LITHOSTRATIGRAPHY

Site 1165 is divided into three lithostratigraphic units (Fig. F5). Lithostratigraphic subdivision is based on a combination of the following data (Fig. F6): (1) visual core description, (2) biogenic and mineralogic composition from smear slides, (3) XRD mineralogy, (4) X-ray photographs, and (5) spectrophotometer reflectance.

Unit I, the uppermost lithostratigraphic unit, consists of structureless brown diatom clay with minor diatom-bearing greenish gray clay with lonestones (Fig. F7A). The upper part of this unit contains foraminifers. The underlying Unit II is composed of interbedded, structureless greenish gray diatom clay and dark gray diatom-bearing clay (Fig. F7B). The uppermost and lower parts of Unit II contain dispersed sand grains, granules, and lonestones. The lowermost unit, Unit III, consists of interbedded, dark gray, thinly bedded, planar-laminated claystones with abundant silt laminae and thin beds of greenish gray bioturbated claystone containing rare dispersed sand grains and granules (Fig. F7C, F7D). Bioturbation is pronounced in Unit I and the upper part of Unit II but decreases in the lower part of Unit II; bioturbation in Unit III is rare.

Intervals: Cores 188-1165B-1H through 6H and Core 188-1165C-1R

Depth: 0-63.8 mbsf

Age: Pleistocene to early Pliocene.

Unit I is characterized by a transition from grayish brown diatom ooze to structureless yellowish gray to brown diatom clay that becomes increasingly interbedded with structureless greenish gray diatom clay downhole. Four facies are recognized in Unit I (Table T3). Facies I-1 consists of structureless yellowish gray (2.5Y 5/2) to brown (10YR 4/2) diatom clay. Siliceous microfossils are abundant and foraminifers are common. Facies I-2 consists of structureless greenish gray (5GY 5/1) diatom clay interbedded with dark grayish brown (2.5Y 4/2) to dark gray (2.5Y 4/0) diatom clay (Fig. F8). Silt laminae and stringers are present in Facies I-2 between 26.6 and 33.5 mbsf (Fig. F9). Facies I-3 consists of dark grayish brown (2.5Y 4/2) diatom ooze. Facies I-4 consists of yellowish gray to brown diatom clay similar to Facies I-1 with scattered mud clasts and sharp flamelike contacts (e.g., Fig. F10; Sample 188-1165C-1R-6, 20-90 cm), and in one bed (Sample 188-1165B-6H-5, 70-75 cm), reworked pseudomorphs of Paleogene foraminifers are present (see "Biostratigraphy and Sedimentation Rates").

In both Facies I-1 and I-2, a reddish brown color (5YR 5/3) with diffuse boundaries is observed in some decimeter-scale intervals. Reddish brown and dark brown mottling is also common. Bioturbation is pervasive, but individual burrows can be identified locally. Diatom ooze (Facies I-3) is present downhole only from 0 to 2.5 mbsf. Based on smear slides, siliceous microfossils (primarily diatoms), sponge spicules, and radiolarians are abundant throughout Unit I. Locally, foraminifers constitute between 5% and 15% of total sediment components in the upper 13.3 m of Unit I. Below 20 mbsf, the diatom-bearing clays contain more silt and the biogenic components gradually decrease downhole. The silt grains are predominantly composed of quartz with minor feldspar and opaques. Dispersed sand grains, granules, and pebble-sized lonestones are present throughout Unit I. Clast lithologies are mainly granite, gneiss, and dolerite.

Interpretation

Facies I-1 has a high biogenic component, is bioturbated, and lacks sedimentary structures that would indicate current activity; therefore, it is a hemipelagic sediment that drapes the seafloor. Facies I-2 is probably of similar hemipelagic origin, but minor current activity during deposition caused the observed laminations and interbedding of different colored beds. Facies I-3 has a high concentration of biogenic material (e.g., diatoms), indicating that it is a pelagic deposit. Facies I-4 displays soft-sediment deformation features, mud clasts, and irregular bedding contacts indicative of mass movement. The presence of reworked pseudomorphs of Paleogene foraminifers, probably derived from erosion of the adjacent continental shelf, suggests mass movement originally on the upper slope or continental shelf break.

Lonestones and dispersed sand grains indicate ice rafting during deposition of Unit I. The increase in biogenic components uphole throughout Unit I suggests a gradual reduction in the relative input of fine siliciclastic material over time. The patchy presence of foraminifers may indicate episodic increases in foraminifer population in the overlying waters or occasional changes in the carbonate compensation depth (CCD).

Interval: Cores 188-1165B-8H through 36X

Depth: 63.8-307.8 mbsf

Age: middle to late Miocene

Subunit IIA

Interval: Cores 188-1165B-8H through 20X

Depth: 63.8-160.1 mbsf

Age: middle to late Miocene

Subunit IIB

Interval: Cores 188-1165B-20X through 28X

Depth: 160.1-252.4 mbsf

Age: middle to late Miocene

Subunit IIC

Interval: Cores 188-1165B-30X through 36X

Depth: 252.4-307.8 mbsf

Age: middle to late Miocene

Unit II, which begins at 63.8 mbsf, is defined by the absence of brown clay at the top of Core 188-1165B-8H. Unit II is characterized by alternations of two main facies. A third facies is recognized in a few beds. Facies II-1 consists of structureless, homogeneous greenish gray (5GY 5/1) diatom clay, whereas Facies II-2 consists of mostly dark gray (5GY 4/1) diatom-bearing clay that shows local thin color banding that may be parallel lamination (Fig. F11). Some intervals of Facies II-2 contain scattered silt laminae, and many lower boundaries of Facies II-2 are sharp. Facies II-1 contains higher amounts of siliceous microfossils and floating sand grains and pebbles than Facies II-2 (Fig. F12). Facies II-1 has rare darker green laminae and up to 1-cm-thick clay beds that contain glauconite, scattered fish teeth, and a higher proportion of diatoms than surrounding sediment. XRD measurements demonstrate that the dominant clay minerals in Facies II-1 are illite, kaolinite, minor chlorite, and illite-smectite mixed-layer clays (see "X-Ray Diffraction Mineralogy"). Facies II-2 has clay composition similar to Facies II-1 but lacks illite-smectite mixed layer clays. Silt grains identified in smear slides include quartz, feldspar, and opaques. Sand and silt-sized glauconite grains are present in trace amounts down to 213.9 mbsf.

Facies II-3 is represented by several 15- to 40-cm-thick beds of nannofossil chalk (e.g., Samples 188-1165B-20X-5, 114-131 cm; 24X-3, 68-105 cm; and 24X-CC). The chalk beds have sharp bases and pass gradually up into Facies II-1 with increasing diatom abundance and decreasing proportions of nannoplankton. Paleontological examination of the best-developed bed of Facies II-3 in Sample 188-1165B-24X-CC reveals an almost monospecific deposit of Reticulofenestra perplexa (see "Biostratigraphy and Sedimentation Rates"). The sediment color changes from white at the base to green gray at the top (5BG 7/1 to 5G 7/1).

The difference in proportions of the two main facies can be used to subdivide Unit II into three subunits. Subunit IIA starts at 63.8 mbsf and is dominated by Facies II-1. The thickness of Facies II-1 beds varies from 1.5 to 5 m, whereas the Facies II-2 beds are <1 m thick. Subunit IIA contains up to 1-cm-thick stiff clay beds in Facies II-1 (e.g., Samples 188-1165B-13H-3, 88-89 cm, and 96-97 cm) and rare planar silt laminae within Facies II-2 (e.g., Sample 188-1165B-13H-2, 80 cm; Fig. F11).

The top of Subunit IIB is placed at 160.1 mbsf, where a general decrease in siliceous microfossils, compared to Subunit IIA, occurs. In Subunit IIB, Facies II-1 and II-2 are of similar thickness, generally 1-2 m. In the upper part of Subunit IIB, two beds of Facies II-3 are present (Fig. F13), and bioturbation is moderate throughout the subunit. The silt fraction of Subunit IIB contains quartz, feldspar, and opaques, and up to 2% garnet. Subunit IIB has fewer pebble-sized lonestones, floating granules, and sand grains than Subunits IIA and IIC.

The Subunit IIB/IIC boundary occurs at 252.4 mbsf, based on a downhole increase in coarse material (>2 mm) and an increase in the proportion of Facies II-1. Some Facies II-2 beds in Subunit IIC exhibit dispersed sand grains, granules, and lonestones and rare planar silt laminae (Fig. F14). Facies II-2 interbeds in this subunit have sharp lower boundaries, and some beds show thin color banding. Overall bioturbation is moderate. The proportion of pebble-sized lonestones, granules, and sand grains in Subunit IIC is similar to that in Subunit IIA and is higher than in Subunit IIB. The main clast lithologies are granite, biotite gneiss, dolerite, and black, high-grade metamorphic rock types.

Interpretation

Facies II-1 has a high biogenic component and contains minimal evidence of current sorting. The laminae containing glauconite and fish teeth indicate periods of very slow terrigenous sedimentation. Therefore, Facies II-1 is interpreted as a hemipelagic sediment. Facies II-2 is a similar hemipelagic sediment, but its higher terrigenous content and silt laminae suggest higher siliclastic input and current activity than Facies II-1. The greater proportion of lonestones and floating sand grains in Facies II-1 than in Facies II-2 reflects a higher relative concentration of IRD. The higher IRD content in Facies I-1 may be a result of lower input of fine terrigenous sediment, so the IRD is less diluted (cf. Barker, Camerlenghi, Acton, et al., 1999). Facies II-3 is a pelagic sediment formed by an influx of nannoplankton caused by a brief period of high productivity and depression of the CCD (see "Biostratigraphy and Sedimentation Rates"). The uphole increase in the proportion of Facies II-1 relative to Facies II-2 from Subunits IIC to IIA indicates a progressive decrease in fine siliciclastic input and bottom-current activity.

Interval: Cores 188-1165B-37X through 76X; 188-1165C-2R through 35R

Depth: 307.8-999.1 mbsf

Age: early Miocene

The Unit II/Unit III boundary is placed at 307.8 mbsf, based on the first downhole occurrence of thinly bedded planar-laminated claystone beds at 307.8 mbsf, a change in XRD mineralogy between 303.3 and 312.7 mbsf, and a change in composition based on smear-slide observations between 301.15 and 307.8 mbsf (Fig. F6). Unit III is characterized by an interbedding of two facies types, which differ in color, composition, and bedding characteristics. Facies III-1 is composed of dark gray (5GY 4/1) thinly bedded planar-laminated clay and claystone that change color to very dark gray or black (N4) below 893.6 mbsf. Facies III-1 contains abundant silt laminae. Facies III-2 consists of greenish gray (5G 4/1) bioturbated, structureless clay and claystone and diatom-bearing clay and claystone with dispersed coarse sand grains and rare granule to small pebble-sized lonestones (Fig. F15). Planar to wavy lamination is present at the bottom of some units in Facies III-2.

The dark gray claystone of Facies III-1 becomes increasingly fissile with depth. Beds are generally <1 cm thick, and silt partings are present along bedding planes, where horizontal planar fractures occur (Fig. F16). Well-preserved sections of core show millimeter-scale silt laminations within the clay beds. Centimeter-scale, structureless, coarse sand beds are present locally between clay laminae (e.g., Samples 188-1165C-30R-5, 35 and 40 cm). Bioturbation is rare and is found in discrete decimeter-sized intervals.

Light-colored silt laminae are a conspicuous feature of Facies III-1. They are mostly flat or wavy 1- to 2-mm-thick single laminae but are also found as packets of several laminae of variable thickness, small lenses up to 3 mm thick, and isolated ripples with loaded bases (Fig. F17). The abundance of silt laminae varies through the section with some 1- to 3-m-thick intervals having several laminae per centimeter. On average, between 150 and 200 laminae are counted per meter, but laminae may be as frequent as 50 per decimeter. Many cross-laminated silt ripples are present. Ripples are more common below 673 mbsf than above. Silt laminae and silt ripples that are larger than elsewhere in the core are most common between 788 and 807 mbsf. Loading of silt ripples into underlying clay laminae is common. Between 788 and 807 mbsf, the thickness of the silt ripples increases up to 1 cm (Fig. F17). In this interval, some cores also display convergence between packages of silt laminae.

Smear slides of Facies III-1 show that siliceous microfossils are rare, and seem to disappear completely below ~600 mbsf. Below 841.5 mbsf, white-colored, calcite-containing laminae are present and sections of core become increasingly carbonate cemented. This carbonate appears to be diagenetic. Two deformed intervals are present, one in Core 188-1165C-10R between 755.2 and 755.6 mbsf and another in Cores 188-1165C-18R and 19R between 828.4 and 839.4 mbsf. The deformation consists of conjugate microfaults, folded silt laminae, and inclined bedding (Fig. F18). Below 893.6 mbsf, a change in color toward darker (N4) planar-laminated claystones occurs and the fracture patterns are curved as well as planar. Between ~675 and 850 mbsf, reflectance values intensify. Over the same interval, OC values increase (see "Organic Geochemistry"), suggesting that sediment geochemistry may affect the reflectivity.

In Facies III-2, laminae are rare and are generally present at the bottom of the beds in which they are found. Bioturbation increases upward. The thickness of the greenish gray intervals is usually <1 m. Upper contacts are generally sharp and in some intervals erosive, with sand grains concentrated as a lag along the contact with overlying Facies III-1 claystone. Arenaceous deep-water benthic foraminifers (see "Biostratigraphy and Sedimentation Rates") and angular mud clasts are present in this facies below 799.5 mbsf (Fig. F19). The siliceous microfossil content is very low and decreases downhole. Some of the greenish gray intervals reveal higher concentrations of up to coarse sand-sized material (Sample 188-1165C-15R-2, 36-37 cm). Several beds near the bottom of the sequence have sharp basal contacts with angular mud clasts along the base.

In both Facies III-1 and III-2, individual horizontal burrows ~0.5 cm in diameter (Zoophycos) can be distinguished as well as clusters of millimeter-sized burrows (Figs. F20, F21). The silt laminae and ripples of Unit III are quartz rich. Small amounts of quartz and feldspar are present within the clay. At 646.1 mbsf (Core 188-1165B-73X), a smear slide displays the presence of highly angular quartz grains (Fig. F22). Amphibole is present in trace amounts with a peak abundance of 2%-7% between 675 and 802 mbsf. Lonestone abundance in Unit III is low and decreases downhole. Pebble-sized lonestones are absent in Facies III-1 and are rare in Facies III-2 beds (Fig. F23). Lonestone composition is mainly dolerite, diorite gneiss, and mudstone. Below 492.2 mbsf, chert beds and nodules are present.

Interpretation

Facies III-1 has many characteristics of contourites (e.g., Gonthier et al., 1984; Rebesco et al., 1997; Heezen et al., 1966; Stow and Piper, 1984). The presence of packages of silt laminae within the claystone reflects the increase in strength of relatively slow-moving, near-continuous bottom currents. Other features characteristic of contourites are lag surfaces that record increased bottom-current velocities and extensively burrowed intervals that reflect comparatively low deposition rates (Wetzel, 1984). The intervals with microfaults and folded laminae were probably remobilized as local slumps. The convergence between packets of silt laminae may indicate deposition on low-angle bedforms or filling of scours.

Facies III-2 shows a high level of bioturbation with only rare laminae, indicating slow hemipelagic deposition accompanied by low current activity. The presence of reworked arenaceous deep-water benthic foraminifers confirms the low current activity. The sand grains and rare pebbles in this facies were either introduced by ice rafting or by sediment gravity flows originating on the upper slope. The few beds with sharp bases and mud clasts are probably local debris flows produced by slumping.

Smear slides were prepared from all major and minor lithologies comprising the sedimentary column of Site 1165. In the lower half of the column, smear slides were replaced by strewn slides, prepared from material scratched from the surface of the lithified sediment. The vast majority of the sedimentary column consists of fine-grained terrigenous material, largely in the clay size range, with various admixtures of silt-sized materials, with the addition of biogenic opal in the upper half of the sediment column (Fig. F12). Foraminifers are found in smear slides only in the first 13.3 m of Hole 1165B. The clay-sized components cannot be sufficiently resolved for analysis under the petrographic microscope but consist of both clay and primary minerals, according to XRD analyses (see "X-Ray Diffraction Mineralogy"). Silt-sized components are mainly quartz, but plagioclase, biotite, amphibole, and other heavy minerals are also present. The silt-sized quartz grains are angular to subangular, as are the few plagioclase grains. Plagioclase grains appear to be remarkably fresh and without any associated alteration products. Opaque minerals are present throughout but seem to be concentrated in the darker lithologies. Glauconite could be identified in a few smear slides, prepared from greenish gray clay (e.g., Sample 188-1165C-23R-6, 36 cm; 882.16 mbsf). These slides also contain a few fish teeth.

Silt persists throughout the entire sediment column; however, in Units I and II, where the contrast between "lighter" and "darker" lithologies is well developed, silt seems to form a higher percentage in the "green" sediment varieties compared to the "dark" ones. Individual silt laminae are particularly conspicuous in Unit III. Smear slides from these laminae reveal the predominance of well-sorted silt-sized quartz, in addition to clay-sized materials that could not be further identified. Near the bottom of Hole 1165C, white, thin (millimeter thick) laminae consist of micrite.

A graph representing the major biogenic opal components of the smear slides prepared from Holes 1165B and 1165C clearly illustrates the trend of diminishing biogenic opal with depth (Fig. F6). Units I and II (0-307.8 mbsf) show relatively high but variable percentages (between 1% and ~30%) of diatom frustules and sponge spicules. At the top of Unit III, the biogenic opal content decreases sharply to an estimated average of ~8%, although major fluctuations in biogenic opal content are clearly identifiable on smear slides. At ~490 mbsf, a further very sharp reduction in opal is evident. This reduction is associated with the first appearance of chert as a nodule or layer at 492.2 mbsf. The association between the disappearance of biogenic opal and the first appearance of identifiable chert probably reflects a diagenetic process (see "Inorganic Geochemistry") and sets depth limitations on the interpretation of siliceous microfossils as paleoproductivity proxies at this site.

Calcium carbonate in the form of micrite appears on smear slides as an admixture to other sediment types, as matrix in thin laminae of silt and clay-sized components, or as pure micrite either in laminae or near-vertical veinlets (in the lower portion of Unit III). It is likely that the micrite is a replacement after nannofossil ooze, but further studies are needed to test this hypothesis.

Although smear slides are invariably biased against coarser (sand sized) components, the overall impression gleaned from the smear slides remains one of relative uniformity of the depositional environment, with grain-size variations largely confined to the silt and clay size ranges (sand-sized components are described in the barrel sheets).

At Site 1165, 127 samples were analyzed for bulk mineralogy, and four samples were taken from the greenish gray and dark gray diatom-bearing clay beds of Facies II-1, II-2, III-1, and III-2 for clay mineralogy analysis. The bulk samples are primarily composed of quartz, calcite, plagioclase, K-feldspar, and a mixture of clay minerals, as well as minor amounts of hornblende and pyrite (Fig. F6). Within the first 140 mbsf, total clay content is relatively high compared to the lower depths. In Subunit IIA, the greenish gray diatom-bearing 1-cm-thick clay beds (Facies II-1) within the 107-110 mbsf interval have a lower relative abundance of feldspars, whereas the clay mineral content is slightly higher. Below 307.8 mbsf (Unit III), the relative abundance of plagioclase increases and the abundance of clay minerals decreases. Facies III-2 contains constant proportions of plagioclase, K-feldspar, and quartz throughout the interval from 307 to 999.1 mbsf.

In Cores 188-1165B-14H (122.01 and 122.20 mbsf) and 37X (315.51 and 317.01 mbsf), four samples were taken from depths where alternations in color suggested a possible change in clay mineralogy. In each sample, the clay consists of illite, kaolinite, and minor chlorite. However, the greenish gray sediments (Facies II-1 and Facies III-2) exhibit a poorly defined phase with a broad and variable diffraction peak between 6° and 9°2 that shifted after glycolation (Figs. F24, F25). This phase is tentatively identified as a minor variable amount of mixed-layer clay, most likely smectite-illite. After heating to 550°C, kaolinite was removed from all samples. Resultant relative abundances of clay minerals vary slightly. The greenish bed (Facies II-1) at 122.2 mbsf contains minor chlorite and has relatively smaller amounts of illite than the dark gray interval immediately above it at 122.01 mbsf (Fig. F24). A greenish clay bed of Facies III-2 at 315.51 mbsf contains illite, minor kaolinite, and mixed-layer illite-smectite, which are not identified in the darker Facies III-1 layer immediately below them at 317.01 mbsf (Fig. F25).

The fine-grained nature of the sediments at Site 1165 facilitates identification of lonestones (isolated clasts >2 mm). Lonestone lithologies are variable and include igneous intrusive (granite), extrusive (dolerite), metamorphic (gneiss and quartzite), and rare sandstone clasts. Most lonestones are subrounded to subangular. A rough estimate of IRD abundance and general characteristics was determined by visual examination of split cores and the use of X-radiographs. Thirty ~20-cm intervals of core were examined for IRD content using a portable veterinarian X-radiograph unit (see "Lithostratigraphy" in the "Explanatory Notes" chapter).

In general, the number of ice-rafted clasts decreases downhole and isolated lonestones become rare below 500 mbsf (Fig. F6). Unit I contains common pebble-sized lonestones including large (>2 cm diameter) granite, dolerite, gneiss, and sandstone clasts. Dispersed pebbles, sand, and granules are particularly common in Cores 188-1165B-1H, 2H, 3H, 5H, and 6H (0-63.8 mbsf) and are rare in Core 4H (25.8-35.7 mbsf).

Scattered grains and granules are common throughout Subunits IIA and IIC but are less common in Subunit IIB. Gravel-sized lonestones are also less abundant within Subunit IIB. Subunit IIC contains an anomalously high number of lonestones within Core 188-1165B-34X (281.20-290.99 mbsf). This core was fractured and biscuited, and the observed lonestones are present within drilling slurry, suggesting that the high lonestone content within this core could be the result of drilling disturbance and downhole contamination.

In Unit III, isolated lonestones are present from 307.8 to ~500 mbsf. Below this depth lonestones are rare. The interval below 500 mbsf contains thin (<50 cm thick) structureless greenish gray claystone units with dispersed granules and sand grains.

Terrigenous IRD in the form of quartz grains (many with breakage features), lithic grains, and heavy minerals are also conspicuous in the coarse-grained fraction (>125 µm) of core-catcher samples used for foraminifer analysis (see "Appendix"). These results demonstrate that ice-rafted mineral and lithic grains are largely restricted to the sediment column above 500 mbsf (Fig. F26) and imply that the sediment delivery system changed at the 500-mbsf level.

X-radiographs were chosen primarily so we could determine the difference in IRD content within greener and darker intervals from selected cores (188-1165B-10H through 188-1165C-5R). Examination of X-radiographs indicates that greener clays have a higher content of IRD than darker clay intervals; however, more green intervals than dark intervals were examined in the upper part of the hole (Units I and II), and some dark intervals studied do contain common IRD (Table T4).

Although downhole contamination of pebble-sized lonestones is a potential problem in unlithified segments of the succession, we suggest that the combined record of pebble-sized lonestones, granules, and sand grains within the sediments represents background IRD sedimentation caused by glacier ice reaching sea level in East Antarctica. Long-term variations are evident in the flux of IRD at Site 1165 and may result from glacial-interglacial climate variations; however, additional studies are needed to determine the details of these fluctuations. IRD content decreases downhole, has a low abundance within Subunit IIB, and has a much lower concentration in the lower part of Unit III below 500 mbsf. The low abundance of IRD in Subunit IIB suggests that few icebergs were nearby during the time of deposition. Within Units I and II, IRD was identified in X-radiographs within both green and dark intervals. In Unit III, dispersed grains are concentrated within Facies III-2 intervals. Some of these grains may have been introduced by mass-wasting processes, but others seem to be IRD. The small-scale changes in IRD concentration may reflect pulses of sedimentation, perhaps associated with rapid and multiple advance/retreat cycles of glaciers from surrounding regions or changes in fine-grained terrigenous input, resulting in dilution of IRD concentrations.

The majority of lonestones are high-grade metamorphic and granitic igneous rocks typical of large areas of East Antarctica and therefore of limited use in tracking provenance; however, two distinctive rock types are present in trace amounts that indicate a probable source in the Lambert Glacier drainage basin. The first is a red quartzose coarse sandstone that resembles the undated nonmarine red beds penetrated at Site 740 located in inner Prydz Bay. These sediments are part of the fill of the Lambert Graben and Prydz Bay Basin and are unlikely to be present anywhere else in the region. The other is a green phyllite or slate. Such low-grade rocks are known only in the region from the southern Prince Charles Mountains (Tingey, 1991); the northern Prince Charles Mountains and coastal areas are dominated by very high grade metamorphic rocks.

The sediments cored at Site 1165 may be generally interpreted as a succession of muddy contourites and hemipelagic deposits. They are fine grained and lack obvious graded bedding. Silt laminae and ripple crossbedding are present in the lower part of the hole, supporting the contourite interpretation.

Unit III is characterized by interbedded, dark gray, laminated claystones and thin (<50 cm thick) greenish gray claystones, which represent cyclical changes in depositional environments during the early to middle Miocene. Average sedimentation rates based on the age model range up to ~15 cm/k.y. for the lower Miocene part of the succession, and the abundance of silt laminae in the dark gray facies is substantial. Bioturbation is rare and lamination is well preserved, but graded beds were not observed. A possible explanation for the relatively high average sediment accumulation rates found in Unit III is high terrigenous input from East Antarctica. The increasing amount of silt and the presence of silt ripples and cross-bedding indicate stronger bottom-current activity for the lower part of the succession. The silt ripples may represent traction surfaces where the finer fraction has been winnowed during short periods of increased current activity. The greenish gray facies in Unit III contains lonestones and dispersed sand grains. Some intervals have angular mud clasts near the base of the structureless interval, which may be interpreted as rip-up clasts. The lonestones and sand grains may represent IRD, or they may have been introduced by local debris flows, as suggested by the rip-up clasts.

Unit II consists of alternating bioturbated dark and greenish gray clay facies, which probably represent cyclical changes from more hemipelagic facies (Facies II-1) to muddy contourites (Facies II-2). Average sedimentation rates based on the age model are up to ~5 cm/k.y. Similar structureless, strongly bioturbated, fine-grained facies are described from Pliocene-Pleistocene drift sediments in the North Atlantic (Stow and Holbrook, 1984). The subunits of Unit II result from variations in the proportion of more terrigenous-rich (II-2) and more hemipelagic (II-1) facies. Subunit IIC signals a change to lower current regimes and terrigenous input than during the deposition of Unit III. This change is partially reversed in Subunit IIB with an increase in the proportion of Facies II-2, but Facies II-2 does not display the same evidence for current activity as Facies III-2. Subunit IIA contains a greater proportion of hemipelagic sediment than Subunit IIB. The Unit III-Unit II transition also marks the beginning of significant IRD input (Fig. F6).

Unit I consists of hemipelagic and pelagic sediments and has several time breaks in the section (see "Biostratigraphy and Sedimentation Rates"). The composition of Unit I is characterized by an upward decreasing terrigenous component. Below 2.5 mbsf, the terrigenous component is up to 50%, silt laminae are locally present, and the sediments are strongly bioturbated. The uppermost strata (0-2.5 mbsf) have a biogenic content of >50%, consistent with hemipelagic deposition with seasonal ice-free conditions. The age model for the hole (Fig. F27) demonstrates average sedimentation rates of 1.5 cm/k.y. for Unit I. The presence of glauconite and pseudomorphs of Paleogene foraminifers suggests that detritus from erosion of sedimentary sequences on the shelf was supplied to the site. The alternation of colored units suggests the possibility of glacial-interglacial cycles (cf. Barker, Camerlenghi, Acton, et al., 1999); however, further study is needed to establish this cyclicity.

The vertical facies distribution and mineralogical composition of the sediments suggests that a shift in depositional environment occurs at ~300 mbsf, the transition from Unit III to Unit II. The IRD content of the sediments rises, whereas average sedimentation rates progressively decrease throughout Units II and I. The shift to a high relative abundance of clay minerals and the presence of glauconite and shelf-derived foraminifers above ~300 mbsf, in contrast to below, reflect a change in provenance during the middle Miocene. Possible explanations for the uphole decrease in terrigenous sedimentation rates and increase in IRD concentrations in Unit I and II are (1) a decrease in bottom-current activity, inhibiting deposition from bottom currents on the continental rise; (2) a reduction in sedimentation rates related to drift development and its changing relief and morphology; or (3) a decrease in input of fine material by East Antarctic ice because of a reduction in basal melting and a transition to polar ice sheet conditions. Further studies, in combination with evidence from other holes closer to the continent (e.g., Site 1167), will evaluate the relationship between Antarctic glaciation, drift development, and bottom-current activity.