Logging data are measurements of physical properties of the formation surrounding a borehole, acquired in situ after completion of coring (wireline logging) or during drilling (Logging-While-Drilling, LWD). The range of data (resistivity, gamma radiation, velocity, density, borehole images,…) in any hole depends on the scientific objectives and operational constraints.

Logging data are measurements of physical properties of the formation surrounding a borehole, acquired in situ after completion of coring (wireline logging) or during drilling (Logging-While-Drilling, LWD). The range of data (resistivity, gamma radiation, velocity, density, borehole images,…) in any hole depends on the scientific objectives and operational constraints.

One of the great stories of geoscience is how Gondwana broke up and the other southern continents drifted northward from Antarctica, which led to major changes in global climate. The recent drilling of Ocean Drilling Project (ODP) Leg 189 addressed in detail what happened as Australia drifted away from Antarctica and the Tasmanian Gateway opened. The drifting contributed to the change in global climate, from relatively warm early Cenozoic “greenhouse” conditions to late Cenozoic “icehouse” conditions. It isolated Antarctica from warm gyral surface currents from the north and provided the critical deepwater conduits that eventually led to ocean conveyor circulation between the Atlantic and Pacific Oceans.

Palaeogene ocean opening south of Tasmania, and palaeoceanographic implications: preliminary results of clay mineral analyses (ODP Leg 189). ODP Leg 189 was designed to test the hypothesis that opening of the Tasmanian Seaway and initiation of circumpolar circulation contributed to the thermal isolation of Antarctica, leading to the development of initial ice-sheet and oceanic thermohaline circulation. The clay assemblages of the Tasmanian region contain the traces of two tectonic stages associated with ocean opening south of the south Tasman Rise near the Palaeocene-Eocene boundary and strike-slip activity between the western Tasmanian land-bridge and Antarctica during the Late Eocene. Earliest Oligocene clays indicate that cooling of Antarctic margins and activity of western boundary circulation progressed with the regional subsidence. (C) 2001 Academie des sciences / Editions scientifiques et medicales Elsevier SAS.

The major ice sheets of the Cenozoic Era are unusual in geological history. Progressive cooling at high latitudes during the Cenozoic eventually formed major ice sheets, initially on Antarctica and later in the Northern Hemisphere. In the early 1970s it was proposed that climate cooled and an Antarctic ice sheet (cryosphere) developed as the Antarctic Circumpolar Current (ACC) progressively isolated Antarctica thermally (Kennett et al., 1975). This current resulted initially from the opening of the Tasmanian Gateway, and the history of this opening was the main focus of ODP Leg 189. Early ocean drilling in the Tasmanian Gateway between Australia and Antarctica (Deep Sea Drilling Project Leg 29: Kennett, Houtz et al., 1975) provided a basic framework of paleoenvironmental changes associated with the opening, but was of insufficient quality and resolution to fully test the potential interrelationships of plate tectonics, circum-polar circulation and global climate. Until now, the timing of events has remained inadequately constrained. The opening of the Tasmanian Gateway in the latest Eocene and the only other important ACC constriction, the Drake Passage, in the earliest Neogene (Fig. 1), had enormous consequences for global climate. These consequences came by isolating Antarctica from warm gyral surface circulation of the Southern Hemisphere oceans, and also by providing the conduits that eventually led to ocean conveyor circulation between the Atlantic and Pacific Oceans. Both factors, in conjunction with positive feedbacks and other changes in the global system, have been crucial in the development of the polar cryosphere, initially in Antarctica in the Paleogene and early Neogene, and later in the Northern Hemisphere in the late Neogene (Shackleton and Opdyke, 1977; Ruddiman et al., 1989). Furthermore, the continued expansion of the Southern Ocean during the Cenozoic because of the northward flight of Australia from Antarctica, has clearly led to further evolution of the Earth’s environmental system and of oceanic biogeographic patterns. The Opening of the Tasmanian Gateway Drove Global Cenozoic Paleoclimatic and Paleoceanographic Changes: Results of Leg 189 N. Exon1, J. Kennett2, M. Malone3,

Palaeogene ocean opening south of Tasmania, and palaeoceanographic implications: preliminary results of clay mineral analyses (ODP Leg 189). ODP Leg 189 was designed to test the hypothesis that opening of the Tasmanian Seaway and initiation of circumpolar circulation contributed to the thermal isolation of Antarctica, leading to the development of initial ice-sheet and oceanic thermohaline circulation. The clay assemblages of the Tasmanian region contain the traces of two tectonic stages associated with ocean opening south of the south Tasman Rise near the Palaeocene-Eocene boundary and strike-slip activity between the western Tasmanian land-bridge and Antarctica during the Late Eocene. Earliest Oligocene clays indicate that cooling of Antarctic margins and activity of western boundary circulation progressed with the regional subsidence. (C) 2001 Academie des sciences / Editions scientifiques et medicales Elsevier SAS.