HR: 0830h
AN: PP31B-0258    [PDF]
TI: Late Cretaceous to Oligocene Geological History of the East Tasman Plateau, a Key Piece of the Tasmanian Gateway Story
AU: * Exon, N F
EM: Neville.Exon@ga.gov.au
AF: Geoscience Australia, GPO Box 378, Canberra, ACT 2601 Australia
AB: A fairly complete Maastrichtian to Holocene marine sequence 776 m thick was drilled at ODP Site 1172 on the East Tasman Plateau. The ETP is a continental fragment that rifted from the adjacent parts of Gondwana (Tasmania, South Tasman Rise and Lord Howe Rise) in the Late Cretaceous and became part of the southwest margin of the proto-Pacific. However, rapid subsidence started only in the late Eocene. The ETP lies just east of the Tasmanian Gateway that formed as Australia and Antarctica separated in the Paleogene, so Site 1172 was ideally located to monitor the effects of its opening. Slow detrital sedimentation (averaging 1.3 cm/ka) persisted until the latest Eocene in this area east of the subsiding `land bridge' (Tasmania and the South Tasman Rise) connecting Australia and Antarctica. When the South Tasman Rise separated from Antarctica at the Eocene-Oligocene boundary (33.5 Ma), the Tasmanian Gateway opened, currents first connected the Indian and Pacific Oceans, and pelagic carbonate sedimentation commenced. At Site 1172, dinocysts, spores and pollen dominate the Maastrichtian through middle Eocene microfossil assemblages, diatoms become abundant in the middle Eocene, and calcareous nannofossils and planktonic foraminifers dominate the Oligocene. The sequences provide evidence of a complex interplay of tectonics, paleoceanography including the onset of the Antarctic Circumpolar Current, and Antarctic cooling and glaciation. The ETP moved northward with Australia, from 65§S in the Late Cretaceous to 60§S in the early Oligocene. At Site 1172, during the Maastrichtian and Paleocene (70-55 Ma) shallow marine mudstone was deposited in reducing conditions, while the onshore climate was humid and seasonally cool and supported a conifer forest. Much of the Paleocene is missing. Cool oceanic conditions characterised the Eocene (dinocysts represent the `Transantarctic Flora'). The early and middle Eocene (55-37 Ma) was marked by slow subsidence, a thick sequence of shallow marine mudstone deposited in increasingly open shelf conditions, and a uniformly wet and cool onshore climate with angiosperms. There was faster subsidence in the late Eocene (37-33.5 Ma), marine shelf mudstone gave way to glauconitic bathyal siltstone as the land bridge between the Indian and Pacific Oceans subsided and strong currents caused condensed sedimentation, and the climate cooled. When the Tasmanian Gateway finally opened at the Eocene-Oligocene boundary, strong currents led to a brief hiatus. Rapid Oligocene subsidence allowed deposition of bathyal chalk and the onset of Antarctic glaciation led to cooling.
DE: 1635 Oceans (4203)
DE: 3022 Marine sediments--processes and transport
DE: 8105 Continental margins and sedimentary basins
SC: Paleoceanography and Paleoclimatology [PP]
MN: 2003 Fall Meeting