Lord Howe Rise, Tasman Sea - preliminary geophysical results and petroleum prospects

The Lord Howe Rise is a major submarine feature in the Tasman Sea; it extends northwest from the New Zealand continental margin and lies about 600 to 1200 km off the east coast of Australia. It is about 2000 km long and 300 km wide, and most of its crest lies in water ranging from 750 to 1200 m deep. It has a continental crustal structure. The general absence of rift-valley or pull-apart basins along the eastern seaboard of Australia has led some authors to speculate that when breakup took place in the Tasman Basin, the rift-valley was breached along its western boundary fault, and is now represented by a zone of horst and graben structures some 200 km in width underlying the western part of the Lord Howe Rise and possibly the Dampier Ridge. Any sedimentary basins associated with rifting would have remained wholly attached to the Lord Howe Plate. Although survey data from the R.V. Sonne cruise show that sediment-filled basins, some of which are grabens, make up much of western Lord Howe Rise, the degree to which they are related to the pre-Tasman Basin rifting remains conjectural. The basins on Lord Howe Rise are 20-40 km wide; some contain up to 4000 m of sediment. Sediments are also relatively thick on the eastern flank of the Middleton and Lord Howe Basins. The seismic profiles indicate that wave-base erosion was taking place on Lord Howe Rise in the Late Cretaceous, and imply that a shallow marine environment was present prior to seafloor spreading in the Tasman Basin. Shallow marine silts and clays penetrated in DSDP Site 207, and palaeogeographical reconstructions, which juxtapose central Lord Howe Rise and the Gippsland Basin, indicate that both marine and non-marine petroleum source rocks may be present. Petroleum traps appear to exist against the boundary faults of the grabens, as internal structures within basins, and in structures interpreted as reefs of Late Cretaceous and Paleocene age. A seal may be provided by pelagic oozes. The eastern flank of Lord Howe Rise was probably the ancient (pre-Maestrichtian) continental margin of the Australian-Antarctic supercontinent. A wedge of Late Cretaceous or older sediment, up to 2000 m thick, which was deposited across the margin, may also be prospective. The presence of potential source rocks, suitable traps, and indications of higher than normal heat flow, suggest that this deep water area, which is now just within range of oil drilling technology, should be considered prospective for petroleum at least in the long term.