Authors / CoAuthors
Exon, N.F. | Brinkhuis, H. | Kennett, J.P. | Hill, P.J. | Quilty, P.G.
Abstract
Continuously cored ODP Leg 189 sites document the marine sequences deposited before, during and after the Tasmanian (Australian-Antarctic) Gateway opened (~33.5 Ma) and deepened. The sites are all on continental crust: one west of Tasmania, three on South Tasman Rise (STR) and one on East Tasman Plateau (ETP). The Tasmanian `land bridge? linked Australia and Antarctica and incorporated parts of Tasmania and STR; one site lay in the gradually widening but restricted Australo-Antarctic Gulf (AAG) in the Indian Ocean, and the others in the more open proto-Pacific Ocean. The main four sites vary in sub-seafloor depth from 776 to 959 m, and their oldest sequence from lowest Upper Eocene (AAG) to Maastrichtian (ETP). The sites are broadly similar, with variations depending on tectono-sedimentary setting. Depositional rates seldom exceeded 4 cm/ky. Until the Oligocene, the region was near the Antarctic margin in very high palaeolatitudes, and dinocyst, diatom and magnetostratigraphic data provide most dating and marine environmental information. From the Late Cretaceous through Late Paleocene (95-55 Ma) left-lateral strike-slip motion moved the Tasmanian region northwest past Antarctica, and Tasman Basin rifting and seafloor spreading occurred in the east. Deltaic sequences filled depocentres with dark, restricted, paralic and marine mudstones (drilled on ETP, STR). At the Paleocene/Eocene boundary (55 Ma) Australia-Antarctic motion changed to north-south along the Tasman Fracture Zone west of STR, and Tasman Basin spreading ceased. South of eastern STR an oceanic basin opened. Fast spreading, beginning in the Middle Eocene, carried this region northward (~43 Ma). In the Early and Middle Eocene, deposition continued of dark, largely deltaic, and broadly similar shallow marine mudstones (thinnest on ETP). Proto-Pacific mudstones become more open marine with time, but AAG mudstones remained restricted. In the Late Eocene (37-33.5 Ma) the continental margins sagged, the water deepened, and some currents may have flowed through shallow seaways. Sedimentation rates declined as winnowing increased and hiatuses formed. On the AAG margin restricted shallow marine mudstone and sandstone were deposited. In the proto-Pacific, as the water deepened in the latest Eocene, marine mudstone gave way to winnowed marine glauconitic siltstone and sandstone. Rapid subsidence followed the final separation of STR and Antarctica. In the proto-Pacific, strong currents swept the shelves and opening straits, and an Early Oligocene hiatus was overlain by Oligocene open marine bathyal carbonates. The AAG margin was now less restricted, but calcareous mudstones had only gradual carbonate increases through into the lower Miocene. From the Oligocene on, calcareous nannofossils, planktonic foraminifers and magnetostratigraphic data provide most dating and marine environmental information. The Neogene sequences, which consist of bathyal chalk and oozes, with limited disconformities in parts of the Miocene, have proved ideal for detailed palaeo-oceanographic/climatic isotope studies - rare in the Southern Ocean.
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nonGeographicDataset
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60408
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- External PublicationAbstract
- ( Theme )
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- marine
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- AU-TAS
- Australian and New Zealand Standard Research Classification (ANZSRC)
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- Earth Sciences
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- Published_Internal
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2004-01-01T00:00:00
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