A nearly continuous 883.5-m-thick section, drilled at three holes at Site 1168, contains a wide range of sediment types, from calcareous biogenic ooze and chalk to siliciclastic claystone and siltstone (Fig. F4). The calcareous biogenic components (nannofossils and foraminifers) predominate from the surface to 260 mbsf (Core 189-1168A-28X), where siliciclastic clay-sized particles first appear. These clays are associated with siliciclastic silty particles below 530 mbsf (Core 189-1168A-56X). Below 750 mbsf (Core 189-1168A-79X), the sediment consists essentially of siliciclastic silt and clay particles.
The sedimentary sequence has been divided into five units, based on lithologic variability. Lithostratigraphic units (Fig. F5) were defined by consideration of core features, smear slides, thin sections, coulometric carbonate analyses, reflectance spectrophotometry, and magnetic susceptibility data. A significant correlation exists between spectrophotometry data (lightness) and coulometric carbonate contents (Fig. F6), indicating that the color of the sediment at Site 1168 is strongly linked to its content in calcareous biogenic, organic, and siliciclastic particles. Although correlation between coulometric analysis and smear-slide observations proved adequate for predominantly calcareous biogenic and siliciclastic sediments, in transitional carbonate sediments an offset exists between quantitative carbonate content and smear-slide estimates (which underestimate the amount of clay-sized siliciclastics). Coulometric carbonate data and visual inspection of the cores provide basic information for determining units and subunits. Smear-slide observations (Fig. F7) were used to describe the lithology from the barrel sheets. Lithostratigraphic units of Site 1168 are compared to those of DSDP Site 282, drilled nearby (Kennett, Houtz, et al., 1975).
Lithostratigraphic Unit I, 260 m thick, is composed of biogenic ooze and chalk divided into two subunits. Subunit IA, 45 m thick, is composed of light greenish gray nannofossil ooze and foraminifer-bearing nannofossil ooze containing macrofossil bioclasts and layers of foraminiferal sand. Carbonate content varies from 75 to 90 wt%. This subunit ranges in age from late Pliocene to the Pleistocene and may correspond to Unit I of DSDP Site 282. Subunit IB, 215 m thick, consists of white nannofossil ooze and foraminifer-bearing nannofossil ooze to chalk, with 85 to 97 wt% carbonate. This subunit ranges in age from middle Miocene to early Pliocene and may correlate to Unit II of DSDP Site 282.
Unit II, 400 m thick, is characterized by siliciclastic clay particles increasing downhole and significant occurrences of silt particles and contains three subunits. Subunit IIA, 150 m thick, ranges from greenish gray clay-bearing nannofossil chalk to nannofossil claystone. Its carbonate content decreases from 75 wt% in the upper part to 30 wt% in the lower part of the subunit. Subunit IIA is early to middle Miocene in age and may correspond to Unit III of DSDP Site 282. Subunit IIB, 130 m thick, includes mostly nannofossil claystone and nannofossil-bearing claystone and is greenish gray to dark greenish gray in color. The carbonate content varies from 10 to 35 wt%. This subunit is late Oligocene in age and may correlate to Unit IV of DSDP Site 282. Subunit IIC, 120 m thick, ranges from silty nannofossil chalks to nannofossil siltstone. The color varies from greenish gray and dark greenish gray to dark gray and olive gray. The carbonate content fluctuates from 18 to 42 wt%. This subunit is early to late Oligocene in age and may correlate to Unit V of DSDP Site 282.
Unit III, 88.6 m thick, is characterized by a variety of pure and mixed siliciclastic sediments and is divided into two subunits. Subunit IIIA, 54 m thick, is early Oligocene in age and varies from clayey nannofossil chalk to nannofossil-bearing silty claystone and nannofossil-bearing organic clayey siltstone. The carbonate content ranges from 0 to 35 wt%, and the color ranges from greenish gray to dark greenish gray. Subunit IIIB, 34.6 m thick, is early Oligocene in age and consists of organic-bearing silty claystone, organic clayey siltstone, and sandy claystone. Sandy layers, shell debris, and pyrite concretions are locally present. The carbonate contents vary from 2 to 18 wt%, and color ranges from dark olive gray and dark gray brown to black. Unit III may correspond to Unit VI of DSDP Site 282.
Unit IV, 13.4 m thick, is latest Eocene to earliest Oligocene in age and is composed of alternating dark grayish brown organic-bearing silty claystone and dark greenish gray glauconite-rich clayey siltstone. This unit is characterized by distinct fluctuations of the carbonate content, from 3 to 38 wt%.
Unit V, 121.5 m thick, is characterized by abundant organic matter and very low carbonate contents (below 8 wt%). This unit ranges from nannofossil-bearing silty claystone to organic silty claystone and clayey siltstone. Unit V is of late Eocene age and may correlate to Unit VII of DSDP Site 282.
Sediments of Site 1168 are generally slightly to moderately disturbed by the drilling process in the APC section. Additional sediment disturbance is common, as breccia at the top of each core and as biscuits in the XCB section of the holes.
The sediments of Unit I are predominantly calcareous biogenic oozes. Unit I is divided into two subunits based on carbonate content, color, and compositional changes.
The sediments of Subunit IA consist of light greenish gray (10GY 8/1) to greenish gray (5GY 6/1) nannofossil ooze and foraminifer-bearing nannofossil ooze and contain abundant bioclast fragments. They are characterized by a high proportion of biogenic carbonates (75 to 90 wt%). Bioturbation is occasional and is rare to present in intensity.
As a minor lithology, thin greenish gray (5GY 6/1) layers (1 to 2 cm thick) and clasts of foraminifers are present throughout this subunit (Fig. F8) (interval 189-1168B-3H-5, 16-30 cm).
An apparent color cyclicity of lighter and darker horizons is observed from spectrophotometer data and visual observation. The basal surface of the darker intervals is typically sharp and contains increased contents of foraminifers. Lightness (L*) was measured on all three holes of Site 1168 in 2-cm resolution. Because of the sampling density of the spectrophotometer, the data include minor sedimentary color changes (e.g., darker bands with a high content of pyritized particles). To investigate possible cyclicity in Subunit IA, moving averages of 10 points and 50 points resolution were calculated (Fig. F9). Both smoothed data sets reveal a strong cyclic pattern, which seems to have very constant cycle lengths. The raw lightness data L* vs. mbsf were then used to perform a spectral analysis applying the Blackman and Tukey method of a Fourier transformation included in the Analyseries software (Paillard et al., 1996). The magnetic susceptibility data set was treated with the same method and parameters. The results imply three major cycle lengths?.71/3.58 m, 1.85/1.79 m, and 0.40/0.55 m—which are detected in both parameters (Fig. F10). At this stage we can only speculate about the length in time of the cycles. There is a possibility that these three cycles represent a 200, a 100, and a 40-k.y. cyclicity. The 200-k.y. cycle and minor peaks then could be explained as a result of interference and harmonic effects. If this is correct, the resulting sedimentation rate in the core interval from 2-23 mbsf of Hole 1168A would then be 1.85 cm/k.y., close to the 1.47 cm/k.y. sedimentation rate deduced from micropaleontological data.
Subunit IB essentially consists of white (N 8) to light greenish gray (10Y 8/1, 10Y 8/2, and 10Y 7/1) nannofossil and foraminifer-bearing nannofossil ooze and chalk. Its carbonate content is very high, between 85 and 97 wt%.
In addition to the major lithology, two minor lithologies are recognized. Light greenish gray (10Y 7/1) to greenish gray (5G 6/1 and 5GY 6/1) layers of nannofossil foraminifer ooze, nannofossil-bearing foraminifer ooze, and foraminifer ooze are found in Cores 189-1168A-6H through 8H and in Core 10H. A foraminifer- and spicule-bearing nannofossil chalk is present in Cores 189-1168A-19X and 21X.
Light greenish gray (5GY 8/1) laminae and clasts enriched in foraminifers are frequent throughout Subunit IB, as well as thin dark beds (N 6/1), nodules, and stains of pyrite. In the lower part of this subunit, infrequent to rare silt-sized calcite suggests that some diagenesis has occurred. Occasional bioturbation is rare to common in intensity and is observed principally as a pale yellow (5Y 7/4) mottled appearance.
An apparent cyclicity of lighter and darker intervals is observed in the lower part of the subunit (Cores 189-1168A-27X and 28X) from spectrophotometer analysis and core description. Major cycles are 1.20 and 2.40 m long.
Unit II is characterized by significant occurrences (20% to 75%) of siliciclastic clay particles increasing with depth in its upper part, in association with siliciclastic silty particles in its lower part. Unit II is divided into three subunits.
Subunit IIA consists of light greenish gray (5GY 7/1) to dark greenish gray (10GY 4/1) clay-bearing nannofossil chalk to nannofossil claystone. The claystone is partially lithified. Color alternations are observed through this subunit although they vary in length. The carbonate content decreases with depth from 75 to 30 wt%, whereas the clay content increases. Occasional rare to common silt-sized calcite grains suggest possible diagenesis of calcareous biogenic material.
Lenses and thin layers of very fine sandy material are occasionally present. Pyrite is present commonly as nodules, laminations, and staining from Core 189-1168A-30X downhole. Alternating beds, light gray (5Y 7/2) and light olive gray (5Y 6/2) to light greenish gray (5GY 7/1) in color, are within Cores 189-1168A-30X and 31X and are identified by both the magnetic susceptibility and photospectrometer in the yellow wavelength (550 nm; Fig. F11). These changes may indicate oscillations in the oxidation levels in the environment. Bioturbation is very frequent, is moderate to common in intensity, and includes Zoophycos and Chondrites burrows.
More clayey intervals are present in Cores 189-1168A-32X, 33X, and 35X. Clays are especially abundant in the darker intervals of Core 189-1168A-33X, where the carbonate content drops to 18 wt% and which correlate with strong increases of the magnetic susceptibility. Both dissolution episodes and increased terrigenous supply may account for the significant clay increase.
The sediments of this subunit are often dominated by a cyclic pattern, as illustrated in the reflectance (Fig. F12). Magnetic susceptibility also shows this cyclicity and is in good agreement with lightness data (Fig. F13). Spectral analysis indicates that the wavelengths of the cycles in both the reflectivity and magnetic susceptibility are in good agreement (Fig. F14).
Prominent fluid-escape structures in Core 189-1168A-36X are closely associated with brecciated sediments up to Core 35X and interrupt a faintly laminated sequence (Fig. F15; interval 189-1168A-36X-3, 35-75 cm). In addition, smaller fluid-escape structures associated with slumps are present in Cores 189-1168A-39X, 41X, and 42X. The soft sediment deformation is very similar to that drilled in the Miocene silty mudstone off Peru during Leg 112 (Lindsey-Griffin et al., 1990). In the upper Miocene Sisquoc Formation of California, similar soft-sediment deformation has been interpreted as resulting from the dissociation of methane hydrates (Kennett and Fackler-Adams, 2000).
In Subunit IIB, the sediment is greenish gray (5GY 5/1 and 10Y 5/1) to dark greenish gray (5GY 4/1 and 10Y 4/1) in color. It mostly varies from nannofossil claystone to nannofossil-bearing claystone, is partially lithified, and its carbonate content fluctuates from 10 to 35 wt%.
Nodules of pyrite, clay concretions, and mud clasts are occasionally present. Convoluted bedding and laminations have been observed locally, but most of this subunit is moderately to abundantly bioturbated. Zoophycos and Chondrites are the most frequently recognized ichnofossils.
The sediment in Subunit IIC varies from silty nannofossil chalk to partially lithified nannofossil siltstone. Its carbonate content is slightly increased by comparison with Subunit IIB and fluctuates from 18 to 42 wt%. Subunit IIC shows a wide range of colors, from greenish gray (10GY 5/1 and 10Y 5/1) and dark greenish gray (10Y 4/1 and 10Y 3/1) to dark gray (5Y 4/1) and dark olive gray (5Y 3/2).
Laminations are commonly observed in this subunit. Convolute bedding and cross bedding have been locally observed. Mud clasts and nodules of pyrite are observed occasionally with the concentration of pyrite nodules increasing in Core 189-1168A-66X. A fluid escape structure and slumps are in Core 189-1168A-65X. Gastropods, bivalves, and undifferentiated mollusc shells are observed in Cores 189-1168A-56X and 58X to 60X. Rare to common bioturbation is present in the upper part (Cores 189-1168A-58X through 61X) and in the lower part (Cores 189-1168A-66X through 70X) of the subunit.
Unit III is characterized by a variety of pure and mixed siliciclastic sediments and by the occurrence of organic matter in significant amounts. Unit III is divided into two subunits.
The sediment varies from clayey nannofossil chalk and nannofossil-bearing silty claystone to organic clayey siltstone and sandy claystone and is partially lithified. The carbonate content fluctuates from 0 to 35 wt%. Its color ranges from greenish gray (10Y 5/1) to dark greenish gray (5GY 4/1 and 10Y 4/1) and very dark greenish gray (10Y 3/1). Rhythmic changes of color are visible.
Some laminations are observed, especially within darker strata. Small nodules of pyrite are present throughout the subunit. Undifferentiated molluscan shells are found in Core 189-1168A-71X. Bioturbation is rare to abundant and mostly consists of Chondrites burrows. Indurated (2.5Y 7/2) clay clasts are observed throughout this subunit. An indurated fine sandstone is observed in interval 189-1168A-75X-6, 56-72 cm.
The sediment in Subunit IIIB consists of partially lithified organic-bearing silty claystone, organic clayey siltstone, and sandy claystone. This subunit is characterized by low carbonate content, between 2 and 18 wt%. Color varies from yellowish brown (2.5Y 3/2 and 5Y 3/1) and dark olive gray (5Y 3/2) to dark gray brown (2.5Y 4/2), very dark greenish brown (2.5Y 3/2), and black (5Y 2.5/1).
As a minor lithology, yellowish brown (2.5Y 3/2 and 5Y 3/1) to dark reddish brown (7.5YR 3/2) sandy layers are present in Section 189-1168A-76X-3 downcore and in Core 79X.
Laminations are visible where bioturbation is absent. Pyrite and coarse sand grains are scattered throughout the subunit, as well as shell debris. Occasional bioturbation is rare to abundant in intensity, with compressed burrows being visible in Cores 189-1168A-77X and 78X.
This unit is characterized by alternating dark grayish brown (2.5Y 3/2) organic-bearing silty claystone and dark greenish gray (5GY 3/1 and 5GY 4/1) clayey siltstone. The latter lithology also contains abundant coarse glauconite grains and some sand-sized siliciclastic particles (glauconitic clayey siltstone) (Fig. F16; interval 189-1168A-79X-6, 25-50 cm). High concentrations of authigenic glauconite are an indicator of low sedimentation rates and sediment-starved depositional environments (McRae, 1972). The frequency and thickness of the dark greenish gray glauconitic clayey siltstone beds decrease with depth in the unit. The dark grayish brown organic-bearing silty claystone contains small grains of pyrite scattered throughout. The carbonate content in Unit IV shows distinct fluctuations, from 3 to 38 wt%. Rare to common bioturbation is visible in Sections 189-1168A-80X-4 to 80X-7. The boundary between Unit III and Unit IV is marked by a sharp contact, whereas the transition to Unit V is gradational.
Unit V consists of silty claystone and nannofossil-bearing organic silty claystone, alternating with organic-bearing clayey siltstone and silty claystone. These lithologies grade to organic clayey siltstone downhole. The sediment of Unit V is partially lithified. The siliciclastic particles are mostly quartz and clay. Very dark brown (10YR 2/2 and 2.5Y 3/2), dark grayish brown (2.5Y 4/2), and very dark grayish brown (2.5Y 4/1 and 10YR 3/1) sediments alternate with black (5Y 2.5/1, 10YR 2/1, and N 3/0) deposits.
Siliciclastic material (mostly very coarse sand to fine gravel) is scattered throughout the unit and is present as occasional layers. Several thin, coarsening upward successions with sharp basal contacts are observed in Core 189-1168A-83X. Sporadic glauconite and pyrite grains are in the upper part of the unit. Laminations are visible in very dark brown to very dark grayish brown sediments, whereas black sediments are generally massive. Where present, laminated sediments consist of darker and lighter layers and contain very small percentages of nannofossils. Shell fragments are present in the upper part of the unit, where gastropods have been recognized, and in Core 189-1168A-92X. Bioturbation is generally absent. However, rare to abundant bioturbation is sporadically observed within small intervals as well as in Cores 189-1168A-91X to 93X.
The purpose of this study is to recognize major paleoenvironmental variations at Site 1168, as expressed by the changing nature and abundance of clay minerals. Forty-three samples have been analyzed, at a resolution of one sample for every two cores.
The clay minerals are dominated by smectite, illite, and kaolinite. Chlorite and random mixed-layered clays, occasionally present in minor amounts, are not included in the percentage estimates. Five units of clay associations were determined for Site 1168, based on the abundances of the minerals. These are designated Units C1 through C5 (Fig. F17).
Unit C1, which extends from the seafloor to 45 mbsf, has a clay association that consists of abundant smectite (40% to 60%) and kaolinite (30% to 50%), accompanied by 10% illite. This unit ranges in age from late Pliocene to the Pleistocene and correlates to lithostratigraphic Subunit IA. Unit C2 extends from 45 to 260 mbsf and is largely dominated by smectite (70% to 90%), with common kaolinite (10% to 25%) and sporadic low contents of illite (0% to 10%). Low clay content in some samples results from greater dilution of the fine siliciclastic fraction by biogenic carbonates. This unit is middle Miocene to early Pliocene in age, and it correlates to lithostratigraphic Subunit IB. Unit C3 extends from 260 to 540 mbsf and is characterized by large fluctuations of the clay association. Smectite (45% to 100%) predominates over kaolinite (0% to 45%), whereas illite is episodically present in minor amounts (0% to 10%). This unit is late Oligocene to middle Miocene in age and correlates to lithostratigraphic Subunits IIA and IIB. Unit C4 extends from 540 to 830 mbsf and contains a clay association dominated by smectite (60% to 75%) decreasing with depth, kaolinite (25% to 35%), and very low amounts of illite (0% to 5%). This unit is late Eocene to late Oligocene in age and encompasses the range of lithostratigraphic Subunit IIIC, Unit IV, and part of Unit V. Unit C5 extends from 830 mbsf to the bottom of Hole 1168A at 883.5 mbsf. Kaolinite (85%) largely dominates the clay association and is associated with illite (5% to 15%) and very low amounts of smectite (0% to 5%). Unit C5 is late Eocene in age.
The extreme dominance of kaolinite in Unit C5 suggests that warm climatic conditions, with high precipitation during at least part of the year, prevailed in the source area of the particles during the late Eocene. In the modern environment, this mineral is typical of tropical to subtropical humid areas with good drainage conditions (Weaver, 1989). This pattern of clay mineral association is unusual for this time interval in most oceanic areas where it is generally marked by largely dominant to exclusive smectite (Chamley, 1989; Robert and Chamley, 1992), including at the adjacent Tasman Sea Sites 592 and 593 (Robert et al., 1985). However, kaolinite has already been observed in the late Eocene at southern high latitudes, in the Atlantic sector (Robert and Kennett, 1992), and in the Prydz Bay sector (Hambrey et al., 1991) of the Southern Ocean. Site 1168 is located on a deltaic lobe, and its late Eocene sediments are predominantly siliciclastic. The clay particles are most probably of local origin, supplied through runoff from the adjacent drainage basin. Identical clay associations in similar sedimentological settings have been observed in the South Atlantic during warm Cretaceous intervals of tectonic activity related to the early stages of ocean opening (Robert, 1987). In these areas, precipitation over steep continental relief ensures continuous leaching of chemical elements from the substrates and intensive chemical weathering and erosion. It is therefore highly probable that the clay association in Unit C5 derives from weathering and erosion of steep continental relief in the adjacent drainage basin during the stage of Eocene tectonic activity that preceded final separation of Tasmania from Antarctica.
The base of Unit C4 is characterized by a drastic increase of smectite and a corresponding decrease of kaolinite. The siliciclastic sediment is still deltaic, and the clay particles are most probably derived from the adjacent drainage basin, as in Unit C5. The clay association becomes very similar to those of late Eocene age observed in most oceanic areas of passive margins, including the adjacent Tasman Sea (Robert et al., 1985; Robert and Chamley, 1992). Smectite dominance still corresponds to warm climatic conditions and intense chemical weathering on the adjacent continent. However, this mineral prevails in areas of low relief with alternating periods of precipitation and aridity, its formation being intensified on volcanic substrates (Chamley, 1989; Weaver, 1989). The transition to predominant smectite at the lower part of Unit C4 therefore suggests that continental relief strongly decreased and the morphology of the adjacent drainage basin drastically changed during the late Eocene. A similar evolution of the clay association is observed in Cretaceous sediments from the South Atlantic, where it is related to the phase of subsidence and transgression of the continental margins that follows the early stages of ocean opening (Robert, 1987). At Site 1168, the transition to dominant smectite is probably related to tectonic relaxation and subsidence of the western Tasmania margin as ocean opening progressed. The content of smectite increased slightly in the upper part of Unit C4 in relation to the continuing subsidence of the passive western Tasmania margin.
Unit C3 is mainly marked by distinct fluctuations of the clay association—smectite increases drastically at ~503 mbsf (NP25, late Oligocene), whereas kaolinite increases at 464 mbsf (NP25, late Oligocene) and from 310 mbsf to 272 mbsf (NN2 to NN4, early Miocene). As the Southern Ocean was already largely open south of Tasmania, the siliciclastic fraction of the sediment decreased progressively through time, as shown from lithostratigraphic Unit III. The clay fraction decreases in abundance, and clay particles most probably originate from different areas of the Australian margin, transported by marine currents to Site 1168. With the subsidence of the passive margin well advanced, the clay association of oceanic margins and basins mostly reflects variations of climate and circulation (Robert, 1987). It is assumed that in Unit C3 at Site 1168, the fluctuations of the clay association mainly result from climatic variability. In the upper Oligocene section, strong increases of smectite and kaolinite in nannofossil Zone NP25 represent intensified chemical weathering in the source areas. Coeval increases of kaolinite have already been observed in different oceanic areas from both hemispheres, where they have been interpreted as intervals of intensified precipitation on most continental areas (Robert and Chamley, 1987). A further increase of kaolinite is in the early Miocene section from Zone NN2 to NN4. Coeval but smaller increases of kaolinite are observed in the adjacent Tasman Sea at DSDP Sites 590, 591, and 592 (Robert et al., 1985; Stein and Robert, 1985). However, such intervals of increased kaolinite have not been observed yet in the North and South Atlantic, where kaolinite decreases in the lower Miocene (Robert and Chamley, 1987). This increase of kaolinite seems of regional importance and may reflect increased warmth and intensified precipitation at middle to high latitudes of the Southwest Pacific. It immediately precedes the development of the East Antarctic Ice Sheet between 14 and 15 Ma (Kennett, 1977).
Unit C2 shows higher contents of smectite, the maximum abundance being recorded between 157 and 138 mbsf (i.e., from Zone NN9 to the lower NN11). Warm climates with alternating intervals of precipitation and aridity prevailed by this time in the source areas of particles (i.e., most probably the southern Australian and the western Tasmanian margins).
Unit C1 is marked by increased contents of kaolinite, beginning during nannofossil Zone NN16 in the late Pliocene. At the same time, the clay content of the predominantly calcareous biogenic sediment slightly increases (Subunit IA). This is probably an interval of intensified erosion of the continental margins. However illite does not increase, which is unusual at this time of further development of physical weathering in most areas. The late Pliocene to Pleistocene clay association of Site 1168 could come from erosion of the central Australian desert by wind activity. At this time in the Tasman Sea, wind supply from arid areas reached its maximum extension to the south (Stein and Robert, 1985). In this case, particles may have been transported to Site 1168 by northwesterly winds similar to those associated with modern nonprecipitating cold fronts (Pye, 1987). However, the clay association derived from central Australia, as observed in the Lord Howe Rise sediments, contains higher proportions of illite (Stein and Robert, 1985). During the late Pliocene, kaolinite increased in several oceanic areas (e.g., at the middle latitudes of the Atlantic and of the North Pacific), whereas it decreased off the northwest African deserts. Differences in the evolution of the clay association have been interpreted as resulting from intensified humidity at middle latitudes of both hemispheres and increased meridional zonation of climate (Robert and Chamley, 1987). At present, both hypotheses may account for the late Pliocene increase of kaolinite at Site 1168, and a higher resolution is necessary to further investigate the climatic processes.
Seismic profiles indicate that the whole area where Site 1168 has been drilled consisted of small sedimentary basins separated by basement highs often capped with Cretaceous or Paleocene poorly sorted sediments (Hill et al., 1997). The Cretaceous to early Eocene sediments were deposited in prograding deltaic environments that kept up with the subsidence caused by crustal thinning as Australia and Antarctica started to rift apart. Unit V of Site 1168 is seismically represented by a body of chaotic reflectors typical of deltaic systems, which give way to well-defined, parallel reflectors to the northwest. The positions of basement highs and sedimentary basins suggest that the late Eocene western Tasmania area consisted of a succession of parallel, most probably subaerial, crests of northwest-southeast orientation separated by troughs of varying depths. At DSDP Site 282 to the west, seismic profiles and sedimentary sequences indicate significantly greater oceanic influence with pillow basalt, dipping reflectors, marine biogenic components, and very low sedimentation rates typical of starved margins. To the southeast, the succession of sedimentary troughs within one of which Site 1168 is located corresponds to a shallow, brackish to marine deltaic setting in the late Eocene, deepening to the northwest (Fig. F18). Subaerial areas were probably located on local basement highs as well as to the east on the Tasmania-Antarctica land bridge, which was exposed to tectonic activity during ocean opening. Apatite fission-track dating indicates tectonic uplift and erosion of western Tasmania during the late Paleocene to early Eocene (O'Sullivan and Kohn, 1997). During the Oligocene, the whole area tilted to the west, possibly in relation to extended subsidence, and siliciclastic sediment supply decreased.
In the lowermost part of the drilled section (lithostratigraphic Unit V), of late Eocene age, an anoxic environment is indicated by the black to very dark gray color of the sediment, abundance of organic remains, and the highest total organic carbon (TOC) values (4 to 5 wt%) for Site 1168. The sediment is predominantly siliciclastic siltstones and claystones, derived from erosion of onshore areas. The organic matter is characterized by a low hydrogen index (HI) from 100 to 200 and a high C/N ratio (25 to 50), representative of terrestrially derived organic matter (see "Organic Geochemistry"). Throughout the whole unit, abundant sporomorphs and plant tissue suggest a nearshore setting, whereas the poorly diversified dinoflagellates are consistent with eutrophic conditions and varying salinity in normal marine to brackish water. The benthic foraminifers present have agglutinated tests typical of highly stressed nearshore, low-oxygen environments (see "Biostratigraphy").
Below 830-840 mbsf, massive siltstone and claystone deposits dominate the sequence. The clay mineral assemblage, dominated by kaolinite, suggests a warm climate and significant precipitation in the source area during at least part of the year (Chamley, 1989; Weaver, 1989). Such assemblages are typically derived from steep continental relief and may correspond to the stage of tectonic activity that preceded the separation of Tasmania from Antarctica. High C/S ratios (2 to 6) are characteristic of a brackish environment. The massive black deposits are sporadically intercalated with very dark gray intervals of alternating dark and light laminations, where the lighter laminae contain abundant nannofossils indicative of open-ocean salinities and showing dissolution traces. These laminated intervals of suboxic conditions most probably correspond to marine incursions and improved oxygenation in the restricted eastern end of the Australo-Antarctic Gulf.
Above 830-840 mbsf, the very dark, predominantly siliciclastic sediment includes episodic occurrences of coarse sand and fine gravel, indicative of intensified erosion in upstream continental areas. The clay mineral assemblage dominated by smectite indicates a warm climate with alternating intervals of humidity and aridity (Chamley, 1989; Weaver, 1989). Such an assemblage is typically derived from continental areas of low relief and is consistent with tectonic relaxation and subsidence associated with passive margin development off Tasmania. Slightly decreased TOC values (between 1.5 and 4.5 wt%), together with fluctuating HI and C/N values, suggest intermittent decreases in terrestrially derived organic components. Lower C/S ratios (1 to 3) indicate brackish to marine conditions (see "Organic Geochemistry"). Foraminifers and nannofossils, as well as laminated intervals, are more frequent than in the lowermost part of the unit.
To summarize, the lower section of upper Eocene Unit V was deposited near a tectonically active hinterland of steep morphology, characteristic of the early stages of ocean opening. Overall, a warm climate with significant precipitation and runoff prevailed in this area, driving erosion of organic and siliciclastic terrigenous particles. This erosion produced sedimentation rates of ~8 cm/k.y. in a restricted basin at the southeastern boundary of the Australo-Antarctic Gulf, where eutrophism and suboxic conditions prevailed. Marine incursions resulted in temporary more oxygen-rich conditions that allowed preservation of marine calcareous planktonic microorganisms (nannofossils). In the upper part of Unit V, intense erosion persisted in upstream areas of the adjacent drainage basin, whereas areas of low-relief increased along the passive margin. Overall, a warm climate persisted, with sporadic or seasonal aridity. Eutrophism and suboxic conditions still prevailed, but oxic marine incursions increased in frequency.
The sediment of the corresponding interval of Site 282 (Kennett, Houtz, et al., 1975), drilled 80 km northwest of Site 1168, overlies pillow basalt, but interpretation of more recent seismic profiles suggests it is possibly on continental crust (Hill et al., 1997). The dominantly siliciclastic sediment of Site 282 is also deposited in comparable suboxic environment, but the clayey silt and silty clay there include up to 20% each of sponge spicules and nannofossils, indicative of greater marine influence than at Site 1168. The Site 282 sedimentation rate of 2.5-3 cm/k.y. is much lower than at the more proximal Site 1168, which has a sedimentation rate of ~8 cm/k.y.
The very dark gray sediment in lithostratigraphic Unit IV, of latest Eocene age, is still dominated by a siliciclastic component and contains organic remains, suggesting the persistence of suboxic conditions. TOC values of ~2 wt% are associated with low HI (150) and relatively high C/N (20) values, which imply predominance of terrestrial organic matter (see "Organic Geochemistry"). Sporomorphs dominate the pollen association and indicate the persistence of a nearshore environment. The low diversity dinocyst assemblage supports eutrophic conditions with varying salinity, and the C/S ratio of ~3 may reflect a somewhat brackish environment. The sediment is sporadically truncated by dark grayish green intervals with abundant glauconite and coarse sand, suggesting low sedimentation rates and active, well-oxygenated bottom currents. Assemblages of nannofossils and planktonic foraminifers also suggest that Unit IV is a condensed section or contains one or several hiatuses (see "Biostratigraphy").
The depositional environment of Unit IV remained suboxic part of the time, with terrestrial input to the restricted basin. Marine incursions were associated with significant current activity, sufficient to produce winnowing, and possibly hiatuses, as marked by intervals of coarse deposits and glauconite formation. These intervals of intensified current activity increased near the Eocene/Oligocene boundary in the upper part of Unit IV. No equivalent of Unit IV has been found at Site 282. However, in DSDP Core 29-282-15 lenses of glauconite have been found near the Eocene/Oligocene boundary and may be the result of downslope transport. It is hypothesized that a shallow connection to the open ocean may have occurred as subsidence progressed in the western Tasmania area.
In the early Oligocene Unit III, an evolving environment is indicated by very dark sediment colors grading to greenish gray, and TOC values decreasing from 2 to 0.5 wt% upsection. The sediment is predominantly siliciclastic in the lower part of the unit (Subunit IIIB), but the particles decrease in size and abundance in the upper part of the unit (Subunit IIIA), suggesting decreased contribution from continental areas. The clay mineral assemblage contains increasing smectite and is indicative of continued subsidence of the passive margin (Chamley, 1989). The continuously decreasing trend of the C/S (from 3 to below 1) and C/N (from 25 to below 10) ratios reflect a transition to more marine conditions (see "Organic Geochemistry"), whereas benthic foraminifer faunas indicate shelf to bathyal water depths but continued low oxygenation (see "Biostratigraphy").
The predominantly siliciclastic sediments of Subunit IIIB show very dark colors and fluctuating TOC values (between 0.5 and 2 wt%) suggesting suboxic to unstable restricted conditions. Dominance of sporomorphs and plant tissue over pollens is consistent with the persistence of a nearshore environment. Transition to more marine conditions is deduced from the C/S ratio decrease to values between 3 and near 0 (see "Organic Geochemistry"). Some alternating intervals of laminated and bioturbated sediments indicate transient suboxic to oxic conditions.
A major step in the paleoceanographic evolution of the western Tasmania margin is present within Subunit IIIA (early Oligocene) where the dominant color of the sediment turns to greenish gray and TOC values decrease below 1 wt%. At the same time, C/N values below 10 and C/S values below 1 suggest the dominance of marine organic matter over terrestrially derived organic components as normal salinity seawater prevailed on the margin (see "Organic Geochemistry"). In most of Subunit IIIA, the sediment is bioturbated with rare laminated intervals. Foraminifers and nannofossils increase in abundance; however, the productivity and/or preservation remain low. Estimated sedimentation rates average 4.3 cm/k.y. and are characteristic of a starved continental margin.
At the beginning of the early Oligocene, marine influences on the western Tasmania margin are more important than in the late Eocene, but poorly oxygenated conditions generally persist at Site 1168. Subunit IIIA of early Oligocene age marks a transition to an increasingly oceanic oligotrophic environment. In the equivalent lower Unit VI of Site 282, a spicule and nannofossil clayey silt indicates greater influence of marine biogenics and less terrestrial contribution than at Site 1168. Intervals of lighter sediment interpreted as being more pelagic represent more open ocean biogenic productivity at Site 282. At this time the sediments of Sites 282 and 1168 converge in composition and include abundant biogenics within dominant siliciclastic clayey silt and silty clay. Similar sedimentation rates and environmental conditions were present in both areas.
The biogenic content of the sediment increases at Site 1168 in Unit II, which consists of nannofossil claystone, siltstone, and chalk, through intensified productivity and/or carbonate preservation. TOC values are low (below 1 wt%) and the organic matter is predominantly of marine origin, as indicated by C/N values below 15. Predominantly marine conditions are deduced from C/S values generally below 2 (see "Organic Geochemistry") and from the benthic foraminifer assemblage, which indicates well-oxygenated conditions at bathyal depths (see "Biostratigraphy").
In Subunit IIC (early to late Oligocene), siliciclastic components are still dominant. Slightly bioturbated sediments alternate with rare laminated intervals that, together with occasional C/S values between 2 and 3, suggest temporary development of suboxic conditions. In the corresponding Unit V of Site 282, similar sediment includes an interval of silty clay enriched in glauconite, indicative of low sedimentation rate and intensified current activity.
Beginning in Subunit IIB in the late Oligocene, the continuous decrease in abundance and size of the siliciclastic component suggests a decreasing influence of erosion and continental processes. This siliciclastic decrease is associated with increasingly abundant calcareous biogenics, which became predominant during the Miocene. Benthic foraminifers suggest increasing water depths and, together with very frequent bioturbation, indicate the persistence of well-oxygenated bottom waters and intensified circulation through time (see "Biostratigraphy").
From the late Oligocene to the Pleistocene, open marine conditions prevailed on the western Tasmania margin and nannofossil chalks and oozes predominate. In the corresponding interval of Site 282, slow sedimentation of biogenic-rich clays and biogenic oozes alternates with several hiatuses spanning parts of the late Oligocene, the early Miocene, and most of the middle Miocene-Pleistocene interval. The hiatuses were caused by dissolution related to strong bottom circulation (Kennett, Houtz, et al., 1975).