Abstract The Ceduna Sub-basin is located within the Bight Basin on the Australian southern margin. Recent structural analysis using newly acquired two-dimensional (2D) and three-dimensional (3D) seismic data demonstrates two Late Cretaceous delta–deepwater fold–thrust belts (DDWFTBs), which are overlain by Cenozoic sediments. The present-day normal fault stress regime identified in the Bight Basin indicates that the maximum horizontal stress ( S Hmax ) is margin parallel; Andersonain faulting theory therefore suggests the delta-top extensional faults are oriented favourably for reactivation. A breached hydrocarbon trap encountered in the Jerboa-1 well demonstrates this fault reactivation. Faults interpreted from 3D seismic data were modelled using the Poly3D © geomechanical code to determine the risk of reactivation. Results indicate delta-top extensional faults that dip 40–70° are at moderate–high risk of reactivation, while variations in the orientation of the fault planes results in an increased risk of reactivation. Two pulses of inversion are identified in the Ceduna Sub-basin and correlate with the onset of rifting and fault reactivation in the Santonian. We propose a ridge-push mechanism for this stress which selectively reactivates extensional faults on the delta-top, forming inversion anticlines that are prospective for hydrocarbon exploration.

Abstract The Jurassic-Cretaceous Bight Basin of offshore southern Australia is a large, under-explored basin with little stratigraphic control. Sequence stratigraphic and structural interpretation of regional-scale 2-D seismic data has revealed the presence of two large progradational delta systems of mid-Late Cretaceous age, which are vastly different in terms of geometry and stratal architecture. The Cenomanian White Pointer Delta is characterized by shale deformation and growth faulting. Growth fault-controlled depocentres occupy a wide band across the Ceduna Sub-basin of the Bight Basin that records the progradation of the delta across the palaeo-shelf during the Cenomanian. The growth faults are generally listric and basinward dipping (regional). Counter-regional faults are uncommon. The updip extensional features are accompanied downdip by a zone of diapirs that coincides with an outer basin high and, in deeper water at the toe of slope, a region of contractional deformation and toe-thrusts. In comparison, the younger (Late Santonian-Maastrichtian) Hammerhead Delta exhibits strongly progradational stratal geometries with little evidence of shale tectonics except in the SW part of the Ceduna Sub-basin. There, sustained progradation under a high sediment supply regime caused gravitational instability and the formation of listric growth faults at the palaeo-shelf margin with associated down-dip contractional deformation.