Geology of the Fairway and New Caledonia Basins in the Tasman Sea: Sediment, Pore Water, Diapirs and Bottom Simulating Reflectors ( Franklin Cruise FR9/01 and Geoscience Australia Survey 232)

Brief description

The Lord Howe Rise and Norfolk Ridge are major north-south structural highs that are separated by the Fairway Basin and the New Caledonia Basin. The Fairway Basin is the shallower of the two basins and is generally in water less than 3000 m deep. The New Caledonia Basin lies further east in water generally deeper than 3600 m. In the area studied on the Franklin research cruise FR9/01, the two basins are about 300 km wide and 900 km long. About 2800 km of 24 channel seismic profiles provided vital information in a very poorly known region. In the Central Fairway Basin in the north, two new east-west cross-sections found more diapirs, and evidence of a bottom-simulating reflector (BSR) indicating the presence of gas hydrates, and young faulting. They increased our knowledge of a part of the basin known to have petroleum potential. In the south, there are six east-west multichannel seismic cross-sections in an area of Australian jurisdiction, where there were none. These seismic profiles show that the deep water depression south of 26.20 S is an extension of the Fairway Basin. It is limited by the Lord Howe Rise to the west and the northern extension of the West Norfolk Ridge to the east, and is roughly 700 km long and 100 km wide: an area of 70,000 km. The basin contains sediments more than 2 seconds thick in places, shallow and deep diapirs (especially north of 29 S), and a BSR in some regions. Water depths are 1200 m to 3600 m. Clearly, the South Fairway Basin has some petroleum potential, although the apparent maximum thickness of sediment (<3 km) found so far is discouraging. Twenty-six Quaternary cores were recovered on the seismic profiles across the basins, partly to further investigate the nature of the BSR in the region (gas hydrate or not). The cores recovered a variety of foram-bearing nanno oozes and nanno oozes, in water depths of 1297-3517 m, and in latitudes of 24-32 S. Uniformly pale core colours in the north suggest purely oxidising conditions, but multicoloured cores in the south, indicating fluctuations in redox conditions, are probably related to inflow of varied bottom water through time. The cores contain negligible quantities of gas. Furthermore, the pore waters do not show the sharp chemical gradients of sulfate, chloride and methane, which would indicate that significant accumulations of gas hydrates were present. This negative result may be because of the highly oxidised nature of the surface sediment, so our results shed no light on the nature of the BSR. Twenty-one samples from low in cores, and three from dredges, have been examined for planktic foraminifera and calcareous nannofossils: both groups are abundant and well preserved, and all core base samples are of Pleistocene age. The foraminiferal assemblages are typically subtropical to warm temperate, dominated largely either by the Globorotalia (Truncorotalia) or the Gr. inflata groups. The nannofossil ages show a clear relationship between location and the age of the core bases. As most cores are of similar lengths, there probably is a systematic variation in sedimentation rates. The four oldest cores, all from the north, have the lowest average sedimentation rates (~4 mm/1000 years), but they are on slopes and may be winnowed. The other northern cores have lower sedimentation rates (~10 mm/1000 years) than the southern cores (25 mm/1000 years), perhaps because productivity has been higher south of 26.45 S than further north. A northern dredge from the Fairway Basin contains chalk and radiolarite with Early Eocene foraminifera and late Early to early Middle Miocene nannofossils. A southern dredge from the eastern Lord Howe Rise contains a volcanic breccia with poorly preserved Early Pliocene foraminifera and Late Miocene to Late Pliocene nannofossils in micrite infillings.

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