Linking Shallow and Deep Features on the Shelf East and West of Stewart Island

Abstract

The sedimentary basins that have developed to the east and west of Stewart Island are linked through similar tectonic and depositional processes during their formation. The sediment supply and tectonic rifting phases are common to both basins, while sedimentation rates and tectonic subsidence define their unique characteristics. To the east, the Great South Basin is the Campbell Plateau's largest sedimentary basin; it has accumulated up to 8.6 km of sediment since it was formed during mid-Cretaceous rifting related to the separation of New Zealand from western Antarctica. West of Stewart Island lies the Solander Basin, which formed during the Cretaceous - late Eocene extensional regime that led to the opening of the Tasman Sea. Hydrocarbon exploration using seismic and well data has suggested that some of the rift basins formed by the spreading of the Tasman Sea and the break-up of Gondwana have become significant petroleum basin depocentres. In this thesis, high-resolution seismic data have been used in conjunction with existing data to further investigate the geological history and hydrocarbon potential of these basins. Several specific features are addressed as outlined below.

On the southeastern side of Stewart Island, a detachment fault associated with late Cretaceous rifting from Gondwana has previously been proposed for the region. Information on the offshore geometry and extent of this, the Sisters Shear Zone, is limited - particularly with regard to its morphology and geological history. Recent work has allowed the dating of rifting episodes across the zone, but its general form is still mostly unknown. This study investigates the shear zone using high-resolution seismic imaging to reconstruct seafloor morphology and shallow fault geometry. High frequency, sub-bottom images were acquired aboard the University of Otago's R/V Polaris II, while seismic processing and interpretation techniques have been employed to investigate the shallow geometry and location of the offshore segment of this proposed detachment fault.

On the west side of Stewart Island, a 240-km-long line of multi-channel seismic reflection data, recorded aboard R/V Maurice Ewing in 1996, has imaged several complex structural features along the shelf adjacent to the Solander Basin. Reprocessing of these data and further high-resolution imaging focuses on faulting that appears to outcrop on the seafloor, as well as inferred erosional features on the shelf margin. The collection of high-frequency boomer data in January 2009 has improved the resolution and interpretability of these features. Direct comparison of single-channel boomer data and multi-channel reflection data has revealed several near surface features which are below the Rayleigh resolution limit of the older low-frequency data, including a Bottom Simulating Reflection and turbidity/contour current erosional features.