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 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 Cisuralian - early Guadalupian, rather than the Lopingian as previously interpreted, but above the Aramac Coal Measures. This older age has implications for understanding the mid-Permian hiatus in the Galilee Basin. With the occurrence of the APP3.2 biozone in the basin, the lacuna has decreased from 30 My to a maximum of 12-13 My, similar to the adjacent Bowen and Cooper basins, suggesting that all three basins were affected by mid-Permian tectonism. Detailed sedimentary logging of four key wells located in the Galilee Basin provides insight into regional palaeo-environmental reconstructions. The decreases in coal seam thickness and pattern of increased splitting toward the south have been attributed to foreland loading in the Bowen Basin. The 'Colinlea Sandstone equivalent' is characterised by cyclic fluvial successions and lacustrine environments, especially toward the centre of the Koburra Trough. One large marine transgression (Black Alley Shale) is recognised in the Galilee Basin splitting the two coal-bearing sequences. This transgression is recognised on the Springsure Shelf and southern Koburra Trough, the latter of which is the northern maximum extent of the transgression. The overlying regression is represented by laterally extensive peat-forming delta plains that are characteristic of the Bandanna Formation. Analysis of 286 concordant U-Pb ages obtained from detrital zircons was conducted as part of an investigation into the provenance of sediment in the Galilee Basin. Cisuralian samples displayed a polymodal distribution of detrital zircon U-Pb ages, with multiple peaks between 300-2000 Ma, while Lopingian samples had much more restricted detrital zircon ages of 250-320 Ma and a minor but consistent population of 1500-1600 Ma. These results suggest that the provenance of Cisuralian sediment in the Galilee Basin was a number of different sources including the nearby craton and volcanic arc, supporting an extensional tectonic setting for the basin. In contrast, the Lopingian provenance was mainly the New England Orogen, suggesting a migration to a platform basin that was combined with the adjacent retro-foreland Bowen Basin.