EXTENSIONAL BASIN — FORMING STRUCTURES IN BASS STRAIT AND THEIR IMPORTANCE FOR HYDROCARBON EXPLORATION

Abstract

The Bass, Gippsland and Otway Basins of southeastern Australia were initiated by north-northeast to south- southwest lithospheric extension, largely during the Early Cretaceous. The extensional stage was followed by a Late Cretaceous to Pliocene thermal subsidence stage and a late stage of compressional tectonic overprinting.The extensional stage was dominated by two orthogonal fault sets - shallow to moderately dipping, rotational, normal faults and steeply dipping, transfer (transform) faults. Thermal subsidence involved vertical rather than horizontal movements, and consequently generated a discrete fault geometry, comprising steep, down-to-basin, normal faults with small displacements. The major extensional structures exerted a range of controls on both sedimentation and structuring during the subsidence stage. Likewise, the location and style of late Tertiary compressional structures overprinted on the Gippsland and, to a lesser extent, Bass and Otway Basins are controlled by reactivation of major early normal and transfer faults. In particular, the Kingfish, Mackerel, Halibut, Flounder and Tuna fields in the Gippsland Basin overlie a single Early Cretaceous transfer fault zone that was a basinwide structural boundary during extension. These fields occupy en echelon compressional structures generated by left-lateral wrench reactivation of the transfer zone during late Tertiary northwest-southeast compression. The major extensional structures have had an important influence on all stages of the evolution of these basins. It is contended that a thorough understanding of their extensional framework is an important factor in hydrocarbon exploration of these and other basins.