LITHOSTRATIGRAPHY

This section outlines the procedures followed to document the basic sedimentology of the deposits recovered during Leg 188, including core description, X-ray diffraction (XRD), X-radiography, color spectrophotometry, and smear-slide description. Only general procedures are outlined, except where they depart significantly from ODP conventions.

Information from macroscopic description of each core was recorded manually for each core section on visual core-description (VCD) forms. A wide variety of features that characterize the sediment were recorded, including lithology, sedimentary structures, color, and sediment deformation. Compositional data were obtained from smear slides. The color (hue and chroma) of the sediments was determined visually using the Munsell soil color charts (1975). This information was condensed and entered into AppleCORE (version 8.1b) software, which generates a simplified, one-page graphical description of each core (barrel sheet). Barrel sheets are presented with split-core photographs (see the "Core Descriptions" contents list). The lithologies of the recovered sediments are represented on barrel sheets by symbols in the column entitled "Graphic Lithology" (Fig. F3).

Primary sedimentary structures, bioturbation parameters, soft-sediment deformation, structural features, and drilling disturbance are indicated in columns to the right of the graphic log. The symbols are schematic but are placed as close as possible to their proper stratigraphic position. For exact positions of sedimentary features, the more detailed VCDs can be obtained from ODP. Deformation and disturbance of sediment that resulted from the coring process are illustrated in the "Drilling Disturbance" column. Blank regions indicate the absence of coring disturbance. Locations of samples taken for shipboard analysis are indicated in the "Samples" column. A summary lithologic description with sedimentologic highlights is given in the "Description" column of the barrel sheet. This description provides information about the major sediment lithologies; important minor lithologies; and an extended summary description of the sediments, including color, composition, sedimentary structures, trace fossils identified and extent of bioturbation, and other notable characteristics. Descriptions and locations of thin, interbedded, or minor lithologies that could not be depicted in the "Graphic Lithology" column are also presented in "Description," where space permits.

The sediment classification scheme used during Leg 188 is descriptive and follows the ODP classification (Mazullo et al., 1988), with some simplifying modifications for sediments that are mixtures of siliciclastic and biogenic components (Fig. F4) and an additional classification of gravel-rich sediments (Fig. F5). Classification is based primarily on macroscopic description of the cores and examination of smear slides. During Leg 188, the total calcium carbonate content of the sediments determined on board (see "Organic Geochemistry" and "Inorganic Geochemistry") was also used to aid in classification. Composition and texture are the criteria used to define lithology. Genetic terms such as pelagic, neritic, hemipelagic, and debris flow do not appear in this classification. The term clay is used only for particle size and is applied to both clay minerals and other siliciclastic material <4 µm in size.

The principal name applied to a sediment is determined by the component or group of components (e.g., total biogenic carbonate) that comprise(s) >50% of the sediment or rock, except for subequal mixtures of biogenic and siliciclastic material. If the total of a siliciclastic component is >50%, the main name is determined by the relative proportions of sand, silt, and clay sizes when plotted on a modified Shepard (1954) classification diagram (Fig. F4A). Examples of siliciclastic principal names are clay, silt, sand, silty clay, sandy clay, clayey silt, sandy silt, clayey sand, and silty sand. However, if the total of biogenic components is >50% (i.e., siliciclastic material <50%), then the principal name applied is ooze (Fig. F4B). Biogenic components are not described in textural terms. Thus, a sediment with 45% sand-sized foraminifers and 55% siliciclastic clay is called foraminifer clay, not foraminifer clayey sand.

In mixtures of biogenic and nonbiogenic material where the biogenic content is 25%-50% (termed mixed sediments in the ODP classification), the name consists of two parts: (1) a major modifier(s) consisting of the name(s) of the major fossil(s), with the least common fossil listed first, followed by (2) the principal name appropriate for the siliciclastic components (e.g., foraminifer clay) (Fig. F4B). If any component (biogenic or siliciclastic) represents <25% of a sediment, it qualifies for minor modifier status and is hyphenated with the suffix -bearing (e.g., nannofossil-bearing clay). In cases of approximately subequal mixtures of calcareous microfossils, the modifiers calcareous or carbonate bearing can be used instead of microfossil names (e.g., calcareous clay). Examples include 11% foraminifers, 34% nannofossils, and 55% clay = foraminifer-bearing nannofossil clay; 20% diatoms and 80% foraminifers = diatom-bearing foraminifer ooze.

The objectives of Leg 188 were to examine depositional processes and the history of climate change along the glacially influenced continental margin of Prydz Bay. In such settings, an important sediment type is represented by poorly sorted diamict facies. The term diamict is used here as a nongenetic term for materials consisting of matrix-supported admixtures of gravel-sized clasts. It should be noted that existing ODP classifications do not adequately address nonsorted or poorly sorted admixtures of siliciclastic sediments, such as diamicts. During Leg 119, poorly sorted admixtures of gravel and fine-grained sediment were covered by one term (diamictite) (Shipboard Scientific Party, 1989). During Leg 188, the classification of poorly sorted sediments containing gravel (Fig. F5) is based on Moncrieff (1989) to allow distinction of clast-poor and clast-rich facies with different sand contents. Matrix grain size in diamicts was described using the Shepard (1954) diagram in Figure F4A. To determine clast content, we adopted the comparison chart for visual percentage estimation as presented by Mazzullo et al. (1988; their fig. 16). A visual percentage estimate of 10% gravel clasts is taken as the boundary between clast-poor and clast-rich lithologies.

The term clast refers to both sand and gravel-sized components, lonestones are gravel-sized (>2 mm) clasts, and grains are floating silt and sand in a fine matrix. Granules are 2-4 mm in size, and pebbles are >4-mm-large clasts (following the Wentworth scale [1922]). The term lonestone is restricted to gravel-sized clasts in a fine-sediment matrix, whereas pebbles or granules can be present without matrix.

The following classes of induration or lithification were adopted and modified from ODP Leg 105 (Shipboard Scientific Party, 1987). They were separated into three classes for biogenic sediments and two classes for nonbiogenic sediments.

For biogenic sediments and sedimentary rocks, the three classes of induration are

1. Soft: ooze; has little strength and is readily deformed under pressure of a finger or broad blade spatula;
2. Firm: chalk, diatomite, radiolarite; partly lithified and readily scratched with a fingernail or the edge of a spatula; and
3. Hard: limestone, porcellanite, chert; well lithified and cemented, resistant or impossible to scratch with a fingernail or the edge of a spatula.

For nonbiogenic clastic sediments, the two classes of induration are

1. Soft: diamicton, gravel, sand, silt, clay; sediment core can be split with a wire cutter; and
2. Hard: diamictite, conglomerate, sandstone, siltstone, claystone; cannot be compressed with finger pressure, or core must be cut with a band saw or diamond saw. The term diamict is used for both soft and hard sediments and includes diamictons and diamictites.

Relative abundances of the main silicate and carbonate minerals were determined semiquantitatively using a Philips APD 3720 X-ray diffractometer with CuK radiation (Ni filter), operated by Philips software PCAPD 3.0. Each bulk-sediment sample was freeze dried, crushed, and mounted with a random orientation into an aluminum sample holder. Instrument conditions were as follows: 40 kV, 35 mA, goniometer scan from 2° to 70°2 for bulk samples, step size = 0.01°2, scan speed at 1.2°2/min, count time = 0.5 s. Peak intensities were converted to values appropriate for a fixed slit width. An interactive software package (MacDiff 4.0.4 PPC) was used on a Macintosh computer to identify the main minerals. Diffractograms were peak corrected to match the main quartz peak at 3.343 Å. In the absence of quartz, no peak correction was applied. Identifications were based on multiple peak matches using the mineral database provided with MacDiff. Relative proportions of quartz, feldspar, clay minerals, and accessory minerals were plotted using the methods of Forsberg et al. (1999). Relative abundances reported in this volume are useful for general characterization of the sediments, but they are not precise quantitative data.

Clay mineralogy was examined by XRD on separate 3-g samples that were placed in a 50-mL centrifuge tube with 10% acetic acid, sonicated for 15 min, and allowed to stand overnight to remove carbonate material. After centrifuging for 15 min at 1500 rpm, the acetic acid was decanted, 25 mL of distilled water was added, the sample was centrifuged again, and the water was decanted. This washing was repeated two more times to remove salt from the sample. After decanting the final wash water, 25 mL of Calgon solution was added to the sample in a 50-mL beaker. The sample was then placed in a sonic dismembrator for as long as 90 s to suspend the clays by ultrasonic disaggregation then centrifuged for 5 min at 1000 rpm to settle the >2-µm particles. The clays that remained in suspension were removed from the top 1 cm of the centrifuge tube and collected by vacuum filtration on a 0.45-µm Millipore filter. The filter was removed and cut in half to prepare two identically oriented clay slides. The clay was transferred to the slide by placing the filter on the slide and rolling the back of the filter with a small roller. The slides used were high-resolution quartz mounts, cut normal to the C-axis for zero-background analysis. One slide was air dried and analyzed, then solvated with ethylene glycol for at least 12 hr and reanalyzed to determine the presence of expandable clays. The second slide was analyzed after being heated to 550°C for 1 hr to collapse kaolinite and smectite. All oriented clay mounts were scanned from 2° to 35°2 in 0.010° increments.

Petrographic analysis of the sand- and silt-sized components of the sediment was primarily by smear-slide description. Tables summarizing data from smear slides are available (see the "Core Descriptions" contents list). These tables include information about the sample location, whether the sample represents a dominant (D) or a minor (M) lithology in the core, and the estimated percentage ranges of sand, silt, and clay, together with all identified components. We emphasize here that smear-slide analysis provides only crude estimates of the relative abundances of detrital constituents. The mineral identification of finer grained particles can be difficult using only a petrographic microscope, and sand-sized grains tend to be underestimated because they cannot be evenly incorporated into the smear. The mineralogy of smear-slide components was validated by XRD. The relative proportions of carbonate and noncarbonate materials estimated from smear slides was validated by chemical analysis of the sediments (see "Inorganic Geochemistry"). Thin-section descriptions were used to verify the composition of gravel-sized clasts in diamicts and the composition of lonestones. For selected clasts, the texture and lithologic composition are determined (see the "Core Descriptions" contents list).

Reflectance of visible light from the surface of cores was routinely measured downcore using a Minolta spectrophotometer (model CM-2002) mounted on the archive multisensor track (AMST). The AMST measures the archive half of each core section. The purpose of measuring the visible light spectra was to provide a continuous stratigraphic record of color variations downcore for visible wavelengths (VIS 400-700 nm). Spectrophotometer readings were taken after cleaning the surface of each core section. The measurements were then automatically taken and recorded by the AMST, which permits measurements only at evenly spaced intervals along each core. Each measurement consists of 31 separate determinations of reflectance in 10-nm-wide spectral bands from 400 to 700 nm. Additional detailed information about measurement and interpretation of spectral data with the Minolta spectrophotometer can be found in Balsam et al. (1997, 1998, 1999) and Balsam and Damuth (2000).

X-radiography was conducted using a portable VR1020 X-ray unit with a highly focused columnator, designed for field use by veterinarians. Before the cruise, the unit was tested by ODP at Texas A&M University for radiation leakage and was found to have no primary leakage. The Texas A&M group established that the unit was safe for operation without radiation protection, substantiating the manufacturer's claim of safety for this standard operating method.

The X-ray unit was mounted semipermanently above a table covered by lead shielding directly inside the doorway from the core-catwalk into the sediment laboratory (Fig. F6). This location allowed quick transport of newly recovered cores to the X-ray table, if needed, for hydrate studies. X-ray images were made using 8 in × 10 in film packs (Kodak Industrex Ready-Pack II-AA film). Either one or two cores could be imaged simultaneously using the specially prepared wood holders on which cores were placed for the X-radiograph. The film was placed under the cores, with lead-letter identification labels. Measurements of the exact core intervals were made, and a photograph was taken using a digital camera for verification of placement of core and film.

The following exposures were most commonly used to give good X-ray images: for whole-round cores, 50 mA/100 kV (full power); for half-round cores, 50 mA/66 kV. The film was developed by an ODP staff photographer using the same general procedures for developing images from the ship's medical X-ray unit. Specifically, Kodak Tmax RS developer was used at 75°F for 4 min, followed by 30 min in a stop bath; then fixed for 4 min, with two washes at 20 and 10 min; and finally dried.

After the images were dried, they were placed on a light table and again photographed with a digital camera. The X-ray films and paper copies of the core and X-ray photos were submitted to ODP as part of the primary data from the cruise.