Quantifying and modelling the effects of pre-existing basement faults on folding of overlying strata in the Surat Basin, Australia: Implications for fault seal potential

Ancient basement fault geometry has a systematic effect on the folding of overlying sedimentary successions. Using aeromagnetic, Bouguer gravity, as well as 2D and 3D seismic datasets, the characteristics of the basement structure underlying the Jurassic-Cretaceous Surat Basin were examined and used to contextualise the nature of its folded strata. Using Fault-Forward modelling software, the deformation of the basement structural layers was modelled and analysed for its influence on the reactivation of faulting. The results show that the major fault trends in the eastern Surat and Bowen basins consist of five key features with N- and NE-trending orientations: the Goondiwindi, Moonie, Undulla Transition Zone, Burunga-Leichhardt, and Cockatoo fault systems. Most of the major fault deformation ceased in the Late Triassic. However, some faults (i.e., the Moonie and Goondiwindi fault systems) were variably reactivated along their lengths during the Jurassic. Three models of pre-existing basement reactivation are proposed to explain the observed variability in folding style with application to different parts of the basin. Ancient fault systems with dip angles greater than 45 degrees were readily reactivated and produced tight monocline structures with high strain (epsilon > 0.35) and fault throw (>60 m) (e.g. Moonie Fault System), while faults with small dip (<30 degrees) or large apical angles generated folds with low strain and uplift of the hanging wall. This variability in fault reactivation according to differences in fault throw potentially influences across-fault juxtaposition of strata and sealing potential for CO2 storage. Modelling such as this should be undertaken elsewhere in situations where trishear structural domains are expected, to better understand and predict the deformation of overlying stratal successions.

Automated summary

This study investigates the impact of ancient basement fault geometry on the folding of overlying sedimentary successions. Through various datasets and modelling techniques, key fault systems were identified in the Surat Basin, with some faults being reactivated during the Jurassic period. Three models of pre-existing basement reactivation were proposed to explain the variability in folding style.