams were only found local to the Hulton-Rand Structure, and do not have an equivalent in the Koburra Trough. They were renamed the 'J and K' seams, and removed from the Betts Creek Group. Chemical Abrasion-Isotope Dilution Thermal Ionization Mass Spectrometry (CA-IDTIMS) and palynological techniques were used to corroborate the proposed correlations. Tuffs within the C coal seams yielded zircon U-Pb dates of 254.31±0.10, 254.41±0.07 and 255.13±0.09 Ma, which are equivalent with a Lopingian date obtained from the Black Alley Shale on the Springsure Shelf, 254.08±0.06 Ma. The correspondence suggests that the C seam is an equivalent to a seam of the Fair Hill Formation in the Bowen Basin and a landward lateral equivalent of the Black Alley Shale, rather than the upper section of the 'Colinlea Sandstone equivalent'. A tuff date from the top of the B coal seam (252.81±0.07 Ma) correlates with published dates of the Yarrabee Tuff in the Bowen III Basin. This places the B seam as an equivalent to the Burngrove Formation and the overlying A seam, the only coal seam within the Bandanna Formation.The age of the 'J and K' seams were determined with palynology and found to contain the biozone APP3.2 index species, Praecolpatites sinuosus, placing them in the late Cisuraliane...