BACKGROUND AND OBJECTIVES

Site 1172 is located on the western side of the East Tasman Plateau ~170 km southeast of Tasmania in a water depth of 2622 m. The site lies in cool subtropical waters just north of the Subtropical Front in an area where both the Subtropical Front and the East Australian Current have had variable influence through time, at a latitude comparable to Site 1168 west of Tasmania. Primary objectives of coring and logging at Site 1172 were to obtain (1) an Oligocene to Holocene pelagic carbonate section in the extreme southwest Pacific under long-term influence of the East Australian Current to construct moderate to high-resolution paleoceanographic and biostratigraphic histories, (2) an Eocene siliciclastic/carbonate sediment sequence for better understanding of paleoceanographic and paleoclimate development in the extreme southwestern sector of the Pacific before Antarctic Circumpolar Current development, and (3) an Eocene-Oligocene transitional sequence to determine effects of the initial opening of the Tasmanian Seaway on the paleoceanography of the Pacific Tasmanian margin region and to compare and contrast (4) changing paleoenvironmental and paleoceanographic conditions on each side of Tasmania as the Seaway opened and the Antarctic Circumpolar Current developed. This site was also expected to provide valuable information about the tectonic history of the East Tasman Plateau in relation to inferred hot spot evolution of the region.

Site 1172 is on thinned continental crust on the western side of the East Tasman Plateau. The plateau is roughly circular, 200 km across, and lies in water depths of 2200-2800 m (Exon et al., 1997). The East Tasman Plateau was located deep within Gondwana early in the Late Cretaceous, when rifting started to separate it from Tasmania and the eastern South Tasman Rise (STR) to the west and the Lord Howe Rise to the east (Exon et al., 1997). Magnetic anomalies suggest that seafloor spreading west of the STR occurred between 75 and 65 Ma (Royer and Rollet, 1997), and the East Tasman Plateau moved eastward ~150 km relative to Tasmania and the STR (Fig. F7 in the "Leg 189 Summary" chapter). At about this time, seafloor spreading started to the east, thus separating the Lord Howe Rise from the East Tasman Plateau. During middle Eocene times, the East Tasman Plateau was at ~65°S, when its fast movement (55 km/Ma) north with Australia commenced.

Continental basement rocks have been dredged from the margins of the East Tasman Plateau (Exon et al., 1997). Seismic profiles and other evidence suggest that they are overlain by Cretaceous sediments, Paleogene sediments, and Neogene hemipelagic and pelagic sediments (Fig. F2). The late Eocene Cascade Seamount consists of volcanic breccia, hyaloclastite, and alkali olivine basalt and is part of the trace of the Balleny mantle plume (hot spot) that extends southward from the East Tasman Plateau to the Balleny Islands near the Ross Sea. The seamount is a guyot whose top lies 650 m below sea level (mbsl). Seismic profiles indicate that there has been subsidence of the central plateau and that as much as 4000 m of sediments fills the resultant depression.

Although there were almost no pre-Quaternary sediment samples from the East Tasman Plateau, circumstantial evidence suggests that above a possible volcanic horizon identified in seismic profiles, presumed to be Late Cretaceous postdrilling, the depression was filled by 500-1000 m of latest Cretaceous to Paleogene sediments and 200-500 m of Neogene carbonate oozes and chalks. Calcareous microfossils in volcaniclastic sedimentary rocks from high on the seamount suggest an increase in water depth from very shallow in the latest Eocene to deep in the early Miocene (Quilty, 1997). Thus, the seamount's relief of 1500 m should approximate the minimum water depth over much of the East Tasman Plateau in the latest Eocene. However, the drilling provided an enigma by proving that the latest Eocene sediments west of the base of the seamount were deposited in upper bathyal depths (much shallower than 1500 m). At the location of Site 1172, AGSO gravity core 147/GC32 recovered 262 cm of light gray clayey foraminifer ooze of late Pleistocene or younger age (<0.46 Ma).

The only relevant Deep Sea Drilling Program (DSDP) location is Site 283 (Kennett, Houtz, et al., 1975), drilled to a depth of 592 m on the abyssal plain, 250 km east of the East Tasman Plateau in a water depth of 4756 m. Drilling at this site recovered entirely deep abyssal Paleocene and younger sediments, with a very poor biogenic record, above heavily weathered and undated pillow basalt. The sediments consist of 16 m of upper Miocene to Holocene zeolitic clay, 164 m of upper Eocene diatom ooze with nannofossils, 225 m of middle Eocene silty clay, and 283 m of Paleocene poorly fossiliferous silty clay and silty pyritic claystone. The site was located above a magnetic anomaly interpreted as Chron 32, suggesting a basement basalt age of Maastrichtian.

Site 1172 is located on multichannel seismic profiles Tasmante 125-4 and AGSO 202-01 and -13 (Fig. F3) in a structural low ~35 km west of the foot of the Cascade Seamount. The site is in an area of almost flat-lying sediments ~1400 m thick and overlying an older section perhaps 1500 m thick at the site. The entire sedimentary section is cut by small normal (compaction) faults. The site was designed to core down well into Paleogene sediments. For conversion from seconds below seafloor to meters below seafloor, velocities were assumed to average 1700 m/s in the Neogene sequence and 2200 m/s in the Paleogene. The upper part of the sedimentary section includes five seismic unconformities (from the top A, B, C, D, and E) whose ages were speculative; three of these unconformities were drilled (Fig. F3). Exon et al. (1997) had considered that the strong reflectors of Unconformity E probably represented middle (or late) Eocene basalt flows and volcaniclastics associated with the Cascade Seamount. Drilling proved this incorrect, and Unconformity E is probably of Late Cretaceous age.

The uppermost semitransparent but bedded sequence is above the downlapped Unconformity A at 0.37 s. Drilling proved this sequence to be carbonate ooze and chalk of Neogene age. Unconformity A proved to be upper Eocene in age. The moderately reflective and well-bedded section below the unconformity, 0.19 s thick, is siliciclastics of Eocene age. The underlying Unconformity B, at 0.5 s, truncates the underlying section. Below Unconformity B are 0.2 s of moderately reflective, thickly bedded sediments of Eocene and Paleocene age. The underlying unconformity C at 0.76 s is a strong reflector with pinch-outs beneath it. It overlies a semitransparent, irregularly bedded sequence ~0.4 s (440 m) thick of Upper Cretaceous siliciclastics.

Unconformity D at 1.19 s (1285 mbsf) is onlapped by the irregularly bedded sequence. Beneath this unconformity is a well-bedded sequence 0.2 s (220 m) thick, believed now to be siliciclastics of Late Cretaceous age. These onlap Unconformity E, the uppermost of a bundle of strong reflectors at 1.4 s (1505 mbsf), which may be within the Upper Cretaceous or the top of the Lower Cretaceous.

Sites 1172 and 1168 represent a pair of sites off the east and west margins of Tasmania, located to allow comparison between environments of sedimentation and biogeographic patterns between the southern Indian and Pacific Oceans at different stages in the development of the Southern Ocean during the Cenozoic. The comparison of the sedimentary environment and biotas in each ocean was expected to provide vital data about the nature of the ocean environment before the opening of the Tasmanian Gateway and the timing and character of developing interconnection between these oceans.

Site 1172 is well located southeast of Tasmania to monitor the paleoceanographic history of the East Australian Current, the major warm surface current of the South Pacific. This western boundary current presently flows southward along the coast of eastern Australia before turning to the east and splitting into two major branches flowing to the north and south of New Zealand. The southern branch flows eastward immediately to the south of Tasmania, where it contacts the powerful Antarctic Circumpolar Current. The strength of the East Australian Current should have varied through time in response to changing strength of the South Pacific gyral circulation. This circulation would have varied in response to changing wind forces, related to global climate change and to plate tectonics. On long time scales, critical changes in plate tectonics in the South and equatorial Pacific that should have contributed to the changing strengths of gyral circulation, and hence of the East Australian Current, include (1) constriction of the Indonesian Seaway beginning near the Eocene-Oligocene transition and continuing through the Neogene, (2) the opening of Drake Passage at the Oligocene-Miocene transition, and (3) the opening of the Tasmanian Seaway and expansion of the Southern Ocean since the Eocene-Oligocene transition.

Site 1172 represents a unique and pioneering opportunity to investigate the history of this boundary current on long time scales. No previous site in the history of ocean drilling has been suitably located to target this problem. Site 1172 appears to have been in sufficiently shallow waters through much of the Cenozoic to ensure calcareous microfossil preservation, which is so vital for isotopic, quantitative micropaleontological, and geochemical investigations of surface-water temperature changes. Changes in surface-water temperatures and in the biogeography of microfossil assemblages are expected to have resulted from changing strength of the East Australian Current.

Site 1172 is also well located to monitor changing interrelations between the East Australian Current and the Antarctic Circumpolar Current. The Antarctic Circumpolar Current was initiated during the Eocene-Oligocene transition far to the south of Tasmania with the opening of the Tasmanian Gateway, and it probably rapidly expanded northward toward Tasmania in the Oligocene because of subsidence of the South Tasman Rise. Site 1172 should have felt the effects of this expansion sometime during the Oligocene.

The East Australian Current has played a long-term role in transporting heat to high latitudes of the South Pacific. In the early Cenozoic, before the inception of the Antarctic Circumpolar Current, it flowed unimpeded to the Antarctic margin and served as a major source of heat transport to the continent (Kennett, 1977; Murphy and Kennett, 1986). With the initiation and later expansion of the circumpolar current, southward flow of the East Australian Current was interrupted and it then flowed to the east well to the north of Antarctica. This had the major effects of limiting heat transport to Antarctica, thermally isolating the continent, and contributing toward cooling and cryospheric development.

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