Previous sampling has shown that basinal Paleocene history was probably similar in two areas, the west Tasmanian margin and the STR. Clearly, there were differences in shallow-marine deltaic deposition during the Eocene, when the northern area experienced more carbonaceous sedimentation while the southern area experienced the deposition of siliceous radiolarian-bearing glauconitic mudstones. Overall, results are summarized by Hill et al. (1997b), Exon et al. (1997a), and Exon et al. (1997b). As Australia cleared Antarctica, submarine erosion in both areas formed an Oligocene unconformity, and both areas subsided steadily. However, the southern area sank vertically as a block, whereas the west Tasmanian margin rotated downward from a hingeline near the coast, increasing water depths with increasing distance from Tasmania. In many southern areas, the Antarctic Circumpolar Current removed most Neogene sediments, but thick late Oligocene to Holocene carbonate sediments are present in depocenters off west Tasmania and on the STR.
The stratigraphy of petroleum exploration wells on the west Tasmanian continental shelf was summarized by Moore et al. (1992). The detrital Upper Cretaceous sequence is probably disconformably overlain by the Cenozoic. Nonmarine to shallow-marine, Paleocene to early Eocene fining-upward sequences are always present. The middle Eocene to lower Oligocene sequence is more calcareous, consisting of shallow-marine sandstone, marl, and limestone. Above a major unconformity, the late Oligocene-age and younger sediments are dominantly shelfal marl and limestone. All of the Late Cretaceous, Paleocene, and Eocene siliciclastic sediments sampled are interpreted as shallow or restricted marine and are commonly deltaic (prograding is marked in seismic profiles).
Existing stratigraphic and sedimentologic information indicates that middle Eocene sequences are different in the northern sites west of Tasmania (DSDP Site 282; Hill et al., 1997b) and in the south in the STR (DSDP Site 281; Exon et al., 1997b), although shallow-marine and deltaic facies are found in both areas. Northern sequences contain abundant organic matter and calcareous temperate microfossil assemblages. Southern sequences contain more siliceous microfossils of colder water character. One occurrence of varves (Exon, et al., 1997b) suggests strong seasonality of the Antarctic climate. The middle Eocene to upper Oligocene sequences are crucial to understanding the opening of the Tasmanian Seaway, initially in shallow and later in deep water. Before the Oligocene, sequences on either side of the STR should have distinctive biogeographic characters.
Study of the uppermost Eocene through Oligocene sequences will be of special importance in examining the timing of the development of the circumpolar circulation both across and south of the STR (~65°S at that time). The opening of this gateway was such a profound event that biotic, sedimentologic, and geochemical parameters would almost certainly have undergone distinct changes. When studied in detail and in unison, changes in these parameters are expected to provide the crucially needed evolutionary information on the gateway. The dating of unconformities or hiatuses will provide critical information on major current activity during the Oligocene, especially in the shallow sequences, although sites have been selected to minimize the effects of sediment erosion. We are especially interested in the timing of initial shallow-water linkage across the STR and deep-water linkage south of the STR.
Sites 1168 and 1170 will provide data about the Indian Ocean paleoenvironment before the opening of the Tasmanian Gateway (middle to late Eocene), whereas Sites 1171 and 1172 will provide information about South Pacific paleoenvironments before the opening. All sites will address the initial shallow-water breakthrough (late Eocene), and most will address the deep-water breakthrough to some extent (early-mid-Oligocene?).
Results from DSDP Leg 29 suggested that a sequential appearance of marine microfossils, from dinocysts and arenaceous foraminifers (early Eocene) to calcareous nannofossils (middle Eocene) to calcareous benthic foraminifers (early late Eocene) and to planktonic foraminifers (late late Eocene), might well be revealed at most of the sites. The order of appearance of major groups is paleoenvironmentally significant and is expected to provide crucial insights about the evolution of the Southern Ocean biota. The upper middle Eocene to the lower Oligocene sequence, where calcareous microfossils are present and sedimentation rates were expected to be 1.5 to 3 cm/k.y., should provide excellent documentation of tectonic, climatic, and oceanographic changes. Planktonic foraminiferal and calcareous nannofossil biostratigraphy, in conjunction with strontium and oxygen isotope stratigraphies should provide a chronology of sufficient resolution. Specific stratigraphic boundary events (e.g., Eocene/Oligocene and Miocene/Pliocene) will be analyzed at high resolution.
Scientific Objectives-Neogene and Quaternary History | Table of Contents