Hole 1166A penetrated to a maximum depth of 381.3 mbsf, and a succession of poorly sorted sands, silt(stones), clay(stones), and diamictons was recovered. These sediments are divided into five lithostratigraphic units (Figs. F3, F4). The uppermost unit (Unit I) is composed of diamicton and poorly sorted clay and silt with dispersed clasts and is divided into four subunits. Unit II is composed of claystone and diatom-bearing claystone with sand beds. Unit III is characterized by coarse to very coarse sand supported by a silty clay matrix. Unit IV consists of stratified sandy silt and sand with organic detritus and black highly carbonaceous clay. Unit V consists only of a 16-cm-thick bed of gray claystone with thin planar laminations.
Unit I is composed of poorly sorted sediments with rare to abundant dispersed clasts. Four subunits are identified (Figs. F3, F4). Subunit IA is restricted to the uppermost 2.74 m of the hole and consists of poorly sorted diatom-bearing sandy silty clay with scattered rock clasts. Subunit IB is composed of diamicton. Subunit IC is characterized by interbedded poorly sorted sandy silt with lonestones along with diatom and diatom-bearing clayey silt containing dispersed granules and sand grains. Subunit ID is composed of diamicton and poorly sorted clayey sandy silt with dispersed clasts. The calcium carbonate content of Unit I ranges from 0.07-1.15 wt% (see "Organic Geochemistry"). A major unconformity occurs between Subunit ID and Unit II (see "Biostratigraphy and Sedimentation Rates").
Subunit IA is composed of homogenous light brownish gray (5Y 4/1) diatom-bearing sandy silty clay with scattered pebble-sized rock clasts. Interval 188-1166A-1R-2, 72-73 cm, is composed of yellowish (5Y 4/3) diatom clay with spicules. Foraminifers are common in intervals 188-1166A-1R-1, 16-21 cm; 1R-1, 117-122 cm; and 1R-2, 9-12 cm (see "Biostratigraphy and Sedimentation Rates"). One large (3 cm × 4 cm) granite clast is present in interval 188-1166A-1R-2, 103-108 cm.
Subunit IB is composed of poorly sorted sediments with rare biogenic material. This subunit is characterized by interbeds of homogeneous dark gray (5Y 4/1) to black (5Y 2.5/2) clast-poor and clast-rich diamicton (Fig. F5). Spectrophotometer light reflectance was measured at 2-cm intervals to characterize the poorly sorted sediments cored at this site. The matrix content is predominantly clayey silt. The lithologies of the rock pebbles and rock granules are variable and include garnet-bearing quartzite, dolerite, diorite gneiss, granite, and rare sandstone. Garnet gneiss is a prominent component of pebbles and granules within this subunit. Clast-rich intervals include fibrous black organic clasts and pyrite granules. A large fractured granodiorite boulder is present within interval 188-1166A-3R-1, 0-31 cm. Planktonic foraminifers were identified in intervals 188-1166A-10R-1, 11-13 cm; 10R-CC; and 11R-1, 55-59 cm (see "Biostratigraphy and Sedimentation Rates"). A bone fragment was observed at Section 188-1166A-11R-1, 49 cm (see "Appendix"). Minor 1- to 2-cm-thick sand and granule beds are present within the diamicton in interval 188-1166A-11R-1, 71-84 cm.
Subunit IC is characterized by interbedded dark gray (5Y 4/1) sandy silt with lonestones and greenish gray (5G 5/1) diatom and diatom-bearing clayey silt containing dispersed granules and sand grains and a few pebbles (Figs. F6, F7). Greenish gray biogenic-rich intervals are slightly bioturbated. Contacts between the dark gray sandy silt and the biogenic-rich greenish gray clayey silt are sharp. Soft-sediment deformation and flame-like structures are present at lithological contacts and within intervals of dark gray sandy silt.
Subunit ID is composed of poorly sorted sediment with rare biogenic material. The upper portion of this subunit is characterized by interbeds of dark gray (5Y 4/1) clast-poor and clast-rich diamicton (interval 188-1166A-14R-1, 0 cm, to 14R-2, 60 cm; 123.00-125.10 mbsf). The lower portion of this subunit is composed of dark gray (5Y 4/1) clayey sandy silt with dispersed rock clasts (interval 188-1166A-14R-2, 60 cm, to 15R-2, 136 cm; 125.10-135.41 mbsf). Minor 1- to 2-cm-thick sand and granule beds are present within the diamicton at Sections 188-1166A-14R-1, 52 cm; 14R-1, 56 cm; and 14R-1, 65 cm. Clast lithologies are variable and include gneiss, granite, and diorite rocks. An isolated mudstone clast is present in Section 188-1166A-14R-5, 59 cm. Black fibrous organic clasts and pyrite fragments are common in the diamicton.
The uppermost biogenic-rich interval of Subunit IA records iceberg-turbated hemipelagic sediments deposited on the outer continental shelf during Holocene interglacial conditions similar to the present day (O'Brien and Leitchenkov, 1997) (see "Background and Objectives"). The presence of scattered pebble-sized clasts suggests deposition of IRD.
The diamicton and poorly sorted clayey sandy silt of Subunit IB suggest subglacial and proximal glaciomarine sedimentation. The rare broken and abraded microfauna contained within this subunit (see "Biostratigraphy and Sedimentation Rates") imply homogenization by subglacial deformation or redeposition of ice-proximal sediments. Intervals containing microfossils, shell fragments, and sand beds suggest current reworking and glaciomarine deposition. A major unconformity occurs at the base of Subunit ID (Section 188-1166A-15R-2, 136 cm; 135.41 mbsf) (see "Biostratigraphy and Sedimentation Rates"; Fig. F4). Below the unconformity, XRD data show shifts in the relative abundances of mica, illite, hornblende, kaolinite, and plagioclase (Fig. F7; see "X-Ray Diffraction Mineralogy"). These shifts within the XRD data suggest an increased amount of physically weathered terrigenous material within Unit I just above the unconformity. XRD data indicate the presence of gibbsite and kaolinite below the unconformity, within Unit II (Fig. F4). Both minerals indicate erosion of chemically weathered material formed in soils.
Soft-sediment deformation and flame-like structures at lithological contacts within Subunit IC suggest loading and rapid dewatering of the underlying sediments during abrupt fluctuations in the environmental setting. The interbedded sandy silt and biogenic-rich clayey silt of Subunit IC suggest fluctuations in the position of the ice margin. Periods of significant glacial retreat from the margin are marked by increased biogenic deposition. These periods are punctuated by deposition of pulses of coarse ice-proximal sediments. Lonestones within the sandy silt of Subunit IC may record deposition of IRD or glacial debris deposited beneath an ice shelf.
Unit II is composed of claystone and diatom-bearing claystone with decimeter-scale sand beds (Figs. F3, F4). The calcium carbonate content of Unit II ranges from 0.40 to 3.34 wt% (see "Organic Geochemistry").
The uppermost 17.24 m of Unit II (interval 188-1166A-15R-3, 8 cm, though 17R-1, 150 cm; 135.63-152.90 mbsf) is characterized by dark greenish gray (5GY 4/1) claystone and diatom-bearing claystone with numerous decimeter-thick light yellowish gray sand beds that contain dispersed rock clasts (<5 mm) (Fig. F8). Within interval 188-1166A-15R-3, 8 cm, to 17R-1, 150 cm (135.63-152.90 mbsf), the sands are poorly sorted. Moderate bioturbation is evident within both the claystone and sand beds. Diatoms are present in the lower portion of the unit in interval 188-1166A-15R-5, 67 cm, to 15R-6, 72 cm (see "Biostratigraphy and Sedimentation Rates"). Shell fragments are found within the greenish gray claystone beds in interval 188-1166A-16R-1, 64-65 cm, and at Section 16R-2, 43 cm. Trace amounts of glauconite are observed in a smear slide of interval 188-1166A-17R-1, 24 cm. Graded sand beds are present in intervals 188-1166A-15R-6, 59-65 cm, and 16R-2, 28-41 cm (Fig. F9). Rare fibrous, black organic clasts are observed within the sand beds.
The lowermost portion of Unit II (interval 188-1166A-17R-2, 0 cm, to 17R-4, 72 cm; 152.90-156.62 mbsf) is composed of centimeter-scale rhythmically interbedded sand and dark gray (5Y 4/1) claystone with dispersed clasts (Fig. F10). The clasts are angular to subangular and are present in both the claystone and the sand beds. Clast lithology is variable and includes metamorphic rocks, dark igneous rocks, and quartzite. Cross-stratification is found in the claystone beds (e.g., intervals 188-1166A-17R-2, 112-116 cm, and 17R-3, 9-14 cm). There is a change in lithology and color at the base of Unit II within a short interval of normal grading (interval 188-1166A-17R-4, 72-77 cm; 156.62-156.57 mbsf).
Unit II is a glaciomarine sequence that records proglacial sedimentation during a marine transgression. The marine transgression interpretation is based on an uphole transition from shallow-water glaciomarine facies to a dominance of diatom-bearing claystones deposited in an open marine environment. At Site 742 (Leg 119), Prydz Bay shelf sediments of similar or younger age (their Unit V) were sampled and the sediments were characterized by homogeneous diamictite (Shipboard Scientific Party, 1989c). The diamictite at Site 742 is interpreted as a proglacial or subglacial deposit. The sediments at Site 742 (Unit V) may be more similar to the lower section of Unit II at Site 1166 than their nomenclature suggests, because different sedimentological classification schemes were used during Legs 119 and 188 (see "Lithostratigraphy" in the "Explanatory Notes" chapter) (Shipboard Scientific Party, 1989a).
Unit III is composed of matrix-supported gray (5Y 5/1) coarse to very coarse sand with abundant granules (Fig. F11). The matrix is composed of silty clay. The calcium carbonate content ranges from 0.35-8.44 wt% (see "Organic Geochemistry"). An isolated mud lamina is present within the uppermost portion of this unit at Section 188-1166A-17R-4, 81 cm. The remainder of Unit III appears ungraded. Rock clasts (>5 mm) are widely dispersed throughout this unit and are predominantly quartzite and garnet-bearing quartzite. Few fibrous, black organic fragments are present (see "Appendix"). Sand and silt grains are angular, whereas granules and pebbles are rounded. A sharp contact between overlying coarse sands and underlying very coarse sands occurs at Section 188-1166A-18R-2, 66 cm (163.16 mbsf), in the uppermost part of this unit. Unit III contains two cemented intervals: the first (interval 188-1166A-22R-1, 101-111 cm; 200.51-200.61 mbsf) is characterized by cemented medium to coarse olive-gray (5Y 5/2) sandstone with thin yellowish and dark brown color banding; the second (interval 188-1166A-26R-2, 70-86 cm; 240.10-240.26 mbsf) consists of cemented dark gray silty fine sandstone containing calcite cement.
The lowermost portion of Unit III is deformed and folded by soft-sediment deformation (interval 188-1166A-27R-1, 83 cm, through 29R-1, 37 cm; 248.43-267.17 mbsf) (Fig. F12). This interval is predominantly gray (5Y 5/1) in color; however, color banding (dark brownish gray, yellowish-bluish gray, light gray, and dark gray) is evident within deformed and folded regions. A deformed bed of black organic material is present in interval 188-1166A-28R-1, 36-60 cm (257.56-257.80 mbsf) (Fig. F13), and angular organic fragments are found within interval 188-1166A-28R-1, 59-62 cm (257.79-257.82 mbsf).
The coarse-grained sands of Unit III record deposition on an alluvial plain or delta. These sands are structureless and have a uniform silty clay matrix. The deformed beds of organic material within the lower portion of Unit III record some reworking of material from organic horizons within the alluvial plain or delta. A sequence of similar appearance was drilled in the last 2 m at Site 742 during Leg 119. Interlayered sands and siltstones and interbedded carbonaceous siltstones in the lowermost 2 m of Site 742 are also characterized by soft-sediment deformation (Shipboard Scientific Party, 1989c). These deformed sediments from Site 742 were tentatively interpreted as fluvial and possibly lacustrine; however, a sequence comparable to the homogeneous coarse sands in the upper two-thirds of Unit III at Site 1166 was not sampled at Site 742. Unit III may have been deposited in a proglacial outwash system similar to that of present-day southern Iceland and Alaska (Eyles and Eyles, 1992), or the sands may record a preglacial alluvial plain or braided delta.
The topmost interval of Unit IV (interval 188-1166A-30R-1, 4-14 cm; 276.44-276.54 mbsf) is homogeneous black (5Y 2.5/1) highly carbonaceous clay. This bed is underlain by stratified sandy silt and sand with organic detritus, with rare to moderate bioturbation (Fig. F14). The stratified interval is characterized by decimeter-scale color alternations from olive brown (2.5Y 4/4) to dark gray (N4) to black (2.5Y N2). The sandy silt contains abundant fine to medium sand-sized mica (Fig. F5) and rare pyrite nodules. Cross-lamination and disturbed and discontinuous laminae are common. A segment of structureless dark gray to black sandy silt with abundant sand-sized mica grains is present in interval 188-1166A-31R-1, 57-139 cm (286.57-287.39 mbsf). Siderite-cemented nodules are present within intervals 188-1166A-32R-1, 82-90 cm (296.12-296.20 mbsf); 32R-1, 96-100.5 cm (296.26-296.30 mbsf); and 32R-1, 114-117 cm (296.44-296.47 mbsf) (see "Physical Properties"). The calcium carbonate content within this unit ranges from 0.33-3.66 wt%. Organic carbon values of up to 5.2% are found in the carbonaceous bed (see "Organic Geochemistry"). Turonian palynomorphs were found in this unit (O'Brien et al., in press).
Unit IV records deposition within a restricted marine or lagoonal environment. Slow sedimentation rates under reducing conditions are indicated by the high organic carbon content and the presence of iron sulfides and preserved laminae. Disturbed and discontinuous laminations are common and may represent possible fluid escape structures.
Unit V consists of gray (5Y 5/1) claystone with thin light gray laminations and gray fine sandy silt. Only 16 cm of sediment (interval 188-1166A-37R-CC, 0-0.20 cm; 342.80-342.96 mbsf) was recovered from this unit.
It is premature to categorize the depositional environment based on sedimentological observations from 16 cm of core. Postcruise pollen analysis of Site 1166 core-catcher sediments indicates a Turonian age for these sediments. From regional seismic stratigraphic correlations, Unit V was recovered below a seismic unconformity that can be traced to Leg 119 Site 741, 110 km away. At Site 741, gray claystone deposits lithologically equivalent to Unit V (Site 1166) were observed. Pollen analysis of the fine-grained light gray siliciclastic sediments at Site 741 indicate an Early Cretaceous (Albian?) age. The sediments from Site 741 were interpreted as sediments deposited on a low-relief alluvial plain (Shipboard Scientific Party, 1989b), and a similar setting is likely for Unit V at Site 1166, although Unit V at Site 1166 is younger than the sediments at Site 741.
Smear slides were
routinely prepared from the fine-grained matrix of silt and sand lithologies
downhole. Diatoms, radiolarians, and sponge spicules are present only from 0 to
151.64 mbsf. Quartz is ubiquitous downhole, and no major fluctuations are
evident. Heavy minerals such as amphibole are omnipresent in amounts of 
5%,
without a recognizable trend in percentages. Feldspars were identified in a
number of slides as minor components, but no significant trend emerged (e.g.,
Samples 188-1166A-6R-CC [47.30 mbsf]; 15R-6, 64 cm [140.69 mbsf]; 30R-1, 10 cm
[276.50 mbsf]) (see "X-Ray Diffraction
Mineralogy") (Fig. F4).
Coal and plant fragments are found in a few slides below 200 mbsf (e.g., Samples
188-1166A-22R-1, 62 cm [200.12 mbsf]; 23R-1, 69 cm [209.79 mbsf]).
A significant trend is discernible in mica (particularly muscovite) percentages. A plot (Fig. F15) of the combined percentages of muscovite and biotite shows that the total abundance remains <5% to a depth of 200 mbsf. In contrast, percentages increase sharply to the range of 20%-35% below 270 mbsf. The large size (up to sand size) of the mica flakes invariably leads to overestimates of mica abundance. Nevertheless, the overall trend is clear and is confirmed by XRD and chemical analyses (see "X-Ray Diffraction Mineralogy" and "Inorganic Geochemistry"). A similar increase in opaque minerals below 270 mbsf is indicated by a few slides.
The high diatom content in the first few meters of Unit I (Subunit IA) reflects the marine origin of the sediment. Similarly, the marine or glaciomarine nature of the sediments of Subunit IC and Unit II is indicated by relatively high (up to ~20%) percentages of diatoms. This relatively high diatom content also causes the low grain densities and increased porosity of Unit II (see "Physical Properties"). The terrestrial (fluvial) to lagoonal depositional environments of Units III (lower part) through V are characterized by high mica content. Muscovite flakes are conspicuous and suggest derivation from granitic and/or gneissic bedrock lithologies in the source areas of the Lambert Glacier drainage system.
At Site 1166, 32 samples were analyzed for bulk mineralogy and six samples were analyzed for clay minerals. The six clay-mineral samples were taken from clay and claystone, overlying clast-poor diamicton and clayey silt with dispersed clasts, and from two clayey matrix-rich sandstones in lower portions of the hole. The bulk mineral samples are primarily composed of quartz, plagioclase, K-feldspar, and a mixture of clay minerals, plus minor hornblende (Fig. F4). Total clay content in the upper 135 mbsf (Unit I) is lower than in deeper portions of the hole (Units II-IV); however, poor recovery, especially between 40 and 60 mbsf, makes determination of downhole trends difficult. The interval between 105 and 136 mbsf in Unit I has a slightly higher total clay content than sediments from the upper part of the hole. From 136 mbsf downhole, the relative abundance of plagioclase decreases as the abundance of clay minerals and quartz increases, except for a claystone-rich interval between 135.6 and 152.9 mbsf, where total clay content is higher compared to total quartz content. Downhole from 152.9 mbsf, the sediments show an increase in quartz content and plagioclase becomes a minor component; however, K-feldspar content remains constant to the bottom of the hole.
Four samples were taken from Cores 188-1166A-12R through 15R to determine clay mineralogy changes between Unit I and Unit II across the Neogene/lower Oligocene unconformity. A very fine grained diatom-bearing silty claystone at 114.73 mbsf in Unit I (Sample 188-1166A-13R-1, 143-145 cm) contains illite, very minor kaolinite and some clay-sized quartz, plagioclase, and K-feldspar (Fig. F16). The clay fraction of the overlying clast-poor diamicton (Sample 188-1166A-12R-1, 45-46 cm) has similar clay-mineral distribution to the claystone, but minor chlorite is also present and the illite content is relatively low (Fig. F16). The clayey silt with dispersed clasts of Unit I at 127.20 mbsf (Sample 188-1166A-14R-3, 120-121 cm) exhibits similar clay minerals as the sediments above, except the relative abundances of kaolinite and illite are higher and minor amounts of chlorite are present. In Unit II, an olive-green claystone directly below the upper Pliocene/lower Oligocene unconformity (Sample 188-1166A-15R-3, 24-25 cm) shows a distinctive change in clay-mineral content as kaolinite forms a major component with some mixed-layer clays and illite. Aluminum hydroxide is present in the form of gibbsite. After heating to 550°C, the kaolinite structure collapsed and gibbsite lost its water content and formed a new phase, as evidenced by the disappearance of distinctive peaks for these minerals on the diffractogram (Fig. F17).
The silty clay matrix of gray sand in Unit III at 219.77 mbsf (Sample 188-1166A-24R-1, 107-108 cm) contains predominantly kaolinite and a minor amount of clay-sized quartz and K-feldspar (Fig. F8). In the dark gray sandy silt at 295.55 mbsf (Sample 188-1166A-32R-1, 25-26 cm), the predominant clay mineral is kaolinite (Fig. F18). Minor gibbsite and clay-sized quartz are also present.
The presence of kaolinite and gibbsite in Unit II marine sediments suggests that the source rocks for these sediments were chemically weathered under a warm, temperate, humid climatic regime. In addition, an abundance of K-feldspar and absence of plagioclase in Unit II and older sediments may indicate chemical weathering of the source rocks. Increased humid conditions in East Antarctica are thought to have prevailed during the late Paleocene to early Eocene (e.g., Robert and Maillot, 1990). An absence of kaolinite in the upper Pliocene section suggests that less chemical weathering occurred in the sediment-source area. By the late Pliocene, most of the chemically weathered crust had been transported to depositional sites and less weathered or unweathered source rocks were exposed. Furthermore, the good preservation of chemical weathering indicators such as kaolinite and gibbsite, along with the absence of other clays in Unit II, may suggest that these components were not transported long distances or that proglacial conditions prevailed during Unit II deposition in the Lambert Glacier drainage system. Cold climatic conditions are characterized by illite- and chlorite-dominated clay mineral assemblages observed in the upper Pliocene.
Despite poor recovery (18.6%) at Site 1166, sediments cored at this site reveal important insights into the sedimentary history of the Prydz Bay outer continental shelf. Unit I records a period of proximal and distal glaciomarine sedimentation on the shelf from the late Pliocene to Holocene. The youngest sediments recovered record interglacial hemipelagic sedimentation subjected to subsequent reworking by iceberg turbation. Periods of proximal glaciomarine and possibly subglacial conditions are recorded within Subunits IB and ID. Intervals of glacial retreat and advance can be identified within the glaciomarine sediments of Subunit IC.
A major unconformity exists between the Pliocene sediments in Unit I and the underlying lower Oligocene/upper Eocene sediments of Unit II. A marine transgression is recorded in the proglacial clays and sands of Unit II, and this early Oligocene- to late Eocene-age event may have been close to the time of the onset of continentwide ice-sheet development in Antarctica. The initial transgression is likely recorded in the uppermost sands of Unit III (see "Downhole Measurements"), culminating in a maximum flooding surface in Unit II. Homogeneous diamictites of similar or slightly younger age (Oligocene to Eocene) than Unit II sediments were penetrated at Site 742 during Leg 119 (Shipboard Scientific Party, 1989c). The diamictites at Site 742 were interpreted as a proximal glaciomarine deposit (Shipboard Scientific Party, 1989c), which is consistent with a proglacial shallow marine interpretation for the sediments observed at Site 1166.
The fluvial-deltaic sands of Unit III may record a downhole transition to sediments deposited in warmer, more temperate conditions in Prydz Bay. The deformed beds of organic material within the lower portion of Unit III resemble the deformed interlayered sands and siltstones drilled in the lowermost 2 m at the base of Site 742 during Leg 119. If the lowermost 2 m at Site 742 is equivalent to the lowermost part of Unit III at Site 1166, then the overlying thick coarse sands sampled at Site 1166 are missing at Site 742. The coarse sands at Site 1166 may indicate a preglacial or early glacial alluvial plain or braided delta. Alternatively, the Unit III deposits recovered at Site 1166 may record a more complete distal glacial outwash system.
Unit IV records deposition within a restricted marine or lagoonal environment. Palynomorphs indicate a Late Cretaceous (Turonian) age. The 16 cm of recovered sediments within Unit V makes interpretation of depositional processes and paleoenvironment speculative; however, the laminated gray claystone of Unit V resembles Lower Cretaceous preglacial fluvial deposits recovered at Site 741 during Leg 119 (Shipboard Scientific Party, 1989b).
