Hole 1167A was drilled to a maximum depth of 447.5 mbsf. We recovered a succession of predominately clayey silty sands with dispersed rock clasts and minor beds of coarse sands, clays, and sandy clays. Two lithostratigraphic units are identified (Figs. F3, F4). Unit I is composed of olive and reddish brown clay and sandy clay with minor admixtures of biogenic components. There is a gradational transition into Unit II, which makes up the majority of the section at Site 1167 and is composed of one major facies (Facies II-1) along with three minor facies. Facies II-1 is composed of interbedded, poorly sorted dark gray sandy silt, silty sand, clayey sand, and clast-poor diamicton. Facies II-2 is composed of moderately sorted gray coarse sand. Facies II-3 is composed of dark gray clay with silt laminations. Facies II-4 is composed of green-gray clay with dispersed clasts, abundant foraminifers, and rare nannofossils. Calcium carbonate is a minor component in the matrix sediments throughout the hole and is slightly more abundant in Unit II than in Unit I.
Unit I comprises a relatively short interval at the top of the hole and is composed of olive (5Y 4/3) clay and sandy clay with isolated beds of fine sand and rare lonestones and brown to reddish brown (10YR 4/3) sandy clay (Figs. F3, F5). The sediments are very low in CaCO3 content, with a maximum of only 0.50 wt% (see "Organic Geochemistry"). Unit I contains up to 2% diatoms and 1% sponge spicules (see "Smear Slides").
Diffuse reddish brown color bands (10YR 4/3) are present in several short intervals in Unit I (188-1167A-1H-1, 143-146 cm; 1H-2, 42-58 cm, and 69-76 cm; 1H-3, 50-59 cm; and 1H-3, 140-145 cm). A normally graded sand bed is found in interval 188-1167A-1H-2, 92-106 cm (2.42-2.56 mbsf), and grades upward from granules and very coarse sand at the bottom to medium sand at the top (Fig. F5).
Unit I sediments record a period of hemipelagic deposition. This interpretation is supported by the fine-grained nature of the sediments as well as the presence of diatoms and sponge spicules (1%-2% recorded from smear slides). The lonestones that are present are likely IRD. The normally graded sand bed can be interpreted as the Ta unit in the subdivision of turbidites by Bouma (1962).
At the lower boundary of Unit I, there is a sharp color change and gradational transition from the clays and sandy clays into the coarser sediments of Unit II. Unit II comprises the rest of the section and is composed of four facies. Facies II-1 is the primary component of the unit and consists of diamictons and sediments characterized by abundant dispersed granules and pebbles in a matrix of varying proportions of sand, silt, and clay. Facies II-2 consists of coarse sands, Facies II-3 consists of gray clays and common silt laminations, and Facies II-4 consists of biogenic-rich green-gray clays. These three facies comprise only thin interbeds and collectively account for <2% of the sediment in Unit II. Calcium carbonate content is generally <1 wt% but is relatively higher than in Unit I and ranges from 0.50 to 1.25 wt%. Smear-slide data indicate that quartz and clay minerals are the primary components of the matrix sediment throughout Unit II. Heavy minerals, opaques, and garnet make up the majority of other components, but their percentages decrease below Section 188-1167A-26X-6, 90 cm.
Facies II-1 consists of dark gray (5Y 4/1), very dark gray (5Y 3/1, 10YR 3/1), and reddish gray (5YR 4/2) poorly sorted sandy silt, silty sand, clayey sand, and clast-poor diamicton with dispersed granules and pebbles of varying sizes and lithologies (Figs. F3, F6). The relative proportions of sand, silt, clay, and clasts change frequently, and few clast-supported beds are present.
Lonestones are common throughout Facies II-1 and consist of variable lithologies including granite, granite gneiss, garnet-bearing gneiss, metaquartzite, and sandstone. Dolerite, schist, conglomerate, various minerals, and rare carbonized wood are also present (Fig. F7). The distribution of granite and sandstone lonestones varies systematically. From 425 to 200 mbsf, sandstone lonestones are more abundant than above 200 mbsf, whereas granite lonestones are more abundant from 200 mbsf to the top of the hole (Fig. F8).
The highest concentrations of rock clasts (>5%) are within interval 188-1167A-19X-2, 0 cm, through 24X-1, 88 cm. Several decimeter- to meter-scale successions of clast-poor diamicton are present in this interval. Clast-poor diamictons are also present in intervals 188-1167A-5H-4, 25-90 cm; 30X-4, 28-41 cm; 37X-CC, 0-10 cm; and 43X-2, 0-90 cm. Diamictons have a sandy silt or silty sand matrix. Gravel beds are interbedded with poorly sorted sandy silt in interval 188-1167A-19X-1, 124 cm, through 19X-2, 130 cm (151.6-153.1 mbsf), and are characterized by subangular to subrounded granules and small pebbles. One gravel bed in interval 188-1167A-19X-2, 50-80 cm (150.8-151.1 mbsf), is composed of ~60% clasts and 40% matrix material (Fig. F9). Additionally, sand and gravel concentrations are present in intervals 188-1167A-13X-2, 60-65 cm; 13X-4, 30-44 cm, and 65-67 cm; and 41X-3, 54-57 cm.
Numerous color alternations of dark gray (5Y 4/1) and dark reddish gray (5YR 4/2) are present in interval 188-1167A-10X-1, 8 cm, through 13X-4, 21 cm (64.38-97.59 mbsf), and minor reddish gray color banding is present in interval 1167A-48X-2, 30-40 cm (430.1-430.2 mbsf). The color contacts are fairly sharp and are both planar and wavy. No apparent lithologic change or sedimentary structures are associated with the color transitions (Fig. F10). Facies II-1 is barren of diatoms and radiolarians, whereas foraminifers are a minor component in this facies (see "Biostratigraphy and Sedimentation Rates").
Facies II-2 consists of moderately sorted gray coarse sand with rare to common dispersed granules (Fig. F3). Grains are subrounded and predominantly composed of quartz, K-feldspar, and mafic minerals. The first occurrence of sand is at the top of the core in interval 188-1167A-22X-1, 0-50 cm (179.2-179.7 mbsf), and drilling operation contamination cannot be ruled out. Dispersed mud clasts as large as 2 cm in diameter are common in the Facies II-2 sand bed in interval 188-1167A-37X-1, 0 cm, through 37X-3, 89 cm (322.5-326.39 mbsf) (Fig. F11). The sand in interval 188-1167A-39X-1, 0 cm, through 39X-2, 70 cm, of Facies II-2 (341.7-343.9 mbsf) displays slight normal grading from very coarse to coarse sand. Rock granules are rare in this sand interval.
Facies II-3 consists of decimeter-scale beds of dark gray (5Y 4/1 and N4) clay and clay with thin (<1 mm) silt laminae and burrowed intervals (Figs. F3, F12). In intervals 188-1167A-25X-1, 88-98 cm (208.88-208.98 mbsf), and 25X-1, 104-136 cm (209.04-209.36 mbsf), the Facies II-3 clay intervals contain rare sand grains and are barren of lonestones. At the top of interval 188-1167A-5H-3, 10-32 cm (36.8-37.02 mbsf), cross-stratification within the clay laminae is observed along with a few discontinuous silt laminae at interval 188-1167A-5H-3, 14-21 cm. There are sharp contacts at the top and the base of this facies (Fig. F13). Within interval 188-1167A-25X-6, 129 cm, through 25X-CC, 21 cm (216.79-217.64 mbsf), there are numerous 1- to 2-mm-thick discontinuous silt laminae. Some silt laminae are subparallel or convergent and indicate cross-bedding (Fig. F12).
Facies II-4 consists of centimeter- to decimeter-scale beds of greenish gray (5GY 4/1) to dark gray (N4) sandy clay with dispersed rock granules. This facies is present in intervals 188-1167A-5H-3, 32-40 cm (37.02-37.1 mbsf); 25X-1, 64-88 cm (208.64-208.88 mbsf); 25X-1, 136-145 cm (209.36-209.45 mbsf); and 25X-CC, 21-25 cm (217.64-217.68 mbsf) (Fig. F3). The upper contact of Facies II-4 is sharp, and the lower contact is gradational to sharp (Fig. F14). In contrast to the other facies, the biogenic content of Facies II-4 is relatively high. Foraminifers are abundant and nannofossils are common.
In Cores 188-1167A-5H and 25X, a succession of alternating coarse- and fine-grained facies is present. In this succession, Facies II-1 dark gray silty sands and clayey sands have sharp lower contacts with the planar, cross-laminated dark gray clays of Facies II-3. At the base of Facies II-3, there is a sharp contact with decimeter-thick greenish gray clays (Facies II-4) (Figs. F3, F15). The succession ends with sharp contacts between Facies II-1 and II-4 sediments. This succession is present in four intervals: 188-1167A-5H-3, 10-40 cm; 25X-1, 64-98 cm, and 104-145 cm; 25X-6, 129 cm; and 25X-CC, 25 cm. In one interval (188-1167A-25X-1, 64-98 cm), a bed of Facies II-4 sediment also overlies Facies II-3 (Fig. F3).
Facies II-1 and II-2 record deposition by mass-transport processes, probably massive debris flows. Debris-flow deposits are evidenced by poor sorting, abundant matrix-supported clasts and mud clasts, a general absence of visible grading, and a lack of pelagic biogenic components. The debris flows may represent deposition during glacial periods when ice extended to the shelf break and could deliver large volumes of sediment to the upper continental slope. At present, the thickness and frequency of individual flows are uncertain. The only apparent breaks in debris-flow deposits are in Cores 188-1167A-5H and 25X, where Facies II-3 and II-4 are found (Fig. F3); however, contacts between additional individual flows may not be megascopically recognizable.
The thin intervals of fine-grained sediments in Facies II-3 and II-4 indicate a change in the mode of sedimentation and a break in debris-flow deposition. Clays with silt laminae are similar in appearance and composition to the sediments of lithostratigraphic Unit III at Site 1165 (see "Lithostratigraphy"), which are interpreted as muddy contourites. Therefore, Facies II-3 may record short intervals when contour currents were active on the fan. The nature of the silt laminae and bioturbation within Facies II-3 would argue against deposition by turbidity currents. The abundance of pelagic foraminifers in Facies II-4 suggests pelagic deposition, whereas the dispersed sand and granules suggest deposition of IRD. This facies thus appears to represent short intervals, possibly interglacials, when mass transport and contour-current deposition were interrupted and pelagic deposition was dominant.
Lonestones from Site 1167 (>1 cm largest visible diameter on the cut face of each core) were cataloged and assigned to one of 16 lithologic varieties (Table T3). Figure F4 illustrates the downhole variation in average size of the lonestone clasts. In general, size variations seem to be random with only a minor increase downhole from 0 to ~200 mbsf. The number of lonestones per meter of core remains fairly consistent from 0 to ~160 mbsf, below which there is a series of downward increases in lonestone concentration from 160 to 210, 300 to 320, and 410 to 420 mbsf. A slight overall downhole increase in lonestones per meter is also noted below 200 mbsf (Fig. F4).
The distribution of the 16 lithologies was plotted against depth to examine possible changes in provenance of the penetrated sediments (Fig. F7). In general, the vertical distribution of lithology groups appears to be random; however, closer inspection of items 6 (granite group) and 7 (sandstone group) suggests that this is not the case. These two lithology groups, granite (including diorites and gabbros for the purposes of binning) and sandstone, show systematic changes in distribution downhole (Fig. F8). The granite group is most abundant in the upper part of the hole and decreases downhole between 0 and ~200 mbsf. Below this point, the abundance of granite group clasts is significantly reduced. In contrast, the sandstone group clasts demonstrate the opposite trend: few or no sandstone clasts appear down to ~180 mbsf, below which the number of sandstone clasts per meter of core increases significantly (Fig. F7). This type of distribution (i.e., an inverse relationship between clast groups) suggests the possibility that two different source areas delivered material to Site 1167 and that one source area (granite) gave way to the other (sandstone). In addition to the variation in sandstone and granite distributions, a greater abundance of schist clasts (item 15 in Fig. F7) and dark clasts of unknown composition (item 14 in Fig. F7) above 180 mbsf may also indicate a change in the predominance of source areas.
Blue gneiss with cordierite is present in Section 188-1167A-46X-CC, 25 cm. Blue gneiss with cordierite is a conspicuous lithology in the Larseman Hills region of Princess Elizabeth Land (Tingey, 1991); therefore, the blue gneiss in Core 188-1167A-46X is possible evidence that this region was a source area for the recovered sediments at Site 1167.
A possible bituminite clast is present in Section 188-1167A-30X-3, 105 cm (~260 mbsf) (Fig. F7). This 2-cm elongate clast is black, rounded, striated, and malleable and exhibits a low specific gravity. Possible sources of this clast are unclear.
The mineralogy of smear slides reveals no trend at this site. Similarly, the grain-size distribution within the mud-sized sediments and the matrix of the coarser grained varieties estimated using smear slides presents no systematic trend (Fig. F16). Several peaks in the abundance of clay-sized and silt-sized components reflect minor changes in sediment type as described in the body of the report.
At Site 1167, 44 samples were analyzed for bulk mineralogy and 13 samples were analyzed for clay minerals. XRD bulk mineralogy data show that the sediments are primarily composed of quartz, plagioclase, K-feldspar, and a mixture of clay minerals (Fig. F4). In Sample 188-1167A-9X-1, 25-26 cm, a minor amount of hornblende and calcite is present. Total clay content remains relatively constant throughout the hole. Total quartz content in the upper 208 mbsf is slightly less than in deeper portions of the hole, whereas from 208 mbsf downhole, the relative abundance of plagioclase decreases and the abundance of K-feldspar stays constant. Poor sorting and differences in abundance of very coarse material in sediments may cause the slight irregular variability in quartz content downhole.
Thirteen samples were chosen for an overview of clay mineralogy changes in Units I and II. These samples were taken from the clay of Unit I, clayey silty sands and diamictons of Unit II, and three thin clay beds in intervals 188-1167A-5H-3, 12-13 cm; 25X-1, 126-127 cm; and 25X-7, 17-18 cm. In Unit I, an olive-gray clay (4.93 mbsf; Sample 188-1167A-1H-4, 43-44 cm) contains kaolinite, smectite, and illite, with some clay-sized quartz, plagioclase, and K-feldspar (Fig. F17). A sample from the underlying dark gray poorly sorted sandy clay with some dispersed granules of Facies II-1 (36.82 mbsf; Sample 188-1167A-5H-3, 12-13 cm) mainly consists of illite, kaolinite, and minor chlorite (Fig. F17). Smectite is absent, and there is more illite and less kaolinite than found in the clays of Unit I. The gray to greenish gray clay bed of Facies II-4 (209.26 mbsf; Sample 188-1167A-25X-1, 126-127 cm) exhibits a very similar clay mineral distribution to the sample at the top of the hole in Unit I; however, a Facies II-3 sample of the dark clay with few silt laminae (217.2 mbsf; Sample 188-1167A-25X-7, 17-18 cm) shows a distinctively different clay-mineral assemblage including kaolinite, less illite, and no smectite (Fig. F18).
The Facies II-1 color-banded reddish gray poorly sorted clayey sand with dispersed clasts (64.61 mbsf; Sample 188-1167A-10R-1, 31-32 cm) contains kaolinite, illite, and smectite (Fig. F19). The most typical lithology of Facies II-1 (Samples 188-1167A-14X-1, 39-40 cm; 27X-1, 39-40 cm; 33X-2, 43-44 cm; 38X-1, 58-59 cm; 43X-2, 104-105 cm; 47X-1, 55-56 cm; and 48X-2, 35-38 cm) demonstrates these same clay minerals; however, the ratio of smectite to illite varies slightly and compared to the abundance of kaolinite remains fairly constant throughout the section (Fig. F19). Below 382 mbsf, the abundance of smectite is higher compared to upper portions of the sediments. The poorly sorted sandy silt to silty sand of Facies II-1 (429.13 mbsf; Sample 188-1167A-48X-1, 83-84 cm) contains predominantly kaolinite, illite, smectite, and also a minor amount of chlorite (Fig. F19).
The presence of smectite in clays of Facies II-4 at two different horizons (4.93 and 209.26 mbsf), coupled with the presence of biogenic components, suggests more hemipelagic sedimentation in these intervals. The clay-rich intervals with silty laminae of Facies II-3 (36.82 and 217.2 mbsf) are dominated by illite and resemble those overlying and underlying poorly sorted sandy silts to silty sands of Facies II-1, which suggests a similar sediment source for both these clays and the debris flows. Slight changes in illite/smectite ratios may relate to variations in sediment sources or may result from different glacial and gravitational flow processes (cf. Ehrmann and Fütterer, 1994). Thus, changes in the clay-mineral assemblages in trough-mouth fan deposits may provide indirect information about periods of glacial advances to the shelf edge during late Pliocene-Pleistocene time; however, smectite dominance in the hole below 382 mbsf is probably more directly linked to sediment-source characteristics, as is the slightly higher total quartz content below 208 mbsf. Overall, the presence of kaolinite relates only to the source area characteristics, where chemically weathered basement and sedimentary rocks were common. During the Pliocene-Pleistocene, less weathered sources were likely available, and these provided the various amounts of illite and smectite observed in the sediments.
Subtle changes in composition downcore at Site 1167 may suggest important implications for the glacial history in the Prydz Bay region. Unit I, the thin Holocene to upper Pleistocene hemipelagite interval, records the most recent deposition on the slope and represents interglacial conditions when fine particles and biogenic material settled out of the water column, and IRD was supplied by icebergs.
Unit II records a thick succession of debris flows on the slope. The relatively thin intervals of Facies II-3 and II-4 clays represent relatively short periods when conditions changed and an alternate form of sedimentation (i.e., current-driven or hemipelagic) was preserved. It is possible that these intervals represent changes from glacial to interglacial conditions or minor fluctuations during a glacial period.
Changes in sediment composition can be identified on different scales at Site 1167. Repetitive changes in sediment composition may be caused by short-term climate cyclicity, perhaps advances and retreats of the grounding line without major shifts in glacial flow patterns. Slight changes in illite/smectite ratios may be related to these grounding-line processes; however, in sediment from the lower part of the hole, smectite dominance is probably related more to source-area characteristics than to grounding-line processes. Large-scale shifts in sediment composition may be related to major rearrangements within the Lambert Glacier-Amery Ice shelf drainage system.
Several lithologic downhole parameters change at 200-210 mbsf. Lonestone data show a significant upward change from sandstone to granite stones at ~200 mbsf, which may suggest a shift in sediment provenance. XRD bulk mineralogy shows a higher abundance of plagioclase above 210 mbsf than below, whereas quartz is more abundant below 210 mbsf than above. Clay mineralogy data shows higher kaolinite/illite ratios for the lower part of the hole than found above 210 mbsf. Additionally, magnetic susceptibility (see "Paleomagnetism"), grain density and porosity (see "Physical Properties"), natural gamma-ray data (see "Downhole Measurements"), and data from foraminifer residues (see "Appendix") all exhibit distinctive changes near 210 mbsf.