A total of 958.8 m of sediments ranging in age from Pleistocene through latest Paleocene was penetrated at Site 1171 (Fig. F4). Of the four holes drilled at Site 1171 (see "Operations"), the sedimentary succession recovered at Holes 1171C and 1171D will be the focus of this lithologic report. Holes 1171A and 1171B recovered strata from the Pleistocene through latest Miocene, which are also present in Hole 1171C. Core recovery at Site 1171 was generally good to excellent (89.4% mean, 93.6% median in Hole 1171C), except for an upper Eocene section in Hole 1171D that contained interbedded lithified and unlithified strata (73.8% total mean, 91.4% mean, and 97.5% median excluding the lithified interval in Hole 1171D). This included an exceptional middle Eocene to upper Paleocene interval, which contained an expanded section, fair to excellent preservation of the calcareous microfossils, and apparent color cyclicity. The triple APC-drilled cores of Holes 1171A, 1171B, and 1171C also contained almost no drilling disturbances, which allows for the construction of a composite section of Holes 1171A, 1171B, and 1171C to a depth of 70 mbsf that contains spectrophotometric reflectivity cycles (See "Composite Depths"). Except in a few intervals, biscuiting and fracturing caused by drilling disturbances varied from absent to moderate (disturbance B0-B3) in the lower part of Hole 1171C and throughout Hole 1171D (Fig. F5).
The sedimentary succession at Site 1171 has been divided into six lithostratigraphic units, with Units I, V, and VI being further subdivided into two, three, and two subunits, respectively (Table T2). These lithostratigraphic units and subunits are at the 10- to 100-meter scale. These units often show an internal cyclic structure at the decimeter to meter scale (in the sense of a systematic repetition of lithologies within the succession). Units and subunits were identified by integrating megascopic core inspection and smear slide examination. The identification of these units was supported by reflectance spectrophotometry, bulk density, magnetic susceptibility, and carbonate content.
Unit I is a biogenic ooze and chalk that was divided into two subunits based on microfossil content. Subunit IA consists of Pleistocene to Pliocene foraminiferal nannofossil ooze and diatom-bearing foraminiferal nannofossil ooze. Subunit IB is composed of nannofossil ooze and foraminifer-bearing nannofossil ooze that represent a fairly continuous section of lower Pliocene to lowermost Miocene succession. The subunit is characterized by having lower foraminifer abundances (ranging between 5% and 15%) than Subunit IA (20%-50%), increased carbonate content (86.5 to 96.8 wt%; average = 94.0 wt%) (Fig. F5A, F5B), and rare to occasional silt-sized detrital glauconite. Unit II is Oligocene in age and represents the transition from the pelagic sediments of Unit I to the predominately siliciclastic sediments in Unit III. Overall, Unit II consists of a foraminifer-bearing nannofossil chalk that downsection contains increasing glauconite, clay, accessory minerals, quartz, radiolarians, bioclasts, sponge spicules, and volcanic glass as well as a decrease in carbonate content. Glauconite generally increases in size and abundance to 272.8 mbsf in Hole 1171C and 269.8 mbsf in Hole 1171D, where the most dramatic lithologic change at this site is found.
Unit III is a thin (6.4 m) uppermost Eocene sequence consisting of glauconitic sandy silt and clayey glauconite silt. Unit IV is an upper Eocene diatomaceous claystone and nannofossil-bearing diatomaceous silty claystone (Fig. F5B). Unit V is a middle Eocene nannofossil claystone to silty claystone to organic-bearing silty claystone. Sediments of Unit V can be grouped into two general types: lighter-colored nannofossil-bearing claystones and darker-colored silty claystones containing more organic debris. Unit V is divided into three subunits based on the distributional patterns of these sediment types at the decimeter scale (Fig. F5C, F5D). However, higher frequency changes at the meter scale also are observed throughout this unit. Subunit VA is a claystone and nannofossil-bearing claystone that increases in nannofossil content and lightens in color downsection. Subunit VB is a claystone, occasionally organic debris-bearing, that contains an overall pattern of lower nannofossil abundance and higher organic debris in the upper and lower sections, separated by a slightly elevated nannofossil content and the absence of organic matter between these two intervals. Subunit VC is a claystone with increasing nannofossils in the lower portion.
In contrast, Unit VI is characterized by nannofossils becoming rare to absent and increasing quartz silt and organic matter. Overall, it can be characterized as nannofossil-bearing claystone, clayey siltstone, and organic matter-bearing siltstone. Subunit VIA consists of nannofossil-bearing claystones overlying silty claystones and is marked by a further downsection decrease in nannofossils and increase in quartz silt and organic matter. Subunit VIB is a silty claystone in the upper portion, grading to an organic matter-bearing clayey siltstone in the lower portion (Fig. F5E). Bioturbation, often common in the overlying siliciclastic units, decreases in the last two cores of Subunit VIB and is replaced by laminated strata. Between Cores 189-1171D-72R and 73R (920.0-939.5 mbsf), the boundary between the Paleocene and Eocene was identified by organic dinocyst biostratigraphy. Little to no apparent sedimentological change occurred across this boundary.
Unit I consists of white, light greenish gray, light bluish gray, and light gray pelagic nannofossil ooze, foraminifer-bearing nannofossil ooze, and occasional intervals of foraminiferal nannofossil ooze with minor siliceous biogenic, calcareous biogenic, and terrigenous materials. Sediments are generally massive, with thin, faintly laminated intervals. This unit is divided into two subunits. Subunit IA contains more foraminifers, less carbonate, and has higher reflectance values than Subunit IB (Fig. F5A, F5B).
Subunit IA is Holocene to Pliocene in age and consists of a nannofossil foraminiferal ooze that grades downsection into a foraminiferal nannofossil ooze and foraminifer-bearing nannofossil ooze. The subunit is characterized by an overall decrease in foraminiferal content (20%-50% in the upper portion to 5%-15% at the base; Fig. F6). Clay minerals, quartz grains, diatoms, radiolarians, and sponge spicules are present in minor amounts. Pyrite stains are frequent, together with occasional silt-sized glauconite grains. From 0 to 19 mbsf, sediment color ranges from white (N 8, 5Y 8/1) to light greenish gray (5Y 7/2, 10Y 8/1 to 7/1) to light bluish gray (10PB 8/1) to light gray (5YR 7/1). From 19 to 41 mbsf, sediments are generally white (N 8) and massive, with occasional faint light greenish gray (5G 7/1) and light bluish gray (5PB 7/1) laminations. A small increase in siliceous material (diatoms and radiolarians; average of 5%-15%) coincides with the change to more massive sediments. Bioturbation is occasionally moderate to common and appears more prevalent in darker intervals; this appearance may relate to the lack of color contrast in the white ooze. The carbonate content (see "Organic Geochemistry") is variable in the upper 27 m, with two major decreases to ~86 wt% at 10.82 and 23.32 mbsf. Gamma-ray attenuation (GRA) bulk density data increase from 0 to 10 mbsf, followed by decreasing values for the rest of the subunit (Fig. F5) (see "Physical Properties"). Subunit IA may correlate to Subunit IA at Site 1168, Subunit IA at Site 1169, and the upper portion of Unit I at Site 1170.
Subunit IB is lower Pliocene to lowermost Miocene in age and consists of white (5Y 8/1 and N 8) nannofossil ooze and foraminifer-bearing nannofossil ooze that grades into chalk at ~211 mbsf. Subunit IB is distinguished from Subunit IA by having a lower foraminiferal content (5%-15%) and a higher and less variable carbonate content (90.2 to 96.9 wt%; average = 94 wt%). Minor variable components include clay minerals, quartz, diatoms, radiolarians, sponge spicules, and bioclasts. Bioturbation is rarely visible. Sediments are generally massive with laminated intervals (10 to 150 cm thick) present that are spaced at meters to decameters. Laminations range in color from light greenish gray (5G 8/1), to light bluish gray (5PB 8/1) and light gray (N 6 and N 7); laminated intervals are occasionally followed by an abrupt color change.
A minor increase in the foraminiferal content coincides with occasional silt-sized pyrite and glauconite grains between 115 and 210 mbsf. GRA bulk density is a relatively constant unit (1.7-1.8 g/cm3) from 41 to 141 mbsf. Silt-sized glauconite is clearly visible throughout the unit. Subunit IB is correlated to Subunits IB and IIA at Site 1168, Subunit IB at Site 1169, Unit II at Site 1170, and the lower portion of Subunit IB at Site 281 (Kennett, Houtz, et al., 1975).
Unit II is late Oligocene in age and is characterized by a foraminifer-bearing nannofossil chalk and a general downsection decrease in carbonate percent (89.7-74.1 wt%) and increases (2-11 wt% increase) in glauconite, clay, accessory minerals, quartz, radiolarians, bioclasts, sponge spicules, volcanic glass, and silt-sized particles. This unit marks the transition from the pelagic sediments of Unit I to the predominately siliciclastic sediments of Unit III. Unit II is distinguished from Subunit IB by (1) lower calcium carbonate percentages (from 89.7 wt% at 252.62 mbsf to 84.2 wt% at 259.30 mbsf), (2) lower reflectivity, and (3) higher abundances of glauconite, radiolarians, bioclasts, sponge spicules, clays, accessory minerals, and volcanic glass. The upper unit boundary also coincides approximately with the Oligocene/Miocene boundary (see "Biostratigraphy").
Smaller-scale features in Unit II may represent paleoenvironmental changes. A relatively thin interval containing alternating light greenish gray (10GY 8/1) to white (N 8) color is observed in Cores 189-1171C-29X and 189-1171D-2R (257-268 mbsf). In two instances, the transition from the lighter to darker strata downsection is abrupt and contains a well-defined surface. In contrast, cycles in Unit I are typically marked by a change from darker to lighter sediments.
A distinct surface marked by foraminifer-bearing chalk above, and nannofossil chalk below, is located 12 cm above the base of Unit II (272.68-272.80 mbsf in Hole 1171C; Fig. F7; interval 189-1171D-3R, 17-30 cm). Glauconite begins to increase downsection both in abundance and size from ~40 cm above this surface (272.28 mbsf in Hole 1171C) often reaching sand size. Rip-up clasts (~1 cm) are observed up to a few centimeters above the surface. The strata below contain considerably less glauconite and consist of massive white (N 8) nannofossil chalk. This thin (12 cm) chalk is terminated by the abrupt lithologic change at 272.68 mbsf in Hole 1171C and 269.8 mbsf in Hole 1171D.
Transitional units are recognized at all of the deeply drilled sites from Leg 189, as well as at sites from Leg 29 (Kennett, Houtz, et al., 1975) and may correspond to Unit II at Site 1171. At Site 1168, Subunits IIB and IIC can be interpreted as an extended transitional unit (130 m thick) dated as late Oligocene (see "Lithostratigraphy" in the "Site 1168" chapter). The transitional unit at Site 1170, Unit III, also roughly correlates to Unit II of Site 1171 and may represent an intermediary between the thicker, more clay-rich transitional Subunits IIB and IIC at Site 1168 (130 and 120 m thick, respectively) and Unit II of Site 1171. Subunit IA and Unit III of DSDP Site 281 may also be equivalent to Unit II of Site 1171 (Kennett, Houtz, et al., 1975).
Unit III is uppermost Eocene in age and consists of 6.4 m of dark greenish gray (5GY 4/1) to blackish green (5G 2.5/1) glauconitic sandy siltstone and clayey glauconitic siltstone. This is a distinct transition (at 269.8 and 272.8 mbsf in Holes 1171C and 1171D, respectively) from the overlying lower Oligocene white nannofossil chalk of Unit II. Visual observations, bulk density, magnetic susceptibility, spectrophotometer, and coulometric carbonate data sets all indicate a major change across this surface (Fig. F5B) (see "Physical Properties"). The interval immediately below this transition (0-0.34 m) is highly bioturbated with Zoophycos visible throughout. Quartz and glauconite increase as the color changes from dark greenish gray (5GY 4/1) to blackish green (5G 2.5/1) downsection. Bioturbation is abundant to common, with a decrease in intensity downsection. Calcium carbonate decreases sharply from 77.3 wt% immediately above the surface to 0.41 wt% below. This agrees with the smear-slide data, where calcareous nannofossils number only a few percent. Small intervals with laminations and individual beds are observed in the upper part of the unit, marked by very high glauconite content, and some clay clasts. Unit III may correlate to Unit IV at Site 1168 and Unit IV at Site 1170.
Unit IV is late to middle Eocene in age, containing 67 m of nannofossil-bearing diatomaceous silty claystone, silty claystone, and diatomaceous claystone. It is distinguished from Unit III by a major lithologic change from clayey glauconitic siltstone to diatomaceous silty claystone, a slight lightening in color, an increase in carbonate content from 0.41 wt% in Unit III to 6.96 wt% at the top of Unit IV (with the unit averaging 4.67 wt%), and bulk density falling from 1.7 g/cm3 at the base of Unit III to 1.35 g/ cm3 at the top of Unit IV. The unit is characterized by an abrupt increase in siliceous biogenic components, with 20%-40% diatoms and lesser sponge spicules and radiolarians. Foraminifers are generally absent. Minor quartz, opaque, and accessory minerals are present throughout. Glauconite content varies (2%-15%) with a marked decrease below 300 mbsf. However, abundant glauconite (20%) reoccurs together with quartz (25%) in a 1-m interval from 317.7 to 318.7 mbsf. Bioturbation is common to abundant with Zoophycos, Chondrites, and burrows of unknown origin.
The color darkens downsection at 276.2 mbsf, from olive-gray (5Y 5/2) to dark gray (5Y 4/1) to black (5Y 2.5/1) silty claystone at ~300 mbsf. The sediment is characterized by increased quartz and glauconite, and there is little biogenic material except for 5% nannofossils. The second downsection darkening succession is slightly lighter in color because of a higher content of diatoms, nannofossils, and sponge spicules. It grades from pale olive (5Y 6/3) at 276.2 mbsf to olive (5Y 5/2 and 4/3) to black (5Y 2.5/2), with quartz and opaque grains at the bottom of the unit.
The lowest values of calcium carbonate (0 and 1.61 wt%) are reached at 294 and 334 mbsf, respectively, in the unit. Between the two minima the values increase to 7.9 and 14.9 wt%; the generally higher values correspond to higher nannofossil content. Unit IV correlates to Unit V at Site 1168, Unit IV at Site 1170, and Units III through V at Site 281 (Kennett, Houtz, et al., 1975).
The sediments of Unit V are middle Eocene claystones and silty claystones with occasional minor amounts of nannofossils. Unit V is differentiated from Unit IV by the general absence of siliceous microfossils, the increase in volcanic glass and opaque and accessory minerals, and occasional mica. This unit is divided into three subunits based on nannofossil content, organic matter content, and visual color.
Subunit VA generally consists of an upper middle Eocene light greenish gray (10Y 6/1), olive (5Y 5/3), dark greenish gray (10Y 4/1), and dark olive-gray (4/13/2) claystone and nannofossil-bearing claystone. Although the upper part of the subunit is interbedded with hard and soft strata, resulting in low drilling recovery (343.50-394.83 mbsf), a satisfactory lithostratigraphic framework was created. Claystone extends from the top to 411.6 mbsf, where nannofossils increase to a minor modifier and carbonate content increases downward. In well-recovered intervals, there is a pervasive alternation in color, from olive (5Y 5/3) to dark olive gray (5Y 3/2) or dark greenish gray (10Y 4/1), with average cycles ranging from 0.6 to 1.5 m long. Typically, darker intervals contain fewer calcareous microfossils than lighter colored intervals, and the cycles may represent high-frequency oscillations (104-105 yr) in the depositional environment. Site 1168 did not penetrate to equivalent-age sediments. This unit roughly correlates to Subunit VA at Site 1170 and to Units IV and V at Site 281 (Kennett, Houtz, et al., 1975).
Subunit VB is a middle Eocene dark greenish gray (10Y 4/1) to very dark gray (5Y 3/1) claystone to an organic-bearing claystone. A bioturbated contact at 421.6 mbsf separates the glauconitic interval at the base of Subunit VA from Subunit VB. This subunit differs from Subunit VA in its lower nannofossil abundance, generally darker color, and higher organic content (Fig. F8). Organic content reaches minor modifier levels in the upper (421.6-440.0 mbsf) and lower sections (469.33-485.70 mbsf). Between these intervals, nannofossils are slightly more abundant (4%-7%). Photospectrometry data indicate that positive or red color in chromaticity coordinate a* (associated with a browner color) correlates with higher total organic carbon (TOC) in the upper and lower parts. A shift to negative values (green color) is associated with the lower organic content and higher nannofossil abundances in the middle part (Fig. F9). Subunit VB roughly correlates to Subunit VB at Site 1170. The nearby DSDP Site 281 records glauconitic silty clays, sand, and basal breccia in the upper Eocene, above a Paleozoic quartz-mica schist basement (Kennett, Houtz, et al., 1975).
Subunit VC is middle Eocene in age and is characterized by silty claystone, with nannofossil content increasing downsection from a minor to occasionally major modifier. The upper boundary of this subunit is identified by (1) an increase in nannofossils, (2) a color change from very dark gray (5Y 3/1) to dark gray (5Y 4/1), and (3) less organic matter (Fig. F10). Subunit VC can be characterized by the following lithologic criteria:
Higher-frequency lithologic changes are observed in most of this unit (521.00-692.49 mbsf), although the upper 40 m of Subunit VC consists of mainly homogeneous olive-gray and dark olive-gray (5Y 4/2 and 5Y 3/2) nannofossil-bearing claystone to silty claystone containing occasional lignite fragments. A cyclic color pattern, of light greenish gray (10Y 5/1 and 5Y 6/2) to dark greenish gray to olive gray (10Y 4/1 to 5Y 4/2), begins at 521 mbsf. The cycle length varies from 65 to 90 cm. The transition from darker to lighter sediments becomes sharper with depth, and an increase in glauconitic silt and fine sand is associated with darker sediment. Carbonate content and color begin to increase at ~521 mbsf (Fig. F10). Bioturbation is common to locally abundant, often obscuring most sedimentary structures as well as the transition between darker and lighter sediments. However, by 530.0 mbsf, sharp surfaces separate the darker strata above from the lighter strata below. There is a particularly distinct surface at 548.90 mbsf with glauconite-filled burrows below the contact surface (Fig. F11).
These cycles continue to 579.47 mbsf, where they are replaced by generally more massive light greenish gray (10Y 5/1) sediments. This change is marked by a thin (6 cm), black (N 2.5) interval containing abundant shells (Fig. F12). Nannofossil abundance generally increases downsection, and calcium carbonate increases from ~1 wt% above 630 mbsf to more than 20 wt% near the base of the subunit (5.8 to 22.5 wt%; average = 14.1 wt%; N = 14). These increases suggest a generally less restrictive and more open marine setting for the lower part of the subunit. Two thin sandstone beds are observed at the base of this subunit in Core 189-1171D-48R (689.90-690.30 mbsf).
The sediments of Unit VI are lower Eocene to upper Paleocene clayey siltstones and silty claystones with organic matter increasing downsection to minor and occasional major modifiers. In contrast to Unit V, carbonate content decreases to a few percent in Subunit VIA and then to trace amounts in Subunit VIB (Fig. F5E). Spectrophotometry data also indicate a sharp color change at the upper boundary of Unit VI. Unit VI was subdivided into Subunits VIA and VIB based on changes in sediment components such as quartz, clay, organic debris, and nannofossils and supported by calcium carbonate content and physical properties data.
Subunit VIA consists mainly of lower Eocene greenish gray to dark greenish gray (5GY 5/1 to 10Y 4/1) nannofossil-bearing silty claystone in the upper part (692.49-729.01 mbsf) and silty claystone in the lower part (729.01-805.10 mbsf). Throughout the subunit, sediment texture is massive with common small-scale bioturbation of unknown origin, and rare Chondrites and Zoophycos are present throughout. Occasionally, burrows contain lighter greenish rings.
Carbonate content in sediment averages 2.1 wt% with a maximum of 9.5 wt% at 801.51 mbsf. The carbonate is mainly derived from nannofossils, which are more frequent between 700 and 730 mbsf (Cores 189-1171D-49R to 52R), with an average of 7% in smear-slide observations. Nannofossil content decreases considerably at 738 mbsf, and nannofossils disappear completely at 780 mbsf. The dominant sediment components are clay (average of 42%) and quartz (average of 38%), with quartz increasing and clay decreasing downsection. Small lignite fragments (<1 cm) are sporadic throughout the unit.
Pressure solution seams are sporadic and filled with quartz, microsparite, or clay. Other postdepositional textural features include a few microfaults (Fig. F13) and calcareous and occasionally siliceous veins. Light yellowish brown (2.5Y 4/2) nodules that range in diameter between 1 and 2.5 cm are sporadic throughout Subunit VIA. According to X-ray diffraction (XRD) results (from Core 189-1171D-50R), the nodules consist mainly of diagenetic minerals (siderite, apatite, and calcite). In the entire subunit, siliceous white tubes of unknown origin are very rare. Two distinct ash layers were found in Cores 189-1171D-51R and 59R.
Subunit VIB consists of lower Eocene dark olive-gray (5Y 3/2) to dark greenish gray (10Y 4/1) silty claystone and clayey siltstone in the upper section, giving way at 870.00 mbsf to lower Eocene to upper Paleocene dark grayish brown (2.5Y 3/2) organic matter-bearing clayey siltstones in the lower part. Sediment texture throughout the entire subunit is massive, and small-scale bioturbation is common. Chondrites, Zoophycos, and burrows of unknown origin, often containing lighter green rings, are sporadic throughout. Pressure solution seams are rare and filled with quartz, microsparite, or clay. Light yellowish brown (2.5Y 4/2) nodules continue into Subunit VIB and are most abundant between 805.1 and 814.65 mbsf in Core 189-1171D-60R. According to XRD results, the nodules consist mainly of calcite, apatite, and siderite, with minor quartz. In the entire subunit, siliceous white tubes of unknown origin are very rare.
Carbonate content is extremely low in Subunit VIB, rarely exceeding 1 wt%. However, from 910 mbsf downward (Core 189-1171D-71R), calcium carbonate increases slightly with spot maxima at 912.01 mbsf (9.2 wt%) and at 949.21 mbsf (5.10 wt%). Clay decreases from 40% to 20% downsection, whereas quartz increases from 38% to 48%. Subunit VIB is uniquely characterized by a downward increase of organic debris (below 865 mbsf; Core 189-1171C-66R). In Section 189-1171D-71R-2, a distinct surface separates a massive, very dark grayish brown (2.5Y 3/2) clay above from a heavily glauconitic, dark grayish brown (2.5Y 4/2) clay below (Fig. F14). Glauconite decreases downward until Section 189-1171D-71R-4, where a lighter colored extensively bioturbated layer is found (Fig. F15). Below this interval, color darkens downsection and laminated sediments are found down to the bottom of the hole. Limestone layers 2 to 12 cm thick are present between 812.35 and 904.12 mbsf. Thin sections show a micritic texture with extremely rare bioclasts (radiolarian ghosts and sponge spicules), a few small, angular quartz grains, and organic matter. Signs of late diagenetic dissolution and deformation are present.
Primary objectives of Site 1171 were to develop a better understanding of the pre-, syn-, and postdepositional history of the opening of the Tasmanian Gateway and its role in Southern Ocean development and climatic processes during the Paleogene and Neogene. Preliminary lithologic results provide evidence that sediments recovered at Site 1171 may hold critical information for unlocking many of the paleoceanographic questions related to the opening of the Tasmanian Gateway. These observations include clear sedimentological evidence of major long-term lithologic changes (scale of tens of meters) during and after the opening and decimeter- to meter-scale cyclic changes in the lithology before the opening of the gateway (lower to middle Eocene), which are suggestive of high-frequency (104-105 yr) cyclicity. They indicate that the paleoenvironmental history of Site 1171 is one of slowly increasing ventilation during the Paleocene and Eocene, a transition to a fully open marine regime during the Eocene-Oligocene transition and throughout the Oligocene, and winnowing caused by vigorous bottom-water circulation during the Neogene.
The depositional history of the late Paleocene to the Eocene at Site 1171 is one of increasing ventilation and water deepening. Embedded within this long-term trend are higher frequency oscillations. The mechanisms of long-term change include local subsidence, opening of the Tasmanian Gateway, deep-water circulation changes, local productivity changes, regional and global climatic changes, and eustasy. The drilling, in combination with the seismic profiles, indicates that most local tectonic activity occurred at the Paleocene/Eocene boundary. This suggests that derivation of sediment from the local fault blocks associated with the Balleny Fracture Zone would have peaked at ~55 Ma. Because of the preliminary nature of this volume, we will not speculate on which are the primary mechanism(s) for the long-term changes. Postcruise studies should be able to address many of these questions. The evidence for increasing ventilation includes an increase of lighter colored claystones with more nannofossils upward and less quartz and organic matter. Lighter colored claystones are interpreted as deposited in more open-marine and less restricted environments, and the darker-colored clayey siltstones and silty claystones in more restricted and less open marine environments.
Late Paleocene sediments are laminated siltstones containing high organic matter and quartz. The presence of laminations and absence of any bioturbation indicate near anoxic conditions. This suggests poor ventilation and may represent a shallow-marine sheltered marginal environment or an extreme shallowing of the oxygen minimum zone. A stratigraphic surface associated with a glauconite layer in Core 189-1171D-71R (912.70-914.90 mbsf) may indicate a larger environmental change or stratigraphic break and corresponds to the Paleocene/Eocene boundary.
The upper part of Subunit VIB (early Eocene) contains increasing clay and decreasing quartz and organic matter, which suggests steadily decreasing input of coarse terrestrial sediment. This trend continues in Subunit VIA, where nannofossil content slowly increases, suggesting a more open ocean regime. Several thin micritic limestone beds between 812.35 and 904.12 mbsf may have been transported into the basin as tempestites, either as a result of tectonic activity or during storm events. If these are storm events, then water depths for Subunit VIA were below fair weather wave base and above storm wave base.
There is a sharp lithologic change at the lower to middle Eocene boundary (Subunit VIA and Unit V boundary). Quartz rapidly decreases and nannofossils increase, indicating a more open marine paleoenvironment. A distinct surface at this boundary also suggests a water-depth change, based on the glauconite above the surface (typical in transgressive phases) and upward increases in nannofossil abundance. In fact, Subunit VC contains the highest carbonate and nannofossil abundances observed in the middle Eocene, indicating more open marine conditions. Generally, from ~500 to 340 mbsf, nannofossil and organic matter abundances vary inversely at the meter to tens of meter scale, indicative of high frequency and longer-term changes in the ventilation history at Site 1171. There is an overall trend within Subunits VA and VB of generally less ventilation than in Subunit VC.
There are several thin sandstone beds in Subunit VC that may represent storms (690.78-692.44 mbsf). Paleogeographic reconstructions and local seismic sections suggest a constricted basin during this time, resulting in relatively low-wave energy. Thus, the storm-wave base level may have been in water as shallow as 30 m. A thin (6 cm) black interval with abundant mollusk shells (Fig. F12) is also interpreted to be a storm lag or sequence boundary (i.e., a change in sea level). In either case, early to middle Eocene water depths were generally above storm-wave base level (30-60 m water depth). These units also had relatively high sedimentation rates, suggesting that subsidence was also high.
The transition from claystone to diatomaceous claystone within the late Eocene (Subunit VA to Unit IV) is associated with diatom assemblages, which suggest an increase in water depth, from inner neritic to outer neritic (see "Biostratigraphy"). The dissolution of the calcareous microfossils in Unit IV is probably caused by the presence of acid pore waters in the sediments, which have 0.2-0.5 wt% TOC. Upper Eocene siliceous biogenic units (i.e., diatoms and radiolarians) are also identified at Sites 1170 and 1172 (see "Lithostratigraphy"), indicating that there were similar depositional conditions on both sides of the Tasmanian Gateway. Similar dissolution of calcareous microfossils occurred in other southern ocean sites (e.g., Sites 689 and 690; Thomas et al., 1995) during the late Eocene.
In Unit III, glauconite increases upsection and becomes a major modifier. In situ glauconite indicates low sedimentation rates and sediment-starved depositional environments (McRae, 1972) and represents a major change at Site 1171 from the high sedimentation rates that characterize the middle Eocene. Possible mechanisms for these lower rates could be decreasing sediment input, reduction in the area of eroding landmasses, decreasing weathering rates caused by climatic cooling, or winnowing by currents.
The lithology of the Eocene/Oligocene transition indicates sweeping changes in both water masses and water depth at Site 1171. Glauconitic silts and clays indicative of neritic to upper bathyal conditions are replaced at 272.8 mbsf (Hole 1171C) by foraminifer-bearing nannofossil chalks. Although this change could be caused in part by a rapid decrease in sedimentation rates and an increase in productivity, benthic foraminiferal studies indicate a rapid water-depth increase across this transition (see "Biostratigraphy").
The depositional history after the opening of the Tasmanian Gateway at Site 1171 is one of winnowing by strong bottom-water currents and the establishment of a well-ventilated environment. Evidence for strong bottom-water currents is found 12 cm above the Eocene-Oligocene transition, where rip-up clasts overlie a distinct, probably erosional, surface (Fig. F7). Detrital glauconite, clay, accessory minerals, quartz, radiolarians, bioclasts, sponge spicules, and volcanic glass content decrease upsection to minor and trace amounts. Silt-sized glauconite is interpreted as detrital based on the small size and roundness of the grains and the lack of glauconite infillings in foraminifers. They continue upsection into the Pliocene-age sediments (up to 41 mbsf) indicating that bottom currents were able to transport silt-sized particles. Evidence for well-ventilated bottom water includes good to excellent preservation of the calcareous microfossils indicating noncorrosive waters and a relatively high diversity (for southern ocean sites) of foraminifers and nannofossils (see "Biostratigraphy"). This pattern has continued into the Holocene. During the Pleistocene and Holocene, foraminifers increase to major modifier, suggesting increased winnowing caused by stronger bottom-water currents of the Antarctic Circumpolar Current.
During the middle Eocene, a persistent alternating pattern was observed at the meter and tens of meters scale. These possible cycles in Unit V contain two basic sedimentary types: lighter-colored nannofossil-bearing claystones interpreted as being deposited during better ventilated, more open marine periods, or marine highstands, and darker-colored silty claystones containing more organic matter, interpreted as being deposited during more restricted, less open marine periods, or marine lowstands.
Several expanded cycles also contain distinct and sharp basal surfaces as well as sedimentological evidence of possible sea level changes, allowing sequence stratigraphic terminology to be applied, albeit at a very preliminary stage. For example, from ~392 to 421.62 mbsf in Subunit VA, a distinct sedimentary succession contains a well-defined basal surface (Fig. F16) and is interpreted as a stratigraphic sequence. This sequence is characterized by the following:
This succession may correspond to a sequence boundary, a transgressive system tract, a condensed section, and a highstand system tract, respectively.
Numerous lithologic successions are bounded by distinct surfaces in Subunit VC. Typically, the base contains a bioturbated surface with glauconite-filled burrows below the contact. This is overlain by abundant glauconitic dark greenish gray strata that lighten upward to light greenish gray strata. The lighter strata often contain abundant nannofossils. An example of this is in Subunit VC between 545.50 and 548.90 mbsf. It contains a sharp, although heavily bioturbated and irregular surface (interval 189-1171D-33R-3, 5-45 cm; Fig F17). The surface at 548.90 mbsf is interpreted as the sequence boundary; the dark-colored glauconitic unit above is the transgressive systems tract, and the lighter strata are assigned to the marine flooding unit (condensed section). Thinner, less-defined successions, containing less distinct surfaces and less contrast in color and glauconite abundance, could be interpreted as parasequences with the surfaces interpreted as flooding surfaces. However, these changes in lithology and biogenic material may also be caused solely by climatically related water-mass changes and may have little to do with changes in sea level (as implied by the sequence stratigraphic terms).
X-ray diffraction analyses were completed on 58 samples from Holes 1171A, 1171C, and 1171D (Fig. F18). The purpose of the clay mineral studies at Site 1171 was to (1) recognize the major variations of the paleoenvironment, as expressed by the clay mineral assemblages at a sampling interval of one every two cores and (2) compare the clay mineral assemblages with those recognized at Sites 1168 and 1170 drilled in similar water depths on the west Tasmania margin, the western STR, and with other areas of the Southern Ocean.
The clay minerals identified include smectite, random mixed-layered clays, illite, chlorite, and kaolinite. Based on the relative abundance of the clay minerals, five units were identified for Site 1171. These were designated Units C1 to C5 (Fig. F18).
Unit C1, which extends from the seafloor to 40 mbsf, has a clay mineral assemblage that consists of abundant smectite (45% to 85%) and common to abundant random mixed-layered clays (15% to 40%) accompanied by illite (0% to 15%) and kaolinite (0% to 5%). Unit C1 ranges in age from late Pliocene to Pleistocene and correlates to lithostratigraphic Subunit IA. Unit C2 extends from 40 to 273 mbsf and is characterized by abundant random mixed-layered clays (10% to 100%) and smectite (0% to 75%) accompanied by illite (0% to 20%) and kaolinite (0% to 15%). Because of the low clay content in the nannofossil oozes and chalks of lithostratigraphic Subunit IA and Unit II, percentages of random mixed-layered clays >50% are probably overestimated. Unit C2 ranges in age from early Oligocene to early Pliocene. Unit C3 extends from 273 to 340 mbsf and exhibits alternation of abundant smectite (25% to 95%) and illite (5% to 50%) accompanied by random mixed-layered clays (0% to 25%) and kaolinite (0% to 5%). Unit C3 is middle to late Eocene in age and correlates to lithostratigraphic Units III and IV. Unit C4 extends from 340 to 690 mbsf and is characterized by largely predominant smectite (85% to 100%) accompanied by minor amounts of illite and kaolinite (0% to 5% each) and sporadic random mixed-layered clays (~10%). Sporadic trace amounts of chlorite are present below 420 mbsf. Unit C4 is middle Eocene in age and correlates to lithostratigraphic Unit V. Unit C5 extends from 690 mbsf to the bottom of Hole 1171D. Unit C5 is characterized by kaolinite (5% to 25%) and illite (5% to 20%) increasing with depth and minor amounts of chlorite (0% to 5%). However, smectite is still abundant in this unit (50% to 85%), and its content decreases with depth. Unit C5 is late Paleocene to early Eocene in age and correlates to lithostratigraphic Unit VI.
The predominance of smectite and kaolinite indicates that warm climatic conditions and intense chemical weathering prevailed in sediment source areas. Smectite predominates in areas of low relief with alternating periods of precipitation and aridity, its formation being enhanced on basic volcanic substrates. Kaolinite is typical of warm areas with high precipitation during at least part of the year and good drainage conditions (Chamley, 1989; Weaver, 1989). Clay assemblages with predominant smectite were widespread in most oceanic areas off passive continental margins during the early Paleogene, including southern high-latitude locations of the Weddell Sea (Robert and Maillot, 1990) and Kerguelen Plateau (Ehrmann, 1991). Illite and chlorite are derived from erosion of substrates and characterize the environments of strong physical weathering. Such conditions prevail in cold and/or dry areas as well as in areas of steep relief, where active erosion prevents full development of soils (Chamley, 1989; Weaver, 1989). As illite and chlorite are associated with dominant smectite and abundant kaolinite, their presence in Paleocene to lower Eocene Unit C5 is probably not climatically induced but rather results from intense erosion of steep relief areas. Although significant amounts of kaolinite have already been observed in calcareous biogenic chalks of Maud Rise (Weddell Sea) in relation with the episode of extreme warmth of the Paleocene/Eocene boundary, it lasted for ~150 k.y. only (Robert and Kennett, 1994). Continuous occurrence of significant amounts of kaolinite from the late Paleocene throughout the entire early Eocene as observed at Site 1171 is unusual in the antarctic region.
Site 1171 is located in one of several transtensional sub-basins that developed on the STR from the Late Cretaceous to the Eocene as Australia moved northward (see "Background and Objectives"). The lithology of the corresponding prograded Paleocene and onlapping Eocene seismic units consists of clayey siltstone and silty claystone. The clay particles are therefore considered of local origin principally, supplied through runoff from adjacent emerged areas. Such emergence was almost certainly caused by the coeval tectonic uplift on nearby faults, associated with the culminating movements on the Balleny Fracture Zone. The clay assemblage would have been derived from weathering and erosion of adjacent steep continental areas occurring at the end of the Cretaceous to Eocene stage of tectonic activity (Royer and Rollet, 1997) that led to the final formation of local transtensional sub-basins in the late Paleocene (see "Tectonic Evolution" in "Discussion and Conclusions" in the "Leg 189 Summary" chapter). Identical clay assemblages in similar structural and sedimentological settings have been observed in the South Atlantic during warm Cretaceous intervals of tectonic activity related to early stages of ocean opening (Robert, 1987). In these areas, tectonic activity resulted in steep relief, whereas precipitation and drainage ensured chemical weathering and erosion of soils (kaolinite) and substrates (illite and chlorite).
The large predominance of smectite characterizes middle Eocene Unit C4, together with decreased contents of kaolinite and illite and the lack of chlorite. The siliciclastic sediment consists of claystone and silty claystone, and the clay particles are most probably derived from adjacent emerged areas as in Unit C5. Largely predominant smectite still points to warm climatic conditions with alternating humid and dry intervals. The clay assemblage of Unit C4 is very similar to that in coeval sediments of Site 1170 (see "Lithostratigraphy" in the "Site 1170" chapter). In the middle Eocene, clay assemblages dominated by smectite also prevailed in areas off the passive Antarctic margins on the Maud Rise (Robert and Maillot, 1990) and Kerguelen Plateau (Ehrmann, 1991). However, the increasing trend of smectite, beginning in the upper part of Unit C5, and correlative decrease of kaolinite, illite, and chlorite suggest that the continental relief decreased from the late early to the middle Eocene. A similar evolution of the clay assemblage 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). The transition to almost exclusively smectite in Unit C4 probably resulted from a stage of tectonic relaxation and subsidence after tectonic activity ceased in that part of the STR (see "Tectonic Evolution" in "Discussion and Conclusions" in the "Leg 189 Summary" chapter).
Distinct increases of illite (and random mixed-layered clays) in middle to upper Eocene Unit C3 are indicative of strong physical weathering and erosion of substrates in the source areas. Coeval increases of illite (up to 100%) also occurred at Site 1170 on the western STR (see "Lithostratigraphy" in the "Site 1170" chapter). However, such an occurrence of illite has not been observed in other areas of the middle and late Eocene Southern Ocean and Tasman Sea, where the clay assemblage consisted largely of smectite (Robert et al., 1985; Robert and Maillot, 1990; Ehrmann, 1991). It is inferred that illite in Unit C3 is derived from physical weathering and erosion of substrates in the emerged parts of the STR area. Lower illite content at Site 1171 than at Site 1170 (see "Lithostratigraphy" in the "Site 1170" chapter) also suggests that physical weathering and erosion were more intense in the western part of the STR. There, steep relief developed during the late middle to late Eocene stage of tectonic activity that preceded the separation of the STR from Antarctica, when strike-slip tectonism formed at the ridge of the Tasman Fracture Zone on the western edge of the STR (Royer and Rollet, 1997). This interval of tectonic activity is associated with a slight decrease of chemical weathering on the emerged Antarctic margins and expansion of cool antarctic waters in the Southern Ocean (Robert and Kennett, 1992; see "Lithostratigraphy" in the "Site 1170" chapter).
A drastic decrease of smectite is the most typical feature of Unit C2, which spans the Oligocene to early Pliocene. Significant amounts of illite and random mixed-layered clays result from erosion of poorly weathered substrates in the sediment's source area. The clay assemblage of Unit C2 at Site 1171 strongly differs from that at Site 1168 on the west Tasmania margin, where relatively high kaolinite content is found together with minor illite and an absence of random mixed-layered clays (see "Lithostratigraphy" in the "Site 1168" chapter). However, it closely resembles that of Unit C2 at Site 1170 (see "Lithostratigraphy" in the "Site 1170" chapter). The C2 clay assemblages at both sites of the STR most probably derived from a range of continental areas and expresses average weathering conditions on the emerged margins of the Southern Ocean.
Unit C1 of late Pliocene to Pleistocene age is marked by significantly increased smectite (up to 80%) and minor amounts of illite and kaolinite (<10%). Coeval sediments at Sites 1168 and 1170 (see "Lithostratigraphy" in the "Site 1168" and "Lithostratigraphy" in the "Site 1170" chapter) and in the adjacent Tasman Sea (Stein and Robert, 1986) contain significantly higher amounts of kaolinite (15% to 45%). Therefore, the clay assemblage of Unit C1 does not principally reflect either dust transport from Australia or weathering conditions of antarctic or subantarctic areas, as illite, chlorite, and random mixed-layered clays account for ~50% of the clay fraction in these areas (Robert and Maillot, 1983, 1990; Ehrmann, 1991). It may rather result from erosion of ancient sediments containing high proportions of smectite. Such sediments are frequent from the Cretaceous to the Paleogene in Southern Ocean margins and basins (Robert and Maillot, 1990; Ehrmann, 1991), including the STR (Fig. F18; see "Lithostratigraphy" in the "Site 1170" chapter). As Cretaceous and early Paleogene sediments outcrop on the flanks of strike-slip basins and ridges of the STR (see "Background and Objectives"), their erosion by submarine currents may have provided ancient reworked smectite to the late Pliocene-Pleistocene Unit C1 of Site 1171.