At Site 1172, sediment was cored in four holes to a maximum depth of 766.50 mbsf. The sediments down to 355 mbsf contain a succession of calcareous biogenic pelagic oozes. Below this calcareous sequence, a distinct glauconitic lithologic unit was penetrated (Cores 189-1172A-39X and 189-1172D-2R) that separates the pelagic sediments from siliciclastic sequences (Unit II).
Based on lithologic changes at Site 1172, the sediments were subdivided into the major Units I to IV with three subunits in Unit I and two subunits in Units III and IV, respectively (Table T2). This subdivision is based on the results of shipboard analyses, which include visual core observations and smear slide-based estimation of sediment components. Measurements of calcium carbonate content, sediment physical properties, lightness, and bulk mineralogy were additionally used as support in describing these units. Unit I contains white calcareous pelagic sediments. Unit II encompasses a complex series of mostly glauconitic siltstones and represents the transition from a carbonate to a siliciclastic facies. Unit III is a diatom-rich claystone with less glauconite content, and Unit IV consists of a variety of silty claystones. A general overview of the principal lithostratigraphic units of the entire cored sequence is presented in Figure F4.
Unit I is characterized by a calcareous succession of mainly nannofossil ooze that represents the entire Neogene and some of the Oligocene at Site 1172. This sedimentary sequence is subdivided into three subunits on the basis of changes in the biogenic calcareous and noncalcareous components, as well as with the support of physical properties data.
Subunit IA is mainly a white (N 8) foraminifer nannofossil ooze that grades occasionally to a foraminifer-bearing nannofossil ooze. In some intervals, sandy clasts contain an increased foraminiferal content. Stained fields of greenish gray to light greenish gray (5Y 7/1 to 5GY 6/1) color were sporadic throughout Subunit IA but locally abundant within layers in Core 189-1172A-3H. In Core 189-1172A-4H, alternating bands of laminated sediment show cycles at every 40 to 50 cm and every 80 to 100 cm. Bioturbation is rarely visible, although Zoophycos ichnofossils were observed.
Smear-slide observations indicate an average nannofossil percentage of 50% in the topmost layers, which increases downward to 90% at 70 mbsf (Fig. F5A, F6). Foraminifers are the second major component and decrease from 45% at the top of the unit to 5% at the bottom of the subunit. This variability in the calcareous content is reflected by large amplitudes in the lightness (L*) and carbonate content. Between 10 and 20 mbsf, clay is a very minor component with a maximum of 10%. Accessory and opaque minerals are rare (2%-3%) between 20 and 45 mbsf, which seems to be reflected by slightly increased values in the magnetic susceptibility record. However, magnetic susceptibility values are generally very low and appear not to be significant.
The sediment of Subunit IB consists entirely of white and light greenish gray (N 8, 10Y 7/1, and 10Y 8/1) nannofossil ooze. Subunit IB is distinguished from Subunit IA by a considerable decrease in foraminiferal content, which is replaced almost entirely by nannofossils. This lithologic succession is not associated with a sharp boundary, but with a gradual change, which was detected in smear-slide analyses.
Subunit IB shows no signs of lithification above Core 189-1172A-29X, below which the ooze-chalk transition begins. Stained bands and thin intervals of light bluish gray color (10GY 7/1 to 5GY 8/1 or 5PB 6/1) are common and occasionally appear as faint laminations (Fig. F7; interval 189-1172A-26X-4, 38-55 cm). Sporadic large burrows of unknown origin are usually filled with pyrite-stained dark gray ooze. Pyrite-stained small black dots <1 mm in size are common. In Core 189-1172A-16H, several pyrite nodules 1 to 4 cm in diameter are present.
Nannofossil content in Subunit IB ranges between 79% and 94% (Fig. F6) and indicates a high degree of lithologic homogeneity. The remaining components show little variation. Foraminifers are consistently present with an average of 4% throughout the subunit, whereas siliceous microfossils (diatoms, radiolarians, and sponge spicules) are rare, except in a few intervals (average percentage of 1.8% in the interval between 117 and 250 mbsf). Clay was noted between 75 and 110 mbsf, between 175 and 200 mbsf, and between 231 and 245 mbsf, with an average of 3% to 6%. Trace amounts of opaque minerals, accessory minerals, and, in particular, volcanic glass are in the interval between 140 and 160 mbsf. These are reflected in slight excursions of the magnetic susceptibility record. The lightness record (L*) shows two phases of decreased values in this interval at 140 and 160 mbsf, which are also reflected in the carbonate content. At 230 mbsf, the carbonate content reaches its absolute maximum of 97 wt% for the entire sediment sequence at Site 1172.
Subunit IC consists of foraminifer-bearing nannofossil chalk and clay-bearing nannofossil chalk of early to middle Miocene age. The boundary between Subunit IB and Subunit IC was set at the top of Core 189-1172A-30X, which follows a short interval of nonrecovery in Core 189-1172A-29X. Between these two cores, there is a considerable change within the calcareous sediment components, which requires separating those two lithologies.
Minor lithologies in Subunit IC include nannofossil-bearing, volcanic glass-bearing clayey siltstone and a diatom- and nannofossil-bearing claystone. Colors range from white (N 8, 5Y 8/2, and 2.5Y 8/1) to pale yellow, light gray, pale olive, or light olive gray (5Y 8/1 to 5Y 6/3). Laminations of bluish gray color (5PB 6/1 to 5PB 5/1 and 5PB 8/1) are occasionally present. Zoophycos, Chondrites, and large burrows of unknown origin are frequent and are particularly abundant in Core 189-1172A-31X. Bioturbation is visible throughout the subunit with a distinct increase downward. Cores 189-1172A-32X and 33X are strongly bioturbated intervals and are dominated by light greenish gray colors (5BG 8/1) (Fig. F8; interval 189-1172A-34X-2, 100-120 cm). Lithification increases downward and pressure solution seams are common throughout the subunit (Fig. F9; interval 189-1172A-34X-5, 69-82 cm).
Smear-slide data indicate a compositional change in two steps through Subunit IC (Fig. F6). The first step is in the upper part of the subunit from Core 189-1172A-30X to 33X and is marked by decreasing nannofossils and increasing foraminifers. The second step shows a distinct increase in clay, quartz, volcanic glass, opaque minerals, and accessory minerals in the lower part of the subunit from Core 189-1172A-34X to 39X. In particular, dark grains, presumably volcanic glass, were observed throughout the subunit, occasionally in thin layers (Core 189-1172A-38X). Carbonate content and, in particular, physical properties data indicate a similar two-step change in Subunit IC (Fig. F5A). The occurrence of a peak in volcanic glass content with an excursion at 310 mbsf, and a distinct decline below 340 mbsf to the lower subunit boundary, is paralleled by a decrease of lightness (L*) values. This interval contains an average of 10% volcanic glass. A parallel variability is noticed in the magnetic susceptibility data, which show two distinct increases at 310 mbsf and between 340 mbsf and the end of the subunit at 355.80 mbsf.
Unit II represents the transition from the pelagic sediments of Unit I to the predominantly siliciclastic sediments of Unit III. Unit II is distinguished from Subunit IC by an increase in glauconite content, a decrease in nannofossils, an abrupt decrease in carbonate content from 69 wt% at 351 mbsf to 0.3 wt% at 357 mbsf, and a sharp change in many of the physical properties data (see "Physical Properties"). Although Unit II is thin, it contains a complex succession of sediments and is characterized downsection by
Unit II is commonly to abundantly bioturbated throughout.
The upper 2.5-m section below 355.8 mbsf contains increased glauconite as well as several distinct surfaces. Glauconite increases down to an irregular surface at 356.24 mbsf, which separates light greenish gray clay-bearing nannofossil chalk above from glauconitic greenish gray diatom- and clay-bearing nannofossil chalk below. There are small rip-up clasts a few centimeters above the contact with small burrows (<1 cm) within a few centimeters below. Glauconite varies from common to abundant (visual inspection) down to a distinct, sharp surface at 357.27 mbsf. Above the surface, pebble-sized rip-up clasts are abundant within a light greenish gray (10Y 7/1) glauconite-, diatom-, and clay-bearing nannofossil chalk, where, as below, there are angular clasts that extend ~40 cm downsection within a greenish gray (10GY 5/1) silty diatomaceous claystone. Green and dark olive glauconite grains begin below the surface (357.27 mbsf) and extend down to 357.74 mbsf. Based on biostratigraphy, a significant hiatus or condensed section of perhaps several million years is present across this distinct surface (see "Biostratigraphy"). There is a 6-cm-thick volcanic ash layer at 357.93-357.99 mbsf. Glauconite abundance does not vary across a sharp color change (greenish gray [10GY 4/1] and blackish green [5GY 2.5/1]) at 358.29 to 360.6 mbsf. Glauconite begins to decrease at 360.6 mbsf and diminishes to very minor amounts at the base of the subunit at 361.12 mbsf.
Unit III is predominantly upper to middle Eocene diatom- and nannofossil-bearing claystone in the upper portion, grading to diatomaceous claystone in the lower portion. It is distinguished from Unit II by its lower glauconite content. Nannofossils decrease downsection from a minor modifier to near 0% by 438.8 mbsf. Based on this change, this unit was divided into two subunits.
Subunit IIIA is a middle to late Eocene greenish gray to dark greenish gray (10GY 5/1 and 4/1) and dark brownish gray (10YR 4/1) diatom- and nannofossil-bearing claystone to very dark brown (10YR 2/2) and very dark grayish brown (2.5Y 3/2) diatomaceous claystone. It is distinguished from Unit II by lower glauconite abundance (15%-25% above the boundary in Unit II to 2%-5% below the boundary in Unit III). A distinct cyclic color pattern is observed in this unit, alternating from greenish gray and dark greenish gray (10Y 5/1 to 4/1) to dark grayish brown (10YR 4/2) in the upper portion (361.12 to 393.40 mbsf). In the lower part of the subunit (393.40-433.89 mbsf), the pattern is generally absent, although occasionally observable from very dark grayish brown (2.5Y 3/2) and very dark brown (10YR 4/2) to greenish gray (10Y 5/1). Bioturbation is common to abundant throughout the subunit with abundant Zoophycos and Chondrites. Glauconite is present in very minor amounts (<6%).
The pervasive color cycles that characterize most of this subunit vary in length downsection. The uppermost part contains an average cycle length of ~1.5 m, whereas at 385 mbsf the length decreases to ~0.4 m until a darker and more homogeneous interval at 412.40 mbsf, which continues down to the base of the subunit. The length of the lighter colored strata tends to decrease downsection, with the darker colored strata becoming predominant by ~400 mbsf. This coincides with a downsection increase in nannofossils from more than 20% to between 4% and 8% (Fig. F10). In the uppermost section, volcanic glass reaches minor modifier status before decreasing below 364 mbsf. There are very minor amounts (<8%) of volcanic glass scattered throughout the core and several thin volcanic beds at 394.45-394.47 and 395.22-395.35 mbsf.
Subunit IIIB consists of a middle Eocene dark gray to very dark gray (5Y 3/1 to 4/1), greenish gray and dark greenish gray (5Y 4/1, 5GY 4/1, and 10Y 4/1), and dark olive-gray (5Y 3/2) diatomaceous silty claystone. It is distinguished from Subunit IIIA above by its decreasing nannofossil content and increasing quartz grains.
Colors show a cyclic pattern on a decameter scale, especially in the upper part of the unit. The color variations are faint, and there is a tendency toward a slightly lighter color below ~450 mbsf (Core 189-1172A-49X) and downsection. Clay, diatoms, and quartz are major components, but minor amounts of opaque minerals, volcanic glass, radiolarians, sponge spicules, and occasionally nannofossils are present throughout (Fig. F10). Planktonic foraminifers were not observed in smear slides, but rare, large benthic foraminifers are present in trace amounts from 447 to 450 mbsf. Glauconite, visible as silt- and sand-sized green/black grains, is present throughout, with occasional intervals of increasing abundance downsection on a 1- to 3-m scale. Pyrite grains appear more abundant in the lower part of the unit. Bioturbation is very common to abundant with several very large burrows, up to 37 cm in length, observed from 480 to 500 mbsf and with Zoophycos being very common in sections with abundant bioturbation (Fig. F11). Most of the distinct burrows are filled with clay of greenish gray or dark greenish gray color (5GY 4/1 and 10GY 5/1). Calcium carbonate values are very low, approaching zero, which is consistent with the very low calcareous nannofossil content.
The spectrophotometer data show little variations until ~450 mbsf and then slightly larger amplitude fluctuations toward the bottom of the subunit. Gamma-ray attenuation (GRA) bulk density data indicate an increase in density until a maximum at 1.72 g/cm3 (440 mbsf) followed by a decrease to 1.28 g/cm3 (450 mbsf) that is maintained downward.
Unit IV extends from 503 mbsf to the bottom of the hole at 765.97 mbsf. This unit consists mainly of claystone with low abundance of nannofossils, diatoms, glauconite, and accessory minerals in changing abundance (Fig. F5C, F5D). These lithologic changes were used to separate Unit IV into two subunits. Physical properties data were employed to support this separation. However, note that lower end GRA bulk density data from the rotary drilled Hole 1172D were not considered reliable because of the incompletely filled liners (see "Physical Properties"). This unit also contains the K/T boundary, at which no major lithologic change was observed.
Subunit IVA extends from 503.40 to 695.99 mbsf and consists largely of claystone. Minor lithologies include silty claystone, nannofossil-bearing claystone, and clayey siltstone. In contrast to the overlying Subunit IIIB, Subunit IVA bears no siliceous microfossils, but it does contain an increasing abundance of opaque minerals and accessory minerals. The exact boundary was defined at changing trends in lightness and magnetic susceptibility.
The claystone of Subunit IVA is olive gray (5Y 4/2, 5Y 3/1, and 5Y 3/2) with some variations to a very dark grayish brown (2.5Y 3/2) and to a greenish gray (5GY 5/1 and 5GY 5/1). Bioturbation is abundant, with well-preserved traces of Zoophycos and Chondrites. Some individual large burrows of unknown origin are filled with pyrite. Pyrite nodules between 1 and 4 cm in diameter were found in Sections 189-1172D-4R-4, 14R-4, 18R-4, and 20R-4. Thin mollusk shell beds were observed in Sections 189-1172D-9R-2 and 10R-4. The dominant clay component varies between 50% and 85% throughout the subunit. Opaque and accessory minerals are abundant and reach a maximum between 536 and 542 mbsf with 10% and 15%, respectively (Fig. F10). Silt-sized grains of glauconite were observed occasionally on the core surface but were rarely found in smear slides. However, between 595 and 609 mbsf, 622 and 636 mbsf, and 691 and 696 mbsf glauconite shows relative peaks in abundance of up to 11%. These peaks correspond to the magnetic susceptibility record with slightly increased values, the older glauconite peak being recorded at the lower boundary of Subunit IVA. Nannofossils are present at 10% abundance at the top of the subunit and decrease downward to trace abundance below 565 mbsf (Fig. F10).
Subunit IVB consists of very dark olive gray (5Y 3/2), very dark gray (10YR 3/1 and 5Y 3/1), and black (5Y 2.5/1 and 5Y 2.5/2) claystone and silty claystone. Subunit IVB is darker in color and less intensely bioturbated and contains fewer glauconite grains than Subunit IVA. The sediments contain organic material (~0.85% total organic carbon [TOC], see "Organic Geochemistry") and opaque minerals (<10% by smear-slide observation; Fig. F10) throughout the section, which are reflected in the dark color of the sediment. Carbonate content averages 1.4 wt%, slightly higher than the averages observed in the lower half of Subunit IVA (see "Organic Geochemistry"). The highest carbonate content (6.5 wt%) of Subunit IVB is observed at 762.9 mbsf (Sample 189-1172D-31R-5, 72 cm). Foraminifers and nannofossils are rare to common with good preservation. The sediment is barren of diatoms and radiolarians; however, siliceous microtubes, already observed in the Eocene sediments in Sites 1170, 1171, and 1172, are present throughout the subunit.
Sediments of this subunit are generally weakly laminated with very thin laminae frequently present throughout the subunit. Bioturbation (thin burrows of ~3 mm in diameter) is common in Cores 189-1172D-27R, 29R, and 31R and rare in other cores. Very fine sand-sized glauconite-rich layers are observed in claystones every ~50-100 cm. Although the amount of glauconite is not significant (i.e., no more than a few percent in smear slides), this periodicity may imply some paleoenvironmental signal. The dark olive-gray silty claystone below the Subunit IVA/IVB boundary, between 696.4 and 697.1 mbsf (interval 189-1172D-24R-5, 20-50 cm; Fig. F12), contains abundant fine to medium sand-sized grains. This sandy interval is reflected in magnetic susceptibility as a distinct positive excursion (see "Physical Properties"). Carbonate particles increase downward in Section 189-1172D-26R-6, and the core catcher of Core 189-1172D-26R contains a pure limestone in which no biogenic particles are recognized. The origin of these nonbiogenic (diagenetic) limestone and carbonate components in the overlying carbonate-rich claystone remains unknown.
The boundary between Subunits IVA (Danian) and IVB (Maastrichtian) is placed at 695.99 mbsf (Sample 189-1172D-24R-5, 29.5 cm). At this horizon, a distinct change from glauconitic siltstone (of Subunit IVA) to underlying claystone (of Subunit IVB) is recognized (Fig. F12; interval 189-1172D-24R-5, 20-50 cm). The claystone layer immediately below the subunit boundary (interval 189-1172D-24R-5, 29.5-31.5 cm) has very little bioturbation and is dark greenish gray (5G 4/1), lighter than the underlying representative black claystones of Subunit IVB. This boundary claystone layer contains silt-sized angular quartz grains and glassy fragments. Above the subunit boundary, between 25 and 29 cm of Section 189-1172D-24R-5, small clasts (~1-3 cm) of claystone, derived from the boundary claystone layer, are observed in the glauconitic siltstone of Subunit IVA. Below the claystone layer of the subunit boundary, in the interval 189-1172D-24R-5, 32-41 cm, a very dark gray siltstone in Subunit IVA facies appears with clasts of glauconitic siltstone.
Micropaleontological evidence (see "Biostratigraphy") suggests that the K/T boundary lies between 44 and 79 cm in Section 189-1172D-24R-5. The sedimentological changes observed at the transition from Subunit IVB to Subunit IVA could be related to K/T boundary events and will be investigated further.
The general trends in lithology of Site 1172 are similar to those found at other sites drilled during Leg 189 with three major phases of sedimentation:
These three major periods are found in nearly all Cenozoic sequences around Antarctica.
This is the first time that Late Cretaceous sediments were drilled by the Ocean Drilling Program (ODP) in this region. The sediment recovered at Hole 1172D, on the East Tasman Plateau, is a very dark brown to black, rarely bioturbated claystone with occasional silt. It suggests a poorly ventilated, restricted shallow-water environment or restricted circulation. These sediments provide a rare window into the Late Cretaceous climate and paleoceanographic history in the extreme southwest Pacific. Postcruise studies will have major implications for Antarctic Late Cretaceous climate reconstruction, as well as the paleoceanographic history prior to the onset of the Antarctic Circumpolar Current.
As at other sites drilled during Leg 189, the Eocene-Oligocene transition at Site 1172 is remarkable sedimentologically. However, excellent recovery at Site 1172 enables a more detailed understanding of the sedimentary pattern and environment during this transition. A gradational change is present within 5 m (Fig. F13) from a diatom- and nannofossil-bearing silty claystone into a diatom- and clay-bearing nannofossil chalk upsection. A high glauconite content suggests a very starved environment. Glauconite increases considerably from the silty claystone upward into glauconite sand and into a glauconitic silt, and it decreases shortly after the transition into a nannofossil chalk. An ash layer at 357.9 mbsf induced a large peak in magnetic intensity and provides an excellent stratigraphic tie point. The changes in lithology and some physical properties (lightness L*, magnetic susceptibility, and GRA bulk density) as well as stratigraphic data at this boundary are documented in detail in Figure F13. The present stratigraphic interpretation of paleomagnetic data was made from Core 189-1172A-39X measurements, although the quality of the recorded signal in the equivalent Core 189-1172D-2R is considered to be substantially better (see "Paleomagnetism").
The Neogene nannofossil oozes of Unit I at Site 1172 indicate a well-oxygenated, open-marine deep-sea environment with high and relatively stable oceanic productivity, which may be affected by incursions of subantarctic surface-water masses or processes associated with frontal movements. Abundant, well-preserved diatom assemblages in the middle Miocene may indicate increased upwelling through a northward shift of the Subtropical Front. This coincides with an increase in sedimentation rates and may be related to the expansion of the East Antarctic Ice Sheet. These higher sedimentation rates may be caused by higher productivity from intensification of the Antarctic Circumpolar Current and indicate the transition from the "Doubthouse" world to the "Icehouse" world. Coarser sections with increased foraminiferal percentages may point to an increase in circulation of intermediate and bottom-water masses and associated winnowing. The variability of interaction between the east Australian current and the Antarctic Circumpolar Current system is of major importance to sedimentation at Site 1172. Results from clay X-ray diffraction (XRD) analyses of slightly darker laminated and thinly bedded intervals in the Neogene sequence indicate variable smectite content, presumably related to sedimentation of eolian dust, from Tasmania and mainland Australia. These intervals occurred particularly during the middle and late Miocene during the expansion of the Antarctic cryosphere and increasing desertification of Australia.
XRD analyses were completed on 41 samples from Holes 1172A and 1172D. The purpose of the clay mineral studies at Site 1172 was to (1) recognize the major variations of the paleoenvironment, as expressed by the clay mineral assemblages at a sampling interval of one for every two cores, and (2) compare the clay mineral assemblages with those recognized at Sites 1168, 1170, and 1171 drilled at comparable water depths on the western Tasmania margin, the STR, and in other areas of the Southern Ocean.
The clay minerals identified include smectite, random mixed-layered clays, illite, and kaolinite. Based on the relative abundance of the clay minerals, five units were identified at Site 1172. These were designated Units C1 to C5 (Fig. F14).
Upper Pliocene to Pleistocene Unit C1, which extends from the seafloor to 50 mbsf, has a clay mineral assemblage that consists of abundant smectite (45% to 70%) and common kaolinite (15% to 25%), accompanied by random mixed-layered clays and illite (up to 15%). Unit C1 correlates to lithostratigraphic Subunit IA. Upper Miocene to upper Pliocene Unit C2, which extends from 50 to 180 mbsf, shows decreased contents of smectite (20% to 60%), increased contents of kaolinite (20% to 30%), and significant amounts of random mixed-layered clays (traces to 40%) and illite (10% to 20%). Because of the low clay content in some nannofossil oozes of lithostratigraphic Subunit IB, percentages of random mixed-layered clays >30% are probably overestimated. Unit C2 corresponds to the upper lithostratigraphic Subunit IB. Middle to upper Miocene Unit C3, which extends from 180 to 305 mbsf, shows increased contents of smectite (50% to 80%) and decreased contents of random mixed-layered clays (0% to 30%), kaolinite (10% to 20%), and illite (0% to 10%). Unit C3 corresponds to lower lithostratigraphic Subunit IB and upper lithostratigraphic Subunit IC. Middle Eocene to middle Miocene Unit C4, which extends from 305 to 410 mbsf, contains predominant smectite (75% to 90%) increasing downhole, associated with kaolinite (10% to 20%), and illite (traces to 5%). Unit C4 corresponds to lithostratigraphic Subunit IC, Unit II, and Subunit IIIA. Upper Maastrichtian to middle Eocene Unit C5, which extends from 410 mbsf to the bottom of Hole 1172D at 766 mbsf, is characterized by almost pure smectite (95% to 100%) associated with sporadic low amounts of kaolinite and illite (up to 5%). Unit C5 correlates to lithostratigraphic Unit IV.
The clay mineral assemblage of Site 1172 is significantly different from those on the South Tasman Rise (see "Lithostratigraphy" in the "Site 1170" chapter and "Lithostratigraphy" in the "Site 1171" chapter) and western Tasmania margin (see "Lithostratigraphy" in the "Site 1168" chapter). Site 1172 is characterized by dominant smectite and an increasing trend of kaolinite, from the late middle Eocene to the early Pliocene. The kaolinite trend is absent from the other sites.
Almost exclusive smectite characterizes upper Maastrichtian to middle Eocene Unit C5. Smectite prevails in areas of low relief with alternating intervals of precipitation and aridity, and its formation is enhanced on basic volcanic substrates (Chamley, 1989; Weaver, 1989). The only visible ash layers and occurrences of volcanic glass, probably from the adjacent Cascade Seamount, were restricted to the upper Eocene and Oligocene intervals. The siliciclastic sediments consist of claystone and silty claystone, and the clay particles are most probably derived from adjacent emerged areas of the Tasmanian landbridge and Campbell Plateau (see "Background and Objectives"). Therefore, the extreme predominance of smectite points to warm climates and intense weathering in the Tasmanian region adjacent to Antarctica. Uppermost Cretaceous to middle Eocene clay assemblages of almost exclusive smectite are present at other southern high-latitude sites on the Falkland Plateau (Robert and Maillot, 1983), Maud Rise (Robert and Maillot, 1990), and Kerguelen Plateau (Ehrmann, 1991), as well as in the middle Eocene of the Lord Howe Rise in the Tasman Sea (Stein and Robert, 1986). No indication of increased erosion of steep continental relief associated with tectonic activity has been found at Site 1172 on the isolated ETP. Early stages of ocean opening in the Tasman Sea were already finished, and the ETP was already separated from the South Tasman Rise and probably under the influence of a western boundary current similar to the modern East Australian Current.
Unit C4 (middle Eocene to middle Miocene) is defined by a continuous decrease of smectite upward and correlative increase of kaolinite, beginning in the late middle Eocene at ~40 Ma (close to the Zone NP16/NP17 boundary). Kaolinite is typical of warm climates with high precipitation during at least part of the year (Chamley, 1989; Weaver, 1989). Kaolinite has already been observed in middle to late Eocene high-latitude areas of the Weddell Sea and subantarctic South Atlantic (Robert and Kennett, 1992), Prydz Bay (Hambrey et al., 1991) and the western Tasmania margin (see "Lithostratigraphy" in the "Site 1168" chapter). Kaolinite formation is enhanced in warm areas of intense precipitation that increase chemical weathering of substrates, especially where uplifted relief ensures good drainage conditions and removal of soluble chemical elements. At Site 1172, there appear to have been no such uplifted source areas during the late middle to late Eocene, as was the case on the Australo-Antarctic Gulf side of the Tasmanian region. The contrast in relief in source areas probably accounts for differences in kaolinite abundance between the western Tasmania margin and the East Tasman Plateau (see "Lithostratigraphy" in the "Site 1168" chapter, "Lithostratigraphy" in the "Site 1170" chapter, and "Lithostratigraphy" in the "Site 1171" chapter). Therefore, the increasing kaolinite may result from increased precipitation on the Australian margin of the Tasman Sea and southern transport by the East Australian Current, probably caused by intensified meridional heat transfer as ocean opening progressed between Antarctica and Tasmania. Some of the kaolinite may also have come from granitic islands forming the rim of the ETP.
The first occurrence and further development of random mixed-layered clays in Unit C3 (middle to upper Miocene) are associated with sporadically increased contents of illite. Illite and random mixed-layered clays are derived from erosion of poorly weathered substrates (Chamley, 1989; Weaver, 1989). They indicate an increase of physical weathering in the source areas during the middle Miocene, at ~13-14 Ma (Zone NN6), probably from cooler climatic conditions that followed the expansion of the East Antarctic Ice Sheet at 14-15 Ma (Kennett, 1977).
A further development of kaolinite (up to 30%) characterizes Unit C2 (upper Miocene to upper Pliocene). Because of the continued spreading in the Southern Ocean during the Oligocene and the Neogene, Australia moved into warmer latitudes and kaolinite formation increased on the emerged Australian passive margin of the Tasman Sea. Increased kaolinite may reflect intensified precipitation starting at ~8.3 Ma (lower Zone NN11). A coeval increase of kaolinite is observed at Site 823 on the northern part of the Australian margin of the Tasman Sea (Chamley et al., 1993). Increased kaolinite contents along the Australian margin of the Tasman Sea were most probably the consequence of intensified meridional heat transfer through the East Australian Current. Regional increases of kaolinite (and associated precipitation) have also been observed in the midlatitudes of the Atlantic Ocean (Robert and Chamley, 1987). Late Miocene increases of precipitation at ~8 Ma preceded the onset of ice buildup in West Antarctica in the latest Miocene (Kennett, 1977).
Unit C1, of late Pliocene and Pleistocene age, shows an increase of smectite. The clay assemblage is very similar to that of Sites 588, 590, and 591 on the Lord Howe Rise (Stein and Robert, 1986). At the same time, the clay content of the calcareous biogenic sediment of Site 1172 slightly increases (Subunit IA). The clay assemblage of Unit C1 may reflect some wind transport from central arid Australia. As observed at Site 1170 on the STR (see "Lithostratigraphy" in the "Site 1170" chapter), this pattern may have commenced during the late Pliocene, when particles from Australian arid areas reached their maximum southward extension and abundance on the Lord Howe Rise (Stein and Robert, 1986).