Capricorn and northern Tasman Basins: structure and depositional systems

Abstract

The Bureau of Mineral Resources completed a major marine multi-channel seismic reflection survey (MCS) off southeast Queensland in December 1989. The research cruise, using RV 'Rig Seismic: was designed to investigate the structure, stratigraphy and petroleum resource potential of offshore basins along this sector of the Australian continental margin. About half of the 2900 km of seismic data acquired were recorded over the Capricorn Basin and margins of the northern Tasman Basin. The survey included seismic ties to the only deep offshore wells in the region, Aquarius-1 and Capricorn-lA, located in the northern Capricorn Basin. It was the first MCS survey shot in the region since 1974. Sonobuoy refraction, gravity, magnetic and bathymetric data were also collected. The Capricorn Basin evolved in the Late Cretaceous as a failed rift arm at the northern end of the Tasman rift system. From the seismic data, basin development in the Capricorn Basin is seen as typically comprising 0.5 ? 1.0 s twt of syn-rift continental/Irestricted marine deposits (Late Cretaceous-early Palaeogene) overlain by about 1.5 s twt of Eocene-Recent, mainly marine, post-rift sediments. Though basement structures generally follow a north-northwest trend, the structural pattern is complex, comprising a mainly discontinuous series of rift basins of various geometries. Complex structure is also indicated by gravity mapping. Shallow-dipping horizons that extend well into 'basement' may be fault-plane reflections from deep extensional structures, but could also be indicative of large wedges of old (Cretaceous+) sediments or volcaniclastics. In the deep-water southeastern part of the basin, at least 3.6 s twt (4 km) of Late Cretaceous ? Palaeogene section is associated with major half-graben development. A mid-Eocene compressional or transpressional event has reactivated basement structures and produced minor faulting and folding in the section. The event, which also produced significant uplift and erosion, is attributed to intraplate stress resulting from major global plate reorganization that occurred at about 43 Ma. Volcanism was active in the southern Capricorn Basin during Late Oligocene. Volcanic edifices several hundred metres high and 3 to 5 km across are exposed on the seafloor. In the Tasman Basin, a 3-km thick pile of Cainozoic post-breakup sediments overlies oceanic basement and highly extended continental crust at the base of the continental slope to the southeast of Fraser Island. A similar thickness of sediment is present at abyssal depths along the northern transform margin. Large-scale mass movements have taken place on the basin's steep and rugged marginal slopes.