INTRODUCTION

During Ocean Drilling Program Leg 189 in the Tasmanian area of the Southern Ocean (Exon, Kennett, Malone, et al., 2001), four sites (Fig. F1; Table T1) were cored on the western Tasmanian margin (WTM), South Tasman Rise (STR), and East Tasman Plateau (ETP). The sites are located on both the Indian side (Sites 1168 and 1170) and the Pacific side (Sites 1171 and 1172) of the Tasmanian area. These sites were designed to test the hypothesis that opening of the Tasmanian Seaway near the Eocene/Oligocene boundary and related initiation of circumpolar circulation contributed to the thermal isolation of Antarctica, leading to the development of the initial ice sheet and oceanic thermohaline circulation (Kennett, 1977).

Four sequences of sediments recovered from sites cored during Leg 189 contain the entire Cenozoic history of continental breakup, ocean opening, oceanographic development, and climate changes south of Australia. Major stages of the Cenozoic history of paleoenvironments in the Tasmanian area are delineated by primary sediment variations that separate three identical lithologic units at all sites (Exon, Kennett, Malone, et al., 2001):

1. A siliciclastic unit (Unit III, from the upper Maastrichtian at Site 1172 to the upper Eocene at all sites) consists of silty claystone and clayey siltstone deposited in restricted, poorly ventilated, neritic (water depth = <200 m) environments. Fluctuating contents of organic matter indicate dysoxic to oxic conditions.
2. The lower part of a transitional unit (Unit II) deposited at neritic depths (late Eocene) coincides with the preservation of abundant diatoms at Sites 1170 and 1171. This is followed by occurrences of slowly deposited glauconitic siltstones and minor sands indicative of intensified bottom water activity in increasing water depths near the Eocene/Oligocene boundary.
3. A biogenic pelagic unit (Unit I) deposited at bathyal depths (water depth = 400–2000 m) begins with lowermost Oligocene nannofossil chalk/limestone at all but Site 1168, where a gradational change from shallow terrigenous to pelagic carbonates occurs in the Oligocene and lower Miocene.

Bulk mineralogy provides a general overview of the sediment composition, which includes minerals of terrigenous, biogenic, and diagenetic origin. Variations in bulk sediment mineralogy yield precise information on the sediment composition, its variations in relation to the history of the Southern Ocean, and postdepositional evolution. Bulk mineralogy also helps in interpreting data using nondestructive methods.

Clay mineralogy provides information on continental weathering conditions in the source area of the particles. Terrigenous clay minerals originate from substrates, sediments, and soils. They are very sensitive to climate, continental morphology, and erosion and are easily transported over great distances by winds and oceanic currents. Clay minerals therefore provide information about different aspects of the paleoenvironments.