Pre-Permian basement and lithological controls on the development of the Jellinbah Thrust Belt: Insight from a new palinspastic reconstruction

Abstract

The complex geological structure known as the Jellinbah Thrust Belt lies between the Jellinbah Fault and Yarrabee Fault on the eastern flank of the Permian Bowen Basin. The impact of the Jellinbah Thrust Belt on the broader scale geometry and evolution of the Bowen Basin and the influence of the basement morphology on the kinematics of the system are important considerations. Partly because it affects the maturity of coal-bearing sediments of economic importance, but also because it provides evidence of structural trends developed during subduction along the eastern margin of Australia during the Hunter-Bowen Orogeny circa 265 Ma to 230 Ma. The pre-deformed extent of the Bowen Basin and amount of shortening that took place during the Hunter-Bowen Orogeny is still uncertain. Likewise the relationship between the faults and basement, the current depth and maximum burial depth of the Bowen Basin and the complexity of its tectonic history still require further examination. For example, there is uncertainty about whether the faults themselves pass through into the underlying basement or if they coalesce to form a decollement at the base of the Bowen Basin. This project attempts to interpret the structural architecture within the system in this part of the Bowen Basin using a palinspastic restoration (essentially restoring the stratigraphy to its pre-deformed state) in order to establish the magnitude of faulting that has taken place within the Jellinbah Thrust Belt. By estimating the major thrust fault movements, we can understand how the basement and stratigraphy influenced the mechanics of the fault system. The morphology of the Carboniferous metasediments and igneous units that form the basement in the Jellinbah Thrust Belt appear to have had an impact on the localisation of strain. Seismic data from the 2007 Dingonose acquisition (07-DNG-01, 07-DNG-02 and 07-DNG08) were depth converted using velocity picks in SKUA TM software. Well data were used to confirm the accuracy of the depth conversion and to estimate stratigraphic boundaries that were traced as horizon reflectors in the seismic data. The fault pattern was manually interpreted based on discontinuities in the 2D seismic. Due to the complexity of the faulting within the regions, conceptual models focussed on the large scale and easily discernible structures. In some instances intensive deformation resulting in a lack of homogeneity in the acoustic impedance or poor quality seismic data produced areas of increased uncertainty. Using the Move TM software allowed for manipulation of the interpretation and various scenarios to be tested. It provided useful analytical tools and accurate empirical measurements to describe and quantify the kinematics of the Jellinbah Thrust Belt. Finally a kinematic reconstruction to restore displaced stratigraphic units to their position prior to faulting was performed in the software. The palinspastic interpretation suggests that the compressional phase of the HunterBowen Orogeny consisted of multiple shortening episodes that were, in some instances, syn-depositional. Inversion of earlier normal faults, slump structures possibly related to the rifting phase observed in the Denison Trough were interpreted near the base of the Basin. The amount of shortening and fault angle permitted the calculation of the burial depth of the late Permian coal measures, with an estimated depth, for the current stratigraphy, of 2.0 km to 2.4 km. Basement high features identified in section 07-DNG-01 act as obstructions to compression, focussing larger reverse displacements in this part of the fault system. Flattening of the basement high to the south-east resulted in less confinement of stresses on specific faults and a more even distribution of displacement within the fault system. Localised dextral transpressional strike-slip displacements incurring out-of section plane movements are also suggested to be a result of the basement morphology. Displacement is interpreted to occur most freely along decollement surfaces associated with numerous clay-rich and carbonaceous units within the Permian coal formations. Fault dip angle maps derived from seismic interpretation suggest that large scale “flats”, where fault dip is comparable to bedding dip, exist at the base of the Bowen Basin. Significant “ramping” is interpreted to coincide with the base of the Macmillan Formation. Associated thrust faults are interpreted to traverse into the older undifferentiated basement of the Thomson Orogen beneath the Bowen Basin. The depth of the Bowen Basin interpreted from seismic data varies between approximately ~1.8 km to 3.5 km within the project area (with the stratigraphic sequence thickening toward the Taroom Trough) due to subsidence related to Mid-Permian foreland loading and thinner deposition on a basement high. An even distribution of fault displacement is observed within the thicker stratigraphic pile to the southeast across the fault system. These observations provide some explanation for the transfer of stress within the fault system. The study also highlighted the need for greater data coverage and higher resolution geophysical data for 3D modelling purposes if the dynamics of the Jellinbah Thrust Belt are to be precisely determined.