SUMMARY OF RESULTS
We recovered a range of biogenic and siliciclastic sediments from the latest Cretaceous to Quaternary (Fig. 16), including the interval of sediments recording the seaway opening south of Australia showing the development of Southern Ocean circulation. The sedimentary sequences allow recognition of three distinct phases of sedimentation (a siliciclastic interval, a transitional unit, and a biogenic carbonate sequence) and three sedimentary provinces (the more restricted west Tasmanian margin [Site 1168], the transitional STR [Sites 1169, 1170, and 1171] and the more ventilated ETP [Site 1172]).
The siliciclastic sequence extends from the late Maastrichtian (drilled on the ETP) to the late Eocene and consists of shallow-water silty claystone and clayey siltstone for the entire Tasmanian region. The siliciclastics are associated with abundant neritic diatoms from the middle Eocene in the Pacific region of the ETP. The sediment is enriched in organic matter in the poorly ventilated western Australo-Antarctic Tasmanian region. Such siliciclastics are widespread on the margins of Australia and Antarctica and have been observed from the Eocene Great Australian Bight (Feary, Hine, Malone, et al., 2000) to the Ross Sea Margin near Cape Adare (Hayes, Frakes, et al., 1975).
The initiation of the transitional unit coincides with the preservation of abundant neritic diatoms in the upper Eocene on the STR, followed by the occurrence of glauconitic siltstones and sands throughout the Tasmanian region. This is indicative of greater bottom-water activity near the Eocene/Oligocene boundary. Siliciclastic sediments decrease rapidly above the glauconitic interval in the lowermost Oligocene of the ETP and the STR but persist in the early Oligocene of the west Tasmanian margin (until ~ 31 Ma.).
The STR and the ETP pelagic carbonate sequence starts abruptly with the first occurrence of nannofossil chalk and limestone in the lowermost Oligocene. The STR sequence also contains common to abundant siliceous microfossils. On the west Tasmanian margin, the pelagic carbonates increase progressively in the lower Oligocene and consist of almost-pure nannofossil chalk by the middle Miocene. The pelagic carbonate ooze contains increased foraminifers and clay content in the whole Tasmanian region from the late Pliocene to the Pleistocene.
Tectonics and Sedimentation
The upper Paleocene and lower Eocene sediments on the STR decrease in grain size upsection as the clay assemblage evolves from a complex assemblage of kaolinite, illite, and smectite to predominant smectite. This pattern reflects decreased erosion and continental relief at the end of the stage of tectonic activity that led to the formation of transtensional basins from Antarctica to much of the STR.
Beginning in the late middle Eocene, increasing grain size of the shallow-marine siliciclastics on the STR and west Tasmanian margin, together with a clay assemblage dominated by illite and kaolinite, resulted from erosion of steep-continental relief. This interval correlates with a stage of late middle and late Eocene tectonism resulting from increased spreading in the Australo-Antarctic Gulf, and strike-slip activity on the western STR. This stage of tectonism ultimately led to a final separation of Australia from Antarctica. The presence of volcanic glass on the Pacific side of the Tasmanian region and ash layers on the ETP indicate that the late Eocene interval of tectonism was associated with intensified volcanic activity that may have persisted into the early Oligocene. How much of this volcanism was associated with the presumed hot spot volcano of Cascade Seamount on the ETP and how much was related to the general tectonism remains to be determined.
Environment of Sedimentation
Extensive bioturbation of the entire late Maastrichtian- to late Eocene-age sequence on the ETP and of Eocene sediments on the STR indicates significant ventilation of bottom waters. However, the absence of carbonate microfossils and the presence of siliceous microfossils suggest that major dissolution occurred and may have been associated with restricted environmental conditions. Darker sediment colors and decreased bioturbation on the western STR, together with abundant organic matter and intervals of lamination on the western Tasmanian margin, indicate poor ventilation of the Australo-Antarctic Gulf during the middle and late Eocene. Anoxic to suboxic conditions prevailed in the sheltered troughs of the Tasmanian margin. Abundant latest Eocene-age glauconite includes foraminifer infillings indicative of in situ formation. Widespread glauconite in the entire Tasmanian region indicates strong bottom-current activity and winnowing at shelf water depths.
Bottom-water ventilation increased sharply in the earliest Oligocene, producing the light colored sediments of the ETP and the STR, whereas ventilation increased slowly from the early Oligocene to the middle Miocene on the western Tasmanian margin as attested by the persistence of laminations and weak bioturbation. The predominance of pelagic carbonates from the earliest Oligocene is unusual at southern high latitudes. The carbonates are likely related to the warm marine influence of subtropical currents transporting heat to the Tasmanian region. This is in sharp contrast with the Ross Sea margin (Site 274), then adjacent to the STR, where biosiliceous microfossils have been predominant since the Oligocene (Hayes, Frakes, et al., 1975).
Climate and Sedimentation
No glacial or ice-rafted deposits were found at any site, even in Oligocene sediments when the Tasmanian region was still close to Antarctica, indicating the absence of significant ice cover in nearby East Antarctica. Smectite or kaolinite dominates in most Eocene sediments of the entire Tasmanian region, indicating that warm climates prevailed on emerged adjacent areas of the Tasmanian land bridge and Antarctic margin. An increasing trend of illite and random mixed layered clays in the lower Oligocene of Site 1170 on the western STR suggests the development of physical weathering on the adjacent emerged areas, like that in other sectors of East Antarctica (Ehrmann, 1991; Robert and Kennett, 1997). This trend immediately follows the strike-slip activity on the western STR during the transition to the more ventilated biogenic sequence.
The continued presence of common to abundant kaolinite on the western Tasmanian margin and the ETP from the lower Eocene to the Pliocene indicates the persistence of relatively warm climates with significant precipitation in the adjacent coastal areas of Australia. Episodes of increased kaolinite content on the western Tasmanian margin in the lower Oligocene and in the upper to middle Miocene correlate with intervals of intensified precipitation at midlatitudes. The early to middle Miocene interval immediately preceded the expansion of Antarctic ice at 14-15 Ma (Kennett, 1977). Kaolinite on the ETP increased continuously from the middle Eocene to the Pliocene as Australia moved north to warmer latitudes. Beginning in the Pliocene, an increase in the sediment clay fraction is associated with a mineralogical change to increased illite and kaolinite, a composition very similar to that in different latitudes of the Lord Howe Rise in the Tasman Sea. This pattern suggests the development of a dust supply from arid central Australia, most probably by tropospheric winds associated with cold fronts (Pye, 1987).
There are pervasive alternations of lighter and darker intervals from the Eocene to the Pleistocene in the entire Tasmanian region. Preliminary spectral analyses have been conducted on lightness and magnetic susceptibility data from the west Tasmanian margin during a chronologically well-constrained Pliocene-Pleistocene interval. Three important cycles of ~200, 100, and 40 k.y. may indicate some orbital control on regional sedimentation. High sedimentation rates of the STR during the Eocene suggest that some variation of the sedimentation at higher frequencies (104-105 yr) might be expected.
Summary of Results-Biostratigraphy | Table of Contents