LITHOSTRATIGRAPHY

Site 1170 consisted of four holes that produced a nearly continuous 779.8-m-thick section of Holocene to middle Eocene sediments (Fig. F4). Nannofossil oozes and chalks characterize the upper interval from 0 to 472 mbsf (Cores 189-1170A-1H through 52X and Cores 189-1170D-1R through 5R), followed by a distinct shift to glauconite-rich, sandy to clayey siltstones between 472 and 497 mbsf (Cores 189-1170D-6R through 9R). This relatively coarse siliciclastic interval overlies relatively finer silty claystones that comprise the remainder of the section (497-779.8 mbsf; Cores 189-1170D-10R through 38R).

The Site 1170 section is divided into five lithostratigraphic units (Units I-V) based on core features, smear slides, thin sections, coulometric-based carbonate content, spectrophotometry, and magnetic susceptibility from Holes 1170A and 1170D (Fig. F5). Units II and V are divided into three and two subunits, respectively. Compositional and textural percentages define the primary lithology of each unit and subunit (Figs. F6, F7). Coulometric-based carbonate content was generally lower than smear slide-based carbonate estimates but did not significantly affect recognition of the major lithostratigraphic units. As demonstrated for Site 1168, Site 1170 spectrophotometric lightness (L*) and carbonate content appear strongly positively correlated. The lithostratigraphic units of Site 1170 are chronostratigraphically correlated to those of ODP Site 1168 (this volume) and DSDP Site 281 (Kennett, Houtz, et al., 1975) (see Table T2).

Unit I (Holocene to early Pliocene) is a light-colored nannofossil ooze that increases downsection in minor modifiers (10%-25%) of clays, foraminifers, and diatoms. Unit II (early Pliocene to early Miocene) is also a nannofossil ooze but contains significantly higher and more consistent carbonate content and spectrophotometric lightness values. Unit III (early Miocene to early Oligocene/late Eocene) is a light-colored clay-bearing nannofossil chalk that increases in clay content downsection, with isolated centimeter- to decimeter-scale gray silicified limestone beds near its base. Unit III marks the transition between the biogenic calcareous regime of Units I and II and the siliciclastic regime of Units IV and V. Unit IV (early Oligocene-late Eocene) is a dark gray glauconitic clayey siltstone with elevated magnetic susceptibility and lower carbonate content. Unit V (middle Eocene) is a finer, dark gray silty claystone with stepped changes in magnetic susceptibility and oscillations in spectrophotometric lightness, carbonate content, and quartz and glauconite content. Site 1170 sediments recovered by the APC and XCB ranged in drilling disturbance from none to severe flow-in because of the high ship heave during heavy weather. To aid in evaluation of physical properties and other data, mean drilling disturbance for each core section in Hole 1170A was rated on a qualitative scale (see "Lithostratigraphy" in the "Explanatory Notes" chapter).

Unit I

Depth: 0 to 93 mbsf

Age: Holocene to early Pliocene

Intervals: Cores 189-1170A-1H to 11H; Cores 189-1170B-1H to 11H; and Cores 189-1170C-1H to 10H

Thickness: 93 m

Unit I is a white (N 8) to light greenish gray (5G 8/1, 5GY 7/1, 10Y 8/1 to 7/1, and 10GY 8/1 to 7/1) nannofossil ooze. Nannofossil abundance decreases downsection, with increasing proportions of minor modifier clays, foraminifers, and diatoms (10%-25% each). Light greenish gray (5G 8/1) to light bluish gray (10B 7/1) laminae and thin beds are common. Bioturbation intensity ranges from absent to common, and pyrite staining is present throughout. Carbonate content ranges between 57 and 93 wt% with an average of 81 wt% (N = 31). A distinct moderately bioturbated light greenish gray (10Y 7-8/1) to light olive-gray (5Y 6/2) interval at ~81 mbsf (Section 189-1170A-10H-3) consists of diatom- and clay-dominated lithologies, with ~10%-30% nannofossils. Unit I is Holocene to early Pliocene in age and likely corresponds to Subunit IA of Site 1168 (see "Lithostratigraphy" in the "Site 1168" chapter) and the upper portion of Subunit IA of DSDP Site 281 (Kennett, Houtz, et al., 1975).

Unit II

Depth: 93 to 373 mbsf

Age: early Pliocene to early Miocene

Intervals: Cores 189-1170A-11H to 41X; Cores 189-1170B-11H to 19H; and Cores 189-1170C-11H to 19H

Thickness: 280 m

Unit II is a white to light greenish gray nannofossil ooze with variable minor modifiers (10%-25%) of clay, foraminifers, and diatoms. The upper unit boundary is based on pronounced increases in carbonate content, spectrophotometric lightness, and bulk density. The unit is divided into three subunits based on color, laminations, minor modifier components, and carbonate content. Pyrite staining and rare pyrite nodules are throughout. Unit II is early Pliocene to early Miocene in age and likely corresponds to the lower portion of Subunits IB and IIA of Site 1168 (see "Lithostratigraphy" in the "Site 1168" chapter) and the lower portion of Subunit IA and Subunit IB of DSDP Site 281 (Kennett, Houtz, et al., 1975).

Subunit IIA

Depth: 93 to 185.9 mbsf

Age: early Pliocene to middle Miocene

Intervals: Cores 189-1170A-11H to 21X; Cores 189-1170B-11H to 19H; and Cores 189-1170C-11H to 19H

Thickness: 92.9 m

Subunit IIA is a foraminifer-bearing white (N 8) nannofossil ooze. Clays and diatoms are common as minor modifiers (10%-25%) in the upper portion (Cores 189-1170A-11H through 15H). Distinct to diffuse, light greenish gray (5G 8/1) to light bluish gray (10B 7/1) laminations of variable thickness (millimeters to centimeters) decrease in overall abundance but increase in localized concentration downsection. Visible bioturbation is difficult to identify because of the subunit's homogeneous composition. Levels with bioturbation ranging from rare to present in intensity are restricted to ~107-123 mbsf and 135-139 mbsf (Cores 189-1170A-13H, 14H, and 16H). Carbonate content ranges between 90 and 96 wt%, with an average of 93 wt% (N = 26) and no stratigraphic trend. Isolated meter-scale intervals of light greenish gray and light bluish gray intervals are found in Cores 189-1170A-12H and 20X, respectively. Subunit IIA is early Pliocene to middle Miocene in age and likely corresponds to Subunit IB of Site 1168 (see "Lithostratigraphy" in the "Site 1168" chapter) and lower portion of Subunit IA of DSDP Site 281 (Kennett, Houtz, et al., 1975).

Subunit IIB

Depth: 185.9 to 290 mbsf

Age: middle Miocene

Interval: Cores 189-1170A-22X to 32X

Thickness: 104.1 m

Subunit IIB is a massive white (N 8) nannofossil ooze with minor modifiers (10%-25%) of foraminifers and clays in Cores 189-1170A-23X, 28X, and 32X. In contrast to Subunits IIA and IIC, laminations are extremely rare and a very diffuse light bluish gray (~10B 8/1). Carbonate content increases markedly by ~3 wt% at the top of the subunit and ranges between 93 and 96 wt%, with an average value of 95 wt% (N = 25) and no stratigraphic trend. Visible bioturbation is absent, even where laminations provide contrast. Subunit IIB is middle Miocene in age and likely corresponds to the upper portion of Subunit IIA of Site 1168 (see "Lithostratigraphy" in the "Site 1168" chapter) and middle portion of Subunit IB of DSDP Site 281 (Kennett, Houtz, et al., 1975).

Subunit IIC

Depth: 290 to 373 mbsf

Age: middle Miocene to early Miocene

Interval: Cores 189-1170A-33X to 41X

Thickness: 83 m

Subunit IIC is a nannofossil ooze to chalk that fluctuates at the meter scale from predominately light greenish gray (5GY 8/1 to 10Y 8/1) to predominately white (N 8) downsection. Minor modifiers (10%-25%) of foraminifers and clays are more prevalent in the upper portion (290-335 mbsf; Cores 189-1170A-32X through 37X). In contrast to Subunit IIB, faint light greenish gray (5GY 7/1) laminations of variable thickness and concentration are relatively common in appearance and prominence, particularly as overall whiteness increases. Carbonate content decreases markedly by ~3 wt% at the top of the subunit, ranges between 91 and 95 wt% with an average of 93 wt% (N = 26), and shows a slight decreasing trend downsection. Visible bioturbation is generally absent, with rare to present levels between ~301 and 310 mbsf and ~330-357 mbsf (Cores 189-1170A-34X and 1170A-37X through 39X). Pyrite staining is less common than in previous subunits. Subunit IIC is middle Miocene to early Miocene in age and likely corresponds to Subunit IIA of Site 1168 (see "Lithostratigraphy" in the "Site 1168" chapter) and lower portion of Subunit IB of DSDP Site 281 (Kennett, Houtz, et al., 1975).

Unit III

Depth: 373 to 472 mbsf

Age: early Miocene to early Oligocene/late Eocene

Intervals: Cores 189-1170A-41X to 52X and Cores 189-1170D-1R to 5R

Thickness: 99 m

Unit III is a light greenish gray (10GY 8/1 to 7/1, 10Y 8/1 to 7/1, 10GY 8/1, and 5GY 8/1) to occasionally white (N 8) nannofossil chalk, with a general downsection increase in clays and decrease in biogenic carbonate. Carbonate content decreases markedly by ~8 wt% at the top of the unit and ranges between 78 and 93 wt%, with an average of 85 wt% (N = 26). The top of the unit is also marked by high magnetic susceptibility and a slight decrease in GRA bulk density. Abundances of diatoms, radiolarians, and sponge spicules are greater overall than in Subunit IIC, occasionally to minor modifier level (10%-25%). Visible bioturbation ranges from absent to abundant in intensity. Distinct grayish green (5G 4/2) to light bluish gray (5B 7/1) laminae are below ~419 mbsf (Core 189-1170A-46X). Isolated intervals of hard limestone greatly reduced core recovery below ~459 mbsf (Cores 189-1170A-51X and 189-1170D-4R). Diagenetic dissolution pressure solution seams enriched in clay are found in Core 189-1170D-5R. Subunit IIIA is early Miocene to early Oligocene-late Eocene and likely corresponds to Subunits IIB, IIC, and Unit III of Site 1168 (see "Lithostratigraphy" in the "Site 1168" chapter) and Unit II and uppermost Unit III of DSDP Site 281 (Kennett, Houtz, et al., 1975).

Unit IV

Depth: 472 to 497 mbsf

Age: early Oligocene/late Eocene to late Eocene

Interval: Cores 189-1170D-5R to 9R

Thickness: 25 m

Unit IV is a dark greenish gray (10Y 5/1 to 4/1) to greenish black (10GY 2.5/1 to 5G 2.5/1), slightly sandy to generally clayey siltstone, containing as much as 35% diatoms and 60% glauconite. High glauconite content is often present within distinct centimeter-scale layers that decrease in abundance and thickness downsection. Nannofossils are rare (<5%), organic matter is present (3%-15%), and bioturbation is generally extensive. Carbonate content decreases markedly at the top of Unit IV by >80 wt% and averages only 2 wt% (N = 7). This pronounced decrease is accompanied by a ~40% decrease in average spectrophotometric lightness (L*) and ~15-fold increase in magnetic susceptibility values compared to Units I to III. Spectrophotometric lightness is relatively constant throughout Unit IV; however, magnetic susceptibility decreases in three minor steps. Unit IV is early Oligocene/late Eocene to late Eocene in age and likely corresponds to Unit IV of Site 1168 (see "Lithostratigraphy" in the "Site 1168" chapter) and Units III to V of DSDP Site 281 (Kennett, Houtz, et al., 1975).

Unit V

Depth: 497 to 779.8 mbsf

Age: late Eocene to middle Eocene

Interval: Cores 189-1170D-9R to 38R

Thickness: 282.8 m

Unit V is a dark green to dark gray silty claystone. The upper boundary is defined by a marked decrease in biosiliceous sediments and increase in silt- to sand-sized quartz. The unit contains prominent long-term oscillations in carbonate content, quartz content, spectrophotometric lightness, and lithologic components. Unit V is middle to late Eocene and likely corresponds to Unit V of Site 1168 (see "Lithostratigraphy" in the "Site 1168" chapter).

Subunit VA

Depth: 497 to 539.1 mbsf

Age: late Eocene

Interval: Cores 189-1170D-9R to 13R

Thickness: 42.1 m

Subunit VA is a dark greenish gray (10Y 3/1 to 5Y 3/2) to dark olive-gray (5Y 3/1) glauconite-bearing silty claystone to rarely clayey siltstone (i.e., only Cores 189-1170D-11R and 12R), with occasional dark olive-gray (5Y 3/1), dark gray (2.5Y 6/1), and black (5Y 2.5/2) intervals. The majority of the silt-sized siliciclastic material is quartz, with less glauconite and more nannofossils relative to Unit IV. Decimeter-scale intervals of higher glauconite content decrease in thickness and frequency downsection. Carbonate content averages 5 wt% (N = 9) with a broad peak through the subunit, consistent with smear-slide nannofossil abundances. Relative to Unit IV, average spectrophotometric lightness (L*) decreases slightly downsection, whereas magnetic susceptibility contains a broad relative maximum and GRA bulk density generally increases. Subunit VA is late Eocene in age and likely corresponds to Unit V of Site 1168 (see "Lithostratigraphy" in the "Site 1168" chapter) and Units IV and V of DSDP Site 281 (Kennett, Houtz, et al., 1975). DSDP Site 281 records glauconitic silty clays, sand, and basal breccia in the upper Eocene above quartz-mica schists (Kennett, Houtz, et al., 1975).

Subunit VB

Depth: 539.1 to 779.8 mbsf

Age: middle Eocene

Interval: Cores 189-1170D-14R to 38R

Thickness: 240.7 m

Subunit VB is a very dark gray (5Y 3/1) to dark olive-gray (5Y 3/2) to dark greenish gray (10Y 3/1) and to very dark greenish gray (10Y 3/1) silty claystone. The upper boundary of the subunit is marked by a 50%-60% decrease in both quartz content and magnetic susceptibility. Notable features within, and exceptions to, this silty claystone lithology include (1) volcanic glass-bearing sediments centered around 549 mbsf (Core 189-1170D-15R), (2) light gray (N 7/1) to light olive-gray (5Y 6/2) decimeter-scale limestones at ~587, ~598, and ~634 mbsf (Cores 189-1170D-19R, 20R, and 23R), (3) a very dark gray (5Y 3/1) to dark olive-gray (5Y 3/2) to very dark gray (5Y 3/1), quartz-rich clayey siltstone from 588 to 615 mbsf (Cores 189-1170D-19R through 21R), (4) a very dark gray (5Y 3/1) to dark greenish gray (10Y 3/1) claystone from ~647 to 653 mbsf (Core 25R), and (5) organic matter-bearing sediments (up to 15% from smear-slide analysis; <3% from geochemical analysis) from 670 to 735 mbsf (Cores 27R through 34R).

Carbonate content averages 2 wt% (N = 73) with three broad maxima at 539-597 mbsf (Cores 189-1170D-14R through 20R), 647-689 mbsf (Cores 25R through 29R), and 727 mbsf to the base of the core (Cores 33R through 38R). This carbonate trend is also reflected in spectrophotometric lightness maxima centered at 580, 675, and 735 mbsf (Cores 189-1170D-18R, 28R, and 34R) and, to a lesser extent, smear slide nannofossil maxima. Magnetic susceptibility is stable within Subunit VB relative to Unit IV and Subunit VA, with only minor incremental increases at ~583 and 686 mbsf (Cores 189-1170D-18R and 29R) and occasional order-of-magnitude higher spikes below 728 mbsf (Core 33R).

Horizontal ~1-mm-diameter hollow tubes of very fine sand- to silt-sized siliceous grains are common at densities of ~1-10/m. Solitary azooxanthellate corals (N = ~5), bivalves (N = ~4), and pyrite nodules are rare but generally are within carbonate-poor intervals. Bioturbation ranges from moderate to abundant in intensity, with larger burrows appearing more common within more carbonate-rich intervals. Faint diffuse laminations are present. Subunit VB is middle Eocene, with the older part constrained by only limited biostratigraphic data. Site 1168 (see "Lithostratigraphy" in the "Site 1168" chapter) did not penetrate to equivalent-age sediments.

Site 1170 is located in the Ninene Basin on the western block of the STR. The basin's boundary with the central block of the STR is a fault scarp ~10 km east of Site 1170. The basin originated as a complex of transtensional sub-basins produced by strike-slip faulting during the Late Cretaceous to Eocene divergence of Australia from Antarctica (Exon et al., 1997). A plate tectonic reconstruction (Royer and Rollet, 1997) suggests that the western block moved with Antarctica from ~95 Ma, colliding with the central block during the latest Cretaceous, and became welded to the central block during the middle Eocene (~43 Ma). Given this paleogeographic location along the southeasternmost part of the developing Australo-Antarctic Gulf, Site 1170 should provide important insights on both regional tectonism and circumpolar ocean-atmosphere evolution.

During the middle to early late Eocene (Subunit VB), silty claystone sediments were deposited at rates of ~10 cm/k.y. or higher. Bioturbation was common to extreme throughout the interval and indicates that bottom waters were generally oxic to dysoxic, supporting metazoans capable of producing millimeter- to centimeter-scale burrow diameters. Burrow cross section populations tend to lack circular end-members, implying that significant compaction and dewatering occurred after bioturbation. This intense bioturbation destroyed primary sedimentary structures, except for extremely rare, faint remnants of irregular bedding at the centimeter scale. Therefore, paleodepth estimates cannot be based on sedimentary structures. Benthic foraminiferal assemblages, although highly altered diagenetically, indicate middle neritic (~50-100 m) paleodepths with a progressively more "restricted" paleoenvironment through time (see "Biostratigraphy").

Three long-term oscillations in carbonate content during the middle Eocene are indicated by both low-resolution carbonate contents (two per core) and high-resolution spectrophotometric lightness values (proxy for carbonate at 2-cm sampling intervals) (Fig. F8). Maxima in carbonate content coincide with smear-slide nannofossil maxima, as well as faunal peaks in the oligotroph dinoflagellate Enneadoxysta partridgei, (see "Biostratigraphy"). Conversely, carbonate minima are higher in clays and silts with quartz more abundant and glauconite appearing and increasing in the two later minima. The driving cause(s) of these long-term oscillations remains uncertain, but the predominance of chemical weathering-produced smectite through the interval does not support significant tectonism. At much higher resolution, gamma-ray logs, and potentially spectrophotometric lightness values, show very regular sedimentation cycles throughout most of the middle Eocene at the orbital frequency of either precession or obliquity (see "Downhole Measurements"), depending on the preferred age model (see "Biostratigraphy").

In the uppermost Eocene (Unit IV), an environment shift to less restricted conditions is inferred from the pronounced increase in neritic diatoms but may be partially controlled by silica dissolution (see "Biostratigraphy" and "Inorganic Geochemistry"). Also associated with this interval is an increase in sand-sized quartz and glauconite, the latter including sand-sized grains often concentrated in centimeter-scale beds. Organic matter also increases and is present throughout the unit, whereas clays show another illite peak and are more abundant overall.

Across the poorly recovered Eocene/Oligocene boundary (Units III/IV), the depositional environment fundamentally shifted from a siliciclastic to carbonate-dominated regime. This shift is best shown by a marked step increase in nannofossils and decrease in glauconite at the boundary, whereas diatoms are absent in the lowermost Unit III but later return as a more open marine fauna. Based on sedimentation rates and carbonate contents, the pronounced shift is driven mostly by increased productivity, particularly as preservation of carbonate microfossils appears consistently moderate to good through the event. This pronounced carbonate productivity increase, together with the paleodepth increase to middle bathyal depths (600-1000 m) based on benthic foraminifers, may reflect the ocean-atmosphere response to the opening of the eastern Tasmanian land bridge of the Australo-Antarctic Gulf to some critical size, shape, latitude, or orientation. Movement along the adjacent scarp was apparently not an integral part in this opening because of the lack of coarse beds at Site 1170, although that depends on the nature of the scarp's lithology.

Through the Oligocene (Unit III), nannofossil sedimentation predominated, whereas biosiliceous sediments were relatively common and clay content decreased through time to less than a few percent. No sedimentological evidence exists for the late early to early late Oligocene hiatus (see "Biostratigraphy"). Through the early to early middle Miocene (Subunit IIC), biosiliceous sediments decrease in abundance and laminations are common. Increased middle Miocene productivity and/or preservation is indicated by higher carbonate content and sedimentation rates. There is seismic evidence of middle Miocene scouring along the scarp to the northeast, which could have provided the quartz found at that level. Biosiliceous sediments began to increase again in the later middle Miocene (upper Subunit IIB) and reached a relative peak at the middle late Miocene boundary (Subunit IIA/IIB boundary). Above this boundary, carbonate content decreases and laminations reappear. The long hiatus in the late Miocene sequence is not recognized lithologically. Following a step increase in biosiliceous sediments, the Pliocene and Quaternary sediments (upper Subunit IIA and Unit I) show no pronounced lithologic patterns other than an apparent late Pliocene reduction in clay content.

X-ray diffraction (XRD) analyses were completed on 47 samples from Holes 1170A and 1170D (Fig. F9). The purpose of the clay mineral studies at Site 1170 was to recognize the major variations of the paleoenvironment as expressed by the clay mineral assemblages at a sampling interval of one every two cores and to compare the clay mineral assemblages with those recognized at Site 1168, which was drilled at similar water depth on the western Tasmania margin and with sites in other areas of the Southern Ocean.

Results

The clay minerals identified include smectite, random mixed-layered clays, illite, and kaolinite. Chlorite has been recognized only in trace quantities in two samples and is not included in the percent estimates. Based on the relative abundance of the clay minerals, four units were identified for Site 1170. These were designated Units C1 to C4 (Fig. F9).

Unit C1, which extends from the seafloor to 100 mbsf, has a clay mineral assemblage that consists of very abundant smectite (50% to 85%) and common kaolinite (10% to 20%) accompanied by random mixed-layered clays and illite (0% to 20% each). Unit C1 ranges in age from the late Pliocene to the Pleistocene and correlates to lithostratigraphic Unit I. Unit C2 extends from 100 to 380 mbsf, is characterized by dominant smectite (50% to 85%) alternating with dominant random mixed-layered clays (0% to 100%), and correlates to lithostratigraphic Unit II. Because of the low clay content in most nannofossil oozes of lithostratigraphic Unit II, percentages of random mixed-layered clays >50% are probably overestimated. Unit C2 is divided into two subunits. Subunit C2A extends from 100 to 250 mbsf and is middle Miocene to early Pliocene in age. Subunit C2A shows decreased contents of smectite (<70%) and significant amounts of random mixed-layered clays (>20%) and illite (up to 25%). Subunit C2B extends from 250 mbsf to 380 mbsf and is early Miocene to middle Miocene in age. Subunit C2B shows increased contents of smectite (up to 100%), low amounts of illite (<10%), and sporadic kaolinite (up to 10%). With the exception of one sample with poor clay content, the random mixed-layered clays are <20%. Unit C3 extends from 380 to 530 mbsf, is characterized by significant occurrences of illite (up to 100%), and correlates to lithostratigraphic Units III and IV and Subunit VA. Unit C3 is divided into two subunits. Subunit C3A extends from 380 to 472 mbsf and is Oligocene in age. Subunit C3A contains abundant smectite (45% to 95%), with random mixed-layered clays (0% to 30%), illite (traces to 15%), and kaolinite (0% to 5%). Subunit C3B extends from 472 to 530 mbsf and ranges in age from the middle Eocene to the late Eocene. Subunit C3B exhibits alternations of dominant illite (5% to 100%) and smectite (25% to 95%), associated with traces of kaolinite (0% to 2%). Unit C4 is characterized by largely predominant smectite (90% to 100%), accompanied by rare illite (0% to 5%), and traces of kaolinite (0% to 2%). Unit C4 is middle Eocene in age and correlates to lithostratigraphic Subunit VB.

Paleoenvironmental Interpretation of Clay Assemblages

The extreme predominance of smectite in middle Eocene Unit C4 supports warm climatic conditions and intense chemical weathering in the sediment source areas. Smectite prevails in areas of low relief with alternating periods of precipitation and aridity, and its formation is enhanced on basic volcanic substrates (Chamley, 1989; Weaver, 1989). Site 1170 was located in a basin on the western block of the STR still attached to Antarctica (see "Background and Objectives"). Sedimentation rates are high, and the sediment is largely fine-grained siliciclastic particles (silty claystone and clayey siltstone). However, the high smectite content at Site 1170 suggests that no significant tectonic activity occurred and that low-relief areas prevailed in adjacent continental areas during the middle Eocene. The clay mineral assemblage is very comparable to those of similar age observed off the passive Antarctic margins on the South Orkney Microcontinent and Maud Rise in the Weddell Sea (Robert and Maillot, 1990; Diester-Haass et al., 1996) and Kerguelen Plateau in the southernmost Indian Ocean (Ehrmann, 1991).

Subunit C3B, of middle and late Eocene age, features distinct increases in illite, which forms up to 100% of the clay fraction. This is indicative of strong physical weathering and erosion of substrates in the source area. Physical weathering and illite formation prevail in modern cold and/or dry environments. Illite is also characteristic of steep relief areas, where tectonic activity and mechanical erosion, together with cold conditions, prevent soil development (Chamley, 1989; Weaver, 1989). Such an occurrence of predominant illite has not been described before in other areas of the middle and late Eocene Southern Ocean and Tasman Sea, where the clay mineral assemblage consisted largely of smectite (Robert et al., 1985; Robert and Maillot, 1990; Ehrmann, 1991). It is therefore assumed that predominant illite in Unit C3 derives from physical weathering and erosion of substrates in adjacent continental areas, where steep relief may have developed during the stage of strike-slip tectonic activity that preceded the separation of the STR from Antarctica to the west. The strike-slip motion probably formed the great ridge of the Tasman Fracture Zone along the western margin of the STR at that time, on the basis of plate tectonic, seismic reflection, and dredge information (Exon et al., 1997). Similar intervals of abundant illite, often associated to significant amounts of kaolinite and random mixed-layered clays, have been observed during active early opening stages in the South Atlantic (Robert, 1987).

Increased contents of illite in Subunit C3B started during the latest middle Eocene at ~38 Ma, in nannofossil Zone NP17. At the same time, slight increases of illite and random mixed-layered clays in the Weddell Sea on Maud Rise (Robert and Maillot, 1990) occurred during an increasing trend in the ¹⁸O values of the benthic and planktonic foraminifers (Kennett and Stott, 1990; Stott et al., 1990). This evolution suggests local development of poorly weathered soils in response to a cooling of the Southern Ocean. Coeval extension of antarctic siliciclastics from the lower to the upper part of the intermediate water mass in the entire Atlantic sector of the Southern Ocean suggests a greater influence of the antarctic environment upon the water column at southern high latitudes (Robert and Kennett, 1992). From clay mineral data, it seems that the peak of tectonic activity in the western STR area was constrained within the latest middle to late Eocene interval. Moreover, Subunit C3B also contains cosmopolitan dinocysts and is limited by possible hiatuses at its lower and upper parts (see "Biostratigraphy"). It is highly probable that the middle late Eocene episode of plate tectonic activity in the STR had significant consequences on the development of cooler conditions at southern high latitudes and expansion of cool antarctic waters.

Illite, random mixed-layered clays, and kaolinite account for 20% to 50% of the clay assemblage of Subunit C3A, which is typical of early Oligocene subantarctic areas (Robert and Chamley, 1992). The conspicuous illite is very similar to that observed in other areas of the Southern Ocean, on the Kerguelen Plateau (Ehrmann, 1991), and on the Maud Rise (Diester-Haass et al., 1993). The trend of illite in Subunit C3A expresses the progression of physical weathering on East Antarctica in relation to the development of cold and/or dry conditions that followed the temporary growth of an ice sheet in the earliest Oligocene (Zachos et al., 1993; Robert and Kennett, 1997).

In Subunit C2B, of early to middle Miocene age, lower values of illite and random mixed-layered clays than in Subunit C3A and sporadic kaolinite suggest slightly decreased physical weathering in the source area. The ratio of the 001 peak intensities for kaolinite and illite (kaolinite/[kaolinite+illite]) provides additional information on Subunit C2B. The ratio is highest (indicative of increased kaolinite) from 350 to 310 mbsf in nannofossil Zones NN2 to NN4 of early Miocene age. This pattern is coeval with a significant increase of the percentage of kaolinite at Site 1168. The regional increase of kaolinite is indicative of increased warmth and intensified precipitation (see "Lithostratigraphy" in the "Site 1168" chapter) and immediately precedes major expansion of the East Antarctic Ice Sheet at 14-15 Ma (Kennett, 1977). However, the clay assemblage of Site 1170 is significantly different from that of Site 1168 on the western Tasmania margin, which is marked by more kaolinite and less illite. As ocean opening progressed, the clay assemblage at Site 1170 most probably derived from a range of continental areas and expresses average weathering conditions on the emerged margins of the Southern Ocean. Further increase of illite and random mixed-layered clays (and less smectite) in Subunit C2A indicates enhanced erosion of poorly weathered substrates, following the expansion of the East Antarctic Ice Sheet during the middle Miocene.

Unit C1 of late Pliocene and Pleistocene age is marked by small increases of kaolinite percentages, whereas the clay content of the calcareous biogenic sediment also slightly increases (Unit I). Such a clay assemblage, containing as much as 20% kaolinite, has not previously been observed in other subantarctic areas (Robert and Maillot, 1983; Ehrmann, 1991), but it is very similar to that observed in late Pliocene sediments from the Lord Howe Rise in the Tasman Sea (Stein and Robert, 1986). Most clay particles of Unit C1 may have been transported southward from arid Australian regions by northwesterly winds (Pye, 1987). This pattern may have commenced during the late Pliocene, when eolian particles from Australian arid areas reached their maximum southward extension and abundance in the Tasman Sea (Stein and Robert, 1986). Unit C1 of Site 1170 is coeval to Unit C1 of Site 1168, but kaolinite is less abundant on the STR than on the west Tasman margin (see "Lithostratigraphy" in the "Site 1168" chapter). Greater distance from the source and/or additional contribution from runoff on the west Tasman margin may account for this difference.