The continental shelf around Tasmania (Fig. 4) is mostly nondepositional at present. The continental slope west of Tasmania falls fairly regularly from 200 to 4000 m. The continental rise lies at 4000-4500 m, and the abyssal plain is generally 4500-5000 m deep. Sampling cruises have shown that the slope is underlain by continental basement and that Late Cretaceous and Paleogene shallow-marine sandstone, siltstone, and mudstone are widespread in deep water west of Tasmania, overlain by Neogene pelagic carbonates. Seismic interpretation shows that basement is generally overlain by several kilometers of Sorell Basin sediments (Fig. 5, P1).
The current-swept STR is a large, northwest-trending bathymetric high that rises to less than 1000 m below sea level and is separated from Tasmania by a west-northwest-trending saddle more than 3000 m deep (Fig. 4). It has a continental core, and seismic profiles show it is cut into basement highs and deep basins with several kilometers of sedimentary section (Fig. 5, P2 and P3). The overlying sequences in faulted basins include known Neogene pelagic carbonates and Paleogene marine mudstones, and seismic evidence suggests they also contain Cretaceous sediments. The top of the rise is a gentle dome with low slopes, but slopes are generally steeper between 2000 and 4000 m. The western slope is more gentle to 3000 m, but below that there is a very steep scarp trending 350°, which drops away to 4500 m as part of the Tasman Fracture Zone.
The East Tasman Plateau is a nearly circular feature, 2500-3000 m deep, separated from southeast Tasmania by a saddle 3200 m deep (Fig. 4). Slopes are generally low, but considerably greater on the plateau's flanks. Atop the plateau is the Cascade Seamount guyot, which formed as the result of hot spot volcanism and has yielded Eocene and younger shallow-water sandstone and volcanics. The plateau has up to 1.5 s two-way traveltime (TWT) of sediments in places (Fig. 5, P4) that are believed to comprise mainly Neogene pelagic carbonates and Eocene mudstones. These are underlain by continental basement rocks.
Deep Sea Drilling Project Results
Leg 29 of the Deep Sea Drilling Project (DSDP) drilled four partially cored sites in the Tasmanian region (Kennett, Houtz, et al., 1975a) (Table 1). The three sites that are relevant for Leg 189 are Site 281 on the STR, Site 282 on the west Tasmanian margin, and Site 280 on the abyssal plain immediately south of the STR (Figs. 4, 6).
Site 282 was drilled on a basement high west of Tasmania. This sequence includes much of the Cenozoic but contains four major unconformities. The sediments rest on a basalt flow of presumed Tertiary age. The sequences are upper Eocene to middle Oligocene mudstone, lower to middle Oligocene mudstone, lower Miocene marl, upper Miocene ooze, and a veneer of Pleistocene ooze. There is little in these sediments to suggest that the earliest sequence was located in deep water until the margin began to subside in the Oligocene. Calcareous microfossils are present throughout and total core recovery was 20%.
Site 281 was drilled on a basement high of quartz-mica schist of latest Carboniferous age, southwest of the tip of the STR. The sequence drilled includes upper Eocene basement conglomerate and glauconitic sandy mudstone, upper Oligocene glauconite-rich detrital sand, Miocene foraminifer-nannofossil ooze, and Pliocene-Pleistocene foraminifer-nannofossil ooze. Evidence from the recovered intervals suggests that the site subsided into deep water after the Miocene. Calcareous microfossils are present throughout, and total core recovery was relatively high (62%).
Site 280 was drilled on a basement high, in deep water southwest of the STR (Fig. 4), and bottomed in an "intrusive basalt," almost certainly associated with oceanic crust. The site penetrated a veneer of upper Miocene to upper Pleistocene clay and ooze underlain, beneath a sampling gap, by 55 m of siliceous lower Oligocene sandy silt and 428 m of middle Eocene to lower Oligocene sandy silt that contains chert in the upper 100 m and glauconite and manganese micronodules in the lower succession. The lower 200 m is rich in organic carbon (0.6%-2.2 %). The younger part of the lower Oligocene to upper Eocene sequence (Unit 5A) contains abundant diatoms, but the lower part (Units 5B and 5C) is almost completely devoid of pelagic microfossils. All sediments are presumed to have been deposited at abyssal depths. A brown organic staining suggests reducing conditions were present in parts of the upper Oligocene and lower Miocene. Total core recovery was only 19%.
Data from Site 281 assisted with the development of a broad globally significant history of the Cenozoic events in the region. Shackleton and Kennett (1975) produced composite foraminiferal oxygen and carbon isotope curves for the late Paleocene to the Pleistocene for Sites 277, 279, and 281. This record exhibits the now classical general increase in oxygen isotopic values reflecting a decrease in bottom- and surface-water temperatures and/or ice buildup during the Cenozoic. There was a general increase in isotopic values during the Paleogene, a rapid increase in the early Oligocene reflecting cryosphere expansion, and steady oxygen isotopic values until the middle Miocene, when there was another rapid oxygen isotopic increase as the Antarctic cryosphere further expanded. This was followed by further increase in oxygen isotopic values reflecting the development of the West Antarctic ice sheet in the late Miocene and the Northern Hemisphere cryosphere in the late Pliocene (Fig. 2). For Site 281 (STR), the sequence studied isotopically was the lower Miocene to the Pliocene, although the hole bottomed in the upper Eocene. In contrast, at Site 277 (Campbell Plateau) the lowest Miocene to upper Paleocene was studied. Although the isotopic sequence developed by Shackleton and Kennett (1975) was pioneering, it is of relatively low resolution, especially within the context of recent investigations.
To 189 Scientific Objectives
To 189 Table of Contents